Patent Publication Number: US-8971702-B2

Title: Method and apparatus for detecting chromatic dispersion, and method and apparatus for compensating chromatic dispersion

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2011/050040 filed Jan. 5, 2011, claiming priority based on Japanese Patent Application No. 2010-000497 filed Jan. 5, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to the measurement and compensation of chromatic dispersion in optical communications, and more particularly to the so-called in-service measurement of chromatic dispersion which simultaneously carries out data transmission and chromatic dispersion measurement in a fiber optic communication system, and to the use of the result of this measurement of chromatic dispersion in the compensation of chromatic dispersion. 
     BACKGROUND ART 
     Data transmission rates in a fiber optic communication system that uses optical fibers to transmit signals are seeing even greater improvements. In a fiber optic communication system, an optical fiber that is a transmission path or transmission line has chromatic dispersion as one of its characteristics, and the waveform distortion produced in optical signals by this chromatic dispersion is a factor that limits transmission rate and transmission distance. Accordingly, there is a need for technology for accurately measuring chromatic dispersion in the optical fiber that is the transmission path and then adjusting in accordance with the measurement result to make the chromatic dispersion substantially zero. Adjustment techniques for making the chromatic dispersion substantially zero are known as, for example, equalization or dispersion compensation. In the following explanation, chromatic dispersion is simply referred to as “CD.” 
     In a fiber optic communication system, because the two ends of a transmission path are typically in separated locations and the chromatic dispersion of the optical fiber varies according to the temperature and external pressure, the measurement and adjustment of CD must be carried out on the far end, i.e., the receiving end of an optical signal, during operation of the system. 
     As a first example of the related art for meeting these demands, a PM-AM conversion method is used as a measurement method on the far end and a monitor light of a different wavelength than the signal transmission is used to detect CD of the transmission path during system operation. The PM-AM conversion method uses the principle that upon transmission of a phase-modulated monitor light, the monitor light that has undergone phase-modulation (PM) is converted to amplitude modulation (AM) under the influence of CD. The first example of the related art is shown in, for example, Kuwahara Shoichiro, et al., “Adaptive dispersion equalization with the detection of dispersion fluctuation using the PM-AM conversion,” Abstract for the Annual Meeting 1998 of Communications Society: Institute of Electronics, Information and Communication Engineers (IEICE), pp. 417 (1998) [NPL1]. The first example of the related art is here described based on the paper by Kuwahara, et al. 
       FIG. 1  shows the system shown by Kuwahara et al. In the transmission end of this system, a signal light from optical transmitter (TX)  1400  to which a data signal having a high bit rate is applied and a monitor light are multiplexed by optical coupler (CPL)  1412 , and the combined light is transmitted to transmission path  1404  such as an optical fiber. Laser light source  1401  of a wavelength that differs from that of the signal light, sine wave generator (SINE GEN)  1402 , and phase modulator (PHASE MOD)  1403  are provided to generate the monitor light. In phase modulator  1403 , the monitor light is generated by subjecting the light from laser light source  1401  to phase modulation by the sine wave signal from sine wave generator  1402 . 
     The signal light and the monitor light that is of a different wavelength than the signal light are propagated on transmission path  1404 . These two light beams propagated on transmission path  1404  come into wavelength demultiplexer (CPL)  1405  at a reception end. Wavelength demultiplexer  1405  separates the received light into the signal light and monitor light. Of these, the signal light is received into optical receiver (RX)  1406 , whereby the data signal is reproduced from the signal light. On the other hand, the monitor light is propagated through chromatic dispersion compensator (CD COMP)  1407  and then comes into photodetector (PD)  1408 . Photodetector  1408  carries out square-law detection of the monitor light, whereby the output of photodetector  1408  is proportional to the amplitude modulation component in the monitor light. Average measurement circuit (AVG)  1411  and band-pass filter (BPF)  1410  are provided at the output of photodetector  1408 , whereby the average level of the detection signal of photodetector  1408  and the intensity of the frequency component of the sine wave signal used in the phase modulation on the transmission side are found. Control circuit  1409  finds the value of CD on transmission path  1404  from the ratio of the average level of the detection signal and the intensity of the frequency component of the sine wave signal and generates a control signal that becomes feedback to the transmission side. 
     In this system, adjustment by a known method is carried out before operation of the system such that residual CD at the wavelength of the signal light becomes zero. At this time, CD at the monitor light wavelength that differs from the wavelength of the signal light typically does not become zero due to the wavelength dependence of CD. Consequently, in order to also set CD at the monitor light wavelength to zero, the amount of compensation is adjusted in CD compensator  1407  with respect to monitor light that has been separated from the signal light. 
     When adjustment is thus carried out before operation to set CD relating to the signal light and monitor light to zero and CD of transmission path  1404  diverges from zero during operation, the monitor signal that is undergoing phase modulation is converted to intensity modulation by CD, whereby a frequency component of the sine-wave signal used in phase modulation appears in the square-law detection output of photodetector  1408  on the receiving side. Control circuit  1409  thereupon judges whether CD relating to the monitor light has diverged from zero based on the ratio of the average level of the detection signal and the intensity of the frequency component of the sine-wave signal. Upon detecting that CD has diverged from zero, control circuit  1409  transmits a control signal to the transmitting side to initiate control for changing the wavelength of the monitor light such that the CD detected at photodetector  1408  relating to monitor light is made zero. 
     When the wavelength of the monitor light is changed and the sine-wave signal frequency component in the detection signal at photodetector  1408  becomes zero, the transmission path CD in the monitor light also becomes zero, whereby the wavelength control of the monitor light is halted. The wavelength of the signal light is then shifted by the amount that the wavelength of the monitor light at this time has been already shifted. In this way, CD relating to the signal light can again be set to zero. Thus, by means of the first example of related art, detecting shift of CD of a monitor light from zero enables control such that the CD of the signal light wavelength becomes zero. 
     As a second example of the related art relating to the present invention, JP-A-2000-346748 [PL1] discloses the monitoring of a CD value by, when transmitting a data signal by means of wavelength multiplexing that uses two different wavelengths, superposing a signal for CD measurement on the optical signal that follows multiplexing and then detecting this signal for measurement on the reception end. In this second example of the related art, an intensity modulation signal is used as an in-service signal used in data transmission and this intensity-modulated signal is wavelength-multiplexed. Then, using a sine-wave signal that has been phase modulated by a pseudo-random code as a CD detection signal, the superposition of the signal is carried out by applying minute intensity-modulation that is driven by the CD detection signal to the optical signal that follows wavelength multiplexing. When there is chromatic dispersion, a difference occurs between the two wavelengths used in signal transmission in the times of arrival at the receiving side of the CD detection signal, whereby the CD detection signals are demodulated at each of the two wavelengths on the receiving side and the time difference in the demodulated codes is detected to enable detection of the CD. Because this technique employs pseudo-random codes, it has the advantage that detection accuracy does not decrease even in cases in which the superposition level of the CD detection signal cannot be made large. 
     As the third example of the related art of the present invention, JP-A-2003-134047 [PL2] discloses the measurement of wavelength dependency of transmission delay of a transmission path and finding chromatic dispersion in an optical transmission system that carries out wavelength multiplexing, by detecting frames belonging to the data signal of each wavelength channel either constantly or at short repeating times and then carrying out a relative comparison of the frame phases for each wavelength channel. 
     SUMMARY 
     Problem to be Solved by the Invention 
     Nevertheless, in the first example of the related art among the above-described examples of the related art, a specific wavelength region is used for the monitor light used in the measurement of CD, and further, the optical intensity of the monitor light must be relatively large to raise the measurement accuracy. As a result, the problem arises that the wavelength region for the monitor light cannot be used for transmission of the signal light, whereby the transmission bandwidth decreases when considered as the entire transmission path. 
     In the second example of the related art, the unconditional superposition of a signal for detecting CD upon data transmission by intensity modulation complicates the avoidance of deterioration of the main signal component by the superposition of the signal for detecting CD. The main signal component is the signal component used in data transmission. In addition, although a high frequency must be used as the signal for detecting CD to increase the accuracy of measuring CD, a low-frequency signal must be used when broadening the range of CD detection, whereby measurement having both high accuracy and broad range becomes problematic when using the second example of the related art. 
     The third example of the prior art presupposes the implementation of wavelength multiplexing, whereby the problem arises that this third example cannot be applied in an optical transmission system in which wavelength multiplexing is not carried out. In addition, even in a wavelength multiplexing system, the measurement of CD is not possible in a system in which frame phases of data signals are not rigorously aligned for each wavelength channel on the transmitting side. 
     It is an exemplary object of the present invention to provide a method and device that can detect chromatic dispersion of a broad range that can occur in an optical transmission path, and to thus provide a method and device for controlling chromatic dispersion. 
     It is another object of the present invention to provide a method and device that can detect and control chromatic dispersion while carrying out actual data transmission. 
     Means for Solving the Problem 
     According to one exemplary embodiment of the present invention, a method of monitoring chromatic dispersion when transmitting an optical signal includes: applying, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2, and transmitting the optical signal to which dips have been applied to a transmission path; receiving the optical signal that is transmitted in by the transmission path and detecting the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1; and based on the detected intensity, generating a monitor signal that represents the chromatic dispersion amount. 
     According to another exemplary embodiment of the present invention, a method of equalizing chromatic dispersion when transmitting an optical signal includes: applying, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2, and transmitting the optical signal to which dips have been applied to a transmission path; receiving the optical signal that is transmitted on the transmission path and detecting the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1; based on the detected intensity, generating a monitor signal that represents the chromatic dispersion amount; and controlling an equalizer that equalizes the optical signal such that the value indicated by the monitor signal is minimized. 
     According to yet another exemplary embodiment of the present invention, a device that monitors chromatic dispersion when transmitting an optical signal includes: a transmitter that applies, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2 and transmits the optical signal to which dips have been applied to a transmission path; and a chromatic dispersion monitor that receives the optical signal that is transmitted on the transmission path, that detects the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1, and that, based on the detected intensity, generates a monitor signal that represents the chromatic dispersion amount. 
     According to yet another exemplary embodiment of the present invention, a device that equalizes chromatic dispersion when transmitting an optical signal includes: a transmitter that applies, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2 and transmits the optical signal to which dips have been applied to a transmission path; a chromatic dispersion monitor that receives the optical signal that is transmitted on the transmission path, that detects the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1, and that, based on the detected intensity, generates a monitor signal that indicates the chromatic dispersion amount; and a chromatic dispersion equalizer that equalizes the optical signal that is received from the transmission path such that the value represented by the monitor signal is minimized. 
