Abstract:
An optical communication system includes a digital signal processer coupled to the coherent receiver, said coherent receiver including a nonlinearity compensation module for compensating for nonlinear effects in fiber in the optical link for increasing capacity or transmission distance of the fiber, the nonlinearity compensation module includes a spectral slicing of the signal into bands, computing nonlinear interaction between the bands with parameters opposite to those of the fiber to reverse the non-linear effects in the fiber, and only certain nonlinear interactions between bands are considered thereby reducing complexity of the nonlinearity compensation.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/711,297, filed Oct. 9, 2012, entitled “Intra-Channel XPM Compensation for Single-Stage Backward-Propagation”, of which the contents are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to optical systems and, more particularly, to inter-band cross-phase modulation compensation for the mitigation of intra-channel nonlinear impairments in optical fiber transmission. 
     Although the following references, articles or publications are referred to in this specification., they are NOT considered relevant to the patentability of the claims herein. They are noted to provide complete information, regardless of their materiality to the claims. [1] Watanabe, S.; Shirasaki, M.; “Exact compensation for both chromatic dispersion and Kerr effect in a transmission fiber using optical phase conjugation,”  Lightwave Technology, Journal of , vol. 14, no. 3, pp. 243-248, Mar 1996. [2] Mateo, E. F.; Xiang Zhou; Guifang Li; , “Electronic phase conjugation for nonlinearity compensation in fiber communication systems,”  Optical Fiber Communication Conference and Exposition  ( OFC/NFOEC ), 2011  and the National Fiber Optic Engineers Conference , vol., no., pp. 1-3, 6-10 Mar. 2011. [3] Ip, E. M.; Kahn, J. M.; , “Fiber Impairment Compensation Using Coherent Detection and Digital Signal Processing,”  Lightwave Technology, Journal of , vol. 28, no. 4, pp. 502-519, Feb. 15, 2010. [4] Lei Li; Zhenning Tao; Liang Dou; Weizhen Yan; Oda, S.; Tanimura, T.; Hoshida, T.; Rasmussen, J. C.; “Implementation efficient nonlinear equalizer based on correlated digital backpropagation,”  Optical Fiber Communication Conference and Exposition  ( OFC/NFOEC ), 2011  and the National Fiber Optic Engineers Conference , vol., no., pp. 1-3, 6-10 Mar. 2011. [5] E. Mateo, M. Huang, F. Yaman, T. Wang, Y. Aono, and T. Tajima, “Nonlinearity compensation using very-low complexity backward propagation in dispersion managed links,” in  Optical Fiber Communication Conference , OSA Technical Digest (Optical Society of America, 2012), paper OTh3C.4. [6] NECLA IR No. 10112 entitled “Equivalent-Link Backward Propagation Method for Nonlinearity Compensation in Fiber Transmission Systems”. [7] Fludger, C. R. S.; Duthel, T.; van den Borne, D.; Schulien, C.; Schmidt, E.-D.; Wuth, T.; Geyer, J.; De Man, E.; Khoe Giok-Djan; de Waardt, H.; , “Coherent Equalization and POLMUX-RZ-DQPSK for Robust 100-GE Transmission,”  Lightwave Technology, Journal of , vol. 26, no. 1, pp. 64-72, Jan. 1, 2008. [8] E. F. Mateo, X. Zhou, and G. Li “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems”  Optics Express  19(2), pp. 570-583. [9] Seb J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16, 804-817 (2008). 
     The capacity of optical fiber is ultimately limited by the Kerr nonlinearity, where refractive index changes with field intensity, causing localized phase shift proportional to power as the signal propagates. This nonlinear phase shift, which accumulates over distance, together with the action of fiber dispersion could severely distort the signal. Such distortion sets an upper limit for fiber capacity at a given transmission distance or alternatively, it limits the transmission distance for a given fiber capacity. 
     Two main approaches have been taken over the years for the nonlinearity compensation (NLC) in optical fiber transmission: Optical techniques and digital signal processing DSP techniques. 
     Optical Techniques: This approach is based on the generation optical phase conjugation. Optical phase conjugation can be used to compensate both fiber dispersion and fiber nonlinearity provided that the transmission link has certain symmetry properties. Typically, optical phase conjugation is implemented by using optical nonlinear processes such as wavelength conversion or Four-wave mixing [1]. Recently, a method to generate optical phase conjugation in the opto-electronic domain was proposed [2]. 
     DSP Techniques: With the advent of coherent detection technologies, the compensation of fiber impairments such as, chromatic dispersion, polarization mode dispersion or fiber nonlinearity, could be now implemented in the digital domain by means of Digital Signal Processing (DSP) methods. Many different methods have been proposed to compensate fiber nonlinearity using DSP techniques. Amongst them, Digital Back-propagation (DBP) has been widely studied and tested in many different transmission links [3]. Although effective, DBP is extremely resource hungry for the current DSP platforms. As a consequence, significant efforts have been made to simplify the DBP algorithms. Some examples of that are the ones published in [4] for dispersion unmanaged links and [5, 6] for dispersion managed links. However, the DSP complexity is still very large and further simplifications have to be made for practical implementation of nonlinear compensation algorithms. This is the main purpose of the invention. 
     Accordingly, there is a need for a solution to compensate nonlinear effects in fiber that can increase fiber capacity and/or transmission distance beyond their limits. 
     BRIEF SUMMARY OF THE INVENTION 
