WO2004041738A1

WO2004041738A1 - Method for dehydration used in optical fiber preform manufacturing process, and method and apparatus for manufacturing optical fiber preform using the same

Info

Publication number: WO2004041738A1
Application number: PCT/KR2003/001139
Authority: WO
Inventors: Ki-Wan Jang; Jae-Sun Kim; Seok-Woo Hwang; Myung-Kyu Lee; Peter Chucksin
Original assignee: Lg Cable Ltd.
Priority date: 2002-11-07
Filing date: 2003-06-10
Publication date: 2004-05-21
Also published as: AU2003237049A1; KR20040040741A; KR100554424B1

Abstract

Disclosed are a method for dehydration used in the optical fiber preform manufacturing process, and method and apparatus for manufacturing an optical fiber using the dehydration method. In the dehydration method, when the dehydrating gas is supplied into a tube during the optical fiber preform manufacturing process, dehydration reaction gas included in the dehydration gas is activated using a light source which emits light having a predetermined frequency capable of activating the dehydration reaction gas included in the dehydrating gas into atoms or ions. Then, the method uses the activated dehydration reaction gas generated as a result of the above process to eliminate moisture or hydroxyl group causing optical losses during the optical fiber preform manufacturing process.

Description

METHOD FOR DEHYDRATION USED IN OPTICAL FIBER PREFORM

MANUFACTURING PROCESS, AND METHOD AND APPARATUS FOR

MANUFACTURING OPTICAL FIBER PREFORM USING THE SAME

TECHNICAL FIELD

The present invention relates to a dehydration method used in an optical fiber preform manufacturing process, and method and apparatus for manufacturing an optical fiber preform using the dehydration method. More particularly, the present invention relates to method and apparatus for effectively removing hydroxyl groups (OH) and moisture included in clad and core while the optical fiber preform is manufactured by adding an external light source for radiating light to an optical fiber preform manufacturing apparatus so that reactivity of dehydration gas is improved.

BACKGROUND ART MCVD (Modified Chemical Vapor Deposition) is one of methods for manufacturing an optical fiber. In MC D, a clad is firstly formed and then a core is formed in the clad.

Referring to FIG. 1 for describing the conventional MCVD more specifically, a composite silicon oxide tube 10 is put on a headstock of a lathe, and then reaction gas for forming soot such as SiCl , GeCl and POCl₃ are flowed into the tube 10 together with oxygen gas. At the same time, a torch 14 for providing a temperature above

1600°C is reciprocated along an axial direction of the tube 10 so that the reaction gas flowed into the tube 10 is sufficiently reacted. Whenever the torch 14 reciprocates once, oxidation of halide gas is induced as expressed in the following Reaction Formula 1 at an area in the tube 10 which is heated up to a reaction temperature, thereby generating fine glass particles 12. While the torch 14 is moving, the soot is deposited on the inner surface of the tube 10 by means of thermophoresis at an area that has a relatively lower temperature than the area heated by the torch 14.

Reaction Formula 1

SiCl₄ + O₂ ^ SiO₂ + 2Cl₂ GeCl₄ + O₂ ^ GeO₂ + 2Cl₂

The layer of soot 12 deposited on the inner surface of the tube 10 is sintered by the heat of the torch 14 adjacently followed and becomes a transparent glass layer. This process is continuously repeated so that a plurality of the clad layers are formed on the inner side of the tube 1 and a plurality of core layers are subsequently formed on the clad layer. FIG. 3 shows a section of the optical fiber preform manufactured by the aforementioned process. In FIG. 3, reference numeral 5 denotes a core, 6 denotes a clad, 7 denotes a tube, d denotes a diameter of the core, and D denotes a diameter of the clad. In the conventional MCVD, while a plurality of clad layers and core layers are formed, there occurs a problem that hydroxyl groups (OH) are included therein as impurities. In fact, the reaction gas flowed into the tube 10 generally contains a small amount of moisture, and this moisture is absorbed on the surface of the deposition layer formed inside the tube 1 and then dispersed into the deposition layer under the high temperature, thereby causing bond of Si and OH. FIG. 2 shows an interatomic bond structure after the soot deposition layer is sintered in the conventional optical fiber preform manufacturing process using MCVD. Referring to FIG. 2, it may be found that a large amount of hydroxyl groups (OH) and Si is bonded therein.

