JPS60246233A - Manufacture of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To obtain the titled preform having high refractive index difference between the core and clad, low transmission loss and high quality, by heating a specific porous preform in a gaseous atmosphere containing deuterium-based gas, and then heating in an atmosphere containing fluorine gas. CONSTITUTION:O2 and H2 are supplied as fuel gases to the upper burner 3A and the lower burner 3B of a multi-core burner through the inlet ports 4 and 5, and are made to react with SiCl4 and Ar supplied through the inlet port 7 of the upper burner 3A forming the part corresponding to the clad, and at the same time, made to react with POCl3 and SiCl4 supplied through the inlet port 7 of the lower burner 3B synthesizing the core part. Fine glass particles are deposited at the end of the starting rotary preform 8 along the axial direction to obtain a porous preform wherein the part 9 corresponding to the core is made of quartz containing P2O5 and the part 10 corresponding to the clad is made of pure quartz. The preform is heated in a gaseous atmosphere containing deuterium- containing gas and chlorine-containing gas, and then heated in a gaseous atmosphere containing a fluorine-containing gas to obtain the titled preform wherein 1-20ppm of OD group is added to the part corresponding to the core, and fluorine is added only to the clad part.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a glass base material for optical fibers, and more specifically, the present invention relates to a method for manufacturing a glass base material for optical fibers. In this method, a small amount of P, 0.5%, or A method for producing a glass base material for an optical fiber, in which fluorine can be easily added selectively to the portion corresponding to the cladding by doping it with heavy water and the refractive index difference between the core and the cladding can be adjusted from the amount of added fluorine. It is related to.

BACKGROUND OF THE INVENTION As shown in FIG. 1, a conventional optical fiber usually consists of a central portion 1 called a core portion through which light passes, and a peripheral portion 2 called a cladding portion. The refractive index n1 of the core portion 1 is higher than the refractive index n of the cladding portion 2, as shown in the refractive index distribution in FIG. 1, for the convenience of transmitting light.

Then, the relative refractive index difference △n1Δn between the core and the cladding: (
n, -nl)/n.

By increasing the angle, the receiving angle for total reflection on the cladding surface can be increased, which has the advantage of reducing power loss when bending the optical glass fiber. As a method to increase Δn, in the production of soot base material by flame hydrolysis method, Goo, A, etc.
4,0. , Tie.

There is a method of increasing the refractive index n1 of the core portion 1 by adding a metal oxide dopant such as, or a method of lowering the refractive index n8 of the cladding portion by adding a fluorine-based gas to a portion of the soot base material corresponding to the cladding. Conceivable. These methods are V
AD method, ovp.

The methods are described in Japanese Patent Publication No. 57-40096 and US Pat. No. 3,757,292, respectively.

However, in the case of the former method of increasing the refractive index nl of the core portion 1 by adding a dopant, the following problem occurs as the amount of the dopant increases.

(1) When the amount of dopant is increased, light scattering (Rayleigh scattering) occurs due to the addition of dopant, and the magnitude of this scattering is proportional to the amount of dopant. This scattering increases transmission loss, which is undesirable for optical transmission.

(2) Adding a large amount of dopants tends to cause bubbles and single crystals in the glass base material.

For example, Ge0. When using Ge02 gas, bubbles may be generated due to Ge02 gas.

When At, O, is used, clusters of At, O, and crystals are likely to occur. The presence of such bubbles and crystal phases causes loss in optical transmission (ie, scattering loss), which is undesirable. In addition, it may cause the optical fiber to break. Also, T
ie.

When using T1″+, coloring may occur due to T1″+.

(3) Structural defects occur in the glass due to the presence of a large amount of dopant, which causes ultraviolet absorption due to the defects and OH group absorption due to the bonding of the defects with H and gas.
The optical properties of the fiber deteriorate. In the dopant, this is A Z3 + , T 1$ 10, Gem+
This is thought to be due to the fact that low-value valences are relatively more stable than tetravalent valences.

On the other hand, the latter method of lowering the refractive index n8 by adding fluorine-based gas to the portion corresponding to the cladding is very effective in solving the above-mentioned disadvantages. This method involves making the core part pure silica or adding a dopant such as Gso to pure silica to give it a high refractive index, and heating only the cladding part at a high temperature for at least a period of time in an atmosphere containing fluorine gas. -C This is a method of adding fluorine to the cladding part to lower the refractive index of the cladding part, and finally obtaining a transparent glass base material.

