Research On Thin-film Lithium Niobate Waveguides And Photonic Devices

Lithium niobate?LiNbO₃,LN?crystal,an optical material with excellent electro-optic and acousto-optic effects,has attracted much attention because of its good physical and chemical stability,wide transparency range?0.4?m⁴.5?m?,large electro-optic coefficient(?₃₃=30 pm/V),and outstanding second-order nonlinearity.Up to now,the photonic devices in bulk LN chip fabricated by annealing proton exchange or Titanium diffusion technique have been widely used in optical communication,quantum communication,fiber-optic gyroscope,and microwave photonics field.However,the refractive index difference of these optical waveguides fabricated by the two processes above is small,which makes it difficult to realize compact photonic devices and hence highly integrated.Therefore,bulk LN chip cannot support the development of the photonic devices towards the integration.Fortunately,LN on insulator?LNOI?thin film can overcome the shortcoming of bulk LN chip.As a newly emerging integrated optical platform,LNOI thin film not only keeps the excellent characteristics of LN crystal in many aspects,including electro-optic,acousto-optic,and nonlinear optical properties,but also supports a compact optical waveguide with submicron size.Therefore,the photonic devices based on LNOI thin film have the advantages of small waveguide cross section,large EO modulation efficiency,strong nonlinear effect,small footprints and so on.In view of these advantages,in recent years there has been a strong push towards the development of high-quality optical waveguide and hence compact and high-performance photonic devices as well as high integration photonic chips with LNOI thin film.With the same purpose,this dissertation focuses on the fabrication technology of high-quality LNOI optical waveguide first and then the theory and technology of the design and fabrication of compact and high-performance LNOI photonic devices.The main contents and innovations are as follows:Firstly,a novel process combing the proton exchange with subsequent dry etching was first used for the fabrication of LNOI optical waveguides with high refractive index difference,which realized the technological innovation for the LNOI optical waveguides and photonics devices.Optical waveguide is the basic component of constructing photonic integrated devices,whose quality determines the performance of photonic devices.As a newly emerging integrated optics platform,these currently reported LNOI optical waveguide fabrication processes are imperfect in the aspect of the waveguide sidewall,the etching depth,and the etching rate.In view of these,a novel deep-etched process combining proton exchange and subsequent dry etching is proposed.With the proposed process,the issue of the shallow etching depth with fluoride gas as etching gas and the issue of the large sidewall tilt angle with argon as etching gas can be addressed.And rectangular optical waveguides with a top width of¹.0?m and a height of⁹00 nm were fabricated in the x-cut LN thin film.The bending losses of the bent waveguides with different radii,the transmission losses of straight waveguides with different lengths,the coupling losses between waveguides and fibers,as well as the polarization rotation of TE or TM polarized input light in waveguides were investigated,which lay a good foundation for the subsequent development of LNOI photonic devices.Secondly,a compact electro-optic tunable optical interleaver with LNOI thin film was designed and demonstrated,which realized high-speed tunable electro-optic interleaver for the first time.Optical interleaver is an important device in optical communication and microwave photonics field.Therein,the compact high-speed tunable optical interleaver is of great significance for reducing the size of system and realizing the function of fast tunability and reconfigurability of the system.To meet this application requirement,an electro-optic tunable optical interleaver employing the configuration of an asymmetric Mach-Zehnder interferometer is designed and fabricated in the x-cut LN thin film using the proposed process.The fabricated device,which employs a push-pull electrode with an eletro-optic length of 1.35 mm,is only 4 mm long and shows a channel spacing of 49.7 GHz in the wavelength range of 1528-1605nm,and the electro-optical tuning efficiencies of 18 pm/V and 16 pm/V for TE and TM polarized input light,respectively.Thirdly,a monolithic 1×4 electro-optic tunable optical interleaver in the LNOI thin film was developed and demonstrated,which explored the integration technology of LNOI thin film photonic devices,and laid a foundation for the development of integrated photonic chips in the LNOI thin film.The monolithic multiplex optical interleaver not only can avoid the performance instability which may be caused by the large volume of discrete devices and more optical interfaces,but also can be used as wavelength selective switch and wavelength router through the electro-optic tunability.Hence,based on the previous research on the compact electro-optic tunable optical interleaver,a monolithic reconfigurable 1×4 electro-optic tunable optical interleaver is designed and fabricated in the LN thin film.The device consists of three asymmetrical Mach-Zehnder interferometers and three independent push-pull electrodes,both are cascaded by two stages,which is one of the most complex photonic devices fabricated in the LNOI thin film up to now.The fabricated interleaver exhibits a channel spacing of 50 GHz between the two adjacent output ports and a channel spacing of 200 GHz for each output port in the wavelength range of 1530 nm-1605 nm,as well as the switch voltages of 4.5 V and 6.7 V for TE and TM polarized input light,respectively.