Abstract:
An optical transmission device includes: an attenuator that attenuates an optical signal from an adjacent optical transmission device; an optical element that is arranged downstream of the attenuator; a detector that detects a change in a characteristic of a transmission path; and a controller that adjusts, when the change is detected, an attenuation of the attenuator to keep the level of the optical signal input to the optical element at a predetermined level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-288724, filed on Sep. 30, 2005, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a technology for adjusting an optical level of a transmission signal to an optimum level in an optical transmission system. 
   2. Description of the Related Art 
   Recently, the optical transmission system for performing transmission of optical signals with a high transmission rate by using an optical fiber for a transmission path, multiplexed by wavelength division multiplexing (WDM), and capable of increasing the information capacity has been popularized and used, instead of electric signals.  FIG. 9  is an explanatory diagram of a configuration example of the optical transmission system. 
   In an optical transmission system  900 , optical add-and-drop multiplexers (OADMs) A, B, E, and D, and in-line amps (ILAs) C and F are provided on a transmission path including an outer ring (upward ring)  910  and an inner ring (downward ring)  920 . Transceivers  901 A,  901 B,  901 D and  901 E are connected to the OADMs A, B, D, and E, respectively, and transmission and reception of optical signals can be performed with an optional communication partner, by adding, dropping, or transmitting transmission light transmitted through the outer ring  910  and the inner ring  920 . The ILAs C and F amplify a WDM beam transmitted through the outer ring  910  and the inner ring  920 . The light transmitted in the optical transmission system  900  is formed of the WDM beam obtained by multiplexing the optical signal and an optical supervisory channel (OSC) beam for supervising the transmission state of the optical signal. 
   In the optical transmission system  900 , it is important to adjust the optical level of the optical signal constituting the WDM beam to an appropriate value by the OADMs A, B, D, and E and the ILAs C and F to transmit the optical signal through the outer ring  910  and the inner ring  920 . 
   As the conventional art relating to the adjustment of the optical level, there is a structure in which in the wavelength multiplexing optical transmission, substantially equal optical output can be obtained in each wavelength, thereby enabling insertion of an optical functional part into an intermediate portion, regardless of the level and the wavelength of the optical signal input to an optical fiber amplifier. In this case, it is important to avoid occurrence of optical surge and determine a connection of parts. Therefore, a technique is disclosed in which feedback control is performed by inserting a variable attenuator in an optical input unit, so that the optical input to the amplifying optical fiber becomes constant. Furthermore, control for changing the overall optical output and optical input to the amplifying optical fiber is performed based on the wavelength information obtained from a supervisory signal, and light to the intermediate optical part and light from the optical part are detected, and when there is no part, pumping is suppressed. By performing such control, occurrence of optical surge at the time of connection can be avoided, and a signal indicating that an optical part is not connected is output (see, for example, Japanese Patent Application Laid-Open No. H11-17259). 
   There is another example in which an optical wavelength multiplexing network can be easily formed. In this technique, it is important to keep constant an optical signal level for each channel, to maintain desired transmission quality. Therefore, a supervisory signal transmitted through the optical fiber transmission path is extracted by a WDM coupler, to obtain the wavelength of the optical signal input to a remote node from the supervisory signal. A feedback controller calculates the wavelength information, which is the sum of the wavelength obtained from the supervisory signal and the wavelength of an optical signal newly added at the remote node, via a supervisory signal processing circuit. Furthermore, by adjusting an attenuation of the variable optical attenuator so that a value obtained by dividing the total optical power of an optical amplifier by the value of wavelength becomes the desired optical power of the optical signal for each channel, feedback control is performed at all times with respect to the attenuation of the variable optical attenuator, to compensate loss fluctuation in the optical fiber transmission path (see, for example, Japanese Patent Application Laid-Open No. 2004-147122). 
   Conventionally, the control of the optical level of the optical signal is performed at the time of startup of the optical transmission system, as in Japanese Patent Application Laid-Open Nos. H11-17259 and 2004-147122. The attenuation of a reception unit is adjusted to control to the optical level to an optimum level, based on the wavelength information of the WDM beam obtained by the OSC controller equipped in the OADMs A, B, D, and E and the ILAs C and F shown in  FIG. 9 . 
