Abstract:
An apparatus and a method for controlling transmissions over an optical signaling network include one or more traffic detectors that are connected to an optical switch to receive incidentally reflected light when the switch is in a transmissive state. The switch is in the transmissive state when a fluid fills a fluid-manipulable chamber, but is in a reflective state when the chamber is free of fluid. The apparatus includes two inputs and two outputs. Typically, the inputs and outputs are waveguides having an index of refraction which approximates the index of refraction of the fluid. The traffic detectors monitor light which is incidentally reflected at interfaces of the waveguides with the chamber. Under fault-free conditions in which signal transmissions are detected to be normal, a first input is connected to a first output and a second input is connected to a second output. However, when a fault condition is detected, the two inputs exchange outputs by reversing the state of the optical switch. In the preferred embodiment, the fault-free condition is one in which the switch is in the reflective state. However, the reflective state is disabled and the transmissive state is established if a fault condition is detected. A prolonged absence of signal transmissions along one of the inputs is indicative of a fault condition. With the switch in the transmissive state, incidentally reflected light is monitored to detect a return to the fault-free condition. The outputs are again exchanged upon recognition of the fault-free condition. Optionally, the outputs are monitored to detect reverse driven “request for resumption” signals, and the reflective state is established if reverse driven signals are detected on both outputs, regardless of the indication of a fault along one or both of the inputs. In a second embodiment, the fault-free condition establishes a transmissive state, with incidentally detected light being monitored to determine if a fault condition requires a change to the reflective state.

Description:
TECHNICAL FIELD 
     The present invention relates generally to fiberoptic communications networks and, more specifically, to a method and an apparatus for controlling transmissions over fiberoptic networks. 
     BACKGROUND ART 
     While signals within telecommunications and data communications networks have traditionally been exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulated laser-generated light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals are often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network. 
     Recently, alternate optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. An isolated first switching element  110  is shown in FIG.  1 . The optical switch of FIG. 1 is formed on a substrate. The substrate may be silicon, but other materials may be used. The silicon substrate includes planar waveguides defined by a lower cladding layer  114 , a core  116 , and an upper cladding layer (not shown). The core material is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers should be formed of a material having a refractive index that is sub-stantially different from the refractive index of the core material, so that optical signals are guided along the waveguides. 
     The core  116  is patterned to form an input waveguide  120  and an output waveguide  126  of a first optical path and to define a second input waveguide  124  and a second output waveguide  122  of a second optical path. The upper cladding layer is then deposited over the patterned core material. A chamber  128  is formed by etching a trench through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the location  130  aligned with the waveguides is filled with vapor or gas. Thus, TIR diverts light (A) from the input waveguide  120  to the output waveguide  122  to exit as light (B), forming route (A→B). When an index-matching fluid resides within the location  130  between the aligned waveguides  120  and  126 , the light (A) propagates through the trench  128  to exit the switching element as light (D), forming route (A→D). The trench  128  is positioned with respect to the four waveguides such that one sidewall of the trench passes through the intersection of the axes of the waveguides. 
     The above-identified patent to Fouquet et al. describes a number of alternative approaches to alternating the first switching element between a transmissive state and a reflective state. One approach is illustrated in FIG.  1 . The first switching element  110  includes two microheaters  150  and  152  that control the position of a bubble within the fluid-containing chamber  128 . The fluid within the chamber has a refractive index that is close to the refractive index of the core material  116  of the four waveguides  120 - 126 . Fluid fill-holes  154  and  156  may be used to provide a steady supply of fluid, but this is not critical. In the operation of the first switching element, one of the heaters  150  and  152  is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In FIG. 1, the bubble is positioned at the location  130  of the intersection of the four waveguides. Consequently, an input signal along the waveguide  120  will encounter a refractive index mismatch upon reaching the chamber  128 . This places the first switching element in a reflecting state, causing the optical signal along the waveguide  120  to be redirected to the output waveguide  122 . However, even in the reflecting state, the second input waveguide  124  is not in communication with the output waveguide  126 . 
     If the heater  150  at location  130  is deactivated and the second heater  152  is activated, the bubble will be attracted to the off-axis heater  152 . This allows index-matching fluid to fill the location  130  at the intersection of the waveguides  120 - 126 . The first switching element  110  is then in a transmissive state, since the input waveguide  120  is optically coupled to the collinear waveguide  126 . 
     A concern with optical switching elements of the type shown in FIG. 1 is that in the transmissive state, a small amount of light is reflected from the switch. Reflection during the transmissive state is a result of the less than precise match between the indices of refraction of the fluid filling the chamber  128  and the waveguide core material  116 . A precise match between the indices is difficult to accomplish because multiple considerations impact the selection of a fluid. For example, because the fluid is manipulated using thermal energy, the thermal properties of the fluid must also be considered. As the mismatch between the refractive index of the fluid and the refractive index of the core material increases, the portion of an optical signal that is reflected at the switch increases. Currently, the incidentally reflected light is not beneficially utilized in switching systems. On the contrary, if the incidentally reflected light leaks into an adjacent waveguide, the result can be undesirable crosstalk. 
