198 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
The influence of non-imaging detector design on heralded ghost-imaging and ghost-diffraction examined using a triggered ICCD came
Ghost imaging and ghost diffraction can be realized by using the spatial correlations between signal and idler photons produced by spontaneous parametric down-conversion. If an object is placed in the signal (idler) path, the spatial correlations between the transmitted photons as measured by a single, non-imaging, “bucket” detector and a scanning detector placed in the idler (signal) path can reveal either the image or diffraction pattern of the object, whereas neither detector signal on its own can. The details of the bucket detector, such as its collection area and numerical aperture, set the number of transverse modes supported by the system. For ghost imaging these details are less important, affecting mostly the sampling time required to produce the image. For ghost diffraction, however, the bucket detector must be filtered to a single, spatially coherent mode. We examine this difference in behavour by using either a multi-mode or single-mode fibre to define the detection aperture. Furthermore, instead of a scanning detector we use a heralded camera so that the image or diffraction pattern produced can be measured across the full field of view. The importance of a single mode detection in the observation of ghost diffraction is equivalent to the need within a classical diffraction experiment to illuminate the aperture with a spatially coherent mode
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
Evidence of slow-light effects from rotary drag of structured beams
Self-pumped slow light, typically observed within laser gain media, is created by an intense pump field. By observing the rotation of a structured laser beam upon transmission through a spinning ruby window, we show that the slowing effect applies equally to both the dark and bright regions of the incident beam. This result is incompatible with slow-light models based on simple pulse-reshaping arising from optical bleaching. Instead, the slow-light effect arises from the long upper-state lifetime of the ruby and a saturation of the absorption, from which the Kramers–Kronig relation gives a highly dispersive phase index and a correspondingly high group index
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
Mechanical Faraday effect for orbital angular momentum-carrying beams
When linearly polarised light is transmitted through a spinning window, the plane of polarisation is rotated. This rotation arises through a phase change that is applied to the circularly polarised states corresponding to the spin angular momentum (SAM). Here we show an analogous effect for the orbital angular momentum (OAM), where a differential phase between the positive and negative modes (±ℓ) is observed as a rotation of the transmitted image. For normal materials, this rotation is on the order of a micro radian, but by using a slow-light medium, we show a rotation of a few degrees. We also note that, within the bounds of our experimental parameters, this rotation angle does not exceed the scale of the spatial features in the beam profile
The Epidemiology of the Soybean/Diaporthe Phaseolorum Var. Caulivora Pathosystem in Louisiana.
Field and greenhouse experiments were conducted during 1988-1991 to investigate the epidemiology of the soybean/Diaporthe phaseolorum var. caulivora (Dpc) pathosystem. The following points were addressed: (i) the effect of inoculation timing on soybean yield and disease severity, (ii) the efficacy of Dpc inoculum during the growing season, (iii) the possibility of alternative hosts for Dpc, and (iv) the relationship between threecornered alfalfa hopper injury and stem canker severity. Inoculation timing had a significant effect on the yield of Dpc susceptible or moderately susceptible soybean cultivars. Soybean yield was reduced most (83 to 93%) and disease severity was greatest (84 to 99%) when cultivars were inoculated at the Vc or V3 growth stage. Yield reduction was not as severe when cultivars were inoculated at the late vegetative or early reproductive growth stages. Significant positive correlations were recorded between rainfall and stem canker severity and relative humidity and stem canker severity. The ability of Dpc to infect soybean during the growing season was investigated. Stem canker incidence was greatest 4-7 weeks after planting. While infection of soybean by Dpc decreased 7 weeks after planting, Dpc inoculum was still infecting soybean 11 weeks after planting. Results from Dpc host range experiments indicated weeds commonly found in south Louisiana soybean fields can serve as alternative hosts for Dpc. Dpc colonized and reproduced in several morning glory species, several leguminous weeds, and wild poinsettia. Threecornered alfalfa hopper injury resulted in increased stem canker severity compared to soybean not injured by this insect. Stem diameter and length and seed and pod yields were reduced compared to noninjured plants infected with Dpc
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
- …