JP2007300496A - Optical transmitter, optical relay, optical transmission system and optical transmission method in wavelength division multiplex transmission - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce signal waveform deterioration generated by a nonlinear optical effect in wavelength division multiplex transmission. <P>SOLUTION: The optical transmitter in a wavelength division multiplex transmission system has a light source which generates light with a plurality of wavelength in which phases of light are synchronized and a compensation means for compensating waveform deterioration of the light of each wavelength generated by the nonlinear optical effect of a transmission path. The compensation means estimates the waveform deterioration on the receiving end to compensate the waveform deterioration based on transmission path parameters such as the transmission parameters. Otherwise, the compensation means estimates the waveform deterioration generated in the transmission path by the nonlinear optical effect by using a transmission data pattern of other wavelength. Furthermore, the compensation means estimates phase variation generation in the transmission path by the nonlinear optical effect based on optical field intensity to compensate the waveform deterioration by the phase variation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength division multiplexing optical modulation method used in a wavelength division multiplexing (WDM) transmission system that multiplexes signal light of a plurality of wavelengths into the same optical fiber to realize a large capacity of optical communication.

  In a conventional wavelength division multiplex transmission system in optical communication, the optical transmission unit prepares lasers each oscillating different wavelengths, for example, distributed feedback laser diodes for the number of channels individually, and each has an individual external optical modulator. The light of each wavelength is modulated by the transmission data series and transmitted. Also, the optical receiver demultiplexes the wavelength-multiplexed signal light into wavelength-modulated signal light using a wavelength filter or the like, and individually demodulates and detects the signal light of each wavelength. As a modulation / demodulation method, an intensity modulation-direct detection method in which 0 and 1 of a transmission data signal are encoded and transmitted as light intensity, a phase modulation method in which 0 and 1 of a transmission data signal are encoded and transmitted as an optical phase, There is a delay phase modulation system that encodes 0 and 1 of a transmission data signal as a phase change of light. Increasing the transmission capacity per optical fiber by increasing the number of wavelength division multiplexing of WDM signals, narrowing the wavelength interval and increasing frequency utilization efficiency is important in constructing a wavelength division multiplexing transmission system economically become.

In general, an optical fiber has dispersion in which a propagation constant varies with frequency. Due to this dispersion, the signal light pulse waveform after propagation through the optical fiber spreads, and signal waveform deterioration due to intersymbol interference occurs on the receiving side. Further, in an optical fiber, nonlinear optical effects such as four-wave mixing (FWM), cross phase modulation (CPM), and self phase modulation (SPM) occur due to the optical Kerr effect whose refractive index changes in proportion to the light intensity. . For example, the FWM uses light of three different wavelengths (optical frequencies: ω ₁ , ω ₂ , ω ₃ ) to generate optical frequencies ω ₄ = ω ₁ + ω ₂ −ω ₃ , ω ₃ + ω ₁ −ω ₂ , ω ₂ + ω _3. -ω new optical (idler light) ₁ is a phenomenon that occurs. In the wavelength division multiplexing transmission system, optical signals are arranged on an optical frequency grid set at equal intervals, so that the optical frequency of idler light generated by the FWM matches the optical frequency of other channels, and the signal light is received on the receiving side. Cause waveform degradation. The SPM in which the light intensity of the own wavelength causes the phase change of the own wavelength and the CPM in which the light intensity of the other wavelengths causes the phase change of the own wavelength cause the pulse time width of the signal light to change, and the signal light on the receiving side Cause waveform degradation. In view of this, a method has been proposed in which a transmission waveform that cancels after dispersion the dispersion due to optical fiber transmission at one wavelength and the effect of SPM is electrically calculated and the waveform is encoded into signal light (Non-patent Document 1). reference).

