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| United States Patent |
6,608,854
|
|
Watanabe
|
August 19, 2003
|
Method, device, and system for waveform shaping of signal light
Abstract
The present invention relates to a method, device, and system for waveform
shaping of signal light. The device for waveform shaping of signal light
according to the present invention includes a distributed feedback (DFB)
laser having a stop band defined as the range of wavelengths allowing
laser oscillation, and a drive circuit for supplying a drive current to
the DFB laser so that the DFB laser oscillates at a first wavelength
included in the stop band. Signal light having a second wavelength not
included in the stop band is input into the DFB laser. In the case that
the signal light is provided by optical pulses each having a high level
and a low level, amplitude fluctuations at the high level of the signal
light can be effectively suppressed by suitably setting the power of the
signal light.
| Inventors:
|
Watanabe; Shigeki (Kawasaki, JP)
|
| Assignee:
|
Fujitsu Limited (Kawasaki, JP)
|
| Appl. No.:
|
571384 |
| Filed:
|
May 15, 2000 |
Foreign Application Priority Data
| May 14, 1999[JP] | 11-133576 |
| Current U.S. Class: |
372/96; 372/31 |
| Intern'l Class: |
H01S 003/08 |
| Field of Search: |
372/25-32,31,96
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Ip; Paul
Assistant Examiner: Nguyen; Tuan
Attorney, Agent or Firm: Staas & Halsey LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on, and claims priority to, Japanese patent
application number 11-133576, filed on May 14, 1999, in Japan, and which
is incorporated herein by reference.
This application is related to U.S. application Ser No. 09/217,018, filed
Dec. 21, 1998, now U.S. Pat. No. 6,424,773, Ser. No. 09/386,847 and Ser.
No. 09/456,821, filed Dec. 8, 1999, now U.S. Pat. No. 6,424,774, which are
incorporated herein by reference.
Claims
What is claimed is:
1. A method comprising:
(a) providing, by a waveform shaper, a distributed feedback (DFB) laser
having a stop band defined as a range of wavelengths allowing laser
oscillation;
(b) driving, by a drive circuit, said DFB laser so that said DFB laser
oscillates at a first wavelength included in said stop band; and
(c) inputting, by the distributed feedback laser, signal light having a
second wavelength not included in said stop band into said DFB laser,
wherein said signal light including a high level and a low level and said
(c) comprising adjusting, by the distributed feedback laser, the power of
said signal light so that amplitude fluctuations at said high level of
said signal light are suppressed in said DFB laser.
2. A method according to claim 1, wherein said (b) comprises supplying, by
the drive circuit, a constant drive current to said DFB laser.
3. A method according to claim 1, further comprising inputting control
light, by a light source, having a third wavelength not included in said
stop band into said DFB laser.
4. A method according to claim 3, wherein said control light includes a
substantially constant power.
5. A method according to claim 1, wherein:
said DFB laser includes an output saturation characteristic; and
said signal light is subjected to waveform shaping according to said output
saturation characteristic to obtain waveform-shaped light, which is output
from said DFB laser.
6. A method according to claim 1, further comprising inputting light output
from said DFB laser into a second DFB laser.
7. A method comprising:
(a) dividing, by an interferometer, signal light into first signal light
and second signal light, wherein each of said first signal light and said
second signal light includes a high level and a low level
(b) inputting, by a first optical path, said first signal light into a
first distributed feedback (DFB) laser having a first output saturation
characteristic, and adjusting, by the first DFB laser, the power of said
first signal light so that amplitude fluctuations at said high level of
said first signal light are suppressed in said first DFB laser;
(c) inputting, by a second optical path, said second signal light into a
second distributed feedback (DFB) laser having a second output saturation
characteristic different from said first output saturation characteristic,
and adjusting, by the second DFB laser, the power of said second signal
light so that amplitude fluctuations at said high level of said second
signal light are suppressed in said second DFB laser; and
(d) combining, by the interferometer, first waveform-shaped light output
from said first DFB laser according to said first output saturation
characteristic and second waveform-shaped light output from said second
DFB laser according to said second output saturation characteristic.
8. A method according to claim 7, further comprising imparting a phase
shift to said second waveform-shaped light so that output signal light as
a difference signal between said first waveform-shaped light and said
second waveform-shaped light is obtained in said (d).
9. A device comprising:
a distributed feedback (DFB) laser having a stop band defined as a range of
wavelengths allowing laser oscillation; and
a drive circuit supplying a drive current to said DFB laser so that said
DFB laser oscillates at a first wavelength included in said stop band;
wherein
signal light having a second wavelength not included in said stop band
being input into said DFB laser;
said signal light includes a high level and a low level; and
the power of said signal light is adjusted so that amplitude fluctuations
at said high level of said signal light are suppressed in said DFB laser.
10. A device according to claim 9, further comprising a light source
inputting control light having a third wavelength not included in said
stop band into said DFB laser.
11. A device according to claim 10, wherein the power of said control light
is set so that an increase in noise at said low level of said signal light
is suppressed.
12. A device according to claim 10, further comprising an optical filter
optically connected to an output of said DFB laser, said optical filter
having a passband including said second wavelength and not including said
first and third wavelengths.
13. A device according to claim 9, further comprising an optical filter
optically connected to an output of said DFB laser, said optical filter
having a passband including said second wavelength and not including said
first wavelength.
14. A device according to claim 9, further comprising a saturable absorber
optically connected to at least one of an input and an output of said DFB
laser.
15. A device according to claim 9, wherein:
said DFB laser comprises a first DFB laser having a first output saturation
characteristic and a second DFB laser having a second output saturation
characteristic; and
said device further comprises:
a first optical coupler dividing said signal light into first signal light
to be input into said first DFB laser and second signal light to be input
into said second DFB laser; and
a second optical coupler combining first waveform-shaped light output from
said first DFB laser according to said first output saturation
characteristic and second waveform-shaped light output from said second
DFB laser according to said second output saturation characteristic.
16. A device according to claim 15, further comprising a phase shifter
imparting a phase shift to said second waveform-shaped light so that
output signal light is obtained as a difference signal between said first
waveform-shaped light and said second waveform-shaped light.
17. A device comprising:
a first optical coupler dividing signal light into first signal light and
second signal light, each of said first signal light and said second
signal light including a high level and a low level;
a first distributed feedback (DFB) laser into which said first signal light
is to be input, said first DFB laser having a first output saturation
characteristic, and adjusting, by the first DFB laser, the power of said
first signal light so that amplitude fluctuations at said high level of
said first signal light are suppressed in said first DFB laser;
a second distributed feedback (DFB) laser into which said second signal
light is to be input, said second DFB laser having a second output
saturation characteristic different from said first output saturation
characteristic, and adjusting, by the second DFB laser, the power of said
second signal light so that amplitude fluctuations at said high level of
said second signal light are suppressed in said second DFB laser; and
a second optical coupler combining first waveform-shaped light output from
said first DFB laser according to said first output saturation
characteristic and second waveform-shaped light output from said second
DFB laser according to said second output saturation characteristic.
18. A device according to claim 17, further comprising a phase shifter
imparting a phase shift to said second waveform-shaped light so that
output signal light is obtained as a difference signal between said first
waveform-shaped light and said second waveform-shaped light.
19. A device according to claim 17, wherein said first and second optical
couplers are provided by a Mach-Zehnder interferometer formed on a
waveguide substrate.
20. A device comprising:
an optical branch dividing signal light into first signal light and second
signal light;
a waveform shaper receiving said first signal light and performing waveform
shaping of said first signal light received to output resultant
waveform-shaped light;
a clock regenerator receiving said second signal light and regenerating
clock pulses according to said second signal light received; and
an optical retiming section receiving said waveform-shaped light and said
clock pulses and correcting the timing of said waveform-shaped light
according to said clock pulses to output resultant regenerated signal
light;
said waveform shaper comprising:
a distributed feedback (DFB) laser into which said first signal light is to
be input, said DFB laser having a stop band defined as a range of
wavelengths allowing laser oscillation; and
a drive circuit supplying a drive current to said DFB laser so that said
DFB laser oscillates at a first wavelength included in said stop band;
said signal light having a second wavelength not included in said stop
band.
21. A device according to claim 20, wherein:
said clock regenerator comprises a mode-locked laser into which said second
signal light is introduced; and
said clock pulses are regenerated by mode locking of said mode-locked laser
according to said second signal light.
22. A device according to claim 20, wherein said waveform shaper comprises
a nonlinear optical loop mirror.
23. A system comprising:
an optical fiber transmission line transmitting signal light; and
at least one optical repeater arranged along said optical fiber
transmission line;
each of said at least one optical repeater comprising:
a distributed feedback (DFB) laser into which said signal light transmitted
by said optical fiber transmission line is supplied, said DFB laser having
a stop band defined as a range of wavelengths allowing laser oscillation;
and
a drive circuit supplying a drive current to said DFB laser so that said
DFB laser oscillates at a first wavelength included in said stop band;
said signal light having a second wavelength not included in said stop
band, wherein said signal light includes a high level and a low level, and
the power of said signal light is adjusted so that amplitude fluctuations
at said high level of said signal light are suppressed in said DFB laser.