However, since the depositing and sintering of the soot 12 is achieved substantially at the same time by using the torch 14 in the MCVD according to the prior art, the removal of the hydroxyl group (OH) included in the clad layer or the core layer as impurities is nearly impossible if a dehydration process is not separately conducted. It is because the hydroxyl group (OH) included as impurities in the soot 12 through chemical reaction is stably bonded to Si and stays in the soot 12 though the MCVD process is conducted at high temperature.

On the other hand, the optical loss, which is the most essential feature for the optical fiber, is composed of the Rayleigh scattering loss caused by the difference of density and constitution of the optical fiber preform, the ultraviolet absorption loss according to electronic transition energy absorption in an atom level, the infrared absorption loss according to energy absorption during lattice vibration, the hydroxyl group absorption loss due to vibration of hydroxyl group (OH), and the macroscopic bending loss. The optical loss should be lowered in order to ensure reliable signal transmission through the optical fiber. The optical fiber generally has an optical loss lower than a predetermined level in the wavelength range between 1280nm and 1620nm, so the two wavelength ranges of 1310nm and 1550nm are currently used as main wavelength ranges for optical communication. However, the optical loss due to the hydroxyl group (OH) absorption is particularly considered as a significant factor in the wavelength 1385nm rather than in other wavelengths, and this wavelength is at present not used due to the high optical loss caused by the hydroxyl group (OH) absorption. Thus, in order to use all of the wavelength range of 1310nm to 1550nm, the optical loss in the wavelength 1385nm due to the hydroxyl group (OH) in the optical fiber should be lower than an average optical loss in 1310nm, namely 0.34dB/Km. Since the core composed of germanium dioxide and silicon dioxide has a Rayleigh loss of about 0.28dB/Km caused by the density and constitution difference of its material itself, the optical fiber can be used in the wavelength range of 1310nm to 1550nm only when the optical loss caused by the hydroxyl group (OH) is controlled to be 0.06dB/Km or less. For this reason, the fabrication of the optical fiber preform should be also controlled so that the concentration of hydroxyl group (OH) in the optical fiber is not more than lppb. However, when just two hydroxyl groups exist on the surface of particle having a diameter of 0.1 μm, the concentration of hydroxyl group comes up to 30ppb and this concentration may be converted into an optical loss of even 0.75dB/Km. This fact shows that it is very difficult to control the concentration of hydroxyl group (OH) contained in the optical fiber preform as impurities in the level of not more than lppb in the conventional MCVD. It is known that making an OH-free single mode optical fiber is possible in OVD

^' (Outside Vapor Deposition) as disclosed in U.S. Pat. No. 3,737,292, U.S. Pat. No. 3,823,995 and U.S. Pat. No. 3,884,550, or in VAD (Vapor Axial Deposition) as disclosed in U.S. Pat. No. 4,737,179 and U.S. Pat. No. 6,131,415. However, it has not ever been reported that an OH-free optical fiber could be manufactured in MCVD. U.S. Pat. No. 5,397,372 merely discloses a technique of making a single mode optical fiber free from hydroxyl group using a plasma heat source, which is an anhydrous heat source, but it is seldom realizable and has little industrial value.

In order to effectively control hydroxyl groups (OH) included in an optical fiber preform manufactured in the MCVD as much as a very small amount of several ppm, a reaction material for cutting off a chemical bond between silicon and the hydroxyl groups (OH) is required. As for the reaction material, gas having chlorine or chloride group is most popularly used in the present.

The method for removing hydroxyl groups using chlorine gas is known as a dehydration process in OVD and VAD. The following chemical formula 2 shows the dehydration reaction for removing moisture and hydroxyl groups respectively by means of reaction with chlorine.

Chemical Formula 2

4Si-OH + 2C1₂ -» 2SiOSi + 4HC1 + O₂ 2H₂O + Cl₂ -> 2HC1 + O₂

On the other hand, hydroxyl groups (OH) should be removed at a temperature below 1200°C at which soot generally starts to be sintered. At a temperature above 1200°C, the soot starts to be partially sintered, so pores between soot particles are reduced, thereby decreasing the space where chlorine gas may exist. In particular, if the surface of soot is melted to cover the pores, dispersion of chlorine into the soot layer may be fundamentally prevented. In addition, in order to ensure that the dehydration reaction according to Chemical Formula 2 is sufficiently progressed, the chlorine gas should stay in the soot layer for a sufficient time. A rate of the reaction depends on a concentration of chlorine gas, a reaction temperature and a reaction time in the dehydration process using the chlorine gas. The Si-OH bond may be more effectively cut as the concentration of chlorine gas increases, as the reaction temperature increases and as the reaction time is elongated.

