Diode laser-induced infrared fluorescence of water vapour

Abstract

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 ν1 + ν3 band of H2O in a static cell, and the resulting fluorescence signal was collected near 2.7 µm (both ν1 and ν3 bands). Experiments were conducted at low pressure in pure water vapour and mixtures of water vapour and N2 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.