Abstract:
The present invention discloses a method for detecting dispersion, overcoming disadvantages of complex configuration and insensitivity to a tiny dispersion of the method and device for detecting dispersion in the prior art. The inventive method includes: obtaining a signal within a predetermined bandwidth range from an optical signal received; obtaining an operated value of power via an operation on the signal within the predetermined bandwidth range; and obtaining amount of system dispersion according to a corresponding relation between the operated value of power and the amount of system dispersion. A device for detecting dispersion is disclosed, including a photoelectric filter operational unit and a processing unit, where an output of the photoelectric filter operational unit is connected to an input of the processing unit. The device for detecting dispersion of the present invention is applicable to an adaptive dispersion compensation system. An optical signal transmission system is further disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/269,502, filed on Nov. 12, 2008, which is a continuation of International Patent Application No. PCT/CN2007/000257, filed on Jan. 24, 2007. The International Application claims priority to Chinese Patent Application No. 200610080030.3, filed May 13, 2006. The afore-mentioned patent applications are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of optical transmission, and in particular to a method and a device for detecting dispersion in an adaptive dispersion compensation system. 
     BACKGROUND OF THE INVENTION 
     Various frequency components or various mode components of a signal transmitted in an optical fiber have different transmission speeds, which results in waveform distortion of the signal, and such phenomena are referred to as dispersion. The impact of the dispersion on the optical transmission causes intersymbol interference between data pulses. Therefore, dispersion compensation is necessary for ensuring transmission performance of a system. For a high speed optical transmission system in a speed of above 40 Gb/s or an optical transmission network desired to be dynamically configured, a solution of adaptively adjustable dispersion compensation is necessary. To implement the adaptive dispersion compensation system, a mechanism of dispersion detection and feedback control is necessary. 
     At present, there are primarily three solutions of adjustable dispersion compensation, i.e. dispersion compensation based on adjustable chirped optical fiber Bragg grating, pre-distortion at the transmission side, and equilibrium at the reception side. 
     Three solutions of dispersion detection in the above dispersion compensation systems are primarily described as follows. 
     In solution 1, the amount of dispersion is determined through comparison of phase difference between clock signals. 
     Referring to  FIG. 1 , a method for detecting dispersion disclosed in a paper entitled “Chromatic Dispersion Monitoring Technique Using Sideband Optical Filtering and Clock Phase-shift Detection”, Journal of Lightwave Technology (JTL), Vol. 20, No. 12, is shown. In this method, a Vestigial side band-Upper (VSB-U) signal or a Vestigial side band-Low (VSB-L) signal on the signal spectrum is filtered out via an optical filter  101  before the photoelectric conversion at the reception side, then the photoelectric conversion is performed on the VSB-U signal or VSB-L signal through a photoelectric converter  102 . A clock signal is extracted by a clock recovery unit  103 , and the phase information of the sideband signal is clarified. Subsequently, the amount of dispersion is determined through the comparison of phase difference between two clock signals corresponding to the sideband signal and a baseband signal respectively. 
     This solution requires two sets of high speed photoelectric conversion and processing configuration, and is complicated in the configuration. In addition, periodical repetition may occur to the clock phase difference, therefore, only the amount of dispersion corresponding to up to the range of one clock cycle may be measured, and the range of the amount of measurable dispersion is limited. 
     In solution 2, a transmission signal is added with a harmonics detection signal which is extracted after the photoelectric conversion at the reception side, and the amount of system dispersion is determined via the intensity of the harmonics detection signal at the reception side. 
     Referring to  FIG. 2 , a solution of dispersion monitoring and compensation using a single in-band sub-carrier tone, disclosed in WH4, OFC2001, is shown. In this solution, a harmonics detection signal is added, via a modulator, to a signal transmitted into a transmission line, spectrum power of the harmonics detection signal is separated by an electric filter after the photoelectric conversion of the optical signal at the reception side, and the amount of system dispersion is determined according to the decrease in the amount of power. 
     In this method, an additional tune device needs to be added at the transmission side, which increases the complexity of system. 
     In solution 3, an optical signal is converted to an electric signal, and the amount of system dispersion is determined by detecting a change of a first minimum point of dispersion in power spectrum. 
