Abstract:
An optical signal quality monitor includes a splitter splitting an input optical signal into two signals; a low-frequency converter converting one split optical signal to a low frequency signal by modulating the optical signal with a frequency offset signal; and an intensity ratio calculator calculating an intensity ratio between the low frequency signal and the other split optical signal, thereby appropriately confirming the quality of a high-bit rate optical signal. The monitor includes plural processing lines, each line including the splitter, the low-frequency converter, and the intensity ratio calculator. At least one line includes an optical noise superimposer superimposing optical noise on the one split signal before inputted to the converter or an optical band-pass filter transmitting the one split signal before inputted to the converter. The monitor includes a polarization state changer changing the polarization state of the input signal before inputted to the splitter.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical signal quality monitor for monitoring, for example, a signal-to-noise ratio, polarization mode dispersion or the like of an optical signal in an optical transmission system. 
         [0003]    2. Description of the Background Art 
         [0004]    When investigating the quality or deterioration factor of a transmitted signal, an estimate of the quality or factor may require the waveform of the signal to be monitored. 
         [0005]    The waveform of an optical signal may be monitored by using devices such as a sampling oscilloscope. Generally, the sampling oscilloscope is however a complicated and expensive device. As opposed to such a device, there is also a proposal for monitoring the quality of an optical signal without using the sampling oscilloscope. 
         [0006]    Japanese patent laid-open publication No. 2005-151597 discloses an optical signal quality monitor that samples an optical signal to be measured having a predetermined bit rate by using an asynchronous timing clock, and uses the resulting histogram to determine the signal-to-noise ratio coefficient Q. This monitor scheme can obtain information on the signal waveform in a short time and configure a monitoring device relatively easily. 
         [0007]    U. K. Lize et al., “Simultaneous Monitoring of Chromatic Dispersion and PMD for OOK and DPSK Using Partial-Bit-Delay-Assisted Clock Tone Detection” Proc. 31st European Conf. on Opt. Commun. (ECOC2003) Mo4. 4 Jul. 2006 proposes that it is possible to detect a frequency component having high intensity from an optical signal and use the frequency component to detect polarization mode dispersion (PMD) and chromatic dispersion. 
         [0008]    However, the monitoring method of the optical waveform taught by the above-mentioned Japanese &#39;597 patent publication is subject to a difficulty that the higher bit rate of the optical signal to be observed the fewer sampling points, thus rendering it difficult to analyze the signal. 
         [0009]    The above-mentioned U. K. Lize, et al., discloses the measuring method of detecting the frequency component corresponding to the bit rate of an optical signal, thus also confronting the difficulty in analyzing the higher-bit rate signal. 
         [0010]    Recently, an optical transmission system for transmitting and receiving an RZ (Return-to-Zero) optical signal of a 160 Gbps bit rate has been researched and developed. However, the methods taught by the above-mentioned Japanese &#39;597 patent publication and U. K. Lize, et al., may not be satisfactory to such a high-bit rate optical signal. 
       SUMMARY OF THE INVENTION 
       [0011]    It is therefore an object of the present invention to provide an optical signal quality monitoring device that may confirm appropriately the quality of a high-bit rate optical signal. 
         [0012]    In accordance with the present invention, an optical signal quality monitor includes a splitter that splits an input optical signal into two split signals; a low-frequency converter that converts one of the split optical signals to a low frequency signal by modulating the optical signal with a frequency offset signal; and an intensity ratio calculator that calculates an intensity ratio between the low frequency signal and the other split optical signal as a reference. 
