XFROG spectrograms of the pulse sequence obtained from FSHG of a pair of delayed pulses with parallel and orthogonal polarization states in type-I and type-II nonlinear crystals. (a) Both fundamental pulses have parallel polarization (arrows indicating directions of polarization). Three second harmonic pulses are generated in type I phase-matching, H1, H12 and H2. (b) Experimental FSHG pulses corresponding to (a). (c) One fundamental pulse has parallel polarization and the other, delayed, has orthogonal polarization. Using a type-II BBO crystal, phase-matching is possible only for orthogonal polarization states. This condition is fulfilled only for the cross-term H12 at the intermediate delay between the two fundamental pulses. (d) Experimental isolated FSHG pulse corresponding to (c).