SYSTEM AND METHOD FOR MONITORING AND CHARACTERIZING OPTICAL LINKS

The present invention provides a system and method for monitoring and characterizing optical links of a communication system by transmitting and receiving optical test signals of a certain wavelength through the optical links where such test signals do not interfere in a significant manner with other optical signals being conveyed through the optical links.

DETAILED DESCRIPTION FIG. 2 depicts a communication system in which Wavelength Division Multiplexers ( 124 , 126 , 128 and 130 ) are attached to both ends of system optical links 106 and 108 such that a test signal having one wavelength is conveyed via optical links 106 and 108 while other optical signals having wavelengths different from the one wavelength are conveyed also via optical links 106 and 108 without interfering with the test signal allowing the system optical links ( 106 , 108 ) to be monitored and characterized by monitoring shelf unit 122 which is coupled to WDM 126 via optical link 118 . It will be readily understood that although the customer equipment is connected to the Central Office equipment via two unidirectional optical links ( 106 , 108 ), the customer equipment can be connected to the Central Office equipment with one or more unidirectional or bi-directional optical links. Wavelength Division Multiplexers (WDM) are devices with at least one input and at least one output through which optical signals of various wavelengths are received and transmitted where said optical signals are processed (e.g., amplify, attenuate, combine, separate) independently of each other. It will be readily understood that other devices or combination of devices which can be configured to perform the functions of Wavelength Division Multiplexers can also be used. For economy of text and for the sake of clarity, the various optical signals discussed herein will be denoted in term of their wavelengths, &lgr; n . Still referring to FIG. 2 , laser test source 120 generates optical signals having wavelength &lgr; 1 which are fed to an input of WDM 124 . Other optical signals of wavelength &lgr; 2 originating from Transmitter 102 are fed to another input of WDM 124 . Both signals (&lgr; 1 , &lgr; 2 ) are transmitted through WDM 124 and onto optical link 106 . The signals are received by WDM 128 which allows the &lgr; 2 signals to pass through to Receiver 112 via local link 107 and routes the &lgr; 1 signals to an input of WDM 130 via loopback link 132 . Transmitter 114 generates optical signals of wavelength &lgr; 3 which is fed to another input of WDM 130 . The signals (&lgr; 1 , &lgr; 3 ) are transmitted through WDM 130 to the central office via system optical link 108 . Note that the &lgr; 2 signals may have the same or substantially the same wavelength as the &lgr; 3 signals as some communication systems transmit and receive optical signals having the same or substantially equal wavelengths. Thus, the &lgr; 2 and &lgr; 3 signals are used by the system and its users for communication and the &lgr; 1 signals are used as test signals for monitoring and characterizing system optical links 106 and 108 . Even though the &lgr; 1 and &lgr; 3 signals are being conveyed simultaneously through system optical link 108 , there is virtually little or no interference between the two signals because their wavelengths are sufficiently different from each other precluding any significant interaction between them that would cause any significant signal degradation. Similarly, there is little or no interaction between the &lgr; 1 and &lgr; 2 signals which are conveyed simultaneously through system optical link 106 . WDM 126 receives the optical signals (&lgr; 1 , &lgr; 3 ) from WDM 130 and allows the &lgr; 1 signals to pass through to monitoring shelf unit 122 via link 118 . The &lgr; 3 signals pass through WDM 126 to receiver 104 via link 108 . It will be readily understood by those skilled in the art to which this invention belongs that the transmitters ( 102 , 114 ) can launch optical signals having more than one wavelength and the receivers ( 104 , 112 ) can are configured to receive such multi-wavelength optical signals. The multi-wavelength signals are chosen such that they substantially do not interfere with the test signal (&lgr; 1 ) Monitoring shelf unit 122 has equipment, well known to those skilled in the art, which is capable of detecting any signal degradation and where in the system optical links such degradation has occurred. Monitoring shelf unit 122 detects signal degradation by monitoring the &lgr; 1 signals after such signals have propagated through the system optical links. Monitoring shelf unit 122 not only has the capability to detect the signal degradation, but it can also analyze the received &lgr; 1 signals to identify the cause of the signal degradation. Through various well known techniques, monitoring shelf unit 122 is thus able to characterize the system optical links based on the received optical signals (&lgr; 1 ). For example, based on the received &lgr; 1 signals, monitoring shelf unit 122 can characterize system optical links 106 and 108 as marginally functional because signal degradation has been detected and found to be due to excessive attenuation of the optical signals at some point in system optical link 108 . The detected signal degradation may also be characterized as the result of improperly functioning customer equipment. Additionally, monitoring shelf unit 122 can document the extent of the signal degradation, the time of occurrence of the signal degradation of certain optical links and record the number of failures occurring in a specific optical link during a certain period of time. The &lgr; 3 optical signals are routed to Receiver 104 via link 108 . Therefore, the Central Office equipment can communicate with customer equipment via optical links while simultaneously allowing the optical links to be monitored and characterized on a continuous, continual, periodic or aperiodic basis to determine the health of the communication link, or cause and location of the signal degradation. Monitoring shelf unit 122 can or in concert with a central processing unit perform statistical analysis of the system and /or local optical links (i.e., one link, a plurality of links, segments of one or a plurality of links) for a short term history (e.g., hourly, daily, weekly, monthly) of the integrity of the system optical links. Thus, for example, once a signal degradation condition is detected, monitoring shelf unit 122 records the time of occurrence, the magnitude of the occurrence and its characterization of the optical link in which the signal degradation condition has occurred. The recorded information can be analyzed by the customer and/or the system provider to determine which of the two entities is responsible for the occurrence of the signal degradation. Thus, the system provider and/or the customer can use a monitor shelf unit to perform statistical analysis on segments of links, on an entire link or on portions of the communication system. The statistical analysis performed by the monitoring shelf unit can be done automatically without having to dispatch a craftsperson thus reducing maintenance costs associated with the system. The results of the statistical analysis can be helpful in establishing measures (e.g., replacing link segments after a certain number of failures) that reduce the occurrence of link failures and consequently help reduce the occurrence of signal degradation. Referring to FIG. 3 , WDM 128 and 130 are located at the system side of demarcation line 110 and are thus owned by the system provider. With this arrangement, the customer need not incur the cost of the test equipment (WDM 128 , 130 , loopback link 132 ) needed to practice the present invention as such equipment would be under the control and ownership of the system provider. The test equipment may be located at an appropriate location that is accessible to the system links ( 106 , 108 ) and the local links ( 107 , 109 ).