PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 630 March 27, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

SUPERCONTINUUM LIGHT IS RED-HOT.  Lasers usually produce a single pure
color, or more precisely, light with a narrow range of wavelengths. But
laser light traveling through special optical fibers can produce
"supercontinuum light," multicolored light with many of the same desirable
properties as ordinary laser light, including a bright, narrow, beam, and
coherence, in which the individual light waves in the beam have a precisely
defined interrelationship.  Many new advances in the production of
supercontinuum light were presented at this week's Optical Fiber
Communication Conference and Exposition (OFC) in Atlanta.  To generate
supercontinuum light at the 1550-nm wavelength used in telecom applications,
Jeff Nicholson (jwn{at}ofsoptics.com) of OFS, Inc. and his colleagues use an
erbium-doped fiber laser, in which erbium atoms in an optical fiber amplify
incoming laser light to the desired level.  With this laser, the researchers
send intense, 100-femtosecond pulses through several meters of highly
nonlinear optical fiber, which interacts with the light to produce the
multiple colors.  With this all-fiber design, based on standard telecom
manufacturing technology, Nicholson and colleagues produced low-noise
supercontinuum light that spans an octave of bandwidth (the frequencies at
the high end of its spectrum are twice that of the low-end). This is the
widest supercontinuum ever produced with an all-fiber laser.
In a different approach that employs continuous streams of laser light
rather than short pulses, Akheelesh Abeeluck of OFS (abeeluck{at}ofsoptics.com)
and co-workers use a Raman fiber laser, in which an optical signal is
amplified by a second, lower-wavelength light source. In their work,
Abeeluck splices the Raman fiber laser to a nonlinear fiber in order to
produce supercontinuum light. With 821 mW of power launched into 4.5 km of
fiber, they achieved a continuum with a bandwidth greater than 247 nm.
While not as wide a continuum as with the all-fiber laser, it is a
potentially high-power continuous source of light.  In a third example,
Zulfadzli Yusoff of the University of Southampton (zuy{at}orc.soton.ac.uk) and
colleagues send intense picosecond pulses through a fiber with a special
geometric pattern of holes running along its length, causing the light to
interact with the fiber in a nonlinear fashion, converting the
single-colored laser light into a broad spectrum of colors.  In his work,
Yusoff then carries out a "spectral slicing" method that uses a filter to
separate these colors. The separated colors then travel through individual
fibers. This approach might reduce the complexity of producing the multiple
colors of light that travel down modern transmission systems, leading to
cost savings.  Among other applications, supercontinuum light could provide
high-quality broadband light for a medical imaging technique known as
"optical coherence tomography" which can yield detailed images of human
tissue.

WATCHING BRICKS AGE.  Civil engineers and materials scientists have long
known that clay bricks and other fired ceramics expand as they age owing to
the absorption of water from the atmosphere. In general, however, studies of
moisture expansion in bricks have been limited to freshly fired bricks over
short timescales. Now researchers from the University of Manchester
Institute of Science and Technology and the University of Edinburgh have
experimentally investigated expansion in bricks over periods extending back
to Roman times, about 1900 years ago. They conclude that brick expansion is
governed by a power law. Specifically, bricks expand in proportion to time,
raised to the quarter power, as opposed to the logarithmic expansion with
time predicted by studies over shorter time scales.  The researchers (M. A.
Wilson, 44-0161-200-4245, moira.wilson{at}umist.ac.uk) propose that the power
law moisture expansion is consistent with the ceramics absorbing water that
diffuses through atomic scale pathways in the material. The new theory
should help in the engineering of brick structures intended to last a
century or more by allowing designers to account for expansion that might
otherwise lead to cracks. The power law may also be handy for archeological
dating of bricks and ceramics. For example, archeologists could measure the
dimensions of a piece of ceramic, and then bake out any moisture it may have
absorbed to determine its size at the time that it was first fired. The age
of the sample can be inferred from the contraction as the ceramic dries out.
(M. A. Wilson et al., Physical Review Letters, 28 March 2003)

GAMMA RAY BURSTERS AND SUPERNOVAS go together, at least in one instance.
GRBs represent some of the most violent events in the universe and have been
the subject of intense study and conjecture.  One theory holds that GRBs are
associated with supernovas
(http://www.aip.org/enews/physnews/2000/split/512-3.html ) This
hypothesis is bolstered by new observations by the Chandra X-Ray Observatory
of GRB 020813, a burster object discovered previously by the High-Energy
Transient Explorer (HETE).  The x-ray spectrum contains characteristic
signs---the presence of ionized silicon and sulphur---of supernovas.  The
new results were announced this week at the meeting of the High Energy
Division of the American Astronomical Society (AAS).  (chandra.harvard.edu
)

THE PHYSICS OF SPEAR THROWING.  Atlatl is the Mexican name for a spear
thrower, a stone age weapon used now not so much for hunting or warfare but
for sport (www.worldatlatl.org ).  As with a bent bow or a stretched rubber
band, an atlatl enhances the hurling power of the human arm by storing
energy in a storage medium (in this case a spring).  It also extends the
lever arm between the spear and wrist, imparting more velocity.  Recently
Richard A. Baugh used high-speed video to study the performance of a modern
atlatl over a variety of values for wrist torque, hand position, and other
factors. The typical speed for a hurled 50-gram dart was 25 m/sec.  The mean
thrown distance in one test was 66 meters.  (Baugh, American Journal of
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