Abstract:
An object of the invention is to provide an optical dispersion monitoring apparatus and an optical dispersion monitoring method, capable of monitoring dispersion accurately with a simple construction, and to an optical transmission system using the same. To this end, the optical dispersion monitoring apparatus comprises: a light receiving section converting an input optical signal into an electrical signal, a signal transition position detecting section detecting the voltage level of a waveform of the output signal from the light receiving section, at a crossing point of a rising edge and a failing edge, and a cumulative dispersion information extracting section comparing the voltage level at the crossing point with a reference signal to extracts cumulative dispersion information.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The present invention relates to a technique for monitoring optical dispersion based on waveforms of transmitted light. In particular, the present invention relates to an optical dispersion monitoring apparatus and an optical dispersion monitoring method, capable of monitoring dispersion accurately with a simple construction, and to an optical transmission system using the same.  
           [0003]    (2) Description of the Prior Art  
           [0004]    In optical communication, as shown at the upper part of FIG. 17 for example, an optical signal sent to a transmission path  101  by an optical transmission apparatus  100  is transmitted for several tens of kilometers to several thousands of kilometers through an optical fiber via optical repeaters  102  using optical amplifiers or signal regenerators, to be received by an optical receiving apparatus  103 . At this time, waveform distortion occurs in the optical signal being transmitted, due to nonlinear optical phenomena occurring in the optical fiber depending on a dispersion characteristic of the optical fiber or the intensity of the optical signal, a change in instantaneous optical frequency of a pulse added in the optical transmission apparatus  100 , and the like.  
           [0005]    To be specific, in the case where a single optical pulse is transmitted through a long distance optical fiber for example, depending on the wavelength of the optical pulse or the characteristic of the optical fiber, “pulse compression” in which the pulse width is narrowed and the peak power is increased, or “pulse spread” in which, conversely, the pulse width is spread and the peak power is reduced, occurs as shown in FIG. 18. Such waveform distortion of optical pulse causes signal interference between adjacent bits in the data transmission, and is therefore a significant problem.  
           [0006]    In order to cope with the above described problem, in a conventional optical transmission system, as shown at the lower part of FIG. 17 for example, there is known a structure in which dispersion compensators  104  are inserted in the transmission path at appropriate spacing to compensate for cumulative dispersion, so that a dispersion characteristic of the whole system is in an optimal condition. Furthermore, in an optical transmission system actually operated, since the dispersion characteristic of optical fiber varies over time, sometimes just a single variable dispersion compensator dynamically compensating for variation over time may be used on its own, or in combination with a fixed dispersion compensator performing a large amount of dispersion compensation. The lower part of FIG. 17 shows an example in which a variable dispersion compensator  104 A and a fixed variable compensator  104 B are connected in series to construct a dispersion compensator  104 . In order to operate the variable dispersion compensator  104 A to perform the dynamic dispersion compensation as described above, an optical dispersion monitoring apparatus  105  is required for determining whether a compensation amount is optimal or not, while the system is operating.  
           [0007]    For a conventional optical dispersion monitoring apparatus, there is for example a structure in which cumulative dispersion is detected by paying attention to the spectral shape or spectral intensity at a specific frequency of a received optical signal. Furthermore, there is also known a structure in which the error rate of a regenerated signal at a required monitoring location is measured to detect cumulative dispersion.  
           [0008]    Moreover, in Japanese Unexamined Patent Publication No. 2001-320329, a technique is proposed in which a received optical pulse signal is converted into an electrical pulse signal, and depending on the voltage level obtained by rectifying and smoothing an AC component of the electrical pulse signal, it is detected whether the occurred waveform distortion is the pulse compression or the pulse spread.  
           [0009]    However, the following problems arise in the conventional optical dispersion monitoring apparatus as described above. Namely, in the system for paying attention to the spectrum of received optical signal, a significantly high quality device is required, since the spectral intensity at a specific frequency is extremely low, and the spectral intensity is easily influenced by frequency characteristics of optical filters, light receiving elements, monitoring circuits, and the like. Consequently, there is a problem in that it is difficult to easily realize an optical dispersion monitoring apparatus.  
           [0010]    Furthermore, in the system for measuring the error rate, there is a drawback in that even if it is possible to detect the existence of cumulative dispersion based on the measured error rate to detect an absolute value of cumulative dispersion, the sign of the cumulative dispersion cannot be extracted. In addition, since a signal regenerator is required to measure the error rate, there is a problem in that the locations where an optical dispersion monitoring apparatus can be installed are limited.  
           [0011]    Moreover, in the technique proposed in Japanese Unexamined Patent Publication No. 2001-320329, since the construction is such that the occurrence of waveform distortion is detected depending on the temporal average power of a mark component of a received optical signal, it is possible to detect whether the waveform distortion is the pulse compression or pulse spread, however, there is a problem in that it is difficult to detect the cumulative dispersion including sign information with high accuracy.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention has been accomplished in view of the above described problems, with an object of providing an apparatus and a method for monitoring optical dispersion, capable of monitoring dispersion accurately with a simple construction, and an optical transmission system using the same.  
           [0013]    In order to achieve the above object, an optical dispersion monitoring apparatus of the present invention, for monitoring dispersion based on a waveform of an input optical signal, comprises: a characteristic amount detecting section selectively detecting a physical amount corresponding to a location where waveform distortion occurring depending on dispersion appears distinctively in the waveform of the input optical signal; and a dispersion information extracting section extracting information related to the dispersion occurred in the optical signal, based on a comparison between the physical amount detected in the characteristic amount detecting section and a reference value indicated by a reference signal, to output the information.  
           [0014]    In such an optical dispersion monitoring apparatus, an input optical signal is supplied to the characteristic amount detecting section, the physical amount corresponding to the location where the waveform distortion occurring depending on dispersion appears distinctively in the signal waveform is detected selectively, and the detection result is transmitted to the dispersion information extracting section. In the dispersion information extracting section, the physical amount detected in the characteristic amount detecting section is compared with the reference value indicated in the reference signal, and information related to the dispersion occurred in the optical signal is extracted based on the comparison result. Thus, it becomes possible to monitor with high accuracy the dispersion including sign information with a simple construction, compared to a conventional monitoring system.  
