An Alternative Demodulation Method for Fiber Bragg Grating Temperature Sensors Using Space-Domain Fiber Cavity Ringdown

A fiber Bragg grating (FBG) sensor utilizing space-domain fiber cavity ringdown (FCRD) scheme is presented and experimentally demonstrated for high-resolution temperature measurement. Different from the existing FBG temperature sensors based on time-domain or frequency-domain FCRD techniques, this scheme senses temperature in the space domain by measuring the ringdown distance rather than the ringdown time or the frequency response of the system over a frequency span, without pulse modulation/microwave modulation nor fast photoelectric detection. It integrates frequency-shifted interferometry (FSI) and FCRD, so that it can achieve high sensitivity, good stability, and thus high measurement resolution. By incorporating a standard single-mode bare FBG in the fiber ringdown cavity (RDC) as the sensing element, the linear response of the sensor to temperature in the range of 25 °C–35 °C was experimentally investigated, and a sensitivity of 0.1641/ (km $\cdot ^{\circ }\text{C}$ ) was attained. Through repetitive testing, the ringdown baseline stability of the sensor was determined to be 0.42%, and the temperature resolution was 0.077 °C, which is the best among the ones of the current bare-FBG-based temperature sensors. In addition, the FBG sensor’s behaviors in other temperature regions were observed by adjusting the operating wavelength of the laser, and the approximately same sensing performances were achieved. The experimental results show that the space-domain-FCRD-based FBG temperature sensor offers a promising candidate for the temperature measurements in civil engineering, environmental protection, biomedicine, and other fields.