Facet phase effects on the coherence collapse threshold of 1.55 μm AR/ HR distributed feedback semiconductor lasers

The sensitivity to optical feedback of 1.55 μm AR/HR distributed feedback semiconductor lasers (DFB) is presented in this paper. The onset of the coherence collapse which is the most critical feedback regime for optical transmissions is theoretically investigated and with a stress on its dependence with facet phase effects (FPE). Taking into account FPE on both facets, the sensitivity to optical feedback is evaluated with respect to both the coupling strength coefficient and the feedback level. The first part of the paper shows that due to the HR-facet, a distribution up to several dB on the coherence collapse thresholds is predicted over the whole DFB laser population. The second part concentrates on the coherence collapse dependence with respect to the antireflection (AR) coating. Calculations show an enhancement of the coherent collapse threshold distribution up to 5 dB due to AR coating impairment. Those simulations are of first importance for optical transmissions since they show that for AR coatings beyond 10^-4, the sensitivity to optical feedback of AR/HR DFB lasers is extremely difficult to evaluate from a laser to another. On the other hand, for AR coatings below 10^-4, all feedback performances are directly connected to the laser wavelength and lasers can be selected for high bit rate isolator-free transmission.