A multimode waveguide interferometer

A multimode waveguide interferometer (MWI) has been developed that can produce fringe spacings as small as λ/9, where λ is the wavelength of the incident light. Interferometers are used to detect tiny changes in length. Typically, a beam of light travels over the distance to be measured and then combines with a reference beam that always travels a fixed distance. The two beams create an interference pattern of bright and dark bands, or fringes, that shift as the distance traveled by the measurement beam changes. Usually, the separation of the fringes (which determines the resolution of the interferometer) is no less than λ/2. Now, two researchers at the University of Stuttgart in Germany have done away with the reference beam. Instead, they have directed a single beam of light obliquely into a waveguide formed by two parallel, movable mirrors. The beam experienced multiple reflections from the mirrors and propagated as a combination of many modes. Each mode interfered with every other mode, which led to a modulation in the light transmitted through the waveguide. The number of reflections within the MWI determines the device’s sensitivity. For their 633-nm light and a mirror separation of about 30 microns, the fringe spacing was only 70 nm. The physicists’ calculations suggest that a more refined MWI could show fringe separations as small as 10 picometers. The duo says that, in addition to opening the door to new, high-precision measurements, MWIs might be useful in optical switches and other communication-related devices. (Y. B. Ovchinnikov, T. Pfau, Phys. Rev. Lett. 87, 123901, 2001 http://dx.doi.org/10.1103/PhysRevLett.87.123901 .)