Novel ultrafast pulse propagation dynamics in hollow-core fibres
Abstract
In this thesis, I describe experimental and numerical work on understanding the complex nonlinear dynamics of the propagation of high-intensity laser pulses in gas-filled
hollow-core fibres (HCFs). The long interaction length and the ability to control the
dispersion and nonlinearity make HCF a great platform for exploring a wide variety of
nonlinear optical phenomena.
By employing high-order soliton dynamics, I experimentally demonstrated compression of µJ-level pulses directly from a 220 fs commercial pump laser to ∼ 13 fs in a single
stage without the need for external elements such as chirped mirrors. Moreover, I demonstrated the generation of wavelength-tunable sub-15 fs pulses through soliton-plasma interactions using the same commercial pump source. I temporally characterized the output
pulses using sum-frequency generation (SFG) cross-correlation frequency-resolved optical gating (XFROG).
Using extreme modulation instability (MI) dynamics, I demonstrated the generation
of a linearly flat supercontinuum (SC) extending from 350 nm up to 2 µm in argon-filled
broadband-guiding HCF. Moreover, I investigated the role of the Raman response on such
dynamics by using nitrogen-filled HCF. I found that due to the close rotational lines in N2,
gain suppression in the fundamental mode causes the pulse to be coupled into higher-order
modes (HOMs), which reduces the energy density of the SC.
Molecules can dissociate due to the high optical intensity of the propagating pulse, as
has been previously observed in filamentation experiments. By using molecular gases, I
was able to observe, for the first time, evidence indicating the dissociation of molecular
gases inside HCF. In particular, I observed the formation of ozone molecules inside the
fibre which is a result of the chemical reactions between the dissociated oxygen molecules
due to the high intensity of the propagating pulse. I studied the effect of such chemical
reactions on pulse propagation dynamics assisted by numerical modeling. In addition, by
using a gas mixture of molecular gases, I observed a novel phenomenon caused by the
chemical reaction between the different gas constituents.