Slow amplitude modulation in the pulse train of a self-mode-locked Ti:sapphire laser

Periodic pulse-train amplitude modulations have been observed in a Kerr-lens self-mode-locked Ti:sapphire laser when the size of an intracavity slit is reduced below its optimal value for stable mode locking. These transient processes are dominated by 100-500 kHz periodic pulse-energy modulations and are inherent to the Kerr-lens mode-locking mechanism based on the intensity dependent transverse-beam profile of the laser and the long gain relaxation lifetime (3.2 ps) of the Ti:sapphire rod. We observe that when the slit width is decreased from 2 mm to 1.4 mm the modulation period increases from 2 ps to 10 mu s and amplitude modulation deepens to almost 100%. An explanation similar to repetitive self-Q-switching and undamped relaxation oscillation is presented, which is consistent with the experimental results. A large low-frequency timing jitter on the order of submicroseconds, associated with the periodic pulse-train modulations, is also observed. The timing jitter is analyzed by incorporating the pulse energy modulation into the phase variation through the Kerr effect and consistent results are obtained. We have determined the cavity parameters to control these detrimental processes for stable mode-locked output. The observation of the amplitude modulation further confirms the Kerr-lens self-focusing model of self-mode locking in Ti:sapphire lasers.<
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