Abstract

We have investigated the question whether time delays between signal and idler photons produced in the parametric downconversion process depend on the length of the nonlinear medium. In an ideal fission process one might expect the correlation times to be independent of this length. Experiments were therefore carried out on KDP crystals of different lengths. The distribution of time delays was measured with the help of two microchannel plate photodetectors and a time-to-digital converter. Electronic transit time spreads in the detector were determined by use of a pulsed diode laser. As a major part of the observed spread of time delays (of the order of 100 ps) is due to the photomultipliers, length-dependent effects are derivable only from the difference between rather similar distributions. The derived time delay dispersion between signal and idler photons was found to be of the order of 100 ps in an 8-cm long crystal with 400-ps transit time, and there are indications that it is slightly smaller in a 1/2-cm long crystal of 25-ps transit time. The experimental results are compared to theoretical calculations based on a parametric coupling Hamiltonian, which indicate that crystal length should play a very small role, of the order of 1 ps, and then only when the relative acceptance bandwidth Δω/ω is ~1 % or greater.

© 1986 Optical Society of America