A proton exchange polarizer with a spatial filter positioned to reduce cross coupling of unguided radiation. A photoconductor substrate is fabricated from LiNbO.sub.3 or LiTaO.sub.3. The substrate has a spatial filter located at a primary reflection point on a bottom of the substrate so as to block unguided TM mode light from reaching the output of the substrate. The spatial filter is fabricated by physical or chemical methods such as saw cutting, diamond machining, etching, micro-machining, laser-machining and/or damaging the surface of the substrate. The unguided TM mode light is attenuated by blockage or interruption of the transmissive region.