CA1155642A

CA1155642A - Thermally aided stripping of optical fiber jackets

Info

Publication number: CA1155642A
Application number: CA000371439A
Authority: CA
Inventors: Glen W.D. Dack; Chu-Chay Tan
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1981-02-20
Filing date: 1981-02-20
Publication date: 1983-10-25

Abstract

THERMALLY AIDED STRIPPING OF OPTICAL FIBER JACKETS

Abstract of the Disclosure A protective jacket around an optical fiber is rendered more easily strippable by heating the fiber to a temperature causing differential expansion of jacket and fiber and consequent breakage or weakening of adhesive bonding between the fiber and its jacket.
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Description

-` 1155642 This invention relates to a method and tool for stripping a jacket from an optical fiber.
Optical fibers for use in telecommunications systems consist usually of a central core of a relatively high refractive index glass, a cladding around the core of a relatively low refractive index glass and a protective jacket. Typically, the fiber jacket is a polymer such as a fluoropolymer, polyurethane, a silicone or an acrylate which is applied to the cladding surface usually either by extrusion or dipping. The coating which may be anything from a few microns to few hundred microns is applied immediately after the fiber is formed and protects the underlying glass from abrasion, contamination and propagation of microcracks.
When terminating an optical fiber at an electro-optic device such as a laser or photodetector, and when interconnecting or splicing fiber ends together, it is necessary first to bare the fiber end by removing an end piece of its jacket.
The polymer jacket bonds to the underlying glass and usually cannot be easily removed by purely mechanical means without stressing the fiber or leaving residual pieces of the jacket sticking to the fiber. The conventional method of removing the jacket is by using a solvent, for

2~ example, tetramethylguanidine for room temperature vulcanizing (RTV) silicone rubber or methyl chloride for ultraviolet curable urethane acrylate. The solvents act to soften the sheath and render it more easily strippable. Liquid chemicals for stripping fiber jackets are difficult to handle and often are toxic. Moreover, the stripping process, if it includes cleansing of the bared fiber ends is time consuming. A method of stripping fibers which is rapid, easy and which renders a clean fiber end is now proposed by the present invention for jacketed fiber in which the ~' jacket material has a greater thermal coefficient of expansion than the material of the fiber. The method comprises heatins a jacketed fiber portion to a temperature at which the consequent thermal mismatch caused by a differential expansion at the interface of the fiber and the jacket promotes breakage and weakening of adhesive bonds between the fiber and the jacket, and the method further comprising gripping the jacket portion and pulling it from said fiber portion.
Usually the fiber portion heated to be bared will be a fiber end portion and after heat treatment, the loosened jacket portion can be merely slid off the end of the fiber. However if an intermediate part of a fiber should need to be bared, then following heat treatment the jacket portion can be ruptured to detach it from the underlying portion of the fiber. According to another aspect of the invention a tool for stripping a part of d protective jacket banding to a portion of an optical fiber comprises a heat conducting body having a bore extending therein, the bore having a diameter marginally greater than the diameter of a jacketed fiber to be stripped, the block having heating means associated therewith for heating the body to a temperature causing differential expansion at the interface of the fiber and the jacket to reduce adhesive bonding therebetween, the tool having a thermally insulating covering around the body permitting the tool to be manually held or clamped in place.
The body can comprise a pair of concentric cylindrical copper members having a resistive heating element located between them.
- The bore can have a belled orifice to aid insertion of a fiber into the bore. The thermally insulating covering is preferably polybenzimidazole (PBI).

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An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:-Figure 1 is a perspective view with part cut away of aheating tool according to the invention; and Figure 2 shows operation of removing a heated optical fiber jacket portion.
Referring in detail to Figure 1, the tool 10 comprises a central copper cylinder 11 having a 450 micron diameter bore extending axially therethrough. The block has a belled orifice 12 for aiding insertion of 300 micron outer diameter jacketed fiber. The cylinder 11 has a resistive heating element 16 wrapped around it. Concentric with the inner cylinder is an outer copper cylinder 18 and a thermally insulating covering 20 of po1ybenzimidazole (PBI).
In operation a jacketed fiber end portion is inserted into the bore and is maintained there while current is supplied to heat the element. For a RTV silicone jacket, 90 ~m in thickness, surrounding a 125 micron outer diameter fiber, a temperature of 350C for a period of 10 seconds has been found sufficient to cause sufficient differential expansion of the jacket 24 around the fiber 26 that adhesive bonds between the jacket and the fiber are weakened or broken sufficiently for the heated jacket portion subsequently to be easily mechanically stripped from the underlying fiber 26. The silicone has a coefficient of thermal expansion of approximately 300 x 10 6/oC whereas that of glass is approximately 0.6 x 10 6/oC. Urethane acrylate which is also a common jacket material has a coefficient expansion of 200 x 10 6/oC which is somewhat less than that of silicone but is still high enough that there will be thermal mismatch between the glass and the urethane acrylate which can weaken or break the adhesive bonds between jacket and fiber. It will be appreciated that there is differential expansion both axially and radially. The actual temperature to which the portion of the optical fiber jacket is raised is not critical but it should be sufficiently high to maximize differential expansion yet not high enough that, for example it becomes brittle since this would lessen the chance of it being pulled cleanly from the fiber as a single jacket section. Additionally, at high temperatures the jacket may ignite or bake onto the fiber thus increasing the difficulty of removing it. The property of the jacket compound to inhibit microcrack propagation within the fiber is destroyed and at such high temperatures there is an increased tendency for such crack propagation.
As shown in Figure 2 the heated jacket portion 22 is readily stripped after extracting the fiber from the tool of Figure 1 simply by gripping the jacketed fiber in one hand and the jacket end portion in the other and pulling the two apart. The exposed bare fiber surface 28 is found to be quite clean and free of contamination. To ensure scrupulous cleanliness, precleaned grips (not shown) may be used.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for stripping a portion of a protective jacket bonded to an optical fiber, the jacket material having a greater thermal coefficient of expansion than the material of the fiber, the method comprising heating the portion of the jacketed fiber, to promote sufficient thermal mismatch at the interface of the fiber and the jacket to cause breakage and weakening of adhesive bonds between the fiber and the jacket along the length of said portion, the method further comprising gripping said jacket portion and pulling it from the underlying fiber portion.

2. A method as claimed in claim 1, specifically for removing an end portion of the fiber jacket, in which, following heating of the jacket end portion, said jacket end portion is pulled axially off the end of the fiber.

3. A tool for stripping a part of a protective jacket bonded to an optical fiber, the jacket material characterized by a higher thermal coefficient of expansion than the material of the optical fiber, the tool comprising a heat conducting body having a linear bore extending thereinto, the bore having a diameter marginally greater than the diameter of a jacketed fiber to be stripped, the block having heating means associated therewith for heating the body to a temperature causing differential expansion at the interface of the fiber and the jacket to markedly reduce adhesive bonding therebetween, the tool having a thermally insulating covering around the body permitting the tool to be manually held.

4. A tool as claimed in claim 3, the body including a first cylindrical block having a bore extending generally axially therethrough.

5. A tool as claimed in claim 3, in which an outer end of the bore is of greater diameter than, and tapers towards, a remaining part of the bore.

6. A tool as claimed in claim 4, including a second cylindrical body concentric with said first cylindrical body, the first and second cylindrical bodies having said heating means disposed therebetween.

7. A tool as claimed in claim 6, in which the heating means is a resistive heating element surrounding said first cylindrical body.

8. A tool as claimed in claim 3, in which said bore extends through the block.

9. A tool as claimed in claim 3, in which the bore is closed at one end.