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| United States Patent |
6,335,814
|
|
Fuse
, et al.
|
January 1, 2002
|
Optical transmission system and optical transmitter and optical receiver
used therefor
Abstract
An angle modulating portion 1 converts an inputted electrical signal into a
predetermined angle-modulated signal. An optical modulating portion 2
converts the angle-modulated signal outputted from the angle modulating
portion 1 into an optical-modulated signal and sends the optical-modulated
signal to an optical waveguide portion 3. An interference portion 6
separates the optical-modulated signal transmitted through the optical
waveguide portion 3 into two optical signals having predetermined
difference in propagation delay and then combines the optical signals. An
optical/electrical converting portion 4 subjects the combined optical
signal to homodyne detection, to acquire a demodulated signal of the
original electrical signal and output the electrical signal. That is, the
interference portion 6 and the optical/electrical converting portion 4
constitute a delayed detection system of an optical signal, so that the
delayed detection system performs conversion processing of an optical
signal into an electrical signal and angle demodulation processing
simultaneously. In this way, a signal with a wide-band and a
high-frequency can be acquired by demodulation without electrical part for
wide-bands and high-frequencies.
| Inventors:
|
Fuse; Masaru (Toyonaka, JP);
Ohya; Jun (Osaka, JP)
|
| Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
136934 |
| Filed:
|
August 20, 1998 |
Foreign Application Priority Data
| Aug 22, 1997[JP] | 9-226291 |
| Mar 11, 1998[JP] | 10-060135 |
| Apr 30, 1998[JP] | 10-121498 |
| Current U.S. Class: |
398/201; 398/9; 398/161; 398/186; 398/188 |
| Intern'l Class: |
H04B 010/04 |
| Field of Search: |
359/182-183,140,173,188
|
References Cited [Referenced By]
U.S. Patent Documents
| 4959826 | Sep., 1990 | Smith | 370/1.
|
| 5541755 | Jul., 1996 | Noe et al. | 359/110.
|
| 5625479 | Apr., 1997 | Suzuki et al. | 359/135.
|
| 6271950 | Aug., 2001 | Hansen et al. | 359/135.
|
| Foreign Patent Documents |
| 7-73639 | Mar., 1995 | JP.
| |
Other References
K. Kikushima et al., "Optical Super Wide-Band FM modulation Scheme and Its
application to Multi-Channel AM Video Transmission Systems", IOOC '95,
PD2-7, pp. 33 and 34.
|
Primary Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Wenderoth, Lind & Ponack, L.L.P.
Claims
What is claimed is:
1. An optical transmission system for optically transmitting an
angle-modulated signal, comprising:
an optical modulating portion for converting said angle-modulated signal
into an optical-modulated signal;
an interference portion for separating said optical-modulated signal into a
plurality of optical signals having predetermined difference in
propagation delay and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
said interference portion into an electrical signal,
said interference portion and said optical/electrical converting portion
constituting a delayed detection system of an optical signal, and the
delayed detection system performing conversion processing of an optical
signal into an electrical signal and angle demodulation processing
simultaneously.
2. The optical transmission system according to claim 1, wherein said
angle-modulated signal is an FM signal obtained by subjecting an analog
signal to frequency modulation.
3. The optical transmission system according to claim 1, wherein said
angle-modulated signal is a PM signal obtained by subjecting an analog
signal to phase modulation.
4. The optical transmission system according to claim 1, wherein said
angle-modulated signal is an FSK modulated signal obtained by subjecting a
digital signal to frequency modulation.
5. The optical transmission system according to claim 1, wherein said
angle-modulated signal is a PSK modulated signal obtained by subjecting a
digital signal to phase modulation.
6. The optical transmission system according to claim 1, wherein said
optical modulating portion generates an optical-intensity-modulated signal
as said optical-modulated signal.
7. The optical transmission system according to claim 6, wherein
said optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from said light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from said optical branch portion respectively, for
subjecting the outputted lights to optical phase modulation using said
angle-modulated signal as an original signal; and
an optical coupling portion for combining the two optical-phase-modulated
signals outputted from said first and second optical phase modulating
portions.
8. The optical transmission system according to claim 6, wherein
said interference portion comprises:
an optical branch portion for branching an inputted optical signal into a
first optical signal and a second optical signal;
an optical delay portion for providing the second optical signal outputted
from said optical branch portion with a predetermined delay; and
an optical combining portion for combining the first optical signal
outputted from said optical branch portion and the second optical signal
outputted from said optical delay portion.
9. The optical transmission system according to claim 6, wherein
said optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from said light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from said optical branch portion respectively, for each
subjecting each of the outputted lights to optical phase modulation using
said angle-modulated signal as an original signal; and
an optical directional coupling portion for combining the two
optical-phase-modulated signals outputted from said first and second
optical phase modulating portions and then dividing the resultant signal
into first and second optical signals in which optical-intensity modulated
components are set in opposite phases to each other, and
said interference portion comprises:
an optical delay portion for providing the second optical signal outputted
from said optical directional coupling portion with a predetermined delay;
and
an optical combining portion for combining the first optical signal
outputted from said optical directional coupling portion and the second
optical signal outputted from said optical delay portion.
10. The optical transmission system according to claim 6, wherein
said interference portion comprises:
an optical waveguide portion for guiding the optical signal outputted from
said optical modulating portion; and
first and second optical transparent/reflecting portions, cascaded on said
optical waveguide portion at a prescribed interval, for respectively
transmitting parts of the inputted optical signals and reflecting the
remained parts, and
propagation time in which an optical signal goes and returns between said
first and second optical transparent/reflecting portions is said
predetermined difference in propagation delay.
11. The optical transmission system according to claim 1, wherein said
optical modulating portion generates an optical-amplitude-modulated signal
as said optical-modulated signal.
12. The optical transmission system according to claim 11, wherein
said optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from said light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from said optical branch portion respectively, for each
subjecting each of the outputted lights to optical phase modulation using
said angle-modulated signals as an original signal; and
an optical coupling portion for combining the two optical-phase-modulated
signals outputted from said first and second optical phase modulating
portions.
13. The optical transmission system according to claim 11, wherein
said interference portion comprises:
an optical branch portion for branching the inputted optical signal into a
first optical signal and a second optical signal;
an optical delay portion for providing the second optical signal outputted
from said optical branch portion with a predetermined delay; and
an optical combining portion for combining the first optical signal
outputted from said second optical branch portion and the second optical
signal outputted from said optical delay portion.
14. The optical transmission system according to claim 11, wherein
said optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from said light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from said optical branch portion respectively, for each
subjecting each of the outputted lights to optical phase modulation using
said angle-modulated signal as an original signal; and
an optical directional coupling portion for combining the two
optical-phase-modulated signals outputted from said first and second
optical phase modulating portions and then dividing the resultant signal
into first and second optical signals in which optical-amplitude-modulated
components are set in opposite phases to each other, and
said interference portion comprises:
an optical delay portion for providing the second optical signal outputted
from said optical directional coupling portion with a predetermined delay;
and
an optical combining portion for combining the first optical signal
outputted from said optical directional coupling portion and the second
optical signal outputted from said optical delay portion.
15. The optical transmission system according to claim 11, wherein
said interference portion comprises:
an optical waveguide portion for guiding the optical signal outputted from
said optical modulating portion; and
first and second optical transparent/reflecting portions, cascaded on said
optical waveguide portion at a predetermined interval, for respectively
transmitting parts of the inputted optical signals and reflecting the
remained parts, and
propagation time in which an optical signal goes and returns between said
first and second optical transparent/reflecting portions is said
predetermined difference in propagation delay.
16. The optical transmission system according to claim 12, wherein
predetermined optical phase modulation is performed in said first and
second optical phase modulating portions so that difference between the
optical phase shift by said first optical phase modulating portion and the
optical phase shift by said second optical phase modulating portion is set
in phase with said angle-modulated signal.
17. The optical transmission system according to claim 14, wherein
predetermined optical phase modulation is performed in said first and
second optical phase modulating portions so that difference between the
optical phase shift by said first optical phase modulating portion and the
optical phase shift by said second optical phase modulating portion is set
in phase with said angle-modulated signal.
18. The optical transmission system according to claim 12, wherein
predetermined optical phase modulation is performed in said first and
second optical phase modulating portions so that difference between the
optical phase shift by said first optical phase modulating portion and the
optical phase shift by said second optical phase modulating portion is set
in opposite phases with said angle-modulated signal.
19. The optical transmission system according to claim 14, wherein
predetermined optical phase modulation is performed in said first and
second optical phase modulating portions so that difference between the
optical phase shift by said first optical phase modulating portion and the
optical phase shift by said second optical phase modulating portion is set
in opposite phases with said angle-modulated signal.
20. The optical transmission system according to claim 1, wherein a product
value of a center angular frequency of said angle-modulated signal and the
predetermined difference in propagation delay in said interference portion
is set to be equal to .pi./2.
21. The optical transmission system according to claim 4, wherein the
predetermined difference in propagation delay in said interference portion
is set to be equal to one symbol length of said digital signal.
22. The optical transmission system according to claim 5, wherein the
predetermined difference in propagation delay in said interference portion
is set to be equal to one symbol length of said digital signal.
23. The optical transmission system according to claim 8, wherein
polarization states of the first optical signal and the second optical
signal to be combined in said optical combining portion are set to be the
same with each other.
24. The optical transmission system according to claim 9, wherein
polarization states of the first optical signal and the second optical
signal to be combined in said optical combining portion are set to be the
same with each other.
25. The optical transmission system according to claim 13, wherein
polarization states of the first optical signal and the second optical
signal to be combined in said optical combining portion are set to be the
same with each other.
26. The optical transmission system according to claim 14, wherein
polarization states of the first optical signal and the second optical
signal to be combined in said optical combining portion are set to be the
same with each other.
27. The optical transmission system according to claim 10, wherein
polarization states of the optical signal transmitting through said first
and second optical transparent/reflecting portions along said optical
waveguide portion and the optical signal transmitting through said first
optical transparent/reflecting portion, reflected at said second optical
transparent/reflecting portion, reflected at said first optical
transparent/reflecting portion and transmitting through said second
optical transparent/reflecting portion are set to be the same with each
other.
28. The optical transmission system according to claim 15, wherein
polarization states of the optical signal transmitting through said first
and second optical transparent/reflecting portions along said optical
waveguide portion and the optical signal transmitting through said first
optical transparent/reflecting portion, reflected at said second optical
transparent/reflecting portion, reflected at said first optical
transparent/reflecting portion and transmitting through said second
optical transparent/reflecting portion are set to be the same with each
other.
29. The optical transmission system according to claim 8, wherein
said optical modulating portion and said interference portion are connected
with a first optical waveguide portion,
said interference portion and said optical/electrical converting portion
are connected with a second optical waveguide portion, and
said first and/or second optical waveguide portions are composed of
single-mode optical fibers.
30. The optical transmission system according to claim 13, wherein
said optical modulating portion and said interference portion are connected
with a first optical waveguide portion,
said interference portion and said optical/electrical converting portion
are connected with a second optical waveguide portion, and
said first and/or second optical waveguide portions are composed of
single-mode optical fibers.
31. The optical transmission system according to claim 9, wherein
said interference portion and said optical/electrical converting portion
are connected with an optical waveguide portion, and
said optical waveguide portion is composed of a single-mode optical fiber.
32. The optical transmission system according to claim 14, wherein
said interference portion and said optical/electrical converting portion
are connected with an optical waveguide portion, and
said optical waveguide portion is composed of a single-mode optical fiber.
33. The optical transmission system according to claim 10, wherein a whole
or a part of the optical waveguide portion in said interference portion is
composed of a single-mode optical fiber.
34. The optical transmission system according to claim 15, wherein a whole
or a part of the optical waveguide portion in said interference portion is
composed of a single-mode optical fiber.
35. The optical transmission system according to claim 1, further
comprising an amplitude adjusting portion for adjusting an amplitude of
said angle-modulated signal and outputting said angle-modulated signal of
a constant amplitude.
36. The optical transmission system according to claim 1, further
comprising a bandwidth limiting portion for limiting a band of said
angle-modulated signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical transmission systems, more
specifically to a system for optically transmitting an angle-modulated
signal.
2. Description of the Background Art
FIG. 30 is a block diagram showing an example of the configuration of a
conventional optical transmission system which transmits an
angle-modulated signal. In FIG. 30, the optical transmission system
includes an angle modulating portion 1, an optical modulating portion 2,
an optical waveguide portion 3, an optical/electrical converting portion
4, an angle demodulating portion 5 and a filter F. Such optical
transmission system is described, for example, in a document (K.
Kikushima, et al., "Optical Super Wide-Band FM Modulation Scheme and Its
Application to Multi-Channel AM Video Transmission Systems", IOOC'95,
PD2-7, 1995, pp. 33-34.).
Next, the operation of the conventional optical transmission system
structured as above will be described. As an electrical signal inputted to
the angle modulating portion 1, assumed is an analog signal such as an
audio or video signal, or a digital signal such as computer data and the
like. The angle modulating portion 1 converts the inputted electrical
signal into an angle-modulated signal with a predetermined frequency and a
predetermined angle modulation scheme to output the angle-modulated
signal. The angle modulation scheme includes FM (frequency modulation) or
PM (phase modulation) for an analog signal and FSK (frequency-shift
keying) or PSK (phase-shift keying) for a digital signal, and is
generically referred to as angle modulation hereinafter. The optical
modulating portion 2 converts the inputted angle-modulated signal into an
optical-modulated signal to output the optical-modulated signal. The
optical/electrical converting portion 4, which includes a photodetector
having square-law-detection characteristics (a pin photo-diode, an
avalanche photo-diode or the like), re-converts the optical-modulated
signal transmitted by the optical waveguide portion 3 into an electrical
signal to output an angle-modulated signal. The angle demodulating portion
5 converts variations in frequency (or variations in phase) of the
angle-modulated signal into variations in amplitude (or variations in
intensity) of an electrical signal, thereby re-generating a signal
correlating with the original electrical signal. The filter F passes only
a signal component corresponding to the original electrical signal (that
is a signal component of the same frequency band as that of the original
electrical signal) among signals outputted from the angle demodulating
portion 5.
