Free-space multipass optical parametric amplifier
Scaling the efficiency of optical parametric amplifiers (OPAs) without degrading spatio-temporal pulse quality is fundamentally limited by spatio-temporal walk-off, intensity dependent gain, and back-conversion. Here, we numerically and experimentally demonstrate an OPA architecture based on a free-propagating quasi-periodic geometry that overcomes these bottlenecks. Operating within a single nonlinear crystal, the system utilizes pass-by-pass dichroic idler rejection to suppress back-conversion, a birefringent crystal for temporal resynchronization, and free-space diffraction to improve the spatial overlap along propagation. Starting from 1.9 {\mu}J 330 fs pulses at 515 nm and a continuous-wave seed at 783 nm, the generation of 0.8 {\mu}J 160 fs signal pulses at the same wave-length is obtained at a repetition rate of 500 kHz. This simple architecture achieves a 64% quantum efficiency, a 42% pump-to-signal conversion efficiency, and 80 dB of gain while maintaining excellent spatial and temporal quality, providing a scalable platform for ultrafast sources with arbitrary emission wavelengths