Primary preform for glass optical fibres and method of its manufacture

Brief description: Method for the manufacture of a primary optical fiber preform, primary preform, final preform, optical fiber.

The present invention relates to a method for the manufacture of a primary preform for an optical fiber using a plasma chemical internal vapor deposition process, wherein glass-forming precursors, whether or not doped, are supplied to the interior of a hollow glass substrate tube, a reaction zone in the form of a plasma is moved back and forth over the length of said hollow glass substrate tube between a reversal point near the supply side and a reversal point near the discharge side of said substrate tube, wherein the substrate tube is positioned in an oven and in said reaction zone such circumstances are created that one or more glass layer packages, composed of at least two separate glass layers, are deposited on the inside of said substrate tube. The present invention furthermore relates to a method for manufacturing a final preform, optical fibers, as well as to the primary preform, final preform and optical fiber obtained therefrom.

With internal vapor deposition techniques, a reaction mixture consisting of glass-forming gases and optional dopants is supplied to the supply side of a hollow glass substrate tube, after which these gases are converted into glass in a reaction zone. Unreacted gases and / or residual products are removed via the discharge side of the hollow glass substrate tube.

In an internal PCVD (Plasma Chemical Vapor Deposition) vapor deposition process, the reaction zone is a plasma which is moved back and forth along the length of the hollow glass substrate tube. In a PCVD process, glass layers are deposited directly on the inside of the hollow glass substrate tube, regardless of the direction in which the reaction zone moves. A PCVD process is known, inter alia, from US 4,741,747, US 5,145,509, US 5,188,648, WO 2004/101458 and US 2008/0044150.

In an internal vapor deposition process of type MCVD (modified chemical vapor deposition) or FCVD (furnace chemical vapor deposition), the reaction of the glass-forming gases and optional dopants is activated by heating the outside of the hollow glass substrate tube by means of a burner or an oven, respectively. . In the reaction zone, which is located at the burner or furnace, the glass-forming gases are converted into so-called soot, which soot is deposited on the inside of the hollow glass substrate tube under the influence of thermophoresis. Under the influence of heating, this soot is then glazed into a glass layer. In an MCVD or FCVD process, glass layers are deposited only if the reaction zone moves toward the discharge side of the hollow glass substrate tube. PCVD, MCVD and FCVD processes are known in the art.

JP 57-51139 discloses an MCVD process in which a base material for an optical fiber is manufactured. In a cycle a number of glass layers are deposited on the inside of a substrate tube, the deposition starting at a position near the supply side and the distance over which the reaction zone moves in the direction of the discharge side is varied per glass layer. The base material is made by performing a number of cycles in succession.

An optical fiber consists of a core and a jacket surrounding the core, which is also referred to as cladding in the English language. The core usually has a higher refractive index than the cladding so that light can be transported through the optical fiber.

The core of an optical fiber can consist of one or more concentric layers, each with a specific thickness and specific refractive index or a specific refractive index curve in the radial direction.

An optical fiber with a core consisting of one or more concentric layers in which the refractive index of the concentric layers is constant in the radial direction is sometimes referred to as a (multiple) step-index optical fiber. The difference n, of the refractive index of a concentric layer with the refractive index of the cladding n_c, can be expressed in a so-called delta value, indicated by Δ ° /_ο and can be calculated according to the formula below:

² _ ²

Δ,.% = ', * 100% where:

n, = refractive index value of low_c, = refractive index value of the cladding

An optical fiber can also be manufactured so that a core with a so-called gradient index refractive index profile is obtained. Such a radial refractive index profile can be described both with a delta value, Δ% and with a so-called alpha value, a. The maximum refractive index in the core is used to determine the Δ% value. The alpha value can be determined according to the formula below:

<td>(</td><td></td><td>oc</td>

<td>1-2Δ%</td><td>-</td><td></td>

<td></td><td>V^a)</td><td>)</td>

where:

ni = refractive index value in the center of the fiber a = radius of the gradient core index [pm] α = alpha value r = radial position in the fiber [pm]

A radial refractive index profile of an optical fiber is to be understood as a representation of the refractive index as a function of the radial position in an optical fiber. It is also possible to graphically represent the refractive index difference with the cladding as a function of the radial position in the optical fiber, which can also be seen as a radial refractive index profile.

