Optical switch changeover controlling method, optical node device and optical switch system

The present invention aims at providing an optical switch changeover controlling method, an optical node device and an optical switch system, capable of avoiding an interruption in an optical output power when changing over an optical path making use of a spatial optical switch arranged with a plurality of optical switch elements. To this end, the optical switch changeover controlling method according to the present invention enables an uninterrupted changeover of an optical path by conducting a reconnection of the optical path after establishing a state where the optical path before the changeover and the optical path after the changeover are simultaneously set concerning the spatial optical switch within the optical node device, when conducting a changeover of the optical path so as to transmit a client optical signal having been transmitted through a working ray path to a protective ray path such as in a case that a fault occurs in the working ray path.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an optical switch changeover controlling technique in conducting a changeover of an optical path by a spatial optical switch comprising a plurality of optical switch elements arranged therein, and particularly to an optical switch changeover controlling method, an optical node device and an optical switch system for realizing an uninterrupted changeover of an optical path.

(2) Related Art

Recently increased information capacities, variation and the like require a flexible and reliable construction of a network as well as an increased capacity of a transmission system. As one way to realize them, there has been demanded a construction of an optical network based on a wavelength division multiplexing (WDM) technique. In constructing such a network, important roles will be played by an optical-switch-adopting optical node device, such as: an optical cross-connect device for setting a bypass route, e.g., when changing over an optical path or when a fault occurs in a transmission path; an optical add drop multiplexer (OADM) for adding/dropping optical signals; and an optical protection device for conducting a recovery from a fault in an optical network.

FIG. 18 is a conceptual diagram for explaining an optical switch changeover controlling method utilizing conventional optical node devices. There is shown an example of a procedure for changing over an optical path from a working ray path to a protective ray path at the time of occurrence of a fault, in which FIG. 18A shows an initial state, FIG. 18B shows a state where a fault occurs, FIG. 18C shows a state where a path release is conducted, and FIG. 18D shows a state where the recovery from the fault has been completed.

In the initial state of FIG. 18A , two optical node devices 1 A, 1 B are interconnected via a working ray path 2 W and a protective ray path 2 P. The working ray path 2 W is input with a client optical signal sc from a client (terminal equipment) 3 A connected to the optical node device 1 A, and the protective ray path 2 P is input with a PCA optical signal sp from a PCA (Protect Channel Access) device 4 A connected to the optical node device 1 A.

When a fault such as a disconnection occurs in the working ray path 2 W as shown in FIG. 18B , the client optical signal sc being connected to the working ray path 2 W is to be changed over to the protective ray path 2 P. Concretely, the connection of the PCA optical signal sp having been connected to the protective ray path 2 P is once released ( path release ) as shown in FIG. 18C , followed by a reconnection of the client optical signal sc to the protective ray path 2 P as shown in FIG. 18D to thereby conduct a changeover from the working ray path 2 W to the protective ray path 2 P at the time of occurrence of the fault.

There will be now briefly explained a changeover operation of optical switches provided in the optical node devices 1 A, 1 B.

As a typical optical switch to be provided in each of the optical node devices 1 A, 1 B, there is used an N N spatial optical switch, for example, which is constituted of matrix-arranged N 2 units of 2 2 optical switch elements (in which N is the number of lines to be changed over at the node), where each 2 2 optical switch element has two inputs and two outputs cooperatively changeable into one of a parallel (bar) state and an interlaced (cross) state.

FIG. 19 is a diagram showing an example of a 2 2 spatial optical switch (i.e., N 2) in the transmission side optical node device 1 A. There is shown a procedure for changing over, the path of the client optical signal sc connected from an input terminal 1i to an output terminal 1o of the optical node device 1 A, to a path from the input terminal 1i to an output terminal 2o.

In a path setting initial state shown in FIG. 19A , an optical path for transmitting the client optical signal sc from the input terminal 1i to the output terminal 1o as shown by a solid line arrow, and an optical path for transmitting the PCA optical signal sp from an input terminal 2i to the output terminal 2o as shown by a dotted line arrow are set. At this time, a 2 2 optical switch element S 11 at the intersection point between the input terminal 1i and output terminal 1o, and a 2 2 optical switch element S 22 at the intersection point between the input terminal 2i and output terminal 2o are brought into parallel states (ON states), respectively.

