Note: Descriptions are shown in the official language in which they were submitted.
CA 02301033 2000-02-15
WO 99/15470 PCT/US98118785
1
DRAW CONSTANT DOWNFEED PROCESS
FIELD OF THE INVENTION
The present invention relates to optical waveguide fibers, and more
particularly, to methods for drawing an optical fiber from an optical fiber
perform whereby the fiber exhibits a more uniform mode filed diameter (MFD)
and reduced polarization mode dispersion (PMD).
BACKGROUND OF THE INVENTION
In the manufacture of optical fiber, a glass core preform is made which
typically comprises SiOz , the axial portion of which is doped with a compound
such as Ge02 to increase the refractive index. When a fiber is drawn from the
glass preform, the doped region will provide the light transmission portion or
core of the fiber.
The above described process is well known in the art and will not be
described in further detail. To obtain optical fiber the glass preform or
blank is
fed into a draw furnace heated to a melting temperature, and a small gob of
glass, with a trailing fiber,
CA 02301033 2000-02-15
WO 99/15470 PCT/US9$/187$5
2
drops from the blank root. The fiber is fed to a tractor and capstan assembly
which draws the fiber from the blank and the fiber is wound on a spool.
As fiber is drawn from a blank, the blank is fed into the furnace, and
fiber diameter is closely monitored. Control of fiber diameter is generally
accomplished by varying certain operating parameters at the draw tower.
Typically, a fiber diameter measuring device is located just below the furnace
outlet to measure the fiber diameter. The measured diameter is compared to a
nominal diameter value and a signal is generated to either increase the
tractor
speed (thus decreasing the fiber diameter), or decrease the tractor speed
(thus increasing the fiber diameter. )
In the 1970's and throughout the mid 1980's, blanks from which fiber
was drawn were relatively small. Draw speeds did not exceed about 8 or 9
meters per second. Because of the blank size and draw speeds used, fiber
diameter was controlled by varying the tractor speed while maintaining the
furnace temperature and blank feed rate relatively constant.
In the mid-1980's a new process control strategy was developed and
introduced as a result of ever increasing draw speeds. Specifically, as draw
speeds approached 10 meters/sec., those skilled in the art abanddned the use
of constant downfeed rates. More specifically, it was believed that in order
to
achieve adequate control at high draw speeds, i.e. speeds approaching and in
excess of 10 meters/sec., it was necessary to resort to a cascade or two-level
process control strategy whereby, in response to an error signal indicating
that
the actual or measured fiber diameter was not equal to the desired diameter,
there would be both a change in the draw speed and a change in the
downfeed rate of the blank into the draw furnace. For example, if the
measured fiber diameter was greater than the desired fiber diameter, then the
control system would increase the tractor speed and, at the same time,
decrease the rate at which the blank was fed into the draw furnace. This
control philos4phy reflected the belief that when operating a fiber draw
process
at a speed greater than 8-9 meterslsec., it was necessary to vary the blank
CA 02301033 2000-02-15
WO 99/15474 PCTNS98/18785
3
downfeed rate when draw speed was varied to maintain a more constant fiber
diameter.
Although this two level control process results in an essentially constant
fiber diameter it has been discovered that other detriments! effects occur as
a
result of this operation. It is believed that oscillations in the draw control
loop,
specifically, oscillations in the blank downfeed rate, can cause variations in
the
core shape during fiber formation. This may be particularly acute at the blank
root from which fiber is drawn. it is believed that oscillations of the blank
root
in the furnace may affect the shape of the core as it is formed at the root of
the
blank, and this is believed to cause poor PMD and nonuniform MFD.
For optical fiber that will be used in telecommunication applications
PMD should be as small as possible, and MFD should be maintained as
uniform as possible. Several solutions have been proposed to address some
of the problems mentioned above. For instance, commonly assigned and co-
pending U.S. Patent Applications Nos. 081858,836 and 081784,574, and PCT
Application No. PCT/US97I02541 disclose various methods and apparatus for
imparting spin to the fiber as it is drawn to reduce PMD. Spinning optical
fiber
as it is drawn causes internal geometric andlor stress asymmetries of the
fiber
to rotate about the fibers axis along the length of the axis; however,
spinning
the fiber does not address the underlying problems in the glass that cause
PMD, nor does spinning entirely eliminate PMD or address the issue of MFD
uniformity.
