Patent Publication Number: US-2006007427-A1

Title: Optical transmission apparatus, method for controlling optical transmission system, and optical relay node equipped with wavelength control function

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is based on and hereby claims priority to Japanese Application No. 2004-199345 filed on Jul. 6, 2004 in Japan, the contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an optical transmission apparatus, a method for controlling an optical transmission system, and an optical relay node equipped with a wavelength control function.  
      2. Description of Related Art  
      In an optical transmission system, when wavelength dispersion is large in a transmission path, it is necessary to perform dispersion compensation. A typical dispersion compensation module is a fiber type module (a so-called DCM), but recently, dispersion compensation modules other than the fiber type module, such as a VIPA (Virtually Imaged Phased Array) type module, an etalon type module, an FBG (Fiber Bragg Grating) type nodule, and a waveguide resonance type module have been realized.  
      Among these, the VIPA type module or an etalon type module using an etalon filter is, in particular, a dispersion compensation device of great possibilities because of being capable of performing dispersion compensation with a simple and compact configuration and of making the amount of dispersion compensation variable. On the other hand, since the configuration needs to utilize resonance, characteristics that the pass band in which dispersion compensation can be performed becomes periodic and, at the same time, that the pass band width for each wavelength is limited (the band becomes narrow) are brought about. One example of the pass band characteristics of the VIPA type module is schematically shown in  FIG. 21 , in which the peak (central wavelength) of the pass band characteristics (hereinafter, simply referred to also as the “passing characteristics”) appears periodically at intervals as narrow as 50, 100, or 200 GHz (giga-hertz) as shown on the upper side in  FIG. 21 . By the way, group delay characteristics for wavelengths are shown on the lower side in  FIG. 21 , in which how the group delay shifts from zero as the wavelength shifts from the peak is shown.  
      Because of this, in a non-WDM (Wavelength Division Multiplexing) system, a periodic wavelength dispersion compensation module (hereinafter, referred to as a periodic dispersion compensation module or a dispersion compensation module of periodic type) such as the VIPA type module and the etalon type module using an etalon filter having characteristics that the pass band in which dispersion compensation can be performed is a narrow band and the peak of the transmittance appears repeatedly at predetermined intervals, is not used but a dispersion compensation module (DCM) having a wide pass band is generally used.  
      On the other hand, when a periodic dispersion compensation module is used in a WDM transmission system, since the pass band in which dispersion compensation can be performed is narrow and periodic, as described above, it is necessary to accurately stabilize not only the transmission wavelength of a light source (an optical transmitter) but also the transmission wavelength (the pass band characteristics) of the periodic dispersion compensation module to a grid wavelength λ Itu  (hereinafter, referred to as the ITU grid wavelength) being compliant with the ITU (International Telecommunication Union) standards. Because of this, as shown in  FIG. 20  for example, an optical transmitter  100  comprises, in addition to an LD module  101  that incorporates a light source (LD)  1010  such as a semiconductor laser diode and a wavelength variation detection circuit  1011  and an LD current control circuit  102 , a wavelength detection circuit  103 , an LD temperature control circuit  104 , etc., for stabilizing wavelength (wavelength lock). And the wavelength detection circuit  103  receives the wavelength variation information from the wavelength variation detection circuit  1011  to detect wavelength variations (errors) and the temperature of the light source  101  is controlled by the LD temperature control circuit  104  so that the detected errors are kept to a minimum (for example, a Peltier element provided in the light source  1010  is controlled). And thus stable matching of the transmission wavelength of the optical transmitter  100  to the corresponding ITU grid wavelength can be realized. On the other hand, the passing characteristics of a periodic dispersion compensation module  200  are also stabilized at the ITU grid wavelength by, for example, stabilizing temperatures.  
      As described above, it is possible to obtain stable dispersion compensation characteristics by accurately matching both the transmission wavelength of the optical transmitter  100  and the pass band characteristics of the periodic dispersion compensation module  200  to the ITU grid wavelength and stabilizing them. In this, in  FIG. 20 , the reference number  105  denotes an external modulator (for example, an LN modulator, etc.) that modulates light from the light source  101  using a transmission signal (data) but the external modulator is not necessary in the case of a direct modulation type. The bold solid line arrow denotes an electric signal line and the thin solid line arrow denotes a light signal line.  
      Conventional technologies relating to stabilization of wavelength further include those proposed in, for example, the following Patent Documents 1 to 3.  
      The technology proposed in Patent Document 1 provides a multi-wavelength stabilizer capable of stabilizing any one of a plurality of wavelengths that can be output from a tunable laser and also capable of widening a drawing band when the tunable laser is used as a spare. Because of this, the multi-wavelength stabilizer described in the Patent Document 1 comprises an interferometer that makes incident light rays interfere with each other in the period corresponding to double the interval between wavelengths of a channel in a WDM system and, at the same time, which outputs the interfering light from two ports by shifting the light half the period from each other, first and second detection means that detect the respective intensities of output light from the respective ports, and a control means that judges whether the channel fixed to a predetermined wavelength is an even number or an odd number and, at the same time, which controls based on the result of the judgment and the outputs of the respective detection means so that the output wavelength of a laser light source becomes a predetermined wavelength.  
      Therefore, in the multi-wavelength stabilizer, it is possible to fix the output wavelength of the laser light source to the predetermined wavelength by judging whether the channel fixed to the predetermined wavelength is an even number channel or an odd number channel and sending a control signal to the laser light source so that a detected value (PDo 1 /PDo 2 ), which is a ratio of the output of the first detection means (PDo 1 ) to the output of the second detection means (PDo 2 ), becomes a target value. Between even number channels or between odd number channels, the same value of PDo 1 /PDo 2  appears in the period double the channel wavelength interval, respectively, therefore, the drawing band of each channel can be widened to double the channel wavelength interval with the predetermined wavelength being at the center.  
      The technology described in Patent Document 2 relates to an optical transmission apparatus that uses an optical fiber grating (FBG) for dispersion compensation. The optical transmission apparatus arranges an FBG for narrow band dispersion compensation in a transmitter, and also arranges an FBG for dispersion compensation, in which its central wavelength has been set in advance so that the central wavelength is matched to the central wavelength of the transmitter side FBG at the central temperature for use, in a receiver. Then, on the transmission side, the wavelength of a transmission light source is stabilized at the central wavelength of the transmission side FBG by a wavelength stabilization circuit, and at the same time, dispersion compensation is performed. And on the reception side, by performing dispersion compensation using the reception side FBG, deterioration due to the SMP (self phase modulation) effect is suppressed. Moreover, by setting the wavelength band width of the transmission side FBG narrower than the wavelength band width of the reception side FBG, it is possible to control the transmission wavelength to within the reflection band of the reception side FBG even if temperatures change independently in the transmission and reception sides, and the wavelength band width required for the reception side FBG can also be reduced.  
      The technology described in Patent Document 3 relates to a method and a system capable of stabilizing wavelength with a simple configuration by using QCSE optical detectors that can serve as a filter and a detector at the same time. And optical currents of light emitted from a single light source are detected respectively by first and second QCSE optical detectors that operate on different bias voltages supplied thereto, and the output wavelength of the light source can be stabilized at a predetermined wavelength by controlling the light source so that the detected optical currents are matched to each other.  
