EP0173659A2 — Method and apparatus for heating thick-walled gla…

Claim 1

Independent Claim 151 words

A method for heating thick-walled glass tubes, particularly quartz tubes, in the manufacture of optical fibres while using microwave energy, characterized by pre-heating the tube (5) to a temperature of about 1000°C-1500°C, preferably by means of a gas flame in a manner known per se; and by subsequently further heating the tube by means of microwave energy generated by a microwave generator (1), by introducing the tube (5) axially into a microwave cavity (2,2') incorporating in its two side surfaces (6,7; 6',7'),or end surfaces,openings (3,4; 3',4') for said insertion of the tube (5), the electrical field strength being given a field image which includes solely one tangential component, according to TE-01n-mode, preferably according to the mode TE-011, whereby the electrical field is so formed as to be tangential to the surfaces of the tube (5), and so formed that the electrical field strength is zero adjacent the surfaces of the cavity.

Claim 2

Independent Claim 151 words

A method for heating thick-walled glass tubes, particularly quartz tubes, in the manufacture of optical fibres while using microwave energy, characterized by pre-heating the tube (5) to a temperature of about 1000°C-1500°C, preferably by means of a gas flame in a manner known per se; and by subsequently further heating the tube by means of microwave energy generated by a microwave generator (1), by introducing the tube (5) axially into a microwave cavity (2,2') incorporating in its two side surfaces (6,7; 6',7'),or end surfaces,openings (3,4; 3',4') for said insertion of the tube (5), the electrical field strength being given a field image which includes solely one tangential component, according to TE-01n-mode, preferably according to the mode TE-011, whereby the electrical field is so formed as to be tangential to the surfaces of the tube (5), and so formed that the electrical field strength is zero adjacent the surfaces of the cavity.

Claim 3

Dependent Claim 72 words

A method according to Claim 1, characterized by using a microwave cavity (2') which has located in its centre part, in a section through its longitudinal axis (24) extending through said openings (3',4'), an extension (23), such that the radial distance from the geometrical centre (25) of the cavity to the wall of the cavity (2') is shorter at the location of the constriction (23) than on both sides of said constriction.

Claim 4

Dependent Claim 28 words

A method according to Claim 2, characterized in that the microwave cavity (2') is spherical, or substantially spherical, with the exception of the location of said constriction (23).

Claim 5

Dependent Claim 24 words

A method according to Claim 1 or 2, characterized by using a microwave cavity (2) which is of cylindrical configuration and made of metal.

Claim 6

Dependent Claim 62 words

A method according to Claim 1,2,3or 4, characterized by using a microwave cavity (2;2') in which the inner surfaces of the walls are highly polished or coated with a highly reflective material which is inert to gases entering the cavity and which will not undergo any form of chemical conversion at the prevailing cavity-wall tempera - tures, said material being preferably gold.

Claim 7

Dependent Claim 42 words

A method according to Claim 1,2,3,4 or 5, characterized by mechanically adjusting the resonance frequency of the cavity (2;2') so that the resonance frequency of said cavity, when loaded with a tube (5), is equal to the frequency ofthe microwave generator (1).

Claim 8

Dependent Claim 52 words

Amethod according to Claim 1,2,3,4 or 5, characterized by controlling the frequency of the microwave generator (1) with the aid of a control unit (16) in a manner such that the said frequency is equal to the resonance frequency of the cavity (2;2') when said cavity is loaded with a tube (5).

Claim 9

Dependent Claim 149 words

Apparatus for heating thick-walled glass tubes, particularly quartz tubes, in the manufacture of optical fibres, including a microwave generator (1) and a microwave cavity, characterized in that the apparatus include known means for heating the glass tube to a temperature of about 1000°C to 1500°C; in that the cavity (2;2') incorporates a centrally located hole (3,4; 3',4') for throughpassage of a tube (5) in the two walls (6,7; 6',7') or end surfaces of the cavity; in that the microwave generator (1) is coupled to the cavity (2) via a waveguide (8), in a manner such that the electrical field strength will only include one tangential component according to a TE-01n-mode, preferably according to the TE-011-mode, whereby the electrical field will be so formed as to be tangential with the surfaces of an inserted tube (5), and so formed that the electrical field strength is zero adjacent the cavity surfaces.

