An oscillator. In the illustrative embodiment, the oscillator includes an ortho-mode feed; a reflective amplifier array adapted to be illuminated by the feed with an input wavefront with a first polarization and to return thereto an amplified wavefront with a second polarization orthogonal to the first wavefront; and a partially reflective plate disposed between the feed and the amplifier. The inventive oscillator provides a Fabry-Perot cavity between the feed and the plate. In the illustrative embodiment, the plate is a polarized plate. The plate may have surfaces with dissimilar polarizations and/or curvatures. The amplifier may be phase locked by an ortho-mode converter or other arrangement.