Transient radiation responses of optical fibers: influence of MCVD process parameters

A dedicated set of fibers elaborated via the Modified Chemical Vapor Deposition (MCVD) technique is used to study the influence of composition and drawing parameters on their responses to an X-ray pulse representative of the radiation environments associated with Megajoule class lasers. These canonical fibers were designed to highlight the impact of these parameters on the amplitude and kinetics of the transient pulsed X-ray Radiation Induced Attenuation (RIA) at room temperature. From pre-forms differing by their core composition, three optical fibers were elaborated by varying the tension and speed during the drawing process. No or only slight RIA change results from the tested variations in drawing process parameters of Ge-doped, F-doped, and pure-silica-core fibers. This study reveals that the drawing process is not the main parameter to be optimized in order to enhance the radiation tolerance of MCVD specialty optical fibers for the LMJ harsh environment. From the hardness assurance point of view, a specialty fiber sufficiently tolerant to this environment should be robust against changes in the drawing process. The origins of the RIA observed in the different fibers are discussed on the basis of spectral decomposition of their measured RIA spectra, using sets of defects from the literature and related to the different core dopants. This analysis highlights the limits of the well-known defect set to reproduce the RIA above 1 for Ge-doped fibers whereas self-trapped holes and chlorine-related species seem responsible for the transient responses of pure-silica-core and F-doped fibers. (authors)