U.S. patent application number 12/656174 was filed with the patent office on 2010-05-20 for multiple core optical fibre.
This patent application is currently assigned to QinetiQ Limited. Invention is credited to Charlotte R. H. Bennett, Laurent Michaille, Terence J. Shepherd, David M. Taylor.
Application Number | 20100124397 12/656174 |
Document ID | / |
Family ID | 34531789 |
Filed Date | 2010-05-20 |
United States Patent
Application |
20100124397 |
Kind Code |
A1 |
Bennett; Charlotte R. H. ;
et al. |
May 20, 2010 |
Multiple core optical fibre
Abstract
A multicore optical fibre includes a microstructured cladding
material formed from a plurality of cladding elements arranged in
an array and each cladding element comprising at least two
different materials each having different refractive indices, and a
plurality of core elements formed within interstitial regions
between adjacent cladding elements. A fibre so formed may have a
large number of cores per unit cross-sectional area as compared
with prior art fibres, and thus allows the fibre to have relatively
short distances between adjacent cores for a given required
inter-core isolation. A fibre so formed has utility in many areas
requiring high core density, such as inter-chip optical
communication, or optical communication between circuit boards.
Inventors: |
Bennett; Charlotte R. H.;
(Malvern, GB) ; Shepherd; Terence J.; (Malvern,
GB) ; Michaille; Laurent; (Malvern, GB) ;
Taylor; David M.; (Malvern, GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
QinetiQ Limited
London
GB
|
Family ID: |
34531789 |
Appl. No.: |
12/656174 |
Filed: |
January 20, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11886844 |
Sep 21, 2007 |
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PCT/GB2006/001061 |
Mar 23, 2006 |
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12656174 |
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Current U.S.
Class: |
385/125 ;
385/126 |
Current CPC
Class: |
G02B 6/02376 20130101;
G02B 6/02338 20130101; G02B 6/02347 20130101; G02B 6/024 20130101;
G02B 6/02333 20130101; C03B 37/0122 20130101; C03B 2203/42
20130101; C03B 2203/12 20130101; C03B 37/02781 20130101; C03B
2203/18 20130101; C03B 37/01222 20130101; C03B 2203/14 20130101;
C03B 2203/34 20130101; G02B 6/02042 20130101; G02B 6/02371
20130101 |
Class at
Publication: |
385/125 ;
385/126 |
International
Class: |
G02B 6/032 20060101
G02B006/032; G02B 6/02 20060101 G02B006/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2005 |
GB |
0506032.2 |
Claims
1-22. (canceled)
23. A multicore optical fibre comprised of a plurality of cladding
elements forming a regular, periodic cladding array, each cladding
element having a higher index region and a lower index region, with
interstitial regions formed between said cladding elements, wherein
core element material is inserted into a plurality of the
interstitial regions during a manufacturing stage so as to form a
plurality of core regions, each core in the fibre being
substantially surrounded by the lower index region of the cladding
elements, such that the regularity of the cladding array is not
affected.
24. A multicore fibre as claimed in claim 23 wherein each cladding
element comprises an inner and an outer region, with the inner
region having a different refractive index to the outer region.
25. A multicore fibre as claimed in claim 24 wherein the inner
region comprises one of a gas, a vacuum and a liquid.
26. A multicore fibre as claimed in claim 24 wherein the outer
region of each cladding element has a higher refractive index than
the inner region.
27. A multicore fibre as claimed in claim 23 wherein the core
material has a refractive index substantially similar to that of
the cladding material having a higher refractive index.
28. A multicore fibre as claimed in claim 23 wherein the cladding
elements comprise cylindrical tubes.
29. A multicore fibre as claimed in claim 23 wherein at least one
core element contains a dopant.
30. A multicore fibre as claimed in claim 23 wherein the cladding
elements are of different overall dimensions in one transverse axis
as compared to another transverse axis.
31. A multicore fibre as claimed in claim 30 wherein the fibre is
birefringent.
32. A multicore fibre as claimed in claim 23 wherein certain
cladding elements are arranged to have different relative
quantities of materials as compared to other cladding elements.
33. A multicore fibre as claimed in claim 23 wherein at least one
core element is of a different size to an adjacent core
element.
34. A multicore fibre as claimed in claim 23 wherein a coupling
coefficient between two adjacent cores is arranged to be not
greater than a required design value.
35. A multicore fibre as claimed in claim 23 wherein a coupling
coefficient between two adjacent cores is arranged to be not less
than a required design value.
36. A multicore fibre as claimed in claim 23 wherein at least one
core element comprises a single mode fibre.
37. A multicore fibre as claimed in claim 23 wherein the fibre is
adapted to taper in diameter along its length.
Description
[0001] This invention relates to the field of optical fibres, and
in particular to fibres having a plurality of cores.
