Sensitivity Enhanced Fiber-Optic Hydrogen Sensor Based on Vernier Effect of Polymer–Air Cascaded FPI

In this article, a fiber-optic hydrogen sensor based on a cascaded Fabry–Perot interferometer (FPI) structure and vernier effect is proposed and experimented. The sensor is mainly composed of a polymer cavity as a reference FPI and an air cavity as a sensing FPI, and platinum/tungsten trioxide (Pt/WO3) and polydimethylsiloxane (PDMS) films successfully coating on the surface of cascaded FPI structure. Due to the utilization of the vernier effect, the sensor has the ability to sense the temperature variations of the exothermic chemical reaction between Pt/WO3 and hydrogen, which results in the envelop spectrum of the sensor shifting with variations of hydrogen concentration. The experiment results demonstrate that the hydrogen sensitivity of the proposed sensor is superior to 5.255 nm/% in the concentration range of 0.25%–1.75%, which is 10.9 times that of the single air-cavity FPI structure. The sensor not only has the advantages of high linearity of more than 0.99 but also merits of better repeatability and stability, easy preparation, compact structure, low cost, and easy long-distance transmission.