JPH1117656A - Wavelength division multiplex type optical transmission system and its method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use an existing distributed sift fiber for an optical transmission line for transmitting wavelength division multiplex optical signals, so as to enlarge allowable optical input power to the distributed shift fiber by arranging the wavelength of signal light to one of plural specified ranges from among the plural wavelength multiplexes signal lights. SOLUTION: An unrelayed point-to-point transmission system is constituted of an optical transmission part 10, an optical transmission line 20 and an optical reception part 30. Then, in the case of using the distributed shift fiber 21, for which a zero distribution wavelength is set around 1.55 μm as the optical transmission line 20, the wavelength of the plural wavelength multiplexed signal lights is arranged either between 1450 nm to 1530 nm or between 1570 nm to 1650 nm. That is, by limiting a wavelength band used, the effects of four- light-wave mixing in the distributed shift fiber 21 is evoided. Thus, the allowable optical input power to the distributed shift fiber 21 is enlarged, and a transmittable distance is substantially extended.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a wavelength division multiplexing type optical transmission system for transmitting a wavelength division multiplexing optical signal using a dispersion shifted fiber.

[0002]

2. Description of the Related Art Wavelength division multiplexing (WDM) transmission technology is a technology in which a plurality of optical signals having different wavelengths (optical frequencies) are multiplexed and transmitted through a single optical fiber transmission line. Here, the optical signal is a signal obtained by directly modulating the output light of the light source with a data signal (direct modulation method) or a signal obtained by modulating an optical carrier output from the light source with a data signal using an external modulator (external modulation). The wavelength is determined by the wavelength of the light source.

On the other hand, an optical amplifier that amplifies an optical signal as light is arranged in the middle of an optical fiber transmission line to compensate for the transmission loss of the optical fiber transmission line, thereby making it possible to perform a reproduction that requires an identification reproduction process in an electric stage. The relay interval can be extended. This optical amplifier has a function of amplifying optical signals of a plurality of wavelengths multiplexed by wavelength division multiplexing. Therefore, only the devices on the transmission side and the reception side are changed to those for wavelength division multiplexing. The transmission capacity of the fiber transmission line can be increased several times the wavelength. For example, the amplification wavelength band of an erbium-doped optical fiber amplifier (EDFA) is 1.53 μm.
m to 1.56 μm, and a wavelength interval of 0.
By multiplexing a plurality of optical signals at 8 nm, optical signals of about 30 channels can be transmitted by one optical fiber.

[0004]

The existing dispersion-shifted fiber is designed to transmit a designed optical signal having a zero dispersion wavelength. When a wavelength division multiplexed optical signal is transmitted through this dispersion-shifted fiber, one of the nonlinear optical effects can be obtained.
Crosstalk due to four-wave mixing occurred, and the input power to the transmission line fiber could not be increased. Hereinafter, this problem will be described in detail.

The transmission loss of a quartz optical fiber is 1.5 μm.
, And becomes minimum around 1.6 μm. The dispersion-shifted fiber is designed so that the chromatic dispersion becomes zero around the wavelength of 1.55 μm, and the transmission distance is extended by suppressing the waveform deterioration due to the chromatic dispersion at this wavelength. The dispersion-shifted fiber is defined by an international standard organization so that the zero-dispersion wavelength is from 1.525 μm to 1.575 μm, but is substantially from 1.535 μm to 1.565 μm with 1.550 μm as the center. Are distributed in
It has been widely laid to date.

On the other hand, when a plurality of lights having different wavelengths are input to an optical fiber, light having a new optical frequency depending on the optical frequency difference is generated based on the third-order nonlinearity in the optical fiber. This is called four-wave mixing, and the optical frequencies f ₁ , f ₂ , f
_This is a phenomenon that, for example, light having an optical frequency of f ₁ + f ₂ −f ₃ is generated from the _three lights of ( ₃ ). This four-wave mixing is more likely to occur as the dispersion value at the input light wavelength is smaller and as the input power per wavelength is larger.

If the optical frequency interval of a wavelength division multiplexed optical signal input to such an optical fiber is constant, the optical frequency of light newly generated by four-wave mixing is one of the original optical signals. It coincides with one optical frequency and interferes with each other to generate intensity noise. Further, even when the optical frequency interval of the wavelength division multiplexed optical signal is not constant, the optical power of the original optical signal is consumed for generating four-wave mixing, which becomes intensity noise. Excess noise due to four-wave mixing is generated when the input power per wavelength is about -5 dBm when the optical frequency interval of the wavelength division multiplexed optical signal is equal, and when the optical frequency interval is unequal, it is one wavelength. The input power per unit is generated from about -2 dBm. Therefore, the optical power that can be input to the optical fiber transmission line cannot exceed the value, and as a result, the transmission distance is limited.

According to the present invention, there is provided a wavelength division multiplexing type optical transmission system in which an existing dispersion shift fiber is used in an optical transmission line for transmitting a wavelength division multiplexed optical signal and the allowable optical input power to the dispersion shift fiber can be increased. The purpose is to provide.

[0009]

In order to achieve the above object, according to the first aspect of the present invention, there is provided a wavelength division multiplexing type optical transmission system which transmits a dispersion shifted fiber having a zero dispersion wavelength near 1550 nm. In the transmission system, the wavelength of at least two of the plurality of wavelength-multiplexed signal lights is between 1450 nm and 1530 nm and 1570 n
A wavelength division multiplexing type optical transmission system characterized by being arranged at any point between m and 1650 nm. According to a second aspect of the present invention, in the wavelength division multiplexing type optical transmission system according to the first aspect, among the wavelengths of the plurality of wavelength-multiplexed signal lights, at least two signal lights have wavelengths from 1450 nm. It is characterized by being arranged between 1530 nm. According to a third aspect of the present invention, in the wavelength division multiplexing type optical transmission system according to the first aspect, among the wavelengths of the plurality of wavelength-multiplexed signal lights, at least two signal lights have wavelengths of 1570 nm or more. It is characterized by being arranged between 1650 nm.

[0010] The invention described in claim 4 is the first invention.
In the wavelength division multiplexing type optical transmission system according to the above, among the plurality of wavelength-multiplexed signal lights, the wavelength of at least two signal lights is arranged between 1450 nm and 1530 nm, and between 1570 nm and 1650 nm. Features. According to a fifth aspect of the present invention, in the wavelength division multiplexing type optical transmission system according to the fourth aspect, the signal light whose wavelength is arranged between 1450 nm and 1530 nm;
Signal light arranged between 50 nm propagates in the dispersion-shifted fiber transmission line in opposite directions.

