NL2006688C2 - METHOD FOR MANUFACTURING A PRIMARY FORM FOR OPTICAL FIBERS. - Google Patents

Description

Brief description: Method for manufacturing a primary optical fiber preform.

The present application relates to a method for manufacturing a primary optical fiber preform using an internal vapor deposition process comprising the following steps: i) providing a hollow glass substrate tube provided with a supply side and a discharge side, ii) to the interior of the hollow glass substrate tube, via the supply side thereof, supplying glass-forming gases, whether or not provided with dopants, iii) establishing a reaction zone in which conditions are created such that deposition of glass on the inside of the hollow glass substrate tube.

Furthermore, the present application provides a hollow glass substrate tube obtained according to the method described above, as well as a primary preform for a microstructured optical fiber.

The present application further relates to a method for the manufacture of a microstructured optical fiber, as well as the special application of such a fiber.

The method mentioned in the preamble is known per se from, for example, US patent application US 2005/0000253. More particularly, the aforementioned patent application discloses an internal vapor deposition process according to PCVD technology, wherein a glass substrate tube along its cylindrical axis is partially surrounded by a resonance space and wherein a gas mixture comprising O2, SiCl4, GeCl4 and optionally other dopants is added to the interior of the hollow substrate tube is supplied. In the above-mentioned resonance space, a local plasma is created, whereby a reaction zone is created, which reaction zone leads to conditions being created on the inside of the hollow substrate tube that, as a result of a reaction between the components of the aforementioned gas mixture, coated or non-doped glass layers are formed. dropped off. The resonance space is moved back and forth over the cylindrical axis of the hollow substrate tube during the deposition process so that said substrate tube 2 is covered on the inside thereof with one or more layers of glass.

An optical fiber consists of a core and an outer layer surrounding the core, which is also referred to as cladding in the English language.

The core of an optical fiber can be made up of one or more concentric layers, depending on the intended optical properties of the fiber. At least a part of the core usually has a higher refractive index than the cladding, so that the light supplied to the optical fiber can be transported mainly through the core of the optical fiber.

For an optical fiber made of glass, the intended, higher refractive index of the core can be obtained by doping the glass of the core with a so-called refractive index-increasing dopant, for example germanium. Germanium is mainly present as GeO2 in the glass obtained by the deposition process. In addition, in special embodiments, it is possible to dope the core with both a refractive index increasing and a refractive index reducing dopant, wherein the mutual ratio between said dopants is adjusted such that the intended refractive index is obtained. Fluoro is mentioned as a suitable refractive index reducing dopant.

It is also known that during the transport of light through the optical fiber the signal strength (optical power) of the light will decrease due to various causes. The aforementioned decrease is indicated by damping. Attenuation is mainly attributed to the effect of so-called Rayleigh scattering and the presence of impurities in the glass, which impurities absorb light at one or more specific wavelengths. From the international application 2009/064381 a method is known in which glass layers are deposited on the interior of a substrate tube using the PCVD technology, which glass layers are provided with so-called "voids", or bubbles. The method used in the aforementioned international application supervises the formation of "soot" or soot, which soot material must be consolidated to form glass, in which glass gas bubbles are trapped as a result of the presence of the gaseous substance used during the consolidation step atmosphere.

The object of the present invention is to provide a method for manufacturing a primary preform for optical fibers 3 in which optical fibers can be obtained which are provided with so-called concentric layers, which concentric layers have gas bubbles which fibers can be used as so-called micro-structured optical fibers marked as.

Another object of the present invention is to provide a method for manufacturing a primary optical fiber preform, wherein glass layers provided with gas bubbles are deposited by adjusting the desired process conditions during the internal deposition process.

The method as stated in the preamble is characterized in that during at least a part of step iii) the pressure in the hollow glass substrate tube is adjusted to a value in the range of 30-80 mbar to form at least one microstructured precursor layer.

