Polarization fluctuation compensation device and optical communication system

A polarization fluctuation compensation device, when WDM light received by, for example, an optical reception device includes a polarization scrambled optical signal and a non-polarization scrambled optical signal, collects information related to whether optical signals having respective wavelengths are polarization scrambled, obtains a target value of control parameters which are different from each other, according to the speed of polarization fluctuations in the non-polarization scrambled optical signal based on the collected information, and performs reception processing of the non-polarization scrambled optical signal by using a control parameter set as the target value. As a result, an influence of fast polarization fluctuations generated resulting from an interaction between optical signals having respective wavelengths can be reliably compensated for, thereby enabling to realize excellent reception characteristics.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-088469, filed on Apr. 7, 2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to; a polarization fluctuation compensation device for compensating for deterioration of reception characteristic due to fluctuations in the polarization state of transmitted light in optical communication, and an optical communication system.

BACKGROUND

In wavelength division multiplexing (WDM) optical communication system, polarization scrambling may be applied to an optical signal in order to suppress the occurrence of polarization dependent gain (or polarization dependent loss) held by an optical repeater, and polarization hole burning (PHB), or to alleviate quality deterioration of a signal waveform due to nonlinearity of an optical fiber (for example, refer to Japanese Laid-Open Patent Publication No. 9-149006).

Moreover, in WDM optical communication system, an optical signal of a modulation format different from an existing channel may be added as a new channel at the time of upgrading the system. In this case, if wavelengths of optical signals of different modulation system are arranged adjacent to each other, performance of the system may be deteriorated resulting from a difference in the cross-phase modulation effect due to the difference of the modulation formats. In order to avoid this, a technique has been proposed where wavelength bands in which each optical signal is arranged, are grouped for each modulation format, and a guard band is provided between respective wavelength bands (for example, refer to Japanese Laid-Open Patent Publication No. 2006-50640).

To realize a large capacity of the optical communication system, research and development of a digital coherent receiver has been currently in progress. In the digital coherent receiver, polarization information included in received light needs to be restructured, following polarization fluctuations in the optical signal, which may occur at a certain rate due to an environmental change, for example, in an optical transmission path. This restructuring of polarization information of the received light is performed by subjecting a photoelectrically converted reception signal to digital signal processing by using a finite impulse response (FIR) filter or the like. In restructuring of polarization information by means of digital signal processing, the arithmetic processing thereof is under a high load, and hence there is a limitation on high speed processing. Therefore, a configuration in which the above-described polarization scrambling is basically not applied to an optical signal received by the digital coherent receiver has been studied.

When the existing optical communication system applied with polarization scrambling is to be upgraded by using the digital coherent receiver, a polarization scrambled optical signal and a non-polarization scrambled optical signal are simultaneously transmitted inside the optical fiber. At this time, the polarization state, phase, and amplitude of the non-polarization scrambled optical signal may fluctuate timewise due to the cross-phase modulation effect and Raman amplification effect resulting from the nonlinearity of the optical fiber. If the polarization state of the optical signal received by the digital coherent receiver fluctuates at a speed faster than a normally assumed speed due to an influence of polarization scrambling applied to other optical signals, digital signal processing for restructuring the polarization information cannot follow the fast fluctuation of the polarization state. Therefore, there is a problem in that quality deterioration of the signal waveform occurs at the time of restructuring the polarization information of the received light.

The above problem is not limited to the case in which the optical signal is received by using the digital coherent receiver, but is common to various types of reception systems using the polarization information of the optical signal. Moreover, the polarization state of the transmitted light may fluctuate faster than the normally assumed speed resulting from an interaction between respective optical signals, due to some sort of phenomenon other than polarization scrambling. Therefore, in reception processing of the optical signal using the polarization information, it is an important issue to reduce the influence of polarization fluctuations faster than the normally assumed speed, on the reception processing.

SUMMARY

Accordingly, the invention provides a polarization fluctuation compensation device that compensates for the quality deterioration of a signal waveform occurring in reception processing of WDM light including a plurality of optical signals having different wavelengths. One aspect of the polarization fluctuation compensation device includes: an information collection circuit adapted to collect information related to the speed of polarization fluctuations in optical signals having respective wavelengths; a parameter calculator circuit adapted to obtain a target value of a control parameter corresponding to each wavelength, based on information collected by the information collection circuit, so that a control parameter in the reception processing related to polarization information of the optical signals having respective wavelengths takes a different value according to the speed of polarization fluctuations in the optical signals having respective wavelengths; and a parameter setting circuit adapted to set the target value obtained by the parameter calculator circuit, as a control parameter in the reception processing corresponding to the respective wavelengths.

DESCRIPTION OF EMBODIMENTS

Hereunder is a detailed description of embodiments of the present invention, with reference to the accompanying drawings.

FIG. 1is a diagram illustrating the configuration of a first embodiment of an optical communication system.

InFIG. 1, in the optical communication system of the first embodiment, WDM light in which, for example, a polarization scrambled optical signal and a non-polarization scrambled optical signal are multiplexed, is transmitted from an optical transmission device1to an optical transmission line2, and the WDM light is repeatedly transmitted, while being amplified by an optical repeater3arranged on the optical transmission line2, and is received by an optical reception device4. The optical communication system includes a polarization fluctuation compensation device5that extracts a part of the WDM light transmitted from the optical transmission device1to the optical transmission line2as monitoring light, and detects whether optical signals having respective wavelengths included in the monitoring light are polarization scrambled, thereby compensating for polarization fluctuations in reception processing corresponding to a non-polarization scrambled optical signal performed by the optical reception device4, based on the detection result thereof.

