Ti:sapphire frequency combs for dual-comb distance metrology

This thesis presents research developing Ti:sapphire lasers for applications in precision absolute distance metrology. Dual Ti:sapphire frequency combs were developed and characterised, each typically generating 64 fs pulses at a wavelength of 780 nm and a pulse repetition rate of 513 MHz. Electronic phase stabilisation using f-to-2f interferometry and direct pump power modulation achieved phase slips of 234 mrad (124 mrad) for the carrier-envelope offset frequencies of the probe (local-oscillator) comb in observation times of 1 second. The comb mode spacings were stabilised with cavity length feedback, achieving a phase noise of 4 mrad in one second for each laser. The developed Ti:sapphire dual-comb system was evaluated for distance metrology. When both combs were fully locked, absolute distance metrology was demonstrated for distances of up to 1.6 m, corresponding to ambiguity ranges of up to order 6. Time-of-flight precision of < λ/4 was achieved in an averaging time of 1 s, allowing handover to interferometric precision, which achieved a precision of 2 nm after an averaging time of 4 s. Cross-calibration of distance measurements using a 100-nm-precision delay stage allowed a direct measurement of the group velocity of air to an uncertainty (0.0026%) consistent with values from established atmospheric dispersion models. Modifying the repetition-rates of probe and local-oscillator combs was shown to potentially extend the effective ambiguity range up to 12.385 km. Methods of reducing the cost and complexity of a Ti:sapphire comb were explored by constructing a Ti:sapphire frequency comb directly pumped by 462 nm and 520 nm laser diodes. The laser was modelocked using a commercial saturable absorber and generated 90 mW average power. Using piezoelectric feedback to the laser cavity length, and current modulation of one of the pump diodes, the laser was fully phase-stabilised to achieve the first example of a directly diode-pumped Ti:sapphire laser with fully stabilised repetition rate and carrier envelope offset frequencies. This comb achieved a phase slip of 860 mrad for the 10 MHz carrier envelope offset frequency and 54 mrad for the 79 MHz repetition frequency, each over 1 second. Pulses with durations of 54 fs were generated at a central wavelength of 803 nm. Single walled carbon nanotubes were explored as potential saturable absorbers. A device was fabricated using spin coating and was shown to achieve modelocking at pump powers as low as 545 mW

Using next-generation sequencing to determine diversity of horse intestinal worms:Identifying the equine ʼnemabiome'

Next generation sequencing of DNA from nematode eggs has been utilised to give the first account of the equine ‘nemabiome’. In all equine faecal samples investigated, multiple species of Strongylidae were detected; ranging from 7.5 (SEM 0.79) with 99+% identity to sequences in the NCBI database to 13.3 (SEM 0.80) with 90+% identity. This range is typical of the number of species described previously in morphological studies using large quantities of digesta per animal. However, the current method is non-invasive, relies on DNA analysis avoiding the need for specialist microscopy identification and can be carried out with small samples providing significant advantages over current methods

Urban stormwater inputs to an adapted coastal wetland: Role in water treatment and impacts on wetland biota

The Lake Pertobe wetland system is a semi-natural wetland that has been modified primarily for recreational use. However, this lake system receives stormwater from much of the central business district of Warrnambool City (Victoria, Australia) and serves as a buffer zone between the stormwater system and the Merri River and Merri Marine Sanctuary. This work considers the impact of stormwater inputs on Lake Pertobe and the effectiveness of the lake in protecting the associated marine sanctuary. Sediment contaminants (including heavy metals and polycyclic aromatic hydrocarbons (PAHs)) and water quality parameters within the lake, groundwater and stormwater system were measured. Water quality parameters were highly variable between stormwater drains and rain events. Suspended solids rapidly settled along open drains and shortly after entering the lake. Groundwater inputs increased both salinity and dissolved nitrogen in some stormwater drains. Some evidence of bioaccumulation of metals in the food chain was identified and sediment concentrations of several PAHs were very high. The lake acted as a sink for PAHs and some metals and reductions in Escherichia coli, biological oxygen demand and total phosphorus were observed, affording some protection to the associated marine sanctuary. Nutrient retention was inadequate overall and it was identified that managing the lake primarily as a recreational facility impacted on the effectiveness of stormwater treatment in the system

High-pressure synthesis, crystal and electronic structures of a new scandium tungstate, Sc0.67WO4

Negative thermal expansion (NTE) materials possess a low-density, open structure which can respond to high pressure conditions, leading to new compounds and/or different physical properties. Here we report that one such NTE material -- white, insulating, orthorhombic Sc2W3O12 -- transforms into a black compound when treated at 4 GPa and 1400 oC. The high pressure phase, Sc0.67WO4, crystallizes in a defect-rich wolframite-type structure, a dense, monoclinic structure (space group P2/c) containing 1-D chains of edge-sharing WO6 octahedra. The chemical bonding of Sc0.67WO4 vis-a-vis the ambient pressure Sc2W3O12 phase can be understood on the basis of the Sc defect structure. Magnetic susceptibility, resistivity, thermoelectric power and IR spectroscopic measurements reveal that Sc0.67WO4 is a paramagnet whose conductivity is that of a metal in the presence of weak localization and electron-electron interactions. Oxygen vacancies are suggested as a potential mechanism for generating the carriers in this defective wolframite material.Comment: 29 pages total, 1 table, 7 figure

