Systematic investigation of the alpha parameter influence on the critical feedback level in QD lasers

The dramatic variation in the linewidth enhancement factor (α _Hfactor) that has been reported for quantum dot lasers makes them an interesting subject for optical feedback studies. A low α _Hfactor combined with a high damping factor is especially interesting because it should increase the tolerance to optical feedback in these devices and may offer potential advantages for direct modulation. In the particular case of QD lasers, the carrier density is not clearly clamped at threshold. The lasing wavelength can switch from the ground state (GS) to the excited state (ES) as the current injection increases meaning that a carrier accumulation occurs in the ES even though lasing in the GS is still occurring. The filling of the ES inevitably enhances the α _Hfactor of the GS above threshold as experimentally and numerically shown. Consequently, this strong variation of the GS α _Hfactor in comparison to QW devices, should theoretically produce a significant variation in the onset of coherence collapse due to feedback. This coherence collapse regime, in which the laser is subject to instabilities, is incompatible with data transmission because of the induced high bit-error rate. One method to investigate the tolerance to optical feedback is to compare experiment with the theoretical work introduced by Petermann. It will be presented that under specific conditions, i.e., in the case of a strong enhancement in the α _Hfactor, the feedback sensitivity of the laser can vary by as much as 10dB within the same device.