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.

This thesis describes the outcome of study to investigate methods of broadband matching to photonic devices such as lasers and high speed detectors. The thesis is divided into two areas of interest relating to the design of broadband fiber optic links. The first area is the application of numerical methods and commensurate line methods to the design of compact equalisers which allow an improved transducer power gain over a wide band. It is shown that physically small equalisers can yield an improvement of 4 dB over a 2 GHz bandwidth. The second area considered is the distortion inherent in a laser diode. Detailed measurements of the second order and intermodulation products are given. A small signal perturbation analysis is presented which helps to explain the observed distortion products. The results of numerical simulation of the distortion using a state variable implementation of the full rate equations and related first, second and intermodulation equations is presented and possible methods of reducing the distortion are explored. It is shown that in principle the distortion could be reduced by pre-generating the distortion and adding an inverted form of the distortion to the transmitted signal. The distortion can then be corrected in the fiber and simulation studies suggest that an improvement of 13 dB optical or 26 dB electrical may be possible.

This paper describes recent progress in a continuing program to develop and apply RM³ (recess mounting with monolithic metallization) technologies for heterogeneous integration. Particular emphasis is placed on the applicability of RM³ integration to in-plane geometries for on-chip optical clock and signal distribution and on the suitability of commercially processed IC wafers for use as substrates for rectangular dielectric waveguides. / Singapore-MIT Alliance (SMA)

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

A Laser Music System has been created, that combines a laser and light sensor system with an infrared distance sensing system that detects the position of a user’s hand when it intersects one or more of the individual laser beam. The laser beams, which are made visible by a small amount of smoke in a dark room, provide visual guidance to the user to reduce the difficulty of using a non-contact instrument as well as enhancing an appealing optical effect for the user. The system uses a number of Sharp distance sensors to map the position of the user’s hand to a variable like pitch. The user should move their hand to different heights to achieve a desired pitch. The laser beam should be broken to trigger the desired note.

Wireless backhaul communication and power transfer can make the deployment of outdoor small cells (SCs) more cost effective; thus, their rapid densification can be enabled. For the first time, solar cells can be leveraged for the two-fold function of energy harvesting (EH) and high speed optical wireless communication. In this thesis, two complementary concepts for power provision to SCs are researched using solar cells – the optical wireless power transfer (OWPT) in the nighttime and solar EH during daytime. A harvested power of 1W is considered to be required for an autonomous SC operation. The conditions of darkness – worst case scenario – are initially selected, because the SC needs to harvest power in the absence of ambient light. The best case scenario of daytime SC EH from sunlight is then explored to determine the required battery size and the additional power from optical sources. As a first approach, an indoor 5m experimental link is created using a white light-emitting diode for OWPT to an amorphous silicon (Si) solar panel. Despite the use of a large mirror for collimation, the harvested power and energy efficiency of the link are measured to be only 18:3mW and 0:1%, respectively. Up to five red laser diodes (LDs) with lenses and crystalline Si (c-Si) cells are used in a follow-up study to increase the link efficiency. A maximum power efficiency of 3:2% is measured for a link comprising two LDs and a mono-c-Si cell, and the efficiency of all of its components is determined. Also, the laser system is shown to achieve an improvement of the energy efficiency by 2:7 times compared with a state-of-the-art inductive power transfer system with dipole coils. Since the harvested power is only 25:7mW, an analytical model for an elliptical Gaussian beam is developed to determine the required number of LDs for harvesting 1W; this shows an estimated number of 61 red LDs with 50mW of output optical power per device. However, a beam enclosure of the developed Class 3B laser system of up to a 3:6m distance is required for eye safety. A simulation study is conducted in Zemax for the design of an outdoor 100m infrared wireless link able to harvest 1W under clear weather conditions. Harvesting 1:2W and meeting eye safety regulations for Class 1 are shown to be feasible by a 1550 nm laser link. The required number of laser power converters is estimated to be 47 with an area of 5 5mm2 per device. Also, the dimensions of the transmitter and receiver are considered to be acceptable for the practical application of SC EH. In the last part of this thesis, two multi-c-Si solar panels are initially used for EH in an outdoor environment during daytime. The power supply of at least 1W is shown to be achievable during hour periods under sunny and cloudy conditions. A maximum average power of 4:1W is measured in the partial presence of clouds using a 10W solar panel. Since the variability of weather conditions induces the harvested power to fluctuate with values of mW, the use of optical sources is required in periods of insufficient solar EH for SCs. Therefore, a hybrid solar/laser based EH design is proposed for a continuous annual SC provision of 1Win ‘darker’ places on earth such as Edinburgh, UK. The 10W multi-c-Si solar panel and the 1550 nm laser link are considered; thus, the feasibility of supplying the SC with at least 1Wper hour monthly using a battery with energy content of only 60Wh is shown through simulations. A maximum monthly average harvested power of 824mW is shown to be required by the 1550 nm laser system that has already been overachieved through simulations in Zemax.

