Optical transmission device for controlling optical level of transmission signal in optical transmission system

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.

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. 9is an explanatory diagram of a configuration example of the optical transmission system.

In an optical transmission system900, 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)910and an inner ring (downward ring)920. Transceivers901A,901B,901D and901E 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 ring910and the inner ring920. The ILAs C and F amplify a WDM beam transmitted through the outer ring910and the inner ring920. The light transmitted in the optical transmission system900is 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 system900, 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 ring910and the inner ring920.

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 inFIG. 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. 10is an explanatory diagram of a startup procedure of the optical transmission system. A reception unit1010includes a variable optical attenuator (VOA)1011, a front photodiode (PD)1014arranged upstream of the VOA1011, a rear PD1015arranged downstream of the VOA1011, an OSC branch coupler1012, and a preamp1013. A transmission unit1050includes a postamp1051, an OSC combination coupler1052, and a 1×2 switch (SW)1054. The reception unit1010and the transmission unit1050further include unit controllers1016and1053, respectively. The unit controller1016of the reception unit1010adjusts the attenuation of the VOA1011based on optical levels detected by the front PD1014and the rear PD1015, to control the optical level of the optical signal input to the preamp1013. The unit controller1016of the reception unit1010and the unit controller1053of the transmission unit1050are connected to an OSC controller1060(for convenience, it is written as “OSC” in the drawings, as well as an OSC controller explained below), to adjust the attenuation of the VOA1011at the time of startup.

An OR1061and an OS1062includes a unit controller1063, an optoelectronic converter (OE)1064, and an electro-optic converter (EO)1065. The unit controller1063controls the OSC controller1060. The OE1064converts an input optical signal to an electric signal and output the electric signal. The EO1065converts an input electric signal to an optical signal and output the optical signal.

The startup procedure of the OADM B connected to the outer ring910and the inner ring920is 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 inFIG. 10).

At first, an output request of amplified spontaneous emission (ASE) beam for optical level control is output from the unit controller1063in the OSC controller1060of the OADM B to the unit controller1016of the OADM B and the unit controller1016of the OADM A (S1). 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)1017arranged upstream of the preamp1013in 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 EO1065of the OADM A and the OE1064of the OADM B (S2). The postamp1051having received the output request of the ASE beam outputs the ASE beam of a level corresponding to one wavelength of the optical signal (S3). At this time, a 1×2 SW1054arranged upstream of the postamp1051in the OADM A is controlled to open.

When the ASE beam is input to the reception unit1010of the OADM B via the outer ring910(S4), and further input to the unit controller1016via the VOA1011, auto-adjustment of the VOA1011is carried out (S5). Specifically, to make the input light of the preamp1013at an appropriate level, the unit controller1016in the OADM B monitors the light-receiving power of the rear PD1015arranged upstream of the preamp1013, and adjust the VOA1011to have an appropriate attenuation.

When the auto-adjustment of the VOA1011has finished, the unit controller1016in the OADM B determines that the input to the preamp1013becomes stable, to release the shut-down state of the preamp1013in the OADM B (S6), and starts up the preamp1013by automatic level control (ALC).

When having confirmed that the preamp1013has been started up, and shifted to automatic gain control (AGC), the unit controller1016in the OADM B suspends the output request of the ASE beam for optical level control from the unit controller1063(S7). When the output of the ASE beam from the postamp1051has stopped, the unit controller1053closes the 1×2 SW1054arranged upstream of the postamp1051in the OADM A, to release the shut-down state of the postamp1051, and starts the operation thereof.

The auto-adjustment of the VOA1011carried out at S5inFIG. 10indicates a process for adjusting the optical level of the optical signal input to the preamp1013(the ASE beam at the time of startup) to be within a dynamic range of the preamp1013.

After the startup operation as described above, the OADM A and the OADM B are in a normal operation state. The VOA1011fixes the attenuation for one wavelength of the optical signal, and the preamp1013carries out automatic gain control (AGC) to control the gain of the multiplexed optical signal to be equalized. This is because in the optical transmission system900, 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 system900in 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 ring910or the inner ring920) 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 VOA1011, 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

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are explained in detail with reference to the accompanying drawings.

