Measurement of the quantum efficiency of photodetectors by parametric fluorescence

An experimental setup has been developed at the Istituto Elettrotecnico Nazionale G. Ferraris (IEN) for the absolute calibration of photodetectors in the photon-counting regime by means of correlated photons in parametric fluorescence. A nonlinear crystal of lithium iodate is pumped by an argon-ion laser at 351.2 nm, generating a broadband source of correlated photons from the incoming wavelength to the far-infrared. Photomultipliers and solid-state, single-photon detectors are employed, operating in the photon-counting regime with very low dark counts. Quantum efficiency is measured at 633 nm. The results show the correlated photons to be a promising technique for radiometry. The advantages of this as opposed to a conventional method at low power levels are shown.