Pulse shaping is the technique which controls the ultra-short pulse shape, and it became of great technological interest because of its potential applications in laser pulse compression, digital communications, microscopy etc. We demonstrate the idea of pulse-shaping technique and pulse propagation with low energy losses in a resonant linear absorbing medium. This thesis presents the results of a study of the propagation of Gaussian and hyperbolic secant ultrashort chirped and chirp-free pulses in homogeneously and inhomogeneously broadened resonant linear absorbers. Changes to the pulse shape and energy loss factor are presented as the pulse propagates in the absorber. The Fast Fourier method is used to numerically determine both the normalized intensity profile and the pulse spectrum. Our results show that, for pulse durations shorter than the relaxation time, chirped pulses in absorbing media obey the area theorem, with their shape changing with the propagation distance. Simulation results of the spectra of chirped pulses clearly show the burning of a spectral ’hole’ as the pulse propagates, with the pulse energy pushed away towards the wings. When compared to chirp-free pulses, chirped pulses reshape faster and develop wings in their tail due to initial phase modulation. Simulation results of the energy loss factor show that chirped pulses propagating in resonant linear absorbers sustain less energy losses than do chirp-free pulses. A comparison of chirped secant and Gaussian pulses shows that secant pulses propagate with lower energy losses. Analytic solutions are presented for long-distance asymptotic expressions of initial rms spectral bandwidth as well as for the attenuation factor of chirped Gaussian pulses. These analytical results are in agreement with numerical simulations. The comparison of energy losses of short chirped Gaussian pulses and long pulses of any profile in linear absorbers is also discussed in the thesis.

First, we investigate the Bouguer-Lambert-Beer (BLB) law as applied to the transmission of ultrashort pulses through water in the linear absorption regime. We present a linear theory for propagation of ultrashort laser pulses, and related experimental results are in excellent agreement with this theory. Thus we conclude that recent claims of the BLB law violations are inconsistent with the experimental data obtained by our group. Second, we study the dynamics of ultrashort pulses in a Lorentz medium and in water via the saddle point method. It shows that the saddle point method is a more efficient and faster method than the direct integration method to study one-dimensional pulse propagation over macroscopic distances (that is, distance comparable to the wavelength) in a general dielectric medium. Comments are also made about the exponential attenuation of the generalized Sommerfeld and Brillouin precursors. By applying the saddle point method, we also determined that the pulse duration estimated by the group velocity dispersion (GVD) approximation is within 2% of the value computed with the actual refractive index for a propagation distance of 6 m in water.

The groundwork has been completed for a large new research initiative involving the development of diode lasers for moderate power ultrashort pulse generation. This thesis reports on the status of three core areas of this initiative: InGaAsP/GaAs diode laser design and characterization, split contact device testing, and thin film interference filter deposition and characterization. Two new short wavelength diode laser designs have been realized and tested. A 980 nm laser was designed, using an InGaAsP barrier/waveguide region. This showed improved far field performance and better contact isolation as compared to an existing 980 nm laser using GaAs barriers. A laser emitting at 850 nm was also designed using GaAs quantum wells surrounded by a new quaternary waveguide region. A test arrangement was developed to facilitate the measurement of IV and LI curves for split contact lasers. Numerous lasers were tested, indicating that short absorber sections and narrow gap widths are preferable for use as saturable absorbing regions in a passively mode locked diode laser. Finally, thin film silicon oxynitride interference filters have been designed, deposited, and characterized for several antireflecting and high reflectance coatings on semiconductor laser facets. A comparison ofsingle layer AR coatings accounting for the modal reflectivity was performed. A four layer high reflectance coating with a peak broadband reflectance of over 90% was deposited on a laser facet. / Thesis / Master of Engineering (ME)

