A technique is described for measuring the displacement of a thin glass diaphragm relative to a fixed glass reference surface. The method employs a single-mode optical fiber to deliver coherent light to the center of the diaphragm and reference surface. The interference fringe pattern caused by partial reflection of light from each surface is focused onto a multimode receive fiber, which conducts the light to a detector. When the diaphragm deflects, the fringe pattern traverses the input end of the multimode fiber causing the detector output signal to follow a cosine-squared time dependence matching the cosine-squared spatial dependence of the fringes. Appropriate up/down counting electronics may be used to count the interference fringe peaks and convert the count to a measure of diaphragm displacement. Results of a preliminary investigation of this diaphragm deflection sensor are presented. Application of the method to pressure measurement is discussed. The technique is inherently insensitive to thermal drift and vibration, and its "all glass" construction makes it applicable to pressure measurements in high-temperature environments.