Calibration of an InGaAs photodiode at 1300 nm with a cryogenic radiometer and a diode laser

A single-element, windowless InGaAs photodetector has been calibrated against a cryogenic radiometer using a diode-laser source of wavelength 1296.9 nm (standard uncertainty u = 0.4 nm). This source differs from the gas lasers usually used with cryogenic radiometers in that the beam has an elliptical profile, is invisible to the naked eye, is not polarized, and is less spatially coherent. We describe the simple adaptations we have made to use this source with the cryogenic radiometer. In particular, we use a fibre-optic delivery system to prepare the beam, and an anti-reflection-coated window to admit the radiation. The relative uncertainty in this method of calibration is estimated to be 0.08% (1σ). The measured response (0.9752 A/W) differs from that obtained from the existing laboratory scale by 0.08%.