     What is needed is an apparatus and a method for advantageously utilizing light that is incidentally reflected from a fluid-manipulable optical switch when the switch is in a transmissive state. What is further needed is such an apparatus and method that introduce little or no crosstalk. 
     SUMMARY OF THE INVENTION 
     An apparatus and a method for controlling transmissions over an optical network include at least one traffic detector connected to an optical switch to receive incidentally reflected light when the switch is in a transmissive state. By utilizing the detection of incidentally reflected light, the traffic detector non-intrusively monitors the conditions of signal transmission capabilities to the switch. Depending upon detection of a fault condition, the optical switch is set in a reflective state or a transmissive state. 
     The apparatus includes a fluid-manipulable chamber, two inputs and two outputs. The inputs and outputs are waveguides that channel optical signals to and from the fluid-manipulable chamber. Under fault-free conditions in which signal transmissions are detected to be normal, a first input is connected to a first output, while a second input is connected to a second output. However, when a fault condition is detected, the two inputs exchange outputs by reversing the fluid-manipulable chamber with respect to its ability to propagate optical signals through the chamber. 
     In a preferred embodiment, the fault-free condition is one in which the fluid-manipulable chamber is in the reflective state. In this state, there is an absence of fluid at the interfaces of the inputs with the walls of the chamber. The resulting mismatch of indices of refraction at the interfaces causes optical signals to be reflected. In this preferred embodiment, the reflected signals from the first input are channeled to the first output, and the reflected signals from the second input are channeled to the second output. At least one of the two outputs is coupled to a traffic detector that monitors traffic along the output. Preferably, each output includes a traffic detector. If one of the traffic detectors recognizes an unexpected absence of signal transmissions, the absence is interpreted as an indication of a fault along the corresponding input. In conventional SONET ring applications, the medium (e.g., a first fiberoptic line) that provides the input for one of the transmission paths is laced or otherwise physically coupled to the medium (e.g., a second fiberoptic line) that forms the output for the other transmission path. Consequently, if the input of the first transmission path is not propagating signals, it is presumed that the output of the second transmission path is also faulty. Likewise, a fault along the second input is likely to coincide with a fault along the first output. For this reason, the fault condition triggers the transmissive state of the fluid-manipulable chamber, thereby coupling the non-faulty input to the non-faulty output. 
     In the transmissive state, the fluid-manipulable chamber is filled with a fluid that has a refractive index that closely matches the refractive index of the light-carrying core material of the inputs and outputs. However, since there will be some mismatch in the indices of refraction, a small amount of each optical signal that is transmitted through the chamber will be reflected at the walls of the chamber. This “incidentally reflected light” is used by the traffic detector or detectors to non-intrusively monitor the condition of the input which was determined to be faulty. When the appropriate traffic detector determines that signals are again flowing through the previously faulty input, the chamber is returned to its normal reflective state. 
     In an alternative embodiment, the transmissive state is the normal condition. The first input is aligned with the first output on opposite sides of the chamber. Similarly, the second input is optically aligned with the second output, so that the two are optically coupled when the fluid-manipulable chamber contains index matching fluid. In this embodiment, the traffic detector or detectors operate in the normal condition to monitor leakage light that is incidentally reflected as a result of a mismatch in the indices of refraction of the fluid and the core material through which optical signals are guided. An unexplained absence of incidentally reflected light is interpreted as an indication of a fault condition at a location upstream from the switch. It is also assumed that the output that is laced or otherwise physically coupled to the faulty input is malfunctioning. In order to minimize signal loss, the fluid-manipulable chamber is switched from the transmissive state to the reflective state, thereby coupling the non-faulty input to the non-faulty output. 
     While the fluid-manipulable chamber is in the reflective state, traffic detectors are used to determine when traffic is again being propagated along the input that was determined to be faulty. The transmissive state is re-established in response to determining that both inputs are functioning properly. 
     An advantage of the invention is that incidentally reflected light is utilized to monitor traffic within an optical network, such as a fiberoptic network. Thus, the monitoring occurs non-intrusively. Another advantage is that the detection of a fault condition automatically reroutes traffic, so as to avoid further signal loss. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of prior art optical switch. 
     FIG. 2 is a schematic diagram of a preferred embodiment of an optical switch according to the present invention. 
     FIG. 3 is a schematic diagram of the switch of FIG. 2 operating in a transmissive state after a fault condition has been detected. 
