Abstract: A wavemeter fabricated using integrated optics technology. A waveguide structure is formed in a substrate, the structure including signal and reference waveguides and means for coupling an optical input signal into both waveguides. Means are provided for varying the optical path length of the signal waveguide with respect to the reference waveguide in response to a control signal. Optical signals passing through the waveguides are coupled to a detection system that produces a feedback signal having a characteristic that is a function of the optical path length difference between the waveguides, and of the wavelength of the optical input signal. A control system receives the feedback signal and produces the control signal such that the feedback signal characteristic is driven towards a predetermined value. The control signal then provides a measure of the wavelength of the optical input signal.