Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-10-29
2001-11-27
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06323981
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical communication system and more particularly to a system for monitoring an optical communication system to locate intermittent faults.
BACKGROUND OF THE INVENTION
In long distance fiber optic communication systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Prior art monitoring techniques include the use of a testing system which generates a test signal and modulating the test signal onto a single channel (or wavelength) with the transmitted data signal. For example, the data signal may be amplitude modulated by the test signal. A loop-back coupler within an optical amplifier or repeater located downstream is used to return a portion of the transmitted signal (data signal plus test signal modulation) to the testing system. The testing system then separates the test signal from the data signal and processes the test signal to examine the health of the transmission system. U.S. Pat. Nos. 4,586,186 and 4,633,464 to C. Anderson et al. discloses a similar technique to modulate test response information from a repeater onto the main data signal to monitor the health of the system.
There are several disadvantages and drawbacks, however, with the prior art monitoring systems. Due to the low signal to noise ratio of the returning test signal, measurements must be taken over a significant period of time. To properly assess the status of the communication system requires that data be collected over periods ranging from about a half hour when the system is out-of-service to about eight hours when the system is in-service. Therefore, if the monitoring system is to detect a fault, the fault must be manifest over the time period that the data is collected. Accordingly, the previously mentioned line-monitoring techniques are most effective in locating static faults. Intermittent faults, however, will go undetected if their duration is substantially less than the time over which monitoring statistics are collected.
Therefore, there is a need for a line monitoring system that can locate intermittent as well as static faults.
SUMMARY OF THE INVENTION
The present invention provides a method for locating an intermittent fault occurring in an optical transmission system that includes a plurality of repeaters extending along first and second optical transmission paths for supporting bi-directional communication. In accordance with the method, a measurement is obtained of the gain imparted to a test signal along a round trip path between its originating point and each repeater. For each repeater there are identified changes in gain beyond a prescribed threshold and the time when the gain changes occur. A record of transmission performance data occurring over time is received. The performance data may be a bit error rate, SONET overhead data, or forward error correcting metrics, for example. For each repeater, a correlation in time is established between the identified gain changes and the transmission performance data. A fault location is identified based on the established correlation.
In accordance with one particular embodiment of the invention, a method is provided for locating an intermittent fault in an optical transmission system that includes first and second optical transmission paths for supporting bidirectional communication. Initially, an optical test signal is generated, which is based on a tone modulated with a pseudo-random sequence. The test signal is combined with a data signal traveling on the first transmission path of the transmission system. A portion of the combined test and data signal is coupled through a loop-back path associated with a repeater situated along the first and second transmission paths so that a returning signal travels along the second transmission path. At least a portion of the returning signal is received, along with a delayed rendition of the test signal that is delayed in time based on the location of the repeater. The returning signal and the delayed rendition of the test signal are correlated to determine the gain imparted to the test signal along its round trip path between its originating point and the repeater. Finally, the gain values are correlated with transmission performance data stored in a receiving terminal to determine the fault location.
REFERENCES:
patent: 5453865 (1995-09-01), Faulkner et al.
patent: 5825515 (1998-10-01), Anderson
Chan Jason
Leung Christina Y
TyCom (US) Inc.
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