Nd:YAG based laser sources for targeting applications

The aim of the research is to improve laser system products manufactured at Selex ES which are used primarily for airborne targeting applications. This is achieved by developments to the design that prevent failures during manufacture or improve beam parameters such as divergence. A Q-switched diode-side-pumped Nd:YAG zig-zag geometry slab laser within a cross Porro prism resonator is investigated. This perturbation insensitive resonator design is used in laser systems operating over the full military environment of vibration and temperature. A number of aspects of the design are computer modelled with experimental verification, such as the effects of thermal lensing in the Nd:YAG slab, and the polarisation states in the resonator. These were used to analyse a number of issues encountered during manufacture, such as the lack of control over the polarisation state for output coupling, pre-lase causing damage to optical elements, and thermal lensing producing variations in beam quality. A number of design changes were made and, after experiments to verify improved performance, they were successfully integrated into a number of laser production programmes. The beam quality of laser systems was found to be affected by thermal lensing. A number of novel solutions were tested experimentally, which affected the thermal lens. Results of the alteration of the pump distribution in the Nd:YAG slab and the profile of conduction cooling are presented. 885 nm pumping instead of the traditional 808 nm pumping produced a reduction of the thermal lens by a factor of two from -0.1 D to -0.05 D, producing an improvement in the laser beam quality from M2 6.5 to 3.5. An enhancement in brightness of 2.2 was demonstrated using a laser resonator incorporating a deformable bimorph mirror. A new concept for a targeting laser source, which incorporated an eye-safe wavelength, was demonstrated using a common resonator intracavity OPO design. A conversion efficiency of 40% was achieved for 36 mJ output of 1573 nm eye-safe light from a 90 mJ laser at 1064 nm. The relative pointing directions of the two wavelength beams was measured to be within 250 μRad angular separation, which will be unaffected by ambient temperature variation. This level of performance is challenging to achieve in the current laser system design incorporating an extracavity OPO

Development of versatile high power bounce geometry lasers

This thesis details an investigation into the development of bounce geometry lasers to achieve a more versatile range of laser characteristics. The bounce geometry has matured in recent years into a useful solid-state pumping scheme, but its performance has to date been limited by a number of factors, as well as largely restricted to neodymium systems. For real-world application, a more versatile range of laser characteristics would be desirable. A new design for a bounce geometry amplifier is presented that achieves a symmetric gain profile and thermal lens by control of the amplifier dimensions. The laser produces a circular stigmatic TEM00 (M2 < 1:11) beam with 14 W power. When Q-switched, the design permits versatile control over the repetition rate (single-shot to 480 kHz) with pulse energies up to 0.45 mJ. The stigmatic design also allows the direct generation of a Laguerre-Gaussian `vortex' beam, and proves favourable for modelocking with the nonlinear mirror method. Several designs are investigated to study power scaling in a master oscillator power amplifier (MOPA) configuration, including a stigmatic MOPA based on the amplifier described above, and a chain of multiple power amplifiers. A folded dual-pumped amplifier design is also demonstrated, which reduces the size and complexity of a multi-stage amplifier and allows power scaling to the 100 W level. Pulse amplification is also investigated, and a MOPA is optimised for energy extraction by a Q-switched oscillator. Finally a 3-micron bounce laser is presented using an erbium-doped YSGG gain medium. Different cavity designs are investigated, and a simple compact cavity is found to be optimum. Thermal effects are investigated and found to be a limiting factor on the laser's performance. Quasi-continuous wave pulse energies of up to 15 mJ are demonstrated, with an average power of up to 430 mW

