Patent Abstract:
A method for an optical communication system and an optical communication system comprising a pump source configured to generate a pump signal having rotating polarization, a polarization sensitive receiver for receiving the optical signal having a polarization tracking cut-off frequency, wherein the polarization of the pump signal is configured to rotate at a predetermined frequency of polarization rotation and the frequency of polarization rotation of the pump signal is higher than the polarization tracking cut-off frequency of the receiver. Suitable for mitigation of cross-polarization modulation (XPolM) related effects in coherent polarization multiplexed quadrature phase shift keying (CP-QPSK) systems.

Full Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. §371 filing of International Application Number PCT/EP2011/071110 which was filed on Nov. 28, 2011, and which claims priority to, and the benefit of, EP patent application number 10 192 953.7, filed on Nov. 29, 2010. The contents of the aforementioned applications are hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention refers to a system and a method for signal processing in a communication system (e.g. an optical communication system). 
     BACKGROUND OF THE INVENTION 
     Driven by growing demand on transmission capacities, sophisticated modulation formats with improved spectral efficiency are becoming the established technology-of-choice for commercial usage in optical communication systems. 
     Relatively robust and price effective on-off keying, widely applied for data rates of up to 10 Gbit/s, is step by step replaced by transmission formats modulated in phase and polarization, enabling further increase of data rates to 40 Gbit/s, 100 Gbit/s and higher. 
     Hereto, combination of polarization multiplexing and coherent signal detection, namely “coherently-detected polarization multiplexed quadrature phase shift keying” (CP-QPSK), has been identified as a modulation format of choice for next generation high capacity transmission. 
     The main CP-QPSK drivers are that it doubles the spectral efficiency and the total capacity, it is part of the 100 G standardization according to the Optical Internetworking Forum (OIF), and all major system houses and component suppliers are currently working on CP-QPSK solutions. 
     On system level, it is of interest to reuse already implemented concepts when introducing new modulation formats. 
     This applies also for optical pump sources that are currently used in context with different applications. 
     As an example of applications for optical pump sources it is worthwhile to mention high power pump lasers for Raman amplifiers, which enable a further increase of span length or idler channels which guarantee optimum EDFA operation in Dense Wavelength Division Multiplexing (DWDM) systems with limited total channel count as well as link stabilization in submarine transmission systems, or dynamically controlled Continuous Wave (CW) channels, which allow for network stabilization in case of sudden power transients (e.g. due to fiber cut). 
     In all of the aforementioned applications, the power level of the pump signal is usually higher than the average power level of the in-service data signals. In order to prevent polarization dependent gain ripple or signal distortions due to FWM interactions or polarization-dependent Raman gain, an unpolarized CW light or a filtered ASE source is conventionally employed. 
     All these techniques are well known in the art and used in current Dense Wavelength Division Multiplexing (DWDM) systems with conventional modulation formats as, for example, On Off Keying (OOK), Optical Duo Binary (ODB), Differential Phase Shift Keying (DPSK) or Differential Quadrature Phase Shift Keying (DQPSK). 
     However, much more than other modulation formats, modulation formats based on polarization multiplexing are sensitive to random or deterministic polarization rotations induced by cross-polarization modulation (XPolM) between the optical signals propagating along a transmission link. 
     In particular, a critical role is played by the spectral region in which such polarization rotations fall. Accordingly, the following classification can be introduced:
         Slow polarization rotations, whose spectral components fall within the adaptive compensation speed of the receiver (&lt;100 kHz). These rotations are typically caused by mechanical and thermal stress effects cumulating during fiber propagation and the receiver is designed to fully compensate for them.   Fast polarization rotations, whose spectral components exceed the adaptive compensation speed of the receiver but fall within its electrical bandwidth (between 100 kHz and the polarization tracking cut-off frequency). These polarization rotations result from XPolM effects cumulating during fiber propagation and the adaptive algorithms within the receiver are not fast enough to compensate for them.   Ultra-fast polarization rotations, whose spectral components fall outside the electrical bandwidth of the receiver (above the polarization tracking cut-off frequency). These polarization rotations also result from XPolM effects cumulating during fiber propagation, but they are so fast that they are rejected by the electrical filter of the receiver and therefore must not be compensated for by the adaptive algorithms of the receiver itself.       

       FIG. 1  is a schematic representation of the application of a conventional depolarization technique. In particular,  FIG. 1  shows a combination of two delayed fractions of a single CW light  11 , the first fraction over a Variable Optical Attenuator  14  and the second fraction over an optical fiber  13  characterized by a fiber length L&gt;L coh , where L coh  is the coherence length. 