In FIG. 31 is shown an example of the structure of the angle demodulating
portion 5 in FIG. 30. In FIG. 31, the angle-modulated signal inputted from
the optical/electrical converting portion 4 is branched into two signals
in a branch portion 51. One signal of the two signals obtained by the
branch is provided with a predetermined delay T.sub.p in a delay portion
52. A mixing portion 53, which is generally constituted by a mixer and the
like, receives the other signal outputted from the branch portion 51 and
the signal outputted from the delay portion 52 to generate a product
signal of these signals and output the product signal.
The conventional optical transmission system of the angle-modulated signal
as described above has an advantage in the following, compared with an
optical transmission system of an amplitude-modulated (AM) signal. That
is, the frequency deviation (or the phase deviation) of the
angle-modulated signal is set larger, so that a larger gain in angle
modulation can be acquired at the optical transmission. As a result, SNR
(signal-to-noise power ratio) of a demodulated signal increases, realizing
transmission of a signal of good quality. Moreover, the frequency
deviation (or the phase deviation) of the angle-modulated signal is
increased to spread a frequency spectrum of the optical-modulated signal
and suppress a peak level of the frequency spectrum, which leads to an
advantage in that deterioration of signal quality due to multipath
reflection on an optical transmission line is reduced.
As described above, in the conventional optical transmission system, an
electrical signal to be transmitted, after being subjected to angle
modulation, is converted into an optical-modulated signal to be optically
transmitted, subjected to square-law-detection on a receiving side to be
re-converted into an angle-modulated signal, and further subjected to
angle demodulation to be the original electrical signal. Therefore, it is
possible, in the conventional optical transmission system, to perform
optical transmission of better quality by increasing the frequency
deviation (the phase deviation) even on an optical transmission line of
poor quality.
However, increasing in the frequency deviation (or the phase deviation) of
the angle-modulated signal makes the frequency and band of the
angle-modulated signal higher and wider. Accordingly, the conventional
optical transmission system as described above, requires electrical parts
for high frequencies and wide-bands in order to constitute the angle
modulating portion 1 and the angle demodulating portion 5. Connection and
matching among such electrical parts for high frequencies and wide-bands
are difficult and multipath reflection among the parts readily occurs.
This causes deterioration of characteristics of the angle modulating
portion 1 and the angle demodulating portion 5, resulting in significant
deterioration of quality of modulated/demodulated signals.
Further, in the case where an expensive electrical part for wide-bands and
high frequencies (for example, the branch portion 51 and the mixing
portion 53 in FIG. 31) is used in the angle demodulating portion 5 which
is installed as a receiving terminal of an optical transmission system,
when configuring an optical subscriber (optical multi-distribution) system
such as a FTTH (Fiber To The Home) system, a CATV network and the like,
the system cost per subscriber becomes very high to significantly degrade
the system from the view point of its economy.
As explained in the above, the conventional optical transmission system is
required, when optically transmitting an angle-modulated signal with a
wider-band and a higher frequency, to use the electrical parts for
wide-bands and high frequencies especially as constituents of the
demodulating portion. Thereby, the conventional optical transmission
system has a specific problem in that group delay characteristics and
modulation/demodulation characteristics are easily deteriorated and
economy of overall system is significantly degraded because of increase in
the cost of the receiving terminal.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an optical
transmission system which realizes good angle demodulation characteristics
by adopting new optical signal processing and is greatly economical by
constituting a receiving terminal at lower cost without electrical parts
for wide-bands and high-frequencies.
The present invention has features described below in order to attain the
above-mentioned object.
A first aspect of the present invention is an optical transmission system
for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal
into an optical-modulated signal;
an interference portion for separating the optical-modulated signal into a
plurality of optical signals having predetermined difference in
propagation delay and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal,
the interference portion and the optical/electrical converting portion
constituting a delayed detection system of an optical signal, and the
delayed detection system performing conversion processing of an optical
signal into an electrical signal and angle demodulation processing
simultaneously.
In the case where an electrical circuit using parts for wide-bands and
high-frequencies is adopted as a demodulation device for an
angle-modulated signal, connection or matching among the parts are
difficult, causing deterioration of linearities of demodulation
characteristics or group delay characteristics easily to degrade the
quality of an demodulated signal. Moreover, the parts for wide-bands and
high-frequencies are generally expensive, so that the cost of the
demodulation device increases, significantly deteriorating the economy of
the system.
Hence, in the above first aspect, an angle-modulated signal is converted
into an optical-modulated signal and the optical-modulated signal is
homodyne detected employing square-law-detection characteristics of a
photodetector, so that demodulation and optical transmission can be
performed only by optical signal processing without using electrical parts
for wide-bands and high-frequencies. Further, when the present aspect is
applied to an optical distribution system, the portions in the
configuration up to the interference portion are installed on a
transmitting equipment side and only the optical/electrical converting
portion is installed on a receiving terminal side, whereby the expensive
constituents are included in only the transmitting equipment. Thus, it is
possible to construct an optical subscriber system which is greatly
economical.
A second aspect is an aspect according to the first aspect, wherein the
angle-modulated signal is an FM signal obtained by subjecting an analog
signal to frequency modulation.
A third aspect is an aspect according to the first aspect, wherein the
angle-modulated signal is a PM signal obtained by subjecting an analog
signal to phase modulation.
A fourth aspect is an aspect according to the first aspect, wherein the
angle-modulated signal is an FSK modulated signal obtained by subjecting a
digital signal to frequency modulation.
A fifth aspect is an aspect according to the first aspect, wherein the
angle-modulated signal is a PSK modulated signal obtained by subjecting a
digital signal to phase modulation.
A sixth aspect is an aspect according to claim 1, wherein the optical
modulating portion generates an optical-intensity-modulated signal as the
optical-modulated signal.
A seventh aspect is an aspect according to the sixth aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from the light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from the optical branch portion respectively, for
subjecting the outputted lights to optical phase modulation using the
angle-modulated signal as an original signal; and
an optical coupling portion for combining the two optical-phase-modulated
signals outputted from the first and second optical phase modulating
portions.
As described in the foregoing, in the seventh aspect, in order to generate
an optical-intensity-modulated signal, an external modulation scheme is
adopted. In place of such external modulation scheme, a direct modulation
scheme can be also adopted.
An eighth aspect is an aspect according to the sixth aspect, wherein
the interference portion comprises:
an optical branch portion for branching an inputted optical signal into a
first optical signal and a second optical signal;
an optical delay portion for providing the second optical signal outputted
from the optical branch portion with a predetermined delay; and
an optical combining portion for combining the first optical signal
outputted from the optical branch portion and the second optical signal
outputted from the optical delay portion.
As described in the foregoing, in the eighth aspect, the inputted optical
signal is branched into two optical signals by the optical branch portion,
the predetermined propagation delay is provided for one of the two optical
signals and then the two optical signals are combined again by the optical
combining portion, which constitutes an interference system necessary for
delayed detection of an optical signal.
A ninth aspect is an aspect according to the sixth aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from the light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from the optical branch portion respectively, for each
subjecting each of the outputted lights to optical phase modulation using
the angle-modulated signal as an original signal; and
an optical directional coupling portion for combining the two
optical-phase-modulated signals outputted from the first and second
optical phase modulating portions and then dividing the resultant signal
into first and second optical signals in which optical-intensity modulated
components are set in opposite phases to each other, and
the interference portion comprises:
an optical delay portion for providing the second optical signal outputted
from the optical directional coupling portion with a predetermined delay;
and
an optical combining portion for combining the first optical signal
outputted from the optical directional coupling portion and the second
optical signal outputted from the optical delay portion.
As described in the foregoing, in the ninth aspect, the external modulation
scheme is adopted in the optical modulating portion and the optical
directional coupling portion is provided, to input the first and second
optical signals, in which optical-intensity-modulated components are set
in opposite phases to each other, to the interference portion. This
eliminates the need for branching the inputted optical signal in the
interference portion.
A tenth aspect is an aspect according to the sixth aspect, wherein
the interference portion comprises:
an optical waveguide portion for guiding the optical signal outputted from
the optical modulating portion; and
first and second optical transparent/reflecting portions, cascaded on the
optical waveguide portion at a prescribed interval, for respectively
transmitting parts of the inputted optical signals and reflecting the
remained parts, and
propagation time in which an optical signal goes and returns between the
first and second optical transparent/reflecting portions is the
predetermined difference in propagation delay.
As described in the foregoing, according to the tenth aspect, two optical
transparent/reflecting portions are provided on the optical waveguide
portion, and a direct light which propagates through both of the optical
transparent/reflecting portions and an indirect light which goes and
returns between the optical transparent/reflecting portions one time and
then propagates are generated, which constitutes an interference system
necessary for delayed detection of an optical signal without physically
branching the optical signal into two. This allows constitution of the
interference system with a simpler configuration.
An eleventh aspect is an aspect according to the first aspect, wherein the
optical modulating portion generates an optical-amplitude-modulated signal
as the optical-modulated signal.
A twelfth aspect is an aspect according to the eleventh aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from the light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from the optical branch portion respectively, for each
subjecting each of the outputted lights to optical phase modulation using
the angle-modulated signals as an original signal; and
an optical coupling portion for combining the two optical-phase-modulated
signals outputted from the first and second optical phase modulating
portions.
A thirteenth aspect is an aspect according to the eleventh aspect, wherein
the interference portion comprises:
an optical branch portion for branching the inputted optical signal into a
first optical signal and a second optical signal;
an optical delay portion for providing the second optical signal outputted
from the optical branch portion with a predetermined delay; and
an optical combining portion for combining the first optical signal
outputted from the second optical branch portion and the second optical
signal outputted from the optical delay portion.
As described in the foregoing, according to the thirteenth aspect, the
inputted optical signal is branched into two optical signals by the
optical branch portion, the predetermined propagation delay is provided
for one of the two optical signals in the delay portion and then the two
optical signals are combined again in the optical combining portion, to
constitute an interference system necessary for delayed detection of an
optical signal.
A fourteenth aspect is an aspect according to the eleventh aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a
given wavelength;
an optical branch portion for branching the light from the light source
into two;
first and second optical phase modulating portions, provided for the two
outputted lights from the optical branch portion respectively, for each
subjecting each of the outputted lights to optical phase modulation using
the angle-modulated signal as an original signal; and
an optical directional coupling portion for combining the two
optical-phase-modulated signals outputted from the first and second
optical phase modulating portions and then dividing the resultant signal
into first and second optical signals in which optical-amplitude-modulated
components are set in opposite phases to each other, and
the interference portion comprises:
an optical delay portion for providing the second optical signal outputted
from the optical directional coupling portion with a predetermined delay;
and
an optical combining portion for combining the first optical signal
outputted from the optical directional coupling portion and the second
optical signal outputted from the optical delay portion.
As described in the foregoing, in the fourteenth aspect, the external
modulation scheme is adopted in the optical modulating portion and the
optical directional coupling portion is provided, to input the first and
second optical signals, in which optical-intensity-modulated components
are set in opposite phases to each other, to the interference portion.
This eliminates the need for branching the inputted optical signal in the
interference portion.
A fifteenth aspect is an aspect according to the eleventh aspect, wherein
the interference portion comprises:
an optical waveguide portion for guiding the optical signal outputted from
the optical modulating portion; and
first and second optical transparent/reflecting portions, cascaded on the
optical waveguide portion at a predetermined interval, for respectively
transmitting parts of the inputted optical signals and reflecting the
remained parts, and
propagation time in which an optical signal goes and returns between the
first and second optical transparent/reflecting portions is the
predetermined difference in propagation delay.
As described in the foregoing, according to the fifteenth aspect, two
optical transparent/reflecting portions are provided on the optical
waveguide portion, and the direct light which propagates through both of
the optical transparent/reflecting portions and the indirect light which
goes and returns between the optical transparent/reflecting portions one
time and then propagates are generated, which constitutes an interference
system necessary for delayed detection of an optical signal without
physically branching the optical signal into two. This allows constitution
of the interference system with a simpler configuration.
A sixteenth aspect is an aspect according to the twelfth aspect, wherein
predetermined optical phase modulation is performed in the first and
second optical phase modulating portions so that difference between the
optical phase shift by the first optical phase modulating portion and the
optical phase shift by the second optical phase modulating portion is set
in phase with the angle-modulated signal.
A seventeenth aspect is an aspect according to the fourteenth aspect,
wherein predetermined optical phase modulation is performed in the first
and second optical phase modulating portions so that difference between
the optical phase shift by the first optical phase modulating portion and
the optical phase shift by the second optical phase modulating portion is
set in phase with the angle-modulated signal.
An eighteenth aspect is an aspect according to the twelfth aspect, wherein
predetermined optical phase modulation is performed in the first and
second optical phase modulating portions so that difference between the
optical phase shift by the first optical phase modulating portion and the
optical phase shift by the second optical phase modulating portion is set
in opposite phases with the angle-modulated signal.
A nineteenth aspect is an aspect according to the fourteenth aspect,
wherein predetermined optical phase modulation is performed in the first
and second optical phase modulating portions so that difference between
the optical phase shift by the first optical phase modulating portion and
the optical phase shift by the second optical phase modulating portion is
set in opposite phases with the angle-modulated signal.
In the sixteenth to nineteenth aspects, phase relation between the
angle-modulated signals inputted into the first and second optical phase
modulating portions is optimally adjusted, to enlarge the
optical-amplitude-modulated component in the optical signal inputted into
the optical coupling portion or the optical directional coupling portion,
which realizes high efficient demodulation and optical transmission with
optical signal processing.
A twentieth aspect is an aspect according to the first aspect, wherein a
product value of a center angular frequency of the angle-modulated signal
and the predetermined difference in propagation delay in the interference
portion is set to be equal to .pi./2.
