JPH08157230A - Production of dispersion-shifted optical fiber preform - Google Patents

Abstract

PURPOSE: To obtain a preform with the refractive index distribution of a stepped core part flattened and without the soot being cracked by forming the soot for the stepped core part with use of plural burners and specifying the maximum surface temp. of the soot. CONSTITUTION: Gaseous H2 , gaseous O2 , glass raw gas and GeCl4 are supplied to a first burner 11, and a soot 21 for a high-refractive index center core part contg. GeO2 as a dopant to increase the refractive index is formed on a starting rod-shaped base material 12. A raw gas having a lower content of GeCl4 than that for the first burner 11 is then supplied to a second burner 13, a raw gas having a slightly lower content of GeCl4 than that for the second burner 13 is supplied to a third burner 14 to form a stepped core part consisting of the soots 22 and 23 having a lower refractive index than the soot 21 around the soot 21, and the maximum surface temp. of the soots 22 and 23 is controlled to 600-700 deg.C. A soot 24 for a clad part having a lower refractive index than for the stepped core part is formed around the stepped core part and then vitrified.

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 an optical fiber preform for a dispersion-shifted optical fiber for the 1.55 μm band, which is suitable for long-distance, large-capacity optical communication.

[0002]

2. Description of the Related Art Conventionally, as a 1.55 μm band dispersion-shifted optical fiber of this type, a dual-shaped structure having a refractive index distribution as shown in FIG. 7 is known.
This is composed of a central core 1 having a high refractive index in the central portion, a staircase core 2 provided around the central core 1 and having a stepwise refractive index distribution having a lower refractive index than the central core 1, and the staircase. The clad 3 is provided around the core 2 and has a refractive index lower than that of the staircase core 2. The central core 1 and the staircase core 2 are made of germania (GeO ₂ ) -doped quartz glass, the cladding 3 is made of quartz glass, and the relative refractive index difference (Δ1) of the central core 1 is 0.75 to 0.85%,
The relative refractive index difference (Δ2) of the staircase core 2 is 0.10 to 0.20.
% And which is in the ratio of the of the step core 2 radius (r ₂₎ and of the central core 1 radius _{(r 1) (r 2 /} r 1) has a 2.5 to 4.

To manufacture such a dispersion-shifted optical fiber, a central core portion corresponding to the central core 1 and a step core 2 are formed.
The step is performed by manufacturing an optical fiber preform having the same refractive index distribution as that of the optical fiber preform having a staircase core part corresponding to the above and a clad part corresponding to the clad 3, and performing ordinary melt spinning of the optical fiber preform.

In order to manufacture this optical fiber preform by the VAD method, a soot forming burner (hereinafter referred to as "soot forming burner") arranged at the lowermost position and inclined with respect to the starting base material 4 as shown in FIG. A soot 5 to be a central core portion is formed on the tip of the starting base material 4 by A, and a soot 6 to be a staircase core portion is formed by a burner B arranged on the side of the starting base material 4 while being deposited and deposited. Generate and deposit.

Further, two burners C and D arranged above the burner B constitute a soot 7 which serves as a clad portion.
To deposit. Predetermined amounts of glass raw material gas SiCl ₄ , GeCl ₄ , fuel H ₂ and oxidant O ₂ are supplied to the burners A and B, respectively, and burner C and burner D are respectively supplied with SiCl ₄ , H ₂ , O ₂ is supplied so that the above-mentioned refractive index profile is obtained. Also,
The maximum temperature of the soot surface at the time of forming the soot is adjusted to a range of 700 to 800 ° C. in which cracking of the soot hardly occurs.

When the soot preform is obtained in this manner, it is dehydrated by a conventional method, and then sintered and vitrified into an optical fiber preform.

In the production of such an optical fiber preform, since the refractive index distribution is complicated and the refractive index distribution is restricted, the usual 1.3 μm band single mode optical fiber preform is used. The production yield was lower than that of the above-mentioned production. That is, as a constraint of the above-mentioned refractive index distribution, it is necessary to make the portion to be the staircase core part flat and horizontal, or to make it flat and the refractive index gradually decreases toward the outside.

Further, in the conventional manufacturing method, as described above, one burner B is used to synthesize the soot which becomes the staircase core portion, so that the radius (R ₂ ) of the staircase core portion and the outside of the central core portion are combined. When the ratio (R ₂ / R ₁ ) to the diameter (R ₁ ) is increased to 3 to 5, the refractive index distribution in the staircase core portion is not flat and there is a defect that it becomes irregular. In addition, cracking of the soot may occur during manufacturing.

