JPH038744A - Rare earth element-doped quartz glass fiber preform and preparation thereof - Google Patents

Abstract

PURPOSE:To prepare the subject optical fiber preform containing a rare earth element homogeneously doped therein in a high concentration and having an excellent optical function, etc., by adding a rare earth element-containing com pound to a porous quartz glass base material, heating the base material in the presence of a fluorine compound and subsequently sintering the treated base material to form glass. CONSTITUTION:Silica glass fine particles produced by a flame hydrolysis method are deposited to prepare a porous glass base material. The base material is impregnated with a rare earth element-containing compound (e.g. erbium chlo ride) and heated in the presence of a fluorine compound to fix the rare earth element compound as a fluorine compound thereof. The treated base material is sintered at a high temperature to form transparent glass, thereby providing a rare earth element-doped quartz glass optical fiber preform comprising a core made of quartz glass containing the rare earth element and the fluorine and a clad made of quartz glass containing the fluorine in an amount layer than that of the fluorine contained in the quartz glass forming the core.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is a rare earth element-doped silica glass base material for optical fibers, and in particular, because it has an optical function, it can be used as an optical fiber laser, optical amplifier, or sensor element. The present invention relates to a base material for a rare earth element-doped silica glass optical fiber and a method for manufacturing the same.

(Prior Art) Regarding quartz glass having optical functions, rare earth element-doped quartz glass, which is obtained by adding a rare earth element to quartz glass, is known.

Therefore, in the production of this rare earth element-doped quartz glass, silicon compounds such as silicon tetrachloride and rare earth element compounds are added to the quartz glass along with germanium, phosphorus, and boronate compounds as dopants, which are added as necessary. Internal CVD method (MCVD method) in which the silica glass particles are introduced into the tube and heated from the outside with an oxyhydrogen flame burner to cause a chemical reaction, and the generated silica glass particles are deposited inside the quartz glass tube.
method (see Japanese Patent Publication No. 63-501711),
A silicon compound such as silicon tetrachloride is fed into an oxyhydrogen flame burner, and the silica glass particles produced by flame hydrolysis here are deposited on a carrier to create a porous glass matrix, which is then injected with rare earth elements. A method in which a compound containing an element is added and then sintered at high temperature to form transparent glass (Special Publication Publication No. 53-3)
980) and the like are known.

(Problems to be Solved by the Invention) However, in this MCVD method, the rare earth element compound is supplied by evaporating the rare earth element chloride by heating at high temperature, so it is difficult to control the supply amount, and it is difficult to control the supply amount. The disadvantage is that it is difficult to obtain wood.

On the other hand, in the method of adding a rare earth element compound to a porous glass base material obtained by flame hydrolysis method and sintering at high temperature, the addition of this rare earth element compound converts the porous glass base material into a solution of the rare earth element compound. Since it is carried out by dipping, it is easy to control the amount of dope, and it has the advantage that it can be applied to compounds with low vapor pressure. There is a drawback that a desired doping amount cannot be obtained because a part of the compound volatilizes.

Therefore, as a method to solve this latter problem, a method has been proposed in which the rare earth element compound added to the porous glass base material is oxidized in an oxidizing atmosphere maintained at a temperature below its melting point ( Japanese Patent Publication No. 83-601
(Refer to Publication No. 21) However, in this method, the rare earth element compound is not completely oxidized, and the rare earth element compound that is not oxidized,
For example, rare earth element chlorides are considerably volatilized during the glass process, and if they remain in the glass without being volatilized, these chlorides cannot enter the glass network and become microcrystals. This has the disadvantage that it becomes cloudy, causing optical transmission loss, and furthermore, the laser oscillation efficiency becomes low.

(Means for Solving the Problems) The present invention relates to a base material for a rare earth element-doped silica glass optical fiber that solves the above-mentioned disadvantages and drawbacks, and a method for manufacturing the same. A base material for a rare earth element-doped silica glass optical fiber, characterized in that the core is quartz glass doped with silica, and the cladding is silica glass containing more fluorine than the fluorine contained in the quartz glass forming the core. A method in which a compound containing a rare earth element is added to a porous glass base material obtained by depositing fine silica glass particles produced by flame hydrolysis, and then sintered at a high temperature to produce transparent glass. The present invention relates to a method for producing a rare earth element-doped silica glass optical fiber base material, which comprises heat-treating the porous glass base material in the presence of a fluorine compound to fix the rare earth element compound as a fluorine compound. be.

