Multimode optical fiber having a structure to reduce...

Details Multimode optical fiber having a structure to reduce... Multimode optical fiber having a structure to reduce...

2001-03-22

2004-01-13

Font, Frank G. (Department: 2877)

Optical waveguides

Optical fiber waveguide with cladding

With graded index core or cladding

Reexamination Certificate

active

06678451

ABSTRACT:

CLAIM OF PRIORITY
This application claims priority to an application entitled, “Multimode Optical Fiber Having a Structure to Reduce Scattering Loss”, filed with the Korean Industrial Property Office on Mar. 22, 2000 and there duly assigned Serial No. 2000-14517.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical fiber, and more particularly to a multimode optical fiber having a structure to reduce scattering loss.
2. Description of the Related Art
In general, an optical fiber is classified into a step index fiber and a graded index fiber. The step index optical fiber is generally comprised of a core with the refractive index n
1
and a cladding with the refractive index n
2
. In step index fiber, the refractive indices of the core and the cladding respectively maintain a constant value. However, the refractive index profile of the core changes gradually with distance from the fiber axis in the graded index optical fiber. The varying characteristics of the refractive indices n(r) from the fiber axis along the radial direction are described according to the following equation 1:
n
(
r
)=
n
1
{square root over (1−2(
r
/&agr;)
&agr;
)}&Dgr;
r≦a,
n
(
r
)=
n
2
{square root over (1−2&Dgr;)}
r>a
  (1)
Herein, n
1
represents the refractive index of the center of the core, n
2
represents the refractive index of the cladding, a represents the radius of the core, &agr; represents the core shape index, and &Dgr; represents the parameter determining the degree of the refractive index variation.
Here, only in the case of n
1
=n
2
, the aforementioned &Dgr; is approximated according to the following equation 2:
Δ
=
(
n
1
-
n
2
)
n
1
,
(
2
)
where &Dgr; is defined as a relative index difference.
FIG. 1
is a sectional view illustrating the refractive index profile of the graded index fiber according to the prior art.
FIG. 1
illustrates the graded index fiber
10
, including the core
11
and the cladding
12
, and the ideal refractive index profile
13
as well as the real refractive index profile
14
of the graded index fiber
10
. In ideal refractive index profile
13
, the region of the core
11
can be defined by the equation 1. In actual practice, however, the center of the core
11
with a recess represented by the refractive profile
14
, as shown in
FIG. 1
, is realized. Here, the recessed shape is caused during the fabrication of an optical fiber preform.
Typically, a chemical vapor deposition method is used in fabricating an optical fiber preform. In this method, after the cladding of the optical fiber preform is prepared, the core is formed by injecting and sintering gasses into the cladding. Here, the core is not filled up to the center, thus a hole having a specified diameter is formed within the glass tube. Thereafter, the optical fiber preform is completed finally through a collapsing. As a consequence of this collapsing process, an optical fiber drawn from the optical fiber preform exhibits a real refractive index profile
14
as shown in FIG.
1
.
Normally, the refractive index variation degree of the graded index fiber
10
can be adjusted by the type and the amount of dopant added to the fiber. Common dopant material used include titanium, germanium, boron, and etc. However, the dopant over the graded index fiber
10
causes scattering loss of the optical signal within the graded index optical fiber
10
. Hence, as the composition rate of the dopant is increased, the scattering loss of the optical signal also increases.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention is to provide an improved multimode optical fiber for eliminating the scattering properties.
In the preferred embodiment, a multimode optical fiber having a structure to reduce scattering loss includes a core divided into a central region with an optimal refractive index and a peripheral region whose refractive index is lower than the central region of the core, and a cladding enclosing the core, wherein the cladding having an refractive index value lower than the lowest refractive index value of the core. Preferably, the refractive index of the central region and the peripheral region range between 0.5 and 2.5.


REFERENCES:
patent: 5993899 (1999-11-01), Robin et al.

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