Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-07-27
2003-02-11
Epps, Georgia (Department: 2873)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S127000, C427S163100
Reexamination Certificate
active
06519404
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for fabricating a coated optical fiber while reducing polarization mode dispersion, and to a coated optical fiber fabricated thereby.
BACKGROUND ART
In a conventional fabrication method of fiber drawing by heating to soften one end of an optical fiber preform and pulling a glass fiber downward therefrom, it was difficult to make the glass fiber with a core portion and a cladding portion around it in perfectly circular and concentric cross section, and their cross section was usually slightly elliptic or of slightly distorted circle shape. For this reason, the refractive index distribution in the cross section of the glass fiber wan not perfectly uniform and this caused the difference between group velocities of two polarized waves in the cross section of the glass fiber, the problem of increase in polarization mode dispersion has occurred.
Large polarization mode dispersion would raise a problem, particularly, when an optical fiber cable is used in practice as a submarine cable or a main cable required performance of which is large-capacity and long-haul transmission. A solution to this problem of polarization mode dispersion is a fabrication method of coated optical fiber, e.g., as disclosed in Japanese Patent Application Laid-open No. H9-243833, in which the glass fiber is drawn and coated to obtain a coated optical fiber and the coated optical fiber is guided by a guide roller periodically swinging the direction of its rotation axis, thereby imparting predetermined twists to the glass fiber.
FIG. 3
shows the fabrication steps of this method. An optical fiber preform
11
placed in a drawing furnace
12
is heated to soften at one end thereof and a glass fiber
13
is pulled vertically downward therefrom under drawing tension of a take-up unit
26
described hereinafter. At this time, the outside diameter of the glass fiber
13
is measured by outside diameter gauge
14
and a controller not shown controls the drawing speed, feed speed of the optical fiber preform, etc. so as to keep the fiber diameter in a prescribed range.
A coating die
15
applies an ultraviolet (UV)-curable resin
16
onto the periphery of glass fiber
13
and a UV emitting unit
17
emits UV light to cure the resin
16
, thereby forming a coating. A coating die
18
further applies a UV-curable resin
19
onto the periphery of the coating and an UV emitting unit
20
emits UV light to cure this resin
19
, thereby forming a second coating and obtaining a coated optical fiber
21
. After that, the coated optical fiber
21
is guided via guide rollers
22
, a swing guide roller
23
, guide rollers
24
,
25
, and the take-up unit
26
to be wound onto a winding-up reel
27
.
The following will explain the principle of imparting the twists to the glass fiber by the swing guide roller.
FIG. 4
is a plan view of the swing guide roller. A roller rotation axis
23
b
of the swing guide roller
23
is always within the horizontal plane even during swinging and swings in a clockwise and a counterclockwise motion of a fixed period within an angular range of ±&thgr; with respect to a reference position about a vertical axis
23
c
passing the center of the swing guide roller
23
.
Accordingly, when the swing guide roller
23
swings from the reference position in the direction A in the figure, it goes into a state of swing guide roller
23
′; when swinging in the opposite direction B, it goes into a state symmetric therewith with respect to the reference position, though not shown. As a result, when the swing guide roller is at the reference position, the coated optical fiber
21
descending from above is in contact with a roller surface
23
a
at a point Pa and travels along the roller surface so that the traveling direction thereof is bent from the vertical direction to the horizontal direction. Then, the coated optical fiber
21
travels in the direction of arrow C.
When the swing guide roller swings to move to the position of
23
′, if the coated optical fiber
21
did not move on the roller surface
23
a
, the coated optical fiber
21
would first contact the roller surface
23
a
at a position of point Qa. However, since the coated optical fiber
21
is held under tension, the coated optical fiber
21
moves on the roller surface
23
a
to take the shortest course. This causes the first contact position of the coated optical fiber
21
with the roller surface
23
a
to move to a position of point Qb on the roller surface
23
a
. At this time, since friction acts between the coated optical fiber
21
and the roller surface
23
a
, the coated optical fiber
21
does not slide to move on the roller surface
23
a
, but the coated optical fiber
21
moves on the roller surface
23
a
while rolling about its axis. Namely, there appears the rolling motion of the coated optical fiber
21
about its axis.
When the coated optical fiber
21
rolls about its axis at the first contact position thereof with the swing guide roller
23
, its rolling force is transmitted immediately above along the coated optical fiber
21
up to the softening position in the lower part of the optical fiber preform
11
from which the glass fiber is being drawn. Since the glass fiber
13
under drawing at the softening position in the lower part of the optical fiber preform
11
is still in a softening state and thus soft, the rolling force transmitted through the coated optical fiber
21
directly acts on the glass fiber
13
in the softening state at the tip of the optical fiber preform
11
, so that a twist is imparted to the drawn glass fiber
13
in the softening portion at the tip of the optical fiber preform
11
. Then the coatings are provided on the glass fiber
13
to make the coated optical fiber
21
.
DISCLOSURE OF THE INVENTION
Since the optical fiber on the optical fiber preform side with respect to the swing guide roller works to transmit the rolling force generated by the rolling about the axis of the coated optical fiber at the position of the swing guide roller, mainly to the drawing portion of the glass fiber, the optical fiber itself is little subject to twisting strain between the tip of the optical fiber preform and the swing guide roller. However, the coated optical fiber
21
is twisted between the swing guide roller
23
and the guide roller
24
because of the rolling about the axis at the position of the swing guide roller
23
. This twist reverses the twist direction according to inversion of the swing direction of the swing guide roller, and thus twists can be averaged in the longitudinal direction to cancel out each other. However, if the cancellation of twists in the longitudinal direction is insufficient because of existence of the guide roller and others, the residual twists will be stored in the coated optical fiber itself and move along with the travel of the coated optical fiber via the take-up unit
26
up to the winding-up reel
27
.
The twists stored in the coated optical fiber are elastic torsion and thus internal stress always acts in directions to return the twists. Therefore, there will arise the problem that during a subsequent step of feeding the coated optical fiber out of the winding-up reel
27
, portions of the coated optical fiber twine round each other to cause a groove state or in the worst case the coated optical fiber is forcibly drawn from the twining portions of the coated optical fiber to cause disconnection. This problem becomes more noticeable, particularly, with increase in the drawing speed in fabrication of the coated optical fiber.
The present invention provides a fabrication method and fabrication apparatus of coated optical fiber capable of relieving the elastic torsion remaining in the coated optical fiber to an unproblematic level even with increase in the drawing speed in fabrication, and also provides a coated optical fiber fabricated thereby.
The inventor had the idea that, in order to adequately cancel out the elastic torsion of the coated optical fiber in the longitudinal direc
Koida Masao
Nagayama Katsuya
Tsuneishi Katsuyuki
Yoshida Motohide
Dinh Jack
Epps Georgia
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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