     A dispersion detection solution is disclosed in U.S. Pat. No. 6,487,352. As shown in  FIG. 3 , the implementation of this detection solution is as follows: After the photoelectric conversion of an optical signal, the signal is filtered via some RF narrowband filters, and then a value of the amount of system dispersion is determined through an analysis of several narrowband spectrum components. In this solution, correspondence relationship between the cosine expansion degree of the power spectrum of a square law receiver and the amount of signal dispersion is utilized. Here, the reference numbers in  FIG. 3  are respectively as follows:  300  represents input optical signal;  301  represents photoelectric converter;  302  represents adjustable amplifier;  303  represents band-pass filter;  304  represents adjustable amplifier;  305  represents band-pass filter;  306  represents square detector;  307  represents low pass filter;  308  represents A/D converter;  309  represents digital signal processor; and  310  represents dispersion control signal. 
     In the above patent, the wave trough position of power spectrum is searched for by high-density sampling for the power spectrum, and the amount of dispersion of a signal is determined with the offset direction and offset amount of the wave trough position. The high-density sampling is necessary for positioning accurately the wave trough position, which complicates the configuration of the detection system, and increases the cost of the same. Further, even a tiny change from the amount of dispersion of a signal needs to be detected and compensated in a high speed system. However, the change of the wave trough position is not sensitive to a tiny variety in the amount of dispersion, which limits the application of this solution in the high speed system. In addition, in this solution, the electric power at each filter&#39;s output needs to be amplified, which increases power consumption of the system. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome disadvantages of the method and device for detecting dispersion, which are complex and insensitive to a tiny dispersion, in the prior art. The present invention provides a detection method and device, which are used for an adaptive dispersion compensation system, with simple implementation, and an optical signal transmission system. 
     The method for detecting dispersion provided in the present invention includes the following steps. 
     Obtaining an electric signal within a predetermined bandwidth range from a received optical signal. 
     Obtaining an operated value of a power via an operation on the electric signal within the predetermined bandwidth range. 
     Obtaining an amount of system dispersion from a correspondence relationship between the operated value of power and the amount of system dispersion. 
     The device for detecting dispersion provided in the present invention includes a photoelectric filter operational unit and a processing unit. 
     The photoelectric filter operational unit is adapted to obtain an electric signal within a predetermined bandwidth range based on an optical signal received, and obtain an operated value of power via an operation on the electric signal within the predetermined bandwidth range. 
     The processing unit is adapted to obtain the amount of system dispersion through a correspondence relationship between the operated value of power and the amount of system dispersion. 
     The optical signal transmission system provided in the present invention includes an adjustable dispersion compensation module and an light splitting module which are connected in serial in an optical fiber path, and a dispersion detecting module adapted to obtain detection light from the light splitting module for detecting the amount of system dispersion, and a control module connected in serial between the adjustable dispersion compensation module and the dispersion detecting module. The dispersion detecting module includes a photoelectric filter operational unit and a processing unit. 
     The photoelectric filter operational unit is adapted to obtain an electric signal within a predetermined bandwidth range based on an optical signal received, and obtain an operated value of power via an operation on the electric signal within the predetermined bandwidth range. 
     The processing unit is adapted to obtain the amount of system dispersion through a correspondence relationship between the operated value of power and the amount of system dispersion. 
     The present invention has advantages as follows. 
     1. Compared with solution 1 in the prior art, the system of dispersion detection and compensation in the present invention is free of the complex and costly clock recovery unit. 
     2. Compared with solution 2 in the prior art, no additional detection light needs to be added at the transmission side in the present invention, so that bandwidth resources are saved, and the impact of the detection light on the signal light is avoided. 
     3. Compared with solution 3 in the prior art, total power or average power is desired to be monitored in the present invention, and the method to obtain them is simple and unique, which is more feasible in a practical system application. 
     4. The present invention is applicable for not only a scenario with an unchanged link, but also a scenario with a changeable link. 
     5. In the present invention, the average power or total power is calculated within a certain bandwidth range, and therefore it is relatively sensitive to tiny dispersion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural diagram of a device for detecting dispersion through comparison of phase difference between clock signals; 
         FIG. 2  is a schematic structural diagram of a device for detecting dispersion through a harmonic detection signal; 
         FIG. 3  is a schematic structural diagram of a device for detecting dispersion through detecting a change of the first minimum point of dispersion in power spectrum; 
         FIG. 4  is a schematic diagram illustrating an electric signal amplitude-frequency characteristic curve that the spectrum intensity is normalized, in which the electric signal is obtained from the square detection of an optical signal pulse transmitted on an optical fiber; 
