Patent Publication Number: US-2007104488-A1

Title: Optical circuit and linear system dedicated node apparatus, linear system WDM network, and tree system WDM network using such

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a U.S. continuation application, filed under 35 USC 111(a) and claiming the benefit under 35 USC 120 and 365(c), of PCT application JP2004/009822 filed Jul. 9, 2004. The foregoing application is hereby incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to optical circuits and, more particularly, to an optical circuit, which converts a ring dedicated node apparatus into a linear system dedicated node apparatus by being attached to the ring dedicated node apparatus and a linear system dedicated node apparatus, a linear system WDM network and a tree system WDM network using such an optical circuit.  
      2. Description of the Related Art  
       FIG. 1A  shows a structure diagram of an example of a ring system WDM (Wavelength Division Multiplexer) network. In the figure, node apparatuses  10   a - 10   d  exclusive for a ring system constitute a ring network and perform add/drop of an optical signal. In the ring network, the optical signal is transmitted in a single direction (a counterclockwise direction).  
       FIG. 1B  shows an explanatory illustration of the node apparatuses  10   a - 10   d  for a ring topology network (hereinafter, referred to as ring system dedicated). Here, a description will be given of the node apparatus  10   a  as an example. In the figure, the node apparatus  10   a  receives a WDM signal from the ring network at an input port  11 , and transmits a WDM signal to the ring network from an output port  12 . An add light input through an input port  13  is multiplexed and transmitted from the output port  12 , and a drop light received and demultiplexed by the input port  11  is output from an output port  14 . The node apparatus  10   a  is provided with a filter at the input port to terminate an add light of its own node so as to prevent the add light of its own node from being input after going round the ring network.  
       FIG. 2A  shows a structure diagram of an example of a tree system WDM network. Node apparatuses  20   a - 20   f  exclusive for a ring system constitute a tree-type network and perform add/drop of an optical signal. In the tree-type network, the optical signal is transmitted in both left and right directions.  
       FIG. 2B  shows an explanatory illustration of the node apparatuses  20   a - 20   f  for a linear topology network (hereinafter, referred to as linear system dedicated node apparatuses) used for a tree system WDM network. Here, a description will be given of the node apparatus  20   a  as an example. In the figure, the node apparatus  20   a  performs reception and transmission by an input/output port  21  with respect to a network connected on the left side, and performs reception and transmission by an input/output port  22  with respect to a network connected on the right side. An add light input through an input port  23  is multiplexed and transmitted to the networks from the input/output ports  21  and  22 , and a drop light received and demultiplexed by the input/output ports  21  and  22  is output from an output port  24 . It should be noted that a node  25  is constituted by a star coupler that is a combination of 1×2 optical couplers, as shown in  FIG. 2C .  
      The node apparatuses  10   a - 10   d  shown in  FIG. 1 (B) and the node apparatuses  20   a - 20   f  are different in their internal structures, and there is a problem in that both cannot be used in common.  
      Meanwhile, Patent Document 1 discloses a node apparatus for bidirectional optical communication performing bidirectional optical communication by transmitting optical signals of different wavelengths in both directions, comprising a unidirectional signal processing part that applies predetermined optical signal processing to an optical signal transmitted in a single direction and a unidirectional/bidirectional conversion processing part that causes a flow of each of optical signals of upward direction and downward direction to be in unidirectional and, on the other hand, causes a flow of optical signals from the unidirectional optical signal processing part to be in bidirectional so that bidirectional wavelength multiplex communication can be performed using an existing node apparatus for unidirectional optical communication.  
      Patent Document 1: Japanese Laid-Open Patent Application No. 11-127121  
      However, the method recited in Patent Document 1 must divide a wavelength band of an optical signal to be transmitted to a network connected on the left side of the node apparatus and a wavelength band of an optical signal to be transmitted to a network connected on the right side of the node apparatus. Thus, there is a problem in that a number of transmitters and a number of occupied wavelengths in the transmission path of the network need to be twice a number of transmission signals.  
