Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-10-24
2001-11-27
Ngo, Hung N. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000
Reexamination Certificate
active
06324327
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical waveguide fiber having a dispersion zero shifted to higher wavelengths, and particularly to such a fiber that has negative total dispersion.
2. Technical Background
In response to demands for high performance waveguide fibers having properties compatible with particular communications systems, investigation of segmented core optical waveguide refractive index profiles has continued. For example in U.S. Pat. No. 5,483,612, Gallagher et al., (the '612 patent) there is disclosed a core profile design which provides low polarization mode dispersion, low attenuation, a shifted dispersion zero, and low dispersion slope. Other core refractive index profiles have been designed to meet the requirements of applications that include the use of higher power signals or optical amplifiers.
A problem that can arise when a core profile is altered in order to arrive at a desired property is that the property is realized at the expense of one or more other essential properties. For example, a certain core refractive index profile design may provide lower total dispersion slope, thus allowing for wavelength division multiplexing over an extended wavelength range. However, in achieving low total dispersion slope, attenuation may be seriously compromised, or cut off wavelength can be moved out of an acceptable range.Thus, core profile design is an exacting task, in which model studies usually precede the manufacturing stage of product development.
There is a need in the art to continue the study of refractive index profiles in order to better understand interaction among profile variables and thus to arrive at profile variable combinations that provide optical waveguide fibers having a set of desired properties.
DEFINITIONS
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments. The definitions of the radii used herein are explained with reference to FIG.
1
. In
FIG. 1
, the radius of the center index segment
10
is the length
2
that extends from the waveguide centerline to the point at which the profile becomes the &agr;-profile of segment
12
, that is, the point at which the refractive index versus radius curve begins to follow the equation, set forth below, for an &agr;-profile. The outer radius
4
of segment
12
extends from the centerline to the radial point at which the extrapolated descending portion of the &agr;-profile crosses the extrapolated extension of profile segment
14
. This definition is readily applied to alternative center segments such as &agr;-profiles or step index profiles. Further, the definition is readily applied to those cases wherein the second segment has a shape other than that of an &agr;-profile. In cases where alternative center segment shapes are used, the radii are illustrated in a separate drawing. The radius
6
of segment
14
extends from the centerline to the radius point at which the &Dgr;% is half the maximum value of the &Dgr;% of segment
16
. The radii of additional segments are defined analogously to that of segment
14
until reaching the final core segment. The midpoint radius
8
of segment
16
, the final segment of the core as illustrated in
FIG. 1
, is measured from the centerline to the midpoint of the width of the segment. The width of a segment such as segment
16
extends between the two half &Dgr;% values at the opposing portions of segment
16
. The clad layer of the fiber is shown as
17
in FIG.
1
. The radii of the next to last segment are found from the geometry of adjoining segments. The inner radius of the next to last segment is the outer radius of the preceding segment. The outer radius of the next to last segment is the center radius of the last segment minus one half the width of the final segment.
The term &agr;-profile refers to a refractive index profile, expressed in terms of &Dgr;(b)%, where b is radius, which follows the equation,
&Dgr;(b)%=&Dgr;(b
o
)(1-[¦b-b
o
¦]/(b
1
-b
o
)]
&agr;
),
where b
o
is the point at which &Dgr;(b)% is maximum, b
1
is the point at which &Dgr;(b)% is zero, and b is in the range b
i
≦b≦b
f
, where delta is defined above, b
i
is the initial point of the &agr;-profile, b
f
is the final point of the &agr;-profile, and &agr; is an exponent which is a real number. The initial and final points of the &agr;-profile are selected and entered into the computer model. As used herein, if an &agr;-profile is preceded by a step index profile or any other profile shape, the beginning point of the &agr;-profile is the intersection of the &agr;-profile and the step or other profile.
Total dispersion is the sum of material dispersion, waveguide dispersion, and inter-modal dispersion. In single mode waveguide fiber, there is no intermodal dispersion. The sign convention used for total dispersion is that total dispersion is positive if shorter wavelengths travel faster in the waveguide fiber than do longer wavelengths. The converse is true for relative speed of light wavelengths in the case of negative total dispersion.
Mode field diameter is found using the Peterman II definition which is known in the art.
Cabled cut off wavelength is the wavelength above which the waveguide fiber propagates a single mode, where the fiber is in cabled form.
SUMMARY OF THE INVENTION
One aspect of the present invention is a single mode optical waveguide fiber having a segmented core region surrounded by and in contact with a clad layer. The relative indexes of the core region and the clad layer are selected to provide a fiber structure that guides light. The core region includes a center segment and at least one annular segment surrounding and in contact with the center segment. The index profiles of the segments of the core region are selected to provide a waveguide fiber having total dispersion slope over a preselected wavelength range less than or equal to about 0.07 ps
m
2
-km, total dispersion at 1560 nm in the range of about −3.4 ps
m-km to −1.0 ps
m-km, mode field diameter in the range of about 7.7 &mgr;m to 8.7 &mgr;m, cabled cut off wavelength less than or equal to about 1480 nm, and, attenuation at 1550 less or equal to about 0.22 dB/km.
In an embodiment of the invention, the waveguide fiber is designed for operation in a wavelength window spanning 1500 nm to 1600 nm. Thus the total dispersion slope is substantially constant over this operating window, and the attenuation does not vary by more than about 0.05 dB/km over this window
In a further embodiment of the invention, the center segment is divided into a first portion which includes the centerline of the waveguide and a second portion which is an annulus surrounding the first portion. The respective profiles of the portions and the at least one annular segment are selected from the group consisting of an &agr;-profile, a step, a rounded step, a trapezoid, and a rounded trapezoid. A preferred profile for the centerline portion is a rounded step. A preferred profile for the annular portion is an &agr;-profile having an &agr; in the range of about 0.8 to 8.
In yet another preferred embodiment of the invention the centerline portion is an &agr;-profile having a in the range of about 0.8 to 1.5 and the center annular portion is an &agr;-profile having an &agr; in the range of about 1 to 6.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skille
Herring James B.
Smith David K.
Chervenak William J.
Corning Incorporated
Ngo Hung N.
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