U.S. patent application number 11/672836 was filed with the patent office on 2007-06-07 for methods for modifying ovality of optical fiber preforms.
This patent application is currently assigned to FITEL USA CORP.. Invention is credited to James William Fleming, Paul Francis Glodis, Siu-Ping Hong, David Kalish, Thomas John Miller, Shunhe Xiong, Zhi Zhou.
Application Number | 20070125127 11/672836 |
Document ID | / |
Family ID | 32990522 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070125127 |
Kind Code |
A1 |
Fleming; James William ; et
al. |
June 7, 2007 |
METHODS FOR MODIFYING OVALITY OF OPTICAL FIBER PREFORMS
Abstract
Methods for modifying preform core ovality during and subsequent
to the formation of an optical fiber preform. After MCVD deposition
forms the core rod, but prior to overcladding of the core rod, the
code rod may be etched to change its ovality. In order to etch the
core rod, the core rod may be mounted to lathe, rotated by at least
two rotors, and subjected to a heat source. Additionally, one of
the at least two rotors may be phase-shifted from another one of
the at least two rotors after the core rod is mounted on the
lathe.
Inventors: |
Fleming; James William;
(Westfield, NJ) ; Hong; Siu-Ping; (Alpharetta,
GA) ; Glodis; Paul Francis; (Atlanta, GA) ;
Miller; Thomas John; (Alpharetta, GA) ; Zhou;
Zhi; (Lawrenceville, GA) ; Kalish; David;
(Roswell, GA) ; Xiong; Shunhe; (Alpharetta,
GA) |
Correspondence
Address: |
SUTHERLAND ASBILL & BRENNAN LLP
999 PEACHTREE STREET, N.E.
ATLANTA
GA
30309
US
|
Assignee: |
FITEL USA CORP.
Norcross
GA
|
Family ID: |
32990522 |
Appl. No.: |
11/672836 |
Filed: |
February 8, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10430779 |
May 5, 2003 |
|
|
|
11672836 |
Feb 8, 2007 |
|
|
|
Current U.S.
Class: |
65/377 ; 65/391;
65/429 |
Current CPC
Class: |
C03B 2205/72 20130101;
C03B 37/01225 20130101; C03B 37/027 20130101; C03B 2203/36
20130101; C03B 37/01228 20130101; C03B 2205/40 20130101; C03B
37/02745 20130101; C03B 37/01861 20130101 |
Class at
Publication: |
065/377 ;
065/391; 065/429 |
International
Class: |
C03B 37/07 20060101
C03B037/07; C03B 37/018 20060101 C03B037/018; C03B 37/01 20060101
C03B037/01 |
Claims
1. A method of modifying an ovality of an optical preform,
comprising: providing a core rod, wherein said core rod comprises a
core and a cladding layer, and wherein said cladding layer has a
non-uniform thickness; mounting said core rod on a lathe having at
least two rotors; rotating said core rod using said at least two
rotors; and etching said core rod to alter the geometry of the
cladding layer such that optical preform ovality is modified;
wherein one of said at least two rotors is phase-shifted from
another one of said at least two rotors after said core rod is
mounted on said lathe.
2. The method of claim 1, wherein the etching of said core rod
comprises etching said core rod using heating means.
3. The method of claim 2, wherein said heating means comprises a
plasma torch.
4. The method of claim 3, wherein etching said core rod comprises
heating the cladding layer of said core rod with the plasma torch;
and wherein an amount of glass removed from a portion of said
cladding layer is based at least in part on an amount of time that
said portion is heated by said plasma torch.
5. The method of claim 1, wherein etching said core rod comprises
etching said core rod where the cladding layer is substantially
oval in cross-section.
6. The method of claim 1, wherein etching said core rod produces a
core rod having less than 5% core ovality.
7. The method of claim 1, wherein etching said core rod produces a
core rod suitable for producing optical fiber having less than 0.1
ps/sqrt (km) of polarization mode dispersion.
8. The method of claim 1, further comprising scanning the core rod
prior to etching said core rod to measure the ovality of said core
rod.
9. The method of claim 8, wherein scanning the core rod comprises
scanning the cladding layer to measure the ovality of said cladding
layer.
10. The method of claim 8 wherein scanning said core rod comprises
an off-line scanning of said core rod.
