Abstract:
An optical path cross connect apparatus employs an economical 1×2 optical switches instead of expensive optical amplifiers, realizing an economical apparatus and suppressing dimensions of the apparatus, a dummy optical signal that realizes a reliable switching from a system in service to a standby system when a fault occurs in the system in service, and a switching method provides a method to replace an optical switch and to insert an optical amplifier, if required, while continuing communication services.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an optical path cross connect apparatus and a switching method thereof, and especially relates to the optical path cross connect apparatus having a redundant configuration, and the switching method thereof.  
           [0003]    2. Description of the Related Art  
           [0004]    With demands for a higher-speed data transmission and a larger volume data, networks and transmission systems need to be capable of handling a wide band, hence, a large capacity and high-speed transmission. To cope with the demands, an optical network based on WDM technology has been desired. The core of the optical network is an optical path cross connect apparatus that divides a wavelength-multiplexed optical signal input from a plurality of input optical fibers by wavelength, carries out cross connection of the divided optical signals, multiplexes the cross connected signals by wavelength, and outputs to output optical fibers.  
           [0005]    Since an optical transmission system handles a large volume of data, a failure in operation causes a massive influence to a large number of users. In this view, optical transmission systems are configured with redundancy such that reliability is enhanced.  
           [0006]    [0006]FIG. 1 shows a block diagram of an example of a conventional optical path cross connect apparatus with a redundant configuration. In this figure, k optical signals, each wavelength-multiplexed by n channels, are input through k optical fibers, that is, there are kxn optical signals. Each of the optical signals is divided into two streams by each of 1×2 optical couplers  10   11 - 10   kn . Each of the two streams is supplied to an OSW (optical matrix switch)  12 , which is a system  0  and in service, and OSW  13 , which is a system  1  and in standby. Each of the OSW  12  and the OSW  13  carries out cross connection. Output signals from the OSW  12  and the OSW  13  are monitored by monitoring units  14   11 - 14   kn  and  15   11 - 15   kn , respectively, such that a failure, if one occurs, is detected, 2×1 optical switches  16   11 - 16   kn  are controlled, and switching between the system  0  and the system  1  is carried out. Here, λ 0  in the figure expresses arbitrary wavelength.  
           [0007]    In the conventional optical path cross connect apparatus, each of the 1×2 optical couplers  10   11 - 10   kn  generates a principle loss of 3 dB, which is a burden to a system. To compensate the loss, insertion of an optical amplifier is needed either before each of the 1×2 optical couplers  10   11 - 10   kn , or after each of the 2×1 switches  16   11 - 16   kn , raising cost and increasing dimensions of the apparatus.  
           [0008]    Further, some matrix type OSWs (optical matrix switches) require an optical input always. In this case, switching from a system in service to a standby system, when a fault occurs, is not correctly performed.  
           [0009]    Furthermore, with the conventional optical path cross connect apparatus shown in FIG. 1, if insertion of an optical amplifier is needed, for example, due to increase in loss, etc., when switch capacity is to be increased, service has to be intercepted in order to insert the optical amplifier, that is, there is a problem of the optical path cross connect stopping communication services.  
         SUMMARY OF THE INVENTION  
         [0010]    It is a general object of the present invention to provide an optical path cross connect apparatus and a switching method thereof that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.  
           [0011]    The present invention made in view of the above-mentioned points aims at providing an optical path cross connect apparatus and a switching method thereof, which dispenses with an optical amplifier, prevents cost and size from increasing, secures continuous operation by a standby system when a failure occurs in a main system, and allows an in-service upgrading.  
