Design of low-chirp directly modulated corrugation-pitch-modulated distributed-reflector lasers with wire-like active regions

Wire-like active-region corrugation-pitch-modulated (CPM) distributed-reflector (DR) lasers with complex-coupled (CC) gratings on silicon are numerically investigated for low-frequency-chirp direct modulation. To suppress frequency chirp arising from stopband width variations in CC distributed-feedback (DFB) structures during injection-current variation, a central CPM segment is incorporated into the DFB section, positioning the lasing mode near the Bragg wavelength. Comparative analysis with index-coupled (IC) lasers reveals a trade-off between adiabatic chirp reduction and bandwidth enhancement. An anti-phase CC-CPM-DR design reduces adiabatic chirp by 20% relative to the IC device. Under 50-Gbaud PAM-4 modulation, total dynamic chirp decreases to 70% of the IC case, yielding a markedly clearer eye diagram.