Optical waveguide fiber draw guiding and coating method and apparatus

A method of uniformly coating an optical waveguide fiber 10 comprises the steps of suspending an elongated glassy optical fiber preform 11 in an upright orientation from a preform holder that includes a collet 14 to which an x-y translator 15 is coupled, heating the preform, and drawing fiber from the bottom of the heated preform. The drawn fiber is passed through a tubular coating applicator 32 located beneath the preform, and the position of the fiber is sensed as it is being drawn. In response to sensed deviations in fiber position from along a preselected path that extends through the tubular coating applicator along the applicator axis, the preform is repositioned by horizontal movement of the preform holder so as to return the drawn fiber to the preselected path and thereby maintain its travel through the coating applicator substantially along the applicator axis.

TECHNICAL FIELD 
This invention relates to methods and apparatuses for guiding and coating 
optical fibers as they are drawn during fiber manufacture from optical 
waveguide fiber preforms. 
BACKGROUND OF THE INVENTION 
In the manufacture of optical waveguides a fiber is typically drawn from a 
symmetrically heated, rod-shaped, glassy preform with the fiber yielding a 
faithful replica of the preform in cross section. This is done by feeding 
the preform downwardly into a furnace and drawing the fiber from molten 
material formed at the bottom of the heated preform. The fiber is then 
passed downwardly through a coating applicator where a thin layer of 
coating material is applied to the fiber. This coating serves to prevent 
airborne particles from impinging upon and adhering to the surface of the 
just drawn glass fiber itself which would weaken it. The coating also 
shields the fiber from surface defects inflicted by subsequent 
manufacturing processes and installation handling. The fiber is then 
routed through a curing oven where the coating is cured and then onto a 
takeup reel via a system of sheaves and capstans. For a more detailed 
description of such a fiber drawing process reference may be had to the 
article titled "Drawing Lightguide Fiber" by David H. Smithgall and Daryl 
L. Myres that appears on pages 49-61 of the Winter 1980 issue of THE 
WESTERN ELECTRIC ENGINEER. 
In conducting the just described fiber draw process it is important that 
the fiber be moved precisely along a preselected path. Deviations from 
this path may result in the fiber actually contacting an edge of the 
relatively small opening at the bottom of the furnace, which opening is 
purposely made quite small to minimize the intake of ambient air or gases. 
Furthermore, deviations from the preselected path through the coating 
applicator may well result in the coating not being applied concentrically 
to the fiber. In other words, the coating may be thicker to one side or 
the other. If the coating is badly nonconcentric the fiber may be directly 
exposed to ambient atmosphere; even where the coating is only marginally 
nonconcentric fiber alignment for connectorization becomes poor creating 
significant transmission losses at interconnect points. In addition, where 
a diameter gauge is employed in the draw machine it too cannot operatively 
tolerate substantial misalignment of the fiber. 
The difficulty in maintaining the movement of the fiber along a preselected 
path beneath the glassy fiber preform arises primarily from the fact that 
the point of emergence of the fiber from the necked down molten mass at 
the bottom of the preform tends to wander or drift. A prime reason for 
this drift is the non-straightness of the starting preform. 
Heretofore, little has been done to alleviate the fiber guidance problem 
just described. As a static alignment measure optical fiber preforms, that 
have been manufactured with some degree of arc, have been straightened by 
heating the preform to its softening point locally and mechanically 
straightening the preform with graphite paddles. Such straightening does 
tend to center the point of emergence better as the preform is consumed in 
the drawing process. However, this method may introduce additional flaws 
on the surface of the preform due to contact with the graphite paddles. 
SUMMARY OF THE INVENTION 
In one form of the invention a method of guiding an optical waveguide fiber 
being drawn from an optical waveguide fiber preform comprises the steps of 
sensing an actual position of the fiber, comparing the sensed position 
with a preselected position located along a preselected path of fiber 
travel, and moving the preform in response to deviations in the sensed 
position from the preselected position so as to maintain the fiber 
travelling substantially along the preselected path. 