     FIG. 4 is a schematic diagram of the switch of FIG. 2 after signal transmission has just resumed over a previously malfunctioning line. 
     FIG. 5 is a schematic diagram of the switch of FIG.  2 ( a ) performing a verification of the fault condition and ( b ) performing an additional diagnosis. 
     FIG. 6 is a process flow of a method for monitoring and controlling signal transmissions utilizing the switch shown in FIG.  2 . 
     FIG. 7 is an alternative embodiment of the switch of FIG. 2 in a transmissive state. 
     FIG. 8 is a schematic diagram of the switch of FIG. 7 in a reflective state after detection of a fault condition. 
     FIG. 9 is a schematic diagram of the switch of FIG. 7 when signal transmission has just resumed on a previously malfunctioning line. 
     FIG. 10 is a schematic diagram of the switch of FIG. 9 as reverse driven “request for resumption” signals are being received over a line that was presumed to be malfunctioning. 
     FIG. 11 is a schematic diagram of the fiberoptic switch of FIG. 7 in a transmissive state at rest in response to a determination that both lines are malfunctioning. 
     FIG. 12 is a process flow of a method for operating the switch of FIG.  7 . 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 2, a fiberoptic network, such as a SONET ring network, includes a fluid-manipulable optical switch  10  for monitoring and controlling signal transmissions over the network. In a preferred embodiment, the SONET ring network includes a first transmission route  13  (here right directed) and a second transmission route  15  (here left directed). Each route carries traffic uni-directionally, but the two routes co-operate to provide bi-directional communication. 
     The first conventional transmission route  13  (i.e., route A→B) includes a first input waveguide  17  connected to a fluid-manipulable chamber  32  of the optical switch  10 . During a reflective state of the optical switch  10 , optical signals (A) received over the first input waveguide  17  are reflected at the interface of the waveguide with the chamber  32 . The reflected energy is directed onto a first intermediate waveguide  25  of the first transmission route  13 . The first intermediate waveguide  25  shares a common connection to the chamber  32  with a first output waveguide  19 . Thus, optical signals (A) received over the first input waveguide exit as light (B) via the first output waveguide  19  when the switch is in a reflective state. 
     The second transmission route  15  (i.e., route C→D) includes a second input waveguide  21  connected to the fluid-manipulable chamber  32 . A second intermediate waveguide  27  shares a common connection to the chamber  32  with the second input waveguide  21 , so that optical signals (C) received over the second input waveguide are reflected at the chamber  32  onto the second intermediate waveguide  27 . The second intermediate waveguide shares a common connection to the chamber  32  with a second output waveguide  23 , so that during the reflective state optical signals (C) received over the second input waveguide  21  are reflected onto the second output waveguide  23  for exit as light (D). 
     A switch controller  46  is provided to control the operating state of the optical switch  10  by selectively enabling the reflective state and a transmissive state of the switch. First and second traffic detectors  41  and  43  monitor signal transmissions along the first route  13  (A→B) and along the second route  15  (C→D) in order to determine whether the transmission routes are functioning properly. For example, when the switch is in the reflective state, the first traffic detector  41  detects clockwise signal transmissions (A) received over the first input waveguide. The presence of clockwise traffic indicates that connectivity with the source of optical signals (A) for the first transmission route is functioning. 
     The function of the first and second traffic detectors  41  and  43  are preferably performed by evanescent couplers. In a preferred embodiment, the evanescent couplers are a bi-directionally coupled pair of detectors which are capable of distinguishing the direction in which monitored traffic (A→B and C→D) is traveling. If the optical switch  10  is located near by a SONET receiver (not shown), it is likely that a receiver will employ a traffic detector which can perform the function of at least one of the traffic detectors of the switch. If the switch  10  is positioned between two closely located SONET receivers, the functions of both the first and the second traffic detectors  41  and  43  can be performed at the SONET receivers. 
     When the first traffic detector  41  observes an unanticipated absence of incoming traffic, the absence is interpreted as being indicative of a malfunction in the upstream source of signals (A) to the first transmission route  13 . Furthermore, if optical fibers of the first and second transmission routes  13  and  15  are closely packaged together, as indicated by groupings  44  and  47 , it is likely that a downstream portion (D) of the second transmission route is also malfunctioning. For example, an optical fiber of the first transmission route  13  may be severed while a trench is dug near fiberoptic lines that connect to the first input waveguide  17  and the second output waveguide  23 . It is likely that the optical fiber connected to conduct signals (D) from the second output waveguide  23  has also been damaged. Consequently, it is undesirable to allow optical signals (C) to be propagated over the second transmission path  15  (C→D), which includes the second output waveguide  23 . Similarly, if the second traffic detector  43  observes an absence of traffic (C) over the second transmission route during the reflective state of the switch  10 , it is likely that optic fibers connected to the second input waveguide  21  and the first output waveguide  19  have been damaged. Thus, it is desirable to avoid transmitting optical signals (B) over the first output waveguide. 
