1,917 research outputs found
Vibrating Optical Fibers to Make Laser Speckle Disappear
In optical systems in which laser illumination is delivered via multimode optical fibers, laser speckle can be rendered incoherent by a simple but highly effective technique. The need to eliminate speckle arises because speckle can make it difficult to observe edges and other sharp features, thereby making it difficult to perform precision alignment of optical components. The basic ideas of the technique is to vibrate the optical fiber(s) to cause shifting of electromagnetic modes within the fiber(s) and consequent shifting of the speckle pattern in the light emerging from the fiber(s). If the frequency of vibration is high enough, a human eye cannot follow the shifting speckle pattern, so that instead of speckle, a human observer sees a smoothed pattern of light corresponding to a mixture of many electromagnetic modes. If necessary, the optical fiber(s) could be vibrated manually. However, in a typical laboratory situation, it would be more practical to attach a vibrating mechanism to the fiber(s) for routine use as part of the fiber-optic illuminator. In experiments, a commercially available small, gentle, quiet, variable- speed vibratory device was used in this way, with the result that the appearance of speckle was eliminated, as expected. Figures 1 and 2 illustrate the difference
Use of a post-asynchronous online discussion assessment to enhance student critical thinking
Asynchronous online discussion (AOD) is used in many tertiary education courses, and assessing it has been shown to enhance critical thinking outcomes. There has, however, been debate on what should be assessed and how the assessment should be implemented. The most common form of assessment involves grading the individual discussion contributions, but it has been suggested that employing a culminating task based on the AOD may be effective. This preliminary study compared the effect on student critical thinking of two approaches to AOD assessment: using a post-AOD assessment, and assessing the discussion contributions themselves. The results, though tentative, showed that while both assessment approaches resulted in significant improvements in student critical thinking, there was no difference in the impact on critical thinking skills between using the post-AOD assessment and assessing the discussion contributions. This result suggests that the form of assessment used in an AOD may be less important than the fact that assessment is included. Interviews with students also provided some insight into ways in which they perceived the discussion environment had contributed to their critical thinking skills. The findings of this study pave the way for further research in this important area
Institutionalizing effective grant funded programs: A success story
As budgets tighten, universities are increasingly struggling with questions of how to institutionalize and sustain successful grant-funded programs. Creative approaches to leveraging existing infrastructure and resources can make it possible. Here, we present our experiences and approaches that could benefit those seeking to institutionalize effective pilot or grant-funded programs
A predicted astrometric microlensing event by a nearby white dwarf
We used the Tycho-Gaia Astrometric Solution catalogue, part of the Gaia Data
Release 1, to search for candidate astrometric microlensing events expected to
occur within the remaining lifetime of the Gaia satellite. Our search yielded
one promising candidate. We predict that the nearby DQ type white dwarf LAWD 37
(WD 1142-645) will lens a background star and will reach closest approach on
November 11th 2019 ( 4 days) with impact parameter mas. This
will produce an apparent maximum deviation of the source position of
mas. In the most propitious circumstance, Gaia will be able to
determine the mass of LAWD 37 to . This mass determination will
provide an independent check on atmospheric models of white dwarfs with helium
rich atmospheres, as well as tests of white dwarf mass radius relationships and
evolutionary theory
Extinction coefficients retrieved in deep tropical ice clouds from lidar observations using a CALIPSO-like algorithm compared to in-situ measurements from the cloud integrating nephelometer during CRYSTAL-FACE
The purpose of this paper is to examine the impact that digital technologies have on sports. Sports organisations are pursuing new technologies in order to boost the quality of their entities, not just on the playing field, but also, they are improving their business performance. I will investigate the opportunities that data analysis and digital technologies offer in sports. The main aim is to show the relationship between the use of technology and data, and business and sporting success. It is not enough to just have said technologies and big data if there is not adequate knowledge to professionally analyze data collected and if the users do not have the ability to use it in their favour. Therefore, the technology and data application has to go through a qualification process to ensure that the applied data co-exist with a context of competent knowledge sharing, individual and organizational learning in order to positively affect sporting and business performance. This paper will help readers understand how clubs and individuals use digital technology and data to their own advantage and with plenty of examples and figures provided, readers will be introduced to key trends in digital technologies in sports
Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4
We present a study of the spatial coherence of the intergalactic medium
toward two pairs of high-redshift quasars with moderate angular separations
observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and
Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted
flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for
both pairs. This strongly suggests that at least some of the absorbing
structures span the 230-300/h_70 proper kpc transverse separation between
sightlines. We also statistically examine the similarity between paired spectra
as a function of transmitted flux, a measure which may be useful for comparison
with numerical simulations. In investigating the dependence of the correlation
functions on spectral characteristics, we find that photon noise has little
