A channel detection system includes an interferometer coupled to a spectrum analyzer
to differentiate additive spontaneous emission (ASE) noise from optical channels
in a dense wave-division multiplex (DWDM) signal. It is assumed that channels,
if present, are centered at frequencies corresponding to a standardized channel
grid. The relative delay of the interferometer is chosen to be greater than the
coherence time of the ASE noise but less than the coherence time of the channels
with the interferometer's free spectral range set to an integer divisor of the
channel-to-channel frequency spacing of the grid such that active channels experience
a high degree of interference. The phase delay of the interferometer is then adjusted
to maximize the interference at each grid-aligned frequency. The spectrum-analyzed
outputs are compared (e.g., subtracted from one another and then thresholded) to
determine the channels present in the DWDM signal.
