Sub-100 fs Kerr-lens Mode-locked femtosecond Yb:CaYAlO4 Laser at GHz repetition rate

Femtosecond lasers with GHz repetition rate play an important role in scientific and industrial applications, such as spectroscopy, optical frequency combs and GHz-Burst pulse trains for micro-machining in the ablation-cooled regime. Kerr-lens Mode-locked (KLM) technique and passively modelocking based on Semiconductor Saturable Absorber Mirror (SESAM) are the primary methods to generate GHz femtosecond all-solid-state lasers (ASSLs). Kerr-lens mode-locked Ti:Sapphire lasers have made a significant progress benefitting from the high-power green pump lasers, repetition rate up to 10 GHz has been obtained with an average power of 1.2 W. In the early 21st century, ytterbium ion (Yb3+) doped laser crystals and ceramics with emission wavelengths near 1 μm gained attention due to their high conversion efficiency and broad gain-bandwidth. Combining with the customized SESAM and high-power multimode fiber-coupled laser diodes (LD), GHz Yb-doped ASSLs with watt-level average power may be easily attained and have made rapid progress. However, GHz KLM lasers have strict requirements for the cavity design and pump sources. For satisfying mode matching and enhancing the soft aperture effect within the gain medium, a high-brightness pump source with excellent beam quality (M2~1) is desired, such as the single-mode fiber coupled LD, however, the maximum pump power of which is only~1 W. As a result, the average power of GHz KLM femtosecond lasers is typically restricted to few tens of milliwatts, which limits the further applications. In this work, we reported the first GHz high-power KLM Yb:CaYAlO4 laser by using a high-power single-mode fiber laser instead of the low-power single-mode fiber coupled LDs as the pump source. On the basis of ABCD matrices, a simple four-mirror bow-tie ring cavity was built such that the laser mode can match well with the focused pump spot in the crystal. At the pump power of 8 W, stable unidirectional KLM was achieved, the laser had an average power of 2.1 W with a pulse duration of 88 fs and a repetition rate of 1.8 GHz, corresponding to a peak power of 11.57 kW. The high peak power and extremely short pulse duration are crucial for coherent octave-spanning supercontinuum generation. The powerful GHz KLM laser with sub-100 fs pulse duration provides an attractive source for optical frequency combs and micro-machining applications.