WO2006106068A2

WO2006106068A2 - Process for the manufacture of a preform for optical fibres

Info

Publication number: WO2006106068A2
Application number: PCT/EP2006/061179
Authority: WO
Inventors: Karsten Braeuer; Richard Schmidt; Andreas Schultheis
Original assignee: Heraeus Quarzglas Gmbh & Co. Kg
Priority date: 2005-04-05
Filing date: 2006-03-30
Publication date: 2006-10-12
Also published as: WO2006106068A3; JP2008534428A; DE102005015706B4; JP5091111B2; DE102005015706A1

Abstract

In a known process for the manufacture of a preform for optical fibres of quartz glass using a plasma burner (1), this is operated in a deposition phase and a smoothing phase, a silicon-containing starting substance being supplied to the plasma burner during the deposition phase, SiO2 being formed therefrom in a plasma flame allocated to the plasma burner and this SiO2 being deposited in layers on the cylinder jacket surface (9) of a substrate body (3) rotating around its own longitudinal axis by reversing movement of the plasma burner along the substrate body and being vitrified directly during this process into quartz glass of the preform and the preform surface being treated during the smoothing phase, by the plasma flame moving at least once along the preform, with a temperature which is higher in comparison with deposition phase such that smoothing of the preform surface and melting of the near-surface bubbles are effected. In order to indicate an economic process on this basis which allows the manufacture of low bubble content or bubble-free preforms with an acceptable time and material expenditure, it is suggested according to the invention that the deposition phase comprises a multiplicity of successive deposition sub-phases in the course of which a quartz glass layer is produced in a thickness of less than 400 µm, successive deposition sub-phases being interrupted by a smoothing phase.

Description

Process for the manufacture of a preform for optical fibres

The invention relates to a process for the manufacture of a preform for optical fibres of quartz glass using a plasma burner which is operated in a deposition phase and a smoothing phase, a silicon-containing starting substance being supplied to the plasma burner during the deposition phase, SiO₂ being formed therefrom in a plasma flame allocated to the plasma burner and this SiO₂ being deposited in layers on the cylinder jacket surface of a substrate body rotating around its own longitudinal axis by reversing movement of the plasma burner along the substrate body and being vitrified directly during this process into quartz glass of the preform and the preform surface being treated during the smoothing phase, by the plasma flame moving at least once along the preform, with a temperature which is higher in comparison with deposition phase such that smoothing of the preform surface and melting of the near-surface bubbles are effected.

The manufacturer of preforms for optical fibres by means of the so-called "POD process" (plasma outside deposition) is generally known and described e.g. in DE 25 36 457 A1. For the manufacture of the preform, a core glass cylinder of undoped quartz glass is provided on whose outer cylinder jacket quartz glass doped with fluorine is deposited as jacket glass layer. To produce the jacket glass layer, an induction coupled plasma burner is used to which a stream of gas is supplied which contains a hydrogen-free silicon compound and oxygen. In addition, a fluorine-containing compound is introduced into the plasma flame allocated to the plasma burner. From the starting substances, SiO₂ is formed which is deposited in layers onto the core glass cylinder rotating around its longitudinal axis and vitrified (sintered) directly on the core glass layer to form the fluorine-containing SiO₂ jacket glass layer. As a rule, the core glass cylinder has a homogeneous radial refractive index profile. It consists mainly of undoped quartz glass but can also contain dopants modifying the refractive index.

Particular attention is paid to avoiding bubbles in the jacket glass layer of the preform. Bubbles are, basically, not permitted or extremely undesirable because fibre defects are produced during drawing of the preform which defects impair the light conduction or lead to fibre breakage. Consequently, an aftertreatment is carried out on completion of the deposition process during which a source of heat is moved at a slow moving speed along the preform jacket surface. The aim of the aftertreatment is not only smoothing of the preform surface but also eliminating particles deposited thereon and near-surface defects and especially the melt fusion of bubbles.

Such a process is known from EP 727 392 A1. It describes a species-appropriate POD process in which the plasma burner is passed, on completion of the deposition process, at least once more, without supplying glass starting material, along the surface of the preform to be produced, in order to smooth the surface and/or to vitrify porous SiO₂ material on the surface. For this purpose, the temperature of the plasma flame is raised and adjusted such that the surface temperature of the preform is below the evaporation temperature of quartz glass but above its softening point during this aftertreatment.

