Solid state and fibre optic laser doppler velocimeters

Abstract

Laser Doppler velocimetry (LDV) has become established as an important technique for the measurement of velocities of macroscopic objects and fluids: the dynamic range is large (∼10 -6-10 5 ms -1) and the measurement is absolute and non-invasive. However, the size and cost of LDV has restricted its use in some areas. This paper presents two separate approaches to reduce these problems: we describe a compact LDV system incorporating a solid state laser diode and also an investigation of the feasibility of a fibre optic LDV system in which the conventional optical components are replaced by fibre optics. The experimental arrangement used for the solid state LDV system was of the Doppler difference type; i.e. a system of parallel interference fringes is focused in the measurement volume, so that a particle passing through this volume produces a scattered light signal which is intensity modulated. In its simplest form, the technique cannot determine the direction of motion of the particle, but this difficulty may be overcome by causing the fringes to ‘move’ within the measurement volume with known velocity. In the present experiments, the laser output frequency was modulated by modulating its drive current; since the path lengths of the two beams interfering in the measurement volume were unequal, fringe motion was achieved. The fibre optic LDV experiment was also of the Doppler difference type, and it was demonstrated that the necessary stabilised interference fringe system could be projected using a fibre optic system. An electronic servo was devised to compensate for the random differential thermal drifts in the fibres which would otherwise have produced unacceptable drifts in the fringe pattern.