Note: Descriptions are shown in the official language in which they were submitted.
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RADIALLY NON UNIFORM AND AZIMUTHALLY ASYMMETRIC OPTICAL WAVEGUIDE FIBER
BACKGROUND OF THE INVENTION
This application is based upon the provisional application S.N.
60/099,535, filed 9/9/98, which we claim as the priority date of this
application.
The invention relates to an optical waveguide fiber and a method of
making a waveguide fiber, having a refractive index profile which varies in
both
the radial and azimuthal directions. The additional flexibility afforded by
the
azimuthal variation provides for index profile designs which meet a larger
number of waveguide fiber performance requirements than is possible using
refractive index variation in only the radial coordinate direction.
Recent development of waveguide fibers having refractive index profiles
which vary in the radial direction has shown that particular properties of the
waveguide can be optimized by adjusting this profile. Varying the refractive
index profile in a more general way than, for example, a simple step, allows
one to select the value of one or more waveguide properties without
sacrificing
a base set of properties including attenuation, strength, or bend resistance.
In addition, certain azimuthal asymmetric core refractive index profiles,
such as those having elliptical, triangular, or square core geometry have been
shown to provide useful waveguide properties such as preservation or mixing of
the polarization modes.
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It is expected, therefore, that core refractive index profiles which vary in
both the azimuthal and radial direction will offer the opportunity to
fabricate
waveguides having new or improved properties for use in telecommunication,
signal processing, or sensor systems.
In U. S. patent 3,909,110, Marcuse, ('110 patent) an azimuthally
asymmetric core of a multimode waveguide is described. A calculation in the
'110 patent indicates that periodic variations in index in both the radial and
azimuthal directions would cause mode coupling, thereby increasing
bandwidth, while limiting losses due to coupling to radiation modes. The
concept was not extended to include single mode waveguides. Also the scope
of the ' 110 patent is quite limited in that it refers only to sinusoidal
azimuthal
variations.
In describing the present azimuthally and radially asymmetric core, the
concept of core sectors is introduced. A core sector is simply a portion of
the
core which is bounded by a locus of points of a first and a second radius
which
form an annular region in the waveguide. Each of the radii are different one
from another and are less than or equal to the core radius. The remaining
boundaries of a sector are two planes oriented at an angle with respect to
each
other and each containing the waveguide fiber centerline. A change in
refractive index along a line within a sector means the retractive index is
different between at least two points along the line.
DEFINITIONS
The following definitions are in accord with common usage in the art.
- A segmented core is a core which has a particular refractive index profile
over
a pre-selected radius segment. A particular segment has a first and a last
refractive index point. The radius from the waveguide centerline to the
location
of this first refractive index point is the inner radius of the core region or
segment. Likewise, the radius from the waveguide centerline to the location of
the last refractive index point is the outer radius of the core segment.
- The relative index, D , is defined by the equation, 0 = (n~2- n22)/2n~2,
where
n~
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is the maximum refractive index of the index profile segment 1, and n2 is a
reference refractive index which is taken to be, in this application, the
minimum
refractive index of the clad layer. The term 0 %, which is 100 X D, is used in
the art.
- The term refractive index profile or simply index profile is the relation
between
0 % or refractive index and radius over a selected portion of the core. The
term a-profile refers to a refractive index proille which follows the
equation,
n(r) = no (1- D[r/a]°) where r is core radius, 0 is defined above, a is
the
last point in the profile, r is chosen to be zero at the first point of the
profile, and
a is an exponent which defines the profile shape. Other index profiles include
a step index, a trapezoidal index and a rounded step index, in which the
rounding is typically due to dopant diffusion in regions of rapid refractive
index
change.
SUMMARY OF THE INVENTION
In a first aspect of the invention, a single mode waveguide has a core
having at least one sector. The refractive index of at least one point within
the
sector is different from that of at least one point outside the sector. In the
case
where the sector is exactly half the core, the choice of what constitutes a
point
inside the sector can be chosen arbitrarily without any loss of precision of
definition of the profile. The core refractive index profile changes along at
least
a portion of one radius to provide radial asymmetry. At a pre-selected radius
the core refractive index within the sector is different from that outside the
sector to provide azimuthal asymmetry.
In one embodiment, the overall core has cylindrical symmetry and thus
is conveniently described in cylindrical coordinates, radius r, azimuth angle
cp,
and centerline height z. The pre-selected radius portion, Dr along which the
refractive index changes is in the range 0 < 0r < r°, where r°
is the core radius.
The pre-selected radius at which the refractive index is different for at
least two
different choices of azimuth angle is within this same range.
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In another embodiment the pre-selected radius portion is a segment
defined as Dr = r2 - r~, where, 0 < r~ < r2 and r2 < ra.