FIG. 1is an explanatory diagram of the configuration of an optical add-and-drop multiplexer (OADM) according to the present invention. The OADM A shown inFIG. 1constitutes the optical transmission system900shown inFIG. 9. As shown inFIG. 1, in the OADM A, the reception unit1010, a demultiplexer1020, an add/drop unit1030, a multiplexer1040, and the transmission unit1050are respectively provided for the outer ring910and the inner ring920. Furthermore, in the OADM A, the OSC controller1060, a controller1070, and a converter1080, which function both for the outer ring910and the inner ring920, are provided.

The reception unit1010includes the VOA1011, the OSC branch coupler1012, and the preamp1013. The transmitted light input from the outer ring910or the inner ring920is attenuated by the VOA1011, and then branched to the WDM beam and the OSC beam by the OSC branch coupler1012.

The OSC beam branched by the OSC branch coupler1012is input to the OR1061in the OSC controller1060. When the input OSC beam indicates a normal optical transmission state, the OSC controller1060is 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 controller1060outputs an instruction to the controller1070to handle the situation by performing a process corresponding to the changed situation.

The OSC beam branched by the reception unit1010in the outer ring910, and instructing a process corresponding to the OSC beam input to the OSC controller1060is output to the transmission unit1050in the inner ring920. Likewise, the OSC beam branched by the reception unit1010in the inner ring920, and instructing a process corresponding to the OSC beam input to the OSC controller1060is output to the transmission unit1050in the outer ring910. 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 system900explained with reference toFIG. 10.

The WDM beam branched by the OSC branch coupler1012is input to the preamp1013. The WDM beam is amplified by the preamp1013, and output to the demultiplexer1020. The demultiplexer1020branches the input WDM beam for each wavelength, and output to the add/drop unit1030.

The add/drop unit1030drops an optical signal addressed to the transceiver901A from the optical signal for each wavelength input from the demultiplexer1020, and outputs the optical signal to the converter1080. Optical signals other than the optical signal dropped by the add/drop unit1030are transmitted directly, and input to the multiplexer1040. On the other hand, the optical signal input from the transceiver901A is added to the add/drop unit1030via the converter1080. The newly added optical signal is output to the multiplexer1040. The multiplexer1040couples the optical signals input from the add/drop unit1030by transmission or addition, and outputs the optical signals as one WDM beam to the transmission unit1050.

The converter1080includes a 2□1 switch1081, an optoelectronic converter (O/E)1082, an electro-optic converter (E/O)1083, and a 1□2 coupler1084. When the optical signal addressed to the transceiver901A is included in the WDM beam of the transmitted light flowing on the outer ring910or the inner ring920, the optical signal is input to the converter1080from the add/drop unit1030. The optical signal input to the converter1080is selected by the 2□1 switch1081, and output to the O/E1082. The O/E1082converts the input optical signal to an electric signal and output the electric signal to the transceiver901A.

When the optical signal is transmitted from the transceiver901A to the transceiver901B, an electric signal is input to the E/O1083in the converter1080. The input electric signal is converted to an optical signal by the E/O1083, and output to the 1□2 coupler1084. The 1□2 coupler1084branches the optical signal input from the E/O1083into two signals, and outputs the respective optical signals to the add/drop unit1030for the outer ring910and the add/drop unit1030for the inner ring920.

The transmission unit1050includes the postamp1051and the OSC combination coupler1052. The WDM beam input from the multiplexer1040is input to the postamp1051. The postamp1051amplifies the input WDM beam and outputs the WDM beam to the OSC combination coupler1052. The OSC combination coupler1052couples the WDM beam input from the postamp1051and the OSC beam input from the OSC controller1060and outputs the OSC beam as a transmission light to the outer ring910or the inner ring920.

