Patent Publication Number: US-11652563-B2

Title: Optical demultiplexer, optical separation device, optical transmission system, and optical transmission method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application under 35 U.S.C. § 371 of International Application No. PCT/JP2019/028084, having an International Filing Date of Jul. 17, 2019, the disclosure of which is considered part of the disclosure of this application, and is incorporated in its entirety into this application. 
     TECHNICAL FIELD 
     The present invention relates to an optical demultiplexer, an optical separator, an optical transmission system, and an optical transmission method for multiplexing optical signals of a plurality of wavelengths to be transmitted through one optical fiber and dividing the multiplexed signal into optical signals of respective wavelengths on a reception side to be received. 
     BACKGROUND ART 
     Currently, there is an optical transmission system in which in the case of transmitting and receiving optical signals of a plurality of wavelengths between communication nodes, the optical signals are multiplexed to be transmitted through an optical fiber, and the multiplexed signal is divided into optical signals of respective wavelengths at a node on a reception side to be received. 
     Many optical transmission systems each use a colored network in which an optical signal of one wavelength is assigned to one transponder of a communication node. In this network, when a set wavelength is changed in a remotely-located transponder connected by an optical fiber as an optical transmission line, a change operation must be performed at the site. 
     Thus, in recent years, it has become popular to make a network colorless as one method for improving flexibility and extensibility of the network. Making a network colorless means a method in which a wavelength of an optical signal can be changed in a transponder by an instruction from a remote location to reduce local work. 
     As this type of optical transmission system, there is an optical transmission system having a star configuration illustrated in  FIG.  10   , for example. Time- and wavelength-division multiplexing (TWDM), which is a hybrid method of time-division multiplexing (TDM) and wavelength-division multiplexing (WDM), is applied to an optical transmission system  10  illustrated in  FIG.  10   . 
     The optical transmission system  10  includes: two optical couplers  12   a  and  12   b  disposed apart from each other in remote locations and connected to each other by one optical fiber  11 ; and a plurality (n) of transmission nodes  14   a ,  14   b ,  14   c , . . . , and  14   n  connected to one of the optical couplers  12   a  by individual optical fibers  13 . Furthermore, the optical transmission system  10  includes: n T filters (tunable filters)  16   a ,  16   b ,  16   c , . . . , and  16   n  connected to the other of the optical couplers  12   b  by individual optical fibers  15 ; and n reception nodes  18   a ,  18   b ,  18   c , . . . , and  18   n  connected to the n T filters  16   a  to  16   n  by individual optical fibers  17 . 
     Here, the transmission nodes  14   a  to  14   n  transmit information in the present example, but can also receive information. The reception nodes  18   a  to  18   n  receive information, but can also transmit information. 
     The transmission node  14   a  includes a transponder  21   a , an optical receiver  22   a , and an optical transmitter  23   a . The transmission node  14   b  includes a transponder  21   b , an optical receiver  22   b , and an optical transmitter  23   b . The transmission node  14   c  includes a transponder  21   c , an optical receiver  22   c , and an optical transmitter  23   c . The transmission node  14   n  includes a transponder  21   n , an optical receiver  22   n , and an optical transmitter  23   n.    
     The reception node  18   a  includes a transponder  25   a , an optical transmitter  27   a , and an optical receiver  26   a . The reception node  18   b  includes a transponder  25   b , an optical transmitter  27   b , and an optical receiver  27   b . The reception node  18   c  includes a transponder  25   c , an optical transmitter  27   c , and an optical receiver  26   c . The reception node  18   n  includes a transponder  25   n , an optical transmitter  27   n , and an optical receiver  26   n.    
     Here, the transponders  21   a  to  21   n , and  25   a  to  25   n  have the same function. The optical transmitters  23   a  to  23   n , and  27   a  to  27   n  have the same function to perform burst transmission or the like, but wavelengths of optical signals that can be transmitted are different as described later. The optical receivers  22   a  to  22   n , and  26   a  to  26   n  have the same function, but wavelengths of optical signals that can be received are different as described later. 
     The transponder  21   a  of the transmission node  14   a  relays information transmitted from a communication terminal (not illustrated) of a user or the like by carrying out relay processing involving mutual conversion between an optical signal and an electrical signal. The optical transmitter  23   a  superimposes the information relayed by the transponder  21   a  on an optical signal of a wavelength λ 1 , stores the superimposed optical signal of the wavelength λ 1  (also referred to as “optical signal λ 1 ”) in a time slot t 1 , and transmits the optical signal λ 1  stored in the time slot t 1  to the optical coupler  12   a  via the optical fiber  13 . 
     The optical transmitter  23   b  of the transmission node  14   b  superimposes information relayed by the transponder  21   b  on an optical signal of a wavelength λ 1 , stores this optical signal λ 1  in a time slot t 2 , and transmits the optical signal λ 1  stored in the time slot t 2  to the optical coupler  12   a  via the optical fiber  13 . 
     The optical transmitter  23   c  of the transmission node  14   c  superimposes information relayed by the transponder  21   c  on an optical signal of a wavelength λ 2 , stores the superimposed optical signal of the wavelength λ 2  (also referred to as “optical signal λ 2 ”) in the time slot t 1 , and transmits the optical signal λ 2  stored in the time slot t 1  to the optical coupler  12   a  via the optical fiber  13 . 
     The optical transmitter  23   n  of the transmission node  14   n  superimposes information relayed by the transponder  21   n  on an optical signal of a wavelength λn, stores the superimposed optical signal of the wavelength λn (also referred to as “optical signal λn”) in the time slot t 1 , and transmits the optical signal λn stored in the time slot t 1  to the optical coupler  12   a  via the optical fiber  13 . 
     The optical coupler  12   a  multiplexes the optical signals λ 1  to λn from the transmission nodes  14   a  to  14   n  by TWDM. The optical signals λ 1  to λn as the multiplexed TWDM signal are transmitted to the other of the optical couplers  12   b  via the optical fiber  11 . Note that the multiplexed optical signals λ 1  to λn are to be referred to as a “multiplexed optical signal λ 1  to λn”. 
     The optical coupler  12   b  branches the multiplexed optical signals λ 1  to λn transmitted from the optical fiber  11  into n pieces and transmits the branched optical signals to the n T filters  16   a  to  16   n  which are active filters, via the optical fiber  15 . 