       FIG. 2  is a schematic representation of the application of a conventional depolarization technique. In particular,  FIG. 2  shows the combination  22  of two orthogonally polarized CW signals  23  and  26  from a single laser source  21 . It shows also the Polarization Beam Splitter PBS  24 , which splits the single laser source  21  in two orthogonally polarized CW signals  23  and  26 , and a Polarization Beam Combiner PBC  25 , which combines the two orthogonally polarized CW signals  23  and  26 . 
       FIG. 3  is a representation  31  of the degradation of the Bit Error Ratio BER  32  of a 40 G CP-QPSK optical signal due to a depolarized CW channel. In particular,  FIG. 3  shows the Bit Error Ratio BER  32  of a 40 G CP-QPSK optical signal versus the difference between the power of the Continuous Wave CW pump signal and the power of CP-QPSK optical signal for three different cases:
         a combination  34  of two delayed fractions of a single CW light, the CW signal being located 50 GHz away from the CP-QPSK optical signal;   a combination  35  of two orthogonally polarized CW signals and from a single laser source, the CW signal being located 50 GHz away from the CP-QPSK optical signal;   a combination  36  of two orthogonally polarized CW signals and from a single laser source, the CW signal being located 2.6 THz away from the CP-QPSK optical signal.       

     In the example, the CW pump and 40 G CP-QPSK co-propagate over a 700 km fiber link. The CW power level has been continuously increased relatively to the data signal. As shown in  FIG. 3 , significant bit-error ratio degradations in the CP-QPSK signal can be observed starting at power differences of 1 dB. When the CW signal is a direct neighbor of the CP-QPSK data signal FEC threshold of 10 −3  is violated when exceeding 5 dB pump-data power difference. Furthermore, significant distortions have been detected even when the CW signal is located 2.5 THz away from the CP-QPSK channel. 
     The example represented in  FIG. 3  clearly shows that a conventional depolarized CW signal causes significant penalties for polarization multiplexed data signals over a very wide spectral region, whose width depends on the power of the CW signal itself. For this reason, due to high power difference, similar distortions can be expected also from co-propagating Raman pumps, thus strongly limiting its application in optical transmission systems with polarization-multiplexed channels. 
     As an alternative, the use of filtered Amplified Spontaneous Emission (ASE) light has been investigated. In contrary to intentionally depolarized CW signals characterized by fast but deterministic polarization rotations, ASE light is completely depolarized, therefore polarization rotations are completely randomized and all polarization states are represented in it with the same probability. 
       FIG. 4  is a representation  41  of the degradation of the Bit Error Ratio BER  42  of a 40 G CP-QPSK optical signal due to a Amplified Spontaneous Emission (ASE) source. In particular,  FIG. 4  shows the Bit Error Ratio BER  42  of a 40 G CP-QPSK optical signal versus the difference between the power of the Amplified Spontaneous Emission (ASE) source and the power of CP-QPSK optical signal for three different cases:
         the Amplified Spontaneous Emission (ASE) source is located 350 GHz away  44  from the CP-QPSK optical signal;   the Amplified Spontaneous Emission (ASE) source is located 850 GHz away  45  from the CP-QPSK optical signal;   the Amplified Spontaneous Emission (ASE) source is located 2.60 THz away  46  from the CP-QPSK optical signal.       

     For this experiment, a wide-band, Amplified Spontaneous Emission (ASE) source filtered by two cascaded 50 GHz optical band-pass filters has been co-propagated together with a 40 G CP-QPSK signal over a 700 km fiber link, and the bit-error ratio of the data signal has been measured for different delta power values. As shown in  FIG. 4 , significant bit-error rate degradations of CP-QPSK signal over a large spectral region could be observed when increasing the power of the ASE source. This is due to the fact that the Cross Polarization Modulation (XPolM) induced random polarization rotations cover all of the above-mentioned categories: slow, fast and ultra fast. In this case, the fast polarization rotations were responsible for the observed impairment. 
     Cross polarization modulation effects are expected to affect not only standard coherent receivers based on digital signal processing, but also direct-detection receivers employing fast polarization controllers for input polarization demultiplexing. Indeed, current active polarization controllers can compensate only relatively slow polarization rotations (in the order of a hundred kHz) but would not be able to cope with fast polarization rotations induced by Cross Polarization Modulation (XPolM). 
     A large number of idler channels schemes, as well as transient suppression channels schemes based either on polarized/depolarized CW signals or on filtered ASE sources are known from the prior art. Cited, for example, is C. Headley, G. Agraval, “ Raman Amplification in Fiber Optics Communication Systems ” Academic Press, Dec. 30, 2004, or J. Chesnoy, G. Agrawal, I. P. Kaminow, and P. Kelley, “ Undersea Fiber Communication Systems ” Academic Press, October 3. 
