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 μs) 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 μs 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.