JP2008244514A - Optical transmitter and relay node, reducing in-phase crosstalk between wdm channels - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce waveform deterioration due to crosstalk between WDM channels. <P>SOLUTION: An optical transmitter in an embodiment of the present invention has a phase adjusting means capable of individually adjusting phases of signal lights of a plurality of channels. In-phase crosstalk due to four-lightwave mixing of the respective channels at a reception terminal is obtained by calculation or measured, and phases of the signal lines of the respective channels are obtained so that a maximum value of in-phase crosstalk of all the channels becomes small, thereby adjusting the phases by the phase adjusting means. Consequently, waveform deterioration due to crosstalk between channels can be reduced. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wavelength division multiplexing optical modulation system used in a wavelength division multiplexing (WDM) transmission system.

  In optical communication, signal data is generated by encoding digital data to be transmitted into light intensity and phase. The signal light transmitted through the optical fiber transmission line is decoded at the receiving end to extract digital data. For example, in the intensity modulation method, digital data “0” and “1” are transmitted in correspondence with light intensity “OFF” and “ON”. At the receiving end, this light is directly converted into an electric signal by a photoelectric conversion element such as a photodiode, and “0” and “1” are determined. In the case of the binary phase modulation / demodulation method, digital data “0” and “1” are transmitted in correspondence with optical phases “0” and “π”. At the receiving end, the phases of the signal light and the local light are compared, and “0” and “π” are read to determine the transmission digital data.

  In a WDM transmission system, light of different wavelengths is encoded with individual digital data, and these are combined by a wavelength filter or the like and transmitted through an optical fiber. At the receiving end, the signal is again demultiplexed for each wavelength by a wavelength filter or the like, and the signal light of each wavelength is received. In the case of a linear transmission path, there is no interaction between light of different wavelengths, so that the signal light of each channel can be transmitted independently.

Andrew R. Charplyvy, “Limitations on lightwave communications imposed by optical-fiber nonlinearities,” J. Lightwave Technology, Vol. 8, No.10., 1990, p.1548. P. P. Mitra et al., “Nonlinear limits to the information capacity of optical fiber communications,” Nature, Vol.411, p.1027, 2001. K. Inoue, “Fiber four-wave mixing in multi-amplifier systems with nonuniform chromatic dispersion,” J. Lightwave technology, vol.13, No.1, 1995, p.88. Y. Miyamoto et al., “High-speed CPFSK WDM signal transmission using PLC-LN hybrid asymmetric MZ modulator,” Proc. OFCNFOEC2005, OTuL2, 2005. Ken Tsuzuki, et al. “Development of n-i-n Mach-Zehnder optical modulator” IEICE Transactions C, Vol. J88-C, No. 2, P.83-90, 2005. Yasumasa Susaki, et al. “Monolithic integrated multichannel optical modulator” IEICE Transactions C, Vol.J88-C, No.6, pp.415-420, 2005. Koji Yamada, et al. “Silicon Wire Waveguide System-Basic Characteristics and Application to Functional Devices-” IEICE Transactions, C, Vol.J-88-C, No.6, pp.374-387. K.O. Hill et al., “Cw three wave mixing in single-mode optical fibers,” J. Appl. Phys. 49 (10) 1978, p.5098. Hiroshi Konno and Hiroshi Yamashita “Nonlinear Programming”, Science and Technology Publishing Co., 1978

  Optical fiber transmission lines induce optical nonlinear effects such as four-wave mixing (FWM), cross-phase modulation (XPM), and self-phase modulation (SPM) by the optical Kerr effect whose refractive index changes in proportion to the light intensity. To do. This phenomenon induces a problem of non-linear inter-channel interference in which the amplitude / phase waveform of the signal light of each channel changes depending on the amplitude / phase waveform of the signal light of other channels. Thereby, since the input signal light power to the optical fiber transmission line is restricted, there is a problem that the transmission distance is restricted (see Non-Patent Documents 1 to 3).

In general, FWM is a phenomenon in which light of _three different frequencies (ω ₁ , ω ₂ , ω ₃ ) is mixed to generate light of a new frequency, for example, ω ₁ + ω ₂ −ω ₃ . This is called non-degenerate FWM, but two of the three frequencies may be degenerated. In WDM transmission, FWM induces inter-channel crosstalk in which FWM light generated by signal light of three or two channels is superimposed on another channel. The generation efficiency of FWM greatly depends on the dispersion characteristics of the optical fiber transmission line, and the phase matching condition is satisfied in the vicinity of the zero dispersion wavelength band, so that the amount of crosstalk between WDM channels caused by FWM increases, resulting in waveform degradation. There is a problem that is large. The dispersion-shifted fiber has a zero dispersion wavelength band in the wavelength band (C band) of 1530 nm to 1565 nm, and has a big problem that it cannot be used despite having a mature optical amplification technique.

Waveform deterioration due to FWM crosstalk in a conventional WDM transmission line is random and unpredictable, and greatly deteriorates the detection waveform at the receiving end. For example, three different frequencies _{(ω 1, ω 2, ω} 3) If the FWM by the new frequency _{ω 4 = ω 1 + ω 2} -ω 3 light is generated, the phase phi ₄ of the optical frequency omega ₄ is It is expressed as φ ₄ = φ ₁ + φ ₂ −φ ₃ using the phases φ ₁ , φ ₂ , and φ ₃ of the _three light sources. In the conventional WDM transmission, since individual lasers are prepared as light sources for the respective channels at the transmission end, their optical phases are generally random, and φ ₁ , φ _{2, and} φ ₃ are not correlated with each other. Therefore, the optical phase φ ₄ = φ ₁ + φ ₂ −φ _{3 of} FWM crosstalk also changes randomly. The optical frequency interval between channels in WDM transmission is uniform, and the optical frequency of the channel mixed with FWM crosstalk matches the optical frequency of the FWM. Therefore, the photocurrent value detected at the receiving end depends on the optical phase relationship between the two. It fluctuates greatly. Since the optical phase of FWM crosstalk changes randomly, the detected photocurrent value changes non-deterministically and is unpredictable.

  The present invention has been made in view of such problems, and an object of the present invention is to reduce waveform deterioration due to crosstalk between WDM channels.

  In order to achieve such an object, the present invention provides an optical transmitter for transmitting a plurality of channels of signal light by wavelength multiplexing, wherein the signal light of the plurality of channels is transmitted. The optical phase adjusting means capable of individually adjusting the phase is provided.

