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PC759 1
OPTICAL FIBER HAVING LOW NON-LINEARITY FOR WDM TRANSMISSION
BACKGROUND OF THE INVENTION
The present invention relates generally to an optical transmission fiber that
has improved characteristics for minimizing non-linear effects, and
specifically to an
optical fiber for use in a wavelength-division-multiplexing (WDM) system that
has two
refractive index peaks with the maximum index of refraction difference located
in an
outer core region.
In optical communication systems, non-linear optical effects are known to
degrade the quality of transmission along standard transmission optical fiber
in
certain circumstances. These non-linear effects, which include four-wave
mixing
(FWM), self-phase modulation (SPM), cross-phase modulation (XPM), modulation
instability (MI), stimulated Brillouin scattering (SBS) and stimulated Raman
scattering
(SRS), particularly cause distortion in high power systems.
The strength of non-linear effects acting on pulse propagation in optical
fibers is
linked to the product of the non-linearity coefficient yand the power P. The
definition
of the non-linearity coefficient, as given in the paper "Nonlinear pulse
propagation in a
monomode dielectric guide" by Y. Kodama et al., IEEE Journal of Quantum
Electronics, vol. QE-23, No. 5, 1987, is the following:
y_ 1 f n(r)nz(r~F(r)I2rdr (1)
lineff Er JF(r)12 rdr]
where r is the radial coordinate of the fiber, nff is the effective mode
refractive index,
A. is a signal wavelength, n(r) is the refractive index radial distribution,
n2(r) is the non-
linear index coefficient radial distribution, and F(r) is the fundamental mode
radial
distribution.
Applicants have identified that equation (1) takes into account the radial
dependence of the non-linear index coefficient n2 which is due to the varying
concentration of the fiber dopants used to raise (or to lower) the refractive
index with
respect to that of pure silica.
If we neglect the radial dependence of the non-linear index coefficient n2 we
obtain a commonly used expression for the coefficient y.
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2
y _ 2m1Z (2)
Mef/
where we have introduced the so called effective core area, or briefly,
effective area,
_ 2'c~ ,~ IF(r)I2 rdr] 2
~an' L~ IF(r)l 4rdr (3)
The approximation (2), in contrast to the definition (1) does not distinguish
between
refractive index radial profiles that have the same effective core area Ae,
value but
different yvalues. While 1/Aeõ is often used as a measure of the strength of
non-
linear effects in a transmission fiber, yas defined by equation (1) actually
provides a
better measure of the strength of those effects.
Group velocity dispersion also provides a limitation to quality transmission
of
optical signals across long distances. Group velocity dispersion broadens an
optical
pulse during its transmission across long distances, which may lead to
dispersion of
the optical energy outside a time slot assigned for the pulse. Although
dispersion of
an optical pulse can be somewhat avoided by decreasing the spacing between '
regenerators in a transmission system, this approach is costly and does not
allow one
to exploit the advantages of repeateriess optical amplification.
One known way of counteracting dispersion is by adding suitable dispersion
compensating devices, such as gratings or dispersion compensating fibers, to
the
telecommunication system.
Furthermore, to compensate dispersion, one trend in optical communications
is toward the use of soliton pulses, a particular type of RZ (Return-to-Zero)
modulation signal, that maintain their pulse width over longer distances by
balancing
the effects of group velocity dispersion with the non-linear phenomenon of
self-phase
modulation. The basic relation that governs soliton propagation in a single
mode
optical fiber is the following:
z
PoTo = cost D~ (4)
where Pa is the peak power of a soliton pulse, To is the time duration of the
pulse, D is
the total dispersion, A is the center wavelength of the soliton signal, and
yis the
previously introduced fiber non-linearity coefficient. Satisfaction of
equation (4) is
necessary in order for a pulse to be maintained in a soliton condition during
propagation.
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A possible problem that arises in the transmission of solitons in accordance
with equation (4) is that a conventional optical transmission fiber is lossy,
which
causes the peak power Po of the soliton pulse to decrease exponentially along
the
length of the fiber between optical amplifiers. To compensate for this
decrease, one
can set the soliton power Po at its launch point at a value sufficient to
compensate for
the subsequent decrease in power along the transmission line. An alternative
approach, as disclosed for example in F.M. Knox et al., paper WeC.3.2, page
3.101-
104, ECOC '96, Oslo (Norway), is to compensate (with dispersion compensating
fiber,
although fibre Bragg gratings can also be used) for the dispersion accumulated
by the
pulses along the stretches of the transmission line where the pulses' peak
power is
below a soliton propagation condition.
Optical fibers having a low non-linearity coefficient are preferred for use in
transmission systems, such as Non-Return-to-Zero (NRZ) optically amplified WDM
systems, as well as non amplified systems, to avoid or limit the non-linear
effects
mentioned above. Furthermore, fibers with a lower non-linearity coefficient
allow an
increase in the launch power while maintaining non-linear effects at the same
level.
An increased launch power in turn means a better S/N ratio at the receiver
(lower
BER) and/or the possibility to reach longer transmission distances by
increasing the
amplifier spacing. Accordingly, Applicants have addressed a need for optical
fibers
having low values of non-linearity coefficient y.
Also in the case of soliton systems, to increase the spacing between
amplifiers one can increase the launch power for the pulses using more
powerful
amplifiers. In this case, however, equation (4) implies that if the launch
power is
increased and the soliton pulse duration remains constant, the ratio DX2/ymust
accordingly be increased. Therefore, lower values of non-linear coefficient
rare
desirable also to provide an increased distance between line amplifiers in a
soliton
transmission system.
