Under the ideal environment condition, the primary fiber attenuation in single mode fiber comes from intrinsic characteristics of the glass and is usually less than 0.2 dB/km. However, when the fiber is exposed to high radiation environment for an extended period, for example in a nuclear facility, the fiber's Radiation-Induced Attenuation (RIA) will accumulate and become fiber's dominant source of loss as the electrons are trapped in the glass due to ionizing radiation. It also takes time for the fiber to recover from the radiation after the radiation source is removed. Depending on the temperature, radiation doses, and radiation sources, the fiber may or may not recover to its original state.
Graded-Index Fiber, also known as G.651.1 under International Telecommunication Union (ITU) standards, is a type of fiber whose refractive index decreases gradually as the radial distance (distance to the core center) increases. In comparison, what we commonly have seen is G.652.D fiber which has a step-index refractive index profile. This article will compare graded-index multimode fiber with traditional step-index fiber, as well as its advantages when dealing with modal dispersion, a common signal distortion error.
The graph below shows the different refractive index profiles of the fiber core and cladding.