![[US Patent & Trademark Office, Patent Full Text and Image Database]](/netaicon/PTO/patfthdr.gif)
| United States Patent |
4,940,306
|
|
Kitayama
, et al.
|
July 10, 1990
|
Star network optical transmission system
Abstract
A star network optical transmission system comprising a star coupler as a
center node, wherein a star coupler is provided with an optical fiber
exhibiting a multimode transmission characteristic to an optical signal
transmitted by said star coupler at an input portion and an output
portion, an optical transmission apparatus having a laser with a
wavelength of .lambda. as a light source, an up optical fiber exhibiting a
single mode transmission characteristic to a wavelength of .lambda.
connecting the optical transmission apparatus to the star coupler, an
optical reception apparatus, and a down optical fiber exhibiting a
multimode transmission characteristic at a wavelength .lambda. and
connecting the optical reception apparatus to said star coupler.
| Inventors:
|
Kitayama; Tadayoshi (Kanagawa, JP);
Matsushita; Kiwami (Kanagawa, JP);
Motoshima; Kuniaki (Kanagawa, JP)
|
| Assignee:
|
Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
300786 |
| Filed:
|
January 23, 1989 |
Foreign Application Priority Data
| Jan 28, 1988[JP] | 63-18083 |
| Mar 04, 1988[JP] | 63-51136 |
| Nov 26, 1988[JP] | 63-298799 |
| Current U.S. Class: |
385/46; 385/24; 398/61; 398/63; 398/143 |
| Intern'l Class: |
G02B 006/28 |
| Field of Search: |
350/96.15,96.16
455/610,612
|
References Cited [Referenced By]
U.S. Patent Documents
| 4089584 | May., 1978 | Polczynski | 350/96.
|
| 4240694 | Dec., 1980 | Pan | 350/96.
|
| 4257124 | Mar., 1981 | Porter et al. | 455/601.
|
| 4366565 | Dec., 1982 | Herskowitz | 455/612.
|
| 4580872 | Apr., 1986 | Bhatt et al. | 350/96.
|
| 4646361 | Feb., 1987 | Usui | 455/601.
|
| 4708424 | Nov., 1987 | Marhic | 350/96.
|
| 4726644 | Feb., 1988 | Mathis | 350/96.
|
| 4850042 | Jul., 1989 | Petronio et al. | 455/612.
|
| 4850043 | Jul., 1989 | Winston | 455/612.
|
Other References
"Optical Star Network Using Wavelength-Division Multiplexing" (by Hiroo
Okuhara et al., Toshiba Review, vol. 40, No. 7, pp. 627-629, 1985).
"Broadcast Television via Passive Optical Networks" (by D. W. Faulkner,
13th ECOC Technical Digest, vol. 1, p. 283, 1987).
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Ngo; John
Attorney, Agent or Firm: Wolf, Greenfield & Sacks
Claims
What is claimed is:
1. A star network optical transmission system having a star coupler as a
center node, said star network optical transmission system comprising: a
star coupler provided with an optical fiber at an input portion and an
output portion, an optical transmission apparatus having a laser with a
wavelength of .lambda. as a light source; an up optical fiber exhibiting a
single mode transmission characteristic at a wavelength of .lambda. and
connecting said optical transmission apparatus to said star coupler; an
optical reception apparatus; and a down optical fiber exhibiting a
multimode transmission characteristic at a wavelength .lambda. and
connecting said optical reception apparatus to said star coupler.
2. A system as claimed in claim 1 wherein the optical fibers at both said
input portion and said output portion exhibit a multimode transmission
characteristic to an optical signal transmitted by said star coupler.
3. A system as claimed in claim 1 wherein said optical fiber at said output
portion exhibits a multimode transmission characteristic to an optical
signal transmitted by said star coupler and wherein the optical fiber at
said input portion exhibits a single mode transmission characteristic at
the wavelength of .lambda..
4. A system as claimed in claim 3 wherein said laser oscillates in a
monovertical mode during modulation.
5. A system as claimed in claim 1 wherein said laser oscillates in a
monovertical mode during oscillation.
6. A star network optical transmission system having a star coupler as a
center node, said star network optical transmission system comprising; a
star coupler provided with an optical fiber at an input portion and an
output portion; an optical transmission apparatus having a laser with a
wavelength of .lambda. which oscillates in a monovertical mode during
modulation and which is used as a light source; an up optical fiber
exhibiting a single mode transmission characteristic at a wavelength of
.lambda. and connecting said optical transmission apparatus to said star
coupler; an optical reception apparatus; and a down optical fiber
exhibiting a multimode transmission characteristic at a wavelength
.lambda. and connecting said optical reception apparatus to said star
coupler.
