Polarization dependences of optical devices in highly-integrated optical systems become a major problem. To overcome this issue, one can implement polarization diversity scheme to achieve a single polarization on-chip network. One of the essential components in a polarization diversity scheme is the polarization beam splitter (PBS). In this thesis, we will a PBS based on a silicon-on-insulator (SOI) platform with reduced device size and broad operation bandwidth. We use the three-dimensional Finite-Difference Time-Domain (3D-FDTD) method to perform the simulation. First, we use two asymmetric waveguides to design an asymmetric directional coupler with only TE-like mode phase matching condition. We then tape the lower waveguide to keep the TE-polarized light, and split the TE- and TM- polarized light. By utilizing an asymmetrical directional coupler with a tapered waveguide, we have achieved a 7.3

The dissertation focuses on the analysis and design for the new fiber-optic passive components based on the photonic-crystal-fiber (PCF). The vector boundary element method (VBEM) and the finite-difference time-domain (FDTD) method are employed to the propagation characteristics of PCF components. A novel octagonal microstructured fiber (OMF) with eight air-holes in the first ring has been proposed. The OMF has significantly wider wavelength range for single-mode operation, more circular-like field distribution, and less confinement loss. In addition, a novel compact polarization beam splitter (PBS) based on the twin-elliptical-core PCF (TEC-PCF) has also been proposed. It behaves with high extinction ration and broad bandwidth with significantly short splitter length. The design concept and the coupling mechanism are presented in this dissertation based on the normal-mode coupling theory and VBEM.

There has been a huge surge in data traffic all over the world due to the rise of streamingmedia services and connected devices. The current demand in data traffic has alreadypushed the optical fiber in the internet architecture to the network edges and the trend isto push it as close as possible, to the CPU. Silicon photonics addresses this challenge byenabling miniaturized optical devices that use light to move huge amounts of data at veryhigh speeds with extremely low power. To further improve the data transmission capacity,one can make use of different polarizations of light. However, to take advantage ofdifferent polarizations, devices with on-chip polarization rotation capability are required.This is achieved by a tunable polarization rotator. Moreover, full control of polarizationrotation can also be utilized to realize a new class of components in integrated photonicsincluding polarization mode modulators, multiplexers, filters, as well as switches foradvanced optical signal processing, coherent communications, and sensing.This thesis introduces a novel tunable polarization rotator that uses microelectromechanicalsystems (MEMS) as its actuation principle. When voltage is applied to a MEMStunable silicon cantilever, a mechanical movement occurs, which in turn affects theoptical mode shape travelling through a waveguide, as a result of which the polarizationis rotated. In this work, a MEMS tunable polarization rotator is designed, fabricated,and characterized with a polarization extinction ratio of 10 dB, which works in 1530nm -1570nm wavelength spectrum. In addition to the MEMS tunable polarization rotator,in this thesis, a free standing polarization beam splitter of length 1.4 μm, the shortestreported to-date to our knowledge, was designed, fabricated, and characterized. Thetunable polarization rotator and beam splitter developed in this thesis have the potentialto increase the bandwidth and flexibility of current optical communication networks, andfind further applications in polarization diversity schemes for sensing. / Mängden datatrafik i världen har växt explosionsartat de senaste åren på grund av detökade antalet uppkopplade enheter samt det snabbt växande tjänsterna för strömmad media. Det stora databehovet har redan gjort det nödvändigt att använda högkapacitiva optiska länkar hela vägen till nätverkets kanter och trenden är att optisk dataöverföring används närmare och närmare själva CPU:erna i datorerna som utgör källa och slutpunkt för all data på Internet. Kiselfotonik möter denna utmaning genom att möjliggöra miniatyriserade optiska system som använder ljus för att snabbt överföra stora mängder data med liten effektförbrukning. För att öka kapaciteten ännu mer kan man använda sig av ljusets polarisation. För att göra detta måste man tillhandahålla system för att vrida polarisation på chipp-nivå vilket man kan åstadkomma med en avstämbar polarisationsvridare. Utöver en ökad kapacitet kan den nya kontrollen över polarisation även användas för att skapa nya typer av integrerade optiska komponenter som polarisationsbaserade modulatorer, multiplexers, filter, såväl som switchar för optisk signalbehandling, koherent kommunikation och avkänning.Denna avhandling presenterar en ny avstämbar polarisationsvridare som använder en mikroelektromekanisk (MEMS) aktuator. När en spänning är applicerad på en MEMS balk skapas en mekanisk rörelse som i sin tur påverkar den optiska mod-bilden som propagerar i en integrerad optisk vågledare vilket resulterar i att polarisationen vrids. Denna avhandling innehåller design, tillverkning och karakterisering av en avstämbar polarisationsvridare med en polariseringsgrad på 10 dB i våglängdsområdet 1530-1570 nm. Utöver det presenteras design, tillverkning och karakterisering av frihängande polarisationsfördelare med en längd på endast 1.4 µm, den kortaste hittills rapporterad. Dessa komponenter har potentialen att öka bandbredden och flexibilite befintligaoptiska kommunikationsnät och hitta nya tillämpningar i sensorsystem.

