JP3027075B2 - Method for producing rare earth element-doped quartz glass fiber preform - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a rare earth element-doped quartz glass fiber preform, and more particularly to a method for producing a functional rare earth element doped quartz glass fiber preform used for an optical fiber amplifier or the like. is there.

[0002]

2. Description of the Related Art Rare earth element-doped quartz glass fibers have an optical function and are therefore used as optical fiber lasers, optical amplifiers, sensor elements and the like. The production of this type of quartz glass is conventionally known by the MCVD method (see Japanese Patent Application Laid-Open No. 63-501711). However, this method involves heating a rare earth element oxide to a high temperature to evaporate it. Since it is supplied to the reaction system, it has a drawback that it is difficult to control the supply amount and it is difficult to obtain a large base material.

On the other hand, in the production of the rare earth element-doped quartz glass, a silicon compound as a glass forming raw material is fed into an oxyhydrogen flame, and silica fine particles generated by the flame hydrolysis are deposited to form a porous glass base material. A method is proposed in which the porous glass base material is immersed in a solution of the rare earth element compound to impregnate the rare earth element compound into the porous glass base material, and then sintered at a high temperature to form a transparent glass. (See Japanese Patent Publication No. 58-3980), which has the advantage that the doping amount can be controlled by the concentration of the solution and that it can be applied to compounds having a low vapor pressure.
However, this involves immersing the porous glass base material in a solution of a rare earth element compound and drying the solute when the solvent moves to the surface of the porous glass base material due to capillary action, which moves near the surface. Due to the accumulation, a concentration distribution of the dopant is formed in the obtained glass, and in an extreme case, there is a problem that the glass is broken due to a difference in a coefficient of thermal expansion between the surface and the inside. Sintering,
At the time of vitrification, a part of the rare earth element compound is volatilized, and there is a disadvantage that a material having a desired doping amount cannot be obtained.However, a core rod manufactured by these methods is used as an optical fiber preform. For example, a method in which the rod is inserted into a quartz glass tube and heated and melted, and a method in which a porous glass body for cladding is sequentially laminated on the outer periphery of the rod are applied.

[0004]

However, in such a method of doping the entire core with a rare earth element, a relatively high concentration of the rare earth element is also doped into the outer peripheral portion having a small optical power. It is known that the amount of light absorbed increases and the amplification efficiency is reduced as compared with the above. To solve this problem, a structure in which the core is doubled and only the central portion thereof is doped with a rare earth element has been proposed. . (IEEE Photo
nics Tech. Lett., Vol. 3, No. 8, 721 (1991)) When such a structure is to be manufactured by a conventionally known method, a second core for a second core is surrounded around a rod for a rare earth element-doped core. The method of coating glass and then coating the glass for cladding is adopted. In such a method, an interface irregular layer is formed at each boundary between the first core, the second core, and the clad, so that scattering loss is caused. And the problem that the amplification characteristics are reduced.
The boundary between the core and the second core has a problem that the optical power is wide and greatly affects the amplification characteristics.

Further, in the known method of doping the core with a rare earth element, the porous glass base material tends to be doped at a high concentration only in the peripheral portion in order to diffuse the rare earth element from the outer periphery. When aluminum is added as a dopant for activation, the rare earth element is trapped in the outer peripheral portion and does not sufficiently penetrate to the central portion due to the interaction therebetween. Therefore, even if the structure is a double core as described above, the effect is sufficiently sufficient. There was also a problem that it was not exhibited.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method for producing a rare earth element-doped quartz glass fiber preform which solves such disadvantages and problems. Flame hydrolysis of the dopant raw material gas and the dopant raw material gas for activating the amplification characteristics, and deposition of the generated silica glass fine particles, the porous glass preform obtained is reduced in an electric furnace.
At least pre-heat the volume to a small extent,
After being immersed in a solution of the compound containing the rare earth element, sufficiently dried, and further immersed in a solvent dissolving the compound containing the rare earth element, the concentration of the compound containing the rare earth element in the porous glass base material After the distribution is made higher in the central part than in the peripheral part, it is dried again, sintered at a high temperature to be transparent vitrified to produce a rare earth element-doped quartz glass rod for the core, and the glass rod for the core is produced. A raw material gas for forming a clad is supplied to the outer periphery of the core to form a porous preform, which is then sintered and turned into a transparent vitreous, so that the center of the core is highly doped with a rare earth element. The obtained quartz glass fiber preform is obtained.

