JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia - 79

Control Potential of in-situ measurements by optical fibres Optical fibers and associated devices gained accuracy for various measurements such as optical index, and light properties which led to sensing abilities in physics and chemistry. This article briefly summarizes the quantities accessible from optical fibres and then focuses on their use as sensors in composite materials. unique link. Two families: distributed and spread sensors are then distinguished. 1- PASCAL CASARI, PROFESSOR, GEM 2- XAVIER CHAPELEAU, PHD IREENA, 3- XAVIER ADURIZ, PHD LTN, 4- CYRIL LUPI, ASSISTANT PROFESSOR IREENA, 5- DOMINIQUE LEDUC, ASSISTANT PROFESSOR IREENA, UNIVERSITY OF NANTES, FRANCE he strong growth of broadband telecommunications has encouraged the development of optical devices such as optical fibres and integrated optical components. However, the latter tend to have characteristics that vary under the influence of external constraints. The properties of optical fibres and their associated components have been used in various industrial sectors to produce sensors suited to diverse problems. T Fig. 1: Optical fibre (250 micrometers in diameter). Distributed sensors The distributed family is made of sensors with a finite dimension which provide local information among which the most famous is the Fiber Bragg Grating Sensor (FBGS). It is manufactured by photo-printing a periodical disturbance of the refractive index of the fiber along a few millimeters, and behaves like a selective wavelength filter: when a broadband light wave propagates along the optical fiber on which a FBGS is inscribed, all the wavelengths are transmitted except the one corresponding to the Bragg wavelength which is reflected. This last wavelength is reflected and reveals both the periodic grating and the refractive index of the fiber. The Bragg wavelength depends on the period of the grating and the effective index of the core of the fiber. Then, any external constraint (mechanical and thermal) affecting the structure of the FBGS is shifting the Bragg wavelength. For example, Figure 2a shows the variation of the Bragg wavelength of a grating loaded in tension: the red and the blue curves represent respectively the spectral response of the sensor without and under external load. Then, once the Optical fibre sensors: Which accessible properties? An optical fiber is a glass cylinder of 125μm of diameter made of an optical cladding which ensures a light wave to propagate inside the core of the fiber (8 to 10 μm diameter for a monomodal one). It is coated by a polymer for mechanical protection. Finally, the component is 250 micrometers in diameter (figure 1). The optical fiber presents very low loss of the transmitted light (reduction of the signal by 0.2 dB/km, or 2% per km), an immunity to electromagnetic disturbances and apparently low intrusivity due to its small dimensions. Consequently, it allows getting measurements from long distances and in harsh environment (radiations, hot spots, explosive atmosphere). Moreover, the same optical fiber can include several sensors leading to the measurement of various parameters (temperature, pressure, strain, optical index) in a No44 October 2008 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia