Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
1999-06-29
2001-02-13
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000, C385S124000, C385S143000, C385S145000
Reexamination Certificate
active
06188824
ABSTRACT:
This application is the national phase under 35 U.S.C § 371 of PCT International Application No. PCT/JP98/00475 which has an International filing date of Feb. 5, 1998 which designated the United States of America.
TECHNICAL FIELD
The present invention relates to an optical fiber. More particularly, it concerns a plastic optical fiber used as an optical signal transmission medium which is arranged around equipment such as personal computers, audio visual equipment, switchboards, telephones, office automation equipment and factory automation equipment.
BACKGROUND OF THE INVENTION
As disclosed in PCT International Patent Publication No. WO95/32442, there have been conventionally employed, as multicore plastic optical fibers for communication, a bare multicore plastic optical fiber comprising a plurality of cores of a transparent core resin having a high refractive index and a cladding arranged so as to surround and bundle the cores, a bare multicore plastic optical fiber prepared by coating each core fiber with a cladding resin to form a cladding layer and surrounding the cladded core fibers with a third resin to bundle them, and a multicore plastic optical fiber cable formed by coating the bare multicore optical fibers with a sheathing resin.
Single core plastic optical fibers formed by coating a core with a cladding resin in two layers are disclosed in Japanese Patent Application Laid-Open Nos. 204209/1987, 51206/1992 and 249325/1993. However, the fibers disclosed therein are single core plastic optical fibers, not multicore plastic optical fibers. Therefore, they have a large core diameter and insufficient light retention upon bending. In case of the conventional multicore plastic optical fibers, when the numerical aperture of the fibers (hereinafter referred to as Fiber NA) is decreased in order to widen signal transmission band width, and when the numerical aperture of incident light source (hereinafter referred to as LNA) is larger than Fiber NA, the fibers cannot receive a light beyond the Fiber NA. Accordingly, the conventional multicore plastic optical fibers have a drawback that the light requirement is small. Another drawback of the conventional multicore plastic optical fibers is light loss caused by bending the fibers. Usually, a multicore plastic optical fiber can reduce light loss caused by bending since each core diameter of the multicore plastic optical fiber can be made very small. Nevertheless, when Fiber NA becomes small, the light loss upon bending unpreferably becomes too large to ignore. Even in the case that Fiber NA is relatively large, a plastic optical fiber having smaller light loss upon bending is more preferable.
An object of the present invention is to provide such a plastic optical fiber with desirable transmission bandwidth that receives a larger quantity of light from a light source and causes less light loss upon bending the fiber.
DISCLOSURE OF THE INVENTION
The present invention relates to a multicore plastic optical fiber for signal transmission comprising 7 or more cores made of a transparent core resin, first cladding layers made of a transparent first cladding resin having a lower refractive index than said core resin, each of said first cladding layers coating said each core, and a second cladding resin surrounding said cores with said first cladding layers, and having a lower refractive index than said first cladding resin, wherein said multicore plastic optical fiber is manufactured by a composite spinning method.
The present invention further relates to a multicore plastic optical fiber, wherein said cores are coated with said first cladding layers to form islands, and said second cladding resin fuses to form a sea.
The present invention further relates to a multicore plastic optical fiber, wherein each of said cores coated with said first cladding layers is further coated with a layer of said second cladding resin to form islands, and a fourth resin surrounds said islands and fuses to form a sea.
The present invention yet further relates to a multicore plastic optical fiber which has the relation represented by the following equations:
Fiber NA≦0.45; and
n
CLAD1
−n
CLAD2
≧0.02
wherein Fiber NA represents a numerical aperture of the multicore plastic optical fiber and is defined by the equation, Fiber NA=(n
CORE
2
-n
CLAD1
2
)
0.5
; and n
CORE
, n
CLAD1
and n
CLAD2
represent refractive indexes of said core resin, said first cladding resin, and said second cladding resin measured at 20° C. using sodium D-line, respectively.
Namely, the difference between the multicore plastic optical fiber of the present invention and the conventional multicore plastic optical fiber is that the multicore optical fiber of the present invention employs two kinds of cladding resins whose refractive indexes differ stepwise. The first cladding directly surrounds cores, and it must have a refractive index corresponding to the band width of the optical fiber. In other words, the band width depends on Fiber NA defined by the square root of difference between the square of the refractive index of the core and the square of that of the cladding. Therefore, the smaller Fiber NA is, the larger the band width is. Although the effect in coating the cores with the first cladding resin surrounded by the second cladding resin is rather complicated, it may be explained as follows. If the cladding resins employed for a plastic optical fiber comprising a core, a first cladding and a second cladding are selected so as to satisfy the following relationship, a refractive index of cores>a refractive index of a first cladding resin>a refractive index of a second cladding resin, an incident light on such an optical fiber is at first transmitted therein at a relatively large angle to the fiber axis as if it proceeds in an optical fiber comprising a core and a second cladding. As the light further travels through the fiber (the fiber lengthens), it changes to a light at a smaller angle to the fiber axis as if the optical fiber comprises a core and a first cladding. Namely, an incident light on the cores at a relatively large angle penetrates the first cladding layer and travels through the optical fiber with full reflection at the boundary between the first and second cladding layers. However, since the light transparency of the first cladding layer is not as high as that of the core resin, the light traveling through the first cladding layer is absorbed and vanished, or is converted to a useful light, which fully reflects at the first cladding layer because of some change in reflecting angles, while the full reflection of the light at the boundary of the first and second claddings is repeated. The multicore plastic optical fiber of the present invention is thought to be such a fiber that changes an incident light thereto at a large angle to one within the predetermined Fiber NA as if it comprises a core and a first cladding while the light travels through a 5 meter long fiber. In this sense, an optical fiber having a single cladding is regarded as the optical fiber having a constant Fiber NA; contrary, a fiber having first and second claddings can be the optical fiber whose Fiber NA becomes smaller in the long direction of the fiber.
The optical fiber of the present invention is structured by coating each of 7 or more cores with a first cladding resin and further surrounding each of the cladded cores with a second cladding resin so as to bundle them and form a fiber. As shown in
FIG. 1
, the optical fiber of the present invention is a multicore plastic optical fiber comprising “islands” formed by coating each core with the first cladding resin layer and a “sea” of the second cladding resin layer surrounding the islands so as to bundle them. Further, for a special occasion, a fourth resin can be employed, for example, in order to improve the heat or chemical resistance of the fiber, and to impart optical shielding to each core. As shown in
FIG. 2
, the optical fiber of the present invention is a multicore plastic optical fiber comprising “islands” for
Asahi Kasei Kogyo Kabushiki Kaisha
Birch, Stewart,Kolasch & Birch, LLP.
Palmer Phan T. H.
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