The OSC controller1060includes the OR1061having a reception function and the OS1062having a transmission function. The OSC controller1060controls the controller1070. The OSC beam branched by the OSC branch coupler1012in the reception unit1010is input to the OSC controller1060by the OR1061. An instruction content of the OSC beam is output from the OSC controller1060to the controller1070. An instruction content to another OADM is input to the OS1062from the controller1070and output to the OSC combination coupler1052in the transmission unit1050as 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 demultiplexer1020, the add/drop unit1030, the multiplexer1040, and the converter1080are 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 transceiver901A and the ILA C or F.

FIG. 2is an explanatory diagram of the configuration of an optical transmission device according to a first embodiment of the present invention. An optical transmission device200A/200B according to the first embodiment has a configuration in which a unit controller116and a preamp115having a PD are included in a reception unit110of a general optical transmission device such as the OADM and the ILA, and is connected to an adjacent optical transmission device200A/200B by the outer ring910.

The reception unit110includes a front PD111, a VOA112, a rear PD113, an OSC branch coupler114, the preamp115with the PD, and the unit controller116. The OSC branch coupler114and the unit controller116in the reception unit110are connected to an OSC controller130.

A transmission unit120includes a postamp121and an OSC combination coupler122. A multiplexer220is arranged upstream of the transmission unit120, and a PD array210is arranged upstream of the multiplexer220. The OSC combination coupler122in the transmission unit120and the PD array210are connected to an OSC controller140.

The operation when the transmission light is transmitted from the optical transmission device200A to the optical transmission device200B is explained. An optical signal branched for each wavelength due to add or drop by the add/drop unit1030shown inFIG. 1is input to the PD array210. The PD array210detects wavelength information of the transmitted optical signal and outputs the wavelength information to the OSC controller140(S21). The optical signal transmitted through the PD array210is input to the multiplexer220. The input optical signal is coupled with optical signals of other wavelengths, and output as a WDM beam to the transmission unit120.

The WDM beam input to the transmission unit120is amplified by the postamp121, and output to the OSC combination coupler122. An OSC beam (S22) has been input to the OSC combination coupler122from the OSC controller140, and the WDM beam input from the postamp121is coupled with the OSC beam, and output as a transmission light to the outer ring910. The OSC beam (S22) output from the OSC controller140includes the wavelength information detected by the PD array210.

The transmission light output from the optical transmission device200A is input to the optical transmission device200B via the outer ring910. The transmission light input to the optical transmission device200B is input, via the front PD111, the VOA112, and the rear PD113, to the OSC branch coupler114to be branched to the WDM beam and the OSC beam (S22). The front PD111and the rear PD113detect the optical level of the transmission light, to calculate the attenuation by the VOA112. The attenuation is input to the unit controller116, and is used for adjustment of the attenuation by the VOA112(at the time of normal operation, the attenuation by the VOA112is fixed to a value adjusted at the time of startup).

The OSC beam (S22) branched by the OSC branch coupler114is input to the OSC controller130. The WDM beam branched by the OSC branch coupler114is input to the preamp115. The preamp115is provided with the PD, and the detection result of the PD is output to the unit controller116at all times. The OSC controller130obtains 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 controller116(S23). A target table (not shown) is stored in the unit controller116, in which information of an optimum optical level (target) of the optical signal multiplexed to the WDM beam input to the respective functional units (111to115) in the reception unit110is recorded. Therefore, the unit controller116calculates the optical level of the optical signal based on the information input from the OSC controller130, 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 controller116instructs adjustment of the attenuation to the VOA112(S24).

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 array210and adding the wavelength information detected by the PD array210to the OSC beam.

FIG. 3is an explanatory diagram of the configuration of an optical transmission device according to a second embodiment of the present invention. As shown inFIG. 3, in an optical transmission device300A/300B according to the second embodiment, a spectrum analyzer unit (SAU)310, which is a wavelength analyzer connected to the postamp121, is provided instead of the PD array210in the optical transmission device200A/200B.

