Pedro Corredera

Coherent Noise Reduction in High Visibility Phase-Sensitive Optical Time Domain Reflectometer for Distributed Sensing of Ultrasonic Waves

Phase-sensitive optical time domain reflectometry (φOTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. Up to now, φOTDRs have been used mostly for the measurement of sub-kHz vibrations, normally in the context of intrusion sensing. In this paper, the authors present an experimental and theoretical description of a high-visibility φOTDR and its performance when used for ultrasonic vibration measurements. The use of a semiconductor optical amplifier in the setup allows to suppress coherent noise and also to improve the spectral response of the pump pulses. These two advantages greatly decrease the detected intra-band noise thus allowing frequency measurements in the limits set by the time of flight of the light pulses while maintaining the simplicity of the scheme, as no post-processing, extremely high coherence lasers or coherent detection methods are required. The sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go up to 1.25 km. This is the first time to our knowledge that a fully distributed measurement of ultrasonic waves was achieved. The statistical behavior of the system was also described theoretically and characterized experimentally.

Distributed Brillouin Fiber Sensor Assisted by First-Order Raman Amplification

Distributed optical fiber Brillouin sensors provide innovative solutions for the monitoring of temperature and strain in large structures. The effective range of these sensors is typically of the order of 20-30 km, which limits their use in certain applications in which the distance to monitor is larger. In this work, we have developed a new technique to significantly extend the measurement distance of a distributed Brillouin Optical Time-Domain Analysis (BOTDA) sensor. Distributed Raman Amplification in the sensing fiber provides the means to enhance the operating range of the setup. Three Raman pumping configurations are theoretically and experimentally investigated: co-propagating, counter-propagating and bidirectional propagation with respect to the Brillouin pump pulse. We show that some of the amplification schemes tested can extend the measurement range and improve the measurement quality over long distances.

Brillouin optical time-domain analysis assisted by second-order Raman amplification.

We propose and experimentally demonstrate a new method to extend the range of Brillouin optical time domain analysis (BOTDA) systems. It exploits the virtual transparency created by second-order Raman pumping in optical fibers. The idea is theoretically analyzed and experimentally demonstrated in a 50 km fiber. By working close to transparency, we also show that the measurement length of the BOTDA can be increased up to 100 km with 2 meter resolution. We envisage extensions of this technique to measurement lengths well beyond this value, as long as the issue of relative intensity noise (RIN) of the primary Raman pump can be avoided.

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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Phase-sensitive optical time-domain reflectometry (φOTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. We show in this Letter that modulation instability (MI) can induce a position-dependent signal fading in long-range φOTDR over conventional optical fibers. This fading leads to a complete masking of the interference signal recorded at certain positions and therefore to a sensitivity loss at these positions. We illustrate this effect both theoretically and experimentally. While this effect is detrimental in the context of distributed vibration analysis using φOTDR, we also believe that the technique provides a clear and insightful way to evidence the Fermi-Pasta-Ulam recurrence associated with the MI process.

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

In this study, the authors present an experimental and theoretical description of the use of first order Raman amplification to improve the performance of a Phase-sensitive optical time domain reflectometer (φOTDR) when used for vibration measurements over very long distances. A special emphasis is given to the noise which is carefully characterized and minimized along the setup. A semiconductor optical amplifier and an optical switch are used to greatly decrease the intra-band coherent noise of the setup and balanced detection is used to minimize the effects of RIN transferred from the Raman pumps. The sensor was able to detect vibrations of up to 250 Hz (close to the limits set by the time of flight of light pulses) with a resolution of 10 m in a range of 125 km. To achieve the above performance, no post-processing was required in the φOTDR signal. The evolution of the φOTDR signal along the fiber is also shown to have a good agreement with the theoretical model.

Modulation instability-induced fading in phase-sensitive optical time-domain reflectometry

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.

Phase-sensitive Optical Time Domain Reflectometer Assisted by First-order Raman Amplification for Distributed Vibration Sensing Over >100 km

We present a new kind of broadband continuous-wave source which outperforms any other broadband superluminescent or amplified spontaneous emission source both in terms of output spectral density and bandwidth. Our source covers the wavelength band of interest for fibre applications (from 1450 to 1625 nm) and has an output power of approximately 1.3 W. The source is obtained by pumping a conventional non-zero dispersion-shifted fibre with a continuous-wave Raman fibre laser tuned to the region of small anomalous dispersion of the fibre. The laser beam undergoes an extreme spectral broadening in the fibre. Our experimental results show clearly that the modulation instability (MI)-induced soliton fission is the key element leading to this spectral broadening. Modulation instability is seeded by fast intensity instabilities present in the laser output. We show that this source features good power stability and we believe that it might have very interesting applications in fibre sensing, for instance to avoid the need of amplification in the interrogation of remote Bragg gratings or to improve the resolution and dynamic range of optical coherence tomography setups.

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

A device composed of commercial optical fibre components and based on a flux-addition method has been designed for the measurement of the linearity of near-infrared detectors over six decades with uncertainties of less than ±0.04%. Non-linear behaviour of commercial InGaAs photodiodes is presented and discussed. Detector non-linearity figures are shown with overfilled illumination, while in underfilled illumination all the photodiodes tested show a very good linearity figure.