Spatiotemporal nonlinear dynamics in graded-index multimode fibers
Author
Teğin, Uğur
Advisor
Ortaç, Bülent
Date
2018-05Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Spatiotemporal pulse propagation in multimode fibers is generally considered as
chaotic. Graded-index multimode fibers reduce the complexity due to its equal
spacing of the modal wave numbers which also introduces a periodic self-imaging
to the propagating beam. This unique phenomenon affects the coupling between
the modes thus graded-index multimode fibers are an ideal testbed to study spatiotemporal
pulse propagation. In this thesis, various spatiotemporal nonlinear
dynamics studied in graded-index multimode fibers to achieve wavelength conversion,
supercontinuum generation triggered by cascaded Raman scattering and
to develop a novel all-fiber all-normal dispersion mode-locked laser cavity.
In normal dispersion regime, spatiotemporal instability of femtosecond pulses
discovered numerically and experimentally by exciting a graded-index multimode
fiber with a Ti:Sapphire laser capable to generate 200 fs pulses at 800 nm. With
90 THz frequency shift, Stokes and anti-Stokes sidebands are observed. The
signature of spatiotemporal instability which allows the sidebands to inherit the
spatial distribution of the pump pulse is observed with the spatial characterization
of the generated sidebands.
Later a high power laser system with adjustable output parameters is developed
as a pump source for spatiotemporal nonlinear pulse propagation studies. By
employing this source, with MHz pump pulse repetition rate high power octavespanning
supercontinuum generation triggered by cascaded Raman scattering is
demonstrated. The results obtained with this novel method is the highest average
power and repetition supercontinuum source with a standard graded-index
multimode fiber in the literature.
Additional spatiotemporal wavelength conversion mechanisms, a small gradedindex
multimode fiber between single mode fiber segments can be used as a
bandpass filter and saturable absorber. These effects are combined in an all-
fiber all normal dispersion laser cavity for the first time in the literature. In the
demonstrated cavity design, mode-locking is achieved by nonlinear multimodal
interference in graded-index multimode fiber segment. All-normal cavity design
supports dissipative soliton pulse formation but it requires bandpass filtering.
This requirement is satisfied with multimode interference reimaging thus a unique
and simple all-fiber cavity design is constructed to generate ultrashort dissipative
soliton pulses. The developed oscillator generates 5 ps pulses at 1030 nm with 44
MHz repetition rate. These pulses are externally compressed to 276 fs. All-fiber
cavity design ensures stability and 70 dB sideband suppression is measured in
radio frequency domain.
Keywords
Fiber LasersGraded-index Multimode bers
Ytterbium Doped Bers
Nonlinear Ber Optics
Spatiotemporal Nonlinear Dynamics