JP2010163329A - Method for manufacturing preform for optical fiber added with rare earth element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing in high productivity an optical fiber having a uniform core NA that enables a solution of a compound containing rare earth elements to be efficiently used. <P>SOLUTION: The method for manufacturing a preform for the optical fiber includes a liquid impregnation step where glass microparticles deposited on the inner peripheral surface of a quartz tube by the MCVD method are impregnated with the solution of a compound containing rare earth elements to add the rare earth elements to the glass microparticles. After a step of clarifying the glass microparticles added with the rare earth elements, the method for manufacturing a preform for the optical fiber includes another step of depositing the glass microparticles and another step of clarifying the glass microparticles. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for manufacturing an optical fiber preform, and more particularly to a method for manufacturing a rare earth element-added optical fiber preform.

  As a manufacturing method for producing a core base material using a modified chemical vapor deposition (MCVD) method, there is a manufacturing method called immersion MCVD in which the formation of glass fine particles and the step of adding a rare earth element-containing compound are performed as separate steps. is there. In the manufacturing method, after depositing glass fine particles on the inner peripheral surface of a quartz tube, a solution containing a rare earth element-containing compound is poured from one end of the quartz tube to penetrate into the fine particles, and then the rare earth element-containing compound, etc. In this method, the glass fine particles are made transparent after being diffused by heat. For example, a glass particulate layer is formed by depositing glass particulates on the inner peripheral surface of a quartz tube, a liquid immersion step in which the glass particulate layer is impregnated with a solution of a rare earth element-containing compound, and a drying step. Patent Document 1 describes a method of manufacturing an optical fiber preform by a forming step and a step of collapsing a quartz tube. In the manufacturing method described above, the optical fiber preform is produced by performing a collapse process for solidifying the quartz tube subsequent to the transparency of the glass fine particle layer.

JP 2004-83399 A

However, when forming or collapsing the fine particle glass, dehydration must be performed by flowing a gas such as Cl ₂ or BCl ₃ because OH groups contained in the raw material are mixed inside the core. This is because the transmission loss of pump light and signal light increases due to the inclusion of OH groups in the core, and the fiber characteristics deteriorate. However, when a gas such as Cl ₂ or BCl ₃ is flowed for dehydration, the reaction between these gases and glass is promoted at a high temperature, so that the core may be selectively etched (evaporated). . Further, since the etching rate of additive elements such as Yb, Al, Er, and Ge is faster than that of glass, the refractive index at the core central portion is lowered, and a dent is generated at the center of the refractive index profile. On the other hand, since the main light is guided through the core center as the fundamental mode (LP01), light scatters due to the depression at the center of the refractive index profile, resulting in an increase in transmission loss and distortion of the mode field. Further, the etching rate of the additive element differs depending on the gas such as Cl ₂ or BCl ₃ , and the glass composition fluctuates and crystal defects are locally generated. In particular, in the case of a Yb-doped fiber laser, the nonlinear problem such as self-excited pulse, Rayleigh scattering (RS), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS) becomes prominent, and the serious problem that the end face of the fiber is destroyed. Occurs. In order to solve this problem, a new collapsing process is required. Since dehydration is an indispensable process in order to reduce the concentration of OH groups, it has been required to improve the optical fiber preform manufacturing method so that the above problems can be solved while performing dehydration. .

  The present invention has been made in view of such circumstances, and a problem to be solved is to provide a method for manufacturing an optical fiber preform in which no dent is generated in the center of a refractive index profile.

That is, the present invention is as follows.
[1] A method of manufacturing an optical fiber preform having a liquid immersion step of adding a rare earth element by impregnating glass fine particles formed on the inner peripheral surface of a quartz tube by an MCVD method with a solution of a rare earth element-containing compound. A rare earth element-doped optical fiber, further comprising a step of depositing glass fine particles and a second transparency step of glass fine particles after the first transparency step of the glass fine particles to which the rare earth element is added. A manufacturing method of a base material.
[2] The manufacturing method according to [1], further including a first collapse step following the first transparency step and a second collapse step following the second transparency step.
[3] In the immersion step, the solution injected into the quartz tube is sealed at both ends of the quartz tube with a plug formed using an elastic material, and the quartz tube with the plug mounted The method according to [1] or [2] above, wherein the method is a step of rotating around a central axis.
[4] Any one of the above [1] to [3], wherein the rare earth element-containing compound includes a rare earth element-containing compound and a co-added compound, and the numerical aperture of the optical fiber preform is 0.05 to 0.2. The manufacturing method as described in.
[5] The method according to [4], wherein the rare earth element is ytterbium (Yb) and the co-added element is aluminum (Al).

  According to the present invention, it is possible to manufacture an optical fiber preform with few dents and crystal defects at the center of the refractive index profile by a simple method without making a significant change in the manufacturing process while utilizing a conventional manufacturing apparatus. it can.

FIG. 1 is a flowchart showing a method for manufacturing an optical fiber preform according to an embodiment of the present invention. FIG. 2 is a cross-sectional view for explaining the liquid immersion process of the embodiment of the present invention. FIG. 3 is a cross-sectional view for explaining the liquid immersion process of the embodiment of the present invention. FIG. 4 is a chart showing the evaluation results of the optical fiber preform manufactured by the method of the example of the present invention. FIG. 5 is a flowchart showing a method for manufacturing an optical fiber preform of a comparative example. FIG. 6 is a chart showing the evaluation results of the optical fiber preform manufactured by the method of the comparative example.

Hereinafter, the present invention will be described in more detail.
The present invention relates to a method for manufacturing an optical fiber preform having a core doped with a rare earth element or the like. FIG. 1 is a flowchart showing the manufacturing method of the present invention. As shown in FIG. 1, the manufacturing method of the present invention includes a first deposition step S1, a liquid immersion step S2, a drying diffusion step S3, a first transparency step S4, a first collapse step S5, a second deposition step S6, 2 transparency process S7 and 2nd collapse process S8. Note that a quartz tube dehydration step S0 may be performed before the deposition step.

[Dehydration step (S0)]
In the MCVD method, it is desirable to suppress the mixing of OH groups in order to increase the transmission loss due to the OH groups contained in the organometallic raw material and the OH groups mixed during core fabrication, and to deteriorate the laser characteristics. Therefore, in the dehydration step (S0), the inner peripheral surface of the quartz tube is dehydrated by introducing a dehydrating gas from one end of the anhydrous quartz tube while heating the anhydrous quartz tube to a high temperature. As the dehydrating gas to be used, Cl ₂ , SiCl ₄ , GeCl ₄ , POCl ₃ , BCl ₃ or the like can be used. At that time, the dehydrating gas may be supplied alone, but it may flow simultaneously with a gas such as O ₂ , Ar, or He. The dehydration temperature is preferably 1200 ° C. to 1500 ° C. in consideration of damage on the inner peripheral surface of the quartz tube. The anhydrous quartz tube means a quartz tube having an OH content of 1 ppm or less (limit of measurement by an infrared spectrometer).

[First deposition step (S1)]
Next, in the first deposition step S1, a glass fine particle (hereinafter sometimes referred to as soot) layer is formed on the inner peripheral surface of the hollow quartz tube. In the first deposition step, while heating the quartz tube, glass source gas, carrier gas, reaction gas, and the like are introduced from one end thereof. Thereby, a soot layer is formed by depositing glass fine particles on the inner peripheral surface of the quartz tube. The temperature for heating the quartz tube is preferably 1000 ° C. to 1600 ° C. When the heating temperature at the time of deposition exceeds 1600 ° C., the size and density of the fine particle glass fluctuate, and the additive element that soaks into the fine particle glass in the liquid immersion step S2 (described later) in which the rare earth element-containing compound is infiltrated into the fine glass particles. And undesirable effects on the doping concentration of rare earth elements. On the other hand, when fine particle glass is deposited at a low temperature of less than 1000 ° C., the soot layer may be peeled off from the quartz tube. Further, when depositing fine particles, it is preferable to control the glass internal pressure so that the internal pressure of the quartz tube is about 4 Pa lower than the atmospheric pressure.
Examples of the glass source gas used in MCVD include SiCl ₄ , SiF ₄ , POCl ₃ , BF ₃ , and BCl ₃ . Further, examples of the carrier gas include He and Ar, and examples of the reaction gas include O ₂ .
In general, the thickness of the soot layer to be formed is about 0.05 mm to about 0.5 mm. The appropriate layer thickness varies depending on the diameter of the quartz tube to be used. However, it is preferable to thicken the soot layer and collapse, so that the fiber production from the fiber preform increases, so the layer thickness is preferably within the above range. .

[Immersion process (S2)]
The next step is a liquid immersion step S2 in which a rare earth element-containing compound solution (hereinafter also referred to as a processing solution) is injected into the inner peripheral surface of the quartz tube on which the soot layer is formed, and the soot layer is impregnated. Here, the rare earth element-containing compound solution is a solution in which the respective chlorides or oxides of the rare earth element and the co-additive with the rare earth element are dissolved. As the rare earth element-containing compound, erbium chloride _{(ErCl 3, ErCl 3 · 6H} 2 O), neodymium chloride (NdCl _3), ytterbium chloride _{(YbCl 3, YbCl 3 · 6H} 2 O), thulium chloride (TMCL _3), lanthanum chloride Examples of the co-addition compound such as (LaCl ₃ ) include AlCl ₃ , AlCl ₃ .6H ₂ O, P ₂ O ₅ , H ₃ PO _{4, and the} like. Examples of the solvent include polar solvents such as alcohols, water, and hydrochloric acid, and ethanol is most preferable from the viewpoint of easy drying and removal.

  The properties of rare-earth-doped fibers are greatly affected by the concentration of rare-earth elements and co-additives. When Yb and Al are co-added, the solution concentration (wt%) is preferably in the range of 0.05 ≦ Yb ≦ 1.5 and 0.05 ≦ Al ≦ 2. Further, since the clustering of Yb affects the photodarkening of glass and adversely affects the fiber laser characteristics, the molar ratio of Al to Yb is also an important parameter for the Yb-doped optical fiber characteristics. That is, in order to suppress the clustering of Yb, it is desirable that the molar ratio R (= Al / Yb) of Al and Yb is 3 ≦ R ≦ 15. By setting the ratio within this range, an optical fiber preform having an NA of 0.05 to 0.2 can be provided.

For this immersion process, various conventional methods can be applied as long as the soot layer formed in the quartz tube can be impregnated with the solution of the co-additive. Among them, the present inventors have used the method shown in FIGS. 2 and 3 as a liquid immersion method capable of making the core NA and the doping concentration distribution of the manufactured optical fiber uniform while realizing a reduction in the amount of processing liquid used. Now, the liquid immersion method of the present invention will be described.
In the embodiment shown in FIG. 2, rubber plugs 41 and 42 are inserted into openings at both ends of the quartz tube 11 in which the soot layer 21 is formed, and a rare earth element-containing compound solution containing a co-added component (hereinafter, referred to as a processing solution may be called. A mode in which the quartz tube 11 is rotated while preventing 31 from flowing out of the quartz tube 11 is described. That is, in this embodiment, the rubber plugs 41 and 42 are used as liquid outflow suppression means. Since the quartz tube 11 is rotated around its central axis, a small amount of the processing liquid 31 stored in the lower part spreads over the entire surface of the soot layer 21 and is effectively impregnated with the fine particle glass. Since the soot layer 21 is not formed with uneven shapes or the like, density unevenness due to unevenness does not occur, and uniform processing is performed. The amount of treatment liquid 31 introduced into the quartz tube 11 varies depending on the diameter of the quartz tube 11 and the thickness of the soot layer 21, but is 2 to 5 mm from the surface of the soot layer 21 located on the bottom surface. Is preferred. This water level corresponds to about 20 to 40% of the internal volume of the quartz tube 11.

