JP2002047027A - Preform for optical fiber and single mode optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a preform and single mode optical fiber in which zero dispersion wavelength, mode field diameter and cutoff wavelength are all within limits of desirable numeric value, and increasing of absorption loss by hydrogen can be controlled, by controlling the generation of micro bubble on the interface of core and clad. SOLUTION: The preform for the optical fiber is characterized in that a core soot body is manufactured by vapor phase synthesis method, and after dehydrating in dehydrating gas atmosphere, the core soot body is fired in inert gas for vitrification, and to the core member obtained by heating and drawing to the stated diameter, the clad part is given for manufacturing the preform, and the core member has low refractive index part around its outer part by 1×10-5-3×10-4 lower than that of the clad part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preform for an optical fiber (hereinafter, simply referred to as a preform) which is a precursor of an optical fiber, and more particularly, to a preform manufactured by a vapor phase synthesis method and a drawing of the same. The present invention relates to a single mode optical fiber obtained by the above method.

[0002]

2. Description of the Related Art One of the methods for manufacturing a preform is VA.
A soot body for the core is formed simultaneously with a part of the clad using a vapor phase synthesis method such as the method D, which is dehydrated and vitrified, and then heated and stretched to a required length and diameter, and Manufacturing a core rod having a part of the clad,
There is a method in which a soot is deposited on the outer periphery of the core rod by a vapor phase synthesis method such as an OVD method, dehydrated and vitrified to produce a preform having a clad portion having a required thickness.

The ideal refractive index distribution shape in the radial direction of a single mode optical fiber having a zero dispersion wavelength λ ₀ around 1,310 nm is such that the core portion is rectangularly higher than the cladding portion. An optical fiber having such an ideal refractive index distribution shape has a mode field diameter MFD [μ
m], cut-off wavelength λ _c [μm], zero dispersion wavelength λ
₀ [nm] satisfies the following equation. −2 ≦ 1,427− (10 × MFD × λ _c + λ ₀ ) ≦ 2 Note that the zero dispersion wavelength is desirably 1,310 ± 5 [nm].

[0004] refractive index profile in the radial direction is formed by adding the raw material gas for forming the core portion to the burner, such as Ge compound having the effect of raising the refractive index to SiCl _4, for example, GeCl _4. However, Ge or GeO ₂ in the soot body deposited at this time diffuses into the clad portion around the core during deposition, dehydration, or vitrification, causing an unintended increase in the refractive index such as tailing of the refractive index distribution. . If there is a tail in the refractive index distribution, the dispersion characteristics of the obtained optical fiber deteriorate. That is, when the zero-dispersion wavelength falls within a desired numerical range, one or both of the mode field diameter and the cutoff wavelength become larger than desirable values. As described above, the increase in the refractive index of the cladding around the core is not preferable in terms of the transmission characteristics of the optical fiber.

[0005] In order to suppress such tailing of the refractive index distribution, heat is applied from outside the deposited core portion,
There is a method of increasing the density near the outer periphery of the core to suppress the diffusion of Ge. However, this method causes a sharp change in density between the core portion and the clad portion, so that there is a problem that, during vitrification, fine bubbles which are observed as bright spots in this portion are easily generated. . Further, for example, Japanese Unexamined Patent Publication No. 9-171120 discloses that G diffused in the clad.
The effect of eO ₂ is described, and it is also found that unintended diffusion of Ge into the cladding suppresses an increase in absorption loss due to hydrogen. That is, if the diffusion of Ge is suppressed by the above method, the absorption loss due to hydrogen may increase.

Further, an optical fiber for signal transmission is connected to a VAD.
In the case of manufacturing by a gas phase synthesis method such as the method, dehydration treatment with chlorine or the like is indispensable. This is 1,385 μm
Since there is an absorption peak due to an OH group near the signal wavelength, it is necessary to remove the OH group by a dehydration treatment.
Chlorine generally used in the dehydration treatment also has the effect of increasing the refractive index in quartz glass. Although the dehydration treatment is performed on both the core and the clad, the density of the soot body formed by the OVD method is generally higher than that formed by the VAD method. Chlorine used in the dehydration treatment hardly remains.

