Lateral Offset Single-Mode Fiber-Based Fabry–Perot Interferometers with Vernier Effect for Hydrogen Sensing

A high-sensitive optical fiber hydrogen (H2) sensor with Vernier effect by cascading three Fabry–Perot interferometers (FPIs) has been proposed. It is prepared by welding three sections of a single-mode fiber with a lateral offset structure and coating polydimethylsiloxane (PDMS) at the offset part to form three cascaded FPIs and generate a Vernier envelope spectrum. To realize H2 sensing, the hydrogen-sensitive material Pd/WO3 is fabricated and coated on the outside of the PDMS. Pd/WO3 will generate local heat when in contact with H2, which will cause the thermal expansion and refractive index change of PDMS and finally lead to the Vernier envelope wavelength shifting with the change of H2 concentration. Due to the Vernier effect, the H2 sensor can achieve a linear measurement sensitivity of 6.214 nm/% in the concentration range of 0–1%, which is 16.11 times that of a single FPI. The proposed H2 sensor possesses advantages of high sensitivity, easy preparation, compact structure, low cost, and easy long-distance transmission.