Abstract:
Disclosed are a digital equalization apparatus for a coherent optical receiver and a digital equalization method for a coherent optical receiver, capable of compensating for chromatic dispersion and polarization impairment through a digital signal processing, and capable of performing a clock recovery and a data recovery through a digital symbol synchronization. The digital equalization apparatus and the method compensate for various impairments occurring on an optical path in a digital manner and achieve synchronization through a simple structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2009-0063202, filed on Jul. 10, 2009, the disclosure of which is incorporated by reference in its entirety for all purposes. 
     BACKGROUND 
     1. Field 
     The following description relates to an equalization technology for a coherent optical receiver used in optical communications. 
     2. Description of the Related Art 
     Coherent optical communication is a type of communication method that performs signal reception in a manner that input optical signals interfere with an optical source of a local oscillator signal and intensity variation and phase variation of the optical signals and phase variation are detected. 
     The coherent optical communication achieves higher receiver sensitivity and is more robust against noise such as amplified spontaneous emission (ASE) as compared to direct detection, and as such coherent optical communication has garnered a large amount of interest and many studies have been undertaken recently on this technology. 
     In general, an optical receiver for a coherent optical communication is provided with an optical phase-locked loop (PLL) or an optical polarization controller to process received optical signals. In addition, the optical receiver may be provided with an equalizer to compensate for impairment occurring on an optical path, such as chromatic dispersion and polarization mode dispersion. 
     In order for the optical receiver to process optical signals, a configuration to control a phase or a polarization of light is required. In general, the phase control or the polarization control has been performed in optical domains. However recently, with the development of digital signal processing technologies, an attempt to control phase or polarization of light has been made in a digital manner. 
     Meanwhile, for the purpose of synchronization of received signals in the optical receiver, a clock signal is recovered from the received signal and the received signal is recovered based on the recovered clock signal. 
     SUMMARY 
     Accordingly, in one aspect, there are provided an equalization apparatus and an equalization method for a coherent optical receiver, in which chromatic dispersion and polarization impairment are compensated for in a digital manner, and a digital symbol synchronization processing is implemented. 
     In one general aspect, there is provided an equalization apparatus for a coherent optical receiver including a photoconverter to convert a received optical signal into a digital signal; and a signal processor to compensate for chromatic dispersion and polarization dispersion in the optical signal by processing the converted digital signal and to perform symbol synchronization in a digital manner by use of a clock different from a sampling clock of the photoconverter. 
     The signal processor performs a clock recovery and a data recovery independent from an optical converter. 
     The signal processor includes a chromatic dispersion compensation unit to receive an output of the photoconverter and compensate for the chromatic dispersion; a digital symbol synchronization unit to receive an output of the chromatic dispersion compensation unit and perform the symbol synchronization; a polarization compensation unit to receive an output of the digital symbol synchronization unit and compensate for the polarization impairment; and a frequency and phase compensation unit to receive an output of the polarization compensation unit and compensate for frequency difference and phase noise between a local oscillation signal of the photoconverter and the optical signal. 
     The digital symbol synchronization unit includes an interpolation unit to perform interpolation on the output signal of the chromatic dispersion unit; and a clock determination unit to determine an operation clock for the interpolation unit by detecting timing error of the interpolation unit and using the detected timing error. The interpolation unit samples data in a middle of a symbol based on the operation clock. 
     In another general aspect, there is provided an equalization method for a coherent optical receiver. The method is performed as follows. A received optical signal is converted into a digital signal. Chromatic dispersion of the optical signal is compensated for. Symbol synchronization is performed on the signal, which has been compensated for chromatic dispersion, in a digital manner. Polarization impairment of the optical signal is compensated for. Frequency difference and phase noise between the optical signal and a local oscillation signal, which is used to convert the optical signal into the digital signal, is compensated for. 
     According to the present invention, various kinds of impairments generated on an optical path are compensated for, thus the efficiency of optical communication is enhanced. In addition, the use of digital equalization scheme simplifies the structure of the equalization apparatus and enhances the equalization efficiency. 
     Other features will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the attached drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an exemplary digital equalization apparatus for a coherent optical receiver. 
         FIG. 2  is a block diagram showing an exemplary signal processor. 
         FIG. 3  is a conceptual diagram of an exemplary synchronization. 
         FIG. 4  is a block diagram showing an exemplary digital symbol synchronization unit. 
         FIG. 5  is a block diagram showing an exemplary polarization compensation unit. 
         FIG. 6  is a view showing an exemplary digital equalization method for a coherent optical receiver. 
         FIG. 7  is a block diagram showing another exemplary digital equalization apparatus for a coherent optical receiver. 
