We describe a dual-beam differential phase interferometer that performs direct phase measurement by using a rotating Ronchi grating filter. Two laser beams derived from a single source are focused onto the sample surface. The reflected light will retrace the same path and intersect at the back focus of the objective lens. Interference between the reflected rays will therefore produce intensity fringes with their spatial location governed primarily by the differential optical phase of the probes. When the fringes are projected onto a rotating grating filter 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 high impedance electronics with shot-noise-limited performance at low laser power.
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