Optical interconnects based on CMOS compatible photonic integrated circuits are regarded as a promising technique to tackle the issues traditional electronics faces, such as limited bandwidth, latency, vast energy consumption and so on. In recent years, plasmonic integrated components have gained great attentions due to the properties of nano-scale confinement, which may potentially bridge the size mismatch between photonic and electronic circuits. Based on silicon nanowire platform, this thesis work studies the design, fabrication and characterization of several integrated plasmonic components, aiming to combine the benefits of Si and plasmonics. The basic theories of surface plasmon polaritons are introduced in the beginning, where we explain the physics behind the diffraction-free confinement. Numerical methods frequently used in the thesis including finite-difference time-domain method and finite-element method are then reviewed. We summarize the device fabrication techniques such as film depositions, e-beam lithography and inductively coupled plasma etching as well as characterization methods, such as direct measurement method, butt coupling, grating coupling etc. Fabrication results of an optically tunable silicon-on-insulator microdisk and III-V cavities in applications as light sources for future nanophotonics interconnects are briefly discussed. Afterwards we present in details the experimental demonstrations and novel design of plasmonic components. Hybrid plasmonic waveguides and directional couplers with various splitting ratios are firstly experimentally demonstrated. The coupling length of two 170 nm wide waveguides with a separation of 140 nm is only 1.55 µm. Secondly, an ultracompact polarization beam splitter with a footprint of 2×5.1 μm2 is proposed. The device features an extinction ratio of 12 dB and an insertion loss below 1.5 dB in the entire C-band. Thirdly, we show that plasmonics offer decreased bending losses and enhanced Purcell factor for submicron bends. Novel hybrid plasmonic disk, ring and donut resonators with radii of ~ 0.5 μm and 1 μm are experimentally demonstrated for the first time. The Q-factor of disks with 0.5 μm radii are                         , corresponding to Purcell factors of . Thermal tuning is also presented. Fourthly, we propose a design of electro-optic polymer modulator based on plasmonic microring. The figure of merit characterizing modulation efficiency is 6 times better comparing with corresponding silicon slot polymer modulator. The device exhibits an insertion loss below 1 dB and a power consumption of 5 fJ/bit at 100 GHz. At last, we propose a tightly-confined waveguide and show that the radius of disk resonators based on the proposed waveguide can be shrunk below 60 nm, which may be used to pursue a strong light-matter interaction. The presented here novel components confirm that hybrid plasmonic structures can play an important role in future inter- and intra-core computer communication systems. / <p>QC 20140404</p>

