Primary preform for optical fibres and method for 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.

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

A.% = "^d* 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:

w (r) = η_λ 1 - 2Δ%

<img file="NL2007447C_D0001.tif" />

where:

= refractive index value in the center of the fiber = radius of the gradient core index [pm] = alpha value = 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:

<td></td><td>CSA, = ^ (dr "-d,?,)</td><td></td>

<td>In which</td><td></td><td></td>

<td>CSA</td><td>= Transverse surface of preform layer i</td><td>[mm²]</td>

<td>di,_you</td><td>= outer diameter of preform layer i</td><td>[mm]</td>

<td>^dij</td><td>= inner diameter of preform layer i</td><td>[mm]</td>

<td></td><td>The usable length of a primary</td><td>preform is in particular</td>

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 is characterized in that the method comprises the following step: defining deposition conditions for the deposition of at least one glass layer of the glass layer package as a function of the position of the reaction zone, viewed in the longitudinal direction of the hollow glass substrate tube, the thus defined deposition conditions within the deposition of the aforementioned glass layer differ from each other.

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 at least two glass layers of the glass layer package differ from each other, without this having an effect on the propagation of the light through the optical fiber.

That is, the present inventors have found that a first fiber, made on the basis of a first primary preform constructed from one or more preform layers, each of which is composed of mutually identical glass layers, has the same optical properties as a second fiber made according to the present invention, wherein the average optical properties of a glass layer package in the second primary preform correspond to the optical properties of the glass layers in the first primary preform. It is noted that the average optical properties are determined in the radial direction.

In other words, the present inventors have found that in an internal vapor deposition process in which relatively thin glass layers are deposited, not every glass layer need have exactly the same refractive index or the same transverse surface, preferably that the thickness of the individual glass layer in the optical fiber manufactured on the basis of the primary preform is significantly smaller than the wavelength of the light propagating through the optical fiber and that the average refractive index of the glass layers is equal to the refractive index of a preform layer in a primary preform in which the glass layers all display the same refractive index. It is further desirable that for the purpose of the refractive index value described above also apply to the transverse surface of the glass layers obtained by deposition within a glass layer package.

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 affected. 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 also provides for the possibility of adjusting the average refractive index and / or the average transverse surface of a preform layer viewed in the radial direction over the length of a preform position. For this purpose, the deposition conditions can be varied as a function of the position of the reaction zone. It is therefore desirable that the deposition conditions of adjacent glass layers within a glass layer package differ from each other.

More specifically, it has been found possible to limit local deviations from the average refractive index and / or the average transverse surface of a preform layer viewed in radial direction by the deposition conditions as a function of the position of the reaction zone viewed in the longitudinal direction of the hollow glass substrate tube for each glass layer in a glass layer package. Thus, it is preferable that within a certain glass layer package the refractive index value and / or the transverse surface of one glass layer obtained by deposition differs from the refractive index value and / or the transverse surface of another glass layer obtained by deposition.

In other words, the present invention is not limited to influencing the geometric and optical taper, but is applicable over the entire length of the primary preform.

In a particular embodiment, it holds that the average refractive index value and / or the average of the transverse surface of a particular glass layer package, composed of a number of separate glass layers obtained by deposition, can be conceived as a combination of the refractive index value or the transverse surface of each individual glass layer, wherein the refractive index value and / or the transverse surface of at least two of such individual glass layers in said glass layer package are mutually different.

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.

Thus at least one of the aforementioned objectives is met.

In a preferred embodiment, the glass layers of a glass layer package each have a thickness viewed in the radial direction in the range of 0.1 to 10 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 low 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:

/ V &lt;Q] * X / jmal 'd * O_nü jibre in which

N = number of glass layers in a glass layer package [-] λ = minimum applied wavelength of the optical fiber [pm] d = thickness of glass layer 3 in glass layer package 4 of a primary preform [pm]

Q_finai = diameter of the final preform made on the basis of the primary preform [mm]

Q_fiber = diameter of the optical fiber [mm]

Defining the deposition conditions preferably includes setting one or more process parameters selected from the group of amount of additional gas to be metered on the feed side, speed of the reaction zone, intensity of the plasma of the reaction zone and the length of the reaction zone. The term quantity should in particular be understood to mean the flow rate, namely the quantity per unit of time.

