Note: Descriptions are shown in the official language in which they were submitted.
CA 02220416 1997-11-06
.. ~
Method o~ Making Optical Fibers
- Background
The invention is directed to a method for making an
optical fiber having optical properties that
systematically vary along its length. This method is
particularly useful for making dispersion managed (DM)
single-mode optical waveguide fibers
The potentially high bandwidth of single-mode optical
fibers can be realized only if the system design is
optimized so that the total dispersion is equal to zero or
nearly equal to zero at the operating wavelength. The
term "dispersion" refers to pulse broadening and is
expressed in ps/nm-km. 'IDispersion Product" refers to
dispersion times length and is expressed in ps/nm.
When telecommunications networks employ multiple
channel c~mmllnications or wavelength division
multiplexing, the system can experience a loss due to four
wave mixing. This loss occurs when the signal wavelength
is at or near the zero dispersion wavelength of the
optical transmission fiber. This has necessitated the
exploration of waveguide fiber designs which can minimize
signal degradation that results from this non-linear
waveguide effect. A dilemma arises in the design of a
waveguide fiber to minimize four wave mixing while
maintaining characteristics required for systems which
CA 02220416 1997-11-06
.
have long spacing between regenerators. That is, in order
to substantially eliminate four wave mixing, the waveguide
fiber should not be operated near its zero of total
dispersion, because four wave mixing occurs when waveguide
dispersion is low, i.e., less than about 0.5 ps/nm-km. On
the other hand, signals having a wavelength away from the
zero of total dispersion of the waveguide are degraded
because of the presence of the total dispersion.
One strategy that has been proposed to overcome this
dilemma is to construct a system using cabled waveguide
fiber segments some of which have a positive total
dispersion and some of which have a negative total
dispersion. If the length weighted average of dispersion
for all the cable segments is close to zero, the
regenerator spacing can be large. However, the signal
essentially never passes through a waveguide length where
the dispersion is close to zero, so that four wave mixing
is prevented.
The problem with this strategy is that each link
between regenerators must be tailored to give the required
length weighted average of dispersion. Maintaining cable
dispersion identity from cabling plant through to
installation is an undesirable added task and a source of
error Further, the need to provide not only the proper
dispersion, but also the proper length of cable having
that dispersion, increases the difficulty of manufacture
and leads to increased system cost. A further problem
arises when one considers the need for replacement cables.
Those problems are overcome by the optical fiber
disclosed in U.S. patent application S.N. 08/584,868
(Berkey et al.) filed January 11, 1996. In accordance
with the teachings of the Berkey et al. application, each
individual fiber is made to be a self contained dispersion
managed system. A pre-selected, length-weighted average
of total dispersion, i.e., total dispersion product, is
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designed into each waveguide fiber. Each waveguide fiber
is interchangeable with any other waveguide fiber designed
for that system link. Thus, the cabled waveguide fibers
all have essentially identical dispersion product
characteristics, and there is no need to assign a
particular set of cables to a particular part of the
system. Power penalty due to four wave mixing is
essentially eliminated, or reduced to a pre-selected
level, while total link dispersion is held to a pre-
selected value, which may be a value substantially equalto zero.
In accordance with the Berkey et al. patent
application, the dispersion of a DM fiber varies between a
range of positive values and a range of negative values
along the waveguide length. The dispersion product,
expressed as ps/nm, of a particular length, 1, is the
product (D ps/nm-km * 1 km). A positive number of ps/nm
will cancel an equal negative number of ps/nm. In
general, the dispersion associated with a length li may
vary from point to point along li. That is, the dispersion
Di lies within a pre-determined range of dispersions, but
may vary from point to point along li. To express the
contribution of li to the dispersion product, expressed in
ps/nm, li is made up of segments dli over which the
associated total dispersion Di is essentially constant.
Then the sum of products dli * Di characterizes the
dispersion product contribution of li. Note that, in the
limit where dli approaches zero, the sum of products dli *
Di is simply the integral of dli * Di over the length li.
If the dispersion is essentially constant over sub-length
li, then the sum of products is simply li * Di.