Note, a 2 2 optical switch element at an intersection point between an input terminal xi and an output terminal yo means such a 2 2 optical switch element in the parallel state: when all 2 2 optical switch elements within a spatial optical switch are once turned into interlaced states (OFF states) and one of the 2 2 optical switch elements is then changed over to a parallel state (ON state) to thereby set an optical path directed from the input terminal xi to the output terminal yo.

FIG. 19B shows a state where the already connected two optical paths are released before conducting a changeover of a path connection, in which all the optical switch elements S 11 , S 12 , S 21 , S 22 are in the interlaced states (OFF states) (path released states).

FIG. 19C shows a path reconnection state where the 2 2 optical switch element S 12 at the intersection point between the input terminal 1i and the output terminal 2o is brought into the parallel state (ON state) so as to set an optical path from the input terminal 1i to the output terminal 2o to thereby connect the client optical signal sc to the protective ray path 2 P.

In the conventional optical node device as described above, there is conducted a consecutive procedure including optical path setting (initial state), optical path release and optical path reconnection, when conducting a connection changeover of an optical signal. Thus, an optical output power (optical output power to the protective ray path 2 P) of the output terminal 2o of the optical node device 1 A is interrupted in the course of the changeover of the optical path, as shown in FIG. 20 . Concretely, this interruption in the optical output power continues over a period of time from the optical path release up to the optical path reconnection, and the interruption period T can be represented by the following equation (1):

T T f T off T r (1)

wherein T f is a falling time of the 2 2 optical switch element, T off is a changeover controlling time (time-lag up to the optical path re-setting), and Tr is a rise time of the 2 2 optical switch element.

As such, the conventional optical node device as described above has a possibility to cause a deterioration of optical signal transmission characteristics or a failure of the device, due to the interruption in the optical output power at the time of changeover of the optical path. Namely, such as when an optical amplifier is arranged on the latter stage side of an optical switch within an optical node device or is arranged within an optical transmission path interconnecting optical node devices, an interruption in optical output power at the time of changeover of an optical path will cause an optical surge in the optical amplifier. This results in a problem of a possibility to cause a deterioration of optical signal transmission characteristics or a failure of the device. Further, the aforementioned optical switch changeover controlling method in the conventional optical node device also has a defect of the time-lag up to the reconnection of the optical path.

SUMMARY OF THE INVENTION

The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide an optical switch changeover controlling method, an optical node device and an optical switch system, capable of avoiding an interruption in optical output power at the time of changeover of an optical path.

To achieve the above object, an optical switch changeover controlling method according to the present invention, for using a spatial optical switch which is provided with a plurality of optical switch elements arranged between a plurality of input terminals and a plurality of output terminals thereof, each of the plurality of optical switch elements being controllable to change over a connection between two input ports and two output ports into one of a parallel state and an interlaced state; and having such a characteristic that a power of optical signal to be output from each of the output ports is continuously changed over from a power of optical signal to be input to one of the two input ports, to a power of optical signal to be input to the other of the two input ports, to thereby change over setting of optical paths interconnecting between the plurality of input terminals and the plurality of output terminals of the spatial optical switch, comprises:

when a first optical path directed from a first input terminal to a first output terminal of the spatial optical switch is changed over to a second optical path directed from a second input terminal to the first output terminal,

an optical path re-setting step for initiating, re-setting of the respective optical switch elements for forming the second optical path, while keeping the setting of the respective optical switch elements for forming the first optical path, to prepare a state where, for one optical switch element participating in forming both of the first optical path and the second optical path, one of the two input ports is input with an optical signal from the first input terminal and the other of the two input terminals is input with an optical signal from the second input terminal; and

an optical path reconnection step for changing over the connection state of the one optical switch element participating in forming both of the first optical path and the second optical path, set in the optical path re-setting step, to the other connection state to thereby release the first optical path and establish the connection of the second optical path.

