U.S. patent application number 09/773696 was filed with the patent office on 2002-01-17 for microstructure optical fibers for dispersion management in optical communication systems.
Invention is credited to Ranka, Jinendra Kumar, Reed, William Alfred, Windeler, Robert Scott.
Application Number | 20020005969 09/773696 |
Document ID | / |
Family ID | 26882589 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005969 |
Kind Code |
A1 |
Ranka, Jinendra Kumar ; et
al. |
January 17, 2002 |
Microstructure optical fibers for dispersion management in optical
communication systems
Abstract
A fiber optic system comprises an optical transmitter, an
optical receiver, and an optical fiber transmission path that
optically couples the transmitter and the receiver to one another.
The transmission path includes a first section that has negative
dispersion at an operating wavelength .lambda..sub.0 greater than
about 1300 nm and a second section that includes a MOF. The MOF has
relatively large anomalous dispersion at .lambda..sub.0 and is
sufficiently long to compensate the accumulated negative dispersion
in the first section. In one embodiment the MOF comprises a core, a
lower index cladding that includes one or more layers of air holes
surrounding the core, characterized in that the diameter of the
core is less than about 8 .mu.m and the difference in effective
refractive index between the core and cladding is greater than
about 0.1 (10%). Preferably, the cladding contains no more than 2
layers of air holes and the distance between the II nearest edges
of adjacent air holes is less than about 1 .mu.m.
Inventors: |
Ranka, Jinendra Kumar;
(Brookline, MA) ; Reed, William Alfred; (Summit,
NJ) ; Windeler, Robert Scott; (Annandale,
NJ) |
Correspondence
Address: |
Michael J. Urbano
1445 Princeton Drive
Bethlehem
PA
18017-9166
US
|
Family ID: |
26882589 |
Appl. No.: |
09/773696 |
Filed: |
February 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60186951 |
Mar 4, 2000 |
|
|
|
Current U.S.
Class: |
398/147 |
Current CPC
Class: |
G02B 6/29377 20130101;
G02B 6/02366 20130101; B82Y 20/00 20130101; G02B 6/02266
20130101 |
Class at
Publication: |
359/161 ;
359/173 |
International
Class: |
H04B 010/00; H04B
010/12 |
Claims
What is claimed is
1. A fiber optic system comprising an optical transmitter, an
optical receiver, an optical fiber transmission path that optically
couples said transmitter and said receiver to one another,
characterized in that said transmission path comprises a first
section that has negative dispersion at an operating wavelength
.lambda..sub.0 greater than about 1300 nm and a second section that
includes a microstructure fiber, said microstructure fiber having a
relatively large anomalous dispersion at said wavelength
.lambda..sub.0 and being sufficiently long to compensate the
accumulated negative dispersion in said first section, said
transmitter generates an optical signal that includes at least one
component at said wavelength .lambda..sub.0, and said
microstructure fiber comprises a core region in which said optical
radiation propagates, an inner cladding region surrounding said
core region and having an effective refractive index lower than
that of said core region, an outer cladding region surrounding said
inner cladding region, said inner cladding region including a
multiplicity of features positioned circumferentially in at least
one relatively thin layer around said core region, said features
being effective to provide index guiding of said radiation, and
said core region and said inner cladding region being mutually
adapted so that said fiber exhibits relatively large anomalous
group velocity dispersion at wavelengths above about 1300 nm, said
core region has a diameter less than about 8 .mu.m and the
difference in effective refractive index between said core region
and said inner cladding region is greater than about 10%.
2. The invention of claim of 1 wherein the outer boundary of said
features is less than about 10-30 .mu.m from the outer boundary of
said core region.
3. The invention of claim of 2 wherein said features are positioned
circumferentially in a multiplicity of relatively thin layers.
4. The invention of claim 1 wherein said inner cladding region
includes capillary air holes that form said features.
