U.S. patent application number 10/234818 was filed with the patent office on 2004-03-04 for inverse dispersion compensating fiber.
Invention is credited to Gruner-Nielsen, Lars, Knudsen, Stig Nissen, Pedersen, Morten Ostergaard.
Application Number | 20040042748 10/234818 |
Document ID | / |
Family ID | 31715290 |
Filed Date | 2004-03-04 |
United States Patent
Application |
20040042748 |
Kind Code |
A1 |
Gruner-Nielsen, Lars ; et
al. |
March 4, 2004 |
INVERSE DISPERSION COMPENSATING FIBER
Abstract
An inverse dispersion fiber is provided that has a relatively
low fiber loss, a relatively low fiber splice loss and a relatively
large effective mode-field area. The inverse dispersion fiber
includes a doped core region with an index of refraction n.sub.1, a
cladding region with an index of refraction n.sub.2, and a trench
region, a first barrier region and a second barrier region with
indices of refraction n.sub.3, n.sub.4, and n.sub.5, respectively,
formed between the doped core region and the cladding region. The
various regions of the inverse dispersion fiber are manufactured in
such a way that the refractive index value ranges are, for example,
approximately 0.709%<(n.sub.1-n.sub.2)/n.sub.2<1.0%,
approximately -0.358%<(n.sub.3-n.sub.2)/n.sub.2<-0.293%,
approximately 0.194%<(n.sub.4-n.sub.2)/n.sub.2<0.237%, and
approximately -0.045%<(n.sub.5-n.sub.2)/n.sub.2<-0.037%. The
inverse dispersion fiber in accordance with the preferred
embodiment has a chromatic dispersion of approximately -44
picosecond/(nanometer-kilomet- er) and a relatively large effective
core area, A.sub.eff, that is, for example, greater than
approximately 30.0 .mu.m.sup.2, both at a wavelength of 1550
nm.
Inventors: |
Gruner-Nielsen, Lars;
(Bronshoj, DK) ; Knudsen, Stig Nissen;
(Frederiksberg, DK) ; Pedersen, Morten Ostergaard;
(Vallensbaek, DK) |
Correspondence
Address: |
GARDNER GROFF, P.C.
PAPER MILL VILLAGE, BUILDING 23
600 VILLAGE TRACE
SUITE 300
MARIETTA
GA
30067
US
|
Family ID: |
31715290 |
Appl. No.: |
10/234818 |
Filed: |
September 4, 2002 |
Current U.S.
Class: |
385/127 |
Current CPC
Class: |
G02B 6/03666 20130101;
G02B 6/29377 20130101; G02B 6/02261 20130101; G02B 6/02009
20130101; G02B 6/0281 20130101 |
Class at
Publication: |
385/127 |
International
Class: |
G02B 006/16; G02B
006/22 |
Claims
What is claimed is:
1. An optical fiber communications system, comprising: at least one
source of optical energy; an optical fiber cable including at least
one positive dispersion optical fiber coupled to said at least one
source, and at least one inverse dispersion optical fiber coupled
to the positive dispersion optical fiber, wherein the inverse
dispersion optical fiber includes a doped core region having an
index of refraction n.sub.1, a cladding region having an index of
refraction n.sub.2, a trench region between the doped core region
and the cladding region and adjacent the doped core region, the
trench region having an index of refraction n.sub.3, a first
barrier region between the doped core region and the cladding
region and adjacent the trench region, the first barrier region
having an index of refraction n.sub.4, and a second barrier region
between the doped core region and the cladding region and adjacent
the first barrier region, the second barrier region having an index
of refraction n.sub.5, wherein the inverse dispersion optical fiber
has a chromatic dispersion between approximately -48
picosecond/(nanometer-kilo- meter) and -38
picosecond/(nanometer-kilometer) at a wavelength of 1550 nanometer
(nm), wherein the optical fiber has a median loss less than or
equal to approximately 0.235 decibels (dB) per kilometer (dB/km) at
1550 nm; and at least one receiver coupled to the inverse
dispersion optical fiber for receiving optical energy from the
source.
2. The system as recited in claim 1, wherein the inverse dispersion
optical fiber has a relative dispersion slope (RDS) that is
approximately 0.0030 nm.sup.-1 at a wavelength of 1550 nm.
3. The system as recited in claim 1, wherein the doped core region,
the cladding region, the trench region, the first barrier region
and the second barrier region are configured in such a way that
approximately 0.709%<(n.sub.1-n.sub.2)/n.sub.2<1%,
approximately -0.358%<(n.sub.3-n.sub.2)/n.sub.2<-0.293%,
approximately 0.194%<(n4-n.sub.2)/n.sub.2<0.237%, and
approximately -0.045%<(n.sub.5-n.sub.2)/n.sub.2<-0.037%,
wherein .DELTA..sub.1=(n.sub.1-n.sub.2)/n.sub.2,
.DELTA..sub.2=(n.sub.3-n.sub.2)/- n.sub.2,
.DELTA..sub.3=(n.sub.4-n.sub.2)/n.sub.2 and
.DELTA..sub.4=(n.sub.5-n.sub.2)/n.sub.2
4. The system of claim 3, wherein .DELTA..sub.1 is approximately
0.788%, .DELTA..sub.2 is approximately -0.326%, .DELTA..sub.3 is
approximately 0.215%, and .DELTA..sub.4 is approximately
-0.041%.
