Influence of the axially varying quasi-Fermi-level separation of the active region on spatial hole burning in distributed-feedback semiconductor lasers

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Abstract
The longitudinal-mode characteristics of distributed-feedback semiconductor lasers subjected to longitudinal spatial hole burning have been investigated using an improved numerical modeling scheme. The main new feature of the model is that it allows for the natural axial variations of the separation between the quasi-Fermi levels (Fermi voltage) in the laser’s active region. This gives rise to a current density injected into the active region that varies along the laser axis, even for uniformly biased lasers. It is found that compared to the results obtained by assuming an uniform current density, the detrimental influence of the longitudinal spatial hole burning on important static characteristics of quarter-wave-shifted distributed-feedback lasers, such as the gain margin and the lasing wavelength stability, is weakened. Therefore, the usual assumption of an uniform injected current density gives rise to an overestimated influence of the spatial hole burning, this overestimation being more important for long-cavity lasers having a grating structure with a large coupling coefficient.