The operation at the time of transmitting the transmission light from the optical transmission device300A to the optical transmission device300B is explained next. At first, the postamp121amplifies the WDM beam input thereto, branches a part of the WDM beam, and outputs the branched part to the SAU310(S31). The SAU310detects the wavelength information and channel level information from the WDM beam, and outputs the detected information to the OSC controller140(S32). The OSC controller140outputs the OSC beam including the wavelength information and the channel level information to the optical transmission device300B (S33).

The transmission light output from the optical transmission device300A is input to the optical transmission device300B via the outer ring910. The transmission light is branched to the WDM beam and the OSC beam (S33) by the OSC branch coupler114, and the OSC beam (S33) is input to the OSC controller130. The OSC controller130obtains 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 controller116(S34). A target table (not shown) is stored in the unit controller116, in which information of an optimum optical level (target) of the optical signal multiplexed to the WDM beam input to the respective functional units (111to115) in the reception unit110is recorded. Therefore, the unit controller116calculates the optical level of the optical signal based on the information input from the OSC controller130, 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 controller116instructs adjustment of the attenuation to the VOA112(S35).

Thus, in the second embodiment, even when the transmission characteristic changes, the optical level can be adjusted to an appropriate level, by having the SAU310and adding the wavelength information detected by the SAU310and the channel level information to the OSC beam.

FIG. 4is an explanatory diagram of the configuration of an optical transmission device according to a third embodiment of the present invention. As shown inFIG. 4, in an optical transmission device400A/400B according to the third embodiment, an SAU410is newly connected to the preamp115in the reception unit110. In the transmission unit120, the SAU310is not provided, and the transmission unit120does 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 device400A to the optical transmission device400B 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 device400A. The transmission light is input to the optical transmission device400B via the outer ring910. The transmission light is branched to the WDM beam and the OSC beam by the OSC branch coupler114. The branched WDM beam is output to the preamp115, and the OSC beam is output to the OSC controller130.

The preamp115amplifies the input WDM beam, branches a part of the WDM beam, and outputs the branched beam to the SAU410(S41). The SAU410detects the wavelength information from the input WDM beam, and outputs the wavelength information to the unit controller116(S42). A target table (not shown) is stored in the unit controller116, in which information of an optimum optical level (target) of the optical signal multiplexed to the WDM beam input to the respective functional units (111to115) in the reception unit110is recorded. Therefore, the unit controller116calculates the optical level of the optical signal based on the information input from the SAU410, 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 controller116instructs adjustment of the attenuation to the VOA112(S43).

Thus, in the third embodiment, since the SAU410analyzes the wavelength of the WDM beam received by the reception unit110, 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. 5is an explanatory diagram of the configuration of an optical transmission device according to a fourth embodiment of the present invention. As shown inFIG. 5, an optical transmission device500A/500B according to the fourth embodiment has the same configuration as that of the optical transmission device400A/400B in the third embodiment. In the optical transmission device500A/500B, when a part of the WDM beam amplified by the preamp115is branched and output to an SAU510(S51), the SAU510detects 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 device500B, a difference occurs between the profile information of the optical signal stored in the SAU510and the newly detected optical level of the optical signal. The SAU510carries out differentiation, and outputs difference information to the unit controller116(S52). The unit controller116instructs adjustment of the attenuation to the VOA112based on the difference information input from the SAU510, 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 (S53).

Thus, in the fourth embodiment, since the SAU510analyzes the wavelength of the WDM beam received by the reception unit110, 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. 6is an explanatory diagram of the configuration of an optical transmission device according to a fifth embodiment of the present invention. As shown inFIG. 6, in an optical transmission device600A/600B according to the fifth embodiment, a SAU610is newly connected to the preamp115in the reception unit110, and a SAU620is connected to the postamp121, as in the optical transmission device300A/300B according to the second embodiment.

The operation at the time of transmitting the transmission light from the optical transmission device600A to the optical transmission device600B is explained next. At first, the postamp121amplifies the WDM beam input thereto, branches a part of the WDM beam, and outputs the branched beam to the SAU620(S61). The SAU620analyzes the optical signal in the WDM beam to detect the profile information, and outputs the detected profile information to the OSC controller140(S62). The OSC controller140outputs the OSC beam including the profile information to the optical transmission device600B (S63).

