Coherent fibre-optic link: applications in Time and Frequency metrology, Geodesy, Radio Astronomy and Seismology

L'abstract è presente nell'allegato / the abstract is in the attachmen

Aerospace Medicine and Biology: A continuing supplement 180, May 1978

This special bibliography lists 201 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1978

Dynamic Optical Networks for Data Centres and Media Production

This thesis explores all-optical networks for data centres, with a particular focus on network designs for live media production. A design for an all-optical data centre network is presented, with experimental verification of the feasibility of the network data plane. The design uses fast tunable (< 200 ns) lasers and coherent receivers across a passive optical star coupler core, forming a network capable of reaching over 1000 nodes. Experimental transmission of 25 Gb/s data across the network core, with combined wavelength switching and time division multiplexing (WS-TDM), is demonstrated. Enhancements to laser tuning time via current pre-emphasis are discussed, including experimental demonstration of fast wavelength switching (< 35 ns) of a single laser between all combinations of 96 wavelengths spaced at 50 GHz over a range wider than the optical C-band. Methods of increasing the overall network throughput by using a higher complexity modulation format are also described, along with designs for line codes to enable pulse amplitude modulation across the WS-TDM network core. The construction of an optical star coupler network core is investigated, by evaluating methods of constructing large star couplers from smaller optical coupler components. By using optical circuit switches to rearrange star coupler connectivity, the network can be partitioned, creating independent reserves of bandwidth and resulting in increased overall network throughput. Several topologies for constructing a star from optical couplers are compared, and algorithms for optimum construction methods are presented. All of the designs target strict criteria for the flexible and dynamic creation of multicast groups, which will enable future live media production workflows in data centres. The data throughput performance of the network designs is simulated under synthetic and practical media production traffic scenarios, showing improved throughput when reconfigurable star couplers are used compared to a single large star. An energy consumption evaluation shows reduced network power consumption compared to incumbent and other proposed data centre network technologies

Frequency Transfer Techniques and Applications in Fiber Optic Communication Systems

Modern society is dependent on communications and the developments of these increases constantly through a seemingly endless demand for communication services and thereby synchronization and time. This is confirmed by a vast range of research on communications, irrespective of technology and protocol. Historically, the national metrology institutes are the distributors of stable accurate time and frequency through national timescales, but that situation has changed with the arrival of Global Navigation Satellite Systems (GNSS) such as GPS (Global Positioning System). The introduction of GNSS-based solutions has resulted in improvement for system users and owners in need of time and frequency. When using a GNSS receiver, sufficient accuracy and precision is often achieved. However, a disadvantage of this development is that GNSS-solutions are based on weak radio signals that can be interfered with. The main objective of the research that forms the groundwork for this thesis is the development of new fiber based methods for time and frequency. The aim is to complement GNSS-based methods for redundancy, with the intention of strengthening the robustness of the Swedish infrastructure. The research has resulted in two unique and innovative transmission technologies, one of which has been patented (two-color, one-way). The first method is based on a non-insertion technology that utilizes passive listening to existing data frames in a fiber optical network and does not require any particular bandwidth. This technology only uses a fraction of the optical signal for time and frequency measurement from an indirect connection to the network. This method has resulted in a precision relative to the GPS carrier phase of less than 1 ns root mean square for distances exceeding 1,100 km. This precision has been achieved for all of the included experiments, conducted within the framework of the thesis, regardless of configuration. The other fiber based technology is a one-way method that uses two wavelengths (colors) for the realization of a correction algorithm and signals thereto. It was developed because the symmetry required for performing two-way time and frequency transfer is rarely precise enough. This optical fiber technique was evaluated with respect to a GPS precise point positioning technique in an urban fiber optical network. The difference in frequency stability between the two systems has been shown to be about 3 × 10-15 over an averaging interval of 10,000 s for a distance of 3 km. The method has also been evaluated in several laboratory experiments with fiber distances up to 160 km. The best performing result is presented as time resolved transit time variations compared with arrival time difference. The standard deviation of the difference between the reference measurement and the one-way, two-color technique result is 3.12 ns and the data showed temperature dependence in transit time of 6 ns / °C

