Patent Publication Number: US-2006002704-A1

Title: Optical switch having an autorestoration feature for switching from a backup optical path to a primary optical path

Description:
This is a divisional application of U.S. application Ser. No. 10/364,825 filed on Feb. 11, 2003. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to optical switches, and more particularly to an optical switch that restores optical traffic from a secondary optical transmission path to a primary optical transmission path after a fault in the primary optical transmission path has been repaired.  
     BACKGROUND OF THE INVENTION  
      Currently, transmission systems employed in the cable television (CATV) industry provide two-way transmission of information; e.g., video, audio, multimedia and/or data; between a head end and a plurality of subscribers. Typically, the head end transmits the information destined for individual subscribers (“downstream information”) in an optical format, via one or more fiber optic links, to one or more optical nodes. Each node converts the optically-formatted downstream information into electrical signals for distribution, typically via a coaxial cable plant having a tree and branch architecture, to individual subscribers. In addition to receiving the downstream information, each individual subscriber may generate information in the form of voice, video, data, or any combination thereof, destined for the bead end. The subscriber- generated information (“upstream information”) is aggregated by the coaxial cable plant and passes to the node for conversion into an optical format for transmission to the head end.  
      CATV service providers and their subscribers are accustomed to high reliability service. One way in which high reliability is achieved is by providing two optical paths between the head end and each optical node, one of which serves as a primary optical path and the other of which serves as a secondary or backup optical path. An optical switch switches the optical information signals from the primary path to the secondary path in the event of an unanticipated failure in the primary path. The optical switches are often located in the head end and the optical nodes.  
      The aforementioned optical switches generally employ an optomechanical switching component that switches between the primary path and the secondary path based on the electrical voltage that is applied to it. A portion of the optical signal in the primary and secondary paths is tapped off and converted to an electrical voltage. The voltages are monitored and if a threshold condition is violated, indicating a failure in the primary path, the switch is activated so that traffic is transferred to the secondary path. Unfortunately, the optical switch does not include any arrangement for switching back from the secondary to the primary path after the primary path has been restored. Rather, an operator or technician must perform a manual power cycle to restart the optical switches in both the head end and the optical node so that the switches return to the primary path. Restoration in this manner can be difficult because the head end and the optical node may be located 50 to 100 km apart from one another. Also, there may be many such optical switches in both the head end and the nodes, thus requiring the operator to take proper care to ensure that the correct combination of switches are power cycled so that there is no interference with traffic on the other paths.  
      Accordingly, it would be desirable to provide a method and apparatus for automatically restoring optical traffic from a secondary optical transmission path to a primary optical transmission path after a fault in the primary optical transmission path has been repaired without the need to perform a manual power cycle.  
     SUMMARY OF THE INVENTION  
      In accordance with the present invention, optical switches are provided in an optical transmission system having at least two optical nodes in optical communication over a primary optical path and a backup optical path. An optical switch is located in each of the optical nodes. Each of the optical switches includes a switching element having an input port and a plurality of output ports coupled to the primary and backup optical paths, respectively. The switching element has a first state optically coupling an optical signal from the input port to the primary path and a second state optically coupling an optical signal from the input port to the backup path. First and second optical taps are located in the primary optical path. Third and fourth optical taps are located in the backup optical path. A first photodetector is optically coupled to the second optical tap for receiving a portion of the optical signal traveling in the primary optical path. A second photodetector is optically coupled to the third optical tap for receiving a portion of the optical signal traveling in the secondary optical path. A first optical path optically couples the first optical tap to the fourth optical tap such that a portion of an optical signal traveling in the secondary path is coupled onto the primary path. Finally, a controller is electrically coupled to each of the optical switches. The controller is configured so that when each of the switching elements are in the second state and the first photodetector in each of the optical switches detects an optical signal, the controller returns the switching elements to the first state.  
      In accordance with one aspect of the invention, the two optical nodes respectively comprise a head end and an optical node in a CATV transmission system.  
      In accordance with another aspect of the invention, the switching element is an optomechanical switching element.  
      In accordance with yet another aspect of the invention, the first and second photodetectors are photodiodes.  
      In accordance with another aspect of the invention, the primary and secondary optical paths are unidirectional paths. Alternatively, the primary and secondary optical paths may be bi-directional paths.  
      In accordance with another aspect of the invention, a method is provided for switching optical traffic from a secondary optical path to a primary optical path, each of which establish an optical communication path between first and second optical nodes. The method begins by: detecting a presence or absence of an optical signal traveling in the primary path through the first optical node; detecting a presence or absence of an optical signal traveling in the primary path through the second optical node; detecting a presence or absence of an optical signal traveling in the secondary path through the first optical node; and detecting a presence or absence of an optical signal traveling in the secondary path through the second optical node. A portion of an optical signal traveling in the first optical node is coupled from the secondary path to the primary path. A switching element is switched in each of the first and second optical nodes from a second state to a first state so that the optical traffic traverses the primary optical path when an optical signal is detected traveling in the primary path through both the first and second optical nodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a simplified block diagram of a conventional arrangement for providing a primary and second optical path between the head end and an optical node in a CATV transmission system.  
