Xabier Angulo-Vinuesa

Raman-Assisted Brillouin Distributed Temperature Sensor Over 100 km Featuring 2 m Resolution and 1.2

Raman assistance in distributed sensors based on Brillouin optical time-domain analysis can significantly extend the measurement distance. In this paper, we have developed a 2 m resolution long-range Brillouin distributed sensor that reaches 100 km using first-order Raman assistance. The estimated uncertainty in temperature discrimination is 1.2°C, even for the position of worst contrast. The parameters used in the experiment are supported by a simple analytical model of the required values, considering the main limitations of the setup.

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The real remoteness of a distributed optical fiber sensor based on Brillouin optical time-domain analysis is considerably extended in this paper using seeded second-order Raman amplification and optical pulse coding. The presented analysis and the experimental results demonstrate that a proper optimization of both methods combined with a well-equalized two-sideband probe wave provide a suitable solution to enhance the signal-to-noise ratio of the measurements when an ultra-long sensing fiber is used. In particular, the implemented system is based on an extended optical fiber length, in which half of the fiber is used for sensing purposes, and the other half is used to carry the optical signals to the most distant sensing point, providing also a long fiber for distributed Raman amplification. Power levels of all signals launched into the fiber are properly optimized in order to avoid nonlinear effects, pump depletion, and especially any power imbalance between the two sidebands of the probe wave. This last issue turns out to be extremely important in ultra-long Brillouin sensing to provide strong robustness of the system against pump depletion. This way, by employing a 240 km-long optical fiber-loop, sensing from the interrogation unit up to a 120 km remote position (i.e., corresponding to the real sensing distance away from the sensor unit) is experimentally demonstrated with a spatial resolution of 5 m. Furthermore, this implementation requires no powered element in the whole 240 km fiber loop, providing considerable advantages in situations where the sensing cable crosses large unmanned areas.

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Sub-meter resolution in long-distance Brillouin Optical Time Domain Analysis (BOTDA) cannot be trivially achieved due to several issues including: resolution-uncertainty trade-offs, self-phase modulation, fiber attenuation, depletion, etc. In this paper we show that combining Raman assistance, differential pulse-width pair (DPP) measurements and a novel numerical de-noising procedure, we could obtain sub-meter resolution Brillouin optical time-domain analysis over a range of 100 km. We successfully demonstrate the detection of a 0.5 meter hot-spot in the position of worst contrast along the fiber.

Extending the Real Remoteness of Long-Range Brillouin Optical Time-Domain Fiber Analyzers

According to recent models, non-local effects in dual-probe-sideband Brillouin Optical Time Domain Analysis (BOTDA) systems should be essentially negligible whenever the probe power is below the Stimulated Brillouin Scattering (SBS) threshold. This paper shows that actually there appear non-local effects in this type of systems before the SBS threshold. To explain these effects it is necessary to take into account a full spectral description of the SBS process. The pump pulse experiences a frequency-dependent spectral deformation that affects the readout process differently in the gain and loss configurations. This paper provides a simple analytical model of this phenomenon, which is validated against compelling experimental data, showing good agreement. The main conclusion of our study is that the measurements in gain configuration are more robust to this non-local effect than the loss configuration. Experimental and theoretical results show that, for a total probe wave power of ~1 mW (500 μW on each sideband), there is an up-shifting of ~1 MHz in the Brillouin Frequency Shift (BFS) retrieved from the Brillouin Loss Spectrum, whereas the BFS extracted from the measured Brillouin Gain Spectrum is up-shifted only ~0.6 MHz. These results are of particular interest for manufacturers of long-range BOTDA systems.

Raman-assisted Brillouin optical time-domain analysis with sub-meter resolution over 100 km

