In this paper we review our recent progress on high performance mode locked InAs quantum dot lasers that are directly grown on CMOS compatible silicon substrates by solid-source molecular beam epitaxy. Different mode locking configurations are designed and fabricated. The lasers operate within the O-band wavelength range, showing pulsewidth down to 490 fs, RF linewidth down to 400 Hz, and pulse-to-pulse timing jitter down to 6 fs. When the laser is used as a comb source for wavelength division multiplexing transmission systems, 4.1 terabit per second transmission capacity was achieved. Self-mode locking is also investigated both experimentally and theoretically. The demonstrated performance makes those lasers promising light source candidates for future large-scale silicon electronic and photonic integrated circuits (EPICs) with multiple functionalities.
Direct epitaxial growth of III-V lasers on silicon provides the most economically favorable means of photonic integration but has traditionally been hindered by poor material quality. Relative to commercialized heterogeneous integration schemes, epitaxial growth reduces complexity and increases scalability by moving to 300 mm wafer diameters. The challenges associated with the crystalline mismatch between III-Vs and Si can be overcome through optimized buffer layers including thermal cyclic annealing and metamorphic layers, which we have utilized to achieve dislocation densities < 7×106 cm-2. By combining low defect densities with defect-tolerant quantum dot active regions, native substrate performance levels can be achieved. Narrow ridge devices with threshold current densities as low as ~130 A/cm2 have been demonstrated with virtually degradation free operation at 35°C over 11,000 h of continuous aging at twice the initial threshold current density (extrapolated time-to-failure >10,000,000 h). At 60°C, lasers with extrapolated time-to-failure >50,000 h have been demonstrated for >4,000 h of continuous aging. Lasers have also been investigated for their performance under optical feedback and showed no evidence of coherence collapse at back-reflection levels of 100% (minus 10% tap for measurement) due to the ultralow linewidth enhancement factor (αH < 0.2) and high damping of the optimized quantum dot active region.
Silicon photonics promises scalable manufacturing of integrated photonic devices through utilization of established CMOS processing techniques and facilities. Unfortunately, the silicon photonics platform lacks a viable light source, which has historically been overcome through heterogeneous integration techniques. To further improve economic viability, the platform must transition to direct epitaxy on Si to bypass the scaling limits imposed by the small sizes and high cost of III-V substrates in heterogeneous integration. InAs quantum dots have demonstrated themselves as the most promising candidate for achieving high performance light emitters epitaxially grown on Si. Using molecular beam epitaxy, we have grown quantum dot lasers composed of InAs dot-in-a-well active layers on industry-standard, on-axis (001) Si substrates. In this report, we utilized p-doping of the quantum dot active region to increase gain for improved dynamic performance and reliability. These devices have been subjected to accelerated aging conditions at 60°C and a bias multiple of twice threshold current density. After 2,750 hours of continuous aging, an extrapolated lifetime of more than 100,000 hours has been calculated.
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