11. The method of claim 8, wherein scanning said core rod comprises
an on-line scanning of said core rod.
12. The method of claim 1, wherein rotating said core rod comprises
rotating said core rod as said core rod is etched.
13. The method of claim 1, wherein rotating said core rod comprises
rotating said core rod in clockwise and counterclockwise
directions.
14. The method of claim 13, wherein the phase-shift between said at
least two rotors is alternated between clockwise and
counterclockwise directions.
15. The method of claim 1, wherein the angular velocity of said
rotors is synchronized.
16. The method of claim 1, wherein the phase-shift between said at
least two rotors is based at least in part on the ovality of the
optical preform.
17. The method of claim 16, wherein the phase-shift between said at
least two rotors is calculated according to the equation
.DELTA..THETA.=K*O.sub.v*sin(.OMEGA.t), where .DELTA..THETA. is the
amount of phase shift in degrees, O.sub.v is the ovality measured
as a percentage as a function of position, K is a conversion
constant, and .OMEGA. is a frequency at which the core rod is
rotating.
18. A method of modifying an ovality of an optical preform,
comprising: providing a core rod, wherein said core rod comprises a
core and a cladding layer, and wherein said cladding layer has a
non-uniform thickness; mounting said core rod on a lathe having at
least two rotors; rotating said core rod using said at least two
rotors; and etching said core rod to alter the geometry of the
cladding layer such that optical preform ovality is modified;
wherein one of said at least two rotors is phase-shifted from
another one of said at least two rotors after said core rod is
mounted on said lathe; and wherein the phase-shift between said at
least two rotors is based at least in part on the ovality of the
optical preform.
19. The method of claim 18, wherein the phase-shift between said at
least two rotors is calculated according to the equation
.DELTA..THETA.=K*O.sub.v*sin(.OMEGA.t), where .DELTA..THETA. is the
amount of phase shift in degrees, O.sub.v is the ovality measured
as a percentage as a function of position, K is a conversion
constant, and .OMEGA. is a frequency at which the core rod is
rotating.
20. A method of modifying an ovality of an optical preform,
comprising: providing a core rod, wherein said core rod comprises a
core and a cladding layer, and wherein said cladding layer has a
non-uniform thickness; mounting said core rod on a lathe having at
least two rotors; rotating said core rod using said at least two
rotors; and etching said core rod to alter the geometry of the
cladding layer such that optical preform ovality is modified;
wherein one of said at least two rotors is phase-shifted from
another one of said at least two rotors after said core rod is
mounted on said lathe; and wherein the phase shift between said at
least two rotors is calculated according to the equation
.DELTA..THETA.=K*O.sub.v*sin(.OMEGA.t), where .DELTA..THETA. is the
amount of phase shift in degrees, O.sub.v is the ovality measured
as a percentage as a function of position, K is a conversion
constant, and .OMEGA. is a frequency at which the core rod is
rotating.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of co-pending U.S.
application Ser. No. 10/430,779, filed May 5, 2003, entitled
"Methods for Modifying Ovality of Optical Fiber Preforms," the
disclosure of which is entirely incorporated by reference
herein.
FIELD OF THE INVENTION
[0002] The present invention relates generally to optical fiber
fabrication, and more specifically, to methods for reducing the
ovality of optical fiber preforms used in manufacturing optical
fiber.
BACKGROUND OF THE INVENTION
[0003] Communications and data transmission systems that transmit
information signals in the form of optical pulses over optical
fiber are now commonplace, and optical fibers have become the
physical transport medium of choice in long distance telephone,
data and video communication networks due to their signal
transmission capabilities, which greatly exceed those of mechanical
conductors. Despite their advantages, however, difficulties in
their manufacture must be overcome in order for lengthy, high-yield
and error-free optical fiber to be produced in mass.
[0004] The manufacture of optical fiber utilizes a glass preform
from which optical fiber is generated. The glass preform reproduces
the desired index profile of the optical fiber in a thick glass
rod. After a preform is created, it is loaded into a fiber drawing
tower. The lower end of the preform is lowered into a furnace so
that the end of the preform is softened until a softened glob falls
don by gravity. As it falls, it forms a thread. The thread cools as
it falls, and undergoes a series of processing steps (e.g.,
application of coating, layers) to form the finished optical fiber.
