WO1990013834A1

WO1990013834A1 - Method in the manufacture of optical fibres

Info

Publication number: WO1990013834A1
Application number: PCT/SE1990/000303
Authority: WO
Inventors: Yngve Hässler
Original assignee: Im Institutet För Mikroelektronik
Priority date: 1989-05-10
Filing date: 1990-05-08
Publication date: 1990-11-15
Also published as: SE463738B; SE8901676L; SE8901676D0; AU5666890A

Abstract

Method of manufacturing optical fibres starting from a thick-walled glass tube (1), preferably a quartz tube, where one or more substances (4) which are to comprise the core of the optical fibre are either deposited and sintered directly on the inside of the thick-walled tube or alternatively on the inside of a thin-walled tube which is subsequently collapsed and inserted in the thick-walled tube, after which the thick-walled tube is heated to its collapsing temperature with the aid of microwave energy supplied to a cavity in which a part of the tube is to be found. The invention is distinguished in that the material from which the thick-walled quartz tube is manufactured is caused to contain contaminants in the form of one or more substances which give rise to a dissipation factor for converting the microwave energy to heat in said material which is sufficient for enabling heating the quartz tube (1) to its collapsing temperature and in that a zone is arranged for braking or substantially preventing diffusion of said contaminants from the quartz tube (1) to the core, said zone being created between the tube and the substance or substances which are to comprise the fibre core.

Description

Method in the Manufacture of Optical Fibres

The present invention relates to a method of manufacturing optical fibres, where the substances forming the core of the fibre are deposited and sintered onto the inside surface of a thick-walled quartz tube, after which the tube is caused to be heated to a temperature at which the tube collapses so that a fibre can be drawn.

In such a procedure, it has been found very advantageous to utilize microwave energy for heating the tube. Microwave heating can be used for all three phases, namely the deposition, sintering and collapsing phases.

In Swedish Patent Specifications Nos. 8403529-4 and No. 8502746-4 there are described advantageous methods for heating thick-walled quartz tubes with the aid of microwave energy to temperatures between 1500 and 2000ºC. An overshadowing problem in the use of microwave energy for heating quartz tubes for the production of optical fibres is that the tubes have a very low dissipation factor. As mentioned, it is, however, from other aspects extremely advantageous to use microwave energy for heating. A very large advantage is that the thick-walled quartz tube is heated uniformly in a radial direction. Another large advantage is that problems with dirtying are avoided, as would be the case when heating with the use of a flame.

The only real difficulty in using microwave energy for heating quartz tubes resides in their low dissipation factor. Part of the question here is that glass in general has a low dissipation factor. The kinds of glass used for producing optical fibres have a considerably lower dissipation factor, however, due to the quartz being extremely pure in order to obtain good optical properties in the fibre.

In principle, glass is an under-cooled form of a liquid phase. The substances in the glass state are not crystaline but amorphous.

Glass material can be manufactured from widely differing substances, but most usual are the so-called oxide glasses. Even these can be based on different substances such as boron, phosphor, sodium or silicon. One of these oxides, silicon dioxide, is called quartz glass in everyday speech, and can be manufactured in a very pure form, in the extreme case with extraneous substance contents below the p.p.m. level. For these reasons, amongst others, quartz glass is therefore suitable for producing optical fibres.

The structure in quartz glass is very stable and comprises a SiO₄ molecule with a silicon atom in the middle of a tetrahedron and with four oxygen atoms in the respective corners. Each oxygen atom further constitutes a corner in an adjacent tetrahedron so that the oxygen atom bonds together two silicon atoms, and each silicon atom thus bonds to four oxygen atoms.

