GB1602052A

GB1602052A - Optical fibre manufacture

Info

Publication number: GB1602052A
Application number: GB21415/78A
Authority: GB
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1977-06-20
Filing date: 1978-05-23
Publication date: 1981-11-04
Also published as: AU521376B2; FR2395232A1; DE2826010A1; AU3709578A; ES470963A1

Description

(54) OPTICAL FIBRE MANUFACTURE (71) We, INTERNATIONAL STANDARD ELECTRIC COR PORATION, a Corporation organised and existing under the laws of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, State of New York, United States of America. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to optical communications fibres, and to the manufacture thereof.

When a force is applied to the linear axis of an optical fibre the tension at the fibre surface increases substantially. Although precautions are taken to keep dust particles and water from its surface, the fibre usually is somewhat abraided and micro-cracks exist on its surface. Since the fibre is under a state of constant tension, the crack propagates readily from the perimeter of the glass surface toward its core. As the crack proceeds from the fibre perimeter the glass fibre becomes substantially weakened and eventually fractures. The presence of water molecules on the glass perimeter enhances crack propagation resulting in early failure.

According to the present invention there is provided a glass optical communications fibre having a glass core surrounded by two or more glass layers, the outer surface of the outermost glass layer of which is in compression at room temperature.

Embodiments of the invention will now be described with reference to the drawings, in which: Figure 1 is a cross section of a glass-onglass optical fibre: Figure 2 is a graphic representation of the relation between the difference in expansion of the outer layers of the fibre of Figure 1 and its tensile strength; Figure 3 is a cross section of a high tensile strength fibre embodying the invention; Figure 4 is a further embodiment of the fibre of Figure 3; Figure 4A is a further embodiment of the fibre of Figure 4.

Figure 5 is a table listing the expansion properties of various materials for use with the fibre of Figures 3-5; Figure 6 is a graphic representation of the tensile and compression forces existing in the fibres of Figures 3-5; and Figure 7 is a graphic represeritation of the improved strength properties of the high tensile strength fibres according to the invention.

Figure 1 shows a typical glass-on-glass optical fibre 10 where the core 11 is a silica compound and the cladding 12 is a silica compound doped with an index of refraction reducing agent or pure silica per se. This invention provides greatly improved tensile properties to glass-on-glass optical fibres by putting the outer (cladding) glass layer in a state of compression. This is accomplished by overcoating the outermost glass layer 12 with a material having a coefficient of thermal expansion lower than that of the outer glass layer 12.

Figure 2 shows the relationship A between the difference in thermal expansion between the two outer-most layers of a glass-on-glass optical fibre and the resulting increase in tensile strength.

Since the thermal expansion coefficient of pure silica is relatively low, having measured values of 5.5 to 8x 10~7cm/ C, it is somewhat difficult to find materials having lower thermal expansion properties and to apply these materials to the surface of the silica layer.

Figure 3 shows a low thermal coefficient of expansion layer 13 over the outer cladding layer 12 of optical fibre 10 to put the cladding layer outer surface into compression. Since the coefficient of thermal expansion of silica has such a low value it is more effective to add more than one layer of materials to the optical fibre outer surface when silica is used as the outermost layer 12.

Figure 4 shows another optical fibre 10 having greater strength properties than that of the embodiment of Figure 3. The fibre 10 of Figure 4 contains a core I 1, a cladding 12, a layer of high thermal expansion material 14 and an outermost layer of low thermal expansion material 13. The presence of the underlying high expansion layer 14 brings the outer low expansion layer 13 into compression.

The optical fibre 10 of Figure 4A is a glass-on-glass optical fibre having a germanate glass core 11 and a borosilicate glass cladding 12 and it has an outer strength layer of pure silica 20 as used with fibres formed by chemical vaport deposition within a silica tube. The high and low expansion layers 14 and 13 are applied as follows: After depositing the germanate and borosilicate glasses within the silica tube and collapsing the tube to form a preform, the preform is kept at an elevated temperature and layer 14 is applied by depositing, for example, borosilicate glass to the outer surface of the preform and then depositing a layer of silica over the borosilicate glass to form the low expansion layer 13.

This method is advantageous for high purity glass-on-glass optical fibres since the high purity core and cladding glasses are deposited on the inside of the silica tube and the high and low expansion glasses are deposited on the outside of the silica tube.

The jacketing glasses do not have to have the same high degree of purity as the core and cladding glasses, so can be readily deposited without the precautions entailed with high purity deposition techneques.

Other methods for forming layers 13 and 14 also include inserting the preform within concentratic boro-silicate and silica tubes and drawing the composite into an optical fibre by the rod-in-tube method. The outer layers 13 and 14 can also be applied to the optical fibre 10 after the fibre drawing process rather than directly on the preform.

The layer 13 and 14 can also be applied separately during manufacture of the fibre 10. The layer 14, for example, can be directly applied to the preform and the layer 13 can be applied to the fibre 10 after drawing from the preform.

Figure 5 shows a table indicating various materials for use with the inner layer designated as 14 in Figures 4, 4A, and the outer layer designated as 13. The published expansion coefficient values are listed along with the difference in expansion that exists when the particular combination of materials are used for inner and outer layers (14, 13) respectively.