5. The invention of claim 4 wherein said air holes are positioned
circumferentially around said core region in no more than two
relatively thin layers and the distance between the nearest edges
of adjacent ones of said holes is less than about 1 .mu.m.
6. The invention of claim 1 wherein the pattern formed by said
features comprises geometric figure selected from the group
consisting of a hexagon and a triangle.
7. The invention of claim 1 wherein said core region and said
cladding regions comprise silica.
8. The invention of claim 8 wherein said features comprise
capillary air holes and said fiber exhibits anomalous group
velocity dispersion greater than approximately 20 ps/nm-km over a
wavelength range from about 1200 nm to about 1700 nm.
9. The invention of claim 8 wherein said features comprise
capillary air holes and said fiber exhibits anomalous group
velocity dispersion greater than approximately 50-200 ps/nm-km over
said wavelength range from about 1200 nm to about 1700 nm.
10. The invention of claim 1 wherein said optical transmitter
comprises a laser-based transmitter.
11. The invention of claim 1 wherein said first section is much
longer than said second section, and the absolute value of
dispersion in ps/nm in said second section is much larger than that
in said first section.
12. The invention of claim 1 wherein said transmitter generates a
multiplicity of signals at different wavelengths greater than about
1300 nm and said microstructure fiber provides relatively large
anomalous dispersion at each of said signal wavelengths.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from provisional
application Ser. No. 60/186,951 filed on Mar. 4, 2000 and was
concurrently filed with application Ser. No. _______ (Ranka 3-13)
entitled Applications of Multimode Microstructure Optical
Fibers.
FIELD OF THE INVENTION
[0002] This invention relates generally to dispersion management in
fiber optic systems and, more particularly, to the use of
microstructure optical fibers (MOFs) to achieve dispersion
compensation.
BACKGROUND OF THE INVENTION
[0003] High bit rate optical transmission systems require periodic
dispersion compensation to correct for the pulse broadening and
distortion that occurs due to the dispersion of the transmission
fiber. The accumulated dispersion over multiple amplifier spans can
often limit the maximum data transmission rate. See, Agrawal, Fiber
Optic Communication Systems, Ch. 9, John Wiley & Sons (1997),
which is incorporated herein by reference. To minimize the
limitation of system performance due to dispersion, a single
wavelength system can be made to operate near the zero-dispersion
wavelength of the transmission fiber span. For state-of-the-art
multiple wavelength systems, this type of operation is not possible
due to the broadband spectral transmission and the detrimental
nonlinear effects that occur between multiple wavelengths when
operating near the zero-dispersion wavelength of a long fiber span.
In order to avoid serious nonlinear impairments, fiber designed for
dense wavelength division multiplexed (DWDM) systems in the 1550 nm
range typically have dispersion values D between +2 and +6 ps/nm-km
for terrestrial systems and between -2 to -10 ps/nm-nkm for
undersea systems. In a terrestrial system the accumulated
dispersion over each transmission span is typically compensated
through the use of a section of dispersion compensating fiber that
provides net dispersion of equal magnitude and of opposite sign to
the dispersion of the span. Dispersion compensating fiber (DCF)
with an anomalous dispersion value D .about.80 ps/nm-km at 1550 nm
is easily achieved requiring only about 10 km of DFC compensate for
an 80 km transmission span. See, Vengsarkar et al., Optics Lett.,
Vol. 18, No. 11,pp. 924-926 (Jun. 1993), which is also incorporated
herein by reference.
[0004] On the other hand, the maximum positive dispersion value
that can be achieved in standard silica optical fibers is limited
to the value of silica material dispersion, which ranges from 0
ps/nm-km at 1290 nm to 20 ps/nm-km at 1550 nm. Conventional optical
fibers can be designed such that the net dispersion can be
significantly lower than this value but not higher.
