Abstract:
An optical switching system is implemented by providing a transmitting section with a preparatory transmitted optical signal selector and a working transmitted optical signal splitter, which are each implemented by a 1×2 optical space switch, and by providing a receiving section with a receiving optical switch which is implemented by a 2×2 optical space switch, and with a preparatory receiving optical gate which is implemented by a 1×2 optical space switch. This makes it possible to switch between the working system and preparatory system without employing any 4×4 optical space switch, thereby implementing a practical optical switching system without causing such problems as communication interruption during maintenance or impairment of transmission path working efficiency.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/JP99/04682 which has an International filing date of Aug. 30, 1999, which designated the United States of America. 
     1. Technical Field 
     The present invention relates to an optical switching system for carrying out path switching on a network that interconnects a plurality of nodes through a working path and a preparatory path using optical signals. 
     2. Background of the Invention 
     FIG. 1 is a block diagram showing a partially redrawn conventionally studied optical switching system as disclosed under the title of “WDM four fiber ring experiment”, B-10-230, page 739, Proceedings of the 1997 IEICE (the Institute of Electronics, Information and Communication Engineers of Japan) General Conference; and FIG. 2 is a partially redrawn operational diagram illustrating a path switching operation in case of a fault described in this paper. 
     In FIG. 1, a reference numeral  101  designates an acoustooptic filter for isolating any desired wavelength component;  102  designates a 4×4 optical space switch for connecting four optical inputs to four output ports in any desired pattern;  103  designates a recombining element for recombining optical signals;  104  designates an optical amplifier for collectively amplifying wavelength multiplexed optical signals;  105  designates a terminal unit for transmitting and receiving optical signals;  106  designates an east side (denoted as “East side” from now on) working input/output port (denoted as “WRK in/out port” from now on) for interconnecting a working path between nodes;  107  designates a west side (denoted as “West side” from now on) WRK in/out port for interconnecting the working path between nodes;  108  designates an East side preparatory input/output port (denoted as “PRT in/out port” from now on) for interconnecting a preparatory path between the nodes; and  109  designates a west side PRT in/out port for interconnecting the preparatory path between the nodes. 
     In FIG. 2, reference numerals  111 - 114  each designate a node corresponding to the optical switching system as shown in FIG. 1;  115  designates an inside transmission path for transmitting optical signals in both directions used as the working path for interconnecting the nodes. The reference numeral  116  designates an outside path for transmitting optical signals in both directions used as the preparatory path for interconnecting the nodes. Reference numerals  117  each designate a 4×4 optical space switch in the node;  118  each designate a working terminal unit connected to the node; and  119  each designate a preparatory terminal unit. 
     Next, the operation will be described. 
     In FIG. 2, the transmission paths  115  and  116  that interconnect the nodes  111 - 114  are wavelength multiplexed, and are connected to the WRK in/out ports  106  and  107  and PRT in/out ports  108  and  109  of the optical switching system of FIG.  1 . Each acoustooptic filter  101 , receiving a drive signal, isolates a particular wavelength signal to be connected to the optical space switch  102 , and passes the remaining wavelength signals without change. Without the drive signal, all the wavelength optical signals pass through the acoustooptic filters  101 . Thus, each acoustooptic filter  101  is used as a switch for switching whether to remove (denoted as “Drop” from now on) the wavelength from the transmission path or not by the drive signal. 
     The isolated signals output from the acoustooptic filters  101  are connected to appropriate terminal units  105  via the optical space switches  102  to be received. The individual nodes are each connected to four terminal units  105 , so that they are connected to East side and West side WRK in/out ports and PRT in/out ports. 
     The optical signals input to the optical switching system through the terminal units  105  are distributed to the appropriate recombining elements  103  at the output ports by the optical space switches  102 , recombined with the optical signals of other wavelengths passing through the acoustooptic filters  101 , and transmitted to adjacent nodes through the output ports as wavelength multiplex signals. 
     The optical space switches  102  switch, if some fault takes place on the transmission path, the communications on the working system to the recombining elements  103  on the same or reverse direction preparatory path in accordance with the fault pattern, thereby saving the communications on the working system at the cost of the communications on the preparatory system. 
     Thus, the network is constructed by providing the nodes of the transmission paths with the optical switching system with the foregoing functions and by connecting the transmission paths in a ring-like pattern. 
     Next, an example of the switching operation in case of a network fault will be described with reference to FIG.  2 . 
     In a normal operation mode in which no fault takes place, communications can be carried out using the working path and preparatory path as shown in FIG.  2 ( 1 ). For example, the working terminal units  118  connected to the node  111  and to the node  113  are bidirectionally interconnected through the working path  115 , and the terminal units  119  connected to the node  113  and to the node  114  are bidirectionally interconnected through the preparatory paths  116 . 
     If a fault takes place on the working path  115  interconnecting the nodes  111  and  113 , and the preparatory path passing through the same route is in a faultless state, the optical space. switches  117  of the nodes  111  and  113  that carry out transmission and reception are switched to the preparatory paths of the same direction as shown in FIG.  2 ( 2 ), so that the communication path used by the working terminal unit  118  is switched to the preparatory path passing through the same route, thereby detouring the signal flowing through the transmission path  115  to the transmission path  116 , and preventing the transmission path from being disconnected. 
     If a fault takes place simultaneously on both transmission paths  115  and  116  interconnecting the nodes, the optical space switches  117  of the nodes  111  and  113  switch the input/output ports for transmission and reception to the opposite preparatory path side as shown in FIG.  2 ( 3 ), thereby detouring the signals to the transmission path  116  passing through the network in the opposite direction, and preventing the working path from being disconnected. In this case, the communications carried out between the nodes  113  and  114  using the transmission path  116  in the faultless state are disconnected to prevent the disconnection of the working transmission path. 
     FIG. 3 is a block diagram showing another partially redrawn conventionally studied optical switching system as disclosed under the title of “Node configuration on OADM ring system”, B-10-85, page 384, Proceedings of the 1997 Society Conference of the Institute of Electronics, Information and Communication Engineers of Japan. 