As a result of the necessarily slow moving speed and the burning loss thus produced, this aftertreatment is accompanied by a considerable time expenditure and material loss. In spite of this, it is not possible to safely and reproducibly produce preforms free from bubbles by means of the known process. Bubbles situated far below the surface, in particular, cannot be removed by means of the known process.

The invention is consequently based on the object of indicating an economic process which allows the manufacture of low bubble content or bubble-free preforms with an acceptable time and material effort.

Starting out from the process mentioned above, this object is achieved according to the invention by the fact that the deposition phase comprises a multiplicity of successive deposition sub-phases in the course of each sub-phase a quartz glass layer is produced in a thickness of less than 400 μm, successive deposition sub- phases being interrupted by a smoothing phase.

A deposition sub-phase comprises at least one deposition pass during which a single vitrified SiO₂ layer is produced by means of the plasma burner moving along the cylinder jacket surface. The SiO₂ layer consists of pure quartz glass or doped quartz glass. As a rule, a deposition sub-phase comprises several deposition passes each one of which contributes to the layer build-up and to strengthening of the quartz glass layer.

According to the invention, the deposition phase is divided into a multiplicity of such deposition sub-phases, on completion of every deposition sub-phase a smoothing phase being introduced during which the quartz glass layer just produced is subjected to thermal treatment.

During the smoothing phase, a further layer build-up is stopped or reduced. Instead, the surface of the preform being formed is subjected to thermal treatment by means of the plasma burner. In this respect, the mode of use of the plasma burner differs during the smoothing phase from its use during the deposition phase in at least two respects.

• On the one hand, the supply of silicon-containing starting substance to the plasma burner is stopped or reduced.

• On the other hand, the temperature of the preform surface is raised. This can be effected by changing the distance between the plasma burner and the preform surface or by reducing the relative speed with which the plasma burner is moved along the preform surface or by an increased flame temperature. An increased flame temperature is obtained, for example, by changing the gas composition in the plasma area or, in the simplest case, by switching off or reducing the supply of silicon-containing starting substance.

It has also been found that the formation of bubbles can be avoided by means of this procedure with a sufficiently high level of reliability insofar as, during the - A -

deposition sub-phase, a quartz glass layer is produced in each case in a thickness of less than 400 μm. As a result of the smoothing phases carried out between the individual deposition sub-phases, the surface is smoothed at relatively short time intervals, any possible near-surface defects, in particular bubbles, being reliably eliminated. A complex and time-consuming aftertreatment of the surface which may, under certain circumstances, also comprise abrasion of the jacket to eliminate deeper lying bubbles, can be avoided in this way. As a result, a shorter process duration overall is obtained, in spite of an additional time expenditure on carrying out the smoothing phase, without the need for any noteworthy abrasion of material layers and, consequently, overall, a cost-effective process.

At least one plasma burner is used for deposition and carrying out smoothing.

The substrate body is a rod-shaped or tubular body of graphite, a ceramic material such as aluminum oxide or glass, in particular doped or non-doped quartz glass. The substrate body is removed after the deposition process or it forms an integral component of the preform.

The preform is a quartz glass body from which optical fibres can be drawn directly or it is a preliminary product, e.g. in the form of a tube, for such a quartz glass body.

It has proved to be advantageous if, in the course of a deposition sub-phase, a quartz glass layer is produced in each case in a thickness in the region between 25 μm and 300 μm, preferably at least 50 μm.

The effectiveness of the smoothing measure increases with the frequency of repetition of the smoothing phases during the deposition process and the probability of blister formation is thus reduced. However, with such a high repetition frequency in the case of which a smoothing phase takes place as early as after the formation of a quartz glass layer in a low thickness of less than 25 μm, the additional effect thus achievable is low; on the other hand, the duration of the process increases with the number of smoothing phases.

The best results are achieved if, in the course of a deposition sub-phase, a quartz glass layer is produced in each case in a thickness of maximum 150 μm. The overall thickness of the quartz glass layer formed during a deposition phase depends on the number of deposition passes carried out. Suitable layer thicknesses are obtained if the reversing movement of the plasma burner comprises fewer than 50 deposition passes in the course of a deposition sub- phase.

