Thesis (MSc (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This thesis consists of two parts; the first part is a discussion on the detailed history of the
development of different types of modelocked lasers, especially the neodymium-doped
lasers. The second part describes the design and development of a modelocked
diode-end-pumped solid state Nd:YVO4 laser using Semiconductor Saturable Absorbers.
The first part of this work will cover the history of modelocking where different types of
lasers were used to generate ultrashort pulses. The discussion will mainly focus on
neodymium-doped lasers such as Nd:YVO4, where we will look at the spectral properties
such as energy levels, absorption and emission wavelengths of such a laser.
The discussion will also look at different types of optical pump sources; such as diode
lasers and flashlamps, where we will see the advantages of using diode lasers as pump
sources due to their better operating conditions and efficiency. We will also look at two
different types of diode pumping setup schemes, which are end-pumping and side
pumping; where we will discover that diode-end-pumping is a better scheme for laser
mode matching resulting in high efficiency and very good beam quality when compared
to side pumping.
The gain bandwidth of the laser material will also be discussed showing that a laser
material with a very large gain bandwidth and broad emission bandwidth is suitable for
generating ultrashort pulses, such as Ti:Sapphire crystal. The discussion will also cover
ultrafast lasers that have a small amplification bandwidth suitable for diode-end-pumping
and that produce high average output power. Ultrafast lasers with low amplification
bandwidth such as Nd:YAG and Nd:YVO4 will be discussed showing that they can
generate very short pulses with durations of down to 19 ps and 20 ps respectively and
average output powers of 27 W and 20 W.
The technique of creating ultrashort pulses which is called modelocking will be
discussed, where passive modelocking will be shown to be more suitable for creating
ultra short pulses in the femtosecond region and active modelocking in the picosecond region. The discussion will also cover saturable absorbers for passive modelocking where
we will discuss the use of semiconductor saturable absorber mirrors to generate reliable
self starting modelocked pulses. We will also cover the instabilities associated with using
saturable absorbers where we will discuss different methods for reducing the instabilities
by using gain media with the smallest saturation fluence.
The second part of the work will deal with the design and development of SESAM
modelocked diode-end-pumped Nd:YVO4 lasers. This part will include a discussion on
the resonator design criteria’s for achieving a stable modelocked diode-end-pumped
solid-state laser. The choice of using Nd:YVO4 as a gain medium will be shown to be
influenced by its large cross sectional area, which is useful in increasing the gain
bandwidth for possible ultrashort pulse generation. The resonator for high power
continuous wave (cw) output has been designed using simulation software developed at
St Andrews University. We will also discuss stability criteria such as the laser spot size
inside the crystal and on the end mirror and how they can be incorporated into the
resonator design software. The discussion will also include the pump setup design and
the efficient cooling method of the crystal using a copper heat sink.
The methodology of obtaining stable, thermal lens invariant, single transverse mode
operation during power scaling of Nd:YVO4 lasers will be discussed. A lens relay
approach is used to extend the cavity length so as to introduce spot size control in the
designed diode-end-pumped Nd:YVO4 laser that will be shown to produce a maximum
average output power of 10.5 W with an average beam quality factor M2 of 1.5. We will
also discuss the incorporation of a single quantum well SESAM within the extended
diode-end-pumped Nd:YVO4 laser resulting in cw-modelocked pulses at an average
output power of 2.8 W with pulse repetition frequency of 179 MHz, equivalent to the
cavity round trip time of 5.6 ns. The incorporation of the double quantum well SESAM
will also be shown to produce stable Q-switched modelocked pulses at an average output
power of 2.7 W with pulse repetition frequency of 208 KHz. / AFRIKAANSE OPSOMMING : Hierdie tesis bestaan uit twee dele. Deel 1 is ‘n indiepte bespreking rondom die
ontwikkelingsgeskiedenis van Modusgebonde lasers, veral van Neodemiumdoteerde
lasers. Deel 2 beskryf die ontwerp en ontwikkeling van ‘n Modusgebonde diodeentgepompde
vastetoestand Nd:YVO4 laser deur van ‘n Halfgeleier Versadigbare
Absorbeerder (SESAM) gebruik te maak.
