
Interestingly enough, there even exist hollow-core fibers (n
core
¼ 1) that
operate based on TIR; for certain frequency ranges, there are materials (e.g.,
sapphire at l ¼ 10:6 mm) that have an index of refraction n < 1 and can,
therefore, serve as a suitable cladding [7]. However, the range of applicability
of such fibers is limited because these materials turn out to have other highly
undesirable properties (e.g., loss, stiffness).
10.1.2 Wave-Guiding by Reflection Off
a Conducting Boundary
This class of waveguides has a hollow core surrounded by a conducting
boundary that acts as a mirror, reflecting the guided light back toward the
core center, thereby ...