24. A system comprising:
an optical fiber transmission line transmitting signal light; and
an optical receiver connected to an output end of said optical fiber
transmission line;
said optical receiver comprising:
a distributed feedback (DFB) laser into which said signal light transmitted
by said optical fiber transmission line is supplied, said DFB laser having
a stop band defined as a range of wavelengths allowing laser oscillation;
and
a drive circuit supplying a drive current to said DFB laser so that said
DFB laser oscillates at a first wavelength included in said stop band;
said signal light having a second wavelength not included in said stop
band, wherein said signal light includes a high level and a low level, and
the power of said signal light is adjusted so that amplitude fluctuations
at said high level of said signal light are suppressed in said DFB laser.
25. A device comprising:
a plurality of distributed feedback (DFB) lasers cascaded through
respective optical filters so that signal light is passed therethrough;
each of said DFB lasers having a stop band defined as a range of
wavelengths allowing laser oscillation; and
each of said DFB lasers being driven, by a corresponding drive circuit, so
as to oscillate at a first wavelength included in said stop band;
said signal light having a second wavelength not included in said stop
band, wherein said signal light includes a high level and a low level, and
the power of said signal light is set so that amplitude fluctuations at
said high level of said signal light are suppressed in said DFB lasers.
26. A method comprising:
providing, by a waveform shaper, a distributed feedback (DFB) laser
including an output saturation characteristic;
inputting, by said DFB laser, signal light into said DFB laser; and
outputting, by said DFB laser, wavelength-shaped light obtained by waveform
shaping of said signal light according to said output saturation
characteristic, wherein said signal light including a high level and a low
level and said inputting comprising adjusting, by the distributed feedback
laser, the power of said signal light so that amplitude fluctuations at
said high level of said signal light are suppressed In said DFB laser.
27. A method comprising:
oscillating by a distributed feedback (DFB) laser at a first wavelength;
inputting, by the DFB laser, signal light having a second wavelength
different from said first wavelength into said DFB laser; and
adjusting, by the DFB laser, the power of said signal light so that said
signal light is subjected to waveform shaping in said DFB laser, said
signal light including a high level and a low level; and the power of said
signal light is adjusted so that amplitude fluctuations at said high level
of said signal light are suppressed in said DFB laser.
28. A device comprising:
a distributed feedback (DFB) laser; and
a drive circuit supplying a drive current to said DFB laser so that said
DFB laser oscillates at a first wavelength;
signal light having a second wavelength different from said first
wavelength being input into said DFB laser;
the power of said signal light being adjusted so that said signal light is
subjected to waveform shaping in said DFB lasers, said signal light
including a high level and a low level, and the power of said signal light
is adjusted so that amplitude fluctuations at said high level of said
signal light are suppressed in said DFB laser.
29. An optical waveform shaping method comprising:
supplying, by a drive circuit, a current to a laser diode so that said
laser diode emits laser light; and
inputting, by the laser diode, light having a wavelength different from the
wavelength of said laser light emitted from said laser diode, into said
laser diode to thereby perform optical waveform shaping, said input light
including a high level and a low level and the power of said input light
is adjusted so that amplitude fluctuations at said high level of said
signal light are suppressed in said laser diode.
30. An optical waveform shaping device comprising:
a laser diode;
current supplying means for supplying a current to said laser diode so that
said laser diode emits laser light; and
light inputting means for inputting light having a wavelength different
from the wavelength of said laser light emitted from said laser diode,
into said laser diode, said input light including a high level and a low
level and the power of said input light is adjusted so that amplitude
fluctuations at said high level of said signal light are suppressed in
said laser diode.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method, device, and system for waveform
shaping of signal light.
2. Description of the Related Art
In an optical fiber communication system that has been put to practical use
in recent years, a reduction in signal power due to transmission line
loss, coupling loss, etc. is compensated by using an optical amplifier
such as an erbium doped fiber amplifier (EDFA). The optical amplifier is
an analog amplifier, which functions to linearly amplify a signal. In this
kind of optical amplifier, amplified spontaneous emission (ASE) noise
generated in association with the amplification is added to cause a
reduction in signal-to-noise ratio (S/N ratio), so that the number of
repeaters is limited to result in the limit of a transmission distance.
Further, waveform degradation due to the chromatic dispersion owned by an
optical fiber and the nonlinear optical effects in the fiber is another
cause of the transmission limit. To eliminate such a limit, a regenerative
repeater for digitally processing a signal is required, and it is
desirable to realize such a regenerative repeater. In particular, an
all-optical regenerative repeater capable of performing all kinds of
signal processing in optical level is important in realizing a transparent
operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are
amplitude restoration or reamplification, timing restoration or retiming,
and waveform shaping or reshaping. Of these functions, special attention
is paid to the reshaping function in the present invention to provide an
ultra high-speed waveform shaping device having a simple configuration by
using a distributed feedback (DFB) laser in its saturated operational
condition.
The most general one of conventional waveform shapers is an optoelectric
(OE) type waveform shaper so designed as to once convert input signal
light into an electrical signal by using a photodetector such as a
photodiode, next subject this electrical signal to electrical waveform
shaping by using a logic circuit, and thereafter modulate laser light by
the waveform-shaped signal. Such an OE type waveform shaper is used for a
regenerative repeater in a conventional optical communication system.
However, the operating speed of the OE type waveform shaper is limited by
an electronic circuit for signal processing, so that the bit rate of an
input signal to the regenerative repeater is fixed to a low rate.
On the other hand, as an all-optical waveform shaper capable of performing
all kinds of processing in optical level, there has already been proposed
various ones including a nonlinear switch accompanying wavelength
conversion, such as a nonlinear optical loop mirror (NOLM) or a Michelson
or Mach-Zehnder interferometer, and a switch employing a saturable
absorber (see Japanese Patent Application No. 10-176316 for the related
art).
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a novel
method, device, and system for waveform shaping independent of the bit
rate, pulse shape, etc. of signal light. Other objects of the present
invention will become apparent from the following description.
In accordance with a first aspect of the present invention, there is
provided a method for waveform shaping of signal light. In this method, a
distributed feedback (DFB) laser having a stop band defined as the range
of wavelengths allowing laser oscillation is first provided. The DFB laser
is driven so as to oscillate at a first wavelength included in the stop
band. Signal light having a second wavelength not included in the stop
band is input into the DFB laser.
The driving of the DFB laser is performed, for example, by supplying a
constant drive current to the DFB laser.
The signal light is provided, for example, by optical pulses each having a
high level and a low level. In this case, amplitude fluctuations at the
high level of the signal light are suppressed in the DFB laser by the
application of the present invention. This suppression effect can be
optimized by adjusting the power of the signal light to be input into the
DFB laser.
According to the first aspect of the present invention, the waveform
shaping of signal light can be performed without the need for
opto/electric conversion or electro/optic conversion, so that it is
possible to provide a novel method for waveform shaping independent of the
bit rate, pulse shape, etc. of signal light.
Preferably, control light having a third wavelength not included in the
stop band is input into the DFB laser. The control light has a
substantially constant power, for example. By inputting the control light
into the DFB laser, an excess increase in noise at the low level of the
signal light is suppressed. This suppression effect can be optimized by
adjusting the power of the control light.
The DFB laser has an output saturation characteristic as will be
hereinafter described. The signal light is subjected to waveform shaping
according to the output saturation characteristic to obtain
waveform-shaped light, which is output from the DFB laser.
In accordance with a second aspect of the present invention, there is
provided a method for waveform shaping of signal light. In this method,
signal light is divided into first signal light and second signal light.
The first signal light is input into a first DFB laser having a first
output saturation characteristic. The second signal light is input into a
second DFB laser having a second output saturation characteristic
different from the first output saturation characteristic. First
waveform-shaped light output from the first DFB laser according to the
first output saturation characteristic and second waveform-shaped light
output from the second DFB laser according to the second output saturation
characteristic are combined.
Preferably, a phase shift is imparted to the first or second
waveform-shaped light so that output signal light as a difference signal
between the first waveform-shaped light and the second waveform-shaped
light is obtained. This phase shift is set so that the difference between
a phase shift generated in the first waveform-shaped light and a phase
shift generated in the second waveform-shaped light becomes .pi. (or an
odd multiple of .pi.). According to the second aspect of the present
invention, a more rigid discrimination characteristic in relation to the
output signal light can be obtained.
In accordance with a third aspect of the present invention, there is
provided a device comprising a DFB laser having a stop band defined as the
range of wavelengths allowing laser oscillation, and a drive circuit for
supplying a drive current to the DFB laser so that the DFB laser
oscillates at a first wavelength included in the stop band. Signal light
having a second wavelength not included in the stop band is input into the
DFB laser.
According to the third aspect of the present invention, it is possible to
provide a device suitable for use in carrying out the method according to
the present invention.
In accordance with a fourth aspect of the present invention, there is
provided a device comprising first and second optical couplers and first
and second DFB lasers. The first optical coupler divides signal light into
first signal light and second signal light. The first signal light and the
second signal light are input into the first and second DFB lasers,
respectively. The first and second DFB lasers have first and second output
saturation characteristics, respectively, wherein the first and second
output saturation characteristics are different from each other. The
second optical coupler combines first waveform-shaped light output from
the first DFB laser according to the first output saturation
characteristic and second waveform-shaped light output from the second DFB
laser according to the second output saturation characteristic.
