Trip-loaded Waveguides And Integrated Optical Devices On Single-Crystal Lithium Niobate Thin Films

Lithium niobate(LN)is a synthetic multifunctional material with excellent electro-optic,nonlinear optic and acousto-optic properties,good transmittance at both visible and telecommunication wavelengths,and is widely used in integrated optics.Recently,single-crystal lithium niobate thin film(lithium niobate on insulator,LNOI)on a low refractive index silica(SiO2)cladding layer fabricated by ion implantation and wafer bonding technologies has been attracted many interests using.Due to the high-refractive-index contrast between LN and SiO2,photonic devices fabricated based on LNOI materials have been greatly improved in both integration and device performances.Multi-device integration on LNOI has become possible.A series of optical devices have been reported so far,such as optical waveguides,electro-optic modulators,microring/microdisk resonators,wavelength conversion devices,and so on.A waveguide is a basic structure for photonic devices,which is the channel for optical transmission,and is one of the main structures of many complex optical devices.Strip-loaded waveguide is an important kind of waveguides on LNOI.The waveguide is formed by covering the surface of the LNOI with a layer of loading strip material.The selection of the material of the loading strip is flexible.A waveguide can be formed as long as the refractive index of the material is greater than 1.Strip-loaded waveguide can make the heterogeneous integration between LN and other materials,and can combine the excellent physical properties of both materials.In this thesis,the heterogeneous integration between LNOI structure and amorphous silicon as well as SiO2 materials were studied,which explored the way to the electro-optic integrated,efficient,and high density integrated optics.Second order optical nonlinearity plays an important role in frequency doubling,parameter down conversion and optical parameter oscillator/amplifier,making it have wide applications in the area of light source,optical communication and quantum optics.In order to achieve efficient wavelength conversion,such as in second harmonic generation(SHG)process,phase matching(PM)or quasi-phasematching(QPM)is required.QPM is generally achieved by periodic polarization inversion of LN.In recent years,periodic polarized lithium niobate(PPLN)films have been prepared on LNOI and effective wavelength conversion has been achieved.PM by the dispersion of the waveguide is another way to achieve efficient wavelength conversion.It has a simpler and more flexible preparation process than that of QPM.By adjusting the size of the device to make the pump light mode and the frequency doubled mode have the same phase velocity,the second harmonic generation can be realized.This thesis studied the PM process in LNOI.A photodetector can convert the optical signal into an electrical signal,realize the electrical detection of the optical signal.The integration ofthe photodetector on the LNOI is of great significance for realizing the LNOI on-chip full-function optical chip.However,LN is a wide-bandgap insulator,which is difficult to prepare directly into a photodetector.The heterogeneous integration of a material with photoelectric conversion properties on the surface of LNOI by deposition can solve this problem.In this thesis,an integrated photodetector is realized by depositing semiconductor material silicon on a LN single crystal film.The main research contents of this paper include three parts.1.The simulation analysis of several types of waveguides based on LNOI and the preparation and the study of strip-loaded optical waveguides.2.The preparation and the study of wavelength conversion devices based on LNOI.3.The integration and the study on the photodetectors on LNOI.The main results are as follows:1.Theoretical Simulation of optical waveguide on LNOIKinds of waveguides on LN thin film,including etching waveguides,proton exchange waveguides,and strip-loaded waveguides were designed and simulated by a full-vectorial finite difference method.The single-mode condition,optical power distribution,and bending loss of these kinds of waveguides were studied and compared systematically.The simulation showed that the effect of proton exchange depth on the optical power distribution in the LN layer is negligible for proton exchange waveguides.For the strip-loaded waveguide,the distribution of optical power in LN layer for quasi-TE(q-TE)and quasi-TM(q-TM)modes in LN layer is related to the thickness of the strip material.The bending loss decreased as the bending radius increase.When the effective refractive index(neff)of the strip waveguide mode is lower than the effective refractive index of the surrounding planar waveguide mode,a part of the loss of the strip waveguide will cause the leakage of the electric field.The LN ridge waveguide achieved low bending loss at a relatively small bending radius(about 20 Vm).These results are useful for the development of waveguide structures,optimization and preparation of high-density integrated optical components.2.Study of amorphous silicon strip-loaded waveguides based on LNOISilicon is the most important material in microelectronics,and also it has important applications in integrated optics and mature processing technology.The heterogeneous integration of silicon