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. Devor attacked Captain Picard from behind with a laser welder, but Picard disabled him using a Vulcan nerve pinch. Later, once the baryon sweep had begun, Picard woke Devor and threatened him with the laser welder, since phasers didn't work due to the sweep. (TNG: "Starship Mine")