U.S. patent application number 12/562749 was filed with the patent office on 2010-01-14 for optical fiber article for handling higher power and method of fabricating or using.
This patent application is currently assigned to NUFERN. Invention is credited to Adrian Carter, Douglas Guertin, Nils Jacobson, Kanishka Tankala.
Application Number | 20100008634 12/562749 |
Document ID | / |
Family ID | 39766508 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100008634 |
Kind Code |
A1 |
Guertin; Douglas ; et
al. |
January 14, 2010 |
OPTICAL FIBER ARTICLE FOR HANDLING HIGHER POWER AND METHOD OF
FABRICATING OR USING
Abstract
An optical fiber preform, and method for fabricating, having a
first core, a second core spaced from the first core and first and
second regions, the first region having an outer perimeter having a
first substantially straight length and the second region having an
outer perimeter having a second substantially straight length
facing the first straight length. One of the regions can comprise
the first core and the other comprises the second core. The preform
can be drawn with rotation to provide a fiber wherein a first core
of the fiber is multimode at a selected wavelength of operation and
a second core of the fiber is spaced from and winds around the
first core and has a selected longitudinal pitch. The second core
of the fiber can couple to a higher order mode of the first core
and increase the attenuation thereof relative to the fundamental
mode of the first core.
Inventors: |
Guertin; Douglas; (Monson,
MA) ; Jacobson; Nils; (Windsor, CT) ; Tankala;
Kanishka; (South Windsor, CT) ; Carter; Adrian;
(Bulli, AU) |
Correspondence
Address: |
NUFERN
7 AIRPORT PARK ROAD
EAST GRANBY
CT
06026
US
|
Assignee: |
NUFERN
East Granby
CT
|
Family ID: |
39766508 |
Appl. No.: |
12/562749 |
Filed: |
September 18, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US08/57950 |
Mar 21, 2008 |
|
|
|
12562749 |
|
|
|
|
60896263 |
Mar 21, 2007 |
|
|
|
Current U.S.
Class: |
385/126 ;
65/402 |
Current CPC
Class: |
C03B 2203/20 20130101;
C03B 37/01222 20130101; G02B 6/02042 20130101; C03B 37/01228
20130101; C03B 37/01231 20130101; C03B 2205/06 20130101; H01S
3/06708 20130101; C03B 2203/34 20130101; C03B 2203/18 20130101;
C03B 2205/07 20130101; C03B 2203/32 20130101; H01S 3/06704
20130101; C03B 37/02763 20130101 |
Class at
Publication: |
385/126 ;
65/402 |
International
Class: |
G02B 6/036 20060101
G02B006/036; C03B 37/02 20060101 C03B037/02 |
Claims
1. An optical fiber preform extending in a longitudinal direction
for drawing an optical fiber therefrom having a first core, and a
second core winding about the first core, comprising: a first core;
a second core spaced from the first core; first and second regions,
said first region when viewed in cross section having an outer
perimeter having a first substantially straight length and the
second region having an outer perimeter having a second
substantially straight length facing said first substantially
straight length; one of said first and second regions comprising
said first core and a cladding disposed about said first core and
the other of said first and second regions comprising said second
core and a cladding disposed about said second core; said preform
being constructed and arranged such that the preform can be drawn
with rotation about an axis passing through the first core to
provide the fiber wherein a first core of the fiber is multimode at
a selected wavelength of operation and a second core of the fiber
is spaced from and winds around the first core of the fiber and has
a selected longitudinal pitch and wherein at said wavelength of
operation the second core of the fiber couples to a higher order
mode of the first core of the fiber and increases the attenuation
thereof relative to the fundamental mode of the first core of the
fiber; and wherein the term "cross section" means cross section
taken perpendicular to the longitudinal direction.
2. The optical fiber preform of claim 1 wherein said optical fiber
preform comprises an outer perimeter bounding the cross section of
the optical fiber preform and defining the geometrical center of
the preform and wherein said first core comprises a cross sectional
area within which the geometrical center lies.
3. The optical fiber preform of claim 1 wherein said first and
second substantially straight lengths are contiguous and
substantially parallel.
4. The optical fiber preform of claim 1 wherein the outer perimeter
of said first region includes a first curved length characterized
by a first radius of curvature and the outer perimeter of said
second region comprises a second curved length that is
characterized by a second radius of curvature that is substantially
the same as said first radius of curvature and wherein said first
radius of curvature originates within the cross sectional area of
said first core and said second radius of curvature does not
originate from within the cross sectional area of said second
core.
5. The optical fiber preform of claim 1 wherein the outer perimeter
of said first region includes a first curved length characterized
by a first radius of curvature and the outer perimeter of said
second region comprises a second curved length that is
characterized by a second radius of curvature that is different
than said first radius of curvature and wherein said first radius
of curvature originates within the cross sectional area of said
first core and said second radius of curvature originates from
within the cross sectional area of said second core.
6. The optical fiber preform of claim 1 wherein said first and
second regions are fused together.
7. The optical fiber preform of claim 1 wherein the outer perimeter
of said second region comprises only substantially straight
lengths.
8. The optical fiber preform of claim 1 comprising a
stress-inducing region for providing a fiber drawn from the preform
with a selected birefringence.
9. An optical fiber preform extending in a longitudinal direction
for drawing an optical fiber therefrom having a first core, and a
second core winding about the first core, comprising: a unitary
core rod comprising a first core and a cladding disposed about said
core; an arrangement of elongate members forming a ring disposed
about said core rod; a second core comprising at least one of the
elongate members of the arrangement forming a said ring; a tube
disposed about the arrangement of elongate members; said preform
being constructed and arranged such that the preform can be drawn
with rotation about an axis passing through the first core to
provide the fiber wherein the first core of the fiber is multimode
at a selected wavelength of operation and a second core of the
fiber is spaced from and winds around the first core of the fiber
and has a selected longitudinal pitch and wherein at said
wavelength of operation the second core of the fiber couples to a
higher order mode of the first core of the fiber and increases the
attenuation thereof relative to the fundamental mode of the first
core of the fiber; and wherein the term "cross section" means cross
section taken perpendicular to the longitudinal direction.
10. The optical fiber preform of claim 9 wherein the optical fiber
preform comprises an outer perimeter bounding the cross section of
the optical fiber preform and defining the geometrical center of
the preform and wherein said first core comprises a cross sectional
area within which the geometrical center lies.
11. The optical fiber preform of claim 9 wherein said core
comprises a diameter and wherein the thickness of the cladding of
the core rod is no greater than 15% of the diameter of the core
rod.
12. The optical fiber preform of claim 9 comprising another tube
interposed between said arrangement of elongate members and said
core rod.
13. The optical fiber preform of claim 9 wherein at least one of
said elongate members comprises a stress-inducing region for
providing a fiber drawn from the preform with a selected
birefringence.
14. A method of fabricating an optical fiber preform extending in a
longitudinal direction for drawing an optical fiber therefrom
having a first core, and a second core winding about the first
core, comprising; providing a first elongate member having when
viewed in cross section an outer perimeter having a first curved
length and a first substantially straight length, the first
elongate member further including, within the outer perimeter of
the first elongate member, a core and a cladding disposed about the
core; providing a second elongate member having when viewed in
cross section an outer perimeter having a second substantially
straight length and a curved length and a second core within the
outer perimeter of the second elongate member; arranging the
preform such that the first substantially straight length faces the
second substantially straight length; the preform being further
constructed and arranged such that the preform can be drawn with
rotation about a longitudinal axis passing through the first core
to provide the fiber wherein the first core of the fiber is
multimode at a selected wavelength of operation and the second core
of the fiber is spaced from and winds around the first core of the
fiber and has a selected longitudinal pitch and wherein at the
wavelength of operation the second core of the fiber couples to a
higher order mode of the first core of the fiber and increases the
attenuation thereof relative to the fundamental mode of the first
core of the fiber; and wherein the term "cross section" means cross
section taken perpendicular to the longitudinal direction.
15. The method of claim 14 wherein arranging the preform comprises
arranging the preform such that the outer perimeter of the cross
section of the preform defines a geometrical center and the
geometrical center lies within the cross sectional area of the
first core.
16. The method of claim 14 wherein arranging the preform comprises
arranging the preform such that the first and second substantially
straight lengths are contiguous and substantially parallel.
17. The method of claim 14 wherein arranging the preform comprises
fusing said first and second elongate members together.
18. The method of claim 14 wherein arranging the preform comprises
disposing a tube about said first and second elongate members.
19. The method of claim 14 wherein said first curved length is
characterized by a first radius of curvature and said second curved
length is characterized by a second radius of curvature, the first
and second radii of curvature being substantially the same and
wherein said first radius of curvature extends from within the
cross sectional area of said first core and wherein said second
radius of curvature does not extend from within the cross sectional
area of said second core.
20. The method of claim 14 wherein said first curved length is
characterized by a first radius of curvature and said second curved
length is characterized by a second radius of curvature, and
wherein the first and second radii of curvature are different and
said first radius of curvature extends from within the cross
sectional area of said first core and wherein said second radius of
curvature extends from within the cross sectional area of said
second core.