     In FIG. 3, reference numerals  121  each designate an optical switching system that receives currently working signals, and switches, in case of a transmission path fault, an optical signal path to an unimpaired transmission path; the reference numeral  122  designates a backup optical switching system used in place of one of the optical switching systems  121  when it is faulty; λ(1)−λ(n+1) each designate an optical signal with a different wavelength; reference numerals  127 - 128  each designate a terminal unit connected to the optical switching system  121  and  122  for transmitting and receiving the optical signals λ(1)−λ(n+1);  123  and  124  each designate a wavelength multiplexer for wavelength multiplexing the optical signals λ(1)−λ(n+1) with different wavelengths transmitted from the terminal units  127 - 128 , and for sending them out to transmitting paths;  125  and  126  each designate a wavelength demultiplexer for demultiplexing the wavelength multiplex signals fed from receiving paths for respective wavelengths, and for inputting them to the optical switching systems corresponding to the optical signals λ(1)−λ(n+1) with different wavelengths;  131  designates intra-office interfaces for inputting signals to be transmitted to the transmission paths;  132  designates intra-office interfaces for outputting signals received from the transmission paths; and  129  and  130  each designate an (n+1):n electric switch for recovering, when a fault takes place in a certain unit associated with a particular wavelength among the terminal units  127 - 128  or optical switching systems  121  provided for each of the n wavelengths, the fault by switching the transmitted and received signals associated with the unit corresponding to that faulty wavelength to the backup unit corresponding to the (n+1) th wavelength. 
     The reference numeral  24  designates a transmitting section installed in each optical switching system  121 ;  23  designates a receiving section installed in each optical switching system  121 ;  11  designates a working transmitted optical signal splitter provided for each transmitting section  24 ; and  22  designates a working received optical signal selector provided for each receiving section  23 . 
     Next, the operation will be described. 
     The conventional example assumes to be applied to a ring network employing the bidirectional transmission paths that enable individual communications for respective wavelengths based on the wavelength multiplexing technique. Communication paths for optical signals of respective wavelengths comprise terminal units  127 - 128  and optical switching systems  121  and  122 , and are connected to the transmission paths through the wavelength multiplexers  123  and  124  and the wavelength demultiplexers  125  and  126 . The n intra-office interfaces  131  or  132  for outputting or inputting the optical signals to be transmitted or received correspond to n+1 wavelengths prepared in advance, and the n working channels have one preparatory channel, constituting a 1:n configuration. 
     If a fault takes place in a terminal unit or in an optical switching system associated with a particular wavelength, the intra-office interface associated with the fault is connected to the preparatory channel through the (n+1):n electric switch  129  or  130 , thereby recovering the fault of the system. 
     If a fault takes place in a transmission path, the fault is recovered by the switching operation of the optical switching systems  121  and  122 . The transmitting section of each of the optical switching systems  121  and  122  associated with respective wavelengths divides the transmitted signal into two with the working transmitted optical signal splitter  11 , and sends them to the two transmitting paths. On the other hand, in each receiving section, the working received optical signal selector  22  selects one of the optical signals fed from the two receiving paths. If a fault takes place on one of the two receiving paths, the working received optical signal selector  22  selects the remaining faultless receiving path to recover the fault of the transmission path. 
     The conventional optical switching systems have the foregoing configurations. The first conventional example as shown in FIGS. 1 and 2 disclosed in the B-10-230 paper employs, when the network is normal, both the working path and preparatory path separately for different communications, whereas in case of a fault it detours the currently working signals to the preparatory path using the 4×4 optical space switches  102  or  117  for the path switching. This presents a problem in that the 4×4 optical space switches  102  or  117  utilizing waveguides have not yet reached a level required for practical applications in the characteristics such as an extinction ratio or loss, or reliability and power consumption. 
     On the other hand, the 4×4 optical space switches  102  or  117  utilizing mechanical optical space switches can meet the foregoing characteristics with the reliability of practical level, and the mechanical switches with a latch function have an advantage of being able to eliminate the power required for holding the present state. These switches, however, have a problem of increasing packaging dimensions because the function of a unit switch is limited to 1×2 or 2×2, which means that  16  unit switches are required to implement the 4×4 optical space switch  102  or  117  by combining the 2×2 unit switches. 
     Furthermore, when a 4×4 optical space switch  102  or  117  in a certain node has to be replaced because of a fault, the conventional example has a problem of interrupting during the recovery job all the communications passing through the terminal units  105 ,  118  or  119  connected to the node because all the transmitted or received signals connected to the terminal units  105 ,  118  or  119  are connected to a single optical space switch  102  or  117 . 
     The conventional example as shown in FIG. 3 disclosed in the, B-10-85 paper divides the output signals from the optical switching systems  121  and  122  to the two transmitting paths so that the two paths transmit the same signals. This presents a problem in that the working efficiency of the transmission paths is limited to ½ because the network always transmits the same signals through the two paths. 
     The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide an optical switching system with high transmission path working efficiency without interrupting all the communications during the recovery job such as component replacement in case of a fault of an optical space switch by decreasing the number of the optical space switches, thereby reducing the packaging size with maintaining the characteristics and reliability of the optical space switches. 
     SUMMARY OF THE INVENTION 
     The optical switching system according to the present invention comprises a receiving section including a receiving optical switch for receiving an optical signal input to a working input port and an optical signal input to a preparatory input port, for outputting as two output signals the two input signals by spatially switching their paths or by passing them through, and for connecting a first output of the two output signals to a working drop port, and a preparatory receiving optical gate for turning on or off a second output signal from the receiving optical switch to be supplied to a preparatory drop port. 
     According to the present invention, a transmitting section comprises a working transmitted optical signal splitter and a preparatory transmitted optical signal selector, and the receiving section comprises the receiving optical switch and the preparatory receiving optical gate. If a fault takes place in the working path while the preparatory path conveys a signal such as an extra traffic signal different from the signal on the working signal in a faultless state, the preparatory receiving optical gate breaks the optical signal to be supplied to the preparatory drop port, thereby preventing the working signal from being erroneously connected to the preparatory drop port when the switching operation is carried out in the fault. This offers an advantage of being able to make full use of the total transmission capacity of both the working path and preparatory path in the faultless state, which increases the working efficiency of the transmission path. 
     Furthermore, because the receiving section can be configured using one 2×2 optical space switch and one optical gate, the packaging dimension can be reduced. Since the number of switches is small, even mechanical optical switches, which have effective characteristics and. functions, are applicable with practical packaging size. 
