High spatial and spectral quality diode-laser-based pump sources for solid-state lasers and optical parametric oscillators

In this thesis the use of high spatial- and spectral-quality diode-laser pump sources for solid-state lasers and continuous-wave optical parametric oscillators (cw OPOs) is investigated. While diode lasers are potentially attractive, compact, low-cost pump sources for solid-state lasers and cw OPOs, the difficulty in obtaining moderate output powers, while retaining high spatial beam quality and spectral purity, often limits the potential of such lasers in these applications. Techniques for obtaining high-power, high spatial- and spectral-quality output from diode lasers are reviewed and the design, development and characterisation of an injection-locked broad-area diode-laser system is described. This system produced output powers of &ap;400mW in a near-diffraction-limited beam (M2&ap;w1.3) and with a spectral width of <30MHz. The injection-locked system was used as the pump source for a quasi-three-level 946-nm Nd:YAG laser. End-pumped solid-state lasers of this type can offer potentially efficient, low-threshold operation if a near-diffraction limited pump source is used allowing optimal overlap with the laser mode. A model, including pump beam quality effects, is developed for such lasers and used to highlight the advantages of a near- diffraction-limited pump source, especially in the case of the 946-nm Nd:YAG transition which suffers from low gain and significant reabsorption losses. A 946-nm Nd: YAG laser pumped by the injection-locked system is described, yielding cw output powers up to 120mW with a 46% slope efficiency, performance comparable to Ti:sapphire- or dye-laser pumping, and 27ns Q-switched pulses having peak powers of 180W. 50W, 20ns pulses at 473nm were obtained by second-harmonic generation in KNbOs. The performance and relative merits of various cw OPO configurations, in the context of diode-laser pumping, are discussed and the development of a doubly- resonant OPO (DRO) based on periodically-poled lithium niobate is described. When pumped by the injection-locked system, this device showed a threshold of 25mW and .tuning of the outputs over 1.15- 1.25 mum at the signal and 2.3-2.65 mum at the idler was obtained by variation of crystal temperature, PPLN grating period and pump wavelength. When pumping with a 100mW single-mode diode laser, a 15mW OPO threshold was observed while retaining a similar tuning range. This represented the first demonstration of a cw DRO directly pumped by a single-mode diode laser. The achievement of such spectral coverage while pumping with this source points to the potential of such systems as compact, tunable sources in the near-to mid-infrared

Incoherently pumped continuous wave optical parametric oscillator broadened by non-collinear phasematching

In this paper, we report on a singly resonant optical parametric oscillator (OPO) pumped by an amplified spontaneous emission (ASE) source. The pump focusing conditions allow non-collinear phasematching, which resulted in a 230 nm (190 cm$^{-1}$) spectral bandwidth. Calculations indicate that such phasematching schemes may be used to further broaden OPO spectral bandwidths.Comment: 7 pages 4 figure

Economic Review of Pasture Development Options for North Australian Beef Enterprises

Beef cattle grazing (~14 million head) native pastures is the dominant economic use of northern grazing lands (2.3 million km2). Few enterprises make positive economic returns in most years or achieve the necessary productivity gains (~2% per annum) to offset an ongoing cost-price squeeze (McCosker et al., 2010). A significant contributor to poor performance is low reproductive performance, management of first calving heifers, calf growth and weaning rates and liveweight gain - linked to nutrition and the low quality of pastures. Pasture development technologies (Gramshaw and Walker, 1988) are available but uptake has been poor. Three pasture development options offering technical promise include (a) mosaic irrigation - small-scale schemes utilising favourable soils and access to water, (b) broad-scale over-sowing of native pastures with improved grasses and legumes, (c) high intensity-short duration (cell) grazing and (c) increasing stock access to underutilised pasture resources by expanding water and fencing infrastructure. How these options might alter the economic performance of enterprises has received limited attention. A formal review employed simulation models and regional case studies to explore the scope for mosaic irrigation to change the production and marketing orientation of northern beef enterprises and deliver economic benefits (MacLeod et al., 2013). Consideration was also given to alternative development options viz. broad-scale pasture sowing, high intensity-short duration (cell) grazing, and additional water and fencing infrastructure. The economic results of these options for three of the regional case studies are summarised in this paper

Shedding Light on Capillary-Based Backscattering Interferometry

Capillary-based backscattering interferometry has been used extensively as a tool to measure molecular binding via interferometric refractive index sensing. Previous studies have analysed the fringe patterns created in the backscatter direction. However, polarisation effects, spatial chirps in the fringe pattern and the practical impact of various approximations, and assumptions in existing models are yet to be fully explored. Here, two independent ray tracing approaches are applied, analysed, contrasted, compared to experimental data, and improved upon by introducing explicit polarisation dependence. In doing so, the significance of the inner diameter, outer diameter, and material of the capillary to the resulting fringe pattern and subsequent analysis are elucidated for the first time. The inner diameter is shown to dictate the fringe pattern seen, and therefore, the effectiveness of any dechirping algorithm, demonstrating that current dechirping methods are only valid for a subset of capillary dimensions. Potential improvements are suggested in order to guide further research, increase sensitivity, and promote wider applicability

An Analysis of Semicircular Channel Backscattering Interferometry Through Ray Tracing Simulations