     The T filters  16   a  to  16   n  each transmit only an optical signal of a predetermined wavelength among the optical signals λ 1  to λn, and transmit the optical signal to a corresponding one of the optical receivers  26   a  to  26   n  of a corresponding one of the reception nodes  18   a  to  18   n , via the optical fiber  17 . A transmission band in which the T filters  16   a  to  16   n  each transmit only an optical signal of a predetermined wavelength is changed by remote instruction. 
     The T filter  16   a  transmits only the optical signal λ 1  among the optical signals λ 1  to λn and transmits the optical signal λ 1  to the optical receiver  26   a  of the reception node  18   a  via the optical fiber  17 . In the present example, the optical signal λ 1  of the time slot t 1  and the optical signal λ 1  of the time slot t 2  are transmitted to the optical receiver  26   a . The T filter  16   b  transmits only the optical signal λ 2  of the time slot t 1  to the optical receiver  26   b . The T filter  16   n  transmits only the optical signal λn of the time slot t 1  to the optical receiver  26   n.    
     The optical receiver  26   a  receives the optical signals λ 1  and λ 1  of the time slots t 1  and t 2 . The transponder  25   a  relays the received optical signals λ 1  and λ 1  of the time slots t 1  and t 2  to a communication terminal. The optical receiver  26   b  receives the optical signal λ 2  of the time slot t 1 , and the transponder  25   b  relays the received optical signal λ 2  to a communication terminal. The optical receiver  26   n  receives the optical signal λn of the time slot t 1 , and the transponder  25   n  relays the received optical signal λn to a communication terminal. 
     In this manner, the optical signals of the respective wavelengths λ 1  to λn transmitted from the transmission nodes  14   a  to  14   n  are received at the reception nodes  18   a  to  18   n  at remote locations, via the optical fiber  11 . 
     In this optical transmission system  10 , the T filters  16   a  to  16   n  are used, but as in an optical transmission system  10 A illustrated in  FIG.  11   , a wavelength selective switch (WSS)  30 , which is an active filter, may be used. 
     The WSS  30  is connected between the other end of the optical fiber  11  having one end connected to the optical coupler  12   a  and the optical receivers  26   a  to  26   n . Each output port of the WSS  30  is connected to each of the optical receivers  26   a  to  26   n.    
     The WSS  30  is an optical switch having a function capable of changing a combination of a wavelength and an output port by remote control, in addition to wavelength multiplexing/demultiplexing functions for connecting time- and wavelength-division multiplexed optical signals λ 1  to λn to be transmitted to the optical fiber  11  to a different output port for each of the wavelengths λ 1  to λn. In other words, when the WSS  30  is used, it is possible to indicate which optical signal of optical signals of the wavelengths λ 1  to λn is to be transmitted to which optical receiver of the optical receivers  26   a  to  26   n.    
     As the technology for configuring the above-described type of optical transmission system  10  or  10 A, there are exemplified technologies described in NPLs 1, 2, and 3. 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: Okada et al., “Verification of Wavelength Routing Function by Wavelength Transfer Matrix networks”, Research Report, Fac. Sci. Tech., Seikei Univ. Vol. 43 No. 2 (2006), pp. 75-81.
     NPL 2: Emile Archambault et al., “Design and Simulation of Filterless Optical Networks: Problem Definition and Performance Evaluation”, J. OPT. COMMUN. NETW./VOL. 2, No. 8/August 2010.   NPL 3: Marij a Furdek et al., “Programmable Filterless Network Architecture Based on Optical White Boxes”, 20th International Conference on Optical Network Design and Modeling (ONDM 2016), May 9-12, 2016, Cartagena, Spain.   

     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, in the above-described optical transmission systems  10  and  10 A, the T filters  16   a  to  16   n  and the WSS  30 , each of which is an active filter, are used. In the active filter, due to aging deterioration or failure, the transmission band shifts or is narrowed as described later, resulting in a variation in transmission wavelength. As a result, there is a problem in that an optical signal of a target wavelength (for example, the wavelength λ 1 ) cannot be appropriately transmitted, resulting in communication interruption. 
       FIG.  12    illustrates an example in which the transmission band of the T filter  16   a  shifts. When there is no deterioration or failure in the T filter  16   a , the T filter  16   a  transmits the optical signal of the target wavelength λ 1  in the transmission band λ 1   a  to λ 1   d , as illustrated on the T filter output side. However, when due to deterioration or failure, the center λ 1  of the transmission band λ 1   a  to λ 1   d  of the T filter  16   a  shifts to λ 1   a  as indicated by an arrow Y 1 , the transmission bandwidth becomes λ 0  to λ 1 , such that it is not possible to properly transmit the optical signal of the target wavelength λ 1 . 
       FIG.  13    illustrates an example of narrowing of the band of the T filter  16   a . It is assumed that due to deterioration or failure of the T filter  16   a , the transmission band λ 1   a  to λ 1   d  of the T filter  16   a  that transmits the optical signal of the target wavelength λ 1  has been narrowed to a transmission band λ 1   b  to λ 1   c  narrower than the bandwidth of the target wavelength λ 1 , as indicated by opposing arrows Y 2  and Y 3 . For example, in a case where the transmission band is narrowed to ½ of the band of the target wavelength λ 1 , it becomes impossible to properly transmit the optical signal of the target wavelength λ 1 . 
     In a flexible network, as illustrated in  FIG.  14   , widths (grid spacings) of transmission bands λa, λb, λc, and λd of the T filter  16   a  becomes narrower. It is assumed that in this case, the transmission band has shifted by shifting or narrowing of the transmission band of the T filter  16   a , as from a dashed frame  71  to a dashed frame  72  indicated by an arrow Y 4 . When only 1/10 of an optical signal of a predetermined wavelength can be transmitted, for example, it becomes more difficult to transmit the optical signal of the target wavelength λ 1 . 
     The present invention has been made in view of such circumstances, and an object of the present invention is to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted, and to properly transmit an optical signal of a target wavelength to suppress communication interruption. 
