Compact Heterodyne Pinhole Point-Diffraction Interferometer with High Repeatability and Adjustable Contrast Based on Dual-Cascaded Acousto-Optic Modulators

Conventional pinhole point-diffraction interferometers (PPDIs) owe their popularity to a simple configuration, but that same configuration limits both the usable measurement aperture and the load the phase shifter can carry. Here, we propose a heterodyne phase-shifting point-diffraction interferometer (HPDI) based on dual-cascaded acousto-optic modulators (AOMs), which removes both limitations while keeping the configuration compact. The dual-cascaded AOM module integrates the functions of frequency shifting, beam splitting, and contrast modulation. By modulating its input RF power, the interference contrast can be flexibly adjusted. Simulation and experimental results verify the feasibility of the proposed HPDI, which achieves a measurement repeatability of 0.045 nm (RMS, with piston, tilt, and defocus removed) and agrees with an absolute shift-rotation calibration to within 3.3 nm (RMS).