In accordance with a fifth aspect of the present invention, there is
provided a device comprising an optical branch, a waveform shaper, a clock
regenerator, and an optical retiming section. The optical branch divides
signal light into first signal light and second signal light. The waveform
shaper receives the first signal light and performs waveform shaping of
the first signal light received to output resultant waveform-shaped light.
The clock regenerator receives the second signal light and regenerates
clock pulses according to the second signal light received. The optical
retiming section receives the waveform-shaped light and the clock pulses
and corrects the timing of the waveform-shaped light according to the
clock pulses to output resultant regenerated signal light. The waveform
shaper may be provided by the device according to the third or fourth
aspect of the present invention.
The clock regenerator comprises a mode-locked laser (MLL) into which the
second signal light is introduced, for example. In this case, the clock
pulses may be regenerated by mode locking of the MLL according to the
second signal light.
The waveform shaper comprises a nonlinear optical loop mirror, for example.
In accordance with a sixth aspect of the present invention, there is
provided a system comprising an optical fiber transmission line for
transmitting signal light, and at least one optical repeater arranged
along the optical fiber transmission line. Each of the at least one
optical repeater may be provided by the device according to the third,
fourth, or fifth aspect of the present invention.
In accordance with a seventh aspect of the present invention, there is
provided a system comprising an optical fiber transmission line for
transmitting signal light, and an optical receiver connected to an output
end of the optical fiber transmission line. The optical receiver may
include the device according to the third, fourth, or fifth aspect of the
present invention.
In accordance with an eighth aspect of the present invention, there is
provided a device comprising a plurality of DFB lasers cascaded so that
signal light is passed therethrough. Each DFB laser has a stop band
defined as the range of wavelengths allowing laser oscillation. Each DFB
laser is driven so as to oscillate at a first wavelength included in the
stop band. The signal light has a second wavelength not included in the
stop band.
According to the present invention, there is provided a method including
the step of providing a DFB laser having an output saturation
characteristic. Signal light is input into the DFB laser. As a result, the
signal light undergoes waveform shaping according to the output saturation
characteristic to obtain waveform-shaped light, which is output from the
DFB laser.
According to the present invention, there is provided a method including
the step of providing a DFB laser oscillating at a first wavelength.
Signal light having a second wavelength different from the first
wavelength is input into the DFB laser. The power of the signal light is
adjusted so that the signal light undergoes waveform shaping in the DFB
laser.
According to the present invention, there is provided a device comprising a
DFB laser and a drive circuit for supplying a drive current to the DFB
laser so that the DFB laser oscillates at a first wavelength. Signal light
having a second wavelength different from the first wavelength is input
into the DFB laser. The power of the signal light is adjusted so that the
signal light undergoes waveform shaping in the DFB laser.
In the present invention as described above, the DFB laser oscillates in a
single mode. By using this, signal light having a wavelength different
from the laser oscillation wavelength of the DFB laser in its oscillating
state is input into the DFB laser, thereby performing waveform shaping.
However, the present invention is not limited by the use of the DFB laser,
but any other lasers such as a semiconductor laser diode and a
gain-clamped optical amplifier may be used. That is, optical pulses or
signal light having a wavelength different from the laser oscillation
wavelength of a laser in its oscillating state are/is input into the laser
to thereby obtain a waveform shaping effect to the optical pulses or the
signal light. For example, in a laser oscillating in multiple modes, such
as a Fabry-Perot laser diode, a plurality of laser oscillation wavelengths
are present, so that the signal light to be subjected to waveform shaping
has a wavelength different from these laser oscillation wavelengths.
In accordance with another aspect of the present invention, there is
provided an optical waveform shaping method comprising the steps of
supplying a current to a laser diode so that the laser diode emits laser
light, and inputting light having a wavelength different from the
wavelength of the laser light emitted from the laser diode, into the laser
diode to thereby perform optical waveform shaping.
In accordance with a further aspect of the present invention, there is
provided an optical waveform shaping device comprising a laser diode,
current supplying means for supplying a current to the laser diode so that
the laser diode emits laser light, and light inputting means for inputting
light having a wavelength different from the wavelength of the laser light
emitted from the laser diode, into the laser diode.
The above and other objects, features and advantages of the present
invention and the manner of realizing them will become more apparent, and
the invention itself will best be understood from a study of the following
description and appended claims with reference to the attached drawings
showing some preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram for illustrating the principle of the present
invention;
FIGS. 2A and 2B are graphs showing examples of the input-output
characteristic of a DFB-LD (distributed feedback laser diode);
FIG. 3 is a block diagram showing a first preferred embodiment of the
waveform shaper according to the present invention;
FIG. 4 is a block diagram showing a second preferred embodiment of the
waveform shaper according to the present invention;
FIG. 5A is a graph showing the input-output characteristic of each DFB-LD
shown in FIG. 4, and FIG. 5B is a graph showing the input-output
characteristic of the waveform shaper shown in FIG. 4;
FIG. 6 is a block diagram showing a third preferred embodiment of the
waveform shaper according to the present invention;
FIG. 7 is a block diagram showing a preferred embodiment of the all-optical
signal regenerating device according to the present invention;
FIG. 8 is a block diagram showing a preferred embodiment of the clock
regenerator shown in FIG. 7;
FIG. 9 is a diagram showing a preferred embodiment of the optical retiming
section shown in FIG. 7;
FIG. 10 is a block diagram showing a preferred embodiment of the system
according to the present invention; and
FIG. 11 is a block diagram showing a fourth preferred embodiment of the
waveform shaper according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Some preferred embodiments of the present invention will now be described
in detail with reference to the attached drawings.
The principle of the present invention will first be described with
reference to FIG. 1. The method according to the present invention
includes providing a DFB laser diode (DFB-LD) 2 as a distributed feedback
(DFB) laser having a stop band defined as the range of wavelengths
allowing laser oscillation. The width of the stop band is 0.5 to 1.0 nm,
for example. The DFB-LD 2 is driven so as to oscillate at a first
wavelength .lambda..sub.0 included in the stop band. Oscillated laser
light obtained as the result of this oscillation is output from the DFB-LD
2. In general, the oscillated laser light is continuous wave (CW) light.
The DFB-LD 2 in its oscillating state has a constant clamped gain with
respect to the oscillated laser light. Signal light having a second
wavelength .lambda..sub.s not included in the stop band is input into the
DFB-LD 2 in the oscillating state. The input signal light is subjected to
waveform shaping in the DFB-LD 2, and resultant waveform-shaped light is
output from the DFB-LD 2. The waveform-shaped light has the second
wavelength .lambda..sub.a.
The DFB-LD 2 may be driven by supplying a constant drive current (bias
current) to the DFB-LD 2. However, the present invention is not limited to
this method, but a DFB laser may be driven by any other methods such as
optical pumping.
In the case that the DFB-LD 2 is driven by a current, which is set to a
constant value, the total number of carriers contributing to laser
oscillation and signal amplification is constant, and the total number of
photons output from the DFB-LD 2 also becomes constant. Accordingly, by
inputting external signal light given as optical pulses into the DFB-LD 2,
the number of photons near the peak of each pulse becomes larger than that
near the leading edge or the trailing edge, thereby exhibiting an effect
that gain saturation is enhanced. By using this effect, amplitude
fluctuations near the peak of each pulse of signal light having amplitude
fluctuations as shown in FIG. 1 can be suppressed to thereby obtain
waveform-shaped light with less amplitude fluctuations.
Thus, according to an aspect of the present invention, a DFB laser having
an output saturation characteristic is used, and waveform-shaped light
obtained by waveform shaping of signal light according to the output
saturation characteristic is output from the DFB laser.
According to the present invention, it is also possible to suppress the
accumulation of amplitude noise due to the accumulation of ASE in a
transmission system using a plurality of cascaded optical amplifiers. The
waveform degradation due to the accumulation of ASE is mainly due to
signal/ASE beat noise, and the waveform degradation becomes most
remarkable near the peak of a signal pulse. Accordingly, the application
of the present invention can effectively compensate for such waveform
degradation.
In general, the response of a laser in its gain clamped condition to a
change in light intensity is as fast as tens of ps or less. Accordingly,
the waveform shaping according to the present invention can be applied
also to a short pulse having a pulse width of tens of ps or less or to an
ultra high-speed signal having a speed of tens to hundreds of Gb/s. This
will now be described more specifically.
In a usual semiconductor optical amplifier (SOA), the speed (modulation
rate or bit rate) of input signal light is limited because of speed
limitation (usually, several GHz) of absorption restoration time or
carrier density variations, and waveform distortion is added to a signal
having a speed equal to or greater than a limited value. To the contrary,
in a gain clamped laser, excess carriers are recombined in an active layer
by stimulated emission, so that the absorption restoration time can be
shortened. In a sufficiently saturated condition, a high-speed response of
tens of ps or less can be achieved to allow the waveform shaping also to
an ultra high-speed signal as mentioned above.
Thus, according to the present invention, the waveform shaping of signal
light can be performed without the dependence on the bit rate and pulse
shape of the signal light.
