Abstract:
The present invention is a method for isolating faults in multi-stage photonic switch networks. Photonic switches in a photonic switch network are first verified individually, using self-test paths built into the photonic switch. Then, interconnecting optical fibers of the photonic switch network are checked. Each photonic switch is equipped with a transmitter and detector. During test, a photonic switch uses its transmitter to transmit light through an optical fiber interconnection to a second photonic switch. Pre-existing pathways within the photonic switches are used to access and route the light. When the second photonic switch detects the transmitted light, the optical fiber interconnection passes the continuity test. When the light cannot be detected, the optical fiber interconnection has a fault that must be repaired. By repeating this process for all optical fiber interconnections between all photonic switches, the photonic switch network can be checked for faults.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to photonic switches, and more particularly to a method for isolating faults in interconnections between photonic switches.  
           [0002]    Optical fibers are increasingly prevalent in the transmission lines of data networks, due to their higher bandwidth capabilities compared to wire transmission lines. Before the photonic switch was invented, light signals switching from one optical fiber to another first were converted to and from electrical impulses using optical-to-electrical-to-optical equipment. The conversion process was time-consuming and slowed the speed of data traveling in the network. The photonic switch provided a way to keep the data network completely optical and thus speed up data transfer rates.  
           [0003]    Many photonic switches are designed to be modular, so that several photonic switches can be connected together using optical fibers to create one larger photonic switch, hereinafter called a photonic switch network. The modularity of the photonic switches gives the customer the flexibility to make a photonic switch network as large or small as desired. The optical fibers in a photonic switch network have to be tested for continuity and proper operation. Typically, an optical fiber is tested by transmitting light through one end of the optical fiber, and checking for the light at the other end with a detector. When the light is detected, the optical fiber is working correctly. When no light is detected, a break in continuity—also known as a fault—exists within the optical fiber, and the optical fiber must either be fixed or replaced.  
           [0004]    In the past, testing the continuity of the interconnecting optical fibers in a photonic switch network was not a simple matter. The optical fibers are connected directly from the data output of one switch to the data input of another, making it difficult to access any of the test light signals. One prior art solution was to use an optical fiber with a light-dividing device, such as a tap or splitter, for each interconnection between photonic switches. A tap or splitter is an optical device that splits the original signal into two or more signals. These split-off signals may or may not differ from each other in signal strength, but are identical in data content. One of the split signals would lead to the normal data path, maintaining the data connection; another signal can be drawn off into a test system. There are drawbacks to this method. First, an optical fiber with a light-dividing device is more expensive than a plain optical fiber. When there are thousands of interconnections to be tested, the additional cost of the light-dividing devices can be quite high. Secondly, the light-dividing device itself can introduce faults into the photonic switch network. This makes it difficult to determine whether a fault lies in an optical fiber, or the associated light-dividing device. Finally, the power of each split-off signal is less than the original, which can cause problems during testing. If the split-off test signal from an optical fiber is too weak, the detector will be unable to detect it, and would instead indicate a fault in that particular optical fiber where none exists. This mistake can cause a flawless optical fiber to be needlessly replaced.  
           [0005]    Accordingly, there remains a need for an improved method for testing interconnecting optical fibers in a photonic switch network.  
         SUMMARY  
         [0006]    The present invention provides a simple and reliable method for isolating faults in interconnections between photonic switches. The photonic switches are first verified individually, using self-test paths built into every photonic switch. Once each individual photonic switch has been verified, the interconnecting optical fibers of the photonic switch network are checked. Each photonic switch is equipped with a transceiver consisting of a transmitter and a receiver. During test, a photonic switch uses its transmitter to transmit light through an optical fiber interconnection to a second photonic switch. Pre-existing pathways within the photonic switches are used to access and route the test light signals, thus eliminating the need for light-dividing devices altogether. When the second photonic switch detects the transmitted light with its receiver, the optical fiber interconnection passes the continuity test. When the second photonic switch cannot detect the light, the optical fiber interconnection has a fault that must be repaired. By repeating this process for all optical fiber interconnections between all photonic switches, the photonic switch network can be tested for proper operation.  
           [0007]    Further features of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.  
