Mode Manipulation In Multimode Optical Fibers And Adiabatic Waveguide Couplers

Mode is not only a fundamental concept describing the spatial distribution of electromagnetic field in optical fibers and waveguides,but also a powerful tool for analyzing the characteristics of light propagation in waveguide devices.In this thesis,we focus on the modes in multimode fibers and supermodes in adiabatic waveguide couplers,study experimentally several techniques for mode selection,mode conversion and mode control in optical fibers,and design a novel adiabatic waveguide device by the supermode shaping approach.The main contents are as follows:Firstly,we studied the mode conversion and control in multimode fibers based on nonlinear effects.Based on the analysis of the phase-matching condition of intermodal four-wave mixing,we proposed a scheme of higher-order-mode supercontinum generation through the cooperative effect of intermodal four-wave mixing and cascaded Raman scattering.Then,we demonstrated experimentally the higher-order-mode visible supercontinuum generation in conventional few-mode fiber and photonic crystal fiber,respectively.Besides,we also studied preliminarily the spatiotemporal nonlinear effects in graded-index multimode fiber.A supercontinuum covering 400~2400 nm was generated and an evident beam cleaning effect was observed.Secondly,the two fiber grating based approaches for mode selection and conversion in fiber lasers were proposed and demonstrated.For all-fiber lasers,we proposed a kind of few-mode fiber Bragg grating pair for mode selection.Based on this scheme,we constructed an all-fiber laser capable of operating in a specific higher-order mode.The slope efficiency and the mode purity are 54% and 95.4%,respectively.Vector and vortex beam output were also demonstrated through external polarization control.For all-fiber amplifier,we designed a mechanically-induced long-period grating based all-fiber mode convertor specially for large-mode-area double-clad fibers.High-efficiency conversion between the fundamental and higher-order modes were successfully realized in a high-average-power picosecond master-oscillator power amplifier system,with the output power reaching as high as 117 W for both modes.Finally,we proposed the use of supermode shaping technique for design of adiabatic waveguide structures and designed a novel adiabatic waveguide coupler that can work bidirectionally.The supermode evolution in such an adiabatic waveguide coupler was analyzed and numerically simulated the modal and beam propagation properties.The numerical results revealed that the bidirectional adiabatic coupling based on both the dark state and the bright state were feasible,efficient and broadband.We also experimentally tested the performance of the coupler,and found that the coupling efficiencies in both directions were above 90% in the design wavelength band.This type of adiabatic waveguide device based on supermode shaping and adiabatic evolution has the advantages of large bandwidth and high tolerance to fabrication errors,thus opening a new way for design of novel integrated photonic devices.