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Skriven 2006-03-09 19:37:07 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 768
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PHYSICS NEWS UPDATE
                                        
The American Institute of Physics Bulletin of Physics News
Number 768   March 9, 2006  by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
                
LIGHT-LIGHT INTERACTIONS IN  VACUUM will be possible soon.
Vacuum---the very name suggests emptiness and nothingness---is
actually a realm rife with potentiality, courtesy of the laws of
quantum electrodynamics (QED).  According to QED, additional (albeit
virtual) particles can be created in the vacuum, allowing
light-light interactions.  Physicists from Umea University (Sweden)
and the Rutherford Appleton Lab (UK) hope to explore the vacuum by
aiming three powerful laser streams at each other.  The laser light
is not aimed at any material target and is not trying to initiate
any nuclear fusion.  Instead the three beams will merge to produce a
fourth stream with a wavelength shorter than any of the input
beams.  This idea of mixing beams has been broached before but the
earlier proposals had the beams all in a single plane.  The
Swedish-British proposal (contact Mattias Marklund, 46-90-786-7717,
mattias.marklund@physics.umu.se), by contrast, foresees a fully
three-dimensional wave mixing process.  The actual experiment is
planned to be carried out over the next year at the Rutherford
Appleton Lab.  By carefully polarizing the incoming light beams, the
number of photons in the output beam can be controlled, providing
valuable information about the interactions that took place in the
vacuum.  What is this "four-wave mixing" good for?  For studying QED
itself, but also for testing theories that propose the existence of
minor departures from Lorentz invariance, which is the proposition
(essential to special relativity) that there is no preferred frame
of reference.  Light-light interactions might also be used to
explore various hypotheses related to dark energy.  (Lundstrom et
al., Physical Review Letters, 3 March 2006)

USING FIBER OPTICS TO PINPOINT STRUCTURAL PROBLEMS EARLY.    At this
week's OFC/NFOEC optical fiber meeting in California, University of
Ottawa physicists reported an optical system that detects problems
in important structures, including natural-gas pipes and concrete
columns, more precisely and potentially earlier than before.
Already being considered for commercial production, the new system
can catch much earlier signs of costly and dangerous structural
failures than previously possible.  Called the Distributed Brillouin
Sensor (DBS), the system uses fiber optics to detect deformation,
cracks, and bending in two structures under real-world conditions.
In one demonstration, conducted with Edmonton-based engineering firm
C-Fer Technologies and Trans-Canada Pipeline Limited, the Ottawa
researchers (Xiaoyi Bao, xbao@uottawa.ca) deployed the DBS system on
a section of steel pipe designed to transport natural gas.  Laying
10 lines of optical fiber along the axis of the pipe, they created
one pulse of laser light and one continuous light wave, each
traveling in opposite directions in the fiber.  When the researchers
applied tension and compression to the pipe to mimic real-world
disturbances, it produced vibrations (sound waves). Through a
phenomenon called the Brillouin effect, these sound waves then
slightly changed the speed of light in the affected part of the
fiber and consequently altered the frequency difference between the
two propagating light waves, providing precise information on the
mechanical strains that were applied to the pipe.  Unlike present
structural health analysis, which is done on a spot-by-spot basis,
DBS can detect problems over all points in the entire structure and
pinpoint the location of a structural deformation to within 5
centimeters, while measuring mechanical strains as low as 20
microstrains (20 millionths of a strain, a dimensionless property
that generally reflects the structure's change in length over its
original length).  This exceeds the 1-meter resolution and 50
microstrains that the construction industry has wanted and expected.
In another demonstration, conducted with civil engineers at the
University of Ottawa, the researchers tested the DBS system on a
concrete column encased with fiber-reinforced rods and sheets.
Subjecting the column to simulated seismic forces such as those that
would occur in an earthquake or tsunami, the researchers could
detect signs of debonding (in which the concrete detached from the
fiber casing) and the cracks (crushing) of concrete as a result of
compression forces.  Unlike competing techniques, the system could
readily tell the difference between debonding and crushing.  The
Ottawa researchers say that DBS can prevent potentially
life-threatening and environmentally damaging accidents and
multimillion-dollar repairs.  In addition, the technique can improve
the testing of structures and materials by providing valuable
information during the testing process.  (Paper OTuL7 at meeting,
www.ofcnfoec.org )

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 * Origin: Big Bang (1:106/2000.7)