Note: Descriptions are shown in the official language in which they were submitted.
~ CA 02221737 1997-11-20
Dispersion Compensating Single Mode Waveguide
Backqround of the Invention
The invention is directed to a single mode optical waveguide fiber
having controlled negative total dispersion and a relatively large effective area.
In particular, the single mode waveguide has a total dispersion which is less
than -100 ps/nm-km.
Several factors have combined to make the wavelength rangel 1500 nm
to 1600 nm, most preferred for telecommunication systems incorporating
optical waveguide fiber. These are:
the availability of reliable lasers in the wavelength window around 1550
nm;
the invention of the optical fiber amplifier having an optimum gain curve
in the wavelength range 1530 nm to 1570 nm;
the availability of systems capable of wavelength division multiplexing of
signals in this wavelength range; and,
the availability of waveguide fibers having a low dispersion to
compliment the very low attenuation over this wavelength range.
These advances in technology make possible very high information
rate, multi-channel telecommunication systems which have a large spacing
between stations where the signal is electronically regenerated.
However, many telecommunication systems installations pre-date the
technological advances which make 1550 nm the most preferred operating
' CA 02221737 1997-11-20 f
window. These earlier systems were designed primarily for use over a
wavelength range centered near 1310 nm. The design includes lasers which
operate at wavelengths near 1310 nm and optical waveguides which have a
zero dispersion wavelength near 1310 nm. The waveguide fiber, in these
5 systems, does have a local attenuation minimum near 1310 nm, but the
theoretical minimum at 1550 nm is about half that at 1310 nm.
A strategy has been developed to make these older systems compatible
with the new laser, amplifier, and multiplex technology. As disclosed in U. S.
patent 5,361,319, Antos et al., ('319, Antos) and discussed further in the
10 references noted therein, an essential feature of this strategy is to overcome
the relatively high total dispersion by inserting into each waveguide fiber link a
length of waveguide fiber which compensates for the total dispersion of the linkat 1550 nm. The term "link" used herein is defined as the length of waveguide
fiber which spans the distance between a signal source, i.e., a transmitter or
15 an electronic signal regenerator, and a receiver or another electronic signal regenerator.
The '319, Antos patent recites a dispersion compensating waveguide
fiber having a core refractive index profile which provides a dispersion at 1550nm of about -20 ps/nm-km. The dispersion sign convention common in the art
20 is that a waveguide dispersion is said to be positive if shorter wavelength light
has a higher speed in the waveguide. Because the dispersion at about 1550
nm of a waveguide fiber, having a zero dispersion wavelength near 1310 nm,
is about 15 ps/nm-km, the length of dispersion compensating waveguide fiber
required to fully compensate for total dispersion at 1550 nm is 0.75 of the
25 original link length. Thus, for example, a 50 km link of waveguide fiber has a
total dispersion at 1550 nm of 15 ps/nm-km x 50 km = 750 ps/nm. To
effectively cancel this dispersion, a length of dispersion compensating
waveguide fiber of 750 ps/nm - 20 ps/nm-km = 32.5 km is required.
The additional attenuation introduced into the link by the dispersion
30 compensating waveguide would have to be offset by means of an optical
amplifier. The introduction of additional electronic regenerators into the link
- CA 02221737 1997-11-20 f
would not be cost effective. Further, the cost of the dispersion compensating
waveguide fiber is a significant fraction of the total waveguide fiber cost. Thelong lengths of dispersion compensating waveguide required must be formed
into an environmentally stable package which can take up considerable space.
Because the compensating waveguide fiber design usually has more
refractive index modifying dopant in the core region, the attenuation is, in
general, higher relative to the standard waveguide fiber in a link.
The higher signal power level, made possible by improved lasers and
by optical amplifiers, as well as wavelength division multiplexing, increases the
possibility that link length or data transmission rate may be limited by non-
linear optical effects. The impact of these non-linear effects can be
limited by increasing the effective area (Aeff) of the fiber. The effective area is
Aeff = 2n (~E2 r dr)2/(JE4 r dr), where the integration limits are 0 to CO, and E is
the electric field associated with the propagated light. The distortion due to
non-linear effects depends upon a term of the form, Pxn2/Aeff, where P is the
signal power, and, n2 is the non-linear index constant. Thus, in the design of adispersion compensating waveguide fiber, care must be taken to insure that
Aeff of the compensating fiber is large enough so that the compensation fiber
does not cause significant non-linear effects in the link. If Aeff of the
compensating fiber is smaller than that of the original fiber in the link, the
compensating fiber may be placed at a link location where signal power is
lower and thus non-linear effects minimum. Also, in many links the smaller Aeff
compensating fiber is a small fraction of the overall link length and so does not
contribute significantly to non-linear distortion of the signal.