Therefore, it will be appreciated that the make-up and length of
optical fiber generated by this process is dependent upon the
characteristics of the preform from which the optical fiber is
drawn.
[0005] The basic manufacturing steps of generating preforms are
well known to those of skill in the art. Three basic forms for the
production of preforms include: Internal Deposition, where material
is grown inside a tube; Outside Deposition, where deposition is
done on a mandrel removed in a later stage; and Axial Deposition,
where deposition is done axially, directly on the glass preform.
One of the most common and widely-used processes in optical fiber
preform production is Modified Chemical Vapor Deposition (MCVD),
which is a type of Internal Deposition. MCVD is a process for
fabricating preforms wherein preform core material is deposited on
the inside surface of a substrate or starting tube (`substrate
tube` and `starting tube` are used interchangeably herein).
Individual layers of deposited material are turned into glass
(vitrified) by a torch that moves back and forth along the length
of the tube. During a deposition process the torch assembly slowly
traverses the length of the starting tube while reactant gasses are
fed into and exhausted from the tube. Following the deposition of
core material and/or cladding material, the starting tube is
collapsed to form a solid core rod by heating it to a higher
temperature than during deposition. After the core rod is
generated, during an overcladding process material such as silica
is added to increase the diameter of the core rod. After
overcladding, the optical fiber perform is complete and ready to be
drawn into optical fiber.
[0006] Although the generation of preforms by the method described
above are commonly utilized in optical fiber manufacturing,
preforms generated by this process often suffer from ovality; that
is, the preforms do not necessarily have a circular cross section
throughout their entire length. Preform ovality is undesirable
because it changes and more often increases the Polarization Mode
Dispersion (PMD) of optical fiber. PMD is a stochastic phenomenon
that leads to the dispersion of the optical pulses transmitted in
an optical fiber. In particular, the dispersion is caused by the
propagation speed difference between the polarization modes of the
fiber. PMD limits the transmission capacity of optical
communication systems by creating inter-symbol interference.
Because low PMD is a desirable characteristic of optical fiber,
reducing preform ovality is a crucial factor in achieving desirable
transmission characteristics of optical fiber.
[0007] Therefore, what is needed is a method for achieving desired
preform core ovality to reduce the PMD of an optical fiber
generated there from.
BRIEF SUMMARY OF THE INVENTION
[0008] The present invention overcomes the disadvantages of the
prior an by providing methods for reducing preform core ovality
during and subsequent to the creation of an optical fiber preform.
According to one embodiment of the present invention, prior to MCVD
deposition on a starting tube, the outer diameter of the starting
tube is altered by etching or a like process to reduce its ovality.
According to another embodiment of the invention, after MCVD
deposition forms the core rod, but prior to overcladding the core
rod, the core rod may be etched, heated and rotated to reduce its
ovality. Both methods may be used independently or in combination
and are advantageous in that they reduce the PMD of optical fiber
drawn from the optical fiber preform. According to a third
embodiment of the present invention, the cladding material of a
core rod having an oval or elliptical core may be etched to mirror
the shape of the oval core. The preform generated there from may
then be placed under a surface tension, or pulled in a manner to
generate a circular optical fiber having low ovality and low
PMD.
[0009] According to one embodiment of the present invention there
is a disclosed method of reducing the ovality of an optical
preform. The method includes the step of providing a starting tube
having a wall, the wall having an exterior surface and an interior
surface and wherein the interior surface defines a hollow region of
the starting tube. The method also includes the step of etching the
wall until the exterior of the wall includes a substantially
circular cross section.
[0010] According to one aspect of the invention, etching the wall
includes etching the wall using heating means, which may include a
plasma torch. According to another aspect of the invention, the
method further includes the step of measuring the ovality of the
starting tube prior to etching the wall of the starting tube to
determine the ovality of the starting tube. According to yet
another aspect of the invention, etching the wall includes etching
the wall only where the exterior of the wall is substantially oval
in cross-section. The method may also include the step of rotating
the starting tube as the starting tube is etched, and/or the step
of removably mounting the starting tube to a lathe to etch the
wall.
[0011] According to another embodiment of the present invention,
there is a disclosed method of reducing the ovality of an optical
preform. The method includes providing a core rod, the core rod
comprising a core and a cladding layer, where the core includes an
oval cross section, mounting the core rod on a lathe having at
least two rotors, and rotating the core rod using the at least one
of the two rotors. The method further includes subjecting the core
rod to a heat source, where one of the at least two rotors is
phase-shifted from another one of the at least two rotors after the
core rod is mounted on the lathe.
