JPH09270769A - Wavelength multiplex light transmitter and light transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce degradation by four light wave mixing(FWM) in a fiber. SOLUTION: Optical signals intensity-modulated by an RZ(return-to-zero) code are sent out by a light transmitter and setting is performed so as to turn the bit phases of respective wavelengths to O or π at the output point 115 of this wavelength multiplex light transmitter. The FWM is not generated between the transmission wavelengths of the different bit phases. Even when the FWM is generated between the transmission wavelengths provided with the same bit phase, when the wavelength of FWM light is made to match only with the optical signals of the different bit phase, a degradation amount is reduced. In such a manner, the degradation by the FWM is suppressed and wavelength multiplex transmission upto about 7 waves in equal wavelength intervals is made possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical information transmission using an optical fiber.

[0002]

2. Description of the Related Art A wavelength division multiplex transmission (WDM) system is being studied as a technique for future large capacity optical communication. The WDM transmission is a method in which a plurality of (for example, N) optical signals having different wavelengths are modulated with independent information signals, which are then multiplexed and transmitted through one optical fiber. It is possible to increase the transmission amount by the number of wavelengths (N) times, even if the transmission rate per wavelength is the same as that of the conventional method of transmitting only an optical signal of one wavelength.

One of the problems of WDM transmission is Four-Wave Mixing or Four-Photon Mixing (FW).
There is a phenomenon called M). This is one of the nonlinear phenomena of the optical fiber used for transmission, and is a phenomenon in which a new optical signal is generated at the sum or difference wavelengths of a plurality of optical signals having different wavelengths. The influence of FWM in WDM transmission is, for example, (Reference 1) IEEE Journal of Lightwave Technology, vo.
l.13, No. 5, May, 1995, pp 841-849.

[0004]

FWM is a phenomenon that appears greatly when the dispersion value in an optical fiber is close to zero, and becomes a serious problem when transmitting using a dispersion shift fiber in the 1.5 μm band, for example. . Although the dispersion-shifted fiber has been widely used in Japan and abroad in recent years, the dispersion value may be completely zero at the wavelength of 1550 nm used for optical transmission. Note that "dispersion" is a phenomenon in which lights having different wavelengths are transmitted at different speeds in an optical fiber. The optical spectrum of an optical signal modulated at high speed contains different wavelength components, and these components arrive at the receiving end at different times due to the influence of dispersion and cause large waveform distortion in the optical waveform after transmission, so the transmission speed It also becomes a factor that limits the transmission distance. Therefore, an optical fiber having a small dispersion value has been required for conventional one-wavelength transmission. However, on the other hand, when the dispersion value is close to zero, the signal light and the FWM light are transmitted over a long distance while keeping the same phase, the "phase matching condition" is satisfied, and the generation efficiency of the FWM light becomes very high. is there.

FIG. 1 shows the configuration of a conventional wavelength division multiplexing optical transmission device (in the case of four wavelength multiplexing), and FIG. 2 shows the optical spectrum of FWM light. In the wavelength multiplexing optical transmission device 100, four optical transmitters 101-1 to 101-4 having different wavelengths are arranged. The output light (wavelengths λ1 to λ4) of each transmitter is multiplexed by the multiplexer 102, amplified by the optical amplifier 103, and input to the optical fiber transmission line 104. In the wavelength multiplexing optical receiver 106, the received optical signal is amplified by the optical amplifier 103, separated by the wavelength demultiplexer 105 for each wavelength, and then converted into an electric signal by the optical receivers 107-1 to 107-4. . FIG. 2A shows a state of four-wave mixing that occurs when the number of light wavelengths is three. f1, f2, f3 are the respective wavelengths λ1,
It is the optical frequency corresponding to λ2, λ3, and FWM is f1, f2,
Three of f3 are generated at the frequency position indicated by the sum and difference of the optical signals. The combination is N ・ N when the number of wavelengths is N.
・ (N-1) / 2 ways. N = 3 shown in FIG. 2 (1)
However, when the number of wavelengths N increases, for example, when N = 8, an extremely large number of FWM signals such as 224 are generated.

