Abstract:
To optimize the number of optical gate switches required by an optical switching apparatus which comprises an optical space switch and wavelength selectors, the following process as a preliminary process of introducing optical signals into the optical space switch, is carried out. In detail, the preliminary process is optically converting the optical signals in relation to the number of input ports of the optical space switch, by optically processing the optical signals. As the results, the number of processed optical signals responding to the input ports and the processed number of wavelengths of each processed optical signal is equal to each other, and thereby, the optimization is achieved.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an optical switching apparatus applied wavelength division multiplexing techniques thereto. 
     The optical switching apparatus switches optical signals without converting them into electrical signals. Recently, such optical switching apparatus might be expected to realize switching with a large amount of capacity which can not be realized by an electrical switching apparatus. In the art of optical switch, there are the following types: an optical space switching (or an optical space-division switching), an optical wavelength switching (or an optical wavelength-division switching), an optical time switching (or an optical time-division switching), and combinations thereof, such as an optical wavelength/space switching (an optical wavelength-division/space-division switching) and an optical wavelength/time switching (an optical wavelength-division/time-division switching). 
     In the optical space switching, channels are assigned only to spaces. FIG. 1 shows an example of an optical space switching apparatus. In the example zeroth through third input channels are assigned to zeroth through third input ports  10 - 0  through  10 - 3 , respectively. On the other hand, zeroth through third output channels are assigned to zeroth through third output ports  11 - 0  through  11 - 3 , respectively. Such optical space switching apparatus  1  can connect between any input channel and any output channel. 
     The optical space switching apparatus which the present invention relates, comprises an existing optical space switch of splitter/combiner type where semiconductor laser amplifiers are used as optical gate switches. For example, such switch is disclosed in Yoshiharu Maeno et al “The Possibility of Optical Switching Technology for Parallel Processing Systems”, IEICE, SB-9-5, 1996. 
     FIG. 2 illustrates an optical switch of splitter/combiner type known to the inventors. The illustrated optical space switch comprises zeroth through third input waveguides  20 - 0  through  20 - 3 , zeroth through third beam splitters  21 - 0  through  21 - 3 , zeroth through fifteenth optical gate switches  22 - 0  through  22 - 15 , zeroth through third beam combiners  23 - 0  through  23 - 3 , and zeroth through third output waveguides  24 - 0  through  24 - 3 . 
     One kind of the existing optical gate switches is a semiconductor laser amplifier, which is turned Into a light-transmitting or an on state and a light absorbing or an off state when an electric current is fed thereto and is not fed thereto, respectively. For example, when the zeroth optical gate switch  22 - 0  turned into the on state, the zeroth input waveguide  20 - 0  is connected to the zeroth output waveguide  24 - 0 . 
     The splitter/combiner type optical switch is strictly nonblocking and serves as a so-called crossbar switch where every pairs of input and output ports have dedicated connection paths. And accordingly, the optical space switching apparatus comprising the above switch also serves as a crossbar network. On the other hand, the optical switch of splitter/combiner type requires optical gate switches, (the number of ports) 2  in number, and therefore, has a fault that it is difficult to be implemented, as the number of ports becomes large. 
     FIG. 3 shows another optical space switching apparatus known to the inventors. The apparatus is applied a wavelength division multiplexing (WDM) technology thereto, and achieves to reduce the number of the optical gate switches as compared with the apparatus illustrated in FIG.  2 . In this apparatus, zeroth through fifteenth input optical signal each having any one of zeroth through third wavelengths λ 0  through λ 3  are supplied from zeroth through fifteenth input ports  10 - 0  through  10 - 15  and combined by zeroth through third beam combiners  31 - 0  through  31 - 3 . 
     In detail, when the zeroth through third input optical signals having zeroth through third wavelengths λ 0  through λ 3  are input to the zeroth beam combiner  31 - 0  from the zeroth through third input ports, the zeroth beam combiner  31 - 0  combines the zeroth through third input optical signals to produce a zeroth WDM optical signal. Likewise, the first beam combiner  31 - 1  combines the fourth through seventh input optical signals having zeroth through third wavelengths λ 0  through λ 3  to produce a first WDM optical signal. The second beam combiner  31 - 2  are input the eighth through eleventh input optical signals having zeroth thereto from third wavelengths λ 0  through λ 3  from the eighth through eleventh input ports, and then, combines the eighth through eleventh input optical signals to produce a second WDM optical signal. The third beam combiner  31 - 3  combines the twelfth through fifteenth input optical signals having zeroth through third wavelengths λ 0  through λ 3  to produce a third WDM optical signal. 
     The optical space switch  32  illustrated in FIG. 3 is of a 4×16 crossbar switch adapted to perform 1-to-4 multicasting at maximum. The illustrated switch  32  has zeroth through third input ports i 0  through i 3  to which the zeroth through third WDM optical signals are supplied and zeroth through fifteenth output ports o 0  through o 15  from which zeroth through fifteenth switched WDM optical signals are outputted. The zeroth through fifteenth output ports of the optical space switch  32  are connected to zeroth through fifteenth wavelength selectors  33 A- 0  through  33 A- 15 , respectively. The zeroth through fifteenth wavelength selectors  33 A- 0  through  33 A- 15  select the optical signal of the desired wavelengths from the zeroth through fifteenth switched WDM optical signals outputted from the optical space switch  32  and produce zeroth through fifteenth selected optical signals. The zeroth through fifteenth wavelength selectors  33 A- 0  through  33 A- 15  are connected to zeroth through fifteenth output ports  11 - 0  through  11 - 15 , respectively. The zeroth through fifteenth output ports  11 - 0  through  11 - 15  transmit the zeroth through fifteenth selected optical signals as zeroth through fifteenth output optical signals, respectively. 
     Thus, the optical space switching apparatus has a function of a 16×16 crossbar network. In the apparatus, the optical space switch  32  may be of splitter/combiner type described above, and may include sixty-four optical gate switches. 
     On the other hand, each of the existing wavelength selectors  33 A (suffixes omitted) comprises optical gate switches, the number of which is equal to the number of wavelengths transmitted into each selector. In the example described above with FIG. 3, the number of wavelengths multiplexed into the switched WDM optical signal is equal to four, and therefore, the number of optical gate switches is also equal to four. Specifically, in each selector, a wavelength demultiplexer demultiplexes switched WDM optical signal into individual optical signals with different wavelengths and transmits the individual optical signals into the optical gate switches, respectively. And then, one of the gate switches corresponding to desired wavelength turns on while the others turn off so that only the optical signal with desired wavelength is outputted from the selector. 
     As understood from the above, the optical switching apparatus of space division type illustrated in FIG. 3 has 128 optical gate switches In total. On the other hand, another 16×16 apparatus consisting of a splitter/combiner type optical switch requires 256 optical gate switches. Thus, the number of optical gate switches which comprise the apparatus illustrated in FIG. 3 is reduced to 1/2 as compared with another apparatus consisting splitter/combiner type optical switch. 
     As against the above optical space switching, optical wavelength/space switching assigns channels to both of wavelengths and spaces. FIG. 4 shows another example of an optical wavelength/space switching apparatus. In the example, zeroth through third input channels are assigned to zeroth and first input ports  10 - 0  and  10 - 1  and zeroth and first wavelengths λ 0  and λ 1  of optical signals transmitting on each of input ports. On the other hand, zeroth through third output channels are assigned to zeroth and first output ports  11 - 0  and  11 - 1  and zeroth and first wavelengths λ 0  and λ 1  of optical signals transmitting on each of output ports. Such optical wavelength/space switching apparatus  2  can connect between any input channel and any output channel. As a related technique, an optical wavelength/space switching apparatus having small sized-hardware is disclosed in Japanese Unexamined Patent Publication No. Hei 3-219793, namely, JP-A 3-219793 and is incorporated herein by reference. 
     One type of such apparatuses known to the inventors is modified the optical space switching apparatus illustrated in FIG.  3 . The optical wavelength/space switching apparatus known to the inventors has no beam combiners  31 - 0  through  31 - 3  as preliminarily processing of the optical space switch  32  and directly are input WDM optical signals to the input ports of the optical space switch through the input ports. Besides, the optical wavelength/space switching apparatus has, as latter stage of the wavelength selectors, wavelength converters corresponding to the wavelength selectors  33 A- 0  through  33 A- 15  and beam combiners. 
