PATENT DOCUMENT

Abstract:
An optical add/drop multiplexer includes a first optical coupler receiving an optical signal including a plurality of multiplexed wavelengths, a wavelength blocker receiving the optical signal from the first optical coupler, and blocking at least one wavelength of the plurality of multiplexed wavelengths, a first wavelength selective switch, having one input port receiving the outputted optical signal from the first optical coupler and a plurality of output ports, demultiplexing a plurality of arbitrarily selected multiplexed wavelengths from the received optical signal, a second wavelength selective switch, having a plurality of input ports, each input port receiving a different optical signal and one output port, multiplexing a plurality of arbitrarily selected wavelength signals on the plurality of input ports, and a second optical coupler receiving the optical signal output from the wavelength blocker and multiplexed wavelength signal from the second wavelength selective switch.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/204,184, filed Aug. 16, 2005, which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-236836 and No. 2004-346685, filed on Aug. 16, 2004, and Nov. 30, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1) Field of the Invention 
         [0003]    The present invention relates to an optical add/drop multiplexer, and more particularly to an optical add/drop multiplexer in which a wavelength cross-connect function in a wavelength multiplexed optical transmission system and an optical add/drop function can be expanded. 
         [0004]    2) Description of the Related Art 
         [0005]    In recent years, with increasing traffic volume, there are demands for a large-capacity network. To meet the demands, an optical network using wavelength division multiplexing (WDM) is applied to a conventional basic network. In the optical network, the needs for a wavelength cross-connect function and an optical add/drop multiplexer (OADM) are increasing. With the wavelength cross-connection function, a destination to which an input light is output is changed for each wavelength of WDM light. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H8-195972. With the OADM, a signal light having an arbitrary wavelength is added to an arbitrary path, and then, dropped. Thus, the signal light is received. The OADM includes a wavelength selective switch (WSS). There are several types of the WSS such as one having a diffraction grating and a matrix switch using a micro electro mechanical system (MEMS) mirror using a MEMS technology, and one having a thin film filter and a matrix switch using the MEMS mirror. 
         [0006]    From the viewpoint of a size and a cost of a device having the functions in the wavelength cross-connect function and of the OADM, it is preferable to make such functions expandable as required while the device is configured as small as possible upon its introduction, not just making the functions advanced. When the device is replaced with another one, optical fibers connected to the device have to be reconnected to the one replaced. However, because the number of optical fibers is as many as thousands, it takes a lot of time for the reconnection. Moreover, to carry out the reconnection, the signals being transmitted have to be disconnected. Therefore, it is desirable to realize a configuration (in-service upgrade) such that the functions can be expanded without disconnecting the signals being transmitted. 
         [0007]    However, in the conventional configuration, a device is prepared by the number estimated, when a device is to be introduced, corresponding to the number of wavelengths and the number of switching routes to be demanded in the future. As a result, a size of the device required at the time of initial introduction becomes large, and introduction cost of the device at the time of initial introduction is increased. 
         [0008]      FIG. 59  is a schematic of a transmission path and a wavelength cross-connect device in a network. Two rings of transmission paths A and B are connected to a wavelength cross-connect device  1300  that forms an optical add/drop multiplexer. The transmission path A includes two optical fibers  1301   a  and  1301   b , while the transmission path B includes two optical fibers  1302   a  and  1302   b . The wavelength cross-connect device  1300  switches a signal in four directions (a total of four routes of #1 to #4) through four lines of the optical fiber  1301   a  to the optical fiber  1302   b . More specifically, the signal can be switched between a route #1 and a route #2, between the route #1 and a route #3, between the route #1 and a route #4, between the route #2 and the route #3, between the route #2 and the route #4, and the between the route #3 and the route #4. 
         [0009]      FIG. 60  is a schematic of a configuration of an optical cross-connect. The case of using an 80×80 matrix switch  1310 , in which the number of inputs and the number of outputs of wavelengths are 80 (λ 1  to λ 80 ), is explained below as an example. If it is predicted that the number of final routes (the number of transmission paths) is four after introduction of the device, the number of fibers for a signal having one wavelength is eight lines as “4 lines (for transmission signals)+4 lines (when all the wavelengths are targeted for adding/dropping)=8 lines”. Therefore, the amount of 80/8=10 wavelengths is assigned to one matrix switch  1310 . 
         [0010]    If the number of routes upon initial introduction is two, input/output ports of the matrix switch  1310  for 40 lines obtained through “(2 lines (for transmission signals)+2 lines (for adding/dropping))×10 wavelengths” are used. Other input/output ports for the remaining 40 lines remain unused, which is wasteful. If prediction made upon the initial introduction is found incorrect and function expansion is required for the number of routes that is above the number predicted, the requirements may not be dealt with. 
       SUMMARY OF THE INVENTION 
       [0011]    It is an object of the present invention to solve at least the above problems in the conventional technology. 
         [0012]    An optical add/drop multiplexer for switching a light path for changing an input light that has multiplexed wavelengths and that is input to an input port to an output light for each wavelength that is led to output ports for a plurality of routes in each transmission path, and for dropping or adding a signal light that has a predetermined wavelength according to one aspect of the present invention includes a core unit. The core unit includes a through path that lets the input light pass through to the output port; a drop port for dropping the input light that has a predetermined wavelength; and an add port for adding the signal light to the input light. 
         [0013]    The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic for explaining function expansion by the optical add/drop multiplexer according to an embodiment of the present invention; 
           [0015]      FIG. 2  is a table for comparing functions of the optical add/drop multiplexers; 
           [0016]      FIG. 3  is a schematic of function expansion from a low count channel DOADM to a high count channel DOADM; 
           [0017]      FIG. 4  is a schematic of function expansion from an ROADM to a DOADM; 
           [0018]      FIG. 5  is a schematic of function expansion from the DOADM to a WXC; 
           [0019]      FIG. 6  is a schematic of a configuration of a core unit; 
           [0020]      FIG. 7  is a schematic of another configuration of the core unit; 
           [0021]      FIG. 8  is a schematic of still another configuration of the core unit; 
           [0022]      FIG. 9  is a schematic of still another configuration of the core unit; 
           [0023]      FIG. 10  is a schematic of a configuration of an add unit; 
           [0024]      FIG. 11A  is a schematic of another configuration of the add unit; 
           [0025]      FIG. 11B  is a schematic of another configuration of the add unit; 
           [0026]      FIG. 12  is a schematic of another configuration of the add unit; 
           [0027]      FIG. 13  is a schematic of another configuration of the add unit; 
           [0028]      FIG. 14  is a schematic of another configuration of the add unit; 
           [0029]      FIG. 15  is a schematic of another configuration of the add unit; 
           [0030]      FIG. 16  is a schematic of another configuration of the add unit; 
           [0031]      FIG. 17  is a schematic of another configuration of the add unit; 
           [0032]      FIG. 18  is a schematic of a configuration of a drop unit; 
           [0033]      FIG. 19A  is a schematic of another configuration of the drop unit; 
           [0034]      FIG. 19B  is a schematic of another configuration of the drop unit; 
           [0035]      FIG. 20  is a schematic of another configuration of the drop unit; 
           [0036]      FIG. 21  is a schematic of another configuration of the drop unit; 
           [0037]      FIG. 22  is a schematic of another configuration of the drop unit; 
           [0038]      FIG. 23  is a schematic of another configuration of the drop unit; 
           [0039]      FIG. 24  is a schematic of another configuration of the drop unit; 
           [0040]      FIG. 25  is a schematic of another configuration of the drop unit, 
           [0041]      FIG. 26  is a schematic of a core unit that changes a wavelength spacing; 
           [0042]      FIG. 27  is a schematic of a core unit that changes a wavelength spacing; 
           [0043]      FIG. 28  is a schematic of a drop unit that changes a wavelength spacing; 
           [0044]      FIG. 29  is a schematic for explaining function expansion of the core unit; 
           [0045]      FIG. 30A  is a schematic of optical power control in the core unit. 
           [0046]      FIG. 30B  is a schematic of another optical power control in the core unit; 
           [0047]      FIG. 31  is a schematic of another optical power control in the core unit; 
           [0048]      FIG. 32A  is a schematic of another optical power control in the core unit; 
           [0049]      FIG. 32B  is a schematic of another optical power control in the core unit; 
           [0050]      FIG. 33  is a schematic of another optical power control in the core unit; 
           [0051]      FIG. 34A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction; 
           [0052]      FIG. 34B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 34A ; 
           [0053]      FIG. 34C  is a schematic for explaining another expansion of the optical add/drop multiplexer shown in  FIG. 34A ; 
           [0054]      FIG. 34D  is a schematic for explaining another expansion of the optical add/drop multiplexer shown in  FIG. 34A ; 
           [0055]      FIG. 34E  is a schematic for explaining another expansion of the optical add/drop multiplexer shown in  FIG. 34A ; 
           [0056]      FIG. 34F  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0057]      FIG. 34G  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 34F ; 
           [0058]      FIG. 34H  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0059]      FIG. 34I  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 34H ; 
           [0060]      FIG. 34J  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0061]      FIG. 34K  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0062]      FIG. 34L  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ) shown in  FIG. 34J ; 
           [0063]      FIG. 34M  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0064]      FIG. 34N  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 34M ; 
           [0065]      FIG. 34O  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 34M ; 
           [0066]      FIG. 34P  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0067]      FIG. 34Q  is a schematic of a colorless AWG that forms the grouping filters (GF 1  to  5 ) shown in  FIG. 34P ; 
           [0068]      FIG. 34R  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
           [0069]      FIG. 34S  is a schematic of a colorless AWG that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 34R ; 
           [0070]      FIG. 34T  is a schematic of the colorless AWG that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 34R ; 
           [0071]      FIG. 35A  is a schematic of the optical add/drop multiplexer at the time of initial introduction (In-service upgrade example 2); 
           [0072]      FIG. 35B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 35A ; 
           [0073]      FIG. 35C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 35A ; 
           [0074]      FIG. 35D  a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 35A ; 
           [0075]      FIG. 35E  is a schematic of a specific configuration the optical add/drop multiplexer shown in  FIG. 35C ; 
           [0076]      FIG. 35F  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 35E ; 
           [0077]      FIG. 35G  is a schematic of a specific configuration of the optical add/drop multiplexer as shown in  FIG. 35C ; 
           [0078]      FIG. 35H  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 35G ; 
           [0079]      FIG. 35I  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 35G ; 
           [0080]      FIG. 35J  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 35C ; 
           [0081]      FIG. 35K  is a schematic of the colorless AWG that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 35J ; 
           [0082]      FIG. 35L  is a schematic of the colorless AWG that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 35J ; 
           [0083]      FIG. 36A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction (In-service upgrade example 3); 
           [0084]      FIG. 36B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A ; 
           [0085]      FIG. 36C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A ; 
           [0086]      FIG. 36D  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A ; 
           [0087]      FIG. 36E  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A ; 
           [0088]      FIG. 36F  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A ; 
           [0089]      FIG. 36G  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F ; 
           [0090]      FIG. 36H  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 36G ; 
           [0091]      FIG. 36I  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F ; 
           [0092]      FIG. 36J  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 36I . 
           [0093]      FIG. 36K  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F ; 
           [0094]      FIG. 36L  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ) shown in  FIG. 36K ; 
           [0095]      FIG. 36M  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ) shown in  FIG. 36K ; 
           [0096]      FIG. 36N  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F ; 
           [0097]      FIG. 36O  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 36N ; 
           [0098]      FIG. 36P  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 36N ; 
           [0099]      FIG. 36Q  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F ; 
           [0100]      FIG. 36R  is a schematic of the colorless AWG that forms grouping filters (GF 1  to  5 ) shown in  FIG. 36Q ; 
           [0101]      FIG. 36S  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F ; 
           [0102]      FIG. 36T  is a schematic of the colorless AWG that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 36S ; 
           [0103]      FIG. 36U  is a schematic of the colorless AWG that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 36S ; 
           [0104]      FIG. 37A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction (In-service upgrade example 4); 
           [0105]      FIG. 37B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 37A ; 
           [0106]      FIG. 37C  is a schematic for explaining the expansion of the optical add/drop multiplexer shown in  FIG. 37A ; 
           [0107]      FIG. 37D  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 37A ; 
           [0108]      FIG. 37E  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 37A ; 
           [0109]      FIG. 37F  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 37D ; 
           [0110]      FIG. 37G  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 37F ; 
           [0111]      FIG. 37H  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 37D ; 
           [0112]      FIG. 37I  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 37H ; 
           [0113]      FIG. 37J  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 37H ; 
           [0114]      FIG. 37K  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 37D ; 
           [0115]      FIG. 37L  is a schematic of the colorless AWG that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 37K ; 
           [0116]      FIG. 37M  is a schematic of the colorless AWG that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 37K ; 
           [0117]      FIG. 38A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction (In-service upgrade example 5); 
           [0118]      FIG. 38B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 38A ; 
           [0119]      FIG. 38C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 38A ; 
           [0120]      FIG. 39A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction (In-service upgrade example 6); 
           [0121]      FIG. 39B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 39A ; 
           [0122]      FIG. 39C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 39A ; 
           [0123]      FIG. 39D  is a schematic for explaining signal switching between transmission paths when the expansion shown in  FIG. 39C  is performed; 
           [0124]      FIG. 40A  is a schematic of a configuration when the interleaver is used on the drop side as the grouping filter; 
           [0125]      FIG. 40B  is a schematic of a configuration when the interleaver is used on the add side as the grouping filter; 
           [0126]      FIG. 41A  is a schematic of a configuration when the band division filter is used on the drop side as the grouping filter; 
           [0127]      FIG. 41B  is a schematic of a configuration when the band division filter is used on the add side as the grouping filter; 
           [0128]      FIG. 42A  is a schematic of a configuration when the colorless AWG is used on the drop side as the grouping filter; 
           [0129]      FIG. 42B  is a schematic of a configuration when the colorless AWG is used on the add side as the grouping filter; 
           [0130]      FIG. 43A  is a schematic of a configuration in which an optical spectrum monitor is used for control of optical power of the drop signal; 
           [0131]      FIG. 43B  is a schematic of a configuration in which an optical spectrum monitor is used for control of optical power of the main signal and the drop signal; 
           [0132]      FIG. 44  is a schematic for explaining extension of the core unit that includes the interleaver; 
           [0133]      FIG. 45A  is a schematic of a wavelength selective switch on the drop side separated as a block; 
           [0134]      FIG. 45B  is a schematic of a wavelength selective switch on the add side separated as a block; 
           [0135]      FIG. 46A  is a schematic of the optical add/drop multiplexer according to an embodiment of the present invention to realize a function of a wavelength cross-connect; 
           [0136]      FIG. 46B  is a graph of a relationship between number of channels for the add unit/drop unit and maximum number of routes for the wavelength cross-connect; 
           [0137]      FIG. 47  is a schematic for explaining expansion of ports for routes of the optical add/drop multiplexer shown in  FIG. 46A ; 
           [0138]      FIG. 48  is a schematic for explaining another expansion of ports for routes of the optical add/drop multiplexer shown in  FIG. 46A ; 
           [0139]      FIG. 49  a schematic for explaining expansion of ports for routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit; 
           [0140]      FIG. 50  is a schematic for explaining the expansion of the ports for the routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit; 
           [0141]      FIG. 51  is a schematic for explaining the expansion of the ports for the routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit; 
           [0142]      FIG. 52  is a schematic for explaining expansion of the ports for the routes of the optical add/drop multiplexer when the 1×6 optical coupler is used on the drop side; 
           [0143]      FIG. 53  is a schematic for explaining the expansion of the ports for the routes of the optical add/drop multiplexer when the 1×6 optical coupler is used on the drop side; 
           [0144]      FIG. 54  is a schematic for explaining expansion of the ports for routes of the optical add/drop multiplexer when the 1×6 optical coupler is used in the drop side of the core unit; 
           [0145]      FIG. 55  is a schematic for explaining expansion of the ports for the routes based on ROADM; 
           [0146]      FIG. 56  is a schematic for explaining expansion of the ports for the routes based on ROADM; 
           [0147]      FIG. 57  is a schematic for expansion of the ports for the routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit; 
           [0148]      FIG. 58  is a schematic for expansion of the ports for the routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit; 
           [0149]      FIG. 59  is a schematic of a configuration of a transmission path and a wavelength cross-connect device in a network; and 
           [0150]      FIG. 60  is a schematic of a configuration of an optical cross-connect. 