  Various methods can be employed for injecting the rare earth element-containing compound solution 31 into the quartz tube 11, and one form thereof is shown in FIG. As shown in FIG. 3, a through hole 44 is formed at the center of one rubber plug 43, and an injection tube 50 is inserted into the through hole 44. The rare earth element-containing compound solution 31 is injected into the quartz tube 11 from the injection tube 50. Since the quartz tube 11 is rotated by a glass lathe, an amount of the rare earth element-containing compound solution 31 reaching the height position of the injection tube 50 in the quartz tube 11 is not necessary. In other words, the rare earth element-containing compound solution 31 does not need to be injected so much that it flows out of the through hole 44. Therefore, when the quartz tube 11 for impregnating the treatment liquid is rotated, the injection tube 50 need only be pulled out, and there is no need to replace it with a rubber stopper in which no through hole is formed. At this time, a stopper may be attached to the through hole 44 after the injection tube 50 has been extracted. Further, depending on the shape of the injection tube 50 outside the quartz tube, the quartz tube 11 can be rotated while being attached. Note that a rubber plug without the through hole 44 may be replaced. Further, if the diameter of the through hole 44 is made larger than the outer shape of the insertion portion of the injection tube 50 so that the injection tube 50 can freely rotate in the through hole 44, the quartz tube 11 can be removed without removing the injection tube 50. Can be rotated. After completion of the immersion, the residual co-added solution in the quartz tube 11 can be removed by removing the rubber stoppers 41 and 42 or 43 stopped at both ends of the quartz tube 11.

  Here, a typical example of the plug formed using an elastic material is a rubber plug, and the plug material is particularly a material that does not dissolve or swell due to the rare earth element-containing compound solution. The type is not limited. Examples thereof include crosslinked isoprene rubber or crosslinked butyl rubber, crosslinked isobutylene / isoprene rubber, thermoplastic elastomer, and thermosetting elastomer. Examples of the thermoplastic elastomer include various elastomers such as urethane, ethylene propylene, EVA, EEA, and styrene, nylon 6, nylon 66, polyester, ethylene vinyl alcohol copolymer, and polyvinylidene chloride. Examples of the thermosetting elastomer include those mainly composed of natural or synthetic isoprene-based, ethylene propylene diene monomer-based, isoprene isobutylene-based, nitrile butadiene-based, chloroprene-based, or silicone-based elastomer. Among these, silicone rubber stoppers are preferably used.

  The appropriate rotation speed of the quartz tube varies depending on the diameter of the quartz tube. For example, in the case of a quartz tube having an outer diameter of 28 mm and an inner diameter of 25 mm, 5-20 revolutions / minute is preferable. Within this rotation speed range, the rare earth element-containing compound solution can be uniformly permeated into the soot layer on the inner surface of the quartz tube.

[Drying diffusion step (S3)]
After the immersion is completed, the process proceeds to the drying diffusion step S3. The quartz tube is naturally dried while flowing an O ₂ gas after removing the residual treatment liquid. The time for natural drying may be about 1 hour.
After drying, in order to raise the temperature of the quartz tube, the temperature of the external heat source is raised stepwise, and the temperature of the quartz tube is heated from 150 ° C. to 1500 ° C. The heating is aimed at dehydration, decomposition of the rare earth element-containing compound and co-additive, and diffusion of these elements, so it is desirable to raise the quartz tube temperature stepwise from 150 ° C. to 1500 ° C. Since decomposition of rare earth element-containing compounds and co-additives occurs at 150 ° C. or higher, rare earth elements such as Er and Yb ions and co-additive ions such as Al and P are decomposed in this dry diffusion step. It is presumed that it is diffused uniformly in the fine particle glass.

[First transparency step (S4)]
Subsequent to the drying diffusion step S3, the first transparency step S4 is performed. In the clearing process, the quartz tube temperature is raised from 1500 ° C. to 1800 ° C. while flowing a dehydration gas such as Cl ₂ and a carrier gas such as He and O ₂ gas into the quartz tube, so that a minute amount of residual moisture and foreign matter are removed. The soot layer to which the rare earth metal element or the like is added can be made transparent.

