Mode locking via delayed orthogonal-polarization reinjection in semiconductor lasers

We demonstrate harmonic mode-locking in a semiconductor laser using orthogonal-polarization reinjection via an external ring cavity integrated with a rotatable $\lambda$/2-plate. By tuning the plate angle $\theta$, we achieve precise control over pulse multiplicity and repetition rates in orthogonal TE/TM modes. For the TE mode, increasing $\theta$ transitions the system from low amplitude and disordered quasi-periodic states to fundamental (single-pulse) mode locking and harmonic dual-pulse regimes. Polarization resolved measurements and cross-correlation analyses reveal polarization-mediated dynamics, achieving coherent pulse alignment at half the cavity roundtrip time. This work establishes orthogonal polarization feedback as a compact, versatile mechanism for tailoring ultrafast pulse trains, enabling applications in polarization-multiplexed communications, dual-comb spectroscopy, and high-repetition-rate photonic systems.