Ultra-short phase-shifted fiber Bragg grating in a microprobe for refractive index sensor with temperature compensation

Ultra-short phase-shifted fiber Bragg grating (PS-FBG) composed of cascaded microchannels is proposed and demonstrated experimentally. The cascaded microchannels are fabricated based on femtosecond (fs) laser micromachining technology in the microprobe and a fiber Bragg grating (FBG) is written in the core of single-mode fiber (SMF) near the microprobe. Ultra-short PS-FBG is sensitive to both refractive index (RI) and temperature, while FBG is only sensitive to temperature, so FBG can be used to compensate temperature crosstalk. The length of the microprobe is less than 150 μm and its diameter is about 40 μm, and the total length of the ultra-short PS-FBG is less than 95 μm, which is about one fiftieth of the traditional FBG. Experimental results show that the RI sensitivity of the PS-FBG is 463.7953 nm/RIU in the range of 1.3323 ∼ 1.3797, the temperature sensitivity is about −28.9 pm/°C from 40 °C to 80 °C, and the temperature crosstalk can be well suppressed based on the transfer matrix method.