Light propagation in tuneable nonlinear periodic photonic structures

The action of light in periodic structures can be quite different to that in a homogenous medium. For example, while a nonlinear beam will spread out in a medium with a negative nonlinearity, in a periodic structure the beam is focused and a localised state is formed. In this thesis I will show my work on light propagation in tuneable nonlinear periodic photonic structures. Nature provides us with dazzling displays of periodic photonic structures in the form of butterfly wings, peacock feathers, and opals. How these magnificent natural spectacles work has been a source of great scientific interest since we mastered the modern scientific method. With new technologies we can utilise periodic photonic structures to control how light propagates, which wavelengths are transmitted or reflected, and how light moves between waveguiding structures. Coupled waveguides provide a platform in which to study the linear and nonlinear light propagation and interaction in periodic photonic structures. Nonlinearity in optics provides a feedback mechanism which allows one beam of light to influence the propagation of another, or even itself. Advancements in our understanding of how light propagates and interacts in nonlinear periodic photonic structures is leading us to new and interesting areas of Physics. It is hoped that one day photons and photonic components can be used in place of electrons in electronic components widely used today. This will propel our computing power and further advance our understanding of the physical universe. In order to fully understand how light behaves in photonic structures and to make use of nonlinear features to allow light to control light, we first must understand the fundamental interactions of light in linear and nonlinear periodic photonic structures. We must be able to tune the properties of the system to investigate the fundamental behaviour of nonlinear beam propagation. In this thesis I investigate light propagation in tuneable nonlinear periodic photonic structures. I begin by introducing relevant concepts and ideas necessary to understand my work (Chapter 1). Included in this introduction is theoretical and experimental work I conducted with two interacting beams in a bulk nonlinear liquid (Sec. 1.4.6). I discover that a high power pump beam influences the nonlinear medium in a way which locally alters its refractive index. This alteration occurs due to a change in temperature of the medium caused by absorption of the pump beam and results in the reflection of a probe beam from the pump beam. I then present my research on the development of two platforms in which liquid is used to guide light in a one-dimensional (1D) periodic array. The first platform is made from photolithographically defined air-filled channels in SU8 polymer (Sec. 2.1). These channels are infiltrated with an index matching oil and the linear diffraction is observed as the temperature of the platform is changed. I find that the discrete diffraction observed matches very well with an accompanying theoretical model of the system, and I am able to estimate the temperature of the liquid in the channels. The second platform for light propagation in a 1D periodic array is developed using selectively infiltrated Photonic Crystal Fibres (Sec. 2.2). I use a simple method of blocking an inverse pattern with oil on one side of the fibre. The other end of the fibre is then submersed in a reservoir of the infiltrating liquid to fill any unblocked holes. I produce a 1D periodic array in a of coupled waveguides and demonstrate temperature tuneable linear diffraction, and nonlinear defocusing. I then move on to present my observation of truncated nonlinear Bloch waves in Lithium Niobate waveguide arrays (Sec. 2.3). Such states are excited with a broad Gaussian input beam in a 1D array of coupled nonlinear waveguides. This state is different from well known solitons and nonlinear Bloch modes because it contains features of both: a constant phase across all guiding waveguides characteristic of a nonlinear Bloch wave, with sharp edges otherwise seen in gap solitons. This work is supported by theoretical modelling, and I am able to show that the width of the soliton is dependant only on the width of the input beam, in contrast to discrete or gap solitons who's width depends on the nonlinearity. Chapter 3 then exhibits my work with liquid infiltrated Photonic Crystal Fibres as a two-dimensional (2D) periodic array of nonlinear waveguides. Firstly I show the existence and excitation conditions of nonlocal gap solitons (Sec. 3.1), where the properties of the system far from the light field influence soliton formation. I find that below a certain refractive index contrast these solitons are no longer excitable and the beam only defocuses. I then present my work on this crossover from focusing to defocusing in nonlinear periodic systems (Sec. 3.2). I show that the bandgap closes before the index contrast reaches zero, and that the system crosses from focusing to defocusing before the bandgap is fully closed. I will finally discuss my theoretical and experimental work on vortex beams propagating around a surface in a nonlinear hexagonal array (Sec. 3.3). I use liquid infiltrated Photonic Crystal Fibres and propagate a vortex beam around the core defect of the fibre. I find that nonlinear vortex modes of charge one are unstable and will focus to occupy a single waveguide on the surface of the core using the discrete model. A continuous model shows that linear and nonlinear charge one vortex modes are unstable and result in an asymmetric output. Linear charge three vortex modes show greater stability due to the staggered phase profile of the input beam, while nonlinear charge three vortex modes lose symmetry at the output due to a loss of this phase profile. I will finish this thesis with conclusions about my work and ideas for future directions this work could take, including specific experimental ideas directly related to this work. I will include some ideas as to the future direction these ideas may provide

