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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.

In real sensors, the crosstalk or undesirable crossed sensitivities must be minimized. Distributed Brillouin sensing is a very useful technique to measure fluctuations of temperature along an optical fiber. However, the later measurement can be influenced by the humidity on the fiber; therefore its effect must be minimized. Because the aforementioned, the Brillouin frequency changes with the humidity. Thus, for a given temperature on a distributed fiber sensor such variations have been investigated. The experimental results obtained using three different types of single mode fibers with 1000m length, at 25ºC are reported in this paper.

A pre-excitation pulse technique in Brillouin optical time domain analysis (PP-BOTDA) for enhancement of the spatial resolution is shown. The technique here exposed is based on the pre-excitation of the stimulated Brillouin scattering and the subtraction of the Brillouin scattering due to the intensity dc level present in the optical pulse. A main optical pulse with 3ns of duration followed by a pulse of 40ns and half the intensity of the main one are used for obtaining 30cm of spatial resolution. The spatial range is 3600m on a standard single mode optical fiber.

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