An Optical Fiber Heterodyne Interferometry Displacement Sensor

An optical non-contacting displacement sensor is introduced for in-process measurements, three dimensional coordinate measuring machines and robots. In the senor, (1) a ramp-shaped frequency-modulated laser diode is used as a light source, (2) a single-mode fiber coupler and a selfoc lens are used to compose a common-pass interferometer, and (3) the displacement of the work surface is detected from the phase change of heterodyne beat signal. The probe of the sensor is simply the selfoc lens itself whose diameter is only 3mm. The sensor has excellent characteristics; (1) the sensitivity, working range and linearity are almost constant for different surface materials and roughnesses, (2) the working range is 26 mm with a resolution of 20 nm, and (3) it is not crucial whether the work surface is exactly perpendicular to the probe or not.