A system for detecting wavelength error that includes one or more ordered
pairs of photodetectors each of which receives an individual optical
signal and is used to detect a wavelength error in that optical signal.
Each individual optical signal is directed towards a corresponding pair
of photodetectors. The pair of photodetectors are positioned around a
region of maximum constructive interference or minimum insertion loss. A
feedback circuit analyzes output signals from the photodetectors and
determines wavelength error based on differences between the output
signals from the photodetectors.