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摘要:
环境折射率和环境温度变化是影响光纤应变测量误差的主要因素。本文利用双模光纤纤芯双模式(LP01和LP11)支持特性设计了一款环境折射率不敏感的双模光纤(DMF)长周期光纤光栅LPFG)应变传感器。设计了传感器模型结构，制作了最优化参数的传感器样品。实验测试了DMF-LPFG传感结构对外部环境中应变、温度和折射率的响应。通过在单模光纤上用紫外激光刻写的布拉格光栅(FBG)解决了环境温度的交叉影响。轴向应变实验结果表明，该新型结构传感器在0 με~840 με应变范围内其轴向应变灵敏度可以达到-5.4 pm/με，该灵敏度值相比较于普通LPFG有很大提高。温度在25 ℃~80 ℃范围内其灵敏度为58.86 pm/℃，表现出较好的线性度。同时，传感器对环境折射率变化表现出不敏感特性。通过采用双参数矩阵对少模LPFG和FBG的应变和温度灵敏度进行处理，可以实现双参数的同时解调。该新型复合光栅结构具有良好的传感性能和工程应用前景。
Abstract:The variation of ambient refractive index and ambient temperature is the main factor affecting the error of optical fiber strain measurement. In this paper, a strain sensor based on the dual-mode fiber (DMF) long period fiber grating (LPFG) is designed. The sensor model structure was designed, and the sensor samples with optimized parameters were produced. The experiment tested the response of the DMF-LPFG sensing structure to the strain, temperature and refractive index in the external environment. Through the Bragg grating (fiber Bragg grating, FBG) written on the single-mode fiber with a UV laser, the cross effect of the ambient temperature is solved. The results of the axial strain experiment show that the axial strain sensitivity of the new structure sensor can reach -5.4 pm/με in the strain range of 0 με~840 με, which is greatly improved compared to the ordinary LPFG. The sensitivity is 58.86 pm/℃ in the temperature range of 25 ℃~80 ℃, showing good linearity. At the same time, the sensor is insensitive to changes in ambient refractive index. The dual-parameter matrix is used to process the strain and temperature sensitivity of the few-mode LPFG and FBG to achieve dual-parameter simultaneous demodulation. The new composite grating structure has good sensing performance and engineering application prospects.
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Key words:
- dual mode fiber /
- LPFG /
- FBG /
- strain sensor /
- temperature compensation
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Overview: Fiber Bragg gratings (FGs) and long period fiber gratings (LPFGs) are novel passive optical devices, which have been investigated for decades due to their unique physical and optical properties. The proposal of high-frequency CO2 laser writing LPFG technology is of great significance to enrich the design and preparation of the LPFG, improve the quality of signal transmission, and realize mass production. Multi-parameter measurement is an inevitable trend in the future development of optical fiber sensing technology. Especially, there are some problems such as the interference of environmental refractive index changes and temperature crosstalk for the application of engineering strain sensing. It is very important to effectively compensate the measurement error caused by temperature changes. The cladding mode of less-mode LPFGs is sensitive to the external refractive index, so the change in ambient refractive index will lead to the measurement deviation. The change in ambient temperature will cause the change of the LPFG period and the effective refractive index of various modes. The core of dual-mode fibers only transmits two low-order modes, so it has higher capacity and flexibility in mode control and analysis than that of single-mode fiber (SMFs). At the same time, compared with the SMF, the all fiber structure of the LPFG is more compact and flexible. Their conversion efficiency between the two modes is very high and the resonant wavelength at the mode conversion is sensitive to the strain and temperature when the phase matching condition is satisfied. Based on this characteristic, a LPFG sensor based on dual-mode fiber is proposed. The variation of ambient refractive index and ambient temperature is the main factor affecting the error of optical fiber strain measurement. In this paper, a strain sensor based on the dual-mode fiber (DMF) LPFG is designed. The sensor model structure was designed, and the sensor samples with optimized parameters were produced. The experiment tested the response of the DMF-LPFG sensing structure to the strain, temperature and refractive index in the external environment. Through the Bragg grating (fiber Bragg grating, FBG) written on the single-mode fiber with a UV laser, the cross effect of the ambient temperature is solved. The results of the axial strain experiment show that the axial strain sensitivity of the new structure sensor can reach -5.4 pm/με in the strain range of 0~840 με, which is greatly improved compared to the ordinary LPFG. The sensitivity is 58.86 pm/℃ in the temperature range of 25 ℃~80 ℃, showing good linearity. At the same time, the sensor is insensitive to changes in ambient refractive index. The dual-parameter matrix is used to process the strain and temperature sensitivity of the few-mode LPFG and FBG to achieve dual-parameter simultaneous demodulation. The new composite grating structure has good sensing performance and engineering application prospects.
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图 1 (a) 传感器结构；(b) 激光经过单模-少模后测量的场分布；(c) 激光经过单模-少模-LPFG后测量的场分布；(d), (e) 激光经过单模-少模-LPFG-偏振片后测量的场分布；(f) LPFG的显微镜图像
Figure 1. (a) The structure of the proposed sensor; (b) The measured field distribution emitted from the DMF without the LPFG; (c) The measured field distribution emitted from the DMF-LPFG; (d), (e) The measured field distribution after passing through the LPFG-polarizer; (f) Microscopic image of the LPFG
图 5 应变测量实验数据。(a) 光谱随应变漂移；(b) 应变灵敏度拟合
Figure 5. Experimental data of strain measurement. (a) Spectrum drift with strain; (b) Sensitivity fitting of strain
图 6 温度测量实验数据。(a) 光谱随温度漂移；(b) 温度灵敏度拟合
Figure 6. Experimental data of temperature measurement. (a) Spectral drift with temperature; (b) Sensitivity fitting of temperature
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