     In order to enable detection of chromatic dispersion on the receiving side in the present invention, for example, dips in optical intensity are applied every n symbols where n is an integer equal to or greater than 2 by means of pseudo-RZ modulation or a pseudo-RZ format to an optical signal in which the symbol rate is f, and then the optical signal to which dips have been applied is transmitted to a transmission path. Compared to a case in which normal RZ modulation is carried out, in pseudo-RZ modulation, the length of time in a dip in which the intensity of an optical signal is extremely small is extremely short, whereby the quality of the optical signal is maintained from the standpoint of data transmission despite the addition of these dips to the optical signal. Accordingly, chromatic dispersion can be detected on the receiving side while actually carrying out data transmission, and moreover, using the wavelength that is used in this data transmission. Equalization of the optical signal such that the residual chromatic dispersion becomes zero can be carried out based on these detection results. By means of this type of method, the bandwidth for data transmission is not decreased for the detection of chromatic dispersion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of the configuration of the optical transmission system in which chromatic dispersion is measured by a PM-AM conversion method; 
         FIG. 2  is a block diagram showing the configuration of a transmitter in the first exemplary embodiment of the present invention; 
         FIG. 3  is a block diagram showing the configuration of a CD monitor in the first exemplary embodiment of the present invention; 
         FIG. 4  is a block diagram showing the configuration of a transmitter in the second exemplary embodiment of the present invention; 
         FIG. 5  is a block diagram showing the configuration of a CD monitor in the second exemplary embodiment of the present invention; 
         FIG. 6  is a block diagram showing an example of the configuration of the optical transmission and reception system that carries out automatic chromatic dispersion equalization for each wavelength channel according to the third exemplary embodiment of the present invention; 
         FIG. 7  is a block diagram showing an example of the configuration of the optical transmission and reception system that carries out automatic chromatic dispersion equalization for a plurality of wavelength channels collectively according to the fourth exemplary embodiment of the present invention; 
         FIG. 8  is a block diagram showing an example of the configuration of a transponder with an automatic chromatic dispersion equalization function in the fifth exemplary embodiment of the present invention; 
         FIG. 9  is a block diagram showing an example of the configuration of an optical transmission system capable of detecting chromatic dispersion; 
         FIG. 10  is a block diagram showing another example of the configuration of an optical transmission system capable of detecting chromatic dispersion; 
         FIG. 11  is a block diagram showing an example of the configuration of an optical transmission system capable of detecting and equalizing chromatic dispersion; 
         FIG. 12A  is a waveform chart showing a 55-gigabit/second (Gb/s) NRZ-QPSK signal; 
         FIG. 12B  is a waveform chart showing the waveform of a 55-Gb/s pseudo-RZ-QPSK signal; 
         FIG. 12C  is a waveform chart showing the waveform of a 55-Gb/s pseudo-RZ-QPSK signal; 
         FIG. 12D  is a waveform chart showing the waveform of a 110-Gb/s pseudo-RZ-QPSK signal; 
         FIG. 13A  is a graph showing the result of simulating an intensity spectrum of an NRZ-QPSK signal having a data rate of 56 Gb/s; 
         FIG. 13B  is a graph showing the result of simulating the intensity spectrum of a PRZ-QPSK signal having a data rate of 56 Gb/s; 
         FIG. 13C  is a graph showing the result of simulating the intensity spectrum of an RZ-QPSK signal having a data rate of 56 Gb/s; 
         FIG. 14A  is a graph showing the result of simulating how the intensity of a frequency component applied by pseudo-RZ modulation changes according to chromatic dispersion; 
         FIG. 14B  is a graph showing the result of simulating how the intensity of a frequency component applied by pseudo-RZ modulation changes according to chromatic dispersion; 
         FIG. 14C  is a graph showing the result of simulating how the intensity of a frequency component applied by pseudo-RZ modulation changes according to chromatic dispersion; 
         FIG. 14D  is a graph showing the result of simulating how the intensity of a frequency component applied by pseudo-RZ modulation changes according to chromatic dispersion; and 
         FIG. 14E  is a graph showing the result of simulating how the intensity of a frequency component applied by pseudo-RZ modulation changes according to chromatic dispersion. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 2  and  FIG. 3  are drawings for describing an optical transmission system according to the first exemplary embodiment of the present invention,  FIG. 2  showing the configuration of transmitter  100  used in this optical transmission system, and  FIG. 3  showing the configuration of CD monitor (CD MON)  200  that can be used in tandem with transmitter  100 . 
     The optical transmission system shown in  FIGS. 2 and 3  carries out optical data transmission to a receiving end from transmitter  100  by way of a transmission path composed of optical fiber or the like and is capable of detecting, at the receiving end, chromatic dispersion on the transmission path when carrying out optical data transmission. A receiver that carries out demodulation and the like of the optical signal to generate a data signal and CD monitor  200  that monitors the level of chromatic dispersion in the received optical signal are provided on the receiving end. 
     As shown in  FIG. 2 , transmitter  100  transmits a single-polarization pseudo-RZ (return-to-zero) signal to a transmission path and includes: laser  110  that is the light source of optical carrier  150 ; modulator (MOD)  111  that modulates optical carrier  150 ; pseudo-RZ carver (PRZ(n))  112  that carries out optical pulse carving to apply a dip for each n symbols to the optical signal; coder  120  that encodes an electric signal, i.e., data signal  101 , that indicates data to be transmitted to generate digital data  160  for modulation; and driver  121  that drives modulator  111  based on digital data  160 . Here, n is any integer equal to or greater than 2, such as 4, 8, or 16. Driver  121  converts digital data signal  160  to a suitable voltage signal  161  and supplies to the modulation input of modulator  111  to drive modulator  111 . Modulator  111  thus modulates optical carrier  150  based on voltage signal  161 . 
     In this configuration of transmitter  100 , clock signal  170  of the same frequency f as the symbol rate (i.e., baud rate) of the signal from coder  120  is conferred to pseudo-RZ carver  112 , and by using this clock signal  170 , pseudo-RZ carver  112  applies a dip for every n symbols to optical signal  151  that follows modulation. These dips make the intensity of optical signal  151  extremely low, for example, make the intensity of the optical signal zero, for extremely short time intervals. The pseudo-RZ carver can be configured by, for example, a frequency divider that frequency-divides clock signal f into n divisions, a D-type flip-flop that takes the output of the frequency divider as D inputs and clock signal f as clock input, and a phase modulator (PSK) that phase modulates the optical signal according to the output of the D-type flip-flop, as described in E. Le Taillandier de Gabory, et al., “Pseudo-Return-to-Zero Modulation Scheme: Application to Compensation of Intra-Polarization Skew for PolMux Signals,” ECOC 2009, paper 3.4.4 (2009) [NPL2]. 
     The addition of a dip for every n symbols by the pulse-carving technique makes the optical output extremely low at the positions of these dips, and these dips can be seen as return-to-zero in the optical signal. In a typical RZ signal, the intensity or polarity of the signal over the length of one-half the time of continuation of one symbol is taken as the intensity or polarity that corresponds to the data value “0,” but in the present exemplary embodiment, dips are applied of a length of time that is sufficiently short compared to the time of continuation of one symbol. As a result, the application of dips of this narrow width to the optical signal using the pulse carving technique is here called pseudo-RZ modulation. In WO2007/004338 [PL3], a technique is shown in which the use of pseudo-RZ modulation enables the easy extraction of a clock signal from an optical signal. Similarly, JP-A-2006-345541 [PL4] shows a technique in which converting an optical signal of NRZ (non-return-to-zero) modulation to an optical signal of pseudo-RZ modulation enables the easy extraction of a clock signal. 
     Optical signal  102  that has been modulated by modulator  111  by data signal  101  that is an electric signal and to which dips have been applied by pseudo-RZ modulation for every n symbols by pseudo-RZ carver  112  is supplied from transmitter  100 , and this optical signal is sent to the receiving end by way of a transmission path. 
     In the configuration shown in  FIG. 2 , transmitter  100  is, for example, a 55-Gb/s (Gb/s) PRZ(8)-QPSK (Quadrature Phase Shift Keying) transmitter. Here, “PRZ(8)” indicates that dips are applied every eight symbols by a pseudo-RZ scheme. A specific example of the configuration of transmitter  100  is next described with the bit rate of data signal  101  as 55 GHz. 
     Data signal  101  being a parallel signal, voltage signals  161  are 27.5 GHz electric signals that correspond to an I-(in-phase) component and Q-(quadrature) component, respectively, the I-component and Q-component being supplied in parallel to modulator  111 . Modulator  111  is a QPSK modulator, and because one symbol is made up from two bits in QPSK, the frequency of the clock signal supplied from coder  120  to pseudo-RZ carver  112  is also 27.5 GHz. Pseudo-RZ carver  112  applies dips to the signal intensity every eight symbols. In other words, n=8 in this example. 
     Graph  1110  of  FIG. 12A  shows the intensity waveform of optical signal  151  that has undergone QPSK modulation immediately after being supplied from modulator  111 . At this stage, optical signal  151  is a signal of the NRZ form. The time of continuation of each symbol is shown as 36.36 ps (picoseconds). In contrast, the optical signal from pseudo-RZ carver  112  is shown by graph  1130  of  FIG. 12C . A dip resulting from pseudo-RZ is shown by reference number  1131  in graph  1130 . The spacing of 290.91 ps between dips in graph  1130  corresponds to the time of continuation of eight symbols. 
     In the example here described, optical signal  102  is a 55-Gb/s PRZ(8)-QPSK signal. In the graph, 55 Gb/s is indicated by “55G.” The result of carrying out simulation for the intensity spectrum of this type of optical signal is shown in graph  1201  of  FIG. 13A . The intensity peak in the vicinity of 28 GHz is seen by the clock component of the symbol rate, and the frequency component in the vicinity of 28 GHz is stronger by 30 dB or more compared to the surrounding frequency area. In addition, due to the addition of dips for every eight symbols by pseudo-RZ, intensity peaks are seen at one-eighth of the frequency of the symbol rate, i.e., 3.44 GHz, as well as at its harmonic frequencies. These intensity peaks are stronger by 20 dB or more than the surrounding frequency components. 
