This thesis presents an investigation of the enhanced intensity-dependent refractive nonlinearity in optical amplifiers biased to transparency. Nonlinearities in an optical amplifier with a bulk active region and in optical amplifiers containing four, eight and sixteen quantum wells were compared using picosecond and femtosecond duration pulses generated using colour-centre lasers. Measurements of nonlinear absorptive and refractive dynamics in the amplifiers were performed using pump-probe and time-division interferometry techniques. A curve fitting function was used to distinguish nonlinearities from the measured dynamics. Carrier-heating, caused predominantly by free-carrier absorption, and virtual effects were found to be the most important nonlinearities in optical amplifiers biased to transparency. A strong correlation of the magnitude of the carrier-heating effect with the thickness of the amplifier active regions was observed. A novel measurement of the intensity dependence of the current required to bias optical amplifiers to transparency was performed using a technique which monitored the opto-electronic voltage across the amplifiers. The measurement showed that the transparency current increased linearly with intensity in the 4 QW and 8 QW amplifiers as a result of carrier-heating. Measurements performed on the 16 QW and bulk amplifiers showed a nonlinear variation of transparency current with intensity. These measurements were supplemented with a pump-probe investigation which revealed a negative trend in the transmission with a time constant in excess of 200 ps. It was suggested that a saturation effect related to the amount of heat added to the carrier distribution through free-carrier absorption was responsible for both of these effects. All-optical switching of picosecond pulses via the investigated enhanced nonlinearity was demonstrated in a polarisation switch, constructed using the bulk amplifier, and a nonlinear directional coupler. The optical powers required to perform optical switching were 1.6 W and 5.8 W respectively.

The main objectives in this research project related to the generation of ultrashort laser pulses using a KC1:T1 colour-centre laser and the study of their propagation in optical waveguides. Coupled-cavity mode-locking of the KC1:T1 colour-centre laser using either monomode optical fibre or passive AlGaAs waveguides as the nonlinear element in the control-cavity have been investigated. With the optical fibre as the nonlinear element, pulses as short as 63 fs have been generated. The large nonlinearity of the AlGaAs waveguides illuminated near the half- bandgap energy has been confirmed through the propagation of ultrashort laser pulses, and nonlinear phase shift in excess of 2mu has been observed. By undertaking the studies described here, the measurements have indicated that the waveguides used have a linear loss of 0.74 cm^-1, a two-photon-absorption coefficient of about 0.1 cm/GW, and a nonlinear refractive index of 0.8 x 10^-13 cm2/W. Coupled-cavity mode-locking with passive AlGaAs waveguides as the control-cavity nonlinear element has been achieved, and two different guiding geometries of the AlGaAs waveguides have been used. With a straight waveguide geometry, pulses having duration of ~ 230 fs have been generated. When a curved waveguide geometry was utilised and appropriate dispersion compensation applied then pulses as short as 160 fs have been produced. By employing waveguides having reduced lengths (down to 1.2 mm), some further shortening of the output pulses was achieved and pulses as short as 150 fs have been recorded. By using a gold coating on rear facet of the waveguide, successful coupled-cavity mode-locking has been achieved at an output power level as low as 4 mW. Some applications using the coupled-cavity mode-locked KC1:T1 colour-centre laser have been performed. Femtosecond pulses at 1.3-mum spectral region have been produced through the process of self-phase-modulation mediated four-wave-mixing in an erbium-doped fibre. Measurement on the group-velocity-dispersion of the AlGaAs waveguide has also been made, and a value of D = -1100 ps/nm/km has been deduced.

Within this thesis the exploitation of the large homogeneously broadened bandwidth of the LiF:F+2 colour-centre laser by production of frequency tunable ultrashort optical pulses over the 0.8-1.0mum spectral region has been presented. A synchronously pumped LiF:F+2 colour-centre laser produced pulses of 700fs duration with average powers of 30mW when a colliding-pulse-modelocked travelling-wave cavity was implemented. Passive modelocking of the LiF:F+2 colour-centre laser was achieved over two spectral operating regions centred around 860nm and 930nm when the saturable absorber dyes IR140 and DaQTeC were employed. Pulse durations as short as 180fs and 130fs were obtained at 860nm and 930nm respectively using colliding-pulse-modelocked group-velocity-dispersion compensated resonators. The laser was pumped at a 10% duty cycle throughout. The use of coupled-cavity- modelocking techniques combined with passive modelocking was found to extend both the tuning range of the laser and useful operating lifetime of the saturable dye. A home built NaCl:OH- colour-centre laser which encorporates the stabilised F+2 colour- centre is presented. Output powers of up to 450mW were obtained for input pump powers of 4W and the laser tuned from 1.4-1.8?m. Electroluminescence studies of NaCl, CsI, CsI:Na, CsI:Tl, and KI crystals are also presented in a study to assess the feasibility of obtaining laser action from such materials by electrical excitation. KI is shown to be the favoured laser candidate by this excitation method and evidence of temporal narrowing and signal enhancement of the electroluminescence output is presented.

