We present a novel optical sensor concept that merges integrated optics and micro-mechanics in a monolithic substrate. This concept pushes microsystems integration and defines a new class of monolithic optical microsystems where only optical signals are processed. As an illustration, we present a high-precision, monolithic, glass-based, micro-displacement sensor. Our displacement sensor is made out of a single piece of glass through a two-step process based on femtosecond laser illumination followed by chemical etching.