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Because of their compatibility with the planar concept of integrated optics, grating couplers offer the most satisfactory means of coupling light into thin film optical waveguides. The purpose of this dissertation has been to study the behaviour, both theoretically and experimentally, and fabrication of grating couplers in nonlinear waveguides. A theory of nonlinear grating couplers is presented based on a coupled-mode approach. The dependence of coupling efficiency on incident beam intensity, beam size, beam position, incident angle, chirp rate, and waveguide losses have been examined all in the presence of nonlinearities in the waveguide. It is reported that, in the presence of nonlinearities, the coupling efficiency decreases with increasing incident power. Different ways of optimizing the coupling efficiency at high incident power levels are presented. These include adjusting the beam size, the coupling angle, and chirping the grating. A new technique is reported for fabrication of regular period, chirped, and curved photoresist gratings. The experimental arrangement is essentially based on Lloyd's mirror fringes and is characterized by its stability, simplicity, and versatility. We also report on successful use of Reactive Ion-Beam Etching (RIBE) with C₂F₆ gas in producing very smooth and deep gratings with high aspect ratios in different waveguide structures. Experimental coupling efficiencies of up to 40% are reported in polystyrene waveguides using etched grating couplers. Experiments are reported in support of the theoretical findings of this dissertation using a polystyrene waveguide with thermal nonlinearity.

The properties of all-optical nonlinear waveguide devices are investigated. In particular, the nonlinear directional coupler (NLDC) and nonlinear Mach-Zehnder interferometer (NLMZ) are analyzed using perturbation theory. The perturbation theory provides differential equations that describe the amplitude of the waveguide modes as a function of the propagation distance. To be practical, these waveguide devices require nonlinear phase shifts of π or more. Therefore, the theoretical investigation of these devices emphasizes their fabrication in bulk and multiple-quantum-well (MQW) gallium arsenide (GaAs). For the first time, absorption, carrier diffusion, and thermal effects are included in the theoretical investigation of the NLMZ and NLDC. The nonlinear dependence of the coupling terms, which has been neglected in all previous work, is shown to be significant for semiconductor based NLDC's. The effects of carrier diffusion on the nonlinear response of a GaAs waveguide is demonstrated using a self-consistent numerical method. The effects are heavily dependent on the waveguide geometry, and, therefore, should be included in the analysis of nonlinear semiconductor waveguide devices. However, if the diffusion length is large compared to the mode width, carrier diffusion simplifies the investigation since the nonlinear absorption and index change are uniform across the mode. This important conclusion is used in the models for the NLMZ and NLDC. The theoretical models predict the NLMZ and NLDC should work in bulk and MQW GaAs. To demonstrate that the required nonlinear phase shifts for the NLMZ and NLDC are indeed possible in bulk and MQW GaAs, the first experimental observation of electronic optical bistability in a MQW GaAs strip-loaded waveguide is recounted. This original research illustrated that phase shifts in excess of 2π are possible in MQW GaAs waveguides and, therefore, the future of all-optical waveguide devices in semiconductors is optimistic.

This dissertation includes three separate studies of related waveguide grating phenomena. These studies deal with a numerical improvement of the integral method of diffraction grating theory, the theoretical analysis of waveguide gratings, and fabrication techniques for photoresist grating masks. The first topic addresses the acceleration of the convergence of the integral kernels. To improve the performance of the integral method for calculating diffraction grating efficiencies, the convergence of the integral kernels is studied. A nonlinear sequence transformation, Levin's u-transformation, is successfully applied to accelerate the convergence of the integral kernels. The computer execution time saving is significant. The application details and many numerical examples are given. The second subject is the ray optics theory of waveguide grating analysis. To establish a linkage between the analysis of diffraction gratings and the analysis of waveguide gratings, a new rigorous ray optics theory is developed. It takes into account phase changes on diffraction, multiple diffraction processes, depletion of the incident guided wave, and lateral shifts. A general characteristic equation that determines the waveguide grating attenuation (coupling) coefficient is derived. The symmetry properties of grating diffraction are applied to waveguide grating analysis for the first time. Lateral shifts of optical rays at a periodically corrugated interface similar to the Goos-Haenchen shift at a planar interface are suggested. The third subject is the in situ control of the development of photoresist grating masks. The existing method for monitoring and modeling photoresist grating development are modified and extended to monitoring and modeling photoresist grating mask development. Experimental examples, detailed theoretical considerations, and computer simulations are presented.

