An output window for a vacuum electron device comprises an output waveguide, an intermediate layer of dielectric material joined to the interior of the output waveguide with a vacuum-tight seal, and upper and lower layers of dielectric material spaced apart from the intermediate layer and arranged above and below, respectively, the intermediate layer in a vertical orientation of the output waveguide. The upper and lower layers including openings. Supports extend inwardly into the output waveguide and support the upper and lower layers. Pillars extend through the openings in the lower layer and support the intermediate layer. The openings in the upper and lower layers permit a venting of a region between the upper layer and the intermediate layer and a region between the lower layer and the intermediate layer during the sealing of the intermediate layer while the intermediate layer is supported by the pillars.