5. The system as recited in claim 1, wherein the inverse dispersion
optical fiber has an effective mode-field area, A.sub.eff, of at
least approximately 30 micrometers squared (.mu.m.sup.2) at a
wavelength of 1550 nm.
6. The system as recited in claim 1, wherein the optical fiber
cable further comprises a plurality of inverse dispersion fibers
spliced together, wherein the splice loss between spliced inverse
dispersion fibers is less than or equal to 0.15 dB at a wavelength
of approximately 1550 nm.
7. The system as recited in claim 1, wherein the splice loss
between the positive dispersion fiber and said at least one inverse
dispersion fiber is less than or equal to 0.40 dB at a wavelength
of approximately 1550 nm.
8. The system as recited in claim 1, wherein the inverse dispersion
optical fiber has a mode-field diameter (MFD) of approximately 6.4
.mu.m at a wavelength of 1550 nm.
9. The system as recited in claim 1, wherein the inverse dispersion
optical fiber has a chromatic dispersion slope of approximately
-0.133 ps nm.sup.-2 km.sup.-1 at a wavelength of 1550 nm.
10. The system as recited in claim 1, wherein the radius of the
doped core region is approximately 2.415 .mu.m, the width of the
trench region is approximately 3.090 .mu.m, the width of the first
barrier region is approximately 3.715 .mu.m, and the width of the
second barrier region is approximately 1.765 .mu.m.
11. An inverse dispersion optical fiber, comprising: a doped core
region having an index of refraction n.sub.1; a cladding region
having an index of refraction n.sub.2, wherein approximately
0.709%<(n.sub.1-n.sub.2)/- n.sub.2<1%, and wherein
.DELTA..sub.1=(n.sub.1-n.sub.2)/n.sub.2, a trench region between
the doped core region and the cladding region and adjacent the
doped core region, the trench region having an index of refraction
n.sub.3, wherein approximately -0.358%<(n.sub.3-n.sub.2)/n.-
sub.2<-0.293%, and wherein
.DELTA..sub.2=(n.sub.3-n.sub.2)/n.sub.2; a first barrier region
between the doped core region and the cladding region and adjacent
the trench region, the first barrier region having an index of
refraction n.sub.4, wherein approximately
0.194%<(n.sub.4-n.sub.2)/n.sub.2<0.237%, and wherein
.DELTA..sub.3=(n.sub.4-n.sub.2)/n.sub.2; and a second barrier
region between the doped core region and the cladding region and
adjacent the first barrier region, the second barrier region having
an index of refraction n.sub.5, wherein approximately
-0.045%<(n.sub.5-n.sub.2)/n.- sub.2<-0.037%, and wherein
.DELTA..sub.4=(n.sub.5-n.sub.2)/n.sub.2.
12. The inverse dispersion optical fiber of claim 11, wherein
.DELTA..sub.1 is approximately 0.788%, .DELTA..sub.2 is
approximately -0.326%, .DELTA..sub.3 is approximately 0.215%, and
.DELTA..sub.4 is approximately -0.041%.
13. The inverse dispersion optical fiber of claim 11, wherein the
optical fiber has a median loss that is less than or equal to
approximately 0.235 decibels per kilometer (dB/km) at a wavelength
of 1550 nm.
14. The inverse dispersion optical fiber of claim 11, wherein the
optical fiber has a relative dispersion slope (RDS) that is
approximately 0.0030 per nanometer (nm.sup.-1) at a wavelength of
1550 nm.
15. The inverse dispersion optical fiber of claim 11, wherein the
optical fiber has an effective mode-field area, A.sub.eff, of at
least approximately 30 micrometers.sup.2 (.mu.m.sup.2) at a
wavelength of 1550 nm.
16. The inverse dispersion optical fiber of claim 11, wherein the
inverse dispersion optical fiber has a mode-field diameter (MFD) of
approximately 6.4 .mu.m at a wavelength of 1550 nm.
17. The inverse dispersion optical fiber of claim 11, wherein the
optical fiber has a chromatic dispersion slope of approximately
-0.133 ps nm.sup.-2 km.sup.-1 at at a wavelength of 1550 nm.
18. The inverse dispersion optical fiber of claim 11, wherein the
radius of the doped core region is approximately 2.415 micrometers
(.mu.m), the width of the trench region is approximately 3.090
.mu.m, the width of the first barrier region is approximately 3.715
.mu.m, and the width of the second barrier region is approximately
1.765 .mu.m.
19. The inverse dispersion optical fiber of claim 11, wherein the
inverse dispersion optical fiber has a chromatic dispersion between
approximately -48 picosecond/(nanometer-kilometer) and
approximately -38 picosecond/(nanometer-kilometer) at a wavelength
of 1550 nanometer (nm).
