A laser welder was a tool used to join two or more parts. It produced a continuous welding beam and was normally used for welding metal together.
In 2369, a group of mercenaries boarded the USS Enterprise-D during preparations for a baryon sweep. The terrorist Devor attacked Captain Picard from behind with a laser welder, but Picard disabled him using a Vulcan nerve pinch. On another occasion, once the baryon sweep had begun, Picard ran across Devor once again; Picard asked Devore what he was doing on the starship and threatened him with the laser welder. (TNG: "Starship Mine")