     In contrast, graph  1202  of  FIG. 13B  shows the result of carrying out simulation for the intensity spectrum of a 56-Gb/s NRZ-QPSK signal. Here, a peak is observed in the vicinity of 28 GHz that corresponds to the symbol rate, but because there are no peaks in lower frequency areas, directly extracting and detecting a frequency component lower than 28 GHz from this signal is problematic. 
     In addition, graph  1203  of  FIG. 13C  shows the result of a simulation for the intensity spectrum of a 56-Gb/s RZ-QPSK signal according to the normal RZ format. In this case as well, a peak is observed in the vicinity of 28 GHz, but because there are no peaks in lower frequency areas, the direct extraction and detection of a frequency component lower than 28 GHz from this signal is problematic. In addition, although there is a peak in the vicinity of 28 GHz, the intensity in the frequency area in the vicinity of this peak is lower than in other frequency areas. 
     CD monitor  200  provided in the receiving end is next described. CD monitor  200  detects the frequency component that has been applied by pseudo-RZ modulation from an incident optical signal and generates electric signal  202  that indicates the chromatic dispersion value. 
     As shown in  FIG. 3 , CD monitor  200  includes: photodiode (PD)  210  that receives optical signal  201  applied as input and converts to electric signal  250 ; divider (DIV)  211  that branches electric signal  250  supplied from photodiode  210  to two signals  251  and  261 ; band-pass filter (BPF)  215  that receives signal  251 ; and monitor circuit  230  that calculates the chromatic dispersion value based on signal  261  and output signal  255  of band-pass filter  215 . Optical signal  201  that comes into photodiode  210  that is a light-receiving element is an optical signal to which dips are applied by pseudo-RZ modulation on the transmitting side. Photodiode  210  supplies electric signal  250  that represents the intensity of optical signal  201 , and this electric signal  250  is both supplied to monitor circuit  230  as signal  261  and supplied to band-pass filter  215  as signal  251  via divider  211 . The branching ratio between the two signals  251  and  261  at divider  211  is set to a fixed value. Other types of light-receiving elements can also be used in place of a photodiode. 
     Band-pass filter  215  extracts from signal  251  a frequency component in which the frequency is represented by k*f/n, where k is a natural number equal to or greater than 1 and f is the frequency of clock signal  170  at transmitter  100 , i.e., the symbol rate in optical signal  201 , and generates signal  255  that indicates the intensity of the extracted frequency component. Here, n corresponds to the insertion of dips every n symbols by means of pseudo-RZ in transmitter  100 . If frequencies of 1/n of the symbol rate f are referred to as dip frequencies, there are peaks at the dip frequencies and the harmonic frequencies in the intensity spectrum of optical signal  201 . as described hereinabove. Band-pass filter  215  therefore extracts the dip frequencies (when k=1) or their k th -order harmonic components (when k&gt;1) in optical signal  201 . 
     However, when the chromatic dispersion amount changes, the intensity ratio of the dip frequency component with respect to the entire optical signal  201  changes. This change of the intensity ratio with respect to the chromatic dispersion amount can be calculated by, for example, simulation. Conversely, the chromatic dispersion amount can be determined if the ratio of the intensity of the dip frequency component (or its harmonic component) with respect to the overall intensity of electric signal  250  that is generated by the reception of optical signal  201  can be found. 
     Here, monitor circuit  230  finds the ratio of the intensity of signal  255  that corresponds to the dip frequency component with respect to the intensity of signal  261  that corresponds to the entire band in optical signal  201 , and based on the value of this ratio, calculates the chromatic dispersion value in the transmission of optical signal  201 . In actuality, the chromatic dispersion value may be determined based on an intensity ratio calculated using a look-up table that is based on the simulation results. The look-up table is stored in a storage medium such as a nonvolatile semiconductor memory incorporated in monitor circuit  230 . Of course, if it is known that the intensity of signal  261  is always a fixed value, or in other words, if it is known that the intensity of optical signal  201  is always fixed, the chromatic dispersion value can be calculated from only signal  255  without using signal  261 . When the average power of optical signal  201  is not fixed, monitor circuit  230  uses signal  261  to calculate the average power of optical signal  201  and calculates the chromatic dispersion value based on this average power. 
     For example, if it is assumed that optical signal  201  is a 56-Gb/s PRZ(4)-QPSK signal, n=4 and symbol rate f is 28 GHz. Here, when k=1, i.e., when the fundamental wave component of the dip frequency is extracted by band-pass filter  215 , CD monitor  200  finds the chromatic dispersion value by means of the frequency component of one-fourth the symbol rate and supplies signal  202  that represents this CD value. Graph  1310  of  FIG. 14A  shows the result of performing simulation that finds how the intensity ratio of the dip frequency component changes with respect to the value of the chromatic dispersion CD. 
     The intensity ratio undergoes monotone change when the chromatic dispersion is in the range of −400 to 0 ps/nm and further undergoes monotone change in the range of 0 to +400 ps/nm. Accordingly, if the absolute value of chromatic dispersion is unknown but the sign is known, monitor circuit  230  is able to monitor the chromatic dispersion within the broad range of ±400 ps/nm. In addition, the sign of chromatic dispersion can be found if the chromatic dispersion is intentionally changed in the positive or negative direction and the change in signal  202  is observed at this time, whereby the actual value of chromatic dispersion can be found by using monitor circuit  230 . 
     The second exemplary embodiment of the present invention is next described. The above-described first exemplary embodiment related to a case in which an optical signal of a single polarization is transmitted, but in the second exemplary embodiment, a case is described in which polarization-division multiplexing is used to transmit an optical signal.  FIG. 4  shows the configuration of the transmitter that is used in the second exemplary embodiment and that supplies a polarization-division-multiplexed pseudo-RZ optical signal.  FIG. 5  shows the configuration of a CD monitor that is the CD monitor used in the second exemplary embodiment, that receives a polarization-division-multiplexed pseudo-RZ optical signal and that detects a plurality of frequency components that are applied in a pseudo-RZ format to generate a signal that represents the chromatic dispersion value. 
     The configuration of transmitter  300  in the second exemplary embodiment is first described using  FIG. 4 . 
     Transmitter  300  supplies a polarization-division-multiplexed signal that has undergone pseudo-RZ modulation to add dips. Transmitter  300  includes: laser  310  that is the light source of optical carrier  350  that is originated; polarization-maintaining coupler (PM CPL)  313  that branches optical carrier  350  while maintaining polarization; modulator (MOD)  311  provided at one of the branches from polarization-maintaining coupler  313 ; pseudo-RZ carver (PRZ(n))  312  that applies dips to optical signal  351  that has undergone modulation by modulator  311 ; modulator (MOD)  316  provided at the other branch from polarization-maintaining coupler  313 ; pseudo-RZ carver  317  that applies dips to optical signal  356  that has undergone modulation by modulator  316 ; polarization rotator (Pol Rot)  315  that rotates the polarization plane of signal light  357  supplied from pseudo-RZ carver  317 ; and polarization combiner (Pol CMB)  316  that combines optical signal  352  from pseudo-RZ carver  312  and optical signal  355  from polarization rotator  315  while maintaining polarization unchanged. Transmitter  300  further includes coder  320  and drivers  321  and  322  for modulation. 
     In this configuration, polarization-maintaining coupler  313  splits optical carrier  350  into two branches and generates optical carriers  353  and  354  having the same polarization state. Coder  320  generates digital data  360  and  362  for modulation for each polarization from an electric signal that indicates data to be transmitted, i.e., data signal  301 , and drivers  321  and  322  convert each of digital data  360  and  362  to voltage signals  361  and  363 , respectively, of appropriate voltages. Modulators  311  and  316  modulate optical carriers  353  and  354  based on voltage signals  361  and  363  to supply optical signals  351  and  356 , respectively. Coder  320  supplies clock signals  370  and  371  of the same frequency as the symbol rate to pseudo-RZ carvers  312  and  317 , respectively. 
     Both n and m are integers equal to or greater than 2 that mutually differ, pseudo-RZ carver  312  applies dips every n symbols to optical signal  351  by means of pseudo-RZ modulation, and pseudo-RZ carver  317  applies dips every m symbols to optical signal  356 . 
     Polarization rotator  315  implements control to rotate the polarization of optical signal  357  such that the polarization of optical signal  357  from pseudo-RZ carver  317  is orthogonal to the polarization of optical signal  352  from pseudo-RZ carver  312  and to supply the result as optical signal  355 . As a result, optical signal  352  and optical signal  355  have mutually orthogonal polarization states, and these signals are subjected to polarization-division-multiplexing by polarization combiner  316  and supplied as optical signal  302 . Accordingly, optical signal  302  supplied as output by transmitter  300  is a signal that has undergone modulation by modulators  311  and  316  according to data signal  301 , that has had pseudo-RZ dips applied to each polarization every n or m symbols by pseudo-RZ carvers  312  and  317 , and then subjected to polarization-division-multiplexing. 
     In the configuration shown in  FIG. 4 , transmitter  300  is a 110-Gb/s PRZ(4, 8)-QPSK transmitter. A specific example of the configuration of transmitter  300  is next described in which the data rate of data signal  301  is 110 GHz, n=8, and m=4. 
     Data signal  301  being a parallel signal, electric signal  361  is a 27.5 GHz electric signal composed of an I-component signal and Q-component signal that corresponds to one polarization, and similarly, electric signal  363  is a 27.5 GHz electric signal composed of an I-component signal and a Q-component signal that correspond to the other polarization. Each of modulators  311  and  316  is a QPSK modulator that takes the I-component signal and Q-component signal as input. Clock signals  370  and  371  that both have frequency of 27.5 GHz are supplied to pseudo-RZ carvers  312  and  317  from coder  320 . 
     Pseudo-RZ carver  312  applies dips to the intensity of optical signal  351  every eight symbols and pseudo-RZ carver  317  applies dips to the intensity of optical signal  356  every four symbols. The intensity waveform of optical signals  351  and  356  that have undergone QPSK modulation by modulators  311  and  316  is shown in graph  1110  of the above-described  FIG. 12A . In addition, the intensity waveform of optical signal  352  from pseudo-RZ carver  312  is shown by graph  1130  of  FIG. 12C , and a dip in this waveform is indicated by reference number  1121 . The intensity waveform of optical signal  355  from pseudo-RZ carver  317  is shown by graph  1120  of  FIG. 12B , and a dip in this waveform is indicated by reference number  1121 . 
