External feedback effects on a quantum dot laterally loss coupled distributed feedback laser

The critical external feedback ratio for coherence collapse in a quantum dot laterally loss coupled distributed feedback (QD LLC DFB) laser is determined to be -14 dB by RIN measurements. Under 2.5 Gbps modulation, the SNR in the QD DFB starts to degrade at a feedback ratio of -30 dB, about 20 dB higher than the value of a typical QW DFB, although the change in RMS jitter is not much different in these two devices. The physical contributions to the high external feedback resistance of the QD LLC DFB would be the low linewidth enhancement factor, high damping rate, HR/HR facet coatings and loss-coupled grating.