JPH11186969A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of optical signals and to minimize the decrease in the entire optical transmission capacity by eliminating unwanted signal light generated in wavelength areas other than a wavelength channel used inside an optical signal group in respective upper and lower transmission directions, and at the same time amplifying and relaying the signal light of upper and lower wavelength channels coexisting inside the same coated topical fiber. SOLUTION: Signal light of a wavelength channel for incoming and of a wavelength channel for outgoing coexisting inside the same coated optical fiber is amplified at the same time altogether for each group of the incoming and outgoing wavelength channels or for each sub group less than an arbitrarily determined wavelength cannel number or for each sub group less than the wavelength channel number inside the group of the incoming and outgoing wavelength channels, and the optical signals are relayed. In such a light amplifier, unwanted light of wavelength areas other than the signal wavelength channel is eliminated, diffusion of a spectrum generated due to the instability in an abnormal dispersion area of a positive dispersion value is suppressed and crosstalks between adjacent wavelengths are suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a communication optical transmission system for transmitting an optical signal using an optical fiber.

[0002]

2. Description of the Related Art In recent years, studies on long-distance wavelength division multiplexing optical transmission systems have been actively conducted. In a wavelength region where dispersion of an optical fiber is small, signal light is deteriorated and different due to nonlinear optical processes, especially four-wave mixing. It has been pointed out that crosstalk between wavelength channels causes a problem, and the following proposals have been made to overcome this problem.

[0003] The first is a method of designing an optical fiber transmission line so that the dispersion of the optical fiber in a wavelength region used for communication is not reduced (TrueWave, dispersion allocation method, etc.) (H. Maeda et al., Technical Report of IEICE, OES
96-57, OPE96-107, LQE96-108 (1996-11)), Part 2
Is to use a fiber with an increased mode field diameter of the optical fiber and a reduced probability of causing a nonlinear optical process of the fiber itself as an optical fiber transmission line (P. Nouchi et al.,
ECOC'96, see MoB.3.2), and Part 3 describes a method of making the intervals between communication wavelength channels uneven (F. Forghieri et al., IE
EE PTL, Vol. 6, No. 6, pp. 754-756, 1994 and O. Ishida
Etc., see QLEO / PacificRim'97, TuJ3).

[0004] Of these methods, the first and second methods require a new optical fiber to be manufactured and laid, which is not effective for existing huge optical fiber transmission lines. Also, part 3
In the method conventionally studied in the method described above, the accumulation of crosstalk cannot be suppressed. In addition, a method of shifting the communication wavelength range from the wavelength range where the dispersion of the existing optical fiber transmission line is small has been studied, but in this case, the optical component for optical signal modulation / demodulation is operated in a wavelength range different from the conventional one. Need to be changed to In addition, the bidirectional optical transmission system has a special configuration in which there is no reflection point on the transmission line optical fiber and an optical circulator is used in the transmission / reception section of the optical signal (K. Aida et al.,
ECOC'96 TuD.2.2) was required.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to use an optical transmission line and an optical communication wavelength band constructed using a conventional dispersion-shifted optical fiber and to use an ITU recommendation (ITU-T Draft).
t Recommendation, G.mcs, Work.Doc.SG15 / Q.25-KYOTO-
59,1996), a long-haul WDM optical transmission system composed of wavelength channels with equally spaced wavelength channels, in which cross-talk between different wavelength channels due to nonlinear optical processes and signal light degradation, and In order to prevent degradation of signal light caused by accumulation of unnecessary signal light generated by non-linear optical processes in the course of transmission of signal light and ASE of the optical amplifier, and to prevent degradation of the signal light at the same time, the deterioration of the signal light is suppressed. It is an object of the present invention to provide an optical transmission system capable of minimizing a reduction in overall optical transmission capacity by taking the above measures.

[0006]

In the optical transmission system of the present invention, in order to achieve the above object, one or both of upstream and downstream wavelength multiplexing optical transmission has three or more wavelength channels. In a wavelength-division multiplexing optical transmission system that performs bidirectional optical transmission using an optical fiber, in a group of optical signals in each of the upstream and downstream transmission directions, in a region where the chromatic dispersion value of the transmission optical fiber used is small, The wavelength channel intervals in the respective groups of optical signals in the upstream and downstream transmission directions are set unequally. Further, the intervals of all the upstream and downstream wavelength channels may be set equally including the idle channels.