Quartz glass can be manufactured in two principally different ways. The starting material in the first case is natural quartz (crystals) which are collected or otherwise extracted directly from mines. These crystals are sorted and pulverized to a size of about 50 μm. Glass tubes of optical quality are then manufactured from this powder via different high temperature steps of over 2000ºC. The contaminants are above all aluminium between 20 and 100 p.p.m., alkali metals Mg, Li, Na and K between 1 and 5 p.p.m. as well as OH ions at about 200 p.p.m. The latter value applies when a hydrogen gas flame is used during manufacture. If electrical heating is used, the OH content can be reduced to under 5 p.p.m. The content of alkali ions and aluminium ions is primarily determined by the starting material, i.e. which mine is utilized. The relationship between these contaminants is fairly constant.

In the glass structure, aluminium (A1) is included as a replacement for silicon (Si). This means that a reduced plus charge is obtained (3+ instead of 4+) and alkali ions in the quartz glass are primarily associated with Al ions.

In the second case, so-called synthetic quartz is manufactured. The initial material here is silicon tetrachloride, which in turn origin- ates from crystals or quartz sand. In principally the same way as in deposition of quartz according to the MCVD technique (Modified Chemical Vapor Deposition), the silicon tetrachloride (SiCl₄) reacts with oxygen (O₂) to form silicon dixode (SiO₂). With this technique, both aluminium and alkali can be almost completely removed and in certain cases the OH content can be reduced to a sub-p.p.m. level.

Research and development are being carried out at the present time on a third manufacturing technique, so-called sun-gel. The starting material here is a water-based solution, containing the ingredients for an SiO₂ network. From this is formed a gel which is allowed to dry and be sintered to transparent quartz glass. At the present time, natural quartz is used to a major extent in the MCVD tehnique. The reason for this is that it is still cheaper than the synthetic quartz.

So far, the use of microwave heating has only been able to be carried out on natural quartz. Only this quartz type has sufficiently large losses for enabling heating without electrical flashover being obtained.

An optical fibre can be divided into a core, in which the light is intended to be conducted and cladding round the core. The core is achieved with a suitable method such as the mentioned so-called MCVD method.

In this case, the core is built up, e.g. by germanium-doped silicon dioxide being deposited and sintered inside the thick-walled quartz tube. When the tube is subsequently collapsed, a rod is formed with a core and surrounding cladding. The problem here is that certain contaminants in the cladding can diffuse into the core during the collapsing phase and subsequent fibre drawing, thus affecting its optical properties negatively, either directly or later. For this reason an attempt is made to start with quartz tubes of extremely high purity. Another problem is that hydrogen gas from the surroundings of the fibre can diffuse into the core of the finished and installed fibre. This also reduces the optical properties of the fibre.

It can thus be established that attempts to start from very pure quartz tubes will result in increased difficulties for applying the advantageous microwave heating. The present invention solves this problem.

The present invention thus relates to a method of manufacturing optical fibres starting with a thick-walled glass tube, preferably a quartz tube, where one or more substances that are to constitute the core of the optical fibre these substances are either deposited and sintered directly onto the inside surface of the tube or alternatively on the inside of a thin-walled tube which is then collapsed and inserted in the thick-walled tube, subsequent to which the latter tube is heated to its collapsing temperature with the aid of microwave energy supplied to a cavity in which a part of the tube is to be found, and is distinguished in that the material from which the thick-walled quartz tube is manufactured is caused to contain contaminants in the form of one or more substances which give rise to a dissipation factor for the conversion of the microwave energy to heat in the material, this factor being sufficient for enabling heating of the quartz tube to its collapsing temperature, and in that a zone intended for braking or substantially preventing diffusion of said contaminants from the tube to the core is created between the tube and the substance or substances which are to comprise the fibre core.

In accordance with a preferred embodiment, the mentioned contaminants include at least one of the metals lithium (Li), sodium (Na), potassium (Ka) or magnesium (Mg). In accordance with a further preferred embodiment, the mentioned contaminants also include aluminium (Al).

In such cases, the contaminants should be present in a concentration of preferably at least 1- 10 p. p.m. With respect to aluminium, a concentration in the order of magnitude 10-100 p.p.m. is required.