Self-Determination, Democracy and Exclusion

This thesis addresses the question of whether the putative group right of collective selfdetermination can be invoked to justify the state’s presumed right to exclude foreigners from its territory. This form of argument represents a growing trend in the literature on exclusion. Such arguments are potentially extremely powerful, as, if successful, they would vindicate a right to exclude largely at the discretion of the receiver state, rather than a right limited to certain cases. It is here argued that theorists who share certain commitments – here referred to as “liberal individualists” - cannot consistently defend a discretionary right to exclude through this approach. The argument proceeds by way of three “case studies”. First, it is shown that the right to collective self-determination cannot be justified as an extension of individual autonomy. Recognising this, theorists with broadly individualist commitments defend the group right to self-determination on the basis of some irreducibly collective value it is said to serve – collective property, and collective freedom of association. Yet it is here argued these theorists are unsuccessful, as a result of their inability to square an irreducibly collective value with their individualistic commitments. It is then suggested that the nature of democratic institutions may be able to explain why we ought to respect group rights accorded to states in the same way we respect the autonomy of individuals. It is argued, however, that no realistically achievable system of democracy would be capable of furnishing such an explanation. An assessment of the reasons these three forms of argument fail provides evidence for a more definite claim: any defence of a right to collective selfdetermination that could justify a discretionary right to exclude must involve the wholesale rejection of moral individualism, the thesis that there are no intrinsically collective goods

Characterising latency for AO optical sensors: An implementation

The latency of electro-optical components is of high importance in the design of Adaptive Optics systems, as it limits the performance of the control loop. There exists a need for a latency measurement method that can be constructed with simple components found in most Adaptive Optics labs that still provides a measurement accurate to sub-microseconds. Through a combination of research and experimentation, potential methodologies were investigated with the aim of producing reliable latency measurements. This document will discuss one such method, involving coupling a LED pulse output and detected pulse input signals to the same clock for easy comparison. For this method, a proof-of-concept was developed using MATLAB and small analogue electronics, and the performance characterised. This characterisation showed that although there is some merit to the method, improvements are necessary to increase the precision of the measurement to a level usable in Adaptive Optics systems

Single detector stereo-SCIDAR for Mount Stromlo

Satellite tracking and imaging is conducted by the ANU Research School of Astronomy and Astrophysics and Electro-Optic Systems (EOS) at Mount Stromlo Observatory, Canberra, Australia, as part of the Space Environment Management Cooperative Research Centre (SERC) to support the development in space situational awareness. Atmospheric turbulence leads to distortions in the measured data. Adaptive optics (AO) systems counteract those distortions and improve the resolution of the tracking and imaging systems. To assist in the design of the AO systems, we need to gather information on the atmosphere at Mount Stromlo: r0, τ 0, and the turbulence Cn2 profile. With the SCIntillation Detection And Ranging (SCIDAR) Technique the scintillation of two stars is measured and their autocorrelation function is computed, providing a measurement of the turbulence profile. This technique usually uses one detector recording the two images of the stars simultaneously. However, the images overlap leading to an underestimation of the Cn2 values. The introduction of stereo-SCIDAR1 over- comes this issue by separating the two stars and imaging them on two separate image sensors. To reduce costs, we introduce a new stereo-SCIDAR system separating the beams from the two stars, but using only one single detector. This has been shown for a Low Layer SCIDAR (LOLAS) system with wide double stars (200 arcsec). We investigate this technique by detecting the scintillation patterns of double stars with separation from 10 to 25 arcsec, allowing some flexibility in the altitude span and resolution, while retaining a simple optical setup. We selected a low noise sCMOS camera as the imager. We show the current design of this system and investigate its feasibility for further development