FIGS. 2A and 2B are graphs showing examples of the input-output
characteristic of a DFB-LD. More specifically, the solid line in each of
FIGS. 2A and 2B shows an example of the relation between input power
P.sub.s-in of signal light and output power P.sub.o-out of waveform-shaped
light in the DFB-LD 2 shown in FIG. 1. The characteristic shown by the
broken line in each of FIGS. 2A and 2B will be hereinafter described.
The characteristic shown by the solid line in FIG. 2A is such that when the
input power P.sub.o-in is equal to or less than a threshold P.sub.th, the
output power P.sub.s-out increases in proportion to the input power
P.sub.s-in, whereas when the input power P.sub.a-in is greater than the
threshold P.sub.th, the output power P.sub.a-out becomes a constant
saturated output power P.sub.aat. The reason for the constant saturated
output power P.sub.aat is that when the input power P.sub.s-in reaches the
threshold P.sub.th, the laser oscillation is stopped, and the gain is
saturated for a higher input power, resulting in a constant output power.
Accordingly, in the case that the signal light is provided by optical
pulses each having a high level and a low level, the amplitude
fluctuations at the high level of the signal light can be effectively
suppressed in the DFB-LD 2 by setting the low level to a zero level and
setting the high level to a value larger than the threshold P.sub.th.
Thus, according to an aspect of the present invention, the power of signal
light is adjusted so that the amplitude fluctuations at the high level of
the signal light are suppressed in the DFB-LD 2. However, the present
invention is not limited to this method, but the high level of the signal
light may be set to a value smaller than the threshold P.sub.th. The
reason for this setting is that there is a case that the output power
tends to be saturated for an input power smaller than the threshold,
depending upon the dynamic characteristics or the like of a DFB laser.
As shown in FIG. 2B, there is a case that when the input power P.sub.s-in
reaches the threshold P.sub.th to stop the laser oscillation, the gain of
signal light rapidly increases. Since the threshold P.sub.th is
substantially equal to the power of oscillated laser light, the gain
increases by about 3 dB, for example. In this case, the input-output
characteristic in the range of input powers larger than the threshold
P.sub.th becomes closer to that of a so-called digital discriminator,
thereby effectively suppressing the amplitude fluctuations at the high
level.
FIG. 3 is a block diagram showing a first preferred embodiment of the
waveform shaper according to the present invention. This waveform shaper
includes a DFB-LD 2 that can provide an output saturation characteristic
according to the present invention, and a drive circuit 4 for supplying a
constant or controlled drive current (bias current) to the DFB-LD 2 so
that the DFB-LD 2 oscillates at a wavelength A.sub.0. Signal light having
a wavelength .lambda..sub.s whose power is adjusted to an optimum value in
accordance with the above-mentioned principle is supplied to the DFB-LD 2,
and the signal light is subjected to waveform shaping according to the
output saturation characteristic in the DFB-LD 2 to obtain waveform-shaped
light having the wavelength .lambda..sub.s. The waveform-shaped light thus
obtained is output from the DFB-LD 2.
To supply the signal light to the DFB-LD 2, an optical fiber for
transmitting the signal light and a lens for optically coupling an output
end of the optical fiber and a first end of the DFB-LD 2 may be used.
Further, to effectively use the waveform-shaped light output from the
DFB-LD 2, an optical fiber to which the waveform-shaped light is to be
introduced and a lens for optically coupling an input end of the optical
fiber and a second end of the DFB-LD 2 may be used. Each lens may be
formed integrally with the corresponding optical fiber by heating the end
of the corresponding optical fiber.
The stop band of the DFB-LD 2 is defined as the range of wavelengths
allowing laser oscillation, so that the wavelength .lambda..sub.0 of the
oscillated laser light is included in the stop band. The wavelength
.lambda..sub.s of the signal light is limited only by the fact that it is
not included in the stop band. For example, in the case that the laser
oscillation wavelength .lambda..sub.0 of the DFB-LD 2 is 1550.0 nm and the
stop band is the range of 1549.5 to 1550.5 nm, the wavelength
.lambda..sub.a of the signal light is set to .lambda..sub.a <1549.5 nm
or .lambda..sub.a >1550.5 nm.
The waveform shaper shown in FIG. 3 is characterized in that it further
includes a probe light source 6 for inputting probe light (control light)
into the DFB-LD 2. The probe light has a third wavelength .lambda.hd p not
included in the stop band of the DFB-LD 2. The wavelength .lambda..sub.p
is not limited by whether or not it coincides with the wavelength
.lambda..sub.a of the signal light. However, in consideration of signal
processing to be performed in the subsequent stage, the wavelength
.lambda..sub.p is preferably different from the wavelength .lambda..sub.s.
As shown in FIG. 3, the probe light is input to the first end of the DFB-LD
2 so as to propagate in the same direction as the direction of propagation
of the signal light in the DFB-LD 2. Alternatively, the probe light may be
input to the second end of the DFB-LD 2 so as to propagate in a direction
opposite to the direction of propagation of the signal light in the DFB-LD
2. To input the probe light into the DFB-LD 2 along the same optical path
as that of the signal light, an optical coupler using a half-mirror or a
fiber fusion type optical coupler or WDM (wavelength division multiplex)
coupler may be used.
By using the probe light source 6, an excess increase in noise at the low
level of the signal light can be suppressed. Further, in the case that the
low level of the signal light continues, undesirable laser oscillation can
be prevented to stabilize the operation of the waveform shaper. If the
probe light source 6 is not used, there is a case that ASE-ASE beat noise
accumulated at the low level of the signal light and perturbation of the
low level due to transmitted waveform degradation cannot be effectively
suppressed. Further, the oscillating state and unoscillating state of the
DFB-LD 2 are repeated with changes of the high level and low level of the
signal light, causing possible instability of the operation of the
waveform shaper in this preferred embodiment, the probe light has a
constant power. Accordingly, the probe light having a constant power is
supplied to the DFB-LD 2 also at the low level of the signal light, thus
obtaining the above-mentioned technical effects. The power of the probe
light is adjusted so as to suppress an increase in noise at the low level
of the signal light.
FIG. 4 is a block diagram showing a second preferred embodiment of the
waveform shaper according to the present invention. This waveform shaper
has a Mach-Zehnder interferometer 10 formed on a waveguide substrate 8.
The Mach-Zehnder interferometer 10 has an input port 12 to which signal
light or signal light and probe light is/are to be input, an output port
14 from which waveform-shaped light is to be output, Y-branches 16 and 18
optically connected to the input port 12 and the output port 14,
respectively, and optical paths 20 and 22 for optically connecting the
Y-branches 16 and 18.
A first DFB-LD 2(#1) is provided on the optical path 20, and a second
DFB-LD 2(#2) and a phase shifter 24 are provided on the optical path 22 so
as to be arranged in this order in a direction from the input port 12
toward the output port 14. The DFB-LDs 2(#1) and 2(#2) are supplied with
drive currents (bias currents) I.sub.b1 and I.sub.b2 from drive circuits
(not shown), respectively, and each DFB-LD accordingly has an output
saturation characteristic according to the present invention. More
specifically, the drive currents I.sub.b1 and I.sub.b2 are set to
different values, for example, so that the DFB-LD 2(#1) has a first output
saturation characteristic and the DFB-LD 2(#2) has a second output
saturation characteristic different from the first output saturation
characteristic.
The signal light supplied to the input port 12 is divided into first signal
light and second signal light at the Y-branch 16. In general, the power of
the first signal light is substantially equal to the power of the second
signal light. The first signal light is supplied through the optical path
20 to the DFB-LD 2(#1). In the DFB-LD 2(#1), the first signal light is
subjected to waveform shaping according to the first output saturation
characteristic to obtain first waveform-shaped light, which is in turn
output from the DFB-LD 2(#1). On the other hand, the second signal light
is supplied through the optical path 22 to the DFB-LD 2(#2). In the DFB-LD
2(#2), the second signal light is subjected to waveform shaping according
to the second output saturation characteristic to obtain second
waveform-shaped light, which is in turn output from the DFB-LD 2(#2). The
first waveform-shaped light and the second waveform-shaped light are
combined at the Y-branch 18 to obtain waveform-shaped light (output signal
light), which is in turn output from the output port 14.
The phase shifter 24 imparts a phase shift .o slashed. to the second
waveform-shaped light so that the waveform-shaped light to be output from
the output port 14 becomes a difference signal between the first
waveform-shaped light and the second waveform-shaped light. As far as such
a difference signal can be obtained, the phase shifter 24 may be omitted,
or it may be provided on the optical path 20 at a position between the
DFB-LD 2(#1) and the Y-branch 18. For example, the phase shifter 24 may be
omitted by setting the optical path lengths of the optical paths 20 and 22
to suitable values. The need for such a phase shift to the first
waveform-shaped light or the second waveform-shaped light is partially due
to the fact that a phase shift imparted to the first signal light in the
DFB-LD 2(#1) and a phase shift imparted to the second signal light in the
DFB-LD 2(#2) are different according to a difference in driving conditions
between the DFB-LDs 2(#1) and 2(#2).
FIG. 5A is a graph showing the input-output characteristic of each DFB-LD
shown in FIG. 4, and FIG. 5B is a graph showing the input-output
characteristic of the waveform shaper shown in FIG. 4. In FIG. 5A, the
horizontal axis represents the input power P.sub.a-in of the waveform
shaper or the input power of each DFB-LD, and the vertical axis represents
the output power P.sub.DFB-LD-out of each DFB-LD. In FIG. 5B, the
horizontal axis represents the input power P.sub.s-in of the waveform
shaper, and the vertical axis represents the output power P.sub.a-out of
the waveform shaper.
