JP5771943B2 - Optical fiber and method for manufacturing glass preform for optical fiber - Google Patents

Description

  The present invention relates to an optical fiber having a structure in which a core and a clad are made of quartz glass, each having a core not containing germanium and an optical clad portion containing fluorine, and a method for producing a glass preform for the optical fiber.

  In the process of manufacturing an optical fiber made of quartz glass, after a glass fine particle deposit is formed, a dehydration treatment is performed, and then sintered to obtain a transparent glass. In the dehydration and sintering processes, it is known that the transmission loss of the optical fiber can be kept low by heating the glass fine particle deposit in an atmosphere of chlorine-containing gas.

  As a method of manufacturing an optical fiber preform having a clad having a two-layer structure, after forming a porous glass intermediate body in which a porous first clad original layer is formed on the outer peripheral surface of a porous core original layer The porous glass intermediate is placed and sintered in a chlorine-based gas-containing atmospheric gas that does not contain a fluorine compound, and the porous glass intermediate is created on the outer peripheral surface of the transparent glass intermediate created by the sintering. It is known that a two-cladding original layer is formed, and the second cladding original layer is dehydrated and sintered to form a transparent glass preform for an optical fiber. And dehydrating the second cladding original layer with a higher chlorine concentration than when dehydrating the first cladding original layer, and sintering the second cladding original layer with a higher chlorine concentration than when sintering the first cladding original layer, The second cladding part on the outer periphery of the first cladding part contains chlorine at a higher concentration than the first cladding part on the outer periphery of the core, so that the refractive index of the second cladding part is higher than the refractive index of the first cladding part. It is known (see, for example, Patent Document 1).

  In addition, in order to increase the refractive index of glass, it is known that after the porous glass body is dehydrated, transparent vitrification (sintering) is performed in a gas atmosphere composed of a mixed gas of an inert gas and silicon tetrachloride. (For example, refer to Patent Document 2). In patent document 2, in order to make the refractive index of the outer cladding higher than the refractive index of the inner cladding in an optical fiber having a two-layered cladding, when adding a fluorine addition It describes that silicon tetrachloride is used when sintering a core of a clad base material. It also describes dehydrating a porous glass body as a core material using silicon tetrachloride.

  Also, in the method of manufacturing a dispersion-shifted optical fiber preform, a pure quartz porous glass body is dehydrated in an atmosphere containing chlorine gas, and then heated while flowing an atmosphere gas containing only silicon tetrafluoride to add fluorine. In addition, it is known that a glass pipe of a portion that becomes a clad of fluorine-added glass is produced by sintering (see, for example, Patent Document 3).

JP 2003-54995 A Japanese Patent Laid-Open No. 10-53423 Japanese Unexamined Patent Publication No. 64-87528

  When manufacturing an optical fiber having an optical cladding part of quartz glass containing fluorine, there is a case where a fluorine-added pipe is produced in the process of manufacturing a glass preform for an optical fiber as described above, and then collapsed after inserting a core rod. However, it has been found that, when producing the pipe, if fluorine is added after the porous glass body is dehydrated with chlorine, a phenomenon that the transmission loss of the optical fiber becomes large occurs. At this time, it has also been found that the residual chlorine concentration is low (for example, below the EPMA detection limit).

As mentioned above, by increasing the concentration of chlorine gas used during dehydration and sintering, a method of increasing the residual chlorine concentration after transparent vitrification is known, but this is intended to increase the refractive index, It does not focus on transmission loss. And when producing a fluorine addition pipe, since the refractive index is lowered by adding fluorine, it is difficult to increase the concentration of chlorine gas having the effect of increasing the refractive index.
It is also known to perform sintering using silicon tetrachloride in order to increase the refractive index as described above, but the object is to make the outer clad of the core and the two-layer clad transparent. This is not intended for the inner cladding (optical cladding) in which fluorine is added to lower the refractive index.

  The present invention relates to an optical fiber having a core and a clad made of quartz glass, a core not containing germanium and an optical clad part containing fluorine, and increasing the chlorine concentration contained in the optical clad part to reduce the transmission loss of the optical fiber. It aims at providing the manufacturing method of the optical fiber which can be restrained low, and the glass preform | base_material for optical fibers.

