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NIST 'Nanotubes on a Chip' Aims To Lead Power Measurements

by Editor1 last modified January 27, 2013 - 12:27

A novel chip-scale instrument made from carbon nanotubes may simplify absolute measurements of laser power. Developers of the unit at the National Institute of Standards and Technology (NIST) say it may prove especially useful for measuring light signals transmitted by optical fibers in telecom networks.

NIST 'Nanotubes on a Chip' Aims To Lead  Power Measurements

NIST’s new cryogenic radiometer. Credit: Tomlin/NIST

The prototype device is a miniature version of an instrument called a cryogenic radiometer. It was made using a silicon chip topped with circular mats of CNTs standing on end, according to NIST project leader John Lehman.

The mini-radiometer builds on NIST's previous work using nanotubes to make an ultra-efficient, highly accurate optical power detector. The work advances NIST's ability to measure laser power delivered through fiber for calibration customers, Lehman added.

"This is our play for leadership in laser power measurements. 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," Lehman said in a statement.

The circular patch of CNTs on a pink silicon backing is one component of NIST’s new cryogenic radiometer. The image shows the device at quarter for scale. Gold coating and metal wiring has yet to be added to the chip. The radiometer will simplify and lower the cost of disseminating measurements of laser power.

The tiny circular forests of CNTs are assembled in a special way to create VANTAs ("vertically aligned nanotube arrays"). This approach, Lehman said, have several desirable properties. Most importantly, they uniformly absorb light over a broad range of wavelengths and their electrical resistance depends on temperature. The VANTAs 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’s tests were performed at a temperature of 3.9 K, using light at the telecom wavelength of 1550 nanometers.

NIST and other institutes 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.

NIST plans to apply for a patent on the chip-scale radiometer. 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.