21. The method of claim 14 wherein providing a selected one of the
first and second members comprises providing the selected member
having when viewed in cross section a substantially circular outer
perimeter and shaping the member to form the first straight length,
wherein the substantially circular outer perimeter defines a
geometrical center lying within the cross sectional area of a core
of the member and wherein shaping comprises shaping such that the
outer perimeter defines a new geometrical center that lies outside
of the cross sectional area of the core of the member.
22. The method of claim 14 wherein providing first elongate member
comprises an axially symmetric deposition process wherein material
to form the first core is deposited on a first cylindrical
substrate and wherein providing the second elongate member
comprises an axially symmetric deposition process wherein material
for forming the second core is deposited on a different cylindrical
substrate.
23. The method of claim 14 comprising forming a hole in a region of
the preform and inserting a stress-inducing elongate member in the
hole for increasing the birefringence of a fiber drawn from the
preform.
24. The method of claim 23 wherein the stress-inducing elongate
member includes a dopant for varying the coefficient of thermal
expansion of the stress-inducing elongate member, and wherein said
stress-inducing elongate member is substantially free of an
outermost region that is free of a concentration of the dopant.
25. The method of claim 24 including providing a stress-inducing
region that includes an outermost region that includes a
concentration of the dopant and substantially removing that
outermost region.
26. The method of claim 23 comprising fusing said first and second
elongate members together to form a fused assembly and forming the
hole in the fused assembly.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of International
Application No. PCT/US2008/057950, filed Mar. 21, 2008, which in
turn claims priority to U.S. Provisional Patent Application No.
60/896,263, filed Mar. 21, 2007. The foregoing applications are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to fiber optic articles, such
as optical fibers, optical fiber preforms and the like, and methods
for making and using fiber optic articles.
BACKGROUND ART
[0003] Fiber lasers and fiber amplifiers capable of handling higher
powers are of considerable interest. Unfortunately, non-linear
effects, such as Simulated Brillioun Scattering (SBS) and
Stimulated Raman Scattering (SRS) can limit the power output of
fiber lasers and amplifiers to lower powers. Because optical fiber
has a rather small cross section, the light propagated by the fiber
is rather concentrated and hence, at higher power operation, the
light can reach a power density that exceeds the power density
threshold for the onset of SBS and/or SRS.
[0004] Fibers having a larger diameter core can lower the power
density and delay the onset of SBS or SRS, thereby allowing higher
power operation. However, a core having a larger diameter is not
typically single mode. Because the larger diameter core will
support modes in addition to the fundamental mode, the fiber
provides a lower beam quality (i.e., a lower M.sup.2 factor) than a
single mode fiber. Higher beam quality is important in many
applications. Certain techniques are known for attenuating and/or
limiting the excitation of such higher order modes such that a
larger core fiber can still provide good beam quality.
[0005] Known techniques include coiling of the fiber to selectively
induce bend loss into the higher order modes, such as is taught in
U.S. Pat. No. 6,496,301, entitled "Helical Fiber Amplifier". US
2006/0024008 A1, entitled "Composite Waveguide", teaches another
technique. With reference to FIG. 1, the '008 application teaches
an optical fiber 100 having a first, centrally located and large
diameter core 122 for propagating power and a side core 124 spaced
from (such as by distance d) and in optical proximity of the
central core 122. The side core 124 winds around the central core
122 and couples to higher order modes of the central core 122. The
higher order modes are attenuated due to the bend loss induced by
the winding of the side core 124. An optical cladding (not shown in
FIG. 1) surrounds the cores 122 and 124 and occupies the space
between them. The winding can be characterized by a pitch P.
Typically the cladding comprises a material having a lower index of
refraction than an index of refraction of material comprised by a
core.
[0006] The '008 publication discloses a method of manufacture of
such an optical fiber comprising drawing while rotating a fiber
preform having a central core and an off-center side core. With
reference to FIG. 2, the '008 publication is understood to teach
that an initial preform 200 having a single central core 222 is
drilled to form a hole 220 for insertion of a side core 224. The
side core 224 can be selected from pre-manufactured glass rods
having a predetermined dopant composition. The preform having the
side core 224 inserted in the hole 220 is then heated to collapse
any air gaps. The preform having the side core can be drawn to a
fiber while rotating the preform about an axis passing through the
central core 222 such that in the resultant drawn fiber the side
core winds about the central core, as shown in FIG. 1.
[0007] While the foregoing applications, patents or techniques may
represent an improvement in the art, each can have drawbacks or
limitations in certain circumstances. Accordingly, it is an object
of the present invention to provide optical articles and methods
for fabricating or using such optical articles that address one or
more of the drawbacks or deficiencies of the prior art.
SUMMARY
[0008] In certain instances, desirable operation of the fiber shown
in FIG. 1 can require a very small spacing d between the side core
124 and the central core 122, and hence a very small spacing S
between the core 222 and drilled hole 220 of FIG. 2 if a preform is
to be made as taught in the '008 published application. A very
small spacing S can be problematic, as the central core 222 can be
damaged during drilling, or the preform otherwise rendered
unusable. For example, due to the stresses (because the material of
the core is typically different than that of the cladding material,
formed by the initial tube), drilling close to the core can cause
crack propagation or shattering, rendering the preform less
desirable or undesirable for use. Yield, an important consideration
in a commercial endeavor, can be reduced unacceptably.
[0009] Accordingly, Applicants teach herein preforms and methods of
making and using such preforms to make fiber that can reduce the
number of holes to be drilled to receive side cores or that can
avoid altogether the need for drilling a hole to receive a side
core. In addressing the difficulty of drilling a hole close to the
core, Applicants discovered that the practical restrictions on
shaping, such as machining, a preform to within a certain distance
of a core to be less onerous.
[0010] In one aspect, the invention provides an optical fiber
preform extending in a longitudinal direction for drawing an
optical fiber therefrom having a first core winding about a second
core. The optical fiber preform can comprise a first core; a second
core spaced from the first core; and first and second regions, the
first region when viewed in cross section having an outer perimeter
having a first curved length and a first substantially straight
length and the second region having an outer perimeter having a
second substantially straight length facing the first straight
length. One of the first and second regions can comprise the first
core and the other of the first and second regions can comprise the
second core. The first and second regions can each include a
cladding disposed about and contiguous with the first and second
cores, respectively. "Disposed about", as used herein, means that a
region, surrounds, at least partially, another region, and may
additionally contact or be contiguous with the other layer or
cladding, if there are no intermediate regions interposed between
the region and other region.
[0011] The preform can be constructed and arranged such that the
preform can be drawn with rotation about an axis passing through
the first core to provide the fiber wherein a first core of the
fiber is multimode at a selected wavelength of operation and a
second core of the fiber is spaced from and winds around the first
core of the fiber and has a selected longitudinal pitch and wherein
at the wavelength of operation the second core of the fiber couples
to a higher order mode of the first core of the fiber and increases
the attenuation thereof relative to the fundamental mode of the
first core of the fiber. It is noted that, as used herein, the term
"cross section" means cross section taken perpendicular to the
longitudinal or elongate direction.
[0012] The optical fiber preform can comprise an outer perimeter
bounding the cross section of the optical fiber preform and
defining the geometrical center of the preform. The first core can
comprise a cross sectional area within which the geometrical center
lies. The first and second substantially straight lengths can be
one or more of spaced apart, contiguous, substantially parallel, or
coextensive at least along a portion of one of the substantially
straight lengths. In certain practices of the invention, outer
perimeter of the second region comprises only substantially
straight lengths. The outer perimeter of the second region can
include a second curved length. The first curved length can be
characterized by a first radius of curvature and the second curved
length is characterized by a second radius of curvature that is
different than the first radius of curvature. The first and second
radii of curvature can have different lengths. The first radius of
curvature can originate within the cross sectional area of the
first core and the second radius of curvature can originate from
within the cross sectional area of the second core. The first and
second radii of curvature can have different lengths.
[0013] The first and second regions can be fused together. The
optical fiber preform can comprise a third region comprising a
D-shaped outer perimeter, as well as a fourth region comprising a
D-shaped outer perimeter. The optical fiber preform can comprise a
region, such as the cross section of a tube, having an inner
perimeter disposed about the first and second regions. The optical
fiber preform can be further constructed and arranged such that the
preform can be drawn to the fiber wherein the first core of the
fiber has a numerical aperture of less than or equal to 0.09. The
optical fiber preform can be further constructed and arranged such
that the preform can be drawn to the fiber wherein the second core
of the fiber has a numerical aperture of less than or equal to
0.09.
[0014] In another aspect, the invention can provide an optical
fiber preform extending in a longitudinal direction for drawing an
optical fiber therefrom having a first core and a second core
winding about the first core, where the preform can comprise a
unitary core rod comprising a first core and a cladding disposed
about the core; an arrangement of elongate members forming a ring
disposed about the core rod; a second core comprising at least one
of the elongate members of the arrangement forming the ring; and an
elongate tubular region (e.g., a commercially available substrate
tube) disposed about the arrangement of elongate members. The
preform can be constructed and arranged such that the preform can
be drawn with rotation about an axis passing through the first core
to provide the fiber wherein a first core of the fiber is multimode
at a selected wavelength of operation and a second core of the
fiber is spaced from and winds around the first core of the fiber
and has a selected longitudinal pitch and wherein at the wavelength
of operation the second core of the fiber can couple to a higher
order mode of the first core of the fiber and increases the
attenuation thereof relative to the fundamental mode of the first
core of the fiber.
