Specialty Optical Fibers Handbook

is an approximate ellipse with the core’s major to minor axis ratio of 0.5. A small

vapor spot or dip in the index profile centered on the fiber axis occurs during the

preform vacuum collapse. Subsequently, some diffusion of the core–cladding

boundary occurs during the fiber pulling.

To produce D-shape fibers and other custom-shaped fibers, the circular

preform is mounted, with the proper angular orientation of the elliptical axis,

inside a grooved surface plate fixture and ground. The initial coarse grind

removes the bulk of the material. Several fine grinds allow for a smooth finish

on the flat of the D-shape preform, thereby reducing the potential for scattering

and generally improving the polarization holding. Usually the flat does not

intersect ...