1,720,972 research outputs found
Hybrid BOFDA/BOCDA system for distributed static and dynamic strain measurements
No full textWe present a distributed optical fiber sensor based on a hybrid Brillouin optical frequency/correlation-domain analysis (BOFDA/BOCDA) configuration for both static and dynamic strain measurements. Distributed static strain (or temperature) measurements are realized using the conventional BOFDA method, i.e., acquiring the baseband transfer function of the fiber through a vector network analyzer (VNA). With little modifications, the same setup can perform dynamic, position-selective measurements synthesizing a correlation peak through a frequency modulation of the laser source while operating the VNA at a single modulation frequency. Experimental tests, carried out at a sampling frequency up to 40 Hz and a spatial resolution of ≈5 cm, demonstrate the validity of the proposed approach
Quasi-distributed refractive index sensing by stimulated Brillouin scattering in tapered optical fibers
No full textIn this paper, we demonstrate that multiple tapers in optical fibers allow for quasi-distributed refractive index sensing via a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) configuration. We first characterize, theoretically and experimentally, the variation of the Brillouin frequency shift (BFS) with the diameter of the tapered fiber. Then, we characterize the dependence of the BFS from the outer refractive index in an optical fiber taper with a waist diameter of 10 micron. Finally, we show that more tapers can be realized along the same fiber, in order to provide multi-point refractive index sensing
Distributed measurement of modal birefringence in a few-mode fiber based on stimulated Brillouin scattering
No full textThis paper presents a novel technique for the distributed measurement of the modal birefringence in a few-mode fiber (FMF). The method exploits two different phenomena observed in distributed Brillouin measurements: the dependence of the Brillouin frequency shift (BFS) on the effective refractive index (ERI) of the interacting optical beams, and the spatial oscillations of the Brillouin gain deriving from multimodal interference. Using both phenomena, a wide range of ERI separations can be measured, from ≈ 10-7 to 10-2 or more. The measurements have been carried out over a two-mode graded-index FMF, using two photonic lanterns to selectively excite the desired spatial modes. We use the BFS measurements to derive the ERI difference between the LP01 and LP11 mode groups, while the spatial oscillations of the Brillouin gain reveal the birefringence between the vector components (TE01, TM01 and HE21) of the LP11 mode group. The experimental measurements are partly supported by full-vector finite-element-method (FEM) simulations. The reported method may also find application in the field of distributed sensing, by taking advantage of the dependence of modal birefringence from physical parameters such as strain and temperature
Brillouin scattering for refractive index sensing in non-adiabatic tapers
Full text linkWe demonstrate the use of non-adiabatic tapers for refractive index sensing in optical fibers based on Brillouin scattering. By exciting higher order optical modes along the taper, the Brillouin gain spectrum becomes multipeaked, where each peak exhibits a different sensitivity to the refractive index of the surrounding medium. By this method, we demonstrate a sensitivity enhancement of the Brillouin frequency shift to refractive index changes by a factor of ≈ 4, compared to an adiabatic taper with the same waist diameter. Furthermore, the use of the spectral difference between two Brillouin gain peaks provides a temperature-independent measurement of the external refractive index
Dynamic and High-Resolution Strain Measurements Using the Brillouin Optical Frequency-Domain Analysis
No full textIn this work, we demonstrate a novel mechanism to localize the stimulated Brillouin interaction over a specific portion of an optical fiber, using a Brillouin Optical Frequency-Domain Analysis (BOFDA) interrogation system and an array of tapers. We show, both numerically and experimentally, that the frequency-domain fiber response, acquired over a narrow range by a vector network analyzer, can be uniquely associated to the multi-taper array whose period matches the swept spectral range. This opens the way to high-resolution (cm-scale) dynamic strain measurements, in addition to fully distributed static strain (or temperature) measurements over the same optical fiber and using the same apparatus
An integrated structural health monitoring system based on Lamb waves
No full textStructural Health Monitoring (SHM) has the potential of providing significant economic benefits through predictive maintenance and identification of damages in structures at an early stage. In particular, active SHM systems based on Lamb wave propagation can be applied to structures of different materials, including composites. In this paper, a portable and totally autonomous SHM system which excites and detects Lamb waves is proposed. Detected signals are processed in-situ and in real time in order to estimate the damage location. The result is then communicated wirelessly to the user, in the form of a spatial map of the damage probability distribution
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
BOFDA sensing over conventional single-mode fibers at cryogenic temperatures
No full textIn this work, we show that high spatial resolution (3-cm) distributed temperature measurements at cryogenic temperatures can be realized using the Brillouin Optical Frequency-Domain Analysis (BOFDA). A large increase of peak gain and narrowing of the Brillouin gain spectrum were observed at temperatures below ≈ 10K under high spatial resolution conditions, thanks to the pre-activation of the acoustic wave involved in the stimulated scattering process. Adopting a slope-assisted configuration, a temperature sensitivity less than 0.1 K is demonstrated over a maximum fiber length of 50 m and at an acquisition rate of 1.75 Hz
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