JP2008079131A - Compensation for crosstalk in wavelength multiplex transmission - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To compensate deterioration of a signal waveform due to a linear crosstalk in wavelength division multiplex transmission. <P>SOLUTION: This wavelength multiplex transmission system, which is one embodiment of this invention, is provided with a compensation portion for compensating deterioration due to a crosstalk from an adjacent channel, on the basis of a transfer characteristic from the adjacent channel to an own channel so that the deterioration due to the crosstalk from the adjacent channel is reduced. For example, this compensation portion reduces the deterioration due to the crosstalk from the adjacent channel by finding the inverse function of a transfer function from the adjacent channel to the own channel and adding it to a transmission optical signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to compensating for linear crosstalk in optical wavelength division multiplexing (WDM) transmission.

  In a conventional WDM transmission system in optical communication, each optical transmission device prepares lasers that oscillate at different wavelengths, for example, distributed feedback laser diodes for the number of channels individually, and uses individual external optical modulators. Is modulated by a transmission data sequence and transmitted. In addition, the optical receiver demultiplexes the optical signal wavelength-multiplexed using a wavelength filter or the like into an optical signal of each wavelength, and individually demodulates and detects the optical signal of each wavelength.

  As a modulation / demodulation method, an intensity modulation-direct detection method in which 0 and 1 of transmission data are encoded and transmitted as light intensity, a phase modulation method in which 0 and 1 of transmission data are encoded and transmitted as an optical phase, and transmission There is a delay phase modulation system that encodes 0 and 1 of data as a phase change of light.

  Here, narrowing the wavelength interval of the WDM signal, increasing the number of wavelength multiplexing, increasing the frequency utilization efficiency, and expanding the transmission capacity per optical fiber can economically construct a wavelength division multiplexing transmission system. It becomes important above.

M.J.Minardi and M.A.Ingram, “Adaptive crosstalk cancellation in dense wavelength division multiplexing network,” Electronics Letter, No.17, Vol.28, p.1621, August 1992

  In the conventional WDM transmission system as described above, when each wavelength is encoded with a high-speed transmission data sequence such as 10 Gbps or 40 Gbps, a spectrum of about a frequency band corresponding to a modulation speed by encoding is included in the optical signal of each wavelength. The upper spread occurs. Therefore, if the wavelength interval between channels is narrowed, the optical spectra of the respective wavelengths overlap each other, and the wavelength separation filter on the receiver side cannot sufficiently attenuate the optical signal of the adjacent channel. The optical signal of the own channel is affected and linear crosstalk occurs (Non-patent Document 1).

  Specifically, a beat that vibrates at the frequency difference is generated by the adjacent wavelength signal and the self-wavelength signal. Since light of each wavelength is output light from different semiconductor lasers, the phase relationship between these wavelengths is random. Therefore, the phase of the generated beat varies randomly with time. Thereby, beats are randomly superimposed on the eye pattern of the detection signal, the opening of the eye pattern is crushed, and transmission characteristics are deteriorated. In addition, adjacent wavelength signals are mixed according to the characteristics of the wavelength separation filter of the receiving apparatus, and the quality of the detection signal itself deteriorates. Thus, the occurrence of linear crosstalk between wavelengths limits the narrowing of the wavelength interval and limits the spectrum utilization efficiency in the WDM transmission system.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a technique for compensating for signal waveform degradation due to linear crosstalk in wavelength division multiplex transmission.

  In order to achieve the above object, according to the present invention, in the wavelength division multiplexing transmission system, the phase of the carrier wave of the second channel which is an adjacent channel of the first channel and the first channel is determined. In synchronism, the amount of deterioration of the signal of the second channel with respect to the signal of the first channel is estimated, and the amount is compensated in the transmission device or the reception device.

  According to a second aspect of the present invention, in the wavelength division multiplexing transmission system according to the first aspect, a carrier wave having the same optical frequency as that of the first channel is encoded by a transmission data sequence of the second channel, and Compensation light in which a transfer characteristic from the second channel transmitter to the first channel receiver is applied is added to the signal of the first channel in the transmission device or the reception device.

  According to a third aspect of the present invention, in the wavelength division multiplexing transmission system according to the second aspect, the compensation light further has an inverse characteristic of a transfer characteristic from a first channel transmitter to a first channel receiver. The applied compensation light is added to the signal of the first channel in the transmitter.

According to a fourth aspect of the present invention, in the wavelength division multiplexing transmission system according to the first or second aspect, the optical frequency of the first channel is ω _k , the optical frequency of the second channel is ω _k + Δω, and the second channel. When the transfer characteristic from the transmitter of the first channel to the receiver of the first channel is H (ω−ω _k ), the carrier wave having the same optical frequency as that of the first channel is encoded by the transmission data sequence of the second channel. Compensation light in which a transfer characteristic H (ω−ω _k + Δω) is further applied to the optical signal is added to the signal of the first channel in the transmission device or the reception device.

