Fiber-coupled electrical-substitution picowatt cryogenic radiometer

We describe a picowatt cryogenic radiometer (PCR) fabricated lithographically for electrical-substitution optical fiber power measurements. The absorber, electrical heater, and thermometer are all on a micro-machined membrane less than 1mm on a side. The PCR is sensitive to input powers from 50fW to 65pW, has a noise equivalent power of 2 × 10^-15W/√Hz, and an effective time constant below 100 ms. A comparison of the response to electrical and optical input powers yields a response standard deviation below ·0.3% (k = 2) for input optical powers of 12pW to 65pW.