Impact of intraband relaxation on the performance of a quantum-dot laser

Measurements on 1.3-μm quantum-dot lasers are presented that reveal a number of interesting effects. 1) At high bias, a second lasing line appears, corresponding to the excited state transition. 2) The linewidth enhancement factor increases dramatically above threshold. 3) The modulation performance is degraded when the second lasing line appears. A comprehensive numerical model is developed to explain this behavior. We attribute it to incomplete gain clamping above threshold. This is caused by a combination of the finite intraband relaxation time and the limited density of states.