Abstract:
The present invention provides a variety of methods and apparatus for monitoring signal quality and switch operation for optical switches in an optical cross connect. The signal quality can be checked by using a beamsplitter to reflect a portion of the light beam carrying the signal onto a photodetector which is connected to a device which can monitor loss of signal, loss of frame, and other transmission characteristics. The position of a switch or waveguide can be monitored by circuits which detect variable levels of resistance and capacitance based on the switch&#39;s position or by reflecting probe light beams off the back of the switching mirror. The present invention also provides a method for determining if the signal quality photodetector is functioning correctly.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/093,385 filed Jul. 20, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a method and apparatus for monitoring the position of an optical switch and detecting errors in the transmission quality of the signal being transmitted through it. The present invention is also directed to a plurality of such apparatuses being used to monitor a plurality of switches within an optical cross connect. 
     BACKGROUND OF THE INVENTION 
     The demand for both greater volume and speed on long distance telecommunications networks has resulted in the rapid improvement of point to point optical transport systems (OTS). These systems can now transport data at rates greater than 20 Gb/s along a single fiber. As a result, demand for systems to provision this traffic and restore it in the event of a network failure has increased as well. The issue of restoration is further complicated by the need for it to occur very rapidly, on the time scale of a few seconds or less, and thus detection of the network faults that make restoration necessary must also be performed very rapidly. 
     One potential solution to this detection problem is through the utilization of “opaque” networks with optoelectronic transponder interfaces to perform fault detection as illustrated in FIG.  1 . In this configuration, a signal is received along optical fiber  101  and fed into transponder  102 . Transponder  102  is an optoelectronic device that translates the optical signal into an electronic signal, performs tests for loss of signal, loss of frame, etc., then translates the signal back into an optical signal, and sends it on to optical cross connect (OXC)  103 . If transponder  102  detects an error, it notifies OTS management system  104  which in turn notifies network management system  105 . Network management system  105  notifies OXC management system  106  which can then begin the restoration process in the OXC  103 . 
     This method suffers from several shortcomings. The communication of error information will have to occur through several elements of the system, most likely using software interfaces. These system components could potentially come from multiple vendors. In this arrangement, the likelihood of achieving the desired response times for network restoration is greatly decreased. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to methods and apparatuses for detecting errors in the signals being transmitted through an optical switch and monitoring the position of the switch in order to improve network restoration time. The optical switch that the present invention operates on is composed of a micromachined mirror that is attached via a hinge to a substrate at an angle to the direction of the light beam so that when the mirror is parallel to and level with the substrate, in the “off” state, the light beam passes by it without disruption but when the mirror is perpendicular to the substrate, in the “on” state, the light beam is redirected to another destination. These switches can be used in combination to form an optical cross connect for routing light beams between multiple destinations. A device of this type is fully described in a co-pending patent application entitled FIBER-OPTIC FREE-SPACE MICROMACHINED MATRIX SWITCHES, Ser. No. 09/001,676, filed Dec. 31, 1997 and is incorporated herein by reference. 
     The present invention monitors the light beam signal exiting the optical switch for errors and, if errors are detected, the invention narrows the possible causes by checking the error detecting sensor for failure and checking the position of the mirrors to ensure that they are set correctly for the proper transmission of the light beam. 
     The present invention monitors the signal for errors by using a beamsplitter placed in the path of the light beam to redirect a portion of the light beam onto a photodetector. The photodetector converts the light into an electronic signal that can be processed to detect loss of signal, loss of frame, and other errors using well known transmission error detection routines. 
     The present invention also provides three methods for detecting the state of the optical switch by monitoring the physical position of the micromachined mirrors. 
     The first method utilizes a circuit formed by conductive material along the mirror, the substrate, and along a probe mounted on the substrate that touches the conductive material on the mirror only when the mirror is substantially perpendicular to the substrate. The state of the mirror in the switch can be determined by the resistance of the circuit. If the circuit has finite resistance, the mirror is perpendicular and the switch is “on.” If the circuit has a nearly infinite resistance, i.e., an open circuit, the mirror is not perpendicular and the switch is “off.” 
     The second method also utilizes a circuit formed by conductive material along the mirror, the substrate, and along a probe mounted on the substrate. However, the probe in this case does not touch the mirror when it is perpendicular to the substrate, but rather the probe is parallel to the mirror a short distance away. The position of the mirror can be determined by measuring the capacitance of the circuit created. The closer the mirror is to the perpendicular “on” state, the higher the capacitance value will be. 
     The third method involves an additional optical input and output for each switch wherein a second light beam is generated by the optical input, reflected off the back of the mirror to the optical output, and the position of the mirror can be monitored based on the information returned by the light beam. If the light beam is reflected to the optical output, then the switch mirror is in the “on” state; if the light beam is not reflected to the optical output, then the switch mirror is in the “off” state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a known configuration for implementing a system to perform fault detection using optoelectronic transponder interfaces. 