     Means for Solving the Problem 
     In order to solve the above problems, an optical demultiplexer according to an aspect of the present invention includes: a plurality of optical gate switches, each of which is configured to transmit, when being turned on, and to block, when being turned off, a multiplexed optical signal, the multiplexed optical signal being obtained by multiplexing optical signals of a plurality of wavelengths by time-division multiplexing or multiplexing optical signals of a plurality of wavelengths by wavelength-division multiplexing in addition to time-division multiplexing; and a cyclic arrayed waveguide grating (cAWG) including a plurality of input ports and a plurality of output ports and configured to input the multiplexed optical signal transmitted through the optical gate switches from the plurality of input ports, demultiplex the input multiplexed optical signal for each wavelength, and cycle and output the demultiplexed optical signals from the plurality of output ports in a predetermined order. 
     Effects of the Invention 
     According to the present invention, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted, and to properly transmit an optical signal of a target wavelength to suppress communication interruption. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating an optical transmission system to which an optical demultiplexer according to a first embodiment of the present invention is applied, the optical transmission system having a star configuration. 
         FIG.  2    is an operating principle diagram of a cAWG. 
         FIG.  3    is a diagram for explaining an input-output operation of the cAWG. 
         FIG.  4    is a block diagram for explaining a first operation by TDM of an optical transmission system according to the first embodiment. 
         FIG.  5    is a block diagram for explaining a second operation by TWDM of the optical transmission system according to the first embodiment. 
         FIG.  6    is a flowchart for explaining the second operation. 
         FIG.  7    is block diagram for explaining a third operation by TWDM of the optical transmission system according to the first embodiment. 
         FIG.  8    is a block diagram illustrating an optical transmission system to which an optical demultiplexer according to a second embodiment of the present invention is applied, the optical transmission system having a link configuration. 
         FIG.  9    is a block diagram illustrating an optical transmission system to which an optical separator according to a third embodiment of the present invention is applied, the optical transmission system having a star configuration. 
         FIG.  10    is a block diagram illustrating a configuration in which a T filter is used in an optical transmission system of a star configuration of the related art. 
         FIG.  11    is a block diagram illustrating a configuration in which a WSS is used in the optical transmission system of a star configuration of the related art. 
         FIG.  12    is a diagram illustrating an example in which a transmission band of the T filter shifts. 
         FIG.  13    is a diagram illustrating an example of narrowing of a band of the T filter. 
         FIG.  14    is a diagram illustrating that a width (grid spacing) of the transmission band of the T filter becomes narrower. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. Here, in all the drawings of the present specification, components having corresponding functions are denoted by the same reference signs and description thereof will be appropriately omitted. 
     Configuration of First Embodiment 
       FIG.  1    is a block diagram illustrating an optical transmission system to which an optical demultiplexer according to a first embodiment of the present invention is applied, the optical transmission system having a star configuration. 
     An optical transmission system  10 B illustrated in  FIG.  1    is different from the related-art optical transmission system  10  described above ( FIG.  10   ) in that the optical transmission system  10 B uses an optical demultiplexer  40  in place of the T filters  16   a  to  16   n  ( FIG.  10   ). The optical demultiplexer  40  includes the same number of optical gate switches  41   a ,  41   b ,  41   c , . . . , and  41   n  as the number of branches n of an optical coupler  12   b  and a cyclic arrayed waveguide grating (cAWG)  42 . 
     The optical gate switches  41   a  to  41   n  utilize, for example, intensity modulation of electro-optic (EO) modulation or LN (phase) modulation to turn on/off (ON/OFF) an optical signal in response to an electric field intensity. The optical gate switches  41   a  to  41   n  transmit, when being turned on, and block, when being turned off, optical signals of wavelengths λ 1  to λn (optical signals λ 1  to λn) from the optical coupler  12   b.    
     The cAWG  42  includes n×n input/output ports, n input ports of which are connected to output sides of the n optical gate switches  41   a  to  41   n , and n output ports of which are connected to n optical receivers  26   a  to  26   n.    
     The cAWG  42  will be described with reference to  FIG.  2   . The cAWG  42  illustrated in  FIG.  2    has n inputs and m outputs and is a passive component, and the interior of which is a wavelength circuit of a wavelength transmission matrix Lmn. Here, it is assumed that m of the m outputs is the same number as n. 
     When optical signals of a plurality of wavelengths (wavelength group) are input from the In side, a wavelength group of a sequence different from that of the wavelength group on the input side In is output from the Om side after processing in the wavelength circuit of the wavelength transmission matrix Lmn of the cAWG  42 . The wavelength transmission matrix Lmn is a two-dimensional matrix and is defined by the following Equation (1):
 
 Om=Lmn×In   (1)
 
     In Equation (1), Om is an output matrix of m rows and I columns, Lmn is a wavelength transmission matrix of m rows and n columns of the wavelength circuit, and In is an input matrix of n rows and I columns. 
     An example of the cAWG  42  of 3×3 in  FIG.  3    is illustrated and described as an example based on the wavelength transmission matrix Lmn. The cAWG  42  has three input ports i 1 , i 2 , and i 3  and three output ports o 1 , o 2 , and o 3 . 
     In the cAWG  42 , in a lateral direction indicated by an arrow Y 6 , optical signals of wavelengths λa 1 , λa 2 , and λa 3  having different frequencies are input to an input port i 1 . In response to this input, in a vertical direction indicated by an arrow Y 7 , an optical signal of a wavelength λa 1  is output from an output port o 1 , an optical signal of a wavelength λa 2  is output from an output port o 2 , and an optical signal of a wavelength λa 3  is output from an output port o 3 . 
     Similarly, optical signals of wavelengths λb 1 , λb 2 , and λb 3  having different frequencies are input to an input port i 2 . In response to this input, an optical signal of a wavelength λb 2  is output from the output port o 1 , an optical signal of a wavelength λb 3  is output from the output port o 2 , and an optical signal of a wavelength λb 1  is output from the output port o 3 . 
     Further, optical signals of wavelengths λc 1 , λca, and λc 3  having different frequencies are input to an input port i 3 . In response to this input, an optical signal of a wavelength λc 3  is output from the output port o 1 , an optical signal of a wavelength λc 1  is output from the output port o 2 , and an optical signal of a wavelength λc 2  is output from the output port o 3 . 