7. A system as claimed in claim 6 wherein the optical fibers at both said
input portion and said output portion exhibit a multimode transmission
characteristic to an optical signal transmitted by said star coupler.
8. A system as claimed in claim 6 wherein said optical fiber at said output
portion exhibits a multimode transmission characteristic to an optical
signal transmitted by said star coupler and wherein the optical fiber at
said input portion exhibits a single mode transmission characteristic at
the wavelength of .lambda..
9. A star network optical transmission system, comprising a transmission
type star coupler having a plurality of input ports and output ports and
an optical transmission apparatus having two optical output terminals,
wherein one of the two optical output terminals of said optical
transmission apparatus is connected to one of the input ports of said
transmission type star coupler via an optical fiber and wherein the other
optical output terminal of said optical transmission apparatus is
connected to one of the output ports of said transmission type star
coupler via an optical fiber and including optical reception apparatus
connected to the other input and output ports of said transmission type
star coupler via optical fibers, wherein there are a plurality of said
transmission type star couplers, and wherein an output from at least one
of the optical reception apparatus connected to a port of one of said star
couplers is applied as the input to the optical transmission apparatus
applying inputs to another of said star couplers.
10. A system as claimed in claim 9 wherein said optical transmission
apparatus includes means for receiving an electrical input, means for
electrically distributing said electrical input to two electrical outputs,
and an electrooptic conversion means connected to each output of said
distributing means, said electrooptic conversion means converting the
electrical input applied thereto into an optical output signal appearing
at an output terminal from said apparatus.
11. A system as claimed in claim 9 wherein said optical transmission
apparatus includes means for receiving an electrical input, an
electrooptic conversion means for converting said electrical input into a
corresponding optical signal, and optical distributor means for splitting
the optical signal from said conversion means into two optical output
signals, one of which is applied to each of said optical output terminals.
12. A star network optical transmission system, comprising:
a plurality of transmission type star couplers, each having a plurality of
input ports and output ports;
a plurality of optical transmission apparatus, each having an input optical
terminal and two output optical terminals, each of the two output optical
terminals connected to one of the input ports of a corresponding one of
the transmission type star couplers via an optical fiber,
a plurality of optical reception apparatus, each having an input connected
to at least one of the remaining input ports and output ports of the
transmission type star couplers via optical fibers and an output;
wherein the output of at least one of the optical reception apparatus which
has an input connected to an output port of one of the transmission type
star couplers is coupled to the input optical terminal of an optical
transmission apparatus which has inputs coupled to another of the
transmission type star couplers.
13. A system as claimed in claim 12 wherein said optical transmission
apparatus includes means for receiving an electrical input, means for
electrically distributing said electrical input to two electrical outputs,
and an electrooptic conversion means connected to each output of said
distributing means, said electrooptic conversion means converting the
electrical input applied thereto into an optical output signal appearing
at an output terminal from said apparatus.
14. A system as claimed in claim 12 wherein said optical transmission
apparatus includes means for receiving an electrical input, an
electrooptic conversion means for converting said electrical input into a
corresponding optical signal, and optical distributor means for splitting
the optical signal from said conversion means into two optical output
signals, one of which is applied to each of said optical output terminals.
Description
BACKGROUND OF THE INVENTION;
1. Field of the Invention:
The present invention generally relates to a star network optical
transmission system, and more particularly to a star network optical
transmission system which employes an optical transmission apparatus
comprising a star coupler as a center node and a laser diode (LD) as a
light source to effect optical fiber transmission and further particularly
to a star network optical transmission system in which optical signal is
branched and transmitted via a star coupler.
2. Prior Art
FIG. 5 is a diagram illustrating the configuration of the prior art star
network optical transmission system as disclosed in "Toshiba Review", vol.
40, No. 7 (1985), p. 627-629 ("Optical Star Network Using
Wavelength-Division Multiplexing"). In FIG. 5, shown at 1, 2, 3 and 4 are
a transmission apparatus, a laser diode (LD), an up optical fiber
comprising a multimode optical fiber and a star coupler for multimode
optical fiber, respectively. Shown at 5, 10 and 11 are optical fibers at
the input portion in the star coupler. Shown at 6 is a mixer portion in
the star coupler 4. Shown at 7, 8 and 9 are optical fibers at the output
portion in the star coupler 4. Shown at 12, 13 and 14 are a down optical
fiber comprising a multimode optical fiber, a reception apparatus and a
light receiving element, respectively.