FIG. 2B shows how crosstalk is generated by the FWM light. The figure shows the case of wavelength-division multiplex transmission of four wavelengths, in which all signals are arranged at equal wavelength intervals. The FWM light is generated at the position indicated by the diagonal lines, and a part of it is generated at the same wavelength as the wavelengths λ1, λ2, λ3 of the optical signal. Generally, the optical phase of the FWM light does not match the phase of the optical signal to be transmitted, so that interference with the optical signal causes random noise and deteriorates the transmission quality of the optical signal. Also, for example, FWM light generated at a frequency of f3 = 2f2-f1 is
Since it occurs only when the optical signals of frequencies f1 and f2 are both ON, it acts as crosstalk on the optical signal of frequency f3 in optical transmission of the intensity modulation system, and causes large transmission deterioration.

Conventionally, several methods have been proposed as measures against FWM. For example (reference 2) IEEE Journal
of Lightwave Technology, vol.13, No. 5, May, 1995,
pp889-897 describes a method of arranging the wavelengths of transmitted light at unequal intervals. FIG. 3 (1) shows an example of the wavelength arrangement, in which optical signals of four wavelengths are arranged at unequal intervals at an interval of 1: 3: 2, and the intervals of any two frequencies do not match,
It is arranged so that the sum and difference of any three optical frequencies do not match the wavelength of the optical signal. The position where the FWM light is generated is the shaded portion in the figure, and since it does not overlap the wavelength of the optical signal, it is possible to suppress the deterioration of the optical signal. In addition to this, the arrangement such as 2: 3: 4 as shown in FIG. 3B is also possible. This example can reduce the band occupied by the wavelength division multiplexed signal compared to the first example.

However, in the case of the conventional non-equidistant arrangement, there is a problem that the wavelength band is required to be 1.5 to 5 times or more as compared with the case of the equidistant arrangement. Further, when the wavelength band is narrowed as in the example of FIG. 3B, the position of the FWM light comes close to the optical signal, so that the wavelength accuracy required for the transmission light, the demultiplexer, the optical filter, etc. There is a problem that the wavelength stability becomes high. Further, there are problems such as the need to stabilize the wavelength of the transmitted light and the need to perform temperature stabilization on the demultiplexer and the like.

Further, as the number of wavelengths increases, the number of FWM lights generated sharply increases, making it difficult to allocate wavelengths. Therefore, in reality, application to about 8 wavelengths is the limit, and theoretically 10 wavelengths are available. Only the placement method has been proposed until.
For this reason, there is a problem that the wavelength difference of the gain of the optical amplifier arranged in the transmission line or the optical transceiver and the wavelength difference of the dispersion value of the optical fiber transmission line are large, which makes it difficult to design the transmission system.

Further, in this method, since the wavelength intervals are unequal, it is difficult to manufacture and test the wavelength multiplexer and demultiplexer. Further, it becomes difficult to use an optical device such as an optical interferometer having wavelength transmission characteristics at equal intervals as wavelength stabilization, dispersion compensation, and a demultiplexer. Further, it becomes difficult to standardize the transmission wavelength and the wavelength interval, which causes problems such as an obstacle to setting an international wavelength standard.

Further, although the crosstalk caused by the overlapping of the wavelength of the FWM light with the wavelength of the signal light can be eliminated by the unequal spacing arrangement, the influence of the loss of the signal light power due to the generation of the FWM light is unavoidable. Each FWM
The light intensity is a few hundredths or less compared to the signal light, but as the number of wavelengths increases, the number of FWM lights sharply increases (for example, 224 in the case of 8 wavelengths), and the light intensity lost by the signal light. Can no longer be ignored. This loss also causes crosstalk, which deteriorates the transmission characteristics.

An object of the present invention is to use F in wavelength division multiplexing transmission.
An object of the present invention is to solve the above-mentioned problems caused by WM and provide a practical wavelength division multiplexing optical transmitter and transmission device.