The optical transmission device1generates optical signals having wavelengths λ1to λMdifferent from each other in M (M is an integer equal to or larger than 1) optical transmitters (TX)11-1to11-M, multiplexes the respective optical signals by a multiplexer12, and scrambles the polarization state of the optical signals having respective wavelengths λ1to λMby providing output light of the multiplexer12to a polarization scrambler13. Moreover, the optical transmission device1generates optical signals having wavelengths λM+1to λM+Ndifferent from each other in N (N is an integer equal to or larger than 1) optical transmitters (TX)14-1to14-N, and multiplexes the respective optical signals by a multiplexer15. Then the optical transmission device1multiplexes output light from the polarization scrambler13and output light from the multiplexer15by a multiplexer16to generate WDM light having (M+N) waves, amplifies the WDM light to a required level by a post-amplifier17, and outputs the WDM light to the optical transmission line2.

The above configuration of the optical transmission device1corresponds to a configuration in which the optical transmitters14-1to14-N and the multiplexers15and16are added to the existing configuration including the optical transmitters (TX)11-1to11-M, the multiplexer12, the polarization scrambler13, and the post-amplifier17, by for example upgrading the optical communication system.

A splitter51of the polarization fluctuation compensation device5is arranged on the optical transmission line2positioned near the output terminal of the optical transmission device1, and a part of the WDM light output from the optical transmission device1to the optical transmission line2is extracted by the splitter51as monitoring light. The WDM light having passed through the splitter51is amplified by the optical repeater3arranged at a required interval on the optical transmission line2, and repeatedly transmitted to the optical reception device4.

The WDM light repeatedly transmitted on the optical transmission line2is input to the optical reception device4, and the optical reception device4amplifies the WDM light to a required level by a pre-amplifier41and demultiplexes the WDM light to optical signals having respective wavelengths λ1to λM+Nby a demultiplexer42. Then the optical reception device4receives the (polarization scrambled) optical signals output from the demultiplexer42and having respective wavelengths λ1to λMby optical receivers (RX)43-1to43-M corresponding to the respective wavelengths, and receives the (non-polarization scrambled) optical signals having respective wavelengths λM+1to λM+Nby respective optical receivers (RX)44-1to44-N corresponding to the respective wavelengths. The respective optical receivers43-1to43-M on the polarization scrambled side have a general configuration corresponding to a reception system that basically does not use the polarization information of the optical signal. On the other hand, the respective optical receivers44-1to44-N on the non-polarization scrambled side correspond to a reception system that uses the polarization information of the optical signal.

The above configuration of the optical reception device4corresponds to a configuration for when the optical receivers44-1to44-N are added by using an unused port of the demultiplexer42, to the existing configuration including the pre-amplifier41, the demultiplexer42, and the optical receivers43-1to43-M, by the aforementioned upgrading of the optical communication system. Specific configuration examples of the respective optical receivers44-1to44-N will be described later.

The polarization fluctuation compensation device5provides monitoring light extracted by the splitter51to a polarization scrambling detector52, and the polarization scrambling detector52detects whether the optical signals having respective wavelengths λ1to λM+Nincluded in the monitoring light have been polarization scrambled. Information related to the presence of polarization scrambling corresponding to the respective wavelengths λ1to λM+Ndetected by the polarization scrambling detector52is collected by an information collection section53. The information collection section53may be circuit. Then in the polarization fluctuation compensation device5, a parameter calculator54obtains a target value of a control parameter to be applied to the processing related to the polarization information of received light performed by the respective optical receivers44-1to44-N corresponding to a reception method using the polarization information of the optical signal, by using the information from the information collection section53, and transmits the result thereof to a parameter setting section55. The parameter calculator54and the parameter setting section55may be circuits. The parameter setting section55sets the target value obtained by the parameter calculator54as a control parameter p in the reception processing by the corresponding optical receivers44-1to44-N inside the optical reception device4. As a result, the reception processing for compensating for polarization fluctuations occurring in the respective wavelengths λM+1to λM+Ndue to the influence of polarization scrambling with respect to the optical signals having respective wavelengths λ1to λMis performed by the respective optical receivers44-1to44-N. A specific configuration example of the polarization scrambling detector52and details of the processing in the parameter calculator54will be described later.

Here a specific configuration example of the respective optical receivers44-1to44-N on the non-polarization scrambled side is described below.

FIG. 2is a block diagram illustrating a configuration example (1) for a case where a digital coherent receiver is used as the respective optical receivers44-1to44-N. In the configuration example (1), an optical signal output from the demultiplexer42(FIG. 1) of the optical reception device4is provided to an input port IN. The input light is, for example, a polarization multiplexing phase modulated optical signal. Input light to the optical receivers is separated to two different polarization components by a polarization beam splitter (PBS)401A. One polarization component (for example, the X polarization component) is output to an optical hybrid circuit (HYB)403X, and the other polarization component (for example, the Y polarization component) is output to an optical hybrid circuit403Y.

Local oscillation light to be output from a local oscillation light source (LOL)402is respectively input to the respective optical hybrid circuits403X and403Y via a polarization beam splitter (PBS)401B. The local oscillation light is continuous light having substantially the same frequency as the frequency of the input light. The optical hybrid circuit403X mixes the X polarization component of input light and the local oscillation light, thereby generating optical signals of an in-phase (I) component and a quadrature-phase (Q) component with optical phases thereof being different by 90 degrees from each other. Moreover the optical hybrid circuit403Y also mixes the Y polarization component of the input light and the local oscillation light, thereby generating optical signals of the I-component and the Q-component, with optical phases thereof being different by 90 degrees from each other.

