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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Advanced polymers for light emitting diodes

Webster, Graham R. January 2000 (has links)
No description available.
32

The laser torquemeter and implications of speckle decorrelation on torque measurement

Tullis, Iain David Charles January 2000 (has links)
Torque measurement of a rotating shaft is a method of monitoring machine performance. Steady transmission of mechanical power from the prime mover to the load is vital to avoid gear and bearing wear, shaft fatigue failure, bearing and coupling failure and noise. Mean and fluctuating torque are fundamental quantities of interest. The laser torquemeter is capable of providing a non-contact measurement of time-resolved torque through a driven system. The laser torquemeter depends upon laser speckle produced from coherent light illuminating a diffuse object and the speckle pattern may be used in determining the angular position of a rotating object. When the object rotates the backscattered speckle pattern, which changes continuously but repeats exactly with every revolution, is sampled by a suitably positioned photo detector. The photo detector output signal is periodic and one period is recorded in memory as a reference and the angular position of a shaft can then be determined by a comparison of this recorded reference signal with the current photo detector output signal. The speckle pattern from two axially separated points on the shaft are monitored and under low or, ideally, zero torque the photo detector outputs are recorded into the laser torquemeter electronics. The laser torquemeter then tracks the live photo detector output and determines the angle at the two points on the shaft. Relative angular displacement in the two angle outputs appears when torque is applied and the shaft twists. When the shaft is displaced, for example by vibration, the backscattered speckle pattern changes on the photo detector and the similarity between the recorded, reference signal and the live, current signal is reduced. In this thesis, the cross-correlation of the real-time photodetector output signal and the recorded reference signal as a function of shaft position is examined. The effects of various shaft motions - rotation, axial translation, pitch and yaw, and radial translation are theoretically and experimentally examined and the results can then be used in the design of an optical head for the laser torquemeter. A review of the current torquemeter technology allows for discussion of the broad spectrum of typical torquemeter operating conditions. The optical head of the laser torquemeter may vary significantly for various torque measurement scenarios. A design procedure for the optical head of the laser torquemeter is summarised. The holy grail of torquemeter manufacturers is to produce a cheap, easy to use, robust, accurate, reliable and non-contacting torquemeter. The laser torque meter has great potential to meet these requirements.
33

Optical fibre long period grating sensors with nanostructured coatings

Topliss, Stephen M. January 2011 (has links)
The dual resonant response of long period fibre gratings (LPG) operating near the phase matching turning point to the deposition of nanostructured coatings is explored. A broad range of LPGs have been fabricated with grating periods ranging from 80m to 180m, and these have been characterized with three different coating materials,-tricosensoic acid, undecyl-calix[4]resorcarene and tert-butyl-calix[8]arene carboxylic acid. The dual resonant response has been exploited with the construction of an LPG based sensor coated with a quinolinium dye forming a pH sensor. The wavelength response of this device was measured with a sensitivity of -0.55pH/nm. Furthermore, length apodised phase shifted long period gratings have been characterized, and the effect on the dual resonant response has been recorded. Partial coating of this device has resulted in the observation of a bandgap feature in the wavelength response. The use of calixarenes as a functional coating for long period fibre gratings is also investigated. Calixarene is applied in a thin layer with a thickness of several hundred nm’s to the cladding of the fibre in the region containing the LPG sensor. The chemical sensing capabilities of a long period fibre grating sensor is presented for the detection of the volatile organic compounds; hexane, cyclohexane, benzene and toluene. The wavelength response was measured and the sensitivity to toluene vapour was recorded at 1600ppmv/nm. Using intensity detection of the central maxima in the wavelength response, the chemical selectivity is demonstrated showing sensitivity to toluene vapour over 13 times greater compared to hexane vapour.
34

Wavefront characterisation and beam correction for high power diode laser arrays

Monjardin-Lopez, Jesus Fernando January 2006 (has links)
No description available.
35