           [0012]    Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by the optical path cross connect apparatus and the switching method thereof particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
           [0013]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a number of variations of an improved optical path cross connect apparatus, such as a variation where a low-loss optical switch is employed, dispensing with insertion of an optical amplifier, thereby cost and size of the apparatus are prevented from increasing; a dummy optical signal is applied such that correct switching to a standby system, hence continuous operation, is ensured; a method to replace an OSW (optical matrix switch) and to insert an optical amplifier, if required, while service continues by redundant components; and so on. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a block diagram of an example of a conventional optical path cross connect apparatus with a redundant configuration;  
         [0015]    [0015]FIG. 2 is a block diagram of the optical path cross connect apparatus with a redundant configuration of a first embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a block diagram of the optical path cross connect apparatus with a redundant configuration of the first embodiment of the present invention;  
         [0017]    [0017]FIG. 4 is a block diagram of the optical path cross connect apparatus with a redundant configuration of a second embodiment of the present invention;  
         [0018]    [0018]FIG. 5 is a block diagram of the optical path cross connect apparatus with a redundant configuration of the second embodiment of the present invention;  
         [0019]    [0019]FIG. 6 is a block diagram of a main part of the optical path cross connect apparatus with a redundant configuration of the third embodiment of the present invention;  
         [0020]    [0020]FIG. 7 is a block diagram of a main part of the optical path cross connect apparatus with a redundant configuration of the fourth embodiment of the present invention;  
         [0021]    [0021]FIG. 8 is a block diagram of a main part of the optical path cross connect apparatus with a redundant configuration of the fifth embodiment of the present invention;  
         [0022]    [0022]FIG. 9 is a block diagram of the optical path cross connect apparatus with a redundant configuration of the sixth embodiment of the present invention;  
         [0023]    [0023]FIG. 10 is a block diagram of the optical path cross connect apparatus with a redundant configuration of the sixth embodiment of the present invention;  
         [0024]    [0024]FIG. 11 is a block diagram of the optical path cross connect apparatus with a redundant configuration of the sixth embodiment of the present invention;  
         [0025]    [0025]FIG. 12 is a block diagram of a main part of the optical path cross connect apparatus with a redundant configuration of the seventh embodiment of the present invention;  
         [0026]    [0026]FIG. 13 is a block diagram of a main part of the optical path cross connect apparatus with a redundant configuration of the eighth embodiment of the present invention;  
         [0027]    [0027]FIG. 14 is a block diagram of a variation of an OSW used in the present invention;  
         [0028]    [0028]FIG. 15 is a block diagram of a WDM interface, to which the optical path cross connect apparatus with a redundant configuration of the present invention is applied;  
         [0029]    [0029]FIG. 16(A), FIG. 16(B), FIG. 16(C) and FIG. 16(D) are figures for explaining a first embodiment of a switching method of the optical path cross connect apparatus with a redundant configuration of the present invention;  
         [0030]    [0030]FIG. 17(A), FIG. 17(B), FIG. 17(C) and FIG. 17(D) are figures for explaining a second embodiment of the switching method of the optical path cross connect apparatus with a redundant configuration of the present invention; and  
         [0031]    [0031]FIG. 18(A), FIG. 18(B), FIG. 18(C) and FIG. 18(D) are figures for explaining a third embodiment of the switching method of the optical path cross connect apparatus with a redundant configuration of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    In the following, embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0033]    [0033]FIG. 2 and FIG. 3 show a block diagram of a first embodiment of an optical path cross connect apparatus with a redundant configuration of the present invention. In FIG. 2, each of k optical fibers (k=8, for example) supplies an optical signal carrying n signals (n=32, for example) by wavelength multiplexing. That is, a total of kxn signals are input to the optical path cross connect apparatus, each of the kxn signals being supplied to each of 1×2 switches  20   11 - 20   kn . Each of the 1×2 switches  20   11-   20   kn  divides the input signal into two branches in one of distribution ratios of 1:p (1&lt;p) and p:1, by control of a control unit  22 , and supplies each of the branched signals to each of OSW (optical matrix switch)  24 , as a serving system  0 , and OSW  25 , as a standby system  1 . Here, λ 0  in the figures expresses arbitrary wavelength.  
