A gas sensing method based on quartz-tuning-fork enhanced photothermal spectroscopy (QEPTS) is reported in this paper. Unlike usually used thermally sensitive elements, a sharply resonant quartz-tuning-fork with the capability of enhanced mechanical resonance was used to amplify the photothermal signal level. Acetylene (C₂H₂) detection was used to verify the QEPTS sensor performance. The measured results indicate a minimum detection limit (MDL) of 718 ppb and a normalized noise equivalent absorption coefficient (NNEA) of 7.63 × 10⁻⁹ cm⁻¹W/√Hz. This performance demonstrates that QEPTS can be an ultra-high sensitive technique for gas detection and shows superiority when compared to usually used methods of tunable diode laser absorption spectroscopy (TDLAS) and quartz-enhanced photoacoustic spectroscopy (QEPAS). Furthermore, when compared to an optical detector, especially a costly mercury cadmium telluride (MCT) detector with cryogenic cooling used in TDLAS, a quartz-tuning-fork is much cheap and tiny. Besides, compared to the QEPAS technique, QEPTS is a non-contact measurement technique and therefore can be used for standoff and remote trace gas detection.

Photoacoustic spectroscopy (PAS) as a highly sensitive and selective trace gas detection technique has extremely broad application in many fields. However, the laser sources currently used in PAS limit the sensing performance. Compared to diode laser and quantum cascade laser, the solid-state laser has the merits of high optical power, excellent beam quality, and wide tuning range. Here we present a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser used as light source in a PAS sensor for trace gas detection. The self-built solid-state laser had an emission wavelength of ~2 μm with Tm:YAP crystal as the gain material, with an excellent wavelength and optical power stability as well as a high beam quality. The wide wavelength tuning range of 9.44 nm covers the absorption spectra of water and ammonia, with a maximum optical power of ~130 mW, allowing dual gas detection with a single laser source. The solid-state laser was used as light source in three different photoacoustic detection techniques: standard PAS with microphone, and external- and intra-cavity quartz-enhanced photoacoustic spectroscopy (QEPAS), proving that solid-state laser is an attractive excitation source in photoacoustic spectroscopy.

A highly sensitive light-induced thermoelectric spectroscopy (LITES) sensor based on a multi-pass cell (MPC) with dense spot pattern and a novel quartz tuning fork (QTF) with low resonance frequency is reported in this manuscript. An erbium-doped fiber amplifier (EDFA) was employed to amplify the output optical power so that the signal level was further enhanced. The optical path length (OPL) and the ratio of optical path length to volume (RLV) of the MPC is 37.7 m and 13.8 cm^-2, respectively. A commercial QTF and a self-designed trapezoidal-tip QTF with low frequency of 9461.83 Hz were used as the detectors of the sensor, respectively. The target gas selected to test the performance of the system was acetylene (C₂H₂). When the optical power was constant at 1000 mW, the minimum detection limit (MDL) of the C₂H₂-LITES sensor can be achieved 48.3 ppb when using the commercial QTF and 24.6 ppb when using the trapezoidal-tip QTF. An improvement of the detection performance by a factor of 1.96 was achieved after replacing the commercial QTF with the trapezoidal-tip QTF.

In this paper, a novel highly sensitive methane (CH₄) and acetylene (C₂H₂) dual-gas light-induced thermoelectric spectroscopy (LITES) sensor based on Lissajous space-division multiplexed (LSDM) technology and trapezoidal-head quartz tuning fork (QTF) detector was reported for the first time. A theoretical LSDM model was established on the basis of three-mirror astigmatic multi-pass cell (MPC) and it was used to design a pair of Lissajous spot patterns with optical path length to volume ratios (OPL/Vs) of 13.5 cm^-2 and 13.3 cm^-2, respectively. Two self-designed trapezoidal-head QTFs with low resonant frequencies of less than 10 kHz and quality factor of ~12000 were adopted to enhance the detection ability. Two kinds of fiber amplifier, erbium doped fiber amplifier (EDFA) and Raman fiber amplifier (RFA), were combined to amplify the output power of two diode lasers to improve the excitation strength. After optimization, minimum detection limit (MDL) of 268.8 ppb and 91.4 ppb for real-simultaneous CH₄ and C₂H₂ sensing were obtained, respectively. When the integration time of the system were 150 s and 100 s, the MDLs could be improved to 54.8 ppb and 26.1 ppb, accordingly. Further improvement methods for such sensor were discussed.

An ultra-high sensitive light-induced thermoelastic spectroscopy (LITES) sensor based on a resonant high $Q$-factor quartz turning fork (QTF) and a Herriot multipass cell was demonstrated for the first time, to the best of our knowledge. The performance of LITES and widely used tunable diode laser absorption spectroscopy (TDLAS) were experimentally investigated and compared at the same conditions. Carbon monoxide (CO) was chosen as the analyte to verify the reported sensors’ performance. With a minimum detection limit (MDL) of 470 ppb for 60 ms integration time and a noise equivalent absorption (NEA) coefficient of $2.0\times {10}^{-7}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}$, and a MDL of 17 ppb with an optimum integration time of 800 s, the reported LITES sensor showed a superior sensing capability compared with a TDLAS sensor and a conventional quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor.

In this paper, a novel four-prong quartz tuning fork (QTF) was designed with enlarged deformation area, large prong gap, and low resonant frequency to improve its performance in laser spectroscopy sensing. A theoretical simulation model was established to optimize the design of the QTF structure. In the simulation of quartz-enhanced photoacoustic spectroscopy (QEPAS) technology, the maximum stress and the surface charge density of the four-prong QTF demonstrated increases of 11.1-fold and 15.9-fold, respectively, compared to that of the standard two-prong QTF. In the simulation of light-induced thermoelastic spectroscopy (LITES) technology, the surface temperature difference of the four-prong QTF was found to be 11.4 times greater than that of the standard QTF. Experimental results indicated that the C₂H₂-QEPAS system based on this innovative design improved the signal-to-noise-ratio (SNR) by 4.67 times compared with the standard QTF-based system, and the SNR could increase up to 147.72 times when the four-prong QTF was equipped with its optimal acoustic micro-resonator (AmR). When the average time of the system reached 370 s, the system achieved a MDL as low as 21 ppb. The four-prong QTF-based C₂H₂-LITES system exhibited a SNR improvement by a factor of 4.52, and a MDL of 96 ppb was obtained when the average time of the system reached 100 s. The theoretical and experimental results effectively demonstrated the superiority of the four-prong QTF in the field of laser spectroscopy sensing.