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

In a particular embodiment of the present invention, it is thus possible for the deposition length, i.e. the length of the substrate tube over which the reaction zone is moved between the two reversal points, to be subdivided into separate deposition areas, the respective deposition condition being determined for each deposition area. In particular, it holds that the deposition condition, determined for a deposition area, is adjustable during the deposition process.

The supply of the additional gas preferably takes place in the form of one or more pulses with a pulse duration and a pulse height. The dopants are added to the reaction mixture. A suitable device for this embodiment is described in EP 2 199 263. Using this device, a main gas stream of glass-forming gases is optionally supplied with a quantity of dopants on the supply side of the hollow glass substrate tube. An additional quantity of dopants can be supplied by means of a by-gas stream. The present invention can thus be applied, for example, by adding pulses of additional dopants to the main gas stream via the secondary gas stream.

The pulse duration is preferably kept relatively small in order to be able to adjust the refractive index of the glass layer in the longitudinal direction as accurately as possible. The pulse height then determines the extent to which the refractive index is affected. The pulse duration is preferably between 1 ms 500 ms milliseconds, more preferably between 1 ms and 200 ms, still more preferably between 5 ms and 100 ms.

Although the device according to EP 2 199 263 is directed to the pulse-shaped administration of additional gases, the method of the present invention is not limited thereto. For example, it is also possible to use a flow controller, such as a mass flow controller.

The amount of additional gas can be both a refractive index increasing and a refractive index reducing agent. Moreover, 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 amount of additional gas can also be a gas that does not directly have a refractive index increasing or refractive index reducing effect, but which can indirectly achieve such an effect. Examples of such gases are O₂, Ar and He. These gases, when fed to the reaction zone, will have an effect on the intensity of the plasma, with the result that the incorporation of dopants 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.

For reasons of controllability of the deposition process, it is preferable to use only a single additional gas in the method according to the present invention. It is noted that the gases additionally supplied to adjust the average refractive index in the radial direction of a glass layer package may or may not be cumulative to a basic amount of dopants that is already supplied to the hollow glass substrate tube as a constant current with the other glass-forming gases.

The definition of the glass layer package preferably further comprises setting a deposition length for each glass layer. 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 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. Methods for drawing optical fibers are known in the art.

Furthermore, the present invention oversees the primary preform obtained with the present method, the final preform produced subsequently, and the optical fibers obtainable 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 shows schematically a radial refractive index profile of a step-index optical fiber

Figure 3 shows a part of a radial refractive index profile of a step index optical fiber according to the prior art

Figure 4 shows a portion of a radial refractive index profile of a step-index optical fiber according to the present invention

Figure 5 shows an embodiment of an implementation of the method according to the present method

Figure 6 shows an example of a response curve

Figure 7 shows an example of some response curves.

Figure 8 shows schematically a preferred embodiment of the method according to the present invention

Figure 9 shows a longitudinal refractive index profile of a primary preform made according to the prior art

Figure 10 shows a longitudinal geometry profile of a primary preform made according to the prior art

Figure 11 shows a longitudinal refractive index profile of a primary preform according to the present invention

Figure 12 shows a longitudinal geometry profile of a primary preform according to the present invention

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. Using the reciprocating reaction zone 8 between reversal points 11 and 12, a number of glass layers 3 (see Figures 3 and 4) are thus 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). With reference to Figure 3, core 1 and any additional preform layers should be construed as preform layers made up of a plurality of glass layers 3. Glass layers 3 are deposited with 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.

Figure 3 schematically shows a part of core 1 and cladding 2 of a contracted primary preform made according to the state of the art, with deposited glass layers 3 of core 1 being shown schematically. Each glass layer 3 of core 1 has the same refractive index difference Δη1 with cladding 2. Because each glass layer 3 has a refractive index difference Δη1 with cladding 2, the average refractive index difference of core 2 is therefore equal to Δη1.

Figure 4 schematically shows a part of core 1 and cladding 2 of a contracted primary preform made according to the present invention, the glass layers 3a - 3f deposited in the internal vapor deposition process being shown schematically. In the manufacture of the primary preform according to Figure 4, at least two glass layer packages 4 are deposited sequentially on the inside of substrate tube 5. Glass layer package 4 consists of the glass layers 3a, 3b, 3c, 3d, 3e and 3f. The refractive index difference of the layers 3b, 3d, 3e and 3f is not equal to Δη1. However, the average refractive index of glass layer package 4 is equal to Δη1.

