Switching the mode of metabolism in the yeast Saccharomyces cerevisiae

The biochemistry of most metabolic pathways is conserved from bacteria to humans, although the control mechanisms are adapted to the needs of each cell type. Oxygen depletion commonly controls the switch from respiration to fermentation. However, Saccharomyces cerevisiae also controls that switch in response to the external glucose level. We have generated an S. cerevisiae strain in which glucose uptake is dependent on a chimeric hexose transporter mediating reduced sugar uptake. This strain shows a fully respiratory metabolism also at high glucose levels as seen for aerobic organisms, and switches to fermentation only when oxygen is lacking. These observations illustrate that manipulating a single step can alter the mode of metabolism. The novel yeast strain is an excellent tool to study the mechanisms underlying glucose-induced signal transduction. © 2004 European Molecular Biology Organization.

Dispersion-modified actively mode-locked erbium fibre laser using a chirped fibre grating

The authors describe the operation of an actively modelocked Er fibre laser incorporating a chrped in fibre Bragg reflection grating as one end mirror to the cavity, acting as a lumped highly dispersive element. In one oreientation the grating shifted the cavity into normal dispersion regime and pulses of -25ps duration were produced. In the opposite oreintation, the cavity dispersion was anomalous and ~8ps pulses were produced with characterisitics typical of solitons propagating in a periodically perturbed system.

Simultaneous demultiplexing and clock recovery using a single electroabsorption modulator in a novel bi-directional configuration

A single electroabsorption modulator was used to demultiplex a 10 Gbit/s channel from a 40 Gbit/s OTDM data stream, whilst simultaneously recovering the 10 GHz electrical clock. This was achieved using a new bi-directional operation of the EA modulator, combined with a simple phase-locked loop feedback circuit. Excellent system performance was achieved, indicating that operation up to and beyond 100 Gbit/s is possible using current technology.

Surface acoustic wave devices with low loss and high frequency operation

This thesis describes an industrial research project carried out in collaboration with STC Components, Harlow, Essex. Technical and market trends in the use of surface acoustic wave (SAW) devices are reviewed. As a result, three areas not previously addressed by STC were identified: lower insertion loss designs, higher operating frequencies and improved temperature dependent stability. A review of the temperature performance of alternative lower insertion loss designs,shows that greater use could be made of the on-site quartz growing plant. Data is presented for quartz cuts in the ST-AT range. This data is used to modify the temperature performance of a SAW filter. Several recently identified quartz orientations have been tested. These are SST, LST and X33. Problems associated with each cut are described and devices demonstrated. LST quartz, although sensitive to accuracy of cut, is shown to have an improved temperature coefficient over the normal ST orientation. Results show that its use is restricted due to insertion loss variations with temperature. Effects associated with split-finger transducers on LST-quartz are described. Two low-loss options are studied, coupled resonator filters for very narrow bandwidth applications and single phase unidirectional transducers (SPUDT) for fractional bandwidths up to about 1%. Both designs can be implemented with one quarter wavelength transducer geometries at operating frequencies up to 1GHz. The SPUDT design utilised an existing impulse response model to provide analysis of ladder or rung transducers. A coupled resonator filter at 400MHz is demonstrated with a matched insertion loss of less than 3.5dB and bandwidth of 0.05%. A SPUDT device is designed as a re-timing filter for timing extraction in a long haul PCM transmission system. Filters operating at 565MHz are demonstrated with insertion losses of less than 6dB. This basic SPUDT design is extended to a maximally distributed version and demonstrated at 450MHz with 9.8dB insertion loss.

Two way power flow using GTO thyristors

Two-way power flow is nothing new and has been in practical use using line commutated converters for at least 50 years. With these types of converters, reversal of power flow can be achieved by increasing the firing angle of the devices beyond 90 degrees thus producing a negative DC voltage. Line commutated converters have several known disadvantages including: the direct current cannot be reversed, the power factor decreases when the firing angle increases and the harmonics are high on the line current. To tackle the above problems a forced commutated converter can be used. The power factor can be unity and the harmonics can be reduced. Many researchers have used PWM with different control techniques to serve the above purposes. In each converter arm, they used a forced commutated device with an antiparallel diode. Under the rectification mode of operation the current path is preponderantly through the diodes and under the inverter operation the current flows preponderantly through the forced commutated devices. Although their results were encouraging and gave a unity power factor with nearly sinusoidal current, the main disadvantage was that there were difficulties in controlling the power factor when the system is needed to operate at lagging or leading power factor. In this work, a new idea was introduced by connecting two GTOs antiparallel instead of a diode and a GTO. A single phase system using two GTO converters which are connected in series was built. One converter operates as a rectifier and the other converter operates as an inverter. In the case of the inversion mode and in each inverter arm one GTO is operated as a diode simply by switching it always on and the other antiparallel GTO is operated as a normal device to carry the inverter current. In case of the rectification mode, in each arm one GTO is always off and the other GTP is operated as a controlled device. The main advantage is that the system can be operated at lagging or leading power factor.