According to such an optical switch changeover controlling method, when changing over the connection setting of the optical path, there can be temporarily realized, by the path re-setting step, a state where the first optical path before the changeover and the second optical path after the changeover are simultaneously set. This enables an uninterrupted changeover of the optical path, as well as suppression of occurrence of optical surge in optical amplifiers. In this way, it becomes possible to achieve stable transmission characteristics of optical signal, and to reduce the frequency of device failures.

The optical switch changeover controlling method according to the present invention as described above can be applied to an optical node device constituted using a spatial optical switch and to an optical switch system, for example. Further, it is also effective to construct an optical network making use of a plurality of optical node devices to which the present invention is applied.

DETAILED DESCRIPTION OF THE INVENTION

There will be described hereinafter embodiments according the present invention, with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram for explaining a basic principle of an optical switch changeover controlling method according to the present invention. Here, an example of a procedure for changing over an optical path from a working ray path to a protective ray path at the time of occurrence of a fault is shown. Further, FIG. 2 is a diagram showing an example of a 2 2 (two inputs and two outputs) spatial optical switch to be used in the transmission side optical node device of FIG. 1 .

In an initial state shown in FIG. 1A , two optical node devices 1 A, 1 B are interconnected via a working ray path 2 W and a protective ray path 2 P. The working ray path 2 W is input with a client optical signal sc from a client (terminal equipment) 3 A connected to the optical node device 1 A, and the protective ray path 2 P is input with a PCA optical signal sp from a PCA device 4 A connected to the optical node device 1 A. As shown in FIG. 2A , a spatial optical switch within the optical node device 1 A is set in this initial state, such that: a 2 2 optical switch element S 11 at an intersection point between an input terminal 1i and an output terminal 1o is set in a parallel state (ON state) so that the client optical signal sc input to the input terminal 1i is output to the working ray path 2 W connected to the output terminal 1o, to thereby set an optical path directed from the input terminal 1i to the output terminal 1o; and a 2 2 optical switch element S 22 at an intersection point between an input terminal 2i and an output terminal 2o is set in a parallel state (ON state) so that the PCA optical signal sp input to the input terminal 2i is output to the protective ray path 2 P connected to the output terminal 2o, to thereby set an optical path directed from the input terminal 2i to the output terminal 2o. This type of 2 2 optical switch element is an optical switch capable of conducting crossbar type switching.

When a fault such as occurs in the working ray path 2 W as shown in FIG. 1B , there is conducted a connection changeover for transmitting the client optical signal sc being transmitted to the working ray path 2 W, to the protective ray path 2 P. In the conventional changeover controlling method, the client optical signal sc is duly connected, after the PCA optical signal sp being connected to the protective ray path 2 P is once released (path release). However, according to the changeover controlling method of the present invention, optical path re-setting is initiated for connecting the client optical signal sc to the protective ray path 2 P, without a connection release (path release) of the PCA optical signal sp. As shown in FIG. 1C , this optical path re-setting temporarily realizes a state where the optical paths before and after the connection changeover are simultaneously set.

Concretely, as shown in FIG. 2B , a 2 2 optical switch element S 12 at an intersection point between the input terminal 1i (second input terminal) and the output terminal 2o (first output terminal) is set in a parallel state (ON state), in preparation for setting an optical path (second optical path) for transmitting the client optical signal sc to the protective ray path 2 P, while keeping the setting of the optical path (first optical path) from the input terminal 2i (first input terminal) to the output terminal 2o. At this time, one input port of the 2 2 optical switch element S 22 at the intersection point between the input terminal 2i and the output terminal 2o is input with the client optical signal sc, and the other input port is input with the PCA optical signal sp, so that the PCA optical signal sp is still output to the protective ray path 2 P at this stage.

There will be now briefly explained characteristics of each 2 2 optical switch element to be provided within the optical node devices.

FIG. 3 is a conceptual diagram showing a state of an input signal and an output signal when the 2 2 optical switch element is changed over from an interlaced state (OFF state) to a parallel state (ON state). FIG. 4 is a graph showing time-wise transitions of powers of optical signals output from respective output ports of the 2 2 optical switch element of FIG. 3 .

