Ruben Ruiz-Lombera

Overcoming Nonlocal Effects and Brillouin Threshold Limitations in Brillouin Optical Time-Domain Sensors

We demonstrate, for the first time to our knowledge, a Brillouin optical time-domain analysis (BOTDA) sensor that is able to operate with a probe power larger than the Brillouin threshold of the deployed sensing fiber and that is free from detrimental nonlocal effects. The technique is based on a dual-probe-sideband setup in which an optical frequency modulation of the probe waves along the fiber is introduced. This makes the optical frequency of the Brillouin interactions induced by each probe wave on the pump vary along the fiber so that two broadband Brillouin gain and loss spectra that perfectly compensate are created. As a consequence, the pulse spectral components remain undistorted, avoiding nonlocal effects. Therefore, very large probe power can be injected, which improves the signal-to-noise ratio (SNR) in detection for long-range BOTDA. Moreover, the probe power can even exceed the Brillouin threshold limit due to its frequency modulation, which reduces the effective amplification of spontaneous Brillouin scattering in the fiber. Experiments demonstrate the technique in a 50-km sensing link in which 8 dBm of probe power is injected.

Proposal of Brillouin optical frequency-domain reflectometry (BOFDR)

We demonstrate a Brillouin optical frequency-domain reflectometry (BOFDR) technique, which can measure the strain and/or temperature along an optical fiber with one-end access, by detecting the spontaneous Brillouin scattering from a sinusoidally modulated pump light. Compared to the Brillouin optical frequency-domain analysis (BOFDA), we show that BOFDR measurements are free from the distorting components related to acoustic wave modulation, thus simplifying data processing.

Structural Damage Identification in an Aluminum Composite Plate by Brillouin Sensing

We report the results of an experimental modal analysis aimed to estimate the location of mechanical changes in an aluminum composite panel. A distributed Brillouin optical fiber sensor was used to retrieve the mode shapes of the first two bending modes of the plate. We show that the position of the defect is retrieved from the strain mode shapes, with a resolution determined by the disposition of the fiber across the structure, as well as by the spatial resolution of the interrogation unit.

Feasibility study of strain and temperature discrimination in a BOTDA system via artificial neural networks

Automatic discrimination between strain and temperature in a Brillouin optical time domain analyzer via artificial neural networks is proposed and discussed in this paper. Using a standard monomode optical fiber as the sensing element, the ability of the proposed solution to detect the known changes that the Brillouin gain spectrum exhibits depending on the applied temperature and/or strain will be studied. Experimental results, where different simultaneous strain and temperature situations have been considered, will show the feasibility of this technique.

Virtual FBGs Using Saturable Absorbers for Sensing with Fiber Lasers

The spectral narrowing of Fiber Bragg Gratings (FBGs) introduced by unpumped Er-doped fiber (EDF) was analyzed for fiber lasers (FL). Owing to spatial hole burning (SHB), the spectral response of a virtual FBG can be employed for narrowing the band pass filter employed to determine the lasing wavelength of laser cavities. A common FL was mounted to analyze the spectral stability of the method, and a FL sensor for strain and temperature measurements was experimentally characterized to determine the stability of the narrowing effect achieved by the unpumped EDF, which acts as a virtual FBG. The results exhibited remarkably good narrowing effects of the spectral response of uniform FBGs.

Experimental demonstration of a Brillouin optical frequency-domain reflectometry (BOFDR) sensor

To measure the strain and/or temperature along an optical fiber with one-end access, a Brillouin optical frequency-domain reflectometry (BOFDR) technique is presented in this paper. It is based on detecting the spontaneous Brillouin scattering from a sinusoidally modulated pump light. Compared to the Brillouin optical frequency-domain analysis (BOFDA), this new BOFDR sensor approach presents the advantage that the measurements are free from the distorting components related to acoustic wave modulation, thus simplifying the associated data processing.

Influence of saturable absorbers on fiber ring laser sensors

In this work, a fiber ring laser sensor has been employed to analyze the influence of passive Er-doped fiber acting as saturable absorber. Lasing modes of fiber ring lasers can be reduced by properly locating a saturable absorber that narrows the spectral width of the FBG that selects the lasing wavelength. Employing commercial Er-doped fiber, different configurations have been evaluated, reaching the SLM regime on a ring cavity of several meters. Thus, the achieved strain response of the whole sensor behaves at its mirror FBG, exhibiting linear response to strain.

Brillouin optical time-domain analyzer with a fiber ring laser working on the SLM regime

In this paper we present the employment of an Erbium Fiber Ring Laser structure working on the Single Longitudinal Mode regime within a Brillouin optical time domain analyzer. An analysis of some key laser parameters will be carried out, proving that a very stable operation is achieved. The associated performance of the BOTDA system, as well as the possible benefits derived from the use of the proposed laser design will also be discussed.

Ultrahigh Temperature Raman-Based Distributed Optical Fiber Sensor With Gold-Coated Fiber

An ultrahigh temperature distributed sensor based on a Raman optical-time-domain-reflectometry (ROTDR) and two types of fibers: a standard multimode fiber and multimode gold-coated fiber are experimentally validated in this paper. A calibration technique has been implemented to correct the dynamic variation of the optical loss in the gold-coated fiber. Distributed temperature measurements up to 600̊ C have been carried out.

Brillouin frequency shift estimation in BOTDA via subpixel processing

In this paper we propose the employment of sub-pixel algorithms for the estimation of the central frequency of the Brillouin Gain Spectrum in a Brillouin Optical Time Domain Analyzer. The experimental results will show that the proposed solution shows a good performance when the chosen frequency step for the required frequency sweep is high. If the improved computational efficiency in comparison to the traditional Lorentzian fitting is also considered, it can be concluded that this approach may be of great interest for dynamic measurement scenarios.