An Electrically Substituted Bolometer as a Transfer Standard Detector
Joseph P. Rice
We have developed an electrically substituted bolometer (ESB) to serve as a portable transfer-standard detector over the full wavelength range from 200 nm to 20 m. The ESB is designed to provide a direct transfer of the optical power scale from the NIST High Accuracy Cryogenic Radiometer (HACR) to NIST spectral comparator facilities in the infrared spectral region where silicon photodiode trap-detectors cannot be used and where the noise of pyroelectric detectors currently limits the uncertainty. The ESB, similar to other liquid-helium-cooled bolometers, uses a gold-black absorber film and silicon temperature sensor on a sapphire substrate. The novel aspect of the ESB is that the gold-black optical absorber film is also used as an electrical heater for chopper-synchronized electrical substitution of optical power at 15 Hz. This operating mode is similar to that used in the commercially-available electrically calibrated pyroelectric radiometer (ECPR). The advantage of the ESB over the ECPR for spectral reference applications is that the noise floor of the 8 mm diameter active-area ESB approaches 10 pW/Hz, corresponding to a detectivity of D* of order 1011 cmHz/W. This is nearly 1000 times better than that attainable from a similar-sized room-temperature ECPR. The advantage of the ESB over traditional helium-cooled bolometers is that, by virtue of the electrical substitution, it is inherently linear over a much wider dynamic range. For example, the ESB is linear from the noise floor to 1 mW (the power range of the HACR), whereas previous helium-cooled bolometers developed at NIST have a similar noise floor but are non-linear above about 10 W and so could not be directly calibrated against the HACR. Because of its low noise, linearity over a wide dynamic range, and spectral and spatial flatness of its response, the ESB is finding other applications beyond that which originally motivated its development.