That is, the present inventors have conducted various studies on rare earth element-topped silica glass-based optical fiber base materials and methods for manufacturing the same, which do not solve the disadvantages associated with the movement and volatilization of rare earth element compounds in quartz glass doped with rare earth element compounds. As a result, after adding a compound containing a rare earth element to a porous glass base material manufactured by flame hydrolysis method, the porous glass base material containing this rare earth element was added before being sintered at high temperature to make it transparent. When a rare earth element compound is fluorinated by heat treatment in the presence of a fluorine compound, this rare earth element compound, for example, a rare earth element chloride which is usually used as this rare earth element compound, becomes a rare earth element fluoride having a boiling point higher than this chloride. When fixed, this porous glass base material becomes doped with fluorine, lowering its softening point, and this rare earth element fluoride does not allow the above-mentioned rare earth element chloride to enter the glass network. On the other hand, we discovered that the rare earth element compound enters into siloxane bonds, stabilizes it, and disperses uniformly in the glass, and when the porous glass base material treated in this way is sintered at high temperature to become transparent glass, the rare earth element compound becomes Since it is fixed as a high rare earth element fluoride and does not move or volatilize, the resulting quartz glass will not have a non-uniform dopant concentration distribution, will not crack, and will not have a high doping amount. It has been found that control is also easier. For this purpose, the base material for the optical fiber has a core made of quartz glass doped with fluorine containing rare earth element fluoride, and the quartz glass containing a larger amount of fluorine than the quartz glass forming this core. The present invention was completed by discovering that the use of a cladding provides the advantage that an optical fiber having a waveguide structure and a core uniformly doped with a rare earth element can be manufactured.

(Function) As described above, the rare earth element-doped quartz glass optical fiber base material of the present invention has a core of fluorine-doped quartz glass containing a rare earth element and fluorine, and the quartz glass that forms this core contains It is characterized by having a cladding part of quartz glass that contains more fluorine than fluorine, and this fluorine-doped quartz glass containing rare earth element fluoride is a porous glass made by flame hydrolysis method. It can be obtained by adding a compound containing a rare earth element to a base material and then heat-treating it in the presence of a fluorine compound to fix the rare earth element as a fluorine compound and doping it with fluorine.

The production of the porous glass preform by this flame hydrolysis method may be carried out by a known method, such as the CVD method or VAD method, which are well known as methods for producing preforms for optical fibers. Therefore, this involves supplying a silicon compound such as silicon tetrachloride to a known oxyhydrogen flame burner, optionally together with a germanium compound such as germanium tetrachloride as a dopant.
It may be produced by depositing silica glass particles or glass particles consisting of silica particles and germanium oxide particles produced by the hydrolysis on a carrier such as a quartz glass rod. However, the porous glass base material obtained in this way must have enough mechanical strength to not break due to loss of cohesive force between fine particles when immersed in a solution containing a rare earth element compound. Therefore, it is better to set the average scraping density to be greater than 0.3 g/cm3, and this also means that when this porous glass base material is immersed in a rare earth element compound solution, the solution will not dissolve into the porous glass base material. It is preferable that the average bulk density is less than 1.0 g/cm' because it is necessary to easily diffuse through the air.

The porous glass base material thus obtained is then immersed in a compound solution containing a rare earth element, so that the rare earth element compound penetrates into the inside thereof. Examples of compounds containing rare earth elements include chlorides, nitrates, and sulfates of rare earth elements such as neodymium, erbium, europium, and cerium, and these are particularly limited as long as they have sufficient solubility in solvents. Although not required, it is generally preferred to use a chloride that is easily available and has sufficient solubility. Also, this solvent is not particularly limited as long as it does not chemically react with the porous glass base material, but water has a strong effect of weakening the cohesive force between fine particles of the porous glass base material. It is not preferable to use a lower alcohol such as methanol or ethanol because of the solubility of the above-mentioned rare earth element compound, its effect on the porous glass base material, and its quick drying rate. Note that this doping with a rare earth element compound may be co-doped using two or more compounds, but in this case it is optional to add a transition metal such as chromium as a photosensitizer. .

In the present invention, the porous glass base material doped with this rare earth element compound is then heated in the presence of a fluorine compound, but according to this method, the rare earth element compound added to the porous glass base material is combined with the fluorine compound. Through the reaction, the above-mentioned rare earth element-containing compound, such as rare earth element chloride, is fixed as rare earth element fluoride, which has a higher boiling point than this, and the porous glass base material is doped with fluorine and has a lower softening point. become. Fluorine compounds used here include fluorocarbon, fluorochloride, sulfur fluoride,
Examples include silicon fluoride, boron fluoride, phosphorus fluoride, sulfur oxyfluoride, and silicon oxyfluoride, specifically C2Fa, CI, j! 2F2. ChCj2.

