Refractive index and temperature sensitivity characterization of excessively tilted fiber grating.

We have investigated experimentally and numerically the temperature and refractive index (RI) sensitivity characteristics of excessively tilted fiber gratings (Ex-TFGs) in detail. Both results have shown that the temperature and RI sensitivities of Ex-TFGs are mode order dependent. For temperature sensitivity, the higher order cladding mode of Ex-TFG exhibited lower temperature sensitivity, quantitatively, the temperature sensitivities of TM cladding modes at the resonance wavelength around 1550nm are 9pm/°C, 6.8pm/°C, 5.6pm/°C and, 4pm/°C for cladding mode 28th, 31st, 35th, 40th, respectively, indicating the overall temperature sensitivity of Ex-TFGs were lower than that of normal FBGs. The SRI sensing results have shown that the RI sensitivity of Ex-TFG at the special index value could be improved by choosing the cladding mode with effective index close to the refractive index of the detecting medium. The SRI sensitivities at the effective mode index were 2250nm/RIU at 1.408, 864nm/RIU at 1.395, 1536nm/RIU at 1.380 and 1360nm/RIU at 1.355, for the cladding mode of 28th, 31st, 35th, 43rd, respectively. The experimental results have also shown the SRI sensitivity of Ex-TFG was increasing with increasing of the resonance wavelength.