The present inventor has determined that by adjusting the pressure in a certain range during at least part of the deposition in step iii), gas bubbles will be formed in the layer thus formed. The term "a microstructure precursor layer" is to be understood to mean a composite glass layer or layer package made up of one or more separate gas layers, gas bubbles being enclosed in the layer package thus formed during deposition. To allow gas bubbles to be included in such a layer package 20, the number of individual glass layers in such a microstructure precursor layer should be at least 20, preferably at least 30. More specifically, if during pressure in step ii) the pressure in the hollow glass substrate tube is set to a value lower than 30 mbar, then no formation of gas bubbles is observed in the thus deposited glass layer. On the other hand, if the pressure in the deposition step in step ii) is set higher than 80 mbar, then the formation of glass layers on the interior of the substrate tube with the inclusion of gas bubbles is not possible. Experiments have shown that the pump side of a deposition set-up closes with SiO2.

In a special embodiment it is desirable that during at least a part of the deposition in step iii) the pressure in the hollow glass substrate tube is set to a value in the range of 30-50 mbar, which will lead to the formation of gas bubbles in the layer thus formed.

The present inventor has further established that for forming at least one preform layer, which preform layer is preferably free of 4 germanium dopants, it is desirable that during at least part of the deposition in step ii) the pressure in the hollow glass substrate tube a value lower than 30 mbar and preferably lower than 20 mbar, in particular lower than 15 mbar, is set. The term "preform layer" used in the present description is to be considered as a composite glass layer composed of one or more separate glass layers, in which preform layer there is no question of enclosing gas bubbles. The term "preform layer" can also be seen as an assembly of glass layers within which the refractive index value is substantially constant, in particular by keeping the process conditions, in particular the pressure, more or less constant during the deposition of such a preform layer. the gas composition. The examples included in the present application indicate that such a preform layer may comprise 50-1000 individual glass layers. According to the present method, use is preferably made of a reaction zone moving back and forth along the length of the substrate tube, wherein a glass layer is deposited per "stroke" of the reaction zone. The absence of germanium has the advantage that a lower damping can be obtained.

In a particular embodiment, it is desirable that a preform layer thus aforementioned is succeeded by the so-called microstructure precursor layer according to the present invention, which microstructure precursor layer is characterized by the presence of one or more gas bubbles therein. If such alternation of forming layer and microstructure precursor layer is desirable, then it is preferable that during at least a part of the deposition in step iii) the pressure in the hollow glass substrate tube is alternately adjusted to a value in the range of 30- 80 mbar and a value lower than 30 mbar. Using such an embodiment, a so-called microstructure precursor layer will be formed during deposition, whereby the pressure is set to a value in the range of 30-80 mbar, while, on the other hand, during the deposition at a pressure below 30 mbar a so-called preform layer, which preform layer is free of enclosed gas bubbles, is formed.

The aforementioned alternately increasing or decreasing the pressure in the hollow glass substrate layer during a part of step iii) will result in a first preform layer being applied to the inside of the hollow substrate tube, followed by a microstructure precursor layer, optionally followed by 5 one or more preform layers and one or more microstructure precursor layers.

From the point of view of the mechanical properties of the deposited glass layers, in particular the expansion coefficient, it is desirable that the first preform layer, namely the layer which directly abuts the inside of the hollow quartz substrate tube, is formed by undoped glass layers.

In a particular embodiment, it is desirable that the pressure conditions during part of the deposition of step iii) be adjusted such that the microstructure precursor layer is embedded between at least two preform layers.

It is also possible in a special embodiment for the pressure conditions during part of the deposition of step iii) to be set such that at least two separate microstructure precursor layers are formed, each separated by at least one preform layer.

The present invention is in particular not limited to a particular number of microstructure precursor layers, wherein the one or more microstructure precursor layers are doped or not.

The process as stated in the preamble is in particular carried out using a plasma as the reaction zone. The plasma process allows precise control of the deposition process, whereby the layer thickness and the number of glass layers can be controlled.

It is preferable that the reaction zone is reciprocated at a speed in the range 10-40 m / min and preferably 15-25 m / min over the length of the hollow glass substrate tube during at least a part of step iii) moved.

After the deposition according to step ii) has been completed, in a specific embodiment it is preferred that the substrate tube thus obtained is subjected to a consolidation step, wherein the substrate tube is converted into a solid primary preform using a heat source.

The hollow glass substrate tube obtained according to the present method is characterized in that one or more microstructure precursor layers, separated by one or more preform layers, are present, which precursor layers and / or preform layers are doped or not.

