SELF-REFERENCED FIBER OPTIC SENSOR FOR MICRODISPLACEMENT MEASUREMENT

A fiber optic microdisplacement sensor is realized by placing a diffra ction grating surface in front of multimode fiber endfaces. This grati ng serves as a dispersion component. The incident white light from a m ultimode fiber is dispersed by the grating and a portion of the optica l power of the color band is picked up by the same fiber or another mu ltimode fiber placed side by side. The spectrum of the light received by the second fiber is a function of the air gap between the fiber end face and the grating surface. Because diffraction angles are different for different wavelength components, the received power of different wavelengths varies differently as the air gap changes. A standard two- wavelengths demodulation technique is employed as a self-referenced sc heme to compensate for the signal power shift that occurs as a result of light source power fluctuations, fiber bending, and other disturban ces. A resolution of 2 mu m has been obtained over the range from 0 to 150 mu m. Potential applications of this device are also discussed.