Advances in solar-pumped laser efficiency and brightness

Advances in both solar-pumped laser efficiency and brightness are herein presented. Several solar laser prototypes with both end-side-pumping and side-pumping configurations were studied and developed to efficiently pump small diameter Nd:YAG laser rods, leading to substantial increase in solar laser collection efficiency and brightness, which have gained international recognitions. All the design parameters were optimized in ZEMAX© non-sequential ray-tracing software. LASCAD© laser cavity analysis software was then used to optimize the laser resonator parameters. Based on the numerically optimization of the solar laser system, the solar laser prototypes were designed and built in Lisbon. Solar energy collection and concentration were achieved through the PROMES-CNRS heliostat-parabolic system, NOVA Fresnel lens system, and the recently new NOVA heliostat-parabolic system. Measurements of the solar input / laser output performance, beam quality M2 factors, and laser beam profiles for both multimode and fundamental mode regime were performed and compared with that of the numerical results. 13.9 W/m2 solar laser collection efficiency was achieved in 2013, through PROMES heliostatparabolic mirror system, by end-side-pumping a 5 mm diameter, 25 mm length Nd:YAG laser rod. This result was further increased to 21.1 W/m2, in 2015, within the same solar facility. In 2016, 25 W/m2 collection efficiency was reported, by end-side-pumping a thinner laser rod through NOVA heliostat-parabolic mirror system. In addition to the enhancement of solar laser collection efficiency, the thermal performance of end-side-pumped solar laser was also substantially improved. In 2017, record solar laser collection efficiency of 31.5 W/m2 was reported by end-sidepumping a 4 mm diameter, 35 mm length Nd:YAG laser rod in PROMES-CNRS heliostatparabolic mirror system. Also, record slope efficiency of 8.9% was achieved. A substantial progress in solar laser beam brightness with Fresnel lens was reported in 2013, through the first TEM00-mode solar laser. 1.9 W solar laser brightness was registered, being 6.6 times more than the previous record. The adoption of an asymmetric laser resonator, for maximum extraction of TEM00-mode solar laser, was also essential for improving significantly the solar laser brightness. By side-pumping a 3 mm diameter, 30 mm length Nd:YAG rod with a double-stage rectangular light guide / 2D-CPC concentrator, 4.0 W solar laser brightness was reported in 2015, doubling the previous record with Fresnel lens. TEM00-mode solar laser collection efficiency of 4.0 W/m2 was obtained by side-pumping a Nd:YAG grooved rod in 2016. Most recently, by endside-pumping a 4 mm diameter, 35 mm length, Nd:YAG rod, the TEM00 mode solar laser collection efficiency was almost doubled, reaching 7.9 W/m2. Record-high solar laser brightness of 6.5 W was also achieved. Advances in solar laser beam stability were also achieved by developing sculptured twisted light guides for efficient uniform redistribution of pump light into a thin and long laser rod. In addition to this, we were also able to demonstrate the first emission of doughnut shaped solar laser beam, which may widen the applications areas of solar-pumped lasers. The research efforts performed during this work for enhancing both solar laser efficiency and beam brightness are explained. Experimental results are discussed and future suggestions are proposed

Power Scaling Feasibility of Chromium-Doped II-VI Laser Sources and the Demonstration of a Chromium-Doped Zinc Selenide Face-Cooled Disk Laser

Tunable lasers in the 2-4 µm wavelength range are needed for Air Force sensor applications. Chromium-doped II-VI materials are a promising class of laser material for tunable operation in this wavelength range, but until recently had not produced enough output power to meet application requirements. This dissertation investigates Cr2+:II-VI material properties and potential laser designs, then experimentally demonstrates and analyzes the performance of a Cr2+:ZnSe disk laser design that can produce sufficient output power. Cr2+:II-VI laser materials are found to be susceptible to overheating and thermal lensing, but are otherwise satisfactory laser materials. The most feasible laser design given a 15 W pump power limit was a face-cooled disk laser design using Cr2+:ZnSe. The experimental implementation of the laser design produced 4.3 W. However, the experimental laser worked well only under a restricted set of conditions, due to thermal lensing caused by a radially non-uniform absorbed power distribution in the laser disk. Design modifications are discussed which should reduce thermal lensing to acceptable levels. The conclusion is that Cr2+:II-VI laser sources can produce enough power for Air Force sensor applications, if enough effort is spent on ensuring adequate thermal management in the laser material

Development of pulsed diode pumped solid state lasers in the bounce amplifier geometry

The work in this thesis focuses on the development of diode pumped solid state (DPSS) lasers, constructed using the bounce amplifier geometry. The bounce amplifier geometry employs a simple side pumping scheme using diode bars and stacks, leading to an efficient and compact system. The laser mode is total internally reflected at the pump surface, spatially averaging gain and thermal non-uniformities giving potential for excellent beam quality from these systems. In this thesis, novel pulsed laser sources based on the 1µm transition of the Nd3+ ion and the 3µm transition of the Er3+ ion are developed, and investigated both experimentally and numerically. An acousto-optically Q-switched Nd:YVO4 laser operating at 1064nm with ultra-high gain (~105) is developed, and using a novel pulse control technique is demonstrated to provide better performance and greater flexibility of the pulsing parameters. Pulsed lasers are useful for many applications, including industrially in laser micromachining and laser marking, where having greater control of laser parameters (e.g. pulse repetition rate, pulse duration and pulse energy) enhances the usefulness of the laser significantly. A problem associated with Q-switching of a high-gain laser system is the difficulty in obtaining clean, single pulse operation from very high (~1MHz) to low (~1kHz) repetition rates. The pulse control technique demonstrated for the first time in this work addresses this issue. The technique uses a secondary laser cavity to control the gain of a primary laser cavity. Prior to implementation of the technique, laser breakthrough occurred at low repetition rates due to the excessive gain and single pulsed operation was not possible below 150kHz. Using the pulse control technique, single pulsed operation was obtained from 800kHz down to 1kHz, with good beam quality across the range, as well as the ability for pulse energy control demonstrated. The development of 3μm laser sources, using Er3+ doped materials is presented. Lasers operating at 3µm are useful directly in applications in medicine and dentistry due to being near the peak of water absorption, as well as indirectly as pump sources for optical parametric generation for production of tuneable mid-IR radiation for spectroscopy, security and defence, and remote sensing applications. In this work, a comparison of the Er:YAG and Er:YSGG laser materials operating at the 3µm transition is undertaken showing superior performance from the less commonly used Er:YSGG material. Different cavity designs are subsequently investigated using the Er:YSGG laser material and an electro-optically Q-switched Er:YSGG laser at 3µm is developed. Numerical modelling of the erbium laser is presented providing greater understanding of laser operation in this complex laser system.Open Acces