[First Collapse Step (S5)]
Subsequent to the first transparency process, the first collapse process may be performed. By providing this first collapse step, the time of the second collapse step can be shortened and glass etching (evaporation) can be more effectively prevented. By executing the first collapse step, the inner diameter of the glass tube can be reduced to some extent. However, if it is too small, the flow of the glass raw material gas becomes worse in the second deposition step, so it is preferable to collapse to about 15 to 20 mm in inner diameter. In order to avoid the core from becoming flat, it is desirable that the internal pressure of the quartz tube be lowered by 0 Pa to 10 Pa with respect to the atmospheric pressure. In addition, the first collapse step can be omitted.

[Second deposition step (S6)]
Thereafter, the external heat source is lowered so that the temperature of the quartz pipe becomes the same as that in the first deposition step S1, and the same glass raw material gas, carrier gas, reaction gas, etc. as those used in the first deposition step A thin film layer (soot layer) of fine particle glass was formed while flowing. In addition, when the thickness of the soot layer is thick, the core refractive index profile is lowered (that is, the refractive index at the core central portion is lowered), so the thickness of the soot layer deposited in the second deposition step is 0.001 mm. To 0.1 mm is preferable.

[Second transparency step (S7)]
Thereafter, the quartz pipe temperature is raised stepwise from 1500 ° C. to 1800 ° C. under the same conditions as in the first clarification step S4, and baking and fine particle glass are clarified.

[Second Collapse Step (S8)]
Subsequent to the second transparency step S7, the quartz tube was collapsed by heating from the outside of the quartz tube at the same temperature. By this second collapse step S8, an optical fiber preform having a solid core doped with rare earth elements and co-additive elements is completed. In order to avoid the core from becoming flat, it is desirable that the internal pressure of the quartz tube be lowered by 0 Pa to 10 Pa with respect to the atmospheric pressure.

[Refractive index measurement method]
The optical characteristics of the optical fiber preform were measured using an optical fiber preform internal refractive index distribution measuring device. The apparatus used is a preform analyzer model P104 manufactured by Photon Kinetics.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by these.

[Example]
The temperature was raised stepwise while flowing 1.4 SLM of O ₂ on the inner wall surface of an anhydrous quartz tube having an outer diameter of 28 mmφ, a tube thickness of 1.5 mm, and a length of 400 mm, and the inner wall surface of the quartz tube was baked at 1200 to 1500 ° C. After dehydration, dehydration was performed at 1130 ° C. while flowing 50 SCCM of Cl ₂ (dehydration step). For holding by a glass lathe, an auxiliary quartz tube having a larger diameter than the quartz tube may be added to both ends of the quartz tube. Although the length of the auxiliary quartz tube is not particularly defined, an auxiliary quartz tube having a length of about 500 mm is used in this embodiment. No soot layer is formed on the auxiliary quartz tube. Here, SLM and SCCM are units of gas flow rate expressed based on 0 ° C., and correspond to L / min and mL / min, respectively.
Thereafter, fine glass was deposited on the inner wall surface of the quartz tube four times at 1180 ° C. while flowing 0.56 SLM SiCl ₄ , 0.4 SLM He, 0.5 SLM O ₂ gas (first). Deposition process). When depositing the fine particles, the internal pressure of the quartz tube was controlled so that the internal pressure of the quartz tube was about 4 Pa lower than the atmospheric pressure.