Claim 10

Dependent Claim 149 words

Apparatus for heating thick-walled glass tubes, particularly quartz tubes, in the manufacture of optical fibres, including a microwave generator (1) and a microwave cavity, characterized in that the apparatus include known means for heating the glass tube to a temperature of about 1000°C to 1500°C; in that the cavity (2;2') incorporates a centrally located hole (3,4; 3',4') for throughpassage of a tube (5) in the two walls (6,7; 6',7') or end surfaces of the cavity; in that the microwave generator (1) is coupled to the cavity (2) via a waveguide (8), in a manner such that the electrical field strength will only include one tangential component according to a TE-01n-mode, preferably according to the TE-011-mode, whereby the electrical field will be so formed as to be tangential with the surfaces of an inserted tube (5), and so formed that the electrical field strength is zero adjacent the cavity surfaces.

Claim 11

Dependent Claim 71 words

Apparatus according to Claim 8, characterized in that a constriction (23) is provided in the centre part of the microwave cavity (2') in a section through its longitudinal axis (24) extending through said opening (3',4'), such that the radial distance from the geometric centre (25) of the cavity to the wall of the cavity (2') is shorter at the location of the constriction (23) than on both sides of the constriction.

Claim 12

Dependent Claim 26 words

Apparatus according to Claim 9, characterized in that the cavity (2') is spherical, or substantially spherical, with the exception of the location of said constriction (23).

Claim 13

Dependent Claim 49 words

Apparatus according to Claim 9 or 10, characterized in that the wall (28) of the cavity (2') at the location of said constriction (23) has an inner surface (29) which is of circular-arcuate configuration and the centre of which coincides with the geometric centre (25) of the cavity (2').

Claim 14

Dependent Claim 38 words

Apparatus according to Claim 9,10 or 11, characterized in that said constriction (23) has an extension, seen in the direction of said longitudinal axis (24), which is 1/4 to 1/2 of the mean diameter of the cavity (2').

Claim 15

Dependent Claim 18 words

Apparatus according to Claim 8 or 9, characterized in that said cavity is cylindrical and made of metal.

Claim 16

Dependent Claim 61 words

Apparatus according to Claim 9,10,11,12 or 13, characterized in that the inner surfaces of the walls of said cavity (2') are highly polished or coated with a highly reflective material which is inert to gases entering the cavity and which will not undergo any form of chemical conversion at the prevailing temperatures of the cavity walls, said material preferably being gold.

Claim 17

Dependent Claim 74 words

Apparatus according to Claim 8,9,10,11,12,13 or 14, characterized in that there is provided a control unit (16) adapted to control a mechanical means (17,18) intended for adjusting the resonance frequency of the cavity (2), said control unit (16) being arranged to control said means (17,18) in a manner such that the resonance frequency of the cavity (2), when a tube (5) is located therein, is equal to the frequency of the microwave generator (1).

Claim 18

Dependent Claim 51 words

Apparatus according to Claim 8,9,10,11,12,13 or 14, characterized in that there is provided a control unit (16) for controlling the frequency of the microwave generator (1) in a manner that said frequency is equal to the resonance frequency of the cavity (2) when a tube (5) is present in said cavity.

Claim 19

Dependent Claim 36 words

Apparatus according to Claim 15 or 16, characterized in that the real value of the control unit (16) comprises a signal obtained from a directional coupler (19), said signal corresponding to the amount of energy reflected.

Claim 20

Dependent Claim 75 words

Apparatus according to any one of Claims 9-17, characterized in that a temperature responsive means (20), preferably a means responsive to infrared radiation, is located in or adjacent to the cavity (2), and is arranged to send to said control unit (16) a signal corresponding to the temperature of the tube (5), said control unit (16) also being arranged to control the microwave generator (1), with respect to its power, in dependence on said temperature.

Method and apparatus for heating thick-walled glass tubes

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