COMPARATIVE THEORY OF WALKOFF-LIMITED TYPE-II VERSUS TYPE-I 2ND HARMONIC-GENERATION WITH GAUSSIAN BEAMS

Starting from a unified treatment for type-I and type-II second harmonic generation (SHG) under the undepleted pump assumption based on the heuristic theory, we compare the effect of walkoff on both types of phase-matching (PM). We show that the beam walkoff between the pump polarization waves in type-II interaction may be regarded as an additional absorption in a type-I interaction experiencing the same walkoff angle. Power saturation is reached at about 2.3 times the aperture length which corresponds to the coherence length. Unlike for type-I SHG, the criterion of optimum focusing becomes meaningless in type-II coupling as the walkoff angle or the crystal length increase. The optimum SHG power or beam shape are then well described by the parallel beam limiting case. Furthermore, it appears that type-II angular far-field intensity distribution remains quasi gaussian whatever the focusing: diffractional effects such as sharp-edged or fringed transverse pattern observed in type-I SH profile under strong focusing do not appear.