     The two optical signals  351  and  356  for which intensity waveforms are shown by graph  1120  of  FIG. 12B  and graph  1130  of  FIG. 12C , respectively, undergo polarization-division-multiplexing to generate polarization-division-multiplexed signal  302 , whereby the intensity waveform of polarization-division-multiplexed signal  302  becomes as shown by graph  1140  of  FIG. 12D . In graph  1140 , reference number  1141  indicates a deep dip that results from the coincidence of the dip (PRZ(4)) for every four symbols and dip (PRZ(8)) for every eight symbols, and reference number  1142  indicates a dip for every four symbols. 
     The configuration of the CD monitor in the second exemplary embodiment is next described using  FIG. 5 . 
     CD monitor (CD MON)  400  detects a plurality of frequency components that have been applied by pseudo-RZ modulation from incident optical signal  401  and generates an electric signal that represents the chromatic dispersion value. Optical signal  401  is a signal to which dips have been added based on a pseudo-RZ format, and, for example, is an optical signal generated by transmitter  300  shown in  FIG. 4  and transmitted on a transmission path such as an optical fiber. Optical signal  401  may be a signal generated by a transmitter other than the transmitter shown in  FIG. 4  as long as it is a signal to which dips have been applied by pseudo-RZ according to a plurality of frequency components. 
     CD monitor  400  includes: photodiode  410  which receives optical signal  401  and converts this optical signal to an electric signal; k+1 pieces of dividers  420  to  42   k ; k pieces of band-pass filters  451  to  45   k ; and monitor circuit  411  that actually generates an electric signal that represents the CD measurement value, where k is an integer equal to or greater than 2. The parameter k indicates the number of types of frequency components by which dips have been applied to optical signal  401 . Band-pass filters  451  to  45   k  each extract a component of mutually different frequencies f 1  to fk. In the following description, each of components of frequencies f 1  to fk are referred to as the f 1  frequency component to the fk frequency component, respectively. 
     When optical signal  401  is received and photodiode  410  that is a light-receiving element issues electric signal  403  that indicates the intensity of optical signal  401 , this electric signal  403  is applied to the initial-stage divider (DIV)  420  and branched into signal  430  and signal  431 . Signal  430  is conferred directly to monitor circuit  411 . In contrast, signal  431  is applied to the next divider (DIV 1 )  421  and branched into signal  441  and signal  432 . 
     Signal  441  is supplied to band-pass filter  451 , and band-pass filter  451  extracts the f 1  frequency component from signal  441  and generates signal  461  that indicates the intensity of this component. Frequency f 1  is the frequency of the dips applied by means of pseudo-RZ in signal  401 . On the other hand, signal  432  is applied as input to the next divider (DIV 2 )  422  and branched into signal  442  and signal  433 . Of these, signal  442  is supplied to band-pass filter  452  and signal  433  is further applied as input to the next divider (DIV 3 )  423 . Band-pass filter  452  extracts the f 2  frequency component from signal  442  and generates signal  462  that represents the intensity of this component. Frequency f 2  is the frequency of the dips applied by pseudo-RZ in signal  401 . 
     Last-stage divider  42   k  similarly receives signal  43   k , extracts the fk frequency component and supplies signal  44   k  to  k   th  band-pass filter  45   k  that generates signal  46   k  that represents the intensity of this component. Because there is no divider following divider  42   k , signal  43   k  may be directly supplied to band-pass filter  45   k  as signal  46   k  without providing last-stage divider  42   k.    
     By means of the above-described configuration, electric signals  461  to  46   k  that represent the intensities of each of the f 1  frequency component to the fk frequency component are generated, and these electric signals are supplied to monitor circuit  411  together with signal  430 . 
     Monitor circuit  411  calculates the chromatic dispersion value in the transmission of optical signal  401  based on the ratio of the intensities of signals  461  to  46   k  with respect to the intensity of signal  430 . The chromatic dispersion values are preferably found through, for example, the use of a look-up table, rather than by actually carrying out computations. The principle for finding the chromatic dispersion values in this case is similar to the case of the first exemplary embodiment. In addition, when the average power of optical signal  401  is not fixed, monitor circuit  411  can eliminate the dependency of reception power in the calculation of chromatic dispersion values by using signal  430  to calculate the average power of optical signal  401  and then using the calculated average power and the intensities of signals  461  to  46   k . Of course, if it is known that the reception power of optical signal  401  is always fixed, the chromatic dispersion value can be determined from the intensities of signals  461  to  46   k  without using signal  430 . 
     For example, if optical signal  401  is assumed to be a polarization-division-multiplexed 112-Gb/s PRZ(4, 8)-QPSK signal, symbol rate f is f=28 GHz, whereby f 1 =f/4 can be selected and f 2 =f/8 can be selected. In other words, it can be assumed that dips are inserted every four symbols, and further, that dips are inserted every eight symbols so as to be realized by the transmitter shown in  FIG. 4 . The result of simulating the change with respect to chromatic dispersion of the f 1  frequency component and the f 2  frequency component in this case is shown in  FIGS. 14A to 14E . 
     Graph  1320  of  FIG. 14B  shows how the intensity ratio of signal  461  of the f 1  frequency component with respect to signal  430  changes with respect to chromatic dispersion, and graph  1330  of  FIG. 14C  shows how the intensity ratio of signal  462  of the f 2  frequency component with respect to signal  430  changes with respect to chromatic dispersion. 
     When it is known whether the sign of the chromatic dispersion amount is positive or negative, change in the intensity ratio of signal  462  is monotone, whereby signal  462  can be used in the detection of chromatic dispersion of the wide range of ±1000 ps/nm and the chromatic dispersion can be monitored. When chromatic dispersion is within the range of ±400 ps/nm, graph  1320  that indicates larger amount of change with respect to chromatic dispersion can be used to monitor chromatic dispersion with still higher accuracy. 
     When the chromatic dispersion is within the range of ±400 ps/nm, the result shown in graph  1320  relating to the f 1  frequency component and the result according to graph  1330  relating to the f 2  frequency component may both be used and monitor circuit  411  may generate electric signal  402  that represents a highly accurate chromatic dispersion value based on a look-up table that uses these two results. 
     When monitoring chromatic dispersion in a still broader range of ±2000 ps/nm, as can be seen from graph  1340  of  FIG. 14D  that shows the simulation result, the chromatic dispersion should be found from the intensity of the f 3  frequency component using optical signal  401  that applies pseudo-RZ(16), i.e., using optical signal  401  in which dips are applied every 16 symbols with f 3 =f/16. In this case, signal  402  supplied from monitor circuit  411  can represent chromatic dispersion up to ±2000 ps/nm. 
     As another example, if optical signal  401  is 56-Gb/s PRZ(8)-QPSK, the symbol rate becomes f=28 GHz, whereby at this time f 1 =f/8 can be selected as the f 1  frequency component that corresponds to signal  461 , f 2 =f/4 can be selected corresponding to signal  462 , and f 3 =f/2 can be selected corresponding to signal  463 . Graph  1350  of  FIG. 14E  shows the simulation result for the intensity of these signals  461  to  463  for chromatic dispersion. Curve  1351  shows the results for signal  461 , curve  1352  shows the results for signal  462 , and curve  1353  shows the simulation results for signal  463 . 
     When the sign of the chromatic dispersion is known, using signal  461  results in monotone change of the signal intensity with respect to chromatic dispersion, whereby chromatic dispersion can be monitored over the broad range of ±1000 ps/nm. When chromatic dispersion is within the range of ±400 ps/nm, chromatic dispersion can be monitored by using signal  462 , and when chromatic dispersion is within the range of ±100 ps/nm, chromatic dispersion can be monitored using signal  463 . 
     The third exemplary embodiment of the present invention is next described. Here, an optical transmission and reception system is described in which chromatic dispersion is detected by means of the monitor method as described above, and based on the detection result, the execution of automatic chromatic dispersion equalization is enabled for each wavelength channel.  FIG. 6  shows the optical transmission and reception system in the third exemplary embodiment. 
     Transponder  500  includes: transmission unit (TX PRZ)  501 ; reception unit (RX)  502 ; CD monitor (CD MON)  503 ; and coupler (CPL)  504 . Transmission unit  501  modulates an optical carrier by means of electric signal  511  that indicates the data to be transmitted, and further, applies dips, by means of pseudo-RZ, in the intensity of the carrier to the optical carrier after the modulation, and transmits optical signal  512 . Transmitter  100  that was described using  FIG. 2  or transmitter  300  that was described using  FIG. 4  can be used as this transmission unit  501 . 
     Optical signal  516  received by transponder  500  is split into two branches by coupler  504 , one branch being distributed to reception unit  502  and the other being distributed to CD monitor  503 . Optical signal  516  is here assumed to have had dips applied every n symbols by pseudo-RZ modulation. Reception unit  502  receives the incident optical signal and demodulates the signal to convert to electric signal  517 . CD monitor  503  monitors the chromatic dispersion of the received optical signal and generates an electric signal that indicates the value of the chromatic dispersion, i.e., monitor signal  521 . As CD monitor  503 , CD monitor  200  that was described using  FIG. 3  or CD monitor  400  that was described using  FIG. 5  can be used. 
     Further, in this transmission and reception system, transponder  530  that is a device similar to transponder  500  is provided. However, transponder  530  supplies optical signal  542  of a wavelength that differs from optical signal  512 , based on electric signal  541  that indicates data to be transmitted. In addition, the wavelength of optical signal  546  that this transponder  530  receives is different from that of optical signal  516 . Based on received optical signal  546 , transponder  530  both generates electric signal  547  that indicates the received data and supplies monitor signal  551  that represents the chromatic dispersion contained in optical signal  546 . 
     Optical multiplexer (MUX)  560  that subjects optical signals  512  and  542  transmitted from transponders  500  and  530 , respectively, to wavelength-multiplexing is further provided, and optical signal  513  that has undergone wavelength-multiplexing is transmitted to the transmission path. Many transponders may be further provided, and the optical signals from these transponders may be subjected to wavelength-multiplexing in optical multiplexer  560 . 
     Optical signal  514  that is produced in a transponder similar to transponders  500  and  530  and subjected to wavelength-multiplexing is sent from the transmission path. Optical demultiplexer (DEMUX)  561  that separates optical signal  514  by wavelength is provided, and optical signals  515  and  545  that have been separated by respective wavelengths pass through variable chromatic dispersion equalizer (VAR CD COMP)  520  and  550  and thus equalized, and finally received by multiplexers  500  and  530  as optical signals  516  and  546 , respectively. Here, variable chromatic dispersion equalizer  520  is controlled by monitor signal  521  from multiplexer  500 , and variable chromatic dispersion equalizer  550  is controlled by monitor signal  551  from multiplexer  530 . 
