Modeling of mechanisms for the creation of an internal clad in sapphire optical fiber using the ⁶Li(n,α)³H reaction

The focus of this work is to model the creation of a cladding, which is internal to a sapphire optical fiber, by irradiating a sapphire fiber, that is surrounded by an annulus of Li-6 enriched lithium carbonate powder, in a nuclear reactor. Such a fiber has been created using the Ohio State University Research Reactor. The ⁶Li(n,α)³H reaction creates high energy tritons and alpha particles that irradiate the fiber simultaneously to a depth of 24 microns, along the entire periphery of the sapphire fiber. The irradiation slightly reduces the index of refraction in the fiber's periphery, thus creating a refractive cladding within the fiber. The Monte Carlo radiation transport code MCNP was used in combination with SRIM/TRIM to predict the modification of the fiber that results from the irradiation. Our analysis predicts that whether it is displacement damage or the density of implanted ions that is responsible for the modification of Ε, or a combination of both, the irradiation yields a graded index in the fiber, with Ε decreasing monotonically from the value of the native sapphire 24 micrometers from the surface of the fiber to a minimum value at the surface of the fiber.