FIG. 3A shows the interlaced state, in which an optical signal A input to an input port Pin 1 of the 2 2 optical switch element is output from an output port Pout 2 , and an optical signal B input to an input port Pin 2 is output from an output port Pout 1 . FIG. 3B shows a state in the course of changeover from the interlaced state to the parallel state, in which both of the optical signal A input to the input port Pin 1 and the optical signal B input to the input port Pin 2 are output from both of output ports Pout 1 , Pout 2 , respectively. FIG. 3C shows the parallel state, in which the optical signal A input to the input port Pin 1 is output from the output port Pout 1 , and the optical signal B input to the input port Pin 2 is output from the output port Pout 2 . As also shown in FIG. 4 A and FIG. 4B , it can be understood that an optical signal power to be output from each of the output ports Pout 1 , Pout 2 is continuously changed over from an optical signal power being input to one of the input ports to an optical signal power being input to the other of the input ports.

In a situation where the aforementioned 2 2 optical switch elements are used in the 2 2 spatial optical switch within the optical node device 1 A shown in FIG. 2 , the optical signal to be output to the protective ray path 2 P is continuously changed over from the PCA optical signal sp to the client optical signal sc when the connection state of the 2 2 optical switch element S 22 (having one input port being input with the client optical signal sc and the other input port being input with the PCA optical signal sp) at the intersection point between the input terminal 2i and the output terminal 2o shown in FIG. 2B is changed over from the parallel state to the interlaced state.

As shown in FIG. 2C , by bringing the 2 2 optical switch element S 22 at the intersection point between the input terminal 2i and the output terminal 2o into the interlaced state (OFF state), the changeover of the client optical signal sc from the working ray path 2 W to the protective ray path 2 P is completed, to thereby complete the recovery from the fault occurred in the working ray path 2 W, as shown in FIG. 1 D.

FIG. 5 is a graph showing a transitional state of the optical signal power to be output to the protective ray path 2 P during the aforementioned consecutive connection changeover.

As shown in FIG. 5 , it can be understood that the optical signal to be output to the protective ray path 2 P is continuously changed over from the PCA optical signal sp to the client optical signal sc, to thereby realize the uninterrupted changeover of the optical power. The time T required from the cutoff (re-setting) of the optical path up to the reconnection of the optical path can be represented by the following equation (2), assuming that T f and T r are a falling time and a rise time of the 2 2 optical switch element, respectively:

T T f , when T f >T r , and

T T r , when T f T r (2).

Thus, comparison of the aforementioned changeover time T according to the conventional method with the changeover time T according to the method of the present invention will show that T<T . This means that the optical switch changeover controlling method according to the present invention can realize the uninterrupted changeover of the optical power while reducing the changeover time of the optical path.

There will be now concretely described exemplary constitutions of a 2 2 optical switch element allowing a continuous changeover of an optical power as explained with reference to FIGS. 3 and 4 .

FIG. 6 is a diagram for showing an exemplary constitution of an optical switch element utilizing a Mach-Zehnder interferometer, and for showing an optical switch characteristic. FIG. 7 is a diagram for showing an exemplary constitution of an optical switch element utilizing a directional coupler, and for showing an optical switch characteristic. Further, FIG. 8 is a diagram for showing an exemplary constitution of a total reflection type optical switch element, and for showing an optical switch characteristic.

Shown in FIG. 6A , FIG. 7 A and FIG. 8A are constitutions of respective optical switch elements. In the optical switch element utilizing the Mach-Zehnder interferometer shown in FIG. 6 , the Mach-Zehnder interferometer is formed by interconnecting opposing ports of optical couplers 10 , 11 each having four ports, by optical waveguides, respectively. The switching operation of the Mach-Zehnder interferometer is controlled by a heater 12 provided on one of arms of the interferometer. In the optical switch element utilizing the directional coupler of FIG. 7 , the directional coupler is constituted of adjacent two optical waveguides. The switching operation of the directional coupler is controlled by changing a ratio between a set coupling length and a full coupling length by a voltage to be applied between electrodes 13 to thereby change the coupling strength. In the total reflection type optical switch element of FIG. 8 , the switching operation thereof is controlled by changing a refractive index of a reflection barrier formed at a binding portion between two optical waveguides, by a voltage to be applied between electrodes 14 .