Thus, there is a need for a dispersion compensating optical waveguide
fiber:
having a length which is a small fraction, e.g., less than 15 %, of the link
length;
which is sufficiently low in attenuation to eliminate the need for
additional signal amplification solely to offset the compensating waveguide
fiber attenuation; and,
CA 02221737 1997-11-20
which has an effective area sufficiently large to preclude non-linear
dispersive effects in the compensating waveguide fiber from being a limiting
factor.
Definition
- The effective area is
Aeff = 2n (JE2 r dr)2/(JE4 r dr), where the integration limits
are 0 to ~, and E is the electric field associated with the propagated light.
- The non-linear discriminator factor is defined by the equation
Gnl =n2/Aeff (exp[D, x L1/Dd/a] -1 )/a, where n2 is the non-linear refraction
coefficient, D1 is the dispersion of the portion of the waveguide optimized for
operation around 1310 nm, L1 is the length corresponding to D1, Dd is the
dispersion of the compensating waveguide fiber and a is the attenuation of the
dispersion compensating fiber. This expression for Gnl derives from a base
definition Gnl ~ n2/Aeff (Effective length x Output Power). The effective lengthand output power are expressed in terms of waveguide fiber length and
attenuation, a. The compensating waveguide fiber is introduced into the
equation via the requirement D1 x L1 = Dd x Ld. Gnl is a useful quantity in
evaluating the efficiency of a link because it is a combination of system factors
such as system architecture, amplifier spacing, Dd/a, and, n2/Aeff.
Summary of the Invention
The invention disclosed herein meets the requirements for an improved
dispersion compensating waveguide fiber. A species of the genus of
segmented core refractive index profiles, introduced in U. S. patent 4,715,679,
Bhagavatula and in U.S. patent application S. N. 08/378,780, Liu, has been
discovered which are uniquely suited for dispersion compensating waveguide
fiber.
A first aspect of the invention is a single mode optical waveguide fiber
having a central core glass region and a surrounding layer of clad glass. The
core glass region has at least three segments, each of which is characterized
CA 02221737 1997-11-20
by a refractive index profile, a radius, r, and a /\ %. The definition of the %
index delta is
% ~ = [(n12 - nC2)/2n,2] x 100, where n, is a core index and nc is the clad index.
Unless otherwise stated, n1 is the maximum refractive index in the core region
characterized by a % ~. The radius of each segment is measured from the
centerline of the waveguide fiber to the point of the segment farthest from the
centerline. The refractive index profile of a segment gives the refractive indexvalue at the radial points of that segment. In this first aspect of the invention,
~, %, the delta percent of the first segment, is positive and the ~ % of at least
one other segment is negative. The radii and ~ %'s of the segments are
chosen to provide a negative total dispersion at 1550 nm which is no greater
than -150 ps/nm-km.
In an embodiment of this first aspect, the core glass region has three
segments and the second segment has a negative /\ %. A preferred
embodiment has respective segments, beginning at the first segment and
proceeding outwardly, having radii in the ranges of about 1 to 1.5 ~m, 5.5 to
6.5 ,um, and, 8 to 9.5,um, and, the respective segments, beginning at the first
segment and proceeding outwardly, having ~ %'s in the ranges of about 1.5 to
2 %, -0.2 to -0.5 %, and, 0.2 to 0.5 to provide an effective area, Aeff~ at 1550nm, no less than about 30,um2. Effective areas higher than 60 ,um2 are
achievable.
in another embodiment of this first aspect, the core glass region has
four segments, and the second and fourth segments have a negative ~ %. A
preferred embodiment has respective radii, beginning at the waveguide center
and proceeding outward, in the ranges of about 1 to 2 ~m, 6 to 8 ~m, 9 to 11
I~m, and, 13 to 17 I~m. The corresponding segment ~ %'s are in the respective
ranges of about 1 to 2%, -0.2 to -0.8%, 0.4 to 0.6%, and -0.2 to -0.8%. These
preferred core profiles provide Aeff at 1550 nm of no less than 30 ,um2. The
dispersion slope of 2 to 15 ps/nm-km provided by these core profiles is
reasonably small.