     Referring to FIGS. 2 and 3, in response to either the first  41  or the second  43  traffic detector (but not both) detecting an absence of incoming traffic (A or C) over the first  13  or the second  15  transmission route, the switch controller  46  disables the normal reflective state and enables the substitute transmissive state of the switch. When the substitute transmissive state of the switch  10  is enabled, fluid fills a first region of the fluid-manipulable chamber  32 . The fluid has an index of refraction which approximately matches the refractive index of the waveguide material. Presuming that the second traffic detector signals a (C) input fault condition, when the fluid fills the first region of the chamber  32 , the first input waveguide  17  and the second output waveguide  23  are optically coupled to form a first transmission path  36  through the chamber  32 . By enabling the transmissive state, traffic (A) is rerouted from the first route  13  to a first substitute transmission route  29  (A→D), which is formed by the first input waveguide, the second output waveguide and the fluid in the chamber. In this manner the switch avoids transmitting optical signals over the potentially faulty external line connected to the first output waveguide  19 . Similarly, if the first traffic detector  41  observes an absence of traffic (A) over the first input waveguide  17 , the switch controller  46  disables the reflective state and enables the substitute transmissive state to redirect traffic (C) from the normal second route  15  to a second substitute transmission route  31  (C→B) to avoid transmifting optical signals over the external line connected to the second output waveguide. 
     The first and second traffic detectors  41  and  43  can be utilized to verify that the substitute transmissive state of the switch  10  is functioning properly. For example, if an undesired bubble forms within the first transmission path  36  at the interface of the first input waveguide  17  and the chamber  32  during the transmissive state, optical signals (A) received over the first input waveguide  17  will be reflected onto the first intermediate waveguide  25 . During proper operation of the transmissive state, the intensity of leakage light incidentally reflected onto the first output waveguide is low in comparison to the intensity of the optical signals. If the undesired bubble has formed in the chamber  32  during the transmissive state, the first traffic detector  41  recognizes that the intensity of light received over the first intermediate waveguide  25  is high and is not consistent with proper functioning of the transmissive state, and an alarm condition is triggered. In an alternative embodiment, external traffic detectors (not shown) can be located on the first output waveguide  19  and the second output waveguide  23  to monitor for proper operation of the transmissive state. For instance, an external traffic monitor on the second output waveguide  23  can detect a bubble as it forms within the first transmission path  36 , because the enlarging bubble blocks propagation of light (A→D) through the switch onto the second output waveguide  23 . 
     When the switch is operating in the substitute transmissive state because the second traffic detector  43  has observed an absence of traffic received over the second input waveguide  21 , the second traffic detector monitors the second input waveguide  21  for incidentally reflected light to determine whether the second input waveguide has returned to an operative state. Referring to FIGS. 2 and 4, if signal propagation (C) resumes over the second input waveguide  21 , the optical signals (C) traverse the fluid-manipulable chamber  32  to form a second transmission path  34  through the chamber. If the index of refraction of the fluid which fills the first region does not precisely match the refractive index of the core material of the second input waveguide  21 , a small amount of leakage from optical signals will be incidentally reflected from the interface of the chamber  32  and the second input waveguide  21 . The incidentally reflected leakage light is represented by arrows having dashed tails. The incidentally reflected optical energy is interpreted as indicating that the condition which caused the malfunction in the upstream region of the second transmission route  15  has been remedied. Furthermore, it is assumed that the downstream region of the first transmission route has also been restored to an operative state as well. As a response to detection of the incidentally reflected optical signals over the second intermediate waveguide  27 , the switch controller  46  disables the substitute transmissive state and re-enables the normal reflective state. 
     In an identical manner, the substitute transmissive state of the switch  10  is enabled if the first traffic detector  41  observes a prolonged absence of signal transmission (A) over the first input waveguide  17  when the switch  10  is in the normal reflective state. After the transmissive state is enabled, the first traffic detector  41  monitors the first intermediate waveguide  25  for incidentally reflected leakage light associated with resumed signal transmission over the first input waveguide  17 . The optical switch  10  interprets the resumption of signal transmission (A) over the first input waveguide as indicating that the first and second transmission routes  13  and  15  have returned to an operational state. In response, the switch controller  46  disables the substitute transmissive state and re-enables the normal reflective state of the switch  10 . 