       
    
    
     DETAILED DESCRIPTION  
       [0008]    [0008]FIG. 1 illustrates a flowchart of the method used to isolate faults in a photonic switch network, made in accordance with the teachings of the present invention. A fault is defined as any problem that would prevent a signal from been routed correctly through the photonic switch network, such as a discontinuity within the waveguide, or an improperly functioning switch point. In step  100 , the individual photonic switches in the photonic switch network are verified to ensure there are no faults within. This verification is performed using self-test mechanisms built into every photonic switch. In step  110 , a test signal is transmitted from the first switch. The test signal may be, but is not limited to being, infrared light. Infrared light is defined as the region of the electromagnetic spectrum having wavelengths between 0.7 micrometer and 1 millimeter, inclusive. In step  120 , the test signal is routed through a redundant path of the first switch. The redundant path is a bypass path through the photonic switch in case one of the other paths should fail. In step  130 , the test signal is routed through an optical fiber connecting the first photonic switch to a second photonic switch. In step  140 , the test signal is routed through a redundant path of the second photonic switch. Finally, in step  150 , a detector at the second photonic switch checks for the test signal. If the test signal is detected, then there are no faults in the optical fiber. Otherwise, a fault exists and the optical fiber must be fixed or replaced.  
         [0009]    [0009]FIG. 2 is an example of a prior art photonic switch  1 , as described by U.S. Pat. No. 6,160,928 to Schroeder and U.S. Pat. No. 6,198,856 to Schroeder et al., both assigned to Agilent Technologies. The photonic switch  1  has a waveguide array  3 , a built-in transmitter  5 , and a built-in detector  7 . The waveguide array  3  has horizontal waveguides A, B, C, D, E, (also designated as inputs  2 , 4 , 6 , 8 , and  10 ) and vertical waveguides  11 ,  13 ,  15 ,  17 ,  19  (also designated as outputs  12 ,  14 ,  16 ,  18 ,  20 ). At the intersection of each waveguide is a switch point, as exemplified by reference number  9 . Each switch point  9  is uniquely identified in FIG. 2 by the combined reference numbers of its intersecting waveguides. For example, the switch point  9  at the intersection of horizontal waveguide A and vertical waveguide  15  is uniquely identified by the coordinate A 15 . The switch point  9  can either be inactive or active, at any given point in time. When the switch point  9  is inactive, a signal in a waveguide that intersects the switch point  9  passes straight through, unchanged. When the switch point  9  is active, the signal in the waveguide passing through the switch point  9  is deflected to an intersecting waveguide. For example, if switch point B 19  is inactive, an incoming signal  21  passing through switch point B 19  will continue straight through as signal  22 . If switch point B 19  is active, an incoming signal  21  will be deflected as signal  23 .  
         [0010]    [0010]FIG. 3 illustrates the same photonic switch  1  as shown in FIG. 2. Within the waveguide array  3 , a few waveguides are reserved for performing special functions. A test path  25 , comprising a vertical and horizontal waveguide, is designated for performing self-tests on the photonic switch  1 . In this example, the test path  25  consists of waveguide  11  and waveguide E. It is optically connected to the built-in transmitter  5  and to the built-in detector  7 . The signal emitted by the built-in transmitter  5  is intended for transmission of optical data, such as infrared light. Another set of waveguides—waveguide D and waveguide  13  in this example—form a redundant path  27 . The redundant path  27  is unused under normal circumstances. It is used as a backup path in case one of the other waveguides or switch points malfunction.  
         [0011]    The test path  25  is used to find faults within the waveguide array  3 . By selectively activating switch points along test path  25 , the waveguide array  3  can be tested for faults. For example, to verify the functionality of switch point C 15 , the switch points C 11  and E 15  on test path  25 , along with switch point C 15  itself, should be activated. As shown in FIG. 3, a signal  29  transmitted from the built-in transmitter  5  travels along test path  25 , reflects off the activated switch points back to the test path  25 , and finally ends at built-in detector  7 . When the built-in detector  7  detects the signal  29 , all the activated switch points are operating correctly, and switch point C 15  in particular has been verified. When the built-in detector  7  cannot detect the signal  29 , a fault must exist somewhere along the path. After methodically testing each waveguide and switch point  9  in this manner, the entire waveguide array  3  can be verified.  