However, different to OVD and VAD, the conventional MCVD executes the deposition process and the sintering process at the same time, so soot is at the nearly same time melted and condensed while the soot is formed. Thus, in the optical fiber preform fabricated by the conventional MCVD, Si-OH included in the glass layer condensed due to the sintering causes a critical hydroxyl group (OH) absorption loss at the wavelength 1385nm. Accordingly, the optical fiber drawn from the preform fabricated by the conventional MCVD has a limitation in the usable optical communication wavelength range

DISCLOSURE OF INVENTION

The present invention is designed to solve the problems of the prior art, and therefore an object of the invention is to provide a dehydration method for realizing a maximum dehydration efficiency by activating dehydration gas from molecular state into atomic or ion state by using photon as a new energy source for the dehydration reaction. An object of the invention is also to provide method and apparatus for manufacturing an optical fiber preform using the dehydration method.

In order to accomplish the above object, the present invention provides a dehydration method using dehydration gas including chlorine gas for removing moisture or hydroxyl group while an optical fiber preform is manufactured using deposition, wherein the dehydration method removes moisture or hydroxyl group adsorbed on or penetrated into a surface of the optical fiber preform by using chlorine reactant activated by radiating light having a wavelength capable of activating chlorine gas in the dehydration gas into atom or ion.

Preferably, the wavelength of the light radiated to the dehydration gas is selected from a wavelength range which induces only a moisture or hydroxyl group removing reaction by activating the chlorine molecules into atoms or ions.

Also preferably, the light radiated to the dehydration gas is ultraviolet rays having a wavelength of 400nm or less, or laser rays having a wavelength of 200nm or less. According to another aspect of the present invention, there is provided a method for manufacturing an optical fiber preform using MCVD (Modified Chemical Vapor Deposition), which includes a sooting process for generating silica soot by heating a silica oxide tube at a temperature lower than the silica sintering temperature with the use of a reciprocating torch with flowing reaction gas and oxygen gas into the tube; a dehydration process for removing moisture absorbed into the silica soot and hydroxyl groups included in the silica soot by heating the tube at a temperature lower than the sooting process with the use of the torch with flowing mixture gas of chlorine gas and carrier gas into the tube; and a sintering process for sintering the silica soot by heating the tube at a temperature higher than the sintering temperature, wherein light having a wavelength capable of activating the chlorine gas into chlorine ion or chlorine atom is radiated to the mixture gas in the dehydration process to make a chlorine reactant, and moisture absorbed in the soot and hydroxyl groups included in the soot are removed with the use of the chlorine reactant.

The method for manufacturing an optical fiber preform according to the present invention may further include a dechlorination process for removing the chlorine reactant remaining in the tube by heating the tube at a temperature higher than the dehydration process and lower than the sooting process with flowing mixture gas including oxygen gas and carrier gas into the tube, after the dehydration process.

Preferably, the sintering process includes the steps of flowing mixture gas including chlorine gas and carrier gas into the tube; radiating light having a wavelength capable of activating the chlorine gas into atoms or ions to the mixture gas to generate an activated chlorine reactant; and removing moisture and hydroxyl groups remaining in the core after the dehydration process by using the activated chlorine reactant.

According to still another aspect of the invention, there is also provided an apparatus for manufacturing an optical fiber preform including a deposition tube in which an optical fiber preform is formed by repeat of generating soot particles, forming a soot layer by means of thermophoresis of the soot particles, and sintering the soot layer; a gas input unit communicated with the deposition tube for flowing reaction gas for deposition of the optical fiber preform into the deposition tube; and a heat supplying unit for periodically supplying a process temperature for forming the optical fiber preform in the deposition tube along an axial direction of the optical fiber preform, thereby being capable of conducting a dehydration process while the optical fiber preform is manufactured, wherein the gas input unit supplies dehydration gas including chlorine gas and carrier gas into the deposition tube while the dehydration process is conducted, wherein the apparatus is provided with a light source for emitting light having a wavelength capable of activating the chlorine gas of the dehydration gas into atoms or ions to make an activated chlorine reactant at a position out of the deposition tube or at a border between the gas input unit and the deposition tube, thereby removing moisture and hydroxyl groups by using the chlorine reactant generated by the emitted light when the optical fiber preform is manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings: FIG. 1 is a diagram for illustrating a silica deposition process while an optical fiber preform is manufactured according to a conventional MCVD (Modified Chemical Vapor Deposition);

FIG. 2 shows a particle structure of a silica soot layer deposited by the process of FIG. 1; FIG. 3 is a sectional view showing a first preform manufactured by the conventional MCVD;

FIGs. 4a to 4d are diagrams for illustrating the process of manufacturing an optical fiber preform according to an embodiment of the present invention; FIG. 5 shows a particle structure of a silica soot layer which is free from moisture and hydroxyl group according to an embodiment of the present invention; and

FIG. 6 is a graph for showing loss along a wavelength range of the optical fiber manufactured by the prior art and the present invention respectively, for comparison.

