All-Fiber Photothermal Gas Sensing Employing Two Types of Cascaded Hollow Core Fibers

In this study, a novel in-line Mach-Zehnder interferometer photothermal spectroscopy gas sensing technique (IM-PTS) is ingeniously proposed. This technology mainly hinges on a unique fiber optic gas sensor design. Specifically, it integrates a 0.4 mm anti-resonant Bragg hollow core fiber (BHCF), a 1.8 mm single hole twin eccentric core fiber (SHTECF), and single mode fiber (SMF), thereby forming an internal Mach-Zehnder interferometer (BSS-MZI). The adoption of this innovative structural design substantially boosts the detection sensitivity of the interferometer. Experimental results demonstrate that the sensor showcases exceptional performance during the detection of acetylene gas. With a 100-second integration time, the minimum detection limit (MDL) reaches 277 parts-per-billion (ppb), and the normalized noise-equivalent absorption coefficient (NNEA) is 3.48 × 10-7W·cm-1·Hz-1/2. This all-fiber in-line MZI photothermal gas sensor effectively surmounts traditional technical bottlenecks. In the realm of trace gas detection, IM-PTS presents a novel and promising solution, attributable to its compact structure, low cost, and high sensitivity.