HIGHLY STABLE DIFFERENTIAL PHASE OPTICAL INTERFEROMETER USING ROTATING RONCHI GRATINGS

We describe a dual-beam differential phase interferometer that perform s direct phase measurement by using a rotating Ronchi grating filter. Two laser beams derived from a single source are focused onto the samp le surface. The reflected light will retrace the same path and interse ct at the back focus of the objective lens. Interference between the r eflected rays will therefore produce intensity fringes with their spat ial location governed primarily by the differential optical phase of t he probes. When the fringes are projected onto a rotating grating filt er with identical periodicity, the transmitted optical signal becomes an oscillating signal from which the optical phase can be measured by standard phase-sensitive techniques. This approach has the advantages of simple system configuration, effective isolation from environmental disturbance, and low-frequency operation, which permit the use of hig h impedance electronics with shot-noise-limited performance at low las er power. (C) 1998 Optical Society of America.