Another optical transmission system according to the present invention is a wavelength multiplexing optical transmission system in which one or both of upstream and downstream have three or more wavelength channels, and a wavelength for performing bidirectional optical transmission using a single optical fiber. In a multiplexed optical transmission system, within a group of optical signals in each of the upstream and downstream transmission directions,
A means for removing unnecessary signal light generated in a wavelength region other than the wavelength channel being used is provided, and the signal light of the upstream wavelength channel and the downstream wavelength channel mixed in the same optical fiber core is collectively collected. Optical amplifying means for simultaneously amplifying and relaying an optical signal, or optical amplifying means for simultaneously amplifying and relaying an optical signal collectively for each group of upstream and downstream wavelength channels, or any Optical amplification means for simultaneously amplifying and relaying an optical signal collectively for each subgroup of the number of wavelength channels less than the number of wavelength channels, or less than the number of wavelength channels in each of the upstream and downstream wavelength channel groups There is provided an optical amplifying means for simultaneously amplifying the optical signals simultaneously for each of the subgroups and relaying the optical signals.

The optical amplifying means used in the optical transmission system of the present invention propagates through a plurality of wavelength channels of three or more channels in which the upward or downward propagation direction is set for each wavelength channel in the same optical fiber core. Optical amplifying means having a number smaller than the number of wavelength channels, and optical isolator means for performing branching coupling of signal light of each wavelength channel and isolation in the propagation direction before, after or before or after the optical amplifying means for the signal light to be transmitted It is desirable to have.

According to the optical transmission system of the present invention, there is provided a method of distributing equally-spaced wavelength channels of the ITU recommendation in a wavelength region where dispersion of a dispersion-shifted fiber is small to channels for an upstream transmission direction and channels for a downstream transmission direction. By distributing the wavelength channels such that the wavelength channel intervals are unequal in each of the upstream and downstream groups, it is possible to prevent mutual influence of signal light caused by four-wave mixing in each propagation direction. Furthermore, in addition to this condition for wavelength channel allocation, by allocating wavelength channels such that the number of unused wavelength channels is minimized, it is possible to prevent a reduction in the overall transmission capacity.

[0010] When the upstream and downstream wavelength channels are arranged as described above, unnecessary signal light that is wavelength-converted by the four-wave mixing process in the process of propagating the signal light and is generated in the transmission wavelength channel in the reverse direction is generated. It can be eliminated by using an optical amplifier as described above.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining the optical transmission system of the present invention. The optical transmission system includes a signal light transmitting section 1, a signal light receiving section 2, an optical multiplexing / demultiplexing section 3 for multiplexing / demultiplexing signal lights of a plurality of wavelength channels at once, a transmission optical fiber 4, and a And a bidirectional optical amplifier 5 which can arbitrarily set up and down transmission directions.

Now, as an example, a dispersion-shifted fiber (dispersion slope 0.07 ps / nm / nm / km / km) is used as the transmission optical fiber 4.
), The wavelength channel arrangement is centered at 1552.52 nm according to the ITU recommendations, and the adjacent wavelength channel spacing is 200 GHz.
Set as At this time, on the assumption that the zero-dispersion wavelength is 1552.52 nm, it is assumed that the group delay amount of the optical fiber can be approximated by the Cellmeier approximation τ (λ) = A + Bλ ² + Cλ ^-2. When the dispersion wavelength characteristic of the transmission optical fiber is estimated from the wavelength λ ₀ = (C / B) ^1/4 dispersion slope S ₀ = 8B and σ (λ) = dτ (λ) / dλ = 2Bλ-2Cλ ^-3 , As shown in FIG.

As can be seen from FIG. 2 showing the dispersion wavelength characteristics of the dispersion-shifted fiber, the absolute value of the chromatic dispersion | σ |
| <0.5 ps / km / nm, where the degradation of the transmitted signal light due to the four-wave mixing process cannot be ignored.
Maeda et al., Technical report of IEICE, OES96-57, OPE96-1
07, LQE96-108 (1996-11)) has a wavelength of about 1545 nm.
And a wavelength range of 15 nm from 1560 nm to about 1560 nm. In this wavelength region, there are approximately 11 wavelength channels at the center.