     Moreover, the transmitting section can be physically divided into optical components through which the working signal passes, and into optical components through which the preparatory signal passes, which presents an advantage of being able to improve the reliability in the system maintenance. 
     In addition, the transmitting section can be configured with one coupler and one 1×2 optical space switch, which offers an advantage of being able to reduce the packaging dimension. Since the number of switches is small, even mechanical optical switches, which have effective characteristics and functions, are applicable with practical packaging size. 
     The optical switching system according to the present invention comprises a receiving section including a preparatory receiving optical switch for receiving an optical signal input to a preparatory input port, for spatially switching a path of the input optical signal to two outputs, and for connecting a first output of the two outputs to a preparatory drop port, and a working received optical signal selector for receiving a second output of the preparatory receiving optical switch and an optical signal input to a working input port, and for selecting one of the two inputs to be supplied to a working drop port. 
     According to the present invention, the receiving section is configured such that it comprises the preparatory receiving optical switch and the working received optical signal selector. If a fault takes place in the working receiving path, the working received optical signal selector carries out switching to select the optical signal from the preparatory receiving path, with breaking the optical signal from the working receiving path. At the same time, the preparatory receiving optical switch stops supplying the optical signal to the preparatory drop port in the fault switching. This enables the communication to make full use of the total transmission capacity of both the working path and the preparatory path in the faultless state, improving the transmission path working efficiency. 
     Furthermore, in a faultless state, the optical components through which the working signal passes can be physically divided from the optical components through which the preparatory signal passes. This makes it possible for the boards mounting the optical switches to be packaged separately for the working path and preparatory path. Thus, even if a fault takes place in an optical switch element, the recovery job can be achieved in some cases by only replacing the board mounting the working path without removing the connection of the preparatory path, for example. This will improve the reliability of the system. 
     Moreover, the switch configuration of the receiving section can be implemented using two 1×2 optical space switches, which offers an advantage of being able to reduce the packaging size. Since the number of the switches is small, even mechanical optical switches, which have effective characteristics and functions, are applicable with practical packaging size. 
     Furthermore, because the transmitting section can be physically divided into optical components through which the working signal passes, and into optical components through which the preparatory signal passes, the reliability in the system maintenance can be improved. 
     In addition, the switch configuration of the transmitting section can be implemented using one coupler and one 1×2 optical space switch, which offers an advantage of being able to reduce the packaging dimension. Since the number of switches is small, even mechanical optical switches, which have effective characteristics and functions, are applicable with practical packaging size. 
     The optical switching system according to the present invention comprises a receiving section including a preparatory receiving optical splitter for dividing into two an optical signal input to a preparatory input, and for outputting them, a working received optical signal selector for receiving a first output of the two outputs of the preparatory receiving optical splitter and an optical signal input to a working input port, and for selecting one of the two inputs to be supplied to a working drop port, and a preparatory receiving optical gate for turning on or off a second output of the preparatory receiving optical splitter to be supplied to a preparatory drop port. 
     According to the present invention, the receiving section is configured such that it comprises the preparatory receiving optical splitter, the working received optical signal selector and the preparatory receiving optical gate. If a fault takes place in the working receiving path, the preparatory receiving optical gate breaks the optical signal sent from the working receiving path to prevent the optical signal from being supplied to the preparatory drop port. This enables the communication to make full use of the total transmission capacity of both the working path and preparatory path in the faultless state, improving the transmission path working efficiency. 
     Furthermore, in the faultless state, the optical components through which the working signal passes can be physically divided from the optical components through which the preparatory signal passes. This makes it possible, even if a fault takes place in an optical switch element or the like, to carry out a recovery job by only replacing the board mounting the working path without removing the connection of the preparatory path, for example. This will improve the reliability of the system. 
     Moreover, the switch configuration of the receiving section can be implemented using one coupler, one 1×2 optical space switches and one optical gate, which offers an advantage of being able to reduce the packaging size. Since the number of the switches is small, even mechanical optical switches, which have effective characteristics and functions, are applicable with practical packaging size. 
     Furthermore, because the transmitting section can be physically divided into optical components through which the working signal passes, and into optical components through which the preparatory signal passes, the reliability in the system maintenance can be improved. 
     In addition, the switch configuration of the transmitting section can be implemented using one coupler and one 1×2 optical space switch, which offers an advantage of being able to reduce the packaging dimension. Since the number of switches is small, even mechanical optical switches, which have effective characteristics and functions, are applicable with practical packaging size. 
     The optical switching system according to the present invention comprises a working add/drop switch with two inputs and two outputs for passing through or for switching optical signals input to a working input port and to a working add port, and for supplying the optical signals to a transmitting section and to a receiving section of the optical switching system; and a preparatory add/drop switch with two inputs and two outputs for passing through or switching optical signals input to a preparatory input port and to a preparatory add port, and for supplying the optical signals to the transmitting section and to the receiving section of the optical switching system. 
     According to the present invention, the optical switching system is configured such that it comprises the working add/drop switch and the preparatory add/drop switch, and these add/drop switches can readily implement the add/drop switching with two 2×2 optical switches. This offers an advantage of being able to construct the optical switching system applicable to a ring network with a small number of unit optical switches, thereby shrinking the packaging dimension. 
     In addition, because it is possible to make full use of the total transmission capacity of both the working path and preparatory path in the faultless state, the transmission path working efficiency can be improved. 
     Moreover, applying the optical switching systems can bidirectionally interconnect any desired nodes in a ring network including a plurality of nodes interconnected. This offers an advantage of being able to improve the flexibility of the network. 
     The optical switching system according to the present invention comprises span switching means provided for each one of sets for switching between a working path and a preparatory path in the same section by switching or by passing through input and output optical signals from and to working input/output ports and from and to preparatory input/output ports; first ring switching means for supplying the span switching means of a first one of the sets with input and output signals associated with a preparatory add port of the first one of the sets, with a working add port of the first one of the sets, with a working drop port of the first one of the sets, and with a preparatory drop port of the first one of the sets by switching or passing through them; and second ring switching means for supplying the span switching means of a second one of the sets with input and output signals associated with a preparatory add port of the second one of the sets, with a working add port of the second one of the sets, with a working drop port of the second one of the sets, and with a preparatory drop port of the second one of the sets by switching or passing through them. 