The usual rates of deposition produce quartz glass layers in thicknesses in the region between 4 and 8 μm per deposition pass. Taking this into consideration, the reversing movement of the plasma burner comprises between 4 and 38 deposition passes, preferably maximum 30 deposition passes in the course of a deposition sub-phase.

The number of deposition passes carried out in successive deposition sub-phases and the thicknesses of the vitrified quartz glass layers thus produced can differ from each other. However, a procedure is particularly preferred in which the same number of deposition passes is carried out in successive deposition sub-phases.

The regularity of the layer build-up in the preform thus effected advantageously influences its optical properties and the reproducibility of the process result is improved. The process can, moreover, be automated in a particularly simple manner. The same number of deposition passes during the deposition sub-phases is usually accompanied by the same duration of the deposition sub-phases and by approximately the same layer thickness per deposition phase.

The process according to the invention has the advantage that any possible bubbles can be removed relatively easily from the thin near-surface quartz glass layers to be smoothed such that a material abrasion is not required during the smoothing phase. On the other hand, it may prove to be advantageous to treate the preform surface with a fluorine-containing etchant during the smoothing phase.

The etchant causes a certain abrasion of the quartz glass layer during the smoothing phase. This procedure has proven itself above all as a preventative measure to remove defects which may later lead to bubble formation, within a short period. In this connection, doping of the quartz glass with fluorine has proved to be advantageous, the etchant serving as source of dopant.

Consequently one and the same substance is supplied to the plasma burner as source of dopant during the deposition phase and as etchant during the smoothing phase. Switching off the supply on changing of the phases is consequently not necessary, simplifying the procedure. SF₆ or CF₄, for example, is used as etchant and dopant.

Preferably, the plasma burner is moved once along the preform surface during the smoothing phase.

The moving speed of the plasma burner is adjusted in such a way during this process that sufficient smoothing is achieved during a single translation along the preform surface. This reduces the duration of the process and, in comparison with repeated heating of the preform surface, leads to a lower energy expenditure.

For this purpose, the plasma burner is preferably moved at a moving speed along the preform surface, which is lower during the smoothing phase than during the deposition phase.

In a further preferred process variant, the plasma burner is fed with the silicon- containing starting substance during the smoothing phase in a lower quantity than during the deposition phase.

A low supply of silicon-containing starting substance also during the smoothing phase has the advantage that a certain layer build-up can take place during this phase leading to a shorter duration of the process as a whole.

In a particularly preferred variant of the process according to the invention, the same plasma burner is used for the forming or SiO₂ during the deposition phase and for smoothing of the preform surface during the smoothing phase.

As a result of the fact that the same plasma burner - although in a different operating mode, as explained above - is also used for smoothing of the preform surface, a lower process expenditure and a simpler, in terms of construction, and in particular cost-effective device is obtained for carrying out the process according to the invention.

In the following, the invention will be explained in further detail by way of practical examples and a patent drawing. In the drawing, the following is shown in detail:

Figure 1 shows the POD process for the manufacture of preform in a schematic view and

Figure 2 shows a statistical evaluation of the results of deposition processes by way of a so-called "box and whisker plot".

In Figure 1 , the process for the manufacture of a preform for so-called multimode fibres is illustrated schematically with a step-wise refractive index profile. For this purpose, a rod 3 of highly pure, non-doped synthetic quartz glass with a diameter of 85 mm is provided and coated by means of a "plasma outside deposition process" (POD process) with a jacket 4 of fluorine-doped quartz glass. For this purpose, SiCI₄, oxygen and SF₆ are supplied to a plasma burner 1 and converted to SiO₂ particles in a burner flame 2 allocated to the plasma burner 1 The main direction of spreading of the plasma flame 2 is indicated by a dotted line 5. By moving the plasma burner 1 in a reversing manner along the rod 3 from one end to the other, the SiO₂ particles are deposited in layers on the cylinder jacket surface 9 of the rod rotating around its longitudinal axes 6 and vitrified directly into quartz glass. In this way, it is possible to incorporate high fluorine concentrations of more than 3% by weight into the quartz glass network of the jacket 4. The plasma flame 2 is produced within a reaction tube 8 of quartz glass which is surrounded by a high frequency coil 7. The high frequency coil 7 has a height of approximately 92 mm and the reaction tube 8 juts out over it by approximately 7.5 mm. A distance of 65 mm is adjusted between the upper end of the high frequency coil 7 and the surface of the rod 3,.

