Structure-dependent damping in quantum-well lasers

The maximum 3 dB modulation bandwidth of a semiconductor laser is determined, if not by RC or power limits, by damping that arises from photon-dependent suppression of the optical gain. In bulk lasers this damping limit is found, both experimentally and analytically, to be relatively constant at 25 - 45 GHz, independent of device design. In contrast, the damping limit is found to vary widely for quantum well lasers. In this paper we will describe experimental results showing the structure dependence of the damping, and we will present evidence for a new model explaining the structure dependence as a result of well-barrier hole burning. This hole burning arises from a buildup of carriers in the barrier layers due to the nonzero carrier capture times of the wells, causing a spatial hole to be burned perpendicular to the active region. This hole can behave like a photon dependent gain suppression, leading to a larger nonlinear gain parameters and a lower effective differential gain. We also suggest ways to optimize quantum well laser structures for maximum modulation bandwidth.