[0013]

SUMMARY OF THE INVENTION In the wavelength division multiplexing optical transmitter including a plurality of optical transmitters having different wavelengths, an optical signal output from each optical transmitter is RZ (Return-to-Zer).
o) Modulate with a code, and at the output end of the wavelength division multiplexing optical transmitter, the bit phase of each optical signal is 0, 2 / m · π, 4 / m · π,
The wavelength spacing of any two optical signals should be different between optical signals set to any value of 2 (m-1) / m · π (m is 2 or more) and set to the same phase. It is achieved by placing.

Further, by limiting m = 2, that is, the bit phase to binary values of 0 and π (1/2 bit shift), the structure of the optical transmitter is simplified and the present invention can be easily implemented.

Furthermore, the object of the present invention can be achieved more effectively by setting the polarization plane of each optical signal to one of two polarization states orthogonal to each other at the output end of the wavelength division multiplexing optical transmitter. it can. The same is true when the wavelength intervals of the transmitted light are all set to be unequal.

Further, in a wavelength division multiplexing optical transmission device using the wavelength division multiplexing optical transmitter, an optical fiber transmission line having a zero dispersion wavelength in the vicinity of the transmission wavelength band is used as an optical fiber transmission line. The use of dispersion shifted fiber (DSF) in transmission, or the use of normal dispersion fiber in 1.3 μm band optical transmission is an important requirement of the present invention.

Further, by arranging an optical repeater using an optical amplifier in the middle of the optical fiber transmission line, the effect of the present invention can be greatly enhanced.

Further, it can be more effectively achieved by performing dispersion compensation in the wavelength division multiplexing optical transmitter or the optical repeater or the wavelength division multiplexing optical receiver. Even when the optical phase adjuster is arranged, it is possible to achieve part of the purpose.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 4 is a block diagram of an optical transmission device according to first to fifth embodiments of the present invention. FIG. 5 shows the wavelength arrangement in the first embodiment of the present invention. The RZ wavelength division multiplexing optical transmitter 110 includes RZ optical transmitters 111-
1-111-4 (wavelengths λ1 to λ4) are arranged. Further, in this example, each RZ optical transmitter has an R of about 50% duty.
The intensity is modulated by a Z (Return-to-Zero) code. The optical signal of each wavelength is multiplexed by the multiplexer 102, and then the optical amplifier 1
It is amplified by 03 and output to the optical fiber transmission line 104.
At this time, the bit phase of the optical signal of each wavelength is set to 0 or π (1/2 bit shift) at the RZ optical transmitter output point 115. This is, for example, the optical delay device 112.
Can be realized by varying the distance from each RZ transmitter 101-1 to 101-4 to the output point 115. The optical signal output from the RZ wavelength division multiplexing optical transmitter 110 is transmitted through the optical fiber transmission line 104, and then amplified by the optical amplifier 103 in the RZ wavelength division multiplexing optical receiver 113.
Then, each wavelength is separated by the demultiplexer 105 and converted into an electric signal by the RZ optical receivers 114-1 to 114-4.

In the present embodiment, as shown in FIG. 5, four transmission wavelengths are arranged at equal intervals, and the bit phase of each signal is 2 of 0 and π.
An example is shown in which any one of the values, here, 0, π, π, 0 is set in order from λ1 on the basis of the bit phase of the transmission signal of λ1. When the duty ratio of the RZ signal is about 50%, the power of the optical signal exists only for about 1/2 time in the bit slot. Therefore, if the bit phases are set to 0 and π, which are shifted by 1/2 bit from each other, the optical signals having different bit phases are not turned on at the same time, and the generation of FWM can be significantly suppressed. For example, in the wavelength arrangement example of FIG. 5, FWM light is generated only in four shaded areas, and in the case of conventional NRZ (Non Return-to-Zero) transmission (
The number of FWM lights generated is significantly smaller than that of 24). The RZ signal may be used in the conventional transmission, but since the bit phase is not set to 0 or π at the output point of the wavelength division multiplexer as in the present invention, such an effect of reducing the FWM is not obtained. Not obtained.