         [0034]    The 1×2 switches  20   11 - 20   kn  are configured by a semiconductor element such as a PLC that performs switching by locally heating an arm of a Mach-Zehnder interferometer structured with a substrate type waveguide, an LN that performs switching by applying an electric field to a directional optical coupler formed in an LiNbO 3  crystal, and a carrier injection type optical switch. A criterion of the 1, that is, the base coefficient of the above-mentioned distribution ratios 1:p and p:1 preferably represents a minimum optical power level that can be monitored by a monitoring unit in a later stage. The other coefficient p of the distribution ratios 1:p and p:1 is several tens to 100 times a large as 1. Usually, an optical signal of the distribution coefficient  1  is supplied to OSW  25 , the standy system  1 , and the optical signal of the distribution coefficient p is supplied to OSW 24 , the working system  0 . Here, as for the OSW  24  and the OSW  25 , MEMS (Micro Electro Mechanical System) is used, for example.  
         [0035]    Optical signals that are cross connected by the OSW  24  and the OSW  25  are supplied to 2×1 switches  26   11 - 26   kn , while being monitored by monitoring units  28   11 - 28   kn  and  29   11 - 29   kn , respectively. When the control unit  22  detects a failure, the control unit  22  causes the 1×2 switches  20   11 - 20   kn  and the 2×1 switches  26   11 - 26   kn  to change routing of the optical signals from the working system  0  to the standby system  1  in an interlocked manner.  
         [0036]    In normal operation, the 1×2 switches  20   11 - 20   kn  and the 2×1 switches  26   11 - 26   kn  are connected to the OSW 24 , the working system  0 , as indicated by a bold solid line in FIG. 2. If a failure is detected by any one of the monitoring units  28   11 - 28   kn , the control unit  22  controls such that the 1×2 switches  20   11 - 20   kn  and the 2×1 switches  26   11 - 26   kn  are connected to the OSW  25 , the standby system  1 , as a bold solid line in FIG. 3 shows.  
         [0037]    In this embodiment, a low loss device such as the 1×2 switches  20   11 - 20   kn  are used instead of 1×2 optical couplers that come with a 3 dB loss, thereby insertion of an optical amplifier to the optical path cross connect apparatus becomes unnecessary, and increase of cost and size are prevented.  
         [0038]    [0038]FIG. 4 and FIG. 5 show a block diagram of a second embodiment of the optical path cross connect apparatus with a redundant configuration of the present invention. Where the same components appear in these figures as FIG. 2, the same reference numbers are given, and explanations are omitted. The second embodiment employs 2×2 optical couplers  30   11 - 30   kn  instead of the 1×2 switches  20   11 - 20   kn .  
         [0039]    In FIG. 4, each of kxn input optical signals is supplied to a first input port of each of the 2×2 optical couplers  30   11 - 30   kn , and is monitored by each of monitoring units  32   11 - 32   kn . When any one of the input optical signals is not present, the monitoring units  32   11 - 32   kn  turn on dummy laser diodes (LD)  34   11 - 34   kn  that have an ON/OFF function, and supply dummy optical signals generated by the turned-on dummy laser diodes (LD)  34   11 - 34   kn  to a second input port of each of the 2×2 optical couplers  30   11 - 30   kn . That is, an optical signal is surely supplied to either of the input ports of the 2×2 optical couplers  30   11 - 30   kn . Each of the input optical signals is branched into two streams by the 2×2 optical couplers  30   11 - 30   kn , and one each of the two streams is supplied to the OSW  24 , the working system  0 , and the OSW  25 , the standby system  1 .  
         [0040]    The optical signals that are cross connected and output from the OSW  24  and the OSW  25  are supplied to the 2×1 switches  26   11 - 26   kn . Moreover, the signals output from the OSW  24  and the OSW  25  are monitored by the monitoring units  28   11 - 28   kn  and  29   11 - 29   kn , respectively. If a fault is detected by the control unit  22 , switching from the OSW  24  to the OSW  25  is performed by switching the 2×1 switches  26   11 - 26   kn .  
         [0041]    For example, if any of the monitoring units  28   11 - 28   kn  detects an absence of an optical signal during normal operation wherein the 2×1 switches  26   11 - 26   kn  are connected to the OSW  24  as a bold solid line shows in FIG. 4, the control unit  22  changes connection of the 2×1 switches  26   11 - 26   kn  to the OSW  25  as a bold solid line of FIG. 3 indicates.  