     In an aspect of the present invention, an optical communication system includes a transmitter; an optical link coupled to the transmitter; a coherent receiver coupled to the optical link for receiving a signal; and a digital signal processer coupled to the coherent receiver, said coherent receiver including a nonlinearity compensation module for compensating for nonlinear effects in fiber in the optical link for increasing capacity or transmission distance of the fiber, the nonlinearity compensation module includes a spectral slicing of the signal into bands, computing nonlinear interaction between the bands with parameters opposite to those of the fiber to reverse the non-linear effects in the fiber, and only certain nonlinear interactions between bands are considered thereby reducing complexity of the nonlinearity compensation. 
     In a similar aspect of the present invention, a method for an optical communication system includes transmitting a signal from a transmitter; coupling an optical link to the transmitter; coupling a coherent receiver to the optical link for receiving a signal; and employing a digital signal processing coupled to the coherent receiver for providing a nonlinearity compensation module for nonlinear effects in fiber in the optical link thereby increasing capacity or transmission distance of the fiber, the nonlinearity compensation including a spectral slicing of the signal into bands with a computing nonlinear interaction between the bands with parameters opposite to those of the fiber to reverse the non-linear effects in the fiber, and only certain nonlinear interactions between bands being considered thereby reducing complexity of the nonlinearity compensation. 
     These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a generic optical fiber communication system with coherent in which the invention can be employed; 
         FIG. 2  is diagram of exemplary digital signal processing DSP stages of a digital coherent receiver in which the inventive nonlinearity compensation  202  is employed; 
         FIG. 3  is a diagram of a multi-stage implementation of inter-band nonlinearity compensation, in accordance with the invention; 
         FIG. 4  is a diagram of the multi-band nonlinear operator  303  shown in  FIG. 3 ; 
         FIG. 5  is a diagram of the module  401  operator for the band k shown in  FIG. 4 ; 
         FIG. 6  is a diagram of the module  501 , shown in  FIG. 5 , for calculation of φ xk  (t) and φ yk  (t); 
         FIG. 7  is a diagram of module  502 , shown in  FIG. 5 , for calculation of θ k  (t); 
         FIG. 8  is a diagram showing application and experimental results of application of the invention to experimental data obtained from fiber transmission over 3400 km of dispersion managed link; and 
         FIG. 9  is a diagram showing an exemplary computer to perform the inventive nonlinearity compensation, in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is directed to a technique that drastically reduces the number of stages and therefore, the power consumption of the DSP chip. Typically, DSP algorithms based on DBP require multiple algorithmic stages. In the classic DBP method, each stage involves two operations, namely: frequency-domain chromatic dispersion compensation (CDC) and time-domain nonlinearity de-rotation (NLdR). The number of required stages depends on the transmission length, dispersion map and channel optical power. In general, the number of stages is chosen to be a compromise between performance and complexity. However, a minimum number of stages are necessary to achieve some performance improvement. Because frequency-domain CDC typically involves fast Fourier transform pairs (FFT/IFFT), the algorithmic complexity of classic DBP is prohibitive. The present invention drastically reduces the number of stages. 
       FIG. 1  is shows a block diagram of a fiber communication system with coherent detection. The transmitter  101  communicates over an optical link  102  for reception by a coherent receiver  103  that outputs to a digital signal processing DSP module  104 . In general, the transmitter could be any modulation format (QPSK, M-QAM, OFDM . . . ) including dual polarization. 
     A conventional coherent receiver is considered for this invention [3,7]. After coherent detection, the signal is digitized and processed through digital signal processing DSP.  FIG. 2  shows a typical flow for the coherent detection DSP stages. The nonlinearity compensation module  20  is the inventive stage. Responsive to a retiming and orthonormalization module  201 , the NLC module outputs to a polarization mode detection compensation and polarization module  203 , followed by a timing recovery module  204 . The timing recovery module outputs to a carrier frequency estimation module  205  that is coupled to a carrier phase estimation module, a de-mapping module  207  and a decoding module  208 . 
     The NLC  202  inventive stage includes a technique based on a spectral slicing of the signal. After slicing, the nonlinear interaction between bands is computed. When such interaction is calculated with parameters opposite to the fiber ones, the nonlinear effects can be reversed. The key of the NLC based invention is that only certain nonlinear interactions between bands are considered, which can simplify the procedural complexity. 
     Physical and Mathematical Background 
     After coherent detection, and optical signal is digitized and re-sampled. The time domain signals can be expressed as  X (t) and  Y (t) for x and y polarizations respectively.  X (t) and  Y (t) can be transformed into multiple bands by sharp filtering in the frequency domain. Such decomposition into N bands can be expressed as, 
                 X   _     ⁡     (   t   )       =       ∑     k   =   1     N     ⁢           ⁢         X   k     ⁡     (   t   )       ⁢     ⅇ     ⅈ   ⁢           ⁢   k   ⁢           ⁢     Ω   k     ⁢   t                     and                 Y   _     ⁡     (   t   )       =       ∑     k   =   1     N     ⁢           ⁢         Y   k     ⁡     (   t   )       ⁢     ⅇ     ⅈ   ⁢           ⁢   k   ⁢           ⁢     Ω   k     ⁢   t                 
where Ω k  is the center frequency of each band.
 
