Method of producing internally coated glass tubes for the drawing of fibre optic light conductors

Abstract

In the reactive deposition of the core material from a gas which is passed through the tube onto the inner wall of the tube by means of a plasma zone, while a relative motion is effected in the axial direction between the tube and a plasma-producing device, the rate of precipitation is increased without impairing the quality of the core material coat, the reactive deposition being effected at a pressure of from 1 to 100 Torr and a temperature zone being superimposed on the plasma zone.

Description

BACKGROUND OF THE INVENTION

This is a continuation of application Ser. No. 610,570, filed Sept. 5, 1975, now abandoned.

The invention relates to a method for producing internally coated glass tubes, consisting of a core and a jacket of glasses which have a mutually different refractive index, by means of a reactive deposition of the coating from a gas mixture which is passed through the tube and which is brought to reaction in the tube.

The tubes produced in this manner are heated to a temperature which is suitable for drawing and thereafter drawn to such an extent that the diameter is reduced until the coating is brought to coincidence and a light conductor of the required diameter is obtained.

Light conductors consist of a light-conducting core which is embedded in a jacket of a lower refractive index. The core may, for example, consist of quartz glass which has been doped with a few percent of a metal oxide which increases the refractive index and the jacket of undoped quartz glass.

For the doping of the core glass TiO₂, GeO₂ and Al₂ O₃ may, for example, be used. In the so-called self-focussing fibre optic light conductors a parabolic change in the refractive index across the radius is obtained by means of a continuous change in the grades of doping. According to a known method such internally coated quartz glass tubes are produced in which gaseous SiCl₄ and oxygen or a mixture of SiCl₄, TiCl₄ and oxygen are passed through a tube brought there to reaction in the gas phase by means of high frequency energization and probably precipitated at least partly as a soot-like glass coat, which must thereafter be melted or sintered. There is a danger that gases are trapped which later on might form light-scattering centers. The heat treatment makes the formation of a doping profile as required for self-focussing fibre optic light conductors difficult, owing to blurring due to diffusion.

The tube may consist of non-doped quartz glass. In this method a uniform relative motion in . The resonator 3 was moved forward and backward along the tube in this test at 60 cm/min.

EXAMPLE III

A mixture of 0.4 volume % AlCl₃, 4 volume % SiCl₄ and .Badd.95.6 volume % oxygen was passed through the quartz tube at a throughput of 42 scm³ per minute (length and diameter as in Example I). The pressure in the tube 1 was 15 Torr. The wall temperature of the tube 1 was kept at 950°C A plasma 4 as in Example I was produced. (Power 180 W, frequency 2.45 GHz). The reaction efficiency was approximately 100%. The tube was passed through the device 2-3 at a speed of 60 cm per minute while the resonator 3 was moved forward and backward along the tube 1. A homogeneous, adhering coat 5 was obtained. The total thickness of the coating was 150 μm.

FIG. 2 shows the total attenuation in dB per km as a function of the wavelength in micrometer of a fiber optic light conductor which was obtained by drawing at 1900°C of an internally coated tube according to Example II. The core diameter was 25 μm and the fiber diameter was 100 μm. The difference in the refractive indexes were approximately 5 o/oo.

By means of the method according to the invention a coating profile which has a certain refractive index in proportion to the doping can be obtained as shown above at a progressive change of the doping share. When a suitable profile is chosen the tube forms in an ideal manner a basic product for the production of monomode, multimode and self-focussing fiber optics.

Dopant-forming compounds which may be used in the method according to the invention are, for example, GeCl₄, TiCl₄, and AlCl₃ which oxidize to form the dopants GeO₂, TiO₂, and Al₂ O₃, respectively.

Claims

1. A method of producing internally coated glass tubes for drawing fiber optic light conductors which consists of a core and a jacket of glasses which have a mutually different refractive index, comprising the steps of introducing into a glass tube surrounded by a resonator a reactive gas mixture consisting of SiCl₄ and oxygen at a pressure of about 1 to 100 Torr, adding GeCl₄ to the gas mixture moving the tube relative to the resonator to form a non-isothermal plasma zone within the tube, and heating the tube to a temperature between 800°C-1200° C. to form a coating free of soot-like particles and consisting of a plurality of layers of sio₂ doped with an increasing content of geo₂.

2. A method as claimed in claim 1 wherein the gas mixture consists of about 96% by volume of oxygen and 4% by volume of SiCl₄.

3. A method as claimed in claim 2 wherein up to 0.4% by volume of germanium tetrachloride (GeCl₄) is added to the reactive gas mixture.

4. A method of producing internally coated glass tubes, for drawing fibre-optic light conductors which consist of a core and a jacket of glasses which have a mutually different refractive index, comprising the steps of introducing into a glass tube surrounded by a resonator a reactive gas mixture comprising SiCl₄ and oxygen at a pressure of about 1 to 100 Torr, moving the tube relative to the resonator 2 and heating the tube to a temperature between 800°C-1200°C while activating the resonator to form a nonisothermal plasma zone within the tube, whereby a coating free of soot-like particles and consisting of a plurality of layers of sio₂ is formed. 5. A method of producing internally coated glass tubes, as claimed in claim 4, further comprising the step of adding a dopant-forming compound to the gas mixture. 6. A method of producing internally coated glass tubes, as claimed in claim 5, wherein the dopant-forming compound is one or more compounds from the group consisting of TiCl₄ AlCl₃, and GeCl₄. 7. A method of producing internally coated glass tubes, as claimed in claim 5 or 6 wherein the dopant-forming compound is added to the gas mixture at a constant rate.

A method of producing internally coated glass tubes, as claimed in claim 9, wherein the dopant-forming compound is added to the gas

mixture at an increasing rate. 9. A method of producing internally coated glass tubes, as claimed in claim 5 or 6 wherein the dopant-forming compound is added to the gas mixture at a varying rate. 10. A method of producing internally coated glass tubes, as claimed in claim 9, wherein the dopant-forming compound is added to the

gas mixture at a decreasing rate. 11. A method of producing internally coated glass tubes, as claimed in claim 9, wherein the dopant-forming compound is added to the gas mixture at a rate which will produce a coating whose index of refraction increases toward a central axis of the tube. 12. A method of producing coatings on walls of glass comprising the steps of:

contacting at least a portion of the wall of the glass with a mixture of a gaseous glass-forming compound and gaseous oxygen at a pressure of about 1 to 100 Torr;

forming a plasma zone in the gas mixture in contact with the glass wall portion;

heating the glass wall portion, to a temperature which is above the temperature necessary to produce substantially stress-free coating layers on the heated tube wall portion but which is below the temperature at which there is substantial reaction of the mixture in the gas phase, to produce a nonisothermal plasma zone; and

thereby causing a heterogeneous reaction to occur on the glass wall resulting in the deposit on the glass wall of a glass coating.

13. A method as claimed in claim 12, characterized in that the glass wall is in the form of a tube and further comprising the step of causing relative movement between the plasma zone and the tube. 14. A method as claimed in claim 13, characterized in that the coating and the gas mixture are on the inside of the tube, and the glass-forming compound is a silicon tetrahalide. 15. A method as claimed in claim 14, characterized in that the tube is heated to a temperature which is not greater than 1200°C and not below 800°C 16. A method as claimed in claim 15, characterized in that the plasma is formed by means of a high frequency field or a microware resonator. 17. A method as claimed in claim 16, characterized in that a dopant-forming compound is added to the gas mixture. 18. A method of producing a fiber-optic light conductor comprising the steps of:

producing an internally coated glass tube as claimed in claim 17; and

drawing the internally coated glass tube to form a a fiber-optic light conductor.