           [0015]    As one aspect of the above described optical dispersion monitoring apparatus, the construction may be such that the characteristic amount detecting section includes: a light receiving section converting the input optical signal into an electrical signal; and a signal transition position detecting section detecting the voltage level corresponding to at least one of a rising edge and a falling edge of waveform of the electrical signal converted in the light receiving section, and the dispersion information extracting section compares the reference value indicated by the reference signal with the voltage level detected in the signal transition position detecting section, and outputs a signal corresponding to the comparison result as dispersion information. In such a construction, the voltage level corresponding to the rising edge or the falling edge of the waveform of the electrical signal converted in the light receiving section is detected as the physical amount corresponding to the location where the waveform distortion appears distinctively, and the dispersion information is extracted based on the comparison of the voltage level and the reference value.  
           [0016]    Furthermore, as another aspect of the above described optical dispersion monitoring apparatus, the construction may be such that the characteristic amount detecting section includes: a light receiving section converting the input optical signal into an electrical signal; and a signal intensity detecting section detecting the average intensity of waveform of the electrical signal converted in the light receiving section, by sampling parts of the waveform at the center of one cycle and locations neighboring the center in accordance with a clock signal synchronized with the input optical signal, and the dispersion information extracting section compares the average intensity detected in the signal intensity detecting section with the reference value indicated by the reference signal, and outputs a signal corresponding to the comparison result as dispersion information. In such a construction, the average intensity of waveform of the electrical signal converted in the light receiving section, at the center of one cycle and the locations neighboring the center, is detected as a physical amount corresponding to the location where the waveform distortion appears distinctively, and dispersion information is extracted based on the comparison of the average intensity and the reference value.  
           [0017]    Moreover, an optical dispersion monitoring method of the present invention, for monitoring dispersion based on a waveform of an input optical signal, comprises: selectively detecting a physical amount corresponding to a location where waveform distortion occurring depending on dispersion appears distinctively in the waveform of the input optical signal; and extracting information related to the dispersion occurred in the optical signal, based on a comparison between the detected physical amount detected and a reference value indicated by a reference signal.  
           [0018]    Furthermore, an optical transmission system of the present invention provided with a variable dispersion compensator on a transmission path through which an optical signal is propagated, for controlling a compensation amount of the variable dispersion compensator to dynamically compensate for dispersion, is constructed so that using the optical dispersion monitoring apparatus of the present invention, dispersion occurred in the optical signal being propagated through the transmission path is monitored and the compensation amount of the variable dispersion compensator is controlled in accordance with the monitored result. In this manner, if the variable dispersion compensator is controlled using the optical dispersion monitoring apparatus of the present invention, it is possible to perform dynamic compensation for dispersion occurred in the optical transmission system easily and reliably.  
           [0019]    Other objects, features, and advantages of this invention will become apparent from the following description of embodiments, in association with the appended drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a block diagram showing a structure of an optical dispersion monitoring apparatus according to a first embodiment of the present invention.  
         [0021]    [0021]FIG. 2 is a block diagram showing an example of a main structure of an optical transmission system in which dynamic dispersion compensation is performed using the optical dispersion monitoring apparatus of FIG. 1.  
         [0022]    [0022]FIG. 3 is a diagram for explaining a change in the optical waveform crossing point relative to cumulative dispersion.  
         [0023]    [0023]FIG. 4 is a diagram for explaining an operation in the first embodiment.  
         [0024]    [0024]FIG. 5 is a diagram for explaining an amplifying operation of a slice amplifier.  
         [0025]    [0025]FIG. 6 is a block diagram showing a structure of an optical dispersion monitoring apparatus according to a second embodiment of the present invention.  
         [0026]    [0026]FIG. 7 is a block diagram showing a structure of an optical dispersion monitoring apparatus according to a third embodiment of the present invention.  
         [0027]    [0027]FIG. 8 is a diagram showing an example of a main structure of an optical transmission system in which dynamic dispersion compensation is performed using the optical dispersion monitoring apparatus of FIG. 7.  
         [0028]    [0028]FIG. 9 is a diagram for explaining an operation of the third embodiment.  
         [0029]    [0029]FIG. 10 is a block diagram showing a structure of an optical dispersion monitoring apparatus according to a fourth embodiment of the present invention.  
         [0030]    [0030]FIG. 11 is a diagram for explaining an operation of the fourth embodiment.  
         [0031]    [0031]FIG. 12 is a block diagram showing a structure of an optical dispersion monitoring apparatus according to a fifth embodiment of the present invention.  
         [0032]    [0032]FIG. 13 is a diagram showing an example of setting a reference signal in the fifth embodiment.  
         [0033]    [0033]FIG. 14 is a block diagram showing a constitutional example in which a phase of a clock signal is made adjustable, in relation to the fifth embodiment.  
         [0034]    [0034]FIG. 15 is a block diagram showing a constitutional example in which there is provided a function for adding an offset signal to a reference signal, in relation to the above embodiments.  
         [0035]    [0035]FIG. 16 is a block diagram showing an example of main structure of an optical transmission system, in which an optical dispersion monitoring apparatus and an error monitoring apparatus are used together, in relation to the embodiments.  
         [0036]    [0036]FIG. 17 shows a structural example of a conventional optical transmission system.  
         [0037]    [0037]FIG. 18 is a diagram for explaining waveform distortion occurs when a single optical pulse is transmitted through an optical fiber. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    Hereunder is a description of embodiments of the present invention based on the appended drawings. Here, identical numerical numbers show identical or equivalent components throughout the figures.  
         [0039]    [0039]FIG. 1 is a block diagram showing a structure of an optical dispersion monitoring apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram showing an example of a main structure of an optical transmission system in which dynamic dispersion compensation is performed using the optical dispersion monitoring apparatus of FIG. 1.  
         [0040]    In the figures, an optical dispersion monitoring apparatus  1  of the present embodiment is provided with, for example, a light receiving section  10  converting an optical signal input thereto into an electrical signal to output it, a signal transition position detecting section  20  detecting the voltage level corresponding to at least one of the rising edge and falling edge of an input light waveform based on the output signal from the light receiving section  10 , and a cumulative dispersion information extracting section  30  extracting information related to cumulative dispersion occurred in the input light, based on the detection result in the signal transition position detecting section  20 .  
