KR20040040741A - Method And Apparatus For Removing OH When Making Optical Fiber Preform - Google Patents

Abstract

PURPOSE: Provided is a dehydration method which is suitably applied to a preparation process of an optical fiber preform with improved dehydration efficiency, by converting dehydration gas in a molecular form to an atomic or ionic form with using a photon as an energy source for the dehydration process in a molecular scale. The method can be used in an extended bandwidth by 100nm or more and draw optical fiber which can be used in the range of 1280-1620nm wavelength. CONSTITUTION: The dehydration method in which the dehydration gas including chloride gas is used for removing moisture or a hydroxide group during the preparation of an optical fiber preform, by using chloride gas as a reactant, which has been converted from a molecular form to an atomic or ionic form by irradiation of light in a certain band to the dehydration gas, or removes impurities containing hydrogen by chemical reaction. The method of producing an optical fiber preform by using modified chemical vapor deposition comprises: a sooting process in which silica soot(12) is generated by heating a silicon oxide tube(10) to a temperature lower than the sintering temperature, with a reciprocally moving torch(14) while feeding the reaction gas and oxygen gas into the tube; a dehydration process in which moisture and hydroxide group are removed from the silica soot by heating the tube at a temperature lower than that of the sooting process while feeding a mixed gas including chloride gas into the tube; and a sintering process of the silica soot by heating the tube at a temperature more than the sintering temperature, in which the dehydration process includes a step of irradiating light in a certain band to the mixed gas.

Description

Method and Apparatus For Removing OH When Making Optical Fiber Preform

The present invention relates to a dehydration method and apparatus for manufacturing optical fiber preforms, and more specifically, to the moisture and hydroxyl groups of the clad and core deposited by irradiating ultraviolet rays to the dehydration gas from the preform manufacturing apparatus in the dehydration process. OH) method and apparatus.

Modified Chemical Vapor Deposition (MCVD) is one of optical fiber manufacturing methods that uses a method of forming a clad first and then a core layer therein.

For the modified chemical vapor deposition (MCVD), as shown in FIG. 1, the silicon oxide tube 10 is mounted on the headstock 8 of the shelf, and then the silicon oxide tube 10 is rotated while the inside of the tube is rotated. Reaction gas such as SiCl ₄ , GeCl ₄ , POCl _{3 is} blown with oxygen gas. At the same time, the flame burner or torch 14 which is heated to a temperature of 1600 ° C. or higher to sufficiently react the reaction gases introduced into the outside of the tube 10 is reciprocated.

Each time the torch reciprocates once, the heated portion generates fine glass particles (called soot) by oxidation of a halogen gas, and the soot powder is produced by the torch. As (14) proceeds, it moves to a portion that has not yet been heated and sticks to the inner surface of the tube (10) by thermophoresis. Scheme 1 below is a reaction scheme of the halogen gas.

SiCl ₄ + O ₂ → SiO ₂ + 2Cl ₂

GeCl ₄ + O ₂ → GeO ₂ + 2Cl ₂

The soot 12 adhering to the surface of the tube or deposition layer is sintered by the heat of the torch 14 immediately following to form a transparent glass layer. If this process is repeated continuously, a plurality of clad layers and a plurality of core layers are deposited on the inside of the tube.

In the conventional oxidation reaction as shown in FIG. 1, since the oxidation reaction is performed at a high temperature of 1600 ° C. or more, the reacted soots 12 are sintered at about the same time by the torch 14 immediately following. Because of this, it is impossible to remove the hydroxyl group (OH) present in the reaction.

At this time, the generated interatomic bonding structure inside the silica soot is shown in FIG. 2. Referring to the drawings, it can be seen that a large amount of hydroxyl groups (OH) are bonded to the soot produced by the conventional method.

FIG. 3 shows a primary preform state of a typical single mode optical fiber, wherein reference numeral 5 denotes a core, 6 a cladding layer, and 7 a one-tube. In addition, d represents the diameter of a core layer, and D represents the diameter of a clad layer, respectively.

