Research On Optical Waveguide Mode Converter

In recent years,the information transmission capacity based on single-mode fiber is approaching shannon limit gradually.In order to improve the optical communication capacity,modulus division multiplexing is considered as the most promising technology.The technology can not only realize large capacity transmission by using optical fiber as medium,but also realize chip level signal transmission by using integrated platform waveguide.A silicon-on-insulator waveguide mode converter is designed to solve the problem of mode field mismatch between single-mode fiber and integrated chip.Aiming at the problems of low conversion rate and narrow bandwidth of traditional all-fiber mode converters,dual-core mode converters and taper-cascaded mode converters based on directional couplers are designed.The main contents of this thesis are as follows:(1)Design a mode converter based on silicon-on-insulator platform that converts the input mode to the output mode using the end coupling mode.The beam propagation method is used to simulate the mode converter.The evolution of the mode field and the conversion process are analyzed.The influence of tip width,boundary curve function and offset of upper and lower waveguides on the performance of mode converter is discussed,and the influence of alignment deviation on the performance of mode converter is analyzed in practical packaging process.The mode converter achieves the conversion rate of 98.7%,the coupling loss of 0.054d B,and the extinction ratio of more than 15d B up to 500nm bandwidth.(2)Design a dual-core mode converter based on directional couplers LP₀₁ mode to LP₁₁mode.The coupling length is calculated by using transverse coupling mode theory and the supermode theory.The influence of other parameters on the performance of mode converter is analyzed.The mode field evolution and mode conversion of mode converter are simulated by beam propagation method.The advantages of S bend fiber such as strong extensibility and low coupling loss are analyzed.The mode converter can achieve the insertion loss of 0.003d B at the wavelength of 1550nm and the target mode conversion rate of 99.7%.Extinction ratio is greater than 20d B with the 320nm bandwidth and the extinction ratio is greater than10d B with the 400nm bandwidth.(3)Design a mode converter based on tapered cascade structure LP₀₁ mode to LP₀₂ mode.The effects of core refractive index,cladding refractive index and section radius on the performance of mode converter were investigated by using the longitudinal non-uniform coupling design.The mode field evolution and mode conversion of mode converter are analyzed.The mode converter achieves a conversion rate of 92.2%.Extinction ratio is greater than 20d B to achieve 178nm bandwidth,extinction ratio is greater than 10d B to achieve 280nm bandwidth.The pattern converter designed in this thesis is suitable for the modular division multiplexing system and provides new ideas for further design and optimization in the future.