[0011] The invention according to claim 6 is the first invention.
3. The wavelength division multiplexing type optical transmission system according to claim 1, wherein the wavelengths of the plurality of wavelength-multiplexed signal lights are between 1450 nm and 1570 nm, and between 1570 nm and 1650 nm.
nm between 1450 nm and 1570 n
signal light having a wavelength between m and 1570 nm
The signal light arranged between 1650 nm and 1650 nm propagates in the dispersion-shifted fiber transmission line in opposite directions, and at least the optical frequency difference of the signal light having a wavelength of 1505 nm or more and 1565 nm or less is arranged at unequal intervals. It is characterized by that. According to a seventh aspect of the present invention, in the wavelength division multiplexing type optical transmission system according to the first aspect, the wavelengths of the plurality of wavelength-multiplexed signal lights are between 1450 nm and 1530 nm, and between 1530 nm and 165 nm.
0 nm, and the 1450 nm to 1530
the signal light whose wavelength is arranged between
The signal light arranged between m and 1650 nm propagates in the dispersion-shifted fiber transmission lines in opposite directions,
At least, the optical frequency difference of the signal light having a wavelength of 1535 nm or more and 1595 nm or less is characterized by being arranged at unequal intervals.

Next, an eighth aspect of the present invention is a wavelength division multiplexing type optical transmission method using a dispersion-shifted fiber having a zero dispersion wavelength near 1550 nm as a transmission line. The wavelength of at least two signal lights is between 1450 nm and 1530 nm, 1570 nm
The wavelength division multiplexing type optical transmission method is characterized in that the wavelength division multiplexing type optical transmission method is arranged at any one of the wavelength division multiplexing wavelengths of 1650 nm to 1650 nm.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the case where a dispersion-shifted fiber having a zero-dispersion wavelength set at around 1.55 .mu.m is used as an optical transmission line, first, the wavelengths of a plurality of wavelength-multiplexed signal lights are 1450 nm to 1510 nm. Between, 157
A wavelength division multiplexing type optical transmission system arranged between 0 nm and 1610 nm (referred to as “first wavelength band limit”) will be described. Thereafter, the wavelength division multiplexing type optical transmission in which the wavelengths of the plurality of wavelength-multiplexed signal lights are arranged between 1450 nm and 1530 nm and between 1570 nm and 1650 nm (referred to as “second wavelength band limitation”). The system will be described.

[First Embodiment Regarding Wavelength Band Limiting] In the following, the wavelengths of a plurality of wavelength division multiplexed signal lights are 1450.
nm to 1510 nm, 1570 nm to 1610
The reasons and system overviews are described first for any placement between nm. Then 5 about the system
One embodiment will be described, and each will be described.

The wavelength division multiplexing type optical transmission system of the present invention uses a dispersion-shifted fiber whose zero dispersion wavelength is set at around 1.55 μm as an optical transmission line, and uses the absolute value of chromatic dispersion when propagating through the dispersion-shifted fiber. Is 0.5 ps / nm
/ Km or more is set for each wavelength of the plurality of optical signals.

References (Fukui et al., "Effect of Fiber Nonlinear Effect in WDM Optical Multi-Relay Transmission Using Dispersion Management", 1996 IEICE General Conference Lecture No. B
1138) states that the transmission distance limitation by four-wave mixing is significantly relaxed if the absolute value of dispersion is 0.5 ps / nm / km or more. On the other hand, the zero dispersion wavelength is 1.5
The actual zero-dispersion wavelength of the dispersion-shifted fiber set near 5 μm is about 1.535 in consideration of manufacturing variations.
Although it is considered that the distribution is from μm to 1.565 μm, the chromatic dispersion value near the wavelength of 1.55 μm is almost a linear function of the wavelength. Therefore, the dispersion slope is set to +0.07 ps /
If nm ² / km, the absolute value of chromatic dispersion is 0.5 ps
The wavelength at which / nm / km or more is 1.53 μm or less or 1.57 μm or more.

In this embodiment, one of these two wavelength ranges is used as the wavelength band of the wavelength division multiplexed optical signal.
Or use both. Specifically, 1.57
A wavelength range of μm to 1.61 μm is used. Or 1.4
A wavelength range of 5 μm to 1.51 μm is used. Alternatively, both wavelength ranges are used. As a result, the phase matching condition required for four-wave mixing generation is not satisfied due to ignorable wavelength dispersion at each wavelength, and four-wave mixing generation can be suppressed. As a result, the allowable optical input power to the dispersion-shifted fiber can be increased, and the transmittable distance can be greatly extended.

By the way, the 1.55 μm band has been conventionally used in the optical fiber communication utilizing the low loss area of the optical fiber. This is because when optical fiber was developed,
In addition to the fact that the low-loss region is reported to be 1.55 μm, the main reason is that an optical fiber amplifier which has significantly improved the performance of an optical communication system in recent years has an amplification band at 1.55 μm. I have. Therefore, in optical fiber communication, use outside the 1.55 μm band was not expected.

However, an optical fiber laid on site for communication has a loss characteristic as shown in FIG.
That is, 1.57 μm to 1.61 μm used in the present invention.
It can be seen that the loss is even lower than 1.55 μm in the wavelength range of m. It can be seen that, in addition to the above-described effects, the use of a wavelength band that has not been assumed in the past provides an effect of further reducing the loss.

In a wavelength division multiplexing type optical transmission system in which a linear optical repeater is arranged in an optical transmission line, optical signals in both wavelength ranges are collectively amplified by one optical amplifier, or each wavelength is The optical signals in the region may be separated and amplified by separate optical amplifiers.

Next, a wavelength division multiplexing type optical transmission system (wavelength division multiplexing type optical transmission system) in which the wavelength of a plurality of wavelength division multiplexed signal lights is arranged between 1450 nm and 1510 nm and between 1570 nm and 1610 nm. )
Are shown in five embodiments, and each is described.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
1 shows the configuration of the embodiment. This embodiment shows an example of a relayless point-to-point transmission system in which an opposing optical transmitting unit and optical receiving unit are connected without a repeater.