An optical fiber according to the present invention capable of solving the above problems is a silica-based optical fiber comprising a core and a cladding,
The core is made of quartz glass not containing germanium,
The clad has an optical clad portion located on the outer circumference of the core, and a jacket portion located on the outer circumference of the optical clad portion,
The optical clad part contains 0.45 mass% or more and 1.50 mass% or less of fluorine, and contains chlorine with an average concentration of 270 ppm or more and 750 ppm or less , and the closer to the core, the higher the chlorine concentration is. .
The “quartz glass not containing germanium” refers to a quartz glass made of pure silica and does not contain germanium (other elements (K, Cl, F, etc.) may be contained in a small amount). .

  The optical fiber according to the present invention preferably has a transmission loss at a wavelength of 1550 nm of 0.176 dB / km or less.

  In the optical fiber according to the present invention, it is preferable that the optical cladding is quartz glass dehydrated using silicon tetrachloride.

The method for manufacturing a glass preform for an optical fiber according to the present invention that can solve the above-described problem is a method of manufacturing an optical fiber by drilling a core material that is made of silica glass that does not contain germanium and is a core of an optical fiber. The core material and the fluorine-added pipe material are integrated by heating and inserted into the central portion of the fluorine-added pipe material to be an optical clad portion, and a glass to be a jacket portion of an optical fiber is formed on the outer periphery of the core. A method of manufacturing a fiber glass preform,
The glass fine particle deposit is dehydrated using silicon tetrachloride, and then sintered in an atmosphere of a fluorine-added gas, and then the center is perforated to form the fluorine-added pipe material,
The dehydration treatment in all parts of the glass particles deposit, not less than ℃ Maximum temperature T _A in the dehydration process is 1110, a total of Δt time be more than the temperature T _eff that acts dewatering action over 100 minutes Meet certain conditions,
The formed fluorine-added pipe material contains 0.45 mass% or more and 1.50 mass% or less of fluorine, contains chlorine with an average concentration of 270 ppm or more and 750 ppm or less , and is close to the core inserted in the center portion It is characterized by having a higher chlorine concentration .

In the method for producing a glass preform for an optical fiber according to the present invention, it is preferable that a bulk density of the glass fine particle deposit during the dehydration treatment is 0.19 g / cm ³ or more and 0.6 g / cm ³ or less.

The optical fiber of the present invention is a quartz glass optical fiber having a core not containing germanium and an optical cladding portion containing fluorine, and the fluorine content of the optical cladding portion is 0.45 mass% or more and 1.50 mass%. It is as follows. This fluorine content provides the necessary relative refractive index difference for the core that does not contain germanium. Furthermore, the optical clad part contains chlorine in an average concentration of 270 ppm or more and 750 ppm or less , and the closer to the core, the higher the chlorine concentration, so that an optical fiber with less transmission loss can be obtained.
According to the manufacturing method for the optical fiber of the present invention, the dehydration process of making the fluoridation pipe material as the optical cladding of the optical fiber, the four highest temperature T _A with using the silicon chloride is 1110 ° C. or more, dewatering action the by a condition to work more than 100 minutes, the addition of fluorine pipe, with fluorine containing less 1.50 mass% to 0.45 mass%, chlorine containing less 750 ppm or higher the average concentration 270 ppm, in the heart The closer to the inserted core, the higher the chlorine concentration . As a result, an optical fiber having a transmission loss with a structure having a core not containing germanium and an optical cladding part containing fluorine can be obtained.

It is sectional drawing which shows one Embodiment of the optical fiber which concerns on this invention. It is a schematic diagram which shows the refractive index distribution of the optical fiber shown in FIG. It is a schematic diagram which shows the dehydration process which manufactures the glass preform | base_material for optical fibers. It is a graph which shows an example of the temperature change of the glass particulate deposit during a dehydration process. It is a graph which shows an example of the temperature change of the glass particulate deposit during a dehydration process. It is a graph which shows an example of the temperature change of the glass particulate deposit during a dehydration process. It is a schematic diagram which shows one process which manufactures the glass preform | base_material for optical fibers. It is a graph which shows the relationship between the chlorine concentration of an optical clad part, and the favorable rate of transmission loss.