Through Einsteins Eyes

We have developed a relativistically accurate computer graphics code, and have used it to produce photo-realistic images and videos of scenes where special relativistic effects dominate, either in astrophysical contexts or in imaginary worlds where the speed of light is only a few metres per second

Lens mounting techniques for precise radial location of fragile lenses in the NGS2 and Veloce instruments

We present novel methods for mounting lenses in a pair of instruments that presented challenging optical and mechanical requirements. The first instrument is the replacement Natural Guide Star Sensor (NGS2) for CANOPUS at Gemini South, which incorporates an objective consisting of a stack of six lenses mounted in a common bore. A compliant radial spacer was used to eliminate lens decentre resulting from the additional radial clearance required to accommodate differential thermal strains between the low thermal expansion lenses and a common bore. In the same instrument, tangent contact toroidal spacers were deployed in place of traditional conical spacers to further reduce contact stresses in fragile calcium fluoride lens elements. The toroidal faces were specified with a 10µm profile tolerance to avoid possible edge contact between the spacers and lenses. We investigated milling and turning machining processes for the production of the spacers by comparing their results via Coordinate Measuring Machine (CMM) measurements. In the second instrument, Veloce, built for the Anglo-Australian Telescope, a lens decentre requirement of 40µm led us to develop a simple means of in-situ centring adjustment of the cell mounted lens. Physical testing of the finished instruments verified the performance of each of these methods. NGS2 produced images at the factory acceptance test in which 94% of encircled energy was captured by a single 16um detector pixel, surpassing the specification of 80%. Bench testing of Veloce during assembly showed that the adjustment mechanism allowed centring of the lens over a range of +/- 0.1mm with a precision of 5µm

AO corrected satellite imaging from Mount Stromlo

The Research School of Astronomy and Astrophysics have been developing adaptive optics systems for space situational awareness. As part of this program we have developed satellite imaging using compact adaptive optics systems for small (1-2 m) telescopes such as those operated by Electro Optic Systems (EOS) from the Mount Stromlo Observatory. We have focused on making compact, simple, and high performance AO systems using modern high stroke high speed deformable mirrors and EMCCD cameras. We are able to track satellites down to magnitude 10 with a Strehl in excess of 20% in median seeing

Availability, outage, and capacity of spatially correlated, Australasian free-space optical networks

Network capacity and reliability for free space optical communication (FSOC) is strongly driven by ground station availability, dominated by local cloud cover causing an outage, and how availability relations between stations produce network diversity. We combine remote sensing data and novel methods to provide a generalised framework for assessing and optimising optical ground station networks. This work is guided by an example network of eight Australian and New Zealand optical communication ground stations which would span approximately $60^\circ$ in longitude and $20^\circ$ in latitude. Utilising time-dependent cloud cover data from five satellites, we present a detailed analysis determining the availability and diversity of the network, finding the Australasian region is well-suited for an optical network with a 69% average site availability and low spatial cloud cover correlations. Employing methods from computational neuroscience, we provide a Monte Carlo method for sampling the joint probability distribution of site availabilities for an arbitrarily sized and point-wise correlated network of ground stations. Furthermore, we develop a general heuristic for site selection under availability and correlation optimisations, and combine this with orbital propagation simulations to compare the data capacity between optimised networks and the example network. We show that the example network may be capable of providing tens of terabits per day to a LEO satellite, and up to 99.97% reliability to GEO satellites. We therefore use the Australasian region to demonstrate novel, generalised tools for assessing and optimising FSOC ground station networks, and additionally, the suitability of the region for hosting such a network.Comment: Accepted in Journal of Optical Communications and Networking. 16 pages, 16 figure

Adaptive optics tracking and pushing system for space debris manoeuvre

As space debris in lower Earth orbits are accumulating, techniques to lower the risk of space debris collisions must be developed. Within the context of the Space Environment Research Centre (SERC), the Australian National University (ANU) is developing an adaptive optics system for tracking and pushing space debris. The strategy is to pre-condition a laser launched from a 1.8 m telescope operated by Electro Optics Systems (EOS) on Mount Stromlo, Canberra and direct it at an object to perturb its orbit. Current progress towards implementing this experiment, which will ensure automated operation between the telescope and the adaptive optics system, will be presented.The authors would like to acknowledge the support of the Cooperative Research Centre for Space Environment Management (SERC Limited) through the Australian Government’s Cooperative Research Centre Programm