BEST MODES FOR CARRYING OUT THE INVENTION

The preferred embodiment of this present invention will now be described more fully hereinafter with reference to the accompanying drawings. However, the terms and the vocabularies used herein should not be construed as limited to general and dictionary meanings but as based on the meanings and concepts in accordance with the spirit and scope of the invention on the basis of the principle that the inventor is allowed to define terms as the appropriate concept for the best explanation. Therefore, the description herein should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. It will be understood that other variations and modifications could be made thereto without departing from the spirit and scope of the invention.

FIGs. 4a to 4d subsequently show the deposition process of an optical fiber preform in MCVD (Modified Chemical Vapor Deposition) to which a dehydration method is applied according to the present invention. In the figures, the same component as one in FIG. 1 is denoted by the same reference numeral.

At first, FIG. 4a shows a sooting process for depositing silica soot 12 on the inner surface of the silica oxide tube 10 in the MCVD. Referring to FIG. 4a, the silica oxide tube 10 is capable of rotating with being installed on the headstock 8 of the lathe. A torch 14 is installed out of the tube 10 so as to reciprocate.

In the sooting process, while flowing reaction gas for forming soot such as SiCl₄, GeCl₄ and POCl₃ into the tube 10 together with oxygen gas, the torch 14 heats the tube with reciprocating along an axial direction of the tube 10. The high temperature heat supplied by the reciprocating torch 14 induces oxidization reaction of halide gas and thus generates soot 12. The soot 12 in a powdered state moves toward an area which is not yet heated by means of thermophoresis and then adhered to the inner surface of the tube 10.

At this time, the process for depositing the silica soot 12 is conducted at a temperature lower than 1600°C, which is generally known as a sintering temperature, and more particularly at 1400 to 1600°C so that the halide gas may have sufficient reaction energy for forming the soot 12 while the silica soot 12 is deposited.

The soot 12 generated by oxidization reaction using the heat of the torch 14 is attached to the inside of the tube 10 by means of thermophoresis. However, since the temperature is kept below the sintering temperature, the soot 12 is not sintered though the torch 14 directly approaches and heat the tube 10, and the pores between the soot particles are still maintained. At this time, since the deposited soot 12 has a positive radius of curvature, hydroxyl groups existing on the surface of the soot particle 12 may be easily removed. Of course, the layer of soot 12 deposited on the inner surface of the tube 10 contains hydroxyl groups as impurities as shown in FIG. 2.

FIG. 4b shows a dehydration process conducted after the sooting process. In the dehydration process, mixture gas such as chlorine, helium, oxygen, nitrogen and argon, which essentially contains chlorine gas as a dehydration gas, is flowed into the tube 10, and the tube 10 is heated at a relatively low temperature by the torch 14 which reciprocates along the axial direction of the tube 10. The heating temperature of the torch 14 is preferably kept to be 500 to 1300°C so that the deposited silica soot 12 is not sintered. In addition, a flow rate of the entire mixture gas and flow rate and partial pressure ratio of each gas in the mixture gas are preferably kept to be constant. More preferably, flow rate and pressure of the entire mixture gas are kept 1 to 10 times of those of the chlorine gas.

During the dehydration process of the present invention, a light source 16 for radiating light of a predetermined wavelength into the tube 10 is installed to a lathe in order to increase a dehydration efficiency. The wavelength of the light supplied by the light source 16 is preferably selected to satisfy the following conditions.

Condition 1

The light generated from the light source 16 should not change geometric and optical features of an optical fiber and a preform such as a core diameter of the preform, a mode field diameter, a ripple structure, a difference of refractive indexes between a core and a clad (ΔN) and a cut-off wavelength.

