Abstract

With ultrashort pulses, parametric gain is predicted to be larger in those directions that, beside phase matching, better satisfy group velocity (GV) matching.^1,2 In the femtosecond regime one then expects the parametric superfluorescence pattern to be quite different than with longer pulses. We evidenced this peculiarity, and hence the role of GV, in a type II phase-matched BBO pumped by 200-fs pulses at 0.6 μm. We define the angles as in Fig. 1. We report the angle of peak parametric superfluorescence as a function of crystal orientation in Fig. 2. We notice that the superfluorescence peak is neither collinear nor in directions related to walk-off. Significative parameters that account for the effect of GV on gain are l_s and l_i, the pulse splitting lengths of signal and idler (l_s is defined as the propagation distance in which the signal and pump pulses separate in absence of gain).² Making use of Sellmeier equations, we evaluated, for every θ_tun, the wavelengths λ_s, λ_i and the related l_s, l_i, as a function of θ. At a given θ_tun, the superfluorescence is expected to peak at the λs and θs at which the largest ls are. The experimental data of Fig. 2 are well fitted by the value of θ (solid line) calculated in this way.

© 1994 IEEE