U.S. patent application number 15/659127 was filed with the patent office on 2017-11-09 for non-circular multicore fiber and method of manufacture.
The applicant listed for this patent is Corning Incorporated. Invention is credited to Douglas Llewellyn Butler, Daniel Warren Hawtof, Rick Charles Layton, III, Gautam Meda, John Stone, III, Pushkar Tandon.
Application Number | 20170322369 15/659127 |
Document ID | / |
Family ID | 57398411 |
Filed Date | 2017-11-09 |
United States Patent
Application |
20170322369 |
Kind Code |
A1 |
Butler; Douglas Llewellyn ;
et al. |
November 9, 2017 |
NON-CIRCULAR MULTICORE FIBER AND METHOD OF MANUFACTURE
Abstract
A multicore fiber is provided. The multicore fiber includes a
plurality of cores spaced apart from one another, and a cladding
surrounding the plurality of cores and defining a substantially
rectangular or cross-sectional shape having four corners. Each
corner has a radius of curvature of less than 1000 microns. The
multicore fiber may be drawn from a preform in a circular draw
furnace in which a ratio of a maximum cross-sectional dimension of
the preform to an inside diameter of the preform to an inside
diameter of the draw furnace is greater than 0.60. The multicore
fiber may have maxima reference surface.
Inventors: |
Butler; Douglas Llewellyn;
(Painted Post, NY) ; Hawtof; Daniel Warren;
(Corning, NY) ; Layton, III; Rick Charles;
(Mansfield, PA) ; Meda; Gautam; (Corning, NY)
; Stone, III; John; (Painted Post, NY) ; Tandon;
Pushkar; (Painted Post, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Family ID: |
57398411 |
Appl. No.: |
15/659127 |
Filed: |
July 25, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15137336 |
Apr 25, 2016 |
|
|
|
15659127 |
|
|
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|
62168125 |
May 29, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/00 20130101; C03B
37/029 20130101; C03B 37/027 20130101; C03B 2203/12 20130101; C03B
2203/04 20130101; G02B 6/02042 20130101; C03B 37/01205 20130101;
C03B 2203/34 20130101; C03B 37/01222 20130101 |
International
Class: |
G02B 6/02 20060101
G02B006/02; C03B 37/012 20060101 C03B037/012; C03B 37/027 20060101
C03B037/027; C03B 37/029 20060101 C03B037/029; C03B 37/012 20060101
C03B037/012 |
Claims
1. A multicore fiber comprising: a plurality of cores spaced apart
from one another and arranged in N.times.M array, wherein N and
M.gtoreq.2; and a cladding surrounding the plurality of cores and
defining a substantially rectangular cross-sectional shape having
four corners, wherein each corner has a radius of curvature less
than 1000 microns.
2. The multicore fiber of claim 1, wherein each corner has a radius
of curvature of less than 500 microns.
3. The multicore fiber of claim 1, wherein each corner has a radius
of curvature of less than 250 microns.
4. The multicore fiber of claim 1, wherein each corner has a radius
of curvature of less than 10 microns.
5. The multicore fiber of claim 1, wherein the rectangular
cross-sectional shape is a substantially square cross-sectional
shape having an array of cores aligned in rows and columns.
6. The multicore fiber of claim 1, wherein the fiber is drawn from
a preform comprising a plurality of rectangular canes each having
one or more flat surfaces that are aligned and consolidated
together to form the preform which is inserted in a furnace to draw
the multicore fiber.
7. A multicore fiber comprising: a plurality of cores spaced apart
from one another; and a cladding surrounding the plurality of cores
and defining a non-circular cross-sectional shape having at least
four corners, wherein the shape has a maxima structure located near
each of at least two of the corners and reduced minima surfaces
between the enlarged maxima structures to allow for alignment of
the multicore fiber with an interconnecting device.
8. The multicore fiber of claim 7, wherein the multicore fiber is
formed from a preform that comprises a plurality of core canes
having one or more flat surfaces that are aligned together and
consolidated to form the preform which is inserted in a furnace to
draw the fiber.
Description
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/137,336 filed Apr. 25, 2016, which claims the benefit
of priority under 35 U.S.C. .sctn.119 of U.S. Provisional
Application Ser. No. 62/168,125 filed on May 29, 2015 the content
of which is relied upon and incorporated herein by reference in its
entirety.