The shape of the radial refractive index profile and in particular the thicknesses of the concentric layers and the refractive index or the refractive index curve in the radial direction of the core determine the optical properties of the optical fiber.

A primary preform comprises one or more preform layers that form the basis for the one or more concentric layers of the core and / or a portion of the cladding of the optical fiber that can be made from a final preform.

A preform layer is made up of a number of glass layers. In an internal vapor deposition process, a glass layer is the layer deposited upon movement of the reaction zone from the feed side to the discharge side or from the discharge side to the feed side.

A final preform as indicated herein is a preform from which an optical fiber is made by a fiber draw process.

To obtain a final preform, a primary preform is provided on the outside with an additional layer of glass, which additional layer of glass comprises the cladding or a part of the cladding. This additional layer of glass can be applied directly to the primary preform. It is also possible to place the primary preform in an already formed glass tube, also referred to as "jacket tube" in the English language. This jacket can optionally be contracted on the primary preform. Finally, a primary preform can include both the core and the cladding of an optical fiber so that the application of an additional layer of glass is not necessary. In that case, a primary preform is identical to a final preform. A radial refractive index profile can be measured on a primary and / or a final preform.

The length and diameter of a final preform determine the maximum length of optical fiber that can be made from the final preform.

In order to reduce the production costs for the production of optical fibers and / or to increase the yield per primary preform, it is therefore desirable to be able to produce the longest possible length of optical fiber that meets the quality requirements on the basis of a final preform.

The diameter of a final preform can be increased by applying a thicker layer of additional glass to a primary preform. Since the optical properties of an optical fiber are determined by the radial refractive index profile, the layer of additional glass must at all times be in the correct proportion to the layer thickness of the preform layers of the primary preform that form the core, more particularly the one or more concentric layers of the core will form in the optical fiber. Therefore, the layer thickness of the additional glass layer applied to the primary preform is limited by the thickness of the preform layers produced by the internal vapor deposition process.

The length of a final preform can be increased by increasing the length, more particularly the usable length, of a primary preform. The usable length is to be understood as the length of the primary preform over which the optical and geometric properties lie within predetermined tolerance limits, which tolerance limits are chosen such that optical fibers are obtained which meet the desired quality requirements.

To determine the usable length of the primary preform, a radial refractive index profile is measured at a number of positions along its length, after which it is possible to determine a so-called longitudinal refractive index profile and longitudinal geometry profile on the basis of these measurements, if desired. .

Thus, a longitudinal refractive index profile can be seen as a graphical representation of the refractive index of a preform layer as a function of the longitudinal position in the primary preform. Of course, instead of the refractive index, the refractive index difference can also be used to determine a longitudinal refractive index profile.

A longitudinal geometry profile can be seen as a graphical representation of the transverse surface of a preform layer as a function of the longitudinal position in the primary preform. The transverse surface, also referred to as CSA, can be determined on the basis of a radial refractive index profile. The CSA can be calculated as follows:

CSA_;

<img file="NL2007448C_D0001.tif" />

in which

CSA;

<img file="NL2007448C_D0002.tif" />

<img file="NL2007448C_D0003.tif" />

= Transverse surface of preform layer i [mm²] = outside diameter of preform layer i = inside diameter of preform layer i [mm]

The usable length of a primary preform is particularly adversely affected by so-called "taper". The term taper is to be understood as a deviation from the optical and / or geometric properties of the primary preform in areas near the ends thereof. A distinction is made between optical taper and geometric taper.

Optical taper refers to deviations from the refractive index (or the refractive index difference), while geometric taper refers to deviations from the transverse surface of the preform layer.

If a primary preform is composed of different preform layers, the optical and geometric taper of the preform layers may differ from each other.

Methods are known in the art for limiting optical and / or geometric taper.