[0002] Multiple core (Multicore) optical fibres have utility in
many different areas of technology. When used in communications
applications they allow a single fibre to carry different optical
signals in parallel, so increasing the data bandwidth of the
communication and potentially decreasing the cost per unit of
information transmitted. Such communications applications may
involve transmission of data across hundreds of miles or across
distances more usually measured in centimetres.
[0003] Another application is in the field of optical image
transfer where a large number of cores are used to transfer light
from an image to a remote viewing location. In this application one
end of a multicore fibre, or more usually a fibre bundle, is placed
such that the image to be transferred falls onto the cores of the
fibre(s). The light falling on the cores is then transported to the
other end of the fibre(s) where each core will then act as a single
pixel for the image at the remote viewing location. See for example
"Imaging with microstructured polymer fibre", Martijn A. van
Eijkelenborg, Optics Express 342, Vol. 12, No. 2, January 2004.
[0004] Another application is in communications between
opto-electronic devices such as vertical cavity surface emitting
lasers (VCSELs). Such devices may be coupled to digital data
streams to enable data transfer between electronic components using
optical fibres. The application offers reliable and quick data
transfer over comparatively large distances as compared to copper
tracks on printed circuit boards. VCSELs are often employed in
arrays, enabling parallel data streams to be employed for even
greater transfer rates. Other devices such as light modulators may
be used as a light source in this application instead of VCESLs.
Such light modulators will typically be used with optical fibres
designed to preserve the polarisation of light launched into
them.
[0005] A multicore fibre is disclosed In Electronics Letters, 23
Jul. 1998, Vol 34 No. 15, this fibre being a four-core step-index
single mode fibre. The fibre is primarily for telecommunications
applications with the purpose of design being "for optical cable
densification". Core density is clearly an important issue for such
a fibre. Core spacing between adjacent cores for the fibre is 51.8
.mu.m.
[0006] Granted U.S. Pat. No. 6,301,420 discloses a multicore
photonic crystal fibre comprising a periodic cladding structure
formed from a plurality of hollow canes in a triangular matrix with
the cores being formed by effectively replacing some of the hollow
rods with solid canes. The spacing of the cores in the cladding
matrix, i.e. the number of cycles of the periodic cladding
structure, is chosen to achieve desired isolation between
neighbouring cores. For many purposes, particularly in the
communications field, the required spacing between cores will
typically be around four or five cycles of the shortest cladding
period to ensure suitable core isolation. As the total number of
cladding periods is limited in practice by the number of canes that
can be stacked in a preform during the fabrication process, this
imposes a limit on the number of cores per unit cross sectional
area that can be achieved, and this limit may restrict the
applications to which the fibre can be put.
[0007] PCF fibres such as U.S. Pat. No. 6,301,420, and other
microstructured fibres can be made by a "stack and draw" method",
wherein the cladding and core canes are stacked together in a
desired formation, and then pulled into a fibre in a temperature
and pressure controlled environment.
[0008] According to a first aspect of the present invention there
is provided a multicore optical fibre comprising a microstructured
cladding material comprising a plurality of cladding elements
arranged in an array, and a plurality of core elements, wherein
each cladding element is comprised of at least two different
materials, each having different refractive indices, characterised
in that each core has associated therewith a plurality of cladding
elements adjacent thereto, each adjacent cladding element having an
apex substantially co-located with every other associated adjacent
cladding element.
[0009] An apex of each associated cladding element is taken to be
the meeting point of extrapolations from two sides of the cladding
element adjacent the core.
[0010] The present invention allows the spatial separation of, and
number of periods between, adjacent cores to be reduced as compared
to the prior art microstructured fibres whilst still maintaining
satisfactory isolation between adjacent cores. Thus, a greater
density and number of cores per fibre may be obtained.
[0011] The cladding elements may comprise cylindrical tubes. When
formed into a fibre the tubes may have a lower refractive index
inner region, the inner region comprising air, or another gas, or a
vacuum. Alternatively the cladding inner region material may
comprise a liquid or solid, with a refractive index lower than that
of the outer material of the cladding element. The cladding
elements preferably form a periodic array, with the core elements
being located at interstitial positions within the cladding
structure such that the regularity of the cladding is not
affected.
[0012] The outer material of the cladding may be made from silica,
or from any other suitable material. The refractive index of the
outer cladding material is greater than the refractive index of the
inner cladding material.
[0013] The core material may be made from similar material to that
of the cladding outer material. Outer region cladding material that
is in proximity to the core material before the stack is pulled to
form the final fibre will, during such pulling, fuse with the core
and become a part of the core material. Each core will then
comprise the original core material, along with material from the
outer region of a part of the cladding. Each core will be largely
surrounded by inner cladding material of lower refractive index,
but will be supported by webs of material comprising parts of the
outer cladding material that have not formed a part of the
core.
