Abstract

A laser velocimeter for short-standoff measurements of wind velocity, gas-flow velocity, and true air speed is described. The instrument uses a narrow-band diode laser directly, instead of a diode-pumped solid-state laser. The optical scheme of a coherent remote sensor (optical heterodyning) is used to measure the Doppler frequency shift produced when the light scattered of atmospheric aerosols is mixed with the local-oscillator beam. Our optical system creates a 15-μm-radius beam waist at a standoff distance of 1.5 m in front of the transmitter/receiver, where all the laser output power is concentrated. The measurement is performed on a single aerosol particle traversing the measurement volume. A 6-in-diameter (15.2-cm.-diameter) lens with a polarization transmitter/receiver switch is used, together with astigmatic-correction and beam-shaping optics. The narrow-band radiation is provided by a 100-mW single-mode diode laser. The local-oscillator beam is generated by splitting off a portion of the transmitter beam by a partial reflector. The instrument includes a high-bandwidth optical detector, a radio-frequency receiver, a digital-signal-processing system, and the corresponding software package. A simple and original approach for the design of the rf receiver has been developed.

© 1995 Optical Society of America