Thus, the primary preform of Figure 3 and the primary preform of Figure 4 exhibit a core 5 with an equal average refractive index difference Δη1 with respect to the cladding. The number of glass layers and glass packages, as shown in Figure 4, cannot be construed as limiting.

The present inventors assume that if the primary preforms of Figures 3 and 4 are processed into optical fibers, there is no measurable difference in optical properties between the optical fibers if the thickness of the glass layers is sufficiently small.

The present inventors assume, without wishing to be bound by this, that there are a number of reasons for this.

First, the layer thickness of a glass layer 3 in a primary preform is reduced many times during the processing of the primary preform into an optical fiber. Layer thicknesses of glass layers 3 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 glass layers 3 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 glass layer 3 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 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 glass layer 3, the present inventors assume that during the processing of the primary preform into an optical fiber, dopants in a glass layer 3 diffuse slightly to adjacent glass layers 3. This results in the mutual differences in refractive index between adjacent glass layers somewhat 3 is reduced.

Figure 5 shows schematically how the method according to the present invention can be implemented in a production process for the production of primary preforms for an optical fiber. It is noted that the present invention is not limited to this method of implementation.

In a first step 100, so-called response curves are determined. A response curve is a graphical representation of the way in which the refractive index of a deposited glass layer 3 reacts in the longitudinal direction of substrate tube 5 if the deposition conditions at a certain position are changed with respect to the deposition conditions over the remaining length of the deposition length 10.

An example of such a response curve is shown in Figure 6. On the horizontal axis, the position of the reaction zone is shown in arbitrary units [au]. The ratio of the refractive index of the deposited glass to the refractive index of pure quartz glass is shown on the vertical axis. The manner of display is in no way limiting to the present invention. The response curve according to Figure 6 can be manufactured by means of an internal vapor deposition process, wherein during the deposition of a preform layer an additional amount of C is pulsed at a position A each time₂F₆is added to the reaction mixture when the reaction zone 8 moves in the direction of discharge side 7. When the reaction zone moves in the direction of feed side 6, no additional amount of gas is supplied. Preferably, the primary preform is contracted after the deposition process. Subsequently, the longitudinal refractive index profile of the preform layer 4 is determined, which corresponds to the response curve according to Figure 6. As can be seen in figure 6, the refractive index drops from a position B almost stepwise, after which the refractive index gradually increases again.

Although the pulse with additional amount of C₂F₆ is added at position A, the effect is only visible from position B. The reason for this is that the gas supply is relatively far away from the reaction zone. It is furthermore observable that despite a pulse-shaped amount of C₂F₆ the shape of the gradient in refractive index is not or less pulse-shaped. The reasons for this include the flow direction and flow speed of the reaction mixture and the direction of movement and speed of reaction zone 8. At a lower movement speed of reaction zone 8, the distance between A and B will become smaller and the increase in the refractive index will also be faster.

It is noted that the response curve according to Figure 6 is determined by adding a pulse amount of C₂F₆ upon movement of reaction zone 8 in the direction of the discharge side. Depending on the type of process and the manner in which the present invention is applied, it may also be desirable to determine response curves upon movement of the reaction zone toward the discharge side and / or to determine response curves in which a pulsed amount of doping with both movement in the direction of the discharge side when feeding in the direction of the supply side. In addition, response curves can also be determined for other dopants and, in addition, the pulse size and pulse duration can be set.

Figure 7 shows a number of response curves, each curve corresponding to a certain amount of pulse-delivered additional amount of C₂F₆. Four response curves a, b, c and d were based on four different additional amounts of C₂F₆ manufactured. The additional amount of C₂F₆ for curve a, the lowest amount was C, the additional amount₂F₆ for curve d the highest. Thus the influence of the magnitude of the additional amount of C can be₂F₆ pulse on the refractive index reduction and the refractive index trend are determined.

The response curves according to figures 6 and 7 are made on the basis of experiments with C₂F₆ as a dopant which were supplied in the form of pulses with a specific pulse height and pulse width. Here, other factors that influence the response curve, such as flow direction and flow speed of the reaction mixture, direction of movement, speed and length of the reaction zone, were kept the same. However, the present invention is not limited thereto. In principle, response curves can be determined for each type of dopant, both refractive index increasing and refractive index reducing, and for each method of addition, whereby the other influence factors can be set freely.

