Abstract: The invention describes devices and methods for determining the wavelength of coherent optical radiation such as is emitted from a laser source. The apparatus is monolithic with no moving parts and consists of optical components that generate signal periodic in the optical frequency of the coherent radiation detected by the component. Each optical component generates a signal with a different period. Differences between the periods of the signals generated by the optical frequency-dependent optical components provides a means of measuring optical wavelengths over a range far exceeding the free spectral range limitations of conventional interferometers. The method of the present invention allows for measurement of optical frequency with an uncertainty of much less than the period of the optical frequency-dependent optical components forming the apparatus.