NL1025476C2 - Rod in tube method for producing optical fibres, comprises reducing pressure inside cavity between rod and tube during heating and flushing with inert gas - Google Patents

Abstract

During the heating step the pressure inside the annular cavity (22) between the rod (11) and tube (12) is reduced and this cavity is flushed with an inert gas (19). A forming method for producing optical fibres (13) comprises placing a tubular mantle around a rod-shaped primary forming part and heating so that the mantle is shrunk onto the rod. During the heating step the pressure inside the annular cavity between the rod and mantle is reduced and this cavity is flushed with an inert gas.

Description

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Brief description: Method for manufacturing a molded part for optical fibers.

The present invention relates to a method for manufacturing a molded part for optical fibers in which a casing bias is placed around a rod-shaped primary molded part while maintaining an annular space between the rod-shaped molded part and the casing tube, after which the assembly of casing tube and rod-shaped molded part is it is heated that the annular space closes and the casing tube 10 comes to lie against the rod-shaped molded part.

Such a method is described in Dutch patent publication 1 021 992, not yet laid open to public inspection, in the name of the present applicant. According to such a method, during the step of heating the rod-shaped molding and the casing tube, a deuterium-containing mixture is applied in the annular space between the casing tube and the rod-shaped molding, which gas mixture serves to exchange the H atoms and the D atoms present in the jacket tube and the rod-shaped molded part. By filling this annular space with a gas mixture containing deuterium, the 20 hydrogen atoms present will be effectively exchanged for deuterium, so that the OH bonds, which show a strong absorption at 1240 nm and 1385 nm, are converted into 0D bonds, which OD -bindings do not have an adverse effect on the absorption spectrum.

From European patent application 1 129 999, it is known to subject a solid glass rod or molded part to the process of "overcladding", wherein a rod-shaped molded part is inserted into a so-called casing tube, which casing tube is contracted on the rod-shaped shaped part using heating and According to such a rod-in-tube (RIT) method, a first casing tube is thus placed within a second casing tube, after which the rod-shaped molded part is positioned within the casing tubes.

I 2 I

I established a pressure gradient between the jacket tubes and finds an I

I heat the rod-shaped molded part and place the jacket tubes around I

I create contradictions. Finally, from the thus obtained I

I preform an optical fiber drawn. I

From U.S. Pat. No. 6,053,013, an I

I apparatus and a method for "overclading" an optical preform

I known, the cancellation space between the primary form part and the I

I mantle tube is stripped of air by turning on the cancellation room I

I close on a vacuum pump. The application of a flushing gas 1n de annulalre I

Space during the contraction of the casing tube on the primary preform is I

I do not know from this. I

I A method is described in U.S. Pat. No. 6,519,974

I for manufacturing an optical fiber through the simultaneous I

I fusion of a primary preform and casing tube and the subsequent I

Drawing of an optical fiber from this is known. According to the I

I method becomes the annular space between the rod-shaped, primary I

I molded part and the casing tube brought under reduced pressure under application I

I of the nitrogen gas flowing along the annular space. For the I

To achieve such an underpressure, nitrogen gas is added to the I

I 20 open top of the cancellation room supplied by an I

I inlet line A and outlet outlet via an intermediate channel I

B. The flow of gas through the channel generates a condition of I

I suppression at the upper ends of the cancellation room, ΐη I

I accordance with Bernoulli's law, and thus the space between I

I I

The casing and the primary optical preform are partially evacuated. I

In this U.S. patent specification 1s it is stated that the flow rate through I

I the channel is the determining factor of the extent to which the gas pressure 1n the I

I annular space is lowered, and thus the I

I flow rate of the nitrogen gas the underpressure in the annular space I

I determine. As special gas, dehydrated nitrogen gas is mentioned because of I

I the low molecular weight thereof. Due to the high speed I

With which the nitrogen gas is used, no nitrogen gas will end up in the annular space between the primary molded part and the casing.

In the production of optical fibers, rod-shaped molded parts or preforms of glass, which are provided on the inside thereof with certain dopants, are drawn into a glass fiber with a diameter of approximately 125 μιη. For the manufacture of such preforms, two separate steps can in principle be distinguished, wherein in a first step the primary preform with 10 dopants for the desired refractive index profile (s) of the optical core is manufactured. In a second step, the primary preform is provided on the outside thereof with an additional jacket of glass to obtain a so-called composite preform which can be pulled out to form an optical fiber under heating.