         [0013]    In accordance with the present invention, it is therefore possible to confirm appropriately the quality of even the high-bit rate optical signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The objects and features of the present invention will become more apparent from consideration of the following detailed description taken inconjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  is a schematic block diagram showing a first embodiment of an optical signal quality monitor according to the present invention; 
           [0016]      FIG. 2  is a graph plotting a curve showing the relationship between the OSNR (Optical Signal-to-Noise Ratio) of an input optical signal and an output division signal from a divider in a no-noise-superimposed line in the first embodiment; 
           [0017]      FIG. 3  is a graph plotting curves showing the relationship between the chromatic dispersion and the output division signal of the no-noise-superimposed line in the first embodiment; 
           [0018]      FIG. 4  is a graph plotting curves showing the relationship between the OSNR of the input optical signal and the output division signals of the no-noise-superimposed and noise-superimposed lines in the first embodiment; 
           [0019]      FIG. 5  is a graph plotting curves showing the relationship between the OSNR of the input optical signal and the output division signals of the no-noise-superimposed and noise-superimposed lines with respect to a chromatic dispersion in the first embodiment; 
           [0020]      FIG. 6  is a schematic block diagram, like  FIG. 1 , showing a second embodiment of an optical signal quality monitor according to the present invention; 
           [0021]      FIG. 7  is a graph plotting curves showing the relationship between the OSNR of the input optical signal and an output division signal from a divider of a system including an optical band-pass filter in the second embodiment; 
           [0022]      FIG. 8  is a schematic block diagram showing a third embodiment of an optical signal quality monitor according to the present invention; 
           [0023]      FIG. 9  is an explanatory diagram useful for understanding a differential group delay (DGD) used in the monitor in the third embodiment; 
           [0024]      FIG. 10  plots outputs from a polarizer in the third embodiment with respect to the DGDs to show the impulse waveform of the outputs when the polarizer has its polarization axis inclining by an angle of 45 degrees with respect to the xi (ξ) axis; 
           [0025]      FIG. 11  is a graph plotting curves showing the relationship between the angle of the polarization axis of the polarizer and the detected intensity of the monitor per se with respect to the DGDs in the third embodiment; 
           [0026]      FIG. 12  explanatorily shows an example of DGD in the third embodiment different in intensity between the ξ and psi (ψ) axes; 
           [0027]      FIG. 13  is a graph plotting curves showing the relationship between the polarizer angle and an intensity ratio of the ξ axis to the ψ axis for a predetermined value of the DGD in the third embodiment; and 
           [0028]      FIG. 14  plots curves of intensity difference (variation) with respect to the DGD in the third embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    With reference to the accompanying drawings, illustrative embodiments of an optical signal quality monitor according to the present invention will be described below. First, reference is made to  FIG. 1 , which is a schematic block diagram showing a first embodiment of an optical signal quality monitor  100 . As shown in the figure, the optical signal quality monitor  100  includes constituent elements dealing with an optical signal and ones dealing with an electrical signal as well as a quality monitoring section  150 . 
         [0030]    The elements dealing with an optical signal are an optical splitter  101 , an amplified spontaneous emission (ASE) generator  102 , two electro-absorption (EA) modulators  103  and  104 , four photo diodes (PD)  105 ,  106 ,  116  and  117 , and two optical distributors  114  and  115 , which are interconnected as illustrated. In the figure, optical connections are depicted with thick lines. The elements dealing with an electrical signal are a local oscillator  107 , two drivers  108  and  109 , two bias-Ts  110  and  111 , a bias drive voltage source  122 , two bandpass filters (BPF)  112  and  113 , two low-pass filters (LPF)  118  and  119 , two dividers  120  and  121 , and two intensity detectors  123  and  124 , which are interconnected as depicted. Electrical connections are shown with thin lines in the figure. 
         [0031]    The optical splitter  101  is adapted to split-the input optical signal into two signals. For example, a received optical signal or an optical signal to be transmitted in an optical transmission system is split as an input optical signal to the optical splitter  101 . The input optical signal may be, for example, an RZ (Return-to-Zero) optical signal of a bit rate of 160 Gbps. 
         [0032]    The optical signal quality monitor  100  in the first embodiment includes a couple of lines or systems, each of which deals with corresponding one of the signals associated with the two split optical signals provided by the optical splitter  101 . One line may hereinafter be referred to as the “first line” and the other the “second line”. Elements in the first or second line may be called by putting the word “first” or “second” before their name respectively. The two lines have the same configuration except that the second line includes the ASE generator  102 . That is to say, the first line consists of the EA modulator  103 , the photo diode  105 , the band filter  112 , the intensity detector  123 , the photo diode  116 , the low-pass filter  118 , and the divider  120 . Similarly, the second line consists of the EA modulator  104 , the photo diode  106 , the band filter  113 , the intensity detector  124 , the photo diode  117 , the low-pass filter  119 , and the divider  121 , as well as the EA modulator  103 . 
