Photobleaching Effects on the Radiation-Induced Attenuation of Germanosilicate Optical Fibers at Low Temperature

Photobleaching (PB) can be employed to reduce the excess losses induced by radiation in silica-based optical fibers (OFs), known as radiation-induced attenuation (RIA). PB consists of a light-annealing of some of the radiation-induced point defects, during or after irradiation. We investigate the PB effects induced by 10-mW power pumps at three different wavelengths (660, 980, or 1550 nm) on the visible to infrared RIA of a standard Ge-doped single-mode OF, the Corning SMF28e+, at room and low temperatures (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 100~^{\circ }$ </tex-math></inline-formula>C). We perform spectral measurements with a home-made acquisition system to investigate the nature of the radiation-induced defects at the two temperatures and which of them are affected by the PB at these three wavelengths. Whereas the RIA at room temperature (RT) can be reproduced only with the contributions of Ge-related point defects, at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 100~^{\circ }$ </tex-math></inline-formula>C, a defect related to the silica matrix significantly contributes to the strong RIA observed: the strain-assisted self-trapped hole (sa-STH2). The pump at 1550 nm does not influence strongly (less than 30%) the RIA in all the spectral ranges at both temperatures. The ones at shorter wavelengths, instead, reduce the RIA at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 100~^{\circ }$ </tex-math></inline-formula>C by at least one order of magnitude, allowing them to reach RIA levels comparable to those measured at RT. The concentrations of all defects decrease when two shorter-wavelength pumps are used, except for the ones responsible for the RIA at longer wavelengths: the Ge-STH and an unidentified contribution of around 0.9 eV (~1400 nm).