In this thesis, novel multi-section laser diodes based on quantum-dot material are designed and investigated which exhibit a number of advantages such as low threshold current density; temperature-insensitivity and suppress carrier diffusion due to discrete nature of density of state of quantum-dots. The spectral versatility in the range of 1.1 µm – 1.3 µm wavelengths is demonstrated through novel mode-locking regimes such as dual-wavelength mode-locking, wavelength bistability and broad tunability. Moreover, broad pulse repetition rate tuning using an external cavity configuration is presented. A high peak power of 17.7 W was generated from the quantum-dot laser as a result of the tapered geometry of the gain section of the laser has led to successful application of such device for two-photon imaging. Dual-wavelength mode-locking is demonstrated via ground (?=1180 nm) and excited (?=1263 nm) spectral bands with optical pulses from both states simultaneously in the 5-layer quantum-dot two-section diode laser. The widest spectral separation of 83 nm between the modes was achieved in a dual-wavelength mode-locked non-vibronic laser. Power and wavelength bistability are achieved in a mode-locked multi-section laser which active region incorporates non-identical QD layers grown by molecular beam epitaxy. As a result the wavelength can be electronically controlled between 1245 nm and 1290 nm by applying different voltages to the saturable absorber. Mode-locked or continuous-wave regimes are observed for both wavelengths over a 260 mA – 330 mA current ranges with average power up to 28 mW and 31 mW, respectively. In mode-locked regime, a repetition rate of 10 GHz of optical pulses as short as 4 ps is observed. Noticeable hysteresis of average power for different bias conditions is also demonstrated. The wavelength and power bistability in QD lasers are potentially suitable for flip-flop memory application. In addition, a unique mode-locked regime at expense of the reverse bias with 50 nm wavelength tuning range from 1245 nm to 1290 nm is also presented. Broad repetition rate tunability is shown from quantum-dot external cavity mode-locked 1.27 µm laser. The repetition rate from record low of 191 MHz to 1 GHz from fundamental mode-locking was achieved. Harmonic mode-locking allows further to increase tuning up to 6.8 GHz (34th-order harmonic) from 200 MHz fundamental mode-locking. High peak power of 1.5 W can be generated directly from two-section 4 mm long laser with bent waveguide at angle of 7° at 1.14 GHz repetition rate without the use of any pulse compression and optical amplifier. Stable mode-locking with an average power up to 60 mW, corresponding to 25 pJ pulse energy is also obtained at a repetition frequency of 2.4 GHz. The minimum time-bandwidth product of 1.01 is obtained with the pulse duration of 8.4 ps. Novel tapered quantum-dot lasers with a gain-guided geometry operating in a passively mode-locked regime have been investigated, using structures that incorporated either 5 or 10 quantum dot layers. The peak power of 3.6 W is achieved with pulse duration of 3.2 ps. Furthermore, the record peak power of 17.7 W and transform limited pulses of 672 fs were achieved with optimized structure. The generation of picosecond pulses with high average power of up to 209 mW was demonstrated, corresponding to 14.2 pJ pulse energy. The improved optical parameters of the tapered laser enable to achieve nonlinear images of fluorescent beads. Thus it is for the first time that QD based compact monolithic device enables to image biological samples using two-photon microscopy imaging technique.

Nonlinear optical microscopy (NLOM) utilizing femtosecond laser pulses is well suited for imaging living tissues. This work reports on the design and development of an optical fiber based multispectral NLOM developed around a laser generating broadband sub-10-fs pulses. An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within the NLOM. Mouse tail tendon was used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an effective method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Reconstruction of ultrashort pulses at the focal plane of the objective allows the excitation of multiple fluorescent probes simultaneously. Multiple fluorescent probe excitation and spectral discrimination is demonstrated using mixtures of fluorescent dye solutions and an in-vitro angiogenesis model containing human umbilical vein endothelial cells (HUVEC’s) expressing multiple fluorescent proteins. Sub-10-fs pulses can be propagated through polarization-maintaining single mode fiber (PMF) for use in NLOM. We demonstrate delivery of near transform-limited, 1 nJ pulses from a Ti:Al2O3 oscillator via PMF to the NLOM focal plane while maintaining 120 nm of bandwidth. Negative group delay dispersion (GDD) introduced to pre-compensate normal dispersion of the optical fiber and microscope optics ensured linear pulse propagation through the PMF. Nonlinear excitation of multiple fluorophores simultaneously and polarization sensitive NLOM imaging using second harmonic generation in collagen was demonstrated using PMF delivered pulses. Two-photon excited fluorescence spectra and second harmonic images taken with and without the fiber indicates that the fiber based system is capable of generating optical signals that are within a factor of two to three of our traditional NLOM.

The main contributions of this thesis to the field of ultrashort pulse measurement are a new set of experimental tools to measure the spatio-temporal fields of femtosecond pulses, and a new simplified formalism to describe such fields in the presence of distortions. More specifically, we developed an experimental technique based on scanning-wavelength digital holography and frequency-resolved optical gating that allows the complete measurement of the electric field E(x,y,t) of trains of identical femtosecond pulses. A related method, wavelength-multiplexed digital holography, is also introduced. It achieves a single-shot measurement of the three-dimensional field E(x,y,t) but at a reduced resolution using a simple experimental apparatus. Both methods can be used to measure various spatio-temporal distortions that often plague femtosecond laser systems, in particular amplified ones. Finally, to unambiguously and intuitively quantify such distortions, we introduce normalized correlation coefficients so that a common language can be used to describe the severity of these effects.

