Abstract:
The present invention relates to chromatic dispersion monitor and method, chromatic dispersion compensator. The chromatic dispersion monitor is used for estimating a chromatic dispersion in accordance with a chromatic dispersion correlation amount sequence, comprising: a phase differential unit, for obtaining a phase difference sequence by performing a phase differential calculation in accordance with the chromatic dispersion correlation amount sequence; a phase difference differential unit, for obtaining a phase difference differential sequence by performing a phase difference differential operation; and a chromatic dispersion estimating unit, for estimating the chromatic dispersion in accordance with the phase difference differential sequence obtained by the phase difference differential unit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present application relates to communication, and the present application relates to chromatic dispersion estimation and compensation in optical communication system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Chromatic dispersion effect in optical fiber will extend width of the transmission pulse, and then cause bit error. In order to eliminate this influence, a chromatic dispersion compensation module shall be added. With respect to a system having time varying chromatic dispersion, real-time monitoring and compensation for the chromatic dispersion are required. The literature OFC2008 OThW2 “Optical Performance Monitoring from FIR Filter Coefficients in Coherent Receivers” provides a real-time chromatic dispersion monitoring method based on filter coefficient analysis.  FIG. 1  illustrates a chromatic dispersion monitor for the real-time chromatic dispersion monitoring method. As shown in  FIG. 1 , the chromatic dispersion monitoring device of the prior art comprises an adaptive filter  101 , a chromatic dispersion correlation amount calculating unit  102  and a chromatic dispersion monitor  105 , wherein the chromatic dispersion monitor  105  comprises a phase extraction unwrapping unit  103  and a chromatic dispersion estimating unit  104 . 
         [0003]    The adaptive filter  101  can be a single FIR adaptive filter (single-polarization system), or a set of adaptive filters having butterfly structure (double-polarization system). The chromatic dispersion correlation amount calculating unit  102  extracts chromatic dispersion correlation component H(ω) with certain algorithm based on the filter coefficient. H(ω) is a set of discrete sequences represented as h 1 , h 2 , . . . , h N  with the length equal to the filter coefficient length N. In the sequence h 1 , h 2 , . . . , h N , respective element has the form of Ae j(ω     2     α+ωb+c) +n, wherein α is in a one-to-one correspondence to the chromatic dispersion amount in the system, A, b and c are constants unrelated to the chromatic dispersion, and n is a noise item. The phase extraction unwrapping unit  103  extracts a phase of H(ω); as the extracted phase is always within [0, 2π), and the actual value of quadratic curve may exceed the range, the phase extraction unwrapping unit  103  is required for unwrapping at the same time. The chromatic dispersion estimating unit  104  obtains the above parameter α through quadratic fitting, and hence estimates the chromatic dispersion. This method balances the system with the adaptive filter, extracts chromatic dispersion correlated portion based on the converged filter coefficient, and estimates the parameter α through quadratic fitting of phase angle, so as to obtain the chromatic dispersion amount in the system. This estimation method has the problem of phase wrap, i.e., once the phase wrap is wrong, the chromatic dispersion amount will not be estimated correctly. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In view of the current state of the prior art, the invention is provided to overcome one or more defects of the prior art, and supply at least one beneficial selection. 
         [0005]    To achieve the above object, the application provides the following aspects. 
         [0006]    Aspect 1, a chromatic dispersion monitor for estimating a chromatic dispersion in accordance with a chromatic dispersion correlation amount sequence, comprising: a phase differential unit, for obtaining a phase difference sequence by performing a phase differential calculation in accordance with the chromatic dispersion correlation amount sequence; a phase difference differential unit, for obtaining a phase difference differential sequence by performing a phase difference differential operation; and a chromatic dispersion estimating unit, for estimating the chromatic dispersion in accordance with the phase difference differential sequence obtained by the phase difference differential unit. 
         [0007]    Aspect 2, the chromatic dispersion monitor according to aspect 1, wherein the phase differential unit is a first conjugate product calculating unit that obtains a first conjugate product sequence in accordance with the chromatic dispersion correlation amount sequence by performing a first conjugate multiplying operation, wherein the first conjugate multiplying operation is to calculate a product between one element in the chromatic dispersion correlation amount sequence and a complex conjugate of another element spaced by a predetermined distance therefrom in the chromatic dispersion correlation amount sequence; the phase difference differential unit is a second conjugate product calculating unit that obtains a second conjugate product sequence in accordance with the first conjugate product sequence by performing a second conjugate multiplying operation, wherein the second conjugate multiplying operation is to calculate a product between one element in the first conjugate product sequence and a complex conjugate of another element spaced by a predetermined distance therefrom in the first conjugate product sequence; and the chromatic dispersion estimating unit estimates the chromatic dispersion in accordance with the second conjugate product sequence. 
