Dielectric apertures, formed by oxidation or wet‐etching of high Al content AlGaAs layers in vertical cavity lasers, have recently been used for improved device performance. In this letter an iterative numerical analysis is used to estimate the excess optical losses as the device diameter is reduced. For smaller sizes, where single‐mode devices are possible, the optical scattering losses become significant unless the aperture is tapered to approximate a perfect lens or is thin to approximate a weaker waveguide. ©1996 American Institute of Physics.