[0009]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to increase the ratio (R ₂ / R ₁ ) to 3 to 5 as well as the case of the above-mentioned ratios (R ₂ / R ₁ ). Even in the case of manufacturing the material, the refractive index distribution of the staircase core portion should be flat and the soot should not be cracked.

[0010]

The object of the present invention is to form a soot, which becomes the staircase core, by using a plurality of burners, and to set the maximum temperature of the surface of the soot to 600 ° C. or more and 700 ° C. or more.
The problem can be solved by setting the temperature within the range of less than ° C. In addition, the thickness of the soot formed by the burners forming the staircase core portion is preferably 50 mm or less for all burners.

[0011]

Since the temperature of the soot surface has a great influence on the doping amount of germania, the larger the temperature difference on the soot surface, the greater the variation of the doping amount of germania.
The maximum temperature of the soot surface is 600 ° C or more and less than 700 ° C, which is about 1 higher than the surface temperature of the soot that becomes the normal cladding.
Since the temperature is lowered by 00 ° C., the variation in the temperature of the soot surface becomes small. In addition, if the soot of the staircase core is synthesized with two or more burners, the thickness of the soot synthesized by each burner can be reduced, and the difference in the surface temperature of the soot synthesized by each burner can be reduced. Can be made smaller.

Therefore, even if the ratio (R ₂ / R ₁ ) is increased, the temperature variation on the surface of the soot is reduced,
The fluctuation of the doping amount of germania does not become so small that the refractive index distribution does not become irregular.

The present invention will be described in detail below. The manufacturing method of the present invention uses germania as a dopant for increasing the refractive index to manufacture an optical fiber preform by the VAD method. FIG. 1 shows an example of the manufacturing method of the present invention. In the figure, reference numeral 11 is a first burner that forms a soot that serves as a central core portion, and the first burner 11 is used for the starting rod-shaped base material 12. It is installed so as to be inclined. A second burner 13 is provided above the first burner to form a soot which will be an inner portion of the stair core portion, and a second soot which will be an outer portion of the stair core portion will be formed above the second burner 13. A three burner 14 is provided.

Further, above the third burner 14, two fourth burners 15 and five burners 16 for synthesizing soot to be the cladding portion are provided, respectively. The second burner 13 to the fifth burner 16 are all starting rod-shaped base materials 1.
The flame is arranged so as to hit the starting rod-shaped substrate 12 substantially horizontally from the side of 2.

In the first burner 11, H ₂ gas as a fuel, O ₂ gas as an oxidizer, SiCl ₄ and G as a glass material gas are used.
eCl ₄ is supplied to form the soot 21 which serves as the central core portion having a high refractive index. The second burner 13 and the third burner 14 have H ₂ gas, O ₂ gas, SiCl ₄ and Ge.
Although Cl ₄ is supplied, the addition ratio of GeCl ₄ is made smaller than that of the first burner 11. Further, the addition ratio of GeCl ₄ to the third burner 14 is made slightly lower than the addition ratio to the second burner 13. The reason is that if this ratio is reversed, a protrusion of the refractive index occurs at the boundary between the soot 22 formed by the second burner 13 and the soot 23 formed by the third burner 14, and a stable refractive index distribution is obtained. Because I can't. The second burner 13 forms a soot 22 which is an inner portion of the staircase core portion having a lower refractive index than the soot 21 which is the central core portion, and the third burner 14 forms a soot 23 which is an outer portion of the staircase core portion.
Is formed.

In this way, the second and third burners 13, 14 are
As a result, the soot that forms the staircase core is divided into two parts, and each burner deposits about half the soot that forms the staircase core. In addition, the soot 22, 23 formed by the second burner 13 and the third burner 14
The maximum temperature of the surface is controlled to be 600 ° C or higher and lower than 700 ° C, preferably 650 ° C or higher and lower than 700 ° C. This temperature control can be easily performed by adjusting the supply amount of H ₂ gas of the fuel supplied to each burner 13, 14.

H ₂ gas, O ₂ gas and SiCl ₄ are supplied to the fourth burner 15 and the fifth burner 16, respectively, so that a soot 24 serving as a clad portion having a low refractive index is formed. The maximum temperature of the soot surface at this time is 700-8
The temperature range is set to 00 ° C., and the temperature range is set so as not to cause soot cracking. When the synthesis of the soot preform by these burner groups is completed, dehydration treatment and transparent vitrification are performed by a conventional method to obtain the intended optical fiber preform.