         [0008]    Aspect 3, the chromatic dispersion monitor according to aspect 2, wherein further comprising a chromatic dispersion correlation amount sequence conversion step for converting respective elements in the chromatic dispersion correlation amount sequence to eliminate influences of amplitudes in the respective elements in the chromatic dispersion correlation amount sequence. This aspect can be completed with algorithms containing an argument taking function and an exponential function, e.g., with equation ĥ i =ce jarg(h     i     ) , h i =ĥ i , wherein c is a fixed constant. 
         [0009]    Aspect 4, the chromatic dispersion monitor according to aspect 2, wherein the first conjugate product calculating unit performs the conjugate multiplying operation with the following equation: p min(ij+j) =h i h* i+j , wherein p i  is the ith element in the conjugate product sequence, h i  is the ith element in the chromatic dispersion correlation amount sequence, when j is an integer greater than 0 and less than N, i=1, 2, . . . , N−j, when j is an integer less than 0 and greater than −N, i=1, 2, . . . , N, N is the length of the chromatic dispersion correlation amount sequence, min(i,i+j) is to get the smaller value between i and i+j, and h* i+j  indicates the complex conjugate of h i+j ; the second conjugate product calculating unit performs second conjugate multiplying operation by using the following equation: q min(i,i+m) =p i p i+m , wherein q i  is the i th  element in the second conjugate product sequence, when m is an integer greater than 0 and less than N−|j|, i=1, 2, . . . , N−|j|−m, when m is an integer less than 0 and greater than −N+|j|, i=1−m, 2−m, . . . , N−|j|, and p* i+m  indicates the complex conjugate of p i+m ; the chromatic dispersion estimating unit obtains a parameter α for chromatic dispersion estimation with one of the following two equations: 
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         [0000]    wherein argil indicates to get an argument of a complex number, mean( ) indicates to get an average, and f s  is a system sampling rate 
         [0010]    Aspect 5, the chromatic dispersion monitor according to aspect 1, wherein the phase differential unit is an argument differential unit that calculates an argument for each chromatic dispersion correlation amount in the chromatic dispersion correlation amount sequence, and calculates an argument difference between the arguments to obtain an argument differential sequence; the phase difference differential unit is an argument difference differential unit that obtains an argument difference differential sequence in accordance with the argument differential sequence by performing a differential operation; and the chromatic dispersion estimating unit estimates the chromatic dispersion in accordance with the argument difference differential sequence. 
         [0011]    Aspect 6, the chromatic dispersion monitor according to aspect 5, wherein the argument differential unit obtains the argument differential sequence with the following equation: p min(i,i+j) =arg(h i )−arg(h i+j ), where p i  is the ith element in the argument differential sequence, h i  is the ith element in the chromatic dispersion correlation amount sequence, when j is an integer greater than 0 and less than N, i=1, 2, . . . , N−j, when j is an integer less than 0 and greater than −N, i=1−j, 2−j, . . . , N, N is the length of the chromatic dispersion correlation amount sequence, and arg( ) indicates to get an argument of a complex number; the argument difference differential unit obtains the argument difference differential sequence with the following equation: q min(i,i+m)=p   i −p i+m , where q i  is the i th  element in the argument difference differential sequence, when m is an integer greater than 0 and less than N−|j|, i=1, 2, . . . , N−|j|−m, and when m is an integer less than 0 and greater than −N+|j|, i=1−m, 2−m, . . . , N−|j|; the chromatic dispersion estimating unit obtains a parameter α for estimating the chromatic dispersion amount with the following equations: {circumflex over (q)} i =mod(q i +π,2π)−π, and 
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         [0000]    where mod(q i +π,2π) indicates to get □q i +π modulo 2π and mean( ) indicates to get an average. 
         [0012]    Aspect 7, a chromatic dispersion compensator, wherein comprising the chromatic dispersion monitor according to any one of aspects 1 to 5. 