      [Patent Document 1]
          Japanese Patent Laid-Open (Kokai) 2000-323784        

      [Patent Document 2]
          Domestic Re-publication of PCT International Publication No. WO97/34379        

      [Patent Document 3]
          Japanese Patent Laid-Open (Kokai) 2003-218461        

      However, as described above, the pass band for wavelength is limited (the band is narrow) in the periodic dispersion compensation module and it is necessary to accurately match the wavelength of the light source to that of the dispersion compensation module. Technologies for the accurate matching include stabilization of temperatures for a dispersion compensation module (VIPA or etalon filter) and incorporation of a wavelength lock function in a light source for stabilizing wavelengths. As a result, a problem arises in that the configurations of both light source and dispersion compensation module become complex and the cost is increased.  
      And in a non-WDM long-distance transmission system, a light source, in which the stabilization does not need to the ITU grid wavelength, is used, therefore, a problem is brought about in that a dispersion compensation module having the periodic pass band characteristics cannot be applied to such a system generally.  
      Moreover, in a WDM transmission system, as described above, it is inefficient for both sides of the light source and the dispersion compensation module to stabilize their wavelengths, and when a plurality of dispersion compensation modules are used in a system, more accurate stability of wavelength will be required. Furthermore, in a WDM long-distance transmission system, when periodic wavelength dispersion compensation modules are used at a plurality of optical relay nodes making up the system, it is necessary to individually stabilize the wavelength of the dispersion compensation modules at all of the nodes and the wavelength of the transmission light source. This is not preferable because the cost of the whole system is increased.  
      Each of the technologies described in the Patent Documents 1 and 3 relates to the stabilization of wavelength on the transmission side alone, therefore, the relationship between the transmission wavelength and the pass band characteristics of the dispersion compensation module is not at all taken into account. In contrast to this, the technology described in the Patent Document 2 is such one that stabilizes the transmission wavelength at the central wavelength of the transmission side narrow band FBG having the dispersion compensation function provided in the transmitter, therefore, it is unlikely that the configurations of the light source and the dispersion compensation module become complex but various problems will be brought about because the output wavelength of the light source is controlled.  
      In other words, to control the output wavelength of a light source, temperatures are generally controlled using a Peltier element, etc., but this not only increases power consumption but also imposes a large load on the light source depending on the variable width of the output wavelength, and as a result, the life of the light source is reduced and anomaly may be caused to occur. On the other hand, if the central emission wavelength is changed, there may be a case where unexpected output power variations may be caused to occur and the entire system may be adversely affected. Moreover, in the case of a WDM transmission system, as described above, the output wavelength of the light source is generally stabilized at the ITU grid wavelength, therefore, the technology described in the Patent Document 2, which changes the output central emission wavelength of the light source, cannot be applied to the system.  
     SUMMARY OF THE INVENTION  
      The present invention has been developed with the above-mentioned problems being taken into account and the object thereof is to make it possible to make the passing characteristics of a dispersion compensation module follow the output wavelength of a light source for highly stable matching only by means of the wavelength stabilization of the output wavelength of the light source without performing the respective wavelength stabilization of both the light source and the dispersion compensation module independently of each other by making it possible to match the peak of the passing characteristics of the dispersion compensation module to the output wavelength of the light source without controlling the output wavelength of the light source.  
      In order to achieve the above-mentioned object, an optical transmission apparatus according to the present invention is characterized in a manner that the optical transmission apparatus comprises an optical transmission section that outputs light having a certain wavelength, a wavelength deviation applying section that controls the optical transmission section to change the wavelength of the output light, an optical device that receives the output light from the optical transmission section and the device has transmission wavelength characteristics that the transmittance changes according to the wavelength of the received light, a monitor section that monitors the intensity of output light from the optical device, and a control means that controls the wavelength deviation applying section and which also controls the transmission wavelength characteristics of the optical device so that the amount of the change in the intensity of the output light from the optical device according to the change in the output light wavelength of the optical transmission section is kept to a minimum.  
      Another optical transmission apparatus according to the present invention is characterized in a manner that the optical transmission apparatus comprises 
          a light source that outputs light having a certain wavelength, a dispersion compensation module that compensates for the wavelength dispersion of light transmitted from the light source and whose passing wavelength characteristics can be controlled, and a control means that controls the passing characteristics of the dispersion compensation module so that the amount of the change in the intensity of light with respect to the change in the wavelength passing through the dispersion compensation module is kept to a minimum.        

      The control means can be composed of a wavelength deviation applying section that gives a wavelength deviation to the transmission light from the light source, a monitor section that monitors the intensity of light having passed through the dispersion compensation module, a detection section that detects the ratio of the amount of the change in the wavelength deviation given by the wavelength deviation applying section to the amount of the change in the intensity of light monitored by the monitor section and the sign thereof, and a dispersion-compensation-module passing-characteristics control section that controls the passing characteristics of the dispersion compensation module based on the ratio and sign detected by the detection section so that the amount of the change in the intensity is kept to a minimum.  
      And also, the dispersion compensation module can be composed of an etalon filter having a light incident surface whose light reflectance is less than 1 and a light reflection surface that reflects light that transmits the light incident surface and whose light reflectance is less than 1, or can be composed of a plurality of stacked etalon filters whose reflectance is less than 1.  
      Moreover, a method for controlling an optical transmission system according to the present invention, in the optical transmission system that provides a light source that transmits light having a certain wavelength, an optical transmission path that transmits light from the light source and a plurality of optical relay nodes interposed in the optical transmission path and having a disperse compensation module of passing characteristics variable type that compensates for the dispersion of the transmission light, is characterized in that the wavelength of the light source is shifted and the wavelength shift information is transferred to each of the optical relay nodes, and each of the optical relay nodes controls the passing characteristics of the dispersion compensation module based on light having passed through the dispersion compensation module of its own node and the transferred wavelength shift information so that the amount of the change in the intensity of light having passed through the dispersion compensation module is kept to a minimum.  
      And also, another method for controlling an optical transmission system according to the present invention, in the optical transmission system being a wavelength-multiplexed optical transmission system that provides a plurality of light sources that transmit light of different wavelengths, an optical transmission path that transmits light from each of the light sources as a wavelength-multiplexed light, and a plurality of optical relay nodes having a periodic dispersion compensation module having periodic passing characteristics that the pass band is a narrow band and the peak of the transmittance appears repeatedly at predetermined intervals and whose passing characteristics are variable, is characterized in that the wavelength of any one of the light sources having a standard wavelength is shifted and the wavelength shift information is transferred to each of the optical relay node, and each of the optical relay nodes controls the passing characteristics of the dispersion compensation module based on the output light of the standard wavelength from the dispersion compensation module of its own node and the transferred wavelength shift information of the standard wavelength so that the amount of the change in the intensity of the light having the standard wavelength having passed through the dispersion compensation module is kept to a minimum, and each of the optical relay nodes controls the transmission wavelength of the light sources other than that having the standard wavelength so that the amount of the change in the intensity of light having a wavelength other than the standard wavelength having passed through the dispersion compensation module is kept to a minimum.  
