This thesis presents the development of photonic microcells for use as the host for coherent optics phenomena and related applications. A photonic microcell consists of a length of hollow-core photonic crystal fibre (HC-PCF) with a gas-filled core that is spliced to conventional optical fibre at either end to seal the gas within the fibre. Towards the goal of demonstrating and assessing the coherence properties of quantum optical effects in photonic microcells, the fabrication of two types of HC-PCF is presented. The established photonic bandgap HC-PCF offers extremely low transmission loss of ~10 dB/km over kilometre distances. However, the fibre has a limited transmission bandwidth of ~50 THz and exhibits modal coupling unfavourable for many applications. Work is presented on the tailoring of this fibre by control and shaping of the core-surround in order to improve its modal properties. A second type of HC-PCF is based on a large-pitch lattice, whose guidance relies on a new mechanism. This fibre exhibits a much improved bandwidth (>1000 THz) and has a relatively higher but still practical loss of ~1 dB/m. The development of photonic microcells at microbar pressure level and with low optical insertion loss is shown, an important step in the improvement of the technology for coherent optics applications which will take advantage of the extreme gas-laser interaction efficiency achieved in HC-PCF. Finally, quantum optical effects are demonstrated in HC-PCF and photonic microcells loaded with both the molecular gas acetylene and atomic vapour rubidium. The observation of electromagnetically induced transparency (EIT) in acetylene-filled HC-PCF represents the first such observation in a molecular gas, while the use of a photonic microcell allows a comparison of many experimental configurations to explore the coherence properties of coherent optical systems in the core of a HC-PCF. Furthermore, EIT is observed unambiguously in a rubidium loaded HC-PCF for the first time, and the anti-relaxation effects of a polymer coating demonstrated in this configuration.
This thesis presents the development of photonic microcells for use as the host for coherent optics phenomena and related applications. A photonic microcell consists of a length of hollow-core photonic crystal fibre (HC-PCF) with a gas-filled core that is spliced to conventional optical fibre at either end to seal the gas within the fibre. Towards the goal of demonstrating and assessing the coherence properties of quantum optical effects in photonic microcells, the fabrication of two types of HC-PCF is presented. The established photonic bandgap HC-PCF offers extremely low transmission loss of ~10 dB/km over kilometre distances. However, the fibre has a limited transmission bandwidth of ~50 THz and exhibits modal coupling unfavourable for many applications. Work is presented on the tailoring of this fibre by control and shaping of the core-surround in order to improve its modal properties. A second type of HC-PCF is based on a large-pitch lattice, whose guidance relies on a new mechanism. This fibre exhibits a much improved bandwidth (>1000 THz) and has a relatively higher but still practical loss of ~1 dB/m. The development of photonic microcells at microbar pressure level and with low optical insertion loss is shown, an important step in the improvement of the technology for coherent optics applications which will take advantage of the extreme gas-laser interaction efficiency achieved in HC-PCF. Finally, quantum optical effects are demonstrated in HC-PCF and photonic microcells loaded with both the molecular gas acetylene and atomic vapour rubidium. The observation of electromagnetically induced transparency (EIT) in acetylene-filled HC-PCF represents the first such observation in a molecular gas, while the use of a photonic microcell allows a comparison of many experimental configurations to explore the coherence properties of coherent optical systems in the core of a HC-PCF. Furthermore, EIT is observed unambiguously in a rubidium loaded HC-PCF for the first time, and the anti-relaxation effects of a polymer coating demonstrated in this configuration.
This thesis describes developments in the fabrication and applications of photonic microcells (PMCs). A PMC is a length of gas-filled hollow core-photonic crystal fibre (HC-PCF) that is hermetically sealed by splicing both ends to standard single mode fibre. A PMC enables integration of gas-filled HC-PCF into all-fibre systems with low insertion loss. PMCs have applications in coherent optics and metrology, where specific HC-PCF designs are advantageous. Two types of state-of-the-art HC-PCF, double bandgap HCPCF and large-pitch Kagome fibre, are fabricated. The double bandgap HC-PCF extends the usable bandwidth of the fibre by providing low loss guidance across two transmission bands. The large-pitch Kagome HC-PCF has a record low attenuation of 0.3 dB/m at 800 nm, while maintaining broadband guidance, which is partially attributed to the core shape. Three distinct developments in the field of PMCs are described. Firstly, a record length 20 metre acetylene-filled PMC is fabricated which is the key component in the first demonstration of an all-fibre slow and fast light system based on electromagnetically induced transparency. Secondly, a technique based on fibre tapering is presented which enables low loss integration of large core Kagome HC-PCFs into PMC form. Thirdly, micromirrors are developed and integrated with HC-PCF to confine light in the longitudinal dimension, providing a means to fabricate multi-pass PMCs. Two uncoated micromirrors are used to form a low finesse microcavity in Kagome HC-PCF, with record high fringe visibility using reflections from a silica/air junction. In collaboration with Kansas State University, an acetylene optical reference with sub-10 KHz accuracy and the first acetylene laser based on population inversion are demonstrated using Kagome HC-PCF.Finally, this thesis reports on rubidium vapour loading in HC-PCF with the ultimate aim being the production of a rubidium-filled PMC for applications in metrology. Preliminary results highlight the limited loading distance of the current technique and modified loading schemes are outlined accordingly.