The invention according to claim 5 is the wavelength division multiplexing transmission system according to claim 1 or 2, wherein the optical frequency of the first channel is ω _k , the optical frequency of the second channel is ω _k + Δω, When the transfer characteristic from the 2-channel transmitter to the first-channel receiver is H (ω−ω _k ), a carrier wave having the same optical frequency as that of the first channel is further transmitted to the second-channel transmission data sequence, Modulation is performed using an electrical signal having a transfer characteristic H (ω−ω _k + Δω) applied thereto to generate compensation light, which is added to the signal of the first channel in the transmission device or the reception device.

  According to a sixth aspect of the present invention, in the wavelength division multiplexing transmission system according to the fifth aspect of the present invention, a first electrical signal obtained by adding a transmission data sequence of a first channel to the electrical signal is used. The carrier wave of the channel is modulated.

  According to a seventh aspect of the present invention, in the wavelength division multiplexing transmission system, the odd-numbered channel and the even-numbered channel are demultiplexed by a Mach-Zehnder delay interferometer having a free spectral interval twice as large as the channel interval. Only one channel is cut out and received using a bandpass filter.

  According to an eighth aspect of the present invention, in the wavelength division multiplexing transmission system according to the seventh aspect, the method according to any one of the first to sixth aspects compensates for waveform deterioration due to adjacent channel crosstalk.

  According to a ninth aspect of the present invention, in the wavelength division multiplexing transmission system according to the seventh aspect, the waveform degradation due to the next adjacent channel is compensated on the same principle as that described in the first to sixth aspects. Features.

  The invention described in claim 10 is a transmitter used in the wavelength division multiplexing transmission system described in claims 1-9.

  The invention described in claim 11 is a receiving apparatus used in the wavelength division multiplexing transmission system described in claims 1-9.

  The invention according to claim 12 is a method in a wavelength division multiplexing transmission system, wherein a phase of a carrier of a second channel which is an adjacent channel of the first channel and the first channel is synchronized, and the second The method includes estimating an amount by which a signal of a channel deteriorates with respect to the signal of the first channel, and compensating for the amount at a transmitting device or a receiving device.

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

  In the WDM transmission system, on the transmission device side, different transmission data sequences are placed on carriers having different frequencies (wavelengths), and these optical signals are combined and transmitted. On the receiving device side, the combined WDM signal is demultiplexed using a wavelength separation filter whose center wavelength of the transmission band matches the center wavelength of each channel, and the optical signal of each channel is received individually. At this time, if the wavelength (frequency) interval of each channel is reduced to about several times the bit rate of the transmission data sequence, the optical signal of the adjacent channel is not sufficiently suppressed in the wavelength separation filter, and the optical field of the adjacent channel is reduced. Mixed. Therefore, in the first embodiment of the present invention, an inverse matrix of the transmission matrix of the WDM transmission line or a compensation matrix similar thereto is obtained, and this inverse matrix is applied to the transmission optical signal, so that it can be detected from the adjacent channel at the receiving end. To reduce the influence of the mixing of the optical electric field.

  FIG. 1 shows a configuration example of a WDM transmission system according to a first embodiment of the present invention. This WDM transmission system includes a transmission device 110 that transmits a WDM signal, a transmission path 120 that propagates the WDM signal, and a reception device 130 that receives the WDM signal. The transmission apparatus 110 includes a phase-locked multi-wavelength light source 112 that generates a carrier wave whose phase is synchronized, M optical modulators 114-1 to 114 -M that modulate the carrier wave with a modulation signal, and an inverse function of a transfer function of a WDM transmission line. Are provided with a pseudo reverse transfer calculation unit 116 that generates a modulated signal for the transmission data series of each channel, and a wavelength multiplexer 118 that combines the modulated optical signals from the optical modulator. The receiving device 130 includes a wavelength separation filter 132 that separates the WDM signal from the transmission path for each wavelength of each channel, and M optical receivers 134-1 to M that receive the wavelength-separated optical signal. .

Here, each wavelength of the light output from the phase-locked multi-wavelength light source is synchronized in phase (that is, the optical phase of the carrier wave is synchronized). This means that the difference from the phase is stably shifted at a constant speed, and the timing at which the optical phases coincide is a constant period. That is, it means that the two optical phases coincide with each other with a period of 1 / (f ₁ −f ₂ ) sec for light having optical frequencies of f ₁ Hz and f ₂ Hz. In other words, when light of different wavelengths is received by the optical receiver, a photocurrent component (beat signal) that oscillates due to the difference between the two optical frequencies is generated. A state where the frequency and phase of the beat signal are stable is called a state where the optical phases of the respective wavelengths are synchronized.

  As a method of generating such a state in which the optical phases are synchronized, there is a method of generating light of other wavelengths from a single wavelength light source having high frequency stability and good coherence. Multi-wavelength light with synchronized optical phase can be obtained by generating other wavelengths by modulating single-wavelength light at a single frequency or by generating other wavelengths by nonlinear optical effects. be able to.

The WDM transmission system shown in FIG. 1 has M channels, and channel numbers are assigned in order from the channel with the lowest frequency. In this case, the optical electric field of a certain channel k, the transmission data sequence _{d k,} n (n is the bit number) dependent envelope function _{E k (t, d k,} n) E using k (t, d _{k, n} ) exp [iω _k t]. Here, the optical angular frequency of the carrier wave of channel k is ω _k = ω ₀ + kΔω, and the transmission data sequence of channel k is d _{k, n} . When this is Fourier transformed, the frequency spectrum ε _k (ω−ω _k ) is expressed by the following equation.