As a result, the cladding formed by the VAD method and the cladding added by the OVD method outside the cladding are:
The degree of increase in the refractive index of the clad portion due to the remaining chlorine is different, and the refractive index distribution of the clad portion is lower on the outer side, so that a step is likely to occur. This step occurs because the residual chlorine in the cladding is smaller in the OVD method as described above. The presence of such a step results in an increase in the transmission characteristics of the obtained optical fiber, in particular, the cutoff wavelength, and promotes the deterioration of the dispersion characteristics due to the tailing of the refractive index distribution. To suppress such a step,
When dehydrating the soot body formed by the OVD method, it is necessary to use a larger amount of a dehydrating gas such as Cl ₂ or to dehydrate over a longer time. In this case, the obtained characteristics are relatively good. However, the manufacturing cost increases.

[0008]

As described above, a soot body is formed by adding only a substance such as Ge that raises the refractive index to a source gas such as SiCl ₄ by a vapor phase synthesis method, and forming a soot body. When the preform is manufactured by processing to a desired diameter after the formation, the transmission characteristics of the optical fiber obtained by drawing this preform, particularly, the values of the zero dispersion wavelength, the mode field diameter, and the cutoff wavelength are all adjusted. It has been difficult to obtain a preform in which the generation of minute bubbles at the interface between the core and the clad is suppressed while maintaining the value within the desired numerical range, and the increase in absorption loss due to hydrogen is suppressed.

The present invention has been made in view of the above circumstances, and when an optical fiber is used, all of the zero dispersion wavelength, the mode field diameter, and the cutoff wavelength are within desired numerical ranges, and the It is an object of the present invention to provide a preform and a single mode optical fiber in which generation of minute bubbles at an interface is suppressed and increase in absorption loss due to hydrogen is suppressed.

[0010]

According to the preform of the present invention, a core soot body is produced by a gas phase synthesis method, the core soot body is dehydrated in a dehydrating gas atmosphere, and then fired and vitrified in an inert gas. Is a core member obtained by heating and stretching to a predetermined diameter, a preform for an optical fiber provided with a clad portion, the core member on the outer peripheral portion of the core portion,
It is characterized by having a low refractive index portion lower by 1 × 10 ^{−5 to} 3 × 10 ⁻⁴ than the refractive index of the cladding portion.

In another aspect of the present invention, in the step of manufacturing the core soot body, a fluorine-containing gas is mixed with any of a raw material gas, a combustion gas, an auxiliary gas, and a sealing gas to form an outer periphery of the core portion previously deposited. In the part, soot containing fluorine is deposited, and a low refractive index part is provided so that the refractive index after dehydration and vitrification is lower than the refractive index of the clad part by 1 × 10 ^{−5 to} 3 × 10 ^−4. It is a preform that was obtained.

The VAD method is adopted as the above-mentioned vapor phase synthesis method,
When depositing soot using a core deposition burner and a plurality of clad deposition burners, a raw material gas, a combustion gas, a combustion supporting gas, and a fluorine-containing gas as one of sealing gases are deposited on any of the clad deposition burners. It is preferred to add and supply. Fluorine diffuses throughout the core soot body during dehydration and vitrification, so that the addition of fluorine in one burner lowers the refractive index of the entire core member. Also, the burner for adding the fluorine-containing gas is a burner for clad deposition adjacent to the burner for core deposition,
This is preferable because the efficiency of fluorine remaining in the soot is high.
The fluorine-containing gas to be added is selected from the group consisting of SiF ₄ , SF ₆ , CF ₄ and CCl ₂ F ₂ , and the number of moles of F contained in the fluorine-containing gas is used to produce a core soot body. 1 / 100-1 of the number of moles of Si in the raw material gas
/ 5, preferably 1/30 to 1/5. Also, the ratio a / b between the outer diameter b of the low refractive index portion and the core diameter a.
Is preferably in the range of 0.1 to 0.25.

The single mode optical fiber according to the present invention is manufactured by drawing an optical fiber by heating the preform having the above structure, and the optical fiber has a mode field diameter MF.
D [μm], cut-off wavelength λ _c [μm], and zero dispersion wavelength λ ₀ [nm] are within a range satisfying the following formula: −2 ≦ 1,427− (10 × MFD × λ _c + λ ₀ ) ≦ 2 1,310 ± 5 [nm], characterized by having a zero dispersion wavelength.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to the drawings. As shown in FIG. 1, the preform of the present invention
When the core member is formed by the VAD method, the outer peripheral portion of the core portion (diameter a) of the core member has an outer refractive index lower than the refractive index n _{0 of the} clad portion formed on the core member by the OVD method. Forming a low-refractive-index portion having a diameter b and a refractive index n _{2 in} the radial direction;
This is heated and baked to be vitrified, and further heated and stretched to a predetermined diameter to form a core member, which is provided with a clad portion by an OVD method. With such a configuration, it is possible to compensate for the deterioration of the dispersion characteristics due to the tailing.

