DE2939659A1 - Multilayer glass optical waveguide fibres - where outer sheath with low coefft. of thermal expansion provides fibres with high tensile strength - Google Patents

Abstract

Fibre consists of a core (a); an outer layer (b), and a sheath (c), where the tensile strength of the fibre is increased due to the difference in the coefft. of thermal expansion (CTE) of layers (a, b). All the layer forming the fibre are deposited successively in the bore of a substrate glass tube, which is then collapsed to make a fibre blank; and the initial substrate glass is removed before drawing the blank into fibre. Materials (a, b) pref. have a high CTE and low glass transformation temp. (GTT), while the thin sheath (c) is quartz with low CTE and high GTT. The substrate is pref. removed to a prescribed depth by grinding, then cleaned, etched and polished to form the blank. One pref. sheath (c) is hard quartz, whereas the substrate contains 96-97% SiO2, the rest being B2O3 and Na2O. Sheath exerts high compressive stress e.g. over 345 N/mm2, so the fibre has a higher axial tensile strength and longer life than conventional fibres.

Description

Method of making a multilayer

prior art glass fiber waveguide The invention relates to a method of manufacturing a multilayer glass fiber optical waveguide with a core, a sheath and an outer shell that are due to their opposite the core and the jacket have different coefficients of thermal expansion, the tensile strength of the glass fiber waveguide increased by coating the inside of a substrate glass tube and collapsing a preform is made from which the fiber optic waveguide is pulled.

Such a method is known from DE-OS 24 19 786 and counts also through the earlier patent application DE-OS 28 26 010 to the state of the art.

Both laid-open documents teach a substrate glass tube to coat the inside with the core material and the material of the jacket adjoining it (in reverse order) . The outer shell necessary to increase the tensile strength is to arise according to DE-OS 24 19 786 in that the internally coated pipe is brought into a tube made of the material of the outer shell, these two tubes together fused and drawn out together to form fibers.

In contrast, according to DE-OS 28 26 010, the outer shell should be deposited by deposition one or two layers on the outer surface of the collapsed inner-coated Substrate Glass tube are generated. The so through internal coating A preform made of a glass tube, collapsing and external coating is then made pulled out to the fiber optic waveguide.

The outer shell consists either of a single layer, whose Thermal expansion coefficient is lower than that of the jacket, or from an internal Layer with a lower coefficient of thermal expansion and one adjacent to it larger layer with a higher coefficient of thermal expansion.

Such outer sheaths have the following meaning: If a glass fiber in Is subjected to a tensile force in the direction of its longitudinal axis, the tension increases on the fiber surface considerably. Even if measures are taken, the fiberglass surface to keep them free of dust particles and moisture, for example by immediate Application of a plastic coating during manufacture is the glass fiber usually somewhat roughened and there are microcracks on their surface. Because the fiberglass is under constant tension, the cracks grow from the outside to the inside. In a short period of time, the total fiber becomes significant weakened and may after a relatively short life, which is for optical purposes Message transmission is not sufficient at all, break it. The presence of water molecules on the outer circumference of the glass fiber also promotes growth the cracks and thus increases the possibility of premature failure.

A known measure to increase the strength of glass fibers for the optical communication consists of a To exert surface pressure. Such a technique is, for example, from one Article by D.A.Krohn and A.R.Cooper known in "Journal of the American Ceramic Society ", December 1969, pages 661-664. This article contains theoretical and experimental data showing that when using a cladding glass a lower coefficient of thermal expansion than that of the core glass is selected, and when the glass transition temperature of the core and the clad is appropriate must be observed, there is a good chance that compressive stresses will arise that increase the fiber strength.

It is known from US Pat. No. 3,849,181 that in certain types of non-optical glass fiber structures, for example in the case of high-strength fibers made of polycrystalline refractory oxides for use in fiberglass products are thereby reinforced you can put an extremely thin cladding of one on the surface of each fiber applies vitreous material. However, there are not those related to optical Fibers dealt with existing problems. It is also stated that for such a constructive fibers used, the outer sheath must be thinner than 0.1 Su. As follows will be made clear, such an extremely thin jacket is not suitable be, a fiber for the optical communication transmission the necessary strength admit.

Task It is the task of the invention to provide another than the known, Specify the method mentioned at the beginning with which a glass fiber light guide is produced can be, whose outer shell has a higher compressive stress than that of the known Process produced glass fibers exerts and thus a greater axial tensile strength and ensures a longer service life of the fiberglass produced.

Solution The problem is solved as indicated in claim 1.

Further developments result from the subclaims.

Advantages According to the new process, either a glass fiber with a single-layer outer shell that has a compressive stress of 345 N / mm² or more, and it can be a fiberglass with a two-layer outer sheath create a compressive stress of 690 N / mm2.

