A cryogenic radiometer was constructed for direct-substitution optical fiber power measurements. The cavity is intended to operate at the 3 K temperature stage of a dilution refrigerator or 4.2 K stage of a liquid cryostat. The optical fiber is removable for characterization. The cavity features micromachined silicon centering rings to thermally isolate the optical fiber as well as an absorber made from micromachined silicon on which vertically aligned carbon nanotubes were grown. Measurements of electrical substitution, optical absorption and temperature change indicate that the radiometer is capable of measuring 10 nW power levels with approximate responsivity of 155 nW/K and 1/e time constant of 13 minutes. An inequivalence between optical and electrical power of approximately 10 % was found, but the difference was largely attributable to unaccounted losses in the optical fiber.
carbon nanotube, cryogenic radiometer, micromachined silicon, optical fiber, optical power meter