         [0041]    The light receiving section  10  converts, for example, an optical signal input to the optical dispersion monitoring apparatus  1  into a current signal using a known light receiving element, and converts the current signal into a voltage signal V IN , to output it to the signal transition position detecting section  20 . The voltage signal V IN  output from this light receiving section  10  is a signal whose level is changed depending on the power of the input light.  
         [0042]    Note, the optical signal input to the optical dispersion monitoring apparatus  1  is an optical signal that has a crossing point in an eye pattern drawn by folding back a time waveform of the optical signal in one cycle, that is, an optical signal applied with a code type in which the signal level has no transition during one bit cycle. A representative example of such an optical signal is an optical signal of NRZ type. However, optical signals capable of being input to the optical dispersion monitoring apparatus  1  are not limited to NRZ type.  
         [0043]    The signal transition position detecting section  20  includes a comparator  21 , a slice amplifier  22  and a low-pass filter  23 . The comparator  21  receives a voltage signal V IN  output from the light receiving section  10  at one input terminal thereof and a feedback signal V X  transmitted through the low-pass filter  23  at the other input terminal, and compares the level of the voltage signal V IN  with the level of the feedback signal V X , to output a voltage signal corresponding to the comparison result to the slice amplifier  22 . A typical analog comparator may be used for this comparator  21 . The slice amplifier  22  is a typical high gain amplifier which amplifies the voltage level of the output signal from the comparator  21  until it reaches the required high level or low level (here “1” or “0” level). The low-pass filter  23  smoothes the voltage signal amplified in the slice amplifier  22  in accordance with a preset time constant. The voltage signal V X , which is transmitted through this low-pass filter  23  to be averaged, is fed back to the other input terminal of the comparator  21 , and also sent to the cumulative dispersion information extracting section  30 .  
         [0044]    The cumulative dispersion information extracting section  30  includes a comparator  31  and a reference signal generating circuit  32 , as shown in FIG. 1 for example. The comparator  31  receives the voltage signal V X  output from the signal transition position detecting section  20  at one input terminal thereof and a reference signal V REF  generated in the reference signal generating circuit  32  at the other input terminal, and compares the level of the voltage signal V X  with the level of the reference signal V REF , to output a voltage signal V OUT  corresponding to the comparison result to outside the optical dispersion monitoring apparatus  1 . Cumulative dispersion here means wavelength dispersion accumulated in an input light. The voltage signal V OUT  output from the comparator  31  is supplied to a variable dispersion compensator  5  as shown in FIG. 2, for example, to be used for a dynamic control of a compensation amount and the like. Here, the reference signal generating circuit  32  applies, for example, an output voltage generated in a variable power source to the other input terminal of the comparator  31  as a reference signal V REF . This variable power source output voltage is set in advance depending on a mark ratio of an optical signal input to the optical dispersion monitoring apparatus  1 , as described later.  
         [0045]    Reference numeral  4  in FIG. 2 denotes an optical amplifier for amplifying an optical signal for repeating transmission. Furthermore, reference numeral  6  denotes an optical coupler for branching a part of the optical signal output from the variable dispersion compensator  5  as a monitoring light and supplying it to the optical dispersion monitoring apparatus  1 . Here, the construction is such that the optical coupler  6  is disposed between the variable dispersion compensator  5  and the optical amplifier  4  to monitor cumulative dispersion. However, on a transmission path  3 , the position where the monitoring light is branched is not limited to the above. Moreover, in the above, there is shown the constitutional example in which cumulative dispersion is compensated using only the variable dispersion compensator  5 . However, it is also possible to apply the optical dispersion monitoring apparatus  1  of the present embodiment to the structure in which the fixed dispersion compensator and the variable dispersion compensator are combined as shown at the lower part of FIG. 17 described above.  
         [0046]    Next is a description of an operation of the optical dispersion monitoring apparatus  1  in the first embodiment.  
         [0047]    Firstly, a change in optical waveform crossing point relative to cumulative dispersion will be described in detail.  
         [0048]    In general, if an optical pulse is propagated through a transmission path using an optical fiber or the like, there is a difference in propagation speed between the rising edge and falling edge of the optical pulse depending on its optical wavelength and a dispersion characteristic of the optical fiber. As a result, in the case where the rising edge is delayed and the falling edge is advanced, the pulse is compressed, and conversely, in the case where the rising edge is advanced and the falling edge is delayed, the pulse is spread. When such pulse compression or pulse spread occurs, since the power of the optical pulse is kept, the peak power is increased when the pulse is compressed, while the peak power being decreased when the pulse is spread.  
         [0049]    The effect as described above is considered to occur only at transition points when an optical signal is switched between the levels of “1” and “0” in the case of an NRZ optical signal. Accordingly, when an optical signal modulated in a random signal of NRZ type as shown in the eye pattern at the left of FIG. 3 for example, is propagated through an optical fiber, waveform distortion as shown in the eye patterns at the right of FIG. 3 occurs depending on an amount of cumulative dispersion.  
         [0050]    When such waveform distortion is compared with each other in paying attention to the crossing points (circled in the figure), it can be seen that positions of crossing points (voltage levels) are changed depending on the state of waveform distortion. To be specific, in a state in which waveform distortion does not occur (cumulative dispersion=0), as shown at the middle right of FIG. 3, the crossing points are positioned centrally between the high level and low level, in a state in which the pulse spread occurs, as shown at the upper right of FIG. 3, the crossing points are positioned on the high level side, and in a state in which the pulse compression occurs, as shown at the lower right of FIG. 3, the crossing points are positioned on the low level side.  
         [0051]    In utilizing this relationship between the voltage level of the crossing points and the state of waveform distortion, in other words, the relationship between the change in optical waveform over time and the cumulative dispersion occurring in the optical signal, the optical dispersion monitoring apparatus  1  of the present embodiment enables cumulative dispersion, including up to positive or negative sign information, to be detected with a simple construction.  