Computer modeling of prism coupling of pulsed laser irradiation at a wavelength of 1064 nm into an organic polymer waveguide with Kerr Law nonlinearities showed that the prism coupling technique was inherent problems that make it difficult to accurately determine the magnitude of the refractive index change. Uncertainty in knowledge of the gap spacing under the prism leads to errors in any estimates of these nonlinear refractive index changes. Results from prism coupling experiments conducted on a polymer waveguide with a pulsed laser are inconclusive.

Integrated photonics is the field of shrinking and simplifying the fabrication of devices that guide and manipulate light. It not only offers to greatly lower the size and cost of systems used in optical communications it also offers a platform on which new physical phenomenon can be explored by being able to fabricate and manipulate structures on the scale of the wavelength of light. One such platform in integrated photonics is that of two-dimensional slab photonic crystals. These structures exhibit a photonic band-gap, a band of optical frequencies that are prohibited from propagating within the medium, that can be used to guide and confine light. When used to create photonic crystal waveguides these waveguides exhibit unique dispersion properties that demonstrate very low optical group velocities, so called "slow-light". This dissertation begins with the practical realization of design and fabrication of such waveguides using the silicon-on-insulator material system using conventional deep-UV photolithography fabrication techniques. It will detail and demonstrate the effect physical dimensions have on the optical transmission of these devices as well as their optical dispersion. These photonic crystal waveguides will then be used to demonstrate the enhancement of nonlinear optical phenomenon due to the slow-light phenomenon they exhibit. First spontaneous Raman scattering will be theoretically demonstrated to be enhanced by slow-light and then experimentally shown to be enhanced in a practical realization. The process of four-wave mixing will be demonstrated to be enhanced in these devices and be shown to be greatly affected by the unique optical dispersion within these structures. Additionally, we will examine the dispersion that exists in silicon nitride microring resonators and the effect it has on the use of these devices to generate optical frequency combs. This is done by leveraging the dispersion measurement methods used to characterize photonic crystal waveguides. We conclude this work by examining the avenues of future work that can be explored in the area of photonic crystal waveguides.

by Lee Yip-Chi. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves [129]-[134]). / Acknowledgments / Abstract / Chapter Chapter 1. --- Introduction --- p.1-1 / Chapter 1.1) --- Recent approaches for wavelength-tunable optical pulse generation --- p.1 -2 / Chapter 1.2) --- Self-seeding a gain-switched Fabry-Perot laser diode --- p.1 -5 / Chapter 1.3) --- About this project --- p.1-8 / Chapter Chapter 2. --- Basic theory --- p.2-1 / Chapter 2.1) --- Basic mechanism of gain-switching --- p.2-1 / Chapter 2.2) --- Mechanism of self-seeding --- p.2-5 / Chapter 2.2.1) --- General principle --- p.2-5 / Chapter 2.2.2) --- Dynamics of singlemode formation --- p.2-7 / Chapter 2.2.3) --- Different cases of modal selection --- p.2-8 / Chapter 2.2.4) --- Reduction of turn-on delay time jitter of optical output --- p.2-10 / Chapter Chapter 3. --- Instrumentation --- p.3-1 / Chapter 3.1) --- Second harmonic autocorrelator --- p.3-1 / Chapter 3.1.1) --- Principle --- p.3-1 / Chapter 3.1.2) --- Description of the 2nd harmonic autocorrelator system --- p.3-3 / Chapter 3.1.3) --- Data acquisition --- p.3-4 / Chapter 3.1.4) --- Alignment and Measurement procedures --- p.3-5 / Chapter 3.1.5) --- Pulsewidth