Fiber-optic force and displacement sensor based on speckle detection with 0.1 nN and 0.1 Å resolution

A new mode of operation for a high-multimode optical-fiber sensor based on the detection of modal interference is described. The main mechanism on which the sensor is based is coupling between different modes due to curvature induced upon bending the fiber. In contrast to previous work, the variation of the length and the average longitudinal strain of the fiber under such curvatures may be neglected in this work. A sensor of this kind has been implemented and shows a resolution of 10 −11 m in displacement and 10 −10 N in force. Such a high resolution in this type of sensor has not been reported previously. The ‘spring’ constant of the sensor is relatively small (5.6 N/m) compared to that of similar sensors. A theoretical analysis of the transduction mechanism is given and some guidelines to understanding its performance are discussed. The potential for this sensor to perform as the sensing element of an atomic force microscope is discussed.