Patents and publications have discussed the design of optical transmission
fibers using a segmented core or double-cladding refractive index profile and
fibers
having a large effective area. For example, U.S. Patent No. 5,579,428
discloses a
single-mode optical fiber designed for use in a WDM soliton telecommunication
system using optical lumped or distributed amplifiers. Over a preselected
wavelength
range, the total dispersion for the disclosed optical fiber lies within a
preselected
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range of positive values high enough to balance self-phase modulation for WDM
soliton propagation. As well, the dispersion slope lies within a preselected
range of
values low enough to prevent collisions between WDM solitons and to reduce
their
temporal and spectral shifts. The proposed fiber of the '428 patent is a
segmented
core with a region of maximum index of refraction in the core of the fiber.
U.S. Patent No. 4,715,679 discloses an optical fiber having a segmented core
of a depressed refractive index for making low dispersion, low loss
waveguides. The
'679 patent discloses a plurality of refractive index profiles including an
idealized
profile having an area of maximum index of refraction at an annular region
outside the
inner core of the fiber but inside an outer core annular region.
U.S. Patent No. 4,877,304 discloses an optical fiber that has a core profile
with a maximum refractive index greater than that of its cladding. U.S. Patent
No.
4,889,404 discloses an asymmetrical bi-directional optical communication
system
including an optical fiber. While the '304 and '404 patents also describe
idealized
refractive index profiles potentially having an outer annular region with an
increased
index of refraction, no specific examples corresponding to those profiles are
disclosed
and the patents are silent as to the non-linear characteristics of optical
fibers having
those profiles.
U.S. Patent No. 5,684,909, EP 789,255, and EP 724,171 disclose single mode
optical fibers having large effective areas made by a segmented refractive
index core
profile. This patent and applications describe computer simulations for
obtaining
fibers with a large effective area for use in long distance, high bit rate
optical systems.
The '909 patent shows a core profile having two non-adjacent profile segments
having a positive index of refraction and two additional non-adjacent segments
having
a negative index of refraction. The '909 patent aims to achieve a fiber with a
substantially zero dispersion slope from the segmented core profile. The
fibers
disclosed in EP 789,255 have extremely large effective areas achieved by a
refractive
index profile with a segmented core but having at least two non-adjacent
segments
with negative refractive difference. EP 724,171 discloses optical fiberswvith
the
maximum index of refraction present at the center of the fiber.
U.S. Patent No. 5,555,340 discloses a dispersion compensating optical fiber
having a segmented core for obtaining dispersion compensation. The '340 patent
discloses a refractive index profile where a resin film surrounding a cladding
has a
higher index of refraction than the inner core of the fiber. This resin,
however, does
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not serve as a low-loss light-conductive layer in the fiber structure.
SUMMARY OF THE INVENTION
Applicants have noticed that the distribution of refractive-index-modifying
5 dopants in the fiber cross-section has a significant impact on the fiber non-
linearity
characteristics. Applicants have determined that the non-linear index n2
contributes to
the non-linearity coefficient ywith a constant term, due to pure silica and
with a
radially varying term, proportional to the concentration of index-modifying
dopants.
Dopants that are added to pure silica glass to increase the refractive index
(e.g.,
Ge02) or to decrease it (e.g., fluorine) both tend to increase the glass non-
linearity
beyond the non-linearity value of pure silica. Applicants have found that
known large-
effective-area fibers, while achieving an overall increase in effective area,
fail to
achieve an optimum decrease in y, due to the effect of dopants in areas of the
fiber
cross-section where the optical field has a relatively high intensity.
Furthermore, Applicants have noticed that refractive-index-modifying-dopants
tend to increase fiber loss, in particular due to an increased scattering
loss. According
to the above, Applicants have afforded the task of developing an optical fiber
with a
low non-linearity coefficient yand a limited loss.
Applicants have developed an optical fiber having a comparatively low dopant
cdncentration where the optical field intensity is relatively high, and a
comparatively
higher dopant concentration where the optical field intensity is relatively
low.
Applicants have found that a low non-linearity coefficient ycan be achieved in
an optical fiber by selecting an index profile for the fiber with a first peak
in the fiber
central cross-sectional area, an outside ring with a second peak value higher
than the
first peak and at least a low-dopant-content region in a cross-sectional
region
between the two peaks. In this fiber the optical field intensity outside the
inner core
region is increased. The presence of a low-dopant-content region in
combination with
a relatively high field intensity achieves a substantial decrease in the non-
linearity
coefficient, together with a limited impact on fiber loss.
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Certain exemplary embodiments may provide an optical
transmission fiber with a low non-linearity coefficient y and high effective
area for use in an optical transmission system, the optical transmission
fiber comprising: a core region comprising: a glass inner core having a first
maximum refractive index difference An1; a first glass layer radially
surrounding the inner core, having a refractive index difference An2 less
than An 1, the first glass layer comprising a low-dopant-content region
having a substantially annular shape and a predetermined and
substantially uniform radius about the core region; a second glass layer
radially surrounding the first layer, having a second maximum refractive
index difference An3 greater than An2; and a low loss cladding
surrounding the core region, wherein an absolute value of Ln2 is less than
or equal to about 15% of an absolute value of An3, and further wherein the
non-linearity coefficient y is less than about 2 V1r' km"', the effective area
is
greater than 45 Nm2, a total dispersion for the fiber in a wavelength range
of 1530 nm to 1565 nm is about 5 to 10 ps/nm/km, and a dispersion slope
for the fiber at a wavelength of 1550 nm is less than or equal to about 0.06
ps/nmZ/km.