[0005] This llritation is inherent in standard fibers used to
compensate for fiber spans where the accumulated dispersion D is
negative. Silica fiber can compensate only for wavelengths above
.about.1290 nm, and long lengths of fiber would be necessary due to
the relatively small maximum value of positive dispersion that can
be achieved. Fiber that is presently being installed for use in
DWDM systems at 1550 nm has normal (negative) D below .about.1450
nm, and there are not practical broadband fiber solutions for
dispersion compensation at shorter wavelengths. For example, a
system operating at 1350 nm over an 80 km span of TrueWave.RTM. (a
trademark of Lucent Technologies Inc.) fiber with D =-10.5 ps/nm-km
would require over 40 km of standard fiber to compensate for the
accumulated dispersion, and it can not compensate for dispersion
slope.
[0006] Microstructure optical fibers (MOFs) have recently been
shown to exhibit large values of anomalous dispersion (positive D)
for wavelengths above 700 nm with peak values greater than 100
ps/nm-km for simple structures. See, U.S. Pat. No. 6,097,870 filed
on May 17, 1999 and issued on Aug. 1, 2000 to J. K. Ranka and R. S.
Windeler (hereinafter the Ranka-Windeler patent) and J. K. Ranka et
al., Optics Lett., Vol. 25, No. 1, pp. 25-27 (Jan. 2000), both of
which are incorporated herein by reference. However, the dispersion
of this type of MOF design at wavelengths above 1300 nm has not
been discussed in the literature.
SUMMARY OF THE INVENTION
[0007] In accordance with one aspect of our invention, a fiber
optic system comprises an optical transmitter, an optical receiver,
and an optical fiber transmission path that optically couples the
transmitter and the receiver to one another. The transmission path
includes a first section that has negative dispersion at an
operating wavelength .lambda..sub.0 greater than about 1300 mn and
a second section that includes a MOF. The MOF has relatively large
anomalous dispersion at .parallel.and is sufficiently long to
compensate the accumulated negative dispersion in the first
section. In one embodiment the MOF comprises a core, a lower index
cladding that includes one or more layers of air holes surrounding
the core, characterized in that the diameter of the core is less
than about 8 .mu.m and the difference in effective refractive index
between the core and cladding is greater than about 0.1 (10%).
Preferably, the cladding contains no more than 2 layers of air
holes and the distance between the nearest edges of adjacent air
holes is less than about 1 .mu.m. Although such a fiber would be
multimode, light (i.e., optical radiation) entering the fiber would
be launched into the fundamental mode and would remain guided in
that mode.
BRIEF DESCRIPTION OF THE DRAWING
[0008] Our invention, together with its various features and
advantages, can be readily understood from the following more
detailed description taken in conjunction with the accompanying
drawing, in which:
[0009] FIG. 1 is a schematic, cross-sectional view of a
microstructure optical fiber in accordance with one embodiment of
our invention in which a single layer of air holes forms a closely
packed hexagon;
[0010] FIG. 2 is an expanded view of the core and air hole regions
of the fiber of FIG. 1;
[0011] FIG. 3 compares calculated dispersion profiles of two MOFs
(circles and squares) with measured dispersion profiles of bulk
silica (dashed line) and TrueWave.RTM. fiber (diamonds); and
[0012] FIG. 4 is a block-diagrammatic view of an optical
communication system in accordance with another embodiment of our
invention.
[0013] In the interest of clarity and simplicity, FIGS. 1, 2 and 4
have not been drawn to scale.
DETAILED DESCRIPTION OF THE INVENTION
[0014] This description will be divided into three sections: first,
we describe the general design of microstructure optical fibers
(MOFs); second we describe modifications of the general design that
render MOFs suitable for generating relatively large, anomalous
dispersion (positive D) at wavelengths above about 1300 nm; and
third, we discuss applications of such fibers.