The present invention further relates to a method for manufacturing a microstructured optical fiber, which method 6 comprises the following steps: a) placing the hollow glass substrate tube obtained after performing step iii), or the primary preform, obtained after the consolidation step in a fiber draw oven, b) heating an end of the hollow glass substrate tube or the primary preform from step a) to a temperature above the softening temperature thereof, and c) drawing an optical fiber from the heated end from step b).

In a special embodiment, it is also possible for an additional amount of glass to be applied to the outer circumference of the hollow glass substrate tube, or the primary preform, in particular by supplying natural silica particles under plasma conditions.

The optical fiber obtained with the present method is particularly suitable for use in a laser.

The present invention will be explained below on the basis of a number of examples and comparative examples, however, it should be noted that the present invention is by no means limited to such experimental data.

Example 1 for comparison

A plasma deposition process was conducted according to a conventional PCVD method, the process conditions listed in Table A below being applicable. It should be noted that clad 1 is deposited as the first layer package on the inside of the glass substrate tube, followed by clad 2, clad 3 and finally the core. Such an order also applies to the experimental results presented below. It is clearly visible from Table A that the process conditions, in particular the pressure of 10 mbar, were identical for clad 1, clad 2, clad 3, etc. and the core, so that in fact no distinction in the preform can be observed between the individual 30 layer packages. The preform obtained in comparative example 1 did not show a bubble structure.

Table A.

7 clad 1 clad 2 clad 3 core

Number of layers 200 100 50 500

Compensation 02 631 631 631 631 5 Response 02 133 133 133 133

SiCl 4 601 601 601 601

GeCl4 0 0 0 0

Freon 0 0 0 0

Power 3600 3600 2000 3600 10 02 excess 2.65 2.65 2.65 2.65 mbar__10__10__10__10

Examples 2-4 for comparison

According to a conventional PCVD method, a plasma deposition process was performed, the process conditions listed in Table B below being applicable. It follows from Table B that experiments with a pressure higher than 80 mbar did not lead to a favorable deposition on the interior of the hollow quartz substrate tube. Such tests were terminated prematurely due to undesirably high pressure increase in which the present inventors found that the pump side was clogged with SiO 2. Therefore, no preforms were made.

Table B

For comparison Pressure (mbar) Deposition 25 serving example clad (number of layers) core (number of layers) 2 85 1000 20 *> 3 90 800 10 *> _4__100__500__ ~ 2_

Note: *> = deposition process terminated prematurely due to blockage on the pump side.

8

Example 1

In Example 1, the deposition process was performed using a deposition rate of 2 g / min, which value applied to both clad 1, clad 2, clad 3 and the core. During the deposition process, as shown in Table 1, for clad 3, the pressure prevailing in the hollow substrate tube was increased to 35 mbar, which increase in pressure led to a clear bubble structure visible in clad 3.

Table 1 10 clad 1 clad 2 clad 3 core

Number of layers 200 100 50 500

Compensation 02 631 631 631 631

Reaction 02 133 133 133 133

SiCl 4 601 601 601 601 15 GeCl 4 0 0 0 0

Freon 0 0 0 0

Power 3600 3600 3600 3600 02 Excess 2.65 2.65 2.65 2.65 mbar 10 10 35 10 20

Example 2

The same operations as in Example 1 were repeated, except that clad 2 was provided with a germane and fluorine doping. The aforementioned doping was also used for the core in varying amounts. During the deposition of 25 clad 3, the pressure of the hollow substrate tube was increased to a value of 35 mbar (see Table 2). Moreover, no dopant was added to clad 3. The aforementioned process conditions for the deposition of clad 3 led to a clear bubble structure in clad 3.

30

Table 2 9 clad 1 clad 2 clad 3 core

Number of layers 200 1430 50 195

Compensation 02 631 574 631 327 5 Response 02 133 75 133 175

SiCl 4, 601, 525, 601, 525

GeCl4 0 318 0 710

Freon 0 405 0 180

Power 3600 3600 3600 3600 10 02 excess 2.65 2.65 2.65 2.65 mbar 10 10 10 10

Example 3

The same operations as in Example 1 were repeated, except that for the deposition of clad 4, the pressure in the hollow substrate tube was increased to 35 mbar (see Table 3). In Example 3, both the core and clad 2 were doped with fluorine and germane. The undoped clad 4 was embedded between an undoped clad 3 and an undoped clad 5, the deposition conditions for both clad 3 and clad 5 being set such that no bubbles were enclosed in clad 3 and clad 5. Thus, in Example 3, clad 4 is embedded between two pure silica cladding layers.

