laserDoppler

Laser Doppler is an optical technique that uses the Doppler shift of coherent light scattered from moving particles or surfaces to measure velocity. A laser beam is directed into a flowing medium or across a vibrating surface, and the light scattered by moving scatterers contains a frequency shift proportional to the velocity component along the light’s propagation direction. To generate a measurable signal, multiple beams are arranged to form an interference fringe pattern, and the scattered light is mixed with a reference beam. The resulting Doppler signal provides information about speed and direction along a defined measurement axis.

Common implementations include two-beam and three-beam arrangements. In two-beam configurations, two laser beams intersect at a measurement volume, creating stationary interference fringes; particles crossing the fringes modulate the scattered light at a Doppler frequency proportional to their velocity. In three-beam or heterodyne LDV, a Bragg cell or frequency shifter introduces a known offset to the reference beam, enabling determination of velocity sign and improving signal quality. The technique is largely non-contact and offers high temporal resolution, with measurement volumes suitable for localized flow or surface motion analysis.

Applications span fluid mechanics, turbidity and blood flow studies, spray and combustion diagnostics, and non-contact vibration measurements in engineering (laser Doppler vibrometry). Advantages include non-invasiveness, high sensitivity, and fast response; limitations include reliance on scattering from particles or surface roughness, sensitivity to optical alignment, and a relatively small sensing volume. Safety considerations arise from laser exposure, necessitating appropriate eye protection and beam control.