[0014] The invention allows a core to be formed at each
interstitial position in the cladding microstructure. For
cylindrical cladding elements stacked in a triangular array, a
given cladding element will have six interstitial regions, each of
which may form a core in a manner as disclosed herein. The
invention therefore provides a means for significantly increasing
the density of cores per unit cross sectional area as compared to
the prior art.
[0015] Each core in a fibre of the current invention will generally
be significantly smaller in effective diameter than the diameter of
each cladding element, such that it may be located in an
interstitial region between adjacent cladding elements.
[0016] The cladding elements may be formed into a triangular array,
or alternatively may be formed into a square array. A square array
has a larger interstitial region in which a core may be located.
For a triangular array the cladding elements adjacent a core will
each be in contact with all other cladding elements adjacent that
core. A triangular array of cladding elements will tend, in a
finished fibre, to produce a substantially tessellated hexagonal
structure, and core elements may be present without significantly
affecting the tessellation. The core elements will tend to be
substantially round in cross section before the fibre is initially
formed during manufacture, but, due to the manufacturing process,
the mechanical and thermal stresses upon it tend to distort the
core so that it resembles a standard vertex in the cladding, but
with a slight bulge. This can be seen in FIG. 13. Note that,
although the core in a finished fibre will not be circular, it will
have an "effective diameter" that may be regarded as circular for
computer modelling purposes.
[0017] Each core element may be incorporated with the cladding
elements such that, before the stack is pulled, the cores do not
disrupt the regular, periodic nature of the cladding elements. The
cores in a finished fibre will also then not disrupt the regularity
of the cladding array.
[0018] One or more of the cores of the present invention may
advantageously be made to transmit radiation in a single mode of
propagation. This has the known advantage that modal dispersion--a
temporal spreading of the radiation travelling through the
fibre--may be significantly reduced.
[0019] Note that although a fibre according to the present
invention is formed from a series of cladding and core elements,
the resulting fibre is more accurately referred to as a
microstructured optical fibre rather than a photonic crystal fibre.
This is because, for the primary means of propagation, guidance of
light in the current invention does not rely on its interaction
with a photonic crystal structure in the cladding (i.e. a
succession of cycles of differing refractive indices), but instead
relies on the difference between the core refractive index and the
inner cladding material refractive index. Fibres according to the
present invention guiding in this way are therefore index guiding
fibres.
[0020] The invention provides the advantage that adjacent cores
within the fibre may be brought closer together whilst still
retaining a given isolation between them. Conversely, sometimes
such coupling is desirable, and hence a particularly close
spacing--which is simple to achieve with the present invention--can
be adopted to give a required coupling coefficient between adjacent
cores, up to a maximum when cores occupy adjacent interstitial
regions of the cladding structure. Other factors may be used to
influence coupling between adjacent cores, such as altering the
volumetric ratio of the different materials forming the cladding
(i.e. changing the cladding filling fraction), altering the size of
the cores themselves, or doping cores to change their refractive
index. Any of these factors may be used to produce a fibre having
adjacent cores of a required coupling coefficient, within a design
range.
[0021] A weakly coupled multicore fibre may typically have a
coupling constant below approximately 0.02 per metre. This will
provide, for a 1 metre fibre with a triangular arrangement of
cores, a bit error rate of less than approximately 1 in 10.sup.12.
For highly coupled cores, a coupling constant may be arranged to be
of the order 100 or 1000 per metre.
[0022] The invention is particularly suited to producing fibres
having the aforementioned properties whilst being able to transmit
light in a single mode of propagation.
[0023] The number of cores in the fibre may be at least 2, such as
at least 4 such as at least 8 such as at least 20 such as at least
50 such as at least 100 such as at least 200 such at least 500
cores.
[0024] The relative core arrangement may advantageously be adapted
such that, in a pulled fibre, the spacing matches that of an array
of light sources, such as VCSELS or light modulators, or an array
of receivers. Such arrays of sources or receivers are often formed
on a single substrate, and have a standardised relative
spacing.
[0025] According to a second aspect of the present invention there
is provided an optical fibre comprised of a plurality of cladding
elements each cladding element having a higher index region and a
lower index region, with interstitial regions formed between said
cladding elements, characterised in that core material is
positioned in a plurality of the interstitial regions so as to form
a plurality of core regions.
[0026] Preferably, the cross-sectional area of each interstitial
region is significantly smaller than the cross-sectional area of
each cladding element. Preferably each core region has a
cross-sectional area significantly smaller than that of each
cladding element.
[0027] According to a third aspect of the present invention there
is provided a method of making a multicore optical fibre comprising
the steps of: [0028] i. assembling a plurality of canes into a
stack, each cane having a higher index region and a lower index
region, the canes having interstitial regions there between when
assembled; [0029] ii incorporating into two or more of the
interstitial regions in the assembly a core material, the core
material not affecting the positions of adjacent canes; [0030] iii
pulling the stack of canes incorporating the core material into a
fibre.
