1,212 research outputs found
An introduction to ghost imaging: quantum and classical
Ghost imaging has been a subject of interest to the quantum optics community for the past 20 years. Initially seen as manifestation of quantum spookiness, it is now recognized as being implementable in both single- and many-photon number regimes. Beyond its scientific curiosity, it is now feeding novel imaging modalities potentially offering performance attributes that traditional approaches cannot match
Generation of Caustics and Spatial Rogue Waves from Nonlinear Instability
Caustics are natural phenomena in which nature concentrates the energy of
waves. Although, they are known mostly in optics, caustics are intrinsic to all
wave phenomena. For example, studies show that fluctuations in the profile of
an ocean floor can generate random caustics and focus the energy of tsunami
waves. Caustics share many similarities to rogue waves, as they both exhibit
heavy-tailed distribution, i.e. an overpopulation of large events. Linear
Schr\"odinger-type equations are usually used to explain the wave dynamics of
caustics. However, in that the wave amplitude increases dramatically in
caustics, nonlinearity is inevitable in many systems. In this Letter, we
investigate the effect of nonlinearity on the formation of optical caustics. We
show experimentally that, in contrast to linear systems, even small phase
fluctuations can generate strong caustics upon nonlinear propagation. We
simulated our experiment based on the nonlinear Schr\"odinger equation (NLSE)
with Kerr-type nonlinearity, which describes the wave dynamics not only in
optics, but also in some other physical systems such as oceans. Therefore, our
results may also aid our understanding of ocean phenomena.Comment: 5 pages, 4 figure
A modular, programmable measurement system for physiological and spaceflight applications
The NASA-Ames Sensors 2000! Program has developed a small, compact, modular, programmable, sensor signal conditioning and measurement system, initially targeted for Life Sciences Spaceflight Programs. The system consists of a twelve-slot, multi-layer, distributed function backplane, a digital microcontroller/memory subsystem, conditioned and isolated power supplies, and six application-specific, physiological signal conditioners. Each signal condition is capable of being programmed for gains, offsets, calibration and operate modes, and, in some cases, selectable outputs and functional modes. Presently, the system has the capability for measuring ECG, EMG, EEG, Temperature, Respiration, Pressure, Force, and Acceleration parameters, in physiological ranges. The measurement system makes heavy use of surface-mount packaging technology, resulting in plug in modules sized 125x55 mm. The complete 12-slot system is contained within a volume of 220x150x70mm. The system's capabilities extend well beyond the specific objectives of NASA programs. Indeed, the potential commercial uses of the technology are virtually limitless. In addition to applications in medical and biomedical sensing, the system might also be used in process control situations, in clinical or research environments, in general instrumentation systems, factory processing, or any other applications where high quality measurements are required
Divergence of an orbital-angular-momentum-carrying beam upon propagation
There is recent interest in the use of light beams carrying orbital angular
momentum (OAM) for creating multiple channels within free-space optical
communication systems. One limiting issue is that, for a given beam size at the
transmitter, the beam divergence angle increases with increasing OAM, thus
requiring a larger aperture at the receiving optical system if the efficiency
of detection is to be maintained. Confusion exists as to whether this
divergence scales linarly with, or with the square root of, the beam's OAM. We
clarify how both these scaling laws are valid, depending upon whether it is the
radius of the Gaussian beam waist or the rms intensity which is kept constant
while varying the OAM.Comment: 4 pages, 2 figure
Imaging with a small number of photons
Low-light-level imaging techniques have application in many diverse fields,
ranging from biological sciences to security. We demonstrate a single-photon
imaging system based on a time-gated inten- sified CCD (ICCD) camera in which
the image of an object can be inferred from very few detected photons. We show
that a ghost-imaging configuration, where the image is obtained from photons
that have never interacted with the object, is a useful approach for obtaining
images with high signal-to-noise ratios. The use of heralded single-photons
ensures that the background counts can be virtually eliminated from the
recorded images. By applying techniques of compressed sensing and associated
image reconstruction, we obtain high-quality images of the object from raw data
comprised of fewer than one detected photon per image pixel.Comment: 9 pages, 4 figure
DichroMatch: a website for similarity searching of circular dichroism spectra
Circular dichroism (CD) spectroscopy is a widely used method for examining the structure, folding and conformational changes of proteins. A new online CD analysis server (DichroMatch) has been developed for identifying proteins with similar spectral characteristics by detecting possible structurally and functionally related proteins and homologues. DichroMatch includes six different methods for determining the spectral nearest neighbours to a query protein spectrum and provides metrics of how similar these spectra are and, if corresponding crystal structures are available for the closest matched proteins, information on their secondary structures and fold classifications. By default, DichroMatch uses all the entries in the Protein Circular Dichroism Data Bank (PCDDB) for its comparison set, providing the broadest range of publicly available protein spectra to match with the unknown protein. Alternatively, users can download or create their own specialized data sets, thereby enabling comparisons between the structures of related proteins such as wild-type versus mutants or homologues or a series of spectra of the same protein under different conditions. The DichroMatch server is freely available at http://dichromatch.cryst.bbk.ac.uk
Limitations to the determination of a Laguerre-Gauss spectrum via projective, phase-flattening measurement
One of the most widely used techniques for measuring the orbital angular
momentum components of a light beam is to flatten the spiral phase front of a
mode, in order to couple it to a single-mode optical fiber. This method,
however, suffers from an efficiency that depends on the orbital angular
momentum of the initial mode and on the presence of higher order radial modes.
