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Multiple quantum well lasers based on the III-V compound semiconductor indium phosphide were studied in this thesis. Systematic investigations were carried out on the effects of quantisation and strain on the temperature sensitivity of threshold current, modulation properties, external cavity operation and the 'gain lever' effect. Trends in the temperature sensitivity of threshold current for lasers of various lengths and quantum well numbers can be understood in terms of the nonlinear gain-current density relation. However, compared to bulk active region devices there was no significant improvement. It is unclear at present whether this is due to the temperature sensitivity of optical gain or Auger recombination. Relative intensity noise measurements were used to compare the modulation properties of lattice-matched and strained MQW lasers. This method gives an estimate of the differential gain and was used to study the properties of devices which were unsuitable for high speed applications. These measurements showed that gain saturation and carrier transport effects may be significant in certain laser structures. Band-filling effects due to the reduced volume and modified density of states were utilised in the demonstration of a grating external cavity laser operating across the 1.3 mum optical fibre window. This configuration results in single-mode operation across the gain spectrum of the laser. High output power was exhibited across a wide tuning range. The 'gain lever' effect uses the nonlinear gain-current density relation to enhance the amplitude modulation efficiency and the signal-to-noise-ratio. MQW and bulk devices of different length and split ratio were compared. It was seen experimentally and numerically that the nonlinearity causing this effect also increases the distortion. Finally, it is discussed that further modifications to laser properties may be seen in active regions incorporating quantum wires and boxes.

This thesis concerns the generation of tunable ultrashort pulses near the 1.55 mum telecommunications window. Two principal laser systems are considered: i) the NaCl:OH colour-centre laser, which employs the technique of synchronously-pumped modelocking to generate tunable picosecond pulses and ii) the self-modelocked Cr4:YAG laser to generate femtosecond pulses tunable from 1.5-1.56 mum. Details are given for an all-solid-state cw and cw-modelocked pump source for Cr4:YAG and colour-centre lasers based on Nd:YAG. Fibre-coupled AlGaAs laser diodes are employed as the solid-state pump source to this laser. When operated cw, up to 8.5 W of linearly polarised output power in a TEM00 beam is obtained. A compact cw actively-modelocked Nd:YAG laser is described having a pulse repetition rate of 194 MHz. Pulse durations down to 34 ps and output powers up to 6.0 W are obtained from this system. An 82 MHz Nd:YVO4 laser is also detailed producing pulsewidths down to 75 ps and average output powers up to 3.5 W. The intrinsic noise source associated with the synchronous modelocking technique is discussed and a simple passive stabilisation scheme, coherent-photon-seeding (CPS), is described and applied to the synchronously-modelocked NaCl:OH laser. Results of a simulation of this laser are reported and a comparison is made with the practical observations of the stabilised laser. For the first time, theoretical and experimental evidence for the presence of high frequency pulse jitter in synchronously-pumped- modelocked (SPML) lasers is presented and the coherent photon seeding technique is shown to eliminate this noise. Details are also given for the construction of a compact, all-solid-state, femtosecond Cr4+:YAG laser. A design prescription for laser resonators having a high propensity for self-modelocking is presented and an unconventional 3-mirror resonator is adopted for optimised self-modelocked operation. Using this design, modelocked output powers up to 300 mW with 120 fs pulses from a compact, regeneratively initiated laser having a pulse repetition rate of 320 MHz is reported for 4.7W incident pump power. Self- modelocking is demonstrated for pump powers down to ~1W with this cavity design. A compact cavity design for self-modelocking is also assessed, with a footprint of just 20 X 25 cm, which places a prism in each cavity arm. 470 fs pulses at 220 mW average output power are reported.