[0015] The optical fiber preform can comprise an outer perimeter
bounding the cross section of the optical fiber preform and
defining the geometrical center of the preform. The first core can
comprise a cross sectional area within which the geometrical center
lies. In one practice of the invention the core rod can comprise a
diameter and the thickness of the cladding of the core rod is no
greater than 15% of the diameter of the core rod. At least some of
the elongate members of the arrangement of elongate members can be
solid rods. The cladding of the core rod can be substantially
solid. The optical fiber preform can comprise another elongate
tubular region interposed between the arrangement of elongate
members and the core rod. The optical fiber preform can comprise
another region (e.g., the cross section of a tube) disposed about
the arrangement of elongate members. The optical fiber preform can
comprise a third core comprising a different one of the elongate
members than the side core.
[0016] The present invention also involves methods. For example, in
one aspect, there is provided a method of fabricating an optical
fiber preform extending in a longitudinal direction for drawing an
optical fiber therefrom having a first core and a second core
winding about the first core. The method can comprise providing a
first elongate member having when viewed in cross section an outer
perimeter having a first substantially straight length, the first
elongate member further including a core within the outer perimeter
of the first elongate member; providing a second elongate member
having when viewed in cross section an outer perimeter having a
second substantially straight length and a second core within the
outer perimeter of the second elongate member; and arranging the
preform such that the first substantially straight length faces the
second substantially straight length. The preform can be further
constructed and arranged such that the preform can be drawn with
rotation about a longitudinal axis passing through the first core
to provide the fiber wherein the first core of the fiber is
multimode at a selected wavelength of operation and the second core
of the fiber is spaced from and winds around the first core of the
fiber and has a selected longitudinal pitch and wherein at the
wavelength of operation the second core of the fiber couples to a
higher order mode of the first core of the fiber and increases the
attenuation thereof relative to the fundamental mode of the first
core of the fiber.
[0017] The preform can be arranged such that the outer perimeter of
the cross section of the preform defines a geometrical center and
the geometrical center lies within the cross sectional area of the
first core, or such that first and second substantially straight
lengths are one or more of spaced apart, substantially parallel, or
contiguous. The first and second elongate members can be fused
together. A tubular region can be disposed about the first and
second elongate members.
[0018] The outer perimeter of the first region can include a first
curved length. The outer perimeter of the second region can include
a second curved length. The first curved length can be
characterized by a first radius of curvature and the second curved
length can be characterized by a second radius of curvature, where
the first and second radii of curvature are different. The first
radius of curvature can originate, or extend, from within the cross
sectional area of the first core and the second radius of curvature
can originate from within the cross sectional area of the second
core. The first and second radii can be of different lengths. The
first elongate member can at one point have when viewed in cross
section a substantially circular outer perimeter and the method can
include shaping the first elongate member to form the first
straight length. The substantially circular outer perimeter can
define a geometrical center lying within the cross sectional area
of the first core and the method can include shaping such that the
outer perimeter defines a new geometrical center that lies outside
of the cross sectional area of the first core. The second member
can at one point have when viewed in cross section a substantially
circular outer perimeter that defines a geometrical center that
lies within the cross sectional area of the second core and the
method can include shaping the second elongate member such that the
outer perimeter defines a new geometrical center that lies outside
the cross sectional area of the second core.
[0019] Many processes that are advantageous in fabricating preforms
include depositing material in an axially symmetric manner on a
substrate, such as a substrate tube or rod. As one example, in the
Modified Chemical Vapor Deposition (MCVD) process, material is
deposited on the inside of a rotating tube that can then be
collapsed to form a solid rod. Material deposited on the inside of
the tube typically forms all or part of a centrally located core in
the solid rod that results from collapsing the tube. Similar
considerations apply, for example, to the Outside Vapor Deposition
(OVD) process, which can also result in a rod having a central
region comprising deposited material surrounded by material of the
tube.
[0020] Providing at least one of the first and second elongate
members can comprise an axially symmetric deposition process
wherein material to form the core of the at least one member is
deposited on a cylindrical substrate. Providing the other of the
first and second elongate members can comprise an axially symmetric
deposition process wherein material for forming the core of the
other elongate member is deposited on a different cylindrical
substrate. The method can comprise fusing the first elongate member
with the second elongate member. The preform can be drawn to form
the fiber while rotating one of the fiber and the preform. Rotating
one of the fiber and the preform can comprise rotating the
preform.
[0021] As noted above, the preform can be constructed and arranged
to provide a fiber having certain properties. One of ordinary skill
in the art understands that a preform is in certain aspects a scale
model of the fiber to be drawn and hence is designed accordingly.
Therefore, given the disclosure herein, it is considered that one
of ordinary skill in the art can construct and arrange a preform
according to the invention so as to provide fibers as described
herein. In one practice of the invention, a preform is constructed
and arranged so as to provide a fiber having a first core having a
diameter of approximately 35 microns and a numerical aperture (NA)
relative to its cladding of approximately 0.07. The spacing d can
comprise approximately 2 microns and the side core can have a
diameter of approximately 12 microns and a NA of approximately 0.09
relative to the cladding. The pitch P can comprise about 6.2
mm.
[0022] For such a fiber, the attenuation for at least one higher
order mode (e.g., LP.sub.11) is understood to be 10-100 dB/meter
greater than the attenuation of the fundamental mode LP.sub.01 at
(or for wavelengths greater than) a selected wavelength of
operation, such as, for example, 1550 nanometers. The attenuation
of several higher order modes (e.g., LP.sub.11, LP.sub.21,
LP.sub.02), or even of all higher order modes, can be 10 to 100
dB/meter higher than the attenuation of the fundamental mode at a
selected wavelength of operation. The loss of the fundamental mode,
at, for example, the same wavelength of operation, can be less than
1 dB/meter, or can be less than 0.5 dB/meter or can even be less
than 0.4 dB/meter.
[0023] The first core of the fiber is usually multimode at the
wavelength of operation. The first core can have a V-number of
greater than 2.405 at the wavelength of operation (multimode and
V-number in this context meaning as considered as if the side core
or cores were absent, as the side core or cores in practice can
make the fiber act as substantially single moded (in the
fundamental mode) or as a fiber having a V-number less than 2.405).
The first core can have a diameter of greater than or equal to 10
microns, greater than or equal to 12 microns, or greater than or
equal to 15 microns. The primary or the side core can have a NA of
less than or equal to 0.12, a NA of less than or equal to 0.10, or
a NA less than or equal to 0.08.
[0024] In certain practices of the invention, the pitch with which
the side core is wound can be less than or equal to 20 mm, less
than or equal to 15 mm, less than or equal to 10 mm, less than or
equal to 8 mm, or less than or equal to 5 mm. The pitch can vary
along the length of the fiber. One way to accomplish varying the
pitch is to change the rate of rotation of the preform and/or the
fiber as the fiber is being drawn from the preform. In certain
practices of the invention, the variation in pitch (i.e., the
difference between the maximum and minimum pitches) can be less
than or equal to 0.5 mm, less than or equal to 1 mm, less than or
equal to 2 mm, or less than or equal to 5 mm. In other practices,
the variation in pitch can be between (i.e., range from) 0.5 mm and
1 mm, can be between 1 mm and 2 mm, or can be between 2 mm and 3
mm. Pitch can be a function of the speed with which the fiber is
drawn and the rotation rate used during draw.
[0025] Further advantages, novel features, and objects of the
invention will become apparent from the following detailed
description of non-limiting embodiments of the invention when
considered in conjunction with the accompanying FIGURES, which are
schematic and which are not drawn to scale. For purposes of
clarity, not every component is labeled in every one of the
following FIGURES, nor is every component of each embodiment of the
invention shown where illustration is not considered necessary to
allow those of ordinary skill in the art to understand the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 schematically illustrates a prior art optical fiber
having a central core and a side core winding about the central
core;
[0027] FIG. 2 schematically illustrates a prior art method of
making a preform from which the fiber of FIG. 1 can be drawn;
[0028] FIG. 3 is a cross section of one embodiment of an optical
fiber preform;
[0029] FIG. 4 is a cross section of another embodiment of an
optical fiber preform;
[0030] FIGS. 5A-5E schematically illustrates steps that can be
employed in fabricating one or both of the preforms of FIGS. 3 and
4;
[0031] FIG. 6A is a cross section of yet a further embodiment of an
optical fiber preform;
[0032] FIG. 6B is cross section of yet an additional embodiment of
an optical fiber preform;
[0033] FIG. 7 schematically illustrates drawing an optical fiber
from a preform while rotating the preform;
[0034] FIG. 8 schematically illustrates a cross section of another
embodiment of an optical fiber preform;
[0035] FIG. 9 schematically illustrates how an elongate member is
to be modified to have a desired shape for use in forming one
embodiment of a preform according to the invention;
[0036] FIG. 10 schematically illustrates a cross section of an
elongate member having the desired shape shown in FIG. 9;
[0037] FIG. 11 schematically illustrates shaping the elongate
member of FIG. 9 to change the shape of at least part of the outer
perimeter thereof; and
[0038] FIG. 12 schematically illustrates a cross section of yet an
additional embodiment of an optical fiber preform.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] FIG. 3 is a cross section of one embodiment of an optical
fiber preform 300 according to the invention, where the cross
section is taken perpendicular to a longitudinal axis along which
the preform extends. The optical fiber preform 300 is formed and
then drawn in the process of fabricating an optical fiber, such as
the optical fiber 100 of FIG. 1, having a first core 122 as well as
a side core 124 that winds about the first core 122. Accordingly,
the optical fiber preform 300 comprises a first core 322 and a
second core 324. The first and second cores 322 and 324 can form,
respectively, the first and side cores 122 and 124 of the optical
fiber 100.
