Research On Optical Fiber Modal Interferometer Filter Modified By Micro/Nano Materials

With the tremendous development of optical fiber manufacturing technology,fiber optics has become one of the important branches of optics.The coupling,loss,dispersion,polarization and nonlinearity of transmission light in optical fiber have attracted extensive attention and a variety of researches have been carried out,which effectively improved the optical waveguide theory and the manufacturing technology of corresponding optical fiber device.As the modal interference theory applied to fiber optics,fiber integrated photonic devices based on modal interferometers with different fibers and structures have appeared and developed rapidly in the fields of fiber lasers,fiber amplifiers and fiber sensors.Due to the advantages of low loss,small size and high stability,fiber integrated photonic devices have shown great vitality in sensing,modulation and other fields.Micro/nano materials such as carbon nanotubes(CNTs)and graphene have good nonlinear light absorption characteristics.Combining carbon nanotubes and graphene materials with modal interferometers(MIs)provides a new idea for the researches.In this thesis,the fiber integrated photonic devices based on few-mode fiber(FMF)and erbium-ytterbium codoped fiber MIs with Down-Taper structure and core-offset structure are studied.Combining with the modification of graphene and CNTs on different structures,the theoretical analyses and experimental researches on sensing,all-optical modulation filtering and wavelength tunable lasers are carried out in detail.The main innovation achievements of this thesis are as follows:Firstly,FMF MI sensors based on cascaded Down-Taper structures are proposed.The interference principle and sensing mechanism of the FMF cascaded Down-Taper MIs are illustrated theoretically,and the feasibility of the FMF cascaded Down-Taper MIs for refractive index sensing and curvature sensing is verified experimentally.The feasibility of adjusting the curvature sensing sensitivities of the MI sensors by selecting the number of cascaded Down-Taper structures is also studied.The experimental results show that the sensitivity of MI with two Down-Taper structures for refractive index sensing is-202.978 nm/RIU,and the corresponding square of the linear correlation coefficient is 0.99276.When the MI is used for curvature sensing,as the number of Down-Taper structures increases from two to three and four,the sensitivity of MI curvature sensor decreases from-29.301 nm/m^-1 to-17.871 nm/m^-1and-5.034 nm/m^-1,respectively.Selecting suitable number of cascaded Down-Taper structures can meet the special curvature sensing applications with different curvature sensitivity requirements,which will greatly extend the application adaptability of the curvature sensor proposed.Secondly,an all-optical tunable fiber filter based on FMF MI coated with graphene/epoxy(GN/EP)resin composite material is proposed.The all-optical tunable filtering mechanism of the FMF MI with core-offset structure modified by GN/EP composite material is expounded theoretically,and the feasibility of FMF MI for all-optical tunable filtering is verified experimentally.The experimental results show that when adjusting the pump power from 0 m W to 300 m W,a wavelength tunable range of the filter based on FMF MI can reach 5.062 nm,and the pump response step is 0.016 nm/m W.Thirdly,an all-optical tunable fiber filter based on Er/Yb codoped fiber(EYDF)MI coated with carbon nanotube/polyvinyl alcohol(CNT/PVA)composite material is proposed.The all-optical tunable filtering mechanisms of the EYDF MIs with core-offset structure unmodified/modified by CNT/PVA composite material are expounded theoretically,and the feasibilities of EYDF MI unmodified/modified by CNT/PVA composites material for all-optical tunable filtering are verified experimentally.The experimental results show that when adjusting the pump power from 0 m W to 210m W,with the modification of CNT/PVA composite material,the pump response wavelength tunable range of the of EYDF MI increases from 2.363 nm to 7.650 nm,which is 3.24 times higher than that unmodified by CNT/PVA composite material.The pump response step of the MI with CNT/PVA composites material increases from 0.011 nm/m W to 0.039 nm/m W,which is 3.54 times higher than that of unmodified MI.The CNT/PVA composite material can significantly improve the pump response wavelength tunable range and pump response step of the EYDF MI filters proposed.Fourthly,all-optical tunable fiber filters based on EYDF cascaded Down-Taper structures MI coated with CNT/PVA composite material and the wavelength tunable optical fiber laser based on the filters above are proposed.The all-optical tunable filtering mechanisms of the EYDF cascaded Down-Taper structure MIs unmodified/modified by CNT/PVA composite material are expounded theoretically.The feasibilities of EYDF cascaded Down-Taper MIs unmodified/modified by CNT/PVA composites material for all-optical tunable filtering are verified experimentally.And the feasibility of applying the MI filters above in the laser cavity to realize the wavelength of the laser tunable is also verified experimentally.The experimental results show that when adjusting the pump power from 0 m W to 300m W,with the modification of CNT/PVA composite material,the pump response tunable range of the EYDF cascaded Down-Taper structure MI filter increases from1.875 nm to 3.112 nm,which is 1.66 times higher than that unmodified by CNT/PVA composite material.The tunable wavelength range of the laser based on the filters above increases from 2.137 nm to 3.638 nm,which is 1.70 times higher than that unmodified by CNT/PVA composite material.