Containerless photoacoustic spectroscopy using photorefractive interferometry

Abstract

Photoacoustic spectroscopy of gases is one of the most sensitive spectroscopic techniques available, often achieving part per million (ppm) or part per billion (ppb) sensitivity. This technique is usually performed by containing the gas sample within a cell coupled to an acoustic transducer. Imaging of photoacoustic response over an area requires scanning of the excitation beam. At the INEEL, research is underway to extend photorefractive dynamic holography to full field spectroscopic imaging without scanning. The photorefractive effect in Bismuth Silicon Oxide is exploited to demodulate the optical phase shift of a signal beam traversing the test gas and coincident with a tunable chopped excitation beam. Molecular absorption at the excitation wavelength produces heat that causes local expansion and subsequent acoustic wave radiation. A model of the photoacoustic absorption and optical phase detection process has been developed. Measurement and modeling results are presented that illustrate the ability of the method to detect water vapor and Hydrogen Fluoride concentrations in nitrogen atmosphere backgrounds near 800 nm, currently producing sensitivities in the 20-1000 ppm. Limitations of the technique and methods for extending to ppb sensitivities with infrared excitation are discussed.