Chapter 21 - Sapphire Optical Fibers

Abstract

This chapter provides an overview of sapphire optical fibers. The use of sapphire fiber as an optical waveguide is driven by its capability of transmitting light radiation at longer wavelengths into the infrared (IR) than glass-based fibers coupled with some of its superior physical and mechanical properties. The two most popular methods employed to grow single-crystal sapphire fiber are laser-heated pedestal growth (LHPG) and the edge-defined film-fed growth (EFG). The EFG method uses a reservoir containing molten source material to continuously supply the growth of a single-crystal sapphire structure. Unlike the LHPG method, the melt in the EFG method is contained in a molybdenum crucible, and the cross-section of the sapphire crystal is determined by the surface configuration of the molybdenum die anchored in the melt. Sapphire is a brittle material, and the flexibility and strength of sapphire fiber decrease at high temperatures. Sapphire is among the hardest of natural minerals and is rated as a 9 on the Mohs scale. Despite this property, because of the brittle nature of sapphire, fiber handling results in surface damage and consequent reduction in strength.