Recent backscattering interferometry studies utilise a single channel microfluidic system, typically approximately semicircular in cross-section. Here, we present a complete ray tracing model for on-chip backscattering interferometry with a semicircular cross-section, including the dependence upon polarisation and angle of incidence. The full model is validated and utilised to calculate the expected fringe patterns and sensitivities observed under both normal and oblique angles of incidence. Comparison with experimental data from approximately semicircular channels using the parameters stated shows that they cannot be explained using a semicircular geometry. The disagreement does not impact on the validity of the experimental data, but highlights that the optical mechanisms behind the various modalities of backscattering interferometry would benefit from clarification. From the analysis presented here, we conclude that for reasons of ease of analysis, data quality, and sensitivity for a given radius, capillary-based backscattering interferometry affords numerous benefits over on-chip backscattering interferometry

Design principles for bifunctional targeted oligonucleotide enhancers of splicing

Controlling the patterns of splicing of specific genes is an important goal in the development of new therapies. We have shown that the splicing of a refractory exon, SMN2 exon 7, could be increased in fibroblasts derived from patients with spinal muscular atrophy by using bifunctional targeted oligonucleotide enhancers of splicing (TOES) oligonucleotides that anneal to the exon and contain a ‘tail’ of enhancer sequences that recruit activating proteins. We show here that there are striking agreements between the effects of oligonucleotides on splicing in vitro and on both splicing and SMN2 protein expression in patient-derived fibroblasts, indicating that the effects on splicing are the major determinant of success. Increased exon inclusion depends on the number, sequence and chemistry of the motifs that bind the activator protein SRSF1, but it is not improved by increasing the strength of annealing to the target site. The optimal oligonucleotide increases protein levels in transfected fibroblasts by a mean value of 2.6-fold (maximum 4.6-fold), and after two rounds of transfection the effect lasted for a month. Oligonucleotides targeted to the upstream exon (exon 6 in SMN) are also effective. We conclude that TOES oligonucleotides are highly effective reagents for restoring the splicing of refractory exons and can act across long introns

The Influence of Motion and Stress on Optical Fibers

We report on extensive testing carried out on the optical fibers for the VIRUS instrument. The primary result of this work explores how 10+ years of simulated wear on a VIRUS fiber bundle affects both transmission and focal ratio degradation (FRD) of the optical fibers. During the accelerated lifetime tests we continuously monitored the fibers for signs of FRD. We find that transient FRD events were common during the portions of the tests when motion was at telescope slew rates, but dropped to negligible levels during rates of motion typical for science observation. Tests of fiber transmission and FRD conducted both before and after the lifetime tests reveal that while transmission values do not change over the 10+ years of simulated wear, a clear increase in FRD is seen in all 18 fibers tested. This increase in FRD is likely due to microfractures that develop over time from repeated flexure of the fiber bundle, and stands in contrast to the transient FRD events that stem from localized stress and subsequent modal diffusion of light within the fibers. There was no measurable wavelength dependence on the increase in FRD over 350 nm to 600 nm. We also report on bend radius tests conducted on individual fibers and find the 266 microns VIRUS fibers to be immune to bending-induced FRD at bend radii of R > 10cm. Below this bend radius FRD increases slightly with decreasing radius. Lastly, we give details of a degradation seen in the fiber bundle currently deployed on the Mitchell Spectrograph (formally VIRUS-P) at McDonald Observatory. The degradation is shown to be caused by a localized shear in a select number of optical fibers that leads to an explosive form of FRD. In a few fibers, the overall transmission loss through the instrument can exceed 80%.Comment: 19 pages, 22 figure

Measuring the Refractive Index and Sub-Nanometre Surface Functionalisation of Nanoparticles in Suspension

Direct measurements to determine the degree of surface coverage of nanoparticles by functional moieties are rare, with current strategies requiring a high level of expertise and expensive equipment. Here, a practical method to determine the ratio of the volume of the functionalisation layer to the particle volume based on measuring the refractive index of nanoparticles in suspension is proposed. As a proof of concept, this technique is applied to poly(methyl methacrylate) (PMMA) nanoparticles and semicrystalline carbon dots functionalised with different surface moieties, yielding refractive indices that are commensurate to those from previous literature and Mie theory. In doing so, it is demonstrated that this technique is able to optically detect differences in surface functionalisation or composition of nanometre-sized particles. This non-destructive and rapid method is well-suited for in situ industrial particle characterisation and biological applications

Biochemical adaptations of the retina and retinal pigment epithelium support a metabolic ecosystem in the vertebrate eye

Here we report multiple lines of evidence for a comprehensive model of energy metabolism in the vertebrate eye. Metabolic flux, locations of key enzymes, and our finding that glucose enters mouse and zebrafish retinas mostly through photoreceptors support a conceptually new model for retinal metabolism. In this model, glucose from the choroidal blood passes through the retinal pigment epithelium to the retina where photoreceptors convert it to lactate. Photoreceptors then export the lactate as fuel for the retinal pigment epithelium and for neighboring Mu ̈ ller glial cells. We used human retinal epithelial cells to show that lactate can suppress consumption of glucose by the retinal pigment epithelium. Suppression of glucose consumption in the retinal pigment epithelium can increase the amount of glucose that reaches the retina. This framework for understanding metabolic relationships in the vertebrate retina provides new insights into the underlying causes of retinal disease and age-related vision loss