Microstructure Fiber Design
[0015] With reference now to FIG. 1, we show a schematic
cross-section of a MOF 10 of the type described in the
Ranka-Windeler patent, supra. The core 12 is surrounded by an inner
cladding 14 and an outer cladding 16. The core may be doped or
undoped; solid or liquid; birefringent or non-birefringent. It may
take on a variety of shapes; e.g., essentially circular or
elliptical. The effective refractive index of the inner cladding 14
is lower than that of the core in order to provide index-guiding of
radiation propagating down the longitudinal axis of the fiber. On
the other hand, the outer cladding 16 provides strength to the
fiber. The inner cladding 14 includes a multiplicity of relatively
low index cladding features 14.1 that serve to lower the effective
refractive index of the inner cladding and to provide index-guiding
of radiation propagating in the core. Illustratively, these
features constitute capillary air holes that have circular cross
sections and are formed in a higher index matrix of, for example,
glass. Typically the core and outer cladding are also made of
glass, and typically also the glass is silica. The outer cladding,
however, need not include features of the type designed into the
inner cladding.
[0016] At least one relatively thin "layer" of inner cladding
features is positioned circumferentially and wrapped around the
core to form a closely packed polygon The Ranka-Windeler patent
defines thin to mean that the outermost, circumferential boundary
of the features is less than about 10-30 .mu.m from the outermost,
circumferential boundary of the core. Features beyond about 10-30
.mu.m play no significant role in the index guiding of radiation
modes at vis-nir wavelengths, defined in the Ranka-Windeler patent
as visible to near infra-red wavelengths. In the case of a single
layer of features, the distance between the two boundaries is
approximately equal to the size (e.g., diameter) of the features.
FIG. 2 illustrates such a single layer design for the case where
the features are air holes and the pattern formed by their
cross-sections is a hexagon; the core, as well as the interstitial
spaces between the air holes, comprise silica. Illustratively, and
as applied to the embodiments of this invention, the air holes and
the core are essentially circular, the effective core diameter is
about 0.5 to 8 .mu.m (e.g., 2-4 em), the effective core area is
about 0.2 to 5 .mu.m.sup.2, the diameter of the air holes is about
0.5 to 7 .mu.m (e.g., 1.8-3.6 .mu.m), and the center-to-center
spacing d.sub.1 of the air holes is about 0.5 to 7 .mu.m (e.g., 1.6
.mu.m). A common outer diameter of the fiber is about 125 .mu.m
although other sizes are suitable. A is relatively large,
illustratively about >10% to 30%, where .DELTA.=(n.sub.eff,core
- n.sub.eff,clad)/ n.sub.eff,core, expressed as a percent. MOFs of
this type can be single mode or multimode.
Preferred Embodiment
[0017] In order to exhibit relatively large anomalous dispersion
(e.g., >20 ps/nm-km) at wavelengths above about 1300 nm (e.g.,
in the wavelength range of about 1200-1700 nm), a MOF in accordance
with one embodiment of our invention comprises core that has
diameter less than about 8 .mu.m and the effective refractive index
difference between the core and the inner cladding is greater than
about 0.1 (10%). Preferably, the inner cladding contains no more
than two layers of air holes, and the distance d.sub.2 between the
nearest edges of adjacent air holes is less than about 1 .mu.m.
Although such a fiber would be multimode, light (i.e., optical
radiation) entering the fiber would be launched into the
fundamental mode and would remain guided in that mode.
[0018] FIG. 3 shows calculated dispersion profiles of two silica
MOFs with different core diameters that provide relatively large
anomalous dispersion in the wavelength range of about 1.2 - 1.7
.mu.m in accordance with illustrative embodiments or our invention.
One of the MOFs had a core diameter of about 2 .mu.m (circles in
FIG. 3), an air hole diameter of about 1.8 .mu.m, .DELTA. of about
27%, and d.sub.2 of about 0.1 .mu.n, whereas the other MOF had a
core diameter of about 4 .mu.m (squares in FIG. 3), an air hole
diameter of about 3.6 .mu.m, A of about 27%and d.sub.2 of about 0.2
.mu.m. The dispersion profiles of bulk silica (dashed line) and an
illustrative silica TrueWave.RTM. fiber (diamonds) are shown for
comparison. The core of the TrueWave.RTM. fiber had a .DELTA. of
about 0.0053, an alpha of about 5, and a radius of about 3.5 .mu.m.