   An example of the method of adjusting the optical signal level at the time of startup (activation) of the OADM or the ILA is shown below.  FIG. 10  is an explanatory diagram of a startup procedure of the optical transmission system. A reception unit  1010  includes a variable optical attenuator (VOA)  1011 , a front photodiode (PD)  1014  arranged upstream of the VOA  1011 , a rear PD  1015  arranged downstream of the VOA  1011 , an OSC branch coupler  1012 , and a preamp  1013 . A transmission unit  1050  includes a postamp  1051 , an OSC combination coupler  1052 , and a 1×2 switch (SW)  1054 . The reception unit  1010  and the transmission unit  1050  further include unit controllers  1016  and  1053 , respectively. The unit controller  1016  of the reception unit  1010  adjusts the attenuation of the VOA  1011  based on optical levels detected by the front PD  1014  and the rear PD  1015 , to control the optical level of the optical signal input to the preamp  1013 . The unit controller  1016  of the reception unit  1010  and the unit controller  1053  of the transmission unit  1050  are connected to an OSC controller  1060  (for convenience, it is written as “OSC” in the drawings, as well as an OSC controller explained below), to adjust the attenuation of the VOA  1011  at the time of startup. 
   An OR  1061  and an OS  1062  includes a unit controller  1063 , an optoelectronic converter (OE)  1064 , and an electro-optic converter (EO)  1065 . The unit controller  1063  controls the OSC controller  1060 . The OE  1064  converts an input optical signal to an electric signal and output the electric signal. The EO  1065  converts an input electric signal to an optical signal and output the optical signal. 
   The startup procedure of the OADM B connected to the outer ring  910  and the inner ring  920  is explained next. The startup of the OADM B is performed by transmitting the OSC beam between adjacent optical transmission devices (that is, OADMs A and B in the example shown in  FIG. 10 ). 
   At first, an output request of amplified spontaneous emission (ASE) beam for optical level control is output from the unit controller  1063  in the OSC controller  1060  of the OADM B to the unit controller  1016  of the OADM B and the unit controller  1016  of the OADM A (S 1 ). The optical level of the ASE beam requested at this time corresponds to one wavelength level of the optical signal. In response to the output request of the ASE beam, a 1×2 switch (SW)  1017  arranged upstream of the preamp  1013  in the OADM B is controlled to open, so that the optical signal from the OADM B is not sent out to the transmission path, thereby shutting down the input light to the OADM B. 
   Subsequently, communication confirmation of the OSC beam is performed in the EO  1065  of the OADM A and the OE  1064  of the OADM B (S 2 ). The postamp  1051  having received the output request of the ASE beam outputs the ASE beam of a level corresponding to one wavelength of the optical signal (S 3 ). At this time, a 1×2 SW  1054  arranged upstream of the postamp  1051  in the OADM A is controlled to open. 
   When the ASE beam is input to the reception unit  1010  of the OADM B via the outer ring  910  (S 4 ), and further input to the unit controller  1016  via the VOA  1011 , auto-adjustment of the VOA  1011  is carried out (S 5 ). Specifically, to make the input light of the preamp  1013  at an appropriate level, the unit controller  1016  in the OADM B monitors the light-receiving power of the rear PD  1015  arranged upstream of the preamp  1013 , and adjust the VOA  1011  to have an appropriate attenuation. 
   When the auto-adjustment of the VOA  1011  has finished, the unit controller  1016  in the OADM B determines that the input to the preamp  1013  becomes stable, to release the shut-down state of the preamp  1013  in the OADM B (S 6 ), and starts up the preamp  1013  by automatic level control (ALC). 
   When having confirmed that the preamp  1013  has been started up, and shifted to automatic gain control (AGC), the unit controller  1016  in the OADM B suspends the output request of the ASE beam for optical level control from the unit controller  1063  (S 7 ). When the output of the ASE beam from the postamp  1051  has stopped, the unit controller  1053  closes the 1×2 SW  1054  arranged upstream of the postamp  1051  in the OADM A, to release the shut-down state of the postamp  1051 , and starts the operation thereof. 
   The auto-adjustment of the VOA  1011  carried out at S 5  in  FIG. 10  indicates a process for adjusting the optical level of the optical signal input to the preamp  1013  (the ASE beam at the time of startup) to be within a dynamic range of the preamp  1013 . 