20. A method for making an optical fiber, comprising the steps of:
forming a doped core region having an index of refraction n.sub.1;
forming a trench region around the doped core region, the trench
region having an index of refraction n.sub.3; forming a first
barrier region around the trench region, the first barrier having
an index of refraction n.sub.4; forming a second barrier region
around the first barrier region, the second barrier region having
an index of refraction n.sub.5; and forming a cladding region
around the second barrier region, the cladding region having an
index of refraction n.sub.2, wherein the doped core region, the
cladding region, the trench region, the first barrier region and
the second barrier region are configured in such a way that
approximately 0.709%<(n.sub.1-n.sub.2)/n.sub.2<1%,
approximately -0.358%<(n.sub.3-n.sub.2)/n.sub.2<-0.293%,
approximately 0.194%<(n.sub.4-n.sub.2)/n.sub.2<0.237%, and
approximately -0.045%<(n.sub.5-n.sub.2)/n.sub.2<0.037%,
wherein .DELTA..sub.1=(n.sub.1-n.sub.2)/n.sub.2,
.DELTA..sub.2=(n.sub.3-n.sub.2)/- n.sub.2,
.DELTA..sub.3=(n.sub.4-n.sub.2)/n.sub.2 and
.DELTA..sub.4=(n.sub.5-n.sub.2)/n.sub.2
21. The method of claim 20, wherein .DELTA..sub.1 is approximately
0.788%, .DELTA..sub.2 is approximately -0.326%, .DELTA..sub.3 is
approximately 0.215%, and .DELTA..sub.4 is approximately
-0.041%.
22. An optical fiber preform, comprising: a doped core region
having an index of refraction n.sub.1; a cladding region having an
index of refraction n.sub.2; a trench region between the doped core
region and the cladding region and adjacent the doped core region,
the trench region having an index of refraction n.sub.3; a first
barrier region between the doped core region and the cladding
region and adjacent the trench region, the first barrier region
having an index of refraction n.sub.4; and a second barrier region
between the doped core region and the cladding region and adjacent
the first barrier region, the second barrier region having an index
of refraction n.sub.5, wherein the doped core region, the cladding
region, the trench region, the first barrier region and the second
barrier region are configured in such a way that approximately
0.709%<(n.sub.1-n.sub.2)/n.sub.2<1%, approximately
-0.358%<(n.sub.3-n.sub.2)/n.sub.2<-0.293%, approximately
0.194% <(n.sub.4-n.sub.2)/n.sub.2<0.237%, and approximately
-0.045% <(n.sub.5-n.sub.2)/n.sub.2<-0.037%, wherein
.DELTA..sub.1=(n.sub.1-- n.sub.2)/n.sub.2,
.DELTA..sub.2=(n.sub.3-n.sub.2)/n.sub.2,
.DELTA..sub.3=(n.sub.4-n.sub.2)n.sub.2 and
.DELTA..sub.4=(n.sub.5-n.sub.2- )/n.sub.2
23. The optical fiber preform of claim 22, wherein .DELTA..sub.1 is
approximately 0.788%, .DELTA..sub.2 is approximately -0.326%,
.DELTA..sub.3 is approximately 0.215%, and .DELTA..sub.4 is
approximately -0.041%.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to an inverse dispersion optical
fiber. More particularly, the invention relates to an inverse
dispersion optical fiber having reduced optical loss relative to
conventional inverse dispersion fiber and that is suitable for
compensating dispersion in large effective area positive dispersion
fiber.
[0003] 2. Description of the Related Art
[0004] Optical fibers are thin strands of glass or plastic capable
of transmitting optical signals, containing relatively large
amounts of information, over long distances and with relatively low
attenuation. Typically, optical fibers are made by heating and
drawing a portion of an optical preform comprising a refractive
core region surrounded by a protective cladding region made of
glass or other suitable material. Optical fibers drawn from the
preform typically are protected further by one or more coatings
applied to the cladding region.
[0005] Advances in transmission over optical fibers have enabled
optical fibers to have enormous bandwidth capabilities. Such
bandwidth enables thousands of telephone conversations and hundreds
of television channels to be transmitted simultaneously over a
hair-thin fiber. Transmission capacity over an optical fiber is
increased in wavelength division multiplexing (WDM) systems wherein
several channels are multiplexed onto a single fiber, with each
channel operating at a different wavelength. However, in WDM
systems, nonlinear interactions between channels occurs, such as
4-photon mixing, which severely reduces system capacity. This
problem has been largely solved by U.S. Pat. No. 5,327,516 (the
'516 patent), which is owned by the assignee of the present
application. The '516 patent discloses an optical fiber that
reduces these nonlinear interactions by introducing a small amount
of chromatic dispersion at the operating wavelengths. As the number
of WDM channels to be transmitted over a single fiber increases,
the optical power carried by the optical fiber also increases. As
the optical power increases, the nonlinear effects caused by
interaction between the channels also increases. Accordingly, it is
desirable for an optical fiber to provide a small amount of
chromatic dispersion to each of the WDM channels in order to reduce
the nonlinear interactions between the channels, especially in view
of ever-increasing bandwidth demands. However, in order to be able
to restore the signal after the transmission link, it is important
that the dispersion introduced vary as little as possible amongst
the different WDM channels.
