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Connecting Theory and Practice in Optoelectronics
There is an increasing interest toward the development of simple and compact comb laser sources. One promising application is the use of a InAs/GaAs Quantum Dot or InAs/InP Quantum Dash single section Fabry-Perot lasers. Many experiments on these devices have demonstrated the possibility of generating a wide optical spectrum of lasing longitudinal modes that are phase-locked. The phase locking is demonstrated by the very narrow RF line at the beat note frequency and by the possibility of getting pulses directly at the laser output or after group delay dispersion compensation with a proper length of dispersive optical fibre. There is however a lack of modelling work providing physical explanations on the capability of the QD lasers of generating phase locked lasing lines.
We have developed a numerical model based on a Time Domain Travelling Wave approach: the spatio-temporal evolution of the optical electric field is described by the slowly varying forward/backward components of the electric field coupled with the slowly varying components of the macroscopic polarization which is the sum of the polarizations of each QD sub-group (ie: QDs with almost the same size) of the inhomogeneous ensemble of dots. In this way we include in the model both the homogeneous gain broadening due to the dephasing time and the inhomogeneous gain broadening due to the non-uniform sizes and shapes of the QDs in the layer. The polarization equations are coupled with electron and hole multi-population rate equations and the whole system is then solved with a finite difference scheme. We have simulated various InAs/GaAs FP lasers of different length and we have found that the gain compression factor (typically high in the case of QD lasers) can explain the origin of the phase locking of the modes and can enable the pulse formation in the FP cavity.
As an example we show in the figure the optical output power versus time at the laser facet (thin solid line) and the pulses (thick solid line) obtained including the group delay dispersion compensation (with GDD=0.4 ps2) of the laser output field . The inset reports the corresponding optical spectrum.
These results are obtained setting the gain compression factor ε=1.5∙10-16 cm3 which is about ten times larger than the typical gain compression factor of the QW case. By a further analysis of these results we have seen that the spectral lines are phased locked and are originated by the contribution of QDs of different sub-groups. Each QD sub-group provide pulse trains directly at the laser output and the GDD compensation (adding a wavelength dependent delay) align these pulse trains generating one single pulse at the fibre output.
More details will be reported at the NUSOD 2016 conference in Sydney (paper MB3).