         [0033]    The first and second lines include the optical distributors  114  and  115 , respectively. Each distributor is adapted to separate the split optical signal provided from the optical splitter  101  further into two signals. It is noted that the optical splitter  101  may split the optical input signal into two signals, for example, equally, i.e. in the ratio of 1:1. The optical distributors  114  and  115  may however distribute the optical signal in a ratio other than 1:1 so long as the distributors,  114  and  115  are configured to distribute the signal in the same ratio as each other. The optical distributors  114  and  115  may be, for example, a directional optical coupler. 
         [0034]    The ASE generator  102  is provided as an optical noise generator for generating an optical noise, such as a broad-spectrum light. The generator  102  is adapted to generate and output the amplified spontaneous emission (ASE) as the broad-spectrum light (optical noise). The generated ASE is superimposed on the split optical signal from the second optical distributor  115 . The resultant optical signal of the distributor  115  is provided to the second EA modulator  104 . The ASE generator  102  may be configured, for example, by including an erbium-doped fiber amplifier (EDFA). 
         [0035]    The local oscillator  107  provided in common to the two lines is adapted to oscillate a signal at 40 GHz-Δf/4 Hz and provide the oscillation signal to the modulator drivers  108  and  109 . The modulator drivers  108  and  109  in the respective lines are adapted to amplify the local oscillation signal. The bias drive voltage source  122  also provided in common to the two lines is adapted to supply a bias drive voltage Vb to the bias-Ts  110  and  111  included in the respective lines. The bias-Ts  110  and  111  in the respective lines are adapted to add negative voltages to the local oscillation signals amplified by the modulator drivers  108  and  109 , respectively, and then, to input the resultant signals to the control terminals (bias input terminals) of the EA modulators  103  and  104 , respectively. 
         [0036]    The EA modulators  103  and  104  in the respective lines are adapted to obtain optical signals (beat signals) having a beat component Δf, e.g. of 1 GHz, between the optical signals from the optical distributors  114  and  115 , respectively, and signals applied to the bias input terminals. The modulators  103  and  104  are also adapted to provide the beat signals to the photo diodes  105  and  106 , respectively. 
         [0037]    The photo diodes  105  and  106  in the respective lines are adapted to photoelectrically convert the beat signals from the EA modulators  103  and  104 , respectively, to corresponding electric signals. The filters  112  and  113  in the respective lines are adapted to filter, i.e. remove the unnecessary components of, the beat components Δf included in the electrical signals converted from the beat signals, respectively. The intensity detectors  123  and  124  in the respective lines are adapted to detect the intensity of the filtered electrical signals and provide the resultant intensity values to the dividers  120  and  121 , respectively. 
         [0038]    The photo diodes  116  and  117  in the respective lines are adapted to photoelectrically convert the input optical signals from the optical distributors  114  and  115 , respectively, to corresponding electric signals. The low-pass filters  118  and  119  in the respective lines are adapted to filter the low-frequency components, including the necessary components of the input optical signals, in the converted electrical signals, corresponding to the respective input optical signals, from the diodes  116  and  117 , respectively. The filters  118  and  119  are also adapted to provide the filtered electrical signals to the dividers  120  and  121 , respectively. On each of the paths from the low-pass filters  118  and  119  to the dividers  120  and  121 , respectively, is provided a device serving as the intensity detector. Those devices are however omitted from  FIG. 1  because they are in practice simple resistors. 
         [0039]    The dividers  120  and  121  in the respective lines are adapted to divide the detection signals (a) from the intensity detectors  123  and  124  by the filtering signals (b) from the low-pass filters  118  and  119 , respectively. Although omitted from  FIG. 1 , an element such as a delay device may be provided before the divider  120  and/or  121  to achieve the same processing-delay before the divider  120  and/or  121 . This allows the set of two signals, the set being provided by the optical distributor  114  and/or  115 , to concurrently have an effect on the dividends and the divisors in the divider  120  and/or  121 . 
