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Complex multilobe far-field emission patterns observed for buried heterostructure (BH) and deep Zn-diffused double heterostructure (DH) lasers are analyzed according to a slab waveguide model with sinusoidal boundary perturbations. This model is applicable because a Fourier analysis of any irregularity in the laser stripe boundary shows spectral components only at integer fractions of the finite laser length. From the angular position of the narrow far-field emission sublobes, sign and magnitude of the effective refractive index step under the stripe can be inferred. From the intensity of the sublobes, the deviation of the stripe boundary from ideal straightness is estimated.