FIG. 6 is a diagram illustrating the operation of the star network optical
transmission system of FIG. 5. In FIG. 6, shown at 3a, 5a, 5b and 7a are
outer diameter of the core of the up optical fiber 3, outer diameter of
the core of one end of the optical fiber 5 at the input portion, outer
diameter of the core of the other end of the optical fiber 5 at the input
portion and outer diameter of the core of the optical fiber 7 at the
output portion, respectively. Like numerals are used where the components
are the same as those of FIG. 5. The detailed description of those
components is omitted.
The operation of the prior art star network optical transmission system
will be described hereinafter. The transmission apparatus 1 converts
transmission signal to optical signal of a wavelength .lambda. through the
laser diode 2, and then outputs it to the up optical fiber 3 comprising a
multimode optical fiber exhibiting a multimode transmission characteristic
to optical signal of a wavelength .lambda.. The laser diode 2 is a
commonly used Fabry-Perot type laser diode and is adapted to oscillate in
a multivertical mode during modulation. The up optical fiber 3 is
connected to the optical fiber 5 at the input portion in the star coupler
4 for multimode optical fiber. The optical signal inputted to the optical
fiber 5 at the input portion is then inputted to the mixer portion 6 where
it is propagated so that beams are equally dispersed and coupled to the
plurality of optical fibers 7, 8 and 9 at the output portion in principle.
If there are further provided optical fibers 10 and 11 at the input
portion, each fiber output is equally dispersed and coupled to the optical
fibers 7, 8 and 9 at the output portion. The optical fibers 5, 10 and 11
at the input portion and the optical fibers 7, 8 and 9 at the output
portion exhibit a multimode transmission characteristic to a wavelength of
.lambda.. The optical fiber 7 at the output portion in the star coupler 4
is connected to the down optical fiber 12 comprising a multimode
transmission characteristic to optical signal of a wavelength .lambda..
The optical signal transmitted to the down optical fiber 12 is then
optoelectrically converted by the light receiving element 14 in the
reception apparatus 13. Thus, the signal from the transmission apparatus 1
is received.
The above described laser diode 2 is a light source having a strong
coherence. When the optical signal outputted from the laser diode 2 is
coupled and transmitted to the up optical fiber 3, the power strength
distribution developed in the core outer diameter 3a is in a speckle
pattern of large particles because of coherence as shown in FIG. 6. If
there is an axis shift or the like between the core outer diameter 3a of
the up optical fiber 3 and the core outer diameter 5a of one end of the
optical fiber 5 at the input portion in the star coupler 4, only a part of
the speckle pattern of the former fiber is coupled to the speckle pattern
of the latter fiber. Also, in the end surface of the connection between
the optical fiber 5 at the input portion and the mixer portion 6, a
speckle pattern is developed in the core outer diameter 5b of the other
end of the optical fiber 5 at the input portion. In the mixer portion 6 of
the star coupler 4, optical signal outputted from the optical fiber 5 at
the input portion is propagated while undergoing the multiple reflection
between the walls of the mixer portion 6 so that a uniform optical power
distribution is developed on the end surface of the output. However, a
speckle pattern is still left. A stable optical power is coupled to the
core outer diameter 7a of the optical fiber 7 at the output portion so far
as such a speckle pattern is stable.
Configured as described above, the prior art star network optical
transmission system is disadvantageous in that the vibration of the up
optical fiber 3 or the temperature characteristics of the laser diode 2
causes a fluctuation in the speckle pattern developed on the end surface
of the connection between the up optical fiber 3 and the optical fiber 5
at the input portion which can cause a deviation in the rate of coupling
to the optical fibers 7, 8 and 9 at the output end surface in the mixer
portion 6, resulting in the generation of so-called modal noise which
deteriorates S/N ratio of transmission signal.
Further, if there is an axis shift in the end surface of the connection
between the optical fiber 5 at the input portion and the optical fiber 3
in the star coupler 4, such a deterioration in S/N ratio becomes
remarkable. Furthermore, since the laser diode oscillates in a
multivertical mode, a phenomenon called mode distribution characteristic
causes a fluctuation in the oscillation spectrum for every pulse of
transmission data during data transmission at a high rate of 100 Mb/s or
more, worsening the effect of modal noise.
The present invention has been attained in order to overcome these
programs. It is an object of the present invention to provide a star
network optical transmission system which reduces the fluctuation in the
speckle pattern developed on the output end surface of a mixer portion in
a star coupler caused by the vibration of an optical fiber or the
temperature characteristics or mode distribution characteristics of a
laser diode to obtain a stable coupling characteristic, enabling signal
transmission with a high S/N ratio.