On the other hand, optical loss, the most important characteristic of optical fiber, is Rayleigh scattering loss due to density difference and composition difference of optical fiber base material, ultraviolet absorption loss due to absorption of electron transfer energy in atoms, infrared absorption loss due to energy absorption during lattice vibration, hydroxyl group It consists of hydroxyl absorption loss and macroscopic bending loss due to vibration of (OH).

For optical transmission, optical loss must be small and optical loss can be used only when the optical loss is less than a certain level in the wavelength band from 1300nm to 1550nm. However, in the 1385nm wavelength band, the optical loss due to hydroxyl (OH) absorption is large, so the 1310nm and 1550nm wavelength bands have been used as the optical communication center wavelength band. That is, in order to use all the wavelength bands from 1310nm to 1550nm, the absorption loss by the hydroxyl group (OH) in the optical fiber should have a value smaller than 0.34dB / Km, which is the average optical loss value of the 1310nm wavelength band. However, since the core layer composed of germanium oxide and silicon oxide has a Rayleigh scattering loss value of about 0.28 dB / Km due to the density difference and the composition difference of the material itself, an optical loss of at least 0.06 dB / Km or less (that is, in the optical fiber The concentration of hydroxyl groups should be controlled to 1 ppb or less).

Regarding the manufacture of OH-free single mode fiber, Outside Vapor Deposition (OVD) known from US Pat. Nos. 3,737,292, US3,823,995, US3,884,550, etc. and US Pat. No. 4,737,179, US6. Vapor Axial Deposition (VAD), known as 131,415, has been known to be possible but has not been reported in Modified Chemical Vapor Deposition (MCVD). However, US Pat. No. 5,397,372 discloses a technique for manufacturing a single mode optical fiber without a hydroxyl group using a plasma heat source that is an anhydrous heat source, but the possibility and commercial value thereof are insufficient.

Gases used to make the core and clad of the optical fiber by Modified Chemical Vapor Deposition (MCVD) include impurities such as water vapor containing hydrogen and hydrogen chloride (HCl) gas. In particular, water vapor is known to be physically or chemically adsorbed on the surface of the soot to form hydroxyl groups or absorbed into the soot to form Si-OH bonds. When the temperature increases above 150 ℃, water vapor that has been physically adsorbed on the soot and pore surface is volatilized, but Si-OH bond is relatively stable, so that it exists in the base material partially even at high temperature above 800 ℃. New Si-OH bonds are produced through chemical reactions with the substrate and are very difficult to remove. Therefore, in order to effectively remove a small amount of hydroxyl group (OH) in ppm unit, a reactant that breaks the chemical bond between silicon and hydroxyl group (OH) is required, and the most widely known substance is a gas including chlorine and chlorine group.

Hydroxide removal using chlorine gas is known as dehydration process by OVD and VAD methods. The following Reaction Scheme 2 represents a dehydration reaction in which water and hydroxyl groups are removed by chemical reaction with chlorine, respectively.

4Si-OH + 2Cl ₂ → 2SiOSi + 4HCl + O ₂

2H ₂ O + Cl ₂ → 2HCl + O ₂

The removal of hydroxyl groups (OH) should usually be done at temperatures below 1200 ° C., the temperature at which sintering begins. At temperatures above 1200 ° C, sintering is partially carried out, and some melting of the soot surface occurs, reducing the voids between the soot, reducing the space for chlorine gas in the soot layer. In particular, if the surface melts to close the pores, the chlorine body may be fundamentally blocked from diffusing into the soot layer. In addition, for the above reaction to proceed sufficiently, the time for the chlorine gas to stay in the soot bed must be ensured. The above reaction indicates that the concentration of chlorine gas, the reaction temperature, and the reaction time are important in the dehydration process using chlorine gas. It can be seen from the above chemical reaction formula that the greater the concentration of chlorine gas, the higher the reaction temperature, and the longer the reaction time, the more effective it is to break the Si-OH bond.