      Moreover, an optical relay node equipped with a wavelength control function according to the present invention is an optical relay node in a wavelength-multiplexed optical transmission system that provides a plurality of light sources that transmit light of different wavelengths, an optical transmission path that transmits light from each of the light sources as a wavelength-multiplexed light, and a plurality of optical relay nodes interposed in the optical transmission path. And the optical relay node is characterized in a manner that the optical relay node provides a periodic dispersion compensation module having periodic passing characteristics that the pass band is a narrow band and the peak of the transmittance appears repeatedly at predetermined intervals and whose passing characteristics are variable, a wavelength-shift-information reception means that receives the wavelength shift information given to any one of the light sources having a standard wavelength, and a control means that controls the passing characteristics of the dispersion compensation module based on the output light of the standard wavelength having passed through the periodic compensation module and the wavelength shift information received by the wavelength-shift-information reception means so that the amount of the change in the intensity of light having the standard wavelength having passed through the dispersion compensation module is kept to a minimum.  
      According to the present invention described above, excellent dispersion compensation characteristics can be obtained because it is possible to make the passing characteristics of a dispersion compensation module follow the output wavelength of a light source for highly stable matching only by means of the wavelength stabilization of the output wavelength of the light source without performing the respective wavelength stabilization of both the light source and the dispersion compensation module independently of each other. In particular, since the passing characteristics of a dispersion compensation module are changed without changing the central emission wavelength of a light source, not only the power consumption but also the load imposed on the light source can be reduced. Moreover, unexpected output power variations due to the change of the central emission wavelength of the light source can be prevented. When the present dispersion compensation system is applied to a WDM transmission system, it will be possible to make the passing characteristics of the dispersion compensation module follow the ITU grid wavelength for stabilization if the output wavelength of the light source is set and stabilized in advance in accordance with the ITU grid wavelength, therefore, application to the WDM transmission system is easy.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing a configuration of a dispersion compensation system (an optical transmission apparatus) in a first embodiment of the present invention;  
       FIG. 2  ( a ) is a diagram showing an example of group delay characteristics and passing characteristics for the wavelength of a dispersion compensation module of VIPA type as a periodic dispersion compensation module shown in  FIG. 1 ;  
       FIG. 2  ( b ) is a diagram showing an example of group delay characteristics and passing characteristics of a dispersion compensation module of etalon type as a periodic dispersion compensation module shown in  FIG. 1 ;  
       FIG. 3  ( a ) and  FIG. 3  ( b ) are diagrams each for explaining the operation principles of the dispersion compensation system shown in  FIG. 1 ;  
       FIG. 4  is a block diagram showing a configuration example in the case where the periodic dispersion compensation module shown in  FIG. 1  is of VIPA type;  
       FIG. 5  is a diagram for explaining a method for changing the wavelength setting of the dispersion compensation module of VIPA type shown in  FIG. 4 ;  
       FIG. 6  is a block diagram showing a configuration example in the case where the periodic dispersion compensation module shown in  FIG. 1  is of etalon type;  
       FIG. 7  is a block diagram showing another configuration example in the case where the periodic dispersion compensation module shown in  FIG. 1  is of etalon type;  
       FIG. 8  is a diagram for explaining a method for changing the wavelength setting of the dispersion compensation module of etalon type shown in  FIG. 6 ;  
       FIG. 9  is a block diagram showing a configuration in which a plurality of dispersion compensation modules of etalon type shown in  FIG. 6  is connected in tandem for a broader band;  
       FIG. 10  is a block diagram showing a modification example of the dispersion compensation system shown in  FIG. 1 ;  
       FIG. 11  is a block diagram showing another modification example of the dispersion compensation system shown in  FIG. 1 ;  
       FIG. 12  is a block diagram showing a configuration of a WDM transmission system in a second embodiment of the present invention;  
       FIG. 13  is a block diagram showing a configuration of an important part of a standard wavelength optical transmitter shown in  FIG. 12 ;  
       FIG. 14  is a block diagram showing a configuration of an important part of an optical relay node shown in  FIG. 12 ;  
       FIG. 15  is a diagram for explaining a method for matching the transmission wavelength of a non-standard wavelength optical transmitter to the central wavelength of each dispersion compensation module in the WDM transmission system shown in  FIG. 12 ;  
       FIG. 16  is a block diagram showing a configuration of an important part of the non-standard wavelength optical transmitter shown in  FIG. 15 ;  
       FIG. 17  is a block diagram showing a configuration of an important part of the optical relay node shown in  FIG. 15 ;  
       FIG. 18  is a flowchart for explaining a method for matching the central wavelength of each dispersion compensation module to the transmission wavelength of the standard wavelength optical transmitter in the WDM transmission system shown in  FIG. 12 ;  
       FIG. 19  is a flowchart for explaining a method for matching the transmission wavelength of the non-standard wavelength optical transmitter to the central wavelength of each dispersion compensation module in the WDM transmission system shown in  FIG. 12 ;  
       FIG. 20  is a block diagram for explaining a conventional wavelength stabilization technology; and  
       FIG. 21  is a diagram schematically showing an example of passing band characteristics and group delay characteristics for the wavelength of the conventional VIPA. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      [A] Explanation of First Embodiment  
       FIG. 1  is a block diagram showing a configuration of a dispersion compensation system (an optical transmission apparatus) in a first embodiment of the present invention. The dispersion compensation system shown in  FIG. 1  comprises an optical transmitter  1  which provides a light source (LD) unit  11  containing a light emitting element  111  such as a semiconductor laser diode (LD) and functioning as an optical transmission section for outputting light having a certain wavelength, an LD current control circuit  12 , an LD temperature control circuit  13 , an external modulator  14 , a modulation section (a repetitive signal generation section)  15 , a phase comparison section  16 , and a control section  17 , and further comprises a dispersion compensation module of periodic type  2 , a dispersion compensation amount setting section  3 , a light reception section  4 , and an optical coupler  5 . The above-mentioned phase comparison section  16  and control section  17  can be arranged inside the optical transmitter  1  or outside the optical transmitter  1  (for example, inside an optical relay node in which the dispersion compensation module of periodic type  2  is provided).  
      The dispersion compensation module of periodic type  2  has periodic passing characteristics for the wavelength (characteristics that the periodic transmittance peak appears repeatedly at predetermined intervals for the wavelength) and dispersion compensation modules having such passing characteristics include, for example, a VIPA type and a reflection filter type employing an etalon filter, etc. Similar to those shown in  FIG. 21 , the VIPA type has the group delay characteristics (shown on the upper side in  FIG. 2  ( a )) and the passing characteristics (shown on the lower side in  FIG. 2  ( a )) repeated at intervals such as 50, 100, and 200 GHz, and the etalon (the reflection filter) type has the group delay characteristics (shown on the upper side in  FIG. 2  ( b )) and the passing characteristics (shown on the lower side in  FIG. 2  ( b )) repeated at intervals of, for example, 0.8 nm (nano meters) for the wavelength.  
       FIG. 4  shows a configuration example of a periodic dispersion compensation module of VIPA type and  FIG. 6  shows that of an etalon type. The dispersion compensation module of VIPA or etalon type has transmission wavelength characteristics that the transmittance varies depending on the wavelength of the received light and functions as an optical device capable of controlling the transmission wavelength characteristics.  
