Abstract:
An optical transmission device includes: an attenuator that attenuates an optical signal before being input into an optical element by a predetermined attenuation amount determined so that a level of an adjustment optical signal input into the optical element falls within a predetermined dynamic range of the optical element; and a controller that adjusts the predetermined attenuation amount so that a level of the optical signal input into the optical element falls within the predetermined dynamic range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-261254, filed on Sep. 8, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a technology for adjusting an optical level of a transmission signal to an appropriate value in an optical transmission system.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, by using optical fibers as transmission lines, diffusion of optical transmission systems which transmit optical signals that can increase data capacity by being multiplexed by means of wavelength division multiplexing (WDM) at a high transmission rate instead of electrical signals has been remarkable.  FIG. 5  is an explanatory diagram of the configuration of an optical transmission system.  
         [0006]     An optical transmission system  500  is provided with optical add and drop multiplexers (OADMs) A, B, E, and D and in-line amps (ILAS: optical repeaters) C and F on a transmission line consisting of an outer ring (upward ring)  510  and an inner ring (downward ring)  520 . To the respective OADMs A, B, D, and E, transceivers  501 A,  501 B,  501 D, and  501 E are connected, and by adding, dropping and transmitting transmission light through the outer ring  510  and the inner ring  520 , optical signals can be transmitted to and received from an arbitrary communications partner. The ILAs C and F amplify WDM light to be transmitted through the outer ring  510  and the inner ring  520 . The transmission light to be transmitted through the optical transmission system  500  consists of WDM light obtained by multiplexing an optical signal and optical supervisory channel (OSC) light to supervise the optical signal transmission state.  
         [0007]     It is important in the optical transmission system  500  that the optical level of an optical signal composing the WDM light is adjusted to an appropriate value by the OADMs A, B, D, and E and ILAs C and F and transmitted through the outer ring  510  and the inner ring  520 .  
         [0008]     As a conventional technique to adjust the optical level, a structure is available in that in wavelength multiplexing optical transmission, regardless of the optical input level and the number of wavelengths to an optical fiber amplifier, optical outputs equal among wavelengths are obtained and an optical functional part can be inserted in an intermediate portion. In such a structure, to perform generation avoidance of optical surge and judgment of parts connection becomes an important issue. Therefore, feedback control is performed by inserting a variable attenuator in an optical input unit to make constant optical inputs to the amplifying optical fibers. Furthermore, based on wavelength number data obtained from supervisory signals, control to change a total optical output and optical inputs to the amplifying optical fibers is performed, light to the intermediate optical part and light from the optical part are detected, and when no part is detected, excitation is restrained. A technique to avoid the generation of optical surge when connecting and emit signals indicating that an optical part has not been connected yet by performing the control explained above has been disclosed (see, for example, Japanese Patent Application Laid-Open No. H11-17259).  
         [0009]     In addition, there is an example in which an optical wavelength multiplexing network is simply constructed. In this case, it becomes an issue that an optical signal level per channel is kept constant and predetermined transmission quality is maintained. Therefore, a supervisory signal transmitted through an optical fiber transmission line is extracted by a WDM coupler and the number of wavelengths of an optical signal to be input to a remote node is obtained from the supervisory signal. The feedback controller calculates the wavelength number data as a sum of the number of wavelengths obtained from the supervisory signal and the number of wavelengths to be newly inserted at the remote node via a supervisory signal processing circuit. Furthermore, a technique is disclosed in that an attenuation amount of the variable optical attenuator is adjusted so that a value obtained by dividing the total optical power of the optical amplifier by the wavelength number data becomes optical power of an optical signal for a desired channel, whereby always feedback-controlling the attenuation amount of the variable optical attenuator and compensating loss fluctuation of the optical fiber transmission line (see, for example, Japanese Patent Application Laid-Open No. 2004-147122).  
