On the linewidth enhancement factor in semiconductor lasers

The linewidth enhancement factor α is known to have an important impact on semiconductor laser frequency stability. We determine under what general circumstances the value of α for a semiconductor laser may depend upon the longitudinal laser structure. A longitudinal modulation of the modal refractive index is shown not to have any influence on α, while our results indicate that a longitudinal modulation of the modal gain such as would be used in a gain-coupled laser will in general change the linewidth enhancement factor.