The transmission light output from the optical transmission device600A is input to the optical transmission device600B via the outer ring910. The transmission light is branched to the WDM beam and the OSC beam (S63) by the OSC branch coupler114, and the OSC beam is output to the OSC controller130. The WDM beam branched by the OSC branch coupler114is amplified by the preamp115, and a part of the WDM beam is branched and input to the SAU610(S64).

The SAU610detects the optical level of a plurality of multiplexed optical signals from the input WDM beam, and outputs the information to the unit controller116as profile information of the optical signal (S65). The OSC controller130extracts the profile information from the OSC beam, and outputs the extracted profile information to the unit controller116(S66).

The unit controller116compares the profile information detected by the SAU620in the optical transmission device600A with the profile information detected by the SAU610in the optical transmission device600B, to calculate a loss in the transmission light in a section630. Based on the calculation result, the unit controller116instructs adjustment of the attenuation to the VOA112, 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 (S67).

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 SAU610and the SAU620to the reception unit110and the transmission unit120, to obtain a loss in a predetermined section630including the outer ring910.

FIG. 7is an explanatory diagram of the configuration of an optical transmission device according to a sixth embodiment of the present invention. As shown inFIG. 7, in an optical transmission device700A/700B according to the sixth embodiment, the OSC controller130is connected to the reception unit, and the OSC controller140is connected to the transmission unit120. 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 device700A to the optical transmission device700B is explained next. At first, to set the attenuation of the VOA112at the time of startup, the ASE beam is transmitted from the OSC controller140to the OSC controller130(S71). The OSC controller130obtains information of the optical level at the time of outputting the optical signal from the OSC controller140in the optical transmission device700A and the optical level at the time of inputting the optical signal to the OSC controller130in the optical transmission device700B from the input OSC beam (for example, the ASE beam), and outputs the information to the unit controller116(S72).

The unit controller116calculates a loss at the time of transmission in the outer ring910from 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 controller130at all times, and the OSC controller130calculates a loss at the time of present transmission in the outer ring910. When the transmission characteristic changes, there is a change in the loss. When there is a change in the loss, the unit controller116calculates a difference in the loss due to the change, and instructs adjustment of the attenuation to the VOA112based 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 (S73).

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. 8is an explanatory diagram of the configuration of an optical transmission device according to a seventh embodiment of the present invention. As shown inFIG. 8, in an optical transmission device800A/800B according to the seventh embodiment has a configuration in which a PD811is added to the transmission unit120in the optical transmission device700A/700B shown in the sixth embodiment. The operation at the time of transmitting the transmission light from the optical transmission device800A to the optical transmission device800B is explained next. At first, the OSC beam output from the OSC controller140for adjusting the VOA112at the time of startup is input to the PD811via the OSC combination coupler122.

The PD811detects the optical level of the OSC beam, and outputs the optical level to the OSC controller140(S81). The OSC controller140outputs the OSC beam including the detected optical level to the optical transmission device800B (S82). The OSC beam (S82) is input to the OSC controller130by the OSC branch coupler114. The OSC controller130extracts the information of the optical level detected by the PD811from the input OSC beam, and outputs the information to the unit controller116(S83).

When the OSC beam is input to the optical transmission device800B, the optical level of the OSC beam is detected by the front PD111, and output to the unit controller116(S84). The unit controller116calculates a loss at the time of transmission in the outer ring910from the information of the optical level of the OSC beam input from the OSC controller130and the information of the optical level of the OSC beam input from the front PD111. Thereafter, when the normal operation is started, the latest information of the optical level of the OSC beam is input to the OSC controller130at all times, and the OSC controller130calculates a loss at the time of present transmission in the outer ring910. When the transmission characteristic changes, there is a change in the loss. When there is a change in the loss, the unit controller116calculates a difference in the loss due to the change, and instructs adjustment of the attenuation to the VOA112based 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 (S85).

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 devices200A/200B to800A/800B, 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.