Optical fiber having improved optics and structure

Details Optical fiber having improved optics and structure Optical fiber having improved optics and structure

2000-11-13

2003-02-25

Nasri, Javaid (Department: 2839)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

C385S126000, C385S127000, C385S028000

Reexamination Certificate

active

06526209

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber suitable as an optical transmission line or dispersion compensator.
2. Related Background Art
The following optical fibers have conventionally been known, for example. A microstructured optical fiber disclosed in Japanese Patent Application Laid-Open No. HEI 10-95628 has a core region, which is usually solid, surrounded by a cladding region that comprises a multiplicity of spaced apart cladding features that are elongate in the axial direction and disposed in a first cladding material. The core region has an effective diameter d
0
and an effective refractive index N
0
. Each cladding feature has a refractive index that differs from that of the first cladding material, and the cladding region has an effective refractive index that is less than N
0
. Further, it is disclosed that a large dispersion is obtained since the cladding region comprises an inner cladding region having an effective refractive index N
cl
and an outer cladding region having an effective refractive index N
co
(where N
cl
<N
co
).
OFC'96 Technical Digest, ThA3 discloses an optical fiber having a W-shaped refractive index profile, and discloses that a low chromatic dispersion (with a large negative value) can be realized in this optical fiber.
Electronics Letters, vol. 18, pp. 824-826 (1982) discloses that, by being provided with “side tunnels” on both sides of a core region, not only high normalized birefringence is realized, but also the cutoff frequency difference between two polarization modes is enlarged, whereby an absolutely single-polarization optical fiber can be realized.
U.S. Pat. No. 5,907,652 discloses the following air-clad optical fiber. Namely, it is a silica-based optical fiber comprising, successively from the fiber center toward the outer periphery, a core region, an inner cladding region, a first outer cladding region, and a second outer cladding region, in which the refractive index of inner cladding is less than that of the core region, and the effective refractive index of first outer cladding region is less than 1.35. Also, the first outer cladding region is selected such that optical characteristics of the optical fiber do not depend on the second outer cladding region. It discloses that the air-clad optical fiber is suitable for cladding-pumped optical fiber lasers and long-period fiber gratings.
SUMMARY OF THE INVENTION
In the microstructured optical fiber disclosed in JP 10-95628A, however, microstructures are distributed over the whole cladding, whereby the number of microstructures is large. For example, the above-mentioned publication states that “Our simulations indicate that at least 4 layers of second capillary features should be provided.” In this case, the number of capillary features would be at least 90, thus becoming large. If the number of microstructures is large as such, then the making becomes difficult. According to the above-mentioned publication, the process of making the microstructured optical fiber is as follows. Namely, silica capillary tubes and a silica rod with no bore are prepared, a tube bundle is formed by arranging a number of silica tubes around the silica rod, the tube bundle and an over cladding tube are collapsed so as to yield a preform, and then an optical fiber is drawn from this preform. However, it takes time and effort to form a tube bundle by arranging small-diameter silica tubes into a bundle without disorder. Also, since there is a strong possibility of the arrangement being disordered, the making with a favorable reproducibility is hard to achieve. The making becomes more difficult as the number of microstructures increases.
On the other hand, a step of boring a preform of a conventional impurity-doped type optical fiber by use of a boring device may be used instead of the above-mentioned making process. Even in the case using this step, however, the conventional microstructured optical fiber has a number of microstructures, whereby the cost of manufacture becomes high.
Also, the optical fiber disclosed in the above-mentioned publication has problems as follows in particular when the microstructures are bores. First, the mechanical strength of optical fiber is lowered due to the bores included therein, whereby strengths against tension and lateral pressures may decrease. Second, there is a possibility of absorption loss occurring due to OH group on surfaces of bores and water vapor within the bores. Therefore, during operations of making or fiber connection, a treatment for lowering the possibility of water vapor entering the bores is necessary, which makes the operations difficult. Third, if glass melts upon fusion splicing and thereby closes bores, then the effective refractive index difference between the core and cladding is lost, so that the optical power leaking out into the cladding remarkably increases, whereby propagation loss becomes greater at the fused part. The first and second problems become more influential as the number of microstructures increases.
On the other hand, the refractive index difference realizable in the impurity-doped type optical fiber disclosed in OFC'96 Technical Digest, ThA3 is small. As a result, realizable value ranges are restricted in terms of the magnitude of absolute value of negative dispersion, magnitude of absolute value of negative dispersion slope, size of effective core area, and reduction of bending loss.
The optical fiber disclosed in Electronics Letters, vol. 18, pp. 824-826 (1982) yields a large linear birefringence with “side tunnels” of air provided on both sides of its core. However, it is desirable that birefringence be smaller for optical transmission application, such as those in which the optical fiber is incorporated in a part of an existing optical transmission line in particular. If the polarization state of light incident on an optical fiber having a large birefringence does not match either of the principal polarization states of fiber, then transmission quality deteriorates due to polarization mode dispersion. Hence, a device for making the polarization state of incident light constant is necessary, which raises the cost. Also, most of existing optical transmission lines have no polarization selectivity, whereby the polarization state of light emitted therefrom is not constant. Thus, the polarization state of light having an inconstant polarization state is hard to keep constant.
In the air-clad optical fiber disclosed in U.S. Pat. No. 5,907,652, a chromatic dispersion having a large negative value and a chromatic dispersion slope having a large negative value are hard to obtain. This is because of the fact that this optical fiber is mainly aimed at lowering the effective refractive index of first outer cladding region, so as to prevent the second outer cladding region from influencing optical characteristics.
In view of such circumstances, it is an object of the present invention to provide an optical fiber which can realize a low chromatic dispersion (having a large negative value), a low chromatic dispersion slope(having a large negative value), a large effective core area, and a low bending loss. It is another object of the present invention to provide an optical fiber facilitating its making, cutting down its cost, improving its strengths against tension and lateral pressures, lowering the possibility of absorption loss occurring due to OH group on surfaces of bores and water vapor within bores, and reducing power loss at fusion splice.
For satisfying the above-mentioned objects, the present invention provides an optical fiber comprising a core region constituted by a substantially homogeneous medium; an inner cladding region surrounding the core region; and an outer cladding region, constituted by a substantially homogeneous medium, surrounding the inner cladding region; wherein the core region, inner cladding region, and outer cladding region are regions extending along a fiber axis and influencing optical characteristics; wherein an average refractiv

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