Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF MAKING OPTICAL FIBERS
Related Applications
This application is a Continuation-In-Part application of U.S. Patent
Application Serial No. 08/844,997, filed April 23, 1997 (and which claims the
benefit of provisional application No. 60/016435, filed April 26, 1996). This
application also claims the benefit of U.S. Provisional Application No.
60/083878, filed May 1, 1998.
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.
"Dispersion Product" refers to dispersion times length and is expressed in
ps/nm.
When telecommunications networks employ multiple channel
communications or wavelength division multiplexing, the system can
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experience a performance degradation due to four wave mixing. This
performance degradation 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 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 local
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, the
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specification of which is hereby incorporated by reference. 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
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
equal to 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, I, is the product (D ps/nm-km * I km). A
positive
number of ps/nm will cancel an equal negative number of pslnm. In general,
the dispersion associated with a length I; may vary from point to point along
I;.
That. is, the dispersion D; lies within a pre-determined range of dispersions,
but
may vary from point to point along I;. To express the contribution of I; to
the
dispersion product, expressed in ps/nm, I, is made up of segments dl; over
which the associated total dispersion D; is essentially constant. Then the sum
of products dl; * D; characterizes the dispersion product contribution of I;.
Note
that, in the limit where dl; approaches zero, the sum of products dl; * D; is
simply
the integral of dl; * D; over the length I;. If the dispersion is essentially
constant
over sub-length I;, then the sum of products is simply I; * D;.
The dispersion of the overall waveguide fiber length is managed by
controlling the dispersion D; of each segment dl;, so that the sum of the
products D; * dl; is equal to a pre-selected value over a wavelength range
wherein signals may be multiplexed. For high rate systems having long
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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 (absolute value) 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 over 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 first 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 positive and negative
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 mixing. 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 allocated power penalty budget. Preferably, the
transition regions between adjacent areas of fiber are less than 10 meters,
preferably less than 5 meters, and most preferably less than 3 meters in
length.
A primary requirement of a process for making 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
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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 sections that will form the fiber sections
of
5 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.
Summary of the Invention
One aspect of the invention relates to a method of making an optical
fiber preform. Briefly, the method comprises arranging alternating regions or
tablets of glass along or within a device for maintaining the alternating
regions
of glass in a desired relationship with respect to one another. At least one
optical characteristic of at least one of the tablets is different than that
of an
adjacent tablet. While the invention can be utilized to make fibers which are
useful at wavelengths, for example, as low as 1300 nm and as high as
1620nm, the invention is particularly advantageous in forming fibers having
alternating dispersion characteristics at wavelengths greater than 1480 nm,
e.g., in the 1550 nm operating window the alternating glass regions comprises
a negative local dispersion and an adjacent one of the glass regions comprises
a positive local dispersion at 1550 nm. This may be achieved, for example, by
assembling alternating regions of glass having different composition or core
index profiles or dopant levels. These alternating glass regions are then
fused
together by heating the glass regions to a temperature sufficient to cause the
glass regions to fuse together and consolidate into a preform, or a precursor
for a preform, which can be used to draw an optical fiber therefrom.
In one embodiment, the alignment device is a glass tube, and the
tablets are inserted into the glass tube. The tablets may, for example, be
constructed of a desired core glass material, and the tube constructed of an
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outer core glass material or a cladding material. Alternatively, the tablets
coutd
be comprised of both core and cladding material.
In another embodiment, the alignment device is a glass rod, and the
tablets are rings or donuts of glass which may be aligned along the rod in a
desired relationship. The rod may be, for example, a core glass material, and
the glass donuts constructed of an outer core material or a cladding material.
In a preferred embodiment, alternating donuts or different dopant or
composition level are used to form at least a portion of an outer core region
which varies along the length of a constant composition inner core region.
The tablets are then exposed to a fusing step in which the resultant
assembly is heated to a temperature sufficient to fuse the tablets to one
another. Prior to the fusing step, the tablets are preferably submifted to a
cleaning step in which the tablets are heated and exposed to a cleaning gas,
such as pure chlorine or chlorine mixed with a diluent gas. If needed or
desired, a vacuum can be applied to facilitate void free fusing of the tablets
together. In the embodiment which employs tablets arranged within a tube,
this may be achieved easily by applying a vacuum to the tube. In the
embodiment which employs rings displaced along a rod, application of a
vacuum may be facilitated by first applying an additional outer glass soot
layer
via CVD and submitting the soot cladding to a consolidation step, then
applying
a vacuum to the outer consolidated glass. The consolidation step also helps
generate a radially-inwardly directed force that causes the assembly to
collapse onto and fuse to the tablets, and causing the assembly to shrink
longitudinally, whereby adjacent tablets are urged toward one another and are
fused to one another.
