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Accurate Brillouin frequency shift measurements using a hybrid Brillouin optical frequency/correlation domain analysis
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Distributed Temperature Sensing through Network Analysis Frequency-Domain Reflectometry
No full textIn this paper, we propose and demonstrate a network analysis optical frequency domain reflectometer (NA-OFDR) for distributed temperature measurements at high spatial (down to approximate to 3 cm) and temperature resolution. The system makes use of a frequency-stepped, continuous-wave (cw) laser whose output light is modulated using a vector network analyzer. The latter is also used to demodulate the amplitude of the beat signal formed by coherently mixing the Rayleigh backscattered light with a local oscillator. The system is capable of attaining high measurand resolution (approximate to 50 mK at 3-cm spatial resolution) thanks to the high sensitivity of coherent Rayleigh scattering to temperature. Furthermore, unlike the conventional optical-frequency domain reflectometry (OFDR), the proposed system does not rely on the use of a tunable laser and therefore is less prone to limitations related to the laser coherence or sweep nonlinearity. Two configurations are analyzed, both numerically and experimentally, based on either a double-sideband or single-sideband modulated probe light. The results confirm the validity of the proposed approach
Distributed Vibration Sensing Based on Optical Vector Network Analysis
No full textWe introduce a novel method for distributed vibration sensing based on extracting the time-domain Rayleigh impulse response of an optical fiber from optical vector network analysis measurements. The optical-frequency-domain transfer function of the fiber is first measured, and then inverse Fourier transformed to provide the bandpass optical time-domain impulse response. Another relevant feature of the technique is that it enables excitation demodulation using the optical frequency dependence of the Rayleigh backscatter signal from the optical fiber, the so-called Rayleigh signature. This is the simplest method to obtain fully linear quantitative measurements of local changes in the strain or temperature experienced by the fiber and it is inherently free from signal fading impairments. Furthermore, the implementation of the technique uses a simple setup based on double-sideband modulation of a laser, self-homodyne detection with an optical hybrid, and narrow-bandwidth electrical signal acquisition and processing. We present proof-of-concept experiments to demonstrate the operation of the method with the measurement of dynamic strain and temperature perturbations in a 115-m optical sensing fiber with 16-cm spatial resolution and a sensitivity of 59 n ∈ / √Hz. This sensing technique has the potential to provide high-sensitivity distributed measurements of tens-of-hertz excitations in hundreds-of-meters fibers, with centimeter spatial resolution. Therefore, it can become a valuable tool for structural health monitoring in application fields such as aerospace, marine, or civil engineering
High Spatial Resolution Distributed Brillouin Measurements at Cryogenic Temperatures
No full textDistributed optical fiber sensors are promising tools for quench detection and protection in superconductor-based magnet systems, as they provide continuous monitoring of the temperature at any location within a magnet with high spatial resolution and immunity to electromagnetic interference. Conventional distributed sensors based on Brillouin scattering, however, fails in reaching adequate performance for quench detection due to limited spatial resolution and/or acquisition rate. In this work, we demonstrate the use of a high-spatial resolution Brillouin Optical Frequency-Domain analysis (BOFDA) system to perform distributed temperature measurements at cryogenic temperatures. A custom-made BOFDA prototype was employed to characterize the Brillouin gain spectrum (BGS) in a standard single-mode fiber, at a temperature ranging from ≈ 260 K down to ≈ 2.6 K. A novel algorithm was devised to extract the temperature from BGS measurements at cryogenic temperatures. Finally, the large increase in the Brillouin gain coefficient at temperatures below ≈ 10 K was exploited to realize slope-assisted BOFDA measurements with a temperature sensitivity less than 0.1 K, a spatial resolution of 3 cm, and an acquisition rate of 1.75 Hz
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Lamb Wave Detection for Structural Health Monitoring Using a ϕ-OTDR System †
Get PDFIn this paper, the use of a phase-sensitive optical time-domain reflectometry (ϕ-OTDR) sensor for the detection of the Lamb waves excited by a piezoelectric transducer in an aluminum plate, is investigated. The system is shown to detect and resolve the Lamb wave in distinct regions of the plate, opening the possibility of realizing structural health monitoring (SHM) and damage detection using a single optical fiber attached to the structure. The system also reveals the variations in the Lamb wave resulting from a change in the load conditions of the plate. The same optical fiber used to detect the Lamb waves has also been employed to realize distributed strain measurements using a Brillouin scattering system. The method can be potentially used to replace conventional SHM sensors such as strain gauges and PZT transducers, with the advantage of offering several sensing points using a single fiber
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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