Effective group velocity and shape of twin beams

The shape and propagation velocities of pulsed twin beams generated in the process of parametric light down-conversion have been theoretically studied in the case where the delays caused by the dispersion of group velocities exceed the length of a pump pulse. It has been shown that the effective group velocity of the scattered pulse is determined by the arithmetic mean of the group velocities at the scattered radiation and pump frequencies. The scattering matrix elements for the large parametric gain have been numerically found. It has been shown that mixing of the spectral components of scattered radiation can cause a significant narrowing of the twin beam pulses. Finally, for the case of generation of broadband twin beams sequentially in two crystals with an aperiodic domain structure, it has been shown that the visibility of the three-frequency interference decreases as the pump pulse lags behind the scattered radiation pulses.