Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: There are many applications where a Gaussian laser beam is not ideal, for
example, in areas such as medicine, data storage, science, manufacturing and so
on, and yet in the vast majority of laser systems this is the fundamental output
mode. Clearly this is a limitation, and is often overcome by adapting the
application in mind to the available beam. A more desirable approach would be
to create a laser beam as the output that is tailored for the application in mind –
so called intra-cavity laser beam shaping. The main goal of intra-cavity beam
shaping is the designing of laser cavities so that one can produce beams directly
as the output of the cavity with the required phase and intensity distribution.
Shaping the beam inside the cavity is more desirable than reshaping outside the
cavity due to the introduction of additional external losses and adjustment
problems. More elements are required outside the cavity which leads to
additional costs and larger physical systems.
In this thesis we present new methods for phase and amplitude intra–
cavity beam shaping. To illustrate the methods we give both an analytical and
numerical analysis of different resonator systems which are able to produce
customised phase and intensity distributions.
In the introduction of this thesis, a detailed overview of the key concepts
of optical resonators is presented.
In Chapter 2 we consider the well–known integral iteration algorithm for
intra–cavity field simulation, namely the Fox–Li algorithm and a new method
(matrix method), which is based on the Fox–Li algorithm and can decrease the
computation time of both the Fox–Li algorithm and any integral iteration
algorithms. The method can be used for any class of integral iteration
algorithms which has the same calculation integrals, with changing integrants.
The given method appreciably decreases the computation time of these
algorithms and approaches that of a single iteration.
In Chapter 3 a new approach to modeling the spatial intensity profile from
Porro prism resonators is proposed based on rotating loss screens to mimic the
apex losses of the prisms. A numerical model based on this approach is presented which correctly predicts the output transverse field distribution found
experimentally from such resonators.
In Chapter 4 we present a combination of both amplitude and phase
shaping inside a cavity, namely the deployment of a suitable amplitude filter at
the Fourier plane of a conventional resonator configuration with only spherical
curvature optical elements, for the generation of Bessel–Gauss beams as the
output.
In Chapter 5 we present the analytical and numerical analyses of two new
resonator systems for generating flat–top–like beams. Both approaches lead to
closed form expressions for the required cavity optics, but differ substantially in
the design technique, with the first based on reverse propagation of a flattened
Gaussian beam, and the second a metamorphosis of a Gaussian into a flat–top
beam. We show that both have good convergence properties, and result in the
desired stable mode.
In Chapter 6 we outline a resonator design that allows for the selection of
a Gaussian mode by diffractive optical elements. This is made possible by the
metamorphosis of a Gaussian beam into a flat–top beam during propagation
from one end of the resonator to the other. By placing the gain medium at the
flat–top beam end, it is possible to extract high energy in a low–loss cavity. / AFRIKAANSE OPSOMMING: Daar is verskeie toepassings waar ʼn Gaussiese laser bundel nie ideaal is
nie, in gebiede soos mediese veld, stoor van data, vervaardiging en so meer, en
tog word die meeste laser sisteme in die fundamentele mode bedryf. Dit is
duidelik ’n beperking, en word meestal oorkom deur aanpassing van die
toepassing tot die beskikbare bundel. ’n Beter benadering sou wees om ʼn laser
bundel te maak wat afgestem is op die toepassing - sogenaamde intra-resonator
bundel vorming. Die hoofdoel van intra-resonator bundel vorming is om
resonators te ontwerp wat direk as uitset kan lewer wat die gewenste fase en
intensiteits-distribusie vertoon. Vorming van die bundel in die resonator is
voordeliger omdat die vorming buite die resonator tot addisionele verliese asook
verstellings probleme bydra. Meer elemente word benodig buite die resonator
wat bydra tot hoër koste en groter sisteme.
In hierdie tesis word nuwe fase en amplitude intra-resonator
bundelvormings metodes voorgestel. Om hierdie metode te demonstreer word
analitiese en numeriese analises vir verskillende resonator sisteme wat
aangepaste fase en intensiteit distribusies produseer, bespreek.
In die inleiding van die tesis word ʼn detailleer oorsig oor die sleutel
konsepte van optiese resonators voorgelê.
In hoofstuk 2 word die bekende integraal iterasie algoritme vir intraresonator
veld simulasie, naamlik die Fox-Li algoritme, en ʼn nuwe metode
(matriks metode), wat gebaseer is op die Fox-Li algoritme, en die
berekeningstyd van beide die Fox-Li algoritme en enige ander integraal iterasie
algoritme verminder. Die metode kan gebruik word om enige klas van integraal
iterasie algoritmes wat dieselfde berekenings integrale het, met veranderde
integrante (waar die integrand die veld van die lig golf is in die geval van die
Fox-Li algoritme, IFTA, en die skerm metode. Die voorgestelde metode
verminder die berekeningstyd aansienlik, en is benaderd die van ʼn enkel iterasie
berekening.
In hoofstuk 3 word ʼn nuwe benadering om die modellering van die
ruimtelike intensiteitsprofiel van Porro prisma resonators, gebaseer op roterende
verliese skerms om die apeks-verliese van die prismas te benader, voorgestel. ʼn Numeriese model gebaseer op hierdie benadering wat die uitset van die
transversale veld distribusie in eksperimentele resonators korrek voorspel, word
voorgestel.
In hoofstuk 4 word ʼn tegniek vir die generering van Bessel-Gauss bundels
deur die gebruik van ʼn kombinasie van amplitude en fase vorming in die
resonator en ʼn geskikte amplitude filter in die Fourier vlak van ʼn konvensionele
resonator konfigurasie met optiese elemente wat slegs sferiese krommings het,
voorgestel.
In hoofstuk 5 word die analitiese en numeriese analises van twee nuwe
resonator sisteme vir die generering van sogenaamde “flat–top” bundels
voorgestel. Beide benaderings lei na ʼn geslote vorm uitdrukking vir die
resonator optika wat benodig word, maar verskil noemenswaardig in die
ontwerptegniek. Die eerste is baseer op die terug voortplanting van plat
Gaussiese bundel, en die tweede op metamorfose van Gaussiese “flat-top”
bundel. Ons toon aan dat beide tegnieke goeie konvergensie het, en in die
gevraagde stabiele modus lewer.
In hoofstuk 6 skets ons die resonator ontwerp wat die selektering van ʼn
Gaussiese modus deur diffraktiewe optiese element moontlik maak. Dit word
moontlik deur die metamorfose van ’n Gaussiese bundel na ʼn “flat-top”
gedurende die voortplanting van die een kant van die resonator na die ander.
Deur die wins medium aan die “flat–top” kant van die bundel te plaas word dit
moontlik om hoë energie te onttrek in ʼn lae verlies resonator.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4018 |
| Date | 03 1900 |
| Creators | Litvin, Igor A. |
| Contributors | Forbes, Andrew, Rohwer, Erich G., University of Stellenbosch. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 85 p. : ill. |
| Rights | University of Stellenbosch |
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