If the wavelength intervals of these wavelength channels can be arranged unequally, it becomes possible to effectively suppress the four-wave mixing process which is a problem in the "wavelength region with small dispersion" (F. Forgrihieri et al., IEEE PTL, Vol. 6, No. 754-75
(See p. 6, 1994)), however, the maximum number of wavelength channels can be multiplexed in a channel arrangement in which the wavelength channel intervals are unequal, under the restriction that the signal light is a one-way transmission wavelength multiplexed signal. The channel arrangement is shown in FIG.
As shown in the figure, five channels 7-1 to 7-5 indicated by arrows are arranged in eleven channels 6-1 to 6-11, and 6 (the number of unused channels V) / 11 (Total number of channels)
× 100 = 54.5% wavelength channels are sacrificed.

On the other hand, assuming that the signal light is a wavelength-division multiplexed signal for bidirectional transmission, a channel arrangement in which the most wavelength channels can be multiplexed in a channel arrangement in which the wavelength channel intervals are unequal is shown, for example, in FIG. , (B), and (c), the wavelength channels that can be used most efficiently are 1 (the number of unused channels V) / 11 (the total number of channels).
X100 = 9.1%, which can be greatly improved. Examples of the number of wavelength channels that can be used are small, such as the arrangement shown in FIGS. 5A, 5B, 5C, and 5D where the number of unused channels is 2.

Further, as is apparent from the following calculation results, even when the wavelength channel intervals are unequal,
In a one-way transmission system using an ordinary optical amplifier, a noise level signal light, for example, ASE of an optical amplifier, is used in a wavelength region that is just equally spaced for any two wavelength channels.
If the noise is generated, this noise light becomes a seed signal, is amplified by the four-wave mixing process, and accumulates as the optical relay is repeated. Finally, the four-wave mixing with the light of the non-signal channel is performed. It can be seen that the signal light is degraded by the process, but in the optical transmission system according to the present invention, the noise light is set to exist in the region where the transmission wavelength channel in the reverse direction of the relay optical amplifier exists. ,
The optical amplifier itself functions as a filter for each relay, and removes noise light, which is a problem. As a result, deterioration of signal light is drastically suppressed.

The light intensity propagating through the optical fiber in consideration of the four-wave mixing process can be obtained by numerical calculation (GPAgrawal, "Nonlinear Fiber Optics", Aca
See demic Presss). That is, Aeff = 50 × 10 ⁻¹² m ² , n
₂ = 3.2 × 10 ^-20 m ² / W, dispersion = 0 ps / km / nm, relative phase =
-Π / 2 (fixed), input signal light intensity = 0 dBm, input noise light intensity = -20 dBm (ASE), optical repeater interval = 100 km, optical amplifier gain = 21 dB, optical fiber loss = 0.21 dB / km so,
It is assumed that continuous signal light of two wavelength channels propagates in the optical fiber, and the signal light is used for the case where noise light is removed to -30 dB (curve 10) and the case where noise light is not removed at all (curve 9) for each relay. FIG. 6 shows a calculation result of a change in the noise light intensity with respect to the propagation distance due to the four-wave mixing process. The former is much better than the latter.

FIG. 7 shows the change in the degree to which the signal light intensity is converted into noise light intensity and deteriorated in the four-wave mixing process with respect to the propagation distance under the above conditions. Compared to the case where noise light is not removed at all (curve 11), the noise light is-
The rejection up to 30 dB (curve 12) is much better.

An optical amplifier having such an operation is capable of transmitting signal light propagating through a plurality of wavelength channels of three or more channels in which the upward or downward propagation direction is set for each wavelength channel in the same optical fiber core. On the other hand, there are provided a number of optical amplifiers smaller than the number of wavelength channels, and optical isolator means for performing branching coupling and isolation in the propagation direction of signal light of each wavelength channel before, after or before or after the optical amplifier. Two examples of such an optical amplifier are shown in FIGS.