This thesis deals with gas contamination problems in atomic fluorine and KrF lasers. Five different versions of transverse electric discharge lasers were constructed using different materials, geometries, and discharge circuits. Three of these were investigated in detail for lifetime performance of a single fill of He-F2 mix (atomic fluorine laser) and of He-Kr-F2 mix (KrF laser). This was done using an on-line quadrupole mass spectrometer. The evolution of the various gas components appearing in the mix was calculated from the mass spectra of the gas mixture recorded at intervals when the laser was operated at one pulse per second. The impurity or contaminant species (ie species other than He, Kr, F2) were found to be the same in all the lasers, differing only in their concentrations. The major contaminants found were CO2, N2, O2, COF2, CF2, SiF4, HF, CO, NO, SF6, and H2O. Of these, CO2 was identified as the most deleterious impurity for both atomic fluorine and KrF lasers, reducing their output energy and the fluorine content, thereby reducing the single fill lifetime of the mix. A simple cold trap decreased the partial pressures of most of the detected impurities in the gas mixture and resulted in a marked increase in the number of shots to half energy and a decrease in the depletion rate of fluorine by half in both KrF and atomic fluorine lasers. To isolate and quantify the effects of individual contaminant species, they were deliberately added singly to the pure gas mix. These experiments confirmed that CO2 was the most important impurity and that H2 and CF4 were the least harmful. Since the impurities were found to influence arcing of the discharge, they must affect preionization and/or discharge processes. Absorption effects were found to be insignificant in the regions of the laser wavelengths (248 nm, 730 nm). In order to quantify the effect of the impurities an effectiveness constant for each contaminant has been defined and used in a simple model which successfully calculates the laser output energy. This model has circumvented the problems arising from the physical complexity of the system and the lack of data on the various kinetic processes. The calculations show that CO2 impurity depletes the energy output of the KrF laser at the rate of 0.9 mJ/ppt(wrt He).

This thesis reports the first comprehensive comparison of the operating regimes of the three major types of Cu halide laser, which oscillate on the 510.6 nm and 578.2 nm resonance-metastable transitions of atomic Cu in pulsed discharges at 10-50 kHz pulse recurrence frequency. The three lasers had similar active volumes (36.8-43.5 cm3) and bores (12.5-13 mm), were excited using the same power supply and circuit and monitored using the same diagnostic apparatus. The CuBr-Ne laser produced an annular output beam, weighted towards the yellow transition, with a maximum average output power of 3.55 W and a maximum efficiency of 0.71 %. When H2 gas was added to this laser at a level of ~5%, the output beam developed an axial (central) peak in intensity, the beam was less constricted, the balance of green and yellow powers was improved, the output power rose to a maximum of 11.4 W and the maximum efficiency reached 1.47 %. In both of these lasers, the CuBr vapour was generated by heating a sidearm of the discharge tube. The vapour was entrained in a flow of Ne buffer gas to seed the active volume. A Cu hybrid laser, where CuBr is generated in the tube in situ by reaction of the discharge products of a Ne-HBr buffer gas with Cu pieces in the tube, has been compared to the two conventional CuBr lasers. The Cu hybrid laser also produced an output beam with a central maximum, little or no constriction and a good balance of green and yellow powers. Maximum average output power reached 12.8 W and the maximum efficiency was 1.66%. In terms of specific average output power, the hybrid laser was clearly superior to the other two, with values of 82 mW.cm-3 (CuBr), 262 mW.cm-3 (CuBr-H2) and 348 mW.cm-3 (Cu hybrid). The specific output power of the Cu hybrid laser obtained in these studies is a record value for any Cu laser (including elemental Cu lasers) of tube bore ~12.5 mm. This result and the general dependences of output power on buffer gas pressure, additive (H2, HBr) pressure, pulse recurrence frequency and charging voltage and capacitances are discussed in detail in terms of the fundamental processes and chemical reactions. The most important processes responsible for the high powers and efficiencies and the Gaussian-like beam profiles in the presence of hydrogen are dissociative attachment of HBr in the interpulse period and at the beginning of the pulse, and the reduction of CuxBrx polymers and monomers by H2 to free Cu atoms in the active volume. This is the first time that the importance of hydrogen reduction in these lasers has been identified. Without it, the filling in of the annular output beam cannot be explained. The mechanism of Cu seeding of the hybrid laser has also been studied in detail, as it is the most obvious difference between the Cu hybrid and conventional CuBr lasers. The basic reactions of the seeding process are described, and it is found that in addition to Cu3Br3 and Cu4Br4 polymers there must be a substantial amount of CuH in the discharge to account for the large density of Cu atoms in free form and locked up in molecular forms. This is the first time that CuH has been suggested as a major Cu-bearing species. The process of Cu dendrite formation in the tube is also discussed. Finally, the properties of the hybrid laser have been considered from the point of view of scaling to very high average output powers. It has been shown that average output powers of 1 kW are possible using current technology.