However, unlike conventional OVD or VAD processes, the conventional modified chemical vapor deposition (MCVD) process proceeds with deposition and sintering to form soot, and at the same time, the soot is melted and densified to form a core and clad layer. Therefore, Si-OH inside the densified glass layer due to sintering always causes hydroxyl (OH) absorption loss in the 1385 nm band in the optical fiber manufactured by conventional chemical vapor deposition (MCVD).

Therefore, the applicant of the present invention adds a dehydration process between deposition and sintering in Modified Chemical Vapor Deposition (MCVD) to remove the hydroxyl group which was difficult to remove in the existing Modified Chemical Vapor Deposition (MCVD) method. (Application No. 10-2002-37360) In addition, the present invention has been applied for the manufacture of an optical fiber preform by a modified chemical vapor deposition method (MCVD) by adding a dechlorination process after the dehydration process. Application No. 10-2002-49108)

However, according to the present invention, since the chlorine gas in the molecular state is used in the dehydration reaction, not only the reaction efficiency is lowered, but also the dependence on the conditions of the reaction such as impurities and temperature is severe, and the reproducibility of the dehydration reaction is ensured by an unstable dehydration reaction. Is difficult.

The present invention has been devised to solve the above problems, using photon as a new energy source for the dehydration reaction in the molecular dehydration process to activate the dehydration gas from the molecular form to the atom or ion form to the maximum dehydration Its purpose is to provide a method and apparatus for realizing efficiency.

It is also an object of the present invention to provide a general method that can be used in all optical fiber preform manufacturing processes (MCVD, VAD, OVD) to remove hydrogen or hydrogen-related impurities using a photochemical reaction.

Other objects and advantages of the invention will be described below and will be appreciated by the practice of the invention. In addition, the objects and advantages of the present invention can be realized by means and combinations indicated in the claims.

1 is a view showing a conventional silica deposition process by the crystal chemical vapor deposition (MCVD) method.

2 is a view showing the particle structure of the silica soot produced through the process of FIG.

3 is a cross-sectional view showing a primary preform manufactured by a crystal chemical vapor deposition (MCVD) method.

4A to 4D are diagrams illustrating a preform manufacturing process according to a preferred embodiment of the present invention.

5 is a view showing the particle structure of the silica soot with water and hydroxyl groups removed according to a preferred embodiment of the present invention.

6 is a graph showing the loss according to the wavelength band of the optical fiber compared with that produced by the prior art.

<Explanation of symbols for the main parts of the drawings>

8 lathe headstock 10 silicon oxide tube

12 silica soot 14 torch

16 light source 18 sintered deposited layer

In order to achieve the above object, the present invention, in the dehydration process using a dehydration gas containing chlorine gas to remove moisture or hydroxyl groups in the manufacture of optical fiber preform by deposition, the light of a wavelength of a specific band to the dehydration gas It is characterized in that the chlorine gas in the molecular form is converted to the atomic or ionic form to be used as a reactant of the dehydration reaction, or impurities containing hydrogen are removed by chemical reaction.

The light irradiated to the dehydration gas is except for other chemical reactions, and is characterized in that the wavelength band to enable the reaction of the separation of chlorine molecules into atoms or ions and the removal of hydrogen-containing impurities.

Preferably, the light irradiated to the dehydration gas is an ultraviolet ray having a wavelength of 400 nm or less or a laser beam having a wavelength of 200 nm or less.

Another aspect of the present invention is a method for manufacturing an optical fiber preform using a modified chemical vapor deposition (MCVD) method, the sintering temperature of the tube using a torch reciprocating while introducing the reaction gas and oxygen gas into the silicon oxide tube A sooting step of producing silica soot by heating to the following temperature; A dehydration step of heating a temperature lower than that of the sooting process while introducing a mixed gas containing chlorine into the tube and removing water and hydroxyl groups in the silica soot; A sintering step of sintering the silica soot by heating the tube to a temperature above the sintering temperature, and in the dehydration step, chlorine molecules are separated into chlorine ions or chlorine atoms by irradiating the mixed gas with light of a specific wavelength band and present in the soot. It characterized in that the reaction occurs with the water and hydroxyl groups.