As described in the foregoing, in the twentieth aspect, the center angular
frequency of the angle-modulated signal and the predetermined difference
in propagation delay in the interference portion are set at optimal
values, to increase demodulation efficiency.
A twenty-first aspect is an aspect according to the fourth aspect, wherein
the predetermined difference in propagation delay in the interference
portion is set to be equal to one symbol length of the digital signal.
A twenty-second aspect is an aspect according to the fifth aspect, wherein
the predetermined difference in propagation delay in the interference
portion is set to be equal to one symbol length of the digital signal.
As described in the foregoing, in the twenty-first and twenty-second
aspects, when the angle-modulated signal is an FSK modulated signal or a
PSK modulated signal obtained by subjecting a digital signal to frequency
modulation or phase modulation, the symbol length of the digital signal
and the predetermined difference in propagation delay in the interference
portion are set to optimal values, thereby increasing the demodulation
efficiency.
A twenty-third aspect is an aspect according to the eighth aspect, wherein
polarization states of the first optical signal and the second optical
signal to be combined in the optical combining portion are set to be the
same with each other.
A twenty-fourth aspect is an aspect according to the ninth aspect, wherein
polarization states of the first optical signal and the second optical
signal to be combined in the optical combining portion are set to be the
same with each other.
A twenty-fifth aspect is an aspect according to the thirteenth aspect,
wherein polarization states of the first optical signal and the second
optical signal to be combined in the optical combining portion are set to
be the same with each other.
A twenty-sixth aspect is an aspect according to the fourteenth aspect,
wherein
polarization states of the first optical signal and the second optical
signal to be combined in the optical combining portion are set to be the
same with each other.
As described in the foregoing, in the twenty-third to twenty-sixth aspects,
the polarization states of the first and second optical signals in the
optical combining portion are set to be the same with each other, thereby
increasing homodyne detection efficiency in the optical/electrical
converting portion, that is demodulation efficiency.
A twenty-seventh aspect is an aspect according to the tenth aspect, wherein
polarization states of the optical signal transmitting through the first
and second optical transparent/reflecting portions along the optical
waveguide portion and the optical signal transmitting through the first
optical transparent/reflecting portion, reflected at the second optical
transparent/reflecting portion, reflected at the first optical
transparent/reflecting portion and transmitting through the second optical
transparent/reflecting portion are set to be the same with each other.
A twenty-eighth aspect is an aspect according to the fifteenth aspect,
wherein
polarization states of the optical signal transmitting through the first
and second optical transparent/reflecting portions along the optical
waveguide portion and the optical signal transmitting through the first
optical transparent/reflecting portion, reflected at the second optical
transparent/reflecting portion, reflected at the first optical
transparent/reflecting portion and transmitting through the second optical
transparent/reflecting portion are set to be the same with each other.
As described in the foregoing, in the twenty-seventh and twenty-eighth
aspects, the polarization states of the direct light and the indirect
light are set to be the same, thereby increasing the homodyne detection
efficiency in the optical/electrical converting portion, that is the
demodulation efficiency.
A twenty-ninth aspect is an aspect according to the eighth aspect, wherein
the optical modulating portion and the interference portion are connected
with a first optical waveguide portion,
the interference portion and the optical/electrical converting portion are
connected with a second optical waveguide portion, and
the first and/or second optical waveguide portions are composed of
single-mode optical fibers.
The thirtieth aspect is an aspect according to the thirteenth aspect,
wherein
the optical modulating portion and the interference portion are connected
with a first optical waveguide portion,
the interference portion and the optical/electrical converting portion are
connected with a second optical waveguide portion, and
the first and/or second optical waveguide portions are composed of
single-mode optical fibers.
As described in the foregoing, in the twenty-ninth and thirtieth aspects,
the first and/or second optical waveguide portions are composed of
single-mode optical fibers, making it possible to perform optical
transmission with optical fibers which are inexpensive.
A thirty-first aspect is an aspect according to the ninth aspect, wherein
the interference portion and the optical/electrical converting portion are
connected with an optical waveguide portion, and
the optical waveguide portion is composed of a single-mode optical fiber.
A thirty-second aspect is an aspect according to the fourteenth aspect,
wherein
the interference portion and the optical/electrical converting portion are
connected with an optical waveguide portion, and
the optical waveguide portion is composed of a single-mode optical fiber.
As described in the foregoing, in the thirty-first and thirty-second
aspects, the optical waveguide portion provided between the interference
portion and the optical/electrical converting portion is composed of a
single-mode optical fiber, making it possible to perform optical
transmission with an optical fiber which is inexpensive.
A thirty-third aspect is an aspect according to the tenth aspect, wherein a
whole or a part of the optical waveguide portion in the interference
portion is composed of a single-mode optical fiber.
A thirty-fourth aspect is an aspect according to the fifteenth aspect,
wherein a whole or a part of the optical waveguide portion in the
interference portion is composed of a single-mode optical fiber.
As described in the foregoing, in the thirty-third and thirty-fourth
aspects, the whole or a part of the optical waveguide portion in the
interference portion is composed of a single-mode optical fiber, allowing
optical transmission with an optical fiber which is inexpensive.
A thirty-fifth aspect is an aspect according to the first aspect, further
comprising an amplitude adjusting portion for adjusting an amplitude of
the angle-modulated signal and outputting the angle-modulated signal of a
constant amplitude.
In the case where delayed detection is performed employing the
square-law-detection characteristics of the optical/electrical converting
portion, as the amplitude of the angle-modulated signal which is the
original signal becomes smaller, the demodulation efficiency decreases.
Further, when the angle-modulated signal has an amplitude fluctuation,
deterioration in signal quality such as waveform distortion and the like
occurs. Hence, in the above thirty-fifth aspect, the amplitude adjusting
portion maintaining the amplitude constant is provided for the inputted
angle-modulated signal, to suppress the above-mentioned deterioration.
A thirty-sixth aspect is an aspect according to the first aspect, further
comprising a bandwidth limiting portion for limiting a band of the
angle-modulated signal.
As described in the foregoing, in the thirty-sixth aspect, the bands of the
angle-modulated signal is previously limited, to lessen the spectrum in
width, thereby preventing deterioration in quality of a demodulated signal
caused by that the part of the spread spectrum of the angle-modulated
signal component is superimposed on the band of the demodulated signal
outputted from the optical/electrical converting portion.
A thirty-seventh aspect is an optical transmitter for optically
transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal
into an optical-modulated signal; and
an interference portion for separating the optical-modulated signal into a
plurality of optical signals having predetermined difference in
propagation delay and then combining the optical signals, and
the optical transmitter transmitting the combined optical signal outputted
from the interference portion.
A thirty-eighth aspect is an aspect according to the thirty-seventh aspect,
wherein the angle-modulated signal is an FM signal obtained by subjecting
an analog signal to frequency modulation.
A thirty-ninth aspect is an aspect according to the thirty-seventh aspect,
wherein the angle-modulated signal is a PM signal obtained by subjecting
an analog signal to phase modulation.
A fortieth aspect is an aspect according to the thirty-seventh aspect,
wherein the angle-modulated signal is an FSK modulated signal obtained by
subjecting a digital signal to frequency modulation.
A forty-first aspect is an aspect according to the thirty-seventh aspect,
wherein the angle-modulated signal is a PSK modulated signal obtained by
subjecting a digital signal to phase modulation.
A forty-second aspect is an aspect according to the thirty-seventh aspect,
wherein the optical modulating portion generates an optical-intensity-
modulated signal as the optical-modulated signal.
A forty-third aspect is an aspect according to the thirty-seventh aspect,
wherein the optical modulating portion generates an optical-amplitude-
modulated signal as the optical-modulated signal.
A forty-fourth aspect is an optical receiver for receiving an
optical-modulated signal and acquiring a demodulated signal of the
optical-modulated signal, comprising:
an interference portion for separating the received optical-modulated
signal into a plurality of optical signals having predetermined difference
in propagation delay and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal, and
the interference portion and the optical/electrical converting portion
constituting a delayed detection system of an optical signal and the
delayed detection system performing conversion processing of an optical
signal into an electrical signal and angle demodulation processing
simultaneously.
A forty-fifth aspect is an aspect according to the forty-fourth aspect,
wherein
the optical-modulated signal is generated from a 2.sup.n -phase (n is an
integer of not less than two) PSK electrical-modulated signal as an
original signal,
the interference portion includes:
a received light dividing portion for dividing an inputted optical signal
into 2.sup.n-1 received lights; and
first to 2.sup.n-1 th optical interference circuits, provided corresponding
to the 2.sup.n-1 received lights respectively, for each branching each of
the received lights into a first optical signal and a second optical
signal, providing the second optical signal with a predetermined delay and
then combining the first and second optical signals, and
the optical/electrical signals are provided corresponding to the first to
2.sup.n-1 th optical interference circuits respectively.
A forty-sixth aspect is an aspect according to the forty-fifth aspect,
wherein
the optical-modulated signal is generated from a quadrature PSK
electrical-modulated signal as an original signal,
the interference portion includes:
a received light dividing portion for dividing an inputted optical signal
into a first received light and a second received light;
a first optical interference circuit for branching the first received light
into a first optical signal and a second optical signal, providing the
second optical signal with a first predetermined delay and then combining
the first and second optical signals; and
a second optical interference circuit for branching the second received
light into a first optical signal and a second optical signal, providing
the second optical signal with a second predetermined delay and then
combining the first and second optical signals, and
the first predetermined delay in the first optical interference circuit and
the second predetermined delay in the second optical interference circuit
are both set to have the absolute magnitude of 1/2 symbol length of the
digital signal and be in opposite phases to each other.
A forty-seventh aspect is an optical transmission system for optically
transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal
into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal
outputted from the optical modulating portion into two signals at least, a
first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal
outputted from the optical branch portion into a plurality of optical
signals having predetermined difference in propagation delay and then
combining the optical signals;
a first optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal; and
a second optical/electrical converting portion, having square-law-detection
characteristics, for converting the second optical-modulated signal
outputted from the optical branch portion into an electrical signal.
As described in the foregoing, according to the forty-seventh aspect, an
angle-modulated signal is converted into an optical signal and branched
into a plurality of optical signals, a part of the optical signals are
subjected to homodyne detection by the interference portion and the first
optical/electrical converting portion to reproduce the original electrical
signal for the angle modulation as described in the first aspect and the
remained part of the optical signals are subjected to direct detection by
the second optical/electrical converting portion to reproduce the
angle-modulated signal. Thereby, if a wired network is constructed by
using an optical fiber as its backbone and the angle-modulated signal
outputted from the second optical/electrical converting portion is sent
out in the air as a radio wave, the optical transmission system can expand
to a wireless network for mobile terminals and the like. Especially, a
high-frequency signal such as a micro wave, a millimetre wave and the
like, which is thought as an suitable signal for a wireless network, is
received and subjected to demodulation, in a wired system, by a low cost
configuration with optical signal processing and at the same time a radio
wave is sent to the mobile terminals and the like, so that a flexible and
greatly economical system can be constructed.
A forty-eighth aspect is an aspect according to the forty-seventh aspect,
further comprising:
a local light source for outputting a light of a predetermined wavelength;
and
an optical combining portion, inserted between the optical branch portion
and the second optical/electrical converting portion, for combining the
second optical-modulated signal outputted from the optical branch portion
and the light from the local light source,
wherein the second optical/electrical converting portion heterodyne detects
the combined optical signal outputted from the optical combining portion
and then converts the optical signal into an electrical signal.
A forty-ninth aspect is an aspect according to the forty-seventh aspect,
further comprising:
a local light source for outputting a light of a predetermined wavelength;
and
an optical combining portion, inserted between the optical modulating
portion and the optical branch portion, for combining the
optical-modulated signal outputted from the optical modulating portion and
the light from the local light source,
wherein the second optical/electrical converting portion heterodyne detects
the second optical-modulated signal outputted from the optical branch
portion and converts the optical-modulated signal into an electrical
signal.
As described in the foregoing, according to the forty-eighth and
forty-ninth aspects, the frequency of the local light source is varied, to
freely up-convert or down-convert the frequency of the angle-modulated
signal outputted from the second optical/electrical converting portion.
A fiftieth aspect is an optical transmission system for optically
transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal
into an optical-modulated signal;
a local light source for outputting a light of a predetermined wavelength;
an optical combining portion for combining the optical-modulated signal
outputted from the optical modulating portion and the light from the local
light source;
an interference portion for separating the combined optical signal
outputted from the optical combining portion into a plurality of optical
signals having predetermined difference in propagation delay and then
combining the optical signals;
an optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal; and
a dividing portion for separating the electrical signal outputted from the
optical/electrical converting portion for each of frequency components and
outputting the electrical signals.
A fifty-first aspect is an optical transmission system for optically
transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal
into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal
outputted from the optical modulating portion into two signals at least, a
first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal
outputted from the optical branch portion into a plurality of optical
signals having predetermined difference in propagation delay and then
combining the optical signals;
a first optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal;
a local oscillation portion for outputting an unmodulated signal of a
predetermined frequency; and
a second optical/electrical converting portion, having square-law-detection
characteristics, in which its bias is modulated with the unmodulated
signal from the local oscillation portion, for converting the second
optical-modulated signal outputted from the optical branch portion into an
electrical signal.
A fifty-second aspect is an optical transmission system for optically
transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal
into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal
outputted from the optical modulating portion into two signals at least, a
first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal
outputted from the optical branch portion into a plurality of optical
signals having predetermined difference in propagation delay and then
combining the optical signals;
a first optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal;
a second optical/electrical converting portion, having square-law-detection
characteristics, for converting the second optical-modulated signal
outputted from the optical branch portion into an electrical signal;
a local oscillation portion for outputting an unmodulated signal of a
predetermined frequency; and
a mixing portion for mixing the electrical signal outputted from the second
optical/electrical converting portion and the unmodulated signal outputted
from the local oscillation portion and outputting the resultant signals.
As described in the foregoing, according to the fiftieth to fifty-second
aspects, the original electrical signal and the angle-modulated signal for
angle modulation can be reproduced only by optical signal processing.