Description of the figures The method according to the invention consists in that in order to produce an improved preform for the optical glass fiber the layer of the core and the layers surrounding it one after the other, but in reverse Sequence within the bore of a substrate glass tube made of quartz glass the well-known technique of chemical vapor deposition (CVD) will. Then, again by a known technique, the internally coated Substrate glass tubes collapse around a solid shape under the influence of strong heat to produce, the innermost layer of which now represents a solid core, and on it the layer formed from the substrate glass tube is essentially removed, one of the methods described below is used for this. By a suitable influencing of the composition and the thickness of the layers and other parameters it is possible, a preform and a resulting glass fiber to produce, which has a thin outer shell under strong pressure. The invention will now be explained in more detail with reference to the drawings, for example.

They show: FIG. 1 the cross section of a typical known optical Glass fiber, FIG. 2 a simplified graphical representation of the relationship between the difference between the coefficients of thermal expansion of the sheath and core and the tensile strength of the fiber, FIG. 3 shows the cross section of a known three-layer optical fiber or preform (not true to scale) that can be produced according to the invention, that it has a high surface compressive stress, Fig.4 shows the cross section of a four-layer optical fiber or preform (not to scale) which is also according to the invention can be produced so that it has a high surface compressive stress having, Fig.5 a table which for some for the two the outermost glass layers of the structure shown in Fix.4 typical thermal expansion coefficients along with some differences in thermal expansion coefficients shows, Fig. 6 and 7 cross-sections (not to scale) of two types of optical Preforms made with the chemical vapor deposition technique in Substrate glass tubes have been produced, FIGS. 8a and 8b are schematic side views, which show certain process steps associated with the production of the in Fig. 6 and 7 related preforms, FIG. 9 a graphic representation of certain Stretching and compressive forces that are applied to the improved fibers or preforms FIGS. 3 and 4 appear as functions of the thickness of the outer layer, FIG graphically the relationship between the service life and the tensile stress at certain Types of layer structures produced according to the invention.

Fig. 11 shows, for example, certain relationships between Thermal expansion coefficients and axial compressive stresses of optical glass fibers with different thicknesses of the outermost layer and the characters 12 and 13 show the course of the coefficients of linear expansion as a function of chemical compositions of two types of glasses that are used in the invention composite optical preforms and glass optical fibers can be used can.

The Fig.1 shows the cross section of a known typical two-layer Glass fiber 10, in which the core 11 is generally made of doped quartz glass and the Cladding 12 generally either made of quartz glass doped with a refractive index reducing dopant or pure quartz glass (SiO2). How out from the cited article by Krohn and Cooper, the tensile strength the fiber can be improved in that for the cladding layer 12 a glass material is used, which has a lower coefficient of thermal expansion than the glass material of the core has. As is well known, the cladding layer is also smaller Refractive index than the core 11, so that the desired light transmission properties given are. FIG. 2 shows a typical relationship A, not necessarily according to some particular measure, between the difference in extent of layers 11 and 12 and the resulting increase in tensile strength of the composite Fiber.

Because the coefficient of thermal expansion of pure silicon dioxide is relative low is measured namely with 5.5 to 8.0 10 7 cm / 0C, it is a little difficult, suitable Find materials for the outer layer that one have significantly lower coefficients of thermal expansion, and such a material to be applied to the outer surface of the quartz layer. Fiq.3 shows a three-layer Fiber structure in which an additional layer, the outer shell 13 with a lower Thermal expansion coefficient than that of the cladding layer applied to the cladding layer is. If the outer shell is applied in a suitable manner, then this third Layer the compressive forces on the surface of the coat layer, which has a larger surface area per unit length than the core alone has. However, how on As will be shown later, it also depends on the layers select suitable materials that have the correct glass transition temperatures so that the optimal tensile strength properties can be achieved. As from the article by Krohn and Cooper mentioned above, is the glass transition temperature Tg, as it is commonly referred to, that temperature at which the glass material when cooling from a viscoelastic state to an elastic state transforms. More precisely, Tg is often defined as the temperature at which the viscosity of the cooling molten glass reaches the value 1013 poise. Although it fluctuates for any given glass this temperature somewhat, depending on the cooling rate, however, given the widely varying temperatures Tg that have actually occurred in practice are insignificant to the present invention.