For example, U.S. Patent No. 4,741,747 discloses a method for manufacturing optical preforms according to the PCVD method wherein glass layers are deposited by moving a plasma on the inside of a glass tube back and forth between two reversal points while adding a reactive tube to the tube gas mixture at a temperature in the range of 1100 ° C and 1300 ° C and at a pressure between 1 and 30 hPa. By causing the plasma to move non-linearly as a function of time in the vicinity of at least one of the reversal points, the area of ​​non-constant deposition geometry at the ends of optical preform is reduced.

The present inventors have found that, although such a method reduces the geometric taper, the optical taper does not improve or even deteriorate. Moreover, the present inventors have found that it is also sometimes necessary at other positions outside the so-called taper areas to influence the refractive index of the deposited glass.

Thus, while the methods of the prior art can increase the usable length of a primary preform, there is a need for a method by which the usable length can be further increased.

It is therefore an object of the present invention to provide a method for manufacturing primary preforms for optical fibers with a large useful length.

Another object of the present invention is to provide a method for manufacturing primary optical fiber preforms wherein influencing optical taper is independent of geometric taper.

Yet another object of the present invention is to provide a method with which the refractive index and / or the transverse surface as a function of the longitudinal position of the primary preform can be accurately adjusted as desired.

The present invention as stated in the preamble is characterized in that the method comprises the following steps:

i) defining deposition conditions for depositing a number of glass layers to be deposited next to each other and forming a glass layer package under said deposition conditions, ii) defining deposition conditions for depositing a successive number of glass layers and depositing them underneath each other said deposition conditions form a successive glass layer package, wherein the deposition conditions thus defined for i) and ii) differ from each other, and iii) optionally repeating steps i) and ii), wherein the deposition conditions defined in iii) may be identical to the process conditions previously defined in i) and ii). On the basis of the aforementioned method, one or more objectives of the present application are thus met.

The present inventors have found that the axial distribution of the refractive index and the layer thickness of the glass layers to be deposited by means of the plasma chemically internal vapor deposition process depend on a number of process factors, wherein, among other things, the size of the deposition, the temperature profile of the around the substrate tube positioned oven, the velocity profile of the movable reaction zone, the amount of oxygen supplied can be mentioned. On the basis of the aforementioned process parameters, it is possible to set a very uniform distribution of both the refractive index profile and the layer thickness over the length of the substrate tube. The present inventors thus strive to set the present method in such a way that both a maximum result is achieved with regard to both the uniformity of the refractive index and the layer thickness.

The present invention is thus based on the assumption that for obtaining a more uniform profile over the length of the substrate tube with regard to refractive index and / or layer thickness, use is made of a combination of glass layer packages which, individually seen, do not have the desired properties, in terms of refractive index or layer thickness, but in combination have the intended properties. Thus, the present inventors have accomplished the present invention by manufacturing glass layer packages via the plasma chemical internal vapor deposition process, involving a combination of layers to produce a result that is more constant as a function of position for both the refractive index and the transverse surface of the glass layer package.

The term glass layer package used in the present application is to be understood as an assembly of adjacent glass layers. In particular, it holds that the deposition conditions within such a glass layer package, consisting of a number of adjacent glass layers, are equal to each other. Thus, the deposition conditions of the glass layer package obtained in step i) are the same for each glass layer present in the glass layer package. According to step ii) of the present method, a subsequent glass layer package is obtained by a plasma chemical internal vapor deposition process, the deposition conditions used for this successive glass layer package being different from those of the deposition conditions of the glass layer package obtained according to step i). The aforementioned steps i) and ii) can be repeated as required, thus obtaining glass layer packages a, b, c, d, etc., wherein the deposition conditions for glass layer packages a, b, c, d differ from each other. It is thus possible to obtain many glass layer packages by means of a plasma chemical internal vapor deposition process, in which any order of deposition conditions can be applied, but the deposition conditions of adjacent glass layer packages must be mutually different. Thus, for example, glass-layer package combinations of, for example, a, b, c, b, a, but also a, b, c, d, a, b, c and so on are possible. The present invention is not limited to the number of glass layer packages and can therefore comprise two, three, four or even more glass layer packages, depending on the special profile conditions of the optical glass fiber. It is important here that the deposition conditions within a glass layer package are identical and, for a subsequent glass layer package, differ from the adjacent glass layer package previously obtained by deposition. It is thus desirable that the deposition conditions of adjacent glass layers within the same glass layer package correspond to each other. More specifically, it is desirable that, within a given glass layer package, the refractive index value of one glass layer obtained by deposition corresponds to the refractive index value of another glass layer obtained by deposition. The number of glass layers within a glass layer package cannot be regarded as limiting. In addition, the number of glass layers in one glass layer package can differ substantially from the number of glass layers in another glass layer package.

