Femtosecond Cr(4+):YAG laser with an L-fold cavity operating at a 1.2-GHz repetition rate

Pulses of 55-fs width and 1.2-GHz repetition rate at 1.52 mum are generated from an L-fold cavity Cr(4+):YAG laser by optimization of the lasing-beam mode inside the gain medium. The pulse width is comparable with that obtained from a standard Z-fold cavity. The pulses are characterized by a second-harmonic-generation frequency-resolved optical gating method. The pulse shape deviates from a sech(2) function because of its broad shoulders. With further miniaturization it should be possible to extend the Cr(4+):YAG mode-locked operation to multigigahertz rates.