[0012] According to one aspect of the invention, the method further
includes the step of etching the core rod. According to another
aspect of the invention, etching the wall includes etching the core
rod using a heating device, which may be a plasma torch. According
to yet another aspect of the invention, the method includes the
step of measuring the ovality of the core rod prior to etching the
core rod. The step of measuring can also include measuring the
ovality of the cladding layer.
[0013] Furthermore, the step of etching can include etching the
core rod only where the cladding layer is substantially oval in
cross-section. Additionally, the step of rotating can include
rotating the core rod as the core rod is etched. According to
another aspect of the invention, the step of rotating includes the
step of rotating the core rod in clockwise and counterclockwise
directions. Additionally, the angular velocity of the rotors may be
equal during rotation.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0014] Having thus described the invention in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0015] FIG. 1 is a diagram showing a general apparatus suitable for
modified chemical vapor deposition (MCVD).
[0016] FIGS. 2A through 2C are diagrams illustrating a general MCVD
method.
[0017] FIGS. 3A through 3C illustrate cross-sections of undesirable
preform geometries generated by a conventional MCVD process,
according to illustrative examples of the present invention.
[0018] FIG. 4 shows a cross-sectional view of a starting tube
having a non-uniform thickness, according to an illustrative
example of the present invention.
[0019] FIG. 5 shows a bisected view of a starting tube ovality
modification apparatus upon which a starting tube is attached,
according to one embodiment of the present invention.
[0020] FIG. 6 shows a cross-section view of a corrected starting
tube having a substantially circular outside diameter and uniform
thickness, according to one aspect of the present invention.
[0021] FIG. 7 shows a bisected view of a core rod ovality
modification apparatus upon which a core rod is attached, according
to one embodiment of the present invention.
[0022] FIGS. 8A through 8C illustrate the ovality modification
performed by the core rod ovality modification apparatus of FIG.
7.
[0023] FIGS. 9A through 9C illustrate the ovality modification
performed by an ovality modification apparatus and draw process of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present inventions now will be described more fully
hereinafter with reference to the accompanying drawings, in which
some, but not all embodiments of the invention are shown. Indeed,
these inventions may be embodied in many different forms and should
not be construed as limited to the embodiments set forth herein;
rather, these embodiments are provided so that this disclosure will
satisfy applicable legal requirements. Like numbers refer to like
elements throughout.
[0025] FIG. 1 shows an apparatus suitable for a MCVD process, as is
known in the prior art. The apparatus includes a shelf 1 which
supports a starting tube 2 and a heating means 4 that applies heat
to a heating zone 3 of the starting tube 2. The starting tube 2
rotates, for example, in the direction indicated by an arrow 5a,
and the heating means 4 reciprocates in the direction indicated by
an arrow 5b by a moving member in track 6, so that the heating zone
3 is shifted along the entire starting tube 2 while the starting
tube rotates. A source gas is introduced via a supply pipe 7 to the
starting tube 2, and the supply pipe 7 is connected to a source
material storage portion 8. The source material storage portion 8
has an inlet 9 for oxygen. Storage tanks 14 and 15 contain reaction
materials 16 and 17, which are usually liquids, and the reaction
materials 16 and 17 enter the starting tube 2, carried by carrier
gases input via inlets 10 and 11. Also, the excited material is
discharged via an outlet 18. A mixing valve (not shown) and a
blocking valve (not shown) measure the flow of gases and perform
other adjustments required for mixing. During a deposition process
the torch assembly slowly traverses the length of the starting tube
2 while the reaction materials and oxygen are fed into and
exhausted from the supply pipe 7. Following the deposition of core
and/or cladding material (in opposite order i.e., cladding material
is deposited before core material is deposited), the starting tube
2 is collapsed to form a solid core rod by heating it to a higher
temperature than during deposition, as is next illustrated in FIGS.
2A through 2C.