The ratio a / b of the outer diameter b of the low refractive index portion formed on the outer peripheral portion of the core portion to the core diameter a is 0.1 to 0.25.
Is desirably within the range. If it is less than 0.1, the production efficiency is remarkably reduced, so that it is not preferable. If it exceeds 0.25, an absorption peak around 1.385 nm cannot be tolerated due to OH groups remaining near the interface between the core member and the clad portion provided thereto. It is not preferable because it increases to the extent.
The difference in refractive index between the low refractive index portion and a cladding portion formed on an outer circumferential portion of the core portion (n ₀ -n ₂₎ is 1 × 10 ^-5 ~3 × 10 ^-4
It is said. Refractive index difference is less than 1 × 10 ^-5 or 3 × 1
When the value exceeds 0 ^-4 , MFD, λ _c , and λ ₀ become −2 ≦ 1,427.
-(10 × MFD × λ _c + λ ₀ ) ≦ 2 cannot be satisfied.

When a preform of the present invention is used to produce a core soot body by a gas phase synthesis method using a VAD method, as shown in FIG. 2, a raw material supplied to a clad deposition burner 2 adjacent to a core deposition burner 1 is used. A fluorine-containing gas is added to any one of the gas, the combustion gas, the auxiliary gas, and the sealing gas, and the outer periphery of the core soot is 1 × 10 ^{−5 to} 3 × 10 ⁻⁴ more than the refractive index of the clad. Form a low low refractive index portion. Further, a part of the clad may be formed thereon by the clad deposition burner 3. After the core soot body 4 thus deposited is dehydrated in a dehydrating gas, for example, a chlorine-based gas atmosphere, it is fired and vitrified in an inert gas atmosphere such as Ar, and heated and stretched to a predetermined diameter to form a core member. A cladding portion is formed on the outer periphery of the core member by a gas phase synthesis method using an OVD method. Reference numeral 5 is a target bar.

The fluorine-containing gas to be added is SiF ₄ , S
It may be used by selecting from the group of F ₆ , CF ₄ and CCl ₂ F ₂ , particularly preferably SiF having a large number of F atoms in one molecule.
₄ , SF ₆ or CF ₄ . The number of moles of F contained in the fluorine-containing gas is 1/100 to 1/100 of the number of moles of Si in the raw material gas used for manufacturing the core soot body.
5 is preferable, and 1/30 to 1/1 is more preferable.
5 If it is less than 1/100, a desired decrease in the refractive index cannot be obtained, and if it exceeds 1/5, the refractive index is undesirably lowered more than desired.

[0018]

EXAMPLES Example 1 Gas-phase synthesis by VAD method
Performed using this burner, the core deposition burner, the SiCl ₄ as a raw material gas 0.11 liters / min, the GiCl ₄ is an additive 0.01 liters / min, cladding adjacent to the core deposition burner 0.40 l / min of SiCl ₄ as a source gas in the deposition burner, 0.10 l / min of SF ₆ as the fluorine-containing gas, and SiCl ₄ as the source gas in the burner for cladding deposition not adjacent to the core deposition burner. ₄ was supplied at 1.0 liter / min to deposit soot produced by the flame hydrolysis reaction, thereby producing a soot deposit. The soot deposit was dehydrated in an atmosphere containing 7% chlorine gas for 4 hours, then fired and vitrified in He gas, and further heated and stretched to have a diameter of 40 mm and a length of 1,30.
A 0 mm core member was obtained.

Next, a soot having a desired thickness is deposited on the outer periphery of the core member by using a gas phase synthesis method by the OVD method.
The obtained soot deposit was dehydrated and fired into glass in an atmosphere containing 10% chlorine gas for 20 hours, and further heated and stretched to a desired diameter and length to obtain an optical fiber preform. This preform has a low refractive index portion lower by 1.0 × 10 ⁻⁴ than the refractive index of the cladding portion between the core portion and the cladding portion. The ratio a / b to a was 0.2. Further, the preform was drawn to obtain an optical fiber. This optical fiber has a zero dispersion wavelength λ ₀ at a wavelength of 1,310 [nm],
MFD of 9.20 [nm] and λ _{c of} 1.27 [μm] satisfied the above relational expression, and had extremely excellent dispersion characteristics as a single mode optical fiber.