CCj2 F! , SF4, SF6, SiF4, Si2F
6, BF3, PF, POF3.5OFs, 502F
2.5i20F,,Si,,0. Examples include F6. By heat treatment in the presence of this fluorine compound,
A rare earth element compound, such as a rare earth element chloride, added to the porous glass base material is fixed as a rare earth element fluoride by the following formula 3 (L represents a rare earth element), and constitutes this porous glass base material. The silica contained in the glass is also fluorine-doped according to the following formula 3. However, if moisture remains in this porous glass matrix and the fluorine compound is sulfur fluoride, this sulfur fluoride The formula % formula % turns into hydrogen fluoride, which corrodes the silica glass particles, causing the porous glass base material to crack. Therefore, it is preferable to set the treatment temperature to 200°C or higher to eliminate adsorbed moisture in the porous glass matrix, and if it is set to 200°C or higher, the reaction rate can be increased, which is advantageous. , this is 1, 2
If the temperature is higher than 0.0°C, the shrinkage of the porous glass base material will become significant and the doping of fluorine elements will be hindered. Since the rare earth element compound present in the glass base material must not be volatilized by this heating, it is necessary to keep the temperature below the temperature at which the rare earth element compound starts volatilizing.

In addition, the rare earth element fluoride beam obtained by this method,
It is represented by lF3, and since 3 moles of fluorine atoms can be bonded to 1 mole of rare earth elements, the weight ratio of fluorine to 1 mole of rare earth elements is calculated to be 0.33 to 1.
However, in order to provide the desired optical function to the quartz glass, at least 0.01% by weight of rare earth elements is required, so fluorine must also be at least 0.003% by weight. Since the fluorine-doped silica glass has a lower glass softening point, the amount of fluorine needs to be at least 0.1% by volume in order to facilitate vitrification by lowering the softening point.

The porous glass matrix containing rare earth element fluoride and doped with fluorine obtained by heat treatment in the presence of the fluorine compound is then sintered at high temperature to become transparent glass. This can be carried out by heating to 1,500°C or higher in a furnace surrounded by an inert gas such as helium, and at this time, a small amount of halogen gas may be mixed for dehydration if necessary. A trace amount of the above-mentioned fluorine compound may be mixed in order to make the oxidation process more complete. The quartz glass thus obtained is transparent, the rare earth fluoride is evenly distributed in the glass network, giving it optical functionality, and it is also doped with fluorine. Therefore, it is considered useful as a base material for optical fibers.

The rare earth element-doped silica glass optical fiber base material of the present invention uses fluorine-doped quartz glass containing a rare earth element and fluorine obtained by the above method as a core, and this cladding is used as a core part. In order to further lower the refractive index and form an optical fiber waveguide structure, it is necessary to use silica glass that contains more fluorine than the amount of fluorine contained in the silica glass that forms this core. The optical fiber base material constructed in this way has a fluorine core and a heat-treated rare earth element that is uniformly dispersed in the glass network, making it a rare earth element-doped silica optical fiber base material with excellent optical functionality. Industrial utility is afforded, which can be advantageously obtained.

(Example) Examples of the present invention and comparative examples will be given below.

Example 5.17 minutes of hydrogen gas and 81 minutes of oxygen gas were supplied to a quartz concentric multi-tube burner, ignited to form an oxyhydrogen flame, and silicon tetrachloride was placed in the center of the burner using oxygen gas as a carrier gas. 0.17J for 27 minutes, and the silica glass fine particles generated by this flame hydrolysis were deposited and grown in the axial direction of the quartz glass rod as a carrier for 8 hours, and the outer diameter was 45 mm, the length was 300 mm, and the average length was 170 g. A porous glass preform having a bulk density of 0.356 g/cm3 was prepared.

Next, this porous glass matrix was coated with 0.1 erbium chloride.
Erbium chloride was immersed in a 00 wt% methanol solution to penetrate into the inside of the solution, and then heated at a room temperature of 25°C at 36°C.
After leaving it for a while to evaporate the methanol, it was placed in a sintering furnace and heated to 700°C, where silicon tetrafluoride (Si
F4) 0.03IL/min, helium (He) 3. O11
The mixture was treated for 3 hours while flowing a mixed gas for 1 minute to fluorinate erbium and dope the porous glass base material with fluorine.

Next, this porous glass base material was heated and sintered at 1,500°C in an electric furnace under a helium gas atmosphere to become transparent glass. Has an outer diameter of 25 mrb with no cracks or the like.
, length 155m1! A + quartz glass lot was obtained, and as a result of chemical analysis, this lot contained 0.67% by weight of fluorine and 0.102% by weight of erbium, and the refractive index was 0.18% with respect to pure silica glass. However, when the doping amount of this material was measured using EPMA, it was found that
The results shown in Figure 1 were obtained, and it was confirmed that the doping was almost uniform in the radial direction.
The erbium fixation rate was 90%.