Various techniques are possible for both the first and the second step, whereby an internal CVD (Chemical Vapor Deposition) technique, such as MCVD, can be used for the manufacture of the primary preform. According to the CVD technique, reactive gases are added to the interior of a hollow substrate tube which react under the influence of heat, the reaction products being deposited on the inner wall of the substrate tube. The heat required for the chemical reaction is obtained by heating the substrate tube on the outside thereof with a heat source, such as hydrogen / oxygen burners, an electric oven or a plasma burner, which heat source is moved along the length of the substrate tube, so that a glass layer is always is deposited.

Another internal CVD technique is PCVD, in which a plasma is generated on the inside of the substrate tube. The reactive gases supplied to the substrate tube react in the plasma and a thin layer is formed on the inner wall of the substrate tube.

Such substrate tubes become 10254 76 under the influence of heat

4 I

contracted to a solid bar, also called the primary preform I

called. Primary preforms can also be obtained using I

an external CVD technique, such as OVD, wherein reactive gases in an I

react and flame on the exterior of a substrate tube. After I

5, when the "soot" is deposited, the substrate is removed and the I

remaining "soot preform sintered by means of heat treatment I

and closed, thus obtaining a primary preform. I

It is known that to increase the yield of I

optical fiber the primary preform is provided with additional glass on the I

10 outside thereof. An I is generally used for this

or more glass tubes that surround the primary preform, also called the I

called "rod-in-tube" technique. I

It is known from European patent publication 1 104 891 I

that primary preforms are contaminated with high I hydroxyl groups

15 damping losses. It should be noted that how I

the lower the damping losses, the greater the distance that the light is I

can lay in a fiber before amplifying the signal

take place. It is thus desirable to have optical I

fibers of which the attenuation losses due to absorption of I

Hydroxyl groups that manifest themselves around a wavelength of 1385 nm, I

be limited as much as possible. I

The object of the present invention is thus the I

providing a preform from which an optical fiber is obtained I

which has low attenuation at a wavelength of 1385 nm. I

Another object of the present invention is the I

providing a preform from which an optical fiber of length I

of at least 300 km. I

Another object of the present invention is the I

providing a method of manufacturing a preform wherein I

The impurities from hydroxyl groups originating from the surface I

of the primary preform. I

1 0254 76 I

5

The present invention is characterized in that the annular space is maintained under reduced pressure during the closing step, which annular space is flushed with an inert gas.

On the basis of the invention described above, it is possible to further reduce the attenuation at a wavelength of 1385 nm of an optical fiber by closing the annular space between this preform and this mantle during the closing of the jacket sleeve on the outside of a primary preform. rinsing the tube with a dry inert gas, while maintaining a negative pressure in the annular space.

It is desirable that the underpressure be in the range between 200 and 900 mbar. A pressure lower than 200 mbar is undesirable because the jacket sleeve will not concentrically contrast around the primary preform. Such a situation should be avoided very much because the optical fiber will show shortcomings in the field of concentricity and circularity. A pressure higher than 900 mbar is undesirable from the point of view of gas inclusions in the optical fiber, which inclusions cause long channels in the optical fiber during the drawing process, which is undesirable.

The inert gases to be used in the present invention are preferably gases or gas mixtures that are not reactive at the applied high temperature of about 2000 ° C. Gases suitable for this purpose are, for example, helium, argon, oxygen or nitrogen or a combination thereof.

In a special embodiment, it is possible that a small amount of gaseous etchant is added to these gases, whereby F6, CZF6 and other gases known as freons can be used.

It is desirable for the inert gas to have a water content of at most 5 ppm. A higher water content leads to contamination with hydroxyl groups, which makes the damping loss undesirable 10254 76

6 I

increase. I

To further increase productivity it is desirable I

to have primary preforms from which, if they become I

combined with a jacket tube to form a composite preform, more than I

5 300 km of single mode fiber can be produced. Primary preforms I

have a characteristic size of 1 meter long and a diameter D I

of approximately 21 mm. If one wants to produce 300 km of fiber from this, the I

core diameter d of a single mode preform to be approximately 4.7 mm. I

It follows that the ratio D / d is about 4.4 for such I

10 primary preforms. If one wants to produce preforms that are more than I

To yield 300 km of fiber, it is desirable that the present invention I

The primary preform used preferably has a D / d ratio of at least I

possesses the highest 4.4, whereby: I

D ** outer diameter of the primary preform, I

15 d core diameter of a single mode preform. I

The attached table also shows that the damping at I

a wavelength of 1385 nm for a standard preform, the I

the present invention has not been applied, just shows an increase at I

lowering the D / d ratio, which increase is undesirable because an I

The aim is to reduce the damping. I

The present process can be carried out off-Hne, i

know to become a heat source in relation to the primary preform and mantle I

moved along its length, with the jacket on the preform I

and thus a composite solid preform is obtained. I

In another embodiment, it is also possible to have the assembly of I

mantle and preform by lelling an oven with the mantle on the I

preform contracts to form the composite solid preform. I

Subsequently transferring this preform to a draw tower makes it I

possibly heating optical I while heating one end of the preform

30 fibers are obtained. I

It is also possible to use the present method directly in an I

102 54 76 * I

7. A device for drawing optical fibers can be provided, wherein the contraction takes place in the draw tower and the preform is drawn into an optical fiber in one operation.