In the figure, the present system comprises an optical transmission unit 1
0, an optical transmission line 20, and an optical receiving unit 30. The method of directly modulating the bias or the like of the light source is also an
0 is applicable. The optical transmitter 10 is based on an external modulation method, and includes a plurality of light sources 11 set to different wavelengths, a plurality of modulators 12 for modulating an optical carrier output from the light source with a data signal, and each of the modulators. An optical multiplexer 13 for multiplexing an optical signal output from the optical signal generator 12, an optical multiplexer 1
The optical post-amplifier 14 collectively amplifies the wavelength division multiplexed optical signal output from the optical amplifier 3. The optical post-amplifier 14 is installed as needed.

The optical transmission line 20 has a zero dispersion wavelength of 1.55 μm.
It is composed of a dispersion-shifted fiber 21 set near m. The optical receiving unit 30 includes an optical preamplifier 31 that collectively amplifies the wavelength division multiplexed optical signal propagated through the dispersion shift fiber 21, an optical demultiplexer 32 that demultiplexes the wavelength division multiplexed optical signal into optical signals of each wavelength, It comprises a plurality of opto-electrical converters (0 / E) 33 for converting optical signals into electric signals, and an electric receiving circuit 34 for demodulating data signals from each electric signal.
If an optical preamplifier is installed before the photoelectric converter 33, the receiving sensitivity can be improved. The optical preamplifier and the optical preamplifier 31 in front of the optical demultiplexer 32 are installed as necessary.

The wavelength of the light source 11 is
The absolute value of chromatic dispersion is 0.5 ps / nm when propagating 1.
/ Km or more. The zero-dispersion wavelength of the dispersion-shifted fiber 21 is considered to be distributed in the range of about 1.535 μm to 1.565 μm due to manufacturing variations, but the dispersion slope is set to +0.07 ps / nm ² / km.
Then, the absolute value of the chromatic dispersion is 0.5 ps / nm / km
The wavelength above is 1.53 μm or less or 1.57 μm
m or more. Therefore, the wavelength band used is 1.5
Wavelength band of 3 μm or less (for example, 1.45 μm to 1.51
μm), or a wavelength band of 1.57 μm or more (for example,
1.571 μm to 1.61 μm), or both wavelength bands.

(Second Embodiment) FIG. 2 shows a second embodiment of the present invention.
1 shows the configuration of the embodiment. The feature of this embodiment is that, in the first embodiment, a linear optical repeater 22 having an optical amplifier as a main component is arranged in the middle of an optical transmission line in order to compensate for the transmission loss of the dispersion-shifted fiber 21. It is in. That is, this is an example of a multi-relay point-to-point transmission system. Thereby, the transmission distance can be significantly increased. The characteristics of the dispersion-shifted fiber 21 and the wavelength band to be used are the same as in the first embodiment. In the case where the transmission distance is long and the optical power is maintained at a high level by the linear optical repeater 22 as in the present embodiment, the deterioration of the transmission quality due to four-wave mixing becomes large in the conventional configuration. In a configuration that limits the wavelength band, the effect is small and the effect is remarkable.

In the first and second embodiments, an optical fiber amplifier or a semiconductor laser amplifier can be used as the optical amplifier, but an optical amplifier having an optimum structure is selected according to the wavelength band to be used. 1.45 μm or more
As an optical amplifier for the 1.51 μm band, there is a Tm-doped optical fiber amplifier (TDFA). As for the gain characteristics, as shown in FIG. 3, the high gain region is particularly in the band of 1.45 μm to 1.48 μm. For the excitation of this TDFA, 1.0 μm
A light source in the band of １．２1.2 μm is used. At present, there are an Nd: YAG laser and an Nd: YLF laser as excitation light sources in this wavelength band. Further, by adopting a configuration in which the amplifying optical fibers are cascaded via an isolator or an optical bandpass filter, an amplifier having a higher gain can be obtained.

As an optical amplifier for the 1.57 μm to 1.61 μm band, there is an Er-doped gain shift optical fiber amplifier (GS-EDFA). This is because a normal ED is improved by optimizing the Er concentration and the like of the amplification optical fiber.
The gain band of the FA (1.53 μm to 1.56 μm) is shifted. FIG. 4 shows the gain characteristics. For excitation of the GS-EDFA, a light source in the vicinity of the 0.98 μm band or in the vicinity of the 1.48 μm band is used.

Wavelength band of 1.53 μm or less and 1.57 μm
When the above wavelength bands are used at the same time, optical signals in both wavelength bands can be collectively amplified by using a semiconductor laser amplifier having a wide gain band. In addition, the development of an optical fiber amplifier that can collectively amplify optical signals in both wavelength bands has been advanced. Alternatively, the optical signals in the respective wavelength bands may be individually amplified and then combined. An example of the configuration will be described below as a third embodiment.

(Third Embodiment) FIG. 5 shows a third embodiment of the present invention.
1 shows the configuration of the embodiment. This embodiment is an example of a multi-relay point-to-point transmission system as in the second embodiment shown in FIG. Components having the same functions as those in FIG. 2 are denoted by the same reference numerals. In the optical transmission unit 10, 1.45 μm to 1.5 μm
The optical signal in the 1 μm band is amplified by, for example, an optical post-amplifier 14A using the TDFA shown in FIG.
The optical signal in the band of 1.61 μm is, for example, a GS-
It is amplified by an optical post-amplifier 14B using EDFA. Then, the optical signals of both bands are multiplexed by the WDM filter for band multiplexing 41 and transmitted to the dispersion shift fiber 21.

In the linear optical repeater 22, the optical signals of both bands are demultiplexed by the WDM filter 42 for band demultiplexing, and 1.45 μm.
The optical signal of the m to 1.51 μm band is, for example, a TDF shown in FIG.
A is amplified by the optical amplifier 43A using A
The optical signal in the 1.61 μm band is, for example, a GS-ED shown in FIG.
The signal is amplified by the optical amplifier 43B using the FA, and the optical signals of both bands are multiplexed again by the band multiplexing WDM filter 41.

In the optical receiving section 30, the optical signals of both bands are split by the WDM filter 42 for band splitting, and are separated from 1.45 μm.
The optical signal in the 1.51 μm band is, for example, a TDFA shown in FIG.
Amplified by an optical preamplifier 31A using
The optical signal in the μm to 1.61 μm band is, for example, a G signal shown in FIG.
It is amplified by an optical preamplifier 31B using an S-EDFA. Thereafter, each optical signal is demodulated in the same manner as in the second embodiment.

