Exploring the feedback limits of quantum dot lasers for isolator-free photonic integrated circuits.

Reflections from on-chip components pose significant challenges to stable laser operation in photonic integrated circuits (PICs). Quantum dot (QD) lasers, with low linewidth enhancement factors and high damping rates, are promising for isolator-free integration, yet earlier feedback studies were capped near -10 dB feedback and never reached coherence collapse (CC). As a result, one could only conclude that QD lasers tolerate feedback up to -10 dB, leaving open whether they remain reliable in practical PICs where lower coupling losses allow much stronger feedback. Here, we optimized QD lasers through advanced epitaxial growth and fabrication and developed a setup that delivers feedback up to 0 dB. Under these conditions, we observed CC at -6.7 dB (21.4% feedback), extending the feedback tolerance by tens of decibels beyond quantum-well (QW) lasers. We further demonstrated penalty-free 10 Gbps operation, robust thermal stability with ±0.5 dB drift across 15-45 °C, >100 h continuous testing, and ~±0.3 dB reproducibility across devices. Modeling indicates even stronger tolerance in realistic PIC cavities, and benchmarking shows our device rivals hybrid DFB-resonator platforms while outperforming other QW, QD, and VCSEL lasers. Together, this work provides the most comprehensive assessment of QD laser feedback tolerance to date and establishes practical design rules for isolator-free PICs.