GB2217037A

GB2217037A - Optical fibre coated with epoxy acrylate, silicone and plastics

Info

Publication number: GB2217037A
Application number: GB8905943A
Authority: GB
Inventors: Francois Fort
Original assignee: Fort Fibres Optiques Rech Tech; Fort Fibres Optiques Recherche et Technologie
Current assignee: Fort Fibres Optiques Rech Tech; Fort Fibres Optiques Recherche et Technologie
Priority date: 1988-03-18
Filing date: 1989-03-15
Publication date: 1989-10-18
Anticipated expiration: 2009-03-15
Also published as: GB2217037B; GB8905943D0; DE3907115A1; FR2628847B1; FR2628847A1; IT8919644A0; IT1228566B; DE3907115C2

Description

1 1 22117 0 3 7 AN OPTICAL FIER INCLUDING A MULTI-LAYER COATING, AND A

METHOD OF MAKING SUCH A FIBER

The present invention relates to an optical fiber with a multi-layer coating, and to a method of making such a fiber. BACKGROUND OF THE INVENTION

The invention relates to optical fibers constituted by a core of doped silica inside optical cladding also made of silica, and it is conventional to provide such a silica fiber with a coating made of epoxy acrylate resin. It is essential for this coating to be applied either immediately after the optical f iber is drawn, or, more generally, simultaneously with the fiber-drawing operation. It is needed to prevent the drawn quartz oxidizing. Simultaneously, the above-mentioned resin serves to plug micropores present in the fiber which become, over time, points from which a break in the fiber may be initiated.

In addition to these two protective functions, i.e. protection against oxidizing and plugging micropores, the epoxy acrylate coating provides an additional mechanical protective function by virtue of the excellent mechanical properties of this material.

However, this coating suffers from the major drawback of being soluble in water and sensitive to humidity, thereby causing the epoxy acrylate protecting the fiber to be destroyed or to swell, thus making the fiber permeable to OH- ions and giving rise to a degree of fragility over a period of time which is unacceptable for fiber use.

As a result, an optical fiber coated with epoxy acrylate is normally disposed in microtubes associated with petroleum jelly in order to protect the fiber from the undesired effects of dampness or humidity.

However, such a structure presents extremely poor mechanical strength by virtue of the protective microtube which is also subject to kinking and which presents a degree of elasticity which lends itself poorly to providing effective protection to the fiber.

2 In addition, the protective system by means of a microtube associated with petroleum jelly is unsuitable for obtaining a very high density of fibers (i.e. a large number of fibers per cable), because of the diameter of the microtube which is needed in order to allow the jelly to be deposited.

By way-of example, for an optical fiber having a diameter of only 250 pm, the tubes have a diameter of 1.5 mm to 2.5 mm.

The present invention thus has the object of providing an optical fiber constituted by a doped silica core with silica optical cladding, and comprising a first coating of epoxy acrylate resin and a second coating for protecting the epoxy acrylate, thereby providing a more satisfactory solution to practical requirements than has been provided by prior art fibers of the same type, and in particular a fiber in accordance with the invention:

is extremely tough and as a result may be treated as though it were a small cable; is completely proof against water and humidity; and provides better exploitation of optical cables made from bundles of fibers in accordance with the invention. SUMMARY OF THE INVENTION

The present invention provides an optical fiber constituted by a core of doped silica with an optical cladding of silica, and which is provided with a first coating of epoxy acrylate resin and a second coating for protecting the epoxy acrylate, wherein the second coating is constituted by a film of silicone which sets under ultraviolet (UV) radiation (i.e. UV radiation causes curing to take place as it does for the epoxy acrylate resin), and the fiber is also provided with a third coating constituted by a film of rigid plastic material having appropriate mechanical and thermal properties and surrounding the silicone coating.

The present invention also provides a method of manufacturing such an optical fiber constituted by a core of doped silica and an optical cladding of silica, and provided with a first coating of epoxy acrylate resin for protecting the fiber and a second coating for protecting the epoxy acrylate, the method comprising the following steps:

3 le 35 the fiber is drawn in a fiber-drawing oven; said first coating of epoxy acrylate resin is deposited and cured (i.e. hardened) using UV radiation; said second coating is deposited in the form of a film of silicone, and said film is UV cured; and a third coating is deposited in the form of a film of rigid plastic material having appropriate mechanical and thermal properties, which film surrounds the second coating of silicone.