[0026] FIGS. 2A through 2C are diagrams illustrating the collapsing
of the starting tube 2 on the deposited materials to form an
optical preform. As illustrated with respect to FIG. 1, in the MCVD
process, a high-purity source gas such as SiCl.sub.4, GeCl.sub.4,
POCl.sub.3, BCl.sub.3 or CF.sub.4, is introduced together with
oxygen into a glass starting tube 21, and the starting tube 21 is
heated by the heating means 23, resulting in soot, an oxidation
deposit, on the inside of the starting tube 21 formed by thermal
oxidation (see FIG. 2A). Here, the concentration of the source gas
is accurately adjusted under the control of a computerized system
to control a refractive index, to thereby deposit a core and/or
cladding layer 22 inside the starting tube 21 (see FIG. 2B). Then,
the starting tube 21 on which the cladding and/or core layer 22
have been deposited is heated by the heating means 23, and
collapsed to form an optical fiber preform 25.
[0027] Though the MCVD process described above is widely used to
generate optical fiber, undesirable ovality of preforms formed
thereby often results. Ovality occurs when one or more of the
successive layers of material comprising the preform have varying
thicknesses or are oval, rather than circular, in their cross
section. FIGS. 3A through 3C illustrate some of the undesirable
preform geometries generated by the MCVD process. FIG. 3A
illustrates the cross-section of a core rod 30 having an oval
cladding layer 34 with a substantially non-oval, or circular, core
32. A core rod is generated by the MCVD process, but prior to the
generation of an optical fiber preform by the subsequent addition
of an overcladding layer, as is known in the art. For the purposes
of this application, core and/or cladding materials deposited
within a starting tube are referred hereinafter collectively as the
core. The core is surrounded by the starting tube, which is
collapsed on the core during the MCVD process, as described above
with respect to FIGS. 2A-2C; thus, the combination of the core and
collapsed starting tube includes the core rod. FIG. 3B illustrates
the cross-section of a core rod 36 having an oval core 38 with a
substantially non-oval, or circular, cladding layer 40. Finally,
FIG. 3C illustrates the cross-section of a core rod 42 having both
an oval core 44 and oval cladding layer 46.
[0028] As noted above, increased ovality of an optical preform
increases the PMD of optical fiber drawn there from. PMD is a
stochastic phenomenon that leads to the dispersion of the optical
pulses transmitted in an optical fiber. In particular, the
dispersion is caused by the propagation speed difference between
the polarization modes of the fiber. PMD limits the transmission
capacity of optical communication systems by creating inter-symbol
interference. Because low PMD is a desirable characteristic of
optical fiber, modifying preform core ovality to reduce PMD is a
crucial factor in achieving desirable transmission characteristics
of optical fiber.
[0029] According to one embodiment of the present invention, one
method of modifying the ovality of an optical preform is by
changing the ovality of the starting tube prior to MCVD deposition.
It will be appreciated by those of skill in the art that starting
tube outer diameter (OD) ovality plays a significant role in
preform core ovality after MCVD deposition. In fact, starting tube
outer diameter ovality correlates directly with preform core
ovality and PMD. Furthermore, it is known that preform core ovality
and PMD are relatively insensitive to the inner diameter (ID) of
starting tubes.
[0030] FIG. 4 shows a cross-sectional view of a starting tube 48
having undesirable high ovality, according to an illustrative
example of the present invention. As illustrated, the starting tube
48 comprises a starting tube wall 49 having a thickness T that
varies around the circumference of the starting tube 48. Thus, not
all portions along the exterior surface 50 of the wall 49 are
equidistant from a center longitudinal axis 51 running the length
of the tube 48. On the other hand, in this example, the interior
surface 52 of the wall 49 is substantially equidistant from the
center longitudinal axis 51.
[0031] FIG. 5 shows a bisected view of the starting tube 48
removably mounted on a starting tube ovality modification apparatus
53, according to one embodiment of the present invention. The
starting tube ovality modification apparatus 53 improves the
starting tube 480D ovality (or starting tube wall 49 uniformity) by
using a heat source on the starting tube 48 to modify its OD
ovality prior to MCVD deposition. More specifically, the apparatus
53 includes a lathe (not illustrated), such as a glassworking
lathe, upon which the starting tube 48 is mounted. The lathe is
operable to rotate 57, 59 the tube 48 around the center
longitudinal axis 51 passing through the length of the tube 48.
According to a preferred embodiment of the present invention, the
lathe's rotation is controlled by two rotors 56, 58. The rotational
velocities of the two rotors 56, 58 are synchronized and locked
together during the etching process so no twist is imparted to the
starting tube 48. An illustrative lathe is described in U.S. Pat.