The configuration shown in FIG. 8 is based on a group of wavelength channels {a1, a2,..., AN, each of which has at least one of the propagation directions of up and down and has three or more wavelength channels having mutually different wavelengths. , B1, b2,..., BM}, the wavelength channel groups {a1, a2,..., AN} whose propagation directions are the same in the forward direction and the wavelength channel groups {b1} whose propagation directions are the same in the reverse direction. , B2,..., BM}, an input / output port A in which A and B are an input port and an output port and an output port and an input port, respectively.
, B, and B, wherein the input / output port A and the output ports {c: a1, c: a2,..., Respectively corresponding to the wavelength channels {a1, a2,. c: aN} and input ports {c: b1, c: b2,..., c: bM} corresponding to the wavelength channels {b1, b2,.
The incident signal light from the input / output port A is divided for each wavelength channel {a1, a2,..., AN}, and each signal light is divided into output ports {c: a1, c: a2,. : aN} and input corresponding to each wavelength channel {b1, b2, ..., bM} which is input from input port {c: b1, c: b2, ..., c: bM} respectively. An optical wavelength multiplexing / demultiplexing unit C for removing unnecessary light existing in a wavelength region other than each wavelength channel, an input / output port E, and each wavelength channel. Input ports {d: a1, d: a2,..., d: aN} and wavelength channels {b1, b2,..., bM} corresponding to a1, a2,. A corresponding output port {d: b1, d: b2,..., D: bM} and a wavelength channel incident from an input port {d: a1, d: a2,. a1, a2, ..., aN｝ , Output from the input / output port E, and divide the incident signal light from the input / output port E for each of the wavelength channels {b1, b2,..., BM}, and divide each signal light into the output port {d. : b1, d: b2, ..., d: bM｝
An optical wavelength multiplexing / demultiplexing unit D and an optical wavelength multiplexing / demultiplexing unit C for simultaneously removing unnecessary light existing in wavelength regions other than the wavelength channels.
, C: a2, ..., c: aN} and input ports {c: b1, c: b2, ..., c: bM} of the optical wavelength multiplexing / demultiplexing unit D Input ports {d: a1, d: a2,..., D: aN} and output ports {d: b1, d: b2,. And the output ports, and the propagation directions of the respective signal lights are determined by the output ports {c: a1, c: a2,..., C: aN} and {d: b1, d: b2,. ..,
Input ports {d: a1, d: a corresponding to d: bM}
2, ..., d: aN} and {c: b1, c: b2, ..., c: bM} optical isolators ｛i: a1, i: a that restrict only to the propagation direction
, I: aN} and {i: b1, i: b2, ..., i: bM}, an optical isolator circuit, an input / output port B, and each wavelength channel {a1, a2,. , AN} and input ports {h: a1, h: a2,..., H: aN} and output ports {h respectively corresponding to wavelength channels {b1, b2,. : b1, h: b2,..., h: bM}, and transmits incident signal light from the input / output port B to each wavelength channel {b1, b2,.
M｝ and split each signal light into output port ｛h: b
1, h: b2,..., H: bM}, and each wavelength channel {a1, a2} which enters from an input port {h: a1, h: a2,. ,..., AN}, an optical wavelength multiplexing / demultiplexing unit that multiplexes all of the incident signal lights and outputs the multiplexed signal light from the input / output port B, and simultaneously removes unnecessary light existing in wavelength regions other than the wavelength channels. H, input / output port F, output ports {g: a1, g: a2,..., G: aN} and wavelength channels {b1 respectively corresponding to the wavelength channels {a1, a2,. , B2,..., BM}, the input ports {g: b1, g: b2,.
, G: b2,..., G: bM}, all the signal lights corresponding to the wavelength channels {b1, b2,. And input signal light from the input / output port F to each of the wavelength channels {a1, a
, AN} and output each signal light from the output ports {g: a1, g: a2,..., G: aN}, and simultaneously to the wavelength region other than each wavelength channel. The output ports {h: a1, h: a2,..., H: aN} and the input ports {h:} of the optical wavelength multiplexing / demultiplexing section G for removing the unnecessary light existing therefrom and the optical wavelength multiplexing / demultiplexing section H. b
, H: bM}, input ports {g: a1, g: a2,..., G: aN} and output ports {g: b of the optical wavelength multiplexing / demultiplexing unit G.
, G: b2,..., G: bM} are connected between the input port and the output port of the corresponding wavelength channel, and the propagation direction of each signal light is set to the output port {g: a1,
g: a2, ..., g: aN} and {h: b1, h: b2, ..., h: bM} and the corresponding input ports {h: a1, h: a2, ..., h respectively :
aN｝ and {g: b1, g: b2, ..., g: bM} optical isolators の み j: a1, j: a2, ..., j: a
An optical isolator circuit comprising N1 and {j: b1, j: b2,..., J: bM}, and between input / output ports E and F;
An optical amplifier K is provided for amplifying output signal light from the input / output ports E and F and outputting the amplified signal light to the input / output ports E and F, respectively.