Further, according to the present invention, crosstalk due to FWM light is also reduced. FIG. 6 shows the FWM according to the conventional NRZ method.
It is a figure which shows the influence of the crosstalk of light. Figure 6 (1),
(2) and (3) show time waveforms of optical signals of wavelengths λ1, λ2, and λ3 at the output point of the wavelength division multiplexing optical transmitter. Further, FIG. 6 (4) shows the influence of the FWM light generated in the optical fiber transmission line 104 due to the interaction between the wavelengths λ1 and λ2 on the received optical signal of the wavelength λ3. The FWM is generated in a section where both the optical signals of wavelengths λ1 and λ2 are on (mark), and the interference with λ3 causes crosstalk. The intensity of the FWM light is very weak, which is several hundredths of that of the signal light, and when λ3 is off (zero), the FWM light is
Degradation due to is not significant. However, when λ3 is on, a beat is generated between the optical signal of λ3 and the FWM light, which acts as a large noise and causes transmission deterioration.

On the other hand, FIGS. 7 and 8 show F in the present invention.
It is a figure which shows the principle of crosstalk reduction of WM light. Figure 7
In the case where the two optical signals λ1 and λ2 causing the FWM have the same bit phase (0) and the optical signal λ3 affected by the FWM light is set to a different bit phase (π). In this case, since the optical signals of λ1 and λ2 are simultaneously turned on (mark), a large FWM is generated on λ3 as shown in FIG. 7 (4). However, λ3 has a bit phase of π
Therefore, the FWM light is always generated in the bit gap of the RZ signal of λ3. However, since the optical signal of λ3 is always off in the bit gap portion, beat between optical signals does not occur and the deterioration due to crosstalk is negligible. The identification time when receiving the RZ signal of λ3 is the position of the arrow in FIG.
Since it does not overlap with the position where the WM light is generated, the deterioration that occurs during reception is extremely small.

Further, FIG. 8 shows a case where the two optical signals λ1 and λ2 causing FWM are set to different bit phases (0, π). In this case, the optical signals of λ1 and λ2 are not turned on at the same time, and even when the FWM occurs, the intensity of the FWM light is very small because it is limited to the bottom of the bit. Further, the position of the generated FWM signal does not coincide with the identification time, and the deterioration due to the crosstalk due to the FWM can be almost ignored.

As described above, according to the present invention, it is possible to reduce the crosstalk due to the FWM by appropriately setting the bit phase for each wavelength. FIG. 9 shows an example of bit phase arrangement in the case where all signal wavelengths are arranged at equal intervals without any space therebetween. FIG. 9 shows a case where two bit phases of 0 and π are used (m = 2), and shows all possible arrangements when the number of wavelengths is 4, 5, 6, and 7. λ1 to λ7 are transmission wavelengths from the short wavelength side, and show the bit phase set for each wavelength with the phase of the shortest wavelength as the reference (0), and are shown excluding those such as left-right symmetry. N = 4, 5,
In the cases of 6 and 7, 4, 3, 4, and 1 wavelength arrangements are possible, respectively. In all of these wavelength arrangements, when signals of the same phase are taken out, the wavelengths are arranged at unequal intervals, so that the FWM is suppressed very efficiently. For example, N
= 7, four wavelengths λ1, λ2, λ5, with bit phase 0 are
λ7 has an unequal wavelength arrangement of 1: 3: 2, and three wavelengths λ3, λ4 and λ6 of the bit phase π have an unequal wavelength arrangement of 1: 2.

The bit phase of the optical signal defined in the present invention is a value immediately after the optical transmitter (or the optical repeater), and even if it is established at the input end of the optical fiber transmission line, the dispersion in the optical fiber. Caused a shift (walk-off) during transmission,
The suppression effect of FWM may be reduced. However, FWM is thought to occur near the input end of an optical fiber with high optical intensity, approximately within (effective length of optical fiber) = 1 / (loss per length of optical fiber), and the bit phase due to dispersion The deviation of only affects those that occur within this distance. Therefore, a sufficient FWM suppression effect can be obtained with a fiber having a small dispersion amount. For example, when a dispersion shift fiber is used, the dispersion amount of the optical fiber is 1.55.
There is a variation of about ± 1 ps / nm / km around μm.
If the wavelength interval is 1 nm, the maximum walk-off amount within the effective length of 20 km is 15 ps. This is 10 Gbit
The bit length of / s is sufficiently smaller than 100 ps. Moreover, the condition for such a large walk-off is when the dispersion value of the optical fiber is large, and since the actual load hardly causes FWM, the influence is further reduced.

