Digital Wavemeter for cw Lasers

Abstract

The wavelength of light can be measured by counting the number of fringes as an interferometer mirror is moved through a known distance. The known distance is best obtained by simultaneously counting the number of fringes from a standard wavelength source for the same change of optical path length. The arrangement in Fig. 1 permits rapid and precise comparison of wavelengths by moving a mirror smoothly over long distances. The interferometer is a two-beam, division of amplitude type related to the Michelson and Sagnac interferometers. In it, the standard and unknown beams travel identical paths in opposite directions. They emerge at separate detectors, so that there is no need for a dichroic beam splitter to separate them, and measurements can be made close to the wavelength of the standard if desired. Moreover, the visible spots of light on the mirrors can be brought into coincidence, ensuring that the two pathlengths are nearly identical, and simplifying the alignment of the unknown beam. The effects of vibrations are also minimized by the use of identical paths.