However, even in this method, the following problems have occurred. That is, simply heating at a high temperature in a fluorine-based gas atmosphere will uniformly add fluorine to the core portion of the soot base material, and as a result, Δn cannot be increased. Therefore, a method of adding fluorine only to the cladding part in the dehydration-sintering process has been proposed and put into practical use (for example, Japanese Patent Application Laid-Open No. 55-67533
Publications, etc.). In this case, the temperature inside the furnace at the time when the fluorine-based gas is introduced is an important condition for adding fluorine only to the cladding portion.

Since fluorine is originally highly reactive, it is extremely difficult to control the temperature in a high-temperature furnace, the concentration of fluorine-based gas, and the treatment time, even if fluorine is added only to the cladding.

Therefore, in order to add fluorine only to the portion corresponding to the cladding of a soot base material with a conventional dopant distribution and bulk density distribution, strict temperature control of approximately ±50°C is required. It was extremely difficult to produce an optical glass fiber with a Δn value of .

On the other hand, in the conventional method, there is a problem with the removal of impurities in the fiber, especially residual water.
It is necessary to suppress the amount of H groups as much as possible and eliminate absorption derived from OH groups. For this reason, a method was proposed in which the OH groups were removed by treating the soot base material in a high-temperature atmosphere containing chlorine (Japanese Patent Publication No. 57-40096, Japanese Patent Publication No. 58-40096,
15503). According to the method described in the above-mentioned publication, it is possible to reduce the amount of OH groups in the glass to about 50 ppb, and it is possible to eliminate the influence of the OH groups on the transmission loss characteristics of Core Ino.

However, in fibers containing dopants such as 〇em, which have been made to have low OH groups in a chlorine-containing high-temperature atmosphere by the method described in the above publication, new problems have arisen due to the low OH groups, as explained below. .

The first is that absorption in the ultraviolet region is likely to occur in the fiber due to radiation such as gamma rays and neutron rays. For example, before radiation irradiation, the transmission loss at a wavelength of 185 μm is 2.
5/'kn, after irradiation, the loss increased to 100 dBAll, and the absorption increased by 10 to 50 times compared to the case where the OH group was reduced by a high temperature atmosphere containing chlorine.

Second, when a fiber containing GeO2 in its core, which has been reduced in OH groups due to a chlorine-containing high-temperature atmosphere, is left in a sealed atmosphere or in a cable structure, OH group absorption occurs again within a period of 1 to 10 years. It's something that happens. In severe cases, the transmission loss increased to an extent that fiber transmission could no longer be tolerated.

These phenomena are thought to be caused by hydrogen in the material that makes up or surrounds the fiber diffusing and moving, reaching the core, and combining with the aforementioned defects present in the core to generate OH groups. It is being

It is clear that the phenomenon described in detail above means that fibers obtained by conventional methods still have many defects and are inferior in long-term reliability.

(Problems to be Solved by the Invention) It is an object of the present invention to solve the problems of the fiber according to the conventional method described above, to minimize the structural defects of the glass in the core-corresponding part, to achieve low loss, high fiber The purpose of the present invention is to provide a method for producing a base material for optical fiber that has high quality and long-term reliability.

(Means for solving the problem) As a result of intensive research, the present inventors found that as a means to solve the above-mentioned problem, some P and O were added to the core-corresponding part of the soot base material.
By adding , to adjust the ease of contraction of the core and cladding part when heated, fluorine is added only to the cladding part, and the refractive index difference between the core and cladding is reduced, and at the same time, The present invention was achieved based on the idea of processing the material so that at least the OH group in the core portion is substituted with a 01) group.

That is, the present invention provides a method for manufacturing a transparent glass base material by sintering a quartz-based porous base material for an optical fiber, in which the core portion is made of quartz containing p, o, and the cladding portion is pure quartz. In this step, the porous base material is heat-treated in a gas atmosphere containing deuterium-based gas, and then heat-treated in a gas atmosphere containing fluorine-based gas, whereby the portion corresponding to the core has OD.
Provided is a method for producing a glass preform for an optical fiber, characterized in that at least 1 to 20 ppm of groups are added and fluorine is added only to the cladding portion.