determination by curve fitting --- p.3-7 / Chapter 3.2) --- Optical pulse detection by high speed photodetector --- p.3-9 / Chapter 3.2.1) --- High speed photodetectors --- p.3-9 / Chapter 3.2.2) --- Data acquisition --- p.3-10 / Chapter 3.2.3) --- Deconvolution of the measured optical pulsewidth --- p.3-11 / Chapter Chapter 4 --- Self-seeding 830 nm laser diode using conventional grating method --- p.4-1 / Chapter 4.1) --- Introduction --- p.4-1 / Chapter 4.2) --- Design parameters --- p.4-2 / Chapter 4.2.1) --- External cavity length --- p.4-2 / Chapter 4.2.2) --- Grating orientation --- p.4-3 / Chapter 4.3) --- Experiment --- p.4-4 / Chapter 4.3.1) --- Experimental setup --- p.4-4 / Chapter 4.3.2) --- Equipment Description --- p.4-5 / Chapter 4.4) --- Results and discussion --- p.4-6 / Chapter Chapter 5. --- Self-seeding 1.3 μm LD using fiber-optic configuration --- p.5-1 / Chapter 5.1) --- Optimized operation of self-seeded laser diode --- p.5-1 / Chapter 5.1.1) --- General Description --- p.5-1 / Chapter 5.1.2) --- Components --- p.5-1 / Chapter 5.1.3) --- Experimental setup --- p.5-6 / Chapter 5.1.4) --- Feedback rate measurement --- p.5-8 / Chapter 5.1.5) --- Results and discussion --- p.5-9 / Chapter 5.2) --- Electrical bias dependence on the self-seeded LD --- p.5-15 / Chapter 5.3) --- An efficient scheme to improve tuning range and provide continuous tuning --- p.5-20 / Chapter 5.3.1) --- General Description --- p.5-20 / Chapter 5.3.2) --- Principle of thermal control scheme --- p.5-20 / Chapter 5.3.3) --- Experimental setup --- p.5-22 / Chapter 5.3.4) --- Results and Discussions --- p.5-23 / Chapter Chapter 6. --- A novel self-seeding configuration --- p.6-1 / Chapter 6.1) --- Principle --- p.6-1 / Chapter 6.2) --- Highly dispersion-shifted fiber --- p.6-2 / Chapter 6.3) --- Optical fiber-mirror --- p.6-3 / Chapter 6.3.1 --- Fabrication --- p.6-4 / Chapter 6.3.2) --- Characterization: --- p.6-6 / Chapter 6.4) --- Experiment --- p.6-10 / Chapter 6.5) --- Results --- p.6-12 / Chapter 6.6) --- Discussions --- p.6-27 / Chapter 6.6.1) --- Electrical tuning characteristic --- p.6-27 / Chapter 6.6.2) --- Sidemode supression ratio characteristics --- p.6-30 / Chapter 6.6.3) --- Thermal tuning characteristics --- p.6-33 / Chapter 6.7) --- Summary --- p.6-36 / Chapter Chapter 7. --- Half-period delayed dual-wavelength picosecond optical pulse generation using a self-seeded laser diode --- p.7-1 / Chapter 7.1) --- Introduction --- p.7-1 / Chapter 7.2) --- Principle --- p.7-2 / Chapter 7.3) --- Experiment --- p.7-4 / Chapter 7.4) --- Results and discussions --- p.7-5 / Chapter Chapter 8. --- A proposed self-seeding configuration for the programmable multi- wavelength optical pulse generation --- p.8-1 / Chapter Chapter 9. --- Conclusion --- p.9-1 / References / Appendix / List of accepted and submitted publications

Liang Tak Keung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references. / Abstracts in English and Chinese. / Abstract --- p.I / Acknowledgement --- p.IV / Table of contents --- p.V / List of figures --- p.VIII / Chapter Chapter 1: --- Introduction --- p.1 / Chapter 1.1 --- Introduction to silicon waveguides --- p.2 / Chapter 1.2 --- Introduction to characterization of silicon waveguides --- p.5 / Chapter 1.3 --- Introduction to applications of silicon waveguides --- p.6 / Chapter 1.4 --- Introduction to chapters --- p.7 / References --- p.9 / Chapter Chapter 2: --- Modal analysis of the single-mode silicon waveguide --- p.12 / Chapter 2.1 --- Waveguide structure --- p.13 / Chapter 2.2 --- Effective Index Method --- p.14 / Chapter 2.3 --- Silicon waveguide modal analysis --- p.20 / Chapter 2.4 --- Conclusion --- p.25 / References --- p.26 / Chapter Chapter 3: --- Optical dispersion --- p.27 / Chapter 3.1 --- Introduction --- p.28 / Chapter 3.1.1 --- Chromatic dispersion --- p.28 / Chapter 3.1.2 --- Polarization-mode dispersion --- p.33 / Chapter 3.2 --- Review of dispersion measurement technique --- p.35 / Chapter 3.2.1 --- Chromatic dispersion measurement --- p.35 / Chapter 3.2.2 --- Polarization-mode dispersion measurement --- p.39 / Chapter 3.3 --- Measurement of chromatic dispersion in silicon waveguide --- p.40 / Chapter 3.3.1 --- Experimental setup --- p.40 / Chapter 3.3.2 --- Measurement theory --- p.41 / Chapter 3.3.3 --- Results and discussions --- p.43 / Chapter 3.4 --- Measurement of polarization-mode dispersion in silicon waveguide --- p.49 / Chapter 3.4.1 --- Experimental setup --- p.49 / Chapter 3.4.2 --- Simulation results --- p.50 / Chapter 3.4.3 --- Results and discussions --- p.51 / Chapter 3.5 --- Conclusion --- p.53 / References --- p.54 / Chapter Chapter 4: --- Nonlinear properties --- p.56 / Chapter 4.1 --- Introduction --- p.57 / Chapter 4.1.1 --- Nonlinear refractive index (optical Kerr effect) --- p.57 / Chapter 4.1.2 --- Self-phase modulation --- p.58 / Chapter 4.1.3 --- Two-photon absorption --- p.59 / Chapter 4.1.4 --- Impact of nonlinearities on waveguides --- p.60 / Chapter 4.2 --- Measurement of nonlinear refractive index n2 and TPA coefficient β2 --- p.61 / Chapter 4.2.1 --- Nonlinear refractive index (n2) --- p.62 / Chapter 4.2.2 --- TPA coefficient (β2) --- p.63 / Chapter 4.2.3 --- Conclusion --- p.65 / References --- p.66 / Chapter Chapter 5: --- Loss in ion-implanted silicon waveguide --- p.67 / Chapter 5.1 --- Introduction to ion implantation --- p.68 / Chapter 5.2 --- Ion-implantation process --- p.70 / Chapter 5.3 --- Loss measurement by Fabry-Perot interferometer --- p.72 / Chapter 5.4 --- Results and discussions --- p.73 / References --- p.75 / Chapter Chapter 6: --- Silicon waveguide autocorrelator --- p.76 / Chapter 6.1 --- Introduction on SHG and waveguide autocorrelation technique --- p.77 / Chapter 6.2 --- Theory of TPA absorption --- p.79 / Chapter 6.3 --- Two-photon-induced photocurrent in silicon waveguide --- p.80 / Chapter 6.3.1 --- Device structure --- p.80 / Chapter 6.3.2 --- Intensity dependent photocurrent generation --- p.81 / Chapter 6.3.3 --- Theoretical modeling of photocurrent generation --- p.83 / Chapter 6.4 --- Autocorrelation measurement of short pulses --- p.87 / Chapter 6.4.1 --- Experimental setup --- p.87 / Chapter 6.4.2 --- Results and discussions --- p.88 / Chapter 6.5 --- Conclusion --- p.92 / References --- p.93 / Chapter Chapter 7: --- Conclusion and future works --- p.94 / Chapter 7.1 --- Conclusion --- p.94 / Chapter 7.2 --- Future works --- p.95 / Appendices --- p.96 / Appendix A: Silicon waveguide fabrication process capability at CUHK --- p.96 / Appendix B: Matlab programs of EIM and TPA calculation --- p.100 / Appendix C: Publications list --- p.104

從上世紀80年代中期開始，矽光子學在研究與工業界都有快速的發展。矽光子學有望實現電子與光子電路的一體化集成，從而使得光電子系統的價格和能耗大大地降低。基於這樣的特性，矽光子學被提出用於當前的光通信系統。 / 在這篇論文中，我們將研究矽波導在光信號的成型和重定時中的應用。首先，我們研究基於自由載流子色散效應的矽調製器。調製器的原理和設計方法將會詳細地討論。我們所設計的調製器是基於馬赫-曾德爾干涉儀。實驗表明，製作好的調製器的3dB電光帶寬達到了5GHz。當調製器進行非歸零開關鍵控調製時，調製器的速率可以達到12.45GHz，且誤碼率在10⁻⁹以下。同時，調製器用於正交頻分複用調製格式的結果也會給出。接著，我們研究矽波導中產生的四波混頻效應如何增強信號消光比和減少時域抖動。我們通過實驗表明了，矽波導中的四波混頻效應可以增強單通道的10和40Gb/s歸零開關鍵控信號的消光比。我們接著將四波混頻效應應用於時域交叉的雙通道歸零開關鍵控信號。實驗結果也表明，兩個通道的消光比都能得到增強。我們也通過實驗表明，四波混頻效應可以減少10Gb/s歸零開關鍵控信號的時域抖動。最後，我們用矽波導和chirped光纖光栅實現了一套可調光延遲系統。當這套系統分別應用於10Gb/s的光脈衝、非歸零開關監控信號和歸零的差分相位鍵控信號時，延遲效果都一致。 / Emerging from the mid-1980s, the field of silicon photonics has been rapidly growing, in both research and industry. Silicon photonics has the great potential of monolithic integration of both electronic and photonic circuits. With monolithic integration, the cost and power consumption of photonic systems can be cut down greatly. Due to these features, silicon photonics is proposed for applications in today’s optical communication systems. / In this thesis, silicon waveguide devices for shaping and retiming of optical signals will be investigated. Firstly, silicon Mach-Zehnder modulators based on free-carrier plasma dispersion effect are explored for amplitude modulation of optical signals. The principle and design of the modulators are discussed in details. Experimental results show that 3 dB electro-optic bandwidth of the modulators is 5 GHz, while 10⁻⁹ bit error rate can be obtained for up to 12.45 Gb/s modulated non-return-to-zero (NRZ) on-off keying (OOK) signal. Also, the results of the modulators for orthogonal frequency division multiplexing modulation will be given. Then, silicon waveguides are used as nonlinear medium of four-wave mixing (FWM) effect for extinction ratio enhancement and timing jitter reduction of optical signals. Extinction ratio enhancement of single channel 10 and 40 Gb/s return-to-zero (RZ) OOK signal is experimentally demonstrated. Following that we extend the scheme for two 40 Gb/s RZ-OOK channels which are time-interleaved and obtain extinction ratio enhancement for both. Timing jitter reduction of 10 Gb/s RZ-OOK signal is also achieved by FWM effect in silicon waveguides. Finally, a tunable delay line incorporating a silicon waveguide and a chirped fiber Bragg grating is realized for timing alignment of optical signals. The tunable delay line is used for 10 Gb/s optical pulse, NRZ-OOK signal and RZ differential phase-shift keying (DPSK) signal, showing consistent performance for all. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Chen, Yimin. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstracts also in Chinese. / Acknowledgements --- p.ii / Abstract --- p.iv / Table of Contents --- p.vii / List of Tables --- p.x / List of Figures --- p.xi / Chapter Chapter 1 --- Introduction --- p.17 / Chapter 1.1 --- Research context --- p.17 / Chapter 1.2 --- Waveguides and modulators in silicon photonics --- p.18 / Chapter 1.2.1 --- Silicon photonics --- p.20 / Chapter 1.2.2 --- Silicon modulator --- p.23 / Chapter 1.2.3 --- Silicon waveguide as nonlinear medium --- p.24 / Chapter 1.3 --- Purpose and outline of this work --- p.26 / References --- p.26 / Chapter Chapter 2 --- Silicon modulators for optical communications --- p.28 / Chapter 2.1 --- Introduction --- p.28 / Chapter 2.1.1 --- Motivation for high capacity transmission system --- p.28 / Chapter 2.1.2 --- Literature review of silicon modulators --- p.30 / Chapter 2.2 --- Design of silicon modulators --- p.38 / Chapter 2.2.1 --- Optical circuit of silicon modulators --- p.38 / Chapter 2.2.2 --- p-n junction --- p.43 / Chapter 2.2.3 --- Electronic circuit --- p.44 / Chapter 2.3 --- Fabrication process --- p.46 / Chapter 2.4 --- Experimental results --- p.47 / Chapter 2.4.1 --- Testing results --- p.48 / Chapter 2.4.2 --- OFDM modulation --- p.58 / Chapter 2.5 --- Summary --- p.61 / References --- p.64 / Chapter Chapter 3 --- Signal quality enhancement using four-wave mixing in silicon waveguides --- p.68 / Chapter 3.1 --- Introduction --- p.68 / Chapter 3.1.1 --- All-optical wavelength conversion and all-optical regeneration --- p.68 / Chapter 3.1.2 --- Generic basics of four-wave mixing --- p.69 / Chapter 3.1.3 --- Four-wave mixing in silicon waveguides --- p.71 / Chapter 3.2 --- Extinction ratio enhancement using FWM in a silicon waveguide --- p.77 / Chapter 3.2.1 --- Extinction ratio enhancement of 10 and 40 Gb/s RZ-OOK signals --- p.77 / Chapter 3.2.1.1 --- Principle --- p.78 / Chapter 3.2.1.2 --- Experimental setup and results --- p.78 / Chapter 3.2.1.3 --- Discussion --- p.82 / Chapter 3.2.2 --- Extinction ratio enhancement of two 40 Gb/s RZ-OOK channels --- p.82 / Chapter 3.2.2.1 --- Principle --- p.83 / Chapter 3.2.2.2 --- Experimental setup and results --- p.84 / Chapter 3.2.2.3 --- Discussion --- p.89 / Chapter 3.3 --- Timing jitter reduction using FWM in silicon waveguides --- p.90 / Chapter 3.3.1 --- Principle --- p.91 / Chapter 3.3.2 --- Experimental setup and results --- p.92 / Chapter 3.3.2.1 --- Timing jitter reduction of RZ-OOK signal --- p.92 / Chapter 3.3.2.2 --- Timing jitter reduction of AMI signal --- p.96 / Chapter 3.3.3 --- Discussion --- p.101 / Chapter 3.4 --- Summary --- p.102 / References --- p.103 / Chapter Chapter 4 --- Optical tunable delay line incorporating a silicon waveguide and a chirped fiber Bragg grating --- p.106 / Chapter 4.1 --- Introduction --- p.106 / Chapter 4.1.1 --- Motivation --- p.106 / Chapter 4.1.2 --- Principle --- p.107 / Chapter 4.1.3 --- Characteristics of the silicon waveguide and chirped FBG --- p.108 / Chapter 4.2 --- Tunable delay line for 10 Gb/s optical signals --- p.111 / Chapter 4.2.1 --- 10 Gb/s optical pulse chain --- p.111 / Chapter 4.2.1.1 --- Experimental setup and results --- p.111 / Chapter 4.2.2 --- 10 Gb/s NRZ-OOK signal --- p.115 / Chapter 4.2.2.1 --- Experimental setup and results --- p.115 / Chapter 4.2.3 --- 10 Gb/s RZ-DPSK signal --- p.119 / Chapter 4.2.3.1 --- Experimental setup and results --- p.119 / Chapter 4.3 --- Summary --- p.124 / References --- p.126 / Chapter Chapter 5 --- Conclusions and future work --- p.129 / Chapter 5.1 --- Conclusions --- p.129 / Chapter 5.2 --- Future work --- p.131 / Chapter Appendix A. --- List of Symbols --- p.133 / Chapter Appendix B. --- Abbreviations --- p.135 / Chapter Appendix C. --- Publications --- p.139

The thesis focuses the design and fabrication of silica-on-silicon multimode interference (MMI) devices for optical communications. Firstly, the relationship between different kinds of multimode interference was established for the first time. This gives a clearer understanding of the multimode interference and helps to design better performance optical MMI devices With the consideration of weak lateral light confinement, different kinds of novel approaches to designing high performance MMI devices are developed. The first new approach is for optimization of silica-on-silicon MMI couplers. It is shown that the length of the multimode section can be varied in a well-defined range to find optimal device performance. The range is linked to the propagation constant spacing of fundamental and higher order modes of the multimode waveguide. The second approach is to introduce a new criterion for designing a MMI coupler with central input. According to overlapping interference analysis, one image space should be left for the adjacent output waveguides because of the lateral distribution of alternatively vanishing and non-vanishing images. This consideration is neglected in previous work and is shown to be important for achieving good uniformity MMI power splitters. Thirdly, a new design of silica-on-silicon multimode interference (MMI) device is proposed. Deeply etched air trenches define the boundaries of the multimode section to achieve strong lateral confinement, resulting in lower loss and imbalance. The access waveguides, however, are buried channel and square core, giving low fibre insertion loss and low polarization dependence. The novel design balanced requirement of the strong lateral confinement of the field in the multimode waveguides and the matching between the fiber and the access waveguides. Then a new approach of designing silica-on-silicon optical switches based on cascaded MMI couplers is presented. The approach combines the transfer matrix method, optimisation of the MMI dimensions, and mode propagation analysis (MPA) for calculation of phase shifts. The feasibility of the large port count switches is also discussed. It is shown that the good performance devices can be realized with a large port count of 32. Finally MMI couplers based on silica-on-silicon optical waveguides were fabricated. The Ge-doped silica waveguides were fabricated by HC-PECVD and RIE. Fabrication processes such as thin film deposition and etching are discussed. Good performance devices have been realized.