Laser-diode distance and displacement sensor with double external cavities

A novel sensor for both distance and displacement, using the feedback effect of a laser diode, has been developed for use in such applications as robot sensing, machining control, expansion monitoring and compensation, and surface profiling in industry. The developed technique uses an extra reflector to form double external cavities among the laser diode, the reflector and the target to be measured for improving in a great extent the stability and precision of the measurement. The sensor has mainly the following two advantages over the previously developed ones. First, it is compact compared to the conventional distance measuring interferometers, because cavity-type interferometer is used. Second, the extra reflector acts as both a reference for absolute distance measurement and another reference for canceling the phase error due to the laser frequency fluctuation. In the measurement of an absolute distance up to a few meters and relative displacement down to several nanometers, the stability was drastically improved mainly against the ambient temperature during the measurement.