In yet another embodiment, the refractive index changes along any or all
radii within a sector, in which the sector has included angle cp greater than
zero
but less than or equal to 180 °.
In another embodiment the radius portion is in the range 0 < 0r <
r°, and
the azimuth angle cp and height z have any value provided the coordinate point
(r, cp, z) is in the core region.
Further embodiments of the invention include those in which the number
of sectors and the angular and radial size of the sectors are specified and
the
functional relationship between radius r and relative index percent 0 % is
specified. Examples of the functional relationships are the a-profile, the
step
and rounded step index profiles, and the trapezoidal profile.
Yet further embodiments of the invention include waveguides having a
segmented core and a specified number of sectors which include areas in
which glass volumes of a particular size and shape have been embedded.
Three and four sector embodiments having a particular core configuration and
embedded portions are described below. In some embodiments, the
embedded portions themselves have a segmented refractive index
configuration.
In general the embodiments of this first aspect of the invention can be
either single mode or multimode waveguide fibers.
A second aspect of the invention is a method of making an azimuthally
and radially asymmetric waveguide fiber. The method may be employed to
make either single mode or multimode waveguide fiber.
One embodiment of the method includes the steps of modifying the
shape of a draw preform and then drawing the preform into a waveguide fiber
having a circular cross section. The shape of the preform is thus transferred
to
the cylindrically symmetric features contained within the preform,
specifically
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the cylindrically symmetric core features. The draw preform shape may be
changed by any of several methods such as etching, sawing, drilling, or
grinding.
In an embodiment of the method, the preform is altered by forming holes
5 or surface indentations therein. Subsequent drawing of the altered preform
into
a waveguide fiber of circular cross section causes a circularly symmetric core
to become radially or azimuthally asymmetry.
In yet another embodiment of the method, two or more core preforms
are fabricated and inserted into a glass tube to form a prefortn assembly. The
waveguide fiber resulting from drawing the preform assembly has the
asymmetry of the assembly. Spacer glass particles or rods may be
incorporated into the tube-core preform assembly.
BR DESCRIPTION O~ TH ,~E DRAWINGS
Fig. 1A is a cross sectional view of an embodiment of the waveguide or prefotm
of the invention, having a central core design.
Fig. 1 B is the index profile taken through the 1 B section of Fig. 1A
Fig. 1 C is the index profile taken through the 1 C section of Fig. 1A.
Fig. 1 D is a cross sectional view of an embodiment of the waveguide or
preform of the invention having a central core design.
Fig. 1 E is the index profile taken through the t E section of Fig. 1 D.
Fig. 1 F is the indox profile taken through the 1 F section of Fig. t D.
Fig. 1 G is a cross sectional view of an embodiment of the waveguide or
prefortn of the invention, having an embedded core design.
Fig. 2A is a cross sectional view of an embodiment of the waveguide or preform
having an embedded core design.
Fig. 2B is the index profile taken through the 28 section of Fig. ZA.
Fig. 2C is a cross sectional view of an embodiment of the waveguide or
preform having an embedded core design.
Fig. 2D is the index profile taken through the 2D section of Fig. 2C.
Fig. 2E is a cross sectional view of an embodiment of the waveguide or
prefortn
having an embedded core design.
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Fig 2F is a cross sectional view of an embodiment of the waveguide or prefomn
having an embedded core design.
Fig. 3. is a cross sectional view of the novel waveguide or preform containing
voids.
Fig. 4A 8. B, and 4C & D show, in cross section, the transfer of the preforrn
outer shape to the core after drawing.
Fig. 5A & B illustrate, in cross section, the affect on the core shape of
preform
voids.
Fig. 6A 8 B, and 7A ~ B illustrate a cross section of a preform core and tube
assembiy and the resulting waveguide after drawing the assembly.
Fig. 8A & B illustrate a cross section of a notched segmented core preform and
the resulting waveguide after draw.
DET_ iLED DESCRIPTION OF Tt~ INVE~~iON
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The core 2 of Fig. 1A is made azimuthally asymmetric by indentations 4.
In this illustration of the novel preform or waveguide fiber, the indentations
comprise the same material as that of the clad layer 6. The perpendicular
sections through the core,1 B and 1 C are set forth in fig. 1 B and Fig. 1 C,
respectively and, show the azimuthal variation in width of the step index
profile.
This particular profile is symmetric in the radial direction.
