Title of Invention

"A METHOD OF MANUFACTURING AN OPTICAL FIBER"

Abstract

An optical fiber preform having a substrate tube, a cladding layer and a core layer further includes a first barrier layer deposited by a material having a low OH diffusion coefficient between the substrate tube and the cladding layer, wherein the first barrier layer is for substantially preventing Oil contained .1 the substrate tube from being diffused into the cladding layer. The optical fiber preform further includes a second barrier layer formed by depositing a material having a low OH diffusion coefficient between the cladding layer and core layer, for substantially preventing OH which has been diffused into the cladding layer from the substrate tube from being diffused further into the core layer. Outer and inner Oil barriers containing no P2O5. are deposited between the substrate tube and the cladding layer and between the cladding layer and the core layer in a deposition process, such that OH can be effectively prevented from being diffused from the substrate tube to the core layer in a core deposition process, a collapsing process or a closing process.

Full Text

Technical Field The present invention relates to a method of manufacturing an general optical fiber preform, and more particularly, to an optical fiber preform for minimizing the diffusion of OH from the substrate tube to the core of an optical fiber, and a manufacturing method thereof. Background Art A single mode optical fiber is made by depositing a cladding layer and a core layer. In a DC-SM (depressed cladding-single mode) type, a cladding layer is deposited by doping SiO with P205, Ge02, and F to lower the deposition temperature and the refractive index, a core layer for transmitting light is deposited by doping Si02 with Ge02 to increase the reflective index, and then an optical fiber preform is manufactured through a collapsing and closing process. In a process for manufacturing an optical fiber preform using modilfied chemical vapor deposition (MCVD), self-collapse of a substrate tube occurs during deposition as the deposition layer becomes thicker, resulting in an increase in the thickness of the tube. Also, a high temperature burner is required to sinte nd consolidate a thick deposition layer, and the time for the collapsing and cleaingg process becomes longer, so that a substrate tube becomes exposed to a high temperature over a long period of time. In this process, while a very small amount of water (H20) (generally about several ppm) contained in the substrate tube is diffused into the deposition layer, diffused water is combined with P205 or S/02 deposited in the cladding region, thus forming a P-O-H or Ge-O-H bond combination. OH diffused up to the core region is combined with S/02 or Ge02 deposited in the core layer, thus forming an Si-O-H or Ge-O-H bond combination while dissolving Si-0 or Ge-0 bond combination. O-H or P-O-H bond combination formed in combination with water in each deposition region as described above results in additional optical loss due to an absorption band at a specific wavelength region. In the case of a single mode optical fiber wavelength bands in which serious optical loss occurs are a 1.24/µm-1.385µm band due to the.O-H bond combination, and a 1.2-1.8/µm band due to the P-O-H bond combination. When OH is diffused into the core region, it forms a non-bridging oxygen (NBO), and the structural homogeneity of glass material of the core layer is thus locally deteriorated, which causes density fluctuation of the core layer. Consequently, scattering loss is increased. The inside and outside diameters of a tube conti ct with an mci.ase in the thickness of the deposition layer during sintering performed simultaneously with deposition, so that it is difficult to obtain an appropriate diameter ratio (that is, cladding diameter/core diameter =D/d). Therefore, a distance sufficient to prevent diffusion-of OH cannot be secured, thus greatly increasing loss due to OH. In the prior art, a method of thickening the cladding layer is used to prevent OH from diffusing from the substrate tube to the core layer. However, when a large-aperture preform is manufactured by this method, contraction of a tube makes it difficult to secure an appropriate diameter ratio, a burner of a higher temperature is requirea during deposition of the core layer since the efficiency of transmitting heat to a core .layer is degraded due to an increase in the thickness of the tube layer. Thus, the tube is exposed to high temperature for a long time, thus increasing loss due to OH. Disclosure of the Invention To solve the above problems, it is an objective of the present invention to provide an optical fiber preform capable of effectively reducing loss due to OH wnne lowering the diameter ratio by forming a barrier layer tor blocking or remarkably alleviating diffusion of OH between a substrate tube and a core layer in order to prevent OH from diffusing from the substrate tube into the core layer. It is another objective of the present invention to provide a method of manufacturing an optical fiber preform having an OH barrier. Accordingly, to achieve the first objc live, there is provided an optical fiber preform having a substrate tube, a cladding layer and a core layer, the optical fiber preform further comprising a first barrier layer deposited by a material having a low OH diffusion coefficient between the substrate tube and the cladding layer, wherein the first barrier layer is for substantially preventing OH contained in the substrate tube from being diffused into the cladding layer. It is preferable that the optical fibi r preform further comprises a second barrier layer formed by depositing a materia, having a low OH diffusion coefficient between the cladding layer and core layer, for substantially preventing OH which has been diffused into the cladding layer frosn the substrain lube from being diffused further into the core layer. To achieve the first objective, there is provided another optical fiber preform having a substrate tube, a cladding layer and a core layer, the optical fiber preform