A coupled-mode analysis of twin-stripe index-guided lasers

An approximated coupled-mode solution to the problem of lateral mode behavior in twin-stripe index-guided injection lasers near threshold is presented. The optical properties are determined by the coupling coefficients between the two waveguides, taking into account the external current-dependent perturbation to the dielectric constant. In the calculations of the external perturbation, a simple model of current spreading under the contacts and carrier diffusion in the active layer is given for a multistripe structure. Emphasis is on the influence of the current ratio on the lateral field shape and modal gain. It is shown that for symmetrical current injection, the fundamental supermode has a slight advantage (higher gain) over the second 180° phase-shifted mode. As the injected current ratio is driven away from the symmetrical point, the second supermode is favored with increasing modal gain difference. The coupled-mode approach is relatively simple and can describe rather accurately this already-known behavior