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.

Design of a monolithic Michelson interferometer for fringe imaging in a near-field, UV, direct-detection Doppler wind lidar

“…The actual speckle grain size depends on the laser beam waist diameter, in our case, wR > 20 mm at R 50 m (see Fig. 2), or on the multimode fiber core diameter [75]. The real speckle pattern properties have to be characterized for the used fibers.…”

Comparison of Ballistocardiogram Processing Methods Based on Fiber Specklegram Sensors

“…Speckle interferometry in a multimode optical fiber is caused by the different phase velocities of each propagation mode. These propagation conditions create an unique interference pattern at the end of the fiber due to the phase delay between modes thus, consequently, any variation of the propagation conditions will provoke differences into the final speckle pattern [20].…”

Image Compression-Encryption Scheme via Fiber Specklegram-Based Compressive Sensing and Double Random Phase Encoding

“…In experiments, the offset excitation was utilized and we scanned the incident wavelength to generate uncorrelated fiber specklegrams for CS. Typically, more propagation modes will induce more speckles in the specklegram [28], which will make the specklegram more uniform distributed. However, in the experiment, we cannot accurately count the number of the propagation modes.…”