     Furthermore, related techniques are disclosed in Japanese Unexamined Patent Publications Nos. Hei 7-59127, Hei 1-109991, Hei 3-100526, and Hei 2-27892, namely, JP-A 7-59127, JP-A 1-109991, JP-A 3-100526, and JP-A 2-27892, respectively. These related techniques are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention provides optical switching apparatuses improved in various aspects, such as the size of hardware and the performance thereof. 
     Research has been directed to relationship between the number of wavelength multiplexed into each WDM optical signal transmitted to the input port of optical space switch and the size of hardware, in particular, the number of optical gate switches. As a result of research, the inventors have found out one thing that the number of optical gate switches required by an optical switching apparatus is optimized if the number of WDM optical signals responding to the input ports of the optical space switch and the number of wavelengths of each WDM optical signal is equal to each other in particular optical switching apparatuses, such as illustrated in FIGS. 2 and 3. This is common to an optical switching apparatus of space division type and that of wavelength-division/space-division type. 
     However, the numbers of WDM optical signals and wavelengths depend on an environment to which the optical switching apparatus is adapted. And furthermore, it is getting things backwards to modify the environment which has already been defined into another environment which corresponds to the numbers of WDM optical signals and wavelength transmitted into the optical space switch. Thus, the numbers are restricted by the environment. 
     Therefore, the present invention provides the following method of delivering a plurality of WDM optical signals to a plurality of input ports of an optical space switch. The method comprises preliminarily processing the WDM optical signals in relation to the number of input ports of the optical space switch, by optically processing the WDN optical signals so that the number of processed WDM optical signals responding to the input ports and the processed number of wavelengths of each processed WDM optical signal is equal to each other. 
     Herein, the numbers of the Input ports, the processed number of wavelength, the WDM optical signals, the wavelengths multiplexed in each of the WDM optical signals are equal to K, J, N, and M, respectively, where K, J, N, and M are integers not less than two. Furthermore, the above method may comprise using the optical space switch which further has K*M*N optical gate switches and which is connected to M*N wavelength selectors each comprising M*N/K additional optical gate switches, after the preliminarily processing. 
     The above method may comprise preliminarily processing the WDM optical signals in relation to the number of input ports of the optical space switch, by optically processing the WDM optical signals so that M*N is equal to J*K. 
     According to one aspect of the present invention, in case where M is larger than N, the preliminary process comprises, responsive to N WDM optical signals each of which has M multiplexed wavelengths., carrying out wavelength routing for the N WDM optical signals to produce K routed WDM optical signals as the K input WDM optical signals, each of which has J(=M*N/K) multiplexed wavelengths. 
     According to another aspect of the present invention, in case where M is smaller than N, the preliminary process comprises three processes: responsive to N WDM optical signals each of which has M multiplexed wavelengths, grouping N WDM optical signals into K sub-groups which comprises N/K WDM optical signals; carrying out optical wavelength shifting for (N−K) ones of N WDM optical signals to make wavelengths of the (N−K) WDM optical signals different from each other at each K sub-group; and then carrying out wavelength division multiplexing for the (N−K) WDM optical signals subjected to the shifting and K WDM optical signals not subjected to the shifting at each K sub-group to produce, as the K input WDM optical signals, K additional WDM optical signals each of which has J(=M*N/K) multiplexed wavelengths. 
     The present invention further provides an optical switching apparatus which comprises an optical converter, an optical space switch, and wavelength selectors, as the followings. Responsive to N WDM optical signals each of which has M multiplexed wavelengths, the optical converter converts the N WDM optical signals into K input WDM optical signals each of which has J multiplexed wavelengths, wherein all of N, M, K and J are integers not less than two and J*K is equal to M*N. The optical space switch comprises K input ports and M*N output ports. The optical space switch responds to K input WDM optical signals to produce M*N switched WDM optical signals through the M*N output ports. The wavelength selectors is M*N. Responsive to the M*N switched WDM optical signals, respectively, the wavelength selectors selects one of J wavelengths multiplexed into the responding switched WDM optical signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of an example of an optical switching apparatus of space division type; 
     FIG. 2 shows an existing optical space switch of splitter/combiner type; 
     FIG. 3 shows a related optical space switching apparatus known to the inventors; 
     FIG. 4 shows a block diagram of an example of an optical switching apparatus of wavelength-division/space-division type; 
     FIG. 5 illustrates an optical switching apparatus according to a first embodiment of the invention; 
     FIGS. 6A through 6D show transmission characteristics of a wavelength router which may be used in the apparatus illustrated In FIG. 5; 
     FIG. 7 shows another optical space switch adaptable to the apparatus illustrated in FIG. 5; 
     FIG. 8 shows a wavelength selector adaptable to the apparatus of the preferable embodiment; 
     FIG. 9 illustrates an optical switching apparatus according to a second embodiment of the invention; 
     FIG. 10 shows a wavelength shifter adaptable to the apparatus illustrated in FIG. 9; 
     FIG. 11 shows a wavelength selector capable of applying to the apparatus illustrated in FIG. 9; 
     FIG. 12 illustrates another optical switching apparatus according to a third embodiment of the present invention; 
     FIG. 13 shows a wavelength shifter adaptable to the apparatus illustrated in FIG. 12; and 
     FIG. 14 shows another optical switching apparatus according to a fourth embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, several preferred embodiments of this invention will be described in detail with reference to the drawing. 
     Referring to FIG. 5, an optical switching apparatus (or an optical exchange) according to a first embodiment of this invention will be described. The optical switching apparatus illustrated in the figure is of a 16×16 space division type where M=8, N=2, K=4 and J=4. That is, M and N are not equal to J and K, respectively. 
     Zeroth through seventh input optical signals having zeroth through seventh wavelengths λ 0  through λ 7  are supplied from zeroth through seventh input ports  10 - 0  through  10 - 7 , respectively and combined (or multiplexed) by a zeroth beam combiner (or optical multiplexer)  31 - 0  into a WDM optical signal to be supplied to a zeroth input port i 0  of a wavelength router  6 . Likewise, eighth through fifteenth input optical signals having zeroth through seventh wavelengths λ 0  through λ 7  are supplied from eighth through fifteenth input ports  10 - 8  through  10 - 15 , respectively, and combined by a first beam combiner  31 - 1  into a WDM optical signal to be supplied to a first input port i 1  of the wavelength router  6 . 
     The wavelength router  6  may be implemented by an silica arrayed-waveguide grating formed on a silicon substrate. For example, such router is disclosed in Hiroshi Takahashi et al “Wavelength Multiplexer Based on SiO 2 —Ta 2 O 5  Arrayed-Waveguide Grating”, Journal of Lightwave Technology, Vol. 12, No. 6, June 1994. 
     Referring to FIGS. 6A through 6D, transmission characteristics of the wavelength router  6  are illustrated as the relationship between the wavelengths of the optical signals and the transmittance. The optical signals have a wavelength channel spacing of 0.8 nm. The wavelength router  6  has a wavelength channel spacing and a free spectral range (FSR) equal to 0.8 nm and 3.2 nm, respectively. Thus, the FSR exactly corresponds to four times the wavelength channel spacing. In this event, the optical signals having the zeroth and the fourth wavelengths λ 0  and λ 4  are transmitted from the zeroth input port i 0  to a zeroth output o 0 . Likewise, the optical signals having the first and the fifth wavelengths λ 1  and λ 5  are transmitted from the zeroth input port i 0  to a first output port o 1 . On the other hand, from the first input port i 1  to the zeroth output port o 0 , the optical signals having the second and the sixth wavelengths λ 2  and λ 6  are transmitted. From the first input port i 1  to the first output port o 1 , the optical signals having the third and the seventh wavelengths λ 3  and λ 7  are transmitted. 
     The above-mentioned characteristics are easily achieved by differing the interval of input waveguides of the arrayed-waveguide grating from that of the output waveguides. From the above, the relationship between the input and the output ports of the wavelength router  6  and the transmitted wavelengths is tabulated in Table 1. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Output 
               