Kim Roberts, et. Al., “Electric Precompensation of Optical Nonlinearity”, Photonics Technology Letters, Vol.18, NO.2, pp.403-405 Nonlinear Fiber Optics 3rd-edition, G.P.Agrawal, Academic Press (2001)

  In the wavelength division multiplexing transmission, in addition to the SPM that is the nonlinear optical effect of the self-wavelength signal light, the FWM and CPM that are the nonlinear optical effects from the other wavelength signal light also cause signal waveform deterioration. In the transmission method that cancels the influence of dispersion and SPM as described above, signal waveform deterioration due to FWM or CPM in wavelength division multiplexing transmission cannot be reduced. Therefore, if the wavelength division multiplexing transmission system is designed so that the influence of FWM and CPM is reduced, the number of wavelength multiplexing and the wavelength interval are limited.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a wavelength division multiplexing optical transmission system that reduces deterioration of a signal waveform due to a nonlinear optical effect in wavelength division multiplexing transmission. .

  In order to achieve such an object, the present invention provides an optical transmitter that modulates a plurality of wavelengths of light and multiplexes and transmits the light, and a plurality of optical phases synchronized with each other. A light source that generates light of a wavelength and a compensation unit that compensates for waveform deterioration of the light of each wavelength generated by the nonlinear optical effect of the transmission line are provided.

  According to a second aspect of the present invention, in the optical transmitter according to the first aspect, the compensating means compensates for waveform deterioration based on a transmission path parameter of the transmission path.

  According to a third aspect of the present invention, there is provided the optical transmitter according to the first or second aspect, wherein the compensating means transmits a transmission data sequence of light of another wavelength in compensating for waveform deterioration of light of one wavelength. Based on the above, the waveform deterioration is compensated.

  According to a fourth aspect of the present invention, in the optical transmitter according to any one of the first to third aspects, the compensating means compensates for waveform degradation based on a nonlinear propagation equation including the transmission path parameter. It is characterized by.

  According to a fifth aspect of the present invention, in the optical transmitter according to any one of the first to fourth aspects, the compensation means corresponds to the transmission data pattern for the light of the plurality of wavelengths and the transmission data pattern. Storage means that associates and stores the compensation waveform of each wavelength to be read, and the compensation means reads the compensation waveform of each wavelength from the storage means according to the transmission data pattern, and performs waveform deterioration of each wavelength. It is characterized by compensating.

  According to a sixth aspect of the present invention, in the optical transmitter according to any one of the first to third aspects, the compensating means performs waveform degradation due to a phase change of light generated by a nonlinear optical effect of the transmission path. It is characterized by compensating.

  The invention according to claim 7 is the optical transmitter according to claim 6, wherein the compensating means modulates and multiplexes light of a plurality of wavelengths with respective transmission data, and then transmits the respective transmission data. The waveform deterioration due to the phase change of the multiplexed light is compensated based on the pattern.

  According to an eighth aspect of the present invention, in the optical transmitter according to the sixth aspect, the compensation means modulates and multiplexes light of a plurality of wavelengths with respective transmission data, and then transmits the multiplexed light. The waveform deterioration due to the phase change of the multiplexed light is compensated based on the optical electric field intensity.

  The invention according to claim 9 is an optical transmission system including the optical transmitter according to any one of claims 1 to 9.

  According to a tenth aspect of the present invention, there is provided an optical repeater that relays multiplexed optical signals in which light of a plurality of wavelengths is modulated, and the multiplexed light based on the optical electric field strength of the multiplexed light. It is characterized in that the waveform deterioration due to the phase change is compensated.

  The invention according to claim 11 is a method for modulating and multiplexing a plurality of wavelengths of light, generating a plurality of wavelengths of light whose phase is synchronized, Compensating for waveform deterioration of light of each wavelength generated by the nonlinear optical effect.

  According to the present invention, by reducing signal waveform deterioration due to nonlinear optical effects in an optical fiber transmission line, the number of wavelength multiplexing and frequency utilization efficiency can be increased, and a wavelength division multiplexing transmission system can be constructed economically. Can do.

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

  In the first embodiment of the present invention, there is provided a method of compensating for the influence from other wavelengths due to the nonlinear optical effect that causes the signal waveform deterioration, using a carrier wave whose optical phase of each wavelength is temporally stable in wavelength division multiplex transmission. Show. Here, the optical phase of each wavelength is stable in time means that the optical phase of each wavelength is synchronized, and the difference between the optical phase of one wavelength and the optical phase of another wavelength is constant. It is a state in which the light phase is shifted stably and the timing at which the optical phases coincide is a constant period. That is, the two optical phases coincide with each other in the 1 / (f1-f2) period with respect to the light having the optical frequencies f1 and f2. In other words, when light of different wavelengths is received by the optical receiver, a photocurrent component (beat component) that oscillates due to the difference between the two optical frequencies is generated. A state where the frequency and phase of the beat component are stable is called a state where the optical phases of the respective wavelengths are synchronized.