      As shown in  FIG. 4 , the dispersion compensation module  2  of VIPA type comprises, for example, a line focus lens  21  that converges output light (collimated light) of the light source  11  into line, an angle dispersion element (a VIPA plate)  22 , both sides of which, that is, both sides of a thin plate having a film thickness t are coated with reflection films, and whose reflectance of the reflection film on the side in opposition to the line focus lens  21  is set to a value slightly less than 100%, and which produces output light having an outgoing angle in accordance with the wavelength of an incident light, a focus lens  23  that converges output light of the VIPA plate  22  into point, and a three-dimensional reflection mirror  24  having a three-dimensional curved shape on the light incidence side. As shown schematically in  FIG. 5 , by making variable the incident angle α of light incident on the VIPA plate, the central wavelength (the transmittance peak) of the periodic passing characteristics can be made variable, and the dispersion compensation amount can be made variable by translating the three-dimensional reflection mirror  24  (translation in the direction perpendicular to the plane of the paper on which  FIG. 4  is shown) to change the position of the mirror curved surface at which the light from the focus lens  23  converges, thereby varying the amount of the change in the optical path length of the reflected light in the wavelength band of the light incident on the three-dimensional mirror.  
      The period of the transmittance peak is determined based on the film thickness t described above. In  FIG. 4 , symbol  20   a  denotes an optical circulator that guides light from a port a to a port b, that is, to the VIPA plate  22  and light from the port b, that is, the reflected light from the VIPA plate  22  to a port c. Since the principles of dispersion compensation (action) by the dispersion compensation module  2  of VIPA type are widely known, no detailed explanation will be given here.  
      On the other hand, as shown in  FIG. 6 , the periodic dispersion compensation module  2  of etalon type can be comprised of, for example, a line conversion lens  25  and an etalon filter (a resonator of reflection type)  26 , on both sides of which, that is, on both sides of a thin plate having a film thickness t, reflection films R 1  and R 2  are formed. If the reflectance of the reflection film R 2  is set to 1 (total reflection), light having all the wavelengths is reflected, therefore, the periodic passing characteristics as shown in  FIG. 2  ( b ) cannot be obtained. In other words, the passing characteristics in this case appear flat in an idealistic situation. On the other hand, since phase characteristics are that the amount of the change in phase differs depending on the wavelength, the periodic group delay characteristics are brought about as a result. By the way, the period thereof is determined based on the film thickness t of the etalon filter  26 .  
      However, if the passing characteristics are flat as described above, the transmittance peak does not exist (in other words, there is no slope part of the passing characteristics), therefore, the control to match the wavelength of the light source unit  11  (the light emitting element  111 ) to the transmittance peak cannot be done any longer, as will be described later.  
      In the first embodiment, therefore, the reflectance of the reflection film R 2  is made less than 1 (substantially equal to 1) purposely so that a part of light will leak out. Due to this, there are produced wavelengths of light that leaks out and other wavelengths of light that does not leak out during the process of internal multiplex reflection depending on the film thickness t, therefore, the dispersion compensation module of periodic type  2  having the periodic passing characteristics for the wavelength as shown in  FIG. 2  ( b ) can be realized.  
      The dispersion compensation module of periodic type  2  having the periodic passing characteristics can also be realized by, as shown in  FIG. 7  for example, combining two etalon filters  26 - 1  and  26 - 2  or more etalon filters having a reflectance less than 1. In  FIG. 7 , the reflectance of the reflection film R 1  of the etalon filter  26 - 1  on the light incidence side is set to a value slightly greater than zero, that is, a value substantially equal to zero, the reflectance of the reflection film R 2  located on the boundary between the etalon filters  26 - 1  and  26 - 2  is set to a value less than 1, and the reflectance of the reflection film R 3  of the etalon filter  26 - 2  on the side in opposition to the reflection film R 2  is set to 1 (total reflection).  
      The setting of wavelength of the periodic dispersion compensation module  2  of etalon type can be performed by making variable the incident angle of light to the etalon filter  26  ( 26 - 1 ,  26 - 2 ) or by making variable the temperature of the etalon filter  26  ( 26 - 1 ,  26 - 2 ) to make the film thickness t variable and thereby, making variable the position of the reflection film R 2  (R 3 ), as shown schematically, for example, in  FIG. 8 .  
      Moreover, in the case of the etalon type, for example as shown in  FIG. 9 , it is possible to widen the band of the periodic passing characteristics by connecting in tandem a plurality of the dispersion compensation module  2  (the line conversion lens  25  and etalon filter  26 ) having the configuration shown in  FIG. 6  (or the dispersion compensation module  2  having the configuration shown in  FIG. 7 ) via the optical circulators  20   a . In this case, however, it is sufficient if there exists at least one of the reflection film R 2  whose reflectance is less than 1.  
      Next, in  FIG. 1 , the dispersion compensation amount setting section  3  sets the amount of dispersion compensation of the dispersion compensation module of periodic type  2 , and in the optical transmitter  1 , the light source unit  11  outputs light having a predetermined wavelength (ITU grid wavelength) by driving the light emitting element  111 , the LD current control circuit  12  supplies and controls the drive current for the light source unit  11  (light emitting element  111 ) (referred to simply as “light source  11 ”, hereinafter), and the LD temperature control circuit  13  is composed of elements such as a Peltier element and prevents shift in wavelength caused by variations in temperature by keeping the temperature of the light source  11  constant. The external modulator  14  modulates the output light of the light source  11  using the main signal (data) to be transmitted and, for example, an LN modulator or the like can be employed.  
      The modulation section (wavelength deviation applying section)  15  changes the output wavelength (transmission wavelength) of the light source  11 , in other words, applies a wavelength deviation of low frequency (slight modulation) by applying a repetitive signal of low frequency (for example, a sinusoidal wave signal of low frequency) to the LD temperature control circuit  13  to control the temperature by the LD temperature control circuit  13 . As described above, when a wavelength deviation of low frequency is applied to the output wavelength of the light source  11 , the passing loss (transmittance) of the dispersion compensation module of periodic type  2  changes and the wavelength deviation is converted into a modulation in intensity (change in intensity).  
      For example, if a case where the VIPA is used is taken as an example for explanation, when the output wavelength of the light source  11  is matched or substantially matched to the peak value (central wavelength) of the periodic passing characteristics of the periodic dispersion compensation module  2  (VIPA) [refer to the symbol A in  FIG. 3  ( a )], the output light of the periodic dispersion compensation module  2  changes into intensity-modulated light to which a slight modulation in intensity has been applied, as shown by the solid line A in  FIG. 3  ( b ). In contrast to this, when the output wavelength of the light source  11  is shifted toward longer wavelengths from the central wavelength of the above-mentioned passing characteristics as shown by the symbol B in  FIG. 3  ( a ), the output light of the periodic dispersion compensation module  2  changes into an intensity-modulated light in phase with the output light of the light source  11 , as shown by the solid line B in  FIG. 3  ( b ) And conversely, when the output wavelength of the light source  11  is shifted toward shorter wavelengths from the central wavelength as shown by the symbol C in  FIG. 3  ( a ), the output light of the periodic dispersion compensation module  2  changes into an intensity-modulated light in opposite phase with the output light, as shown by the dotted line C in  FIG. 3  ( b ).  
