Research On Thin-film Lithium Niobate Microring Resonator For Microwave Photonics Applications

At present,communication systems are facing huge challenges in terms of energy consumption and bandwidth.Microwave photonic technology is a major technical solution to the challenges in the communication field,but it still faces the problems of large system volume,poor stability and high power consumption.The application of photonic integration technology can not only significantly reduce the size and power consumption of the system,but also significantly improve its performance,which is expected to achieve a single microwave photonic processing chip.The microring resonator is one of the main building blocks of integrated optics.It is compact in structure,sensitive to wavelength,capable of resonance enhancement and easy to integrate with other optical components.It is widely used and has strong research value.Lithium niobate on insulator(LNOI)is a new generation of low-loss and multifunctional photonic integration platform,which has great application potential in the fields of integrated optics and microwave photonics.In this paper,the working principle and design method of microring resonators are comprehensively and systematically studied.To meet the application requirements of microwave photonics,theoretical analysis and experimental research on on-chip optical frequency comb,thermo optical tunable narrow band flat top filter and sideband selector based on LNOI micro ring resonator are carried out are developed.The thesis mainly includes the following aspects:1.The theoretical basis for generating Kerr optical frequency combs based on on-chip microring resonators is comprehensively reviewed and discussed.A vertical grating-coupled LNOI microring with high quality factor(Q)for generating Kerr optical frequency combs is designed and fabricated.By optimizing the waveguide structure,anomalous dispersion in the communication band is achieved.High-precision micro-nano processing technology and the Q value of the fabricated LNOI microring resonator has reached 9.4×10~5.The single-end coupling efficiency is-7.4 d B.The Kerr optical frequency comb was successfully excited by a single pump.The optical frequency comb with a repetition rate of200 GHz and a comb range of about 22 nm and the optical frequency comb with a repetition rate of 100 GHz and a comb range of about 13 nm are demonstrated experimentally.2.The LNOI thermo-optic tunable narrow-bandwidth flat-top filter was designed and fabricated with structure of two identical LNOI microring resonators in series.By using the thermo-optic effect of lithium niobate,the resonant peak of the double ring is tuned to compensate for the process error.For the first time,a flat passband filtering effect and a tunable central wavelength spectral response based on the microring structure are realized on the LNOI platform.The fabricated filter exhibits an on-chip insertion loss of 0.84 d B,a3 d B bandwidth of 4.8 GHz,a fluctuation of less than 0.3 d B in the passband,an out-of-band rejection ratio of over 34 d B,and a power of 89.4 m W required to shift the center wavelength by one free spectral range.The response time of thermo-optic tuning is in the sub-millisecond order.The design method of the filter is mature and the structure is relatively simple.It is an important supplement to the LNOI integrated optical device library and is expected to be applied in microwave photonics.3.A kind of monolithic integrated sideband selector which combines LNOI tunable microring filter and LNOI asymmetric Mach-Zehnder interferometer(MZI)type electro-optic modulator was designed and fabricated.The sidebands generated by the intensity modulation of LNOI electro-optical intensity modulator can be selected by tuning the passband of the microring filter.The sideband selector realizes the function of reserving the target sideband and effectively suppressing the optical carrier and the remaining sidebands.The device retains the transmission characteristics of an asymmetric MZI-type electro-optic modulator and a microring filter,with an on-chip insertion loss of about 11 d B and an extinction ratio of over 35 d B.Based on it,a design scheme of broadband microwave photonic mixing chip using on-chip microring optical filtering to reduce spurious mixing and improve spectral purity is proposed.