Preferably, the low-dopant-content region has a refractive index
difference, in absolute value, equal to or lower than 10% of the second
maximum refractive index difference An3. Preferably, the low-dopant-
content region has a refractive index difference, in absolute value, equal to
or lower than 5% of the second maximum refractive index difference An3.
Preferably, a fiber wavelength cutoff is less than 1480 nm. Preferably, a
macrobending attenuation coefficient for the fiber at a wavelength of 1550
nm is less than 1 dB/km.
Certain other exemplary embodiments may provide an optical
transmission system, comprising: an optical transmitter for outputting an
optical signal; and an optical transmission line for transmitting the signal,
the optical transmission line including an optical transmission fiber
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comprising: a core region having a first refractive index peak, an outside
ring having a second refractive index peak, and a low-dopant-content
region having a third refractive index set at a value between the first and
second refractive index peaks, the low-dopant-content region having a
substantially annular shape and a predetermined and substantially uniform
radius with the low-dopant-content region between said core region and
said outside ring, wherein the low-dopant-content region has a refractive
index difference, in absolute value, equal to or lower than about 15% of the
second refractive index peak, and further wherein the optical transmission
fiber has a non-linearity coefficient y less than about 2 W"' km-', an
effective area greater than 45 Nm2, a total dispersion in a wavelength
range of 1530 nm to 1565 nm of about 5 to 10 ps/nm/km, and a dispersion
slope at a wavelength of 1550 nm of less than or equal to about 0.06
ps/nm2/km.
Preferably, the low-dopant-content region has a refractive index difference,
in absolute value, equal to or lower than 10% of the second refractive index
peak. Preferably, the low-dopant-content region has a refractive index
difference, in absolute value, equal to or lower than 5% of the second
refractive index peak. Preferably, the optical transmission system further
comprises a plurality of optical transmitters for outputting a plurality of
optical signals, each signal having a particular wavelength; and an optical
combiner for combining the optical signals to form a wavelength division
multiplexed optical communication signal and outputting the combined
signal onto said optical transmission line. Preferably, the optical
transmission fiber has a length greater than 50 km. Preferably, the optical
transmission line comprises an optical amplifier.
It is to be understood that both the foregoing general description and the
following detailed description are exemplary and explanatory only and are
not restrictive of the invention, as claimed. The following description, as
well as the practice of the invention, set forth and suggest additional
advantages and purposes of this invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of the
invention, and together with the description, explain the advantages and
principles of the invention.
Fig. 1 is a cross-section of an optical transmission fiber consistent
with the present invention;
Fig. 2 is a graph of the refractive index profile of the cross-section of
the fiber in Fig. 1 consistent with a first embodiment of the present
invention;
Fig. 3 is a graph of computer simulations of dispersion vs. inner-core
radius for the first embodiment of the present invention;
Fig. 4 is a graph of computer simulation of effective area vs. area of
refractive index profile for inner core for the first embodiment of the
present
invention;
Fig. 5 is a graph of computer simulation of non-linearity coefficient y
vs. inner peak area for the first embodiment of the present invention;
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Fig. 6 is a graph of computer simulations of effective area vs. index of
refraction for the second glass layer for the first embodiment of the present
invention;
Fig. 7 is a graph of computer simulation of electric field vs. optical fiber
radius
for the first embodiment of the present invention;
Fig. 8A is a graph of computer simulations of non-linearity coefficient vs.
effective area for the first embodiment of the present invention;
Fig. 8B is a graph of computer simulations of non-linearity coefficient vs.
effective area for conventional dual-shape dispersion-shifted optical fibers;
Fig. 9 is a graph of the refractive index profile of the cross-section of the
fiber
in Fig. 1 consistent with a second embodiment of the present invention;
Fig. 10 is a graph of the refractive index profile of the cross-section of the
fiber
in Fig. 1 consistent with a third embodiment of the present invention;
Fig. 11 is a refractive index profile of the optical fiber in Fig. 1
consistent with a
fourth embodiment of the present invention;
Fig. 12 is a graph of total dispersion vs. wavelength for a fiber according to
the
fourth embodiment of the present invention;
Fig. 13 is a refractive index profile of the fiber in Fig. 1 consistent with a
fifth
embodiment of the present invention;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made to various embodiments according to this
invention, examples of which are shown in the accompanying drawings and will
be
obvious from the description of the invention. In the drawings, the same
reference
numbers represent the same or similar elements in the different drawings
whenever
possible.
Optical fibers consistent with the present invention have a refractive index
profile that includes two areas of peak refractive index difference in a
radial dimension
of the fiber, where the greater of the two peaks is positioned radially
outward from the
first peak. Applicants have discovered that optical fibers having refracttve
index
profiles of this nature can produce optical characteristics in a wavelength
operating
range of 1520 to 1620 nm that includes a relatively low non-linearity
coefficient rand
a relatively high effective area. Due to their characteristics, the invention
fibers can
be advantageously used, in particular, in long length (e.g., greater than 50
km) optical
transmission lines and/or with high power signals (e.g., in optical
transmission lines
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with optical amplifiers). Moreover, Applicants have discovered that optical
fibers
including this refractive index profile can effectively operate as nonzero
dispersion
fibers to minimize the non-linear effect of four-wave mixing in WDM systems,
both for
nonzero positive dispersion and nonzero negative dispersion. Furthermore,
Applicants have determined that optical fibers including this refractive index
profile
can effectively operate as dispersion shifted fibers to minimize the non-
linear effects
in optical transmission systems.