The basic principles of generation, characterisation and propagation of ultrashort laser pulses and related experimental studies of mode locking of a coupled-cavity KC1:Ti colour-centre laser are presented. Extensive experimental investigations of mode locking with a two-cavity arrangement similar to a previously reported soliton laser has indicated that such a scheme is a very general technique for the generation of picosecond- femtosecond pulses. With or without the formation of optical solitons in the control fibre a variety of spectral and temporal features for the laser output pulses can be obtained. This new type of pulse generation technique has thus been named as "coupled-cavity mode locking" and it has been shown in this work that the coupled-cavity mode-locked KC1:Ti colour-centre laser is a very useful femtosecond laser source. Through replacing the traditional electron-optical streak camera tube by an optical Kerr material and using the optical (pulse) beam to scan the pulse(s) to be measured, a novel pulse measurement method have been proposed. For this new measurement scheme no electron-photon transformation and any other electronic systems are involved, and therefore it is expected to have the advantages of easy operation and high resolution. Study of the relationship between the temporal and the spectral features of an optical field suggests that a Fourier transform of a first-order spectral autocorrelation of a femtosecond pulse will give rise to the intensity profile of the pulse. A schematic for the measurement of the first-order spectral autocorrelation is also configurated. A strong similarity between the impact of dispersion on the temporal feature and that of self-phase modulation on the spectral feature of optical pulses has been found. This leads to two important inferences: first, the spectral extension induced by self-phase modulation is linearly proportional to the peak optical intensity only when the associated nonlinear phase shift is sufficiently large, second, if the incident pulses are appropriately chirped a suitable length of SPM-medium can be used to compress the incident spectrum (rather than broaden it). To initiate coupled-cavity mode locking a minimum optical power coupled into the control cavity is required. The specific value of this threshold is not only dependent on the properties of the nonlinear medium such as the length of the fibre, the magnitude of the Kerr coefficient, but also on the main cavity parameters, e. g. the main cavity power level and the effective gain bandwidth. Such a fact simply illustrates that to understand the physical mechanisms involved in the coupled-cavity mode locking the interactive function of various factors in both cavities must be considered. The upper-limit of the nonlinear phase shift, phiNL, experienced by the pulses coupled into the control cavity is found to be phiNL ≤ 2TC. Above this value, further increase of the intrafibre power would lead to the laser operation in an unstable regime. In addition, if the peak optical intensity inside the fibre core is sufficiently high the other nonlinear effects such as modulational instability and self-Raman action can occur besides self-phase modulation. In these cases the process of mode locking enhancement may be frustrated. By incorporating an Er-doped mono-mode optical fibre in the control cavity of such a coupled-cavity mode-locked KCl:Ti laser has led to the generation of 75 fs pulses at 1.5 mum. This result represents the shortest pulses produced with this method in a non-soliton regime.

A continuous-wave, frequency-doubled, diode-pumped Nd:YLF laser, capable of generating 1-W of single-frequency radiation at 523.5 nm, with a linewidth of 10 kHz and frequency tunability of 9 GHz has been developed. By using the tunable green laser as pump source, 4.5 GHz continuous smooth tuning in the range of 10²⁰ - 10⁷⁰ nm has been demonstrated in a low threshold doubly-resonant optical parametric oscillator. The investigation of thermal effect in the end-pump Nd:YLF laser crystal and the consideration of diode array end-pump geometry have led to an optimum folded-cavity design. Such an optical resonator can eliminate the astigmatism in the laser output, which is induced by the cavity folding on a curved mirror and the anisotropic thermal effect in Nd:YLF, resulting in a circular fundamental laser mode. In addition, a tightly focused beam waist is produced inside the cavity so that efficient intracavity SHG can be achieved. Over 30% optical conversion efficiency from diode to TEM00 10⁴⁷ nm laser output and 10% conversion efficiency from diode to single-frequency SHG green radiation has been demonstrated. A novel intracavity birefringent filter frequency selection technique has been applied in the standing-wave laser resonator to achieve the single-frequency operation. The performances of the laser in the fundamental wave and in second harmonic generation are investigated in detail in this thesis. The tuning behaviour and stability requirements of type-I and type-II CW doubly-resonant OPO have been compared by using the above green laser as the pump source and LBO, KTP as nonlinear crystals. Through utilisation of a simple cavity length servo, type-II phase matching allows single signal-idler mode-pair operation. By means of pump frequency tuning, and cavity length servo control, the output of signal and idler frequency can be tuned continuously over the KTP crystal phase-matching range. Theoretical analyses on diode-end-pumped Nd:YLF lasers, intracavity frequency doubling, the novel intracavity birefringent filter, and the timing behaviour and stability requirements of single cavity DRO are also presented in this work.

This thesis is concerned with the design and construction of an all-solid-state, tuneable, pulsed Cr:LiSAF laser and its evolution into an all-solid-state, single-frequency, pulsed Ti:sapphire laser. The initial investigation into the pulsed operation of the vibronic medium CrrLiSAF led to the development of a novel 1.3mum Nd:YLF pump laser. The development of this diode-pumped, Q-switched, 1.3mum Nd:YLF laser is described in detail. This included a comparison of the stimulated emission cross-sections of the 1.321(alphamum and 1.313(0,m lines, which were shown to be equally strong. The frequency-doubling of the 1.321(mum laser in LBO and KTP is compared and the performance of LBO is shown to be superior in this situation. The resulting 660.5nm laser was used as the pump source for a gain-switched Cr.LiSAF laser. The operation of a tuneable, gain-switched CnLiSAF laser with line-narrowing is described in detail. Specifically, the factors limiting its suitability as a diode-pumped, tuneable, single- frequency, pulsed laser medium are addressed. This was achieved, in part, by the construction of an equivalent gain-switched Ti:sapphire laser which was pumped by a frequency-doubled 1mum Nd:YLF laser. The performance of the two gain-switched vibronic lasers is compared when line-narrowed by interferometric means. Ti:sapphire was established as the superior option in this regard and it was developed further, culminating in an all-solid-state, high repetition rate, gain-switched, single-frequency laser. Single-frequency operation was achieved by the technique of injection- seeding the Ti: sapphire resonator with a single-frequency diode laser. The development of this laser, specifically in its high repetition rate form, is considered to be of great interest and potential.

This work contains theoretical and experimental studies of the properties of three-level atoms subjected to two single-frequency, continuous-wave laser sources with special emphasis on magnetic-field-induced sum frequency mixing (SFM) and electromagnetically-induced transparency (EIT). In sodium vapour, two resonant-enhancement routes for SFM were experimentally studied. In one route (3S-3P-3D) the output is generated on a quadrupole-allowed transition and on the other (3S-3P-4P), a dipole-allowed transition. Phase matching conditions are studied in detail. On the second route, the two contributions to the phase mismatch can combine to heavily distort the spectroscopic lineshapes observed. Doubly-resonant SFM is considered and two paths for excitation of the output coherence are identified. Control of their relative strength, via the input laser strengths, is predicted, as well as interference between them. These are both experimentally observed on the 3S1/2-3P1/2-3D3/2 transition scheme in sodium. Theory is presented to show how the use of a strong upper laser in resonant SFM can, by EIT, greatly reduce the absorption on the lower transition without similarly reducing the nonlinear process. A significant increase in conversion efficiency is predicted. Theory is also presented on the inclusion of the local field correction, which is relevant for high vapour densities, in the Bloch equations for the three-level atom. Nonlinear coherence cross-coupling terms arise and the generalised equations are then used for two case studies; one concerning EIT, the other SFM. EIT is experimentally studied using continuous-wave lasers in rubidium vapour. Focusing and de-focusing were observed on the probe laser beam as it tunes through the transparency window at close, but separate, detunings. This is attributed to the radial intensity profile of the coupling laser which imposes a spatial refractive index modulation on the medium at the probe laser wavelength.

The design, construction, evaluation and development of an all solid state, narrow linewidth laser source is presented. The narrow linewidth laser system was based on a miniature standing wave Nd:YAG laser cavity, end-pumped with 100mW of 809nm light from a fibre coupled GaAlAs diode laser array. This basic CW laser generated up to 30mW at 1064nm in a single, diffraction limited transverse mode (TEM00) but multi-longitudinal mode output beam. The laser had a pump power threshold of 24mW and an optical to optical slope efficiency of 39%. A simple rate equation based numerical model of this laser was developed to allow various design parameters such as length of Nd:YAG gain medium and amount of output coupling to be optimised. Excellent agreement between the numerical model predictions of the output power as a function of input pump power and experimental data from the optimised multi-longitudinal mode laser was obtained. To restrict this laser to operate on a single longitudinal mode, twisted cavity mode and intracavity etalon, mode selecting techniques were investigated. Both methods were found to produce reliable single mode laser operation and resulted in output powers at the 10mW level. The relative free running frequency stability between a pair of single longitudinal mode diode laser pumped Nd:YAG lasers was investigated. By isolating these lasers from environmental noise using a small, custom built anechoic chamber the linewidth of the optical heterodyne signal between the two free running lasers was reduced from tens of megahertz to around 10kHz measured on a millisecond time scale. Further improvement in linewidth was achieved by actively locking the laser frequency to a novel ultra high finesse (F~12,500, free spectral range ~500MHz) spherical mirror Fabry-Perot reference interferometer using the technique of Pound-Drever locking. The locked laser displayed a maximum frequency deviation of only 1kHz from the centre of the reference cavity transmission and a frequency noise spectral density of ~20Hz/ √Hz at 1kHz. In one of the first reported demonstrations of an all solid state injection seeded laser system, this single frequency laser was used to injection seed a diode laser array, transversely pumped, Q-switched Nd:YAG laser to produce 0.25mJ, 35ns pulses in a single longitudinal, single transverse mode beam. Preliminary results on injection locking between two single frequency diode laser pumped Nd:YAG laser are also reported. A novel frequency stabilisation scheme based on resonant optical feedback locking iproposed and some preliminary experimental work on this technique is presented.

The work reported in this thesis is primarily concerned with the generation of ultrashort pulses from a NaCl:OH- colour-centre laser. Active mode locking of the NaCl:OH- laser by synchronous pumping and acousto-optic loss modulation was characterised in detail. Synchronously mode-locked pulses of 8 ps duration were compressed to 250 fs in an anomalously-dispersive optical fibre and a novel soliton-effect pulse compressor was constructed from a nonlinear fibre-loop mirror. Using this device, the synchronously mode-locked pulses were compressed to 300 fs with no discernible background radiation. The NaChOH- laser was coupled-cavity-mode locked using both nonlinear Fabry- Perot and Michelson cavity configurations, and pulses of 110 fs duration were obtained. A sawtooth amplitude modulation of the laser output was observed. This modulation, which arose from the beating of the mode-locked Nd:YAG pump laser and NaCl:OH- laser pulse trains, was avoided by frequency-referencing the colour-centre laser to the pump laser. A characterisation of the phase noise of the frequency-referenced, coupled-cavity mode- locked laser was performed. By replacing the frequency synthesiser for the pump laser mode locker with a crystal oscillator, the phase noise of both the Nd:YAG pump laser and the NaCl:OH- colour-centre laser were reduced by two orders of magnitude. The technique of self-mode locking was successfully applied to the NaCl:OH- laser. For this laser, it was necessary to include a rod of high-nonlinearity lead-silicate glass in the laser cavity to achieve sufficient self focusing for self-mode locking. Stable mode-locking was initiated by a regenerative acousto-optic modulation and pulses of 95 fs were obtained. The thesis concludes with descriptions of some experiments performed using a mode-locked NaCl:OH- laser and a KCl:TiO(1) colour-centre laser. Efficient pulsed Raman amplification in an optical fibre was achieved by using an optical fibre thats group-velocity dispersion was the same for both the pump and signal wavelengths. By co-propagating pulse trains from the KCl:TiO(1) and NaCl:OH- lasers through a semiconductor optical amplifier cross phase modulation was observed. Ultrafast all-optical switching using the nonlinearity at half the bandgap was demonstrated for an GaA1As integrated interferometer. This nonlinearity was subsequently used to coupled-cavity mode lock the KCl:TiO(1) colour-centre laser.

This thesis is concerned with the generation of ultrashort pulses from vibronic lasers. The two laser active materials used in the course of the work were Ti:sapphire and Cr:LiSAF. A self-modelocked Ti:sapphire laser has been described which generated pulses as short as 2ps, tunable over the wavelength range 730 - 850nm. The average output power was 400mW corresponding to a peak pulse power of 1kW. Using a prism sequence to implement intracavity dispersion-compensation resulted in the generation of near-transform limited pulses as short as 53fs with a peak pulse power of ~100kW. Two initiation techniques have been developed for the generally non-self-starting self- modelocking process, based upon intracavity insertion of either a regeneratively driven acousto-optic modulator or a solid-state saturable absorber. A cw Cr:LiSAF laser pumped by the 476.5nm line of the argon-ion laser output, was demonstrated; this generated a maximum output power of 300mW with a slope efficiency of 20% at 825nm. A dispersion-compensated self-modelocked Cr:LiSAF laser has been described that generated pulses as short as 45fs over the tuning range 770-910nm. The peak pulse power generated was 40kW. The phase noise of a modelocked Ti: sapphire laser has been reduced by referencing the cavity frequency to an ultrastable crystal oscillator. The phase noise of the frequency locked laser was 410fs (100-500Hz), 305fs (500Hz-5kHz) and 263fs (5-50kHz). By referencing two modelocked Ti:sapphire lasers to a common crystal oscillator two independently tunable pulse sequences with a relative timing jitter of ~1ps have been generated. A novel laser based upon a single Ti:sapphire gain element generating synchronised pulses at two different wavelengths has been demonstrated. Cross-correlation data recorded between the two output pulse sequences indicated a relative timing jitter of 26fs.