On the effects of an antireflection coating impairment on the sensitivity to optical feedback of AR/HR semiconductor DFB lasers

The sensitivity to optical feedback of 1.55-μm antireflection (AR)/ high-reflection (HR) DFB semiconductor lasers is presented in this paper. The onset of the coherence collapse, which is the most critical feedback regime for optical transmissions, is theoretically investigated with a stress on its dependence with facet phase effects (FPEs). Taking into account FPEs 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 decibels 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 AR coating. Calculations show an enhancement of the coherent collapse threshold distribution up to 10 dB due to the AR coating impairment. These 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 one laser to another. On the other hand, for AR coatings below 10^-4, all feedback performances are directly connected to the laser wavelength, and DFB lasers can be easily selected for high bit rate isolator-free transmission.