The axial compressive stress caused by an outer pressure layer is very important for preventing or reducing surface tension, though the together set fiber of a tensile or bending stress is exposed. This in turn reduces the surface tension caused Corrosion and can significantly extend the life of the fiber. For example, if the surface compressive stress on an optical glass fiber is 690 N / mm², see above The fiber will theoretically have an unlimited lifespan, as far as one can only the stress-induced damage is considered when the applied stresses are below 690 N / mm² can be maintained. On the other hand, when the tensile stress is about 1035 N / mm2 increases, the service life for the same fiber decreases rapidly by a factor from (- c) to n is a fatigue parameter that is typically between 15 and 25 for a Glass fiber with a quartz surface. 6s is the tensile stress and sc is the compressive stress, to which the fiber is exposed.

Thus, in this example, the service life would be significantly reduced will. Using the manufacturing process described below and of the various physical parameters considered, it is possible to optically Fibers with a surface compressive stress of the order of 690 N / mm² or get higher.

Figure 4 shows another embodiment of an optical Fiberglass 10 with four layers, again not necessarily to scale. These Fiber and its preform can have better tensile properties than the one after Fig. 3 have. As shown, the fiber 10 does not only include the ordinary core 11 and jacket 12, but also a second outermost layer 14 of material with one relatively high coefficient of thermal expansion. Like the fiber or preform according to Fig.3 this fiber also has an outermost layer 13 of a material with a relatively low Expansion coefficient. The two layers 13 and 14 form the so-called outer sheath of the fiber.

The glass of layer 14 as well as the core and cladding layer a higher coefficient of thermal expansion and a lower glass transition temperature than the glass of layer 13. Layer 14, although not drawn to scale, is preferably considerably thicker than the cladding layer 12. As a result, the surface is per unit length passed through the printing layer 13 of the finished four-layer fiber is enclosed, considerably larger than the surface area per unit length that in the three-layer fiber, which is drawn from the preform according to Figure 3, from the Layer 13 is enclosed. This means that the four-layer fiber has a larger one Surface compressive stress can have, increasing the tensile strength of the fiber, its Fatigue strength and its service life is improved.

If the core 11 is covered by a suitable cladding layer 12 with less Refractive index is surrounded, so the outer layers 14 and 13 do not need in With regard to their optical properties but only with regard to their effectiveness the generation of compressive forces on the composite fiber or preform will.

Fig. 5 is a table showing various types of glass, for example for the second outermost layer 14 and for the outermost layer 13 in the layer arrangement can be used according to Fig.4. There are typical values of the coefficients of thermal expansion listed together with differences in these values from selected pairs of glasses, used for layers 14 and 13.

In the following, the inventive method for production will now be described of high tensile strength optical preforms and optical fibers with the help of technology the inner coating of the glass tube is described. To the different techniques of interior coating, which are of course possible for the production of optical glass fibers, belongs to the technology chemical vapor deposition (CVD) and their various modifications. Examples of various so-called CVD processes can be found in U.S. Patents 3,982,916 and 4,009,014.

The CVD process can be very briefly described in connection with the typical Describe the inner coating as follows: A hollow tube made of natural or synthetic Quartz glass is held in a suitable manner as a substrate tube, for example in a glass lathe, and after flame polishing the inner surface of the substrate glass tube is, the glass-forming materials of the different layers are sequentially introduced into the heated substrate glass tube in the form of halide vapors. if a substance such as silicon tetrachloride is entrained in oxygen and this Mixture is introduced into the hollow substrate-glass tube, while sufficient heat is applied is, a layer is deposited within the glass tube, which is essentially consists of pure silicon dioxide. In the layer structure shown in Figure 6 is the layer formed from the substrate glass tube with 34 and the deposited one Quartz glass layer denoted by 33. Because they are both made of quartz in this example can, they are only indicated by a dashed line Circle from each other differentiated. For reasons of cost, the substrate tube 34 can normally be of lesser value Quality than the thinner deposited layer 33, so this as one The ion barrier layer serves to reduce the contamination of the next Layer to be deposited coat layer. The thickness of the ion barrier layer 33 can easily control yourself using known measures. This includes the setting the mixing ratio and the control of the relative movement of the substrate tube and the applied localized heat zone and its regulation.

The layer to be deposited as the next layer is that which will result in the cladding layer 32 in Figure 6. When boron trichloride and silicon tetrachloride be transported by oxygen as a carrier gas, a layer made of borosilicate glass be deposited as a coat layer. Finally, a third can be annular Layer of germanosilicate glass are deposited, which the core 31 in Fig.6 forms. For this purpose, the incoming chemical vapors contain germanium tetrachloride and silicon tetrachloride vapors in oxygen, possibly with something extra Phosphorus oxide chloride vapor.

The last step in the process to produce the one shown in Fig.12 Layer structure consists in the temperature of the substrate and glass tube to increase its concentric inner layers effective heat source so far, that the glass tube under the Oberin surface tension a solid preform of lesser Collapsed outside diameter. However, if you go to the invention one Fiber fabric to be produced <(if you want to achieve tensile strength, you have to further edit the structure shown in Figure 6 to get most or all of the original To remove substrate material 24.