     However, such conventional schemes have a severe impact on next generation transmission systems based on coherently detected polarization-multiplexed optical signals such as CPQPSK. 
     The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution that minimize the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems such as CP-QPSK. 
     SUMMARY OF THE INVENTION 
     In order to overcome the above-described need in the art, the present invention discloses an optical communication system for transmitting an optical signal, comprising a pump source configured to generate a pump signal having rotating polarization, a polarization sensitive receiver for receiving the optical signal having a polarization tracking cut-off frequency, wherein the polarization of the pump signal is configured to rotate at a predetermined frequency of polarization rotation and the frequency of polarization rotation of the pump signal is higher than the polarization tracking cut-off frequency of the receiver. 
     The polarization tracking cut-off frequency can be defined as a frequency of polarization variations that leads to factor of two higher BER at the polarization sensitive receiver comparing to case without. 
     In a next embodiment of the invention the optical signal is a coherently-detected polarization-multiplexed quadrature phase shift keying (CP-QPSK) signal. 
     It is also an embodiment, that the system further comprises a continuous wave (CW) laser source for generating a continuous wave (CW) laser signal. 
     In other alternative embodiments of the present invention, the system further comprises a polarization modulator for modulating the continuous wave (CW) laser signal and a radio frequency (RF) source connected with the polarization modulator for generating the polarization rotation of the pump signal. 
     In a further embodiment, the system further comprises a first polarization rotator for rotating the polarization of the continuous wave (CW) laser signal and thereby generating a first component of the pump signal. 
     In a next embodiment, the system further comprises an optical phase modulator for phase-modulating the continuous wave (CW) laser signal and a radio frequency (RF) source connected with the phase modulator, the optical phase modulator being configured to generate a second component of the pump signal. 
     In an alternative embodiment, the pump signal is a combination of the first component and the second component of the pump signal. 
     It is also an embodiment, that the first component and the second component of the pump signal have a different polarization with respect to each other. 
     In other alternative embodiments of the present invention, the radio frequency (RF) source is configured to adjust the frequency of polarization rotation of the pump signal. 
     In a further embodiment, the system further comprises a first continuous wave (CW) laser source for generating a first continuous wave laser signal and a second continuous wave (CW) laser source for generating a second continuous wave (CW) laser signal, the first and the second continuous wave (CW) laser signals being frequency detuned with respect to each other. 
     In a next embodiment, the system further comprises a second polarization rotator coupled with the second continuous wave (CW) laser source for rotating the polarization of the second continuous wave (CW) laser signal and thereby generating a second laser signal. 
     In an alternative embodiment, the pump signal is a combination of the first continuous wave (CW) laser signal and the second laser signal, the first continuous wave (CW) laser signal and the second laser signal having a different polarization with respect to each other. 
     It is also an embodiment, that the frequency of polarization rotation of the pump signal is adjustable by adjusting the frequency detuning of the first and the second continuous wave (CW) laser signals with respect to each other. 
     In a further embodiment, the apparatus further includes an optical fiber link for propagating the pump signal and the optical signal. 
     In an alternative embodiment, the polarization sensitive receiver is a coherent receiver. 
     The problem stated above is also solved by a method for transmitting an optical signal which includes: generating a pump signal having rotating polarization, transmitting the pump signal, transmitting the optical signal, receiving the optical signal by means of a polarization sensitive receiver having a polarization tracking cut-off frequency, rotating the polarization of the pump signal at a predetermined frequency of polarization rotation, the frequency of polarization rotation of the pump signal being higher than the polarization tracking cut-off frequency of the receiver. 
     The method, the apparatus and the system provided, in particular, bears the following advantages:
         a) They minimize the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems such as CP-QPSK.   b) They allow the generic usage of optical pump sources independently on modulation format of data channels.   c) They do not require necessarily the employment of high RF electronics.   d) They are easy to implement.   e) They provide a benefit related to power budget, mechanical footprint as well as cost efficiency.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained by way of example in more detail below with the aid of the attached drawings. 
         FIG. 1  is a schematic representation of the application of a conventional depolarization technique. 
         FIG. 2  is a schematic representation of the application of a conventional depolarization technique. 
         FIG. 3  is a representation  31  of the degradation of the Bit Error Ratio BER  32  of a 40 G CP-QPSK optical signal due to a depolarized CW channel. 
         FIG. 4  is a representation  41  of the degradation of the Bit Error Ratio BER  42  of a 40 G CP-QPSK optical signal due to an Amplified Spontaneous Emission (ASE) source. 