         [0013]    Aspect 8, the chromatic dispersion compensator according to aspect 7, wherein further comprising: a static primary chromatic dispersion compensating unit, for compensating a chromatic dispersion in accordance with a chromatic dispersion estimation value obtained by the chromatic dispersion monitor; an adaptive residual chromatic dispersion compensating unit, for compensating a residual chromatic dispersion after the chromatic dispersion compensation by the primary chromatic dispersion compensating unit, and provides an input of a chromatic dispersion correlation amount calculating unit; and the chromatic dispersion correlation amount calculating unit, for calculating a chromatic dispersion correlation amount in accordance with a filter coefficient of the residual chromatic dispersion compensating unit; wherein the chromatic dispersion monitor monitors the chromatic dispersion in accordance with the chromatic dispersion correlation amount. 
         [0014]    Aspect 9, a chromatic dispersion monitoring method for estimating a chromatic dispersion in accordance with a chromatic dispersion correlation amount sequence, comprising: a phase differential step, for obtaining a phase difference sequence by performing a phase differential calculation in accordance with the chromatic dispersion correlation amount sequence; a phase difference differential step, for obtaining a phase difference differential sequence by performing a phase difference differential operation; and a chromatic dispersion estimating step, for estimating the chromatic dispersion in accordance with the phase difference differential sequence obtained in the phase difference differential step. 
         [0015]    Aspect 10, the chromatic dispersion monitoring method according to aspect 9, wherein the phase differential step obtains a first conjugate product sequence in accordance with the chromatic dispersion correlation amount sequence by performing a first conjugate multiplying operation, wherein the first conjugate multiplying operation is to calculate a product between one element in the chromatic dispersion correlation amount sequence and a complex conjugate of another element spaced by a predetermined distance therefrom in the chromatic dispersion correlation amount sequence; the phase difference differential step obtains a second conjugate product sequence in accordance with the first conjugate product sequence by performing a second conjugate multiplying operation, wherein the second conjugate multiplying operation is to calculate a product between one element in the first conjugate product sequence and a complex conjugate of another element spaced by a predetermined distance therefrom in the first conjugate product sequence; and the chromatic dispersion estimating step estimates the chromatic dispersion in accordance with the second conjugate product sequence. 
         [0016]    Aspect 11, the chromatic dispersion monitoring method according to aspect 9, wherein the phase differential step calculates an argument for each chromatic dispersion correlation amount in the chromatic dispersion correlation amount sequence, and calculates an argument difference between the arguments to obtain an argument differential sequence; the phase difference differential step obtains an argument difference differential sequence in accordance with the argument differential sequence by performing a differential operation; and the chromatic dispersion estimating step estimates the chromatic dispersion in accordance with the argument difference differential sequence. 
         [0017]    Aspect 12, a computer executable software, comprising such an instruction that when the instruction is executed by the computer or other logical device, the computer or other logical device is enabled to implement the steps in the above method. Herein the execution includes an execution after the instruction is explained or compiled. 
         [0018]    Aspect 13, a computer readable storage medium stored with the computer executable software of aspect 12; the computer readable storage medium can be, e.g., CD, DVD, soft disk, magnetic tape, hard disk driver, flash memory, ROM, RAM, etc. 
         [0019]    By means of the device and method of the present invention, the processes of unwrapping and quadratic fitting can be eliminated, so that the estimation of chromatic dispersion is more reliable. 
         [0020]    In reference to the following descriptions and drawings, above and further aspects and features of the present invention will be clearer. The descriptions and drawings concretely disclose the specific embodiments of the present invention, and specify the principle of the invention and the ways that can be adopted. It shall be understood that the scope of the present invention is not limited thereby. Within the range of the spirit and provisions of the accompanied claims, the present invention includes various changes, modifications and equivalents. 
         [0021]    Features described and/or illustrated with respect to one embodiment can be used in one or more other embodiments in the same or similar way, by combining features in other embodiments or replacing features in other embodiments. 
         [0022]    To be emphasized, the term “include/comprise” in the text means the existence of feature, integral, step, or component, but not excluding the existence or addition of one or more other features, integrals, steps or components. 
         [0023]    The aspects of the present invention can be understood better with reference to the drawings. The components in the drawings are not proportionally drafted, but for illustrating the principle of the present invention. For the convenience of illustrating and describing some portions of the present invention, corresponding components in the drawings may be amplified, i.e., they appear larger than other components in the examplary device actually fabricated according to the present invention. The elements and features described in one drawing or embodiment of the present invention can be combined with elements and features illustrated in one or more other drawings or embodiments. In addition, similar reference numerals represent corresponding components in the drawings, and can be used to indicate corresponding components in more than one embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The drawings illustrate the preferred embodiments of the present invention, constitute a part of the Specification, and are used to further specifically describe the principle, feature and advantage of the present invention in combination with the Description. 