             
          
           
               
                   
                 o0 
                 o1 
                 o2 
                 O3 
               
               
                   
                   
               
             
          
           
               
                   
                 input 
                 i0 
                 λ0, λ4 
                 λ1, λ5 
                 λ2, λ6 
                 λ3, λ7 
               
               
                   
                   
                 i1 
                 λ2, λ6 
                 λ3, λ7 
                 λ0, λ4 
                 λ1, λ5 
               
               
                   
                   
               
             
          
         
       
     
     From Table 1, it will be understood that, when the WDM optical signal including the zeroth through the seventh wavelengths λ 0  through λ 7  is supplied through each of the zeroth and the first input ports i 0  and i 1 , a WDM optical signal including the zeroth, the second, the fourth, and the sixth wavelengths λ 0 , λ 2 , λ 4 , and λ 6  (hereinafter referred to as a wavelength group A) is produced from each of the zeroth and the second output ports o 0  and o 2 . In addition, another WDM optical signal including the first, the third, the fifth, and the seventh wavelengths λ 1 , λ 3 , λ 5 , and λ 7  (hereinafter referred to as a wavelength group B) is produced from each of the first and the third output ports o 1  and o 3 . 
     The WDM optical signals having the wavelength groups A and B are delivered from the wavelength router  6  to an optical space switch (or an optical space division switch)  32 . The optical space switch  32  of this embodiment is a 4×16 optical crossbar switch adapted to perform 1-to-16 multicasting at maximum, and has zeroth through third input ports i 0  through i 3  and zeroth through fifteenth output ports o 0  through o 15 . The optical space switch  32  directs the WDM optical signal supplied through each input port to a desired output port. Herein, arbitration control is carried out so that those WDM optical signals supplied through different input ports are not outputted from a common output port. 
     The optical space switch  32  according to the embodiment is of splitter/combiner type illustrated in FIG.  7  and comprises zeroth through third input waveguides  20 - 0  through  20 - 3 , zeroth through third beam splitters  21 - 0  through  21 - 3 , zeroth through fifteenth beam combiners  23 - 0  through  23 - 15 , and zeroth through fifteenth output waveguides  24 - 0  through  24 - 15 , zeroth through sixty-third optical gate switches or semiconductor laser amplifiers  22 - 0  through  22 - 63 . In this embodiment, the input waveguides, beam splitters, beam combiners, and output waveguides are all formed on a common silicon substrate while the optical gate switches are mounted on the substrate. Each of the optical gate switches  22  (suffixes omitted) is turned into a light-transmitting or an on state and a light-absorbing or an off state when an electric current is supplied and is not supplied, respectively. By turning on and off the optical gate switches  22  (suffixes omitted), a crossbar switch function is achieved. 
     Zeroth through fifteenth switched WDM optical signals produced from the optical space switch  32  are supplied to zeroth through fifteenth wavelength selectors  33 - 0  through  33 - 15 , respectively, as illustrated in FIG.  5 . 
     Referring to FIG. 8, each of the wavelength selectors  33  (suffixes omitted) comprises an input waveguide  40 , a wavelength demultiplexer  41  of an arrayed-waveguide grating type, a beam combiner  43 , an output waveguide  44 , and zeroth through third optical gate switches  42 - 0  through  42 - 3   3  as semiconductor laser amplifiers. In this embodiment, the input waveguide, wavelength demultiplexer, beam combiner, and output waveguide are all formed on a common silicon substrate while the optical gate switches are mounted on the substrate. Each wavelength selector  33  serves as a 4×1 wavelength selector for selecting and outputting a desired one of four wavelengths. 
     The switched WDM optical signal having the zeroth, the second, the fourth, and the sixth wavelengths (or the first, the third, the fifth, and the seventh wavelengths) λ 0 , λ 2 , λ 4 , and λ 6  (or λ 1 , λ 3 , λ 5 , and λ 7 ) is supplied to an input port i 0  of the wavelength demultiplexer  41 . The wavelength demultiplexer  41  demultiplexes the switched WDM optical signal into individual wavelengths to produce an optical signal having the zeroth (or the first) wavelength λ 0  (or λ 1 ) through a zeroth output port o 0 , an optical signal having the second (or the third) wavelength λ 2  (or λ 3 ) through a first output port o 1 , an optical signal having the fourth (or the fifth) wavelength λ 4  (or λ 5 ) through a second output port o 2 , and an optical signal having the sixth (or the seventh) wavelength λ 6  (or λ 7 ) through a third output port o 3 . As described above, the optical space switch  32  carries out arbitration control so that the WDM optical signals having the wavelength groups A and B are not simultaneously supplied to any single wavelength selector  33 . Therefore, one of the wavelength groups A and B is supplied through the input port i 0  and each of the four output ports produces an optical signal of a single wavelength to be delivered to a corresponding one of the optical gate switches  42  (suffixes omitted). One of the optical gate switches  42  (suffixes omitted) which is supplied with a desired wavelength is turned on while the others are turned off. Thus, the optical signal of the desired wavelength is selected from the four wavelengths and passes through the beam combiner  43  to be delivered through the output waveguide  44 . 
     Zeroth through fifteenth selected optical signals produced from the zeroth through the fifteenth wavelength selectors  33 - 0  through  33 - 15  are delivered to zeroth through fifteenth output ports  11 - 0  through  11 - 15 , respectively, as illustrated in FIG.  5 . 
     By collective switching of the WDM optical signals and multicasting by the optical space switch  32  and wavelength selection by the wavelength selectors  33  (suffixes omitted), the optical switching apparatus of space division type has a function of 16×16 crossbar network. 
     For example, consideration will be made about the case where connection between the zeroth input port  10 - 0  and the zeroth output port  11 - 0  and connection between the tenth input port  10 - 10  and the first output port  11 - 1  are simultaneously carried out. In this event, the optical signal having the zeroth wavelength λ 0  and supplied from the zeroth input port  10 - 0  passes through the beam combiner  31 - 0  and then through the wavelength router  6  (from the zeroth input port i 0  to the zeroth output port o 0 ) to be supplied to the zeroth input port i 0  of the optical space switch  32 . On the other hand, the optical signal of the second wavelength λ 2  supplied from the tenth input port  10 - 10  passes through the beam combiner  31 - 1  and then through the wavelength router  6  (from the first input port i 1  to the zeroth output port o 0 ) to be supplied to the zeroth input port i 0  of the optical space switch  32 . The optical space switch  32  multicasts to the zeroth and the first output ports o 0  and o 1  the WDM optical signal having the zeroth and the second wavelengths λ 0  and λ 2  supplied from the zeroth input port i 0 . The zeroth wavelength selector  33 - 0  selects the zeroth wavelength λ 0  for delivery to the zeroth output port  11 - 0 . The first wavelength selector  33 - 1  selects the second wavelength λ 2  for delivery to the first output port  11 - 1 . In the above-mentioned manner, the connection from the zeroth input port  10 - 0  to the zeroth output port  11 - 0  and the connection from the tenth input port  10 - 10  to the first output port  11 - 1  are simultaneously carried out. 
     In the first embodiment described above, the wavelength router  6  serves as an optical converter to reduce the number of multiplexed wavelengths. Specifically, at the input of the wavelength router  6 , the number of multiplexed wavelengths is equal to 8 while the number of multiplexed spaces is equal to 2. Since the WDM optical signals of the wavelength group A produced from the zeroth and the second output ports o 0  and o 2  of the wavelength router  6  are different in wavelength from the WDM optical signals of the wavelength group B produced from the first and the third output ports o 1  and o 3 , the number of wavelengths used in the whole apparatus is equal to 8 in total. However, the number of multiplexed wavelengths on a single route is equal to 4 at maximum and the wavelength selectors  33  (suffixes omitted) similarly operate for both of the wavelength groups. Therefore, the number of multiplexed wavelengths is reduced to 4. 