     By applying the XPM formalism for WDM channels [8], the nonlinear interaction between bands can be described by the following set of coupled equations, 
                         -       ∂     X   k         ∂   z         +       (       α   2     +     L   CD     +     ⅈ   ⁢           ⁢     C   xk         )     ⁢     X   k       +     ⅈ   ⁢           ⁢     Q   k     ⁢     Y   k         =   0     ,     
     ⁢         -       ∂     Y   k         ∂   z         +       (       α   2     +     L   CD     +     ⅈ   ⁢           ⁢     C   yk         )     ⁢     Y   k       +       ⅈQ   k   *     ⁢     X   k         =   0     ,           (   1   )               
Where c (x,y)k  and Q k  represent the intra-channel XPM (iXPM) contribution and intra-channel polarization mixing (iPolM) term, respectively, i.e:
 
                     C       (     x   ,   y     )     ⁢   k       =     -     γ   (              X   k          2     +            Y   k          2     +       ∑     m   ≠   k               ⁢           ⁢     R       (     x   ,   y     )     ⁢   m           )               (   2   )                   Q   k     =     -     γ   (       ∑     m   ≠   k               ⁢           ⁢       X   m   *     ⁢     Y   m         )         ,           (   3   )               
where, and R (x)k =2|X k | 2 +|Y k | 2  and R (y)k =2|Y k | 2 +|X k | 2 . The first two terms on the right hand side of Eq. (2) represent the intra-band nonlinearity whereas R (x,y)m  includes the inter-band contribution. The operator L CD  represents the chromatic dispersion of the fiber and depends on the dispersion map. Therefore, in general, the CD operators are time and distance dependent, i.e. L CD (t, z). The parameter γ is the effective nonlinear parameter and its value is obtained through optimization. Equations (1) are a set of partial differential equations that have no analytical solution. Typically, Eqs. (1) are solved using the split-step method. This method uses several stages in which the system can be uncoupled into its linear and nonlinear parts. This is the typical multi-stage implementation for back-propagation.  FIG. 3  shows an schematic of the multi-stage solution of Eqs. (1).
 
