Gain-induced modes in planar structures

The properties of modes in a slab structure with gain in the center region and loss in the surrounding medium are investigated. The propagation constant and field distribution of the lowest-order modes are determined. The cutoff frequencies and propagation constants of the next-higher mode are given. Furthermore, the effect of a refractive index depression or increase in the center region is determined. The depression does not destroy the mode, as may be expected, but causes it to have a cutoff frequency. Comparison of these results to the experimental data shows that gain-induced modes play an important part in the lateral confinement in stripe-geometry GaAs lasers.