         [0040]    The quality monitoring section  150  is adapted to obtain a monitoring result of the quality of the input optical signal on the basis of the division results of the divider  120  and/or  121 . The quality monitoring section  150  may be, for example, a personal computer incorporating desired software, such as quality monitoring program. The processing of the quality monitoring section  150  will be described in more detail in connection with the description of the operations below. 
         [0041]    Now, the operation of the optical signal quality monitor  100  of the first embodiment will be described as an example of monitoring the quality of an optical signal. The optical signal quality monitor  100  according to the first embodiment receives an RZ optical signal of a bit rate of 160 Gbps. The input RZ optical signal is split into two signals by the optical splitter  101  to be forwarded to the corresponding lines. Each split optical signal is additionally divided into two signals by the optical distributor  114  or  115  in the corresponding line. 
         [0042]    One of the optical signals distributed by the one optical distributor  115  in the second line is superimposed with the ASE generated by the ASE generator  102 . In turn, the resultant optical signal from the ASE superimposition enters the EA modulator  104 . One of the optical signals distributed by the other optical distributor  114  in the first line directly enters the EA modulator  103 . 
         [0043]    The oscillation signal of 40 GHz-Δf/4 Hz is outputted from the local oscillator  107  to the two lines and amplified by the respective modulator drivers  108  and  109 . The amplified signals are applied with a negative voltage by the bias-Ts  110  and  111  and then fed to the EA modulators  103  and  104  in the respective lines. 
         [0044]    Thus, in the respective lines, the beat signals of Δf are outputted from the EA modulators  103  and  104  and converted to electrical signals by the respective photo diodes  105  and  106 . The converted electrical signals pass through the respective narrow band filters  112  and  113  having the passband center frequency of Δf and then are fed to the intensity detectors  123  and  124  in the respective lines. The detectors  123  and  124  detect the intensities of the Δf (Hz) components of the electrical signals to feed the detected intensity signals to the dividers  120  and  121 , respectively. 
         [0045]    Furthermore, the other split optical signals in the distributors  114  and  115  are outputted to the photo diodes  116  and  117  and then converted to electrical signals by the diodes  116  and  117 , respectively. The converted electrical signals pass through the low-pass filters  118  and  119  to remove the unnecessary components therefrom. The filtered electrical signals then enter the dividers  120  and  121 , respectively. 
         [0046]    In the dividers  120  and  121 , the detection signals (a) from the intensity detectors  123  and  124  are divided by the filtering signals (b) from the low-pass filters  118  and  119 , in the respective lines. Both of the two division signals are then fed to the quality monitoring section  150 . 
         [0047]      FIG. 2  is a graph plotting the division signal a/b (Intensity Ratio) from the divider  120  with respect to the optical signal-to-noise ratio (OSNR) of the input optical signal to show a characteristic curve of the signal a/b. 
         [0048]    The input a to the divider  120  (the output from the intensity detector  123 ) has a frequency limited within the frequency band of the beat signal. Regardless of the OSNR of the input optical signal, therefore, the input a is not generally affected by the noise. The input b to the divider  120  (the output from the low-pass filter  118 ) includes a frequency lower than the necessary frequency band for the input optical signal. The OSNR of the input optical signal, therefore, only has to be small enough to reduce the effect from the noise sufficiently. Outside the range, the OCNR may take a large value depending on the noise. With reference to  FIG. 2 , therefore, the division signal a/b from the divider  120  decreases as the OSNR of the input optical signal degrades. 
         [0049]      FIG. 3  is a graph plotting the division signal a/b (Intensity Ratio) from the divider  120  with respect to the chromatic dispersion (Dispersion) of the input optical signal to show a characteristic curve of the signal a/b. In  FIG. 3 , a parameter To defines the initial pulse. The pulse width or duration of the initial pulse has an intensity of 1/e for no dispersion. It is assumed here that the pulse is a Gaussian pulse without chirp. 