Further, the frequency of the local light source or the local oscillation
portion is varied, to freely up-convert or down-convert the frequency of
the angle-modulated signal to be reproduced.
A fifty-third aspect is an optical transmission system for optically
transmitting two signals at least, a first electrical signal and a second
electrical signal simultaneously, comprising:
an angle modulating portion for converting the first electrical signal into
an angle-modulated signal;
a combining portion for combining the angle-modulated signal and the second
electrical signal;
an optical modulating portion for converting the combined signal outputted
from the combining portion into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal
outputted from the optical modulating portion into two signals at least, a
first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal
outputted from the optical branch portion into a plurality of optical
signals having predetermined difference in propagation delay and then
combining the optical signals;
a first optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal; and
a second optical/electrical converting portion, having square-law-detection
characteristics, for converting the second optical-modulated signal
outputted from the optical branch portion into an electrical signal.
As described in the foregoing, according to the fifty-third aspect, a
digital signal and an analog signal, for example, which are different
types of electrical signals, can be optically transmitted simultaneously
and individually reproduced.
A fifty-fourth aspect is an aspect according to the fifty-third aspect,
wherein an occupied frequency band of the first electrical signal, an
occupied frequency band of the second electrical signal and an occupied
frequency band of the angle-modulated signal do not overlap with each
other.
A fifty-fifth aspect is an aspect according to the fifty-third aspect,
further comprising:
a first signal processing portion for limiting the occupied frequency band
of the first electrical signal; and
a second signal processing portion for limiting the occupied frequency band
of the second electrical signal.
A fifty-sixth aspect is an aspect according to the fifty-fifth aspect,
further comprising:
a third signal processing portion for passing only a frequency component
corresponding to the occupied frequency band of the first electrical
signal as to the electrical signal outputted from the first
optical/electrical converting portion and reproducing waveform information
which was lost by the band limitation in the first signal processing
portion; and
a fourth signal processing portion for passing only a frequency component
corresponding to the occupied frequency band of the second electrical
signal as to the electrical signal outputted from the second
optical/electrical converting portion and reproducing waveform information
which was lost by the band limitation in the second signal processing
portion.
As described in the foregoing, according to the fifty-sixth aspect, the
waveform distortion caused by the band limitation performed on the
transmitting side can be corrected on the receiving side.
A fifty-seventh aspect is an optical transmission system for optically
transmitting a plurality of electrical signals, comprising:
a plurality of angle modulating portions for converting each of the
plurality of electrical signals into an angle-modulated signals;
a combining portion for combining the angle-modulated signals outputted
from the plurality of angle modulating portions;
an optical modulating portion for converting the combined signal outputted
from the combining portion into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal
outputted from the optical modulating portion into a plurality of
optical-modulated signals; and
an plurality of optical signal processing portions, provided corresponding
to the plurality of optical-modulated signals outputted from the optical
branch portion respectively, for each performing predetermined optical
signal processing and then individually reproducing the plurality of
electrical signals, and
each of the optical signal processing portions including:
an interference portion for separating the optical-modulated signal
outputted from the optical branch portion into a plurality of optical
signals having difference in propagation delay decided according to
frequencies of angle-modulated signals to be acquired by demodulation and
then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal.
As described in the above, according to the fifty-seventh aspect, a digital
signal and an analog signal, for example, which are different types of
electrical signals, can be optically transmitted simultaneously and
individually reproduced.
A fifty-eighth aspect is an aspect according to the fifty-seventh aspect,
wherein occupied frequency bands of the plurality of electrical signals
and occupied frequency bands of the plurality of angle-modulated signals
do not overlap with each other.
A fifty-ninth aspect is an aspect according to the fifty-seventh aspect,
further comprising a plurality of signal pre-processing portions for
limiting the occupied frequency bands of the plurality of electrical
signals.
A sixtieth aspect is an aspect according to the fifty-ninth aspect, wherein
each of the plurality of optical signal processing portions further
includes a signal post-processing portion for passing a frequency
component corresponding to an occupied frequency band of an electrical
signal to be reproduced and reproducing waveform information which was
lost by the band limitation in the signal pre-processing portion as to the
electrical signal outputted from the optical/electrical converting
portion.
As described in the foregoing, according to the sixtieth aspect, the
waveform distortion caused by the band limitation performed on the
transmitting side can be corrected on the receiving side.
A sixty-first aspect is an optical transmission system for optically
transmitting a multichannel angle-modulated signal obtained by subjecting
plurality-channel electrical signals to angle modulation respectively and
frequency-division multiplexing, comprising:
an optical modulating portion for converting the multichannel
angle-modulated signal into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal
outputted from the optical modulating portion into a plurality of
optical-modulated signals; and
a plurality of optical signal processing portions, provided corresponding
to the plurality of optical-modulated signals outputted from the optical
branch portion respectively, for each performing predetermined optical
signal processing and then reproducing an electrical signal on an
individual channel, and
each of the optical signal processing portions including:
an interference portion for separating the optical-modulated signal
outputted from the optical branch portion into a plurality of optical
signals having difference in propagation delay decided according to
frequencies of electrical signals on channels to be reproduced and then
combining the optical signals; and
an optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal outputted from
the interference portion into an electrical signal.
As described in the foregoing, according to the sixty-first aspect, the
multichannel angle-modulated signal obtained by
frequency-division-multiplexing can be optically transmitted
simultaneously.
These and other objects, features, aspects and advantages of the present
invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the configuration of an optical
transmission system according to a first embodiment of the present
invention.
FIG. 2 is a block diagram showing a first specific example of configuration
of the optical transmission system according to the first embodiment of
the present invention.
FIGS. 3a to 3c are diagrams for explaining FM demodulation operation in the
optical transmission system in FIG. 2.
FIG. 4 is a block diagram showing a first operational example of the
optical transmission system in FIG. 2.
FIG. 5 is a block diagram showing a second operational example of the
optical transmission system in FIG. 2.
FIG. 6 is a block diagram showing a second specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention.
FIGS. 7a to 7c are diagrams for explaining FM demodulation operation in the
optical transmission system in FIG. 6.
FIG. 8 is a bock diagram showing a third specific example of configuration
of the optical transmission system according to the first embodiment of
the present invention.
FIG. 9 is a block diagram showing a first operational example of the
optical transmission system in FIG. 8.
FIG. 10 is a block diagram showing a s econd operational example of the
optical transmission system in FIG. 8.
FIG. 11 is a block diagram showing an example of the structure of an
optical receiver used in a system for optically transmitting a QPSK
modulated signal.
FIG. 12 is a block diagram showing a fourth specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention.
FIG. 13 is a block diagram showing a fifth specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention.
FIGS. 14a to 14d are diagrams for explaining FM demodulation operation in
the optical transmission system in FIG. 13.
FIG. 15 is a block diagram showing a sixth specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention.
FIGS. 16a to 16d are diagrams for explaining FM demodulation operation in
the optical transmission system in FIG. 15.
FIG. 17 is a block diagram showing the configuration of an optical
transmission system according to a second embodiment of the present
invention.
FIG. 18 is a block diagram showing the configuration of an optical
transmission system according to a third embodiment of the present
invention.
FIG. 19 is a block diagram showing the configuration of an optical
transmission system according to a fourth embodiment of the present
invention.
FIG. 20 is a block diagram showing the configuration of an optical
transmission system according to a fifth embodiment of the present
invention.
FIG. 21 is a block diagram showing the configuration of an optical
transmission system according to a sixth embodiment of the present
invention.
FIG. 22 is a block diagram showing the configuration of an optical
transmission system according to a seventh embodiment of the present
invention.
FIG. 23 is a block diagram showing the configuration of an optical
transmission system according to an eighth embodiment of the present
invention.
FIG. 24 is a block diagram showing the configuration of an optical
transmission system according to a ninth embodiment of the present
invention.
FIG. 25 is a block diagram showing the configuration of an optical
transmission system according to a tenth embodiment of the present
invention.
FIG. 26 is a block diagram showing the configuration of an optical
transmission system according to an eleventh embodiment of the present
invention.
FIG. 27 is a block diagram showing the configuration of an optical
transmission system according to a twelfth embodiment of the present
invention.
FIG. 28 is a block diagram showing the configuration of an optical
transmission system according to a thirteenth embodiment of the present
invention.
FIG. 29 is a block diagram showing the configuration of an optical
transmission system according to a fourteenth embodiment of the present
invention.
FIG. 30 is a block diagram showing the configuration of a conventional
optical transmission system.
FIG. 31 is a block diagram showing the structure of an angle demodulating
portion in FIG. 30.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(First Embodiment)
FIG. 1 is a block diagram showing the configuration of an optical
transmission system according to a first embodiment of the present
invention. FIG. 1 also shows schematic diagrams of frequency spectrums of
signals in respective portions. In FIG. 1, the optical transmission system
of the present embodiment includes an angle modulating portion 1, an
optical modulating portion 2, a first optical waveguide portion 3, an
interference portion 6, a second optical waveguide portion 7, an
optical/electrical converting portion 4 and a filter F. Depending on the
required structure of a transmission side and a receiving side from the
view point of the whole system, the first and second optical waveguide
portions 3 and 7 are both needed in some cases, or either one of the
optical waveguide portions is needed in other cases.
It is to be noted in FIG. 1 that the angle demodulating portion 5 shown in
FIG. 30 is not provided. That is, the present embodiment is characterized
by that a demodulated signal can be acquired with performing a new and
peculiar optical signal processing without performing electrical
demodulation processing.
Next, the operation of the embodiment shown in FIG. 1 will be explained.
The angle modulating portion 1 receives an analog signal such as an audio
signal, a video signal and the like, or a digital signal such as computer
data and the like as an electrical signal to be transmitted and outputs an
angle-modulated signal originated from the above-mentioned signal. The
optical modulating portion 2 receives the angle-modulated signal outputted
from the angle modulating portion 1 and outputs an
optical-intensity-modulated signal with, for example, a direct modulation
scheme, or outputs an optical-intensity-modulated signal or an
optical-amplitude-modulated signal with an external modulation scheme. The
optical signal is transmitted through the first optical waveguide portion
3. The interference portion 6 separates the inputted optical signal into
two optical signals having predetermined difference in propagation delay
and then combines the optical signals again. The combined optical signal
is transmitted through the second optical waveguide portion 7. The
optical/electrical converting portion 4, which includes a photodetector
having square-law-detection characteristics (a pin photo-diode, an
avalanche photo-diode or the like), re-converts the inputted combined
optical signal into an electrical signal and re-generates an electrical
signal (an analog signal or a digital signal) correlating with the
original electrical signal with angle demodulation operation to output the
electrical signal. The filter F passes only a signal component
corresponding to the original electrical signal (that is, a signal
component of the same frequency band as that of the original electrical
signal) among signals outputted from the optical/electrical converting
portion 4. Described below are more specific examples of configuration of
the present embodiment.
(1) First specific example of configuration in the first embodiment
FIG. 2 is a block diagram showing a first specific example of configuration
of the optical transmission system according to the first embodiment of
the present invention. In FIG. 2, the optical transmission system of the
present example of configuration includes an FM portion 100 as an example
of the angle modulating portion 1. The optical modulating portion 2
includes a light source 201, a first optical branch portion 202, a first
optical phase modulating portion 203, a second optical phase modulating
portion 204 and an optical coupling portion 205. The interference portion
6 includes a second optical branch portion 601, an optical delay portion
602 and an optical combining portion 603.
Next, the operation of the specific example shown in FIG. 2 will be
explained below. To the FM portion 100 is inputted an electrical signal
with a high frequency and a wide-band, for example, a multichannel
frequency-division-multiplexed signal and the like as an original signal
to be subjected to FM. The FM portion 100 converts the inputted electrical
signal into an FM signal with a predetermined frequency. The optical
modulating portion 2 of the present configuration example has t he
configuration of an external modulation scheme and the light source 201
outputs an unmodulated light. The first optical branch portion 202
branches the unmodulated light outputted from the light source 201 into
two. The first and second optical phase modulating portions 203 and 204,
which are provided for each of the two lights outputted from the first
optical branch portion 202, perform predetermined optical phase modulation
with the FM signals outputted from the FM portion 100. The
optical-phase-modulated signals outputted from the first and second
optical phase modulating portions 203 and 204 are each combined in the
optical coupling portion 205 to be converted into an
optical-amplitude-modulated signal. The optical-amplitude-modulated signal
is inputted to the interference portion 6 through the first optical
waveguide portion 3. In the interference portion 6, the second optical
branch portion 601 branches the inputted optical signal into first and
second optical signals. The optical delay portion 602 provides the second
optical signal outputted from the optical branch portion 601 with a
predetermined delay T.sub.p. The optical combining portion 603 combines
the first optical signal outputted from the second optical branch portion
601 and the second optical signal outputted from the optical delay portion
602 to output the resultant signal to the second optical waveguide portion
7. The optical/electrical converting potion 4 creates a product of the
first and second optical signals transmitted through the second optical
waveguide portion 7 (such operation is commonly called as homodyne
detection).
Description will be made of operation of the first specific example below
using equations. It is assumed that as to the first and second
optical-amplitude-modulated signals outputted from the second optical
branch portion 601, an electric field component Ea(t) of the first optical
signal is expressed by the following equation (1) and an electric field
component Eb(t) of the second optical signal passing through the optical
delay portion 602 is expressed by the following equation (2),
respectively.
Ea(t)=m cos(2.pi.f.sub.t t).times.cos(2.pi.f.sub.0 t) (1)
Eb(t)=-m cos{2.pi.f.sub.t (t-T.sub.p)}.times.cos{2.pi.f.sub.0 (t-T.sub.p)}
(2)
In the above-described equations (1) and (2), m represents an amplitude in
the electric field, f.sub.t represents an (instantaneous) frequency of the
FM signal, f.sub.0 represents an optical frequency and T.sub.p represents
the predetermined delay in the optical delay portion 602. After these are
combined to be subjected to square-law-detection in the optical/electrical
converting portion 4, an optical current I.sub.0 (t) after the
square-law-detection is expressed by the following equation (3).
