Abstract:
An optical crossconnect selectively connects at least one input terminal with at least one output terminal. The optical crossconnect includes: an optical switch connecting at least one input port connected to the at least one input terminal with at least one output port connected to the at least one output terminal; at least one supervisory light generating circuit for sending a supervisory light from each of the at least one output port to the optical switch; and at least one supervisory light receiving circuit for receiving the supervisory light output from the at least one input port.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical crossconnect that uses an optical switch to set optical signal paths, an optical network device, and a connection status supervisory method. More particularly, the present invention relates to an optical crossconnect capable of supervising optical signal paths set in the optical crossconnect without affecting the service signal, an optical network device, and a connection status supervisory method. 
     2. Description of the Related Art 
     An optical crossconnect system, which uses an optical switch, can process a large number of optical signals because it can switch signals regardless of their formats. The introduction of Wavelength-Division Multiplexing (WDM) technology allows the optical crossconnect to process and switch an even larger number of optical signals. Because this technology makes signal-path setting easier and protection more efficient, much effort has been made at the research and development of this field. This is described in detail, for example, in “Examining an integrated solution to optical transport networking.”, Wavelength Division Multiplexing: (The first ever European meeting place for WDM Systems, Network, Marketing &amp; Engineering Professionals), pp. 18-23, November 1997, London. 
     Conventionally, several methods for supervising optical paths in an optical crossconnect have been studied. For example, the supervisory signal is superimposed over the service signal. This method is described, for example, in “A study of the method for supervising optical paths using the pilot tone signal in an optical path network” by Hamazumi et. al., pp. 45-52, in Technical Report of IEICE, OCS96-66 or in “Inter-Network interface for photonic transport networks and SDH transport network” by Satoru Okamoto et. al., pp. 850-855, in GLOBECOM&#39;97 paperS24.1, Phoenix Ariz. November 1997. 
     Conventionally, the supervision of the optical paths of an optical crossconnect has been done in one of the methods described below. In the first method, the input signal of the optical crossconnect is modulated, and the power of this signal is supervised at the output of the optical crossconnect. When the magnitude of the signal is abnormal, the corresponding optical path is assumed to be abnormal. In the second method, the power of the input signal of the optical crossconnect is compared with that of the output signal. When the optical power has been attenuated significantly, the corresponding optical path is assumed to be abnormal. 
     However, in the optical crossconnect in accordance with the first method, the input signal is modulated and therefore the optical signal output from the optical crossconnect is also modulated. This modulation affects the quality of the optical signal. 
     In the optical crossconnect in accordance with the second method, the loss of the optical switch can be supervised but the service signal cannot be identified. Therefore, this method is not appropriate for supervising whether or not the optical paths are normal. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to solve the problems associated with the prior art described above. It is an object of the present invention to provide an optical crossconnect, an optical network device, and a connection status supervisory method that can supervise, without affecting the service signal, the optical signal paths that are set in the optical crossconnect. 
     The optical crossconnect according to the present invention is an optical crossconnect selectively connecting at least one input terminal with at least one output terminal, the optical crossconnect comprising an optical switch connecting at least one input port connected to the at least one input terminal with at least one output port connected to the at least one output terminal; at least one supervisory light generating circuit for sending a supervisory light from each of the at least one output port to the optical switch; and at least one supervisory light receiving circuit for receiving the supervisory light output from the at least one input port. 
     An optical network device according to the present invention is an optical network device transferring optical signals among a plurality of optical node devices interconnected via optical transmission lines, wherein each of the plurality of optical node devices comprises the optical crossconnect described above. 
