Integrated Optical Waveguide Devices Design Based On Lithium Niobate Thin Film And Application In Microwave Photonic Filter

With the rapid development of the new generation of information technologies,such as big data,cloud computing,internet of things(IoT)and high-definition(HD)video,"Cloud Living" has become a trendy lifestyle.The huge data throughput and data transmission rate have put forward higher requirements for the future communication industry.Benefit from the advantages of small size,low power consumption,low cost and high level of integration,the integrated photonic devices have been widely used in communication,sensing,computing and even artificial intelligence.Among the materials that realize integrated photonic devices,lithium niobate enjoys the reputation of "optical silicon" due to its good electro-optic effect,acousto-optic effect,piezoelectric effect,birefringence characteristic and nonlinear effect,and occupies a very important position in the field of integrated photonics.Lithium niobate thin film(Lithium niobate on insulator,LNOI)has brought the technical innovation for the development of lithium niobate industry.LNOI retained the excellent characteristics of lithium niobate materials.With high refractive index difference(0.7)between lithium niobate and silica materials,the optical waveguide devices based on LNOI has very big improvement both on device performance and integration,therefore attracted a lot of researchers.In recent years,the photonic devices based on LNOI have been emerging one after another.At present,low-loss optical waveguides,electro-optical modulators,acousto-optical modulators,resonators,photonic crystals and nonlinear optical devices have been reported.LNOI has successfully achieved mixed integration with Si,SiN and other platforms,integrating the advantages of various materials in one.LNOI will play an important role in the field of large-scale photonic integrated circuits and integrated microwave photonic systems in the future,and become a platform for the wide application of photonic integrated circuits in the future.However,LNOI is still in the stage of rapid development.To make LNOI an attractive and competitive integrated optical platform,more photonic devices need to be researched,especially the polarization control devices and optical delay lines based on LNOI.Microwave photonics,which take advantage of photonics to achieve the generation,processing,transmission and measurement of high-speed microwave signals,has attracted a large number of photonics and microwave field researchers since it was proposed.Specifically,the microwave photonic filter,which can overcome the electronic bottleneck of traditional filters,is the research hotspot.The rapid development of integrated photonics technology has pushed the microwave photonic filter to a new development height,which not only reduces the volume,cost and complexity of the microwave photonic filter,but also greatly improves the performance including bandwidth,spectral resolution,noise performance,tunability and reconfigurability.In addition,the research progress of monolithic integration and hybrid heterogeneous integration is conducive to the realization of monolithic microwave photonic filter.The excellent electro-optic effect of lithium niobate,makes it has a unique advantage in the microwave photonic system.Therefore the research of photonic devices based on LNOI,especially polarization control devices and optical delay line,will fill the gaps in research on LNOI platform at this area,broaden the development prospects of LNOI integrated photonic platform,and can also explore road for the highly integrated microwave photonic filter and microwave photonic system based on LNOI to provides reliable theoretical basis and research basis.A polarization beam splitter based on directional coupler and a tunable optical delay line based on waveguide grating are proposed on LNOI,and a tunable notch microwave photonic filter and a tunable bandpass filter are proposedThe main research contents and innovation of this dissertation are summarized as follows:(1)A compact polarization beam splitter based on directional coupler is proposed on LNOI,making full use of birefringence characteristic of lithium niobate.Based on the effective refractive index analysis of the lithium niobate strip waveguide,the theoretical analysis and modeling simulation of the device are carried out.The length of the device and extinction ratio as the evaluation index to optimize the device performance.Numerical results show that,when TE(TM)mode input,the extinction ratio of polarization beam splitter can reach 38 dB(38.8 dB),the working bandwidth 135 nm(50 nm),the fabrication tolerance for waveguide width>100 nm(40.5 nm),and the fabrication tolerance for the thickness of lithium niobate film is about 160 nm.(2)A tunable optical delay line based on waveguide grating is proposed on LNOI,which consists of an LNOI uniform waveguide Bragg grating and flexible electrodes covering along the grating.Taking advantage of the electro-optic effect of lithium niobate,the optical delay line breaks through the traditional working mode of grating delay line.Tunable time delay can be realized by changing the position of the applied voltage,and through different applied voltage flexible reconfigurable delay line can be realized.Through numerical simulation and analysis,the maximum time delay of the delay line is up to 310 ps,the tunable range is 300 ps,the tuning accuracy is 10 ps,and the tuning range of the central reflection wavelength is 1.66 nm.(3)A tunable notch microwave photonic filter based on a polarization beam splitter and waveguide grating true-time delay line is proposed and demonstrated experimentally.The simulation(experimental)results show that the depth of the notch filter can reach 48.72 dB(22.54 dB).The interference signal suppression and the center frequency tuning performance of the microwave photonic filter are also tested.The center frequency tuning range is about 1.57 GHz,the filter has a simple structure,low driving voltage,low power,and fast response speed.(4)Based on the waveguide grating tunable delay line,a tunable band-pass microwave photonic filter is proposed.An array waveguide grating(AWG)is used to cutting a wide spectrum light source for a multi-tap microwave photonic filter.Bandpass is realized by phase modulation to intensity modulation transfer.Benefit from the low power consumption and multi-wavelength time delay tuning actively,the flexible tuning of the pass band filter can be realized by changing the applied voltage and the applied position of the delay line.