The respective optical signals of the I-component and the Q-component respectively output from the respective optical hybrid circuits403X and403Y are converted to digital electric signals by respectively corresponding optical receivers (O/E)404XI,404XQ404YI, and404YQ, and AD converters (ADC)405XI,405XQ,405YI, and405YQ. As a result, I-component data and Q-component data corresponding to the X polarization of the input light, and I-component data and Q-component data corresponding to the Y polarization of the input light can be acquired, and respective data are output to a waveform equalization section406. One set of I-component data and Q-component data corresponds to one complex number, and the I-component data expresses a value of a real part of a certain complex number, and the Q-component data expresses a value of an imaginary part of the complex number. An optical electric field (optical amplitude and optical phase) corresponding to one polarization component of the input light is expressed by one set of I-component data and Q-component data.

The waveform equalization section406performs arithmetic processing for compensating for dispersion or the like of the optical transmission path2, with respect to output data from the AD converters405XI,405XQ405YI, and405YQ thereby performing waveform equalization of the optical signal. The respective data processed by the waveform equalization section406is output to a polarization information restructuring section407. The polarization information restructuring section407performs arithmetic processing for restructuring information corresponding to the X polarization and the Y polarization with respect to respective output data from the waveform equalization section406, according to the polarization fluctuations in the optical signal input to the digital coherent receiver.

FIG. 3is a circuit diagram illustrating a specific configuration example of the polarization information restructuring section407. In the configuration example, output data corresponding to the X polarization from the waveform equalization section406is provided to an input port INx, and output data corresponding to the Y polarization from the waveform equalization section406is provided to an input port INy. An FIR filter (FIRxx)407A and an FIR filter (FIRxy)407C are connected in parallel to the input port INx, and an FIR filter (FIRyx)407B and an FIR filter (FIRyy)407D are connected in parallel to the input port INy.

FIG. 4is a circuit diagram illustrating a specific example of the FIR filters407A to407D. The FIR filter includes; 2n delay elements (T/2)4071connected in series to the input port IN, multipliers4072connected to the input port IN and output ends of the respective delay elements4071, and an adder (Σ)4073to which outputs of the respective multipliers4072are provided. The respective delay elements4071delay the input signal by time T/2. Accordingly, a signal delayed stepwise by T/2 to n×T is acquired by the 2n delay elements4071. The respective multipliers4072multiply the input signal by a filter coefficient w corresponding to the number of delay steps. A calculation result of an algorithm calculation circuit407E inFIG. 3is applied as the filter coefficient w.

The algorithm calculation circuit407E calculates the filter coefficient w according to a well-known adaptive equalization algorithm by using the control parameter p provided from the polarization fluctuation compensation device5. Specific examples of the adaptive equalization algorithm include a least mean square (LMS) algorithm expressed by equation (1) below, a constant modulus algorithm (CMA) expressed by equation (2) below, and the like.LMS algorithm:
w(n+1)=w(n)−μr*(n)(yn−sn)  (1)CMA
w(n+1)=w(n)−μr*(n)(|yn|2−γ)yn(2)

where, in equations (1) and (2), w denotes a filter coefficient, r denotes a reception signal, yndenotes a filter output signal, sndenotes a training signal, γ denotes a constant, and μ denotes a step size. In these adaptive equalization algorithms, the control parameter p from the polarization fluctuation compensation device5is provided as the step size μ.

The adder4073adds up the multiplication results acquired by the respective multipliers4072. A signal indicating the addition result of the adder4073is output from an output port OUT of the FIR filter. Moreover output signals from the two FIR filters407A and407B in the polarization information restructuring section407(FIG. 3) are added, and output as data corresponding to the restructured X polarization. Moreover, output signals from the remaining two FIR filters407C and407D are added, and output as data corresponding to the restructured Y polarization.

Returning toFIG. 2, data calculated by the polarization information restructuring section407is provided to a signal processing section408. The signal processing section408uses the output data from the polarization information restructuring section407to perform digital signal processing such as frequency offset compensation, phase synchronization, and signal identification, and outputs the received data.

FIG. 5is a block diagram illustrating a configuration example (2) for a case where a direct detection receiver is used as the respective optical receivers44-1to44-N inFIG. 1. In the configuration example (2), as in the case of the configuration example (1) illustrated inFIG. 2toFIG. 4, an optical signal (for example, polarization multiplexing phase modulated optical signal) output from the demultiplexer42(FIG. 1) of the optical reception device4is provided to the input port IN. Input light to the optical receiver is input to a polarization controller411, and the polarization direction of the input light with respect to an optical axis of a polarization beam splitter (PBS)412connected to an output end of the polarization controller411, is controlled by the polarization controller411.

Output light from the polarization controller411is separated into two different polarization components by the PBS412. One polarization component (for example, the X polarization component) is output to an optical receiver (O/E)413X, and the other polarization component (for example, the Y polarization component) is output to an optical receiver (O/E)413Y. At this time, a part of the respective output lights from the PBS412is branched and provided to a controller415. The controller415monitors the power of the respective branched lights, and feed-back controls the polarization controller411according to the monitoring results thereof. Here the control parameter p from the polarization fluctuation compensation device5is provided to the controller415as a loop gain in the feed-back control of the polarization controller411by the controller415.

