Dual-wavelength interferometer for surface profile

Abstract

The design and first experimental results of a dual-wavelength heterodyne interferometer (DWHI) for precise surface profile measurements are presented. The DWHI is based on a Mach-Zehnder type interferometer with the surface to be measured in the sensing arm of the interferometer. Two adjacent wavelengths are simultaneously used in the interferometer; the range information is gained from the beat frequency of both heterodyne interferometer signals. Thus, the extreme range ambiguity of classical interferometers and their sensitivity to mechanical distortions are reduced by the ratio of laser frequency to beat frequency. The DWHI is expected to have the good SIN performance a of coherent detection system but also the capability of measuring to diffusely scattering targets. The main features of the system presented here are:

use of a single diode laser in combination with a high frequency Bragg cell to generate two adjacent wavelengths use of an all-fiber concept for the optomechanical build-up