The configuration of the optical amplifier shown in FIG. 9 has a group of wavelength channels {a1, a2,..., Consisting of three or more wavelength channels, each channel having one of the upstream and downstream propagation directions and different wavelengths from each other. , AN, b1, b2, ...
, BM} and the same wavelength channel group {a1, a2,..., AN} in the forward direction and the same wavelength channel group {b1, b2,.
An optical amplifier comprising two optical isolator circuits each having an input port, an output port, and input / output ports A and B serving as output ports and input ports, respectively, for each of M｝. A, output ports {c: a1, c: a2,..., C: aN} corresponding to the respective wavelength channels {a1, a2,..., AN} and respective wavelength channels {b1, b2,. , BM} and input / output port A with input ports {c: b1, c: b2,..., C: bM} respectively.
The incident signal light from each of the wavelength channels ｛a1, a2, ...,
aN｝ and split each signal light into output port ｛c: a
1, c: a2,..., C: aN}, and each wavelength channel {b1, b2} that enters from an input port {c: b1, c: b2,. ,..., BM｝, an optical wavelength multiplexing / demultiplexing unit that multiplexes all of the incident signal lights and outputs the multiplexed signal light from the input / output port A, and simultaneously removes unnecessary light existing in a wavelength region other than each wavelength channel. C, an output port E-1, and input ports {d: a1, d: a2,..., D: aN} corresponding to the respective wavelength channels {a1, a2,..., AN}. All the signal lights corresponding to the wavelength channels {a1, a2,..., AN} incident from {d: a1, d: a2,..., D: aN} are multiplexed and output port E -1 and simultaneously removes unnecessary light existing in a wavelength region other than each wavelength channel, an optical wavelength multiplexing / demultiplexing unit D-1, an input port E-2, and each wavelength channel # b1, b2, ... , BM｝ output ports respectively d: b1, d: b2,..., d: bM} and input port E
-2, the incident signal light from each wavelength channel ｛b1, b2, ...
, BM}, and outputs each signal light from the output ports {d: b1, d: b2,..., D: bM}, and at the same time, unnecessary light existing in a wavelength region other than each wavelength channel. An optical isolator i which is connected to the output port E-1 and limits the propagation direction of the signal light to only the propagation direction from the output port E-1 to the input port F-1. : EK and an optical isolator i: KE that is connected to the input port E-2 and restricts the propagation direction of the signal light to only the propagation direction from the output port F-2 to the input port E-2. Optical isolator circuit, input / output port B, each wavelength channel # a1, a
The input ports {h: a1,
h: a2, ..., h: aN} and each wavelength channel {b1, b
Output ports {h: b1,
h: b2,..., h: bM}, and divides the incident signal light from the input / output port B for each wavelength channel {b1, b2,. Port ｛h: b1, h: b
2, ..., output from h: bM} and input port {h: a
1, h: a2,..., H: aN}, multiplex all the incident signal lights corresponding to each wavelength channel {a1, a2,. At the same time, the optical wavelength multiplexing / demultiplexing unit H for removing unnecessary light existing in the wavelength region other than each wavelength channel, the input port F-1, and the wavelength channels {a1, a2,..., AN} are respectively supported. , G: aN} and input signal light from the input port F-1 to each of the wavelength channels {a1, a2,.
N｝ and split each signal light into output port 出力 g: a
1, g: a2,..., G: aN}, and an optical wavelength multiplexing / demultiplexing unit G-1 for simultaneously removing unnecessary light existing in a wavelength region other than each wavelength channel, and an output port F-2. And input ports {g: b1, g: b2,..., G: bM} respectively corresponding to the wavelength channels {b1, b2,..., BM}, and input ports {g: b
1, g: b2,..., G: bM}, all the signal lights corresponding to the wavelength channels {b1, b2,. An optical isolator circuit having an optical wavelength multiplexing / demultiplexing unit G-2 for outputting and simultaneously removing unnecessary light existing in a wavelength region other than each wavelength channel, which is connected to an input port F-1 and a signal light propagation direction. Output port E
-1 which is connected to an output port F-2 and an optical isolator i: K-F for limiting only the direction of propagation from the output port F-1 to the input port F-1. Optical isolator i: F that restricts only to the direction of propagation to 2
-K optical isolator circuit, optical isolator i: E
-K and an optical isolator i: K-F, an optical amplifier K-1 for amplifying an output signal light from the output port E-1 and outputting the amplified signal light to the input port F-1, and an optical isolator i: An optical amplifier K-2 is provided between the FK and the optical isolator i: KE and amplifies the output signal light from the output port F-2 and outputs the amplified signal light to the input port E-2.