[0040] The preform 300 can comprise an outer perimeter 328 bounding
the cross section of the optical fiber preform 300 and defining the
geometrical center of the preform 300. The geometrical center 330
of the outer perimeter 328 is the location determined by the
average x coordinate and the average y coordinate of the set of x
and y coordinates that describes the perimeter 328 in the cross
sectional plane. (For a perfect circle, the geometrical center is
what is normally referred to as the center of the circle).
Typically, the first core 322 comprises a cross sectional area
(defined by the outer perimeter 334 of the first core 322) within
which the geometrical center 330 lies. Preferably, the outer
perimeter 328 is substantially circular in shape.
[0041] The preform 300 can also comprise first and second regions,
338 and 342, respectively. The first region 338 can include an
outer perimeter 344 having a first curved length 346 and a first
substantially straight length 348. The second region 342 can have
an outer perimeter 356 having a second substantially straight
length 358 facing the first substantially straight length 348. The
first substantially straight length 348 can be substantially
parallel to the second substantially straight length 358. The first
substantially straight length 348 can be spaced from the second
substantially straight length 358. In the embodiment shown in FIG.
3, the substantially straight lengths are substantially parallel
and the second substantially straight length 358 is shown as
coextensive with at least part of the first substantially straight
length 348. The first substantially straight length 348 and the
second substantially straight length 358 are contiguous in the
embodiment shown in FIG. 3. The outer perimeter 356 of the second
region 342 can include the curved length 359, which can be referred
to as the second curved length to distinguish it from the first
curved length 346 of the first region 338.
[0042] The preform 300 can include a second core 324 spaced from
the first core 322, (where, as is evident to one of ordinary skill
in light of the disclosure herein, the spacing between the cores
refers to the edge-to-edge distance between the outer perimeters
thereof along a diameter passing through the geometric centers of
each of the outer perimeters). One of the first and second regions
338 and 342 can comprise the second core 324. In the embodiment
shown in FIG. 3, the second region 342 comprises the second core
324. The preform 300 can be constructed and arranged such that the
preform 300 can be drawn with rotation (See FIG. 7 and accompanying
description) to a fiber, such as the fiber 100 of FIG. 1, wherein
the central core 122 is multimode at a selected wavelength of
operation and the side core 124 is wound about and spaced from the
central core 122 and has a selected longitudinal pitch P and such
that at the wavelength of operation the side core 124 couples to a
higher order mode of the central core 122 and selectively increases
the attenuation thereof relative to the fundamental mode of the
central core 122.
[0043] The first curved length 346 of the first region 338 can be
characterized by (i.e., substantially conform to) a first radius of
curvature R.sub.1. The second curved length 359 of the second
region 342 can be characterized by a radius R.sub.2. R.sub.1 and
R.sub.2 are typically different in that they can originate from
different points and/or can have different lengths. Typically they
both originate from different points and have different lengths.
R.sub.1 typically originates from within the cross sectional area
of the first core and R.sub.2 originates from within the cross
sectional area of the second core. R.sub.1 typically has a greater
length than R.sub.2.
[0044] The preform 300 can include an outer region 360, which can
be annular as shown in FIG. 3 and which can be a tube, disposed
about the first region 338 and the second region 342. The tube can
have the inner perimeter 362 and the outer perimeter of the tube
can be the outer perimeter 328 of the preform 300. The inner
perimeter can be contiguous with at least part of the curved
lengths 346 and 359. The outer region 360 (as well as the first and
second regions, except typically for the cores thereof) can
comprise, consist essentially of, or consist of, for example,
glass, such as silica glass. As noted elsewhere herein, many
techniques are suitable for fabricating all or parts of a preform
according to the invention. A vapor deposition process, such as the
Outside Vapor Deposition process, described elsewhere herein, can
be used in forming all or part of the preform 300, including
fabricating the outer region 360. For example, glass soot
comprising a suitable index of refraction (e.g., lower than an
index of refraction of the material of the first and second
regions, 338 and 342, disposed about the cores of those regions),
can be deposited over the first and second regions 338 and 342. The
soot is then consolidated to form a clear glass. The preform 300
can comprise glass tubes or rods, selected ones of which are
designated by reference numeral 364. The tubes or rods can be sized
(e.g., have diameter selected) to fill the gaps between the inner
perimeter 362 of the tube 360 and the curved length 359 of the
perimeter 356 of the second region 342 to help prevent voids or
inclusions in fiber drawn from the preform. The outer region 360,
the first and second regions 338 and 342 and the tubes or rods 364
can comprise substantially similar material, such as, for example,
silica glass.
[0045] The outer perimeter 328 of the preform 300 can comprise a
diameter, indicated by arrows 393, that intersects the first and
second cores, 322 and 324, and that is substantially perpendicular
to one or both of the substantially straight lengths 348 and 358.
The first and second regions, 338 and 342, such as when arranged in
the preform 300, can comprise the diameter indicated by reference
numerals 395. The diameter 395 can intersect the first and second
cores, 322 and 324. The diameter 395 can be substantially
perpendicular to one or both of the substantially straight lengths
348 and 358 and/or can be the maximum diameter that can be taken
across the first and second regions 334 and 342. The diameter 395
can be coextensive with at least part of the diameter 393.
[0046] FIG. 4 is a cross section of another embodiment of an
optical fiber preform according to the invention, where the cross
section is taken perpendicular to a longitudinal axis along which
the preform extends. The optical fiber preform 400 comprises a
first core 422 and a second core 424, and is in many respects
similar to the preform 300 of FIG. 3. The first and second cores
422 and 424 can form, respectively, the first and centrally located
core 122 and the side core 124 of the optical fiber 100.
[0047] The preform 400 can comprise an outer perimeter 428 bounding
the cross section of the optical fiber preform 400 and defining the
geometrical center of the preform 400. The first core 422 comprises
a cross sectional area (defined by the outer perimeter 434 of the
first core 422) within which the geometrical center 430 lies.
[0048] The preform 400 can also comprise first and second regions,
438 and 442, respectively. The first region 438 can include an
outer perimeter 444 having a first curved length 446 and a first
substantially straight length 448. The second region 442 can have
an outer perimeter 456 having a second substantially straight
length 458 facing the first substantially straight length 448. The
first substantially straight length 448 can be substantially
parallel to the second substantially straight length 458. The first
substantially straight length 448 can be spaced from the second
substantially straight length 458. In the embodiment shown in FIG.
4, the substantially straight lengths are substantially parallel
and the second substantially straight length 458 is shown as
coextensive with at least part of the first substantially straight
length 448. The first substantially straight length 448 and the
second substantially straight length 458 are contiguous in the
embodiment shown in FIG. 4.
[0049] The preform 400 can include a second core 424 spaced from
the first core 422. One of the first and second regions 438 and 442
can comprise the second core 424. In the embodiment shown in FIG.
4, the second region 442 comprises the second core 424. The
preform, including the first and second cores 422 and 424, can be
constructed and arranged such that the preform 400 can be drawn
with rotation to a fiber, such as the fiber 100 of FIG. 1, wherein
the central core 122 is multimode at a selected wavelength of
operation and the side core 124 is wound about and spaced from the
central core 122 and has a selected longitudinal pitch P and such
that at the wavelength of operation the side core 124 couples to a
higher order mode of the central core 122 and selectively increases
the attenuation thereof relative to the fundamental mode of the
central core 122.
[0050] The first curved length 446 of the first region 438 can be
characterized by (i.e., substantially conform to) a first radius of
curvature R.sub.3. The second curved length 459 of the second
region 442 can be characterized by a radius R.sub.4. R.sub.3 and
R.sub.4 are typically different, that is, they can originate from
different locations and/or can have different lengths. In the
embodiment shown, the radius R.sub.3 originates from within the
cross sectional area of the first core, the radius R.sub.4
originates from within the cross sectional area of the second core,
and the radius R.sub.3 has a longer length than the radius R.sub.4.
In certain instances it may be possible for R.sub.3 and R.sub.4 to
be substantially the same.
