4  MHz Kerr-lens mode-locked Ti:sapphire oscillator for high-repetition-rate ultrafast electron diffraction technology

In non-relativistic ultrafast electron diffraction systems, the spatiotemporal characteristics of the electron beam deteriorate significantly due to space-charge effects. As a result, the number of electrons per pulse must be kept very low, which in turn necessitates high-repetition-rate operation to ensure efficient data acquisition. Here, we designed a Herriott multi-pass cavity with an anomalously negative total group delay dispersion to develop a laser oscillator capable of operating at a stable repetition rate in the MHz regime. The resulting oscillator delivered 76 nJ pulses with a duration of 135 fs at a repetition rate of 4.31 MHz and maintained stable mode-locking for over 100 h.