As generally referenced as 10 in Fig. 1, the optical transmission fiber with a
low non-linearity coefficient ycomprises a plurality of light conducting
layers of glass
with varying indices of refraction. As shown in the cross-section of fiber 10
in Fig. 1,
the axial center of the fiber is an inner core 12 that has a first maximum
refractive
index difference On1 and a radius r1. As readily known to those of ordinary
skill in the
art, refractive index difference refers to the difference in refractive index
between a
given layer of glass and the cladding glass. That is, the refractive index
difference
On1 of inner core 12, having a refractive index n1, equals n1 - ncladding.
Glass core
12 preferably is made of Si02 doped with a substance that increases the
refractive
index of pure Si02, such as Ge02. Other dopants increasing the refractive
index are
for example A1203, P205, TiO2i Zr02 and Nb203.
A first glass layer 14 surrounds the inner core 12 and is characterized by an
index of refraction across its width that is less than the indices of
refraction along the
radius r1 of inner core 12. Preferably, and as discussed in more detail below,
first
layer 14 is made of pure SiOZ that has a refractive index difference An2
substantially
equal to 0.
A second glass layer 16 surrounds the first glass layer 14 along the length of
the fiber 10. Second glass layer 16 has a maximum index of refraction An3
within its
width that exceeds the maximum index of refraction of the glass An1 within
inner core
12. Finally, a low loss cladding 18 surrounds the second glass layer 16 in a
conventional manner to help guide light propagating along the axis of fiber
10.
Cladding 18 may comprise pure Si02 glass with a refractive index difference
substantially equal to 0. If cladding 18 includes some refractive-index-
modifying
dopant, the cladding should have an index of refraction across its width that
is less
than the maximum indices of refraction within both inner core 12 and second
layer 16.
Fig. 2 illustrates a refractive index profile across the radius of fiber 10
for a first
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embodiment of the present invention. As generally shown, fiber 10 has two
refractive
index peaks 20 and 22 positioned respectively within inner core 12 and second
layer
16. First layer 14, which is disposed radially between inner core 12 and
second layer
16, provides a refractive index dip relative to its two adjacent layers 12 and
16.
5 Consequently, the combination of inner core 12, first layer 14, and second
layer 16
generally provides an optical fiber profile having a segmented core with an
outer layer
having the highest index of refraction within the cross-section of the fiber.
As shown in Fig. 2, according to a first embodiment of the present invention,
inner core 12 has a radius r1 that is about 3.6 m to 4.2 m, but preferably
is about
10 3.9 m. Between the center of the fiber and the radial position at 3.9 m,
inner core
12 includes a refractive-index-increasing dopant such as Ge02 or the like that
produces a peak index of refraction at or near the axial center of fiber 10
and a
minimum for the inner core at its outer radius. At the peak, the refractive
index
difference for inner core 12 is about 0.0082 to 0.0095, but preferably is
about 0.0085.
The concentration of the refractive-index-increasing dopant decreases from the
center of inner core 12 to the outer radius at about 3.9 m in a manner to
produce a
profile having a curved slope that resembles a substantially parabolic shape.
The
preferred substantially parabolic shape corresponds to a profile a of between
about
1.7 and 2.0, but preferably of about 1.9. In general, the profile of inner
core 12 is a
profile a corresponding to the following:
a
On = On, 1- r )],r E[O,rl] (5)
rl
As is readily known to one of ordinary skill in the art, the profile a
indicates the
amount of roundness or curvature to the profile of the core, where a = 1
corresponds
to a triangular shape for the glass core, and a= 2 corresponds to a parabola.
As the
value of a becomes greater than 2 and approaches 6, the refractive index
profile
becomes more nearly a step index profile. A true step index is described by an
a of
infinity, but an a of about 4 to 6 is a step index profile for practical
purpbses. The
profile a may have an index depression, in the shape of an inverted cone,
along its
centerline, for example if the fiber is produced by the OVD or MCVD methods.
First glass layer 14 has a refractive index difference On2, referenced as 24,
that is less than An1. As shown in Fig. 2, the preferred refractive index
difference An2
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for first glass layer 14 has a constant value of about 0, which corresponds to
a layer
of pure SiOZ glass. However, the refractive index difference An2 of first
glass layer
may differ from zero, due to the presence of refractive index modifying
dopants,
provided that the dopant content of first glass layer 14 is low. It is
envisaged that the
refractive index difference varies across first glass layer. In any case,
refractive-
index-modifying dopants from inner core 12 or from second glass layer 16 may
diffuse into first glass layer 14 during fiber fabrication.
Applicants have determined that, in order to achieve the above described
advantages in combination with a relatively high field intensity in first
glass layer 14,
e.g., in terms of fiber low loss and low non-linearity, a low-dopant content
in first glass
layer 14 corresponds to a dopant content such as to cause a refractive index
difference An2 for first glass layer 14 (in absolute value) of about, or
preferably lower
than, 15% of the fiber peak refractive index difference, i.e. of the
refractive index
difference An3 of second glass layer 16. The skilled in the art can adapt this
value so
that the resulting optical fiber has non-linear and/or loss characteristics
matching the
characteristics of an optical system that he/she intends to make, such as
length of the
optical transmission line, amplifier number and spacing and/or power, number
and
wavelength spacing of the transmission signals.
According to a preferred embodiment, improved fiber characteristics can be
achieved by a dopant concentration in first glass layer 14 such as to cause a
refractive index difference On2 that is lower in absolute value than 10% of
refractive
index difference An3 of second glass layer 16. This low dopant content in
first glass
layer, in combination with a relatively high field intensity in that region,
gives a very
limited contribution to the fiber non-linearity coefficient and loss.
Still more preferred fiber characteristics can be achieved by a refractive
index
difference An2 lower, in absolute value, than 5% of refractive index
difference An3 of
second glass layer 16.
First glass layer 14 has an outer radius r2 which, as shown in Fig. 2, is
between about 9.0 m and 12.0 m, but preferably is 9.2 m. As a result, first
glass
layer 14 has a width extending from about 4.8 m to about 8.4 m for a first
embodiment of the present invention.
Second glass layer 16, like inner core 12, has its refractive index difference
increased by doping the width of the glass layer with Ge02 and/or other well-
known
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dopants. Second glass layer 16 has a substantially parabolic profile across
its radius
that culminates in a maximum refractive index difference On3, depicted as 22
in Fig.
2, that exceeds the maximum refractive index difference An1 of glass core 12
and
refractive index difference On2 of first layer 14. Index profiles other than
parabolic,
e.g., rounded or step like, are also envisaged for second glass layer 16.
Preferably the index of refraction On3 of second glass layer 16 at its peak
exceeds the peak index of refraction An1 for inner core 12 by more than 5%.
The
index of refraction An3 of second glass layer 16 at its peak is about 0.009 to
0.012,
but preferably is about 0.0115. Second glass layer 16 has a width w that
equals
about 0.6 m to 1.0 m, but preferably is about 0.9 m.
Cladding 18 of optical fiber 10 has a refractive index profile 26 that has a
refractive index difference substantially equal to 0. As mentioned, cladding
26
preferably is pure Si02 glass but may include dopants that do not raise its
index of
refraction above that of the maximum indices of refraction 20 and 22 of inner
core 12
and second layer 16.
Applicants have found that optical transmission fiber 10 with the refractive
index profile of Fig. 2 has several desirable optical characteristics for use
in WDM
transmission. Preferably, optical transmission fiber 10 is used in a
transmission
system that operates over a wavelength range of 1530 nm to 1565 nm where the
fiber
provides a total dispersion of 5 to 10 ps/nm/km across that operating
wavelength
range. More particularly, fiber 10 exhibits in the above wavelength range the
following optical characteristics, with the characteristics of the most
preferred
embodiment in parentheses:
Dispersion = 5-10 ps/nm/km (5.65 ps/nm/km @1550 nm)
Dispersion Slope @ 1550 nm <_ 0.06 ps/nm2/km (0.056 ps/nm2/km)
Macrobending Attenuation Coefficient @1550 nm < 1 dB/km
Effective Area > 45 mZ
y< 2 W' km-' (1.4 W' km-' @1550 nm)
ka,tar < 1480 nm (fiber cutoff wavelength according to ITU.T G.650)
These optical characteristics satisfy desired qualities for a transmission
fiber for
WDM systems both of soliton and non-soliton type.
As mentioned, the non-linearity coefficient yprovides an indication of the
susceptibility of a fiber to non-linear effects. With yof less than 2 V1r' km-
', fiber 10
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exhibits favorable response in high power optical transmission systems that
may
otherwise initiate severe problems from self-phase modulation, cross-phase
modulation, and the like. As well, fiber 10 includes a nonzero dispersion
value across
the operating range of 1530 nm to 1565 nm, which helps to deter detrimental
four-
wave mixing. Moreover, the relatively small slope of total dispersion across
the
operating wavelength range enables fiber 10 to provide relatively small
differences of
dispersion between carrier wavelengths in a WDM system.
Figs. 3-6 more fully illustrate the relationships between the physical and
optical characteristics of fiber 10. These figures present results from
computer
simulations for fiber 10 for various physical and optical relationships when
considering
six parameters: radius r1 of inner core 12, maximum index of refraction An1 of
inner
core 12, profile shape a for inner core 12, outer radius r2 of first layer 14,
width w of
second layer 16, and maximum index of refraction An3 of second layer 16. In
the
simulations represented by each of the graphs of Figs. 3-6, these six
parameters
were varied essentially at random substantially across the ranges for the six
parameters outlined above, i.e. r1 of 3.6-4.2 m, On1 of 0.0082-0.0095, a of
1.7-2.0,
r2 of 9.0-12.0 m, w of 0.6-1.0 m, and An3 of 0.009-0.012. Each dot
represents a
different set of the six parameters. The simulation considered only parameter
sets
having On1 <An3. Accordingly, all dots correspond to fibers having an outer
refractive
index peak higher than the inner peak.
As shown in the simulation results of Figs. 3-6, to achieve an optical fiber
having a low non-linearity factor, the area of the refractive index profile
for inner core
12 should be lowered. An outer ring of increased refractive index,
specifically second
glass layer 16, is added to help obtain a high effective area and a low non-
linearity
coefficient for fiber 10. In particular, Applicants have found that the
addition of the
second glass layer of increased index of refraction heightens the electrical
field
distribution in the cross-section of the fiber in regions with low dopant
content,
lowering it at the center of the fiber, so that the non-linearity coefficient
yremains low.
Furthermore, Applicants have found that the addition of the second glass layer
of increased index of refraction has low influence on overall fiber
dispersion, and that
fiber dispersion is essentially determined by the radial dimension r1 of
refractive index
profile of inner core 12.
