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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.
(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
Experimental writing device of the DMF-LPFG
Transmission spectrum of the DMF-LPFG
The strain measurement set-up
Experimental data of strain measurement. (a) Spectrum drift with strain; (b) Sensitivity fitting of strain
Experimental data of temperature measurement. (a) Spectral drift with temperature; (b) Sensitivity fitting of temperature
Experimental data of refractive index measurement