KR100661692B1 - Method of preparing preforms for optical fibers - Google Patents

Abstract

The present invention provides a method for preparing a preform for an optical fiber. The present invention seeks to eliminate or greatly reduce undesirable refractive index variations at the fiber center. The method of preparing a preform having a central duct includes a first collapsing step, an etching step, and a second depression step. In the first depression step, the preform is heated at the first preform depression temperature to reduce the size of the center conduit without sealing the center conduit. In the etching step, the etchant gas is introduced through the center conduit at a temperature lower than the first preform depression temperature described above to etch the final deposited layer portion of the core glass layer. Finally, this preform is recessed at a second depression temperature to form a solid rod by sealing the central conduit of the preform.

Description

METHODS OF PREPARING PREFORMS FOR OPTICAL FIBERS}             

1 shows a refractive index profile for a multimode fiber of the prior art prepared according to a conventional method,

2 shows a refractive index profile for a multimode fiber prepared according to the present invention;

3 is a differential mode delay diagram for an optical fiber of the prior art,

4 is a differential mode delay diagram for an optical fiber produced in accordance with the present invention.

This application claims the priority of US Provisional Application No. 60 / 123,136, filed March 8, 1999, entitled "Method to Manufacture Optical Fibers."

The present invention relates to a method for preparing a preform for an optical fiber. More particularly, the present invention relates to a method for eliminating or greatly reducing undesirable refractive index changes in the center of an optical fiber.

One of the processes for making glass optical fibers is to recess the preform with a central duct into a solid glass rod. During the process of sealing the central conduit, volatile dopants such as germanium are desorbed or released from one location of the preform structure. These dopant molecules are then redeposited at other locations or carried out of the preform. Due to the redeposition and / or transport of these dopants, undesirable refractive index deflections occur at the center of the core. 1 illustrates an index of refraction of a multimode glass optical fiber of the prior art. Due to the refractive index deflection, spikes or dips are formed. Such refractive index deflection is adversely affected in high speed systems with limited oscillation conditions.

Therefore, efforts have been made to solve such problems in the related art. U. S. Patent No. 4,793, 843 discloses a method of making an optical fiber preform. In the process described there, a gas etchant consisting of an oxygen compound and a fluorocarbon compound C ₂ F ₆ flows through the central conduit of the preform when the preform is heated for depression. That is, the preform is recessed and etching is performed at the same time. Thus, during etching, etching is performed at a depression temperature, so that fluorine diffuses into the glass structure and germanium is volatilized. U. S. Patent No. 4,793, 843 describes a central conduit size of about 1 millimeter before the gas etchant passes through the central conduit. If the center conduit is small, process control is difficult due to the risk of conduit closure due to surface tension. If the conduit is closed too quickly while the preform is being formed, there is a fear that the glass becomes thinner and undesirable airlines are generated inside the preform. Although the refractive index profile of the final solid preform and the optical fiber derived from the preform is improved, the technique disclosed in US Pat. No. 4,793,843 has a limitation of etching with C ₂ F ₆ . It is also particularly described that etching with SF ₆ at the depression temperature is less effective than etching with C ₂ F ₆ at the depression temperature. Other prior art (eg, US Pat. No. 5,761,366 No. 4,557,561 and Proceedings of ECOC 1982 by Scheneider et al. Disclose etching with SF ₆ while being depressed, but only performed as a separate step from the sinking step.

The present invention provides a method for eliminating or greatly reducing undesirable refractive index changes at the center of an optical fiber. The method for preparing a preform for an optical fiber according to the present invention includes the following steps. The preform with the center conduit is heated to the first depression temperature to reduce the size of the center conduit. The surface of the central conduit is then etched at a temperature below the minimum depression temperature of the preform to remove the deposited core material portion. Finally, the preform is reheated to a second depression temperature to completely retract the center conduit of the preform.

The etching step is performed by flowing etchant gas through the central conduit. The etchant gas contains a mixture of oxygen and SF ₆ . The etching process is preferably carried out at a temperature of about 200 to 400 ° C. below the minimum depression temperature of the preform. In the present invention, the etching process is performed in the temperature range of about 1600 to 2000 ℃.

