A practicable fiber optic displacement sensor with subnanometre resolution is described . It consists of a He-Ne laser source , a directional fiber coupler as a beam splitter , a probe made up of optical fibers and a photodetector setup . Its principle is based on the highly sensitive detection of power change at a " zero " working point in light reflected from an object surface to be measured when the surface is subject to a small displacement . In operation , the probe of sensor carries the incident light to the object surface and receives the reflected light . The probe divides the reflected light into two signals . The two light signals are then transmitted to two photodetectors through the optical fibers . By employing a differential amplifier , we can acquire a highly sensitive displacement signal at the " zero " working point . The result of measurement is not sensitive to the power fluctuation of the light source . The measurement is noncontact . The working range is about 6Oum . The probe is small with diametre less than 5mm . It is also easy for the sensor to be used to measure pressure , strain , vibration , etc