BACKGROUND
[0002] This invention generally pertains to a multicore fiber that
typically includes a cladding having a plurality of cores which
allows for space division multiplexing (SDM) and enhanced signal
carrying capacity, multiple spatial paths for data communication
are constructed within a single transmission fiber. The multicore
fiber is typically connected to a transceiver having transmitters
and receivers at opposite ends of the fiber. Typically, the
multicore fiber is formed in a generally circular shape which may
result in drawbacks in aligning the individual cores to the signal
paths in connecting devices. Accordingly, it is desirable to
provide for a multicore fiber that offers alternative alignment and
connectivity.
SUMMARY
[0003] In accordance with one embodiment, a method of forming a
multicore fiber is provided. The method includes the step of
forming a preform having a plurality of cores and cladding
surrounding the cores, wherein the preform has a non-circular cross
section with a plurality of corners and a maximum dimension across
the cross section of the preform. The method also includes the step
of inserting the preform in a draw furnace having a substantially
circular cross section such that a ratio of the maximum dimension
of the preform to an inside diameter of the draw furnace is greater
than 0.60. The method further includes the step of drawing a
multicore fiber from the preform to achieve a reduction in
cross-sectional size as the fiber is drawn while substantially
maintaining a non-circular cross-sectional shape and the plurality
of corners of the preform.
[0004] In accordance with another embodiment, a multicore fiber is
provided. The multicore fiber includes a plurality of cores spaced
apart from one another in a N.times.M array wherein N and
M.gtoreq.2, and a cladding surrounding the plurality of cores and
defining a substantially rectangular cross-sectional shape having
four corners, wherein each corner has a radius of curvature less
than 1000 microns.
[0005] In accordance with a further embodiment, a multicore fiber
is provided that includes a plurality of cores spaced apart from
one another, and a cladding surrounding the plurality of cores and
defining a non-circular cross-sectional shape having at least four
corners. The non-circular cross-sectional shape has a maxima
structure located near each of at least two of the corners and
reduced minima surfaces between the enlarged maxima structures to
allow for alignment of the multicore fiber with an interconnecting
device.
[0006] Additional features and advantages will be set forth in the
detailed description which follows, and in part will be readily
apparent to those skilled in the art from that description or
recognized by practicing the embodiments as described herein,
including the detailed description which follows, the claims, as
well as the appended drawings.
[0007] It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary, and are intended to provide an overview or framework to
understanding the nature and character of the claims. The
accompanying drawings are included to provide a further
understanding, and are incorporated in and constitute a part of
this specification. The drawings illustrate one or more
embodiments, and together with the description serve to explain
principles and operation of the various embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an end view of a rectangular multicore fiber
having a linear array of six cores according to one embodiment;
[0009] FIG. 1A is an end partial perspective view of a plurality of
starting canes used to form a preform for forming the multicore
fiber of FIG. 1;
[0010] FIG. 1B is an end partial perspective view of the canes
following the step of machining flat side walls into a generally
square shape;
[0011] FIG. 1C is an end partial perspective view of a preform
formed after consolidating the canes shown in FIG. 1B which is used
for forming the multicore fiber of FIG. 1;
[0012] FIG. 2 is an end view of a multicore fiber having a
generally square shape with sixteen cores shown in a
two-dimensional 4.times.4 array according to another
embodiment;
[0013] FIG. 2A is an end partial perspective view of a plurality of
starting canes used to form a preform for forming the multicore
fiber of FIG. 2;
[0014] FIG. 2B is an end partial perspective view of the canes
following the step of machining flat side walls on each cane;
[0015] FIG. 2C is an end partial perspective view of a preform
formed by consolidating the canes shown in FIG. 2B which is used to
form the multicore fiber of FIG. 2;
[0016] FIG. 3 is a schematic diagram illustrating an optical fiber
production system useful for forming the multicore fiber;
[0017] FIG. 4 is a cross-sectional view of the cylindrical draw
furnace containing a preform for forming the multicore fiber,
according to one embodiment;
[0018] FIG. 4A is a cross-sectional view taken through line IVA-IVA
of FIG. 4 showing the preform relative to the furnace in an upper
draw furnace position;
[0019] FIG. 4B is a cross-sectional view taken through line IVB-IVB
of FIG. 4 further illustrating the preform shape at a lower
position within the draw furnace;
[0020] FIG. 4C is a cross-sectional view taken through line IVC-IVC
of FIG. 4 illustrating the preform at yet a lower position within
the draw furnace;
[0021] FIG. 4D is a cross-sectional view taken through line IVD-IVD
of FIG. 4 further illustrating the preform at yet a lower position
within the draw furnace;
[0022] FIG. 5 is a top view of the preform in the draw furnace
illustrating relative dimensions;
[0023] FIG. 6 is an end view of a multicore fiber having a linear
array of cores and maxima reference surfaces formed near opposite
ends;
[0024] FIG. 6A is an exploded end view of a preform used to form
the multicore fiber of FIG. 6 showing the machined canes and the
assembly thereof;
[0025] FIG. 6B further illustrates the preform of FIG. 6A following
machining of the maxima and minima surfaces;
[0026] FIG. 7 is an end view of a multicore fiber having a
rectangular two-dimensional array of cores and maxima reference
surfaces near the corners, according to a further embodiment;
[0027] FIG. 7A is an exploded end view of a preform used to form
the multicore fiber of FIG. 7 showing the machined canes and the
assembly thereof;
[0028] FIG. 7B is an end view of the preform of FIG. 7A further
showing maxima and minima surfaces; and
[0029] FIG. 8 is an end view of a multicore preform containing a
plurality of openings that results in air channels in the drawn
multicore fiber, according to yet a further embodiment.