[0051] The preform 400 can include an outer region 460, which can
be the cross section of a tube, disposed about the first region 438
and the second region 442. The region 460 can comprise the inner
perimeter 462 and the outer perimeter of the region 460 can be the
outer perimeter 428 of the preform. The inner perimeter can be
contiguous with at least part of one or both of the curved lengths
446 and 459. The outer region 460 (as well as the first and second
regions, except typically for the cores thereof) can comprise,
consist essentially of, or consist of, for example, glass, such as
silica glass. As noted above in the discussion of FIG. 3, the outer
region can be formed, at least in part, by a vapor deposition
process such as, for example, the Outside Vapor Deposition process.
The preform 400 can comprise glass tubes or rods, the cross
sections of selected ones of which are designated by reference
numeral 464. The tubes or rods can be sized (e.g., have diameter
selected) to fill the gaps between the inner perimeter 462 of the
tube 460 and the curved length 459 of the perimeter 456 of the
second region 442 to help prevent inclusions or voids in fiber
drawn from the preform. The outer region 460, the first and second
regions 438 and 442 and the tubes or rods 464 can all be comprised
of, consist essentially of, or consist of substantially similar
material, such as, for example, silica glass.
[0052] The preform 400 can include third and fourth regions, 470
and 474, respectively. The fourth region 474 can be substantially
similar to the third region 470. One or both of the third and
fourth regions can have a D-shaped cross section. The third region
470 can include an outer perimeter 478 having a curved length 480
and a substantially straight length 482. The outer perimeter 444 of
the first region 438 can include another substantially straight
length 484 that is substantially parallel to the substantially
straight length 482. In the embodiment of the preform 400 shown in
FIG. 4, the substantially straight lengths 482 and 484 are depicted
as coextensive, though one of ordinary skill in the art, cognizant
of the disclosure herein, understands that the substantially
straight lengths 482 and 484 can be spaced from each other. As
compared with the embodiment shown in FIG. 3, the embodiment shown
in FIG. 4 can use fewer tubes or rods 464 (if any at all) to fill
any gaps between the inner perimeter 462 of the outer region 460
and the outer perimeter 456 of the second region 442.
[0053] The outer perimeter 428 of the preform 400 can comprise a
diameter, indicated by arrows 493, that intersects the first and
second cores, 422 and 424, and that is substantially perpendicular
to one or both of the substantially straight lengths 448 and 458.
The first and second regions, 438 and 442, such as when arranged in
the preform 400, can comprise the diameter indicated by reference
numerals 495. The diameter 495 can intersect the first and second
cores, 422 and 424. The diameter 495 can be substantially
perpendicular to one or both of the substantially straight lengths
448 and 458 and/or can be the maximum diameter that can be taken
across the first and second regions 438 and 442. The diameter 495
can be coextensive with at least part of the diameter 493.
[0054] In the embodiments shown in FIGS. 3 and 4, the outer regions
360 and 460, which can be the cross sections of commercially
available substrate tubes, surround the regions (e.g., the first,
second, third and fourth region and the regions 364 and 464
corresponding to rods or tubes) so as to hold them together.
However, one or more of the regions shown in FIGS. 3 and 4 as
surrounded by the outer region can be fused to each other such that
the outer region need not be included. The outer perimeter of the
preform is thus made up of the appropriate lengths of the
boundaries of the various regions. Typically it is desirable that
the outer perimeter be substantially circular. If the preform is to
be rotated during draw, it can be advantageous to balance the
preform about the longitudinal axis so as to avoid undue vibration
during draw.
[0055] In one practice of the invention, the outer region can be
included, as shown in FIGS. 3 and 4, and the first and second
regions fused together, including fusing to the outer region via,
for example, heating.
[0056] Though not explicitly indicated by reference numbers in FIG.
4, the outer perimeter of the second region 442 can include at
least one substantially straight length that is substantially
parallel to the straight length 482 of the outer perimeter 478 of
the third region 470. In the embodiment of the preform 400 shown in
FIG. 4, the outer perimeter of the second region 442 also includes
a substantially straight length that is substantially parallel to a
substantially straight length of the fourth region 474.
[0057] One of ordinary skill in the art, aware of the disclosure
herein, understands that a preform according to the invention can
include additional regions. For example, the first region 438 could
comprise two regions with a vertical boundary between them,
located, for example, to the left of the first core 422. In such an
embodiment the region including the first core 422 would include a
perimeter having only substantially straight lengths. Such
considerations apply equally to the second region 442.
[0058] Furthermore, a preform according to the disclosure can
include one or more stress-inducing regions for providing an
optical fiber drawn from the preform with a selected birefringence.
For example, a fiber having sufficient birefringence can better
maintain, for a certain input polarization, the polarization state
of the light as it propagates along a length of the fiber, such
that the polarization state of the light exiting the other end of
the fiber is identical to or more closely resembles the input
polarization state. Such a fiber is often referred to as
"polarization maintaining" or "PM" fiber. A stress-inducing region
creates a non-uniform stress field in the fiber for inducing
birefringence via the stress-optic effect. One type of
stress-inducing region comprises material having a different
coefficient of thermal expansion than the material of the preform
(or resultant drawn fiber) that is disposed about the
stress-inducing region. Differences in dimensional changes during
the drawing process (which involves heating, softening and flow of
preform constituents and subsequent solidification) causes
permanent stresses to be induced in the resultant drawn fiber and
hence birefringence. With reference to FIGS. 3 and 4, the preform
300 can include stress-inducing region 396 and the preform 400 can
include stress-inducing region 496.
[0059] Although preforms 300 and 400 are illustrated in FIGS. 3 and
4, respectively, as each including one stress-inducing region, one
of ordinary skill in the art, apprised of the present disclosure,
will appreciate that a preform according to the disclosure can
include a plurality of stress-inducing regions, such as, for
example, a pair of stress-inducing regions. For example, with
reference to FIG. 3, the pair of stress-inducing regions could be
diametrically opposed about the first core 330. In this instance
one of the pair of stress-inducing regions would be located within
the second region 342. FIGS. 3 and 4 are exemplary, and the
stress-inducing region or regions can be located in any region or
regions of the preform. The location and arrangement of the
stress-inducing region or regions is typically determined, at least
in part, by the amount of birefringence desired in the optical
fiber drawn from the preform, and, as is known in the art, the
amount of birefringence can be a function of at least the distance
of the stress-inducing regions from the core, the size and shape of
the stress-inducing region, as well as the difference between the
coefficient of thermal expansion of the stress-inducing region and
the material between the stress-inducing region and the core.
[0060] FIGS. 5A-5E schematically illustrate steps that can be
employed in fabricating one or both of the preforms of FIGS. 3 and
4. Each of FIGS. 5A through 5E is a cross sectional view, taken
along a longitudinal axis, of an elongate member that can be used
to form a preform to draw an optical fiber having first and side
cores. In one sense, each member can be considered a preform as it
could be drawn to form an optical fiber, though the resultant fiber
would have just a single, centrally located core.
[0061] FIG. 5A shows an elongate member 501 that includes a core
522 and a cladding 537 disposed about the core 522. The elongate
member 501 can include an outer perimeter 544 that substantially
conforms to a circle having the radius R.sub.5. The elongate member
501 can be shaped, such as by being machined (e.g., grinding) to
remove material to the right of the dotted line A in FIG. 5A, such
that the outer perimeter 544 of the elongate member 501 includes
the curved length 546 and the substantially straight length 548, as
shown in FIG. 5B. The core 522 remains within the outer perimeter
544, and the edge of the core is shown as being spaced from the
substantially straight length 548 by the distance S1.
[0062] FIG. 5C shows a second elongate member 503 that includes a
core 524 and a cladding 547 disposed about the core 524. The second
elongate member 503 can include an outer perimeter 556 that can
substantially conform to a circle having the radius R.sub.6. One of
the radii R.sub.5 and R.sub.6 is typically different than the other
of the radii, such as by being smaller or larger. In the embodiment
shown in FIG. 5E, the radius R.sub.6 is smaller than the radius
R.sub.5. The elongate member 503 can be shaped, such as by being
machined (e.g., grinding) to remove material to the left of the
dotted line B in FIG. 5C, such that the outer perimeter 556 of the
elongate member 503 includes the curved length 559 and the
substantially straight length 558, as shown in FIG. 5D. The core
524 remains within the outer perimeter 556, and the edge of the
core is shown as being spaced from the substantially straight
length 558 by the distance S2.
[0063] The first elongate member 501 and the second elongate member
503 can be arranged as illustrated in FIG. 5E, such that the
substantially straight length 548 of the first elongate member 501
faces the substantially straight length 558 of the second elongate
member 503. The substantially straight length 548 can be
substantially parallel to the substantially straight length 558.
The substantially straight length 548 can be spaced from the
substantially straight length 558. In the embodiment shown in FIG.
5E, the substantially straight length 548 is substantially parallel
to and contiguous with the substantially straight length 558. The
substantially straight length 558 is shown as coextensive with part
of the substantially straight length 548 in FIG. 5E. The
arrangement shown in FIG. 5E can be disposed within an outer
region, such as the tube 360 of FIG. 3, along with rods or tubes to
fill gaps as desired, to form the preform 300 having the cross
section shown in FIG. 3. One way to arrange the elongate members
501 and 503 as shown in FIG. 5E is to fuse the elongate member 501
to the elongate member 503.