         FIG. 5  is a flowchart of a method for detecting dispersion according to an embodiment of the present invention; 
         FIG. 6  is a structural diagram of a device for detecting dispersion according to an embodiment of the present invention; 
         FIG. 7   a  is a structural diagram of a device for detecting dispersion in which an integrator is employed in an operational module according to the present invention; 
         FIG. 7   b  is a structural diagram of a device for detecting dispersion in which an average power calculating module is employed in an operational module according to the present invention; 
         FIG. 7   c  is a structural diagram of a device for detecting dispersion in which a power ratio calculating module is employed in an operational module according to the present invention; 
         FIG. 8  is a structural diagram of a device for detecting dispersion according to another embodiment of the present invention; 
         FIG. 9  is a diagram showing a structure of the photoelectric filter operational unit provided in the present invention; 
         FIG. 10  is a structural diagram of an adaptive compensation system, employing the device for detecting dispersion, which is applicable to the transmission side and the line according to an embodiment of the present invention; 
         FIG. 11  is a structural diagram of an adaptive compensation system, employing the device for detecting dispersion, which is applicable to the reception side according to an embodiment of the present invention; and 
         FIG. 12  is a diagram showing a simulation result of power spectrum density. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention are described below with reference to accompanying drawings. 
     Principals of the present invention are explained firstly by taking an example of a single frequency raised cosine signal. 
     Assume a signal I=I 0  (1+m cos(2πft)), m&lt;&lt;1, where, I 0  denotes the average power of signal, and m denotes modulation depth. The power spectrum density outputted after square detection is I f =I 0 m|cos(πλ 2 D r f 2 /c)|, where, λ denotes a carrier wavelength, D r  denotes transmission system dispersion, and c denotes a velocity of light in the vacuum. 
     The first extreme value point of dispersion indicates a first zero point in the system impulse response spectrum of dispersion, and the second extreme value point of dispersion indicates a second zero point in the system impulse response spectrum of dispersion, and the like. A frequency point F corresponding to the first extreme value point of dispersion is obtained by calculating, where, F=(c/2D r λ 2 ) 1/2 . It can be seen from the correspondence relationship between the first extreme value point F of dispersion and the system dispersion D r  that, when the system dispersion D r  increases, the value of frequency that corresponds to the first minimum point F of dispersion decreases, and when the system dispersion D r  decreases, the value of frequency that corresponds to the first minimum point F of dispersion increases. 
     With respect to the spectrum intensity I f =I 0 m|cos(πλ 2 D r f 2 /c)|, a sum may be calculated in a selected bandwidth range, and a relationship between total power and the system dispersion D r  in the selected bandwidth range may be obtained from the expression of spectrum intensity above. One-to-one correspondence relationship exists between the total power and the system dispersion D r . The total power decreases when the system dispersion D r  increases, and increases when the D r  decreases. 
     Referring to  FIG. 4 , it is a schematic diagram illustrating an electric signal amplitude-frequency characteristic curve that the spectrum intensity is normalized, in which the electric signal is obtained from the square detection of an optical signal pulse transmitted on an optical fiber. The first minimum point of dispersion of the spectrum intensity is located at F (the second minimum point of dispersion and other minimum points are not shown). Because of the effect of dispersion, this amplitude-frequency characteristic is different from the ideal amplitude-frequency characteristic without dispersion. The position of F is fixed if the system dispersion is constant. Otherwise, the position of F changes with the system dispersion. When the dispersion of a signal increases, the first minimum point of dispersion shifts from the frequency point F towards lower frequency, as shown by frequency point F′ in  FIG. 4 . When the dispersion of the signal decreases, the first minimum point F of dispersion shifts towards higher frequency, as shown by frequency point F″ in  FIG. 4 . A change in the spectrum density causes the arc MN moves towards M′N′ or M″N″, and thus the total power in the area surrounded by characters M, N, P and Q is changed. In other words, when the system dispersion D r  increases, the total power in the area surrounded by characters M, N, P and Q decreases, and when the system dispersion D r  decreases, the total power in the area surrounded by characters M, N, P and Q increases. 
     With respect to the spectrum intensity I f =I 0 m|cos(πλ 2 D r f 2 /c)|, if an average value is calculated in a selected bandwidth range, the relationship between average power and the system dispersion D r  in the selected bandwidth range may be obtained from the expression of spectrum intensity above. One-to-one correspondence relationship exists between the average power and the system dispersion D r . The average power decreases when the system dispersion D r  increases, and increases when the D r  decreases. 
