3,725 research outputs found
ΔNp63α a key factor of epithelial differentiation controls the activity of YB-1 oncoprotein: potential implications in carcinogenesis
No full textMy thesis unveils a novel protein-protein association involving ΔNp63αand YB-1 and a mechanism through which a factor essential for epithelial development and differentiation can control the activity of an oncoprotein
Erbium:ytterbium co-doped large-core fiber laser with 297W continuous-wave output power
No full textWe have demonstrated a high-power and high-efficiency erbium:ytterbium (Er:Yb) co-doped fiber laser that produces 297 W of continuous-wave output at 1567 nm. The slope efficiency with respect to the launched pump power changed from 40% to 19% at higher output power due to the onset of Yb co-lasing at 1067 nm. However, the Yb co-lasing was essential for the suppression of catastrophic pulsation at high pump powers that otherwise results if the Yb band gain is allowed to build up. Spectroscopic characteristics of the fiber and the impact of the Yb co-lasing on the 1567 nm slope efficiency are also discussed
High power pulsed ytterbium doped fibre lasers and their applications
No full textThe aim of my project is to develop pulsed Ytterbium (Yb) doped fibre master oscillator power amplifier (MOPA) systems seeded by semiconductor lasers. I was principally focused on two specific projects aligned to sponsored programs of research within the ORC pulsed fibre laser group: the first project, TSB funded project LAMPS, aimed to develop an important class of next generation laser system capable of average output powers of more than 100 W when operating in both the nanosecond and picosecond regimes. The goal was to develop a fully fiberized, polarisation maintaining, single transverse mode system. The full project included the development of the necessary diode & micro-optic systems, fibre beam delivery technology and with application focused evaluations in collaboration with our industrial partners. The main project partners were BAE Systems, Selex, Ceram, Intense Photonics, ORC, Herriot Watt University, Power Photonics, OptoCap and Rofin Sinar. I contributed to the development of the single transverse mode Ytterbium (Yb)-doped fibre system and achieved the full target specifications of 100 W of output power with single mode and single polarisation operation in both the nanosecond and picosecond regimes. In addition, second harmonic generation pumped by the fundamental beam at 1.06 µm was also achieved. In order to transfer from picosecond pulses to nanosecond pulses it is only necessary to switch the seed laser, the power amplifier system remaining unchanged making for a highly flexible system. Both fundamental and second harmonic beam were successfully used to do material processing and various high power frequency conversion experiments (visible, broadband supercontinuum and mid-IR). The second project, called HEGAC (also funded by the TSB), was a collaboration with the University of Cambridge and SPI Lasers Ltd. The aim of the HEGAC project was to develop a high power nanosecond fibre laser with an active pulse shaping capability suitable for cutting metals. This project targeted mJ pulses with more than 100 W average power at the final output – with a 200 W stretch objective. We first achieved more than 310 W using a free space seeding and pumping configuration in our laboratories proving power scaling of our proposed approach. I subsequently rebuilt and improved this system and developed a fully- fiberized version (including all pump launches). The laser was capable of generating >100 W of output power and pulse energies up to 2.5 mJ. This project also involved spatial mode as well as temporal pulse shaping. Using a pair of axicon lenses the normal Gaussian beam profile was converted to a ring shaped profile as required and the system tested up to average powers of 100 W. In addition to the normal temporal pulse shapes required using our pulse shaping system (square, triangle and step), I also achieved high average power pulses with smooth shaped pulses (Parabolic and Gaussian) using an adaptive pulse shaping technique. The laser was transported and successfully used in materials processing experiments at Cambridge, proving the robustness of the design and implementation. I also did some novel experiments on high efficiency Raman conversion exploiting the square shaped pulses possible using this laser
Yb-doped femtosecond lasers and their frequency doubling
No full textUltralow threshold, compact and highly efficient femtosecond lasers based on Yb³⁺-doped
potassium yttrium tungstate (Yb:KYW) and Yb³⁺-doped vanadium yttrium oxide
(Yb:YVO 4 ) have been demonstrated within this PhD-research project. For a continuous
wave unmode-locked Yb:KYW laser a threshold as low as 101 mW was obtained with a
slope efficiency of 74%. By employing a single prism for dispersion control, the laser was
tunable between 1012 nm to 1069 nm. When operated in the mode-locked regime, this
laser produced transform-limited pulses having durations of 210 fs at a central wavelength
of 1044 nm. Stable mode locking was observed for an optimised incident pulse fluence on
the SESAM between 140 μJ/cm²
to 160 μJ/cm²
which was 2-3 times higher than the
designed energy pulse fluence of the SESAM (70 μJ/cm²).
