Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
2000-06-27
2001-11-13
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
C385S126000
Reexamination Certificate
active
06317551
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical waveguide fibers having improved resistance to bending, and particularly to waveguide fibers having large effective area, and negative total dispersion in the 1550 nm operating window, and improved resistance to macro-bend and micro-bend
2. Technical Background
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, which can cause degradation of signals in high power systems. In general, a mathematical description of these non-linear effects includes the ratio, P/A
eff
, where P is light power. For example, a non-linear optical effect can follow an equation containing a term, exp [PxL
eff
/A
eff
], where L
eff
is effective length. Thus, an increase in A
eff
produces a decrease in the non-linear contribution to the degradation of a light signal propagating in the waveguide.
The requirement in the telecommunication industry for greater information capacity over long distances, without regenerators, has led to a reevaluation of single mode fiber refractive index profile design.
The focus of this reevaluation has been to provide optical waveguides that reduce non-linear effects such as those noted above and are optimized for the lower attenuation operating wavelength range around 1550 nm, i.e., the range from about 1250 nm to 1700 nm. In addition the waveguide should be compatible with optical amplifiers, and, retain the desirable properties of optical waveguides now deployed, such as, high strength, fatigue resistance, and bend resistance.
A waveguide fiber having at least two distinct refractive index segments has been found to have sufficient flexibility to meet or exceed the criteria for a high performance waveguide fiber system. The genera of segmented core designs are disclosed in detail in U.S. Pat. No. 4,715,679, Bhagavatula.
The effective area of a waveguide is in general increased by designing refractive index profiles that cause the light power distribution in the fiber to be shifted outwardly from the centerline of the waveguide fiber, thus reducing the power density. In moving the power distribution outwardly toward the core edge, however, the waveguide is made more susceptible to power losses due to bending of the fiber.
Bending losses have been found to occur in the cabling process as well as in the installation process. In some waveguide fiber uses, at least a part of the waveguide is installed as a coil, for example, in a junction box.
Thus there is a need for an optical waveguide fiber that reduces the non-linear term of refractive index by increasing effective area, A
eff
, while maintaining a desired resistance to macrobend and microbend.
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 definitions set forth herein are in accord with a computer model that was used to predict functional waveguide properties given a refractive index profile. The model can also be used in the inverse to provide a family of refractive index profiles that will have a pre-selected set of functional properties.
The effective area is
A
eff
=2&pgr;(∫E
2
r dr)
2
/(∫E
4
r dr),
where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide. An effective diameter, D
eff
, may be defined as,
A
eff
=&pgr;(D
eff
/2)
2
.
The relative refractive index percent, &Dgr; %=100×(n
i
2
−n
c
2
)/2n
i
2
, where n
i
is the maximum refractive index in region i, unless otherwise specified, and n
c
, is the average refractive index of the cladding region unless otherwise specified.
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
)]
&Dgr;
),
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 a 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 &Dgr;-profile is preceded by a step index profile or any other profile shape, the beginning point of the &Dgr;-profile is the intersection of the &agr;-profile and the step or other profile.
In the model, in order to bring about a smooth joining of the &agr;-profile with the profile of the adjacent profile segment, the equation is rewritten as;
&Dgr;(b)%=&Dgr;(b
a
)+[&Dgr;(b
o
)−&Dgr;(b
a
)]{(1−[|b−b
o
|/(b
1
−b
o
)]
a
},
where b
&Dgr;
is the first point of an adjacent segment.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins.
Alternate bend tests include wrapping of the fiber around one or more mandrels of pre-se
Mitchell Brian E.
Smith David K.
Bovernick Rodney
Chervenak William J.
Connelly-Cushwa Michelle R.
Corning Incorporated
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