Condition 2 The light generated from the light source 16 should not change composition, density and geometric structure of a soot layer, a vitrified deposition layer, a preform after sintering, a preform after collapsing, and a matrix constituting a core and a clad of an optical fiber after drawing. Condition 3

The light generated from the light source 16 should not induce any other reaction (secondary reaction) except the dehydration reaction such as activating chlorine molecules into atoms or ions and removing hydrogen-contained impurities such as hydroxyl group.

According to experiments, an ultraviolet lamp for supplying ultraviolet rays having a wavelength of 400nm or less or a laser light source for supplying laser rays having a wavelength of 200nm or less is preferably adopted, but not limited to those cases.

In case the light source 16 is an ultraviolet lamp, the light source 16 is preferably installed adjacent to a gas input hole or directly to the headstock 8 of the lathe so that the distance between a light emitting unit of the ultraviolet lamp and the tube 10 is less than 1000mm. However, the light source 16 may be installed to all area near the tube

(e.g., the headstock of the lathe, an end shaft of the lathe or the like) except a hot zone which corresponds to a center of the torch 14.

In addition, a frequency wavelength of the ultraviolet lamp is 400nm or less, and the ultraviolet lamp preferably has an output of 5W or more and supplies an ultraviolet ray having an arc length of 1cm or more. Selectively, a glass or glass composite material capable of enduring a temperature over 100°C may be installed in front of the ultraviolet lamp and spaced apart from the ultraviolet lamp not so long as 500mm in order to cut off the light having a wavelength of 400nm or more. By the light supplied from the light source 16, neutral chlorine gas is reacted so that chlorine molecules are separated into chlorine atoms or chlorine ions as shown in the following chemical formula 3.

Chemical Formula 3

CI2 -> 2C1

The chlorine ion or chlorine atom generated by the chemical formula 3 is physically or chemically adsorbed to the surface of the soot particle 12 and acts as a nucleation causing a chain reaction with hydroxyl groups, thereby increasing probability that the dehydration reaction is generated in series. Thus, the hydroxyl groups contained in the deposition layer of soot 12 as impurities may be effectively removed. Of course, the chlorine ion or atom is also combined with moisture adsorbed to the surface of the soot 12 so that the moisture may not be dispersed into the deposition layer of soot 12, thereby preventing the formation of Si-OH.

FIG. 5 shows an atom structure of the soot particle 12 in case the dehydration process is conducted with radiating ultraviolet rays to the mixture gas including dehydration gas. Referring to FIG. 5, it would be seen that the hydroxyl groups (OH) having included in the silica soot 12 as impurities in FIG. 2 and the moisture having adsorbed to the surface of the soot 12 in FIG. 2 are removed.

Additionally, if the light is radiated from the light source 16 into the tube 10, hydrogen molecules, which may exist as much as a very small amount in the tube 10, may be split into hydrogen atoms and reacted with chlorine ion as shown in the following chemical formula 4, so the hydrogen gas may be easily removed in a shape of hydrogen chloride (HC1).

Chemical Formula 4 H₂ + Cl₂ -» 2HC1

In case chlorine and hydrogen which are diatomic molecules are existing together in the tube 10, a significant amount of activating energy is generally required for forming hydrogen chloride. If the temperature is not sufficiently high in the tube 10, an average motional velocity of gas molecules is low, so the molecules have very little possibility of being split into atoms by collision. However, if the light is radiated into the tube 10 by using the light source 16 according to the present invention, chlorine and hydrogen are activated to be atoms or ions, thereby increasing the number of generated nucleations and thus increasing the number of generated hydrogen chloride molecules. The number of generated hydrogen chloride is proportionally increased as output and intensity of the light supplied through the light source 16 increase. Experiments show that the concentration of hydrogen ion may be lowered less than lppb in the tube 10 when the present invention is applied.

On the other hand, the chlorine molecule gas may cause the combination scattering loss caused by mismatch of core-clad viscosity, which is a factor of fine bubbles acting as a border between the core and the clad. In addition, the chlorine molecule gas may be a cause of increasing reverse reaction of GeO₂ formation in the core so that Ge distribution in the core becomes microscopically irregular. However, the present invention may solve such problems since the chlorine molecule gas is activated to be atoms or ions in the dehydration process.

In the dehydration process of the present invention, an effective molecular number of chlorine gas participating in the dehydration reaction is doubled, and an efficiency of removing moisture and hydroxyl group is maximized by using chlorine cathode ion having high activity. In addition, many probable drawbacks may be overcome by using the mixture gas including chlorine molecule gas. Furthermore, the dehydration process using the light source may be applied to various methods for making an optical fiber preform such as MCVD, VAD and OVD in order to improve efficiency of the dehydration reaction.