Direct real-time cavity mode assessment and optimization technique for fiber lasers

A simple technique for direct real-time assessment of a fiber laser cavity-mode condition during operation is demonstrated. Mode stabilization and optimization with this cavity-mode monitoring and conditioning feedback scheme shows significant improvements to the output performance.

Direct cavity assessment and mode optimisation technique for DFB fibre laser

A technique for direct real-time assessment of a distributed feedback fibre laser cavity conditions during operation is demonstrated and used to provide a cavity mode conditioning feedback mechanism to optimise output performance. Negligible wavelength drift is demonstrated over a 52 mW pump power range.

Further studies on flow distribution on distillation sieve trays

The thesis describes experimental work on the possibility of using deflection baffles in conventional distillation trays as flow straightening devices, with the view of enhancing tray efficiency. The mode of operation is based on deflecting part of the liquid momentum from the centre of the tray to the segment regions in order to drive stagnating liquid at the edges forward. The first part of the work was a detailed investigation into the two-phase flow patterns produced on a conventional sieve tray having 1 mm hole size perforations. The data provide a check on some earlier work and extend the range of the existing databank, particularly to conditions more typical of industrial operation. A critical survey of data collected on trays with different hole sizes (Hine, 1990; Chambers, 1993; Fenwick, 1996; this work) showed that the hole diameter has a significant influence on the flow regime, the size of the stagnant regions and the hydraulic and mass transfer performance. Five modified tray topologies were created with different configurations of baffles and tested extensively in the 2.44 m diameter air-water pilot distillation simulator for their efficacy in achieving uniform flow across the tray and for their impact on tray loading capacity and mass transfer efficiency. Special attention was given to the calibration of the over 100 temperature probes used in measuring the water temperature across the tray on which the heat and mass transfer analogy is based. In addition to normal tray capacity experiments, higher weir load experiments were conducted using a 'half-tray' mode in order to extend the range of data to conditions more typical of industrial operation. The modified trays show superior flow characteristics compared to the conventional tray in terms of the ability to replenish the zones of exceptionally low temperatures and high residence times at the edges of the tray, to lower the bulk liquid gradient and to achieve a more uniform flow across the tray. These superior flow abilities, however, tend to diminish with increasing weir load because of the increasing tendency for the liquid to jump over the barriers instead of flowing over them. The modified tray topologies showed no tendency to cause undue limitation to tray loading capacity. Although the improvement in the efficiency of a single tray over that of the conventional tray was moderate and in some cases marginal, the multiplier effect in a multiple tray column situation would be significant (Porter et al., 1972). These results are in good agreement with an associated CFD studies (Fischer, 1999) carried out by partners in the Advanced Studies in Distillation consortium. It is concluded that deflection baffles can be used in a conventional distillation sieve tray to achieve better liquid flow distribution and obtain enhanced mass transfer efficiency, without undermining the tray loading capacity. Unlike any other controlled-flow tray whose mechanical complexity impose stringent manufacturing and installation tolerances, the baffled-tray models are simple to design, manufacture and install and thus provide an economic method of retrofitting badly performing sieve trays both in terms of downtime and fabrication. NOTE APPENDICES 2-5 ARE ON A SEPARATE FLOPPY DISK ONLY AVAILABLE FOR CONSULTATION AT ASTON UNIVERSITY LIBRARY WITH PRIOR ARRANGEMENT

Gas-fired radiant heaters

This dissertation covers four areas of particular interest for the successful application of radiant heating in industrial environments. In it the author tackles the problem of how to predict the thermal comfort produced both by single heaters and also systems of heaters; proposes a method for modelling the mechanisms by which heaters interact with the buildings in which they are installed, in the static and dynamic cases; explores techniques for measuring the radiation produced by heaters; and presents experiments concerned with finding the temperatures and power balances prevailing during normal operation. It is contended that, whilst the generally accepted guides for sizing and operating space heating plant were a good first approximation, there are intrinsic subtleties arising from the fact that the primary mode of heat transfer in this instance is radiative. These nuances are concerned with how best to maximise the heat transfer from the heat source to the heated object; the placement of heaters within a system; and an assessment of the various techniques and strategies involved in controlling a radiant heating system. The conclusions reached are that: if sized and controlled correctly radiant heating systems offer considerable operational advantages over other types of space heating systems in certain applications, in terms of both economy and controllability. The efficacy of radiant heating systems is affected primarily by the control strategy implemented; secondarily, by the structure of the building into which it is installed; and only marginally by all other factors.

