Note: Descriptions are shown in the official language in which they were submitted.
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SINGLE MODE OPTICAL WAVEGUIDE
Backctround of the Invention
The invention is directed to a single mode optical waveguide fiber
designed for long repeater spacing, high data rate telecommunication systems.
In particular, the single mode waveguide combines excellent bend resistance,
low dispersion slope, and large effective area, AeK.
A waveguide having large effective area reduces non-linear optical
effects, including self phase modulation, four wave mixing, cross phase
modulation, and non-linear scattering processes, all of which can cause
degradation of signals in high power systems. In general, a mathematical
description of these non-linear effects includes the ratio, PIA$n, where P is
optical power. For example, a non-linear optical effect usually follows an
equation containing a term, exp [P x L~ff/AeK], where Lerr is effective
length.
Thus, an increase in Aerr produces a decrease in the non-linear contribution
to
the degradation of a light signal.
The requirement in the telecommunication industry for greater
information capacity over long distances, without electronic signal
regeneration,
has led to a reevaluation of single mode fiber index profile design. The
genera
of these profile designs, which are called segmented core designs in this
application, are disclosed in detail in U. S. patent 4,715,679, Bhagavatula.
The focus of this reevaluation has been to provide optical waveguides
which:
- reduce non-linear effects such as those noted above;
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- are optimized for the lower attenuation operating wavelength range
around 1550 nm;
- are compatible with optical amplifers; and,
- retain the desirable properties of optical waveguides such as high
strength, fatigue resistance, and bend resistance.
The definition of high power and long distance is meaningfut only in the
context of a particular telecommunication system wherein a bit rate, a bit
error
rate, a multiplexing scheme, and perhaps optical amplifiers are specified.
There are additional factors, known to those skilled in the art, which have
impact upon the meaning of high power and long distance. However, for most
purposes, high power is an optical power greater than about 10 mw. In some
applications, signal power levels of 1 mW or less are still sensitive to non-
linear effects, so that AeK is still an important consideration in some lower
power
systems. A long distance is one in which the distance between electronic
regenerators can be in excess of 100 km. The regenerators are to be
distinguished from repeaters which make use of optical amplifiers. Repeater
spacing, especially in high data density systems, can be less than half the
regenerator spacing.
To provide a suitable waveguide for multiplexed transmission, the total
dispersion should be low, but not zero, and have a low slope over the window
of operating wavelength.
A typical application for such a waveguide fiber is undersea systems
that, in order to be economically feasible, must carry high information
densities
over long distances without regenerators and over an extended window of
wavelengths. The present invention describes a novel profile which is
singularly suited to meeting the stringent requirements of this kind of use.
The
detailed requirements of the use system are set forth below.
Definitions
The following definitions are in accord with common usage in the art.
- The radii of the segments of the core are defined in terms of the index of
refraction. A particular segment has a first and a last refractive index
point.
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The radius from the waveguide centerline to the location of this first
refractive
index point is the inner radius of the core region or segment. Likewise, the
radius from the waveguide centerline to the location of the last refractive
index
point is the outer radius of the core segment.
The segment radius may be conveniently defined in a number of ways,
as will be seen in the description of Figs. 1 & 2 below. In the case of Fig.
2,
from which Tables 1 & 2 are derived, the radii of the index profile segments
are
defined as follows, where the reference is to a chart of D % vs. waveguide
radius:
~ * the radius of the central core segment, r~, is measured from the axial
centerline of the waveguide to the intersection of the extrapolated central
index
.profile with the x axis, i.e., the D °r6 = 0 point;
* the outer radius, r2, of the first annular segment is measured from the
axial centerline of the waveguide to the intersection of the first annular
segment
profile with the line representing the ~ % of the second annular segment
profile;
*the outer radius, r3, of the second annular segment is measured from
the axial centerline of the waveguide to the point at which the relative index
is
midway between the relative indexes of the second and third annular
segments; and,
*the outer radius, r4, of the third annular segment is measured from the
axial centerline of the waveguide to the point at which the relative index is
midway between the relative indexes of the third annular segment and the clad
layer.
In the more general refractive index profile of Fig. 1, alternative
definitions are used. No particular significance is attached to the definition
of
index profle geometry. Of course, in carrying out a model calculation the
definitions must be used consistently as is done herein.
- The effective area is
Aerr = 2rr (jE2 r dr)21(jE4 r dr), where the integration limits
are 0 to ~, and E is the electric field associated with the propagated light.
An
effective diameter, Derv, may be defined as,
AeK = n(Dett~2)2 .
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- The relative index, a%, is defined by the equation,
D% = 100 x (n~2- n22)/2n~2, where n~ is the maximum refractive index of
the index profile segment 1, and n2 is a reference refractive index which is
taken to be, in this application, the refractive index of silica.
