17.1 INTRODUCTION

After years of playing ensemble roles, large-core multimode fiber has stepped into the spotlight of fiber optic technology and innovation. From the smallest of veins in the human body, to the vastness of the universe, when the need for every photon matters, the advantages of large-core (>200 micron) multimode specialty fibers are taking the lead. As the name implies, multimode fibers are those types of fibers designed to carry multiple rays of light or modes. There are two types of multimode fibers: step index and graded index. For purposes of this chapter, we discuss the types and applications of large-core step-index multimode optical fibers.

Many industrial and medical applications require a range of geometries, clad-core ratios, and numerical apertures (NAs) for step-index multimode fibers depending on whether the end-use is for laser surgery, illumination, or sensing. Fiber core geometries can range from 100 m to more than 1000 m, and the clad-core ratios can range from 1.05 to more than 1.20. In general, the larger the NA available, the smaller the clad-core ratio or the smaller the fiber core can be. Smaller cores and core-clad ratios lead to lesser expense for materials and more flexible fibers.

Smaller dimensioned optical fibers also permit the use of smaller catheters, enabling associated surgery procedures to be less invasive. Small systems also can require broader illumination from optical fibers that may be minimized in number or in size. For...