The dispersion of the overall waveguide fiber length
is managed by controlling the dispersion Di of each segment
dli, so that the sum of the products Di * dli is equal to a
pre-selected value over a wavelength range wherein signals
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may be multiplexed. For high rate systems having long
regenerator spacing, the wavelength range in the low
attenuation window from about 1525 nm to 1565 nm may be
advantageously chosen. In this case, the sum of the
dispersion products for the DM fiber would have to be
targeted at zero over that range of wavelengths. The D
magnitudes are held above 0.5 ps/nm-km to substantially
prevent four wave mixing and below about 20 ps/nm-km so
that overly large swings in the waveguide fiber parameters
are not required.
The length ov~ which a given total dispersion
persists is generally greater than about 0.1 km. This
lower length limit reduces the power penalty (see FIG. 5),
and simplifies the manufacturing process.
The period of a DM single-mode waveguide is defined
as a first length having a total dispersion which is
within a first range, plus a second length having a
dispersion which is in a second range, wherein the first
and second ranges are of opposite sign, plus a transition
length over which the dispersion makes a transition
between the ~irst and second range. To avoid four wave
mixing and any associated power penalty over the
transition length, it is advantageous to keep the part of
the transition length which has an associated total
dispersion less than about 0.5 ps/nm-km as short as
possible.
If the transition regions between the regions of
higher and lower dispersion are too long, the dispersion
in the central portions of the transition regions will be
near zero for some finite length of fiber. This will
result in some power penalty due to four wave mi xi ng The
longer the transition regions are, the higher the power
penalty. The transition regions should therefore be
sufficiently sharp that the fiber power penalty does not
cause the total system power penalty to exceed the
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., ,
allocated power penalty budget.
A primary requirement of a process for m~king DM
fibers is that it be able to form short transition
regions. Moreover, the process of making the DM fiber
should not be one that itself induces an excess loss that
is unrelated to four wave mixing. Also, the process
should be simple and be sufficiently flexible that it can
be implemented with a variety of fiber designs and
materials. Thus, the DM fiber must be a unitary fiber
that is formed by drawing a draw preform or draw blank
that includes sect ~ns that will form the fiber sections
of different dispersion. Such a unitary fiber does not
include splices between separately drawn fiber sections,
as each splice would introduce additional loss. Ideally,
the total attenuation of the unitary fiber is no greater
than the composite of the weighted attenuation of each of
the serially disposed sections of which it is formed.
An attempt was made to form a DM fiber core rod by
fusing together core cane sections by the lathe and torch
method. In addition to being difficult to implement, that
method suffered from core misalignment, and the flame-
caused core wetting problems.
Summary of the Invention
Therefore, an object of the invention is to provide
an optical fiber having distinctly different optical
characteristics along its length and an improved m~thod
for making such a fiber. Another object is to provide a
method for making optical fiber of the aforementioned type
wherein the transition lengths between sections of
different characteristics are very short. A further
object is to provide a method for making fiber of the
aforementioned type wherein the attenuation is
sufficiently low for use as long distance transmission
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fiber. Another object is to provide a method for making
low loss single-mode DM optical fiber having short
transition lengths. Yet another object is to provide a
method for making optical fibers exhibiting low
polarization mode dispersion.
One aspect of the invention concerns a method of
making an optical fiber preform. Briefly, the method
comprises the following steps. A coating of cladding
glass particles is deposited on the outer surface of a
cladding glass tube, and a plurality of tablets is
inserted into the cladding glass tube. At least one
optical characteristic of at least one of the tablets in
the tube is different than that of an adjacent tablet, and
each tablet has at least a central region of core glass.
While the coated assembly is heated to a temperature less
than the sintering temperature of the cladding glass
particles, a centerline gas is flowed through the tube.
The centerline gas is selected from the group consisting
of pure chlorine and chlorine mixed with a diluent gas.
Thereafter, the coated assembly is heated to sinter the
coating, thereby generating a radially-inwardly directed
force that causes the tube to collapse onto and fuse to
the tablets, and causing the cladding glass tube to shrink
longitudinally, whereby adjacent tablets are urged toward
one another and are fused to one another.
