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NIST develops fibre-coupled cryogenic radiometer

Posted: 23 Dec 2011     Print Version  Bookmark and Share

Keywords:optical  fibre  cryogenic  radiometer 

Scientists from the U.S. National Institute of Standards and Technology (NIST) have developed a device that can take absolute measurements of optical power transmitted via an optical fibre.

The protoype device is a fibre-coupled cryogenic radiometer that links optical fibre power measurements to fundamental electrical units and national standards. It uses a microscopic forest of carbon nanotubes—the world's darkest material—to measure values that are about one-thousandth of the levels typically attained with a cryogenic radiometer lacking direct fibre input capability. With improvements in temperature control and speed, the device could meet the needs for ultraprecise calibrations at ultra low power in telecommunications, medical devices and other industries.

Optical power and energy are traceable to fundamental electrical units. Radiometers absorb optical energy and convert it to heat. Then the electrical power needed to induce the same temperature increase is measured. Because optical and electrical heating are not exactly equivalent, measurement uncertainties can be relatively large from a metrology point of view.

The demonstration is also a step towards converting radiometry from a classical practice based on electrical units to a quantum practice based on single particles of light (photons).

"We have many customers who request optical power measurements in fibre, mainly for optical communications," project leader John Lehman said. "Also, our single-photon measurements are done in fibre."

The new radiometer is about 70mm long and incorporates a 1.45mm-thick optical fibre capped by a light-trapping cavity at one end with the nanotube absorber and a heater. The ultra-dark nanotubes are grown on a tiny X-shaped piece of micromachined silicon. Light absorption was so high it was difficult to determine measurement uncertainties; Lehman travelled to a special facility at the British National Physical Laboratory to make some measurements.

Experiments and calculations indicate the new radiometer can measure a power level of 10nanowatts with an uncertainty of 0.1 per cent. By comparison, typical measurements of optical power delivered through fibre have an uncertainty of 3 per cent or more at similar power levels. More importantly, these commercial devices rely on a series of calibrations to establish traceability to national standards.

NIST aims to develop an absolute quantum standard for optical power and energy based on single photons. The effort includes development of sources and detectors spanning a wide range of optical power measurements, from single photon counts to trillions of photons. Single photons are already used in quantum communications systems, which offer novel capabilities such as detecting extremely weak optical signals and providing quantum guarantees on security.

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