Abstract
A right-angle prism was used to enhance the acoustic signal of a quartz-enhanced photoacoustic spectroscopy (QEPAS) system. The incident laser beam was parallelly inverted by the right-angle prism and passed through the gap between two tuning fork prongs again to produce another acoustic excitation. Correspondingly, two pairs of rigid metal tubes were used as acoustic resonators with resonance enhancement factors of 16 and 12, respectively. The QEPAS signal was enhanced by a factor of 22.4 compared with the original signal, which was acquired without resonators or a prism. In addition, the system noise was reduced a little with double resonators due to the Q factor decrease. The signal-to-noise ratio (SNR) was greatly improved. Additionally, a normalized noise equivalent absorption coefficient (NNEA) of 5.8 × 10−8 W·cm−1·Hz−1/2 was achieved for water vapor detection in the atmosphere.
Highlights
Since its first introduction by Kosterev et al in 2002 [1], quartz-enhanced photoacoustic spectroscopy (QEPAS) has been widely demonstrated for trace gas detection with outstanding performance [2,3]
The acoustic resonator (AR) is a pair of two short rigid metal tubes, placed on two sides of tuning fork closely [6], which is called the on-beam structure, or the tubes are placed on a relatively long tube with a small slit in the middle [7], and such a structure is known as an off-beam QEPAS
Borri et al presented an intracavity QEPAS (I-QEPAS) technique for trace gas detection [8,9]; the method combined QEPAS with a buildup optical cavity based on quantum cascade lasers, and it is suitable for cases where a long absorption distance can be achieved by the lens
Summary
Since its first introduction by Kosterev et al in 2002 [1], quartz-enhanced photoacoustic spectroscopy (QEPAS) has been widely demonstrated for trace gas detection with outstanding performance [2,3]. The valid work distance is very limited when the collimated laser beam needs to successfully pass through the tuning fork prongs. In such a case, Zheng et al utilized two tuning forks which increased the signal intensity with an enhancement factor of 1.6 [10]; they presented a double-pass QEPAS sensor with a low-cost, high-reflection concave mirror [11]. Where C(ω) denotes the parameter of the AR unit, related to the resonant enhancement factor and modulation angular frequency ω; α is the absorption coefficient related to the detected gas concentration and absorption line characteristics; W is the laser power operating at a specific wavelength overlapping with the target gas absorption line.
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