Dans cette thèse, j'étudie la dynamique non linéaire résultant d’une compétition entre deux modes dans des systèmes lasers à boites quantiques.D’abord, je considère le cas de la compétition entre deux modes de polarisation apparaissant dans les diodes laser nanostructurées à cavité verticale et émettant par la surface (VCSELs). Il est connu que ces composants peuvent avoir une polarisation instable menant à des dynamiques riches. Récemment, un surprenant saut de mode entre deux états polarisés elliptiquement a été récemment découvert dans les VCSELs à boites quantiques. Ce comportement montre des propriétés intrigantes qui nécessitent une interprétation alternative. Dans cette thèse, je montre que ce comportement dynamique peut-être reproduit en utilisant le modèle spin-flip (SFM). En particulier je démontre et confirme expérimentalement que les sauts de modes sont en réalité des fluctuations chaotiques de faible dimension : un chaos en polarisation. Je démontre ensuite la pertinence de la dynamique chaotique observée pour les applications exploitant le chaos optique, en réalisant un générateur de nombres aléatoires à grande vitesse basé sur le chaos en polarisation.Deuxièmement, j'étudie les effet d'une rétroaction optique à délai sur les lasers à boites quantiques émettant simultanément depuis l'état fondamental et le premier état excité. Je clarifie l'impact the cette rétroaction optique ainsi que les mécanismes et bifurcations correspondantes. Je montre théoriquement qu'une rétroaction optique favorise globalement l'émission par l'état fondamental, mais aussi qu'un tel montage peut être utilisé pour commuter entre ces deux modes d'émission lorsque l'on change le taux ou le délai de la rétroaction. Enfin, je confirme ces observations expérimentalement, en rapportant des commutations entre l'état fondamental et l'état excité. / In this thesis, I study the nonlinear dynamics induced by the competition between two modes in quantum dot laser systems.First, I focus on the competition between polarization modes that takes place in quantum dot vertical-cavity surface-emitting lasers (VCSELs). It is well-known that these devices can exhibit polarization instabilities leading to rich dynamical evolution. Recently, a new peculiar random-like hopping between two non-orthogonal elliptically polarized states has been highlighted in QD VCSELs. This behavior shows intriguing features which clearly call for a different interpretation. In this thesis, I show that the dynamical behavior reported experimentally can accurately be reproduced within the spin-flip model (SFM) framework. In particular, I demonstrate and confirm experimentally that the peculiar random-like hoppings are in fact deterministic low-dimensional chaotic fluctuations, i.e. ``Polarization Chaos''. I then make a proof-of-concept demonstration of a high-speed random bit generator based on polarization chaos, hence demonstrating that the chaotic dynamics uncovered is relevant for optical chaos-based applications.Secondly, I investigate the effects of an external optical feedback on quantum dot lasers emitting simultaneously from the ground and the excited states. I bring new light on the impact of optical feedback and the corresponding mechanisms and bifurcations. I highlight theoretically that optical feedback globally favors the ground state emission, but also that it can be used to switch from one mode to the other when changing the feedback rate and/or the time-delay. In addition, I experimentally report switching between the ground and excited states when varying the external cavity length at the micrometer scale, which supports the theoretical predictions.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18281号 / 工博第3873号 / 新制||工||1594(附属図書館) / 31139 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 川上 養一, 教授 藤田 静雄, 准教授 須田 淳 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The FP multimode semiconductor laser has lightened up a new field of optical communication technology in the past two decades. Numerical modeling of its physical behaviours and transient response has been discussed previously in literature, mostly by constructing the multimode rate equations. Rate equations are very helpful in studying and predicting the average photon and carrier transient response and relaxation oscillation. However, their deficiency in statistical photon fluctuation limits the function of describing stochastic power shifted from main mode to other side modes. Therefore, a noise driven model with conjunction of optical fibre and photodiode is built to form an optical communication system in the simulation scope. The multimode nature of FP lasers causes several problems such as mode partitioning noise (MPN), intersymbol interference (ISI), and frequency chirping, among which mode partitioning noise is the most serious of the concern in this discussion. The stereotype analytical measurement of MPN power penalty is based on several assumptions on the received waveform shape and power distribution spectrum, which limits its fields of application and accuracy. This work develops a numerical solution to power penalty due to MPN, and it can be employed to any multimode laser diode models regardless of the received signal shape and power distribution spectrum. In conclusion, the MPN power penalty is a significant profile of evaluating system perform in fibre-optic communication links. It highly depends on shape of power distribution spectrum, number of modes, length of fibre, and pattern of signal waveform. / Thesis / Master of Applied Science (MASc)