[0064] The various regions of a preform according to the invention
can be cross sections of various elongate members. According to one
practice of the invention, it can be advantageous in certain
instances to fuse some or all of the members together prior to
drawing an optical fiber from the preform. However, such fusing is
not considered necessary. For example, members can simply be
arranged in proximity to each other within a tubular outer member
(e.g., the member having the cross section 360 in FIG. 3) as shown
in FIGS. 3 and 4. The term "preform," as used herein, is not
limited to the particular, final structure from which a particular
fiber is drawn. For example, in the instance where all of the
regions or members are fused, the unfused arrangement is also
considered a preform. An outer region can be disposed about the
arrangement 505 using an Outside Vapor Deposition process, as noted
above.
[0065] To form the preform 400 illustrated in FIG. 4, elongate
members 501 and 503 can be shaped such that material above the
dotted line C and below the dotted line D in FIG. 5B is removed
from elongate member 501 and material above the dotted line E and
below the dotted line F in FIG. 5D is removed from elongate member
503. The outer perimeter of each of the elongate members 501 and
503 now includes two additional substantially straight lengths,
with, in the case of the elongate member 501, the two additional
straight lengths each being substantially parallel to each other
and hence substantially perpendicular to the substantially straight
length 548. In the case of the elongate member 503, each of the two
additional substantially straight lengths is substantially parallel
to each other and substantially perpendicular to the substantially
straight length 558. As appreciated by one of ordinary skill in the
art, in light of the disclosure herein, the elongate members can be
arranged with additional D shaped elongate members (which form the
regions 470 and 474) and disposed within a suitable tubular outer
region 460 to form the preform 400 shown in FIG. 4. Tubes or rods
464 can be included as appropriate, and may not be needed at all in
certain practices of the invention.
[0066] Elongate members suitable to provide D-shaped regions 470
and 474 can be formed from circular elongate members that are
shaped to remove material and provide the outer perimeters of the
regions with appropriate substantially straight lengths (e.g., the
substantially straight lengths 482 and 486 shown in FIG. 4. The
additional elongate members can be drilled to form a hole to
accommodate additional side cores, such as, for example, the side
core 491 shown in FIG. 4. For example, the '008 published
application teaches that additional side cores can be used.
[0067] Returning briefly to FIG. 5B, a stress-inducing region 596
can be added to the preform 300 or 400, by, for example, forming a
hole (e.g., ultrasonically drilling a hole) in the first elongate
member 501 and inserting a suitable stress inducing elongate member
(e.g., a stress rod, such as borosilicate stress rod) into the
hole. The hole can be formed and/or the rod inserted, at any
appropriate step in the process of fabricating the preform. For
example the hole can be drilled prior to removing material to the
right of dotted line "A," the stress rod inserted and the first
elongate member 501 collapsed about the elongate stress member, and
the material to right of dotted line "A" then removed. Or, instead,
the hole can be formed prior to material removal, material then
removed, and the stress rod inserted into the hole subsequent to
material removal. If the first and second elongate members 501 and
503, respectively, are fused together, the hole and/or the stress
rod can be inserted after the fusing. It may not be necessary to
collapse the elongate member 501 about the elongate stress member
prior to drawing a fiber from the preform, though this can have
certain advantages (e.g., preventing the elongate stress member
from falling out of the hole). If desired, fusing and collapse can
be avoided to various extents, and all or part of the preform
assembled as inter fitting parts and drawn to a fiber directly from
the assembled preform, as is discussed above. Some expedient should
be taken to keep the stress rod in the hole, such as, for example,
adding a cap to the bottom of the preform or refraining from
drilling the hole all the way through the longitudinal extent of
the preform or preform member.
[0068] Basic information regarding the formation of stress-inducing
elongate members (e.g., stress rods) is known in the art. For
example, a modified chemical vapor deposition (MCVD) process can be
used to deposit silica-based glass inside a silica or doped silica
(e.g., fluorine down doped) substrate tube. The deposited silica
glass can include a selected dopant for increasing or decreasing,
as appropriate, the coefficient of thermal expansion of the
deposited silica glass. Known suitable dopants include boron and
phosphorus. After deposition of a sufficient amount of doped
silica, the tube is then collapsed to form a substantially solid
stress rod. At this point the tube includes a region (e.g., that
part of the rod that is formed by the tube) that is typically free
of the dopant included in the deposited glass. A typical stress rod
can include, for example an inner region having, for example, a
diameter of from about 9 to about 12 mm, with an outermost diameter
of, for example, about 15 mm (the region in between typically
corresponds to the region formed by the tube). It can be
advantageous to remove most or all of the tube-derived region such
that the stress rod is substantially free of an outermost region
that is free of the selected dopant. This can allow for higher
birefringence in the fiber drawn from the preform, as doped
deposited silica having the different thermal coefficient of
thermal expansion (due to the selected dopant alone or in
combination with other dopants) will be closer to the core of the
drawn fiber. Also, removal of most or all of the region
corresponding to the tube can be advantageous where the substrate
tube is pre-doped with fluorine, as the removal can help avoid
formation of undesired light-guiding regions in the resultant drawn
fiber.
[0069] It can be advantageous to initially form the hole so as to
have an internal diameter that is less than the desired final
diameter and to increase the diameter from the smaller diameter to
the final diameter by, for example, one or both of honing and
etching. For example, an etchant solution (e.g., hydrofluoric acid)
can be pumped through the hole to etch away the walls and increase
the diameter. In fact, generally speaking, etching or honing to a
final dimension from an initially machined dimension that is
slightly different that the final dimension can be a useful
technique that can be applied in other circumstances as well. For
example, rather than grinding away all of the material to the right
of the dotted line A in FIG. 5A, most can be removed by grinding
and then the final desired dimension (i.e., shaping such that all
of the material to the right of the dotted line A is removed)
achieved by etching. Accuracy can be improved, as etching gives
finer control of the material removal process.
[0070] It is understood that one of ordinary skill in the art is
aware of the teachings of the '008 patent publication and other
publications pertaining to fibers having side cores, as well as the
art of preform manufacture, wherein dimensions of a preform and
draw parameters are selected so as to fabricate a fiber having
selected core and cladding dimensions so as to provide for a
desired optical behavior. Accordingly, details of the foregoing are
not repeated here.
[0071] According to the present invention, the primary and side
cores of preforms, such as the preforms noted above in FIGS. 3, 4
and 6 (and FIGS. 8 and 12 below) can be constructed and arranged
such that the preform can be drawn with rotation to a fiber wherein
the first core is multimode at a selected wavelength of operation
and the second core is spaced from and winds around the first core
so as to have a selected longitudinal pitch and such that at said
wavelength of operation said side core couples to a higher order
mode of the first core and selectively increases the attenuation
thereof relative to the fundamental mode of the first core.
[0072] The spacing S3 indicated in FIG. 5E is substantially equal
to the sum of S1 and S2. Applicants consider that, for a spacing
between the first and the side cores of less than a certain value,
it is safer, in terms of the likelihood of damage to the preform,
to shape the preform as shown in FIGS. 5A-5E to achieve the spacing
S3 than it is to drill a hole next to the core 222 so as to achieve
a spacing S (shown in FIG. 2) that is substantially the same as
S3.
[0073] Note that, regarding one or both of the elongate members 501
and 503, prior to shaping the members to remove material to the
side of the lines A and B, the geometrical center defined by the
outer perimeter of the member lies within the cross section area of
a core of the member (e.g., within the cross section area of the
core 522 or 524, where the cross sectional area core is that area
within the outer perimeter of the core). This is typically because
the initial forming of the member including a core is by a
substantially axially symmetric process. The core is typically
located substantially at the center of the elongate member (other
cores, such as, for example, additional side cores, could be added
to the member, such as by drilling a hole and inserting a core rod
into the hole). After removal of the material to the right of
dotted line A (in the case of elongate member 501) or to the left
of dotted line B (in the case of elongate member 503), the
geometric center of the member as now defined by the outside
perimeter of the member typically is no longer located within the
cross sectional area of the core.
[0074] The elongate members 501 and 503 are each typically made of
glass, such as, for example, silica glass. The claddings 537 and
547 can each consist essentially of, or consist of undoped silica
glass. The elongate members can be formed by a variety of general
preform fabrication processes known in the art of fiber optic
preparation, including, but not limited to, MCVD, OVD, VAD, and the
like.
