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RESEARCH PRODUCT

Irradiation Effects in Optical Fibers

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Intrinsic and extrinsic optical fiber-based sensors are promising devices to be used in very different and complex environments, by their very nature: capabilities to work under electromagnetic fields; possibility to carry multiplexed signals (time, wavelength multiplexing); small size and low mass; ability to handle multi-parameter measurements in distributed configuration; possibility to monitor sites far away from the controller. In the case of the optical fibers, the possibility to be incorporated into various types of sensors and actuators, free of additional hazards (i.e. fire, explosion), made them promising candidates to operate in adverse conditions as those required by space applications and terrestrial nuclear facilities (Alam et. al.a, 2006; Alam et al.b, 2006; Berghmans et al., 2008; Ott, 2002). In nuclear environments optical fibers found an application niche in optical communication links, embedded into various all-fiber or hybrid sensors or as light-guides for control and diagnostics (Alfeeli et al., 2007; Ahrens et al., 2001; Fernando et al., 2005; Fielder et al., 2005; Florous et al., 2007; Gan et al. 2008; Henschel et al., 2001; Kimurai et al. 2002; O'Keeffe et al. 2008; Reichle et al., 2007; Troska et al., 2003). For applications related to fusion installations the requirements are quite demanding because of the exposure to (Campbell, 2005; Griscom, 1998; Hodgson, 2006; ITER Physics Expert Group on Diagnostics, 1999; Shikama, 2003; Zabezhailov, 2005): ionising radiation, high temperature, and high electromagnetic disturbances. One of the major drawbacks for optical fibers use under ionizing radiation is related to the development of colour centres, which affect dramatically the optical transmission in UVvisible-NIR spectral ranges (Griscom, 1998; Karlitschek, 1995). For this reason, optical fibers are by more than 30 years in the focus of colour centres research (Friebele, 1976; Kaiser, 1974). Research on radiation induced colour centres in pure and doped bulk silica materials has a long history of over 50 years (Weeks, 1956), but it is still actual (Radiation effects, 2007; Devine et al., 2000; Pacchioni et al., 2000), as new materials and devices (optical fibers, waveguides, multiplexers, or fiber lasers) are continuously devised and evaluated. Apart from the diversity of the investigated materials and devices new challenges are presented by the various irradiation conditions to which such materials and devices are subjected. The complexity of the colour centres dynamics lead to the use of complementary methods to individuate these centres (electron paramagnetic resonance: EPR, luminescence) besides the