No. 6,178,779, the entire contents of which are incorporated herein
by reference. Because the structure and operation of lathes are
well known to those of ordinary skill in the art, they are not
described further herein.
[0032] Referring again to FIG. 5, the starting tube 48 is removably
mounted adjacent to an isothermal plasma torch 54, as is well known
in the art, which generates a plasma fireball 55. The plasma torch
54 is mounted on a movable support which permits it to traverse the
length of the starting tube 48. Therefore, in combination with the
rotation provided by the rotors 56, 58, the plasma torch can heat
any outside portion of the starting tube 48. A variety of
isothermal plasmas may be used by the apparatus 53 of the present
invention. Examples include oxygen and oxygen-containing plasma,
e.g., oxygen/argon. The plasma is typically hydrogen-free, such
that OH impurities in the resulting article are substantially
avoided. The plasma fireball 55 heats and etches the exterior
surface 50 of the starting tube wall 49, thereby removing glass
from the starting tube wall 49. Generally, the longer the plasma
fireball 55 heats a particular portion of the starting tube wall
49, the greater the amount of glass etched there from. It will be
appreciated that the etching does not affect the interior surface
52 of the wall 49.
[0033] Selectively etching the outside of the starting tube wall 49
where it is too thick, such as in the 12 o'clock and 6 o'clock
positions in the illustrative cross-sectional view of a starting
tube 48 in FIG. 4, reduces the thickness of the wall 49 such that a
constant or near constant wall thickness, and reduced ovality, is
achieved. According to one aspect of the invention, up to 1 mm of
material may be removed from the OD of the wall 49 to make the OD
perfectly or near perfectly uniform.
[0034] The reduced ovality of the starting tube OD reduces the PMD
of an optical fiber drawn there from, thereby enhancing the
transmission characteristics of the resulting optical fiber.
[0035] It will be appreciated by those of ordinary skill in the art
that the preferential etching of the starting tube wall 49 or OD
ovality is achieved by varying the rotational velocity of the tube
48 as a function of the starting tube ovality. Therefore, the
slower the rotation of the starting tube 48, the greater the amount
of glass is etched away. To control the portions of the tube 48
that are etched, the OD ovality of the starting tube 48 can be
scanned prior to etching either on-line or off-line. Off-line
scanners are well known to those of ordinary skill in the art for
measuring starting tube dimensions and ovality. However, such
scanners have not been integrated with a system to control a plasma
torch or other heat source in a closed loop system to modify OD
ovality. Alternatively, on-line scanning can be done with laser
devices, as are well known in the art. According to one embodiment
of the present invention, the ovality of the starting tube 48 is
calculated by scanning equipment configurable to measure the
ovality of the tube 48 at any cross section along the entire length
of the tube 48. The scanning equipment is in electrical
communication with the starting tube ovality modification apparatus
53 to communicate the requisite rotational speed and location of
the plasma torch 54 to reduce the OD of the starting tube 48 where
necessary to produce a substantially non-oval and circular OD.
Alternatively, measurements can be made manually and entered into
the starting tube ovality modification apparatus 53 using an input
means well known in the art.
[0036] FIG. 6 shows an illustrative cross-section of the starting
tube of FIG. 4 after its OD ovality is reduced by the starting tube
ovality modification apparatus 53. The corrected starting tube 60
has a substantially uniform thickness D between the interior
surface 54 of the starting tube wall 64 and the exterior surface 62
of the starting tube wall 64. Therefore, the exterior surface 62 of
the tube 60 is substantially equidistant, along its entire surface,
to the center longitudinal axis 53 passing through the length of
the tube 60. Once the starting tube ovality is achieved using the
apparatus and method described above, MCVD deposition using the
modified starting tube can proceed, the MCVD deposition utilizing
the modified starting tube ensuring lower PMD, and hence enhanced
optical fiber transmission characteristics, than would otherwise be
achieved if the starting tube OD ovality was left unchanged prior
to MCVD deposition.
[0037] According to another embodiment of the invention, after MCVD
deposition forms the core rod, but prior to overcladding the core
rod to produce the full preform, the code rod is etched to modify
its ovality. This method may be used independently or in
combination with the methods for eliminating or reducing starting
tube ovality, discussed in detail above. Etching the core rod
properly can also reduce the PMD of optical fiber drawn from an
optical preform.