         FIG. 5  is a schematic representation of the application of a depolarization technique for application in polarization multiplexed transmission system according to an embodiment of the invention. 
         FIG. 7  is a schematic representation of the application of a depolarization technique for application in polarization multiplexed transmission system according to an embodiment of the invention. 
         FIG. 8  is a representation  81  of the degradation of the Bit Error Ratio BER  82  of a 40 G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources. 
         FIG. 9  is a representation  91  of the degradation of the Bit Error Ratio BER  92  of a 40 G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     As regards the description of  FIGS. 1 to 4 , reference is made to the background of the invention. 
     Illustrative embodiments will now be described with reference to the accompanying drawings to disclose the teachings of the present invention. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
       FIG. 5  is a schematic representation of the application of a depolarization technique for application in polarization multiplexed transmission system according to an embodiment of the invention. In particular,  FIG. 5  shows a pump source  51 , for example a Raman pump source, which includes a single CW light source  53 , for example a laser source. The CW light source  53  generates a CW laser signal  56  which is modulated by a polarization modulator  55 , in particular a fast polarization modulator, that generates the fast polarization rotation of the pump signal  52 . The required speed of polarization rotations can be generated using the RF source  54 , connected to the polarization modulator  55 , which is configured to adjust the frequency of polarization rotation of the pump signal  52 . The so generated pump signal  52  can be propagated in a fiber link together with an optical signal, for example a CP-QPSK signal. The optical signal and the pump signal  52  can be received by a polarization sensitive receiver having a given polarization tracking cut-off frequency. The ultra-fast polarization of the pump signal  52  can be adjusted in such a way that the frequency of polarization rotation of the pump signal  52  is higher than the polarization tracking cut-off frequency of the receiver. As a consequence, the spectral components of the polarization rotation can be naturally rejected by the polarization sensitive receiver, which may also be a coherent receiver. The polarization tracking cut-off frequency can be defined as a frequency of polarization variations that leads to factor of two higher BER at the polarization sensitive receiver comparing to case without. In this way the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems can be efficiently minimized. 
       FIG. 6  is a schematic representation of the application of a depolarization technique for application in polarization multiplexed transmission system according to an embodiment of the invention. In particular,  FIG. 6  shows a pump source  61 , for example a Raman pump source, which generates a pump signal  62 , combination of a phase modulated component  68  and a phase un-modulated component  67 , having different polarization, derived from a single CW laser source  63 . An optical coupler, for example a 3 dB coupler (not shown in  FIG. 6 ) may split the single laser source  63  in two differently polarized CW signals  623  and  626 . According to a different embodiment of the invention, the two CW signals  623  and  626  may be orthogonally polarized. An optical phase modulator  66 , phase-modulates the CW signals  626 , while a polarization rotator  65 , rotates the polarization of the CW signal  623  relative to the CW signal  626 . An optical coupler, for example a 3 dB coupler (not shown in  FIG. 6 ), may combine the two differently polarized CW signals  67  and  68 . The required speed of polarization rotations can be generated using the RF source  64 , connected to the optical phase modulator  66 , which is configured to adjust the frequency of polarization rotation of the pump signal  62 . The so generated pump signal  62  can be propagated in a fiber link together with an optical signal, for example a CP-QPSK signal. The optical signal and the pump signal  62  can be received by a polarization sensitive receiver having a polarization tracking cut-off frequency. The ultra-fast polarization of the pump signal  62  can be adjusted in such a way that the frequency of polarization rotation of the pump signal  62  is higher than the polarization tracking cut-off frequency of the receiver. As a consequence, the spectral components of the polarization rotation can be naturally rejected by the polarization sensitive receiver, which may also be a coherent receiver. In this way the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems can be efficiently minimized. 
       FIG. 7  is a schematic representation of the application of a depolarization technique for application in polarization multiplexed transmission system according to an embodiment of the invention. In particular,  FIG. 7  shows a pump source  71 , for example a Raman pump source, which generates pump signal  72 , combination of two frequency detuned, differently polarized CW laser signals  75  and  78  from two CW laser sources  73  and  74 . According to a different embodiment of the invention, the two CW signals  75  and  78  may be orthogonally polarized. A polarization rotator  77 , coupled with the second continuous wave (CW) laser source  74 , which rotates the polarization of the continuous wave (CW) laser signal  76  relative to the CW signal  75  thereby generating the CW signal  78 . The fast polarization rotation is generated by combining the two laser signal  78  and  75 . The frequency of polarization rotation of the pump signal  72  can be adjustable by adjusting the frequency detuning of the first  75  and the second  76  continuous wave (CW) laser signals with respect to each other. Hereto, speed of polarization rotation is directly related to the frequency detuning of the two CW channels. 