Fig. 3 illustrates the relationship between the radius r1 and the dispersion
for
CA 02274361 1999-06-14
14
fiber 10. The value of r1 is preferably less than 3a., to achieve a monomodal
behavior at a given wavelength X. For a given range of dispersion, a proper
range of
radial dimension r1 for the refractive index profile may be determined.
For deterring non-linear effects and enabling larger power the effective area
of
fiber 10 should be kept relatively high, preferably in excess of 45 m2. It is
possible to
lower the non-linearity coefficient in two ways: either reducing the area of
the
refractive index profile for the inner core (i.e., the area of the region
between peak 20
and the co-ordinate axes in Fig. 2) (Figs 4-5), or increasing the refractive
index of the
second outer peak (Fig. 6). Figs. 4 and 5 show the former effect for a series
of
computer simulations. For the sake of clarity, the radial dimension r1 in the
simulations was kept constant, and so dispersion is essentially determined, in
these
figures. In order to reduce the area of the refractive index profile for the
inner core, it
is useful to reduce the refractive index difference An1 for a given radial
dimension r1.
An increase in effective area as the refractive index An1 is lowered occurs as
shown
in Fig. 4 because electric field confinement in inner core 12 becomes weaker.
Because a decrease in the area of the refractive index profile for the inner
core leads to an increased effective area for the fiber, the decrease in area
also
provides a lower non-linearity coefficient y, as shown in Fig. 5. Thus, the
fiber 10 with
a lower non-linearity coefficient ycan handle increased power and/or have
decreased non-linear effects.
As well, Applicants have recognized that the addition of a lateral area of
higher index of refraction positioned radially outward from the inner core
will help to
obtain a relatively large effective area and therefore low non-linearity
coefficient y.
The addition of this lateral peak refractive index zone helps to make the
electrical field
distribution larger but does not substantially affect dispersion.
The radial position of second layer 16, its width, and its peak index of
refraction all affect the overall effective area of the fiber. For example,
Fig. 6 shows
results of a computer simulation comparing effective area and the peak index
of
refraction difference for second layer 16, where other fiber parameters are
kept
constant for the sake of clarity. As is evident from Fig. 6, an increasing
index of
refraction difference for the outer ring 16 generates an increasing effective
area for
fiber 10.
Fig. 7 illustrates the spread of electric field within the cross-section of
fiber 10
CA 02274361 1999-06-14
due to the addition of outer ring 16. In Fig. 7, references 20 and 22 denote
an inner
core and an outer ring, respectively, while reference 23 denotes the
electrical field
distribution across the fiber radius. The presence of the outer peak enlarges
the
electric field distribution in the fiber.
5 Applicants have also determined that optical fibers having a maximum
refractive index region in an outer ring of the core as in the profile of Fig.
2 exhibit a
low ABõ = yproduct, i.e., a lower yin comparison with other fibers having the
same
effective area. For example, Fig. 8A illustrates the simulated relationship
between y
and effective area for fibers 10 according to the first embodiment. In
contrast, Fig. 8B
10 illustrates the simulated relationship between yand effective area for
conventional
dual-shape dispersion-shifted optical fibers, which exhibit a less desirable
(i.e., a
higher) ABõ - yproduct.
In short, fiber 10 provides an optical waveguide with a unique refractive
index
profile for transmitting optical WDM signal with nonzero dispersion and a
relatively
15 low non linearity coefficient. These features enable fiber 10 to minimize
signal
degradation due to four-wave mixing and/or to permit the use of higher power.
Fig. 9 illustrates a second embodiment of the present invention for optical
fiber
10 of Fig. 1. In this second embodiment, inner core 12 has a radius r1 that is
about
2.3 m to 3.6 m, but preferably is about 2.77 m. Between the center of the
fiber
and the radial position at 2.77 m, inner core 12 includes one or more
refractive-
index-increasing dopants, such as Ge02 or the like, that produce a peak index
of
refraction at or near the axial center of fiber 10 and a minimum for the inner
core at its
outer radius. At the peak, the index of refraction An1 for inner core 12 in
the second
embodiment is about 0.010 to about 0.012, and preferably is about 0.0113. As
with
the first embodiment, the concentration of the refractive-index-modifying
dopant in the
core 12 decreases from the center to the outer radius at about 2.77 m in a
manner
to produce a profile having a profile a of about 1.4 to about 3.0, but
preferably of
about 2.42. First glass layer 14 in the second embodiment has a substantially
constant refractive index difference An2, referenced as 24, that is about 0,
due to
undoped silica glass. However, as previously explained with reference to the
first
embodiment of Fig. 2, low dopant concentrations can be present in first glass
layer
14. The first layer 14 extends to an outer radius r2 equal to between about
4.4 m
and 6.1 m, but preferably equal to 5.26 m. As a result, first glass layer 14
has a
CA 02274361 1999-06-14
16
width extending from about 0.8 m to about 3.8 m, but preferably of about
2.49 m,
for a second embodiment of the present invention.
As with the first embodiment, the second embodiment includes a second glass
layer 16, like inner core 12, with its refractive index difference increased
by doping the
width of the glass layer with Ge02 and/or other well-known refractive-index-
increasing
dopants. Second glass layer 16 has a substantially parabolic profile across
its radius
that culminates in a maximum refractive index difference An3, depicted as 22
in Fig.
9. Index profiles other than parabolic, e.g., rounded or step like, are also
envisaged
for second glass layer 16.
Preferably the index of refraction On3 of second glass layer 16 at its peak
exceeds the peak index of refraction On1 for inner core 12 by more than 5%.
