Speckle characterization in multimode fibers for sensing applications

Abstract

This paper proposes the characterization of speckle patterns of multimode fibers in view of sensing applications and particularly for detection of vibration or seismic activity. Plastic optical fibers are used in this work due to its excellent flexibility and adaptability to build sensor heads. We are interested in the response to vibration, for which we use a short cylindrical piezoelectric transducer (PZT) vibrating in radial direction. The multimode fiber was coiled as tightly as possible around the mandrel of the PZT and periodic stretching effect was caused by the radial oscillations of the actuator. The PZT is modulated with a frequency generator by applying a sinusoidal signal in the range of 0 to 20 Hz, so the speckle patterns can be time averaged. The fiber extreme is attached to a high speed camera with a plastic adaptor, centering the speckle pattern into the CCD. Maintaining the fiber position, a region of interest is selected to capture the video sequence and it is captured to detect the variations in the speckle pattern. Once having the video sequence, it is processed by averaging the pixel differences between two consecutive frames. This processed sequence is also filtered in order to reduce the high frequency noise component. In this work we report the results of the characterization of 3 types of multimode fibers, with core diameters of 50 μm, 240 μm and 980 μm.