Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-11-06
2001-07-24
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
Reexamination Certificate
active
06266172
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical communication systems and more particularly to techniques for determining the bit rate and monitoring the performance of a channel signal in an optical system, without requiring complete recovery of the channel signal.
BACKGROUND OF THE INVENTION
A conventional receiver in a multi-channel optical communication system typically includes a clock recovery circuit designed to operate at a particular predetermined bit rate. The receiver is therefore bit-rate specific. Performance monitoring of a given channel signal in the receiver is generally done only after the channel signal data has been completely recovered, and thus after a bit-rate specific clock recovery process. This presents a significant problem in applications such as wavelength division multiplexed (WDM) optical networks in which the transmission medium is generally bit-rate independent. More particularly, the nodes of such a network often do not have any information regarding the bit rate of particular channel signals which are routed through them. As a result, it is generally not possible to monitor the performance of a channel signal at a network node without including the above-noted bit-rate specific receiver circuitry at that node.
Including such bit-rate specific circuitry for each channel signal at each node of a network would substantially increase the cost and complexity of the network, and is therefore not a viable solution for most practical applications. Unfortunately, there are apparently no suitable alternative techniques which may be used to provide a bit-rate independent channel signal monitoring capability in a node of an optical network. A need therefore exists for a device which can identify a channel signal and measure its performance at a node of an optical network, without requiring prior information about the specific signal bit rate, without completely recovering the channel signal data, and thus without the need for bit-rate specific clock recovery circuitry.
SUMMARY OF THE INVENTION
The invention provides methods and apparatus for determining the bit rate of an optical signal and measuring the performance of the optical signal, without any prior information regarding the signal bit rate and without the need for completely recovering the signal data. In an illustrative embodiment, one of several non-return-to-zero (NRZ) modulated channel signals in a multi-channel optical signal is selected using a tunable optical filter. The selected channel signal is then applied to a front-end optical receiver or photodetector which generates a corresponding electrical signal. The electrical signal is processed in an autocorrelator which generates an autocorrelation function of the electrical signal, and the width of a peak in the autocorrelation function provides a measure of the channel signal bit rate. The autocorrelator may include a splitter for separating the electrical signal into first and second parts, a delay line for introducing a delay between the first and second parts, a multiplier for generating the product of the first and second parts, and an integrator for integrating the multiplier output over a suitable time period in order to provide the autocorrelation function. The autocorrelator may be part of an optical receiver which uses the determined bit rate to generate a clock signal for recovering the channel signal data.
In accordance with another aspect of the invention, the performance of the selected channel signal may be measured after the bit rate has been determined in the manner described above. The delay line is set to provide a delay corresponding to the inverse of the determined bit rate, and the output of the multiplier is digitized to provide an indication of voltage level. The voltage levels falling within a specified range corresponding to a desired eye margin are then counted to determine an approximate bit error rate for the channel signal. Unlike performance measures generated using conventional techniques, this performance measure can be generated in a bit-rate independent manner, and therefore without the need for bit-rate specific clock recovery circuitry. As a result, the invention is well suited for implementation in nodes of a WDM optical network and in numerous other important applications.
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Agere Systems Optoelectronics Guardian Corp.
Pascal Leslie
Ryan & Mason & Lewis, LLP
Singh Dalzid
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