Linewidth enhancement factor and chirp in quantum dot lasers

Abstract

We have made a comparative study of the linewidth enhancement factor (LEF) and chirp in quantum dot (QDL’s) and quantum well lasers (QWL’s). The simulations are based on the quasiequilibrium approximation and on semiempirical transition energies and amplitudes of InGaAs quantum pyramid structures. We have accounted for the carriers confined in the active material as well as for the carriers in all the other material layers. It is found that in the quasiequilibrium approximation inhomogeneous broadening leads to asymmetric population of the quantum dot ground state. If the QDL is operated at the gain maximum, the asymmetry leads to nonzero chirp even for a single bound resonance state located at a large distance from other resonances. Our calculations show that, by detuning the laser emission to ∼15 nm shorter wavelengths with a frequency selective cavity and by tailoring the resonance energies and inhomogeneous broadening, the LEF and chirp of a QDL can be made very small. This detuning does not add a substantial penalty to the efficiency of the laser. For QWL’s, a similar reduction of chirp is generally not feasible due to the fundamentally different density of states. Therefore QDL’s have an important advantage over QWL’s as directly modulated light sources in applications where the stability of the emission wavelength is critical.