  FIG. 1 shows a configuration example according to a first embodiment of the wavelength division multiplexing optical transmission system of the present invention. As shown in FIG. 1A, a wavelength division multiplexing transmission system 100 includes an optical transmitter 200 that multiplexes and transmits a plurality of wavelengths of modulated signal light, an optical fiber transmission line 300, and a plurality of wavelengths of modulated signal light. And an optical receiver 400 that receives and demodulates. In this wavelength division multiplexing transmission system, as shown in FIG. 1B, the light generated from the light sources having a plurality of wavelengths of the optical transmitter 200 includes k, k + 1, k + 2, and k + 3 carriers having a wavelength multiplexing number m and a frequency axis. It is assumed that the optical phases of the respective wavelengths are synchronized with each other at equal intervals.

Four-wave mixing is a phenomenon in which new light is generated by optical frequency mixing of three or two lights having different frequencies. For example, as shown in FIG. 1B, channels k-1, k, and k + 1 If the optical frequencies are ω _k−1 , ω _k , and ω _{k + 1} , idler light is generated at ω _k + ω _{k + 1} −ω _k−1 = ω _{k + 2} by frequency mixing between these signal lights. The optical phase ψ _{k + 2} of the generated idler light is expressed as ψ _{k + 2} = φ _k-1 + φ _k − using the optical phases φ _k−1 , φ _k , φ _{k + 1} of the signal lights of the channels k−1, k, k + 1. It is expressed as φ _{k + 1} . When the optical phases between the carrier waves are not synchronized, the optical phases φ _{k + 2} and ψ _{k + 2} of the signal light transmitted at the optical frequency ω _{k + 2} have no correlation. Therefore, the idler light generated by the four-wave mixing and the channel k + 2 The optical amplitude and optical phase of the signal light generated by the interference of the signal light vary randomly. On the other hand, the amplitude and phase of the signal light generated by the interference are stabilized by locking the optical phase of the carrier wave of each wavelength. Further, the compensation signal light having the same amplitude as that of the idler light generated by the four-wave mixing is added in advance to the signal light of ω _{k + 2} and transmitted, thereby canceling the crosstalk caused by the four-wave mixing on the receiving side. Is possible.

  Here, for the sake of simplicity, the description has been given focusing on one channel, but it will be understood that the effect of waveform deterioration due to four-wave mixing can be canceled in the same manner for other channels as well. However, it is necessary to compensate in consideration of the influence of the added compensation light on other channels. In addition, by locking the optical phase between the carrier waves of each wavelength, crosstalk between channels due to self-phase modulation and cross-phase modulation can also be predicted, thus compensating for all nonlinear optical effects in the optical fiber transmission line. Can do. Although the pre-compensation method using the optical transmitter has been described here, it is also possible to estimate and compensate for changes in the optical amplitude and optical phase waveform received during optical fiber transmission from other channels by the optical receiver. It is.

  Hereinafter, a configuration example in which the compensation signal light is superimposed on the signal light will be described using the intensity modulation-direct detection method as an example. FIG. 2 shows an example of the configuration. In FIG. 2, an optical transmitter 200 modulates a carrier wave of each channel with transmission data to generate signal light, and modulates a carrier wave of each channel with a modulation signal for compensation to generate a compensation signal light. , A compensation signal calculation circuit 230 that generates a modulation signal for compensation from transmission data of each channel, a phase shifter 240 that adjusts the phase of the compensation signal light of each channel, and a signal of each channel And an optical multiplexer 250 that combines and outputs the light and the compensation signal light.