         [0010]     Conventionally, as shown in Japanese Patent Application Laid-Open Nos. H11-17259 and 2004-147122, optical level control of an optical signal is performed when starting the optical transmission system  500 , and an attenuation amount of a reception unit is adjusted based on wavelength number data of the WDM light acquired by OSC controllers installed inside the OADMs A, B, D, and E and ILAs C and F shown in  FIG. 5 , whereby controlling to an optimum optical level.  
         [0011]     An example of a method of adjusting the optical signal level when starting OADMs or ILAs is explained below.  FIG. 6  is an explanatory diagram for explaining starting procedures of the optical transmission system. A reception unit  610  includes a front photodiode (PD)  614  and a rear PD  615  in the front and rear of a variable optimal attenuator (VOA)  611 . The reception unit  610  and a transmission unit  650  include unit controllers  653 . In the reception unit  610 , a unit controller  616  adjusts the attenuation amount of the VOA  611  based on optical levels detected by the front PD  614  and the rear PD  615  and controls an optical level of an optical signal to be input into a preamp  613 . Furthermore, in the reception unit  610  and the transmission unit  650 , the unit controllers  616  and  653  are connected to an OSC controller  660  to adjust the attenuation amount of the VOA  611  when starting.  
         [0012]     In  FIG. 6 , an OR  661  and an OS  662  include a unit controller  663 , an optical-electrical converter (OE)  664 , and an electrical-optical converter (EO)  665 . The unit controller  663  controls the interior of the OSC controller  660 . The OE  664  converts an input optical signal into an electrical signal and outputs it. The EO  665  converts an input electrical signal into an electrical signal and outputs it.  
         [0013]     Next, starting procedures of the OADM B connected to the outer ring  510  and the inner ring  520  is explained. To start the OADM B, OSC light is transmitted between optical transmission devices adjacent to each other (between OADMs A and B in the example shown in  FIG. 6 ).  
         [0014]     First, from the unit controller  663  of the OSC controller  660  of the OADM B, an optical level controlling amplified spontaneous emission (ASE) light output request is output to the unit controller  616  of the OADM A (S 1 ). The optical level of the ASE light requested at this point corresponds to an optical signal  1  wave level. In response to the ASE light output request, to prevent the optical signal from the OADM B from being sent to the transmission line, a 1×2 switch (SW)  617  disposed at the stage before the preamp  613  of the OADM B is controlled to open and shut down the input light to the OADM B.  
         [0015]     Next, OSC light communication confirmation is made in the EO  665  of the OADM A and the OE  664  of the OADM B (S 2 ). A postamp  651  that has received the ASE light output request outputs ASE light at a level corresponding to the optical signal  1  wavelength (S 3 ). At this point, to prevent the optical signal from the OADM A from being sent to the transmission line, a 1×2 SW  654  disposed at the stage before the postamp  651  of the OADM A is controlled to open.  
         [0016]     When the ASE light is input to the reception unit  610  of the OADM B via the outer ring  510  (S 4 ) and further input into the unit controller  616  via the VOA  611 , the VOA  611  is automatically adjusted (S 5 ). Specifically, the unit controller  616  of the OADM B adjusts the VOA  611  to an appropriate attenuation amount by monitoring light receiving power of the rear PD  615  provided at the stage before the preamp  613  so that the input light of the preamp  613  becomes an appropriate level.  
         [0017]     When the automatic adjustment of the VOA  611  is ended, the unit controller of the OADM B judges that the input into the preamp  613  has become stable, releases the shut-down state of the preamp  613  of the OADM B (S 6 ), and starts the preamp  613  by means of output constant control (ALC).  
         [0018]     When the unit controller  616  of the OADM B confirms that the preamp  613  has started and transited to automatic gain control (AGC), the unit controller stops the ASE light output request for optical level control from the unit controller  663  (S 7 ). When the output of ASE light from the postamp  651  is stopped, the unit controller  653  closes the 1×2 SW  654  disposed at the stage before the postamp  651  of the OADM A, releases the shut-down state of the postamp  651 , and starts operation.  