As shown in FIG. 5A, the first output saturation characteristic of the
DFB-LD 2(#1) is given by a saturated output power P.sub.ast1, and the
second output saturation characteristic of the DFB-LD 2(#2) is given by a
saturated output power .sub.ast2 (.noteq.P.sub.ast1) In this example shown
in FIG. 5A, P.sub.ast1 <P.sub.ast2 is obtained by suitably setting the
drive currents for the DFB-LDs 2(#1) and 2(#2). A partial characteristic
until reaching the saturated output power P.sub.ast1 (a characteristic in
a proportional region) in the first output saturation characteristic is
substantially coincident with that in the second output saturation
characteristic. Accordingly, the output signal light given as the
difference signal between the first waveform-shaped light and the second
waveform-shaped light (i.e., the waveform-shaped light output from the
waveform shaper shown in FIG. 4) is determined by a rigid discrimination
characteristic as shown in FIG. 5B.
Thus, according to the second preferred embodiment of the waveform shaper
shown in FIG. 4, the output saturation characteristic becomes a
discrimination characteristic as shown in FIG. 5B, thereby allowing better
waveform shaping. Further, since the difference signal between the first
waveform-shaped light and the second waveform-shaped light is obtained, an
increase in noise can be effectively prevented also in the case of using
no probe light.
FIG. 6 is a block diagram showing a third preferred embodiment of the
waveform shaper according to the present invention. In contrast to the
first preferred embodiment shown in FIG. 3, the third preferred embodiment
is characterized in that an optical filter 26 optically connected to the
output of the DFB-LD 2 and a saturable absorber 28 optically connected to
at least one of the input and the output of the DFB-LD 2 are additionally
provided. While the saturable absorber 28 is optically connected through
the optical filter 26 to the output of the DFB-LD 2 as shown, a saturable
absorber optically connected to the input of the DFB-LD 2 may be used.
The optical filter 26 has a passband including the wavelength
.lambda..sub.s of signal light and not including the wavelength
.lambda..sub.0 of oscillated laser light in the DFB-LD 2 and the
wavelength .lambda..sub.p of probe light. By adopting the optical filter
26, the oscillated laser light and the probe light both unnecessary and
rather harmful in the subsequent signal processing can be removed, so that
it is possible to provide a waveform shaper highly valuable for use in an
all-optical regenerative repeater or the like. In the case that the probe
light source 6 is not adopted, the optical filter 26 has a passband
including the wavelength .lambda..sub.a of signal light and not including
the wavelength .lambda..sub.0 of oscillated laser light in the DFB-LD 2.
The saturable absorber 28 is provided by a reverse-biased semiconductor
device (e.g., laser diode and semiconductor optical amplifier) to suppress
noise due to amplitude fluctuations at the low level of signal light
according to its saturable absorption characteristic. In general, a
saturable absorber exhibits a nonlinear input-output characteristic by an
absorption effect to input light having a level equal to or less than a
saturation level. Accordingly, when the light corresponding to the signal
light output from the DFB-LD 2 and passed through the optical filter 26 is
passed through the saturable absorber 28, the input-output characteristic
of this waveform shaper has a partial characteristic shown by the broken
line in FIG. 2A or FIG. 2B, thus obtaining an operation closer to that of
a discriminating circuit. As a result, amplitude fluctuations at the high
level and the low level of the signal light can be suppressed to allow
higher-quality waveform shaping.
To further improve the response of the saturable absorber 28, a light
source for inputting other probe light into the saturable absorber 28 may
be additionally provided. Further, to stabilize the operation of the
saturable absorber 28, an optical isolator may be optically connected to
the input or output of the saturable absorber 28.
In this preferred embodiment, the Y-branches 16 and 18 each operating as an
optical coupler are provided by the Mach-Zehnder interferometer 10 formed
on the waveguide substrate 8, so that the waveform shaper can be made
compact.
FIG. 7 is a block diagram showing a preferred embodiment of the all-optical
signal regenerating device according to the present invention. This device
includes an optical branch 30 provided by an optical coupler, for example,
a waveform shaper 32 provided by any one of the various preferred
embodiments of the present invention, a clock regenerator 34, and an
optical retiming section 36. The optical branch 30 divides supplied signal
light into first signal light and second signal light. The first signal
light is supplied to the waveform shaper 32. The waveform shaper 32
performs waveform shaping of the supplied signal light to output resultant
waveform-shaped light. The second signal light is supplied to the clock
regenerator 34. The clock regenerator 34 regenerates clock pulses (optical
clock) according to the supplied second signal light. The waveform-shaped
light and the clock pulses are supplied to the optical retiming section
36. The optical retiming section 36 corrects the timing of the
waveform-shaped light according to the clock pulses to output resultant
regenerated signal light.
In obtaining the waveform-shaped light to be output from the waveform
shaper 32, the waveform shaping is performed on the high level and/or the
low level of the signal light according to the present invention. However,
the pulse spacing of the wavelength-shaped light possibly becomes
nonuniform. To the contrary, the pulse spacing of the clock pulses output
from the clock regenerator 34 is uniform. Accordingly, by configuring the
optical retiming section 36 so that it functions as an optical AND
circuit, for example, it is possible to obtain regenerated signal light
subjected to waveform shaping and having a uniform pulse spacing.
The clock regenerator 34 may include a MLL into which the second signal
light is introduced. In this case, the clock pulses are regenerated by
mode locking of the MLL according to the second signal light. This will
now be described more specifically for a ring MLL.
FIG. 8 is a block diagram showing a preferred embodiment of the clock
regenerator 34 shown in FIG. 7. This clock regenerator 34 includes an
optical path 46 provided between an input port 42 and an output port 44,
and an optical loop 48 as a ring laser optically coupled to the optical
path 46. Each of the optical path 46 and the optical loop 48 is provided
by an optical fiber, for example. In this case, the optical coupling
between the optical path 46 and the optical loop 48 may be made by a fiber
fusion type optical coupler 50. Accordingly, a part of the optical path 46
and a part of the optical loop 48 are provided by the optical coupler 50.
The optical loop 48 includes an optical amplifier 52 for compensating for a
loss in the optical loop 48 so that laser oscillation occurs in the
optical loop 48, an adjuster 54 configured by a delay circuit having a
variable delay time .tau., and a nonlinear medium (nonlinear optical
medium) 56. Particularly in this preferred embodiment, the optical loop 48
further includes an optical bandpass filter 58 having a passband including
a wavelength .lambda..sub.c of the laser oscillation by the ring laser.
Signal light having a wavelength .lambda..sub.a (the second signal light)
modulated at a speed (or bit rate) f.sub.s is supplied to the input port
42, and a part of the supplied signal light is introduced through the
optical coupler 50 into the optical loop 48. The optical path length L of
the optical loop 48 is preliminarily adjusted by the adjuster 54 so that
the modulation rate (corresponding to frequency) f.sub.s of the signal
light becomes equal to an integral multiple of the reciprocal
.DELTA..nu.=c/L (c: light velocity) of a recirculation period of the
optical loop 48. The optical amplifier 52 may be provided by an EDFA
(erbium doped fiber amplifier), for example.
Particularly in this preferred embodiment, the nonlinear medium 56 is
provided by a third-order nonlinear medium. When the signal light is
introduced into the nonlinear medium 56, amplitude modulation (AM) or
frequency modulation (FM) occurs in the nonlinear medium 56 to mode-lock
the laser oscillation by the optical loop 48. As a result, clock pulses
having a wavelength .lambda..sub.c and a frequency f.sub.s are generated
or regenerated, and the clock pulses are output through the optical
coupler 50 from the output port 44. This will now be described more
specifically.
Continuous wave (CW) laser light having a wavelength .lambda..sub.c is
preliminarily oscillated by a ring laser configured by the optical loop
48, and signal light having a wavelength .lambda..sub.s and a frequency
(bit rate or speed) f.sub.a is input into the optical loop 48. At this
time, four-wave mixing (FWM) employing this signal light as pump light
(excitation light) is generated in the nonlinear medium 56, so that the CW
light having the wavelength .lambda..sub.c is amplitude-modulated by the
signal light. The amplitude-modulated CW light includes a component of the
fundamental frequency f.sub.a. Accordingly, by setting the optical path
length of the optical loop 48 as mentioned above, clock pulses having a
frequency f.sub.a are generated.
Thus, clock pulses can be obtained without the need for opto/electric
conversion in this preferred embodiment, so that it is possible to provide
an all-optical clock regenerator not depending on the bit rate, pulse
shape, etc. of signal light.
The nonlinear medium 56 may be provided by a semiconductor optical
amplifier (SOA), a single-mode fiber, or a dispersion shifted fiber (DSF).
It is effective to use a high-nonlinear DSF (HNL-DSF) having high
nonlinear effects as the DSF. The HNL-DSF will be hereinafter described.