[0006] Important advances have been made in the quality of the
material used in making optical fibers. In 1970, an acceptable loss
for glass fiber was in the range of 20 dB/km, whereas today losses
are generally about 0.25 dB/km. The theoretical minimum loss for
glass fiber is about 0.16 dB/km, and it occurs at a wavelength of
about 1550 nanometers (nm). Dispersion in a glass fiber causes
pulse spreading for pulses that include a range of wavelengths, due
to the fact that the speed of light in a glass fiber is a function
of the transmission wavelength of the light. Pulse broadening is a
function of the fiber dispersion, the fiber length and the spectral
width of the light source. Dispersion for individual fibers is
generally illustrated using a graph (not shown) having dispersion
on the vertical axis (in units of picoseconds (ps) per nanometer
(nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the
horizontal axis. There can be both positive and negative
dispersion, so the vertical axis may range from, for example, -250
to +25 ps/nm km. The wavelength on the horizontal axis at which the
dispersion equals zero corresponds to the highest bandwidth for the
fiber. However, this wavelength typically does not coincide with
the wavelength at which the fiber transmits light with minimum
attenuation.
[0007] For example, typical single mode fibers generally transmit
best (i.e., with minimum attenuation) at 1550 nm, whereas
dispersion for the same fiber would be approximately zero at 1310
nm. Also, the aforementioned theoretical minimum loss for glass
fiber occurs at the transmission wavelength of about 1550 nm.
Because minimum attenuation is prioritized over zero dispersion,
the wavelength normally used to transmit over such fibers is
typically 1550 nm. Also, Erbium-doped amplifiers, which currently
are the most commonly used optical amplifiers for amplifying
optical signals carried on a fiber, operate in 1530 to 1565 nm
range. Because dispersion for such a fiber normally will be closest
to zero at a wavelength of 1310 nm rather than at the optimum
transmission wavelength of 1550 nm, attempts are constantly being
made to improve dispersion compensation over the transmission path
in order to provide best overall system performance (i.e., low
optical loss and low dispersion).
[0008] In order to improve dispersion compensation at the
transmission wavelength of 1550 nm, it is known to couple the
transmission fiber, which normally is a positive dispersion fiber
(PDF), with an inverse dispersion fiber (IDF). The positive
dispersion transmission fiber typically comprises a single mode
fiber designed to introduce dispersion in order to reduce the
nonlinear interactions between channels. The inverse dispersion
fiber has a negative dispersion and negative dispersion slope that
match the dispersion characteristics of the positive dispersion
transmission fiber (but are opposite in sign) in order to
compensate dispersion in a broad wavelength range and minimize the
residual dispersion (i.e., dispersion on wavelength channels other
than the center wavelength channel being compensated).
[0009] A transmission PDF is coupled to a length of IDF by
splicing. The combination of the PDF and the IDF has both an
intrinsic fiber loss and a splicing loss. Of course, overall
optical loss for a transmission link should be kept at a minimum.
This is especially true over long transmission links because more
amplifiers are needed in order to prevent transmission quality
degeneration when the transmission link has larger losses. For
example, in trans-oceanic communications systems it is advantageous
to use a combination of large effective area PDF and an IDF having
matching dispersion and dispersion slope characteristics that are
of opposite sign to those of the PDF. This combination results in
the minimal accumulation of residual dispersion over the
transmission wavelength range. Conventional IDF has a median loss
of, for example, approximately, 0.246 db/km at 1550 nm. One way to
decrease the overall loss of the transmission link would be to
utilize an IDF that has a lower fiber loss than the conventional
IDF that is currently being used.
[0010] Many features of a fiber, such as an IDF, can be ascertained
from the refractive index profile of the fiber. The refractive
index profile shows how the index of refraction of the fiber varies
as a function of distance from its central axis. Parameters used
for describing the refractive index profile generally are
referenced to the index of refraction of the outermost layer of
glass. Idealized models of refractive-index profile typically
comprise axially symmetric rings or regions of different refractive
index. However, changing the number, size and/or shape of any one
of these regions generally impacts more than one characteristic of
the fiber (e.g., dispersion slope is reduced, but bending loss is
increased or effective area is decreased). Thus, it is a
significant design effort to create a refractive index profile that
provides most if not all of the desired features for the fiber, and
yet still be readily manufacturable.
[0011] It would be desirable to have an IDF with a refractive index
profile that provides the IDF with a lower fiber loss than the
fiber loss of the conventional IDF currently being used and that
provides minimal accumulation of residual dispersion over a
transmission link comprising a combination of a PDF and an IDF. In
addition, the loss when splicing this IDF to a PDF should be kept
as low as possible. It would also be desirable to provide such an
IDF that is suitable for compensating dispersion of a large
effective area PDF, such as a super-large effective area PDF.
