Research On Optial Mode Selection And Controlling Based On Microfiber Devices

Microfiber is a special kind of fiber waveguide with a diameter of several micro/nano meters,which has many characteristics different from ordinary fiber.Its diameter is close to or smaller than the lightwave wavelength transmitted.The refractive index difference between the cladding and the core is large,resulting in special mode field distribution and strong evanescent field coupling characteristics.Microfiber has broad application prospects in optical communication,fiber sensing,fiber laser,quantum optics and nonlinear optics,etc.In this paper,firstly,the optical characteristics,fabrication methods and the latest research progress of microfiber are reviewed.The strong evanescent field coupling and mode field characteristics of microfiber are analyzed theoretically.Based on these special optical characteristics,two microfiber devices are introduced: a mode selective coupler and a microfiber knot resonator.The mode selective coupler and microfiber knot resonator devices and their mode selection and regulation characteristics in the fiber laser cavity are studied.The detailed contents are as follows:1.Based on the optical waveguide theory,the strong evanescent field characteristics and mode characteristics of microfiber are studied theoretically.The phase matching condition for the fundamental mode(in the single mode fiber)conversion to higher order mode(in the few mode fiber)is analyzed using the coupled mode theory.Based on the finite element method,the relationship between the mode effective refractive index and the fiber diameter is calculated.The matching condition of the fiber diameter is obtained when the phase-matching is achieved in the fundamental mode(in the single mode fiber)and the higher order mode(in the few mode fiber).Secondly,the phase matching conditions are verified by the beam propagation method.The mode field distribution and the power conversion relationship between modes are simulated under satisfying the phase matching condition.2.The mode selective coupler is applied to the fiber laser.Since the mode selective coupler can be used as both a mode converter and a mode separator,the transverse mode of the fiber laser can be accurately selected and regulated.The cylindrical vector beam has a mode purity of more than 90% for both radial polarization beam and azimuthal polarization beam.Combined with mode-locked and Q-switching techniques in fiber lasers,a figure-8 cavity mode-locked cylindrical vector beam fiber laser based on the mode selective coupler,we propose and present an figure-8 cavity mode-locked cylindrical vector beam fiber laser based on a mode selective coupler,an actively Q-switched cylindrical vector beam fiber laser based on a mode selective coupler,and a passively mode-locked fiber laser with cylindrical vector beam based on a mode selective couple.3.According to the coupled mode theory and the ring waveguide cavity theory,the transmission function of the microfiber knot resonator is theoretically studied.The transmission function of the microfiber knot resonator is simulated by MATLAB,and its transmission characteristics are studied.The fused tapered method is used to study fabrication of the microfiber knot resonator,and analyze the performance of the fabricated microfiber knot resonator.The microfiber knot resonator is applied to the fiber laser.Due to the Vernier effect of the the microfiber knot resonator,the longitudinal mode of the fiber laser can be precisely adjusted and selected.A novel tunable single longitudinal mode fiber laser is proposed,and demonstrated stable single longitudinal mode laser is achieved in the range from 1545 to 1565 nm.