| As the best wired medium,optical fiber has the advantages of antielectromagnetic interference,low cost,light weight and wide coverage.The distributed optical fiber sensor based on stimulated Brillouin scattering can make full use of the advantages of optical fiber and has broad application prospects.However,both temperature and strain will lead to a linear change in Brillouin frequency shift.Generally,there is only a single Brillouin scattering peak in ordinary single-mode optical fiber,which can not distinguish the effect of temperature and strain on Brillouin frequency shift,resulting in the cross-sensitivity problem of Brillouin distributed optical fiber sensor.Few-mode fiber supports more than two optical modes.The contribution of different optical modes in the process of stimulated Brillouin scattering makes it have different characteristics compared with single-mode fiber.Therefore,in order to study the coupling contribution mechanism of different optical modes to stimulated Brillouin scattering in few-mode fiber and explore the potential of its application in twoparameter sensing,this paper studies the Brillouin gain spectrum characteristics of a step-type dual-mode fiber by combining theoretical simulation and experiment,and carries out fiber design and performance optimization for temperature-strain dual-parameter sensing.The main research contents are as follows:(1)The Brillouin gain spectrum characteristics of step-type dualmode fiber are studied by combining theoretical simulation and experiment.Firstly,the intra-mode Brillouin gain spectrum and inter-mode Brillouin gain spectrum of the step-type dual-mode fiber are simulated and calculated by theoretical analysis.Then,an experimental platform was built to study the effect of higher-order modes on stimulated Brillouin scattering of multimode fiber.The Brillouin gain spectrum of dual-mode fiber was measured at 1550 nm wavelength under the condition that only the fundamental mode was incident and the higher-order mode and fundamental mode was both incident.It is found that when only the fundamental mode is incident,the Brillouin scattering in the LP01-LP01 mode is generated in the dual-mode fiber,and the Brillouin gain spectrum obtained by simulation and experiment has only one scattering peak.Compared with the experimental results,the simulation calculation error of the Brillouin frequency shift is 0.3%;When both the fundamental mode and the higher-order mode are incident,the intra-mode Brillouin scattering and inter-mode Brillouin scattering occur simultaneously in the fiber under the coupling of various acoustooptic modes.The experimental Brillouin gain spectrum has both main peak and gentle slope,which is the same as the spectrum type calculated by simulation.And the comparison error between simulation calculation and experiment is as follows:the simulation calculation error of main peak Brillouin frequency shift is 0.3%,the simulation calculation error of gentle slope Brillouin frequency shift is 0.5%,and the simulation calculation error of main peak-gentle slope power difference is 12.7%.Within the allowable range of error,the theoretical simulation and experimental results are in good agreement,indicating that the Brillouin gain spectrum of the dual-mode fiber when transmitting multiple modes simultaneously comes from the linear superposition of the Brillouin gain spectrum within and between modes.Finally,the influence of the structural parameters of the step-type dual-mode fiber on its intramode and inter-mode Brillouin gain spectrum is investigated.(2)The response characteristics of Brillouin frequency shift of dualmode fiber to bending in quasi-basic mode are studied theoretically and experimentally.Theoretical analysis shows that compared with singlemode fiber,dual-mode fiber has larger core size,which will lead to greater strain of dual-mode fiber in the same bending state,making the Brillouin frequency shift of dual-mode fiber more sensitive to bending response.The bending sensitivity of Brillouin frequency shift calculated by theoretical simulation is 8.53 MHz/cm-2.In addition,the Brillouin frequency-shift bending sensitivity of the dual-mode fiber in the quasi-basic mode state is 10.16 MHz/cm-2,and the simulation error is 18.8%compared with the experimental results.The experimental and simulation results show that the bending sensing sensitivity based on Brillouin frequency shift can be increased to 3.6 times of that of ordinary single-mode fiber by using the quasi-basic mode state of dual-mode fiber.The simulation and experimental results show that the dual-mode fiber based on the quasiprimal-mode state is expected to realize high-sensitivity bending sensing.(3)A kind of hetero-core dual-mode fiber is designed for temperature and strain two-parameter sensing.First of all,aiming at the problem of dual-parameter sensing and anti-bending design,a kind of anti-bending hetero-core dual-mode fiber with bimodal mode Brillouin gain spectrum is proposed.The refractive index of the core part is stepped.The anti-bending performance of the fiber is improved by adding multiple grooves in the cladding.By adjusting the structure parameters of the optical fiber,it can be realized under the determined optical fiber structure:the Brillouin gain peak between the two modes is high and the peak is close,and its gain coefficients are 0.0838/mw and 0.0845/mw respectively;The peak-topeak frequency shift difference is 117 MHz,which can effectively avoid the aliasing of the two peaks;Bending loss of LP01 mode is 3.25×10-11 dB/m,bending loss of LP11 mode is 1.42×10-04 dB/m;The temperature and strain sensing coefficients of the two peaks are CTL11=4.37 MHz/℃,C SL11=0.2 MHz/με,CTL12=3.58 MHz/℃,CSL12=0.16 MHz/με,The uncertainty of temperature and strain sensing is 0.24 0C,4.7με。Then,in order to further reduce the sensing uncertainty of temperature strain and improve the anti-measurement error ability of the optical fiber sensor,by adjusting the refractive index distribution of the two layers of fiber core,the bimodal gain coefficients of the inter-mode Brillouin gain spectrum of the fiber structure determined by simulation analysis are 0.0677/mw and 0.0646/mw respectively;The peak-to-peak frequency shift difference is 96 MHz;The temperature strain coefficients of the two peaks are CTL11=3.39 MHz/℃,CSL11=0.16 MHz/με,CTL12=2.83 MHz/℃,CSL12-0.12 MHz/με,The uncertainty of temperature and strain sensing is 0.08℃,1.1 με。The uncertainty of temperature and strain sensing can be significantly reduced without losing too much Brillouin bimodal performance and sensing sensitivity.To sum up,this paper studied the Brillouin scattering characteristics of step-type dual-mode fiber by combining theoretical simulation and experiment,and designed a hetero-core dual-mode fiber whose inter-mode Brillouin gain spectrum can make full use of the optical mode distribution characteristics of dual-mode fiber to achieve better Brillouin bimodal performance,and optimize the anti-bending performance and antimeasurement error performance.The simulation and experimental research results can not only provide new ideas for the application of Brillouin sensing in multimode fiber,but also have certain reference significance in the fields of mode division multiplexing fiber communication systems,fiber lasers,fiber amplifiers,etc. |
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