Comparative Example 1 A preform and an optical fiber were manufactured under the same conditions as in Example 1 except that no fluorine-containing gas was added to the clad deposition burner adjacent to the core deposition burner. did. The obtained preform had a step of 1.5 × 10 ^{−4 in} the clad portion, and the optical fiber obtained by drawing this preform had poor dispersion characteristics and was not preferable as a single mode optical fiber. .

(Comparative Example 2) No fluorine-containing gas was added to the burner for clad deposition adjacent to the burner for core deposition, and the atmosphere for dehydrating and firing vitrified soot deposits obtained by the OVD method was 20 times. % Except for containing chlorine gas, a preform and an optical fiber were manufactured under the same conditions as in Example 1 except for the above. The obtained preform has a low refractive index portion of 0.1 × 10 ^{−4 in} the core portion and the cladding portion, and the optical fiber obtained by drawing this preform is relatively unfavorable as a single mode optical fiber. However, the production required about 1.9 times the amount of chlorine.

[0022]

According to the preform of the present invention, the generation of fine bubbles at the interface between the core and the clad is suppressed, and the increase in the absorption loss due to hydrogen is also suppressed. The optical fiber has a zero dispersion wavelength, a mode field diameter, and a cutoff wavelength all within desired ranges, and has extremely excellent dispersion characteristics as a single mode optical fiber.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a refractive index distribution in a radial direction of a preform of the present invention.

FIG. 2 is a schematic view showing a state in which a core member is manufactured by a VAD method.

[Explanation of symbols]

1. Burner for core deposition 2. 2. Burner for clad deposition Burner for clad deposition 4. Core suit body 5. Target rod a. Core diameter b. Outer diameter of low refractive index part n ₀ Refractive index of cladding part n ₁ Refractive index of core part n ₂ Refractive index of low refractive index part

Claims (9)

[Claims] Claims 1. A core soot body is produced by a gas phase synthesis method, and the core soot body is dehydrated in a dehydrating gas atmosphere.
An optical fiber preform in which a core member obtained by firing and vitrifying in an inert gas and heating and stretching the core member to a predetermined diameter is provided with a clad portion, wherein the core member has an outer peripheral portion of the core portion. In addition, the refractive index of the cladding is 1 × 10 ^{−5 to} 3
An optical fiber preform having a low refractive index portion lower by × 10 ^-4 . 2. After producing a core soot body by a gas phase synthesis method and dehydrating the core soot body in a dehydrating gas atmosphere,
Firing in an inert gas, vitrified, and a core member obtained by heating and stretching this to a predetermined diameter, a preform for an optical fiber obtained by providing a clad portion, in the step of manufacturing the core soot body, The fluorine-containing gas is mixed with any of the raw material gas, the combustion gas, the auxiliary gas, and the sealing gas, soot containing fluorine is deposited on the outer periphery of the previously deposited core, and the refractive index after dehydration vitrification is reduced. An optical fiber preform comprising a low refractive index portion lower by 1 × 10 ^{−5 to} 3 × 10 ⁻⁴ than a refractive index of a cladding portion. 3. A vapor phase synthesis method is a VAD method, and when soot containing fluorine is deposited using a core deposition burner and a plurality of clad deposition burners, a source gas, 3. The optical fiber preform according to claim 2, wherein a fluorine-containing gas is added to any one of the combustion gas, the auxiliary gas, and the sealing gas. 4. The optical fiber preform according to claim 3, wherein the burner to which the fluorine-containing gas is added is a clad deposition burner adjacent to the core deposition burner. 5. The method according to claim 1, wherein the fluorine-containing gas is SiF ₄ , SF ₆ , C
F ₄ and preform for optical fiber according to claim 3 or 4 are those selected from the group of CCl ₂ F _2. 6. The molar number of F contained in the fluorine-containing gas to be added is determined by the number of moles of S in the raw material gas used for manufacturing the core member.
6. The number of moles of i is 1/100 to 1/5.
The preform for an optical fiber according to any one of the above. 7. The ratio a / b of the outer diameter b of the low refractive index portion to the core diameter a is in the range of 0.1 to 0.25.
Or the preform for optical fibers according to 2. 8. An optical fiber obtained by heating and drawing the optical fiber preform according to claim 1. Description: 9. The optical fiber according to claim 8, wherein a mode field diameter MFD [μm] and a cut-off wavelength λ _c [μ
m], and the zero dispersion wavelength λ ₀ [nm] is within a range satisfying the following formula: −2 ≦ 1,427− (10 × MFD × λ _c + λ ₀ ) ≦ 2 1,310 ± 5 [nm] A single mode optical fiber having a zero dispersion wavelength.