In view of the disadvantages in the art, it would be desirable to provide
a method for maintaining or increasing MFD uniformity while at the same time
reducing PMD. There is an explicit need for such when drawing optical fiber at
high draw rates, i.e. greater than 10 meterslsecond, which may contribute to
increased downfeed oscillation in the root, there by increasing PMD in the
fiber.
*rB
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
4
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method for the high
speed drawing of optical fiber that alleviates one or more of the problems due
5. to limitations and disadvantages of the related prior art. The principal
advantage of the present invention is the provision of a method for
controlling
the diameter of a drawn optical fiber while reducing PMD in the fiber and
maintaining uniform MFD when drawing the fiber at high speed. The method
comprises drawing fiber at a high speed while keeping the blank downfeed rate
constant. It is believed that constant downfeed rate avoids oscillation of the
blank root in the furnace which causes variability in the core shape during
fiber
formation. Such variations are believed to contribute to poor PMD and MFD in
the final fiber.
To achieve these and other advantages and in accordance with the
purpose of the invention, as embodied and broadly described, the invention is
a method for reducing polarization mode dispersion in drawn optical fiber
comprising the steps of feeding an optical fiber preform of a predetermined
size into a furnace at a predetermined downfeed rate, drawing an optical fiber
from the optical fiber preform at a draw rate of at least 10 meters per
second.
and varying the draw rate to maintain a substantially constant fiber diameter
while maintaining the predetermined downfeed rate constant. Preferably, the
draw rate is greater than 14 meters per second and most preferably, greater
than 20 meters per second.
In a preferred embodiment, the downfeed rate is constant for a first
zone or range of draw speeds and is then changed to a different constant
downfeed rate for a second zone or range of draw speeds. As the draw speed
varies in each zone, the downfeed rate remains constant within each zone. In
addition, the downfeed rate may be different for each zone. The method may
also include the step of decreasing the downfeed rate as the draw rate
changes from one zone to another having a higher rate of draw s~~ds, or
increasing the downfeed r~fi~ as the draw rate changes from one tn another
*rB
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
S
having a lower range of draw speeds. The invention may also include the
step of spinning the fiber as it is being drawn to further reduce PMD.
In accordance with another embodiment of the invention, a method for
drawing optical fiber from an optical fiber preform is provided, comprising
the
steps of feeding the optical fiber preform of a predetermined size into a draw
furnace at a constant downfeed rate and drawing optical fiber from the optical
fiber preform at a draw rate of at least 10 meters per second. The method
further comprises the steps of measuring the drawn fiber diameter and
generating a signal representative of the measured diameter and comparing
the generated signal to a nominal fiber diameter. A second signal
representative of the difference of the comparison is generated and used to
vary the draw rate to adjust the drawn fiber diameters. The method also
includes the step of sensing the draw rate to determine if it is within a zone
of
predetermined speeds and changing the downfeed rate to another
predetermined rate if the sensed draw rate is outside of the zone. The
downfeed rate is constant for a first zone or range of draw speeds and is then
changed to a different constant downfeed rate for a second zone or range of
draw speeds. Preferably, the downfeed rate is maintained constant within
each zone and as the draw rate is varied between the plurality of zones, the
downfeed rate is change accordingly. The method according to this
embodiment may include the further step of spinning the optical fiber as it is
drawn.
It is to be understood that both the foregoing general description and
the following detailed description are exemplary and explanatory and are
intended to provide further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of a fiber drawing apparatus.
DETAILED DESCRIPTION
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
6
The present invention is directed to method for reducing polarization
mode dispersion in drawn optical fiber wherein an optical fiber preform of a
predetermined size is fed into a furnace at a predetermined downfeed rate.