And a dechlorination step of removing chlorine in the silica soot by heating to a temperature higher than the dehydration step and lower than the sooting step while introducing a mixed gas including an oxygen gas into the tube after the dehydration step.

In addition, in the sintering process, a mixed gas containing chlorine gas is injected into the tube, light of a specific wavelength band is irradiated to the mixed gas, and chlorine molecules are separated into chlorine ions or chlorine atoms to remain in the soot after the dehydration process. It is also possible to allow the reaction with moisture and hydroxyl groups to occur.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms or words used in the specification and claims should not be construed as having a conventional or dictionary meaning, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention. Therefore, 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.

4A to 4D sequentially illustrate the deposition process of the optical fiber base material by the crystal chemical vapor deposition (MCVD) method. The same reference numerals as in FIG. 1 indicate the same members having the same function.

First, FIG. 4A shows a sooting process in which silica soot 12 is deposited on the inner surface of silicon oxide tube 10 by quartz chemical vapor deposition (MCVD). The silicon oxide tube 10 is rotated while being mounted on the headstock 8 of the shelf, and the torch 14 is installed outside the tube 10 to reciprocate.

In this sooting process, a reaction gas such as SiCl ₄ , GeCl ₄ , POCl _{3 is} blown together with oxygen gas into the tube 10, and the torch 14 reciprocates to heat the tube 10. When the torch 14 reciprocates, the soot is generated by oxidation of a halogen gas by the high heat supplied, and the soot powder is moved to an unheated portion and the tube is subjected to thermophoresis. It will stick to the inner surface of 10). It is confirmed that the reaction in which the soot is produced is shown in Scheme 1.

In this case, the process of depositing the silica soot is generally performed at a temperature lower than 1600 ° C., which is generally known as a sintering temperature, and more preferably 1400 to 1600 so that the halogenated gases have sufficient reaction energy to form the soot during silica soot deposition. It is carried out at a temperature of ℃. Accordingly, the soot reacted by the heat of the torch 14 adheres to the inside of the tube 10 by the thermophoretic phenomenon, but does not sinter since it maintains a temperature below sintering and maintains a state of voids. Therefore, the radius of curvature of the soot has a positive value so as to maintain a good condition for removing hydroxyl groups mainly present on the surface of the soot. In this state, hydroxyl groups are bonded to the inside of the silica soot, and such an atomic bonding structure is the same as that shown in FIG.

4B shows the dehydration process performed after the sooting process. In this process, a mixture of chlorine, helium, oxygen, nitrogen, argon, and the like, which necessarily includes chlorine gas, is introduced into the tube 10, and the tube 10 is relatively held by the torch 14 continuously reciprocating. Heated to low temperature. At this time, it is preferable to maintain the heating temperature of the torch 14 at 500 to 1300 ° C. so as not to sinter the silica soot. In addition, the total flow rate of the mixed gas, the flow rate of each gas, and the relative partial pressure ratio are preferably maintained at a constant ratio. In particular, the total flow rate, the individual flow rate, and the partial pressure ratio of chlorine and helium or chlorine compound and the mixed gas are injected at least 1 times and at most 10 times.

In addition, in the dehydration process of the present invention, a light source 16 is provided on the shelf 8 to irradiate light of a specific wavelength into the tube. Light supplied by the light source 16 causes the core diameter of the preform, the mode field diameter, the ripple structure, the refractive index difference (ΔN) between the core and the clad, and the cut-off wavelength. The geometry and optical structure of the preform and optical fiber, etc., should be similar to those of the prior art. In addition, a matrix forming a soot layer, a vitrified deposition layer, a preform after sintering, a preform after collapsing, an optical fiber core and a clad layer after drawing by light generated by the light source 16. There should be no change in the composition, density, or geometry of the polymer and no secondary reactions other than the separation of chlorine molecules into atoms or ions and the removal of hydrogen containing impurities. According to the experiment, it is most preferable to use an ultraviolet lamp for supplying ultraviolet rays of 400 nm or less or a laser light source source having a wavelength band of 200 nm or less to the light source 16 satisfying the above conditions.