I.sub.0 (t)=1/2+L [m.sup.2 cos.sup.2 (2.pi.f.sub.t t).times.cos.sup.2
(2.pi.f.sub.0 t)+m.sup.2 cos.sup.2 {2.pi.f.sub.t
(t-T.sub.p)}.times.cos.sup.2 {2.pi.f.sub.0 (t-T.sub.p)}-
2m.sup.2 cos(2.pi.f.sub.t t).times.cos{2.pi.f.sub.t
(t-T.sub.p)}.times.cos{2.pi.f.sub.0 t}.times.cos{2.pi.f.sub.0
(t-T.sub.p)}] (3)
Considering that, in the above equation (3), signals corresponding to the
terms of periodic functions depending on the optical frequency f.sub.0 are
not outputted due to a frequency response limit in the optical/electrical
converting portion 4, a signal component I.sub.s (t) derived from the
optical/electrical converting portion 4 is expressed by the following
equation (4) by expanding only a third term.
##EQU1##
Since m, f.sub.0, T.sub.p are constant values, the magnitude of the fourth
term in the second expanded equation (4b) of the equation (4) is changed
depending on the instantaneous frequency f.sub.t of the FM signal. That
is, it is possible to derive an optical current whose magnitude is changed
according to variations in frequency of the FM signal.
While, in the first specific example, the optical modulating portion 2
outputs an optical-amplitude-modulated signal, the optical modulating
portion 2 may output an optical-intensity-modulated signal. The operation
of this case will be described below using equations. As is the case with
the above description, as for the first and second
optical-intensity-modulated signals outputted from the second optical
branch portion 601, the electric field component Ea(t) of the first
optical signal is expressed by the following equation (5) and the electric
field component Eb(t) of the second optical signal passing through the
optical delay portion 602 is expressed by the following equation (6),
respectively.
##EQU2##
The optical current I.sub.0 (t) obtained by subjecting the combined signal
to square-law-detection in the optical/electrical converting portion 4 is
expressed by the following equation (7).
##EQU3##
Considering that, in the above equation (7), signals corresponding to the
terms of periodic functions depending on the optical frequency f.sub.0 are
not outputted due to the frequency response limit of the
optical/electrical converting portion 4, the signal component I.sub.s (t)
derived from the optical/electrical converting portion 4 is expressed by
the following equation (8).
##EQU4##
It is found that in a second expanded equation (8b) of the above equation
(8), the magnitude of the fourth term is changed depending on the
instantaneous frequency f.sub.t of the FM signal, making it possible to
derive an optical current whose magnitude is changed according to
variations in frequency of the FM signal as in the case with the second
expanded equation (4b) of the above-described equation (4).
FIGS. 3a and 3b schematically show amplitude fluctuation components (or
intensity fluctuation components) of electric fields of the first and
second optical signals, respectively. FIG. 3c shows a waveform of an
optical current, which is outputted from the optical/electrical converting
portion 4, corresponding to t he amplitude fluctuation components or
intensity fluctuation component. As shown in FIG. 3c, the
optical/electrical converting portion 4 outputs a pulse-like signal
comprising of negative differential pulses. Each pulse duration of each
differential pulse is constant corresponding to the predetermined delay
T.sub.p in the optical delay portion 602, and occurrence intervals of the
differential pulses correspond to the variations in frequency of the FM
signal outputted from the FM portion 100. The filter F receives the
pulse-like signal to pass only a signal component (a low-frequency
component) of a band corresponding to that of the electrical signal
inputted to the FM portion 100. I n this way, the electrical signal can be
obtained.
FIG. 4 is a diagram showing a first operational example of the optical
transmission system in FIG. 2. In FIG. 4, according to the present
operational example, the FM portion 100, the optical modulating potion 2,
the optical waveguide portion 3 and the interference portion 6 constitute
an optical transmitter PT, and the optical/electrical converting portion 4
constitutes an optical receiver PR. Further, an optical transmission
medium such as an optical fiber and the like is used as the second optical
waveguide portion 7 to expand a physical distance between the optical
transmitter PT and the optical receiver PR.
In the operational example in FIG. 4, the first and second optical signals
transmitted through the second optical waveguide portion 7 (the optical
fiber) are both created from the light source 201, so that the optical
wavelengths of the optical signals are the same. Therefore, even when not
a special optical fiber having polarization maintaining features but a
normal single-mode optical fiber is employed as the second optical
waveguide portion 7, the relative polarization states of the two optical
signals outputted from the optical combining portion 603 can be always
maintained constant even after the optical signals are transmitted through
the second optical waveguide portion 7. Accordingly, the two optical
signals are adjusted so that the polarization states of the optical
signals become the same, and then inputted to the optical combining
portion 603, thereby enabling the polarization states of the two optical
signals to be maintained the same even after the optical signals are
transmitted through the second optical waveguide portion 7. As a result,
homodyne efficiency in the optical/electrical converting portion 4 reaches
its maximum, which makes it possible to realize high FM demodulation
efficiency with high stability.
As described in the above, in the operational example in FIG. 4, a
constituent required for the optical receiver PR is only the
optical/electrical converting potion 4 which is relatively inexpensive,
and expensive parts are all accommodated in the optical transmitter PT.
Accordingly, the present configuration can provide the optical receiver PR
(the receiving terminal) at low costs and especially in the case of an
optical distribution system, the system cost is reduced to construct the
greatly economical system.
FIG. 5 is a diagram showing a second operational example of the optical
transmission system in FIG. 2. In FIG. 5, according to the present
operational example, the FM portion 100 and the optical modulating portion
2 constitute the optical transmitter PT, and the interference portion 6
and the optical/electrical converting portion 4 constitute the optical
receiver PR. Further, an optical transmission medium such as an optical
fiber and the like is used as the first optical waveguide portion 3 to
expand the physical distance between the optical transmitter PT and the
optical receiver PR. The operational example in FIG. 5 has a feature that
the optical receiver PR can be constituted by relatively inexpensive parts
(since an electric demodulation circuit is not required) and especially in
the case of an optical distribution system, the system cost is reduced to
construct the greatly economical system, although the feature is not so
remarkable as that of the operational example in FIG. 4.
(2) Second specific example of configuration in the first embodiment
FIG. 6 is a block diagram showing a second specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention. In FIG. 6, the optical transmission
system of the present specific example includes an optical directional
coupling portion 206 in place of the optical coupling portion 205, the
first optical waveguide portion 3 and the second optical branch portion
601 in the first operational example of the first specific example (refer
to FIG. 4), and the other configuration is the same as that in FIG. 4.
Accordingly, description will be made of the operation below with an
emphasis on the difference from the first operational example of the first
specific example.
In the second specific example, the optical directional coupling portion
206 combines the optical-phase-modulated signals outputted from the first
and second optical phase modulating portions 203 and 204 to convert the
resultant signal into an optical-amplitude-modulated signal and then
branches the optical-amplitude-modulated signal into first and second
optical signals that have optical-modulated components being set in
opposite phases to each other. In this case, as shown in FIG. 7c, a
waveform of an optical current outputted from the optical/electrical
converting portion 4 becomes a pulse-like signal being in opposite phase
with respect to that of the first operational example (refer to FIG. 3c),
and the number of occurrence of positive differential pluses included in
the pulse-like signal uniquely corresponds to the variations in frequency
of the FM signal. Accordingly, the pulse-like signal is inputted to the
filter F, whereby only a signal component of a band (a low-frequency
component) corresponding to that of an electrical signal inputted to the
FM portion 100 is derived and as a result, the electrical signal can be
acquired. Since equations of the operation are the same as those of the
first operational example except that the phases of the signal waveforms
are different, description of the equations is omitted here.
In the second specific example, the FM portion 100, the optical modulating
portion 2 and the interference portion 6 constitute the optical
transmitter PT, and the optical/electrical converting portion 4
constitutes the optical receiver PR. Further, an optical transmission
medium such as an optical fiber is used as the second optical waveguide
portion 7 to expand the physical distance between the optical transmitter
PT and the optical receiver PR.
As described above, the optical transmission system in the second specific
example requires as a constituent of the optical receiver PR only the
optical/electrical converting portion which is relatively inexpensive, as
in the case with the operational example in FIG. 4, and expensive parts
are all accommodated in the optical transmitter PT. Accordingly, it is
possible to provide the optical receiver PR (the receiving terminal) at
low costs and especially in the case of an optical distribution system,
the system cost is reduced, to construct the greatly economical system.
(3) Third specific example of configuration in the first embodiment
FIG. 8 is a block diagram showing a third specific example of configuration
of the optical transmission. system according to the first embodiment of
the present invention. In FIG. 8, the optical transmission system of the
present specific example includes first and second optical
transparent/reflecting portions 606 and 607 and optical waveguide portion
605 in place of the first and second optical waveguide portions 3 and 7,
the second optical branch portion 601, the optical delay portion 602 and
the optical combining portion 603 in the first specific example (refer to
FIG. 2), and the other configuration is the same as that in FIG. 2.
Therefore, the operation will be explained below with an emphasis on the
difference from the first specific example.
In the third specific example, an optical signal outputted from the optical
coupling portion 205 is guided through the optical waveguide portion 605
to the optical/electrical converting portion 4. The first and second
optical transparent/reflecting portions 606 and 607 are cascaded on the
optical waveguide portion 605 at a prescribed interval. As shown in FIG.
8, a part of the optical signal outputted from the optical coupling
portion 205 is transmitted through the first optical
transparent/reflecting portion 606 and then through the second optical
transparent/reflecting portion 607 and reaches the optical/electrical
converting portion 4 (such optical signal is referred to as a direct
light, hereinafter). Another part of the optical signal outputted from the
optical coupling portion 205 is transmitted through the first optical
transparent/reflecting portion 606, reflected at the second optical
transmitting/reflecting portion 607, further reflected at the first
optical transparent/reflecting portion 606, transmitted through the second
optical transparent/reflecting portion 607 and reaches the
optical/electrical converting portion 4 (such optical signal is referred
to as an indirect light, hereinafter). The optical/electrical converting
portion 4 subjects the direct light and the indirect light to homodyne
detection with the square-law-detection characteristics and creates a
product of the two lights. Propagation time in which the indirect light
goes and returns between the first and second optical
transparent/reflecting portions 606 and 607 installed at the predetermined
interval (hereinafter referred to as round-trip propagation time)
corresponds to the delay T.sub.p in the optical delay portion 602 in the
first and second specific examples.
FIG. 9 is a block diagram showing a first operational example of the
optical transmission system in FIG. 8. In FIG. 9, according to the present
operational example, the FM portion 100, the optical modulating portion 2
and the interference portion 6 constitute the optical transmitter PT, and
the optical/electrical converting portion 4 constitutes the optical
receiver PR. An optical transmission medium such as an optical fiber is
used as the optical waveguide portion 605 to expand the physical distance
between the optical transmitter PT and the optical receiver PR. That is,
in the present operational example, the optical waveguide portion 605
functions as the second optical waveguide portion 7 in FIG. 1 as well.
In the operational example in FIG. 9, t he direct light and the indirect
light transmitted through the optical waveguide portion 605 (the optical
fiber) are both created from the light source 201, so that the optical
wavelengths are the same. Therefore, even when not a special optical fiber
having polarization maintaining features but a normal single -mode optical
fiber is employed as the optical waveguide portion 605, relative
polarization states of the direct light and the indirect light outputted
from the second optical transparent/reflecting portion 607 can be always
maintained constant even while the lights are transmitted through the
optical waveguide portion 605. Accordingly, the polarization states of the
two lights are maintained the same, if the polarization states in the
first and second optical transparent/reflecting portions 606 and 607 are
adjusted so as to be the same, even after the lights are transmitted
through the optical waveguide portion 605. As a result, the homodyne
efficiency in the optical/electrical converting portion 4 reaches its
maximum, thereby making it possible to realize high FM demodulation
efficiency with high stability.
As described in the above, in the operational example in FIG. 9, a
constituent required for the optical receiver PR is only the
optical/electrical converting potion 4 which is relatively inexpensive and
expensive parts are all accommodated in the optical transmitter PT as in
the case with the operational example in FIG. 4. Accordingly, the optical
receiver PR (the receiving terminal) can be provided at low costs and
especially in the case of an optical distribution system, the system cost
decreases to construct the greatly economical system.
FIG. 10 is a block diagram showing a second operational example of the
optical transmission system in FIG. 8. In FIG. 10, according to the
present operational example, the FM portion 100 and the optical modulating
portion 2 constitute the optical transmitter PT, and the interference
portion 6 and the optical/electrical converting portion 4 constitute the
optical receiver PR. Further, an optical transmission medium such as an
optical fiber and the like is used as the optical waveguide portion 605 to
expand the physical distance between the optical transmitter PT and the
optical receiver PR. That is, In the present operational example, the
optical waveguide portion 605 functions as the first optical waveguide
portion 3 in FIG. 1 as well.
As described above, the operational example in FIG. 10 has a feature that
the optical receiver PR can be structured by relatively inexpensive parts
and especially in the case of an optical distribution system, the system
cost decreases to construct the greatly economical system, although the
feature is not so remarkable as that of the operational example in FIG. 9.
The variations and operation requirements of the first to third specific
examples (FIGS. 2, 6 and 8) will be explained in detail below.
A. About modulation schemes
While the first to third specific examples are configured so that an analog
signal is subjected to FM to be optically transmitted, the present
embodiment can be applied to a system in which an analog signal is
subjected to PM to be optically transmitted, and in this case, the effect
is the same as those of the specific examples. As to the configuration in
this case, it is only necessary to replace the FM portion 100 with a known
PM portion and the other configuration of the optical transmission system
may be completely the same as those in the first to third specific
examples.
Moreover, the present embodiment can naturally subject a digital signal, in
place of an analog signal, to frequency modulation or phase modulation for
optical transmission, and in this case, the effect is the same as those of
the specific examples. As to the configuration of this case, it is only
necessary to replace the FM portion 100 with a known FSK portion or PSK
portion and other configuration may be completely the same as those in the
first to third specific examples.