The respective optical receivers413X and413Y convert the respective output lights from the PBS412to electric signals, and output the electric signals to respective signal processing sections414X and414Y. The respective signal processing sections414X and414Y perform processing such as signal identification or error correction with respect to the output signals from the respective optical receivers413X and413Y, to thereby generate and output received data.

Next is a description of a specific configuration example of the polarization scrambling detector52illustrated inFIG. 1.

FIG. 6is a block diagram illustrating specific configuration examples (A) to (D) of the polarization scrambling detector52. The configuration example (A) illustrated at the top inFIG. 6includes a demultiplexer501, M+N polarimeters5021to502M+N, and M+N fluctuation detection circuits5031to503M+N. Monitor light obtained by extracting a part of the WDM light output from the optical transmission device1(FIG. 1), by the splitter51, is input to the demultiplexer501, and the demultiplexer501demultiplexes the monitor light to optical signals having respective wavelengths λ1to λM+Nand outputs the optical signals. The respective polarimeters5021to502M+Nmonitor a Stokes parameter expressing the polarization state of the respective optical signals demultiplexed by the demultiplexer501. The respective fluctuation detection circuits5031to503M+Ndetect the speed (frequency) of polarization fluctuations in the optical signal, based on a change in the Stokes parameter monitored by the respective polarimeters5021to502M+N, to determine whether polarization scrambling has been performed, and transmit the determination result thereof to the information collection section53.

In the configuration example (B) illustrated in the second stage inFIG. 6, a wavelength variable filter504is used instead of the demultiplexer501in the configuration example (A) so that a central wavelength of a transmission band of the wavelength variable filter504is matched with the respective wavelengths λ1to λM+Nand switched sequentially, thereby decreasing the installed number of the polarimeters502and the fluctuation detection circuits503to realize downsizing and cost reduction.

The configuration example (C) illustrated in the third stage inFIG. 6includes a demultiplexer511, M+N splitters5121to512M+N, 2×(M+N) polarizers513A1,513B1to513AM+N, and513BM+N, photodiodes (PD)514A1,514B1to514AM+N, and514BM+N, and M+N fluctuation detection circuits5151to515M+N. Monitor light obtained by extracting a part of WDM light output from the optical transmission device1(FIG. 1), by the splitter51is input to the demultiplexer511, and the demultiplexer511demultiplexer the monitor light into optical signals having respective wavelengths λ1to λM+Nand outputs the optical signals. The respective splitters5121to512M+Nbranch the respective optical signals demultiplexed by the demultiplexer511into two and output these optical signals.

One set of polarizers513A1and513B1corresponding to wavelength λ1respectively have an optical axis with the direction thereof being different from each other, and cut out a specific polarization component along the direction of the optical axis from the respective optical signals branched by the splitter5121into two, and output the polarization components to the PDs514A1and514B1. The respective PD514A1and514B1convert the light cut out by the respective polarizers513A1and513B1to electric signals and output the electric signals to the fluctuation detection circuit5151. The fluctuation detection circuit5151detects the speed (frequency) of the polarization fluctuations in the optical signal of wavelength λ1, based on the output signals from the respective PDs514A1and514B1to determine whether polarization scrambling has been performed, and transmits the determination result thereof to the information collection section53. Polarizers513A2,513B2to513AM+N, and513BM+N, PDs514A2,514B2to514AM+N, and514BM+N, and fluctuation detection circuits5152to515M+Ncorresponding to other wavelength λ2to λM+Nhave the same configuration as those of the polarizers513A1and513B1, the PDs514A1and514B1, and the fluctuation detection circuit5151corresponding to the wavelength λ1.

In the configuration example (D) illustrated in the fourth stage inFIG. 6, a wavelength variable filter516is used instead of the demultiplexer511in the configuration example (C), and a central wavelength of a transmission band of the wavelength variable filter516is matched with respective wavelengths λ1to λM+Nand switched sequentially, thereby decreasing the installed number of the splitters512, the polarizers513A and513B, the PDs514A and514B, and the fluctuation detection circuits515to realize downsizing and cost reduction.

In the configuration examples (C) and (D), for an optical signal of one wavelength, different polarization components are cut out by using a set of polarizers513A and513B. However, only a specific polarization component can be cut out by a single polarizer to determine the presence of polarization scrambling based on a change in the power. In this case, the detection accuracy for the presence of polarization scrambling decreases compared to the case of using one set of polarizers, however, further downsizing and cost reduction can be realized.

Next is a description of an operation of the optical communication system of the first embodiment.

In the optical communication system of the above configuration, an optical signal having wavelengths λ1to λMpolarization scrambled by the polarization scrambler13inside the optical transmission device1, and an optical signal having wavelengths λM+1to λM+Nnon-polarization scrambled, pass through the same optical transmission path2from the optical transmission device1and are repeatedly transmitted to the optical reception device4, while being amplified by the optical repeater3. At the time of repeated transmission of the WDM light, temporal fluctuations occur in the polarization state, phase, and amplitude of the non-polarization scrambled optical signals having wavelengths λM+1to λM+N, due to the cross-phase modulation (XPM) effect and Raman amplification effect by the polarization scrambled optical signals having wavelengths λ1to λM.

Here the influence of the XPM effect by the polarization scrambled optical signals is specifically explained. When it is assumed that a polarization scrambling frequency in the polarization scrambler13is f, and a polarization rotation frequency of the polarization scrambled optical signal is ω=2πf, an electric field Ep of the optical signal is expressed by the following equation (3).

where Epxdenotes the electric field of the X polarization component, and Epydenotes the electric field of the Y polarization component. Moreover E0denotes the electric field of the (linearly-polarized) optical signal before polarization scrambling, and t denotes time.