The optical amplifier suitable for the optical transmission system of the present invention is not limited to the above two examples, but collectively collects the signal light of the upstream wavelength channel and the downstream wavelength channel mixed in the same optical fiber. An optical amplifier that simultaneously amplifies and relays an optical signal, or an optical amplifier that amplifies and relays an optical signal simultaneously and collectively for each group of upstream and downstream wavelength channels, or arbitrarily divided Optical amplifier that simultaneously amplifies and relays an optical signal collectively for each subgroup of less than the number of wavelength channels, or a number of wavelength channels less than the number of wavelength channels in each of the upstream and downstream wavelength channel groups. Various types of optical amplifiers, including optical amplifiers with optical isolator circuits that have a configuration that mixes the two examples, such as optical amplifiers that amplify and simultaneously relay optical signals for each subgroup at once Those of the configuration can be realized.

By using such an optical amplifier to remove unnecessary light in a wavelength region other than the signal wavelength channel, in the anomalous dispersion region where the dispersion value is positive, the spread of the spectrum caused by the influence of modulation instability is reduced. Thus, crosstalk between adjacent wavelength channels and deterioration of signal light can be effectively suppressed.

[0024]

As described above, according to the optical transmission system of the present invention, a relay amplifier capable of collectively amplifying and relaying optical signals of a plurality of wavelength channels having arbitrary upstream and downstream transmission directions. In the wavelength region where the dispersion of the transmission optical fiber in the wavelength channel group in each of the upstream and downstream transmission directions is small, the wavelength channel intervals are uneven and the unused wavelength channels are minimized. By adopting a simple configuration, it is possible to prevent crosstalk between channels and deterioration of signal light due to nonlinear optical processes that occur in the signal light propagation process of long-haul optical wavelength division multiplexing transmission systems with equal wavelength channels, and to achieve the conventional unevenness. In the optical transmission system proposed as a wavelength channel arrangement type, the ASE of the optical amplifier and the nonlinear optical process in the transmission process of the signal light Unnecessary signal light is accumulated by repeating the relay transmission, and eventually the deterioration of the signal light due to the nonlinear optical process promoted by the accumulated unnecessary signal light is prevented by effectively preventing the accumulation of the unnecessary signal light. By taking measures to prevent the signal light deterioration, the effect of minimizing the decrease in the transmission capacity of the entire optical transmission system can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an optical transmission system of the present invention.

FIG. 2 is a diagram illustrating dispersion wavelength characteristics of a dispersion-shifted fiber.

FIG. 3 is a diagram illustrating a manner in which wavelength channels in one direction are allocated to each wavelength channel in a wavelength region where dispersion is small.

FIG. 4 is a diagram illustrating a state in which bidirectional wavelength channels are allocated to each wavelength channel in a wavelength region where dispersion is small.

FIG. 5 is a diagram illustrating a state in which bidirectional wavelength channels are allocated to each wavelength channel in a wavelength region where dispersion is small.

FIG. 6 is a diagram illustrating a change in noise light intensity with respect to a propagation distance due to a four-wave mixing process with signal light.

FIG. 7 is a diagram showing a change in signal light intensity deterioration due to a four-wave mixing process with noise light with respect to a propagation distance.

FIG. 8 is a diagram illustrating a configuration example of an optical amplifier used in the optical transmission system of the present invention.

