Time Domain Travelling Wave Approach to Investigate the Multi-Mode Dynamics and the Self-Mode Locking in 1.3 µm InAs/GaAs Fabry-Perot Quantum Dot Lasers

We report, for the first time, simulations of mode-locked pulse generation at 1.3μm in single-section QD lasers using a TDTW approach. With parametric analysis we find the conditions for obtaining pulses in agreement with experiments.

In the last years several experimental results have reported the generating of very short-pulses in single section Quantum Dash [1] or Quantum Dot [2] FP lasers either directly at the laser output facet or after group delay compensation with propagation in a proper length of dispersive single mode fiber. This technique is attractive because it allows getting rid of the saturable absorber (SA) section usually present in passive mode-locked lasers. QDash and QDot based lasers appear as good candidates for this application because, thanks to the inhomogeneous broadening of the gain spectrum, a wide optical spectrum of lasing modes above threshold can be achieved [1], [2]. The theoretical study and the simulation of the multi-mode dynamics in these inhomogeneously broadened nanostructure lasers is a complicate problem that was partially undertaken in [3] to explain qualitatively the large chaotic oscillations of the longitudinal mode amplitudes measured in some QD FP lasers [3]. An initial attempt to simulate this self-mode locking (SML) behavior has been done in [4], where the mechanism responsible for self-mode-locking was completely attributed to a “Kerr-effect” modelled as an equivalent lumped ultra-fast SA included in the model in the last slice of the device. Because of this ideal and artificial SA, stable and locked pulses were always obtained [4]. This SML operation was also experimentally found in the past for QW lasers [for ex. 5 and 6] and it was studied using coupled cavity mode theory, assuming the coupling due to third order polarization effects [5], [7]. Due to the complexity of the QD material in terms of discrete carrier states in conduction and valence band and of inhomogeneous broadening of the gain spectrum caused by the QD size dispersion, we investigate this problem using a Time Domain Travelling Wave approach developed in our group and already used to simulate QD passive mode-locking and QD-DFB lasers [8]. This approach includes intrinsically all the main non-linear effects (FWM, SPM, XPM, spectral hole burning) as well spatial hole burning consisting in carrier gratings due to the standing wave patterns caused by counter-propagating forward and backward fields. We show in this work that these non-linear effects (without any additional equivalent SA element introduced to model the Kerr-effect as in [4]) we can reproduce the self-mode locking in QD FP laser as measured in [2]. We limit here our simulation analysis to the case of InAs/GaAs QD FP lasers, even if the TDTW simulator presented could be used to extend this analysis also at OD-FP lasers emitting at .