SUMMARY OF THE INVENTION
[0012] The invention is embodied in an optical communications
system including one or more optical transmission devices, one or
more optical receiving devices, and at least one optical fiber
cable coupled therebetween that includes at least one positive
dispersion optical fiber and corresponding inverse dispersion
optical fiber. According to embodiments of the invention, the
inverse dispersion fiber has negative dispersion and a negative
dispersion slope around the wavelength 1550 nm. The inverse
dispersion fiber includes a doped core region with an index of
refraction n.sub.1, a cladding region with an index of refraction
n.sub.2, and a trench region, a first barrier region and a second
barrier region with indices of refraction n.sub.3, n.sub.4, and
n.sub.5, respectively, formed between the doped core region and the
cladding region.
[0013] Inverse dispersion fiber according to embodiments of the
present invention preferably has a chromatic dispersion of
approximately -44 picosecond/(nanometer-kilometer) and a relatively
large effective mode-field area, A.sub.eff, e.g., greater than
approximately 30.0 .mu.m.sup.2, both at a wavelength of 1550
nanometers. The various regions of the inverse dispersion fiber are
manufactured in such a way that the refractive index value ranges
preferably are, e.g., 0.709%<(n.sub.1-n.sub.2)/n.sub.2<1%,
-0.358%<(n.sub.3-n.sub.2)/n- .sub.2<-0.293%,
0.194%<(n4-n.sub.2)/n.sub.2<0.237%, and -0.045% -0.037%. In
accordance with the preferred embodiment of the present invention,
manufacture of the optical fiber includes manufacture of a core
region having a diameter of approximately 4.83 .mu.m, a trench
region having a diameter of approximately 11.01 .mu.m, a
first-barrier region having a diameter of approximately -18.44
.mu.m, and a second barrier region having a diameter of
approximately 21.97 .mu.m.
[0014] The refractive index of the core preferably is graded to
follow a power law, with an exponent of .gamma., where
1<.gamma.<7. The core region is doped with, for example,
germanium or other suitable material. The trench region, the first
barrier region, the second barrier and the cladding region are
doped with, for example, germanium and/or fluorine, and/or other
suitable material(s). Inverse dispersion optical fiber according to
embodiments of the invention provides improved compensation of
positive dispersion optical fibers, including existing positive
dispersion optical fibers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1A is a cross-sectional end view of an inverse
dispersion optical fiber in accordance with an embodiment of the
present invention.
[0016] FIG. 1B is a graphical diagram of a refractive index
graded-core profile of the inverse dispersion optical fiber shown
in FIG. 1.
[0017] FIG. 2A is the same as FIG. 1A and is repeated to
demonstrate the relationship between the refractive indices of the
different layers of the fiber as a function of the radius of the
various regions;
[0018] FIG. 2B is a refractive index difference profile that
represents the differences between the refractive indices of the
layers of the fiber shown in FIG. 2A as a function of the radius of
the various regions.
[0019] FIG. 3 is a graphical diagram that represents the residual
dispersion of a span of an optical fiber link that includes a
length of super-large effective area (SLA) fiber and a length of
the inverse dispersion optical fiber of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] The IDF described herein will be described as having
particular properties and a particular refractive index profile.
However, it should be noted that the IDF described herein is an
example of the IDF of the present invention. Those skilled in the
art will understand, in view of the discussion provided herein,
that the IDF of the present invention is not limited to any
particular IDF. IDFs having properties and refractive index
profiles different from those of the IDF described herein are also
within the scope of the present invention.
[0021] The IDF of the present invention is suitable for
compensating dispersion in many types of optical fibers such as,
for example, positive dispersion, pure silica core fiber from
Sumitomo Electric Industries, Ltd., as described in "Ultra Low
Nonlinearity Low Loss Pure Silica Core Fiber," Electronics Letters
Online No: 19991094, Aug. 3, 1999, Vascade 100 fiber from Corning,
large effective area fiber from Fujitsu and UltraWave.RTM. SLA
fiber from Fitel USA Corporation.
[0022] As discussed above, the overall dispersion of an optical
fiber generally results from the combination of material
dispersion, which depends on the actual material(s) used in making
the optical fiber, and waveguide dispersion, which is a function of
the refractive-index profile of the fiber. FIG. 1A shows a
cross-sectional view of an IDF 10 and FIG. 1B shows its
corresponding refractive index profile in accordance with the
preferred embodiment of the present invention. The IDF 10 includes
a plurality of layers 11, 12, 13, 14 and 15, each of which has a
different index of refraction. The region 11 corresponds to the
central core of the optical fiber 10 and has a nominal index of
refraction n.sub.1. The central core region 11 is surrounded by a
first annular ring or region 12 (trench region) that has a nominal
refractive index n.sub.3. The trench region 12 is surrounded by a
second annular region 13 (first barrier region), which has a
nominal refractive index n.sub.4. The second region 13 is
surrounded by a third annular region 14 (second barrier region)
that has a nominal refractive index n.sub.5. An outer cladding 15
of nominal refractive index n.sub.2 surrounds region 14.
[0023] It should be noted that the optical fiber 10 is not drawn to
scale (the outer diameter of cladding layer 15 preferably is
approximately 125 .mu.m, while the diameter of the core region 11
preferably is less than approximately 6 .mu.m). Also, as discussed
in greater detail below, because of the relative refractive index
values of the various regions, and due to their functions, the
first region 12 will be referred to hereinafter as a trench region,
the second region 13 will be referred to hereinafter as a first
barrier region, and the third region 14 will be referred to
hereinafter as a second barrier region. The region 15 will be
referred to hereinafter as the outer cladding.