Hereinafter, an example of an embodiment of an optical fiber according to the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view of the optical fiber of the present embodiment cut along a plane perpendicular to the optical axis, and FIG. 2 is a schematic diagram showing a refractive index distribution of the optical fiber of FIG.

  As shown in FIG. 1, the optical fiber 1 has a core 2 at the center and a clad 5 on the outer periphery thereof. The clad 5 has an optical clad portion 3 located on the outer circumference of the core 2 and a jacket portion 4 located on the outer circumference of the optical clad portion 3. The core 2 is quartz glass that does not contain germanium, and is substantially pure quartz. However, the core 2 may contain only a small amount of chlorine or the like during the manufacturing process.

As shown in FIG. 2, in the optical fiber 1, the central core 2 has the highest refractive index, and the surrounding cladding 5 has a low refractive index by a relative refractive index difference Δn with respect to the core 2.
As specific examples of the size of each part, the diameter d1 of the core 2 is 9.6 μm, the diameter d2 of the optical cladding part 3 is 48 μm, and the diameter d3 of the jacket part 4 is 125 μm. By appropriately setting the diameter of the core 2 and the shape of the refractive index distribution, the optical fiber 1 can be a single mode fiber or a multimode fiber. Further, the core 2 may be formed of a plurality of layers having different refractive indexes.

  The optical clad portion 3 is a region near the core 2 in the clad 5, and is a portion greatly related to the optical transmission characteristics of the optical fiber 1. The optical clad part 3 of the present embodiment is made of quartz glass containing 0.45 mass% or more and 1.50 mass% or less of fluorine and containing chlorine with an average concentration of 270 ppm or more and 2000 ppm or less. The optical cladding 3 is adjusted so that the refractive index of the optical cladding 3 becomes smaller than that of the core 2 by adding fluorine. By setting the fluorine content of the optical cladding part 3 to 0.45 mass% or more and 1.50 mass% or less, the relative refractive index difference Δn with respect to the core 2 is set to 0.15% to 0.50%. For example, the relative refractive index difference Δn of the optical cladding 3 with respect to the core 2 is 0.4%.

  The optical cladding 3 contains 270 ppm or more of chlorine on average. Thereby, the transmission loss of the optical fiber 1 can be reduced. For example, when the average chlorine concentration of the optical cladding part 3 is 270 ppm or more, the transmission loss of the optical fiber 1 at a wavelength of 1550 nm is 0.176 dB / km or less. Moreover, when the average chlorine concentration of the optical cladding part 3 exceeds 2000 ppm, the increase in the refractive index due to the inclusion of chlorine cannot be ignored, and it is necessary to increase the fluorine content accordingly.

  Further, the closer to the core 2 in the optical cladding part 3, the higher the chlorine concentration may be. For example, the average chlorine concentration may be in the range of 300 ppm or more and 3000 ppm or less in a portion from the boundary with the core 2 in the optical cladding portion 3 to a diameter twice as large as the MFD (mode field diameter) having a wavelength of 1550 nm. Since the MFD of a normal single mode optical fiber is about 10 μm, the range of about 20 μm in diameter corresponds to this.

  The optical cladding 3 is made of quartz glass containing chlorine and fluorine as described above by dehydrating the glass particulate deposit with silicon tetrachloride and then adding fluorine and sintering. Yes.

  The jacket part 4 preferably has the same refractive index as that of the optical cladding part 3, and is preferably quartz glass containing fluorine at the same level as the optical cladding part 3. However, since the jacket portion 4 has a smaller influence on the optical transmission characteristics of the optical fiber 1 than the optical cladding portion 3, the refractive index may be different from that of the optical cladding portion 3.

