Twofold Repetition Frequency Using Colliding‐Pulse Mode‐Locked Laser Diode With Multistacked Quantum Dots

We compare conventional mode‐locked laser diodes (MLLD) and colliding‐pulse mode‐locked laser diodes (CP‐MLLD) with a quantum dot (QD) gain medium using a highly stacked InAs QD. Both devices were fabricated using 15‐layer strain‐compensated QDs grown via solid‐source molecular beam epitaxy on InP(311)B substrates. The CP‐MLLD structure incorporated a saturable absorber (SA) section positioned at the center of the cavity, enabling second‐harmonic mode locking. The results showed that the CP‐MLLD laser achieved a higher repetition rate of 137 GHz and a shorter pulse width of 1.11 ps, compared to the 70 GHz and 3.35 ps for MLLD. These improvements are attributed to the effective doubling of the longitudinal mode spacing and broader gain spectrum enabled by the CP‐MLLD with multilayer QD. Additionally, applying a forward bias to the SA region reduced the absorption loss, resulting in broader spectra and narrower pulses. These findings suggest that QD CP‐MLLD is highly promising for ultrafast optical communication and precision photonic systems requiring stable, high‐repetition‐rate pulse sources.