      Therefore, it will be possible to detect whether the output wavelength of the light source  11  is matched or substantially matched to the central wavelength of the passing characteristics of the dispersion compensation module  2  and whether the output wavelength is shifted toward longer wavelengths or shorter lengths with respect to the central wavelength by comparing the phase of light having passed through the periodic dispersion compensation module (referred to simply as a “dispersion compensator”, hereinafter)  2  with the phase of light before passing therethrough.  
      In the first embodiment, therefore, the light reception section (monitor section)  4  that receives (monitors) a part of the output light of the dispersion compensator  2  branched by the photo coupler  5 , and the phase comparison section  16  that compares the phase of a signal that applies a modulation to the output light of the light source  11  from the modulation section  15  with the phase of light monitored by the light reception section  4  are provided so that whether the output light of the dispersion compensator  2  is an intensity-modified light in phase or in opposite phase with the output light of the light source  11  can be detected. However, an object whose phase which is to be compared with the output light of the dispersion compensator  2  by the phase comparison section  16  can be the output light of the light source  11  before passing through the dispersion compensator  2 . In other words, the phase comparison section  16  functions as a detection section that detects the ratio of the amount of the change in the above-mentioned wavelength deviation applied by the modulation section  15  to the amount of the change in the intensity of light monitored by the light reception section  4  and the sign thereof (in phase/the opposite phase).  
      Then, the control section (dispersion compensator passing characteristics control section)  17  adaptively controls the passing characteristics of the periodic dispersion compensator  2  based on the detection results (the above-mentioned ratio and sign) by the phase comparison section  16 . For example, in the case where the dispersion compensator  2  is of VIPA type, the control section  17  controls the incident angle α of light to the VIPA plate  22  to a greater value so as to shift the passing characteristics (the transmittance peak) of the dispersion compensator  2  toward longer wavelengths, when “in phase” is the detection result of the phase comparison section  16 , as described in  FIG. 5 . On the other hand, when “the opposite phase” is the detection result, the control section  17  conversely controls the above-mentioned incident angle α to a smaller value so as to shift the passing characteristics toward shorter wavelengths. Thus, the control section  17  operates in such a manner that the amount of the change in the intensity of light with respect to the change in the wavelength passing through the dispersion compensator  2  is kept to a minimum [state shown by the symbol A in  FIG. 3  ( b )], in other words, in such a manner that the central wavelength of the passing characteristics of the dispersion compensator  2  is matched to the output wavelength of the light source  11 .  
      On the other hand, in the case where the dispersion compensator  2  of etalon type is used, the control section  17  operates in such a manner that, as in the case of the VIPA type, the central wavelength of the passing characteristic of the dispersion compensator  2  is matched to the output wavelength of the light source  11  by means of the shift of the passing characteristics (the transmittance peak) of the dispersion compensator  2  by controlling the incident angle of light to the etalon filter  26  ( 26 - 1 ,  26 - 2 ) or controlling the film thickness t by the temperature control of the etalon filter  26  ( 26 - 1 ,  26 - 2 ) based on the detection results of the phase comparison section  16 , as described above with reference to  FIG. 8 .  
      In other words, when “in phase” is the detection result by the phase comparison section  16 , the control section  17  shifts the passing characteristics (the transmittance peak) of the dispersion compensator  2  toward longer wavelengths by controlling the incident angle of light to the etalon filter  26  ( 26 - 1 ,  26 - 2 ) to a greater value or controlling the film thickness t to a greater value by the control of the temperature of the etalon filter  26  ( 26 - 1 ,  26 - 2 ) toward higher temperatures. On the other hand, when “opposite phase” is the detection result, the control section  17  shifts the passing characteristics of the dispersion compensator  2  toward shorter wavelengths and matches the central wavelength of the passing characteristics of the dispersion compensator  2  to the output wavelength of the light source  11  by conversely controlling the above-mentioned incident angle to a smaller value or controlling the film thickness t to a smaller value by the control of the temperature of the etalon filter  26  ( 26 - 1 ,  26 - 2 ) toward lower temperatures.  
      It is, therefore, possible to highly stably match the output wavelength of the light source  11  to the passing characteristics of the dispersion compensator  2  and obtain excellent dispersion compensation characteristics, because the central wavelength of the passing characteristics of the dispersion compensator  2  can be made to follow the output wavelength of the light source  11  for matching with each other. As a result, in the first embodiment also, when the dispersion compensation system is applied to a WDM transmission system, it will be possible to stabilize the passing characteristics of the dispersion compensator  2  at the ITU grid wavelength if the output wavelength of the light source  11  is set and stabilized in advance in accordance with the ITU grid wavelength.  
      By the way, it is possible to configure the dispersion compensation system shown in  FIG. 1  in the manner as shown in  FIG. 10 . In other words, in the system shown in  FIG. 1 , the output light of the light source  11  is modulated by the external modulator  14  using the main signal, but it is also possible to directly modulate the light source  11  (light emitting element  111 ) using the main signal as shown in  FIG. 10 . In this case also, as in the configuration employing the external modulator  14 , it will be possible for the control section  17  to make the central wavelength of the passing characteristics of the dispersion compensator  2  follow the output wavelength of the light source  11  for stabilization based on the phase comparison results by the phase comparison section  16 .  
      In other words, the light reception section  4 , the modulation section  15 , the phase comparison section  16 , and the control section  17  described above function as a wavelength lock mechanism (control means) that adjusts the passing characteristics of the dispersion compensator  2  adaptively in accordance with the output wavelength of the light source  11  and which stabilizes (locks) the output wavelength of the light source  11  in the vicinity of the passing characteristics peak of the dispersion compensator  2 , and control, to be more specific, the transmission wavelength characteristics of the dispersion compensator  2  by controlling the LD temperature control circuit  13  that serves as a wavelength deviation applying section so that the amount of the change in the intensity of the output light of the dispersion compensator  2  in accordance with the change in the output light wavelength of the light source  11  is kept equal to a predetermined threshold value or less.  
      Although a dispersion compensator having periodic characteristics is taken as an example for explanation of the first embodiment, examples are not limited to this, and it is obvious that the central wavelength in the transmission band can be controlled in accordance with the output wavelength of a light source by the method shown in the first embodiment provided that a dispersion compensator has characteristics that the passing characteristics change in the band near the central wavelength and the central wavelength in the transmission band can be controlled.  
      In the dispersion compensation system in the first embodiment, which is configured in such a manner as described above, a slight modulation (wavelength deviation) is applied in advance to the output light of the light source  11  from the modulation section  15 , the degree to which the output wavelength of the light source  11  is shifted from the central wavelength of the passing characteristics of the dispersion compensator  2  is detected by the phase comparison section  16  based on the comparison between the phase of the output light before passing through the dispersion compensator  2  and the phase after passing therethrough, and the passing characteristics of the dispersion compensator  2  are controlled adaptively for stabilization by the control section  17  in such a manner that the shift is eliminated.  
      Therefore, it is not necessary to stabilize the respective wavelengths of both the light source  11  and the dispersion compensator  2  independently of each other and excellent dispersion compensation characteristics can be obtained, because the passing characteristics of the dispersion compensator  2  can be made to follow the output wavelength of the light source  11  for highly stable matching by the wavelength stabilization of only the output wavelength of the light source  11 . Particularly, in the case of the first embodiment, the passing characteristics of the dispersion compensator  2  are changed by means of a mechanical control without changing the central emission wavelength of the light source  11 , therefore, it will be possible to reduce not only power consumption but also the load of the light source  11 . Moreover, it is also possible to prevent unexpected variations in output power due to the change of the central emission wavelength of the light source  11 .  
