Optical fibers having crystalline material and method of making same

DETAILED DESCRIPTION OF THE INVENTION Referring to the accompanying drawing, an optical fiber or optical fiber preform according to the present invention consists of a glass tube 1 surrounding a crystalline rod whose composition is radially graded to provide its central region 2 with a higher refractive index than that of its peripheral region 3.
This grading may be provided by lining the bore of the tube with an index-reducing dopant which is subsequently diffused into the crystalline material.
An alternative method of providing the requisite grading makes use of the partition effects when a crystal mixture is subjected to controlled cooling.
For this purpose a pair of structurally compatible crystals with similar physical properties are required which will form solid solutions, and in which the one having the higher melting point, the solvent, also has the lower refractive index.
Controlled cooling of a long cylinder of the liquid mixture under conditions providing cylindrical symmetry of the temperature distribution will cause the material to solidify from the outside in.
At an interface between solid and liquid there is, under equilibrium conditions, a greater concentration of solute in the liquid than in the solid.
Hence controlled cooling results in a solid cylinder with a radial grading of composition in which the solute concentration decreases with distance from the axis.
This principle can of course be extended to producing a graded index distribution using the partition effects occuring with a mixture of three or more components.
Examples of possible pairs of components for producing index grading are:
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Salt Melting Point (.
degree.
C.
)
Refractive Index
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LiF 870 1.
35
NaF 999 1.
31
BaF.
sub.
2 1280 1.
45
CaF.
sub.
2 1360 1.
41
NaCl 730 1.
52
KCl 776 1.
49
CaBr 636 1