A team of researchers from the U.S. Army Research Laboratory in
Adelphi, Md., and the University of Maryland in Baltimore, captured
reflected photons from a highly specialized laser beam to create a VGI
image of a remote target.
In the case of VGI, reflection does not refer to a mirror image of an
object. Rather it is merely the individual reflected photons of light
that are counted with a single-pixel camera known as a light bucket.
"Virtual ghost imaging is an amazing tool," says Ronald Meyers, a
quantum physicist with the U.S. Army Research Laboratory, in a paper
published in the American Institute of Physics' journal Applied Physics
Letters. "Because we are no longer bound by the need to collect spatial
information – as is necessary in a typical camera – we can
produce an image in some rather adverse and highly obscured conditions."
In normal ghost imaging, harnessing information to make an image is a
two-step process. First, you analyze the light source, which could be
the sun or a lamp, with a charge-coupled device (CCD) camera. You then
use a second detector, a light bucket, to count the reflected photons.
By combining the data from the light source with the properties of the
collected photons, an image can be created.
The trick to making an image from photons that contain no spatial
information lies in physics related to "entanglement," a property of
light that Einstein referred to as "spooky action at a distance."
Through entanglement, photons (individual packets of light) can share a
certain degree of information. This property is already being developed
for specialized communications and computers.
Virtual ghost imaging is a more self-contained and robust application
of this phenomenon. For example, in VGI, one light source was a laser
that produced an incredibly coherent beam of light known as a Bessel
beam. Bessel beams, unlike normal laser beams, produce
concentric-circle patterns. If a portion of the beam is blocked or
obscured along its trajectory, the original pattern eventually reforms.
"Bessel beams are self-healing and provide an important tool in virtual
ghost imaging," said Meyers. "Even after passing through distortions or
a mask, the same well-defined ring shapes reemerge." So long as enough
photons make it to the target and back to the single-photon detector,
it's possible to construct an image.
In their proof-of-concept demonstration, the researchers compared a
Bessel beam's VGI imaging capabilities with that of a normal "Gaussian"
laser beam. Their target was the letters "ARL." The light was then
reflected back to the single pixel bucket detector. The researchers
conducted this same test several times, placing different objects or an
obscuring medium in the paths of the two light beams. In each case
– whether passing through an offset aperture, cloudy water, or
heat distortion – the Bessel beam reformed to produce a
recognizable VGI image. The Gaussian beam produced a much less faithful
image, and, in the case of the offset aperture, produced virtually no
image at all.
"What this demonstrates is that by combining virtual ghost imaging with
a highly diffraction-free coherent light source like a Bessel beam,
it's possible to probe through conditions that would normally thwart
other imaging technologies," Meyers says.
According to the researchers, potential spin-offs of ghost imaging and
virtual ghost imaging include applications in
Intelligence-Surveillance-Reconnaissance (ISR), medical imaging, and
Article: "Virtual Ghost Imaging through Turbulence and Obscurants using
Bessel Beam Illumination" is published in Applied Physics Letters.
Authors: Ronald E. Meyers (1), Keith S. Deacon (1), Arnold D. Tunick (1), and Yanhua Shih (2).