      Moreover, in a linear system WDM network shown in  FIG. 3 , when performing transmission from a node apparatus  32  to networks connected on left and right sides using the same wavelength, an optical signal added in the node apparatus  32  is dropped by each of node apparatuses  31  and  33 . For example, when a malfunction occurs in the network connected on the right side of the node apparatus  33  and a reflection of an optical signal occurs at the position where the malfunction occurs, an optical signal added in the node apparatus  32  is reflected at the position where the malfunction occurs and is dropped at each of the node apparatuses  31  and  33 . Thus, a coherent cross-talk occurs, which deteriorates signals. Additionally, even if the above-mentioned problem does not occur, there is a problem in that a Rayleigh scattered light in a transmission path or a reflected light at an end surface of a connector causes a coherent cross-talk, which deteriorates optical signals.  
     SUMMARY OF THE INVENTION  
      It is a general object of the present invention to provide an optical circuit in which the above-mentioned problems are eliminated.  
      A more specific object of the present invention is to provide an optical circuit that causes a ring system dedicated node apparatus to be used in common as a part of a linear system dedicated node apparatus by attaching to a ling system dedicated node apparatus to convert into a linear system dedicated node apparatus, and a linear system dedicated node apparatus, a linear system WDM network and a tree system WED network using such an optical circuit.  
      In order to achieve the above-mentioned objects, there is provided according to the present invention an optical circuit that is attachable to a ring system dedicated node apparatus, which performs add/drop on an optical signal received from a first network and transmits to a second network, so as to convert into a linear system dedicated node apparatus that performs add/drop on an optical signal received from the first or second network and transmit to the second or first network, the optical circuit comprising an optical filter having a characteristic of reflecting an optical signal band supplied from the second network and transmitting an optical signal band supplied from the first network, and transmitting an occupied band of an add light in the ring system dedicated node apparatus to which the own circuit is attached at a predetermined transmission rate and reflecting the reminder.  
      According to the above-mentioned optical circuit, the ring system dedicated node apparatus can be used in common with a part of the linear system dedicated node apparatus by attaching the optical circuit to the ring system dedicated node apparatus to convert the ring system dedicated node apparatus into the linear system dedicated node apparatus. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1A  is a structure diagram of a ring system dedicated WDM network;  
       FIG. 1B  is an illustration of a ring system dedicated node apparatus;  
       FIG. 2A  is a structure diagram of a tree system WDM network;  
       FIG. 2B  is an illustration of a linear system dedicated node apparatus;  
       FIG. 2C  is a structure diagram of a node shown in  FIG. 2A ;  
       FIG. 3  is an illustration for explaining a malfunction in a linear system WDM network;  
       FIG. 4  is a structure diagram of a ring system dedicated node apparatus according to an embodiment of the present invention;  
       FIG. 5  is a structure diagram of a ring system WDM network having the ring system dedicated node apparatus shown in  FIG. 4 ;  
       FIG. 6  is a structure diagram of a linear system WDM network according to a first embodiment of the present invention using an optical circuit of the present invention;  
       FIG. 7A  is a side view of an optical filter shown in  FIG. 6 ;  
       FIG. 7B  is a side view of an optical filter shown in  FIG. 6 ;  
       FIG. 8A  is a structure diagram of an embodiment of a linear system WDM network constituted by the linear system dedicated node apparatuses of the present invention;  
       FIG. 8B  is a transmission/reflection characteristic diagram of an optical filter shown in  FIG. 8A ;  
       FIG. 9A  is a structure diagram of a node apparatus for explaining optical paths when a failure occurs in the node apparatus;  
       FIG. 9B  is a structure diagram of the node apparatuses shown in  FIG. 8A  for explaining optical paths when a failure occurs in the node apparatuses;  
       FIG. 10A  is a structure diagram of a tree system WDM network constituted by connecting three linear system WDM networks using a star coupler;  
       FIG. 10B  is an illustration for explaining nodes that transmit a signal light from the left side and nodes that transmit a signal light from the right side, viewed from each node;  
       FIG. 11  is a transmission/reflection characteristic diagram of an optical filter of each node apparatus of the tree system WDM network of FIG.  10 A;  
       FIG. 12  is a block diagram of a node apparatus of a simplified structure which does not need to receive a WDM signal from a network on the right side;  
       FIG. 13A  is a structure diagram of a network for explaining prevention of coherent cross-talk when using the node apparatus shown in  FIG. 12 ;  
       FIG. 13B  is a structure diagram of another network for explaining prevention of coherent cross-talk when using the node apparatus shown in  FIG. 12 ;  
       FIG. 14  is a block diagram of a linear system dedicated node apparatus according to a second embodiment using an optical circuit according to the present invention; and  
       FIG. 15  is a block diagram of a linear system dedicated node apparatus according to a third embodiment using an optical circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A description will now be given of embodiments according to the present invention.  