Additional layers of glass may be added as desired. Preferably, CVD
methods are employed to deposit these additional layers, after which these
layers may be consolidated into glass. Prior to consolidation, these
additional
layers are first exposed to a cleaning step, wherein the coated assembly is
heated to a temperature less than the sintering temperature of the cladding
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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.
A further aspect of the invention concerns a unitary (i.e., which is not
made up of fiber sections which are spliced together) 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 of
which preferably is less than 10 meters.
Brief 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 chart 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. 4 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
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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.
FIGS. 12 and 13 are refractive index profiles of dispersion shifted optical
fibers.
FIGS. 14 and 15 illustrate exemplary alternating index of refraction
profiles which may be formed along a single optical fiber using the methods of
the present invention to facilitate controlling dispersion characteristics
over a
wide wavelength range.
FIG. 16 illustrates an alternative method in accordance with the present
invention.
FIGS. 17 and 18 illustrate alternative exemplary alternating index of
refraction profiles which may be formed along a single optical fiber using the
methods of the present invention.
FIGS. 19 and 20 illustrate alternative exemplary alternating index of
refraction profiles which may be formed along a single optical fiber using the
methods of the present invention.
FIGS. 21 and 22 illustrate alternative exemplary alternating index of
refraction profiles which may be formed along a single optical fiber using the
methods of the present invention.
FIGS. 23 and 24 illustrate alternative exemplary alternating index of
refraction profiles which may be formed along a single optical fiber using the
methods of the present invention.
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FIGS. 25 and 26 illustrate alternative exemplary alternating index of
refraction profrles which may be formed along a single optical fiber using the
methods of the present invention.
Detailed Description of 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 plurality 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.
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 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
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to be 1540 nm 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 the range of about 0.5 to 4
5 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
10 0.5 and 4 ps/nm-km.
Although, in FIG. 2, only waveguide fibers having positive dispersion
slopes are illustrated, the present invention is not limited thereto. For
example,
in addition to alternating sections of positive and negative dispersion, in
one
preferred embodiment of the invention, the fiber of the present invention is
constnrcted so that it alternates between areas of positive slope dispersion
adjacent to areas of negative slope dispersion. The fiber may also be
constructed of alternating sections having different effective areas. In
another
preferred embodiment, the fiber is constructed so that adjacent sections
alternate between areas of negative total dispersion having a negative
dispersion slope and areas of positive total dispersion having a positive
dispersion slope. In another preferred embodiment, the fiber is constructed so
that adjacent sections alternate between areas of negative total dispersion
having a low or zero dispersion slope and areas of positive total dispersion
having a low or zero dispersion slope, e.g. less than .02 ps/nm2km, more
preferably less than .01 ps/nm2km.
The design of the DM fiber depends strongly on the details of the
telecommunication 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
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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 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 significantly, 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 and 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/nrn-km. Curve 70 represents power
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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 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 length. 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, but preferably they are prepared using chemical vapor
deposition (CVD) methods, wherein the glass is deposited in a soot form, and
thereafter heated and consolidated into a glass. While any such CVD method
can be employed, examples of preferred CVD 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 consist of a core region
and
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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. Glass tablets, as used herein, means any glass component region
which is assembled together with other glass component regions to form
alternating sections of glass which vary with respect to one another in at
least
one optical characteristic or composition (dopant level).
The lengths of the tablets depend upon the specific type of fiber being
made. In the process of making DM 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. In an alternative
embodiment, the tablets are formed by sawing tablets having desired lengths,
for example, by using a diamond abrasive loaded wheel saw. The ends of the
resultant tables are then polished. In one embodiment, illustrated in Fig. 8,
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 joint 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
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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.
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 if.
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 consolidation 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 flows
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 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
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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
5 that portion of tube 90 at the end of soot layer 91 collapses 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
10 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
15 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 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. While the step of applying vacuum after the blank tip fuses may
not always be necessary, it is in many embodiments preferred, as this step
seems to greatly facilitate formation of fiber having low attenuation,
particularly
at the interface between adjacent glass component or tablet regions.
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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.
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.
In the present invention, these glass tablets are preferably formed using
chemical vapor deposition methods. Thus, glass soot is deposited by CVD,
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17
after which the glass soot is consolidated into a relatively void-free glass.
The
resultant glass may then be cut into tablets 81 and 82 having a desired
composition, shape and size, and assembled inside the tube 90 as desired.
By using chemical vapor deposition techniques to form the glass, it is
possible
to form glass tablets having complex index of refraction profile, by varying
the
amount or type of dopant deposited during the CVD soot deposition process.
For example, in the embodiment just described, tablets 81 and 82 can
be constructed to have different index of refraction profiles. In one such
preferred embodiment, tablets 81 and 82 are made from glass rods which were
formed using OVD methods in which the amount of and type of dopant added
during the soot deposition step or a subsequent soot doping step is selected
to
result in a particularly desired refractive index profile. The resultant soot
glass
preforms are then heated and consolidated into glass rods.
In one embodiment, glass tablets 81 and 82 are assembled which have
index of refraction profiles similar to those illustrated in Figs. 14 and 15,
respectively. The first rod, the profile of which is illustrated in Fig. 14,
has an
index of refraction profile which, when described from the axis outward,
comprises a high index central core region having index n,, followed by a
fluorine doped "moat" region having index n2 which is less than index n, and
has an index less than pure Si02, followed by a third region having an index
n3
which is in between that of index n1 and n2. This may optionally be followed
by
a cladding region which is comprised substantially of Si02, but may be
slightly
up-doped or down-doped.
The second rod, the profile of which is illustrated in Fig. 15, has an index
of refraction profile which, when described from the axis outward, comprises a
high index central core region having index n,, followed by a °moat"
region
having index n2 which is not less than Si02, followed by a third region having
an index n3 which is intermediate between that of index n, and n2. This may
optionally be followed by a cladding region which is comprised substantially
of
SiO2, but which may be slightly up-doped or down-doped.
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Cylindrically shaped glass tablets 81 are then cut from the first rod and
cylindrically shaped glass tablets 82 are cut from the second rod. Tablets 81
and 82 having index of refraction profiles as illustrated in Figs. 14 and 15,
respectively, may then be inserted into tube 90 as described above in
alternating fashion. This embodiment is particularly useful for forming fibers
having alternating sections of positive dispersion having a positive slope
(vs.
wavelength) and negative total dispersion having a negative slope. When
operated near, but not at, the Zero dispersion wavelength, such fibers having
been constructed which exhibit total attenuations of less than .5 dB/km. If
desired, additional glass soot (doped or undoped) can then be deposited onto
the assembly which includes tube 90 and tablets 81 and 82, for example if
additional cladding is desired.
In an alternative embodiment, glass tablets 81 and 82 are assembled
which have index of refraction profiles similar to those illustrated in Figs.
17
and 18. The first rod, the profile of which is illustrated in Fig. 17, has an
index
of refraction profile which, when described from the axis outward, comprises a
high index central core region having index n,, followed by a lower index
"moat"
region having index n2 which is less than index n, . In the embodiment
illustrated the first high index region is comprised of Si02 doped with GeOz,
and
the lower index of the lower index region is at or about the index of
refraction of
pure silica. This may optionally be followed by an additional cladding region
which is comprised substantially of Si02, but may be slightly up-doped or
down-doped. The presence of a lower index moat region within the core
region is particularly preferred in order to form regions having negative
dispersion having negative dispersion slope.
The second rod has an index of refraction profile which, as illustrated in
Fig. 18, when described from the axis outward, comprises a high index central
core region having index n,, followed by a lower region having index n2 which
is not less than Si02. In the embodiment illustrated the first high index
region is
comprised of Si02 doped with Ge02 (of course, any other dopant selected to
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19
increase the index of refraction to greater than that of SiOz may be employed
instead of Ge02), and the lower index of the lower index region is also
comprised of SiOz doped with Ge02, but with less dopant that the high index
region. In the embodiment illustrated, a trough is present in the region
corresponding to Si02 tube 90 used to assemble the tablets 81 and 82. This
trough index region is not expected to have much impact on the fiber
performance. Of course, if desired, a Ge02 doped Si02 tube could be
employed which matches the index of the second lower index region, thereby
avoiding formation of the index trough altogether. This may optionally be
followed by a cladding region which is comprised substantially of Si02, but
which may be slightly up-doped or down-doped.
Cylindrically shaped glass tablets 81 are then cut from the first rod and
cylindrically shaped glass tablets 82 are cut from the second rod. Tablets 81
and 82 having index of refraction profiles as illustrated in Figs. 17 and 18,
respectively, may then be inserted into tube 90 as described above in
alternating fashion. As illustrated in Figs. 17 and 18, the tube is then
overcladded with an additional layer of Ge02 doped Si02 which matches the
index of the second lower index region in Fig. 18. This overcladding serves as
the clad region for the fiber.
This embodiment is particularly useful for forming fibers having
alternating sections of positive dispersion having a positive slope (vs.
wavelength) and negative total dispersion having a negative slope. It is also
advantageous in that it avoids the use of expensive fluorine doping. When
operated near, but not at, the zero dispersion wavelength, such fibers having
been constructed which exhibit total attenuations of less than .5 dB/km. If
desired, additional glass soot (doped or undoped) can then be deposited onto
the assembly which includes tube 90 and tablets 81 and 82, for example if
additional cladding is desired.
Of course, the invention can be carried out in alternative and additional
manners. For example, rather than inserting tablets into a tube, ring or donut
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shaped tablets 81 and 82 could be employed and iocated along a rod 112, as
illustrated in Fig. 16. For example, to formulate a fiber preform similar to
the
one formed in the example set forth immediately hereinabove, rather than
assembling alternating glass tablets 81 and 82 inside a tube, a single glass
rod
5 having index n, can be used to form the high index central core region (this
could be, for.example, Ge-doped Si02. In this manner, a rod having
continuous, relatively uniform composition along its length may be employed to
form a high index inner core portion, and the donuts employed to form lower or
different outer core portions of the core refraction index profile. The
resultant
10 assembly 112, which includes rod 112 and.donut shaped tablets 81 and 82,
can then be consolidated and/or have additional glass applied by CVD as
desired.
In an alternative embodiment, glass tablets 81 and 82 are assembled
which have index of refraction profiles similar to those illustrated in Figs.
19
15 and 20. This combination can be used to form a step index fiber, which for
example, has a diameter of 125 ~m and alternates between segments
consisting of step design with a delta of ~0.5% and a core diameter equal to
about 8 p.m followed by a segment with same delta but with core diameter
equal to about 4 p,m. Such a design would result in alternating sections
having
20 widely differing dispersions of negative and positive magnitude,
respectively.
Such a dispersion managed fiber can be fabricated by assembling pellets of
two different core canes inside a tube, as described above. Such pellets could
be sliced or diced using a saw, and afterwards polished using flame polishing
or mechanical polishing techniques. Alternatively, this type of fiber can be
used
using the process illustrated in Fig. 16, wherein donut shaped tablets 81 and
82 are assembled onto a core cane110.
In this case, a single continuous core cane of appropriate diameter may
be employed to assemble the donut shaped tablets 81 and 82. In this
embodiment, core cane 110, which has the index profile 120 illustrated in
Figs.
19 and 20, will form part of the final core when the blank is fully assembled.
Tablets 81 and 82, which have index profiles 122 and 123, respectively (again
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21
as illustrated in Figs. 19 and 20}, are diced into sizes that fit snuggly
(preferably
with enough clearance between components to enable cleaning via a cleaning
gas such a chlorine) onto central core cane 110, and are assembled thereon to
make up the alternate sections of the DMF. Tablet 82 is made of pure silica in
this example. A blank assembled in this fashion alternates between the
profiles illustrated in Figs. 19 and 20 and would result in alternating
sections
having widely differing dispersions of negative and positive magnitude,
respectively. After the core cane is assembled in this fashion, more soot is
preferably deposited to finish the outer cladding portion of the predraw
blank.
The blank is then consolidated to form a blank for making dispersion managed
fiber. Prior to or during the consolidation process, the tablets 81 and 82 and
core 110 are preferably exposed to a cleaning step employing chlorine.
Vacuum can be applied to facilitate complete radial fusing of the tablets 81
and
82 to the central core 110 as well as longitudinally to each other. There are
a
number of advantages to this assembly/fabrication process. First is that the
majority of the core is made of continuous cane and is not exposed to tablet
forming processes which may impart contaminants. Second is that, using
dicing or sawing techniques, tablets 81 and 82 may be thin (e.g. having an
aspect ratio of the outer diameter of the donut or tube to the thickness of
the
donut is greater than 1, more preferably greater than 5). This enables the
formation of small (e.g. less than 5, more preferably less than 2, and most
preferably less than 1 km) period dispersion managed fibers. The cleaning
process prior to fusing or consolidation helps to eliminate contaminants
between surfaces in the tablet/core rod assembly, particularly in the polished
areas to eliminate the contaminants. In addition, any-contaminant on these
sleeved sections is away from the core center where the mode intensities are
higher. Another advantage is that bigger core canes can be made using this
process. Also, the sleeves can be cut with non-contaminating processes like
water jet cutting or C02 laser cutting. One of the drawbacks of this approach
is
that the designs cannot be completely arbitrary. Alternate sections preferably
have significant profile overlaps in the central part of the core. But this
limiting
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22
is not very critical, since variations of the above concept can be used to
generate a variety of profiles.
For example, in an alternative embodiment, glass tablets 81 and 82 are
assembled which have index of refraction profiles similar to those illustrated
in
Figs. 21 and 22, respectively, to form alternating sections having
significantly
different dispersion. In this case, the central continuous cane still has a
delta
of about 0.5% step profile. As above, tablet 81 has a delta -0.5% profile 132
shown in Fig. 21 to generate a fiber having a step index core of about 8 ~m
diameter and zero dispersion in the 1300 nm window. Tablet 82 has a
depressed index profile 131 as shown in Fig. 22 leading to a w-type profile
with
dispersion zero around 1630 nm and a dispersion slope of 0.025 ps/km.nm2.
This simple example shows the capability of this approach to not only achieve
alternating sections of widely separated positive and negative dispersions,
but
which also have low slopes useful for wide operating wavelength ranges in the
1300-1620 nm range, and especially useful in the 1550 nm operating window
(e.g. 1525-1565nm).
A still further alternative embodiment involves assembling glass tablets
81 and 82 which have index of refraction profiles similar to those illustrated
in
Figs. 23 and 24, respectively. In this embodiment, two different triangular
segmented core designs are employed. The index profile 136 for donut-
shaped tablet 81 illustrated in Fig. 23 leads to a 1300 nm zero dispersion
wavelength and Fig. 24 for longer zero dispersion wavelength designs (e.g.
1600 or higher). This example illustrates the flexibility of this fabrication
approach where the central core is common, but the ring location, its delta,
and
width are different for the alternating sections. This example also
illustrates
another aspect of the disclosure wherein the interface between the central
core
cane and the sleeve can be set based on process requirements. For example,
the separation can be in the silica moat area as shown in Figs. 23 and 24 near
the ring location with germania doped silica. For processing ease (seed
formation), and heat aging considerations, this flexibility is quite useful.
Another variation of the invention involves assembling donut shaped
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23
sections 81 having axial grooves 140 which are serrated into locations around
the inner peripheral surface of the donut shaped tablet 81, as shown in Figs.
25 and 26. The serrations can be made using a dicing saw or by cutting with
non-contaminating C02 laser. Alternatively, the grooves can be made in the
central continuous core cane also. Because of these voids, consolidation and
fiber draw preferably is done under special conditions. For example, the draw
should be done at low enough temperatures that the voids do not collapse
during draw. The dispersion values of the sections having voids will be
significantly different compared to the sections without voids. By making the
cross sections of these serrations very narrow and short and traveling the
length of the tablet, it should also be possible to draw them without the
voids
collapsing. This enables the periodicity of the dispersion managed fiber to be
quite small, on the order of a few meters if need be. Also, other interesting
properties, such as for example, polarization control, may be combined with
dispersion management in these fibers by the design of voids/ serration's in
the
azimuthal direction.
Of course, it is not necessary that all of the process steps described in
the above embodiments be conducted in exactly the same sequence as is
described above. For example, in the embodiment described above, in which
cylindrical tablets 81 and 82 are inserted into a glass tube 90, if desired,
layer
91 of cladding glass particles or soot may be deposited onto tube 90 prior to
the tablets 81 and 82 being deposited into the tube 90. Coating 91 could then
again be built up to any sufficient outside diameter that will result after
consolidation and fiber draw into an optical fiber having the desired optical
characteristics. In this embodiment, the tube and soot assembly is preferably
consolidated prior to the tablets 81 and 82 being deposited within tube 90, in
which case the soot layer 91 and tube 90 may be cleaned and consolidated,
for example, by using the cleaning and consolidation techniques discussed
above. Alternatively, tablets 81 and 82 may be inserted into the tube 90 prior
to the resultant assembly being consolidated. Preferably, one end of tube 90
is
closed off and a vacuum is applied to the other end of the tube 90, as was
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24
discussed above. This embodiment results in a heavier walled silica tube
within
which the tablets 81 and 82 may be deposited. Of course, rather than starting
with a relatively thin walled tube and depositing soot thereon, a thicker
silica
walled tube could be employed if desired, and the soot deposition step avoided
entirely.
