High gain, multi-pass, femtosecond Yb: KYW crossed-crystals amplifier pumped by high-brightness tapered laser diodes

In this work we present the development and characterization of a high-gain, double-pass and four-pass femtosecond Yb: KYW amplifier pumped by two polarization-coupled tapered laser diodes emitting a total power of 9 W at 979 nm. The amplifier utilized relies on a crossed-crystals configuration, employing two 3-mm-long 10%-at. doped N g -cut Yb: KYW crystals with orthogonal orientation of the respective N m -axis in order to optimize pump absorption and minimize gain narrowing. The amplifier was seeded either with almost Fourier-transform limited ∼300-fs pulses from an in-house-made Yb: KYW oscillator, or with 5-ps-long, chirped pulses (∼15 nm full-width at half-maximum bandwidth around 1030 nm) from a commercial Yb-doped fiber laser. In both cases, we achieved a four-pass small-signal gain G 0 > 100 and an optical-to-optical extraction efficiency of about 30%, even at low seed average power levels. The amplifier showed excellent spatial beam quality preservation up to the full pump power (M 2 < 1.1) and good preservation of the pulse duration with negligible gain narrowing in case of the 300-fs bulk seeder and pulses \:≤\: 200 fs after compression in case of the fiber laser seeder.