Sub-30-fs Yb:CALGO laser oscillator based on cross-polarized multi-mode diode pumping

Ytterbium (Yb)-doped materials are an excellent choice for efficient and powerful ultrafast lasers. They exhibit favorable emission properties, which include a low quantum defect and compatibility with cost-effective high-power pump diodes. While being strongly beneficial for efficiency, the low quantum defect is a challenge for operation in the few-cycle regime. In the standard collinear pumping scheme, the optical pump light passes a dichroic mirror, whose spectral transmission and dispersion properties can substantially perturbate the oscillating broad-band pulse. Recently, we introduced a broadband cross-polarization pumping scheme that avoids this issue and enables significant improvements in optical-to-optical efficiency and output average power of modelocked few-cycle Yb-bulk laser oscillators. However, our initial prototype relied on a high-power, diffraction-limited fiber laser for pumping. Here, we demonstrate that the broadband cross-polarization pumping scheme is also well-suited for cost-effective multimode diode pumping of Kerr-lens modelocked Yb-bulk lasers. In doing so, we developed a hard-aperture Kerr-lens modelocked Yb:CALGO bulk laser oscillator delivering ultrashort pulses in the sub-30 fs regime. We present two configurations, which both operate at 85 MHz repetition rate. The first delivers 22-fs pulses at 0.3 W of average output power with a 4% optical-to-optical efficiency, while the second produces 28-fs pulses at 0.5 W of average power with a 12% optical-to-optical efficiency. This study validates the high potential of cross-polarization pumping for cost-effective, high-efficiency Yb-lasers operating in the few-cycle regime, achieving shorter pulse durations and higher power levels than previously demonstrated for multimode diode pumping.