A Research of Intermodulation Components in Laser Vibrometers for Precise Measurement of Oscillations

Abstract

This paper presents the results of theoretical research of signals of laser vibrometers, with the surface of the object of interest orthogonal to the direction of the beam. The article shows that the Michelson interferometer based vibrometer may yield erroneous results when measuring the amplitude of oscillations. The errors arise due to the phase shift between the interfering beams, which affects the measurement of the absolute value of surface shift. The article presents theoretical analysis of the spectra of the output signals of interferometers of various designs. For the Michelson interferometer based vibrometer, the algorithm for the measurement of frequencies and amplitudes by reading the oscillograms of interference signals suggests that the amplitude be much larger than the laser's wavelength. It is proved that in this case the result does not depend on the mismatch in phases between the beamsplitter and each mirror. The output signal of the heterodyne interferometer has been analyzed. The property of spectral components of being independent of the beams' phases helps eliminate errors when calculating the amplitude. A special design of the Michelson interferometer is proposed, where the beam's frequency is shifted by using the optical modulator that operates in the Bragg diffraction mode.