Phase-Shifting Interferometry Measures Surfaces

Hank Hogan

There are no good vibrations when it comes to precise surface measurements. Mirrors, for instance, are often manufactured to specifications of fractions of a wavelength, and that requires surface measurement resolutions of better than 100 nm. Sources of vibration such as air handlers and passing trucks can render those precise measurements difficult or impossible.

A team from the Optical Sciences Center at the University of Arizona in Tucson has found a way to make such measurements despite the world's shake, rattle and roll. Jim Burge, an assistant professor, and graduate student Chunyu Zhao developed the technique and an initial implementation, which has already proved its worth.

"We use this instrument routinely in our shop for characterizing prototype mirrors for space," Burge said.

The method is an extension of the phase-shifting interferometer, in which a beam of light is reflected off a surface, such as a mirror. Shifting the phase of the beam creates an interference pattern of light and dark fringes that highlight surface irregularities and height differences. Vibrations wash out these fringes and, if severe enough, can render the interferometer useless.

Burge and Zhao got around this problem by running what are essentially two phase-shifting systems simultaneously. Both work with the 633-nm output of a helium-neon laser.

One system operates at a high frequency, using a photodiode to capture fringe images in a 100-µs cycle. When it detects vibration, it feeds that information into an electrooptic modulator from ConOptics Inc. of Danbury, Conn., which acts as a servomechanism and adjusts the phase shift of the second, more precise system. The phase shift ramp for that component is sped up or slowed down 4000 times a second, a much faster rate than typical vibrations.

"This allows high-quality measurements in environments that have large amounts of vibration," Burge stated.

The vibration-compensated instrument performed well in tests, yielding results in a vibrationally noisy environment similar to those obtained in a quiet setting.

John Hayes, a co-worker of Burge's at the university, implemented further refinements of the technique in an instrument that he built for NASA.