Abstract

A photoacoustic (PA) sensor based on higher-order acoustic modes is demonstrated. The PA sensor is designed to enhance the gas detection performance and simultaneously suppress ambient noise sources (e.g., flow noise, electrical noise, and external acoustic noise). Two microphones are used and positioned such that the PA signals are $\pi$ out of phase. Ambient acoustic noise are approximately in the same phase and will be subtracted and thus improve the SNR. In addition, by placing the gas inlets and outlets so that the gas flows through the node of the first higher-order membrane mode the coupling of flow noise is approximately 20 dB lower compared with flow through the fundamental mode at 5 L/min. The noise reduction and thus the increase in sensitivity is demonstrated by measuring vibrational lines of methanol and methane using a broadband interband cascade laser emitting radiation at 3.38 μm. A signal-to-noise improvement of 20 (26 dB) using higher-order modes is demonstrated compared with the fundamental mode. The maximum normalized noise equivalent absorption coefficient is $1.1\times {10}^{-6}\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{1/2}$ for a flow of 5 L/min. It is anticipated that the noise cancellation strategy may find use in many practical industrial and environmental PA sensors, where ambient noise sources plays a crucial role for the absolute sensitivity.

© 2015 Optical Society of America

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