In addition, when the quartz glass rod obtained in this way was stretched to form a core, porous silica glass was deposited on the outer periphery of the core, and fluorine was doped during sintering to form a cladding. The index difference is 0.48% lower than that of silica glass, and the refractive index difference between the core and cladding is 0.
．． 3%, this clad/core ratio was set to L2.9.
A single mode fiber preform with an outer diameter of 24 mm and a length of 250 mm was manufactured by adjusting the outer diameter of 12 mm.
When we spun it into a 5 μm fiber and examined its absorption wavelength characteristics, we obtained the results shown in Figure 2.
．． There was no increase in loss due to scattering, except that an absorption peak peculiar to erbium was observed at 54 μva.

Comparative Example In the method of the above-described example, the porous glass base material impregnated with erbium chloride was not subjected to the fluoridation step of heat treatment in the presence of silicon tetrafluoride, but was immediately heated in an electric furnace under a helium gas atmosphere. A quartz glass rod was made and used as a core in the same manner as in the example except that it was heated to 1,800° C. in an ambient atmosphere to make it transparent. An optical fiber base material was prepared by depositing porous silica glass on the outer periphery of this core and doping it with fluorine during sintering so that the refractive index of this cladding decreased by 0.3%. The average concentration of erbium in quartz glass is 0.0
Fr is 2% by weight, and this erbium doping amount is EP
When measured with MA, the results shown in Figure 3 were obtained, and this is because erbium tends to be unevenly distributed in the outer periphery of the lot, and a thin sushi-like shape is formed on the surface layer of the glass rod due to the precipitation of erbium. It was confirmed that cracks had occurred. Due to evaporation during sintering, the fixation rate of erbium is as low as 55%, and the absorption wavelength characteristics of the optical fiber made from this also show scattering loss over a wide wavelength range due to the roughness of the core surface, except for the absorption peak of erbium. Ta.

(Effects of the Invention) As described above, the rare earth element-doped silica-based optical fiber base material of the present invention contains a rare earth element and fluorine, and has a core made of quartz glass doped with fluorine. The cladding is made of quartz glass that contains more fluorine than that contained in
The quartz glass that forms this core is made by adding a rare earth element compound to a porous glass base material obtained by flame hydrolysis, and then heat-treating it in the presence of a fluorine compound to fix the rare earth element compound as a fluoride and create a porous glass. It is manufactured by doping the base material with fluorine, and according to this method, 8'! l], since volatilization is eliminated, it is possible to easily obtain highly concentrated and uniformly doped rare earth element fluoride, and since the rare earth element fluoride stably exists in the glass network, this Optical fiber preforms using quartz glass have the advantage of superior optical functionality.

[Brief explanation of the drawing]

Fig. 1 is an EPMA measurement graph of the erbium concentration distribution of the erbium-doped quartz glass obtained in the example of the present invention, and Fig. 2 is a fluorine-doped quartz glass with the erbium-topped quartz glass obtained in this example as the core. FIG. 3 shows a graph of the erbium concentration distribution (EP) and 1A measurement of the erbium-doped quartz glass obtained in the comparative example. o0〇- Figure 3

Claims (1)

[Claims] 1. The core is quartz glass containing rare earth elements and fluorine, and the cladding is quartz glass containing more fluorine than the fluorine contained in the quartz glass forming the core. Rare earth element-doped silica glass base material for optical fiber. 2. The quartz glass that forms the core contains 0.01 rare earth elements.
2. The rare earth element-doped silica glass optical fiber base material according to claim 1, which contains fluorine in an amount of 3 times the mole % or more of the rare earth element. 3. In a method in which a compound containing a rare earth element is added to a porous glass base material obtained by depositing silica glass particles produced by flame hydrolysis, and then sintered at high temperature to become transparent glass, the sintering step is 3. The rare earth element-doped silica glass optical fiber according to claim 1, wherein the porous glass base material is first heat-treated in the presence of a fluorine compound to fix the rare earth element compound as a fluorine compound. Method of manufacturing base material. 4. The method for producing a rare earth element-doped silica glass optical fiber base material according to claim 3, wherein the heat treatment in the presence of a fluorine compound is performed in a temperature range below the temperature at which volatilization of the rare earth element compound starts. 5. The heat treatment temperature in the presence of a fluorine compound is 200-1
4. The method for producing a rare earth element-doped quartz glass optical fiber base material according to claim 3, wherein the temperature range is 200°C. 6. Porous glass base material has an average bulk density of 0.3 to 1.0 g
4. The method for producing a base material for a rare earth element-doped quartz glass optical fiber according to claim 3, wherein the base material has a diameter of /cm^3.