When manufacturing the soot base material, P, 0. When you add
When such a soot base material is subjected to high temperature heat treatment, P, 0. Since the added portion shrinks faster than the P2O-unadded portion, the core portion shrinks faster than the cladding portion (that is, the time for vitrification becomes shorter). Therefore, in order to add fluorine only to the cladding in order to increase the refractive index difference △n between the core and the cladding, the core-corresponding portion should be doped with P, O, and then subjected to high-temperature heat treatment when creating the soot base material. Even if fluorine gas is introduced into the atmosphere at some point during the subsequent high-temperature heat treatment, the outermost periphery of the early sintered core portion of the soot base material will shrink inside the fluorine core. In order to suppress penetration, fluorine is not added inside the core, but is selectively added only to the portion corresponding to the cladding, and this is already known.

This more P! The effect of adding Os will be explained based on the findings obtained through experiments by the present inventors. FIG. 10 shows experimental data obtained by the present inventors regarding the relationship between the amount of P, O, (motX) in the glass base material and the transparent vitrification temperature. From this figure, the contraction of the silica-based porous base material doped with P and OI occurs at a lower temperature than that of pure quartz, and it becomes transparent vitrified quickly. It can be seen that the more P□Os there is, the lower the temperature becomes. From this, when it is desired to shrink only the core portion quickly, at least P and O are required to be at least α025motX, thereby shrinking at a temperature 50° C. lower than that of the cladding portion and making it transparent vitrified. However, P, 0. If the amount is large, the shrinkage is rapid and bubbles are likely to remain in the base material, and in the range of experiments conducted by the present inventors, (L 5 rnol X or more) was not preferable.

Note that P, which causes only the core portion to contract, is 0. The amount is preferably in the range of 101 to 5% by weight.

However, in places where P and O are added in this way, defects are likely to occur, and there are also many defects. This causes radiation resistance and reabsorption of OH groups. This fact is already known.

P in glass, 0. The present inventors have confirmed through experiments that when there are many OH groups, the ultraviolet absorption resulting from the defects increases, and when OH groups are added, the absorption resulting from the defects decreases. This is because the defect and OH are combined.

OOH Defect 10H,O-→Defect EliminationIt was found in the course of developing the present invention that in order to reduce the number of defects, it is preferable for at least 1 PPm or more of OH groups to exist. In fact, when it was fabricated into a fiber and the transmission loss was investigated, absorption due to defects in the vicinity of α65 μm almost disappeared. This indicates that the OH group eliminates defects.

However, when the amount of OH groups is large, there is a large absorption at wavelengths of 095 μm, 1.24 μm, and 1.39 μm derived from OH groups, and the wavelengths of 1.5 μm used for optical fiber communication are
Increase the transmission loss value at 0 μm. For this reason, the permissible amount of OH is limited to 0.3 ppm. Therefore, although increasing OHi can eliminate defects,
Conversely, the transmission loss at 1.30 μm is increased.

On the other hand, in the glass doped with Plol, an additional 1
．． It is known that absorption derived from P-OH exists near 55 μm. This also has an adverse effect on optical transmission in the 1.55 μm band.

It has been found that the ease of production can be overcome by substitution with an OD group. The reason for this is as follows. Table 1 shows the vibrational absorption wavelengths of OH and OD and 1000p at each vibrational absorption wavelength.
It shows the absorption intensity (dB/kII) of pm. α9 of OH group
The absorptions of the OD group corresponding to the absorptions at 4, 1.24° and 1.39μm are 1.2B and 1.68B, respectively, as shown in Table 1.
Although the absorption is at 1.88 μm, the intensity of the absorption of OD group, which generally corresponds to the absorption of OH group, is about 0.57 μm of OH group.
It is known that twice as much.

For example, at 1.50 μm, the absorption increase for 1 ppm of OH group is α6 dB/ki+, but for 1 p of OD group, the increase in absorption is α6 dB/ki+.
At pm, it becomes [1L01dB/km. Therefore, the OD base value that is the same as the absorption increase at 1.30 μm at OH group αS ppm is approximately 20 ppm. Therefore, the preferable range of the amount of OD groups is 1 to 20 ppm.