IT\u27S NOT RAINBOWS AND UNICORNS : REGULATED COMMODITY AND WASTE PRODUCTION IN THE ALBERTA OILSANDS

This dissertation examines the regulated oilsands mining industry of Alberta, Canada, widely considered the world’s largest surface mining project. The industrial processes of oilsands mining produce well over one million barrels of petroleum commodities daily, plus even larger quantities of airborne and semisolid waste. The project argues for a critical account of production concretized in the co-constitutional relations of obdurate materiality and labor activity within a framework of regulated petro-capitalism. This pursuit requires multiple methods that combine archives, participant observation, and semi-structured interviews to understand workers’ shift-to-shift relations inside the “black box” of regulated oilsands mining production where materiality co-constitutes the processes and outcomes of resource development and waste-intensive production. Here, the central contradiction pits the industry’s colossal environmental impact and its regulated environmental relations, which – despite chronic exceedances – are held under some control by provincial and federal environmental agents, further attenuated by firms’ selective voluntary compliance with global quality standards as well as whistleblowers and otherwise “troublesome” employees. ‘It’s not rainbows and unicorns,’ explains one informant, distilling workers’ views of the safety and environmental hazards they simultaneously produce and endure as wage laborers despite pervasive regulation. In addition to buttressing geographical conceptualizations of socionatural resource production, contributions arise in the sympathetic engagement with workers, which may hold useful insights for activism against the industry’s environmental outcomes

Effects of climate change and fishing on demersal ecosystems : an approach to human impacts on North East Atlantic and Spanish Mediterranean communities

Las comunidades marinas están expuestas a ciertas presiones de origen ambiental y antropogénico, entre las cuales destacan el calentamiento global y la presión pesquera. Para conservar el buen funcionamiento de los ecosistemas marinos y facilitar un uso sostenible de los servicios que proveen es necesario que su gestión se realice desde un enfoque ecosistémico. Este tipo de gestión debe apoyarse en estudios científicos que aspiren a entender los procesos que subyacen a las reorganizaciones ecológicas que suceden cuando los impactos de la pesca y el cambio climático actúan de forma combinada. Esta tesis contiene varias aproximaciones en esta línea de investigación, analizando la conexión entre los cambios composicionales observados en las comunidades y el desplazamiento de las especies en busca de sus condiciones ambientales óptimas, atendiendo también otros mecanismos ecológicos que subyacen a estos cambios, en concreto, aquellos relacionados con la sensibilidad y la resiliencia de las comunidades marinas.Global warming is at present an undeniable driver of marine communities’ reorganizations, but marine ecosystems are also exposed to other pressures of environmental and anthropogenic origin, among which the many forms of fishing stand out. In order to protect the functioning of marine ecosystems and allow a sustainable access to the services they provide, a well informed ecosystem-based management depends on the sustained generation of scientific assessments. This thesis aims at a better understanding of the processes that underlie marine communities’ reorganizations related to both warming and fishing, so as to ultimately facilitate the prevention of catastrophic shifts. Through different approaches this thesis analyzes the connection between observed compositional changes and the species tracking of their preferred niche conditions, and also examines the ecological mechanisms behind such changes, in particular those related to the marine communities’ sensitivity and resilience