FIG. 9 is a diagram showing another configuration example of the optical amplifier used in the optical transmission system of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Signal light transmission part 2 Signal light reception part 3 Optical multiplexing / demultiplexing part 4 Transmission optical fiber 5 Optical amplifier 6 Wavelength channel 7 Signal light transmission direction 8 Unused wavelength channel 9 Propagation of noise light when noise light is not removed at all Curve showing change with respect to distance 10 Curve showing change with respect to propagation distance of noise light when noise light is removed to −30 dB for each relay 11 Change with respect to propagation distance of deterioration of signal light intensity when noise light is not removed at all 12 Curves A, B, E, and F showing changes in signal light intensity degradation with respect to propagation distance when noise light is removed to −30 dB for each relay. Signal light input / output ports C, D, G, H Optical wavelength multiplexer / demultiplexer c: b, d: a, g: b, h: a Signal light input port c: a, d: b, g: a, h: b Signal light output port i, j Optical isolator

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/18

Claims (6)

[Claims] 1. A wavelength-division multiplexing optical transmission system that performs wavelength-division multiplexing optical transmission in which one or both of upstream and downstream has three or more wavelength channels and performs bidirectional optical transmission using a single optical fiber. Means for removing unnecessary signal light generated in a wavelength region other than the wavelength channel being used within a group of optical signals in each of the downstream transmission directions, and further, for upstream mixed in the same optical fiber. Optical amplification means for simultaneously amplifying the signal light of the wavelength channel and the downstream wavelength channel simultaneously and relaying the optical signal, or simultaneously amplifying the optical signal simultaneously for each of the upstream and downstream wavelength channel groups. Amplifying means for relaying optical signals by means of an optical signal, or simultaneously amplifying optical signals simultaneously for each subgroup of less than the number of arbitrarily divided wavelength channels to relay optical signals Amplifying means, or optical amplifying means for simultaneously amplifying and relaying an optical signal simultaneously and collectively for each subgroup of the number of wavelength channels less than the number of wavelength channels in each of the upstream and downstream wavelength channel groups. An optical transmission system characterized by the above. 2. An optical amplifying means for transmitting a signal light having a wavelength of three or more wavelength channels, each of which has an upward or downward propagation direction set for each wavelength channel in the same optical fiber core, to a wavelength. It is characterized by comprising an optical amplifying means of a number smaller than the number of channels, and an optical isolator means for performing branching coupling and isolation in the propagation direction of signal light of each wavelength channel before, after or before or after the optical amplifying means. The optical transmission system according to claim 1. 3. A wavelength division multiplexing optical transmission system in which one or both of upstream and downstream has three or more wavelength channels in a bidirectional optical transmission system using a single optical fiber. An optical transmission system characterized in that, in a region where the chromatic dispersion value of a transmission optical fiber is small, wavelength channel intervals in groups of optical signals in the upstream and downstream transmission directions are unequally set. 4. An optical system according to claim 1, wherein all wavelength channels are set on a wavelength channel grid in which wavelength intervals are equally spaced, and light in each of upstream and downstream propagation directions is in a region where a chromatic dispersion value of a transmission optical fiber to be used is small. 4. The optical transmission system according to claim 3, wherein the wavelength channels in the signal group are set unequally. 5. A device for removing unnecessary signal light generated in a wavelength region other than a wavelength channel used in a group of optical signals in each of upstream and downstream transmission directions, further comprising: Optical amplification means for simultaneously and simultaneously amplifying the signal light of the upstream wavelength channel and the downstream wavelength channel mixed in the core and relaying the optical signal, or for each group of the upstream and downstream wavelength channels Optical amplification means for simultaneously amplifying the optical signal and relaying the optical signal, or simultaneously amplifying the optical signal simultaneously for each subgroup of less than the number of arbitrarily divided wavelength channels to relay the optical signal Optical amplifying means or optical amplification at the same time for each subgroup of the number of wavelength channels less than the number of wavelength channels in each of the upstream and downstream wavelength channels, and relaying optical signals The optical transmission system according to claim 3, further comprising: an optical amplifying unit that performs the following. 6. An optical amplifying means for applying a wavelength to a signal light propagating through a plurality of wavelength channels of three or more channels for which an up or down propagation direction is set for each wavelength channel in the same optical fiber core. It is characterized by comprising an optical amplifying means of a number smaller than the number of channels, and an optical isolator means for performing branching coupling and isolation in the propagation direction of signal light of each wavelength channel before, after or before or after the optical amplifying means. The optical transmission system according to claim 5, wherein