KR100490135B1 - Method of making optical fiber preform having ultimate low PMD - Google Patents

Abstract

The present invention relates to an optical fiber base material having a cladding / core deposition layer formed in a base material tube, wherein the optical fiber base material is characterized in that the collapsing process is performed several times, and the disintegration rate is 0.01 to 0.06 mm / min. The method of Colebs is disclosed.

According to the present invention, the specific ratio and polarization mode dispersion characteristics of the optical fiber base material can be improved.

Description

Method for manufacturing optical fiber base material for improving polarization mode dispersion characteristics by improving optical fiber specific ratio {Method of making optical fiber preform having ultimate low PMD}

The present invention relates to a method for manufacturing an optical fiber base material for improving the polarization mode dispersion characteristics by improving the specific ratio of the optical fiber, in particular, the temperature, torch feed rate and in the tube during the preparation of the fiber preform (hereinafter referred to as the base material) A method for improving specific ratio and polarization mode dispersion characteristics of an optical fiber by optimizing the collapse rate associated with an external pressure difference.

In addition, the present invention relates to a method for improving the polarization mode dispersion characteristics by minimizing the refractive index digging phenomenon of the optical fiber base material by collibbling at the same time with the etching during the Colebs process.

In general, optical fibers widely used as waveguides for optical transmission are manufactured by cutting a base material composed of a core and a cladding at a high temperature.

As known methods for manufacturing an optical fiber base material, it can be divided into two methods, an external deposition method and an internal deposition method.

Here, the internal deposition method injects a raw material gas such as SiCl₄, GeCl₄, POCl₃ together with oxygen through a technique such as Modified Chemical Vapor Deposition (MCVD) inside the tube, and heats the tube by using a torch. Allow for deposition due to thermal oxidation to form cladding and cores.

When the cladding and the core is formed by the above process, an empty space exists in the tube, and thus undergoes a Collapse process of condensing the tube by applying heat from the outside.

The Collebs process is a major process that has a great influence on the geometry of the optical fiber base material. For example, the base tube 10, the core 1, and the cladding 2, as shown in the cross section of the base material shown in FIG. In the case where the ratio of the non-periphery characteristic of the) is poor, the polarization mode dispersion (PMD) increases, which adversely affects the optical transmission characteristics.

The specific ratio is largely dependent on the viscosity or surface tension of the heated base tube, and the viscosity and surface tension are sensitive to factors such as temperature. It is important to get the optimal data.

In addition, in the Collebs process, the core vaporized tube is heated to a higher temperature (2000-2300 ° C.) than the vapor deposition, thereby reducing the outer diameter and the inner diameter of the tube. At this temperature, the inner and outer walls of the tube simultaneously reach the softening temperature, thereby causing viscous flow. In this case, it is known that viscous flow occurs in the inner diameter of the tube due to the surface tension in the direction of minimizing the free surface of the tube and the pressure difference in and out of the tube. As a result, the rate of collapse is the difference in pressure inside and outside the tube. It is greatly influenced by the viscosity and surface tension of the base tube being heated and the viscosity and surface tension is improved sensitively depending on factors such as temperature, heating time, inner diameter, outer diameter, etc. Has found a close relationship.

On the other hand, in this Collebs process, the core vaporized tube is heated to a higher temperature (2000-2300 ° C.) than the vapor deposition, thereby causing the volatilization of GeO ₂ , which is one of the additives in the core.

As a result, the concentration of GeO ₂ decreases at the inner surface of the deposited core, thereby causing an index dip in which the refractive index is lowered at the center of the core, as shown in FIG. 9. Recondensation back to ₂ , diffused into the core, resulting in an index peak at which the refractive index rises again at the core center.

The axial nonuniformity of index dips and index peaks, and the resulting refractive indices, can lead to poor PMD characteristics due to increased losses due to microbending and potential stresses due to asymmetry of the refractive indices in single mode, and significantly reduced bandwidth in multimode. Can be.

Therefore, in order to etch the portion having such low refractive index, after the etching process of flowing the etching gas such as C ₂ F ₆ , C ₃ F ₈ , C ₄ F ₁₀ , the last Colebs is made of glass rod to remove the hole inside. Through the process, the optical fiber base material is manufactured.

However, volatilization of GeO ₂ due to high temperatures may also occur in the last Colebs process, so it is desirable to minimize the surface area inside the tube immediately before the last Colebs process.