         [0052]    To be specific, the operation of the present optical dispersion monitoring apparatus  1  will be described in detail with reference to FIG. 4. Firstly, the optical signal branched in the optical coupler  6  disposed on the transmission path  3  is sent to the light receiving section  10  to be converted into the voltage signal V IN , and supplied to the signal transition position detecting section  20 . In the waveform of the voltage signal V IN  input to the signal transition position detecting section  20 , distortion occurs depending on cumulative dispersion as shown in (A) of FIG. 4 for example. In addition, the waveform shown on the left side in (A) of FIG. 4 is one example of when the pulse is compressed, the waveform in the center is one example of when no distortion occurs (cumulative dispersion=0), and the waveform on the right side is one example of when the pulse is spread.  
         [0053]    In the signal transition position detecting section  20 , the comparator  21  compares the voltage signal V IN  output from the light receiving section  10  with the voltage signal V X  fed back through the low-pass filter  23 , to output a voltage signal corresponding to the comparison result to the slice amplifier  22 . Note, in an initial state, the voltage signal V X  from the low-pass filter  23  is set to the ground level or the like for example. In the slice amplifier  22 , the voltage signal output from the comparator  21  is amplified to the required level. The amplification operation in this slice amplifier  22  differs from an amplification operation in a linear amplifier as shown in a conceptual diagram of FIG. 5 for example, and the input signal thereto is amplified until it reaches the “1” or “0” level. The voltage signal amplified in the slice amplifier  22  is sent to the low-pass filter  23 , smoothed (averaged) in accordance with a required time constant, and the voltage signal V X  transmitted through the low-pass filter  23  is fed back to the comparator  21 .  
         [0054]    As described above, the output signal of the comparator  21  is fed back to the comparator  21  via the slice amplifier  22  and the low-pass filter  23 , so that the voltage level of the feedback signal becomes stable following the voltage level at the crossing points of the signal V IN  input to the signal transition position detecting section  20  as shown in (B) of FIG. 4. As a result, in branching the voltage signal V X  fed back from the low-pass filter  23  to the comparator  21 , a change in the rising edge or the falling edge of the input pulse over time is detected as a change in the voltage level at the crossing points. This voltage signal V X  corresponding to the voltage level at the crossing points is sent to the cumulative dispersion information extracting section  30  as an output of the signal transition position detecting section  20 .  
         [0055]    In the cumulative dispersion information extracting section  30 , the comparator  31  compares the voltage signal V X  output from the signal transition position detecting section  20  with the reference signal V REF  output from the reference signal generating circuit  32 , to output a voltage signal V OUT  corresponding to the comparison result as cumulative dispersion information. To be specific, the reference signal V REF  supplied to the comparator  31  is set in advance with the fixed voltage level depending on the mark ratio of the optical signal input to the optical dispersion monitoring apparatus  1  as shown in (C) of FIG. 4. Here, the fixed voltage level is set to approximately match the voltage level at the crossing points when the cumulative dispersion is 0. The fixed reference signal V REF  set in this manner is supplied to the comparator  31 , so that the voltage level of the voltage signal V OUT  output from the comparator  31  corresponds to the cumulative dispersion as shown in (D) of FIG. 4. To be specific, in the example of (D) of FIG. 4, a negative value voltage signal V OUT  is output as the cumulative dispersion information when the pulse is compressed, while a positive value voltage signal V OUT  being output as the cumulative dispersion information when the pulse is spread.  
         [0056]    The relationship between the state of waveform distortion and the sign of the cumulative dispersion is that in the case where a chirp characteristic of a modulator on a transmission side of the optical transmission system is positive for example, the cumulative dispersion is negative when the pulse is compressed, while the cumulative dispersion being positive when the pulse is spread. Furthermore, in the case where the chirp characteristic is negative for example, the cumulative dispersion is positive when the pulse is compressed, while the cumulative dispersion being negative when the pulse is spread. Accordingly, in making the chirp characteristics of the system to correspond to the value of the above described voltage signal V OUT , it is possible to determine the cumulative dispersion including the sign information.  
         [0057]    According to the optical dispersion monitoring apparatus  1  of the first embodiment as described above, the signal transition position detecting section  20  detects the voltage level at the crossing points of the optical signal to which a code type represented in NRZ type is applied, and the cumulative dispersion information extracting section  30  extracts the cumulative dispersion information based on the detection result. Thus, it is possible to detect the cumulative dispersion including up to the sign information with high accuracy using the simpler structure than the conventional monitoring system having paid attention to spectrum intensity. Furthermore, the present optical dispersion monitoring apparatus  1  does not require a signal regenerator as in the conventional system in which an error rate is measured. Hence, it is possible to reduce restrictions to the installation location in the optical transmission system. If the variable dispersion compensator  5  disposed in the optical transmission system is feedback controlled using such an optical dispersion monitoring apparatus  1 , it is possible to perform easily and reliably dynamic compensation for cumulative dispersion occurring in the system.  
         [0058]    In the first embodiment, the wavelength dispersion accumulated in the input light has been considered as the cumulative dispersion. However, the present invention is not limited thereto. The present invention may be applied to other optical dispersion, such as polarization dispersion and the like, as in the case of the wavelength dispersion, if a relationship with the occurring state of waveform distortion can be specified.  
         [0059]    Next is a description of an optical dispersion monitoring apparatus according to a second embodiment of the present invention.  
         [0060]    [0060]FIG. 6 is a block diagram showing a structure of the optical dispersion monitoring apparatus of the second embodiment.  
         [0061]    In FIG. 6, the structure of the present optical dispersion monitoring apparatus  1 ′ differs from that of the first embodiment shown in FIG. 1 in that a gain control amplifier  33  and a low-pass filter  34  are disposed instead of the variable power source that has been used as the reference signal generating circuit  32 , in the cumulative dispersion information extracting section  30 . Components other than the above are the same as those in the first embodiment, and hence the descriptions thereof are omitted here.  
         [0062]    The gain control amplifier  33  receives the voltage signal V IN  output from the light receiving section  10  at an input terminal thereof, and amplifies the input signal to the required level, to output it to the low-pass filter  34 . The low-pass filter  34  averages the voltage signal amplified in the gain control amplifier  33  in accordance with a preset time constant. The voltage signal transmitted through the low-pass filter  34  is supplied to the comparator  31  as the reference signal V REF .  
         [0063]    Here, the gain control amplifier  33  is disposed in a former stage of the low-pass filter  34 , but may be disposed in a latter stage of the low-pass filter  34 . Furthermore, if the voltage signal V IN  output from the light receiving section  10  is of the sufficient level, the gain control amplifier  33  may be omitted.  
