Research On Mode Converter And Optical Fiber Laser Based On Few-Mode Optical Fiber

Mode division multiplexing technology is one of the effective means to greatly increase the transmission capacity of communication systems.Mode converters are the key components in mode division multiplexing systems using linear polarization modes,orbital angular momentum modes and vector modes.Mode converters based on few-mode fibers have a simple structure,small size,high conversion efficiency and good compatibility with fiber optic systems,etc.,are good choices for mode division multiplexing system.Multi-wavelength fiber lasers are ideal for dense wavelength division multiplexing systems,which can reduce system complexity and cost compared to semiconductor laser arrays.Single-longitudinal-mode(SLM)narrow linewidth fiber lasers can also be applied to dense wavelength division multiplexing systems due to their good beam quality and narrow linewidth.They also have broad application prospects in long-distance fiber sensing,laser radar,and coherent optical communication.Few-mode fiber lasers which can output high-order linear polarization modes or vector modes,can reduce system complexity and improve mode multiplexing flexibility when applied to a mode division multiplexing system.By combining a few-mode fiber laser with a multi-wavelength fiber laser or a narrow linewidth fiber laser,we can fabricate a fiber laser for a wavelength division-mode division hybrid multiplexing system.This thesis is supported by the Central Universities'Basic Research Funds Project,the'973'Project and the National Natural Science Foundation,the main research achievements of this thesis are listed as follows:(1)The principle and characteristic of mode selective coupler is studied from the coupled mode theory.The influence of coupler structural parameters on mode coupling is analyzed.We have fabricated dual-mode fibers using the MCVD(Modified Chemical Vapor Deposition)devices,and fabricated a broadband mode selective coupler consisiting of a single-mode fiber and a dual-mode fiber using fused biconical tapering method.Switchable output with tunable orbital angular momentum mode and linear polarization mode is realized under the action of a linear polarizer.(2)The mode coupling characteristics of few-mode fiber Bragg gratings are studied using coupled mode theory.We have fabricated a kind of few-mode fiber and made a grating on it.Multi-wavelength fiber laser based on a few-mode fiber Bragg grating and a mode selective coupler is constructed.The few-mode fiber grating works as a wavelength selector.Switchable outputs between triple-wavelength,dual-wavelength and single-length are realized based on offset splicing and polarization controlling method.The outputs of the tunable orbital angular momentum mode are implemented in every single-wavelength operation state.(3)Compound ring cavity structure is fabricated with single-mode fiber couplers,the narrow linewidth fiber laser with cylindrical vector mode output is constructed based on offset splicing and few-mode fiber Bragg grating.Radial polarized beam and azimuthally polarized beam output are realized.The output wavelength locates at the fundamental mode resonance wavelength of the few-mode fiber Bragg grating.The measured results show that the Lorentz fitted 20dB linewidth of the laser output is 7.1kHz,corresponding to the 3dB linewidth about 355Hz.(4)Using a mechanical long period grating as a mode converter,the narrow linewidth fiber laser with cylindrical vector mode output is constructed based on few-mode fiber Bragg grating and compound cavity structure.Switchable output of two wavelength is realized.The measured results show that the Lorentz fitted 20dB linewidths of two output wavelentgths are 5.7kHz and 6.8 kHz,corresponding to the 3dB linewidths about 285Hz and 340Hz.