Ultrahigh-sensitivity fiber Bragg grating refractive index sensor based on thin cladding and mode transition

This paper presents an ultrahigh-sensitivity fiber Bragg grating (FBG) refractive index (RI) sensor, which combines the high sensitivity of mode transition (MT) and thin cladding FBG. According to the derivatives of the effective refractive index (ERIs) of different cladding modes versus overlay thickness when surrounding refractive index (SRI) is 1.33, we found the sensitivity to SRI increases as the order of cladding mode increases. The 21st cladding mode is selected for sensing, which has the strongest coupling with core mode. For the 21st cladding mode, we obtain the optimum overlay thickness (OOT) in MT under different cladding radii by calculating derivative of ERI versus overlay thickness, and we approve that the sensitivity to SRI increases as the overlay thickness approaches the OOT. In addition, as the cladding radius decreases, the OOT gradually decreases, and the sensitivity at the OOT gradually increases, but the detecting range decreases. We give the matching relationship between cladding radius and OOT, which is meaningful and given by y = 303-903*exp(-0.098*x), where y is OOT(nm), x is cladding radius(mu m). Then, we calculate the sensitivity to SRI under different cladding radii and matched OOTs. The results show the sensitivity to SRI is available to 1.115 x 10(5)pm/RIU, corresponding resolution is 8.97 x 10(-6)RIU, when the cladding radius is 14.4 mu m and the matched OOT is 84nm. Such a FBG SRI sensor is competitive with other FBG-based refractometers and the resolution is about two orders of magnitude higher than those FBG-based refractometers used for RI sensing around 1.33 with thin cladding or MT.