In particular, when using an ultraviolet lamp as the light source 16, it is preferable to maintain the distance between the light emitting portion of the ultraviolet lamp and the surface of the tube 10 at 1000 mm or less and install it near the gas inlet or on the headstock 8 of the shelf. Moreover, it is preferable to use the ultraviolet lamp which emits the ultraviolet-ray which the frequency wavelength of an ultraviolet lamp is 400 nm or less, the output is 5 W or more and a luminous field is 1 cm or more.

In addition, it is preferable to block the light of the wavelength band of 400nm or more emitted from the light emitting part of the ultraviolet lamp, and to install a glass or glass composite that can withstand temperatures of 100 ° C or more at 500 mm or less in front of the light emitting part of the ultraviolet lamp.

By the light supplied from the light source 16, the neutral chlorine gas separates chlorine molecules into chlorine ions or chlorine atoms as in Scheme 3.

Cl ₂ → 2Cl

In this case, chlorine ions or chlorine atoms act as nucleation to cause a chain reaction with hydroxyl groups on the silica surface, and increase the probability of the dehydration reaction occurring in series, thereby increasing the dehydration effect.

5 is a view showing soot particles from which moisture and hydroxyl groups are removed by irradiating a mixed gas with ultraviolet rays. Referring to the drawings it can be seen that unlike the hydroxyl group (OH) and moisture in the silica soot is removed.

In addition, by irradiating the light supplied from the light source 16 into the tube, hydrogen molecules which may be present in traces in the tube may be split into hydrogen atoms and reacted with chlorine ions as shown in Scheme 4 to easily remove hydrogen chloride (HCl) gas.

H ₂ + Cl ₂ → 2HCl

When the diatomic molecules chlorine and hydrogen are present in a mixture in the tube, high activation energy is required to form hydrogen chloride. If the temperature inside the tube is not high enough, the rate of motion of the gas molecules is small, so the possibility of atomization through the collision of molecules is very small. Therefore, the supply of light for activating energy increases the number of collisions between photons and reactant gas molecules as the light output and intensity increases. This supply of light energy increases the number of generated reaction nuclei in the chain reaction and thus increases the number of hydrogen chloride molecules formed. Therefore, according to Scheme 4, the concentration of hydrogen ions in the tube may be 1 ppb or less by weight.

On the other hand, the chlorine molecular gas may cause defect scattering loss due to the mismatch of the core-clad viscosity, which causes fine bubbles that serve as the core-clad boundary, and the chlorine molecular gas may cause reverse reaction of GeO _{2 in} the core. It can increase, causing microscopic non-uniformity of Ge distribution in the core. Therefore, this problem can be solved by irradiating the chlorine molecular gas with a specific wavelength of light to chlorine atom or ionization.

The light source 16 may be installed in the entire tube area (shelf headstock, shelf end shaft area, etc.) except for the hot zone area corresponding to the center of the torch 14.

The dehydration process using the light source of the present invention doubled the effective mole number of the chlorine gas participating in the conventional dehydration reaction, and maximized the removal efficiency of water and hydroxyl groups by using a chemically active chlorine anion, molecular form It overcomes several disadvantages that can be caused by using mixed gas containing chlorine gas. In addition, the dehydration process using the light source can be applied to all the methods (MCVD, VAD, OVD) for manufacturing the optical fiber preform can improve the dehydration reaction efficiency.

In order for the above dehydration reaction to proceed sufficiently, the time for which chlorine gas stays in the soot layer should be ensured. For this purpose, the feed rate of the torch 14 is preferably maintained at 700 mm / min or less.