  The invention according to claim 2 is the optical transmitter according to claim 1, wherein in-phase crosstalk determining means for determining in-phase crosstalk due to four-wave mixing of each channel, and in-phase cross of all channels. In-phase crosstalk reducing means for obtaining the phase of the signal light of each channel so that the maximum value of the talk is reduced is provided.

  The invention according to claim 3 is the optical transmitter according to claim 2, wherein the common-mode crosstalk reducing means is configured to reduce the maximum value of all the common-mode crosstalks so that the maximum value of each channel is reduced. The phase and polarization of signal light are obtained.

  The invention according to claim 4 is the optical transmitter according to claim 2 or 3, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by calculation. Features.

  The invention according to claim 5 is the optical transmitter according to claim 4, wherein the in-phase crosstalk determining means includes a loss factor, chromatic dispersion, dispersion slope, nonlinear coefficient, and signal light of the transmission line. The in-phase crosstalk by four-wave mixing of each channel is obtained by calculation using the wavelength and the interval between channels as parameters and the phase of the signal light of each channel as a variable.

  The invention according to claim 6 is the optical transmitter according to claim 2 or 3, wherein the in-phase crosstalk determining means measures in-phase crosstalk by four-wave mixing of each channel at the receiving end. It is characterized by obtaining by.

  The invention according to claim 7 is the optical transmitter according to any one of claims 2 to 6, wherein the common-mode crosstalk reducing means has a large value among the common-mode crosstalk of all channels. The above-mentioned channels are selected, and the in-phase crosstalk for each of the selected one or more channels is obtained using a continuous function representing the relationship between the in-phase crosstalk of the selected one or more channels and the phase of the signal light of each channel. The phase range of the signal light of each channel to reduce the phase is obtained, and the phase range common to the selected one or more channels is obtained.

  The invention according to claim 8 is the optical transmitter according to claim 1, wherein the phase of the signal light of the plurality of channels is adjusted to the same phase or opposite phase by the optical phase adjusting means. Features.

  The invention according to claim 9 is a relay node that relays wavelength multiplexed light obtained by wavelength multiplexing signal light of a plurality of channels, and is capable of individually adjusting the phase of the signal light of the plurality of channels. An adjustment means is provided.

  The invention described in claim 10 is the relay node according to claim 9, wherein the common-mode crosstalk determining means for determining the common-mode crosstalk by the four-wave mixing of each channel, and the common-mode crosstalk of all the channels. And an in-phase crosstalk reducing means for obtaining the phase of the signal light of each channel so that the maximum value becomes smaller.

  The invention according to claim 11 is the relay node according to claim 10, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by calculation. .

  The invention according to claim 12 is the relay node according to claim 10, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by measurement at the receiving end. It is characterized by.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 shows a configuration example of a wavelength division multiplexed light transmitter according to the first embodiment of the present invention. This optical transmitter includes a light source 10 that generates multi-wavelength light whose phases are synchronized, a filter 12 that separates the generated light for each wavelength, and a modulator 14-1 that modulates light of each wavelength with respective modulation signals. -4, phase adjusting units 16-1 to 16-4 that adjust the phase of light of each wavelength, and a multiplexer 18 that multiplexes these lights. Here, a case where the number of wavelength multiplexing is four will be described, but it should be noted that this embodiment can be applied to an arbitrary number of wavelength multiplexing.

  The optical transmitter uses phase-locked multi-wavelength light as a carrier wave of WDM signal light. The phase-synchronized multiwavelength light is assumed to have a highly accurate control of the optical frequency difference and phase difference between adjacent modes, which are stable in time. Therefore, when a beat signal between adjacent modes is detected, its frequency and phase are stable in time. Each optical frequency mode matches the optical frequency of the WDM signal light, and is generally a uniform frequency interval. As a method of realizing the phase-locked multi-wavelength light source 10, there are a method of modulating the phase or intensity of light from a laser light source that oscillates in a single mode to convert it into a plurality of modes, a method of using a mode-locked laser diode, and the like. . There is also a method of expanding the spectrum of the phase-locked multi-wavelength light generated by using these by the supercontinuum effect of the optical fiber.

  The phase-locked multi-wavelength light is separated into individual wavelengths by the wavelength separation filter 12, and the light modulators 14-1 to 14-4 modulate the light of the respective wavelengths with the transmission data, and the optical phase adjusters 16-1 to 16-4. After adjusting the phase of the light, the light is multiplexed by the multiplexer 18 to obtain WDM transmission light. Here, the light of each channel is individually modulated and the phase of the light of each channel is also adjusted. Examples of the realization method include an electro-optic (EO) effect, a thermo-optic (TO) effect, a phase adjustment method using a refractive index change by magneto-optic (MO), and a method of mechanically adjusting an optical path length. For phase adjustment, a Mach-Zehnder interferometer using an EO crystal such as a lithium niobate crystal or an indium phosphorus crystal is often used as a modulator. In this case, it is necessary to add an electrode for adjusting the optical phase of the entire channel separately from the conventional electrode for data modulation. It is also possible to use a bias voltage application port of a conventional modulator. Even when an electro-absorption modulator (EAM) is used, an electrode is provided separately from the EAM portion, and the optical phase is shifted using the refractive index change or EO effect by carrier control of the optical semiconductor waveguide. You can also. Phase adjustment is also possible by separately preparing a medium having an EO effect, a TO effect, an MO effect and the like separately from the modulation unit and allowing light to pass therethrough.

  Furthermore, in the individual modulation arms of each channel from the wavelength separation filter 12 to the multiplexer 18, it is necessary to maintain the optical phase relationship of each arm. Therefore, the wavelength separation filter to the multiplexing unit are integrated. May be. For example, there are an optical planar circuit (PLC) mainly composed of glass and an indium phosphorous as a main component as an integration substrate, and an integration technology with a Mach-Zehnder type EO modulator has already been developed. (Non-Patent Document 4). Further, a technique for integrating EAMs for a plurality of channels on an InP substrate and a technique for producing a waveguide / functional element on a silicon substrate have also been developed (Non-Patent Documents 5 and 6). When an optical integrated circuit is used in this way, electrodes can be provided in the waveguides of the respective channels manufactured on the integrated substrate, and the EO effect and the TO effect can be used.

(Second Embodiment)
FIG. 2 shows a configuration example of a wavelength multiplexing optical relay node according to the second embodiment of the present invention. This relay node includes a filter 22 that separates the WDM signal light for each wavelength, a phase adjustment unit 26-1 to 4-4 that adjusts the phase of the light of each wavelength, and a multiplexer 28 that combines these lights. I have. This relay node can adjust the optical phase in each channel of the WDM signal light. Here, a case where the number of wavelength multiplexing is four will be described, but it should be noted that this embodiment can be applied to an arbitrary number of wavelength multiplexing.

