JP2938605B2 - Method of manufacturing preform for single mode optical fiber - 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 manufacturing a single-mode optical fiber preform, and more particularly to a method for manufacturing a single-mode optical fiber preform having a stable relative refractive index difference in the longitudinal direction. Things.

[0002]

2. Description of the Related Art A single-mode optical fiber preform is manufactured by introducing a gaseous glass material such as silicon tetrachloride and a doping agent such as germanium tetrachloride into an oxyhydrogen flame burner. The glass microparticles generated by this flame hydrolysis are deposited on a carrier and grown in the axial direction to produce a porous glass base material, which is manufactured by the so-called VAD method, and then dehydrated and sintered at a high temperature. It is manufactured by vitrification.

[0003]

However, the base material for single-mode optical fiber manufactured by the VAD method has a large relative refractive index difference (Δn) in the rod due to fluctuations in the pulling speed. There is a disadvantage of sticking. This is because the growing porous glass preform is gradually pulled upward so that a porous glass preform having a constant diameter can be obtained by deposition and growth of glass fine particles. In the early and late stages of production of a material, the pulling speed is more likely to change than the pulling speed of the steady portion or the straight body portion, and the pulling speed may change during work due to other factors.

[0004] When the pulling speed changes in this way, the relative refractive index difference (△ n) of the glass base material obtained by dehydrating and sintering it.
And it is difficult to obtain a stable glass base material having a constant relative refractive index difference (△ n), and the ratio of a portion having a desired refractive index from one glass base material is 60% or less. There is a disadvantage that it also becomes.

To stabilize the pulling speed, a method has been proposed to correct the fluctuation of the supply amount of the raw material gas due to the atmospheric pressure. The present inventors also control the amount of hydrogen supplied to the cladding burner. (Refer to Japanese Patent Application No. 2-38039), it is difficult to make the pulling rate completely constant, and even if it can be made constant, it is determined based on the ordinary method. The optical fiber preform obtained by dehydration treatment and sintering at a constant temperature at
n), as shown in FIG. 4, there is a problem that only the start portion and the end portion of the preform are high and are not uniform, and the relative refractive index in the longitudinal direction of the preform for an optical fiber is long. There is a disadvantage that the difference (△ n) is not constant, lacks stability, and reduces the yield.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method of manufacturing a preform for a single-mode optical fiber which has solved the above disadvantages. This method uses a germanium-doped glass fine particle as a target substrate. In a method of producing a porous glass preform by depositing and pulling it up in the axial direction to produce a porous glass preform, and then dehydrating and sintering this at a high temperature, a dehydration treatment temperature of the porous glass preform is produced. Is set at a higher temperature at the start and end portions of the porous glass base material than at the intermediate portion.

That is, the present inventors have determined the relative refractive index difference (Δn).
As a result of various studies on a method for manufacturing a single-mode optical fiber preform in which the longitudinal direction is constant, a germanium-doped porous glass preform made by the VAD method was dehydrated at a constant temperature and then fired. As described above, the relative refractive index difference between the start portion and the end portion of the base material increases as described above, but in this dehydration process, the processing temperature at the start portion and the end portion of the porous glass member is set to an intermediate value. It was found that if the temperature was higher than the processing temperature in the part, the relative refractive index difference (△ n) distribution would be stable in the longitudinal direction of the base material. Was completed. This will be described in more detail below.

[0008]

The present invention relates to a method for manufacturing a single-mode optical fiber preform having a specific refractive index difference (Δn) that is constant and stable in the longitudinal direction.

The production of the optical fiber preform in the present invention is basically performed by a known VAD method. Therefore, this uses one core oxyhydrogen flame burner and a plurality of cladding oxyhydrogen flame burners, and this core oxyhydrogen flame burner has silicon tetrachloride. Silica glass fine particles doped with germanium oxide generated by flame hydrolysis of silicon tetrachloride and germanium tetrachloride by introducing a raw material gas consisting of germanium tetrachloride as a doping agent, oxygen gas, and hydrogen gas Is deposited on a target substrate to grow a core portion in the axial direction, and silicon tetrachloride, oxygen gas and hydrogen gas are supplied to an oxyhydrogen flame burner for cladding to produce a silicon tetrachloride flame. Silica glass fine particles generated by hydrolysis are deposited and grown on the core to form a clad, thereby producing a porous glass base material, which is then dehydrated and sintered at a high temperature.

However, various studies have been made on the dehydration process of the porous glass base material in this case. The dehydration process is usually performed in the presence of a halogen gas such as Cl _2. Is carried out at a constant speed through a heating zone having a constant temperature as shown in FIG. 3 in a He gas atmosphere containing Cl ₂ gas, for example, and the resultant is doped into the porous glass base material. Some of the crystalline germanium oxide (GeO ₂ ) reacts with Cl ₂ gas to
l ₄ a certain amount of volatilization becomes, but porous glass preform Star - for a lot of GeO ₂ in the crystallinity in the up side and the end side unsteady portion occurs, subsequent as shown in FIG. 4 It has been found that the relative refractive index difference (Δn) in the longitudinal direction of the optical fiber preform obtained in the sintering step becomes high and nonuniform at its start portion and end portion.