       
    
    
     DETAILED DESCRIPTION 
       [0151]    Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
         [0152]    Recently, instead of the matrix switch, a wavelength selective switch and a wavelength blocker are actively studied and developed. The wavelength selective switch can be used to switch an arbitrary wavelength in an arbitrary direction, and the wavelength blocker can block an arbitrary wavelength from an arbitrary wavelength. These have such advantages as compact size, low cost, low insertion loss, a smaller number of fibers required when being mounted. 
         [0153]    The wavelength selective switch or the wavelength blocker is used in an optical add/drop multiplexer according to an embodiment of the present. The function is expanded from a Dynamic OADM (DOADM) that supports a small number of wavelengths (LCC: Low Count Channel) to a DOADM that supports a multiple wavelength (HCC: High Count Channel). Furthermore, the function is expanded to Wavelength Cross-Connect (WXC). It is thereby possible to realize the function expansions without disconnecting a transmission signal. 
         [0154]      FIG. 1  is schematic for explaining function expansion by the optical add/drop multiplexer according to an embodiment of the present invention. An example of the function expansion (in-service upgrade) is shown therein such that the function of the optical add/drop multiplexer is expanded from a low count channel (LCC) DOADM to a high count channel (HCC) DOADM and then to the WXC, depending on changes in network requirements. 
         [0155]    At the time of initial introduction, a DOADM  2   a  is arranged for one ring network (metro ring  1   a ). This is based on prediction such that the ring network may be expanded up to three ring networks  1   a  to  1   c  five years later. 
         [0156]    Since there are add/drop requests only for some wavelengths upon the initial introduction, a low count channel (LCC) DOADM  2   a  that has a necessary minimum function is arranged. As shown in  FIG. 1 ,  3   a  represents an “add” unit, and  3   b  represents a “drop” unit. The DOADM  2   a  arranged upon the initial introduction has an expandable configuration so as to support network requirements expected five years later. 
         [0157]    Referring to “Two years later”, for example, the configuration is expected to support an increase in the required number of wavelengths in one ring network  1   a . A DOADM  2   b  uses available ports of the add unit  3   a  and the drop unit  3   b . Alternatively, by adding an add/drop module to an available port, the function is expanded to a high count channel (HCC) DOADM  2   b  without disconnecting transmission signals during operation. 
         [0158]    Referring to “Five years later”, for example, the function is expanded from the DOADM  2   b  to a wavelength cross-connect (WXC)  2   c  without disconnecting existing transmission signal so that communications are possible between three ring networks  1   a  to  1   c  that correspond to metro ring #1 to metro ring #3, respectively. The change from the DOADM  2   b  to the WXC  2   c  indicates not an exchange of devices but function expansion. With the function expansion, the name is changed from the DOADM  2   b  to the WXC  2   c . The WXC  2   c  allows the function of a wavelength cross-connect device to be performed in the transmission path. 
         [0159]      FIG. 2  is a table for comparing functions of the optical add/drop multiplexers with each other. The diagram describes a configuration example, presence or absence of the function for adding/dropping an arbitrary wavelength to an arbitrary port, and permission or prohibition of reconfiguration for each of the OADM, an ROADM (Reconfigurable OADM), the DOADM, and a DOADM with limitation on wavelength. As explained with reference to  FIG. 1 , by using the DOADM, the function of adding/dropping an arbitrary wavelength to an arbitrary port can be provided in the future, and reconfiguration becomes possible. 
         [0160]    Referring to the function expansion of the present invention, it is also possible to use any configuration example other than the OADM, i.e., the ROADM and the DOADM with limitation on wavelength. Reconfiguration becomes possible with the ROADM. In the DOADM with limitation on wavelength, the function of adding/dropping an arbitrary wavelength to an arbitrary port is limited on wavelength as compared with the DOADM, but reconfiguration is possible in the same manner as the DOADM. If there are a small number of wavelengths that are to be added or dropped, the DOADM with limitation on wavelength obtained at cost lower than the DOADM can be used. 
         [0161]      FIG. 3  to  FIG. 5  are schematics of function expansions in the respective optical add/drop multiplexers. As shown in the figures, the optical add/drop multiplexer includes a core unit that includes the wavelength selective switch or the wavelength blocker, a drop unit that drops signal light from the core unit to be led to an output port for dropping (drop port), and an add unit that outputs signal light to be added to the core unit from an input port for adding (add port). 
         [0162]      FIG. 3  is a schematic of function expansion from a low count channel DOADM to a high count channel DOADM. An input signal in which N wavelengths are multiplexed over the transmission path passes through a core unit  11   a  and is output. The core unit  11   a  includes a wavelength selective switch (WSS) or a wavelength blocker (WB), and causes a drop unit  12   a  to drop a signal having a predetermined wavelength. Furthermore, the core unit  11   a  multiplexes a signal from an add unit  13   a  on a main signal. 
         [0163]    In a low count channel (LCC) DOADM  10   a , wavelengths “i” of the signal dropped from the core unit  11   a  are output to receivers (Rx) through ports “i” of the drop unit  12   a . Signals from transmitters (Tx) are input through ports “i” of the add unit  13   a , and are added in the core unit  11   a . Although the number of ports i of the drop unit  12   a  is the same as the number of ports i of the add unit  13   a , they may be different from each other. 
         [0164]    When the function is expanded to a high count channel (HCC) DOADM  10   b  and the number of wavelengths is increased from i to k (the number of ports i&lt;k), the core unit  11   a  is used as it is, and the number of ports is increased to k using available ports of the drop unit  12   a  and the add unit  13   a . In addition, another drop unit and add unit (not shown) are further added to available ports. With this addition, the function can be expanded to the high count channel DOADM  10   b.    
         [0165]      FIG. 4  is a schematic of function expansion from an ROADM to a DOADM. In a ROADM  20   a , ports of a drop unit  22   a  and an add unit  23   a  that are connected to a core unit  21   a  correspond only to fixed wavelengths (λ 1  to λn) decided respectively upon initial introduction. When the function is expanded to a DOADM  20   b , the core unit  21   a  is used as it is without replacement, but the drop unit  22   a  is replaced with a drop unit  22   b  and the add unit  23   a  is replaced with an add unit  23   b , each in which ports correspond to arbitrary wavelengths. Each of the drop unit  22   b  and the add unit  23   b  includes an optical switch or an optical filter, and any one of wavelengths (one wavelength of λs 1  to λsn) out of the wavelengths λ 1  to λn can be selected for each port. With the selection, it is possible to expand the function without disconnecting a signal in the transmission path in the core unit  21   a.    
         [0166]      FIG. 5  is a schematic of function expansion from the DOADM to a WXC, and depicts an example of expanding the function of the DOADM  20   b  of  FIG. 4  to the WXC  20   c . A core unit  21   a  includes a drop-side port  25   a  and an add-side port  25   b . The core unit  21   a  is additionally provided corresponding to an increase in the number of transmission paths based on network requirements. In the example of  FIG. 5 , the number of routes (the number of transmission paths) increases from 1 to 3, and a core unit  21   b  and a core unit  21   c  are added accordingly. 
         [0167]    Although the drop unit and the add unit are omitted from the WXC  20   c  of  FIG. 5 , the drop unit  22   b  and the add unit  23   b  described in the DOADM  20   b  are connected to the core units  21   a ,  21   b , and  21   c . Ports of the drop-side port  25   a  and ports of the add-side port  25   b  that are provided in the core units  21   a ,  21   b , and  21   c  are connected to each other in the interior of the WXC  20   c.    
         [0168]    The drop-side port  25   a  of the core unit  21   a  is connected to the add-side port  25   b  of the core unit  21   b  and to the add-side port  25   b  of the core unit  21   c . The drop-side port  25   a  of the core unit  21   b  is connected to the add-side port  25   b  of the core unit  21   a  and to the add-side port  25   b  of the core unit  21   c . Furthermore, the drop-side port  25   a  of the core unit  21   c  is connected to the add-side port  25   b  of the core unit  21   a  and to the add-side port  25   b  of the core unit  21   b.    
         [0169]    By the examples of connections, the functions can be expanded corresponding to the number of routes in the three metro rings (#1 to #3) as explained with reference to  FIG. 1 . Therefore, it is possible to expand the function such that the number of core units that forms the WXC  20   c  is increased and the number of routes is increased without disconnecting a main signal passing through the core unit. 
         [0170]    Various configuration examples of the core unit are explained below with reference to  FIG. 6  to  FIG. 9 .  FIG. 6  is a diagram of configuration example 1 of the core unit. A core unit  30  as shown in  FIG. 6  includes a core  1  ( 30   a ) and a core  2  ( 30   b ). The core  1  ( 30   a ) includes a 1×2 (hereinafter, the number of inputs versus the number of outputs is expressed as “the number of inputs×the number of outputs”) optical coupler  31 , a wavelength blocker (WB)  32  connected to one of the outputs of the optical coupler  31 , and a 2×1 optical coupler  33  of which one of the inputs is connected to the output of the wavelength blocker  32 . The core  2  ( 30   b ) includes a 1×N-port wavelength selective switch (WSS)  34  for dropping connected to the other output of the optical coupler  31 , and an M×1-port wavelength selective switch (WSS)  35  for adding connected to the other input of the optical coupler  33 . 
         [0171]    A multiple-input and single-output optical coupler couples a plurality of signal lights input, and outputs them as a multiplexed wavelength. A single-input and multiple-output optical coupler drops a multiplexed signal light input as it is, and outputs the signal lights. A multiple-input and single-output wavelength selective switch multiplexes a plurality of arbitrary wavelengths input, and a single-input and multiple-output wavelength selective switch demultiplexes a signal light having an arbitrary wavelength from the multiplexed signal light input, and outputs the signal lights (if there are N outputs, N wavelengths are output). Therefore, when the signal passes through the optical coupler and is dropped, the whole signal light multiplexed is dropped, which causes attenuation to increase as compared with the wavelength selective switch. An optical amplifier or the like is provided to take measures against the attenuation. 
         [0172]    A wavelength selective switch (WSS) and so on (not shown) are further connected to ports of the wavelength selective switches  34  and  35  that are arranged in the drop unit and the add unit, respectively. With the connection, the function can be expanded from the low count channel DOADM to the high count channel DOADM. Furthermore, by combining the wavelength selective switches with each other, the function is expanded to the WXC, which allows the loss to be suppressed without upsizing the device. As shown in  FIG. 6 , there are a small number of fibers to be connected between the components in the core unit  30 , which makes it easy to conduct the connections. Moreover, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. Furthermore, it is possible to realize a “drop and continue” function used to transmit the same wavelength signal as a main signal to the drop side while a certain wavelength is transmitted as the main signal. 
         [0173]      FIG. 7  is a schematic of another configuration of the core unit. A core unit  30  of  FIG. 7  includes a 1×2 optical coupler  41 , an M×1-port wavelength selective switch (WSS)  42  connected to one output of the optical coupler  41 , and a 1×N-port wavelength selective switch (WSS)  43  for dropping connected to the other output of the optical coupler  41 . 
         [0174]    A wavelength selective switch and a grouping filter or so (not shown) are further connected to ports of the wavelength selective switch  43  for dropping, and an optical coupler or so (not shown) is connected to the add unit. Based on the connections, the function is expanded from the low count channel DOADM to the high count channel DOADM. Furthermore, by combining the wavelength selective switches with each other, the function is expanded to the WXC, which allows the loss to be suppressed without upsizing the device. As shown in  FIG. 7 , there are a small number of fibers to be connected between the components in the core unit  30 , which makes it easy to conduct the connections. Moreover, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. Furthermore, it is possible to realize the drop and continue function used to transmit the same wavelength as a main signal also to the drop side while a certain wavelength is transmitted as the main signal. 
         [0175]      FIG. 8  is a schematic of still another configuration of the core unit. A core unit  30  of  FIG. 8  includes a 1×N-port wavelength selective switch (WSS)  51 , a 2×1 optical coupler  52  whose one of inputs is connected to one of a plurality of output ports of the wavelength selective switch  51 , and an M×1-port wavelength selective switch (WSS)  53  for adding connected to one input of the optical coupler  52 . 
         [0176]    A wavelength selective switch and a grouping filter or so (not shown) are further connected to ports of the wavelength selective switch  51  for dropping, and an optical coupler or so (not shown) is connected to the add unit. Based on the connections, the function can be expanded from the low count channel DOADM to the high count channel DOADM. Furthermore, by combining the wavelength selective switches with each other, the function is expanded to the WXC, which allows the loss to be suppressed without upsizing the device. As shown in  FIG. 8 , there are a small number of fibers to be connected between the components in the core unit  30 , which makes it easy to conduct the connections. Moreover, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0177]      FIG. 9  is a schematic of still another configuration of the core unit. A core unit  30  of  FIG. 9  includes a 1×N-port wavelength selective switch (WSS)  61 , and an M×1-port wavelength selective switch (WSS)  62  whose one of input ports is connected to one of a plurality of output ports of the wavelength selective switch  61 . 
         [0178]    A wavelength selective switch, a grouping filter, an optical coupler, and so on (not shown) are further connected to ports of the wavelength selective switches  61  and  62  that are arranged in the drop unit and the add unit, respectively. Based on the connections, the function is expanded from the low count channel DOADM to the high count channel DOADM. Furthermore, by combining the wavelength selective switches with each other, the function is expanded to the WXC, which allows the loss to be suppressed without upsizing the device. As shown in  FIG. 9 , there are a small number of fibers to be connected between the components in the core unit  30 , which makes it easy to conduct the connections. Moreover, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0179]    Various configuration examples of the add unit are explained below with reference to  FIG. 10  to  FIG. 17 .  FIG. 10  is a schematic of a configuration of an add unit. An add unit  70  of  FIG. 10  includes an optical multiplexer  71  for a fixed wavelength. When the optical multiplexer  71  is used, the function can be expanded to the OADM (ROADM) that is reconfigurable because input ports (1 to M) provided in the optical multiplexer  71  support a fixed wavelength. The add unit  70  is connected to the add-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), a part of the input ports of the optical multiplexer  71  is used for reception, and another part thereof is used for the WXC. The function is thereby expanded to the ROADM including the WXC. The add unit  70  of  FIG. 10  is connected to the add-side port of the core unit  30 , which allows a simple OADM to be constructed at low cost. 
