Theoretical And Experimental Study Of The Fused Tapered Optical Fiber Mode Selective Coupler

Optical fiber mode converter can realize the conversion between different transverse modes in optical fibers,which has important and extensive applications in mode division multiplexing optical fiber communication system and high-order transverse mode light field generation.Its common implementation schemes include long-period fiber grating,mechanical pressure fiber grating,photonic lantern,optical fiber mode selective coupler,etc.Optical fiber mode selective coupler has attracted wide attention for its simple structure,low insertion loss and wide working bandwidth.In this paper,the fused tapered optical fiber mode selective coupler is studied,and its theoretical and experimental research is carried out.The main research results are as follows:(1)The concept,classification and properties of modes in optical fibers are introduced.The power density distributions of LP01 mode,LP11 mode,LP21 mode and LP02 mode are discussed.The coupled mode equation is deduced by perturbation method,which lays a foundation for the analysis and design of mode selective couplers.(2)The integral formulas for calculating the coupling coefficients between different waveguides are derived,and the accuracy of the mode field integral method based on the coupled mode theory is discussed in combination with the supermode theory.The mode conversion between different types of optical fibers,including single-mode with dual-mode and single-mode with four-mode,is simulated and analyzed.The effects of operating wavelength and core spacing on mode coupling characteristics are discussed.(3)RSoft is used to model and simulate the mode selective coupler in optical fibers,and the energy flow in the process of mode coupling in optical fibers is analyzed.By reasonably designing the parameters of the optical fibers,the different high-order modes of LP01 mode in single-mode optical fiber and few-mode optical fibers are coupled,and the energy flow process from the fundamental mode to the high-order mode is simulated.(4)Mode selective couplers are fabricated by using ordinary single-mode fibers,self-made two-mode fibers and ordinary single-mode fibers,commercial four-mode fibers,respectively.Through reasonable design,the conversion from LP01 mode to LP11 mode is realized.The performance parameters were tested:in the wavelength range of 1530nm to 1560nm,the two mode selective couplers achieve an effective conversion from LP01 mode to LP11 mode;The additional loss of single-mode with two-mode fiber mode selective coupler is reduced from 1.9 dB to 1.4 dB,and the mode conversion efficiency is increased from 1.2%to 3.2%.The additional loss of single-mode with four-mode fiber mode selective coupler is reduced from 2.23 dB to 1.68 dB,and the mode conversion efficiency is increased from 0.58%to 2%.