A further aspect of the invention concerns a unitary
optical fiber that results from the above-described
method. The fiber comprises a plurality of serially
disposed optical fiber sections, each fiber section having
a glass core and a glass outer cladding. The core of a
first fiber section is different from the core of each
fiber section that is adjacent to the first section. The
cladding of the first fiber section is identical to the
cladding of the adjacent fiber sections. Between each two
adjacent fiber sections is a transition region, the length
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of which is less than 10 meters.
Brie~ Description of the Drawings
FIG. 1 is an illustration of total dispersion varying
along the waveguide fiber length.
FIG. 2 shows how the zero dispersion of a waveguide
fiber may vary to maintain total dispersion of the
waveguide within a pre-selected range over a pre-
determined wavelength window.
FIG 3a is a ehart illustrating the power penalty vs.
input power for a system comprised of particular waveguide
sub-lengths having a low total dispersion magnitude.
FIG. 3b is a chart illustrating the power penalty vs.
input power for a system comprised of particular waveguide
sub-lengths having a higher total dispersion magnitude.
FIG. ~ is a chart of total dispersion vs. power
penalty.
FIG. 5 is a chart of dispersion variation period
length vs. power penalty.
FIG. 6 is a chart of transition region length vs.
power penalty.
FIG. 7 is a schematic representation of a process of
making an optical fiber, adjacent sections of which have
distinctly different characteristics.
FIG. 8 is an enlarged cross-sectional view of the
tablets of FIG. 7.
FIG. 9 illustrates the application of a layer of
cladding glass particles to a tube.
FIG. 10 is a cross-sectional view of the fused
assembly resulting from the consolidation/fusion step
illustrated in FIG. 7.
FIG. 11 is a partial cross-sectional view of a
modification of the embodiment of FIG. 7.
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FIGS. 12 and 13 are refractive index profiles of
dispersion shifted optical fibers.
Detailed Description o~ the Invention
Dispersion Managed Fiber Design
The total dispersion of a DM fiber is charted vs.
waveguide length in FIG. 1. The total dispersion is seen
to alternate between positive values 2 and negative values
4. Whereas FIG. 1 illustrates a plurality of sublengths
exhibiting negative -dispersion and a pIurality of
sublengths exhibiting positive dispersion, only one
negative dispersion sublength and one positive dispersion
sublength are required. The spread in total dispersion
values indicated by line 6 illustrates that total
dispersion varies with the wavelength of light propagated.
The horizontal lines of the spread 6 represent total
dispersion for a particular light wavelength. In general,
the length of waveguide 8, characterized by a particular
total dispersion, is greater than about 0.1 km. There is
essentially no upper limit on length 8 except one which
may be inferred from the requirement that the sum of
products, length x corresponding total dispersion, is
equal to a pre-selected value.
2~ The chart of total dispersion vs. wavelength shown in
FIG. 2 serves to illustrate design considerations for a DM
single-mode waveguide fiber. Lines 10, 12, 14 and 16
represent total dispersion for four individual waveguide
fibers. Over the narrow wavelength range considered for
each waveguide, i.e., about 30 nm, the dispersion may be
approximated by a straight line as shown. The wavelength
range in which multiplexing is to be done is the range
from 26 to 28. Any waveguide segment which has a zero
dispersion wavelength in the range of 18 to 20 may be
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g
combined with a waveguide segment having a zero dispersion
wavelength in the range 22 to 24, to yield a waveguide
having a pre-selected total dispersion over the operating
window 26 to 28.
The following example is based on FIG. 2. Take the
operating window to be 1540 n to 1565 nm. Assume that
the single-mode waveguide fiber has a dispersion slope of
about 0.08 ps/nm2-km. Let line 30 be the 0.5 ps/nm-km
value and line 32 the 4 ps/nm-km value. Apply the
condition that the total dispersion within the operating
window must be in t~e range of about 0.5 to 4 ps/nm-km. A
simple straight line calculation then yields zero
dispersion wavelength range, 18 to 20, of 1515 nm to 1534
nm. A similar calculation yields a zero dispersion
wavelength range, 22 to 24, of 1570 nm to 1590 nm.