     A connection status supervisory method according to the present invention is a connection status supervisory method for supervising a connection status of an optical crossconnect, the connection status supervisory method comprising the steps of generating a supervisory light to supply it to each of output ports of the optical crossconnect, the supervisory light being superimposed over by a signal that is unique; receiving the supervisory light output from each of input ports of the optical crossconnect; and determining whether or not an optical path is normal by referencing the signal superimposed over the supervisory light received in the step of receiving the supervisory light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a diagram showing the configuration of an optical crossconnect according to a first prior art; 
     FIG. 2 is a diagram showing the supervisory operation of the first prior art; 
     FIG. 3 is a diagram showing an example of waveform of the supervisory signal superimposed over the service signal; 
     FIG. 4 is a diagram showing the configuration of a supervisory signal superimposing circuit, FIG.  4 ( a ) is a diagram showing the configuration in which an optical demodulator is used, and FIG.  4 ( b ) is a diagram showing the configuration in which an input optical signal is converted to an electric signal; 
     FIG. 5 is a diagram showing the configuration of an optical crossconnect according to a second prior art; 
     FIG. 6 is a diagram showing the configuration of an optical crossconnect in a first embodiment of the present invention; 
     FIG. 7 is a diagram showing the supervisory operation of the first embodiment; 
     FIG. 8 is a diagram showing an example of the configuration of a 4×4 optical switch; 
     FIG. 9 is a diagram showing an example of the configuration of a 8×8 optical switch; 
     FIG. 10 is a diagram showing an example of the configuration of a 32×32 optical switch; 
     FIG. 11 is a diagram showing an example of the configuration of optical reflector; 
     FIG. 12 is a diagram showing the configuration of an optical crossconnect in a second embodiment of the present invention; 
     FIG. 13 is a diagram showing another configuration of an optical modulator and other components in the second embodiment; 
     FIG. 14 is a diagram showing the configuration of an optical crossconnect in a third embodiment of the present invention; 
     FIG. 15 is a diagram showing the supervisory operation of the third embodiment; 
     FIG. 16 is a diagram showing the supervisory operation of the third embodiment; 
     FIG. 17 is a diagram showing the configuration of an optical crossconnect in a fourth embodiment of the present invention; 
     FIG. 18 is a diagram showing the configuration of an optical crossconnect in a fifth embodiment of the present invention; 
     FIG. 19 is a diagram showing the supervisory operation of the fifth embodiment; 
     FIG. 20 is a diagram showing the supervisory operation of the fifth embodiment; 
     FIG. 21 is a diagram showing the configuration of an optical crossconnect in a sixth embodiment of the present invention; 
     FIG. 22 is a diagram showing the configuration of an optical crossconnect in a seventh embodiment of the present invention; 
     FIG. 23 is a diagram showing an example of the configuration of an optical network using optical crossconnect; and 
     FIG. 24 is a diagram showing an example of the configuration of an optical network using optical crossconnects. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing an optical crossconnect, an optical network device, and a connection status supervisory method according to the present invention, conventional optical crossconnects will be described to help understand the present invention. 
     FIG. 1 shows an example of the configuration of a conventional optical crossconnect. 
     FIG. 1 shows an example of an optical crossconnect according to a first prior art. The optical crossconnect shown in FIG. 1 comprises optical signal input terminals  701 - 1 - 701 -n, an n×n optical switch  702 , optical signal output terminals  703 - 1 - 703 -n, optical path control means  704 , optical path supervisory means  705 , optical couplers  706 - 1 - 706 -n, photo-diodes  707 - 1 - 707 -n, supervisory signal detecting means  708 - 1 - 708 -n, supervisory signal superimposing circuits  709 - 1 - 709 -n, and supervisory signal generating means  710 - 1  - 710 -n. 
     In FIG. 1, optical signals received by the optical signal input terminals  701 - 1 - 701 -n are sent to the n×n optical switch  702  via the supervisory signal superimposing circuits  709 - 1 - 709 -n respectively. Normally, the n×n optical switch  702  selectively connects n input ports to n output ports in any combination in response to the control signal from the optical path control means  704 , and outputs the optical signals supplied to the input ports to the output ports. N output signals from the n×n optical switch  702  are output to the optical signal output terminals  703 - 1 - 703 -n via the optical couplers  706 - 1 - 706 -n respectively. That is, an optical signal received by the optical signal input terminals  701 - 1 - 701 -n may be output to any desired terminal out of the optical signal output terminals  703 - 1 - 703 -n via one of the paths that are set by the n×n optical switch  702 . 