         [0042]    In this embodiment, an optical signal is always supplied to the 2×2 optical couplers  30   11 - 30   kn , and branched into two streams such that the optical signal is always supplied to the OSW  24  and the OSW  25 . In this manner, stable operation of OSW  24  and the OSW  25  is secured, even if the OSW  24  and the OSW  25  are matrix type switches.  
         [0043]    [0043]FIG. 6 shows a block diagram of a main part of the optical path cross connect apparatus with a redundant configuration of a third embodiment of the present invention. In FIG. 6, the same reference numbers are given to the same components as FIG. 4, and explanations thereof are omitted. The third embodiment uses dummy laser diodes (LD)  36   11 - 36   kn  that are always turned on, and gates  38   11 - 38   kn  that open and close according to an output from the monitoring units  32   11 - 32   kn , instead of the dummy laser diodes (LD)  34   11 - 34   kn  that have the ON/OFF function. Except for this point, the third embodiment is the same as the second embodiment shown in FIG. 4.  
         [0044]    In FIG. 6, when absence of an optical signal is detected concerning any one of the first input ports of the 2×2 optical couplers  30   11 - 30   kn , dummy optical signals generated by the dummy laser diodes (LD)  36   11 - 36   kn  are supplied to the second input ports of the 2×2 optical couplers  30   11 - 30   kn  through the gates  38   11 - 38   kn  that are opened by control of the monitoring units  32   11 - 32   kn .  
         [0045]    [0045]FIG. 7 shows a block diagram of a main part of a fourth embodiment of the optical path cross connect apparatus with a redundant configuration of the present invention. In FIG. 7, the same reference numbers are given to the same components as FIG. 6, and explanations thereof are omitted. This embodiment employs higher-output laser diodes, such as laser diodes having a  4  times as high output power as the dummy laser diode (LD)  36   11 , instead of the dummy laser diodes (LD)  36   11 - 36   kn . For example, dummy laser diodes (LD)  40   1-40   h  are capable of outputting an output 4 times as high as the dummy laser diodes (LD)  36   11 - 36   kn . The output is divided into 4 streams by 1×4 optical couplers  42   1 - 42   h , and provided to the gates  38   11 - 38   kn  that are controlled by the monitoring units  32   11 - 32   kn . Except for this point, the fourth embodiment is the same as the second embodiment shown in FIG. 4.  
         [0046]    In FIG. 7, when input optical signals are not present at the first input ports of the 2×2 optical couplers  30   11 - 30   kn , the gates  38   11 - 38   kn  are turned on by the monitoring units  32   11 - 32   kn , and the dummy optical signals from the 1×4 optical couplers  42   1-42   h  are supplied to the second input port of the 2×2 optical coupler  30   11 - 30   kn  through the turned-on gates.  
         [0047]    [0047]FIG. 8 shows a block diagram of a main part of a fifth embodiment of the optical path cross connect apparatus with a redundant configuration of the present invention. In FIG. 8, the same reference numbers are given to the same components as FIG. 7, and explanations thereof are omitted. Instead of the dummy laser diodes (LD)  40   1-40   h  of the higher output power, dummy laser diodes (LD)  44   1 - 44   h  that are capable of a lower power output and always turned on, and optical amplifiers  46   1 - 46   h  are employed in this embodiment. Outputs of the optical amplifiers  46   1 - 46   h  are branched into four streams by 1×4 optical couplers  42   1 - 42   h , and supplied to the gates  38   11 - 38   kn . Except for this point, the fifth embodiment is the same as the second embodiment shown in FIG. 4.  
         [0048]    [0048]FIG. 9, FIG. 10, and FIG. 11 show a block diagram of a sixth embodiment of the optical path cross connect apparatus with a redundant configuration of the present invention. In these figures, the same reference numbers are given to the same components as FIG. 2, and explanations thereof are omitted. In the sixth embodiment, 2×2 switches  50   11 - 50   kn  are used instead of the 1×2 switches  20   11 - 20   kn    
         [0049]    In FIG. 9, kxn input optical signals are supplied to first input ports of the 2×2 switches  50   11 - 50   kn . Further, dummy optical signal signals that dummy laser diodes (LD)  52   11 - 52   kn  that are always turned on output are supplied to second input ports of the 2×2 switches  50   11 - 50   kn  2×2.  
