Abstract

Group-velocity (GV) mismatch severely limits the efficiency of three-wave mixing processes with ullrashort pulses. Special conditions were found in the latest 80’s by Martinez, by Szabo' et al. and by Zhang et al. for obtaining GV matching in second-harmonic generation via achromatic or non-collinear phase-matching. More recently, we demonstrated that GV matching can be achieved in the more general case of tunable parametric generation by using suitable non-collinear phase-matching configurations [1,2]. Here we show that this can be obtained also in collinear parametric interactions, if the interacting pulses are made to propagate in the biréfringent nonlinear crystal with the pulse fronts suitably tilted respect to the phase fronts (Fig. 1). In fact, tilting the pump pulse by an angle γ modifies its collinear GV by the factor tan(ρ)tan(ργ), where ρ is the walkoff angle. Figure 2 shows the calculated γ values for which the GV of a 400 nm pump in type I BBO equals either the idler GV (dashed line) or the signal GV (dotted line); the full line gives the lilt angle γ_opt for which the signal- and idler-pulse GV's in the reference frame of the pump pulse are opposite, a condition that allows signal and idler to be amplified while staying locked to the pump, without broadening or gain saturation [3]. The relevant fact is that, since γ_opt is virtually constant over almost the entire tuning range, such a locked amplification can be obtained with a single set-up at any wavelengths.

© 1996 IEEE