     Next, optical path supervision performed by this optical crossconnect will be described with reference to FIG.  2 . FIG. 2 shows how the n×n optical switch  702 , shown in FIG. 1, is set up to allow the service signal received at the optical signal input terminal  701 - 1  to be output to the optical signal output terminal  703 - 2 . In FIG. 2, the service signal is intensity-modulated by the supervisory signal f 1  in the supervisory signal superimposing circuits  709 - 1 . FIG. 3 shows the service signal over which the supervisory signal f 1  has been superimposed. 
     FIG. 4 shows an example of the supervisory signal superimposing circuit. The supervisory signal superimposing circuit comprises an optical signal input terminal  801 , an optical modulator  802 , an optical signal output terminal  803 , a supervisory signal input terminal  804 , an optical signal input terminal  811 , opto-electric conversion means  812 , electric-photo conversion means  813 , an optical signal output terminal  814 , and a supervisory signal input terminal  815 . 
     The function shown in FIG.  4 ( a ) uses the optical modulator to amplitude-modulate the received optical signal using the supervisory signal. The function shown in FIG.  4 ( b ) converts the received optical signal to the electric signal and, when the electric signal is reproduced back to the optical signal, converts the service signal, over which the supervisory signal is superimposed, back to the optical signal. As a result, both the function shown in FIG.  4 ( a ) and the function shown in FIG.  4 ( b ) give the service signal over which the supervisory signal is superimposed. The optical waveform is shown in FIG.  3 . 
     Then, the service signal, over which the supervisory signal is superimposed, passes through the n×n optical switch  702 . At the optical coupler  706 - 2 , a part of the service signal branches off into the photo-diode  707 - 2 . The supervisory signal detecting means  708 - 2  extracts the supervisory signal with the frequency of f 1  from the optical current in the photo-diode  707 - 2  and sends the extracted supervisory signal to the optical path supervisory means  705 . The optical path supervisory means  705  checks the frequency and the level of the signal detected as the supervisory signal to identify the path through which the optical signal has actually passed as well as the loss. Then, the optical path supervisory means  705  determines whether or not the optical path that is set by the optical path control means  704  is operating normally. 
     The optical crossconnect according to the first prior art, with the configuration described above, has means for superimposing the supervisory signal over the service signal. However, because the supervisory signal remains superimposed over the service signal output from the optical crossconnect, the quality of the service signal is affected. 
     On the other hand, another configuration, shown in FIG. 5, has been proposed as a supervision method that does not affect the service signal. The optical crossconnect shown in FIG. 5 comprises optical signal input terminals  901 - 1 - 901 -n, an n×n optical switch  902 , optical signal output terminals  903 - 1 - 903 -n, optical path control means  904 , optical path supervisory means  905 , optical couplers  906 - 1 - 906 -n, photo-diodes  907 - 1 - 907 -n, signal detecting means  908 - 1 - 908 -n, optical couplers  909 - 1 - 909 -n, photo-diodes  910 - 1 - 910 -n, and signal detecting means  911 - 1 - 911 -n. 
     The optical crossconnect shown in FIG. 5 has means for branching a part of the service signal at the input and the output of the n×n optical switch  902 . With these means, the optical crossconnect supervises the service signal level and thereby the connection paths of the optical switch. 
     However, the optical crossconnect according to the prior art described above can monitor the loss of the optical switch but cannot identify the service signal. Therefore, this optical crossconnect is not suitable for determining that the optical paths are correct. 
     Next, the configuration and the operation of the optical crossconnect according to the present invention will be described with reference to FIGS. 6-13. 
     First embodiment 
     FIG. 6 shows the configuration of an optical crossconnect in a first embodiment of the present invention. The optical crossconnect shown in FIG. 6 comprises optical signal input terminals  1 - 1 - 1 -n, an n×n optical switch  2 , optical signal output terminals  3 - 1 - 3 -n, optical path control means  4 , optical path supervisory means  5 , optical couplers  6 - 1 - 6 -n, photo-diodes  7 - 1 - 7 -n, supervisory signal detecting means  8 - 1 - 8 -n, optical couplers  9 - 1 - 9 -n, optical reflector  10 - 1 - 10 -n, and supervisory signal generating means  11 - 1 - 11 -n. 
