Chapter 16 - Elliptical Core and D-Shape Fibers

Abstract

This chapter provides an overview of elliptical core and D-shape fibers. The D-shape elliptical core fiber satisfies two important requirements—the accurate angular location of the birefringent axes and access to the optical fields—for polarization-preserving fiber. The axes of birefringence are positioned by locating the flat of the D typically against a flat surface or by the glint from the reflection off of the flat. This inherent feature of the D-shape fiber locates the birefringence axes no matter how far down the fiber from a reference point and circumvents the lack of angular reference exhibited by circular clad commercial polarization-maintaining fibers that exhibit some degree of internal twist. One method to manufacture an elliptical core preform is to fabricate a tube with nonuniform thickness and collapse the tube by heating it to the softening point. The surface tension in the shaped walls during the collapse and subsequent draw cause the fiber core to be nearly elliptical in cross-section. The field of the propagating wave distributed in the elliptical core extends beyond the core–cladding interface decaying exponentially from the interface into the cladding. By modifying the cladding material close to the core–cladding interface, the waveguide characteristics are changed, and consequently the propagation characteristics are also changed.