         [0180]      FIG. 11A  is a schematic of another configuration of the add unit. An add unit  70  includes an M×1-port wavelength selective switch (WSS)  81 .  FIG. 11B  is a schematic of another configuration of the add unit. In an example as shown in  FIG. 11B , a plurality (two in the example of  FIG. 11B ) of M×1-port wavelength selective switches (WSS)  81 , each of which is the basic configuration as shown in  FIG. 11A , are provided to connect outputs of the wavelength selective switches  81  to inputs of the 2×1 optical coupler  82 , respectively. 
         [0181]    The optical coupler  82  having the configuration as shown in  FIG. 11B  is provided to increase the number of channels of the add unit  70 . Such configuration example allows an arbitrary wavelength type DOADM to be realized. These add units  70  are connected to the add-side ports of the core units  30  (see  FIG. 6  to  FIG. 9 ), which makes it possible to expand the function from the low count channel DOADM to the high count channel DOADM. Based on the configuration, the add unit  70  can be easily connected to the add-side ports of the core unit  30 , and a signal having an arbitrary wavelength can be transmitted to each of the add-side ports of the core unit  30 . 
         [0182]      FIG. 12  is a schematic of another configuration of the add unit. An add unit  70  includes an M×1 optical coupler  91 . An optical amplifier  92  that amplifies an output of the optical coupler  91  may be provided if necessary. Such an add unit  70  allows the arbitrary wavelength type DOADM to be realized, and is connected to the add-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), which makes it possible to expand the function from the low count channel DOADM to the high count channel DOADM. By connecting the add unit  70  to the add-side port of the core unit  30 , a simple OADM can be constructed at low cost. 
         [0183]      FIG. 13  is a schematic of another configuration of the add unit. An add unit  70  includes an M×M matrix switch  96  and an optical multiplexer  97  that multiplexes inputs from M pieces of ports. An optical amplifier  98  that amplifies an output of the optical multiplexer  97  may be provided if necessary. This provision allows the arbitrary wavelength type DOADM to be constructed. Such an add unit  70  is connected to the add-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), which makes it possible to expand the function from the low count channel DOADM to the high count channel DOADM. By connecting the matrix switch  96  having the required number of wavelength ports to the add-side ports of the core unit  30 , a signal having an arbitrary wavelength can be transmitted to each of the add-side ports. In this case, there is no need to prepare a plurality of matrix switches even including some pieces that are not used upon initial introduction. 
         [0184]      FIG. 14  to  FIG. 17  are schematics of configurations of the add unit. A grouping filter is applied to each of the add unit. The grouping filter can be realized by using a filter that is manufactured comparatively easily. The grouping filter is connected to the add-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), which allows the function to be expanded to the DOADM in a simple manner at low cost. 
         [0185]      FIG. 14  is a schematic of another configuration of the add unit. An add unit  100  includes an M×1 grouping filter  101 . Based on the configuration, the ports of the grouping filter  101  correspond to a plurality of assigned wavelengths to realize the DOADM with limitation on wavelength. 
         [0186]      FIG. 15  is a schematic of another configuration of the add unit. An add unit  100  includes an interleaver (IL)  102  that serves as the M×1 grouping filter. The internal configuration of the interleaver  102  is explained in detail later. Input to each of M ports of the interleaver  102  are wavelengths one by one out of the wavelengths assigned to each of the M ports, and M pieces of signals having the wavelengths input are multiplexed and are output. 
         [0187]      FIG. 16  a schematic of another configuration of the add unit. An add unit  100  includes a band division filter (BDF)  103  that serves as the M×1 grouping filter. The internal configuration of the band division filter  103  is explained in detail later. Input to each of M ports of the band division filter  103  are wavelengths one by one out of the wavelengths assigned to each of the M ports, and M pieces of signals having the wavelengths input are multiplexed and are output. 
         [0188]      FIG. 17  a schematic of another configuration of the add unit. An add unit  100  includes a colorless AWG (Colorless Arrayed Waveguide Grating)  104  that serves as the M×1 grouping filter. The colorless AWG  104  is configured by using the cyclic property of AWG, and allocates an optical signal with wavelengths multiplexed input into an input port, to different output ports according to each wavelength. Input to each of M ports of the colorless AWG  104  are wavelengths one by one out of the wavelengths assigned to each of the M ports, and M pieces of signals having the wavelengths input are multiplexed and are output. A specific product of the colorless AWG  104  is an AWG router manufactured by NEL. As compared with other systems, the colorless AWG has a higher degree of design flexibility, and a compact size and low cost are possible to be achieved (Reference: “Press Release” [online], Mar. 20, 2003, NTT Electronics Corp., [Search: Jul. 15, 2004], Internet &lt;URL:http://www.nel.co.jp/new/information/2003 — 03 — 20.html&gt;) 
         [0189]    Various configuration examples of the drop unit are explained below with reference to  FIG. 18  to  FIG. 25 .  FIG. 18  is a schematic of a configuration of a drop unit. A drop unit  110  of  FIG. 18  includes an optical demultiplexer  111  for a fixed wavelength that has N pieces of output ports. When the optical demultiplexer  111  is used, the function can be expanded to the DOADM with limitation on wavelength because the output ports provided in the optical demultiplexer  111  support a fixed wavelength. The drop unit  110  is connected to the drop-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), a part of the ports of the optical demultiplexer  111  is used for transmission, and another part thereof is used for the WXC. The function is thereby expanded to the ROADM including the WXC. The drop unit  110  of  FIG. 18  is connected to the drop-side port of the core unit  30 , which allows a simple OADM to be constructed at low cost. 
         [0190]      FIG. 19A  is a schematic of another configuration of the drop unit. A drop unit  110  of  FIG. 19A  includes a 1×N-port wavelength selective switch (WSS)  121 .  FIG. 19B  is a schematic of another configuration of the drop unit. In an example as shown in  FIG. 19B , a plurality (two in the example in the figure) of 1×N-port wavelength selective switches (WSS)  121 , each of which is the basic configuration as shown in  FIG. 19A , are provided to connect outputs of a 1×2 optical coupler  122  to ports in the input side of these wavelength selective switches  121 . 
         [0191]    The optical coupler  122  having the configuration as shown in  FIG. 19B  is provided to increase the number of channels of the drop unit  110 . Such configuration example allows an arbitrary wavelength type DOADM to be realized. These drop units  110  are connected to the drop-side ports of the core unit  30  (see  FIG. 6  to  FIG. 9 ), which makes it possible to expand the function from the low count channel DOADM to the high count channel DOADM. Based on the configuration, the drop unit  110  can be easily connected to the drop-side ports of the core unit  30 , and a signal having an arbitrary wavelength can be transmitted to each of the drop-side ports of the core unit  30 . 
         [0192]      FIG. 20  is a schematic of another configuration of the drop unit. A drop unit  110  includes a 1×N optical coupler  131  and a plurality of wavelength variable light filters  132  that are connected to the N pieces of output ports of the optical coupler  131 . An optical amplifier  133  may be provided in the input side of the optical coupler  131  if necessary. Such a configuration allows the arbitrary wavelength type DOADM to be realized. The drop unit  110  is connected to the drop-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), which makes it possible to expand the function from the low count channel DOADM to the high count channel DOADM. By connecting the drop unit  110  to the drop-side port of the core unit  30 , a simple OADM can be constructed at low cost. 
         [0193]      FIG. 21  is a schematic of another configuration of the drop unit. A drop unit  110  includes an optical demultiplexer  141  that includes N pieces of output ports, and an N×N matrix switch  142 . An optical amplifier  143  may be provided in the input side of the optical coupler  141  if necessary. Such a configuration allows the arbitrary wavelength type DOADM to be realized. The drop unit  110  is connected to the drop-side port of the core unit  30  (see  FIG. 6  to  FIG. 9 ), which makes it possible to expand the function from the low count channel DOADM to the high count channel DOADM. By connecting the matrix switch  142  having the required number of wavelength ports to the drop-side ports of the core unit  30 , a signal having an arbitrary wavelength can be transmitted to the each of the drop-side ports. In this case, there is no need to prepare a plurality of matrix switches even including some pieces that are not used upon initial introduction. 
         [0194]      FIG. 22  to  FIG. 25  are configuration examples each in which a grouping filter is used in the drop unit.  FIG. 22  is a schematic of another configuration of the drop unit. A drop unit  150  includes a 1×N grouping filter  151 . Based on the configuration, the ports of the grouping filter  151  correspond to a plurality of wavelengths assigned to realize the DOADM with limitation on wavelength. 
         [0195]      FIG. 23  is a schematic of another configuration of the drop unit. A drop unit  150  includes an interleaver  152  that serves as the 1×N grouping filter. The internal configuration of the interleaver  152  is explained in detail later. The interleaver  152  realizes the function of the drop unit by allocating wavelengths of a drop signal one by one, out of the wavelengths assigned to N ports of the interleaver, to each of the N ports. 
         [0196]      FIG. 24  is a schematic of another configuration of the drop unit. A drop unit  150  includes a band division filter (BDF)  153  that serves as the 1×N grouping filter. The internal configuration of the band division filter  153  is explained in detail later. The band division filter  153  realizes the function of the drop unit by allocating wavelengths of a drop signal one by one, out of the wavelengths assigned to N ports of the band division filter, to each of the N ports. 
         [0197]      FIG. 25  is a schematic of another configuration of the drop unit. A drop unit  150  includes a colorless AWG  154  that serves as the 1×N grouping filter. The colorless AWG  154  realizes the function of the drop unit by allocating wavelengths of a drop signal one by one, out of the wavelengths assigned to N ports of the colorless AWG, to each of the N ports. 
         [0198]      FIG. 26  is a schematic of a core unit that changes a wavelength spacing. A core unit  160  includes a BHz/2 BHz input-side interleaver  161 , two 1×2 optical couplers  162   a  and  162   b  that are connected to the interleaver  161 , two 1×N-port 2 BHz-spacing wavelength selective switches (WSS)  163   a  and  163   b  for dropping, a BHz/2 BHz output-side interleaver  164 , two M×1-port 2 BHz-spacing wavelength selective switches (WSS)  165   a  and  165   b  for adding. The core unit  160  can support transmission signals at a BHz (e.g., 50 GHz) spacing. The output-side interleaver  164  returns the transmission signals at a 2 BHz spacing to those at the BHz spacing and outputs the transmission signals. It is noted that 2 BHz represents a frequency as twice as BHz (if B=50 G, 2 BHz=100 GHz). 
         [0199]    A wavelength selective switch or a grouping filter or so (not shown) is further connected to the ports of the wavelength selective switches  163   a  and  163   b  for dropping in the core unit  160 , and an optical coupler or so is connected to the port for adding, which allows the function expansion from the low count channel DOADM to the high count channel DOADM. Furthermore, a combination of a plurality of wavelength selective switches allows the function to be expanded to the WXC. When the wavelength spacing is narrowed in terms of design or manufacturing of the wavelength selective switch in particular, the number of ports has sometimes been limited. According to the core unit  160  having the configuration, the expansion can be easily realized by using the wavelength selective switches  163   a ,  163   b ,  165   a , and  165   b  that support a spacing (2 BHz) that is twice as wide as the wavelength spacing (BHz) of signals. 
         [0200]      FIG. 27  is a schematic of a core unit that changes a wavelength spacing. An add unit  170  includes a BHz/2 BHz interleaver  171  and an M×1-port 2 BHz-spacing wavelength selective switch (WSS)  172 . This configuration allows the wavelength spacing handled by the wavelength selective switch  172  to be widened (loosened) to 2 BHz even if the transmission signal is at BHz. The add unit  170  is connected to the add-side port of the core unit  160  of  FIG. 26  to allow the function expansion from the low count channel DOADM to the high count channel DOADM. 
         [0201]      FIG. 28  is a schematic of a drop unit that changes a wavelength spacing. A drop unit  180  includes a BHz/2 BHz interleaver  181  and a 1×N-port 2 BHz-spacing wavelength selective switch (WSS)  182 . This configuration allows the wavelength spacing handled by the wavelength selective switch  182  to be widened (loosened) to 2 BHz even if the transmission signal is at BHz. The drop unit  180  is connected to the drop-side port of the core unit  160  of  FIG. 26  to allow the function expansion from the low count channel DOADM to the high count channel DOADM. 
         [0202]      FIG. 29  is a schematic for explaining function expansion of the core unit. A core unit  190   a  is provided before the function expansion (upon initial introduction), and at this time a transmission signal is at BHz. At the time of the initial introduction with little communication capacity, a 1×2 optical coupler  193   a , a 1×N-port 2 BHz-spacing wavelength selective switch (WSS)  194   a , and an M×1-port 2BHz-spacing wavelength selective switch (WSS)  195   a  are arranged between a pair of interleavers  191  and  192 , and the device is started to be operated. 
         [0203]    When the communication capacity increases and the addition of the device is needed, the function is to be expanded. At this time, a core unit  190   b  may be configured by additionally providing another group of 1×2 optical coupler  193   b , a 1×N-port 2 BHz-spacing wavelength selective switch (WSS)  194   b , and an M×1-port 2 BHz-spacing wavelength selective switch (WSS)  195   b  between the pair of interleavers  191  and  192 . This configuration allows the extension while operating the transmission signal, which makes it possible to increase the number of add/drop ports using a general-purpose wavelength selective switch. Moreover, there is no need to replace the internal configuration with another one, which makes it possible to achieve function expansion at low cost. 
         [0204]    The control of optical power in portions of the core unit is explained below.  FIG. 30A  is a schematic of optical power control in the core unit. A core unit  200  includes a 1×2 optical coupler  201 , a 1×N-port wavelength selective switch (WSS)  202  for dropping, and an M×1-port wavelength selective switch (WSS)  203  for adding. A branch portion for power monitor and a monitor  204  for optical power are arranged in an output portion of the M×1-port wavelength selective switch (WSS)  203 . The monitor  204  includes a photodetector such as PD and detects the intensity of each channel in the optical WDM signal or total optical signal power. The wavelength selective switch  203  adjusts photo-coupling of a through signal (main signal) passing through the core unit  200  and an add signal for each channel to perform optical power control. 
         [0205]      FIG. 30B  is a schematic of another optical power control in the core unit. A core unit  210  includes a 1×N-port wavelength selective switch (WSS)  211  for dropping, and an M×1-port wavelength selective switch (WSS)  212  for adding. A branch portion for power monitor and a monitor  213  for optical power of each channel, or total optical power are arranged in an output portion of the M×1-port wavelength selective switch (WSS)  212 . With this arrangement, photo-coupling of a through signal (main signal) passing through the core unit  210  and an add signal is adjusted for each channel to perform optical power control. 