     Specific examples of the optical transmission and reception system shown in  FIG. 6  are next described. 
     In the first example, transmission unit  501  of transponder  500  is of the same configuration as transmitter  100  according to pseudo-RZ modulation shown in  FIG. 2  and produces a 56-Gb/s PRZ(8)-QPSK signal. In addition, CD monitor  503  is of the same configuration as CD monitor  200  shown in  FIG. 3 , and is set to f/n=f/8 with the frequency of the symbol rate as f. Transponder  530  may also be configured from a similar transmitter and CD monitor. 
     Variable chromatic dispersion equalizer  520  is controlled such that monitor signal  521  that indicates chromatic dispersion is minimized. As shown in graph  1330  of  FIG. 14C , if residual chromatic dispersion in optical signal  515  from demultiplexer  561  is within ±1000 ps/nm, the amount of CD can be determined accurately, and by controlling variable chromatic dispersion equalizer  520  based on this amount of CD, the residual chromatic dispersion in the optical signal that is received by reception unit  502  can be made zero. Accordingly, the quality of data signal  517  supplied from reception unit  502  is optimized. Data signal  517  in this case is an electric signal. In addition, even if the chromatic dispersion should change during transmission in optical signal  514  from the transmission path, the residual chromatic dispersion in optical signal  515  can be equalized. The residual chromatic dispersion in optical signal  545  can also be equalized by executing similar control over variable chromatic dispersion equalizer  550 . 
     In the second example, transmission unit  501  of transponder  500  is of the same configuration as transmitter  100  according to pseudo-RZ modulation that was shown in  FIG. 2  and produces a 56-Gb/s PRZ(8)-QPSK signal. CD monitor  503  is of the same configuration as CD monitor  400  shown in  FIG. 5  and is set to f 1 =f/8, f 2 =f/4, and f 3 =f/2. Transponder  530  may also be configured from a similar transmitter and CD monitor. 
     In this case as well, variable chromatic dispersion equalizer  520  is controlled such that monitor signal  521  that indicates chromatic dispersion is minimized. As shown by graph  1350  of  FIG. 14E , if the residual chromatic dispersion in optical signal  515  is within ±1000 ps/nm, the amount of CD can be determined accurately, the residual chromatic dispersion in the optical signal received by reception unit  502  can be made zero through the control of variable chromatic dispersion equalizer  520  based on this amount of CD, and the quality of data signal  517  can be optimized. In order to execute this type of control, information from the f 1  frequency component is used in the initial stage of control to generate monitor signal  521  and the residual chromatic dispersion is first suppressed to within ±400 ps/nm. Next, monitor signal  521  is generated based on information from the f 2  frequency component to suppress the chromatic dispersion to ±100 ps/nm. Then, using information from the f 3  frequency component, minute change in the residual chromatic dispersion is tracked to enable highly accurate compensation of the chromatic dispersion. The adoption of range switching of this type enables the selection of the optimum dynamic range for equalizing chromatic dispersion. In addition, the residual chromatic dispersion in optical signal  515  can be equalized even when there is change in the chromatic dispersion over time in optical signal  514  received from the transmission path. The residual chromatic dispersion in optical signal  545  can be equalized through the execution of similar control over variable chromatic dispersion equalizer  550 . 
     In the third example, transmission unit  501  of transponder  500  is of the same configuration as polarization-division-multiplexing pseudo-RZ transmitter  300  that was shown in  FIG. 4  and generates a polarization-division-multiplexed 112-Gb/s PRZ(4, 8)-QPSK signal. CD monitor  503  is of the same configuration as the CD monitor  400  shown in  FIG. 5  and is set to f 1 =f/8 and f 2 =f/4. Transponder  530  may also be configured from a similar transmitter and CD monitor. 
     In this case as well, variable chromatic dispersion equalizer  520  is controlled such that monitor signal  521  that indicates the chromatic dispersion is minimized. As shown by graph  1350  of  FIG. 14E , if the residual chromatic dispersion in optical signal  515  is within ±1000 ps/nm, the amount of CD can be accurately determined, the residual chromatic dispersion can be made zero, and the quality of data signal  517  can be optimized. In order to execute this type of control, information from the f 1  frequency component is used in the initial stage of control to generate monitor signal  521  and the residual chromatic dispersion is first suppressed to within ±400 ps/nm. Monitor signal  521  is then generated based on information from the f 2  frequency component to carry out finer control. The adoption of this type of range switching enables selection of the optimum dynamic range. Equalization of the residual chromatic dispersion in optical signal  515  can be realized even if the chromatic dispersion should change over time in optical signal  514  received from the transmission path. The residual chromatic dispersion in optical signal  545  can also be equalized by implementing similar control over variable chromatic dispersion equalizer  550 . 
     The fourth exemplary embodiment of the present invention is next described. An optical transmission and reception system is here described that is capable of detecting chromatic dispersion by the monitor method such as described hereinabove, and based on this detection result, implementing automatic equalization of chromatic dispersion collectively for a plurality of wavelength channels.  FIG. 7  shows the optical transmission and reception system in the fourth exemplary embodiment. 
     Transponder  600  is of the same configuration as transponder  500  that was shown in  FIG. 6  and modulates an optical carrier by electric signal  611  that indicates data to be transmitted to a partner, applies dips in the intensity of the carrier by means of pseudo-RZ to the modulated optical carrier, transmits optical signal  512 , and finally, receives optical signal  615  from the partner side to supply data signal  617  as an electric signal. Monitor signal  660  that indicates the measured CD value is also supplied from transponder  600 . In addition, transponder  630  that is a device similar to transponder  600  is also provided. However, transponder  630  supplies, based on electric signal  641  that indicates data to be transmitted, optical signal  642  of a wavelength that differs from optical signal  612 . In addition, transponder  630  receives optical signal  645  in which the wavelength differs from that of optical signal  615 , and based on this optical signal  645 , both generates electric signal  647  that indicates the received data and supplies monitor signal  661  that represents the chromatic dispersion contained in optical signal  645 . 
     Optical multiplexer (MUX)  660  is provided that subjects optical signals  612  and  642  transmitted from transponders  600  and  630 , respectively, to wavelength-multiplexing, and the wavelength-multiplexed optical signal  613  is transmitted to the transmission path. Many transponders may be further provided and the optical signals from these transponders may be subjected to wavelength-multiplexing in optical multiplexer  660 . 
     Optical signal  633  that has been generated by a transponder similar to transponders  600  and  630  and subjected to wavelength-multiplexing is transmitted from the transmission path. Optical demultiplexer (DEMUX)  661  is provided that separates optical signal  633  by wavelength, and optical signals  615  and  645  that have been separated by wavelength are received by multiplexers  600  and  630 , respectively. 
     In this exemplary embodiment, variable chromatic dispersion equalizer (VAR CD COMP)  664  is provided between demultiplexer  651  and the transmission path, and optical signal  633  from the transmission path undergoes compensation of chromatic dispersion in groups relating to a plurality of wavelength channels by passing through variable chromatic dispersion equalizer  664 . This type of variable chromatic dispersion equalizer  664  can employ a device such as is disclosed in S. Sohma, et al., “40λ WDM Channel-by-Channel and Flexible Dispersion Compensation at 40 Gb/s Using Multi-channel and Flexible Dispersion Compensator,” ECOC 2009, paper 3.3.1 (2009)) [NPL3]. 
     Control circuit  662  is provided to control variable chromatic dispersion equalizer  664 . Control circuit  662  controls variable chromatic dispersion equalizer  664  by means of control signal  663  such that the residual chromatic dispersion in both the wavelength of the received optical signal of transponder  600  and the wavelength of the received optical signal of transponder  630  become zero based on monitor signal  660  from transponder  600  and monitor signal  661  from transponder  630 . 
     Specific examples of the optical transmission and reception system shown in  FIG. 7  are next described. 
     In the first example, the transmission unit of transponder  600  is of the same configuration as transmitter  100  according to pseudo-RZ modulation shown in  FIG. 2  and produces a 56-Gb/s PRZ(16)-QPSK signal. The CD monitor of transponder  600  is of the same configuration as CD monitor  200  shown in  FIG. 3 , and f/n=f/16 is set with the frequency of the symbol rate being f. Transponder  630  may also be configured from a similar transmitter and CD monitor. 
     In this case, control circuit  662  generates control signal  663  such that monitor signal  661  that indicates chromatic dispersion is minimized to control variable chromatic dispersion equalizer  664 . From graph  1340  of  FIG. 14D , if the residual chromatic dispersion in optical signal  615  is within ±2000 ps/nm, the amount of CD can be determined accurately, and by controlling variable chromatic dispersion equalizer  664  based on this amount of CD, the residual chromatic dispersion can be made zero and the quality of data signal  617  can be optimized. In addition, the residual chromatic dispersion in optical signal  615  can be equalized even if the chromatic dispersion should change during transmission in optical signal  633  from the transmission path. The residual chromatic dispersion in optical signal  645  can also be equalized by implementing similar control. 
     In the second example, the transmission unit of transponder  600  is of the same configuration as transmitter  100  according to pseudo-RZ modulation shown in  FIG. 2  and produces a 56-Gb/s PRZ(8)-QPSK signal. The CD monitor of transponder  600  is of the same configuration as CD monitor  400  shown in  FIG. 5  and is set to f 1 =f/8, f 2 =f/4, and f 3 =f/2. Transponder  630  may also be configured from a similar transmitter and CD monitor. 
     In this case as well, variable chromatic dispersion equalizer  664  is controlled such that monitor signal  660  is minimized. As shown by graph  1350  of  FIG. 14E , if the residual chromatic dispersion in optical signal  615  is within ±1000 ps/nm, the amount of CD can be accurately determined, the residual chromatic dispersion can be made zero, and the quality of data signal  617  can be optimized. In order to implement this type of control, information from the f 1  frequency component is used to generate monitor signal  660  in the initial stage of control and the residual chromatic dispersion is first suppressed to within ±400 ps/nm. Monitor signal  660  is next generated based on information from the f 2  frequency component and the chromatic dispersion is suppressed to ±100 ps/nm, following which information of the f 3  frequency component is used to carry out tracking or the like of minute changes in the residual chromatic dispersion whereby the chromatic dispersion can be compensated with high accuracy. By adopting this type of range switching, the optimum dynamic range for equalizing the chromatic dispersion can be selected. In addition, the residual chromatic dispersion in optical signal  615  can be equalized even when the chromatic dispersion changes over time in optical signal  633  received from the transmission path. The residual chromatic dispersion in optical signal  645  can also be equalized by implementing the same type of control. 