This thesis describes developments in atomic vapour loading in hollow core photonic crystal fibre (HC-PCF) for fabrication of atomic vapour loaded photonic microcells (PMC). These developments have been targeted at addressing some of the issues associated with loading atomic vapours in confined waveguiding geometries such as increased dephasing and physio-chemical wall absorptions. Atomic vapour loaded HC-PCF and PMC’s have applications in laser metrology, coherent optics and magneto optical spectroscopy. State of the art HC-PCF have been fabricated for loading with atomic vapour including both photonic bandgap (PBG) guiding and inhibited coupling (IC) hypocycloidal core shape Kagome HC-PCF. Record loss of 70 dB/km has been achieved in IC hypocycloid core shape Kagome HC-PCF in the spectral region centred at 800 nm. This fibre retains excellent single mode propagation combined with large core and increased optical bandwidth in comparison with specialist PBG HC-PCF optimised for operation around 800 nm. Aluminosilicate sol-gel coatings have been developed and successfully applied to the inner core wall of HC-PCF’s to reduce the atomic vapour surface interaction. Confining atomic vapours in micron scaled HC-PCF results in increased dephasing rates because of the frequent atom wall collisions. Anti relaxation coating materials have been applied to the inner core wall and the longitudinal relaxation time has been measured in coated and uncoated fibres utilising a magneto optical technique. Additionally sub Doppler transparencies are investigated in anti relaxation coated and uncoated HC-PCF.
A photonic microcell (PMC) is a length of gas-filled hollow core-photonic crystal fiber (HC-PCF) which is hermetically sealed at both ends by splicing to standard single mode fiber. We describe advances in the fabrication technique of PMCs which enable large core Kagome-lattice HC-PCFs to be integrated into PMC form. The modified fabrication technique uses fiber-tapering to accommodate the large dimensions of the fiber and enables low loss splices with single mode fiber by reducing mode field mismatch. Splice losses as low as 0.6 dB are achieved between 1-cell defect Kagome HC-PCF and single mode fiber. Relative to the previously reported PMCs, which were based on photonic bandgap HC-PCF, the present Kagome HC-PCF based PMC provides broad optical transmission, surface mode-free guidance and larger core at the cost of slightly increased fiber attenuation (~0.2 dB/m). Therefore, the integration of this fiber into PMC form opens up new applications for PMC-based devices. The advantage of the large core dimensions and surface mode free guidance for quantum optics in gas-filled HC-PCF are demonstrated by generation of narrow sub-Doppler features in an acetylenefilled large core PMC.
This thesis reported on atomic spectroscopy for the development of alkaline atomic vapor photonic microcell (PMC). The work is motivated by reproducing the outstanding laboratory environment based performances achieved in the fields of frequency standard and coherent optics in highly compact and stand-alone devices that can be accessible to a wider scientific community or to a commercial market. In our case these future devices will be based a Hollow-core photonic crystal fiber (HC-PCF) filled with a gas phase material to form a PMC, and outstands with an ultra-long long interaction length and micrometric modal area. However, the micrometric scale of the fiber core harboring the atoms raises several technical and scientific challenges. Among the technical challenges, we list the development of efficient process for atom loading inside long hollow fiber with small core diameter, the suppression or mitigation of physio-chemical reactivity of the atoms (i.e. Rubidium) with the fiber core inner-surface silica etc. In parallel, the large surface-to-volume ratio of the fiber-core raises questions like the coherence relaxation dynamics and the nature and effect of the atom-surface interaction. The thesis reports on fiber-core inner surface coating with different materials to mitigate the physio-chemical reactions of the confined atoms with the surface, on tapering large core inhibited coupling Kagome HC-PCF, and splicing technique that ensures low splice loss and no atomic reactivity during the splicing process. In parallel, the thesis reports on a set of spectroscopy experiments to assess the relaxation dynamics of atoms inside HC-PCF and to report on novel sub-Doppler transparencies.