In the present invention, in order to progress the dehydration reaction sufficiently, the time that chlorine atoms or ions stay in the deposition layer of soot 12 should be sufficiently ensured. In this point, the movement velocity of the torch 14 is preferably kept to be 700mm/min or less. FIG. 4c shows the dechlorination process conducted after the dehydration process according to the present invention. Though the chlorine gas flowed into the tube 10 is mostly reacted with hydroxyl groups in the dehydration process, a part of the chlorine gas is penetrated or scattered into pores in the soot and exists therein in a form of a chemical bond (e.g., Si-Cl) or or an atom (e.g., Cl). In addition, hydrochloric acid (HCl) gas remaining in the silica as an impurity after the dehydration process is sometimes reacted with the silica to form a Si-Cl bond. This process is to eliminate such chlorine gas and hydrochloric acid gas. In this dechlorination process, after a mixture gas essentially having oxygen gas is flowed into the tube 10, the tube 10 is heated using the torch 14 at a temperature lower than a sintering temperature. The mixture gas preferably adopts oxygen-helium gas or oxygen-nitrogen gas.

FIG. 4d shows the sintering process conducted after the sooting, dehydration and dechlorination processes. Reference numeral 18 donates a sintered deposition layer. In the sintering process, while the torch 14 keeps a high temperature over

1700°C, the silica particles are dehydrated and sintered at the same time. At this time, a mixture gas having chlorine, helium and oxygen is flowed into the tube 10, and a flow rate of the entire mixture gas and flow rate and partial pressure ratio of each gas are preferably kept to be constant. Similar to the dehydration process, the sintering process may adopt the light source 16 so that moisture and hydroxyl groups remaining after the dehydration process are removed by radiating light of a predetermined wavelength from the light source 16.

On the other hand, since the processes of FIGs. 4a to 4c are conducted at a temperature lower than the sintering temperature, the porous particles are sintered due to the heat of a high temperature over 1700°C in this sintering process, so the soot 12 is vitrified. At this time, the movement velocity of the torch 14 is preferably kept to be 700mm/min or less.

If the aforementioned sooting, dehydration, dechlorination and sintering processes are performed, one clad layer is formed. These processes are repeated until the clad layer reaches a desired thickness.

In addition, if the clad layer reaches a desired thickness, a core layer is then formed by conducting the sooting, dehydration, dechlorination and sintering processes with setting proportions of reaction materials differently. If the core layer is generated up to a desired thickness, a first preform is completed. A diameter ratio (D/d) of the completed clad layer and core layer is preferably determined in the range of 1.0 to 5.0.

The first preform manufactured through the aforementioned processes is then made into an optical fiber through a collapse process and a drawing process. At this time, the surface of the first preform is preferably etched to a depth of 0.7mm or more. The outer surface of the first preform generally shows the highest contents of hydroxyl groups due to the heating of torch and is exposed to various impurities. Thus, if the chemical etching is conducted using hydrofluoric acid (HF) which melts silica on the outer surface of the preform as mentioned above, the impurities on the preform surface are removed.

FIG. 6 is a graph for showing the optical loss generated in the optical fiber core in the range of llOOnm ~ 1700nm, in which a dotted line shows the optical loss of a conventional optical fiber, and a solid line shows the optical loss of an optical fiber fabricated according to the present invention. Referring to FIG. 6, it would be understood that the optical loss is relatively high over the entire wavelength range in the optical fiber fabricated using the conventional MCVD. However, in case of the optical fiber made by the method of the present invention, a peak of the OH absorption loss at the wavelength of 1385nm is dramatically decreased less than 0.30dB/Km, and the optical losses caused by scattering at the wavelengths 1310nm and 1550nm are also decreased respectively less than 0.34dB/Km and less than 0.20dB/Km, when compared with the conventional single-mode optical fiber.

The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In particular, though the dehydration process using optical chemical reaction of the present invention is described on the basis of the MCVD, the present invention is not limited to that case but may be applied to other methods for making an optical fiber preform such as OVD and VAD.

INDUSTRIAL APPLICABILITY

The dehydration process of the present invention may dramatically improve efficiency of the dehydration reaction when being applied to all kinds of optical fiber preform manufacturing methods including OVD and VAD as well as MCVD. In case of the MCVD, it is possible to manufacture a first preform having an optical absorption loss of 0.30dB/Km or less caused by hydroxyl group at 1385nm by lowering the hydroxyl group below lppb. Thus, a usable frequency range of the optical fiber may be expanded as much as at least lOOnm rather than the conventional one, and the optical fiber may be used at any wavelength in the range of 1280 to 1620nm.

