Investigation For The Optical Loss Of Butt-coupling Between SOI Waveguide And Fiber

In recent years,the silicon-on-insulator（SOI）waveguide photonic integrated chip has presented the impressive establishments in research and the applications are becoming wider and wider,accordingly the SOI chip-fiber coupling loss naturally becomes a hot research topic in the silicon photonic integrated circuits（PIC）field.In this dissertation,a guided-wave mode convertor（MC）composed of silica-on-silicon（SOS）waveguide and SOI waveguide tapered structure is exploited to implement a low-loss butt-coupling between SOI PIC chip and fiber.The SOS waveguide is formed by a hyperthermal oxidation of Si waveguide using local oxidation of silicon（LOCOS）technology.According to the Deal-Grove model,a thermodynamic theoretical model of Henry’s law gas constant is established,and the dependences of Si O₂growth rate and growth thickness on oxidation temperature and oxidation time are systematically simulated,and then testified by the existing statistical physical empirical model.The SOI waveguide size,the oxidation temperature and the oxidation time were simulated to fabricate three types of chip samples,and the performances of the mode converters were tested.In this dissertation,LOCOS technique is used to make the Si O₂core layer and etching technique is used to make the SOI waveguide taper structure as the research object to explore the theory and technology of reducing low-loss SOI waveguide-fiber butt-coupling.The main research work and results are as follows:First,based on the fiber-SOS waveguide-SOI waveguide system,the conversion process of the guided wave mode through the waveguide channels is studied,and then the Fresnel reflection loss theory of SOI waveguide taper structure and the mode conversion theory of SOI waveguide taper structure are studied.The analysis of the fiber-SOS waveguide-SOI waveguide system shows that because the fiber-SOS waveguide coupling is the gap mode conversion between two low refractive-index contrast waveguides of the same type of materials,the mode conversion between is ultra-low loss.Therefore,the fiber-chip coupling loss of the fiber-SOS waveguide-SOI waveguide system is mainly co-determined by the Fresnel reflection loss of the SOS waveguide-SOI waveguide taper interface and the mode conversion loss of the SOI waveguide taper.The theoretical models of the Fresnel reflection loss and the SOI waveguide tapered structure were established.The guided wave mode conversion efficiency of near 80%was reached by the simulations of MATLAB programming software and FD-BPM professional software,and the optimized length and equivalent refractive index of SOI waveguide taper structure were obtained.The simulated results of the fiber-SOI waveguide butt-coupling losses were 0.8-1.0 d B/facet.Second,according to the Deal-Grove model of mass conservation and oxidative growth dynamics in chemical reactions,a thermodynamic theoretical model was established for the gas constants of Henry’s Law.The Pfeffer/Ohring model and Kostinski model were used to select the lower activation energy and the higher diffusion coefficient,then the two-dimensional dependence of Si O₂growth rate and growth thickness on oxidation temperature and time was simulated.In addition,for comparison,the statistical data of six sets of dipole solvents were selected to calculate the Henry’s law gas constants,and the relationship between the Henry’s law gas constants and the temperature was used to establish a statistical physical empirical model,then the diffusion coefficient of H₂O in Si O₂was set as D₀=1.0×10^-10m²·s^-1/E_a=0,and the dependence of Si O₂growth rate and growth thickness on oxidation temperature and time was systematically simulated using the Pfeffer/Ohring model of the diffusion coefficient and the activation energy.Comparison and analysis of the theoretical model of thermodynamics and the empirical model of statistical physics show that the theoretical model of thermodynamics can be used to simulate the oxidation process of silicon at high temperature,and the empirical model of statistical physics is used to verify the numerical simulation results of the theoretical model of thermodynamics.When the diffusion coefficient and activation energy are D₀=1.0×10^-10m²·s^-1/E_a=0,the Pfeffer/Ohring model can be used to simulate the oxidation temperature and time of Si O₂core layer with a thickness less than 3.0μm,while when the diffusion coefficient and activation energy are D₀=8.4×10^-7m²·s^-1/E_a=1.47×10^-19J,Kostinski model can be used to simulate the oxidation temperature and time of Si O₂core layer with a thickness greater than 3.0μm.Third,the SOI strip waveguide was fabricated by LOCOS technology,and SOI taper waveguide and SOI ridge waveguide were fabricated by electron beam lithography,then the SOI strip waveguide,the SOI taper waveguide and the SOI ridge waveguide form an SOI waveguide with taper mode converter.In order to ensure that the Si strip waveguide is completely oxidized,an excessive water vapor is introduced into the oxidation reaction.Therefore,the thickness of the Si O₂core layer is 3.0μm after being oxidized at 1240℃for 3 h.By measuring the butt-coupling losses of three types of the SOI ridge waveguide chips:the SOI ridge waveguide,the SOI ridge waveguide with non-tapered mode converter and the SOI waveguide with tapered mode converter,it is found that the SOI waveguide with tapered mode converter has the lowest butt-coupling loss with fiber.It is proved that the mode converter composed of Si O₂waveguide core and SOI waveguide conical structure from LOCOS technology plays an important role in reducing the coupling loss between SOI waveguide and fiber.