         [0206]      FIG. 31  is a schematic of another optical power control in the core unit. A core unit  220  includes a 1×2 optical coupler  221 , a 1×N-port wavelength selective switch (WSS)  222  for dropping, and an M×1-port wavelength selective switch (WSS)  223  for adding. A branch portion for power monitor and a monitor  224  are arranged in an output portion of the wavelength selective switch  222  for dropping. Photo-coupling is adjusted for each channel in the wavelength selective switch  222  to adjust an optical power level to be output from the wavelength selective switch  222 . This adjustment allows the optical power level of drop signals for each channel to be controlled. 
         [0207]      FIG. 32A  is a schematic of another optical power control in the core unit. A core unit  230  includes a 1×N-port wavelength selective switch (WSS)  231  for dropping, a 2×1 optical coupler  232 , and an M×1-port wavelength selective switch (WSS)  233  for adding. A branch portion for power monitor and a monitor  234  are arranged in an output portion of the wavelength selective switch  231 . Photo-coupling is adjusted for each channel in the wavelength selective switch  231  to adjust an optical power level at the output portion of the wavelength selective switch  231 . This adjustment allows optical power control for a through signal (main signal) passing through the core unit  230  and for a drop signal to be performed for each channel. 
         [0208]      FIG. 32B  is a schematic of another optical power control in the core unit. A core unit  240  includes a 1×N-port wavelength selective switch (WSS)  241  for dropping, and an M×1-port wavelength selective switch (WSS)  242  for adding. A branch portion for power monitor and a monitor  243  for optical power are arranged in an output portion of the wavelength selective switch  241 . Photo-coupling is adjusted for each channel in the wavelength selective switch  241 , which allows optical power control for a through signal (main signal) passing through the core unit  230  and for a drop signal to be performed for each channel. 
         [0209]      FIG. 33  is a schematic of another optical power control in the core unit. A core unit  250  includes a 1×N-port wavelength selective switch (WSS)  251  for dropping, a 2×1 optical coupler  252 , and an M×1-port wavelength selective switch (WSS)  253  for adding. A branch portion for power monitor and a monitor  254  are arranged in an output portion of the wavelength selective switch  253 . Photo-coupling is adjusted for each channel in the wavelength selective switch  253  to allow optical power control for a drop signal to be performed for each channel. 
         [0210]    An optical spectrum monitor can be used instead of the monitor  204  to the monitor  254  in the configuration examples 1 to 6 ( FIG. 30A  to  FIG. 33 ) of the optical power control in the core units. Alternatively, an optical power monitor array can be used as the monitor. 
         [0211]    In-service upgrade example 1 of the optical add/drop multiplexer according to the present invention is explained below.  FIG. 34A  is a diagram of a configuration of an optical add/drop multiplexer upon initial introduction. An optical add/drop multiplexer  300   a  forms the low count channel (LCC) DOADM. As shown in the figure, a core unit  301   a  of the optical add/drop multiplexer  300   a  includes a 1×2 optical coupler  310 , a 1×8-port 50-GHz-spacing wavelength selective switch (WSS)  311  for dropping, and a 9×1-port 50-GHz-spacing wavelength selective switch (WSS)  312  for adding. The core unit  301   a  is connected with a drop unit  302   a  and an add unit  303   a . Based on the configuration, the number of signals to be dropped to the drop unit  302   a  by the core unit  301   a  corresponds to eight ports at maximum, and the number of signals to be added from the add unit  303   a  corresponds to nine ports at maximum. A part of the signals to be dropped or added can be dropped to or added from the wavelength cross-connect device (not shown) or the like. 
         [0212]      FIG. 34B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 34A . The core unit  301   a  of an optical add/drop multiplexer  300   b  has the same configuration as that of  FIG. 34A . That is, no part is changed in the core unit  301   a . However, each configuration of the drop unit  302   a  and the add unit  303   a  is changed. A new drop unit  302   b  includes an optical demultiplexer (DeMux)  321 , and an add unit  303   b  includes an optical multiplexer (Mux)  322 . This configuration allows the optical add/drop multiplexer  300   b  to expand the function to the ROADM that supports the wavelength cross-connect. 
         [0213]      FIG. 34C  is a schematic for explaining another expansion of the optical add/drop multiplexer shown in  FIG. 34A . The core unit  301   a  of an optical add/drop multiplexer  300   c  has the same configuration as that of  FIG. 34A . That is, no part is changed in the core unit  301   a . However, the drop unit  302   a  and the add unit  303   a  are changed to a drop unit  302   c  and an add unit  303   c , respectively. The drop unit  302   b  includes a 1×8-port 50-GHz-spacing wavelength selective switch (WSS)  331 , and the add unit  303   c  includes a 16×1-port optical coupler (CPL)  333 . As shown in  FIG. 34C , by providing a 1×2 optical coupler  332  in the drop unit  302   c , a signal dropped from one of the ports of the core unit  301   a  can also be dropped to a plurality of 1×8-port 50-GHz-spacing wavelength selective switches (WSS)  331 . A plurality of 16×1-port optical couplers  333  can be arranged in the add unit  303   c . This configuration allows the optical add/drop multiplexer  300   c  to expand the function to the high count channel (HCC) DOADM. 
         [0214]    Furthermore, a part of the 1×8-port 50-GHz-spacing wavelength selective switches (WSS)  311  of the core unit  301   a  is connected with the 1×8-port 50-GHz-spacing wavelength selective switches (WSS)  331  of the drop unit  302   c , and the rest of the ports are connected to the wavelength cross-connect device (not shown), which allows the function to be expanded to the high count channel DOADM that supports the wavelength cross-connect. 
         [0215]      FIG. 34D  is a schematic for explaining another expansion of the optical add/drop multiplexer shown in  FIG. 34A . The core unit  301   a  of an optical add/drop multiplexer  300   d  has the same configuration as that of  FIG. 34A , but the number of the core unit  301   a  is increased to four (core unit  1  to core unit  4 ). This configuration allows the number of routes to be increased from 1 to 4 and the function to be expanded to the WXC configuration. The function can be expanded to that of  FIG. 34D  after the function is expanded to the ROADM (see  FIG. 34B ), or can be expanded after the function is expanded to the high count channel (HCC) DOADM (see  FIG. 34C ). It is noted that the drop unit and the add unit are omitted in  FIG. 34D  for simplicity. 
         [0216]      FIG. 34E  is a schematic for explaining another expansion of the optical add/drop multiplexer shown in  FIG. 34A . An optical add/drop multiplexer  300   e  is an example of modifying the drop unit  302   c  and the add unit  303   c  a shown in  FIG. 34C . A 1×10 grouping filter (GF)  341  is provided in a drop unit  302   e , and a 16×1-port optical coupler (CPL)  342  is provided in an add unit  303   e . This configuration allows the optical add/drop multiplexer  300   e  to expand the function to the high count channel (HCC) DOADM. The grouping filter  341  is less expensive than WSS  331  (see  FIG. 34C ), which allows reduction in cost. 
         [0217]    The grouping filter  341  of the drop unit  302   e  is connected to a part of the ports of the 1×8-port 50-GHz-spacing wavelength selective switches (WSS)  311  in the core unit  301   a , and the rest of the ports are connected to the wavelength cross-connect device (not shown). It is thereby possible to expand the function to the DOADM with limitation on wavelength that supports the wavelength cross-connect. 
         [0218]    The configurations of the function expansions as shown in  FIG. 34B  to  FIG. 34E  can be provided without replacement of the core unit  301   a . Therefore, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0219]    In-service upgrade example 2 of the optical add/drop multiplexer according to the present invention is explained below.  FIG. 35A  is a schematic of the optical add/drop multiplexer at the time of initial introduction. An optical add/drop multiplexer  350   a  forms the low count channel (LCC) DOADM. As shown in the figure, a core unit  351   a  of the optical add/drop multiplexer  350   a  includes a pair of 50 GHz/100 GHz interleavers (IL)  352   a  and  352   b  in the input side and the output side thereof. The interleaver  352   a  includes two 1×2 optical couplers  353   a  and  353   b , two 1×8-port 100-GHz-spacing wavelength selective switches (WSS)  354   a  and  354   b  for dropping, and two 9×1-port 100-GHz-spacing wavelength selective switches (WSS)  355   a  and  355   b  for adding. 
         [0220]    The core unit  351   a  is connected with a drop unit  361   a  and an add unit  362   a . Based on the configuration, the number of signals to be dropped to the drop unit  361   a  by the core unit  351   a  corresponds to 16 ports at maximum, and the number of signals to be added from the add unit  362   a  corresponds to 18 ports at maximum. A part of the signals dropped or added can be dropped or added to the wavelength cross-connect device (not shown) or the like. 
         [0221]      FIG. 35B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 35A . The core unit  351   a  of an optical add/drop multiplexer  350   b  has the same configuration as that of  FIG. 35A . That is, no part is changed in the core unit  351   a . However, each configuration of the drop unit  361   a  and the add unit  362   a  is changed. A drop unit  361   b  includes two optical demultiplexers (DeMux)  363   a  and  363   b , and an add unit  362   b  includes optical multiplexers (Mux)  364   a  and  364   b . This configuration allows the optical add/drop multiplexer  350   b  to expand the function to the ROADM that supports the wavelength cross-connect. 
         [0222]      FIG. 35C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 35A . The core unit  351   a  of an optical add/drop multiplexer  350   c  has the same configuration as that of  FIG. 35A . However, each configuration of the drop unit  361   a  and the add unit  362   a  is changed. A drop unit  361   c  includes two 1×16 grouping filters (GF)  371   a  and  371   b , and an add unit  303   e  includes two 16×1-port optical couplers (CPL)  372   a  and  372   b . This configuration allows the optical add/drop multiplexer  350   c  to expand the function to the high count channel (HCC) DOADM with limitation on wavelength that supports the wavelength cross-connect. A larger number of grouping filters can be provided in the drop unit  361   c  corresponding to the required number of channels for dropping. Likewise, a larger number of optical couplers can be provided in the add unit  362   c  corresponding to the required number of channels for adding. A part of the signals to be dropped or added can be dropped or added to the wavelength cross-connect device (not shown) or the like. 
         [0223]      FIG. 35D  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 35A . The core unit  351   a  of an optical add/drop multiplexer  350   d  has the same configuration as that of  FIG. 35A , but the number of core unit  351   a  is increased to four (core unit  1  to core unit  4 ). This configuration allows the number of routes to be increased from 1 to 4 and the function to be expanded to the WXC configuration. The function can be expanded to that of  FIG. 35D  after the function is expanded to the ROADM (see  FIG. 35B ), or can be expanded after the function is expanded to the high count channel (HCC) DOADM (see  FIG. 35C ). It is noted that the drop unit and the add unit are omitted in  FIG. 35D  for simplicity. 
         [0224]    The configurations of the function expansions as shown in  FIG. 35B  to  FIG. 35D  can be provided without replacement of the core unit  351   a . Therefore, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0225]    In-service upgrade example 3 of the optical add/drop multiplexer according to the present invention is explained below.  FIG. 36A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction. An optical add/drop multiplexer  380   a  forms the ROADM. A core unit  381   a  of the optical add/drop multiplexer  380   a  includes a 1×2 optical coupler  391 , a 50-GHz-spacing wavelength blocker (WB)  392 , and a 2×1 optical coupler  393 . A drop unit  382   a  includes an optical demultiplexer (DeMux)  400 , and an add unit  383   a  includes an optical multiplexer (Mux)  401 . 
         [0226]      FIG. 36B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A . The core unit  381   a  of an optical add/drop multiplexer  380   b  has the same configuration as that of  FIG. 36A . That is, no part is changed in the core unit  381   a . However, a 1×8-port 50-GHz-spacing wavelength selective switch (WSS)  395  for optical demultiplexing is provided in a drop-side port of the core unit  381   a . An 8×1-port 50-GHz-spacing wavelength selective switch (WSS)  396  for optical multiplexing is provided in an add-side port of the core unit  381   a . These portions are configured as a unit different from the core unit  381   a , and the unit is additionally arranged as a core unit  381   b . This arrangement allows the optical add/drop multiplexer  380   b  to achieve function expansion as low count channel (LCC) DOADM. In this configuration, the optical demultiplexer  400  provided in the drop unit  382   a  and the optical multiplexer  401  provided in the add unit  383   a  as shown in  FIG. 36A  can be detached and used for another device. A part of the output ports of the wavelength selective switch  395  and a part of the input ports of the wavelength selective switch  396  can also be dropped or added to a wavelength cross-connect device (not shown). 
         [0227]      FIG. 36C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A . The core unit  381   a  of an optical add/drop multiplexer  380   c  has the same configuration as that of  FIG. 36A . That is, no part is changed in the core unit  381   a . However, the 1×8-port 50-GHz-spacing wavelength selective switch (WSS)  395  for optical demultiplexing is provided in the drop-side port of the core unit  381   b . The 8×1-port 50-GHz-spacing wavelength selective switch (WSS)  396  for optical multiplexing is provided in the add-side port of the core unit  381   b . At least one of the output ports of the wavelength selective switch  395  in the drop side is connected to the optical demultiplexer (DeMux)  400  of the drop unit  382   a , and at least one of the input ports of the wavelength selective switch  396  in the add side is connected to the optical multiplexer (Mux)  401  of the add unit  383   a . This arrangement allows the optical add/drop multiplexer  380   c  to achieve function expansion as the ROADM that supports the wavelength cross-connect. The optical add/drop multiplexer  380   c  can also be configured by expanding the functions of the optical add/drop multiplexer  380   b  (see  FIG. 36B ). 
         [0228]      FIG. 36D  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A . A functional state of an optical add/drop multiplexer  380   d  as shown in  FIG. 36D  immediately before it is configured is equivalent to the optical add/drop multiplexer  380   b  (see  FIG. 36B ) based on the (LCC) DOADM. The configurations of the core units  381   a  and  381   b  are not changed. However, a drop unit  382   b  includes a 1×2 optical coupler  411 , and two 1×8-port 50-GHz-spacing wavelength selective switches (WSS)  412 . An add unit  383   b  includes a 16×1 optical coupler (CPL)  413 . This configuration allows the function to be expanded to the high count channel (HCC) DOADM. The number of pieces of the optical coupler  411  and of the wavelength selective switch  412  provided in the drop unit  382   b  and the number of pieces of the optical coupler  413  provided in the add unit  383   b  can be increased by the number required. A part of the output ports of the wavelength selective switch  395  and a part of the input ports of the wavelength selective switch  396  can also be dropped or added to a wavelength cross-connect device (not shown). 
