Overtone resonance enhanced single-tube on-beam quartz enhanced photoacoustic spectrophone

Abstract

A single-tube on-beam quartz enhanced photoacoustic spectroscopy (SO-QEPAS) spectrophone, which employs a custom-made quartz tuning fork (QTF) having a prong spacing of 700 lm and operating at the 1st overtone flexural mode, is reported. The design of QTF prong geometry allows the bare QTF to possess twice higher Q-factor values for the 1st overtone resonance mode falling at $17.7 kHz than in the fundamental resonance mode at $2.8 kHz, resulting in an 8 times higher QEPAS signal amplitude when operating in the 1st overtone resonance mode. Both the vertical position and length of the single-tube acoustic micro-resonator (AmR) were optimized to attain optimal spectrophone performance. Benefiting from the high overtone resonance frequency and the quasi 1st harmonic acoustic standing waves generated in the SO-QEPAS configuration, the AmR length is reduced to 14.5 mm. This allows the realization of compact spectrophone and facilitates the laser beam alignment through the QTF þ AmR system. The signal enhancement in the overtone resonance mode and the high acoustic coupling efficiency between the AmR and QTF in the SO-QEPAS configuration yields an overall sensitivity enhancement factor of $380 with respect to the bare custom QTF operating in the fundamental resonance mode. Published by AIP Publishing.

Review of Recent Advances in QEPAS-Based Trace Gas Sensing

“…If the fundamental resonance frequency of custom QTF is small (a few kHz), the resonance frequency of the first overtone flexural mode should not be too high. Therefore, the first overtone flexural mode of QTF can be used in a QEPAS sensor [71][72][73][74]. The schematic of the fundamental flexural mode and the first overtone flexural mode of QTF is shown in Figure 7.…”

Quartz–enhanced photoacoustic spectrophones exploiting custom tuning forks: a review

“…The SO-QEPAS configuration with an ID = 0.65 mm and l = 38 mm results in a maximum SNR, 128 times higher than of the bare QTF#2, with a quality factor of ~6,700, ~20% lower than that of the bare QTF#2. The SO-QEPAS configuration was also employed with QTF#1 operating in the first overtone flexural mode at ~17.7 kHz [18]. The tube length was reduced due to the high overtone resonance frequency.…”

“…For these measurements, we used a slit width of ~100 μm, similar to that reported in [17] and [18]. Since the prongs width for QTF#4 is 800 μm, considerably wider than that of QTF#1 and QTF#2 (250 μm), we tested slit widths Table 3.…”

“…For example, the use of QTF with prongs spacing ≥700 μm allowed extending QEPAS operation to powerful but with poor beam quality laser sources, such as near-IR fiber amplified lasers [16]. Furthermore, the use of QTFs with such large prongs spacing allowed the realization of a novel spectrophone configuration in which a single tube is inserted between the prongs of the QTF (single-tube spectrophone) [17,18].…”

Damping Mechanisms of Piezoelectric Quartz Tuning Forks Employed in Photoacoustic Spectroscopy for Trace Gas Sensing

“…Several QTFs geometries have been proposed for QEPAS, with resonance frequency spanning from 2.8 to 32 kHz for the fundamental flexural mode . By lowering the fundamental resonance mode below 5 kHz, the first overtone mode (having a frequency about x6 larger than the fundamental one) also become suitable for QEPAS sensing . In a QTF, fundamental and first overtone modes exhibit substantially different resonance properties, mainly because loss mechanisms are strongly dependent on the related vibrational dynamics.…”