       FIG. 4  is a block diagram of a ring system dedicated node apparatus according to an embodiment of the present invention. The ring system dedicated node apparatus shown in  FIG. 4  performs add/drop on an optical signal received from a network connected on the left side and transmits the optical signal to a network connected on the right side.  
      In  FIG. 4 , a ring system dedicated node apparatus  40  receives a WDM signal from a ring network by an input port  41 , and transmits the WDM signal to a ring network from an output port  42 . Add lights of wavelengths of λ 1 , λ 2 , λ 3  and λ 4  are supplied to and multiplexed by a 1×4 optical coupler  45 , and are supplied to a reject and add filter  46 .  
      The reject and add filter  46  removes occupied bands of the wavelengths λ 1 , λ 2 , λ 3  and λ 4  of the add light in the WDM signal supplied from the input port  41 , and thereafter multiplexes the multiplexed add light (wavelengths of λ 1 , λ 2 , λ 3  and λ 4  supplied from the 1×4 optical coupler  45  and supplies the add light to a 1×2 optical coupler  47 . The 1×2 optical coupler  47  branches the WDM signal from the reject and add filter  46 , and outputs one of the branched signal from an output port  42  and supplies the other branched signal to a 1×4 optical coupler  48 . It should be noted that the 1×2 optical coupler  47  outputs from an output port  42  75% of the WDM signal from the reject and add filter  46 , and supplies 25% to the 1×4 optical coupler  48 .  
      The 1×4 optical coupler  48  branches the above-mentioned WDM signal into four signals and supplied the signals to a variable optical filters  49   a - 49   d . The variable optical filters  49   a - 49   d  demultiplex drop lights of wavelength of λi, λj, λk, and λl from the WDM signal, respectively, and output them from an output port  44 . Although λ 1 -λ 4 ≠λi-λl in a usual state where communication with other node apparatuses is performed, the wavelengths λi-λl may be set to one of the wavelength λ 1 -λ 4  in order to identify a cause of a malfunction.  
      The above-mentioned ring system dedicated node apparatus  40  is used as node apparatuses  40   a ,  40   b  and  40   c  of the ring system WDM network shown in  FIG. 5 . Here, occupied areas of the add lights of the node apparatuses  40   a ,  40   b  and  40   c  are different from each other. For example, an add light of the wavelength of λ 8  added by the node apparatus  40   b  is dropped by each of the node apparatuses  40   c  and  40   a , and, thereby received by the node apparatuses  40   c  and  40   a  simultaneously.  
       FIG. 6  is a block diagram of a linear system dedicated node apparatus according to a first embodiment that is configured using an optical circuit according to the present invention. The linear system dedicated node apparatus performs add/drop on an optical signal received from a network connected on the left side so as to send the optical signal to a network connected on the right side, and performs add/drop on an optical signal received from the network connected on the right side so as to send the optical signal to the network connected on the left side.  
      In  FIG. 6 , the linear system dedicated node apparatus comprises a ring system dedicated node apparatus  40  having the same structure as that shown in  FIG. 4  and an optical circuit  50 .  
      The optical circuit  50  comprises circulators  51  and  54  and optical filters  52  and  53 . The circulator  51  has a first port a connected to the network on the left side, a second port b connected to the optical filter  52 , and a third port c connected to the filter  53 , wherein an optical signal input through the first port a is output from the second port b and an optical signal input through the third port c is output from the first port a.  
