Measurements of Low Optical Power with Cryostat-Based Predictable Quantum Efficient Detector at Liquid Nitrogen Temperature

We have validated optical power measurements with a Predictable Quantum Efficient Detector (PQED) at liquid nitrogen temperature (77 K) at low optical power from 130 fW to 3.3 pW. Two laser wavelengths at 514 nm and 785 nm were used. The lowest measured optical power corresponds to a photon flux of 0.5·10⁶ photons per second (785 nm). The PQED’s responsivity is linear within the relative measurement uncertainties of 8% at 0.5·10⁶ ph/s and 1.4% at 10·10⁶ ph/s (95% confidence level), which enables the calibration of other low photon flux detectors directly against a primary standard of optical power.