7/ Table 1 Using the burner 3B, oxygen is supplied from the supply port 4 and hydrogen is supplied from the supply port 5 as combustion gases, and they are ejected from the outermost annular boat of the multicore tube burner and its inner boat. At the same time, 5iCt4 as a raw material gas is supplied from the supply lower using an inert gas such as Ar gas as a carrier gas, and is fed from the center boat of the multi-core tube burner to cause a reaction. In addition, Ar gas is supplied from the supply port 6 for shielding so that the raw material gas reacts in a space several steps away from the tip of the burner.
It will be ejected from the circular boat next to the central boat. In this state, glass particles are deposited in the axial direction from the tip of the rotating starting base material 8 so as to obtain a rod of glass fine particles, that is, a soot base material.

Next, as shown in FIG. 3, in order to locally increase the amount of dopant PtOs or bulk density in the core-corresponding portion 9, oxygen is supplied from the supply port 4 of the upper burner 5A at a rate of 8 t/min to the supply port 6. Ar is supplied at a rate of 21/min from
6t~8t/min of hydrogen from supply port 5, 5i from supply lower
cz, is supplied in the range of 10 to 50 ccZ. The upper burner 3A adjusts the dopant concentration or bulk density locally (
11A and 11B), it serves not only to increase K but also to synthesize the cladding equivalent portion 10. In order to locally increase the bulk density, it is recommended to increase the flow rate of hydrogen and increase the surface temperature of the portion corresponding to the core.

On the other hand, the lower burner 5B has the function of synthesizing the core part, and supplies Pool from the supply lower with 20 cc7% and 5i
oz, is simultaneously supplied at a rate of 200cc/min, O,/
H. It can be burned at the flame point. Moreover, when GeO2 is added, GeCl4 may be supplied at the same time. Note that the above conditions are just an example and do not limit the present invention.

In the manner described above, a soot base material in which the p, o I concentration or bulk density of the core-corresponding portion is higher than that of the cladding portion can be manufactured.

Next, the soot base material is inserted into a heat-resistant core tube such as a pure quartz core tube or an alumina core tube and heated to a high temperature to perform heavy water dope dehydration and sintering.

At this time, the main focus is first on doping the soot with heavy water, for example, in a temperature range of 600°C to 1100°C.
It is preferable to process in an atmosphere where θ gas is supplied at about 5 t/min and heavy water gas at about 51/min. The purpose of using heavy water is OD replacement, and deuterium gas such as Dl + 1 JDl rcD4 * can be used. . It is still preferable that the purity of the heavy water is 995X or higher. This is because impurities in heavy water are mainly H and O, which causes OH groups to remain. If the deuteration temperature is below 600° C., the HjO and OH groups in the soot base material cannot be removed, and deuteration also takes time, which is disadvantageous. In addition, 11
When heated above 00°C, the soot base material in the jacket part begins to shrink, and even if fluorine-based gas is introduced in the second stage, the bulk density of the soot base material increases throughout, so fluorine is not added to the soot base material. It is difficult to add it to the base material jacket.

Following the first-stage heat treatment, a second-stage heat treatment is performed that focuses on the addition of fluorine. In this case, the temperature is p of the core inside the soot base material obtained by the flame hydrolysis reaction,
Although it depends on the distribution of os additives and bulk density distribution, a range of approximately 1150°C and 150°C is preferable. That is, by configuring the soot base material as described above, at least the outermost peripheral portion 11B of the core-corresponding portion is sintered at around 1000°C, and then heated at 1600°C or higher until the entire soot f and d4J are sintered. During processing, the atmosphere is fluorine-based gas,
Fluorine can be added only to the cladding portion 10.

In other words, in order to inject fluorine-based gas under conditions such that fluorine is not added into the core-corresponding part 9 of the soot base material, at least the outermost periphery of the core part must be added during the sintering process of the soot base material. It is advisable to start introducing the fluorine-based gas when the relative density of the portion 11B becomes high. Specifically, the relative density of at least the outermost peripheral portion 11B of the core portion is preferably 0.45 or more.

Therefore, when heat treatment is performed in a furnace tube with a uniform temperature distribution and a heating rate of α (°C/min), the temperature T1 at which the relative density of at least the outermost peripheral part 11B of the core-corresponding part becomes 045, and the cladding-corresponding part The fluorine gas may be introduced until the temperature T3 at which the relative density becomes α45. In this case, the larger the temperature difference ΔT = T, −T, the larger the tolerance for the temperature at which the fluorine-based gas is introduced, making temperature control easier. Conversely, the smaller ΔT is, the more precise control of the charging temperature is required, which is disadvantageous.

In order to increase ΔT, p and o shown in FIG.

It is preferable that the height Δ1 of the concentration core portion is higher.

However, 1. If f is too high, bubbles are likely to be generated during sintering, which is not preferable. Therefore Δ1−α01
It is preferable to carry out flame hydrolysis so that the ratio of P, O, to the glass base material is in the range of -5% by weight.

For example, two cases in which the dopant P801 concentration distribution is (A) and CB) as shown in FIG. 4 will be compared. Here, r is the radial distance from the central axis of the soot base material,
rl is the radius of the core equivalent part of the soot base material, that is, the position of the outermost peripheral part 11B of the core equivalent part 9, Δr is a minute distance, and rI+Δr is the innermost peripheral part of the cladding equivalent part 10 in FIG. It has a radius of 11A. Incidentally, Δr is approximately 1 to 2 cm, but is not limited to this value.

Figure 5 shows two soot base materials (A) and (■) with different p, o, and concentrations of these additives, which are heated in a heating furnace with a heating rate of 16㎜ (A)). .

CB) shows the change in relative density with respect to temperature. Curves Ar1 and Br1 in Fig. 5 are the soot base material (,k), respectively.
The curves ArI+Δr1Brl+Δr show the changes in the relative density of the innermost periphery 11A of the cladding-equivalent parts of the soot base materials (A) and CB), respectively. .

As shown in FIG. 5, the temperature difference from when the relative density of the outermost peripheral part 11B of the core equivalent part in the soot base material reaches cL45 until the relative density of the innermost peripheral part 11A of the cladding equivalent part reaches (L45) ΔT is cA) soot base material ΔTA is (B
) is larger than the temperature difference ΔTB of the soot base material. Accordingly, the temperature control of the soot base material (A) becomes easier. The same can be said about the bulk density distribution.

Although it is most advantageous to simultaneously increase the pcOs concentration and the bulk density at least in the outermost peripheral portion 11B of the core portion, the effects of the present invention can be achieved by selecting either one.

Note that the effects shown below can be achieved regardless of whether or not Gem and a constant refractive index adjusting agent are added to the core portion.

In addition, if it is necessary to adjust the amount of OD groups in the core-corresponding part, use an atmosphere containing chlorine-based gas before or during deuteration to cause rapid contraction of the soot base material at a temperature of 1000 to 1200°C. It is best to take measures to prevent this from happening.

Example 1 P, 0. A soot base material for a single mode fiber having a concentration and relative bulk density distribution was put into a heating furnace at 800°C, and as shown in the graph of Figure 7, D and O gases were fed at 100cc/min and H6 gas was heated at 10t/min. Hold for 60 minutes in an atmosphere supplied at a rate, raise the temperature to 1000°C, stop only the O gas halfway through, then flow chlorine gas at 200 cc/min and hold for 60 minutes, then 87.gas at 150 cc/min. The temperature was raised to 1300°C over 1 hour while further adding H, and then pure H
Transparent vitrification was performed by raising the temperature to 1500° C. or higher in an atmosphere of θ gas.

The refractive index distribution of the obtained glass base material is shown in FIG. 6(C). The core equivalent part has a refractive index distribution of pure quartz,
On the other hand, the portion corresponding to the cladding showed a decrease in refractive index corresponding to the amount of fluorine added. When XMA elemental analysis (X-ray microanalyzer) was performed, it was confirmed that the portion corresponding to the core did not contain any fluorine element. The core part of the product fiber had a Δn value of OX, and the cladding part had a -[130X. OD groups were present at i ppm. Also, Nagoe 63
There was no ultraviolet absorption in the μm region, and no increase in OH group absorption was observed even after high-temperature heat treatment at 200°C.

