This thesis describes work on fibre transitions made in photonic crystal fibres (PCF) and conventional standard fibres. Three post-processing techniques were used to make the transitions: fibre tapering, ferrule drawing and a new technique – PCF hole inflation. All these methods change the fibre dimensions on a centimeter scale while maintaining very low loss. In the hole inflation technique, cladding holes are pressurized and can be enlarged while heat-treating, unlike other techniques where the holes can only be reduced in size. Controlled hole expansion was used to produce devices for applications such as supercontinuum generation. Furthermore, differential pressurization of holes could create a diversity of core shapes in a PCF section. For example they were investigated to improve interfacing of laser diodes to fibres. Differential pressurization was also used to introduce new cores into PCFs. Introducing a larger core asymmetrically by the original core resulted in a fundamental to second-order mode conversion with a high extinction ratio. Alternatively, similar mode conversion was demonstrated by fusing two unequal standard fibres. Also with standard fibres, low-loss multimode to single-mode fibre transitions were made using a modified fibre fabrication technique. These fibre transitions and optical devices have a wide range of potential applications, for example in supercontinuum generation and low-loss interfacing of fibres to other optical systems.

No description available.

M.Ing. (Electrical and Electronic Engineering) / This research project sets out to investigate passive L-C coupling circuits as an alternative to transformer-capacitor couplers, with the prime intention to reduce the cost of coupling in power-line communications, hence the title “Compact, Economical Coupling Circuits for Low-Voltage Power-line Communications”. This chapter first presents a short introduction to Power-Line Communications, and then briefly reviews relevant topics surrounding this project. Also discussed, in general, are the standards and regulations for power-line communications i.e. the CENELEC EN50065.1 Standard and the IEEE 1901.2 Standard. This information intends to give the reader guidelines and relevant protocols and/or rules when designing and building communication devices for PLC. This information will also help us in our designs and experimental set-ups as we proceed with this project. The reader will also be introduced to the concept of filtering and impedance matching in communication systems. Emphasis will be laid on the required output signal frequency profile of a coupling band-pass filter. The chapter will define filtering in terms of communications and also discuss the more significant and important parameters in filter circuits. The last section of this chapter will touch on impedance matching and the concept of maximum power transfer, so as to attain maximum signal transfer to the receiver-end.

The phenomenon of dopant diffusion as a viable means of coupler fabrication is investigated. It is well known that the diffusion of dopants can improve the uniformity of multimode star couplers manufactured by the fused biconical taper technique. The theoretical basis for the same phenomenon in a single mode coupler is developed, on the basis of the theory of diffusion and the Gaussian approximation for circular fibers. A novel technique to manufacture and design single mode optical fiber couplers with a minimization of the manufacturing complexity is demonstrated. Traditionally fused biconical tapered couplers have been manufactured by twisting, fusing and elongating optical fibers at elevated temperatures. Usually, high temperature oxy-hydrogen flames are used for such purposes and some degree of skill is needed for a human operator. The complexity of control procedures for automation of the process is greatly increased by the fact that the tapering process is an integral part of the feedback loop. This can be eliminated if a constant tension is maintained on the fibers in the heating process while heat is applied uniformly from a source such as a platinum wire furnace. Since the refractive index differentials responsible for the guiding phenomenon at optical frequencies are directly dependent on concentration of dopants like fluorine and germania, radial diffusion of such dopants causes the fiber cores that are heated in a platinum wire furnace to come closer together. Such proximity leads to the phenomenon of evanescent field interaction or coupling of optical power from one arm of the coupler to the other. The time evolution of the coupling process can be predicted in theory. While initial results are promising, the ability to automate the manufacture of couplers will be successful only after greater control over the variables is established. It is the intention of this work to understand the physics behind the mechanism as well as to prove the feasibility of modeling real world phenomena under controlled conditions. / Master of Science

This thesis deals with coplanar directional and hybrid couplers. Firstly, the thesis describes the theory of hybrid microwave integrated circuits with their advantages and disadvantages in comparison with other types of microwave circuits. Next, the thesis deals with basic microstrips and individual types of coplanar strips and waveguides are also more closely analyzed. The thesis also explains a theory of directional and hybrid couplers and analyses particular kinds of directionality and attributes of the real directional couplers. The following chapter shows a method of design of a few coplanar directional coupler types. Attributes of the types are then verified in CST Microwave Studio. The last chapter includes measured parameters of the couplers and their comparison with the theoretical values from the previous part. The next aim of the thesis is to create laboratory tasks which deals with coplanar directional couplers. The laboratory task is found in the appendix.

