Dependence of Linewidth Enhancement Factor on Duty Cycle in InGaAs–GaAs Quantum-Dot Lasers

The linewidth enhancement factor (alpha-factor) of InGaAs-GaAs quantum-dot (QD) lasers is studied as a function of the current pulse duty cycle. In this letter, we demonstrate the impact that current pulse duty cycle has on measured -factor in QD lasers due to increased thermal effects. Below threshold, while comparable quantum-well lasers blue-shift under all conditions, the QD lasers demonstrate an unusual sensitivity of chirp to duty cycle of the current, with a blue-shift at low duty cycles, and a red-shift at high duty cycles and dc current, which may lead to negative -factor. The assumption of Gaussian gain distribution and linear gain shift with temperature is used to analyze the -factor at different duty cycle currents. The result agrees well with the positive, zero, and negative alpha-factors reported for QD lasers. The effect of duty cycle is quite significant and can shift the alpha-factor substantially going from pulse to dc. This is due to the low thermal conductivity of QD regions and increased thermal effects in those devices.