Diode laser-induced infrared fluorescence of water vapour

Infrared laser-induced fluorescence (LIF) of water vapour was investigated for its potential as a spatially resolved gasdynamic diagnostic. A cw diode laser operating near 1392 nm was scanned across a single absorption transition in the ν 1 + ν 3 band of H 2 O in a static cell, and the resulting fluorescence signal was collected near 2.7 µm (both ν 1 and ν 3 bands). Experiments were conducted at low pressure in pure water vapour and mixtures of water vapour and N 2 using a 20 mW laser in a double-pass arrangement. A simple analytical model was developed to relate LIF intensity to gas properties as a function of laser power. The spectrally resolved, single-line excitation spectrum was fitted with a Voigt profile, allowing inference of the water vapour temperature from the Doppler-broadened component of the measured fluorescence lineshape. A two-line excitation scheme was also investigated as a means of measuring temperature with reduced measurement time. From these initial measurements, we estimate that a practical sensor for atmospheric pressure applications would require a minimum of 1–2 W of laser power for two-line, fixed-wavelength temperature measurements and a minimum of about 70 W of power for scanned-wavelength measurements.