         [0050]    The first input ports of the 2×2 switches  50   11 - 50   kn  are monitored by monitoring units  54   11 - 54   kn , and the monitored signals are supplied to a control unit  56 . Under normal operation, the 2×2 switches  50   11 - 50   kn  supply the input optical signals supplied to the first input ports to the OSW  24 , the working system  0 , by control of the control unit  56 , and supply the dummy optical signals to the OSW  25 , the standby system  1 . When an abnormality is present, the dummy optical signals supplied to the second input ports are switched to the OSW  24 , the working system  0 , and the optical signals supplied to the first input ports are switched to the OSW  25 , the standby system  1 .  
         [0051]    The optical signals cross connected by the OSW  24  and the OSW  25  are supplied to the 2×1 switches  26   11 - 26   kn . Further, the output signals of the OSW  24  and the OSW  25  are monitored by the monitoring units  28   11 - 28   kn  and  29   11 - 29   kn , respectively, and supplied to the control unit  56 . The control unit  56  is performs switching of the OSW  24  and the OSW  25  by switching the 2×1 switches  26   11 - 26   kn  and the 2×2 switches  50   11 - 50   kn , when a fault is detected by the signals supplied from the monitoring units  28   11 - 28   kn ,  29   11 - 29   kn , and  54   11 - 54   kn .  
         [0052]    If a fault is detected by any one of the monitoring units  28   11 - 28   kn  during normal operation, that is, while the input optical signals provided to the first input ports of the 2×2 switches  50   11 - 50   kn  are supplied to the OSW  24 , and the dummy optical signals provided to the second input ports of the switches are supplied to the OSW  25 , as two bold solid lines show in FIG. 9, the control unit  22  switches such that the 2×1 switches  26   11 - 26   kn  output signals from the OSW  25 , and the input optical signals to the first input ports of the 2×2 switches  50   11 - 50   kn  are provided to the OSW  25 , and the dummy optical signals provided to the second input ports of the 2×2 switches  50   11 - 50   kn  are provided to the OSW  24  as shown in FIG. 10.  
         [0053]    Further, if absence of an optical signal is detected by a monitoring unit, for example, if the monitoring unit  54   11  detects absence of an optical signal to the first input port of the 2×2 switch  50   11  under the normal operating condition as described above, the control unit  22  switches such that the dummy optical signals provided to the second input ports of the 2×2 switches  50   11 - 50   kn  are supplied to the OSW  24 , as a bold solid line shows in FIG. 11, while providing the OSW  25  with the optical signals provided to the first input ports. In this manner, stable operation of the OSW  24  is assured, when an optical signal returns to the first input port of the 2×2 switch  50   11 , and the 2×2 switches  50   11 - 50   kn  are also resumed to the status shown in FIG. 9.  
         [0054]    [0054]FIG. 12 shows a block diagram of a main part of a seventh embodiment of the optical path cross connect apparatus with a redundant configuration of the present invention. In FIG. 12, the same reference numbers are given to the same components as FIG. 9, and explanations thereof are omitted. This embodiment employs high-power laser diodes  70   1 - 70   h  that are capable of outputting, for example, 4 times as high output power as a dummy laser diode (LD)  52   11 , instead of the dummy laser diodes (LD)  52   11 - 52   kn . The high-power laser diodes  70   1 - 70   h  are always turned on, and generate dummy optical signals, each of which is branched into four streams by 1×4 optical couplers  72   1-72   h . The dummy optical signals output from the 1×4 optical couplers  72   1 - 72   h  are supplied to the second input ports of the 2×2 switches  50   11 - 50   kn . Except for this point, other composition is the same as the sixth embodiment shown in FIG. 9.  
