A technique has been developed for the non-contact measurement of the lateral motion of a diffusely reflecting surface. This technique, which is based on the use of a commercially available stabilized 2-frequency laser, is capable of sub-micron resolution, and operates with a stand-off distance of up to 50 cm. The principle of the measurement is coherent heterodyne detection of Doppler-shifted light scattered by the diffuse surface. By making two such measurements at spatially separated points on the surface, extension of the surface can be monitored. The resultant extensometer provides a non-contact replacement for conventional strain-gage extensometers, and is particularly applicable to measurements in harsh environments (high temperature, corrosive atmospheres, etc.). Frequency response in excess of 25 KHz has been confirmed, making the sensor particularly useful for closed-loop operation in fatigue testing.