     To prevent nonlinearity, both local dispersion and dispersion of a system may not be zero. Otherwise, no first minimum point of the dispersion exists, or the first minimum point of the dispersion is at an infinite distance. If the standard system dispersion is D rs ≠0, a standard first minimum point of dispersion is ensured to exist in a real system, and a real first minimum point of dispersion varies with the system dispersion around the standard first minimum point of dispersion. 
     If the standard dispersion is D rs , the corresponding normalized intensity curve is shown as the dot-and-dash line in  FIG. 4 . Assuming that the real dispersion of a system is D r , where, D r  satisfies D rl ≦D r ≦D rh . If the dispersion is D rl , the corresponding normalized intensity curve is shown as the dotted line in  FIG. 4 . If the dispersion is D rh , the corresponding normalized intensity curve is shown as the solid line in  FIG. 4 . 
     In the present invention, the amount of system dispersion is determined by measuring the power of a signal within a bandwidth range Δf1 with upper and lower borders corresponding to frequency points P and Q. With the correspondence relationship between total power or average power within the bandwidth range Δf1 and the amount of system dispersion, the amount of system dispersion is obtained, and the amount of system dispersion compensation is obtained from the calculation of a difference between the amount of system dispersion and that of standard dispersion, and then a control amount for dispersion compensation in the system is obtained according to a specific dispersion compensation solution of the system, thereby the dispersion compensation is performed for the system. 
     Referring to  FIG. 5 , the method for detecting dispersion according to an embodiment of the present invention includes the following steps. 
     In step  201 , an electric signal is obtained through photoelectric conversion of an optical signal. In the embodiment, taking an intensity modulated raised cosine pulse for example, assuming that the optical pulse intensity I=I 0 (1+m cos(2πft)), m&lt;&lt;1, where, I 0  denotes the average power of pulse, and m denotes the modulation depth. The power spectrum outputted after square detection in the photoelectric conversion is expressed as I f =I 0 m|cos(πλ 2 D r f 2 /c)|, where, λ denotes the carrier wavelength, D r  denotes the transmission system dispersion, and c denotes the velocity of light in the vacuum. 
     In the embodiment, the intensity modulation is used as a light modulation approach. Other modulation approaches may also be used, such as the phase modulation, frequency modulation and polarization modulation. 
     In step  202 , a bandwidth range Δf1 is selected, and the upper and lower frequency borders of which are selected under the principles that the upper frequency value P is lower than a frequency value corresponding to the first minimum point of the power spectrum density dispersion of the electric signal, and the lower frequency value Q is selected as such a frequency value that enables the power most sensitive to a dispersion change, with the frequency value at the optimal point being 0. 
     In practice, a guard band Δf2 which is smaller than the frequency difference between the point P and the point F may be set. The Δf2 is selected under the principle that the frequency point of P is as small as possible without transferring to a side of the F with higher frequency. 
     In step  203 , the power spectrum that is within the bandwidth range Δf1 and has upper and lower frequency borders of Q and P is separated out by filtering the obtained power signal, and thereby a band-pass or low pass signal is obtained. 
     The step  201  above may be performed after the step  203 , in other words, firstly, an optical signal with bandwidth range Δf1 is obtained by filtering the optical signal with an optical band-pass filter, and then an electric signal is obtained through photoelectric conversion on the filtered optical signal. In the embodiment, the electric signal is the power signal. 
     In step  204 , the band-pass or low pass signal obtained through separation via filter is summed up, so that the total power of the signal having upper and lower frequency borders of Q and P and bandwidth range of Δf1 is obtained. 
     In step  205 , the amount of system dispersion is obtained based on the correspondence relationship between the total power and the amount of system dispersion. 
     There is a fixed correspondence relationship between the obtained total power and the amount of system dispersion, and the fixed correspondence relationship may be deduced from I f =I 0 m|cos(πλ 2 D r f 2 /c)|. Therefore, the amount of system dispersion may be obtained through detection of the total power. Further, the obtained total power may be converted to the amount of dispersion compensation and amount of control, which are used as the amount of control of a dispersion compensation system for compensating the dispersion. 
     In the implementation, the relationship among the total power, the amount of dispersion, the amount of dispersion compensation and the control amount of the dispersion compensation may be built into a lookup table that is stored in a storage device such as RAM. In practice, this lookup table may be invoked in real time for obtaining the control amount of dispersion compensation which is then inputted to a control module, to control a dispersion compensation module to generate corresponding amount of dispersion compensation. 
     Before the lookup table is invoked, an analog signal of total power may be converted into a digital signal, which is used as an index for searching for the corresponding control amount of dispersion compensation in the lookup table. Referring to Table 1, an example of the lookup table is shown. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Amount 
                   
