The passive and coupled-cavity mode locking of a LiF:F2+ colour-centre laser was studied. Pulses of less than 180 fs were obtained in the 0.85 mum spectral region by passive mode-locking using the saturable absorber dye IR140. Coupled-cavity mode locking, where a length of optical fibre was incorporated in an external control cavity, resulted in the generation of 1.5 ps duration pulses. Active and coupled-cavity techniques were applied to a KCl:TiO(1) colour-centre laser operating in the 1.5 mum spectral region. Coupled-cavity mode locking was obtained using either Fabry-Perot or Michelson interferometer arrangements. Although either arrangement generated similar pulse durations, the Michelson scheme was found to be more stable. Pulse durations of less than 100 fs were routinely obtained from either arrangement by using small-core fibre in which the dispersion minimum coincided with the laser operating wavelength. The KCl:TiO(1) laser was used to study various absorptive and refractive nonlinearities in a 1.5 mum InGaAsP optical amplifier. Pulse distortion caused by gain saturation and loss saturation was studied in the temporal and spectral domains. In addition, cross-phase modulation related to gain saturation was investigated. Self-phase modulation caused by an ultrafast refractive nonlinearity has been observed for the first time, and its nonlinear coefficient n2 was deduced to be -2 x 10^-11cm2W-1. Coupled-cavity mode locking of the KCl:TiO(1) laser was also obtained when the InGaAsP amplifier was used, with pulses as short as 280 fs. Self-starting operation was achieved by eliminating parasitic optical feedback from the device facets. Mode locking was observed for amplifier drive currents either above or below transparency, suggesting that saturable gain and saturable loss respectively were the dominant nonlinearities exploited. The synchronous and coupled-cavity mode locking of a NaCl:OH- laser operating near 1.57 mum was investigated. Coupled-cavity mode locking using an optical fibre was achieved, but was accompanied by a sawtooth modulation in power output (or pulse duration) related to the short gain storage time of NaC1;OH-. This was circumvented by substituting the InGaAsP amplifier for the fibre to obtain self-starting mode locking.

A study of magnetic field induced sum frequency mixing (SFM) in sodium vapour was carried out using continuous-wave lasers as the sources of the fundamental radiation. The three-wave mixing nonlinear optical process was resonantly enhanced by tuning the laser frequencies close to single and two-photon resonances in the sodium atoms. The coherent ultraviolet radiation at the sum frequency of the two input laser frequencies was emitted by the coherently driven 3S-4D electric-quadrupole, which was rotated by the transverse magnetic field to allow collinear generation of the sum frequency wave. Two single-frequency dye lasers were used to examine in detail the role of the intermediate 3P atomic states in the coherent two-photon absorption. Resonant single photon transitions were investigated for the first time in a nonlinear optical process in an atomic vapour. High resolution SFM line profiles were obtained which illustrated the complicating contributions of hole-burning, velocity selection, optical pumping, saturation and frequency dependent phase mismatching to the three-wave mixing effect. The use of additional single photon resonant enhancement and control over the refractive index of the sodium vapour showed that large effective nonlinear X(2) susceptibilities were possible in atomic vapours which could exceed those of nonlinear crystals. The variation of SFM power with atomic particle density due to bulk phase mismatching reflected the wavelength dependence of the sodium dispersion with the 3P intermediate state off-resonance. Phase shifts of the focused Gaussian laser beams led to an asymmetric behaviour of the phase matching with respect to the sign of the phase mismatch k. Saturation spectroscopy was utilised for the first time to examine the Zeeman spectra of the sodium 3S-3P D line resonances in a transverse magnetic field. A novel experimental method was used to restrict the detrimental effects of velocity changing collisions on the resolution of the nonlinear laser spectroscopic technique. The possibility of using optical pumping with a transversal, resonant light beam to induce the second-order nonlinearity necessary for second harmonic generation in sodium vapour was experimentally investigated.