For size miniaturization and cost reduction, the design of dual band devices has become an emerging research area in recent years. A desirable dual-band solution should offer size compactness, high performance (e.g. low insertion loss) and compatible with conventional printed circuit broad (PCB) technology, especially microstrip lines. / In this research, several new devices, including rat-race coupler, power divider and crossover junction, capable of operating at dual frequency bands are proposed. These structures involve only simple branch-line sections and a minimal number of shunt stubs. All characteristic impedances are ranged from 20 O to 100 O. Most designs can operate with wide frequency spacing between the two bands. These designs offer low insertion loss as well as good return loss performances, and are small in size, in compared to the broadband approach. For design purposes, explicit closed-form equations are derived for the evaluation of circuit parameters. In addition, the usable range of these devices with respect to frequency band separation is examined. For verification, various prototypes are constructed by using microstrip technology and in-house fabrication facilities. Both simulated and measured results are presented and compared with state-of-the-art examples. / Microwave passive couplers are widely used in microwave and millimeter-wave applications and communication systems. Common examples are branch line coupler, rat race coupler, power divider, and crossover junction. They are used for the dividing, combining and re-directing of signal power. / Very often, a passive coupler utilizes simple quarter-wavelength transmission lines for implementation which will lead to narrow-band operation. Therefore, it is difficult to deploy such circuit for wide-band or multi-band applications. Multi-section topologies may be used to broaden the operating bandwidth, with which the major drawbacks are enlarged circuit size and the requirement of extreme high (or low) branch-line characteristic impedances. Both are not attractive for mass and low cost production. Conventional design approaches are, therefore, not suitable for modern communication systems with multi-band operation. / Wong, Fai Leung. / Adviser: Michael Cheng. / Source: Dissertation Abstracts International, Volume: 73-06, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 118-122). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

