Simultaneous Measurement of Temperature and Strain Based on Compact Parallel Core–Cladding FBGs Inscribed by Femtosecond Laser Point-by-Point Method

Fiber Bragg grating (FBG) has been widely used in many fields for the measurement of temperature, strain, and other parameters, but single-mode FBG has a cross-sensitivity problem between temperature and strain. In this article, a new scheme is proposed to solve the problem by using parallel core–cladding FBGs, whose grating areas are located at the interface region of the core and cladding of the fiber. The sensing principle of parallel core–cladding FBGs for the simultaneous measurement of temperature and strain is theoretically analyzed. The femtosecond laser point-by-point method of the parallel core–cladding FBGs is experimentally investigated. Two parallel FBGs, namely, core FBG and cladding FBG, are formed efficiently and conveniently by a single time inscription. Two transmission peaks have been observed, which are excited by the core mode and the cladding mode. The experiment results demonstrated that the proposed compact sensor can detect the temperature with sensitivity of 11.043 pm/°C and strain with sensitivity of 1.2101 pm/
$\mu \varepsilon $
up to
$1.114\times 103\,\,\mu \varepsilon $
, and the relative errors of temperature and strain are less than 5.8% and 8.3%, respectively. The proposed parallel core–cladding FBGs are much more compact as one FBG element compared with the conventional structure with two cascaded FBGs or parallel FBGs. The parallel core–cladding FBGs can overcome the cross-sensitivity problem of single-mode FBG, which has great application value and potential in fiber sensing fields for dual-parameter.