DETAILED DESCRIPTION
[0030] Reference will now be made in detail to the present
preferred embodiments, examples of which are illustrated in the
accompanying drawings. Whenever possible, the same reference
numerals will be used throughout the drawings to refer to the same
or like parts.
[0031] The following detailed description represents embodiments
that are intended to provide an overview or framework for
understanding the nature and character of the claims. The
accompanied drawings are included to provide a further
understanding of the claims and constitute a part of the
specification. The drawings illustrate various embodiments, and
together with the descriptions serve to explain the principles and
operations of these embodiments as claimed.
[0032] Referring to FIGS. 1-2C, the terminal end of a bare
(uncoated) multicore fiber 10 having a non-circular cross-sectional
shape is illustrated, according to first and second embodiments.
The multicore fiber 10 illustrated in FIG. 1 is a linear array
embodiment having a plurality of glass cores 12 arranged in a
1.times.N array and surrounded by a glass cladding 14 and having a
generally rectangular cross-sectional shape. In this example, a
linear array of six cores 12 are illustrated, each spaced apart
from one another and separated by the cladding 14. The multicore
fiber 10 illustrated in FIG. 2 is a two-dimensional array
embodiment having a plurality of cores 12 arranged in an N.times.M
array and surrounded by a cladding 14 and having a generally square
cross-sectional shape. In this example, the N.times.M array is a
4.times.4 array of sixteen cores 12, each spaced apart from one
another and separated by the cladding 14. It should be appreciated
that the multicore fiber 10 may have any number of two or more
cores 12.
[0033] The multicore fiber 10 employs a plurality of glass cores
spaced from one another and surrounded by a cladding having a shape
that is non-circular in cross section. The cores and cladding may
be made of glass or other optical fiber material and may be doped
suitable for optical fiber. In one embodiment, the shape of the
multicore fiber 10 may be a rectangular cross-sectional shape as
shown in FIG. 1. In another embodiment, the shape may be a square
cross-sectional shape as shown in FIG. 2. According to other
embodiments, other non-circular cross-sectional shapes and sizes
may be employed including hexagonal and various polygonal forms.
The multicore fiber 10 employs a plurality of cores, each capable
of communicating light signals between transceivers including
transmitters and receivers which may allow for parallel processing
of multiple signals. The multicore fiber 10 may be used for
wavelength division multiplexing (WDM) or multi-level logic or for
other parallel optics of spatial division multiplexing. The
multicore fiber 10 may advantageously be aligned with and connected
to the various devices in a manner that allows for easy and
reliable connection so that the plurality of cores 12 are
accurately aligned at opposite ends with light communication paths
in connecting devices.
[0034] The multicore fiber 10 is produced by drawing a preform
having a plurality of cores and cladding configured in a shape
similar to the shape of the resulting fiber that is drawn
therefrom. In the embodiment shown in FIGS. 1-2C, the multicore
fiber 10 has a plurality of cores 12 spaced apart from one another
and a cladding 14 surrounding the plurality of cores and defining a
substantially rectangular or square cross-sectional shape having
four approximately right angle, i.e., ninety degree (90.degree.)
corners 16 that are relatively sharp. Each corner 16 has a radius
of curvature of less than 1000 microns, according to one
embodiment. More specifically, the radius of curvature of each of
the corners 16 is less than 500 microns, and more preferably less
than 250 microns, and most preferably less than 10 microns. The
rectangular cross-sectional shape may have two pairs of unequal
side walls or may be a substantially square cross-sectional shape
with two pairs of equal side walls having a two-dimensional array
of cores 12 aligned in rows and columns as shown in FIG. 2. The
non-circular shape of the multicore fiber 10 includes substantially
flat surfaces, such as the four lateral outside walls defining the
cross-sectional circumference of the fiber 10 which allows for the
orientation and alignment of the fiber 10 with a connecting device,
such that the cores 12 accurately align with light communication
paths in the connecting device. The flat surfaces thereby serve as
reference surfaces for alignment purposes to align and connect the
multicore fiber 10 to a connecting device. In addition, the
multicore fiber 10 may include one or more additional cores (not
shown) that may serve as an alignment core to align the cores 12
with communication paths presented with the connecting device at
one or both ends of the fiber 10.