The
index of refraction An3 of second glass layer 16 at its peak is about 0.012 to
0.014,
but preferably is about 0.0122.
Second glass layer 16 has a width w that equals about 1.00 m to about 1.26
m, but preferably is about 1.24 m.
Preferably, optical transmission fiber 10 is used in a transmission system
that
operates over a wavelength range of 1530 nm to 1565 nm where the fiber
provides
positive nonzero dispersion characteristics. Nonzero dispersion fibers are
described
in ITU-T Recommendation G.655.
Fiber 10 constructed according to the second embodiment of Fig. 9 exhibits
the following preferred optical characteristics (the values are given for a
value of 1550
nm, unless otherwise indicated):
Chromatic Dispersion @ 1530 nm _ 0.5 ps/nm/km
0.07 ps/nm2/km <_ Dispersion Slope < 0.11 ps/nm2/km
45 m2 <_ Ae1 _< 100 m2
1 V1r' km-' <_ y<_ 2 W' km-'
Macrobending Attenuation Coefficient <_ 0.01 dB/km (fiber loosely wound-in
100 turns with a bend radius of 30 mm)
Microbending Sensitivity < 10 (dB/km)/(g/mm)
7,,tff < 1600 nm (fiber cutoff wavelength according to ITU.T G.650)
The second embodiment for fiber 10 having the above-listed optical
characteristics provides acceptable conditions for the transmission of both
solitons
and non-soliton WDM systems.
CA 02274361 1999-06-14
17
Fig. 10 depicts a refractive index profile of a third embodiment of the
present
invention for optical fiber 10, whose cross-section is shown in Fig. 1. The
third
embodiment, like the first and second embodiments, includes an inner core with
a
heightened refractive index difference Anl and a profile shape a, together
with a first
layer of glass having a lower refractive index difference An2 and a second
layer of
glass having the maximum index of refraction difference On3 in the cross-
section of
the fiber. The following sets forth the preferred physical parameters for
fiber 10
according to the third embodiment of the present invention as illustrated in
Fig. 10.
Inner Core Radius r1 = 2.387 m
Inner Core Refractive Index Difference On1 = 0.0120
First Layer Radius r2 = 5.355 m
First Layer Refractive Index Difference An2 = 0.0
Second Layer Width w = 1.129 m
Second Layer Refractive Index Difference An3 = 0.0129.
Of course, variations from these optimal structural values do not alter their
general
inventive features. Fiber 10 according to the third embodiment of the present
invention advantageously obtains the following optimal optical characteristics
(at a
wavelength of 1550 nm):
Dispersion = 3.4 ps/nm/km
Dispersion Slope = 0.11 ps/nmZ/km
Mode Field Diameter = 9.95 m
Effective Area = 90 m2
y= 1.00 V1r' km-'.
The third embodiment for fiber 10 having the above-listed optical
characteristics
provides acceptable conditions for the transmission in both solitons and non-
soliton
WDM systems.
Fig. 11 illustrates a fourth refractive index profile for optical fiber 10
that
generates optical characteristics of nonzero positive dispersion. The physical
characteristics of the inventive fiber of Fig. 11 include a radius r1 for
inner core 12 of
about 3.2 m, an index of refraction profile a for inner core 12 of about 2.9,
a
maximum refractive index difference on1 at reference 20 for inner core 12 of
about
0.0088, an outer radius of first glass layer 14 of about 7.2 m with an index
of
refraction An2 at reference 24 of about 0, a width of second glass layer 16 of
about
CA 02274361 1999-06-14
18
0.8 m, and a maximum index of refraction On3 at reference 22 of the second
glass
layer 16 of about 0.0119. As with the refractive index profile of Fig. 2, the
profile of
Fig. 11 for the nonzero positive dispersion fiber has the characteristic
multiple peaks
high refractive index, where the outer peak is present in the second glass
layer 16,
has a substantially parabolic shape, and at its maximum 22 exceeds the maximum
index of refraction 20 within inner core 12.
Fiber 10 having the refractive index profile of Fig. 11 provides positive
total
fiber dispersion across the operating wavelength band of 1530 nm to 1565 nm.
Such
a performance has desirable application in optical systems that have a
relatively high
optical power and would otherwise generate deleterious four-wave mixing
products.
Fig. 12 depicts the simulated total dispersion vs. wavelength for optical
fiber 10
having the refractive index profile of Fig. 11. As shown in this figure, the
refractive
index profile of Fig. 11 produces dispersion across the wavelength band of
about
1530 nm to 1565 nm that stretches between about 0.76 ps/km/nm and 3.28
ps/km/nm. Specifically, the fiber with the refractive index profile shown in
Fig. 11
provides the following optical characteristics at 1550 nm:
Dispersion = 2.18 ps/nm/km
Dispersion Slope = 0.072 ps/nm2/km
Macrobending Attenuation Coefficient = 0.01 dB/km
Mode Field Diameter = 9.0 m
Effective Area = 62 m2
y= 1.8V1r1 km-'.
All of these characteristics fall within the ranges stated by ITU-T G.655
Recommendation for nonzero dispersion fibers.