  In this embodiment, in addition to the optical amplification function and the Add / Drop function in the conventional relay node, a function for adjusting the optical phase of each channel of the WDM signal light is added. The WDM signal light input to the node is separated into individual wavelengths by the wavelength separation filter 22, and after the phases of the respective wavelengths are individually adjusted by the phase adjustment units 26-1 to 26-4, they are combined again by the multiplexer 28. The WDM transmission light is output. As means for separating the WDM signal light into individual wavelengths, wavelength dispersion devices such as gratings, AWGs, wavelength selective switches (WSS), and the like can be used. Examples of a method for realizing phase adjustment include an electro-optic (EO) effect, a thermo-optic (TO) effect, a phase adjustment method using a refractive index change by magneto-optic (MO), and a method of mechanically adjusting an optical path length. It is done.

  In addition, in the individual modulation arms of each channel from the wavelength separation filter 22 to the multiplexer 28, it is necessary to maintain the optical phase relationship of each arm. Therefore, the wavelength separation filter to the multiplexing unit are integrated. May be. For example, there are an optical planar circuit (PLC) mainly composed of glass and an indium phosphorous as a main component as an integration substrate, and an integration technology with a Mach-Zehnder type EO modulator has already been developed. . In addition, a technique for integrating EAMs for a plurality of channels on an InP substrate and a technique for producing a waveguide / functional element on a silicon substrate have also been developed. When an optical integrated circuit is used in this way, electrodes can be provided in the waveguides of the respective channels manufactured on the integrated substrate, and the EO effect and the TO effect can be used.

  It is also possible to integrate a series of functions that spatially separate wavelengths using a wavelength dispersion element such as a grating, adjust each element of the liquid crystal array, adjust the optical phase, and re-combine. It is.

  In addition to the function of adjusting the optical phase, the function of adjusting the bit delay of data is also added, so that the FWM crosstalk in the first transmission span and the second transmission span can be canceled more efficiently. Can do.

(Third embodiment)
FIG. 3 shows a configuration example of the in-phase crosstalk suppression method for wavelength multiplexed light according to the third embodiment of the present invention. In the present embodiment, waveform degradation due to FWM crosstalk is reduced by controlling the optical phase of transmission light after synchronizing the optical phase of each channel of WDM signal light. As shown in FIG. 3, the in-phase crosstalk suppression method according to the present embodiment compares the in-phase crosstalk amount of each channel with the function 32 for evaluating the in-phase crosstalk amount by the FWM of the wavelength multiplexed light and sets the maximum value. A selection function 34 and an algorithm 36 for minimizing the maximum value of the in-phase crosstalk.

The optical electric field envelope of the main signal light of the channel of the optical angular frequency ω _k is E _k (t), the optical phase is φ _k (t), and FWM crosstalk light of the same frequency mixed in the channel is ΔE _F−k (T), the optical phase is θ _F−k (t). Here, ΔE _F-k (t) and θ _F-k (t) are obtained by synthesizing FWM crosstalk generated by a combination of various WDM channels mixed in channel k in optical fiber transmission of WDM signal light. To do.

Both the main signal light and the FWM crosstalk light are input to the receiver, and the optical electric field E _k-tot (t) is expressed by the following equation.

The first term of this equation is a desired detection signal that depends only on the main signal light. On the other hand, the second term and the third term depend on the optical electric field of the FWM crosstalk light and vary depending on the data pattern of the other channel, so that the detection signal light of the channel k is deteriorated. Comparing the magnitude of the second term and the third term, as compared with the optical field amplitude E _k of the main signal light _(t), the optical field amplitude ΔE _{F-k (t)} of the FWM crosstalk light because small, The third term is a more major factor of deterioration.

In general, a factor that causes optical signals having different frequencies to leak and deteriorates the waveform is called power crosstalk, while a factor that causes signal light having the same frequency to leak and deteriorates the waveform is called coherent crosstalk. In power crosstalk, since the frequencies of the two lights are separated, the third term of equation (2) oscillates at that frequency difference, so it can be removed by a low-band transmission filter or the like, and signal degradation in power crosstalk is small. FWM crosstalk light is generally coherent crosstalk because it has the same frequency as the main signal light, but it should be noted here that the third term in equation (2) depends on the phase difference between the main signal light and the FWM crosstalk light. If the phase difference can be controlled, the third term can be reduced. The third term of the equation (2), the amplitude Delta] E _F-k of the main signal light and the in-phase component of the FWM crosstalk _{(t) cos (φ k (} t) -θ F-k (t)) and the main signal It is the product of the light amplitude E _k (t). Therefore, only the in-phase component of FWM crosstalk (in-phase FWM crosstalk) contributes to the third term, and the quadrature component does not contribute to the third term. That is, if the optical phase of FWM crosstalk can be orthogonalized with respect to the main signal light, it is possible to suppress the waveform degradation like power crosstalk.

  In this embodiment, as shown in FIG. 3, waveform deterioration due to the FWM crosstalk light is suppressed to a minimum by controlling the phase difference between the FWM crosstalk and the main signal light. In WDM optical fiber transmission, FWM crosstalk mixed in channel n includes all combinations of channel i, channel j, and channel k that satisfy i + j = k + n. Therefore, the case where all of these FWM crosstalks can be set in a quadrature phase relationship with the main signal light of channel n is limited to the case where the number of WDM channels is small. When the number of WDM channels is large, the sum of the in-phase components of main FWM crosstalk (referred to as in-phase crosstalk) is taken and set to the phase relationship of the WDM signal light that minimizes it.

Here, D _{i, j} is a degeneration coefficient, D = 3 (i = j), D = 6 (i ≠ j), χ ₁₁₁₁ : third-order nonlinear susceptibility, c: speed of light, L: fiber length, n: Refractive index. Further, .DELTA.k is defined by the wave number difference _{Δk = -k i -k j + k} k + k n in FWM generation affects the deviation from the phase matching. Here, although the optical electric field amplitude E (t) in optical communication varies with time depending on the data pattern, in order to determine the amount of crosstalk in the worst case, all the channels are set to the maximum value E ₀ here. The coefficient is 2πω _n χ ₁₁₁₁ / nc = κ.

