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.
A novel principle for absolute position measurements of rough surfaces is presented. A measurement object with a rough surface acts as an external reflector for an antireflection coated laser diode. Determining the longitudinal mode spacing yields the distance from the laser diode to the measurement object. Synchronous pumping of the laser diode results in locking of the modes of the built-up Fabry-Perot resonator. Due to resonance enhancement the mode locked external cavity laser sensor allows highly resolved displacement measurements of rough surfaces. The influence of the object as well as the active gain medium on the accuracy of the measurements is investigated by experiments and simulations.