Integration of optical fibers in radiative environments: Advantages and Limitations
Résumé
Optical fibers are now considered for use as part of systems, like control-command, diagnostics, sensors that will have to operate in harsh environments associated with future nuclear power plants, high energy physics facilities, nuclear waste repository,... For these applications, optical fibers present a number of advantages compared to coaxial cables, due to their electromagnetic immunity. However, it has been shown that radiations affect the fiber properties mainly through three macroscopic degradation mechanisms [1]. First, radiations generate point defects in the silica-based matrix of the fibers resulting in an increase of its linear attenuation resulting in the degradation or loss of propagated signal (Radiation-Induced Attenuation, RIA). Second, some of these defects can also emit light that will add to the signal, degrades the signal to noise ratio and some times saturates the detectors (Radiation-Induced Emission). Finally, for some environments, radiations can change the glass structure by a compaction phenomenon, resulting in degradation of fiber guiding or sensing properties. In this paper, we will review the main limiting degradation phenomenon for the fiber integration in the different environments studied by our group: inertial and magnetic fusion, space, nuclear waste repository,... More particularly, the intrinsic and extrinsic fiber parameters that influence the amplitude of radiation-induced effects will be presented. A very important one is the composition of the glass used for the making of the fiber core and optical cladding [2]. Radiations response of different classes of optical fibers: pure-silica core, fluorine, germanium, phosphorus-doped optical fibers will be presented for various harsh environments associated with military, civil applications [1,3]. Spectral dependence of RIA and RIE in these fibers will be discussed as the response of the fiber mainly depends on the wavelength of interest for the application: eg the ultraviolet-visible part of the spectrum for diagnostics, the infrared for telecommunications or sensing. Furthermore, we will also present the influence of the manufacturing process on the fiber vulnerability. We will show that its influence strongly depends on the considered application and harsh environment. Finally, different techniques will be presented that have been shown to be able to reduce the fiber radiation sensitivity. Their interest and limitations of use will be discussed for various environments