Development and characterisation of holmium and erbium lasers for the ablation of biological tissue

The development of pulsed laser systems operating in the infrared at 2.1 µm and 2.94µm, based on Cr:Tm:Ho:YAG and Er:YAG laser crystals, and the ability of these lasers to ablate biological tissue is reported. Thermal lensing in the laser crystals has been investigated and found to be the main factor restricting the operating ranges of these lasers. Additionally, increases in the threshold of holmium lasers due to thermal population of the lower laser level increases the amount of heat dissipated in the crystal lattice, leading to increased thermal lensing. Thus, the divergence properties of resonators containing these crystals depends, additionally, on the operating temperature. Modelling of the divergence behaviour of resonators based around Cr:Tm:Ho:YAG and Er:YAG laser crystals in simple resonator geometries is demonstrated using computer based ray tracing algorithms. The temporal behaviour of these lasers has been experimentally assessed and compared to a 'rectangular pump pulse' theory. Using this theory it is possible to predict the delay between the start of the excitation pulse and the start of the laser pulse but not the duration of the output pulse. The reasons for this are discussed. Pulses of 2.94µm radiation ablate soft tissue more efficiently than similar pulses of 2.1µm. Mass loss due to laser radiation is shown to be linear with dose for the 2.1µm radiation. However, at 2.94µm mass removal is impeded at high doses by the extension of a charred zone into the ablation crater. Operation at high fluences is required to overcome this problem. However, there is an increase in mechanical damage to surrounding tissue and a change in crater shape at fluences greater than 0.085 J mm-2 coinciding with a significant impulse being imparted to the tissue. The maximum mass loss per unit of delivered energy at 2.1µm and 2.94µm are approximately 48% and 60% of that expected for ablation of a pure water target. Routes for the energy loss are discussed. The energy lost in the form of kinetic energy is determined experimentally to be less than 1% of the total energy delivered. A linear model was found to best described the ablation performance at both wavelengths. The implications of these findings are discussed

Development of an Efficient Ti:Sapphire Laser Transmitter for Atmospheric Ozone Lidar Measurements

The impetus of this work was to develop an all solid-state Ti:sapphire laser transmitter to replace the current dye lasers that could provide a potentially compact, robust, and highly reliable laser transmitter for differential absorption lidar measurements of atmospheric ozone. Two compact, high-energy pulsed, and injection-seeded Ti:sapphire lasers operating at a pulse repetition frequency of 30 Hz and wavelengths of 867 nm and 900 nm, with M2 of 1.3, have been experimentally demonstrated and compared to model results. The Ti:sapphire lasers have shown the required output beam quality at maximum output pulse energy, 115 mJ at 867 nm and 105 mJ at 900 nm, with a slope efficiency of 40% and 32%, respectively, to achieve 30 mJ of ultraviolet laser output at 289 run and 300 nm with two LBO nonlinear crystals

Nonlinear optical functionalities of VO2- and GaN-based nanocomposites

This thesis presents fundamental research and concepts for active photonic elements operating in the telecom wavelength regime. The aim of the study is to determine the characteristics of the investigated nanostructures and to evaluate the implementation of the proposed materials in potential optical devices. In the first part of this thesis the optical properties as well as the photonic application of vanadium dioxide (VO2) nanocrystals (NCs) are studied. VO2 exhibits an easily accessible insulator-to-metal phase transition (IMT) near ambient temperatures. Upon excitation it undergoes an atomic rearrangement that is accompanied by a substantial modification of the complex dielectric function. When VO2 undergoes the IMT, the near-infrared transmission peaks of a moderate-finesse etalon containing a sub-wavelength layer of VO2 NCs are found to markedly shift in their spectral position and peak transmissivity. Both heat deposition and optical excitation permit to actively control the etalon’s functionality. Much less is known about the nonlinear optical properties of VO2 beyond the established IMT. To this end the nonlinear optical response of a thin film of VO2 NCs is investigated with open aperture z-scans involving femtosecond near-infrared pulses. A pronounced saturable absorption on the short-wave side of the resonance as well as a marked reverse saturable absorption in the telecom window are observed. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers. In the second part a semiconductor heterostructure based on hexagonal ultranarrow GaN/AlN multi-quantum wells (MQWs) is investigated. The tailored inter-miniband (IMB) transition is characterized in terms of its linear as well as ultrafast nonlinear optical properties using the established pump-probe scheme. In line with theoretical predictions for LO-phonon scattering, a fast relaxation is found for resonant IMB excitation. In stark contrast, significantly larger relaxation times are observed for photon energies addressing the above barrier continuum. The last section reports on a new type of nonlinear metasurface taking advantage of these telecom-range IMB transitions. The heterostructure is functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second harmonic generation at normal incidence which is completely absent for an antenna array without the heterostructure underneath

