7.1 INTRODUCTION

Rare earth (RE) doping of optical fibers dates back to the 1960s and was one of the forces driving development of guided wave optical fibers. The goal was to exploit the long path length provided by wave-guiding media to improve operation of Nd, Er, and Er/Yb fiber lasers. Then, as now, the fiber consisted of regions with raised refractive index to guide light and some distribution of RE ions that interacted with this light. Very simply, the goal remains to exploit the optical activity of the RE elements to create a laser or amplifier. From its inception to the late 1980s, such fibers were primarily a research platform for study of various optical phenomena, which, though interesting, did not mature into commercial products. There were no compelling applications or needs that were filled by fiber lasers. This changed very quickly and very dramatically with the discovery of the erbiumdoped fiber amplifier (EDFA) because a critical need for optical amplification arose, and because the EDFA was able to fulfill that need exceptionally well.

The success of the erbium-doped fiber spawned an industry driven to improve and surpass its performance. This included the search for alternative dopants and hosts, as well as improved fiber designs. Naturally, these efforts morphed into other uses and applications for amplifier fiber...