     First, the signal is sliced into bands in the frequency domain to perform the operations, 
                 X   _     ⁡     (   t   )       =       ∑     k   =   1     N     ⁢           ⁢         X   k     ⁡     (   t   )       ⁢     ⅇ     ⅈ   ⁢           ⁢   k   ⁢           ⁢     Ω   k     ⁢   t                     and                 Y   _     ⁡     (   t   )       =       ∑     k   =   1     N     ⁢           ⁢         Y   k     ⁡     (   t   )       ⁢     ⅇ     ⅈ   ⁢           ⁢   k   ⁢           ⁢     Ω   k     ⁢   t                 
Module  301  performs such operation in the frequency domain, where digital filters are used to cut the original spectrum create the bands. Then, the linear operator ( 302 ) performs CD compensation to each band. CD compensation can be performed either in the frequency domain using a FFT/IFFT pair or in time domain using FIR filters [9]. Both operations can be combined using look-up-table (LUT) methods in order to simplify the DSP (see [10]). The amount of dispersion to be compensated at each stage depends on the transmission link and the number of stages. In dispersion unmanaged links, the amount of CDC per stage is the total CD of the link divided by the number of stages [3]. In dispersion managed links, the amount of CDC per stage is the residual CD of the link divided by the number of stages [5,6]. In order to obtain the nonlinear operator ( 303 ), Eqs. (1) have to be solved for L CD =0. We have used a perturbation approach together with an dispersive walk-off factorization approach [8]. Finally, the nonlinear operators can be expressed as:
 
 X   k   out ( t )= X   k   in ( t ) e   iφ     xk     (t)   −iY   k   in ( t ) e   iφ     ym     (t) δ k ( t )
 
 Y   k   out ( t )= Y   k   in ( t ) e   iφ     yk     (t)   −iX   k   in ( t ) e   iφ     xm     (t) δ k *( t )  (4)
 
Where the functions φ (x,y)k (t) and δ k (t) are given by,
 
     
       
         
           
             
               
                 
                   
                     
                       
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     The functions h km (t) are the impulse response of the filters H km (ω). Filters H km (ω) account for the dispersive walk-off effect. The filters H km (ω) are given by the following expression, 
                       H   km     ⁡     (   ω   )       =     γ   ⁢           ⁢           ⅇ       -   ⅈ     ⁢           ⁢     d   km     ⁢   ω   ⁢           ⁢     L   /   M         -   1       α   -     ⅈ   ⁢           ⁢     d   km     ⁢   ω         .               (   6   )               
where d km =β 2 (ω k −ω m ) is the walk-off parameter. The parameter β 2  is the chromatic dispersion constant of the fiber and L is the transmission distance. For dispersion managed links, β 2  is replaced by  β   2  which is the dispersion parameter of the equivalent link [5,6]. Following are diagrams for the operations inside the module  303 .
 
