1,721,017 research outputs found
Measurement and compensation of laser-induced wavefront deformations and focal shifts in near IR optics
No full textWe demonstrate the feasibility of passive compensation of the thermal lens effect in fused silica optics, placing suitable optical materials with negative dn/dT in the beam path of a high power near IR fiber laser. Following a brief overview of the involved mechanisms, photo-thermal absorption measurements with a Hartmann-Shack sensor are described, from which coefficients for surface/coating and bulk absorption in various materials are determined. Based on comprehensive knowledge of the 2D wavefront deformations resulting from absorption, passive compensation of thermally induced aberrations in complex optical systems is possible, as illustrated for an F-Theta objective. By means of caustic measurements during high-power operation we are able to demonstrate a 60% reduction of the focal shift in F-Theta lenses through passive compensation. (C) 2014 Optical Society of AmericaGerman federal ministry of economics and technology [KF2411810DF2
Characterization of an ArF excimer laser beam from measurements of the Wigner distribution function
No full textAn ArF excimer laser beam at a wavelength of 193 nm has been characterized by a quantitative determination of the Wigner distribution function. The setup, comprising a spherical lens, a rotating cylindrical lens and a moveable ultraviolet-sensitive CCD detector, enabled the mapping of the entire four-dimensional phase space within less than 20 min. Experiments yielded complete information about second-order moment-based parameters, spatial coherence, wavefront, beam profiles, as well as beam propagation and local distributions of radiant intensities
Photo-thermal measurement of absorptance losses, temperature induced wavefront deformation and compaction in DUV-optics
No full textA measurement system for quantitative registration of transient and irreversible lens effects in DUV optics induced by absorbed UV laser radiation was developed. It is based upon a strongly improved Hartmann-Shack wavefront sensor with an extreme sensitivity of similar to lambda/10000 rms @ 193nm, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted through the laser-irradiated site of a sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test laser is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. This transient wavefront distortion yields a quantitative measure of the absorption losses in the sample. In the case of fused silica, an additional permanent change indicates irreversible material compaction. Results for both fused silica and CaF2 are presented and compared. (C) 2009 Optical Society of Americ
Measurement of the Wigner distribution function of non-separable laser beams employing a toroidal mirror
No full textWe present a new experimental approach to determine the Wigner distribution function of non-separable laser beams. A rotatable toroidal mirror is employed to focus an IR laser beam at a wavelength of 1064 nm. For different rotation angles of the mirror, intensity profiles of the reflected beam are captured. The resulting data suffices to fully reconstruct the four-dimensional Wigner distribution from which the global degree of coherence is derived. A comparison to theoretical expectations reveals validity of the applied formalism. Since the setup dispenses with any transmissive optics it can be applied to a broad spectral range, especially to extreme UV and soft x-ray radiation.Deutsche Forschungsgemeinschaft [SFB755
Absolute measurement of surface and bulk absorption in DUV optics from temperature induced wavefront deformation
No full textA measurement system for quantitative determination of both surface and bulk contributions to the photo-thermal absorption in DUV optics was developed. It is based upon a Hartmann-Shack wavefront sensor with a sensitivity of similar to lambda/10000 rms, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted perpendicular through the excimer laser-irradiated site of a cuboid sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test laser is distorted into a cylindrical shape, with bending ends towards the surface. Sign and magnitude depend on index change and expansion. By comparison with thermal theory, this transient wavefront distortion yields a quantitative absolute measure of bulk and surface absorption losses in the sample. First rresults for fused silica are presented. (C) 2010 Optical Society of Americ
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
Wigner distribution measurements of the spatial coherence properties of the free-electron laser FLASH
No full textThe four-dimensional Wigner distribution function is determined from intensity profiles measured in the focused photon beam of FLASH (Free-electron laser in Hamburg) for a variety of photon beamline settings. The Wigner formalism results in comprehensive coherence information without the requirement of simplifying assumptions on the beam. The entire four-dimensional spatial mutual coherence function, horizontal and vertical coherence lengths and the global degree of coherence are derived and compared to Young’s double pinhole measurement
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