Pulsed microwave excitation of noble gas ion lasers at frequencies between 3 and 17 GHz is investigated. The advantages of using microwaves instead of conventional DC sources to pump a laser are explained. These include the lower electrode and discharge tube wear due to the oscillating nature of a microwave electric field. The propagation of microwave radiation in an ionised gas is examined. At the frequencies used, the skin depth of an Argon ion laser discharge is shown to be approximately 1 mm, indicating good microwave power absorption. The dependence of the microwave power absorption on the frequency is shown to be weak. Microwave transmission at a dielectric/gas-discharge boundary, similar to those in the laser coupling structures used, is found to be around 1% of the incident power. It is suggested that for maximum laser efficiency, microwave power should be introduced directly into the gas discharge. Two microwave coupling structure designs for supplying microwave power to the laser discharges are described. The first of these, a waveguide coupler device based on the 3dB branch guide coupler, produces a transverse electric field across the laser tube. The procedure used to design a branch guide coupler using a Chebyshev impedance taper and T- junction discontinuity corrections is outlined, and a description of the entire laser coupling structure is given. The second design comprises a helix wrapped round the laser tube and produces an axial electric field. The electric field distribution around a helix is calculated as a function of helix parameters and the effects of surrounding objects are considered. The best helix dimensions are found for optimum laser operation. The characteristics of conventional Argon ion and Helium-Krypton ion lasers are given. No significant differences between conventional noble gas ion lasers and the microwave excited lasers reported here are observed. At the input powers used (~100 kW peak, 1 uS pulses, 1000 pps), 100 mW, 1 uS and 30 mW, 5 uS laser pulses are observed from Argon and Helium-Krypton gas mixtures, respectively. The transverse and axially excited lasers perform equally well.

Infrared, ultraviolet and mass spectrometric techniques have been used to investigate the short gas-life of discharge excited XeF lasers, for He/Xe/NF3 mixtures. Infrared absorption studies provided initial information on the changes which occur in the laser gas composition during pulsing. The information was used to complement those of mass spectrometric studies, in which the chemical composition of laser gas mixtures were determined as a function of the number of laser pulses. Ultraviolet absorption spectroscopy was used to study optical absorption at the laser wavelength in fresh and used gas mixtures. The effects of several contaminants on laser performance were studied by adding small concentrations of these contaminants to the basic gas mixture of He/Xe/NF3. The results provided information on the identity of the main contaminants. Cold traps were used to extend the gas-life, and to identify dominant contaminants. The laser device used in this work is excited by a conventional blumlein circuit, which is triggered by a pressurised spark-gap switch. For a single gas fill of the basic mixture (He/Xe/NF3), the number of laser pulses to half-energy is approximately 150/litre atm. Infrared and mass spectrometric studies show that the fast deterioration of laser performance is due to the depletion of NF3, and to the accumulation of contaminants in the laser. The contaminants have been identified as N2, O2, NO, NO2, N2O, CO, CO2, NF2, N2F2, HF and CF4. Of these, NO2 absorbs at the laser wavelength (351nm), but the absorption coefficient in used gas mixtures is small compared to small signal gains of laser devices similar to the one used in this work. There is strong evidence that water vapour, which is one of the main impurities in fresh gas mixtures, may be the source of oxygen in the formation of oxides of nitrogen (NO, NO2, N2O) and carbon (CO, CO2) For fresh gas mixtures, the laser pulse energy is insensitive to the addition of small concentrations of N2, H2 and CF4. In contrast, the addition of 0.05% of CO2, CO and O2 results in approximately 60, MO and 20% reductions in the laser pulse energy respectively. The estimated change in laser output after 1000 shots due to the accumulation of CO2, O2 and CO is 20, 10 and 5% respectively. The addition of small concentrations ( < 1 torr) of N2 CO2, CO, O2 and H2 results in negligible changes in the gas-life. However, when 2 torr of CF4 is added to the basic mixture of He/Xe/NF3 a threefold increase in the gas-life is observed. The improved gas-life is attributed to lower rates of formation of O2, NO2 and NO. After using He/Xe/NF3/H2 mixtures, the gas-life of the basic mixture increased by a factor of five to about 700 shots/litre atm. Mass spectrometric analysis of the gas mixture before and after lasing shows that the improvement in the gas-life is mainly due to lower levels of O2, NO2 and NO, and to a significant reduction in the rate of depletion of NF3. The eventual deterioration of laser performance is mainly attributed to the accumulation of CO and CO2 in the laser. For the laser device and gas mixture used in this work, the optimum trap temperature lies in the region of -100 to -150&deg;C. For a trap temperature of -150&deg;C, the gas-life is 1500 shots/litre atm for a single gas fill. This is about 2.5 times the best result obtained without the use of cold traps. The eventual termination of laser action is due to NF3 depletion and the accumulation of contaminants in the laser. By boiling-off the contaminants sequentially, CO2 has been identified as a major contaminant.