In the case of using an SOA as the nonlinear medium 56, gain is generated
in the nonlinear medium 56. Accordingly, the optical amplifier 52 for
maintaining the laser oscillation in the optical loop 48 may be
eliminated. More generally, in the case that the linear or nonlinear gain
in the nonlinear medium 56 is sufficiently large, the optical amplifier 52
may be eliminated.
In the case that an HNL-DSF is used as the nonlinear medium 56, the
wavelength .lambda..sub.a of signal light is preferably set substantially
equal to the zero-dispersion wavelength .lambda..sub.0 of the HNL-DSF, so
as to most effectively generate FWM in the nonlinear medium 56. With this
setting, optimum phase matching can be attained, and a broadest conversion
band and a maximum conversion efficiency can be obtained. The term of
"conversion" used herein means conversion from signal light to clock
pulses. Further, by managing the zero-dispersion wavelength A.sub.0 of the
HNL-DSF to a constant value with high accuracy, the conversion band can be
broadened. This will also be hereinafter described.
FIG. 9 is a diagram showing a preferred embodiment of the optical retiming
section 36 shown in FIG. 7. In this preferred embodiment, the optical
retiming section 36 is provided by a nonlinear optical loop mirror (NOLM).
The NOLM includes a first optical coupler 66 including a first optical
path 62 and a second optical path 64 directionally coupled to each other,
a loop optical path 68 for connecting the optical paths 62 and 64, and a
second optical coupler 72 including a third optical path 70 directionally
coupled to the loop optical path 68. A part or the whole of the loop
optical path 68 is provided by a nonlinear optical medium. Particularly in
this preferred embodiment, an HNL-DSF is used as the nonlinear optical
medium. The coupling ratio of the first optical coupler 66 is set to 1:1.
The waveform-shaped light from the waveform shaper 32 (see FIG. 7) is
supplied as control pulses having a wavelength .sub.o to the third optical
path 70. The clock pulses from the clock regenerator 34 (see FIG. 7) are
supplied as probe pulses having a wavelength .lambda..sub.c to the first
optical path 62.
The operation of this NOLM will now be described in brief. When the probe
pulses and the control pulses are input into the optical paths 62 and 70,
respectively, regenerated pulses (regenerated signal light) are obtained
in accordance with the operation of an optical AND circuit, and they are
output from the second optical path 64 of the optical coupler 66. The
regenerated pulses have the same wavelength .lambda..sub.o as that of the
probe pulses.
The probe pulses are split into two components having equal powers by the
optical coupler 66. These two components propagate clockwise and
counterclockwise in the loop optical path 68, respectively. During the
propagation in the loop optical path 68, the two components undergo equal
phase shifts .PHI. by the nonlinear optical medium, and thereafter they
are combined by the optical coupler 66. This phenomenon occurs in the case
that the control pulses are not input into the optical path 70. In this
case, the light obtained by the combination of the two components at the
optical coupler 66 is output from the first optical path 62 as if it is
reflected by a mirror, but not output from the second optical path 64,
because the two components are equal in power and phase.
When the control pulses are introduced into the loop optical path 68 by the
optical coupler 72, the control pulses propagate in only one direction
(e.g., clockwise as shown) in the loop optical path 68, and the nonlinear
refractive index of the nonlinear optical medium changes for the light
propagating in this direction only when the control pulses are at the high
level. Accordingly, in combining the two components of the probe pulses at
the optical coupler 66, the phases of the two components of the probe
pulses synchronous with the high level of the control pulses are different
from each other, and the phases of the two components of the probe pulses
synchronous with the low level of the control pulses are coincident with
each other. Letting .DELTA. .PHI. denote a phase difference in the former
case, an output proportional to {1-cos(.DELTA..PHI.)}/2 is obtained from
the second optical path 64 of the optical coupler 66. Accordingly, by
adjusting the power of the control pulses (i.e., the waveform-shaped light
from the waveform shaper 32) so that the phase difference .DELTA..PHI.
becomes .pi., it is possible to carry out an operation such that the light
having a wavelength .lambda..sub.c is output from the second optical path
64 only when the control pulses and the probe pulses overlap each other,
and otherwise the output level becomes a low level.
Thus, the regenerated signal light synchronous with the clock pulses can be
obtained according to the waveform-shaped light supplied to the optical
retiming section 36 (see FIG. 7). Accordingly, the conversion from the
waveform-shaped light into the regenerated signal light accompanies the
wavelength conversion from .lambda..sub.s into .lambda..sub.c.
Accordingly, the all-optical signal regenerating device shown in FIG. 7
can be used also as a wavelength converter used at a node in an optical
network, for example. In this case, it is possible to obtain a converted
wavelength corresponding to the wavelength .lambda..sub.c of the clock
pulses regenerated in the clock regenerator 34 (see FIG. 7).
The NOLM itself has a saturable absorption characteristic and accordingly
also has a waveform shaping function. That is, waveform distortion near
the leading edge (low-power region) and the peak (high-power region) of a
pulse can be compressed by the nonlinear (saturation) effects at these
regions, so that the NOLM itself can perform an operation of waveform
shaping similar to that in each preferred embodiment of the waveform
shaper mentioned above.
Further, a four-wave mixer may also be used as the optical retiming section
36 having the function of an optical AND circuit (see FIG. 7). In this
case, the waveform-shaped light or the clock pulses is/are used as pump
light, and four-wave mixing is generated according to the ON/OFF state of
the pump light, thereby performing the optical retiming. In the case that
an optical fiber is used as a nonlinear optical medium for the four-wave
mixer, it is effective to make the zero-dispersion wavelength of this
optical fiber coincident with the wavelength of the pump light in
optimizing a phase matching condition.
As a nonlinear optical effect applicable to optical signal processing in an
optical communication system, it is considered to apply three-wave mixing
in a second-order nonlinear optical medium or an optical Kerr effect such
as self-phase modulation (SPM), cross-phase modulation (XPM), and
four-wave mixing (FWM) in a third-order nonlinear optical medium. Examples
of the second-order nonlinear optical medium include InGaAs and
LiNbO.sub.3. Examples of the third-order nonlinear optical medium include
an optical fiber and a semiconductor medium such as a semiconductor
optical amplifier (SOA) and a distributed feedback laser diode (DFB-LD).
In the preferred embodiment of the clock regenerator 34 shown in FIG. 8 or
the preferred embodiment of the optical retiming section 36 shown in FIG.
9, the optical Kerr effect in an optical fiber is particularly effective.
A single-mode fiber is suitable as the optical fiber, and especially a
dispersion-shifted fiber (DSF) having a relatively small chromatic
dispersion is preferable.
In general, the third-order nonlinear coefficient .gamma. of an optical
fiber is expressed as follows:
.gamma.=.omega.n.sub.2 /cA.sub.off (1)
where .omega. is the optical angular frequency, c is the velocity of light
in a vacuum, and n.sub.2 and A.sub.off are the nonlinear refractive index
and the effective core area of the optical fiber, respectively.
The nonlinear coefficient .gamma. of a conventional DSF is as small as
about 2.6 W.sup.-1 km.sup.-1, so a fiber length of several km to 10 km or
more is necessary to obtain sufficient conversion efficiency. If a shorter
DSF can be used to realize sufficient conversion efficiency, the
zero-dispersion wavelength can be managed with high accuracy, thereby
realizing high-speed and wide-band conversion. The term of "conversion"
used herein means conversion from signal light into clock pulses or
conversion from waveform-shaped light into regenerated signal light.
In general, for enhancement of the third-order nonlinear effect of an
optical fiber, it is effective to increase a light intensity by increasing
the nonlinear refractive index n.sub.2 in Eq. (1) or by reducing a mode
field diameter (MFD) corresponding to the effective core area A.sub.off in
Eq. (1).
The nonlinear refractive index n2 can be increased by doping the clad with
fluorine or the like or by doping the core with a high concentration of
GeO.sub.2, for example. By doping the core with 25 to 30 mol % of
GeO.sub.2, a large value of 5.times.10.sup.-20 m.sup.2 /W or more (about
3.2.times.10.sup.-20 m.sup.2 /W for a usual silica fiber) can be obtained
as the nonlinear refractive index n2.
On the other hand, the MFD can be reduced by designing a relative
refractive-index difference .DELTA. between the core and the clad or by
designing the core shape. Such design of a DSF is similar to that of a
dispersion compensating fiber (DCF). For example, by doping the core with
25 to 30 mol % of GeO.sub.2 and setting the relative refractive-index
difference .DELTA. to 2.5 to 3.0%, a small value of less than 4 .mu.m can
be obtained as the MFD. Owing to the combined effects of increasing the
nonlinear refractive index n.sub.2 and reducing the MFD, an optical fiber
(HNL-DSF) having a large value of 15 W.sup.-1 km.sup.-1 or more as the
nonlinear coefficient .gamma. can be obtained.
As another important factor, the HNL-DSF having a large nonlinear
coefficient .gamma. as mentioned above has a zero dispersion in a
wavelength band used. This point can also be satisfied by setting each
parameter in the following manner. That is, in general, a dispersion in a
usual DCF increases in a normal dispersion region with an increase in
refractive index difference .DELTA. under the condition that the MFD is
set constant. On the other hand, the dispersion decreases with an increase
in core diameter, whereas the dispersion increases with a decrease in core
diameter. Accordingly, the dispersion can be reduced to zero by increasing
the core diameter under the condition that the MFD is set to a certain
value in a wavelength band used.
