A diode-laser-to-waveguide butt-coupling model is described. The model takes into consideration the Fabry–Perot reflection and transmission of the étalon formed by the laser front facet and the waveguide entrance facet. The model predicts coupling efficiency and the coupled-power fluctuations that occur versus the separation between the laser and the waveguide. Calculations performed for Fabry–Perot-laser-to-KTP waveguide coupling show that the transverse and the angular alignment tolerances of the waveguide can be increased when the waveguide entrance facet is antireflection coated. The longitudinal alignment tolerance of the waveguide can be increased by use of an index-matching gel between the laser and the waveguide.