The natures of most radiation-induced point defects in amorphous silicon dioxide (a-SiO2) are well known on the basis
of 55 years of electron spin resonance (ESR) and optical studies of pure and doped silica in bulk, thin-film, and fiberoptic
forms. The self-trapped holes (STHs), discovered only in 1989, appear to be responsible for most radiationinduced
red/near-IR optical absorption in silica-based photonics. However, accelerated testing of a-SiO2-based devices
slated for space applications must take into account the highly supralinear dependence of the initial STH creation rate on
ionizing dose rate...and the possibility to permanently reduce the created numbers of STHs by high-dose pre-irradiation.
The level of second harmonic generation (SHG) in fused silica can be altered by the addition and removal of water by wet and dry annealing in N2. The magnitude of the effect is sensitive to the lot of material. Maximum SHG occurs with annealing temperatures near 500 degree(s)C with smaller values of SHG occurring for annealing temperatures on either side of 500 degree(s)C. For annealed samples, the SHG has been found to have both surface and bulk components. Preliminary EPR analysis has shown a growth in both the Si and Ge E' centers on either side of a 500 degree(s)C - 600 degree(s)C annealing temperature. Finally, from depoling current data, evidence for a second order `bond effect' along with a third order `hole filling effect' has been found.
Thermal stabilities of photoinduced paramagnetic defects in second-harmonic (SHG) generation Ge-P-doped silica fibers and in Ge- and Ge-P-doped silica preforms are studied by electron-spin-resonance and compared with that of the reported self-organized gratings. Our data suggests that the charge trapping sites for the electric-field-induced SHG are Ge(1), Ge(2) and Ge E'd1 centers. The defects responsible for the Bragg gratings are Ge E'-type centers. Thermal darkening in Ge-doped silica core fibers reported is likely due to the thermal induced Ge E'd1 center.
Rare earth-doped glasses exhibit high initial photosensitivity but their response saturates at relatively modest values of (Delta) n (approximately 5 X 10-7), which greatly limits their usefulness for device applications. In the context of our model, saturation results from either exhaustion of photosensitive rare earth sites, trap sites, or through competition between two photon creation and one photon bleaching processes. In this paper we report the results of new experiments designed to further elucidate the photosensitivity process with specific emphasis on the saturation mechanisms(s). Based on these new experimental results we present a refinement of our earlier model.
The survivability of optical fibers for data bus and gyroscope applications in the natural space radiation environment has been analyzed using radiation-induced loss data of single mode, multimode, and polarization-maintaining fibers. Since it is virtually impossible to simulate the dynamic conditions of space, extrapolations have been made from measurements at dose rates, temperatures, and total doses different from those onboard spacecraft. The anticipated degradation of most Ge-doped silica core fibers and all pure silica core fibers appears to be well within allowable margins in fibers for data bus applications, while the radiation sensitivity of polarization-maintaining fibers could result in a significant decrease in fiber gyro performance.
Photoinduced defect centers by harmonics of 1.06 micrometers photons in Ge-doped silica are discussed and proposed defect models reviewed in relation to the self-organization phenomena, such as Hill gratings and second harmonic generation (SHG) in Ge-doped silica core fibers. In particular, we will show that the reported preparation kinetics of SHG in both seeded and unseeded processes can be explained in terms of defect centers induced by the fourth harmonic of 1.06 micrometers photons. The SHG erasure kinetics can also be understood by the reaction of defect centers responsible for SHG with free excitons.