The present invention is related to optical communications systems. 
There is a general need in optical communications systems for a function referred to as, “performance monitoring”.  Performance monitoring generally involves monitoring the quality of optical communications signals at various points in a system to identify signals whose characteristics indicate the need to take some kind of corrective action.  Examples of such monitored signal characteristics include signal power, optical signal-to-noise ratio (OSNR), and bit error rate.  Corrective actions that can be taken include adjusting transmitter power to improve signal power or OSNR, adjusting or replacing optical filters that may be contributing to noise or excessive signal attenuation, replacing or repairing faulty connections, etc.  The need for performance monitoring is particularly great in modern wavelength division multiplexed (WDM) optical communications systems.  In WDM systems, many optical communications signals are carried together through optical fibers, optical amplifiers, and other system components.  The signals are placed as close together as possible in the frequency domain to obtain high overall capacity of data transmission.  Due to the close spacing of many signals, there is a relatively high potential for signals to suffer excessive crosstalk or nonlinear optical pulse distortion, and other deleterious effects.  Thus, it is important to monitor signal quality to detect such problems so as to permit corrective action to be taken. 
It has been known to use a piece of test equipment referred to as an “optical spectrum analyzer” or OSA to monitor optical signal quality.  OSAs provide for accurate measurement of optical signal characteristics such as optical power level and optical noise level.  OSAs are intended for use in activities such as initially setting up a communications link, diagnosing faults, and routine maintenance.  OSAs are also very expensive. 
It has also been known to use a piece of test equipment referred to as optical sampling oscilloscope to monitor optical signal quality.  This instrument provides a complete set of measurement capabilities including live waveforms and histograms.  Optical sampling oscilloscopes aid designers of high speed transmitter devices and systems.  Optical sampling oscilloscopes are also very expensive. 
A relatively inexpensive technique for performance monitoring in optical communications system would be desirable.