The snowshoe hare filter to spruce establishment in boreal Alaska

Thesis (Ph.D.) University of Alaska Fairbanks, 2018Interior Alaska is a heterogeneous landscape within the circumpolar boreal forest and is largely composed of black and white spruce (Picea mariana and P. glauca). Improving our understanding of the factors affecting patterns in spruce regeneration is particularly important because these factors ultimately contribute to shaping the boreal forest vegetation mosaic. Herbivory by snowshoe hares (Lepus americanus) is one factor that likely drives patterns in spruce establishment. The interaction between spruce and snowshoe hares provides an opportunity to study how plant-herbivore interactions can affect succession, vegetation community composition, and consequently, how herbivory influences landscape heterogeneity. I explored how herbivory by snowshoe hares alters the survival and growth of spruce seedlings across Interior Alaska's boreal forest. I hypothesized that the survival and growth rate of regenerating spruce is significantly reduced by snowshoe hare herbivory and that snowshoe hare herbivory influences the pattern of spruce establishment across time and space. To address this hypothesis, I conducted research in three distinct vegetation communities across the region: productive lowland floodplains (Chapters 1 and 2), treeline (Chapters 3 and 4), and recently burned stands of black spruce (Chapter 5). Together these five chapters reveal that snowshoe hares affect spruce establishment across much of boreal Alaska. Where and when hares are abundant, spruce can be heavily browsed, resulting in suppressed seedling growth and increased seedling mortality. The results of these studies also reveal a consistent and predictable pattern in which this plant-herbivore interaction takes place. The snowshoe hare filter acts as a 'spatially aggregating force' to spruce establishment, where the potential for optimal regeneration is highest during periods of low hare abundance and where hares are absent from the landscape.Introduction -- Chapter 1. Stage-dependent effects of browsing by snowshoe hares on successional dynamics in a boreal forest ecosystem -- Chapter 2. Asynchronous recruitment dynamics of snowshoe hares and white spruce in a boreal forest -- Chapter 3. Can snowshoe hares control treeline expansions? -- Chapter 4. Functional responses of white spruce to snowshoe hare herbivory at treeline -- Chapter 5. Herbivory by snowshoe hares on regenerating black spruce foreshadows future capacity to influence postfire succession -- Conclusion

Fire Regimes of Lower-elevation Forests in Great Smoky Mountains National Park, Tennessee, U.S.A.

Disturbance is a natural part of any forest ecosystem. When disturbance regimes are altered, the forest stands will reflect those changes. Southern Appalachian xeric pine-oak woodlands are one forest type that has experienced such change, primarily in the form of fire suppression. The western side of Great Smoky Mountains National Park contains stands of large trees that escaped earlier intensive logging, show evidence of past fire, and provide an ideal setting for reconstructing stand histories. For three lower-elevation (ca. 500 m ASL) study sites, I used crossdated yellow pine tree-ring chronologies and records from cross-sections taken from living and dead pines to reveal historical patterns and relationships of wildfire, climate, and human activity. Cores and vegetation data collected at three 20 x 50 m plots per site provided age structure, stand structure, and stand composition. All three chronologies displayed a high degree of sensitivity to yearly environmental fluctuations and extended back through the 1700s. Yellow pine growth was strongly and positively correlated with winter temperatures, which were primarily influenced by the North Atlantic Oscillation. The tested climate variables displayed relationships that appeared to shift over time, or across an ambiguous boundary on which the park resides. Climate oscillations in both the Atlantic Ocean and Pacific Ocean modulated wildfire frequency and events. Wildfire events occurred frequently prior to park establishment in 1934 and were primarily anthropogenic in origin. Most fires burned during dormancy or early in the growing season, but widespread and more recent fires tended to occur later. Fire frequency peaked in the 1800s with an average return interval of two years. Absence of wildfire during suppression was associated with establishment of fire-sensitive species, such as red maple and eastern white pine. Yellow pine regeneration was weak and dominated by Virginia pine. Results from this study can be used by park personnel to plan and manage fires to restore ecosystem processes to a pre-suppression state. The chronologies provided three centuries of data that can be used to reconstruct climate variables and to enhance our understanding of climate dynamics

Data transport over optical fibre for ska using advanced modulation flexible spectrum technology