The present inventors have further found that the average refractive index value of glass layer packages, composed of a combination of a number of separate glass layer packages arranged next to each other, can be interpreted as a combination of the refractive index value of each individual glass layer package, the refractive index value of at least two of such individual glass layer packages in said composite glass layer package are mutually different.

Furthermore, it holds that the cross sectional area (CSA) of glass layer packages, composed of a combination of a number of separate glass layer packages arranged side by side, can be interpreted as a combination of the CSA of each individual glass layer package, the CSA of at least two of such individual glass layer packages in the aforementioned composite glass layer package are different from each other.

However, the present invention is by no means limited to a particular number of glass layer packages, nor the number of glass layers in a given glass layer package.

The present invention is based on the insight that when drawing a final preform, the layer thicknesses of the glass layers as deposited by an internal vapor deposition process in the final preform are drastically reduced. A typical optical fiber has a diameter of 125 µm. A final preform for single mode fibers, for example, has a diameter of approximately 100 to 150 mm or even more. The thickness of the preform layers and therefore also the thickness of the glass layers in the final preform are therefore reduced by a factor of the order of about 800 to 1200 or even more during the manufacture of the optical fiber.

The present inventors have realized that the way in which the light propagates through the optical fiber is influenced by the average properties of a number of adjacent glass layers rather than by the properties of each individual glass layer. Thus, the present inventors have found that it is possible to build up a preform layer from glass layer packages, wherein each glass layer package consists of at least two glass layers and wherein the optical properties of the glass layer packages differ from each other, but that the combination of glass layer packages has no effect on propagation. of the light through the optical fiber.

Using the present invention, it is therefore possible to set the deposition conditions of the glass layers in such a way that geometric taper is kept to a minimum while the optical taper is not or hardly influenced. It is also possible, using the present invention, to set the deposition conditions of the glass layers in such a way that optical taper is limited to a minimum while the geometric taper is not or hardly influenced by this. In other words, using the present invention, it has been found possible to adjust the optical taper and geometric taper independently of each other. Therefore, the useful length of a primary preform can be increased relative to methods known in the art.

The present invention further provides the possibility of manufacturing a primary preform wherein the ratio of the transverse surfaces of the different preform layers is substantially constant over the length of the primary preform, but wherein the layer thicknesses of the preform layers are not constant over the length of the primary preform. In a further processing step, such a primary preform can be provided with an additional layer of glass, the layer thickness of the additional layer of glass being chosen such that the ratio between the transverse surface of the additional layer of glass and the transverse surface of the preform layers is constant over the length. of the primary preform. A final primary preform is thus created in which the proportions of the additional layer of glass and the preform layers are constant along the length of the final preform. The outer diameter of such a final preform is generally not constant in the longitudinal direction. The aforementioned technology is referred to as profile overloading. A final preform made on the basis of profile overcladding technology results after drawing into a fiber with a constant outer diameter to a fiber in which the layer thicknesses of the concentric layers of the core and cladding are substantially constant over the length, which in turn results in a fiber with virtually constant longitudinal optical properties.

In a preferred embodiment, the glass layers of a glass layer package each have a thickness seen in the radial direction in the range of 0.1 to µm, preferably 0.5 to 5 µm.