The first, second, and third embodiments described above are examples of point-to-point transmission systems, each of which avoids deterioration of transmission quality due to four-wave mixing and uses a dispersion-shifted fiber. It has been explained that the transmission distance can be dramatically improved. However, the present invention is not limited to the point-to-point transmission system, but is applicable to any network type wavelength division multiplexing type optical transmission system. For example, a signal converted into an electric signal after being demultiplexed by the optical receiving unit 30 of the second embodiment is digitally reproduced, and if necessary, subjected to an electrical routing process, and then converted into an optical signal again. It can also be applied to a multi-relay optical transmission system in which the signal is wavelength division multiplexed and transmitted to an optical transmission line, or this procedure is repeated a plurality of times.

Furthermore, a specific wavelength 1
The present invention is also applicable to a wavelength division multiplexing type optical transmission system in which an optical node for dropping / adding one or a plurality of optical signals is arranged. Examples of the configuration will be described below as fourth and fifth embodiments. (Fourth Embodiment) FIG. 6 shows a configuration of a fourth embodiment of the present invention.

In the figure, the system comprises a center node 50, a plurality of remote nodes 60, and a dispersion-shifted fiber 21 connecting these in a ring. Each remote node 60 is assigned at least one different wavelength, and communicates with the center node 50 using each wavelength. The remote node 60 includes an optical preamplifier 61 installed as necessary, an optical add / drop multiplexer that splits an optical signal of an assigned wavelength from the wavelength division multiplexed optical signal, and inserts the optical signal of the wavelength into the wavelength division multiplexed optical signal. Circuit 6
2. It is composed of an optical post-amplifier 63 installed as needed.

The center node 50 comprises an optical transmitter 51 corresponding to each wavelength assigned to each remote node, an optical multiplexer 52 for combining optical signals of each wavelength, and an optical post-amplifier 53 installed as required. It comprises a transmission system, an optical preamplifier 54 installed as required, an optical demultiplexer 55 for demultiplexing a wavelength division multiplexed optical signal into optical signals of each wavelength, and a receiving system including an optical receiver 56 corresponding to each wavelength. Is done.

The wavelength division multiplexed optical signal multiplexed by the center node 50 propagates through the dispersion shift fiber 21 and reaches the remote node 60. The remote node 60 branches only the optical signal of the wavelength assigned from the wavelength division multiplexed optical signal, and inserts the optical signal of the wavelength into the wavelength division multiplexed optical signal. The optical signal passing through each remote node 60 reaches the center node 50, where it is demultiplexed for each wavelength. As described above, although the configuration of the present embodiment is physically a ring network configuration, the center node 50 and the plurality of remote nodes 6 are logically connected by the path identified by the wavelength.
0 has a star network configuration connected in a star shape. A linear optical repeater for compensating for transmission loss may be inserted in the dispersion-shifted fiber 21 as needed.

(Fifth Embodiment) FIG. 7 shows a fifth embodiment of the present invention.
1 shows the configuration of the embodiment. The feature of this embodiment is that the center node that aggregates information is removed from the configuration of the fourth embodiment, a path in which a dedicated wavelength is allocated is formed between each remote node, and each remote node is connected in a mesh. Where it is.

The remote node 60 branches an optical signal of an assigned wavelength from an optical preamplifier 61 and a wavelength division multiplexed optical signal installed as necessary, and inserts the optical signal of that wavelength into the wavelength division multiplexed optical signal. It comprises an optical add / drop circuit 62 and an optical post-amplifier 63 installed as required. Each communication between the remote nodes is assigned a wavelength. For example, if the total number of remote nodes is N, the remote node # 1 has wavelengths λ12, λ13,.
Using the 1N light, remote nodes # 2, # 3,.
Communicates with When transmitting by one optical fiber,
N (N-1) / 2 wavelengths are required. If two optical fibers are used, the number of wavelengths can be reduced to about N (N-1) / 8. A linear optical repeater for compensating for transmission loss may be inserted in the dispersion-shifted fiber 21 as needed.

If the present invention is applied to a wavelength division multiplexing type ring network constituted by dispersion shifted fibers as shown in the fourth or fifth embodiment, the influence of four-wave mixing can be avoided. In addition, it is easy to increase the transmission distance of the node, narrow the channel interval, increase the number of channels, etc., and can expect a great effect. The wavelength band to be used is, for example, a band of 1.45 μm to 1.51 μm or 1.57 μm to
Use the 1.61 μm band or both. The linear optical repeater using both wavelength bands may have the same configuration as that of the third embodiment.

As described above, the wavelength division multiplexing type optical transmission system of the present invention can avoid the influence of four-wave mixing in the dispersion-shifted fiber by limiting the wavelength band to be used. As a result, the allowable optical input power to the dispersion-shifted fiber can be increased, and the transmittable distance can be greatly extended. In addition, 1.57 μm
When a wavelength range of 1.61 μm is used, 1.55 μm
Since the transmission loss can be further reduced than in the m band, the transmittable distance can be extended as compared with the conventional 1.55 μm band transmission.

[Embodiment Regarding Second Wavelength Band Limiting] In the following, the wavelengths of a plurality of wavelength division multiplexed signal lights are 1450.
nm to 1530 nm, 1570 nm to 1650
A wavelength division multiplexing type optical transmission system disposed anywhere between nm will be described.

