Research On The Temperature/strain Sensor Based On Few-mode Fiber Bragg Grating

Fiber Bragg grating（FBG）is capable of sensing temperature,strain,bending,refractive index and other parameters,and has important applications in aerospace,civil engineering,power systems and biomedical fields.Usually,when temperature and strain are applied to fiber Bragg grating at the same time,the temperature-strain cross sensitivity problem prevents the dual-parameter measurement of temperature and strain,which restricts the further development and application of fiber grating sensors.Compared with FBG,few-mode fiber Bragg grating（FM-FBG）has multiple reflection peaks,which has great potential and application value in sensing field.The main innovative points and findings of this paper are as follows:（1）In order to address the natural narrow bandwidth of wide-bandwidth optical sources with Er³⁺-doped fiber amplified spontaneous emission（ASE）,the ASE light source was optimized by studying the effects of each parameter on the spectrum,to achieve a highly flat output spectrum.The effects of the length of the erbium-doped fiber,the reflection coefficient of the fiber ring mirror,the power of the pump laser,and the length of the external unpumped pigtail on the output spectrum were studied in simulations using a dual-way backward ASE light source.The simulation results showed that a highly flat erbium-doped ASE light source,with a 3 d B bandwidth of 51.10 nm,was obtained when the erbium-doped fiber length was 6 m,the external unpumped pigtail length was 2 m,the reflection coefficient of the fiber ring mirror was 99%,and the pumping power was 1 W.The optimized ASE light source can meet the high-bandwidth requirements of fiber optic sensing applications（2）To address the problem of low output power of the designed Erbium-doped fiber ASE light source,a bi-directionally pumped,two-way structured ASE light source is designed to achieve high power and high flatness output.Compared with the forward single-pass ASE light source,the bi-directionally pumped,dual-pass structure of the ASE light source improves pumping efficiency by8.24%,spectral flatness by 1 d B,and 3 d B bandwidth by 24.56 nm.This broadband ASE light source can be applied to subsequent measurements of few-mode fiber Bragg grating sensing.（3）To address the problem of temperature/strain cross sensitivity in fiber grating,wavelength-temperature coefficient and wavelength-strain coefficient measurements of fiber grating（SM-FBG and FM-FBG）are carried out,and it is found that the difference between the two correlation coefficients of LP₀₁ mode and LP₁₁ mode in few-mode fiber is relatively small,which limits the temperature and strain resolution in dual parameters sensing;Subsequently,the experiments are carried out by using the LP₁₁ mode of FM-FBG and the LP₀₁ mode of SM-FBG to achieve simultaneous temperature/strain measurements,where the former is subjected to the combined effect of temperature and strain and the latter is subjected to the effect of temperature only,and the dual parametric measurements were achieved by the dual matrix method.The temperature measurement error is±1.6°C and the strain measurement error is±20.04με,which provides a feasible solution to the temperature-strain cross sensitivity problem.