4C shows the dechlorination process carried out after the dehydration process. Most of the chlorine gas injected into the tube 10 in the dehydration process reacts with hydroxyl groups, but some of the chlorine gas penetrates or diffuses into the pores in the soot to exist as Si-Cl bonds or Cl atoms. In addition, regardless of the dehydration process, hydrochloric acid (HCl) gas, which is present as an impurity, reacts with the silica to form Si—Cl bonds in the silica. The present step is to remove such chlorine gas and chlorine gas, and after introducing a mixed gas containing oxygen gas into the tube 10, the tube (with a torch 14) in a temperature range lower than the sintering start temperature. 10) is heated. The mixed gas may be an oxygen-helium mixed gas or an oxygen-nitrogen mixed gas.

4D shows the sintering process performed after the sooting, dehydration, and dechlorination process. Reference numeral 18 denotes a sintered deposited layer.

In the sintering process, the silica particles are dehydrated and sintered while maintaining the torch 14 at a high temperature of 1700 ° C or higher. At this time, a mixed gas containing chlorine, helium, and oxygen is introduced into the tube 10, and the total flow rate of the mixed gas, the flow rate of each gas, and the partial pressure ratio thereof are preferably maintained at a constant ratio.

In the sintering step, as in the dehydration step, by irradiating the light of a specific wavelength band by the light source 16 into the tube 10, the water and hydroxyl groups remaining or recontaminated after the dehydration step can be removed.

Meanwhile, since the processes of FIGS. 4A to 4C are all performed at a temperature below the sintering temperature, the silica particles maintain porosity. In this process, the porous particles are sintered by receiving high temperature heat of 1700 ° C. or more, and the soots are vitrified. At this time, the feed rate of the torch 14 is preferably maintained at 700 mm / min or less.

This sooting, dehydration, dechlorination, and sintering process form a layer of cladding, which is continuously repeated until the cladding layer has a desired thickness.

In addition, after the cladding layer has a desired thickness, the input ratio of the reactants is set differently to perform a sooting, dehydration, dechlorination, and sintering process to form a core layer, and when the core layer is formed to a certain thickness, a first preform is formed. ) Is completed. The diameter ratio (D / d) of the finished clad layer and the core layer is preferably determined within the range of 1.0 to 5.0.

The first preform completed through the above-described process is then completed into an optical fiber through a collapsing process. At this time, it is preferable to etch 0.7 mm or more of the outer surface of the primary preform. In general, the outer surface of the preform has the highest hydroxyl content in the preform due to the heating of the torch and is exposed to various impurities. Therefore, when chemical etching is performed using hydrofluoric acid (HF), which is a chemical that dissolves silica on the outer surface of the preform, various impurities and many hydroxyl groups on the surface of the preform are removed.

Figure 6 is a graph showing the loss in the entire 1100 nm to 1700 nm region occurring in the optical fiber core, the loss of the optical fiber core manufactured by the prior art in the figure is a dotted line, the loss of the optical fiber core manufactured by the present invention is It is shown by the solid line.

Referring to the drawings, it can be seen that the optical loss is high over the entire wavelength band when manufacturing the optical fiber by conventional chemical vapor deposition (MCVD). However, in the optical fiber manufactured by the method of the present invention, it can be seen that the OH absorption loss peak of the 1385 nm band is significantly reduced to 0.30 dB / Km or less, and the optical loss due to scattering of the 1310 nm and 1550 nm bands is 0.34, respectively. dB / Km, 0.20 dB / Km or less, compared with the existing single-mode fiber can be seen that.

As described above, although the present invention has been described by way of limited embodiments and drawings, the present invention is not limited thereto and is intended by those skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the scope of the claims to be described. In particular, the dehydration process by the photochemical reaction of the present invention has been described based on the modified chemical vapor deposition method (MCVD), but is not limited to this and can be used in all other methods (OVD, VAD) for manufacturing the optical fiber preform.

The dehydration process according to the present invention can be applied not only to the modified chemical vapor deposition method (MCVD) but also to all the methods (OVD, VAD) for manufacturing the optical fiber preform, which can significantly improve the dehydration reaction efficiency, and the modified chemical vapor deposition method (MCVD). In the case of lowering the hydroxyl group to 1ppb or less it is possible to manufacture a primary preform having a light absorption loss of 0.30dB / Km or less at 1385nm. Therefore, compared with the conventional optical fiber, the usable band of 100 nm or more can be expanded and the optical fiber usable at any wavelength between 1280-1620 nm can be drawn out.

