Optical fiber gas sensor based on thermal lens spectroscopy

This article describes how to design an optical fiber gas sensor based on a thermal lens (TL) spectroscopy instrument composed of spherical lensed fibers. This sensor features a detection head composed of two spherical lensed fibers assembled across from each other. The distance between the two fibers is just under 100 μm, with the gas in between. In other words, the optical absorption path length for our gas sensor is extremely short compared with that for other optical spectroscopic techniques (e.g., 10–20 mm∼1–2 m). In addition, our gas sensor is extremely simple, small (e.g., 20 mm in length), adjustment-free and robust. These points are extremely important for actual use in the field. The measurement principle and theoretical analysis of the design are first described. Acetylene (¹²C₂H₂) gas is used as a sample gas to evaluate the sensitivity of our sensor. The limit of detection was estimated to be approximately 5200 Pa in our experimental condition. In the conclusion, the capability of our technique is discussed, and a technique for enhancing the sensitivity in our TL technique is proposed.