by Choi Kup Sze. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references. / Acknowledgement / Abstract / Chapter 1. --- Introduction --- p.1-1 / Chapter 1.1 --- Introduction --- p.1-1 / Chapter 1.2 --- References --- p.1-6 / Chapter 2. --- Theory of Optical Waveguides --- p.2-1 / Chapter 2.1 --- Theory of Planar Slab Waveguides --- p.2-2 / Chapter 2.2 --- Theory of Channel Dielectric Waveguides --- p.2-13 / Chapter 2.2.1 --- Marcatili's Method --- p.2-13 / Chapter 2.2.2 --- Effective Index Method --- p.2-20 / Chapter 2.3 --- References --- p.2-24 / Chapter 3. --- A Numerical Method for Optical Waveguides --- p.3-1 / Chapter 3.1 --- Introduction --- p.3-1 / Chapter 3.2 --- two-dimensional Fourier Series Expansion Method --- p.3-2 / Chapter 3.3 --- References --- p.3-13 / Chapter 4. --- Theory of Directional Couplers --- p.4-1 / Chapter 4.1 --- Dual-Channel Coupler --- p.4-1 / Chapter 4.2 --- Multi-channel Directional Coupler --- p.4-8 / Chapter 4.3 --- References --- p.4-9 / Chapter 5. --- Waveguide Formation by Ion Implantation --- p.5-1 / Chapter 5.1 --- Introduction --- p.5-1 / Chapter 5.2 --- Physics of Ion Implantation --- p.5-3 / Chapter 5.3 --- Lattice Damage and Annealing --- p.5-5 / Chapter 5.3.1 --- Lattice Damage --- p.5-5 / Chapter 5.3.2 --- Annealing --- p.5-6 / Chapter 5.4 --- Index Change due to Implantation --- p.5-8 / Chapter 5.5 --- Waveguide Processing Techniques --- p.5-10 / Chapter 5.5.1 --- Photolithography --- p.5-10 / Chapter 5.5.2 --- Processing Techniques --- p.5-11 / Chapter 5.6 --- References --- p.5-13 / Chapter 6. --- Optical Loss in Waveguides --- p.6-1 / Chapter 6.1 --- Loss Mechanisms in Optical Waveguides --- p.6-1 / Chapter 6.2 --- Principle of Propagation Loss Measurement --- p.6-4 / Chapter 6.2.1 --- Cut-back Method --- p.6-5 / Chapter 6.2.2 --- Scattering Light Method --- p.6-7 / Chapter 6.2.3 --- Fabry-Perot Interference Technique --- p.6-9 / Chapter 6.3 --- References --- p.6-16 / Chapter 7. --- Fabrication and Measurement of Optical Waveguides --- p.7-1 / Chapter 7.1 --- Fabrication of Optical Waveguides --- p.7-1 / Chapter 7.1.1 --- Fabrication of waveguides in GaAs by MeV oxygen ion implantation --- p.7-1 / Chapter 7.1.2 --- Waveguide End Facet Preparation --- p.7-4 / Chapter 7.2 --- Measurement of Optical Waveguides --- p.7-7 / Chapter 7.2.1 --- Laser Sources --- p.7-7 / Chapter 7.2.2 --- Guided Wave Excitation --- p.7-10 / Chapter 7.2.3 --- Intensity Profile Measurement --- p.7-17 / Chapter 7.2.4 --- Coupling Coefficient Measurement --- p.7-20 / Chapter 7.2.5 --- Propagation Loss Measurement --- p.7-25 / Chapter 7.3 --- References --- p.7-34 / Chapter 8. --- Results and Discussions --- p.8-1 / Chapter 8.1 --- Near Field Pattern Measurement --- p.8-1 / Chapter 8.2 --- Discussion on the Index Change of the Implanted GaAs --- p.8-5 / Chapter 8.3 --- Propagation Loss Measurement --- p.8-8 / Chapter 8.4 --- Observation of Optical Coupling in Directional Coupler --- p.8-14 / Chapter 8.5 --- References --- p.8-19 / Chapter 9. --- Conclusion --- p.9-1 / Chapter 10. --- Improvement and Extension --- p.10-1 / Appendix 1 Evaluation of the product〈n2 φuvφu'v'〉 --- p.A1-1 / Appendix 2 Transmission of Lossy Fabry-Perot Cavity --- p.A2-1 / Appendix 3 Effective Index versus Index Difference --- p.A3-1 / Appendix 4 Effect of Temperature on the Transmission of a Fabry-Perot Cavity --- p.A4-1 / Appendix 5 Evaluation of An from the Near Field Pattern --- p.A5-1