The preform or waveguide core of Fig. 1 D is both radially and
azimuthally asymmetric. In this illustration of the novel waveguide or
preform,
the core is divided into four sectors. Each of the two diagonally opposed
sectors, 8 and 10 are mirror images of each other as is shown by the sections
1 F and 1 E taken through the core. In Fig. 1 E, the radial dependence of the
1 E
section is shown as 16, a rounded step or an cl-profile. In Fig. 1 F, the
profile 18
of the 1 F section is a step index profile. The clad portions 12 and 14 may
comprise any material having a refractive index lower than that of the
adjacent
core region. That is, the composition of the clad layer is generally limited
only
by the condition that the core clad structure guide rather than radiate tight
launched into the waveguide.
Fig. 1 G is an example of a more complex structure in accord with the
novel prefomn and waveguide. In this illustration waveguide core or core
preform 20 comprises a segmented core having central region 22, and
adjoining annular regions 28, 24, and 26. Each region is characterised by a
respective relative refractive index ~ %, an index profile and an area
determined by radii 32, 34, 36, 38 and 40. For example, central region 22 and
annular region 24 may comprise respective germanium doped silica glasses
and annular regions 28 and 28 may comprise silica and the relative sizes of
the
areas may be as shown. The asymmetry is introduced into the core prefonn by
embedded glass volumes 30, which in general have a refractive index different
from that of either annular segment 24 or 26 contacted by the glass volumes
30.
The glass volumes 30 can be formed by sawing or grinding, for example,
followed by filling of the volumes with a glass by any of a number of means
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including deposition. The distribution of light energy carried by core 20 will
be
determined by the relative refractive indexes and sizes of the segments
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22, 28, 24, 26, end 30. The functional properties of the waveguide are
detemnined by the distribution of light energy across ttie core preform or
core
20.
In another embodiment of the novel preform or waveguide, the core is
comprised of a matrix glass 50 having embedded glass volumes 42, 44, and 48
as illustrated in Fig. 2A. The glass volumes extend from end to end of the
preform or the waveguide drawn from the preform. The clad glass layer 52
surrounds the cure 50. The refractive index of core glass 50 is higher than
that
of clad layer 52. Section 2B through one of the embedded volumes is shown in
Fig. 2B as a step index profile. The sizes of cross sectional area of the
embedded glass volumes 42. 44 and 48 can be the same or different and a
number of relative orientations relative to the clad glass layer are possible.
The structure of Fig. 2A can made by drilling a preform, smoothing the
walls of the resulting holes, and filling the holes with glass powder or rods.
As
an alternative, the core can be formed of rods which are then inserted into a
holding tube, either with or without the use of spacer glass rods or
particles.
The need for a holding tube can be eliminated by welding the rods together
using appropriate glass spacer material. Tha overclad layer can be deposited
over the welded assembly of rods or can be fabricated as a tube which is
shrunk onto the assembly before or during draw.
Another embodiment which includes a matrix glass and a plurality of
embedded glass volumes is shown in Fig. 2C. Hero the gross structure of
waveguide 54 is similar to that of Fig. 2A, except that the embedded glass
volumes 56, 58 and 60 each have a segmented core refractive index profile.
An example of the segmented core profile is shown in Fig. 2D, which is the
cross section through one of the embedded volumes in which a central region
of relatively high D % is surrounded by two annular regions, 62 and 64. In the
illustration, the first annulus 62 is lower in a % than the second annulus 64.
It
is understood that each of the segments may have a radial dependence
selected from a plurality of possibilities, such as an o-profile or a rounded
step
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profile, and the relative o %'s of the segments can be adjusted to provide
different waveguide functional properties.
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The methods of making the prefonn or waveguide of Fig. 2C are
essentially identical to the method of making the preform or waveguide of Fig.
2A.
Two additional embodiments of this preform or waveguide type are
illustrated in Figs. 2E 8 2F. The embedded glass volumes 66, 68, and 70 in
Fig. 2E have a rectangular cross section and are arranged substantially at the
apexes of an equilateral triangle. Other arrangements of the embedded glass
volumes are contemplated such as arrangement along a diameter of the core
region. The core region 72 can comprise a number of shapes and
compositions. In the simple example illustrated in Fig. 2E, the core glass 72
is
a step index profile and, as is required to guide light, has a higher
refractive
index than at least a portion of the clad layer 74.
In Fig. 2F a configuration comprising fNe embedded glass volumes is
illustrated. Here, four glass volumes of diamond cross section 76, 78, 80 and
82 are symmetrically arranged about a circular central core region 84. It is
evident that numerous variations of this design are possible. For example the
refractive indexes of the embedded volumes 76, 78, 80, 82, and 84 can each
have a different relative index as compared to that of the core 86.
