| Lithium niobate has excellent electro-optic,acousto-optic,and optical nonlinear effects,which is well-known as "silicon of photonics".With the development of its fabrication processes in recent years,thin film lithium niobate on insulator(LNOI)has emerged as a new integrated photonic platform,which is attracting widespread attention with its component library growing fast.However,the chemical inertness of lithium niobate makes it a difficult material to etch.Although pure physical etching based on argon milling has been comparatively mature,which is incompatible with standard complementary metal-oxide semiconductor processes,resulting in limited scalability and large-scale integration capabilities of LNOI devices.To address this issue,a silicon nitride loaded LNOI platform has been proposed as an attractive alternative of direct etching.Silicon nitride has a similar refractive index and optical transparency window to lithium niobate but possesses mature waveguide fabrication processes.Thus,the waveguide structures can be formed by etching the silicon nitride layer,which means that we can not only avoid the direct etching of lithium niobate,but also use the excellent material properties of lithium niobate to achieve high-performance electro-optic,acousto-optic,and optical nonlinear devices.Recently,researchers have developed a series of high-performance active devices based on the silicon nitride loaded LNOI platform.Although these experimental demonstrations are impressive,to realize the great potential of LNOI as an integrated photonic platform,it is still essential to develop some important passive components for monolithic integration with the active devices to achieve high-performance,multi-functional,and low-cost photonic integrated circuits(PICs).Optical(de)multiplexer is one of the most important passive components,which can provide significant parallelism for optical interconnection,optical computing,and optical sensing.In the field of data communication,the monolithic integration of high-speed optical modulators and optical(de)multiplexers is conducive to the realization of high-speed,large-capacity,and low-cost PICs.Therefore,the implementation of optical multiplexing devices in LNOI is particularly important.Based on this,this thesis aims to study the optical multiplexing technologies in the silicon nitride loaded LNOI platform and develop optical multiplexing/demultiplexing devices with compact size,low power consumption and good scalability by using three different physical dimensions: optical wavelength,mode,and polarization.The main research includes:First,the dielectrically loaded LNOI waveguides.We briefly review the recent research progresses in dielectrically loaded LNOI platforms,and summarize the advantages and disadvantages of different optical loading materials,including silicon,silicon nitride,polymers,etc.By comparing the performance of different platforms,the silicon nitride loaded LNOI waveguide was finally selected as the research object of this thesis.Second,the theory and design of optical waveguides.Firstly,the optical waveguide theories are introduced,including the structures of optical waveguides,the formation of optical modes,the definitions of transverse electric mode(TE)and transverse magnetic mode(TM),and the coupled mode theory.Based on the fundamentals of optical waveguides,we design the structural parameters of the silicon nitride loaded LNOI waveguide.After that,two commonly used interfaces between the optical fiber and waveguide are introduced,the first one is inverse taper,and the second one is grating coupler.The advantages and disadvantages of different optical interfaces are analyzed.To carry out the experimental characterization of the fabricated devices,we design,simulate,and experimentally demonstrate the grating couplers as our optical interfaces in this thesis.Third,the wavelength-division multiplexing devices.We firstly propose and demonstrate a subwavelength grating assisted contra-directional coupler.By introducing the subwavelength grating waveguide and multimode normal waveguide,the effective refractive index difference between the Bloch mode in the grating waveguide and the spatial mode in the normal waveguide is increased.The contra-coupling wavelength is avoided to be located in the photonic bandgap of the grating waveguide,and thus successfully achieving single-band and low-loss coupling around 1550 nm.Then,a wavelength-division(de)multiplexer is experimentally demonstrated based on the contra-directional couplers.The device can implement the multiplexing and demultiplexing of three independent wavelength channels,with a narrow channel spacing of 3.2 nm(400 GHz).The experimental results show that the device insertion loss for each channel is about 2.5 d B,with the inter-channel crosstalk below-20 d B.Fourth,the mode-division multiplexing devices.We firstly analyze the mode behaviors along different crystallographic directions of lithium niobate,which indicate that the crystallographic Z direction is more suitable for realizing mode-division multiplexing.According to the phase matching condition,a mode-division(de)multiplexer is proposed and experimentally demonstrated based on the directional coupler structure,realizing the multiplexing and demultiplexing of four optical mode channels.The experimental results show that the device insertion loss is lower than 1.46 d B and the inter-channel crosstalk is below-13.03 d B,at the 40 nm wavelength range from 1525 nm to 1565 nm.In addition,the 40 Gbps data transmission experiment has observed clear and open eye diagrams,indicating that the device has good data transmission capabilities.Moreover,to address the signal degradation in mode-division multiplexing systems as the device number increases,we propose and demonstrate the high-order mode pass filters based on subwavelength gratings.Based on the Bragg conditions of grating waveguides,the devices can filter undesired modes out and improve the crosstalk performance of the system.The experimental results show that the device insertion loss is lower than 3.1 d B and the mode extinction ratio is larger than 34 d B,at the wavelength of 1550 nm.Fifth,the polarization-division multiplexing devices.According to the mode behavior analysis,we realize the polarization rotation by utilizing the mode hybridization at a specific waveguide width.Then,a polarization splitter-rotator is proposed and experimentally demonstrated based on the directional coupler structure,which can implement multiplexing and demultiplexing of two light polarizations.The experimental results show that the device insertion loss is lower than 1.49 d B and the inter-channel crosstalk is below-17.75 d B,at the wavelength range from 1525 nm to1565 nm.Similarly,in order to improve the crosstalk performance of polarization-division multiplexing systems,we propose and demonstrate a TM polarizer based on the subwavelength gratings.The experimental results show that the device insertion loss is about 1.3 d B,and the polarization extinction ratio is as high as30.6 d B,at the wavelength of 1550 nm. |