           [0025]      FIG. 1  illustrates a chromatic dispersion monitor of the prior art. 
           [0026]      FIG. 2  illustrates a conceptual graph of a chromatic dispersion monitor according to the present invention. 
           [0027]      FIG. 3  illustrates an explanatory block diagram of a chromatic dispersion monitor according to an embodiment of the present invention. 
           [0028]      FIG. 4  illustrates an explanatory block diagram of a chromatic dispersion monitor according to another embodiment of the present invention. 
           [0029]      FIG. 5  illustrates an explanatory block diagram of a chromatic dispersion compensator of a chromatic dispersion monitor according to the present invention. 
           [0030]      FIG. 6  illustrates a flow diagram of a chromatic dispersion monitoring method according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The embodiments of the present invention are described as follows in combination with the drawings. The present invention avoids the problem of phase unwrapping by means of phase differential, so that the estimation of chromatic dispersion is more reliable. 
         [0032]      FIG. 2  illustrates a chromatic dispersion monitor according to an embodiment of the present invention. As shown in  FIG. 2 , the chromatic dispersion monitor according to an embodiment of the present invention comprises an adaptive filter  101 , a chromatic dispersion correlation amount calculating unit  102  and a chromatic dispersion monitor  204 . 
         [0033]    The adaptive filter  101  and the chromatic dispersion correlation amount calculating unit  102  are same as those in the prior art, and herein are not described. In the embodiment of the present invention, the chromatic dispersion monitor  204  comprises a phase differential unit  201 , a phase difference differential unit  202  and a chromatic dispersion estimating unit  203 . 
         [0034]    It is assumed that an AD sampling rate of a system, (e.g., receiver system) is f s , the chromatic dispersion correlation amount calculating unit  102  extracts a chromatic dispersion correlation amount H(ω), which is a set of discrete sequences represented as h 1 , h 2 , . . . , h N  with the length equal to the filter coefficient length N. 
         [0035]    The chromatic dispersion monitor  204  which is composed of the phase differential unit  201 , the phase difference differential unit  202  and the chromatic dispersion estimating unit  203 , allows two operation ways, i.e., processing conjugate product of the sequence h 1 , h 2 , . . . , h N , or processing differential of argument sequence arg(h 1 ), arg(h 2 ), . . . , arg(h N ), wherein arg(*) indicates to obtain an argument of a complex number, and the detailed descriptions are given as follows. 
         [0036]      FIG. 3  illustrates an embodiment in which the chromatic dispersion monitor  204  processes conjugate product of the sequence h 1 , h 2 , . . . , h N , wherein a first conjugate product calculating unit  301 , a second conjugate product calculating unit  302  and a chromatic dispersion estimating unit  303  correspond to the phase differential unit  201 , the phase difference differential unit  202  and the chromatic dispersion estimating unit  203  in  FIG. 2 , respectively. 
         [0037]    As shown in  FIG. 3 , the chromatic dispersion correlation amount H(ω) outputted by the chromatic dispersion correlation amount calculating unit  102  is a sequence  205  represented as h 1 , h 2 , . . . , h N , and the sequence  205  is inputted to the first conjugate product calculating unit  301 . 
         [0038]    Through conjugate multiplying operation, the first conjugate product calculating unit  301  obtains a conjugate product sequences of the inputted sequence  205 , i.e., a first conjugate product sequence  306  corresponding to the phase differential sequence. The first conjugate product sequence  306  can be represented as wherein p 1 , p 2 , . . . , p N−|j| , wherein p min(i,i+j) =h i h* i+j , p i  is the i th  element in the first conjugate product sequence  306 , and min(i,i+j) is the smaller value between i and i+j. In one embodiment, N&gt;j&gt;0, and at that time, i=1, 2, . . . , N−j. In another embodiment, −N&lt;j&lt;0, and at that time, i=1−j, 2−j, . . . , N. From the above descriptions, it can be seen that the conjugate multiplying operation is a multiplication of a certain chromatic dispersion correlation amount in a chromatic dispersion correlation amount sequence with a conjugation of another chromatic dispersion correlation amounts spaced therefrom by one or more other chromatic dispersion correlation amounts in that sequence. 