         [0019]     The automatic adjustment of the VOA  611  performed at S 5  of  FIG. 6  means processing to adjust an optical level of an optical signal input in the preamp  613  (ASE light when starting) so as to fall within the dynamic range of the preamp  613 .  
         [0020]     Herein, the VOA  611  and the dynamic range of the preamp  613  are explained.  FIG. 7  is an explanatory diagram for explaining a VOA control when starting the optical transmission system. In  FIG. 7 , the vertical axis indicates the optical level, and the horizontal axis indicates the time of detection of the optical level indicated on the vertical axis. The PD input time of  FIG. 7  indicates an optical level when ASE light is input into the front FD  614  of the reception unit  610 . The ASE light to be input into the front PD  614  is controlled to an optical level within a dynamic range (20 decibels) by the optical transmission device of the front stage (OADM A in the example of  FIG. 6 ).  
         [0021]     The unit controller  616  adjusts the attenuation amount of the VOA  611  based on detected values of the front PD  614  and the rear PD  615  so that the optical level at the PD input time becomes a target optical level. As the target optical level, an optical level resultant of attenuating 3 decibels from an upper limit of a dynamic range (10 decibels) of the input into the preamp  613  is appropriate. By the adjustment to the target, the attenuation amount of the VOA  611  can be fixed and WDM light at an appropriate optical level can be output from the reception unit  610 .  
         [0022]     However, as shown in  FIG. 7 , when an input rise of ASE light for adjustment occurs after adjusting the VOA  611 , the preamp  613  does not normally start.  FIG. 8  is an explanatory diagram for explaining a VOA control when the input level of the optical signal rises. In  FIG. 8 , the vertical axis indicates the optical level, and the horizontal axis indicates the time of detection of the optical level indicated on the vertical axis. At the VOA adjusting level El indicated on the vertical axis, the optical level input into the PD is attenuated by VOA, and when inputting it into the preamp  613 , the optical level is adjusted so as to fall within the dynamic range of the preamp input.  
         [0023]     At this point, when the connection of the connector of the cable to input ASE light is insufficient and the connector is properly connected later, the input level of the ASE light may rise. After being adjusted to the VOA adjusting level E 1 , when the optical level rises to the final optical input level E 2 , the VOA attenuation amount is adjusted based on the VOA adjusting level E 1 , so that when being input into the preamp  613 , the AMP input level of the optical level of E 2  deviates from the dynamic range.  
         [0024]     When the input level of the optical signal input into the preamp  613  is out of the dynamic range, the preamp  613  does not start, or when the preamp  613  is in gain constant control, the output of the preamp  613  rises by following input fluctuation and adversely influences the optical signal.  
       SUMMARY OF THE INVENTION  
       [0025]     It is an object of the present invention to at least solve the problems in the conventional technology.  
         [0026]     An optical transmission device according to an aspect of the present invention, which adjusts a level of an optical signal input into an optical element having a predetermined dynamic range, includes: an attenuator that attenuates the optical signal before being input into the optical element by a predetermined attenuation amount, wherein the predetermined attenuation amount is determined so that a level of an adjustment optical signal input into the optical element falls within the predetermined dynamic range of the optical element; and a controller that adjusts the predetermined attenuation amount so that a level of the optical signal input into the optical element falls within the predetermined dynamic range of the optical element.  
         [0027]     A method according to another aspect of the present invention is a method of adjusting a level of an optical signal input into an optical element having a predetermined dynamic range in an optical transmission device. The method includes: attenuating the optical signal before being input into the optical element by a predetermined attenuation amount, wherein the predetermined attenuation amount is determined so that a level of an adjustment optical signal input into the optical element falls within the predetermined dynamic range of the optical element; and adjusting the predetermined attenuation amount so that a level of the optical signal input into the optical element falls within the predetermined dynamic range of the optical element.  