[0075] FIGS. 6A and 6B are cross sections of yet further
embodiments of an optical fiber preform according to the present
invention. With reference to FIG. 6A, the optical fiber preform
600A comprises a first core 622A and a side core 624A. The first
core comprises an outer perimeter 634A. The preform 600A comprises
an outer perimeter 628A bounding the cross section of the optical
fiber preform 600A and defining the geometrical center of the
preform. The geometrical center can lie within the cross sectional
area of the first core 622A, which is the area within the outer
perimeter 634A of the first core 622A. The cladding 637A can be
disposed about the core 622A and can have an annular shape, as
shown in FIG. 6A. The first core 622A and cladding 637A can be part
of a core rod having as its outer perimeter the outer perimeter
644A of the cladding 637A. The core rod can be unitary, that is,
the first core 622A can adheringly contact the cladding 637A. One
way to form such a unitary core rod is to deposit core material on
the inside of a tube, such as by using the aforementioned MCVD
technique, where at least part of the tube can form at least part
of the cladding. The tube can be etched or machined to have its
outside diameter reduced, if necessary, to provide a core rod
having a selected outside diameter. Typically, the cladding 637A
will form a rather thin layer around the first core 622A of the
unitary core rod. In various practices of the invention, the
thickness T of the cladding is no greater than 15% of the diameter
D.sub.1 of the first core 622A, no greater than 10% of the diameter
D.sub.1, or no greater than 8% of the diameter D.sub.1. The first
core 622A and the cladding 637A of the core rod can be
substantially solid, that is, substantially free of longitudinally
extending voids, such as would be the case, for example, for the
cladding 637A if it were formed by packing rods or tubes together
(such as is often done in forming a preform for photonic bandgap or
microstructured cladding fiber).
[0076] An annular arrangement of elongate members 653A, which are
typically circular (e.g., tubes or rods), is disposed about the
core 622A and cladding 637A, along with the side core 624A, which
can be a tube or rod, typically including a material that is
different than a material included by one or more of the tubes or
rods 653A. The preform 600A can include a first annular region
673A, having the outer perimeter 683A, disposed about the annular
arrangement of elongate members 653A, and a second annular region
660A, having the outer perimeter 628A, disposed about the first
annular region 673A. The annular regions can be cross sections of
first and second substrate tubes, respectively, as is appreciated
by one of ordinary skill in the art, in light of the disclosure
herein. If available, a single tube having the appropriate
thickness can be used instead of two tubes. As noted above in
conjunction with the discussion of FIGS. 3, 4 and 5A-5E, all or
selected parts of the preform 600A can be fused prior to drawing
optical fiber from the preform.
[0077] The preform 600A can include one or more stress-inducing
regions, such as the two regions 696A shown in FIG. 6A, which, in
the embodiment shown in FIG. 6A, are arranged diametrically about
the core 622A. The stress-inducing regions can be formed as note
above, that is, by drilling holes and inserting appropriate stress
rods. The holes can be drilled after fusing some or all of the
parts of preform 600A together (if it is desired to fuse the parts)
or, by way of another example, can be drilled on the tube that
forms annular region 673A prior to incorporating the tube into the
preform 600A. Stress rods can be inserted in holes at various steps
in the process of forming the preform.
[0078] With reference to FIG. 6B, the optical fiber preform 600B
comprises a first core 622B and a side core 624B. The first core
comprises an outer perimeter 634B. The preform 600B comprises an
outer perimeter bounding the cross section of the optical fiber
preform 600B and defining the geometrical center of the preform.
The geometrical center can lie within the cross sectional area of
the first core 622B, which is the area within the outer perimeter
634B of the first core 622B. The cladding 637B can be disposed
about the core 622B and can have an annular shape, as shown in FIG.
6B. The first core 622B and cladding 637B can be part of a core rod
having as its outer perimeter the outer perimeter of the cladding
637B. The core rod can be unitary, that is, the first core 622B can
adheringly contact the cladding 637B. The first core 622B and the
cladding 637B of the core rod can be substantially solid, that is,
substantially free of longitudinally extending voids, such as would
be the case, for example, for the cladding 637B if it were formed
by packing rods or tubes together (such as is often done in forming
a preform for photonic bandgap or microstructured cladding
fiber).
[0079] An annular array of elongate members 653B, which are
typically circular (e.g., tubes or, more preferably, substantially
solid rods), can be disposed about the core 622B and cladding 637B,
along with the side core 624B, which can be a tube or rod,
typically including a material that is different than a material
included by one or more of the tubes or rods 653B. The tubes or rod
653B can, as one of ordinary skill having read this disclosure will
readily understand, form part of the cladding of the fiber drawn
from the preform. The preform 600B can include a first annular
region 673B, having an outer perimeter, disposed about the annular
arrangement of elongate members 653B. All or selected parts of the
preform 600B can be fused prior to drawing optical fiber from the
preform. For brevity, not all features of the preform 600B are
described in detail, as they are similar to those described above
for the preform 600A of FIG. 6A, as one of ordinary skill,
cognizant of the disclosure herein, will appreciate.
[0080] Note the preform 600B can include one or more
stress-inducing regions, which have been described above. A
stress-inducing region or regions can be formed from an elongate
stress-inducing member 697B included in the preform, as shown in
FIG. 6B. A plurality of stress-inducing members 697B can be
arranged as indicated by reference number 698B so as to form a
stress-inducing region in the fiber drawn from the preform. An
elongate stress-inducing member can comprise, for example, a
borosilicate stress rod. Phosphosiliate stress rods, as well as
stress rods that include both boron and phosphorus are also known
in the art, as are other constituents for an appropriate elongate
stress member.
[0081] As noted above, general preform fabrication methods and
drawing a fiber from a preform using a draw tower having a furnace
for heating one end of the preform are well known in the art, and,
accordingly, are not discussed in a lot of detail herein.
[0082] A preform according to the invention can be drawn to a fiber
while rotating one or both of the fiber and the preform.
Preferably, only the preform is rotated. FIG. 7 schematically
illustrates drawing an optical fiber from a preform while rotating
the preform. The motor 711 rotates, as indicated by reference
numeral 715, the preform 717. The furnace 725 heats the end 727 of
the preform 717 to facilitate drawing of the fiber 731. The coating
apparatus 741 can add a coating to the fiber that can serve to
protect the fiber as well as a cladding, depending, as is known in
the art, on the characteristics of the cladding (e.g., the
refractive index of the cured cladding relative to the refractive
index of the region of the optical fiber to which the coating is
applied. The coating apparatus 741 shown in FIG. 7 includes nested
reservoirs 745 and 747 for applying a dual coating to the optical
fiber 731. As is known in the art, the inner coating reservoir can
apply a soft coating and the outer coating reservoir can apply a
hard coating over the soft coating. A UV lamp (not shown) typically
cures the coatings before the fiber 731 is wound on a spool. In
some practices of the invention the coating has a lower index to
confine pump light to the region of the fiber surrounded by the
cured coating. Such fibers are known in art as "cladding pumped" or
"double clad" fibers. It can be advantageous for the region of the
fiber surrounded by the coating to have a non-circular outer
perimeter. For example, the outer region 360 of FIG. 3 or the outer
region 460 of FIG. 4 can be shaped, such a by grinding, to have a
non-circular perimeter (e.g., a flat can be ground into the outer
perimeter 328 of outer region 360 to provide the desired
non-circularity).
[0083] It can be desirable to apply a vacuum to a preform, such as,
for example, to the region of the preform 300 of FIG. 3 inside of
the inner perimeter 362 of the outer region 360, while drawing the
preform to a fiber, as is know in the art.
[0084] A preform can be made using one or more of a variety of
suitable methods, including vapor phase methods such as outside
vapor deposition (OVD), Modified Chemical Vapor Deposition (MCVD),
Chemical Vapor Deposition (CVD) and Vapor Axial Deposition (VAD)
and combinations thereof. Vapor phase methods usually employ
suitable gas precursors that are introduced to a hot substrate, a
hot zone, or directly into a flame. The latter technique is known
as flame hydrolysis. In the flame hydrolysis technique, precursor
gases are introduced to a flame to form soot that is deposited on a
substrate, such as the inside, outside or end of a tube or rod. The
soot is subsequently heated and sintered using an oven or furnace.
The tube or rod can form a part of the resultant optical fiber
preform, or can be removed. The OVD and VAD processes typically
involve flame hydrolysis. In other vapor phase techniques, such as
CVD and MCVD, precursor gases are introduced to a hot zone and/or a
heated substrate, which can again be a tube or rod. One supplier of
MCVD lathes and of draw towers is Nextrom Technologies of Finland.
Nextrom Technologies has a U.S. office located at 2150 Northmont
Pkwy--suite F, Duluth, Ga. 30096.
[0085] Each of the foregoing techniques can include one or more
overjacketing steps wherein a member formed by one of the foregoing
techniques is overjacketed with a glass tube that will typically
form additional cladding. Overjacketing can be used to add a
region, such as the region 360 of FIG. 3, to a preform. Glass tubes
are available in a variety of sizes, and are often specified by an
inner and outer diameter. Tubes can be etched to adjust the outer
and inner diameters as necessary to facilitate various desired
glass working steps, as is know in the art. Glass tube and rods
suitable for deposition of soot, the deposition of glass, or for
use as an overjacket are available from Heraeus Amersil, Inc. 3473
Satellite Blvd., Duluth, Ga., 30096. The glass rods and/or tubes
can include various types of glasses, such as, for example, silica
glass, borosilicate glass, a fluorinated silica glass, a phosphate
glass and other types of glasses.
[0086] Rods and tubes can also be made by casting molten glass into
appropriate molds. For example, one technique for providing a tube
is to cast molten glass into a mold that is spun on a lathe.
