Optical Injection-Enhanced Tunable Quantum-Dot Lasers for High-Speed Direct Modulation

Direct modulation of quantum dot (QD) lasers is valued for its high energy efficiency and low cost. However, its modulation bandwidth is limited by finite intraband relaxation and gain compression. Here, we investigate the dynamic characteristics and demonstrate the high-speed capability of a monolithic dual-ring Vernier quantum-dot (QD) laser. The free running laser shows a 3-dB small-signal bandwidth of 5.5 GHz, which is increased beyond 9 GHz under optical injection locking (OIL) while maintaining a high side-mode suppression ratio (SMSR) and stable operation. In an isolator-free directly modulated laser and direct detection (DML-DD) link, we achieve 12 Gb/s non-return-to-zero (NRZ) transmission without OIL and 23 Gb/s NRZ under OIL, both with bit-error-rate (BER) below the hard-decision forward error correction (HD-FEC) threshold and without electronic equalization or bandwidth-optimized epitaxy. Furthermore, the device remains stable under external optical feedback (EOF) up to −10 dB. These results indicate the monolithic dual-ring Vernier QD laser as a robust, reflection tolerant transmitter platform, demonstrating that optical injection effectively overcomes its intrinsic bandwidth bottleneck to enable high-speed, isolator-free operation.