     According to the present invention, the optical switching system is configured such that it comprises for each of the two sets the span switching means and the first and second ring switching means. The ring switching means can configure anther preparatory path in a reverse direction even if both the working path and preparatory path fall into a fault at the same time in a particular section of the ring network. The span switching means can detour the signals to the preparatory path in the same section when only the working path falls into a fault and the preparatory path is normal in the section. Providing the two switching modes can improve the reliability of the network. 
     Furthermore, because both the ring switching means and span switching means are composed of the optical space switches, the number of the optical switches required is small, offering an advantage of being able to reduce the packaging size. 
     Moreover, because the total transmission capacity of both the working path and preparatory path can be utilized in the faultless state, the working efficiency of the transmission paths can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a configuration of a conventional optical switching system; 
     FIGS.  2 ( 1 )-( 3 ) are operational diagrams illustrating path switching operations in case of a fault in the conventional optical switching system; 
     FIG. 3 is a block diagram showing a configuration of another conventional optical switching system; 
     FIG. 4 is a block diagram showing a configuration of an embodiment 1 of an optical switching system in accordance with the present invention; 
     FIG. 5 is a block diagram showing a configuration of an embodiment 2 of the optical switching system in accordance with the present invention; 
     FIG. 6 is a block diagram showing a configuration of an embodiment 3 of the optical switching system in accordance with the present invention; 
     FIG. 7 is a block diagram showing a configuration of an embodiment 4 of the optical switching system in accordance with the present invention; 
     FIGS.  8 ( 1 )-( 2 ) are diagrams illustrating connection configurations of a ring network formed using the embodiment 4 of the optical switching system; 
     FIG. 9 is a block diagram showing a switching pattern in case of a fault taking place on a working transmitting path of the ring network formed by using the embodiment 4 of the optical switching system; 
     FIG. 10 is a block diagram showing a switching pattern in case of a fault taking place on a working receiving path of the ring network formed by using the embodiment 4 of the optical switching system; 
     FIG. 11 is a block diagram showing a configuration of an embodiment 5 of the optical switching system in accordance with the present invention; and 
     FIG. 12 is a block diagram showing a switching state of a ring on the ring network employing the embodiment 5 of the optical switching system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments of the invention will now be described. 
     EMBODIMENT 1 
     FIG. 4 is a block diagram showing a configuration of an embodiment 1 of an optical switching system in accordance with the present invention. In this figure, the reference numeral  1  designates an input port of a working system (denoted as “WRKin port” from now on) of the present embodiment 1 of the optical switching system;  2  designates an output port of the working system (denoted as “WRKout port”);  3  designates an input port of a preparatory system (denoted as “PRTin port” for now on);  4  designates an output port of the preparatory system (denoted as “PRTout port” from now on);  5  designates an add port of the working system (denoted as “Add(WRK) port” from now on) for inserting in or transmitting to a transmission path a signal through an optical switching system  18 ;  6  designates a drop port of the working system (denoted as “Drop(WRK) port” from now on) for discarding or receiving a signal from the transmission path through the optical switching system  18 ;  7  designates an add port of the preparatory system (denoted as “Add(PRT) port” from now on); and  8  designates a drop port of the preparatory system (denoted as “Drop (PRT) port” from now on). 
     The reference numerals  18  and  26  each designate the optical switching system of the present embodiment 1;  24  designates a transmitting section that transfers in faultless state the input optical signal to the Add(WRK) port  5  to the WRKout port  2 , and the input optical signal to the Add(PRT) port  7  to the PRTout port  4 , and that transfers, if a fault takes place on a transmitting side working path which is supplied with the optical signal from the WRKout port  2 , the input optical signal to the Add(WRK) port  5  to both the WRKout port  2  and PRTout port  4 ;  23  designates a receiving section that supplies in the faultless state the input optical signal to the WRKin port  1  to the Drop (WRK) port  6 , and the input optical signal to the PRTin port  3  to the Drop(PRT) port  8 , and that supplies, if a fault takes place on a receiving side working path transmitting the optical signal to the WRKin port  1 , the input signal to the PRTin port  3  to the Drop(WRK) port  6 . 
     The reference numeral  9  designates a receiving optical switch for spatially switching the paths of the optical signals input through the WRKin port  1  and PRTin port  3 , thereby outputting one of them to the Drop(WRK) port  6 ;  10  designates a preparatory receiving optical gate interposed between the remaining output of the receiving optical switch  9  and the Drop(PRT) port  8  for switching between the optical transmissive state and untransmissive state. The receiving optical switch  9  and preparatory receiving optical gate  10  are installed in the receiving section  23 . 
     The reference numeral  11  designates a transmitted optical signal splitter of the working system for dividing the optical signal input to the Add(WRK) port  5  into two, and for supplying one of them to the WRKout port  2 ; and  12  designates a preparatory transmitted optical signal selector for selecting one of the other output of the working transmitted optical signal splitter  11  and the optical signal input through the Add(PRT) port  7 , and supplies the selected signal to the PRTout port  4 . The working transmitted optical signal splitter  11  and preparatory transmitted optical signal selector  12  are both installed in the transmitting section  24 . 
     The reference numerals  13  and  19  each designate a working terminal unit for carrying out communications through the working paths when the transmission paths are normal;  14  and  20  each designate a preparatory terminal unit for carrying out the communications through the preparatory paths in the similar manner; the reference numeral  15  designates a working receiving path for connecting the WRKin port  1  of the optical switching system  18  with the WRKout port of the optical switching system  26 ;  16  designates a preparatory transmitting path for connecting the PRTout port  4  of the optical switching system  18  with the PRTin port of the optical switching system  26 ;  17  designates a preparatory receiving path for connecting the PRTin port  3  of the optical switching system  18  with the PRTout port of the optical switching system  26 ; and  25  designates a working transmitting path for connecting the WRKout port  2  of the optical switching system  18  with the WRKin port of the optical switching system  26 . 
     Next, the operation will be described. 
     In the present embodiment  1 , two opposite optical switching systems  18  and  26  are interconnected through the working transmitting path  25  and working receiving path  15 , and through the preparatory transmitting path  16  and preparatory receiving path  17 . The optical switching systems  18  and  26  are connected with the working terminal units  13  and  19 , and with the preparatory terminal units  14  and  20 , respectively. 