In another form of the invention a method of uniformly coating an optical 
waveguide fiber comprises the steps of positioning an elongated glassy 
optical fiber preform in an upright orientation, heating the preform, 
drawing fiber from the bottom of the heated preform and passing the drawn 
fiber through a tubular coating applicator located beneath the preform. 
The method further comprises the steps of sensing the position of the 
fiber being drawn from the preform and repositioning the preform in 
response to sensed deviations in fiber position from along a preselected 
path that extends through the tubular coating applicator along the 
applicator axis so as to return the drawn fiber to the preselected path 
and thereby maintain its travel through the coating applicator 
substantially along the applicator axis. 
In another form of the invention apparatus for guiding an optical waveguide 
fiber as it is drawn from a heated optical fiber preform comprises means 
of sensing an actual position of the fiber, means for comparing the sensed 
position with a preselected position located along a preselected path of 
fiber travel, and means for repositioning the preform in response to 
deviations in the sensed position from the preselected position so as to 
maintain the fiber travelling substantially along the preselected path. 
In yet another preferred form of the invention apparatus for uniformly 
coating an optical waveguide fiber as it is drawn from an optical fiber 
preform comprises means for sensing the position of the fiber as it is 
drawn from the preform and tubular applicator means for applying liquid 
coating material to the fiber as it passes therethrough. The apparatus 
further comprises means for repositioning the preform in response to 
sensed deviations in fiber position from along a preselected path that 
extends through the tubular coating applicator along the applicator axis 
so as to return the drawn fiber to the preselected path and thereby 
maintain its travel through the coating applicator substantially along the 
applicator axis.

DETAILED DESCRIPTION 
Referring now in more detail to the drawing, there is shown in FIG. 1 
apparatus being used in drawing an optical waveguide fiber 10 from an 
optical waveguide fiber preform 11. The apparatus comprises an upright 
tower 12 which supports various elements of the drawing apparatus. These 
elements include a collet 14 mounted to the x-slide member of an x-y 
translator 15 shown in greater detail in FIG. 2. The x-y translator 
conventionally comprises a motor 16 having mounted to its output shaft 18 
a pinion 17 in mesh with a rack 19 that extends from a y-slide. The 
translator has another motor 20 mounted to the y-slide itself with its 
output shaft connected to the x-slide slidably supported on the y-slide. 
So constructed, the collect, from which the preform 11 is suspended, may 
be moved about by motors 16 and 20 in a horizontal plane. 
The optical fiber draw apparatus further comprises means for feeding the 
preform downwardly into a zirconia RF induction heating furnace 22. This 
feeding means includes a motor 24 mounted to the tower 12 which rotates a 
screw 25 that slides a carriage 26 upon a pair of rails 28. Power 
transmission between the screw and motor is provided by a pair of pulleys 
30 and a belt 31. Operation of the motor causes the screw 25 to turn and 
thereby lower the carriage 26 to which the x-y translator, collet and 
preform are supported. 
The apparatus further includes a pyrometer 30 used in measuring and 
controlling the temperature of the zirconia furnace 22. An optical fiber 
position detector 31 is mounted to the tower beneath the furnace. The 
detector here is conventionally comprised of two Model 1233 United 
Detector Technology position sensing photodetectors mounted along x and y 
axes beside the fiber and a Uniphase Model 1103 He-Ne laser for emitting a 
beam of light onto the fiber at a 45.degree. angle with respect to each 
axis. Each photodetector generates an electronic signal proportional to 
the position of the fiber along its designated axis. 
The apparatus further includes a coating applicator 32 mounted to another 
x-y positioner 33 supported upon the tower 12 beneath the furnace 22 and 
the fiber position detector 31. An ultraviolet curing oven 35 is mounted 
to the tower beneath the coating applicator 32. A sheave 36 is rotatably 
mounted to the tower beneath the curing oven while a capstan 37 is located 
beside the tower. 
