The objective of the proposed research is to develop multiwavelength lasers as cost-efficient sources for broadband optical access networks. Todays telecommunications networks have widely adopted optical fiber as the backbone transmission medium. Optical fiber systems are promising candidates for the broadband access networks to offer high-speed and future-proof services. To harness the available bandwidth in fiber and to meet the ever-growing bandwidth demand, wavelength division multiplexing (WDM) techniques have been investigated. There have been intense research activities for the creation of new low-cost laser sources for such emerging applications. In this context, multiwavelength fiber ring lasers have been significantly investigated as they present many advantages, including simple structure, low-cost, and selectable multiwavelength operation. We propose a new laser system architecture that emits alternate multiwavelength picosecond pulses operating at room temperature. Optical signal generation is based on a single active component, an unbalanced Mach-Zehnder interferometer, inserted in an actively mode-locked erbium-doped fiber ring laser to provide both intensity modulation and wavelength-selective filtering. Time and frequency controls of the light emission are reached by inserting an additional modulator and a periodic filter in the cavity. This approach focuses on the application of multiwavelength lasers as sources for WDM passive optical networks.

We present a passive and robust mode-locking scheme for a Vertical External Cavity Surface Emitting Laser (VECSEL). We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide a stable colliding pulse operation. With this cavity geometry, the two counter propagating pulses synchronize on the SESAM to saturate the absorber together. This minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counter-propagating pulses in the SESAM is shown to extend the range of the modelocking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195fs with an average power of 225mW per output beam at a repetition rate of 2.2GHz, giving a peak power of 460W per beam. The remarkable robustness of the modelocking regime is discussed and a rigorous pulse characterization is presented.

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.

1.55-µm high-speed modelocked semiconductor lasers are theoretically and experimentally studied for various coherent photonic system applications. The modelocked semiconductor lasers (MSLs) are designed with high-speed (>5 GHz) external cavity configurations utilizing monolithic two-section curved semiconductor optical amplifiers. By exploiting the saturable absorber section of the monolithic device, passive or hybrid mode-locking techniques are used to generate short optical pulses with broadband optical frequency combs. Laser frequency stability is improved by applying the Pound-Drever-Hall (PDH) frequency stabilization technique to the MSLs. The improved laser performance after the frequency stabilization (a frequency drifting of less than 350 MHz), is extensively studied with respect to the laser linewidth (~ 3 MHz), the relative intensity noise (RIN) (< -150 dB/Hz), as well as the modal RIN (~ 3 dB reduction). MSL to MSL, and tunable laser to MSL synchronization is demonstrated by using a dual-mode injection technique and a modulation sideband injection technique, respectively. Dynamic locking behavior and locking bandwidth are experimentally and theoretically studied. Stable laser synchronization between two MSLs is demonstrated with an injection seed power on the order of a few microwatt. Several coherent heterodyne detections based on the synchronized MSL systems are demonstrated for applications in microwave photonic links and ultra-dense wavelength division multiplexing (UD-WDM) system. In addition, efficient coherent homodyne balanced receivers based on synchronized MSLs are developed and demonstrated for a spectrally phase-encoded optical CDMA (SPE-OCDMA) system.

Un nombre croissant d’applications telles que le micro-usinage ou le diagnostique de composants électroniques nécessitent de fortes puissances moyennes dans différentes gammes de longueurs d’onde (infra-rouge à 1030 nm, vert à 515 nm ou ultra-violet à 343 nm). Ces fortes puissances moyennes lasers ne sont généralement atteignables qu’à l’aide d’architecture laser de type MOPA (Master Oscillator Power Amplifier). C’est dans cette optique que la société Eolite Systems veut développer ses propres oscillateurs car elle maitrise déjà l’amplification à l’aide de fibres de type barreau à large aire modale. Le développement d’oscillateurs picosecondes de fortes puissances moyennes est ainsi une brique essentielle dans la chaîne d’amplification globale. Dans le cadre d’un contrat CIFRE entre Eolite Systems et le Laboratoire Onde et Matière d’Aquitaine de l’Universitéde Bordeaux 1, nous avons développé différents laser dont la puissance moyenne est supérieure à 10W, à une cadence de 74 MHz. La durée des impulsions générées s’étend de 20 ps à 130 fs. Le fonctionnement de ces différents lasers repose sur l’utilisation de la rotation non-linéaire de polarisation dans la fibre optique qui, dans le régime de fonctionnement à dispersion normale, permet d’atteindre le verrouillage en phase des modes et ainsi la génération d’impulsions d’énergie de l’ordre de 150nJ. Nous avons développé un code de simulation numérique afin de rendre compte de la dynamique des impulsions dans la cavité. Les résultats obtenus à l’aide de ce code sont en bon accord avec ceux obtenus lors des différentes expériences. / A growing number of applications such as micro-machining of electronical components need high average power in a various range of wavelengths (infrared at 1030 nm, green at 515 nm and ultravioletat 343 nm). These high average power lasers are generally design using a MOPA (MasterOscillator Power Amplifier) scheme. Following this idea, Eolite systems wants to develop its own oscillators since it already masters the different difficulties when trying to reach high average power,especially by using rod-type fibers with a large mode area. The development of high power picosecondfiber oscillators is a essential piece in the global amplification scheme. In the frame of a CIFRE contract between Eolite Systems and the Laboratoire Onde et Matière d’Aquitaine of the Universitéde Bordeaux 1 we have developed different laser oscillators with an average power of more than10 W at a repetition rate of 74 MHz. The pulse duration generated is ranging from 20 ps down to130 fs. The pulsed regime was initiated using non-linear polarization evolution in the fiber and in anormal dispersion laser cavity. This lead to the generation of pulse energies in the range of 150 nJ for the different architectures. We also developed a numerical code in ordre to fully understand the influence of the different elements in the laser cavity. The results are in good agreement with those obtained with the experimental setup.

