Directional spatial-resolved laser Doppler velocimeter using a two-colour fibre laser

A concept for the realisation of directional laser Doppler velocimeters (LDV), without having to use opto-electronic modulators like Bragg cells is presented. The concept is based on the use of a diode-pumped fibre laser, emitting at two different wavelengths. The two laser colours are used to generate two interference fringe systems in the measuring volume of the LDV system. The fringe systems have the same spacing d, but a phase shift of about d/4, see Fig. 1. Opposite motions of the tracer particle result in an opposite phase relation of the generated burst signals. The evaluation of the sign of the phase shift between these two signals gives the direction of the fluid flow. The main advantage of this directional discrimination principle is its complete passive arrangement. In consequence, a high miniaturization degree of the LDV measuring head can be achieved. Besides the directional discrimination, the use of a two-colour laser allows the measurement of velocity gradients inside the measuring volume. A similar LDV system like in Fig. 1 was realised in order to obtain a spatial resolution of the velocity measurement. Due to the chromatic dispersion of the imaging optics, different spatial dependences of the spacings of two fringe systems are obtained. Employing two Doppler frequency measurements, a spatial resolved velocity measurement is achieved experimentally by a back-scattering differential LDV system for the first time to our knowledge. Both concepts will be combined for directional velocity measurements with spatial resolution, e.g. for the measurement of boundary layers. Fig. 1. Principle of the directional laser Doppler velocimeter using a two colour fibre laser. Top: Two equal distant fringe systems of different colour are generated by means of an achromatic imaging of a phase diffraction grating. Due to chromatic dispersion of a glass plate, employed in the lens focal plane, a defined phase shift between the two fringe systems is achieved. The scattering light is chromatical separated and generates a complex quadrature signal pair, i.e. sine-cosine signals. Their evaluation gives the directional Doppler frequency. Bottom: Mach-Zehnder interferometer scheme. After splitting the beam, different phase shifts of the two colours are generated. The combination of the beams in the measuring volume of the LDV systems results in a sine-cosine signal pair.