Flexible Spectrum Dense Wavelength Division Multiplexed (DWDM) optical fibre networks are next-generation technology for handling extremely high data rates of the kind produced by MeerKAT and SKA.We optimise the flexible spectrum for real-time dynamic channel wavelength assignment, to ensure optimum network performance. We needed to identify and develop novel hardware and dynamic algorithms for these networks to function optimally to perform critical tasks. Such tasks include wavelength assignment, signal routing, network restoration and network protection. The antennas of the Square Kilometre Array (SKA) network connect to the correlator and data processor in a simple point-to-point fixed configuration. The connection of the astronomer users to the data processor, however, requires a more complex network architecture. This is because the network has users scattered around South Africa, Africa and the whole world. This calls for upgrade of the classical fixed wavelength spectrum grids, to flexible spectrum grid that has improved capacity, reliable, simple and cost-effectiveness through sharing of network infrastructure. The exponential growth of data traffic in current optical communication networks requires higher capacity for the bandwidth demands at a reduced cost per bit. All-optical signal processing is a promising technique to improve network resource utilisation and resolve wavelength contention associated with the flexible spectrum. Flexible Spectrum Dense Wavelength Division Multiplexed (DWDM) optical fibre networks are next-generation technology for handling extremely high data rates of the kind produced by MeerKAT and SKA. Each DWDM channel is capable of 10 Gbps transmission rate, which is sliceable into finer flexible grid 12.5 GHz granularity to offer the network elastic spectrum and channel spacing capable of signal routing and wavelength switching for the scalability of aggregate bandwidth. The variable-sized portions of the flexible spectrum assignment to end users at different speeds depend on bandwidth demand, allowing efficient utilisation of the spectrum resources. The entire bandwidth of dynamic optical connections must be contiguously allocated. However, there is an introduction of spectrum fragmentation due to spectrum contiguity related to the optical channels having different width. Thus large traffic demands are likely to experience blocking regardless of available bandwidth. To minimise the congestion and cost-effectively obtain high performance, the optical network must be reconfigurable, achievable by adding wavelength as an extra degree of freedom for effectiveness. This can introduce colourless, directionless and contentionless reconfigurability to route individual wavelengths from fibre to fibre across multiple nodes to avoid wavelength blocking/collisions, increasing the flexibility and capacity of a network. For these networks to function optimally, novel hardware and dynamic algorithms identification and development is a critical task. Such tasks include wavelength assignment, signal routing, network restoration and network protection. In this work, we for the first time to our knowledge proposed a spectrum defragmentation technique through reallocation of the central frequency of the optical transmitter, to increase the probability of finding a sufficient continuous spectrum. This is to improve network resource utilisation, capacity and resolve wavelength contention associated with a flexible spectrum in optical communication networks. The following chapter provides details on a flexible spectrum in optical fibre networks utilising DWDM, optimising transmitter-receivers, advanced modulation formats, coherent detection, reconfigurable optical add and drop multiplexer (ROADM) technology to implement hardware and middleware platforms which address growing bandwidth demands for scalability, flexibility and cost-efficiency. A major attribute is tunable lasers, an essential component for future flexible spectrum with application to wavelength switching, routing, wavelength conversion and ROADM for the multi-node optical network through spectrum flexibility and cost-effective sharing of fibre links, transmitters and receivers. Spectrum slicing into fine granular sub-carriers and assigning several frequency slots to accommodate diverse traffic demands is a viable approach. This work experimentally presents a spectral efficient technique for bandwidth variability, wavelength allocation, routing, defragmentation and wavelength selective switches in the nodes of a network, capable of removing the fixed grid spacing using low cost, high bandwidth, power-efficient and wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) transmitter directly modulated with 10 Gbps data. This to ensure that majority of the spectrum utilisation at finer channel spacing, wastage of the spectrum resource as caused by the wavelength continuity constraint reduction and it improves bandwidth utilisation. The technique is flexible in terms of modulation formats and accommodates various formats with spectrally continuous channels, fulfilling the future bandwidth demands with transmissions beyond 100 Gbps per channel while maintaining spectral efficiency