     Such reduction in number of the multiplexed wavelengths provides the following two advantages. First, the number of the optical gate switches required in the apparatus is reduced. If the wavelength router  6  is not provided, the optical space switch  32  is of a 2×16 type while the wavelength selectors  33  is of a 8×1 type. Therefore, the number of the optical gate switches  22  required in the optical space switch  32  is equal to 32. The number of the optical gate switches  42  required per each wavelength selector  33  is equal to 8. Thus, the optical switching apparatus requires 160 optical gate switches in total. On the other hand, in this embodiment, the optical space switch  32  requires 64 optical gate switches while each wavelength selector  33  requires 4 optical gate switches. Thus, the optical switching apparatus according to this embodiment requires 128 optical gate switches in total. Thus, the number is reduced to 4/5 as compared with the case where the wavelength router  6  is not provided. 
     Second, the number of multiplexed wavelengths of the WDM optical signal supplied to the optical gate switch  22  of the optical space switch  32  is reduced so that the optical gate switch  22  as the semiconductor laser amplifier is hardly saturated. In absence of the wavelength router  6 , eight wavelengths are supplied to the optical gate switch  22  at maximum. On the contrary, in this embodiment, four wavelengths are supplied at maximum. Thus, a light intensity (or an optical signal intensity) for each wavelength can be increased to twice as compared with the case where the wavelength router  6  is not provided. Therefore, a light intensity of the optical signal delivered to the output port  11  (suffixes omitted) is increased to twice also. 
     Referring to FIG. 9, an optical switching apparatus according to a second embodiment of this invention is of a 16*16 space division type where M=2, N=8, K=4, and J=4. That is, M and N are not equal to J and K, respectively. 
     The zeroth through the fifteenth input optical signals are supplied through the zeroth through the fifteenth input ports  10 - 0  through  10 - 15 , respectively. Herein, each of the zeroth, the second, the fourth, the sixth, the eighth, the tenth, the twelfth, and the fourteenth input optical signals has the zeroth wavelength λ 0  while each of the first, the third, the fifth, the seventh, the ninth, the eleventh, the thirteenth, and the fifteenth input optical signals has the first wavelength λ 1 . 
     The zeroth and the first input optical signals are supplied to a zeroth primary-stage beam combiner  31 A- 0 . The second and the third input optical signals are supplied to a first primary-stage beam combiner  31 A- 1 . The fourth and the fifth input optical signals are supplied to a second primary-stage beam combiner  31 A- 2 . The sixth and the seventh input optical signals are supplied to a third primary-stage beam combiner  31 A- 3 . The eighth and the ninth input optical signals are supplied to a fourth primary-stage beam combiner  31 A- 4 . The tenth and the eleventh input optical signals are supplied to a fifth primary-stage beam combiner  31 A- 5 . The twelfth and the thirteenth input optical signals are supplied to a sixth primary-stage beam combiner  31 A- 6 . The fourteenth and the fifteenth input optical signals are supplied to a seventh primary-stage beam combiner  31 A- 7 . Each of the first through the seventh primary-stage beam combiners  31 A- 0  through  31 A- 7  combines two input optical signals supplied thereto to produce a WDM optical signal of two wavelengths. Specifically, the zeroth through the seventh primary-stage beam combiners  31 A- 0  through  31 A- 7  produce zeroth through seventh primary-stage WDM optical signals, respectively. 
     Among the zeroth through the seventh primary-stage WDM optical signals, the first, the third, the fifth, and the seventh primary-stage WDM optical signals are supplied to zeroth through third wavelength shifters  7 - 0  through  7 - 3 , respectively. Each of the zeroth through the third wavelength shifters  7 - 0  through  7 - 3  wavelength-shifts the zeroth and the first wavelengths λ 0  and λ 1  of the primary-stage WDM optical signal supplied thereto into a WDM optical signal of the second and the third wavelengths λ 2  and λ 3 . Specifically, the zeroth wavelength shifter  7 - 0  wavelength-shifts the first primary-stage WDM optical signal to produce a zeroth wavelength-shifted WDM optical signal. The first wavelength shifter  7 - 1  wavelength-shifts the third primary-stage WDM optical signal to produce a first wavelength-shifted WDM optical signal. The second wavelength shifter  7 - 2  wavelength shifts the fifth primary-stage WDM optical signal to produce a second wavelength-shifted WDM optical signal. The third wavelength shifter  7 - 3  wavelength-shifts the seventh primary-stage WDM optical signal to produce a third wavelength-shifted WDM optical signal. 
     For example, each of the zeroth through the third wavelength shifters  7 - 0  through  7 - 3  comprises a wavelength shifter using four-wave-mixing (FWM) in the semiconductor laser amplifier (SLA). Such wavelength shifter is disclosed, for example, in R. Schnabel et al, “Polarization Insensitive Frequency Conversion of a 10-channel OFDM Signal Over 275 GHz in a Semiconductor Laser Amplifier”, Proceeding of 19th European Conference on Optical Communication, Vol. 13, ThP 12.4, September 1993. 
     Referring to FIG. 10, the wavelength shifter  7  (suffixes omitted) according to this embodiment comprises an input optical fiber  50 , zeroth and first pump light sources  51 - 0  and  51 - 1 , first and second beam combiners  52  and  53 , a semiconductor laser amplifier  54 , a wavelength filter  55 , and an output optical fiber  56 . 
     Each of the zeroth and the first pump light sources  51 - 0  and  51 - 1  comprises a semiconductor laser. The zeroth pump light source  51 - 0  produces a zeroth pump light having a zeroth pump wavelength λp 0  while the first pump light source  51 - 1  produces a first pump light having a first pump wavelength λp 1 . The zeroth and the first pump lights are combined by the first beam combiner  52  to produce a combined pump light having the zeroth and the first pump wavelengths λp 0  and λp 1 . The combined pump light is supplied to the second beam combiner  53 . The second beam combiner  53  is also supplied from the input optical fiber  50  with the primary-stage WDM optical signal having the zeroth and the first wavelengths λ 0  and λ 1 . 
     Specifically, the second beam combiner  53  is supplied on one hand with the primary-stage WDM optical signal supplied through the input optical fiber  50  and having the zeroth and the first wavelength λ 0  and λ 1  and on the other hand with the combined pump light having the zeroth and the first pump wavelengths λp 0  and λp 1  produced from the zeroth and the first pump light sources  51 - 0  and  51 - 1  as semiconductor lasers. The second beam combiner  53  combines the primary-stage WDM optical signal and the combined pump light to produce a WDM optical signal, having the zeroth and the first wavelengths λ 0  and λ 1  and the zeroth and the first pump wavelengths λp 0  and λp 1 . 
     The WDM optical signal produced by the second beam combiner  53  is supplied to the semiconductor laser amplifier  54 . Since four-wave-mixing occurs within the semiconductor laser amplifier  54 , a wavelength-shifted WDM optical signal is newly produced which are wavelength-shifted from the zeroth and the first wavelengths λ 0  and λ 1  to the second and the third wavelengths λ 2  and λ 3  shifted by Δλ (where Δλ=|λp 1 −λp 0 |). Specifically, the semiconductor laser amplifier  54  produces the WDM optical signal having the zeroth through the third wavelengths λ 0 , λ 1 , λ 2 , and λ 3  and the zeroth and the first pump wavelengths λp 0  and λp 1 . The WDM optical signal is supplied to the wavelength filter  55 . The wavelength filter  55  transmits the second and the third wavelengths λ 2  and λ 3  of the WDM optical signal among the six wavelengths λ 0 , λ 1 , λ 2 , λ 3 , λp 0 , and λp 1  to produce a wavelength-shifted WDM optical signal which is delivered to the output optical fiber  56 . 
     Turning to FIG. 9, the zeroth through the third wavelength-shifted WDM optical signals are supplied to zeroth through third secondary-stage beam combiners  8 - 0  through  8 - 3 , respectively. The zeroth through the third secondary-stage beam combiners  8 - 0  through  8 - 3  are supplied with the zeroth, the second, the fourth, and the sixth primary-stage WDM optical signals produced by the zeroth, the second, the fourth, and the sixth primary-stage beam combiners  31 - 0 ,  31 - 2 ,  31 - 4 , and  31 - 6  and having the zeroth and the first wavelengths λ 0  and λ 1 . The zeroth secondary-stage beam combiner  8 - 0  combines the zeroth primary-stage WDM optical signal and the zeroth wavelength-shifted WDM optical signal to produce a zeroth secondary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . Likewise, the first secondary-stage beam combiner  8 - 1  combines the second primary-stage WDM optical signal and the first wavelength-shifted WDM optical signal to produce a first secondary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . The second secondary-stage beam combiner  8 - 2  combines the fourth primary-stage WDM optical signal and the second wavelength-shifted WDM optical signal to produce a second secondary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . The third secondary-stage beam combiner  8 - 3  combines the sixth primary-stage WDM optical signal and the third wavelength-shifted WDM optical signal to produce a third secondary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . The zeroth through the third secondary-stage WDM optical signals are supplied to the optical space switch  32 . 