         [0064]    In the optical dispersion monitoring apparatus  1 ′ with the above construction, the reference signal V REF , which is a reference for when the cumulative dispersion information is extracted in the cumulative dispersion information extracting section  30  based on the voltage level at the crossing points detected in the signal transition position detecting section  20 , is set following a change in the optical signal input to the optical dispersion monitoring apparatus  1 ′.  
         [0065]    To be specific, the voltage signal V IN  converted photoelectrically in the light receiving section  10  is gain controlled by the gain control amplifier  33 , and then transmitted through the low-pass filter  34  to be averaged. As a result, the reference signal V REF  following the change in the input signal is generated. At this time, even if the waveform distortion occurs corresponding to the cumulative dispersion in the optical signal input to the optical dispersion monitoring apparatus  1 ′, since the optical signal power is stored irrespectively of the waveform distortion, the voltage level of the reference signal V REF  averaged by the low-pass filter  34  is constant independently of the occurring state of cumulative dispersion. As a result, the reference signal V REF  generated in the above manner can be used as the reference for when the cumulative dispersion is determined based on the voltage level at the crossing points. On the other hand, in the case where the power setting of optical signal input to the optical dispersion monitoring apparatus  1 ′ is changed due to a change in operating conditions of the system, the voltage level of the reference signal V REF  is changed following the change in the power setting. As a result, in the case where the fixed reference signal V REF  is used as in the first embodiment, it is necessary to reset the reference signal V REF  according to the change in operating conditions. However, by using the reference signal V REF  that follows the change in the input signal as in the present embodiment, it is possible to realize the automatic setting to the optimum level.  
         [0066]    In the case where the cumulative dispersion is determined using the reference signal V REF  that follows the change in the input signal as described above, it is desirable to pay attention to a change in the mark ratio of the input optical signal. The following is a description of this using a specific example.  
         [0067]    In general, not only in optical communications but also in most data communications, by using a data signal in compliance with a format based on a standard set in advance, interconnection is possible between a plurality of systems. For example, an international standard of 10 Gbit/s in the optical communication field corresponds to “ITU-T G.707”. According to the standard above, the mark ratio of most (about 99.999950/%) of data is 1/2. However, strictly speaking, there is a part called “header” for frame synchronization normally or STM identification in the rest (about 0.00005%) of the data. In this “header” part, the mark ratio is defined to be 3/4 or 1/4, and its average power is changed in proportion to the mark ratio.  
         [0068]    Accordingly, since the level of the voltage signal output from the low-pass filter  34  in the cumulative dispersion information extracting section  30  is changed depending on the mark ratio due to the input of the header part, there is a possibility that an error occurs in the determination of cumulative dispersion in the cumulative dispersion information extracting section  30 . In order to prevent such an error due to a change in the mark ratio, it is effective to have an influence of level change due to the input of the header part masked with the level of when other data part is input, by increasing the time constant of the low-pass filter  34  for example.  
         [0069]    According to the optical dispersion monitoring apparatus  1 ′ of the second embodiment as described above, the voltage signal V IN  output from the light receiving section  10  is averaged using the gain control amplifier  33  and the low-pass filter  34 , and the reference signal V REF  that follows the change in the input signal is supplied to the comparator  31 . Thus, even in the case where the transmitted optical power is changed due to the change in operating conditions of the system, it is possible to set the reference signal V REF  to the optimal level automatically following the change in the transmitted optical power. Therefore, it is possible to monitor the cumulative dispersion stably. Furthermore, if the time constant of the low-pass filter  34  is set considering the change in the mark ratio of the optical signal, it is possible to monitor the cumulative dispersion more reliably.  
         [0070]    Next is a description of an optical dispersion monitoring apparatus according to a third embodiment of the present invention.  
         [0071]    [0071]FIG. 7 is a block diagram showing a structure of the optical dispersion monitoring apparatus of the third embodiment. Furthermore, FIG. 8 is a block diagram showing an example of a main structure of the optical transmission system in which dynamic dispersion compensation is performed using the optical dispersion monitoring apparatus of FIG. 7.  
         [0072]    In the figures, an optical dispersion monitoring apparatus  2  of the present embodiment, for example, includes the light receiving section  10  converting an input optical signal into an electrical signal to output it, a signal intensity detecting section  40  sampling a part of the signal output from the light receiving section  10 , in which a waveform change due to cumulative dispersion appears distinctively, to detect its intensity (power), and the cumulative dispersion information extracting section  30  extracting cumulative dispersion information based on the detection result in the signal intensity detecting section  40 . The structures of the light receiving section  10  and the cumulative dispersion information extracting section  30  are the same as those in the first embodiment, and hence the description thereof is omitted here.  
         [0073]    The signal intensity detecting section  40  includes, for example, a selector circuit  41 , a clock generation circuit  42 , a duty adjusting circuit  43  and a low-pass filter  44 . The selector circuit  41  receives the voltage signal V IN  output from the light receiving section  10  at an input terminal thereof, and performs a switching operation in accordance with a clock signal CLK output from the duty adjusting circuit  43 , to take a part out of the voltage signal V IN  at the center of one cycle and the locations neighboring the center, to output it to the low-pass filter  44 .  
         [0074]    The clock generation circuit  42  generates a clock signal synchronized with a data frequency of the optical signal input to the optical dispersion monitoring apparatus  2 . As a specific example of this clock generation circuit  42 , a circuit extracting a clock signal component from an electrical or optical data signal can be adopted. Furthermore, in the case where the present optical dispersion monitor  2  is disposed in a regenerative repeater, a clock signal obtained from a data clock regeneration circuit can also be utilized without change.  
         [0075]    The duty adjusting circuit  43  adjusts a duty of the clock signal output from the clock generation circuit  42 , to supply it to a control terminal of the selector circuit  41 . The low-pass filter  44  averages the voltage signal sampled by the selector circuit  41  in accordance with a preset time constant. A voltage signal V P  transmitted through this low-pass filter  44  is supplied to the one input terminal of the comparator  31  in the cumulative dispersion information extracting section  30 .  