CA 02221737 1997-11-20 ~
In another embodiment of this aspect of the invention, the core glass
region has four segments, numbered 1 to 4, beginning at the waveguide fiber
center. The corresponding relative refractive index percent of the segments
are ordered as 4 % ~ /\3 % > ~4 % > ~2 %, where ~2 % iS negative. The
respective ~ %'s are, 1.5 to 2 % for ~, %, -0.2 to -0.45 % for ~2 %, 0.25 to 0.45
% for ~3 %, and, 0.05 to 0.25 % for 4 %, and the respective radii associated
with these ~ %'s are in the ranges of about 0.75 to 1.5 l~m for r1, 4.5 to 5.5 ,um
for r2, 7 to 8 ,um for r3, and, 9 to 12 ,um. In this embodiment, the total dispersion
slope is negative which serves to cancel with the positive slope of the
waveguide fiber of the original link operating in the 1310 nm window.
Typically, the negative slope of the total dispersion is in the range of about -0.1
to-5.0 ps/nm2-km.
A second aspect of the invention is a single mode optical waveguide
fiber link made of a first length of single mode fiber designed for operation inthe 1310 nm window and a length of dispersion compensating single mode
waveguide fiber. The dispersion compensating fiber length and total
dispersion product at 1550 nm are chosen to add algebraically with the length
times dispersion product of the first length of waveguide fiber to produce a pre-
selected value of total dispersion for the link. The pre-selected value may
advantageously be chosen zero at 1550 nm to provide the lowest total
dispersion over this window. If four wave mixing or self phase modulation is an
anticipated problem for 1550 nm window operation, the total dispersion at
1550 nm may be selected to be a small positive number.
The attenuation of the dispersion compensating waveguide fiber is held
to a low value so that attenuation does not become a data rate limiter for the
link. In addition, Aeff should be large enough, at least 30 ,um2, so that
significant non-linear dispersive effects are not introduced by the dispersion
compensating waveguide fiber. The ratio of the compensating fiber total
dispersion and attenuation, together with Aeff are combined in a function which
describes a discriminating factor, denoted Gnl in the art and defined above,
CA 02221737 1997-11-20
which is a measure of the properties of the compensating waveguide fiber with
regard to non-linear dispersive effects.
An embodiment of this aspect of the invention includes a dispersion
compensating waveguide fiber which has a total dispersion, Dd no greater than
-150 ps/nm-km, Aeff > 30 I-m2, and the magnitude of Dd/a > 150 ps/nm-dB. In a
preferred embodiment, the magnitude of Dd/a is > 250 ps/nm-dB.
Because the total dispersion of the compensating fiber is a large
negative number, the length of compensating fiber required to arrive at a pre-
selected value of total dispersion for the link is generally less than 15 % of the
link length and may be less than 5 % of the link length.
A third aspect of the invention is a method of making a single mode
optical waveguide which compensates at 1550 nm for dispersion in a link
originally designed for operation in the 1310 nm window. The draw preform,
comprising a central core glass region and a surrounding clad glass layer, the
core glass region having the properties described in the first aspect of the
invention, may be made by any of several techniques in the art. These include,
inside and outside chemical vapor deposition, axial chemical vapor deposition
and any of the modifications of these techniques in the art. The core regions
having positive relative refractive index may be formed using a dopant such as
germania in a silica glass matrix. The core regions of negative relative index
may be formed using a dopant such as fluorine.
A drawing tension greater than about 100 grams has been found to
yield better total dispersion to attenuation ratios than similar waveguide fibers
drawn at lower tension. To limit loss due to bending, an outside diameter
greater than about 125 ~m is preferred. The upper limit on outside diameter is
set by practical limitations such as cost and required cable size. A practical
upper limit is about 170 ,um.
To limit attenuation due to residual coating stress, the coated
waveguide fiber may be loose wrapped on a spool and heat treated. For most
effective stress relief, the spool size should be greater than about 45 cm. The
winding tension used to wrap the waveguide fiber onto the spool is less than
, CA 02221737 1997-11-20
about 20 grams. A preferred winding method is one in which the waveguide
fiber is allowed to assume a catenary configuration just prior to being wound
onto the spool.
A heat treatment at a temperature at least 30 ~C greater than the glass
5 transition temperature, Tg, of the polymer coating and continued for 1 to 10
hours has been found effective to relieve residual coating stresses for the
coating types and thicknesses used in testing. A holding time of about 5 hours
was found to be effective for the thickness, about 60 lum, of UV cured acrylate
coating used in the manufacture of the waveguide fiber described herein.
It is understood that the heat treating method recited herein includes
temperature and time limitations suited to any of the several polymer coatings
types and thicknesses suitable for use in the manufacture of optical waveguide
fiber.