         [0028]     A computer-readable recording medium according to still another aspect of the present invention stores a computer program that causes a computer to execute the above method.  
         [0029]     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  
       [0030]      FIGS. 1A and 1B  are explanatory diagrams of the configuration of an optical transmission device according to an embodiment of the present invention;  
         [0031]      FIG. 2  is an explanatory diagram for explaining a starting abnormality that occurs in the optical transmission device;  
         [0032]      FIG. 3A  is an explanatory diagram of the configuration of a unit controller;  
         [0033]      FIG. 3B  is a flowchart of a re-adjustment process performed by the unit controller shown in  FIG. 3A ;  
         [0034]      FIG. 4A  is an explanatory diagram of another configuration of the unit controller;  
         [0035]      FIG. 4B  is a flowchart of a re-adjustment process performed by the unit controller shown in  FIG. 4A ;  
         [0036]      FIG. 5  is an explanatory diagram of the configuration of a conventional optical transmission system;  
         [0037]      FIG. 6  is an explanatory diagram for explaining starting procedures of the conventional optical transmission system;  
         [0038]      FIG. 7  is an explanatory diagram for explaining a VOA control when starting the optical transmission system; and  
         [0039]      FIG. 8  is an explanatory diagram for explaining a VOA control when an input level of an optical signal rises. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     Exemplary embodiments of the present invention are explained in detail with reference to the accompanying drawings.  
         [0041]      FIG. 1A  is an explanatory diagram of the overall configuration of an optical transmission device according to an embodiment of the present invention. The optical transmission device is an optical add and drop multiplexer (OADM) in an optical transmission system, such as the conventional optical transmission system  500  shown in  FIG. 5 . As shown in  FIG. 1A , the OADM includes reception units  110 , demultiplexers  620 , add/drop units  630 , multiplexers  640 , and transmission units  120  that are arranged on the outer ring  510  and the inner ring  520 , respectively. The OADM further includes an optical supervisory channel (OSC) controller  660 , a controller  670 , and a converter  680  that are commonly used for an optical transmission on the outer ring  510  and an optical transmission on the inner ring  520 .  
         [0042]     The reception unit  110  includes the VOA  611 , an OSC branching coupler  612 , and the preamp  613 . A transmission light on the outer ring  510  or inner ring  520  is attenuated by the VOA  611  and then branched into WDM light and OSC light by the OSC branching coupler  612 .  
         [0043]     The OSC light branched by the OSC branching coupler  612  is input into the OR  661  of the OSC controller  660 . The OSC controller  660  remains in a standby mode as long as the OSC light indicates a normal transmission state. On the other hand, when the OSC light indicates a change in the transmission state, such as an abnormality of the transmission line or an increase or decrease in the wavelengths of signals multiplexed into the WDM light, the OSC controller  660  outputs an instruction for coping with the change to the controller  670 .  
         [0044]     The OSC light indicating the change in the transmission state is output to the transmission unit  120  of the inner ring  520 /the outer ring  510 , after being branched by the reception unit  110  of the outer ring  510 /the inner ring  520  and input into the OSC controller  660 . Thus, by outputting the OSC light from the outer ring  510 /the inner ring  520  to the inner ring  520 /the outer ring  510 , the OSC light, which functions as a control signal, is transmitted back to the upstream optical transmission device (such as OADM and ILA). As a use example of the OSC light, there is the starting procedures of the optical transmission system  500  explained above with reference to  FIG. 6 .  
         [0045]     The WDM light branched by the OSC branching coupler  612  is input into the preamp  613 . The WDM light is amplified by the preamp  613  and output to the demultiplexer  620 . The demultiplexer  620  demultiplexes the WDM light into optical signals of different wavelengths, and outputs the optical signals to the add/drop unit  630 .  