25 30 clad 1 clad 2 clad 3 clad 4 clad 5 core

Number of layers 200 1230 100 50 100 195

Table 3 10

Compensation 02 631 574 631 631 631 327 5 Response 02 133 133 133 133 133 175

SiCl4 601 601 601 601 601 525

GeCl4 0 318 0 0 0 710

Freon 0 405 0 0 0 180

Power 3600 3600 3600 3600 3600 3600 10 02 Excess 2.65 2.65 2.65 2.65 2.65 2.65 mbar__10__10__10__35__10__10

Example 4

In Example 4, the same operations as in Example 1 were repeated, except that dopants were added for different cladding layers, the exact data being included in Table 4 below. In Example 4, a pressure increase to 35 mbar was applied for the deposition of clad 3, which led to the formation of a bubble structure in clad 3. The clad 3 thus formed is embedded between clad 2 and clad 4, wherein both clad 2 and clad 4 were doped with freon. Additional research has shown that clad 2 and clad 4 have no bubble structure.

25 30 clad 1 clad 2 clad 3 clad 4 clad 5 core

Number of layers 800 100 50 100 800 195

Table 4 11

Compensation 02 631 597 631 597 631 327 5 Response 02 133 133 133 133 133 175

SiCl4 601 601 601 601 601 525

GeCl4 0 0 0 0 0 710

Freon 0 500 0 500 0 180

Power 3600 3600 3600 3600 3600 3600 10 02 oversize 2.65 2.65 2.65 2.65 2.65 2.65 mbar__10__10__35__10__10__10

Example 5

The operations used in Example 5 were identical to the operations listed in Example 1, except that the core in Example 5 was doped with fluorine and germane. For the deposition of clad 3, a pressure of 35 mbar was applied, which led to the formation of a bubble structure in clad 3. In Example 5 there is also talk of a clad 4, composed of a layer package of 400 layers, which means that clad 3, composed of a 20 layer package of 50 layers, is positioned reasonably close to the core. Such a positioning has led to the bubbles appearing somewhat larger than in Example 4, for example, whereby the arrangement of the bubbles also appears to be less uniform.

25 30 clad 1 clad 2 clad 3 clad 4 core

Number of layers 200 1140 50 400 55 12

Table 5

Compensation 02 631 631 631 631 427 5 Response 02 133 133 133 133 133

SiCl4 601 601 601 601 525

GeCl4 0 0 0 0 650

Freon 0 0 0 0 150

Power 3600 3600 3600 3600 3600 10 02 excess 2.65 2.65 2.65 2.65 2.65 mbar 10 10 35 10 10

Example 6

In Example 6, a deposition process was carried out such that there are four concentric layers provided with bubbles, in particular clad 2, clad 4, clad 6 and core 2 (see Table 6). For the special process settings, reference is made to Table 6 below, where only core 3 had a fluorine and germane doping.

20 Table 6 clad 1 clad 2 clad 3 clad 4 clad 5 clad 6 core 1 core 2 core 3 Number of layers 200 35 200 35 200 35 200 35 900

Compensation 631 631 631 631 631 631 631 631 560 O * __________ 25 Response 02 133 133 133 133 133 133 133 133 133

SiCl4 601 601 601 601 601 601 601 601 601

GeCl4 00000000 390

Freon 00000000 509

Power 3600 3600 3600 3600 3600 3600 3600 3600 3600 30 02 excess 2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.65 2.65 mbar 10 35 10 35 10 35 10 35 10 13

In summary, it can be stated that increasing the internal pressure prevailing in the hollow substrate tube during the deposition process to a value of 30-80 mbar leads to the formation of at least one microstructure precursor layer. If during the deposition process the pressure is lowered to a value lower than 30 mbar, preferably lower than 20 mbar, a preform layer is formed in which there is no question of the enclosure of gas bubbles. The microstructure precursor layer thus formed can be regarded as a concentric layer package in the preform, wherein during the drawing process to form the optical fiber the gas bubbles present in a preform are converted into channels filled with gas. If a preform layer according to the present invention is embedded between two separate microstructure precursor layers, the light entering into said preform layer will not be able to exit the preform layer, the microstructure precursor layer acting as a kind of reflection layer for the incident light. It is thus possible to manufacture an optical fiber in which light from different laser sources can be coupled into the optical fiber. The present application is therefore particularly suitable for use in a laser construction.