[0038] FIG. 7 shows a bisected view of a core rod ovality
modification apparatus 73 upon which a core rod 69 is removably
attached, according to one embodiment of the present invention. The
apparatus 73 is substantially similar to the apparatus discussed
with reference to FIG. 5, above. The core rod ovality modification
apparatus 73 improves the core rod 69 ovality by using a heat
source on the core rod in combination with spinning of the core rod
69 to improve the ovality of both the core 72 and overcladding
layer 70 of the core rod 69. As in FIG. 7, the apparatus 73
includes a lathe (not illustrated), such as a glassworking lathe,
upon which a core rod 69 having undesirable ovality is mounted. The
lathe is operable to rotate 77, 79 the core rod 69 around the
center longitudinal axis 71 passing through the length of the core
rod 69. According to one illustrative embodiment of the present
invention, the core rod 69 initially has the geometry illustrated
in FIG. 8A including an oval core 72 and an oval cladding layer
70.
[0039] Referring again to FIG. 7, the core rod 69 is removably
mounted adjacent to an isothermal plasma torch 74, as is well known
in the art, which generates a plasma fireball 75 as described
above. Like the apparatus of FIG. 7, the plasma torch 74 is mounted
on a movable support which permits it to traverse the length of the
core rod 69. The plasma fireball 75 heats and etches the exterior
surface of the core rod 69, thereby removing glass from exterior
portions of the cladding layer 70. Generally, the longer the plasma
fireball 75 heats a particular portion of the cladding layer 70,
the greater the amount of glass etched there from. It will be
appreciated that the etching does not affect the core 72 of the
core rod 69.
[0040] More specifically, according to one embodiment of the
invention, the apparatus 73 preferentially etches the OD 68 of the
core rod 69 until the cladding layer 70 is substantially similar to
the shape of core in cross-section. The results of the preferential
etching are illustrated in FIG. 5B. The preferential etching
reduces the thickness of some or select portions of the
overcladding layer 70 of FIG. 8A such that the thickness of the
overcladding layer 81 is substantially consistent or constant
around the core 82. Next, while the core rod 69 continues to be
heated along the length of its oval sections, the rotors 76, 78,
which control the lathe's rotation, are phase-shifted as a function
of the core rod's 69 ovality along its entire length.
[0041] According to one aspect of the present invention, the phase
shift is calculated as a function of its ovality:
.DELTA..THETA.=f(O.sub.v)=A sin(.OMEGA.t)=K*O.sub.v*sin(.OMEGA.t)
where .DELTA..THETA. is the amount of phase shift in degrees or
radius as a function of ovality, position and time; O.sub.v is the
ovality in % (e.g., (max-min)/(max+min)/2) as a function of
position; K is a conversion constant; and .OMEGA. is a frequency at
which the core rod spins (clockwise and counter-clockwise) are
alternated. This phase-shift operation provides a "spin motion" of
the core rod to improve its ovality.
[0042] According to one aspect of the invention, the angular
velocity of the two rotors 76, 78 are synchronized but for
transmission time periods during the phase shifting. Additionally,
it will be appreciated that it is advantageous to alternate the
phase-shift between positive and negative--i.e., between clockwise
and counterclockwise directions. This prevents any permanent twist
from being imparted in the core rod. The effect of the phase shift
or spinning motion is to average out and improve the core 72
ovality. Furthermore, when combined with surface tension created by
preferential etching, phase-shifting or spinning the core rod 69
will perturb the core 72 to redistribute it to a more circular
shape. Therefore, phase-shifted rotation of the oval core rod 80
having an oval core 82 will result in the core rod 83 illustrated
in FIG. 8C, which includes a substantially circular core 85 and
cladding layer 84. Therefore, selectively etching the outside of
the core rod 69 to match the oval or elliptical shape of the core,
followed by phase-shifted rotation of the core rod 69, reduces the
ovality of the core rod 69 thereby reducing the PMD of an optical
fiber drawn there from. Therefore, methods of the present invention
enhance the transmission characteristics of the resulting optical
fiber. It will also be appreciated that where the cladding layer
substantially circular, the core rod ovality correction apparatus
73 need perform the initial step of etching the cladding layer 70
before rotating the core rod to achieve a perform having low
ovality.
