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.