Optical EngineeringSingle sideband techniques for laser Doppler velocimeter frequency offset
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Frequency shifting is a commonly used technique to determine the direction of motion and enable simpler processing of Doppler signals generated by laser Doppler velocimeter (LDV) and Doppler lidar systems. This shift is most commonly achieved by passing the reference, measurement, or injection-locking laser beam though an acousto-optic modulator (AOM). However, AOMs are expensive and require specialized drive electronics, hence methods of generating a frequency shift without the use of an AOM are desirable to reduce the overall cost and size of LDV systems. Various methods of frequency shifting without AOMs have been reported, along with direction discriminating processing systems that do not require a frequency shift. However, these systems are either not applicable to or are of limited use for pulsed Doppler lidar. We have developed a new frequency shifting technique that is applicable to pulsed Doppler lidar and does not require an AOM. The method is based on the principles of radio frequency single-sideband modulation. Two implementations are presented, along with the results from concept validation experiments that illustrate successful decoding, including direction discrimination, of Doppler signals. The first implementation is a fully electronic method that uses an electronic modulator fed with quadrature signals generated optically, and in the second, semioptical implementation the modulation function is performed optically.