A compact resonant Π-shaped photoacoustic cell with low window background for gas sensing

Abstract: A resonant photoacoustic cell capable of detecting the traces of gases at an amplitude-modulation regime is represented. The cell is designed so as to minimize the window background for the cell operation at a selected acoustic resonance. A compact prototype cell (the volume of acoustic cavity of ~ 0.2 cm^3, total cell weight of 3.5 g) adapted to the narrow diffraction-limited beam of near-infrared laser is produced and examined experimentally. The noise-associated measurement error and laser-initiated signals are studied as functions of modulation frequency. The background signal and useful response to light absorption by the gas are analyzed in measurements of absorption for ammonia traces in nitrogen flow with the help of a pigtailed DFB laser diode operated near a wavelength of 1.53 um. The performance of absorption detection and gas-leak sensing for the prototype operated at the second longitudinal acoustic resonance (the resonance frequency of ~ 4.38 kHz, Q-factor of ~ 13.9) is estimated. The noise-equivalent absorption normalized to laser-beam power and detection bandwidth is ~ 1.44x10^{-9} cm^{-1} W Hz^{-1/2}. The amplitude of the window-background signal is equivalent to an absorption coefficient of ~ 2.82x10^{-7} cm^{-1}.