A resonant grating mirror (RGM) that combines a single layer planar waveguide and a subwavelength grating is used to simultaneously control the beam quality, the spectral bandwidth, and the polarization state of an Er:YAG laser. This simple device is compared to classical methods using several intracavity components: an etalon for wavelength selection, a thin film polarizer for polarization selection, and an aperture for spatial filtering. It is demonstrated that the RGM provides the same polarization purity, an enhanced spectral filtering, and a significant improvement of the beam quality. In CW operation, the Er:YAG laser with a RGM emits an output power of 1.4 W at 1617 nm with a M2 of 1.4.