The Effect Of Facets On The Stability Of DFB Lasers

Optical communications systems increasingly need to make use of single longitudinal mode (SLM) lasers so that fibre dispersion does not degrade system performance. At gigabaud rates over long distances with conventional Fabry Perot lasers dispersion will limit performance even in the low dispersion 1.3 μm windows, or at 1.5 μm with dispersion-shifted fibre. Various single frequency laser structures have been used, but the distributed feedback (DFB) laser and variants of it using internal gratings for feedback have become the most popular SLM devices. It is desirable that an SLM laser should operate stably to high output powers; we show here for the first time that high power DFB operation is limited by asymmetric longitudinal spatial hole burning effects which are critically dependent on facet reflectivity and phase.