Here, a configuration example in which the four-wave mixed light of ω _k−1 , ω _k , and ω _{k + 1} superimposed on ω _{k + 2} is compensated by paying attention to the signal light of frequency ω _{k + 2} is shown. When the transmission data d _k−1 , d _k , and d _{k + 1} are (1, 1, 1), four-wave mixed light is generated at the frequency ω _{k + 2} . Therefore, the compensation signal calculation circuit 230 reads the data patterns of the channels k−1, k, k + 1, and drives the ω _{k + 2} compensation optical modulator 220 based on the values to generate compensation signal light. The optical phase of the compensation signal light is adjusted in advance by the phase shifter 240 (k + 2) so that the optical phase is inverted with respect to the idler light generated by the four-wave mixing, and is combined with the signal light of ω _{k + 2} by the optical multiplexer (250). To wave. Here, an example in which light generated at the frequency ω _{k + 2} is compensated by the frequencies ω _k−1 , ω _k , and ω _{k + 1} is shown, but the four-wave mixed light superimposed on the frequency ω _{k + 2} is shown in FIG. It occurs even when the frequency combination is different. Therefore, by generating the compensation signal light shown in FIG. 2 for each four-wave mixed light, it is possible to further reduce the influence due to nonlinear waveform deterioration. Further, the transmission path parameter used in the compensation signal calculation circuit 230 is unique to each optical fiber and fluctuates with time. Therefore, the compensation signal calculation circuit 230 of the optical transmitter 200 has a desired waveform. It is also possible to generate a compensated transmission waveform by performing feedback control.

  In the present embodiment, a desired received signal waveform is given as a final state in a nonlinear propagation equation using a transmission line parameter depending on an optical fiber transmission line as a coefficient, using a carrier wave in which the optical phase of each wavelength is synchronized. Thus, a transmission waveform for suppressing nonlinear waveform deterioration is obtained, and based on this, the optical amplitude and optical phase of each wavelength are controlled to generate and transmit a precompensated transmission signal waveform.

The operation of this embodiment will be described with reference to FIG. As shown in FIG. 3, the optical transmitter 200 according to this embodiment includes a modulator 210 that generates a pre-compensated transmission signal light by modulating a carrier wave of each channel with a modulator driving signal, and transmission data of each channel. And a waveform calculation circuit 232 that generates a modulator drive signal that is compensated based on the nonlinear propagation equation. In this optical transmitter 200, light of a plurality of wavelengths λ ₁ , λ ₂ , λ ₃ ,... Λ _m whose frequency and initial phase are stable in time are input to each optical modulator 210 as a carrier wave. In the optical modulator 210 of each wavelength, the transmission data patterns d _{1, n} , d _{2, n} , d _{3, n} ,... _{Dm, n} assigned to each channel are changed to the wavelengths λ ₁ , λ ₂ , λ _3. , ... lambda-phase phase component of the carrier of the optical amplitude and the calculated modulator drive signal waveform calculation circuit 232 for encoding the light phase of the _{m M i1, M i2, M} i3, ... M im and, quadrature phase The components M _q1 , M _q2 , M _q3 ,... M _qm are input. In the optical modulator 210, each wavelength _{λ 1, λ 2, λ 3} , ... λ light of _m-phase phase component of the modulator drive signal _{M i1,} M _{i2, M} i3, a ... _{M im,} quadrature phase component M _{q1, M q2, M} q3, by ... _{M qm,} its amplitude and phase are modulated, and output to the transmission path optical fiber 300 as a transmission signal light.

  When the optical phase between the carrier waves of the transmission signal light is synchronized, the waveform after optical fiber transmission can be calculated by giving the optical amplitude and optical phase of the optical transmitter to the nonlinear propagation equation as boundary conditions . Similarly, the waveform on the transmission side can be calculated by giving the optical amplitude / phase waveform on the reception side as the boundary condition of the nonlinear propagation equation. Therefore, by giving a desired waveform as the optical amplitude / phase waveform of the optical receiver and calculating, it is possible to obtain a transmission waveform in which the waveform change received in the optical fiber transmission line is pre-compensated.

In recent years, optical fiber transmission simulation technology has been improved, but a general method is to obtain a waveform of a receiver by giving a waveform of a transmitter as a boundary condition. In order to obtain a pre-compensated transmission waveform using this existing simulation technique, the transmission path parameter used for the simulation may be calculated by applying a negative sign. The principle is shown below. The nonlinear propagation equation is expressed by the following equation as photoelectric field A, distance z, time t, nonlinear coefficient γ, dispersion β ₂ , dispersion slope β ₃ , and loss coefficient α (see page 49 of Non-Patent Document 2). .