[0035] In the embodiment shown in FIGS. 1-1C, the linear array
configuration of the multicore fiber 10 has six cores 12 linearly
spaced from one another and surrounded by cladding 14. The spacing
between the cores 12 may be 20 to 200 microns, according to one
embodiment. The core-to-core spacing between cores varies by less
than 1 micron, and preferably less than 0.2 microns. The multicore
fiber 10 has four generally flat surfaces around the circumference
of the end view. One or more of the generally flat surfaces serves
as a reference surface to orient and align the fiber 10 and its
multiple cores 12 with a connecting device. Each of the four
corners 16 has a radius of curvature that is sufficiently small so
as to provide a generally sharp corner. In one embodiment, the
radius of curvature of each corner 16 is less than 1000 microns,
preferably less than 500 microns, more preferably less than 250
microns, and most preferably less than 10 microns. The multicore
fiber 10 may have a width of 240 microns, a height of 40 microns,
and may extend a length of in the range of 10 cm to several hundred
kilometers. The multicore fiber 10 may be formed in a draw furnace
by employing a preform having a much larger size in height and
width and similar shape and is drawn according to a process that
maintains the non-circular cross-sectional shape of the fiber 10
and the shape of each of the plurality of corners 16.
[0036] A method of forming a preform that is used to form the
multicore fiber 10 of FIG. 1 is illustrated in FIGS. 1A-1C. In FIG.
1A, a plurality of generally cylindrical starting canes 20 are
provided. The glass canes 20 may be constructed of any glass or
other optical fiber material and may be doped suitable for the
manufacture of optical fiber. Each cane 20 has a cylindrical glass
core 22 and a surrounding cladding 24. The glass core 22 may be
formed of Germania doped silica or other suitable glass. The
cladding 14 may be formed of silica or fluorine doped silica or
other suitable glass. The canes 20 may each have a starting minimum
diameter of 25 millimeters and a minimum core diameter size of 8
microns, according to one embodiment. The length of the canes 20
may be 1 meter or greater. Each of the starting canes 20 is
processed to form one or more flat outer surfaces 28. As seen in
FIG. 1B, four flat outer surfaces 28 are formed as square defining
side walls in each of the canes 20 such that the cane 20 has a
square cross section. The flat surfaces 28 may be formed by
machining the cylindrical canes into a square cross-sectional shape
having four relatively sharp approximately right angle corners each
having a radius of curvature of less than 1000 microns, and
preferably less than 500 microns, more preferably less than 250
microns, and more preferably less than 10 microns. The individual
square shaped canes 20 are then stacked together side-by-side in a
linear array and accurately aligned and consolidated together to
form the preform 30 as shown in FIG. 1C. The consolidation may
include heating both ends of the machined canes 20 with a hand
torch or other heater to melt the individual canes 20 together.
Optionally, a redraw handle may be attached to the preform 30. The
preform 30 may be redrawn in a two-step process of down driving the
preform at 15 millimeters per minute at 2000.degree. furnace
temperature to bond the glass together, increased to 20 millimeters
per minute at temperatures of 1900.degree. C. down to 1850.degree.
C. In addition, the preform 30 may be twisted as redrawn to enable
a skew in the drawn fiber of greater than one rotation per 100
kilometers. The preform 30 may be returned to the top of the hot
zone of the draw furnace and redrawn at a temperature of about
2150.degree. C., twisting the cane assembly together, and down
feeding the preform at 10 millimeters per minute at 2000.degree. C.
to 1820 millimeters square canes. In one example, a 7 inch (17.78
cm) inside diameter consolidation furnace may be used to heat the
preform to consolidate it together. The preform may be the down
driven at 5 to 15 millimeters per minute through 1490.degree. C. to
1550.degree. C. in the hot zone of the draw furnace. The preform 30
is heated in the draw furnace to melt the glass and draw the bare
optical fiber 10 shown in FIG. 1. The bare optical fiber 10 may
also be coated.
