1,720,968 research outputs found
Convex approach to covariance control with application to stochastic low-thrust trajectory optimization
No full textThis paper outlines a novel approach to the design of optimal space trajectories under significant uncertainty. Finite-horizon covariance control, i.e., the steering of a system from an initial probability distribution to a desired one at a prescribed time, is employed to plan an optimal nominal path along with a robust feedback controller that compensates for exogenous in-flight disturbances. A mindful convexification strategy is devised to recast the nonlinear covariance control problem as a deterministic convex optimization problem. The convexification is based on a convenient change of variables that allows for relaxing the covariance matrix discrete-time propagation into a set of semidefinite cone constraints. While featuring a larger feasible space, the relaxed problem shares the same optimal solution as the original one. The lossless property of the relaxation is analytically proven by leveraging optimal control theory and discussed on the basis of numerical experiments. Monte Carlo campaigns are carried out to validate the in-flight performance of the attained control policies
Assessment of broad usability of a simple analytic model for passively Q-switched lasers with Cr:YAG saturable absorbers
No full textAnalytic approximations for key parameters of passively Q-switched (PQS) lasers, such as pulse energy and duration, were developed and widely tested in particular for microchip lasers with semiconductor saturable absorbers. However, the same approach can be readily extended to PQS lasers using Cr:YAG as the saturable absorber, including excited-state absorption (ESA) in the model. ESA causes a reduction of pulse energy and an increase in pulse duration. Interestingly, performances of many previously reported PQS lasers using Cr:YAG, from microchips to more powerful multi-millijoule lasers, are correctly predicted by this simple model. Unlike energy estimation, which is intrinsically robust, pulse width requires a more careful evaluation of the influence of the ratio (total linear loss)/(saturable loss) as a correction of the asymptotic result
Microchip Laser Started, 1-MW Peak Power Mamyshev Oscillator at 1 μm
No full textA hybrid single/double-clad Mamyshev ring-oscillator was reliably started by a 1064-nm PQS microchip laser. At 5-W pump-power we obtained 12-MHz repetition rate, ∼115-nJ mode-locked pulses with >65-nm FWHM spectrum and 50-fs duration after compression
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
25 W continuous-wave Yb:LiLuF4 single-crystal-fiber laser oscillator
No full textWe present the first demonstration of a solid-state laser oscillator based on a Yb-doped, birefringent single crystal fiber grown by the micro-pulling-down method. Pumping a 42-mm-long, 2%-at. Yb3+:LiLuF4 crystal with a 105-W maximum power fiber coupled laser diode, we obtained a maximum output power of 10.4 W, with diffraction limited beam quality (M2<1.1), in a simple plane-concave resonator. By modifying the resonator layout to increase the laser efficiency, we obtained output power up to 25 W, with still fairly good beam quality (M2≃2). Thermal lensing effect and laser intrinsic slope efficiency and losses were characterized in both resonator configurations
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
1-mJ multi-kHz nanosecond pulses from a single-crystal Yb:LiLuF4 amplifier seeded by a passively-Q-switched laser
No full textSingle-crystal fiber Yb-doped LiLuF4 grown by the micro-pulling-down method was tested in a high-power pumped amplifier setup seeded by a passively Q-switched Yb:YLF laser. With 82-W absorbed pump power and 130-μJ injected pulse energy, the amplifier yielded 1.05-mJ pulses with 110-ns duration at 5.2 kHz, linearly polarized with excellent beam quality. These results prove the high optical quality and promising performance of this new material technology exploiting birefringent Yb-doped materials at very high pump power
Femtosecond Mamyshev fiber oscillator started by a passively Q-switched microchip laser
No full textA femtosecond Mamyshev fiber oscillator in normal dispersion mode at 1 μm was started reliably and safely by an inexpensive diode-pumped passively Q-switched monolithic microchip laser emitting 300-ps pulses. Four-wave mixing spectral broadening is shown to play a pivotal role in starting the Mamyshev oscillator, owing to the random short and intense temporal fluctuations allowed by its ∼10-nm bandwidth. Systematic studies of the starting dynamics show that a success rate of 100% of the attempts is achieved with modest seed energy, as low as ∼30 nJ from the sub-nanosecond laser, corresponding to ∼100 pJ for the total four-wave mixing signal required to start the oscillation
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