         [0039]    Through conjugate multiplying operation, the second conjugate product calculating unit  302  obtains a further conjugate product sequence of the first conjugate product sequence  306 , which is called as a second conjugate product sequence  307  and represented by q 1 , q 2 , . . . , q N−|j|−|m|  in the drawing, wherein q min(i,i+m) =p i p* i+m , q i  is the ith element in the second conjugate product sequence  307 , in one embodiment, N−|j|&gt;m&gt;0, at that time, i=1, 2, . . . , N−|j|−m. In another embodiment, −N+|j|&lt;m&lt;0, at that time, i=1−m, 2−m, . . . , N−|j|. 
         [0040]    In the present invention, the sequence  205  corresponds to the chromatic dispersion correlation amount sequence, the first conjugate product sequence  306  corresponds to the phase differential sequence, and the second conjugate product sequence  307  corresponds to the phase difference differential sequence. The operation using the equation p min(i,i+j) =h i h* i+j  corresponds to the phase differential operation, and the operation using the equation q min(i,i+m) =p i p* i+m  corresponds to the phase difference differential operation. 
         [0041]    The chromatic dispersion estimating unit  303  estimates α by calculating an average and taking phase angle. For example, the chromatic dispersion estimating unit  303  can estimate a by firstly taking the phase angle and then calculating the average. At that time, 
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         [0000]    i.e., firstly taking the argument for q i , then calculating the average, finally dividing with a normalization constant. Alternatively, the chromatic dispersion estimating unit  303  can estimate α by firstly calculating the average and then taking the argument. At that time, 
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         [0000]    i.e., firstly calculating the average for q i , then taking the argument, finally dividing with a normalization constant. Wherein arg(*) indicates to get an argument of a complex number, mean(*) indicates to get an average, and the chromatic dispersion amount can be estimated directly based on a with the prior art. 
         [0042]    The above method is with respect to the complex number operation, but it is only related to the phase angle, and is unrelated to the module value A. Actually, it can take ĥ i =ce jarg(h     i     ) , c is any constant. Preferably, c=1, at that time, ĥ i =e jarg(h     i     ) , and the subsequent first conjugation calculation is performed by replacing h i  with ĥ i . 
         [0043]    The sequence h 1 , h 2 , . . . , h N  has form of Ae j(αω     1       2     +bω     1     +c) , Ae j(αω     2       2     +bω     2     +c) , . . . , Ae j(αω     N       2     +bω     N     +c) . But in actual work, the module values A of the respective elements in the sequence h 1 , h 2 , . . . , h N  may be not completely equal to each other, i.e., there are slight differences. With the above equation ĥ i =ce jarg(h     i     ) =ce j(αω     i       2     +bω     i     +c) , the influence of A is eliminated, so that the estimation of the chromatic dispersion is more reliable. The influence of A also can be eliminated with other equations. 
         [0044]    The principle of the method is described herein by taking an example that j=1 and m=1. 
         [0045]    The sequence h 1 , h 2 , . . . , h N  has form of Ae j(αω     1       2     +bω     1     +c) , Ae j(αω     2       2     +bω     2     +c) , . . . , Ae j(αω     N       2     +bω     N     +c) , wherein 
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         [0000]    then the sequence p 1 , p 2 , . . . , p N−1  has form of |A| 2 e −j(α(ω     1     ω     2     )Δω+bΔω) , |A| 2 e −j(α(ω     2     ω     3     )Δω+bΔω) , . . . , |A| 2 e −j(α(ω     N−1     ω     N     )Δω+bΔω) , wherein c is eliminated; sequence q 1 , q 2 , . . . , q N−2  has form of |A| 4 e jα(2Δω     2     ) , |A| 4 e jα(2Δω     2     ) , . . . , |A| 4 e jα(2Δω     2     ) , wherein b is eliminated and the sequence is constant, without the necessary of being unwrapped. During the estimation of α, the average is calculated to eliminate the influence of the noise, so as to obtain more accurate estimation. 
         [0046]      FIG. 4  illustrates an embodiment in which the chromatic dispersion monitor  204  processes the argument sequence arg(h i ), arg(h 2 ), . . . , arg(h N ). Wherein an argument differential unit  401 , an argument difference differential unit  402  and a chromatic dispersion estimating unit  403  correspond to the phase differential unit  201 , the phase difference differential unit  202  and the chromatic dispersion estimating unit  203  in  FIG. 2 , respectively. 