     The optical space switch  32  illustrated in the figure is adapted to perform 1-to-16 multicasting at maximum and has the zeroth through the third input ports i 0  through i 3  and the zeroth through the fifteenth output ports o 0  through o 15 . The optical space switch  32  directs the secondary-stage WDM optical signal supplied through each input port to a desired output port. Herein, arbitration control is carried out so that those WDM optical signals supplied through different input ports are not outputted from any common output port. The optical space switch  32  in this embodiment is similar in structure and operation to that of the first embodiment and will not be described any longer. The optical space switch  32  delivers the zeroth through the fifteenth switched WDM optical signals through the zeroth through the fifteenth output ports, respectively. 
     The zeroth through the fifteenth switched WDM optical signals produced by the optical space switch  32  are supplied to the zeroth through the fifteenth wavelength selectors  33 A- 0  through  33 A- 15 , respectively. 
     Referring to FIG. 11, each of the wavelength selectors  33 A (suffixes omitted) comprises the input waveguide  40 , a wavelength demultiplexer  41 A of an arrayed-waveguide grating type, the beam combiner  43 , the output waveguide  44 , and the first through the third optical gate switches  42 - 0  through  42 - 3  as semiconductor laser amplifiers. In this embodiment, the input waveguide, wavelength demultiplexer, beam combiners, and output waveguide are all formed on a common silicon substrate while the optical gate switches are mounted on the substrate. The wavelength selector  33 A serves as a 4×1 wavelength selector for selecting and outputting a desired one of four wavelengths. 
     A switched WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3  is supplied from the input waveguide  40  to an input port i 0  of the wavelength demultiplexer  41 A. The wavelength demultiplexer  41 A demultiplexes the switched WDM optical signal into individual wavelengths to produce optical signals of the zeroth wavelength λ 0 , the first wavelength λ 1 , the second wavelength λ 2 , and the third wavelength λ 3  through zeroth through third output ports o 0  through o 3 , respectively. One of the optical gate switches  42  (suffixes omitted) which is supplied with the desired wavelength is turned on while the others are turned off. Thus, the optical signal of the desired wavelength is selected from the four wavelengths and sent through the beam combiner  43  to be outputted from the output waveguide  44 . 
     Turning to FIG. 9, the zeroth through the fifteenth wavelength selectors  33 A- 0  through  33 A- 15  select the optical signals of the desired wavelengths from the zeroth through the fifteenth switched WDM optical signals and produce zeroth through fifteenth selected optical signals. 
     The zeroth through the fifteenth selected optical signals produced by the zeroth through the fifteenth wavelength selectors  33 A- 0  through  33 A- 15  are delivered to zeroth through fifteenth output ports  11 - 0  through  11 - 15 , respectively. 
     By collective switching of the WDM optical signals and multicasting by the optical space switch  32  and wavelength selection by the wavelength selectors  33 A (suffixes omitted), the optical switching apparatus of space division type has a function of a 16×16 crossbar network. 
     For example, it is assumed that connection between the zeroth input port  10 - 0  and the zeroth output port  11 - 0  and connection between the second input port  10 - 2  and the first output port  11 - 1  are simultaneously carried out. In this event, the zeroth input optical signal having the zeroth wavelength λ 0  and supplied through the zeroth input port  10 - 0  is sent through the zeroth primary-stage beam combiner  31 A- 0  and the zeroth secondary-stage beam combiner  8 - 0  to be supplied to the zeroth input port i 0  of the optical space switch  32 . On the other hand, the second input optical signal having the zeroth wavelength λ 0  and supplied from the second input port  10 - 2  is sent through the first primary-stage beam combiner  31 A- 1  to the zeroth wavelength shifter  7 - 0  to be wavelength-shifted into the second wavelength λ 2  which is sent through the zeroth secondary-stage beam combiner  8 - 0  to be supplied to the zeroth input port  10  of the optical space switch  32 . The optical space switch  32  multicasts to the zeroth and the first output ports o 0  and o 1  the WDM optical signal having the zeroth and the second wavelengths λ 0  and λ 2  and supplied through the zeroth input port i 0 . The zeroth wavelength selector  33 A- 0  selects the zeroth wavelength λ 0  for delivery to the zeroth output port  11 - 0 . The first wavelength selector  33 A- 1  selects the second wavelength λ 2  for delivery to the first output port  11 - 1 . In the above-mentioned manner, the connection from the zeroth input port  10 - 0  to the zeroth output port  11 - 0  and the connection from the tenth input port  10 - 10  to the first output port  11 - 1  are simultaneously achieved. 
     In the second embodiment, the wavelength shifters  7  (suffixes omitted) and the secondary-stage beam combiners  8  (suffixes omitted) serve as another optical converter to increase the number of multiplexed wavelengths. Specifically, at the output of the primary-stage beam combiners  31  (suffixes omitted), the number of multiplexed wavelengths and the number of multiplexed spaces are equal to two and eight, respectively. On the other hand, at the output of the secondary-stage beam combiners  8  (suffixes omitted), the number of multiplexed wavelengths and the number of multiplexed spaces are equal to four and four, respectively. 
     Because of increase in number of the multiplexed wavelengths, the number of the optical gate switches required in the apparatus is reduced. In absence of the wavelength shifters  7  (suffixes omitted) and the secondary-stage beam combiners  8  (suffixes omitted), the optical space switch  32  is of a 8×16 type while the wavelength selector  33  is of a 2×1 type. Therefore, the optical gate switches  22  required in the optical space switch  32  are equal in number to 128. The number of the optical gate switches  42  required per each wavelength selector  33  is equal to two. Thus, the optical switching apparatus requires 160 optical gate switches in total. On the other hand, in this embodiment, the optical space switch  32  requires 64 optical gate switches while each wavelength selector  33  requires 4 optical gate switches. Thus, the optical switching apparatus according to this embodiment requires 128 optical gate switches in total. Thus, the number is reduced to 4/5 as compared with the case where the wavelength shifters  7  (suffixes omitted) and the secondary-stage beam combiners  8  (suffixes omitted) are not provided. 
     Referring to FIG. 12, an optical switching apparatus according to a third embodiment of this invention is of a 16×16 wavelength-division/space-division type where M=8, N=2, K=4, and J=4. That is, M and N are not equal to J and K, respectively. 
     A zeroth input WDM optical signal supplied through the zeroth input port  10 - 0  and having the zeroth through the seventh wavelengths λ 0  through λ 7  is sent to the zeroth input port i 0  of a wavelength router  6 . Likewise, a first Input WDM optical signal supplied through the first input port  10 - 1  and having the zeroth through the seventh wavelengths λ 0  through λ 7  is sent to the first input port i 1  of the wavelength router  6 . 
     The wavelength router  6  in this embodiment is similar in structure and operation to that of the first embodiment and will not be described any longer. If the zeroth and the first WDM optical signals having the zeroth through the seventh wavelengths λ 0  through λ 7  are supplied to the zeroth and the first input ports i 0  and i 1  of the wavelength router  6 , respectively, each of the zeroth and the second output ports o 0  and o 2  produces the WDM optical signal having the zeroth, the second, the fourth, and the sixth wavelengths λ 0 , λ 2 , λ 4 , and λ 6  (the wavelength group A). On the other hand, each of the first and the third output ports o 1  and o 3  produces the WDM optical signals having the first, the third, the fifth, and the seventh wavelengths λ 1 , λ 3 , λ 5 , and λ 7  (the wavelength group B). 