In this thesis instabilities which occur in the plasma tube of an argon ion laser are examined. Coherent oscillations in the discharge potential across the plasma tube are found to be a usual feature of an argon laser. The light output of the laser is shown to be modulated by these oscillations only if the discharge current is modulated. It is found that these oscillations are due to instabilities in the anode zone of the gas discharge. The oscillations occurring within the anode zone of an argon laser have been studied using optical and Langmiuir probe techniques. The oscillations are discussed in terms of a phenomenological model due to Pupp. Many of the observed features of the oscillations can be described in terms of this model. The values of the anode fall that have been observed in the argon laser are substantially less than would be expected on the basis of earlier work on glow discharges in argon. this is discussed and an estimate of the anode fall is made which is closer to the observed value than that predicted by Von Engel's theory of the anode fall. A simple mathematical model of the anode zone is presented and it is shown that under the conditions appropriate to an argon laser the anode fall is not expected to be stable. These instabilities in the anode zone of the plasma tube, while they appear to be a normal feature of the argon laser discharge, do not necessarily modulate the laser light. It is shown that if the power supply for the laser has a high output impedance at the frequency of the oscillations then the modulation of the light may be reduced to a negligible level.

This thesis describes the development of a continuous-wave (cw), diode pumped, repetitively Q-switched Nd:YLF laser, and its application as a pump source for non-linear optical frequency converters and optical parametric oscillators (OPO's), with typical pulse widths of several tens of nanoseconds and repetition rates in the kHz region. At the core of this thesis is the proposal and realisation of a novel rhombic Nd:YLF slab geometry, which is pumped by two 20W cw AlGaAs diode arrays, each focused by means of 3 lenses into a folded resonator path. The novelty of this adaptation is that it uses Yttrium Lithium Fluoride (YLF), known for its comparatively low tensile strength, as the Neodymium host crystal, by addressing and solving the thermal stress fracture problem in YLF for pump powers up to 20W per surface in an end-pumped configuration. With the excellent beam quality, high average output power and short pulses achieved, Nd:YLF has been established as an excellent material for high density, diode pumped, Q-switched lasers at 1047nm. With identical slabs coated for 1321nm operation also available, the performance and laser parameters such as crossection, linewidth and thermal lensing for these two transitions were compared. The pronounced change in the thermal lensing of the 1321nm transition led to an investigation of excited state upconversion (ESA) in Nd:YLF at 1047 and 1321nm under cw and Q-switched operation. The excellent access to the pumped volume made not only a time dependent spectral analysis of the visible and near UV fluorescence bands possible, but also permitted a spatial analysis of the different up-conversion areas. This pump density and laser transition dependent loss mechanism has implications on the end-effect governed lensing, upper state lifetime and Q-switched pulse form. The low threshold, high repetition rate conversion schemes realised during this thesis expanded the spectral cover of the 1mum pump laser from the near UV (349nm) at one end of the spectrum over the green (523.5nm) and 1.5mum region up to the near infrared at 3.445mum. The progress in this area lies not in the well known phase match configuration of the materials used such as KTP, KTA and LBO, but in the adaptation of these nonlinear converters and non-critically phase matched (NCPM) OPO's to achieve high conversion efficiencies at low pulse energies (typically less than 1mJ, 40kW). With average output powers in the green of 3W (~60% conversion from the IR at 6kHz repetition rate), 750mW in the UV, IW at 1.54mum and 280mW at 3.44mum it has been demonstrated that the high average output power of the repetitively Q-switched system can be preserved in the up- and down-converted radiation.

In this thesis describes experimental and theoretical work is described for a class of solid-state lasers which are classified by the term "microchip". The work presented here may be grouped into two parts. Chapters 2 and 5 describe work directed towards novel frequency modulation and up-conversion and are therefore of a more developmental flavour. The work presented in chapters 3 and 4 are of a more fundamental nature and pertains to transverse mode definition and spectral selectivity in these devices. In chapter 2 a laser diode array pumped, electro-optically tunable microchip laser is described. Based on the combined gain/non-linear material neodymium doped magnesium oxide lithium niobate (Nd:MgO:LiNbO3) tuning of the six oscillating axial modes was achieved at a sensitivity of 8.9 MHz V-1 mm. Chapter 3 describes the mechanisms observed to define the transverse mode of operation in lithium neodymium tetraphosphate, LiNdP4O12, or LNP, microchip lasers. This material was of interest in this particular study in that the dn/dT of the material was negative and would clearly preclude the guiding mechanisms postulated for the Nd:MgO:LiNbO3 microchip laser as described in chapter 2. The LNP microchip laser was excited by a Ti:sapphire laser and a HeNe based Fizeau interferometer was used to map out the pump induced changes in optical thickness between the cavity mirrors. Further investigation using a modified interferometer revealed that definition of the transverse mode of operation was primarily due to pump induced input surface deformation. The single frequency properties of a laser diode pumped LNP device, as a function of gain length, are the subject of chapter 4. The diode pumped LNP device output exhibited the same excellent spatial characteristics of operation as the Ti:sapphire pumped device. Fourteen milliwatts of single frequency output was obtained. A model, based on spatial dephasing of longitudinal modes, was modified appropriately to predict the single frequency performance of the LNP device as a function of gain length within the resonator. Chapter 5 deals with developmental work on a laser diode array pumped, composite material microchip laser for green output. The device consisted of a neodymium doped yttrium vanadate (Nd:YVO4) gain section in intimate optical contact with a non-linear material (potassium titanyl phosphate, or KTP) for frequency doubling of the 1.064 ?m line in Nd:YVO4. Initial experiments on the device were performed using a laser diode and coupling optics. Six milliwatts of C.W. green power were obtained for 150 mW of incident pump radiation. The device operated in a near diffraction limited spatial mode (M2=1.1) at all the incident pump powers. The oscillating waist within the device was found to be pump power dependent. The conversion of the fundamental into the green was observed to be only slightly reduced by the variable elliptical eigenpolarisation within the cavity.