According to such a manufacturing method of the optical fiber preform, the maximum surface temperature of the soots 22 and 23, which are the stair core portions, is as low as 600 ° C. or higher and less than 700 ° C., so that the temperature of the soot surface varies. And the fluctuation of the germania doping amount becomes small. In addition, since the soots 22 and 23 are synthesized by the two burners, the second burner 13 and the third burner 14, the temperature variation on the surface of the generated soot is small, and therefore the amount of germania to be doped is small. Fluctuation becomes small, and the refractive index distribution of the staircase core becomes flat. Thus, even if the above ratio (R ₂ / R ₁ ) is increased to 3 to 5 and the manufacturing allowable range is expanded, the burner 1
The soot forming part per book does not expand so much,
It is possible to prevent the cracking of the soot due to the increase of the ratio (R ₂ / R ₁ ).

Further, according to the present invention, when a large-sized optical fiber preform is produced, the amount of the soot that becomes the staircase core portion increases and the soot diameter increases, but in this case, The number of burners that combine the soot that forms the staircase core can be three or more. In the present invention, the maximum surface temperature of the soot that forms the staircase core is 600 ° C.
Soot cracking does not occur even if the temperature is set as low as less than 700 ° C., and it is considered that the reason is that the soot is doped with germania.

Specific examples will be shown below. (Example 1) As shown in FIG. 1, five burners were used to form soot, and the obtained soot was dehydrated and then made into transparent glass to obtain an optical fiber preform with an outer diameter of 62 mm. The supply amounts of SiCl ₄ and GeCl ₄ to each burner and the thickness (mm) of the soot formed by each burner were as follows. However, the unit of the supply amount is SCCM. Burner formation soot site SiCl ₄ GeCl ₄ soot thickness 1st burner central core part 70 8 10 (radius) 2nd burner staircase core part 140 4.5 40 3rd burner staircase core part 175 4.5 40 4th burner clad part 700 0 30 5th burner clad part 850 0 30 Further, the maximum surface temperature of the soot which becomes the staircase core part is the second,
Adjust the H ₂ gas supply to the third burner to 670-68
It was set to 0 ° C. The refractive index distribution of the obtained optical fiber preform is
As shown in FIG. 2, there was almost no irregularity in the refractive index distribution in the staircase core. In addition, the ratio (R ₂ / R ₁ )
Was about 4.0.

(Example 2) An optical fiber was prepared in the same manner as in Example 1, except that the maximum surface temperature of the soot, which is the staircase core formed by the second burner and the third burner, was 610 to 620 ° C. did. The refractive index distribution of the obtained optical fiber preform was the same as that of Example 1 shown in FIG. 2, and the ratio (R ₂ / R ₁ ) was about 4.0.

Example 3 An optical fiber was prepared in the same manner as in Example 1 except that the thickness of each soot formed by the second burner and the third burner was 50 mm. The refractive index distribution of the obtained optical fiber preform is
Similar to that of Example 1 shown in FIG. 2, the ratio (R ₂ /
R ₁ ) was about 5.0.

(Conventional Example 1) An optical fiber preform having a diameter of 60 mm was similarly obtained by using four burners manufactured by the conventional method shown in FIG. The supply amounts of SiCl ₄ and GeCl ₄ to each burner and the thickness (mm) of the soot formed by each burner are as follows. Burner formation soot site SiCl ₄ GeCl ₄ soot thickness Burner A Central core part 70 7 10 (radius) Burner B Stair core part 280 770 Burner C Clad part 700 0 30 Burner D Clad part 850 0 30 Also becomes stair core part The maximum surface temperature of the soot was adjusted to 720 to 730 ° C. by adjusting the supply amount of H ₂ gas to the burner B. As shown in FIG. 3, the refractive index distribution of the obtained optical fiber preform changed greatly in the staircase core portion and was irregular. The ratio (R ₂ / R ₁ ) was about 3.5.

