Distributed feedback fibre laser strain and temperature sensors

This thesis presents the development of two new types of polarimetric distributed feedback (DFB) fibre laser sensors for simultaneous strain and temperature measurements. These fibre Bragg grating (FBG) based sensors offer strain and temperature measurement accuracies of ±0.3 - ±15 µ.epsilon and ±0.04 - ±0.2°C which are suitable for many applications. The main advantage of these DFB fibre laser sensors over other FBG based sensors is the simplicity of their interrogation system. The first type of sensor operates stably in a single longitudinal mode which splits into two orthogonally polarised modes. This sensor utilises the wavelength of one polarisation mode and the RF beat frequency between the two polarisation modes. The system complexity is reduced to a minimum in the dual longitudinal mode polarimetric DFB fibre laser sensor which utilises the RF beat frequencies between two longitudinal modes and their associated orthogonal polarisations, therefore requiring only a simple and cost effective frequency counter.The ability of measuring strain and temperature simultaneously is demonstrated with prototype sensors embedded in a concrete test specimen. Miniature sensor packages are used to protect the sensors from their harsh environment and to enable handling by personnel who is inexperienced with fibre optic sensors. The reduction of measurement resolution caused by external feedback into DFB fibre laser sensors, which is likely to occur in serially multiplexed sensor networks and remote sensors, is investigated. Furthermore, operation of DFB fibre lasers up to 400 °C is demonstrated. This yields important information about grating decay in sensor and telecommunication applications, the required annealing temperatures to prevent this decay and the initial grating strength to obtain the maximum output power of DFB fibre lasers. This experimental work is supported by extensive theoretical modelling. For the first time an Er3+:Yb3+ DFB fibre laser model is presented which takes homogeneous upconversion of paired Er3+-ions and pump excited state absorption into account. An extended version of this model incorporates, for the first time, self-heating in DFB fibre lasers which is caused by non-radiative decays. The performance of DFB fibre lasers employed in telecommunication applications is likely to benefit from these modelled results, which are also verified by experimental data

Development of competitive capabilities in mature, slow moving international niche markets : the case of the Portuguese Port Wine Industry

Nowadays, globalization affects every industry. Globalization has likewise accelerated the development speed of alcoholic beverage companies, necessitating any business that wants to remain competitive to adapt. In the international context, the rather unexploited Port wine industry is noticeably stable. Founded in the 16th century, the industry has gone through numerous industry cycles and is currently present in over 100 countries. However, the industry faces several threats, appearing out of its minor size and an increasingly hostile and dynamic environment. This thesis aims to examine the challenges of a mature, slow-moving, yet highly international niche industry, and analyses which strategic moves are specifically necessary in the Port wine industry to develop the dynamic capabilities to remain competitive. To explore these topics, an inductive exploratory case study was conducted. Eight experts were interviewed to derive three hypotheses about the reasons for the historic stability, the current agility and future prospective of this industry. The results display that (1) in historic context the Port wine industry was able to respond promptly and in consensus to market changes, (2) the core-capabilities of the industry are derived mainly from past-experience and have a shortage in the development and release of new resources, (3) the industry is aware of key threats and opportunities but incapable to transform the knowledge accumulated into a new value-creating strategy. The findings implicate that collaboration and a more modern approach to marketing is needed to regain visibility in saturated markets and capture new customers.Atualmente a globalização afeta todas as indústrias. Como tal, a globalização tem vindo a acelerar a velocidade de desenvolvimento das empresas de bebidas alcoólicas, induzindo à necessidade de adaptação das empresas para se manterem competitivas. Notoriamente estável no contexto internacional encontra-se a pouco explorada indústria do Vinho do Porto. Fundada no século XVI, esta indústria passou por diversos ciclos e encontra-se presentemente em mais de 100 países. Porém, a indústria enfrenta diversas ameaças resultantes da sua reduzida dimensão do ambiente cada vez mais hostil e dinâmico. Esta tese pretende examinar os desafios desta indústria madura e de mudança lenta, ainda que corresponda a uma indústria de nicho substancialmente internacionalizada, e analisa quais os movimentos estratégicos necessários para o desenvolvimento das capacidades dinâmicas necessárias à manutenção da sua competitividade. Para explorar estes tópicos, realizámos um estudo de caso indutivo e exploratório. Entrevistámos oito especialistas para formular três hipóteses relativas à indústria: estabilidade histórica, agilidade atual e prospetivas futuras. Os resultados obtidos evidenciam que: (1) no contexto histórico a indústria do Vinho do Porto foi capaz de responder atempada e consensualmente às mudanças do mercado; (2) as capacidades centrais da indústria resultam essencialmente das experiências passadas e a indústria demonstra dificuldades na criação e no desenvolvimento de novos recursos; (3) a indústria está ciente das ameaças e das oportunidades-chave, mas é incapaz de transformar o conhecimento acumulado numa nova estratégia de criação de valor. Estes resultados sugerem que colaboração e uma mais moderna abordagem de marketing são necessárias para que a indústria reganhe visibilidade em mercados saturados e capture novos clientes