First, the wavelength of the wavelength-multiplexed signal light is set to 14
50 nm to 1530 nm, 1570 nm to 16
The reason for setting the distance to 50 nm will be described in detail. First, the relationship between the chromatic dispersion of signal light and the intensity of four-wave mixing light will be described. Here, “four-wave mixing” means that three lights having optical frequencies f ₁ , f ₂ , and f ₃ are nonlinearly interacting with a propagation medium as described above, and the optical frequency is f _FWM = f _i + f _j −f _k. Is a phenomenon that generates four-wave mixing light. Where i, j,
It is assumed that k takes a value from 1 to 3 and j ≠ k. Four-wave mixing light is also generated when f _i and f _j coincide, that is, even with two lights. In wavelength division multiplexing optical communication using a wavelength region with small dispersion, the generation efficiency of four-wave mixing light increases as the amount of phase matching Δβ decreases. Here, the phase matching amount [Delta] [beta] ^{is, Δβ = (- λ 4 π} / c 2) · (dD / dλ) · {(f i -f
₀ ) + (f _j −f ₀ )｝ · (f _i −f _k ) · (f _j −f _k ), according to K. Inoue's paper "Fiber four-wav
e mixing in the zero-dispersion wavelength regio
n ", J. Lightwave Technol., Vol. 10, pp. 1553-1561, 19
92. Here, f ₀ is a value obtained by converting the zero dispersion wavelength into a frequency. Λ is the wavelength of light, c
Represents the speed of light, and D represents chromatic dispersion. From this equation, when the optical frequency of one of the wavelength-multiplexed signal lights coincides with f ₀ (f _i = f _j = f ₀ ),
Alternatively, the optical frequency of the two signal lights is f _{0 in the} frequency space.
(F _i −f ₀ = f ₀ −f _j ), Δβ becomes zero and the generation efficiency of four-wave mixing becomes maximum.
If the difference between the frequency of the generated four-wave mixing light and the optical frequency of any of the signal lights is within the receiving band of the receiver, the four-wave mixing light becomes interference noise with the signal light.
When the optical frequencies of the signal light are arranged on the optical frequency grid arranged at equal intervals, that is, in the case of the equidistant optical frequency arrangement, the optical frequency of the generated four-wave mixing light is always located on this grid. Will be. For this reason, in the case of the equally spaced optical frequency arrangement, the influence of interference noise due to the four-wave mixing light becomes serious.

FIG. 9 is a simulation result of the relationship between the wavelength dispersion of signal light and the intensity of four-wave mixing light. The simulation conditions are as described in the upper right of FIG. The power of four-wave mixing light in this simulation is based on the paper by K. Inoue and H. Toba, "Fiber four-wave mi
xing in multi-repeater systems with nonuniform chr
omatic dispersion ″, J. Lightwave Technol., 13, pp.88-
93,1995. In FIG. 9, the abscissa represents the chromatic dispersion of the signal light of the channel with the smallest chromatic dispersion among the signal lights of 16 waves at 200 GHz intervals, and when these signal lights propagate through the optical fiber, the wavelength of a certain signal light The vertical axis indicates the ratio (dB) of the intensity of four-wave mixing light to the intensity of signal light, which is generated in conformity with FIG. The ratio of the four-wave mixing light intensity to the signal light intensity is-
It is known that signal light degrades at 30 dB or more. From FIG. 9, it can be seen that signal light whose chromatic dispersion is almost zero has a chromatic dispersion of 0.35 ps / km / nm or less. Can be read. The dispersion slope of an optical fiber is generally 0.07 ps / nm ² /
km, the wavelength distance of the signal light closest to the zero dispersion wavelength from the zero dispersion wavelength is 5 nm (= 0.35 /
If it is less than 0.07), deterioration occurs. In other words, if the wavelength distance of the signal light closest to the zero-dispersion wavelength from the zero-dispersion wavelength is 5 nm or more, the problem caused by four-wave mixing light can be avoided.

The zero-dispersion wavelength of the dispersion-shifted fiber, which is currently widely manufactured and laid, is approximately 1535 n around 1550 nm due to manufacturing variations.
It is distributed from m to 1565 nm. Therefore, when the currently manufactured and laid dispersion-shifted fiber is used as the optical transmission line, the wavelength of the signal light is set to 1530 nm (1535).
-5) or less, or 1570 nm or more (1535+
By setting 5), it is possible to avoid the problem due to the deterioration due to the four-wave mixing light.

Next, the wavelength of the signal light is changed from 1450 nm to 1
The reason for setting the range to 650 nm will be described. FIG.
FIG. 3 is a diagram showing a typical example of a loss-wavelength characteristic of a dispersion-shifted fiber. When a dispersion-shifted fiber is used as an optical transmission line, its span is generally 100 km. Also,
The gain of the optical amplifier constituting the repeater is generally 30 [dB].
It is. Therefore, the fiber loss is set to 0.3 dB / km (=
30/100), the wavelength from 1450 nm to 165
It can be seen that 0 nm may be used. As described above, when the currently manufactured and laid dispersion-shifted fiber is used as the optical transmission line, the wavelength of the signal light is between 1450 nm and 1530 nm indicated by the reference numeral 112 in FIG. 11 or 1570 nm to 1650 nm indicated by the reference numeral 111 in FIG. It can be seen that long-distance WDM transmission can be realized without using the wavelengths between the four wavelengths without deteriorating the transmission characteristics due to four-wave mixing. Reference numeral 110 represents a zero-dispersion wavelength distribution of a dispersion-shifted fiber serving as an optical transmission line.

Next, the wavelength band 11 of the optical signal shown in FIG.
The validity of 1,112 is proved by experiment. FIG.
It is an experimental result showing the relationship between the average transmission intensity per channel and the power penalty. In the figure, the horizontal axis represents the average transmitted light intensity per channel, and the vertical axis represents the power penalty due to four-wave mixing. The length of the dispersion-shifted fiber used was 40 km, and the bit rate of the transmitted optical signal was 10 G
b / s, the number of wavelengths is 8 wavelengths, the optical frequency interval is 200 GHz
It is. Here, the wavelengths of the signal light tested are two wavelength bands, 1543 to 1556 nm, which have been conventionally used, and 1581 to 1589 nm, according to the present embodiment. The power penalty used here is defined as follows. Power penalty [dB] = 10 × log (Pt / P
b) Pb is the average received light power required to achieve a bit error rate of 10 <-9> when the transmitter is directly connected to the receiver without transmitting the dispersion-shifted fiber for the optical transmission line.
In addition, Pt propagates 40 km through the dispersion-shifted fiber,
This is the average received light power required to achieve a bit error rate of 10-9. As is apparent from FIG. 6, when the signal light wavelength of 1543 to 1556 nm, which has been conventionally used, is used, if the intensity of the transmitted light per channel is increased, the power penalty increases due to the influence of four-wave mixing.

On the other hand, the wavelength bands 1581 to 1581 according to this embodiment
When 1593 nm is used, the power penalty does not increase because the effect of four-wave mixing does not matter. Also, being able to increase the transmission power per channel means that the input power of the linear repeater can be increased, thereby reducing the effect of noise of the optical amplifier and increasing the transmission distance, that is, the interval between repeaters. Will be able to do it.