         [0050]    When the chromatic dispersion distorts the waveform of the input signal, the power of the input signal at the desired frequency decreases. In other words, the input a to the divider  120  (the output from the intensity detector  123 ) decreases in comparison to the input b to the divider  120  (the output from the low-pass filter  118 ). The division signal a/b thus decreases. Because the division signal a/b decreases in correspondence with variation of the wavelength or waveform of the input signal, when the chromatic dispersion is constant, the division signal a/b remains constant even if the OSNR of the input optical signal changes. 
         [0051]    As has been discussed above, the division signal a/b from the divider  120  is processed in the first line without superimposing the noise on the signal a/b. In the same manner, a division signal a/b from the divider  121  is also processed in the second line with superimposing the noise on the signal a/b. Particularly, in the second line, the noise is superimposed on the signal a/b. Because the division signal a/b from the divider  121  is superimposed with noise (ASE from the ASE generator  102 ), the signal a/b from the divider  121  starts to decrease at a smaller OSNR than the signal a/b from the divider  120  in correspondence with a degree of the superimposition. 
         [0052]      FIG. 4  is a graph plotting the division signal a/b from the divider  120  in the first line (“Monitor A” in the figure) and the division signal a/b from the divider  121  in the second line (“Monitor B” also in the figure) in relation to the optical signal-to-noise ratio (OSNR) of the input optical signal to show the characteristic curves of both the signals a/b. 
         [0053]    According to  FIG. 4 , since, in the second line, the ASE generator  102  adds noise (ASE) to the signal, the division signal a/b from the divider  121  in the second line starts to decrease at approximately 35 dB of the OSNR of the input optical signal. By contrast, since, in the first line, no noise is added to the signal, the division signal a/b from the divider  120  in the first line starts to decrease at approximately 25 dB of the OSNR of the input optical signal. For instance, when the OSNR of the input optical signal degrades from 27 dB to 25 dB, the division signal a/b from the divider  120  in the first line decreases little, whereas the division signal a/b from the divider  121  in the second line decreases by 0.2 dB. 
         [0054]      FIG. 5  is a graph plotting the division signal a/b from the divider  120  in the first line (“Monitor A” in the figure) and the division signal a/b from the divider  121  in the second line (“Monitor B” also in the figure) in relation to the OSNR of the input optical signal so as to show the characteristic curves of both the signals a/b with and without chromatic dispersion. 
         [0055]    On the other hand, as seen in  FIG. 3 , the division signal a/b decreases with a regular inclination regardless of the OSNR in either case of the division signal a/b (Monitor A in  FIG. 5 ) from the divider  120  in the first line and the division signal a/b (Monitor B in  FIG. 5 ) from the divider  121  in the second line. With reference to  FIG. 5 , therefore, when the chromatic dispersion changes from no dispersion (0 ps/nm) to some dispersion (0.2 ps/nm, for example, in  FIG. 5 ), the decrease in the division signal a/b (Monitor A) from the divider  120  in the first line is the same as the decrease in the division signal a/b (Monitor B) from the divider  121  in the second line. 
         [0056]    As described above, the two detection signals, i.e. the division, signals show different variations when the OSNR degrades, but show the same variation when the chromatic dispersion distorts the waveform of the input signal. The waveform distortion caused by the OSNR degradation may thus be distinguished in observation from the waveform distortion caused by the chromatic dispersion. 
         [0057]    The first and second line dividers  120  and  121  may output the division signals a/b (Monitor A and Monitor B) to a display device (the quality monitoring section  150  in this embodiment) that displays the division signals, by which the observers may determine the quality of the input optical signal. As the background of the display, it is preferable to show the characteristic curves at the display device as shown in FIGS.  2 ,  4 , and  5  obtained in advance on the optical transmission system so as to facilitate the determination of the quality of the input optical signal by the observer. 