[0031] After the pulling process, the canes act as an array of
cladding elements, whereas the core material forms core regions
within the cladding element array.
[0032] Known modern fibre pulling methods are able to substantially
eliminate the interstitial regions of microstructured fibres, such
that material surrounding an interstitial region closes in to fill
these regions, these regions then going on to become a vertex in
the cladding structure. Inserting core element material into the
cladding interstitial regions during the manufacturing phase may
create further, secondary interstitial regions around this core
element material. During the pulling process these secondary
interstitial regions may be eliminated using the same known
techniques, such that the core material may fuse with a part of the
outer cladding material to form a larger core region substantially
free from artefacts of the secondary interstitial regions.
[0033] Note that herein the term "interstitial" refers to the
regions between adjacent cladding elements in the fibre in its
unpulled state. When used in the context of a fibre that has been
pulled, it refers to those parts of the fibre where the
interstitial regions would have been in the unpulled fibre.
[0034] For certain applications it is advantageous for the fibre to
preserve the polarisation of light passing along it. To this end,
during the production of the fibre the structure may advantageously
be arranged to be birefringent.
[0035] The fibre may be formed such that the cladding elements are
each of a different overall dimension in one transverse axis as
compared to another transverse axis. Preferably, the structure may
be elliptical. One technique for forming such a structure is to
squeeze the fibre in one axis during pulling process to produce an
elliptical structure of the desired characteristic. Alternatively,
the fibre may be formed using elements that are of a non-circular
cross-section so that any additional shaping during the pulling
phase is not necessary. Alternatively, the fibre may be formed
using cladding canes that have different ratios of higher index
region to lower index region, i.e. having different filling
fractions. By arranging the fibre to have cladding materials of
differing filling fractions in different regions of the
cross-sectional area of the fibre, birefringence can be
obtained.
[0036] The current invention has an advantage over the prior art in
that a multicore fibre can be produced having a greater number of
cores per unit cross sectional area using conventional fibre
pulling equipment. Furthermore, the optical isolation between
adjacent cores is greater for a given physical separation distance
as compared to the prior art. This makes the invention more
suitable to particularly high density packing of the fibre
cores.
[0037] A fibre according to the present invention may be made from
any suitable material. Typically the fibre will be made from
silica, but other materials, such as chalcogenide, fluoride or lead
glasses, or suitable polymer materials may also be used.
[0038] The invention will now be described in more detail, by way
of example only, with reference to the following Figures, of
which:
[0039] FIG. 1 diagrammatically illustrates a multicore optical
fibre of the prior art;
[0040] FIG. 2 diagrammatically illustrates a second multicore
optical fibre of the prior art, this fibre employing a photonic
crystal cladding structure;
[0041] FIG. 3 diagrammatically illustrates a detailed view of the
cladding and core of a prior art photonic crystal fibre both before
and after the pulling process;
[0042] FIG. 4 diagrammatically illustrates a preform and final core
and cladding structure of a first embodiment of the current
invention, the Figure showing a transverse cross-sectional
representation of the fibre;
[0043] FIG. 5 diagrammatically illustrates two embodiments of the
present invention, showing different core layouts;
[0044] FIG. 6 diagrammatically illustrates an embodiment of the
present invention wherein the fibre is arranged to exhibit
birefringence;
[0045] FIG. 7 diagrammatically illustrates an embodiment of the
present invention wherein the fibre is arranged to have different
sized cores;
[0046] FIG. 8 diagrammatically illustrates an embodiment of the
present invention wherein the cladding structure of the fibre is
arranged in a square formation;
[0047] FIG. 9 shows a graph indicating the range of physical
properties a prior art
[0048] PCF fibre must have in order to remain single mode, as
modelled by an "effective index" method;
[0049] FIG. 10 shows a graph indicating the range of physical
properties the fibre of the current invention as shown in FIG. 5
needs to have in order to remain single mode, as modelled by an
"effective index" method;
[0050] FIG. 11 diagrammatically illustrates an embodiment of the
present invention wherein a fibre is arranged to have a tapered
profile along its longitudinal axis with a single mode fibre having
been positioned in an interstitial region during manufacture;
[0051] FIG. 12 diagrammatically illustrates another embodiment of a
tapered fibre according to the present invention wherein a large
diameter end of the fibre is arranged to have cores of a size to
allow convenient coupling to a conventional single mode fibre;
and
[0052] FIG. 13 shows a Scanning Electron Microscope (SEM) image of
a fibre of the present invention.
[0053] FIG. 1 shows a fibre preform used in the manufacture of a
prior art index guiding multicore fibre, in a state before a
drawing or pulling process, in FIG. 1a, and in a state after the
drawing, in FIG. 1b, this being a finished fibre. The fibre is made
by a process of mechanically cutting a single fibre preform 1 into
four, and arranging the four conventional and identical preforms 1
as shown to produce a 2.times.2 fibre preform 2, with the single
preforms 1 in contact as shown, and leaving an interstitial gap 3.
