Increasing the Modulation Instability Threshold in a Direct-diode-pumped Kerr-lens Mode-locked Ti:Sa Oscillator

At high pump powers and intracavity laser power of Kerr-lens mode-locked lasers, modulation instability inevitably arises, limiting pulse energy and duration; it has been studied for Diode Pumped Solid-State pumping but not for direct-diode pumping. We implement a direct-diode-pumped Ti:sapphire oscillator using two GaN diodes at 455/465 nm. Varying the pump-lens focal length F suppresses modulation instability. For F = 80 mm, we obtain stable 12 fs pulses (~100 nm bandwidth) with average power up to 300 mW at 8 W of pump; further pump increase leads to noise and breakup into sub-pulses. For F = 60 mm, the spectrum broadens to 110–120 nm with 450 mW at 11 W. For F = 50 mm, the spectrum is 127–140 nm (10 fs pulse duration) and the power is 500 mW at 11 W, with modulation instability absent. This result provides a practical route to improving the efficiency and stability of direct-diode-pumped Kerr-lens mode-locked Ti:sapphire oscillators.