Next, the fine particle glass at the end was removed from the inner surface of the quartz tube, and both ends of the auxiliary quartz tube were sealed using a rubber stopper. Separately, a rare earth element-containing compound solution obtained by dissolving 0.6 g of YbCl ₃ .6H ₂ O (rare earth element-containing compound) and 2 g of AlCl ₃ .6H ₂ O (co-added compound) in 300 mL of ethanol (also referred to as a treatment liquid). Was prepared. After injecting 150 mL of a rare earth element-containing compound solution into the quartz tube, the quartz tube is rotated around its central axis for 1 hour while being held on the chuck of the glass lathe, and the entire inner surface of the quartz tube is uniformly contained. The compound solution was infiltrated. At this time, the water level of the injected processing liquid was about 5 mm from the surface of the lowest soot layer. The rotation speed of the chuck of the glass lathe capable of holding and rotating the quartz tube was 10 rpm. YbCl ₃ · 6H ₂ O and AlCl ₃ · 6H ₂ O are both water-soluble and soluble in water and alcohols, but alcohol was used as a solvent in consideration of the drying process. The co-addition amount of YbCl ₃ · 6H ₂ O and AlCl ₃ · 6H ₂ O varies depending on the properties of the target optical fiber, but in this example, the numerical aperture (NA) is 0.06 to 0.08, Co-addition was performed at a Yb concentration of about 0.5 wt% and an Al concentration of about 0.6 wt% so that the core absorption coefficient at a wavelength of 915 nm would be 100 to 120 dB / m (immersion step).

After the immersion treatment, the rubber stoppers stopped at both ends of the quartz tube were removed, and the residual co-added solution in the quartz tube was poured and discarded, followed by natural drying for about 1 hour while flowing O ₂ gas. Then, in order to raise the temperature of the quartz tube, the temperature of the external heat source was raised stepwise, and heat drying was performed at a temperature of the quartz tube of 150 ° C. to 1500 ° C. (drying diffusion step). In the heat drying, dehydration, decomposition of YbCl ₃ · 6H ₂ O, AlCl ₃ · 6H ₂ O, and Al and Yb were diffused, and the heating temperature of the quartz tube was raised stepwise from 150 ° C. to 1500 ° C. Decomposition of YbCl ₃ · 6H ₂ O and AlCl ₃ · 6H ₂ O occurs at 150 ° C. or higher, and the generated Yb ions and Al ions are uniformly diffused in the fine particle glass by heating.
Thereafter, the internal pressure in the quartz tube is maintained to be 4 Pa lower than the atmospheric pressure, the O ₂ flow rate is 0.3 SLM, the He flow rate is 0.7 SLM, the Cl ₂ flow rate is 20 SCCM, and the quartz tube temperature is 1500 while flowing the mixed gas. The temperature was raised from 1 ° C. to 1800 ° C. to make the fine particle glass transparent (first transparent step), and the inner diameter of the glass tube was reduced to 15 mmφ (first collapse step).
Thereafter, an external heat source is controlled so that the temperature of the quartz pipe can be maintained at 1180 ° C., and a soot layer is supplied while flowing 0.56 SLM SiCl ₄ , 0.4 SLM He, 0.5 SLM O ₂ gas. Formed. When the thickness of the soot layer is thick, the refractive index profile at the center of the core is lowered. Therefore, in the present embodiment, 0.001 mm to 0.1 mm is preferable (second deposition step).
Thereafter, the quartz pipe temperature was gradually increased from 1500 ° C. to 1800 ° C. while flowing 0.3 SLM O ₂ , 0.7 SLM He, and 20 SCCM Cl ₂ gas, and baked again to make the fine particle glass transparent (first glass). 2 transparency process).
Finally, a second collapsing process for the quartz tube was performed. When collapsing, bubbles and cavities are not generated in the center of the core due to insufficient heating, and concaves are not formed in the refractive index profile. On the other hand, excessive heating causes the quartz tube to sag or Yb clusters to form. A solid core having no longitudinal variation was produced by appropriately moving the heater so as not to occur. As a result of measurement using an optical fiber preform internal refractive index distribution measuring apparatus (Photon Kinetics, model P104), the core diameter of the produced optical fiber preform was about 2 mmφ.

  In FIG. 4, the evaluation result by the preform analyzer model P104 (made by Photon Kinetics) of the optical fiber preform produced in the Example is shown. In FIG. 4, the horizontal axis indicates the radial position (mm) from the central axis of the optical fiber preform, and the vertical axis indicates the refractive index difference of the preform with respect to the matching oil used at the time of measurement. A refractive index profile having no refractive index dent at the core central portion was obtained. This is an effect of suppressing the additive element from being etched during the collapse. In addition, since the crystallinity of the core central portion is also improved, light transmission loss due to light scattering is reduced, and nonlinear characteristics can be suppressed, fiber characteristics can be reproducible, and manufacturing stability can be expected.

