Phase-Sensitive Noise Suppression in a Photoacoustic Sensor based on Acoustic Circular Membrane Modes

摘要

A photoacoustic (PA) sensor based on higher order acoustic modes isdemonstrated. The PA sensor is designed to enhance the gas-detectionperformance and simultaneously suppress ambient noise sources (e.g. flow noise,electrical noise and external acoustic noise). Two microphones are used andpositioned such that the PA signals are ($\pi$) out of phase. Ambient acousticnoise are approximately in the same phase and will be subtracted and thusimprove the SNR. In addition, by placing the gas in- and outlets so that thegas flows through the node of the first higher order membrane mode the couplingof flow noise is approximately 20 dB lower compared with flow through thefundamental mode at 5 L/min. The noise reduction and thus the increase insensitivity is demonstrated by measuring vibrational lines of methanol andmethane using a broadband interband cascade laser emitting radiation at 3.38$\mu$m. A signal-to-noise improvement of 20 (26 dB) using higher order modesare demonstrated compared with the fundamental mode. The maximum normalizednoise equivalent absorption coefficient is 1.1 $\times 10^{-6}$ W cm$^{-1}$Hz$^{1/2}$ for a flow of 5 L/min. It is anticipated that the noise cancelationstrategy may find use in many practical industrial and environmental PAsensors, where ambient noise sources plays a crucial role for the absolutesensitive.