Next, a method for manufacturing an optical fiber preform on which such an optical fiber 1 is drawn will be described.
First, a core material to be the core 2 of the optical fiber 1 is prepared. The core material is made of quartz glass that does not contain germanium. In order to produce the core material, for example, a glass fine particle deposit of pure silica is produced by the VAD method, dehydrated and sintered in a chlorine atmosphere, transparentized, and then stretched to have a desired outer diameter.

  In addition, a fluorine-added pipe material that becomes the optical cladding portion 3 of the optical fiber 1 is prepared. In order to produce a fluorine-added pipe material, first, a glass fine particle deposit of pure silica is produced by, for example, the VAD method. In the VAD method, glass particles are deposited on a starting rod suspended in a reaction vessel by a glass particle generating burner. For example, it is possible to grow the glass particulate deposit in the axial direction of the starting bar by detecting the deposition surface of the glass particulate and pulling up the starting rod so that the position of the deposition surface becomes constant.

The glass fine particle deposit thus produced is dehydrated in the heating furnace 17 as shown in FIG. In this dehydration process, the entire glass particulate deposit 16 is heated with the atmosphere of a silicon tetrachloride (SiCl ₄ ) gas and an inert gas (such as helium) inside the heating furnace 17 that houses the glass particulate deposit 16.

  As a form of the heating furnace 17, there is a case where a heating area by a heater is provided in a part in the longitudinal direction, and a case where a heating furnace which heats the entire inside of the heating furnace with a heater 18 and a core tube 19 at a substantially uniform temperature as shown in FIG. There is. In the case where a heating furnace having a heating region in part is used, the entire glass particle deposition body 16 is heated while moving the glass particle deposition body 16 in the axial direction with respect to the heating region in the vicinity of the heater in the heating furnace. In the case of using a soaking furnace as shown in FIG. 3, the glass fine particle deposit 16 accommodated in the furnace is heated without moving. In the heating furnace 17 of FIG. 3, the glass particulate deposit 16 is accommodated and heated inside the furnace tube 19, and the atmosphere gas is introduced into the heating space through the supply pipe 20 and appropriately passed through the exhaust pipe 21. Exhaust atmospheric gas.

For example, by using a heating furnace having a heating region in a part of the longitudinal direction, the glass particulate deposit 16 is moved in the axial direction from top to bottom with respect to the heating region of the heating furnace, and the lower end of the glass particulate deposit 16 is moved. When heating sequentially from the upper end to the upper end, the temperature change during the dehydration process at an arbitrary location A (see FIG. 3) of the glass particulate deposit 16 is schematically shown in the graph of FIG. Temperature T _A along the vertical axis of the graph, the highest temperature during the dehydration process, the temperature T _eff along the vertical axis of the graph represents a lower limit temperature which acts dewatering action, dehydrated at a temperature T _eff more time domain Δt Indicates that will be done. The temperature T _eff is 1000 ° C., for example. In the example of FIG. 4, any point A with the movement from the top to the bottom of the glass particle deposited body 16 it is heated above the temperature T _eff enters the heating zone, after reaching the maximum temperature T _A, heating The temperature falls outside the region and falls below the temperature T _eff . The time over which the temperature T _{eff is} exceeded is Δt.

Further, the glass particulate deposit 16 is reciprocated in the axial direction from top to bottom and from bottom to top with respect to the heating region of the heating furnace, and the glass particulate deposit 16 is sequentially heated from the lower end to the upper end and from the upper end to the lower end. In this case, when the temperature change during the dehydration process at an arbitrary location A (see FIG. 3) of the glass particulate deposit 16 is schematically shown, the graph of FIG. 5 is obtained. In the example of FIG. 5, any point A with the movement from the top to the bottom of the glass particle deposited body 16 it is heated above the temperature T _eff enters the heating zone, after reaching the maximum temperature T _A, heating The temperature falls outside the region and falls below the temperature T _eff . At this time, the time during which the temperature becomes _{equal to} or higher than T _eff is Δt ₁ . Then, after the upper end of the glass fine particle deposit 16 is heated in the heating region, the moving direction of the glass fine particle deposit 16 is reversed, and the arbitrary position A is accompanied by the movement of the glass fine particle deposit 16 from the bottom to the top. There is heated above the temperature T _eff entered again heating zone, after reaching the maximum temperature T _a, going down below the temperature T _eff off the heating region. At this time, the time during which the temperature is _{equal to} or higher than T _eff is Δt ₂ . That is, in this example, the total time Δt = Δt ₁ + Δt ₂ when the temperature is equal to or higher than T _eff .