Further, FIG. 12 shows another example of the prior art star network
optical transmission system as described in D. W. Faulkner, "Broadcast
Television via Passive Optical Networks", 13th ECOC Technical Digest vol.
1, p. 283 (1987). Shown at 122, 122, 123 and 124 in FIG. 12 are an optical
transmission apparatus, a one-input 2N (N is an integer) output star
coupler (hereinafter simply referred to as "[1.times.2N] star coupler"),
and a 1st and 2Nth optical reception apparatus, respectively. Shown at
125, 126a and 126b are optical fibers. Shown at 127 and 128 are an input
optical fiber and a mixer portion. Shown at 129a and 129b are output
optical fibers. FIG. 13 is a diagram illustrating the operation of the
star coupler 122. Optical signal inputted from the input optical fiber 127
is diffused at the mixer portion 128, and then distributed into the output
optical fibers 129a and 129b.
The operation of the conventional star network optical transmission system
will be described with reference to FIG. 12. In this optical transmission
system, optical signal transmitted by the optical transmission apparatus
121 is branched by the star coupler 122 to 2N branches, and then
transmitted to the optical reception apparatus 123 and the 2Nth optical
reception apparatus 124.
Optical signal transmitted by the optical transmission apparatus 121 is
transmitted to the star coupler 122 via the optical fiber 125. In the star
coupler 122, the transmitted optical signal is then propagated by the
input optical fiber 127 to the mixer portion 128 where it is diffused and
then equally distributed to 2N output optical fibers 129a to 129b. The
optical signals thus equally distributed are transmitted to the 1st to
2Nth optical reception apparatus 123 to 124 via the optical fibers 126a to
126b connected to the output optical fibers 129a to 129b, respectively.
Configured as described above, the conventional star network optical
transmission system requires the use of a star coupler having the same
number of output ports as that of the branches or a plurality of star
couplers having a less number of output ports to form a multibranch
system. In this configuration, as the number of branches increases, the
number of branch systems increases, making the system larger in size and
expensive to implement. Furthermore, in order to manufacture a [1.times.N]
star coupler having a plurality of output ports, a [2N.times.2N] star
coupler having a plurality of input ports and output ports is actually
manufactured in the manufacturing process. These input ports of the star
coupler thus manufactured are used singly. Thus, such a star coupler
exhibits a low cost performance.
The present invention has been attained in order to overcome these
problems. It is an object of the present invention to provide a star
network optical transmission system which uses an [N+1].times.N+1] star
coupler to enable 2N branching of optical signal.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, a star network
optical transmission system of the present invention is provided which
comprises an optical fiber at the input portion of a star coupler and a
transmission apparatus are connected to each other via an up optical fiber
comprising a single mode optical fiber exhibiting a single mode
transmission characteristic to a wavelength of .lambda..
In this star network optical transmission system, the optical fiber at the
input portion in the star coupler and the transmission apparatus are
connected to each other via an up optical fiber comprising a single mode
optical fiber. In this configuration, no speckle pattern is developed on
the output end surface of the up optical fiber. Furthermore, since the
single mode optical fiber as the up optical fiber has a smaller core
diameter than a multimode optical fiber, stable coupling of speckle
patterns is made on the end surface of the connection between the up
optical fiber and the star coupler, reducing the generation of modal
noise.
As described above, the star network optical transmission system according
to the present invention has a configuration in which an optical fiber at
the input portion of a star coupler and a transmission apparatus are
connected to each other via an up optical fiber comprising a single mode
optical fiber exhibiting a single mode transmission characteristic to a
wavelength .lambda.. In this configuration, stable coupling of speckle
patterns is made on the surface of the connection between the up optical
fiber and the star coupler, making it possible to transmit signal with
less modal noise.
In accordance with a second aspect of the present invention of a star
network optical transmission system of the present invention is provided
which comprises a dynamic single mode laser diode of a wavelength .lambda.
which oscillates in a monovertical mode during modulation, is used as a
light source in an optical transmission apparatus, and an optical fiber at
the input portion of a star coupler and an optical transmission apparatus
are connected to each other via an up optical fiber comprising a single
mode optical fiber exhibiting a single mode transmission characteristic to
a wavelength of .lambda..
In this star network optical transmission system, a distribution feedback
type laser of a wavelength .lambda. which oscillates in a monovertical
mode even during high rate modulation is used as a light source in an
optical transmission apparatus so that modal noise caused by the
fluctuation in the oscillation spectrum due to mode distribution
characteristic can be eliminated. Furthermore, the optical fiber at the
input portion in the star coupler and the optical transmission apparatus
are connected to each other via an optical fiber comprising a single mode
optical fiber so that no speckle pattern is developed on the output end
surface of the up optical fiber. Moreover, since the single mode optical
fiber as up optical fiber has a smaller core diameter than a multimode
optical fiber, a stable coupling of speckle pattern is made on the end
surface of the connection between the up optical fiber and the star
coupler, reducing the generation of modal noise.