In a second step 101, a primary preform must be manufactured according to the prior art. The primary preform may optionally be contracted into a solid bar, after which the longitudinal refractive index profile and the longitudinal geometry profile are determined in step 102.

On the basis of the longitudinal refractive index profile and longitudinal geometry profile determined in step 102 and on the basis of the predetermined response curve (s), a layer package 4 is defined in step 103 for the manufacture of subsequent primary preforms, with deviations from the refractive index and / or the geometry relative to a desired one is corrected.

The correction of the refractive index is carried out by changing the deposition conditions, in particular the amount of doping in the reaction mixture, depending on the position of reaction zone 8. Depending on the degree of the deviation (s), a larger number of glass layers 3 in a glass layer package 4 may be required to obtain the desired result.

The correction of the geometry, in particular the layer thickness of the preform layer, can be corrected by setting the speed of reaction zone 8 as a function of the position. A lower speed of reaction zone 8 generally results in a thicker glass layer and the thickness of a glass layer will decrease with a higher set speed. Glass layer package 4 and the associated deposition conditions are thus defined in step 103.

In a practical embodiment, the definition of the glass layer package 4 and the associated deposition conditions for correcting the longitudinal refractive index profile results in a so-called matrix. An example of such a matrix is ​​shown in Figure 8. In Figure 8, deposition length 10 is subdivided into a number of deposition areas. In figure 8 there are twenty-one, but this can also be more or less. In a preferred embodiment, deposition length 10 is divided into 10 to 100 deposition areas, more preferably 2050 deposition areas. Such a subdivision into predetermined deposition areas is practical, but not necessary. Subsequently, the number of glass layers 3 of glass layer package 4 is defined, with ten glass layers being chosen in the embodiment according to Figure 8. The arrows on the left and right sides of the matrix are indicative of the direction of movement of the reaction zone. Thus, the reaction zone for a first glass layer 3 of glass layer package 4 moves from supply side 6 to discharge side 7 (in Figure 8 from left to right). The reaction zone for the second glass layer 3 of glass layer package 4 then moves from the discharge side 7 to the supply side 6, etc. Depending on the position of the reaction zone, a pulsed additional amount of C becomes₂F₆added to the reaction mixture. Such a pulse is indicated by | in the matrix while □ means that no additional amount of C2F6 is supplied. Thus, it has been found that the deposition length, i.e. the length of the substrate tube over which the reaction zone is moved between the two reversal points, is subdivided into separate deposition areas, the respective deposition condition being determined for each deposition area, in particular the deposition condition being determined for a deposition area, is adjustable during the deposition process.

It is noted that the matrix of Figure 8 relates to the addition of pulses C₂F₆, so that correction can be made for too high a refractive index. However , it is also possible to define a plurality of matrices based on pulses with a refractive index-increasing dopant, such as GeCI₄or with gases that have no direct effect on the refractive index, such as oxygen or argon. Finally, it is still possible to manufacture similar matrices for other deposition conditions such as, for example, the speed and power of the reaction zone. Using the present invention, different matrices can thus be run simultaneously and the properties of a glass layer 3 in glass layer package 4 are influenced simultaneously in a number of different ways. For process control reasons, it is preferable to carry out the present method with a maximum of two variables, ie two types of deposition conditions.

It is noted that step 100 on the one hand and steps 101 and 102 on the other are independent of each other and can therefore be carried out in any order.

After glass layer package 4 and the deposition conditions have been defined in a glass layer 3, a subsequent primary preform is then produced in step 104 using the glass layer package 3 defined in step 4 and the associated deposition conditions. From this primary preform, in step 105, optionally after contraction to a solid primary preform, the longitudinal refractive index profile and the longitudinal geometry profile are determined. If the longitudinal refractive index profile and the longitudinal geometry profile meet the requirements, further primary preforms can then be manufactured using the already determined glass layer package. If the longitudinal refractive index profile and / or the longitudinal refractive index profile do not or insufficiently meet the requirements, the previously defined glass layer package can be adapted, expanded or replaced, which means that steps 103, 104 and 105 are performed again.

Comparative example

Figure 9 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. This has the result that the usable length of the primary preform according to the comparative example is a maximum of approximately 820 mm. The numbers used in Figure 9 are for illustrative purposes and should not be construed as limiting.