A phase shift due to the optical Kerr effect in an optical fiber having a
length L is proportional to .gamma.P.sub.p L where P.sub.P is the average
pump light power. Accordingly, the fiber having a nonlinear coefficient
.gamma. of 15 W.sup.-1 km.sup.-1 can achieve the same conversion
efficiency as that by a usual DSF even when the fiber length is reduced to
about 2.6/15.apprxeq.1/5.7 as compared with the usual DSF. As mentioned
above, the usual DSF requires a length of about 10 km for sufficient
conversion efficiency. To the contrary, the HNL-DSF having a large
nonlinear coefficient .gamma. as mentioned above can obtain a similar
effect with a reduced length of about 1 to 2 km. Practically, loss in the
fiber is reduced in an amount corresponding to a decrease in fiber length,
so that the fiber can be further shortened to obtain the same efficiency.
In such a short fibers controllability of the zero-dispersion wavelength
can be improved, and ultra wide-band conversion can be achieved as will be
hereinafter described. Further, when the fiber length is several km,
polarization can be fixed, that is, a polarization maintaining ability can
be ensured. Therefore, application of the HNL-DSF to the present invention
is greatly effective in achieving high conversion efficiency and wide
conversion band and removing polarization dependence.
To effectively produce an optical Kerr effect, especially XPM in the NOLM
shown in FIG. 9, for example, and thereby improve the efficiency of
conversion from the waveform-shaped light into the regenerated signal
light, phase matching between the probe pulses and the control pulses must
be achieved. The phase matching will now be described with reference to
FIG. 9. The probe pulses are branched to first probe pulses propagating
clockwise in the loop optical path 68 and second probe pulses propagating
counterclockwise in the loop optical path 68 by the optical coupler 66.
The control pulses are passed through the optical coupler 72 and propagate
clockwise in the loop optical path 68.
A phase matching condition in the loop optical path 68 is given by timing
coincidence of the control pulses and the first probe pulses both
propagating clockwise in the loop optical path 68. If the timing
coincidence of the control pulses and the first probe pulses is not
achieved, optical Kerr shift by XPM is limited to cause a difficulty of
effective operation of an optical AND circuit.
Since the wavelength of the control pulses and the wavelength of the first
probe pulses are different from each other, the group velocity of the
control pulses and the group velocity of the first probe pulses in the
loop optical path 68 are different from each other, resulting in
occurrence of timing deviation proportional to the length of the loop
optical path 68. To avoid this possibility, wavelength location is
preferably selected so that the group velocity of the control pulses and
the group velocity of the first probe pulses become equal to each other.
The most effective wavelength location for minimizing the timing deviation
is obtained by locating the wavelength .lambda..sub.s of the control
pulses and the wavelength .lambda..sub.c of the first probe pulses in
substantially symmetrical relationship with respect to the zero-dispersion
wavelength of the loop optical path 68. Over a wide band near the
zero-dispersion wavelength, the chromatic dispersion changes substantially
linearly, so that a good phase matching condition can be obtained by
making the group velocity of the signal pulses and the group velocity of
the first probe pulses coincide with each other by the above-mentioned
wavelength location.
However, if there are variations in the zero-dispersion wavelength itself
along the fiber, the group velocities become different from each other in
spite of the above wavelength location, causing a limit to a conversion
band and a convertible signal rate. Thus, a conversion band by the fiber
is limited by dispersion. If dispersion along the fiber is perfectly
controlled, for example, if a fiber having a zero-dispersion wavelength
uniform over the entire length (exactly, the nonlinear length) is
fabricated, a conversion band infinite in fact (unlimitedly wide in a
range where the wavelength dependence of dispersion is linear) could be
obtained by locating the wavelength .lambda..sub.s of the waveform-shaped
light and the wavelength .lambda..sub.c of the clock pulses in symmetrical
relationship with respect to this uniform zero-dispersion wavelength.
Actually, however, the zero-dispersion wavelength varies along the fiber,
causing a deviation of the phase matching condition from an ideal
condition to result in a limit of the conversion band.
A first method for realizing a wide conversion band is to use an HNL-DSF.
In the case that the HNL-DSF is used, sufficient conversion can be
achieved with a length of about 1 to 2 km, so that dispersion
controllability can be improved to easily obtain a wide-band
characteristic. In particular, by suppressing variations in the
zero-dispersion wavelength near an input end where the efficiency of
production of an optical Kerr effect is high, the conversion band can be
widened most efficiently. Further, by cutting the fiber into a plurality
of small sections and next joining any of the small sections similar in
zero-dispersion wavelength by splicing or the like (in an order different
from the initial order counted from a fiber end), a wide conversion band
can be obtained although an average dispersion over the entire length is
unchanged.
Alternatively, many fibers each having a length (e.g., hundreds of meters
or less) allowing high-accuracy dispersion control required to obtain a
sufficiently wide conversion band may be prepared in advance, and any of
these fibers having a required zero-dispersion wavelength may be combined
to be spliced, thereby fabricating a fiber having a length required to
obtain a required conversion efficiency.
In the case of widening the conversion band as mentioned above, it is
effective to gather the sections of the fiber having less variations in
zero-dispersion wavelength near an input end where the light intensity is
high. Further, the conversion band can be further widened by increasing
the number of sections of the fiber as required, or by alternately
arranging the positive and negative signs of dispersion at a relatively
large-dispersion portion separate from the input end to thereby suitably
combine the small sections.
The degree of reducing the length of each section in cutting the optical
fiber may be based on the nonlinear length, for example. The phase
matching in FWM in a fiber sufficiently shorter than the nonlinear length
may be considered to depend on the average dispersion of the fiber. As an
example, in FWM using a pump light power of about 30 mW in a fiber having
a nonlinear coefficient .gamma. of 2.6 W.sup.-1 km.sup.-1, the nonlinear
length is about 12.8 km. In this example, the length of each section is
set to about 1/10 of 12.8 km, i.e., about 1 km. As another example, in FWM
using a pump light power of about 30 mW in a fiber having a nonlinear
coefficient .gamma. of 15 W.sup.-1 km.sup.-1, the nonlinear length is
about 2.2 km. In this example, the length of each section is set to about
1/10 of 2.2 km, i.e., about 200 m. In any case, a wide conversion band can
be obtained by measuring an average zero-dispersion wavelength of fiber
sections each sufficiently shorter than the nonlinear length and combining
any of the fiber sections having almost the same zero-dispersion
wavelength to thereby configure a fiber achieving a required conversion
efficiency.
In the preferred embodiment of the optical retiming section 36 shown in
FIG. 9, the dispersion of the HNL-DSF used as the loop optical path 68 is
preferably set so that no walk-off of two pulses (one of the pulses of the
waveform-shaped light and one of the clock pulses) is generated. As an
example, the zero-dispersion wavelength of the HNL-DSF is set near the
middle between the wavelength .lambda..sub.o of the waveform-shaped light
and the wavelength .lambda..sub.c of the clock pulses. Alternatively, the
zero-dispersion wavelength may be set longer or shorter than the
wavelengths of the two pulses. In the case that the zero-dispersion
wavelength is set longer than the wavelengths of the two pulses, the
dispersion of the HNL-DSF falls in a normal dispersive region, so that
modulation instability effects can be suppressed. In the case that the
zero-dispersion wavelength is set shorter than the wavelengths of the two
pulses, the dispersion of the HNL-DSF falls in an anomalous dispersive
region, so that soliton effects can be used. How the zero-dispersion
wavelength is set may be determined according to actual system conditions.
Further, an optical filter, an optical amplifier, and an optical isolator
may be provided upstream or downstream of the configuration shown in FIG.
7 or inside of the configuration shown in FIG. 7, as required.
FIG. 10 is a block diagram showing a preferred embodiment of the system
according to the present invention. This system includes an optical fiber
transmission line 70 for transmitting signal light, and a plurality of
optical repeaters (R) 72 (two being shown) arranged along the optical
fiber transmission line 70. A single optical repeater may be used instead.
Signal light is supplied from an optical sender (OS) 74 to the optical
fiber transmission line 70, and the signal light transmitted by the
optical fiber transmission line 70 is received by an optical receiver (OR)
76.
At least one of the above-mentioned preferred embodiments of the present
invention may be applied to each optical repeater 72, thereby obtaining
so-called 3R functions or 2R functions without the need for opto/electric
conversion. The term of "3R" means Reshaping, Retiming, and Regeneration,
and the term of "2R" means Reshaping and Regeneration. Particularly in the
case that each optical repeater 72 includes an optical amplifier such as
an EDFA, the ASE generated in the optical amplifier is accumulated in the
system shown in FIG. 10. Therefore, the application of the present
invention to each optical repeater 72 allows effective waveform shaping or
all-optical signal regeneration.
Alternatively, the waveform shaping or all-optical signal regeneration
according to the present invention may be performed at an output end of
the optical fiber transmission line 70. In this case, at least one of the
various preferred embodiments of the present invention is applied to the
optical receiver 76. For enhancement of a receiver sensitivity, an optical
amplifier such as an EDFA may be provided as an optical preamplifier in
the optical receiver 76.