B. Definitions of optical amplitude modulation In the first to third
specific examples, the phase of each optical phase modulation operation in
the first and second optical phase modulating portions 203 and 204, that
is, the phase of each FM signal inputted to the first and second optical
phase modulating portions 203 and 204 is preferably set to a phase which
enlarges an optical-amplitude-modulated component in the optical signal
combined in the optical coupling portion 205 or the optical directional
coupling portion 206. This will be described below.
Here, it is assumed that an electric field component E.sub.1 (t) of an
optical signal outputted from the first optical phase modulating portion
203 is expressed by the following equation (9) and an electric field
component E.sub.2 (t) of an optical signal outputted from the second
optical phase modulating portion 204 is expressed by the following
equation (10).
##EQU5##
In the above equations (9) and (10), fo is an optical frequency, d.sub.1
and d.sub.2 are the phase shift by the first and second optical phase
modulating portions 203 and 204, respectively.
After detecting a combined electric field obtained by combining the
electric field components E.sub.1 (t) and E.sub.2 (t), an optical current
I.sub.0 (t) corresponding to the electric field is expressed by the
following equation (11).
##EQU6##
Here, as expressed by the following equation (12), d.sub.b and d(t) are
introduced as parameters representing relative phases between d.sub.1 and
d.sub.2.
d.sub.b +d(t)=d.sub.1 -d.sub.2 (12)
The above db and d(t) correspond to a bias level (a voltage) and a
modulated signal for a Mach-Zehnder type optical modulator constituted by
the first optical branch portion 202, the first and second optical phase
modulating portions 203 and 204, and the optical coupling portion 205 (or
the optical directional coupling portion 206), respectively.
When the bias level db satisfies the following equation (13), that is, when
phase difference between the first optical phase modulating portion 203
and the second optical phase modulating portion 204 is in phase with the
FM signal, the above equation (11) is expressed by the following equation
(14). As is clear from the following equation (14), the optical current
I.sub.o (t) outputted from the optical/electrical converting portion 4 has
a component proportional to a square of the modulated signal d(t) and an
optical-amplitude-modulated component is generated.
##EQU7##
When the bias level db satisfies the following equation (15), that is, when
the phase difference between the first optical phase modulating portion
203 and the second optical phase modulating portion 204 is in opposite
phase with the FM signal, the above equation (11) is expressed by the
following equation (16). As is clear from the following equation (16), the
optical current I.sub.0 (t) outputted from the optical/electrical
converting portion 4 is proportional to the square of the modulated signal
d(t) and an optical-amplitude-modulated component is generated.
##EQU8##
As explained above, the phase of each FM signal inputted to the first and
second optical phase modulating portions 203 and 204 is adjusted to an
optimal state, which can enlarge an optical-amplitude-modulated component
in the optical signal outputted from the optical coupling portion 205 or
the optical directional coupling portion 206. As a result, it is possible
to perform efficient FM demodulation.
As in the above, in the second and third specific examples, the optical
modulating portion 2 is structured so as to convert an FM signal into an
optical-amplitude-modulated signal and output the
optical-amplitude-modulated signal. The optical modulating portion 2,
however, may adopt an optical intensity modulation scheme in place of the
optical amplitude modulation as described in the first specific example,
and the operation and effect are almost the same as those in the
above-described specific examples.
Moreover, while description was made as to the optical modulating portion
2, mainly to the structure which adopts the "external optical modulation
scheme" using a Mach-Zehnder interferometer structure in the above
specific examples, in the case where the optical modulating portion 2 uses
"optical intensity modulation" in the above first and third specific
examples, it is also possible to adopt a "direct optical modulation
scheme" which is more popular as an optical modulation scheme, that is a
structure in which an injection current to a semiconductor laser element
is direct modulated with an FM signal. In this case, the optical
transmission system can be configured more readily at lower costs.
C. About delay
In the above first to third specific examples, the predetermined delay Tp
in the optical delay portion 602 or the round-trip propagation time Tp
between the first and second optical transparent/reflecting portions 606
and 607 installed at the predetermined interval is preferably set so as to
satisfy the relation in the following equation (17) with respect to a
center angular frequency .omega..sub.c (=2.pi..multidot.f.sub.c) of the FM
signal.
.omega..sub.c.times.T.sub.p =.pi./4 (17)
By satisfying the above-described relation, as is clear from the second
expanded equation (4b) of the equation (4) or the second expanded equation
(8b) of the equation (8) shown in the first specific example, improved are
linearity and demodulation efficiency of the outputted optical current
I.sub.0 (t) from the optical/electrical converting portion 4 relative to
the instantaneous frequency f.sub.t of the FM signal centering on the
frequency f.sub.c (=f.sub.c +.DELTA.f(t)). That is, the FM demodulation
characteristics improve to acquire a demodulated signal with better
quality.
Moreover, while an analog signal is subjected to FM to be optically
transmitted in the first to third specific examples, in the case where a
digital signal is subjected to modulation to be optically transmitted in
place of the analog signal, the predetermined delay T.sub.p in the optical
delay portion 602 or the round-trip propagation time T.sub.p between the
first and second optical transparent/reflecting portions 606 and 607
installed at the predetermined interval is preferably set so as to satisfy
the relation in the above equation (17) with respect to the center
frequency f.sub.c of an FSK modulated signal, or the relation in the
following equation (18) with respect to one symbol length (symbol time) L
of the digital signal.
T.sub.p =L (18)
When the above relation first expanded equation (4a) of the equation (4) or
the first expanded equation (8a) of the equation (8) shown in the first
specific example, a delayed detection system of the FSK (or PSK) modulated
signal is structured to perform demodulation with higher efficiency.
Also, when a digital signal is subjected to phase modulation to optically
transmit a quadrature PSK modulated signal (a QPSK modulated signal), the
interference portion 6 and the optical/electrical converting portion 4
have preferably a double parallel structure shown in FIG. 11. In FIG. 11,
an optical dividing portion 608 divides an optical signal outputted from
the optical modulating portion 2 into first and second received lights. A
first optical interference circuit 6a and a first optical/electrical
converting portion 4a perform homodyne detection for the first received
light, and a second optical interference circuit 6b and a second
optical/electrical converting portion 4b perform homodyne detection for
the second received light. A filter Fa and a filter Fb derive the original
digital signal component from an outputted signal from the first
optical/electrical converting portion 4a and an outputted signal from the
second optical/electrical converting portion 4b, respectively. Further, a
predetermined delay T.sub.1 in the first optical interference circuit 6a
and a predetermined delay T.sub.2 in the second optical interference
circuit 6b are preferably set so as to satisfy the relations in the
following equations (19) and (20), respectively, with respect to one
symbol length (symbol time) L of the digital signal.
T.sub.1 =L/2+L (19)
T.sub.2 =L/2+L (20)
By satisfying the above relations, a delayed detection system is configured
for each of an I signal component and a Q signal component of the QPSK
modulated signal, making it possible to favorably subject the QPSK
modulated signal to demodulation.
While description was made of the case where the QPSK modulated signal is
optically transmitted in the above, more generally speaking, when an
electrical signal inputted to the angle modulating portion 1 is a digital
signal and a PSK modulated signal with 2.sup.n -phase (n is a natural
number) are outputted in place of the FM signal, FIG. 11 is constituted by
a received light dividing portion which divides the inputted optical
signal into 2.sup.n-1 received lights, first to 2.sup.n-1 th interference
portions which are provided for each of the 2.sup.n-1 received lights,
branch each of the received lights into first and second optical signals,
give a predetermined delay to the second optical signal and then combine
the first and second optical signals, and optical/electrical converting
portions provided for each of the first to 2.sup.n-1 th interference
portions.
D. About polarization states
In the first to third specific examples, for example, two optical
propagation paths between the second optical branch portion 601 and
optical combining portion 603 in FIG. 2, two optical propagation paths
between the optical directional coupling portion 206 and optical combining
portion 603 in FIG. 6, or an optical propagation part existing between the
first and second optical transparent/reflecting portions 606 and 607 on
the optical waveguide portion 605 in FIG. 8 are preferably constituted by
an optical transmission medium capable of maintaining polarization such as
a polarization maintaining fiber, an optical waveguide on the substrate of
crystal or glass and the like. This enables the two optical signals (the
first and second optical signals) outputted from the second optical branch
portion 601 or the optical directional coupling portion 206 to be combined
in the optical combining portion 603 with the polarization states of the
two optical signals maintained the same. In other case, the polarization
states of the direct light and the indirect light can be maintained the
same in the optical waveguide portion 605 in FIG. 8. Thereby, the homodyne
efficiency in the optical/electrical converting portion 4 becomes always
maximum to realize high FM demodulation efficiency with high stability.
Further, in the first to third specific examples, the first optical branch
portion 202, the first and second optical phase modulating portions 203
and 204 and the optical coupling portion 205 (or the optical directional
coupling portion 206) are preferably constructed on a same crystal
substrate. Such structure is the same as that of an optical intensity
modulator of normal Mach-Zehnder type. Adopting such structure makes
construction of an optical transmitter more readily.
Moreover, in order to downsize the apparatus, the second optical branch
portion 601, the optical delay portion 602 and the optical combining
portion 603 in the first specific example; the optical delay portion 602
and the optical combining portion 603 in the second specific example; and
a part of the optical waveguide portion 605 and the first and the second
optical transparent/reflecting portions 606 and 607 in the third specific
example may be constructed on the above-described crystal substrate.
Additionally, in the first or second specific example, the optical
propagation paths between the second optical branch portion 601 or the
optical directional coupling portion 206 and the optical combining portion
603 are structured by optical waveguides on the substrate of crystal or
glass, thereby maintaining the polarization states of the two optical
signals to be inputted to the optical combining portion 603 the same and
stable to realize high FM efficiency with stability.
Still further, in the third specific example, the optical propagation paths
consist of the optical waveguide portion 605 and the first and second
optical transparent/reflecting portions 606 and 607 are structured by
optical waveguides on the substrate of crystal or glass, whereby the
polarization states of the direct light and the indirect light are
maintained the same and stable to realize high FM efficiency with
stability.
(4) Fourth specific example of configuration in the first embodiment
FIG. 12 is a block diagram showing a fourth specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention. In FIG. 12, in the optical
transmission system of the present specific example, to one of the two
optical propagation paths between the optical directional coupling portion
206 and the optical combining portion 603 is inserted a polarization
adjusting portion 609. The other configuration of the present specific
example is the same as that of the optical transmission system of the
second specific example (refer to FIG. 6). The operation of the fourth
specific example will be described below with an emphasis on the
difference from the second specific example.
In the present specific example, a polarization state of one of two optical
signals outputted from the optical directional coupling portion 206 is
adjusted in the polarization adjusting portion 609 to equate the
polarization states of the first and second optical signals in the optical
combining portion 603. This makes the homodyne efficiency in the
optical/electrical converting portion 4 maximum, which realizes high FM
demodulation efficiency.
While FIG. 12 shows the case where the polarization adjusting portion 609
is inserted to the optical propagation path on the other side of the
optical propagation path provided with the optical delay portion 602, the
polarization adjusting portion 609 may be inserted to the optical
propagation path provided with the optical delay portion 602, further, to
both of the two optical propagation paths.
In addition, also in the first specific example (refer to FIG. 2), the
polarization adjusting portion 609 may be inserted to both of the two
optical propagation paths between the second optical branch portion 601
and the optical combining portion 603 or to either one of the optical
propagation paths. In this case, the same effect can be obtained as that
of the fourth specific example.
(5) Fifth specific example of configuration in the first embodiment
FIG. 13 is a block diagram showing a fifth specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention. In FIG. 13, in the optical
transmission system of the present specific example, between the FM
portion 100 and the first and second optical phase modulating portions 203
and 204 is additionally inserted an amplitude adjusting portion 8. The
other configuration of the present specific example is the same as that of
the optical transmission system in the second specific example (refer to
FIG. 6). The operation of the fifth specific example will be described
below with an emphasis on the difference from the second specific example.
In the present specific example, the amplitude adjusting portion 8 receives
an FM signal outputted from the FM portion 100 and subjects the FM signal
to waveform shaping so that the amplitude is constant to output the FM
signal to the first and second optical phase modulating portions 203 and
204. In the second specific example, as shown in FIG. 14c, as the
amplitude of the FM signal is smaller, the amplitude of the pulse-like
signal outputted from the optical/electrical converting portion 4
decreases. Therefore, the FM demodulation efficiency is degraded. In
addition, the amplitude of the FM signal fluctuates with time, causing the
FM demodulation efficiency to vary with time to occur distortion of a
demodulated waveform.
Accordingly, by providing the amplitude adjusting portion 8 as in the
present specific example, the amplitude of the FM signal is maintained
constant, making it possible to suppress the degradation or variation of
the FM demodulation efficiency and the distortion of the demodulated
signal.
Similarly, in the first or third specific example (refer to FIG. 2 or FIG.
8), the amplitude adjusting portion 8 may be additionally inserted between
the FM portion 100 and the first and second optical phase modulating
portions 203 and 204. In this case, the same effect as that in the fifth
specific example can be obtained.
(6) Sixth specific example of configuration in the first embodiment
FIG. 15 is a block diagram showing a sixth specific example of
configuration of the optical transmission system according to the first
embodiment of the present invention. In FIG. 15, in the optical
transmission system of the present specific example, a bandwidth limiting
portion 9 is additionally inserted between the FM portion 100 and the
first and second optical phase modulating portions 203 and 204. The other
configuration of the present specific example is the same as that of the
optical transmission system of the second specific example (refer to FIG.
6). Description will be made of the operation of the sixth specific
example below with an emphasis on the difference from the second specific
example.