     As noted above with reference to FIG. 2, if traffic (A or C) ceases over one of the input waveguides  17  and  21 , it is assumed that the downstream region (B or D) of the other transmission route is inoperative. However, it is desirable to be able to verify this assumption, since the condition which has caused traffic over an input to one of the transmission routes (e.g., the second route  15 ) to cease might not have affected the downstream region of the other transmission route (i.e., the first transmission route  13 ). Referring now to FIG. 5, the first traffic detector  41  monitors the first intermediate waveguide  25  for a reverse driven signal which indicates that the downstream region of the first transmission route  13  is functioning properly. Such a reverse driven signal could be a “request for resumption” to the normal state of the switch. The first  41  and second  43  traffic detectors both preferably include a pair of directionally coupled evanescent detectors which distinguish the direction in which optical signals travel. In a preferred embodiment, the reverse driven signal is a remotely transmitted test signal (e.g., the request for resumption signal) having a different wavelength than the standard data bearing optical signals (A and C) in order to differentiate the reverse driven signals from data bearing optical signals. 
     The reverse driven signals received over the first output waveguide  19  travel through the second transmission path  34  during the transmissive state. However, as can be seen with reference to FIG. 5, a small amount of leakage light is incidentally reflected at the interface of the chamber  32  and the first output waveguide  19 . In response to the first traffic detector  41  detecting the incidentally reflected leakage light of the reverse driven signal, the switch controller  46  disables the substitute transmissive state and re-enables the normal reflective state of the switch  10 , returning the switch to the fault-free condition of FIG.  2 . Similarly, when the substitute transmissive state was originally enabled because traffic (A) ceased over the first transmission route  13 , the second traffic detector  43  monitors for incidentally reflected leakage light from reverse driven signals received over the second output waveguide  23 . The switch controller  46  re-enables the normal reflective state of the switch  10  in response to detection of those reverse driven signals. 
     Preferably, a remote device (not shown) which generates the reverse driven signals continues to transmit the reverse driven signals after the reflective state has been enabled, thereby providing reassurance that the downstream region of the first  13  or the second  15  transmission route is still functioning properly. 
     Referring to FIGS. 2-6, a method for monitoring and controlling optical signal transmissions over a SONET ring network is described. At startup  50 , the switch  10  is enabled in its normal reflective state, as indicated by step  52 . In step  54 , the first and second transmission routes  13  and  15  are monitored to confirm the continuation of incoming traffic (A and C). As long as the traffic detectors  41  and  43  continue to determine that traffic (A and C) is progressing as expected, steps  54  and  56  operate as a loop. The decision step  56  initiates a fault-response condition only if an unexpected absence of incoming traffic is detected along at least one route  13  or  15 . When an unexpected absence is detected, the decision step  58  determines whether the absence is isolated to one of the routes or is a concern for both routes. When the signals are not present on both routes, the controller  46  leaves the switch in the normal reflective state and returns the process to the monitoring step  54 . On the other hand, if incoming traffic has ceased over only one of the transmission routes, the switch controller  46  enables the substitute transmissive state at step  60 . 
     In a first example, if step  60  was performed in response to an absence of signal reception (C) over only the second transmission route  15 , enabling the transmissive state results in signal traffic (A) being switched from the first transmission route  13  to the substitute transmission route  29  (A→D). During the substitute transmissive state, the second traffic detector  43  monitors the second intermediate waveguide  27  at step  62  to determine at step  64  whether a forward driven signal has been detected. If the refractive index of the fluid within the chamber  32  does not precisely match the refractive index of the light-bearing core material of the second input waveguide  21 , a small portion of each forward driven signal will be incidentally reflected onto the second intermediate waveguide  27 . When a resumption of signal reception (C) over the second input waveguide  21  is detected, the switch controller  46  disables the substitute transmissive state and re-enables the normal reflective state in the switch, because it is presumed that the condition which caused a malfunction in the second transmission route  15  and which potentially caused a malfunction in the downstream portion of the first transmission route  13  has been remedied. 
     There is an important case in which a downstream party has a failure in that party&#39;s sending line only. Then, the party can request resumption of conventional operation. The “request for resumption” can be made with a reverse driven signal along that party&#39;s receiving line. 
     Continuing the first example of the method of FIG. 6, the first traffic detector  41  monitors the first intermediate waveguide  25  at step  66  for incidentally reflected light from a “request for resumption” reverse driven signal. Receipt of the reverse driven signal over the first intermediate waveguide indicates that the condition which caused the malfunction in the second transmission route  15  has not adversely affected the operation of the downstream portion of the first transmission route  13 . In response, the switch controller  46  re-enables the normal reflective state at step  68 . If neither a resumption of signal transmission over the second input waveguide nor a reverse driven signal over the first output waveguide  19  is detected, the switch  10  returns to step  62 . After step  68 , the switch returns to step  54 , where the first and second transmission routes  13  and  15  are again monitored to determine if incoming traffic has ceased over either route. 