     In the third example, the transmission unit of transponder  600  is of the same configuration as polarization-division-multiplexing pseudo-RZ transmitter  300  shown in  FIG. 4  and produces a polarization-division-multiplexed 112-Gb/s PRZ(4, 8)-QPSK signal. The CD monitor of transponder  600  is of the same configuration as CD monitor  400  shown in  FIG. 5  and is set to f 1 =f/8 and f 2 =f/4. Transponder  630  may also be configured from similar transmitter and CD monitor. 
     In this case, variable chromatic dispersion equalizer  664  is controlled such that monitor signal  661  that indicates the chromatic dispersion is minimized. As shown in by graph  1350  of  FIG. 14E , if the residual chromatic dispersion in optical signal  615  is within ±1000 ps/nm, the amount of CD can be accurately determined, the residual chromatic dispersion can be made zero, and the quality of data signal  617  can be optimized. In order to implement this type of control, information from the f 1  frequency component is used to generate monitor signal  660  in the initial stage of control and the residual chromatic dispersion is first suppressed to within 400 ps/nm. Monitor signal  660  is then generated based on information from the f 2  frequency component and finer control is implemented. The adoption of this type of range switching enables selection of the optimum dynamic range. The residual chromatic dispersion in optical signal  615  can be equalized even when chromatic dispersion changes over time in optical signal  633  received from the transmission path. The residual chromatic dispersion in optical signal  645  can also be equalized by implementing the same control over variable chromatic dispersion equalizer  664 . 
     Next, as the fifth exemplary embodiment of the present invention, an example of the configuration of a transponder with an automatic chromatic dispersion equalizing function is next described.  FIG. 8  shows an example of the configuration of the transponder in this exemplary embodiment. 
     In general terms, transponder  700  includes transmission unit (TX PRZ)  701  and reception unit  702 . Transmission unit  701  modulates an optical carrier by data signal  711 , which is an electric signal that indicates the data to be transmitted, and by using a pseudo-RZ modulation scheme on the modulated optical carrier, applies dips in the intensity of the optical carrier to generate optical signal  712  and then transmits optical signal  712 . Reception unit  702  receives optical signal  716 , carries out coherency detection, demodulation, and supplies electric signal  717  as received data as well as monitors the chromatic dispersion in optical signal  716  to equalize the chromatic dispersion based on results of the monitoring. This type of reception unit includes: coherency reception module (COH RX)  750 ; laser  751  provided as a local oscillator (LO); analog-to-digital converter (ADC)  752  that converts the analog electric signal supplied from coherency reception module  750  to a digital signal; and digital signal processing unit  752  that carries out signal processing of the received signal that has been digitized. As in the case of each of the above-described exemplary embodiments, the pseudo-RZ modulation method is used to apply a dip in intensity for every n symbols to optical signal  716  that is received by reception unit  702 . 
     Coherency reception module  750  includes a 90-degree hybrid and four balanced photodiodes, and incident optical signal  716  is mixed with local oscillation light from laser  751  and subjected to coherency detection. Four output analog signals from coherency reception module  750 , i.e., the received signals, are converted to a digital signal by analog-to-digital converter  752  and supplied to digital signal processing unit  760 . 
     Digital signal processing unit  760  includes: CD compensation unit (CD COMP)  720  that compensates chromatic dispersion by FIR (Finite Impulse Response) filtering calculation; chromatic dispersion monitor unit  703  that detects chromatic dispersion based on the output from CD compensation unit  720 ; polarization separation calculation unit (CMA)  761  that carries out polarization-separation calculation for the output from CD compensation unit  720 ; clock extraction unit (CR)  762  that is connected to the output of polarization separation calculation unit  761  and that extracts clocks from the signal after the polarization-separation; frequency difference compensation unit (CPE)  763  that compensates the carrier frequency difference for the signal after the clock extraction; and demodulation unit (DEC)  764  that demodulates the signal for which compensation of carrier frequency difference has been carried out. The output of CD compensation unit  720  is split and conferred to chromatic dispersion monitor unit  703  and polarization separation calculation unit  761 . Polarization separation calculation unit  761  carries out polarization separation calculation by means of a CMA (Constant Modulus Algorithm). Frequency difference compensation unit  763  implements compensation of the carrier frequency difference between local oscillation light  751  and received optical signal  716  by means of a CPE (Carrier Phase Estimation) algorithm. The output from demodulation unit  764  is data signal  717  that is an electric signal representing the received data. 
     Chromatic dispersion monitor unit  703  is next described in detail. Chromatic dispersion monitor unit  703  detects residual chromatic dispersion, generates monitor signal  733  that represents the residual CD value, and controls the chromatic dispersion compensation in CD compensation unit  720  by means of this monitor signal  733 . The tap coefficients of the FIR filter in CD compensation unit  720  are set such that the residual CD value indicated by monitor signal  733  is minimized. In addition, a signal identical to monitor signal  733  is supplied to the outside of transponder  700  as monitor signal  732 . Monitor signal  732  can, for example, be used for carrying out chromatic dispersion compensation outside transponder  700 . Chromatic dispersion monitor unit  703  in this configuration includes: absolute value calculation unit (MODULUS)  730  that calculates the absolute value, and FFT analysis unit  731  that analyzes the output of absolute value calculation unit  730  by means of Fast Fourier Transform (FFT) and generates monitor signal  733 . Absolute value calculation unit  730  first calculates the absolute value of the signal that is applied as input to chromatic dispersion monitor unit  703 . This calculated value represents the amplitude of received optical signal  716 . FFT analysis unit  731  then performs Fourier transform by means of a FFT algorithm upon the output of absolute value calculation unit  730  to calculate the intensity of a frequency component that is the same as the frequency of dips realized by pseudo-RZ modulation that have been applied to optical signal  716  or the intensity of the frequency component of its harmonic. FFT analysis unit  731  then calculates the value of the chromatic dispersion by the same method as in the above-described exemplary embodiments, and generates monitor signal  733 . FFT analysis unit  731  may hold a look-up table that indicates the relations between the magnitude of chromatic dispersion and the intensity of the selected frequency component, and the value of residual chromatic dispersion may be found by retrieving this look-up table based on the intensity of the selected frequency component. The look-up table may be stored, for example, in a memory medium such as a memory device belonging to digital signal processing unit  760 . 
     When the tap coefficients of the FIR filter in CD compensation unit  720  are set such that the value of monitor signal  732  supplied to CD compensation unit  720  from chromatic dispersion monitor unit  703  is a minimum, the influence of residual chromatic dispersion becomes zero in the signal that is the object of demodulation, whereby the quality of data signal  717  supplied from demodulation unit  764  is optimized. 
     This type of transponder  700  can be used as, for example, transponders  500  and  530  of the optical transmission and reception system shown in  FIG. 6 , or can be used as transponders  600  and  630  of the optical transmission and reception system shown in  FIG. 7 . 
     Transmitters, CD monitors, and transponders based on exemplary embodiments of the present invention have been described hereinabove. By arranging these transmitters or transponders on the transmitting side, arranging the CD monitors or transponders on the receiving side, and connecting the receiving side and transmitting side by a transmission path such as an optical fiber, an optical transmission and reception system can be configured that can detect on the receiving side chromatic dispersion that occurs on the transmission path. 
       FIG. 9  shows an example of the configuration of the optical transmission system that can detect chromatic dispersion that can occur on the transmission path in this way. 
     Transmitter (TX PRZ)  810  is provided on the transmitting side. Transmitter  810  modulates the optical carrier by means of electric signal  801  that represents data to be transmitted, and further, applies a dip every n symbols by means of pseudo-RZ modulation identical to that described above to the optical carrier after the modulation. Optical signal  820  that has thus undergone modulation and the application of dips is transmitted to the receiving side by way of transmission path  821 . Transmitter  810  is configured to enable changing the transmission wavelength in this output signal  820  that is transmitted, i.e., the wavelength of the optical carrier. Transmitter  100  shown in  FIG. 2  or transmitter  300  shown in  FIG. 4  can be used as this transmitter  810 . 
     Transmission path  820  includes one or a plurality of spans that are connected in a series, reference number  83   k  being used to represent these spans. Each span  83   k  includes: optical fiber  84   k ; and optical amplifier  85   k  that amplifies the optical signal transmitted in on optical fiber  84   k  and transmits the optical signal to the receiving side. 
     On the receiving side, coupler (CPL)  811  that branches optical signal  822  that has been transmitted on transmission path  821  into two optical signals  803  and  804 , CD monitor (CD MON)  812  that receives optical signal  804 , detects the chromatic dispersion, and supplies monitor signal  802  according to the detected value are provided. Optical signal  803  is supplied to, for example, receiver (RX)  850  that receives and demodulates the optical signal and supplies data signal  851 . Receiver  850  is typically used in an optical communication system or optical communication network and detailed explanation regarding its configuration is therefore here omitted. Alternatively, optical signal  803  may be transmitted by way of another transmission path. As CD monitor  812 , CD monitor  200  shown in  FIG. 3  or CD monitor  400  shown in  FIG. 5  can be used. 
     An operator or system that monitors transmission path  821  can use monitor signal  802  for implementing operation, optimization, or monitoring of a transmission path. By using as transmitter  810  a device that can vary transmission wavelength, information relating to chromatic dispersion on each wavelength can be obtained. 
       FIG. 10  shows another example of the configuration of an optical transmission system that can detect chromatic dispersion that can occur on a transmission path. The optical transmission system shown in  FIG. 10  is a system in which only the configuration on the receiving side has been altered in the optical transmission system in  FIG. 9 , and the configuration on the transmitting side and transmission path  821  are identical to the system shown in  FIG. 9 . 