Centrifugal force causes the molten glass to press outward against
the walls of the mold such that the glass cools to form a tube.
Rods and tubes can also be made by Sol-Gel techniques.
[0087] Most typically, silica is the host glass of the optical
fiber or preform, to which other materials are added. Common dopant
materials used with silica include aluminum, boron, fluorine,
germanium, phosphorus, titanium, the rare earths (such as, for
example, erbium, ytterbium and lanthanum) and transition metals,
which can be used to provide selected attenuation. However, other
types of glass, such as, for example, chalcogenide glass, ZBLAN
glass, phosphate glass, fluoride glass, germanium based glass and
the like, as well as any of the single crystal or polycrystalline
materials such as thallium bromoiodide, germanium, zinc selenide,
and the like, may be found suitable. By way of example, and not of
limitation, an optical fiber according to the invention may
comprise any of these or other materials, or variants thereof,
singly or in combination for the core, cladding or other layers.
Dopants can be added using, for example, vapor phase processes
and/or solution doping processes.
[0088] FIG. 8 schematically illustrates in more detail an example
of the preform 300 illustrated in FIG. 3. In most instances the
reference numerals of FIG. 8 correspond to those used in
conjunction with FIG. 3. To allow for separate discussion of the
FIGURES without engendering confusion, the suffix "A" is added to
the reference numerals of FIG. 8. To avoid undue repetition,
however, a full description corresponding to each of the reference
numerals of FIG. 8 is not repeated here, and for those not
discussed here reference may be made to the discussion accompanying
FIG. 3 and the reference numeral as used there without the suffix.
Similar considerations apply to other FIGURES below, as will be
readily appreciated by one of ordinary skill in the art.
[0089] In the embodiment shown in FIG. 8, the first region 338A can
comprise an outer perimeter 344A having a first curved length 346A
that is characterized by a radius of curvature R.sub.1A that is
substantially the same as R.sub.2A, where the outer perimeter 356A
of the second region 342A includes a second curved length 359A that
is characterized by the radius of curvature R.sub.2A. That is, the
radius of curvature R.sub.1A and the radius of curvature R.sub.2A
can originate from the substantially the same location (e.g., the
geometrical center of the outer perimeter 334A of the first core
322A) and have substantially the same length. The first and second
curved lengths 346A and 359A can form substantially the entire
outer perimeter of the first and second regions, which outer
perimeter can be the outer perimeter 328A of the preform, as the
substantially circular nature of the outer perimeter of the first
and second regions can obviate the need to dispose the region 360A
about the first and second regions. If the region 360A is disposed
about the first and second regions 338A and 342A, there may be
little or no need to include tubes or rods 364 shown in FIG. 3, as
any gaps between the outer perimeters of the first and second
regions and the inner perimeter of the outer region 362A can be
considerably reduced in size (e.g., see the gap between the outer
perimeter 356 of the second region 342 and the inner perimeter 362
of the outer region 360 as illustrated in FIG. 3). Though not shown
in FIG. 8, the preform 300A can include one or more stress-inducing
regions. Stress-inducing regions can be included with the preform
300A according to variety of techniques, such as, for example,
according to one or more of the techniques relating to stress rods
that are disclosed elsewhere herein.
[0090] The preform 300A can be fabricated generally according to
the procedures shown in FIGS. 5A through 5E and the accompanying
description thereof provided herein. However, the elongate member
503, which can form region 342A shown in FIG. 8, is shaped to have
an outer perimeter having a curved section substantially
characterized by the radius of curvature of R.sub.2A. With
reference to FIG. 9, the elongate member 503A includes a core 524A,
a cladding 547A, and an outer perimeter 556A that substantially
conforms to a circle having a radius R.sub.6A, where R.sub.6A is
longer than R.sub.2A.
[0091] With reference to FIGS. 8 and 9, the distance C is the
distance between the geometric centers of the outer perimeters of
the typically circular cores 322A and 324A taken along a line
through the centers (e.g., the line H in FIG. 9). With reference to
FIG. 9, the material between the outer perimeter 556A and the
dotted line 559A, namely the material of region 913 of FIG. 9, is
removed. The dotted line 559A substantially conforms to a radius of
curvature R.sub.2A where the radius originates at location 919
spaced along a diameter of the outer perimeter 556A by a distance C
from the geometric center of the outer perimeter of the core 524A.
Typically, the elongate member 503A is made from an axially
symmetric process and has a circular core 524A, and hence the
aforementioned geometric center is simply the center of the
circular core. The material above the line B is then removed, such
as by grinding or etching so as to form the second elongate member
503A shown in FIG. 10 having an outer perimeter 556A comprising the
substantially straight length 558A and the curved length 556B that
is characterized by the radius R.sub.2A. The first elongate member
501 can be arranged with the second elongate member 503A such that
the substantially straight lengths are contiguous and the entire
outer perimeter thereof is characterized by a radius R.sub.2A. The
line B shown in FIG. 9 and the line A shown in FIG. 5A can be
selected such that the edges of the cores of the elongate members
are spaced by the appropriate distance (e.g., the distance S3 shown
in FIG. 5E) when the first and second elongate members are arranged
as shown to form the preform 300A. This distance is in turn
selected to provide the desired spacing d (see FIG. 1) in the
resultant fiber drawn from the preform with rotation, as
illustrated schematically in FIG. 7.
[0092] FIG. 11 illustrates one technique for removing the region
913 from the elongate member 503A of FIG. 9 so as to provide the
curved length 556B shown in FIG. 10. The elongate member 503A is
mounted on a polishing or grinding apparatus so as to pivot about
the axis Z shown in FIG. 11. The axis Z can be moved downward, as
indicated by arrow 1103, such that the elongate member 503A
contacts the grinding or polishing surface 1105. Axis Z can be
selected so as to pass through locations 1119, on each end face of
the elongate member, corresponding to location 919 shown in FIG. 9.
Arrows 1101 indicate the pivoting motion about the axis Z. The axis
Z can be moved downward until it is spaced from the grinding or
polishing surface by a distance substantially equal to R.sub.2A. In
one practice of the invention, reference numeral 1105 can represent
the surface of an etchant bath.
[0093] FIG. 12 illustrates another example of a preform 300B
according to the invention. With reference to FIG. 10, the material
above line E can be removed from elongate member 503A so that
elongate region 503A may form second region 342B of the preform
300B of FIG. 12. The second region 342B can comprise the outer
perimeter 356B having a substantially straight length 1284 formed
by the removal of the material above line E in FIG. 10. The preform
300B can comprise a third region 1270 comprising a third core 1291
that is spaced from the cores 322B and 324B. The spacings between
the cores can be selected such that the resultant spacings between
the corresponding cores of the fiber drawn from the preform can, as
taught in the '008 application noted above, attenuate higher order
modes in the fiber core formed from the core 322B of the preform
300B. The third region 1270 can be derived from an elongate member
according to the teachings herein. The outer perimeter of the third
region 1270 can comprise the curved length 1280, which can be
characterized by a radius R.sub.12 that has a shorter length than
the radius R.sub.1B, where the outer perimeters of the first and
second regions each include curved lengths characterized by the
radius R.sub.1B. By proper selection of the foregoing radii, in
certain circumstances the gap 1221 can remain small enough such
that the preform 300B need not include rods or tubes to help fill
the gap. In other circumstances such rods or tubes may help fill
the gap. The preform 300B can include one or more stress-inducing
regions, such as, for example, the stress-inducing region indicated
by reference numeral 397. Stress-inducing regions can be included
with the preform 300B according to variety of techniques, such as,
for example, according to one or more of the techniques relating to
stress rods that are disclosed elsewhere herein.
[0094] It is noted that in many instances the reference numerals of
FIG. 12 correspond to those used in conjunction with FIG. 3. To
allow for separate discussion of the FIGURES without engendering
confusion, the suffix "B` is added to the reference numerals of
FIG. 12. To avoid undue repetition, however, not all the reference
numerals of FIG. 3 are provided with a corresponding reference
numeral in FIG. 12 and not all those shown are discussed here. For
those reference numerals not discussed here reference may be made
to the discussion accompanying FIG. 3 and the reference numeral as
used there without the suffix.
[0095] Several embodiments of the invention have been described and
illustrated herein. Those of ordinary skill in the art will readily
envision a variety of other means and structures for performing the
functions and/or obtaining the results or advantages described
herein and each of such variations or modifications is deemed to be
within the scope of the present invention. More generally, those
skilled in the art would readily appreciate that all parameters,
dimensions, materials and configurations described herein are meant
to be exemplary and that actual parameters, dimensions, materials
and configurations will depend on specific applications for which
the teachings of the present invention are used. It is therefore to
be understood that the foregoing embodiments are presented by way
of example only and that within the scope of the appended claims
and equivalents thereto, the invention may be practiced otherwise
than as specifically described.
[0096] In the claims as well as in the specification above all
transitional phrases such as "comprising", "including", "carrying",
"having", "containing", "involving" and the like are understood to
be open-ended. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the U.S. Patent
Office Manual of Patent Examining Procedure .sctn.2111.03, 7.sup.th
Edition, Revision 1.
* * * * *