Claims

What is claimed is:

1. A dehydration method using dehydration gas including chlorine gas for removing moisture or hydroxyl group while an optical fiber preform is manufactured using deposition, wherein the dehydration method removes moisture or hydroxyl group adsorbed on or penetrated into a surface of the optical fiber preform by using chlorine reactant activated by radiating light having a wavelength capable of activating chlorine gas in the dehydration gas into atom or ion.

2. A dehydration method according to claim 1, wherein the wavelength of the light radiated to the dehydration gas is selected from a wavelength range which gives no influence on a geometric and optical feature of a preform and an optical fiber, including a core diameter of the preform, a mode field diameter, a ripple structure, a difference of refractive indexes between a core and a clad

(ΔN) and a cut-off wavelength.

3. A dehydration method according to claim 1, wherein the wavelength of the light radiated to the dehydration gas is selected from a wavelength range which does not change composition, density and geometric structure of a soot layer, a vitrified deposition layer, a preform after sintering, a preform after collapsing, and a matrix constituting a core and a clad of an optical fiber after drawing.

4. A dehydration method according to claim 1, wherein the wavelength of the light radiated to the dehydration gas is selected from a wavelength range which induces only a moisture or hydroxyl group removing reaction by activating the chlorine molecules into atoms or ions.

5. A dehydration method according to claim 1, wherein the light radiated to the dehydration gas is ultraviolet rays having a wavelength of 400nm or less.

6. A dehydration method according to claim 1, wherein the light radiated to the dehydration gas is laser rays having a wavelength of 200nm or less.

7. A method for manufacturing an optical fiber preform using MCVD

(Modified Chemical Vapor Deposition), comprising: a sooting process for generating silica soot by heating a silica oxide tube at a temperature lower than the silica sintering temperature with the use of a reciprocating torch with flowing reaction gas and oxygen gas into the tube; a dehydration process for removing moisture absorbed into the silica soot and hydroxyl groups included in the silica soot by heating the tube at a temperature lower than the sooting process with the use of the torch with flowing mixture gas of chlorine gas and carrier gas into the tube; and a sintering process for sintering the silica soot by heating the tube at a temperature higher than the sintering temperature, wherein light having a wavelength capable of activating the chlorine gas into chlorine ion or chlorine atom is radiated to the mixture gas in the dehydration process to make a chlorine reactant, and moisture absorbed in the soot and hydroxyl groups included in the soot are removed with the use of the chlorine reactant.

8. A method for manufacturing an optical fiber preform according to claim 7, wherein hydrogen existing in the tube is removed by activating hydrogen molecules existing in the tube into hydrogen atoms or hydrogen ions with the use of the light radiated in the dehydration process so that the hydrogen is reacted with the chlorine reactant into hydrogen chloride.

9. A method for manufacturing an optical fiber preform according to claim

7, after the dehydration process, further comprising: a dechlorination process for removing the chlorine reactant remaining in the tube by heating the tube at a temperature higher than the dehydration process and lower than the sooting process with flowing mixture gas including oxygen gas and carrier gas into the tube.

10. A method for manufacturing an optical fiber preform according to claim 7, wherein the sooting process is conducted at a temperature in the range of 1400 to 1600°C.

11. A method for manufacturing an optical fiber preform according to claim 7, wherein the dehydration process is conducted at a temperature in the range of 500 to 1300°C.

12. A method for manufacturing an optical fiber preform according to claim 7, wherein the light radiated to the mixture gas in the dehydration process is ultraviolet rays having a wavelength of 400nm or less.

13. A method for manufacturing an optical fiber preform according to claim 12, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp is installed out of the tube so that a distance between a light emitting unit of the ultraviolet lamp and a surface of the tube is kept to be 1000mm or less.

14. A method for manufacturing an optical fiber preform according to claim 12, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp is installed out of the tube except a hot zone which corresponds to a center of the torch.

15. A method for manufacturing an optical fiber preform according to claim 12, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp has an output of 5W or more and supplies an ultraviolet ray having an arc length of lcm or more.

16. A method for manufacturing an optical fiber preform according to claim 12, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp is provided with a cut-off member installed in front of the ultraviolet lamp and spaced apart from the ultraviolet lamp not so long as 500mm, the cut-off member being made of glass or glass composite material capable of enduring a temperature over 100° C, the cut-off member cutting off light having a wavelength of 400nm or more.