  Further, the variation in NA in the longitudinal direction of the core of the optical fiber preform manufactured in this example was as small as about 5%. As a result, it was possible to secure 240 mm as an effective length that can be used as a base material. The variation in the NA of the core in the longitudinal direction is measured by measuring the NA value at positions 20, 60, 100, 140, 180, and 220 mm from the end of the 240 mm long base material, and the standard deviation of the NA value at those positions. Was doubled, and the value obtained by dividing it by the average NA was calculated as the variation. The obtained effective length (240 mm) was more than twice as compared with the case of manufacturing by the conventional liquid immersion method. Thus, by improving the variation in characteristics, it becomes possible to ensure the reproducibility of characteristics and the stability of manufacturing.

[Comparative example]
In the comparative example, an anhydrous quartz tube having an outer diameter of 28 mmφ, a tube thickness of 1.5 mm, and a length of 400 mm is used, and the second deposition step, the second transparency step, and the second collapse step are omitted. An optical fiber preform with a core diameter of about 2 mmφ was manufactured under the same manufacturing conditions as in the Examples except that the collapse process was performed following the first transparency process. FIG. 5 is a flowchart showing the manufacturing process of the comparative example.

  FIG. 6 shows an evaluation result of an optical fiber preform manufactured by the manufacturing method of the comparative example using a preform analyzer model P104 (manufactured by Photon Kinetics). In FIG. 6, the horizontal axis indicates the radial position (mm) from the central axis of the optical fiber preform, and the vertical axis indicates the refractive index difference of the preform with respect to the matching oil. A clear recess with a refractive index could be confirmed in the core central part.

  As described above, by the immersion MCVD method of the present invention, it is possible to provide an optical fiber preform that does not have a refractive index dent at the core central portion. In addition, in the immersion process, the rare earth element-containing compound solution is sealed in the quartz tube using a stopper made of an elastic material, and the quartz tube is rotated, thereby doping the soot layer with rare earth elements and co-additive elements. Thus, a uniform optical fiber preform can be produced in a high yield with a small amount of processing liquid and small variations in NA and the like.

  According to the manufacturing method of the present invention, it is possible to provide an optical fiber that has no refractive index dent at the core central portion, has a small optical transmission loss due to improved crystallinity at the core central portion, and has nonlinear characteristics suppressed. In addition, it is possible to improve the reproducibility of fiber characteristics and the stability of manufacturing, and it is possible to manufacture a homogeneous optical fiber with a small variation in NA and the like with a high yield.

11 Quartz tube 21 Soot layer 31 Rare earth element-containing compound solution (treatment liquid)
41, 42, 43 Rubber stopper 44 Through hole 50 Injection pipe

Claims (5)

A method of manufacturing an optical fiber preform having a liquid immersion step of adding a rare earth element by impregnating a glass fine particle formed on the inner peripheral surface of a quartz tube by an MCVD method with a solution of a rare earth element-containing compound,
A rare earth element-added optical fiber preform characterized by further comprising a step of depositing glass fine particles and a second transparency step of glass fine particles after the first transparency step of the glass fine particles to which the rare earth element is added. Production method.   The manufacturing method according to claim 1, further comprising: a first collapse process following the first transparency process; and a second collapse process following the second transparency process.   In the immersion step, the solution injected into the quartz tube is sealed with plugs formed at both ends of the quartz tube and formed of an elastic material, and the quartz tube with the plug is attached to a central axis. The manufacturing method according to claim 1, wherein the manufacturing method is a step of rotating around the substrate.   4. The production according to claim 1, wherein the rare earth element-containing compound includes a rare earth element-containing compound and a co-addition compound, and the numerical aperture of the optical fiber preform is 0.05 to 0.2. 5. Method. The method according to claim 4, wherein the rare earth element is ytterbium (Yb) and the co-added element is aluminum (Al).

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