As described above, the star network optical transmission system according
to the present invention has a configuration in which a dynamic single
mode laser diode which oscillates in a monovertical mode during modulation
is used as a light source in an optical transmission apparatus and an
optical fiber at the input portion of a star coupler and an optical
transmission apparatus are connected to each other via an up optical fiber
comprising a single mode optical fiber exhibiting a single mode
transmission characteristic to a wavelength of .lambda.. In this
configuration, stable coupling of speckle patterns is made on the end
surface of the connection between the up optical fiber and the star
coupler, making it possible to transmit a signal with less modal noise.
In accordance with a third aspect of the present invention, a star network
optical transmission system is provided wherein an optical transmission
apparatus outputs signals to two optical fibers. One of the two optical
fibers is connected to an input port of a star coupler, and the other to
an output of the star coupler. The other input and output ports which are
not connected to the optical transmission apparatus are connected to the
optical reception apparatus via optical fibers.
In this star network optical transmission system according to the present
invention, signals inputted from the optical transmission apparatus to the
input and output ports can be coupled to an output port and an input port,
respectively, without interferring with each other due to the directional
coupling characteristics of the star coupler. Thus, with an
[N+1].times.[N+1] star coupler, a 2N branch transmission system can be
obtained.
As described above, the present invention enables star couplers to operate
bidirectionally. Thus, star couplers required to obtain the same number of
branches can be obtained in a smaller size at a lower cost as compared to
the conventional system.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and many of the attendant advantages of the present
invention will be readily appreciated as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying drawings wherein like parts
in each of the figures are designated by the same reference characters and
in which:
FIG. 1A is a diagram illustrating the configuration of an embodiment of the
star network optical transmission system according to the present
invention;
FIG. 1B is a diagram illustrating the configuration of another embodiment
of the star network optical transmission system according to the present
invention;
FIGS. 2A and 2B are diagrams illustrating the operation of the star network
optical transmission systems of FIGS. 1A and 1B respectively;
FIG. 3A is a diagram illustrating the configuration of still another
embodiment of the star network optical transmission system according to
the present invention;
FIG. 3B is a diagram illustrating the configuration of another embodiment
of the star network optical transmission system according to the present
invention;
FIG. 4A is a diagram illustrating the operation of the star network optical
transmission system of FIG. 3A;
FIG. 4B is a diagram illustrating the operation of the star network optical
transmission system of FIG. 3B;
FIG. 5 is a diagram illustrating the conventional star network optical
transmission system;
FIG. 6 is a diagram illustrating the operation of the star network optical
transmission system of FIG. 5;
FIG. 7 is a configuration diagram of an embodiment of the star network
optical transmission system according to the present invention;
FIG. 8 is a diagram illustrating the operation of a star coupler in the
present embodiment;
FIG. 9 is a configuration diagram of an optical transmission apparatus in
the present embodiment;
FIG. 10 is a configuration diagram of an optical transmission apparatus in
another embodiment;
FIG. 11 is a configuration diagram of further embodiment of the present
star network optical transmission system;
FIG. 12 is a configuration diagram of the conventional star network optical
transmission system; and
FIG. 13 is a diagram illustrating the operation of the conventional star
coupler of FIG. 12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1A is a diagram illustrating the configuration of an embodiment of the
star network optical transmission system according to the present
invention. FIG. 2A is a diagram illustrating the operation of the star
network optical transmission system of FIG. 1A. Where the parts are the
same as those of FIGS. 1A and 2A, the same numbers are used in FIGS. 5 and
6. Thus, detailed explanation is omitted in FIGS. 5 and 6. In FIG. 1A
shown at 15 is an up optical fiber comprising a single mode optical fiber.
In FIG. 2A, shown at 15a' is a section of the core 15a of the up optical
fiber 15. Further, shown at 5a' and 5b' are sections of the cores 5a and
5b of an optical fiber 5.
The operation of an embodiment of the star network optical transmission
system of the present invention will be described hereinafter. In order to
suppress the generation of modal noises when a signal is transmitted from
a transmission apparatus 1 comprising a laser diode 2 as a light source
via a star coupler 4 for a multimode optical fiber, the speckle pattern
which appears on the output terminal surface of a mixer portion 6 of the
star coupler 4 needs to be kept stable against the vibration of an up
optical fiber 3 and the temperature characteristics of the laser diode 2.