     In this manner, the cAWG  42  performs processing (cyclic processing or wavelength routing processing) to demultiplex optical signals of a plurality of wavelengths input from one input port and output the demultiplexed optical signals while the n output ports are cycled. The processing is similarly performed for a plurality of optical signals input from the n input ports. Here, the wavelength routing processing is performed by the wavelength circuit of the wavelength transmission matrix Lmn. 
     Next, in the optical transmission system  10 B illustrated in  FIG.  1   , a transponder  21   a  of a transmission node  14   a  relays information transmitted from a communication terminal (not illustrated) of a user or the like and outputs the information to an optical transmitter  23   a . The optical transmitter  23   a  superimposes the relayed information on the optical signal of the wavelength λ 1 , stores the optical signal (optical signal λ 1 ) of the wavelength λ 1  after superposition in a time slot t 1 , and transmits the optical signal λ 1  stored in the time slot t 1  to an optical coupler  12   a  (one optical coupler) via an optical fiber  13 . This transmission is burst transmission. In other words, the transmission node  14   a  stores the optical signal  1 , on which the information received from the communication terminal is superimposed, in the time slot t 1  and transmits the optical signal λ 1  stored in the time slot t 1  to the optical coupler  12   a.    
     Similarly, transmission nodes  14   b  to  14   n  also transmit optical signals λ 1 , λ 2 , and λn to the optical coupler  12   a . At this time, the transmission node  14   b  stores the optical signal λ 1  in a time slot t 2  and transmits the optical signal λ 1  stored in the time slot t 2 , the transmission node  14   c  stores the optical signal λ 2  in the time slot t 1  and transmits the optical signal λ 2  stored in the time slot t 1 , and the transmission node  14   n  stores the optical signal λn in the time slot t 1  and transmits the optical signal λn stored in the time slot t 1 . 
     The optical coupler  12   a  multiplexes the optical signals λ 1  to λn from the transmission nodes  14   a  to  14   n  by TWDM, and transmits the multiplexed optical signal (TWDM signal) λ 1  to kn to the other optical coupler  12   b  via an optical fiber  11 . 
     The optical coupler  12   b  branches the multiplexed optical signal λ 1  to λn from the optical fiber  11  into n pieces, and transmits the branched optical signals to the n optical gate switches  41   a  to  41   n  via an optical fiber  15 . 
     The optical receivers  26   a  to  26   n  of reception nodes  18   a  to  18   n  receive optical signals output from the cAWG  42  in response to ON/OFF of the optical gate switches  41   a  to  41   n . Transponders  25   a  to  25   n  relay the received optical signals to communication terminals. 
     First Operation of First Embodiment 
     Next, a first operation by TDM of the optical transmission system  10 B according to the first embodiment illustrated in  FIG.  4    will be described. 
     Optical signals λ 1 , λ 1 , . . . , and λ 1  on which information received from a communication terminal is superimposed is transmitted to the optical coupler  12   a  by the transmission nodes  14   a  to  14   n . At this time, the transmission node  14   a  stores an optical signal λ 1  in the time slot t 1  and transmits the optical signal λ 1  stored in the time slot t 1 , the transmission node  14   b  stores an optical signal λ 1  in a time slot t 2  and transmits the optical signal λ 1  stored in the time slot t 2 , the transmission node  14   c  stores an optical signal λ 1  in a time slot t 3  and transmits the optical signal λ 1  stored in the time slot t 3 , and the transmission node  14   n  stores an optical signal λ 1  in a time slot tn and transmits the optical signal λ 1  stored in the time slot tn. 
     The optical coupler  12   a  time-division-multiplexes the optical signals λ 1  of the time slots t 1  to tn from the transmission nodes  14   a  to  14   n  and transmits the multiplexed optical signal λ 1  of the time slots t 1  to tn to the other optical coupler  12   b  via the optical fiber  11 . 
     The optical coupler  12   b  branches the multiplexed optical signal λ 1  of the time slots t 1  to tn from the optical fiber  11  to n pieces, and transmits the branched optical signals to the n optical gate switches  41   a  to  41   n  via the optical fiber  15 . 
     Here, it is assumed that all the optical gate switches  41   a  to  41   n  are turned on. In this case, the multiplexed optical signal λ 1  of the time slots t 1  to tn is transmitted through each of the optical gate switches  41   a  to  41   n  and is input to each of the input ports of the cAWG  42 . 
     The cAWG  42  performs wavelength routing processing on the optical signal λ 1  of the time slots t 1  to tn input from each of the input ports to output the optical signal λ 1  of the time slots t 1  to tn from each of the output ports. This optical signal λ 1  of the time slots t 1  to tn is received at each of the optical receivers  26   a  to  26   n  of the reception nodes  18   a  to  18   n , and is relayed to a communication terminal by each of the transponders  25   a  to  25   n.    
     Second Operation of First Embodiment 
     Next, a second operation by TWDM of the optical transmission system  10 B according to the first embodiment illustrated in  FIG.  5    will be described with reference to the flowchart illustrated in  FIG.  6    together. 
     In step S 1  illustrated in  FIG.  6   , optical signals λ 1 , λ 2 , . . . , and λn of a plurality of wavelengths on which the information received from a communication terminal is superimposed are transmitted to the optical coupler  12   a  by the transmission nodes  14   a  to  14   n  illustrated in  FIG.  5   . At this time, the transmission node  14   a  stores an optical signal λ 1  in the time slot t 1  and transmits the optical signal λ 1  stored in the time slot t 1 , the transmission node  14   b  stores an optical signal λ 1  in the time slot t 2  and transmits the optical signal λ 1  stored in the time slot t 2 , the transmission node  14   c  stores an optical signal λ 2  in the time slot t 1  and transmits the optical signal λ 2  stored in the time slot t 1 , and the transmission node  14   n  stores an optical signal λn in the time slot t 1  and transmits the optical signal λn stored in the time slot t 1 . 
     In step S 2 , the optical coupler  12   a  multiplexes the optical signals λ 1  of the time slots t 1  to tn from the transmission nodes  14   a  to  14   n  by TWDM. In the TWDM, two optical signals each having an identical wavelength λ 1  are time-division-multiplexed in the time slots t 1  and t 2  and optical signals of the plurality of wavelengths λ 1 , λ 2 , . . . , and λn are wavelength-division-multiplexed to be multiplexed. This multiplexed optical signal λ 1  of the time slots t 1  to tn is transmitted to the other optical coupler  12   b  via the optical fiber  11 . 
