Miniaturized Fiber-Optic Photoacoustic Gas Sensor for Sub-ppb-Level Detection of Carbon Monoxide Based on Quantum Cascade Laser and Multipass Cell

A high-sensitivity fiber-optic photoacoustic carbon monoxide (CO) sensor based on quantum cascade laser (QCL) and nonresonant multipass cell is proposed. By leveraging the mid-infrared fundamental band absorption, multipass absorption enhancement, and cantilever resonance frequency detection, a multimechanism synergy has been achieved to enable highly sensitive detection of CO. In the mid-infrared band, CO exhibits a strong absorption coefficient, thereby eliminating the need for a high-power optical amplifier. Furthermore, by integrating a miniaturized multipass cell, the photoacoustic signal is remarkably enhanced, enabling the miniaturization and ultrahigh sensitivity of the sensor. A pair of spherical reflectors are symmetrically installed at both ends of the photoacoustic cell to form a Herriott multipass cell. The light beam reflects 20 times within the multipass cell, creating 10 elliptically distributed light spots. The gas chamber volume of the sensor is only 1.28 mL, with an optical path length of 510 mm. The generated photoacoustic signals are measured by a fiber-optic Fabry–Perot (FP) cantilever microphone, which can detect weak signals with high sensitivity at the resonance frequency of the cantilever. The measured signal amplitude is 8.7 times that of a single reflection. When the averaging time is 100 s, the minimum detection limit of the system for CO is 0.8 ppb, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 4.94 × 10–9 Wcm–1/Hz1/2.