In Equation (2) representing the photocurrent of channel n, the ratio R _F-n of the crosstalk component ΔI corresponding to the third term to the desired signal component I _s-n represented by the first term is minimized. Therefore, it is possible to suppress the deterioration of the received waveform. This crosstalk amount R _F-n is represented by R _F-n = ΔI _F-n / I _s-n = 2Re {ΔE _F-n / E _s-n }. Here, the main signal _{E s-n} channels n in the transmission fiber output end is _{E s-n = E 0 exp} (-αL / 2 + i (ω n t-k n L + φ n)), the FWM crosstalk field When the crosstalk amount R _F-n divided by the main signal is obtained, the following equation is obtained.

  Equation (5) shows that the optical phase relationship between the FWM crosstalk and the main signal depends on the optical fiber propagation distance L. However, when L is infinite, it converges to exp (−αL−iΔkL) → 0. It can be simplified as an expression.

Equation (6) is applied to all channels, and in order to improve the performance of the entire system, the phase {φ ₁ , φ of each channel is set so that the maximum value of the in-phase crosstalk of all channels is reduced. ₂ ,..., Φ _N } may be obtained.

Specifically, referring to FIG. 3, the functional block 32 evaluates the in-phase crosstalk amount by FWM using the wavelength, dispersion slope, dispersion value, fiber length, loss factor, and frequency interval as given parameters. . Next, the maximum value Max {R _F−1 , R _F−2 ,..., R _F−N } is selected from the in-phase crosstalk amount obtained by comparing all channels by the function block 34. The phase {φ ₁ , φ ₂ ,..., Φ _N } for reducing the selected maximum value Max {R _F−1 , R _{F− 2} ,..., R _F−N } by the function block 36. Ask for.

(Fourth embodiment)
FIG. 4 shows a configuration example of the in-phase crosstalk suppression method for wavelength multiplexed light according to the fourth embodiment of the present invention. In this embodiment, waveform deterioration due to FWM crosstalk is reduced by controlling not only the phase of the transmitted light but also the phase shift amount of the relay node. As shown in FIG. 4, the in-phase crosstalk suppression method according to the present embodiment compares the amount of in-phase crosstalk of each channel with the function 42 for evaluating the in-phase crosstalk amount by the FWM of wavelength multiplexed light, and sets the maximum value. A selecting function 44 and an algorithm 46 for minimizing the maximum value of the in-phase crosstalk.

  In optical fiber transmission, optical fiber transmission lines may be multistaged through a relay node having an optical amplification function. In such a case, the FWM crosstalk light generated in the first transmission span of each channel and the FWM crosstalk light generated in the second transmission span are added and input to the receiver. Therefore, if control can be performed so that the optical phases of the two are reversed, it is possible to cancel the FWM crosstalk of the first span and that of the second span. Even when there is a bit delay due to transmission path dispersion, the optical phase of both FWM crosstalks can be controlled to add in a phase relationship that causes less signal degradation. In order to effectively suppress and cancel, the optical phase of each channel can be adjusted at the relay node.

If the optical phases of the channels 1, 2,..., N at the optical fiber input end of the first transmission span are φ ₁ , φ ₂ ,..., Φ _N , the FWM light ΔE _{F-n, 1 (1 )} Divided by the main signal light is expressed by the following equation.

Since the FWM crosstalk ΔE _{F-n, 1} generated in the first transmission span propagates through the second optical fiber transmission line, the optical electric field ΔE _{F-n, 1 (2} at the output end of the second transmission line) ₎ Is expressed as follows: Here, it is assumed that the optical amplification gain at the relay node after the first transmission is G1, and the optical phase shift amounts of the respective channels are Δφ _1-1 , Δφ _2-1 ,..., Δφ _N-1 .

On the other hand, the value obtained by dividing the optical electric field ΔE _{F−n, 2 (2)} at the output end of the FWM crosstalk light generated in the second optical fiber transmission line by the main signal light at the output end of the second transmission span is It is expressed by the following formula.

Therefore, the FWM generated in the first transmission span and the FWM generated in the second transmission span are added, and the total FWM crosstalk amount ΔE _{F−n, 2} / / E at the optical fiber output end of the second transmission span. _{Sn (2)} is represented by the following formula.

Therefore, the crosstalk amount R _{F-n, 2} in the photocurrent is expressed by the following equation.

As shown in this equation, in addition to the case described in the first embodiment, the optical phase shift amount {Δφ _{1 (1)} , Δφ _{2 (1)} ,..., Δφ _{N ( 1)} } has been added as a freely adjustable variable. Even in this case, similarly to the initial phase adjustment of the transmitted light, the phase {φ ₁ , φ ₂ ,..., Φ _N } of each channel and the maximum value of the in-phase crosstalk of all the channels are reduced. The phase shift amounts {Δφ _{1 (1)} , Δφ _{2 (1)} ,..., Δφ _{N (1)} } may be set.

Specifically, referring to FIG. 4, the function block 42 evaluates the in-phase crosstalk amount by FWM using the wavelength, dispersion slope, dispersion value, fiber length, loss factor, and frequency interval as given parameters. . Next, from the in-phase crosstalk amount obtained by comparing all the channels by the function block 44, the maximum value Max {R _F-1,2 , R _F-2,2 ,..., R _{F-N, 2} } is selected. Then, by the function block 46, the phase {φ ₁ , φ ₂ ,... That reduces the selected maximum value {R _F-1,2 , R _F-2,2 ,..., R _{F-N, 2} }. , Φ _N } and phase shift amounts {Δφ _{1 (1)} , Δφ _{2 (1)} ,..., Δφ _{N (1)} } are obtained.

Here, it is assumed that the optical phases of the respective channels of the WDM signal light are synchronized. However, when the phases are not synchronized, {φ ₁ , φ ₂ ,..., Φ _N } in Expression (11) This corresponds to the case where the mutual phase relationship varies randomly depending on time. It should be noted here that in the FWM crosstalk generated in the first transmission span and the second transmission span as shown in the equations (10) and (11), the initial optical phase {φ ₁ , φ of each channel is used. _2, ..., it includes a phi _N} as a common term, the initial optical phase of each channel {phi _1, phi _2, ..., independent of the _{φ N} {Δφ 1 (1} ), Δφ 2 ₍₁₎ ,..., Δφ _{N (1)} } can be adjusted to cancel FWM crosstalk generated in the first transmission span and the second transmission span. Therefore, even when the optical phase of the WDM signal light output from the transmission end is not synchronized, the FWM crosstalk amount can be suppressed by adjusting the optical phase shift amount at the relay node.

