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In this paper, an on-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a custom quartz tuning fork (QTF) acting as a photoacoustic transducer, was realized and tested. The QTF is characterized by a resonance frequency of 28 kHz, ~15% lower than that of a commercially available 32.7 kHz standard QTF. One-dimensional acoustic micro resonator (AmR) was designed and optimized by using stainless-steel capillaries. The 28 kHz QTF and AmRs are assembled in on-beam QEPAS configuration. The AmR geometrical parameters have been optimized in terms of length and internal diameter. The laser beam focus position and the AmR coupling distance were also adjusted to maximize the coupling efficiency. For comparison, QEPAS on-beam configurations based on a standard QTF and on the 28 kHz QTF were compared in terms of H 2 O and CO 2 detection sensitivity. In order to better characterize the performance of the system, H 2 O, C 2 H 2 and CO 2 were detected for a long time and the long-term stability was analyzed by an Allan variance analysis. With the integration time of 1 s, the detection limits for H 2 O, C 2 H 2 and CO 2 are 1.2 ppm, 28.8 ppb and 2.4 ppm, respectively. The detection limits for H 2 O, C 2 H 2 and CO 2 can be further improved to 325 ppb, 10.3 ppb and 318 ppb by increasing the integration time to 521 s, 183 s and 116 s

Radial-cavity quartz-enhanced photoacoustic spectroscopy (RC-QEPAS) was proposed for trace gas analysis. A radial cavity with (0,0,1) resonance mode was coupled with the quartz tuning fork (QTF) to greatly enhance the QEPAS signal and facilitate the optical alignment. The coupled resonance enhancement effects of the radial cavity and QTF were analyzed theoretically and researched experimentally. With an optimized radial cavity, the detection sensitivity of QEPAS was enhanced by > 1 order of magnitude. The RC-QEPAS makes the acoustic detection module more compact and optical alignment comparable with a bare QFT, benefiting the usage of light sources with poor beam quality.

With the rapid development of laser technology, information technology, and materials, the detection of trace gas has been taken to a new level. Advanced analytical methods were invented, for example, gas chromatography, mass spectrometry, chemical sensors, absorption spectrum, and so forth. With the detailed data gathered from new high-tech equipment, we can predict and diagnose diseases such as diabetic ketoacidosis, irritable bowel syndrome, lung cancer and so on. However, there is still a large amount of exhaled gas that has not been discovered, which means that breath analysis of exhaled gas is a very promising field in the future. Non-invasive breath diagnosis and monitoring technology have attracted huge attention from all over the world. Using exhaled gas to diagnose and monitor human disease has numerous advantages for being non-invasive, convenient and environment friendly.

A high-power near-infrared (NIR) quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for part per billion (ppb) level acetylene (C2H2) detection was reported. A 1536 nm distributed feedback (DFB) diode laser was used as the excitation light source. Cooperated with the laser, a C-band 10 W erbium-doped fiber amplifier (EDFA) was employed to boost the optical excitation power to improve QEPAS detection sensitivity. A pilot line manufactured quartz tuning fork (QTF) with a resonance frequency of 28 kHz was used as the photoacoustic transducer. In the case of high excitation power, gas flow effect and temperature effect were found and studied. Benefitting from the low QTF resonance frequency, high excitation power, and vibrational-translational (V-T) relaxation promoter, a detection limit of ∼7 ppb was achieved for C2H2 detection, corresponding to a normalized noise equivalent absorption coefficient of 4.4×10−8 cm−1 · W · Hz−1/2.