      The optical filter  52  has 100% transmittance with respect to an optical signal band supplied from the network connected on the left side and 100% reflectance with respect to an optical signal band supplied from the network connected on the right side. Any transmittance from 0% through 100% (100% -0% reflectance) may be used with respect to occupied band of an add light of the node apparatus  40 . Thereby, it is possible to select the same transmittance with the optical filter  53  mentioned later. The optical signal transmitted through the optical filter  52  and the optical signal reflected by the optical filter  52  are supplied to a reject and add filter  46  of the node apparatus  40 .  
      The optical filter  53  has 100% transmittance with respect to an optical signal band supplied from the network connected on the left side and 100% reflectance with respect to an optical signal band supplied from the network connected on the right side, and has a predetermined transmittance with respect to an occupied band of an add light of the node apparatus  40 . The predetermined transmittance is determined by a position of a network on which the node apparatus is provided. If it is located near the center of the network, the transmittance is set to 50% (50% transmittance). If the right side of the node apparatus  40  is neat an end of the network, the transmittance is decreased (increasing reflectance) so as to set the filter characteristic to increase an add light transmitted to the center of the network is increased since a number of node apparatuses connected on the lift side of the node apparatus  40  is large.  
       FIG. 7A  is a side view of the optical filter  52 . In  FIG. 7A , an optical transmission film  56  is provided on a surface of a transparent substrate  55 . An optical signal from the circulator  54  is supplied to a port P 1 , and an optical signal band supplied from the network connected on the right side transmits through the transparent film  56  and the optical transmission film  56  and multiplexed with an optical signal band supplied from the network connected on the right side, which is reflected by the optical transmission film  56 , and exits toward the ring system dedicated node apparatus  40  from a port P 3 .  
       FIG. 7B  is a side view of the optical filter  53 . In  FIG. 7B , an optical transmission film  58  is provided on a surface of a transparent substrate  57 . An optical signal from the ring system dedicated node apparatus  40  is supplied to a port P 4 . An optical signal band of the optical signal supplied from the network connected on the left side and a part of an add light of the node apparatus  40  transmit through the transparent substrate  57  and the optical transmission film  58  and exit from a port P 5  toward the circulator  54 . On the other hand, an optical signal band supplied from the network connected on the right side and the rest of the add light of the node apparatus are reflected by the optical transmission film  58 , and exit from a port P 6  toward the circulator  51 .  
      The circulator  54  has a first port a connected to the optical filter  53 , a second port b connected to the network on the right side, and a third port c connected to the optical filter  52 , wherein an optical signal input through the second port b is output from the third port c, and an optical filter input through the first port a is output from the second port b.  
      An optical signal supplied from the left side in  FIG. 6  is supplied to the reject and add filter  46  of the node apparatus  40  through the circulator  51  and the optical filter  52  so as to remove an add light occupied band of the node apparatus  40 . Then, a part of the optical signal is split by the 1×2 optical coupler  47  toward a 1×2 optical coupler  48 , and the rest of the optical signal transmits through the optical filter  53  and sent to the network on the right side through the circulator  54 .  
      On the other hand, an optical signal supplied from the right side in  FIG. 6  is supplied to the reject and add filter  46  of the node apparatus  40  through the circulator  54  and the optical filter  52  so as to remove an add light occupied band of the node apparatus  40 . Then, a part of the optical signal is split by the 1×2 optical coupler  47  toward the 1×2 optical coupler  48 , and the rest of the optical signal is reflected by the optical filter  53  and sent to the network on the left side through the circulator  51 .  
      A WDM signal added by the reject and add filter  46  of the node apparatus  40  is supplied to the optical filter  53  through the 1×2 optical coupler  47 , and a part of the optical signal reflected by the optical filter  53  is sent to the network on the left side through the circulator  51 . A part of the optical signal transmitted through the optical filter  53  is sent to the network on the right side through the circulator  54 .  