Die eerste afdeling fokus op Modusbinding om ultrakort pulse te ontwikkel in
verskillende tipes lasers. Die bespreking sentreer rondom Neodemiumdoteerde lasers
soos Nd:YVO4. In hierdie geval beskou ons ook die spektraaleienskappe van die laser vir
beide die absorpsie en emissie golflengtes. Verder word verkillende tipes pompbronne
ondersoek (soos diodelasers en flitslampe). Die voordele van diodelasers kom sterk na
vore a.g.v. beter werking en effektiwiteit. Verskillende pompopstellings word ook
ondersoek naamlik ent-en kantpomping. Entpomping kom hier na vore as die beter opsie
i.t.v. laser-moduspassing. Dit lei tot ‘n hoër effektiwiteit wat ‘n beter straalkwaliteit tot
gevolg het, in vergelyking met kantgepompde opstellings. Die versterkingsbandwydte
word ook bespreek: ‘n groot versterkingsbandwydte en breë emissiebandwydte is geskik
om ultrakort pulse te ontwikkel. Ti:Saffier is ‘n goeie voorbeeld. Ultravinnige lasers met
‘n klein versterkingsbandwydte word ook bespreek aangesien dit geskik is vir diodeentpomping
wat dan ‘n hoë gemiddelde uitsetdrywing lewer. Nd:YAG en Nd:YVO4 word
ondersoek en daar word getoon dat hul pulse van so kort as 19 ps en 20 ps onderskeidelik
teen ‘n gemiddelde uitsetdrywing van 27 W en 20 W kan lewer. Die tegniek waarmee
ultrakort pulse geskep word is Modusbinding: passiewe modusbinding is meer geskik vir
femtosekonde pulse en aktiewe modusbinding is meer geskik vir pikosekonde pulse.
Verder word versadigbare absorbeerders bespreek, vir hul gebruik in die betroubare
selfinisiërende modusgebonde pulse. Die onstabiliteite geassosieer met versadigbare
absorbeerders word ook bespreek asook verskillende metodes om dit te minimaliseer.
Die tweede afdeling behandel die ontwerp en ontwikkeling van ‘n SESAM
modusgebonde diode-entgepompde Nd:YVO4 laser. Die resonator ontwerpspesifikasies
vir stabiele werking word ook bespreek. Die keuse van Nd:YVO4 as versterkingsmedium is a.g.v. die groot deursnitarea wat die versterkingsbandwydte verhoog, om ultrakort
pulse te genereer. Die resonator vir hoë drywing kontinuestraal werking is ontwerp deur
van St Andrews sagteware gebruik te maak. ‘n Bespreking van stabiliteitsspesifikasies
soos die laser kolgrootte, binne die kristal asook op die entspieël volg, asook die
pompmetodiek en effektiewe verkoeling van die kristal. Die totale metodiek rondom die
verkryging van ‘n stabiele, termieselens invariante, enkele transversale modus laser word
bespreek met die oog op drywingsverhoging. Die geval onder bespreking is waar die
laser se kolgrootte beheer kan word op die entspieël deur die resonatorlengte aan te pas.
Dit word getoon dat dit ‘n kontinuestraal laser van 10.5 W drywing kan lewer teen die
maksimum gemiddelde straalkwaliteit van M2 = 1.5. Die byvoeging van ‘n enkele
kwantumput SESAM in die laser het modusgebonde pulse tot gevolg. Die gemete
waardes was 2.8 W gemiddelde drywing met ‘n pulsherhalingstempo van 179 MHz wat
in lyn is met die pulsbewegingstyd in die resonator van 5.6 ns. Deur van ‘n dubbele
kwantumput SESAM gebruik te maak word Q-geskakelde modusgebonde pulse verkry,
teen ‘n gemiddelde uitsetdrywing van 2.7 W en ‘n pulsherhalingstempo van 208 KHz.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4225 |
| Date | 03 1900 |
| Creators | Ngcobo, Sandile |
| Contributors | Von Bergmann, Hubertus M., Bollig, Christoph, 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 | 106 p. : ill. |
| Rights | University of Stellenbosch |
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