That is, the present inventors have conducted various studies on a method for producing a rare earth element-doped quartz glass fiber preform having excellent amplification characteristics, and found that a rare earth element-doped quartz glass rod for a core is made of silicon tetrachloride. Silica glass produced by flame hydrolysis of a glass-forming material gas such as a dopant material gas for increasing the refractive index, such as germanium tetrachloride, and a dopant material gas for activating amplification characteristics, such as aluminum chloride. Fine particles are deposited, and the obtained porous glass base material is reduced in an electric furnace to at least a small volume.
Pre-heated to the extent that it shrinks, and then immersed in a solution of a compound containing a rare earth element such as a solution of erbium chloride in methanol or ethanol, and then dried sufficiently to further dissolve the compound containing the rare earth element in methanol or ethanol. By immersing in a solvent such as the above, the concentration distribution of the compound containing a rare earth element in the porous glass base material is dried again so that the concentration is higher in the central part than in the peripheral part, and sintered at a high temperature. A rare earth element-doped quartz glass rod for a core is manufactured by vitrification, and a glass forming raw material gas is supplied to the outer periphery of the core glass rod to form a porous preform body. By sintering and turning into a transparent glass, it is possible to obtain a quartz glass fiber preform in which the core is heavily doped with rare earth elements. It found Rukoto was confirmed to complete the present invention that it is possible to obtain an excellent rare earth element doped silica glass fiber amplification characteristics according to this.
This is described in more detail below.

[0008]

The present invention relates to a method for producing a rare earth element-doped quartz glass fiber preform, and more particularly to a method for producing a functional rare earth element doped quartz glass fiber preform used for an optical fiber amplifier or the like. However, this deposits silica glass fine particles generated by flame hydrolysis and transfers the obtained porous glass base material to an electric furnace.
At least to the extent that its volume is slightly reduced.
Preheating , immersing in a solution of a compound containing a rare earth element, such as a solution of erbium chloride in methanol or ethanol, and then thoroughly drying, and further dissolving the compound containing the rare earth element in a solvent such as methanol or ethanol. By immersing, the concentration distribution of the compound containing a rare earth element in the porous glass base material is made higher in the central part than in the peripheral part, dried again, and sintered at a high temperature to form a transparent vitrified glass. To produce a rare earth element-doped quartz glass rod for the core, and then to produce a quartz glass fiber preform by a known method.

The production of the rare earth element-doped quartz glass rod for the core is performed by using a glass forming raw material gas such as silicon tetrachloride, a dopant raw material gas such as germanium tetrachloride to increase the refractive index, and an amplification characteristic such as aluminum chloride. is flame hydrolysis and dopant material gas for activated with oxyhydrogen flame, and the resulting silica glass soot was deposited on a quartz glass starting material to make a porous glass preform, an electric furnace which At least its body
Pre-heated to a degree that the volume is slightly reduced , immersed in a solution of a compound containing a rare earth element such as a solution of erbium chloride in methanol or ethanol, and then dried thoroughly.
Further, by immersing in a solvent such as methanol or ethanol, the concentration distribution of the compound containing a rare earth element in the porous glass base material is made higher in the central part than in the peripheral part, and dried again. 1,400 to 1,5 in the furnace
What is necessary is just to heat it to 00 degreeC, to sinter, and to make it into transparent glass.

In this case, the production of the porous glass body may be performed by a VAD method or an OVD method which is well known as a production method of a preform for an optical fiber. Therefore, the VAD method is preferred. In addition, since it is necessary to increase the refractive index, germanium tetrachloride is preferably used as a dopant. Since the porous glass base material is immersed in the solution of the compound containing the rare earth element and the solvent as described above in order to dope the rare earth element, the cohesive force between the fine particles when immersed in the solution or the solvent is reduced. The average bulk density is 0.3 g, as it is required to have enough mechanical strength to not be lost and destroyed
/ cm ³ or more, but the average bulk density is 1.0 g / cm ³ or less because this solution and solvent are required to easily diffuse and move in the porous base material. Good.