In a further preferred embodiment, the number of glass layers from which a glass layer package is composed is in the range 2-100, preferably 2 - 50 and more preferably 4 - 30. A relatively high number of glass layers makes an accurate control of the average optical properties of the glass layer package. possible. A small number of glass layers is relatively easy to control, but limits the possibility of adjusting the average optical properties of the glass layer package. A practically manageable process can be conducted with a glass layer package of approximately 10 to 20 glass layers.

Preferably the number of glass layers of a glass layer package is set such that the following condition is met:

N &lt;0.1 * final fiber where = number of glass layers in a glass layer package [-] = minimum applied wavelength of the optical fiber [pm] = thickness of glass layer in glass layer package of a primary preform [pm]

Q_fina= diameter of the primary preform [mm] = diameter of the final preform made from

Qfibre = diameter of the optical fiber [mm]

The definition of the deposition conditions preferably comprises the setting of one or more process parameters selected from the group of flow rates of the glass-forming precursors to be dosed on the supply side, content of dopant (s), speed of the reaction zone, intensity of the plasma of the reaction zone and the length of the reaction zone.

It is noted that the direction of movement of the reaction zone should not be considered as a deposition condition.

In the present invention, it is desirable that the deposition length, i.e. the length of the substrate tube over which the reaction zone is moved between the two reversal points, the relevant deposition condition, be kept constant during the deposition of glass layers to form a glass layer package. Thus, in a special embodiment, it holds that the deposition condition determined for the deposition of the one glass layer package composed of a number of glass layers is constant during the deposition of the one glass layer package, and wherein the deposition condition determined for the deposition of the other glass layer package composed of a number of glass layers is also constant during the deposition of the other glass layer package, but wherein the deposition condition used for one glass layer package differs from the deposition condition used for the other glass layer package.

The dopants can be both a refractive index increasing and a refractive index reducing agent. The present invention is not limited to the use of a single dopant, but a combination of dopants can also be used. The dopants used may furthermore differ per glass layer of a glass layer package. Suitable dopants are, for example, GeCl₄, PO₂CI₅, N₂ CF₄SiF₄, C₂F₆, C₄F₈, CCI₂F₂SiF₄Si₂F₆, SF₆, NF₃ and f₂.

By adjusting the amount of dopant over the length of the primary preform, it is possible to obtain a desired longitudinal refractive index profile. If necessary, the speed of the reaction zone can also be adjusted as a function of the position to influence the thickness of the glass layer in the longitudinal direction. The geometric taper can thus also be influenced.

The gases such as O₂Ar and He, when fed to the reaction zone, can have an effect on the intensity of the plasma, with the result that dopant incorporation efficiency can be increased or decreased. The total amount of glass deposited and thus the layer thickness of a glass layer can also be influenced somewhat.

The deposition length is to be understood as the distance between a reversal point of the reaction zone near the supply side and a reversal point of the reaction zone near the discharge side of the hollow glass substrate tube. The adjustment of the deposition length can therefore be carried out by adjusting the position of the reversal points of the reaction zone for the glass layers of a glass layer package. Varying the deposition length is a possibility to influence the layer thickness of the glass layer package near the supply and / or discharge side. The deposition length on the supply side is preferably reduced by no more than the length of the plasma. Also, the deposition length on the discharge side is preferably reduced by no more than the length of the plasma. The length of the plasma in a PCVD process is approximately 5 cm - 60 cm, preferably 15 cm - 25 cm. It is noted that setting the deposition length should not be construed as defining deposition conditions.

The reaction zone is preferably a plasma generated by means of microwaves and preferably moves at an average speed in the range 2 m / min - 40 m / min, preferably 15 m / min - 25 m / min in the longitudinal direction of the hollow glass substrate tube back and forth between the two turning points.

Preferably, the primary preform comprises at least one preform layer, which preform layer is at least partly composed of glass layer packages and wherein the preform layer has seen a substantially constant average refractive index in the radial direction. The principle of the present invention applies both to preform layers with a (medium) constant refractive index, indicated by step index preform layers, and also to preform layers with a non-constant refractive index. For example, the present invention can also be applied to the manufacture of optical fiber preforms with a gradient index type core, or a core with a triangular refractive index profile.

