Optical fiber

Details Optical fiber Optical fiber

2001-02-27

2003-10-21

Nguyen, Khiem (Department: 2839)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

C385S125000

Reexamination Certificate

active

06636677

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber which can be suitably used as an optical transmission path and a dispersion compensator.
2. Description of the Related Art
FIG. 14
is a cross-sectional view showing a central portion of an optical fiber including microstructures which has been known conventionally. This optical fiber has a structure in which silica glass
61
constitutes a main medium and a large number of voids (vacant holes)
62
are disposed in a cross section thereof. A central portion in cross section having no voids
62
constitutes a core region
63
and a portion surrounding the core region
63
and including a large number of voids
62
constitutes a cladding region
64
.
The principle of light confinement of the optical fiber including such microstructures is explained qualitatively using a concept called effective refractive indices (for example, T. A. Birks et al. Optics Letters Vol. 22 p.961 (1997)). Due to the presence of the microstructures, in a strict sense, the refractive index should show a complicate distribution in the cladding region
64
. However, on the assumption that the optical waveguiding characteristics can be approximated by replacing the inside of the cladding region with a homogeneous medium, the refractive index of the homogeneous medium is called the effective refractive index. The effective refractive index n
eff
satisfies a following equation.
1
(
f
1
n
1
2
+
f
2
n
2
2
)
≤
n
eff
2
≤
f
1
⁢
n
1
2
+
f
2
⁢
n
2
2
(
1
)
where, n is the refractive index and f is the volume fraction. Further, a suffix 1 represents a main medium (silica glass) and a suffix 2 represents a sub medium (air). With respect to the volume refraction, f
1
+f
2
=1 is held. Usually, since n
1
>n
2
, the both side members in the equation (1) become smaller corresponding to the increase of f
2
. Accordingly, the effective refractive index of the cladding region
64
including a large number of voids
62
becomes smaller than the effective refractive index of the core region
63
so that the light confinement is realized in the same manner as the usual optical fiber.
Further, a microstructured optical fiber having a greater negative dispersion than the optical fiber shown in
FIG. 14
is disclosed in U.S. Pat. No. 5,802,236. As shown in FIG.
15
, in this optical fiber, the cladding region is constituted by an inner cladding region and an outer cladding region and by making the void diameter in the inner cladding region greater than the void diameter in the outer cladding region, the effective refractive index of the inner cladding region is made smaller than the outer cladding region.
The previously-mentioned model assumed to define the effective refractive index is considered to be reasonable as long as the optical wavelength is sufficiently long compared to the scale of the microstructure. However, as the optical wavelength becomes shorter, the light is locally concentrated at portions having the high refractive index and hence, it is considered that the assumption that the structure having a non-uniform refractive index distribution can be replaced by a homogeneous medium will lose the validity. As a result, it should be noted that specification of the structure based on effective refractive index is inevitably ambiguous.
In the conventional microstructured optical fiber in which the void diameters in cross section are not uniform, it is difficult to securely realize desired characteristics. This is because, although the void diameters are changed in response to the glass surface tension and/or the internal stress at the time of fiber drawing, the amount of the change depends on the void diameters. For example, when the void diameters are small, the surface tension strongly acts and hence, the contraction is liable to occur compared with the case in which the void diameters are large. As a result, it is difficult to perform the fiber drawing such that each of the voids having different diameters is formed in the fiber with the desired diameters.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances to provide an optical fiber which has sub-medium regions and is capable of securely realizing desired characteristics even when cross sectional areas of the sub-medium regions are changed at the time of fiber drawing.
To solve the problems, according to the present invention, in an optical fiber composed of a core and a cladding surrounding the core, where a given cross section of the cladding includes a plurality of regions made of sub mediums having refractive indices different from the refractive index of the main medium constituting this cladding, wherein the regions made of the sub mediums are arranged in one given or a plurality of given circular annular regions and the centers of the regions made of the sub mediums in each of the circular annular regions are arranged on the same circumference centered at the center of the core and having a diameter predetermined for each of the circular annular regions.
Here, the main medium is a material which can constitute an optical fiber by itself and the regions of the main medium are connected together. On the other hand, the sub mediums may be materials which cannot constitute the optical fiber by themselves and are scattered in a plurality of regions in the optical fiber. A typical main medium is silica glass and a typical sub medium is air or an inert gas.
According to the finding of the inventors of the present inventions, structures containing regions of sub mediums can be specified with less ambiguity using mean refractive index which is expressed by a following equation. Here, assuming that a region can be divided into M sub regions and each of them is formed by a homogeneous medium, the mean refractive index n
avg
of the region can be expressed by the following equation (2).
n
avg
=
∑
i
=
1
M
⁢
f
i
⁢
n
i
2
(
2
)
That is, the mean refractive index n
avg
is the RMS (Root Mean Square) average of the refractive indices of respective mediums weighted by the volume fraction of each medium. Here, n
i
is the refractive index of the i-th medium and f
i
is its volume fraction and a following equation holds.
∑
i
=
1
M
⁢
f
i
=
1
(
3
)
Accordingly, provided that the regions are determined, the mean refractive index is unambiguously determined. In other words, this implies that the value of the mean refractive index depends on the determination of the regions. According to the optical fiber of the present invention, the sub-medium regions are arranged such that they are positioned in the inside of the given circular annular region and their centers are positioned on given circumferences centered at the center of the core. Due to such a constitution, it becomes easy to design the mean refractive index in each of the circular annular regions so that the optical fiber possesses the desired optical characteristics.
The sub-medium regions can be arranged on the circumferences centered at the center of the core so that the arrangement has the four-fold rotational symmetry with respect to the center. Such a constitution is preferable to decrease the mode birefringence and the polarization mode dispersion.
At least one of cross-sectional areas and the refractive indices of these regions made of the sub mediums may change along the fiber axis. Due to such a constitution, the mean refractive index of the circular annular region including the sub mediums can be changed along the fiber axis. Accordingly, the optical fiber whose optical characteristics are changed along its axis can be easily realized.
Further, it is preferable that sections where the cladding does not contain the sub mediums are spaced along the fiber axis. Thus, by providing the sections having no sub-medium regions in cross section, it becomes possible to cleave the optical fiber and splice it to another optical fiber by fusion without excess transmission loss at the splice due to deformation of the structure and conta

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