     Optical signals received by the optical signal input terminals  1 - 1 - 1 -n of the optical crossconnect shown in FIG. 6 are sent to the n×n optical switch  2  via the optical couplers  6 - 1 - 6 -n, respectively. Normally, the n×n optical switch  2  selectively connects n input ports with n output ports, in response to the control signal from the optical path control means  4 , to output the optical signals supplied to the input ports. The n output signals from the n×n optical switch  2  are output to the optical signal output terminals  3 - 1 - 3 -n via the optical couplers  9 - 1 - 9 -n, respectively. That is, an optical signal received by one of the optical signal input terminals  1 - 1 - 1 -n may be output to any desired terminal of the optical signal output terminals  3 - 1 - 3 -n via a path that is set in the n×n optical switch  2 . 
     Next, referring to FIG. 7, the operation of the optical crossconnect in this embodiment will be described. FIG. 7 shows the operation that is performed when the optical path is set so that the service signal received by the input terminal  1 - 1  is output to the output terminal  3 - 2  by the n×n optical switch  2  in the embodiment shown in FIG.  6 . 
     In FIG. 7, a part of the service signal is branched at the optical coupler  9 - 2  into the optical reflector  10 - 2 . The optical reflector  10 - 2  changes the reflectivity of the received optical signal according to the signal (signal with frequency of f 2  shown in FIG. 7) received from the supervisory signal generating means  11 - 2 . The optical signal, which will be sent back to the optical coupler  9 - 2 , is intensity-modulated by the frequency of f 2 . The modulated signal is then sent to the photo-diode  7 - 1  via the n×n optical switch  2  and the optical coupler  6 - 1  as the supervisory signal. The supervisory signal detecting means  8 - 1  extracts the supervisory signal with the frequency of f 2  from the optical current of the photo-diode  7 - 1  and sends the extracted supervisory signal to the optical path supervisory means  5 . The optical path supervisory means  5  checks the frequency and the level of the signal detected as the supervisory signal to identify the path through which the optical signal has actually passed as well as the loss. Then, the optical path supervisory means  5  determines whether or not the optical path that is set by the optical path control means  4  is operating properly. 
     The optical crossconnect performs the operation described above to supervise that an optical path that is set in the optical crossconnect is normal. When an abnormal condition is detected, the optical crossconnect informs the operator of the condition or sends the optical signal to some other path. 
     In this embodiment, when the service signal is already optically intensity-modulated, the supervisory signal f 2  should be set to about one thousandth of the service signal frequency or the bit rate to avoid an interference with the service signal. 
     The n×n optical switch  2  used in the embodiment is implemented by generating switching devices, each having optical waveguides, 2-inputs/2-outputs, and so on, on a substrate of LiNbO 3  (lithium niobate) or quartz. 
     FIG. 8 shows an example of a 4×4 optical switch fabricated on a LiNbO 3  substrate. The optical switch shown in FIG. 8 is composed of 2×2 switches connected by optical waveguides. Combining 2×2 switching devices allows an input optical signal received from one of four input ports to be output to any desired port. FIG. 8 also shows an example of switching characteristics. The switching characteristics shown in this figure indicates that applying voltage to the 2×2 switching devices causes the optical matrix switch to perform switching operation. 
     FIG. 9 shows an example of the configuration of an 8×8 optical switch, while FIG. 10 shows an example of the configuration of a 32×32 optical switch. The principle and the operation are the same as those of the optical switch shown in FIG.  8 . 
     FIG. 11 shows an example of the configuration of the optical reflector used in the embodiment. The optical signal sent through the optical fiber reflects on the mirror. The mirror rotation angle θ determines the reflection direction as well as the optical power which will be sent back to the optical fiber. This means that, in this embodiment, rotating the mirror in FIG. 11 according to the supervisory signal frequency allows the optical power, which will be returned to the optical fiber, to be intensity-modulated. 
