Research Of Novel Device Base On MMI

With the demand of the bandwidth is increasing,time,frequency,polarization,multilevel modulation format,multi-optical fiber,and other dimensions have already reach the transmission limit.As space mode is an additional degree of freedom that has not been utilized,increasing and fully utilizing space mode is an effective way to improve the capacity of optical communication.Mode division multiplexing(MDM)is an important part of space division multiplexing(SDM),it can increase transmission capacity of one fiber and improve the utilization rate of light spectrum effectively,so it is one of the most promising methods to increase channel capacity in optical communication.Mode conversion,which is the most basic component in mode division multiplexing system,is a technical challenge to be realized.Among large numbers of mode conversion techniques,mode convertors based on SOI attract the most attention beacause of the advantages of high flexibility,loss loss,small space and easy to integrate,but its bandwidth and fabrication tolerance needs to be improved.In general,the problem that exists in fabrication tolerance,flexibility,bandwidth and the limited numbers of modes needs to be solved.Therefore,this dissertation mainly focuses on the novel devices based on MMI,especially in the applications in mode conversion.The main research efforts are summarized as follow.1.The principle of mode convertors based on directional coupler has been studied theoretically,which is that the number of mode and mode effective refractive index varies when the width of waveguide changes.Based on this principle,we realized mode conversion from fundamental mode to 1st mode,2nd mode and 3rd mode respectively by designing the width of coupling waveguides and the length of coupling region properly in simulation.At last,the influence of the length of the S bend on the coupling length has been investigated as well.2.A novel scheme to realize mode conversion based on coupler and phase shifter is proposed for the first time.The basic principle of the scheme is that the combination of N mode fields of fundamental mode with adjacent phase difference of ? can generate N-1-th order mode.Therefore,a 1 xN coupler with even splitter ratio can be used to split a fundamental mode into N fundamental modes,while phase shifters are used to adjust the phase differences between adjacent fundamental mode fields to be ?;at last,the N fundamental modes with adjacent phase difference of ? are combined by a wide waveguide,in turn,a N-1-th mode is generated.In order to prove the validity of principle,we design and realize a broadband 1st-order mode converter and two kinds of broadband 2nd-order mode converters based on MMI and butterfly phase shifter in simulation.MMI and butterfly phase shifter are chosen as the coupler and the phase shifter respectively due to the fact that they have relatively broad bandwidth and large fabrication tolerance.The mode conversion efficiency,operation bandwidth and fabrication tolerance of the devices are anylized.Simulation results show that the fabrication tolerance is large and the operation bandwidth is wide.For small size devices(the lengths of the devices are about 100?m),mode conversion efficiency of all devices is between 83%and 89%,the devices can work well in the whole C-band with minimum insertion loss of 0.05dB.Then the 2nd-order mode device is successfully fabricated on a SOI platform with observation of a clear 2nd-order mode at the output,which further proves the validity of our scheme.Devices designed based on our scheme can solve the problems of low fabrication tolerance,low flexibility,narrow bandwidth and limited orders of mode in mode conversion,which has potential applications in the future high capacity networks.3.The influence of boundary on MMI optical field distribution is investigated.We deduce the self-imaging effect in theory,introduce performance index of MMI briefly.The the influences of boundary on MMI optical field distribution,especially in the case of boundary changed periodly in proportion to the light propagation constant in the waveguide,are discussed.Simulations are mainly focused on the field of MMI with sin border.It is verified by simulation that boundary shape change will affect multimode interference region.