Efficient generator of a two-photon field of visible radiation

A description is given of an optimal system for parametric conversion of one-photon light into a two-photon field. This system is suitable for absolute calibration of photodetectors throughout the visible range without the use of a standard source for comparison. Tuning curves of such a parametric converter are calculated and verified experimentally. The efficiency achieved in this method of generation of a two-photon field (10⁶ of two-photon pairs per second in a spectral band Δν = 10 cm^–1 wide) makes it possible to calibrate photodetectors to within at least 3–4% after accumulation of a signal for a time of the order of 100 sec.