Algebraic addition of the total dispersion of waveguide
fiber segments having dispersion zero within the stated
ranges will yield a total dispersion between 0.5 and 4
ps/nm-km.
The design of the DM fiber depends strongly on the
details of the telecommllnication system as can be seen in
FIGS. 3a and 3b which show power penalty charted vs. input
power for a 120 km link having 8 channels, wherein the
frequency separation of channels is 200 GHz. In this case
the power penalty is that due primarily to four wave
mixing. Curve 62 in FIG. 3a rises steeply to a penalty
near 1 dB for an input power of about 10 dBm. The penalty
is about 0.6 dB for an input power of 10 dBm (curve 64).
For both curves the magnitude of the total dispersion is
about 0.5 ps/nm-km. However, for the steeper curve 62 the
sub-length for total dispersion of a given sign is 10 km.
The corresponding sub-length of the dispersion in curve 64
is 60 km. The extra penalty results from the additional
transitions through zero dispersion for the shorter, 10 km
sub-length case. An alternative statement is for the 10
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km case, the phase separation of the signals, which is
proportional to the oscillation sub-length, is not large
enough to substantially prevent four wave mixing. An
"oscillation sub-length" is either the positive or
negative dispersion sub-length of a period. Where there
is no sign associated with oscillation sub-length, the
positive and negative oscillation sub-lengths are taken as
equal.
However, magnitude of the total dispersion also has
an impact upon phase separation and thus upon power
penalty. Curve 66 in FIG. 3b shows the power penalty for
a system identical to that shown in FIG. 3a, except that
the sub-length is shorter, about 1 km, but the total
dispersion magnitude is 1.5 ps/nm-km. Causing the
waveguide total dispersion to make wider positive to
negative swings reduces power penalty signi~icantly, from
0.6 dB to less than 0.2 dB. The penalty difference of
about 0.4 dB~120 km is large enough to be the difference
between a functional and non-functional link, especially
for long unregenerated links of 500 km or more.
FIG. 4 is interpreted in essentially the same manner
as FIG. 3a an 3b. Curve 68 shows power penalty charted
vs~ total dispersion magnitude. The sub-length of the
waveguide is chosen as about 1 km because the length of
the shortest cables in general use is about 2 km. Again
there are 8 channels having a frequency separation of 200
GHz, a total length of 120 km, and the input power is 10
dBm. Again the power penalty rises steeply when total
dispersion magnitude falls below about 1.5 ps/nm-km.
System design is shown from another viewpoint in FIG.
5. In this case, the dispersion magnitude is fixed at 1.5
ps/nm-km. Curve 70 represents power penalty vs. sub-
length magnitude for a system having 8 channels with 200
GHz frequency separation and 10 dBm input power. The
length is chosen to be 60 dispersion sub-lengths and the
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11
sub-length is allowed to vary. Lower power penalties
result when the sub-length is above 2 km. But with the
relatively large total dispersion magnitude, little is
gained by lengthening the sub-length beyond 2 km. Note
the generally lower four wave mixing penalty paid when the
number of channels used is reduced to 4 as shown by curve
72.
Another design consideration is the sharpness of the
transition length over which the total dispersion changes
sign. Here also, the signal phase separation is affected
by the transition L~ngth. Thus, a shallow transition
would cause the signal to travel a waveguide region of
near zero total dispersion, and this adversely impacts
power penalty caused by four wave mixing.
The following example illustrates the effect of
transition length on power penalty. Assume that the input
power is 10 dBm. Four channels are used having a
frequency separation of 200 GHz. The magnitude of total
dispersion is 1.5 ps/nm-km and the oscillation sub-length
of the total dispersion is taken to be 2 km. The chart of
power penalty vs. transition length, shown as curve 74 in
FIG. 6 , shows that shorter transition lengths are
preferred.
Fiber Fabrication
A method that produces very short transition regions
is illustrated in FIGS. 7 and 8. To practice this method,
core preforms can be prepared by any known process.