         [0076]    The optical signal input to the optical dispersion monitoring apparatus  1  may be not only the NRZ optical signal or the like, which has the crossing points existing in the eye pattern drawn by folding back a time waveform of the optical signal in one cycle, but also an RZ signal or the like, which has no crossing points, in other words, an optical signal of code type in which there is the signal level transition during one bit cycle.  
         [0077]    In the optical dispersion monitoring apparatus  2  with the above construction, a monitor light branched by the optical coupler  6  disposed on the transmission path  3  of the optical transmission system (FIG. 8) is sent to the light receiving section  10 , and converted into the voltage signal V IN , to be supplied to the signal intensity detecting section  40 . Here, assuming the case where an optical signal of RZ type is repeatedly transmitted in the system, the waveform distortion occurs in the waveform of the voltage signal V IN  input to the signal intensity detecting section  40  depending on the cumulative dispersion as shown in (A) of FIG. 9 for example. The waveform shown on the left side in (A) of FIG. 9 is an example of when the pulse is compressed, the waveform in the center is an example of when no distortion occurs (cumulative dispersion=0), and the waveform on the right side is an example of when the pulse is spread.  
         [0078]    In the signal intensity detecting section  40 , the voltage signal V IN  output from the light receiving section  10  is input to the selector circuit  41 . The clock signal CLK as shown in (B) of FIG. 9 is supplied to this selector circuit  41  from the clock generation circuit  42  via the duty adjusting circuit  43 , and a connection state between input and output terminals of the selector circuit  41  is switched in synchronization with the clock signal CLK. Here, when the clock signal CLK is at the high level, the signal input to the input terminal is output from the output terminal. By such a switching operation of the selector circuit  41 , a part of the signal at the center of one cycle and the locations neighboring regions of the center is taken out, and the sampled signal is output to the low-pass filter  44 . The signal sampled in the selector circuit  41  is averaged in the low-pass filter  44  according to a required time constant. As a result, the voltage signal V P  indicating the average intensity as shown in (D) of FIG. 9 is generated to be output to the cumulative dispersion information extracting section  30 .  
         [0079]    It is effective to adjust, by the duty adjusting circuit  43 , the duty of the clock signal CLK generated in the clock generation circuit  42  to be supplied to the selector circuit  41  for signal sampling, so as to reduce a period of time when the input and output terminals of the selector circuit  41  are in a closed circuit condition. That is to say, by taking a part out of the signal with narrower width at the center of one cycle and the locations neighboring the center for sampling, there is caused a significant difference in the voltage level output from the low-pass filter  44  even if there is a small difference in cumulative dispersion, hence it becomes possible to achieve an improvement in the accuracy of monitoring cumulative dispersion in the latter staged cumulative dispersion information extracting section  30 .  
         [0080]    Furthermore, similarly to the aforementioned case, it is also effective to have the influence of level change due to the input of the header part masked with the level of when other data part is input, by increasing the time constant of the low-pass filter  34  for example, considering the change in the mark ratio of the input optical signal.  
         [0081]    In the cumulative dispersion information extracting section  30 , the voltage signal V P  output from the signal intensity detecting section  40  is supplied to the one input terminal of the comparator  31 , and similarly to the first embodiment, the level of the voltage signal V P  is compared with the level of the reference signal V REF , and the voltage signal V OUT  corresponding to the comparison result is output to outside as the cumulative dispersion information. However here, regarding the reference signal V REF  supplied to the comparator  31 , the fixed voltage level is preset depending on the mark ratio of the optical signal input to the optical dispersion monitoring apparatus  2 , and the duty of the clock signal supplied to the selector circuit  41 . (E) of FIG. 9 shows an example in which the average voltage level for when the cumulative dispersion is 0 and the reference signal V REF  are set to be almost identical, as specific setting of the reference signal V REF . By supplying the reference signal V REF  set in this manner to the comparator  31 , the voltage level of the voltage signal V OUT  output from the comparator  31  corresponds to the cumulative dispersion as shown in (F) of FIG. 9. To be specific, in the example of (F) of FIG. 9, a positive value voltage signal V OUT  is output as the cumulative dispersion information when the pulse is compressed, and a negative value voltage signal V OUT  is output as the cumulative dispersion information when the pulse is spread.  
         [0082]    According to the optical dispersion monitoring apparatus  2  of the third embodiment as described above, the part of the input optical signal at the center of one cycle and the locations neighboring the center is sampled to detect the average intensity, and the cumulative dispersion information is extracted based on the detection result. Thus, only the part of the input signal, where the waveform change due to the cumulative dispersion appears most distinctively during one cycle, is utilized for detecting the cumulative dispersion. Therefore, it is possible to detect the cumulative dispersion including the sign information with high accuracy. Furthermore, similarly to the effect in the case of the first embodiment, since the present optical dispersion monitoring apparatus  2  according to the present embodiment differs from the conventional monitoring system in which the error rate is measured, it is possible to reduce restrictions to the installation location in the optical transmission system. If the variable dispersion compensator  5  disposed in the optical transmission system is feedback controlled using such an optical dispersion monitoring apparatus  2 , it becomes possible to perform dynamic compensation for the cumulative dispersion occurring in the system easily and reliably.  
         [0083]    Next is a description of an optical dispersion monitoring apparatus according to a fourth embodiment of the present invention.  
         [0084]    [0084]FIG. 10 is a block diagram showing a structure of the optical dispersion monitoring apparatus of the fourth embodiment.  
         [0085]    In FIG. 10, the structure of the present optical dispersion monitoring apparatus  2 ′ is different from the structure in the third embodiment shown in FIG. 7 in that a comparator  45  and a sample and hold circuit  46  are disposed in the signal intensity detecting section  40 , instead of the selector circuit  41  and the duty adjusting circuit  43 . Other structures than the above, namely, the clock generation circuit  42 , the low-pass filter  44 , the light receiving section  10  and the cumulative dispersion information extracting section  30 , are the same as those in the third embodiment, and hence the descriptions thereof are omitted here.  