               
               
                   
                   
                 of system 
                 Control amount 
               
               
                 Output power of 
                   
                 dispersion  
                 of dispersion 
               
               
                 A/D converter 
                 Amount of system 
                 compensation 
                 compensation 
               
               
                 (dBw) 
                 dispersion (ps/nm) 
                 (ps/nm) 
                 (a.u.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 −54.48 
                 15 
                 10 
                 10 
               
               
                 −54.70 
                 10 
                 5 
                 5 
               
               
                 −54.90 
                 5 
                 0 
                 0 
               
               
                 −55.10 
                 0 
                 −5 
                 −5 
               
               
                 −55.20 
                 −5 
                 −10 
                 −10 
               
               
                 −55.28 
                 −10 
                 −15 
                 −15 
               
               
                 −55.30 
                 −15 
                 −20 
                 −20 
               
               
                   
               
             
          
         
       
     
     For obtaining the data in the above table, the following experiment conditions are included. 
     A 40 Gbit/s Gauss pulse sequence is used, Full Width at Half Maximum (FWHM) of a pulse is 6.25 ps, peak power of an input signal is 10 dBm, standard dispersion of system is 42 ps/nm, a first minimum point of dispersion is at 36.8 GHz, and P=36 GHz, Q=0 GHz. Output power of an A/D converter is the total power within the bandwidth. 
     Referring to  FIG. 12 , a diagram illustrating the simulation result of power spectrum density with the above data used as experiment conditions is shown. Where, curve 1 represents the power spectrum density of a source signal, while curve 2 represents the power spectrum density of the signal in the case that the dispersion is 55 ps/nm, and curve 3 represents the power spectrum density of the signal in the case that the dispersion is 41.6 ps/nm. 
     The control amount of dispersion compensation depends on a specific dispersion compensation technology. The above data is generated via the Dispersion Compensation Fiber (DCF). 
     In the case that the link is unchanged, there is a unique standard dispersion. In step  205 , an analog signal of power may not be converted into a digital signal before the lookup table is invoked, instead, the lookup table may be searched directly. Referring to Table 2, the lookup table is searched in a multiple-to-one approach in stead of the one-to-one approach for searching Table 1. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Amount 
                 Control 
               
               
                   
                   
                 of system 
                 amount of 
               
               
                 Output 
                   
                 dispersion 
                 dispersion 
               
               
                 power from 
                 Amount of system 
                 compensation 
                 compensation 
               
               
                 integrator (dBw) 
                 dispersion (ps/nm) 
                 (ps/nm) 
                 (a.u.) 
               
               
                   
               
             
             
               
                 x 1 ~x 2   
                 y 1   
                 z 1   
                 w 1   
               
               
                 x 2 ~x 3   
                 y 2   
                 z 2   
                 w 2   
               
               
                 x 3 ~x 4   
                 y 3   
                 z 3   
                 z 3   
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 x n−1 ~x n   
                 y n   
                 z n   
                 w n   
               
               
                   
               
             
          
         
       
     
