Generation and stability of optical bullets in quadratic nonlinear media

We show numerically that modulational instability of two-dimensional solitary waves in quadratic media leads to the formation of a train of quasi-stable three-dimensional light bullets of mutually trapped fundamental and second harmonic pulses. The dynamics and stability of these bullets are studied both numerically and variationally. Small deviations fi om the stationary bullet profile generally lead to long lived pulsations. Regions of unstable behaviour (decay) are found for both positive and negative mismatches. (C) 1998 Elsevier Science B.V.