Prior to any of the consolidation steps or the step of fusing the tablets 81
and 82 with tube 90, the cleaning operations discussed above are also
preferably applied to the resultant,assembly.
In another alternative embodiment, tablets 81 and 82 are again
deposited into a silica tube 90, as was discussed above. However, prior to
deposition of the soot layer 91, the tablets in tube 90 are redrawn. This may
be done, for example, by exposing the assembly which consists of tube 90 and
tablets 81 and 82 to a temperature sufficient to consolidate and fuse the
tablets
81 and 82 in tube 90 into a monolithic, seed-free preform, and then drawing
the
resultant precursor or preform into a somewhat thinner diameter. The resultant
consolidated preform may then be overclad with soot, if desired, to achieve
any
particular desired amount of additional glass prior to having the resultant
preform drawn into a fiber.
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, the specifications of which are all hereby
incorporated by reference.
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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
5 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. Optical fibers having this type of refractive index profile can be
easily
10 manufactured.
A simple DM fiber refractive index profile is the step index profile. Two
core preforms could be formed of the same core and ctadding 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
15 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
positive to negative dispersion variation. A range of core radii variation of
5
to 10 % is, in general, sufficient for most applications.
The following example describes the formation of a single-mode DM
20 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
25 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 ~m OD, it would exhibit zero
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26
dispersion at 1520 nm. The second preform is such that if it were similarly
formed into a 125 pm 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
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 N~m 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 centerline 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, draw blanks formed
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27
by this process were drawn to form DM optical fibers having an OD of 125
Vim. Single-mode DM optical fibers made by this process have been drawn
without upsets; attenuation has typically been 0.21 dBlkm or less. 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 wavelength 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 of making 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 minimized by providing a
fiber with alternate lengths that exhibit significantly different values of D,
wherein 0 is defined as (n,2 - n22)12n,2 (n, 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 0, and the other type of tablet
exhibits
a significantly different value of D. The A-value of a fiber core can be
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28
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 purpose 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 heady 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 erbium-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 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 its
core
has an azimuthally asymmetrical refractive index 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).
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29
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.
Using the methods of the present invention, dispersion managed fibers
which employ alternating sections of positive and negative dispersion have
been constructed which are suitable for telecommunication appiications such
as wavelength division multiplexing (WDM) systems, in which case the
invention can be employed to reduce cumulative dispersion substantially to
zero. Likewise, fibers have been constructed having alternating regions of
positive slope and negative slope dispersion.
Using the methods herein, low loss dispersion managed fibers have
been manufactured. By low loss, it is meant that the fibers have having
attenuations of less than .5 dB/km, more preferably less than .25 dB/km and
most preferably less than .22 dB/km over the operating wavelength range of
about 1550nm.
In addition, the preferred fibers in accordance with the invention are
single mode optical waveguide fibers whose operating range (i.e., the source
wavelength range is between about 1300 to 1700 nm, more preferably
between about 1500 nm to about 1580 nm, and most preferably between about
1525 nm to 1565 nm, have been achieved. These fibers exhibit no attenuation
at the interface between adjacent areas of having different optical
characteristics.
Such fibers, as well as methods for their making, are further disclosed in
US Patent Application No. 081844,997, filed April 23, 1997; 081423,656, filed
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January 11, 1996, the specifications which are all hereby incorporated by
reference.
Although the invention has been described in detail for the purpose of
illustration, it is understood that such detail is solely for that purpose and
.
5 variation can be made therein by those skilled in the art without departing
from
the spirit and. scope of the invention which is defined by the following
claims.
For example, although in Fig. 7 the method illustrated employs a soot
layer on the outside of tube 90, the soot layer is optional, and several
variations of this approach may be employed. For example, in one example,
10 an assembly comprised of the tube 90 and~the tablets 81 and 82 (but no soot
layer 91 ) is fused together and redrawn into a fused monolithic assembly
having a thinner diameter than tube 90. This redrawn assembly may then be
coated with further glass soot, which may be doped or undoped, as desired.
Alternatively, this fused monolithic assembly may, if desired, be drawn
directly
15 into a fiber. In either case, a vacuum is preferably applied to the tube 90
during the fusing step.
