As design algorithms for diffractive optical elements improve, the limiting factor becomes the fabrication process. It is hoped a better understanding of fabrication errors will allow elements with greater tolerance to be designed. This is important for high-power laser fiber coupling, where hot spots lead to failure. We model seven different fan-out gratings applying misetch, misalignment, and feature rounding. Our main findings are that misetch can lead to improved results, misalignment is strongly asymmetric, and both the p and p/2 masks can dominate misalignment. Rounding has a r2 dependence and potentially can be incorporated into the design stage. Finally we present some experimental data for misalignment. © 2007 Optical Society of America.