Liquid-core long-period grating temperature sensor based on femtosecond writing

Long-period fiber grating (LPFG) based temperature sensors have a wide range of applications in biological and chemical sensing. However, conventional LPFGs exhibit low sensitivity because of the low thermo-optic coefficient of silica. In this paper, a novel liquid-core LPFG device is proposed. The sensor is fabricated by using femtosecond laser to periodically inscribe long-period grating onto the silica material near the air core of the hollow-core fiber (HCF). Liquid with a high refractive index (RI) is filled into the HCF to form a liquid-core waveguide, and transmission peaks resulting from the grating resonance are observed because of the coupling between the fundamental mode and cladding modes. Experimental results show that the temperature sensitivity of the LPFG is highly dependent on the RI of the filled liquid. When liquid with a refractive index of 1.476 is filled in the fiber core, the temperature sensitivity is approximately 6.34 nm/°C in the range of 27-40°C. The device has great potential for applications in areas such as medicine, and chemical industry.