A THz optical modulator for a continuous-wave laser using a hollow-core photonic crystal fiber (HC-PCF) was proposed and theoretically analyzed. Lightwaves traveling through the HC-PCF is modulated by interactions with coherently driven Raman-active molecules in the core. The coherent molecular motion is excited by a pulse train having an interval between successive pulses shorter than the molecular dephasing time. In principle, a rotational transition of molecular hydrogen (S 0 (1)) at a pressure of 1 atm has a long enough dephasing time to maintain molecular coherence during a 1 GHz commercially available mode-locked pulse train. Optimization of the waveguide dispersion would enable phase-matching between the probe beam and generated sidebands during optical modulation. The proposed scheme would be achievable with a reasonable pump beam power and HC-PCF length, and with a feasible pressure of molecules in the core.
A THz optical modulator for a continuous-wave laser using a hollow-core photonic crystal fiber (HC-PCF) was proposed and theoretically analyzed. Lightwaves traveling through the HC-PCF is modulated by interactions with coherently driven Raman-active molecules in the core. The coherent molecular motion is excited by a pulse train having an interval between successive pulses shorter than the molecular dephasing time. In principle, a rotational transition of molecular hydrogen (S 0 (1)) at a pressure of 1 atm has a long enough dephasing time to maintain molecular coherence during a 1 GHz commercially available mode-locked pulse train. Optimization of the waveguide dispersion would enable phase-matching between the probe beam and generated sidebands during optical modulation. The proposed scheme would be achievable with a reasonable pump beam power and HC-PCF length, and with a feasible pressure of molecules in the core.
We report on the efficient, tunable, and selective frequency up-conversion of a supercontinuum spectrum via molecular modulation in a hydrogen-filled hollow-core photonic crystal fiber. The vibrational Q(1) Raman transition of hydrogen is excited in the fiber by a pump pre-pulse, enabling the excitation of a synchronous, collective oscillation of the molecules. This coherence wave is then used to up-shift the frequency of an arbitrarily weak, delayed probe pulse. Perfect phase-matching for this process is achieved by using higher order fiber modes and adjusting the pressure of the filling gas. Conversion efficiencies of ~50% are obtained within a tuning range of 25 THz.
We report detailed experimental results on the optical nutation effect in acetylene-filled hollow-core photonic crystal fibers (HC-PCFs). This coherent quantum optical effect is manifested by the appearance of attenuated oscillations in the profiles of transmitted step-like coherent optical pulses. The experiments used 15 ns optical pulses with peak powers up to ∼4 W and centered at 1530.37 nm, which corresponds to the acetylene (12C2H2) vibration-rotation resonance absorption line P9. The all-fiber cell made from a HC-PCF with internal diameter ∼10.3 μm contained a gas pressure of about 0.12 Torr. The Maxwell-Bloch equations are used to analyze the influence of the homogeneous relaxation, the inhomogeneous Doppler broadening of the absorption line, and the random orientation of the acetylene molecules. By comparing the experimental results with numerical simulations, we obtain the longitudinal and the transverse relaxation times to be close to 10 ns and find that the transition dipole moment of the acetylene P9 absorption line is ∼1.36×10−32 Cm.
Summary form only given. The development of the Raman gas-filled hollow core photonic crystal fibers allow to exacerbate the light-matter interaction with extremely high efficiency. Among the nonlinear optical effect that were extensively explored we count stimulated Raman scattering, with demonstrations like over 5-octaves wide Raman comb using H2-filled inhibited coupling guiding HC-PCF pumped with a picosecond pulsed laser [1]. Furthermore, Raman-active gases are known to exhibit very weak susceptibility outside the Raman frequencies, and thus are considered to be poor media for the generation of new frequencies through parametric four-wave mixing (FWM).Here we demonstrate, for the first time to our knowledge, the generation of pairs of sidebands via parametric FWM in strongly driven Raman molecular gas. The experimental set-up used a 1030 nm Yb-fiber laser with average power of 15 W, a repetition rate of 1 MHz and pulse duration of 27 ps. The light is then coupled into a photonic microcell composed by a D2-filled inhibited coupling tubular-lattice single-ring HC-PCF [2]. The fiber exhibits a low-loss broadband guidance with a minimum of 10 dB/km around the pump wavelength and operates in almost a single mode regime with a mode field diameter of 28 μm. The D2 molecule has a vibrational Raman resonance around 89.8 THz, and rotational resonances of 12.4 THz for the ortho-D2 and 7.1 THz for the para-D2. The pressure of D2 was set at 40 bar to experimentally maximize the Raman lines from ortho-D2. The results are shown in the figure 1. The generated spectrum presents around 100 laser lines over a 200 THz wide spectral span with a dense cluster whose frequency-difference is under 2 THz by a ro-vib SRS/FWM combination. Four FWM sidebands can be identified (plotted in red color), generated in both sides of two para-rotational lines reproduced eight times around the pump radiation and three times around the first vibrational Stokes.
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
Experimental observations of nonstationary coherent optical phenomena, i.e., optical nutation, free induction, and photon echo, in the acetylene (12C2H2) filled hollow-core photonic-crystal fiber (PCF) are reported. The presented results were obtained for the acetylene vibration-rotational transition $P$9 at wavelength 1530.37 nm at room temperature under a gas pressure of 0.5 Torr. An all-fiber pumped-through cell based on the commercial 2.6-m-long PCF with a 10-\ensuremath{\mu}m hollow-core diameter was used. The characteristic relaxation time T2 during which the optical coherent effects were typically observed in our experiments was estimated to be \ensuremath{\approx}8 ns. This time is governed by the limited time of the acetylene molecules' presence inside the effective PCF modal area and by intermolecule collisions. An accelerated attenuation of the optical nutation oscillations is explained by a random orientation of acetylene molecules.