Mode Selective Coupler Based On Hole-Assisted Dual-Core Fiber

Mode division multiplexing(MDM)has become an effective technique to overcome the bottleneck of single-mode optical fiber transmission capacity caused by Shannon nonlinear limit and“fiber fuse”influence.MDM systems provide a new multiplexing dimension for optical transmission.The mode selective coupler is one of the important key components to implement MDM transmission.In this paper,a compact mode selective coupler has been demonstrated based on tapered hole-assisted dual-core fiber(HADCF).HADCF is a kind of special fiber,which is composed of a central single-mode core,a suspended few-mode core and a large eccentric air hole.The designed mode selective coupler is composed of light source,single-mode fiber and the tapered HADCF.In order to realize the phase matching condition of the two cores,the structure of the HADCF was reduced proportionally by the hot melt tapering technique.According to the gradual mode evolution mechanism,during a slow cross section change process,energy can be transferred between the fundamental mode of the single-mode core and the higher-order mode of the suspended few-mode as long as the propagation constants of the two cores are matched at a certain point in the tapered region.This kind of mode selective coupler can realize the mode conversion from O band to C band only by changing the length of the pull cone.The coupling peak has a blue shifts with the increase of the tapered length.The innovation of this device is to realize the conversion of high-order mode inside a single fiber,the device is small in size,high in integration,easy to make,and the conversion of high-order mode is high in purity.The wavelength controllable mode conversion can be realized simply by changing the tapered length.The main research contents of this paper are as follows:(1)The finite element method was used to calculate the mode characteristics of the tapered HADCF.The transmission characteristics were calculated by using the beam propagation method,and the theoretical coupling transmission spectrum was obtained.The coupling length at different tapering rates was calculated,and the linear relationship between the coupling wavelength and the coupling wavelength was found.(2)Experiment preparation and test research of the mode selective coupler based on tapered HADCF were carried out through simulation experiment,and the tunable mode conversion of the wavelength from O band to C band is realized.Then the mode selective coupler was tested,including polarization correlation,model design,LP₁₁mode purity and the environment temperature and refractive index stability.At last,the bandwidth and conversion efficiency of the mode selective coupler are studied,we taper the HADCF twice which can improve the bandwidth and conversion efficiency of the mode selective coupler.