     The receiving side includes: coupler (CPL)  911  that branches optical signal  822  that has been transmitted on transmission path  821  into two optical signals  903  and  904 ; variable chromatic dispersion equalizer (VAR CD COMP)  913  that compensates the chromatic dispersion in optical signal  904 ; and CD monitor (CD MON)  912  that receives output optical signal  905  from variable chromatic dispersion equalizer  913 , detects chromatic dispersion, and supplies monitor signal  906  that accords with the detected value. Similar to the case of  FIG. 9 , optical signal  903  may be supplied to the receiver, or may be transmitted to another transmission path. CD monitor  200  shown in  FIG. 3  or CD monitor  400  shown in  FIG. 5  can be used as CD monitor  912 . CD monitor  912  controls variable chromatic dispersion equalizer  913  such that monitor signal  906  that indicates chromatic dispersion is minimized. The setting values in variable chromatic dispersion equalizer  913  are supplied to the outside as signal  902  that shows the chromatic dispersion value. The operator or system that monitors transmission path  821  can use signal  902  to implement operation, optimization, or monitoring of a transmission path. 
       FIG. 11  shows an example of the configuration of an optical transmission system that can detect chromatic dispersion that can occur in a transmission path and that can equalize this chromatic dispersion. The optical transmission system shown in  FIG. 11  is a system in which only the configuration of the receiving side has been altered in the optical transmission system shown in  FIG. 9 , and the configuration of transmitting side and transmission path  821  are identical to the configuration shown in  FIG. 9 . 
     On the receiving side, optical signal  822  transmitted on transmission path  821  is first sent to chromatic dispersion equalizer (VAR CD COMP)  1013 . The output optical signal  1023  from chromatic dispersion equalizer  1013  is next applied as input to coupler (CPL)  1011  and branched into optical signal  1003  and optical signal  1004 . Optical signal  1003  may be supplied to the receiver as in the case shown in  FIG. 9  or may be transmitted to another transmission path. On the other hand, optical signal  1004  is sent to CD monitor (CD MON)  1012  that receives optical signal  1004  to detect chromatic dispersion, and supplies monitor signal  1002  that accords with the detected value. CD monitor  200  shown in  FIG. 3  or CD monitor  400  shown in  FIG. 5  can be used as CD monitor  1012 . CD monitor  1012  controls chromatic dispersion equalizer  1013  such that monitor signal  1006  that indicates the chromatic dispersion is minimized. As a result, the residual chromatic dispersion in the output of chromatic dispersion equalizer  1013  relating to optical signal  823  transmitted on transmission path  821  becomes zero. As a result, it can be guaranteed that the effect of relay chromatic dispersion relating to transmission path  821  in optical signal  1003  supplied from coupler  1011  will be zero. 
     The graphs shown in  FIGS. 12A to 12D ,  FIGS. 13A to 13C , and  FIGS. 14A to 14E  that were used when explaining the above-described exemplary embodiments are next described in greater detail. 
       FIGS. 12A to 12C  are for showing the waveform of a 55-Gb/s NRZ-QPSK signal and a 55-Gb/s pseudo-RZ-QPSK signal. Graph  1110  of  FIG. 12A  shows a 55-Gb/s NRZ-QPSK signal. Graph  1120  of  FIG. 12B  shows a 55-Gb/s pseudo-RZ(4)-QPSK signal in which dips  1121  in optical intensity have been applied by pseudo-RZ modulation every four symbols. Graph  1130  of  FIG. 12C  shows a 55-Gb/s pseudo-RZ(8)-QPSK signal in which dips  1131  have been applied by pseudo-RZ modulation every eight symbols. Graph  1140  of  FIG. 12D  shows the waveform of the signal obtained by applying polarization-division-multiplexing to the signal of the waveform shown in graph  1120  and the signal of the waveform shown in graph  1130 . Accordingly, graph  1140  shows the waveform of a 110-Gb/s pseudo-RZ(4, 8)-QPSK signal. In graph  1140 , deeper dip  1141  appears at the location at which dips resulting from the pseudo-RZ(4) format coincide with dips resulting from the pseudo-RZ(8) format. In contrast, relatively shallow dips  1142  are dips resulting from only the pseudo-RZ(4) format. 
     By means of pseudo-RZ modulation, dips of a narrow time width are applied to the optical intensity every n symbols in a signal. The application of these dips is not only realized by a format that differs from normal RZ driven by a clock of 1/n the symbol rate, but also adds to the optical signal a frequency component that is lower than the symbol rate. On the other hand, in contrast to a case in which the intensity modulation of an optical signal is carried out at low frequency, deterioration of signal quality does not occur in this type of application of dips. 
       FIGS. 13A to 13C  each show graphs that show the simulation results of the intensity spectrums relating to an NRZ-QPSK signal, a PRZ-QPSK signal and an RZ-QPSK signal, all of these signals having a 56-Gb/s data rate. In each graph, the vertical axis represents the spectral power density (SPD). Graph  1201  of  FIG. 13A  shows the simulation results of an intensity spectrum of an electric signal obtained by directly receiving a 56-Gb/s pseudo-RZ(8)-QPSK signal. By carrying out pseudo-RZ(8) modulation, the frequency component of ⅛ the symbol rate becomes strong. This frequency is the dip production frequency realized by pseudo-RZ modulation. By using dips having a narrow time width, the harmonic components of the dip frequency also appear at high intensity. 
     As a comparison, graph  1202  of  FIG. 13B  shows the simulation result of the intensity spectrum for a 56-Gb/s NRZ-QPSK signal. The frequency component of the symbol rate appears strong, but there is no place where the spectrum is particularly strong on the lower-frequency side. Graph  1203  of  FIG. 13C  shows a similar simulation result for a 56-Gb/s RZ-QPSK signal. In this case as well, the frequency component of the symbol rate appears strong, but there is no place on the lower-frequency side in which the spectrum is particularly strong. 
     All of  FIGS. 14A to 14E  show graphs indicating the simulation results of how the intensity of the frequency component applied in the pseudo-RZ format changes according to chromatic dispersion. In these graphs, the vertical axes show the values of signals by the peak-to-peak voltage (Vpp) of the normalized clock. 
     Graph  1310  of  FIG. 14A  shows, in a case in which a 56-Gb/s pseudo-RZ (4)-QPSK signal is influenced by chromatic dispersion during transmission, the change of monitor signal  202  supplied from CD monitor  200  when this type of optical signal is received at CD monitor  200  shown in  FIG. 3 . In this case, monitor signal  202  is produced using the intensity of the frequency component of ¼ the symbol rate. When the chromatic dispersion is within ±400 ps/nm, the only minimum point in monitor signal  202  is at the time that chromatic dispersion is zero. Monitor signal  202  shows symmetrical change around the zero-dispersion point. Accordingly, regarding the signal that is represented in graph  1310 , if control is implemented such that the signal becomes a minimum, chromatic dispersion control can be implemented such that the residual chromatic dispersion becomes zero. In addition, the absolute value of chromatic dispersion can be found from the value of the monitor signal. If the sign of chromatic dispersion is already known, the actual value of the chromatic dispersion value can be found from the value of the monitor signal. 
     Similar to the case of graph  1310 , graph  1320  of  FIG. 14B  shows the change due to chromatic dispersion of monitor signal  202  supplied from CD monitor  200  relating to a 56-Gb/s pseudo-RZ(4)-QPSK signal, but the range of chromatic dispersion differs from that of graph  1310 . In graph  1320 , the change of the monitor signal is shown over a wider range for positive chromatic dispersion. 
     As can be seen from the results of graphs  1310  and  1320 , using monitor signal  202  from CD monitor  200  enables control of the chromatic dispersion over the broad range of ±400 ps/nm. 
     Graph  1330  of  FIG. 14C  shows, in a case in which a 56-Gb/s pseudo-RZ (8)-QPSK signal is influenced by chromatic dispersion during transmission, the change of monitor signal  202  when this type of optical signal is received at CD monitor  200  shown in  FIG. 3 . The intensity of a frequency component that is ⅛ the symbol rate is used to produce monitor signal  202 . The change in intensity of the monitor signal is similar to the change shown in graph  1320 . In graph  1330 , only the positive region of the chromatic dispersion is shown, but the change of the monitor signal with respect to chromatic dispersion was a symmetrical shape around the zero-dispersion point. As can be seen from graph  1330 , in the case of a pseudo-RZ(8)-QPSK signal, use of monitor signal  202  enables control of chromatic dispersion over a still broader range of ±1000 ps/nm. 
     Graph  1340  of  FIG. 14D  shows, in a case in which a 56-Gb/s pseudo-RZ (16)-QPSK signal is influenced by chromatic dispersion during transmission, the change in monitor signal  202  when this type of optical signal is received at CD monitor  200  that is shown in  FIG. 3 . The intensity of the frequency component of 1/16 the symbol rate is used to produce monitor signal  202 . The change in the intensity of the monitor signal is similar to the change shown in graph  1320 . Graph  1330  shows only the positive region of chromatic dispersion, but the change of monitor signal with respect to chromatic dispersion was a symmetrical shape around the zero-dispersion point. As can be seen from graph  1340 , in the case of a pseudo-RZ(16)-QPSK signal, the use of monitor signal  202  enables control of chromatic dispersion over a still broader range of ±2000 ps/nm. 
     When detection of chromatic dispersion over an even still broader range is desired, dips should be added to the optical signal every n symbols by pseudo-RZ modulation and the value of n should be made greater than 16. The production of the monitor signal should employ the intensity of the dip frequency component when this value of n is large. 
     Graph  1350  of  FIG. 14E  shows, in a case in which a 56-Gb/s pseudo-RZ(8)-QPSK signal is influenced by chromatic dispersion during transmission, the change in monitor signal  402  when this type of optical signal is received at CD monitor  400  shown in  FIG. 5 . The dip frequency in this case is ⅛ the symbol rate. It is assumed that, by enabling the generation of the monitor signal based on a plurality of frequency components in CD monitor  400 , the three types: dip frequency component (i.e., ⅛ the symbol rate), and the secondary and quaternary harmonic components (a component of ¼ the symbol rate and a component of ½ the symbol rate) are used in the generation of monitor signal. Graph  1350  shows the change in intensity of the monitor signal for each of these frequency components. The signal realized by the frequency component of ⅛ the symbol rate is shown by curve  1351 , the signal realized by the frequency component of ¼ the symbol rate is shown by curve  1352 , and the signal realized by the frequency component of ½ the symbol rate is shown by curve  1353 . The shape of change of all of the signals is the same as the shape in the above-described graph  1320 , and the change is a symmetrical shape with respect to the zero-dispersion point. 