Shown in FIG. 6B , FIG. 7 B and FIG. 8B are optical output power characteristics relative to parameters when the respective optical switch elements are controlled, respectively. As understood, there can be obtained the same optical output power characteristic in any of the optical switch elements. Further, shown in FIG. 6C , FIG. 7 C and FIG. 8C are transitions of optical output powers when the respective optical switch elements are changed over from interlaced states to parallel states, respectively. As understood, there is shown a characteristic in which the optical output power is continuously changed over from an input signal power of one of the two inputs to that of the other, in any of the optical switch elements.

There will be now described a concrete example of an N N spatial optical switch utilizing the aforementioned type 2 2 optical switch elements.

FIG. 9 is a diagram for explaining the optical switch changeover controlling method according to the present invention in a matrix type 4 4 spatial optical switch. Further, FIG. 10 is a diagram for explaining the optical switch changeover controlling method according to the present invention in a constant interlace type (known as a PI-Loss architecture) 4 4 spatial optical switch. It is noted that each figure shows a procedure at the time of when the 4 4 spatial optical switch is changed over from a state where an optical path directed from an input terminal 1i to an output terminal 1o and another optical path directed from an input terminal 3i to an output terminal 3o, for example are set in initial states, to a state where an optical path directed from the input terminal 1i to the output terminal 3o is set to leave the input terminal 3i to be unconnected.

In a path setting initial state as shown in each of FIG. 9 A and FIG. 10A an optical switch element S 11 at an intersection point between an input terminal 1i and an output terminal 1o and an optical switch element S 33 at an intersection point between an input terminal 3i and an output terminal 3o are set in parallel states (ON states), respectively, to thereby set the optical path directed from the input terminal 1i to the output terminal 1o and the optical path directed from the input terminal 3i to the output terminal 3o. From this state, there is initiated the setting of an optical path directed from the input terminal 1i to the output terminal 3o.

Concretely, as shown in each of path re-setting states of FIG. 9 B and FIG. 10B , an optical switch element S 13 at an intersection point between the input terminal 1i and the output terminal 3o is brought into an ON state, so as to set an optical path directed from the input terminal 1i to the output terminal 3o, while the optical switch element S 33 setting the optical path directed from the input terminal 3i to the output terminal 3o is kept in the ON state. At this time, one input port of the optical switch element S 33 is input with the optical signal from the input terminal 1i, and the other input port of the optical switch element S 33 is input with the optical signal from the input terminal 3i. By switching over the optical switch element S 33 from the ON state of this path re-setting state to the OFF state in a path reconnection state as shown in each of FIG. 9 C and FIG. 10C , the optical signal to be output to the output terminal 3o is changed over from the optical signal of the input terminal 3i to the optical signal of the input terminal 1i in an uninterrupted manner for the optical power, similarly to the aforementioned situation shown in FIG. 5 .

FIG. 11 is a diagram for explaining the optical switch changeover controlling method according to the present invention in a tree type 4 4 spatial optical switch. It is noted that 24 units of 2 2 optical switch elements are required for constituting the 4 4 spatial optical switch, in case of the tree type. It is also noted that this kind of tree type spatial optical switch is so typical.

In a path setting initial state of FIG. 11A , there are set: an optical path directed from an input terminal 1i to an output terminal 1o by bringing an optical switch element S A to a parallel state (ON state); and an optical path directed from an input terminal 3i to an output terminal 3o by bringing optical switch elements S I , S K , S L to parallel states (ON states), respectively.

A connection changeover is conducted from this state. However, the number of optical switch elements to be controlled is increased as compared with the situations of spatial optical switches shown in FIGS. 9 and 10 , respectively. Thus, the changeover controlling is slightly complicated. Firstly, there is conducted the control of optical switch elements required for establishing the connection from the input terminal 1i to the output terminal 3o, while keeping the current states of the optical switch elements S I , S K , S L , respectively. Namely, as shown in the re-setting state of FIG. 11B , the optical switch element S A is brought from the parallel state into an interlaced state, and an optical switch element S C is brought from an interlaced state into a parallel state. At this time, the optical switch element S L is input with the optical signals from the input terminal 1i and the input terminal 3i.

