1,720,962 research outputs found
Developments in the generation and measurement of vortex modes in solid-state lasers
No full textWe have explored several methods of generating Laguerre-Gaussian doughnut modes with the eventual application of laser processing directing our research. These modes have potential in this field due to their increasing mode steepness with azimuthal phase and their potential for carrying orbital angular momentum (OAM). A key research goal has been to obtain such modes with high purity and the ability to select the handedness of the OAM, in a system with robust potential for power scaling. All of which are key in order to fully understand the impact of these modes on laser processing.Initially we explored intra-cavity, gain shaping methods in Nd:YAG to achieve these goals. This maintains all the advantages of power scaling normally associated with solid state lasers, and has the potential for excellent beam quality. To the best of our knowledge, however, no concrete method has been presented for selecting the handedness within the cavity. We have selectively excited doughnut modes by coupling multi-modal laser diode pump light into capillary fibres and re-imaging the fibre end into a Nd:YAG gain medium. By using a fibre with a 0.8 aspect ratio between its inner and outer radius, we would be able to select higher order doughnut modes that have a steeper intensity profile and more OAM per photon. Through this we have generated Laguerre-Gaussian 'petal' modes up to an azimuthal order of 23. These high order modes exhibit greater intensity steepness but no OAM. These modes have little practical application.In order to better investigate what governs the selection of the OAM handedness we moved to a ring laser design pumped using a capillary fibre with a 0.5 aspect ratio. This, whilst operating unidirectionally at a single frequency, produces a doughnut mode with an azimuthal order of one and a clear presence of the helical phase and therefore OAM. This has allowed us to examine such a mode in isolation. Through this we observed that reversing beam propagation direction in the ring cavity also reversed the handedness of the helical phase. We found this effect associated with the Faraday Rotator as it is the only known non-reciprocal effect in the cavity. We also found a similar effect in the highly chiral material, tellurium dioxide. The effects were replicated in a slightly multi-modal ring laser but were not observed in a standing wave laser. Tests in free space were unable to detect this effect, so either another mechanism is responsible or the effect was too weak for our detection methods.We therefore moved to external methods for creating doughnut modes. We chose to focus on astigmatic mode converters as these can produce perfectly pure beams unlike methods using spiral phase plates or spatial light modulators. We have developed on the previous design by Beijersbergen et al. by replacing the cylindrical lenses with off-axis spherical mirrors. We have also expanded the theory of mode converters to apply to off-axis spherical mirrors. The mirrors allow for greater power scaling and wavelength flexibility. The key to this is producing high purity HG01 modes. Initially these were produced by placing a slit in a cavity that naturally oscillated on the LG01 mode. However there was a significant fundamental mode impurity. We improved this by developing a Nd:YVO4 laser cavity pumped by two separate circular pump beams. This greatly improved the output power to 175mW as well as the mode quality. This method allows for control of the OAM by rotating the HG01 though 90°. Feeding this through the converter produced a high purity LG01 mode with negligible loss.We also developed a diagnostic technique for analysing the purity of modes generated by a mode converter. As the most likely impurity is a HG01 mode from misaligning the converter this can be picked up by measuring the azimuthal symmetry of the Mach-Zehnder spiral interference pattern. A pure mode will generate a spiral with an even azimuthal intensity distribution. A HG01 impurity will disrupt this. This can be measured with a virtual rotating slit. Our theoretical predictions of the amount of azimuthal variation a given level of impurity will cause matched extremely well when this method was tested with a real system. The need for high purity modes has led us to develop further diagnostic techniques to more quantitatively assess the quality of HG01 and LG01 modes. For both modes this involves analysing its intensity profile as recorded on a CCD camera. It provides quantitative analysis of key properties of the intensity profile in order to identify impurities. For the HG01 mode it compares the evenness of the two intensity maxima and how close to zero the central node is. For the LG01 it again measures how close to zero the central null is, the circularity of the beam, and the azimuthal symmetry. Further analysis generates a matching theoretical mode to the image and subtracts it to look at the residual power distribution. This allows us to identify specific modal impurities incoherently combined with the beam and quantify them. Further work is needed to take into account of distortions created by the CCD camera itself as well as general noise.Finally we Q switched the HG01 seed laser. This allowed for greater output power of 460mW without losing mode purity. It produced 24ns pulses with a 147kHz repetition rate. The pulses could be shortened further simply by shortening the cavity. Passing this through a double-pass bounce geometry Nd:YVO4 slab amplifier; pumped with up to 55W of power we obtained an maximum amplified power of 17W. However some significant mode impurity was added as well as large amount of astigmatism. It was noticed there was damage on the amplifier crystal that may have been the cause of some of this. However amplification is clearly possible, whether by amplifying the HG01 or LG01 mode
Simple technique for high-order ring-mode selection in solid-state lasers
No full textHigh-order ring-shaped laser modes are attractive for applications such as laser micromachining and optical particle trapping due to their increased transverse intensity gradients compared to a fundamental Gaussian mode of the same size. In the case of micromachining, the steeper intensity profile of a single high-order ring-mode will yield a “sharper” cutting profile, while presenting a cutting face that is spatially identical for all cutting directions. The peak of the intensity gradient for a ring-shaped Laguerre-Gaussian (LG0n) mode, when focussed to a given beam size, increases dramatically with increasing azimuthal mode order, n, so efficient high-order LG mode generation at high powers is very desirable
Method for generating high purity Laguerre-Gaussian modes
No full textGeneration of a donut-shaped first-order Laguerre-Gaussian (LG01) vortex mode via a method designed to yield high mode purity is reported. Our approach utilises a novel twin-spot end-pumping scheme to directly excite the first order Hermite-Gaussian (HG01) mode in a solid-state laser, followed by a novel astigmatic mode converter based on spherical (concave) mirrors aligned at oblique incidence. A simple theoretical model for the mode converter is derived and from this the design approach is explained along with the potential benefits compared to conventional schemes based cylindrical-lens astigmatic mode converters, particularly for power scaling. As a proof-of-principle and to confirm the benefits of this scheme in terms of high mode purity we have applied it to an end-pumped Nd:YVO4 laser to generate a (LG01) beam with a controllable sense of azimuthal phase and hence orbital angular momentum. A method for characterising the resulting beam based on analysis of the spiral interference pattern derived with the aid of a Mach-Zehnder interferometer is described and yields a value for the LG01 mode purity of 94%. Common sources of mode impurity are identified and the prospects for scaling to higher power whilst maintaining high mode purity are considered
Dataset for Method for generating high purity Laguerre-Gaussian vortex modes paper
No full textDataset for:
Clarkson, W. et al (2019). Method for Generating High Purity Laguerre-Gaussian Modes. IEEE Journal of Quantum Electronics 55(5).</span
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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