[0037] The square multicore fiber 10 illustrated in FIG. 2 is
formed to include a two-dimensional N.times.M array of cores 12
each separated by a distance and surrounded by cladding 14, where N
and M are greater than or equal to 2. The distance between the
cores 12 within each row and column may be in the range of 20 to
200 microns. The multicore fiber 10 shown in FIG. 2 has four
approximately right angle ninety degree (90.degree.) corners 16
each having a radius of curvature less than 1000 microns, more
preferably less than 500 microns, more preferably less than 500
microns, more preferably less than 250 microns, and most preferably
less than 10 microns. As a result, the square configuration of the
multicore fiber 10 has relatively sharp corners 16. The square
multicore fiber 10 may have a width of 125, a height of 125, and
may extend a length of in the range of 10 cm to hundreds of
kilometers.
[0038] The multicore fiber 10 of FIG. 2 is formed by drawing the
fiber 10 from a preform 30 which is shown formed in FIGS. 2A-2C. In
FIG. 2A, a plurality of cylindrical starting canes 20 are
illustrated, each having a cylindrical glass core 22 and a
surrounding glass cladding 24. Each cane 20 may have a minimum
width of 25 millimeters in diameter, a minimum core diameter of 8
microns, and a length of 1 meter or more. In all, sixteen canes are
arranged in a two-dimensional 4.times.4 array. Each of the sixteen
canes 24 are processed to include at least one flat surface, and
more particularly to include four flat surfaces 28 defining a
square shape as shown in FIG. 2B. The four flat surfaces 28
provided on each cane 20 may be formed by machining the cylindrical
wall to form a square cross-sectional shape having four generally
sharp corners 26. The plurality of machined canes 20 are aligned in
a 4.times.4 array as seen in FIG. 2B and then stacked together and
consolidated to form a preform 30 as shown in FIG. 2C. The preform
30 may be consolidated by heating both ends of the canes 20 with a
hand torch or other heater to melt the individual canes 20 together
and by redrawing the preform in the two-step process as described
above in connection with the linear array preform shown in FIG. 1C.
As a result, the preform 30 shown in FIG. 2C has a non-circular
cross-sectional shape shown as a square shape having four
substantially flat outer side walls and four sharp corners 26 that
define the square shape. Each corner 26 has a radius of curvature
less than 1000 microns, and more preferably less than 500 microns,
more preferably less than 250 microns, and most preferably less
than 10 microns. The preform 30 is then inserted into a draw
furnace and used to draw the multicore fiber 10 shown in FIG.
2.
[0039] Referring to FIG. 3, an optical fiber production system 40
is generally shown, according to one embodiment. The optical fiber
production system 40 includes a draw furnace 42 that may be heated
to a temperature of about 2000.degree. C. The glass optical fiber
preform 30 is placed in the draw furnace 42 and the multicore fiber
10 is heated and drawn therefrom, as shown by the bare optical
fiber 10 output exiting the bottom of the furnace 42. Once the bare
optical fiber 10 is drawn from the preform 30, the bare optical
fiber 10 may be cooled as it exits the bottom of draw furnace 42.
After sufficient cooling, the bare optical fiber 10 may be
subjected to a coating unit 44 where a primary protective coating
layer is applied to the outer surface of the bare optical fiber 10.
After leaving the coating unit 44, the coated optical fiber 10'
with a protective layer can pass through a variety of processing
stages within the production system 40 such as tractors or rollers
46 and 48 and onto a fiber storage spool 50. One of the rollers 46
or 48 may be used to provide the necessary tension in the optical
fiber as it is drawn through the entire system and eventually wound
onto the storage spool 50.
[0040] The preform 30 is used to draw the multicore fiber 10 such
that the preform 30 shown in FIG. 1C is redrawn into the fiber 10
as shown in FIG. 1 and the preform 30 of FIG. 2C is redrawn into
the fiber 10 shown in FIG. 2. In doing so, the preform 30 is
inserted in a cylindrical draw furnace which may have a
substantially circular cross-sectional shape on the inside surface
and is heated to a temperature to melt the preform 30 and draw the
multicore fiber from the preform to achieve a reduction in
cross-sectional size as the fiber is drawn while substantially
maintaining the non-circular cross-sectional shape and the
plurality of corners of the preform. The draw process may include
drawing the multicore fiber 10 at a draw tension of greater than
100 grams to enable the square or rectangular geometry and the
plurality of corners to maintain their shape. According to one
embodiment, the multicore fiber 10 is drawn at a draw tension in
the range of 100 grams to 300 grams, and more preferably in the
range of 150 grams to 300 grams. The multicore fiber 10 may be
drawn at a peak draw furnace temperature ranging between
1900.degree. C. and 2150.degree. C. It should be appreciated that
the linear and two-dimensional embodiments of the multicore fiber
10 may be formed using the preforms shown in FIGS. 1C and 2C and
inserted into the draw furnace 40 of the fiber production system 40
to form the multicore fiber 10 shown in FIGS. 1 and 2,
respectively.
