Edwin Jan Klein - Universiteit Twente

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Chapter 3 3.3 Geometrical design choices After the most suitable values for the micro-resonator parameters, such as the roundtrip losses and the coupling coefficients, have been found through careful examination of the system parameters, the actual micro-resonator can be designed. However, before the resonator can be designed, the decision has to be made on what kind of resonator to use. Up to this point the micro-resonators used in the analytical discussion and the various figures assumed a micro-resonator geometry where the resonant cavity is formed by a ring. This is not the only useful geometry, however. Depending on the application in which the micro-resonator is used other cavity geometries are also possible and may actually be superior to that of the ring resonator. 3.3.1 Micro-resonator geometry The three most commonly used resonator types are shown in Figure 3.13. These are the previously introduced ring resonator, the disc and the racetrack resonator. Figure 3.13a. Disc resonator. Figure 3.13b. Ring resonator. 50 Figure 3.13c. Racetrack resonator. Disc resonators, due to their single edge geometry, have inherently low losses compared to the other geometries. Disc resonators are therefore attractive for use in sensing systems [27] that, in order to achieve a high sensitivity, require many roundtrips of the light within the resonator. A downside to the disc resonator compared to the other geometries is that it is more susceptible to higher order modes which make it a less attractive option in telecom applications. In a ring-resonator these higher order modes can be suppressed by choosing the width of the ring in such a way that, while the fundamental mode is still propagating as a whispering gallery mode, higher order modes are suppressed by losses incurred through lower waveguide confinement and edge scattering. A problem that occurs for both the disc and ring geometries is that for small radii the coupling length between the resonator and port waveguides becomes very short, resulting in often undesirable low resonator coupling. Although this problem can in part be solved by reducing the coupling gap between the resonator and its port waveguides it is eventually limited by lithographical resolution and losses within the coupling section due to modal overlap losses [51]. A racetrack shaped resonator is created by elongating the coupling region of a ring resonator through the addition of a straight waveguide. This makes the racetrack resonator more effective than a ring resonator at small ring radii due its comparatively higher coupling with the port waveguides. In addition the coupling strength of a racetrack resonator is easier to adjust since it requires a change in the length of the
51 Design straight waveguide instead of a lithographically more challenging change in coupling gap width. A downside to using a racetrack resonator is that it can have substantially higher losses compared to disc or ring resonators. This is caused by transition losses that occur at the interfaces between the straight and ring waveguides where the propagating modes have to adapt to the changing geometry. These transition losses are of most concern at low contrasts where achieving a good modal overlap between straight and ring modes can be challenging. In high index materials such as silicon nitride and silicon, however, creating a low loss transition is less problematic because of better modal confinement. 3.3.2 Vertical or lateral coupling Besides deciding on the general geometry of the resonator a choice has to be made on how the resonator will be coupled to its port waveguides. The two options that can be considered for this are vertical and lateral coupling where each option has its own advantages and disadvantages. Ring Gap Figure 3.14. Cross-section, top-view and fabricated example of a vertically coupled resonator. Figure 3.15. Cross-section, top-view and fabricated example of a laterally coupled resonator. For vertical coupling the resonator and the port waveguides are placed in different layers as is shown for a ring resonator in Figure 3.14. For lateral coupling the Ring Gap
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DENSELY INTEGRATED MICRORING- RESON
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DENSELY INTEGRATED MICRORING- RESON
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To my parents…
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esonators is limited by the spacing
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schakelaar “aan” is, dan is de
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3.6.2 Overlap loss reduction.......
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Chapter 4 will propagate through th
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Chapter 4 As demonstrated by these
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Chapter 4 What is more interesting
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Chapter 4 method it does allow time
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Chapter 4 complex than the interact
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Chapter 5 5.1 Introduction The mate
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Chapter 5 wafer is used (step 1). T
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Chapter 5 range of 0.9 to 1.1 µm.
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Chapter 5 The process flow with the
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Chapter 5 5.3.3 Mask layout The big
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Chapter 5 5.4. Fabrication and mask
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Chapter 5 1. Definition of the vert
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Chapter 5 5.4.2 Fabrication The fab
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Chapter 5 Figure 5.21. Process flow
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Chapter 5 removed using lift-off of
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Chapter 6 6.1 Introduction About ha
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Chapter 6 In early, comparatively s
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Chapter 6 that the TEOS has a very
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Chapter 6 The most important aspect
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Chapter 6 6.2.3 Chromium heater des
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Chapter 6 Therefore, even if the si
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Chapter 6 Next, the heater was heat
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Chapter 6 6.3 Characterization of s
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Chapter 6 polarization maintaining
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Chapter 6 the resonators in this gr
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Chapter 6 Although the fitted coupl
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Chapter 6 a cladding thickness of 3
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Chapter 6 6.4 A wavelength selectiv
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Chapter 6 A possible explanation fo
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Chapter 7 Densely integrated device
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Densely integrated devices for WDM-
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7.2.2 Spectral measurements Densely
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Power Densely integrated devices fo
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7.3.4 Characterization Densely inte
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Figure 7.31a. Eye pattern of the re
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Chapter 8 8.1 Introduction In most
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Chapter 8 amount of power will be d
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Chapter 8 Another problem that may
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Chapter 8 halves that are each comp
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Chapter 9 Discussion and Conclusion
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Appendix A. Mason’s rule Several
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Appendix B. Crosstalk Derivation Th
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List of Acronyms ADSL - Asymmetric
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List of Symbols Q - Quality Factor
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Bibliography [1] Dan Schiller, “E
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Regions,” IEEE Photonics Technolo
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[66] B. E. Little, S. T. Chu, J. V.
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[97] H. Haeiwa, T. Naganawa and Y.
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[135] A. Driessen, D. H. Geuzebroek
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[165] T. Barwicz, M. R. Watts, M. A
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Publication List Patents (pending)
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R. Dekker, L.T.H.Hilderink, M.B.J.
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Networks (BB Photonics),” Interna
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Dankwoord/Acknowledgements Zo, dit
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Edwin Jan Klein - Universiteit Twente

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