Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The concept of broadband coherent supercontinuum (SC) generation in all-normal dispersion
(ANDi) fibers in the near-infrared, visible and ultraviolet (UV) spectral regions
is introduced and investigated in detail. In numerical studies, explicit design criteria are
established for ANDi photonic crystal fiber (PCF) designs that allow the generation of
flat and smooth ultrabroad spectral profiles without significant fine structure and with
excellent stability and coherence properties. The key benefit of SC generation in ANDi
fibers is the conservation of a single ultrashort pulse in the time domain with smooth and
recompressible phase distribution. In the numerical investigation of the SC generation
dynamics self-phase modulation and optical wave breaking are identified as the dominant
nonlinear effects responsible for the nonlinear spectral broadening. It is further demonstrated
that coherence properties, spectral bandwidth and temporal compressibility are
independent of input pulse duration for constant peak power. The numerical predictions
are in excellent agreement with experimental results obtained in two realizations of ANDi
PCF optimized for the near-infrared and visible spectral region. In these experiments,
the broadest SC spectrum generated in the normal dispersion regime of an optical fiber
to date is achieved. The exceptional temporal properties of the generated SC pulses are
verified experimentally and their applicability for the time-resolved study of molecular
dynamics in ultrafast transient absorption spectroscopy is demonstrated. In an additional
nonlinear pulse compression experiment, the SC pulses obtained in a short piece of
ANDi PCF could be temporally recompressed to sub-two cycle durations by linear chirp
compensation. Numerical simulations show that even shorter pulse durations with excellent
quality can be achieved by full phase compensation. The concept is further extended
into the UV spectral regime by considering tapered optical fibers with submicron waist
diameter. It is shown that coherent SC spectra with considerable spectral power densities
in the usually hard to reach wavelength region below 300 nm can be generated using
these freestanding photonic nanowires. Although technological difficulties currently prevent
the fabrication of adequate nanofibers, the concept could be experimentally verified
by coherent visible octave-spanning SC generation in tapered suspended core fibers with
ANDi profile. The work contained in this thesis therefore makes important contributions
to the availability and applicability of fiber-based broadband coherent SC sources with
numerous high-impact applications in fundamental science and modern technology. / AFRIKAANSE OPSOMMING: Die konsep van breëband koherente superkontinuum (SK) in alles-normaal dispersiewe
(ANDi) vesels in die naby-infrarooi, sigbare en ultraviolet (UV) spektrale gebiede word
voorgestel en in detail ondersoek. In numeriese studies word eksplisiete ontwerpskriteria
vasgestel vir ANDi fotoniese kristal vesel (FKV) ontwerpe wat dit moontlik maak om plat
en gladde ultra-breë spektrale profiele te genereer sonder noemenswaardige fynstruktuur
en met uitstekende stabiliteit en koherensie eienskappe. Die sleutel voordeel van SK
genering in ANDi vesels is die behoud van ’n enkele ultrakort puls in tyd met ’n gladde
en saamdrukbare fase distribusie. In die numeriese ondersoek van die SK generering is die
dinamika van fase selfmodulering geïdentifiseer as die dominante nie-lineêre effek wat verantwoordelik
is vir die nie-lineêre spektrale verbreding. Daar word voorts aangetoon dat
die koherensie eienskappe, spektrale bandwydte en saamdrukbaarheid in tyd onafhanklik
is van die inset pulsduur vir konstante drywing. Die numeriese voorspellings stem uitstekend
ooreen met die eksperimentele resultate wat verkry is met twee ANDi FKVÕs
wat optimeer is vir die naby-infrarooi en sigbare spektrale gebied. In hierdie eksperimente
is die breedste SK spektrum gegenereer wat tot hede in die normaal dispersiewe regime
met ’n optiese vesel behaal is. Die besondere eienskappe van die genereerde SK pulse is
eksperimenteel bevestig en die toepasbaarheid vir tyd opgelosde studie van molekulêre
dinamika is gedemonstreer. In ’n addisionele nie-lineêre puls kompressie eksperiment is
SK pulse verkry in ’n kort stuk ANDi FKV wat in tyd saamgedruk kon word tot sub-twee
siklus tydsduur deur liniêre tjirp kompensering. Numeriese simulasies toon aan dat selfs
korter pulse met uitstekende kwaliteit behaalbaar is met volledige fase kompensasie. Die
konsep is verder uitgebrei na die UV spektrale gebied deur ’n koniese vesel te beskou met
sub-mikron diameter. Daar is aangetoon dat koherente SK spektra met noemenswaardige
spektrale drywing in die golflengte gebied onder 300 nm, wat gewoonlik as moeilik toeganklik
beskou word, bereik kan word deur hierdie vrystaande fotoniese nano-vesels aan
te wend. Alhoewel tegnologiese probleme die vervaardiging van voldoende nano-vesels
verhinder, kon die konsep eksperimenteel bewys word deur koherente sigbare oktaafspannende
SK te genereer in koniese gesuspendeerde kern vesels met ’n ANDi profiel
aan te wend. Die werk wat in die tesis vervat is, maak dus belangrike bydraes tot die
beskikbaarheid en toepasbaarheid van vesel gebaseerde breëband koherente SK bronne
met verskeie hoë impak toepassings in fundamentele wetenskap en moderne tegnologie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/17780 |
| Date | 12 1900 |
| Creators | Heidt, Alexander Matthias |
| Contributors | Rohwer, Erich G., Bartelt, Hartmut, Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 103 p. : ill. |
| Rights | Stellenbosch University |
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