      Thus, the ring system dedicated node apparatus can be used as a part of a ring system dedicated node apparatus.  
      In the meantime, as for the optical filters  52  and  53 , a variable optical filter such as disclosed in the following Document 1 may be used, which is of a single input and double output type and the two outputs have inverse characteristics to each other.  
      Document 1: “wide band programmable optical frequency filter”, Shiyo Jingu, electronic-intelligence communication society papers, C-I Vol. J81-C-I No. 4, pp. 254-263, April 1998  
      If such a variable optical filter is used as the optical filters  52  and  53 , a wavelength characteristic in each of linear system dedicated node apparatuses in a tree system WDM network can be changed by a management apparatus which manages the tree system WDM network so that add light occupied bands do not overlap with each other, and also positions of the linear system dedicated node apparatuses in the tree system network can be changed freely by changing transmittance of the add light occupied bands.  
       FIG. 8A  is a block diagram of an embodiment of a linear system WDM network constituted by the linear system dedicated node apparatus according to the present invention. In  FIG. 8A , each node apparatus has the structure shown in  FIG. 6 , which comprises the ring system dedicated node apparatus  40  and the optical circuit  50 . The node apparatus  61  and the node apparatus  62  are connected with each other by an optical fiber transmission path  64 , and the node apparatus  62  and the node apparatus  63  are connected with each other by an optical fiber transmission path  65 . Optical fiber transmission paths are connected to the left side of the node apparatus  61  and the right side of the node apparatus  63  so as to form a linear system WDM network. Add light occupied bands of the node apparatuses  61 ,  62  and  63  are set so that they do not overlap with each other.  
       FIG. 8B  shows a transmission/reflection characteristic of the optical filter  53  of each of the node apparatuses  61 ,  62  and  63 . The optical filters  52  and  53  of the node apparatus  61  have 50% transmittance and 50% reflectance in the add light occupied band of its own so as to exhibit a transmittance indicated by a solid line and a reflectance indicated by a dashed line in an upper part of  FIG. 8B . Additionally, the optical filters  52  and  53  of the node apparatus  61  have 0% transmittance and 100% reflectance with respect to an optical signal (add light occupied bands of the node apparatuses  62  and  63 ) supplied from the network on the right side.  
      The optical filters  52  and  53  of the node apparatus have, as shown in the middle of  FIG. 8B , 50% transmittance and 50% reflectance in the add light occupied band of its own, and have 100% transmittance and 0% reflectance in the band of the optical signal supplied from the network on the left side and 0% transmittance and 100% reflectance with respect to an optical signal (add light occupied band of the node apparatus  63 ) supplied from the network on the right side.  
      The optical filters  52  and  53  of the node apparatus  63  have, as shown in a lower part of  FIG. 8B , 50% transmittance and 50% reflectance in the add light occupied band, and have 100% transmittance and 0% reflectance in a band (add light occupied bands of the node apparatuses  61  and  62 ) of an optical signal supplied from the network on the left side.  
      Accordingly, an add light of the node apparatus  62  is branched a indicated by an arrow in  FIG. 8A  by the optical filter  53  of the node apparatus  62 . One of the branched add lights is sent to the optical fiber transmission path through the circulator  54 , and, then, transmitted to the node apparatus  63  and the optical fiber transmission path beyond the node apparatus  63 . The other of the branched add lights is sent to the optical fiber transmission path  64  through the circulator  51  of the node apparatus  62 , and, then, transmitted to the node apparatus  61  and the optical fiber transmission path beyond the node apparatus  61 . Thus, the ring system dedicated node apparatuses  40  constituting the node apparatuses  61  and  63  are capable of simultaneously receiving optical signals added by the node apparatus  62 .  
      When a failure occurs in the network connected on the right side of the optical circuit  50  and a reflection of the optical signal occurs at a position where the failure occurs as shown in  FIG. 9A , the optical signal added by the node apparatus  40  is reflected (as indicated by a dashed line) at the position where the failure occurs, and supplied to the optical filter  52  through the circulator  54 . A part of the optical signal reflected by the optical filter  52  is removed by the reject and add filter  46 , and the optical signal transmitted through the optical filter  52  is output to a non-coupled port and removed.  