         [0086]    The comparator  45  receives the voltage signal V IN  output from the light receiving section  10  at one input terminal thereof and the reference signal V REF  generated in the reference signal generating circuit  32  in the cumulative dispersion information extracting section  30  at the other input terminal, and compares the level of the voltage signal V IN  with the level of the reference signal V REF , to output a voltage signal corresponding to the comparison result to the sample and hold circuit  46 . A typical analog comparator may be used for this comparator  45 . The sample and hold circuit  46  samples the signal output from the comparator  45  in synchronization with the clock signal CLK from the clock generation circuit  42 , to output it to the low-pass filter  44 . As a specific example of this sample and hold circuit  46 , a delay flip-flop (D-FF) circuit or the like may be used.  
         [0087]    In the optical dispersion monitoring apparatus  2 ′ with the above construction, the voltage signal V IN  converted photoelectrically in the light receiving section  10  is supplied to the comparator  45  in the signal intensity detecting section  40 . Here, assuming a case where an optical signal of NRZ type is repeatedly transmitted in the system, the level of the voltage signal V IN  input to the comparator  4  is changed depending on the cumulative dispersion as shown in (A) of FIG. 11.  
         [0088]    The comparator  45  compares the level of the voltage signal V IN  from the light receiving section  10  with the reference signal V REF  as shown in (B) of FIG. 11, to output the voltage signal corresponding to the comparison result to the sample and hold circuit  46 . The sample and hold circuit  46  samples the voltage signal from the comparator  45  in accordance with the clock signal CLK as shown in (C) of FIG. 11 in synchronization with the data frequency of input signal. To be specific, as shown in (D) of FIG. 11, the sample and hold circuit  46  samples the voltage signal from the comparator  45  at the time of rising edge of the clock signal CLK, and thereafter, holds the level of the voltage signal until the time of next rising edge. As a result, the level of the signal output from the sample and hold circuit  46  is changed differently depending on the state of waveform distortion. In (D) of FIG. 11, a portion of the output level of the sample and hold circuit  46  shown by dotted lines when the cumulative dispersion=0 indicates a possibility in that the level of the voltage signal V IN  reaches the reference signal V REF , and hence the output level becomes unstable due to an influence of noise and the like.  
         [0089]    The output signal from the sample and hold circuit  46  is sent to the low-pass filter  44 , to be smoothed in accordance with a required time constant. As a result, a voltage signal V P  indicating averaged intensity as shown in (E) of FIG. 11 is generated to be output to the cumulative dispersion information extracting section  30 . In the cumulative dispersion information extracting section  30 , similarly to the third embodiment, the voltage signal V P  output from the signal intensity detecting section  40  is supplied to the one input terminal of the comparator  31 , the level of the voltage signal V P  is compared with the level of the reference signal V REF  as shown in (F) of FIG. 11, and a voltage signal V OUT  corresponding to the comparison result is output to outside as the cumulative dispersion information. The voltage level of this voltage signal V OUT  corresponds to the cumulative dispersion as shown in (G) of FIG. 11. To be specific, in one example of (G) of FIG. 11, a positive value voltage signal V OUT  is output as the cumulative dispersion information when the pulse is compressed, and a negative value voltage signal V OUT  is output as the cumulative dispersion information when the pulse is spread.  
         [0090]    According to the optical dispersion monitoring apparatus  2 ′ of the fourth embodiment as described above, it is possible to achieve the same effect as in the third embodiment, by performing sampling of signal using the comparator  45  and the sample and hold circuit  46 .  
         [0091]    In the above third and fourth embodiments, the structure is such that the fixed reference signal V REF  is supplied to the comparator  31  in the cumulative dispersion information extracting section  30 . However, similarly to the second embodiment shown in FIG. 6, the structure may also be adopted in which the reference signal V REF  that follows the change in the input signal is supplied to the comparator  31 .  
         [0092]    Next is a description of an optical dispersion monitoring apparatus according to a fifth embodiment of the present invention. Here, the description will be made on an improved example of the optical dispersion monitoring apparatus in the fourth embodiment, wherein stability of operation is achieved.  
         [0093]    [0093]FIG. 12 is a block diagram showing a structure of the optical dispersion monitoring apparatus of the fifth embodiment.  
         [0094]    In FIG. 12, the optical dispersion monitoring apparatus  2 ″ of the present embodiment includes a comparator  45 A and a sample and hold circuit  46 A disposed in parallel with each other, and a comparator  45 B and a sample and hold circuit  46 B disposed in parallel with each other, in the signal intensity detecting section  40 . Furthermore, similarly to the second embodiment described above, in order to generate a reference signal V REF  that follows the change in the input signal, the gain control amplifier  33  and the low-pass filter  34 , and potentiometers  35 A and  35 B, are disposed in the cumulative dispersion information extracting section  30 , and also a NAND circuit  36  and a switch circuit  37  are disposed to disconnect a monitor, to stabilize an operation as described later. The structures other than the above are the same as those in the fourth embodiment.  
         [0095]    The comparators  45 A and  45 B, and the sample and hold circuits  46 A and  46 B are the same as the comparator  45  and the sample and hold circuit  46  used in the fourth embodiment. Here, each of the comparators  45 A and  45 B receives the voltage signal V IN  output from the light receiving section  10  at one input terminal thereof. Furthermore, a voltage from a sliding terminal of the potentiometer  35 A is applied to the other input terminal of the comparator  45 A as a high level side reference signal V REF-H , and a voltage from a sliding terminal of the potentiometer  35 B is applied to the other input terminal of the comparator  45 B as a low level side reference signal V REF-L . The comparator  45 A compares the level of the voltage signal V IN  with the level of the reference signal V REF-H  and the comparator  45 B compares the level of the voltage signal V IN  with the level of the reference signal V REF-L . Then, the comparators  45 A and  45 B output voltage signals indicating the comparison results to data input terminals of the sample and hold circuits  46 A and  46 B, respectively.  
         [0096]    The sample and hold circuits  46 A and  46 B sample the signals output from the comparators  45 A and  45 B in synchronization with the clock signal CLK from the clock generation circuit  42 . The sample and hold circuit  46 A outputs the sampled signal to the low-pass filter  44 , and outputs an inversion signal of the sampled signal to the NAND circuit  36  in the cumulative dispersion information extracting section  30 . Furthermore, the sample and hold circuit  46 B outputs the sampled signal to the NAND circuit  36  in the cumulative dispersion information extracting section  30 .  