Brief Description of the Drawinqs
FIG. 1 is a general illustration of the novel core region refractive index profile.
FIG. 2 is a particular embodiment of the novel core region refractive index
profile.
FIG. 3 is a measurement made on a draw preform which incorporates an
20 embodiment of the novel core profile.
FIG. 4a shows a family of curves which relate the discriminator factor to the
ratio of total dispersion and attenuation.
FIG. 4b shows the dependence of the system loss, introduced by the
compensating waveguide fiber, on the ratio of total dispersion and attenuation.
Detailed Description of the Invention
The wide applicability of segmented core waveguide fiber designs to
particular telecommunication system requirements derives from the flexibility
provided by the segmented core concept. The number of core segments is
30 limited only by the core diameter and the narrowest core segment which can
affect the propagation of light in a waveguide. Also, it is known that the width,
CA 02221737 1997-11-20 ~
placement, refractive index profile, and the relative location of the core
segments, with reference, for example, to the waveguide long axis centerline,
affect the properties of the segmented core waveguide fiber. The large
number of permutations and combinations of the segments accounts for the
5 flexibility of the segmented core design.
The problem solved by the invention, disclosed and described herein, is
that of upgrading a telecommunication system, designed for operation in the
1310 nm window, to operate in the 1550 nm wavelength window, without
resorting to a major overhaul of the system. The solution to this problem is a
10 dispersion compensating waveguide fiber which can be readily inserted into a
communications link and which has a total dispersion characteristic, an
attenuation, and, an Aeff to allow high data rate transmission in the operating
window around 1550 nm. In particular, the compensating fiber must have a
dispersion characteristic which essentially cancels 1550 nm window dispersion
15 of the 1310 nm section of the link. The compensating fiber should have an
attenuation low enough to allow insertion, into the link, of the compensating
fiber without causing a need for signal regeneration. In some case optical
amplification of the signal may be required. The Aeff of the compensating fiber
should be large enough that the compensating fiber does not become the data
20 rate limiting component with regard to non-linear effects.
A general core region refractive index profile which meets these
requirements is shown in FIG. 1. Four segments, 2, 4, 6, and 8, are shown in
the illustration. In one embodiment of the invention, segment 8 is equal in
refractive index to that of clad 10, so that the core glass region has three
25 segments. The invention is not limited to three or four segment core refractive
index profiles. However, in terms of manufacturing cost, the simplest profile
which meets the system requirements is preferred.
Dashed lines 7 indicate alterations which can be made in the segment
index profiles without substantially changing the waveguide fiber properties.
30 The corners of the profile may be rounded. The central profile shape may be,
for example, triangular or parabolic. Only one segment need have a negative
CA 02221737 1997-11-20
%. An alternative statement of the impact of small profile alterations or
perturbations is, the ~ %'s, the widths at the bases, and the outer radii of a
segments are more important factors in determining waveguide fiber
characteristics.
Table 1. shows a computer model study done to evaluate the sensitivity
of waveguide fiber properties to core segment placement and ~ %. Index
profiles 1 through 5 follow the FIG. 1 four segment core region refractive indexprofile illustration. Index profile 6 is a three segment profile which has all the
features of FIG. 1 except for the final segment 8.
Table 1
Index 1 Index2 Index3 Index4 Index5 Index6
Disp. -430 -549 -475 -220 -310 -327
ps/nm-
km
Disp. 6.3 9.8 10.6 2.4 13.6 4.2
Slope
ps/nm2-
km
Aeff ~m2 78 104 132 58 208 72
Cutoff 2.2 2.3 1.9 2.0 1.9 1.9
~um
4 % 1.5 1.5 1.45 1.5 1.5 2.0
r1 llm 1.5 1.5 1.5 1.5 1.45 1.05
~2 % -0-5 ~0-5 ~0 5 ~0-5 ~0-5 ~0-3
r2~m 6.5 7 8 5.8 8 6
/\3 % 0.5 0.5 0.5 0.6 0.6 0.35
r3,um 10.5 11 11 9 11 8.8
~4 % -0.5 -0.5 -0.5 -0.5 -0.5 0
r4,um 13 13 17 13 17
f- CA 02221737 1997-11-20 ~
Several of the advantageous features of the design are shown in Table
1. These are:
- very large negative dispersions are achievable together with large Aeff
5 for all of the index profiles studied;
- the cut off wavelength is relatively insensitive to segment parameter
changes;
- reducing the radius of segment 2 is effective in reducing the total
dispersion slope; and,
- a three segment core can meet the requirements of many system
configurations.
