Polarized optical fiber and method of forming same

DETAILED DESCRIPTION In accordance with the present invention, an optical fiber having an oriented microcrystalline structure causing inherent, fixed birefringence is provided without resorting to a non-symmetrical core or stress-induced birefringence, although the oriented structure may, of course, be usefully combined with such other means of inducing birefringence.
In the method for forming polarization-locking optical waveguide fibers, the microcrystalline structure of the waveguide core is modified to exhibit different indices of refraction on two mutually orthogonal lateral axes by subjecting the heated glass fiber to a preferentially oriented electric field as the fiber is cooled.
In the practice of the preferred method, an optical fiber preform, having a central core region and a surrounding cladding region, is fed to a heating zone in which the temperature of the preform is elevated to drawing temperatures in conventional fashion.
As the preform is drawn down to waveguide dimensions in one or more drawing stages and while the material thereof is in and being cooled from its plastic temperature, it is subjected to a preferentially oriented electric field to align its microcrystalline structure; the alignment of the microcrystalline structure being retained until the fiber being drawn is cooled to a temperature at which the mobility of its molecular structure is at a minimum or to the temperature at which it solidifies.
As a result of the microcrystalline alignment, the resulting fiber exhibits an inherent birefringence by which the indices of refraction on two mutually orthogonal lateral axes differ.
A principal objective of the present invention is, therefore, the provision of an improved method for forming optical fibers, particularly optical fibers of a single-mode waveguide type, having birefringent characteristics that provide polarization locking or retaining transmission charactersitics.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow taken in conjunction with the accompanying drawings in which like parts are designated by like reference characters