  In addition, a propagation delay difference occurs in the bits of channel i, channel j, and channel k, which are the sources of FWM due to chromatic dispersion of the first transmission span, and the first transmission span and the second transmission span It is assumed that the optical electric field strength is different. That is, the in-phase FWM crosstalk that occurs in the first transmission span and the in-phase FWM crosstalk that occurs in the second transmission span can be canceled because the data pattern is different even if the phase is inverted. There are cases where it is not possible. Since such a bit shift occurs depending on the chromatic dispersion, Equation (11) can be minimized after compensating for the bit delay by dispersion compensation. In addition, by controlling the data bit delay in the optical phase adjustment function of the relay node, the data pattern of the WDM signal light in the first transmission span and the second transmission span is adjusted to coincide with each other efficiently. It is also possible to cancel FWM crosstalk.

Further, in order to suppress the in-phase FWM regardless of the pattern, it corresponds to assuming that the electric field amplitudes E _{0 of} the first transmission span and the second transmission span are different. Therefore, in addition to the equation (11), the following equations (12) and (13) are also defined as objective functions, and these R _{F−n (2)} and R ′ _F− are determined for all channels. {φ ₁ , φ ₂ ,..., φ _N } and {Δφ _{1 (1)} , Δφ _{2 (1)} so as to reduce the maximum value of _{n (2)} , R ″ _{F−n (2)} . ,..., Δφ _{N (1)} } may be set.

Here, only two spans have been described as examples. However, even when there are two or more spans, the phase shift amounts {Δφ _{1 (m)} , Δφ _{2 (m)} , ..., Δφ _{N (m)} } (m: number of relay node) is provided, and each relay node has a function to minimize the maximum value of the in-phase FWM crosstalk amount in reception of each channel. Optical phase shift amount Phase shift amount {Δφ _{1 (m)} , Δφ _{2 (m)} ,..., Δφ _{N (m)} } and optical phase {φ ₁ , φ ₂ ,. φ _N } may be set.

  In addition, although the loss coefficient and the nonlinear coefficient of the first and second transmission line optical fibers are the same, there are cases where this value is actually different so that the values of the first and second spans can be handled individually. The values depending on the span can be used for α and γ in the above discussion. Moreover, although it is assumed that the optical electric field amplitudes of all the channels are the same, it is not a particularly necessary condition, and when they are different, the above argument can be extended by using individual values. Furthermore, for the sake of simplicity, it is assumed that the fiber loss is equal to the gain of the optical amplifier. However, this is not a particularly necessary condition, and if it is different, the discussion can be expanded to consider each.

(Fifth embodiment)
FIG. 5 shows a configuration example of the in-phase crosstalk suppression method for wavelength multiplexed light according to the fifth embodiment of the present invention. In this embodiment, in addition to controlling the optical phase of each channel of WDM input light to the optical fiber transmission line as in the third embodiment, by controlling the polarization of each channel, FWM crosstalk is further increased. The waveform deterioration due to is reduced. As shown in FIG. 5, the in-phase crosstalk suppression method according to the present embodiment selects the maximum value by comparing the in-phase crosstalk amount of each channel with the function 52 for evaluating the in-phase crosstalk amount by the FWM of the wavelength multiplexed light. Function 54 and an algorithm 56 for minimizing the maximum value of the in-phase crosstalk.

The photocurrent equation (2) and the FWM crosstalk equations (5) and (6) generated in the optical fiber transmission line are for an optical electric field whose polarization is in a parallel state. By introducing the polarization axis and individually handling the optical electric field component of each polarization axis, it is possible to expand the expression to take into account the polarization (Non-patent Document 8). As a result, as in the third and fourth embodiments, the crosstalk amount {R _F−1 , R _F−2 ,..., R _F−N } in the photocurrent for each channel is the initial value. phase _{{φ 1, φ 2, ···} , φ N}, the polarization angle _{{ψ 1, ψ 2, ···} , ψ N} is expressed as a function of. Therefore, the initial phase {φ is reduced so that the maximum value of the in-phase crosstalk {R _F-1,2 , R _F-2,2 ,..., R _{F-N, 2} } of all channels becomes smaller. _{1, φ 2, ···, φ} N}, the polarization angle _{{ψ 1, ψ 2, ···} , sets the [psi _N}. Further, the phase shift of each channel at the relay node can be used together, and the phase shift amount {Δφ _{1 (1)} , Δφ _{2 (1)} ,..., Δφ _{N (1)} } at the relay node, and further at the relay node By providing a rotation function capable of individually adjusting the polarization of each channel, the rotation angles {ψ _{1 (1)} , ψ _{2 (1)} ,..., Ψ _{N (1)} } are also used as optimization parameters. Can be adjusted.

Specifically, referring to FIG. 5, the function block 52 evaluates the in-phase crosstalk amount by FWM using the wavelength, dispersion slope, dispersion value, fiber length, loss factor, and frequency interval as given parameters. . Next, the maximum value Max {R _F-1,2 , R _F-2,2 ,..., R _F-N, from the in-phase crosstalk amount obtained by comparing all the channels by the function block 54 _{. 2} } is selected. Then, by the function block 56, the maximum value selected phase {phi ₁ to minimize the _{{R F-1,2, R F} -2,2, ···, R F-N, 2}, φ 2, · , Φ _N }, phase shift amounts {Δφ _{1 (1)} , Δφ _{2 (1)} ,..., Δφ _{N (1)} } and rotation angles {ψ _{1 (1)} , ψ _{2 (1)} ,. .., Ψ _{N (1)} } is obtained.

(Sixth embodiment)
FIG. 6 shows a configuration example of an in-phase crosstalk estimation method for wavelength multiplexed light according to the sixth embodiment of the present invention. FIG. 6 shows an in-phase FWM crosstalk calculation algorithm in wavelength division multiplexing transmission with N channels. As shown in the figure, the in-phase crosstalk estimation method according to this embodiment includes a function 102 for selecting a combination of channels, a function 104 for calculating a phase mismatch amount, and a function 106 for calculating a phase mismatch phase shift amount. The function 108 for calculating the amplitude of the FWM crosstalk, the function 110 for selecting the initial phase, the function 112 for calculating the initial phase, the function 114 for calculating the degeneracy coefficient, and the photocurrent generated by the FWM crosstalk for each channel. And a selector 116 for outputting the fluctuation.

  In the combination selection function block 102, a combination (i, j, k) satisfying 1 ≦ i + j−k ≦ N is obtained. In the phase mismatch amount calculation block 104, the selected combination (i, j, k) and the dispersion value D of the optical fiber transmission line, dispersion slope ∂D / ∂λ, wavelength λ, channel frequency interval Δf, loss factor α The phase mismatch amount Δk is obtained using transmission path parameters such as the fiber length L. This calculation method is given by the following equation using the parameters shown on the left.