Thus, as shown in the path reconnection state of FIG. 11C , there is established an optical path directed from the input terminal 1i to the output terminal 3o, by finally bringing the optical switch element S L from the parallel state into the interlaced state. Thus, the optical signal to be output to the output terminal 3o is changed over from the optical signal of the input terminal 3i to the optical signal of the input terminal 1i in an uninterrupted manner for the optical power, similarly to the aforementioned situation shown in FIG. 5 .

There will be described hereinafter an optical node device utilizing the optical switch changeover controlling method according to the present invention.

FIG. 12 is a diagram showing an exemplary constitution of the optical node device according to an embodiment of the present invention. Here, as the optical node device, there is assumed an optical cross-connect (optical XC) device using an N N spatial optical switch. Further, consideration is made for a situation where optical signals (not wavelength multiplexed) of single wavelength are transmitted through ray paths to be connected to input and output ports, respectively.

In FIG. 12 , the optical XC device 20 of this embodiment is constituted to comprise: a spatial optical switch 21 provided with N N (N K L) pieces of input and output ports connected to K threads of inter-station input ray paths and K threads of inter-station output ray paths as well as L threads of intra-station input ray paths and L threads of intra-station output ray paths, respectively; and optical amplifiers (pre-amplifiers 22 A and post-amplifiers 22 B) provided between the spatial optical switch 21 and input and output ray paths, respectively, and to connect the input ray paths to desired output ray paths, respectively, making use of the optical switch changeover controlling method according to the present invention. Although not shown in this figure, the spatial optical switch 21 is supposed to incorporate therein a functional constitution corresponding to controlling means.

In the optical XC device 20 having such a constitution, the connection changeover (optical cross-connect) between the input ray paths and output ray paths in this node is conducted in the optical spatial switch 21 , similarly to the aforementioned controlling procedure to be conducted when changing over the optical path from the working ray path to the protective ray path at the time of occurrence of a fault. Thus, there is eliminated any interruption at the time of connection changeover, to thereby avoid an occurrence of an optical surge in the optical amplifiers 22 . In this way, it becomes possible to stabilize the transmission characteristics of optical signals and to reduce the frequency of device failures.

In the optical XC device 20 of the aforementioned embodiment, there has been considered the situation where the optical signals that are not wavelength multiplexed are transmitted through respective ray paths. However, the present invention is not limited thereto, and can be applied to a situation where wavelength multiplexed optical signals are transmitted through ray paths, respectively. FIG. 13 is a diagram showing an exemplary constitution of an optical XC device where wavelength multiplexing is conducted.

In the exemplary constitution of FIG. 13 , there are provided: optical demultiplexers 23 for demultiplexing respective WDM optical signals (containing optical signals of wavelengths 1 to m ) transmitted through inter-station input ray paths into optical signals of respective wavelengths; and optical multiplexers 24 for multiplexing the optical signals of wavelengths 1 to m output from respective output ports of an spatial optical switch 21 to thereby transmitting the multiplexed optical signals to inter-station output ray paths, respectively. The spatial optical switch 21 is provided with N N (N (K m) L) pieces of input and output ports for receiving the optical signals of (K m) waves output from the respective optical demultiplexers 23 and outputting the optical signals of (K m) waves to the respective optical multiplexers 24 , and being connected to L threads of intra-station input ray paths and L threads of intra-station output ray paths. Also, in the optical XC device 20 of the aforementioned constitution, the connection changeovers between input ray paths and output ray paths are conducted in an uninterrupted manner. Thus, there can be obtained the same function and effect as the situation of the optical XC device 20 shown in FIG. 12 .

There will be now described an optical network constructed of optical node devices utilizing the optical switch changeover controlling method according to the present invention.