Next, the wavelength of the signal light is changed from 1450 nm to 1
A description will be given of a usage form of the wavelength band in the case where the wavelength band is between 530 nm and 1570 nm to 1650 nm. (First Usage) The wavelength of the signal light is arranged on the long wavelength side where the problem due to four-wave mixing can be avoided.
There is a usage form of a wavelength band that is arranged between 1570 nm to 1650 nm indicated by reference numeral 111. Further, the wavelength of the signal light is arranged on the short wavelength side which can avoid the problem due to the four-wave mixing, that is, 145 shown by reference numeral 112 in FIG.
There is a usage form of a wavelength band that is arranged between 0 nm and 1530 nm. Further, the wavelength of the signal light is 1450n.
m to 1530 nm, as well as 1570 nm to 1
It is also possible to use wavelengths between 650 nm simultaneously. In this case, the transmission capacity of the fiber can be doubled. It should be noted that, in the above-described application, there is no limitation on the propagation direction of the optical signal. Therefore, all the signal lights may propagate in the same direction, or some of the optical signals may have different propagation directions from the other optical signals.

(Second mode of use) As a second mode of use, the range from 1450 nm to 153 indicated by reference numeral 112 in FIG.
The propagation direction of the signal light whose wavelength is disposed between 0 nm and the optical transmission path is between 1570 nm and 165 indicated by reference numeral 111.
There is a use form in which the propagation directions of the optical transmission lines of the signal light arranged between 0 nm are opposite to each other. Hereinafter, the reason for this will be described. In the first mode of use, the wavelength of the signal light ranges from 1450 nm to 1530 nm.
It is assumed that wavelengths between 1 nm and 1570 nm to 1650 nm are simultaneously used, and that the propagation directions of all signal lights are the same. In this case, when the bit rate per wavelength is relatively small, the wavelength becomes 1450.
signal light between 1 nm and 1570 nm
The walk-off between signal lights between m and 1650 nm is in the same order as the time of one time slot. As a result, crosstalk occurs due to stimulated Raman scattering, and there is a problem that deterioration of transmission quality cannot be ignored. Note that “walk-off” means that two optical signals having different wavelengths are shifted in relative time position according to fiber propagation due to a difference in group delay time. “Stimulated Raman scattering” refers to a phenomenon in which the energy of signal light on the short wavelength side shifts to signal light on the long wavelength side via the vibration of molecules constituting the fiber.

Since stimulated Raman scattering occurs only when both the signal light on the short wavelength side and the signal light on the long wavelength side are present, the decrease in the power of the signal light on the short wavelength side is caused by the sign of the two. It changes depending on the combination and the relative time position, which causes crosstalk and deteriorates transmission characteristics. To avoid this problem, the wavelength of the signal light must be 1
Between 450 nm and 1530 nm, and 157
It is effective to make the wavelength between 0 nm and 1650 nm propagate in the opposite direction. Thereby, the walk-off between the signal light on the short wavelength side and the signal light on the long wavelength side can be increased, and the attenuation of the power of the short wavelength side signal due to stimulated Raman scattering can be averaged. Such bidirectional transmission includes crosstalk caused by non-degenerate four-wave mixing generated between signal light having a wavelength between 1450 nm and 1530 nm and signal light having a wavelength between 1570 nm and 1650 nm, and a waveform caused by cross-phase modulation. It is also useful in avoiding deterioration. This is because the walk-off increases due to the bidirectional transmission, the phase matching condition of non-degenerate four-wave mixing is not satisfied, and the cross-phase modulation is averaged. Note that “cross-phase modulation” means that the local refractive index of the transmission fiber changes due to an optical pulse wave, the instantaneous frequency of another optical pulse wave changes, and the phase of an optical signal changes. For the above reasons, the propagation directions of the signal light whose wavelength is arranged between 1450 nm and 1530 nm in the optical transmission line and the propagation directions of the signal light arranged between 1570 nm and 1650 nm are opposite to each other. Good to do.

(Third mode of use) As described above, between 1450 nm and 1530 nm, and between 1570 nm and 165 nm
It has been described that the wavelength of the optical signal is arranged between 0 nm. Here, the reason why the wavelength band from 1530 nm to 1570 nm is not used is to avoid signal deterioration due to four-wave mixing as described above. Signal degradation due to four-wave mixing can be suppressed by unequally spaced wavelength arrangements, as detailed in these papers. Here, the “unequally spaced wavelength arrangement” means that the frequency f _FWM = f _i + f _j −f _{k of} the four-wave mixing light generated from any three waves of the optical frequencies f ₁ , f ₂ and f ₃ is multiplexed. The arrangement is such that the optical frequency differences of the signal lights are unequally spaced so that the optical frequencies of any of the signal lights thus obtained have a difference equal to or greater than the reception band of the receiver. Where i,
j and k take any value from 1 to 3, and j ≠ k
It is assumed that For example, the frequency intervals are 125,
300, 200, 375, 150, 175, 350, 2
Four-wave mixing light in which any three of the twelve wavelength-multiplexed signal lights arranged at 50, 150, 325 and 225 GHz are at least 25 GHz from any signal light
This will occur at a distant frequency position, and will not be interference noise.

Therefore, in an optical signal having a wavelength close to the zero-dispersion wavelength of a dispersion-shifted fiber serving as an optical transmission line, the wavelength range that can be used can be expanded by partially using the unequally spaced frequency arrangement. An example of this usage is shown in FIG.
It is shown in FIG. The wavelength of a plurality of signal lights to be wavelength multiplexed is code 1
It is arranged between 1450 nm and 1570 nm shown at 20 and between 1570 nm and 1650 nm shown at 111. And from 1450 nm to 1570 mm
The signal light whose wavelength is arranged between them and the signal light arranged between 1570 nm and 1650 nm propagate through the dispersion-shifted fiber transmission lines in mutually opposite directions. Even in the worst case where the zero dispersion wavelength of the fiber is at 1535 nm, at least the wavelength indicated by reference numeral 130 is 1505 nm (= 15
The optical frequency difference of the signal light of 35- (1565-1535)) or more and 1565 nm or less is arranged at unequal intervals. In addition,
Reference numeral 120 denotes a signal light arranged between 1450 nm and 1570 nm, and a certain signal light has a wavelength of 15
When it is close to 70 nm, 1500 nm (= 1535− (1
570-1535)) The optical frequency difference of the signal light of not less than 1570 nm and not more than 1570 nm may be arranged at unequal intervals.

