Traditional gas sensing methods, such as semiconductor and electrochemical sensors, face significant limitations in harsh and confined environments due to cross-sensitivity, poor electromagnetic compatibility, and their inability to meet stringent intrinsic safety standards. Although photoacoustic spectroscopy offers high selectivity and sensitivity, its reliance on resonant cavity dimensions and electromagnetic microphones restricts its use in space-constrained settings. To address these limitations, this study develops a gas sensing device that integrates high sensitivity, compactness, intrinsic safety, and electromagnetic immunity for reliable trace gas monitoring in critical industrial safety and process control applications.

The proposed sensor employs a dual-enhancement mechanism within a miniaturized 1 mL nonresonant photoacoustic cell. A Herriott-type multipass configuration, featuring two coaxially aligned concave mirrors, extends the effective optical path length to approximately 440 mm. This represents an order-of-magnitude enhancement over a single-pass configuration, significantly amplifying light-gas interaction and improving photoacoustic excitation efficiency without increasing the physical dimensions of the cell. For high-sensitivity signal detection, a cantilever-enhanced fiber-optic Fabry-Perot acoustic sensor is integrated into the photoacoustic cell. The cantilever’s vibration modulates the Fabry-Perot cavity length, formed between its surface and the end face of the optical fiber ferrule, enabling all-optical detection of the photoacoustic signal. The complete measurement system employs a distributed-feedback laser as the excitation source, wavelength-modulated at half the cantilever’s resonant frequency. This enables second-harmonic detection, effectively suppressing fundamental frequency noise.

Comprehensive experiments were conducted to evaluate the sensor’s performance using methane as the target analyte. Frequency response measurements demonstrated clear resonant enhancement, with the second-harmonic photoacoustic signal amplitude peaking sharply at the cantilever’s resonance frequency of 1970 Hz, confirming the optimal operating conditions. Spectral analysis of the time-domain signal acquired at a methane concentration of 50 µL/L revealed a distinct frequency component that matched the second harmonic of the modulation frequency, validating successful signal excitation and detection. The sensor exhibited an excellent linear response across methane concentrations ranging from 10 to 50 µL/L, with a calibration sensitivity of 36.73 pm/(µL·L^–1), demonstrating high measurement repeatability and linearity. Based on the characteristic absorption coefficient of methane at 1650.9 nm, the normalized noise equivalent absorption coefficient was calculated to be 9.96 × 10^–10 cm^–11 W/Hz^1/2, representing state-of-the-art performance for non-resonant photoacoustic systems. Allan-Werle deviation analysis further revealed that extending the integration time to 100 s could improve the minimum detection limit to 7.24 nL/L, demonstrating excellent long-term stability.

This study successfully demonstrates a miniaturized all-optical photoacoustic gas sensor that enhances optical absorption via a Herriott-type multipass cell with mechanical resonance amplification via cantilever-enhanced fiber-optic acoustic detection. The sensor exhibits excellent methane-detection performance within a compact 1 mL photoacoustic cell volume. Its all-optical architecture ensures immunity to electromagnetic interference, intrinsic safety in explosive environments, low transmission loss for remote monitoring, and passive operation at the sensing end. Moreover, the requirement for only a 1 mL sample volume makes it particularly valuable for trace gas analysis when sample conservation is essential, as well as for online laboratory testing. Educationally, this sensor system integrates fundamental principles such as the photoacoustic effect, multi-pass optical path design, fiber-optic interferometry, and mechanical resonance. It provides an ideal experimental platform for students in optical system design, signal processing, and interdisciplinary applications. The demonstrated dual-enhancement strategy offers a promising technological pathway for developing next-generation high-performance miniature gas sensors suitable for harsh environments.