NIST Develops Fiber-Coupled Radiometer Capable of Measuring Absolute Optical Power

By using a microscopic forest of carbon nanotubes, the National Institute of Standards and Technology (NIST) scientists have developed a first-of-its-kind fiber-coupled cryogenic radiometer that is capable of delivering absolute optical power measurements via an optical fiber.

The prototype device relates the measurements of optical fiber power directly to national standards and basic electrical units. The carbon nanotube forest allows the device to measure values down to one-thousandth the values normally measured with a cryogenic radiometer requiring the capability of direct fiber input. With improved speed and temperature control, the radiometer can deliver ultra-accurate measurements at ultralow power in medical devices, telecommunications and other industries.

The 70-mm long radiometer comprises a 1.45 mm thick optical fiber sealed by a light-capturing cavity integrated with a heater and a nanotube absorber at one end. The researchers grew the ultra-dark nanotubes over microscopic x-form micromachined silicon piece. It was hard to identify measurement aberrations due to the high absorption of light. Hence, the researchers used the National Physical Laboratory’s special facility to record certain measurements.

Calculations and experiments proved that the novel radiometer is capable of measuring a power level down to 10 nW with 0.1% of uncertainty when compared to 3% of uncertainty of standard optical power measurements through optical fiber at the same power levels. These demonstrations pave the way to the transition of radiometry from an electrical unit-based traditional practice to a photon-based quantum practice.

NIST plans to design an absolute quantum standard for optical energy and power based on individual photons, which requires development of detectors and sources having a broad scope of optical power measurements, ranging from counts of individual photons to trillions of photons.