The invention is described below in more detail, partially In connection with the accompanying drawing, where

Figure 1 is a longitudinal section of a thick-walled quartz tube, which is provided with a layer on its inside which shall constitute the core of the fibre; - Figures 2a and 2b illustrate a quartz tube in accordance with Figure 1 in the collapsing and drawing phase, respectively; - Figure 3 is a longitudinal section of a quartz tube treated in accordance with a first embodiment of the invention; - Figure 4 illustrates the concentration of a substance in a

quartz tube which has been treated in accordance with another method in accordance with the invention; and - Figure 5 illustrates a second embodiment of the invention.

Figures 1 and 2 illustrate the prior art, where the numeral 1 denotes the quartz tube which in the finished fibre 2 comprises the cladding 3 thereof. Deposited and sintered germanium-doped silicon dioxide is denoted by the numeral 4, which constitutes the core 5 after collapsing and drawing the fibre, and is illustrated by dashed lines in Figure 2a. At the present time, it is not entirely clarified what substances individually or in combination give a large effect on the increase of the dissipation factor. However, it has been established that lithium has substantial effect on the dissipation factor. It also appears that aluminium in combination with lithium provides an important effect on the dissipation factor.

What has been definitely established is that the quartz tube containing both aluminium and lithium has given a sufficiently high dissipation factor for heating the tube to the collapsing temperature, with the aid of microwave energy .

However, it has been found that just Li and probably also Na have a negative effect on the aging properties of the fibre. If these substances are in the fibre core or in its immediate vicinity, the future properties of the fibre are deteriorated considerably. What occurs is that the attenuation of the fibre increases notably. It has also been established that both Li and Na can diffuse from the cladding into the core during the fibre drawing operation. Of course, it may be so that there are different contaminating substances from the one mentioned above which have the property of increasing the dissipation factor of the quartz tube and simultaneously have the property of negatively affecting the optical properties of the fibre in the immediate or distant future.

The present invention is based on the concept of causing the material from which the quartz tube is manufactured to contain contaminants increasing the dissipation factor but at the same time of braking or preventing the optical properties of the fibre becoming deteriorated thereby due to one or more of the contaminants diffusing into the core of the fibre. Such diffusion can either take place during the fibre drawing operation or later.

The invention shall therefore not be regarded as limited to any special substances, but includes all the substances which increase the dissipation factor and the diffusion of which into the core, it Is simutaneously possible to brake or substantially prevent. In accordance with the invention, there is thus no need to start with the relatively expensive highly pure quartz tubes as is the case at present, but it is possible to start with tubes which are less pure. In addition, it may be necessary in the use of certain kinds of quartz to add given contaminants for obtaining a desired dissipation factor.

However, the quartz production method will be altered, e.g. so that only highly pure synthetic quartz Is produced for producing quartz tubes for optical fibres. In this case, substantially all contaminants must be added to the material. An advantage with starting from highly pure synthetic quartz is that optimal contaminants can be selected with reference to their capacity for increasing the dissipation factor and the possibility of preventing their diffusion into the core.

With respect to the above-mentioned substances, it has been found that aluminium prevents diffusion of lithium very greatly when the content of aluminium is approximately 100 p.p.m. or higher. For this reason, one of the preferred contaminants is aluminium, which both increases the dissipation factor and simultaneously prevents diffusion of lithium which is a substance increasing the dissipation factor.

In accordance with the present invention, a zone is created which is arranged for braking or substantially preventing diffusion of the mentioned contaminants from the quartz tube to the core. The zone is created between the thick-walled glass tube and the substance or substances which are to comprise the fibre core.

In accordance with the first embodiment of the invention, this zone is created at or on the inside of the thick-walled tube.

In accordance with a first preferred method in accordance with the invention, this zone is formed by depositing aluminium-doped quartz on the inside of the tube, this doping being relatively heavy and preferably at least 5000-10000 p.p.m.

