Dynamic properties of quantum dot distributed feedback lasers: high speed, linewidth and chirp

The dynamic properties of distributed feedback lasers (DFBs) based on InAs/InGaAs quantum dots (QDs) are studied. The response function of QD DFBs under external modulation is measured, and the gain compression with photon density is identified to be the limiting factor of the modulation bandwidth. The enhancement of the gain compression by the gain saturation with the carrier density in QDs is also analysed for the first time. The linewidth of the QD DFBs is found to be more than one order of magnitude narrower than that of conventional quantum well (QW) DFBs at comparable output powers. The figure of merit for the narrow linewidth is compared between different semiconductor materials, including bulk, QWs and QDs. Linewidth re-broadening and the effects of gain offset are also investigated. Finally, the chirp of QD DFBs is studied by time-resolved-chirp measurements. The wavelength chirping of the QD DFBs under 2.5 Gbps modulation is characterized. The strong dependence of the linewidth enhancement factor on the photon density is explained by the enhancement of gain compression by the gain saturation with the carrier density, which is related to the inhomogeneous broadening and spectral hole burning in QDs