      On the other hand, when a failure occurs in the network connected on the left side of the optical circuit  50  and a reflection of the optical signal occurs at a position where the failure occurs, the optical signal added by the node apparatus  40  is reflected (as indicated by a dashed line) at the position where the failure occurs, and supplied to the optical filter  52  through the circulator  51 . A part of the optical signal transmitted through the optical filter  52  is removed by the reject and add filter  46 , and the optical signal reflected by the optical filter  52  is output to the non-coupled port and removed.  
      When a failure occurs in the network connected on the right side of the ode apparatus  63  in the linear system WDM network constituted by the node apparatuses  61 ,  61  and  63  and a reflection of the optical signal occurs at the position where the failure occurs as shown in  FIG. 9B , the optical signal added by the node apparatus  61  is reflected (as indicated by a dashed line) at the position where the failure occurs, and supplied to the optical filter  52  through the circulator  54  of the node apparatus  63 . A reflected component of the add light of the ode apparatus  62  is removed by causing occupied band of the add light of the node apparatus  62  to transmit therethrough and output to a non-coupled port.  
      Moreover, when a failure occurs in the network connected on the left side of the node apparatus  61  and a reflection of an optical signal occurs at a position where the failure occurs, the optical signal added by the node apparatus  62  is reflected (indicated by a dashed line) at the position where the failure occurs and supplied to the optical filter  52  through the circulator  51  of the node apparatus  61 . The optical filter  52  of the node apparatus  61  reflects the whole occupied band of the add light of the ode apparatus  62  and output it to a non-coupled port, and, thereby, the reflection component of the add light is removed.  
      Therefore, the optical signal is prevented from being deteriorated due to a coherent cross talk caused by a Rayliegh scattered light in a transmission path or an obstacle and a reflection light at an end surface of a connector. Additionally, there is no need to separate a wavelength band of an optical signal to be sent to the network connected on the left side of the node apparatus from a wavelength band of an optical signal to be sent to the network connected on the right side of the node apparatus, and, hereby, a number of transmitters and a number of occupied wavelengths in the transmission path of the network can be equal to a number of signals to transmit.  
      A description will now be given of a tree system WDM network constituted by connecting three linear system WDM networks L 1 , L 2 , L 3  using a star coupler SP as shown in  FIG. 10A . In the figure, the network comprises node apparatuses  1  through  9 . Each of the node apparatuses  1  through  9  has the same structure as that shown in  FIG. 6 , and an add light occupied bands of the node apparatuses are set so that they do not overlap with each other. The network is terminated on the right side of each of the node apparatuses  3 ,  6  and  9 . Thus, each of the node apparatuses  3 ,  6  and  9  does not need to receive a signal from the network on the right side.  FIG. 10B  shows a node that transmits a signal light (WDM signal) from the left side viewed from each node X (X=1 through 9) and a node that transmits a signal light from the right side. It should be noted that the star coupler has the same structure as that shown in  FIG. 2C .  
       FIG. 14  is a block diagram of a linear system dedicated node apparatus according to a second embodiment using an optical circuit according to the present invention. It should be noted that numbers indicated along the horizontal axis are the numbers of the node apparatuses.  
      For example, as shown in  FIG. 11 -(A), the optical filters  52  and  53  of the node apparatus  1  have 50% transmittance and 50% reflectance in the add light occupied band of its own, 100% transmittance and 0% reflectance with respect to optical signal bands (add light occupied bands of the node apparatuses  4  through  9 ) supplied from the left side, and 0% transmittance and 100% reflectance with respect to optical signal bands (add light occupied bands of the node apparatuses  2  and  3 ) supplied from the right side.  