     In the normal mode, the optical signal output from the working terminal unit  13  is supplied to the optical switching system  18  through the Add(WRK) port  5 , and is transferred to the working terminal unit  19  through the working transmitted optical signal splitter  11 , WRKout port  2 , working transmitting path  25  and optical switching system  26 . In contrast, the working optical signal output from the working terminal unit  19  is supplied to the receiving optical switch  9  of the optical switching system  18  through the optical switching system  26 , working receiving path  15  and WRKin port  1 . The receiving optical switch  9 , which is provided for switching the receiving path by bringing its internal optical transmission paths into a cross state if a fault takes place on the working receiving path  15 , is in a through state in a faultless state so that the signal from the WRKin port  1  is transferred to the working terminal unit  13  through the Drop (WRK) port  6 , thus establishing the bidirectional communication path between the working terminal units  13  and  19 . 
     On the other hand, it is possible in a faultless state to establish communication path between the preparatory terminal units  14  and. 20  using the preparatory transmitting path  16  and preparatory receiving path  17 . The optical signal supplied from the preparatory terminal unit  14  to the optical switching system  18  is transferred to the preparatory terminal unit  20  through the preparatory transmitted optical signal selector  12  which is set in a through state, PRTout port  4 , preparatory transmitting path  16  and optical switching system  26 . On the other hand, the optical signal output from the preparatory terminal unit  20  is supplied to the PRTin port  3  through the optical switching system  26  and preparatory receiving path  17 , and is connected to the preparatory receiving optical gate  10  through the receiving optical switch  9  in the through state, arriving at the preparatory terminal unit  14  through the preparatory receiving optical gate  10  in the transmissive state, thereby establishing the bidirectional communication path between the preparatory terminal units  14  and  20 . Because the preparatory terminal units  14  and  20  can exchange signals different from those between the working terminal units  13  and  19 , the total transmission capacity-of both the working transmitting path  25  and receiving path  15  and of both the preparatory transmitting path  16  and receiving path  17  can be utilized in a faultless state. 
     If some fault takes place on a working transmission path, the communication is saved by switching the optical switching systems  18  and  26  to detour the working path communications to the preparatory path side. 
     First, a case will be considered in which a fault takes place on the working receiving path  15  connected to the WRKin port  1 . 
     On the opposite working terminal unit  19  side, the optical switching system  26  divides the transmitted optical signal into two, and supplies them to both the working receiving path  15  and preparatory receiving path  17 . In this case, the transmitted signal from the preparatory terminal unit  20  is discarded. The receiving section  23  in the optical switching system  18 , detecting the faulty state of the signal from the working receiving path  15 , places the receiving optical switch  9  in the cross state in order to connect the signal from the preparatory receiving path  17  to the working terminal unit  13 , thus recovering the fault. 
     The switching of the receiving optical switch  9  entails the path switching of the optical signal fed through the working receiving path  15  to the preparatory receiving optical gate  10  in the untransmissive state so that the optical signal is discarded by the gate function of the preparatory receiving optical gate  10 . This can prevent the preparatory terminal unit  14 , which receives the optical signal from the preparatory terminal unit  20  in a faultless state, from receiving the optical signal sent from the working terminal unit  19  during the recovery of the fault of the working receiving path  15 . In particular, the gate function is essential to avoid erroneous connection in the case where the communication between the working terminal units  13  and  19  differ from that between the preparatory terminal units  14  and  20  in the faultless state. 
     Next, a case will be considered in which a fault takes place on the working transmitting path  25  connected to the WRKout port  2 . The transmitting section  24  in the optical switching system  18  places in a faultless state the preparatory transmitted optical signal selector  12  at the through state, thereby transferring the transmitted signal from the preparatory terminal unit  14  to the preparatory transmitting path  16  through the Add(PRT) port  7  and PRTout port  4 . However, if a fault takes place on the working transmitting path  25 , the transmission signal from the preparatory terminal unit  14  is discarded to save the communication of the working terminal unit  13 . To achieve this, the preparatory transmitted optical signal selector  12  is brought in the cross state so that the transmitted signal from the working terminal unit  13  is divided by the working transmitted optical signal splitter  11  and delivered to both the WRKout port  2  and PRTout port  4  (bridging operation). The optical signal transmitted through the two paths, the working transmitting path  25  and the preparatory transmitting path  16 , is received by the optical switching system  26  which selects the optical signal via the preparatory transmitting path  16  and connects it to the working terminal unit  19 , thus recovering the communication between the working terminal units  13  and  19  suffering from the fault of the working transmitting path  25 . 
     As described above, the present embodiment 1 can make full use of the total transmission capacity of both the working transmitting path  25  and working receiving path  15  and of both the preparatory transmitting path  16  and preparatory receiving path  17  in the faultless state. In addition, even if a fault takes place on the working transmitting path  25  or working receiving path  15  conveying the currently working signals, the communications of the preparatory system flowing through the preparatory transmitting path  16  and preparatory receiving path  17  are discarded, and the communications of the working system are detoured to the preparatory transmitting path  16  and preparatory receiving path  17 . This offers an advantage of being able to save the communications of the working system. 
     In addition, the number of the optical space switches the embodiment 1 requires is small because the transmitting section  24  requires one 1×2 optical space switch (preparatory transmitted optical signal selector  12 ), the receiving section  23  requires one 2×2 optical space switch (receiving optical switch  9 ) and one optical gate switch (preparatory receiving optical gate  10 ). This offers an advantage of being able to reduce the packaging size. 
     EMBODIMENT 2 
     FIG. 5 is a block diagram showing a configuration of an embodiment 2 of the optical switching system in accordance with the present invention. Incidentally, in the drawings accompanying the following embodiments, the same components as those of the previous embodiments are designated by the same reference numerals, and the description thereof will be omitted. 
     In FIG. 5, the reference numeral  21  designates a preparatory receiving optical switch for carrying out spatial path switching of the optical signal input from the PRTin port  3 , and for supplying a first optical output to the Drop(PRT) port  8 ; and  22  designates a working received optical signal selector for selecting one of a second optical output of the preparatory receiving optical switch  21  and the optical signal supplied from the WRKin port  1 , and for supplying the selected signal to the Drop(WRK) port  6 . The reference numeral  23   a  designates a receiving section including the preparatory receiving optical switch  21  and the working received optical signal selector  22 ; and  18   a  designates an optical switching system of the present embodiment 2 comprising the transmitting section  24 , receiving section  23   a , WRKin port  1 , WRKout port  2 , PRTin port  3 , PRTout port  4 , Add(WRK) port  5 , Drop(WRK) port  6 , Add(PRT) port  7  and Drop(PRT) port  8 . 