[0011] In this case, the concentration of Cl ₂ gas in the atmosphere in this GeCl ₄ volatiles the dehydration step, the dehydration treatment temperature, to differ by each of the processing speed, the concentration of Cl ₂ in the atmosphere, and a constant processing speed It has been found that the higher the processing temperature, the greater the degree of volatilization of GeCl ₄ becomes. If the processing temperature in the start part and the end part is higher than the processing temperature in the other parts as shown in FIG. 1, the amount of GeCl ₄ volatilized increases, and this unsteady part disappears, and the subsequent firing It was confirmed that the relative refractive index difference (Δn) of the optical fiber preform obtained in the binding step was uniform, constant and stable as shown in FIG.

Since the processing temperature at the start portion and the end portion of the porous glass preform in the step of dehydrating the porous glass preform needs to be higher than the other portions as described above. But this temperature is higher than other parts
There is no effect when the temperature is higher than 10 ° C, and when the temperature is higher than 50 ° C, △ n becomes too low at both ends, so it is necessary that the temperature be higher by 10 to 50 ° C than other parts. The preferred temperature is preferably in the range of 20 to 40 ° C.

[0013]

Next, examples of the present invention and comparative examples will be described. Examples, Comparative Examples Silicon tetrachloride 30.0 cc / min. For oxyhydrogen flame burner for core,
3.5 cc / min of germanium tetrachloride, 3.0 liter / min of oxygen gas and 1.0 liter / min of hydrogen gas are supplied, and 400 cc / min of silicon tetrachloride is supplied to the oxyhydrogen flame burner for cladding.
And oxygen gas at 10.0 liters / minute and hydrogen gas at 8.0 liters / minute to supply core glass fine particles doped with germanium oxide generated by the flame hydrolysis and cladding glass fine particles containing no germanium oxide. It is deposited on a synthetic quartz glass rod rotating at 20 rpm installed at the tip of the material, grown in the axial direction to make a porous glass base material consisting of a core part and a clad part, and this porous glass base material is pulled up Using a device, the porous glass base material having a diameter of 100 mm and a length of 800 mm was produced by pulling up at a constant speed of 0.65 ± 0.005 mm / min.

Then, the porous glass base material is placed in a He gas atmosphere containing 10% by volume of Cl ₂ gas in a quartz glass furnace having a length of 2,000 mm and a furnace temperature set as shown in FIG. Using a core tube, dehydrate at a speed of 5.0 mm / min.
The temperature was raised to 1,600 ° C and sintering was performed to produce an optical fiber preform with an outer diameter of 40 mm and a length of 400 mm. The relative refractive index difference (△ n) in the longitudinal direction of the preform was shown in Fig. 2. As shown, the value was uniform throughout the longitudinal direction.

[0015] However, quartz temperature of the glass furnace core tube addition was as shown in FIG. 3 is dehydrated under the same conditions as in the above example, the mother for an optical fiber subjected to sintering in the above for comparison The material is made, and the relative refractive index (の 長
n) When the distribution was examined, it was found that the relative refractive index (Δn) was large only in the start portion and the end portion as shown in FIG.

[0016]

The present invention relates to a method for producing a single-mode optical fiber preform in which the relative refractive index difference (Δn) is uniform and stable in the longitudinal direction, which is known as described above. In a method for producing a preform for optical fiber, a porous glass preform is prepared by a VAD method, and then dehydrated and sintered at a high temperature. The temperature of the start part and the end part is set higher than that of the intermediate part.

According to this, the relative refractive index difference (Δn) in the longitudinal direction of the optical fiber preform obtained by the subsequent sintering process becomes large between the start portion and the end portion in the conventional method, and becomes uniform. The absence of this provides the advantage of being constant and uniform stabilized throughout the longitudinal direction.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in processing temperature in a dehydration and sintering step of a porous glass base material according to the present invention.

FIG. 2 is a distribution diagram of a relative refractive index difference (Δn) in a longitudinal direction of a preform for an optical fiber obtained by the present invention.

FIG. 3 is a graph showing a change in processing temperature in a dehydration and sintering step of a porous glass base material according to a conventional method.

FIG. 4 is a distribution diagram of a relative refractive index difference (Δn) in a longitudinal direction of an optical fiber preform obtained by a conventional method.

────────────────────────────────────────────────── ─── of the front page continued (72) inventor Osamu Kuriyama Gunma Prefecture Annaka Isobe 2-chome 13th No. 1 Shin-Etsu chemical Co., Ltd. Isobe in the factory (58) investigated the field (Int.Cl. ^6, DB name) C03B 37/00-37/16

Claims (1)

(57) [Claims] 1. An optical fiber obtained by depositing germanium-doped glass fine particles on a target substrate, pulling it up in the axial direction to produce a porous glass base material, and then dehydrating and sintering at a high temperature. In the method for manufacturing a base material for use, the single-mode light is characterized in that the dehydration temperature of the porous glass base material is set to be higher at the start portion and the end portion of the porous glass base material than at the intermediate portion. Manufacturing method of fiber preform.