[0024] Although the rings in FIG. 1A suggest that the changes
between the refractive indices of the regions 11-15 are abrupt,
this is not the case. In accordance with the preferred embodiment
of the present invention, the fiber 10 is a graded-index fiber and
the refractive index changes between adjacent layers are gradual.
However, abrupt changes are shown in FIG. 1A to enable distinctions
between the regions to be easily made.
[0025] FIG. 1B is a graphical representation of the refractive
index profile 20 of the fiber 10 shown in FIG. 1A. The Y-axis
corresponds to refractive index.times.100 and the X-axis
corresponds to positions along a radius from the center of the core
11 of the fiber 10 to the outer edge of the cladding 15 of the
fiber 10. The refractive index values shown in FIG. 1B are actually
relative refractive index values, i.e., they are relative to the
refractive index of the outer cladding 15. Therefore, the index
values given in FIG. 1B should be regarded as the difference
between the index value for the particular region and that of the
outer cladding 15 divided by that of the outer cladding (i.e.,
(n.sub.region-n.sub.cladding)/n.sub.cladding)). Therefore, when the
indices of refraction of the various regions of the fiber 10 are
discussed herein, it should be understood that they are actually
relative indices of refraction.
[0026] The core region 11 has an index of refraction n.sub.1. The
trench region 12 has an index of refraction n.sub.3, which is less
than n.sub.1. The first barrier region 13 has an index of
refraction n.sub.4, which is greater than n.sub.3. The second
barrier region 14 has an index of refraction n.sub.5, which is less
than n.sub.4 (that of the first barrier region) but greater than
n.sub.3 (that of the trench region). The cladding region 15 has an
index of refraction n.sub.2, which is greater than n.sub.3 (that of
the trench region) and n.sub.5 (that of the second barrier region),
but less than n.sub.4 (that of the first barrier region) and
n.sub.1 (that of the core).
[0027] The core region 11 has a radius b.sub.1. The trench region
12 has an outer radius b.sub.2 and an inner radius is b.sub.1. The
first barrier region 13 has an outer radius b.sub.3 and an inner
radius b.sub.2. The second barrier region 14 has an outer radius
b.sub.4 and an inner radius b.sub.3. The cladding region 15 has an
outer radius b.sub.5 and an inner radius b.sub.4.
[0028] The fiber refractive index profile 20 shown in FIG. 1B
represents a fiber in accordance with the preferred embodiment of
the present invention, which comprises a germanium-doped silica
(SiO.sub.2) core 11 (e.g., SiO.sub.2 doped with an appropriate
amount of GeO.sub.2), a fluorine (F) and/or germanium (Ge)-doped
trench region 12 surrounding the core region 11 (e.g., SiO.sub.2
doped with an appropriate amount of GeO.sub.2 and F), a germanium
and/or fluorine and/or phosphorous-doped first barrier region 13
surrounding the trench region 12 (e.g., SiO.sub.2 doped with an
appropriate amount of GeO.sub.2, F and P), a germanium and/or
fluorine-doped and/or phosphorous-doped second barrier region 14
surrounding the first barrier region 13 (e.g., SiO.sub.2 doped with
an appropriate amount of GeO.sub.2 and F, and P) and a pure silica
outer cladding 15 surrounding the second barrier region 14.
[0029] In the refractive index profile 20 shown in FIG. 1B, the
nominal refractive indices n.sub.1, n.sub.3, n.sub.4 and n.sub.5,
are all relative to the refractive index n.sub.2 of the cladding
15, which corresponds to the X-axis in FIG. 1B. The nominal
refractive index n.sub.1 of the core region 11 is approximately
0.788%. The nominal refractive index n.sub.3 of the trench region
12 is approximately -0.326%. The nominal refractive index n.sub.4
of the first barrier region 13 is approximately 0.215%. The nominal
refractive index n.sub.5 of the second barrier region 14 is
approximately -0.041%. According to embodiments of the invention,
the refractive index profile provides negative dispersion, inverse
dispersion, or dispersion compensating optical fiber with
relatively large effective transmission area (i.e., effective
mode-field area, A.sub.eff) and transmission characteristics that
provide an improved dispersion and dispersion slope match with
super-large effective area (SLA) positive dispersion fibers, such
as those discussed previously herein. It should be noted that the
fiber of the present invention is not limited to these refractive
index values. Those skilled in the art will understand, in view of
the discussion provided herein, that these refractive indices
correspond to the preferred fiber configuration (and thus
correspond to the preferred refractive index values) and that other
refractive index values are suitable for providing a fiber that
meets the goals of the present invention.
[0030] The portion of the profile 20 labeled with the refractive
index of n.sub.1 corresponds to the core region 11 of the fiber 10.
