Rare Earth-Doped Fibers

This chapter discusses two main categories of rare earth (RE)-doped fibers—fibers for telecommunications applications and fibers for high-power source applications. Fabrication of low-loss optical fiber requires very pure precursor materials and a method to produce the cylindrically symmetrical preform from which the fiber is drawn. To achieve the current level of transparency for optical communications transmission fiber, the concentration of impurities such as iron must be reduced to less than one part per billion. The use of vapor methods precludes transport of transition metal ions and other contaminants to the reaction zone during deposition and allows for the routine fabrication of fiber approaching the theoretical minimum loss in silica. All-vapor delivery avoids contamination of the system, accelerates the glass formation process, and allows superior control of the refractive index profile. Vapor delivery may be accomplished by heating RE chlorides to around 1000 °C to generate sufficient vapor. The surface plasma chemical vapor deposition technique is a modification of the modified chemical vapor deposition technique that allows a lower deposition temperature.