further comprising "a first barrier layer deposited by a material having a low OH diffusion coefficient between the substrate tube and the cladding layer, wherein the first barrier layer is for substantially preventing 04 contained in the substrate tube from being diffused into the cladding layer, wherein the refractive index of the core layer is greater than the refractive index of the cladding layer and gradually increases in the direction from the outside of the core layer to the center of the core layer. It is preferable that this optical fiber preform further comprises a second barrier layer deposited by a material having a low OH diffusion coefficient between the cladding layer and core layer, wherein the second barrier layer is for substantially preventing OH diffused into the cladding layer from being diffused further into the core layer. To achieve the second objective, there is provided a method of manufacturing an optical fiber preform having a substrate tube, a cladding layer and a core layer, the method comprising the steps of: forming a first barrier layer by depositing a material having a low OH diffusion coefficient; forming a cladding layer by doping a material suitable for lowering a process temperature and increasing deposition efficiency; and forming a core layer being a region through which an optical signal is transmitted. It is preferable that a second barrier layer is further formed by depositing a material having a low OH diffusion coefficient, before the core layer is formed after the cladding layer is formed. Also, it is preferable that the core layer is Formed so that the refractive index gradually increases in the direction from the outside to the centre of the core layer. In accordance with the present invention relates to a method of manufacturing an optical fiber preform having a substrate tube, a cladding layer and a core layer, characterized in that the method comprising the steps of: forming a first barrier layer by depositing a material having a low OH diffusion coefficient; forming a cladding layer by doping a material suitable for lowering a process temperature and increasing deposition efficiency; and forming a core layer being a region through which an optical signal is transmitted. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is a view illustrating a general signal mode optical fiber; Fig. 2 is a view illustrating a signal mode optical fiber according to the present invention; Fig. 3 is a view illustrating another signal mode optical fiber according to the present invention; and Fig. 4A, 4B and 4C are views illustrating a method of manufacturing a signal mode optical fiber according to the present invention using a modified chemical vapor deposition (MCVD) method. BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will now be described in more detail with reference to the attached drawings. Referring to Fig. 1 showing a general depressed cladding-single mode] (DCSM) optical fiber, reference numerals 11 denotes a substrate tube, reference numeral 12 denotes a cladding layer, and reference numerals 13 denotes a core layer. Also, A + represents the refractive index of the core layer and A -represents the refractive index of the cladding layer, relative to the refractive index of the substrate tube, respectively. Also, d represents the diameter of the core layer, and D represents the diameter of the cladding layer. P2Os is deposited to form the cladding layer 12. P2Os has a relatively low melting point of about 570 ° C, so when it is used together with a different source material, the process temperature can be lowered and deposition efficiency can be increased. On the other hand, since the P2Os doped on the cladding layer 12 has a large hygroscopicity, it acts as an OH bridge for transmitting OH contained in the substrate tube 11 to the core layer 13.] therefore, loss due to OH in the core layer 13 is increased. Fig. 2 is a view illustrating single mode optical fiber according to the present invention. In Fig. 2, reference numerals 21 denotes a substrate tube, reference numeral 22 denotes a first barrier layer (outer cladding layer), reference numeral 23 denotes a middle cladding layer, reference numeral 24 denotes a second barrier layer (inner cladding layer), and reference numeral 25 denotes a core layer. Also, 1+ represents the relative refractive index of the core layer 25, and represents the refractive index of the middle cladding layer 23, which are relative indices to that of the substrate tube 21. 10 represents the refractive index of the first barrier layer 22, and 1 represents the refractive index of the second barrier layer 24, which are relative indices to that of the middle barrier layer 23. d represents the diameter of the core layei 25, D, represents the diameter of the second barrier layer 24, D represents ine diameter of the middle cladding layer 23, and D0 represents the diameter of the first barrier layer 2?.. As described above, the cladding layer of the optical fiber preform according to the present invention is comprised of three layers each having a different chemical composition rate. In other words, the cladding layer is comprised of the first barrier layer (outer cladding layer) 22, the middle cladding layer 23, and the second barrier layer (inner cladding layer) 24. The first barrier layer (outer cladding layer) 22 is positioned between the substrate tube 21 having a large OH concentration and the middle cladding layer 23 containing the OH carrier P205, and prevents OH contained in the substrate tube 21 from being diffused into the middle cladding layer 23. The second barrier layer (inner cladding layer) 24 is positioned between the middle cladding layer 23 and the core layer 25, and prevents OH diffused from the substrate tube 21 into the middle cladding layer 23 in spite of the first barrier layer 22 from further penetrating into the core layer 25. The first and second barrier layers 22 and 24 do not contain P205 which acts as an OH bridge, the refractive indices thereof arc controlled using Si02, GeOz, and F, and the thicknesses thereof are appropriately controlled