The reason is that once the phase has been flattened, the field retains its
ringed intensity pattern and is therefore a nontrivial superposition of purely
radial modes, of which only the fundamental one couples to a single mode
optical fiber. In this paper, we study the efficiency of this technique both
theoretically and experimentally. We find that even for low values of the OAM,
a large amount of light can fall outside the fundamental mode of the fiber, and
we quantify the losses as functions of the waist of the coupling beam of the
orbital angular momentum and radial indices. Our results can be used as a tool
to remove the efficiency bias where fair-sampling loopholes are not a concern.
However, we hope that our study will encourage the development of better
detection methods of the orbital angular momentum content of a beam of light.Comment: 5 pages, 4 figure
Direct Measurement of a 27-Dimensional Orbital-Angular-Momentum State Vector
The measurement of a quantum state poses a unique challenge for
experimentalists. Recently, the technique of "direct measurement" was proposed
for characterizing a quantum state in-situ through sequential weak and strong
measurements. While this method has been used for measuring polarization
states, its real potential lies in the measurement of states with a large
dimensionality. Here we show the practical direct measurement of a
high-dimensional state vector in the discrete basis of orbital-angular
momentum. Through weak measurements of orbital-angular momentum and strong
measurements of angular position, we measure the complex probability amplitudes
of a pure state with a dimensionality, d=27. Further, we use our method to
directly observe the relationship between rotations of a state vector and the
relative phase between its orbital-angular-momentum components. Our technique
has important applications in high-dimensional classical and quantum
information systems, and can be extended to characterize other types of large
quantum states.Comment: 8 pages, 3 figure
Photocycloaddition Of 4,5\u27,8-trimethylpsoralen And Oleic-acid Methyl-ester - Product Structures And Reaction-mechanism
The stereochemical structures of the four adducts formed between oleic acid methyl ester (OAME) and 4,5\u27,8-trimethylpsoralen (tmPso) have been determined. Assignment of the tmPso \u27H NMR spectrum was accomplished by analogy to two coumarin model compounds and with the use of homonuclear decoupling and resonance enhancement. Assignment of the lH NMR spectra for the OAME-tmPso adducts was made by analogy to the spectra of OAME and tmPso and using 2D &resolved and COSY analyses. The configurations of the cyclobutyl rings in these adducts was determined by The stereochemical structures of the four adducts formed between oleic acid methyl ester (OAME) and 4,5\u27,8-trimethylpsoralen (tmPso) have been determined. Assignment of the tmPso \u27H NMR spectrum was accomplished by analogy to two coumarin model compounds and with the use of homonuclear decoupling and resonance enhancement. Assignment of the lH NMR spectra for the OAME-tmPso adducts was made by analogy to the spectra of OAME and tmPso and using 2D &resolved; and COSY analyses. The configurations of the cyclobutyl rings in these adducts was determined by MM2 energy minimization calculations, homonuclear \u27H NOE analysis, and comparison of products obtained with cis-OAME and trans-EAME (elaidic acid methyl ester). Only four of the eight possible disastereomeric adducts are detected. These adducts have the cis-cis- , cis-cis-HT, trans-cis- , and trans-cis-HT configurations. The lack of formation of the other isomers may be due to the geometric requirements of exciplex formation. The mechanism of the reaction was established to involve initial bond formation at the 4 position of tmPso, most likely to form a diradical intermediate. The rate of dissociation of the trans diradical is much faster than ring closure, in contrast to the cis diradical whose rate of ring closure is at least as fast as dissociation. The rate of cis-trans isomerization of the 9,lO-bond of the fatty ester portion of the diradical is faster than ring closure for the cis diradical and slower than ring closure for the trans diradical
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