     The WDM optical signals having the wavelength groups A and B and produced from the wavelength router  6  are supplied to the optical space switch  32 . The optical space switch  32  comprises a 4×16 optical crossbar switch adapted to perform 1-to-16 multicasting at maximum, and has the zeroth through the third input ports i 0  through i 3  and the zeroth through the fifteenth output ports o 0  through o 15 . The optical space switch  32  directs the WDM optical signal supplied through each input port to a desired output port. Herein, arbitration control is carried out so that those WDM optical signals supplied through different input ports are not outputted through any common output port. 
     The optical space switch  32  in this embodiment is similar in structure and operation to that in FIG. 7 described in conjunction with the first embodiment and will not be described any longer. In this embodiment, the zeroth through the fifteenth switched WDM optical signals produced by the optical space switch  32  are also supplied to the zeroth through the fifteenth wavelength selectors  33 - 0  through  33 - 15 , respectively. Each of the zeroth through the fifteenth wavelength selectors  33 - 0  through  33 - 15  selects and outputs a desired one of the four wavelengths of the switched WDM optical signal supplied thereto. The wavelength selectors  33  (suffixes omitted) in this embodiment are similar in structure and operation to those of the first embodiment and will not be described any longer. Specifically, the zeroth through the fifteenth selected optical signals produced by the wavelength selectors  33 - 0  through  33 - 15  are supplied to zeroth through fifteenth wavelength converters  36 - 0  through  36 - 15 , respectively. 
     Each of the wavelength converters  36  (suffixes omitted) has a function of converting the wavelength of the optical signal supplied thereto into a predetermined specific wavelength. 
     Referring to FIG. 13, the possible wavelength converter  36  comprises an Input optical fiber  60 , an optical receiver  61 , and an optical transmitter  62 , and an output optical fiber  63 . An optical signal of a desired wavelength is supplied through the input optical fiber  60  to the optical receiver  61  to be converted into an electrical signal. The electrical signal is converted by the optical transmitter  62  back into the optical signal. In the above-mentioned manner, if the optical transmitter  62  has the zeroth wavelength λ 0  as a transmission wavelength by way of example, wavelength conversion from the desired wavelength into the zeroth wavelength λ 0  is achieved by the possible wavelength converter  36 . 
     The zeroth through the fifteenth wavelength converters  36 - 0  through  36 - 15  convert zeroth through fifteenth selected optical signals into zeroth through fifteenth wavelength converted optical signals, respectively. The zeroth through the seventh wavelength converted optical signals are supplied to the zeroth beam combiner  37 - 0 . The eighth through the fifteenth wavelength converted optical signals are supplied to the first beam combiner  37 - 1 . The zeroth beam combiner  37 - 0  combines the zeroth through the seventh wavelength converted optical signals to produce a zeroth WDM optical signal which is delivered to the zeroth output port  11 - 0 . Likewise, the first beam combiner  37 - 1  combines the eighth through the fifteenth wavelength converted optical signals to produce a first WDM optical signal which is supplied to the first output port  11 - 1 . 
     Thus, collective switching of the WDM optical signals and multicasting by the optical space switch  32 , and wavelength selection by the wavelength selectors  33  (suffixes omitted), the optical switching apparatus of this embodiment has a function of a 16×16 crossbar network. 
     For example, it is assumed that connection between the zeroth wavelength λ 0  of the zeroth input port  10 - 0  and the zeroth wavelength λ 0  of the zeroth output port  11 - 0 , and connection between the second wavelength λ 2  of the first input port  10 - 1  and the first wavelength λ 1  of the zeroth output port  11 - 0  are simultaneously carried out. In this event, the optical signal having the zeroth wavelength λ 0  and supplied through the zeroth input port  10 - 0  passes through the zeroth input port i 0  of the wavelength router  6  to the zeroth output port o 0  to be supplied to the zeroth input port i 0  of the optical space switch  32 . On the other hand, the optical signal having the second wavelength λ 2  and supplied through the first input port  10 - 1  passes through the first input port i 1  of the wavelength router  6  to the zeroth output port o 0  to be supplied to the zeroth input port i 0  of the optical space switch  32 . The optical space switch  32  multicasts to the zeroth and the first output ports o 0  and o 1  the WDM optical signal having the zeroth and the second wavelengths λ 0  and λ 2  and supplied from the zeroth input port i 0 . The zeroth wavelength selector  33 - 0  selects the optical signal having the zeroth wavelength λ 0 . This optical signal is converted by the zeroth wavelength converter  36 - 0  into the zeroth wavelength λ 0  to be delivered through the zeroth beam combiner  37 - 0  to the zeroth output port  11 - 0 . On the other hand, the first wavelength selector  33 - 1  selects the optical signal having the second wavelength λ 2 . This optical signal is converted by the first wavelength converter  36 - 1  into the first wavelength λ 1  to be delivered through the first beam combiner  37 - 0  to the first output port  11 - 0 . In the above-mentioned manner, the connection from the zeroth wavelength λ 0  on the zeroth input port  10 - 0  to the zeroth wavelength λ 0  on the zeroth output port  11 - 0  and the connection from the second wavelength λ 2  on the first input port  10 - 1  to the first wavelength λ 1  on the zeroth output port  11 - 0  are simultaneously carried out. 
     In the third embodiment, the wavelength router  6  serves as another optical converter to reduce the number of multiplexed wavelengths. Specifically, at the input of the wavelength router  6 , the number of multiplexed wavelengths is equal to 8 and the number of multiplexed spaces is equal to 2. On the other hand, at the output, the number of multiplexed wavelengths is equal to 4 and the number of multiplexed spaces is equal to 4. There is a difference in wavelength between the WDM optical signal having the wavelength group A and produced from each of the zeroth and the second output ports o 0  and o 2  of the wavelength router  6  and the WDM optical signal having the wavelength group B and produced from each of the first and the third output ports o 1  and o 3 . Therefore, the number of wavelengths used by the apparatus is equal to 8 in total. However, the number of multiplexed wavelengths on a single route is equal to 4 at maximum and the wavelength selectors  33  (suffixes omitted) similarly operate for both of the wavelength groups. Thus, the number of multiplexed wavelengths Is reduced to 4. 
     Such reduction in number of the multiplexed wavelengths provides the following two advantages. First, the number of the optical gate switches required in the apparatus is reduced. If the wavelength router  6  is not provided, the optical space switch  32  is of a 2×16 type while the wavelength selectors  33  is of a 8×1 type. Therefore, the number of the optical gate switches  22  required in the optical space switch  32  is equal to 32. The number of the optical gate switches  42  required per each wavelength selector  33  is equal to 8. Thus, the optical switching apparatus requires 160 optical gate switches in total. On the other hand, in this embodiment, the optical space switch  32  requires 64 optical gate switches while each wavelength selector  33  requires 4 optical gate switches. Thus, the optical switching apparatus of wavelength-division/space-division type according to this embodiment requires 128 optical gate switches in total. Thus, the number is reduced to 4/5 as compared with the case where the wavelength router  6  is not provided. 
     Second, the number of multiplexed wavelengths of the WDM optical signal supplied to the optical gate switch  22  of the optical space switch  32  is reduced so that the optical gate switch  22  as the semiconductor laser amplifier is hardly saturated. In absence of the wavelength router  6 , eight wavelengths are supplied to the optical gate switch  22  at maximum. On the contrary, in this embodiment, four wavelengths are supplied at maximum. Thus, an intensity of optical signal for each wavelength can be increased to twice as compared with the case where the wavelength router  6  is not provided. Therefore, received power at the optical receiver  61  of the wavelength converter  36  (suffixes omitted) is increased to twice also 
