M. E. Gingerich

Optical fiber waveguides in radiation environments, II

Abstract This paper will review recent progress in understanding the behavior of optical fiber waveguides when they are exposed to ionizing radiation. Not only have the growth and recovery of the radiation-induced attenuations been thoroughly characterized, in some cases the defect centers which cause these absorptions have been identified, and means for reducing the radiation sensitivity of the fibers have become apparent. The behavior of the radiation-induced loss is described in terms of parameters such as fiber composition and dopants, fiber structure, wavelength and intensity of the light source, temperature, total dose, time after irradiation, dose rate, and radiation history.

Overview Of Radiation Effects In Fiber Optics

The optical properties of fiber waveguides can be degraded by exposure to nuclear radiation, primarily through the generation of color centers in the fiber core. This paper will review recent studies of the radiation-induced absorption in state-of-the-art fiber optics. Particular emphasis is placed on the development of more radiation resistant pure and doped silica core waveguides and on the understanding of the damage processes in these materials. The results of studies of radiation damage in high birefringent, polarization-maintaining fibers and in heavy metal fluoride glasses and fibers will also be reviewed.

Compositional effects on the radiation response of Ge-doped silica-core optical fiber waveguides

The effect of variations in the Ge, B, P, and OH content on the radiation response of Ge-doped silica-core optical fiber waveguides has been measured at 0.82 and 1.3 microm. It has been established that variations in neither OH content nor Ge concentration affect the response of these fibers. B doping increases the loss at short times following pulsed irradiation, whereas P suppresses the intense Ge-related transient absorption at both 0.82 and 1.3 microm while inhibiting long-term recovery. Since there is a minimum in the induced loss near 1.1-1.2 microm in P-doped fibers, the damage at 1.55 microm is significantly higher than that at 1.3 microm.

Photobleaching effects in optical fiber waveguides

The effect of optical signal intensity at the wavelength of system operation (0.85 microm) on the recovery of the radiation-induced attenuation in optical fiber waveguides following exposure to a 3700-rad dose of ionizing radiation has been investigated. Photobleaching has been observed in both pure and doped silica core fibers, although the effect is more pronounced in the former.

Radiation damage of optical fiber waveguides at long wavelengths.

Measurements of the radiation-induced optical attenuation at 1.3 microm and the induced absorption spectra (0.4-1.7 microm) of state-of-the-art pure synthetic silica and doped silica core optical fiber waveguides have been undertaken to characterize their radiation response at long wavelengths. It has been observed that radiation-induced absorption bands at long wavelengths can give rise to substantial induced losses at both 1.3 and 1.55 microm in some fibers, especially those doped with P or B; the ratio of the damage at 1.3 and 1.55 microm to that at 0.82 microm in these fibers has been found to be only ~0.29 and 0.71, respectively. In contrast, pure fused silica and binary Ge-doped silica core fibers have shown the greatest hardness at long wavelengths. Suggestions have been made for the optimum wavelengths and preferred fiber compositions to minimize the effects of nuclear radiation in fiber-optic communications systems operating at long wavelengths.

Model for the dose, dose-rate and temperature dependence of radiation-induced loss in optical fibers

We propose, and verify in the case of a Ge-doped-silica-core optical fiber, a general explanation for the power-law dependencies on dose frequently observed for the radiation-induced attenuation in optical fibers. This insight permits detailed prediction of the post-irradiation recovery curves, given just the empirical exponent of the power law, 0 >

Effect of low dose rate irradiation on doped silica core optical fibers.

The optical attenuation induced in multimode doped silica core optical fiber waveguides by a years exposure to low dose rate (1 rad/day) ionizing radiation was studied, allowing a characterization of fibers deployed in these environments and a determination of the permanent induced loss in the waveguides. Variations in the induced attenuation at 0.85 microm have been observed with changes in the dose rate between 1 rad/day and 9000 rads/min. These dose rate dependences have been found to derive directly from the recovery that occurs during the exposure; the recovery data predict little or no dose rate dependence of the damage at 1.3 microm. The low dose rate exposure has been found to induce significant permanent attenuation in the 0.7-1.7-microm spectral region in all fibers containing P in the core, whether doped uniformly across the diameter or constrained to a narrow spike on the centerline. Whereas permanent loss was induced at 0.85 microm in a P-free binary Ge-doped silica core fiber by the years exposure, virtually no damage was observed at 1.3 microm.

Survivability of optical fibers in space

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.

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.

Correlation of single-mode fiber radiation response and fabrication parameters

Statistically significant correlations have been established between certain fabrication parameters of matched clad, single-mode optical fiber waveguides and their response to an ionizing radiation dose of 2000 rad. The reCOVE:ry data measured at -35 degrees C following exposure have been fit to nth-order kinetic behavior where the adjustable parameters are the initial and permanent incremental losses (A(o) and A(f), respectively), the half-life of attenuation tau, and the order of kinetics n. The set of fibers chosen for analysis had Ge-doped silica cores. In fibers with Ge-F-doped silica clads, A(o) correlates with the concentration of Ge-doped into the fiber core; A(f) correlates with the ratio of oxygen to reagents used during core deposition; and tau and n correlate with a two-way interaction of core oxygen and fiber draw speed. In P-F-doped clad fibers, the P concentration has been found to correlate with the order of the kinetics of recovery.