Note also that one may achieve a decreased total dispersion slope, if a
system requires a lesser amount of negative total dispersion.
Table 2.
Index 21 Index 22 Index 23
Disp. ps/nm-km -310 -280 -273
Disp. Slope -0.1 -2.4 -1.2
ps/nm2-km
Af,ff ,um2 25 19 22
Cut Off ~m 2.0 1.9 1.9
% 2.0 2.0 2.0
r"um 1.1 1.1 1.1
~2 % -0.3 -0.3 -0.3
r2,um 5.5 - 6 5.5
~3 % 0.35 0.35 0.35
r31~m 8 8.8 8.3
~4 % 0.1 0 . 0
r4 ,um 10
( CA 02221737 1997-11-20
The embodiment of the novel profile illustrated in FIG. 2 again shows a
four segment, 12, 14, 16, and 18 core glass region. The clad glass layer is
shown as structure 20. The main features of this design are: the central
segment relative index is high in comparison to the design of FIG. 1; only one
5 negative relative index portion, 14, is present; and, the radii of segments 14,
16, and 18 are reduced relative to the design illustrated in FIG. 1. One effect
of moving the segment locations closer to the waveguide centerline is to
reduce Aeff
The design of the core glass region refractive index profile 21 follows
that illustrated in FIG. 2. Index profiles 22 and 23 are similar to the illustration
of FIG. 2 except that the ~ % of segment 18 is zero for these two cases. Table
2. shows the results of a computer model study to evaluate the properties of
core region index profiles which yield a negative total dispersion slope in the
dispersion compensating waveguide fiber. A negative total dispersion slope in
15 the compensating waveguide fiber serves to cancel at least a part of the
positive slope of the remainder of the link, thereby lowering the link dispersion
slope over the wavelengths of the 1550 nm window of operation. The data in
Table 2. indicates that Aeff is low when negative dispersion slope is achieved.
Thus this compensating waveguide design is to be used in cases where only a
20 short length of compensating fiber is required or where non-linear dispersiveeffects are not important, such as parts of a link whereat signal power density
is low.
Example - A Three Segment Profile having Large Dd/a
An optical waveguide fiber preform was prepared having a three
25 segment core glass region refractive index profile as shown in FIG. 3. Central
segment 22 had a ~ % of 1.83. Segment 24 had a negative l~ % of -0.32 %.
Segment 26 had a relative refractive index of 0.32 %. The segment radii may
be read in millimeters from the horizontal axis and converted to their
waveguide fiber equivalents using the final waveguide fiber outside diameter
which was 155 ,um. The draw tension averaged about 200 gm. The resulting
f CA 02221737 1997-11-20 f
waveguide fiber was loose wound on 46 mm diameter spool and annealed for
about 10 hours at 50~C.
The total dispersion was -214 ps/nm-km and the attenuation was 0.6
dB/km to yield a Dd/a of 356 ps/nm-dB. The effective area was 50 I~m2.
5 Advantageously, the dispersion slope for waveguides having this core
configuration is in the range -2 to +2 ps/nm2-km.
The non-linear discriminator factor, Gnl, defined above, is charted vs.
Dd/a in FIG. 4a. The resulting family of curves 32 allows one to readily predictsystem performance, given the ratio Dd/a. Referring to the equation for G
10 above, it is clear that Gnl becomes small as Dd/a becomes large. Thus,
waveguide fiber performance, from a system point of view, can be estimated
from the Dd/a ratio. Also, the trade off of dispersion against attenuation, in the
dispersion compensating fiber can be read directly form the chart in FIG. 4a.
For example, if a particular system can operate only if Gnl is less than about
30, the dispersion of the compensating fiber càn vary between -150 and
-400 ps/nm-km as attenuation varies between 0.29 dB/km and 3.2 dB/km.
The chart shown in FIG. 4b may also be used to evaluate the
performance of a dispersion compensating waveguide fiber. The y axis is the
total loss introduced into the link by the dispersion compensating waveguide
fiber. The x axis is the Dd/a ratio. Curve 34 is drawn assuming that the
original system designed for 1310 nm window operation has a length of 100
km and a dispersion at 1550 nm of 17 ps/nm-km. The dramatic improvement
in contributed loss as Dd/a increases illustrates the value of this ratio in
estimating the performance of the dispersion compensating waveguide fiber.
Although particular embodiments of the invention are disclosed and
described hereinabove, the scope of the invention is nonetheless limited only
by the following claims.