according to the overall thickness of the cladding layer. In particular, only the first barrier layer 22 can be interposed between the substrate tube 21 having a large concentration of OH and the middle cladding layer 23, or only the second barrier layer 24 can be interposed between the middle cladding layer 23 and the core layer 25. Referring to the refractive index characteristics of the optical fiber preform, the refractive index of the core layer 25 is greater than that of the cladding layers 22, 23 and 24. Thus, the refractive index of each of the outer and inner cladding layers 22 and 24 is controlled to be the'same as or similar to the refractive index of the middle cladding layer 23. Also, the refractive indices of these three layers can be controlled to be the same. In general, the concentration of OH in the deposition layer is about 1/1000 or les-; of the concentration of OH in the substrate tube. However, the cladding layer is deposited by doping P205 in order to lower the process temperature in the cladding deposition process Here, the P205 has a large hygroscopicity. Accordingly, the P205 deposited in the cladding layer acts as a bridge for transmitting OH from the substrate tube to the core layer, thus increasing loss due to OH in the core layer. Hence, in the present invention, an OH barrier doped with materials ha ng low OH diffusion coefficients is formed between the substrate tube having a large concentration of OH and the cladding layer containing the OH carrier P205, or/and between the cladding layer and the core layer. The thus-formed OH barrier can prevent the diffusion of OH from the substrate tube 21 to the core layer 25. FIG. 3 is a view illustrating another single mode optical fiber according to the present invention. In FIG. 3, reference numeral 31 denotes a substrate tube, reference numeral 34 denotes a first barrier layer (outer cladding layer), reference numeral 32 cL-notes middle claddinp layer, reference numeral 35 denotes a second barrier layer (inner cladding layer), an 1 reference numeral 33 denotes a core layer. Also. AN + represent; the refractive inc ex of the core layer 33, and AN" represents" the refractive index of the middle cladding layer 32, which are relative indices to that of the substrate tube 31. As described above, the cladding layer of the optical fiber preform according to the present invention is comprised of three layers each having a different chemical composition rate. In other words, the cladding layer is comprised of the first barrier layer (outer cladding layer) 34, the middle cladding layer 32. and the second barrier layer (inner cladding layer) 35. The first barrier layer (outer cladding layer) 34 is positioned between the substrate tube 31 having a large OH concentration and the middle cladding layer 32 containing the OH carrier P205, and prevents OH contained in the substrate tube 31 from being diffused into the middle cladding layer 32 The second barrier layer (inner cladding layer ) 35 is positioned between the middle cladding layer 32 and the core layer 33, and prevents OH diffused from the substrate tube 31 into the middle cladding layer 32 or OH resulting from water contained in a chemical material during deposition of the middle cladding layer 32, from penetrating into the core layer 33 which is an optical waveguiding region. The refractive index of each of the outer and inner cladding layers 34 and 35 is controlled to be the same as or similar to the refractive index of the middle cladding layer 32, and not to be greater than the refractive index of the substrate tube 31 or core layer 33. The amount of OH" contained in the substrate tube is relatively high compared to that of silica for deposition. Silica is the mosi stable deposition chemical material against an OH component in structure and can effectively block the diffusion of OH at a high temperature. Hence', the first and second barrier layers 34 and 35 do not contain P205 acting as an OH bridge, the refractive index of the cladding is controlled using Si02, Ge, or F, and the thicknesse i these barrier layers are appropriately controlled according to the overall thickness of the cladding layer. Referring to the refractive index characteristics of the optical fiber preform, the refractive index of the core layer 33 is greater than that of the cladding layers 32, 34 and 35. and the refractive index of the core layer 33 increases at a constant rate toward the center of the core layer. Thermal stress due to quick freezing is generated when an optical fiber is drawn out from the preform at high speed. Accordingly, the refractive index of the core layer 33 gradualh increases from the refractive index AN0 of the boundary toward the center thereof, thereby finally making the refractive index AN at the center the greatest. By doing this, the optical loss of the optical fiber due to thermal stress, and degradation of the mechanical characteristics of the optical fiber can be prevented, and thus an optical fiber having a low loss and a low diameter ratio can be drawn out at high speed. For example, it is preferable that the refractive index of the outermost portion of the core layer is 75 to 99% of that of the center of the core layer. FIGS. 4A, 4B and 4C are views illustrating a method of manufacturing the single mode optical fiber according to the present invention shown in FIG. 2 or 3 using a modified chemical vapor deposition (MCVD) method. In the MCVD method, high purity carrier gases such as SiCIA, GeCIA, POCI3, or BCI3 are introduced together with oxygen into a substrate tube 41 made of glass, and heat is then applied to the substrate tube 41 by a heating means 43, whereby soot, an oxidized deposit, is formed on the inside of the substrate tube by thermal oxidation, in FIG. 4A. Here, the concentration of the sc rce gas is accurately controlled by a computer to adjust the refractive index, thereby depositing a cladding layer/core layer 42. The heating means 43 applies heat to the substrate tube 41 which rotates in the direction indicated by a rotating arrow, while the heating means moves in the direction indicated by