     Second Embodiment 
     FIG. 12 shows the configuration of an optical crossconnect in a second embodiment of the present invention. The optical crossconnect shown in FIG. 12 comprises optical signal input terminals  101 - 1 - 101 -n, an n×n optical switch  102 , optical signal output terminals  103 - 1 - 103 -n, optical path control means  104 , optical path supervisory means  105 , optical couplers  106 - 1 - 106 -n, photo-diodes  107 - 1 - 107 -n, supervisory signal detecting means  108 - 1 - 108 -n, optical couplers  109 - 1 - 109 -n, optical reflector  110 - 1 - 110 -n, optical modulation means  111 - 1 - 111 -n, and optical modulator driving means  112 - 1 - 112 -n. 
     The optical crossconnect shown in FIG. 12 is similar to the optical crossconnect shown in FIG. 6 except the supervisory signal generating means. The supervisory signal paths in FIG.  12  are the same as those in FIG.  6 . However, the supervisory signal is generated by the optical modulation means  111 - 1 - 111 -n which intensity-modulate the optical signal supplied from the optical couplers  109 - 1 - 109 -n according to the signal from the optical path supervisory means  105 . The supervisory signal detecting means  108 - 1 - 108 -n demodulate the intensity modulated signals and send them to the optical path supervisory means  105 . The optical path supervisory means  105  checks whether the received signal matches the signal that was sent to determine whether the optical path is normal. As described above, a signal, generated by the optical path supervisory means  105  and different from that of any other path that is set, is used in this embodiment as supervisory signal. 
     In this embodiment, the supervisory signal is sent back to the input side of the n×n optical switch  102  using the path via which the optical signal was supplied from the optical coupler  109 - 1 - 109 -n to the optical modulation means  111 - 1 - 111 -n. The supervisory signal may be sent back in some other way. For example, optical circulators  120 - 1 - 120 -n may be used as shown in FIG.  13 . 
     Third embodiment 
     FIG. 14 shows the configuration of an optical crossconnect in a third embodiment of the present invention. The optical crossconnect shown in FIG. 14 comprises optical signal input terminals  201 - 1 - 201 -n, an n×n optical switch  202 , optical signal output terminals  203 - 1 - 203 -n, optical path control means  204 , optical path supervisory means  205 , optical couplers  206 - 1 - 206 -n, photo-diodes  207 - 1 - 207 -n, supervisory signal detecting means  208 - 1 - 208 -n, optical couplers  2091 - 209 -n, variable optical attenuators  210 - 1 - 210 -n, supervisory signal generating means  211 - 1 - 211 -n, a 1:n optical coupler  212 , and a supervisory light source  213 . 
     The optical crossconnect in the first embodiment described above generates the supervisory signal by intensity-modulating a part of the service signal, while the optical crossconnect in this embodiment has one supervisory light source  213  for generating the supervisory signal. More specifically, the light output from the supervisory light source  213  is distributed to the variable optical attenuators  210 - 1 - 210 -n by the 1:n optical coupler  212 . Each variable optical attenuator changes the optical attenuation amount by its own supervisory signal generating means  211 - 1 - 211 -n. As a result, the light from the supervisory light source is intensity-modulated by signals each with one of n different frequencies f 1 -fn. The intensity-modulated supervisory signal passes through the n×n optical switch  202  in the opposite direction of the service signal. It is sent via the optical coupler  206 - 1 - 206 -n and then opto-electric converted by the photo-diode  207 - 1 - 207 -n. The supervisory signal is extracted by the supervisory signal detecting means  208 - 1 - 208 -n and is sent to the optical path supervisory means  205 . The optical path supervisory means  205  checks the frequency and the level of each signal detected as the supervisory signal to identify the path through which the optical signal has actually passed as well as the loss. Then, the optical path supervisory means  205  determines whether or not the optical path that is set by the optical path control means  204  is operating normally. 