In the above example, the wavelength band used for wavelength division multiplexing transmission is assumed to be, for example, the 1.5 μm band, and the fiber used is a dispersion shift fiber, but the present method can be applied to other cases. For example, the currently widely used ordinary dispersion fiber has a zero dispersion wavelength in the vicinity of 1.3 μm, and therefore, when performing wavelength division multiplexing transmission in this wavelength band, crosstalk due to FWM becomes a problem as in the above example. The present invention can also be applied to this case.

Further, in the above embodiment, an example in which the wavelength intervals are arranged at equal intervals has been shown, but it is not always necessary to arrange them at equal intervals. FIG. 10 is a diagram showing the arrangement of bit phases in the second embodiment of the present invention. In this figure, the wavelength positions for 9 waves of λ1 to λ9 are secured at equal intervals, the wavelength of λ4 (center) is unused, and the wavelength division multiplexed signal of 8 wavelengths is arranged at the remaining position.
The example which suppressed WM is shown. That is, the wavelengths of the bit phase 0 (λ1, λ2, λ6, λ8) are arranged at unequal intervals of 1: 4: 2, and the wavelengths of the bit phase π (λ3, λ4, λ8).
7, λ9) are arranged in a 1: 3: 2 unequal spacing. As described above, if the optical signals having the same bit phase are unequal to each other, the FWM can be suppressed. Therefore, there are many examples of possible wavelength arrangements other than the present embodiment. For example,
The wavelength position to be unused is not limited to the center, and two or more wavelengths may be unused. With such an arrangement,
Since the wavelength slots for arranging the optical signals are equally spaced, the compatibility with the interference type optical multiplexer / demultiplexer using an optical waveguide or the like is high. Further, it is also possible to make all wavelengths unequal by combining with the completely unequal arrangement as shown in FIG. In this case, compared to the conventional non-equidistant arrangement, the FWM
As a result, the number of combinations in which the signal is generated is reduced, so that the crosstalk due to the loss of the signal light is reduced. Further, it is possible to suppress deterioration of transmission quality even with respect to leakage of FWM light caused by imperfections of an optical demultiplexer or the like.

Further, in this method, the smaller the duty ratio of the pulse, that is, the narrower the pulse width of the RZ signal, the more F
It is possible to enhance the effect of WM suppression. The reason for this is that if the pulse width is made narrower, the FWM generated by overlapping the vicinity of the tail of the RZ pulse is reduced, and the FWM suppressing effect is not reduced even when the bit phase walk-off occurs in the optical fiber. When such a thin RZ pulse is used, the number of bit phases can be increased to 3 or more. The more bit phases used, the more FWM
And the number of wavelengths can be further increased. For example, FIG. 11 shows a third embodiment of the present invention, which uses three bit phases (m = 3; phase is 0, 2π / 3, 4π / 3).
In this example, 12 WDM signals are arranged at equal intervals. The wavelength of bit phase 0 is 1: 3: 2, the wavelength of bit phase 2π / 3 is 1: 4: 2, and the wavelength of bit phase 4π / 3 is 6: 3:
The two are arranged at unequal intervals to suppress the FWM.
When m = 3, a maximum of about 15 waves can be achieved, and when m is increased, a combination with a larger number of wavelengths can be easily realized.

FIG. 12 shows a fourth embodiment of the present invention, which is an example in which wavelengths are arranged with a guard band in between.
Generally, the FWM in an optical fiber has a certain wavelength (for example, 5
It becomes almost negligible for optical signals separated by more than (nm). Therefore, by setting the width of the guard band to this value or more and not placing the signal wavelength between them,
It becomes possible to independently transmit the wavelength division multiplexed signal in both the upper band and the lower band of the guard band. Therefore, independent FWM in each band
It becomes possible to suppress and increase the number of transmission wavelengths.