As shown in FIG. 1, this optical electric field that can be defined for each channel is transmitted after being frequency (wavelength) multiplexed and then demultiplexed and received. The transfer characteristic from the port of the k-th channel of the transmitting apparatus 110 to the port of the l-th channel of the receiving apparatus 130 is expressed as H _{l, k} (ω−ω _l ). The reception electric field E ′ _k (ω−ω _k ) of each channel is expressed by the following determinant.

  Here, let us consider a process in which a certain optical electric field spectrum F (ω) that has been Fourier transformed as a continuous function is subjected to inverse Fourier transform, sampled at a time interval ΔT, and Fourier transformed again. A time function f (t) obtained by inverse Fourier is expressed by the following equation.

A function F _s (ω) that is Fourier-transformed after the time function is sampled at the time interval ΔT is expressed by the following equation.

  Here, a part D (ω′−ω) other than F (ω ′) in the integral symbol will be considered. Since D (ω′−ω) is a periodic function of 4π / ΔT, the focus is on only the range of −2π / ΔT <ω′−ω ≦ + 2π / ΔT.

  First, paying attention to the range of 2π / ΔT <ω′−ω ≦ 0, the numerator is a vibration function having an amplitude of 1 with a period of 4π / (2N + 1) ΔT, and the following inequality holds.

  Therefore, the amplitude of D (ω′−ω) is an inversely proportional function of ω′−ω, and becomes a singular point where the denominator is zero in the vicinity of ω′−ω = 0, and the right side of the inequality of equation (5) is infinite. However, in other cases, it becomes a finite value equal to or less than the value in the vicinity of ω′−ω = 0. When the limit value at ω′−ω = 0 is obtained by returning to the equation (4), the following equation is obtained.

  On the other hand, in the range of 0 <ω′−ω ≦ + 2π / ΔT, the deformation can be made as follows.

  Similar to the above discussion, it becomes a singular point at ω′−ω = 2π / ΔT, and its limit is given by (2N + 1) where N is infinite, otherwise the amplitude is less than the limit value. It becomes. From the above, it can be seen that D (ω′−ω) can be treated as a delta function with a period of 2π / ΔT.

  Substituting this into equation (3) yields:

From the above discussion, when an optical electric field of a certain spectrum F (ω) is given, the Fourier spectrum F _s (ω) obtained by sampling this with a time interval ΔT in the time domain is obtained by changing F (ω) to 2π. It can be seen that the function is repeated at / ΔT.

In the following, a transmission coding method for compensating for crosstalk due to the frequency characteristic of the WDM transmission system represented by Expression (2) will be described. In digital communication, the value at the center of the bit observed at the receiving end is desired to be equal to the value transmitted from the transmitting end, so that the influence of adjacent crosstalk represented by the transfer matrix is suppressed at the receiving end. It is desirable. The outline of the method shown here is that a transmission spectrum vector multiplied by an inverse matrix of the matrix H (ω) shown in Expression (2) is transmitted. However, since the optical signal input from the transmission end port of the channel k includes a frequency other than ω _{k as} it is, it is not realistic. Therefore, it is necessary to compensate for the crosstalk from the adjacent channel in the value sampled at the bit center of the received signal, and to obtain a periodic function of 2π / ΔT which is a characteristic of the sampled function. the use, method of compensating for crosstalk only the signal of the frequency omega _k optical signal input from the transmission-end port of the channel k is derived. This is shown below. The Fourier transform F _{s, k} (ω−ω _k ) of each channel obtained by sampling Equation (2) at the center of the bit is a periodic function of Δω = 2π / ΔT, and thus is transformed as the following equation. be able to. However, here, it is assumed that the symbol rate (sampling rate) and the inter-channel frequency interval are equal.

Here, let the matrix of equation (10) be H. Since the vector of the optical electric field spectrum of each channel in the equation (10) has a spectrum distributed around ω ₀ as a center, a vector obtained by multiplying this vector by an inverse matrix of H or a compensation matrix H _C similar thereto is also ω. It becomes an optical electric field centered on _zero .

When the transmission signal electric field after compensation is obtained by applying _HC to the vector of the transmission optical electric field spectrum of each channel as the compensated transmission signal electric field, when this transmission electric field of channel k is input from the transmission side port of channel k, optical field output from the receiving port becomes the electric field obtained by operating the matrix equation H of formula (10), it can be seen that crosstalk is compensated by H and H _C is canceled.

