On-chip laser diodes based on quantum dot (QD) and quantum well (QW) semiconductor materials are strong candidates for applications that rely on stable laser emissions with narrow linewidth. Examples include lidar, remote sensing, and coherent communications. For these applications, the light sources deliver high power efficiency, high temperature operation, and small form factors. The heterogeneous integration of III-V lasers with SiN microresonators, aided by self-injection locking, further offers compactness, high-volume production capabilities, and enhanced stability. To facilitate future engineering designs for narrowing the linewidth in lasers, an international research team from Saudi Arabia, Europe, and the U.S. performed a parametric investigation of the physics underlying the spectral narrowing of self-injection-locked, on-chip lasers. Unlike studies that address linewidth narrowing via external cavities in lasers by focusing on the cavity-coupling dynamics, the researchers investigated the enablement of linewidth narrowing by quantum-confined active mediums. The researchers identified the trade-offs involved with tailoring the linewidth, output power, and injection current for different device configurations. They compared QD and QW active medium archetypes, looking at the differences in injection locking and linewidth narrowing arising from carrier quantum confinement, and the associated density of state functions, gain saturation, and carrier-induced refractive index fluctuations. They found that although both QD and QW devices showed similar linewidth-narrowing capabilities, QD devices were more energy efficient while QW devices emitted at a higher optical power in the self-injection-locked state. Schematic of the integrated III-V/SiN composite cavity lasers. They are composed of III-V QD/QW DFB lasers and SiN micro-ring resonators. Courtesy of E. Alkhazraji et al. The researchers used a composite-cavity structure to analyze heterogeneously integrated III-V/silicon nitride (SiN) lasers with QD QW active regions. They focused on the effects of carrier quantum confinement on the dynamic and spectral characteristics of the locked composite cavity device. They examined the emission spectral refinement, or linewidth narrowing, that could be attained by integrating III-V QD or QW distributed feedback (DFB) lasers with SiN micro-ring resonators. “When properly tuned and locked to one or more of the micro-ring’s whispering-gallery modes, optical feedback in the form of Rayleigh backscattering can enable drastic reductions in the lasing linewidth of a laser diode to the Hz-level,” said Emad Alkhazraji, a researcher at King Abdullah University of Science and Technology (KAUST). Increasing the QD layers or density in each layer increased the output power without significantly increasing the threshold current of the QD laser. The researchers found that minimizing the number of QW layers decreased the threshold current of the QW laser without significantly reducing the output power. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-023-01172-9).