FIG. 13 shows a fifth embodiment of the present invention, which is an example in which the methods for suppressing the FWM by setting the polarization state are used in combination. In this embodiment, by setting the polarization state of each signal light at the output end 115 of the RZ wavelength optical transmitter to either of two polarization states (P or S) orthogonal to each other, the suppression effect of the FWM of the present invention is obtained. Is increasing. That is, in this embodiment, the four-wave optical signal of phase 0 has a wavelength interval of 2:
Originally, the effect of suppressing FWM cannot be obtained because of the 3: 2 arrangement, but by making the polarization states of the two waves on both sides orthogonal to the polarization states of the two inner waves, even if FWM light is generated, reception is possible. The FWM is suppressed by setting the polarization state orthogonal to the signal to be processed. In this example, an example in which eight WDM signals are arranged at equal intervals is shown, but a larger number of optical signals can be arranged in the same manner. It is also possible to use it in combination with the above-mentioned unequal spacing arrangement.

In the present invention, the bit phase of each signal is
It is necessary to set it with an accuracy of a fraction of the transmission bit rate to 1/10. If the bit rate is 10 Gbps, the bit length will be about 2 cm in the fiber. As a method of adjusting the bit phase, there is a method of using the optical delay device 112 as in the example of FIG. Further, a method of adjusting the length of the optical fiber from the RZ optical transmitter 111 to the output point 115 of the RZ wavelength division multiplexing optical transmitter, a phase of an electric signal or a clock signal input to each RZ optical transmitter 101, a delay line, etc. It is also possible to use a technique such as adjustment by using. When a VCO (variable voltage oscillator) or the like is arranged in the transmitter 101, it can be adjusted by the voltage of this input signal.

FIG. 14 is a block diagram showing a sixth embodiment of the present invention, which is an example in which the dispersion compensator 120 is arranged in the RZ wavelength division multiplexing optical receiver 113. By disposing the dispersion compensator 120, it is possible to compensate for the influence of XPM (cross phase modulation) which is a problem in wavelength division multiplexing transmission in addition to FWM. At the same time, it is possible to prevent deterioration of transmission quality due to SPM (self-phase modulation effect) and dispersion itself. The dispersion compensation amount is most effective when it is equal to the dispersion amount after the effective length of the optical fiber transmission line 104. Further, dispersion compensation may be performed individually in each RZ optical receiver 114.

FIG. 15 is a block diagram showing a seventh embodiment of the present invention, which is an example in which an optical repeater 121 using an optical amplifier 103 is arranged in the middle of an optical fiber transmission line 104 to perform relay transmission. is there. When the range of dispersion of the dispersion value of the optical fiber is small or the repeater interval is short, the bit phase does not change much even immediately after each optical repeater 121, so that the present invention is effective. If not, the optical repeaters 121, R
Dispersion compensation is performed in the Z wavelength multiplexing optical receiver 104, and dispersion of the optical fiber (correctly, the amount of walk-off that occurs between signals)
The present invention can be applied by setting to be substantially zero.
16 is a configuration diagram showing an eighth embodiment of the present invention, in which an optical phase adjuster 122 is used instead of the dispersion compensator to adjust the bit phase of each optical signal. Such an optical phase adjuster can be realized by, for example, combining a wavelength demultiplexer that decomposes a WDM signal for each wavelength and a variable optical delay line. Further, by arranging the clock extraction circuit inside each optical repeater, the shift amount of the bit phase can be detected, so that the bit phase can be automatically adjusted.

