An apparatus and method for estimating a parameter of interest using values of a beam property from two or more electromagnetic beams that both pass through at least part of an optical displacement device. The apparatus may include a Fabry-Perot interferometer, a collimated light source, and a detection array. At least one mirror of the interferometer may be operably coupled to an element receiving an external stimulus, such as pressure, force, and/or acceleration. The method includes using the apparatus.