Each preform 1 comprises a higher index region 4 forming the core,
and a lower index region 5 that forms the cladding. The 2.times.2
preform is then drawn in a conventional manner into a fibre having
the required dimensions. This drawing process also fuses the
individual preforms 1 together, and also ensures that the
interstitial gap 3 is either totally or substantially eliminated.
The drawn fibre is shown in FIG. 1b. The overall diameter of the
pulled fibre is 125 .mu.m--a standard size in optical fibre
production--with a diagonal separation of the cores of 62.5
.mu.m.
[0054] The production technique described in relation to FIG. 1 can
be adapted to produce more cores, although this gets more difficult
as the number of cores is increased. Standard fibre pulling
equipment incorporates jigs designed to take certain sized
preforms, and so if preforms are made that do not match the pulling
jigs then the production process becomes more costly as specialised
jigs will be required. This imposes an economic and practical limit
on the number of cores that can be incorporated into a fibre of
this type while still being capable of being formed into a fibre of
125 .mu.m overall diameter.
[0055] FIG. 2 shows a prior art photonic crystal fibre as described
in U.S. Pat. No. 6,301,420. The fibre comprises a plurality of
cores 6, each core 6 surrounded by a plurality of cladding elements
7. Each cladding element 7 comprises a hollow cylindrical tube. A
plurality of such tubes 7 are positioned in a triangular
arrangement as shown to form a cladding. Interstitial regions 8
exist between adjacent cladding elements at this stage. Each core
region 6 is formed by positioning a solid cylindrical cane in place
of a cladding tube 7, the cane having a diameter equal to that of a
single cladding element.
[0056] During manufacture an assembly of core 6 and cladding
elements 7 as described above forms a preform 9, which is then
pulled in conventional manner to produce a microstructured fibre of
a desired size. During the pulling process the forces on the
materials tends to result in the cladding tubes 7 forming a
hexagonal structure, with the interstitial regions being completely
or substantially eliminated due to the pressures present on the
fibre during the pulling. Optical isolation between adjacent cores
6 is dependent upon the number of cladding elements that separate
the cores. If the application to which the fibre is to be put does
not require a great deal of isolation then more cores per unit area
may be employed; however, for many applications three or four
cladding element repetitions are required between core elements
6.
[0057] It is relatively straightforward to generate multicore
fibres with this technology, as described above--hollow tubes are
replaced by solid rods when the fibre preform is assembled.
However, there is a limit on the ratio of cores to cladding tubes
that can be used in such a fibre whilst still maintaining a
required degree of isolation between adjacent cores. This results
in a relatively modest maximum number of cores being achievable
using standard fibre pulling machinery and standard 125 .mu.m
overall fibre diameters.
[0058] FIG. 3 shows a detailed view of a core of the type of fibre
shown in FIG. 2, and the surrounding cladding structure both before
(in FIG. 3a), and after (FIG. 3b) the pulling process. As can be
seen, the fibre is a high air filling fraction fibre. Clearly the
latter figure is not shown to the same scale as the former, due to
the significant reduction in fibre diameter that takes place during
the pulling process. FIG. 3a shows a core 6 surrounded by cladding
tubes 7, with interstitial gaps 8 existing between adjacent
elements. This represents a part of a preform. FIG. 3b shows the
same elements, but after having been passed through a fibre pulling
process. Core 6' has been transformed from a circular form to
hexagonal form due to the heat and physical forces involved during
the process. Also, cladding elements 7' have likewise been
transformed into hexagonal structures for the same reasons. It will
be noticed that the interstitial regions 8 present in FIG. 3a have
completely disappeared as a result of the pulling process.
[0059] FIG. 4 shows a first embodiment of the present invention.
FIG. 4a shows a part of a preform for a fibre, and FIG. 4b shows
the same part after the pulling process has been done. Again the
two parts are shown at differing scales.
[0060] FIG. 4a shows an end-on view of a part of a preform that,
when pulled in a conventional manner, will result in a multicore
fibre of the first embodiment. Circular hollow silica canes 9 are
shown stacked into a regular, triangular array. Each cane 9
comprises a lower refractive index material inner region 100,
surrounded by a higher refractive index outer region 101.
Interstitial regions 10 exist between adjacent canes. Into some of
these interstitial regions, e.g. 11, have been placed solid rods of
core material that partially fill the interstitial region. The
partially filled interstitial regions 11 will, when the preform is
pulled into a fibre, result in a core region being formed. As
described above, the pulling process substantially removes the
interstitial regions, and adjacent parts of the preform will fuse
together to form a fibre.
