Ultra-stable 50 GHz high peak power colliding-pulse mode-locked lasers with spot size converter

50 GHz colliding-pulse mode-locked lasers with an integrated front-side spot-size converter have been investigated. These devices achieve sub-400 fs pulse durations after external compression, timing jitters as low as 135 fs, and optical peak power up to 5 W. The integrated spot-size converter provides a circular, narrow far field for efficient fiber coupling. The colliding-pulse mode-locked lasers exhibit superior characteristics, achieving an RF linewidth of 3.5 kHz and a Lorentzian optical linewidth of 740 kHz. To evaluate the impact of the cavity design, a direct comparison is performed with a conventional mode-locked laser featuring two separate gain sections. Both devices have identical InGaAsP multi-quantum well active layer structures, and both have electrically isolated gain sections for fair comparison. A threshold current of 8 mA and a time-averaged output power of 110 mW are obtained in both cases, even though the colliding-pulse configuration has double the cavity length.