1,720,967 research outputs found
Development of rare-earth doped microstructured optical fibres
No full textThis thesis describes the development of novel optical fibres, microstructured optical fibres (MOFs), and demonstrates device applications based on these structures. A particular emphasis is made on incorporating rare-earth ions within these fibres in order to realise novel active devices. Together with the development of the fabrication technique, characterisation and applications of these radically different fibre types are presented. First, the fabrication techniques of MOFs, which heavily rely upon fibre drawing, are studied. A mathematical model developed for the capillary drawing process is experimentally examined. Good agreement is obtained whilst it is also found that the model provides useful physical insights for determining the fibre draw parameters even for MOFs with complex geometries. Details of the fabrication techniques developed to optimise fibre structures are also presented. Transmission properties of highly nonlinear MOFs are then studied experimentally. It is found that the transmission losses are strongly influenced by the core dimensions due to the high Rayleigh scattering coefficient that originates from the holey cladding. A simple model is used to explain the observations. In addition, a continuous effort towards reducing OH-induced losses of this fibres type is outlined. Rare-earth doped highly nonlinear MOFs are fabricated and characterised. Then, three device demonstrations are carried out for the first time. These include a mode-locked ytterbium doped MOF laser, a nonlinear amplifier based on an ytterbium doped MOF, and a continuous wave erbium doped MOF laser with a very low threshold and high efficiency. Using the ytterbium doped MOF, wide tunability of ultrashort pulses from 1µm to 1.58µm is demonstrated using the soliton self frequency shift effect. For the erbium doped MOF, a pump power threshold of 0.5mW and a slope efficiency of 57% are demonstrated. Novel cladding pumped fibres, air clad MOFs, which use a conventional inner cladding and a holey outer cladding, are developed aiming at improved performance of cladding pumped fibre lasers. Wide tunability over 110nm and pure three level operation at 980nm of ytterbium doped cladding pumped fibre lasers are demonstrated. Finally, the fabrication and characterisation of large mode area microstructured fibres (LMA-MOFs) are described, and a comparison with conventional counterparts is made in terms of bend losses and corresponding effective mode areas. The results show that a slight refractive index difference introduced in the core region of this fibre type strongly modifies its waveguide characteristics. By applying this knowledge, a novel ytterbium doped cladding pumped fibre, which uses different sizes of air holes to define the inner and outer cladding, is developed. A continuous wave output power in excess of 1W is obtained. Results concerning various forms of pulsed laser operation using this fibre are presented and future possibilities are discussed
Improved performance of erbium doped holey fiber amplifiers
No full textWe report performance improvements of holey-fiber erbium-doped amplifiers. By optimizing the device-length and splicing-losses we achieve saturation powers in excess of ~20dBm for booster-amplifier implementations and noise figures close to the quantum-limit for preamplifier designs
Erbium doped holey fiber devices
No full textWe fabricated an erbium-doped holey fiber, and demonstrate an all-fiber laser with ultra-low threshold (~0.5mW) and amplifier providing 47 dB of internal gain. Incorporating this amplifier within a ring-laser we demonstrate a tuning range over 100nm
Fundamentals and applications of silica and non-silica holey fibers
No full textMicrostructured optical fibers are of increasing interest since they offer unique optical properties and design flexibility that cannot be achieved in conventional forms of fiber. We review structural features, fabrication, optical properties and potential applications of index-guiding holey optical fibers. The state-of-the-art is presented for both silica and compound glass holey fibers
Cladding pumped Ytterbium-doped fiber laser with holey inner and outer cladding
No full textWe have fabricated an ytterbium doped all-glass double-clad large mode area holey fiber. A highly efficient cladding pumped single transverse mode holey fiber laser has been demonstrated, allowing continuous-wave output powers in excess of 1W with efficiencies of more than 80%. Furthermore both Q-switched and mode-locked operation of the laser have been demonstrated
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Ultra-short pulse Yb<sup>3+</sup> fiber based laser and amplifier system producing >25W average power
No full textWe have demonstrated an ultra-short-pulse Yb-fiber laser and amplifier system that produces >25W average power.The output pulses were recompressed to a duration of 110 fs using a standard bulk grating-based compressor
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