         [0229]      FIG. 36E  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A . A functional state of an optical add/drop multiplexer  380   e  as shown in  FIG. 36E  immediately before it is configured is equivalent to the optical add/drop multiplexer  380   c  (see  FIG. 36C ) in the functional state of the ROADM or to the optical add/drop multiplexer  380   d  (see  FIG. 36D ) in the functional state of the (HCC) DOADM. A plurality pairs of the core units  381   a  and  381   b  are connected to allow the function to be expanded to the optical add/drop multiplexer  380   e  including the WXC. The functions of the pair of core units  381   a  and  381   b  are described in the one core unit as shown in  FIG. 36E  for simplicity. The configurations of the drop units  382   a  and  382   b  and the add units  383   a  and  383   b  are not shown, but these units are connected to the core units, respectively. 
         [0230]      FIG. 36F  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 36A . An optical add/drop multiplexer  380   f  as shown in  FIG. 36F  is in a function expanded state of the (HCC) DOADM, and is another configuration example in which it can be replaced for the configuration of  FIG. 36D . In the optical add/drop multiplexer  380   f  as shown in  FIG. 36F , a 1×16-port grouping filter (GF)  416  is arranged in the drop unit  382   c . The add unit  383   b  uses the 16×1-port optical coupler (CPL)  413 . In the configuration example of  FIG. 36F , the function can be further expanded to the WXC as shown in  FIG. 36E . 
         [0231]    The configurations of the function expansions as shown in  FIG. 36B  to  FIG. 36F  can be provided without replacement of the core unit  381   a . Therefore, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0232]    In-service upgrade example 4 of the optical add/drop multiplexer according to the present invention is explained below.  FIG. 37A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction. An optical add/drop multiplexer  430   a  forms the ROADM. A core unit  431   a  of the optical add/drop multiplexer  430   a  includes a 1×2 optical coupler  432 , a 50-GHz-spacing wavelength blocker (WB)  433 , and a 2×1 optical coupler  434 . A core unit  431   b  is formed as a module differently from the core unit  431   a . The core unit  431   b  includes a 50 GHz/100 GHz interleaver (IL)  435  connected to a drop-side port thereof, and a 50 GHz/100 GHz interleaver (IL)  436  connected to an add-side port thereof. A drop unit  432   a  includes two optical demultiplexers (DeMux)  441 , and an add unit  433   a  includes two optical multiplexers (Mux)  442 . 
         [0233]      FIG. 37B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 37A . The core units  431   a  and  431   b  of an optical add/drop multiplexer  430   b  have the same configuration as that of  FIG. 37A . That is, no parts are changed in the core units  431   a  and  431   b . However, the core unit  431   b  is further connected with a core unit  431   c  that is configured as another unit. The core unit  431   c  includes a plurality of 1×8-port 100-GHz-spacing wavelength selective switches (WSS)  451  for dropping, and a plurality of 8×1-port 100-GHz-spacing wavelength selective switches (WSS)  452  for adding. This arrangement allows the optical add/drop multiplexer  430   b  to achieve function expansion as the low count channel (LCC) OADM. A part of the output ports of the wavelength selective switches (WSS)  451  or a part of the input ports of the wavelength selective switches (WSS)  452  can also be dropped or added to a wavelength cross-connect device (not shown). In this configuration, the optical demultiplexer  441  provided in the drop unit  432   a  and the optical multiplexer  442  provided in the add unit  433   a  as shown in  FIG. 37A  can be detached and used for another device. 
         [0234]      FIG. 37C  is a schematic for explaining the expansion of the optical add/drop multiplexer shown in  FIG. 37A . Function expansion from the function of the low count channel (LCC) DOADM as shown in  FIG. 37B  is explained below. Each of the core units  431   a ,  431   b , and  431   c  of an optical add/drop multiplexer  430   c  has the same configuration as that of  FIG. 37B . That is, no parts are changed therein. 
         [0235]    At least one of the output ports of the wavelength selective switch  451  in the drop side is connected to the optical demultiplexer (DeMux)  441  of the drop unit  432   a . At least one of the input ports of the wavelength selective switch  452  in the add side is connected to the optical multiplexer (Mux)  442  of the add unit  433   a . This arrangement allows the optical add/drop multiplexer  430   c  to achieve function expansion as the ROADM that supports the wavelength cross-connect. The optical add/drop multiplexer  430   c  can be configured by expanding the functions of the optical add/drop multiplexer  430   a  (see  FIG. 37A ). When the function is to be changed from the initial state of  FIG. 37A , the core unit  431   c  may be additionally arranged in the above manner. 
         [0236]      FIG. 37D  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 37A . A functional state of an optical add/drop multiplexer  430   d  as shown in  FIG. 37D  immediately before it is configured is equivalent to the optical add/drop multiplexer (see  FIG. 37B ) based on the low count channel (LCC) DOADM. The configurations of the core units  431   a ,  431   b , and  431   c  are not changed. The drop unit  432   b  includes a 1×10-port grouping filter (GF)  455 . The add unit  433   b  includes a 16×1-port optical coupler (CPL)  456 . This configuration allows the function to be expanded to the high count channel (HCC) DOADM. A part of the output ports of the wavelength selective switches  451  or a part of the input ports of the wavelength selective switches  452  can also be dropped or added to a wavelength cross-connect device (not shown). The number of grouping filters  455  provided in the drop unit  432   b  and the number of optical couplers  456  provided in the add unit  433   b  can be additionally provided by the number of ports required. 
         [0237]      FIG. 37E  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 37A . A functional state of an optical add/drop multiplexer  430   e  as shown in  FIG. 37E  immediately before it is configured is equivalent to the optical add/drop multiplexer (see  FIG. 37C )  430   c  in the functional state of the ROADM or to the optical add/drop multiplexer (see  FIG. 37D )  430   d  in the functional state of the (HCC) DOADM. A group of three units such as the core units  431   a ,  431   b , and  431   c  is connected in plurality, which allows the function to be expanded to the optical add/drop multiplexer  430   e  including the WXC. As shown in  FIG. 37E , the three units such as the core units  431   a ,  431   b , and  431   c  are described in one core unit for simplicity. The configurations of the drop units  432   a  and  432   b  and the add units  433   a  and  433   b  are not shown therein, but they are connected to the core units  431   a ,  431   b , and  431   c , respectively. 
         [0238]    The configurations of the function expansions as shown in  FIG. 37B  to  FIG. 37E  can be provided without replacement of the core unit  431   a . Therefore, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0239]    In-service upgrade example 5 of the optical add/drop multiplexer according to the present invention is explained below.  FIG. 38A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction. An optical add/drop multiplexer  500   a  forms the ROADM. A core unit  501   a  of the optical add/drop multiplexer  500   a  includes a 1×2 optical coupler  511 , and a 4×1-port wavelength selective switch (WSS)  512 . A drop unit  502   a  includes a 1×N-port optical demultiplexer (DeMux)  515 , and an add unit  503   a  includes an M×1-port optical multiplexer (Mux)  516 . 
         [0240]      FIG. 38B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 38A . The core unit  501   a  of an optical add/drop multiplexer  500   b  has the same configuration as that of  FIG. 38A . That is, no part is changed in the core unit  501   a . However, the core unit  501   a  is further connected with a core unit  501   b  that is configured as another unit. The core unit  501   b  includes a 1×3 optical coupler (CPL)  520  for dropping. One of the output ports of the optical coupler  520  is connected to the drop unit  502   a , and the functions of the other output ports can be expanded so as to have the wavelength cross-connect. 
         [0241]      FIG. 38C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 38A . An optical add/drop multiplexer  500   c  includes a plurality pairs of the core units  501   a  and  501   b  as shown in  FIG. 38B  (four pairs shown in  FIG. 38C ) to expand the function to the WXC. In the configuration example, a signal can be switched between the two rings of the transmission paths A and B as shown in  FIG. 59 . 
         [0242]    Different core units are connected to each other between the output ports of the optical couplers  520  for dropping and the input ports of the wavelength selective switches  512  for adding, as shown in  FIG. 38C . For example, some of the output ports of the optical coupler  520  in a core unit  1  are connected to the input ports of the wavelength selective switches  512  in a core unit  3  and a core unit  4 . Some of the output ports of the optical coupler  520  in a core unit  2  are connected to the input ports of the wavelength selective switches  512  in the core unit  3  and the core unit  4 . Some of the output ports of the optical coupler  520  in the core unit  3  are connected to the input ports of the wavelength selective switches  512  in the core unit  1  and the core unit  2 . Some of the output ports of the optical coupler  520  in the core unit  4  are connected to the input ports of the wavelength selective switches  512  in the core unit  1  and the core unit  2 . The routes of the transmission paths input or output to or from the core units are described using sign “#”. The core unit  1  outputs the input of the route #1 to the route #2. The core unit  2  outputs the input of the route #2 to the route #1. The core unit  3  outputs the input of the route #3 to the route #4. The core unit  4  outputs the input of the route #4 to the route #3. 
         [0243]    The optical add/drop multiplexer  500   c  is configured as a wavelength cross-connect including four routes, and can switch a signal between the route #1 and the route #2, the route #1 and the route #3, the route #1 and the route #4, the route #2 and the route #3, the route #2 and the route #4, and the route #3 and the route #4 as shown in  FIG. 59 . 
         [0244]    The configurations of the function expansions as shown in  FIG. 38B  and  FIG. 38C  can be provided without replacement of the core unit  501   a . Therefore, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signal. 
         [0245]    In-service upgrade example 6 of the optical add/drop multiplexer according to the present invention is explained below.  FIG. 39A  is a schematic of a configuration of the optical add/drop multiplexer at the time of initial introduction. An optical add/drop multiplexer  530   a  forms the ROADM. A core unit  531   a  of the optical add/drop multiplexer  530   a  includes a 1×2 optical coupler  531 , and a 3×1-port wavelength selective switch (WSS)  532 . A drop unit  532   a  includes a 1×N-port optical demultiplexer (DeMux)  541 , and an add unit  533   a  includes an M×1-port optical multiplexer (Mux)  542 . 
         [0246]      FIG. 39B  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 39A . The core unit  531   a  of an optical add/drop multiplexer  530   b  has the same configuration as that of  FIG. 39A . That is, no part is changed in the core unit  531   a . However, the core unit  531   a  is further connected with a core unit  531   b  that is configured as another unit. The core unit  531   b  includes a 1×2 optical coupler (CPL)  544  for dropping. One of the output ports of the optical coupler  544  is connected to the drop unit  532   a , and the function can be expanded so that the other output port has the wavelength cross-connect. 
         [0247]      FIG. 39C  is a schematic for explaining expansion of the optical add/drop multiplexer shown in  FIG. 39A . An optical add/drop multiplexer  530   c  includes a plurality pairs of the core units  531   a  and  531   b  as shown in  FIG. 39B  (four pairs in  FIG. 39C ) to expand the function to the WXC. 
         [0248]    Different core units are connected to each other between the output ports of the optical couplers  544  for dropping and the input ports of the wavelength selective switches  532  for adding, as shown in  FIG. 39C . For example, one of the output ports of the optical coupler  544  in a core unit  1  is connected to one of the input ports of the wavelength selective switches  532  in a core unit  4 . One of the output ports of the optical coupler  544  in a core unit  2  is connected to one of the input ports of the wavelength selective switches  532  in a core unit  3 . One of the output ports of the optical coupler  544  in the core unit  3  is connected to one of the input ports of the wavelength selective switches  532  in the core unit  2 . One of the output ports of the optical coupler  544  in the core unit  4  is connected to one of the input ports of the wavelength selective switches  532  in the core unit  1 . The routes of the transmission paths input or output to or from the core units are described using sign “#”. The core unit  1  outputs the input of the route #1 to the route #2. The core unit  2  outputs the input of the route #2 to the route #1. The core unit  3  outputs the input of the route #3 to the route #4. The core unit  4  outputs the input of the route #4 to the route #3. 
         [0249]    The configurations of the function expansions as shown in  FIG. 39B  and  FIG. 39C  can be provided without replacement of the core unit  531   a . Therefore, even during system operation, the functions can be expanded without reconnecting the fibers and disconnecting the main signals. 
         [0250]      FIG. 39D  is a schematic for explaining signal switching between transmission paths when the expansion shown in  FIG. 39C  is performed. There are two rings of a transmission path A (optical fibers  1301   a  and  1301   b ) and a transmission path B ( 1302   a  and  1302   b ) formed by the optical add/drop multiplexer  530   c  including the WXC, and signal switching is performed between the transmission paths A and B as shown in  FIG. 39D . The optical add/drop multiplexer  530   c  as explained with reference to  FIG. 39C  is configured as a wavelength cross-connect including four routes, and can switch a signal between a route #1 and a route #2, between the route #1 and a route #4, between the route #2 and a route #3, and between the route #3 and the route #4. The optical add/drop multiplexer  530   c  has a function such that the number of routes that is selectable is limited as compared with the optical add/drop multiplexer  500   c  (see  FIG. 38C ), but has an advantage of achieving simplified configuration. 
         [0251]      FIG. 40A  is a schematic of a configuration when the interleaver is used on the drop side as the grouping filter. An interleaver  551  is connected to one of the output ports of a 1×N-port wavelength selective switch (WSS)  550 . As shown in  FIG. 40A , the number of wavelengths (λ) of a transmission signal is 80 waves at maximum, and a 1×8-port interleaver  551  is used as the grouping filter (GF). An input signal to the interleaver  551  has eight waves at maximum at a 50 GHz-spacing. 
         [0252]    In the example as shown in  FIG. 40A , the eight waves are λ 1 , λ 2 , λ 14 , λ 23 , λ 27 , λ 52 , λ 69 , and λ 80 . One 100 GHz/50 GHz interleaver  551   a , two 200 GHz/100 GHz interleavers  551   b , and four 400 GHz/200 GHz interleavers  551   c  are sequentially connected in the interleaver  551 . This connection allows the signals input at a 50 GHz-spacing to be demultiplexed from the outputs of the eight ports in total, and 10 waves (10λ) are assigned to each of the ports. Upon actual operation, one wave out of the 10 waves is output (e.g., port  1  outputs λ 23 ). 
         [0253]      FIG. 40B  is a schematic of a configuration when the interleaver is used on the add side as the grouping filter. An 8×1-port interleaver (IL)  553  is used as the grouping filter (GF). Input signals to the interleaver  553  are λ 1 , λ 2 , λ 14 , λ 23 , λ 27 , λ 52 , λ 69 , and λ 80  in the example as shown in  FIG. 40B . Four 400 GHz/200 GHz interleavers  553   a , two 200 GHz/100 GHz interleavers  553   b , and one 100 GHz/50 GHz interleaver  553   c  are sequentially connected in the interleaver  553 . This connection allows inputs to the eight ports in total, and 10 waves are assigned to each of the ports. Upon actual operation, one wave out of the 10 waves is input (e.g., λ 23  is input to port  1 ). The output of the interleaver  553  is set as signals at a 50 GHz-spacing, and is connected to one of the input ports of the N×1-port wavelength selective switch (WSS)  554 . These interleavers  551  and  553  are excellent in transmission characteristics as compared with another system in which they are used as grouping filters. 