         [0058]    The quality monitoring section  150  may incorporate some functions of determining the signal quality. For example, the quality monitoring portion  150  may be achieved as a personal computer incorporated desired software or a quality monitoring program sequence for executing the determination functions. In the software, the characteristic curve information as shown in  FIGS. 2 ,  4 , and  5  may be obtained in advance on the target optical transmission system to be stored in a database. In the subsequent observation, the division signals a/b (Monitor A and Monitor B) of the first and second line dividers  120  and  121  may be checked against the stored characteristic curve information to obtain the value of a desired quality parameter, such as OSNR. For example, when information on the characteristic curve as shown in  FIG. 4  is stored and used, the quality monitoring section  150  outputs the OSNR of 25 dB for the Monitor A of −7.5 dB and the Monitor B of −8.05 dB, or the OSNR of 27 dB for the Monitor A of −7.5 dB and the Monitor B of −7.8 dB. 
         [0059]    The observers or the software may store suitably the information on the characteristic curve in correspondence with the observation environment and the observation target. 
         [0060]    For example, in the optical transmission system for receiving an optical signal with the chromatic dispersion generally fixed or hardly varying, when the observed center of the ONSR is generally constant and varies slightly, it is sufficient to store only the characteristic curve information shown in  FIG. 2 . For instance, in case where the observed center of the ONSR is 15 dB and has a variation of 10 to 20 dB, the observers or the software may store and process the characteristic curve information as shown in  FIG. 2  only which is obtained by observing in advance the division signal a/b (Monitor A in  FIG. 4 ) from the divider  120  in the first line. By contrast, in case where the observed center of the ONSR is of an appropriate value, the observers or the software may store and process the characteristic curve information only which is obtained by observing in advance the division signal a/b (Monitor B in  FIG. 4 ) from the divider  121  in the second line. 
         [0061]    Alternatively, in the optical transmission system for receiving an optical signal with the chromatic dispersion generally fixed or hardly varying, when the observed center of the ONSR is generally constant and varies considerably, it is also possible to store the characteristic curve information as shown in  FIG. 4 . For instance, in case where the observed center of the ONSR is 22 dB and has a variation of 15 to 30 dB, the observers or the software may store and process the characteristic curve information shown in  FIG. 4  which is obtained by observing in advance the division signal a/b from the divider  120  in the first line (Monitor A) and the division signal a/b from the divider  121  in the second line (Monitor B). In this case, for example, if the ONSR is around 15 dB, the characteristic curve of the Monitor A is used to determine the observation, but if the ONSR is around 30 dB, the characteristic curve of the Monitor B is used to determine the observation. 
         [0062]    Furthermore, in the optical transmission system for receiving an optical signal with the chromatic dispersion which varies, it is possible to store the characteristic curve information as shown in  FIG. 5 . For instance, when the observation of the division signal a/b (Monitor A) from the divider  120  in the first line is −7.8 dB, a combination of the chromatic dispersion and the OSNR cannot be fixed because such a combination may be the chromatic dispersion of 0 ps/nm and the OSNR of 17 dB, or the chromatic dispersion of 0.2 ps/nm, and the OSNR of 20 dB. However, if the information on the observation of the division signal a/b (Monitor B) from the divider  121  in the second line may be used, it is possible to determine a combination of the chromatic dispersion and the OSNR. 
         [0063]    Thus, according to the first embodiment, the input optical signal is split into two signals, one of the split optical signals is modulated with a signal as offset to thereby be converted to a low frequency signal, and the ratio of the low frequency signal intensity to the other split optical signal intensity is used to monitor the waveform quality of the input optical signal. It is thus possible to appropriately confirm the quality of even the high-bit rate optical signal. 
         [0064]    In addition, the two split signals may be monitored in such a couple of lines, and the signal in one line is superimposed with noise. The two signals may thus be monitored in the different ranges in respective lines, thereby distinguishing the signal quality in a wide range. 
         [0065]    Moreover, by using the two lines, the OSNR may be detected regardless of the variation of the chromatic dispersion. 