Nonlinear optical microscopy (NLOM) utilizing femtosecond laser pulses is well suited for imaging living tissues. This work reports on the design and development of an optical fiber based multispectral NLOM developed around a laser generating broadband sub-10-fs pulses. An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within the NLOM. Mouse tail tendon was used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an effective method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Reconstruction of ultrashort pulses at the focal plane of the objective allows the excitation of multiple fluorescent probes simultaneously. Multiple fluorescent probe excitation and spectral discrimination is demonstrated using mixtures of fluorescent dye solutions and an in-vitro angiogenesis model containing human umbilical vein endothelial cells (HUVEC’s) expressing multiple fluorescent proteins. Sub-10-fs pulses can be propagated through polarization-maintaining single mode fiber (PMF) for use in NLOM. We demonstrate delivery of near transform-limited, 1 nJ pulses from a Ti:Al2O3 oscillator via PMF to the NLOM focal plane while maintaining 120 nm of bandwidth. Negative group delay dispersion (GDD) introduced to pre-compensate normal dispersion of the optical fiber and microscope optics ensured linear pulse propagation through the PMF. Nonlinear excitation of multiple fluorophores simultaneously and polarization sensitive NLOM imaging using second harmonic generation in collagen was demonstrated using PMF delivered pulses. Two-photon excited fluorescence spectra and second harmonic images taken with and without the fiber indicates that the fiber based system is capable of generating optical signals that are within a factor of two to three of our traditional NLOM.

We simulate ultrashort pulse propagation through water by numerical methods, which is a kind of optical communication research. Ultrashort pulses have been known to have non Beer-Lambert behavior, whereas continuous waves (CW) obey the Beer-Lambert law. People have expected that the ultrashort pulse loses less intensity for a given distance in water than CW which implies that the pulse can travel over longer distances. In order to understand this characteristic of the pulse, we model numerically its spectral and temporal evolution as a function of traveling distance through water. We achieve the pulse intensity attenuation with traveling distance, obtain the temporal envelope of the pulse and compare them with experimental data. This research proves that the spectral and temporal profile of a pulse can be predicted knowing only the intensity spectrum of the input pulse and the refractive index spectrum of water in the linear regime. The real feasibility and the advantage of using an ultrashort pulse as a communication carrier will also be discussed.

Pages 10, 46, 126, 142 and 146 have been omitted because they were completely blank. / <p> This thesis describes the development of a tunable, ultrashort-pulse semiconductor-based laser system operating in the 1 μm wavelength region. The design of the oscillator is based on a two-contact long-wavelength InGaAs-GaAs quantum-well semiconductor device containing integrated gain and saturable absorber sections. A key design component of the oscillator is the fabrication of a curved ridge-waveguide in the gain section of the device, which allows the laser to be operated in a compact, linear external cavity. Under conditions of passive or hybrid mode-locking, the semiconductor oscillator can generate pulses of 1 to 10 ps in duration, which are tunable from 1030 to 1090 nm. The oscillator is also capable of being passively mode-locked at harmonics of the cavity round-trip frequency, allowing tuning of the pulse repetition rate from 0.5 to over 5 GHz. Noise measurements on two independently hybridly mode-locked semiconductor lasers reveal that the absolute noise of each laser is dominated by phase noise at frequencies below 10^5 Hz, while amplitude noise dominates at higher frequencies.</p> <p>Semiconductor and fibre optical amplifiers are used to scale the average power level of the mode-locked pulses. Semiconductor optical amplifiers consisting of narrow-stripe and flared-waveguide designs have been fabricated using the same material structure as that of the mode-locked semiconductor oscillator. Narrow-stripe devices with a length of 800 μm have produced amplified average signal powers of 13 mW, while 1700-μm-long, 2° flared-waveguide devices have produced amplified average signal powers of 50 mW. A fibre-based system consisting of a single-mode double-clad Yb-doped fibre has been constructed to investigate the suitability of a mode-locked diode laser as a seed-source for a Yb:fibre amplifier. Amplified average signal powers of up to 1.4 W have been obtained at the output of the fibre for a launched pump power of 2.1 W. Compression of the amplified pulses using a modified dual-grating compressor yields pulse durations as low as 500 fs and a peak power of up to 1.5 kW.</p> <p> Preliminary work is reported on the development of a novel dual-wavelength optical source consisting of two synchronized mode-locked diode lasers and a polarization-maintaining Yb:fibre amplifier. Numerical simulations based on a rate-equation model for the amplifier gain are conducted to investigate the performance characteristics of a Yb:fibre amplifier when operated under dual-wavelength signal amplification. The simulations are used to predict and optimize the performance of the fibre amplifier for two mode-locked semiconductor-seed-oscillators operating at wavelengths of 1040 and 1079 nm. Good agreement is obtained between the simulations and experimental results. </p> / Thesis / Doctor of Philosophy (PhD)

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.