As is shown in Fig. 3, the embedded volumes 88 in a prefomn or a
waveguide can be voids. A waveguide having elongated voids along the long
axis can be made by forming elongated voids, for example, by drilling or
etching, in a core or draw preform. The index of the core glass 90 is
necessarily diff~rent from that of the voids, thus providing an asymmetrical
axe
region. In the case in which Fig. 3 represents a draw preform, the voids may
be collapsed during the draw process to produce an asymmetric core. The
shape of the core region after collapse of the voids is determined by the
relative
viscosity of core material 90 and clad layer material 92. Control of the
relative
viscosity of the glasses is maintained by control of temperature gradient in
the
portion of the preforrn being drawn. The relative viscosity also depends upon
core and clad glass composition.
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Figs. 4A and 4B illustrate the transfer of a prefonn shape, 98 in Fig, 4A,
from the clad layer portion 84 of the preform, to the core portion 102 in Fig.
4B
of a waveguide 100 drawn from preform 98. The transfer ocdrrs as shown in
Figs. 4A and 4B when the inidal symmetry of the preform core 88 is the same
as the symmetry of the waveguide clad layer 104. Cylindrical symmetry is
shown because this is the symmetry most compatible with current preform
fabrication and draw processes. Other symmehies are possible, for example,
by partial transfer of the pceforrn shape to the waveguide core shape. i.e.,
the
final shape of the waveguide departs from cylindrical symmetry.
A cross section of a segmented core prefomt having a square shape is
shown in Fig. 4C. After heating and drawing the preform into a cylindrical
waveguide, the segmented core, 106 in Fig. 40, takes on square shape due to
the viscous flow of the core material which takes place to accommodate the
cylindrically shaped surface of the clad layer.
In an analogous manner, the pnrform of Fig. 5A, having core 190, clad
layer 112 and elongated voids 108, will produce an asymmetric core when
drawn into a cylindrically shaped waveguide. However, in this case the preform
is cylindrical, and the movement of the core material is due to the filling of
the
voids during draw. As long as the preform shape is preserved as the prefomi is
drawn into a waveguide, the core must distort, i.e., become asymmetric, to
fill
the voids.
Example
A preform of the type shown in Fig. 5A was made using the outside
vapor deposition process. The core region 110 was germanium doped silica
and the clad layer 112 was silica. Voids 108 were formed in the preform by
drilling followed by smoothing of the walls of the void using an etching
solution.
The preform was drawn into a waveguide fiber having the Zero dispersion
wavelength in the 1500 nm operation window, i.e., the waveguide was
dispersion shifted. The waveguide had an unusually large mode field diameter
of 10.4 pm as compared to mode field diameters in the range of 7 pm to 8 um
for dispersion shifted waveguides having an azimuthally symmetric core.
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A method of making an asymmetric core is illustrated in Figs. 6A and 8B.
Segmented core preforms 114, 116 and 118 are fabricated using any of several
known methods including, outside vapor deposition, axial vapor deposition,
plasma deposition, or modified chemical vapor deposition. The core prefortns
5 are inserted into tube 122 where they are held in place by spacer rods 120.
The rods may be made of silica, doped silica or the like. If needed, a clad
layer
124 may be deposited on the tube. The preform assembly may now be drawn
into a waveguide fiber having cores 130, 132, and 134 embedded in core glass
128 and surrounded by clad glass layer 128 as shown in Fig. 6B. The
10 assembly as shown in Fig. 6A may be drawn directly. As an altemafrve, the
deposited clad layer may be consolidated prior to draw. In addition, before
clad
deposition, the tube, core prefonn and spacer rod assembly may be heated
sufficiently to soften the surfaces thereof to cause them to adhere to each
other, thereby forming a more stable structure for use in the overdad or draw
process.
The method of making an asymmetric core shown in Figs. 7A and 7B is
closely related to that illustrated in Figs. 6A and 8B'. In Fig. 7A the core
is
bounded by annulus 136 which serves to better contain light propagating in
step index core preforms 138, 140, and 142. As described above, spacer rods
or glass powder may be used to stabilize the relative positions of the core
preforms within the annulus. The assembly of core preforms, optional spacer
material, annulus and overclad material may be drawn directly or first
consolidated~and then drawn. The resulting wavegufde fiber is shown in Fig.
7B.
A final example of a method of forming an asymmetric core is shown in
Figs. 8A and 88. In Fig. 8A a preform has a segmented core having central
region 144, first annular region 146, and second annular region 148. The
preform has been ground or sawed or the like to form notches 152. The
notches may be empty or filled with material 150 which is a material different
in
composition from that of dad layer 154. The preform assembly is drawn to
form a waveguide having an asymmetric core as shown in Fig. 8B. Here again
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the assembly may be drawn din3ctly or deposition, consolidation, or tacking
steps may be carried out before draw to hold the parts of the preform in
proper
relative registration.
Although particular embodiments of the invention have been disclosed
and described herein, the invention is nonetheless limited only by the
following
claims.
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