Multiplex communications – Diagnostic testing
Reexamination Certificate
1998-12-21
2001-12-04
Ngo, Ricky (Department: 2731)
Multiplex communications
Diagnostic testing
C359S199200, C359S199200
Reexamination Certificate
active
06327250
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a line monitoring system employed in an optical communication system, and more specifically to a line monitoring system in which crosstalk is suppressed between the monitoring channel and the data channels.
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 modulates 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.
One problem that arises from the use of an amplitude modulated test signal is that crosstalk is generated between the test signal and data channels. It has been determined that the crosstalk increases as the frequency separation between the test signal and the data channels increase. That is, data channels nearest the test signal exhibit the least crosstalk while data channels farthest from the test signal exhibit the greatest crosstalk. Moreover, crosstalk occurs among data channels that are both greater and less than the frequency of the test signal. Based on these observations, the source of the crosstalk is Raman gain that results in a transfer of power between the test signal and the data channels.
Accordingly, it would be desirable to reduce the cross talk that arises between the test signal and the data channels in a wavelength division multiplexed optical communication system.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for monitoring an optical transmission path through an optical transmission system supporting bi-directional communication between first and second terminals along first and second optical transmission paths. The first transmission path includes at least one optical amplifier located therein. In accordance with the method, a test signal is generated, which is formed by a superposition of first and second optical tones located at first and second wavelengths, respectively. The first and second wavelengths are within the bandwidth of the optical amplifier. The amplitude and phase of the first and second optical tones are arranged so that the test signal has a substantially constant intensity over a modulation cycle of the first and second optical tones. The test signal is transmitted from the first terminal along the first optical transmission path and through the optical amplifier. A portion of the test signal is received at the first terminal after it traverses the optical amplifier, an optical loop-back path, and a second transmission path. The received portion of the test signal may be compared to a delayed rendition of the generated test signal to assess transmission path performance.
The present invention advantageously employs a test signal with a constant intensity so that the Raman gain transferred between the test signal and any given data channel will also be constant. By suppressing the intensity variations in the data channels in this manner crosstalk between the test signal and the data channels will not arise.
REFERENCES:
patent: 4586186 (1986-04-01), Anderson
patent: 4633464 (1986-12-01), Anderson
patent: 5654816 (1997-08-01), Fishman
patent: 5960833 (1999-10-01), Jensen
patent: 6124960 (2000-09-01), Garthe et al.
Chen Chien-Jen
Golovchenko Ekaterina A.
Pilipetskii Alexei N.
Ngo Ricky
TyCom (US) Inc.
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