Design and analysis of long-period fiber gratings in tapered multimode chalcogenide glass fiber for temperature measurement

A highly sensitive temperature sensor based on long-period fiber gratings (LPFGs) in tapered multimode chalcogenide fiber was designed. The transmission characteristics of these LPFGs and their temperature sensitivities of different cladding modes with variations in the waist diameters and surrounding refractive indices (SRIs) were theoretically studied. Simulation results showed that the temperature sensitivity of LPFGs could be effectively increased by reducing the waist diameter. The temperature sensitivity of the proposed LPFGs with a 75 μm waist diameter at the lowest LP02 cladding mode was calculated to be 1.89 nm/°C at 1.55 μm. When the grating period of the designed LPFG is selected at its dispersion-turning point, the temperature sensitivity can achieve a maximum absolute value of 15.2 nm/°C at 1.55 μm, which is approximately 120 times higher than that of tapered silica LPFGs. The influences of variations in SRIs on resonant wavelength and temperature sensitivity of these LPFGs were very weak due to the relatively large cladding refractive index of As–Se fiber, which indicated that this sensor showed a high stability against changes in SRIs. Therefore, this designed sensor can be used in complex environments where high-precision temperature measurement is required.