Heterodyne frequency shift laser doppler anemometer

Laser Doppler anemometry is a non-intrusive method for the study of flow processes by measuring the velocity of tracer particles carried in the flow. The direction, as well as the magnitude of the flow velocity can be obtained if frequency shift techniques are employed. Conventional techniques involve a single laser source and a beam splitter to generate two LDA beams which cross in the measuring volume. A frequency shift is then introduced into one of these two beams. The alternative approach presented here involves the use of two independent laser sources, which differ in their emission frequency by a set value, giving the frequency shift. The emission line width and stability of the laser frequencies directly affect the bandwidth of the measuring signal. We show that such effects can be eliminated by correlating the measuring signal with a reference signal. The reference signal consists of a beat signal of the two laser sources which is processed to produce a reference signal pair with a phase difference of 90 between the two components. By mixing the measuring signal with this reference signal pair we obtain a sine-cosine quadrature signal which is modulated only by the Doppler frequency. This quadrature signal is independent of the shift frequency and its fluctuations. The requirements for the stability and emission line width of the laser sources are therefore significantly reduced. The quadrature signal demodulation then allows directional measurements of quasi-static flow processes with velocities of less than 0.05 m/s corresponding to very small Doppler frequencies of 10 kHz or less. (orig.)Available from TIB Hannover: RO 3132(53) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman