Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2001-03-05
2004-09-07
Glick, Edward J. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000
Reexamination Certificate
active
06788865
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization maintaining optical fiber capable of transmitting optical signals while maintaining their polarization state, which can be utilized as a transmission medium in optical communication networks and optical signal processings.
2. Description of the Background Art
FIG. 1
shows a conventional optical fiber
10
in which a core
11
in a form of a hollow hole is surrounded by a photonic crystal structure cladding
12
, and this photonic crystal structure cladding
12
is further covered by a jacket
13
. Note that, in the literature, a term “photonic crystal structure cladding” is used regardless of a material used for the core while a term “photonic band gap cladding” is used specifically in the case of using a hollow hole core or the core with the refractive index lower than that of the cladding, and a term “photonic crystal structure cladding” will be used throughout this specification in the former sense so that the “photonic crystal structure cladding” should be construed as including the “photonic band gap cladding”.
The photonic crystal structure cladding
12
has a diffraction grating (represented by blank dots in the figure), which is usually formed by hollow holes but it can also be formed by a material with a different refractive index in circular cross sectional shapes.
Next, the principle of optical waveguiding by the optical fiber
10
in such a configuration will be described. In this optical fiber
10
, in the case where the material of the core
11
is glass, the equivalent refractive index of the photonic crystal structure cladding
12
is lower than that of the core
11
, so that lights are waveguided within the core
11
by the confinement due to the total reflection of the refractive index (similar to the confinement in the general single mode fiber).
On the other hand, in the case where the refractive index of the core
11
is lower than that of the photonic crystal structure cladding
12
or in the case where the core
11
is a hollow hole so that its refractive index is equal to that of the air which is approximately 1, the photonic crystal structure cladding
12
and the jacket
13
in the surrounding are formed by silica materials same as those used in the ordinary optical fiber, so that their refractive indexes are higher than that of the core (hollow hole)
11
. Consequently, if the cladding in the same structure as that of the conventional optical fiber is used, the refractive index of the hollow hole core
11
would become lowest and therefore it would be impossible to confine the energy of lights within the core
11
in this structure.
For this reason, the confinement of lights is realized by adopting a structure called photonic crystal structure in part of the cladding. Namely, the photonic crystal structure cladding
12
having a diffraction grating with an appropriate lattice interval for confining lights within the core
11
is provided in the surrounding of the core (hollow hole)
11
.
FIG. 2
shows a configuration of the photonic crystal structure cladding
12
. In general, the three-dimensional photonic crystal structure is a diffraction grating capable of causing the Bragg reflection of lights in all directions, which is realized by setting the lattice constant (lattice interval) d of the diffraction grating d approximately equal to the wavelength of lights to be propagated through the medium (core=air), as shown in FIG.
2
.
There are many possible configurations for a crystal lattice constituting the photonic crystal structure besides a square lattice shown in FIG.
2
.
FIG. 3
shows some exemplary configurations for a crystal lattice constituting the photonic crystal structure.
FIG. 3A
shows a square shaped lattice structure (black portions in the figure) with a higher refractive index which is embedded in a medium (white portion in the figure) with a lower refractive index.
FIG. 3B
shows a square shaped lattice structure (white portions in the figure) with a lower refractive index which is embedded in a medium (black portion in the figure) with a higher refractive index.
FIG. 3C
shows a triangular shaped lattice structure (black portions in the figure) with a higher refractive index which is embedded in a medium (white portion in the figure) with a lower refractive index.
FIG. 3D
shows a triangular shaped lattice structure (white portions in the figure) with a lower refractive index which is embedded in a medium (black portion in the figure) with a higher refractive index.
FIG. 3E
shows a honeycomb shaped lattice structure (black portions in the figure) with a higher refractive index which is embedded in a medium (white portion in the figure) with a lower refractive index.
According to J. D. Joannopoulos et al., “Photonic Crystals”, Princeton University Press, pp. 122-126, 1995, it is known that the photonic crystal structure is present in these configurations so that the confinement of lights can be realized.
Note also that the lattice structure with cylindrical or circular hole shaped lattice holes is assumed here, but the shape of the lattice holes is not necessarily limited to the cylindrical or circular hole shape, and the photonic crystal structure can also be realized by using the lattice structure with the lattice holes in a triangular prism or triangular hole shape, a rectangular bar or rectangular hole shape, a hexagonal bar or hexagonal hole shape, etc.
When the core in a form of a hollow hole is provided in this photonic crystal structure, lights are strongly confined within this core. Consequently, when it is desired to waveguide lights through some structure, it is possible to propagate lights while confining them within that structure (the core
11
) by providing the photonic crystal structure (the photonic crystal structure cladding
12
) in the surrounding of that structure (the core
11
), as shown in FIG.
2
.
This photonic crystal structure is provided in the surrounding of the optical fiber core to realize the confinement such that lights do not propagate in a radial direction from a center of the optical fiber core. Namely, as shown in
FIG. 1
, the cross section of the optical fiber
10
has a lattice shaped structure, and this same structure is maintained along the length direction. In other words, the cross section of the optical fiber
10
has a uniform structure throughout (except for a fluctuation of shape due to the fabrication process of the optical fiber) and there is no structure that is perpendicular or oblique to the length direction of the optical fiber
10
. By adopting this structure, it is possible to propagate lights entered into the core
11
while confining them within the core
11
.
In the conventional optical fiber
10
described above, although it is possible to transmit lights through the optical fiber while confining lights within the core
11
, the following problem arises in the case of carrying out optical communications using this optical fiber
10
. Namely, in the case where the shape of the core
11
of the optical fiber is circular, there is no mechanism for determining a polarization direction of lights propagating within the core
11
, so that a fluctuation in the polarization direction within the core
11
can be caused by a slight fluctuation in this circular share of the core
11
. Consequently, the polarization state of optical signals after transmission through the optical fiber
10
can vary due to causes such as temperature variation or vibration of the polarization maintaining optical fiber
10
, and it has been necessary for a receiving side to use a polarization independent structure that is not affected by the variation of the polarization of optical signals.
The currently existing polarization maintaining optical fibers include the PANDA fiber which does not use the photonic crystal structure. The fabrication process of this PANDA fiber requires a sophisticated technique of forming holes at two locations in a vicinity of the core made by the material of the
Kawanishi Satoki
Okamoto Katsunari
Glick Edward J.
Holmes Brenda O.
Kao Chih-Cheng Glen
Kilpatrick & Stockton LLP
Nippon Telegraph and Telephone Corporation
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