[0061] The cross sectional area of the interstitial region between
adjacent stacked canes in a preform will depend, for a given cane
diameter, on the stacking arrangement used. The fibre of FIG. 4 has
a triangular stacking arrangement, which gives a minimal
interstitial area. If a larger area is desired, so as to allow
larger cores to be made, then an alternative stacking arrangement,
such as a square stacking arrangement, may be used.
[0062] FIG. 4b shows the fibre after having been through the
pulling process. Each of the silica canes 9' have been transformed
into a hexagonal form due to the pressures and temperatures present
during the pulling process. Interstitial regions e.g. 10 present in
the preform have disappeared. Interstitial regions e.g. 11
containing a core rod have formed into a core 12 of the fibre,
formed from the core rod material itself, and also the immediately
adjacent cladding material. The result is a core region 12 capable
of radiation guidance. Each such core region 12 is substantially
surrounded by the region of lower refractive index of the cladding
material 100', but each core 12 has three supporting webs formed
from a part of the cladding higher index material 101'.
[0063] As each interstitial region of the original preform is able
to be formed into a core in this manner, the density of cores per
unit cross sectional area can be considerably increased over that
of the prior art. However, for some applications it may not be
advantageous to use every possible interstitial position as a core,
as this may result in cross-talk between adjacent cores above
acceptable limits for a given application.
[0064] The diameter of the cores and their relative spatial
separation will vary depending upon the application to which a
fibre is to be put. FIG. 5 shows two fibre designs each having a
different core arrangement. FIG. 5a shows a fibre 102 comprising a
triangular arrangement of canes 103 forming the cladding and having
a core 104 positioned in the interstitial region of two opposite
corners of every other cane 103 in a row for each alternate row of
canes 103 in the fibre 102. The fibre 102 has a minimum distance
between adjacent centres of cladding elements (periodicity,
.LAMBDA.) of 3.5 .mu.m and an air filling fraction f of 0.9. The
total core diameter (including the part of the core made up by what
would be cladding material before pulling) is 740 nm, and core
separation is 4 .mu.m. Such a fibre has a core packing density of
approx. 47000 cores per square mm.
[0065] FIG. 5b shows a second fibre 105 having a sparser array of
cores 104 per unit area, and per number of cladding elements 103.
This again has a triangular arrangement of cladding elements 103,
forming rows, but has a single core 104 positioned adjacent to
every third cladding element 103 on each row. Thus the cores 104
are in a triangular lattice arrangement. The fibre 105 has a
minimum distance between adjacent centres of cladding elements
(periodicity, .LAMBDA.) of 9 .mu.m and an air filling fraction f of
0.65. The total core diameter (including the part of the core made
up by what would be cladding material before pulling) is 3.2 .mu.m.
Core separation is nominally 15.625 .mu.m, equal to three times the
side of the hexagon formed by each cladding element. Such a fibre
has a core packing density of approx. 4730 cores per square mm.
[0066] FIGS. 6 to 8 shows three further embodiments of the present
invention. FIG. 6 shows a birefringent fibre both before (FIG. 6a)
and after (FIG. 6b) the pulling stage. FIG. 6a shows a matrix of
cladding canes 200 arranged in a triangular lattice, with some
interstitial regions 201 having core elements 202 therein. Each
cladding cane 203 is elliptical in cross-section before the pulling
stage. The non-circular cross-section gives a degree of
birefringence to the completed fibre. FIG. 6b shows the stack of
canes 200 at FIG. 6a after the pulling process. The elliptical
cross-section of the unpulled canes 203 becomes an irregular
hexagon 203' after pulling. In effect, the hexagonal lattice of the
embodiment shown in FIG. 4 has effectively been distorted in one
axis. A similar effect can be achieved by using circular canes
during the stacking phase of manufacture, and then squeezing the
fibre in one axis during the pulling phase. The core elements 202
in FIG. 6a become cores 202' in the pulled fibre in the usual way
as described above.
[0067] A further advantage of the fibre of FIG. 6 is that, by
selecting an appropriate degree of distortion from the circular
cross-section, the distribution of cores in the finished fibre can
be arranged to be on a square lattice. This arrangement is
particularly convenient for aligning the cores with optical sources
or detectors that themselves are on a square lattice. For the core
arrangement shown in FIG. 6, a reduction in one axis of a factor 3
will give a square lattice of cores.
[0068] FIG. 7 shows a further embodiment of the present invention.
Again FIG. 7a shows a stack of canes 204 before pulling, and FIG.
7b shows the equivalent fibre cross-section after the pulling
process. Referring to FIG. 7a, cladding canes 204 are arranged in a
triangular matrix. Core elements 205, 206, 207 are positioned in
interstitial regions between cladding elements as described above.
Two of the core elements 205, 206 are of the same size, whilst the
third core 207 has a smaller diameter. Clearly, after the pulling
process the core 207' is smaller than the two other cores 205',
206'. The provision of different sizes of cores in this manner
gives an extra degree of freedom in the design of the fibre, and
can be used to change the modal properties of the cores, the
coupling properties of adjacent cores, or can adapt the non-linear
behavioural properties of cores.