     Next, the operation will be described. 
     The optical switching system  18   a  of the present embodiment 2 is identical to the optical switching system of the embodiment 1 in that it can utilize in a faultless state the total transmission capacity of both the working transmitting path  25  and working receiving path  15  and of both the preparatory transmitting path  16  and preparatory receiving path  17 , and in that it has the switching function inclusive of the function of breaking the optical signal in a fault switching to prevent the preparatory terminal unit  14  from being erroneously connected. Thus, the present embodiment differs from the embodiment 1 only in the configuration of the receiving section  23   a.    
     The receiving section  23   a  comprises two 2×1 optical space switches (the preparatory receiving optical switch  21  and working received optical signal selector  22 ). First, in a faultless state, both the preparatory receiving optical switch  21  and working received optical signal selector  22  are in the through connection state, establishing the connection between the WRKin port  1  and Drop(WRK) port  6 , and the connection between the PRTin port  3  and Drop(PRT) port  8 . 
     A fault taking place on the working receiving path  15  will bring both the preparatory receiving optical switch  21  and working received optical signal selector  22  into the cross connection state, connecting the PRTin port  3  with the Drop (WRK) port  6 . Thus, the working received optical signal selector  22  does not output the optical signal supplied from the WRKin port  1 , and the Drop(PRT) port  8  connected to the preparatory terminal unit  14  does not output any optical signal. This can prevent the signal sent from the working terminal unit  19  from being connected to the preparatory terminal unit  14  when the fault takes place on the working receiving path  15 , thereby preventing the erroneous connection. 
     As described above, the present embodiment 2 can make full use of the total transmission capacity of both the working transmitting path  25  and working receiving path  15  and of both the preparatory transmitting path  16  and preparatory receiving path  17  in the faultless state. In addition, if a fault takes place on a working path conveying the currently working signals, the communications of the preparatory system flowing through the preparatory transmitting path  16  and preparatory receiving path  17  are discarded, and the communications of the working system are detoured to the preparatory transmitting path  16  or to the preparatory receiving path  17 . This offers an advantage of being able to save the communications of the working system. 
     In addition, the number of the optical space switches required is small because the transmitting section requires one 1×2 optical space switch (the preparatory transmitted optical signal selector  12 ), and the receiving section requires two 1×2 optical space switches (the preparatory receiving optical switch  21  and working received optical signal selector  22 ). This offers an advantage of being able to reduce the packaging size. 
     Furthermore, considering the optical components in the optical switching system  18   a , through which the optical signals pass which are input or output through the ports, it is found that each optical component transmits only one optical signal in the faultless state. For example, the working transmitted optical signal splitter  11  transmits only the input signal fed from the Add (WRK) port  5 , and the working received optical signal selector  22  transmits only the input signal fed from the WRKin port  1 . This enables the signal paths in the faultless state to be mounted on separate boards, which in turn can prevent the services from being interrupted during the maintenance such as replacing the boards. For example, when the preparatory receiving optical switch  21  and the working received optical signal selector  22  are mounted on different boards, and only the preparatory receiving optical switch  21  must be replaced because of a fault, removing the faulty board will leave the working received optical signal selector  22 . This offers an advantage of being able to prevent the working receiving path  15  from being disconnected. 
     EMBODIMENT 3 
     FIG. 6 is a block diagram showing a configuration of an embodiment 3 of the optical switching system in accordance with the present invention. In this figure, the reference numeral  31  designates a preparatory receiving optical splitter for dividing into two the optical signal supplied from the PRTin port  3 ;  32  designates a preparatory receiving optical gate installed between a first optical output of the preparatory receiving optical splitter  31  and the Drop(PRT) port  8  for switching between the optical transmissive state and untransmissive state. In the present embodiment 3, the working received optical signal selector  22  is mounted such that it selects one of a second optical output of the preparatory receiving optical splitter  31  and the optical signal supplied from the WRKin port  1 , and outputs the selected signal to the Drop(WRK) port  6 . The reference numeral  23   b  designates a receiving section comprising the preparatory receiving optical splitter  31 , preparatory receiving optical gate  32  and working received optical signal selector  22 ; and  18   b  designates an optical switching system of the present embodiment 3 comprising the transmitting section  24 , receiving section  23   b , WRKin port  1 , WRKout port  2 , PRTin port  3 , PRTout port  4 , Add (WRK) port  5 , Drop (WRK) port  6 , Add(PRT) port  7  and Drop(PRT) port  8 . 
     Next, the operation will be described. 
     The optical switching system  18   b  of the present embodiment 3 is identical to the optical switching systems of the embodiments 1 and 2 in that it can utilize in the faultless state the total transmission capacity of both the working transmitting path  25  and working receiving path  15  and of both the preparatory transmitting path  16  and preparatory receiving path  17 , and in that it has the switching function inclusive of the function of breaking the optical signal in a fault switching to prevent the preparatory terminal unit  14  from being erroneously connected. Thus, the present embodiment differs from the embodiments 1 and 2 only in the configuration of the receiving section  23   b.    
     The optical signal supplied from the preparatory receiving path  17  to the PRTin port  3  is divided by the preparatory receiving optical splitter  31 . The working received optical signal selector  22  selects one of the optical signals fed from the WRKin port  1  and PRTin port  3 , and outputs the selected signal to the Drop(WRK) port  6 . Thus, the working terminal unit  13  can receive the optical signal either from the working receiving path  15  or from the preparatory receiving path  17 . In addition, the Drop(PRT) port  8  can either be connected to the optical signal input to the PRTin port  3 , or be disconnected from the optical signal by closing the preparatory receiving optical gate  32 . This enables the signal sent from the working terminal unit  19  through the preparatory receiving path  17  to be connected to the preparatory terminal unit  14  in the faultless state, and to be interrupted in case of a fault of the working receiving path  15 , thereby preventing the optical signal from being erroneously connected to the preparatory terminal unit  14 . 