     Next, by referring to FIGS. 15 and 16, the supervisory operation in this embodiment will be described. FIG. 15 shows the flow of the service signal and the supervisory signal when the service signal flows from the optical signal input terminal  201 - 1  to the optical signal output terminal  203 - 2 . Because the supervisory signal (f 2 ) passes through the n×n optical switch  202  in the opposite direction of the service signal, the supervisory signal detecting means  208 - 1  can receive the signal with the frequency of f 2  and check the frequency and the level to check if the optical path is normal. On the other hand, it is also possible to supervise some other path through which no service signal passes (for example, the path “ 201 -n→ 203 -n”) and reserve it as a standby path. 
     FIG. 16 shows an example of path switching. As shown in FIG. 15, it is necessary to confirm, in advance, that the path “ 201 -n→ 203 -n” operates properly as a standby path. When an error occurs in the service signal passing through the path “ 201 - 1 → 203 - 2 ”, the signal is switched to the standby path “ 201 -n→ 203 -n”. Supervising the standby path as described above increases the reliability of the network. 
     In this embodiment, the service signal need not be supplied to the n×n optical switch  202  when supervising the optical paths because the supervisory light source is provided as an additional component. Therefore, before the service signal is supplied, the optical path to be used may be checked in advance. 
     Fourth Embodiment 
     FIG. 17 shows the configuration of an optical crossconnect in a fourth embodiment of the present invention. The optical crossconnect shown in FIG. 17 comprises optical signal input terminals  301 - 1 - 301 -n, an n×n optical switch  302 , optical signal output terminals  303 - 1 - 303 -n, optical path control means  304 , optical path supervisory means  305 , optical couplers  306 - 1 - 306 -n, photo-diodes  307 - 1 - 307 -n, supervisory signal detecting means  308 - 1 - 308 -n, optical couplers  3091 - 309 -n, optical modulators  310 - 1 - 310 -n, optical modulator driving means  311 - 1 - 311 -n, a 1:n optical coupler  312 , and a supervisory light source  313 . 
     The optical crossconnect in this embodiment is similar to the optical crossconnect in the third embodiment described above except the supervisory signal generating means. The supervisory signal paths in FIG. 17 are the same as those in the third embodiment. However, the supervisory signal is generated by each of the optical modulators  310 - 1 - 310 -n, which intensity-modulate the light from the supervisory light source  313 , using the signal from the optical path supervisory means  305 . The supervisory signal detecting means re-generates the intensity-modulated signal and sends the re-generated signal back to the optical path supervisory means  305 . The optical path supervisory means  305  checks whether the received signal matches the signal that was sent to determine whether the optical path is normal. Therefore, in this embodiment, the signals, each generated by the optical path supervisory means for each optical path that is set and each different from those of other paths, are used as supervisory signals. 
     In this embodiment, the service signal need not be supplied to the n×n optical switch  302  when supervising the optical paths because the supervisory light source is provided as an additional component as in the third embodiment. Therefore, before the service signal is supplied, the optical path to be used may be checked in advance. 
     Fifth Embodiment 
     FIG. 18 shows the configuration of an optical crossconnect in a fifth embodiment of the present invention. The optical crossconnect shown in FIG. 18 comprises optical signal input terminals  401 - 1 - 401 -n, an n×n optical switch  402 , optical signal output terminals  403 - 1 - 403 -n, optical path control means  404 , optical path supervisory means  405 , optical couplers  406 - 1 - 406 -n, photo-diodes  407 - 1 - 407 -n, supervisory signal detecting means  408 - 1 - 408 -n, optical couplers  409 - 1 - 409 -n, optical gate switches  410 - 1 - 410 -n, gate switch driving means  411 - 1 - 411 -n, a 1:n optical coupler  412 , a supervisory light source  413 , and a supervisory signal generating means  414 . 
     The optical crossconnect shown in FIG. 18 is similar to the optical crossconnect in the third embodiment described above except the supervisory signal generating means. The supervisory signal paths in FIG. 18 are the same as those in the third embodiment described above. However, the supervisory signal is generated by intensity-modulating the light from the supervisory light source  413  using the signal from the supervisory signal generating means  414 . The supervisory signal distributed to the optical gate switches  410 - 1 - 410 -n via the optical coupler  412  passes through one of n gate switches, under control of the optical path supervisory means  405 , for supervising the optical path (FIG. 19, FIG.  20 ). That is, the optical path supervisory means  405  enables only one of the optical gate switches corresponding to the path to be supervised. When the supervisory signal returns from the supervisory signal detecting means, the optical path supervisory means determines that the optical path is normal. By performing this operation for n paths, all paths may be supervised. 