     The so generated pump signal  72  can be propagated in an fiber link together with an optical signal, for example a CP-QPSK signal. The optical signal and the pump signal  72  can be received by a polarization sensitive receiver having a polarization tracking cut-off frequency. The ultra-fast polarization of the pump signal  72  can be adjusted in such a way that the frequency of polarization rotation of the pump signal  72  is higher than the polarization tracking cut-off frequency of the receiver. As a consequence, the spectral components of the polarization rotation can be naturally rejected by the polarization sensitive receiver, which may also be a coherent receiver. In this way the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems can be efficiently minimized. 
     The embodiment of the invention comprising the two detuned CW laser signal  75  and  78  does not require necessarily the employment of high RF electronics; furthermore, it provides a benefit related to power budget, mechanical footprint as well as cost efficiency. 
       FIG. 8  is a representation  81  of the degradation of the Bit Error Ratio BER  82  of a 40 G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources. 
     In particular,  FIG. 8  shows the Bit Error Ratio BER  82  of a 40 G CP-QPSK optical signal versus the frequency offset  83  between the two orthogonally polarized, frequency detuned CW laser sources. 
     The 40 G CP-QPSK receiver is characterized by a polarization tracking cut-off frequency of around 8 GHz. 
     For frequency offset below 10 GHz, significant degradations of data signal can be observed when co-propagating with the pump in a fiber link (optical power variation ΔP=11.5 dB). However once the frequency offset is larger than a critical minimum detuning (in the example shown in  FIG. 8  ca. 12 GHz), the distorting influence of the high power pump signal is effectively suppressed. 
       FIG. 9  is a representation  91  of the degradation of the Bit Error Ratio BER  92  of a 40 G CP-QPSK optical signal due to two orthogonally polarized, frequency detuned CW laser sources. 
     In particular,  FIG. 9  shows the Bit Error Ratio BER  92  of a 40 G CP-QPSK optical signal versus the spacing  93  between the CW pump signal and the CP-QPSK signal. 
     Moreover,  FIG. 9  shows that a frequency offset Δf of 15 GHz between the two CW signals leads to distortion-free performance almost independently of the spacing between the CW pump and the CP-QPSK channel. 
     The experimental analysis confirmed that a minimum frequency detuning (polarization rotation speed) is required in order to effectively suppress impairments on polarization multiplexed data signals. Thus, controlled frequency spacing between the two CW lasers may be needed. 
     The present invention allows for generic usage of optical pump sources independently on modulation format of data channels. 
     For each modulation format and data rate, a critical minimum frequency detuning has to be identified individually. Theoretically, a fix value for the maximum frequency detuning cannot be established. Larger frequency offset may be beneficial for reducing of pump beating, however this may reduce the optical bandwidth available for useful channels&#39; transmission and may leads to higher sensitivity to frequency dependent polarization changes (due to second order polarization mode dispersion, SOPMD or polarization dependent loss, PDL). 
     As far as the CW sources are concerned, fixed laser with wavelength locker or a tunable laser can be used. The two CW lights have to be combined at different polarizations (preferably orthogonally polarized). The power level of the lasers should preferably be similar, but slight mismatches do not affect the performance significantly. 
     The present invention is not limited to the details of the above described principles. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. Mathematical conversions or equivalent calculations of the signal values based on the inventive method or the use of analogue signals instead of digital values are also incorporated. 
     LIST OF ABBREVIATIONS 
     
         
         ASE Amplified Spontaneous Emission 
         BER Bit Error Ratio 
         CW Continuous Wave 
         CP-QPSK Coherent Polarization-multiplexed Quadrature Phase Shift Keying 
         DPSK Differential Phase Shift Keying 
         DQPSK Differential Quadrature Phase Shift Keying 
         DWDM Dense Wavelength Division Multiplexing 
         EDFA Erbium Doped Fiber Amplifier 
         FWM Four Wave Mixing 
         ODB Optical Duo Binary 
         OIF Optical Internetworking Forum 
         OOK On Off Keying 
         OTSC Optical Transient Suppression for C-band 
         OSA Optical Spectrum Analyzer 
         PBC Polarization Beam Combiner 
         PBS Polarization Beam Splitter 
         PDL Polarization Dependent Loss 
         SOPMD Second Order Polarization Mode Dispersion 
         VOA Variable Optical Attenuator 
         XPolM Cross Polarization Modulation

Technology Classification (CPC): 7