         [0254]    Specific examples of the configurations using the interleaver as the grouping filter are explained below.  FIG. 34F  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E .  FIG. 34G  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 34F . In the configuration of  FIG. 34E , if the number of wavelengths of a main signal input to the core unit  301   a  is 40 wavelengths, an interleaver  343  (see  FIG. 34G ) as the grouping filter (GF)  341  is connected to each of the five ports out of the eight output ports of the wavelength selective switch  311 , and different wavelengths are assigned to the output ports of all the interleavers  343 . By connecting the remaining three ports to the wavelength cross-connect device, it is possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 40 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0255]      FIG. 34H  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E .  FIG. 34I  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 34H . In the configuration of  FIG. 34E , if the number of wavelengths of a main signal input to the core unit  301   a  is 80 wavelengths, a 1×2 optical coupler  346  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  311 , and a 1×8-port interleaver  343   a  (see  FIG. 34I ) as the grouping filter (GF)  341  is connected to two output ports of the optical coupler  346 . Thus, different wavelengths are assigned to all the output ports of the interleavers  343   a . By connecting the remaining three ports to the wavelength cross-connect device, it is possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0256]      FIG. 36G  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F .  FIG. 36H  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 36G . In the configuration of  FIG. 36F , if the number of wavelengths of a main signal input to the core unit  381   a  is 40 wavelengths, a 1×8-port interleaver  417  (see  FIG. 36H ) as the grouping filter  416  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  395 . Different wavelengths are assigned to all the output ports of the interleavers  417 , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 40 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0257]      FIG. 36I  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F .  FIG. 36J  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 36I . If the number of wavelengths of a main signal input to the core unit  381   a  as shown in  FIG. 36F  is 80 wavelengths, a 1×2-port optical coupler  418  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  395 , and a 1×8-port interleaver  417  (see  FIG. 36J ) as the grouping filter (GF)  416  is connected to two output ports of the optical coupler  418 . Different wavelengths are assigned to all the output ports of the interleavers  417 , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0258]      FIG. 35E  is a schematic of a specific configuration the optical add/drop multiplexer shown in  FIG. 35C .  FIG. 35F  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 35E . In the configuration of  FIG. 35C , if the number of wavelengths of a main signal input to the core unit  351   a  is 80 wavelengths, a 1×8-port interleaver  373  (see  FIG. 35F ) as the grouping filters  371   a / 371   b  is connected to each of the five ports out of the eight output ports of the respective wavelength selective switches (WSS)  354   a  and  354   b . Different wavelengths are assigned to all the output ports of the interleavers  373 , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0259]      FIG. 37F  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 37D .  FIG. 37G  is a schematic of the interleaver that forms a grouping filter (GF) shown in  FIG. 37F . In the configuration of  FIG. 37D , if the number of wavelengths of a main signal input to the core unit  431   a  is 80 wavelengths, a 1×8-port interleaver  457  as the grouping filter (GF)  455  is connected to each of the five ports out of the eight output ports of the respective wavelength selective switches (WSS)  451 . Different wavelengths are assigned to all the output ports of the interleavers  457 , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0260]      FIG. 41A  is a diagram of a configuration example of using a band division filter as a grouping filter in the drop side. A band division filter (BDF)  561  is connected to one of the output ports of a 1×N-port wavelength selective switch (WSS)  560 . As shown in  FIG. 41A , the number of wavelengths (λ) of a transmission signal is 80 waves at maximum, and a 1×8-port band division filter  561  is used as the grouping filter (GF). Eight wavelengths (8×) are assigned respectively to eight output ports of the band division filter  561 , and one of the eight wavelengths is used for actual operation. 
         [0261]      FIG. 41B  is a diagram of a configuration example of using a band division filter as a grouping filter in the add side. Eight wavelengths each are assigned respectively to eight input ports of an 8×1-port band division filter (BDF)  563 , and one of the eight wavelengths is used for actual operation. The output of the band division filter  563  is connected to one of the input ports of an N×1-port wavelength selective switch (WSS)  564 . It may be necessary to ensure a guard band that is unavailable, depending on the band division filters  561  and  563 . This guard band may cause an available guide band to be limited. However, the band division filters  561  and  563  can be realized at low cost as compared with some other system in which the band division filters are used as the grouping filters. 
         [0262]    Specific examples of the configurations using the band division filter as the grouping filter are explained below.  FIG. 34J  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E .  FIG. 34K  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E .  FIG. 34L  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ) shown in  FIG. 34J . In the configuration of  FIG. 34E , if the number of wavelengths of a main signal input to the core unit  301   a  is 40 wavelengths, the band division filters (BDF)  344   a  and  344   b  as the grouping filter  341  are connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  311 . Different wavelengths are assigned to all the output ports of the band division filters  344   a  and  344   b , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to loosen the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to the 40 wavelengths to be dropped and at the same time to realize the wavelength cross-connect. 
         [0263]      FIG. 34M  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E ; 
         [0264]      FIG. 34N  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 34M .  FIG. 34N  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 34M .  FIG. 34O  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 34M . If the number of wavelengths of a main signal input to the core unit  301   a  as shown in  FIG. 34E  is 80 wavelengths, a 1×2-port optical coupler  344  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  311 , and the (1×8-port) band division filters  344   c  and  344   d  as the grouping filter (GF)  341  are connected to each of the two output ports of the optical coupler  344 . Different wavelengths are assigned to all the output ports of the band division filters  344   c  and  344   d , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0265]      FIG. 36K  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F .  FIG. 36L  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ) shown in  FIG. 36K .  FIG. 36M  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ) shown in  FIG. 36K . If the number of wavelengths of a main signal input to the core unit  381   a  as shown in  FIG. 36F  is 40 wavelengths, the (1×8-port) band division filters  419   a  and  419   b  as the grouping filter (GF)  416  are connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  395 . Different wavelengths are assigned to all the output ports of the band division filters  419   a  and  419   b , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 40 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0266]      FIG. 36N  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F .  FIG. 36O  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 36N .  FIG. 36P  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 36N . If the number of wavelengths of a main signal input to the core unit  301   a  as shown in  FIG. 36F  is 80 wavelengths, a 1×2-port optical coupler  418  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  395 , and the (1×8-port) band division filters  419   c  and  419   d  are connected to each of the two output ports of the optical coupler  418 . Different wavelengths are assigned to all the output ports of the band division filters  419   c  and  419   d , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0267]      FIG. 35G  is a diagram of another specific configuration of the optical add/drop multiplexer as shown in  FIG. 35C .  FIG. 35H  is a schematic of the band division filter  373   a  that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 35G .  FIG. 35I  is a schematic of the band division filter  373   d  that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 35G . In the configuration of  FIG. 35C , if the number of wavelengths of a main signal input to the core unit  351   a  is 80 wavelengths, the (1×8-port) band division filters  373   a  and  373   b  as the grouping filters (GF)  371   a / 371   b  are connected to each of the five ports out of the eight output ports of the wavelength selective switches (WSS)  354   a  and  354   b . Different wavelengths are assigned to all the output ports of the band division filters  373   a  and  373   b , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0268]      FIG. 37H  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 37D .  FIG. 37I  is a schematic of the band division filter that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 37H .  FIG. 37J  is a schematic of the band division filter that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 37H . In the configuration of  FIG. 37D , if the number of wavelengths of a main signal input to the core unit  431   a  is 80 wavelengths, the (1×8-port) band division filters  458   a  and  458   b  as the grouping filters (GF)  455  are connected to each of the five ports out of the eight output ports of the respective wavelength selective switches (WSS)  451 . Different wavelengths are assigned to all the output ports of the band division filters  458   a  and  458   b , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0269]      FIG. 42A  is a schematic of a configuration when the colorless AWG is used on the drop side as the grouping filter. A colorless AWG  571  is connected to one of the output ports of a 1×N-port wavelength selective switch (WSS)  570 . As shown in the figure, the number of wavelengths (λ) of a transmission signal is 80 waves at maximum, and a 1×10-port colorless AWG  571  is used as the grouping filter (GF). Four wavelengths (4λ) as a group are assigned to each of the 10 output ports of the colorless AWG  571 , and one of the four wavelengths is used for actual operation. 
         [0270]      FIG. 42B  is a schematic of a configuration when the colorless AWG is used on the add side as the grouping filter. Four wavelengths as a group are assigned to each of the input ports of a 10×1-port colorless AWG  573 , and one of the four wavelengths is used for actual operation. The output of the colorless AWG  573  is connected to one of the input ports of a N×1-port wavelength selective switch (WSS)  574 . 
         [0271]      FIG. 34P  is a diagram of another specific configuration of the optical add/drop multiplexer as shown in  FIG. 34E .  FIG. 34Q  is a schematic of a colorless AWG that forms the grouping filters (GF 1  to  5 ) shown in  FIG. 34P . If the number of wavelengths of a main signal input to the core unit  301   a  of  FIG. 34E  is 40 wavelengths, a 1×8-port colorless AWG  345  as the grouping filter (GF)  341  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  311 . Different wavelengths are assigned to all the output ports of the colorless AWG  345 , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 40 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0272]      FIG. 34R  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 34E .  FIG. 34S  is a schematic of a colorless AWG that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 34R .  FIG. 34T  is a schematic of the colorless AWG that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 34R . If the number of wavelengths of a main signal input to the core unit  301   a  of  FIG. 34E  is 80 wavelengths, a 1×2-port optical coupler  344  is connected to each of the five ports out of the eight output ports of the wavelength selective switches (WSS)  311 , and the (1×8-port) colorless AWGs  345   a  and  345   b  are connected to each of the two output ports of the optical coupler  344 . Different wavelengths are assigned to all the output ports of the colorless AWGs  345   a  and  345   b , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0273]      FIG. 36Q  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F .  FIG. 36R  is a schematic of the colorless AWG that forms grouping filters (GF 1  to  5 ) shown in  FIG. 36Q . If the number of wavelengths of a main signal input to the core unit  381   a  of  FIG. 36F  is 40 wavelengths, a 1×8-port colorless AWG  420  as the grouping filter (GF)  416  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  395 . Different wavelengths are assigned to all the output ports of the colorless AWG (CMDX)  420 , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 40 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0274]      FIG. 36S  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 36F .  FIG. 36T  is a schematic of the colorless AWG  420   a  that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 36S .  FIG. 36U  is a schematic of the colorless AWG  420   b  that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 36S . If the number of wavelengths of a main signal input to the core unit  381   a  of  FIG. 36F  is 80 wavelengths, a 1×2-port optical coupler  418  is connected to each of the five ports out of the eight output ports of the wavelength selective switch (WSS)  395 , and the (1×8-port) colorless AWGs (CMDX)  420   a  and  420   b  are connected to each of the two output ports of the respective optical couplers  418 . Different wavelengths are assigned to all the output ports of the colorless AWGs (CMDX)  420   a  and  420   b , and the remaining three ports are connected to the wavelength cross-connect device. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0275]      FIG. 35J  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 35C .  FIG. 35K  is a schematic of the colorless AWG  374   a  that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 35J .  FIG. 35L  is a schematic of the colorless AWG  374   b  that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 35J . If the number of wavelengths of a main signal input to the core unit  351   a  of  FIG. 35C  is 80 wavelengths, 1×8-port colorless AWGs  374   a  and  374   b  as the grouping filters (GF)  371   a / 371   b  are, connected to each of the five ports out of the eight output ports of the respective wavelength selective switches (WSS)  354   a  and  354   b . Different wavelengths are assigned to all the output ports of the colorless AWGs  374   a  and  374   b , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0276]      FIG. 37K  is a schematic of a specific configuration of the optical add/drop multiplexer shown in  FIG. 37D .  FIG. 37L  is a schematic of the colorless AWG that forms grouping filters (GF 1 ,  3 ,  5 ,  7 , and  9 ) shown in  FIG. 37K .  FIG. 37M  is a schematic of the colorless AWG that forms grouping filters (GF 2 ,  4 ,  6 ,  8 , and  10 ) shown in  FIG. 37K . In the configuration of  FIG. 37D , if the number of wavelengths of a main signal input to the core unit  431   a  is 80 wavelengths, 1×8-port colorless AWGs  374   c  and  374   d  as the grouping filter (GF)  455  are connected to each of the five ports out of the eight output ports of the respective wavelength selective switches (WSS)  451 . Different wavelengths are assigned to all the output ports of the colorless AWGs (CMDX)  374   c  and  374   d , and the each remaining three ports are connected to the wavelength cross-connect devices. It is thereby possible to overcome the limitation on the number of wavelengths for use, which is a problem occurring upon using the grouping filter, and to allow signals corresponding to all the 80 wavelengths of the main signal to be dropped and at the same time to realize the wavelength cross-connect. 
         [0277]    An example of using an optical spectrum monitor for optical power control is explained below.  FIG. 43A  is a schematic of a configuration in which an optical spectrum monitor is used for control of optical power of the drop signal. A core unit  580  includes a 1×2 optical coupler  581 , an M×1-port wavelength selective switch (WSS)  582 , and a 1×N-port wavelength selective switch (WSS)  583  for dropping. Optical couplers  584   a  to  584   n  are provided in the output ports in the drop side, respectively. Optical signals branched by the optical couplers  584   a  to  584   n  are combined by an N×1 optical coupler  585 , and the optical signals combined are input to an optical spectrum monitor  586 . The optical spectrum monitor  586  adjusts an optically combined state of each of the ports of the wavelength selective switch (WSS)  583  so that the optical power at each of the ports is a required value. It is thereby possible to control the optical power of an optical signal in the drop side. 
         [0278]      FIG. 43B  is a schematic of a configuration in which an optical spectrum monitor is used for control of optical power of the drop signal. A core unit  590  includes a 1×N-port wavelength selective switch (WSS)  591  for dropping, a 2×1 optical coupler  592 , and an M×1-port wavelength selective switch (WSS)  593  for adding. Optical couplers  594   a  to  594   n  are provided in the output ports in the main signal side and the drop side of the wavelength selective switch  591 , respectively. Optical signals branched by the optical couplers  594   a  to  594   n  are combined by an N×1 optical coupler  595 , and the optical signals combined are input to an optical spectrum monitor  596 . The optical spectrum monitor  596  adjusts an optically combined state of each of the ports of the wavelength selective switch (WSS)  591  so that the optical power at each of the ports is a required value. It is thereby possible to control the optical power of the main signal and the optical signal in the drop side. 
         [0279]    Examples of a configuration when a core unit using an interleaver is extended are explained below.  FIG. 44  is a schematic for explaining extension of the core unit that includes the interleaver. A core unit  600   a  upon initial introduction of an optical add/drop multiplexer  600  is switchably configured among four routes (#1 to #4). 
         [0280]    The core unit  600   a  includes four 50/100 GHz interleavers  601   a  to  601   d  provided in its input side corresponding to the four routes, and four 100/50 GHz interleavers  604   a  to  604   d  provided in its output side. Arranged between the input-side interleavers and the output-side interleavers are four 1×4-port 100-GHz-spacing wavelength selective switches (WSS)  602   a  to  602   d  and four 4×1-port 100-GHz-spacing wavelength selective switches (WSS)  603   a  to  603   d . The output ports of the wavelength selective switches  602   a  to  602   d  are mutually connected to the input ports of the wavelength selective switches  603   a  to  603   d  according to switching for each required route. Transmission signals are input or output to or from the optical add/drop multiplexer  600  at a 50 GHz-spacing. At the time of initial introduction of the device with little communication capacity, the core unit  600   a  starts the operation of the device using the channel of an even number. A wavelength spacing of the transmission signal in this case is 100 GHz. 