      When the dispersion compensation system is applied to a WDM transmission system, it will be possible to make the passing characteristics of the dispersion compensator  2  follow the ITU grid wavelength for stabilization if the output wavelength of the light source  11  is set and stabilized in advance in accordance with the ITU grid wavelength, therefore, application to the WDM transmission system is easy.  
      (A1) Explanation of Modification Example  
       FIG. 11  is a block diagram showing a modification example of the dispersion compensation system described above. The dispersion compensation system shown in  FIG. 11  differs from the system described above with reference to  FIG. 1  in that the control section  17  is configured so as to control the output wavelength of the light source  11 , instead of the output wavelength of the dispersion compensator  2 , based on the result of the phase comparison by the phase comparison section  16 .  
      In other words, when the result of the phase comparison by the phase comparison section  16  is “in phase”, the control section  17  in this modification example shifts the output wavelength of the light source  11  toward shorter wavelengths by reducing the temperature of the LD temperature control circuit  13  and, on the other hand, when the detection result is “opposite phase”, the control section  17  conversely shifts the output wavelength of the light source  11  toward longer wavelengths by raising the temperature of the LD temperature control circuit  13 , and thus the control section  17  operates so that the output wavelength of the light source  11  is matched to the central wavelength of the passing characteristics of the dispersion compensator  2 .  
      In other words, the light reception section  4 , the LD temperature control circuit  13 , the modulation section  15 , the phase comparison section  16 , and the control section  17  described above in this modification example function as a wavelength lock mechanism that stabilizes (locks) the output wavelength of the light source  11  in the vicinity of the passing characteristics peak of the dispersion compensator  2  by making the output wavelength of the light source  11  follow the passing characteristics of the dispersion compensator  2 .  
      In this case also, therefore, it is not necessary to stabilize the respective wavelengths of both the light source  11  and the dispersion compensator  2  independently of each other and excellent dispersion compensation characteristics can be obtained, because the output wavelength of the light source  11  and the passing characteristics of the dispersion compensator  2  can be matched each other highly stably by only the stabilization of the passing characteristics of the dispersion compensator  2 . Then, when the dispersion compensation system is applied to a WDM transmission system, it will be possible to stabilize the output wavelength of the light source  11  at the ITU grid wavelength if the passing characteristics of the dispersion compensator  2  are set and stabilized in advance in accordance with the ITU grid wavelength.  
      Moreover, in the dispersion compensator  2  of VIPA type, as described above, it is possible to make variable the peak (the central wavelength) of the periodic passing characteristics by changing, for example, the incident angle α of light to the VIPA plate  22 , the physics-optical length, that is, the film thickness t of the VIPA plate  22  (when there exists an air gap sandwiched by the mirrors making up the VIPA plate  22 , the gap length is changed), or by changing the index of refraction when there exists a dielectric between the mirrors making up the VIPA plate  22 , etc. In the dispersion compensator  2  of etalon type, on the other hand, it is possible to make variable the central wavelength of the periodic passing characteristics by changing the incident angle of light to the etalon filter  26  ( 26 - 1 ,  26 - 2 ) or the film thickness t. However, in this modification example, what is necessary is only to control the output wavelength of the light source  11 , therefore, it is not necessarily needed to make variable the periodic passing characteristics of the dispersion compensator  2 .  
      [B] Explanation of Second Embodiment  
       FIG. 12  is a block diagram showing a configuration of a WDM transmission system according to a second embodiment of the present invention. The WDM transmission system shown in  FIG. 12  comprises a transmission side terminal station node  10  having a plurality of optical transmitters  1  that each transmits light having a different wavelength respectively and a WDM coupler  5 ′ that multiplexes the wavelength of each output light of the respective optical transmitters  1  and outputs to an optical transmission path as WDM light, a reception side terminal station node  30  having a WDM coupler  6  that branches the WDM light from the optical transmission path for each wavelength and a plurality of optical receivers  7  that each receives the signal light of each wavelength branched by the WDM coupler  6 , and optical relay nodes (OADM nodes)  20 - 1  to  20 -N (N is an integer equal to or greater than 1) interposed in the transmission path, the number of which corresponds to a distance through which the WDM light is transmitted as it is between the terminal station nodes  10  and  30 . This distance is called as 3R span.  
      The optical relay nodes  20 - i  (i=1 to N) are each provided with an optical amplifier  8  such as an EDFA (Erbium Doped Fiber Amplifier) and a periodic dispersion compensator (DC)  2  of VIPA or etalon type, described above, and thereby, the WDM light is transmitted between the above-mentioned terminal station nodes  10  and  30  while being amplified at a time and dispersion is compensated for.  
      In the transmission side terminal station node  10 , an optical transmitter (a standard wavelength optical transmitter)  20 - 1  (in  FIG. 13 , optical transmitter  1 ) which is one of the optical transmitters  1  and which transmits light having a wavelength that serves as a standard when the central wavelength of the passing characteristics of the dispersion compensator  2  in each optical relay node  20 - i  is controlled in the same manner as that described in the first embodiment, comprises a light source  11  (light emitting element  111 ), an LD current control circuit  12 , an LD temperature control circuit  13 , and an external modulator  14 , which are the same as or similar to those already described above, and further comprises a wavelength offset setting section  18  and an OSC (optical service channel) transmission section  19   a , as shown, for example, in  FIG. 13 .  
      The wavelength offset setting section  18  changes the LD temperature of the light source  11  and applies an amount of wavelength offset Δλ m  (m is an integer variable equal to or greater than zero and increased by one each time the wavelength offset is given, as will be described later) to the output light of the light source  11  by giving a predetermined amount of wavelength offset (shift) (the initial value is zero) to the LD temperature control circuit  13 . And the OSC transmission section  19   a  provides a function to notify each optical relay node  20 - i  on the downstream side of information about the amount of wavelength offset Δλ m  by the wavelength offset section  18  as wavelength offset information through OSC.  
      On the other hand, each optical relay node  20 - i  comprises a dispersion compensator  2 , a dispersion compensation amount setting section  3 , a light reception section  4 , an optical coupler  5 , and a control section  17 , which are the same as or similar to those already described above, and further comprises a division circuit  16  that functions as the already-described phase comparison section  16  and a wavelength variable optical filter  9 , as shown, for example, in  FIG. 14 .  
      The wavelength variable optical filter  9  transmits only the light having the standard wavelength as monitor light among the output light (WDM light) of the dispersion compensator  2  and the monitor light is to be received by the division circuit  16  through the light reception section  4 . An OSC reception section (wavelength shift information reception means)  19   b  receives the wavelength offset information sent through OSC and supplies the information to the division circuit  16 .  
      The division circuit  16  detects the shift of the standard wavelength from the central wavelength of the passing characteristics of the dispersion compensator  2  corresponding to the standard wavelength based on the monitor light having the standard wavelength sent from the light reception section  4  and the wavelength offset information (the amount of wavelength offset Δλ m ) received by the OSC reception section  19   b , and the control section  17  controls the central wavelength of the dispersion compensator  2  based on the detection result so that the above-mentioned shift is eliminated.  
