Optical fiber

Details Optical fiber Optical fiber

2002-07-09

2003-09-16

Lee, John D. (Department: 2874)

Optical waveguides

Having particular optical characteristic modifying chemical...

Of waveguide core

C385S123000, C522S178000

Reexamination Certificate

active

06621977

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved optical fiber comprising a core made of a polymer obtained by polymerizing a polymerizable stock material containing a polymerizable hydroxy monomer, and specifically it relates to an optical fiber with excellent coloring resistance and humidity resistance properties.
BACKGROUND OF THE INVENTION
Optical fibers comprising polymer cores have been utilized in the prior art not only as optical signal transmission media for optical communications, but also as optical transmission media or light emission media for decorative and illumination purposes. In particular, for decorative purposes it has become common to use large-aperture optical fibers, having relatively large core apertures, i.e. large dimensions in the direction orthogonal to the lengthwise direction of the core (for example, a diameter of 3 mm or greater).
Such optical fibers are usually constructed of a hollow tube-shaped cladding and a core made of a light-permeable polymer with a higher refractive index than the cladding. The core is situated within the cladding, and they are usually bonded together. Suitable materials for the core include (meth)acrylate-based polymers (and copolymers) that have high transparency. The cladding may be formed from a fluorine-based polymer. The core aperture, i.e. the core dimension in the direction orthogonal to the lengthwise direction, refers to the diameter of a near-circle of the cross-sectional diagram in the direction orthogonal to the lengthwise direction of the core.
Such optical fibers comprising a core and cladding and production processes therefor are disclosed, for example, in Japanese Unexamined Patent Publication No. 57-45502, Japanese Unexamined Patent Publication No. 7-168029 and elsewhere.
For example, in Japanese Unexamined Patent Publication No.7-168029 there is disclosed a “sequential polymerization method” whereby the cladding is packed with a (meth)acrylic monomer and sequential polymerization is carried out from one end to the other in the lengthwise direction of the cladding. In this sequential polymerization method, the monomer is usually pressure-packed into a tube-shaped cladding that is sealed at one end, and the monomer-packed cladding is fixed in a heated tank and heated to polymerize the monomer for production. Here, the heating temperature (i.e. the temperature of the heat medium in the heating tank) is increased from the one end to the other for sequential polymerization of the monomer.
Incidentally, for large-aperture optical fibers, it is preferred for the glass transition temperature (T
g
) of the core to be as low as possible in order to improve the flexibility and improve the handling properties such as bending processability and bending workability. However, using a polymer with a relatively low T
g
in order to reduce the T
g
of the core tends to result in a hygroscopic property phenomenon under high temperature and high humidity. Cores that have thus absorbed humidity, when placed in areas of low temperature, produce condensation that results in clouding that can be externally seen as white fogginess. Clouding hampers transmission of light, i.e. it lowers the transmission efficiency. This is commonly known as the phenomenon of reduced humidity resistance.
In order to improve the humidity resistance, Japanese Unexamined Patent Publication No. 64-15707 discloses an optical fiber comprising a core with improved humidity resistance, formed by copolymerizing (i) an n-butylmethacrylate monomer and (ii) a polymerizable hydroxy(meth)acrylate monomer with a hydroxyl group in the molecule. In this patent publication, the n-butylmethacrylate is used as an essential component of the copolymerizable monomer. In cores consisting of a copolymer obtained from a starting substance (stock material) containing the hydroxy(meth)acrylate monomer, the hydroxyl group introduced into the copolymer has the effect of improving the humidity resistance.
SUMMARY OF THE INVENTION
In optical fibers with a relatively low aperture (for example, a diameter of less than 3 mm), it has been necessary to add comparatively large amounts of the hydroxy monomer (for example, at least 15 wt % in the stock material in the aforementioned publication). If the hydroxy monomer is added in too small an amount, the improvement in humidity resistance is inadequate. This is because the smaller minimum dimension in the direction orthogonal to the lengthwise direction of the core compared to those of relatively larger aperture results in more rapid humidity absorption (moisture penetration) to the center of the core, causing earlier saturation by humidity absorption. However, addition of the hydroxy monomer in a large amount was found to cause coloration of the core. When the core interior is visually observed from one end of the core of an optical fiber with a length of 10 m or greater, this coloration is a phenomenon which a colored light is seen even if the light incident (introduced) to the core from the other end is white light. If such coloration phenomenon is considerable, it becomes a particular problem for practical use in the fields of decoration and illumination.
On the other hand, in optical fibers with a relatively large aperture, it has been necessary to control the T
g
of the core to a comparative low range to match the aperture, in order to improve the manageability (to ensure relative flexibility). When such control of the T
g
to a lower range is attempted by decreasing the proportion of the main monomer such as n-butylmethacrylate and increasing the amount of the hydroxy monomer which is a relatively low T
g
homopolymer, the same sort of core coloration occurs.
The present invention provides an optical fiber with improved coloring resistance (a property of resistance to the aforementioned coloration), without lowering humidity resistance or flexibility. The present invention provides an optical fiber comprising a core capable of transmitting incident light from one end in the lengthwise direction toward the other end, the core consisting of a polymer of a polymerizable stock material that contains a polymerizable hydroxy monomer with a hydroxyl group in the molecule, the optical fiber being characterized in that the content of the polymerizable hydroxy monomer is 0.5 to 9 wt % with respect to the total weight of the polymerizable stock material, and the minimum dimension (R) in the direction orthogonal to the lengthwise direction of the core and the glass transition temperature (T
g
) of the core have a relationship represented by the following Inequality 1:
117/
R+
23
<T
g
<1050/
R
2
+39  (Inequality 1)
where the unit of R is mm, and the unit of T
g
is ° C.
DETAILED DESCRIPTION OF THE INVENTION
Optical Fiber
The function and effect of the optical fiber of the invention will now be explained in detail.
The optical fiber of the invention comprises a polymer formed by polymerizing a polymerizable stock material containing a polymerizable hydroxy monomer (throughout the present specification this will be referred to either as “polymerizable hydroxy monomer” or “hydroxy monomer”) having a hydroxyl group in the molecule, wherein the hydroxy monomer content is controlled to 0.5 to 9 wt % with respect to the total of the polymerizable stock material. This can provide effective improvement in the coloring resistance without excessively lowering the humidity resistance or flexibility. If the hydroxy monomer content is too low it will not be possible to effectively improve the humidity resistance, whereas if it is too high the core will be more susceptible to coloration, thus lowering the coloring resistance. With these considerations, the hydroxy monomer content is preferably controlled to the range of 0.7 to 8 wt %, and especially 0.9 to 7.5 wt %, with respect to the total of the polymerizable stock material.
The optical fiber of the invention is particularly suitable as a large-aperture optical fiber having a relatively large aperture. The aperture, or diameter, of the core is normally

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