The work in this thesis is concerned with the characterisation and development of modelocked solid-state lasers covering the 700 - 1000 nm region. Results are presented for a passively modelocked LiF:F+2 colour-centre laser, however, most of the work has concentrated on the Ti: sapphire (Ti:Al2O3) laser. In chapter 2, the operation of a cw LiF: F+2 colour-centre laser is discussed. This laser was passively modelocked using the saturable absorber dye, DaQTeC and pulses as short as 170 fs were generated with average output powers of ~10 mW. Pulses as short as 127 fs were generated in a dispersion compensated, colliding-pulse modelocked geometry over a wavelength range of 925 - 950 nm. A nonlinear external cavity was added to the basic laser configuration in an attempt to extend the modelocked tuning range and the saturable absorber dye lifetime. The technique of coupled-cavity modelocking was applied to a Ti:Al2O3 laser and enabled pulses as short as 1.3 ps to be generated. These pulses were frequency chirped and could be directly compressed to 290 fs outside the laser. By using the technique of intracavity dispersion compensation in both the main and coupled cavities, pulses as short as 90 fs were generated, having average powers of ~200 mW and peak powers of more than 20 kW. The simpler technique of self-modelocking is described in chapter 5 and allowed the generation of pulses as short as 60 fs from a dispersion compensated cavity configuration. Average output powers of ~600 mW were measured, which corresponded to peak powers of 110 kW. This laser had a modelocked tuning range which spanned the 750 - 950 nm region. Using fibre/prism pulse compression techniques pulses as short as 45 fs were produced. In chapter 6, the measurement and suppression of phase noise on the self-modelocked Ti:Al2O3 laser are discussed. The technique for noise reduction was also applied to two similar self-modelocked lasers in an attempt to synchronise the two laser pulse sequences.

A miniature Nd:YAG laser that was transversely pumped by a quasi-cw laser-diode array has been designed and constructed. This laser was injection seeded by a continuous wave single-frequency Nd:YAG laser that was also pumped by a laser- diode array. This was the first reported holosteric, or all-solid-state, laser that was capable of generating single frequency pulses in a high quality single transverse mode beam which it achieved at peak powers up to 7 kW. Two different types of laser-diode array were used in this work, and both have been characterised with respect to their use as pump sources for solid-state lasers. A fibre-coupled type SDL-2430-H2 laser-diode array, which emitted 100 mW from the end of a 100 mum core diameter fibre at the Nd:YAG absorption wavelength of 809 nm, was used to longitudinally pump a continuous wave Nd:YAG laser. Spatial hole-burning encouraged this laser to operate on several longitudinal modes, with an output power of up to 31 mW. With the addition of an etalon and a Brewster angled plate to the cavity of this laser, single longiuidinal mode operation was achieved at an output power level of 10 mW. The frequency from this laser was stabilised against thermal drift by phase sensitive locking to the resonance of an external reference cavity. The pump source for the transversely pumped Nd:YAG laser was an SDL-922-J quasi-cw laser-diode bar, which emitted from its 1 cm wide aperture pulses of 200 mus duration with energies up to 5 mJ at reperition rates between 10 - 100 pps. The characteristics of this laser-bar that were measured include a frequency chirp of 5 nm through the pulse, which was found to have a significant effect on the pumping of the Nd:YAG medium. An analysis of the pump-rate distribution throughout the Nd:YAG laser-rod was undertaken with reference to the transverse mode structure of this laser. This was used as the basis of a full rate-equation-based model of the transversely pumped laser. Fundamental transverse mode pulses with energies of 1.1 mJ were generated when the aperture provided by the Nd:YAG rod itself was used to suppress higher order modes. Upon Q-switching the transversely pumped laser using an acousto-optic modulator, single transverse mode pulses with peak power of up to 13 kW were produced. This laser was induced to operate on a single longitudinal mode by injection seeding with the single frequency diode-pumped laser. Operating at a peak power of 7 kW, this holosteric laser was particularly "well-behaved" with a pulse-to-pulse intensity stability of 6% over 100 pulses. A LiNbO3 electro-optic Q-switch configuration was implemented as optical damage to the acousto-optic component had limited the intracavity flux intensity. Development of these lasers towards higher peak power microlaser systems for applications including range-finding and surgery is continuing.

