高效铌酸锂薄膜波导模斑转换器设计

Design of high-efficiency lithium niobate thin film waveguide mode size converter

铌酸锂薄膜光子集成技术在高速光电子领域不断凸显,被广泛用于各种片上功能实现,如电光调制、光频梳、滤波器、非线性光学频率转换器、非线性量子光源、激光器等。在铌酸锂薄膜光子集成技术发展过程中,目前面临的一个重要的技术瓶颈就是铌酸锂薄膜纳米波导与单模光纤的高效耦合。针对这一问题,设计了一种基于SiO_(2)、SiON锥形结构以及双层铌酸锂锥形结构的模斑转换器,实现铌酸锂薄膜纳米波导与单模光纤之间模式和能量的高效传递与转换。采用三维有限差分光束传播法对器件结构进行了模拟仿真,并优化了结构参数,可实现与铌酸锂薄膜波导与单模光纤的高效耦合,耦合效率在82.2%~89.0%之间,同时,得到了±1.8μm光纤耦合对准容差,可为下一步制备出高效耦合的铌酸锂薄膜光子器件提供参考。

Objective The photonic integration technology based on lithium niobate thin films has become increasingly prominent in the field of high-speed optoelectronics,and is widely used for various on-chip functions,such as electro-optical modulation,optical frequency comb,filter,nonlinear optical frequency converter,nonlinear quantum light source,laser etc.In the development of lithium niobate film photonic integration technology,there is an important technical bottleneck which is the effective coupling of lithium niobate film nanowaveguides and single-mode fibers,which is also the key to hinder the practical application of lithium niobate thin film photonic devices.On-chip mode size converter is widely used in mode field transformation to realize waveguide mode field transformation.Although the existing researches have improved the coupling efficiency by using bilayer tapered waveguides or composite structures,they are all coupled with tapered fiber or thin diameter fiber,which still cannot achieve effective coupling with single-mode fiber.To solve this problem,a mode size converter based on SiO_(2) waveguide,SiON tapered waveguide and bilayer LN tapered waveguide is designed to achieve efficient mode and energy transfer and conversion between lithium niobate film nanowaveguide and single-mode fiber.Methods The structure of the mode size convertor composed of SiO_(2) waveguide,SiON tapered waveguide and bilayer LN tapered waveguide is simulated by using the three-dimensional finite difference beam propagation method,and the structural parameters of each section are sequentially optimized through optical pattern matching design and adiabatic mode transmission design,and the optical coupling efficiency and adiabatic mode conversion efficiency of each section are simulated.Results and discussions The research results show that when the refractive index difference between the core layer and cladding layer of the SiO_(2) waveguide is 0.75%and the size of SiO_(2) waveguide is 6μm×6μm,the coupling efficiency between SiO_(2) waveguide and single-mode fiber is about 93%(Fig.6).When the mode field size of the wide end of SiON tapered waveguide is 2.5μm×2.5μm-3.5μm×3.5μm,the refractive index of the corresponding core layer is 1.48-1.51,the length of the SiON tapered waveguide(L1)is greater than 250μm and the width of the tapered tip W3 is 0.1-0.3μm,the optical mode is gradually converted from the SiO_(2) waveguide to the SiON waveguide,and the conversion efficiency of the SiON tapered waveguide is 93%-97.2%(Fig.8).The bilayer LN tapered waveguide includes the LN tapered planar waveguide and the LN tapered ridge waveguide.In the LN tapered planar waveguide,when the tapering length(L2)changes in the range of 200-300μm,the width of the tapered tip W4 changes within 0.1-0.15μm,and the width of the wide end(W5)changes in the range of 0.8-1.4μm,the optical mode profile in LN tapered planar waveguide increases with the increase of the inverse taper width of LN tapered planar waveguide,while that in SiON layer decreases,and the conversion efficiency of the LN tapered planar waveguide is 96%-98.5%(Fig.9).In the LN tapered ridge waveguide,when the length of LN ridge tapered waveguide(L3)varies from 40 to 100μm,and the width of the tapered tip of LN ridge tapered waveguide W6 varies from 0.1μm to 0.3μm,the optical mode is gradually converted into LN ridge waveguide optical mode,and conversion efficiency of the LN tapered ridge waveguide exceeds 99%(Fig.10).Through the above design,effective coupling with lithium niobate film waveguide and single-mode fiber can be realized,and the coupling efficiency is 82.2%-89.0%(Fig.11).At the same time,±1.8μm fiber coupling alignment tolerance is obtained(Fig.12).Conclusions The proposed mode size converter based on SiO_(2) waveguide,SiON tapered waveguide and bilayer LN tapered waveguide provides a new method for the coupling and integration of lithium niobate thin film photonic devices,which can provide a reference for the next step of preparing highly efficient coupling lithium niobate thin film photonic devices,and is beneficial to further realize the integrated application of lithium niobate devices.