     FIG. 2 is a perspective view of an optical cross connect apparatus with fault detection using the beam splitting method and apparatus of the present invention. 
     FIG. 3 is a side view of a micro lens positioned between the photodetector and the beamsplitter. 
     FIG. 4 is a perspective view of a switching mirror with position monitoring via the resistance method and apparatus of the present invention. 
     FIG. 5 is a perspective view of a switching mirror with position monitoring via the capacitance method and apparatus of the present invention. 
     FIG. 6 is a top view of an optical cross connect apparatus with mirror position monitoring via the optical method of the present invention. 
     FIG. 7 is a top view of the apparatus of the present invention for testing the photodetector for failure. 
     FIG. 8 is a top view of a waveguide switch with position monitoring via the resistance method and apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is shown in FIG. 2 wherein optical cross connect  201  is constructed of a plurality of individual optical switches, each having error detection capabilities provided by the present invention. A data transmission carried via light beam  202  enters OXC  201  through fiber  203 . Light beam  202  is then redirected by switch mirror  204  and exits the switch through fiber  205 . Before exiting OXC  201 , however, light beam  202  passes through beamsplitter  206  which redirects a small portion of the light beam onto photodetector  207 . Photodetector  207  translates the reflected portion of light beam  202  into an electrical signal which can then be analyzed by a processor unit to determine characteristics of the data transmission. These characteristics can include, but are not limited to, loss of signal, loss of frame, signal quality, and verification of switching path. Beamsplitter  207  as shown in FIG. 2 is a 45 degree beamsplitter, but this should not be construed as a limiting factor. Switch mirror  204 , beamsplitter  206 , and photodetector  207  are all integrated onto the substrate  208 . 
     As shown in FIG. 3, the efficiency of photodetector  301  can be enhanced by mounting micro lens  302  on the substrate so that the portion of the light beam reflected by the beamsplitter passes through micro lens  302  and is focused on photodetector  301 . Photodetector  301  can be of various types based on the transmission characteristics to be gathered. For detection of only loss of signal, a low speed photodetector can be employed. A higher speed and more sophisticated photodetector can be employed to gather loss of signal, loss of frame, and other signal quality indicators. Photodetector  301  may also be used to read the header information in the signal to verify that the light beam is on the correct communication path based on its final destination or to perform parity checking on the data being transmitted. 
     FIG. 4 shows an embodiment of an apparatus to monitor the position of the switch mirror using electrical resistance measurements. Switch mirror  401 , consisting of conductive plate  402  with reflective surface  403 , is attached to substrate  404  via hinge mechanism  405 . Switch mirror  401  can rotate from a position which is parallel to and level with substrate  404  to a position substantially perpendicular to substrate  404 . It is in this perpendicular position that switch mirror  401  is “on”, that is, the mirror at this position reflects and redirects the light beam carrying the data transmission through the switch. A smaller conductive plate  406  with probe  407  protruding from one edge of it is also connected to substrate  404  via a hinge mechanism and is positioned such that the end of probe  407  touches conductive plate  402  when switch mirror  401  is in the perpendicular position. Conductive plate  406  can be affixed permanently in position on substrate  404  or can be hinged and moved into position when required. Circuit  408  on substrate  404  connects conductive plate  402  with probe  407 , and on this circuit sensor  409  measures the resistance between probe  407  and conductive plate  402 . When switch mirror  401  is in the perpendicular position, probe  407  will touch the conductive plate  402 , changing the resistance from infinity to a finite value, and sensor  409  will register this change. When switch mirror  401  is parallel to and level with substrate  404 , probe  407  will not touch conductive plate  402 , creating an open circuit, and sensor  409  will register an extremely large resistance (approaching infinity). 
     FIG. 5 shows an embodiment of an apparatus to monitor the position of the switch mirror using electrical capacitance measurements. Switch mirror  501 , consisting of conductive plate  502  with reflective surface  503 , is attached to substrate  504  via hinge mechanism  505 . Switch mirror  501  can rotate from a position which is parallel to and level with substrate  504  to a position perpendicular to substrate  504 . It is in this perpendicular position that switch mirror  501  is “on”, that is, the mirror at this position reflects and redirects the light beam carrying the data transmission through the switch. A smaller conductive plate  506  is also connected to substrate  504  via a hinge mechanism and is positioned parallel to conductive plate  502  such that there is a small distance between conductive plate  502  and conductive plate  506  when switch mirror  501  is in the perpendicular position. Conductive plate  506  can be affixed permanently in position on substrate  504  or can be hinged and moved into position when required. Circuit  507  connects conductive plate  502  with plate  506 , and on this circuit sensor  508  measures the capacitance between plates  502  and  506 . When switch mirror  501  is in the perpendicular position, plates  502  and  506  will be close together, resulting in an effective capacitor, and sensor  508  will register a high capacitance value. When switch mirror  501  is parallel to and level with substrate  504 , plates  502  and  506  will be perpendicular to one another and further apart, resulting in a poor capacitor, and sensor  508  will register an extremely low capacitance value (approaching zero). From the measured capacitance value, the angle of switch mirror  501  can be determined. 