The amount of phase shift due to XPM applied to optical signals of other wavelengths by the polarization scrambled optical signal is expressed by the following equation (4) (for example, refer to G. P. Agrawal, Nonlinear Fiber Optics, 4th ed., Academic Press, San Diego, 2007, Chapter 6.2). Loss in the optical transmission path2and walk-off due to wavelength dispersion is ignored.

where ΦXPM—xdenotes the phase shift amount due to XPM with respect to an X-polarized optical signal, and ΦXPM—ydenotes the phase shift amount due to XPM with respect to a Y-polarized optical signal. Moreover, γ denotes the nonlinear coefficient of the optical transmission path2, and L denotes the overall length of the optical transmission path2.

It is seen from the relation in equation (4) that the phase of the X polarization component and the phase of the Y polarization component in the optical signals of other wavelengths fluctuate at frequency 2f due to the influence of XPM by the polarization scrambled optical signal. That is, the polarization scrambled optical signal and the non-polarization scrambled optical signal are transmitted together on the optical transmission path2, thereby adding a polarization fluctuation with frequency 2f to the polarization scrambled optical signal.

The polarization fluctuation with frequency 2f occurring in the non-polarization scrambled optical signal due to the influence of the above polarization scrambled optical signal is faster than the polarization fluctuations occurring due to an environmental change in the optical transmission path, and signal processing performed by the optical receiver that receives the optical signal cannot follow the fast polarization fluctuation. The level of polarization fluctuations occurring in the non-polarization scrambled optical signal due to the influence of polarization scrambling is related to the relative arrangement between the wavelength of the polarization scrambled optical signal and the wavelength of the non-polarization scrambled optical signal.

Therefore in the first embodiment, the polarization fluctuation compensation device5detects whether polarization scrambling has been performed with respect to the optical signals having respective wavelengths λ1to λM+Ntransmitted from the optical transmission device1to the optical transmission path2, and optimizes the control parameter p in the signal processing in the optical receivers44-1to44-N corresponding to the non-polarization scrambled optical signal, taking the relation of relative wavelength arrangement into consideration based on the detection result thereof, thereby compensating for polarization fluctuations due to the influence of polarization scrambling.

FIG. 7is a flowchart illustrating one example of an operation in the polarization fluctuation compensation device5.

When the optical communication system is activated, in step110inFIG. 7(indicated by S110inFIG. 7, and similarly hereunder), the polarization scrambling detector52detects whether the optical signals having respective wavelengths λ1to λM+Nincluded in the WDM light transmitted from the optical transmission device1to the optical transmission path2have been polarization scrambled. In the processing performed by the polarization scrambling detector52, for example, when the speed of the polarization fluctuation detected for wavelengths λ1to λM+Naccording to the respective configuration examples (A) to (D) illustrated inFIG. 6exceeds a threshold level preset according to the frequency f of the polarization scrambler13, it is determined that the wavelength is polarization scrambled. When the speed of the polarization fluctuation is lower than the threshold level, it is determined that the wavelength is non-polarization scrambled.

In step120, information related to whether the optical signals having respective wavelengths λ1to λM+Ndetected by the polarization scrambling detector52have been polarization scrambled is collected by the information collection section53.

The relation set in the table inFIG. 8is explained in detail. In the combination #1of the non-polarization scrambled optical signal of wavelength λ1and the polarization scrambled optical signals of wavelengths λ2to λ8, comparatively fast polarization fluctuations occur in the optical signal of wavelength λ1, due to the optical signal of wavelength λ1receiving a strong influence of polarization scrambling with respect to the optical signals of adjacent wavelength λ2and the next wavelength λ3. Therefore, in the setting p(#1) of the control parameter p corresponding to the combination #1, α1capable of compensating for the polarization fluctuation is set as a target value of the control parameter p corresponding to wavelength λ1. Because wavelengths λ2to λ8correspond to polarization scrambled optical signals, setting of the control parameter p is not required.

In the combination #2of the non-polarization scrambled optical signals of wavelengths λ1and λ2and the polarization scrambled optical signals of wavelengths λ3to λ8, comparatively fast polarization fluctuations occur in the optical signals of wavelengths λ1and λ2due to receiving a strong influence of polarization scrambling with respect to the optical signals of wavelength λ3to λ5, as in the case of the combination #1. Therefore, α1is respectively set as a target value of the control parameter p corresponding to wavelengths λ1and λ2.

Furthermore in the combination #3of the non-polarization scrambled optical signals of wavelengths λ1to λ3and the polarization scrambled optical signals of wavelengths λ4to λ8, comparatively fast polarization fluctuations occur in the optical signals of wavelengths λ2and λ3due to receiving a strong influence of polarization scrambling with respect to the optical signals of wavelengths λ4to λ6, and the influence of polarization scrambling also reaches the optical signal of wavelength λ1, and polarization fluctuations also occur in the optical signal of wavelength λ1, although slower than that of wavelengths λ2and λ3. Therefore, in the setting p(#3) of the control parameter p corresponding to the combination #3, α1is set as a target value of the control parameter p corresponding to wavelengths λ2and λ3, and α2smaller than α1is set as a target value of the control parameter p corresponding to wavelength λ1.

