Silicon-based On-chip Polarization Manipulating And Multiplexing/Demultiplexing Devices

Recently,the rapid development of the emerging technologies,including cloud computing,Internet of things and so on,requires higher demands for the communication networks and signal processing systems.By virtue of the remarkable advantages of compatibility of CMOS processing,compact construction,low cost,high reliability,and realization of hybrid optoelectronic integration,silicon-based photonic integrated devcies are increasingly favored in optical communication,optical interconnection,data center,5G wireless communication,novel radar,high-performance computing and many other fields.At present,on-chip polarization manipulating and multiplexing transmission technologies are the research hotspots of silicon photonics.The former one can reasonably manipulate the polarization states of the signal in photonic integrated circuits(PICs)for polarization-transparent propagation,while the latter can effectively improve the transmission capacity in PICs to break the bottleneck in terms of bandwidth.In order to further expand the library of high-performance silicon-based photonic integrated functional devices,this dissertation mainly carries out some research work regarding the two directions above.First of all,the development of integrated photonics is summarized,mainly focusing on the commonly used material platforms,silicon-based micro-/nano-scale waveguide structures,including nanowires,slot waveguides,hybrid plasmonic waveguides,and subwavelength grating waveguides,and research progress of the polarization manipulating and multiplexing technologies for on-chip optical interconnection.Besides,the key performance parameters of the corresponding devices are then described,and the main contents and innovations of this dissertation are also given.Secondly,the basic theory of optical waveguide and numerical analysis methods employed in this dissertation for waveguides and devices to investigate their mode characteristics and transmission features are briefly introduced,including finite element method,finite difference frequency domain method,finite difference time domain method,etc.Besides,the basic calculation processes,applicable ranges and boundary conditions are involved as well.Then,a polarization beam splitter(PBS)based on an asymmetrical directional coupler(DC)for hybrid plasmonic waveguides is proposed.The used DC consists of a hybrid plasmonic waveguide and a thin strip waveguide,and the latter one is designed to only support a single polarized mode.As a result,the gap width between the two coupled waveguides can be shortened to 0,shrinking the device size considerably and reducing the fabrication difficulty to some extent.The coupling length of the suggested PBS is only ? 1.2 ?m,making it one of the shortest polarization beam splitters currently known.Next,silicon-based on-chip polarizers are studied systematically,and a variety of efficient design schemes are proposed for different waveguide structures.A compact polarizer for strip waveguides using a three-waveguide DC is presented,which has compact footprint,very low insertion loss(less than 0.1 d B)and can obtain better performance by cascading multiple DCs.Three different polarizers are designed for silicon-based slot waveguides,e.g.,a compact polarizer based on a DC,a compact and easily fabricated polarizer based on a DC and an efficient polarizer using subwavelength grating(SWG)waveguides.The first two adopt the principle of mode attenuation while the last one uses the principle of mode cut-off.After optimization,all the above devcies have advantages of high extinction ratio,low insertion loss and large operating bandwidth.Next,on the basis of the research of the polarization manipulating devices mentioned above and in combination with the demand of power distribution in the specific applications,a new photonic integrated device having both functions of polarization selection and power division is proposed by using SWG waveguides and hybrid plasmonic Bragg grating.The device is mainly composed of the SWG-based optical divider and the periodically varied metal layer above it.In this dissertation,the device is designed and optimized in detail,and the key performance parameters are given.With the features of flexible design,compact construction and excellent performance,this device can be used to construct a variety of silicon-based optical systems,including optical modulation.Next,research work of silicon-based multiplexing/demultiplexing devices is performed.As to the wavelength-and mode-divison-multiplexing technologies,two compact devices are proposed,namely,a compact near-infrared/mid-infrared wavelength(de)multiplexer based on the SWG-inserted DC,and a compact mode(de)multiplexer using an asymmetrical DC.Their structures and operating principles are described in detail.Moreover,the optimal device performance is obtained,and fabrication tolerance to the key parameters is also analyzed.The two devices have comparatively certain advantages in performance and overall size,and can be applied into the optical interconnections to improve the data capacity.Finally,the summary of the whole dissertation is drawn and the prospect of the following work is given.