Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2001-04-26
2003-08-12
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000, C385S124000, C385S126000, C385S127000
Reexamination Certificate
active
06606440
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber suitable as an optical transmission path, a dispersion compensator, an optical filter, an optical power equalizer and an optical amplifier.
2. Description of the Related Art
Conventionally, optical fibers composed of alternately arranged annular layers of high and low refractive indices have been known and disclosed in J. Marcou, et al., “Monomode photonic band gap fibers for dispersion shifting towards short wavelengths” ECOC'99, I-pp.24-25 (hereinafter called D1) and Y. Fink, et al., “Guiding optical light in air using an all-dielectric structure” Journal of Lightwave Technology, vol.17, No.11, November, 1999 pp, 2039-2041 (hereinafter called D2).
In these optical fibers, light is confined in the center region, which is called the core region, surrounded by annular layers, which is called the cladding region. The refractive index of the core region is lower than the refractive indices of the annular layers in the cladding region. Accordingly, the confinement of light in the core is not based on total internal reflection but on Bragg reflection due to the regularity in the radial profile of refractive index. That is, a diverging cylindrical wave centered at the fiber axis is strongly coupled to a converging cylindrical wave centered at the fiber axis because of the regularity in the radial profile of refractive index in the cladding region. As a result, the diverging cylindrical wave is reflected by the annular layers in the cladding, and is confined in the core.
The thicknesses of the annular layers in the cladding are not necessarily uniform. In D1, the refractive index distribution is designed to have the periodicity based on the Bessel functions, while in the D2, the thicknesses of the layers are determined in accordance with the zero points of the Bessel functions.
Further, in R. F. Cregan, et al “Single-Mode photonic band gap guidance of light in air”, Science, vol.285, pp.1537-1539 (September, 1999)(hereinafter called D3), an optical fiber having a cross-sectional structure in which a defect is introduced in a refractive index periodic structure having a two-dimensional translational symmetry is disclosed. In the cross section of this optical fiber, small regions (cells) having given refractive index distribution are regularly arranged, and some of the cells are replaced with cells having different refractive index distribution, resulting in breaking of the translational symmetry of the cross-sectional refractive index distribution. Those symmetry-breaking cells are called defects.
The two-dimensional periodic structure of the refractive index, if properly designed, reflects light belonging to a given wavelength band regardless of angle of incidence. Such a wavelength band is called a full PBG (full Photonic Band Gap). The light having the wavelength within the PBG is confined in the defect in the periodic structure. The periodic structure and the defect extend along the fiber axis and hence, the light propagates along the fiber axis.
Further, U.S. Pat. No. 5,802,236 discloses an optical fiber which includes a core and a cladding, wherein the effective refractive index of the core is higher than the effective refractive index of the cladding and the cladding has cladding feature structures which are arranged non-periodically. In such an optical structure, since the effective refractive index of the core is higher than the effective refractive index of the cladding, the light is confined in the core by total internal reflection. Here, assuming that a non-uniform region having spatially varying refractive index can be replaced with a homogeneous medium with maintaining the same optical characteristics, the effective refractive index is defined as the refractive index of such a homogeneous medium.
It is also conventionally known that Bragg reflection mirror can be formed by regularly laminating planar thin films consisting of media having different refractive indices, and that a high reflection efficiency is achieved by meeting the quarter wavelength condition where the optical thicknesses of the thin films are equal to a quarter wavelength.
SUMMARY OF THE INVENTION
However, in the optical fiber disclosed in D1, the refractive index difference between neighboring two annular layers is small (relative refractive index difference being 0.5%) because it is formed by doping Ge into silica glass.
Accordingly, the reflection efficiency of the annular layers in the cladding becomes small, and hence optical confinement to the core becomes weak. As a result, the optical power leaks to the outside of the fiber so that the transmission loss, particularly the transmission loss due to the bending of the fiber, increases.
On the other hand, in the optical fiber described in D2, the cladding region is composed of tellurium (refractive index being 4.6) and polystyrene (refractive index being 1.59). Due to the large difference in refractive index between the media, a high reflection efficiency can be obtained. However, the fabrication of this optical fiber is difficult for the following reason. According to the fabrication method disclosed in D2, this optical fiber is obtained by alternately depositing a tellurium film having a thickness of approximately 0.8 &mgr;m and a polystyrene film having a thickness of approximately 1.6 &mgr;m on an outer periphery of a glass tube of a diameter of 1.92 mm. However, it is difficult to fabricate a long fiber uniformly by this method. This is because if the optical fiber is wound in a coil while the films are deposited on it, it is difficult to deposit the films with uniform thickness. On the other hand, if the fiber is not wound in a coil shape, it is difficult to fabricate a long fiber because the length is limited by the size of the depositing facility. For example, the fiber length which is reported in the above-mentioned literature is as short as 10 cm. Further, since the films are deposited on a cylindrical surface, the control of the film thickness is difficult compared with the conventional thin film forming where the films are deposited on a planar surface. This also makes it difficult to fabricate a fiber which is uniform along its axis.
Further, in the optical fiber described in D3, the size of the defect is limited to integer times of the size of the cells of the periodic structure in the cross-sectional refractive index distribution. Accordingly, the size of the core is also limited to integer times of the size of the cells. The diameter of the core affects the number of guided modes and the degree of the optical confinement of the guided modes. Accordingly, the limited range of selection of the core diameter results in the limited range of achievable optical characteristics of the optical fiber. Particularly, it becomes difficult to deliberately control the wavelength range for single-mode operation and the tolerance to bending.
The present invention has been made in view of the above and it is an object of the present invention to provide an optical fiber based on confinement by Bragg reflection which exhibits strong optical confinement to the core, facilitates the fabrication of a long fiber, and realizes a high freedom in selection of the core diameter.
To achieve such an object, the optical fiber according to the present invention is the optical fiber consisting of a core region and a cladding region which surrounds the core region and has a plurality of regions spaced apart in cross section and made of sub mediums, whose refractive indices differ from that of a main medium constituting the cladding region, wherein the core region has lower mean refractive index than that of the cladding region, and wherein the arrangement of the regions made of sub mediums has such a regularity in the radial direction of the optical fiber that the light with given wavelength, propagation coefficient and electric field distribution propagates along the fiber axis and has not less than 50% of its total propagating power in the core region, and
Hasegawa Takemi
Nishimura Masayuki
Sasaoka Eisuke
Doan Jennifer
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Ullah Akm Enayet
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