     As described above, the present embodiment 3 can make full use of the total transmission capacity of both the working transmitting path  25  and working receiving path  15  and of both the preparatory transmitting path  16  and preparatory receiving path  17  in the faultless state. In addition, if a fault takes place on one of the working paths conveying the currently working signals, the communications of the preparatory system flowing through the preparatory transmitting path  16  or preparatory receiving path  17  are discarded, and the communications of the working system are detoured to the preparatory transmitting path  16  or to the preparatory receiving path  17 . This offers an advantage of being able to save the communications of the working system. 
     In addition, the number of the optical space switches required is small because the transmitting section  24  requires one 1×2 optical space switch (the preparatory transmitted optical signal selector  12 ), and the receiving section requires one 1×2 optical space switch (the working received optical signal selector  22 ) and one optical gate switch (the preparatory receiving optical gate  32 ). This offers an advantage of being able to reduce the packaging size. 
     Furthermore, considering the optical components in the optical switching system  18   b , through which the optical signals pass which are input or output through the ports, it is found that each optical component transmits only one optical signal in the faultless state. This enables the signal paths in the faultless state to be mounted on separate boards, which can prevent the services from being interrupted during the maintenance such as replacing the boards. 
     EMBODIMENT 4 
     FIG. 7 is a block diagram showing a configuration of the embodiment 4 of the optical switching system in accordance with the present invention. In this figure, the reference numeral  41  designates a 2×2 working Add/Drop switch that has its two inputs connected to the WRKin port  1  and the Add (WRK) port  5 , respectively, and its two outputs connected to the working received optical signal. selector  22  and the working transmitted optical signal splitter  11 , respectively, and that switches the connections between the two inputs and the two outputs with taking the two sates, the through state and the cross state. The reference numeral  42  designates a 2×2 preparatory Add/Drop switch that has its two inputs connected to the PRTin port  3  and the Add(PRT) port  7 , respectively, and its two outputs connected to the preparatory receiving optical splitter  31  and the preparatory transmitted optical signal selector  12 , respectively, and that switches the connections between the two inputs and the two outputs with taking the two sates, the through state and the cross state. 
     Next, the operation will be described. 
     Although the description of the foregoing embodiments 1, 2 and 3 is made about the transmission path switching between the two opposite optical switching systems, the conventional Add/Drop ring network as shown in FIGS. 1 and 2 interconnects two or more systems in a ring configuration, and establishes the communication path between any two optical switching systems that perform the Add/Drop operation. This is carried out by inserting or transmitting the optical signal into or to the transmission path of the network (this operation will be denoted as “Add” from now on), or by placing the system at the path through state in which the signal from the transmission path is not remove or receive (Drop), but is passed through the system without change. The object of the present embodiment 4 is to apply its system to such an Add/Drop ring network. It interconnects the WRKin port  1  with the WRKout port  2 , and the PRTin port  3  with the PRTout port  4  in order to implement the path through state in which no connections are established between the WRKin port  1  and Drop(WRK) port  6 , between the Add(WRK) port  5  and WRKout port  2 , between the PRTin port  3  and Drop(PRT) port  8 , and between the Add(PRT) port  7  and PRTout port  4 . 
     FIG. 8 shows an example of a connection configuration of a ring network. In this figure, reference numerals  51 - 54  each designate a working terminal unit;  55  designates a bidirectional working path constituting a working transmitting/receiving path; and  56  designates a bidirectional preparatory path constituting a preparatory transmitting/receiving path. In the connection configuration the Add/Drop ring network comprises four optical switching systems  18   c . A bidirectional communication is carried out between the working terminal unit  51  and working terminal unit  52  through the working path  55 , for example. 
     The optical switching systems  18   c  installing the working terminal units  51  and  52  involved in the communication are in the Add/Drop state in which the signals are transmitted to or received from the working path  55 . The remaining optical switching systems  18   c  connected with the working terminal units  53  and  54  not involved in the communication are in the path through state in which the sections of the working path are interconnected, and the sections of the preparatory path  56  are interconnected without establishing the Add/Drop connection. 
     If a fault takes place on the working path  55  interconnecting the working terminal unit  51  and working terminal unit  52 , the two optical switching systems  18   c  in the Add/Drop state undergo fault switching so that the communication path is switched to the preparatory path  56  side to save the communication. In this case, the optical switching systems  18   c  connected with the working terminal units  53  and  54  not involved in the communication keep the path through state. 
     The switching to place the optical switching system  18   c  in the path through state or in the Add/Drop state is carried out by the working Add/Drop switch  41  and preparatory Add/Drop switch  42  as shown in FIG.  7 . When the working Add/Drop switch  41  and preparatory Add/Drop switch  42  are in the cross state, the optical signals input to the WRKin port  1  and PRTin port  3  pass through the Add/Drop switches  41  and  42 , respectively, and are supplied to the transmitting section  24 . When the preparatory transmitted optical signal selector  12  is in the through state, the input signals are transferred to the WRKout port  2  and PRTout port  4 , thereby establishing the connections between the WRKin port  1  and WRKout port  2 , and between the PRTin port  3  and PRTout port  4 . Thus, the optical switching system  18   c  is placed in the path through state. 
     To bring the optical switching system  18   c  in the Add/Drop state, the Add/Drop switches  41  and  42  are brought in the through state. The switching of the working Add/Drop switch  41  can change the state between the Add/Drop state and the path through state of the working signal, and the switching of the preparatory Add/Drop switch  42  can change the state between the Add/Drop state and path through state of the preparatory signal. 
     If a fault takes place in such a ring network, the switching pattern must be changed in accordance with the fault position. FIG. 9 illustrates a switching pattern when a fault takes place on the working transmitting path  25 , whereas FIG. 10 illustrates a switching pattern when a fault takes place on the working receiving path  15 . In either case, switching of the preparatory transmitted optical signal selector  12  and/or of the working received optical signal selector  22  can implement the operation of the different switching patterns. 