         [0066]    With reference further to the accompanying drawings, a description will now be given of an optical signal quality monitor according to an alternative, second embodiment of the present invention.  FIG. 6  is a schematic block diagram of the configuration of an optical signal quality monitor  100 A according to the second embodiment. The optical signal quality monitor  100 A replaces the ASE generator  102  in the first embodiment with an optical band-pass filter (OBPF)  102 A. The optical band-pass filter  102 A maybe inserted between the optical distributor  115  and the EA modulator  104  in the second line. The remaining constituent elements may be the same as in the first embodiment, and like components are designated with the same reference numerals. 
         [0067]    With reference to  FIG. 7 , a description will be given on why the optical band-pass filter  102 A is used in the optical signal quality monitor  100 A in the second embodiment. The graph shown in  FIG. 7  plots the division signal a/b (Power decrease) from the divider  121  in the second line including the optical band-pass filter  102 A with respect to the optical signal-to-noise ratio (OSNR) of the input optical signal to show a characteristic curve of the signal a/b. 
         [0068]    As seen in  FIG. 7 , when using the optical band-pass filter  102 A to receive the signal of 3 dB and then pass the frequency of band width Δf therethrough, it is possible to provide different intensity ratios a/b for the same OSNR depending on the band width. In other words, by suitably selecting the band width Δf of the optical band-pass filter  102 A in inputting the 3 dB signal, it is possible to accurately detect the OSNR that could not be accurately detected only using the division signal a/b from the divider  120  in the first line, by referring to the division signal a/b from the divider  121  in the second line. 
         [0069]    For example, the band width Δf for the 3 dB signal of the optical band-pass filter  102 A may be selected to handle the characteristic curves shown in  FIG. 7  like the characteristic curve of the Monitor B in  FIG. 4 . 
         [0070]    The second embodiment may have advantages similar to those in the first embodiment. The second embodiment may additionally provide the different intensity depending on the variation of the OSNR without using the ASE generator  102  that requires power. 
         [0071]    With reference further to the accompanying drawings, a description will be now given of an optical signal quality monitor according to another alternative, third embodiment of the present invention.  FIG. 8  is a schematic block diagram of the configuration of an optical signal quality monitor  100 B according to the third embodiment. The optical signal quality monitor  100 B includes a monitoring device  203  that has a similar configuration to the optical signal quality monitor  100  or  100 A in the first or second embodiment, a polarizer  201  rotatably provided on the input side of the monitoring device  203 , and an oscillator  202  that outputs an oscillation signal, for example, a signal with a frequency of 10 Hz or less, to rotate the polarizer  201  at a predetermined speed. 
         [0072]    With reference to  FIG. 9 , the primary polarization mode dispersion (PMD) is generally expressed by a differential group delay (DGD) generated between a given xi (ξ) axis component and another psi (ψ) axis component perpendicular to the ξ axis in the optical signal received from an optical fiber  204 . According to the third embodiment, the waveform distortion due to the DGD may be detected. As shown in  FIG. 9 , the input light has a delay difference caused by the DGD between the given ξ axis component and the other ψ axis component perpendicular to the ξ axis. 
         [0073]    The polarizer  201  is adopted to polarize the input optical signal and to provide the resultant polarized signal to the monitoring device  203 . The polarizer  201  rotates in response to a sinusoidal wave signal at a low frequency (here 10 Hz) generated by the oscillator  202 . When the polarization axis of the polarizer  201  is parallel with the ξ axis or the ψ axis, the monitoring device  203  detects the same pulse shape as the transmitted waveform. 
         [0074]    In contrast, when the polarization axis of the polarizer  201  is at an angle of 45 degrees to the ξ axis, the monitoring device  203  detects a pulse shape that is distorted by the DGD as shown in  FIG. 10 . In the figure, it is assumed that the pulse is Gaussian without chirp and has a pulse width of 2.5 ps for no DGD. 
         [0075]    When the polarizer  201  rotates, the monitoring device  203  detects different pulse widths depending on the angle between the polarization axis and the ξ axis. This thus causes the difference of the signal intensity, between the division signals a/b from the dividers  120  and  121 , as shown in  FIG. 11 . Because the intensity difference depends on the DGD, the waveform distortion caused by the DGD may be detected by observing the intensity difference. As seen in  FIG. 11 , for example, when the intensity difference is within range from 0 dB to −2.5 dB, the DGD is 2 ps. 