As shown in FIG. 1A, in the up optical fiber 15 comprising a single mode
optical fiber, no speckle pattern is developed on the surface of the
connection between the star coupler and an optical fiber 5 at the input
portion because the mode excited by the laser diode 2 is a single mode. On
the other hand, if the optical fiber 5 at the input portion of the star
coupler 4 is a multimode optical fiber, a speckle pattern is developed on
the surface of the connection between the optical fiber 5 at the input
portion and the mixer portion 6. However, since the up optical fiber 15 is
a single mode optical fiber, the speckel pattern appearing on the surface
of the connection between the mixer portion 6 and the optical fiber 5 at
the input portion exhibits no change even if the up optical fiber 15
vibrates or the temperature characteristics of the laser diode 2 exhibit
fluctuation due to temperature. Therefore, stable coupling characteristics
can be obtained without changing the speckle pattern developed on the
surface of the connection between the mixer portion 6 and the output
optical fibers 7, 8 and 9, making it possible to suppress the generation
of modal noise.
FIG. 3A is a diagram illustrating the configuration of another embodiment
of the star network optical transmission system according to the present
invention. FIG. 4A is a diagram illustrating the operation of the star
network optical transmission system of FIG. 3A. In the embodiment shown in
FIG. 3A a single mode optical fiber is used as an optical fiber 16 at the
input portion in the star coupler 4A. In accordance with this embodiment,
the generation of the above described modal noise is further reduced. As
shown in FIG. 4A, with a single mode optical fiber as an optical fiber 16
at the input portion in the star coupler 4, no speckle pattern is
developed on the surface of the connection between the optical fiber 16 at
the input portion and the mixer portion 6. Therefore, even if the star
coupler 4 is subject to vibration, the mixer portion 6 stably excites from
the optical fiber 16 at the input portion. This stabilizes the speckle
pattern developed on the surface of the connection between the mixer
portion 6 and the optical fibers 7, 8 and 9 at the output portion,
preventing the generation of modal noise. Shown at 16a' in FIG. 4A is a
section of the core 16a of the optical fiber 16 at the input portion.
Further, shown at 15a' is a section of a core 15a of the optical fiber 15.
FIG. 1B is a diagram illustrating the configuration of an embodiment of the
star network optical transmission system according to the present
invention. Where the parts are the same as those of FIGS. 1B and 2B, the
same numbers are used as in FIGS. 5 and 6. Thus, detailed explanation is
omitted in FIGS. 1B and 2B. In FIG. 1B shown at 15 is an up optical fiber
comprising a single mode optical fiber. In FIG. 2B, shown at 5a', 5b' and
15a' are sections of the core 5a, 5b and 15a, respectively.
Shown at 116 is a dnyamic single mode laser diode which oscillates in a
monovertical mode during modulation.
The operation of this embodiment of the star network optical transmission
system of the present invention will be described hereinafter with
reference to FIG. 2B. One of the main causes of modal noise during the
transmission of a signal from the optical transmission apparatus 1 via the
star coupler 4 for multimode optical fiber is the speckle pattern
fluctuation due to the vibration of an optical fiber and the fluctuation
in the emission spectrum of the light source.
The effect developed when the up optical fiber 15 vibrates will be first
discussed hereinafter with reference to FIG. 1B.
As shown in FIG. 1B in the up optical fiber 15 comprising a single mode
optical fiber, no speckle pattern is developed on the surface of the
connection between the star coupler 4 and the optical fiber 5 at the input
portion because the mode excited by the dynamic single mode laser diode
116 is a single mode. On the other hand, if the optical fiber 5 at the
input portion in the star coupler 4 is a multimode optical fiber, a
speckle pattern is developed on the end surface of the connection between
the optical fiber 5 at the input portion and the mixer portion 6. However,
since the up optical fiber 15 is a single mode optical fiber, the speckle
pattern developed on the end surface of the connection between the mixer
portion 6 and the optical fiber 5 at the input portion exhibits no
fluctuation even if the up optical fiber 15 vibrates. Therefore, stable
coupling characteristics can be obtained without changing the speckle
pattern developed on the end surface of the connection between the mixer
portion 6 and the optical fiber 7 at the output portion, making it
possible to suppress the generation of modal noise.
