An evaluation of the performance of a lens-based plastic optical fiber strain sensor

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One of the key parameters monitored in structural health monitoring applications is strain. In this work an extrinsic plastic optical fiber (POF) sensor developed previously has been optimised to increase its strain sensitivity. The extrinsic POF sensor basic design used here and previously comprises of two fiber units. As the distance of separation between the cleaved surfaces of the two fibers increases, the intensity of light detected by the receiving end of the fiber changes due to the loss of light associated with the increased separation. The two factors affecting the sensitivity of the POF sensor are the diameter of the fiber and the angle at which the light exits the emitting fiber. Here, in the new sensing scheme a ball lens is introduced at the emitting end of the optical fiber. This lens converges the light from the fiber into a spot. The cone of light emitted from this source depends on the numerical aperture and the size of the spot. For this sensor design, approximately six fold increase in strain sensitivity is achieved compared to the previous strain sensor design without the ball lens. Experiments were carried out to test the new sensor design for repeatability and cyclic response. The free vibration response was also evaluated. Based on the results of the experiments it is concluded that the sensor is able to detect strains as small as 30 με for a 15 mm gauge length.