Ultra-Compact Optical Fiber Gas Sensor With High Sensitivity, Fast Response and Large Dynamic Range

We report a miniature optical fiber photothermal (PT) gas sensor with high sensitivity, fast response and large dynamic range. The sensing region is an air gap formed between the cleaved ends of two single mode fibers (SMFs). Theoretical formulations of photothermal phase modulation and interferometric phase detection are presented. Numerical simulation and experimental investigation are carried out to optimize the system parameters to maximize the photothermal signal. A gas sensor with an air gap of 130 µm demonstrates a noise equivalent concentration of 45 parts per billion and dynamic range of $\text{2} \times \text{10}^7$ for acetylene detection, with a response time of 0.9 s. The sensor is simple to construct and may be used for real-time gas detection in a confined space.