In the first embodiment of the invention, in the first heating / partial depression step, three depression paths are formed in the direction of the outlet at the inlet to the torch transverse speed and the inlet of the preform which decrease in the temperature range of 2150 ± 100 ° C. do. The etching process is performed by flowing an etchant gas through the central conduit at about 1800 ° C. The etchant gas contains a mixture of SF ₆ 6 sccm and oxygen 194 sccm. Then, the preform is finally recessed in the temperature range of 2200 ± 100 ° C.

In a second embodiment of the invention, three recessed paths are formed through the crossover torch in the direction of the outlet from the inlet to the outlet of the preform and the torch crossing speed decreasing in the temperature range of 2250 ± 150 ° C. The etching process is performed by flowing an etchant gas through the central conduit at about 1800 ° C. The etchant gas contains a mixture of SF ₆ 6 sccm and oxygen 194 sccm. Finally, the preform is recessed in the temperature range of 2220 ± 100 ° C.

In a third embodiment of the invention, three recessed paths are formed through the transverse torch from the discharge end of the preform to the suction end, whereby the discharge end is plugged into the device to prevent gas from escaping from the preform. (plugging) Then, after removing the device at the discharge end, the etching process is performed by flowing an etchant gas of SF ₆ 6sccm and oxygen 194sccm while heating the preform in a temperature range of 1800 ± 150 ° C. Finally, the preform is recessed in the temperature range of 2220 ± 100 ° C. to form a solid rod.

The features and components of the present invention will be better understood by the following detailed description of preferred embodiments of the invention.

The present invention discloses a method for preparing preforms without substantial refractive index changes such as spikes, settlements and / or plateaus at the center of the refractive index profile. It is an object of the present invention to partially recess a preform having a central conduit, etch a portion of the final deposited layer of the core at a temperature below the minimum depression temperature of the preform, and then finally recess the preform to form a solid rod. Is achieved. When the optical fiber is derived from the preform prepared according to the invention, the optical fiber shows a refractive index profile with a significantly reduced central defect. When the procedure of the present invention is optimized, the refractive index is smoothed at the center of the optical fiber.

According to one aspect of the invention, a mixture of oxygen and SF ₆ is used as etchant gas during the etching process. This gas mixture decomposes higher concentrations of F atoms produced at lower temperatures than the fluorinated hydrocarbons proposed in the prior art (US Pat. No. 4,793,843) because of the low bonding energy of SF ₆ molecules. Therefore, in the present invention, the etching process is performed at a temperature of 200 to 400 DEG C lower than the minimum depression temperature. In addition, due to low temperature etching, the present invention significantly reduces the diffusion of fluorine into the glass and the volatility of germanium during the etching process. The flow rate of the etchant gas is SF ₆ is 3.0 to 60.0 sccm, and oxygen is 50 to 1500 sccm. The etching rate during the etching process is 0.003 cm 2 / min to 0.08 cm 2 / min.

According to another aspect of the present invention, since the processing temperature of the present invention is lower than the processing temperature in the prior art, a larger size bubble line is allowed. Gas mixtures decompose at relatively low temperatures and can be eaten in much larger center conduits than are used in the prior art, thereby making the process much more robust. Lower temperatures reduce the length of fluorine diffusion into the glass matrix. Thus, the absolute size of the bubble line is less important than the threshold described in the prior art. The optical preform produced according to the present invention has no apparent central settlement even when the central conduit is as large as 5 mm. Etching is preferred at a temperature range of approximately 1600 to 2000 ° C., in particular 1800 ° C. By separating the low temperature etching step from the high temperature depression process in the present invention, it is possible to more easily maintain the center conduit open during the etching process. This is a great advantage when mass producing preforms.

The present invention is used to prepare both multimode and single mode preforms prepared by a modified chemical vapor deposition (MCVD) process (see, eg, US Pat. No. 4,334,903). An MCVD preform without a central settlement in the refractive index profile is prepared by the following steps.

1. A barrier layer and a core glass layer are deposited on the inner surface of the quartz substrate tube using an MCVD process to form a preform having a central conduit.