and LNOI combines the excellent electrical properties of silicon,the mature micromachining process and the excellent optical properties of LN.A layer of amorphous silicon film was deposited on the surface of LNOI by plasma enchanced chemical vapor doposition(PECVD)method,and then,a set of waveguides with widths of 2-7 μm were prepared by etching of the a-Si thin film.Light transmission was observed after the waveguide was annealed in air at 300 °C for 1 h.The 2 μm-wide waveguide showed propagation losses of 20 dB/cm for the q-TM mode and 42 dB/cm for the q-TE mode at 1550 nm.The root-mean-square(RMS)surface roughness of the a-Si thin film before and after annealing was 1.04 and 0.35 nm,respectively.High-resolution transmission electron microscopy(HRTEM)was performed to investigate the interface morphologies.A well-defined interface was clearly observed,and the structure of the a-Si thin film was proved to be amorphous.The main cause of the waveguide loss is the absorption of internal defects and surface dangling bonds in the amorphous silicon film.Since hydrogen atom can passivate the point defects and surface dangling bonds inside the material and reduce the absorption of the film,we deposited the amorphous silicon film with PECVD containing hydrogen.The effects of preparation conditions such as substrate temperature,RF power,silane concentration and reaction gas pressure on the absorption loss of the film were investigated.The amorphous silicon loaded strip waveguides on LNOI was prepared.Transmission losses in q-TE and q-TM modes were 6 dB/cm and 5.5 dB/cm,respectively.Magnetron sputtering is another method of depositing amorphous silicon films.The influence of sputtering power and the distance between the sample and the target on the absorption loss of the film were briefly analyzed.The amorphous silicon film was fabricated on the LN film by magnetron sputtering.The optical properties of the film were studied by end-face coupling test.The amorphous silicon loaded strip waveguides with a width of 4-7 μm were prepared on LNOI.The optical absorption caused by the point defects and dangling bonds in amorphous silicon film was found to be mainly at the 1550 nm.The optical properties of silicon film at another communication band,the 1310 nm band,were studied.The loaded strip waveguides were fabricated and measured.The transmission losses for a 4 μm wide waveguide were 11 dB/cm and 10 dB/cm for the q-TE and q-TM modes,respectively.3.Study on SiO2-strip-loaded waveguide based on LNOISiO2 is one of the most important and commonly used materials in planar optical waveguide(PLC)technology,with high transmission and easy preparation.The SiO2 strip-loaded waveguide is a fundamental study to achieve the combination of LN and SiO2 PLC.A SiO2 thin film was deposited on the surface of LNOI by magnetron sputtering and the surface morphology and composition of the film were measured.A 2-5 μm wide loaded strip waveguides were prepared using lift-off process.For a 2 μm wide waveguide,the transmission losses of q-TE and q-TM modes were 0.2 dB/cm and 0.8 dB/cm,respectively.This study provides a new material for the preparation of strip-loaded devices on the LNOI platform.The combination of SiO2 and LN provides a new idea for the development of more advanced photonic integrated devices and circuits.4.Fabrication and study of Y-Branch on LNOIThe Y-branch is a basic integrated optical component that can split or combine light.It is an important component of Mach-Zehnder modulators,fiber optic gyroscopes,and wavelength division multiplexing systems.Based on the SiO2 strip-loaded waveguide,the Y-branchs on LNOI were prepared,and the light intensity distribution was measured.The transmittance of the Y-branch waveguide with a bend radius of 700 μm and a waveguide width of 4 μm at the wavelength of 1550 nm was 70%-80%.The split ratio is nearly 1:1.5.Fabrication and study of wavelength conversion devices based on LNOISecond harmonic generation is achieved via an proton exchange waveguides on the LNOI.The interaction between the fundamental frequency light and the doubled frequency light which have the same polarization direction was realized via mode phase matching.The largest second-order nonlinear coefficient d33 of LN was fully used,and the distribution of χ(2)in LN film was modulated by means of proton exchange.These methods resulted in a significant increase of overlap integral between interactive modes.The PM conditions for different waveguide widths and film thicknesses were studied.The theoretical conversion efficiency is 58%W-1 cm-2,and the waveguides have been experimentally prepared with a conversion efficiency of 48%W-1 cm-2.6.Fabrication and study of photodetectors on LNOIPhotodetector is an important component to the LNOI integrated optical platform We fabricated the photodetector on LNOI by heterogeneously integrating amorphous silicon film,and detected the dark current,responsiveness and response time of the photodetector by vertical incidence of light.The highest response is 1.35 A/W.The switching characteristics of the device at wavelengths of 520 nm and 650 nm were detected respectively.The test of the photodetector on the LNOI planar waveguide is realized by the end face coupling.The switching characteristics,and the relationship between the photocurrent and illumination intensity of the detector are studied.