Silica Optical Fiber Technology for Devices and Components: Design, Fabrication, and International Standards

CHAPTER EIGHT

Optical Fibers Based on Air–Silica Guiding Structure

8.1   REVIEW OF AIR–SILICA GUIDANCE IN OPTICAL FIBERS

In silica optical fibers, the refractive index control is one of key technologies in mode confinements, chromatic dispersion controls, and birefringence manipulation. The control should be in both ways—increasing and decreasing the refractive index. In comparison to relatively sufficient and efficient doping of the index-raising GeO₂ into silica, the index-lowering agents in silica glass such as B₂O₃ and F have shown severe limitation of doping concentration due to the huge excess loss for high B₂O₃ and chemically unstable glass forming for high F. The lowest possible refractive index is that of air and holey regions have been incorporated in optical fiber to maximize the refractive index contrast, and various air–silica guiding structures have been proposed in various forms [1–3].

However, it is since 1996 when Knight et al. [4] first reported a transverse periodic air hole structure in silica cladding that intensive and explosive research efforts have begun for innovative nano- and micro-scale air–silica structured optical fibers and their applications. There are various names for these air–silica guiding optical fibers such as photonic crystal fibers (ASHF), microstructured optical fiber (MOF), and holey fiber (HF). In this chapter, we will use the term air–silica holey fiber (ASHF) to be more inclusive in classification of similar fibers.

These ASHFs have ...