Here, the compensation transmission vector generated by multiplying the inverse matrix as H _C is ideal compensation solutions, which may not be a realistic transmission signal light. This is because not only the crosstalk from the adjacent channel is compensated, but also the waveform distortion due to its own demultiplexing filter characteristic is compensated. Therefore, to generate a compensation matrix H _C as a target only compensation of waveform change due to the crosstalk, it is conceivable to generate a compensation signal light by multiplying the original transmission signal light. As a compensation matrix H _C for the purpose of compensating only for crosstalk, for example, H · H _C has H _{k, k} (ω−ω ₀ ) as a diagonal component of k rows and k columns, and components other than those on the diagonal line are included. The _HC is a diagonal matrix that is all zero. That is, when allowed to act H _C to H, all other components of the diagonal is zero features will become necessary to achieve compensation of crosstalk. When the compensated transmission signal light of the k-th channel generated by applying _HC is E _{Tx, k} (ω−ω ₀ ), it is expressed by the following equation.

Here, the transmission light of all the channels in Expression (11) is distributed around ω ₀ , and is not a transmission signal of WDM signal light as it is. This is because the frequency distribution of the transmission signal light needs to be distributed around ω _k . Therefore, using the periodicity of Δω = 2π / ΔT, which is a characteristic of the Fourier spectrum of the sampling function, Equation (11) is modified so that the spectrum is distributed around ω _k . When attention is paid to a certain channel k component E _{Tx-C, k} (ω−ω ₀ ) in the transmission signal light of Expression (11), the spectrum is shifted by kΔω so that the spectrum is distributed around ω _k . Therefore, it is expressed as the following formula.

However, H _C (H ₁₁ (ω−ω _k ),...) Represents a compensation matrix of the inverse transfer function matrix H. In addition, since the k channel component of the transmission electric field vector is shifted by kΔω in the modification to the equation (12), it is necessary to shift the k-th column of the transfer matrix H of the equation (10) acting in association therewith by kΔω. Using the shifted transmission determinant and compensated transmission signal light, the received signal light is expressed by the following equation.

  If the compensated transmission signal light represented by Expression (12) is transmitted, the spectrum obtained by multiplying the transfer matrix is output from the receiving end port, and the value at the center of the bit that is the sampling point is from the adjacent channel. Crosstalk is compensated.

  Actually, sampling is performed after photoelectric conversion. However, since the electrical band in the sampling is limited by photoelectric conversion and a threshold determination circuit, a value obtained by convolution integration with a certain response function is output at the time of sampling. Therefore, it may be necessary to correct the compensation method represented by the equation (12) by taking that amount into consideration.

Although the case where the data symbol rate is equal to the frequency difference between the channel carriers has been described here, the same description can be made even if both are in a multiplying relationship. When the channel interval is A times the symbol rate, Δω in equations (10) to (13) may be replaced with AΔω. Even if the relationship is not a multiplication relationship, the deviation from the multiplication of the channel spacing is included in the optical electric field envelope E _k (t, d _{k, n} ) defined by the equation (1) as a constant phase rotation, Since the above description can be applied, the compensation method is as shown in Expression (12). In this case, since the constant phase rotation is included in E _k (t, d _{k, n} ), the spectrum E _j (ω−ω _k ) of each channel is different from the above, and the reverse transfer calculation is complicated. become.

  1 generates a modulator driving waveform for modulating a carrier wave of each wavelength based on Expression (12).

  Further, by modulating the signal light input to the receiving end with an electrical signal having a channel frequency interval Δω, signal light in which the carrier light frequency of the signal light of the adjacent channel is shifted by the channel interval Δω can be generated. . When the change in the time waveform due to the transfer characteristic from the adjacent channel to the self-confidence channel is small, this frequency shift signal light can also be used as compensation light. Furthermore, if an optical filter that reproduces the waveform change received by the envelope of the adjacent channel signal light with the transfer characteristic from the adjacent channel to the self-confidence channel, desired compensation light can be obtained. Further, similar processing can be performed electrically using an analog electric signal processing circuit or a digital electric signal processing circuit after photoelectric conversion by an optical receiver and conversion to an electric signal.

  In addition, it is premised on synchronizing carrier wave phases of adjacent channels, but the purpose is to make waveform degradation deterministic by synchronization. Therefore, by providing a function for detecting the carrier phase relationship between adjacent channels even in the asynchronous case, waveform deterioration can be estimated on the basis of the phase relationship. Therefore, it is possible to compensate even in the asynchronous case.

  In the second embodiment of the present invention, compensation light is generated as an optical signal. By adding the compensation light to the transmission optical signal or the reception optical signal, it is possible to reduce the influence due to the mixing of the optical electric field from the adjacent channel.

  FIG. 2 shows a schematic diagram of a WDM transmission system according to a second embodiment of the present invention. Unlike FIG. 1, FIG. 2 is a schematic diagram expressing necessary functions by paying attention to signals in the WDM transmission system. Accordingly, components or functions that are not necessary for describing this embodiment are omitted. As shown in FIG. 2, the WDM transmission system according to the present embodiment includes transmitters 211-1 to 211 -M that output transmission optical signals of each channel, and an adder 213 that adds compensation light to the transmission optical signals of each channel. -1 to M, a WDM transmission line 220 for propagating a WDM signal obtained by wavelength-multiplexing the optical signal of each channel, and receivers 234-1 to M to receive the WDM signal after separating the wavelength of the WDM signal into optical signals of each channel. I have.

