Second harmonic generation in a quasi-phase matched structure containing a saturable absorber

Quasi-phase matching has achieved wide recognition as a powerful and versatile tool for efficient second harmonic generation (SHG). In such media, for imperfect phase matching. the process of frequency up-conversion evolves periodically with distance because the residual pump nave is parametrically amplified is regions of high conversion efficiency. This results in frequency down-conversion. Even when perfect phase-matching is met, any noise power at the fundamental frequency will be strongly amplified in regions of high harmonic power, causing down conversion. In practice, these effects limit the SHG efficiencies that can be achieved under stable conditions