Multiplexed far-field diffractive optical elements for anti-counterfeiting

Caley, Adam J. January 2007 (has links)
No description available.
36

Mid-infrared diode-pumped solid-state lasers

Esser, M. J. Daniel January 2010 (has links)
The concept of a diode-end-pumped Tm3+ laser pumping a Ho3+ laser was utilised to develop mid-infrared solid-state laser devices. After presenting an in-depth literature study of the promising material Tm:GdVO4, it was theoretically predicted and experimentally verified that a diode-end-pumped Tm:GdVO4 laser can be operated over a 100 nm wavelength range merely through the appropriate selection of the resonator output coupling value. The output at 1818 nm is the shortest wavelength demonstrated for a multi-watt Tm:GdVO4 laser, while the quasi-continuous-wave output power of 8.7 W at 1915 nm is the highest reported for a diode-end-pumped Tm:GdVO4 laser. A dual-end-pumped Tm:GdVO4 laser operating at 1892 nm was subsequently designed as pump source for a Ho:YLF laser, the implementation of which was demonstrated for the first time. The Q-switched Ho:YLF laser, pumped with a 83 mJ quasi-continuous-wave pulse of 19 ms duration at 5 Hz repetition rate, produced 1.9 mJ in a 17.6 ns pulse at 2051 nm on the π-polarisation, and 2.1 mJ in a 47.7 ns pulse on the σ-polarisation. This initial research work directed the way towards the development of high-energy Ho3+ lasers and amplifiers pumped with alternative high average power Tm3+ laser devices.
37

Resonators for slab-waveguide lasers

Shackleton, Christian John January 1993 (has links)
No description available.
38

Development of bioprocessor chip technology

Hayden, Christopher John January 2002 (has links)
No description available.
39

Optical properties of InP/AlGaInP quantum dot laser heterostructures

Lutti, Julie January 2005 (has links)
Optical characterisation of InP/AlGaInP quantum dot laser structures, involving laser threshold and wavelength, optical modal gain and absorption spectra, and radiative efficiency are presented. The samples were grown by MOVPE in Sheffield University on (100) 10 off and (211)B GaAs substrates, and consist of 5 layers of self-assembled InP dots, with each layer grown on Alo.3Gao.7InP and placed in a GaInP quantum well. A record low threshold current density of 290A/cm2 at a wavelength of 740nm for a 1.6 mm-long device with uncoated facets is obtained from one of the sample grown on (100) 10 off substrate at 690 C. This sample has an internal optical mode loss of 4 1 cm"1 and an internal quantum spontaneous emission efficiency of 30% for current densities corresponding to the threshold. The ground state modal gain is shown to saturate at 17cm"1 at room temperature, which is about lA of the full population inversion limit, and the saturation level increases with decreasing temperature. A sample grown at a lower temperature of 650 C has higher optical mode loss (7.5 1 cm"1) and quantum efficiency under 15%. A sample grown on (211)B substrate exhibits optical transitions at higher photon energies, consistent with smaller dot sizes. The thesis also presents an analysis of the segmented contact technique used to measure gain and absorption spectra, determining criteria for the excitation and detection geometries required to ensure accurate measurement. It is shown that the collection angle and device nearfield should be limited so that rays that intercept the stripe edges are not collected. If this is not satisfied, the measurement underestimates the modal gain. An exponential variation of the measured ASE upon stripe length cannot be taken as evidence for a correct collection geometry.
40

Experimental investigation of fluorescence resonance energy transfer using quantum dots

Chong, Ee Zhuan January 2007 (has links)
Quantum dots (QDs) are light emitting nanoscale semiconductor crystals with novel optical properties that are markedly different from the conventional organic dyes. Their size tunable emission, broad absorption and resistance to photo- and chemical degradation have afforded a new route to biomolecular studies, especially in long term live cell imaging that is hardly feasible with the utility of organic dyes. With the advent of highly luminescent core-shell QDs with quantum yields over 0.5, sensitive and robust optical sensing assays can be realised in which organic dyes rarely have much success. In this research, I incorporated these core-shell QDs of variable core compositions that are tailored to emit in the visible and the far-red spectral regimes into the development of multiplexed QD-dye assays via a ligand-receptor binding scheme. The close proximity of QDs to energetically resonant dyes is commensurate to Forster/fluorescence resonance energy transfer (FRET) from which course we set to evaluate the potential of QDs as energy donors through the use of the steady state and the time-resolved spectroscopic techniques. Both approaches not only complement each other but also can provide substantive evidence in verifying the suitability of QDs in the design of FRET based assays where the efficiency of energy transfer is generally governed by the donor-acceptor separation and in turn, the size of QDs. Henceforth from that notion, the relevance of FRET efficiency to the proximity relationships in QD-dye self-assemblies are examined theoretically as a function of acceptor-to-donor ratio. Besides the experimental studies of QDs in proximity-induced energy coupling, I take the initiative to develop a theoretical treatment of the multilayer structure of QDs based on the parabolic band approximation to offer a simple analytical route to investigate the size dependency of energy gap and the localisation of carriers which in essence underpin the QD optical behaviour. The analyses are further extended to the type-II QDs with a slight different band profile in which the conduction and the valence band extrema centre on different regions of the heterostructure leading to the separation of carriers and thus, contributing to the distinctive optical characteristics that strongly depart from typical type-I QDs.

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