We demonstrate the application of a novel type of distributed fiber optic sensors (DFOSs) to dynamically monitor the effects of wind on solar tracker structures used in photovoltaic power stations. This DFOS is based on the stimulated Brillouin scattering nonlinear optical effect in optical fiber, which can be used to measure the distribution of strain and temperature along a given structure. However, contrary to existing solutions, the sensor provides dynamic real-time measurements with hundreds or even thousands of full simultaneous measurements for all positions in the fiber each second. Moreover, high-precision and high spatial resolution are obtained. This so-called dynamic Brillouin optical time-domain analysis (D-BOTDA) sensor provides real-time monitoring of the bending and torsion of the structure of solar trackers in response to wind load. This helps the solar tracker manufacturer asses and improve the mechanical designs so as to introduce corrective measures and develop cost-effective components that properly withstand the effects of wind at any given location. We experimentally demonstrate the application of a D-BOTDA sensing system to measure distributed bending and, for the first time to our knowledge, also distributed torsion along the stressed beam of the solar tracker. For this purpose, we have developed a procedure to instrument the torsion beam with two optical sensing fibers that are fixed helically wound along the beam in opposite directions, so that any common-mode thermal or bending effects are removed. We initially performed tests in a laboratory facility in which sections of the torsion beam could be subjected to controlled moments. Static and dynamic loads were applied and the measured deformations were compared to those obtained with fiber Bragg gratings, which just provide point measurements of strain. In both cases, full agreement was demonstrated. Finally, the system was installed in an operational solar park.
KEYWORDS: Distortion, Frequency modulation, Optical tracking, Sensors, Scattering, Temperature metrology, Modulation, Signal to noise ratio, Spatial resolution, Raman spectroscopy
We demonstrate a technique to mitigate the residual second-order non-local effects in Brillouin optical time-domain analysis (BOTDA) sensors in which the Brillouin frequency shift (BFS) profile is not uniform along the fiber. It is based on adding a wavelength modulation to the probe wave that makes it track the average BFS found along its way. Using this method we are able to inject a total probe wave power of 15 dBm in a 120-km sensing fiber link, which, to the best of our knowledge, is the highest probe power ever demonstrated in a long-range BOTDA sensing fiber link. The enhancement in the detected signal-to-noise ratio brought by the use of such power provides 2-MHz BFS measurement precision at the end of the 120-km sensing link with 3-m spatial resolution, all without the need to resort to additional means such as the use of coding or Raman gain.
KEYWORDS: Sensors, Signal attenuation, Tolerancing, Modulation, Distortion, Multiplexing, Distributed computing, Frequency modulation, Signal to noise ratio, Distance measurement, Temperature metrology, Raman spectroscopy
We demonstrate a simple technique to provide conventional Brillouin optical time-domain analysis sensor with compensation for pump pulse attenuation and tolerance to non-local effects. The technique is based on operating the sensor in loss configuration so that energy is transferred from the probe wave to the pump pulse that becomes amplified as it counter-propagates with the probe wave. Furthermore, the optical frequency of the probe wave is modulated along the fiber so that the pump pulse experiences a flat gain spectrum that equally amplifies all the spectral components of the pulse, hence, preventing distortion. The method is experimentally demonstrated in a 100-km fiber link, obtaining a measurement uncertainty of 1 MHz at the worst-contrast position.
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