[0034]

EFFECTS OF THE INVENTION By setting the bit phase of the wavelength-division-multiplexed signal so as to be shifted by 1/2 bit from 0 and π, the optical signals having different bit phases are not turned on at the same time, so that the FWM is suppressed. . FWM light is generated between signals having the same bit phase. However, if optical signals having overlapping FWM light wavelengths have different bit phases, deterioration due to crosstalk hardly occurs. That is, if the wavelength intervals of the optical signals having the same bit phase are arranged so as to be unequal to each other, there is an effect that the deterioration due to the FWM can be completely suppressed. Further, in the present invention, the FWM generated
Since the number of lights is significantly reduced, the crosstalk caused by the loss of the optical signal due to the FWM is also significantly reduced. Similarly, the present invention has an effect of reducing crosstalk generated between wavelength-division multiplexed signals due to the Raman effect.
This is because the number of combinations of optical signals that are turned on at the same time is reduced.

In particular, when two bit phases of 0 and π are used, there is an effect that the configuration of the optical transmitter and the like becomes the simplest. In this case, if the wavelength arrangement shown in FIG. 9 is used,
There is an effect that wavelength multiplexing of up to 7 waves becomes possible with a wavelength arrangement at equal intervals in which the required wavelength band is minimum. For this reason, it becomes difficult to be influenced by in-band deviation of the gain of the optical fiber amplifier and wavelength dependence of the dispersion value of the optical fiber, and the transmission distance can be extended. The effect of the present invention can be maximized by using a dispersion shift fiber in the wavelength band of 1.5 μm or a normal dispersion fiber in the wavelength band of 1.3 μm.

When this system is applied to non-relay transmission,
The walk-off of the bit phase occurs only in the effective length portion of the optical fiber transmission line of about 20 km, and there is an effect that the FWM suppressing effect does not deteriorate.

Further, by arranging the wavelength intervals at unequal intervals, there is an advantage that the suppression effect of the FWM is enhanced and the number of wavelengths that can be transmitted is significantly increased at the same time. In particular, in a method in which some wavelengths are unused while keeping the wavelength intervals constant, there is an effect that the compatibility with an optical interferometer type optical multiplexer / demultiplexer using an optical waveguide or the like is high. Further, as the number of bit phases used is increased, the effect of suppressing the FWM is increased, and the number of wavelengths can be increased.

Further, by arranging the wavelengths with the guard band in between, it becomes possible to independently transmit the wavelength-multiplexed signal to both the upper band and the lower band of the guard band, and it is possible to increase the number of wavelengths.

Further, the polarization states of the signal light are orthogonal to each other 2
The F of the present invention can be set by setting one of the two polarization states.
This has the effect of increasing the WM suppression effect.

Further, in this method, the smaller the pulse duty ratio is and the narrower the pulse width of the RZ signal is, the more the effect of suppressing the FWM is increased. There is also an effect that the influence of XPM can be secondarily reduced by lowering the duty ratio. This is because as the bit width is narrower, the bits pass each other due to the effect of dispersion in the optical fiber, and the XPM effect is more easily canceled.

Further, by disposing the dispersion compensator in the optical receiver, it is possible to prevent the deterioration of the transmission quality due to XPM (cross phase modulation), SPM (self phase modulation effect) and dispersion itself. Further, there is an effect that the transmission distance is increased by performing the relay transmission using the optical repeater. Especially by performing dispersion compensation in the optical repeater, the above XPM, S
In addition to preventing the deterioration of PM and the like, the walk-off amount generated in each optical fiber section can be made substantially zero to enhance the effect of the present invention. The same applies when the optical phase adjuster 122 is used instead of the dispersion compensator.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a conventional wavelength division multiplexing optical transmission device.

FIG. 2 is a diagram showing an optical spectrum of FWM light in conventional wavelength division multiplexing transmission.

FIG. 3 is a diagram showing an optical spectrum of FWM light in a conventional non-equidistant wavelength arrangement.

FIG. 4 is a diagram showing a configuration of a wavelength division multiplex transmission device according to first to fifth embodiments of the present invention.

FIG. 5 is a diagram showing a wavelength arrangement in the first embodiment of the present invention.

FIG. 6 is a diagram showing an influence of crosstalk of FWM light by a conventional NRZ method.

FIG. 7 is a first diagram showing the principle of crosstalk reduction of FWM light in the present invention.

FIG. 8 is a second diagram showing the principle of crosstalk reduction of FWM light in the present invention.

