High-precision temperature sensor using phase-shifted fiber Bragg grating written directly by femtosecond laser.

A high-quality π-phase-shifted fiber Bragg grating (PSFBG) was inscribed in conventional single-mode fiber by use of femtosecond laser direct writing technology to demonstrate a high-precision temperature sensor. During grating inscription, the slit beam shaping technique was employed to reduce the 3 dB bandwidth of the grating's phase-shifted peak from 12.0 pm to 4.8 pm, thereby significantly enhancing the wavelength resolution of the fiber Bragg grating. Such a PSFBG exhibited a temperature sensitivity of 10.71 pm/°C, with an average standard deviation range of 0.005-0.016 °C for temperature measurements. This PSFBG represents a highly promising temperature sensor, featuring a minimum temperature resolution of 0.01 °C and a maximum temperature uncertainty of 0.016 °C. This outstanding performance is primarily attributed to the reduction of the PSFBG's birefringence from 5.607 × 10^-5 to 2.247 × 10^-6 using the slit beam shaping technique, which substantially enhances the stability of the resonant wavelength of the grating's phase-shifted peak and the measurement accuracy of the fiber Bragg grating temperature sensor.