Moreover, in the combination #4of the non-polarization scrambled optical signals of wavelengths λ1to λ4and the polarization scrambled optical signals of wavelengths λ5to λ8, comparatively fast polarization fluctuations occur in the optical signals of wavelengths λ3and λ4due to receiving a strong influence of polarization scrambling with respect to the optical signals of wavelengths λ5to λ7, and the influence of polarization scrambling also reaches the optical signal of wavelength λ2, and polarization fluctuations also occur in the optical signal of wavelength λ2, although slower than that of wavelengths λ3and λ4. Furthermore the influence of polarization scrambling also reaches the optical signal of wavelength λ1, though slightly, and a polarization fluctuation slower than that of wavelength λ2occurs in the optical signal of wavelength λ1. Therefore, in the setting p(#4) of the control parameter p corresponding to the combination #4, α1is set as a target value of the control parameter p corresponding to wavelengths λ3and λ4, α2smaller than α1is set as a target value of the control parameter p corresponding to wavelength λ2, and α3smaller than α2is set as a target value of the control parameter p corresponding to wavelength λ1.

When the target value of the control parameter p corresponding to the presence of polarization scrambling is obtained by the parameter calculator54in this manner, control proceeds to step140inFIG. 7, and the parameter setting section55sets the control parameters in the reception processing in the optical receivers44-1to44-N on the non-polarization scrambled side to the target values obtained by the parameter calculator54. As a result, in the case of the digital coherent receiver in the configuration example (1) illustrated inFIG. 2toFIG. 4, a step size μ to be used in the arithmetic processing according to the adaptive equalization algorithm in the polarization information restructuring section407is optimized. Moreover, in the case of the direct detection receiver in the configuration example (2) illustrated inFIG. 5, the loop gain in the feed-back control of the polarization controller411by the controller415is optimized. Accordingly, the non-polarization scrambled optical signal and the polarization scrambled optical signal are simultaneously transmitted, thereby enabling to reliably compensate for the influence of fast polarization fluctuations occurring in the non-polarization scrambled optical signal, and the reception processing of the optical signal using the polarization information can be performed with high accuracy.

FIG. 9illustrates one example in which the relation between the control parameter p and signal quality (Q value) is calculated by changing the polarization scrambling frequency f. It is seen inFIG. 9that the value of the control parameter p at which the signal quality becomes the best changes corresponding to the polarization scrambling frequency f. Accordingly, by optimizing the control parameter p corresponding to the speed of polarization fluctuations occurring due to the influence of polarization scrambling, excellent signal quality can be realized in the respective optical receivers44-1to44-N.

In the explanation of the operation of the polarization fluctuation compensation device5, the presence of polarization scrambling of the optical signals having respective wavelengths it is expressed by using a binary of “1” and “0” (in the upper stage inFIG. 8). However, for example, as illustrated inFIG. 10, polarization fluctuations occurring due to the influence of polarization scrambling can be expressed by multiple values. Here the speed (frequency) of the polarization fluctuations detected by the polarization scrambling detector52is expressed stepwise between 0 and 1, based on the polarization scrambling frequency f.

Moreover, when the target value of the control parameter p corresponding to the combination of the polarization scrambled optical signal and the non-polarization scrambled optical signal is determined by using the tables illustrated inFIG. 8andFIG. 10, if the number of wavelength multiplexing of the WDM light increases, combination of the polarization scrambled optical signal and the non-polarization scrambled optical signal becomes complicated and a large scale table is required. In such a situation, the target value of the control parameter can be determined based on calculation according to a procedure illustrated, for example, in the flowchart inFIG. 11, instead of using the table.

Specifically, as in steps110and120illustrated inFIG. 7, it is detected whether the optical signals having respective wavelengths λ1to λM+Nhave been polarization scrambled, and the results thereof are collected by the information collection section53. Then in step131inFIG. 11, the parameter calculator54determines the relation in the following equation (5) by using the information from the information collection section53.

where λSCR[k] denotes an index indicating whether a wavelength channel k has been polarization scrambled, with “0” indicating “non-polarization scrambled”, and “1” indicating “polarization scrambled”. Moreover, i denotes a wavelength channel for which a target value of the control parameter p is to be calculated. Ntotaldenotes the number of wavelength multiplexing of the WDM light transmitted on the optical transmission path2. N1is a first determination parameter to be used for determining the control parameter p, and indicates a wavelength range in which a search is made of whether there is a polarization scrambled optical signal. For example, in the case of N1=3, a search is made of whether there is a polarization scrambled optical signal in a range of the wavelength channel i±3 based on the wavelength channel i to be calculated.

For the determination processing in step131, combinations #5to #7of the polarization scrambled optical signals and the non-polarization scrambled optical signals having the respective wavelengths as illustrated for example inFIG. 12are assumed, to consider a case in which wavelength λ1is to be calculated. In this case, i=1, and Ntotal=8. When it is assumed that N1=3 is set, because i−N1=−2<0, m=1 is set, and because i+N1=4<Ntotal=8, n=4 is set. In the case of combination #5inFIG. 12, because the left-hand value in equation (5) is “0”, control proceeds to step141in the flowchart inFIG. 11, to set “α1” as a target value of a control parameter p[1]. Moreover in the case of combination #6inFIG. 12, because the left-hand value in equation (5) is “1”, control proceeds to step132in the flowchart inFIG. 11. Furthermore in the case of combination #7inFIG. 12, because the left-hand value in equation (5) is “3”, control proceeds to step132in the flowchart inFIG. 11. That is, in the determination processing in step131, a value is obtained by totaling indexes indicating whether adjacent wavelength channels in a range of ±N1have been polarization scrambled, based on the wavelength channel i to be calculated, and it is determined whether the value is equal to or larger than 1.

In step132, the following relation in equation (6) is determined.

