Mitigation of on-chip reflections in hybrid-integrated quantum dot coherent comb lasers via 3D-printed microlens structures

We present a novel method for mitigating detrimental chip-scale reflections in hybrid-integrated quantum dot (QD) coherent comb lasers using 3D-printed micro-lens structures. On-chip reflections can severely degrade laser coherence and comb stability, creating significant challenges in photonic integrated circuits (PICs). Traditional anti-reflection coatings often fall short when applied to the complex geometries encountered in densely integrated photonic systems. Our solution utilizes direct-write two-photon polymerization lithography to fabricate micro-lenses directly on critical laser facets and sensitive fiber interfaces. These customized micro-optics suppress parasitic back-reflections by preserving mode matching and redirecting reflected light away from the laser cavity and fiber interface. Experimental results confirm stable comb mode-locking, narrow optical linewidths, and low relative intensity noise, while maintaining compact integration. We further successfully demonstrate the hybrid-integrated InAs/InP QD comb laser module integrated with 3D-printed micro-lenses within a converged optical/mmWoF fronthaul system, achieving satisfactory and reliable system end-to-end performance. The proposed solution is compact, scalable, and compatible with standard photonic manufacturing process. This work provides a practical pathway toward more robust hybrid photonic platforms and enables stable deployment of QD-based comb lasers across diverse applications in quantum information processing, metrology, and optical communications.