Seed Spectral Shaping for 90 fs Multi-mJ 1-kHz Laser Pulses in Yb:CaAlYO₄ Amplifier

Ytterbium-doped crystals are promising materials for solid-state lasers that can produce high energy and average power at a low cost and compact design. The main problem with Yb-based systems is that their pulse durations are longer in the ultrafast range than those of Ti-sapphire. While Yb-based femtosecond oscillators working in the sub-30 fs region have been successfully demonstrated [1], when scaling the energy to the mJ level, the pulse duration of the reported systems has inevitably degraded significantly due to a narrowing of the spectrum. One way to mitigate the gain narrowing effect is to shape the spectrum of the seed pulse to be the inverse of the accumulated amplification curve; this approach results in a far broader spectrum than is usually possible with the classical approach. While there are many techniques for spectral modulation, we wanted to use the simplest and most compact, taking into account that we are to integrate it into a regenerative amplifier. To modulate the spectrum, we have decided to use a couple of birefringent quartz plates, a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda/2$</tex> waveplate, and a polarizer. This approach required only the insertion of the birefringent plates and a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda/2$</tex> waveplate, as we already had a polarizer, which, combined with the Faraday rotator, was used as output for the amplifier.