     In step S 3 , the optical coupler  12   b  branches the multiplexed optical signal λ 1  of the time slots t 1  to to from the optical fiber  11  into n pieces and transmits the branched optical signals to the n optical gate switches  41   a  to  41   n  via the optical fiber  15 . 
     Here, at step S 4 , it is determined whether the optical gate switches  41   a  to  41   n  each are turned on. As a result, when all the optical gate switches  41   a  to  41   n  are not turned on but turned off, a transmission operation of the optical signal is terminated. 
     On the other hand, when it is determined that only the optical gate switch  41   a  is turned on, in step S 5 , the optical gate switch  41   a  transmits the multiplexed optical signals λ 1 , λ 1 , . . . , and λn of the time slots t 1  and t 2  from the optical coupler  12   b  and outputs the optical signal to a first input port of the cAWG  42 . 
     In step S 6 , the cAWG  42  performs wavelength routing processing on the multiplexed optical signals λ 1 , λ 1 , . . . , and λn of the time slots t 1  and t 2  input from the first input port. This wavelength routing processing causes the cAWG  42  to output the optical signals λ 1  of the time slots t 1  and t 2  from an output port connected to the optical receiver  26   a  of the reception node  18   a . The cAWG  42  also outputs the optical signal λ 2  of the time slot t 1  from an output port connected to the optical receiver  26   b  of the reception node  18   b . The cAWG  42  also outputs the optical signal λn of the time slot t 1  from an output port connected to the optical receiver  26   n  of the reception node  18   n.    
     In step S 7 , in the reception nodes  18   a  to  18   n , the optical signals λ 1  of the time slots t 1  and t 2  received by the optical receiver  26   a  are relayed and transmitted to communication terminals. Similarly, the optical signal λ 2  of the time slot t 1  received by the optical receiver  26   b  and the optical signal λn of the time slot t 1  received by the optical receiver  26   n  are relayed and transmitted to communication terminals. 
     Third Operation of First Embodiment 
     Next, a third operation by TWDM of the optical transmission system  10 B according to the first embodiment illustrated in  FIG.  7    will be described with reference to the flowchart illustrated in  FIG.  6    together. 
     This third operation performs similar processing from step S 1  to step S 4  illustrated in  FIG.  6   . 
     When it is determined from the determination result in step S 4  that only the optical gate switch  41   b  is turned on, in step S 5 , the optical gate switch  41   b  transmits the multiplexed optical signals λ 1 , λ 1 , . . . , and λn of the time slots t 1  and t 2  from the optical coupler  12   b  and inputs the optical signal to a second input port of the cAWG  42 . 
     In step S 6 , the cAWG  42  performs wavelength routing processing on the multiplexed optical signals λ 1 , λ 1 , . . . , and λn of the time slots t 1  and t 2  input from the second input port. This wavelength routing processing causes the cAWG  42  to output the optical signal λ 2  of the time slot t 1  from an output port connected to the optical receiver  26   a . The cAWG  42  outputs the optical signal λn of the time slot t 1  from an output port connected to the optical receiver  26   c . The cAWG  42  also outputs the optical signals λ 1  of the time slots t 1  and t 2  from an output port connected to the optical receiver  26   n.    
     In step S 7 , in the reception nodes  18   a  to  18   n , the optical signal λ 2  of the time slot t 1  received by the optical receiver  26   a  is relayed and transmitted to a communication terminal. Similarly, the optical signal λn of the time slot t 1  received by the optical receiver  26   c  and the optical signals λn of the time slots t 1  and t 2  received by the optical receiver  26   n  are relayed and transmitted to the communication terminals. 
     In such an optical transmission system  10 B, the optical demultiplexer  40  including the optical gate switches  41   a  to  41   n  that performs only simple on/off operation and the cAWG  42  that is a passive component in the subsequent stage is used. Due to this, in the optical demultiplexer  40 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. In addition, the optical demultiplexer  40  having a simple configuration can realize such processing, such that the cost can be reduced. 
     Configuration of Second Embodiment 
       FIG.  8    is a block diagram illustrating an optical transmission system to which an optical demultiplexer according to a second embodiment of the present invention is applied, the optical transmission system having a link configuration. 
     An optical transmission system  10 C illustrated in  FIG.  8    has a dual ring configuration using two optical fibers  31   a  nd  32  each having a ring shape. A plurality of optical couplers  31   a ,  31   b ,  31   c ,  31   d  (first optical couplers) and a plurality of relays  31   e ,  31   f ,  31   g  each including an optical amplifier are interposed and connected in the outer optical fiber  31 . A plurality of optical couplers  32   a ,  32   b ,  32   c ,  32   d  (second optical couplers) and a plurality of relays  32   e ,  32   f  each including an optical amplifier are connected in the inner optical fiber  32 . 
     A transmission device  51  of a node  50 A (first node) for transmitting and receiving an optical signal and performing predetermined processing is connected to the optical coupler  31   a  connected in the outer optical fiber  31 . A transmission device  51  of a node  50 B is connected to the optical coupler  31   b , a transmission device  51  of a node  50 C is connected to the optical coupler  31   c , and a transmission device  51  of a node  50 D (second node) is connected to the optical coupler  31   d.    
     A reception device  52  of the node  50 A is connected to the optical coupler  32   a  connected in the inner optical fiber  32 , a reception device  52  of the node  50 B is connected to the optical coupler  32   b , a reception device  52  of the node  50 C is connected to the optical coupler  32   c , and a reception device  52  of the node  50 D is connected to the optical coupler  32   d  (connection lines are not illustrated). 
     The transmission device  51  includes n optical transmitters  51   a ,  51   b , . . . , and  51   n , as representatively illustrated for the node  50 A. 
     The reception device  52  includes n optical receivers  52   a ,  52   b , . . . , and  52   n  and the optical demultiplexer  40  described above, as representatively illustrated for the node  50 D. The optical gate switches  41   a  to  41   n  of the optical demultiplexer  40  are connected to the optical coupler  31   d . The output ports of the cAWG  42  are connected to the optical receivers  52   a ,  52   b , . . . , and  52   n . Here, the other nodes  50 A to  50 C each also include the optical demultiplexer  40 , although not illustrated. 