         [0012]    [0012]FIG. 4 illustrates the same photonic switch  1  as shown in FIGS. 2 and 3, with a fault  31  located at switch point A 19 . The redundant path  27  is used as a detour when a problem exists somewhere in the waveguide array  3 . For instance, due to the location of fault  31 , switch point A 19  cannot be activated, and a signal cannot be directly deflected from horizontal waveguide A to vertical waveguide  19 . However, by using switch points along redundant path  27 , the fault  31  can be bypassed. When switch points A 13 , D 13 , and D 19  are activated, a signal  33  can still be routed from waveguide A to waveguide  19 , as illustrated in FIG. 4. The redundant path  27  is a useful feature that provides robustness to the waveguide array  3 .  
         [0013]    [0013]FIG. 5 depicts a preferred embodiment for testing a photonic switch network  41 , made in accordance with the teachings of the present invention. Stage  1  has a single photonic switch  1 A, stage  2  has a single photonic switch  1 B, and each photonic switch is illustrated with only the relevant waveguides visible. Photonic switch  1 A has a redundant path  27 A intersected by a waveguide M, and a switch point  9 A located at their intersection. A transmitter  45 A and a detector  47 A are optically connected to the redundant path  27 A through a self-test loop  49 A. The transmitter  45 A and detector  47 A may be separate components, or combined into a single transceiver  43 A. Photonic switch  1 B has a redundant path  27 B intersected by a waveguide N, and a switch point  9 B 1  located at their intersection. The redundant path  27 B intersects itself at switch point  9 B 2 . A transmitter  45 B and a detector  47 B are optically connected to the redundant path  27 B through a self-test loop  49 B. The transmitter  45 B and detector  47 B may be separate components, or combined into a single transceiver  43 B. An optical connection  45  connects the output of waveguide M to the input of waveguide N, and must be verified to ensure proper operation of the photonic switch network  41 .  
         [0014]    Before verifying optical connection  45 , each photonic switch must first check for faults within itself. Each photonic switch runs an internal self-test using its built-in transmitter  5 , built-in detector  7 , and test path  25 , as shown in FIG. 3. Each photonic switch also verifies its own self-test loop  49  by transmitting a test signal from its transmitter  45 . When the detector  47  detects the test signal, the self-test loop  49  is working correctly. These first two steps constitute step  100  of FIG. 1, eliminating the individual photonic switches as possible sources of faults.  
         [0015]    Finally, the interconnecting optical connection  45  can be tested, as described in steps  110  through  150  of FIG. 1. Referring back to FIG. 5, a test signal  51  is transmitted from the transmitter  45 A to redundant path  27 A. The test signal  51  is routed from the redundant path  27 A to waveguide M by activating switch point  9 A. The test signal  51  is output from waveguide M to the optical connection  45 . When the test signal  51  reaches photonic switch  1 B, it continues on to waveguide N. By activating switch points  9 B 1  and  9 B 2 , the test signal  51  is deflected onto the redundant path  27 B of photonic switch  1 B. It then travels through self-test loop  49 B, where it finally reaches the receiver. When the detector  47 B detects the test signal  51 , no faults exist in optical connection  45 . When the detector  47 B cannot detect the test signal  51 , a fault must lie in the optical connection  45 . By repeating this process for any optical connections that exist between any two photonic switches, the entire photonic switch network  41  can be verified.  
         [0016]    While FIG. 5 shows a photonic switch network  41  with only two stages, and only one photonic switch per stage, there are many other possible arrangements for interconnecting photonic switches. FIG. 6 depicts a few alternative arrangements for a photonic switch network  41 ′. A few examples are listed in Table 1.  
                   TABLE 1                       Alternative Arrangement   Example in FIG. 6                   More than one photonic switch per   Stage 2 has photonic switches 1D       stage   and 1E           Stage N has photonic switches 1F           and 1G       A single photonic switch connected   Photonic switch 1C is connected to       to more than one photonic switch   photonic switches 1D and 1E       More than two stages in a photonic   Stage N represents the last stage of       switch network   any number of stages greater than           two       A photonic switch connected to other   Photonic switch 1F is connected to       photonic switches within the same   photonic switch 1G within Stage N       stage                  
 
         [0017]    There are many other methods, not illustrated due to space considerations, for interconnecting the photonic switches. One arrangement is a multi-stage Clos, a method for networking switches well known in the art. The photonic switches can be connected to other photonic switches that are not in adjacent stages. The photonic switches do not have to be grouped into stages, either.