         [0047]    As shown in  FIG. 4 , the chromatic dispersion correlation amount H(ω) outputted by the chromatic dispersion correlation amount calculating unit  102  is a sequence  205  (h 1 , h 2 , . . . , h N ), and the sequence  205  is inputted to the argument differential unit  401 . 
         [0048]    Through the difference operation, the argument differential unit  401  obtains a sequence  406  called as the argument differential sequence  406  and represented as p 1 , p 2 , . . . , p N−|j| , wherein p min(i,i+j) =arg(h i )−arg(h i+j) . In an embodiment, N&gt;j&gt;0, at that time, i=1, 2, . . . , N−j. In another embodiment, −N&lt;j&lt;0, at that time, i=1−j, 2−j, . . . , N. p i  is the i th  element in the argument differential sequence  406 , i.e., to get a difference between an argument of an element in the sequence  205  and an argument of an element spaced therefrom by a specified distance. 
         [0049]    Through the difference operation, the argument difference differential unit  402  obtains an argument difference differential sequence  407  represented by q 1 , q 2 , . . . , q N−|j|−|m| , wherein q min(i,i+m) =p i −p i+m , q i  is the i th  element in the argument difference differential sequence  407 . In one embodiment, N−|j|&gt;m&gt;0, at that time, i=1, 2, . . . , N−|j|−m. In another embodiment, −N+|j|&lt;m&lt;0, at that time, i=1−m, 2−m, . . . , N−|j|. 
         [0050]    The chromatic dispersion estimating unit  403  adds it to respective element in the sequence q 1 , q 2 , . . . , q N−|j|−|m| , calculates modulus of them with respect to 2π (also called as modulo 2π), so that each of the elements is limited in (−π, π], so as to obtain a sequence {circumflex over (q)} 1 , {circumflex over (q)} 2 , . . . , {circumflex over (q)} N−|j|−|m| ,i.e. {circumflex over (q)} i =mod(q i +π,2π)−π, q i  is the i th  element in the argument difference differential sequence  407 . Wherein mod(q i +π,2π) indicates q i +π modulo 2π. Then the average is calculated to estimate 
         [0000]    
       
         
           
             
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         [0000]    wherein mean( ) indicates to get average, and the chromatic dispersion amount can be estimated directly based on α with the prior art. 
         [0051]    In addition, similarly, the influence of A can also be eliminated with certain algorithms as mentioned previously. 
         [0052]    The principle of the method is described herein by taking an example that j=1 and m=1. 
         [0053]    The sequence h 1 , h 2 , . . . , h N  has form of Ae j(αω     1       2     +bω     1     +c) , Ae j(αω     2       2     +bω     2     +c) , . . . , Ae j(αω     N       2     +bω     N     +c) , wherein 
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         [0000]    then the sequence p 1 , p 2 , . . . , p N−1  has form of −j(α(ω 1 +ω 2 )Δω+bΔω), −j(α(ω 2 +ω 3 )Δω+bΔω), . . . , −j(α(ω N−1 +ω N )Δω+bΔω), c is eliminated; the sequence q 1 , q 2 , . . . , q N−2  has form of jα(2Δω 2 ), jα(2Δω 2 ), . . . , jα(2Δω 2 ), wherein b is eliminated and the sequence is constant, without the necessary of being unwrapped. During the estimation of α, the average is calculated to eliminate the influence of the noise, so as to obtain more accurate estimation. 
         [0054]    The chromatic dispersion monitor of the present invention may have many applications, e.g., being applied in the chromatic dispersion compensating device, the system state detecting device, etc. 
         [0055]    OFC/NFOEC2007 OTuA1 “Transmission of 42.8 Gbit/s Polarization Multiplexed NRZ-QPSK over 6400 km of Standard Fiber with no Optical Dispersion Compensation” mentions a structure that uses two-stage filter to compensate the loss of an optical fiber transmission system. In this system, usually a FIR of preset coefficient is used to compensate the dispersion, but the concrete preset value is unknown. The preset value can be estimated with the chromatic dispersion monitor of the present invention by utilizing the following FIR. 
         [0056]      FIG. 5  illustrates a chromatic dispersion compensator according to an embodiment of the present invention. As shown in  FIG. 5 , the chromatic dispersion compensator according to an embodiment of the present invention comprises a first stage filter  501 , a second stage filter  502 , a chromatic dispersion correlation amount calculating unit  102  and a chromatic dispersion monitor  204 . 
