Multiwalled carbon nanotubes, each measuring 40 µm long, absorb more than 99.9% of the light inside NIST’s prototype fiber-coupled radiometer. (Huang/NanoLab, colorized by Talbott/NIST)

Researchers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have developed a prototype device that can perform absolute measurements of optical power delivered through an optical fiber. The world's first fiber-coupled cryogenic radiometer, the device is based on microscopic carbon nanotubes—the world's darkest material—and is used to link optical-fiber power measurements directly to fundamental electrical units and national standards. It can measure values that are about one-thousandth of the levels typically attained using cryogenic radiometers lacking direct fiber input capability. If scientists can improve the device's temperature control and speed, it could perhaps be used for ultraprecise calibrations at ultralow power in medical device applications.

Optical power and energy are traceable to fundamental electrical units. Radiometers absorb optical energy and convert it into 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.

Measuring about 70 mm in length, the radiometer incorporates a 1.45-mm-thick optical fiber capped by a light-trapping cavity at one end with the nanotube absorber and a heater. The nanotubes are grown on a tiny X-shaped piece of micromachined silicon. Because the high light absorption rendered it difficult to determine measurement uncertainties, project leader John Lehman performed some measurements at the National Physical Laboratory in the UK.

Experiments and calculations indicate that the new radiometer can measure a power level of 10 nW with an uncertainty of 0.1%. In contrast, typical measurements of optical power delivered through fiber have an uncertainty of 3% or more at similar power levels. More importantly, commercial devices with that capacity rely on a series of calibrations to establish traceability to national standards.

NIST's goal is to develop an absolute quantum standard for optical power and energy based on single photons. The effort includes the 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.