     Referring to FIG. 14, an optical switching apparatus according to a fourth embodiment of this invention is of a 16×16 wavelength-division/space-division type where M=2, N=8, K=4 and J=4. That is, M and N are not equal to J and K, respectively. 
     The zeroth through the seventh input WDM optical signals each of which has the zeroth and the first wavelengths λ 0  and λ 1  are supplied through the zeroth through the seventh input ports  10 - 0  through  10 - 7 , respectively. 
     Among the zeroth through the seventh input WDM optical signals, the first, the third, the fifth, and the seventh input WDM optical signals are supplied to the zeroth through the third wavelength shifters  7 - 0  through  7 - 3 , respectively. Each of the zeroth through the third wavelength shifters  7 - 0  through  7 - 3  wavelength-shifts the zeroth and the first wavelengths λ 0  and λ 1  of the input WDM optical signal supplied thereto into a wavelength-shifted WDM optical signal having the second and the third wavelengths λ 2  and λ 3 . Specifically, the zeroth wavelength shifter  7 - 0  wavelength-shifts the first input WDM optical signal to produce a zeroth wavelength-shifted WDM optical signal. The first wavelength shifter  7 - 1  wavelength-shifts the third input WDM optical signal to produce a first wavelength-shifted WDM optical signal. The second wavelength shifter  7 - 2  wavelength-shifts the fifth input WDM optical signal to produce a second wavelength-shifted WDM optical signal. The third wavelength shifter  7 - 3  wavelength-shifts the seventh input WDM optical signal to produce a third wavelength-shifted WDM optical signal. The wavelength shifters  7  (suffixes omitted) are similar in structure and operation to those of the second embodiment and will not be described any longer. 
     The zeroth through the third wavelength-shifted WDM optical signals are supplied to one input ports of the zeroth through the third primary-stage beam combiners  8 - 0  through  8 - 3 , respectively. On the other hand, the other input ports of the zeroth through the third primary-stage beam combiners  8 - 0  through  8 - 3  are also supplied with the zeroth, the second, the fourth, and the sixth input WDM optical signals each of which has the zeroth and the first wavelengths λ 0  and λ 1 . The zeroth primary-stage beam combiner  8 - 0  combines the zeroth input WDM optical signal and the zeroth wavelength-shifted WDM optical signal to produce a zeroth primary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . Likewise, the first primary-stage beam combiner  8 - 1  combines the second input WDM optical signal and the first wavelength-shifted WDM optical signal to produce a first primary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . The second primary-stage beam combiner  8 - 2  combines the fourth input WDN optical signal and the second wavelength-shifted WDM optical signal to produce a second primary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . The third primary-stage beam combiner  8 - 3  combines the sixth input WDM optical signal and the third wavelength-shifted WDM optical signal to produce a third primary-stage WDM optical signal having the zeroth through the third wavelengths λ 0  through λ 3 . The zeroth through the third primary-stage WDM optical signals are supplied to the optical space switch  32 . 
     The optical space switch  32  illustrated in the figure comprises a 4×16 optical crossbar switch adapted to perform 1-to-16 multicasting at maximum, and has the zeroth through the third input ports i 0  through i 3  and the zeroth through the fifteenth output ports o 0  through o 15 . The optical space switch  32  directs the primary-stage WDM optical signal supplied through each input port to a desired output port. Herein, arbitration control is carried out so that those WDM optical signals supplied through different input ports are not outputted from any common output port. The optical space switch  32  in this embodiment is similar in structure and operation to that of the first embodiment and will not be described any longer. The optical space switch  32  produces the zeroth through the fifteenth switched WDM optical signals from the zeroth through the fifteenth output ports o 0  through o 15 , respectively. 
     The zeroth through the fifteenth switched WDM optical signals produced by the optical space switch  32  are supplied to the zeroth through the fifteenth wavelength selectors  33 A- 0  through  33 A- 15 , respectively. 
     Each of the wavelength selectors  33 A selects and outputs a desired one of the four wavelengths of the switched WDM optical signal supplied thereto. The wavelength selectors  33 A in this embodiment are similar in structure and operation to those In the second embodiment and will not be described any longer. 
     The zeroth through the fifteenth wavelength selectors  33 A- 0  through  33 A- 15  select the optical signals of desired wavelengths from the zeroth through the fifteenth switched WDM optical signals and produce the zeroth through the fifteenth selected optical signals, respectively. The zeroth through the fifteenth selected optical signals produced by the zeroth through the fifteenth wavelength selectors  33 A- 0  through  33 A- 15  are supplied to the zeroth through the fifteenth wavelength converters  36 A- 0  through  36 A- 15 , respectively. 
     Each of the wavelength converters  36 A (suffixes omitted) has a function of converting the wavelength of the input optical signal supplied thereto into a predetermined specific wavelength. The wavelength converters  36 A are similar in structure and operation to the wavelength converters  36  in the third embodiment and will not be described any longer. 
     The zeroth through the fifteenth wavelength converters  36 A- 0  through  36 A- 15  convert the zeroth through the fifteenth selected optical signals into the zeroth through the fifteenth wavelength-converted optical signals. In the example illustrated in the figure, each of the zeroth, the second, the fourth, the sixth, the eighth, the tenth, the twelfth, and the fourteenth wavelength-converted optical signals has the zeroth wavelength λ 0  while each of the first, the third, the fifth, the seventh, the eleventh, the thirteenth, and the fifteenth wavelength-converted optical signals has the first wavelength λ 1 . 
     The zeroth and the first wavelength-converted optical signals are supplied to a zeroth final-stage beam combiner  37 A- 0 . The second and the third wavelength-converted optical signals are supplied to a first final-stage beam combiner  37 A- 1 . The fourth and the fifth wavelength-converted optical signals are supplied to a second final-stage beam combiner  37 A- 2 . The sixth and the seventh wavelength-converted optical signals are supplied to a third final-stage beam combiner  37 A- 3 . The eighth and the ninth wavelength-converted optical signals are supplied to a fourth final-stage beam combiner  37 A- 4 . The tenth and the eleventh wavelength-converted optical signals are supplied to a fifth final-stage beam combiners  37 A- 5 . The twelfth and the thirteenth wavelength-converted optical signals are supplied to a sixth final-stage beam combiner  37 A- 6 . The fourteenth and the fifteenth wavelength-converted optical signals are supplied to a seventh final-stage beam combiner  37 A- 7 . 
     The zeroth final-stage beam combiner  37 A- 0  combines the zeroth and the first wavelength-converted optical signals to produce a zeroth WDM optical signal which is delivered to the zeroth output port  11 - 0 . Likewise, the first final-stage beam combiner  37 A- 1  combines the second and the third wavelength-converted optical signals to produce a first WDM optical signal which is delivered to the first output port  11 - 1 . The second final-stage beam combiner  37 A- 2  combines the fourth and the fifth wavelength-converted optical signals to produce a second WDM optical signal which is delivered to the second output port  11 - 2 . The third final-stage beam combiner  37 A- 3  combines the sixth and the seventh wavelength-converted optical signals to produce a third WDM optical signal which is delivered to the third output port  11 - 3 . The fourth final-stage beam combiner  37 A- 4  combines the eighth and the ninth wavelength-converted optical signals to produce a fourth WDM optical signal which is delivered to the fourth output port  11 - 4 . The fifth final-stage beam combiner  37 A- 5  combines the tenth and the eleventh wavelength-converted optical signals to produce a fifth WDM optical signal which is delivered to the fifth output port  11 - 5 . The sixth final-stage beam combiner  37 A- 6  combines the twelfth and the thirteenth wavelength-converted optical signals to produce a sixth WDM optical signal which is delivered to the sixth output port  11 - 6 . The seventh final-stage beam combiner  37 A- 7  combines the fourteenth and the fifteenth wavelength-converted optical signals to produce a seventh WDM optical signal which is delivered to the seventh output port  11 - 7 . 
     Thus, by collective switching of the WDM optical signals and multicasting by the optical space switch  32 , and wavelength selection by the wavelength selectors  33  (suffixes omitted), the optical switching apparatus of wavelength-division/space-division type has a function of a 16×16 crossbar network. 