   After the startup operation as described above, the OADM A and the OADM B are in a normal operation state. The VOA  1011  fixes the attenuation for one wavelength of the optical signal, and the preamp  1013  carries out automatic gain control (AGC) to control the gain of the multiplexed optical signal to be equalized. This is because in the optical transmission system  900 , it is assumed that the wavelengths of optical signals multiplexed in the WDM beam on the transmission path changes corresponding to the communication state. Therefore, even at the time of increase or decrease in the wavelengths of optical signals, the OADMs A and B can keep the level of the optical signal at an appropriate level. 
   However, even in the optical transmission system  900  in which optical level control is carried out by the OADMs A, B, D, and E and the ILAs C and F, if bending or an excessive temperature change occurs in the transmission path (for example, the outer ring  910  or the inner ring  920 ) itself, the transmission characteristic of the transmission path changes, thereby affecting the optical level of the WDM beam. When the transmission characteristic has changed, the attenuation fixed at the time of startup is attenuated as usual in the VOA  1011 , since there is no change in the wavelengths of the optical signals multiplexed in the WDM beam. 
   As a result, the WDM beam, whose optical level has changed as compared with the optical level at the time of startup or at the time of normal operation, due to a change in the transmission characteristic of the transmission path, is input to the OADMs A, B, D, and E and the ILAs C and F. Such a WDM beam is output directly to the transmission path, with the change in the optical level being not corrected. When the optical level changes of the WDM beam are accumulated, the changes cannot fall within the dynamic range of the pre-designed input level to the OADMs A, B, D, and E and the ILAs C and F, thereby causing an error. 
   When the dynamic range of the input level is designed to be large, taking the changes in the transmission characteristic into consideration, the production cost of the OADMs A, B, D, and E and the ILAs C and F increases. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to at least solve the problems in the conventional technology. 
   An optical transmission device according to an aspect of the present invention adjusts a level of an optical signal from an adjacent optical transmission device that is arranged upstream of the optical transmission device on a transmission path. The optical transmission device includes: an attenuator that attenuates the optical signal; an optical element that is arranged downstream of the attenuator; a detector that detects a change in a characteristic of the transmission path; and a controller that adjusts, when the change is detected, an attenuation of the attenuator to keep the level of the optical signal input to the optical element at a predetermined level. 
   A method according to another aspect of the present invention is a method for an optical transmission device to adjust a level of an optical signal from an adjacent optical transmission device that is arranged upstream of the optical transmission device on a transmission path. The method includes: attenuating the optical signal by an attenuator; detecting a change in a characteristic of the transmission path; and adjusting, when the change is detected, an attenuation of the attenuator to keep the level of the optical signal after being attenuated by the attenuator at a predetermined level. 
   A computer-readable recording medium according to still another aspect of the present invention stores a computer program for an optical transmission device to adjust a level of an optical signal from an adjacent optical transmission device that is arranged upstream of the optical transmission device on a transmission path. The computer program causes the optical transmission device to execute: attenuating the optical signal by an attenuator; detecting a change in a characteristic of the transmission path; and adjusting, when the change is detected, an attenuation of the attenuator to keep the level of the optical signal after being attenuated by the attenuator at a predetermined level. 
   The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an explanatory diagram of the configuration of an optical add-and-drop multiplexer (OADM); 
       FIG. 2  is an explanatory diagram of the configuration of an optical transmission device according to a first embodiment of the present invention; 
       FIG. 3  is an explanatory diagram of the configuration of an optical transmission device according to a second embodiment of the present invention; 
       FIG. 4  is an explanatory diagram of the configuration of an optical transmission device according to a third embodiment of the present invention; 
       FIG. 5  is an explanatory diagram of the configuration of an optical transmission device according to a fourth embodiment of the present invention; 
       FIG. 6  is an explanatory diagram of the configuration of an optical transmission device according to a fifth embodiment of the present invention; 
       FIG. 7  is an explanatory diagram of the configuration of an optical transmission device according to a sixth embodiment of the present invention; 
       FIG. 8  is an explanatory diagram of the configuration of an optical transmission device according to a seventh embodiment of the present invention; 
       FIG. 9  is an explanatory diagram of a configuration example of an optical transmission system; and 
       FIG. 10  is an explanatory diagram of a startup procedure of the optical transmission system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of the present invention are explained in detail with reference to the accompanying drawings. 