- The term refractive index profile or simply index profile is the relation
between
0 % or refractive index and radius over a selected portion of the core. The
term
alpha profile refers to a refractive index profile which follows the equation,
n(r) = no (1- D[rlaj°) where r is core radius, 0 is defined above, a is
the
last point in the profile, r is chosen to be zero at the first point of the
profile, and
a is an exponent which defines the profile shape. Other index profiles include
a
step index, a trapezoidal index and a rounded step index, in which the
rounding
is typically due to dopant diffusion in regions of rapid refractive index
change.
- Total dispersion is defined as the algebraic sum of waveguide dispersion and
material dispersion. Total dispersion is sometimes called chromatic dispersion
in the art. The units of total dispersion are ps/nm-km.
- The bend resistance of a waveguide fiber is expressed as induced attenuation
under prescribed test conditions. Standard test conditions include 100 turns
of
waveguide fiber around a 75 mm diameter mandrel and 1 turn of waveguide
fiber around a 32 mm diameter mandrel. In each test condition the bend
induced attenuation, usually in units of dB/(unit length), is measured. In the
present application, the bend test used is one turn of the waveguide fiber
around a 20 mm diameter mandrel, a more demanding test which is required
for the more severe operating environment of the present waveguide fiber.
Summary of the invention
The novel single mode waveguide fiber of this application meets the high
performance telecommunication system requirements set forth herein.
A first aspect of the invention is a single mode optical waveguide fiber
having a segmented core surrounded by a cladding glass layer. The core has
at least four segments, at least one of which has a negative relative index, -
4
%. The segmented core is defined in terms of the relative index percents, the
refractive index profiles and the radii of the segments. The radii are
measured
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from the centerline of the waveguide fiber and extend to a point of the
segment
defined in terms of the relative refractive indexes as stated in the
"Definitions"
section and as shown in Figs. 1 8~ 2. Throughout this application, the core
extent, i.e., the outer radius of the core, is defined in terms of the segment
5 geometry. The largest part of the light energy is carried in the core, but
it is
understood that the portion of the cladding layer adjacent the core does carry
a
significant amount of light. The portion of the cladding layer adjacent the
core
in the novel waveguide preferably contains a refractive index increasing
dopant.
In one embodiment of the invention, the central segment is made to
have a negative relative index, -0~ °~.
In another embodiment of the invention, the core region has four
segments, all having positive relative indexes except for the central segment
which has a negative relative index. For this case, the 0 %'s follow the
inequality, 02 % > ~4 % > ~3 % > ~~ %, in which the numbering of segments is
consecutive and begins with 1 at the central segment. In this embodiment the
refractive index profiles of the first and third annular segments may be an a-
profile, a step index, a trapezoid, or a rounded step or trapezoid. The second
annular region may have the form of a step index profile, a term used to
identify
an index segment consisting of a constant horizontal portion. In addition, the
portion of the cladding layer which contains an index increasing dopant, thus
providing a cladding layer portion having a refractive index greater than that
of
silica, may have a step index profile.
Particular ranges of values for D, °r6, e2 %, ~ %, ~ %, the
relative
indexes, and radii, r~, r2, r3, r4, of a core region having four segments,
which
provide the set of target properties of the novel waveguide are set forth in
the
tables below. The appropriate range of the relative index, 05 %, of the
preferred doped cladding layer portion is also given in the tables. A radius
for
the doped cladding layer portion is not required. In effect the doped portion
of
the clad layer extends to a radius whereat the light intensity carried in the
waveguide is negligible. This radius value is typically determined by testing
methods known in the art, such as measurement near field intensity.
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This aspect of the invention, including its embodiments of profile
segment shape and size, is capable of providing a single mode optical
waveguide having an effective area > 70 ~m2 and a total dispersion slope <
0.08 pslnm2-km over a pre-selected range operating of operating wavelengths.
As is noted above, the window about 1550 nm to 1560 nm is at present
preferred because of the low attenuation in this range and its correspondence
with the gain curve of erbium doped optical amplifiers. The minimum effective
area can be increased and the total dispersion slope can be decreased
substantially by tuning the radii, D %'s, and shape of one or more profile
segment. The effect of such tuning is seen by comparing the data in Table 1 to
that in Table 2 below. The ranges of Table 2 provide a waveguide fiber having
Aeff > 80 p,m2 and a total dispersion slope < 0.07 ps/nm2-km.
A second aspect of the invention is a waveguide fiber having at least
four segments. A portion of the cladding layer adjacent the core contains an
index increasing dopant. The 0 's, radii and profile shapes are chosen to
provide the waveguide fiber properties listed in Table 3.
Brief Description of the Drawin4s
Fig. 1 is a chart of O % vs. radius illustrating a refractive index profile in
accord
with the invention and the definitions of 0, and r;.
Fig. 2 is a chart showing an alternative embodiment of the refractive index
profile.