However, even if the inner diameter is made smaller after the collebs, the inner diameter of the tube is increased due to the internal hydraulic pressure during the etching process, so there is a limit to minimizing or preventing the volatilization of GeO ₂ during the last collebs process.

The present invention has been made in view of the above, and the call to improve the specific ratio and polarization mode dispersion characteristics of the optical fiber by optimizing the collapse rate associated with the temperature of the optical fiber base material, the torch transfer speed and the pressure difference inside and outside the tube Its purpose is to provide a RESP method.

It is another object of the present invention to provide a Collebs method to improve the polarization mode dispersion characteristics by minimizing the refractive index digging phenomenon of the optical fiber base material by the simultaneous at the same time the etching during the Collebs process.

In order to achieve the above object, the Collebs method for improving the specific ratio and the polarization mode dispersion characteristics of the optical fiber according to the present invention, performing several Colebs process, each collapse rate is 0.01 ~ 0.06mm / min It is characterized by.

Preferably, the number of times is 3 to 5 times, and the torch movement speed is set to 2 to 24 mm / min to set the time for which the torch stays so that the base tube exerts sufficient surface tension and exhibits a good specific ratio. It is characterized by setting.

The tube outer circumferential surface is heated to a tube surface temperature of 2000 to 2300 ° C., the pressure difference between the inside and outside of the tube is 0 to 10 mmWC, and the torch moving speed is 2 to 24 mm / min.

In addition, according to another aspect of the present invention is disclosed an optical fiber base material Colebps method characterized in that while the inner diameter of the tube just before the last Colebs at the same time as the etching while maintaining the same.

According to the present invention as described above, the specific power characteristic of the optical fiber base material is improved, and the polarization mode dispersion characteristic can be improved.

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

First, Figure 2 is a view from above of a cross-sectional view of forming a deposition layer of the source gas on the inner wall of the base tube as a pretreatment process of the Colebs process according to the present invention. Referring to the drawings, a raw material gas 11 such as SiCl₄, GeCl₄, POCl₃, etc. is injected together with oxygen in a state where the base tube 10 made of quartz glass is rotated at constant speed in the circumferential direction thereof, and the tube 10 Transferring the torch 13 of the semi-cylindrical shape along the longitudinal direction of the heating the tube 10 is a transparent glass film is deposited on the inner wall of the tube (10). At this time, by controlling the concentration of the source gas 11 to adjust the refractive index of the deposition layer to form a cladding and core layer.

Here, the shape of the torch 13 may be variously modified. For example, various heating means such as an oxygen / hydrogen burner and a plasma torch may be employed.

3 is a view showing a cross section of the base material obtained by repeatedly performing the above process until the tube 10 is blocked. As shown in the figure, a cladding / core deposition layer 12 is formed on the inner wall of the tube 10 and an empty space is formed in the center thereof.

At this time, the Colebs process is performed to fill the empty space by condensing the optical fiber base material as a whole.

Figure 4 is a schematic cross-sectional view showing one embodiment of this Colebs process. Referring to the drawings, in the state in which the tube 10 is rotated at constant speed in the circumferential direction, the torch 13 heats the outer circumferential surface of the tube 10 and moves along the longitudinal direction of the tube 10 to condense the heated portion. The empty space inside the tube 10 is filled.

Here, of course, the process may be configured to transfer the tube 10 based on the torch 13.

In addition, it is a matter of course that the structure of the torch 13 in a ring shape without rotating the tube 10 may be configured to surround the tube 10 and to heat.

In the present invention, both sides of the torch 13 are provided with anti-radiation plates 14 made of at least one of SUS, Quartz, Al ₂ O ₃ , and ZrO ₂ , which are materials having properties such as heat resistance and oxidation resistance. By reducing the heat radiation loss to both sides of the torch 13 to improve the collebs rate.

In addition, according to the present invention to improve the specific raw material characteristics of the base material through the process as shown in FIG. Referring to the drawings, first, the process of setting the surface temperature of the tube to 2000 ~ 2300 ℃ is performed (step S10). At this time, it is also possible to include a plurality of torch 13 as disclosed in Korean Patent Application No. 1998-0032447 filed by the present applicant so as to increase the heat transfer to the tube (10).

