2,761 research outputs found

    Modulation induced frequency shifts in a CPT-based atomic clock

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    We investigate systematic errors associated with a common modulation technique used for phase sensitive detection of a coherent population trapping (CPT) resonance. In particular, we show that modification of the CPT resonance lineshape due to the presence of off-resonant fields leads to frequency shifts which may limit the stability of CPT-based atomic clocks. We also demonstrate that an alternative demodulation technique greatly reduces these effects.Comment: 14 pages, 7 figure

    Performance of a prototype atomic clock based on lin||lin coherent population trapping resonances in Rb atomic vapor

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    We report on the performance of the first table-top prototype atomic clock based on coherent population trapping (CPT) resonances with parallel linearly polarized optical fields (lin||lin configuration). Our apparatus uses a vertical cavity surface emitting laser (VCSEL) tuned to the D1 line of 87Rb with current modulation at the 87Rb hyperfine frequency. We demonstrate cancellation of the first-order light shift by proper choice of rf modulation power, and further improve our prototype clock stability by optimizing the parameters of the microwave lock loop. Operating in these optimal conditions, we measured a short-term fractional frequency stability (Allan deviation) 2*10^{-11} tau^{-1/2} for observation times 1s<tau< 20s. This value is limited by large VCSEL phase noise and environmental temperature fluctuation. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated lin||lin CPT resonance clock with environmental impacts minimized.Comment: 6 pages, 8 fugure

    Doing Democracy Differently

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    Transnational civil society networks have become increasingly important democratizing actors in global politics. Still, the exploration of democracy in such networks remains conceptually and methodologically challenging. Practice theory provides a framework to study democracy as routinized performances even in contexts of fluid boundaries, temporal relations and a diffuse constituency. The author attempts to understand how new forms of democratic practice emerge in the interaction between political actors and their structural environments

    Barbara Morgan's Photographic Interpretation of American Culture, 1935-1980

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    In 1935, Barbara Morgan, a recent arrival in Depression-era New York, reinvented her career as an artist when she abandoned painting and adopted the medium of photography. In the four-and-a-half decades that followed, Morgan witnessed the remaining years of the Great Depression, World War II, the Korean Conflict, the Cold War, the Vietnam War, and Three Mile Island. This dissertation will trace the photographic oeuvre of Morgan as she responded to these events both directly and indirectly, while simultaneously tracking the important artistic and cultural trends of each decade. The first chapter discusses Morgan's early photomontage work, in which she pushed the boundaries of American photography while exploring diverse metaphors for metropolitan splendor and urban isolation as well as the anxieties of the Great Depression and hope for a better future. Morgan's 1941 book Martha Graham: Sixteen Dances in Photographs anchors the second chapter. The influential dance photographs that comprise this publication highlight Morgan's modernist interpretations of Martha Graham's early dances and allow Morgan to examine beauty, strength, and a complex series of emotions through simple gestures and movement. The third chapter uses the light abstraction Morgan employed as a tailpiece for Sixteen Dances as the starting point to investigate her connections to broader artistic trends in the United States during and after the Second World War. In 1951, Morgan published Summer's Children, a photographic account of life in a summer camp that marked a major departure for the artist. Chapter four examines this book in the context of the Cold War and considers such diverse topics as summer camps, progressive education, fear-mongering, and the rise of the photo-spread. In the last two decades of her career, Morgan returned to the medium of photomontage. The fifth chapter examines this period, in which Morgan protested nuclear proliferation, environmental indifference, a perceived lack of scientific morality, and violent entertainment through her montages

    Phosphor Thermometry on Surfaces - A Study of its Methodology and its Practical Applications

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    Phosphor Thermometry is a term describing an optical measurement technique for remote temperature sensing. Its working principle is based on the temperature-sensitive emission characteristics of certain ceramic substances termed thermographic phosphors. These inorganic materials can either be coated on objects for surface thermometry or be seeded into the gas phase or into liquid flows as solid particles. After optical excitation, often achieved using pulsed laser systems, the phosphor emits an extended and typically red-shifted afterglow referred to as phosphorescence. As the temperature changes, either the temporal or the spectral composition of the phosphorescence emission can be used to determine temperatures through comparison with the results of temperature calibration, carried out earlier. In many applications, temperatures both at various points and in two-dimensional fields have been characterised with a high degree of temporal and spatial resolution by use of thermographic phosphors. The combined sensitivities of different phosphors span a temperature range extending from cryogenic temperatures up to approximately 2000 K. In the present study, the reader is introduced to the physical basis of phosphor luminescence and to utilization of the optical properties involved for temperature measurement. The thesis also examines various means of reducing measurement uncertainty in surface phosphor thermometry. This is done in a series of experimental studies concerned with the characterization and treatment of various error sources during temperature calibration, signal detection and data evaluation. A major factor considered here is that of the coating thickness. It appears to have an intrusive effect on surface temperatures in applications involving both high local and temporal thermal gradients. The effects of instrumentation on signal detection are also investigated. The measurement accuracy was found to depend very much upon the consistency, achieved in the reproduction of the operating conditions from the temperature calibrations carried out to the experiments. This can be attributed to non-linear signal transformations that occur during detection. Even two detectors nominally identical were shown to exhibit large differences in the linearity of the signal response. Unfortunately, the linear workspace of many detectors is confined to very low signal values, the measurement precision being comparably poor due to the low signal-to-noise ratios involved. In order to improve the measurement precision without reducing the accuracy of the results, higher signal levels could be accessed through measures to compensate for detector-specific non-linearities. The signal responses to variations in operating conditions of several different point detectors and imaging devices were characterized, providing a basis for effective means of signal correction. Interest in uncertainty reduction here also led to the investigation of means of signal processing enhancement. Temperature sensitivity was found to be a quantity which is not determined exclusively by the phosphor itself, it is also depending on the operator's choice of conditions for detection and evaluation. For evaluation schemes based on temporal decay transients, the proper choice of a time window for evaluation was found to play an important role. Finally, the versatility of phosphor thermometry as applied to surfaces was demonstrated in several industry-relevant applications, including a car engine, an aircraft turbine and a large-bore two-stroke diesel engine for marine vessels

    Doing democracy differently: political practices and transnational civil society

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    Transnational civil society networks have become increasingly important democratizing actors in global politics. Still, the exploration of democracy in such networks remains conceptually and methodologically challenging. Practice theory provides a framework to study democracy as routinized performances even in contexts of fluid boundaries, temporal relations and a diffuse constituency. The author attempts to understand how new forms of democratic practice emerge in the interaction between political actors and their structural environments
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