The method of manufacturing just described by means of the technique the CVD inner coating avoids losses due to absorption and impurities, which are otherwise very likely due to hydroxyl compounds and Transition metal contamination may occur.

Various suitable heat sources are suitable for use in this process known, e.g. hydrogen-oxygen flames, plasma torches and C02 lasers higher Power; these heat sources are not shown here.

It is important that the different layers of glass are deposited are uniform in their radius and in their layer thickness. With an optical Preform with a structure as shown in Fig. 6 are the layer radii and layer thicknesses for example shown and not necessarily to scale. But when When the preform is drawn into optical fiber, the outer layer is usually much thinner than the type of fiber with the structure shown in Figure 3. It is typically that with a 125 ou thick fiber produced by this inner coating process is made, the thickness of the outer layer is of the order of 25 µ. As already in connection with the description of the fibers with the structures after 3 and 4 indicated, the outer layer must be much thinner so that the huge Compressive forces arise for the improved fibers produced according to the invention are characteristic.

For example, it must have a thickness of 1 to 10 µm, preferably ranging from 1 to 5 p. This is true even if all the other important parameters are carefully selected, including the coefficient of thermal expansion and the glass transition temperatures, as previously described. Therefore, there is a need for further processing of the structure shown prior to fiber drawing, as will be briefly described.

There is a similar problem with making a four-layer one Preform by this process of CVD inner coating. This shows the in Fig.7 fiber layer structure shown. This figure shows a collapsed preform or the resulting fiber with the same layers as in Figure 6, however with an intermediate layer 35. Except for the much greater thickness of the outer layer 33-34 in Fig.7, this structure generally corresponds to that shown in Fig.4. Again, must be careful with the layer materials and physical parameters are proportioned in accordance with the principles previously described Thickness of the outer layer 33-34 can be reduced in order to take full advantage of the invention Process to achieve.

The goal of further processing the preforms with the construction of the The type shown in Fig.12 or 13 is most, and preferably almost to remove all of layer 34 from the original substrate material and only the thin ion barrier layer 33 consist of quartz as the outer printing layer permit. With it you'll get various additional process steps, as will now be described in connection with Figures 8t and RB.

In the process shown in FIG. 8A, the internal coating and collapsing produced layer structure according to Figure 6 or 7 in a normal glass lathe clamped and carefully aligned. When this preform rotates, how schematically is indicated by the dashed circle 41, the material of the substrate layer 34, which is to be removed, abraded by means of a precision grinding wheel 42, which is indicated schematically with 43 and 44 during its rotation against the preform is reciprocated along an axis parallel to the axis of the preform. After a desired thickness is ground off, the preform is cleaned and etched to remove any polishing compound. A good final treatment the preform surface can then be made by flame polishing. at Preforms machined using this method of grinding and polishing it was found that the compressive stress in the outer layer of a low value of a few N / mm2 to a much higher compressive stress of the order of magnitude can be increased by 345 N / mm2 or more.

Thus the method steps described in connection with FIG. 8A the treatment of the preform can be carried out successfully, it depends indicates that the raw preform is carefully controlled during the collapse, to ensure that the layers are concentric and the preform is axial Direction is straight. if essentially the whole of the original Substrate material 34 can be removed, so becomes the very thin ion barrier layer 33, which has previously been melted, is actually the outer quartz layer that exerts the high pressure. This technique also eliminates any cracks and defects, which can arise in the substrate glass tube, rendered ineffective.

Another method for removing the unwanted substrate layer 34 is shown schematically in Figure 8B.

After the raw preform, as previously described, in a suitable manner is clamped in the glass lathe, a CO2 gas laser 45 with high power can be used to remove the undesired substrate material. As in Fix 89 indicated schematically, the laser beam 46 can in the usual way with a Metal mirror 47 is reflected and focused with a lens 48 made of germanium dioxide will. By not shown, but indicated by the dashed arrow 49 Mean is the laser beam along an axis parallel to the axis of the rotating Preform moved back and forth. The ones required for removing material with a laser precise setting means are known to the person skilled in the art and need not be explained further here will.

If the method according to Fig.8A or 8B is applied to the preform shown in Fig.6 is applied, it has been experimentally found that the surface pressure can be increased to a value of about 690 N / mm2. It's theoretical too possible to generate even higher compressive forces in the surface of the preform, such as is known if these compressive stresses are maintained when the The preform is later drawn out into the composite optical fiber and with a Plastic sheath is coated.

Combinations of those shown in FIGS. 8A and 8B can of course also be used Process steps are used, as well as chemical etching to assist the initial removal of the substrate material 34 with the aim of removing the undesired Remove material as accurately and quickly as possible.

