Noise-Modulated Optomechatronic Distance-Measuring System

A novel measuring system for optical distance sensing of solid targets is described. The system simply applies a noise-frequency-modulated laser diode for illuminating the target and an interferometer/photodetector device for coherent beam detection. This configuration is treated here as an optomechatronic correlator. The delay in travel time of the target beam in the interferometer yields a stochastically modulated electrical beat frequency in the photodetector output. Its mean frequency value is a measure for the target distance. Additionally to the required stochastic modulation of the injection current, the natural phase noise modulation of the laser diode also has to be taken into account. Theory delivers nonlinear measurement characteristics with strongly increased slope at the short-distance range. The experimental verification in a distance range up to 4 m shows a relative distance measurement error from to for an averaging time range from 10 to 1 s. The system is potentially qualified for approach sensing in mechatronic devices like tool machinery and robotics.