This thesis describes the development of all-solid-state self-modelocked Cr:LiSAF and Cr:LiSGaF lasers. Diode-pumped colquiriite lasers compare favourably with mainframe-pumped Ti:sapphire lasers, but the conflicting requirements of self-modelocking and pumping by broad-area diodes can cause problems. Two alternatives were investigated. Firstly, high Cr3+ doping permitted pumping in the Cr:LiSAF green absorption trough. Frequency-doubled Nd:YLF and Nd:YAG minilasers offer high-power, diffraction-limited beams in the green. The 'green problem' plagued both minilasers, however, and an Ar-ion pump laser was therefore also used. A laser utilising a 10%-doped AR-coated Cr:LiSAF crystal produced 30 mW output power at 320 mW pump, and 90 fs pulses at 859 nm and 86 MHz repetition rate. The self-modelocking threshold was 280 mW. A laser utilising a 22%-doped Brewster-angled crystal produced 120 mW output power at 1.1 W pump, and 72 fs pulses at 76 mW output. The self-modelocking threshold was 360 mW. 160 mW output was obtained by pumping the crystal at both ends to overcome thermal problems. Secondly, factors affecting CW and self-modelocking thresholds were considered; in particular, the effects of waist size, beam brightness, cavity configuration and intracavity dispersion. A Cr:LiSAF laser, pumped by a self-injection-locked diode, produced sub-100-fs pulses for just 73 mW pump. Its tuning curve was modulated by birefringence effects. The excellent free-running noise properties of the laser permitted a sub-picosecond streak camera evaluation. Two low-threshold lasers utilising low-loss Cr:LiSGaF were also developed. 87 fs pulses at 11.5 mW output power were produced from a Z-cavity laser for 170 mW pump. A laser utilising a compact 'retroreflector' three-minor cavity produced 84- fs pulses at 173 MHz repetition rate for 116 mW pump. Subsequent optimisation led to the production of sub-100-fs pulses for 40 mW pump, with self-modelocked operation demonstrated at pump powers as low as 21 mW.

The development of a pulsed all-solid-state blue laser system is described in this thesis. The laser system was constructed on the basis of the latest diode laser pumping technology and the availability of the nonlinear material, potassium niobate, in single crystals of high optical quality. With the advantages of diode laser pumping, the 946-nm line in Nd:YAG was successfully made to lase at room temperature, both in cw and Q-switched operation. By frequency doubling the Q-switched 946-nm pulses in a KNbO3 nonlinear crystal, pulses of coherent blue light at a wavelength of 473 nm were obtained with high conversion efficiency. The 946 nm laser line differs from the more usual transitions used in the Nd:YAG laser in having a lower laser level in the ground state manifold. As a result it experiences significant reabsorption loss due to the (thermally excited) lower level population. Because of this reabsorption loss, the length of the crystal is an important parameter, and as a result of a trade-off between the pumping efficiency and the reabsorption loss, there is an optimum crystal length for every pump power level. A theoretical model was developed to describe this system, and with output coupler transmission, pump beam size and cavity mode size as parameters, optimum conditions for laser oscillation were obtained numerically. The theoretical model was tested with both a diode laser and a Ti:Sapphire laser as a pump source for the 946-nm laser. The theory was found to be in excellent agreement with the results. When pumping with the Ti:Sapphire laser, a slope efficiency of 58 % was obtained with a 2-mm long Nd:YAG crystal, but this dropped to about 11 % with diode laser pumping, which shows the advantage of pumping with a beam of high optical quality. The Q-switched version of the 946-nm Nd:YAG laser was also studied theoretically, and a modified four level model was developed to describe the dynamic behaviour of the laser in the presence of lower state reabsorption. The relation of this quasi-three level model to the ideal four level model used in the absence of such absorption was explored. The experiments on Q-switching described here were the first to be reported on the application of this technique to a diode laser pumped 946-nm Nd:YAG laser. Q-switching was achieved by a Brewster-angled acoustic-optic Q-switch, which also acted as a linear polariser. When the laser was pumped by a 0.5 W diode-laser array, pulses with an energy of 4.9 muJ and pulse duration of 68.5 ns were observed at room temperature. Cooling the Nd:YAG crystal to 5 °C improved the laser performance to give pulses of 5.6 muJ pulse energy and 62 ns pulse duration, corresponding to peak powers of 76 W. The pulse repetition rate was kept at 1.5 kHz throughout. Blue light was generated by passing the 946-nm laser pulses along the a-axis of a 5.1-mm long potassium niobate nonlinear crystal, which was kept at 185 °C to obtain noncritical phase matching. The nonlinear coefficient used in this case was d32 (~19.5 pm/V 946 nm). Problems connected with crystal depoling were successfully solved both by the use of a uniform and well controlled temperature environment for the crystal and by providing a poling electric field during heating/cooling. Pulses of coherent blue light at 473 nm with 42 ns duration and 22 W peak power were generated from the output of the 946 nm laser described above, corresponding to an internal peak-power conversion efficiency of 40%, which was in excellent agreement with the second-harmonic-generation theory.

