Network Reference - Laser-Doppler Velocimeter

First introduced in 1964, laser-Doppler velocimetry (LDV) has been recognized as a very reliable method in measuring velocity of complex flow field including turbulent, separating, re-circulating and vortical flows. It can be used under hostile conditions as in chemically reacting flows, combustion, flames and flows with radiation. The instrument is capable of operating over wide velocity, density, temperature and composition ranges. LDV provides means of non-intrusive measuring technique for flows that are sensitive to disturbance. The size of the measurement volume is small enough for essentially point measurements. Having the ability for rapid response, LDV is also suitable for measurements of high-frequency fluctuations. No calibration is required since the velocity is measured directly from light scattered by particles in the flow. However, LDV systems are expensive and complex to setup (both in electrical and optical components). In addition, extracting velocity information can be a tedious and complex process. Conventional LDV systems commonly employ ion gas lasers because of their output power, coherence length and beam quality. The cost, size and power consumption of these lasers and their associated optics are all relatively large and this has prevented the use of LDV in many industrial applications. Commercially available diode lasers and avalanche photodiode detectors can be used to replace gas lasers and photomultipliers tubes. It is, therefore, possible to construct an LDV system, which is substantially reduced in cost and power consumption and can fit in the palm of a hand.

Figure below displays NAL Research's miniature LDV systems. It consists of two current-controlled, index-guided diode lasers in the visible range (780-880nm). The diode lasers are driven by a single current oscillator at a pre-determined base frequency. The beams are divided into four with a beam-splitter. The laser pulses are subsequently focused into two measurement volumes separated by a precisely pre-determined distance by collimating optics, mirrors and focusing lens and resulted in two sequentially pulsed fringe systems.

A receiver system consisted of a single avalanche photodiode and focusing lens is used to detect the scattered Doppler signals from both measurement volumes. The sampling rate of the avalanche photodiode is synchronized with the current oscillator base frequency. Doppler signal from each measurement volume is de-coupled, from the signal sampled by the photodiode, to obtain velocity information of a particle.