[0069] FIG. 8 shows an example of an alternative to the triangular
lattice cladding structure shown in the above examples. Here,
cladding elements 208 have been arranged in a square lattice. This
arrangement gives a larger interstitial region 209, thus allowing
larger core elements 210 to be used without disrupting the
periodicity of the cladding. FIG. 8a shows the stack before
pulling, and FIG. 8b shows the resultant fibre after the pulling
process. FIGS. 7b and 8b are shown to an approximately similar
scale, and it can clearly be seen that the core 210' diameter of
FIG. 8b is greater than that of FIG. 7b. Of course, different sized
cores can be used with the square configuration of this embodiment,
as described above in relation to a triangular cladding
arrangement. Thus a larger variation of core size is possible with
this cladding arrangement. A further difference with the embodiment
of FIG. 8 is that each core now has four webs supporting it,
compared to the three webs present in a fibre having a triangular
cladding lattice.
[0070] The optical fibres of the current invention may be modelled
using known modelling techniques. Prior art PCF fibres, such as
those shown in FIG. 2, are often modelled using a step-index fibre
model with an approximate effective cladding index given by a
scalar theory. See for example section 3.2 of the book "Photonic
Crystal Fibres", Bjarklev et al, Kluwer Academic Publishers, 2003.
For a given structure it is possible to calculate the number of
modes supported by a core, and also the coupling between adjacent
cores, given two main parameters, these being the periodicity
.LAMBDA. and the air filling fraction f. For maximum isolation
between cores, single mode transmission is desirable, which
generally places an upper limit on the periodicity for a given
wavelength of operation. If the transmission mode is kept
unchanged, the coupling between cores scales inverse exponentially
with the distance between the cores.
[0071] A particular prior art PCF fibre modelled using the above
technique required cores with a sufficiently small coupling that
five periods were required between cores for an air-filling
fraction of approximately 0.2, and with a periodicity of 6.25
.mu.m. For low air filling fractions, the effective index of the
cladding a PCF fibre of this type varies as the operating
wavelength changes so as to maintain single mode transmission. This
also means that if the structure (core size and periodicity) is
scaled in size, the transmission mode remains almost unchanged.
However, the coupling between cores does not remain the same as the
structure scales. Thus reducing the scale of a fibre of the prior
art will tend to increase the coupling between adjacent cores. Thus
prior art fibres cannot have the core density increased by scaling
without having undesired effects on the fibre properties. Reducing
the scale would therefore mean increasing the number of periods
between cores or an increase in air filling fraction to produce
more mode confinement.
[0072] FIG. 9 shows this in graphical form, with the shaded regions
indicating where single mode transmission may occur. It can be seen
that at least five cladding periods are needed between adjacent
cores, but there is some flexibility in adapting the air filling
fraction so as to be able to reduce the periodicity. However, the
range of periodicities over which such a fibre would stay single
mode narrows, so making fabrication difficult. The figure represent
a PCF prior art fibre designed to have a bit error rate of
10.sup.-12 over a 1 m length at an operating wavelength of 850
nm.
[0073] FIG. 10 shows a similar graph, but this time for a fibre
according to the present invention. The fibre of FIG. 5b has been
modelled using similar techniques used in the prior art. The core
diameter was taken to be the diameter of the interstitial rod used
to form the core, added to the thickness of a silica bridge. This
gives a core diameter of approximately (1.16- f).LAMBDA.. Thus the
core size depends on the air filling fraction, with smaller cores
for higher values. As with the prior art fibre, a single mode of
transmission will give the least amount of coupling between cores,
leading to an upper limit on the periodicity as shown in the
figure. However, as the cores in the present invention are small in
relation to the cladding periodicity, it is possible to achieve the
necessary optical isolation with a distance between cores of less
than twice the periodicity, as shown in FIG. 10. Increasing the air
filling fraction allows smaller cores with a smaller number of
periods between adjacent cores.
[0074] The effective index model of analysis and modelling can not
yet provide a rigorous analysis of either the PCF prior art, or of
the current invention. One thing it does not model well is the webs
that support each core. However, it is accurate enough to provide a
good indication as to the expected performance of a fibre of the
present invention.
[0075] A multicore fibre of the present invention may incorporate
cores doped with a material to change the optical properties of the
core. In particular, a dopant may be used to alter the normal
refractive index of the core material, or may be used to enhance
non-linear properties of the core. Doping of the core may be
conveniently done by doping the core rods (11 of FIG. 4a) before
assembly of the preform.