For the determination processing in step132, subsequent steps after step131in combinations #5to #7inFIG. 12described above are explained. When N2=1 is set, because i−N2=0, x=1 is set, and because i+N2=2<Ntotal=8, y=2 is set. Moreover in the case of combination #6inFIG. 12, because the left-hand value in equation (6) is “0”, control proceeds to step142in the flowchart inFIG. 11, to set “α2” as a target value of the control parameter p[1]. Furthermore in the case of combination #7inFIG. 12, because the left-hand value in equation (6) is “1”, control proceeds to step143in the flowchart inFIG. 11, to set “α3” as a target value of the control parameter p[1]. That is, in the determination processing in step132, a value is obtained by totaling indexes indicating whether adjacent wavelength channels in a range of ±N2have been polarization scrambled, based on the wavelength channel i to be calculated, and it is determined whether the value is equal to or larger than 1. The target values to be set in steps141to143have a relation of α1>α2>α3, which is the same as in the case of using the table illustrated inFIG. 8.

In the above manner, by determining the target value of the control parameter p corresponding to the respective wavelengths by the calculation processing according to the flowchart inFIG. 11, even when the number of wavelength multiplexing of the WDM light transmitted on the optical transmission path2increases, a large scale table need not be prepared, thereby enabling to easily realize the polarization fluctuation compensation device5.

The number of conditional equations, and the value and number of determination parameters in the arithmetic processing according to the flowchart inFIG. 11can be appropriately set and changed according; to the type of optical fiber used in the optical transmission path2, the input power of the optical signal to the optical transmission path2, the number of wavelength multiplexing and the wavelength interval of the WDM light, the frequency f of the polarization scrambler13, and the system parameter such as of the modulation format of the respective optical signals.

Next is a description of a second embodiment of an optical communication system.

In the above first embodiment, an example is described where, in the case when a non-polarization scrambled optical signal is transmitted together with a polarization scrambled optical signal by upgrading the optical communication system, fast polarization fluctuations occurring in the non-polarization scrambled optical signal due to the influence of polarization scrambling are compensated for. However, the source occurrence of the fast polarization fluctuations is not limited to polarization scrambling, and the polarization state of transmission light may fluctuate faster than the normally assumed speed due to some factors other than polarization scrambling. Here one example of an optical communication system that can accommodate such a situation is described in the second embodiment.

FIG. 13is a diagram illustrating the configuration of the second embodiment of the optical communication system. Parts the same as or corresponding to the configuration of the first embodiment, are denoted by the same reference symbols, and description thereof is omitted. InFIG. 13, in the optical communication system of the second embodiment, an optical transmission device1includes optical transmitters14-1to14-N, a multiplexer15, and a post-amplifier17. Moreover an optical reception device includes a pre-amplifier41, a demultiplexer42, and optical receivers44-1to44-N. In the optical communication system, polarization scrambling is not performed basically with respect to optical signals having respective wavelengths repeatedly transmitted from the optical transmission device1to the optical reception device4via an optical transmission path2and an optical repeater3.

A polarization fluctuation compensation device5applied to the optical communication system extracts a part of the WDM light input to the optical reception device4as monitor light by a splitter51, and provides the monitor light to a polarization fluctuation detector56. The polarization fluctuation detector56detects the speed of polarization fluctuations in optical signals having respective wavelengths λ1to λNincluded in the monitor light extracted by the splitter51. The respective configuration examples (A) to (D) illustrated inFIG. 6can also be applied to the polarization fluctuation detector56. An information collection section53collects information related to the speed of polarization fluctuations corresponding to respective wavelengths λ1to λNt detected by the polarization fluctuation detector56.

In the optical communication system, the polarization fluctuation detector56detects the speed of polarization fluctuations occurring in the optical signals having respective wavelengths λ1to λNtransmitted on the optical transmission path2and input to the optical reception device4, and the information collection section53collects the information. Then a parameter calculator54determines a target value of the control parameter p corresponding to the information obtained by the information collection section53, by referring to a table in which the speed of polarization fluctuations is expressed by multiple values as illustrated, for example, inFIG. 10, and a parameter setting section55sets the target value as the control parameter p for the optical receivers44-1to44-N corresponding to the respective wavelengths λ1to λN.

According to the optical communication system of the second embodiment, the influence of fast polarization fluctuations can be reliably compensated for by the polarization fluctuation compensation device5, even if the polarization state of the optical signals having respective wavelengths λ1to λNtransmitted on the optical transmission path2fluctuates faster than the normally assumed speed due to some factors, thereby enabling to perform the reception processing of the optical signals using the polarization information, with high accuracy.

Next is a description of a third embodiment of an optical communication system.

In the first embodiment, a part of the WDM light transmitted from the optical transmission device1to the optical transmission path2is extracted by a splitter51to detect whether optical signals having respective wavelengths have been polarization scrambled by the polarization scrambling detector52. However, when the optical transmission device1or the like has a function of outputting information related to the presence of polarization scrambling corresponding to respective wavelengths, the information related to the presence of polarization scrambling can be collected instead of directly monitoring optical signals having respective wavelengths, to determine the speed of polarization fluctuations in the non-polarization scrambled optical signal based on the information, thereby enabling to obtain a target value of the control parameter p for the optical receiver corresponding to each wavelength. The third embodiment corresponds to a configuration example in this case.

FIG. 14is a diagram illustrating a configuration example of the optical communication system of the third embodiment.