In addition to the above dispositions, the invention includes other dispositions which appear from the following description.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from the additional description below which is made with reference to the accompanying drawing in which the sole figure is a cut-away perspective view of an optical fiber including coatings in accordance with the invention.

MORE DETAILED DESCRIPTION

Reference 1 designates an optical fiber whose diameter is about 125 pm, for example, and which is constituted by a core of doped silica in an optical cladding of silica (not shown in detail in the drawing).

The fiber 1 has a first coating constituted by a film 2 of epoxy acrylate resin with the thickness of the film being such that the diameter of the above-described fiber when fitted with said first coating is about 250 pm, for example, with epoxy acrylate being known for its excellent mechanical properties, as means for protecting the fiber-constituting quartz from oxidizing (while fiber-drawing is taking place), and as an effective means for plugging micropores present in the fiber, as well as being curable, i.e. hardenable, on exposure to ultraviolet (UV) radiation.

In accordance with the invention, the layer of epoxy acrylate 2 is surrounded by a second coating (intermediate coating) oonstituted by a film of silicone 3 whose thickness is such that the diameter of the fiber 1 provided with both coatings 2 and 3 is about 410 pm, for example.

4 In addition, the fiber carries a third coating 4 constituted by an outer sheath of plastic material enabling the overall fiber to be extremely tough, such that it may be treated like a genuine small cable (or "microcable"). To this end, it is advantageous, but not essential, to use polyetherimide whose traction strength is 160 N/mm2 and whose shock strength (Gardner impact test) is 36 N/mm, and which withstands temperatures lying in the range -55"C to +150"C. This material is also remarkable for its extremely high resistance to creep.

The thickness of the layer of plastic material is such that the fiber when fitted with all three above-described coatings has a total diameter of about 1000 pm, i.e. 1 mm, which diameter is considerably smaller than the diameter obtained in the prior art by protecting the same fiber with its epoxy acrylate coating by means of a microtube and petroleum jelly (see the above-mentioned description of the prior art).

As a result, a fiber with a multilayer coating in accordance with the present invention:

is extremely tough due to the excellent mechanical properties of the layer of epoxy acrylate resin; is completely proof against humidity and water, by virtue of the layer of silicone which protects the fiber from humidity (i.e. against OH- ions contained in damp agents), and also against other possible external chemical agents; and is extremely strength, thereby enabling a f iber having a total outside diameter of 1 mm (as explained above) to withstand a traction of close to 19.6 daN (20 kg) and a radial load of 29.4 daN (30 kg) over 1 cm of fiber generator line.

In order to complete the picture of the mechanical proper- ties of a fiber in accordance with the invention (and having the geometrical characteristics defined above), it should also be specified that when subjected to traction of 15 daN, the change in transmitted signal loss is not greater than 0.1 dB/km and is reversible. Cable elongation is less than 0.5%.

In addition, increasing loss and elongation continue to remain reversible up to a maximum force of 20 daN.

Further, a transverse crushing force of 20 daN per cm of generator line of the fiber gives rise to a change of loss of less than 0.1 dB when the force is applied for 15 minutes.

The minimum static radius of curvature without degrading the optical characteristics of the fiber is 10 mm, and for twisting at one turn per meter of jumper, loss variation remains less than 0.1 dB and is reversible.

The materials used for coating the fiber in accordance with the invention satisfy technical specification CM26, i.e.

they do not propagate flame or fire.

Friction force testing was performed by winding contiguous turns of a sample of the fiber of the invention helically about a diameter of 100 mm. A second sample was passed over this sheet of jumper lines with a load of 50 grams at one of its ends. Thereafter, displacement relative to applied load was noted.

The optical properties of the fiber of the invention in base band and its attenuation per unit length are such that it is possible, for example, to meet the technical specifications of distribution fibers, namely:

a passband of 200 MHz/km, and attenuation of not more than 4 dB/km, at a wavelength of 850 nm; and a passband of 200 MHz/km and an attenuation of not more than 1.5 dB/km, at a wavelength of 1300 run.

In general, it can be said that the optical properties concerning attenuation and passband of fibers fitted with coating in accordance with the invention are not changed by the presence of said coating, which coating not only protects the, fiber against any kind of stress, but is equally applicable to monomode fibers and to multimode fibers.