Continuous Ultraviolet to Blue-Green Astrocomb

The characterization of Earth-like exoplanets and precision tests of cosmological models using next-generation telescopes such as the ELT will demand precise calibration of astrophysical spectrographs in the visible region, where stellar absorption lines are most abundant. Astrocombs--lasers providing a broadband sequence of ultra-narrow, drift-free, regularly spaced optical frequencies on a multi-GHz grid--promise an atomically-traceable, versatile calibration scale, but their realization is challenging because of the need for ultra-broadband frequency conversion of mode-locked infrared lasers into the blue-green region. Here, we introduce a new concept achieving a broad, continuous spectrum by combining second-harmonic generation and sum-frequency-mixing in an aperiodically-poled MgO:PPLN waveguide to generate gap-free 390-520 nm light from a 1 GHz Ti:sapphire laser frequency comb. We lock a low-dispersion Fabry-Perot etalon to extract a sub-comb of bandwidth from 392-472 nm with a spacing of 30 GHz, visualizing the thousands of resulting comb modes on a high resolution cross-dispersion spectrograph. Complementary experimental data and simulations demonstrate the effectiveness of the approach for eliminating the spectral gaps present in second-harmonic-only conversion, in which weaker fundamental frequencies are suppressed by the quadratic \{chi}^((2)) nonlinearity. Requiring only ~100 pJ pulse energies, our concept establishes a practical new route to broadband UV-visible generation at GHz repetition rates.Comment: 14 pages; 4 figure

Linked Open Data Vocabularies and Identifiers for Medieval Studies

This paper examines the use of Linked Open Data in the research field of medieval studies. We report on a survey of common identifiers and vocabularies used across digitized medieval resources, with a focus on three internationally significant collections in the field. This survey has been undertaken within the “Mapping Manuscript Migrations” (MMM) project since 2017, aimed at aggregating and linking disparate datasets relating to the history of medieval manuscripts. This has included reconciliation and matching of data for five main classes of entities: Persons, Places, Organizations, Works, and Manuscripts. For each of these classes, we review the identifiers used in MMM’s source datasets, and note the way in which they tend to rely on generic vocabularies rather than specialist medieval ones. As well as discussing some of the major issues and difficulties involved in conceptualizing each of these types of entity in a medieval context, we suggest some possible directions for building a more specialized Linked Open Data environment for medieval studies in the future.Peer reviewe

Effectiveness and cost-effectiveness of a very brief physical activity intervention delivered in NHS Health Checks (VBI Trial): study protocol for a randomised controlled trial.

BACKGROUND: Physical activity interventions that are targeted at individuals can be effective in encouraging people to be more physically active. However, most such interventions are too long or complex and not scalable to the general population. This trial will test the effectiveness and cost-effectiveness of a very brief physical activity intervention when delivered as part of preventative health checks in primary care (National Health Service (NHS) Health Check). METHODS/DESIGN: The Very Brief Intervention (VBI) Trial is a two parallel-group, randomised, controlled trial with 1:1 individual allocation and follow-up at 3 months. A total of 1,140 participants will be recruited from 23 primary care practices in the east of England. Participants eligible for an NHS Health Check and who are considered suitable to take part by their doctor and able to provide written informed consent are eligible for the trial. Participants are randomly assigned at the beginning of the NHS Health Check to either 1) the control arm, in which they receive only the NHS Health Check, or 2) the intervention arm, in which they receive the NHS Health Check plus 'Step It Up' (a very brief intervention that can be delivered in 5 minutes by nurses and/or healthcare assistants at the end of the Health Check). 'Step It Up' includes (1) a face-to-face discussion, including feedback on current activity level, recommendations for physical activity, and information on how to use a pedometer, set step goals, and monitor progress; (2) written material supporting the discussion and tips and links to further resources to help increase physical activity; and (3) a pedometer to wear and a step chart for monitoring progress. The primary outcome is accelerometer counts per minute at 3-month follow-up. Secondary outcomes include the time spent in the different levels of physical activity, self-reported physical activity and economic measures. Trial recruitment is underway. DISCUSSION: The VBI trial will provide evidence on the effectiveness and cost-effectiveness of the Step It Up intervention delivered during NHS Health Checks and will inform policy decisions about introducing very brief interventions into routine primary care practice. TRIAL REGISTRATION: ISRCTN Registry, ISRCTN72691150 . Registered on 17 July 2014