     In the embodiment above, the amount of system dispersion is determined by measuring a change in the total power of a signal within the bandwidth range Δf1 having upper and lower borders corresponding to frequency point P and Q. Further, the amount of system dispersion may be determined by calculating a change in the average power, and the other steps are substantially the same as those in the above embodiment and thus are not described in detail here. 
     The above embodiment is suitable for the case in which nonlinearity effect in a system may be ignored. If the nonlinearity effect in the system is relatively significant, to reduce the impact of the nonlinearity effect, firstly, the total power (or average power) within the selected bandwidth range is calculated according to the above embodiment, and then a further total power (or average power) within a nonlinear sensitive region is calculated, where the upper frequency border of the nonlinear sensitive region is smaller than the lower frequency border of the selected bandwidth range to which the nonlinear sensitive region belongs. Because the nonlinear sensitive region is located in a lower frequency region, the lower frequency point is at 0. A ratio of the total powers or average powers of the two regions is obtained. The ratio also has a one-to-one correspondence relationship with the amount of system dispersion, and therefore, with the ratio, the amount of system dispersion may also be obtained and used as the control amount of dispersion compensation system. The subsequent step is the same as the step  205 , and thus will not be described in detail here. 
     In another embodiment of the method of the present invention, firstly, an optical signal may also be selected within a predetermined bandwidth range, of which an upper border is smaller than the value of frequency corresponding to the first minimum point of dispersion of the power spectrum density, and then an electric signal is obtained through photoelectric conversion of an optical signal in the predetermined bandwidth range. And further processing for the electric signal may be performed with reference to the embodiment shown in  FIG. 5 . Further, an embodiment of a device for detecting dispersion provided in the present invention is shown in  FIG. 6 , and the device for detecting dispersion includes a photoelectric filter operational unit  61 , an A/D converter  62 , and a processing unit  63 . 
     The photoelectric filter operational unit  61  is adapted to separate out an electric signal in the selected bandwidth range, obtain an operated value of power via an operation on the electric signal within the predetermined bandwidth range, and convert an optical signal to an electric signal. 
     The A/D converter  62  is adapted to convert an analog signal into a digital signal. 
     The processing unit  63  is adapted to obtain the amount of system dispersion through a correspondence relationship between the operated value of power and the amount of system dispersion, and further obtain the amount of system dispersion compensation and control amount of system dispersion compensation. 
     An electric signal  64  outputted from the output of the photoelectric filter operational unit  61  is inputted to the input of the A/D converter  62 , and an digital signal  65  outputted from the output of the A/D converter  62  is inputted to the input of the processing unit  63 . 
     Referring to  FIG. 7   a , the photoelectric filter operational unit includes a photoelectric converting module  71 , a filter  72 , and an operational module. In the embodiment, the operational module is a frequency integrator  73 , and the processing unit is a total power lookup module  74 . 
     An optical signal having an optical carrier of f c  is inputted into the photoelectric converting module  71  to obtain an electric signal  81 ; then power spectrum that is within the bandwidth range Δf1 and has upper and lower frequency borders of Q and P is separated out via the filter  72  to obtain a band-pass or low pass signal  82 , which is then inputted to the frequency integrator  73 . An analog signal  83  of the total power that indicates a total power of a signal having upper and lower frequency borders of Q and P and having a bandwidth range of Δf1 is outputted from the frequency integrator  73 . Through the A/D converter  62 , the analog signal  83  of the total power is converted into a digital signal  84  of the total power, which is used as an index for searching for the corresponding amount of system dispersion in the total power lookup module  74 , and thereby the amount of system dispersion compensation and control amount of system dispersion compensation  66  is obtained. An example of the lookup table is shown in Table 1. The photoelectric converting module  71  may be implemented with a PIN transistor or other photoelectric detection devices. Because there is a fixed correspondence relationship between the detected energy and the amount of signal dispersion, the total power lookup module is a storage device, such as RAM, in which the relationship between the detected power and the amount of system dispersion is stored. In practice, the lookup table in the total power lookup module is invoked in real time. In addition, a correspondence relationship among the power, the amount of dispersion, the amount of dispersion compensation and the control amount of dispersion compensation may be stored in the total power lookup module. In this way, the amount of system dispersion compensation and control amount of system dispersion compensation  66  may be obtained directly from the total power lookup module  74 . 
     Referring to  FIG. 7   b , the frequency integrator  73  in the operational module may be replaced by an average power calculating module  75 , and the total power lookup module  74  in the processing unit may be replaced by an average power lookup module  76 . 
