Application Of Supermode Theory In The Analysis And Design Of Fiber Mode Selective Couplers

The fiber mode selective coupler(MSC)is a new type of fiber coupler that can realize the conversion of different transverse modes in optical fibers.It can be used for mode multiplexing/demultiplexing in the mode division multiplexing(MDM)optical fiber communication system,or used to generate specific high-order transverse mode lasers in optical fiber lasers.With the research and development of MDM optical fiber communication technology,optical field control and application technology,optical fiber MSC has received more and more attentions,so related analysis theories and manufacturing processes have become more mature,which greatly support and promote the development of related devices and systems.On the basis of the previous studies on fiber MSCs,this thesis focuses on the theoretical analysis and design of fiber MSC and mainly studies the application of supermode theory in the analysis and design of fiber MSCs.The main research results obtained are as follows:(1)Combined with numerical simulation,the working principle of symmetric single-mode twin-core optical fiber couplers are studied,whose coupling lengths under different structural parameters are calculated by using the coupled mode theory(CMT)and supermode theory respectively.The results are compared and discussed.(2)The asymmetric twin-core optical fiber MSC is analyzed using the supermode theory.The analytical expression of the propagation constant and mode field distribution of the supermodes in the asymmetric twin-core fiber MSC structure are obtained using the CMT.Simulation results show that in the case of strong coupling,there exit small errors between the analytical and numerical results of propagation constants and optical field distributions of supermodes in the asymmetric twin-core optical fiber MSC.(3)The coupling length and maximum power coupling efficiency of the asymmetric twin-core optical fiber MSC are expressed as functions of the propagation constant of the supermode.Using the analytical and numerical results of the propagation constants of supermodes,the coupling length and maximum power coupling efficiency of the symmetrical twin-core optical fiber MSC are calculated under different structural parameters.Simulation results show that in the case of strong coupling,the calculation error of the coupling length and the maximum power coupling efficiency are small.In practice,the propagation constant of the supermode in the asymmetric twin-core optical fiber MSC can be obtained directly using the COMSOL software based on the finite element method,and then the coupling length and maximum power coupling efficiency required can be calculated.(4)The images of supermodes in the symmetrical twin-core optical fiber MSC and asymmetric fusion tapered optical fiber MSC are obtained and analyzed using the COMSOL software based on the finite element method.The research results show that the images of supermodes in the symmetrical twin-core optical fiber MSC and asymmetric fusion tapered optical fiber MSC have the same characteristics as those in the asymmetric twin-core optical fiber MSC,so corresponding characteristic parameters such as the coupling length can be calculated using the propagation constant of the supermode obtained.(5)Based on the supermode theory,the asymmetric twin-core optical fiber MSC and asymmetric fusion tapered optical fiber MSC for the 1550 nm waveband are designed using the SMF-28 single-mode fiber made in Corning and the SM2000dual-mode fiber.