Origins of radiation-induced attenuation in pure-silica-core and Ge-doped optical fibers under pulsed x-ray irradiation

Abstract

We investigated the nature, optical properties, and decay kinetics of point defects causing large transient attenuation increase observed in silica-based optical fibers exposed to short duration and high-dose rate x-ray pulses. The transient radiation-induced attenuation (RIA) spectra of pure-silica-core (PSC), Ge-doped, F-doped, and Ge + F-doped optical fibers (OFs) were acquired after the ionizing pulse in the spectral range of [∼0.8-∼3.2] eV (∼1500-∼380 nm), from a few ms to several minutes after the pulse, at both room temperature (RT) and liquid nitrogen temperature (LNT). Comparing the fiber behavior at both temperatures better highlights the thermally unstable point defects contribution to the RIA. The transient RIA origin and decay kinetics are discussed on the basis of already-known defects absorbing in the investigated spectral range. These measurements reveal the importance of intrinsic metastable defects such as self-trapped holes (STHs), not only for PSC and F-doped fibers but also for germanosilicate optical fibers as clearly evidenced by our LNT measurements. Furthermore, our results show that fluorine co-doping seems to decrease the RIA related to the strain-assisted STHs absorption bands in both types of optical fibers. Regarding Ge-doped glasses, besides a description of the defects responsible of the RIA, highlighting the STHs' role in their transient response, we provide a clear correlation between the GeX and GeY centers' kinetics. In conclusion, the presented results improve our understanding of the transient RIA origin in the ultraviolet and visible domains. The lack of knowledge about the defects causing the RIA in the near-infrared domain will require future studies.

Extreme Radiation Sensitivity of Ultra-Low Loss Pure-Silica-Core Optical Fibers at Low Dose Levels and Infrared Wavelengths

“…Ge-STHs and GeY defects are still not sufficient to fully reproduce the measured RIA spectra, at least another source of optical losses around 0.9 eV has to be identified for continuous irradiation [ 21 ]. This is also true for pulsed irradiation, even if in this case, a recent study pointed out the STH role on the steady state and transient response of germanosilicate fibers [ 24 , 25 ].…”

Pulsed X‐Ray Radiation Response of Ultralow Loss Pure‐Silica‐Core Optical Fibers

“…Also interesting is the comparison at LNT (Figure 1b): with the exception of the P-doped fiber, which is characterized by a similar radiation response, [25] the canonical samples are much more sensitive at low temperature, especially, the Ge-doped fiber, which is the most radiation sensitive at LNT. [24] Low OH PSC and Ge-doped exhibit also different spectra shapes, with the appearance of new absorption bands, especially in the visible domain, in agreement with the greater STHs' stability at LNT. [1,24,29] Accordingly, the ULL-SMF has also a very different response: its RIA magnitude is comparable to the other samples in the near-IR spectral domain, with the unexpected absence of the STH's contribution around %1 eV.…”

“…This behavior strongly differs compared with the low-OH radiation-resistant PSC OF. [24] Indeed, such high near-IR RIA values are not expected in PSC OFs under these experimental conditions, making those fibers no longer automatically considered as resistant to radiation. This IR feature, according to the previous observations under steady state X-rays or γ-rays, is attributed to a particular class of self-trapped holes (STHs), [19,26,27] explaining the strong postirradiation bleaching already observed in ref.…”

“…Figure 1 shows the comparison between the RIA spectrum, 1 s after the X-ray pulse, of the ULL-SMF with the ones of canonical samples with other core compositions [21][22][23] measured under the same conditions at RT (Figure 1a) and LNT (Figure 1b) [24,25] : low OH PSC, Ge-and P-doped OFs. At RT (Figure 1a), as already observed in literature, [1,24,25] the P-doped fiber is the most sensitive to irradiation, just as the PSC and the Ge-doped fibers are the most tolerant, in the ms timescale after the X-ray pulse.…”

“…Figure 1 shows the comparison between the RIA spectrum, 1 s after the X-ray pulse, of the ULL-SMF with the ones of canonical samples with other core compositions [21][22][23] measured under the same conditions at RT (Figure 1a) and LNT (Figure 1b) [24,25] : low OH PSC, Ge-and P-doped OFs. At RT (Figure 1a), as already observed in literature, [1,24,25] the P-doped fiber is the most sensitive to irradiation, just as the PSC and the Ge-doped fibers are the most tolerant, in the ms timescale after the X-ray pulse. This is in accordance with the classification of the fibers' vulnerability under X-/γ-ray steady-state irradiation, where the P-doped fiber has RIA levels several orders of magnitude higher than the PSC and Ge-doped ones, which are comparable, over the whole investigated spectral range.…”