Thus, optical preforms and fibers have been shown to be of high quality can be produced that have a significantly greater tensile strength and longer Have service life, provided that in addition to the glass compositions and Dopants, the relative coefficients of thermal expansion and the glass transition temperatures is carefully observed. In the three-layer structure according to Figure 3 and in the The finished structure made from the structure of Fig. 12 consists of the following physical relationships: (1) The glass compositions used for the core and cladding, which together form the inner part of the preform or fiber must have high coefficients of thermal expansion and have relatively low glass transition temperatures.

(2) In addition, they must contain dopants selected in this way are that the desired optical properties result for the core and cladding, i.e.

a lower index of refraction for the cladding than for the core. In In other words: the relative refractive indices must be chosen so that the required numerical aperture and the acceptable optical scattering losses can be achieved, which the resulting fiber should have.

(3) Usually, as has been shown, these requirements are not incompatible with each other. For example, the core can be made from a 20% germanium dioxide (GeO2) doped quartz glass, whereas the cladding layer consists of a 4% Boron oxide (B203) doped quartz glass can exist. Alternatively, the coat can be a borosilicate layer containing 40% germanium dioxide and 15% boron oxide. Such compositions have an average coefficient of thermal expansion, which is significantly higher than that of pure silicon dioxide, i.e. even by one or 2 orders of magnitude higher. They also have a much lower glass transition temperature namely in the range from 6000 to 7500 C compared to about 12000 C for pure silicon dioxide.

(4) The outer printing layer, which is essentially made of pure silicon dioxide exists, has a relatively low coefficient of thermal expansion and a lot higher glass transition temperature Tg, as required.

In the four-layer structure according to Figure 4 and in the completed Structure from the layer structure shown in Fig.7, are the important physical ones Relationships essentially the same as before also for them three-tier structure listed plus the following relationships: (5) The additional layer 14 in Fig.4 or the layer 35 in Fig.7 should be a relative high coefficient of thermal expansion and a relatively low glass transition temperature Have tg.

For example, a glass composition of 40 z GeO2 / SiO2 for this layer can be used.

In order to achieve these desired physical parameters, you can other mixtures of GeO2, B203 and SiO2 can also be used.

(6) The glass transition temperature Tg of the intermediate layer with the large one The coefficient of thermal expansion is not critical, but there is a practical limit which limits its glass transition temperature Tg downwards. The melting points of the three inner layers must not be so low that they cause premature deformation effect of the preform during the collapse. This will make the allowable percentage Share of B203 in any borosilicate layer to a value of less than 50% limited.

The relative thicknesses of the core, cladding, and any intermediate layer are not critical as long as the optical requirements are met. For example the layer radii for the core 25u, for the cladding 30 u and for the intermediate layer 59 u. With an outer pressure layer made of silicon dioxide with a thickness between 1 and 5 F, surface pressures should be of the order of magnitude 345 to 690 N / mm2 can be achieved.

The thickness of the cladding layer can also be increased without the to significantly influence the optical properties of light propagation. this means an advantage of the invention in that it increases the radius of the fiber area with the high coefficient of thermal expansion and the low transformation temperature the surface pressure at a given thickness of the fiber region with the low Thermal expansion coefficient and the high glass transition temperature is increased> ~ Dependent the type of light transmission desired, the cladding layer of the finished Fiber thickness up to 20 or 30> a.

A modification of the method given above will now be described, in which the material of the substrate glass tube on a different than previously shown Kind is removed.

Using the technique of sanding or abrading described above using a laser beam to evenly remove the substrate material, difficulties arise if the collapsed preform is not perfectly straight is or when the core and the various layers are not exactly concentric are.

In addition, the desired outer layer of the finished preform must be obtained and fibers have a thickness of only a few y, so that the necessary high compressive stress arises. To achieve a maximum compressive stress in the outer layer 33 of a finished preform or fiber, the substrate material 34 should be as complete be removed as possible, and the layer 33 should be a uniform final radial thickness in the range of 1 to 10 microns, preferably no more than 5 microns and better still have 2 to 3 P on the finished fiber. When the substrate layer 34 is physically is removed by grinding or by ablating with a laser, so was found that it is difficult to such optimal thin To achieve layer thicknesses of layer 33, if only the collapsed preform is slightly odd or if the core and surrounding layers are not accurate are concentric. In this case some of the substrate layer is likely to be lost 34 remains, or that some parts of the thin layer 33 are removed. Through this the size and evenness of the desired pressure on the surface adversely changed. In any case, the optimal uniform radial Thickness of 2 to 3 P of the outer layer 13 (Fig.3, Fig.4) of a finished fiber only very difficult to achieve.

