Exploring optical nonlinearity in gas-filled hollow core fibre

Abstract

The growing need for novel light sources in variety applications increases the demand for laser sources operating in many different range of spectrum. Despite the success in development of mid-infrared (mid-IR) lasers, which are essential in many applications such as: environmental science, bio-science and physics, there are still lack of reliable lasers in this range with existing fibre laser technology compatibility. Meanwhile the nonlinearity in gases has been explored extensively from the very beginning of nonlinear optics, however, new developments in pulsed lasers and fibre design provide opportunities for more applications. The introduction of Hollow Core Photonic Crystal Fibres (HC-PCF) has revolutionised the area of nonlinearity in gaseous media by offering a single-mode confined light beam for very long distances. In this thesis, the focus was on mid-IR pulse generation by Raman frequency conversion in a gas-filled HC-PCF. Due to reliable performance and compatibility of fibre lasers with HC-PCFs, and towards fully fiberized source, an erbium-doped fibre laser (1.55 μm) has been selected as the pump for this project. In order to reach as far as possible into the mid-IR region, hydrogen has been selected as the filling gas of fibre, due to its large frequency shift and high Raman gain. The large frequency shift and mid-IR operating range required a new fibre design with a broadband transmission window and relatively low loss in mid-IR. After studying conventional HC-PCF structures, the recently proposed Nested Anti-resonant Nodeless Fibre (NANF) has been selected as the most suitable option for the purpose of this thesis [71]. Two NANFs, made of silica and tellurite, have been designed and optimized through the use of the developed Finite Element Method (FEM) toolbox in this thesis for operating wavelengths at pump (1.55 μm) and 1st Stokes (4.35 μm). A novel design has also been introduced in NANFs which shows polarization maintaining feature as good as the latest state-of-the-art HC-PBGF type [77]. The proposed design also shows polarizing capability in addition to its polarization maintaining by presenting a large loss ratio (~30 dB) between different polarizations of propagating light through it. The pulse propagation throughout the hydrogen-filled NANFs has been investigated by modelling the Raman response of hydrogen and numerically solving the Generalized Nonlinear Schrödinger Equation (GNLSE). Simulations show promising results for frequency conversion towards mid-IR and the possibility of Raman lasers in this region by considering different gas and using the readily available air in HC fibres. Furthermore, in this work, the nonlinear dynamics of atmospheric air-filled HC fibres have been studied, ranging from Raman down conversion process to a high spectral power density supercontinuum spanning from 850 to 1600 nm. A semi-quantum model for air has been adopted and integrated into the GNLSE, which surpasses the limitations of simple model. Using the adopted model, the experimental results have been reproduced without any extra computational cost. A rigorous study has been performed on nonlinear dynamics of pulse propagation in air-filled HC fibres and the origin of many nonlinear phenomenon are identified.

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Identifiers

ORCID for Seyed Mohammad Abokhamis Mousavi: orcid.org/0000-0002-5250-2630

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

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