Preferably, the downfeed rate is kept constant throughout the entire draw
process in order to minimize oscillation of the preform root in the furnace in
order to maintain MFD uniformity and reduce PMD in the drawn optical fiber.
Fig. 1 illustrates a well known optical fiber draw system, designated
generally by reference numeral 1. Preform 10 disposed vertically in muffle 11
of a draw furnace. Preform 10 includes a handle (not shown} that attaches to
a holding device (not shown) in a known manner. The holding device is part of
preform feed drive 22, which controls the rate at which preform 10 is fed into
the furnace. Heating element 12 supplies heat to at least the bottom portion
of preform 10. The temperature of heating element 12 is controlled by
temperature controller 49 in a known manner. After a well known start up
procedure is employed, preform feed drive 22 feeds preform 10 into the
furnace. As preform 10 is fed into the furnace, the end portion of preform 10,
commonly referred to as the root, melts and fiber 14 is drawn from root
portion
13 of perform 10 by tractor 20.
After leaving muffle 11, fiber 14 passes through diameter monitor 15
which produces a signal that is used in a feedback control loop to regulate
the
speed of tractor 20 and preform feed drive 22, as well as to regulate
temperature in the furnace through temperature controller 49. After diameter
monitor 15, fiber 14 passes through a cooling tube 17 and a coater 18 by
which a curable protective coating is applied to fiber 14. The coated fiber
may
also pass through a coating curing apparatus and if desired additional waters
(not shown). The feedback control of perform feed drive 22, tractor drive 21
and temperature controller 49 can be implemented by known control
algorithms. Tractor drive 21 is provided with an input from control algorithm
48
which is part of draw control computer 47. Given the demand for optical fiber,
it is advantageous to run tractor 20 at a rate of at least 10 meters per
second.
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
7
Preferably, tractor 20 produces a draw speed of greater than 14
meterslsecond, and more preferably greater than 20 meters per second.
The present invention is directed to a method for reducing polarization
mode dispersion in drawn optical fiber comprising the step of feeding a glass
preform and drawing an optical fiber at a speed greater than 10
meterslsecond. The size of prefonn 10 can be measured by weight or by its
diameter. The downfeed rate of perform 10 is selected based on the size of
perform 10. Preferably, the downfeed rate, once selected, remains constant
throughout the fiber drawing process. Alternatively, the downfeed rate may
remain constant within a predetermined zone or range of draw speeds. There
may be any number of zones of draw speeds and the range of draw speeds
within each zone may also vary. However, each zone has a predetermined
downfeed rate associated with it and the downfeed rate remains constant
within the given zone.
If the draw speed, which is controlled through tractor drive 21, increases
or decreases out of a specific zone of draw speed, a signal is sent from
control
algorithm 48 to preform feed drive 22 to change the downfeed rate to the
appropriate downfeed rate for the particular zone of draw speed. Control
algorithm 48 is set up so that as the tractor speed changes from one zone to
another, the downfeed rate changes by small increments until the
predetermined downfeed rate is reached. This allows the downfeed rate to
adjust back to the original rate quickly if the tractor speed were to suddenly
return to the original zone.
According to another aspect of the invention, the method may comprise
the further steps of sensing the draw rate to determine if it is within a zone
of
predetermined speed and varying the downfeed rate if the sensed draw rate is
outside of the zone. In this embodiment, a draw rate sensor (not shown)
continually monitors draw rate at draw control computer 47. If the draw speed
changes from one zone to another, control algorithm 48 sends a signal to
preform feed drive 22 to increase or decrease the downfeed rate to the
predetermined constant rate associate with the zone of draw speed.
