G. H. Sigel

Oxygen-associated trapped-hole centers in high-purity fused silicas

Abstract Two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature γ-irradiated, high-purity fused silica. One, which we label the “wet” OHC, predominates in the high-OH-content (wet) silicas while the other, the “dry” OHC, is more prevalent in low-OH (dry) silicas. Excellent computer simulations of the low-temperature electron-spin-resonance spectra are obtained for both wet and dry silicas using only the relative abundance of the “wet” and “dry” OHCs as an adjustable parameter. Analysis of the 17 O-hyperfine structure which occrs in samples of wet silica enriched in 17 O provides direct confirmation that the “wet” OHC is a hole trapped in a single nonbonding 2p-orbital of an oxygen (presumed nonbridging). Correlation of optical absorption and electron spin resonance via isochronal pulse anneals indicates that the “dry” OHC has an optical transition ay 7.6 eV. In addition, it is reported that the “dry” OHC can be induced in the dry silicas by the fiber drawing process. From the present results, an O 2 − molecular ion model appears most attractive for the “dry” OHC.

Photoluminescence in as-drawn and irradiated silica optical fibers: an assessment of the role of non-bridging oxygen defect centers

Abstract The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber-optic waveguides have been investigated. The extended pathlength available with a fiber-optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectivity doped bulk glasses suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the SiO network, specifically, dangling non-bridging oxygens ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions such as alkali.

Drawing‐induced defect centers in a fused silica core fiber

The process of fiber drawing has been found to produce defect centers in optical fibers with a silica core of low OH content (Suprasil W‐1), but not in fibers with a silica core of high OH content (Suprasil 1). At least one center gives rise to the drawing‐induced optical absorption, which is identical to the 630‐nm band created by ionizing radiation in bulk samples of Suprasil W‐1. By means of electron‐spin‐resonance techniques, additional drawing‐induced defect centers have been observed; these centers are identical to the radiation‐induced oxygen‐associated hole centers in bulk fused silica.

Radiation protection of fiber optic materials: Effects of oxidation and reduction

The response to ionizing radiation of a suite of high‐purity silicas, a titanium‐doped silica, high‐purity germania, and silica fibers with germanium‐doped and titanium‐doped cores has been measured. The radiation sensitivity of these materials was found to depend upon the atmosphere used during a heat treatment prior to irradiation.

Effects of ionizing radiation on transmission of optical fibers

The spectral character of the optical absorption produced by ionizing radiation in several fibers and fiber materials has been measured. The nature of the damage has been identified in some of the samples and preliminary work has been successful in synthesizing radiation‐protected soda lime silicate glasses for fiber applications.

Effect of ionizing radiation on the optical attenuation in doped silica and plastic fiber-optic waveguides

The radiation‐induced optical attenuation has been measured over a wide range of dose and time in state‐of‐the‐art step‐ and graded‐state 60Co and pulsed electron irradiation. Results are reported for previously uncharacterized silica fibers containing dopants such as Ge,P,B,F,Tl, and Cs. At short times following the irradiation, fibers which contain Ge exhibit an extremely high attenuation that is not observed when these fibers contain P. Recovery is typically more complete in the silica core fibers than in the doped silica core fibers, which can have losses 25–1000 times the intrinsic loss 3 months after an irradiation of 105–106 rad.

Effect of ionizing radiation on the optical attenuation in polymer‐clad silica fiber‐optic waveguides

The optical attenuation induced in polymer‐clad silica fiber‐optic waveguides by low‐dose ionizing radiation (0.5–14 MeV) has been found to be much greater than expected on the basis of previous high‐dose measurements. The radiation sensitivity is dependent upon the purity and OH content of the silica and upon the radiation history of the fibers, but it does not appear to depend upon the drawing conditions nor on the limited radiation‐induced loss in the polymer cladding. The damage is greater at short times following the irradiation, and the decay kinetics of different synthetic silica fibers are qualitatively the same.

Radiation response of fiber optic waveguides in the 0.4 to 1.7 μ region

The response of fiber optic waveguides to ionizing radiation has been studied. Measurements of the growth and decay of the radiation-induced loss at 0.82μ have revealed that fibers with low OH are more susceptible to damage than those with high OH. The addition of P to Ge-doped silica core fibers has been found to suppress an intense transient absorption. Spectral measurements of the radiation-induced absorption between 0.4 - 1.7μ have shown an increase in the OH overtone and combination band intensity with irradiation so that the induced loss at 1.3μ is actually less in the low OH content silica core fibers than in the high OH content fibers. Real time spectral measurements of the damage following a pulsed irradiation have lead to an identification of the absorption bands and damage mechanisms responsible for the radiation-induced absorption.

The Fiber Optic Dosimeter on the Navigational Technology Satellite 2

A fiber-optic radiation dosimeter has been developed that utilizes the darkening induced in silicate glasses by ionizing radiation. A small, light weight, low-power consuming dosimeter package was deployed on the Navigational Technology Satellite 2. Observed real-time accumulated absorbed dose behind three shield thicknesses for the period July 1977 - June 1978 are compared with both the AE4 and the AEI7 particle fluence models for outer zone trapped electrons. The observations indicate little shielding improvement above three g/cm2 due to the presence of bremsstrahlung generated within the shielding and give evidence of a harder energy spectrum than predicted by these models. Occasional large dose rate fluctuations may correlate with sunspot activity during late 1977 and early 1978.

An Analysis of Photobleaching Techniques for the Radiation Hardening of Fiber Optic Data Links

The effects of light signals propagating within a fiber optic waveguide on the magnitude, spectral character and stability of radiation-induced optical absorption have been investigated. In particular, the prospects for the utilization of photobleaching techniques to increase the radiation hardness of fiber optic links has been assessed for typical state-of-the-art optical fibers. Both steady state and pulsed light sources have been employed. Fibers fabricated both from high purity synthetic silica and selectively doped silicas have been utilized. The nature of the photo-bleaching process in silica-based glasses is discussed and the characteristics of the photoluminescence accompanying the bleaching process are reported. It is concluded that the selective use of optical bleaching in irradiated fiber optic waveguides provides a useful method for increasing the operational hardness of a data link. This can be accomplished at modest power levels achievable with present day solid state light sources, and the system can be configured such that the transmitted and bleach signals are identical. The technique is most effective in pure silica core materials and promises to primarily impact systems with modest length requirements in which high light intensities can be launched throughout the link. Second generation waveguides with lower intrinsic losses will make the photobleaching approach useful even for extended ranges. The possibility of self-photobleaching waveguides containing luminescent ions is also discussed.