Generally applicable laser Doppler anemometers have become commercially available and are now widely used for fluid flow studies. When used for high velocity measurements, e.g., in excess of 100 m/sec, laser Doppler anemometers usually comprise a high power argon-ion laser and a counter signal processor. A literature survey showed that most high speed LDA measurements have been carried out at Doppler frequencies much below the frequency capabilities of modern LDA frequency counters. This paper suggests that this might be due to the multiaxial-mode output of cw lasers employed for high speed velocity measurements. The theory is discussed and experimental investigations are described that verify the theoretical results. These results suggest the use of single-mode lasers for high speed velocity measurements. The multiaxial-mode output of cw lasers is also claimed to be responsible for signal-to-noise ratio differences between LDA signals obtained with the blue and green lines of argon-ion lasers. The differences are only observed, however, when broadband detection systems are employed. Consideration is also given to the use of photodetectors for high frequency LDA systems. It is shown that photomultipliers that permit high anode currents are advantageous in laser Doppler anemometry; they are essential when measurements are made at high Doppler frequencies. Their use permits available high power lasers to be employed for LDA measurements and the high and basically noise-free gain of photomultipliers still to be used.
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