         [0281]    If the communication capacity increases, a core unit  600   b  is extended to achieve function expansion. The core unit  600   b  includes 1×4-port 100-GHz-spacing wavelength selective switches (WSS)  610   a  to  610   d  of which input ports are connected to the interleavers  601   a  to  601   d  in the input side of the core unit  600   a , and 4×1-port 100-GHz-spacing wavelength selective switches (WSS)  611   a  to  611   d  of which output ports are connected to the interleavers  604   a  to  604   d  in the output side of the core unit  600   a . Upon extension of the core unit  600   b , the core unit  600   a  handles the channel of an even number for a transmission signal, while the core unit  600   b  handles the channel of an odd number for a transmission signal. According to the example of the function expansion based on the configuration, cost reduction upon initial introduction becomes possible. 
         [0282]    Examples of configurations in which the internal configuration of the core unit is broken into blocks are explained below.  FIG. 45A  is a schematic of a wavelength selective switch on the drop side separated as a block. A core unit  620  includes a 1×2 optical coupler  621  and an M×1 wavelength selective switch (WSS)  622 . Furthermore, a core block  620   a  including a 1×N wavelength selective switch (WSS)  623  for dropping can be connected to the core unit  620  according to the number of ports that allow signals to be dropped. It is thereby possible to change only the block according to whether the wavelength selective switch  623  for dropping is required. 
         [0283]      FIG. 45B  is a schematic of a wavelength selective switch on the add side separated as a block. A core unit  630  includes a 1×N wavelength selective switch (WSS)  631  and a 2×1 optical coupler  632 . Furthermore, a core block  630   a  including an M×1 wavelength selective switch (WSS)  633  for adding can be connected to the core unit  630  according to the number of ports that allow signals to be added. It is thereby possible to change only the block according to whether the wavelength selective switch  633  for adding is required. The block formed in the drop side or the add side of the core unit can be used as a configuration of the core unit upon function expansion as explained in the in-service upgrade examples. 
         [0284]    In the optical add/drop multiplexers, the remaining ports out of the ports for adding/dropping of the add unit or the drop unit are used as ports for routes for wavelength cross-connect, but expansion examples of a port for a WXC route in order to ensure the fixed number of routes are explained below with reference to the drawings. 
         [0285]      FIG. 46A  is a schematic of the optical add/drop multiplexer according to an embodiment of the present invention to realize a function of a wavelength cross-connect. An optical add/drop multiplexer  700   a  includes a core unit  701   a , a drop unit  702   a , and an add unit  703   a . The core unit  701   a  includes a 1×2 optical coupler  710   a , a 1×7-port wavelength selective switch (WSS)  711   a  for dropping connected to one of the outputs of the 1×2 optical coupler  710   a , and an 8×1-port wavelength selective switch (WSS)  712   a  for adding connected to the other output of the 1×2 optical coupler  710   a.    
         [0286]    The 1×7-port wavelength selective switch (WSS)  711   a  is connected with the drop unit  702   a , and the 8×1 port wavelength selective switch (WSS)  712   a  is connected with the add unit  703   a . Furthermore, in order to realize the wavelength cross-connect (WXC), two ports in the output side of the 1×7-port wavelength selective switch (WSS)  711   a  and two ports in the input side of the 8×1 port wavelength selective switch (WSS)  712   a  are connected to other routes (#3, #4). The number of input ports of the wavelength selective switch (WSS)  712   a  and the number of output ports of the wavelength selective switch (WSS)  711   a  of  FIG. 46A  are the minimum number to realize the wavelength cross-connect for four routes. Therefore, the wavelength selective switches can be replaced with another wavelength selective switch including a larger number of ports. All the wavelength selective switches as shown hereinafter are configured with the necessary minimum number of ports. 
         [0287]    The drop unit  702   a  includes a plurality of 1×8-port wavelength selective switches (WSS)  721 . Each of the wavelength selective switches (WSS)  721  can drop the wavelength to eight wavelengths. If 40 wavelengths (λ 1  to λ 40 ) are multiplexed as shown in this embodiment, five pieces of the wavelength selective switches (WSS)  721  are necessary to drop signal lights having all the wavelengths. The add unit  703   a  includes a plurality of 8×1 optical couplers (CPL)  731  and a plurality of optical amplifiers  732  to recover attenuation due to the 8×1 optical couplers (CPL)  731 . In the 8×1 optical couplers (CPL)  731 , eight wavelengths can be added to each of them, and five pieces of the 8×1 optical couplers (CPL)  731  are required to add signal lights having all the wavelengths. The optical amplifier  732  is provided to amplify the signal light attenuated due to the 8×1 optical coupler (CPL)  731 . 
         [0288]    Referring to the ports for output or input of the wavelength selective switch provided in the core unit  701   a , the required number of ports are used for ports for adding and ports for dropping such that one port is required if a signal light having 8 wavelengths is to be added or dropped and two ports are required if a signal light having 16 wavelengths is to be added or dropped. The remaining ports are used as a wavelength cross-connect switch. Therefore, the number of ports that can be used as the WXC is changed depending on the required number of ports for adding or for dropping. In other words, the number of routes depends on the number of wavelengths to be added or dropped. 
         [0289]      FIG. 46B  is a diagram of a relationship between the number of channels for the add unit/drop unit and the maximum number of routes for wavelength cross-connect. The x-axis indicates the number of add/drop channels and the y-axis indicates the maximum number of routes for the wavelength cross-connect. Values obtained when 8×1 (1×8) elements are used in the add unit/drop unit are shown therein. Therefore, the relationship between the number of add/drop channels and the maximum number of routes for the wavelength cross-connect becomes [the maximum number of routes=(the number of output ports not for adding, out of the output ports of the wavelength selective switch for dropping in the core unit)+2]. The value “+2” in the right side indicates a through (main signal) port to a route #2 through which the main signal is caused to pass as shown in  FIG. 46A , and indicates a port for a route #1 in which a signal is not directly output to the input port. 
         [0290]      FIG. 47  and  FIG. 48  are schematics for explaining expansion of ports for routes of the optical add/drop multiplexer shown in  FIG. 46A . In an optical add/drop multiplexer  700   b  of  FIG. 47  and an optical add/drop multiplexer  700   c  of  FIG. 48 , the number of ports of wavelength selective switch (WSS) in each core unit is indicated by the necessary minimum number to realize the optical add/drop function. Therefore, the number is different depending on the expansion examples. In actual cases, the optical add/drop multiplexer employs a 1×8-port wavelength selective switch (WSS) for dropping and a 9×1-port wavelength selective switch (WSS) for adding, and therefore, the optical add/drop multiplexer  700   b  and the optical add/drop multiplexer  700   c  are configured with the same core unit. 
         [0291]    A core unit  701   b  of the optical add/drop multiplexer  700   b  of  FIG. 47  includes a 1×6-port wavelength selective switch (WSS)  711   b  for dropping of which five ports in the output side are connected to the drop unit  702   a , and a 9×1-port wavelength selective switch (WSS)  712   b  for adding of which five ports in the input side are connected to the add unit  703   a . The number of ports for connection from the 1×6-port wavelength selective switch (WSS)  711   b  to the drop unit  702   a  and the number of ports for connection from the add unit  703   a  to the 9×1-port wavelength selective switch (WSS)  712   b  are fixed to five ports (for 40 wavelengths), respectively. It is thereby possible to drop or add all the signal lights (λ 1  to λ 40 ) multiplexed. In order to increase the number of ports for connection to routes, a 1×6-port wavelength selective switch (WSS)  742  as an expansion element  741  for output to a route is connected to one of the outputs of the 1×6-port wavelength selective switch (WSS)  711   b  for dropping. Furthermore, 2×1 optical couplers  752  as an expansion element  751  for input from a route are connected to three ports in the input side of the 9×1-port wavelength selective switch (WSS)  712   b  for adding. 
         [0292]    As shown in  FIG. 47 , an optical amplifier  743  that amplifies a signal light to be output to a route is provided between the 1×6-port wavelength selective switch (WSS)  711   b  and the expansion element  741 . However, the optical amplifier  743  may be provided in either one of the ports for output to and input from a route. Therefore, the optical amplifier  743  may also be provided between the expansion element  751  and the 9×1-port wavelength selective switch (WSS)  712   b.    
         [0293]    A core unit  701   c  of the optical add/drop multiplexer  700   c  includes a 1×8-port wavelength selective switch (WSS)  711   c  for dropping of which five ports in the output side are connected to the drop unit  702   a , and a 7×1-port wavelength selective switch (WSS)  712   c  for adding of which five ports in the input side are connected to the add unit  703   a . The number of ports for connection from the 1×8-port wavelength selective switch (WSS)  711   c  to the drop unit  702   a  and the number of ports for connection from the 7×1-port wavelength selective switch (WSS)  712   c  to the add unit  703   a  are fixed to five ports (for 40 wavelengths), respectively. It is thereby possible to drop or add all the signal lights (λ 1  to λ 40 ) multiplexed. 
         [0294]    In the optical add/drop multiplexer  700   c , three 1×2 optical couplers  744  as the expansion element  741  are connected to three ports in the output side of the 1×8-port wavelength selective switch (WSS)  711   c  for dropping, and a 6×1-port wavelength selective switch (WSS)  753  as the expansion element  751  is connected to one of the inputs of the 7×1-port wavelength selective switch (WSS)  712   c  for adding. These points are different from the optical add/drop multiplexer  700   b  (see  FIG. 47 ). By using the optical couplers  744  for the expansion element  741 , an unnecessary signal light may be input depending on a route. Therefore, the 7×1-port wavelength selective switch (WSS)  712   c  for adding in the core unit  701   c  is controlled so as to cut off the unnecessary signal light. 
         [0295]    As explained above, in the expansion examples of  FIG. 47  and  FIG. 48 , the expansion elements ( 741 ,  751 ) for routes are provided in the wavelength selective switches (WSS) for dropping and add for connection to another route, which allows independent six ports to be ensured. In other words, the wavelength cross-connect for eight routes can always be configured, irrespective of the number of wavelengths to be added or dropped. If the optical couplers ( 744 ,  752 ) are used for the expansion elements ( 741 ,  751 ), a plurality of signal lights having the same wavelength are multiplexed, which may cause signal degradation due to optical interference to occur therein. If the optical coupler is used, it is exclusively provided in either one of the expansion element  741  for output and the expansion element  751  for input, and the wavelength selective switch (WSS) is arranged in the other one of the expansion elements ( 741 ,  751 ) as shown in  FIG. 47  or  FIG. 48 . As explained above, when the optical coupler is used for the expansion element ( 741 ,  751 ), it is also exclusively provided only in either one of the expansion elements in optical add/drop multiplexers as explained below with reference to the drawings. 
         [0296]      FIG. 49  to  FIG. 51  are schematics for explaining expansion of ports for routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit. The core unit ( 701   d ,  701   e ,  701   f ) of each optical add/drop multiplexer as shown in  FIG. 49  to  FIG. 51  is obtained by adding a 1×2 optical coupler  710   b  as an expansion element  713 . The optical coupler  710   b  is added between the output of the 1×2 optical coupler  710   a  to the drop unit  702   a  and the 1×7-port wavelength selective switch (WSS)  711   a  that drops a signal to the drop unit  702   a , of the core unit  701   a  in the optical add/drop multiplexer  700   a  (see  FIG. 46A ). 
         [0297]    The number of ports of the wavelength selective switch (WSS) for dropping or adding in the core unit of each of the optical add/drop multiplexers as shown in  FIG. 49  to  FIG. 51  is indicated by the necessary minimum number to realize the function. In actual cases, the optical add/drop multiplexer employs a 1×8-port wavelength selective switch (WSS) for dropping and a 9×1-port wavelength selective switch (WSS) for adding. Therefore, the core units ( 701   d ,  701   e ,  701   f ) of  FIG. 49  to  FIG. 51  have the same configuration as one another. The optical amplifier  743  that amplifies a signal light to be output to a route is provided between the expansion element  713  of the core unit and the expansion element  741  for routes. The optical amplifier  743  may be provided in either one of the add side and the drop side of the route. 
         [0298]    In an optical add/drop multiplexer  700   d  of  FIG. 49 , a 1×5 port wavelength selective switch (WSS)  711   d  for dropping is connected to one of the outputs of the expansion element  713  in the core unit  701   d . Furthermore, five ports in the output side of the 1×5-port wavelength selective switch (WSS)  711   d  for dropping are connected to the drop unit  702   a . One 1×2 optical coupler  744  as the expansion element  741  for routes is connected to the other port of the expansion element  713 . Five ports for input of the 8×1-port wavelength selective switch (WSS)  712   a  for adding are connected from the add unit  703   a  and two ports thereof are connected from other routes to form the wavelength cross-connect for four routes. 
         [0299]    As explained above, the port for connection to the drop unit  702   a  is separated from the port for connection to the expansion element  741  for the routes in the core unit  701   d . With the separation, the increase or decrease in the number of wavelengths to be added or dropped is performed mutually independently from the increase in the number of routes for the cross-connect. Furthermore, the optical coupler  744  is used for the expansion element  741 , and this case is compared with the case of using the wavelength selective switch to allow simplification and cost reduction of the configuration. Moreover, if necessary, the optical amplifier  743  may be provided in the upstream or the downstream of the optical coupler  744  as the expansion element  741  so as to compensate for optical loss due to the expansion element  713  of the core unit  701   d.    
         [0300]    An optical add/drop multiplexer  700   e  of  FIG. 50  includes the core unit  701   e  the same as that of the optical add/drop multiplexer  700   d  (see  FIG. 49 ). However, the optical add/drop multiplexer  700   e  has a difference in that a 1×6-port wavelength selective switch  742  that serves as the expansion element  741  for routes is connected to one of the outputs of the expansion element  713 , five ports in the input side of the 9×1-port wavelength selective switch (WSS)  712   b  for adding are connected from the add unit  703   a , and three ports thereof are connected with three 2×1 optical couplers  752  that serves as the expansion element  751  with signals input from routes. 
         [0301]    The wavelength cross-connect for eight routes is configured in the above manner, and the number of routes can further be increased. Moreover, if necessary, the optical amplifier  743  may be provided in the upstream or the downstream of the 1×6-port wavelength selective switch  742  as the expansion element  741  so as to compensate for optical loss due to the expansion element  713  of the core unit  701   e.    
         [0302]    An optical add/drop multiplexer  700   f  of  FIG. 51  includes the core unit  701   f  the same as that of the optical add/drop multiplexer  700   d  (see  FIG. 49 ). However, the optical add/drop multiplexer  700   f  has a difference in that a 1×6 optical coupler (CPL)  745  that serves as the expansion element  741  for routes is connected to one port of the expansion element  713 , five ports in the input side of the 7×1-port wavelength selective switch (WSS)  712   c  for adding are connected from the add unit  703   a , and one port thereof is connected from one 6×1-port wavelength selective switch  753  that serves as the expansion element  751 . 
         [0303]    The wavelength cross-connect for eight routes is configured in the above manner. Using the optical coupler  745  for the expansion element  741  may cause unnecessary signal light to be input depending on a route. Therefore, the 7×1-port wavelength selective switch (WSS)  712   c  for adding in the core unit  701   f  is controlled so as to cut off the unnecessary signal light. Furthermore, the 1×6 optical coupler (CPL)  745  used as the expansion element  741  for routes has a larger optical loss as compared with the 1×2 optical coupler  744  (see  FIG. 49 ). Therefore, if the output for the routes in the same level as that of the optical add/drop multiplexers  700   d  and  700   e  is required, the optical amplifier  743  needs to be provided in the upstream or the downstream of the 1×6 optical coupler  745  so as to compensate for the optical loss. 
         [0304]    If the optical couplers ( 744 ,  745 ,  752 ) are used for the expansion elements ( 741 ,  751 ), a plurality of signal lights having the same wavelength are multiplexed, which may cause signal degradation due to optical interference to occur therein. Therefore, as shown in  FIG. 50  or  FIG. 51 , the wavelength selective switch (WSS) has to be arranged in either one of the expansion elements ( 741 ,  751 ). 
         [0305]      FIG. 52  to  FIG. 54  are schematics for explaining expansion of the ports for the routes of the optical add/drop multiplexer when the 1×6 optical coupler is used on the drop side. Each of core units ( 701   g ,  701   h , and  701   i ) of the optical add/drop multiplexers as shown in  FIG. 52  to  FIG. 54  includes the 1×6 optical coupler  745  that serves also as the expansion element  713  in the drop side, instead of the 1×7-port wavelength selective switch (WSS)  711   a  for dropping of the core unit  701   a  in the optical add/drop multiplexer  700   a  (see  FIG. 46A ). 
         [0306]    The number of ports of each of wavelength selective switches for dropping and add of each core unit in the optical add/drop multiplexers of  FIG. 52  to  FIG. 54  is the necessary required number of ports to realize the functions. A 1×8-port wavelength selective switches (WSS) for dropping and a 9×1-port wavelength selective switch (WSS) for adding are used to allow realization of the same functions. Therefore, the core units ( 701   g ,  701   h , and  701   i ) of  FIG. 52  to  FIG. 54  have the configurations actually the same as one another. Furthermore, the 1×6 optical coupler has a larger optical loss as compared with the 1×2 optical coupler. Therefore, if necessary, the optical amplifier  743  may be provided in the input side of each of the wavelength selective switches  721  in the drop unit  702   a  so as to compensate for the optical loss. 
         [0307]    In an optical add/drop multiplexer  700   g  of  FIG. 52 , five ports in the output side of the 1×6 optical coupler  745  as the expansion element  713  that is provided for dropping of the core unit  701   g  are fixed for dropping and connected to the drop unit, and the remaining one port is connected to the 1×2 optical coupler  744  as the expansion element  741  for routes. In the 8×1-port wavelength selective switch (WSS)  712   a  for adding, five ports in the input side thereof are connected from the add unit  703   a , and two ports thereof are connected from other routes. 
         [0308]    The drop unit  702   a  is separated from the expansion element  741  for routes in the above manner to configure the wavelength cross-connect for four routes. By limiting the number of routes to four, the routes can be formed independently from one another at low cost without using the wavelength selective switch (WSS) for the expansion element  741  for routes. 
         [0309]    An optical add/drop multiplexer  700   h  of  FIG. 53  includes the core unit  701   h  the same as that of the optical add/drop multiplexer  700   g  (see  FIG. 52 ). However, the optical add/drop multiplexer  700   f  has a difference in that the 1×6-port wavelength selective switch (WSS)  742  as the expansion element  741  for routes is connected from the expansion element  713  for dropping, five ports in the input side of the 9×1-port wavelength selective switch (WSS)  712   b  for adding are connected from the add unit  703   a , and three ports thereof are connected with 2×1 optical couplers  752  as the expansion element  751  with signals input from routes. 
         [0310]    The wavelength cross-connect for eight routes is configured in the above manner, and the number of routes can further be increased. Furthermore, the optical amplifier  743  may be provided in the upstream or the downstream of the 1×6-port wavelength selective switch  742  as the expansion element  741  so as to compensate for optical loss due to the expansion element  713  of the core unit  701   h.    
         [0311]    An optical add/drop multiplexer  700   i  of  FIG. 54  includes the core unit  701   i  the same as that of the optical add/drop multiplexer  700   g  (see  FIG. 52 ). However, the optical add/drop multiplexer  700   i  has a difference in that the 1×6 optical coupler  745  as the expansion element  741  for routes is connected from the expansion element  713  for dropping, five ports in the input side of the 7×1-port wavelength selective switch (WSS)  712   c  for adding are connected from the add unit  703   a , and one port thereof is connected with the 6×1-port wavelength selective switch (WSS)  753  as the expansion element  751  with signals input from routes. 
         [0312]    The wavelength cross-connect for eight routes is configured in the above manner. Using the optical coupler  745  for the expansion element  741  may cause unnecessary signal light to be input depending on a route. Therefore, the 7×1-port wavelength selective switch (WSS)  712   c  for adding in the core unit  701   i  is controlled so as to cut off the unnecessary signal light. Furthermore, if necessary, the optical amplifier  743  may be provided in the upstream or the downstream of the 1×6 optical coupler (CPL)  745  as the expansion element  741  so as to compensate for optical loss due to the expansion element  713  in the core unit  701   i.    
         [0313]    If the optical couplers ( 744 ,  745 ,  752 ) are used for the expansion elements ( 741 ,  751 ), a plurality of signal lights having the same wavelength are multiplexed, which may cause signal degradation due to optical interference to occur therein. Therefore, as shown in  FIG. 53  or  FIG. 54 , the wavelength selective switch (WSS) is arranged in either one of the expansion elements ( 741 ,  751 ). 
         [0314]    An optical coupler, a matrix switch, or a grouping filter, instead of the wavelength selective switch, may be used for the drop unit  702   a  in each of the optical add/drop multiplexer of  FIG. 47  to  FIG. 53 . Furthermore, a wavelength selective switch, a matrix switch, or a grouping filter, instead of the optical coupler, may be used for the add unit  703   a  therein. 
         [0315]      FIG. 55  to  FIG. 56  are schematics for explaining expansion of the ports for the routes based on ROADM. In all of the optical add/drop multiplexers of  FIG. 46A  to  FIG. 54 , the functions are based on add and drop of an arbitrary wavelength as the DOADM. Optical add/drop multiplexers  700   j  and  700   k  as shown in  FIG. 55  and  FIG. 56  are formed as the ROADM, and add and drop a signal light having a fixed wavelength. In this case, a fixed wavelength device such as the AWG is used as an optical demultiplexer for adding or dropping to allow signals of all wavelengths to be added or dropped by a single device. 
         [0316]    Therefore, in the configuration based on the ROADM, more ports out of ports of a 4×1-port wavelength selective switch (WSS)  712   j  for adding in a core unit  701   j  can be assigned for routes.  FIG. 55  and  FIG. 56  depict the necessary minimum number of ports to realize the functions. In actual cases, the optical add/drop multiplexers ( 700   j ,  700   k ) employ a 9×1-port wavelength selective switch (WSS) for adding. Therefore, the core unit  701   j  of  FIG. 55  and a core unit  701   k  of  FIG. 56  are configured with the same core unit. 
         [0317]    The optical add/drop multiplexer  700   j  of  FIG. 55  includes the core unit  701   j , a drop unit  702   j , and an add unit  703   j . The core unit  701   j  includes the 1×2 optical coupler  710   a , the 1×2 optical coupler  710   b  as the expansion element  713  that is connected to one port of the 1×2 optical coupler  710   a  and is used for connection for dropping, and a 4×1-port wavelength selective switch (WSS)  712   j  for adding connected to the other port of the 1×2 optical coupler  710   a . A drop unit  702   j  including an optical demultiplexer  722  is connected to one port of the 1×2 optical coupler  710   b  as the expansion element  713 , and the 1×2 optical coupler  744  as the expansion element  741  for routes is connected to the other port thereof. One port in the input side of the 4×1-port wavelength selective switch (WSS)  712   j  for adding is connected from the add unit  703   j  including an optical multiplexer  733 , and two ports thereof are connected from other routes. 
         [0318]    The wavelength cross-connect for four routes is configured in the above manner. The expansion element  713  in the core unit  701   j  separates the signal connected to the drop unit  702   j  from the signal connected to routes. Furthermore, the routes are limited to four to allow the functions to be realized with simple configuration so that the signal light for the routes is less attenuated. 
         [0319]    The optical add/drop multiplexer  700   k  of  FIG. 56  includes a core unit  701   k  the same as that of the optical add/drop multiplexer  700   j  (see  FIG. 55 ). However, the optical add/drop multiplexer  700   k  has a difference in that the 1×6 optical coupler (CPL)  745  as the expansion element  741  for routes is connected to the core unit  701   k , one port in the input side of the 8×1-port wavelength selective switch (WSS)  712   a  is connected from the add unit  703   j , and six ports thereof are connected from other routes. 
         [0320]    The wavelength cross-connect for eight routes is configured in the above manner. Using the optical coupler  745  for the expansion element  741  may cause unnecessary signal light to be input depending on a route. Therefore, the 8×1-port wavelength selective switch (WSS)  712   a  for adding in the core unit  701   k  is controlled so as to cut off the unnecessary signal light. Furthermore, the optical amplifier  743  may be provided to compensate for optical loss due to the 1×6 optical coupler (CPL)  745  provided in the upstream or the downstream of the expansion element  741 . 
         [0321]    As explained above, the optical add/drop multiplexers  700   j  and  700   k  of  FIG. 55  and  FIG. 56  need only one port each for connection to the add unit and the drop unit, unlike the configuration based on the DOADM, which makes it possible to realize the function at low cost because there is no need to provide the wavelength selective switch (WSS) for the expansion element  741  for routes. Furthermore, as compared with the core units ( 701   d ,  701   e ,  701   f ) of the optical add/drop multiplexers  700   d ,  700   e , and  700   f , each of the core units ( 701   d ,  701   e ,  0701   f ) has a difference in that the 1×5 port wavelength selective switch (WSS)  711   d  is added to one port in the output side of the 1×2 optical coupler  710   b  as the expansion element  713  for dropping (see  FIG. 49  to  FIG. 51 ). Therefore, referring to a main signal passing from #1 in to #2 out, a signal input from another route, or a signal output to another route, it is possible to perform the function expansion (in-service upgrade) from the optical add/drop multiplexers  700   j  and  700   k  to the optical add/drop multiplexers  700   d ,  700   e , and  700   f  without disconnecting the signals. 
         [0322]    In the expansion examples of each port for WXC routes of the optical add/drop multiplexers as explained with reference to  FIG. 47  to  FIG. 56 , the number of routes can be fixed and ensured, and the port for WXC route can be expanded without disconnecting the through path passing from the input port to the output port of the optical add/drop multiplexer. 
         [0323]      FIG. 57  is a schematic for explaining expansion of the ports for the routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit. The core unit of each optical add/drop multiplexer is shown in  FIG. 57 . The core unit of each optical add/drop multiplexer shown in  FIG. 57  is obtained by adding a 1×2 coupler C 2  as an expansion element  713 . The optical coupler C 2  is added between the output of the 1×2 optical coupler C 1  and each of the unit D 1  for dropping and the unit D 2  to other routes. The number of port of AWG for dropping or adding in the core unit of each of the optical add/drop multiplexer shown  FIG. 57  is indicated by the necessary minimum number to realize the function. 
         [0324]    In the drop unit D 1 , the AWG AWG 1  for dropping is connected to one of the outputs of the expansion element C 2 . Each port in the output side of the AWG AWG 1  for dropping is connected to the receiver for dropping. The 1×N WSS WSS 1  is connected to one of the outputs of the expansion element C 2 . Each port of the 1×N WSS WSS 1  is connected to other routes to form the wavelength cross-connect. Here, these ports to realize the wavelength cross-connect carry out the function for dropping. 
         [0325]    As explained above, the port for connection to the drop unit D 1  is separated from the port for connection to other routes. With the separation, the increase or decrease in the number of wavelengths to be added or dropped is performed independently from the increase or decrease in the number of routes for the wavelength cross-connect. Furthermore, to use the AWG for dropping and adding allow simplification and cost reduction of the node configuration. 
         [0326]      FIG. 58  is a schematic for illustrating expansion of the ports for the routes of the optical add/drop multiplexer when the 1×2 optical coupler is added to the core unit. The core unit of each optical add/drop multiplexer is shown in  FIG. 58 . The core unit of each optical add/drop multiplexer shown in  FIG. 58  is obtained by adding a 1×2 coupler C 2  as an expansion element  713 . The optical coupler C 2  is added between the output of the 1×2 optical coupler C 1  and each of the drop unit D 1  and D 2 . The number of port of the wavelength selective switch (WSS) for dropping or adding in the core unit of each of the optical add/drop multiplexer shown  FIG. 2  is indicated by the necessary minimum number to realize the function. In actual case, the optical add/drop multiplexer employs a 1×N-port WSS for dropping and an M×1-port WSS for adding. 
         [0327]    In the drop unit D 1  shown in  FIG. 58 , a 1×N port WSS WSS 1  for dropping is connected to one of the outputs of the expansion element C 2 . Each port in the output side of the 1×N WSS WSS 1  for dropping is connected to the receiver. The 1×N WSS WSS 2  is connected to one of the outputs of the expansion element C 2 . Some ports of the 1×N WSS WSS 2  are connected to the receiver for dropping, and other ports are connected to other routes to form the wavelength cross-connect. Here, these ports to realize the wavelength cross-connect carry out the function for dropping. 
         [0328]    As explained above, the port for connection to the drop unit D 1  is separated from the port for connection to other routes. In the case of the DOADM function, the number of required drop signals up to N can prepare only the 1×N WSS WSS 1 . When the number of required drop signals is over N, it is possible to realize the configuration by arranging the empty port of 1×N WSS WSS 2  to other routes. It is possible to realize the dropping and adding configuration corresponding to the required the number of wavelengths (ports) by a minimum composition. Furthermore, it is possible to realize the configuration to routes other network by a minimum composition in proportion to the number of demands. 
         [0329]    As explained above, according to the optical add/drop multiplexers, the device is configured with minimum components upon initial introduction when a small number of wavelengths are to be dropped and added. Thereafter, when the multiple wavelengths are to be dropped and added and the number of routes is increased, a configuration corresponding to each case is added to allow the function expansion. In this case, there is no need to replace the add unit with another one through which a transmission signal passes. This allows the in-service upgrade such that the function is expanded without disconnecting a transmission signal. 
         [0330]    According to the present invention, it is possible to expand the optical add/drop function corresponding to the change in network requirements. 
         [0331]    Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.