However, when the average bulk density of the porous glass base material deposited by the flame hydrolysis method is 0.3 g / cm ³ or less, or when the density of a part of the surface layer is low, a compound containing a rare earth element is used. Prior to immersion in the above solution, it is effective to perform preheating in an electric furnace at least to such an extent that its volume is slightly reduced. As described above, the porous glass base material is immersed in the solution of the rare earth element compound to allow the rare earth element compound to penetrate into the interior of the porous glass base material. Chloride, nitrate, sulfate and the like may be used, and it is also possible to co-dope two or more of these rare earth element compounds. Further, as this compound, it is preferable to use a chloride which is easily available and has sufficient solubility in a solvent, and water can be used as the solvent. Since it has a strong effect of weakening the cohesive force of methanol, it is preferable to use methanol, ethanol or the like.

In addition, aluminum or phosphorus can be used as a dopant for activating the amplification characteristics used here, but a transition metal such as chromium can also be added as a photosensitizer. . In the case where aluminum is contained in the porous glass base material, after immersing in a solution of a compound containing a rare earth element, drying is performed.
If the sintering is further performed at a high temperature, the rare earth element compound is likely to be deposited at a high concentration near the surface of the porous glass body, and after vitrification, the surface is crystallized or fine cracks are generated on the surface. Therefore, it is preferable that the porous glass base material is immersed in a solution of a compound containing a rare earth element, dried, and further immersed only in the solvent used in the solution to allow the rare earth compound near the surface to penetrate into the inside. .

In this step, when the penetration of the rare earth element compound into the inside is insufficient, the rare earth element is doped at a high concentration only in the central portion of the porous glass base material by repeating drying and immersion only with a solvent. be able to.
Then, after drying the porous glass base material, a heat treatment is performed in an electric furnace to produce a quartz glass rod for a core which is vitrified and doped with a rare earth element. Although helium is preferable as the atmosphere gas, a small amount of halogen gas may be mixed for the purpose of dehydration, or a small amount of oxygen gas may be mixed for complete conversion to oxide.

The rare-earth-element-doped quartz glass fiber preform intended for the present invention is prepared by stretching the rare-earth-doped quartz glass rod for a core, if necessary, and then cleaning the surface. Then, a glass material gas, for example, only silicon tetrachloride is supplied to the outer peripheral portion to form a porous glass layer serving as a clad, which is then sintered to form a transparent glass. Can be obtained by Incidentally, in order to obtain a rare earth element-doped quartz glass fiber from the rare earth element-doped quartz glass fiber preform, it may be formed into a fiber by drawing according to a known method. This provides an advantage that a rare earth-doped silica glass fiber, which is useful as a sensor element for an optical amplifier, can be easily obtained.

[0015]

Next, examples of the present invention will be described. EXAMPLE A hydrogen gas 4 liter / min and an oxygen gas 9 liter / min were supplied to a quartz concentric multi-tube burner to form an oxyhydrogen flame,
2.0 liters of argon gas in an evaporator containing silicon tetrachloride
Gaseous silicon tetrachloride in a minute
Pass through an anhydrous aluminum chloride evaporator heated to 20 ° C to generate a gas containing silicon tetrachloride and anhydrous aluminum chloride, and further pass this through a germanium tetrachloride evaporator to produce silicon tetrachloride. A mixed gas of aluminum chloride and germanium tetrachloride is prepared and supplied to the oxyhydrogen flame, and glass fine particles generated by the flame hydrolysis of the raw material gas are deposited for 6 hours in the axial direction of the starting material made of synthetic quartz glass. The porous silica matrix was made to grow.

Then, the porous glass base material is heated at 1,100 ° C.
Preliminary sintering in an electric furnace heated to a bulk density of 0.40 g / cm ³
Erbium chloride is immersed in a 0.1% by weight methanol solution to penetrate the inside of the porous glass base material, and is allowed to dry in air, and then immersed in methanol to sufficiently remove erbium chloride. After infiltration, it was air-dried in air and heated in an electric furnace under a helium gas atmosphere at 1,500 ° C to form a vitrified transparent glass.As a result, a quartz glass rod containing 500 ppm of erbium oxide was obtained. When this was analyzed by EPMA, it was found that erbium had the highest concentration at the center and almost no erbium was detected at the periphery as shown in FIG.

