Application of acoustic resonators in photoacoustic trace gas detection and metrology

Abstract

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics will be considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes will be discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology will be covered. The main characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification will be discussed including setup properties and noise features. Recent results will be presented for a state-of-the-art dual-resonator differential cell suitable for sensitive trace gas detection with low electronic and acoustic noise. With this resonant cell and a near-infrared DFB diode laser radiating at 1.53 μm (42 mW) polar ammonia molecules have been detected with a sensitivity of 200 ppbv under flow conditions to reduce adsorption effects. By excitation of fundamental vibrations in methane using a pulsed optical parametric oscillator (OPO), with 60 mW output power, a sensitivity of 1.2 ppbv has been achieved employing this resonant cell. This setup allows accurate detection of the greenhouse gas methane which has a concentration of about 1.7 ppmv in ambient air.