A down-doped ring (having a .DELTA. of about -0.0017 and a radius
of about 5.4 .mu.m) surrounded the core, and an up-doped ring
(having a .DELTA. of about 0.0012 and a radius of about 7.9 .mu.m)
surrounded the down-doped ring.
[0019] As the data indicate, a MOF having a 4 .mu.m core in
accordance with one embodiment of our invention can exhibit
anomalous dispersion of about 50-100 ps/nm-km in this wavelength
range. Moreover, the amount of such dispersion can be increased
significantly by making the core diameter smaller. Thus, a MOF
having 2 .mu.m core can exhibit anomalous dispersion of about
175-200 ps/nm-km in the same wavelength range.
[0020] In general an MOF in accordance with our invention can
readily be designed to have anomalous dispersion in excess of 20
ps/nm-km, which is approximately the maximum dispersion of silica
glass.
Applications
[0021] As shown in FIG. 4 a communication system 80 includes an
optical fiber that provides a communication link between an optical
transmitter 82 and utilization device 84. The transmitter is
typically a laser-based transmitter. The utilization device may be
a piece of terminal equipment, an optical receiver, a
photodetector, an optical amplifier, etc. The link may include one
or more optical devices 88 well known in the art such as optical
amplifiers, couplers, multiplexers, isolators etc. that couple a
first fiber section 86 to a second fiber section 89. In accordance
with one embodiment of our invention, at least one of the segments
(e.g., segment 86) comprises a single mode optical fiber that has
negative dispersion (e.g., standard optical fiber) at a wavelength
.lambda..sub.0 above about 1300 mn and at least one segment (e.g.,
segment 89) comprises a MOF, in accordance with one embodiment of
our invention, that has anomalous dispersion at .lambda..sub.0, and
the transmitter includes an optical source (e.g., a laser) that
generates a signal that includes at least one component at
.lambda..sub.0. Illustratively, segment 86 comprises a relatively
long span of standard fiber that tends to have a relatively small
negative dispersion, but over the entire length of the span (e.g.,
50 km) the accumulated dispersion in the segment can be very large
(e.g., -100 ps/nm). Yet, segment 89 may be a relatively short span
of MOF that has relatively high anomalous dispersion (e.g., 100
ps/nm-km) and, therefore, is able to compensate the entire
accumulated dispersion in only a 1 km length of MOF. In order to
achieve practical compensation we do not require that the
accumulated negative dispersion be reduced to exactly zero; slight
undercompensation or over-compensation may be tolerable depending
on the system specifications.
[0022] In those applications where the transmitter generates a
multiplicity of signals at different wavelengths above about 1300
nm (e.g., as the channel carrier signals in a DWDM system), the MOF
segment provides relatively high anomalous dispersion at all of the
wavelengths and hence compensation, at least to some extent, for
all of the channels. Of course, in a multi-wavelength system the
compensation cannot be perfect at every wavelength since the
dispersion of a MOF is itself wavelength dependent. However, it is
well known that practical fiber optic systems have an acceptable
range of dispersion error, and the length of the MOF can be chosen
so that dispersion is compensated over as wide a wavelength range
as possible. For example, the length of the MOF could be chosen to
produce essentially perfect compensation at the center wavelength
of a WDM system, in which case wavelengths nearer the edges of the
range will experience imperfect dispersion compensation.
[0023] It is to be understood that the above-described arrangements
are merely illustrative of the many possible specific embodiments
that can be devised to represent application of the principles of
the invention. Numerous and varied other arrangements can be
devised in accordance with these principles by those skilled in the
art without departing from the spirit and scope of the
invention.
* * * * *