       FIG. 2  shows a simplified block diagram of an arrangement for providing a primary and second optical path between the head end and an optical node in a CATV transmission system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  shows a simplified block diagram of a conventional arrangement for providing a primary and second optical path between the head end and an optical node in a CATV transmission system. Optical switches  110  and  120  are located in the head end and the optical node, respectively. Optical transmission path  112  serves as the primary path while optical transmission path  114  serves as the secondary or backup optical path. Traffic along the primary path  112  and the secondary path  114  may be unidirectional or bidirectional. Optical switch  110  includes an optomechanical switching element  116 , tap couplers  117  and  118  and photodiode  121  and  122 . Likewise, optical switch  120  includes an optomechanical switching element  124 , tap couplers  125  and  126  and photodiodes  127  and  128 . A switch controller  127  controls the operation of switches  110  and  120 .  
      Referring to switch  110 , tap couplers  118  and  121  respectively couple a small portion of the optical signals traveling in paths  112  and  114  to photodiodes  121  and  122 . Controller  123  receives the electrical signals from the photodiodes  121  and  122  and determines the position of the optomechanical switching element  124 . As shown, switch  120  is configured in a manner similar to switch  110 .  
      In operation, switches  110  and  120  are initially in states A and B, respectively. That is, the switches  110  and  120  provide a continuous optical path to points A and B on the primary path so that the signals are transmitted along the primary path  112 . If transmission along the primary path  112  is now lost because of a fiber break, the controller  127  will respectively force the optomechanical switching elements  116  and  124  to switch from positions A and B to positions A′ and B′, respectively. As a result, traffic is now transported along the secondary path  114 . As previously mentioned, transmission will continue along the secondary path  114  even after the primary path  112  has been restored. The only way to restore switches  110  and  120  to states A and B, respectively, is to perform a manual power cycling in which the optomechanical switching elements  116  and  124  return to their initial states. This limitation is overcome with the inventive optical switches depicted in  FIG. 2 .  
       FIG. 2  shows a simplified block diagram of an arrangement for providing a primary and second optical path between the head end and an optical node in a CATV transmission system in accordance with the present invention. Optical switches  210  and  220 , which are located in the head end and the optical node, respectively, switch optical traffic between primary transmission path  212  and secondary transmission path  214 . Optical switch  210  includes an optomechanical switching element  216 , tap couplers  217 ,  218 ,  230  and  232  and photodetectors  221  and  222 . Likewise, optical switch  220  includes an optomechanical switching element  224 , tap couplers  225 ,  226 ,  240  and  242  and photodetectors  227  and  228 . A switch controller  227  controls the operation of switches  210  and  220 . Each switch  210  and  220  has its own controller because the switches are often located 50-100 km apart.  
      Optomechanical switching elements  216  may be any arrangement that employs physical motion of one or more optical elements to perform optical switching. In this way, a spatial displacement of a reflected beam is affected. Photodetectors  221 ,  222 ,  227  and  228  may be any component that converts an optical signal received from the tap couplers to an electrical signal such as a photodiode, for example.  
      Referring to switch  210  in more detail, tap couplers  218  and  221  respectively couple a small portion of the optical signals traveling in paths  212  and  214  to photodiodes  221  and  222 . In addition, tap couplers  230  and  232  are also located in the primary path  212  and the secondary path  214 , respectively. Tap coupler  230  couples a small portion of the optical traffic traveling along the primary path  212  and directs it along optical fiber  234  to tap coupler  232 . Tap coupler  232 , in turn, couples the portion of the optical traffic received from primary path  212  onto the secondary path  214 . That is, a portion of the traffic traveling along the primary path  212  is placed on the secondary path  214 . Likewise, tap coupler  232  couples a small portion of the optical traffic traveling along the secondary path  214  and directs it along optical fiber  234  to tap coupler  230 . Tap coupler  230 , in turn, couples the portion of the optical traffic from the secondary path  214  onto the primary path  212 . That is, a portion of the traffic traveling along the secondary path  214  is placed on the primary path  212 . As shown, switch  220  is configured in a manner similar to switch  210 .  
      In operation, assume switches  210  and  220  are in states A′ and B′, respectively, as a result of a fiber break along the primary path  212 . A small portion of the optical signal traveling in the secondary path  214  is coupled to the primary path  212 . Photodetector  222  in switch  210  detects the signal but, because of the fiber break, photodetector  228  in switch  220  will not detect the portion of the signal tapped from the secondary transmission path  214 . However, when the primary path  212  has been restored, both photodetectors  222  and  228  will detect the portion of the signal tapped from the secondary transmission path  214 . In response to the signals detected by both photodetectors  222  and  228 , controller  227  activates the optomechanical switching elements  216  and  224  so that the switches  210  and  220  are returned to state A and B. That is, the transmission is automatically restored to the primary state. Power cycling is not required at either the head end or the optical node.  
      The state of the optomechanical switching elements  216  and  224  is determined by the voltage that is applied to them via electrical boards incorporated into the switches  216  and  224 . A threshold condition is established for the switches  216  and  224  in software that determine the value of the voltage pulse (or current pulse) that changes their state. The threshold condition can be adjusted either by the operator or can be factory-set based on the distance over which the signal is transmitted and customer requirements.  
      The analog voltages generated by the photodetectors are directed to the controller  227  via logarithmic amplifiers, which are used by firmware to determine the appropriate state of the optomechanical switching elements. The firmware then sends a voltage pulse (or current pulse) to the optomechanical switching elements  216  and  224  to switch them from the primary path to the secondary path, or visa versa.  
      Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, while the invention has been described in terms of an optical switch that provides a secondary or backup path in a CATV system, the optical switch more generally may be employed in any optical transmission system in which a backup path is to be provided.