Optical Heterodyning And Doppler Effect Applied To Laser Vibrometers And Anemometers

Elastic diffusion of light on moving objects is affected by Doppler effect ; coherent detection of this effect gives the opportunity to reach high resolutions (1010 to 1011). This light beating spectroscopy provi-des a frequency modulated signal proportional to the velocity of the diffusing object. The main characteristics of this optical heterodyne detection are : . a coherent mixing of two colinear waves of different intensities and frequencies, . a detected waves etendue limited by the coherence Otendue, . no sensitivity to thermal noise. Laser noise (mode beating and plasma noise) which usually limits coherent detection sensitivity is removed with a balanced heterodyne detection which improves greatly the signal to noise ratio. Common mode noise is removed in the same time the signal is multiplied by a factor two. Application of these methods to different optical measurements is presented. The first one, an infrared 8 watts CO2 laser anemometer is able to perform remote measurements of atmospheric wind up to 500 meters. The second one, a vibrometer using a 1 mW red laser is able to perform vibration measurements on a poor diffu-ser at distances of several meters with a resolution of half a wavelenght. Main advantages and limitations of these methods are discussed. Industrial applications are presented.© (1980) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.