  As shown in Expression (6), the FWM of the fiber output has an effect of phase rotation by the phase mismatch amount Dk, and the phase shift amount δθ is obtained in the function block 106 for calculating the phase mismatch phase shift amount. This value is expressed by the following equation using the loss coefficient α and the phase mismatch amount Dk.

The phase of the FWM light depends on the phase of the light that is the source. Assuming that n = i + j−k at the optical fiber output end, the phase difference Δθ of the FWM light with respect to the main signal of the n-th channel is the initial phase φ _i , φ _j , φ _k , φ _n and phase mismatch at the optical fiber input end It depends on the phase shift amount δθ and is expressed by the following equation.

  On the other hand, in the functional block 108 for FWM amplitude calculation, the optical electric field amplitude of FWM crosstalk is obtained. The arithmetic expression is as follows.

Here, the parameters common to all channels are excluded and simplified. There is no problem when comparing differences in FWM crosstalk amount by channel. If an absolute value is required, 2πωE ₀ ² / (nc) may be multiplied. Where ω is the optical angular frequency, E _{0 is the} input optical electric field amplitude, n is the refractive index, and c is the speed of light. Moreover, in an actual transmission line, there is a possibility that a difference in generation efficiency between the theoretical value and the calculated value may occur, and it may be corrected as necessary.

In the initial phase of the selection block 110, the initial optical phase _{{φ 1, φ 2, ···} , φ N} based on a combination selected from (i, j, k), and selects the initial phase, the initial phase computing At block 112, an initial phase is determined. In the degeneration coefficient calculation block 114, a degeneration coefficient is obtained based on the selected combination (i, j, k).

With the calculation method shown in FIG. 6, photocurrent fluctuation due to FWM crosstalk of each channel is output via the selector 116. That is, the calculation of N input and N output for outputting the photocurrent fluctuation due to the FWM crosstalk light of each channel using the optical phase {φ ₁ , φ ₂ ,..., Φ _N } at the transmission fiber input end as input variables. realizable. Therefore, with respect to various channel combinations (i, j, k), it is possible to obtain FWM crosstalk estimates for each channel for a predetermined transmission path parameter.

Here, the optical field amplitude for all channels is assumed to be constant, if each has a different value E _1, E _2, · · ·, the E _N is a combination of (i, j, k) since FWM amplitude generated by different channel-dependent _{E 1,} E 2, · · ·, it is necessary to calculate considering _{E N.} FIG. 7 shows an in-phase FWM crosstalk calculation algorithm in this case. This in-phase crosstalk estimation method has a function 118 for selecting an initial amplitude as compared with FIG.

  In this case, Formula (4) is represented by the following formula.

The main signal EF-n of the channel n is _{E F-n = E 0 exp} (-αL + i (ω n t-k n L + φ n)), the crosstalk amount obtained by dividing the FWM crosstalk field in the main signal _{R F- When n} is obtained,

It becomes. Here, the expression R _F-n indicates that the optical phase relationship between the FWM crosstalk and the main signal depends on the optical fiber propagation distance L, but converges to exp (−αL−iDkL) → 0 when L is infinite. Therefore, it can be simplified as the following equation.

Equation (20) is applied to all channels, and in order to improve the performance of the entire system, in-phase crosstalk of all channels {R _F-1 , R _F-2 ,..., R _F-N }, The phase {φ ₁ , φ ₂ ,..., Φ _N } of each channel may be obtained so that the maximum value becomes smaller. The optical electric field amplitude of each channel and the propagation constant of the optical fiber transmission line are given as constants.

  In FIG. 7, functional blocks such as the combination selection 102, the phase mismatch amount calculation 104, the phase mismatch phase shift amount calculation 106, the initial phase selection 110, the initial phase calculation 112, the degeneration coefficient calculation 114, and the selector 116 are shown in FIG. Is the same as Unlike the case of FIG. 6, the FWM amplitude calculation function block 108 performs processing represented by the following equation.

(Seventh embodiment)
FIG. 8 shows a configuration example of the in-phase crosstalk suppression method for wavelength multiplexed light according to the seventh embodiment of the present invention. As shown in the figure, the in-phase crosstalk suppression method according to the present embodiment includes an optical phase adjustment unit 202-1 to N that adjusts the phase of light of each channel at the transmission end, and a multiplexer that multiplexes these lights. 204, a filter 206 that separates wavelength-division-multiplexed light propagated through the transmission path for each wavelength, a measuring unit 208-1 to N that monitors and measures in-phase crosstalk of each wavelength at the receiving end, and a measured in-phase cross An algorithm 210 for minimizing talk. Although it is possible to realize the function of evaluating the in-phase FWM crosstalk of FIG. 4 or FIG. 5 using the in-phase FWM crosstalk calculation as shown in the sixth embodiment, in this embodiment, the in-phase FWM crosstalk is realized. Talk evaluation is realized by monitoring the waveform at the receiving end of the actual transmission path.

  Specifically, the monitor value of the in-phase crosstalk at the receiving end obtained by the measuring unit 208 is fed back to the function block 210 of the minimization algorithm at the transmitting end or the relay node. At the transmitting end, the signal light of the channel to be measured is turned off, the main signal component input to the receiving end is removed, and the FWM crosstalk amount mixed in the channel can be measured. In this case, in order to measure only the light having the same phase component as the main signal light, it is necessary to use a coherent detection method such as heterodyne reception or homodyne reception. Since the phase relationship between adjacent channels of WDM signal light is controlled stably in time, local light used for coherent detection can be realized by a method synchronized with the optical phase of the adjacent channel. Also, at the transmitting end, the main signal is not completely turned off, but a part of the output transmission light power is reduced, but it is used as a reference for the phase of the main signal light, and in-phase crosstalk is performed. You may measure. Further, by turning off the data modulation of the measurement target channel at the transmission end, the fluctuation of the main signal light itself can be stopped. At this time, the fluctuation of the signal light at the receiving end is due to the common-mode crosstalk. Therefore, if the fluctuation width is measured, the common-mode crosstalk can be measured.

  Furthermore, since the signal quality such as the error rate and Q value of the main signal light changes due to in-phase crosstalk even during normal operation, this received signal quality is used as an indirect evaluation index for in-phase crosstalk. It is also possible to feedback control the initial phase of the transmitting end and the phase shift amount of the relay node. Thus, by setting the phase of each channel at the transmission end to the phase value obtained by the minimization algorithm 210 by the phase adjustment unit 202, in-phase crosstalk due to FWM at the reception end can be suppressed.