         [0055]    [0055]FIG. 13 shows a block diagram of a main part of an eighth embodiment of the optical path cross connect apparatus with a redundant configuration of the present invention. In FIG. 13, the same reference numbers are given to the same components as FIG. 12, and explanations thereof are omitted. In the eighth embodiment, instead of the high-power dummy laser diodes  70   1 - 70   h , dummy laser diodes  74   1 - 74   h  that are always turned on and optical amplifiers  76   1 - 76   h  are employed. Outputs from the optical amplifiers  76   1 - 76   h  are provided to the second input ports of the 2×2 switches  50   11 - 50   kn . Except for this point, other compositions are the same as the sixth embodiment shown in FIG. 9.  
         [0056]    [0056]FIG. 14 shows a block diagram of a variation of the OSW used in the present invention. An OSW  78  that cross connects 256×256 waves includes OSW  79 , OSW  80 , OSW  81  and OSW  82 , arranged into a two-step configuration, and each of which being capable of cross connecting 128×128 waves. A multi-step configuration, such as this, enables relatively small OSWs to structure a relatively large OSW.  
         [0057]    [0057]FIG. 15 shows a block diagram of a WDM interface to which the optical path cross connect apparatus with a redundant configuration of the present invention is applied. In FIG. 15, each of k optical fibers (k=8, for example) provides an optical signal that includes n optical signals (n=32, for example) that are wavelength multiplexed to each of optical dividers  84   1 - 84   k . Thus, there are kxn (8×32=256, in this example) optical signals that are supplied to an optical path cross connect apparatus (OXC)  86 . The kxn optical signals are cross connected, and supplied to fixed wavelength converters  88   11 - 88   kn  that convert the supplied optical signals into predetermined wavelength, and output to adders  89   1 - 89   k . The adders  89   1 - 89   k  assemble the output signals into k WDM signals, and output to k optical fibers.  
         [0058]    In order to facilitate path tracing, a direct modulation or an indirect modulation may be applied to each of the dummy laser diodes (LD)  34   11 - 34   kn ,  36   11 - 36   kn ,  40   1 - 40   h ,  44   1-44   h ,  52   11 - 52   kn ,  70   1 - 70   h , and  74   1 - 74   h . In this manner, identifying an input port, optical signal of which has an abnormality, is facilitated.  
         [0059]    As described above, this embodiment enables to reduce loss in the entire apparatus and to suppress increases in cost and dimensions of the apparatus. In addition, operation of an OSW that requires a constant supply of an optical signal is stabilized.  
         [0060]    Following embodiments relate to a switching method that realizes an in-service modification of an optical path cross connect apparatus. Conventionally, when insertion of an optical amplifier is needed due to increase in loss, etc., for example, in making switch capacity increase, a conventional optical path cross connect apparatus as shown in FIG. 1 has to stop service during insertion of the optical amplifier and upgrading. This problem is solved by following embodiments of the switching method.  
         [0061]    [0061]FIG. 16(A), FIG. 16(B), FIG. 16(C), and FIG. 16(D) show figures for explaining a first embodiment of the switching method of the present invention, relative to an optical path cross connect apparatus with a redundant configuration. This embodiment applies to the case where an in-service upgrading is performed, accompanied with insertion of an optical amplifier on an input side of an OSW system  0  that is in service.  
         [0062]    As shown in FIG. 16(A), each of wavelength-multiplexed optical signals supplied by k optical fibers is divided into n signals based on wavelength, resulting in kxn optical signals. The kxn optical signals are supplied to 1×2 switches  100   11 - 100   kn . First output ports of the 1×2 switches  100   11 - 100   kn  supply the input optical signals to 2×2 optical couplers  110   11 - 110   kn  during normal operation. The 2×2 optical couplers  110   11 - 110   kn  divide the input optical signals into two streams, and supplies one of the streams to the system  0  OSW  112 , which is in service, and the other of the streams to a system  1  OSW  113 , a standby system. The optical signals are cross connected by the OSW  112  and OSW  113 , and then supplied to 2×1 switches  116   11 - 116   kn . The 2×1 switches  116   11 - 116   kn  select signals from the system  0  OSW  112 , the system in service, during the normal operation.  