In accordance with a second preferred method, the mentioned zone is formed by depositing silicon nitride on the inside of the thick- walled tube (1). In accordance with both the first and second method, there is obtained a heavy diffusion-inhibiting layer on the inside of the tube. The substance or substances which are to form the core of the fibre are subsequently deposited and sintered onto the layer of aluminium- doped quartz in accordance with the first method, or onto the layer of silicon nitride in accordance with the second method.

A particular advantage with using a silicon nitride layer is that it effectively prevents the above-mentioned diffusion of hydrogen from the surroundings of the finished and installed fibre into its core. There is thus also prevented deterioration of the optical properties of the fibre due to hydrogen diffusion. This embodiment of the invention is illustrated in Figure 3, where the numeral 6 denotes the mentioned layer of aluminium-doped silicon dioxide or silicon nitride. The remaining Figures denote the same things as in Figures 1 and 2.

When the quartz tube contains lithium and aluminium, for example, these substances will cause the tube to have a sufficiently high diffusion factor. Simultaneously, the zone mentioned will prevent lithium from diffusing into the core during both collapsing and drawing phases, as well as later during use of the fibre.

According to a third preferred method in accordance with the invention, the mentioned zone is formed by the tube being treated internally so that the contaminant or contaminants which are to have their diffusion into the core braked are caused to be removed so that a decreasing concentration gradient is formed, starting from the outside of the tube and in a direction towards its inside with respect to the substance or substances in question.

In this case, the technique is thus to achieve a zone close to the inside of the tube which has a very low concentration of the contaminants affecting the properties of the core negatively in the immediate or distant future, while the same contaminants are to be found in a higher concentration in the remaining parts of the tube for their accomplishing a sufficiently high dissipation factor.

In accordance with a preferred embodiment of the invention, the mentioned contaminating substance or substances are caused to be removed on the inside of the tube by taking chlorine gas through the tube at a high temperature. With regard to lithium for example, the introduction of chlorine gas results in that lithium chloride is formed and taken away with the gas flow. After a given treatment time, the lithium concentration has fallen heavily on the inside of the tube and has also penetrated into the tube wall to some extent.

Accordingly, a zone is formed close to the inside of the tube where the concentration of the contaminating substance or substances increases successively from zero or a very low concentration on the inside of the tube to a higher value further in the tube.

An example of a concentration profile is illustrated in Figure 4, where the x-axis is in the radial direction of the tube 1. The wall of the tube is illustrated in the diagram, where the numeral 7 denotes the outside thereof and the numeral 8 its inside. Of course, the mentioned first and second methods of achieving a zone can be combined. In this case the concentration, e.g. of lithium is first lowered on the inside of the tube by taking chlorine gas through the tube, after which aluminium-doped quartz is deposited on the inside of the tube. In this way, there is created an extremely effective zone against the diffusion of lithium into the core.

According to another embodiment of the invention, see Figure 5, the zone is formed on the outside or inside of a thin-walled glass tube 9. This tube is intended to form a so-called pre-mould intended for insertion into the thick-walled glass tube 1.

A preform 9 is formed so that the substance or substances which are to form the core 5 of the fibre being deposited and sintered on the inside of a thin-walled quartz tube. This tube is subsequently heated to its collapsing temperature and caused to collapse.

A thin-walled tube 9 is illustrated collapsed in Figure 5 and has a given radial play 11 to a surrounding thick-walled tube 1. The thick- walled tube 1 is subsequently caused to collapse about the pre-mould 9 inserted in the thick-walled tube.

In accordance with this second embodiment, the mentioned zone can be formed on the outside or inside of the thin-walled tube 9 before or after collapsing the thin-walled tube. What is applicable to the case where the zone is created on the inside of the thin-walled tube is also applicable in the same way when the zone is created on the inside of the thick-walled tube.