Further, when the glass fine particle deposit 16 is housed in a soaking furnace and the entire glass fine particle deposit 16 is heated at the same time, the glass fine particle deposit 16 is being dehydrated at an arbitrary position A (see FIG. 3). A schematic representation of the temperature change is as shown in the graph of FIG. In the example of FIG. 6, the temperature of any point A with increasing temperature of the soaking furnace is heated to above the temperature T _eff increases, reaches a maximum temperature T _A. Is maintained as it is the highest temperature T _A, dehydration is completed. The time over which the temperature T _{eff is} exceeded is Δt. In the dehydration process in the soaking furnace, the supply of silicon tetrachloride gas is stopped at the end of the dehydration, so that the dehydration reaction does not proceed after this even if the temperature is _{equal to} or higher than T _eff .

In either example shown in FIGS. 4 to 6, in order to exert a dewatering effect of silicon tetrachloride enough, as 1110 ° C. or higher the maximum temperature T _A in the dehydration process, above the temperature T _eff that acts dewatering action The total time Δt is set to be 100 minutes or more. A stronger dehydrating action can leave more chlorine. After dehydrating the glass particulate deposit 16, the glass particulate deposit 16 is heated in a fluorine gas (eg, SiF ₄ ) atmosphere to add fluorine and sinter to make it transparent. Thus, the average chlorine concentration of the transparent glass body to which fluorine is added can be set to 270 ppm or more. When the glass fine particle deposit 16 after dehydration is sintered by adding fluorine, for example, SiF ₄ having a concentration of about 5% is mixed with He and heated to 1500 ° C. while flowing to add fluorine. Heat to 1600 ° C. in the same atmosphere. Thereby, 0.45 mass% or more and 1.50 mass% or less of fluorine can be contained.

Further, the bulk density of the glass particulate deposit 16 during the dehydration treatment is preferably 0.19 g / cm ³ or more and 0.6 g / cm ³ or less. If the bulk density is less than 0.19 g / cm ³ , the glass particulates are soft, so that the glass particulate deposit 16 is likely to break during dehydration, and conversely if the bulk density is greater than 0.6 g / cm ³ , the glass particulates However, since the dehydrating material and additives are difficult to penetrate, it is difficult to dehydrate, and it is difficult to add fluorine.

  Thus, the central axis part of the transparent glass body containing chlorine and fluorine is perforated to obtain a fluorine-added pipe material. The fluorine-added pipe material is made of quartz glass containing 0.45 mass% or more and 1.50 mass% or less of fluorine and chlorine with an average concentration of 270 ppm or more and 2000 ppm or less.

  As shown in FIG. 7, the core material 12 is inserted into the hole 14 at the center of the fluorine-added pipe material 13. In this state, the fluorine-added pipe material 13 is heated, and the core material 12 and the fluorine-added pipe material 13 are integrated by rod in collapse. Thereby, the intermediate body of the optical fiber preform which has the part used as the core 2 of the optical fiber 1 and the part used as the optical clad part 3 of the optical fiber 1 is formed.

And the glass of the part used as the jacket part 4 of the optical fiber 1 is formed in the outer periphery of this intermediate body. For example, glass particles of pure silica are deposited on the outer periphery of the intermediate by the OVD method, and the deposit is grown in the radial direction. It is heated in a heating furnace and dehydrated. In the dehydration treatment, the inside of the heating furnace is made an atmosphere of chlorine-containing gas (chlorine gas, silicon tetrachloride gas, etc.) and an inert gas. Thereafter, the transparent and sintered together with fluorine gas (e.g., SiF ₄₎ is heated in an atmosphere adding fluorine. Thereby, the glass used as the jacket part 4 of the optical fiber 1 is formed, and the glass base material for the optical fiber having the part that becomes the core 2, the part that becomes the optical cladding part 3, and the part that becomes the jacket part 4 is formed. can get.

