Open Access
Pulse coding linearization for Brillouin optical time-domain analysis sensors
(Optical Society of America, 2018) Mariñelarena Ollacarizqueta, Jon; Iribas Pardo, Haritz; Loayssa Lara, Alayn; Institute of Smart Cities - ISC
We introduce a simple method to extend the performance of pulse coding techniques in their application to Brillouin optical time-domain analysis sensors (BOTDA). It is based on applying a simple logarithmic processing on the detected probe wave that compensates the deviation from linearity of the sensor response for long code lengths. The technique ensures that the accumulated effect of a sequence of pulses is equal to the linear addition of the effects of the individual components, which is the essential condition to ensure a correct decoding of the probe gain measurement. We experimentally demonstrate the compensation of the Brillouin frequency shift error induced by the accumulated gain nonlinearity. Furthermore, a proof-of-concept 80 km sensing link within a total 200 km fiber loop demonstrated a better than 2 MHz precision with 2 m spatial resolution.
Open Access
Linearly-configured BOCDA system with large modulation amplitude using dual-probe wave
(Optical Society of America, 2018) Mariñelarena Ollacarizqueta, Jon; Kim, Yong Hyun; Loayssa Lara, Alayn; Song, Kwang Yong; Institute for Advanced Materials and Mathematics - INAMAT2; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
We present and experimentally demonstrated a linearly-configured Brillouin optical correlation domain analysis (LC-BOCDA) system that allows large modulation amplitude (Δf) based on a dual probe wave and the control of correlation-order. The adoption of dual probe and the control of correlation-order can suppress the effects of two main origins of the noise - the Brillouin loss of the pump and the beat noise between the pump and probe waves. We experimentally confirm that Δf and the number of resolving points, N, can be increase to 1.8 times without significant noise.
Open Access
Structural health monitoring of solar trackers using distributed fiber optic sensors
(SPIE, 2019) Mariñelarena Ollacarizqueta, Jon; Mompó Roselló, Juan José; Zurita Gabasa, Jesús; Urricelqui Polvorinos, Javier; Júdez Colorado, Aitor; López-Amo Sáinz, Manuel; Jiménez Romero, Sergio; Achaerandio, Álvaro; Loayssa Lara, Alayn; Institute of Smart Cities - ISC; Gobierno de Navarra / Nafarroako Gobernua, 0011-1365-2017-000122; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
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.
Open Access
Gain dependence of measured spectra in coherent Brillouin optical time-domain analysis sensors
(SPIE, 2016) Mariñelarena Ollacarizqueta, Jon; Urricelqui Polvorinos, Javier; Loayssa Lara, Alayn; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent detection. It is found that the large Brillouin gain that comes from the use of high power pulses induces a narrowing of the RF phase-shift spectrum that is measured in these sensors. This narrowing leads to a Brillouin frequency shift measurement error when the sensor is configured for dynamic measurements. However, the effect has been found to be less significant than that observed in dynamic slope-assisted BOTDA sensors based on amplitude.
Open Access
Gain dependence of the phase-shift spectra measured in coherent Brillouin optical time-domain analysis sensors
(IEEE, 2016) Mariñelarena Ollacarizqueta, Jon; Urricelqui Polvorinos, Javier; Loayssa Lara, Alayn; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent self-heterodyne detection. These sensors are particularly suitable to perform dynamic strain and temperature measurements because the radio-frequency (RF) signal that is obtained when the probe wave is detected has a phase-shift spectrum that is independent to first order of the Brillouin gain. Therefore, a fixed optical frequency separation between pump and probe wave can be deployed and uses the stable RF phase-shift spectrum to obtain the Brillouin frequency shift (BFS) from measured changes in the probe RF phase-shift. However, in this paper, it is found that there is a narrowing of the RF phase-shift spectrum that depends on the Brillouin gain. This effect becomes significant when very high power pulses are used and the resultant large gain induces a narrowing of the RF phase-shift spectrum. This narrowing leads to a BFS measurement error when the sensor is configured for dynamic measurements. We analyze, theoretically and experimentally, the origins and the magnitude of the narrowing of RF phase-shift spectra for high pump pulses in a coherent BOTDA sensor. Furthermore, this spectral shape change is compared to the broadening of the gain spectrum that has been recently discovered in conventional direct-detection BOTDA sensors, which is linearly dependent on the pulse peak power injected to the fiber, finding that the spectral shape change is less significant in coherent BOTDA sensors. Finally, we quantify the BFS measurement error that it can induce and find the trade-offs to keep it below a certain threshold. It is found that, from a practical point of view, this effect is significant for short fibers, where nonlinear effects are negligible and large pump pulses can be used.
Open Access
Enhancement of the dynamic range in slope-assisted coherent brillouin optical time-domain analysis sensors
(IEEE, 2017) Mariñelarena Ollacarizqueta, Jon; Urricelqui Polvorinos, Javier; Loayssa Lara, Alayn; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
We present two techniques that provide an extension of the dynamic range of coherent Brillouin optical time-domain analysis (BOTDA) sensors for dynamic measurements. This type of BOTDA sensors rely on self-heterodyne detection of a phase-modulated probe wave and the dynamic range for fast measurements is limited to the linear region of the RF phase-shift spectrum measured. The first method for range extension that we introduce is based on launching pump pulses containing multiple frequency components. This makes the Brillouin spectra generated by each component to overlap, providing a wider linear region of the detected RF phase-shift spectrum and allowing to measure larger Brillouin frequency shift variations. The second method relies on shortening the length of the pump pulses, which leads to the broadening of the detected RF spectra. The theoretical fundamentals of both range enhancing techniques are presented. Moreover, we experimentally demonstrate that they provide a threefold to fourfold enhancement in the dynamic range. Finally, the factors limiting their performance are determined: for the multi-frequency pump pulse technique, it is the worsening of Kerr non-linear effects due to the simultaneous propagation of multiple spectral components in the fiber, and, for the pulse-shortening method, it is the signal-to-noise ratio penalty linked to the reduction of the magnitude of the Brillouin interaction.