       FIG. 4  shows the multiband operator  303 . The operator comprises block for each of the bands.  FIG. 5  show the operator for band k,  401 . In  FIG. 5 , the module  501  calculates the phase shifts φ (x,y)k  whereas the module  402  carries out the calculation of the function θ k . Details of  502  and  503  are shown in  FIGS. 6 and 7  respectively. Module  503  performs an exponential operation using look-up table (LUT). Module  504  carries out phase conjugation. 
       FIG. 6  show the block diagram for the operator  501 . The module  601  performs the convolution between |X k (t)| 2 , |Y k (t)| 2 , with the impulse responses, h km (t). Convolution can be performed using FIR filters in time domain or FFT/IFFT pair in frequency domain. The coefficients of the filters h km (t) are obtained by performing inverse Fourier (IFFT) transform of the filters, H km (ω) which depend on the chromatic dispersion of the link through the walk-off parameters d km . The values of the filters are static and can be stored in a memory location so they cannot be calculated every time. 
     Example of Application and Experimental Results 
     As an example of application,  FIG. 8  shows results from the application of this invention to experimental data obtained from fiber transmission over 3400 km of dispersion managed link. Results are shown for M=1 stages, showing that an improvement of 0.5 dB can be obtained with only one stage. This is the maximum improvement obtained from a single-stage algorithm to our knowledge. 
     As  FIG. 8  shows, there are an optimum number of bands for best performance in a single-stage operation. The reason is that for large number of bands, the FWM between bands have to be considered for better performance. This significantly increases the algorithm complexity and it is not included in this invention. For a small number of bands, performance becomes limited by the chromatic dispersion. The optimum number of bands depends on the link characteristics and the trade-off between performance and complexity. 
     The invention may be implemented in hardware, firmware or software, or a combination of the three. Where parts of the invention are implemented in a computer program, they may be executed on a programmable computer having a processor, a data storage system, volatile and non-volatile memory and/or storage elements, at least one input device and at least one output device. 
     By way of example, a block diagram of a computer to support the system is discussed next in  FIG. 9 . The computer preferably includes a processor, random access memory (RAM), a program memory (preferably a writable read-only memory (ROM) such as a flash ROM) and an input/output (I/O) controller coupled by a CPU bus. The computer may optionally include a hard drive controller which is coupled to a hard disk and CPU bus. Hard disk may be used for storing application programs, such as the present invention, and data. Alternatively, application programs may be stored in RAM or ROM. I/O controller is coupled by means of an I/O bus to an I/O interface. I/O interface receives and transmits data in one of or combination of analog or digital form over one or a number of communication links such as a serial link, local area network, wireless link, optical link and parallel link. Optionally, a display, a keyboard and a pointing device (mouse) may also be connected to I/O bus. Alternatively, separate connections (separate buses) may be used for I/O interface, display, keyboard and pointing device. Programmable processing system may be preprogrammed or it may be programmed (and reprogrammed) by downloading a program from another source (e.g., a floppy disk, CD-ROM, or another computer). 
     Each computer program is tangibly stored in a machine-readable storage media or device (e.g., program memory or magnetic disk) readable by a general or special purpose programmable computer, for configuring and controlling operation of a computer when the storage media or device is read by the computer to perform the procedures described herein. The inventive system may also be considered to be embodied in a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein. 
     From the foregoing it can also be appreciated that the invention, unlike previous solutions, provides significant competitive/commercial value. Nonlinearity compensation is a fundamental aspect in future transceivers based on coherent detection and it has been identified as a key technology by the fiber communications community. However, the difficulties in terms of complexity have been preventing its implementation in real products. This invention significantly simplifies the complexity of DSP-based NLC by giving up some performance. By implementing a NLC method in the current product, a significant commercial advantage can be obtained with respect to competitors. 
     The foregoing is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. Additional details are provided in the accompanying “Appendix to Specification”. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that those skilled in the art may implement various modifications without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.