  Each transmitter outputs a transmission optical signal obtained by modulating a predetermined carrier wave with a transmission data sequence. Each adder adds compensation light described below to the transmission optical signal. The transmission optical signal to which the compensation light is added is transmitted as a WDM signal via the WDM transmission line. Each receiver receives the WDM signal from the WDM transmission line with wavelength separation for each channel.

Here, as an optical signal received by a receiver of a certain channel k, an optical signal electric field of its own channel is E _k (t), and an optical signal electric field due to crosstalk from an adjacent channel is ΔE _k (t). WDM channel spacing is assumed to be Δω, and for simplicity, only crosstalk from one of adjacent channels is assumed.

Here, the optical signal output of the transmitter of channel k is E _k (t, d _k ) exp [iω _k t + φ _k ] as the optical angular frequency ω _k of the carrier wave, and the optical signal output of the transmitter of channel k + 1 is the carrier wave. E _{k + 1} (t, d _{k + 1} ) exp [i (ω _k + Δω) t + φ _{k + 1} ] as the optical angular frequency ω _k + Δω of Further, the transfer characteristic (transfer function) of the own channel from the transmitter of channel k to the receiver of channel k is H ₀ (ω−ω _k ), and the transfer characteristic from the transmitter of channel k + 1 to the receiver of channel k. Is ΔH _XT (ω−ω _k ). The optical electric field spectrum of the transmission optical signal of channel k is expressed by the following equation by Fourier transform.

  Further, the optical electric field spectrum at the receiving end of the channel k is expressed as the following equation.

  Similarly, the optical electric field spectrum of the transmission optical signal of the adjacent channel k + 1 is expressed by the following equation.

Here, transfer functions of crosstalk components such as a wavelength multiplexer, a WDM transmission line, and a wavelength separation filter that are received until the optical signal of the adjacent channel k + 1 is input to the receiver of the own channel k are expressed as H _xt (ω− ω _k ), the optical electric field spectrum of the crosstalk component is expressed by the following equation.

  Furthermore, it is assumed that the optical field of compensation light defined by the following equation is input to the receiver of channel k.

  Then, since all these optical electric field components are input to the receiver of channel k, Equation (15), Equation (17), and Equation (18) are summed to obtain the following equation.

  When the equation (19) is inverse Fourier transformed into a function of time, the following equation is obtained.

  This expression represents the optical electric field vibration of the carrier wave of the channel k, and the square bracket on the right side represents the envelope of the optical electric field vibration. Focusing on this envelope portion, when the symbol rate of the transmission data sequence and the channel frequency interval are equal, the symbol time interval Δt satisfies Δt = Δω / 2π, so the electric field at the center of the data symbol is t = nΔt Is substituted and is expressed by the following equation.

Therefore, when φ _{k + 1} −θ _k = π is satisfied, the second term of Equation (21) becomes zero, and the crosstalk component is present for all transmission path characteristics and wavelength multiplexer / wavelength separation filter characteristics. It can be seen that it is compensated.

  FIG. 3 shows an example of a method for generating the compensation light shown in Expression (18). 4 shows the spectrum at each point in FIG. 3. FIG. 4 (a) shows the point a in FIG. 3, FIG. 4 (b) shows the point b in FIG. 3, and FIG. The point c in FIG. 3, FIG. 4D shows the spectrum at the point d in FIG. 3, and FIG. 4E shows the spectrum at the point e in FIG.

Referring to FIG. 3, the carrier wave of channel k is modulated with the transmission data sequence of channel k by optical modulator 214a (FIG. 4 (a)). Further, the carrier wave of channel k is modulated by the transmission data sequence of adjacent channel k + 1 by optical modulator 214b (FIG. 4B), and the same characteristic as the characteristic received by the crosstalk envelope at transfer characteristic calculation unit 215 As a result, compensation light is generated (FIG. 4C). Here, the transfer characteristic calculation unit 215 receives the transfer characteristic H _xt (ω−ω _k ) received until the optical signal of the adjacent channel k + 1 is input to the receiver of the channel k, and the channel interval Δω (= ω _{k + 1} −ω _k). ) _Has a characteristic H _xt (ω−ω _k + Δω) shifted to the lower frequency side. Such characteristics can be measured in advance. Further, transmission characteristics can be estimated by transmitting a pilot signal from an adjacent channel and measuring signal light input to the receiver of channel k. Also, transfer characteristics may change depending on the conditions such as the temperature and stress of the transmission path and multiplexer / demultiplexer, and the change is monitored from changes in the transmission signal and pilot signal to generate a compensation matrix and compensation light. There is also a way to feed back to the department. The generated compensation light is added to the modulated optical signal of the channel k by the adder 213 (FIG. 4 (d)) and transmitted to the transmission line via the wavelength multiplexer 218 (FIG. 4 (e)). With such a configuration, the compensation light shown in FIG. 2 can be generated, added to the transmission optical signal, and transmitted.

  In addition, as shown in FIG. 5, before the photoelectric conversion at the receiving end, the received light electric field of the adjacent channel that causes crosstalk can be shifted by the carrier frequency interval between the channels to obtain the compensation light. Can be added to the optical signal electric field of the own channel to compensate for the crosstalk component.