Here, T is the time when the center of the optical waveform propagating at the group velocity vg is time 0.

  Next, the length of the transmission line optical fiber is set to L, and a new variable z ′ is introduced as z ′ = L−z. At this time, A (z ′) satisfies the following nonlinear equation.

Since z ′ = 0 and z ′ = L correspond to z = L and z = 0, respectively, A (z ′ = 0) and A (z ′ = L) respectively represent waveforms at the receiver and the transmitter. To express. Equation (2) matches the equation obtained by multiplying the transmission line parameters γ, β ₂ , β ₃ , and α of Equation (1) by −1. Therefore, a desired waveform can be given as A (z ′ = 0), and the pre-compensated transmission waveform can be obtained using the existing transmission line simulator technology with the transmission line parameters γ, β ₂ , β ₃ , α as negative signs. it can. γ, β _2, β _3, obtained in advance of the transmission path parameters, such as alpha, the transmission data pattern _{d 1, n, d 2,} n, d 3, n, ... d m, depending on _n, for example Split Using the Step Fourier method (see page 51 of Non-Patent Document 2), a modulator drive signal for generating a pre-compensated transmission waveform can be obtained from Equation (2). When the output of the waveform calculation circuit 232 is a digital signal, modulator driving signals M _i1 , M _i2 , M _i3 ,... M _im and M _q1 , M _q2 , M _{q3,. qm} can be generated and the optical amplitude and optical phase of the carrier wave of each wavelength can be modulated.

As a method for modulating the optical amplitude and optical phase of the carrier wave of each wavelength from the modulated signals M _i1 , M _i2 , M _i3 ,... M _im and M _q1 , M _q2 , M _{q3 ,.} There is a method of obtaining arbitrary amplitudes and phases by modulating the amplitudes of the quadrature phase components and combining the respective modulated waveforms. A configuration example of such a modulator is shown in FIG. This optical modulator 210 is configured such that Mach-Zehnder interferometer type intensity modulators 212 and 214 are arranged in respective optical paths branched into two, and each modulator can be independently modulated with different drive signals. . The in-phase component and the quadrature component of the transmission waveform obtained from the equation (2) are converted into modulator drive signals M _i1 , M _i2 , M _i3 ,... M _im and M _q1 , M _q2 , M _{q3 ,.} And input to the optical modulators 212 and 214, respectively. By recombining the two optical paths through the phase shifter 216 so that the phase difference between the two optical paths is π / 2 ± nπ (n: integer), the in-phase component and the quadrature component of the pre-compensated transmission waveform can be obtained. It is possible to generate signal light having the same.

  FIG. 5 shows signal waveforms in each part of the wavelength division multiplex transmission system. FIG. 5 shows an example in which the number of wavelengths is 4, the bit rate is 10 Gbps, the channel interval is 25 GHz, and the transmission wavelength is in the vicinity of the zero dispersion wavelength of the optical fiber. FIG. 5A shows the data series of channel 4, FIG. 5B shows the in-phase component of the transmission waveform of channel 4, and FIG. 5C shows the quadrature component. FIG. 6 shows a received waveform after transmission of an 80 km optical fiber. FIG. 6A shows a received waveform in which the waveform deterioration due to the nonlinear optical effect and dispersion is compensated and the eyes of the eye pattern are opened. For comparison, FIG. 6B shows a received waveform when compensation according to the present invention is not performed.

When the transmission waveform is obtained, the group velocity varies depending on the channel due to chromatic dispersion. Therefore, the influence from other channels due to the nonlinear optical effect is caused by the transmission data pattern d _{1, n} , d _{2, n} , d _{3, n} to be transmitted. ,..., D _{m, n} before and after several bits of transmission data patterns can be used for the calculation.

