JPH038743A - Optical fiber preform and preparation thereof - Google Patents

Abstract

PURPOSE:To provide an optical fiber preform having reduced structural defects, low transmission loss, excellent heat resistance, good hydrogen resistance, etc., by preparing a porous quartz glass base material, heating the base material at a specific temperature in a fluorine atmosphere for the doping of a prescribed amount of the fluorine and subsequently sintering the doped base material to form the glass. CONSTITUTION:A gaseous silicon compound (e.g. carbon tetrachloride) is hydro lyzed with an oxygen-hydrogen flame to produce glass fine particles, thereby preparing a porous glass base material deposited on a carrier. The porous glass base material is placed in a fluorine atmosphere and heated at 200-1000 deg.C to dope the fluorine into the base material. The treated base material is sintered to form glass, thereby providing an optical fiber preform whose core portion and/or clad portion each contains 0.01-0.2wt.% of the fluorine. Since the fluorine is doped after the preparation of the porous glass base material but before the sintering thereof, the optical fiber preform having reduced structured defects can be prepared.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a base material for optical fibers, which has extremely few structural defects related to transmission loss, and which is suitable for chemical treatment, heat treatment, and hydrogen fibre.
The present invention relates to an optical fiber base material that does not cause an increase in transmission loss compared to fibers, and a method for manufacturing the same.

[Prior art] In order to obtain a desired refractive index distribution, the base material for optical fiber is doped with germanium in the core, the cladding is made of pure quartz, and the core is made of pure quartz. Optical fibers manufactured using these methods have structural defects in the core or cladding, resulting in deterioration in transmission characteristics, and fiber spinning conditions or hydrogen treatment may cause structural defects. Increased absorption loss related to defects occurs, and 1
It is also known that loss increases in the , 3 μm, and 1.55 am bands.

In addition, various studies are being conducted on the addition of dopants to fill in structural defects, and it is known that fluorine-doped silica glass is chemically stable and has excellent heat and hydrogen resistance properties. Regarding the doping of fluorine into the cladding part, there are two methods: a method of adding a fluorine compound during gas phase hydrolysis of a silicon compound, a method of adding a fluorine compound during the glass sintering process of a porous glass base material, and a method of adding a fluorine compound during the sintering process of a porous glass base material. A method is known in which a fluorine compound is added during a dehydration step at a temperature of 1,000 to 1,200°C.

[Problems to be Solved by the Invention] However, if this fluorine doping is performed during gas phase hydrolysis of a silicon compound, structural defects will occur at the same time as the fluorine doping reaction in the process of producing silica particles.
Structural defects cannot be effectively eliminated by fluorine doping during the production of this porous glass base material. In addition, when a fluorine compound is added during the sintering process of the porous glass base material, transparent vitrification occurs at high temperatures, so the fluorine doping reaction occurs very quickly due to the diffusion of fluorine, but the shrinkage of the porous glass base material fluorine doping occurs at the same time, so when a small amount of fluorine compound is added, this fluorine dope tends to be large at the periphery of the base material and less at the center, and at the same time a convex refractive index distribution is formed. It has the disadvantage that structural defects remain in the center, and when fluorine is doped during the dehydration process during sintering, the fluorine doping reaction is faster than the fluorine diffusion, so the vitrification temperature of the fluorine-doped quartz base material decreases. Since the porous glass base material shrinks during the process, there is a disadvantage that it is difficult to eliminate structural defects uniformly over the entire area of the base material with a small amount of fluorine.

Although chlorine compounds, which are commonly used for dehydration, can be considered as an effective doping agent to eliminate these structural defects, the bond between silicon and chlorine is weaker than the bond between silicon and fluorine, making it difficult to stretch the optical fiber base material. It is difficult to effectively eliminate structural defects with this chlorine compound because the bonds between silicon and chlorine are likely to be broken and structural defects are likely to be regenerated depending on high-temperature treatment conditions such as fiber spinning or fiber spinning.

