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Turning sound into light

Actors who perform in musicals often sweat in torrents when they
have to zap around the stage on roller-skates or sing a ballad under a
burning spotlight, dressed in bearskin. To allow the audience to hear
them clearly, the artists wear cleverly hidden microphones underneath
their make-up and costume. But as soon they break into a sweat, the
microphone can short-circuit within seconds. This is because conventional
microphones convert sound waves directly into electrical energy,
by vibrating a diaphragm. Dynamic microphones employ a principle
similar to a bicycle dynamo, in which the diaphragm displaces a coil
inside a magnetic field. Condenser microphones convert sound by
means of charge transfer and pass the signal on to an amplifier. When
sound is transmitted over optical fibers, however, moisture has no
effect at all on the microphone. Researchers at the Fraunhofer Institute
for Applied Optics and Precision Engineering IOF in Jena have been
working on just such an optical microphone since 2002 on behalf of
Sennheiser electronic GmbH.
“With this optical microphone, a beam of light from an LED is transmitted
through an optical fiber and a set of lenses”, explains Dr. Andreas
Bräuer, who heads the IOF Microoptics Department. “This array
of optical components splits the beam and focuses it onto a reflective
membrane. The reflected light passes through the lenses to a fiberoptic
receiver.” At each change in the optical signal, the membrane
vibrates and its movement is converted into electrical current by a remote
photodetector. “The IOF has been especially helpful in working
with us to improve the signal-to-noise ratio”, says Dr. Wolfgang
Niehoff, Research Director at Sennheiser electronic. “In other words,
how can we make the process of transforming sound into modulated
light even more efficient?”
Because the optical microphone can be made without metal parts, it
is totally immune to electromagnetic and magnetic interference. This
allows the technology to be used in other applications such as scanners
that use magnetic resonance imaging for medical examinations.
Nervous patients can maintain contact with the attending physician via
the microphone without affecting the results of the examination. “The
optical microphone can also be used to attenuate the noise level inside
the scanner”, adds Niehoff. “Computer-generated anti-noise cancels
out the noise recorded with the microphone. This makes the examination
more bearable. We’re already supplying to a manufacturer of such
systems.”
From FHG Research News

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