     As in the third embodiment, the service signal need not be supplied to the n×n optical switch  402  when supervising the optical paths because the supervisory light source is provided as an additional component. Therefore, before the service signal is supplied, the optical path to be used may be checked in advance. 
     Sixth Embodiment 
     FIG. 21 shows the configuration of an optical crossconnect in a sixth embodiment of the present invention. The optical crossconnect shown in FIG. 21 comprises optical signal input terminals  501 - 1 - 501 -n, an n×n optical switch  502 , optical signal output terminals  503 - 1 - 503 -n, optical path control means  504 , optical path supervisory means  505 , optical couplers  506 - 1 - 506 -n, photo-diodes  507 - 1 - 507 -n, supervisory signal detecting means  508 - 1 - 508 -n, optical couplers  509 - 1 - 509 -n, supervisory light sources  510 - 1 - 510 -n, and supervisory signal generating means  511 - 1 - 511 -n. 
     The optical crossconnect shown in FIG. 21 is similar to the optical crossconnect in the third embodiment except the supervisory signal generating means. In FIG. 21, the supervisory signal paths are the same as those in the third embodiment. However, in this embodiment, one supervisory light source and one supervisory signal generating means are provided for each output port of the n×n optical switch  502 . The supervisory signals are generated by intensity-modulating the light from the supervisory light sources  510 - 1 - 510 -n using the signal from the supervisory signal generating means  511 - 1 - 511 -n. Because n supervisory signal generating means  511 - 1 - 511 -n generate signals with frequencies f 1 -fn respectively, the supervisory signals are also intensity-modulated by frequencies f 1 -fn respectively. 
     Each of the intensity-modulated supervisory signals passes through one of the optical paths of the n×n optical switch  502  in the opposite direction of the service signal. It is sent via the corresponding optical coupler  506 - 1 - 506 -n and then opto-electric converted by the corresponding photo-diode  507 - 1 - 507 -n. The supervisory signal is extracted by the supervisory signal detecting means  508 - 1 - 508 -n and is sent to the optical path supervisory means  505 . The optical path supervisory means  505  checks the frequency and the level of each signal detected as the supervisory signal to identify the path through which the optical signal has actually passed as well as the loss. Then, the optical path supervisory means  505  determines whether or not the optical path that is set by the optical path control means  504  is operating normally. 
     In this embodiment, the service signal need not be supplied to the n×n optical switch  502  when supervising the optical paths because the supervisory light sources are provided as additional components as in the third embodiment. Therefore, before the service signal is supplied, the optical path to be used may be checked in advance. 
     Seventh Embodiment 
     FIG. 22 shows the configuration of an optical crossconnect in a seventh embodiment of the present invention. The optical crossconnect shown in FIG. 22 comprises optical signal input terminals  601 - 1 - 601 -n, an n×n optical switch  602 , optical signal output terminals  603 - 1 - 603 -n, optical path control means  604 , optical path supervisory means  605 , optical couplers  606 - 1 - 606 -n, photo-diodes  607 - 1 - 607 -n, supervisory signal detecting means  608 - 1 - 608 -n, optical couplers  609 - 1 - 609 -n, supervisory light sources  610 - 1 - 610 -n, and optical source driving means  611 - 1 - 611 -n. 
     The optical crossconnect shown in FIG. 22 is similar to the optical crossconnect in the third embodiment described above except the supervisory signal generating means. In FIG. 22, the supervisory signal paths are the same as those in the third embodiment. However, in this embodiment, one supervisory light source and one supervisory signal generating means are provided for each output port of the n×n optical switch  602 . The supervisory signals are generated by intensity-modulating the light from the supervisory light sources  610 - 1 - 610 -n using the signal from the optical path supervisory means  605 . 