         [0097]    The gain control amplifier  33  and the low-pass filter  34  disposed in the cumulative dispersion information extracting section  30  are the same as those used in the second embodiment. The potentiometers  35 A and  35 B are variable resistors each having three terminals, and are connected in series between an output terminal of the low-pass filter  34  and the ground terminal. A voltage at the common connection node of the potentiometers  35 A and  35 B is supplied to the comparator  31  as the reference signal V REF  for detecting the cumulative dispersion based on the voltage signal V P  output from the signal intensity detecting section  40 .  
         [0098]    The NAND circuit  36  calculates the NAND of the inverse sampled signal output from the sample and hold circuit  46 A and the sampled signal output from the sample and hold circuit  46 B, to output the calculated result to the switching circuit  37 . The switching circuit  37  is disposed at an output stage of the comparator  31 , to perform a switching operation according to the output signal from the NAND circuit  36 .  
         [0099]    In the optical dispersion monitoring apparatus  2 ″ with the above construction, as described in the description of the state (state after sampling) of (D) of FIG. 11 in the fourth embodiment, it is considered that in the case where the cumulative dispersion is approximately 0, the levels of the voltage signal V IN  and the reference signal V REF  to be compared with each other by the comparator  45  in the signal intensity detecting section  40  are the same, and hence the signal level after sampling becomes unstable. Therefore, if such a state occurs, the signal V OUT  indicating the monitored result of the cumulative dispersion is not output to outside.  
         [0100]    To be specific, in the fourth embodiment, signal sampling is performed based on one reference signal V REF  in the signal intensity detecting section  40 , whereas in the present embodiment, as shown in FIG. 13 for example, the reference signal V REF-H  whose level is ΔH higher than the reference signal V REF  and the reference signal V REF-L  whose level is ΔL lower than the reference signal V REF  are obtained using the potentiometers  35 A and  35 B, and signal sampling is performed based on both of the reference signals V REF-H  and V REF-L . As a result, when the level of the voltage signal V IN  input to each of the comparators  45 A and  45 B is between V REF-L  and V REF-H  (shaded portion in FIG. 13), the signal sampled by the sample and hold circuit  46 A is at the low level, and the signal sampled by the sample and hold circuit  46 B is at the high level.  
         [0101]    Accordingly, in the above case, the voltage signals sent from the sample and hold circuits  46 A and  46 B to the NAND circuit  36  are both at high levels, and the low level signal is output from the NAND circuit  36  to the switching circuit  37 , so that the switching circuit  37  becomes open circuit. As a result, in a condition where the voltage signal V IN  is in the vicinity of V REF  (the cumulative dispersion is about 0), and hence the operation is unstable, the cumulative dispersion information output from the comparator  31  is prevented from being sent to outside. On the other hand, in cases other than the above, since the output signal from the NAND circuit  36  is at the high level, the switching circuit  37  becomes closed circuit, and hence the cumulative dispersion information is sent to outside.  
         [0102]    According to the optical dispersion monitoring apparatus  2 ″ of the fifth embodiment as described above, since the monitored result obtained when a monitoring state of cumulative dispersion becomes unstable due to the influence of noise and the like, is not output to outside, it becomes possible to stabilize the monitoring operation. If the variable dispersion compensator  5  disposed in the optical transmission system as shown in FIG. 8 is feedback controlled using such an optical dispersion monitoring apparatus  2 ″, the optical dispersion monitoring apparatus  2 ″ is disconnected depending on the state of cumulative dispersion compensation, and hence it becomes possible to prevent noise and the like from being propagated from the monitoring system. Therefore, it becomes possible to perform dynamic compensation for the cumulative dispersion occurred in the system reliably.  
         [0103]    In the third to fifth embodiments, the input optical signal at the center of one cycle and the locations neighboring the center is sampled. However, for example, if the location where the waveform change due to the cumulative dispersion appears distinctively is shifted from the center of one cycle, it is also possible that a phase of the clock signal CLK supplied to the selector circuit  41  and the sample and hold circuit  46  is adjusted using a phase adjuster or the like, to be shifted from the center of the cycle for optimization. To be specific, FIG. 14 shows an example of the case where a phase adjuster  47  is disposed in the optical dispersion monitoring apparatus  2 ″ of the fifth embodiment.  
         [0104]    Furthermore, in the first to fifth embodiments, the level of the reference signal V REF , being a reference for when the cumulative dispersion is detected in the cumulative dispersion information extracting section  30 , is set to be approximately coincident with the voltage level of when the cumulative dispersion is 0. However, for example, considering the dispersion characteristics of the transmission path and the like on the latter stages of the position where the optical dispersion monitoring apparatus is disposed in the optical transmission system, identification characteristics of an optical receiving apparatus, and the like, then in the case where the dispersion compensation is performed intentionally so as to occur the required cumulative dispersion, without setting the monitor reference in the optical dispersion monitoring apparatus for cumulative dispersion=0, it is possible to respond to such a case by providing a function for adding an offset signal V OFFSET  to V REF  set corresponding to cumulative dispersion=0, as shown in FIG. 15. FIG. 15 shows a constitutional example corresponding to the optical dispersion monitoring apparatus  1  of the first embodiment, but such a function is also applicable to the other embodiments.  
         [0105]    Moreover, in the first to fifth embodiments, as shown in FIG. 2 and FIG. 8, the example is described in which the variable dispersion compensator  5  in the optical transmission system is feedback controlled based on the cumulative dispersion information output from the optical dispersion monitoring apparatus. However, as shown in FIG. 16 for example, the construction may be such that the optical dispersion monitoring apparatus in each embodiment and a known error monitoring apparatus  7  are used together, to feedback control the variable dispersion compensator  5 , while switching the monitored results of each monitoring apparatus using a monitor switching apparatus  8 . This type of construction is effective in the following condition. Namely, when the system is powered up, or in the case where the variation in the dispersion characteristics is significantly large, a condition is assumed in which the waveform distortion is so high that the optical dispersion monitoring apparatus of the present invention cannot operate normally. In such a condition, it is effective to feedback control the variable dispersion compensator  5  using the monitored result of the error monitoring apparatus  7  having a wide operating range for coarse control, and the monitored result of the optical dispersion monitoring apparatus of the present invention for fine control.