  Also, since the transmission path parameter is unique to each optical fiber and fluctuates with time, the waveform calculation circuit of the optical transmitter is feedback-controlled so that the received waveform becomes a desired waveform, thereby generating a transmission waveform. You can also

In the second embodiment of the present invention, it is necessary to calculate the transmission waveform of each wavelength according to the transmission data pattern in real time. However, when the bit rate of transmission data increases, it is necessary to increase the calculation speed. Therefore, in this embodiment, a transmission waveform corresponding to a combination of bit patterns of a plurality of channels is calculated in advance using transmission path parameters such as γ, β ₂ , β ₃ , α, and the transmission waveform is stored in a waveform data table. A method is shown in which the transmission waveforms that have been stored and matched with the waveform data table in accordance with the combination of bit patterns are extracted.

This embodiment will be described with reference to FIG. The transmission data patterns d _{1, n} , d _{2, n} , d _{3, n} ,... _{Dm, n} recognize the transmission data pattern of each channel to be transmitted by the data pattern matching circuit 234, and according to the data pattern combination. A pointer signal P _i to the transmission waveform in the memory is output. The waveform data memory _{236, γ, β 2, β} 3, pre-computed transmit waveform using a transmission line parameters, such as α is are stored respectively pointer P _i. Transmission waveform corresponding to the pointer signal P _i that is input from the verification circuit 234 or modulator drive waveform is outputted from the waveform data memory 236, the modulator 210 of each wavelength are driven.

When obtaining the transmission waveform, the group velocity differs depending on the channel due to chromatic dispersion, and therefore the influence from other channels due to the nonlinear optical effect is caused by the transmission data pattern d _{1, n} , d _{2, n} , d _{3, Since} it is necessary to use a transmission data pattern of several bits before and after _{n 1} ,... dm _{, n} for calculation, the data pattern verification circuit also recognizes a data pattern of several bits before and after and outputs a pointer signal P _ij to the waveform data table. You may make it do. In this case, the transmission waveform that suppresses nonlinear waveform deterioration accumulated in the waveform data table is calculated in advance in consideration of a data pattern of several bits before and after, and is sequentially generated according to the pointer signal P _ij. .

  Also, since the transmission path parameter is unique to each optical fiber and fluctuates with time, the transmission waveform can be generated by feedback controlling the data table of the transmitter so that the received waveform becomes a desired waveform. it can.

Next, some specific examples according to this embodiment will be described. FIG. 8 is an example of a transmitter according to this embodiment. Referring to FIG. 8, a short wavelength laser light source is used as light source 202, and light of a plurality of wavelengths is generated by optical modulator 206 driven at frequency f. The generated light of a plurality of wavelengths has a synchronized frequency mode at a wavelength interval c / f (c is the speed of light). The light of a plurality of wavelengths is distributed to the optical path for each wavelength by the optical demultiplexer 208 and input to the modulators 210 arranged in parallel. The data pattern matching circuit 234 recognizes the transmission data patterns d _{1, n} , d _{2, n} , d _{3, n} ,... _{Dm, n} of each channel to be transmitted, and looks up the table according to the data pattern combination. A pointer signal P _ij to the transmission waveform in 236 is output. In the lookup table 236, transmission waveforms calculated in advance using transmission parameters such as γ, β ₂ , β ₃ , α and the like are stored for the pointer P _ij . The modulator driving waveform corresponding to the pointer signal P _ij input from the verification circuit 234 is converted by the digital-analog converter, and then the modulator 210 of each wavelength is driven. The modulator driving waveform is composed of modulation signals M _i and M _q that respectively modulate the amplitudes of the in-phase component and the quadrature component of the light of each wavelength.

Arbitrary amplitude and phase can be obtained by combining the waveforms modulated by the modulation signals M _i and M _q , respectively, and the transmission waveform can be encoded on the carrier wave of each wavelength. The encoded carrier wave of each wavelength is multiplexed by the multiplexer 250 and output.

  Even if the frequency and the initial phase between the wavelengths until the light is split from the light source 202 are stable in time, the optical path length of each wavelength varies due to thermal and mechanical fluctuations, and beats of different wavelengths The signal may become unstable in time on the output side. Therefore, a beat signal between carriers of different wavelengths can be stabilized on the output side by using a phase lock loop circuit, a voltage controlled oscillator (VCO), and a phase shifter. Such an example is shown in FIG.