In the same manner, 1 shown at 112 in FIG.
When dividing between 450 nm and 1530 nm and between 1530 nm and 1650 nm shown by reference numeral 121,
The signal light having a wavelength between 1450 nm and 1530 nm and the signal light having a wavelength between 1530 nm and 1650 nm are propagated in the dispersion-shifted fiber transmission lines in opposite directions. In the worst case where the zero dispersion wavelength of the fiber is at 1565 nm, at least the symbol 1 is used to avoid deterioration due to four-wave mixing.
31 is not less than 1535 nm and not more than 1595 nm (= 1
The optical frequency difference of the signal light of 565+ (1565-1535) or less is arranged at unequal intervals. In addition, in the signal light arranged between 1530 nm and 1650 nm indicated by reference numeral 121, when the wavelength of the signal light in the signal light is close to 1530 nm, it is 1530 or more and 1600 nm (= 1565 + (15
65-1530)) The following optical frequency differences of the signal light may be arranged at unequal intervals.

Next, an example of the wavelength division multiplexing type optical transmission system for limiting the wavelength band used in the signal light described in the first to third utilization modes will be described with reference to FIGS.
0 will be described. FIG. 16 is a block diagram of the first wavelength division multiplexing type optical transmission system. As shown in FIG. 16, this system is composed of optical transmission / reception devices 212 and 213 each including a transmission circuit 210 and a reception circuit 211, and one optical fiber transmission line 224 connecting the two optical transmission / reception devices 212 and 213. The transmission circuit 210 includes a plurality of transmitters 220 for generating signal lights having different wavelengths from each other and a multiplexer 221 for wavelength-multiplexing the plurality of signal lights. The reception circuit 211 includes a demultiplexer for separating the plurality of signal lights. And a plurality of receivers 223 for demodulating an electric signal from the demultiplexed signal light. The optical transmission / reception devices 212 and 213 include the transmission circuit 211, the reception circuit 212, and a filter or circulator 225. The transmitter 220 includes a light source 11 set to different wavelengths in FIG. 1, a modulator 12 for modulating an optical carrier output from the light source with a data signal, and the like.
Comprises an opto-electric converter (0 / E) 33 in FIG. 1, an electric receiving circuit 34 for demodulating a data signal from each electric signal, and the like.

FIG. 17 is a block diagram of a second wavelength division multiplexing type optical transmission system. Compared to the system of FIG. 16, all signal lights are transmitted or received or relayed.
A non-repeated or multi-repeated point-to-point optical wavelength division multiplexing bidirectional transmission system characterized by being collectively amplified by at least one bidirectional optical amplifier 270. In addition, in FIGS. 17 to 20 including this figure, parts corresponding to the respective parts in FIG. 16 are denoted by the same reference numerals, and description thereof will be omitted. As described above, in a system in which signal light is transmitted to the optical fiber transmission line 224 with high power using the bidirectional optical amplifier 270, four-wave mixing or Raman crosstalk occurs in a conventional wavelength band where optical signals are arranged. Deterioration increases. However, these can be avoided by using the wavelength of the optical signal described above.

FIG. 18 is a block diagram of a third wavelength division multiplexing type optical transmission system. In this system, when a signal light is separated by a filter or a circulator 225 according to its propagation direction during transmission / reception or relaying,
It is characterized in that it is amplified by different optical amplifiers 280 and 281 depending on the propagation direction. This system is a system for transmitting signal light to an optical fiber transmission line with a high puff using an optical amplifier, similar to the system shown in FIG. 17, and the effect of the above-described use of the wavelength of the optical signal is great.

FIG. 19 is a block diagram of a fourth wavelength division multiplexing type optical transmission system. This system is shown in FIG.
Alternatively, as compared with the system shown in FIG. 18, before or after all the signal light propagates through the optical fiber transmission line 224, the optical fiber transmission line 224 has a dispersion slope of the opposite sign to that of the optical fiber transmission line 224 and the zero dispersion wavelength is almost zero. The dispersion compensation is performed collectively by the equal dispersion compensation fiber 290.

FIG. 20 is a block diagram of a fifth wavelength division multiplexing type optical transmission system. This system is shown in FIG.
Alternatively, as compared with the system of FIG. 18, when the signal light is separated for each transmission direction by the filter or circulator 225 at the time of transmission / reception or relaying, the average dispersion of the plurality of signal lights in each propagation direction is substantially equal to
Dispersion compensating fibers 2100 and 2 having dispersion of opposite sign
101 is characterized by being dispersion-compensated.

In the first mode of use of the above-mentioned wavelength band, when the propagation directions of all optical signals are equal,
The systems of the first to fifth embodiments described in the first wavelength band limitation may be used. In addition, the three usage modes in the wavelength allocation of the optical signal described above can be applied not only to the point-to-point transmission system but also to any network type wavelength division multiplexing type optical transmission system.

In this embodiment, since it is assumed that a dispersion-shifted fiber, which is currently widely manufactured and laid, is used as an optical transmission line, the wavelength arrangement of an optical signal may be set in the above three use modes. ,It turns out that.
Needless to say, this technical idea can be applied to an optical transmission line having characteristics different from those of the above-described dispersion-shifted fiber. For example, if the zero-dispersion wavelength of a dispersion-shifted fiber serving as an optical transmission line is, for example, 1550 nm, the wavelength at which an optical signal is arranged is changed from 1450 nm to 1545 nm as shown in FIG.
(1550-5) or 1555 nm (1550)
Any value between +5) and 1650 nm may be used.
That is, the wavelength band in which the signal light is arranged may be determined based on the above-described technical idea according to the characteristics of the optical transmission line.

As described above, by limiting the wavelength band used in a plurality of wavelength-multiplexed signal lights, it is possible to avoid the influence of four-wave mixing in a dispersion-shifted fiber already installed. Therefore, the allowable optical input power to the dispersion-shifted fiber can be increased, and the transmittable distance can be greatly extended.

[0063]

As described above, the wavelength division multiplexing type optical communication network of the present invention can avoid the influence of four-wave mixing in the dispersion-shifted fiber by limiting the wavelength band to be used. As a result, the allowable optical input power to the dispersion-shifted fiber can be increased, and the transmittable distance can be greatly extended.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment in a first wavelength band limitation.

FIG. 2 is a block diagram showing a configuration of a second embodiment in a first wavelength band limitation.

FIG. 3 is a diagram illustrating gain characteristics of a Tm-doped optical fiber amplifier (TDFA).