         [0046]     The add/drop unit  630  branches an optical signal destined to the transceiver  501 A from the optical signals input from the demultiplexer  620 , and outputs the optical signal to the converter  680 . Other optical signals are transmitted as it is and input into the multiplexer  640 . On the other hand, an optical signal from the transceiver  501 A is input into the add/drop unit  630 , through the converter  680 , and further input into the multiplexer  640 . The multiplexer  640  multiplexes optical signals from the add/drop unit  630  into WDM light, and outputs the WDM light to the transmission unit  120 .  
         [0047]     The converter  680  includes a 2×1 switch (SW)  681 , an optical-electrical converter (OE)  682 , an electrical-optical converter (EO)  683 , and 1×2 coupler  684 . An optical signal, which is destined to the transceiver  501 A and included in the WDM light on the outer ring  510  or the inner ring  520 , is input into the converter  680  through the add/drop unit  630 . The optical signal is input, through the 2×1 SW  681 , into the OE  682  that converts the optical signal into an electrical signal and outputs the electrical signal to the transceiver  501 A.  
         [0048]     On the other hand, when the transceiver  501 A transmits an optical signal to another transceiver  501 A, an electrical signal from the transceiver  501 A is input into the EO  683  of the converter  680  to be converted into an optical signal and output to the 1×2 coupler  684 . The 1×2 coupler  684  branches the optical signal from the EO  683  into two, and outputs the optical signals to the add/drop unit  630  on the outer ring  510  and the add/drop unit  630  on the inner ring  520 , respectively.  
         [0049]     The transmission unit  120  includes the postamp  651  and an OSC multiplexing coupler  652 . The WDM light from the multiplexer  640  is input into the postamp  651  that, amplifies the WDM light and outputs the amplified WDM light to the OSC multiplexing coupler  652 . The OSC multiplexing coupler  652  multiplexes the WDM light from the postamp  651  and the OSC light from the OSC controller  660 , and outputs the multiplexed light as a transmission light to the outer ring  510  or the inner ring  520 .  
         [0050]     The ILA has a similar configuration as that of the OADM explained above, except that the ILA does not include the demultiplexer  620 , the add/drop unit  630 , the multiplexer  640 , and the converter  680 . With such a configuration, the ILA does not add/drop optical signals.  
         [0051]      FIG. 1B  is an explanatory diagram of the detailed configuration of the optical transmission device.  FIG. 1B  illustrates a transmission unit  120  of an optical transmission device  100 A and a reception unit  110  of an optical transmission device  100 B that are connected to each other by the outer ring  510 .  
         [0052]     As shown in  FIG. 1B , the reception unit  110  includes a front PD  111 , a VOA  112 , a rear PD  113 , an OSC branching coupler  114 , a PD  118 , a preamp  115 , an OSC  130 , and a unit controller  116 / 117 . The OSC branching coupler  114  and the unit controller  116 / 117  are connected to the OSC  130 .  
         [0053]     On the other hand, the transmission unit  120  includes a 1×2 switch (SW)  124 , a postamp  121 , an OSC multiplexing coupler  122 , and a unit controller  123 . The unit controller  116  (explained in detail below with reference to  FIGS. 3A and 3B ) and  117  (explained in detail below with reference to  FIGS. 4A and 4B ) have a function unique to the present invention, that is, a function of adjusting the attenuation amount of the VOA  112  independently of the OSC  130 .  
         [0054]     A starting procedure of the optical transmission device  100 B shown in  FIG. 1B  is explained below. The postamp  121  of the optical transmission device  100 A outputs an ASE light for optical level adjustment to the optical transmission device  100 B, after controlling the 1×2 SW  124 , which is arranged upstream of the postamp  121 , to open to prevent an optical signal from the optical transmission device  100 A from being output to the outer ring  510 .  
         [0055]     The above process can be performed, for example, by a control signal included in the OSC light that is transmitted from a device controller of the optical transmission device  100 B to the optical transmission device  100 A. Upon receiving the control signal, a device controller of the optical transmission device  100 A sends an instruction to the unit controller  123  to control the 1×2 SW  124  and/or the postamp  121 .  
