Research Center of Optoelectronic Sensing Engineering, School of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11547031) and the Nanjing University of Posts and Telecommunications Research Funds, China (Grant No. NY217111)
Received Date:06 July 2019
Accepted Date:27 September 2019
Available Online:27 November 2019
Published Online:01 December 2019
Abstract:The stress-sensitive and temperature-insensitive characteristics of the tapered fiber grating can be used effectively to suppress the cross-sensitive problem of temperature and stress. In this paper, a fiber grating with a symmetric double-cone shape is proposed, which is made by using a fused taper technology. The theoretical model of sensing characteristics is established and analyzed by the transfer matrix method. Firstly, the factors affecting the change of radon coefficient are studied, and the relationship between the radon coefficient and the amount of grate length change is obtained, and then the spectral characteristics of the symmetric fused-tapered fiber grating are analyzed to discuss the origin of dense modulation at the short wavelength of the spectrum. The effects of temperature and stress on the reflection spectrum of symmetrically fused-tapered fiber grating are studied, and the relationship between the corresponding center wavelength and spectral bandwidth is obtained. In order to solve the problem of low stress sensitivity of the fiber grating, a scheme is presented that the radius difference of the optical fiber in the sensing cone region is enhanced by using polymer to coat the tapered area. Finally, a fused taper technology is used to prepare the symmetrically molten fiber grating, and verify the correctness of theoretical simulation in experiment, indicating that its stress sensitivity is 0.11391 nm/N. Firstly, the ripple coefficient of the symmetrically fused-tapered fiber grating is linearly related to the amount of change in the length of the grating. Secondly, because the grating cycle is small at the end of the symmetrically melt-pull-cone fiber-optic grating, and the reflectivity is less than 1, the left-hand transmission light and the right-hand reflected light will produce interference, so the spectral short wavelength will present dense modulation phenomenon. Thirdly, the center wavelength shifts to long wavelength region and the reflection bandwidth is broadened as stress is raised, and the center wavelength and reflection bandwidth are both linearly changed with the applied stress. Finally, the center wavelength shifts to long wavelength region as the temperature rises gradually, and the effect on the spectral bandwidth can be ignored. The stress sensitivity of the fiber grating increases hundreds of times by increasing the difference in fiber optic grating radius in the sensing tapered area, and the stress sensitivity can be further improved by increasing the fused taper variation of the grating. The spectral bandwidth of the symmetrical fused tapered fiber grating is only sensitive to stress but not to temperature. The characteristics can be used to realize the double-parameter measurement of temperature and stress. Keywords:tapered fiber grating/ fused-biconical taper/ effective index/ grating period/ sensing
在研究应力与温度对光纤光栅反射谱影响之前, 先讨论啁啾系数C与FBG半光栅长度$L_1$以及光栅长度变化量$\Delta L$的关系. 从图3可以得出, FBG半栅区长度相同时, 光栅长度变化量与啁啾系数满足线性关系; 光栅长度变化量相同时, FBG半栅区长度越大, 啁啾系数越小. 图 3 啁啾系数随锥区长度及其变化量分布图 Figure3. Distribution of the chirp coefficient with the length of the cone and its variation.
选用FBG向两边各拉伸$\Delta L = 1~$ μm制成的对称熔融拉锥型光纤光栅, 并对其实施轴向应力进行应力传感模拟仿真, 研究应力大小对反射谱的影响. 其中应力为0 N (未施加应力) 和2 N时得到的反射光谱如图4所示. 图 4 光栅长度变化量1 μm的反射光谱图 Figure4. Reflectance spectra with the grating length variation of 1 μm.
图 6 光谱带宽半宽度随轴向应力变化分布图 Figure6. Distribution of half-width of the spectral bandwidth with axial stress
23.2.温度与光谱宽度的关系 -->
3.2.温度与光谱宽度的关系
把上述对称熔融拉锥型光纤光栅引入不同温度场中进行温度传感模拟仿真分析, 仿真结果如图7所示. 图 7 光纤光栅在不同温度下的反射谱 Figure7. Reflection spectra of fiber grating at different temperatures.
由图7可知, 随着温度逐渐增大, 反射谱中心波长向长波长方向移动, 这与普通光纤光栅型温度传感特性是一致的. 而其光谱半宽度$\lambda _r - \lambda _m$与温度的关系如图8所示. 图 8 光谱带宽半宽度随温度变化分布图 Figure8. Distribution of spectral bandwidth half-width with temperature
类似地, 得到不同光栅长度变化量$\Delta L$所对应的光谱半宽度${\lambda _r} - {\lambda _m}$与轴向应力P关系曲线图, 如图11所示. 图 11 光谱带宽半宽度随轴向应力变化分布图 Figure11. Distribution of half-width of the spectral bandwidth with axial stress