An external cavity laser is assembled by using an antireflection coated laser diode together with the surface of a measurement object. The automatic evaluation of the longitudinal modespacing yields the distance between the laser diode and the measurement object. The measurement resolution is increased by utilizing the resonance effect due to synchronous pumping of the laser diode current. Thus, a distance sensor with interesting properties for industrial applications is set up. Nevertheless, a systematic measurement deviation arises as a result of the nonlinear properties of the laser diode. A fundamental understanding of the processes inside the laser diode is necessary for achieving a measurement uncertainty in the micrometer range. Seen applications are in-situ measurements at grinding processes or the focus control at laser material processing.