Introdução: A psoríase é uma doença inflamatória crônica com prevalência estimada em 2% da população mundial. As alterações ungueais estão presentes em cerca de 50% dos casos e chegam a acometer até 80% dos pacientes em algum momento da vida. Elas costumam ser resistentes aos tratamentos convencionais e possuir caráter recidivante. O laser de corante pulsado (LCP) tem surgido como alternativa terapêutica. Objetivos: Avaliar a eficácia do LCP na psoríase ungueal e o impacto do tratamento na qualidade de vida. Métodos: Para este estudo foram selecionados quatorze pacientes com diagnóstico de psoríase das unhas provenientes do ambulatório de Dermatologia do Hospital de Clínicas de Porto Alegre ou de clínicas privadas, tratados com aplicação do LCP com fluência de 6J/cm2 e duração de pulso de 0,45ms nas unhas acometidas em sessões mensais por três meses e uma nova visita, para seguimento, após 90 dias era realizada. A avaliação objetiva dos resultados foi realizada pelo índice de gravidade da psoríase ungueal (NAPSI) e avaliação subjetiva através de registro fotográfico das unhas acometidas na primeira e na terceira sessões e 90 dias após a última sessão. O índice de qualidade de vida para dermatologia (DLQI) foi respondido na primeira e 90 dias após a última sessão. Resultados: A mediana de melhora nas contagens do NAPSI total, do NAPSI do leito e do NAPSI da matiz ungueais foram, respectivamente, de 44,15% (P=0,002), 50% (P=0,033) e 65,1% (P=0,024). Os pacientes apresentaram também melhora significativa no DLQI após o tratamento, com mediana de 40%. Conclusões: O LCP pode ser considerado uma alternativa de tratamento para psoríase ungueal. / Introduction: Psoriasis is a chronic inflammatory disease with an estimated prevalence of 2% of the world population. The nail changes are present in about 50% of cases and affect up to 80% of patients at some point in life. Nail psoriasis is often refractory to conventional treatments. Pulsed dye laser (PDL) has emerged as an alternative therapy. Objectives: To evaluate the efficacy of PDL in nail psoriasis and the impact of treatment on quality of life. Methods: For this study, were selected eleven patients diagnosed with nail psoriasis from the Dermatology clinic of the Hospital de Clínicas de Porto Alegre or private clinics. PDL was applied in affected nails with fluency of 6J/cm2 and pulse duration of 0,45ms in monthly sessions for three months. A follow-up visit after 90 days was done. The outcome assesment was made by the Nail Psoriasis Severity Index (NAPSI) and photographic records in the basement, in the third session and in the follow-up. The Dermatology Life Quality Index (DLQI) was answered in the baseline and in the follow-up. Results: The median improvement in the scores of the overall NAPSI, nail bed NAPSI and nail matrix NAPSI were respectively 44.15% (P = 0.002), 50% (P = 0.033) and 65.1% (P = 0.024). The patients also had significant improvement in DLQI after treatment with a median of 40%. Conclusions: PDL can be considered an alternative treatment for nail psoriasis.