Sub-30-Fs Kerr-Lens Mode-Locked Yb.CaF₂ Laser

Ytterbium (Yb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>+) doped calcium fluoride (Yb:CaF<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) is a highly promising laser crystal for the development of ultrashort-pulse, high-power / energy solid-state oscillators and amplifiers at ~ 1 μm [1]. It possesses several advantageous properties, including wide band-gaps that result in relatively low linear and nonlinear refractive indices, high thermal conductivity with moderate dependence on the Yb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>+ doping level, and low phonon energies, which lead to relatively long fluorescence lifetime. These characteristics contribute to excellent energy storage capabilities in Q-switched lasers and ultrafast amplifiers. Additionally, Yb:CaF2 features a negative thermo-optic coefficient, effectively suppressing thermal lensing [2]. The growth of large-volume Yb:CaF2 crystals is well-established using Bridgman-Stockbarger or Czochralski methods, enabled by its congruent melting and relatively low melting point. The clustering of Yb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>+ ions within the crystal results in a “glassy-like” spectroscopic behavior, characterized by extremely broad, flat, and smooth gain profiles, which can support the generation of sub-30 fs pulses in mode-locked (ML) lasers. However, despite its broad gain bandwidth, the full potential of Yb:CaF2 for ultrashort-pulse generation remains underexploited. Previously, using a 12-W, single-transverse-mode Yb fiber laser at 979.5 nm as a pump source, an Yb:CaF2 laser generated 48 fs pulses at 1046 nm via soft-aperture Kerr-lens mode-locking (KLM), reaching an average output power of 2.7 W at a repetition rate of 73 MHz [3]. This work demonstrates further shortening of the pulse duration of the Yb.CaF<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> laser using the soft-aperture KLM.