FIG. 14 is a diagram showing an exemplary constitution of an optical network applied with the present invention. Shown in this figure is an example of a 4-fiber ring network constructed of four units of optical XC devices each capable of accommodating four threads of inter-station input ray paths and four threads of inter-station output ray paths, for example. To simplify the explanation, this optical network is assumed to be of no wavelength multiplexing.

In FIG. 14 , the 4-fiber ring network is constituted of optical XC devices 30 A, 30 B, 30 C, 30 D interconnected in a ring shape via two threads of clockwise ray paths (from West to East in the figure) and two threads of counterclockwise ray paths (from East to West in the figure). In this 4-fiber ring network, one of the same directional two ray paths is used as a working ray path (solid line arrow in the figure) so as to transmit the client optical signal sc, and the other is used as a protective ray path (dotted line arrow in the figure) so as to transmit the PCA optical signal sp.

There will be now considered a situation, for example, as shown in FIG. 15 , where a fault such as a fiber disconnection has occurred in the clockwise working ray path between the optical XC device 30 A (node 1 ) and the optical XC device 30 B (node 2 ), in the 4-fiber ring network having the aforementioned constitution. In this case, the client optical signal sc having been transmitted through the working ray path between the node 1 and node 2 is changed over from the working ray path to the protective ray path by a span switch between the node 1 and node 2 .

FIG. 16 is a diagram for explaining a changeover operation of a 4 4 spatial optical switch in the optical XC device 30 A (node 1 ).

The path connection (initial state) shown in FIG. 16A is set such that: the clockwise client optical signal sc is transmitted through an optical path directed from the working path (from W) to the working path (to E); the counterclockwise client optical signal sc is transmitted through an optical path directed from the working path (from E) to the working path (to W); the clockwise PCA optical signal sp is transmitted through an optical path directed from the protective path (from W) to the protective path (to E); and the counterclockwise PCA optical signal sp is transmitted through an optical path directed from the protective path (from E) to the protective path (to W).

When a fault occurs in the clockwise working ray path between the node 1 and node 2 : the clockwise client optical signal having been connected to the working path (to E) which is one of the output side ports is changed over to the protective path (to E); and the optical signal having been connected to the protective path (from E) is changed over from the protective path (to W) to the working path (to W).

In this case, as shown in FIG. 16B under protection (path re-setting), there is initiated such path re-setting that an optical switch element S 13 is brought into a parallel state (ON state) so as to set an optical path directed from the working path (from W) to the protective path (to E), and an optical switch element S 42 is brought into a parallel state (ON state) so as to set an optical path directed from the protective path (from E) to the working path (to W), while keeping the connected states of the counterclockwise optical path directed from the working path (from E) to the working path (to W) and the clockwise optical path directed from the protective path (from W) to the protective path (to E).

Then, as shown in the completion of the protection of FIG. 16C (path re-connection), the optical path from the protective path (from W) to the protective path (to E) is released by changing over an 2 2 optical switch element S 33 from an ON state to an OFF state, to thereby set an optical path directed from the working path (from W) to the protective path (to E). Simultaneously, the optical path directed from the working path (from E) to the working path (to W) is released by changing over an optical switch element S 22 from an ON state to an OFF state, to thereby set an optical path directed from the protective path (from E) to the working path (to W). In this way, the recovery from the fault is completed (i.e., the span switch is completed).

FIG. 17 is a graph showing a transition of optical power to be output from the protective path (to E) at the output side of the 4 4 spatial optical switch at the time of connection changeover as described above. As shown in FIG. 17 , the optical power to be output from the protective path (to E) is continuously changed over from the power of the PCA optical signal sp input to the protective path (from W) to the power of the client optical signal sc input to the working path (from W). Thus, there can be avoided an interruption at the time of connection changeover, which has been caused in the conventional.

In the above, there has been illustrated the constant interlace type 4 4 spatial optical switch as shown in FIG. 16 . However, the constitution of the optical node device to be used in a ring network is not limited thereto, and a matrix type or tree type constitution can be adopted. Further, there has been described the 4-fiber ring network having four units of nodes. However, the present invention is not limited to such a number of nodes and the configuration thereof in the optical network.