As will now be described in detail, the completion of the Preform not by any non-linearity or by a lack of concentricity its core and the surrounding layers are impaired. Hence the layer 13 of the finished fiber is thin enough in any case, namely 2 to 3 p thin will.

So far it has been indicated that layer 13 is essentially the same Glass should have the same composition as the substrate glass layer, i.e. undoped silicon dioxide, however, preferably a higher purity. So when these two layers melted together so they essentially behave like a single layer so far it concerns the process steps of physical grinding or removal. In the process steps described below, an alternative to this Much finer control of the removal of the substrate material is possible by easily different glass compositions for the layers 34 and 33 can be used. Such slightly different glass compositions have different compositions Etching speeds, when the structure shown in Fig. 6 is embedded in a commercially available glass etchant such as e.g. a hydrofluoric acid solution is immersed. You can, as before, also for the dejected Printing layer 13 use essentially pure silicon dioxide, but then one Substrate glass made of a doped silicon dioxide material with a substantially greater etching speed.

It was found that the under the trade name "VYCOR" known silicate glass is well suited for this purpose. These glass compositions, which is widely used in the glass industry is from Corning Glass Works, Corning, New York and is composed of a sodium borosilicate glass from 96 to 97% silicon dioxide with a 3 to 4 percent mixture of boron oxide and sodium oxide, i.e. 1 to 2% boron oxide and the remainder sodium oxide. It is known, that this sodium borosilicate glass is somewhat softer than pure quartz glass, and that in hydrofluoric acid it is about three times the etching speed than pure quartz glass Has. The etching speeds can of course be increased to a certain extent or be scaled down by changing the concentration and temperature of the etchant changes. Other glasses that are similar to the "VYCOR" glass can also be inserted by inserting them Appropriate modifiers can be made as opposed to the etching rate Magnify quartz glasses. An essential feature of the modification described here of the manufacturing process consists in choosing a suitable glass for the substrate tube, For example, select "VYCOR" or a similar glass that is opposite to pure Quartz glass or another glass suitable for the thin printing layer a higher one Has etching speed.

Thereby the relationship between the substrate layer 34 and the Printing layer 33 determined in the partially finished preform according to Figure 6.

In order to shorten the etching time, if the one shown in FIG. 6 or that shown in FIG The preform is finally processed, a substantial part of the excess can initially Substrate material by mechanical grinding or ablation with a laser beam can be removed as described above, but this step is not essential.

The subsequent etching process can be easily performed with accuracy control by regulating the concentration of the etchant, as well as the Temperature and immersion time. The etchant is preferably known per se Wise magnetically stirred in order to achieve an even etching. The etching time is set depending on the thickness of the substrate layer so that the whole substrate layer is removed and only the printing layer 13 as the outer layer of the finished preform remains behind. Details of the etching process are known per se and need here not to be further described. Because the softer substrate glass is much faster as the underlying layer is etched away, the etching process can easily be carried out with be stopped within a reasonable time tolerance before the thin layer 13 is attacked will. As an example, Table 1 below shows some glass compositions, which are suitable for the described etching process, together with brief information the desired physical properties of each glass: Tabel I Layer No. Glass Composition Relative Physical Properties 31 Quartz glass with 20% GeO2 Suitable refraction (optionally with traces of index for fiber core Boron or phosphorus) Relatively high coefficient of thermal expansion Relatively low glass transition temperature T g 32 4% borosilicate glass (or suitable refractive index, optionally a mixture of 40% for fiber cladding germanium silicate glass and relatively high heat-15% borosilicate glass) expansion coefficient expansion coefficient Relatively low glass transition temperature Tg 35 (only with 40% germanium silicate glass, relatively high heat - Fig. 7) (or optionally Mixtures of expansion coefficients of germano- and borosilicate (relative glasses) transformation temperature Glass-9 Tg 33 Pure fused quartz- Relatively low glass (Si02) thermal coefficient Relatively high glass transition temperature T g Relatively low etching rate 34 VYCOR glass or similar. Relatively high etching speed in hydrofluoric acid As before Mentioned can be the exact dimensions of the core layer and the various layers vary considerably depending on the particular purpose for which the finished fiber is used is provided and depending on whether it is a single mode or multimode optical fiber should be.

Table II below gives some for explanation purposes only exemplary dimensions of finished multimode optical fibers, which are made from the Layer structures according to Fig. 6 and 7 completed preforms can be drawn. It should be noted that in each case the outer layer is a high compressive stress has almost optimal thickness of 2 to 3, u, as with the manufacturing process described above is easily accessible.

