Research On Coupling Mechanism And Structural Optimization Design Of Silicon Optical Waveguide And Single Mode Fiber

With the arrival of the big data era,the requirements for information transmission capacity,bandwidth,processing speed,and other aspects are becoming increasingly high,especially for the transmission and exchange of optical information,which puts forward higher requirements for optical interconnection between chips.Silicon photonic technology based on silicon substrate has become a research hotspot in optical communication field due to its high transmission rate,compatibility with standard CMOS technology and high integration.However,silicon photonic chips based on silicon photonic technology face challenges in standardized design,manufacturing,coupling packaging,and other aspects,especially the coupling problem between silicon photonic chip and singlemode fiber in optical packaging,which can significantly reduce the performance of silicon photonic devices.The typical size of silicon optical waveguide used for optical transmission in silicon photonic chip is one order of magnitude smaller than the fiber core diameter.The large size difference between the two leads to problems such as optical mode field mismatch,which brings a series of challenges to the coupling between silicon optical waveguide and single mode fiber.Therefore,the study of efficient coupling between silicon optical waveguide and single mode fiber is one of the key factors to improve the performance of silicon photonic devices.Supported by the National Natural Science Foundation of China(52175445)and the Natural Science Foundation of Hunan Province(2022JJ30743),this thesis studies the coupling mechanism and structural optimization design of silicon optical waveguide and single mode fiber,reveals the coupling mechanism and law of inverted taper end coupling and grating vertical coupling,and optimates the coupling structure.The high efficiency and high precision coupling between silicon optical waveguide and single mode fiber is realized.The specific research work of this thesis is as follows:1.Based on the optical waveguide transmission theory,the coupling model between silicon optical waveguide and single mode fiber is constructed,and the influence mechanism and law of coupling parameters on coupling efficiency are studied.In order to solve the problem of efficient coupling between silicon optical waveguide and single mode fiber,two coupling methods,namely end coupling and vertical coupling,are analyzed for silicon optical waveguide and single mode fiber.The basic theory of optical waveguide is analyzed,including Maxwell’s equations,basic optical theory of optical waveguide,single mode condition of SOI waveguide,etc.The theoretical analysis methods required for design and optimization of coupling structure are described.2.Based on the finite-difference time-domain method,the inverted taper end coupling model between silicon optical waveguide and single mode fiber is established.The coupling mechanism and law are obtained,and the reciprocal conical coupling structure is optimized.In order to solve the transmission loss problem,the influence of tip width and taper length on the transmission efficiency of inverted taper waveguide is studied.The results show that the maximum transmission efficiency can reach 99.26%when the tip width is 0.15μm and taper length is 200μm.Taking the transmission efficiency and coupling efficiency as the research objectives,the dimensions of the rectangular and ridge shaped upper cladding structures and the refractive index of the upper cladding materials are optimized.The results show that the theoretical coupling efficiency between the structure and the standard single mode fiber is 46.11% when the rectangular upper cladding width is 2.9μm and the height is 2.7μm.When the upper ridge cladding width is 1.5μm and the ridge height is 1μm,the theoretical coupling efficiency between the structure and the single mode fiber is 58.28%.When the relative refractive index difference of the core cladding is 2.694%～3.667%,the transmission efficiency is greater than 99%.According to the simulation results,the important parameters of the end coupling structure are obtained,and the mechanism and law of the influence of the alignment deviation between the fiber and the inverted taper waveguide on the coupling performance are studied.3.The grating vertical coupling model between silicon optical waveguide and single mode fiber is established,the coupling mechanism and law are obtained,and the grating vertical coupling structure is optimized.The working mechanism of grating coupling based on Bragg diffraction conditions is analyzed,and the structural parameters of grating coupling are optimized for coupling performance.According to the optimization results,the grating vertical coupling structure is designed.The theoretical coupling efficiency of the grating vertical coupling is 56.60%and the 3d B bandwidth is 82 nm when the grating period is 660 nm,the duty ratio is 0.47,the etching depth is 85 nm,the incident Angle is 21°,the horizontal distance is 4μm and the vertical height is 1.2μm.The focusing grating coupling method is analyzed,which can not only reduce the device size,but also ensure the coupling performance.4.The inverted taper end coupling structure and the grating vertical coupling structure are fabricated by micro nano manufacturing process of planar optical circuit.The boundary image tracking method is proposed to analyze the sidewall roughness of optical waveguide,and the forming rule of sidewall roughness is obtained.The technological processes of the inverted taper coupling structure and the grating vertical coupling structure are analyzed.According to the technological processes,the samples are manufactured and the structural dimensions are measured.The silicon optical waveguide and fiber coupling structure approximate to the designed value is obtained.At the same time,based on the boundary image curve tracking method,the relationship between the electron beam exposure dose and the sidewall roughness of the structure boundary in the electron beam lithography process is discussed.The relationship among the structure design value,the sidewall roughness and the actual structure size is obtained,which provides reference significance for the process with different mask design values and different boundary contour requirements.The relationship between the profile roughness of the structure before and after the reactive ion etching process is studied and the variation range of the profile roughness of the structure before and after the reactive ion etching process under different lengths is calculated.At the same time,laser confocal microscope,infrared spectrometer and atomic force microscope are used to study the morphology comparison between the etched region and the non-etched region of the sample,and the intuitive bottom morphology of the etched region is given.The process quality of the bottom-etched region is calculated according to the roughness distribution.5.The coupling testing platform for silicon optical waveguide and optical fiber is built,and the effectiveness of the inverted taper coupling and grating vertical coupling methods is experimentally verified.Based on the six degrees of freedom coupling platform,the coupling experiments of silicon optical waveguide and single mode fiber are carried out.The coupling efficiency of the inverted taper coupling structure is 5.33%(insertion loss is 12.726 d B),and the 3d B bandwidth is greater than 50 nm.The coupling efficiency of grating vertical coupling structure is 2.5%,2.25%and 2.22%,respectively(insertion loss is 16.003 d B,16.483 d B and16.546 d B,respectively).At the same time,the experimental error is included in the theoretical analysis.When the alignment deviation of horizontal direction and height direction is 2μm,the theoretical coupling efficiency between silicon optical waveguide and fiber is less than 20%.