The employment of several combinations of chirped mirror designs for control of
intracavity group velocity dispersion led to excellent results. The threshold for mode
locking was satisfied for a pump power of 255 mW where the slope efficiency was
measured to be 62%. This is the most efficient SESAM-assisted femtosecond laser yet
reported and the highest optical-to-optical efficiency of 37% is exceptional. Transform-limited pulses with durations as short as 90 fs were produced in a spectral region centred
on 1052 nm. The success of this research thus represents a good foundation on which to
design and build more compact configurations that will incorporate just one chirped mirror
for dispersion compensation.
A relatively high nonlinear refractive index, n₂ , of 15 x 10⁻¹⁶ cm²/W was measured
in Yb:YVO₄ and this affords particular potential for this candidate material in Kerr-lens
mode locking. In fact, for operation in the femtosecond domain, the threshold power was
190 mW with a slope efficiency of 26% and near-transform-limited pulses as short as 61fs were generated at a centre wavelength of 1050 nm. The main objectives in developing
this type of laser relate to a demonstration of high peak power operation in thin disc laser
configurations.
The deployment of a diode-pumped Yb:KYW femtosecond laser as a pump source
for frequency doubling in a periodically-poled LiTaO₃ crystal was realised. The maximum
realized output power of 150 mW corresponded to an impressive second harmonic
conversion efficiency of 43%. 225-fs duration green pulses (centred at 525 nm) were
generated under the condition of strong focusing in the nonlinear crystal
Topological states of thermoelectric Yb-filled skutterudites
No full textThe effects of topological states on the thermoelectric performance of a highly efficient thermoelectric Yb-filled CoSb3 skutterudite are investigated through combined ab initio calculations and electrical transport measurements. The nontrivial topological states are revealed by ab initio calculations and inferred from anoma-lous Hall conductivity and magnetoresistance. The linear bands associated with the topological states lead to low single-band effective mass and high carrier mobility, and consequently high power factor. Furthermore, the additional band minima due to filling the voids with Yb atoms raise the valley degeneracy, which favors the density-of-states effective mass and thus the Seebeck coefficient but scarcely changing the carrier mobility. These effects together contribute to the high power factor of Yb-filled CoSb3 skutterudite. Our results show that topological states play a crucial role in improving the performance of thermoelectric materials
Towards the tumble resistant microlight
No full textThe tumble mode is a pitching departure from controlled flight which leads to a pitch autorotation that is generally unrecoverable – resulting in vertical ground impact, usually preceded by in-flight breakup (the mechanism for which, surprisingly, can sometimes prevent loss of life). This was identified in work led by the British Microlight Aircraft Association beginning in 1997 as a response to a number of fatal accidents in Rogallo winged microlight aeroplanes, although the tumble is also known to occur to hang-gliders. This paper explains how this class of aeroplane is controlled, and how it has been found that they can enter the tumble mode. The mechanism by which the tumble can be entered is described. This has led to work showing how flight testing can be used to establish and demonstrate resistance to tumble entry – particularly important with increasing number of very high performance flexwings. These flight tests will be explained, together with the significance of the results. Recent accident investigation work has also shown a new mechanism of tumble entry, through partial failure of the A-frame structure and the pitch-trimmer mechanism. Also described is a possible relevance to well known historical accidents to flying wing aeroplanes– specifically the YB-49 and dH-108, and discovered data on the characteristics of the BKB-1flying wing glider; are also described
200W gain-switched-diode-seeded single-polarization narrow linewidth all-fiber picosecond MOPA
No full textWe report a fully fiberized, single-polarization, gain-switched, diode-seeded fiber MOPA delivering 28ps pulses at a repetition frequency of 214MHz with 200W of average output power and up to 0.93µJ pulse energy
Revascularization of turnover sternum: A definitive treatment for intractable funnel chest.
