Radiation effects in optics
The main effect of ionizing radiation on optics is the production of color centers. The radiation produces electrons which populate defects in the glass matrix. These lead to absorption in the energy gap and therefore to a darkening of the optical material.
Additional effects are the change of the index of refraction, the production of light (fluorescence, luminescence, scintillation), and surface effects at optical boundaries.
Optical technologies investigated at Fraunhofer INT
The radiation effects group at Fraunhofer INT has an experience of more than 40 years in investigating the effects of radiation in optical fibers.
The Co-60 source TK1000 (Fig. 1) is a dry point source that has optimal properties for the investigation of radiation effects in optical fibers.
It is a point source giving an extremely homogeneous field for spherically symmetric objects like fiber spools. Irradiations are possible from dose rates of several mGy/s up to 2 Gy/s. With a heating element it is possible to irradiate the fibers at temperatures up to 150 °C. A helium cooled cryostat (Fig. 3) makes it possible to irradiate the fibers at temperatures as low as -253 °C (20 K).
Another optical technology that was tested extensively at the TK1000 at Fraunhofer INT are fiber Bragg gratings (FBG). Here the main effect is the change of the index of refraction, which leads to a shift of the reflected wavelength. With our own precision interrogators and a stress-free temperature-stable environment, radiation induced wavelength shifts of only a few pm can be measured. The sensitivity of FBGs can range over several decades.
We can also test bulk optical materials at the TK1000 (Fig. 5), as well as thin optical films at several external irradiation sources.