  In the second embodiment of the present invention, the influence of the transmission characteristics of the own channel is not taken into consideration. Therefore, in the third embodiment of the present invention, compensation that takes into account the transmission characteristics of the own channel is made possible by applying the inverse characteristic of the transmission characteristics of the own channel to the compensation light.

FIG. 6 shows a schematic diagram of a WDM transmission system according to a third embodiment of the present invention. 6 includes, in addition to the configuration of FIG. 2, reverse transfer characteristic calculators 314-1 to M that cause the reverse characteristic of the transfer characteristic of each channel to act on the compensation light. The reverse transfer characteristic calculation unit 314 calculates the reverse transfer characteristic H _k ⁻¹ (ω−ω _k ) of the transfer characteristic H _k (ω−ω _k ) of the own channel of the WDM transmission line including the wavelength multiplexer and the wavelength separation filter. , Work with the compensation light of that channel. By adding the compensation light with this characteristic to the transmission optical signal, the reception end compensates for the influence of the transmission characteristics of the own channel, and the reception end receives the more ideal optical waveform of Equation (18). Therefore, a further compensation effect can be expected.

  In the fourth embodiment of the present invention, compensation light is generated using a Mach-Zehnder modulator. In a modulator in which two light intensity modulators are integrated in parallel, or a modulator in which an intensity modulator and a phase modulator are arranged in series, the in-phase component and the quadrature component of light can be modulated independently. Optical electric field amplitude and phase waveform can be generated. Therefore, when the optical electric field time waveform of Expression (18) is subjected to inverse Fourier transform to obtain the time waveform, the following expression is obtained.

  Therefore, when this time waveform is decomposed into an in-phase component and a quadrature component, it is expressed as the following equation.

Here, H ′ _xt (ω−ω _k ) is a transmission path transfer characteristic such as a transfer function H _xt (ω−ω _k ) for the crosstalk component and an inverse transfer function H ⁻¹ (ω−ω _k ) of the own channel. The transfer characteristics used when generating compensation light in the optical region are shown. The optical waveform of the compensation light is expressed by Expression (23) and Expression (24) using this transfer characteristic of the optical region.

  The envelope portions excluding the phase rotation of the carrier wave in these two formulas are real numbers, and represent the optical field amplitudes of the in-phase component and the quadrature component, respectively. Therefore, in order to generate compensation light, these in-phase component and quadrature component are input to two optical intensity modulators juxtaposed with an optical path length difference of 90 degrees in phase difference, and the carrier wave of the own channel is input. Modulate.

FIG. 7 shows a configuration example for generating compensation light using an integrated modulator in which such two light intensity modulators are arranged in parallel. As shown in the figure, the waveforms of the envelopes of the equations (23) and (24) are input to the two modulators 413a and 413b, and the carrier optical signal of the own channel is modulated, and the phase difference of 90 degrees is added. By combining them, compensation light can be generated. Further, since H ′ _XT (ω) is a function distributed around the angular frequency ω = 0, the envelope indicated by the equations (23) and (24) is the frequency component of the channel interval or about twice that. Have An electric signal of such a band can be generated, and the optical signal processing necessary for generating the compensation light can be realized by analog and digital operations of the electric signal. When the modulator is driven by the electric signal thus generated, desired compensation light is directly generated.

Further, in this embodiment, the inverse characteristics of the transmission characteristics of the own channel are applied to the modulation input signals to the two modulators 413a and 413b, and the transmission characteristics of the own channel are taken into consideration as in the third embodiment. The compensated light may be generated. Here, the method of generating only the compensation light by the electric signal processing has been described, but it is also possible to realize the final transmission signal light by adding the compensation light and the main signal by the electric signal processing. The compensated signal light as represented by the expression (12) in the first embodiment is expressed in a form including both the main signal and the compensation light. Even such an optical signal is decomposed into an in-phase component and a quadrature component of light after returning to a time waveform as shown in the equations (22), (23), and (24) of the fourth embodiment. This is because the electrical signal processing is described using the transfer function H _{k, l} (ω) included in the equation (12). The amplitudes of the in-phase component and the quadrature component are given in the form as shown in Equation (23) and Equation (24), and the two modulators are driven using this to directly generate a compensated transmission waveform. Is possible.

  In the above description, it is assumed that an optical signal having an amplitude linearly proportional to the drive voltage input to the modulator is output. Need to be generated.

  Even when the frequency interval between channels and the bit rate are close to each other, if the 1-bit delay Mach-Zehnder interferometer is operated when the WDM optical signal is wavelength-separated at the receiving end, the spectrum of the own channel Although the transmission band is narrow compared to the spread, the optical field of the adjacent channel can be removed while suppressing the waveform deterioration, so that the beat signal from the adjacent channel can be suppressed. Such a configuration is shown in FIG.

  After the delay Mach-Zehnder interferometer 531, a band pass filter (BPF), wavelength separation filters 132a and 132b, etc. are used to further demultiplex each channel. There is an optimum value for the filter width for demultiplexing each subsequent channel.