Referring to FIG. 9, a part of the WDM signal light from optical multiplexer 250 is input to phase-locked loop circuit 262 by optical demultiplexing means 260, and the other is output as WDM signal light. The WDM signal light input to the phase-locked loop circuit 262 is demultiplexed into two wavelengths or multiple wavelengths including adjacent wavelengths in the phase-locked loop circuit, and each is optically / electrically converted and oscillates with the optical frequency difference between them. Get beat signal. By frequency-mixing the beat signal component and the component oscillating with the data clock, the difference between the beat component phase Δφ ₁ , Δφ ₂ ,... Δφ _m and the data clock phase θ (Δφ ₁ −θ), (Δφ ₂ −θ),... (Δφ _m −θ) is detected. These detected phase difference signals are input to the VCO 264 to change the phase shift amount Φ of the phase shifter 266 by −ΔΦ ₁ , −ΔΦ ₂ ,... −ΔΦ _m . Phase shift amount changes ΔΦ ₁ , ΔΦ ₂ ,... ΔΦ _{m work so} that Δφ ₁ −θ = 0, Δφ ₂ −θ = 0,... Δφ _m −θ = 0, and a signal of a certain wavelength and its adjacent wavelength. It is possible to temporally stabilize the phase difference between the beat signal obtained by detecting light and the data clock.

  In addition to the above method, by integrating the duplexer 208 on the input side to the multiplexer 250 on the output side, fluctuations in the thermal and mechanical optical path length can be reduced, and the beat signal and The phase difference of the data clock can be stabilized in time.

The nonlinear phenomenon in the optical fiber is a phenomenon in which phase modulation proportional to the square of the optical electric field strength occurs. Furthermore, this phase modulation also changes the waveform amplitude through the chromatic dispersion of the optical fiber. Therefore, using the transmission data of a plurality of channels, the optical electric field intensity | A (t, z = 0) | ² during the optical fiber transmission is calculated, and the phase modulation opposite to the nonlinear phase modulation received in the optical fiber transmission line is performed. The phase is modulated by the transmitter and transmitted. FIG. 10 shows the basic configuration. Referring to FIG. 10, the carrier wave of each wavelength is modulated using the respective transmission data. The modulated optical signal is multiplexed by the multiplexer 250 and then subjected to phase modulation of γ | A (t, z = 0) | ² L _eff using the phase modulator 270 and transmitted. Here, γ is the nonlinear coefficient of the optical fiber, L _eff is the effective fiber length, and L _eff is adjusted so that the received signal quality of the receiver becomes high.

Further, as shown in FIG. 11, since a signal obtained by photoelectrically converting a part of the modulated signal light is proportional to the square of the optical electric field intensity, this is used as a drive signal for the phase modulator 270. May be used. Here, the phase modulation signal generation unit 276 multiplies the signal of | A (t, z = 0) | ² obtained by photoelectric conversion of the signal light using the values of γ and L _eff as coefficients, A phase modulator drive signal is generated. The values of γ and L _eff are adjusted in advance so that the received signal quality of the receiver becomes high.

  Further, as shown in FIG. 12, a part of the output light of the linear amplifier 320 is extracted for each linear relay node, and the signal obtained by the photoelectric conversion 344 can be used as a drive signal for the compensation phase modulator 340. . Thereby, the nonlinear phase modulation in the fiber transmission line 350 can be compensated. In the above method, the nonlinear compensation effect can be enhanced by using an optimum frequency filter for the phase modulator drive signal.

  Although the present invention has been described specifically with reference to several embodiments, the embodiments described herein are merely illustrative in view of many possible forms to which the principles of the present invention can be applied. It does not limit the scope of the present invention. The embodiments illustrated herein can be modified in configuration and details 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.

