Abstract: A method and apparatus may be used to determine the wavelength of an input optical signal. The wavelength of an input optical signal may be determined by dividing the input optical signal into a plurality of individual optical signals and inducing the individual optical signals to interfere with each other to form a plurality of output optical signals, such that the powers of the output optical signals have a ripple dependence on the wavelength of the input optical signal. The peak amplitude of each of the powers depends monotonically on the wavelength of the input optical signal over a range of wavelengths to be monitored for said input optical signal. A plurality of electrical output signals may be produced from the output optical signals, each electrical output signal corresponding to the power of each output optical signal. The electrical output signals may then be compared to a predetermined set of signal values in order to determine the wavelength of the input optical signal.