      Moreover, each optical relay node  20 - i  is also provided with an OSC transmission section  19   c  that notifies each optical relay node  20 - i  on the upstream side and the transmission side terminal station node  10  of information about the intensity of the output light (selected by the wavelength variable optical filter  9 ) whose wavelength is other than the standard wavelength of the dispersion compensator  2  received by the light reception section  4  through OSC, as shown, for example, in  FIG. 17 .  
      Next, an optical transmitter (a non-standard wavelength optical transmitter)  1  that transmits light having a wavelength other than the standard wavelength (denoted by a symbol  1 ′ for simplification of explanation, hereinafter) comprises, as shown, for example, in  FIG. 16 , a light source  11  (a light emitting element  111 ), an LD current control circuit  12 , an LD temperature control circuit  13 , and an external modulator  14 , which are the same as or similar to those already described above, and further comprises a division circuit  16   a  having a function equivalent to that of the already-described phase comparison section  16 , a control section  17   a  having a function equivalent to that of the already-described control section  17 , and an OSC reception section  19   d , and the wavelength offset amount Δλ m  can be set to the LD temperature control circuit  13  and the division circuit  16   a.    
      The OSC reception section  19   d  receives the intensity information about the standard wavelength transferred from the OSC transmission section  19   c  of the optical relay node  20 - i  on the downstream side via OSC and supplies the information to the division circuit  16   a . And the division circuit  16   a  detects the shift of the output wavelength of the light source  11  from the central wavelength of the passing characteristics of the dispersion compensator  2  corresponding to the output wavelength based on the intensity information and the wavelength offset amount Δλ m .  
      Then, the control section  17   a  makes the output wavelength of the light source  11  follow the central wavelength of the dispersion compensator  2  by controlling the temperature of the light source  11  using the LD temperature control circuit  13  in such a manner that the wavelength shift detected by the division circuit  16   a  is eliminated.  
      Due to the configuration described above, in the WDM transmission system according to the second embodiment, it will be possible to set the central wavelength of the standard wavelength of the periodic dispersion compensator  2  to the center of the transmission wavelength of the standard wavelength optical transmitter  1  as follows: the wavelength of the light source  11  of the standard wavelength optical transmitter  1  is shifted (offset) purposely and the information is transferred to each optical relay node  20 - i  via OSC; and in each optical relay node  20 - i , the power of light having the standard wavelength after having passed through the periodic dispersion compensator  2  is monitored, the ratio of the wavelength offset amount to the amount of the change in the intensity and the magnitude thereof are calculated, and the calculated ratio of the setting of the central wavelength to the standard wavelength of the periodic dispersion compensator  2  is adjusted to become smaller. Moreover, after the above-mentioned adjustment of the standard wavelength, it will be possible to make settings so that the transmission wavelengths of the respective light sources  11  of the other non-standard wavelength optical transmitters  1 ′ can be matched to the central wavelength of the passing characteristic of the dispersion compensator  2 , respectively.  
      A detailed procedure is explained below with reference to  FIG. 18  and  FIG. 19 .  
      First, the action to match the central wavelength of the dispersion compensator  2  of each optical relay node  20 - i  to the output wavelength (the standard wavelength) of the standard wavelength optical transmitter  1  in a WDM transmission system is explained (refer to  FIG. 18 ). In the following explanation, the variable k represents, as shown in  FIG. 12 , a set counter value that is increased by one each time the node  20 - i  (having the periodic dispersion compensator  2 ), the wavelength of which is the adjustment target, moves to the next one on the downstream side, where the initial value is zero.  
      As shown in  FIG. 18 , in step S 1  for initial settings, the LD temperature of the light source  1  of the standard wavelength optical transmitter  1  is set to the initial value by the LD temperature control circuit  13  [at this time, the wavelength offset amount Δλ m  (m=0) is zero] (step S 1 - 1 ), and the fact that the wavelength offset amount Δλ m  is zero is transferred from the upstream side to each optical relay node  20 - i  on the downstream side by, for example, the OSC transmission section  19   a  via OSC (step S 1 - 2 ).  
      In the optical relay node  20 - 1 , in step S 2  for wavelength adjustment, a part of light having passed through the periodic dispersion compensator  2  is branched by the optical coupler  5 , only the component of the standard wavelength is extracted by the wavelength variable optical filter  9  (step S 2 - 1 ), and the intensity of the extracted component of the standard wavelength is monitored by the light reception section  4  (step S 2 - 2 ). At this time, the modulated component of the main signal is averaged.  
      When k=0 and the initial state m=0, the optical relay node  20 - 1  records the intensity of the monitored standard wavelength as I 0, 0  (that is, the initial value of power at the first node is recorded) (step S 2 - 3 ) When k=0 and m≠0 (that is, in a state in which the wavelength offset has been given once or more times), the intensity of the standard wavelength monitored by the light reception section  4  is recorded as I m, 0  (the value at the first node when the wavelength is shifted is recorded) (step S 2 - 4 ).  
      After this, when k=0, the present block (step S 2  for wavelength adjustment) is quitted (step S 2 - 5 ), m is increased by 1 (m←m+1) (step S 3 ) and, at the same time, k is increased by 1 (step S 4 ) and step S 5  for wavelength offset is performed in the standard wavelength optical transmitter  1  by the wavelength offset setting section  18 . In other words, the LD temperature of the light source  11  is changed, the offset Δλ m  is given to the current value of the standard wavelength (step S 5 - 1 ) and the wavelength offset amount Δλ m  is transferred from the upstream side to each optical relay node  20 - i  by the OSC transmission section  19   a  via OSC (step S 5 - 2 ).  
      Next, step S 2  for wavelength adjustment is performed again but now k=1, therefore, in the first optical relay node  20 - 1 , the amount of the change ΔI, which is caused by the wavelength offset, in the intensity of light having the standard wavelength is obtained by the following expression (1) (step S 2 - 6 ). In the following expression (1), L=1 to N−1: 
 
α I=I   m, N −Σ( I   m, L   −I   m−1, L )− I   m−1, N   (1) 
 
 where N=1. 
 
      Then, the wavelength offset amount Δλ m  transferred via OSC is obtained (step S 2 - 7 ) and when not in the initial state (that is, m≠0), the amount of the change from the wavelength offset amount of the immediately previous one, that is Δλ m −Δλ m−1 , is obtained (step S 2 - 8 ), and when k=N, the ratio of the amount of the change in the intensity to the amount of the change in the wavelength offset amount R m =ΔI/(Δλ m −Δλ m−1 ) is obtained by the division circuit  16  (step S 2 - 9 ).  
      As a result, when R m &gt;0 (in phase), the control section  17  shifts the central wavelength of the periodic dispersion compensator  2  toward longer wavelengths and conversely, when R m &lt;0 (opposite phase) the central wavelength is shifted toward shorter wavelengths (step S 2 - 10 ). Due to this, it will be possible to match the central wavelength (the peak) of the passing characteristics of the dispersion compensator  2  in the optical relay node  20 - 1  to the wavelength of the light source  11  of the standard wavelength optical transmitter  1 .  