2. Heat the preform to reduce the size of the center conduit at the first depression temperature.

3. Etchant gas is flowed through the center conduit at a temperature about 200-400 ° C. below the minimum depression temperature to etch the core portion deposited from the center conduit of the preform.

4. Completely recess the center conduit of the preform at the second depression temperature.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

It should be noted that the depression temperature is defined as the temperature that results in a slow viscous flow of molten glass to the center of the tube, driven by a force due to surface tension causing a decrease in tube diameter.

Example 1

Gradient index MCVD multimodal preforms are made from Heraeus Synthetic Quartz substrate tubes. The glass layer for the core is deposited on the inner surface of the tube through an MCVD process to form a preform having a central conduit. The preform is heated and partially recessed to reduce the size of the central conduit using three recessed paths. The partial recession path is conducted from the inlet to the outlet of the preform at a decreasing torch crossing speed through the outer transverse torch. As an example the crossing speed of these three paths, ie the crossing speed for the first path, is about 3.7 cm / min, the crossing speed for the second path is about 2.9 cm / min, and the crossing speed for the third path Is about 1.7 cm / min. These three pathways are recessed in the temperature range of about 2150 ± 100 ° C. Oxygen flows through the central conduit at a flow rate of at least 200 sccm during the three depression paths. During the partial depression step, volatile dopants such as Ge and P diffuse out of the glass matrix. The central conduit diameter is approximately 3 mm.

Now, the glass region depleted of the dopant in the center of the preform is etched. While heating the central conduit to a temperature of about 1800 ° C., the etching process is performed by flowing an etchant gas containing a mixture of SF ₆ 6 sccm and oxygen 194 sccm through the central conduit. While the etching process is performed, the torch traverses from the suction end to the discharge end direction of the preform at a torch crossing speed of approximately 1.2 to 21.0 cm / min, preferably 2.1 cm / min. In this embodiment, the etching rate is approximately 0.01 cm 3 / min. The center conduit of the preform is not recessed under these etching conditions. Low temperature is important because it minimizes fluorine diffusion into the glass and minimizes dopant volatilization. When the etching process of the present invention is carried out, the inner diameter of the tube can be as large as 5 mm.

After the etching process is complete, the central conduit is sealed at the discharge end of the preform, with 300 sccm of oxygen flowing into the central conduit. When the central conduit is sealed, the oxygen flow rate is reduced to zero. Finally, the preform is recessed into a solid rod. The temperature at which the preform part is recessed is in the temperature range of approximately 2200 ± 100 ° C. The central conduit is sealed along the length of the preform using the final recessed path from the outlet to the inlet direction. The crossing speed for the final reverse depression path is about 1.9 cm / min. However, faster traversal speeds can be used as long as the central conduit in the desired reverse depression path is sealed. The entire depression and etching step takes about 3 or 5 hours depending on the diameter and length of the tube.

Example 2

The MCVD single mode preform is made of a 19 × 25 mm Heraeus synthetic quartz substrate tube. MCVD is used to deposit a glass layer as a core on the inner surface of the tube to form a preform having a central conduit.

The preform is partially recessed, so three recessed paths are used to reduce the size of the central conduit. The partial recession path is conducted from the inlet to the outlet of the preform at a decreasing torch crossing speed through the outer transverse torch. In one example, the individual traverse speeds of these three paths, ie the traverse speed for the first path, is about 2.6 cm / min, the traverse speed for the second path is about 1.8 cm / min, and the traverse speed for the third path. Is about 1.0 cm / min. These three pathways are recessed in the temperature range of about 2250 ± 150 ° C. Oxygen flows through the central conduit at a flow rate of at least 200 sccm during all three depression paths. During the partial depression process, volatile dopants such as Ge and P diffuse out of the glass matrix. The final center conduit diameter is approximately 1.5 mm.

Etch the dopant depleted glass region in the center of the preform. While heating the central conduit to a temperature of about 1800 ° C., the etching process is performed by flowing an etchant gas containing a mixture of SF ₆ 6 sccm and oxygen 194 sccm through the central conduit. While the etching process is performed, the torch traverses from the suction end to the discharge end of the preform at a torch crossing speed of approximately 3.8 cm / min. In this embodiment, the etching rate is approximately 0.01 cm 3 / min. The center conduit of the preform is not recessed under these etching conditions. Low temperature is important because it minimizes fluorine diffusion into the glass and minimizes dopant volatilization.