在現代通信系統，無線服務的需求不斷增加，帶動了通信系統，支持多標準的操作需要。 雙波段或多波段操作幾乎都是必要的，能夠提供這些操作的微波器件已成為減小尺寸和降低成本有吸引力的解決方案。 / 分支線耦合器是用於微波和毫米波應用的最流行的無源電路之一。它們通常用於平衡放大器和混頻器去實現良好的回波損耗以及隔離。其中一個至關重要的部份是設計一個可以靈活作多波段分配的分支線耦合器。 / 傳統上，完全平面的實施，雙波段分支線耦合器可以通過短截線，階梯阻抗線，耦合線等不同的分佈式結構實現。不同的設計方案已在這幾年來出現。窄帶操作和複雜的電路設計，是以前的設計的主要缺點。雖然，在理論上，多節技術可以拓寬帶寬，但它的主要缺點是電路的面積變大了及使用極端低/高傳輸線阻抗。因此，它不是一個大量和低成本生產的解決方案。 / 在這項研究中，設計了全新的並增強了性能的雙波段分支線耦合器(零分貝和三分貝的功率分裂)。這些設計能在兩個指定的頻帶有不平等的工作帶寛。通過正確選擇雙頻四分之一波長阻抗變換器的電氣長度和線路阻抗，傳輸相位斜率將能夠被控制並給出帶寬不對稱的特點，其性能可以進一步擴展，涵蓋了廣泛的應用。 / 以上所有設計都只需要單層線路版的制作及可實現的傳輸線阻抗。應用奇/偶模式分析所給出設計公式。這些設計具有低損耗，佈局靈活，緊密的尺寸大小的特性。這些設計己經使用標準微帶的結構實現其特點，其結果得到了實驗結果的進一步驗證。分支線耦合器只需要更小的節數就能實現相同的性能。 / In modern communication systems, the increasing demand for wireless services has driven the need for communication systems that support multi-standard operations. Dual-/Multi- band operation is almost a necessity and the adoption of microwave multi-band devices has become an attractive solution towards size and cost reduction of RF frontend designs. / Branch-line coupler is one of the most popular passive circuits used for microwave and millimeter-wave applications. They are commonly used in balanced amplifiers, phase-shifter, mixer and frequency multipliers for achieving good return loss, as well as isolation. It is therefore essential to have a branch-line coupler with multi-band operation. / Traditionally, for fully planar implementation, the construction of dual-band branch-line couplers are usually accomplished by distributed structures based upon shunt-stub, stepped-impedance line, coupled line etc. Narrow-band operation and circuit complexity are the major drawbacks for these previous designs. Although, in theory, the available bandwidth may be broadened by multi-section configurations, its major tradeoffs are the enlarged circuit size as well as the extreme line dimensions involved. Therefore, it is not preferable to mass and low cost production. / In this research, advanced designs of dual-band branch-line coupler (0 dB and 3 dB power splitting) with enhanced performances are presented. By proper selection of the number of sections, electrical lengths and line impedances of appropriate branch-lines of the coupler, its performance can be further extended to cover a wide range of applications. / All the proposed circuits require only single-layer fabrication and realizable line impedance. Closed form design formulas are made available by the application of even/odd- mode formulation. They feature low loss, flexible layout and compact size. The designs have been implemented and characterized using standard microstrip, and verified experimentally. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Yeung, Sung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 92-95). / Abstracts also in Chinese. / Abstract --- p.ii / 摘要 --- p.iii / Acknowledgement --- p.iv / Table of Content --- p.v / Lists of Figures --- p.viii / Lists of Tables --- p.xii / Chapter Chapter 1 --- : Introduction --- p.1 / Chapter 1.1 --- Research Motivation and Objective --- p.1 / Chapter 1.2 --- Original Contribution --- p.3 / Chapter 1.3 --- Research Approach, Assumptions and Limitations --- p.4 / Chapter 1.4 --- Overview of the Thesis Organization --- p.5 / Chapter Chapter 2 --- : Review of Microwave Coupler Design --- p.6 / Chapter 2.1 --- Coupler Design Fundamental --- p.6 / Chapter 2.1.1 --- Coupler Design with Equal Power Splitting --- p.7 / Chapter 2.1.2 --- Coupler Design with Unequal Power Splitting --- p.12 / Chapter 2.1.3 --- 0-dB Coupler or Crossover --- p.16 / Chapter 2.1.4 --- Coupler Design with Size Miniaturization --- p.18 / Chapter 2.1.5 --- Wide Band Coupler Design --- p.21 / Chapter 2.2 --- Dual-Band and Multi-Band Branch-line Coupler --- p.25 / Chapter 2.2.1 --- Dual-Band Couplers Based on Composite Right/Left-Handed Transmission Line --- p.25 / Chapter 2.2.2 --- Dual-Band Couplers with Shunt Stubs --- p.28 / Chapter 2.2.3 --- Dual-Band Coupler Based on Stepped-Impedance-Stub-Line --- p.30 / Chapter 2.2.4 --- Dual-Band Coupler with Port Extensions --- p.33 / Chapter 2.2.5 --- Tri-Band Coupler Based on Matching Network --- p.35 / Chapter 2.2.6 --- Multi-passband Branch-line Coupler Design --- p.37 / Chapter 2.3 --- Summary --- p.39 / Chapter Chapter 3 --- : A Novel Dual-band 0-dB Branch-line Coupler Design --- p.40 / Chapter 3.1 --- Proposed Circuit --- p.40 / Chapter 3.2 --- Analysis of Single-band 0-dB Branch-line Coupler --- p.43 / Chapter 3.3 --- Single- to Dual-band Conversion --- p.52 / Chapter 3.4 --- Experimental Results --- p.55 / Chapter 3.5 --- Summary --- p.58 / Chapter Chapter 4 --- : A Novel Dual-band 3-dB Branch-line Coupler with Unequal Bandwidth --- p.59 / Chapter 4.1 --- Proposed Dual-band Impedance Transformer: --- p.59 / Chapter 4.2 --- Single-band 3-dB Coupler Design --- p.65 / Chapter 4.3 --- Dual-band 3-dB Coupler Design --- p.70 / Chapter 4.4 --- Experimental Results --- p.76 / Chapter 4.4.1 --- Equal bandwidth design --- p.76 / Chapter 4.4.2 --- Unequal bandwidth design --- p.78 / Chapter 4.5 --- Summary --- p.81 / Chapter Chapter 5 --- : A Novel Dual-band 0-dB Branch-line Coupler Design with Unequal Bandwidth --- p.82 / Chapter 5.1 --- Proposed Circuit --- p.82 / Chapter 5.2 --- Analysis and Formulation --- p.84 / Chapter 5.3 --- Simulation Results --- p.85 / Chapter 5.4 --- Experimental Results --- p.87 / Chapter 5.5 --- Summary --- p.89 / Chapter Chapter 6 --- : Conclusion and Recommendation for Future Work --- p.90 / Chapter 6.1 --- Conclusion --- p.90 / Chapter 6.2 --- Recommendation for future work --- p.91 / References --- p.92 / Author’s Publications --- p.96 / Chapter Appendix 1: --- Brief Summary of Design Approaches of Hybrids Couplers --- p.97 / Chapter Appendix 2: --- Transformation between S- and ABCD- parameters for two-port network --- p.99