Computer modeling of ultrashort pulsed laser ablation of diamond and graphite with experimental verification

Ultrashort pulsed lasers create a fundamentally different ablation mechanism than conventional pulsed lasers because of the ultrashort laser pulse\u27s extreme intensity (\u3e 1012W/cm2) and time duration (\u3c 10--12s), which is shorter than the electron-lattice transfer time (\u3e10--12S). Consequently, a thermally excited plasma is generated in a cool lattice. Assumptions used in conventional pulsed laser ablation are invalid for ultrashort pulses. In this work, computer modeling of ultrashort pulsed ablation was performed for diamond. A two-step model was developed in which a heat transfer, finite-difference model was formulated and tight-binding molecular dynamics simulations were performed to evaluate the dynamics of ablation events. The heat transfer model incorporated absorption of the laser light by the electrons and predicted the thermal profiles within the electrons from the start of a laser pulse to 1 ps. The tight-binding molecular dynamics predicted the threshold electron temperatures (room temperature lattice) and overall equilibrium temperature (electron and lattice at the same temperature) required for changes in structure and ablation to occur in the material. The results of both simulations were then used to predict ablation threshold, ablation volume, and the size of the heat-affected zone within the material;Ultrashort pulsed laser ablation experiments were performed on chemical vapor deposited and on single crystal diamonds, as well as on highly-oriented pyrolytic graphite, in order to verify the model predictions. Scanning electron microscopy, atomic force microscopy, profilometry, and micro-Raman spectroscopy were employed to characterize the ablated surfaces. Results showed that ultrashort pulses, compared with nanosecond laser pulses, yield lower threshold fluences, higher material removal rates, and much more precise ablation, all of which are attributed to the increased absorption coefficient and improved energy coupling. The most significant observation is that the surfaces of diamond and graphite did not undergo phase transformation, demonstrating that chemical cleanliness is increased with use of ultrashort pulses rather than nanosecond or longer pulses. In addition, thermal damage and the associated debris and recast layer formation were non-existant with ultrashort pulses. The investigation further showed that ultrashort pulsed lasers significantly reduced the feature size and improved the feature resolution, leading to sub-micron machining, which is not achievable in nanosecond or longer pulsed lasers. These experimental observations are consistent with predictions base on the finite difference and molecular dynamics models

End-pumped solid-state lasers.

Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2010.This dissertation consists of four sections, with the focus on near- and mid-infrared lasers using Yttrium Lithium Fluoride (YLF) crystals doped with various rare-earth ions as a gain medium. As introduction a general overview of the concepts pertaining to end-pumped solid- state lasers are presented. The basic principles, components and operation of lasers are discussed. Stimulated emission, laser gain media, pump sources and pump geometries are elaborated upon. Three-, four-, quasi-three- and quasi-four-level laser schemes are described. Finally, the advantages and disadvantages of end-pumping as opposed to side-pumping schemes for solid-state lasers are discussed. Thereafter, the design and results of a high-powered diode-end-pumped Nd:YLF laser is presented. In conjunction with previously demonstrated methods, the thermal fracture issues of Nd:YLF were addressed by utilizing the natural doping gradient along the boule of the crystal. This, in addition to a novel crystal mounting technique, resulted in the highest reported output power from a diode-end-pumped Nd:YLF laser as well as record pumping powers. In the third section, a compact Ho:YLF oscillator-ampli er system is reported. The novel setup utilised the unpolarised pump power from a bre-laser e ciently by using the pump light transmitted by the oscillator crystal to pump the ampli er crystal, which produced 21.3mJ at 1 kHz, with an M2 better than 1.1. Lastly, the conclusion is drawn that YLF as a host material can be used in a highly successful manner for high-power applications. Additionally, the novel pumping scheme implemented in the Ho:YLF oscillator-amplifier has been shown to be scalable by a subsequent system which delivered record performance