  FIG. 9 is a graph showing the relationship between the filter width and the quality (Q value) of the received optical signal in the configuration of FIG. This is a result when a delayed quadrature phase modulation (DQPSK) signal having a channel interval of 12.5 GHz and a bit rate of 12.5 Gbps is received. Regarding the quality of the received optical signal of channel 1, when channel 2 is turned off and channel 3 is turned on (black circle plot), when channel 2 is turned on and channel 3 is turned off (black square plot) Is shown. Here, channel 2 is an adjacent channel separated from channel 1 by one channel, and channel 3 is an adjacent channel separated from channel 1 by two channels. The optimum point when both channel 2 and channel 3 are turned on is considered to be the optimum point of the BPF width near the intersection of both plots. Although slightly deviated from the BPF width at the intersecting point, the calculation result of the Q value when both channel 2 and channel 3 are ON is indicated by a triangle.

  This embodiment can be combined with any of the first to fourth embodiments described above. That is, as shown in FIG. 10, a delay Mach-Zehnder interferometer 531 is placed in front at the receiving end, and the reception optical signals of each channel are received by each receiver 134 via the wavelength separation filters 132a and 132b. be able to.

As a specific example, FIG. 11 shows a WDM transmission in which compensation light is generated using an optical filter and added to a transmission optical signal of the own channel and a reception device having a delayed Mach-Zehnder interferometer in a WDM demultiplexing unit. An example of a system configuration is shown. Here, description will be made by paying attention to the transmission optical signal of channel k. The carrier wave of wavelength λ _k is modulated by the optical modulator 514-k with the transmission data sequence of channel k, while the carrier wave of wavelength λ _k is adjacent to the adjacent channel k−1 by the optical modulators 514 a-k and 514 b-k, respectively. And k + 1 transmission data sequences. The optical signal modulated by the transmission data sequence of the adjacent channel is processed by the optical filters 517a and 517b so as to obtain desired compensation light, and transmitted by the wavelength multiplexer 118 using the transmission data sequence of channel k. And transmitted to the receiving apparatus 130 via the transmission path 120. At the receiving end, the received WDM optical signal is separated into an odd channel and an even channel through a delay Mach-Zehnder interferometer 531, and further separated into received optical signals for each channel by a BPF 533.

  Here, the embodiment using the receiving device having the delay Mach-Zehnder interferometer when the compensation light is generated by the optical signal processing using the optical filter has been described. However, when the compensation light is generated by the electrical signal processing, It can be used in combination with all the compensation signal generation methods shown in the first to fourth embodiments, such as a method for directly generating a compensated transmission waveform by signal processing.

  FIG. 12 shows a compensation light generation method for compensating for crosstalk mixed from channel k + 1 to channel k in this WDM transmission system. 12A shows a spectrum of signal light obtained by modulating a carrier wave of channel k with data of channel k + 1, for generating compensation light indicated by a solid line and a dotted line. An optical filter is operated to generate compensation light. The broken line is the BPF, and the solid line is the filtering waveform of the delay Mach-Zehnder interferometer. FIG. 12B shows a spectrum waveform of the compensation light after processing the compensation light of the channel k by the delay Mach-Zehnder interferometer and the BPF. In this case, the compensation light is added in the opposite phase to the optical signal mixed from the adjacent channel at the receiving end, thereby reducing the influence of the mixed optical signal from the adjacent channel.

  FIG. 13 shows an example of the calculation result of the compensation effect in this configuration example. As a result, when a delayed quaternary phase modulation (DQPSK) signal having a channel interval of 12.5 GHz and a bit rate of 12.5 Gbps is received, the transmission width of the BPF is taken on the horizontal axis, and the received signal quality (Q value) is calculated. Is. Regarding the quality of the received optical signal of channel 1, when channel 2 is turned off and channel 3 is turned on (black circle plot), when channel 2 is turned on and channel 3 is turned off (black square plot) Is shown. Here, channel 2 is an adjacent channel separated from channel 1 by one channel, and channel 3 is an adjacent channel separated from channel 1 by two channels. The optimum point when both channel 2 and channel 3 are turned on is considered to be the optimum point of the BPF width near the intersection of both plots, and the Q value at this time is about 14 dB.

  On the other hand, the gray dot represents the case where the BPF width is set to about twice the bit rate, the compensation light is generated and added to the main signal by the method shown in FIG. 11, and the crosstalk from the adjacent channel is compensated. The value of is shown. In this case, even when both of the adjacent channels 2 and 3 are ON, the Q value is improved to about 16 to 17 dB, and the effect of the compensation can be confirmed.

