Cascading of second-order processes in a type II phase-matched SHG crystal applied to mode-locking of a CW Nd:YAG laser

Abstract

It has been shown recently that a quadratic non-linear crystal may be used for all-optical signal processing. It was demonstrated that amplitude modulation, phase modulation, transistor action, phase conjugaison was feasible thanks to parametric interactions occurring in crystals cut for second harmonic generation (SHG). The main part of these applications was considered under phase mismatched condition1,2,3,4 and relied on the cascading of sum and difference frequency generation (SFG+DFG). With the use of type II crystal a cascading effect still happens in the case of perfect phase-matching provided the two orthogonally polarized inputs at the fundamental frequency carry different intensity5,6,7. This is even one of the most efficient configuration to achieve all-optical processing with the lowest control powers. The interaction between the two waves at the fundamental frequency and the second harmonic wave may be used in specific arrangement to achieve ultrafast switching, pulse compression8, and saturable absorption or transparency9,10. In particular, we have already demonstrated that a type II SHG crystal excited by unbalanced fundamental intensities on its neutral axis gives rise at the output to an electromagnetic field whose state of polarization is intensity dependent (figure 1). In the case of high imbalance between the two fundamental inputs, the weakest fundamental input may completely vanish during propagation because of the SHG process and be further regenerated by difference frequency generation with an opposite phase. The nonlinear evolution of the polarization was previously exploited in a polarization gate geometry to realize a device with self induced transparency9. It was suggested that the same set-up could be used intracavity to achieve mode-locking of a laser. This forms the subject of the present communication since we report the mode locking operation of a diode pumped CW Nd:YAG laser by means of intensity dependent polarization evolution in a KTP crystal.