NIST's 'nanotubes on a chip' may simplify optical power measurements
Tomlin/NIST |
The
National Institute of Standards and Technology (NIST) has demonstrated a novel
chip-scale instrument made of carbon nanotubes that may simplify absolute
measurements of laser power, especially the light signals transmitted by
optical fibers in telecommunications networks. The prototype device, a
miniature version of an instrument called a cryogenic radiometer, is a silicon
chip topped with circular mats of carbon nanotubes standing on end. The
mini-radiometer builds on NIST's previous work using nanotubes, the world's
darkest known substance, to make an ultraefficient, highly accurate optical
power detector, and advances NIST's ability to measure laser power delivered
through fiber for calibration customers.
"This
is our play for leadership in laser power measurements," project leader
John Lehman says. "This is arguably the coolest thing we've done with
carbon nanotubes. They're not just black, but they also have the temperature
properties needed to make components like electrical heaters truly multifunctional."
NIST
and other national metrology institutes around the world measure laser power by
tracing it to fundamental electrical units. Radiometers absorb energy from
light and convert it to heat. Then the electrical power needed to cause the
same temperature increase is measured. NIST researchers found that the
mini-radiometer accurately measures both laser power (brought to it by an
optical fiber) and the equivalent electrical power within the limitations of
the imperfect experimental setup. The tests were performed at a temperature of
3.9 K, using light at the telecom wavelength of 1550 nanometers.
The
tiny circular forests of tall, thin nanotubes called VANTAs ("vertically
aligned nanotube arrays") have several desirable properties. Most importantly,
they uniformly absorb light over a broad range of wavelengths and their
electrical resistance depends on temperature. The versatile nanotubes perform
three different functions in the radiometer. One VANTA mat serves as both a
light absorber and an electrical heater, and a second VANTA mat serves as a
thermistor (a component whose electrical resistance varies with temperature).
The VANTA mats are grown on the micro-machined silicon chip, an instrument
design that is easy to modify and duplicate. In this application, the
individual nanotubes are about 10 nanometers in diameter and 150 micrometers
long.
By
contrast, ordinary cryogenic radiometers use more types of materials and are
more difficult to make. They are typically hand assembled using a cavity painted
with carbon as the light absorber, an electrical wire as the heater, and a
semiconductor as the thermistor. Furthermore, these instruments need to be
modeled and characterized extensively to adjust their sensitivity, whereas the
equivalent capability in NIST's mini-radiometer is easily patterned in the
silicon.
NIST
plans to apply for a patent on the chip-scale radiometer. Simple changes such
as improved temperature stability are expected to greatly improve device
performance. Future research may also address extending the laser power range
into the far infrared, and integration of the radiometer into a potential
multipurpose "NIST on a chip" device.
Source: National
Institute of Standards and Technology (NIST)
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Posted by Unknown
on Saturday, January 26, 2013.
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