Threshold and Dynamics in Semiconductor Quantum Well Lasers

Abstract

The application of semiconductor lasers to optical communications and interconnects requires low threshold current, high-frequency modulation, and low-noise characteristics. Quantum well (QW) lasers have received considerable attention due to the demonstrated low threshold current, predicted superior modulation and spectral dynamics due to the reduction of active layer thickness and corresponding modification of density of states for the injected carriers in the extremely thin active region. However, in comparison with their bulk counterparts - double heterostructure (DH) lasers, quantum well lasers have not experimentally demonstrated significant improvement in the modulation bandwidth especially in the case of single quantum well (SQW) lasers. In a practical quantum well structure, the separate confinement heterostructure (SCH) is usually used to confine the optical field in the waveguide and the injected carriers in the quantum well region. The fundamental Fermi-Dirac statistics results in that, in addition to the carrier population in the quantum well region, there is also a significant carrier population in the optical confining region. In the previous differential gain evaluations, the carrier population in the energy states in the optical confining region of the separate confinement heterostructure (referred as the state/band filling effects in QW lasers) was omitted. The state filling effects are, in principle, inherent in any QW structure due to the Fermi distribution of the injected carriers. A re-evaluation of differential gain for typical GaAs/AIGaAs QW and DH bulk lasers with consideration of state filling shows that (i) there is no differential gain enhancement in SQW lasers in comparison to the bulk lasers; (ii) there is an additional differential gain enhancement in multiple quantum well (MQW) lasers stemming from the reduction of state filling. These conclusions are consistent with the experimental results of high speed modulation bandwidth in semiconductor lasers. These theoretical and experimental investigations provide useful guides in design of QW lasers of ultra-high performance. Using these design criteria, strained InGaAs MQW buried heterostructure (DH) lasers have been fabricated. These lasers have demonstrated record low lasing threshold currents (0.25 mA) and high speed at low operation current (3dB bandwidth of 5 GHz at 2.1 mA). These lasers are potentially important for optical interconnects and local area network communication systems.