1 11 A fiber in accordance with the invention withstands a temperature range of -55C to +150C without permanent degradation of its optical and mechanical properties, with a total variation of - 0.70 dB to + 0.15 dB, as has been demonstrated by tests performed using a fiber in accordance with the invention having a length of 50 meters (m) and wound helically about a diameter of 80 cm.

6 The method of manufacturing a fiber in accordance with the invention consists in implementing the following steps.

the fiber is drawn in a fiber-drawing oven; a first coating of epoxy acrylate resin is deposited thereon (in particular by immersion coating) and the first coating is cured (i.e. hardened) under UV radiation; a second coating is deposited (in particular by immersion coating),said second coating being constituted by a film of silicone and being cured under UV radiation; and a third coating constituted by a film of rigid plastic material is deposited, said material surrounding the second silicone coating.

The advantage of this method is that the coating for protecting the epoxy acrylate is made of a material which is cured by exposure to UV radiation as is epoxy acrylate, thereby enabling the fiber to be drawn at high speed by virtue of the immediate polymerization which takes place in the presence of a source of UV light.

Thus, as can be seen from the above, the invention is not limited to the particular embodiment and implementation specifically described above. On the contrary, the invention covers any variant that may occur to the person skilled in the art without going beyond the scope of the present claims. In particular, it should be understood that the numbers specified relating to optical fiber diameter and corresponding to various different protection layers building up the coating of the invention (and thus relating to the thicknesses of these layers for a fiber of specified diameter) are given merely by way of non-limiting examples of the invention. 30 Further, although the sources of UV radiation have not been specified, they could naturally be constituted by at least me UV lighting strip operating at an appropriate power level. Further, it should be underlined that because of the good mechanical and thermal strength of the "microcable" constituted by an optical fiber fitted with composite coating in accordance with the invention, the mmicrocable" can be conveyed as such in an optical cable comprising a bundle of fibers which are coated 1 7 in accordance with the invention (i. e. a bundle of "microcables") without there being any need to provide each fiber ("microcable") of the bundle with additional coating. This means that these "microcables" can be taken immediately to electronic equipment and connected thereto without requiring welding or branching boxes, thereby obtaining considerable savings in time and money.

8

Claims

CLAIMS l/ An optical fibe3; constituted by a core of doped silica and an

optical cladding of silica and fitted with a first coating of epoxy acrylate resin and a second coating for protecting the epoxy acrylate,includes the improvement whereby the second coating is constituted by a film of silicone which is cured by the application of ultraviolet (UV) radiation, as is the epoxy acrylate resin, and whereby the fiber is also fitted with a third coating constituted by a film of plastic material whose traction strength is not less than 160 N/mm2 and whose impact strength is not less than 36 N/mm; and which withstands low temperatures down to at least -550C and high temperatures up to at least +150"C, which film surrounds the silicone coating.

2/ A fiber according to claim 1, wherein the plastic material constituting the third coating is polyetherimide.

3/ A method of manufacturing an optical fiber constituted by a doped silica core with optical cladding of silica and fitted with a first coating of epoxy acrylate resin for protecting the fiber and a second coating for protecting the epoxy acrylate, the method comprising the following steps: drawing the fiber in a fiber-drawing oven; depositing said first coating of epoxy acrylate resin and curing said first coating by means of UV radiation; depositing said second coating which is constituted by a film of silicone, and curing said film using UV radiation; and depositing a third coating constituted by a film of plastic material having a traction strength of not less than 30 160 N/mm2 and an impact strength of not less than 36 N/mm, and which withstands low temperatures down to at least -550C and high temperatures up to at least + 150"C, which film surrounds the second coating of silicone.

4/ A method according to claim 3, characterized in that the plastic material constituting the third coating is polyetherimide.

9 5/ An optical cable constituted by a bundle of microcables each of which is constituted by a coated optical fiber according to claim I and obtained using the method of claim 3.

Published 1989 atThe Patent O:Moe. State House, 66171 High HoIbornLondonWC1R 4TP. Purther copies maybe obtainedfrom The Patent Office. Sales Branch, St. Mary Cray, Orpington, Kent BR5 3RD. PrInted by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87