Quantum-efficiency measurement of photodetectors by means of correlated photons

We report the first results obtained at the Istituto Elettrotecnico Nazionale Galileo Ferraris of quantum-efficiency measurements of actively quenched avalanche photodiodes in the photon-counting regime. The measurements were performed with the technique of correlated photons generated by a parametric downconversion process ina nonlinear crystal of lithium iodate, an absolute technique that does not require any standard. We pay special attention to the problem of alignment of the optical bench and to the implementation of a parametric amplifier scheme with a diode-laser source. The technique is described, and results of quantum-efficiency measurements of two photodiodes are presented. (C) 1999 Optical Society of America [S0740-3224(99)00810-3].