No full textAchieving aerospace standard porosity levels when welding thin and thick-section aluminium using fibre-delivered lasers : executive summary
No full textEnvironmental and commercial pressures have forced the aerospace industry to look at
alternatives to riveting for the manufacture of aluminium aircraft structures. This resulted, at
the end of last century, in an extensive study by Airbus into the possiblities of using CO2
lasers, which led to the process being implemented for a (small) number of stringer-to-skin
fuselage panels in the newer Airbus models. Since this initial commercial success, new laser
sources have become available that are more suitable for the welding of aluminium than CO2
lasers, in the form of Nd: YAG and Yb-fibre lasers. Both produce a wavelength that is
absorbed more efficiently by aluminium alloys than the CO2 laser wavelength, resulting in an
improved keyhole stability, as demonstrated in the late nineties for Nd: YAG lasers. In addition,
Yb-fibre lasers have become available at output powers higher than available for Nd: YAG
lasers, allowing thicker sections of aluminium to be welded in a single pass. However, despite
their claimed advantages, no efforts were made to demonstrate the potential of these lasers
for (aluminium) aircraft manufacture. For this reason, the author initiated a series of studies in
2001, with the overall aim to develop procedures to laser weld both thin (3.2mm) and thicksection
(12.7mm) aerospace aluminium alloys using these fibre-delivered lasers to a weld
quality, in particular related to weld metal porosity, suitable for aerospace service. The focus
in this research was on weld metal porosity, because this is a particular problem when laser
welding aluminium, either in the form of fine (hydrogen) porosity or larger porosity associated
with an unstable keyhole behaviour. The benchmark weld metal porosity for this study was
obtained from the stringent weld quality classes defined in BS EN 13919-2 and AWS D17.1.
The approach to this research was in three parts, with work in the first aimed at demonstrating
that a 3kW Nd: YAG laser was capable of producing low-porosity welds in 3.2mm thickness
2024 aluminium alloy, and thus can be considered for replacing the CO2 laser currently used
for the stringer-to-skin fuselage application. Prior to the final part of the research, in which a
7kW Yb-fibre laser was used to demonstrate that these benchmark porosity levels could also
be achieved in thicker section (aerospace-grade) aluminium, a comparison study was carried
out to quantify the difference in welding performance between the Nd: YAG and the Yb-fibre
laser. At an output power of 4kW focused in a 0.4mm diameter spot, the Yb-fibre laser was
capable of a 30% higher welding speeds in 4mm (5083) aluminium alloy, or a 20% increase in
depth of penetration for welding speeds between 1 and 15m/min, compared with the Nd: YAG
laser. This improvement in welding performance, together with an output power of 7W,
produced full penetration in 12.7mm thickness (aerospace-grade) AI-Zn-Mg-Cu aluminium
alloy using the Yb-fibre laser autogenously, or in a hybrid configuration with a MIG arc. Both
the autogenous laser and hybrid laser-MIG process were capable of producing welds with a
weld metal porosity in line with the BS EN 13919-2 and AWS D17.1 benchmark conditions, at
welding speeds of 0.55 and 0.75m/min, respectively. At these production rates, the 248
metres of stringer incorporated in a typical aluminium wing structure can be welded in 7.5 and
5.5 hours, in case of autogenous laser and hybrid laser-MIG, respectively, compared with
37.6 hours currently needed for the riveting process
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