With conventional fibers, O is reduced by heat treatment under the above conditions.
Increased absorption of H groups is observed.

Comparative Example 1 In order to confirm the effect of Example 1, P, 0. The soot base material was manufactured without supplying any soot.

This soot base material was transparently vitrified under the same sintering conditions as in Example 1, and XMA analysis of the obtained glass base material revealed that fluorine was uniformly distributed regardless of the core-corresponding part and the cladding-corresponding part. Was. Therefore, it was confirmed that the refractive index distribution cannot effectively increase Δn.

Example 2 A soot base material for a single mode fiber having the dopant concentration and relative bulk density distribution shown in FIGS. 6(a) and 6(b) was heated to 800°C, and chlorine gas was
cc/min and He gas was supplied at a rate of 5t/min, and heated to 1050°C at a temperature increase rate of 5°C/min in an atmosphere containing 2% by volume of O, and then only O was supplied. The tube was stopped, 8N+' gas was introduced at a flow rate of 150 cc/min and heated to 1300° C., and then heated to 1600° C. or higher in a pure He 2 atmosphere to perform transparent vitrification.

The refractive index distribution of the obtained glass base material is shown in Figure 8.
The core equivalent part has a refractive index corresponding to pure quartz, and △n
On the other hand, the portion corresponding to the cladding showed a decrease in refractive index corresponding to the amount of fluorine added, and the Δn value was −α15%.

When XMA analysis was performed, it was confirmed that the core-corresponding portion contained no fluorine element at all.

Example 3 P, 0. A soot base material for single mode fiber with concentration and relative bulk density distribution was put into a heating furnace at 800℃, and D, 0 was set to 1.
cc/min, chlorine gas was supplied at a rate of 100 cc/min, He gas was supplied at a rate of 101/min, and the temperature was raised to 1100''C at a rate of 5°C/min. - Inject gas at a rate of 150 CC/min and raise the temperature to 1300 °C, then pure Ha gas 101
1500 in an atmosphere containing 1ace/min of 8F/min.
Transparent vitrification was performed by raising the temperature above ℃.

The refractive index distribution of the obtained glass base material is shown in FIG. The peripheral portion of the glass base material was vertical as shown by the dotted line in FIG. This means that when making the glass transparent, the volatilization of fluorine in the glass can be suppressed by creating a fluorine-added atmosphere.

The portion corresponding to the core had a refractive index of pure silica, while the portion corresponding to the cladding showed a decrease in refractive index corresponding to the amount of fluorine added. When XM analysis was carried out, it was confirmed that the core portion did not contain any fluorine element. The core part of the product fiber had a molar value of 0%, and the cladding part had a value of -a25.

Further, the amount of heavy water in this fiber was about 2 ppm.

Comparative Example 3 To confirm the effect of Example 3, hydrogen was supplied from the supply port 5 of the upper burner 3m at 24/min, while P!
0. The soot base material was manufactured without supplying any soot. As a result, the bulk density distribution was as shown in FIG. 6(b). This soot base material was transparently vitrified under the same sintering conditions as in Example 1, and XMA analysis of the obtained glass base material revealed that fluorine was uniformly present in the core-corresponding part and the cladding-corresponding part. It was distributed.

Example 4 A single-mode fiber Sou-F base material having the P2O concentration (a) and relative bulk density distribution shown in FIGS. 6(a) and 6(b), respectively, was placed in a heating furnace at 800°C.
In an atmosphere where gas was supplied at a rate of 50,007 minutes, D, O gas was supplied at a rate of 10 cc/min, and He gas was supplied at a rate of 10 t/min,
After holding for 15 minutes, the temperature was increased to 100°C at a heating rate of 40°C/min.
The temperature is raised to 0°C, and when it reaches 1000°C, only the supply of O gas is stopped. Then at 200 c of gas
The temperature was raised to 1100°C with c/splitting flow, and then the temperature was raised to 1300°C over 1 hour while adding sy gas at 150cc/min, and then the temperature was further raised to 1500°C or higher in a pure He gas atmosphere. Transparent vitrification was performed.