Comparative Example 1 As shown in FIG. 1, five burners were used to obtain an optical fiber preform having a diameter of 62 mm. The supply amounts of SiCl ₄ and GeCl ₄ to each burner and the thickness (mm) of the soot formed by each burner are as follows. Burner formation soot site SiCl ₄ GeCl ₄ soot thickness 1st burner central core part 70 8 10 (radius) 2nd burner staircase core part 140 4.5 40 3rd burner staircase core part 175 4.5 40 4th burner clad part 700 0 30 5th burner clad part 850 0 30 Maximum surface temperature of soot by the 2nd and 3rd burners is 720
˜730 ° C. The refractive index distribution of the obtained optical fiber preform is irregular as shown in FIG. 4, and the ratio (R ₂ / R ₁ )
Was about 4.0. Even if the soot that forms the staircase core is formed with two burners, the maximum surface temperature of the soot is 70
If the temperature is higher than 0 ° C., the variation in temperature becomes large, the fluctuation of the doping amount of germania becomes large, and the refractive index distribution is disturbed.

(Comparative Example 2) A soot preform was produced in the same manner as in Comparative Example 1, using five burners. The supply amounts of SiCl ₄ and GeCl ₄ to each burner and the thickness (mm) of the soot formed by each burner were as follows. Burner formation soot site SiCl ₄ GeCl ₄ soot thickness 1st burner central core part 70 8 10 (radius) 2nd burner staircase core part 140 4.5 40 3rd burner staircase core part 175 4.5 40 4th burner clad part 700 0 30 5th burner clad part 850 0 30 Maximum surface temperature of soot by the 2nd and 3rd burners is 570
When the temperature was adjusted to ˜580 ° C., the soot preform cracked during the soot deposition. The surface temperature of the soot was too low, and the difference in bulk density between the soot and the soot caused by the fourth burner fluctuated significantly, causing cracking.

Comparative Example 3 An optical fiber preform was prepared in the same manner as in Example 1 except that the thickness of the soot formed by the second burner and the third burner was 60 mm. The refractive index distribution of the obtained optical fiber preform is shown in FIG.
It was the same irregularity as that of Comparative Example 1 shown in FIG.

[0027]

As described above, according to the method of manufacturing the preform for dispersion-shifted optical fiber of the present invention, the soot to be the staircase core is formed by two or more burners, and the surface of the soot is formed. Maximum temperature is over 600 ℃ 70
Since the temperature is lower than 0 ° C, the variation in the temperature of the soot becomes small, and the doping amount of germania becomes uniform,
The irregularity of the refractive index distribution in the staircase core is extremely small. Also, because it forms a soot comprising a step core portion with two or more burners, the ratio of the radius of the step core portion (R ₂₎ and the central core of radius _{(R 1) (R 2 /} R 1) 5 It can be reasonably expanded to a certain degree, the manufacturing allowable range is widened, and the yield is improved.

Further, since it is not necessary to deposit a large soot with one burner, it is possible to prevent the soot from cracking due to the temperature difference and the bulk density difference of the soot. Further, since the supply ratio of GeCl ₄ supplied to the burner can be changed respectively, the refractive index of the staircase core part can be adjusted so as to gradually decrease toward the outside.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an example of a manufacturing method of the present invention.

FIG. 2 is a diagram showing a refractive index distribution of an optical fiber preform obtained in an example.

FIG. 3 is a diagram showing a refractive index distribution of an optical fiber preform obtained in a conventional example.

FIG. 4 is a diagram showing a refractive index distribution of an optical fiber preform obtained in a comparative example.

FIG. 5 is a diagram showing a refractive index distribution of a 1.55 μm band dispersion-shifted optical fiber.

FIG. 6 is a configuration diagram showing a conventional method for producing a preform for a dispersion-shifted optical fiber for the 1.55 μm band.

[Explanation of symbols]

13 ... 2nd burner, 14 ... 3rd burner, 22 ... Inner part of soot which becomes a staircase core part, 23 ... Outer part of soot which becomes a staircase core part

Claims (2)

[Claims] 1. A central core portion having a high refractive index located at the center, a staircase core portion having a refractive index lower than that of the central core portion, and a staircase core portion having a refractive index lower than that of the central core portion. However, when a dispersion-shifted optical fiber preform having a clad portion having a refractive index lower than that of the staircase core portion is manufactured by the VAD method using germania (GeO ₂ ) as a dopant for increasing the refractive index, A soot to be formed is formed by using a plurality of soot forming burners, and the maximum surface temperature of the soot is set to 600 ° C. or higher and lower than 700 ° C., a process for producing a base material for a dispersion shift optical fiber. 2. The method for producing a preform for a dispersion-shifted optical fiber according to claim 1, wherein each of the soot-forming burners forming the staircase core has a soot thickness of 50 mm or less.