         [0076]      FIG. 11  plots the characteristic curves of the intensity ratio when the waveform is distorted by the DGD for the intensity ratio between the ξ axis and the ψ axis of 1:1. Actually, however, the intensities are often different between the ξ axis and the ψ axis.  FIG. 13  plots characteristic curves of the intensity ratio x/y for the DGD of 1 ps in relation to the angle of the polarizer  201 , where y and x respectively denote the intensities on the ξ axis and the ψ axis, as shown in  FIG. 12 . According to  FIG. 13 , it maybe seen that the intensity variation is caused depending on the angle of the polarizer  201  even when the intensity ratio x/y is not equal to unity. 
         [0077]      FIG. 14  a graph plotting intensity difference (variation) with respect to the DGD to show a characteristic curve of the intensity. The intensity difference may be detected to monitor the waveform distortion caused by the DGD. 
         [0078]    The monitoring device  203  receives the oscillation signal from the oscillator  202  as the angle information of the polarizer  201  so as to refer to the angle information as appropriate depending on the monitored subject. 
         [0079]    In addition to above-mentioned case depending on the DGD, for example, in case where the first and second line division signals may be incorporated and be processed when the angle of the polarizer is an integral multiple of π/2, it is possible to detect the OSNR as the first and second embodiments. Moreover, the maximum intensity may also be used to detect the OSNR and chromatic dispersion. 
         [0080]    In a method for detecting the waveform distortion caused by the DGD, at least, the target optical transmission system is observed in advance to store information thereon or observing information of the reference system is stored in advance, and the stored information is compared with the current observation of the system so as to obtain the observation of the desired characteristics. Alternatively, in the method, the reference information may be displayed and the observation may be overlapped on the displayed image, thereby facilitating the observer&#39;s understanding of the observation. 
         [0081]    Thus, the third embodiment may monitor the waveform distortion caused by the DGD as the optical signal quality. The OSNR and chromatic dispersion of the optical signal may also be monitored at the same time. 
         [0082]    Although the above embodiments use the Gaussian pulses without chirp that form a short pulse train with a repetition frequency of 160 GHz as the input signal, in other embodiments, modulated signals or other pulse shapes may also be used. The low frequency Δf may not be limited to 1 GHz. 
         [0083]    With respect to the optical signal quality monitor of the present invention, the first embodiment is specifically featured by the two-line monitoring configuration. However, the two-line monitoring configuration is not essential for the monitor and one-line or three or more line monitoring configuration may be used. For instance, only one line may be used to monitor the desired characteristics. Alternatively, third-line may be added and a monitoring section may be arranged in the third-line. In the third-line, an amount of the superimposed noise (level) differs from that in the second line or the 3 dB band width Δf differs from that of the optical band-pass filter in the second line. 
         [0084]    The configurations of the second lines in the first and second embodiments may be mixed in the same optical signal quality monitoring device. 
         [0085]    The third embodiment is specifically featured in that the polarization state of the input optical signal is varied by the rotational polarizer. Alternatively, an optical element exerting the electro-optical effect or the magneto-optical effect may be used to vary the polarization state of the input optical signal. Alternatively, the input optical signal may be split into two signals, and be passed through a plurality of polarizers having different polarization angles, and then be input to monitoring sections in lines corresponding to the respective polarizers. 
         [0086]    Furthermore, in-the optical signal quality monitor of the present invention, the elements dealing with electric signals and those dealing with optical signals may be changed or modified from those described in the above embodiments to any elements that may exert the essential functions of the present invention. For example, the functions of the electrical BPFs  212  and  213  and LPFs  218  and  219  in the first embodiment may be exerted by optical devices. 
         [0087]    The entire disclosure of Japanese patent application No. 2007-184608 filed on Jul. 13, 2007, including the specification, claims, accompanying drawings and abstract of the disclosure is incorporated herein by reference in its entirety. 
         [0088]    While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.