[0041] Referring to FIGS. 4-5, the preform 30 is illustrated
disposed in the draw furnace 42 during the fiber draw process. The
draw furnace 42 has a substantially circular cross section with
circular inside diameter D.sub.f The inside diameter D.sub.f of the
draw furnace may range from 3 to 12 inches (7.62-30.48 cm)
according to one embodiment, and may be approximately 7 inches
(17.78 cm), according to one specific example. The preform 30 has a
non-circular cross section with a plurality of corners as described
herein. The non-circular cross section of the preform 30 may be a
rectangular cross section or a square cross section having four
corners 26. The preform 30 has a maximum dimension D.sub.p across
the cross section of the preform 30 as seen in FIG. 5. The maximum
dimension of a rectangular or square preform is defined by the
diagonal line extending between opposite corners. The preform 30 is
inserted into the draw furnace 42 and may be heated to a
temperature of approximately 2000.degree. C. The heat generated by
the draw furnace causes the preform to melt and to draw the preform
into the multicore fiber which is drawn from the bottom of the
preform and exiting the bottom of the draw furnace 42.
[0042] The relative dimensions of the preform 30 to the inside
diameter of the furnace 42 are illustrated near the top of the
furnace 42 in FIG. 4A and FIG. 5. FIGS. 4B-4D illustrate the
preform 30 at lower portions of the furnace as the multicore fiber
is drawn from the draw furnace 42. Initially, near the top of the
draw furnace 42, the maximum cross-sectional dimension of the
preform 30 is shown by dimension D.sub.p as a diagonal between
opposite corners of the rectangular preform. The inside diameter of
the draw furnace 42 is shown by line D.sub.f and the preform 30 is
preferably central within the draw furnace 42. The ratio of maximum
dimension D.sub.p of the preform 30 to the inside diameter D.sub.f
of the draw furnace 42 is greater than 0.60, more preferably
greater than 0.70, more preferably greater than 0.80, more
preferably greater than 0.90, and more preferably greater than
0.95.
[0043] The square or rectangular preform 30 when drawn at the high
temperatures in the draw furnace can undergo rounding of the
corners or edges due to the surface energy driven viscous flow
restructuring. The degree of rounding of the edges can be described
by a non-dimensional draw parameter X which is the ratio of the
residence time and the characteristic time of restructuring, and
may be represented by the following equation:
X = ( length of hot zone .times. surface tension draw speed .times.
glass viscosity .times. maximum cross - sectional dimensional Dp )
. ##EQU00001##
[0044] The glass surface tension may be calculated using the
following relationship: surface tension (in
dynes/centimeter)=233.28+0.35.times.T.sub.peak, where T.sub.peak is
the peak temperature in Kelvin at the draw. The glass viscosity may
be calculated using the following relationship: glass viscosity (in
Poise)=Exp[-13.738+(60604.7/T.sub.peak)]. In some embodiments, the
preform may have an acceptable rounding of the edges when drawn
under conditions corresponding to the draw parameter X having
values preferably less than 5.times.10.sup.-6, more preferably less
than 2.times.10.sup.-6, and even more preferably less than
1.times.10.sup.-6. By employing a non-circular preform 30 such as a
rectangular or square preform having corners and defining a maximum
dimension of a size sufficiently large enough such that the corners
are near the inner walls of the draw furnace 42 results in a heat
distribution that melts the preform in a manner that maintains the
shape of the preform, particularly the corners of the preform, as
it is drawn into the multicore fiber 10. As a result, the preform
does not undergo excessive rounding of the corners as it
transitions to the fiber such that the resulting multicore fiber 10
has a substantially similar shape to the shape of the preform
30.
[0045] In accordance with another embodiment, the multicore fiber
is drawn at a draw speed of V.sub.draw from a preform having
maximum cross section dimension D.sub.p in a draw furnace having
peak temperature in Kelvin of T.sub.peak and hot zone length of
L.sub.draw under conditions that result in non-dimensional draw
parameter
X=(L.sub.draw.times..sigma.)/(V.sub.draw.times..mu..times.D.sub.p)
to be less than 5.times.10.sup.-6; wherein a is the glass surface
tension defined as .sigma. (dynes/cm)=233.28+0.035.times.T.sub.peak
and .mu. is the glass viscosity defined as .mu.