Claims (31)

In the dehydration process using a dehydration gas containing chlorine gas to remove moisture or hydroxyl groups in the manufacture of optical fiber preform by deposition, By irradiating the dehydration gas with light of a wavelength of a specific band to convert the chlorine gas of the molecular form into an atom or ion form to use as a reactant of the dehydration reaction, or to remove impurities containing hydrogen by chemical reaction Dewatering method in the manufacture of optical fiber preforms. The method of claim 1, The light irradiated to the dehydration gas is a preform such as a core diameter of a preform, a mode field diameter, a ripple structure, a refractive index difference (ΔN) between the core and the clad, and a cut-off wavelength. And a wavelength band having a low influence on the geometry and optical structure of the optical fiber and a mounting position of the light source device. The method of claim 1, The light irradiated to the dehydration gas may be composed of a soot layer, a vitrified deposition layer, a preform after sintering, a preform after collapsing, a matrix of the fiber core and the clad layer after the draw, a density, Dewatering method for manufacturing optical fiber preform, characterized in that selected from the wavelength band does not change the geometry. The method of claim 1, The light irradiated to the dehydration gas is a wavelength band for separating the chlorine molecules into atoms or ions and only causing a reaction for removing hydrogen-containing impurities. The method of claim 1, The light irradiated to the dehydration gas is a dehydration method for manufacturing an optical fiber preform, characterized in that the ultraviolet ray of 400nm or less. The method of claim 1, The light irradiated to the dehydration gas is a dewatering method for manufacturing an optical fiber preform, characterized in that the laser beam of 200nm or less wavelength. In the method for manufacturing an optical fiber preform using a crystal chemical vapor deposition method (MCVD), A sooting step of producing a silica soot by heating the tube to a temperature below a sintering temperature by using a torch reciprocating while introducing a reaction gas and oxygen gas into the silicon oxide tube; A dehydration step of heating a temperature lower than that of the sooting process while introducing a mixed gas containing chlorine into the tube and removing water and hydroxyl groups in the silica soot; A sintering step of sintering the silica soot by heating the tube to a temperature above the sintering temperature, And irradiating the mixture gas with light in a specific wavelength band in the dehydration process to separate chlorine molecules into chlorine ions or chlorine atoms to react with water and hydroxyl groups present in the soot. The method of claim 7, wherein The optical fiber, characterized in that the concentration of hydrogen ions in the preform is 1ppb or less by performing a sufficient reaction with the chlorine by hydrogen atoms or hydrogen ionization of the hydrogen molecules present in the tube by the light irradiated in the dehydration process Preform manufacturing method. 8. The method of claim 7, wherein And a dechlorination step of removing chlorine in the silica soot by heating the mixture gas including the oxygen gas into the tube at a temperature higher than the dehydration step and lower than the sooting step. . The method of claim 7, wherein The sooting process is an optical fiber preform manufacturing method, characterized in that performed at a selected value in the temperature range of 1400-1600 ℃. The method of claim 7, wherein The dehydration process is an optical fiber preform manufacturing method, characterized in that performed at a selected value in the temperature range of 500-1300 ℃. The method of claim 7, wherein The light irradiated to the mixed gas in the dehydration step is an optical fiber preform manufacturing method, characterized in that the ultraviolet ray of 400nm or less wavelength. The method of claim 12, The ultraviolet light is supplied by an ultraviolet lamp, and the optical fiber preform manufacturing method characterized in that installed on the outside of the tube so that the distance between the light emitting portion of the ultraviolet lamp and the surface of the tube is maintained at 1000mm or less. The method of claim 12, The ultraviolet light is supplied by an ultraviolet lamp, the ultraviolet lamp is a fiber preform manufacturing method, characterized in that installed in the outer region of the tube other than the hot zone (hot zone) area corresponding to the center of the torch. The method of claim 12, The ultraviolet