     The outer optical fiber  31  transmits an optical signal in a clockwise direction, as indicated by an arrow Y 9 . The inner optical fiber  32  transmits an optical signal in a counter-clockwise direction opposite to the clockwise direction. During these transmissions, the relays  31   e ,  31   f , and  31   g  and the relays  32   e  and  32   f  amplify and relay the optical signal. The optical couplers  31   a  to  31   d  and the optical couplers  32   a  to  32   d  perform coupling (multiplexing) or branching of an optical signal. 
     It is assumed that in such a configuration, optical signals of wavelengths λ 1  to λn (optical signals λ 1  to λn) are transmitted from the optical transmitters  51   a ,  51   b , . . . , and  51   n  of the node  50 A toward the node  50 D as follows. 
     That is, the optical transmitter  51   a  stores an optical signal λ 1  in a time slot t 1  and transmits the optical signal λ 1  stored in the time slot t 1  to the optical coupler  31   a . The optical transmitter  51   b  stores an optical signal λ 2  in a time slot t 2  and transmits the optical signal λ 2  stored in the time slot t 2  to the optical coupler  31   a , and the optical transmitter  51   n  stores an optical signal λn in the time slot t 1  and transmits the optical signal λn stored in the time slot t 1  to the optical coupler  31   a.    
     The optical coupler  31   a  multiplexes the transmitted optical signals λ 1 , λ 2 , . . . , and λn by TWDM, and transmits the multiplexed optical signal λ 1  to λn in the clockwise direction indicated by the arrow Y 9  via the outer optical fiber  31 . At this time, the multiplexed optical signals λ 1  to λn are amplified and relayed by the relays  31   f  and  31   g.    
     The relayed multiplexed optical signal λ 1  to λn is branched into the number (n) of the optical gate switches  41   a  to  41   n  at the optical coupler  31   d , and transmitted to the optical gate switches  41   a  to  41   n  via an optical fiber. 
     Here, when it is assumed that only the optical gate switch  41   a  is turned on, the optical gate switch  41   a  transmits the multiplexed optical signal λ 1  to λn from the optical coupler  31   d  and outputs the optical signal to a first input port of the cAWG  42 . 
     The cAWG  42  performs wavelength routing processing on the multiplexed optical signals λ 1 , λ 2 , . . . , and λn of the time slots t 1  and t 2  input from the first input port. This wavelength routing processing causes the cAWG  42  to output the optical signal λ 1  of the time slot t 1  from an output port connected to the optical receiver  52   a . A reception device  52   a  receives the optical signal λ 1 . 
     Similarly, the cAWG  42  outputs the optical signal λ 2  of the time slot t 2  from an output port connected to the optical receiver  52   b . A reception device  52   b  receives the optical signal λ 2 . In addition, the cAWG  42  outputs the optical signal λn of the time slot t 1  from an output port connected to the optical receiver  52   n . A reception device  52   n  receives the optical signal λn. In this way, the optical signals λ 1  to λn transmitted from the node  50 A are received at the node  50 D. 
     In the optical transmission system  10 C, the optical demultiplexer  40  used in a network having a ring configuration includes only the optical gate switches  41   a  to  41   n  that performs only a simple on/off operation and the cAWG  42  which is a passive component in the subsequent stage. Due to this, in the optical demultiplexer  40 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. In addition, the optical demultiplexer  40  having a simple configuration can realize such processing, such that the cost can be reduced. 
     Configuration of Third Embodiment 
       FIG.  9    is a block diagram illustrating an optical transmission system to which an optical separator according to a third embodiment of the present invention is applied, the optical transmission system having a star configuration. 
     An optical transmission system  10 D illustrated in  FIG.  9    is different from the optical transmission system  10 B according to the first embodiment (see  FIG.  1   ) in that an optical separator  60  is used in place of the optical demultiplexer  40 . The optical separator  60  includes n optical gate switches  61   a ,  61   b ,  61   c , . . . , and  61   n  that perform an on/off operation at a high speed, and the cAWG  42  as described above. 
     The optical gate switches  61   a  to  61   n  are capable of transmitting an optical signal of a single wavelength in a burst unit (one packet). In other words, the optical gate switches  61   a  to  61   n  are capable of separating wavelengths λ 1  to λn of the optical signal in a time unit. For example, the optical gate switch  61   a  is turned on only during a time slot t 1  and transmits only an optical signal λ 1  of the time slot t 1  of time-division-multiplexed optical signals λ 1 , λ 2 , . . . , and λn by TDM transmitted from the optical coupler  12   b.    
     Similarly, the optical gate switch  61   b  is turned on only during a time slot t 2  and transmits only an optical signal λ 1  of the time slot t 2 . The optical gate switch  61   c  is turned on only during a time slot t 3  and transmits only an optical signal λ 2  of the time slot t 3 . The optical gate switch  61   n  is turned on only during a time slot tn and transmits only an optical signal λn of the time slot tn. 
     The cAWG  42  performs wavelength routing processing on the optical signals λ 1 , λ 2 , . . . , and λn transmitted as described above. This wavelength routing processing causes the cAWG  42  to output the optical signals λ 1  of the time slot t 1  from an output port connected to the optical receiver  26   a . The cAWG  42  also outputs the optical signal λ 2  of the time slot t 2  from an output port connected to the optical receiver  26   b . The cAWG  42  also outputs the optical signal λn of the time slot tn from an output port connected to the optical receiver  26   n.    
     In such a configuration, first, the optical signals λ 1 , λ 1 , λ 2 , . . . , and λn on which information received from a communication terminal is superimposed are transmitted to the optical coupler  12   a  by the transmission nodes  14   a  to  14   n . At this time, the transmission node  14   a  stores the optical signal λ 1  in the time slot t 1  and transmits the optical signal λ 1  stored in the time slot t 1 , the transmission node  14   b  stores the optical signal λ 1  in the time slot t 2  and transmits the optical signal λ 1  stored in the time slot t 2 , the transmission node  14   c  stores the optical signal λ 2  in the time slot t 3  and transmits the optical signal λ 2  stored in the time slot t 3 , and the transmission node  14   n  stores the optical signal λn in the time slot tn and transmits the optical signal λn stored in the time slot tn. 
