Theoretical And Experimental Studies On Lithium Niobate Photonic Wires And Related Devices

With the development of information and internet era,people have put forward higher requirements for signal transmission and processing ability,which promotes the rapid development of integrated photonics technology.Among many integrated optical materials platforms,lithium niobate plays an important role in the fields of nonlinear wavelength conversion,optical frequency comb generation and efficient electro-optic modulation due to its wide-band transparent window,high second-order nonlinear coefficient and excellent electro-optic properties.However,traditional Li Nb O₃ optical waveguide fabrication techniques,such as titanium diffusion or proton exchange,only yield subtle index contrast,which makes the size of Li Nb O₃integrated devices too large to achieve large-scale integration.In recent years,the lithium niobate-on-insulator（LNOI）technology has brought a new life to Li Nb O₃crystal,facilitating the resurgence of integrated photonics based on the lithium niobate.The LNOI platform combines the multifunctional properties of lithium niobate with high refractive index contrast,which enhances the optical field confinement ability of the lithium niobate photonic wires（nanowaveguides）.It allows smaller cross section size and bending radius,greatly improving the performance and integration of related devices.Photonic wire is an important basic structure in LNOI integrated photonics,and its waveguide and dispersion properties are important basic knowledge.In this thesis,a theoretical analysis of the lithium niobate photonic wire is carried out,which provides guidance for the design and fabrication of micro-photonic devices based on the LNOI platform.In addition,the LNOI platform is currently in a stage of rapid development.Although a variety of high-performance photonic devices have been developed on this platform,some key building blocks,such as polarization control and active devices,are still developing relatively slowly,resulting in the lack of polarization control and optical amplification functions.In order to promote the monolithic integration of passive and active photonic devices on LNOI platform and make LNOI a more attractive and competitive integrated optical platform,polarization control devices and erbium-doped active photonic wires are also studied.The specific contents of this study include:1.The waveguide properties of rib-type photonic wires（PWs）based on LNOI platform are studied.Aiming at 1.55μm wavelength,effective refractive indices of several lower-order transverse-electric/magnetic（TE/TM）modes in the central rib region and the fundamental mode in the slab waveguide in the side region are calculated as a function of geometric parameters of the PWs.By comparing effective refractive index of the first-order mode in the central rib region with that of the fundamental mode in the slab waveguide,single-mode condition of rib-type lithium niobate PWs are determined in terms of geometric parameters.Electric field distributions of fundamental TE and TM modes are also simulated and discussed.In addition,the effective group refractive indices of the fundamental modes propagated in the central rib region are also calculated in relation to the wavelength.2.The dispersion characteristics of photonic wires based on LNOI platform are studied.The flexibility of dispersion tailoring,zero dispersion wavelength,dispersion flatness and zero modal birefringence（ZMB）are studied.The dispersion tailoring of lithium niobate photonic wires by geometrical parameters and applied electric field is analyzed.The dispersion flatness of lithium niobate photonic wire is calculated quantitatively and compared with the results of silicon photonic wires with the comparable size.The influence of etch depth on the group velocity dispersion of TE₀₀and TM₀₀ modes propagated in photonic wires is also studied.The realization of ZMB is discussed,and geometric parameters are proposed to realize single mode propagation and ZMB simultaneously.3.Two kinds of polarization-handling passive photonic devices based on LNOI platform are studied.A fundamental mode polarization converter based on a ridge-type Li Nb O₃ photonic wire is proposed for the first time.The device achieves transverse asymmetrical refractive index distribution by introducing a semi-infinite dielectric cover layer on one side of LNOI PW.The interplay of material and waveguide birefringence enables to realize mode hybridization,and a taper structure is used to achieve the mode conversion.The mode hybridization and conversion characteristics of tapered laterally asymmetrical ridge LNOI photonic wires are simulated at 1.55μm wavelength.The results show that the device can achieve efficient conversion between TM₀₀ and TE₀₀ modes,and the length is expected to be less than 100μm.The conversion efficiency can be optimized by choosing an appropriate cover layer material and/or adjusting its thickness.In addition,a polarizing beam splitter based on asymmetric directional coupler is proposed to separate TE and TM polarized light on LNOI platform.The device length is only about 11μm,and the polarization extinction ratio is more than 20 dB.4.The active optical amplification building block based on LNOI platform is studied.Er:LNOI thin film with a diameter of 3 inches and a doping concentration of0.5 mol%was prepared through the growth of doped crystals,ion cutting technology.Then,LNOI active photonic wires were successfully fabricated on the surface of LNOI films by micro-nano processing technology,and the surface morphology,mode field distribution,loss,spectrum and amplification performance were characterized.The results show that the erbium-doped active photonic wires can achieve a signal enhancement factor of more than 5 dB/cm at 1531 nm when 1480 nm pumping.High efficiency LNOI active photonic wires could become an important fundamental element in future lithium niobate photonic integrated chip.