Method for fabricating optical fibers with enhanced mode coupling

A method of fabricating an optical fiber having an asymmetric refractive index profile that varies periodically along the length of the fiber is described. The prescribed variations are obtained by means of a fabrication process that includes periodically varying the source of at least one of the fiber materials in a direction transverse to the fiber-drawing direction. This transverse periodicity is translated into the desired longitudinal periodicity along the fiber by the fiber-drawing process.

TECHNICAL FIELD 
This invention relates to methods for fabricating multimode optical fibers 
having prescribed spatial variations for introducing controlled mode 
coupling. 
BACKGROUND ART 
In U.S. Pat. No. 3,687,514 it is shown that the mode dispersion in a 
multimode waveguide can be reduced by deliberately enhancing the mode 
conversion processes in the waveguide. This is accomplished by introducing 
"imperfections" in the waveguide, such as changes in the cross-sectional 
dimensions of the guide and/or changes in the orientation of the guide 
axis. Arrangements for fabricating optical fibers having such variations 
are described in U.S. Pat. Nos. 4,038,062; 3,666,348; 3,912,478; and the 
previously cited U.S. Pat. No. 3,687,514. However, the periodicity of the 
resulting spatial variations produced by all of these techniques is much 
greater than one would like. As a result, the net coupling length for the 
fiber is relatively large and the reduction in the dispersion is 
correspondingly small. 
In an alternative technique, described in U.S. Pat. No. 3,909,110, 
refractive index variations are built into the preform from which the 
fiber is to be pulled. This, however, requires considerable care in the 
preparation of the preform and, hence, tends to be a relatively expensive 
method. 
SUMMARY OF THE INVENTION 
The method of fabricating optical fibers in accordance with the present 
invention comprises the steps of: providing a supply of fiber forming 
materials that includes at least a core material and a cladding material 
of lower refractive index; heating said materials to a molten state; and 
drawing a fiber from said supply; characterized in that the location of 
the source of at least one of said fiber forming materials is caused to 
change as the fiber is drawn. This can be accomplished in a number of 
ways. If a solid preform is used, the preform is provided with a 
refractive index asymmetry and is rotated as the fiber is drawn. Using the 
double-crucible method of fiber fabrication, the same effect is produced 
by causing the source of core material to move in a direction transverse 
to the fiber-drawing direction, or by providing the inner crucible, 
containing the core material, with an asymmetric outlet aperture, and 
rotating the crucible. In either case, a transverse periodicity of 
circular asymmetry is translated into the desired longitudinal periodicity 
by the fiberdrawing process. 
It is an advantage of one aspect of the present invention that the 
refractive index profile of the starting preform is relatively simple and, 
hence, the preform is correspondingly inexpensive to make. It is an 
advantage of another aspect of the invention that the spatial variations 
in the resulting fiber are inherent in the method of fabrication, rather 
than impressed upon the fiber after it is drawn.

DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 shows the cross section of an optical 
fiber preform 10 comprising an inner core region 11 surrounded by an outer 
cladding 12 of lower refractive index. Great care is usually taken in the 
manufacture of such preforms to insure that the core region is uniformly 
circular throughout the length of the preform, and that it is coaxial with 
the cladding. A fiber drawn from such a preform will maintain this 
circular symmetry and, in general, the mode coupling will be minimal. 
In accordance with one aspect of the present invention, the above-described 
symmetry is deliberately disrupted by the inclusion in the preform of an 
index irregularity comprising a third region of different refractive 
index, as illustrated in FIGS. 2 and 3. In FIG. 2 the preform 20 includes 
a core region 21 surrounded by a cladding 22 of lower refractive index, as 
in the preform of FIG. 1. However, a third region 23, contiguous with the 
core, is included within the cladding. The refractive index of the third 
region can be different than that of both the core and host cladding, or 
it can be an extension of the core. For the purposes of the present 
invention it is not important whether the index of region 23 is greater or 
less than the cladding index. Nor is the shape of the region particularly 
significant. These factors will be determined primarily by the particular 
method used to fabricate the preform, and by the dopants used to obtain 
the desired index profile. What is significant, however, is that the 
preform index profile no longer has axial symmetry. 
In the preform of FIG. 3, the asymmetry is obtained by the inclusion of a 
third region 31 within the host core 32, rather than within the cladding 
33. As above, the refractive index of region 31 is different, either 
greater or less than that of the core material. 
Using either type of preform, or one including both irregularities, a fiber 
having enhanced mode coupling is obtained by pulling the fiber while, 
simultaneously, rotating the preform about its longitudinal axis, as 
illustrated in FIG. 4. Specifically, the preform 40 is placed in an oven 
41 (or heated by means of a CO.sub.2 laser) so as to reduce it to a molten 
state, and rotated by a motor 42 as the fiber is pulled. The effect is to 
create a helically-shaped irregularity in the fiber whose pitch is a 
function of the pulling rate, v, and the rotational rate of the preform. 
For example, if the fiber is pulled at the rate of one meter/sec, and a 
coupling period of 5 mm is desired, the rotation rate of the preform 
should be 
EQU RPM=60.times.(1000)/(5) =12,000. 
It should be noted that the important parameters determining the coupling 
period are the pulling velocity and the rotational rate. The shape of the 
irregularity in the preform is not critical, nor is the longitudinal 
uniformity of the preform. Indeed, the very nonuniformities that result in 
preforms being rejected for normal use may be enough to produce the 
desired mode coupling if the preform is rotated as the fiber is drawn. 
The technique described above can be extended to other forms of fiber 
fabrication as, for example, the double-crucible method, illustrated in 
FIG. 5. In the double-crucible method, the fiber materials are contained 
within a pair of crucibles. The outer crucible 50 contains the cladding 
material 51. The inner crucible 52 contains the core material 52. Both 
crucibles are located within a suitable oven 55 which maintains the 
materials in their molten state. 
In the usual arrangement, the outlets of the two crucibles are coaxially 
aligned. In the particular embodiment shown in FIG. 5, on the other hand, 
the outlet aperture of the inner crucible is offset, as indicated by the 
center line Z'--Z'. The effect of this offset is to displace the source of 
the core material. Thus, if the inner crucible is rotated about the axis 
Z--Z of the outer crucible by means of a motor 54 as the fiber is drawn, a 
helical discontinuity is created along the fiber by virtue of the 
resulting meandering of the core 60 about the cladding axis, as 
illustrated in FIG. 6. The pitch of the helical core and, hence, the 
periodicity of the mode coupling is controlled by the appropriate 
selection of the fiber-drawn rate and the rotational rate of the inner 
crucible outlet aperture. 
In an alternate arrangement, illustrated in FIG. 7, the outlet aperture of 
the inner crucible 61 is coaxially aligned with the outlet aperture of the 
outer crucible 62. However, by providing the inner crucible outlet with an 
asymmetric cross section, such as, for example, an elliptical cross 
section, the effect is to continuously change the location of the source 
of core material as the inner crucible is rotated. The result is to form a 
helical discontinuity as the fiber is drawn similar to that shown in FIG. 
6. 
Substantially the same result can be obtained by means of a rotating rod 
disposed within the outlet aperture of the inner crucible, as illustrated 
in FIG. 8. In this arrangement both crucibles 80 and 81 remain stationary, 
with their outlet apertures coaxially aligned along a common axis Z--Z. 
The flow of core material is controlled by a hollow rod 82 which extends 
into the outlet of crucible 81, and whose longitudinal axis is colinear 
with the crucible axis Z--Z. To vary the location of the source of core 
material, the lower end of rod 82 is provided with an opening that is off 
center. Thus, as the rod is rotated about its axis by a motor 83, the core 
material, flowing through the off-centered opening, assumes the helical 
configuration shown in FIG. 6. 
FIG. 9 shows, in cross section, the end of rod 82 located in the outlet 
aperture of crucible 81. This particular rod configuration, formed by 
cutting a longitudinal groove in the rod, illustrates one means of 
obtaining the desired off-centered aperture which effectively produces a 
constantly changing outlet location for crucible 81 as the rod is rotated. 
Another means of producing a periodic change in the location of the source 
of core material is to employ a linear displacement of a rod 101 that 
extends into the outlet aperture of the core material crucible 100, as 
illustrated in FIG. 10. Rod 101 is caused to oscillate back and forth by 
suitable means, such as the solenoid drive 102-103. 
FIG. 11 shows a bottom view of crucible 100 with the end of rod 101 
blocking a portion of the outlet aperture. As the rod moves back and 
forth, the source of core material is displaced periodically, causing a 
periodic core perturbation in the drawn fiber. 
As noted above, the periodicity perturbation is a function of the fiber 
pulling rate and the source rotation or translation rate. Both of these 
parameters can remain constant or, alternatively, one or both can be 
varied in a prescribed manner to produce a fiber having perturbation 
periods which are either constant, or which vary in a defined manner. 
SUMMARY OF THE INVENTION 
A method of fabricating an optical fiber having an asymmetric refractive 
index profile that varies periodically along the length of the fiber is 
described. The method is characterized in that the location of the source 
of at least one of the fiber forming materials is caused to change as the 
fiber is pulled. The one material can be the core material itself, or a 
material that is embedded within the core and/or cladding portion of a 
solid preform.