Mode-locking in 25-km fibre laser

We demonstrate mode-locking and single-pulse generation in fibre laser with record-setting cavity length of 25 km. Substantial increase in the pulse round trip duration leads to ultra-low repetition rate of 8.097 kHz and pulse energy of 3.7 uJ.

Intracavity dynamics in high-power mode-locked fiber lasers

A theoretical model is developed which characterizes the intracavity pulse evolutions in high-power fiber lasers. It is shown that experimentally observed dynamics of the key pulse parameters can be described by a reduced model of ordinary differential equations. Critical in driving the intracavity dynamics is the amplitude and phase modulations generated by the discrete elements in the laser. The theory gives a simple geometrical description of the intracavity dynamics and possible operation modes of the laser cavity. Furthermore, it provides a simple and efficient method for optimizing the performance of complex multiparametric laser systems.

Longitudinal dispersion in spiral wound RO modules and its effect on the performance of batch mode RO operations

Operation of reverse osmosis (RO) in cyclic batch mode can in principle provide both high energy efficiency and high recovery. However, one factor that causes the performance to be less than ideal is longitudinal dispersion in the RO module. At the end of the batch pressurisation phase it is necessary to purge and then refill the module. During the purge and refill phases, dispersion causes undesirable mixing of concentrated brine with less concentrated feed water, therefore increasing the salt concentration and energy usage in the subsequent pressurisation phase of the cycle. In this study, we quantify the significance of dispersion through theory and experiment. We provide an analysis that relates the energy efficiency of the batch operation to the amount of dispersion. With the help of a model based on the analysis by Taylor, dispersion is quantified according to flow rate. The model is confirmed by experiments with two types of proprietary spiral wound RO modules, using sodium chloride (NaCl) solutions of concentration 1000 to 20,000 ppm. In practice the typical energy usage increases by 4% to 5.5% compared to the ideal case of zero dispersion.

Passive mode-locked lasing by injecting a carbon nanotube-solution in the core of an optical fiber

In this paper, we propose a saturable absorber (SA) device consisting on an in-fiber micro-slot inscribed by femtosecond laser micro fabrication, filled by a dispersion of Carbon Nanotubes (CNT). Due to the flexibility of the fabrication method, efficient and simple integration of the mode-locking device directly into the optical fiber is achieved. Furthermore, the fabrication process offers a high level of control over the dimensions and location of the micro-slots. We apply this fabrication flexibility to extend the interaction length between the CNT and the propagating optical field along the optical fiber, hence enhancing the nonlinearity of the device. Furthermore, the method allows the fabrication of devices that operate by either a direct field interaction (when the central peak of the propagating optical mode passes through the nonlinear media) or an evanescent field interaction (only a fraction of the optical mode interacts with the CNT). In this paper, several devices with different interaction lengths and interaction regimes are investigated. Self-starting passively modelocked laser operation with an enhanced nonlinear interaction is observed using CNT-based SAs in both interaction regimes. This method constitutes a simple and suitable approach to integrate the CNT into the optical system as well as enhancing the optical nonlinearity of CNT-based photonic devices.

Low loss depressed cladding waveguide inscribed in YAG:Nd single crystal by femtosecond laser pulses

A depressed cladding waveguide with record low loss of 0.12 dB/cm is inscribed in YAG:Nd(0.3at.%) crystal by femtosecond laser pulses with an elliptical beam waist. The waveguide is formed by a set of parallel tracks which constitute the depressed cladding. It is a key element for compact and efficient CW waveguide laser operating at 1064 nm and pumped by a multimode laser diode. Special attention is paid to mechanical stress resulting from the inscription process. Numerical calculation of mode distribution and propagation loss with the elasto-optical effect taken into account leads to the conclusion that the depressed cladding is a dominating factor in waveguide mode formation, while the mechanical stress only slightly distorts waveguide modes.

Alternate wavelength-switching in a dual-wavelength mode-locked fibre laser

We report a stable, dual-wavelength mode-locked fiber laser at 1 GHz with a wavelength spacing of 0.7 nm. Alternate switching between single and dual wavelength output was also achieved by simple adjustment of the pump power.