Research On Key Technology Of Mode Selective Couplers Based On Side-polished Six Mode Fiber

With the rapid development of 5G Internet of Things and other information technology,people's demand for network traffic is increasing day by day.In order to overcome the huge pressure of urgent bandwidth consumption on optical transmission network,it is urgent to improve the transmission capacity of optical fiber communication system.As a kind of space division multiplexing technology,the mode division multiplexing technology based on few mode fiber has great potential to improve the optical fiber communication capacity in the future.Mode division multiplexing refers to the use of independent mode in the few mode optical fiber for signal transmission.Each mode transmission channel constitutes an independent channel to carry different information and transmit in the optical fiber at the same time,so the optical fiber communication capacity has been increased.It is very important to selectively excite the higher order mode in the few mode fiber.The mode selective coupler plays an important role in obtaining the higher order mode.Based on the design of single-mode fiber and six-mode fiber,six-mode selective couplers with all fiber side polishing and bonding structure are designed and manufactured.Based on the design of single-mode fiber and six-mode fiber,six modes of all-fiber side-polished fiber bonding structure mode selective couplers are fabricated in this paper.The LP₀₁,LP₁₁,LP_21,LP₀₂,LP₃₁ and LP₁₂ modes are implemented in the six-mode fiber selective excitation.The device has the characteristics of simple manufacturing process,high coupling efficiency,and high mode purity.The mode characteristics and mode coupling theory of optical fiber are analyzed theoretically.Based on the mode coupling theory of optical fiber,the mode selection coupler of side polished fiber is designed,and the simulation model of mode selection coupler based on side polished fiber is established.Based on the finite element method,the mode effective refractive index of side polished single-mode fiber and side polished six mode fiber are calculated,and the phase matching condition is obtained.Based on the beam propagation method,the transmission characteristics of the coupler are analyzed,the structure of the coupler is optimized,and a set of optimal parameters are obtained to realize high coupling efficiency,low crosstalk and low loss excitation of the mode selective coupler for high-order modes.The function of the device is based on the phase matching between the fundamental mode of the single-mode fiber and the high-order mode of the six mode fiber.By optimizing the core radius and the cladding residual thickness of the three single-mode fibers,the LP₀₁,LP₁₁,LP₂₁,LP₀₂,LP₃₁ and LP₁₂modes are coupled with high efficiency and low crosstalk,respectively.At 1550 nm,the mode selective coupler has a high coupling ratio,and the coupling ratios of the six modes are more than 95%;This device also has the advantage of large effective bandwidth.In C band(1530nm-1570nm),the coupling ratios of LP₀₁,LP₁₁,LP₂₁,LP₀₂,LP₃₁ and LP₁₂ modes are all higher than 80%;And at 1550nm,the crosstalk between target mode and non target mode is less than-17.6d B.The theoretical model of measuring the mode effective refractive index of optical fiber based on prism coupling method is established,and the experimental platform is built.Six kinds of single-mode fibers with specific cladding residual thickness and core radius are measured,and the mode effective refractive index of side polished six mode fibers with residual thickness of 0?m is measured.The experiment verifies the feasibility and accuracy of prism coupling method to measure the mode effective refractive index.In this paper,the fabrication process of the device is improved.The core radius of the tapered single-mode fiber is made to meet the design parameters by drawing the tapered single-mode fiber.Then,the side grinding of the tapered single-mode fiber and the non tapered few mode fiber is carried out by using the wheel type polishing mill.After finishing the polishing and fine grinding,the polishing surface is smooth.The polishing surface of the device is bonded on the built device bonding platform,and finally the packaging test is carried out.The mode selective coupler is composed of side cast single-mode fiber and side cast six mode fiber.LP₀₁ mode is input at the single-mode input end of the coupler,than LP₀₁,LP₁₁,LP₂₁,LP₀₂,LP₃₁ and LP₁₂ modes are selectively output at the output end of the six mode fiber.The coupling efficiency at 1550 nm is greater than 85.0%,and the mode purity is higher than 88%.The coupling efficiency is higher than81%and the maximum loss is 1.8 d B in the broadband range of 1530 nm-1600 nm.The designed devices can be widely used in the wideband optical communication system based on mode division multiplexing.The innovation of this paper are sa follows:1.The prism coupling method is designed to measure the higher order mode effective refractive index of six mode fiber.2.The minimum diameter of single-mode fiber is tapered to 76?m.Then the side of fiber is polished by a wheel grinder,and the polishing process is innovative..3.The structure and fabrication technology of the device are proposed for the first time at home and abroad.The device has the advantages of low cost and high efficiency.In addition,the device has the advantages of high coupling ratio,wide bandwidth and high mode purity.