the-straight arrow. The source gases to be deposited are introduced into the substrate tube 41 through an inlet connected to a source material storage unit. A mixing valve and a blocking valve measure the flow of the source materials introduced into the substrate tube and perform adjustments necessary for mixture of the source materials. In a process for depositing a cladding layer in the present invention, first, an outer cladding layer (a first barrier) is formed by depositing a material having a low OH diffusion coefficient excluding an OH carrier material such as P205 having a large hygroscopicity. Another material suitable lor lowering the process temperature and increasing deposition efficiency is doped, thereby forming a middle cladding layer. A material having a low OH diffusion coefficient is deposited excluding an OH carrier material such as P205, thereby forming an inner cladding layer (a second barrier). A core layer, a region whei .• an optical signal is transmitted, is then formed. Therefore, the mixing of soune gases introduced into the substrate tube 41 becomes different according to each deposition layer, and this mixing can be accomplished by appropriately controlling the mixing valve and the blocking valve. In a process for depositing the core layer, the core layer s deposited so that the refractive index is constant from the outside to the center thereof, or so that the refractive index gradually increases in the direction from the outside to the center thereof. FIG. 4B shows a cladding layer/core layer 40 deposi' within the substrate tube 41. In FIG. 4B, reference numeral 43 denotes an outer cladding layer, reference numeral 44 denotes a middle cladding layer, reference numeral 45 denotes an inner cladding layer, and reference numeral 46 denotes a core layer. Referring to FIG. 4C, the deposited layers as shown in FIG. 4B are collapsed and closed by applying heat to the substrate tube 41, on which the cladding layer/core layer 40 has been deposited, using the heating means 43, thereby forming an optical fiber preform 47. In a deposition process, the outer and inner OH barriers 4". and 4; which have the middle cladding layer 44 therebetween and do not contain P205 acting as an OH bridge, are deposited, thereby effectively preventing OH from being diffused from the substrate tu6e 41 into the core layer 46 during a core deposition process, a collapsing process or a closing process. Accordingly, the loss due to an OH absorption band in the core layer can be minimized while an appropriate diameter ratio (D/d) is maintained. Also, the diameter ratio can be made small, and thus the frequency of deposition can be reduced, thereby shortening the processing time. Here, it is preferable that a ratio (D/d) of the diameter (D) of the cladding layer to the diameter (d) of the core layer is 1.1 to 3.0. Meanwhile, in a sintering process performed simultaneously with deposition, self-collapse due to internal surface tension occurs in a process for sintering and consolidating soot particles. A buffer layer having a similar viscosity to the substrate tube exists between the substrate tube having a high viscosity and the cladding layer having a relatively low viscosity, such that the deterrent power of the tube is improved, and contraction of the tube can thus be r -duced. When an optical fiber preform is manufactured using tie MCVD method, the total processing time becomes shorter as the diameter ratio becomes smaller, and a small diameter ratio is very favorable to the manufacture of a preform having a large aperture. In the prior art, when a diameter ratio becomes small, the OH loss is suddenly increased, thus deteriorating the quality of an optical fiber. Thus, it is commonly known that the diameter ratio is about 3.0. However, according to the present invention, even when the diameter ratio is reduced to less than 3.0. for example, to about 1.1 to 3.0, the OH absorption loss can be reduced, and loss due to thermal stress can also be minimized. Industrial Applicability In the present invention, according to optical fiber preforms having an OH bariier and a manufacturing method thereof as described above, outer and inner OH barriers containing no P2O5 are deposited between a substrate tube and a cladding layer and between the cladding layer and a core layer in a deposition process, such that OH is effectively prevented from being diffused from the substrate tube to the core layer in a core deposition process, a collapsing process or a closing process. Hence, loss due to OH in the core layer can be prevented. Also, the core layer is formed to increase- its refractive index in the direction from the outside to the center, such that degradation of characteristics due to high-speed drawing-out of an optical fiber from the preform can be prevented. We claim 1. A method of manufacturing an optical fiber preform having a substrate tube, a cladding layer and a core layer, characterized in that the method comprising the steps of: forming a first barrier layer by depositing a material having a low OH diffusion coefficient; forming a cladding layer by doping a material suitable for lowering a process temperature and increasing deposition efficiency; and forming a core layer being a region through which an optical signal is transmitted. 2. The method as claimed in claim 1, wherein a second barrier layer is formed by depositing a material having a low OH diffusion coefficient, before the core layer is formed after the cladding layer is formed. 3. The method as claimed in claim 1, wherein the core layer is formed so that the refractive index gradually increases in the direction from the outside to the center of the core layer. 4. The method as claimed in claim 1 or 2, wherein the refractive index of the first or second barrier layer is controlled by doping SiO2, GeO2, or F and does not contain P2O5 having a relatively large hygroscopicity. 5. A method of manufacturing an optical fiber preform having a substrate tube, a cladding layer and a core layer substantially as herein described with reference to the foregoing description and the accompanying drawings.