         FIG. 8  is a block diagram showing still another exemplary digital equalization apparatus for a coherent optical receiver. 
     
    
    
     Elements, features, and structures are denoted by the same reference numerals throughout the drawings and the detailed description, and the size and proportions of some elements may be exaggerated in the drawings for clarity and convenience. 
     DETAILED DESCRIPTION 
     The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses and/or systems described herein. Various changes, modifications, and equivalents of the systems, apparatuses and/or methods described herein will suggest themselves to those of ordinary skill in the art. Descriptions of well-known functions and structures are omitted to enhance clarity and conciseness. 
     Hereinafter, an exemplary equalization apparatus will be described with reference to accompanying drawings. 
       FIG. 1  is a block diagram showing an exemplary digital equalization apparatus for a coherent optical receiver. 
     As shown in  FIG. 1 , the equalization apparatus  100  includes a photoconverter  110  and a signal processor  120 . 
     The photoconverter  110  converts received optical signals into digital signals. For example, if the optical signal is a coherent optical signal modulated in a phase shift keying (PSK) scheme, the photoconvertor  110  splits the received optical signal into polarization component and generates I-channel signals (In phase signal) and Q-channel signals (Quadrature phase signal) from each polarization component. As shown in  FIG. 1 , the digital signals generated by the photoconverter  110  are represented in the form of I x , Q x , I y  and Q y . In this case, I and Q indicate an I-channel signal and a Q-channel signal, respectively, and x and y indicate an x-polarization signal and a y-polarization signal, respectively. 
     The photoconverter  110  includes a local oscillation unit  111 , an optical splitting unit  112 , an optical mixing unit  113 , a photo-receiver unit  114  and an analog-digital converter (ADC)  115 . 
     A received optical signal is split into two polarization components by the optical splitting zo unit  112 , and a local oscillation signal generated from the local oscillation unit  111  is also split into two polarization components by the optical splitting unit  112 . The optical splitting unit  112  may be implemented using a polarization beam splitter (PBS). The optical splitting unit  112  splits a received signal into an x-polarization component and a y-polarization component. 
     The x-polarization component and the y-polarization component of each of the optical signal and the local oscillation signal are input into the optical mixing unit  113 . The optical mixing unit  113  mixes each polarization component of the optical signal with each polarization component of the local oscillation signal. The optical mixing unit  113  may be implemented using an optical hybrid which mixes signals to output signals having a phase difference of 90°. 
     The output of the optical mixing unit  113  is input into the photo-receiver unit  114 . The photo-receiver unit  114  may be implemented as a photo-receiver which converts received signals into electric signals. Since I-channel signals and Q-channel signals are generated corresponding to both polarization components, the output from the photo-receiver unit  114  are represented as four types of signals including I x , Q x , I y  and Q y . 
     The signals I x , Q x , I y  and Q y  are input into the ADC  115 , and the ADC  115  samples the signals (I x , Q x , I y  and Q y ), so that the signals I x , Q x , I y  and Q y  are converted into digital signals. In the present embodiment, the sampling rate of the ADC  115  is independent from the symbol rate of the optical signals. For example, the sampling rate of the ADC  115  is set as a value close to a general sampling rate, that is, a Nyquist rate. 
     The digital signals of the ADC  115  are input into the signal processor  120 . The signal processor  120  demodulates and decodes the optical signals through a digital signal processing. 
     In detail, the signal processor  120  processes the digital signal converted in the photoconverter  110  to compensate for chromatic dispersion and polarization impairment of the optical signals. In addition, the signal processor  120  performs a symbol synchronization in a digital manner by use of a clock signal independent from a sampling clock signal of the photoconverter  110  (hereinafter the term “clock signal” may be referred simply to as “clock” for ease of reference). 
     For example, in the symbol synchronization, the signal processor  120  performs clock recovery and data recovery independent from a sampling clock of the ADC  115 . 
       FIG. 2  is a block diagram showing an exemplary signal processor. 
     As shown in  FIG. 2 , the exemplary signal processor  120  includes a chromatic dispersion compensation unit  201 , a digital symbol synchronization unit  202 , a polarization compensation unit  203 , a frequency and phase compensation unit  204  and a decoding unit  205 . 
     Each of the chromatic dispersion compensation unit  201 , the digital symbol synchronization unit  202  and the frequency and phase compensation unit  204  is subdivided corresponding to each of the polarization component. In addition, a signal processing for the I-channel signals is separately performed from a signal processing for the Q channel signals. 