         [0063]    In order to upgrade the apparatus, in the first place, the 2×1 switches  116   11 - 116   kn  are switched to receive the optical signals from the system  1  OSW  113 , the standby system. Then, the system  0  OSW  112  is removed as shown in FIG. 16(B).  
         [0064]    Next, as shown in FIG. 16(C), while the OSW  112  is replaced and upgraded, optical amplifiers  102   11 - 102   kn  are inserted between second output ports of the 1×2 switches  100   11 - 100   kn  and the 2×2 optical couplers  110   11 - 110   kn .  
         [0065]    Then, as shown in FIG. 16(D), the 2×1 switches  116   11 - 116   kn  are switched to receive the optical signals from the replaced system  0  OSW  112 , and, in this manner, the in-service upgrade is completed.  
         [0066]    [0066]FIG. 17(A), FIG. 17(B), FIG. 17(C), and FIG. 17(D) show figures for explaining a second embodiment of the switching method of the present invention, applicable to an optical path cross connect apparatus with a redundant configuration. This embodiment shows the case where optical amplifiers are inserted on an output side of a system  0  OSW that is in service, in connection with an in-service upgrade.  
         [0067]    As shown in FIG. 17(A), each of wavelength-multiplexed optical signals supplied by k optical fibers is divided into n signals based on wavelength, resulting in kxn optical signals. The kxn optical signals are supplied to 1×2 optical couplers  118   11 - 118   kn . The 1×2 optical couplers  118   11 - 118   kn  divide the input optical signals into two streams, and supply one stream to the system  0  OSW  112 , a system in service, and the other stream to the OSW  113 , a standby system  1 . The OSW  112  and the OSW  113  cross connect the optical signals, and supply outputs to first input ports and second input ports of 2×2 switches  120   11 - 120   kn , respectively.  
         [0068]    First output ports of the 2×2 switches  120   11 - 120   kn  output the optical signals from the OSW  112 , while second output ports outputting the optical signal from the OSW  113  during normal operation. The both output ports are connected to two input ports of 2×1 switches  116   11 - 116   kn . The 2×1 switches  116   11 - 116   kn  select and output the optical signal from the OSW  112  during the normal operation.  
         [0069]    In order to upgrade the apparatus, in the first place, the 2×2 switches  120   11 - 120   kn  are switched such that the optical signals from the OSW  112  are output from the second output ports, while the optical signals from the OSW  113  are output from the first output ports. The, the OSW  112  is removed as shown in FIG. 17(B),  
         [0070]    Next, as shown in FIG. 17(C), while exchanging and upgrading the OSW  112 , optical amplifiers  122   11 - 122   kn  are inserted between the second output port of the 2×2 switches  120   11 - 120   kn  and the 2×1 switches  116   11 - 116   kn ,  
         [0071]    Then, as shown in FIG. 17(D), the 2×1 switches  116   11 - 116   kn  are switched such that the optical signal from the OSW  112  are selected, and, in this manner, the in-service upgrade is completed.  
         [0072]    [0072]FIG. 18(A), FIG. 18(B), FIG. 18(C), and FIG. 18(D) show figures for explaining a third embodiment of the switching method of the present invention, relative to an optical path cross connect apparatus with a redundant configuration. This embodiment shows the case where optical amplifiers are inserted on both input and output sides of a system  0 , an OSW in service, in connection with an in-service upgrade.  
         [0073]    As shown in FIG. 18(A), each of wavelength-multiplexed optical signals supplied by k optical fibers is divided into n signals based on wavelength, resulting in kxn optical signals. The kxn optical signals are supplied to 1×2 switches  100   11 - 100   kn . The optical signals input to the 1×2 switches  100   11 - 100   kn  are output from first output ports of the 1×2 switches  100   11 - 100   kn  to 2×2 optical couplers  110   11 - 110   kn  during normal operation. The 2×2 optical couplers  110   11 - 110   kn  divide the input optical signals into two streams, one of which is supplied to a system  0  OSW  112 , a system in service, with the other stream being supplied to a system  1  OSW  113 , a standby system  1 . The optical signals are cross connected by the OSW  112  and the OSW  113 , and supplied to first and second input ports, respectively, of 2×2 switches  120   11 - 120   kn .  
