The bridge-contacted ridge-waveguide (BCRW) laser structure is used to investigate the influence of the lateral refractive-index step Δneffon lasing characteristics at 1.55 μm wavelength. For this purpose the dependence of δneffon the device parameters is numerically calculated by solving the slab-waveguide problem for the guiding region. It is proved that lateral refractive-index steps up to 10−1can be obtained with the BCRW structure and result in strongly index-guided lasers even for small stripe widths. These BCRW lasers show low CW-threshold currents of 33 mA and quantum efficiencies of 29%. A maximum quantum efficiency of 45% is obtained for δneff$ 8 × 10−3at which the applied index step just compensates carrier-induced antiguiding. It is demonstrated that BCRW lasers can be operated in a stable longitudinal single mode at a side-mode suppression exceeding 20 dB.