Highly Sensitive CO-LITES Sensor Based on a Tapered Fiber Focusing and a Fiber-Coupled MPC

In this paper, a highly sensitive carbon monoxide (CO) light-induced thermoelastic spectroscopy (LITES) sensor based on tapered-fiber focusing and a fiber-coupled multipass cell (MPC) is demonstrated for the first time. An MPC with a fiber-coupled structure and an optical length of 40 m is employed to significantly increase the effective absorption path length, reduce the difficulty of optical alignment, and improve the robustness of the sensor system. By comparing five different beam shaping and focusing schemes at the MPC output, the influence of the excitation spot size on the generation of the LITES signal is thoroughly investigated. Experimental results demonstrate that the system response is significantly enhanced as the spot size is compressed. Owing to the micron-scale focused spot produced by the tapered fiber end face, the tapered-fiber configuration exhibits the strongest sensing response among the five schemes. This configuration achieves the highest signal-to-noise ratio (SNR) and exhibits a good linear response to CO concentration. The minimum detection limit (MDL) of the proposed CO-LITES sensor is 3.86 ppm. Based on Allan deviation analysis, when the system integration time reaches 300 s, the minimum detection limit can be further improved to 304 ppb, demonstrating excellent long-term stability.