ИСТИНА

We consider parametric down-conversion (PDC) in a chirped quasi-phase-matched second-order nonlinear crystal with undepleted quasimonochromatic pump. Using a firstorder approximate quantum solution for the down-converted field, we calculate the squeezing spectra and the squeezing angles for arbitrary, sufficiently slowly varying poling profiles. We compare the approximate solutions with the exact and numerical ones and find a very good agreement, even in the regime of moderate gain, where the existing approaches are inapplicable. We discuss also a recent experiment for high-gain PDC in an aperiodically poled crystal, where an ultrashort correlation time between bright twin beams was observed.