A Laser Heterodyne Interferometer For Measuring Surface Roughness

A laser heterodyne interferometer for measuring surface microtopography is described in this paper. A frequency difference between reference arm and measuring arm is obtained by utilizing two acousto-optic modulators in the interferometer. Phase demodulation technique is employed to improve the resolution up to 0.36 degrees, corresponding to one thousandth wavelength. Height variations of 10 nm can be resolved, while the theoretical height resolution is 10 angstroms. The simplest layout and the less optics are used in the interferometer without any polarizes and lens designed specially as other interferometers.