Distributed sensing using polarization sensitive optical low-coherence reflectometry and fiber gratings

Optical low coherence reflectometry (OLCR) is non-destructive interferometric technique that allows measuring amplitude and phase of the light reflected from the device under test. OLCR is a powerful tool for the characterization of the various optical devices such as fiber gratings and optical waveguides. This thesis work had explored possibilities to combine the OLCR technique with fiber gratings to perform distributed strain and temperature measurements, and to develop novel sensing techniques as an alternative to classical spectral methods. Special attention is devoted to polarization sensitive measurements and techniques, and OLCR techniques that use only amplitude information, as an alternative to more complicated phase sensitive measurements. Using a polarization sensitive system two methods for the measurement of local birefringence of fiber Bragg gratings (FBG) using OLCR were developed. The first technique uses oscillations in the OLCR amplitude signal to directly obtain the beat length and the birefringence. The second method is based on the measurement of the OLCR phase and inverse scattering algorithm. Both methods were compared with birefringence obtained from spectral measurements, and very good agreement was obtained. The indirect method, based on local Bragg grating determination requires two independent measurements and mathematical reconstruction, but provides a very high spatial resolution of ≈ 25 µm. The grating length limits the minimum measurable birefringence in the direct method, but a good sensitivity of 4×10-6 was obtained, which corresponds to a Bragg wavelength shift of 4 pm. The spatial resolution is in the millimeter range in this case. The novel methods were successfully applied for the distributed measurement of birefringence of FBG under diametric load. New types of tunable devices and sensors – fiber Bragg gratings written in high attenuation fibers (HAF) were developed. Active tuning by heating was achieved by optical pumping of a pure single mode fiber without any mechanical or electrical parts, or deposited light absorption coatings. Tuning can be controlled by the applied pump power, the position of the grating, the HAF attenuation level, and the pumping configuration. The proposed design assures a high repeatability and long lifetime. Using OLCR these devices were successfully applied to measure the liquid level with a spatial resolution of 100 micrometers. Liquid level measurements that use both phase and amplitude, or only amplitude were demonstrated. The same principles could be easily applied to design other fiber grating devices (long-period, tilted etc), and sensors based on hot-wire anemometry, like flow or vacuum sensors

High-speed internal strain measurements in composite structures under dynamic load using embedded FBG sensors

Internal strain measurements in cross-ply carbon-epoxy composite plates under dynamic loads are carried out using embedded FBG sensors. The principle of the FBG interrogation is based on intensity demodulation achieved via a Fabry-Perot filter. To account for the non-linearity of the filter, the system is calibrated and the amplitude of the strain data is validated. Strains are acquired at a rate of 100 kHz with a noise level as low as 2 mu epsilon and used for modal analysis and strain monitoring in low energy impact. The experimental results under impact and modal analysis compare very well with pertinent numerical models and modal analysis obtained from laser vibrometer measurements. (C) 2010 Elsevier Ltd. All rights reserved

High-speed internal strain measurements in composite structures under dynamic load using embedded FBG sensors

Internal strain measurements in cross-ply carbon-epoxy composite plates under dynamic loads are carried out using embedded FBG sensors. The principle of the FBG interrogation is based on intensity demodulation achieved via a Fabry-Pérot filter. To account for the non-linearity of the filter, the system is calibrated and the amplitude of the strain data is validated. Strains are acquired at a rate of 100 kHz with a noise level as low as 2 με and used for modal analysis and strain monitoring in low energy impact. The experimental results under impact and modal analysis compare very well with pertinent numerical models and modal analysis obtained from laser vibrometer measurements

Light-controlled reconfigurable fiber bragg gratings written in attenuation fiber

A novel type of light controlled fiber Bragg gratings written in attenuation fiber is demonstrated. The spectral reconfiguration can be controlled by the pump power, the pumping configuration, the grating position and the fiber attenuation. © Optical Society of America

Burn-up determination and accident testing of HTR-PM fuel elements irradiated in the HFR Petten

The current paper reports the results of gamma spectroscopic burn-up determination and KüFA safety testing at JRC Karlsruhe on spherical high-temperature reactor fuel elements, which were fabricated by the Institute of Nuclear and New Energy Technology of the Tsinghua University, Beijing. The fuel elements were irradiated in the High Flux Reactor, Petten, in the frame of the HFR-EU1 and HTR-PM campaigns and transported to JRC Karlsruhe for post-irradiation examination and accident testing in the KüFA device in the frame of a bilateral safety research study. Burn-up determination was performed on seven fuel elements based on the quantitative measurement of their Cs-137 inventories using an established gamma spectroscopy set-up in the JRC hot cell facilities. Accident testing was conducted on three HTR-PM fuel elements in several phases at simulated accident temperatures between 1620 °C and 1770 °C for 150 h to simulate hypothetical depressurization and loss-of-forced circulation accidents. The release of Kr-85 was measured during the tests in a cold trap and solid fission product release was determined by gamma spectroscopic analyses of exchangeable cold plates in a low background environment. After successful completion of the KüFA tests the fuel elements are currently undergoing further post-irradiation examinations including profile disintegration as well as ceramography and scanning electron microscopy of individual coated particles.JRC.G.I.3-Nuclear Fuel Safet

Jedan kritički i umjetnički projekt između Italije i Hrvatske: Nove tendencije kroz korespondenciju Vere Horvat Pintarić i Umbra Apollonija

Tra la seconda metà degli anni '50e i primi anni '60, la neoavanguardia italiana e croata, grazie anche ai critici d'arte Vera Horvat Pintarić e Umbro Apollonio, ebbe importanti momenti di contatto nel campo dell'astrazione geometrica e cinetica, definita come "nuova tendenza", che fece la sua apparizione con le esposizioni di Zagabria intitolate Nove tendencije. Il presente intervento, da una prospettiva finora inedita, vuole ripercorrere la storia della "nuova tendenza", ponendo in luce le intuizioni critiche sull'astrazione di Mondrian che ebbe Pintarić, in parallelo con la coeva attenzione che l'artista olandese aveva ricevuto anche in Italia. L'eredità di Mondrian, la nascita e lo sviluppo della neoavanguardia, sono episodi che oggi si possono leggere attraverso gli scambi epistolari intercorsi tra Pintarić e Apollonio, che fu tra i maggiori sostenitori dell'arte cinetica in Italia e diretto interlocutore con l'ambiente artistico e culturale croato