  While the present invention has been described with respect 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 is not intended to limit the scope of the invention. For example, the embodiment of the present invention can be applied to an arbitrary modulation system regardless of intensity modulation or wrinkle modulation. As described above, the configuration and details of the embodiment exemplified here can be changed without departing from the gist 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 schematic diagram which shows the structural example of the WDM transmission system which concerns on 1st Example of this invention. It is a schematic diagram which shows an example of the WDM transmission system which concerns on 2nd Example of this invention. It is a schematic diagram which shows the structural example of the receiver of the WDM transmission system which concerns on 2nd Example of this invention. It is a figure which shows the spectrum in each point in the structural example of FIG. It is a schematic diagram which shows another example of the WDM transmission system which concerns on the 2nd Example of this invention. It is a schematic diagram which shows an example of the WDM transmission system which concerns on the 3rd Example of this invention. It is a schematic diagram which shows the structural example which produces | generates compensation light using a Mach-Zehnder modulator in the WDM transmission system which concerns on the 4th Example of this invention. It is a schematic diagram which shows the structural example of the WDM transmission system which concerns on the 5th Example of this invention. It is a graph which shows the relationship between the filter width in the example of a structure of FIG. 8, and received optical signal quality (Q value). It is a schematic diagram which shows the structural example of the receiver of the WDM transmission system which concerns on the 5th Example of this invention. It is a schematic diagram which shows an example of the WDM transmission system comprised combining the 5th Example of this invention and another Example. It is a figure which shows the spectrum in each point in the structural example of FIG. 14 is a graph showing the relationship between the filter width and the received optical signal quality (Q value) in the configuration example of FIG. 13.

Explanation of symbols

DESCRIPTION OF SYMBOLS 110 Transmitter 112 Phase-synchronized multi-wavelength light source 114 Optical modulator 116 Pseudo reverse transfer calculation unit 118 Wavelength multiplexer 120 Transmission path 130 Receiver 132 Wavelength separation filter 134 Receiver 211 Transmitter 213 Adder 214 Optical modulator 215 Transmission characteristics Calculation unit 218 Wavelength multiplexer 220 Transmission path 233 Adder 234 Receiver 314 Reverse transfer characteristic calculation unit 413 Modulator 514 Modulator 517 Optical filter 531 Delayed Mach-Zehnder interferometer

Claims (12)

In a wavelength division multiplexing transmission system,
Synchronize the phase of the carrier of the second channel, which is an adjacent channel of the first channel and the first channel, and estimate the amount by which the signal of the second channel deteriorates relative to the signal of the first channel, and transmit An apparatus or a receiving apparatus compensates for the amount thereof. In the wavelength division multiplexing transmission system according to claim 1,
Compensation light in which a carrier wave having the same optical frequency as that of the first channel is encoded by a transmission data sequence of the second channel, and a transfer characteristic from the transmitter of the second channel to the receiver of the first channel is applied. Is added to the signal of the first channel in the transmitting device or the receiving device. The wavelength division multiplexing transmission system according to claim 2,
The compensation light further includes a compensation light, which is obtained by applying a reverse characteristic of a transfer characteristic from a first channel transmitter to a first channel receiver, to the first channel signal in the transmitter. Wavelength multiplex transmission system. The wavelength division multiplexing transmission system according to claim 1 or 2,
When the optical frequency of the first channel is ω _k , the optical frequency of the second channel is ω _k + Δω, and the transfer characteristic from the transmitter of the second channel to the receiver of the first channel is H (ω−ω _k ), Transmitting a compensation light in which a transfer characteristic H (ω−ω _k + Δω) is further applied to an optical signal obtained by encoding a carrier wave having the same optical frequency as that of the first channel with a transmission data sequence of a second channel In the apparatus or the receiving apparatus, the wavelength division multiplexing transmission system is added to the signal of the first channel. The wavelength division multiplexing transmission system according to claim 1 or 2,
When the optical frequency of the first channel is ω _k , the optical frequency of the second channel is ω _k + Δω, and the transfer characteristic from the transmitter of the second channel to the receiver of the first channel is H (ω−ω _k ), Modulating a carrier wave having the same optical frequency as that of the first channel using an electrical signal in which a transmission characteristic H (ω−ω _k + Δω) is applied to the transmission data sequence of the second channel to generate compensation light A wavelength division multiplexing transmission system, wherein a transmission apparatus or a reception apparatus adds the signal to the first channel signal. In the wavelength division multiplexing transmission system according to claim 5,
A wavelength division multiplexing transmission system, wherein a second carrier signal obtained by adding a transmission data sequence of a first channel to the electrical signal is used to modulate a carrier wave of the first channel. In a wavelength division multiplexing transmission system,
Divide odd and even channels with a Mach-Zehnder delay interferometer having a free spectral interval twice the channel interval, and cut out and receive only one channel using a bandpass filter with an appropriate transmission bandwidth. A wavelength division multiplexing transmission system. The wavelength division multiplexing transmission system according to claim 7,
7. A wavelength division multiplexing transmission system according to claim 1, wherein waveform degradation due to adjacent channel crosstalk is compensated. The wavelength division multiplexing transmission system according to claim 7,
7. A wavelength division multiplexing transmission system that compensates for waveform degradation due to the next adjacent channel on the same principle as described in claim 1.   A transmitter used in the wavelength division multiplexing transmission system according to claim 1.   A receiving device used in the wavelength division multiplexing transmission system according to claim 1. A method in a wavelength division multiplexing transmission system, comprising:
Synchronizing the phase of the carrier of the second channel which is an adjacent channel of the first channel and the first channel;
Estimating the amount by which the second channel signal degrades with respect to the first channel signal;
Compensating for the amount in a transmitting device or a receiving device.

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