The portion of the profile 20 labeled with the nominal refractive
index of n.sub.3 corresponds to the trench region 12 of the fiber
10. The portion of the profile 20 labeled with the nominal
refractive index of n.sub.4 corresponds to the first barrier region
13 of the fiber 10. The portion of the profile 20 labeled with the
nominal refractive index of n.sub.5 corresponds to the second
barrier region 14 of the fiber 10. The portion of the profile 20
labeled with the nominal refractive index of n.sub.2 corresponds to
the cladding region 15 of the fiber 10. It can be seen from the
profile 20 that the core 11 has a nominal index of refraction
(n.sub.1) that is positive, that the trench region 12 has an index
of refraction (n.sub.3) that is negative, and that the first
barrier region 13 has a nominal index of refraction (n.sub.4) that
is positive, but less than the refractive index n.sub.1of the core
region 11. Therefore, the first barrier region 13 has a nominal
index of refraction n.sub.4 that is greater than that of the trench
region n.sub.3. The second barrier region 14 has a nominal index of
refraction n.sub.5, which is less than n.sub.4 (that of the first
barrier region) but greater than n.sub.3 (that of the trench
region). The cladding region 15 has an index of refraction n.sub.2,
which is greater than n.sub.3 (that of the trench region) and
n.sub.5 (that of the second barrier region), but less than n.sub.4
(that of the first barrier region) and n.sub.1(that of the
core).
[0031] In addition to graphing the refractive index profile of the
optical fiber 10 of the present invention using the actual values
of the index of refraction, as shown in FIG. 1B, it is useful to
show a refractive index difference profile as a function of
normalized refractive index value differences .DELTA..sub.1,
.DELTA..sub.2, .DELTA..sub.3 and .DELTA..sub.4, which are defined
as: .DELTA..sub.1=(n.sub.1-n.sub.2)/n.su- b.2.times.100%,
.DELTA..sub.2=(n.sub.3-n.sub.2)/n.sub.2.times.100%,
.DELTA..sub.3=(n.sub.4-n.sub.2)/n.sub.2.times.100%, and
.DELTA..sub.4=(n.sub.5-n.sub.2)/n.sub.2.times.100%. This can be
seen with reference to FIG. 2A and FIG. 2B. FIG. 2A is identical to
FIG. 1A and is repeated to demonstrate how the refractive index
differences shown in FIG. 2B correspond to the regions 11-15 of the
optical fiber 10 of the present invention shown in FIG. 2A.
[0032] FIG. 2B is a refractive index difference profile 30. The
dashed lines between FIG. 2A and FIG. 2B show how the refractive
index differences .DELTA..sub.1, .DELTA..sub.2, .DELTA..sub.3 and
.DELTA..sub.4 relate to regions 11-15 of the fiber 10. Using the
above difference equations with the refractive index values given
above, the following delta values are obtained:
.DELTA..sub.1.apprxeq.0.788%; .DELTA..sub.2.apprxeq.-0.326%;
.DELTA..sub.3.apprxeq.0.215%; and .DELTA..sub.4.apprxeq.0.041%.
Preferably, the ranges for the delta values are as follows:
0.709%<.DELTA..sub.1<1%; -0.358%<.DELTA..sub.2&l-
t;-0.293%; 0.194%<.DELTA..sub.3<% 0.237; and
-0.045%<.DELTA..sub.- 4<-0.037%. In accordance with the
preferred embodiment of the present invention, be is approximately
2.41 .mu.m (i.e., the core region diameter is approximately 4.83
.mu.m), b.sub.2 is approximately 5.50 .mu.m (i.e., the trench
region diameter is approximately 11.01 .mu.m), b.sub.3 is
approximately 9.22 .mu.m (i.e., the first barrier region diameter
is approximately 18.44 .mu.m) and b.sub.4 is approximately 10.98
.mu.m (i.e., the second barrier region diameter is approximately
21.97 .mu.m). Thus, the width of the trench region is approximately
3.09 .mu.m (5.50 .mu.m-2.41 .mu.m), the width of the first barrier
region is approximately 3.72 .mu.m (9.22 .mu.m-5.50 .mu.m) and the
width of the second barrier region is approximately 1.76 .mu.m
(10.98 .mu.m-9.22 .mu.m).
[0033] The following table shows the refractive indices for each of
the regions 11-14 as well as the diameters of each of the regions
11-14 of the optical fiber of the present invention in accordance
with the preferred embodiment.
1 Refractive index (x1000) [absolute index difference Fiber Region
Diameter [micron] compared to SiO.sub.2] Second Barrier 21.97 -0.59
Region First Barrier Region 18.44 3.12 Trench 11.01 -4.72 Core 4.83
11.43 Index profile exponent Core .gamma. = 4
[0034] It should be noted that the refractive index values given in
Table 1 are the absolute values as opposed to relative values. The
fiber refractive index profile 20 of FIG. 1B is a graded index
profile and follows an exponent profile given by the following
equation:
n(r)=n.sub.0.multidot.(1-(r/r.sub.0).sup.r) r<r.sub.0 Equation
1
[0035] where n(r) is the core refractive index as a function of the
fiber radius and .gamma., the core index profile exponent,
(.gamma.=4 in Table 1 for this example) is an exponent that
determines the core shape. The term no is the maximum core
refractive index and the term r.sub.0 is the maximum core radius.