         [0056]     The ASE light output from the postamp  121  is input into the reception unit  110  of the optical transmission device  100 B. The reception unit  110  adjusts the attenuation amount of the VOA  112  to an appropriate value. Specifically, the PD  118  of the reception unit  110  detects the level of the ASE light input into the preamp  115 , and the unit controller  116  compares the detected level with a target input level and adjusts the attenuation amount of the VOA  112  to the appropriate value, that is, a value within the dynamic range of the preamp  115 .  
         [0057]      FIG. 2  is an explanatory diagram for explaining a starting abnormality that occurs in the optical transmission device. When the input level to the optical transmission device  100 B rises after the attenuation amount of the VOA  112  is temporarily fixed by the procedures described above, the optical transmission device  100 B detects the rise by the PD  118  arranged upstream of the preamp  115  and re-adjusts the attenuation amount of the VOA  112 .  
         [0058]     The detection of the rise is performed, for example, in a predetermined time period after adjusting the attenuation amount of the VOA  112  based on the ASE light. When no rise is detected within the time period, the unit controller  116  shifts to ALC control of the preamp  115 , which is a next starting procedure.  
         [0059]      FIG. 3A  is an explanatory diagram of the configuration of the unit controller  116 . As shown in  FIG. 3A , the unit controller  116  includes a target input level storage unit  201 , a preamp input level storage unit  202 , a comparator  203 , and a VOA controller  204 .  
         [0060]     The target input level storage unit  201  stores, as a target input level, an optical level of the ASE light detected by the PD  118  (in other words, input into the preamp  115 ) when the attenuation amount of the VOA  112  is adjusted based on the ASE light. The target input level storage unit  201  outputs the target input level to the comparator  203 .  
         [0061]     The preamp input level storage unit  202  stores, as a preamp input level, an optical level of an optical signal detected by the PD  118 . Storing of the preamp input level is periodically performed, and the preamp input level stored in the preamp input level storage unit  202  is periodically updated to the latest preamp input level. The preamp input level storage unit  202  outputs the preamp input level to the comparator  203 .  
         [0062]     The comparator  203  calculates the difference between the target input level input from the target input level storage unit  201  and the preamp input level input from the preamp input level storage unit  202 , and judges whether the difference exceeds a predetermined threshold (for example, ±0.5 decibel). The judgment results of the comparator  203  are input into the VOA controller  204 .  
         [0063]     The VOA controller  204  outputs an adjustment instruction to the VOA  112  according to the judgment results input from the comparator  203 .  
         [0064]      FIG. 3B  is a flowchart of a re-adjustment process of the attenuation amount of the VOA  112  performed by the unit controller  116  shown in  FIG. 3A . The optical level detected by the PD  118  when the attenuation amount of the VOA  112  is adjusted based on the ASE light is stored in the target input level storage unit  201  (step S 301 ). The preamp input level is periodically stored in the preamp input level storage unit  202  (step S 302 ).  
         [0065]     The comparator  203  calculates the difference between the target input level and the preamp input level (step S 303 ), and judges whether the difference exceeds the threshold of ±0.5 decibel (step S 304 ).  
         [0066]     The judgment results at step S 304  are input into the VOA controller  204 , and when the difference is less than or equal to ±0.5 decibel (step S 304 : No), the process is ended there because the attenuation amount of the VOA  112  is appropriate and therefore does not need to be re-adjusted.  
         [0067]     On the other hand, when the difference exceeds ±0.5 decibel (step S 304 : Yes), the attenuation amount of the VOA  112  is inappropriate and therefore re-adjustment is necessary. Therefore, the VOA controller  204  re-adjusts the attenuation amount of the VOA  112  by outputting a re-adjustment instruction to the VOA  112  (step S 305 ).  
