Off-axis QEPAS using a pulsed nanosecond Mid-Infrared Optical Parametric Oscillator

TL;DR

This paper presents an off-axis QEPAS sensor utilizing a pulsed nanosecond MIR OPO for methane detection, achieving high sensitivity and low detection limits through optimized micro-resonator design and spectroscopic analysis.

Contribution

It introduces a novel off-axis QEPAS setup with coupled micro-resonators and a pulsed MIR OPO, demonstrating improved sensitivity and detection limits for trace gas analysis.

Findings

Detection limit of 0.8 ppm for methane

Optimal detection limit of 32 ppbv at 190s integration time

Thermal noise of QTF is the main background noise

Abstract

A trace gas sensor, based on quartz-enhanced photoacoustic spectroscopy (QEPAS), consisting of two acoustically coupled micro-resonators (mR) with an off-axis 20 kHz quartz tuning fork (QTF) is demonstrated. The complete acoustically coupled mR system is optimized based on finite element simulations and experimentally verified. The QEPAS sensor is pumped resonantly by a nanosecond pulsed single-mode mid-infrared optical parametric oscillator (MIR OPO). The sensor is used for spectroscopic measurements on methane in the 3.1 um to 3.5 um wavelength region with a resolution bandwidth of 1 cm^-1 and a detection limit of 0.8 ppm. An Allan deviation analysis shows that the detection limit at optimum integration time for the QEPAS sensor is 32 ppbv@190s and that the background noise is solely due to the thermal noise of the QTF.