Research On Lithium Niobate Photonics Devices Based On Long Period Waveguide Grating

With the rapid development of optical communication and information processing technology,various applications such as e-commerce,e-government,cloud computing,high-definition video,and artificial intelligence,as well as data centers supporting these applications,have emerged and promoted human civilization to quickly enter the information age.In this process,photonic devices,as key components of optical communication and optical information processing systems,play an indispensable role.They push the development of the systems and determine their performance.Therefore,it is of great significance to research and develop various high-performance photonic devices to continuously advance optical communication and optical information processing technologies,thereby meeting the ever-increasing needs of the information society.Therefore,countries around the world,from the government to the private sector,have throwed in lots of manpower and material resources in the research and development of various high-performance photonic devices.In the mean time,the integration of grating,an important optical device,with optical waveguide and hence development of gratingtype photonic devices for optical communication and optical information processing systems have also attracted extensive attention,and long-period waveguide grating(LPWG)came into being.LPWG can be used to realize efficient coupling between the core modes or between the core modes and the cladding modes and,hence,realize functions such as wavelength filtering,mode filtering,mode conversion,modulation,sensing,and etc.It has the advantages of versatility,compact structure,and easy integration.Up to now,various photonic devices based on LPWG have been successfully developed using silicon,polymer,silicon nitride,lithium niobate(LN),and other materials.Among them,LPWG photonic devices based on LN platform have attracted much attention.This is because LN has unique advantages in development of high-speed tunable photonic devices due to its large electro-optic coefficient,wide transparent window,and mature waveguide fabrication technology.Tunability of photonic devices is the premise of using photonic devices to modulate optical wave parameters,the basis of realizing device reconfiguration,and the key to make up for the device fabrication process errors,reduce fabrication process requirements,and then improve device performance.The realization of tunability usually depends on electro-optic effect and thermo-optic effect,and electro-optic tuning based on electro-optic effect has the advantages of high speed and low power consumption.Under this background,this dissertation carries out study on electro-optic tunable LPWG-type photonic devices based on LN platform.The main innovative research work of this dissertation includes:1.Based on the traditional LN wafer,an electro-optically tunable long-period grating(LPG)-assisted optical waveguide directional coupler(DC)is developed.The device is fabricated on x-cut LN substrate by annealing proton exchange process,which has the advantages of mature optical waveguide fabrication process,low waveguide loss,and easy coupling with optical fiber.The device can realize efficient coupling of two fundamental modes in asymmetric DC composed of waveguides with different widths by forming LPG on the side wall of wide waveguide to compensate the phase mismatch between the two fundamental modes.And further,high-speed electro-optical tuning with low driving voltage can be realized by fabricating push-pull electrodes on both sides of waveguide.Based on the above principles,the device is designed by investigating the impact of the structural parameters of the waveguide and electrode on the device performance,and several proof-of-principle devices are fabricated.One of the devices which exhibits the best performance is packaged,and its performance,including electrooptical tuning,thermo-optical tuning,small signal modulation,and direct-current(DC)drift are investiagted.The results show that the device can achieve a rejection band with a contrast as high as 34.0 d B at the wavelength of 1532.9 nm for the TE-polarized input light,and a second rejection band will be generated during electro-optic tuning.The electro-optic tuning efficiencies of the center wavelengths of the two rejection bands are1.20 nm/V(1526.4-1549.1 nm)and 1.74 nm/V(1576.1-1602.2 nm),respectively.While there is only one rejection band with a tuning efficiency of-0.128 nm/℃(22 ℃-60℃)during Thermo-optic tuning.2.Based on the emerging lithium niobate on insulator(LNOI),an electro-optically tunable LPG-assisted LNOI optical waveguide DC is developed.The working principle of the device is the same as that of the grating-assisted DC based on the traditional LN,but it is made of LNOI,so it has the advantages of compact structure,small size,and easy integration.The fabricated best proof-of-principle device,which has a grating coupling region length of 816 μm,can achieve a rejection band with a contrast of 14.8 d B at the wavelength of 1595.3 nm for the TE-polarized input light.And the rejection band with a maximum contrast of 20.7 d B can be achieved at the wavelength of 1594.5 nm during electro-optic tuning.Electro-optical and thermo-optical tuning efficiencies of the center wavelengths of the rejection band are 0.38 nm/V(1595.3 nm-1599.0 nm)and 0.14 nm/℃(25℃-50℃),respectively.Finally,the small signal modulation and DC drift characteristics of the device are investigated.3.Based on LNOI optical waveguide,an electro-optically tunable LPWG featuring compact structure,small size,and easy integration is developed.By forming asymmetric LPG on both sides of LNOI two-mode ridge waveguide,the efficient coupling between the fundamental mode and the first-order mode in the two-mode ridge waveguide is realized.And then the first-order mode is filtered by using the tapered ridge waveguide and the metal absorption band.Therefore,the proposed grating can achieve high-contrast transmission spectrum.Further,by fabricating tuning electrodes above the LPWG,different electro-optic modulation for the fundamental mode and the first-order mode can be achieved,which help to realize efficient tuning for the transmission spectrum.Based on the above principles,the device is designed by investigating the impact of the structural parameters of the waveguide and electrode on the device performance,and several proofof-principle devices are fabricated.One of the fabricated devices with the best performance,which has a grating coupling region length of 648 μm,can achieve a rejection band with a contrast of 16.32 d B for the TE-polarized input light.The electrooptical tuning efficiency of the device is 0.34 nm/V,and the thermo-optical tuning efficiency of the device is 0.137 nm/℃.Finally,the small signal electro-optic modulation and DC drift characteristics of the device are also investigated.The LPG-assisted traditional LN optical waveguide DC and LNOI optical waveguide DC developed above can be used as tunable filters,mode filters,electro-optic modulators in the fields of reconfigurable wavelength division multiplexing,mode division multiplexing and electro-optical modulation,while the LNOI-based LPWG can be used as tunable filters.