In the present specific example, the bandwidth limiting portion 9 receives
an FM signal outputted from the FM portion 100 and limits an occupied
bandwidth of the FM signal to output the FM signal to the first and second
optical phase modulating portions 203 and 204. In the above second
specific example, under the influence of non-linearities in FM
demodulation characteristics and the like, a component of the FM signal
passes through the optical/electrical converting portion 4 to remain in
the optical current outputted from the optical/electrical converting
portion 4. At this time, as shown in FIGS. 16a and 16b, in the case where
a modulation index or the frequency deviation of the FM signal is large,
there is possibility that a spectrum of the FM signal component remained
in the output of the optical/electrical converting portion 4 spreads out
in the frequency band of the demodulated signal, to interfere the
demodulated signal. Therefore, as in the present specific example, the
bandwidth limiting portion 9 is provided to previously eliminate a part of
a lower sideband of the spectrum of the FM signal before the FM signal is
inputted to the first and second optical phase modulating portions 203 and
204. Thus, it is possible to prevent the spectrum of the FM signal
component remained in the output of the optical/electrical converting
portion 4 from being superimposed on the frequency band of the demodulated
signal, resulting in improvement in the quality of the demodulated signal.
Also in the first or third specific example (refer to FIG. 2 or FIG. 8),
the bandwidth limiting portion 901 may be additionally inserted between
the FM portion 100 and the first and second optical phase modulating
portions 203 and 204. In this case, the same effect as that of the sixth
specific example can be achieved.
(Second Embodiment)
FIG. 17 is a block diagram showing the configuration of an optical
transmission system according to a second embodiment of the present
invention. FIG. 17 also shows schematic diagrams of frequency spectrums of
signals in respective portions. In FIG. 17, the optical transmission
system of the present embodiment includes an angle modulating portion 1,
an optical modulating portion 2, an optical waveguide portion 3, an
optical branch portion 10, an interference portion 6, a first
optical/electrical converting portion 4, a second optical/electrical
converting portion 4', a filter F and a filter F', and is different from
the first embodiment (refer to FIG. 1) in that the optical branch portion
10, the second optical/electrical portion 4' and the filter F' are added.
Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the first embodiment and the
detailed description thereof are omitted. The difference from the first
embodiment will be mainly described below.
The optical branch portion 10 branches an optical signal (an
optical-intensity-modulated signal or an optical-amplitude-modulated
signal), which is outputted from the optical modulating portion 2 and then
guided by the optical waveguide portion 3, into two. One optical signal of
the two optical signals is subjected to angle demodulation by the
interference portion 6 and the first optical/electrical converting portion
4 and further subjected to filtering processing by the filter F, to be
re-converted into an electrical signal corresponding to an electrical
signal inputted to the angle modulating portion 1. The other optical
signal of the two optical signals, for example, is subjected to square-law
detection in the second optical/electrical converting portion 4'. Thereby,
an optical-intensity-modulated component or an optical-amplitude-modulated
component in the other optical signal is re-converted into an electrical
signal. After that, the signal outputted from the second
optical/electrical portion 4' is subjected to filtering processing in the
filter F', so that the same angle-modulated signal as an angle-modulated
signal outputted from the angle modulating portion 1 can be derived.
As described in the foregoing, the optical transmission system in FIG. 17
converts an angle-modulated signal into an optical signal to branch the
optical signal into a plurality of optical signals, reproduces an original
electrical signal for the angle modulation from each of some of these
optical signals, using the interference portion 6 and the first
optical/electrical converting portion 4, as described in the first
embodiment and subjects the other of these optical signals respectively to
square-law detection in the second optical/electrical converting portion '
to reproduce an angle-modulated signal. This can construct a wired network
using an optical fiber as a backbone and can also integrate the optical
transmission system, if, for example, the angle-modulated signal outputted
from the second optical/electrical converting portion 4' is sent out in
the air as a radio wave, with a wireless network for mobile terminals and
the like. Especially, in the case of utilizing a high-frequency, a
microwave, a millimetre wave and the like, which is thought as an
effective signal for a wireless network, the angle-modulated signal is
received and subjected to demodulation to be the original electrical
signal by a low cost configuration with optical signal processing in a
wired system and at the same time it is sent to mobile terminals and the
like as a radio wave. Thereby, a flexible and economical system can be
constructed.
(Third Embodiment)
FIG. 18 is a block diagram showing the configuration of an optical
transmission system according to a third embodiment of the present
invention. In FIG. 18, the optical transmission system of the present
embodiment includes an angle modulating portion 1, an optical modulating
portion 2, an optical waveguide portion 3, an optical branch portion 10,
an interference portion 6, a first optical/electrical converting portion
4, a local light source 11, an optical combining portion 12, a second
optical/electrical converting portion 4', a filter F and a filter F', and
is different from the second embodiment (refer to FIG. 17) in that the
local light source 11 and the optical combining portion 12 are added.
Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the second embodiment and the
detailed description thereof is omitted here. The difference from the
second embodiment will be mainly described below.
One optical signal of two optical signals, which are obtained by a branch
and outputted in/from the optical branch portion 10, is subjected to angle
modulation by the interference portion 6 and the first optical/electrical
converting portion 4, and further subjected to filtering processing by the
filter F, to be re-converted into an electrical signal corresponding to an
electrical signal inputted to the angle modulating portion 1. The other
optical signal of the two optical signals, which is obtained by the branch
and outputted, is combined with a light outputted from the local light
source 11 by the optical combining portion 12, to be inputted to the
second optical/electrical converting portion 4'. The second
optical/electrical converting portion 4' performs heterodyne detection
with the combined two lights. Thereby, from the second optical/electrical
converting portion 4' is outputted a beat signal of a frequency
corresponding to difference in wavelength between the two lights. The
filter F' derives only beat signal component from the signal outputted
from the second optical/electrical converting portion 4', to output the
beat signal component.
(Fourth Embodiment)
FIG. 19 is a block diagram showing the configuration of an optical
transmission system according to a fourth embodiment of the present
invention. In FIG. 19, the optical transmission system of the present
embodiment includes an angle modulating portion 1, an optical modulating
portion 2, a local light source 13, an optical combining portion 14, an
optical waveguide portion 3, an optical branch portion 10, an interference
portion 6, a first optical/electrical converting portion 4, a second
optical/electrical converting portion 4', a filter F and a filter F', and
is different from the second embodiment (refer to FIG. 17) in that the
local light source 13 and the optical combining portion 14 are added.
Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the second embodiment and the
detailed description thereof is omitted. The difference from the second
embodiment will be mainly described below.
An optical signal outputted from the optical modulating portion 2 is
combined with a light outputted from the local light source 13 by the
optical combining portion 14, to be transmitted to the optical branch
portion 10 by the optical waveguide portion 3. One optical signal of two
optical signals, which are obtained by a branch and outputted in/from the
optical branch portion 10, is subjected to angle demodulation by the
interference portion 6 and the first optical/electrical converting portion
4 and further subjected to filtering processing by the filter F, to be
re-converted into an electrical signal corresponding to an electrical
signal inputted to the angle modulating portion 1. The other optical
signal of the two optical signals, which is obtained by the branch and
outputted, is subjected to heterodyne detection in the second
optical/electrical converting portion 4'. Thereby, outputted from the
second optical/electrical converting portion 4' is a beat signal of a
frequency corresponding to difference in wavelength between the optical
signal outputted from the optical modulating portion 2 and the light from
the local light source 13. The filter F', derives the beat signal
component from the signal outputted from the second optical/electrical
converting portion 4', to output the beat signal component.
(Fifth Embodiment)
FIG. 20 is a block diagram showing the configuration of an optical
transmission system according to a fifth embodiment of the present
invention. In FIG. 20, the optical transmission system of the present
embodiment includes an angle modulating portion 1, an optical modulating
portion 2, a local light source 13, an optical combining portion 14, an
optical waveguide portion 3, an interference portion 6, an
optical/electrical converting portion 4 and a dividing portion 15, and is
different from the first embodiment (refer to FIG. 1) in that the local
light source 13, the optical combining portion 14 and the dividing portion
15 are added. Therefore, the same reference numbers are assigned to the
portions operating in the same manners as those in the first embodiment
and detailed description thereof is omitted. The difference from the first
embodiment will be mainly described below.
An optical signal outputted from the optical modulating portion 2 is
combined with a light outputted from the local light source 13 on the
optical combining portion 14 and then transmitted to the interference
portion 6 by the optical waveguide portion 3. The optical signal is
subjected to angle demodulation and heterodyne detection by the
interference portion 6 and the optical/electrical converting portion 4. At
this time, the outputted signal from the optical/electrical converting
portion 4 includes an angle-demodulated signal component corresponding to
an electrical signal inputted to the angle modulating portion 1 and a beat
signal component of a frequency corresponding to difference in wavelength
between the optical signal outputted from the optical modulating portion 2
and the light outputted from the local light source 13. The dividing
portion 15 branches the outputted signal from the optical/electrical
converting portion 4 into two and subjects the two signal obtained by the
branch to predetermined filtering processing, respectively to separate the
angle-demodulated signal component and the beat signal component and
output the two signals.
As described in the above, the optical transmission systems in FIG. 18,
FIG. 19 and FIG. 20 can provide different kind of networks (for example, a
wired network using an optical fiber and a wireless network) at the same
time, as in the case with the optical transmission system in FIG. 17.
Moreover, regardless of a value of the frequency of the angle-modulated
signal outputted from the angle modulating portion 1, the optical
transmission systems can suitably set the wavelength of the optical signal
from the optical modulating portion 2 and the wavelength of the light from
the local light source 11 or 13, to freely convert the frequency of the
angle-modulated signal which is a beat signal outputted from the second
optical/electrical converting portion 4', thereby making it possible to
generate an angle-modulated signal of the frequency suitable for each
network connected to the second optical/electrical converting portion 4'
and thereafter. Thus, a more flexible system can be configured.
(Sixth Embodiment)
FIG. 21 is a block diagram showing the configuration of an optical
transmission system according to a sixth embodiment of the present
invention. In FIG. 21, the optical transmission system of the present
embodiment includes an angle modulating portion 1, an optical modulating
portion 2, an optical waveguide portion 3, an optical branch portion 10,
an interference portion 6, a first optical/electrical converting portion
4, a second optical/electrical converting portion 4', a local oscillation
portion 16, a filter F and a filter F', and is different from the second
embodiment (refer to FIG. 17) in that the local oscillation portion 16 is
added. Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the second embodiment and the
detailed description thereof is omitted. The difference from the second
embodiment will be mainly described below.
The optical branch portion 10 branches an optical signal, which is
outputted from the optical modulating portion 2 and guided by the optical
waveguide portion 3, into two. One optical signal of the two optical
signals is subjected to angle demodulation by the interference portion 6
and the first optical/electrical converting portion 4 and further
subjected to filtering processing by the filter F, to be re-converted into
an electrical signal corresponding to an electrical signal inputted to the
angle modulating portion 1. The other optical signal of the two optical
signals is inputted to the second optical converting portion 4'. A bias
voltage of the second optical/electrical converting portion 4' is
modulated with a local signal (an unmodulated signal) outputted from the
local oscillation portion 16. Accordingly, the second optical/electrical
converting portion 4' square-law detects the receive d optical signal and
thereby generates a beat signal induced by the angle-modulated signal
outputted from the angle modulating portion 102 and the local signal, to
output the beat signal. The filter F' derives only beat signal component
from the outputted signal from the second optical/electrical converting
portion 4'.
(Seventh Embodiment)
FIG. 22 is a block diagram showing the configuration of an optical
transmission system according to a seventh embodiment of the present
invention. In FIG. 22, the optical transmission system of the present
embodiment includes an angle modulating portion 1, an optical modulating
portion 2, an optical waveguide portion 3, an optical branch portion 10,
an interference portion 6, a first optical/electrical converting portion
4, a second optical/electrical converting portion 4', a local oscillation
portion 16 and a mixing portion 17, and is different from the second
embodiment (refer to FIG. 17) in that the local oscillation portion 16 and
the mixing portion 17 are added. Therefore, the same reference numbers are
assigned to the portions operating in the same manners as those in the
second embodiment and the detailed description thereof is omitted. The
difference from the second embodiment will be mainly described.
The optical branch portion 10 branches an optical signal, which is
outputted from the optical modulating portion 2 and guided by the optical
waveguide portion 3, into two. One optical signal of the two optical
signals is subjected to angle demodulation by the interference portion 6
and the first optical/electrical converting portion 4 and further
subjected to filtering processing by the filter F, to be re-converted into
an electrical signal corresponding to an electrical signal inputted to the
angle modulating portion 1. The other optical signal of the two optical
signals is subjected to square-law detection in the second
optical/electrical converting portion 4' and an
optical-intensity-modulated component or an optical-amplitude-modulated
component of the optical signal is re-converted into an electrical signal.
Thereby, from the second optical/electrical converting portion 4' is
outputted the same angle-modulated signal as the angle-modulated signal
outputted from the angle modulating portion 1. The mixing portion 17 mixes
the angle-modulated signal and a local signal (an unmodulated signal)
outputted from the local oscillation portion 16, to generate a beat signal
induced by the angle-modulated signal and the local signal and output the
beat signal. The filter F' derives only beat signal component from the
outputted signal of the second optical/electrical converting portion 4'
and outputs the beat signal component.
As described in the above, the optical transmission system in FIG. 21 and
FIG. 22 can provide different kind of networks at the same time, as in the
case with the optical transmission system in FIG. 17. Further, regardless
of a value of the frequency of the angle-modulated signal outputted from
the angle modulating portion 1, the optical transmission system can
suitably set the frequency of the local signal outputted from the local
oscillation portion 16 to freely convert the frequency of an
angle-modulated signal which is the beat signal induced by the
angle-modulated signal and the local signal, thereby making it possible to
generate an angle-modulated signal of a frequency suitable for each
network connected to the second optical/electrical converting portion 4'
and thereafter and send the angle-modulated signal. Thus, a more flexible
system can be configured.