(Poise)=Exp[-13.738+(60604.7/T.sub.peak)]. In another embodiment,
the multi-core fiber is drawn under process conditions
corresponding to the non-dimensional draw parameter of less than
2.times.10.sup.-6. In yet another embodiment, the multi-core fiber
is drawn under process conditions corresponding to the
non-dimensional draw parameter of less than 1.times.10.sup.-6.
[0046] The preform 30 starts out with the largest maximum dimension
D.sub.p near the top of the draw furnace 42 and maintains the
cross-sectional size up until the furnace 42 melts the glass and
draws the glass downward in an approximately tapered shape as shown
in FIG. 4. As the glass melts and begins to taper at a lowered
position within the furnace 42 shown by cross section IVB-IVB of
FIG. 4B, the width and height dimensions of the preform 30 are
reduced and the four side walls are generally shown having a slight
concave curvature with the corners being substantially sharp as
shown in FIG. 4B. As the preform 30 further is drawn into the
furnace 40 to the lower position shown in FIG. 4C, the width and
height dimensions of the preform 30 are further reduced and the
corners remain relatively sharp. As the preform 30 is drawn further
into the draw furnace 42 to the position shown in FIG. 4D, the
height and width of the preform 30 are further reduced and the four
walls are shown substantially straight and the four corners are
relatively sharp. The final drawn multicore fiber 10 resulting from
drawing the preform 30 is shown in FIG. 1 having four substantially
flat surfaces defining a rectangular cross-sectional shape and four
sharp corners 16. The multicore fiber 10 may have slight dimples in
the side walls as seen in FIG. 1 which are the result of multiple
canes used to form the preform. It should be appreciated that the
preform 30 shown in FIG. 2C may likewise be placed in the draw
furnace 42 and heated to draw the multicore fiber 10 shown in FIG.
2 which similarly maintains its shape with flat surfaces and sharp
corners.
[0047] As a result, a method of forming a multicore fiber 10 is
provided that includes the step of forming a preform 30 having a
plurality of cores 12 and cladding 14 surrounding the cores,
wherein the preform has a non-circular cross section with a
plurality of corners and a maximum dimension across the cross
section of the preform. The method includes the step of inserting
the preform in the draw furnace 42 having a substantially circular
cross section, such that a ratio of the maximum dimension D.sub.p
of the preform to an inside diameter of the draw furnace D.sub.f is
greater than 0.60. The method further includes the step of drawing
a multicore fiber 10 from the preform to achieve a reduction in
cross-sectional size as the fiber is drawn while substantially
maintaining a non-circular cross-sectional shape and the plurality
of corners of the preform. The maximum dimension of the preform to
the inside diameter of the draw furnace is greater than 0.60,
preferably greater than 0.70, preferably greater than 0.80, more
preferably greater than 0.90, and most preferably greater than
0.95.
[0048] Referring to FIGS. 6-8, a bare multicore fiber 110 and the
formation of a preform 130 for forming the bare multicore fiber 110
having maxima reference surfaces and minima surfaces is illustrated
according to both a linear array embodiment and a two-dimensional
array embodiment. The multicore fiber 110 illustrated in FIG. 6 is
a linear array embodiment having a plurality of cores 12 arranged
in a 1.times.N and surrounded by a cladding 14. In this example,
the linear array includes six cores 12 each spaced from one another
and separated by the cladding 14. The left and right side of the
end of the multicore fiber 110 are shown having a generally rounded
surface in this embodiment. The top and bottom sides of the
multicore fiber 110 each have flat surfaces that define maxima
surfaces 162 and minima surfaces 160 located between the maxima
surfaces 162 on opposite the ends. The maxima surfaces 160 have an
extended height greater than the minima surfaces by height H.sub.f.
The maxima surfaces 162 are located on opposite side of the fiber
preferably near the corners and serve as reference structures for
orienting the multicore fiber 110 into proper alignment with a
connecting device. It should be appreciated that the cladding 114
surrounding the plurality of cores 112 defines a non-circular
cross-sectional shape and that the multicore fiber 110 generally
has four corners which are generally shown as rounded in this
embodiment. The maxima surface structures 162 are located near each
of at least two of the corners or the left and right ends of the
fiber and the reduced height minima surface structures 160 are
located between the enlarged maxima structures 162 to allow for
accurate alignment of the multicore fiber with an interconnecting
device. As such, the multicore fiber 110 will rest against the
ground or other surface by making contact with maxima reference
structures 162.
