High efficiency broadband parametric amplification assisted by second harmonic generation

In typical scenarios, quadratic nonlinear processes such as second harmonic generation (SHG) and sum frequency generation (SFG) that accompany optical parametric amplification (OPA) are often considered unfavorable parasitic effects. These effects can lead to a reduction in energy conversion efficiency during OPA. In this study, we demonstrate an approach to enhance OPA efficiency by eliminating the signal pulse through SHG in a simple collinear geometry while maintaining a broadband output using a femtosecond pump laser. The complete setup features a three-stage OPA system, with the final stage employing a hybrid OPA-SHG process that effectively suppresses back conversion, achieving up to 60.8 % pump depletion and 26.2 % pump-to-idler energy conversion, resulting in an idler energy of 0.97 mJ. By harnessing the capabilities of a broadband femtosecond pump and Type-I phase matching to enhance phase-matched bandwidth, we generated a 1.85-μm idler pulse with a near-transform-limited (TL) duration of 33.5 fs through compression in a silicon window. Furthermore, we maintained excellent beam quality and output power stability, with only 0.23 % root mean square (RMS) fluctuations over the course of one hour. This mid-IR laser source, operating at the millijoule (mJ) level and characterized by its high beam quality, is exceptionally well-suited for advancing attosecond and strong-field research.