Drawing optical waveguides by heating with laser radiation

In drawing an optical waveguide, the blank is heated by a beam of laser radiation which has substantially zero power from the axis of the blank to a prescribed radial position and substantially uniform power from that position to a greater radial position. A rotating spinner splits radiation from the laser into two rotating beams, the paths of which are made to be non-intersecting with the axis of the blank being drawn. A conical reflector forms the radiation into a circumferential beam through which the blank passes.

BACKGROUND OF THE INVENTION 
This invention relates to methods of and apparatus for drawing optical 
waveguides, and more particularly to the use of laser radiation for 
heating the waveguide blank to the drawing temperature. 
Waveguides used in optical communication systems are hereinafter referred 
to as "optical waveguides", and are normally constructed from a 
transparent dielectric material, such as glass or plastic. U.S. Pat. Nos. 
3,711,262--Keck and Schultz and 3,775,075--Keck and Maurer describe 
techniques for making a blank which is heated and drawn into a waveguide 
having a uniform diameter. 
U.S. Pat. No. 3,865,564--Jaeger and Logan suggests the use of laser 
radiation to heat the waveguide blank to the drawing temperature. The use 
of laser radiation has many advantages for this purpose. It is uniform, 
easily controlled and easily focused. In the Jaeger and Logan patent, an 
annular beam of laser radiation is formed. The annular beam is directed to 
a conical reflector which focuses the energy on the waveguide blank. A 
conical reflector is particularly suitable for focusing the energy on the 
blank. However, with the optical system of this patent, the energy is 
focused on the axis of the waveguide blank. This results in an inherent 
instability in the draw. As the glass diameter is reduced, the power 
density on the surface increases, thus lowering the viscosity and 
encouraging further diameter reduction. Furthermore, in this patent the 
blank is heated with a beam which has a power density distribution along 
the axis of the blank which is the same as the power density distribution 
of the original laser source. Heating of the blank can be accomplished 
more efficiently by having the power density highest where the blank 
diameter has not begun to attenuate. 
It is an object of the present invention to provide laser radiation heating 
which is more stable and more efficient. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a waveguide blank is heated to the 
drawing temperature by an annular beam of laser radiation which has 
substantially zero power from the axis of the blank to a prescribed radial 
position. 
In order to produce such a beam, radiation from a laser is directed to a 
rotating reflector which splits the radiation into two beams which rotate 
about the axis of the waveguide blank. These beams are reflected onto 
paths which do not intersect the axis of the waveguide blank. The rotating 
beams of laser radiation are directed to a conical reflector through which 
the waveguide passes. The conical reflector focuses the energy into a 
circumferential beam which has substantially zero radiation from the axis 
of the waveguide blank to a prescribed radius. Such a beam provides more 
stable heating than the prior art wherein the energy is focused on the 
axis of the blank. 
In accordance with another important aspect of this invention, two rotating 
reflecting surfaces in the optical path form a beam having a substantially 
uniform power distribution from the prescribed radius to a greater radius. 
This uniform power distribution promotes efficient heating. The curvature 
of these reflective surfaces can be circular, hyperbolic or any shape 
which produces a desired radial power density distribution. 
In accordance with another important aspect of the invention, the laser 
radiation from the source is split into two beams by a rotating reflector 
which has two reflecting surfaces which join in a line intersecting the 
axis of the blank. This is referred to as a "rooftop" reflector. The shape 
of the surfaces are changed to produce a desired axial power distribution 
along the waveguide blank. One particularly suitable axial power 
distribution has a half Gaussian shape wherein the power is a maximum at 
the point where the blank diameter is reduced and falls in a Gaussian 
curve to zero on one side of this point. 
The foregoing and other objects, features and advantages of the invention 
will be better understood from the following more detailed description and 
appended claims.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIGS. 1A-1D, a substantially continuous glass blank 11 is 
transported through the laser optics of this invention by conventional 
means which are not shown. A source of laser radiation 12 emits a beam of 
laser radiation 20. 
The beam of laser energy 20 is assumed to be collimated to within the 
normal divergence of the laser for which the invention is designed. One 
laser suitable for use is a continuous-wave CO.sub.2 laser emitting at 
10.6 .mu.m, but the invention is not limited to this type of source. Only 
the principal (central) ray of the laser beam is used for illustration in 
FIGS. 1A-1D and 2. 
A rotating reflector 13 splits the radiation into two beams which are 
rotating and which are on paths which do not intersect divergent the axis 
of the blank 11. The rotating beams are reflected from the annular mirrors 
14 and 15 to the conical reflector 25. Conical reflector 25 forms a beam 
of laser radiation in a plane normal to the blank 11. This beam is 
circumferential with substantially zero power density from the axis of 
blank 11 to a prescribed radial position. 
In FIGS. 1B and 1D, the rotating reflector 13 is broken away to show that 
it includes an annular element 21 which has reflective and transmissive 
rings. The rotating reflector 13 also includes a rooftop reflector 22 and 
reflective surfaces 24 and 28 which can be seen in FIGS. 2 and 3. 
FIG. 2 shows the optics of the system without the two annular mirrors 14 
and 15 which permit separation of the laser beam axis and the blank 
drawing axis. In FIG. 2, these two axes appear coincident which is an 
impossible situation but this facilitates understanding of the operation. 
The beam 20 passes through a hole in optical element 21, which is, for 
example, a 4" diameter by 1/4" thick piece of zinc selenide. The beam 
strikes the "rooftop" reflector 22 (shown in section), which directs 
one-half of the beam off-axis at a given angle and directs the other half 
off-axis at a similar but opposite angle. For clarity in this explanation, 
only the portion of the incoming laser beam which is directed upward will 
be considered, but it should be understood that the path to be described 
is duplicated by the lower half of the beam. 
