Abstract: An apparatus for determining the position of the focal point of a laser beam in a laser machining system in which the laser beam is transmitted via an optical fiber to the work-piece to be machined by the laser beam. The z coordinate position of the focal point is determined by a mirror for reflecting some of the laser radiation back towards the end surface of the optical fiber, and a detector is arranged between the laser and the outgoing fiber for measuring the radiation reflected back through the fiber.