(Eighth embodiment)
FIG. 9 shows a configuration example of the in-phase crosstalk suppression method for wavelength multiplexed light according to the eighth embodiment of the present invention. As shown in the figure, the common-mode crosstalk suppression method according to the present embodiment has a function 302 for evaluating common-mode crosstalk by FWM, a function 304 for selecting an objective function, and a function 310 for selecting a variable for minimizing a continuous function. And a function 312 for calculating the common range. This embodiment relates to an algorithm for efficiently minimizing common-mode crosstalk.

  In the in-phase crosstalk evaluation function block 302, feedback input of the in-phase crosstalk amount output with respect to the input variable is performed, and the phase, polarization angle, and relay of each channel of the transmitter in a direction to reduce the in-phase crosstalk. The phase shift amount of each channel of the node is changed.

Of the in-phase crosstalk, the channel having the maximum value is selected, and the variables φ ₁ , φ ₂ ,... Such as the phase of each channel, the phase shift amount of each channel of the relay node, and the polarization angle are reduced so as to reduce it. ..., give the change to φ _N. In order to efficiently search for variables such as the optimum transmitter phase, relay node phase shift amount, and polarization angle at high speed, an algorithm for selecting the amount of change given to the variable is important.

  Methods for solving the minimization problem include simplex method, Lagrange method, Kuhn-Tucker method, quasi-Newton method, steepest descent method, and conjugate gradient method. However, since there are a plurality of functions here and the problem is to minimize the maximum value, these methods may not be applied as they are. In that case, as a method using the variable selection method in the method of minimizing the differentiable function, the value fed back from the in-phase FWM crosstalk evaluation 302 is compared by the comparator 306, and one or more of the large in-phase crosstalk is detected. Detect channels. A function that gives the amount of in-phase FWM crosstalk for the channel is selected in function block 304, focusing on that function and minimizing in function block 310. This minimization processing is performed for each channel by the buffer 308 for one or more channels having a large in-phase crosstalk detected by the comparator 306. That is, the processing of the function block 314 is performed for the number of channels detected by the comparator 306.

  Here, since the selected objective function is differentiable, the variable selection method in the minimization method as described above can be applied in the function block 310. For the variable of the next step given by the variable selection method, the value fed back from the block of the in-phase FWM crosstalk evaluation is detected whether the channel giving the minimum value is the same as the previous step. Then, the FWM crosstalk amount is calculated by continuously giving the variables of the next step. If the channel number giving the maximum value is changed, the objective function used in the minimizing method of the differentiable function is replaced with that of the channel, and the variable of the next step is selected. Furthermore, when two or more functions show the same value, the variable selection function block pays attention to both functions, calculates a desired variable range by applying a differentiable minimization method, In 312, the in-phase FWM crosstalk amount is evaluated using the variable in the common range of both functions as the next variable.

Another _{method, Max {R F-1,} R F-2, ···, R F-N} that is potentially minimized _{{φ 1, φ 2, ···} , φ N } Are selected as candidates, and Max {R _F−1 , R _F−2 ,..., R _F−N } at each candidate point is evaluated, and the optimum {φ ₁ , φ ₂ ,. A solution of φ _N } can be obtained. As candidate points, there exists a curved surface where R _F-n1 = R _F-n2 =... = R _F-nk holds for an arbitrary number k of integers n1, n2 _,. , the independent variables _{{φ '1, φ' 2} , ···, φ 'n-k + 1} as a variable ▽ _{R F-n} becomes = 0 of the newly introduced independent variable point _{{φ' 1 (0)} , Φ ′ _{2 (0)} ,..., Φ ′ _{N−k + 1 (0)} } are candidates. The independent variable candidate points correspond to the phase variables {φ _{1 (0)} , φ _{2 (0)} ,..., Φ _{N (0)} } of each channel. _Max at the point where the candidate _{{R F-1, R F} -2, ···, R F-N} by evaluating the solution of the optimal _{{φ 1, φ 2, ···} , φ N} Can be requested.

In order to realize this, an optimization algorithm using the Kuhn-Tucker condition is effective. In Kuhn-Tucker nonlinear programming, when a certain condition is imposed on a variable, a solution to the problem of minimizing the objective function in the variable is given (Japanese Patent Document 9). Here, R _F-n1 = R _F-n2 , R _F-n1 = R _F-n3 , R _F-n1 = R _F-n4 ,..., R _F-n1 = R _F-nk is k−. There will be one condition. The variable is a phase parameter such as {φ ₁ , φ ₂ ,..., Φ _N }, and in some cases, a polarization angle.

In addition, differentiable evaluation parameters are set separately from the original objective function, and based on these evaluation parameters, a differentiating objective function minimization algorithm that has been proposed in the past is applied to transmit data more efficiently. There is a method of selecting input variables such as the initial phase at the end, the phase shift amount of the relay node, and the polarization angle. In the description so far, Max {R _F−1 , R _F−2 ,..., R _F−N } which takes the maximum value in all the channels with respect to the in _- phase crosstalk R _{F−N of} each channel. Although it was set as the objective function, for example, the variance giving R _F-N variation is set as one evaluation parameter, and the initial value of the transmitting end is set using a minimization algorithm that can differentiate the evaluation parameter as the objective function. It is also possible to select variables such as phase, relay node phase shift amount, and polarization angle. At that time, Max {R _F-1 , R _F-2 ,..., R _FN }, which is the original objective function, needs to be evaluated, but there is a possibility that the optimum solution can be efficiently searched. is there. Further, as an evaluation parameter, a function that varies is multidimensional variables x1, x2, · · ·, relative to the size of xn nonlinearly, for example exp (x1 + x2 + ··· + xn) , etc., the maximum value Max _{{R F- 1} , R _F−2 ,..., R _F−N } are differentiable functions and are effective as evaluation parameters for efficiently selecting variables.

(Ninth embodiment)
FIG. 10 shows an example of phase arrangement according to the ninth embodiment of the present invention. When the dispersion value is very small, for example, when WDM transmission is performed in the vicinity of the zero dispersion band, the phase shift due to phase mismatch is small, so the phases of all channels are adjusted to be the same phase at the fiber input end. If the phases are arranged, the FWM has a quadrature phase relationship with the main signal, and waveform deterioration can be reduced. Since positive and negative may be irrelevant, some channels may be in reverse phase as shown by the broken line in FIG. In this case, the calculation result of the fluctuation range of the photocurrent value by FWM is indicated by a circle in FIG. The number of WDM wavelengths is five waves. The calculation was performed by changing the dispersion value D [ps / nm / km] with φ ₁ = φ ₂ = φ ₃ ,..., φ _N = 0. In addition, the fluctuation range of the photocurrent when the phase was not adjusted was plotted with ● marks. As can be seen from the figure, when the dispersion value is very small, the fluctuation range of the photocurrent is suppressed to about −20 dB, and a large improvement effect is expected by comparing the circles. The number of channels was 10 wavelengths, and the input optical power was 0.1 mW to give an absolute value.