Resolvent Positive Linear Operators Exhibit the Reduction Phenomenon

The spectral bound, s(a A + b V), of a combination of a resolvent positive linear operator A and an operator of multiplication V, was shown by Kato to be convex in b \in R. This is shown here, through an elementary lemma, to imply that s(a A + b V) is also convex in a > 0, and notably, \partial s(a A + b V) / \partial a <= s(A) when it exists. Diffusions typically have s(A) <= 0, so that for diffusions with spatially heterogeneous growth or decay rates, greater mixing reduces growth. Models of the evolution of dispersal in particular have found this result when A is a Laplacian or second-order elliptic operator, or a nonlocal diffusion operator, implying selection for reduced dispersal. These cases are shown here to be part of a single, broadly general, `reduction' phenomenon.Comment: 7 pages, 53 citations. v.3: added citations, corrections in introductory definitions. v.2: Revised abstract, more text, and details in new proof of Lindqvist's inequalit

Neutral delay equations from and for population dynamics

For a certain class of neutral differential equations it is shown that these equations can serve as population models in the sense that they can be interpreted as special cases or caricatures of the standard Gurtin-MacCamy model for a population structured by age with birth and death rate depending on the total adult population. The delayed logistic equation does not belong to this class but the blowfly equation does. These neutral delay equations can be written as forward systems of an ordinary differential equation and a shift map. There are several quite distinct ways to perform the transformation to a system, either following a method of Hale or following more closely the renewal process. Similarly to the delayed logistic equation, the neutral equation (and the blowfly equation as a special case) exhibit periodic solutions, although only for a restricted range of parameters

Dual Fronts Propagating into an Unstable State

The interface between an unstable state and a stable state usually develops a single confined front travelling with constant velocity into the unstable state. Recently, the splitting of such an interface into {\em two} fronts propagating with {\em different} velocities was observed numerically in a magnetic system. The intermediate state is unstable and grows linearly in time. We first establish rigorously the existence of this phenomenon, called ``dual front,'' for a class of structurally unstable one-component models. Then we use this insight to explain dual fronts for a generic two-component reaction-diffusion system, and for the magnetic system.Comment: 19 pages, Postscript, A

Reduction of systems of first-order differential equations via Lambda-symmetries

The notion of lambda-symmetries, originally introduced by C. Muriel and J.L. Romero, is extended to the case of systems of first-order ODE's (and of dynamical systems in particular). It is shown that the existence of a symmetry of this type produces a reduction of the differential equations, restricting the presence of the variables involved in the problem. The results are compared with the case of standard (i.e. exact) Lie-point symmetries and are also illustrated by some examples.Comment: 12 page

Gradients versus Cycling in Genetic Selection Models

We review the hierarchy of (continuous time) selection models starting with the classical Fisher's viability selection model, and its generalizations when allowing mutations, recombination, sex-dependent viabilities, fertility selection and different mortality rates. We analyse the question in which way Fisher's "Fundamental Theorem of Natural Selection" and Kimura's Maximum Principle can be extended to these more general situations. It turns out that in many cases this is principally impossible since the dynamics becomes cycling or even chaotic

Occupation time distributions for the telegraph process

For the one-dimensional telegraph process, we obtain explicit distribution of the occupation time of the positive half-line. The long-term limiting distribution is then derived when the initial location of the process is in the range of sub-normal or normal deviations from the origin; in the former case, the limit is given by the arcsine law. These limit theorems are also extended to the case of more general occupation-type functionals.Comment: 23 pages, 4 figure