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arXiv:1703.04543 · Physical Review A

Control of ultrafast pulses in a hydrogen-filled hollow-core photonic-crystal fiber by Raman coherence

Federico Belli, A. Abdolvand, John C. Travers, Philip St.J. Russell

Published Jan 12, 2018• 25 citations
arXivHTMLPDF

Abstract

We present the results of an experimental and numerical investigation into temporally nonlocal coherent interactions between ultrashort pulses, mediated by Raman coherence, in a gas-filled kagome-style hollow-core photonic-crystal fiber. A pump pulse first sets up the Raman coherence, creating a refractive index spatiotemporal grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second, probe, pulse allows a high degree of control over its evolution as it propagates along the fiber through the grating. Of particular interest are soliton-driven effects such as self-compression and dispersive wave (DW) emission. In the experiments reported, a DW is emitted at $\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and exhibits a wiggling effect, with its central frequency oscillating periodically with pump-probe delay. The results demonstrate that a strong Raman coherence, created in a broadband guiding gas-filled kagome photonic-crystal fiber, can be used to control the nonlinear dynamics of ultrashort probe pulses, even in difficult-to-access spectral regions such as the deep and vacuum ultraviolet.

DOI
10.1103/physreva.97.013814

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Physical Review A•Feb 9, 2018•2 citations
We present the results of an experimental and numerical investigation into temporally nonlocal coherent interactions between ultrashort pulses, mediated by Raman coherence, in a gas-filled kagome-style hollow-core photonic-crystal fiber. A pump pulse first sets up the Raman coherence, creating a refractive index spatiotemporal grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second, probe, pulse allows a high degree of control over its evolution as it propagates along the fiber through the grating. Of particular interest are soliton-driven effects such as self-compression and dispersive wave (DW) emission. In the experiments reported, a DW is emitted at \ensuremath∼300nm\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}\ensuremath∼300nm and exhibits a wiggling effect, with its central frequency oscillating periodically with pump-probe delay. The results demonstrate that a strong Raman coherence, created in a broadband guiding gas-filled kagome photonic-crystal fiber, can be used to control the nonlinear dynamics of ultrashort probe pulses, even in difficult-to-access spectral regions such as the deep and vacuum ultraviolet.
The sidebands of stimulated Raman scattering initiated from vacuum fluctuations in hollow-core photonic crystal fiber filled with Raman gas exhibit surprisingly strong self- and mutual coherence [1] as well as persistence of coherence for much longer than the dephasing time T2[2]. This opens new prospects in a number of applications such as attosecond pulse generation and optical waveform synthesis. However, the spatiotemporal coherence dynamics underlying these results is not fully investigated and no rigorous theoretical simulation is available. This contribution discusses theoretical and experimental results that study this issue and explains the long-lived effect of the coherence on a delayed probe pulse in the pump-probe setup.
We report the first demonstration of group delay tuning with stimulated Raman scattering-induced dispersion in a hydrogen-filled hollow-core optical fiber.A pump laser induces a sharp refractive index change near the S 0 (0) Raman transition of hydrogen molecules, enabling the control of the group velocity of signal pulses around the Stokes wavelength.Experiments with an 80-m-long hollow-core fiber filled with 2.5 bar hydrogen achieved continuous tuning of the pulse delay up to 1.42 ns by varying the Raman amplification from 0 to 10 dB.The tunable pulse delay is realized by changing the pump power as well as the hydrogen pressure.This work provides a new technique for controlling the pulse propagation in optical fibers with high flexibility.
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
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.
We present an experimental and theoretical study of the stimulated Raman emission in hollow-core kagome waveguides in a pump-probe arrangement. We perform an experimental investigation of the power of the Stokes signal from the probe, which is below the stimulated Raman scattering threshold, as a function the pump-probe delay time. The results show the Stokes power to increase with pump-probe delay, reaching a maximum at 10 ns, and to decrease afterward. In view of a coherence decay time of only 0.25 ns, we demonstrate a surprisingly slow reduction of Stokes signal with the characteristic time much longer than the coherence decay time by a factor of up to 40. The numerical investigations explain the observed phenomenon as a result of the spatiotemporal dynamics of the probe pulse and Raman coherence. We show that the increase of the characteristic time can be related to the spatial position of the intense sideband generation event and its dependence on the pump-probe delay.