Examples of processes that can be employed to make the
core preforms are outside vapor deposition (OVD), vapor
axial deposition (VAD), modified chemical vapor deposition
(MCVD) wherein a core layer is formed inside a glass tube,
and plasma chemical vapor deposition (PCVD) wherein the
reaction within the tube is plasma induced. The core
preform can consist entirely of core glass or it can
CA 022204l6 l997-ll-06
12
consist of a core region and a cladding region.
There is initially formed two or more cylindrical
preforms that are capable of being overclad and formed
into optical fibers having disparate optical
characteristics. For most applications only two different
types of core preforms are required; two preforms are
utilized in the embodiment illustrated in FIGS. 7 and 8.
The first and second preforms are cut into tablets 81
and 82, respectively. The lengths of the tablets depend
upon the specific type of fiber being made. In the
process of making ~fibers, the lengths of the tablets 81
and 82 are selected to yield in the resultant optical
fiber the desired sub-lengths. The tablets can be made by
the simple score and snap method. Tablet 81 has a core
region 83 and a cladding region 84; tablet 82 has a core
region 85 and a cladding region 86.
A tubular glass handle 92 having an annular
enlargement 97 is fused to one end of an elongated glass
tube 90. Handle 92 is part of a ball ~oint type gas feed
system of the type disclosed in U.S. patent 5,180,410.
Enlargement 97 is adapted to rest on a slotted base of a
support tube (not shown) that suspends handle 92 in a
consolidation furnace. Tube 90 is heated and a dent 98 is
formed near handle 92. Alternatively, that part of handle
92 adjacent tube 90 could be dented. The assembly
including tube 90 and handle 97 is inserted into a lathe
(not shown)and rotated and translated with respect to
burner 100 which deposits on tube 90 a layer 91 of
cladding glass particles or soot (see FIG. 9). Coating 91
can be built up to a sufficient outside diameter (OD) that
the resultant preform can be consolidated and drawn into
an optical fiber having the desired optical
characteristics. Layer 91 can overlap handle 92 as shown
in FIG. 7.
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13
Tube 90 is oriented so that the end affixed to handle
92 is lower than the other end, and tablets 81 and 82 are
alternately inserted into the upper end of tube 90. The
tablets cannot fall beyond dent 98. Tube 90 is heated
and a dent 99 is formed near that end opposite dent 98.
When tube 90 is inverted, dent 99 prevents the tablets
from falling from it.
Handle 92 is suspended from a support tube (not
shown) which is lowered to insert assembly 94 into
consolidation furnace muffle 95. While assembly 94 is
heated in the cons-~ldation furnace, a drying gas flows
upwardly through the furnace (arrow 93). The drying gas
conventionally comprises a mixture of chlorine and an
inert gas such as helium. A chlorine-containing gas
stream (arrow 96) is flowed from tube 92 into tube 90.
Although gas stream 96 could contain a diluent such as
helium, pure chlorine is preferred for cleaning purposes.
Since the diameter of each of the tablets 81 and 82 is
slightly smaller than the inner diameter of tube 90, the
chlorine flows downwardly around the entire periphery of
each of the tablets; it also fIows or diffuses between
adjacent tablets. The chlorine then exhausts through the
bottom of tube 90. The chlorine functions as a hot
chemical cleaning agent. During this hot chlorine
cleaning step, the temperature is below the consolidation
temperature of soot coating 91 so that the space between
tablets 81 and 82 and tube 90 remains open for a
sufficient length of time for the required cleaning to
occur. The chlorine cleaning step is more effective at
high temperatures. It is preferred that the temperature
of the cleaning step be at least 1000~C, since at lower
temperatures, the duration of the step would be
sufficiently long that the step would be undesirable for
commercial purposes. Obviously, lower temperatures could
CA 02220416 1997-11-06
be employed if processing time were not a concern. The
flow of hot chlorine between the tube 90 and tablets 81
and 82 is very beneficial in that it allows the surfaces
of adjacent tablets and of tube and tablets to be brought
together without the formation of seeds at their
interface. Seeds include defects such as bubbles and
impurities that can produce attenuation in the resultant
optical fiber.