In accordance with the invention the optical fiber position detector 31 is 
coupled by interface electronics with the preform x-y positioner 15. FIG. 
3 illustrates the interface electronics as being conventionally comprised 
of two independent circuits, one for use in maintaining proper x-axis 
positioning of the fiber and one for maintaining proper y-axis positioning 
although a single LM 339 integrated circuit (IC) is used for electronic 
comparisons. The fiber position detector's photodetector for the x-axis is 
connected with pins 6 and 5 of the IC while its photodetector for the 
y-axis is connected with pins 8 and 11. Pins 7 and 9 are connected with a 
+15 VDC power supply through fixed and variable resistors while pins 4 and 
10 are connected with a -15 VDC power supply through serially connected 
fixed and variable resistors. The outputs of the four comparators 40, 41, 
44 and 45 are coupled with the -x, +x, -y and +y inputs of the drivers of 
motors 16 and 20, respectively as shown. 
In operation the optical fiber preform 11 is slowly fed into the furnace 22 
where it is heated so as to cause the bottom portion of the preform to 
become molten from which fiber is drawn under some 20 gms of tension. The 
fiber 10 is drawn down through the bottom of the furnace past the position 
sensing photodetector and then through the coating applicator 32 where it 
is coated with a thin film, e.g. 80 microns radially thick, with a uv 
curable ethyl acrylate coating material. After the fiber passes through 
the bottom of the coating applicator it is directed through the curing 
oven 35 which cures the coating. The fiber is then routed about the sheave 
36 and drawn by capstan 37 to an unshown takeup spool. 
As the fiber passes through the position sensing photodetector bipolar 
signals are emitted from each of its photodetectors to the interface 
electronics. As shown in FIG. 3 the output from the x-axis sensor is fed 
to pin 6 of the LM339 integrated circuit here functioning as analog 
comparator 40. Pin 7 of this comparator is connected to a 100 ohm 
potentiometer that is connected in series with 150K ohm resistor between 
ground and a +15 VDC power supply. The output from the x-sensor is also 
connected to pin 5 of the IC integrated circuit, here functioning as 
analog comparator 41, whose pin 4 is similarly connected to a 100 ohm 
potentiometer that is connected in series with the 150K ohm resistor 
between ground and -15 VDC power supply. The potentiometers here establish 
dead zones to provide a small range of plus or minus signal values that 
are ignored. Should, for example, a minus voltage exists between pins 6 
and 7 the comparator outputs a signal from pin 1 causing motor 20 to drive 
the x-slide in a direction opposite that from the fiber off axis position 
detected by the photodetector. This continues until the preform is moved 
in this x-direction sufficiently to bring the fiber back into the dead 
zone whereupon the comparator ceases to have a minus voltage across pins 6 
and 7 and switches its output signal to the drive-inactive state. 
Conversely, should a plus voltage be outputted from the x-sensor the 
comparator 41 drives the x-slide in the opposite direction until the fiber 
is returned sufficiently close to its preselected path as to cause the 
electronics to enter the dead zone value. Similarly, for the y-sensor the 
comparator 44 serves to drive motor 16 in one direction in moving the 
y-slide while the comparator 45 moves the motor 16 in the opposite 
direction. 
The above action serves to maintain the fiber 10 moving substantially along 
the preselected linear path extending beneath the furnace 22 through the 
coating applicator and to the pulley 36. As the point of the fiber 
emergence from the preform drifts about the detector 31 detects the 
corresponding movement of the fiber off of its preselected linear path. 
Once it moves beyond a selected threshold distance, as established 
electronically by the dead zones, the x-y translator repositions the 
preform sufficiently to bring the fiber back to its preselected path. In 
this manner the fiber is maintained travelling substantially along its 
preselected axis and the linear path extending between the periphery of 
the pulley 36 and the preform through the coating applicator. 
It should be understood that the just described embodiment merely 
illustrates principles of the invention in one preferred form. Many 
modifications, additions and deletions may, of course, be made thereto 
without departure from the spirit and scope of the invention as set forth 
in the following claims.