If a primary preform comprises several different preform layers, the glass layer packages from which these different preform layers are composed can differ from each other. A first preform layer can for instance be composed of glass layer packages with ten glass layers, while a second preform layer is composed of glass layer packages of sixteen glass layers. The deposition conditions of the glass layer packages may also differ, but the deposition conditions within a glass layer package are identical for the glass layers present in such a package.

The present invention further relates to a method for manufacturing a final preform for an optical fiber comprising the following steps:

i) producing a primary preform according to the present invention ii) contrasting the primary preform obtained in step i) to a solid primary preform under the influence of a heat source, iii) optionally applying an additional amount of glass to the outside of the solid primary preform obtained in step ii) to form the final preform.

An optical fiber can then be made by heating one end of the final preform and pulling the optical fiber therefrom.

The present invention will be explained in more detail below with reference to a number of figures and an example, although it should be noted, however, that the present invention is by no means limited thereto.

Figure 1 shows schematically a plasma chemical internal vapor deposition process

Figure 2 schematically shows a radial refractive index profile of a step-index optical fiber.

Figure 3 shows two examples of the refractive index profile of a so-called single mode core.

Figure 4 shows an example of the cross-sectional profile (CSA) as a function of the position of different glass layer packages.

Figure 5 shows an example of the refractive index value as a function of the position of different glass layer packages.

Figure 6 shows the refractive index value of a single mode core as a function of the position in the preform bar.

Figure 7 shows the cross-sectional profile of a single mode core as a function of the position in the preform bar.

Figure 8 shows the refractive index profile as a function of the position for a core composed of three glass layer packages.

Figure 9 shows the average refractive index profile as a function of the position for a core composed of three glass layer packages.

Figure 10 shows the average cross-sectional profile as a function of the position for a core composed of three glass layer packages.

Figure 11 shows a longitudinal refractive index profile of a primary preform made according to the prior art with a PCVD process.

Figure 12 shows a longitudinal geometry profile of the primary preform according to the comparative example.

Figure 1 schematically depicts an internal vapor deposition process for the manufacture of a primary optical fiber preform. A hollow glass substrate tube 5 is provided with a supply side 6 and a discharge side 7. Supply side 6 and discharge side 7 can be positioned between a gas supply and a gas discharge, respectively (not shown). Supply side 6 and discharge side 7 can be clamped, for example, by means of a cylindrical passage with o-ring seal, so that the internal volume of hollow glass substrate tube 5 is isolated from the atmosphere on the outside thereof. Such a construction makes it possible to perform an internal vapor deposition process under reduced pressure when a pump (not shown) is coupled to the gas outlet. The reaction mixture containing the glass-forming gases and optional dopants are supplied on the supply side 6 during the vapor deposition process. Any additional dopants that are supplied in the method according to the present invention can either be supplied directly on feed side 6 or mixed with the reaction mixture prior to feed.

Figure 1 furthermore shows a reaction zone 8 which, during the internal vapor deposition process, reaction zone 8 moves back and forth between a reversal point 11 located near supply side 6 and a reversal point 12 located near discharge side 7. Reaction zone 8 has a length 9 seen in longitudinal direction of substrate tube 5. , which is relatively small in relation to the deposition length. For a PCVD process, length 9 is approximately 5 cm - 60 cm.

The distance between both reversal points is the deposition length 10, which deposition length 10 corresponds to the length over which glass layers are deposited on the inside of hollow glass substrate tube 5. In a PCVD type internal vapor deposition process, at least the deposition length 10 and the two reversal points can be surrounded by an oven (not shown) set to a temperature of about 800 ° C - 1300 ° C, preferably 950 ° C - 1100 ° C.