Next, the pressure difference between the inside and the outside of the tube 10, that is, the difference between the pressure due to the temperature or the fluid flow inside the tube 10 and the torch flame pressure applied outside the tube 10 is maintained at 0 to 10 mmWC. The flow rate inside the tube is adjusted and added to the tube 10 (step S20). Here, oxygen (O ₂ ) is the most common material for controlling the flow rate inside the tube. In addition, pressure is generated by the torch used for heating, and the torch flame pressure is determined as a function including factors such as the shape of the torch and the gas flow rate.

Subsequently, for example, when the torch 13 is transferred at a speed of 2 to 24 mm / min along the longitudinal direction of the tube 10, the collebs are sequentially formed along the longitudinal direction of the tube 10 (step S30).

The surface tension and the pressure difference inside / outside the tube are used to collide the hollow base material in the deposited state. The decay rate is inversely proportional to the time of the Colebs process. And the collapse rate is proportional to (internal and external pressure difference + surface tension) / (viscosity of the tube). However, the specific ratio is also proportional to (internal and external pressure difference + surface tension) / (viscosity of the tube), so that the appropriate pressure difference and tube viscosity should be selected to obtain a base material with a small specific ratio as much as possible. do. Tube viscosity changes exponentially with temperature, and tube temperature is affected by heating time, that is, the speed of the torch's movement, the time the torch stays, and the tube thickness. Therefore, the heating temperature and running speed of the torch and the pressure inside the tube should be set according to the tube thickness and the deposition layer thickness.

8 is an example of the implementation, when the outer diameter and inner diameter of the tube before the implementation of the Colebs process is 30.5mm, 22.5mm, respectively, and the thickness of the deposited layer is a graph showing the change in specific ratio according to the breakdown rate of the ash.

Referring to FIG. 8, when the breakdown ratio of the synapse is 0.06 mm / min or less, a specific ratio of 0.3% or less can be obtained. According to an experiment, the optical fiber drawn from the base material of this or less has a polarization mode dispersion of 0.05 ps / √km or less. Indicates a value.

A value of 0.01 mm / min or less of the breakdown rate of extremely low productivity is not included in the present invention even though the specific ratio characteristic is good.

Experimental Example

In this experiment, the tube was set to rotate 20 minutes per minute, and the collaps was controlled differently so that the decay rate decreased with each cycle by adjusting the temperature, the internal / external pressure difference, and the oxygen gas flow rate, which are the main factors in the Colebs process. , Repeating the Coleps process three times.

Temperature (℃) Internal / external pressure difference (mmWC) Oxygen gas flow rate (sccm) Disintegration Rate (mm / min) Specific ratio (%) 1 time 2000 10 3000 0.05-0.06 0.9 Episode 2 2150 5 1500 0.03-0.04 0.6 3rd time 2300 0 20 0.01 to 0.02 0.2

Even when the number of rotations of the tube was 30, 40 revolutions per minute, the specific ratio of the optical fiber was improved.

On the other hand, in order to remove or minimize the refractive index defect layer inside the tube through etching after the Collebs process, the surface area inside the tube should be minimized to prevent volatilization of GeO ₂ . For this purpose, the size of the void space inside the tube should be made to a level of 2 to 4 mm after the Colebs process, that is, before the last Colebs process made of glass rods, to minimize the refractive index defects in the core.

Therefore, in the present invention, by collapsing simultaneously with the etching, the increase in the inner diameter of the tube due to the etching is prevented to minimize or eliminate refractive index defects.

In one embodiment of the present invention was implemented by using a method of maintaining a constant tube inner diameter by changing the moving speed of the torch while maintaining a constant pressure and temperature inside the tube, it is possible to minimize the base material manufacturing defects 2 mm was made into a lower limit, and 4 mm in which refractive index dig phenomenon is not found in an optical fiber state at the time of edge line was made into an upper limit.

FIG. 10 is made according to an embodiment in which the flow rate ratio of the introduced etching gas (O ₂ / C ₂ F ₆ ) is 5.7 while maintaining the inner diameter of the tube at 2 mm in the fourth Collebs process of the total Collebs processes. The refractive index of the center of the core of the optical fiber is shown.

Unlike FIG. 9, FIG. 10 is a refractive index diagram of a base material core from which refractive index defects that appeared at the center of a core according to an embodiment of the present invention are removed.