While assembly 94 is lowered further into the furnace
muffle, the wall of that portion of tube 90 at the end of
soot layer 91 coll~ses and fuses together, thereby
cutting off the centerline chlorine flow. As an optional
step, a valve can then be switched to pull a vacuum within
tube 90. As assembly 94 continues its movement into the
furnace muffle, first its tip and then the remainder of
the assembly is subjected to the maximum furnace
temperature which is sufficient to sinter coating 91.
Soot coating 91 shrinks both radially and longitudinally
as it sinters.
As soot coating 91 shrinks longitudinally, it causes
tube 90 to decrease in length. This causes adjacent
tablets 81 and 82 to be forced together while they are
subjected to sintering temperature, whereby they fuse
together without forming seeds. Without this longitudinal
shrinking of tube 90, adjacent tablets could not become
sufficiently fused to form low loss optical fibers.
As soot coating 91 shrinks radially, it exerts a
force radially inwardly on tube 90. This urges tube 90
inwardly against tablets 81 and 82 to form a fused
assembly 98 (see FIG. 10) in which the three regions 81,
90' and 91' are completely fused. Region 90' is the
collapsed tube, and region 91' is the sintered porous
coating. A relatively low density soot provides a greater
inwardly directed force; however, the soot coating must be
CA 02220416 1997-11-06
sufficiently dense to prevent cracking.
The consolidation of the tablet-filled overclad tube
to yield a seed free preform is a crucial processing step.
For the tablets to fuse together without seeds, it is
necessary to flow chlorine through the tube to chemically
clean all the surfaces. However, the step of applying
vacuum after the blank tip fuses is not necessary.
The fused assembly is removed from the consolidation
furnace. Regions 90' and 91' of fused assembly 98
function as cladding in the resultant optical fiber.
Assembly 98 can be-used as a draw blank-and can be drawn
directly into an optical fiber. Fused assembly 98 can
optionally be provided with additional cladding prior to
the fiber drawing step. For example, a coating of
cladding soot can be deposited onto assembly 98 and then
consolidated. Alternatively, assembly 98 can be inserted
into a cladding glass tube. If additional cladding were
added, the diameters of the core regions of tablets 81 and
82 would have to be suitably adjusted.
As compared to fusing the core canes or tablets prior
to inserting them into a cladding glass tube, the present
method is simple to perform, and it enables the fusion to
be carried out in a dry environment. The method is self
aligning in that adjacent core canes of different diameter
will be centered on the axis of the resultant draw blank
when tube 90 collapses inwardly during the sintering of
porous glass coating 91.
The method of this invention brings new degrees of
freedom in the tailoring of fiber properties. It results
in the formation of optical fibers having adjacent regions
or lengths of disparate properties. Very abrupt
transition regions connect the adjacent fiber lengths.
The attenuation of this fiber is identical to that of
standard long distance telecommunication fiber, i.e. less
than 0.25 dB/km and preferably less than 0.22 dB/km.
CA 02220416 1997-11-06
16
In the embodiment shown in FIG. 11, dents 98 and 99
are not formed in tube 90. A short length 104 of glass
capillary tubing is fused to one end of tube 90, and the
glass handle is fused to the opposite end of tube 90.
Tablets 81 and 82 are inserted through the handle and into
tube 90. The tablets cannot fall beyond tube 104 since
that tube has a relatively small bore. When the assembly
is lowered into the consolidation furnace to initiate the
sintering process, tube 104 initially fuses to cut off the
chlorine flow.
Forming DM Fibers
A dispersion managed fiber is formed from core
preforms that are capable of forming single-mode optical
fibers having different zero dispersion wavelengths. The
dispersion of a waveguide length can be changed by varying
various waveguide parameters such as geometry, refractive
index, refractive index profile, or composition. Any of a
large number of refractive index profiles provide the
required flexibility for adjusting waveguide dispersion
and thereby varying the total dispersion. These are
discussed in detail in U.S. patent 4,715,679, Bhagavatula,
and applications S.N. 08/323,795, S.N. 08/287,262, and
S.N. 08/378,780.
