Optical systems are scalable under low-intensity illumination since their governing equations are linearly dependent of the optical signal strength. Nonetheless, in high-intensity regimes, the induced polarization becomes nonlinear, rendering the simple scalability of the previous systems invalid. Despite this, canonical nonlinear phenomena such as filamentation and high-harmonic generation in free space have recently been demonstrated to be scalable. Here we will discuss the extension of the scale-invariance paradigm to stimulated Raman scattering and molecular modulation in hollow anti-resonant fibers filled with Raman-active gases. We have found that the complex in-fiber dynamics can be accurately reproduced under very different conditions by keeping the so-called gain reduction factor, that accounts for the coupling of the interacting fields, as well as the dephasing time unaltered. Such scaling strategy enables access to equivalent nonlinear propagation scenarios without sacrificing performance, laying the foundations for the design of nonlinear devices operating in exotic frequencies, like the ultraviolet, or quantum frequency convertors of non-classical light.
Optical systems are scalable under low-intensity illumination since their governing equations are linearly dependent of the optical signal strength. Nonetheless, in high-intensity regimes, the induced polarization becomes nonlinear, rendering the simple scalability of the previous systems invalid. Despite this, canonical nonlinear phenomena such as filamentation and high-harmonic generation in free space have recently been demonstrated to be scalable. Here we will discuss the extension of the scale-invariance paradigm to stimulated Raman scattering and molecular modulation in hollow anti-resonant fibers filled with Raman-active gases. We have found that the complex in-fiber dynamics can be accurately reproduced under very different conditions by keeping the so-called gain reduction factor, that accounts for the coupling of the interacting fields, as well as the dephasing time unaltered. Such scaling strategy enables access to equivalent nonlinear propagation scenarios without sacrificing performance, laying the foundations for the design of nonlinear devices operating in exotic frequencies, like the ultraviolet, or quantum frequency convertors of non-classical light.
Backward stimulated Raman scattering in gases provides a promising route to compression and amplification of a Stokes seed-pulse by counter-propagating against a pump-pulse, as has been already demonstrated in various platforms, mainly in free-space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backward Raman gain is comparable with, but lower than, the forward gain, we report quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm from a narrowband 532-nm-pump. The efficiency increases to 65% when the backward process is seeded by a small amount of back-reflected forward-generated Stokes light. At high pump powers the backward Stokes signal, emitted in a clean fundamental mode and spectrally pure, is unexpectedly always stronger than its forward-propagating counterpart. We attribute this striking observation to the unique temporal dynamics of the interacting fields, which cause the Raman coherence (which takes the form of a moving fine-period Bragg grating) to grow in strength towards the input end of the fiber. A good understanding of this process, together with the rapid development of novel anti-resonant-guiding hollow-core fibers, may lead to improved designs of efficient gas-based Raman lasers and amplifiers operating at wavelengths from the ultraviolet to the mid-infrared.
Backward stimulated Raman scattering in gases provides a promising route to the compression and amplification of a Stokes seed pulse by counter-propagating against a pump pulse, as has been demonstrated already in various platforms, mainly in free space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report, to the best of our knowledge, the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backward Raman gain is comparable to, but lower than, the forward gain, we report quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm from a narrowband 532 nm pump. Efficiency increases to 65% when the backward process is seeded by a small amount of back-reflected forward-generated Stokes light. At high pump powers, the backward Stokes signal, emitted in a clean fundamental mode and spectrally pure, is unexpectedly always stronger than its forward-propagating counterpart. We attribute this striking observation to the unique temporal dynamics of the interacting fields, which cause the Raman coherence (which takes the form of a moving fine-period Bragg grating) to grow in strength toward the input end of the fiber. A good understanding of this process, together with the rapid development of novel anti-resonant-guiding hollow-core fibers, may lead to improved designs of efficient gas-based Raman lasers and amplifiers operating at wavelengths from the ultraviolet to the mid-infrared.
Raman scattering underlies a broad range of spectroscopic and light-generation techniques, yet its conventional description, based on the Raman gain spectrum, accurately describes only long-pulse, steady-state dynamics. We employ a time-domain theoretical approach to account for spectrotemporal aspects of continuous frequency-shifting, which are not addressed by prior theories. In particular, molecules with strong Raman responses do not produce an efficient soliton self-frequency shift in gas-filled hollow-core fibers. The time-domain analysis exposes temporal and spectral distortions from the Raman response that impact frequency-shifting detrimentally, and identifies how these distortions can be suppressed by reducing the Raman interaction to a perturbation on the electronic response. Experiments that employ gas mixtures with tunable Raman fractions of the nonlinear response demonstrate up to a four-fold increase in quantum efficiency (from 20% to 80%) compared to the pure molecular gas, and unity-efficiency Raman shifting will be possible. The time-domain framework uncovers phenomena that are inaccessible through the frequency-domain treatment of Raman scattering, and it applies to Raman interactions in solids, liquids, and gases across Raman temporal regimes.
Anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers is discussed. It is shown that the efficient anti-Stokes generation observed under conditions of significant wave mismatch is caused by phase locking of the interacting fields. This leads to the establishment of a phase difference that is independent of the optical path. An optimization technique, based on the adjustment of the wave mismatch along a gas-filled hollow fiber using pressure control, is proposed. Anti-Stokes conversion efficiencies close to the theoretical maximum of 50% are predicted.
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
In 1964 Bloembergen and Shen predicted that Raman gain could be suppressed if the rates of phonon creation and annihilation (by inelastic scattering) exactly balance. This is only possible if the momentum required for each process is identical, i.e., phonon coherence waves created by pump-to-Stokes scattering are identical to those annihilated in pump-to-anti-Stokes scattering. In bulk gas cells, this can only be achieved over limited interaction lengths at an oblique angle to the pump axis. Here we report a simple system that provides dramatic Raman gain suppression over long collinear path lengths in hydrogen. It consists of a gas-filled hollow-core photonic crystal fiber whose zero dispersion point is pressure adjusted to lie close to the pump laser wavelength. At a certain precise pressure, stimulated generation of Stokes light in the fundamental mode is completely suppressed, allowing other much weaker phenomena such as spontaneous Raman scattering to be explored at high pump powers.