The present invention will be explained below with reference to a number of examples, but it should be noted, however, that the present invention is by no means limited to such special examples.

In addition, the present invention is schematically shown in the attached figure.

The attached figure shows an embodiment of a draw tower in which furnace 10 contains a composite preform consisting of a primary preform 11 and a jacket tube 12, with an optical fiber 13 from the heated end at a temperature of approximately 2000 * C is drawn. The primary preform 11 and the casing tube 12 are extended with an extension tube 15 and an extension rod 14. At the top of extension pipe 15 and extension rod 14 there is a sealing element 17 which closes the assembled preform. This sealing element also provides axial fixation of the preform 11 in the casing tube 12. The annular space 22, formed between the preform 11 and the casing tube 12, is flushed with an inert gas 19 which via a line 18 enters the annular space is led. Also arranged in the sealing element 17 is a line 16 which is connected to a vacuum system 21 so that a reduced pressure can be applied in the annular space. It is desirable that the annular space of the composite preform is already flushed with an Inert gas before it is placed in oven 10. During the drawing of the optical fiber 13, the conditions of rinsing with inert gas and applying the reduced pressure must be maintained.

Examples.

Using the various techniques, primary preforms have been produced, each provided with a different 1 02 54 76

I I

I 8 I

I size of the D / d ratio. I

Primary preform PCVD PCVD PCVD PCVD PCVO MCVD MCVD I

I D / d 5 4.2 3.8 3.4 2.9 4.4 3.9 I

I 5 km fiber 200 300 400 550 600 300 380 I

I Number of preforms 4574876 I

Attenuation 1310 nm 0.33 0.33 0.32 0.33 0.33 0.33 0.33 0.33 I

Attenuation 1385 nm untreated 0.37 0.40 0.38 0.43 0.44 0.40 0.43 I

Reduction attenuation 1385 nm 0 0.005 0.01 0.02 0.05 0 0.01 I

Damping 1550 nm0.19 0.18 0.19 0.2 0.19 0.2 0.19 I

I can clearly see from the above table that I

I optical fibers obtained from composite preforms with a ratio of I

I D / d <4.4 show a clear reduction in damping at an I

I wavelength of 1385 nm due to the rinsing of the annular I

I room with inert gas and the presence of a negative pressure in the annular I

I space. A further reduction to a value of <4.0 leaves a further I

I see a larger reduction. I

I 20 I

10254 76 "I

Claims (10)

1. Method for manufacturing an optical fiber molded article wherein a jacket tube is placed around a rod-shaped primary molded article while maintaining an annular space between the rod-shaped molded article and the jacketed tube, after which the assembly of jacketed pipe and rod-shaped shaped article is heated such that the annular space closes and the casing tube comes to lie against the rod-shaped molded part, characterized in that during the closing step the annular space is maintained under a reduced pressure, which annular space is flushed with an inert gas. Method according to claim 1, characterized in that the inert gas is selected from the group consisting of helium, argon, nitrogen and oxygen, or a combination thereof. Method according to one or more of claims 1-2, characterized in that the inert gas has a water content of at most 5 ppm. 4. Method according to one or more of the preceding claims, characterized in that an etchant is added to the inert gas. The method according to claim 4, characterized in that the etchant is selected from the group of SF6, C2F6 and freon compounds. Method according to one or more of the preceding claims, characterized in that the primary preform has a ratio D / d 25 of at most 4.4, wherein: D = outer diameter of the primary preform, d = core diameter of the single mode preform. 7. Method as claimed in one or more of the foregoing claims, characterized in that the heating required for closing the annular space is carried out in a device for drawing optical fibers, whereafter in the same device 10254 725 The end of the molded part thus formed is heated and an optical fiber is drawn therefrom II. I 8. Method according to one or more of claims 1-6, characterized in that the heating is carried out in an oven to form a composite solid preform. I Method according to one or more of the preceding claims, characterized in that the annular space is flushed with the Inert gas before the heating step takes place. I Method according to one or more of the preceding claims, characterized in that the reduced pressure has a value of I 200-200 mbar. I I 15 I I 1025476