Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) is of importance for gas detection in environmental monitoring. This review summarizes the unique properties, development and recent progress of the IBBCEAS technique. Principle of IBBCEAS for gas sensing is described, and the development of IBBCEAS from the perspective of system structure is elaborated, including light source, cavity and detection scheme. Performances of the reported IBBCEAS sensor system in laboratory and field measurements are reported. Potential applications of this technique are discussed.

A near-infrared methane (CH4) sensor system was demonstrated by using off-axis integrated cavity output spectroscopy (OA-ICOS), which was an effective gas-phase analytical technique and was adopted for fast, real-time and in-situ ambient atmospheric CH4 monitoring. A 60cm-long cavity with an effective absorption path length of 2150 m was fabricated with a fast response time of ~ 0.8 s. An Allan deviation analysis yielded a minimum detection limit of 13 parts-per-billion in volume (ppbv) using wavelength modulation spectroscopy (WMS) technique, which corresponds to a noise equivalent absorption sensitivity (NEAS) of 9.7×10-10 cm-1·Hz-1/2. By applying multiple denoising scheme of wavelet transform (WT), Kalman filtering (KF) and empirical mode decomposition (EMD) method, the precision levels of CH4 measurements were improved by a factor of 4, 5 and 6, respectively. For field application, dual-gas sensing was implemented for simultaneous measurement of CH4 and water vapor (H2O) in a single sensor system. Also, a three-day continuous monitoring of atmospheric CH4 concentration levels was carried out to verify the validity and reliability of the demonstrated ppbv-level CH4 sensor system.

Integrated photonic sensors have attracted significant attention recently for their potential for high-density integration. However, they face challenges in sensing gases with high sensitivity due to weak light-gas interaction. Slow light, which dramatically intensifies light-matter interaction through spatial compression of optical energy, provides a promising solution. Herein, we demonstrate a dual slow-light scheme for enhancing the sensitivity of photothermal spectroscopy (PTS) with a suspended photonic crystal waveguide (PhCW) on a CMOS-compatible silicon platform. By tailoring the dispersion of the PhCW to generate structural slow light to enhance pump absorption and probe phase modulation, we achieve a photothermal efficiency of 3.6 × 10⁻⁴ rad·cm·ppm⁻¹ · mW⁻¹ · m⁻¹, over 1 − 3 orders of magnitude higher than the strip waveguides and optical fibers. With a 1-mm-long sensing PhCW incorporated in a stabilized on-chip Mach-Zehnder interferometer with a footprint of 0.6 mm², we demonstrate acetylene detection with a sensitivity of 1.4 × 10⁻⁶ in terms of noise-equivalent absorption and length product (NEA · L), the best among the reported photonic waveguide gas sensors to our knowledge. The dual slow-light enhanced PTS paves the way for integrated photonic gas sensors with high sensitivity, miniaturization, and cost-effective mass production.