In the system shown in FIG. 10, there is a possibility that the waveform of
the signal light may be distorted by dispersion or nonlinear optical
effects in the optical fiber transmission line 70 or may be perturbed by
the accumulation of ASE noise in the optical amplifiers during repeatered
transmission. However, the waveform distortion of the signal light due to
the dispersion or nonlinear optical effects can be prevented by providing
a dispersion compensator or a nonlinear compensator in each optical
repeater 72 or in the optical receiver 76. Further, the waveform
perturbation due to the accumulation of ASE noise can be effectively
prevented by performing the waveform shaping in accordance with the
present invention. Accordingly, the combination of the present invention
and a dispersion compensator or a nonlinear compensator is greatly
effective in constructing a long-haul, ultra high-speed, and high-quality
optical transmission system. The nonlinear compensator may be provided by
a device employing phase conjugate conversion, for example.
The chromatic dispersion that is often referred to simply as dispersion is
a phenomenon such that the group velocity of an optical signal in an
optical fiber changes as a function of the wavelength (frequency) of the
optical signal. In a standard single-mode fiber, for example, an optical
signal having a longer wavelength propagates faster than an optical signal
having a shorter wavelength in a wavelength region shorter than 1.3 .mu.m,
and the resultant dispersion is usually referred to as normal dispersion.
In contrast, an optical signal having a shorter wavelength propagates
faster than an optical signal having a longer wavelength in a wavelength
region longer than 1.3 .mu.m, and the resultant dispersion is usually
referred to as anomalous dispersion.
In recent years, the nonlinearities of an optical fiber have received
attention in association with an increase in optical signal power due to
the use of an EDFA. The most important nonlinearity that limits a
transmission capacity is an optical Kerr effect occurring in an optical
fiber. The optical Kerr effect is a phenomenon such that the refractive
index of an optical fiber changes with the intensity of an optical signal.
A change in the refractive index modulates the phase of an optical signal
propagating in an optical fiber, resulting in the occurrence of frequency
chirping which changes a signal spectrum. This phenomenon is known as
self-phase modulation (SPM). Spectral broadening due to SPM occurs to
cause further enlargement of the waveform distortion due to chromatic
dispersion.
In this manner, the chromatic dispersion and the optical Kerr effect impart
waveform distortion to an optical signal with an increase in transmission
distance. Accordingly, to allow long-haul transmission by an optical
fiber, the chromatic dispersion and the nonlinearity must be controlled,
compensated, or suppressed.
As a technique for controlling the chromatic dispersion and the
nonlinearity, the use of a regenerative repeater including an electronic
circuit for a main signal is known. For example, a plurality of
regenerative repeaters are arranged along a transmission line. Each
regenerative repeater performs opto/electric conversion, electric
regeneration, and electro/optic conversion in this order before the
waveform distortion of an optical signal becomes excessive. However, this
method has a problem that the regenerative repeater required is expensive
and complicated, and that the electronic circuit included in the
regenerative repeater limits the bit rate of a main signal.
As a technique for compensating for the chromatic dispersion and the
nonlinearity, optical soliton is known. An optical signal pulse having an
amplitude, pulse width, and peak power each accurately specified with
respect to a given anomalous dispersion is generated, thereby balancing
pulse compression due to both SPM induced by the optical Kerr effect and
the anomalous dispersion and pulse broadening due to dispersion, so that
an optical soliton propagates as maintaining its waveform.
As another technique for compensating for the chromatic dispersion and the
nonlinearity, the application of optical phase conjugation is known. For
example, a method for compensating for the chromatic dispersion of a
transmission line has been proposed by Yariv et al. (A. Yariv, O. Fekete,
and D. M. Pepper, "Compensation for channel dispersion by nonlinear
optical phase conjugation" Opt. Lett., vol. 4, pp. 52-54, 1979). An
optical signal is converted into phase conjugate light at the midpoint of
a transmission line, and the waveform distortion due to chromatic
dispersion in the front half of the transmission line is compensated by
the waveform distortion due to chromatic dispersion in the rear half of
the transmission line.
In particular, if the causes of phase fluctuations of electric fields at
two points are identical with each other, and an environmental change
inducing these causes is gentle during a light propagation time between
the two points, the phase fluctuations can be compensated by locating a
phase conjugator (phase conjugate light generator) at the midpoint between
the two points (S. Watanabe, "Compensation of phase fluctuation in a
transmission line by optical conjugation" Opt. Lett., vol. 17, pp.
1355-1357, 1992). Accordingly, the waveform distortion due to SPM can also
be compensated by adopting the phase conjugator. However, in the case that
the optical power distributions on the upstream and downstream sides of
the phase conjugator are asymmetrical with respect thereto, the
compensation for nonlinearity becomes incomplete.
The present inventor have proposed a technique for overcoming the
incompleteness of the compensation due to the asymmetry of optical powers
in the case of using a phase conjugator (S. Watanabe and M. Shirasaki,
"Exact compensation for both chromatic dispersion and Kerr effect in a
transmission fiber using optical phase conjugation" J. Lightwave Technol.,
vol. 14, pp. 243-248, 1996). The phase conjugator is located in the
vicinity of a point on a transmission line such that a total dispersion or
total nonlinear effect in a portion of the transmission line upstream of
this point is equal to that in a portion of the transmission line
downstream of this point, and various parameters are set in each minute
section of the upstream and downstream portions.
By using a third-order nonlinear optical medium such as an optical fiber
and a semiconductor optical amplifier, phase conjugate light can be
generated by nondegenerate four-wave mixing. When signal light having an
angular frequency .omega..sub.s and pump light having an angular frequency
.omega..sub.p (.omega..sub.p.noteq..omega..sub.o) are supplied to the
nonlinear optical medium, phase conjugate light (converted signal light)
having an angular frequency 2.omega..sub.p -.omega..sub.o is generated by
four-wave mixing based on the signal light and the pump light in the
nonlinear optical medium, and this phase conjugate light is output
together with the signal light and the pump light from the nonlinear
optical medium.
The above term of "nondegenerate" used herein means that the wavelength of
the signal light and the wavelength of the pump light are different from
each other. Since the wavelength of the signal light, the wavelength of
the pump light, and the wavelength (angular frequency) of the phase
conjugate light satisfy the above-mentioned relation, wavelength
conversion is performed simultaneously with the generation of the phase
conjugate light. Accordingly, in the case that an HNL-DSF is used as the
nonlinear optical medium for phase conjugate conversion and wavelength
conversion, the above-mentioned discussion for obtaining a high conversion
efficiency and a broad conversion band is applicable as it is, by adapting
the term of "conversion" mentioned above to these conversions.
FIG. 11 is a block diagram showing a fourth preferred embodiment of the
waveform shaper according to the present invention. This waveform shaper
includes the DFB-LD 2, the drive circuit 4, the probe light source 6, and
the optical filter 26 each shown in FIG. 6, and further includes a DFB-LD
2', a drive circuit 4', a probe light source 6', and an optical filter 26'
respectively corresponding to the components 2, 4, 6, and 26. The DFB-LDs
2 and 2' are cascaded (or connected in tandem) in such a manner that the
light output from the optical filter 26 is input into the DFB-LD 2'.
Since the two DFB-LDs 2 and 2' are cascaded in this preferred embodiment,
the degree of waveform shaping can be improved as compared with the case
of using a single DFB-LD.
Probe light is supplied from the probe light source 6 to the DFB-LD 2, so
that the generation of oscillated laser light having a wavelength
.lambda..sub.0 can be effectively suppressed. In this case, the optical
filter 26 may be omitted to input the light output from the DFB-LD 2
directly into the DFB-LD 2'. Accordingly, the probe light from the probe
light source 6 is passed through the DFB-LD 2 and thereafter supplied also
to the DFB-LD 2', so that the probe light source 6' may be omitted. If the
generation of oscillated laser light having a wavelength .lambda..sub.0 in
the DFB-LD 2' is effectively suppressed in this case, the function
required for the optical filter 26' is to remove the probe light having a
wavelength .lambda..sub.p.
Thus, it is possible to provide a waveform shaper which can effectively
perform waveform shaping, by cascading a plurality of DFB-LDs to carry out
the present invention.
While various preferred embodiments of the present invention using a DFB
laser have been described, the present invention is not limited by the use
of a DFB laser. That is, also in the case of using any lasers other than a
DFB laser, a gain clamped condition can be obtained in relation to optical
amplification, so that the waveform shaping of signal light can be
performed as in the case of using a DFB laser. For example, a Fabry-Perot
laser diode oscillating in multiple modes may be used to carry out the
present invention. In this case, the laser diode has a plurality of laser
oscillation wavelengths, and the wavelength of signal light to be
subjected to waveform shaping is therefore set different from these laser
oscillation wavelengths.
According to the present invention as described, above, it is possible to
provide a novel method, device, and system for waveform shaping
independent of the bit rate and pulse shape of signal light. As a result,
various performance limits in the existing linear optical communication
system can be broken down. The effects obtained by the specific preferred
embodiments of the present invention have been described above, so the
description thereof will be omitted herein.
The present invention is not limited to the details of the above described
preferred embodiments. The scope of the invention is defined by the
appended claims and all changes and modifications as fall within the
equivalence of the scope of the claims are therefore to be embraced by the
invention.
* * * * *
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