Optical devices based on planar lightwave circuit (PLC) technology have the advantages of small size, high reliability, possibility for large scale production, and potential integration with electronics. These devices are widely employed in optical telecommunications, sensing, data storage, imaging, and signal processing. This thesis focuses on some selected PLC based devices, such as power splitters, demultiplexers, triplexers and polarization beam splitters. First, the basic principle of the waveguides and the simulation methods for PLC devices are discussed. A novel effective index method is introduced to reduce a two-dimensional structure to a one-dimensional one, and can be implemented for arbitrarily shaped waveguides. Numerical methods, such as finite-difference mode solver, beam propagation method, finite-difference time-domain method are introduced to analysis the mode profile of the waveguides, and the propagation properties of light in PLC devices. Multimode interference (MMI) couplers are widely used in many PLCs, such as power splitters, ring lasers, optical switches, and wavelength division multiplexers/demultiplexers. In this work, concepts for improving the self-imaging quality of MMI couplers are analyzed and new designs are proposed. A significant improvement in performance together with compact sizes were obtained with taper sections at the input/output of MMI couplers based on SOI, and deeply etched ridges in MMI couplers based on SiO2. A polarization insensitive dual wavelength demultiplexer based on sandwiched MMI waveguides was presented. Novel devices including triplexers and polarization beam splitters were realized by using photonic crystal (PhC) structures. Two stages of directional couplers based on PhC waveguides are cascaded to form an ultracompact triplexer. The special decoupling property of the PhC waveguide based directional coupler was utilized in the design. A novel polarization beam splitter was realized by combining a MMI coupler and a PhC which works as a polarization sensitive reflector. Finally, fabrication and optical characterization of an ultra-compact directional coupler and PhC structures in InP are presented. In a single etching step, by using the lag-effect in inductively coupled plasma reactive ion etching, a compact directional coupler (55 μm) is demonstrated. Carrier life times in PhC structures etched by chemically assisted ion beam etching were investigated, for emitter and switching applications. / QC 20100909

Nowadays, silicon photonics is a widely studied research topic. Its high-index-contrast and compatibility with the complementary metal-oxide-semiconductor technology make it a promising platform for low cost high density integration. Several general problems have been brought up, including the lack of silicon active devices, the difficulty of light coupling, the polarization dependence, etc. This thesis aims to give new attempts to novel solutions for some of these problems. Both theoretical modeling and experimental work have been done. Several numerical methods are reviewed first. The semi-vectorial finite-difference mode solver in cylindrical coordinate system is developed and it is mainly used for calculating the eigenmodes of the waveguide structures employed in this thesis. The finite-difference time-domain method and beam propagation method are also used to analyze the light propagation in complex structures. The fabrication and characterization technologies are studied. The fabrication is mainly based on clean room facilities, including plasma assisted film deposition, electron beam lithography and dry etching. The vertical coupling system is mainly used for characterization in this thesis. Compared with conventional butt-coupling system, it can provide much higher coupling efficiency and larger alignment tolerance. Two novel couplers related to silicon photonic wires are studied. In order to improve the coupling efficiency of a grating coupler, a nonuniform grating is theoretically designed to maximize the overlap between the radiated light profile and the optical fiber mode. Over 60% coupling efficiency is obtained experimentally. Another coupler facilitating the light coupling between silicon photonic wires and slot waveguides is demonstrated, both theoretically and experimentally. Almost lossless coupling is achieved in experiments. Two approaches are studied to realize polarization insensitive devices based on silicon photonic wires. The first one is the use of a sandwich waveguide structure to eliminate the polarization dependent wavelength of a microring resonator. By optimizing the multilayer structure, we successfully eliminate the large birefringence in an ultrasmall ring resonator. Another approach is to use polarization diversity scheme. Two key components of the scheme are studied. An efficient polarization beam splitter based on a one-dimensional grating coupler is theoretically designed and experimentally demonstrated. This polarization beam splitter can also serve as an efficient light coupler between silicon-on-insulator waveguides and optical fibers. Over 50% coupling efficiency for both polarizations and -20dB extinction ratio between them are experimentally obtained. A compact polarization rotator based on silicon photonic wire is theoretically analyzed. 100% polarization conversion is achievable and the fabrication tolerance is relatively large by using a compensation method. A novel integration platform based on nano-epitaxial lateral overgrowth technology is investigated to realize monolithic integration of III-V materials on silicon. A silica mask is used to block the threading dislocations from the InP seed layer on silicon. Technologies such as hydride vapor phase epitaxy and chemical-mechanical polishing are developed. A thin dislocation free InP layer on silicon is obtained experimentally. / QC20100705