         [0057]    The first stage filter  501  filters with a long-coefficient filter of a static coefficient, so as to compensate a relatively large chromatic dispersion. The second stage filter  502  filters with an adaptive short-coefficient filter, so as to track the system change in real-time, compensate residual dispersion and polarize relevant dispersion. The second stage filter  502  further completes demultiplexing in a polarization multiplexing system. The second stage filter  502  corresponds to the adaptive filter  101  in  FIGS. 1 and 2 . When the system change is large, the residual dispersion will approach to or exceed the compensation limit of the second stage filter  502 , and then the filter coefficient of the first stage filter  501  shall be adjusted, so that the residual dispersion is within the compensation range of the second stage filter  502 . The determination of the coefficient of the first stage filter  501  requires a dispersion value in the system, and the dispersion value can be determined based on the filter coefficient of the second stage filter  502 . Namely, in an embodiment of the present invention, the second stage filter  502  outputs its filter coefficient to the chromatic dispersion correlation amount calculating unit  102 . 
         [0058]    The chromatic dispersion monitor of the present invention monitors the filter coefficient of the short-coefficient filter, then determines the dispersion value in the system and adjusts the dispersion compensation amount of the long-coefficient filter, so that even if the system dispersion changes, the long-coefficient filter is still capable of compensating most dispersion, and the residual dispersion of the system is still within the compensation range of the short-coefficient filter. 
         [0059]    In addition, although the above descriptions are just with respect to a chromatic dispersion compensator that performs filtering compensation with two stages, a person skilled in the art can conceive that the chromatic dispersion monitor of the present invention can also be used for a chromatic dispersion compensator that performs the filtering compensation with only one stage. At that time, the monitored dispersion value is directly estimated, and the estimated dispersion value is sent to other modules upon request. 
         [0060]      FIG. 6  illustrates a flow diagram of a chromatic dispersion monitoring method according to an embodiment of the present invention. 
         [0061]    As shown in  FIG. 6 , firstly in step S 601 , the adaptive filter  101  performs an adaptive filtration. Next, in step S 602 , the chromatic dispersion correlation amount calculating unit calculates the chromatic dispersion correlation amount and obtains a chromatic dispersion correlation amount sequence h 1 , h 2 , . . . , h N . After that, in step S 603 , a phase differential calculation is performed. In this step, as shown in  FIG. 3 , a first conjugate calculation can be carried out by the first conjugate product calculating unit in accordance with the algorithm described previously, so as to obtain a first conjugate product sequence p 1 , p 2 , . . . , p N−|j|  (p min(i,i+j) =h i h* i+j ) in accordance with the chromatic dispersion correlation amount sequence h 1 , h 2 , . . . , h N . Optionally, in this step, a differential calculation can be performed by the argument differential unit  401 , as shown in  FIG. 4 , so as to obtain an argument differential sequence p 1 , p 2 , . . . , p N−|j|  in accordance with the chromatic dispersion correlation amount sequence h 1 , h 2 , . . . , h N , wherein p min(i,i+j) =arg(h i )−arg(h i+j ). Next, a phase difference differential calculation is carried out in step S 604 . When the first conjugate product sequence is calculated in step  603 , a second conjugate product sequence will be calculated in step S 604 , and this is carried out, e.g., by the second conjugate product calculating unit  302  in  FIG. 3  with the previous algorithm. When the argument difference sequence is calculated in step  603 , an argument difference differential sequence will be calculated in step S 604 , and this is carried out, e.g., by the argument difference differential unit  402  in  FIG. 4 . 
         [0062]    After that, in step S 605 , an estimation of chromatic dispersion is carried out. The estimation of chromatic dispersion can be carried out by the chromatic dispersion estimating unit  303  in  FIG. 3 , or by the chromatic dispersion estimating unit  403  in  FIG. 4 , in accordance with the calculation result of step S 604 . 
         [0063]    To be noted, the descriptions of the present invention are just exemplary, so that a person skilled in the art can clearly know and then implement the present invention. For this reason, some components, which may exist in actual conditions but are obvious for a person skilled in the art, are omitted in the descriptions of the invention. 
         [0064]    The specific embodiments of the present invention are described as above. But a person skilled in the art will understand that any change, modification or replacement to the present invention is possible. Thus any change, modification or replacement not deviating from the spirit and scope of the claims of the present invention shall be explained as falling within the protection scope of the present invention,