Laser Doppler vibration measurement using a polarisation-based device

Abstract

A non-contacting laser-based displacement and vibration measuring device is presented. It is based on the coherent detection of the Doppler shift introduced into light scattered by an optically rough target object. Geometric manipulation of the beams within the system compensates for the speckle in the scattered light. The sense of the target motion is derived using a passive technique based on controlled polarisation of the reference beam.In its simplest mode of operation it has a resolution of half of the laser wavelength, but this may be increased by a factor of four. Continuous measurements of displacement of up to 10 cm have been made with an experimental system, with less than 1% error; vibrations of smaller amplitude have been measured with greater accuracy. In the existing configuration the maximum target velocity in the direction of the measuring beam is limited by the 100 kHz analogue signal processing roll-off to 32 mm/s; however, this may easily be extended by using more sophisticated electronics.