      Additionally, as shown in  FIG. 11 -(B), the optical filters  52  and  53  of the node apparatus  2  have 50% transmittance and 50% reflectance in the add light occupied band of its own, 100% transmittance and 0% reflectance with respect to optical signal bands (add light occupied bands of the node apparatuses  1  and  4  through  9 ) supplied from the left side, and 0% transmittance and 100% reflectance with respect to an optical signal band (add light occupied band of the node apparatus  1 ) supplied from the right side. Similarly, other node apparatuses  3  through  9  have transmittance and reflectance shown in  FIG. 11 -(C) through  11 -(I).  
      With respect to the node apparatuses  3 ,  6  and  9  that do not need to receive a WDM signal from the network on the right side, the simplified structure shown in  FIG. 12  can be used. In  FIG. 12 , an optical circuit  65  is constituted by the circulator  51  alone. The circulator has the first port a connected to the network on the left side, the second port b connected to the reject and add filter  46  of the ring system dedicated node apparatus  40 , the third port c connected to the 1×2 optical coupler  47  of the node apparatus  40 .  
      Although optical signals added by other node apparatuses and supplied from the network on the left side make a round and return to the network on the left side, if the optical filters  52  and  53  of the node apparatuses other than the node apparatuses  3 ,  6  and  9  have ideal characteristics, a coherent cross talk can be prevented.  
      As shown in  FIG. 13A , an optical signal added by the node apparatus  2  makes a round in the optical circuit  65  of the node apparatus  3  and returns to the node apparatus  2 . Then the optical signal goes through the circulator  54  and supplied to the optical filter  52  in the node apparatus  2 . A part of the optical signal reflected by the optical filter  52  is removed by the reject and add filter  46 , and a part of the optical signal transmitted through the optical filter  52  is output to the non-coupled port and removed.  
      As shown in  FIG. 13B , an optical signal added by the node apparatus  1  and passed through the node apparatus  2  makes a round in the optical circuit  65  of the node apparatus  3  and returns to the node apparatus  2 . Then, the optical signal goes through the circulator  54  and supplied to the optical filter  52  in the node apparatus  2 , and is transmitted through the optical filter  52  and output to the non-coupled port and removed.  
       FIG. 14  is a block diagram of a linear system dedicated node apparatus according to a second embodiment using an optical circuit according to the present invention. The optical circuit  70  shown in  FIG. 14  differs from the optical circuit  50  shown in  FIG. 6  in that a variable optical attenuator  71  is connected between the circulator  51  and the optical filter  52  and a variable optical attenuator  72  is connected between the optical filter  52  and the circulator  54 .  
      In this embodiment, if a light intensity of a WDM signal received from the network on the left side differs from a light intensity of an add light of its own, the WDM signal received from the network on the left side is attenuated by the variable optical attenuator  71  so as to match the light intensity of the add light of its own. Additionally, if a light intensity of a WDM signal received from the network on the right side differs from a light intensity of an add light of its own, the WDM signal received from the network on the right side is attenuated by the variable optical attenuator  72  so as to match the light intensity of the add light of its own. It should be noted that the optical circuit may be provided with only one of the variable optical attenuators  71  and  72 .  
       FIG. 15  is a block diagram of a linear system dedicated node apparatus according to a third embodiment using an optical circuit according to the present invention. The optical circuit  80  shown in  FIG. 15  differs from the optical circuit  50  shown in  FIG. 6  in that an optical amplifier  81  is connected between the circulator  51  and the optical filter  52  and an optical amplifier  82  is connected between the optical filter  52  and the circulator  54 .  
      In this embodiment, if a light intensity of a WDM signal received from the network on the left side differs from a light intensity of an add light of its own, the WDM signal received from the network on the left side is amplified by the optical amplifier  81  so as to match the light intensity of the add light of its own. Additionally, if a light intensity of a WDM signal received from the network on the right side differs from a light intensity of an add light of its own, the WDM signal received from the network on the right side is amplified by the optical amplifier  82  so as to match the light intensity of the add light of its own. It should be noted that the optical circuit may be provided with only one of the optical amplifiers  81  and  82 .  
      It should be noted that network connected on the left side corresponds to a first network in the present invention and the network connected on the right side corresponds to a second network in the present invention.  
      The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.