     The chromatic dispersion compensation unit  201  processes the output of the photoconverter  110  to compensate for chromatic dispersion in the optical signals. Since the chromatic dispersion has a linear characteristic, the chromatic dispersion can be compensated for based on a chromatic dispersion value of optic fiber constituting a transmission path. For example, the chromatic dispersion may be compensated for through a finite impulse response (FIR) filter having a filter coefficient derived from a chromatic dispersion value of optical fiber. 
     The digital symbol synchronization unit  202  processes the output of the chromatic dispersion compensation unit  201  to perform digital symbol synchronization. In a conventional optical receiver, a clock signal is recovered from a received signal for the purpose of synchronization, and the recovered clock signal is used to recover the received signal. In this case, a process of recovering a clock signal is referred to as clock recovery, and a process of recovering a received signal is referred to as data recovery. The digital symbol synchronization unit  202  simultaneously performs the clock recovery and the data recovery in a digital manner. 
     In the present embodiment, the digital symbol synchronization unit  202  samples a predetermined data in a symbol. In this case, sampling timing of the digital symbol synchronization unit  202  is independent from sampling timing of the ADC  115 , and is determined based on timing error detection and a feedback of detected timing error. 
     The polarization compensation unit  203  processes signals, which have been subject to digital symbol synchronization, to compensate for polarization impairment. The polarization impairment may refer to polarization mode dispersion (PMD) or polarization dependent loss (PDL). For example, when an optical signal is split into two polarization components by the optical splitter  112 , one of the polarization components may contain a modulated x-polarization component (referred to as ‘x′’) and a modulated y-polarization component (referred to as ‘y′’). The polarization compensation unit  203  separates the modulated polarization components (x′ and y′) from an x-polarization signal or a y-polarization signal. 
     The output of the polarization compensation unit  203  is input into the frequency and phase compensation unit  204 . According to the present embodiment, a received optical signal may interfere with a local oscillation signal generated from the local oscillation unit  111  (see  FIG. 1 ). In that case, a laser frequency difference between the optical signal and the local oscillation signal may be generated. The frequency and phase compensation unit  204  compensates for the laser frequency difference by estimating a laser frequency offset. In addition, since the optical signal and the local oscillation signal have a finite laser linewidth, a phase noise may be generated. The frequency and phase compensation unit  204  compensates for such phase noise. 
     In this regard, output signals of the frequency and phase compensation unit  204  may have phase information identical to phase information of signals that are originally output from a sending end. The output signals of the frequency and phase compensation unit  204  are input into the decoding unit  205 , and the decoding unit  205  extracts a bit sequence from phase information of the signals. 
       FIG. 3  is a conceptual diagram of exemplary symbol synchronization. 
     An analog signal is converted into a digital signal by a sampler such as an analog-digital converter (ADC). In this case, the sampler needs to determine a clock for a symbol synchronization between a sending party and a receiving party. Methods of determining a clock and performing symbol synchronization are classified into three types including an analog method, a hybrid method and a digital method. 
     In the analog method (a), a clock is generated in an analog manner, and sampling is performed in an analog manner based on the generated clock. In the hybrid method (b), a clock is generated in a digital manner, and sampling is performed in an analog manner based on the recovered clock. In the digital method (c), a clock used in a sampler is arbitrarily provided, and clock recovery and sampling are simultaneously performed. 
     For example, in the exemplary digital equalization apparatus  100  employing the digital method (c), the sampling rate of the ADC  115  is set as a predetermined value satisfying Nyquist Theory. The symbol synchronization is performed in the signal processor  120  independent from the sampling rate of the ADC  115 . 
       FIG. 4  is a block diagram showing an exemplary digital symbol synchronization unit. 
     As shown in  FIG. 4 , the digital symbol synchronization unit  202  includes an interpolation unit  410 , a clock determination unit  420  and a decimation filter unit  430 . 
     The interpolation unit  410  performs an interpolation on the output signal of the chromatic dispersion compensation unit  201 . For example, the interpolation unit  410  calculates values between sample values and generates a sequence of digital signals. The generated digital signal sequence passes through the decimation filter unit  430  and then input into the polarization compensation unit  203  in the form in which each symbol corresponds to one sample. 
     The clock determination unit  420  determines a clock and applies the determined clock to the interpolation unit  410 . The clock determination unit  420  calculates a clock allowing the interpolation unit  410  to sample data in the middle of the symbol. 
     For example, if an optical signal is a PSK-modulated signal, a timing error detector  421  calculates timing error by use of PSK signal characteristics. A loop-filter  422  having a proportional-and-integral structure controls a generation period of the clock in a timing processor  423  by use of the calculated timing error. A clock generated from the timing processor  423  is input into the interpolation unit  410 , and the interpolation unit  410  samples data in the middle of a symbol based on the applied clock. 
