Effect of mirror imperfections on phase-locked semiconductor laser arrays

Phase-locked semiconductor laser arrays normally have far-field-radiation patterns broader than that predicted by theory. We propose a possible broadening mechanism caused by steps on the cleaved mirror facets and by misalignment of the laser guide with respect to the laser facet. In the presence of facet steps, supermodes are coupled upon reflection. To satisfy the round-trip threshold condition, a laser cavity mode has to be an admixture of supermodes. This results in a broader far-field pattern (FFP) than that of a single supermode. Asymmetry of the pattern can also be predicted by this model. For an array of 10 lasers with center-to-center spacing of 8μm, the FWHM angle in the FFP of the lowest cavity mode is broadened by 1.5 and 2.5 folds, respectively, for random steps of average heights 17 and 90 Å between adjacent lasers. The broadening factor is 2.3 when a single 204 Å step exists between the third and the fourth laser. If the laser cavity is not aligned perpendicular to the facet, significant change in the FFP occurs only for a civity length not much shorter than the coupling length. A general consequence of the supermode mixing is the diminution of the threshold gain difference among the cavity modes, encouraging their simultaneous excitation.