One type of refractive index profile which is useful
for forming optical fibers having zero dispersion at
predetermined wavelengths is that having a relatively high
index central region surrounded by an annular region of
depressed index which is in turn surrounded by an outer
annular region of index higher than that of the depressed
index region (see FIG. 12). The index profile of another
embodiment (see FIG. 13) includes an essentially constant
index central portion having a refractive index
substantially equal to the clad glass refractive index and
an adjacent annular region of increased refractive index.
-
CA 02220416 1997-11-06
17
Optical fibers having this type of refractive index
profile can be easily manufactured.
A simple DM fiber refractive index profile is the
step index profile Two core preforms could be formed of
the same core and cladding materials, the radius of one
core region being larger than that of the other. The draw
blank is drawn to a fiber having lengths of a first core
radius interspersed between lengths of a second core
radius that is larger than the first radius A core
diameter difference of about 5 % to 25 % is sufficient to
produce the desired~eositive to negative dispersion
variation. A range of radii variation of 5 % to 10 % is,
in general, sufficient for most applications.
The following example describes the formation of a
single-mode DM fiber suitable for providing zero
dispersion at 1545-1555 nm. Two different core preforms
were made by a method similar to that disclosed in U.S.
patent 4,486,212 which is incorporated herein by
reference. Briefly, the method of that patent includes
the steps of (a) depositing glass particles on a mandrel
to form a porous glass preform, (b) removing the mandrel
and consolidating the porous preform to form a dry,
sintered preform, (c) stretching the sintered preform and
closing the axial aperture therein. The core preform
included a central region of core glass surrounded by a
thin layer of cladding glass. Both of the core preforms
had core refractive index profiles of the type shown in
FIG. 12. The first core preform was such that if it were
provided with cladding and drawn into a single-mode fiber
having a 125 um OD, it would exhibit zero dispersion at
1520 nm. The second preform is such that if it were
similarly formed into a 125 ~m OD single-mode fiber, its
zero dispersion wavelength would be 1570 nm. The core
preforms were stretched to diameters of 7 mm and 7.1 mm.
The first and second stretched preforms were scored and
CA 02220416 1997-11-06
1~
snapped to form tablets 81 and 82 of substantially equal
length. Tablets 81 had core regions 83 and cladding
regions 84; tablets 82 had core regions 85 and cladding
regions 86.
A one meter length of silica tube 90 was employed; it
had an inside diameter (ID) of 7.5 mm and an O.D. of 9 mm.
The technique described in conjunction with FIG. 7 was
employed to load tablets 81 and 82 into tube 90. Coating
91 was built up to a sufficient OD that the resultant
preform could be consolidated and drawn into a 125 ~um OD
single-mode fiber.
The resultant assembly 94 was suspended in a
consolidation furnace. While assembly 94 was rotated at 1
rpm, it was lowered into consolidation furnace muffle 95
at a rate of 5 mm per minute. A gas mixture (arrow 93)
comprising 50 sccm chlorine and 40 slpm helium flowed
upwardly through the muffle. A centerline flow of 0.3
slpm chlorine flowed downwardly around tablets 81 and 82
and exhausted from the bottom of tube 90. The maximum
temperature in the consolidation furnace was about 1450~C.
As assembly 94 moved downwardly into the furnace, the
centçrline chlorine flow chemically cleaned the surfaces
of tablets 81 and 82 and the inner surface of tube 90. As
assembly 94 moved further into the furnace muffle, that
region of tube 90 below the tablets fused and cut off the
centerline chlorine flow. A valve (not shown) was then
switched to pull a vacuum within tube 90. Assembly 94
continued its movement into the furnace muffle, and
coating 91 was sintered. Tube 90 was forced inwardly
against tablets 81 and 82, and the contacting surfaces of
all of the glass elements became fused. As soot 91
sintered, tube 90 became shorter, and seed-free fused
joints were formed between adjacent tablets.
After being removed from the consolidation furnace,
CA 02220416 1997-11-06
19
draw blanks formed by this process were drawn to form DM
optical fibers having an OD of 125 um. Single-mode DM
optical fibers made by this process have been drawn
without upsets; attenuation has typically been 0.21 dB/km.