     The optical coupler  12   a  time-division multiplexes the optical signals λ 1  to λn of the time slots t 1  to tn from the transmission nodes  14   a  to  14   n , and transmits the multiplexed optical signal λ 1  to λn of the time slots t 1  to tn to the other optical coupler  12   b  via the optical fiber  11 . However, in the present example, the optical signals λ 1  to λn are time-division multiplexed, but may be time- and wavelength-division multiplexed. 
     The optical coupler  12   b  branches the multiplexed optical signal λ 1  to λn of the time slots t 1  to tn from the optical fiber  11  into n pieces, and transmits the branched signals to the n optical gate switches  61   a  to  61   n  via the optical fiber  15 . 
     The optical gate switches  61   a  to  61   n  each are turned on only during a predetermined time slot to transmit an optical signal, as follows. That is, the optical gate switch  61   a  is turned on only during the time slot t 1  to transmit only the optical signal λ 1 , and the optical gate switch  61   b  is turned on only during the time slot t 2  to transmit only the optical signal λ 1 . The optical gate switch  61   c  is turned on only during the time slot t 3  to transmit only the optical signal λ 2 . The optical gate switch  61   n  is turned on only during the time slot tn to transmit only the optical signal λn. 
     The cAWG  42  performs wavelength routing processing on the optical signals λ 1  to λn transmitted as described above to output the optical signal λ 1  of the time slot t 1  to the optical receiver  26   a  and to output the optical signal λ 1  of the time slot t 2  to the optical receiver  26   b . The cAWG  42  also outputs the optical signal λ 2  of the time slot t 3  to the optical receiver  26   c  and outputs the optical signal λn of the time slot tn to the optical receiver  26   n . The optical signals λ 1 , λ 2 , . . . , and λn received by the optical receivers  26   a  to  26   n  are relayed to communication terminals by the transponders  25   a  to  25   n.    
     The optical separator  60  used in such an optical transmission system  10 D includes only the optical gate switches  61   a  to  61   n  that perform only a simple on/off operation, the optical gate switches capable of transmitting and separating the multiplexed optical signal λ 1  to λn for each optical signal of a single wavelength (one packet), and the cAWG  42  that is a passive component in the subsequent stage. 
     Due to this, in the optical separator  60 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, it is possible to eliminate a variation in transmission wavelength when an optical signal having a predetermined wavelength is transmitted and to properly transmit an optical signal having a target wavelength, such that communication interruption can be suppressed. In addition, the optical separator  60  having a simple configuration can realize such processing, such that the cost can be reduced. It is possible to realize such an effect in a flexible network. 
     However, also when the optical separator  60  is used in place of the optical demultiplexer  40  of the optical transmission system  10 C having a ring configuration illustrated in  FIG.  8   , the same effects as the optical transmission system  10 D can be obtained. 
     Effects 
     (1) The optical demultiplexer  40  includes the plurality of optical gate switches  41   a  to  41   n  configured to transmit, when being turned on, and block, when being turned off, a multiplexed optical signal, the multiplexed optical signal obtained by multiplexing optical signals of a plurality of wavelengths by time-division multiplexing or wavelength-division multiplexing in addition to time-division multiplexing, and the cAWG  42  including a plurality of input ports and a plurality of output ports and configured to input the multiplexed optical signal transmitted through the optical gate switches  41   a  to  41   n  from the plurality of input ports, demultiplex the input multiplexed optical signal for each wavelength, and cycle and output the demultiplexed optical signals from the plurality of output ports in a predetermined order. 
     According to this configuration, the optical demultiplexer  40  can be configured by including the optical gate switches  41   a  to  41   n  that perform only a simple on/off operation and the cAWG  42  that is a passive component in the subsequent stage. According to this optical demultiplexer  40 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, when the optical demultiplexer  40  is used on the input side of the reception nodes  18   a  to  18   n  of an optical network, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. In addition, the optical demultiplexer  40  can be realized with the simple configuration described above, such that the cost can be reduced. 
     (2) The optical separator  60  includes the plurality of optical gate switches  61   a  to  61   n  configured to perform an on operation that transmits an optical signal of a single wavelength in a time unit and an off operation that blocks an optical signal, the plurality of optical gate switches  61   a  to  61   n  being configured to, at the time of inputting a multiplexed optical signal obtained by time-division multiplexing optical signals of a plurality of wavelengths, perform the on operation only during a time slot of a predetermined time in performing the time-division multiplexing to separate an optical signal of a single wavelength stored in the time slot, and the cAWG  42  including a plurality of input ports and a plurality of output ports and configured to input the optical signal of the single wavelength transmitted through the optical gate switches  61   a  to  61   n  from the plurality of input ports, and cycle and output the input optical signal of the single wavelength from the plurality of output ports in a predetermined order. 
     According to this configuration, the optical separator  60  can be configured by including the optical gate switches  61   a  to  61   n  configured to performs only a simple on/off operation consisting of an on operation that transmits an optical signal of a single wavelength in a time unit and an off operation that blocks an optical signal, and the cAWG  42  that is a passive component in the subsequent stage. According to this optical separator  60 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, when the optical separator  60  is used on the input side of the reception nodes  18   a  to  18   n  of the optical network, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. 
     (3) When a multiplexed optical signal obtained by multiplexing optical signals of a plurality of wavelengths by wavelength-division multiplexing in addition to time-division multiplexing is input, the optical gate switches  61   a  to  61   n  according to (2) above perform the on operation only during a time slot of a predetermined time when the time-division multiplexing is performed to separate an optical signal of a single wavelength stored in the time slot, and the cAWG  42  inputs the optical signal of the single wavelength transmitted through the optical gate switches  61   a  to  61   n  from the plurality of input ports, demultiplexes the input optical signal of the single wavelength, and cycles and outputs the demultiplexed optical signals from the plurality of output ports in a predetermined order. 
     According to this configuration, even if the multiplexed optical signal is a signal obtained by multiplexing optical signals of a plurality of wavelengths by wavelength-division multiplexing in addition to time-division multiplexing, an optical signal of a single wavelength can be separated by the optical gate switches  41   a  to  41   n , and the multiplexed optical signal can be demultiplexed for each wavelength by the cAWG  42 . Thus, an optical signal of a predetermined single wavelength can be output from the optical separator  60 . 