light is supplied by an ultraviolet lamp, the ultraviolet lamp is an optical fiber preform manufacturing method characterized in that for supplying ultraviolet light of the output is 5W or more and the luminous field is 1cm or more. The method of claim 12, The ultraviolet light is supplied by an ultraviolet lamp, installed at a distance of 500 mm or less from the light emitting part of the ultraviolet lamp, and composed of a glass or glass composite that can withstand temperatures of 100 ° C. or more, and blocks light in a wavelength band of 400 nm or more. The optical fiber preform manufacturing method characterized by further comprising a shielding film. The method of claim 7, wherein The light irradiated to the mixed gas in the dehydration process is a laser having a wavelength of 200nm or less and the laser is supplied by a laser light source source. The method of claim 7, wherein Optical fiber preform manufacturing method, characterized in that the conveying speed of the torch in the dehydration process is maintained at 700mm / min or less. The method of claim 7, wherein In the dehydration process, the mixed gas includes chlorine and helium gas, and the total flow rate, the individual flow rate, and the partial pressure ratio of the mixed gas are introduced at a ratio of 1 to 10 times less than that of chlorine. The method of claim 7, wherein In the sintering process, a mixed gas containing a chlorine gas is injected into the tube, light of a specific wavelength band is irradiated to the mixed gas to separate chlorine molecules into chlorine ions or chlorine atoms, and water remaining in the soot after the dehydration process; A method for producing an optical fiber preform, characterized in that the reaction occurs with a hydroxyl group. The method of claim 20, The method of producing an optical fiber preform, characterized in that the hydrogen molecules present in the tube by the irradiated light to a hydrogen atom or hydrogen ionization to sufficiently react with the chlorine so that the concentration of hydrogen ions in the preform is 1ppb or less. . The method of claim 20, The sintering process is an optical fiber preform manufacturing method, characterized in that carried out at a temperature of 1700 ℃ or more. The method of claim 20, Optical fiber preform manufacturing method characterized in that the conveying speed of the torch in the sintering process is maintained at 700mm / min or less. The method of claim 20, In the sintering process, the mixed gas includes chlorine and helium gas, and maintains the total flow rate, the individual flow rate, and the partial pressure ratio of the mixed gas at a constant ratio. The method of claim 20, And an etching process of etching the finished first preform outer surface after the sintering process by 0.7 mm or more. The method of claim 20, The light irradiated to the mixed gas in the sintering process is an optical fiber preform manufacturing method, characterized in that the ultraviolet ray of 400nm or less wavelength. The method of claim 26, The ultraviolet light is supplied by an ultraviolet lamp, and the optical fiber preform manufacturing method characterized in that installed on the outside of the tube so that the surface distance of the tube and the light emitting portion of the ultraviolet lamp is maintained at 1000mm or less. The method of claim 26, The ultraviolet light is supplied by an ultraviolet lamp, the ultraviolet lamp is a fiber preform manufacturing method characterized in that it is installed in the outer region of the tube other than the hot zone (hot zone) area corresponding to the center of the torch. The method of claim 26, The ultraviolet light is supplied by an ultraviolet lamp, the ultraviolet lamp is an optical fiber preform manufacturing method characterized in that for supplying ultraviolet light of the output is 5W or more and the luminous field is 1cm or more. The method of claim 26, The ultraviolet light is supplied by an ultraviolet lamp, installed at a distance of 500 mm or less from the light emitting part of the ultraviolet lamp, and composed of a glass or glass composite that can withstand temperatures of 100 ° C. or more, and blocks light in a wavelength band of 400 nm or more. The optical fiber preform manufacturing method characterized by further comprising a shielding film. The method according to any one of claims 7 to 30, The sooting, dehydration and sintering process is continued until a cladding layer and a core layer having a desired thickness are obtained, and the diameter ratio (D / d) of the finished cladding layer and the core layer is determined within a range of 1.0 to 5.0. Optical fiber preform manufacturing method.