FREQUENCY-LOCKED LIGHT-SCATTERING - REAL-TIME DOPPLER VELOCIMETRY WITH CLOSED-LOOP FEEDBACK-CONTROL

Real-time measurement capability of a frequency-modulated filtered lig ht-scattering- (FM FLS) Doppler velocimeter has been demonstrated. Dop pler-shifted light from a frequency-modulated Ti:sapphire laser scatte red from a supersonic flow is imaged through a potassium vapor cell an d is detected by FM spectroscopy. The FM signal is used in closed-loop feedback control of the laser frequency to lock the Doppler-shifted s cattered light to the resonance frequency of the filter. The differenc e between the filter resonance frequency and the laser frequency when the scattered light is frequency locked to the filter resonance is the flow-induced Doppler shift. Changes in flow velocity are tracked by c hanges in laser frequency, which is subsequently measured to obtain th e Doppler shift. The frequency-locking capability of the technique was achieved with use of a simple analog controller. The random Doppler s hift measurement errors (2 sigma) were approximately 20 MHz, which cor respond to velocity measurement errors for the real-time measurement o f less than 3% in a 10-Hz bandwidth. (C) 1998 Optical Society of Ameri ca.