Fig. 13 illustrates a fifth refractive index profile for optical fiber 10 that
generates optical characteristics of nonzero negative dispersion with
relatively low
non-linearity coefficient. The physical characteristics of the inventive fiber
of Fig. 13
include a radius r1 for inner core 12 of about 2.4 m to 3.2 m and preferably
of.
about 2.6 m, an index of refraction profile a for inner core 12 of about 1.8
to 3.0 and
preferably of about 2.48, a maximum refractive index difference An1 at
reference 20
for inner core 12 of about 0.0106-0.0120 and preferably of about 0.0116, an
outer
radius of first glass layer 14 of about 5.3 m to 6.3 m and preferably of
about 5.9 m
with an index of refraction An2 at reference 24 preferably of about 0, a width
of
CA 02274361 1999-06-14
19
second glass layer 16 of about 1.00 m to 1.08 m and preferably of about 1.08
m,
and a maximum index of refraction On3 at reference 22 of the second glass
layer 16
of about 0.0120 to 0.0132 and preferably of about 0.0129. As previously
explained,
low dopant concentrations can be present in first glass layer 14. As with the
refractive
index profile of Figs. 2, 9, 10, and 11, the profile of Fig. 13 for the
nonzero negative
dispersion fiber has the characteristic multiple peaks of high refractive
index, where
the outer peak is present in the second glass layer 16, has a substantially
parabolic
shape, and at its maximum 22 exceeds the maximum index of refraction 20 within
inner core 12. Index profiles other than parabolic, e.g., rounded or step
like, are also
envisaged for second glass layer 16. Preferably the index of refraction An3 of
second
glass layer 16 at its peak exceeds the peak index of refraction On1 for inner
core 12
by more than 5%.
Fiber 10 having the refractive index profile of Fig. 13 provides negative
total
fiber dispersion across the operating wavelength band of 1530 nm to 1565 nm.
Such
a performance has desirable application in optical systems used in underwater
transmission systems that have a relatively high optical power and would
otherwise
generate deleterious four-wave mixing products. Specifically, the fiber with
the
refractive index profile shown in Fig. 13 provides the following optical
characteristics
at 1550 nm, with the characteristics of the most preferred embodiment in
parentheses:
Dispersion _ - 0.5 ps/nm/km (-2.46 ps/nm/km)
0.07 ps/nmZ/km <_ Dispersion Slope < 0.12 ps/nm2/km (0.11 ps/nm2/km)
Macrobending Attenuation Coefficient _ 0.01 dB/km (0.0004 dB/km)
Mode Field Diameter = 9.1 m
45 m2 < Effective Area <_ 75 mZ (68 m2)
1.2 V1t' km-' <_ y< 2 W' km-' (1.3 V1r*' km-' )
1600 nm (fiber cutoff wavelength according to ITU.T G.650)
A sixth refractive index profile for optical fiber 10 will now be described
that
generates optical characteristics of shifted dispersion with relatively low
non-linearity
coefficient. Dispersion shifted fibers are described in ITU-T Recommendation
G.653.
The physical characteristics of the fiber according to the sixth embodiment
include a
radius rl for inner core 12 of about 3.2 m, an index of refraction profile a
for inner
core 12 of about 2.8, a maximum refractive index difference en1 at reference
20 for
CA 02274361 1999-06-14
inner core 12 of about 0.0092, an outer radius of first glass layer 14 of
about 7.8 m
with an index of refraction On2 of about 0, a width of second glass layer 16
of about
0.8 m, and a maximum index of refraction An3 of the second glass layer 16 of
about
0.0118. As with the refractive index profile of Figs. 2, 9, 10, 11 and 13 the
profile
5 according to the sixth embodiment for the dispersion shifted fiber has the
characteristic multiple peaks of high refractive index, where the outer peak
is present
in the second glass layer 16, has a substantially parabolic shape, and at its
maximum
22 exceeds the maximum index of refraction 20 within inner core 12. Index
profiles
other than parabolic, e.g., rounded or step like, are also envisaged for
second glass
10 layer 16. Preferably the index of refraction An3 of second glass layer 16
at its peak
exceeds the peak index of refraction On1 for inner core 12 by more than 5%.
Fiber 10 having the refractive index profile of Fig. 13 provides low absolute
value total fiber dispersion across the operating wavelength band of 1530 nm
to 1565
nm.
15 Specifically, the fiber provides the following optical characteristics,
given at
1550 nm unless otherwise indicated:
Dispersion = 0.42 ps/nm/km
Dispersion Slope = 0.066 ps/nm2/km
Dispersion @ 1525 nm = -1.07 ps/nm/km
20 Dispersion @ 1575 nm = +2.22 ps/nm/km
Macrobending Attenuation Coefficient = 0.6 dB/km
Mode Field Diameter = 8.8 m
Effective Area = 58 m2
y = 1.56 W' km-'
X,,ff = 1359 nm (fiber cutoff wavelength according to ITU.T G.650)
It will be apparent to those skilled in the art that various modifications and
variations can be made to the system and method of the present invention
without
departing from the spirit or scope of the invention. For example, the
refractive index
profiles depicted in the figures are intended to be exemplary of preferred
embodiments. The precise shape, radial distance, and refractive index
differences
may readily be fluctuated by one of ordinary skill in the art to obtain the
equivalent
fibers as disclosed herein without departing from the spirit or scope of this
invention.
Although fiber operation in a wavelength range of between 1530 nm and 1565 nm
CA 02274361 1999-06-14
21
has been disclosed for the given embodiments, signals in different wavelength
ranges
can be transmitted in a fiber according to the invention, if specific
wavelength
requirements arise in present or future optical communication systems. In
particular
the skilled in the art may envisage use of the described fibers, or of
straightforward
modifications thereof, to operate in an extended wavelength range of between
about
1520 nm and about 1620 nm, where silica keeps low attenuation properties.
The present invention covers the modifications and variations of this
invention
provided they come within the scope of the appended claims and their
equivalents.