Table II Fiber Layer From Preform Thickness in µm Added Through Layer knife in um A. Finished three-layer fiber (from Fig. 6) core 31 50 (diameter) 50 Jacket 32 15 80 outer print layer 33 2-3 84-86 B. Finished four-layer fiber (from Fig. 7) core 31 50 (diameter) 50 cladding 32 15 80 intermediate layer 35 30 140 outer Printing layer 33 2-3 144-146 It can thus be seen that by a suitable choice of material various advantages for layers 13 and 14 and the preferred etching process over other Procedures arise. These benefits include: (1) It is desirable that the substrate glass tube have a precision bore has, however, its wall thickness does not have to be particularly precise.

(2) If the excess material of the substrate glass tube first is removed by grinding or some other physical method, this is it Process step not critical at all, since it does not affect the substrate material until the thin layer of pressure that is to remain has to be removed.

(3) In the described method of selective etching there is no risk of the etchant penetrating the thin printing layer and destroying it.

(4) In the event that the partially finished preform is slightly out of round, for example, due to non-uniform collapse, the method may the selective etching can still be used to create a high tensile strength preform and a resulting optical glass fiber with an outer pressure layer more uniform To manufacture thickness.

Some of the relationships between the properties are discussed below of the glass materials used and those of the glass fibers produced.

The Fig.9 shows graphically the relationship between the pressure and Tensile forces at the interface 15 of the in 4 shown fiber structure exist as a function of the thickness of the outer layer. The compressive force on the outer surface of the layer 13 extends exponentially as shown as a function of the thickness of the outer Layer (curve 16) and its value becomes asymptotic at the interface 15.

To actually increase the tensile strength of optical fibers To determine with outer layers with layers 14 and 13 of Figure 4 is the service life as a function of the tension exerted on the composite fiber in Fig. 10 shown schematically for three curves, each with a different difference in Thermal expansion between the materials of layers 14 and 13. Curve A shows schematically that if there is a relatively small difference between the coefficients of thermal expansion of layers 14 and 13, the fiber is made in a short time at a given time effective tensile stress will fail. If there is a greater difference in the coefficient of thermal expansion could the fiber in the manner shown generally by curve B. fail and, if the difference is still greater, according to that shown by curve C. Art. It should be noted that the curves of Fig.10 are shown purely by way of example and are not related to any particular standard.

In addition, they are not necessarily linear for every case.

The glass materials listed in Fig.5 are used for the layers 14 and 13 in Figure 4 preferred because of the easy manufacturing costs, which in a Glass-on-glass coatings are created in optical fibers. It was also found that the surface pressure can be increased significantly if the thickness of the relative thin outer layer controls and if you have a second extreme Applying a layer that has a higher coefficient of thermal expansion and a lower one Glass transition temperature as the corresponding parameters of the outermost layer Has.

If you choose silicon dioxide for the outer layer, you can the surface pressure for a range of internal thermal expansion coefficients Calculate the area which is enclosed by the silicon dioxide layer. The Fig.11 shows the results of such a set of calculations. The figure shows the Course of the axial compressive stress as a function of the weighted average of the Coefficients of thermal expansion of the materials forming the inner area for four different thicknesses of the outer layer, which are between 1 and 10 μm. It can be seen that for a given coefficient of thermal expansion the axial compressive stress in the fiber increases faster when you consider the thickness of the outer Layer diminished.

In the case of the optical glass fibers described above, both the Refractive indices and coefficients of thermal expansion of the core 11 and the cladding 12 determined by the choice of glass composition. For example, with typical commercially produced glass optical fibers the core of a glass composition of GeO2 / SiO2 and the cladding consists of a glass composition of B203 / Si02. Typical curves of the coefficient of thermal expansion as a function of the glass compositions are shown in Figures 12 and 13 for these two types of glass.

Mixtures can also be used.

So far, the description has largely been based on that it is important to setting for the Glass core, the cladding and the outer layer or layers to select and click on the relative thickness of the layers must be observed, so that there is one in the thin outer layer great compressive stress arises.

It also emphasized the known facts that the core and the cladding must have the appropriate indices of refraction for the finished fiber has the high-quality optical transmission properties.

However, there is also an additional parameter that is also should also be noted, namely the relationship between the glass transition temperatures T g of the core and the various layers. It was found that a maximum Tensile strength can be achieved if you put a thin outer layer of pressure on it Silicon dioxide is used, which not only has a lower coefficient of thermal expansion than the inner part of the fiber, but also has a much higher glass transition temperature Tg In addition, the outer layer of silicon dioxide should preferably be a substantial one have higher glass transition temperatures Tg than the average of the glass transition temperatures of the core and the layers forming the inner part of the composite fiber. Just like the refractive indices and the coefficients of thermal expansion are also the glass transition temperatures Tg functions of the glass compositions.