[Means for Solving the Problems] The present invention relates to an optical fiber base material that solves the above-mentioned disadvantages and drawbacks, and a method for producing the same. The obtained glass particles are deposited on a carrier to form a porous glass base material,
An optical fiber base material obtained by sintering and vitrifying the same, wherein the core portion and/or cladding portion substantially contains 0.01 to 0.2% by weight of fluorine. A porous glass base material is produced by hydrolyzing a glass base material and a gaseous silicon compound with an oxyhydrogen flame, and deposited on a carrier to create a porous glass base material, which is then heated in a fluorine atmosphere to The present invention relates to a method for producing an optical fiber base material, which is characterized in that it is heat-treated at a temperature of .000° C. and then sintered and vitrified.

That is, as a result of various studies conducted by the present inventors on methods for obtaining optical fiber preforms with few structural defects, it was found that the structural defects present in optical fiber preforms are mainly due to the manufacturing process of the porous glass preform and its sintering. This occurs during the process, but in order to prevent the formation of these structural defects, it is effective to fill them with fluorine dope, which has a strong bond with silicon and is thermally and chemically stable. Fluorine is added to the core and/or cladding of the optical fiber base material at a rate of 0.
They found that it is effective to add fluorine in an amount of 0.01 to 0.2% by weight, and completed the present invention by conducting research on this method of doping fluorine.

This will be explained in further detail below.

[Function] The present invention produces a porous glass base material by depositing glass particles obtained by hydrolyzing a gaseous silicon compound with an oxyhydrogen flame on a carrier, and then sintering and vitrifying the glass particles. Regarding fiber preforms, this material is a porous glass preform obtained by flame hydrolysis and doped with fluorine to produce an optical fiber preform with extremely few structural defects.

The porous glass base material made by this flame hydrolysis already has structural defects, and these structural defects are caused by E′ center (=Si・), NBOHC (Mi5t-0・), and peroxy radical. It is known that there are three forms of glass (Mi5t-0-0・), and siloxane bonds with such structural defects are more reactive than siloxane bonds in other stable glasses. Therefore, when this is doped with fluorine, this structural defect is easily cleaved by fluorine to form a stable 5t-F bond through the reaction of the following formula %, and thus this structural defect disappears.

The fluorine compounds used in this fluorine dope include carbon fluoride, carbon fluoride chloride, sulfur fluoride, silicon fluoride, silicon oxyfluoride, and specifically CF4. C2F6
．． C3FB, CCN2F2. CClF2゜SF4
．． SF6. SOF2. SO2F2. SiF4.
5i2FB, 5i20F6゜5i30. Examples include Fa, but compounds containing carbon and sulfur may generate new structural defects due to residual carbon and sulfur, so silicon fluoride and silicon oxyfluoride are preferably used. good. This fluorine compound can be mixed with helium gas and co-supplied so that the fluorine compound is in the range of 0.1 to 1% by volume. It is not preferable to carry out the process in the presence of adsorbed moisture, since this moisture reacts violently with the fluorine compound to produce hydrogen fluoride, which damages the porous glass. In addition, when doping this porous glass base material with fluorine, the bulk density of the porous glass base material affects the diffusion rate of fluorine and the reactivity of the silica particles. The bulk density of the material is preferably 0.3 g/cm3 or less.

The amount of fluorine required to dope the porous glass base material with this fluorine compound is less than 0.01% by weight, which makes it difficult to dope uniformly, but it is 0.2% by weight to fill in structural defects. % is sufficient, and if it exceeds 0.2% by weight, the vitrification temperature will drop and dehydration in the subsequent process will become difficult, so it is necessary to set it in the range of 0.01 to 0.2% by weight. Therefore, this fluorine doping temperature is 20
If the temperature is lower than 0℃, fluorine reacts violently with the moisture remaining in the porous glass base material and damages the porous glass base material, and if the temperature is higher than 1,000℃, the porous glass base material starts to shrink. Since diffusion of fluorine is hindered and it becomes difficult to dope uniformly, it is necessary to keep the doping temperature in the range of 200 to 1,000°C, but this doping time depends on the reactivity of the fluorine compound and the outsideness of the porous glass base material. It may be selected appropriately by considering the diffusion time depending on the diameter and bulk density.

The porous glass base material doped with fluorine in this way has extremely small structural defects, so it is then heated in a sintering furnace to vitrify it and dehydrated to form the desired optical fiber base material. It can be used as a material, but
According to this method, it is possible to advantageously obtain an optical fiber preform that has extremely few structural defects, has extremely low transmission loss, and has excellent thermal stability and hydrogen resistance.