     Now turning to a second example, if the substitute transmissive state was enabled at step  60  because signal transmission (A) ceased over the first transmission route  13 , then at step  62  the first traffic detector  41  monitors for the resumption of traffic (A), as evidenced by incidentally reflected leakage light. The first traffic detector  41  determines whether incidentally reflected leakage light is detected from a revived forward driven signal (A) received over the first input waveguide  17 . Simultaneously, the second traffic detector  43  monitors for a reverse driven signal (D) received over the second output waveguide  23 . Should neither the forward nor reverse driven signals be detected, the process returns to step  62 . If either the forward or the reverse driven signal are detected, at step  68  the switch controller re-enables the normal reflective state and the first and second traffic detectors  41  and  43  resume monitoring the first and second transmission routes at step  54  to determine if signal reception has ceased over either route. In a third example, at decision step  58 , it is determined that signal reception (A and C) has ceased over both transmission routes  13  and  15 . In this situation, the normal reflective state is maintained until signal transmission resumes on one of the transmission routes. Thus, the process returns to the monitoring step  54 . 
     With reference to FIG. 7, in a second embodiment, the switch  10  operates in a normal transmissive state in the absence of a detected fault condition. The switch monitors and controls signal transmission over a first linear path  12  (A→B) and a second linear path  14  (C→D). Each path carries network traffic uni-directionally, so that the first path  12  carries traffic in a first direction and the second path  14  carries traffic in the opposite direction. The first linear path  12  (A→B) includes an input waveguide  16  which connects to a first region of the fluid-manipulable chamber  32  of the switch. In the normal transmissive state of the switch, when the first region of the chamber  32  is filled with fluid having a refractive index which closely matches the refractive index of the waveguide core material, optical signals (A) propagate into the chamber from the first input waveguide  16 . The light propagation forms the first transmission path  36  within the chamber and exits via a first output waveguide  18 . In the normal transmissive state, light (C) which enters the fluid-manipulable chamber  32  from a second input waveguide  20  forms the second transmission path  34  through the chamber. The second transmission path exits the chamber  32  through a second output waveguide  22  of the second linear path  14 . 
     Although it is desirable to have as close a match as possible for the refractive indices of the waveguides and the fluid within the chamber, it is likely that a small mismatch will occur. Consequently, a small amount of light will be reflected from the interfaces of the chamber  32  and the first and second output waveguides  18  and  22  and from the interfaces of the chamber  32  and the first and second input waveguides  16  and  20 . For example, a fiberoptic signal (A) traveling along the first transmission path  36  will be partially reflected at the rear face of the chamber  32 . A first monitoring waveguide  28  is connected to the chamber to receive the incidentally reflected signal and to direct it toward a first forward traffic detector  38  for the first linear path  12 . The incidentally reflected light is represented as arrows having dashed tails. A second monitoring waveguide  30  is connected to the chamber  32  to receive light incidentally reflected from the second transmission path  34  and to direct the light toward a second forward traffic detector  44 . The amount of light which is reflected from the rear face of the chamber is small, so the first and second forward traffic detectors are configured to recognize only power, not the information content of the reflected signals. 
     The first and second forward traffic detectors  38  and  44  can be utilized to monitor the operational state of the SONET ring network. However, first and second reverse detectors  42  and  40  provide more specific data regarding the operational state of the network and are therefore the preferred devices for performing the monitoring function. During normal operation of the network, the switch  10  is in the normal transmissive state and the first forward traffic detector  38  receives leakage light incidentally reflected from the rear face of the chamber  32 . In addition, the first forward traffic detector  38  receives leakage light which is incidentally reflected from the second input waveguide  20  onto intermediate waveguide  26 . Similarly, the second forward traffic detector  44  receives leakage light incidentally reflected from the rear face of the chamber  32  within the second transmission path  34  and receives leakage light incidentally reflected from the front face of the chamber  32  onto a first intermediate waveguide  24 . Optical signals (A) which travel over the first linear path  12  will contribute an equal amount of leakage light to the first and second traffic detectors  38  and  44 . Likewise, optical signals (C) which travels over the second linear path  14  will contribute equal amounts of leakage light to the first and second forward traffic detectors. 
     If signal propagation (C) ceases over the second transmission path  34 , the first and second forward traffic detectors  38  and  44  will both observe a drop in the intensity of incidentally reflected leakage light. It may be known beforehand that optical signals (A or C) on one of the linear paths have a higher signal intensity than optical signals on the other linear path. In this case, it will be possible to ascertain which of the linear paths has ceased to carry traffic. For example, if signal intensity of traffic (A→B) on the first linear path  12  is three times as high as the signal intensity of traffic (C→D) on the second linear path  14 , a 25 per cent decrease in the intensity of leakage light observed by both the first and second forward traffic detectors  38  and  44  indicates that signal transmission has ceased over the second linear path  14 . However, if signal intensity, or some other difference between signal traffic on the first and second linear paths is not known beforehand, it will not be possible to identify which linear path has ceased to carry signal transmissions by utilizing the first and second forward traffic detectors  38  and  44 . 