     FIG. 1  is an explanatory diagram of the configuration of an optical add-and-drop multiplexer (OADM) according to the present invention. The OADM A shown in  FIG. 1  constitutes the optical transmission system  900  shown in  FIG. 9 . As shown in  FIG. 1 , in the OADM A, the reception unit  1010 , a demultiplexer  1020 , an add/drop unit  1030 , a multiplexer  1040 , and the transmission unit  1050  are respectively provided for the outer ring  910  and the inner ring  920 . Furthermore, in the OADM A, the OSC controller  1060 , a controller  1070 , and a converter  1080 , which function both for the outer ring  910  and the inner ring  920 , are provided. 
   The reception unit  1010  includes the VOA  1011 , the OSC branch coupler  1012 , and the preamp  1013 . The transmitted light input from the outer ring  910  or the inner ring  920  is attenuated by the VOA  1011 , and then branched to the WDM beam and the OSC beam by the OSC branch coupler  1012 . 
   The OSC beam branched by the OSC branch coupler  1012  is input to the OR  1061  in the OSC controller  1060 . When the input OSC beam indicates a normal optical transmission state, the OSC controller  1060  is turned to a waiting state. On the other hand, when the input OSC beam indicates abnormality in the transmission path, or a change in the transmission state such as an increase or decrease in the wavelengths of optical signals multiplexed in the WDM beam, the OSC controller  1060  outputs an instruction to the controller  1070  to handle the situation by performing a process corresponding to the changed situation. 
   The OSC beam branched by the reception unit  1010  in the outer ring  910 , and instructing a process corresponding to the OSC beam input to the OSC controller  1060  is output to the transmission unit  1050  in the inner ring  920 . Likewise, the OSC beam branched by the reception unit  1010  in the inner ring  920 , and instructing a process corresponding to the OSC beam input to the OSC controller  1060  is output to the transmission unit  1050  in the outer ring  910 . This is for transmitting the OSC beam, which is a control signal, to the optical transmission unit (OADM, ILA) on a preceding stage by outputting the OSC beam to the ring network in the opposite direction. As a specific use example of the OSC beam, it can be used at the time of a startup process of the optical transmission system  900  explained with reference to  FIG. 10 . 
   The WDM beam branched by the OSC branch coupler  1012  is input to the preamp  1013 . The WDM beam is amplified by the preamp  1013 , and output to the demultiplexer  1020 . The demultiplexer  1020  branches the input WDM beam for each wavelength, and output to the add/drop unit  1030 . 
   The add/drop unit  1030  drops an optical signal addressed to the transceiver  901 A from the optical signal for each wavelength input from the demultiplexer  1020 , and outputs the optical signal to the converter  1080 . Optical signals other than the optical signal dropped by the add/drop unit  1030  are transmitted directly, and input to the multiplexer  1040 . On the other hand, the optical signal input from the transceiver  901 A is added to the add/drop unit  1030  via the converter  1080 . The newly added optical signal is output to the multiplexer  1040 . The multiplexer  1040  couples the optical signals input from the add/drop unit  1030  by transmission or addition, and outputs the optical signals as one WDM beam to the transmission unit  1050 . 
   The converter  1080  includes a 2□1 switch  1081 , an optoelectronic converter (O/E)  1082 , an electro-optic converter (E/O)  1083 , and a 1□2 coupler  1084 . When the optical signal addressed to the transceiver  901 A is included in the WDM beam of the transmitted light flowing on the outer ring  910  or the inner ring  920 , the optical signal is input to the converter  1080  from the add/drop unit  1030 . The optical signal input to the converter  1080  is selected by the 2□1 switch  1081 , and output to the O/E  1082 . The O/E  1082  converts the input optical signal to an electric signal and output the electric signal to the transceiver  901 A. 
   When the optical signal is transmitted from the transceiver  901 A to the transceiver  901 B, an electric signal is input to the E/O  1083  in the converter  1080 . The input electric signal is converted to an optical signal by the E/O  1083 , and output to the 1□2 coupler  1084 . The 1□2 coupler  1084  branches the optical signal input from the E/O  1083  into two signals, and outputs the respective optical signals to the add/drop unit  1030  for the outer ring  910  and the add/drop unit  1030  for the inner ring  920 . 
   The transmission unit  1050  includes the postamp  1051  and the OSC combination coupler  1052 . The WDM beam input from the multiplexer  1040  is input to the postamp  1051 . The postamp  1051  amplifies the input WDM beam and outputs the WDM beam to the OSC combination coupler  1052 . The OSC combination coupler  1052  couples the WDM beam input from the postamp  1051  and the OSC beam input from the OSC controller  1060  and outputs the OSC beam as a transmission light to the outer ring  910  or the inner ring  920 . 
