Laser vibrometry: pseudo-vibrations

The application of Laser Doppler Velocimetry (LDV) to the measurement of normal-to-surface vibration of a solid surface is now established as a technique complementary to the use of an accelerometer. Several practical systems have been developed and a number are now commercially available. Each velocirneter relies on the same principle of operation, namely the detection of a Doppler shift, fD, in the light scattered from a vibrating target. Fig. 1 shows a typical vibrometer arrangement. Since the photodetector cannot respond quickly enough to detect the light frequency directly, scattered light from the vibrating surface is mixed with a reference beam and heterodyned on the detector surface. In addition, in order to resolve the sign of the vibration velocity, it is necessary to pre-shift the reference beam by a known amount, fR, resulting in an optical beat at the detector of frequency (fR ± fD). An appropriate Doppler signal processor then demodulates the detector signal to produce a time-resolved analogue of the target vibration velocity (in the direction of the incident beam). Systems differ in the method adopted to produce the reference beam frequency shift. Bragg cells [1], diffraction gratings [2] rotating scattering discs [3] and frequency modulation of the laser beam itself [4] have all been used successfully... (continues).