Optical fibers having crystalline material and method of making same

An optical fiber preform comprising:

• a core having a radial grading of composition and a radially inwardly increasing refractive index and being formed from a mixture of a first two-element compound crystalline material with a predetermined melting point and a second two-element compound crystalline material with a lower melting point and a higher refractive index than the first crystalline material so that when the two materials are heated and then cooled in incremental steps while the tube is horizontally positioned and rotated, the first material begins to solidify before the second material thereby forming the outer portion of the core and the second material solidifies after the first material thereby forming the inner portion of the core, to cause a partition layer to be formed at the interface between the outer and inner sections of the core;
• and
• a cladding layer surrounding the core and having a melting point higher than the first crystalline material.

A method of making an optical fiber preform comprising the steps of:

• lining the interior surface of a glassy tube which forms a cladding with a dopant material having a melting temperature below that of the tube;
• filling the lined tube with a crystalline material having a higher refractive index than the dopant material and a melting temperature below that of the tube;
• heating the tube to a temperature below the melting temperature of the tube so that the tube has sufficiently good mechanical properties to be capable of acting as a container for the heated mixture and to a temperature sufficient to cause the dopant material to diffuse into the crystalline material in order to produce a solid solution core possessing an optical waveguiding radial gradient of composition and a refractive index that increases radially inwardly.

A method of making an optical fiber preform having a glassy tube filled with a mixture of at least two crystalline materials existing in solid solution, which mixture possesses an optical waveguiding radial grading of composition and refractive index, and melts at a temperature at which the glassy tube still has sufficiently good mechanical properties to be capable of acting as a container for the molten mixture, wherein said graded mixture is made by forming a mixture of at least two materials, fusing them together within the bore of the tube, and then cooling the tube to solidify its contents from the outside in whilst maintaining substantially no temperature gradient in the tube contents in the axial direction and while the tube is rotated about its axis held horizontal.