   The OSC controller  1060  includes the OR  1061  having a reception function and the OS  1062  having a transmission function. The OSC controller  1060  controls the controller  1070 . The OSC beam branched by the OSC branch coupler  1012  in the reception unit  1010  is input to the OSC controller  1060  by the OR  1061 . An instruction content of the OSC beam is output from the OSC controller  1060  to the controller  1070 . An instruction content to another OADM is input to the OS  1062  from the controller  1070  and output to the OSC combination coupler  1052  in the transmission unit  1050  as the OSC beam. 
   The basic configuration of the OADM A, B, D, or E is as described above. The ILA C or F has a configuration in which the demultiplexer  1020 , the add/drop unit  1030 , the multiplexer  1040 , and the converter  1080  are removed from the configuration of the OADM A, B, D, or E, that is, a configuration in which transmission of optical signals is not carried out between the transceiver  901 A and the ILA C or F. 
     FIG. 2  is an explanatory diagram of the configuration of an optical transmission device according to a first embodiment of the present invention. An optical transmission device  200 A/ 200 B according to the first embodiment has a configuration in which a unit controller  116  and a preamp  115  having a PD are included in a reception unit  110  of a general optical transmission device such as the OADM and the ILA, and is connected to an adjacent optical transmission device  200 A/ 200 B by the outer ring  910 . 
   The reception unit  110  includes a front PD  111 , a VOA  112 , a rear PD  113 , an OSC branch coupler  114 , the preamp  115  with the PD, and the unit controller  116 . The OSC branch coupler  114  and the unit controller  116  in the reception unit  110  are connected to an OSC controller  130 . 
   A transmission unit  120  includes a postamp  121  and an OSC combination coupler  122 . A multiplexer  220  is arranged upstream of the transmission unit  120 , and a PD array  210  is arranged upstream of the multiplexer  220 . The OSC combination coupler  122  in the transmission unit  120  and the PD array  210  are connected to an OSC controller  140 . 
   The operation when the transmission light is transmitted from the optical transmission device  200 A to the optical transmission device  200 B is explained. An optical signal branched for each wavelength due to add or drop by the add/drop unit  1030  shown in  FIG. 1  is input to the PD array  210 . The PD array  210  detects wavelength information of the transmitted optical signal and outputs the wavelength information to the OSC controller  140  (S 21 ). The optical signal transmitted through the PD array  210  is input to the multiplexer  220 . The input optical signal is coupled with optical signals of other wavelengths, and output as a WDM beam to the transmission unit  120 . 
   The WDM beam input to the transmission unit  120  is amplified by the postamp  121 , and output to the OSC combination coupler  122 . An OSC beam (S 22 ) has been input to the OSC combination coupler  122  from the OSC controller  140 , and the WDM beam input from the postamp  121  is coupled with the OSC beam, and output as a transmission light to the outer ring  910 . The OSC beam (S 22 ) output from the OSC controller  140  includes the wavelength information detected by the PD array  210 . 
   The transmission light output from the optical transmission device  200 A is input to the optical transmission device  200 B via the outer ring  910 . The transmission light input to the optical transmission device  200 B is input, via the front PD  111 , the VOA  112 , and the rear PD  113 , to the OSC branch coupler  114  to be branched to the WDM beam and the OSC beam (S 22 ). The front PD  111  and the rear PD  113  detect the optical level of the transmission light, to calculate the attenuation by the VOA  112 . The attenuation is input to the unit controller  116 , and is used for adjustment of the attenuation by the VOA  112  (at the time of normal operation, the attenuation by the VOA  112  is fixed to a value adjusted at the time of startup). 
   The OSC beam (S 22 ) branched by the OSC branch coupler  114  is input to the OSC controller  130 . The WDM beam branched by the OSC branch coupler  114  is input to the preamp  115 . The preamp  115  is provided with the PD, and the detection result of the PD is output to the unit controller  116  at all times. The OSC controller  130  obtains the wavelength information from the input OSC beam and the supervisory information indicating whether the transmission path is normal, and output these pieces of information to the unit controller  116  (S 23 ). A target table (not shown) is stored in the unit controller  116 , in which information of an optimum optical level (target) of the optical signal multiplexed to the WDM beam input to the respective functional units ( 111  to  115 ) in the reception unit  110  is recorded. Therefore, the unit controller  116  calculates the optical level of the optical signal based on the information input from the OSC controller  130 , by referring to the target table, and when the optical level of the optical signal increases or decreases as compared to the optical level at the time of startup or at the time of normal operation, the unit controller  116  instructs adjustment of the attenuation to the VOA  112  (S 24 ). 
