EFFECTS OF STRAIN ON THE HIGH-SPEED MODULATION OF GAAS-BASED AND INP-BASED QUANTUM-WELL LASERS

We have performed a small-signal numerical analysis of pseudomorphic GaAs- and InP-based Fabry-Perot quantum-well lasers using optical gain spectra calculated from 4 X 4 k . p band structure, with strain effects included via the deformation potential theory. A number of intrinsic factors affecting high speed direct modulation are examined. Examination of the effect of lifetime broadening shows that the resonance frequency will increase at a rate of approximately 250 MHz per meV reduction in the lifetime broadening for a GaAs-based strained layer quantum-well laser, as compared to a rate of 80 MHz/meV for the lattice-matched case. The laser modulation speed is limited by either device heating or facet damage, for a particular construction and biasing scheme. We performed analyses by varying the cavity length and the well number, and found that if the limitation is imposed by the optical power then the modulation speed increases as the laser cavity becomes shorter and the number of quantum wells increases. If the limitation is imposed by the injection current density, however, then the modulation speed decreases for the laser with shorter cavity length. Also, the highest modulation speed is given by an optimum well number, which is 4 for the In0.3Ga0.7As active region. A resonance frequency of approximately 16 GHz is predicted from our analysis, for a pseudomorphic GaAs-based quantum-well laser with 30% excess indium, cavity length approximately 50 mum, width approximately 5 mum, and at an average output power approximately 5 mW.