impact for S/N >~ 10 per resolution element. However, the agreement between the
autocorrelation along the line sight and the crosscorrelation between
sightlines, a potential test of cosmological geometry, depends significantly on
instrumental resolution. Finally, we present an inventory of metal lines. These
include a a pair of strong C IV systems at z ~ 3.4 appearing only toward
Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.Comment: 33 pages, 13 figures, submitted to Ap
Lidar System for Airborne Measurement of Clouds and Aerosols
A lidar system for measuring optical properties of clouds and aerosols at three wavelengths is depicted. The laser transmitter is based on a Nd:YVO4 laser crystal pumped by light coupled to the crystal via optical fibers from laser diodes that are located away from the crystal to aid in dissipating the heat generated in the diodes and their drive circuits. The output of the Nd:YVO4 crystal has a wavelength of 1064 nm, and is made to pass through frequency-doubling and frequency-tripling crystals. As a result, the net laser output is a collinear superposition of beams at wavelengths of 1064, 532, and 355 nm. The laser operates at a pulse-repetition rate of 5 kHz, emitting per-pulse energies of 50 microJ at 1064 nm, 25 microJ at 532 nm and 50 microJ at 355 nm. An important feature of this system is an integrating sphere located between the laser output and the laser beam expander lenses. The integrating sphere collects light scattered from the lenses. Three energy-monitor detectors are located at ports inside the integrating sphere. Each of these detectors is equipped with filters such that the laser output energy is measured independently for each wavelength. The laser output energy is measured on each pulse to enable the most accurate calibration possible. The 1064-nm and 532-nm photodetectors are, more specifically, single photon-counting modules (SPCMs). When used at 1064 nm, these detectors have approximately 3% quantum efficiency and low thermal noise (fewer than 200 counts per second). When used at 532 nm, the SPCMs have quantum efficiency of about 60%. The photodetector for the 355-nm channel is a photon-counting photomultiplier tube having a quantum efficiency of about 20%. The use of photon-counting detectors is made feasible by the low laser pulse energy. The main advantage of photon-counting is ease of inversion of data without need for complicated calibration schemes like those necessary for analog detectors. The disadvantage of photon-counting detectors is that they inherently have narrow dynamic ranges. However, by using photon-counting detectors along with a high-repetition rate laser, it is possible to obtain wide dynamic range through accumulation of counts over many pulses
Renormalized Energies of Superfluorescent Bursts from an Electron-Hole Magneto-plasma with High Gain in InGaAs Quantum Wells
We study light emission properties of a population-inverted 2D electron-hole
plasma in a quantizing magnetic field. We observe a series of superfluorescent
bursts, discrete both in time and energy, corresponding to the cooperative
recombination of electron-hole pairs from different Landau levels. The emission
energies are strongly renormalized due to many-body interactions among the
photogenerated carriers, exhibiting red-shifts as large as 20 meV at 15 T.
However, the magnetic field dependence of the lowest Landau level emission line
remains excitonic at all magnetic fields. Interestingly, our time-resolved
measurements show that this lowest-energy burst occurs only after all upper
states become empty, suggesting that this excitonic stability is related to the
`hidden symmetry' of 2D magneto-excitons expected in the magnetic quantum
limit.Comment: 5 pages, 4 figure
Variability Tests for Intrinsic Absorption Lines in Quasar Spectra
Quasar spectra have a variety of absorption lines whose origins range from
energetic winds expelled from the central engines to unrelated, intergalactic
clouds. We present multi-epoch, medium resolution spectra of eight quasars at
z~2 that have narrow ``associated'' absorption lines (AALs, within 5000 km
s^{-1} of the emission redshift). Two of these quasars were also known
previously to have high-velocity mini-broad absorption lines (mini-BALs). We
use these data, spanning ~17 years in the observed frame with two to four
observations per object, to search for line strength variations as an
identifier of absorption that occurs physically near (``intrinsic'' to) the
central AGN.
Our main results are the following: Two out of the eight quasars with narrow
AALs exhibit variable AAL strengths. Two out of two quasars with high-velocity
mini-BALs exhibit variable mini-BAL strengths. We also marginally detect
variability in a high-velocity narrow absorption line (NAL) system, blueshifted
\~32,900 km s^{-1}$ with respect to the emission lines. No other absorption
lines in these quasars appeared to vary. The outflow velocities of the variable
AALs are 3140 km s^{-1} and 1490 km s^{-1}. The two mini-BALs identify much
higher velocity outflows of ~28,400 km s^{-1} and ~52,000 km s^{-1}. Our
temporal sampling yields upper limits on the variation time scales from 0.28 to
6.1 years in the quasar rest frames. The corresponding minimum electron
densities in the variable absorbers, based on the recombination time scale, are
\~40,000 cm^{-3} to ~1900 cm^{-3}. The maximum distances of the absorbers from
the continuum source, assuming photoionization with no spectral shielding,
range from ~1.8 kpc to ~7 kpc.Comment: 16 pages, 4 figures, ApJ, accepte
Charge order, dynamics, and magneto-structural transition in multiferroic LuFeO
We investigated the series of temperature and field-driven transitions in
LuFeO by optical and M\"{o}ssbauer spectroscopies, magnetization, and
x-ray scattering in order to understand the interplay between charge,
structure, and magnetism in this multiferroic material. We demonstrate that
charge fluctuation has an onset well below the charge ordering transition,
supporting the "order by fluctuation" mechanism for the development of charge
order superstructure. Bragg splitting and large magneto optical contrast
suggest a low temperature monoclinic distortion that can be driven by both
temperature and magnetic field.Comment: 4 pages, 3 figures, PRL in prin
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