
When drawn, the high surface tension forces within the molten glass force the
preform to circularize, thereby creating the characteristic elliptical jacket shape.
8.5.4 Elliptical Core, Form-Birefringent Fiber
Some degree of ellipticity may be induced in the core of any single-mode fiber
if process controls are inadequate. For example, in MCVD, if the surface tension
forces within the collapsing preform are not accurately balanced by pressurizing
the substrate during the final stages of the fabrication process, then the preform
will ‘‘pull flat’’ to some degree and create an elliptical core. To fabricate a core
that is deliberately elliptical, it