The effect of the emission spectrum of the light source will be discussed
hereinafter. The emission spectrum fluctuation can be classified into two
groups: emission spectrum fluctuation caused by mode distribution
characteristics during high rate modulation and emission wavelength
fluctuation due to temperature. The speckle pattern fluctuation caused by
the emission wavelength fluctuation due to temperature is normally slow
and causes little modal noise. Therefore, this fluctuation will be
neglected herein. In FIG. 1B, since as a light source in the optical
transmission apparatus 1 the dynamic single mode laser diode 116 which
oscillates in a monovertical mode during modulation is used, there is no
emission spectrum fluctuation due to mode distribution characteristic
during high rate modulation. Therefore, the speckle pattern developed on
the connection between the mixer portion 6 and the optical fiber 5 at the
input portion and between the mixer portion 6 and the optical fiber 7 at
the output portion exhibits no fluctuation. Thus, stable coupling
characteristics can be provided between the mixer portion 6 and the
optical fiber 7 at the output portion, suppressing the generation of modal
noise.
FIG. 3B is a diagram illustrating the configuration of another embodiment
of the star network optical transmission system according to the present
invention. FIG. 4B is a diagram illustrating the operation of the star
network optical transmission system of FIG. 3B. In the embodiment shown in
FIG. 3B, a single mode optical fiber is used as an optical fiber 17 at the
input portion in the star coupler 4. In accordance with this embodiment,
the generation of the above described modal noise is further reduced. As
shown in FIG. 4B, with a single mode optical fiber as an optical fiber 17
at the input portion in the star coupler 4, no speckle pattern is
developed on the surface of the connection between the optical fiber 17 at
the input portion and the mixer portion 6. Therefore, even if the star
coupler 4 is subject to vibration, the mixer portion 6 stably excites from
the optical fiber 16 at the input portion. This stabilizes the speckle
pattern developed on the surface of the connection between the mixer
portion 6 and the optical fiber 7, 8 and 9 at the output portion,
preventing the generation of modal noise. Shown at 17a, in FIG. 4B is a
section of the core 17a of the optical fiber 17 at the input portion.
While the invention has been described with reference to embodiments in
which only one set of the transmission apparatus 1 is used, the same
effects can be attained even when it applies to an embodiment in which a
plurality of transmission apparatus 1 and a star coupler 4 are connected
to each other via optical fibers 15 comprising a plurality of single mode
optical fibers.
In these embodiments, the transmission apparatus 1 may be a transmission
portion of a repeater, or the reception apparatus 13 may be a reception
portion of a repeater. Thus, the transmission of signal via the star
coupler 4 can be similarly effected with less modal noise.
Another embodiment of the present invention will be described hereinafter
with reference to the drawings. FIG. 7 shows the configuration of an
optical star network transmission system which is an embodiment of the
present invention. Shown at 710 in FIG. 7 is an optical transmission
apparatus having two outputs. Shown at 711 is an input terminal. Shown at
712 and 713 are optical fibers. Shown at 714 is an [N+1].times.[N+1] star
coupler. Shown at 715a, 715b and 715c are star coupler input optical
fibers. Shown at 716 is a mixer portion. Shown at 717a, 717b and 717c are
output optical fibers. Shown at 718a and 718b are optical fibers. Shown at
719 is an Nth optical reception apparatus. Shown at 720a and 720b are
optical fibers. Shown at 721 is an [N+1]th optical reception apparatus.
FIG. 8 is a diagram explaining the operation of the star coupler 714.
FIG. 9 is a configuration diagram illustrating an embodiment of the optical
transmission apparatus 710 having two outputs. Shown at 722 in FIG. 9 is a
distributor. Shown at 723 and 725 are light emitting elements. Shown at
724 and 726 are electrooptic conversion portions.
FIG. 10 is a configuration diagram illustrating a still further embodiment
of the optical transmission apparatus 710 having two outputs. Shown at 727
in FIG. 10 is a light emitting element. Shown at 728 is an electrooptic
conversion portion. Shown at 729 is an optical distributor. Shown at 730
is an input port. Shown at 731 and 732 are output ports.
The operation of the present embodiment will be described hereinafter with
reference to FIGS. 7 and 8. The optical transmission apparatus 710 makes
an electrooptic conversion of transmission signal inputted from the input
terminal 711 to produce optical signal which is then outputted to two
optical fibers 712 and 713. The optical fiber 712 is connected to the
input optical fiber 715a of the star coupler 714. The optical signal
outputted from the optical fiber 712 is transmitted by the input optical
fiber 715a, diffused at the mixer portion 716, and then distributed into
the output optical fibers 717a, 717b, 717c, etc. On the other hand, the
optical signal outputted from the optical fiber 713 is similarly
transmitted by the output optical fiber 717a in the direction opposite to
that of the optical signal outputted from the fiber 12, diffused at the
mixer portion 716, and then distributed into the input fibers 715a, 715b,
715c and so on. In principle, the optical signal inputted from the optical
fiber 712 to the mixer portion 716 is not coupled to the optical fibers
715b and 715c due to the unidirectional diffusivity of optical signal at
the mixer portion 716. Similarly, the optical signal inputted from the
optical fiber 713 to the mixer portion 716 is not coupled to the optical
fiber 717b and 717c in principle. Therefore, two signals transmitted by
different paths in the transmission system are not crossed by each other.