     In contrast, the first and second reverse traffic detectors  42  and  40  monitor the operational state of the network and provide specific information regarding which linear path has ceased supporting signal propagation (A or C). During the normal transmissive state, the first reverse traffic detector  42  receives leakage light incidentally reflected from the first input waveguide  16  at the front face of the fluid-manipulable chamber  32  onto the first intermediate waveguide  24 . When the first reverse traffic detector  42  fails to detect any incidentally reflected light for a predetermined time interval, the absence of incidentally reflected light is interpreted as an indication of a fault condition associated with a failure in the first linear path  12  (A→B). The second reverse traffic detector  40  receives leakage light incidentally reflected from the second input waveguide  20  during the normal transmissive state of the switch. When the second reverse traffic detector  40  observes an absence of incidentally reflected leakage light, this is indicative of a fault condition associated with the second linear path  14  (C→D). Moreover, because optical fibers of the first and second linear paths are packaged closely together, it is possible that an event which caused a malfunction at a source of one of the paths also caused a malfunction at a destination end of the other path. For example, if an optical fiber of the first linear path is broken while a trench is dug near fiberoptic lines connected to the first input waveguide  16  and the second output waveguide  22 , it is likely that the optical fiber connected to the second path  14  has also been broken. 
     The switch controller  46  communicates with the first  38  and second  44  forward traffic detectors and the first  42  and second  40  reverse traffic detectors to determine whether to maintain the normal transmissive state or to enable the substitute reflective state of the switch  10 . Typically, this is achieved by selectively activating and deactivating heaters, as described with reference to FIG.  1 . However, other methods of manipulating the index-matching fluid may be used without departing from the invention (e.g., inkjet techniques). 
     For purposes of illustration, the first and second input waveguides  16  and  20  are shown as being coaxially aligned with the first and second output waveguides  18  and  22 . Since the refractive index of the fluid is not precisely matched to that of the waveguide material, there may be an advantage to providing a slight offset in the linear alignment of the input and output waveguides. As a separate issue, the drawings have shown the ninety degree angles between waveguides that exchange signals by reflection at the chamber. However, ninety degree angles are not critical. 
     Referring to FIG. 8, if the second reverse traffic detector  40  fails to detect incidentally reflected light for a predetermined time interval, the second reverse traffic detector communicates the absence of detected traffic to the switch controller  46 . Alternatively, the second forward traffic detector  44  can be utilized to monitor for traffic (C) received at the switch over the second input waveguide  20 . As previously noted, the absence of incidentally reflected light detected by either the second forward traffic detector  44  or the second reverse traffic detector  40  indicates that the second path  14  (C→D) has been damaged somewhere upstream of the switch chamber  32 . Furthermore, there is a likelihood that the optical fiber for exiting light (B) of first path  12  has been damaged downstream of the switch at the same location as the site where the second path  14  has been damaged. The switch controller  46  enables the substitute reflective state in the switch  10 , thereby diverting the traffic away from the first output waveguide  18  by reflecting the traffic onto the first intermediate waveguide  24 . The first intermediate waveguide  24  shares a common connection to the switch chamber  32  with the second output waveguide  22 , so that traffic (A) is reflected onto the second output waveguide  22  of the second path  14  (i.e., a path A→D is formed). It should be understood that the switch controller  46  is also responsive to the first reverse traffic detector  42  to enable the substitute reflective state (to form a path C→B) in response to detection of an extended absence of incoming traffic (A) over the first path  12 . 
     Enablement of the substitute reflective state in the switch  10  when incoming traffic (A or C) is not detected on either the first or second input waveguides  16  and  20  is a temporary fault-response measure to ensure delivery of information carried on one of the paths when a portion of one of paths  12  and  14  is suspected of malfunctioning. Consequently, the switch continuously verifies that the fault condition is still present. Referring to FIG. 9, after the substitute reflective state has been enabled and incoming traffic (A) on the first input waveguide  16  is redirected to the second output waveguide  22  (A→D), the second reverse traffic detector  40  monitors for traffic (C) received over the second input waveguide  20 . Because the switch is in the reflective state, any such traffic will be reflected onto the second intermediate waveguide  26 , where it is detected by the second reverse traffic detector  40 . If incoming traffic (C) is detected, it is presumed that the fault condition for the input for the second path  14  has been repaired. It is further assumed that the fault condition for the output (B) along the first path  12  has also been repaired. Consequently, the switch controller  46  enables the normal transmissive state and the optical switch  10  resumes operation over paths (A→B) and (C→D) as shown in FIG.  7 . 