After the etching process is complete, the central conduit is sealed at the discharge end of the preform, with 300 sccm of oxygen flowing into the central conduit. When the central conduit is sealed, the oxygen flow rate is reduced to zero. Finally, the preform is recessed into a solid rod. The central conduit is sealed along the length of the preform using the final recessed path from the outlet to the inlet direction. The temperature at which the preform part is recessed is in the temperature range of approximately 2200 ± 100 ° C. Faster is preferred as long as the center conduit in the desired reverse depression path is sealed, but the transversal velocity for the final reverse depression path is about 0.5 cm / min. The entire depression and etching process takes about 5 to 7 hours depending on the diameter and length of the tube.

Example 3

Gradient index MCVD multimodal preforms are made using a Heraeus synthetic quartz substrate tube. A glass layer for the core is deposited on the inner surface of the tube using an MCVD process to form a preform having a central conduit.

After depositing the glass layer, the discharge end of the preform is plugged into the device to prevent gas from escaping from the preform. The preform is then partially recessed to reduce the center conduit using three recessed paths. The partial recessed path is conducted from the inlet to the outlet of the preform at a torch crossing speed which is reduced by an external transverse torch. As an example the crossing speed of these three paths, ie the crossing speed for the first path, is about 2.0 cm / min, the crossing speed for the second path is about 2.0 cm / min, and the crossing speed for the third path Is about 1.4 cm / min. These three pathways are recessed in the temperature range of about 2150 ± 100 ° C. During the depression process, the inside of the tube is maintained in a non-flowing oxygen atmosphere. During the partial depression step, volatile dopants such as Ge and P diffuse out of the glass matrix. After this partial depression, the diameter of the final center conduit is approximately 3 mm.

Etch the dopant depleted glass region at the center of the preform. The preform is heated to a temperature of about 1800 ± 150 ° C. while the etching process is carried out by flowing an etchant gas containing a mixture of SF ₆ 6sccm and oxygen 194sccm through its central conduit. The device plugged at the discharge end of the preform is removed to allow gas to flow through the central conduit tube. While the etching process is performed, the torch traverses from the suction end to the discharge end of the preform at a torch crossing speed of approximately 2.1 cm / min. The center conduit is not recessed under these etching conditions. Low temperature is important because it minimizes fluorine diffusion into the glass and minimizes dopant volatilization. In this embodiment, the etching rate is approximately 0.01 cm 3 / min. Under the etching conditions of the present invention, the inner diameter of the center conduit can be as large as 5 mm.

After the etching step is completed, the central conduit is sealed at the discharge end of the preform, with 300 sccm of oxygen flowing into the central conduit. When the central conduit is sealed, the oxygen flow rate is reduced to zero. Finally, the preform is recessed into a solid rod. The final depression path from the outlet to the inlet is used to seal the central conduit along the length of the preform. The temperature at which the preform part is recessed is in the temperature range of approximately 2200 ± 100 ° C. The crossing speed for the final reverse depression path is about 1.9 cm / min. However, faster traversal speeds can be used as long as the central conduit in the desired reverse depression path is sealed.
2 shows the refractive index profile for multimode fibers made in accordance with the present invention. Under the production method of the present invention, it can be clearly seen that there is no settlement of the refractive index at the center of the preform.
FIG. 3 shows a differential mode delay for the fibers produced by the prior art MCVD process and FIG. 4 shows the differential mode delay for the fibers produced by the present invention. These are actual data from the fibers produced according to the invention and from fibers not related to the invention. The horizontal X axis is the time axis in nanoseconds. The Y axis in the vertical direction is the radial axis in microns with a bold line that is the fiber center. The Z axis perpendicular to the plane of the figure is the relative intensity axis. The double pulse shown in FIG. 3 is an unwanted effect, which will adversely affect high speed systems with limited oscillation conditions. This effect can be minimized when the profile shape is optimized as shown in FIG.
This effect may be minimized by minimizing certain defects such as central dip, bump and / or plateau at the center of the refractive index profile. These defects can be eliminated by the present invention. Multimode fibers without central settlement have the additional advantage that when the fibers are used in a 1000 BASE-LX system as described in the IEEE 802.3z standard, no longer an offset patch cord is required.
The method described herein can be used to minimize deflection at the center of the refractive index profile. Although this process has been developed for MCVD processes, all glass optical waveguide preforms requiring sealing of the central conduit, including those made using plasma-enhanced chemical vapor deposition (PCVD) processes and outside vapor deposition (OVD) processes, are required. Applicable to By controlling the center of the profile, it is possible to produce fibers closer to the theoretically calculated profile. This additional capability makes it easy to produce multimode fibers and single mode fibers with improved performance. One example of the use of the present invention is to reduce pulse splitting at the center of a gradient index multimode fiber utilizing the present invention. Another example is the control of waveguide properties, including mode field diameter and dispersion properties, for single mode fibers using the present invention.