FIG. 1A is a schematic diagram of a wavelength division multiplexing transmission system, and FIG. 1B is a diagram for explaining compensation of four-wave mixing. FIG. 1 is a schematic diagram showing a configuration example of a transmitter according to a first embodiment of the present invention in a wavelength division multiplexing transmission system. It is the schematic which shows the structural example of the transmitter by the 2nd Example of this invention in a wavelength division multiplexing transmission system. It is the schematic which shows the structural example of the modulator in the transmitter of the wavelength division multiplexing transmission system by this invention. FIG. 5A is a diagram showing signal waveforms in each part of the wavelength division multiplex transmission system according to the present invention, FIG. 5A is a data pattern of a certain channel in the transmitter, and FIG. 5B is an in-phase modulation signal in the modulator of the transmitter. FIG. 5C shows the quadrature phase component of the modulation signal in the modulator of the transmitter. FIG. 6A shows a received waveform after optical fiber transmission in a wavelength division multiplex transmission system, FIG. 6A shows a received waveform when the waveform deterioration according to the present invention is compensated, and FIG. 6B does not compensate the waveform deterioration. Is the received waveform. It is the schematic which shows the structural example of the transmitter by the 3rd Example of this invention in a wavelength division multiplexing transmission system. It is the schematic which shows the specific structural example of the transmitter by the 3rd Example of this invention in a wavelength division multiplexing transmission system. It is the schematic which shows the specific structural example of the transmitter by the 3rd Example of this invention in a wavelength division multiplexing transmission system. It is the schematic which shows the structural example of the transmitter by the 4th Example of this invention in a wavelength division multiplex transmission system. It is the schematic which shows the structural example of the transmitter by the 4th Example of this invention in a wavelength division multiplex transmission system. It is the schematic which shows the structural example of the transmitter by the 4th Example of this invention in a wavelength division multiplex transmission system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Wavelength division multiplexing transmission system 200 Optical transmitter 208 Optical demultiplexer 212, 214 Mach-Zehnder interferometer type intensity modulator 216 Phase shifter 250 Optical multiplexer 260 Optical demultiplexing means 330 Optical demultiplexing means

Claims (11)

In an optical transmitter that modulates and multiplexes light of a plurality of wavelengths,
A light source that generates light of a plurality of wavelengths whose light phases are synchronized;
An optical transmitter comprising: compensation means for compensating for waveform deterioration of light of each wavelength generated by a nonlinear optical effect in a transmission line. The optical transmitter according to claim 1,
The optical transmitter is characterized in that the compensation means compensates for waveform deterioration based on a transmission path parameter of the transmission path. The optical transmitter according to claim 1 or 2,
The compensation means compensates for waveform degradation based on a transmission data sequence of light of another wavelength when compensating the waveform degradation of light of one wavelength. The optical transmitter according to any one of claims 1 to 3,
The optical transmitter is characterized in that the compensation means compensates for waveform degradation based on a nonlinear propagation equation including the transmission path parameter. The optical transmitter according to any one of claims 1 to 4,
The compensation means includes storage means for storing a transmission data pattern for light of the plurality of wavelengths and a compensation waveform for each wavelength corresponding to the transmission data pattern in association with each other, and the compensation means includes the transmission data pattern In accordance with the optical transmitter, the compensation waveform of each wavelength is read from the storage means to compensate for the waveform deterioration of each wavelength. The optical transmitter according to any one of claims 1 to 3,
The optical transmitter is characterized in that the compensation means compensates for waveform deterioration due to a phase change of light generated by a nonlinear optical effect of the transmission path. The optical transmitter according to claim 6.
The compensation means modulates and multiplexes light of a plurality of wavelengths with respective transmission data, and then compensates for waveform deterioration due to a phase change of the multiplexed light based on the respective transmission data patterns. Optical transmitter. The optical transmitter according to claim 6.
The compensation means modulates and multiplexes light of a plurality of wavelengths with respective transmission data, and then compensates for waveform deterioration due to a phase change of the multiplexed light based on an optical electric field intensity of the multiplexed light. An optical transmitter.   An optical transmission system comprising the optical transmitter according to claim 1. In an optical repeater that modulates light of a plurality of wavelengths and relays multiplexed optical signals,
An optical repeater that compensates for waveform deterioration due to a phase change of the multiplexed light based on an optical electric field intensity of the multiplexed light. A method for modulating and multiplexing light of a plurality of wavelengths,
Generating light of multiple wavelengths whose phases are synchronized;
Compensating for waveform degradation of light of each wavelength generated by a nonlinear optical effect in a transmission line.

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