Ultrabroadband 12-fs mode-locked Yb:CALGO laser oscillator.

Pulse compression in Yb-doped lasers remains constrained by two critical bottlenecks: the narrow fluorescence spectrum of gain crystals and the inherently weak intracavity nonlinear spectral broadening effects. Here, through spectral-domain optimization of output coupler transmittance curves and using Yb:CALGO crystal as the gain medium, we have engineered enhanced output spectral bandwidth, enabling a 3.5-fold expansion in spectral bandwidth (Δλ = 323 nm at -10 dB) compared to the fluorescence spectrum. As a result, we achieved a Kerr-lens mode-locked Yb:CALGO oscillator with a pulse duration of 12.3 fs, approaching 3 optical cycles. It is the shortest, to the best of our knowledge, pulse duration ever generated directly from any type of Yb laser oscillators and rare-earth laser oscillators.