A Novel Interferometer

Interferometers are sensitive measuring devices that use the wave properties of light to detect tiny changes in length. Physicists Michelson and Morley ushered in the age of modern physics when their interferometer helped show that there is no aether filling the voids between the stars and planets. Countless other experiments have relied on interferometers of various designs.

Now, researchers at the Universität Stuttgart have developed a new type of interferometer that may exceed the measurement sensitivity of older designs by as much as five hundred times. In most interferometers, a beam of light is allowed to travel over some distance that a physicist would like to measure. The measurement beam is then combined with a reference beam that always travels a fixed distance. The two beams create an interference pattern that consists of a number of bright and dark bands, or fringes, that shift as the distance traveled by the measurement beam changes.

The new multimode waveguide interferometer (MWI), on the other hand, does not have a fixed reference beam. Instead, a single beam of light enters a waveguide formed by two movable parallel mirrors. The beam propagates as a combination of many modes, effectively following numerous paths simultaneously through the waveguide. Each mode interferes with every other mode, leading to a modulation in the light transmitted through the waveguide, and a sensitive measurement of the distance between the waveguide mirrors.

A laboratory test of the MWI resulted in detection of motion as little as a ninth as large as the wavelength of the light that entered the interferometer, and theoretical calculations suggest that a more refined version could detect motion a thousand times smaller than the light wavelength used. Conventional interferometers, by contrast, are capable of accurately measuring distance changes only half the wavelength of the input light or larger.

In addition to opening the door to new, high precision measurements, MWIs may serve as fast optical switches and other communication-related devices. But perhaps most importantly, the researchers point out, the MWI shows that significant innovations can be surprisingly simple even in refined and fundamental fields like classical optics. (Ovchinnikov and Pfau, Physical Review Letters, 17 September 2001.)