In accordance with a first method, the mentioned zone is formed by depositing aluminium-doped quartz 10 on the outside or inside of the thin-walled tube 9, which doping being relatively heavy and preferably at least 5000-10000 p.p.m. In accordance with a second method, the mentioned zone is formed by depositing a layer 10 of silicon nitride on the outside or inside of the thin-walled tube 9. In accordance with a third method, the mentioned zone is formed by the thin-walled tube being internally treated so that the contaminant or contaminants which are to have their diffusion into the core braked shall be caused to be removed such that a falling concentration gradient of the substance or substances in question is formed from the outside of the tube in a direction towards its inside, i.e. in the same way as described above relating to the thick-walled tube.

Irrespective of whether the first embodiment or second embodiment is applied, i.e. respectively creating the zone on the inside of the thick-walled tube or creating the zone on the outside or inside of the thin-walled tube, the same result is achieved.

A number of embodiment examples has been described above. It is obvious that the selection of different substances and the concentration of them can be varied.

The present invention shall thus not be regarded as limited to the embodiment examples described above, but may be varied within the scope of the accompanying Claims.

Claims

1. Method of manufacturing optical fibres starting from a thick- walled glass tube (1), preferably a quartz tube, where one or more substances (4) which are to comprise the core (5) of the optical fibre are either deposited and sintered directly on the inside of the thick-walled tube or alternatively on the inside of a thin-walled tube, the latter subsequently being collapsed and inserted in the thick-walled tube, whereafter the thick-walled tube is heated to its collapsing temperature with the aid of microwave energy which is supplied to a cavity, in which a part of the tube is to be found, c h a r a c t e r i z e d in that the material from which the thick- walled quartz tube is manufactured is caused to contain contaminants in the form of one or more substances giving rise to a dissipation factor, for the conversion of the microwave energy into heat in said material, this factor being sufficient for enabling heating the quartz tube (1) to its collapsing temperature, and in that a zone is created between the quartz tube and the substance or substances which are to comprise the core of the fibre, this zone being arranged to brake or substantially prevent diffusion of said contaminants from the quartz tube (1) to the core (5).

2. Method as claimed in Claim 1, c h a r a c t e r i z e d in that said contaminants include at least one of the metals lithium (Li), sodium (Na), potassium (Ka) or magnesium (Mg).

3. Method as claimed in Claim 2, c h a r a c t e r i z e d in that said contaminants include aluminium (Al).

4. Method as claimed in Claim 1, 2 or 3 c h a r a c t e r i z e d in that said zone is created on the inside of the quartz tube (1) before depositing the substance or substances which are to form the core (5) of the fibre.

5. Method as claimed in Claim 1, 2 or 3, c h a r a c t e r i z e d in that said zone is created on the inside of the thin-walled tube (9) before depositing the substance or substances which are to form the core (5) of the fibre.

6. Method as claimed in Claim 1, 2 or 3, c h a r a c t e r i z e d in that said zone is created on the outside of the thin-walled tube (9).

7. Method as claimed in Claim 4, 5 or 6, c h a r a c t e r i z e d in that said zone is formed by depositing aluminium- doped quartz (6; 10), said doping being relatively heavy and preferably at least 5000-10000 p.p.m.

8. Method as claimed in Claim 4, c h a r a c t e r i z e d in that said zone is formed by depositing silicon nitride (6; 10).

9. Method as claimed in Claim 1, 2, 3, 4 or 5, c h a r a c t e r i z e d in that said zone is formed by coating the inside of the quartz tube (1; 9) such that the contaminant or contaminants, the diffusion of which into the core (5) shall be braked, is/are caused to be removed so that a decreasing concentration gradient counted from the outside of the tube (1; 9) towards the inside thereof is formed by the substance or substances in question.

10. Method as claimed in Claim 9, c h a r a c t e r i z e d in that said substance or substances are caused to be removed by taking chlorine gas through the tube at high temperature.