     The resistance and capacitance methods of monitoring the position of a switching mirror can also be applied to waveguide switches. FIG. 8 shows an embodiment of the apparatus to monitor the position of a waveguide switch using electrical resistance measurements. Waveguide  801  has movable portion  802  that can be set to at least two positions so as to transmit the signal being carried by waveguide  801  to waveguide  803  or  804 . Waveguide  802  is constructed with a conductive material and a circuit connects waveguide  802  to probe  805 . Probe  805  is positioned so that it touches waveguide  802  when waveguide  802  is positioned to transmit the signal to waveguide  803  but does not touch waveguide  802  when the signal is being transmitted to waveguide  804 . Sensor  806  on the circuit measures the resistance between waveguide  802  and probe  805 . When waveguide  802  is in position to transmit the signal to waveguide  803 , probe  805  touches waveguide  802  and sensor  806  measures a finite resistance. When waveguide  802  is in position to transmit the signal to waveguide  804 , probe  805  does not touch waveguide  802 , creating an open circuit, and sensor  806  measures a resistance approaching infinity. 
     Similarly, the waveguide switch in FIG. 8 could be configured to use the capacitance method for determining the position of waveguide  802  by positioning a conductive plate to be close to waveguide  802  when it is in position to transmit the signal to waveguide  803  and to be further away from waveguide  802  when it is in position to transmit the signal to waveguide  804 . 
     FIG. 6 illustrates a method and apparatus for monitoring the position of a switch mirror by using a second light beam directed at the back of the optical switch mirror. A signal light beam enters the switch on path  601 , reflects off mirror  602  and exits the switch on path  603 , accomplishing the signal redirecting task of the switch. To monitor the position of the mirror, a probe light beam enters the switch on path  604 , reflects off the back of mirror  602 , and exits the switch on path  605  where it can be received by a probe beam detector. The information received by the probe beam detector can be interpreted to determine if the switch mirror is in the correct position. If mirror  602  was incorrectly positioned, the probe beam would be reflected off in the wrong direction and this would be detected by the probe beam detector. This method and apparatus can be applied to an entire optical cross connect as shown in FIG. 6 by having multiple probe beam detectors positioned at the monitoring outputs from the switch. The input probe beams could be from a common light source to enable very simple checking of a row of switch mirrors or the probe beams could originate from separate light sources having different projection characteristics, such as pilot tones or low-speed bit sequences, that would distinguish the probe beams from one another and allow monitoring of individual switch mirrors. 
     FIG. 7 illustrates a method and apparatus for determining if the integrated photodetector has failed. Since the current-voltage curve of a photodetector shows diode behavior when it is functioning correctly and shows an open or short circuit when malfunctioning, comparing its dark current under a reverse bias and diode current under a forward bias can determine if the photodetector has failed. Photodetector  701  is connected to coplanar waveguide  702  providing a ground signal and is rotatably connected to in-plane cantilever  705  which can be positioned to provide a connection to waveguide  703  carrying a forward bias signal or waveguide  704  carrying a reverse bias signal. Cantilever  705  is switched between these positions by the use of comb drive actuator  706 , and cantilever probe tip  707  is brought into contact with waveguide  703  or  704  by electrostatic force. To determine if photodetector  701  is functioning correctly, cantilever  705  is switched to connect to waveguide  704 , thus applying a reverse bias to photodetector  701 . The dark current of photodetector  701  is then measured. Cantilever  705  is then switched to connect to waveguide  703 , thus applying a forward bias to photodetector  701 . The forward diode current is measured and compared with the dark current measurement. If the measurements show correct diode behavior, then photodetector  701  is functioning correctly. The two waveguide transmission lines  703  and  704  and cantilever  705  could be replaced by a single transmission line and the status of the photodiode could be checked by changing the polarity of the applied electrical bias on the single transmission line. 
     This method of checking the photodetector for correct operation can be combined with the methods for monitoring switch mirror position to allow the management module of an OXC to quickly identify the likely source of any problems in the switches. If an error such as loss of signal or loss of frame is detected by a photodetector, the OXC management can check both the position of the switch mirrors and the functionality of the photodetector in order to attempt to reroute traffic around the problem as well as to inform repair personnel of the likely source of the problem. 
     The present invention is not limited to the specific implementations described. It is expected that those skilled in the art will be able to devise other implementations which embody the principles of the present invention and remain within its scope.