In addition, in the last Colebs process made of glass rods for cutting, a negative sound pressure, preferably about -5 to -7.5 mmWC, is applied to the inside of the tube 10 so as not to affect the deformation of the base metal structure and to promote the Cobbs. Be sure to

Here, according to the present invention, inert gas, which has a relatively high thermal diffusivity, is introduced into the tube 10 in order to prevent the reduction of the collebs rate in the tube 10. It is desirable to minimize the external temperature difference. Examples of such gases include He (Helium) or Ar (Argon).

The cross section of the base material subjected to the Colebs process performed by the above method has a form in which the cladding / core deposition layer 12 is filled inside the tube 10 with good specific ratio characteristics as shown in FIG. 6.

The cladding / core deposition layer 12 may be classified into a cladding region and a core region according to refractive index. When the cladding / core deposition layer 12 is schematically illustrated, an optical fiber base material having superior specific ratio characteristics is obtained as shown in FIG. 7.

In the above, preferred embodiments of the present invention have been described with reference to the accompanying drawings. Here, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

According to the Collebs method of the optical fiber base material according to the present invention, by adjusting the temperature, pressure, etc. applied to the base material so that the viscosity or surface tension that has a great influence on the geometry of the base material during the Collebs process is optimized.

Accordingly, the present invention improves the specific power ratio at a specific ratio of 0.3% or less, compared to the conventional conventional numerical values of 2.0% or higher Ovality and 0.5ps / √km or more polarization mode dispersion characteristics, and 0.05ps / The polarization mode dispersion characteristic of √ km or less is obtained.

In addition, according to the present invention has an effect of increasing the rate of Colebps by injecting a gas having a high thermal diffusion coefficient into the tube during the Colebps process.

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to

1 is a view showing a cross section of the optical fiber base material through the conventional Colebs process.

Figure 2 is a schematic cross-sectional view showing the formation of the deposition layer of the source gas on the inner wall of the base tube as a pretreatment process of the Colebs process according to the present invention.

3 is a view showing a cross section of the optical fiber base material obtained through the process of FIG.

Figure 4 is a cross-sectional view showing a state that the Coleps process is performed according to an embodiment of the present invention.

5 is a flowchart illustrating a process for improving specific ratio according to the present invention.

6 is a cross-sectional view showing a cross-section of the base material obtained by the Coleps process according to the process of FIG.

FIG. 7 is a view illustrating an improvement in the specific power characteristic of the optical fiber base material through the process of FIG. 5.

FIG. 8 is a graph showing the relationship between the breakdown rate of a sugar and the specific ratio in the case where the collabs step is performed on a base tube having a specific specification. FIG.

FIG. 9 is a diagram illustrating refractive index defects (Index Dip) occurring in a base material after the Collebs process.

10 is a refractive index diagram of a base material core from which refractive index defects have been removed by the practice of the present invention.

<Description of main reference numerals in the drawings>

 1: fiber optic base core 2: fiber optic base cladding 10: tube

 11: source gas 12: cladding / core deposition layer 13: torch

 14: radiation prevention plate

Claims (7)

Forming a cladding / core deposited layer inside the base tube; Condensing the base material tube to a desired inner diameter by heating the outer circumferential surface of the tube by using a moving heat source while injecting an inflow gas into the base material tube in which the deposition layer is formed; In the manufacturing method of the optical fiber base material comprising the step of producing a base rod by heating and finally collapsed the base tube, The condensation step is The number of processes is 3 to 5 times, and the disintegration rate per time is 0.01 to 0.06 mm / min, The feed rate of the heat source is 2 ~ 24mm / min, the surface temperature of the tube is 2,000 ~ 2,300 ℃, the pressure difference between the inside and outside the tube is 0 ~ 10mmWC manufacturing method of the optical fiber base material. delete delete delete The method of claim 1, The pressure difference between the inside and outside of the tube is adjusted to the flow rate of the inner inlet gas, The flow rate of O 2, the internal inflow gas, is 20 to 3,000 sccm. The method of claim 1, The manufacturing method of the optical fiber base material, characterized in that the decay rate of the ash is gradually reduced as the number of times. Forming a cladding / core deposited layer inside the base tube; Condensing the inner circumferential surface of the tube by using a moving heat source while injecting the inflow gas into the inside of the base tube on which the deposition layer is formed such that the inner diameter of the base tube becomes 2 to 4 mm; And disintegrating at the same time by heating the outer circumferential surface while injecting the etching gas into the inside of the base material tube.