     The timing error represents an extent by which sampling timing deviates from optimum sampling timing. For example, since samples may have inaccurate values in a symbol transition region, the timing error indicates an extent by which samples deviate from the middle of the symbol. The timing error detector  421  calculates timing error by searching for a symbol transition location and a symbol center. 
     In this manner, the symbol rate for signals is determined independent from the sampling rate of the ADC  115 . Accordingly, even if the sampling frequency is different from the symbol rate due to external conditions, symbol synchronization can be stably achieved. 
       FIG. 5  is a block diagram showing an exemplary polarization compensation unit. 
     As shown in  FIG. 5 , the polarization compensation unit  203  extracts a polarization component of modulated signals. For example, in  FIG. 5 , I xd  and I yd  represent outputs of the decimation filter  430  of the digital symbol synchronization unit  202 , respectively, I x0  and I y0  represents polarization components of modulated signals, respectively. 
     In the case that an input optical signal is a PSK modulated signal, the polarization compensation unit  203  adaptively obtains coefficients of a finite impulse response (FIR) filter  501  in a constant modulus algorithm (CMA) scheme. 
       FIG. 6  is a view showing an exemplary digital equalization method for a coherent optical receiver. The digital equalization method for a coherent optical receiver will be described with reference to  FIG. 6 . 
     First, a received optical signal is converted into a digital signal (operation  601 ). For example, the photoconverter  110  splits each of an optical signal and a local oscillation signal into an x-polarization component and a y-polarization component. An I-channel signal and a Q-channel signal for each polarization component are generated. The photo-receiver unit  114  detects the I-channel signals and Q-channel signals to generate analog signals. The analog signals are converted into digital signals through the ADC  115 . 
     After that, the chromatic dispersion of optical signals is performed (operation  602 ). For example, the chromatic dispersion compensation unit  201  compensates for chromatic dispersion based on chromatic dispersion values of optic fiber constituting a transmission path. 
     Then, symbol synchronization is performed on signals, which have been subject to the chromatic dispersion, in a digital manner (operation  603 ). For example, the digital symbol synchronization unit  202  simultaneously performs a clock recovery and a data recovery in a digital manner. In the case that the optical signal is a PSK signal, the symbol synchronization is performed as follows. Interpolation is performed on the signals such that data is sampled in the middle of a symbol. Timing error of an interpolation signal is detected. Interpolation timing is determined by use of the detected timing error. 
     After that, polarization impairment of the optical signals is compensated for (operation  604 ). For example, the polarization compensation unit  203  separates polarization components of modulated signals. 
     Finally, a frequency difference and phase noise are compensated for (operation  605 ). For example, the frequency and phase compensation unit  205  estimates and compensates for a laser frequency offset between an optical signal and a local oscillation signal and phase noise due to the finite laser linewidth. 
       FIG. 7  is a block diagram showing another exemplary signal processor. As shown in  FIG. 7 , a signal processor  700  may further include a signal conditioning unit ( 701 ) in addition to the components shown in  FIG. 2 . If necessary, the signal conditioning unit  701  may perform various kinds of pre-processing. For example, the signal conditioning unit  701  may perform a normalization, a compensation for IQ-mismatch and a linear transformation on input signals. 
       FIG. 8  is a block diagram showing still another exemplary signal processor. 
     As shown in  FIG. 8 , a signal processor  800  includes the chromatic dispersion compensation unit  201 , the polarization compensation unit  203 , the digital symbol synchronization unit  202 , the frequency and phase compensation unit  204  and the decoding unit  205 . 
     Since the description of the components is identical to that described with reference to  FIG. 2 , details will be omitted in order to avoid redundancy, and will be described in conjunction with  FIG. 2 . In  FIG. 2 , the signal processor  120  compensates for chromatic dispersion, performs symbol synchronization and then performs polarization compensation. However, in  FIG. 8 , the signal processor  800  compensates for chromatic dispersion, performs polarization compensation and then performs symbol synchronization. 
     The sequence of operations including the chromatic dispersion compensation, the symbol synchronization and the polarization compensation is not limited to the present embodiment. For example, in  FIGS. 6 and 7 , the chromatic dispersion compensation, the symbol synchronization and the polarization compensation may be performed in a predetermined order suitable for the system performance and the required use. 
     The disclosure can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. 
     Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves such as data transmission through the Internet. The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion 
     Also, functional programs, codes, and code segments for accomplishing the present invention can be easily construed by programmers skilled in the art to which the present invention pertains. A number of exemplary embodiments have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.