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Numerical modelling and characterization of quantum-dot vertical-cavity surface-emitting lasers and semiconductor optical amplifiers using realistic quantum-dot wave functions and carrier distributions

Final Activity Report Summary - QDVCSELSOA (Numerical modeling and characterization of quantum-dot vertical-cavity surface-emitting lasers and semiconductor optical amplifiers ...)

Quantum dot (QD) semiconductor optical amplifiers (SOAs) have been intensively studied as a promising technological solution to provide high-speed optical amplifiers for next-generation optical access networks such as 100 Gbit Ethernet. Many experimental results have demonstrated the unique properties and outstanding performance of QD SOAs compared to conventional quantum well or bulk SOAs, but the comprehensive theoretical understanding of physical mechanisms and operation principles should be further investigated to facilitate the commercial deployment of QD SOAs. In this project, they developed an elaborate numerical model for QD SOAs, considering unique electronic band structure and inhomogeneous gain broadening of self-assembled QDs.

The carrier dynamics of QDs is calculated using six hundred coupled rate equations, where a novel and efficient method to include the effect of p-type doping on the enhanced hole occupation is proposed. In the first, we calculate high-speed small-signal cross-gain modulation (XGM) responses of QD SOAs, which agree to the experimental results. We theoretically and experimentally demonstrate that the main gain saturation mechanism of QD SOAs is shifted from slow total carrier density depletion with a 3-dB bandwidth of less than 20 GHz to spectral hole burning with a 3-dB bandwidth of more than 100 GHz as the injection current increases.
Secondly, a simple analytical formula based on density matrix theory is derived to calculate saturation output power induced by spectral hole burning. We propose to change the functionality of a QD SOA between linear amplifiers and XGM-based all-optical wavelength convertors by controlling device parameters such as doping density and barrier potential.
Finally, we calculate the time-dependent gain and refractive index changes when an ultra-short pulse with the pulse width of 130 fs is injected into a QD SOA. The effect of free carrier absorption on gain and phase changes is added in this numerical model. The fast and slow components of gain and phase recovery are identified as a function of injected currents. The theoretical predictions based on calculated results are well matched with the measured results from ultra-fast pump-probe experiments.

The completed research results have a significant impact on comprehensive physical understanding and numerical modelling of self-assemble QD structures. Technically, the developed numerical model will help to optimize the device structure of QD-based optical amplifiers and facilitate practical deployment of next-generation 100 Gbit Ethernet applications.