The design of a diffraction grating used as an output coupler for a Fabry-Perot resonator is presented. We then consider the problem of determining the distortion and the cross-polarization of the incident Gaussian beam. Different types of gratings, plane with straight or curvilinear grooves and elliptical, have been designed and built. The output pattern from a resonator using such a grating has been measured experimentally. We found that the elliptical grating generates the least distortion and cross-polarization. This result is attributed to the geometry of the grooves and is in excellent agreement with numerical calculations. (author) 19 figs., 2 tabs., 18 refs.