A novel tapered quartz tuning fork (QTF) was designed to enhance its stress magnitude and charge distribution in QTF-based laser spectroscopy, which had a low resonant frequency of 7.83 kHz and a wide fork gap for long energy accumulation time and easy optical alignment. Compared to the reported rectangular QTF, this tapered QTF transfers the maximum stress position from the root to the middle to improve its sensing performance. Furthermore, the unique design eliminates the 90° right angles typically found in standard QTFs, which often lead to undesired “webs” and “facets” during the etching process. This design minimizes performance degradation by reducing the presence of residual unexpected materials. QTF-based laser spectroscopy of quartz-enhanced photoacoustic spectroscopy (QEPAS) and light-induced thermoelastic spectroscopy (LITES) were adopted to verify its performance. Compared with the widely used standard QTF, the total surface charge of the tapered QTF was improved 5.08 times and 5.69 times in QEPAS and LITES simulations, respectively. Experiments revealed that this tapered QTF-based QEPAS sensor had a 3.02 times improvement in signal-to-noise-ratio (SNR) compared to the standard QTF-based system. Adding an acoustic micro-resonator to this tapered QTF-based QEPAS sensor improved the signal level by 97.20 times. The minimum detection limit (MDL) for acetylene (C2H2) detection was determined to be 16.45 ppbv. In the LITES technique, compared to the standard QTF, this tapered QTF-based sensor had a 3.60 times improvement in SNR. The MDL for C2H2 detection was determined to be 146.39 ppbv.

In this paper, the performance of two self-designed lithium niobate tuning forks (LiNTF), round-head and tapered LiNTFs, was systematically explored in lithium niobate-enhanced photoacoustic spectroscopy (LiNPAS) and light-induced thermoelastic spectroscopy (LITES) sensors. Finite element analysis results revealed that the stress and surface charge density of the LiNTFs were higher than those of the standard quartz tuning fork (QTF), owing to the high piezoelectric coefficient and electromechanical coupling coefficient of the LiNbO3. The sensing performance of the two LiNTFs was experimentally evaluated, and acetylene (C2H2) was used as the test gas for performance validation. In the C2H2–LiNPAS system, the 2f signal peak values of the round-head LiNTF and the tapered LiNTF were 3.47 times and 4.29 times higher than those of the standard QTF, respectively. When the average time reached 1000 s, the minimum detection limits (MDLs) of the sensor based on round-head LiNTF and the tapered LiNTF were 723 and 450 ppb, respectively. In the C2H2–LITES system, the 2f signal peak values of the round-head LiNTF and the tapered LiNTF were found to be 3.79 times and 5.13 times higher than that of the standard QTF. The MDLs of the LITES sensor based on the round-head LiNTF and the tapered LiNTF were determined to be 101 and 52 ppb, respectively.

A tunable diode laser absorption spectroscopy (TDLAS) sensor with a highly sensitive dual-component for methane (CH₄) and acetylene (C₂H₂) detection is reported in this paper for the first time. A multi-pass cell (MPC) design model was established employing a vector-based ray-tracing method. A dual-channel MPC with an interlaced dual hexagonal star pattern was designed to improve gas absorption and realize real-time synchronous detection of CH₄ and C₂H₂. During the simultaneous continuous monitoring of CH₄ and C₂H₂, the sensor exhibited an excellent linear response to concentration variations. The minimum detection limit (MDL) for CH₄ reached 132.08 ppb, improving to 77.32 ppb when the average time was increased to 300 s. In the case of C₂H₂, the MDL was measured at 20.19 ppb and further reduced to 3.50 ppb under the same extended average time.