Abstract: A modal domain optical fiber sensor for vibration monitoring and as a mechanical motion detector. A laser that focuses coherent light through a lens into one end of a multimode optical fiber. The fiber is secured to a mechanical mechanism for exciting vibrations. The other end of the optical fiber passes its output through a spatial filter and into a photodetector. A variable voltage signal from the photodetector is fed to a monitoring unit, such as an oscilloscope or a microcomputer, for analysis. Of particular significance is the intentional mismatch between the wavelength of the light 18 from the laser and the single mode cut-off wavelength of the glass optical fiber. In a preferred embodiment, the optical fiber has a wavelength of 850 nm and the coherent light has a wavelength of 633 mm.