      After this, it will be possible to match the central wavelength of each dispersion compensator to the wavelength of the light source  11  sequentially from the upstream side by increasing m and k by 1 for the other optical relay nodes  20 - 2  to  20 - n  (steps S 3  and S 4 ) and performing step S 5  for wavelength offset and step S 2  for wavelength adjustment (calculation is performed using the expression (1) on the assumption that k=N).  
      Next, the action to match the transmission wavelength of the optical transmitters  1 ′ (the non-standard wavelength optical transmitters) having a wavelength (a channel) other than the standard wavelength to the central wavelength of each dispersion compensator  2  that has been matched to the standard wavelength as described above is explained below (refer to  FIG. 19 ).  
      As shown in  FIG. 19 , each non-standard wavelength optical transmitter  1 ′ sets the LD temperature of the light source  11  to the initial value using the LD temperature control circuit  13  as step S 6  for initial settings [at this time, the wavelength offset amount Δλ m  (m=0) is zero] (step S 6 - 1 ), and the fact that the wavelength offset amount Δλ m  (m=0) is zero is transferred from the upstream side to each optical relay node  20 - i  on the downstream side by, for example, the OSC transmission section  19   a  via OSC (step S 6 - 2 ).  
      In each optical relay node  20 - 1 , in step S 7  for wavelength adjustment, first, a part of light having passed through the periodic dispersion compensator  2  is branched by the optical coupler  5 , and only the components of the wavelengths, which are the adjustment targets having a wavelength other than the standard wavelength, are extracted by the wavelength variable optical filter  9  (step S 7 - 1 ), and the intensity of the extracted component of the standard wavelength is monitored by the light reception section  4  (step S 7 - 2 ). At this time, the modulated component of the main signal is averaged.  
      Then, when k=0 and the initial state m=0, the optical relay node  20 - 1  records the intensity of the monitored standard wavelength as I 0, 0  (that is, the initial value of power at the first node is recorded) (step S 7 - 3 ). When k=0 and m≠0 (that is, in a state in which the wavelength offset has been given once or more times), the intensity of the standard wavelength monitored by the light reception section  4  is recorded as I m, 0  (the value at the first node when the wavelength is shifted is recorded) (step S 7 - 4 ).  
      After this, when k=0, the present block (step S 7  for wavelength adjustment) is quitted (step S 7 - 5 ), m is increased by 1 (m←m+1) (step S 8 ) and, at the same time, k is increased by 1 (step S 9 ) and step S 10  for wavelength offset is performed in the non-standard wavelength optical transmitter  1 ′ by the wavelength offset setting section  18 . In other words, the LD temperature of the light source  11  is changed, the offset Δλ m  is given to the current value of the transmission wavelength (step S 10 - 1 ) and the wavelength offset amount Δλ m  is transferred from the upstream side to each optical relay node  20 - i  by the OSC transmission section  19   a  via OSC (step S 10 - 2 ).  
      Next, step S 7  for wavelength adjustment is performed again but now k≠0, therefore, in the k-th optical relay node  20 - k , the amount of the change ΔI, which is caused by the wavelength offset, in the intensity of light having the standard wavelength is obtained by the following expression (2) (step S 7 - 6 ) In the following expression (2), L=1 to N−1: 
 
Δ I=I   m, N −Σ( I   m, L   −I   m−1, L )− I   m−1, N   (2) 
 
      Then, the optical relay node  20 - 1  notifies the non-standard wavelength optical transmitter  1 ′ of information about the amount of the change ΔI from the OSC transmission section  19   c  via OSC (step S 7 - 7 ).  
      In the non-standard wavelength optical transmitter  1 ′, if not in the initial state (m=0), the amount of the change from the wavelength offset amount of the immediately previous one, that is Δλ m −Δλ m−1 , is obtained (step S 7 - 8 ), and when k=N, the ratio of the amount of the change in the intensity to the amount of the change in the wavelength offset amount R m =ΔI/(Δλ m −Δλ m−1 ) is obtained by the division circuit  16   a  (refer to  FIG. 16 ) (step S 7 - 9 ).  
      As a result, when R m &gt;0 (in phase), the control section  17   a  shifts the transmission wavelength of light source  11  toward shorter wavelengths by controlling the LD temperature using the LD temperature control circuit  13  and conversely, when R m &lt;0 (opposite phase), the transmission wavelength of the light source  11  is shifted toward longer wavelengths (step S 7 - 10 ). Due to this, it will be possible to match the transmission wavelength of the light source  11  in the non-standard wavelength optical transmitter  1 ′ to the central wavelength (the peak) of the passing characteristics of the dispersion compensator  2  in the optical relay node  20 - 1 .  
      After this, it will be possible to match the transmission wavelength (the channel other than the standard wavelength) of the light source  11  in each non-standard wavelength optical transmitter  1 ′ to the peak of the passing characteristics of the dispersion compensator  2  in each node  20 - k  by increasing m and k by 1 (steps S 8  and S 9 ) and by performing step S 10  for wavelength offset and step S 7  for wavelength adjustment (refer to  FIG. 15 ).  
      As described above, according to the second embodiment, it will be possible to match the peak of the passing characteristics of the dispersion compensator  2  in each optical relay node  20 - k  to the center of the transmission wavelength of the light source  11  in a system having a plurality of the periodic dispersion compensators  2  in a WDM transmission system as follows: the wavelength of the light source  11  is shifted purposely and the information is transferred to each optical relay node  20 - k ; and in each optical relay node  20 - k , the power of light after having passed through the periodic dispersion compensator  2  in the node  20 - k  is monitored, the ratio of the wavelength shift amount to the amount of the change in the intensity and the magnitude thereof are calculated, the wavelength setting of the periodic dispersion compensator  2  is adjusted so that the calculated ratio of the change in intensity becomes smaller.  
      As a result, even when many dispersion compensators  2  are used in a system such as in a WDM long-distance transmission system, it is not necessary to adjust the central wavelength of the dispersion compensator  2  for each node  20 - k  and the cost of the whole system can be reduced. Particularly, in the case of the second embodiment, the central wavelength of the dispersion compensator  2  is adjusted sequentially from the node  20 - k  on the upstream side, therefore, a more accurate wavelength setting can be realized.  
      Moreover, in the process described above, adjustment is first made for the standard wavelength and after the matching of the central wavelength of the dispersion compensator  2  was performed, the transmission wavelength of the light source  11  of other channels are adjusted so as to be matched to the passing characteristics of the dispersion compensator  2 , therefore, it is possible to highly accurately match the transmission wavelength of the light source  11  of each channel and the peak of the passing characteristics of each dispersion compensator  2  to the ITU grid wavelength, and also to realize excellent dispersion compensation characteristics in a WDM long-distance transmission system.  
      The present invention is not limited to each of the embodiments described above, and it is obvious that various modifications can be done without departing from the scope of the present invention.  
      As described above, according to the present invention, it is not necessary to stabilize the respective wavelengths of both the light source and the dispersion compensator independently of each other, and excellent dispersion compensation characteristics can be obtained because the passing characteristics of the dispersion compensator can be made to follow the output wavelength of the light source for highly stable matching only by the wavelength stabilization of the output wavelength of the light source, and therefore, it can be thought that the present invention is extremely useful in the optical transmission technology field.