A spiral optical-frequency shifter for the laser Doppler velocimeter (LDV) is described, which offers lower shifted frequencies of 50 Hz-50 kHz manually. These lower shifted frequencies cannot be easily obtained from conventional optical-frequency shifters (Bragg cell, rotating diffracting grating, etc.). However, these lower shifted frequencies are useful for LDV measurement of lower-velocity flow such as blood flow in microvessels. A spiral optical element consisting of acrylic resin (refractive index, n=1.61) is inserted into one of the dual incident beams of a differential LDV, in which an optical frequency shift is brought about by rotation of the spiral element. The shifted frequency is changed by rotational frequency mechanically. The performance of the shifter is discussed; the experimental results are in good agreement with the theoretical values of shifted frequencies (error <1%).
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