Documents:

in-pct-2000-417-del-abstract.pdf

in-pct-2000-417-del-assignment.pdf

in-pct-2000-417-del-claims.pdf

in-pct-2000-417-del-complete specification (granted).pdf

in-pct-2000-417-del-correspondence-others.pdf

in-pct-2000-417-del-correspondence-po.pdf

in-pct-2000-417-del-description (complete).pdf

in-pct-2000-417-del-drawings.pdf

in-pct-2000-417-del-form-1.pdf

in-pct-2000-417-del-form-19.pdf

in-pct-2000-417-del-form-3.pdf

in-pct-2000-417-del-form-5.pdf

in-pct-2000-417-del-gpa.pdf

in-pct-2000-417-del-pct-101.pdf

in-pct-2000-417-del-pct-105.pdf

in-pct-2000-417-del-pct-202.pdf

in-pct-2000-417-del-pct-210.pdf

in-pct-2000-417-del-pct-301.pdf

in-pct-2000-417-del-pct-304.pdf

in-pct-2000-417-del-pct-308.pdf

in-pct-2000-417-del-pct-401.pdf

in-pct-2000-417-del-pct-402.pdf

in-pct-2000-417-del-pct-409.pdf

in-pct-2000-417-del-pct-416.pdf

in-pct-2000-417-del-petition-137.pdf

in-pct-2000-417-del-petition-138.pdf