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
8
The present inventive method also includes the step of varying the draw
rate in response to the measured fiber diameter to maintain a substantially
constant frber diameter while maintaining the predetermined downfeed rate
constant. In order to maintain a constant fiber diameter, fiber 14 is
constantly
monitored by diameter monitor 15. Diameter monitor 15 produces a signal
representative of the measured fiber diameter. That signal is sent to draw
control computer 47. At draw computer 47, the measured signal is compared
to a predetermined nominal fiber diameter value. A second signal is generated
based on any difference between the measured fiber diameter value. The
second signal sent to the tractor drive 21 and the tractor speed is varied to
maintain a constant fiber diameter. This process is carried out hundreds of
times per minute and the downfeed rate remains constant throughout the draw
process during all ranges of tractor speed.
It may also be advantageous to spin the fiber as it is drawn. Spin in
fiber has been demonstrated to further reduce PMD. Various methods and
apparatus have been developed to impart spin in a fiber as it is drawn.
Reference is made to commonly assigned and co-pending U. S. Patent
Applications Nos. 081858,836 and 081784,574 and PCT application no.
PCTIUS97102541; and U.S. Patent No. 5,298,047, far a more detailed
understanding of methods and apparatus used for spinning fiber, each of
which is herein incorporated by reference.
The advantages associated with the invention are numerous. In the
prior art draw systems, fiber diameter is controlled by tractor speed. The
control loop involves a two step process control at the draw . If the tractor
speed varies, the downfeed rate responds to variation in tractor speed.
Although not wanting to be bound by any theory or explanation as to why the
present invention functions, we believe that this in turn produces an
oscillation
of root 13 in the furnace. It is believed that oscillation of the root portion
of
preform 10 in the furnace causes variability in the core shape of the draw
optical fiber and that the variations in core shape lead to higher PMD and
nonuniformity in MFD, both of which adversely affect fiber performance.
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
9
The present invention helps to reduce preform oscillation by providing a
constant down feed rate during the draw process. Contrary to the well-
recognized two-step control approach to drawing frber at high rates of speed,
control algorithm 48 is set up to maintain the preform downfeed rate constant
even as the tractor speed varies to maintain fiber diameter. It is believed
that
this control mechanism reduces or perhaps eliminates oscillations in the draw
control loop that can cause variations in the core shape during fiber
formation,
and results in reduced PMD and improves MFD uniformity.
EXAMPLES
The invention will be further described by the following examples, which
are intended to be exemplary of the invention.
EXAMPLE 1
An unspun optical fiber was produced using a draw system similar to
that illustrated in Fig. 1. The tractor speed was allowed to vary up to a
maximum of 19 meters per second to maintain a constant fiber diameter, while
the downfeed rate was kept constant at about 2.75 millimeters per minute. The
resulting frber was tested for PMD and MFD uniformity. The results as
compared to a fiber drawn under a standard process (i.e. variable dowr~feed
rate), are shown in Table 1 below:
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
Tabie 1
CONSTANT
DOWNFEED
RATE
REDUCTION
PMD 71 %
~IMPROVEMENT
MFD 83%
UNIFORMITY
As the results indicate, PMD was significantly reduced and MFD was
improved in the draw process according to the present invention compared to
5 a standard process.
EXAMPLE 2
A fiber was drawn using an apparatus similar to that depicted in Fig. 1
The fiber was also spun during the draw process. The downfeed rates were
10 set according to the zone embodiment of the present invention as describe
above to achieve a 15.5 meters per second nominal draw speed. The drawn
fiber was tested and the results of PMD and MFD uniformity were compared to
a fiber drawn using a standard draw process. Several different runs were
undertaken and the results are shown in Table 2 below.
Table 2
CONSTANT
DOWNFEED
RATE
r6 REDUCTION
PMD 80%
~6 IMPROVEMENT
MFD 76r6
UNIFORMITY
As Table 2 shows, there is a significant reduction in PMD in fibers
drawn according to the present invention as compared to the fibers drawn
CA 02301033 2000-02-15
WO 99/15470 PCT/US98/18785
11
according to a standard process. MFD uniformity is also significantly
improved.
It will be apparent to those skilled in the art that various modifications
and variations can be made in the method of the present invention which are
nevertheless within the scope of the appended claims and their equivalents.