Next, the core glass rod is stretched by a glass lathe to smooth the surface, and silica glass fine particles generated by flame hydrolysis of a glass forming raw material gas containing silicon tetrachloride are deposited on the outer peripheral portion. To form a porous glass layer of sufficient thickness as a cladding, sintering it to form a transparent vitreous glass to produce an erbium-doped quartz glass fiber preform, and then drawing it to produce an optical fiber. When a 1.55 μm signal light input to the fiber was -10 dBm, a 1.48 μm pump light was pumped at 40 mW.This showed good results with an amplification gain of 37 dB and a noise figure of 5.2 dB at this time. Was.

[0018]

The present invention relates to a method for producing a rare earth element-doped quartz glass fiber preform, which activates a glass forming raw material gas, a dopant raw material gas for increasing the refractive index, and amplification characteristics as described above. The raw material gas is flame-hydrolyzed and the resulting silica glass fine particles are deposited, and the resulting porous glass base material is at least slightly reduced in volume in an electric furnace.
Preheating, immersing in a solution of a compound containing a rare earth element, drying it sufficiently, and further immersing it in a solvent that dissolves the compound containing the rare earth element, After the concentration distribution of the compound containing the rare earth element is made higher in the central part than in the peripheral part, it is dried again, sintered at a high temperature to be transparent vitrified to produce a rare earth element-doped quartz glass rod for a core, and the core is produced. The raw material gas for cladding is supplied to the outer periphery of the glass rod for forming a porous preform body, and then these are sintered to form a transparent glass. According to the above, it is possible to easily obtain a rare earth element-doped quartz glass fiber base material in which the core portion is highly doped with a rare earth element, and this is an optical fiber having excellent amplification characteristics. It becomes one with the advantage of giving.

[Brief description of the drawings]

FIG. 1 shows a distribution of a rare earth element-doped quartz glass fiber preform obtained by the method of the present invention in a dopant radial direction.

[Explanation of symbols]

1 ... distribution of germanium, 2 ... distribution of erbium,
3 ... Clad.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Tadakatsu Shimada 2-3-1-1, Isobe, Annaka-shi, Gunma Shin-Etsu Chemical Co., Ltd. Precision Functional Materials Research Laboratories (56) References JP-A-56-73638 (JP) , A) T.I. Kashiwada, M .; Shigematsu, T .; Kougo, H. Kanamori, and M.K. Nishimura, "Erbium-Dopped Fiber Amplifier Pumped at 1.48 µm with Extremely High Efficiency", IEEE PHOTONICS TECHNOLOGY, TEL. 3, No. 8, p. 721-723 (58) Field surveyed (Int. Cl. ⁷ , DB name) C03B 37/00-37/16

Claims (2)

(57) [Claims] 1. A porous material obtained by flame hydrolysis of a glass forming raw material gas, a dopant raw material gas for increasing the refractive index, and a dopant raw material gas for activating amplification characteristics, and depositing generated silica glass fine particles. Reduce the volume of glass preform at least slightly in electric furnace
Pre-heated to the extent that it is immersed in a solution of a compound containing a rare earth element, then dried sufficiently, and further immersed in a solvent that dissolves the compound containing a rare earth element to thereby form a porous glass base material. After making the concentration distribution of the compound containing the rare earth element higher in the central part than in the peripheral part, it is dried again, sintered at a high temperature and turned into a transparent glass to produce a rare earth element doped quartz glass rod for the core Then, a raw material gas for cladding is supplied to the outer peripheral portion of the glass rod for the core to form a porous preform body, and then these are sintered and transparently vitrified, so that the central portion of the core is formed. A method for producing a rare earth element-doped quartz glass fiber preform, characterized by obtaining a quartz glass fiber preform heavily doped with a rare earth element. 2. A step of immersing a porous glass base material in a solution of a compound containing a rare earth element, drying it sufficiently, and further immersing it in a solvent dissolving the compound containing the rare earth element is repeated at least once. A method for producing a rare earth element-doped quartz glass fiber preform according to claim 1 .