FIG. 4 shows an Er-doped gain-shifted optical fiber amplifier (G
FIG. 5 is a diagram illustrating gain characteristics of an S-EDFA.

FIG. 5 is a block diagram showing a configuration of a third embodiment in the first wavelength band limitation.

FIG. 6 is a block diagram showing a configuration of a fourth embodiment in a first wavelength band limitation.

FIG. 7 is a block diagram showing a configuration of a fifth embodiment in a first wavelength band limitation.

FIG. 8 is a diagram showing the loss characteristics of an optical fiber laid on site for communication.

FIG. 9 is a diagram illustrating a simulation result of a relationship between chromatic dispersion of signal light and four-wave mixing light intensity.

FIG. 10 is a diagram showing a typical example of a loss-wavelength characteristic of a dispersion-shifted fiber.

FIG. 11 is a diagram illustrating a second wavelength band limitation.

FIG. 12 is a diagram illustrating an experimental result of a relationship between an average transmission power per channel and a power penalty.

FIG. 13 is a diagram showing a second use mode in the second wavelength band limitation.

FIG. 14 is a diagram illustrating a third use mode in the second wavelength band limitation.

FIG. 15 is a diagram showing another example in the third mode of use with the second wavelength band limitation.

FIG. 16 is a block diagram of a first wavelength division multiplexing type optical transmission system in a second wavelength band limitation.

FIG. 17 is a block diagram of a second wavelength division multiplexing type optical transmission system in a second wavelength band limitation.

FIG. 18 is a block diagram of a third wavelength division multiplexing type optical transmission system in the second wavelength band limitation.

FIG. 19 is a block diagram of a fourth wavelength division multiplexing type optical transmission system in the second wavelength band limitation.

FIG. 20 is a block diagram of a fifth wavelength division multiplexing type optical transmission system in the second wavelength band limitation.

FIG. 21 shows that the zero-dispersion wavelength of the dispersion-cyst fiber is 155.
FIG. 4 is a diagram for explaining wavelength band limitation of an optical signal in the case of 0 nm.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical transmission part 11 Light source 12 Modulator 13 Optical multiplexer 14 Optical post-amplifier 20 Optical transmission line 21 Dispersion shift fiber 22 Linear optical repeater 30 Optical receiving part 31 Optical preamplifier 32 Optical demultiplexer 33 Optoelectric converter 34 Electric reception Circuit 41 WDM filter for band multiplexing 42 WDM filter for band multiplexing 43A, 43B Optical amplifier 50 Center node 51 Optical transmitter 52 Optical multiplexer 53 Optical post amplifier 54 Optical preamplifier 55 Optical demultiplexer 56 Optical receiver 60 Remote node Reference Signs List 61 optical preamplifier 62 optical add / drop circuit 63 optical post-amplifier 210 transmission circuit 211 reception circuit 212, 213 optical transceiver 220 transmitter 221 multiplexer 222 demultiplexer 223 receiver 224 optical fiber transmission line 225 filter or circulator 270 optical amplifier 280, 281 light Width 290 dispersion compensating fiber 2100,2101 dispersion compensating fiber

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Seiji Norimatsu 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Junichi Kani 3- 192-1 Nishi-Shinjuku, Shinjuku-ku, Tokyo No. Japan Telegraph and Telephone Corporation

Claims (8)

[Claims] 1. A wavelength division multiplexing type optical transmission system in which a transmission line has a dispersion-shifted fiber having a zero-dispersion wavelength near 1550 nm.
The wavelength of at least two signal lights is 1450 nm to 15
A wavelength division multiplexing type optical transmission system, wherein the wavelength division multiplexing type optical transmission system is arranged between 30 nm and 1570 nm to 1650 nm. 2. A wavelength of at least two of the plurality of wavelength-multiplexed signal lights is 1450 nm.
2. The wavelength division multiplexing type optical transmission system according to claim 1, wherein the wavelength division multiplexing type optical transmission system is disposed between the wavelength division multiplexing and the wavelength division multiplexing. 3. A wavelength of at least two of the plurality of wavelength-multiplexed signal lights is 1570 nm.
The wavelength division multiplexing type optical transmission system according to claim 1, wherein the wavelength division multiplexing type optical transmission system is arranged between the wavelength division multiplexed wavelength and the wavelength range of 1650nm. 4. The wavelength-multiplexed plurality of signal lights, wherein at least two signal lights have wavelengths between 1450 nm and 1530 nm and between 1570 nm and 1650 nm.
2. The wavelength division multiplexing type optical transmission system according to claim 1, wherein the wavelength division multiplexing type optical transmission system is arranged between nm. 5. The signal light having a wavelength between 1450 nm and 1530 nm and the signal light having a wavelength between 1570 nm and 1650 nm propagate in the dispersion-shifted fiber transmission lines in opposite directions. 5. The wavelength division multiplexing type optical transmission system according to claim 4, wherein: 6. The wavelength-multiplexed plurality of signal lights have a wavelength between 1450 nm and 1570 nm, and a wavelength of 157 nm.
The signal light arranged between 0 nm and 1650 nm, and the signal light arranged between 1450 nm and 1570 nm, and the signal light arranged between 1570 nm and 1650 nm are directed in opposite directions through the dispersion-shifted fiber transmission line. The wavelength division multiplexing type optical transmission system according to claim 1, wherein the optical frequency differences of the signal light having a wavelength of 1505 nm or more and 1565 nm or less are arranged at unequal intervals. 7. The wavelength of the plurality of wavelength-multiplexed signal lights is between 1450 nm and 1530 nm, and 153.
The signal light disposed between 0 nm and 1650 nm, and the signal light disposed between 1450 nm and 1530 nm, and the signal light disposed between 1530 nm and 1650 nm are directed in opposite directions through the dispersion-shifted fiber transmission line. 2. The wavelength division multiplexing type optical transmission system according to claim 1, wherein the optical frequency differences of the signal lights having wavelengths of 1535 nm or more and 1595 nm or less are arranged at unequal intervals. 8. A wavelength division multiplexing type optical transmission method using a dispersion-shifted fiber having a zero-dispersion wavelength near 1550 nm as a transmission line, wherein at least two of the plurality of wavelength-multiplexed signal lights have a wavelength of 1450 nm. Between 1530 nm and 1
A wavelength division multiplexing type optical transmission method, wherein the wavelength division multiplexing type optical transmission method is arranged at any one of between 570 nm and 1650 nm.