  The optical fiber glass preform manufactured in this manner is drawn by a drawing apparatus, thereby having a structure having a core 2 not containing germanium and an optical cladding part 3 containing fluorine, and having a small transmission loss. The optical fiber 1 can be obtained. The details of the mechanism by which transmission loss is reduced by chlorine remaining at a certain concentration in the optical clad part 3 containing fluorine in this way are unknown, but chlorine atoms relieve residual stress on the core. It is presumed that it is.

(About chlorine concentration and transmission loss of optical cladding)
The relationship between the concentration of chlorine contained in the optical cladding 3 shown in FIG. 1 and the transmission loss of the optical fiber 1 was examined.
The optical clad part 3 prepared several samples from which the average residual density | concentration of chlorine differs as quartz glass containing 0.45 mass% or more and 1.50 mass% or less of fluorine. The transmission loss of the optical fiber 1 at a wavelength of 1550 nm was examined for each sample, and the good rate of transmission loss was calculated. The result is shown in FIG. The good rate of transmission loss was calculated as good rate = (good length / evaluation length) × 100 (%), assuming that the transmission loss of the optical fiber 1 at a wavelength of 1550 nm is 0.176 dB / km or less. .

  As shown in FIG. 8, the good rate is almost 0% up to an average residual concentration of chlorine up to 200 ppm, the good rate changes abruptly around 270 ppm, and the good rate exceeds 50% and is almost 100% above 270 ppm. It turns out that it becomes. That is, it can be seen that the transmission loss of the optical fiber 1 at a wavelength of 1550 nm is 0.176 dB / km or less when the average chlorine concentration of the optical cladding 3 is 270 ppm or more.

(Dehydration conditions of the glass particulate deposit that will be the optical cladding)
Change the total Δt maximum temperature T _A and dewatering action is above the temperature T _eff acting time for performing the dehydration treatment of the soot glass deposit body 16 shown in FIG. 3, the relationship between the residual chlorine concentration and transmission loss Examined.
The diameter of the glass fine particle deposit 16 is 100 mm, the length of the glass fine particle deposit 16 is 750 mm, and the whole is heated while moving the glass fine particle deposit 16 in the axial direction using a heating furnace having a heater length of 400 mm, Dehydration treatment was performed. Silicon tetrachloride was used for the dehydration treatment and supplied into the heating furnace at a flow rate of 1.0 liter / min. After the dehydration treatment, SiF ₄ having a concentration of about 5% was mixed with He and flowing while adding fluorine to sinter and make transparent. The residual chlorine concentration of the transparent glass body was measured. In addition, an optical fiber glass base material was prepared using the optical fiber, and an optical fiber was prepared by drawing to measure a transmission loss at a wavelength of 1550 nm. These results are shown in Table 1. Incidentally, the maximum temperature T _A dealing with minimum values at different measurement points of the glass particle deposited body 16 as a representative value. In Comparative Example 4, silicon tetrachloride was not used for the dehydration treatment, and chlorine was supplied into the heating furnace at a flow rate of 2.0 liters / minute. The heating furnace temperature is 1200 ° C., and the temperature T _eff at which the dehydrating action works is 1000 ° C.

In Comparative Examples 1 and 2 and Example 1, the glass fine particle deposit was traversed in one direction with respect to the heating furnace to perform dehydration.
Comparative Example 1, the traverse speed is fastest (30 mm / min), the highest temperature _{T A} is 1020 ° C., the time Δt to be above the temperature _{T eff} was only 30 minutes. As a result, the residual chlorine concentration was as low as 140 ppm, and the good rate of transmission loss was 0%.
Comparative Example 2, to the traverse speed 20 mm / min, the maximum temperature _{T A} became 1120 ° C.. The time Δt at which the temperature T _{eff was} exceeded was 60 minutes, but the residual chlorine concentration was 240 ppm, and the good rate of transmission loss was 5%.
Example 3, to the traverse speed 10 mm / min, the maximum temperature _{T A} became 1180 ° C.. The time Δt at which the temperature T _{eff is} exceeded is as long as 110 minutes. As a result, the residual chlorine concentration increased to 350 ppm, and the good rate of transmission loss was 100%.