This is the same attenuation that would have been
exhibited by a single-mode dispersion shifted optical
fiber drawn from a preform formed by overcladding one of
the 7 mm core canes.
The two different types of tablets that were employed
in the fiber making process combined to provide a zero
dispersion wavelengt~h of 1545-1555 nm. The zero
dispersion wavelength was determined by the total lengths
of each kind of core in the fiber. The zero dispersion
wavelength of the fiber could be changed by cutting off a
portion at one end of the fiber, thus changing the ratio
of the lengths of each kind of core in the fiber.
The oscillation sub-lengths and the period are
controlled by the lengths of the core preform tablets.
Fibers having oscillation sub-lengths of 1.2 to 2.5 km
were drawn.
Other Fiber Types
The method of the invention has been specifically
described in connection with the manufacture of DM single-
mode optical fibers, and a description of a method ofmaking such a fiber is set forth in the preceding specific
example. However, it can be employed to make many other
types of optical fibers having optical properties that
systematically vary along the fiber's length. In each
instance, the fiber can be made by inserting the
appropriate tablets into a tube and processing the tube as
described above.
Spontaneous Brilluoin Scattering (SBS) can be
minimi zed by providing a fiber with alternate lengths that
exhibit significantly different values of ~, wherein ~ is
CA 02220416 1997-11-06
defined as
(nl2 - n22)/2n12 (nl and n2 are the refractive indices of the
core and cladding, respectively. One of the types of
tablets that is used to make the fiber preform exhibits a
given ~, and the other type of tablet exhibits a
significantly different value of ~. The ~-value of a
fiber core can be controlled by controlling the amount of
a dopant in the core or by changing the composition of the
core, i.e. by adding other dopants to the core. Numerous
dopants including oxides of tantalum, aluminum, boron can
be employed for the--~urpose of changing refractive index
and other properties such as viscosity.
A fiber that provides a filtering function could be
made by alternately disposing in a tube a plurality of
tablets that are capable of forming optical fiber having a
filtering function and a plurality of tablets that are
capable of forming standard, non-filtering optical fiber.
The tablets need not be of equal or nearly equal
lengths. For example, a fiber could include relatively
short sections, the cores of which are doped with active
dopant ions capable of producing stimulated emission of
light when pumped with light of suitable wavelength.
Dopant ions of a rare earth such as erbium are
particularly suitable for this purpose. Thus, a fiber
having sections of erbium-doped core located at spaced
intervals along its length could be made by employing
relatively long tablets of standard, erbium-free core and
relatively short tablets of erb~ doped core.
A fiber where the core systematically decreases in
size such as that employed in Soliton fibers could be made
by inserting into the tube a plurality of tablets, each
having a core diameter smaller than the previous one or
each having a core diameter larger than the previous one.
Alternatively, some other core characteristic that affects
dispersion could be varied in the tablets so that the
CA 02220416 1997-11-06
21
dispersion of the resultant fiber monotonically decreased
from one end of the fiber to the other.
The above-described examples employ alternately
disposed tablets that have disparate optical properties.
In one embodiment, a single core preform could be used to
form all tablets. A single preform is formed such that
i ~s core has an azimuthaiiy asymmetrical refractive lndex
profile. For example, the core could be slightly out of
round, i.e. the core cross-sectional shape of the core is
an ellipse having a major axis and a minor axis (see U.S.
patent 5,149,349) ~Alternatively, the fiber could contain
stress rods on opposite sides of the core as disclosed in
U.S. patent 5,152,818. An elliptical core fiber can be
formed as follows. Tablets are severed from the preform.
A cladding glass tube is provided with a coating of
cladding glass soot. The tablets are inserted into the
cladding glass tube such that the major axis of the
elliptical core of one tablet is rotated with respect to
the major axes the cores of adjacent tablets. After the
cladding soot is consolidated and the tablets are fused to
the tube and to each other, the resultant draw blank is
drawn into an optical fiber having low polarization mode
dispersion.
Although particular embodiments of the invention have
been discussed in detail, the invention is nevertheless
limited only by the following claims.
