In this work, a photoacoustic (PA) gas sensor with a micro-embedded acoustic resonator for gas leakage detection was demonstrated. The micro-embedded acoustic resonator was fabricated by putting a leaky hollow-core fiber (L-HCF) into a cylindrical buffer chamber. The L-HCF was utilized as the PA cavity and the light transmission media simultaneously. The optimal inner diameter of the L-HCF was 1.7 mm. The embedded acoustic resonator was experimentally proven to be equivalent to a T-type half-open acoustic resonator, but the structure became much more compact. The volume of the amount of gas in the cell was only ∼0.3 mL, and the gas diffusion time to fill the sensor under room temperature (25°C) and ambient pressure (101 kPa) was ∼44 s. Trace acetylene (C₂H₂) in pure nitrogen (N₂) was chosen as the target gas, and the minimum detectable limit (MDL) reached 29 ppb when the lock-in integration time was 1 s. The normalized noise equivalent absorption (NNEA) coefficient was calculated to be 3.0 × 10⁻⁹ W·cm^-1·Hz^-1/2. The micro-resonant PA gas sensor, with merits of compactness, low gas consumption, and low cost, has the potential to be a remote gas sensing scheme in fields of environmental protection, industrial process monitoring, and so on.

A flexible elliptical-gradient metallic hollow waveguide (EGMHW) is proposed, fabricated, and characterized. It is based on an elliptical-gradient polycarbonate (PC) base tube with inner silver film coating. The property of dynamical mode filtering from large to small port was theoretically analyzed and experimentally demonstrated, and it was tuned that only TE_11y remained at the small port. Nonreciprocally, the EGMHW has a relatively low straight transmission loss of 1.2 dB/m at operation frequency 0.14 THz from small to large port, compared to 5.3 dB/m of the inversed direction. This phenomenon and the loss characteristics for different bent angles and radii were analyzed in detail. Measured results show that the waveguide has a great polarization-maintaining performance with a polarization ratio of 99.4% as well as robustness for bending and twisting. Finally, the property of mode filtering was experimentally verified through imaging superposed field distribution of the remained mode and comparing with the corresponding numerical results. This work provides an effective way for the design of flexible low-loss mode-tunable THz waveguides.

A multi-gas sensing system based on mid-infrared spectral absorption was developed for the detection of CH₄, C₂H₆, and C₂H₄. The system utilized a broadband infrared source, a hollow waveguide (HWG) absorption cell, and a tunable Fabry–Pérot (FP) detector. The limits of detection (LODs) of CH₄, C₂H₆, and C₂H₄ were 7.33 ppm, 2.13 ppm, and 8.09 ppm, respectively. For multi-gas measurements, the support vector machine (SVM) algorithm model was employed to calculate the concentration of each component. The root mean square error of prediction (RMSEP) values for CH₄, C₂H₆, and C₂H₄ were 15.91 ppm (1.26%), 1.64 ppm (0.57%), and 6.95 ppm (0.55%), respectively. The generation of stimulated absorption spectra of mixed gases was realized, and the sample selection of measurement for accurate concentration calculation of each gas was optimized. The system proposed in this work provides a simple, miniaturized, and cost-effective solution for multi-gas sensing.

This study presents a multi-gas detection system based on low-loss hollow waveguide, demonstrating significant improvements in both detection sensitivity and accuracy. The system utilizes 4 hollow waveguides optimized for different mid-infrared bands, coupled with a broadband thermal infrared light source and tunable Fabry-Pérot detectors. The hollow waveguides, which are inner-coated with silver iodide and silver films, exhibit low transmission loss and high flexibility, enabling efficient gas sensing. Experiments were conducted to detect greenhouse gases (CH₄, CO₂), toxic gases (CO, NO, SO₂), and atmospheric pollutants (CCl₂F₂) at various concentrations. The system achieved detection limits of 58.7 ppm for CH₄, 5.18 ppm for CO₂, 85.1 ppm for CO, 70.1 ppm for NO, 5.79 ppm for SO₂, and 30.9 ppm for CCl₂F₂. The system demonstrates high sensitivity and selectivity, with potential for further improvement through noise reduction and data fusion techniques. This cost-effective and compact system offers a robust solution for real-time multi-gas detection in environmental monitoring, industrial safety, and medical diagnostics.

In this study, the transmission properties of flexible dielectric coated metallic hollow waveguides at 4.28 THz were studied. Waveguide samples was fabricated, characterized, and quantitatively analyzed including loss and mode structure. High-reflective silver and low-absorptive cyclic olefin polymer (COP) were selected and plated on the inner wall of flexible manufactured polycarbonate (PC) base tubes through dynamic liquid-phase coating method. Waveguides with a theoretically optimized dielectric film thickness and different inner diameters were characterized by using terahertz quantum cascade laser. Their mode structure and energy coupling efficiency with fundamental mode were analyzed. And distinctive results were obtained. The waveguide with 2mm inner has a quasi-single-mode property with coupling efficiency of 90.2%, corresponding to a gaussian field distribution. Moreover, the efficiency is above 89% even as the waveguide is bent to 45°. The transmission loss are 2.5 and 1.7 dB/m for the waveguides with 1mm and 1.5mm inner radius, respectively.