Detailed Description of the Invention
The invention described herein is a family of single mode optical
waveguide fibers defined by the parameters of a family of refractive index
profiles. The refractive index profiles include at least four core segments,
one of
which has a negative relative index percent, -0; %, and a cladding layer which
preferably contains a refractive index increasing dopant at least in the
cladding
portion adjacent the core region.
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The refractive index profile of the novel waveguide may be described in
terms of the d %'s and radii shown in Fig. 1. Thus relative index values,
indicated as 2, 4, 6, 8, 10 and 12, in Fig. 1 are the respective relative
index
values of the central segment, the first, second, third, and nth annular
segment
of the core. Relative index 14 is that of the cladding layer portion, adjacent
the
outermost segment of the core, which contains a refractive index increasing
dopant. The respective radii, r;, i=1, 2, 3, ... , n are shown as 16, 18, 20,
and 22.
Radius 16 is measured from the waveguide fiber centerline to the point of
intersection of the central segment with the first annular segment. Radius 18
is
measured from the centerline to the point of zero relative index, i.e., the
intersection of the second annular segment profile with the x-axis.
Dashed lines 24, 26, 28, and 30 show alternative shapes of the index
profile of the respective segments. What these dashed lines represent are
alternative members of the family of profiles which provide the pre-selected
set
of waveguide properties set forth in Table 3. These alternatives are regarded
as perturbations of the base profile not large enough to appreciably change
the
energy distribution in the waveguide fiber of the light. carried therethrough.
The embodiment of the novel profile illustrated in Fig. 2 is used to
calculate the refractive index profile geometry set forth in Tables 1 and 2. A
waveguide fiber having a profile as set forth in Tables 1 or 2 can have the
waveguide fber the corresponding performance requirements stated in Table
3. The definitions of r~, r2, r3, and r4 illustrated in Fig. 2 follow exactly
those
given in the °Definitions" section above. The relative index percents,
0~, ~2, A3,
Aa, and t1s are shown in Fig. 2 as 32, 34, 36, 38, and 40, respectively. It
will be
understood that small variations of this profile will not appreciably change
the
waveguide properties. For example, the horizontal profiles of segments 32, 36,
or 40 could be slightly concave or convex, or contain a small dip or rise in
relative index without having an effect on the calculated waveguide
properties.
However, a comparison of the two tables shows that sub-micron
changes in certain of the radii, for example the lower limit of radius r~, can
markedly affect the total dispersion slope. Other small changes in certain of
the profile variables can impact the waveguide performance.
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Table 1
Slope< ~~ 92 ~ % ~ % as r~ r2 r3 R4 ~,m
% ~ % ~,m ~m ~,m
i
Ae~r>70
Low -0.32 1.24 -0.02 0.40 0.09 1.69 3.72 8.32 9.30
'
Limit
High -0.24 1.39 0.03 0.52 0.11 1.82 3.87 8.63 9.65
t
Limit
In Table.l, the refractive index profile segments are constrained by the
requirement that the total dispersion slope be less than or equal to 0.08
ps/nm2-km and the effective area be greater than 70 ~m2, over a wavelength
range centered about 1550 nm. The effective wavelength range is set by the
limit on the total dispersion slope and the total dispersion value at 1555 nm,
which in the embodiments of Tables 1 and 2 is taken to be less than about -3
pslnm-km.
Table 2
Slope< J ~ I
0.07 e~ D2 ~ % ~ % As r~ r2 ( r3 ra
% % % um ~m ~.m ~.m
>80
Low
-0.32 1.26 -0.02 0.41 0.09 1.76 3.72 8.32 9.30
Limit
High
-0.25 1.33 0.01 0.52 0.11 1.82 3.82 8.60 9.65
Limit
To improve the total dispersion slope to a value less than or equal to
0.07 ps/nm2-km and the effective area to a value greater than or equal to 80
~m2, as in Table 2, a comparison of tabulated values show that the overall
core
radius r4, and the cladding layer relative index may be held constant, while
incremental changes are made in the other profile variables. The values of ~2
%, Os %, and r, would seem to be more important than the other variables in
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reaching target Aan and total dispersion slope. However, the variables
interact
to provide a profile which satisfies all waveguide performance requirements
set
forth in Table 3. The overall profile geometry must be considered in each
case.
Table 3
AeK Disp. Att. Disp.1560 7~.~ Macro-bend
i 550
(~m2) Slope (dBlkm) (pslnm- (nm) (dB/m)
(ps/nm2 km)
-km)
Table >80 <0.07 <0.25 -2.0 <1500 <10
1 ~
fi ber
Table >70 <0.08 <0.25 -2.0 <1500 <10
2 ~
fiber
For example the low limit of 0~, 02, and A3 in Table 1 are set by the
requirement that the total dispersion be less negative than -3 pslnm-km in the
operating window about 1555 nm. The edges of the profile family envelope are
found by changing a variable or set of variables until the model predicts a
performance parameter that is out of specification.
Although particular embodiments of the invention have been herein
disclosed and described, the invention is nonetheless limited only by the
following claims.