     The band-pass or low pass signal  82  outputted from the filter  72  is inputted to the average power calculating module  75  to obtain an analog signal  85  of the average power of an electric signal in the selected bandwidth range. Through the A/D converter  62 , the analog signal  85  of the average power is converted into a digital signal  86  of the average power, which is used as an index for searching for the corresponding amount of system dispersion in the average power lookup module  76 , and thereby the amount of system dispersion compensation and control amount of system dispersion compensation  66  is obtained. The other relationships between signals are the same as those in the above embodiment, and thus are not described in detail here. 
     Referring to  FIG. 7   c , the frequency integrator  73  in the processing unit may be replaced by a power ratio calculating module  77 , and the total power lookup module  74  in the processing unit may be replaced by a power ratio lookup module  78 . 
     The band-pass or low pass signal  82  outputted from the filter  72  is inputted to the power ratio calculating module  77  to obtain a ratio  87  between the average power or total power of an electric signal in the selected bandwidth range and that in the nonlinear sensitive region. Through the A/D converter  62 , the analog signal  87  of the power ratio is converted into a digital signal  88  of the power ratio, which is used as an index for searching for the corresponding amount of system dispersion in the power ratio lookup module  78 , and thereby the amount of system dispersion compensation and control amount of system dispersion compensation  66  is obtained. The other relationships between signals are the same as those in the above embodiment, and thus are not described in detail here. 
     For a case in which the link is unchanged, the device does not need to include the A/D converter  62 . As shown in  FIG. 8 , the device includes a photoelectric filter operational unit  61  and a processing unit  63 . The output of the photoelectric filter operational unit  61  is connected to the input of the processing unit  63 . An analog signal  64  outputted from the photoelectric filter operational unit  61  is directly inputted into a lookup module of the processing unit for performing searching. Accordingly, a lookup table in the lookup module is searched in a multiple-to-one approach. This case is suitable for the structure in which an integrator, an average power calculating module or a power ratio calculating module is employed in the operational module, and the A/D converter may not be included. This has a similar structure with that in the above embodiment and is not described in detail here. 
     In the above embodiments, the photoelectric converting module  71  is located in front of the filter  72  in the photoelectric filter operational unit. In practice, as shown in  FIG. 9 , the filter  72  may be located in front of the photoelectric converting module  71  with the output of the filter  72  connected to the input of the photoelectric converting module  71 , and then the filter is an optical band-pass filter. 
     The device for detecting dispersion provided in the present invention may be used to construct an adjustable dispersion compensation system. 
     Referring to  FIG. 10 , an embodiment of a system in which an adjustable dispersion compensation module is applicable to the transmission side and the line is shown. In  FIG. 11 , an embodiment of a system in which an adjustable dispersion compensation module is applicable to the reception side is shown. The systems shown in  FIG. 10  or  11  includes an adjustable dispersion compensation module and an light splitting module (i.e. a light splitter) which are connected in serial in an optical fiber path, a dispersion detecting module adapted to obtain detection light from the light splitting module for detecting amount of system dispersion, and a control module connected in serial between the adjustable dispersion compensation module and the dispersion detecting module. The dispersion detecting module includes a photoelectric filter operational unit and a processing unit. The photoelectric filter operational unit is adapted to obtain an electric signal within a predetermined bandwidth range from a received optical signal, and obtain an operated value of power from the operation on the electric signal within the predetermined bandwidth range; the processing unit is adapted to obtain the amount of system dispersion by using a correspondence relationship between the operated value of power and the amount of system dispersion. 
     In the system as shown in  FIG. 10 , an optical signal is inputted into an adjustable dispersion compensation module  4   a   1  after transmission over a line. Part of the output light on which the dispersion compensation has been performed is split, by a light splitter  4   a   2 , to be used as detection light which is then inputted into a dispersion detecting module  4   a   3 . A control module  4   a   4  feeds back a control signal to the adjustable dispersion compensation module according to the result of dispersion amount detected by the dispersion detecting module, so as to adjust the amount of dispersion compensation to achieve the optimal performance of the system. A transmission segment may or may not be provided between the adjustable dispersion compensation module  4   a   1  and the light splitter  4   a   2 . 
     In the system as shown in  FIG. 11 , after being transmitted over a line, part of the optical signal is split firstly, by a light splitter  4   a   2 , to be used as detection light which is then inputted into a dispersion detecting module  4   a   3 . A control module  4   a   4  feeds back a control signal to the adjustable dispersion compensation module  4   a   1  according to the result of dispersion amount detected by the dispersion detecting module, so as to adjust the amount of dispersion compensation to achieve the optimal performance of the system. 
     The foregoing descriptions have explained the preferred embodiments of the present invention. It will be apparent to those skilled in the art that modifications and alternations can be made to the invention without departing from the scope and spirit of the present invention, and all the modifications and alternations are intended to be within the scope of the present invention.