     Each of the intensity-modulated supervisory signals passes through one of the optical paths of the n×n optical switch  602  in the opposite direction of the service signal. It is sent via the corresponding optical coupler  606 - 1 - 606 -n and then opto-electric converted by the corresponding photo-diode  607 - 1 - 607 -n. The supervisory signal is re-generated by the supervisory signal detecting means  608 - 1 - 608 -n and is sent to the optical path supervisory means  605 . The optical path supervisory means  605  checks whether the received signal matches the signal that was sent to determine whether the optical path is normal. As described above, the signals, each generated by the optical path supervisory means for each path that is set and each different from those of other paths, are used in this embodiment as supervisory signals. 
     Again, in this embodiment, the service signal need not be supplied to the n×n optical switch  602  when supervising the optical paths because the supervisory light sources are provided as additional components as in the third embodiment. Therefore, before the service signal is supplied, the optical path to be used may be checked in advance. 
     In the embodiments described above, the optical couplers (for example,  6 - 1 - 6 -n) are provided on the input side of the n×n optical switch (for example,  2 ) to branch the supervisory signal transmitted from the output side into the photo-diodes (for example,  7 - 1 - 7 -n). It should be noted that the branching of the supervisory signal is not limited to this configuration. For example, the optical couplers  6 - 1 - 6 -n may be replaced by optical circulators. The use of optical circulators prevents the supervisory signal from leaking into the input side of the optical crossconnect, further improving the transmission characteristics. 
     Interconnecting a plurality of optical node devices, each with the optical crossconnect in any of the embodiments described above, via optical transmission lines allows a network shown in FIG. 23 to be built. In this optical network, optical signals are transmitted via stations (optical node devices). For example, in the optical crossconnect in the first embodiment, the optical signal input terminals  1 - 1 - 1 -n and the optical signal output terminals  3 - 1 - 3 -n shown in FIG. 6 are each connected to other desired stations (optical node devices). Therefore, setting paths in the n×n optical switch  2  allows any transmission path of an optical signal to be set up. 
     Recently, an optical network using the wavelength-division multiplexing (WDM) technology is being studied. FIG. 24 shows the configuration of an optical network using the WDM technology. In the optical network shown in FIG. 24, optical signals, generated by wavelength-division multiplexing the signals with the wavelength ranging from λ1 to πw, are transmitted between stations (optical node devices). An optical crossconnect in any of the above embodiments may be used as an optical node device of the optical network shown in FIG.  24 . The configuration of such an optical node device is shown in FIG.  24 . An optical de-multiplexer, which de-multiplexes a wavelength-division multiplexed signal from the optical network into components of various wavelengths, is connected to the input of the optical crossconnect of the optical node device. Also, an optical multiplexer, which multiplexes optical signals of various wavelengths, is connected to the output of the optical crossconnect. 
     For example, assume that the optical crossconnect in the first embodiment (FIG. 6) is used as the optical node device. In this case, the optical signals with the wavelength ranging from λ1 to λw, which are not yet wavelength-multiplexed, are connected to the optical signal input terminals  1 - 1 - 1 -n and the optical signal output terminals  3 - 1 - 3 -n. The transmission path of each optical signal is set by the n×n optical switch  2  on a wavelength basis. Wavelength multiplexed signals are transmitted through transmission lines between the stations (optical node devices). Using wavelength-multiplexed signals with some specific wavelengths for protection purposes improves network reliability. 
     The present invention has the advantages described below. The optical crossconnect according to the present invention propagates the supervisory signal from the output side to the input side of the optical switch in the opposite direction of the service signal. This supervisory signal is monitored on the input side. 
     Using the detected supervisory signal, the optical crossconnect checks the path through which the optical signal has actually passed as well as the loss and determines whether or not the optical path that is set by the optical path control means is operating normally. In addition, because no intensity-modulation is performed for the service signal output from the optical crossconnect, there is no concern for quality degradation. 
     While this invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of this invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications, and equivalents as can be included within the spirit and scope of the following claims.