Tuning forks with optimized geometries for quartz-enhanced photoacoustic spectroscopy.

Pietro Patimisco,Hubert Rossmadl,Angelo Sampaolo,Verena Mackowiak,Frank K Tittel,Alex Cable,Stefano Dello Russo,Marilena Giglio,Vincenzo Spagnolo

doi:10.1364/oe.27.001401

Abstract

We report on the design, realization, and performance of novel quartz tuning forks (QTFs) optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). Starting from a QTF geometry designed to provide a fundamental flexural in-plane vibrational mode resonance frequency of ~16 kHz, with a quality factor of 15,000 at atmospheric pressure, two novel geometries have been realized: a QTF with T-shaped prongs and a QTF with prongs having rectangular grooves carved on both surface sides. The QTF with grooves showed the lowest electrical resistance, while the T-shaped prongs QTF provided the best photoacoustic response in terms of signal-to-noise ratio (SNR). When acoustically coupled with a pair of micro-resonator tubes, the T-shaped QTF provides a SNR enhancement of a factor of 60 with respect to the bare QTF, which represents a record value for mid-infrared QEPAS sensing.

Highlights

Optical techniques operating in the mid-infrared spectral regions are capable of excellent trace gas sensing performances, together with high sensitivity and selectivity [1,2] due to the presence of strong ro-vibrational absorption bands of many molecules
We report on the design, realization, and performance of novel quartz tuning forks (QTFs) optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS)
Lowering the resonance frequency while keeping the quality factor high is a straightforward approach to enhance the performance of a QEPAS sensor

Summary

Introduction

Optical techniques operating in the mid-infrared spectral regions are capable of excellent trace gas sensing performances, together with high sensitivity and selectivity [1,2] due to the presence of strong ro-vibrational absorption bands of many molecules. For slow relaxing gases, such as CO, CO2 and NO a QEPAS sensor operational frequency as high as 32.7 kHz, like in standard QTFs, can limit the sound wave generation efficiency [12] These considerations suggested directions for the realization of improved QTFs: i) reduction of the QTF fundamental frequency, ii) increase the prongs spacing in order to facilitate the optical alignments and minimize the photo-thermal noise level. This paper reports an investigation of the influence of prong sizes on both the resonance frequency and on the quality factor of the fundamental flexural mode, leading to the design of a quartz tuning fork optimized for QEPAS sensing Starting from this design, two novel geometries were proposed: one with T-shaped prongs to optimize the strain field between the prongs and their support and the other one having prongs with grooves carved on the central sides in order to reduce the QTF electrical resistance. The influence of the geometrical parameters on the photoacoustic response, namely the internal diameter and the length of the two tubes together with the spacing between the tube and the QTF, was investigated to determine the optimal micro-resonator geometry

Guidelines for the design of quartz tuning forks

T-shaped tuning forks

Photoacoustic response

Findings

Conclusions