(Eighth Embodiment)
FIG. 23 is a block diagram showing the configuration of an optical
transmission system according to an eighth embodiment of the present
invention. FIG. 23 also shows schematic diagrams of frequency spectrums of
signals in respective portions. In FIG. 23, the optical transmission
system of the present embodiment includes an angle modulating portion 1, a
combining portion 18, an optical modulating portion 2, an optical
waveguide portion 3, an optical branch portion 10, an interference portion
6, a first optical/electrical converting portion 4, a second
optical/electrical converting portion 4', a filter F and a filter F', and
is different from the first embodiment (refer to FIG. 1) in that the
combining portion 18, the optical branch portion 10, the second
optical/electrical converting portion 4' and the filter F' are added.
Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the first embodiment and the
detailed description thereof is omitted here. The difference from the
first embodiment will be mainly described below.
The combining portion 18 combines an angle-modulated signal outputted from
the angle modulating portion 1, of which original signal is the first
electrical signal, and the second electrical signal, to output the
resultant signal. The optical modulating portion 2 converts the combined
signal into an optical-modulated signal, to output the optical-modulated
signal. The optical branch portion 10 branches the optical signal guided
by the optical waveguide portion 3 into two. One optical signal of the two
optical signals is subjected to angle demodulation with the interference
portion 6 and the first optical/electrical converting portion 4 and
further subjected to filtering processing by the filter F, to be
re-converted into an electrical signal corresponding to the first
electrical signal inputted to the angle modulating portion 1. The second
optical/electrical converting portion 4' receives the other optical signal
of the two optical signals and re-converts the optical-intensity-modulated
component or the optical-amplitude-modulated component of the optical
signal into an electrical signal with square-law detection, to output an
electrical signal which corresponds to the second electrical signal
inputted to the combining portion 18. The filter F' derives the second
electrical signal component from the outputted signal of the second
optical/electrical converting portion 4' and outputs the second electrical
signal component.
In the eighth embodiment, it is preferable that occupied frequency bands of
the first electrical signal, the angle-modulated signal and the second
electrical signal do not overlap each other as shown in FIG. 23, which
allows to separate each signal with filtering processing on the receiving
side. Hence, a ninth embodiment described below devises a method of
avoiding the overlaps of the occupied frequency bands of the
above-mentioned signals.
(Ninth Embodiment)
FIG. 24 is a block diagram showing the configuration of an optical
transmission system according to the ninth embodiment of the present
invention. The ninth embodiment is an application of the above-mentioned
eighth embodiment and therein first, second, third and fourth signal
processing portions 19, 20, 21 and 22 are added to the configuration of
the eighth embodiment. Each of the third and fourth signal processing
portions 21 and 22 also has a function of filter and therefore the filters
F and F' are not provided in the present embodiment.
The first and second signal processing portions 19 and 20 limit the
frequency bands of a first electrical signal and a second electrical
signal so that the frequency bands occupied by the first and second
electrical signals do not overlap each other, to output the electrical
signals. For example, when the occupied frequency bands of the two
electrical signals overlap each other, either one or both of the bands are
limited. Though this bandwidth limitation causes a distortion of a
reproduced waveform on the receiving side, such waveform distortion is
corrected in the third and fourth signal processing portions 21 and 22.
Furthermore, a carrier frequency in the angle modulating portion 1 is set
to a frequency which prevents the occupied frequency band of the
angle-modulated signal from overlapping with both occupied frequency bands
of the first and second electrical signals.
The third signal processing portion 21 passes only a frequency component
corresponding to the occupied frequency band of the first electrical
signal among signals outputted from the first optical/electrical
converting portion 4. The third signal processing portion 21 also
reproduces waveform information, which was lost in the signal processing
by the first signal processing portion 19, as required. The fourth signal
processing portion 22 passes only a frequency component corresponding to
the occupied frequency band of the second electrical signal among signals
outputted from the second optical/electrical converting portion 4'. The
fourth signal processing portion 22 also reproduces waveform information,
which was lost in the signal processing by the second signal processing
portion 20, as required. Waveform information lost in the first signal
processing portion 19 or the second signal processing portion 20 is a
low-frequency component such as a DC component and the like as an example,
and in this case, the signal waveform becomes, for example, a differential
waveform of the original signal. Accordingly, differential reproduction
(integration) processing is performed in the third signal processing
portion 21 or the fourth signal processing portion 22, thereby making it
possible to reproduce the original signal waveform.
As described in the above, the optical transmission system in FIG. 23 or
FIG. 24 converts a first electrical signal into an angle-modulated signal,
optically transmits the angle-modulated signal, subjects the signal to
angle demodulation using the interference portion 6 and the first
optical/electrical converting portion 4 to produce the first electrical
signal, and at the same time optically transmits a second electrical
signal other than the first electrical signal, to derive the second
electrical signal by the second optical/electrical converting portion 4'.
This makes it possible, for example, to simultaneously transmit different
types of signals such as an analog signal and a digital signal with one
optical fiber. Even in the case where the transmitted signal includes a
high-frequency signal such as a microwave, a millimetre wave and the like,
it is possible to construct a flexible and greatly economical system which
allows reception and demodulation in a low cost configuration with optical
signal processing. While in the eighth and ninth embodiments, the case
where two electrical signals are simultaneously transmitted is described,
three or more electrical signals can, of course, be simultaneously
transmitted.
(Tenth Embodiment)
FIG. 25 is a block diagram showing the configuration of an optical
transmission system according to a tenth embodiment of the present
invention. In FIG. 25, the optical transmission system of the present
embodiment includes a first angle modulating portion 1, a second angle
modulating portion 1', a combining portion 18, an optical modulating
portion 2, an optical waveguide portion 3, an optical branch portion 10, a
first interference portion 6, a second interference portion 6', a first
optical/electrical converting portion 4, a second optical/electrical
converting portion 4', a filter F and a filter F', and is different from
the first embodiment (refer to FIG. 1) in that the second angle modulating
portion 1', the combining portion 18, the optical branch portion 10, the
second interference portion 6', the second optical/electrical converting
portion 4' and the filter F' are added. Therefore, the same reference
numbers are assigned to the portions operating in the same manners as
those in the first embodiment and the detailed description thereof is
omitted. The difference from the first embodiment will be mainly described
below.
The combining portion 18 combines a first angle-modulated-signal outputted
from the first angle modulating portion 1 performing angle modulation
using a first electrical signal as the original signal and a second
angle-modulated signal outputted from the second angle modulating portion
1' performing angle modulation using a second electrical signal as the
original signal, to output the resultant signal. The optical modulating
portion 2 converts the combined signal into an optical-modulated signal,
to output the optical-modulated signal. The optical branch portion 10
branches the optical signal guided by the optical waveguide portion 3 into
two. One optical signal of the two optical signals is subjected to angle
demodulation by the first interference portion 6 and the first
optical/electrical converting portion 4 and further subjected to filtering
processing by the filter F, to be re-converted into an electrical signal
corresponding to the first electrical signal inputted to the first angle
modulating portion 1. The other optical signal of the two optical signals
is subjected to angle demodulation by the second interference portion 6'
and the second optical/electrical converting portion 4' and further
subjected to filtering processing by the filter F', to be re-converted
into an electrical signal corresponding to the second electrical signal
inputted to the second angle modulating portion 1'.
In the present embodiment, it is preferable that occupied frequency bands
of the first and second electrical signals and the first and second
angle-modulated signals do not overlap each other, which allows to
separate each signal with filtering processing on the receiving side.
Hence, an eleventh embodiment described below devises a method of avoiding
overlaps of occupied frequency bands of the above-mentioned signals.
(Eleventh Embodiment)
FIG. 26 is a block diagram showing the configuration of an optical
transmission system according to the eleventh embodiment of the present
invention. The eleventh embodiment is an application of the tenth
embodiment, and therein first, second, third and fourth signal processing
portions 23, 24, 25 and 26 are added to the configuration of the tenth
embodiment. The third and fourth signal processing portions 25 and 26 also
have functions of filter and therefore the filters F and F' are not
provided in the present embodiment.
The first signal processing portion 23 and the second signal processing
portion 24 limit the frequency bands of a first electrical signal and a
second electrical signal so that frequency bands occupied by the first
electrical signal and the second electrical signal do not overlap each
other, to output the two electrical signals. For example, when the
occupied frequency bands of the two electrical signals overlap each other,
either one or both of the bands of the two signals are limited. Though
this bandwidth limitation causes a distortion of a reproduced waveform on
the receiving side, such waveform distortion is corrected by the third and
fourth signal processing portions 25 and 26. Furthermore, carrier
frequencies in the first angle modulating portion 1 and the second angle
modulating portion 1' are set to frequencies which prevent the occupied
frequency bands of the first electrical signal, the second electrical
signal, the first angle-modulated signal and the second angle-modulated
signal from overlapping each other.
The third signal processing portion 25 passes only a frequency component
corresponding to the occupied frequency band of the first electrical
signal among signals outputted from the first optical/electrical
converting portion 4. The third signal processing portion 25 also
reproduces waveform information, which was lost in the signal processing
by the first signal processing portion 23, as required. The fourth signal
processing portion 26 passes only a frequency component corresponding t o
the occupied frequency band of the s econd electrical signal among signals
outputted from the second optical/electrical converting portion 4'. The
fourth signal processing portion 26 also reproduces waveform information,
which was lost in the signal processing by the second signal processing
portion 24, as required. Waveform information lost in the first signal
processing portion 23 or the second signal processing portion 24 is a
low-frequency component such as a DC component and the like, and in this
case, the signal waveform becomes, for example, a differential waveform of
the original signal. Accordingly, differential reproduction (integration)
processing is performed in the third signal processing portion 25 or the
fourth signal processing portion 26, thereby making it possible to
reproduce the original signal waveform.
As described in the above, the optical transmission system in FIG. 25 or
FIG. 26 convert s first and second electrical signals into angle-modulated
signals, then combines and optically transmits the angle-modulated signals
and subjects the resultant signal to angle demodulation using the
interference portion and the optical/electrical converting portion, to
reproduce the first and second electrical signal. This makes it possible
to simultaneously transmit different types of signals such as an analog
signal and a digital signal with one optical fiber. Even in the case where
the transmitted signal includes a high-frequency signal such as a micro
wave, a millimetre wave and the like, it is possible to construct a
flexible and greatly economical system which can receive and subject the
signal to demodulation in a low cost configuration with optical signal
processing. While in the tenth and eleventh embodiments, the case where
two electrical signals are simultaneously transmitted is described, three
or more electrical signals can, of course, be simultaneously transmitted.
(Twelfth Embodiment)
FIG. 27 is a block diagram showing the configuration of an optical
transmission system according to a twelfth embodiment of the present
invention. The optical transmission system of the present embodiment has a
configuration in which the configuration of the first embodiment shown in
FIG. 1 is extended in order to transmit a multichannel
frequency-division-multiplexed signal. In FIG. 27, to an angle modulating
portion 1 is inputted a n-channel frequency-division-multiplexed signal
which is obtained by frequency-division-multiplexing n-channel electrical
signals. The optical branch portion 10 branches an inputted optical signal
into n optical signals. A plurality of optical signal processing portions
constituted by interference portions 6, optical/electrical converting
portions 4 and filters F are provided in parallel, corresponding to each
of the n optical signals outputted from the optical branch portion 10. The
optical signal processing portions each subject the electrical signals on
different channels to demodulation. Therefore, the delay for an optical
signal in the interference portion 6 in an optical signal processing
portion is set to a value most suitable for a frequency of an electrical
signal on a channel to be subjected to demodulation in the optical signal
processing portion. A passband of each of the filters F is designed so as
to pass only an electrical signal on a channel to be subjected to
demodulation. Since the other configuration of the present embodiment is
the same as that in the first embodiment shown in FIG. 1, the same
reference numbers are assigned to the corresponding portions and the
description thereof is omitted here. Such extension as shown in the
present embodiment can, of course, be made in the other above-described
embodiments.
(Thirteenth Embodiment)
FIG. 28 is a block diagram showing the configuration of an optical
transmission system according to a thirteenth embodiment of the present
invention. The present embodiment has a configuration in which the third
embodiment shown in FIG. 18 is extended in order to derive a plurality of
beat signals of different frequencies. In FIG. 28, a plurality of optical
heterodyne portions constituted by local light sources 11, optical
combining portions 12, second optical/electrical converting portions 4'
and filters F' are provided in parallel. Wavelengths of lights outputted
from the local light sources 11 are set so as to be different from each
other depending on frequencies of beat signals to be derived in the
optical heterodyne portions. A passband of each of the filters F' is
designed so as to pass only a beat signal of a frequency to be derived.
Since the other configuration of the present embodiment is the same as
that in the third embodiment shown in FIG. 18, the same reference numbers
are assigned to the corresponding portions and the description thereof is
omitted here.
(Fourteenth Embodiment)
FIG. 29 is a block diagram showing the configuration of an optical
transmission system according to a fourteenth embodiment of the present
invention. The present embodiment has a configuration in which the fourth
embodiment shown in FIG. 19 is extended in order to derive a plurality of
beat signals of different frequencies. In FIG. 29, an optical combining
portion 14 combines an optical signal outputted from an optical modulating
portion 2 and a light outputted from each of a plurality of local light
sources 13. Wavelengths of lights outputted from the local light sources
13 are each set so as to be different from each other. A plurality of
optical detecting portions constituted by second optical/electrical
converting portions 4' and filters F' are provided in parallel. A passband
of each of the filters F' is designed so as to pass only a beat signal of
a frequency to be derived. Since the other configuration of the present
embodiment is the same as that in the fourth embodiment shown in FIG. 19,
the same reference numbers are assigned to the corresponding portions and
the description thereof is omitted here.
Extension for the same purpose as that of the extension performed in the
thirteenth and fourteenth embodiments (that is extension in order to
derive a plurality of beat signals of different frequencies) can be made
in the third to seventh embodiments, although those are not shown.
The more specific examples of configuration and the operational examples of
each portions described for the first embodiment (refer to FIGS. 2 to 16)
can be applied to the second to fourteenth embodiments (FIGS. 17 to 29) as
it is.
While the invention has been described in detail, the foregoing description
is in all aspects illustrative and not restrictive. It is understood that
numerous other modifications and variations can be devised without
departing from the scope of the invention.
* * * * *
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