         [0068]     Thus, the VOA  112  is re-adjusted by the unit controller  116 . Subsequently, as in the same manner as the conventional starting procedures, the shut-down state of the preamp  115  of the optical transmission device  100 B is released and the preamp  115  is started. When confirming that the preamp  115  has started, the unit controller  116  stops sending the ASE light output request. When the ASE light stops, the unit controller  123  closes the 1×2 SW  124  arranged upstream of the postamp  121  and starts operation.  
         [0069]      FIG. 4A  is an explanatory diagram of the configuration of the unit controller  117 . As shown in  FIG. 4A , the unit controller  117  includes an alarm monitor  211 , an AMP controller  212 , and a VOA controller  213 .  
         [0070]     The alarm monitor  211  monitors whether the preamp  115  has been started. When the preamp  115  does not start in a predetermined time period, the alarm monitor  211  outputs an alarm indicating a starting abnormality of the preamp  115  to the AMP controller  212  and the VOA controller  213 .  
         [0071]     Upon receiving the alarm from the alarm monitor  211 , the AMP controller  212  outputs a re-start instruction to the preamp  115 . Upon receiving the alarm from the alarm monitor  211 , the VOA controller  213  outputs an adjustment instruction to the VOA  112 .  
         [0072]      FIG. 4B  is a flowchart of a re-adjustment process of the attenuation amount of the VOA  112  performed by the unit controller  117  shown in  FIG. 4A . The alarm monitor  211  judges whether a predetermined time has elapsed (step S 401 ). When the predetermined time elapses (step S 401 : Yes), the alarm monitor  211  judges whether the preamp  115  has normally started (step S 402 ).  
         [0073]     When the preamp  115  normally starts (step S 402 : Yes), the process is ended there because the re-adjustment of the VOA  112  is not necessary. On the other hand, when the preamp  115  does not normally start (step S 402 : No), an alarm is generated by the alarm monitor  211  because the re-adjustment of the VOA  112  is necessary (step S 403 ).  
         [0074]     The AMP controller  212  and the VOA controller  213  judge whether they have received the alarm (step S 404 ). Upon receiving the alarm (step S 404 : Yes), the VOA controller  213  outputs an instruction for re-adjustment to the VOA (step S 405 ), and the AMP controller  212  outputs an instruction to start the preamp  115  (step S 406 ). The process of step S 405  and the process of step S 406  can be performed simultaneously.  
         [0075]     Thus, the VOA  112  is re-adjusted by the unit controller  117 . Subsequently, as in the same manner as the conventional starting procedures, the shut-down state of the preamp  115  of the optical transmission device  100 B is released and the preamp  115  is started. When confirming that the preamp  115  has started, the unit controller  117  stops sending the ASE light output request. When the ASE light stops, the unit controller  123  closes the 1×2 SW  124  arranged upstream of the postamp  121  and starts operation.  
         [0076]     According to the embodiment explained above, even when the level of the optical signal fluctuates after the attenuation amount of the VOA is adjusted by the OSC, the attenuation amount is re-adjusted by the unit controller without the aid of OSC, thereby preventing erroneous operation, which is due to a level change of the optical signal, in starting procedure of the optical transmission device.  
         [0077]     According to the embodiment explained above, the level of the optical signal input into the preamp  115  is adjusted to fall within the dynamic range of the preamp  115 . However, for example, an input level to the optical reception unit can be adjusted if the optical transmission device does not include any amplifier.  
         [0078]     According to the embodiment explained above, the optical transmission device  100 A/ 100 B includes the OSCs  130  and  140 , the OSC branching coupler  114 , and the OSC multiplexing coupler  122  for an optical transmission of an OSC signal. However, these components are not indispensable to the optical transmission device  100 A/ 100 B.  
         [0079]     The optical level control method explained in the above embodiment can be realized, for example, by storing a prepared computer program in the FPGA in the AMP unit.  
         [0080]     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.