According to the invention, one and the same plasma burner 1 is operated during the deposition process for the manufacture of the jacket glass (4) in 2 different modes. During the deposition phase, SiCI₄, oxygen and SF₆ are supplied to the plasma burner, as mentioned above, and it is moved in a reversing manner at a moving rate of 500 mm/min along the preform surface 9. The rate of rotation of rod 3 and the rate of translation of the plasma burner 1 result in an average thickness of the individual jacket glass layers of approximately 6 μm.

In the deposition phase, no SiCI₄ is supplied to the plasma burner 1 and it is moved at a markedly lower moving rate of 300 mm/min once from one end of the preform to the other. No material is deposited during the smoothing phase. The supply of oxygen and SF₆ is not modified in comparison with the deposition phase.

The deposition phases and smoothing phases alternate each other. A deposition phase of a certain duration which leads to the formation of a quartz glass layer of a given thickness and which will be explained in further detail in the following, is followed by a smoothing phase during which the preform surface is more strongly heated, smoothed and freed from defects.

The preform obtained according to the process of the invention consists of a core of pure quartz glass which has a refractive index at 633 nm of 1.4571 and of a jacket of fluorine-doped quartz glass which has a refractive index of 1.440 at a wavelength of 633 nm. The fluorine content of the jacket glass is 5% by weight. The content of hydroxyl groups in the core is 700 wt.-ppm. The core has a diameter of 70 mm and the jacket an outside diameter of 77 mm.

In the following, the process according to the invention is described in further detail by way of practical examples and Figure 2. Five series of experiments were carried out for the manufacture of preforms according to the process explained in general terms above. The figure shows a statistical evaluation, for each series of experiments, by way of a so-called "box and whisker plot" 20, the build-up time t_A up to a predetermined final diameter of the preform being reached being plotted in h on the Y axis.

The star-shaped ends 21 of each plot 20 show the shortest and the longest duration of the build-up time of each series of experiments respectively. 50% of the build-up times are within the area comprised by the rectangular box 22. The subdivision of box 22 by a horizontal line 23 provides the median value above and below which 25% of the build-up times are situated respectively. The small square 24 symbolises the arithmetic mean of all the build-up times of the series of experiments. In the series of experiments A and B, the deposition phase was broken off in each case, by a "burnout run ", after 4 hours in the course of which an increase in the preform diameter by approximately 2000 μm was produced. During a burnout run, the plasma burner 1 was moved once along the preform surface 9 at a moving rate of 20 mm/min, the supply of SiCI₄ being cut off. As a result of a burnout run, a decrease in the outside diameter of the preform by approximately 300 μm takes place by etch attack. On completion of a burnout run, the quality of the quartz glass layer treated was checked with the naked eye, particular attention being paid to still existing bubbles. Where necessary, the burnout run was continued until a satisfactory quality of the quartz glass layer was reached. In the series of experiments A, the first burnout run took place after a deposition phase of 4 hours, in the second series of experiments B after 2 hours.

As a result of the passing, in terms of time, of the burnout runs having a low reproducibility respectively, substantial variations, in terms of time, with respect to the complete build-up of the desired thickness of the jacket glass layer and, additionally, partially long build-up times of more than 16 or 19 hours arise. Although, overall, this procedure leads to useful results, it has the further substantial disadvantage that it cannot be easily automated.

The preforms of the series of experiments C were produced by way of an automated process, one smoothing pass being carried out, after a deposited layer thickness of 405 μm, in which the plasma burner 1 was operated in the deposition phase. During the smoothing pass, the plasma burner 1 is moved at a reduced moving rate once along the preform surface 9, it being operated in a way as described above for the smoothing phase. As a result of the lower translation speed, a higher temperature of the preform 9 is obtained.

As a result of the regular short smoothing passes after comparatively short deposition phases, low bubble content quartz glass layers were obtained. However, it is apparent that complete freedom from bubbles could not be achieved in all cases such that an additional burnout run became necessary to remove bubbles, as described above for the series of experiments A and B. The subsequent removal of the bubbles formed leads to long build-up times with great time variations. The preforms of the series of experiments D and E were also produced by way of an automated process, a simple smoothing pass being carried out once after 1 1 deposition passes during which the plasma burner 1 was operated in the deposition phase.

