Simultaneous measurement of temperature and surrounding refractive index by superimposed coated long period fiber grating and fiber Bragg grating sensor based on mode barrier region

In this paper, simultaneous measurement of temperature and surrounding refractive index (SRI) by the superimposed coated long period fiber grating and fiber Bragg grating (SCLBG) sensor based on mode barrier region (MBR) is proposed. The sensor makes use of the different characteristics between long period fiber grating (LPFG) and fiber Bragg grating (FBG) in response to temperature and SRI, which makes it very convenient to realize simultaneous measurement of temperature and SRI. In addition, the sensitivity of the sensor to SRI is further improved by selecting an appropriate overlay thickness within the range of MBR. The simulation results show that the temperature can be accurately measured by the resonant wavelength of FBG in SCLBG. The variation of the LPFG resonant wavelength in SCLBG is caused by both SRI and temperature, and the change caused by temperature is the same as that of FBG. Therefore, the change of LPFG resonant wavelength caused by temperature can be eliminated from the change of FBG resonant wavelength, and the accurate SRI change can be obtained. According to the analysis, the sensitivity of SCLBG to temperature and SRI can reach 10pm/degrees C and 2326.7nm/RIU, respectively. The linearity of the measured temperature and SRI is up to 0.999, and SRI sensitivity is at least four times higher than reported superimposed LPFG and FBG sensors. Therefore, the sensor proposed in this paper has the characteristics of high sensitivity and high linearity, which is expected to be applied in the field of sensing.