During the internal vapor deposition process, a gas mixture of glass-forming gases, whether or not provided with dopants, is supplied via supply side 6 of hollow glass substrate tube 5, these glass-forming gases being converted into glass in reaction zone 8. Thus, using the reciprocating reaction zone 8 between reversal points 11 and 12, a number of glass layers 3 (see Figures 3 and 4) are deposited on the inside of hollow glass substrate tube 5.

The present invention relates to an internal vapor deposition process of the PCVD type in which microwaves are partially coupled into the interior of hollow glass substrate tube 5 through a resonance space, also referred to as a resonator, in the longitudinal direction to form reaction zone 8, namely a plasma. Length 9 of the reaction zone 9 depends in particular on the construction of the resonator and the process settings. The ratio between the length 9 of the reaction zone and the length of the resonator, viewed in the longitudinal direction, is approximately 0.5-3.

The resonance space is moved back and forth over the length of the hollow glass substrate tube in a PCVD process between the reversal points 11 and 12. Resonators are known in the art, for example from US patent applications published under the numbers US 2007/0289532, US 2003/0159781 and US 2005/0172902, and US patents US 4,844,007 US 4,714,589 and US 4,877,938. The PCVD process is a so-called low-pressure process, which means that the pressure during the internal vapor deposition process is set to a value in the range 1 - 40 mbar, preferably in the range 5-30 mbar.

Figure 2 schematically shows a radial refractive index profile of a contracted primary preform for an optical fiber. The primary preform is provided with a core 1 and a cladding 2. The refractive index difference between core 1 and cladding 2 is shown as Δη1. The refractive index of core 1 and cladding 2 have both seen a constant value in the radial direction. There is thus a primary preform for a step-index type optical fiber. In the manufacture of a primary preform according to Figure 2, cladding 2 comprises a substrate tube 5 and optionally one or more additional preform layers (not shown), and any additional preform layers to be understood as preform layers which are composed of several glass layers 3. Glass layers 3 are deposited at a reciprocating movement of reaction zone 8 during the internal vapor deposition process. It is noted that substrate tube 5 in the primary preform should not be construed as a preform layer.

The layer thickness of a glass layer in a primary preform is reduced many times during the processing of the primary preform into an optical fiber. Layer thicknesses of individual glass layers in a glass layer package for a PCVD type process are in the range 0.1 µm - 10 µm per glass layer. A final preform has an outer diameter, depending on the type of optical fiber being produced, in the range 50 mm - 200 mm, so that the layer thickness of a glass layer in the primary preform is reduced by a factor of 400 to 1600, respectively. As a result, for primary preforms produced by a PCVD process, the layer thickness of a glass layer in the optical fiber will be many times smaller than the wavelength of the light propagating through the fiber, so that this light is joined to each other by a relatively large number of adjacent glass layers is influenced rather than by the individual glass layers.

An optical fiber is mainly used in the wavelength range between approximately 850 nm and 1700 nm. A typical optical fiber further has a diameter of about 125 µm, more generally the diameter is in the range 80 µm - 250 µm.

In addition to the effect of the layer thickness of a glass layer, the present inventors assume that during the processing of the primary preform into an optical fiber, dopants in a glass layer diffuse slightly to adjacent glass layers. As a result, the mutual differences in refractive index between adjacent glass layers is somewhat reduced.

Figure 3 shows two examples of the refractive index profile of a so-called single mode core. The profile shown on the left is of an ideal single mode core where the refractive index value displays a constant value over the entire radius. The principle according to the present invention is schematically shown in the right-hand side. The deposition of the core is achieved by forming different glass layer packages, also called stacks, with the glass layer packages showing different values ​​for the refractive index. However, within such a glass layer package, the refractive index value is constant. By applying different deposition conditions for different glass layer packages and combining the refractive index values ​​of the glass layer packages thus obtained, an average refractive index value of the core is obtained which is identical to the value as shown in the left-hand side of the figure. Although seven different glass layer packages can be distinguished in figure 3, it should be clear that the application is not limited to such a number. Furthermore, in the right-hand side, the refractive index value for the "high" and "low" stacks is always the same, so in fact the combination of stack A and stack B. It should also be noted that the application is not limited to the combination of stacks A and B but that many embodiments are possible, for example ABCBCA etc.