     In the operation of the optical switch of the present invention, it is assumed that if incoming traffic is not detected on one of the input waveguides, for example signals (C) of the second input waveguide  20 , then the other path, namely the path  12 , is also damaged downstream of the switch. Although this assumption can prevent transmission of data down the malfunctioning destination line, it is possible that the malfunction of the second path is unrelated to the operational condition of the first path. Therefore, it is desirable to provide a mechanism whereby the functional status of the downstream portion of the first path  12  can be verified after an absence of incoming traffic (C) has been observed on the second path. To this end, the second reverse traffic detector  40  monitors for a reverse driven “request for resumption” signal received over the first output waveguide  18  of the first path  12 . Preferably, the reverse driven signal is of a different wavelength than the standard data-bearing fiberoptic signals (A and C) transmitted over the network, so as to differentiate the reverse driven signal from a forward driven signal (C) received over the second input waveguide  20 . As can be seen with reference to FIG. 10, the reverse driven signal received over the first output waveguide  18  will be reflected onto the second intermediate waveguide  26  during the substitute reflective state. When the second reverse traffic detector  40  detects the reverse driven “request for resumption” signal, the switch controller  46  re-establishes the normal transmissive state in the switch to allow signal transmission to proceed over the first output waveguide  18  of the first linear path  12  (A→B). 
     Preferably, a remote device (not shown) which transmits the reverse driven “request for resumption” signals continues to transmit the reverse driven signals after the transmissive state has been enabled in order to provide reassurance that the portion of the first path  12  downstream of the switch  10  is still functioning. During the normal transmissive state, a small portion of the reverse driven signals will be incidentally reflected from the first output waveguide  18  onto the second intermediate waveguide  26  of the second path  14 , where it will be detected by the second reverse traffic detector  40 . 
     With reference to FIG. 11, if no incoming traffic (A and C) is detected over either of the paths  12  or  14 , then the normal transmissive state of the switch is maintained. 
     Referring to FIGS. 7 and 12, after a startup step  76 , a method for monitoring and controlling data propagating over a fiberoptic network includes the step  78  of establishing the normal transmissive state of the switch  10 . Then, in step  80 , incidentally reflected light is received from interfaces of the switch chamber  32  with the first and second output waveguides  18  and  22 . In one embodiment, the step of receiving the incidentally reflected light exclusively utilizes light reflected off the rear faces of the switch chamber  32 . For example, light which is incidentally reflected from the first transmission path  36  within the switch chamber is captured by the first monitoring waveguide  28 , where it is received and detected by the first forward traffic detector  38 . Similarly, a small amount of light that is incidentally reflected from the rear face of the switch chamber  32  within the second transmission path is  34  captured by the second monitoring waveguide  30  and received by the second forward traffic detector. Alternatively or additionally, incidentally reflected light reflected at the front faces of the switch chamber  32  is captured by the first and second reverse traffic detectors  42  and  40 . 
     Step  82  determines whether an absence of incidentally reflected light has been observed by the traffic detectors. As previously noted, the absence of incoming traffic on one path  12  and  14  (A→B) or (C→D) throws into question the reliability of the portion of the other path downstream of the switch  10 . If incoming traffic along both paths is continuously detected, a loop is formed by steps  80  and  82 , as the switch controller  46  maintains the normal transmissive state of the switch  10 . On the other hand, if an absence of incidentally reflected light has been detected, in step  84  the switch controller  46  enables the substitute reflective state of the switch. During the reflective state, the switch reverses the connections for the incoming traffic (A and C). For example, if no incoming traffic (C) is detected on the second path  14 , incoming fiberoptic signals (A) received over the first input waveguide  16  are rerouted onto the second output waveguide  22  of the second path  14 . In this manner, traffic is prevented from traveling over the downstream portion of the potentially severed first fiberoptic line  12 . 
     Step  86  is a decision step that determines whether the “dead” fiberoptic line, namely the line for which no incoming traffic has been detected, has come alive. For example, if the second path  14  is the dead line, the second reverse traffic detector  40  monitors for incoming traffic. When incoming traffic is detected, it is presumed that the second path has been repaired and that the first path  12  has been repaired as well. Therefore, the switch controller  46  reverts the switch to the normal transmissive state in step  88  and the process returns to step  80 . 
     If no incoming traffic is detected over the second path  14 , the second reverse traffic detector  40  monitors for a reverse driven “request for resumption” signal received over the first output waveguide  18  of the first path  12  in step  90 . When the reverse driven signal is detected, the step  88  of reverting to the normal transmissive state is performed. If no reverse driven signal is detected, in step  92  the substitute reflective state of the switch is maintained. The process then returns to step  86 .