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

TL;DR

This paper demonstrates a photoacoustic sensor utilizing higher order acoustic modes and phase-sensitive noise suppression to significantly improve gas detection sensitivity amidst ambient noise.

Contribution

The study introduces a novel phase-sensitive noise suppression technique in a photoacoustic sensor using higher order acoustic modes and optimized gas flow placement.

Findings

Achieved 20 dB noise reduction compared to fundamental mode.

Demonstrated 26 dB signal-to-noise improvement with higher order modes.

Maximum normalized noise equivalent absorption coefficient of 1.1×10⁻⁶ W cm⁻¹ Hz¹/² at 5 L/min flow.

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 in- 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 $\mu$m. A signal-to-noise improvement of 20 (26 dB) using higher order modes are demonstrated compared with the fundamental mode. The maximum normalized noise 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 cancelation strategy may find use in many practical industrial and environmental PA sensors, where ambient noise sources plays a crucial role for the absolute sensitive.