Figure 4 shows an example of the cross-sectional profile (CSA) as a function of the position of different glass layer packages. The line with the highest value for CSA on the left side of the figure can be considered as the CSA of a first stack. The line with the lowest value for CSA on the left side of the figure can be considered as the CSA of a second stack. The remaining line is half the sum of the two aforementioned lines.

Figure 5 shows an example of the refractive index value as a function of the position of different glass layer packages. The line with the highest value for the refractive index on the left side of the figure can be considered as the refractive index of a first stack. The line with the lowest value for the refractive index on the left side of the figure can be considered as the refractive index of a second stack. The remaining line is half the sum of the two aforementioned lines.

It is clear from both Figure 4 and Figure 5 that the average values ​​show a more uniform behavior as a function of the position.

Fig. 6 shows the refractive index value of a single mode core as a function of the position in the preform bar, with regions recognizing at both ends of the preform bar deviating from the center region, namely between 200 and 1000 mm. Such deviating values ​​limit the usable length of the preform bar from which optical fibers can be obtained.

Figure 7 shows the cross-sectional profile of a single mode core as a function of the position in the preform bar. Here too, just as in Figure 6, areas at the two ends can be recognized that deviate from the central area, namely between 200 and 1000 mm.

Figure 8 shows the refractive index profile as a function of the position for a core according to the present invention composed of three glass layer packages. The usable length of the preform bar from which optical fibers can be obtained is greater than that of Figure 6 and Figure 7.

Figure 9 shows the average refractive index profile as a function of the position for a core according to the present invention composed of three glass layer packages. Here, too, there is an increase in the usable length of the preform bar compared to that of Figure 6 and Figure 7.

Figure 10 shows the average cross-sectional profile as a function of the position for a core according to the present invention composed of three glass layer packages. The increase in the usable length of the preform bar compared to that of Figure 6 and Figure 7 is clearly perceptible.

Comparative example

Figure 11 shows a longitudinal refractive index profile of a primary preform made according to the prior art with a PCVD process. On the horizontal axis the position along the length of the primary preform is shown, while the refractive index is shown on the vertical axis. The primary preform according to the comparative example has three preform layers, namely a preform layer 13 with a high refractive index, which will form the core in the fiber, a preform layer 14 and a preform layer 15. Around the refractive index value of each preform layer 13, 14 and 15 are two horizontal lines corresponding to the tolerance limits within which the refractive index of the preform layer should lie.

The refractive index measurements of the preform layers of the primary preform according to the comparative example begin at a position of approximately -175 mm and end at a position of approximately 950 mm.

It can be seen that the refractive index of in particular preform layer 13 does not fall within the tolerance limits over a part of the length. This results in that the usable length of the primary preform according to the comparative example is limited, as far as the refractive index is concerned, to the length part between the positions of approximately -20 mm and 800 mm or even lower. As a result, the usable length of the primary preform according to the comparative example is a maximum of approximately 820 mm.

Figure 12 shows a longitudinal geometry profile of the primary preform according to the comparative example. The position along the length of the preform is shown on the horizontal axis, while the diameters of the preform layers are shown on the vertical axis. The diameters of the preform layers 13, 14 and 15 are shown as a function of the position in the primary preform. Two horizontal lines are shown around the diameter value of each preform layer 13, 14 and 15 which correspond to the tolerance limits within which the diameter of the preform layer should lie.

It can be seen that the diameter of, in particular, preform layer 14 does not fall within the tolerance limits over a part of the length. This leads to the usable length of the primary preform according to the comparative example being limited in terms of geometric properties to the length part between positions of approximately Omm and 1125 mm.

Since the usable length of the primary preform according to the comparative example has already been limited because of the refractive index between positions -20mm and 800mm, the usable length of the primary preform according to the comparative example will be limited to the length part between positions Omm and

800 mm. In other words, the usable length of the primary preform according to the comparative example is approximately 800 mm.