Low-Noise Tunable Quantum-Dot Lasers for Coherent FMCW Ranging and High-Speed Optical Communications

Overview

Photonic integrated circuits (PICs) are pivotal for the advancement of next-generation coherent communications and emerging sensing technologies, particularly frequency-modulated continuous-wave (FMCW) LiDAR. In these systems, laser linewidth and frequency noise are critical factors defining detection sensitivity, phase stability, and ranging precision. However, achieving narrow linewidth and low noise on-chip lasers typically requires complex grating structures, epitaxial regrowth,or low-loss external cavities, which compromise manufacturability and cost-efficiency.

By exploiting the unique gain physics of quantum-dot (QD) media, this work establishes a practical route toward low-noise and highly linear frequency-chirped tunable lasers. The discrete density of states in QDs gives rise to a near-zero linewidth enhancement factor (α-factor), which suppresses the coupling between carrier-density fluctuations and refractive-index variations and thereby intrinsically reduces phase noise. Building on this advantage, we present a novel tunable laser architecture incorporating dynamic population gratings (DPGs), which avoids process-intensive gratings or regrowth while delivering state-of-the-art coherence on a monolithic QD platform. The fabricated tunable lasers demonstrate a tuning range exceeding 50 nm, a side-mode suppression ratio (SMSR) above 52 dB, and a record intrinsic linewidth of 12.6 kHz, together with strong tolerance to optical feedback.

Beyond static coherence, this dissertation further investigates the dynamic response of QD lasers for both sensing and communication applications. For FMCW LiDAR, we analyze laser chirp dynamics and develop a pre-distortion algorithm that enables MHz-level sweep rates, GHz-class chirp bandwidth, a high modulation efficiency of 29.5 GHz/V, and a sweep linearity on the order of 10⁻⁵. For communication applications, we further employ optical injection locking (OIL) for direct modulation, increasing the laser small-signal modulation bandwidth from 5.5 GHz to 9 GHz and enabling error-free 23 Gb/s NRZ transmission.

Finally, by integrating self-injection locking, we further suppress frequency noise and achieve an ultra-narrow linewidth below 20 Hz. Together, these results establish a scalable and accessible route toward ultra-coherent, fast-tunable, and robust on-chip light sources for future photonic integrated systems.