1,324 research outputs found
Near-field to far-field transition of photonic crystal fibers: symmetries and interference phenomena
The transition from the near to the far field of the fundamental mode
radiating out of a photonic crystal fiber is investigated experimentally and
theoretically. It is observed that the hexagonal shape of the near field
rotates two times by pi/6 when moving into the far field, and eventually six
satellites form around a nearly gaussian far-field pattern. A semi-empirical
model is proposed, based on describing the near field as a sum of seven
gaussian distributions, which qualitatively explains all the observed phenomena
and quantitatively predicts the relative intensity of the six satellites in the
far field.Comment: 7 pages including 6 figures. Animated version of Fig. 5 is available
at http://www.crystal-fibre.com/technology/movie.gi
Nitrogen and Other Compounds in Rain and Snow
The importance of regularly analyzing the rain and snow for nitrogen and other compounds is widely recognized, both for its agricultural and hygienic significance. Messrs. F. T. Shutt and R. L. Dorrance of Ottawa, Canada, carried on a systematic investigation of the rain and snow through a period of ten years, 1908 to 1917, the results of which study have added much to our knowledge of the importance of the nitrogen compounds as an agricultural factor
Reduced micro-deformation attenuation in large-mode area photonic crystal fibers for visible applications
We consider large-mode area photonic crystal fibers for visible applications
where micro-deformation induced attenuation becomes a potential problem when
the effective area A_eff is sufficiently large compared to lambda^2. We argue
how a slight increase in fiber diameter D can be used in screening the
high-frequency components of the micro-deformation spectrum mechanically and we
confirm this experimentally for both 15 and 20 micron core fibers. For typical
bending-radii (R~16 cm) the operating band-width increases by ~3-400 nm to the
low-wavelength side.Comment: Accepted for Optics Letter
Optimal power extraction from active particles with hidden states
We identify generic protocols achieving optimal power extraction from a
single active particle subject to continuous feedback control under the
assumption that the instantaneous net velocity, but not the fluctuating
contribution originating from the self-propulsion, is accessible to direct
observation. Our Bayesian approach draws on the Onsager-Machlup path integral
formalism and is exemplified in the cases of free run-and-tumble and active
Ornstein-Uhlenbeck dynamics in one dimension. Such optimal protocols extract
positive work even in models characterised by time-symmetric positional
trajectories and thus vanishing informational entropy production rates. We
argue that the theoretical bounds derived in this work are those against which
the performance of realistic active matter engines should be compared.Comment: 6 pages (main) + 10 pages (SM), 4 figure
Charles W. Knight Correspondence
Entries include a typed letter on personal stationery, a letter on Maine State Library stationery, and a newspaper clipping with a photographic image of Knight
Experimental investigation of cut-off phenomena in non-linear photonic crystal fibers
The modal cut-off is investigated experimentally in a series of high quality
non-linear photonic crystal fibers. We demonstrate a suitable measurement
technique to determine the cut-off wavelength and verify it by inspecting the
near field of the modes that may be excited below and above the cut-off. We
observe a double peak structure in the cut-off spectra, which is attributed to
a splitting of the higher order modes. The cut-off is measured for seven
different fiber geometries with different pitches and relative hole size, and a
very good agreement with recent theoretical work is found.Comment: 3 pages including 1 table and 4 figures. Accepted for Optics Letter
Indiana Nonprofit Employment: 2009 Update
Analyzes recent trends in the state's nonprofit employment, including proportion of overall employment; payroll growth, distribution, and wages by sector; nonprofit and for-profit wage gap; and proportion of nonprofit workers employed by charities
A Taxonomy of Fallacies in System Safety Arguments
Safety cases are gaining acceptance as assurance vehicles for safety-related systems. A safety case documents the evidence and argument that a system is safe to operate; however, logical fallacies in the underlying argument may undermine a system s safety claims. Removing these fallacies is essential to reduce the risk of safety-related system failure. We present a taxonomy of common fallacies in safety arguments that is intended to assist safety professionals in avoiding and detecting fallacious reasoning in the arguments they develop and review. The taxonomy derives from a survey of general argument fallacies and a separate survey of fallacies in real-world safety arguments. Our taxonomy is specific to safety argumentation, and it is targeted at professionals who work with safety arguments but may lack formal training in logic or argumentation. We discuss the rationale for the selection and categorization of fallacies in the taxonomy. In addition to its applications to the development and review of safety cases, our taxonomy could also support the analysis of system failures and promote the development of more robust safety case patterns
The arithmetical hierarchy in the setting of
We continue work from (Greenberg and Knight) on computable structure theory in the setting of , where the countable ordinals play the role of natural numbers, and countable sets play the role of finite sets. In the present paper, we define the arithmetical hierarchy through all countable levels (not just the finite levels). We consider two different ways of doing this—one based on the standard definition of the hyperarithmetical hierarchy, and the other based on the standard definition of the effective Borel hierarchy. For each definition, we define computable infinitary formulas through all countable levels, and we obtain analogues of the well-known results from (Ash and Knight, 1989) and (Chisholm, 1990) saying that a relation is relatively intrinsically just in case it is definable by a computable formula. Although we obtain the same results for the two definitions of the arithmetical hierarchy, we conclude that the definition resembling the standard definition of the hyperarithmetical hierarchy seems preferable
Low-loss criterion and effective area considerations for photonic crystal fibers
We study the class of endlessly single-mode all-silica photonic crystal
fibers with a triangular air-hole cladding. We consider the sensibility to
longitudinal nonuniformities and the consequences and limitations for realizing
low-loss large-mode area photonic crystal fibers. We also discuss the
dominating scattering mechanism and experimentally we confirm that both macro
and micro-bending can be the limiting factor.Comment: Accepted for Journal of Optics A - Pure and Applied Optic
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