The refractive index distribution of the obtained glass base material is shown in FIG. 6(C). The core-corresponding part showed a refractive index distribution of pure silica, while the cladding-corresponding part showed a decrease in refractive index corresponding to the amount of fluorine added. When XMA elemental analysis (X-ray microanalyzer) was performed, it was confirmed that the portion corresponding to the core did not contain any fluorine element. The core part of the product fiber is Δn
The value of the OX1 cladding part was -0.30X.

OD groups were present at 1 ppm. Also, the wavelength α65μm
There was no ultraviolet absorption present in the region, and no increase in absorption of OH groups was observed even after high-temperature heat treatment at 200°C. Conventional fibers show increased absorption of OH groups by heat treatment under the above conditions.

Example 5 A soot base material for single mode fiber having the dopant concentration shown in FIG.
At a heating rate of 5°C/min in an atmosphere supplied at a rate of 1.
Heat to 1,000℃, leave at 1,000℃, and add 1
After supplying cc/min for 1 minute, remove the chlorine gas, stop the O, and start raising the temperature again, but at this time, the temperature is 8F and the gas is 150℃.
It was charged at a flow rate of cc/min and heated to 1500°C, and then heated to 1600°C or higher in a pure Hθ atmosphere to perform transparent vitrification.

In the above embodiments, the soot base material for a single mode fiber was described, but the method of the present invention can be applied to a soot base material for a multimode fiber to obtain the same effect.

As is clear from the above, according to the method for manufacturing a glass base material for optical fiber of the present invention, fluorine is not added to the core-corresponding portion of the soot base material during high-temperature heating, and fluorine is added only to the cladding-corresponding portion of the soot base material. Can be easily added. Therefore, it is possible to effectively manufacture an optical fiber with a large Δn.

However, according to the method for producing a glass base material for optical fiber of the present invention, when fluorine-based gas is used for at least one time to add fluorine to a portion corresponding to the cladding when the soot base material is heated and sintered at high temperature, Adjustment of manufacturing conditions is extremely easy.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the refractive index distribution structure of an optical fiber. FIG. 2 is a schematic diagram of a method for producing a soot base material by a flame hydrolysis method according to the present invention. FIG. 5 is a diagram illustrating the dopant distribution within the soot base material. FIGS. 4(A) and 4(B) show P in the soot base material, 0. A diagram showing an example of concentration distribution. FIG. 5 shows the change in the relative density in the radial direction within the soot base material in the cases of (4) and (0) due to temperature increase (16° C./min). FIGS. 6(a), (b), and (C) show the P, 0. Graph showing concentration distribution, relative density distribution, and refractive index difference distribution. FIG. 7 is a diagram illustrating processing conditions for the soot base material in Example 1 of the present invention. FIG. 8 is a diagram showing the refractive index difference distribution of the soot base material of Example 2 of the present invention. FIG. 9 is a diagram showing the refractive index difference distribution of the soot base material of Example 5 of the present invention. FIG. 10 is a graph showing the relationship between the amounts of p, o, and (motX) in the glass base material and the transparent vitrification temperature. Agents 1) Osamu Akiyo Ryo Hagiwara - Ngo ('14

Claims (4)

[Claims] (1) The core equivalent part is P, 0. In a method for manufacturing a transparent glass base material by sintering a quartz-based porous base material for an optical fiber, the porous base material is made of quartz containing quartz and the cladding portion is pure quartz. After heat treatment in a gas atmosphere containing fluorine-based gas, heat treatment is performed in a gas atmosphere containing fluorine-based gas, thereby adding at least 1 to 20 ppm of OD groups to the core-corresponding portion, and adding fluorine only to the cladding portion. A method for producing a glass base material for optical fiber, characterized by: (2) Claim (1) in which the porous base material is heat-treated in a gas atmosphere containing deuterium-based gas and chlorine-based gas
) A method for producing a glass base material for optical fibers as described in item 1. (3) Heat treatment in an atmosphere containing chlorine gas before or after heat treating the porous base material in an atmosphere containing deuterium gas, or in an atmosphere containing deuterium gas and chlorine gas. A method for manufacturing a glass preform for optical fiber according to claim (1) or (2). (4) Any one of claims (1) to (3), in which the core portion is heat-treated until it reaches a temperature at which it contracts before being heat-treated in a gas atmosphere containing a fluorine-based gas. A method for producing a glass base material for optical fiber described in .