   Thus, in the first embodiment, even when the transmission characteristic changes, the optical level can be adjusted to an appropriate level, by having the PD array  210  and adding the wavelength information detected by the PD array  210  to the OSC beam. 
     FIG. 3  is an explanatory diagram of the configuration of an optical transmission device according to a second embodiment of the present invention. As shown in  FIG. 3 , in an optical transmission device  300 A/ 300 B according to the second embodiment, a spectrum analyzer unit (SAU)  310 , which is a wavelength analyzer connected to the postamp  121 , is provided instead of the PD array  210  in the optical transmission device  200 A/ 200 B. 
   The operation at the time of transmitting the transmission light from the optical transmission device  300 A to the optical transmission device  300 B is explained next. At first, the postamp  121  amplifies the WDM beam input thereto, branches a part of the WDM beam, and outputs the branched part to the SAU  310  (S 31 ). The SAU  310  detects the wavelength information and channel level information from the WDM beam, and outputs the detected information to the OSC controller  140  (S 32 ). The OSC controller  140  outputs the OSC beam including the wavelength information and the channel level information to the optical transmission device  300 B (S 33 ). 
   The transmission light output from the optical transmission device  300 A is input to the optical transmission device  300 B via the outer ring  910 . The transmission light is branched to the WDM beam and the OSC beam (S 33 ) by the OSC branch coupler  114 , and the OSC beam (S 33 ) is input to the OSC controller  130 . The OSC controller  130  obtains the wavelength information and the channel level information from the input OSC beam, and the supervisory information of the transmission path, and output these pieces of information to the unit controller  116  (S 34 ). A target table (not shown) is stored in the unit controller  116 , in which information of an optimum optical level (target) of the optical signal multiplexed to the WDM beam input to the respective functional units ( 111  to  115 ) in the reception unit  110  is recorded. Therefore, the unit controller  116  calculates the optical level of the optical signal based on the information input from the OSC controller  130 , by referring to the target table, and when the optical level of the optical signal increases or decreases as compared to the optical level at the time of startup or at the time of normal operation, the unit controller  116  instructs adjustment of the attenuation to the VOA  112  (S 35 ). 
   Thus, in the second embodiment, even when the transmission characteristic changes, the optical level can be adjusted to an appropriate level, by having the SAU  310  and adding the wavelength information detected by the SAU  310  and the channel level information to the OSC beam. 
     FIG. 4  is an explanatory diagram of the configuration of an optical transmission device according to a third embodiment of the present invention. As shown in  FIG. 4 , in an optical transmission device  400 A/ 400 B according to the third embodiment, an SAU  410  is newly connected to the preamp  115  in the reception unit  110 . In the transmission unit  120 , the SAU  310  is not provided, and the transmission unit  120  does not have a function of detecting the information such as the wavelength information. 
   The operation at the time of transmitting the transmission light from the optical transmission device  400 A to the optical transmission device  400 B is explained next. At first, a transmission light in which the WDM beam and the OSC beam are combined is output from the optical transmission device  400 A. The transmission light is input to the optical transmission device  400 B via the outer ring  910 . The transmission light is branched to the WDM beam and the OSC beam by the OSC branch coupler  114 . The branched WDM beam is output to the preamp  115 , and the OSC beam is output to the OSC controller  130 . 
   The preamp  115  amplifies the input WDM beam, branches a part of the WDM beam, and outputs the branched beam to the SAU  410  (S 41 ). The SAU  410  detects the wavelength information from the input WDM beam, and outputs the wavelength information to the unit controller  116  (S 42 ). A target table (not shown) is stored in the unit controller  116 , in which information of an optimum optical level (target) of the optical signal multiplexed to the WDM beam input to the respective functional units ( 111  to  115 ) in the reception unit  110  is recorded. Therefore, the unit controller  116  calculates the optical level of the optical signal based on the information input from the SAU  410 , by referring to the target table, and when the optical level of the optical signal increases or decreases as compared to the optical level at the time of startup or at the time of normal operation, the unit controller  116  instructs adjustment of the attenuation to the VOA  112  (S 43 ). 
