Statistical-mode sensor for fiber optic vibration sensing uses

Abstract

A method of sensing vibration using the detection of changes in the spatial distribution of energy in the output of a multimode optical fiber has been demonstrated. Two implementations of the sensor have been built and tested. The first implementation involved simple optical processing of the output fiber speckle pattern using spatial filtering. The second implementation involved projecting the pattern on a CCD array and digitally processing observed changes in the intensity distribution. A mathematical model has been developed which has shown good agreement with observed sensor behavior. The sensor technique has been used to detect induced structural vibration in laboratory test specimens. Simple field testing has also demonstrated the ability of the technique to detect personnel and vehicles passing over a buried and electrically undetectable sensing cable. The sensing technique is compatible with off-the-shelf components and fiber cable and even allows for simultaneous telecommunication and sensing using the same optical fiber cable. Near term application of this technology could provide significant benefits for vibration sensing, intrusion detection, and acoustic sensing.

Influence of the refractive index of liquids in the speckle pattern of multimode fibers

“…This transmission is governed by Fresnel reflection coefficients. In equation (2), it can be clearly seen that the bent fiber is influenced by the refractive index of the external environment, so it implies that the system is sensitive to liquid media and their concentrations.…”

“…Since the speckle pattern in the output fiber varies slowly with time, this pattern is sensitive to disturbances such as temperature, pressure or vibration in the fiber, which can be obtained by a suitable speckle pattern processing [1]. By taking advantage of this characteristic, various schemes and assemblies of sensors based on the speckle generated in multimode optical fibers have been proposed [2][3][4][5].Those configurations have advantages over conventional systems due to their low cost, insensitive to electromagnetic interference, flexible, lightweight and easy installation. Spillman et al demonstrated a method using vibration sensing changes in the spatial distribution of energy in the exit end of a multimode fiber [2].…”

“…By taking advantage of this characteristic, various schemes and assemblies of sensors based on the speckle generated in multimode optical fibers have been proposed [2][3][4][5].Those configurations have advantages over conventional systems due to their low cost, insensitive to electromagnetic interference, flexible, lightweight and easy installation. Spillman et al demonstrated a method using vibration sensing changes in the spatial distribution of energy in the exit end of a multimode fiber [2]. The authors implemented two experimental setups, the first speckle pattern is spatially filtered and the second speckle pattern is projected onto CCD array, where intensity changes are observed and related to vibration.…”

Reflection-based fiber specklegram sensor

“…Therefore, fiber specklegrams can be used for sensing purposes. Since the 1980s, numerous designs and arrangements of sensors based on fiber specklegram have been proposed to measure vibration, displacement, strain, and acoustic waves [3][4][5][6]. These first sensors using bulky assemblies and signal processing was very extensive and laborious.…”

Normalization of laser-induced breakdown spectroscopy spectra using a plastic optical fiber light collector and acoustic sensor device

“…In this paper, the use of a bare polymer optical fiber is proposed as an acoustic sensor for normalization purposes, based on the sensing principle of speckle interferometry [16]. This effect is extremely sensitive to mechanical perturbations of the optical fiber and has the potential to surpass the performance of conventional microphones.…”