Differential laser Doppler vibrometry for displacement measurements down to 1 mHz with 1 nm amplitude resolution in harsh environments

Abstract

Environmental disturbances increasingly limit laser Doppler vibration measurements with decreasing frequency of the moving surface. Particularly limiting for the detection of small displacements of a specimen in a furnace are refractive index fluctuations in the air from heat haze, optical path length fluctuations from thermal expansions in the measurement setup, and mechanical vibrations from the building and other sources. In this paper, we demonstrate how these disturbances can be largely compensated for. In our application, we require a displacement resolution of less than 10 nm to measure the chemical expansion of volume crystals and thin films at an ambient temperature of up to 800 °C. The required sensitivity cannot be achieved with state-of-the-art commercially available laser Doppler vibrometers since they are not designed to operate at frequencies below 0.1 Hz. In order to reduce the dominating low-frequency environmental noise, we employ a differential laser Doppler vibrometer in a setup optimized to minimize low-frequency displacement noise. We prove that the concept of differential measurement can eliminate disturbances by optical turbulence and parasitic mechanical vibrations with a newly constructed laser Doppler vibrometer. The elimination of these noise contributions allows for the first time a displacement sensitivity of about 1 nm for frequencies in the mHz range in a 600 °C environment.