Compact Hybrid Interferometer Based on Twin- Core Side Hole Fiber and Harmonic Vernier Effect for Gas Pressure Measurement

A novel and compact fiber-optic gas pressure sensor using a chitosan-coated twin-core side hole fiber (TCSHF)-based structure is presented. This sensor comprises single-mode fiber (SMF), coreless fiber (CLF), and TCSHF connected in series. The center core (CC) of TCSHF and the side hole (SH) with chitosan coating form the Michelson interferometer (MI), while the side core (SC) forms the Fabry–Pérot interferometer (FPI). By leveraging the hybrid interferometer within the TCSHF segment, the second harmonic Vernier effect (HVE) is generated. For the first time, the internal envelope demodulation method is practically introduced for precise HVE envelope dip localization. The sensor detects changes in gas pressure based on the deformation of the chitosan film. This configuration offers exceptional sensitivity, with a pressure sensitivity of 589.41 nm/MPa and a temperature sensitivity of −0.47 nm/°C. Different sensitivities from Vernier envelope dips enable simultaneous dual-parameter sensing. The sensor demonstrates stability over pressure cycles and time, affirming its potential for practical applications. The approach provides a straightforward yet effective means for achieving ultrasensitive gas pressure and temperature measurements.