     (4) The optical transmission system  10 B includes: a plurality of transmission nodes  14   a  to  14   n  configured to transmit optical signals of a plurality of wavelengths; one optical coupler  12   a  configured to multiplex the optical signals of a plurality of wavelengths transmitted from the plurality of transmission nodes  14   a  to  14   n  by time-division multiplexing or wavelength-division multiplexing in addition to time-division multiplexing; the other optical coupler  12   b  connected to the one optical coupler  12   a  via an optical transmission line and configured to branch the multiplexed optical signal transmitted in the optical transmission line; and a plurality of reception nodes  18   a  to  18   n  connected to the optical demultiplexer  40  described in (1) above to which the other optical coupler  12   b  is connected, in which the multiplexed optical signal branched by the other optical coupler  12   b  is transmitted through each of the optical gate switches  41   a  to  41   n  of the optical demultiplexer  40  and optical signals obtained by demultiplexing the transmitted multiplexed optical signal for each wavelength by the optical demultiplexer  40  are received at the reception nodes  18   a  to  18   n.    
     According to this configuration, in the optical demultiplexer  40 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, in the optical transmission system, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted, and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. 
     (5) The optical transmission system  10 C includes: an optical transmission line (optical fibers  31 ,  32 ) having a ring shape; a first optical coupler  31   a  connected to the optical transmission line and configured to multiplex optical signals of a plurality of wavelengths by time-division multiplexing or wavelength-division multiplexing in addition to time-division multiplexing; a second optical coupler  31   d  connected to the optical transmission line and configured to branch an optical signal multiplexed by the first optical coupler  31   a ; a first node  50 A connected to the first optical coupler  31   a  and having a plurality of optical transmitters  51   a  to  51   n  configured to transmit optical signals of a plurality of wavelengths; and a second node having the optical demultiplexer  40  described in (1) above connected to the second optical coupler  31   d  and a plurality of optical receivers  52   a  to  52   n  configured to receive optical signals from the plurality of optical transmitters  51   a  to  51   n  via the optical demultiplexer  40 , wherein optical signals of a plurality of wavelengths transmitted from the plurality of optical transmitters  51   a  to  51   n  are multiplexed by the first optical coupler, the multiplexed optical signal is transmitted through each of the optical gate switches  41   a  to  41   n  of the optical demultiplexer  40 , and optical signals obtained by demultiplexing the transmitted multiplexed optical signal for each wavelength by the optical demultiplexer  40  are received by the optical receivers  52   a  to  52   n.    
     According to this configuration, in an optical transmission system having a ring configuration as well, similarly to the optical transmission system of (4), it is possible by the optical demultiplexer  40  to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted, and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. 
     (6) The optical transmission system  10 D includes: a plurality of transmission nodes  14   a  to  14   n  configured to transmit optical signals of a plurality of wavelengths; one optical coupler  12   a  configured to transmit a multiplexed optical signal obtained by multiplexing the optical signals of a plurality of wavelengths transmitted from the plurality of transmission nodes  14   a  to  14   n  by time-division multiplexing; the other optical coupler  12   b  connected to the one optical coupler  12   a  via an optical transmission line and configured to branch the multiplexed optical signal transmitted in the optical transmission line; and a plurality of reception nodes  18   a  to  18   n  connected to the optical separator  60  described in (2) above to which the other optical coupler  12   b  is connected, wherein the multiplexed optical signal branched by the other optical coupler  12   b  is transmitted through each of the optical gate switches  61   a  to  61   n  of the optical separator  60 , and optical signals obtained by separating the transmitted multiplexed optical signal for each time slot in the optical separator  60  are received at the reception nodes  18   a  to  18   n.    
     According to this configuration, in the optical separator  60  used in the optical transmission system, shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, in the optical transmission system, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted, and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. 
     (7) The optical transmission system  10 D includes: the optical separator  60  described in (3) above; a plurality of transmission nodes  14   a  to  14   n  configured to transmit optical signals of a plurality of wavelengths; one optical coupler  12   a  configured to transmit a multiplexed optical signal obtained by multiplexing the optical signals of a plurality of wavelengths transmitted from the plurality of transmission nodes  14   a  to  14   n  by wavelength-division multiplexing in addition to time-division multiplexing; the other optical coupler  12   b  connected to the one optical coupler  12   a  via an optical transmission line and configured to branch the multiplexed optical signal transmitted in the optical transmission line; and a plurality of reception nodes  18   a  to  18   n  connected to the optical separator  60  to which the other optical coupler  12   b  is connected, wherein the multiplexed optical signal branched by the other optical coupler  12   b  is transmitted through each of optical gate switches  41   a  to  41   n  of the optical separator  60 , and optical signals obtained by demultiplexing the transmitted multiplexed optical signal for each time slot in the optical separator  60  are received at the reception nodes  18   a  to  18   n.    
     According to this configuration, similarly to the optical transmission system  10 D described in (6) above, in the optical separator  60 , shifting or narrowing of the transmission band as in an active filter is eliminated. Accordingly, in the optical transmission system, it is possible to eliminate a variation in transmission wavelength when an optical signal of a predetermined wavelength is transmitted, and to properly transmit an optical signal of a target wavelength, such that communication interruption can be suppressed. 
     In addition, a specific configuration can be appropriately changed without departing from the gist of the present disclosure. 
     REFERENCE SIGNS LIST 
     
         
           10 B,  10 C,  10 D Optical transmission system 
           11 ,  31 ,  32  Optical fiber 
           12   a ,  12   b  Optical coupler 
           14   a  to  14   n  Transmission node 
           18   a  to  18   n  Reception node 
           21   a  to  21   n ,  25   a  to  25   n  Transponder 
           22   a  to  22   n ,  27   a  to  27   n  Optical receiver 
           23   a  to  23   n ,  26   a  to  26   n  Optical transmitter 
           31   a  to  31   d ,  32   a  to  32   d  Optical coupler 
           31   e ,  31   f ,  31   g ,  32   e ,  32   f  Relay 
           50 A to  50 D Node 
           40  Optical demultiplexer 
           41   a  to  41   n ,  61   a  to  61   n  Optical gate switch 
           42  cAWG 
           51  Transmission device 
           51   a  to  51   n  Optical transmitter 
           52  Reception device 
           52   a  to  52   n  Optical receiver 
           60  Optical separator