Highly Sensitive Temperature and Gas Pressure Sensor Based on Long-Period Fiber Grating Inscribed in Tapered Two-Mode Fiber and PDMS

A long period fiber grating (LPFG) was inscribed in a tapered two-mode fiber (TMF) by using a CO ₂ laser pulse point-to-point direct writing technique. TMF is spliced between two segments of single mode fiber (SMF), then TMF is tapered to a waist diameter of $55 \mu \text{m}$ , and this structure cannot form a fiber optic interferometer. After writing an LPFG on a tapered TMF, many resonance dips appeared. These resonance dips are formed by the coupling between different core modes and cladding modes; therefore, they have different sensitivity responses to environmental parameters. In order to further improve the sensitivity of LPFG resonance dips and the mechanical strength of LPFG, the polydimethylsiloxane (PDMS) optical material is coated on the surface of LPFG. The experiment found that two resonance dips wavelength of $\lambda _{{1}}$ and $\lambda _{{2}}$ of LPFG are very sensitive to temperature and gas pressure, and the temperature and gas pressure sensitivities of $\lambda _{{1}}$ reach −0.412 nm/°C and −12.16 nm/MPa respectively, while the temperature and gas pressure sensitivities of $\lambda _{{2}}$ reach −0.484 nm/°C and −18.26 nm/MPa respectively. Due to its simple manufacturing, low cost, and robustness, this LPFG has enormous potential in highly sensitive temperature and gas pressure measurement applications.