Abstract: There is provided a method, apparatus and system for calibrating and operating an optical wavemeter. In calibration, training optical signals with known wavelengths are input to a wavemeter, and corresponding photodetector measurements are obtained. Optical parameters of the wavemeter are then estimated based on the measurements. The optical parameters are indicative of a length difference ?L between two unequal-length waveguides in an optical delay line of the wavemeter; and scattering parameters of a multi-mode interferometer (MMI) coupler of the wavemeter. The estimation process involves a (e.g. golden-section) search to determine one or more output values for at least one of the optical parameters, based on an objective function which indicates a difference expected and actual measurements. The expected measurements are generated based on a numerical model incorporating candidate values for the optical parameters.