[0076] A fibre having one or more cores doped in this manner opens
up the applications to which the fibre may be put. It is known that
PCF fibres with high air-filling fractions and small cores may be
used in non-linear and soliton physics to generate, for example,
white light. This requires the positioning of the zero group
velocity dispersion to near the pump wavelength. This design may
have advantages for such applications at visible wavelengths. With
a multiplicity of cores it may be possible to induce coupling with
a precision required to allow the transfer of energy between
cores.
[0077] Non linear behaviour within a fibre can be induced without
doping, if the intensity of the illumination source is high enough
to excite the materials within the fibre sufficiently. For example,
"white light" or super-continuum generation can occur if the
optical intensity in the fibre is greater than a threshold
determined by the material properties. Four-wave mixing can also be
done in a similar manner. The present invention is particularly
suited to applications requiring non-linear behaviour, as the
coupling between cores can be controlled to a high degree using a
number of methods as described above. Design degrees of freedom
that influence the coupling include the spacing of adjacent cores,
the core size, core dopants, and the ratio of the quantities of the
different materials making up the cladding elements.
[0078] Fibres may also be made according to the present invention
with differing core densities in different regions of the fibre
cross section. This can allow the same fibre to be used for
different purposes. For example, one part of the fibre could be
used in a non-linear mode, whereas another part could be used to
transfer data along relatively highly isolated channels.
[0079] FIG. 11 shows a further embodiment of the present invention.
A microstructured fibre is formed as following the techniques as
described above. A stack is created comprising cladding elements
and core elements. However, the difference with this embodiment is
that at least one of the core elements comprises a single mode
fibre. This fibre is positioned at an interstitial region as
before, but takes the place of the core element at that region. The
stack incorporating the single mode fibre is then drawn down in a
tapered fashion to form a structure similar to that shown in FIG.
11. Here, a fibre 212 is shown diagrammatically in cross section.
The fibre 212 has a diameter that tapers from a large diameter end
213 to a small diameter end 214. The taper is generally not linear
along the length of the fibre. The wider end 213 has regions 215,
216 corresponding to areas of high and low refractive index in the
cladding and core elements. One such low refractive index region is
interstitial region 217. A conventional single mode fibre 218, such
as an index guiding fibre itself having a core defined by a doped
region 219 is placed, during manufacture, into interstitial region
217. A pulling process adapted to produce a taper along the fibre
is carried out, with the smaller end of the fibre 214 becoming
essentially a fibre as described in relation to FIGS. 4-8. The
single mode conventional fibre 218 protrudes from the end face of
the microstructured fibre structure 213 and, as at that point it is
a standard single mode fibre, may be coupled to an optical source,
or spliced to another fibre, in a known manner. This allows
convenient coupling to sources or receivers that may not be in a
convenient format for other types of connection. The tapered
portion of the fibre 220 is arranged to taper adiabatically to
ensure that light entering core 219 remains single mode during its
passage along the fibre 212. After the taper the fibre 218 acts as
a core according to the present invention as described in relation
to FIGS. 4-8 above. A cross-section of the end 214 of the fibre is
shown 221, having a core and cladding structure similar to that of
FIG. 5. Although only a single conventional single mode fibre has
been shown incorporated into the fibre 212, generally there will be
many such fibres, so allowing conventional coupling arrangements
from them to a plurality of cores formed in the interstitial
regions of the other end 214 of the fibre 212. Further details of
this technique but as applied to conventional photonic crystal
fibres can be found in "Splice-free interfacing of photonic crystal
fibres", Leon-Saval et al, Optics Letters, 1.7.05, Vol. 30 No.
13.
[0080] FIG. 12 shows another embodiment of a tapered fibre. Here, a
fibre 222 is formed in a manner described in relation to FIG. 4a
above, but wherein the fibre 222 is pulled in a tapered fashion as
described in relation to FIG. 11. A narrow diameter end 223 of the
fibre 222 is as described in relation to FIG. 4b, whilst a large
diameter end 224 has a diameter such that each core region e.g. 225
is of a size such that it may conveniently be coupled to a
conventional single mode fibre 226. Such a fibre 222 has benefits
in allowing convenient coupling to standard single mode fibres, and
also has application in photonic signal processing applications.
See for example "Microstructure Fibre Array for RF Photonic Signal
Processing Applications", Electronics Letters, Vol 42, No. 5.
[0081] FIG. 13 shows an SEM of a portion of an end-face of a fibre
made according to the present invention. The fibre is made entirely
from silica and has cladding elements 227 arranged in a hexagonal
lattice, with a cladding periodicity of 9 .mu.m. A hexagonal
lattice in a pulled fibre equates of course to a triangular lattice
in an unpulled fibre. Cores 228 are positioned at various points
throughout the lattice in a regular array, each being spaced
nominally 15.625 .mu.m from adjacent cores. In this embodiment all
cores 228 are the same size.
[0082] The skilled person will be aware that other embodiments
within the scope of the invention may be envisaged, and thus the
invention should not be limited to the embodiments as herein
described.
* * * * *