Described herein are devices and techniques for suppressing parasitic modes in planar waveguide amplifier structures. One or more of the side and end facets of a planar waveguide amplifier are angled with respect to a fast axis defined in a transverse plane perpendicular to a core region. A relationship between glancing in-plane angles of incidence and threshold bevel angles θT can be used to select side bevel angles θS to suppress parasitics by redirecting amplified spontaneous emission (ASE) from the core. It is possible to select the one or more bevel angles θS to be great enough to substantially redirect all but ballistic photons of any guided modes, effectively narrowing a numerical aperture of the planar waveguide amplifier along a slow axis, defined in a transverse plane perpendicular to the fast axis. Beneficially, such improvements can be realized for three part waveguide structures (e.g., cladding-core-cladding), with substantially smooth edge facets.