Abstract: Disclosed is an electron beam drilling system wherein an optical fiber link is used to prevent deviant drilling. The position of fixtures within a vacuum chamber is monitored by alignable fiber segments. The junctions of the segments form couples which transmit light from a source to detectors when the fixtures are properly positioned. The alignable fiber segments are mounted on and under the fixtures in a manner which integrally provides protection of the junctions from metal spatter. The light source and detectors are mounted external to the chamber; seals are provided where the fiber segments pass through the chamber wall. The detectors signal a control system to only allow beam output when the fixtures and parts thereon are properly positioned.