Experimental and theoretical aspects of the formation of excimer molecules in gas discharges are described. Experimental studies are of the emission spectrum of rare-gas halide molecules, produced in a microwave discharge, and cover aspects relevant to rare-gas halide lasers and to the possibility of pumping these lasers by microwave discharge. The experimental work consists mainly of an extensive programme testing mixtures of a rare-gas (Xe, Kr, or Ar) with a halogon donor (SF6, NF3, or HCl) and a buffer gas (He, Ne, or Ar). The majority of work concerns KrF and XeF but some results are also presented for ArF, XeCl, and KrCl. The spectrum of XeO is observed. The main variables are mixture ratio and total pressure and the principal measurements are available either from integrated spectra or from temporal records of the intensity at the spectrum peak. The results form an empirical optimisation study. One of the aims of this study was to find the mixture ratios and total pressures for which the fluorescent emission, for any given excimer system, was a maximum. These ratios and pressures could then be used as a starting point in attempts to obtain lasing. Considerable use was made of SF6, as the donor because it is a cheap alternative to NF3 and has not been studied in detail as a donor elsewhere. Results are extracted from the main body of results in order to compare the performance of these two donors and to compare the performance of the three buffers. It is shown conclusively that greater lifetimes are obtainable with SF6 but at the cost of lower intensities. In the buffer comparision much lower fluorescent emission is observed when Ar is used, in both XeF and KrF mixtures, and that discharges are only formed below about 100Torr. Below the optimum dilutions the results for He and Ne aresimilar. However, at higher dilutions in XeF and KrF mixtures, fluorescent intensities are significantly greater, and discharges may be obtained at greater total pressure, with Ne buffer. In XeF mixtures it is found that gas lives are 4 to 8 times as long with Ne buffer as they are with He buffer. The temporal records provide gas life information on a scale ranging from 300 to 10^6 pulses. To reveal behaviourover fewer pulses and within single pulses a high speed system (5ns response time) was developed using a monochromator, fast photomultiplier, and storage oscilloscope. Because of the very small ratio of the pulse width to the inter-pulse period a blanking system was devised to remove the over-bright base line occurring during the inter-pulse period. The high repetition rate of 1100pps and much reduced volume enabled gas life studies to be carried out in times which are orders of magnitude smaller than those required using the 10pps of a conventional excimer laser system with much greater volume.

The vibrational kinetics of the CO2 laser system are studied experimentally and theoretically. A sequence (00°2) band/ fundamental (00°1) band gain ratioing technique is used to measure the CO2 asymmetric stretch mode temperature (T3) in low-pressure cw laser discharges; the relationship of discharge current to electron density is determined by X-band microwave cavity resonance. The experimental measurements are compared to theory using a comprehensive computer model of CO2 laser kinetics, based on the vibrational temperature approximation. It is demonstrated that the observed saturation of vibrational temperature with increasing discharge current is caused by the de-activation of excited molecules by electron superelastic collisions, at a rate predicted by the principle of detailed balance. Superelastic collisions crucially determine the attainable vibrational temperatures, and limit T3 to values below the optimum for 00°1 or 00°2 band laser action. The associated laser gain limitations are investigated, and it is shown that superelastic collisions inflict efficiency losses on pulsed TE CO2 lasers even at moderate input energies. The operating characteristics of CO2 sequence band lasers are also examined. A comparison of oscillator performance with corresponding small-signal gain measurements indicates a sequence band saturation intensity which, is higher than that of the fundamental band. This observation is supported by model computations, which predict that the. extractable 00°2 band laser power (alphao Is) is typically 60% of that available on the 00°1 band.

This thesis describes the theoretical and experimental investigation into many aspects of circuit and component design for high average power short pulse systems. The primary objective of this work is to develop both circuit design criteria and pulsed components for long life rare gas halide lasers. A pulsed system consists of three major components, energy storage, switches, and load. This investigation considers the type of circuit which uses capacitive energy storage in combination with a closing switch to transfer electrical power to a load. Specific loads are not addressed but the implications of load characteristics that affect circuit/component design and life are considered. The investigation reported, describes the physical and electrical characteristics and analysis of phenomena that adversely affect the performance and life of pulse duty components. In the area of capacitive storage, lifetimes of one particular design was improved by 3 orders of magnitude and a means of detecting the imminent failure of oil filled capacitors was devised and patented. In the area of switching, methods are described by which hydrogen thyratrons can be operated in parallel with equal current sharing without the need for inductive or resistive ballasting. Finally, the design and testing of a 200 watt XeCl laser modulator is presented.