     More specifically, when a fault takes place on the working transmitting path  25  as illustrated in FIG. 9, the optical signal from the working terminal unit  13  is transmitted through the Add (WRK) port  5 , working Add/Drop switch  41 , working transmitted optical signal splitter  11 , preparatory transmitted optical signal selector  12  and PRTout port  4 , and is supplied to the preparatory transmitting path  16 . On the other hand, the optical signal is received from the working receiving path  15  by the working terminal unit  13  through the WRKin port  1 , working Add/Drop switch  41 , working received optical signal selector  22  and Drop (WRK) port  6 , or is received from the preparatory receiving path  17  by the preparatory terminal unit  14  through the PRTin port  3 , preparatory Add/Drop switch  42 , preparatory receiving optical splitter  31 , preparatory receiving optical gate  32  and Drop (PRT) port  8 . 
     Likewise, when a fault takes place. on the working receiving path  15  as illustrated in FIG. 10, the optical signal transmitted via the preparatory receiving path  17  is received by the working terminal unit  13  by capturing it from the PRTin port  3 , and by conveying it to the working terminal unit  13  through the preparatory Add/Drop switch  42 , preparatory receiving optical splitter  31 , working received optical signal selector  22  and Drop (WRK) port  6 . On the other hand, the optical signal is sent from the working terminal unit  13  to the working transmitting path  25  through the Add(WRK) port  5 , working Add/Drop switch  41 , working transmitted optical signal splitter  11  and WRKout port  2 , or is sent from the preparatory terminal unit  14  to the preparatory transmitting path  16  through the Add(PRT) port  7 , preparatory Add/Drop switch  42 , preparatory transmitted optical signal selector  12  and PRTout port  4 . 
     Although the present embodiment 4 employs the receiving section  23   b  of the embodiment 3 as the receiving section, using the receiving section  23  or  23   a  of the embodiment 1 or 2 can implement the same operation. 
     As described above, the present embodiment 4 can make full use of the total transmission capacity of both the working transmitting path  25  and working receiving path  15  and of both the preparatory transmitting path  16  and preparatory receiving path  17  in the faultless state. Furthermore, in case of a fault, the communications of the working transmitting path  25  or the working receiving path  15  are detoured to the preparatory transmitting path  16  or preparatory receiving path  17 . This offers an advantage of being able to save the communications of the working system. 
     In addition, although the Add/Drop function is added in the present embodiment 4, the optical switching system  18   c  can be constructed by adding to the number of the optical space switches required in the embodiments 1-3 the two 2×2 optical space switches (the working Add/Drop switch  41  and preparatory Add/Drop switch  42 ). This offers an advantage of being able to reduce the number of unit switches as compared with the conventional example 1, thereby shrinking the packaging size. 
     Furthermore, the working Add/Drop switch  41  and preparatory Add/Drop switch  42  added to carry out the Add/Drop functions in the present embodiment 4 are composed of the separate optical space switches corresponding to the working and preparatory systems each. This enables the two systems to be mounted on separate boards. Accordingly, even if a fault takes place in the Add/Drop switch of one of the systems, and a recovery job involving the board removal is required, it is unnecessary to remove the optical space switch of the other system. This offers an advantage of being able to prevent the services of the faultless system from being interrupted. 
     EMBODIMENT 5 
     FIG. 11 is a block diagram showing a configuration of the embodiment 5 of the optical switching system in accordance with the present invention. In this figure, each reference numeral  18   d  designates an optical switching system of the present embodiment 5;  81  designates a span switching means for switching between a working path and a preparatory path in the same section on a West side transmission path;  82  designates an East side span switching means;  83  and  84  each designate a ring switching means (first and second ring switching means) for switching between the sections themselves on the transmission path;  85  and  86  each designate a working terminal unit;  87  designates a West side PRT in/out port;  88  designates a West side WRK in/out port;  89  designates an East side PRT in/out port;  90  designates an East side WRK in/out port;  91  designates a West side Add/Drop(PRT) port (Add(PRT) port and Drop(PRT) port);  92  designates a West side Add/Drop(WRK) port (Add(WRK) port and Drop(WRK) port);  93  designates an East side Add/Drop(PRT) port (Add(PRT) port and Drop(PRT) port);  94  designates an East side Add/Drop(WRK) port (Add(WRK) port and Drop (WRK) port);  95  designates a working path; and  96  designates a preparatory path. 
     Next, the operation will be described. 
     In the present embodiment 5, the working terminal unit  85  connected to the West side Add/Drop(WRK) port  92  of the optical switching system  18   d  is bidirectionally connected with the working terminal unit  86  via the ring switching means  83 , West side span switching means  81  and working path  95 . 
     If a fault takes place on the working path  95  between the optical switching system  18   d  connected with the working terminal unit  85  and the optical switching system  18   d  connected with the working terminal unit  86 , and if the preparatory path  96  in the same section is normal, the span switching means  81  carries out the span switching through the steps as described in the embodiments 1-4. The span switching can be carried out between the two optical switching systems  18   d  adjacent to the fault section. Even if a plurality of working path faults take place between the end nodes carrying out the communications, their recovery can be implemented by the span switching in that section. In the conventional example as illustrated in FIG.  2 ( 2 ), the path switching is carried out by the end nodes when the fault takes place on the transmission path  115  only. In this case, if a fault takes place on the transmission path  115  between the nodes  113  and  112 , and another fault takes place on the transmission path  116  between the nodes  112  and  111 , a communication path passing through the node  112  cannot be secured. The present embodiment 5 carries out the span switching independently for each path between the adjacent nodes. This offers an advantage of being able to secure the path even in the fault pattern as described above. 
     FIG. 12 illustrates a switching state of the ring switching. If faults take place at the same time on the working path  95  and preparatory path  96  in the same section, the ring switching means  83  carries out the switching to the reverse direction preparatory path  96  to save the communication. The ring switching function is equivalent to the switching of the conventional example as illustrated in FIG.  2 ( 3 ). 
     INDUSTRIAL APPLICABILITY 
     As described above, the optical switching system according to the present invention comprises in the transmitting section the preparatory transmitted optical signal selector and working transmitted optical signal splitter, each of which can be implemented using a 1×2 optical space switch, and in the receiving section the receiving optical switch implemented using a 2×2 optical space switch, and the preparatory receiving optical gate implemented using a 1×2 optical space switch. This enables the switching between the working and preparatory systems without using a 4×4 optical space switch. Thus, the optical switching system is preferably applied to the path switching of the network interconnecting a plurality of nodes through the working path and preparatory path conveying optical signals.