An analysis of the linewidth and spectral behavior of DBR lasers

Using recently developed techniques for calculating the linewidths of lasers with arbitrary external feedback, we demonstrate that DBR lasers with long grating lengths can exhibit sharply reduced linewidths. We also discuss several effects of the dependence of the refractive index on carrier density such as the large sensitivity of the linewidth and threshold current to the roundtrip phase change in the active region. Such linewidth and threshold variations are especially pronounced for long grating lengths.