[Example code] Next, examples of the present invention will be given.

Example 1 Silicon tetrachloride is gasified and sent to an oxyhydrogen flame burner,
The glass particles generated by the flame hydrolysis here are deposited on a quartz glass rod as a starting material, with a diameter of 80 m + n and a length of 45 mm.
A porous glass base material having a thickness of 0 mm and a bulk density of 0.230 g/cm3 was prepared.

Next, this porous glass base material was set in a tubular electric furnace, heated to 500°C in a helium gas atmosphere, and silicon fluoride (SiF4
) in a gas atmosphere mixed at a ratio of 0.006 IL/min for 8 times in a temperature range of 450°C to 500°C for a residence time of 30 minutes, and then treated with fluorine. , dehydrated at 1,150°C in a helium gas atmosphere containing chlorine gas, and 1,45% in a helium gas atmosphere.
It was heated to 0°C to make it transparent and a quartz glass rod was made.

Next, we investigated the fluorine content of this quartz glass iron, and found that it was 0.11% by weight.When we stretched it and covered it with a pure quartz jacket tube, we examined the refractive index, and found that it was 0.11% by weight. The refractive index decrease was 0.03%. In addition, after using this quartz glass rod as a core and depositing a porous material on its outer periphery, we doped it with fluorine during sintering to form a cladding with a refractive index lowered by 0.35% based on quartz. When a single-mode optical fiber preform was formed by repeating the section formation twice, the refractive index distribution of this optical fiber preform was as shown in FIG. 1, with a refractive index difference Δn of 0.32% and a clad/ The core diameter ratio is 13
．． It was 4.

This optical fiber base material was spun into a fiber with an outer diameter of +25 IJm and a length of 6 KITl.
When we measured the characteristics of this fiber, we found that the wavelength! , 3. u+
The transmission loss at n and 1.55 μm is 0.35 dB, respectively.
/Km.

The absorption by the hydroxyl group of 1.394 m at 0.18 dB/1 is also good at 0.7 dB/KrQ, which includes 0.64
1 from on. [No unique absorption peak due to structural defects was observed in the i4m wavelength range. When this optical fiber was heat-treated at 200°C for 4 hours in a hydrogen gas atmosphere of 1 atm, it was found that due to the diffusion of hydrogen, l,
No new absorption peaks due to structural defects were observed, except that the hydrogen gas absorption at 24 μm increased by only 0.03 dB/m, and there was no change in the loss value at both wavelengths of 1.3 μm and 1.55 μm.

Example 2 Silicon tetrachloride is gasified and sent to an oxyhydrogen flame burner, and the silica fine particles generated by flame hydrolysis here are deposited on a quartz glass rod as a starting material. At this time, the core portion is doped with germanium. , the cladding part is made of pure quartz with a diameter of 100 mm.
mm, length [i00+nm and bulk density 0.21 g/
A porous glass base material consisting of a core portion and a cladding portion having a size of cm3 was manufactured by integral synthesis.

Next, set this porous glass base material in a tubular electric furnace,
Silicon fluoride (SiF+) was heated to 600℃ in a helium gas atmosphere, and silicon fluoride (SiF+)
The residence time in the temperature range from 550℃ to 600℃ under 7 atmospheres is 35 minutes.
After moving the porous glass base material to a temperature of 300°C and subjecting it to fluorine treatment, it was dehydrated at 1,200°C in a helium gas atmosphere containing chlorine gas, and then heated to 1,430°C in a helium gas atmosphere.
A quartz glass rod was made by heating it to a transparent vitrification.

Next, when we examined the refractive index of this quartz glass rod stretched and covered with a pure quartz jacket, we found that the refractive index increase in the core part was 0.31%, and the refractive index decrease in the cladding part was 0.02%. The clad/core diameter ratio was 4.8, and the fluorine content in both the core and clad parts was 0.075% by weight as a result of chemical analysis. In addition, after depositing a porous material made of fine silica particles on the outer periphery of this quartz glass rod, the fluorine doping method was carried out in the same manner as in this example, and the material was made into transparent glass to form a base material for a single mode optical fiber. As shown in Figure 2, the refractive index distribution of this optical fiber base material is such that the refractive index increase in the core part is 0.31% and the refractive index decrease in the cladding part is 0.02%.
, the relative refractive index difference between the core part and the cladding part is 0.33%
The cladding/core diameter ratio of this product is 14.
It was 5.

