White-cell multipass optical parametric amplifier with 91.9% quantum efficiency at 1490 nm

Pushing the conversion efficiency of optical parametric amplification (OPA) is a central goal in ultrafast science, traditionally addressed by complex schemes such as quasi-parametric, cross-polarized, and cascaded amplification. Recently, dispersion-engineered multipass OPA demonstrated 32% pump-to-signal conversion. Here, we significantly advance this architecture using a white-cell-based geometry that enables eight crystal passes with only seven mirror reflections, drastically reducing passive losses. The result is a record pump-to-signal conversion efficiency of 55.7% at 1490 nm with 91.9% pump depletion. Remarkably, this near-complete energy extraction is achieved alongside a small-signal gain exceeding 80 dB while maintaining high beam quality (M²<1.5). By combining high energy transfer with extreme gain in a single compact cell, this work provides a new, to the best of our knowledge, efficiency benchmark for the next generation of high-power ultrafast sources.