Finally, a fundamentally different manufacturing process should be briefly mentioned a tensile optical glass fiber mentioned. This is that on the end of a rotating mandrel is known to have a core glass rod with the required physical properties Properties is formed, wherein the mandrel is moved slowly axially. Simultaneously will on the resulting rod from the core material the various concentric Layers by similar chemical vapor deposition processes applied in a controlled order. The coated rod is on top sintered, so that it is a solid, already provided with a printing layer optical Preform results from which the tensile optical glass fiber can then be drawn.

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Claims (12)

INTERNATIONAL STANDARD ELECTRIC CORPORATION, NEW YORK claims 1. A method of manufacturing a multilayer glass fiber optical waveguide with a core, a sheath and an outer shell that are due to their opposite the core and the jacket have different coefficients of thermal expansion, the tensile strength of the glass fiber waveguide increased by coating the inside of a substrate glass tube and collapsing a preform is made from which the fiber optic waveguide is drawn, characterized in that all layers of the glass fiber waveguide are successively applied to the inner wall of the substrate glass tube, on it the substrate glass tube collapses and then the material of the original substrate glass tube removed from the collapsed layer structure and the resulting layer from the material (34, Fig.6, Fig.7) of the original substrate glass tube freed preform to the glass fiber waveguide is pulled out. 2. The method according to claim 1, characterized in that fiir the Core (11) and the jacket (12) materials with a relatively high mean coefficient of thermal expansion and a relatively low glass transition temperature and that at a single-layer outer shell (Fig. 3) this (13) and with a two-layer outer shell (Fig. 4) the outermost layer (13) is much thinner than the inner layers (11, 12, (14)) and made of a quartz material with a relatively low coefficient of thermal expansion and a relatively high glass transition temperature compared to the inner layers consists. 3. The method according to claim 2 or 1, characterized in that for the outer shell (Fig.3, 12) or its outermost layer (Fig.4, 13) melted Quartz glass is used. 4. The method according to any one of claims 1 to 3, characterized in that that the removal of the material (34) of the original substrate glass tube by Precision grinding of the collapsed layer structure (Fig.6 or Fig.7) up to one uniform sufficient depth is made (Fig.8A). Method according to claim 4, characterized in that after the precision grinding the ground surface is cleaned, etched and polished to complete the preform, from which the fiber optic waveguide is drawn. 0 ,. Method according to one of Claims 1 to 3, characterized in that that the removal of the material (34) of the original substrate glass tube by Precision trimming of the outer layer (34) of the collapsed layer structure (Fig. 6 or Fig. 7) by means of a highly precisely controlled high power laser beam up to a uniform sufficient depth is made (Fig.8B). 7. The method according to any one of claims 1 to 3, characterized in that that a material (34) with a greater etching speed for the substrate glass tube is used as the material of the layer (33) deposited thereon which has collapsed Layer structure (Fig.6 or Fig.7) to remove the material (34) of the original Substrate glass tube is immersed in a glass etchant and this material is affected the etching speed etched away and the etching process is ended when the whole Material (34) of the original substrate glass tube is removed. 8. The method according to claim 7, characterized in that for the thin outer shell (33, Fig.6) or its outermost thin layer (33, Fig.7) essentially pure fused quartz glass and a sodium borosilicate glass for the substrate glass tube is used. 9. The method according to claim 8 or 7, characterized in that as Etchant hydrofluoric acid (= hydrofluoric acid) is used. 10. The method according to claim 8, characterized in that for the thin outer shell (33, Fig. 6) or its outermost layer (33, Fig. 7) a hard one Quartz glass and a sodium borosilicate glass for the substrate glass tube 96 to 97% silicon dioxide and 3 to 4% mixture of boron oxide (B203) and sodium oxide (Na2O) is used. 11. The method according to any one of the preceding claims, characterized in, that for the core (11), the jacket (12) and optionally an intermediate layer (14) such materials are chosen that they have a weighted average coefficient of thermal expansion which is one or two orders of magnitude higher than that of the outer shell (Fig. 3) or its outermost layer (Fig.4) and that these materials are a weighted Glass transition temperature in the order of 600 ° C to 7250C compared to a Have a glass transition temperature of about 1200C of the outermost quartz layer. 12. The method according to any one of the preceding claims, wherein the outer shell consists of two layers, the inner layer a relatively high one and the outer one Layer has a relatively low coefficient of thermal expansion, characterized in that that a quartz glass is used for the inner layer (14) of the outer shell (13, 14, Figure 4) which is primarily made with germanium or a mixture of germanium and Boron is doped in such a way that it has a relatively high coefficient of thermal expansion still has a relatively low glass transition temperature.