Analysis and experiment on simultaneous measurement of strain and temperature by etched and un-etched FBG pair

Abstract

This paper presents analysis and experiment on simultaneous measurement of strain and temperature using pair of etched and un-etched fiber Bragg grating (FBG) fabricated in a single mode glass optical fiber. The intrinsic sensitivity for strain and temperature of FBG explicitly depends on the clad diameter. The mechanical properties of glass optical fiber such as Young’s modulus and Poisson’s ratio change with cladding diameter. This in turn will change the strain optic coefficient and thermal expansion coefficient of the optical fiber. Analytically, it is shown that the change in refractive index, Young’s modulus, Poisson’s ratio and thermal expansion coefficient was 0.04, 58.2 GPa, 0.29 and 0.99 × 10−6/°C respectively as the cladding diameter was decreased from 125 µm to 11 µm. The results obtained for strain and temperature sensitivity from the analysis are verified with experimental results. Experimentally, as the fiber clad diameter is reduced from 125 µm to 25 µm via hydrofluoric (HF) etching, the average strain sensitivity increases from 1.06 pm/µɛ to 1.20 pm/µɛ for the maximum applied strain of around 5500 µɛ and also the average temperature sensitivity increases from 11.48 pm/°C to 11.80 pm/°C for the maximum applied temperature of 150 °C. The temperature and strain can be measured simultaneously with high accuracy by using these sensitivity coefficients for two gratings.