The optical signals outputted from the output optical fibers 717b and 717c
in the star coupler 714 are transmitted by the optical fibers 718a and
718b to a first optical reception apparatus 73 and the Nth optical
reception apparatus 719, respectively. Similarly, the optical signals
outputted from the output optical fibers 715b and 715c in the star coupler
714 are trasmitted by the optical fibers 720a and 720b to the [N+1]th
optical reception apparatus 721 and the 2Nth optical reception apparatus
74, respectively.
The operation of the above described star coupler will be described
hereinafter with reference to FIG. 8. As can be seen in the explanatory
view, the execution of 2N branchings can be accomplished by an
[N+1].times.[N+1] star coupler. The [N+1].times.[N+1] star coupler is
about half the [1.times.2N] star coupler in the width of the mixer portion
and thus can be easily prepared. Furthermore, as compared to two
[1.times.N] star couplers, the [N+1].times.[N+1] star coupler is half in
the size of housing or mixer portion and thus is inexpensive. an
embodiment wherein the optical transmission apparatus 710 output optical
signals to two optical fibers is shown in FIGS. 9 and 10. In FIG. 9,
signal from the input terminal 711 is divided by the distributor 722 into
two signals on an electrical level which are then converted by the light
emitting element 723 and the electrooptic conversion portion 724 and by
the light emitting element 725 and the electrooptic conversion portion 726
to optical signals which are outputted to the optical fibers 712 and 713,
respectively. In FIG. 10, transmission signal inputted from the input
terminal 711 is converted by the light emitting element 727 and the
electrooptic conversion portion 728 to an optical signal which is then
inputted to the input port 730 of the optical distributor 729 from which
it is distributed to the output ports 731 and 732 and then outputted to
the optical fibers 712 and 713, respectively.
As compared to the conventional system, the embodiment shown in FIG. 9
further comprises the light emitting element 725, the electrooptic
conversion portion 726, and the distributor 722. As compared to the
conventional system, the embodiment shown in FIG. 10 further comprises the
optical distributor 729. On the other hand, in the conventional system, a
star coupler having 2N port outputs is formed, or a (1.times.N) star
coupler is added in order to obtain 2N branchings. Above-mentioned system
according to the present invention can realize a smaller and cheaper
branch system.
In FIG. 7, optical signal outputted from the optical transmission apparatus
710 to the optical fiber 712 returns to the optical transmission apparatus
710 via the star coupler 714 and the optical fiber 713. However, the
amount of light coupled to the optical fiber 713 in the star coupler is
about 1/N of the initial amount. Therefore, if the number of branches is
great, the amount of light coupled to the optical fiber 713 is small,
exerting no effect on the optical transmission operation. Optical signal
outputted from the optical transmission apparatus 710 to the optical fiber
713 returns to the optical transmission apparatus 710 via the star coupler
714 and the optical fiber 712. However, this too, exerts no effect on the
optical transmission operation.
While these embodiments have been described with reference to the case
where a single 2N branch system is formed, they have similar effects even
when applied to a relay branched system in which the output of an optical
reception apparatus 74 is relayed to an optical transmission apparatus 710
in a plural star network optical transmission system as shown in FIG. 11.
By using an [N+1].times.[N+1] star coupler, 2N branching are enable per
one relay.
While the present invention has been described with reference to the star
coupler comprising a slub type mixer shown in FIG. 8, similar effects can
be obtained even when it is applied to a fiber-fused star coupler.
* * * * *
![[Image]](/netaicon/PTO/image.gif)
![[Home]](/netaicon/PTO/home.gif)
![[Boolean Search]](/netaicon/PTO/boolean.gif)
![[Manual Search]](/netaicon/PTO/manual.gif)
![[Number Search]](/netaicon/PTO/number.gif)
![[Help]](/netaicon/PTO/help.gif)
![[Bottom]](/netaicon/PTO/bottom.gif)
![[View Shopping Cart]](/netaicon/PTO/cart.gif)
![[Add to Shopping Cart]](/netaicon/PTO/order.gif)
![[Top]](/netaicon/PTO/top.gif)