Figure 10 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 due to 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 800mm. In other words, the usable length of the primary preform according to the comparative example is approximately 800 mm.

Example

Figure 11 shows a longitudinal refractive index profile of a primary preform made in accordance with the present invention. Apart from the use of the method of the present invention, all other conditions during the manufacture of the primary preform were the same as those of the primary preform of the comparative example.

In the method of the present invention, response curves were determined for pulsing with C₂F₆, corresponding to the curves in Figures 6 and 7.

Based on the longitudinal refractive index profile and longitudinal geometry profile of the comparative example, a matrix was then defined for a glass layer package 4 consisting of 10 glass layers. The matrix consisted of 20 deposition areas. Subsequently, the primary preform of the example was prepared using the present invention.

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 example has three preform layers, namely a preform layer 16 with a high refractive index, which will form the core in the fiber, a preform layer 17 and a preform layer 18. Two horizontal lines are shown around the refractive index value of each preform layer 16, 17 and 18 which correspond to the tolerance limits within which the refractive index of the preform layer should lie. These tolerance limits are equal to the limits according to the comparative example.

The measurements of the refractive index of the preform layers of the primary preform according to the example start at a position of approximately -180 mm and end at a position of approximately 1000 mm. The length of the primary preform according to the comparative example is thus approximately 1180 mm. This length is slightly higher than the length of the primary preform according to the comparative example, but is not necessarily due to the effects of the present invention.

It can be seen in Figure 11 that the refractive index of all preform layers over practically the entire length of the primary preform falls within the tolerance limits. Only from a position of approximately 900 mm does the refractive index of preform layer 16 no longer meet the specification. This results in the usable length of the primary preform according to the example being, as regards the refractive index, between positions -180 mm and 900 mm.

Figure 12 shows a longitudinal geometry profile of the primary preform according to the example. The position along the length of the preform is shown on the horizontal axis, while the diameters of the preform layers 16, 17 and are shown on the vertical axis. The diameters of the preform layers 16, 17 and 18 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 16, 17 and 18 which correspond to the tolerance limits within which the diameter of the preform layer should lie. It is noted that the tolerance limits at the ends of the primary preform show a decrease. This is a deliberate setting in which the proportions of the transverse surfaces of the different preform layers 16, 17 and 18 are kept constant over the length of the primary preform. In a further step, such a primary preform is provided with an additional layer of glass to form a final preform, the layer thickness of this 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 final 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. Those skilled in the art will appreciate that the consequence will be that such a final preform does not have a constant diameter as seen in length. The optical fiber obtained after withdrawal of the final preform exhibits constant geometric properties as seen in the longitudinal direction.

The diameter of all preform layers 16, 17 and 18 all fall within the tolerance limits over the entire length of the primary preform according to the example. This results in that the usable length of the primary preform according to the example is not limited by geometric properties, with the result that the usable length of the primary preform according to the example is approximately 1080 mm, which is an improvement of approximately 35% compared to the comparative example.

The primary preform according to the example is manufactured based on the method according to the present invention and wherein the profile overcopping technology is applied. However, the present invention is not limited to this combination. It is also possible to choose to produce a primary preform in which the diameters of the preform layer (s) are constant along the length of the primary preform. However, the combination of the method according to the present invention with the profile overcopping technology offers more flexibility in the manufacture of a primary preform, which may be particularly desirable if a primary preform is to be produced with three or more preform layers, ie primary preforms with a relatively complex radial refractive index profile.

It is further to be noted that while the present invention is particularly aimed at obtaining a substantially constant average refractive index and preferably also a substantially constant layer thickness of the glass layer package viewed in the longitudinal direction, it is also conceivable to apply the present invention in other ways to fit.

The present method can, for example, also be used to manufacture optical fibers with a controlled and desired taper of optical and / or geometric applications. Thus, for example, the speed of the reaction zone and the amount of dopant can be adjusted such that the average refractive index and or thickness of a glass layer package decreases in a certain longitudinal direction.

In another example, the primary preform in the longitudinal direction can be understood to be composed of segments, each segment having a specific radial refractive index profile, which radial refractive index profile is constant across the width of the segment, but where the radial refractive index profiles of two adjacent segments Differ from one another. In this way it is possible, for example, to manufacture a fiber with alternating sign segments for the chromatic dispersion.