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Claims (17)

A method for manufacturing a primary optical fiber preform using an internal vapor deposition process comprising the following steps: i) providing a hollow glass substrate tube provided with a supply side and a discharge side, ii) attaching it to the interior of the hollow glass substrate tube, via the supply side thereof, supplying glass-forming gases, whether or not provided with dopants, iii) establishing a reaction zone in which conditions are created such that deposition of glass takes place on the inside of the hollow glass substrate tube, with characterized in that during at least part of step iii) the pressure in the hollow glass substrate tube is adjusted to a value in the range of 30-80 mbar to form at least one microstructure precursor layer. 2. Method according to claim 1, characterized in that during at least a part of the deposition in step iii) the pressure in the hollow glass substrate tube is set to a value in the range of 30-50 mbar. Method according to one or more of the preceding claims 1-2, characterized in that during at least a part of the deposition in step iii) the pressure in the hollow glass substrate tube is at a value lower than 30 mbar, preferably lower than 20 mbar, in particular lower than 15 mbar, is set to form at least one preform layer. Method according to one or more of the preceding claims, characterized in that during at least a part of the deposition in step iii) the pressure in the hollow glass substrate tube is alternately adjusted to a value in the range of 30-80 mbar and a value lower than 30 mbar. Method according to one or more of the preceding claims, characterized in that a first preform layer is applied to the inside of the hollow substrate tube, followed by a microstructure precursor layer, optionally followed by one or more preform layers and one or more microstructure precursor layers. Method according to claim 5, characterized in that the first preform layer is formed by undoped glass layers. Method according to one or more of the preceding claims, characterized in that the pressure conditions during a part of the deposition of step 5 iii) are set such that the microstructure precursor layer is embedded between at least two preform layers. Method according to one or more of the preceding claims, characterized in that the pressure conditions during part of the deposition of step iii) are set such that at least two separate microstructure precursor layers are formed, each separated by at least one preform layer . Method according to one or more of the preceding claims, characterized in that the one or more microstructure precursor layers are doped or not, in particular that the one or more preform layers are free from germanium. Method according to one or more of the preceding claims, characterized in that the reaction zone in step iii) is a plasma. 11. A method according to claim 10, characterized in that the reaction zone at a speed in the range of 10-40 m / min and preferably 15-25 m / min over the length of the hollow glass substrate tube during at least a part from step iii) is moved back and forth. Method according to one or more of the preceding claims, characterized in that after the completion of step iii) a consolidation step of the substrate tube obtained after step iii) is carried out, which consolidation step is obtained using a heat source of a solid primary preform. A hollow glass substrate tube obtainable by the method according to one or more of the preceding claims 1-12, characterized in that one or more microstructure precursor layers, separated by one or more preform layers, are present, which precursor layers and / or preform layers are not provided with dopants. Primary preform for a microstructured optical fiber obtainable by the method according to claim 12 A method for manufacturing a micro-structured optical fiber comprising the steps of: a) placing the hollow glass substrate tube obtained after performing step iii), or the primary preform obtained after the consolidation step of claim 12 in a fiber drawing furnace, b) heating one end of the hollow glass substrate tube or the primary preform from step a) to a temperature above its softening temperature, and c) pulling an optical fiber from the heated end of step b). 16. A method according to claim 15, characterized in that the method further comprises applying an additional layer of glass 10 to the outer circumference of the hollow glass substrate tube obtained after performing step iii) prior to step a), or the primary preform obtained after the consolidation step of claim 12. Use of an optical fiber obtainable by means of the method according to one or more of the preceding claims 15-16 in a laser. 15