Study On Dual-parameter Sensing Characteristics Of Few-mode Fiber

Fiber-optic sensors based on the few-mode fiber technology have attracted more and more attention due to its advantages such as high sensing sensitivity,resistance to electromagnetic interference,high temperature resistance,and dual-parameter sensing.At the same time,they are facing severe challenges.High deployment costs are one of the main challenges.The few-mode optical fiber can support multiple modes of optical signals and can be used for sensing multiple physical quantities simultaneously.It has the potential for a high degree of integration and can greatly reduce operating costs.Aiming at the requirements of multi-parameters,high cross-sensitivity discrimination,and high sensitivity in the few-mode optical fiber sensors,this paper proposes a few-mode optical fiber based bending and distortion specklegram sensor and three types of few-mode optical fiber temperature and strain dual-parameter sensors.The major work of the thesis as follows:1.The spatially and spectrally resolved imaging(S2)technology is used to analyze the characteristics of lateral offset,polarization,bending,and twisting in a few-mode fiber,and a bend and twist specklegram sensor based on S2 imaging technology is proposed.The S2 imaging method is used to reconstruct the LP11 specklegram,eliminating the step of using a mode converter to separate the pure LP11 mode,simplifying the experiment and reducing the cost,and analyzing the all higher-order modes in parameters such as differential mode group delay(DMGD),relative intensity(MPI),and mode field distribution.S2 imaging technology is used to analyze polarization-maintaining few-mode fibers(PM-FMF)and the case where the dominant modes are higher-order modes,which expands the application range of S2 imaging technology.2.The mode coupling characteristics of polarization-maintaining few-mode Bragg gratings were studied by selective polarization excitation,and the temperature and strain sensing characteristics of polarization-maintaining few-mode Bragg gratings(PM-FMF-FBG)were comprehensively analyzed.For the first time,the sinusoidal variation of intensity in PM-FMF-FBG was observed as a function of temperature and strain.Based on this,utilizing both wavelength demodulation and phase demodulation,we achieved simultaneous measurement of temperature and strain in PM-FMF-FBG.The temperature-strain cross-sensitivity discrimination is one of the best available methods.3.The parameters of the few-mode fiber and the long-period fiber are optimized.Theoretical and experimental results prove the existence of the turning point in the few-mode long-period grating(FMF-LPFG)and the double response peak in the transmission spectrum.The strain and temperature sensitivity are maximized near the turning point.Based on this,simultaneous measurement of temperature and strain based on FMF-LPFG is realized,and the sensitivity is improved by an order of magnitude over the parameter without optimization.4.The theory and experiment prove that the critical wavelength phenomenon of LP041 and LP11 mode interference in the few-mode fiber is a supplement to the existing theory.A high-sensitivity,high-modulation long-period grating-assisted inter-mode interference sensor is proposed.Since the two interference peaks have opposite sensitivity coefficients,the sensor has a good ability to distinguish between temperature and strain.