An analysis of the linewidth and spectral behaviour of DBR lasers

Using recently developed techniques for calculating the linewidths of lasers with arbitrary external feedback, it is shown that distributed Bragg reflector lasers with long grating lengths can exhibit sharply reduced linewidths. 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, are discussed. Such linewidth and threshold variations are especially pronounced for long grating lengths.