Linear and Nonlinear Optics in Coupled Waveguide Arrays

The following thesis is comprised of four main areas of work. These are centred around the experimental observation of phenomena associated with both linear and non-linear optics in silicon photonic-wires. As a comparison, I also discuss a similar coupled-waveguide system; dual-core hollow-core photonic crystal fibre. To introduce the reader to this work, the first chapter will recap some undergraduate level theory; a general introduction to optical waveguides. It is not intended to be a complete theoretical picture, as many beautiful texts on optics already exist [1–3]. This chapter concerns itself only with the aspects of optics with which the author was intimately aware of throughout the completion of this thesis. Thereafter, the chapters become specific to the particular experiments undertaken. Each one follows a simple framework: examination of the relevant theory, extending upon that already discussed in the first chapter, a literature review and finally a discussion of the work I completed within this thesis. Chapter 2 is the only chapter not related to silicon based photonics. Here I discuss dual-core hollow-core photonic crystal fibres; including guidance mechanisms, fabrication methods and the numerical modelling techniques employed in my work. I will compare these numerical results to experimental results taken by colleagues at the university of Bath. Chapter 3 analyses linear propagation in arrays of silicon photonic wires. I extend the simple picture of light propagating in waveguides to discuss the di↵erent types of dispersion inherent in this system and how dispersion tailoring can be achieved; with reference to the other literature on this topic. Experimental results are examined and discussed. Chapters 4 and 5 discuss non-linear propagation in silicon photonic wire arrays; modulation instability and spatio-temporal solitons respectively. In each case I extend the ideas on non-linearity presented in Chapter 1 to explain both modulation instability and optical solitons. Detailed descriptions of the experiments undertaken, and associated numerical modelling completed are then discussed. Whilst the work I present is incomplete, I will discuss subsequent work performed by my colleagues at the University of Bath based on my initial work. Finally, Chapter 6 draws together my conclusions.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Modulational instability in a silicon-on-insulator directional coupler: Role of the coupling-induced group velocity dispersion

We report frequency conversion experiments in silicon-on-insulator (SOI) directional couplers. We demonstrate that the evanescent coupling between two subwavelength SOI waveguides is strongly dispersive and significantly modifies modulational instability (MI) spectra through the coupling induced group velocity dispersion (GVD). As the separation between two 380-nm-wide silicon photonic wires decreases, the increasing dispersion of the coupling makes the GVD in the symmetric supermode more normal and suppresses the bandwidth of the MI gain observed for larger separations

Supermode dispersion and waveguide-to-slot mode transition in arrays of silicon-on-insulator waveguides

In this Letter, we report group index measurements of the supermodes of an array of two strongly coupled silicon-on-insulator waveguides. We observe coupling-induced dispersion that is greater than the material and waveguide dispersion of the individual waveguides. We demonstrate that the system transforms from supporting the two supermodes associated with two coupled waveguides to the single mode of a slot waveguide within the investigated spectral range. During the cutoff of the antisymmetric supermode, an anti-crossing between the symmetric TM and antisymmetric TE supermodes has been observed

Evanescent coupling assisted four-wave mixing in a silicon-on-insulator directional coupler

Four-wave mixing (FWM) has been extensively explored in optical fibers and more recently in on-chip silicon-oninsulator (SOI) waveguides. A phase-matched FWM with a pair of degenerate pump photons generating and amplifying signal and idler photons is referred as modulational instability (MI). Following theory of FWM in waveguide arrays, we utilize evanescent couplings between neighboring waveguides to control the phase-matching condition in FWM. In experiments, a set of single-channel SOI nanowaveguides with the waveguide width decreasing from 380nm to 340nm demonstrate that changing the waveguide group velocity dispersion (GVD) at the pump wavelength from being anomalous to being normal makes MI gain gradually disappear. We also perform the same experiment with an array of two 380nm-wide SOI waveguide, and demonstrate that for the large separation of 900nm and 800nm, MI gain is present as for the single waveguide; while for the small separation of 400nm, the MI gain disappears. This transformation of phase-matching in FWM is attributed to the fact that the coupling induced dispersion changes the net GVD of the symmetric supermode from being anomalous for large separation to being normal for small separation. Our observation illustrates that the coupling-induced GVD can compete and exceed in value the GVD of a single SOI nanowaveguide. This creates a new previously unexplored degree of freedom to control FWM on chips