We report on progress in different hollow core photonic crystal fiber (HC-PCF) design and fabrication for atomic vapor based applications. We have fabricated a Photonic bandgap (PBG) guiding HC-PCF with a record loss of 107dB/km at 785nm in this class of fiber. A double photonic bandgap (DPBG) guiding HC-PCF with guidance bands centred at 780nm and 1064nm is reported. A 7-cell 3-ring Kagome HC-PCF with hypocycloid core is reported, the optical loss at 780nm has been reduced to 70dB/km which to the best of our knowledge is the lowest optical loss reported at this wavelength using HC-PCF. Details on experimental loading of alkali metal vapours using a far off red detuned laser are reported. This optical loading has been shown to decrease the necessary loading time for Rb into the hollow core of a fiber. The quantity of Rb within the fiber core has been enhanced by a maximum of 14% through this loading procedure.
In this thesis I study coherent light-matter interactions via stimulated Raman scattering (SRS) in a gas-filled hollow-core photonic crystal fiber (HC-PCF).The HC-PCF constitutes the foundation of the experimental results presented in this thesis, as without its unique properties realization of these experiments would not have been possible.These unique properties include tight confinement of laser light and matter in the small core of the fiber, which leads to extremely high conversion efficiencies in the SRS, and spectral filtering of unwanted nonlinear waves.This setup creates a clean system of two optical fields interacting via Raman medium inside the core of the fiber.This thesis consists of three parts.The first part (chapter 2) includes a general overview of the different types of HC-PCF, their fabrication techniques and guidance mechanisms.Two types of HC-PCF are considered: photonic bandgap HC-PCF and kagomé-HC-PCF.Guidance mechanisms of these two types of fibers are considered in detail.Of particular interest to me is the broadband guidance in kagomé-HC-PCF.Up to now, the mechanism responsible for this broadband guidance is not understood.A simplified semi-analytical model is derived which accounts for this broadband guidance in kagomé-HC-PCF and explain many of its guidance properties.In particular, using the model, the fiber loss as a function of wavelength is reproduced fairly well.The model also explains an important feature of kagomé-HC-PCF, namely, the insensitivity of its loss to the number of cladding layers.In the second part of the thesis (chapter 3), I set the basic theoretical formalism of SRS to be used in the rest of the thesis.The aim of the chapter is to derive the coupled wave equations that govern the evolution of the pump, Stokes and coherence fields using classical as well as semi-classical approaches.The third part of the thesis (chapters 4, 5, 6 and 7) discusses the results of experimental and theoretical investigations of my studies on SRS in gas-filled HC-PCF.I start with an overview of the already know results about SRS in HC-PCF and explain the mechanism of phase-locking for efficient generation of anti-Stokes SRS.This work clarifies high conversion efficiencies to anti-Stokes frequencies observed in early experiments.Most of these early experiments are focused on lowering the threshold for SRS generation and are done at relatively high pressures.In chapters 5 and 6, I use some of the unique properties of the HC-PCF to explore the coherent light-matter SRS interaction regimes not previously accessible.This includes the first experimental observation of backward superluminal solitary waves and self-similar solutions of SRS coupled-wave equations.These results represent a significant advance in the study of coherent effects and point to a new generation of highly engineerable gas cells for studying complex nonlinear phenomena.In chapter 7, I briefly overview the results presented in previous chapters and conclude my thesis by explaining possible improvements as well as new interesting directions in studying coherent light-matter interactions in gas-filled HC-PCF.In particular, I introduce a simple scheme for generating (purely rotational) broadband coherent frequency comb at low pump pulse energies using a gas-filled HC-PCF.Generation of such a frequency comb is quite interesting for synthesizing ultrashort pulses as well as a broad coherent tunable light source with high spectral brightness.The classic system of education is mainly based on the ability of deductive reasoning and knowledge of the classics.Quite often, in the course of learning that, we lose the joy and excitement of thinking.Scientific research, on the other hand, is not just about knowing and reasoning, but also thinking, creativity and courage of realizing new ideas.For that, I am hugely indebted to my supervisor, Prof. Philip Russell.Philip, thank you for teaching me the divergent way of thinking, to see many solutions to one problem, for giving me the courage to express my ideas and for providing excellent facilities and environment to realize them.I would also like to thank Prof. Curtis Menyuk for very nice discussions and comments about my thesis and specially the work he has done on self-similarity.Your deep insight and understanding of the physics and the beauty of mathematics behind it, is truly amazing and I really hope that in future I would have the opportunity to learn more from you.Many thanks go to Dr. Johannes Nold for who he is and