     For example, it is assumed that connection between the optical signal having the zeroth wavelength λ 0  on the zeroth input port  10 - 0  and the optical signal having the zeroth wavelength λ 0  on the zeroth output port  11 - 0 , and connection between the optical signal having the zeroth wavelength λ 0  on the first input port  10 - 1  and the optical signal having the first wavelength λ 1  on the zeroth output port  11 - 0  are simultaneously carried out. In this event, the optical signal having the zeroth wavelength λ 0  and supplied from the zeroth input port  10 - 0  passes through the zeroth primary-stage beam combiner  8 - 0  to be supplied to the zeroth input port of the optical space switch  32 . On the other hand, the optical signal having the zeroth wavelength λ 0  and supplied from the first input port  10 - 1  is wavelength-shifted by the zeroth wavelength shifter  7 - 0  into an optical signal having the second wavelength λ 2  which is delivered through the zeroth primary-stage beam combiner  8 - 0  to be supplied to the zeroth input port i 0  of the optical space switch  32 . The optical space switch  32  multicasts to the zeroth and the first output ports o 0  and o 1  the WDM optical signal having the zeroth and the second wavelengths λ 0  and λ 2  and supplied from the zeroth input port i 0 . The zeroth wavelength selector  33 A- 0  selects the optical signal having the zeroth wavelength λ 0 . This optical signal is converted by the zeroth wavelength converter  36 A- 0  into the optical signal having the zeroth wavelength λ 0  which is delivered through the zeroth final-stage beam combiner  37 A- 0  to the zeroth output port  11 - 0 . On the other hand, the first wavelength selector  33 A- 1  selects the optical signal having the second wavelength λ 2 . The optical signal is converted by the first wavelength converter  36 A- 1  into the optical signal having the first wavelength λ 1  which is delivered through the zeroth final-stage beam combiner  37 A- 0  to the zeroth output port  11 - 0 . In the above-mentioned manner, the connection from the optical signal having the zeroth wavelength λ 0  on the zeroth input port  10 - 0  to the optical signal having the zeroth wavelength λ 0  on the zeroth output port  11 - 0 , and the connection from the optical signal having the zeroth wavelength λ 0  on the first input port  10 - 1  to the optical signal having the first wavelength λ 1  on the zeroth output port  11 - 0  are simultaneously achieved. 
     In the fourth embodiment, the wavelength shifters  7  (suffixes omitted) serve as another optical converter to increase the number of multiplexed wavelengths. Specifically, in the input ports  10  (suffixes omitted), the number of multiplexed wavelengths is equal to 2 while the number of multiplexed spaces is equal to 8. On the other hand, at the output of the primary-stage beam combiners  8  (suffixes omitted), the number of multiplexed wavelengths is equal to 4 and the number of multiplexed spaces is equal to 4. 
     The increase in number of multiplexed wavelengths results in reduction of the optical gate switches required in the apparatus. Specifically, in absence of the wavelength shifters  7  (suffixes omitted) and the primary-stage beam combiners  8  (suffixes omitted), the optical space switch  32  is of an 8×16 type while the wavelength selector  33  is of a 2×1 type. Therefore, the optical gate switches  22  required in the optical space switch  32  is equal to 128 while the optical gate switches  42  required per each wavelength selector  33  is equal to 2. Thus, the optical switching apparatus requires 160 optical gate switches in total. On the other hand, in this embodiment, the optical space switch  32  requires 64 optical gate switches while each wavelength selector  33  requires 4 optical gate switches. Therefore, according to the embodiment, the optical switching apparatus of wavelength-division/space-division type requires 128 optical gate switches in total. Thus, the number is reduced to 4/5 as compared with the case where the wavelength shifters  7  (suffixes omitted) and the next-stage beam combiners  8  (suffixes omitted) are not provided. 
     It will be understood that this invention is not restricted to the foregoing embodiments and can be modified in various other manners within the scope set forth in the appended claims. For example, the values M, N, K and J can be selected within the range specified in the claims. 
     In the first and the third embodiments, the wave-length routers comprise optical converters. On the other hand, in the second and fourth embodiments, the wavelength shifters and the secondary-stage beam combiners comprise optical converters. However, the optical converter is not restricted to the above-mentioned structure but may be another structure which has a function adapted to the present invention. 
     In the first through the fourth embodiments, the optical space switch  32  is of splitter/combiner type using the optical gate switches. However, the optical space switch is not restricted to the above-mentioned structure but may be an optical matrix switch made of lithium niobate or the like and having an electro-optic effect or an acousto-optic effect, a mechanical optical switch, or a liquid-crystal optical switch. 
     In the first through the fourth embodiments, each of the wavelength selectors  33  (suffixes omitted) and  33 A (suffixes omitted) comprises the wavelength demultiplexer, the optical gate switch, and the beam combiner. However, the wavelength selector is not restricted to the above-mentioned structure but may comprise a combination of the wavelength demultiplexer and the optical matrix switch, or a wavelength tunable optical filter such as an acousto-optic optical filter, a fiber Fabry-Pérot optical filter, and a angle-tuned interference optical filter. 
     In the first through the fourth embodiments, the optical gate switch in the optical space switch  32  and the wavelength selector  33  or  33 A comprises a semiconductor laser amplifier. However, the optical gate switch is not restricted to the above-mentioned structure but may comprise a electro-absorption optical modulator formed by a semiconductor, an optical gate switch made of lithium niobate having the electro-optic effect or the acousto-optic effect, a mechanical optical switch, or a liquid-crystal optical switch. 
     In the first and the third embodiments, the wavelength router  6  comprises a silica arrayed-waveguide grating-type device formed on the silicon substrate. However, the wavelength router is not restricted to the above-mentioned structure but may be an arrayed-waveguide grating type device formed on a semiconductor substrate, a reflection-type grating device, or a Fabry-Pérot optical filter, a Mach-Zehnder optical filter, an interference optical filter, and a combination thereof. 
     In the second and the fourth embodiments, each of the wavelength shifters  7  (suffixes omitted) utilizes four-wave-mixing in the semiconductor laser amplifier. However, the wavelength shifter is not restricted to the above-mentioned structure but may utilize four-wave-mixing in the optical fiber. 
     In the third and the fourth embodiments, each of the wavelength converters  36  (suffixes omitted) and  36 A (suffixes omitted) comprises a combination of the optical receiver and the optical transmitter. However, the wavelength converter is not restricted to the above-mentioned structure but may utilize four-wave-mixing in a medium such as a semiconductor and silica or may utilize a nonlinear optical effect such as a cross gain modulation. 
     In the third and the fourth embodiments, the optical switching apparatus of wavelength-division/space-division type comprises a combination of the optical space switch, the wavelength selector, the wavelength converter, and the beam combiner. However, the apparatus of wavelength-division/space-division type is not restricted to the above-mentioned structure but may comprise a combination of the wavelength demultiplexer, the optical space switch, the wavelength converter, and the beam combiner. 
     As thus far been described, according to the third and the fourth embodiments of this invention, it is possible to reduce the number of multiplexed wavelengths in the optical switching apparatus of space division type or in the optical switching apparatus of wavelength-division/space-division type. In addition, according to the second and the fourth embodiments of this invention, it is possible to increase the number of multiplexed wavelengths. By such increase or decrease, the number of the multiplexed wavelengths can be optimized. This provides the following practical advantages. First, it is possible to minimize a hardware amount such as the optical gate switch required in the exchange. Second, if the number of the multiplexed wavelengths is reduced, the laser amplifier used as the optical gate switch is hardly saturated.