A two-component gas sensor based on Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) and Light-Induced Thermoelastic Spectroscopy (LITES) was realized by the time-division multiplexing technique. Two off-beam acoustic microresonator (AMRs) were used in the experiment for twice excitation and enhancement of QEPAS signal. The four fiber collimators were connected with the two off-beam AMRs to be responsible for laser propagation in the two AMRs. After passing through the second AMR, the laser beam was incident to the sidewall of the quartz tuning fork (QTF) by the fiber collimator 5 for the excitation of the LITES signal, which realized the superposition of QEPAS signals and LITES signal on a single QTF. The time-division operation of distributed feedback lasers was realized by optical switches to accomplish the detection of C ₂ H ₂ and CH ₄ . The experimental results showed that the minimum detection limits of C ₂ H ₂ and CH ₄ were 2.06 ppmv and 7.14 ppmv, respectively, which proved that the gas sensor had good sensing performance.

A four-off-beam acoustic micro-resonator (AmR) structure is proposed to improve the photoacoustic signal intensity in quartz-enhanced photoacoustic spectroscopy (QEPAS) for trace gas sensing. Two vertical-array-based double-off-beam AmR structures are arranged on each side of the quartz tuning fork (QTF) horizontally to construct a four-off-beam AmR structure using a rectangular prism. This QEPAS scheme based on the four-off-beam AmR is evaluated in the water-vapor detection of air at normal atmospheric pressure and room temperature (27C). Comparing with the bare QTF scheme, the new QEPAS configuration enhances the photoacoustic spectroscopy signal by a factor of 25.84 times, achieving a minimum detection limit of 278 ppbv with a normalized noise equivalent absorption coefficient of 1.27x10 $^-9$ W/Hz $1/2$ in water vapour detection at a wavelength of 1368.597 nm.

A multipoint gas sensing scheme based on photoacoustic spectroscopy was proposed. Multiple photoacoustic spectroscopy (PAS) gas cells (resonant frequency $f_0=5.0\mathrm{kHz}$) were connected in series for the multipoint gas sensing with wavelength modulation technique. The PAS signal was excited by modulating the tunable distributed feedback laser diode wavelength at $f_0/2$ using a changing driving current. The gas concentration of each gas cell was obtained by the PAS signal, which was demodulated by the lock-in amplifier. A multipoint PAS experiment to detect the water vapor at 1368.597 nm was implemented to verify the scheme we presented. With the three PAS gas cells, the linear response to the water vapor concentration of our sensors achieved 0.9978, 0.99591, and 0.99617, and their minimum detection limits were 479, 662, and 630 ppb, respectively.

A dual-gas intra-cavity quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on frequency division multiplexing (FDM) of a quartz tuning fork (QTF) using two acoustic microresonators (AmRs) was demonstrated in this paper. The two AmRs were constructed into two fiber ring lasers with C-band and L-band erbium-doped fibers (EDFs) and two pump sources, eliminating the model competition of dual-wavelength and improving the stability of the wavelength. Two laser beams modulated with different frequencies f₁ and f₂ from two fiber ring lasers with different wavelengths are focused at two different positions between the QTF prongs to excite two photoacoustic signals, and two AmRs of on-beam structure achieved the enhancement of two photoacoustic signals respectively. The piezoelectric transducer (PZT) and acousto-optic modulator (AOM) are used to scan and modulate the gas absorption wavelength respectively, and different modulation frequencies f₁ and f₂ ensure that the QEPAS signals generated by different target gases will not be interfered with each other. Under 1 atm and 296 K atmosphere, the acetylene (C ₂ H ₂ ) and carbon dioxide (CO ₂ ) selected as target gases were simultaneously detected at 1530.37 nm, 1572.66 nm to demonstrate the performance of the sensor. The experimental results achieved the minimum detection limits (MDLs) of 492.71 ppbv, 173.36 ppmv, and R² of 0.9986, 0.9992 by the linear fits of the signal with different gas concentrations for the C ₂ H ₂ and CO ₂ , respectively. It shows that the dual-gas QEPAS sensor system proposed in the paper possessed high sensitive and real-time detection of various gases.

In this paper, a multicomponent gas detection system based on photoacoustic spectroscopy (PAS) is proposed with a combination of frequency division multiplexing (FDM) and time division multiplexing (TDM), combining a resonance photoacoustic cell and broadband microphone. A PAS gas cell with a wide frequency response bandwidth was used to achieve the FDM by selecting a specific modulation frequency of each component gas. The sawtooth wave driver current of each laser was output at a constant time interval for achieving the TDM. Compared with the laser channel control using a photoswitch, the driver current control was a simpler and more convenient means to implement TDM. The four gas components of methane ( C H 4 ), water ( H 2 O ) vapor, carbon dioxide ( C O 2 ), and acetylene ( C 2 H 2 ) were selected as sample gases for testing the feasibility of the method. The experimental results showed that the gas detection limits of C H 4 , H 2 O vapor, C O 2 , and C 2 H 2 were 75.435, 2.502, 341.960, and 4.284 ppm, respectively. In addition, the linear fittings of gas concentration were 0.99386, 0.99772, 0.98995, and 0.98955, respectively.