Creation of an Internal Cladding in Sapphire Optical Fiber Using the $^{6}$ Li(n, $\alpha)^{3}$ H Reaction

Abstract

The focus of this work is to extend optical frequency domain reflectometry sensing to sapphire optical fiber, by creating a cladding in the fiber. A cladding was created by irradiating a sapphire optical fiber, which was surrounded by an annulus of Li-6 enriched lithium carbonate (Li2CO3) powder, in The Ohio State University Research Reactor. The 6Li(n, $\alpha)^{3}\text{H}$ reaction created high energy alpha particles and tritons that irradiated the fiber simultaneously to a depth of 24 microns along the entire periphery of the sapphire fiber, thereby slightly reducing the index of refraction in the fiber’s periphery and creating a cladding within the fiber. Transmitted light intensity profiles show that the ion implanted cladding made the fiber’s intensity profile nearly single mode. The cladding survived to the highest temperature that was tested (1500 °C). The cladding made the fiber sufficiently single mode that the cladded sapphire fiber, when read out with an optical backscatter reflectometer, produced distributed temperature measurements along the entire length of the fiber.