     As shown in graph  1350 , if the monitor signal is generated using curve  1351 , i.e., using the dip frequency component, the control range of chromatic dispersion can be set wide. However, because the rate of change of the monitor signal is small in the vicinity of the zero-dispersion, fine control is problematic in the vicinity of zero-dispersion. The case in which the rate of change is great in the vicinity of zero-dispersion is, as shown in waveform  1353 , the monitor signal that is generated using the quaternary harmonic of the dip frequency. By selecting which frequency component to use when generating the monitor signal, an appropriate dynamic range can be set for each control range, and the control of chromatic dispersion can be carried out over a broad range and with high accuracy. 
     Exemplary embodiments have been described hereinabove. Exemplary technological features of the present invention are noted below. 
     (Supplementary Note 1) A method of monitoring chromatic dispersion when transmitting an optical signal, the method including: 
     applying, to an optical signal in which the symbol rate is f, a dip in the optical intensity for every n symbols by pseudo-RZ modulation where n is an integer equal to or greater than 2, and then transmitting the optical signal to which dips have been applied to a transmission path; 
     receiving the optical signal that has been transmitted on the transmission path and detecting the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1; and 
     based on the detected intensity, generating a monitor signal that represents the chromatic dispersion amount. 
     (Supplementary Note 2) The method as set forth in Supplementary Note 1, wherein a band-pass filter is used to detect the intensity of the frequency component of k*f/n. 
     (Supplementary Note 3) The method as set forth in Supplementary Note 1, wherein the received signal obtained by receiving the optical signal from the transmission path is converted to a digital signal, and the digital signal is subjected to a digital signal processing to detect the intensity of the frequency component of k*f/n. 
     (Supplementary Note 4) The method as set forth in any one of Supplementary Notes 1 to 3, wherein, when detecting the intensity of the frequency component of k*f/n, a plurality of different integers k are used to determine the intensity for each of a plurality of frequency components, and the monitor signal is generated for each of the plurality of frequency components. 
     (Supplementary Note 5) A method of equalizing chromatic dispersion when transmitting an optical signal, including: 
     applying, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2, and transmitting the optical signal to which dips have been applied to a transmission path; 
     receiving the optical signal that has been transmitted on the transmission path and detecting the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1; 
     based on the detected intensity, generating a monitor signal that represents the chromatic dispersion amount; and 
     controlling an equalizer that equalizes the optical signal such that the value that is represented by the monitor signal is minimized. 
     (Supplementary Note 6) The method as set forth in Supplementary Note 5, wherein: 
     the dips are applied to each of a plurality of optical signals of different wavelengths; 
     the plurality of optical signals to which the dips are applied are subjected to wavelength-multiplexing and transmitted to the transmission path; and 
     the received optical signal undergoes wavelength separation to generate the monitor signal for each wavelength and the optical signal is equalized for each wavelength. 
     (Supplementary Note 7) The method as set forth in Supplementary Note 5, wherein: 
     the dips are applied to each of a plurality of optical signals of different wavelengths; 
     the plurality of optical signal to which the dips have been applied are subjected to wavelength-multiplexing and transmitted to the transmission path; 
     the monitor signal is generated for each wavelength; 
     the optical signal before wavelength-separation which is received from the transmission path is equalized such that a value shown by the monitor signal for each wavelength which is obtained by carrying out the wavelength-separation of the optical signal is minimized. 
     (Supplementary Note 8) The method as set forth in Supplementary Note 5, wherein: 
     dips are applied at different repeating frequencies to each of a plurality of optical signals of different polarization states; 
     the plurality of optical signals to which the dips have been applied are polarization-division-multiplexed and transmitted to the transmission path; 
     the monitor signal is generated for each of the different repeating frequencies. 
     (Supplementary Note 9) The method as set forth in any one of Supplementary Notes 5 to 8, wherein a band-pass filter is used to detect the intensity of the frequency component of k*f/n. 
     (Supplementary Note 10) The method as set forth in any one of Supplementary Notes 5 to 8, wherein the received signal obtained by receiving the optical signal from the transmission path is converted to a digital signal, and the digital signal is subjected to a digital processing to detect the intensity of the frequency component of k*f/n. 
     (Supplementary Note 11) The method as set forth in any one of Supplementary Notes 5 to 10, wherein, when detecting the intensity of the frequency component of k*f/n, a plurality of differing integers k are used to detect the intensity for each of a plurality of frequency components; and the monitor signal is generated for each of the plurality of frequency components. 
     (Supplementary Note 12) The method as set forth in Supplementary Note 11, wherein any one monitor signal among the monitor signals for each of the plurality of frequency components is used to control the equalizer. 
     (Supplementary Note 13) The method as set forth in Supplementary Note 11, wherein the monitor signals for each of the plurality of frequency components are switched according to the chromatic dispersion amount to control the equalizer. 
     (Supplementary Note 14) The method as set forth in any one of Supplementary Notes 1 to 13, wherein a look-up table that shows the relation between chromatic dispersion amounts and the intensities of the frequency components of k*f/n is prepared in advance and the monitor signal is generated by retrieving the look-up table based on the detected intensity. 
     (Supplementary Note 15) A device that monitors chromatic dispersion when transmitting an optical signal, the device including: 
     a transmitter that applies, to an optical signal in which the symbol rate is f, a dip in the intensity for every n symbols by means of pseudo-RZ modulation, where n is an integer equal to or greater than 2, and that transmits the optical signal to which dips have been applied to a transmission path; and 
     a chromatic dispersion monitor that receives the optical signal that is transmitted on the transmission path, that detects the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1, and that, based on the detected intensity, generates a monitor signal that indicates the chromatic dispersion amount. 
     (Supplementary Note 16) The device as set forth in Supplementary Note 15, wherein the transmitter includes: a modulator that modulates an optical carrier by a signal that indicates data to be transmitted; and a pseudo-RZ carver that applies the dips to the optical signal after modulation. 
     (Supplementary Note 17) The device as set forth in Supplementary Note 15 or 16, wherein the chromatic dispersion monitor includes: a light-receiving element that receives the optical signal from the transmission path; a band-pass filter that detects the intensity of the frequency component of k*f/n; and a monitor circuit that, based on the detected intensity, generates a monitor signal that represents the chromatic dispersion amount. 
     (Supplementary Note 18) The device as set forth in Supplementary Note 15 or 16, wherein the chromatic dispersion monitor includes a digital signal processing unit that converts a received signal obtained by receiving the optical signal from the transmission path to a digital signal, that subjects the digital signal to a digital signal processing to detect the intensity of the frequency component of k*f/n, and that generates the monitor signal. 
     (Supplementary Note 19) A device that equalizes chromatic dispersion when transmitting an optical signal, the device including: 
     a transmitter that applies, to an optical signal in which the symbol rate is f, a dip in optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal or greater than 2 and that transmits the optical signal to which dips have been applied to a transmission path; 
     a chromatic dispersion monitor that receives the optical signal that is transmitted on the transmission path, that detects the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1, and that, based on the detected intensity, generates a monitor signal that represents the chromatic dispersion amount; and 
     a chromatic dispersion equalizer that equalizes the optical signal that is received from the transmission path such that the value represented by the monitor signal is minimized. 
     (Supplementary Note 20) The device as set forth in Supplementary Note 19, wherein the transmitter includes: a modulator that modulates an optical carrier by a signal that indicates the data to be transmitted; and a pseudo-RZ carver that applies the dips to the optical signal after modulation. 
     (Supplementary Note 21) The device as set forth in Supplementary Note 19 or 20, wherein the chromatic dispersion monitor includes: a light-receiving element that receives the optical signal from the transmission path; a band-pass filter that detects the intensity of the frequency component of k*f/n; and a monitor circuit that, based on the detected intensity, generates a monitor signal that indicates the chromatic dispersion amount. 
     (Supplementary Note 22) The device as set forth in Supplementary Note 19 or 20, wherein the chromatic dispersion monitor includes: an analog/digital converter that converts a received signal obtained by receiving the optical signal from the transmission path to a digital signal; and a digital signal processing unit that subjects the digital signal to a digital signal processing to detect the intensity of the frequency component of k*f/n and that generates the monitor signal. 
     (Supplementary Note 23) The as set forth in any one of Supplementary Notes 19 to 22, wherein the chromatic dispersion monitor includes a look-up table that shows the relation between the chromatic dispersion amounts and the intensities of the frequency components of k*f/n, and that retrieves the look-up table based on the detected intensity to generate the monitor signal. 
     (Supplementary Note 24) A transmitter that transmits an optical signal to a transmission path, the transmitter including: 
     a modulator that modulates an optical carrier by means of a signal that indicates data that to be transmitted; and 
     a pseudo-RZ carver that applies, to the optical signal after the modulation, a dip in the optical intensity for every n symbols by means of pseudo-RZ modulation where n is an integer equal to or greater than 2. 
     (Supplementary Note 25) A chromatic dispersion monitor that monitors chromatic dispersion in an optical signal that is transmitted on a transmission path, in which dips in optical intensity have been applied to the optical signal by pseudo-RZ modulation for every n symbols, the chromatic dispersion monitor including: 
     a light-receiving element that receives the optical signal and converts to a received signal; 
     a band-pass filter that detects the intensity of a frequency component of k*f/n from the received signal where k is an integer equal to or greater than 1; and 
     a monitor circuit that, based on the detected intensity, generates a monitor signal that shows the chromatic dispersion amount. 
     (Supplementary Note 26) A chromatic dispersion monitor that monitors chromatic dispersion in an optical signal in which dips in optical intensity have been applied in the optical signal by pseudo-RZ modulation for every n symbols and that is transmitted on a transmission path, the chromatic dispersion monitor including: 
     a light-receiving element that receives the optical signal and converts to a received signal; 
     an analog/digital converter that converts the received signal to a digital signal; 
     a digital signal processing unit that subjects the digital signal to a digital signal processing to detect the intensity of a frequency component of k*f/n and that, based on the detected intensity, generates a monitor signal that shows the chromatic dispersion amount. 
     Although the present invention has been described with reference to exemplary embodiments, the present invention is not limited to the above-described exemplary embodiments. The configuration and details of the present invention are open to various modifications within the scope of the present invention that will be clear to one of ordinary skill in the art. 
     This application is based upon and claims the benefits of priority from Japanese Patent Application No. 2010-000497, filed on Jan. 5, 2010, the disclosure of which is incorporated herein in its entirety by reference. 
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