Abstract

Tunable semiconductor lasers are attractive sources for measuring molecular species’ absorption lineshapes in flowfields. Their wide modulation bandwidth allows fast time response suitable for studying transient flows. For a weak absorption such as the oxygen A-band near 760 nm, laser intensity modulation which limits measurement sensitivity can be eliminated by combining laser wavelength modulation and second-harmonic detection. This technique is used in the measurement of velocity and temperature of oxygen in transient high temperature and high speed flows generated in a shock tub. A 3-mW GaAlAs laser diode is used to monitor oxygen absorption lineshapes at a 10-kHz rate in the end section of a shock tube along two optical paths with a 30° relative angle. Laser wavelength markers are obtained by passing a third beam through a low finesse etalon. The small amplitude 10-MHz modulation is directly coupled to the laser, and second-harmonic signals are demodulated by frequency mixers in a 500-kHz bandwidth. The oxygen velocity behind the incident shock is derived from the Doppler shift between the lines recorded along the two beam directions. Typical absorptions in the range 0.2–1% are observed. Temperature is extracted by fitting the second-harmonic experimental lineshapes to a spectral model.

© 1991 Optical Society of America