NIST researchers have developed a prototype device that can measure absolute optical power in optical fibers at the nanowatt level. The device, a fiber-coupled cryogenic radiometer, uses a microscopic forest of carbon nanotubes – the world’s darkest material – to achieve measurements that are about one-thousandth of the levels typically attained with conventional cryogenic radiometers.
The new radiometer is about 70mm long and incorporates a 1.45mm-thick optical fiber 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 National Physical Laboratory (the British equivalent of NIST) to make some measurements.
Experiments and calculations indicate the new radiometer can measure a power level of 10 nanowatts with an uncertainty of 0.1 percent. By comparison, typical measurements of optical power delivered through fiber have an uncertainty of 3 percent 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.
Keywords: Single-Photon, Nanotubes, Radiometry, Photons, Metrology