Figure 6 is a graphic representation of the relationship between the compressive and tensile forces existing at the interface between the outer layer 13 and the inner layer 14 of the optical fibre 10 of Figures 4 and 4A. The relationship between the compressive forces on the outer surface 13 is shown at 16, and indicates that the compression increase exponentially as a function of the thickness of coating 13, becoming asymptotic in value at the interface 15. The relationship between the tensile forces 17 existing in the inner coating 14 is shown as a function of the thickness of coating 14 relative to the interface indicated at 15 in Figure 6.

To determine the effective increase in tensile strength for optical fibres containing the dual coatings indicated and shown in figures 4 and 4A, the time to failure as a function of stress is indicated for contours of increasing differences in the thermal expansion coefficient between the materials constituting the outer layer 13 and the inner layer 14 of the embodiments of Figures 4 and 4A. Figure 7 shows that for a fixed stress applied to the optical fibre 10 of figures 4 and 4A, for relatively small differences existing in the thermal expansion coefficients of layers 13 and 14 (indicated at A) the fibre fails in a relatively short period of time. Larger differences existing in the coefficient of thermal expansion between layers 13 and 14 are shown at B where for the same stress applied to the fibre 10 the time to failure is increased. Figure 7 indicates that the longest time to failure for a given applied stress to the optical fibre 10 occurs at C having the greatest difference between the inner and outer layers 14, 13.

Figure 5 lists glass-like materials for use with glass-on-glass optical fibres.

Glass materials are conveniently chosen for the outer 13 and inner 14 layers of glasson-glass optical fibres because of the convenience and expense involved in the use of glass materials as listed. Various nonglass compositions can also be employed.

Metals generally have a higher coefficient of thermal expansion than that of glass.

When the metal is applied to the glass optical fibre outer surface, the metal will cause the underlying glass material to be forced into a state of high compression.

Combinations of outer layers and subjacent layers of metals and glass materials can also be employed. For the embodiment of Figure 4, for example, the higher coefficient of expansion layer 14 can comprise a metal and the outer lower thermal of expansion layer 13 could comprise glass.

Although the methods and structures of this invention are described as being applied to optical communication fibre to improve the tensile properties of these fibres, this is by way of example only. The invention finds application wherever high strength glass fibres may be employed.

WHAT WE CLAIM IS: 1. An optical communications fibre having a glass core surrounded by two or more glass layers, the outer surface of the outermost glass layer of which is in compression at room temperature.

2. An optical fibre as claimed in claim 1 wherein the outer surface of said outmost glass layer is maintained in compression at room temperature at least in part by the use of a glass composition for the outermost layer which has a lower coefficient of thermal expansion than that of one or more of the glass layers that it encloses.

3. An optical fibre as claimed in claim 2 wherein the outermost glass layer encloses a layer of higher coefficient of thermal expansion which in its turn encloses a silica layer.

4. An optical fibre as claimed in claim 1 wherein the outer surface of said outermost glass layer is maintained in compression at room temperature at least in part by the use of a glass composition for the outermost glass layer that has a lower coefficient of thermal expansion than that of a non-glass layer within which it is enclosed.

5. An optical fibre as claimed in claim 4 wherein said non-glass layer is a metal layer.

6. An optical fibre as claimed in claim 5 wherein said metal layer is made of aluminium or tin.

7. A method of making an optical fibre including the step of applying, to the outer surface of an optical fibre preform including a glass core surrounded by a layer of cladding glass of lower refractive index, a compression layer of glass having a coefficient of thermal expansion lower than that of the glass providing said outer surface of the preform, and the step of drawing the coated fibre preform into optical fibre, wherein the relative thicknesses and coefficients of thermal expansion of the compression layer glass and the component glasses of the preform are such that the outer surface of the compression layer, which forms the outermost glass layer of the fibre, is maintained in compression at room temperature.

8. A method of making an optical fibre including the steps of applying a coating of optical cladding glass to the bore of a silica substrate tube and then a layer of optical core glass having a higher refractive index than that of the cladding glass, of collapsing the bore of the internally coated tube to form a solid cross-section optical fibre preform, of coating the outer surface of the preform with first and second glass layers having respectively relatively higher and relatively lower coefficients of thermal expansion, and of drawing fibre from the coated preforms, wherein the relative thicknesses and coefficients of thermal expansion of the layers forming the coated preform are such that the outer surface of the drawn fibre is in compression at room temperature.

9. A method of making an optical fibre as claimed in claim 7 and substantially as hereinbefore described with reference to the accompanying drawings.

10. An optical fibre made by the method claimed in claim 7, 8 or 9.

11. An optical fibre as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the tensile properties of these fibres, this is by way of example only. The invention finds application wherever high strength glass fibres may be employed. WHAT WE CLAIM IS:

1. An optical communications fibre having a glass core surrounded by two or more glass layers, the outer surface of the outermost glass layer of which is in compression at room temperature.

10. An optical fibre made by the method claimed in claim 7, 8 or 9.