         [0074]    The 2×2 switches  120   11 - 120   kn  output the optical signal supplied from the system  0  OSW  112  from first output ports during the normal operation, while outputting the optical signals from the system  1  OSW  113  from second output ports. Each of the output signals is supplied to each of two input ports of 2×1 switches  116   11 - 116   kn . The 2×1 switches  116   11 - 116   kn  select and output the optical signal from OSW  112  during the normal operation.  
         [0075]    In order to upgrade the apparatus while in service, at the first instance, the 2×2 switches  120   11 - 120   kn  are controlled such that the optical signal from the system  0  OSW  112  are output from the second output ports, and the optical signal from the system  1  OSW  113  are outputted from the first output ports. Then, the system  0  OSW  112  is removed as shown in FIG. 18(B).  
         [0076]    Next, as shown in FIG. 18(C), while exchanging and upgrading the system  0  OSW  112 , optical amplifiers  102   11 - 102   kn  are inserted between the second output ports of the 1×2 switch  100   11 - 100   kn  and the 2×2 optical coupler  110   11 - 110   kn . Further, optical amplifiers  122   11 - 122   kn  are inserted between the second output ports of the 2×2 switch  120   11 - 120   kn , and the 2×1 switches  116   11 - 116   kn .  
         [0077]    Then, as shown in FIG. 18(D), 2×1 switch  116   11 - 116   kn  are switches such that the optical signals from the system  0  OSW  112  are selected, and, in this manner, the in-service upgrade is completed.  
         [0078]    Thus, this embodiment realizes upgrading that includes insertion of optical amplifiers without stopping operation of optical path cross connection. By not installing the optical amplifiers directly to an OSW, the number of the optical amplifiers is halved. While a 1×2 switch and the like are installed to each channel, an overall cost is suppressed, because the optical amplifiers are more expensive than the 1×2 switches. Dimension of an optical path cross connect apparatus is also suppressed, according to this embodiment.  
         [0079]    It is remarked that each of the 1×2 switches  20   11 -20 kn  and  100   11 - 100   kn  corresponds to a 1×2 optical switch described in a claim, each of the OSW  24 , and the OSW  112  corresponds to the optical switch of a system in service and each of the OSW  25  and the OSW  113  corresponds to the optical switch of a reserve system in a claim. Further, each of the 2×2 optical couplers  30   11 - 30   kn  and  110   11 - 110   kn  corresponds to a 2×2 optical coupler, and each of the 2×2 switches  50   11 - 50   kn  and  120   11 - 120   kn  corresponds to a 2×2 optical switch in a claim. Each of the monitoring units  28   11 - 28   kn  and  29   11 - 29   kn  corresponds to the first monitoring units in a claim, and the control unit  22  corresponds to a control unit in a claim. Each of the monitoring units  32   11 - 32   kn  corresponds to the second monitoring unit, each of the monitoring units  54   11 - 54   kn  corresponds to the third monitoring unit in a claim. Each of the 2×1 switches  116   11 - 116   kn  corresponds to a 2×1 optical switch, and each of the 1×2 optical couplers  118   11 - 118   kn  corresponds to a 2×2 optical coupler in a claim.  
         [0080]    As mentioned above, according to the present invention, insertion of an optical amplifier is dispensed with by using a low loss 1×2 or 2×2 optical switch, and increase of cost and dimensions of an optical path cross connect apparatus can be suppressed.  
         [0081]    Further, continuous cross connect operation is realized when a fault occurs in a system in service by automatically switching from the system in service to a standby system. Providing a dummy optical signal to a standby system ensures a smooth switching.  
         [0082]    The present invention further provides a method to upgrade the optical path cross connect apparatus, which may include insertion of an optical amplifier, without stopping service.  
         [0083]    Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.  
         [0084]    The present application is based on Japanese priority application No. 2001-396247 filed on Dec. 27, 2001 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.