Preferred embodiments of the present invention described above are for illustration and description, and are not intended to limit the present invention to only the embodiments. Clearly, one of ordinary skill in the art will appreciate that many modifications and variations are possible. The embodiments have been selected and described in order to best explain the principles and practical applications of the invention, and those skilled in the art will understand the various embodiments of the invention, and various modifications suitable for a particular use will be contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

According to the present invention, it is possible to provide a method for eliminating or greatly reducing undesirable refractive index changes in the center of an optical fiber.

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Claims (21)

Preforms for optical fibers, the preforms having a central conduit, an intake end and an exhaust end, having a core layer last deposited on the surface of the duct; , A first heating step of heating the preform at a first temperature of at least about 2150 +/− 100 ° C. sufficient to initiate partial collapse of the central conduit; Partially collapsing the central conduit while performing the first heating step until the central conduit has a diameter between approximately 3 and 1.5 mm; At a second temperature of approximately 1600 ° C. to 2000 ° C. lower than the first temperature by an amount sufficient to prevent further collapse of the partially collapsed conduit while removing a portion of the last deposited core layer on the surface of the conduit Etching the partially collapsed conduit by flowing an etchant gas through the conduit; A second heating step of heating the preform at a temperature sufficient to collapse the partially collapsed conduit of the preform after the etching step is completed, thereby forming a solid preform rod. Method for preparing preforms for optical fibers. delete The method of claim 1, And providing the etchant gas as a mixture of oxygen and SF ₆ . The method of claim 3, wherein Wherein the oxygen has a flow rate of about 50 to 1500 sccm and the SF ₆ has a flow rate of about 3.0 to 60.0 sccm. The method according to claim 1 or 3, A method of preparing a preform for an optical fiber comprising performing the etching step at a rate of about 0.003 to 0.08 cm ³ / min. The method according to claim 1 or 3, Wherein said etching step is performed by traversing an external heating source relative to said preform at a relative transversal velocity of about 1.2 cm / min to 21.0 cm / min. The method according to claim 1 or 3, A method of preparing a preform for an optical fiber comprising closing the discharge end of the preform while performing the second heating step. The method according to claim 1 or 3, The partially collapsing step is performed by traversing the preform at a decreasing transversal speed for each path, using an external torch of multiple paths, in the direction from the suction end to the discharge end. How to prepare preforms for The method of claim 8, The torch of the three paths is about 1.0 to about the third path, at a crossing speed of about 2.0 to 3.7 cm / min for the first path, and about 1.8 to 2.9 cm / min for the second path. A method of preparing a preform for an optical fiber, the method comprising using at a crossing speed of 1.7 cm / min. The method of claim 9, Flowing oxygen through the central conduit of the preform at a rate of at least 200 sccm during the three successive paths. The method according to claim 1 or 3, And wherein the partially collapsed conduit is etched while the second temperature is approximately 1800 ° C. The method according to claim 1 or 3, Wherein the preform is completely collapsed at a temperature of approximately 2200 +/- 100 ° C. during the second heating step. delete delete delete delete delete delete delete delete delete

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