The effect of making a core according to the refractive index
profile defined by Equation 1 is to lower the anomalous fiber loss
term, .alpha..sub.anamalous, which is given by the following
equation: 1 anomalous 2 ( + 2 ) 2 Equation 2
[0036] Using a core exponent .gamma. of 4, as is the case with the
preferred embodiment of the present invention, enables the
anomalous loss term .alpha..sub.anamalous to be lowered by more
than a factor of 2 compared to a similar fiber that has a stepped
core refractive index profile with a core exponent .gamma. of 30 or
more. As stated above, in accordance with the preferred embodiment
of the present invention, the range of .gamma. is approximately
1<.gamma.<7.
[0037] The following table lists the median values for the optical
parameters of the optical fiber of the present invention having the
refractive index profile and other characteristics discussed above
with reference to FIGS. 1A-2B.
2TABLE 2 Parameter Unit Value Total fiber length. [km] 2300 OTDR
attenuation @ 1550 nm. [dB/km] 0.234 Attenuation spike dB] 0.02
Maximum 1 km. individual section [dB/km] 0.242 loss @ 1550 nm.
Chromatic dispersion @ 1550 nm. [ps/nm -44.16 km] Chromatic
dispersion slope @ 1550 nm. [ps/nm.sup.2 -0.133 km] Relative
dispersion slope @ 1550 nm. [1/nm] 0.00303 Cable cutoff wavelength
[nm] 1341 Mode-field diameter @ 1550 nm. [micron] 6.4 Spool PMD
[ps/km.sup.0.5] 0.044 Attenuation @ 1385 nm. (Water peak) [dB/km]
0.437 Avg. splice loss (IDF - SLA) [dB] <0.40 Avg. splice loss
(IDF - IDF) [dB] <0.15
[0038] The relative dispersion slope (RDS) specified in Table 2 is
defined as RDS=(.differential.D/.differential..lambda.)/D, where D
is the chromatic dispersion of the fiber and the derivative of D
with respect to .lambda. is the chromatic dispersion slope of the
fiber.
[0039] FIG. 3 is a graph that illustrates the obtainable residual
dispersion as a function of wavelength for a span of an optical
fiber link comprising a length of SLA fiber and a length of the IDF
fiber of the present invention. It can be seen that the residual
dispersion variation can be kept very low in a broad wavelength
range around 1550 nm and the curve 30 can be made zero, if desired,
at the target transmission wavelength of 1550 nm. The Fiber 10 of
the present invention has improved fiber loss characteristics
compared to other IDFs, as shown below in Table 3. The fiber 10 of
the present invention has a median loss of 0.234 decibels per
kilometer (dB/km) at 1550 nm. The "median loss" is the loss value
where 50% of the loss measurements fall below the median loss value
and 50% of the loss measurements exceed the median value. This is a
large improvement over other conventional IDF fibers (not shown)
that have been used in the past and that generally have a median
loss of at least approximately 0.246 decibels/km (dB/km) at 1550
nm. Table 3 shows the loss for a full 45 km span of combined
UltraWave.RTM. SLA fiber and a conventional IDF fiber, for a full
45 km span of combined UltraWave.RTM. SLA fiber and the IDF fiber
of the present invention and for a full 45 km span of combined,
conventional non-zero-dispersion submarine fibers, one of which has
a large effective area and the other of which has a low dispersion
slope. The equivalent effective area is defined as the effective
area of a non-zero dispersion shifted fiber with attenuation 0.21
dB/km, which will result in the same non-linear phase shift from
self-phase modulation as the transmission span of interest. As a
requirement in the calculations, the launched input power is
adjusted so as to keep a constant output power.
3TABLE 3 Equivalent Loss [dB/km] Fiber combination effective area
including splices SLA/conventional IDF 63.9 0.233 SLA/IDF in
accordance 73.3 0.218 with the preferred embodiment of the
invention Conventional NZDF solution 64 0.218
[0040] It can be seen in Table 3 that the span that utilizes the
IDF of the present invention has a larger equivalent effective area
and lower loss than with the other spans. It can be seen from Table
2 that the chromatic dispersion of the fiber of the present
invention having the index profile of FIG. 1B is -44.16 ps/nm km at
1550 nm, which is greater (numerically) than that of typical IDFs
at 1550 nm. The reduction in loss and increase in absolute
dispersion (-44.16) leads to lowered non-linear penalties in the
transmission span (the optical fiber link). Therefore, the fiber 10
of the present invention allows an optical fiber link to be made
that has very low residual dispersion over the transmission band
while, at the same time, providing a large equivalent effective
area (and thus low non-linear penalties and low optical loss in the
optical fiber link). These improvements, in turn, enable the span
length to be increased and the number of amplifiers along the
optical fiber link to be decreased, thereby enabling the overall
system costs to be reduced.
[0041] It will be apparent to those skilled in the art that many
changes and substitutions can be made to the embodiments of the
optical fibers herein described without departing from the spirit
and scope of the invention as defined by the appended claims and
their full scope of equivalents. Such changes and substitutions
include, but are not limited to, the use of different doping
materials to achieve the same general profile shapes, and the use
of plastic materials (rather than glass) in making the optical
fiber.
* * * * *