[0049] The multicore fiber 110 of FIG. 6 is formed from a preform
130 which is shown formed in FIGS. 6A and 6B. In FIG. 6A, a
plurality of starting canes 120 are shown each having a glass core
122 and a cladding 124. The starting canes 120 may be cylindrical
and may be machined to include one or more flat surfaces. In the
embodiment shown, the left and right end canes are formed with one
flat surface on the inner side and the middle two canes 120 are
formed with left and right flat surfaces. The plurality of canes
120 are aligned and joined together and consolidated to form a
preform as described above. Further, maxima surface structures 172
and minima surface structures 170 are machined into the top and
bottom surfaces of the preform 130 as shown in FIG. 6B. In doing
so, the maxima surfaces 172 are formed at a height that is greater
than the minima surfaces 170 by distance H.sub.p on both the top
and bottom sides. The maxima surfaces 172 are formed near the
corners or opposite ends on both the top and bottom sides. The
minima surfaces 170 are formed between the opposite ends. The
preform 130 is then placed into the draw furnace and used to draw
the bare multicore fiber 110 shown in FIG. 6. The multicore fiber
110 may also be coated.
[0050] Referring to FIG. 7, a bare multicore fiber 110 is shown
having a plurality of maxima and minima surface structures 162 and
160, respectively, for a two-dimensional core array embodiment. In
this embodiment, a two-dimensional N.times.M array of sixteen glass
cores 112 are shown surrounded by cladding 114. The multicore fiber
110 is generally rectangular shaped having four side walls defining
a rectangular or square shape and has somewhat rounded corners.
Each of the four walls has maxima surfaces 162 formed near the
corners or ends and minima surfaces 160 between the maxima surfaces
162. The maxima surfaces 162 have a height greater than the minima
surfaces 160 by a distance H.sub.f. As such, the maxima surfaces
162 serve as standoffs to allow for orientation and alignment of
the multicore fiber 110 with a connecting device so that the core
112 align with the light communication paths. It should be
appreciated that the multicore fiber 110 may have one or more side
walls which include the maxima surfaces 162 for alignment purposes,
but in this example all four side walls are configured as such.
[0051] The formation of a preform 130 that is used to form the
multicore fiber 110 of FIG. 7 is illustrated in FIGS. 7A and 7B.
The preform 130 is formed by starting with a plurality of starting
canes 120 that may be cylindrical and may be machined to include
one or more flat surfaces. In the example shown, the four corner
canes are each formed having two flat surfaces, while the remaining
canes 120 are formed having three or four flat surfaces as shown.
The machined canes 120 are then aligned and assembled together and
consolidated to form a preform. The preform 130 is then machined to
form the flat surfaces on the side walls which include the maxima
surfaces 172 and minima surfaces 170 as shown in FIG. 7B. The
maxima surfaces 172 have a height greater than the minima surfaces
170 by a distance H.sub.p. The preform 130 is then placed in the
draw furnace and used to draw the multicore fiber 110 shown in FIG.
7. As can be seen, the resulting shape of the multicore fiber 110
including the maxima and minima surfaces and the corners or ends
are substantially the same as the shape of the preform shown in
FIG. 7B.
[0052] While approximately square or rectangular shaped preforms
are shown for drawing like shaped multicore fibers, it should be
appreciated that other shapes and sizes may be produced with
reference surfaces according to other embodiments.
[0053] Referring to FIG. 8, a multicore preform 130' is shown
according to yet another embodiment including a plurality of
openings 180 which may be used to form a multicore fiber that
results in air channels or air holes in the drawn multicore fiber.
The openings 180 may be formed by machining openings into adjoining
canes during the assembly of the preform 130 such that the
resulting shape of the assembled canes results in the shape of the
opening 180. Alternatively, the plurality of openings 180 may be
formed by drilling holes within the preform. The presence of
openings 180 in the multicore fiber that is produced from the
preform 130' allows for air channels which may provide for signal
isolation.
[0054] Various modifications and alterations may be made to the
examples within the scope of the claims, and aspects of the
different examples may be combined in different ways to achieve
further examples. Accordingly, the true scope of the claims is to
be understood from the entirety of the present disclosure in view
of, but not limited to, the embodiments described herein.
[0055] It will be apparent to those skilled in the art that various
modifications and variations can be made without departing from the
spirit or scope of the claims.
* * * * *