10.1 INTRODUCTION

The history of the development of optical fibers has been largely determined by various constraints stemming from the materials used for the fiber core. For example, in the case of long-haul telecom applications, it is essential to have very low losses at the wavelength of a reliable and commercially available laser. Because silica has very low losses over wavelengths ranging from the visible to the near infrared (IR), which coincide with the operating wavelengths of a number of readily available lasers, it became the material of choice for the fiber core; most of the optical power travels through the core, so the properties of the core material determine the loss properties of such a fiber. Furthermore, the carrier wavelength in telecom applications has migrated from 1.31 to 1.55 m, to match the wavelength for which the loss of the (mostly) silica core material is minimized. For other important applications, such as surgery, sensing, and industrial welding, it is often desirable to operate at longer wavelengths (>2 m) where the losses of silica are prohibitively high. Consequently, many other potential core materials that have moderate losses at mid-IR wavelengths have been explored and used, including chalcogenide glasses [1, 2], heavy-metal fluoride glasses [3], and polycrystalline materials [4, 5].

However, instead of looking for a fiber core material with the...