In Examples 2 and 3 and Comparative Example 3, dehydration was performed by reciprocating the glass fine particle deposit in a reciprocating manner with respect to the heating furnace.
Comparative Example 3, the traverse speed is fastest (30 mm / min), the highest temperature _{T A} is 1020 ° C., the time Δt to be above the temperature _{T eff} was 60 minutes. Although the time Δt was longer than that of Comparative Example 1, the residual chlorine concentration was as low as 170 ppm, and the good rate of transmission loss was 0%.
Example 2, to the traverse speed 20 mm / min, the maximum temperature _{T A} became 1120 ° C.. The time Δt for _{reaching the} temperature T _eff was 120 minutes. The time Δt was longer than that of Comparative Example 2, the residual chlorine concentration was 300 ppm, and the good rate of transmission loss was 92%.
Example 3, to the traverse speed 10 mm / min, the maximum temperature _{T A} became 1180 ° C.. The time Δt at which the temperature T _{eff was} exceeded was 220 minutes. As a result, the residual chlorine concentration increased to 420 ppm, and the good rate of transmission loss was 100%.

  In Comparative Example 4 using chlorine gas, the dehydration treatment was performed under the same conditions as in Example 1 in which the good rate was 100%. As a result, the residual chlorine concentration was as low as 140 ppm, and the good rate of transmission loss was 0%.

Thus, dried over silicon tetrachloride, the maximum temperature T _A is 1180 ° C., the temperature T _eff above becomes time Δt is equal to or more than 110 minutes Example 1 and Example 3, the residual chlorine concentration at that time It became clear that the good rate of transmission loss could be made 100%.

  Further, in Examples 1 to 3 and Comparative Examples 1 to 4, the glass fine particle deposit had a diameter of 100 mm, but this was set to 150 mm, and the dehydration treatment was performed under the same conditions as in Example 3. The results are shown in Table 2.

Thus, compared with Example 3 above, the diameter of the glass particle deposited body is larger in Example 4, the maximum temperature T _A is low, the time Δt to be above the temperature T _eff has become shorter, because the As a result, the residual chlorine concentration and the good rate of transmission loss decreased. As the diameter of the glass particulate deposit increases, it is necessary to maintain the temperature Δt above the maximum temperature T _A and the temperature T _eff so as not to decrease by increasing the temperature of the heating furnace or slowing the traverse speed. I understand that.

  1: optical fiber, 2: core, 3: optical clad part, 4: jacket part, 5: clad, 12: core material, 13: fluorine-added pipe material, 16: glass fine particle deposit

Claims (2)

A core material made of quartz glass not containing germanium and serving as a core of an optical fiber is inserted into the central portion of a fluorine-added pipe material in which a central portion is perforated to be an optical cladding portion of the optical fiber, and the core material and the core A method of manufacturing a glass preform for an optical fiber by heating and integrating the fluorine-added pipe material and forming a glass serving as a jacket portion of the optical fiber on its outer periphery,
The glass fine particle deposit is dehydrated using silicon tetrachloride, and then sintered in an atmosphere of a fluorine-added gas, and then the center is perforated to form the fluorine-added pipe material,
The dehydration treatment in all parts of the glass particles deposit, not less than ℃ Maximum temperature T _A in the dehydration process is 1110, a total of Δt time be more than the temperature T _eff that acts dewatering action over 100 minutes Meet certain conditions,
The formed fluorine-added pipe material contains 0.45 mass% or more and 1.50 mass% or less of fluorine, contains chlorine in an average concentration of 270 ppm or more and 750 ppm or less, and is closer to the core inserted in the central portion. A method for producing a glass preform for an optical fiber, wherein the chlorine concentration is high. 2. The method for producing a glass preform for an optical fiber according to claim 1, wherein a bulk density of the glass particulate deposit during the dehydration treatment is 0.19 g / cm ³ or more and 0.6 g / cm ³ or less. .

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