  FIG. 11 shows that the fluctuation range of light due to the FWM suddenly increases around the dispersion value D = 0.2 to 1, and it is necessary to optimize the phase of each channel in the fiber input WDM signal. Tables 1 to 3 show calculation examples of the optimum phase arrangement when the dispersion value is set to 1 ps / nm / km and the number of WDM channels is 3 to 5 waves. This calculation example is a result of calculation using the calculation algorithm shown in the third embodiment, and the phase angle is optimized and calculated with a step width of 15 degrees. FIG. 12 shows the dispersion value dependence of the maximum value and the minimum value of the in-phase crosstalk amount due to the optical phase arrangement of each channel at the WDM wavelength number of 5 wavelengths. Since this phase arrangement depends on the propagation constant of the transmission line fiber, the channel interval of the transmitted light, etc., it is actually necessary to calculate with the algorithm according to the present invention according to these values.

(Tenth embodiment)
FIG. 13 shows an example of phase arrangement according to the tenth embodiment of the present invention. In this embodiment, the WDM signal light is divided into several blocks, and waveform degradation due to FWM crosstalk is suppressed using the initial phase control according to the present invention in each block. This is effective when it is difficult to realize phase synchronization for all channels of WDM transmission light and to calculate the optimum phase arrangement. Since FWM occurs between the blocks, a guard band can be provided between the blocks in such a case. Moreover, in order to suppress the non-linear interaction between the blocks, the polarizations may be orthogonalized for each block.

  While the present invention has been described with respect to several specific embodiments, the embodiments described herein are merely illustrative in view of the many possible embodiments to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. The configuration and details of the embodiment exemplified here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a figure which shows the structural example of the transmitter of the wavelength multiplexing light by the 1st Embodiment of this invention. It is a figure which shows the structural example of the relay node of the wavelength division multiplexing light by the 2nd Embodiment of this invention. It is a figure which shows the structural example of the in-phase crosstalk suppression method of the wavelength division multiplexing light by the 3rd Embodiment of this invention. It is a figure which shows the structural example of the in-phase crosstalk suppression method of the wavelength division multiplexing light by the 4th Embodiment of this invention. It is a figure which shows the structural example of the in-phase crosstalk suppression method of the wavelength division multiplexing light by the 5th Embodiment of this invention. It is a figure which shows the structural example of the in-phase crosstalk estimation method of the wavelength division multiplexing light by 6th Embodiment of this invention. It is a figure which shows another structural example of the in-phase crosstalk estimation method of the wavelength division multiplexing light by 6th Embodiment of this invention. It is a figure which shows the structural example of the in-phase crosstalk suppression method of the wavelength division multiplexing light by the 7th Embodiment of this invention. It is a figure which shows the structural example of the in-phase crosstalk suppression method of the wavelength division multiplexing light by the 8th Embodiment of this invention. It is a figure which shows the example of the phase arrangement | positioning by the 9th Embodiment of this invention. It is a figure which shows the calculation result by the 9th Embodiment of this invention. It is a figure which shows another calculation result by the 9th Embodiment of this invention. It is a figure which shows the example of the phase arrangement | positioning by the 10th Embodiment of this invention.

Explanation of symbols

12, 22 Wavelength separation filter 14 Modulator 16, 26 Phase adjustment unit 18, 28 Multiplexer 116 Selector 202 Phase adjustment unit 204 Multiplexer 206 Wavelength separation filter 208 Measurement unit

Claims (12)

An optical transmitter that multiplexes and transmits signal light of a plurality of channels,
An optical transmitter comprising optical phase adjusting means capable of individually adjusting the phase of the signal light of the plurality of channels. The optical transmitter according to claim 1, wherein
In-phase crosstalk determining means for determining in-phase crosstalk by four-wave mixing of each channel;
An optical transmitter comprising: in-phase crosstalk reducing means for obtaining a phase of signal light of each channel so that a maximum value of in-phase crosstalk of all channels is reduced. The optical transmitter according to claim 2, wherein
The in-phase crosstalk reducing means obtains the phase and polarization of signal light of each channel so that the maximum value of all in-phase crosstalk is reduced. The optical transmitter according to claim 2 or 3,
The optical transmitter according to claim 1, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by calculation. The optical transmitter according to claim 4, wherein
The in-phase crosstalk determining means uses transmission line loss coefficient, chromatic dispersion, dispersion slope, nonlinear coefficient, wavelength of signal light, interval between channels as parameters, and phase of signal light of each channel as a variable. An optical transmitter characterized by calculating in-phase crosstalk by four-wave mixing by calculation. The optical transmitter according to claim 2 or 3,
The optical transmitter according to claim 1, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by measurement at a receiving end. The optical transmitter according to any one of claims 2 to 6,
The common-mode crosstalk reducing means selects one or more channels having a large value from the common-mode crosstalk of all channels, and determines the relationship between the common-mode crosstalk of the selected one or more channels and the phase of the signal light of each channel. For each of the one or more selected channels, a phase range of the signal light of each channel that reduces in-phase crosstalk is obtained using the continuous function to represent the phase range common to the one or more selected channels An optical transmitter characterized in that: The optical transmitter according to claim 1, wherein
An optical transmitter characterized in that the phase of the signal light of the plurality of channels is adjusted to the same phase or opposite phase by the optical phase adjusting means. A relay node that relays wavelength multiplexed light obtained by wavelength multiplexing signal light of a plurality of channels,
A relay node comprising optical phase adjusting means capable of individually adjusting the phase of the signal light of the plurality of channels. The relay node according to claim 9, wherein
In-phase crosstalk determining means for determining in-phase crosstalk by four-wave mixing of each channel;
A relay node comprising: common-mode crosstalk reduction means for obtaining a phase of signal light of each channel so that a maximum value of common-mode crosstalk of all channels is reduced. The relay node according to claim 10, wherein
The relay node according to claim 1, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by calculation. The relay node according to claim 10, wherein
The relay node according to claim 1, wherein the in-phase crosstalk determining means obtains in-phase crosstalk by four-wave mixing of each channel by measurement at a receiving end.

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