Research On Sensing Characteristics Of LPFG Inscribed In The Few-modes Fiber

Long-period fiber grating?LPFG?has some advantages,such as small volume,light weight,anti-electromagnetic interference and no backward reflection.It has been widely studied and applied in optical fiber communication and optical fiber sensing field for the past two decades.With the emergence of few-mode fiber,few mode fiber grating?FM-LPFG?has been researched a lot.The few-modes fiber?FMF?can support several core modes,so LPFG based on FMF may has different characteristics compared with single-mode long-period fiber grating?SM-LPFG?.FM-LPFG can be used as a mode selection filter to filter the core-mode by coupling the energy of the fundamental core mode into cladding modes,leaving higher-order core mode;The coupling between higher-order core mode and co-propagating cladding mode or cross-coupling between several core modes and cladding modes may increase the sensing sensitivity of the FM-LPFG;The coupling between the two core modes can also be used as a mode converter.Based on the coupling model theory,this thesis mainly studies FM-LPFG in the following aspects:?1?The mode coupling mechanism and the sensing characteristics of FM-LPFG are analyzed in detail.The phenomenon of mode-reorganization in four-layer fiber is studied,and the effect of film-layer parameters on the mode-reorganization is also analyzed by Matlab and Comsol software.?2?The LPFG is written on the four-mode and two-mode fiber using high-frequency CO2 laser pulse.The process of writing LPFG on the FMF fiber is analyzed.At the same time,the effect of the length of the FMF on the transmission spectrum of LPFG is also analyzed.The length of the FMF should be controlled within 15cm.So,the interference between core modes will not have a significant impact on the transmission spectrum.?3?The experimental study on the temperature,bending,refractive index and twist sensing characteristics of four-mode and two-mode LPFG are carried out,and are compared with SM-LPFG at the same time.The result find that the temperature sensitivity of LPFG inscribed in the four-mode fiber can reach up to 93.7pm/?.The LPFGs inscribed in both four-mode and two mode fiber have higher refractive index sensitivity compared with SM-LPFG.The average refractive index sensitivity of the four-mode LPFG can reach up to-863nm/RIU which is almost 9 times higher than the refractive index sensitivity of SM-LPFG at an ambient refractive index of 1.443 to1.4502.The shift of the resonant wavelength sensitivity of LPFG inscribed in the two-mode fiber is-32.88nm/m^-11 when the fiber curvature is in the range of1.0m^-1-2.8m^-1,and the shift of the resonant wavelength sensitivity can reach-0.34nm/?rad/m?when the distortion rate of fiber is in the range of 36rad/m-56rad/m,improved by 148%and 43.7%compared with SM-LPFG,respectively.?4?Both SM-LPFG and FM-LPFG are coated with a layer of ZnO film by sol-gel method.Both SM-LPFG and FM-LPFG have higher refractive index and distortion sensitivity than before.Especially,when the ambient refractive index is between1.4456¹.4497,the refractive index sensitivity of FM-LPFG after coating is improved by 171.47%.