Abstract

We present the first proof-of-concept results which will lead, ultimately, to a new non-intrusive velocimetry diagnostic which combines the spectral resolution of narrow bandwidth atomic vapor filters, with the inherent sensitivity of Frequency Modulated (FM) spectroscopy. The experimental approach is illustrated in Fig. 1. One hundred MHz (100 MHz) side-bands are imposed upon the output of a single frequency, continuously tunable Ti: Sapphire laser at 767 nm, using a resonant E/O modulator. The combined Rayleigh and elastic scattering is transmitted through an optically thin potassium vapor filter, and imaged onto a photomultiplier tube, where it is de-modulated with a phase-sensitive detection scheme, producing a characteristic FM 1st derivative spectrum. By standard 1st derivative nulling, the laser has effectively been locked to the flow. If a portion of the excitation laser beam is heterodyned against a reference laser which is locked to an identical potassium absorption filter, then the beat frequency is a direct measure of the flow field Doppler shift, and, therefore, the velocity. Since the Doppler shift is independent of the scattering species, the velocity measurement is independent of the flow composition.

© 1995 Optical Society of America