Coated optical fiber and method of making the same

Details Coated optical fiber and method of making the same Coated optical fiber and method of making the same

1999-07-19

2001-01-30

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

C385S123000, C385S145000, C428S378000, C522S096000

Reexamination Certificate

active

06181859

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical fiber coated with two resin layers, and a method of making the same.
BACKGROUND ART
From the viewpoint of low transmission loss, silica glass fibers have been used as optical fibers for long-distance transmission. In order to maintain their transmission and mechanical characteristics in such optical fibers, a primary coating made of a relatively soft resin and a secondary coating made of a relatively hard resin are applied to the surface of the glass fiber.
Conventionally used is a coated optical fiber in which, at the same time when a glass fiber is drawn from an optical fiber preform, a primary coating resin is applied thereto and cured thereon so as to form a primary coating, and then a secondary coating resin is applied to and cured on the outer periphery of the primary coating so as to form a secondary coating. On the other hand, a recent study from the viewpoint of improving productivity is directed to a method in which both primary and secondary coating resins are simultaneously applied to the outer periphery of the glass fiber and are subsequently cured.
As such a method, for example, proposed is a method in which a plasticizer transferable between the primary and secondary coatings is added such that its transferred amount from the secondary coating to the primary coating is at least equivalent to the transferred amount in the opposite direction, thereby restraining the glass fiber and the primary coating from peeling off from each other (Japanese Patent Application Laid-Open Gazette No. Sho. 62-129805). Also proposed is a method in which the curing temperature of the primary coating resin is set higher than that of the secondary coating resin so that the primary coating resin cures before the secondary coating resin begins to cure, thereby restraining the glass fiber and the primary coating from peeling off from each other (Japanese Patent Application Laid-Open Gazette No. Sho. 63-315542).
In the former method of these conventional methods, however, the plasticizer preferentially added to the secondary coating may bleed, whereby the adhesion force at the interface between the secondary coating and coloring layer in the coated optical fiber tends to decrease. As a result, in a tape-like coated optical fiber, it becomes difficult to remove the coatings simultaneously, thus remarkably deteriorating the workability. In the latter method, on the other hand, since the resin is cured at a high temperature, its termination reaction becomes prevalent, thus increasing uncrosslinked low molecule ingredients. When its crosslinking density does not rise, its modulus of elasticity and breaking extension may decrease, whereby the function as a coating for attaining stable long-term protection tends to deteriorate.
It is an object of the present invention to provide a coated optical fiber in which primary and secondary coatings are simultaneously formed around the outer periphery of a glass fiber and in which the occurrence of of peeling can sufficiently be prevented at the interface between the primary coating resin and the glass fiber without preferentially adding a plasticizer into the secondary coating or without making the curing temperature of the primary coating resin higher than that of the secondary coating resin; and a method of making the same.
DISCLOSURE OF THE INVENTION
The coated optical fiber in accordance with the present invention is a coated optical fiber comprising a drawn glass fiber, and a primary coating (inner coating layer) and a secondary coating (outer coating layer) which are formed by photo-curing a primary coating resin (inner resin) and a secondary coating resin (outer resin) which are simultaneously applied to an outer periphery of the glass fiber, wherein an adhesion force S (g/cm) at the interface between the primary coating resin after curing and the glass fiber is at least {fraction (1/13)} ((g/cm)/° C.) with respect to a glass transition temperature Tg (° C.) of the secondary coating resin after curing.
The inventors have found that, in an optical fiber provided with primary and secondary coatings formed around the outer periphery of a glass fiber, in order to suppress peeling which occurs at the interface between the glass fiber and the primary coating, it is necessary to satisfy a relationship which will be explained in the following.
Namely, due to the heat of polymerization generated therein, usually the temperature of the coating resin is higher than the glass transition temperature thereof. When such a coating resin is cooled, it changes into a glassy state in the vicinity of the glass transition temperature as indicated in FIG.
1
. As it is further cooled from the glass transition temperature to room temperature; due to the difference in coefficient of linear expansion between the primary and secondary coating resins, stress is generated in the primary coating in the direction moving away from the glass fiber. This residual thermal stress causes the primary coating to separate from the glass fiber.
In accordance with the present invention, it is necessary for the adhesion force S at the interface between the primary coating and the glass fiber to be at least {fraction (1/13)} with respect to the glass transition temperature Tg of the secondary coating resin [(S/Tg)≧({fraction (1/13)})]. Namely, when the glass transition temperature of the secondary coating resin is low, the adhesion force between the primary coating and the glass fiber can be relatively small; whereas, the higher is the glass transition temperature of the secondary coating resin, the greater becomes the adhesion force needed between the primary coating and the glass fiber. The inventors have found that peeling is sufficiently prevented from occurring at the interface between the primary coating and the glass fiber when the adhesion force of the primary coating with respect to the glass fiber is thus selected in response to the glass transition temperature of the secondary coating resin. Accordingly, in the coated optical fiber of the present invention, a primary coating having an adhesion force greater than the peeling force occurring at the interface between the glass fiber and the primary coating is employed, thereby sufficiently restraining the peeling from occurring.
More specifically, at the point of time when a resin in a high-temperature state being polymerized is cooled to its glass transition temperature, a glassy-state portion appears at a certain part in the resin, thus restraining its molecular chain from moving. Consequently, the lower the glass transition temperature is, the smaller becomes the change in size during the process in which the resin is cooled from the glass transition temperature to room temperature, whereby a relatively smaller adhesion force can sufficiently restrain the peeling from occurring due to the residual thermal stress. On the other hand, the higher the glass transition temperature is, the greater becomes the change in size during the process in which the resin is cooled from the glass transition temperature to room temperature, whereby a relatively greater adhesion force is necessary for sufficiently restraining the peeling from occurring due to the residual thermal stress.
The method of making a coated optical fiber in accordance with the present invention comprises:
a first step of obtaining a glass fiber by heating, melting, and drawing a glass preform;
a second step of preparing a primary coating resin and a secondary coating resin such that an adhesion force S (g/cm) at the interface between the primary coating resin after curing and the glass fiber is at least {fraction (1/13)} ((g/cm)/° C.) with respect to a glass transition temperature Tg (° C.) of the secondary coating resin after curing;
a third step of simultaneously applying the primary coating resin and secondary coating resin to an outer periphery of the glass fiber; and
a fourth step of photo-curing the primary coating resin and secondary coating resin to yield the coated optical fiber.
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