In this thesis, we have analyzed the coupling gap dependent micro-ring loss in a single ring all-pass filter configuration using the two dimension (2D) finite difference time domain (FDTD) and EIM (effective index method). We utilized a new analysis scheme by calculating the transmission signal as a function of input wavelength and fitting the transmission spectrum with a phenomenological ring loss parameter. This novel scheme circumvents the complex waveguide mode analysis process, when the coupling gap is narrow and the all-pass coupling region becomes multi-mode. We first find that the radiation loss increases rapidly with decreasing coupling gap width. Our results show that the intrinsic bending losses of silicon micro-rings (on oxide) with the radius of 1.5 £gm, 2.5£gm, and 5£gm are about 20 dB/cm, 3 dB/cm, 1 dB/cm for TE polarization modes, respectively. For TM modes, the intrinsic bending losses with the radius of 1.5£gm, 2.5 £gm, and 5£gm are about 2573 dB/cm, 64 dB/cm, and 0.8 dB/cm, respectively. Next, we find that power coupling coefficients of the single ring all-pass filter configuration using the ring to ring couplers are much higher than the bus to ring couplers. The radiation losses of the ring to ring couplers for TM modes are improved significantly in all coupling gap widths. In a high-density integrated optics circuit, specially designed ring coupling region device structure is needed to address this serious optical loss issue.

There is continued interest in nonlinear devices for different types of optical signal processing, such as Raman or parametric amplifiers. The small nonlinearity of conventional single-mode fibers sets a major limitation for these devices. A large nonlinearity can be achieved by having a large nonlinear coefficient, a small effective area, or both. Having a small effective area, however, requires efficient coupling to very small core fibers. A novel technique for splicing conventional single-mode fibers to small core fibers is proposed and demonstrated. The coupling efficiency obtained by this technique is considerably improved over that obtained by the butt-joint splice. This technique uses a highly tapered splice in which the field leaves the core and propagates as a fundamental cladding mode before it couples back to the core mode of the small core fiber. At the beginning of the taper the fundamental core mode carries most of the power. Over the down-taper region, the core mode couples to the fundamental cladding mode for which the cladding-air interface plays a major role in guiding the light. Over the up-taper region, the cladding mode is coupled back to the core mode. Fabrication of such a device involves many constraints. Alignment of the cores, the slope of the taper, and the taper length are important issues to ensure that excessive radiation loss does not take place. The theory of tapered single-mode fiber is discussed including adiabaticity criteria, length considerations, mode coupling and wavelength dependence. We use a computational simulation to examine how the field changes from one part of the taper to the other. Variations of the fiber and the field properties along the taper are studied. In this simulation, the tapered region is approximated as a sufficiently large number of cascaded uniform fiber segments of decreasing or increasing diameters. Another analysis based on the conservation of power flow is also provided. Tapered splices were fabricated using two different experimental setups. The experimental setup to verify our theoretical results is shown. The tapering process is thoroughly discussed. The spectrum of a tunable laser passing through a splice shows how modes interact with each other during the tapering process. We successfully fabricated very low loss tapers with extremely small diameters. Tapered splices showed a lower loss than their butt-joint counterparts. Experimental measurements of these tapered splices are presented and discussed. / Ph. D.