Femtosecond Cr^4-:YAG laser with an L-fold cavity operating at a 12-GHz repetition rate

Pulses of 55-fs width and 1.2-GHz repetition rate at $1.52 \mathit{\mu }\mathrm{m}$ are generated from an L-fold cavity ${\mathrm{Cr}}^{4+}:\mathrm{YAG}$ 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 ${\mathrm{sech}}^2$ function because of its broad shoulders. With further miniaturization it should be possible to extend the ${\mathrm{Cr}}^{4+}:\mathrm{YAG}$ mode-locked operation to multigigahertz rates.