In the optical communication system inFIG. 14, the splitter51and the polarization scrambling detector52of the polarization fluctuation compensation device5in the first embodiment are omitted, and an information collection section53collects flag information indicating the presence of polarization scrambling to be output from respective optical transmitters11-1to11-M, and14-1to14-N inside the optical transmission device1to the outside. For example, it is assumed that the flag information indicates “1” when polarization scrambling is performed with respect to the optical signal output from the optical transmitter, and indicates “0” when polarization scrambling is not performed. The information collection section53collects the flag information matched with the respective wavelengths, thereby obtaining information related to the presence of polarization scrambling corresponding to all the wavelengths of the WDM light transmitted from the optical transmission device1to the optical transmission path2.

In the third embodiment, an example in which the presence of polarization scrambling is determined by using the flag information output from the respective optical transmitters11-1to11-M and14-1to14-N is described. However, information available for determining the presence of polarization scrambling is not limited to the example described above. For example, when reception processing is performed with respect to a non-polarization scrambled optical signal by the digital coherent receiver as illustrated inFIG. 2toFIG. 4, the respective optical receivers inside the optical reception device4can output to the outside, flag information indicating the presence of output of local oscillation light, and collect the flag information from the respective optical receivers by the information collection section53. Because polarization scrambling is not performed basically with respect to the optical signal to be received by the digital coherent receiver, when the flag information indicates that there is an output of local oscillation light, it can be determined that there is no polarization scrambling, and when the flag information indicates that there is no output of local oscillation light, it can be determined that there is polarization scrambling.

Moreover, for example, for a meshed network as illustrated inFIG. 15, when the polarization fluctuation compensation device5is used for a reception section of respective nodes1A to1D, information related to the presence of polarization scrambling corresponding to the respective wavelengths, transmitted from a network management system (NMS)6which manages the operation status of the entire network, to respective nodes1A to1D, can be used. In the meshed network inFIG. 15, because a transmission path for the optical signal received by the respective nodes1A to1D is different for each wavelength, the polarization fluctuations can be compensated for more reliably by determining the presence of polarization scrambling corresponding to respective reception wavelengths by using information managed in the NMS6, than by directly detecting the polarization state of the optical signal at the transmission ends of the respective nodes1A to1D.

Next is a description of a fourth embodiment of an optical communication system. In the fourth embodiment, an application example for improving compensation accuracy of polarization fluctuations in the first to third embodiments is described.

FIG. 16andFIG. 17are diagrams illustrating the configuration of the main part of the polarization fluctuation compensation device applied to an optical communication system of the fourth embodiment, together with a configuration example of optical receivers44-1to44-N.FIG. 16is an application example corresponding to the digital coherent receiver illustrated inFIG. 2, andFIG. 17is an application example corresponding to the direct detection receiver illustrated in FIG.5. The configuration of the entire optical communication system is the same as that illustrated inFIG. 1,FIG. 13, orFIG. 14.

InFIG. 16andFIG. 17, a polarization fluctuation compensation device5′ applied to the fourth embodiment further includes a signal quality monitor57in addition to the configuration of the polarization fluctuation compensation device5in any one of the first to third embodiments. The signal quality monitor57monitors information related to the quality of a reception signal output from a signal processing section408(FIG. 16) or signal processing sections414X and414Y (FIG. 17) of the optical receivers44-1to44-N, and feeds back the monitoring result to a parameter setting section55. As the information related to the signal quality monitored by the signal quality monitor57, for example; information of the number of error corrections detected at the time of error correction, a bit error rate (BER), or a ratio between average power and standard deviation of signal light monitored by a digital coherent receiver as described in Japanese Laid-Open Patent Publication No. 2009-198364 can be used.

FIG. 18is a flowchart illustrating one example of an operation performed by the polarization fluctuation compensation device5′. In the polarization fluctuation compensation device5′, at first, as in the case of the first to third embodiments, an information collection section53collects information related to the presence of polarization scrambling (or the speed of polarization fluctuations) in the optical signals having respective wavelengths (S210), and a parameter calculator54obtains a target value of the control parameter p by referring to the table illustrated inFIG. 8or the like, or according to the relational expression shown in the flowchart ofFIG. 11, by using the information from the information collection section53(S220). Then a parameter setting section55sets a control parameter in the reception processing performed by the respective optical receivers44-1to44-N, to the target value (S230).

In the subsequent step240, the signal quality monitor57monitors the information related to the quality of the reception signal output from the signal processing sections in the respective optical receivers44-1to44-N. In step250, the parameter setting section55determines whether the signal quality of the respective wavelengths monitored by the signal quality monitor57satisfies a preset reference value. When the signal quality satisfies the reference value, the value of the current control parameter value is maintained. On the other hand, when the signal quality does not satisfy the reference value, control proceeds to step260.

In step260, the parameter setting section55changes the value of the control parameter set in the optical receiver so as to improve the signal quality monitored by the signal quality monitor57, and control returns to step240. As the change of the control parameter, when the target value of the control parameter obtained in step220is αj, for example, the control parameter is sequentially changed to αj+1, αj−1, αj+2, αj−2, and so on, to search for a value at which the signal quality satisfies the reference value near the target value αj. Moreover, for example, when the number of candidate values of the control parameter is limited, such a value at which the signal quality satisfies the reference value for all the candidate values can be searched.

According to the optical communication system of the fourth embodiment, because setting of the control parameter is feedback controlled according to the actual reception signal quality in the respective optical receivers44-1to44-N, in addition to setting the control parameter with respect to the respective optical receivers44-1to44-N in a feed forward manner according to the target value obtained by the parameter calculator54of the polarization fluctuation compensation device5′, compensation of the polarization fluctuations can be more reliably performed.