When this optical fiber base material was spun into a fiber with an outer diameter of 125 IJm and a length of 8 m, the fiber characteristics were measured, and the transmission loss at a wavelength of 1.3411111.1.55 μm was 0.37 dB/m, respectively. m, 0.19d
B/Km, and the absorption by hydroxyl group of 1.39μm is also o
, 2 da/m, and no peculiar absorption peak due to structural defects was observed in the wavelength range from 0.6 pm to 1.61 Jm.

Furthermore, this optical fiber was heat-treated at 200°C for 4 hours in a hydrogen gas atmosphere of 1 atm, but the absorption by hydrogen gas of 1.24 μm due to diffusion of hydrogen gas increased by only 0.04 dB/m. No new absorption peaks due to structural defects were observed at 1.3 pm.

There was no change in the loss value at both wavelengths of 1.55 tJm. Comparative Example A porous glass base material was made in the same manner as in Example 1, but this porous glass base material was dehydrated and sintered in the same manner as in Example 1 without fluorine treatment during sintering to form quartz. A glass rod is made, this quartz glass rod is stretched to form a core part, a porous material is deposited on the outer periphery of the rod, and when this is sintered, it is doped with fluorine so that the refractive index decrease is 0 compared to pure quartz.
．． When a single-mode optical fiber base material was made by forming a cladding portion with a 32% concentration and repeating the deposition of the porous material and the fluorine doping during sintering twice, the refraction of this optical fiber base material was As shown in FIG. 3, the refractive index difference was 0.32%, and the cladding/core diameter ratio was 13.4.

Next, this optical fiber base material was spun into a fiber with an outer diameter of 125 μm and a length of 6 m, and the fiber characteristics were measured. As a result, the transmission loss at wavelengths of 1.3 μm and 1.55 μm was 0.55 dB/m, respectively. m, 0.32 dB/K
m is 1.397, absorption by hydroxyl group at im is also 11.
2 dB/Km, which also shows an absorption peak at 0.63 μm due to structural defects, and 0.63 μm. The transmission loss at um was also lo, 8 dB/Km.

In addition, when this optical fiber was heat-treated at 200°C for 4 hours in a hydrogen gas atmosphere of 1 atm, the optical fiber was heated to 2.5 dB/2 at wavelengths of 1.24 μm and 1.39% m, respectively.
In addition to the increase in loss of 4.0 dB/Km,
1.52. @ due to structural defects in am, collection peak Q, 1 (
m occurs in ldB/, 1.3pm, 1.554II1
The transmission loss at 1 is also 0.62 dB/Km, respectively.
increased to 0.40 dB/m.

[Brief explanation of the drawing]

FIG. 1 is a refractive index distribution diagram of the optical fiber base material manufactured in Example 1 of the present invention, FIG. 2 is a refractive index distribution diagram of the optical fiber base material manufactured in Example 2 of the present invention, FIG. 3 shows a refractive index distribution diagram of an optical fiber base material manufactured in a comparative example. Figure 1 Figure 2 D7”13.4

Claims (1)

[Claims] 1. Fine glass particles obtained by hydrolyzing a gaseous silicon compound with an oxyhydrogen flame are deposited on a carrier to form a porous glass base material, which is then sintered and vitrified. 1. A preform for an optical fiber, characterized in that the core portion and/or the cladding portion substantially contain 0.01 to 0.2% by weight of fluorine. 2. Gaseous silicon compounds are hydrolyzed with an oxyhydrogen flame to produce glass particles, which are deposited on a carrier to create a porous glass matrix, which is then placed in a fluorine atmosphere to produce glass particles.
2. The method for manufacturing an optical fiber preform according to claim 1, wherein the preform is heat-treated at a temperature of 00 to 1,000[deg.] C., and then sintered and vitrified. 3. The method for manufacturing an optical fiber preform according to claim 2, wherein the fluorine atmosphere consists of a fluorine compound and helium. 4. The bulk density of the porous glass base material is 0.3 g/cm^3
The method for manufacturing an optical fiber preform according to claim 2, which is as follows.