During the smoothing phases, no SiCI₄ was supplied to the plasma burner 1 but only SF₆ and oxygen, as explained above. The first smoothing pass took place after an initial build-up time of 2 hours. In the series of experiments D, a quartz glass layer thickness of approximately 70 μm was obtained after every deposition phase, which was completed by a smoothing pass.

Bubbles were thus completely avoided; reproducible short build-up times were obtained with very low variations such as they are desirable for industrial production.

In the series of experiments E, the rate of build-up was increased by increasing the supply of SiCI₄ by approximately 10% compared with the series of experiments D, as a result of which a layer thickness increased by approximately 10% is also obtained during the deposition phase. As a result, it was possible to further reduce the build-up time without this being accompanied by an unforeseen and irreparable bubble formation. This series of experiments, too, thus resulted in a bubble-free build-up and a correspondingly short build-up time.

The refractive index profiles of the preforms made by the process according to the invention do not differ, in spite of the smoothing passes, from refractive index profiles of preforms obtained according to the standard process. In the case of sequences of SiCI₄-free smoothing passes at time intervals of 15 minutes, in particular, no radial fluctuation in the refractive index is discernible. The fibres drawn from the preform are characterised by a high transmission.

Claims

Patent claims

1. Process for the manufacture of a preform for optical fibres of quartz glass using a plasma burner (1 ) which is operated in a deposition phase and a

5 smoothing phase, a silicon-containing starting substance being supplied to the plasma burner during the deposition phase, SiO₂ being formed therefrom in a plasma flame (2) allocated to the plasma burner (1 ) and this SiO₂ being deposited in layers on the cylinder jacket surface of a substrate body (3) rotating around its own longitudinal axis (6) by reversing O movement of the plasma burner (1 ) along the substrate body (3) and being vitrified directly during this process into quartz glass (4) of the preform and the preform surface (9) being treated during the smoothing phase, by the plasma flame (2) moving at least once along the preform, with a temperature which is higher in comparison with deposition phase such that 5 smoothing of the preform surface (9) and melting of the near-surface bubbles are effected, characterised in that the deposition phase comprises a multiplicity of successive deposition sub-phases in the course of which a quartz glass layer is produced in each sub-phase in a thickness of less than 400 μm, successive deposition sub-phases being interrupted by a 0 smoothing phase.

2. Process according to claim 1 characterised in that, in the course of a deposition sub-phase, a quartz glass layer is produced in each case in a thickness in the region between 25 μm and 300 μm, preferably at least 50 μm.

5 3. Process according to claim 2 characterised in that, in the course of a deposition sub-phase, a quartz glass layer is produced in each case in a thickness of maximum 150 μm.

4. Process according to one of the preceding claims characterised in that the reversing movement of the plasma burner (1 ) comprises fewer that 50 0 deposition passes in the course of a deposition sub-phase.

5. Process according to claim 4 characterised in that the reversing movement of the plasma burner (1 ) comprises between 4 and 38 deposition passes, preferably maximum 30 deposition passes, in the course of a deposition sub-phase.

6. Process according to one of the preceding claims characterised in that in successive deposition sub-phases the same number of deposition passes is carried out.

7. Process according to one of the preceding claims characterised in that the preform surface (9) is treated with a fluorine-containing etchant during the smoothing phase.

8. Process according to claim 7 characterised in that the quartz glass is doped with fluorine, the etchant serving as source of dopant.

9. Process according to one of the preceding claims characterised in that the plasma burner (1 ) is moved once along the preform surface during the smoothing phase.

10. Process according to one of the preceding claims characterised in that the plasma burner (1 ) is moved along the preform surface (9) at a moving speed which is lower during the smoothing phase than during the deposition phase.

1 1 . Process according to one of the preceding claims characterised in that, during the smoothing phase, the silicon-containing starting substance is supplied to the plasma burner (1 ) in a lower quantity than during the deposition phase.

12. Process according to one of the preceding claims characterised in that the same plasma burner (1 ) is used for the forming or SiO₂ during the deposition phase and for smoothing of the preform surface (9) during the smoothing phase.