   Thus, in the third embodiment, since the SAU  410  analyzes the wavelength of the WDM beam received by the reception unit  110 , detects the wavelength information, and calculates the optical level of the optical signal by referring to the target table, even when the transmission characteristic changes, the optical level can be adjusted to an appropriate level. 
     FIG. 5  is an explanatory diagram of the configuration of an optical transmission device according to a fourth embodiment of the present invention. As shown in  FIG. 5 , an optical transmission device  500 A/ 500 B according to the fourth embodiment has the same configuration as that of the optical transmission device  400 A/ 400 B in the third embodiment. In the optical transmission device  500 A/ 500 B, when a part of the WDM beam amplified by the preamp  115  is branched and output to an SAU  510  (S 51 ), the SAU  510  detects the optical level of a plurality of multiplexed optical signals from the input WDM beam, and stores the information as profile information of the optical signal. This detection process is performed continuously during operation of the optical transmission device. 
   When the transmission characteristic of the transmission path changes, to change the optical level of the optical signal input to the optical transmission device  500 B, a difference occurs between the profile information of the optical signal stored in the SAU  510  and the newly detected optical level of the optical signal. The SAU  510  carries out differentiation, and outputs difference information to the unit controller  116  (S 52 ). The unit controller  116  instructs adjustment of the attenuation to the VOA  112  based on the difference information input from the SAU  510 , so that the optical level of the optical signal becomes equal to the optical level at the time of startup or at the time of normal operation (S 53 ). 
   Thus, in the fourth embodiment, since the SAU  510  analyzes the wavelength of the WDM beam received by the reception unit  110 , and stores the analysis result as profile information of the optical signal, the optical level can be adjusted to an appropriate level by using a difference when there is a change in the optical level, even when the transmission characteristic changes. 
     FIG. 6  is an explanatory diagram of the configuration of an optical transmission device according to a fifth embodiment of the present invention. As shown in  FIG. 6 , in an optical transmission device  600 A/ 600 B according to the fifth embodiment, a SAU  610  is newly connected to the preamp  115  in the reception unit  110 , and a SAU  620  is connected to the postamp  121 , as in the optical transmission device  300 A/ 300 B according to the second embodiment. 
   The operation at the time of transmitting the transmission light from the optical transmission device  600 A to the optical transmission device  600 B is explained next. At first, the postamp  121  amplifies the WDM beam input thereto, branches a part of the WDM beam, and outputs the branched beam to the SAU  620  (S 61 ). The SAU  620  analyzes the optical signal in the WDM beam to detect the profile information, and outputs the detected profile information to the OSC controller  140  (S 62 ). The OSC controller  140  outputs the OSC beam including the profile information to the optical transmission device  600 B (S 63 ). 
   The transmission light output from the optical transmission device  600 A is input to the optical transmission device  600 B via the outer ring  910 . The transmission light is branched to the WDM beam and the OSC beam (S 63 ) by the OSC branch coupler  114 , and the OSC beam is output to the OSC controller  130 . The WDM beam branched by the OSC branch coupler  114  is amplified by the preamp  115 , and a part of the WDM beam is branched and input to the SAU  610  (S 64 ). 
   The SAU  610  detects the optical level of a plurality of multiplexed optical signals from the input WDM beam, and outputs the information to the unit controller  116  as profile information of the optical signal (S 65 ). The OSC controller  130  extracts the profile information from the OSC beam, and outputs the extracted profile information to the unit controller  116  (S 66 ). 
   The unit controller  116  compares the profile information detected by the SAU  620  in the optical transmission device  600 A with the profile information detected by the SAU  610  in the optical transmission device  600 B, to calculate a loss in the transmission light in a section  630 . Based on the calculation result, the unit controller  116  instructs adjustment of the attenuation to the VOA  112 , so that the optical level of the optical signal becomes equal to the optical level at the time of startup or at the time of normal operation (S 67 ). 
   Thus, in the fifth embodiment, even when the transmission characteristic changes, the optical level can be adjusted to an appropriate level, by respectively connecting the SAU  610  and the SAU  620  to the reception unit  110  and the transmission unit  120 , to obtain a loss in a predetermined section  630  including the outer ring  910 . 
