| In recent years,to meet the needs of social development,the data traffic growth rapidly.But at the same time,the increase of transmission rate of the optical fiber,which plays a key role in data transmission as a kind of transmission medium,is becoming more and more difficult.One of the reasons is the limitation of the exchange capacity of switching nodes,another reason is that increasing the transmission rate of the optical fiber itself is becoming increasingly difficult.So,the study about all-optical switching and large capacity transmission technology will be very helpful to solve these problems.The optical vortex is a kind of structured light which has a spiral wave front.It can carry orbital angular momentum(OAM)and it can be widely used in optical tweezers,particle manipulation,microscope,sensing,optical communication and so forth.Recently,the optical fiber-based generation,control,measurement and application of optical vortex has become a hot issue of research.Especially,optical vortex can be used in large capacity optical fiber communication.The combination of optical vortex and optical fiber can improve the performance of optical fibers and extend the application area of optical fibers.Under the support of related research projects,some researches about the above mentioned contents including special optical fiber and optical vortex are successfully implemented in this doctoral thesis.The main outcomes are as follows:(1)Low-concentration erbium-doped fibers are fabricated for the use of amplifiers.Single mode and few mode high-concentration erbium-doped fibers are fabricated for the use of optical fiber lasers.Fibers with large mode area and low bending loss are fabricated which can be used for fiber to the home(FTTH).Several few-mode fiber are fabricated for the subsequent research.(2)Three kinds of vortex fibers are theoretically analyzed,manufactured and measured.A method based on S2 technique is presented to measure the vector modal content.This method is practically implemented in measuring the vector modal content in a vortex fiber and the characterization of the vortex fiber is obtained successfully.(3)Two classical methods of optical fiber-based OAM generation are introduced,and the differences and relations between the two methods are explained clearly.A new method based on the combination of one even mode and one odd mode which belong to different vector modes within the same mode group is presented to generate continually tunable OAM and realize a mode switch between linearly polarized OAM mode and linearly polarized mode.A new method based on thecombination of one even and one odd linearly polarized modes which have orthogonal polarization directions within the same mode group is presented to generate continually tunable OAM and realize a mode switch between linearly polarized OAM mode and linearly polarized mode.The evolution of a linearly polarized mode when propagating in a vortex fiber is investigated,two dimensional tunable OAM and mode switch can be achieved by taking advantage of the evolution process.The evolution of a vector mode when propagating in a polarization-maintaining-few mode fiber is theoretically investigated,two dimensional tunable OAM and mode switch can be achieved by taking advantage of the evolution process.The theoretical system of optical-fiber-based OAM generation is enriched.(4)Inspired by the Ring Method,a method called scalar intensity analysis method(SIAM)is presented for measuring the mode purity of linearly polarized OAM mode.The feasibility,precision,sensitivity and error of this method are theoretically analyzed,the application condition and occasion of the method are introduced detailedly too.The method is practically applied to the experiments.(5)The mechanical long-period grating based mode converter and the fiber Bragg grating based mode converter are investigated.A mode switchable three-wavelength optical fiber laser based on the cascaded long-period grating and fiber Bragg grating is presented.Switchable three wavelengths with single mode output and switchable two wavelengths with six transverse modes output are achieved in the optical fiber laser and this work lays the foundation for the development of the vortex fiber laser. |
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