Specialty Optical Fibers Handbook

8.6.2 Numerical Aperture (NA)

To cope with the small-diameter bends often encountered in sensor coils,

many PM fibers are designed with higher NAs than generally found in telecom-

munications fibers. Typical NA values of 0.16 or 0.20 are achieved by incorpo-

rating higher levels of germania (GeO

2

) within the fiber core. Whereas germania,

like silica, is a transparent amorphous glass, it is also susceptible to the forma-

tion of defects when subjected to the high (1800–2200 8C) temperatures of both

preform fabrication and fiber drawing. These defects are created when thermal

energy knocks electrons from the material lattice to create empty sites, the

presence of which intensifies the contribution from the electronic absorption

edge (or Urbach edge