The beam, upon reflection from reflector 22, strikes an annular portion 23 
of element 21 which has been coated with a reflective coating, such as 
aluminum. The reflected beam then strikes a cylindrical reflective surface 
24. This surface is oriented such that the beam is reflected along a path 
which is not parallel to the original axis and, more importantly, can 
never intersect the original axis. This is the means by which a line focus 
at the axis of the conic reflector 25 is avoided. 
The reflective surface 24 provides the desired axial power density 
distribution along the waveguide blank. 
Upon reflection from reflective surface 24, the beam passes through 
transmissive annular portion 26 of element 21. The beam continues along 
this path until it strikes conic reflector 25 (shown in partial section). 
It is directed in the general direction of, but not precisely toward, the 
axis 27 of conic reflector 25. It eventually arrives at some point of 
nearest approach to the axis, which is depicted in FIGS. 2 as the 
termination of the path. Unless absorbed in this region, it continues in 
some unspecified path and is eventually dissipated due to diffuse 
reflections. 
The optical waveguide and the glass blank from which it is drawn are 
arranged to be coaxial with the conic reflector. Since the invention 
precludes rays of laser energy from intersecting this axis, the system can 
be made to generate a cylindrical, or circumferential, zone centered on 
this axis in which negligible energy is present. 
FIG. 3 shows rotating reflector, or spinner, 13 which includes rooftop 
reflector 22, cylindrical reflective surface 24 and an identical 
cylindrical reflective surface 28 located 180.degree. from 24. Rooftop 
reflector 22 has two surfaces which join in a line 29 which intersects the 
axis. 
The rotation of this element at a high speed (typically 10,000 rpm) causes 
the beam path shown in FIG. 2 and its complementary path to be projected 
to all points on the conic reflector 25. Thus, a completely 
circumferential and essentially constant illumination of the glass blank 
is obtained. 
The reflective surfaces 24 and 28 are intended to be the primary influence 
in the transformation of the laser energy. Therefore, the geometry of each 
surface is cylindrical. If reflective surfaces 24 and 28 are circular 
cylinders, an incoming laser beam having a Gaussian power density 
distribution as a function of radius is transformed into a radial power 
density distribution in the vicinity of the conic reflector's axis having 
an essentially Gaussian distribution times some scaling factor determined 
by system geometry, as shown in FIG. 4A. Specifically, the radial distance 
from the axis to the center of the lobes is determined by the orientation 
of cylindrical reflective surfaces 24 and 28 with respect to the system 
axis. 
The power density distribution as a function of radius which is shown in 
FIG. 4B represents a more desirable condition in terms of process 
stability and blank-position insensitivity. This distribution is obtained 
when the surfaces 24 and 28 are hyperbolic cylinders. This uniform power 
distribution produces very efficient heating. 
The axial power density distribution in the vicinity of the conic 
reflector's axis is influenced primarily by the shape of the surfaces of 
the rooftop reflector 22. If, as is generally the case, the incoming beam 
is circularly symmetric with a Gaussian distribution having a diameter d 
at the 1/e.sup.2 intensity points, the rooftop reflector 22 divides this 
beam into two beams each having a half Gaussian profile with a width of 
d/2. This profile is transmitted by the optical elements to the vicinity 
of the axis of conic reflector 25, where it appears as the power density 
distribution as a function of axial position z. A half Gaussian axial 
distribution is shown in FIG. 5. 
The half Gaussian axial distribution appears to be most efficient for the 
drawing of optical waveguide fiber. However, modification of this 
characteristic is possible by the proper specification of non-planar 
surfaces on the rooftop reflector 22. 
The present invention offers means for complete control of the size and 
shape of the axial power density distribution in the vicinity of the glass 
blank. While a half Gaussian distribution has been described, many others 
are possible. The system described in the cited Jaeger and Logan patent 
provides only a Gaussian distribution; its size can apparently be varied 
by certain adjustments and/or redesign. 
The optical element 21 can be fabricated of any stable material which is 
relatively transparent at the laser's emitting wavelength and upon which a 
reflective coating can be applied. Examples are zinc selenide, sodium 
chloride, potassium chloride, and germanium. 
The reflective surface 23 of element 21 can be a thin film of any material 
which is highly reflective at the laser's emitting wavelength. Examples 
are aluminum, silver, gold, and nickel. 
The rooftop reflector 22 and the cylindrical reflecting surfaces 22 and 28 
can be of any material which is highly reflective at the laser's emitting 
wavelength either because of its intrinsic reflectivity or through the 
application of a suitable reflective coating. 
The assembly consisting of the rooftop reflector 22 and the cylindrical 
reflecting surfaces can rotate at any physically realizable speed which is 
sufficiently high to produce an effectively constant blank illumination. 
The reflective surfaces of the rooftop reflector can be planar or any 
cylindrical shape which, by design, will yield some desired axial power 
density distribution. 
The curvature of the reflective surfaces of the cylindrical reflectors can 
be circular, hyperbolic, or any shape which, by design, will yield some 
desired radial power density distribution. 
The orientation of the cylindrical reflecting surfaces 22 and 28 can be of 
any design to yield a desired radial position of the lobes of the radial 
power density distribution. 
The techniques embodied in the present invention are not limited to the 
production of optical waveguide by drawing from a glass blank. They are 
applicable to any process requiring controllable heating of a relatively 
small zone on the surface of a raw-material blank or pre-form, such as 
cutting, heat-treating, etc. 
While a particular embodiment of the invention has been shown and 
described, various modifications are within the true spirit and scope of 
the invention. The appended claims are intended to cover all such 
modifications.