[0043] Like the embodiment described with respect to FIG. 7, it
will be appreciated by those of ordinary skill in the art that the
preferential etching of the cladding layer 70 of core rod 69 is
achieved by varying the rotational velocity of the core rod 69 as a
function of the core rod ovality. Therefore, the slower the
rotation of the core rod 69, the greater the amount of glass is
etched away. To control the portions of the core rod 69 that are
etched, the OD 68 ovality of the core rod 69 can be scanned prior
to etching either on-line or off-line. According to one embodiment
of the present invention, the ovality of the core rod 69 is
calculated by scanning equipment configurable to measure the
ovality of the rod 69 at any cross section along its entire length.
The scanning equipment is in electrical communication with the core
rod ovality modification apparatus 73 to communicate the requisite
rotational speed and location of the plasma torch 74 to match the
OD 68 of the core rod 69 with the core 72 such that the relative
exterior surfaces of the respective layers are substantially
congruent. Alternatively, measurements can be made manually and
entered into the core rod ovality modification apparatus 73 using
an input means well known in the art.
[0044] According to yet another embodiment of the present
invention, the cladding material of a preform having an oval or
elliptical core may be etched to mirror the shape of the oval core.
Thereafter the perform may be placed under a surface tension, or
pulled in a manner to generate a circular optical fiber having low
ovality and low PMD. FIGS. 9A through 9C illustrate the ovality
modification performed by an ovality modification apparatus and
method of the present invention. According to one illustrative
embodiment of the present invention, the core rod 85 initially has
the geometry illustrated in FIG. 9A. The core rod geometry is
similar to that geometry shown in FIGS. 3B and 8B, including an
oval core 87 and a substantially circular cladding layer 89.
[0045] Using the core rod ovality modification apparatus 73, OD of
the core rod is preferentially etched to match the shape of the
core 87. The results of the preferential etching are illustrated in
FIG. 9B. As shown in FIG. 9B, the preferential etching reduces the
thickness of some or select portions of the overcladding layer 93
of FIG. 9A such that the thickness of the overcladding layer is
substantially consistent or constant around the core 95. Therefore,
in the core rod 91 illustrated in FIG. 91B the overcladding layer
93 is congruent with the core 91. After this modification is made,
the core rod 91 is prepared for and subjected to overcladding (not
illustrated), as is well known in the art.
[0046] Next, the resulting perform is subjected to the draw
process. During the drawing of optical fiber from the draw tower a
surface tension is used, in combination with a drawing speed
effected by a longer melt zone to draw the preferentially etched
perform to effect optical fiber having a substantially circular
cross section of core 99 and cladding layer 98 illustrated in the
cross-sectional view of the core and cladding layers of an optical
fiber 97 illustrated in FIG. 9C. It is well known to those of
ordinary skill in the art how a surface tension may be applied to
create substantially circular core and cladding layers (i.e., in
cross-section) from an elliptically-shaped core rod and cladding
layer such as those illustrated in FIG. 9B. According to one aspect
of the invention, the temperature at which the fiber is typically
drawn (i.e., drawn without the need for a geometry modification)
may be increased to encourage the creation of the cross-section
shown in FIG. 9C. Furthermore, during drawing, the fiber may be
spun in clockwise and counterclockwise directions to encourage the
geometry modification taking place between FIGS. 9B and 9C. Like
the above-described methods, this process results in an optical
fiber having low ovality and low PMD, and thus, advantageous
transmission capabilities.
[0047] Many modifications and other embodiments of the inventions
set forth herein will come to mind to one skilled in the art to
which these inventions pertain having the benefit of the teachings
presented in the foregoing descriptions and the associated
drawings. Thus, it will be appreciated by those of ordinary skill
in the art that the present invention may be embodied in many forms
and should not be limited to the embodiments described above. As an
example, although the above apparatus and methods are disclosed
with respect to a MCVD process, the inventions disclosed herein may
be used with a variety of optical fiber fabrication processes, such
as VAD processing. Therefore, it is to be understood that the
inventions are not to be limited to the specific embodiments
disclosed and that modifications and other embodiments are intended
to be included within the scope of the appended claims. Although
specific terms are employed herein, they are used in a generic and
descriptive sense only and not for purposes of limitation.
* * * * *