Systematic errors induced by distortions in the pump pulse of conventional Brillouin distributed fiber sensors are thoroughly investigated. Experimental results, supported by a theoretical analysis, demonstrate that the two probe sidebands in standard Brillouin optical time-domain analyzers provide a non-zero net gain on the pump pulse, inducing severe distortions of the pump when scanning the pump-probe frequency offset, especially at high probe power levels. Compared to the impact of non-local effects reported in the state-of-the-art, measurements here indicate that for probe powers in the mW range (below the onset of amplified spontaneous Brillouin scattering), the obtained gain and loss spectra show two strong side-lobes that lead to significant strain/temperature errors. This phenomenon is not related to the well-known spectral hole burning resulting from pump depletion, but it is strictly related to the temporal and spectral distortions that the pump pulse experiences when scanning the Brillouin gain/loss spectrum. As a solution to this problem, a novel scanning scheme for Brillouin sensing is proposed. The method relies on a fixed frequency separation between the two probe sidebands, so that a flat zero net gain is achieved on the pump pulse when scanning the pump-probe frequency offset. The proposed technique is experimentally validated, demonstrating its ability to completely cancel out non-local effects up to a probe power ultimately limited by the onset of amplified spontaneous Brillouin scattering. The method allows for one order of magnitude improvement in the figure-of-merit of optimized long-range Brillouin distributed fiber sensors, enabling measurements along a 100 km-long sensing fiber with 2 m spatial resolution and with no need of added features for performance enhancement.

Non-local effects in dual-probe-sideband Brillouin optical time domain analysis

Raman-assistance (RA) has been identified as a promising technique to extend the measurement range of Brillouin optical time-domain analysis (BOTDA)-based distributed sensors. Unfortunately, Raman amplification introduces a great amount of relative intensity noise (RIN) to the detected low-frequency probe wave. This RIN transfer problem has been widely identified as a major limitation in RA-BOTDA. In vector BOTDA (VBOTDA), the detected signal is transferred to a high-frequency carrier where the Raman RIN transfer turns out to be much less harmful. In addition, a VBOTDA can also provide information about the phase-shift induced by the local stimulated Brillouin scattering gain curve, paving the way for dynamic measurements. In this letter, we demonstrate, for the first time to our knowledge, the RA in a VBOTDA obtaining gain and phase measurements. Our results show a significant reduction of the RIN transfer effect in RA-VBOTDA compared with standard RA-BOTDA, making this type of scheme particularly interesting for long-range and dynamic distributed sensing.

Novel scanning method for distortion-free BOTDA measurements

We evaluate the Brillouin frequency shift (BFS) determination error when utilizing the Brillouin phase spectrum (BPS) instead of the Brillouin gain spectrum (BGS) in BOTDA systems. Systems based on the BPS perform the determination of the BFS through a linear fit around the zero de-phase frequency region. An analytical expression of the error obtained in the BFS determination as a function of the different experimental parameters is provided and experimentally validated. The experimental results show a good agreement with the theoretical predictions as a function of the number of sampling points, signal-to-noise ratio (SNR) and Brillouin spectral linewidth. For an equal SNR and linewidth, the phase response only provides a better BFS estimation than the gain response when the fit is performed over a restricted frequency range around the center of the spectral profile. This may reduce the measurement time of specific BOTDA systems requiring a narrow frequency scanning. When the frequency scan covers most of the Brillouin spectral profile, gain and phase responses give very similar estimations of the BFS and the BPS offers no crucial benefit.

Relative Intensity Noise Transfer Reduction in Raman-Assisted BOTDA Systems

Along the last years, much debate has been done on the efficiency of slow-light phenomena in order to enhance light-matter interactions, especially for sensing purposes. This improvement could be key to develop more compact and sensitive devices. In this work we develop an all-fiber sub-micrometric displacement sensor using slow-light sensitivity enhancement in a lossy ring resonator. In the proposed structure the losses produced by the displacement of a mechanic transducer can be translated into strong variations of group index and therefore strong transmittance variations. We show that this effect is strictly related to slow light, and not related to confinement effects or any other.

Evaluation of the accuracy of BOTDA systems based on the phase spectral response.

Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. In particular, Brillouin optical time-domain analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation, or environmental applications. The extension of the measuring range in these sensors has, therefore, become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the signal-to-noise ratio of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as preamplification before detection, pulse coding, or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results.

Slow-Light and Enhanced Sensitivity in a Displacement Sensor Using a Lossy Fiber-Based Ring Resonator

In this paper we combine the use of optical pulse coding and seeded second-order Raman amplification to extend the sensing distance of Brillouin optical time-domain analysis (BOTDA) sensors. Using 255-bit Simplex coding, the power levels of the Raman pumps and the Brillouin pump and probe signals were adjusted in order to extend the real physical sensing distance of a BOTDA sensor up to 120 km away from the sensor interrogation unit, employing a 240-km long loop of standard single-mode fiber (SSMF) with no repeater. To the best of our knowledge, this is the first time that distributed measurements are carried out over such a long distance with no active device inserted into the entire sensing loop, constituting a considerable breakthrough in the field.