     FIG. 7  is an explanatory diagram of the configuration of an optical transmission device according to a sixth embodiment of the present invention. As shown in  FIG. 7 , in an optical transmission device  700 A/ 700 B according to the sixth embodiment, the OSC controller  130  is connected to the reception unit, and the OSC controller  140  is connected to the transmission unit  120 . Different from the first to the fifth embodiments, a new functional unit is not added. 
   The operation at the time of transmitting the transmission light from the optical transmission device  700 A to the optical transmission device  700 B is explained next. At first, to set the attenuation of the VOA  112  at the time of startup, the ASE beam is transmitted from the OSC controller  140  to the OSC controller  130  (S 71 ). The OSC controller  130  obtains information of the optical level at the time of outputting the optical signal from the OSC controller  140  in the optical transmission device  700 A and the optical level at the time of inputting the optical signal to the OSC controller  130  in the optical transmission device  700 B from the input OSC beam (for example, the ASE beam), and outputs the information to the unit controller  116  (S 72 ). 
   The unit controller  116  calculates a loss at the time of transmission in the outer ring  910  from the information of the optical levels of the obtained two OSC beams. Thereafter, when the normal operation is started, the latest information of the optical level of the OSC beam is input to the OSC controller  130  at all times, and the OSC controller  130  calculates a loss at the time of present transmission in the outer ring  910 . When the transmission characteristic changes, there is a change in the loss. When there is a change in the loss, the unit controller  116  calculates a difference in the loss due to the change, and instructs adjustment of the attenuation to the VOA  112  based on the calculation result, so that the optical level of the optical signal becomes equal to the optical level at the time of startup or at the time of normal operation (S 73 ). 
   Thus, in the sixth embodiment, on the assumption that the OSC beam is in the normal operation state, by determining a change between the OSC beam used at the time of startup and the OSC beam at the time of normal operation, the optical level can be adjusted to an appropriate level even when the transmission characteristic changes. 
     FIG. 8  is an explanatory diagram of the configuration of an optical transmission device according to a seventh embodiment of the present invention. As shown in  FIG. 8 , in an optical transmission device  800 A/ 800 B according to the seventh embodiment has a configuration in which a PD  811  is added to the transmission unit  120  in the optical transmission device  700 A/ 700 B shown in the sixth embodiment. The operation at the time of transmitting the transmission light from the optical transmission device  800 A to the optical transmission device  800 B is explained next. At first, the OSC beam output from the OSC controller  140  for adjusting the VOA  112  at the time of startup is input to the PD  811  via the OSC combination coupler  122 . 
   The PD  811  detects the optical level of the OSC beam, and outputs the optical level to the OSC controller  140  (S 81 ). The OSC controller  140  outputs the OSC beam including the detected optical level to the optical transmission device  800 B (S 82 ). The OSC beam (S 82 ) is input to the OSC controller  130  by the OSC branch coupler  114 . The OSC controller  130  extracts the information of the optical level detected by the PD  811  from the input OSC beam, and outputs the information to the unit controller  116  (S 83 ). 
   When the OSC beam is input to the optical transmission device  800 B, the optical level of the OSC beam is detected by the front PD  111 , and output to the unit controller  116  (S 84 ). The unit controller  116  calculates a loss at the time of transmission in the outer ring  910  from the information of the optical level of the OSC beam input from the OSC controller  130  and the information of the optical level of the OSC beam input from the front PD  111 . Thereafter, when the normal operation is started, the latest information of the optical level of the OSC beam is input to the OSC controller  130  at all times, and the OSC controller  130  calculates a loss at the time of present transmission in the outer ring  910 . When the transmission characteristic changes, there is a change in the loss. When there is a change in the loss, the unit controller  116  calculates a difference in the loss due to the change, and instructs adjustment of the attenuation to the VOA  112  based on the calculation result, so that the optical level of the optical signal becomes equal to the optical level at the time of startup or at the time of normal operation (S 85 ). 
   Thus, in the seventh embodiment, on the assumption that the OSC beam is in the normal operation state, by determining a change between the OSC beam used at the time of startup and the OSC beam at the time of normal operation, the optical level can be adjusted to an appropriate level even when the transmission characteristic changes. 
   According to the optical transmission devices  200 A/ 200 B to  800 A/ 800 B, even when the transmission characteristic changes, the optical level can be automatically adjusted to an optimum level. 
   The optical level control method explained in the embodiments is realized by installing a program prepared beforehand in a computer, for example, an FPGA, or firmware in an AMP unit. 
   Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.