17. A method for manufacturing an optical fiber preform according to claim

7, wherein the light radiated to the mixture gas in the dehydration process is a laser having a wavelength of 200nm or less, and the laser is supplied from a laser light source.

18. A method for manufacturing an optical fiber preform according to claim

7, wherein a velocity of the torch is kept at 700mm/min or less in the dehydration process.

19. A method for manufacturing an optical fiber preform according to claim 7, wherein the mixture gas in the dehydration process includes chlorine and helium gas, and flow rate and pressure of the entire mixture gas are 1 to 10 times of those of the chlorine gas.

20. A method for manufacturing an optical fiber preform according to claim 7, wherein the sintering process includes the steps of: flowing mixture gas including chlorine gas and carrier gas into the tube; radiating light having a wavelength capable of activating the chlorine gas into atoms or ions to the mixture gas to generate an activated chlorine reactant; and removing moisture and hydroxyl groups remaining in the core after the dehydration process by using the activated chlorine reactant.

21. A method for manufacturing an optical fiber preform according to claim 20, wherein the hydrogen molecules existing in the tube are activated into hydrogen atoms or hydrogen ions by the radiated light, thereby inducing the hydrogen to be reacted with the chlorine reactant and become hydrogen chloride in order to remove hydrogen from the tube.

22. A method for manufacturing an optical fiber preform according to claim 20, wherein the sintering process is conducted at a temperature of 1700°C or above.

23. A method for manufacturing an optical fiber preform according to claim 20, wherein a velocity of the torch is kept to be 700mm/min or less in the sintering process.

24. A method for manufacturing an optical fiber preform according to claim 20, wherein the mixture gas in the sintering process includes chlorine and helium gas, and a flow rate of the entire mixture gas and flow rate and partial pressure ratio of each gas in the mixture gas are kept constant.

25. A method for manufacturing an optical fiber preform according to claim

20, further comprising an etching process for etching an outer surface of a first preform made after the sintering process.

26. A method for manufacturing an optical fiber preform according to claim 20, wherein the light radiated to the mixture gas is ultraviolet rays having a wavelength of 400nm or less.

27. A method for manufacturing an optical fiber preform according to claim 26, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp is installed out of the tube so that a distance between a light emitting unit of the ultraviolet lamp and a surface of the tube is kept to be 1000mm or less.

28. A method for manufacturing an optical fiber preform according to claim 26, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp is installed out of the tube except a hot zone which corresponds to a center of the torch.

29. A method for manufacturing an optical fiber preform according to claim 26, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp has an output of 5W or more and supplies an ultraviolet ray having an arc length of lcm or more.

30. A method for manufacturing an optical fiber preform according to claim

26, wherein the ultraviolet rays are supplied from an ultraviolet lamp, and the ultraviolet lamp is provided with a cut-off member installed in front of the ultraviolet lamp and spaced apart from the ultraviolet lamp not so long as 500mm, the cut-off member being made of glass or glass composite material capable of enduring a temperature over 100°C, the cut-off member cutting off light having a wavelength of 400nm or more.

31. A method for manufacturing an optical fiber preform according to any of claims 7 to 30, wherein the sooting, dehydration and sintering processes are repeated until obtaining desired thickness of a clad layer and a core layer, and a diameter ratio of the clad layer to the core layer (D/d) is selected from the range of 1.0 to 5.0.

32. An apparatus for manufacturing an optical fiber preform including a deposition tube in which an optical fiber preform is formed by repeat of generating soot particles, forming a soot layer by means of thermophoresis of the soot particles, and sintering the soot layer; a gas input unit communicated with the deposition tube for flowing reaction gas for deposition of the optical fiber preform into the deposition tube; and a heat supplying unit for periodically supplying a process temperature for forming the optical fiber preform in the deposition tube along an axial direction of the optical fiber preform, thereby being capable of conducting a dehydration process while the optical fiber preform is manufactured, wherein the gas input unit supplies dehydration gas including chlorine gas and carrier gas into the deposition tube while the dehydration process is conducted, wherein the apparatus is provided with a light source for emitting light having a wavelength capable of activating the chlorine gas of the dehydration gas into atoms or ions to make an activated chlorine reactant at a position out of the deposition tube or at a border between the gas input unit and the deposition tube, thereby removing moisture and hydroxyl groups by using the chlorine reactant generated by the emitted light when the optical fiber preform is manufactured.