Much study has been made of the effects of ionizing radiation, both Gamma and from source natural to the space flight environment, on optical fiber. Information can be found in the literature documenting the dependency of a fiber’s performance in a radiation environment on the materials used to make the glass, the processes used, coatings used, dose type and rate and total dose. Recovery times, self-annealing and photobleaching effects have also been well documented. Manufacturers interested in the space flight market are aware of these dependencies and have developed manufacturing processes which produce products which can withstand tens to hundreds of kRads(Si) total dose with less than one dB increase in loss per kilometer of fiber. Flight projects using multimode fiber, use lengths much shorter than a kilometer and have tended to consider radiation sensitivity a non-issue. With the emergence of the use of single mode fiber, the radiation issue is being revisited to examine the compatibility of the recovery times (³ 2 sec) with the high data rates being applied.

Optical fiber will darken due to ionizing radiation creating centers of absorption where unwanted elements and other optical defects occur in the fiber. Generally a fiber will experience defects during the drawing process making them hard to isolate and eliminate, regardless of the purity of the glass pre-form. Radiation performance can also be affected by the coatings. There is evidence that suggests that the primary coating on the fiber has a much greater impact on the radiation performance of a fiber than does the secondary coating. Polyimide coatings undergo a heat cure while acrylates undergo a ultraviolet (UV) cure. It has been surmised that the high temperature cure inherent in the polyimide coating process can actually anneal the defects induced in the fiber by the drawing process. [4]

Germanium, used to dope fiber cores in order to raise the waveguide index of refraction, causes radiation sensitivity. Phosphorous doped fiber (for the core or cladding) has been well documented as not acceptable for use in space environments due to its radiation sensitivity[5]. Lower temperatures produce the largest radiation induced attenuation in fibers. The radiation induced loss sensitivity can vary by a factor of 25 between temperature extremes where the worst case is at the low end of the temperature range. At lower temperatures the annealing of color centers decreases. In general it is best to use pure silica in space applications when total dose requirements exceed 5kRads(Si) [6].

Dose rate will have an effect on the results of radiation testing when they are above 960 rads(Si)/hour [6,7]. Often bit error rate has been used as a pass/fail criterion making dose rate dependence much less obvious. When attenuation is measured, special attention should be given to dose rate and kept as low as feasible Test data has been generated for many fibers with a variety of material formulations to total dose levels through and beyond 1 MRads(Si) [6]. An algorithm has been established by NRL which allows extrapolation of radiation performance data for varying values of total dose, temperature, and dose rate [8].

---

Table of Contents

Next Section: Long Term Reliability