We present a model to compute the linewidth in vertical-cavity surface-emitting
lasers, accounting for the 3D-structure of real devices. To this aim
we include the noise source in the field equations and treat both the noise
and the structural characteristics by means of coupled mode theory.
In this way we obtain an expression for the linewidth that is given as the
relation, modified by two correction factors that account for spatial effects
and modal dispersion of the resonator.
In the numerical results we give some guidelines towards devices aimed to
narrow-line emission and, as a significant example, we study the transition
from gain to index-guided devices in oxide-confined VCSEL's.