Made available in DSpace on 2016-03-15T19:38:49Z (GMT). No. of bitstreams: 1 2012_Lucia Akemi Miyazato Saito.pdf: 7872621 bytes, checksum: 4898bb89f5642047c4e878b18fc4f31a (MD5) Previous issue date: 2012-11-01 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / We investigated the mechanisms responsible for pulse formation and evolution in Erbium-doped fiber lasers with cavity lengths varying from 16.4 m to 100.8 km that were actively mode-locked at repetition rate of 1 GHz. The variation of pulse widths and the peak powers in these lasers defined three regimes of propagation inside the cavities and we identified the mechanisms responsible for the pulse formation and evolution in each one of these regimes depending on the ratio between the cavity length (Lcav) and the dispersion length (LD) and nonlinear length (LNL). When Lcav is shorter than LD and LNL, there is neither dispersive nor nonlinear effect during pulse evolution (pulse has a duration of approximately 30 ps). In this regime, its final duration is determined by the standard theory of active modelocking. For Lcav shorter than LD but ~ LNL, the pulse evolution is in the nonlinearity-dominant regime where the self-phase modulation leads to spectral broadening. In addition, for cavities longer than 12.6 km, Lcav ~ LD and also longer then LNL. The pulse evolution is in the dispersion-dominant regime with its duration depending on the accumulated dispersion. In this regime the soliton effect takes place and the final pulse duration depends on the cavity length. Also, we present a comparison between an in-field and in-laboratory 50 km ultralong Erbium fiber lasers actively mode-locked with repetition rate varying from 1 to 10 GHz generating pulses from 35.2 to 68.7 ps. The pulse widths generated at higher frequencies are in agreement with Kuizenga-Siegman theory. However, for lower frequencies the pulses have higher intracavity peak power which allows the soliton effect to take place. Depending on the pump power level, the repetition rate and the cavity length, both lasers can operate in active mode-locking or under the influence of the soliton regime that locks the pulse duration according to the dispersion and cavity length. Due to the soliton robustness, this condition eliminates most of the environmental influence in the in-field mode-locking regime and makes both lasers very similar. / O laser de fibra dopada com Érbio é apresentado nesta tese numa nova configuração: cavidade ultralonga, com dimensão da ordem de dezenas de quilômetros. Foram investigados os mecanismos responsáveis pela formação e evolução do pulso em cavidades com comprimentos de 16,4 m a 100,8 km operando em regime de acoplamento de modos ativo na taxa de repetição de 1 GHz. A variação da largura dos pulsos e da potência de pico nestes lasers definem três regimes de propagação no interior das cavidades sendo possível identificar os mecanismos responsáveis pela formação e evolução em cada um destes regimes, dependendo da relação entre o comprimento da cavidade (Lcav) e o comprimento de dispersão (LD) e também em relação ao comprimento não-linear (LNL). Quando Lcav é menor que LD e LNL, não há efeito dispersivo e não-linearidades apreciáveis na evolução do pulso, obtendo-se aproximadamente 30 ps. Neste regime, a duração final é determinada pela teoria de acoplamento de modos ativo. Para Lcav menor que LD mas ~LNL, a evolução do pulso é dominado pela não-linearidade onde a automodulação de fase resulta em alargamento espectral. Além disso, para cavidades mais longas que 12,6 km, Lcav ~ LD e Lcav mais longo que LNL, a duração do pulso na saída aumenta proporcionalmente com a dispersão acumulada e ocorre a propagação de pulsos sob efeito solitônico. Complementando o estudo, o laser na configuração em laboratório é comparado com o laser montado em campo, sendo utilizado em ambos 50 km de fibra padrão. O laser à fibra dopada com Érbio é modulado ativamente com taxa de repetição variando entre 1 e 10 GHz gerando pulsos de 35,2 a 68,7 ps. A duração dos pulsos gerados em frequências altas está de acordo com a teoria de Kuizenga-Siegman. No entanto, em baixas frequências os pulsos têm potência de pico intracavidade alta o que permite que haja a formação de pulsos solitônicos na propagação. Dependendo do nível da potência de bombeamento, da taxa de repetição e do comprimento da cavidade, os dois lasers podem operar em acoplamento de modos ativo ou sob a influência do regime solitônico que trava a duração do pulso de acordo com a dispersão e comprimento da cavidade. Devido à robustez do sóliton, esta condição elimina a maior parte da influência do ambiente no acoplamento de modos do laser na configuração em campo, tornando os dois lasers muito similares.