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A novel optical sensor for absolute position measurements of rough surfaces is presented. The technical surface acts as reflector of an external cavity for an antireflection-coated laser diode. Evaluation of the resulting mode spacing determines the laser cavity length, i.e. the distance to the rough surface. Modelocking by synchronous pumping of the laser diode allows highly-resolved measurements with position resolution of 5 μm and temporal resolution of 200 μs.