Decades of efforts have pushed the replacement of electrical interconnects by optical links to the interconnects between computers, racks and circuit boards. It may be expected that optical solutions will further be used for inter-chip and intra-chip interconnects with potential benefits in bandwidth, capacity, delay, power consumption and crosstalk. Silicon integration is emerging to be the best candidate nowadays due to not only the dominant status of silicon in microelectronics but also the great advantages brought to the photonic integrated circuits (PICs). Regarding the recent breakthroughs concerning active devices on silicon substrate, the question left is no longer the feasibility of the optical interconnects based on silicon but the competitiveness of the silicon device compared with other alternatives. This thesis focuses on the study of two key components for the optical interconnects, both especially designed and fabricated for silicon platform. One is a high speed electroabsorption modulator (EAM), realized by transferring an InP-based segmented design to the hybrid silicon evanescent platform. The purpose here is to increase the speed of the silicon PICs to over 50  Gb/s or more. The other one is a high performance grating coupler, with the purpose to improve the optical interface between the silicon PICs and the outside fiber-based communication system. An general approach based on the transmission line analysis has been developed to evaluate the modulation response of an EAM with a lumped, traveling-wave, segmented or capacitively-loaded configuration. A genetic algorithm is used to optimize its configuration. This method has been applied to the design of the EAMs on hybrid silicon evanescent platform. Based on the comparison of various electrode design, segmented configuration is adopted for the target of a bandwidth over 40 GHz with as low as possible voltage and high extinction ratio. In addition to the common periodic analysis, the grating coupler is analyzed by the antenna theory assisted with an improved volume-current method, where the directionality of a grating coupler can be obtained analytically. In order to improve the performance of the grating coupler, a direct way is to address its shortcoming by e.g. increasing the coupling efficiency. For this reason, a nonuniform grating coupler with apodized grooves has been developed with a coupling efficiency of 64%, nearly a double of a standard one. Another way is to add more functionalities to the grating coupler. To do this, a polarization beam splitter (PBS) based on a bidirectional grating coupler has been proposed and experimentally demonstrated. An extinction ratio of around -20 dB, as well as a maximum coupling efficiency of over 50% for both polarizations, is achieved by such a PBS with a Bragg reflector underneath. / QC 20100906

<p>The fiber-to-the-x (FTTX) has been widely investigated as a leading access technology to meet the ever growing demand for bandwidth in the last mile. The passive optical network (PON) provides a cost-effective and durable solution. In this thesis, we investigate different aspects of the PON, in the search for cost-effective and high-performance designs of link system and devices.</p> <p>In Chapter 2, we propose a novel upstream link scheme for optical orthogonal frequency division multiplexing (OOFDM)-PON. The colorless laser diodes are used at the optical network units (ONUs), and the overlapped channel spectrum of orthogonal subcarrier multiplexing provides high spectral efficiency. At the optical line terminal (OLT), optical switch and all optical fast Fourier transform (OFFT) are adopted for high speed demultiplexing. The deterioration caused by the laser perturbation is also investigated.</p> <p>In Chapter 3, we design a novel polarization beam splitter (PBS), which is one of the most important components in polarization-controlled optical systems, e.g. the next-generation PON utilizing polarization multiplexing. Our PBS is built on a slab waveguide platform where the light is vertically confined. Planar lenses are formed to collimate and refocus light beam by converting the phase front of the beam. A planar subwavelength grating of a wedge shape induces the form birefringence, where the transverse electric (TE) and transverse magnetic (TM) waves have different effective refractive indices, and are steered to distinct directions. This design provides low insertion loss (< 0.9 dB) and low crosstalk (< -30 dB) for a bandwidth of 100 nm in a compact size, and can be realized by different material systems for easy fabrication and/or monolithic integration with other optical components.</p> <p>In Chapter 4, we study the mode partition noise (MPN) characteristics of the Fabry-Perot (FP) laser diode using the time-domain simulation of noise-driven multi-mode laser rate equation. FP laser is cheaper than the widely used distributed feedback (DFB) laser diode in PON, but its MPN is the major limiting factor in an optical transmission system. We calculate the probability density functions for each longitudinal mode. We also investigate the k-factor, which is a simple yet important measure of MPN. The sources of the k-factor are studied with simulation, including the intrinsic source of the laser Langevin noise, and the extrinsic source of the bit pattern.</p> / Doctor of Philosophy (PhD)