FIG. 9 is a diagram showing an arrangement example (in the case of equal intervals) of bit phases in the first embodiment of the present invention.

FIG. 10 is a diagram showing an arrangement example (in the case of unequal intervals) of bit phases according to the second embodiment of the present invention.

FIG. 11 is a diagram showing an arrangement example of bit phases (when the number of phases is m = 3) in the third embodiment of the present invention.

FIG. 12 is a diagram showing an arrangement example of bit phases (when a guard band is used) in the fourth example of the present invention.

FIG. 13 is a diagram showing an example of arrangement of bit phases (when polarization setting is used together) in the fifth embodiment of the present invention.

FIG. 14 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 15 is a configuration diagram showing a seventh embodiment of the present invention.

FIG. 16 is a configuration diagram showing an eighth embodiment of the present invention.

[Explanation of symbols]

100 ... Wavelength multiplex optical transmitter, 101 ... Optical transmitter, 102 ... Multiplexer, 103 ... Optical amplifier, 104 ... Optical fiber transmission line, 105 ...
Demultiplexer, 106 ... Wavelength multiplexed optical receiver, 107 ... Optical receiver, 11
0 ... RZ wavelength division multiplexing optical transmitter, 111 ... RZ optical transmitter, 112 ...
..Optical delay devices, 113 ... RZ wavelength division multiplexing optical receivers, 114 ... RZ
Optical receiver, 115 ... Output point of RZ wavelength division multiplexing optical transmitter, 120 ...
Dispersion compensator, 121 ... Optical repeater, 122 ... Optical phase adjuster.

Claims (8)

[Claims] 1. A wavelength division multiplexing optical transmitter including a plurality of optical transmitters having different wavelengths, wherein an optical signal output from each of the optical transmitters is modulated by an RZ (Return-to-Zero) code, and the wavelengths are modulated. At the output end of the multiplex optical transmitter, the bit phase of each optical signal is 0, 2 / m · π, 4 / m · π, ... 2 (m-1) / m · π
(M is 2 or more), and the optical signal wavelengths are arranged so that the wavelength intervals of any two optical signals are not equal between the optical signals set in the same phase. Characteristic wavelength division multiplexing optical transmitter. 2. A wavelength division multiplexing optical transmitter according to claim 1, wherein m = 2. 3. Of two polarization states in which the polarization planes of each optical signal are orthogonal to each other at the output end of the wavelength division multiplexing optical transmitter,
3. The wavelength division multiplexing optical transmitter according to claim 1, wherein the wavelength multiplexing optical transmitter is set to either one. 4. The wavelength division multiplexing optical transmitter according to claim 1, wherein all wavelength intervals of the transmitted light are set to be unequal intervals. 5. A wavelength-multiplexed optical receiver including the wavelength-multiplexed optical transmitter, an optical fiber transmission line, and a plurality of optical receivers corresponding to transmission wavelengths of the optical transmitter. In a wavelength division multiplexing optical transmission apparatus for transmitting information from a transmitter to the wavelength division multiplexing optical receiver via the optical fiber transmission line, an optical fiber having a zero dispersion wavelength near the transmission wavelength band is used as the optical fiber transmission line. Yes, especially 1.5μ
4. A dispersion shift fiber (DSF) is used in m band optical transmission, or a normal dispersion fiber is used in 1.3 μm band optical transmission.
Or the wavelength division multiplexing optical transmission device according to any one of 4). 6. The wavelength division multiplexing optical transmission apparatus according to claim 5, wherein an optical repeater using an optical amplifier is arranged in the middle of an optical fiber transmission line. 7. The wavelength division multiplexing optical transmitter or the optical repeater,
Alternatively, the wavelength division multiplexing optical transmission device according to claim 5 or 6, wherein dispersion compensation is performed in the wavelength division multiplexing optical receiver. 8. The wavelength division multiplexing optical transmitter or the optical repeater,
Alternatively, an optical phase adjuster is arranged in the wavelength division multiplexing optical receiver, and the optical transmission device according to any one of claims 5, 6 and 7.