Method and apparatus for monitoring wavelength multiplexed optical

Abstract

One method of determining the signal to noise ratio of an optical signal models the noise floor based on amplified spontaneous emission (ASE) which is accumulated along the link. The ASE wavelength dependence is represented by a known mathematical function with a limited number of parameters. The parameters of this function are derived from measurements of the noise floor at a limited number of points by computational best fit means. A number of fiber grating filters at wavelengths between ITU allocated wavelengths are used to sample the optical noise and reflect the light back. An optical circulator is used to direct the reflected ASE light to an optical switch which allows the detector to select between transmitted and reflected light. The signal transmitted through the switch is monitored by a scanning filter with the appropriate resolution to resolve the signal or noise peaks. A first scan of the transmitted peaks is carried out and the signal is digitized and stored. A second scan of the reflected noise is carried out and digitized. A microcomputer computes the peak values and wavelengths, calibrates the wavelength scale from the known grating wavelengths, recomputes the ASE shape and the SNR, and communicates the results to outside agents when queried.