JP2000063148A - Optical fiber core member, optical fiber preform and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for flattening the refractive index distribution of the clad of an optical fiber core member and an optical fiber preform, to obtain the flattened core member and preform, to widen an area of a fiber to be designed, to improve quality and yield and to better stabilization of operation and productivity. SOLUTION: In this a quartz-based optical fiber core member, the refractive index distribution in the radial direction of a clad by chlorine is made high at the outer periphery and reduced toward the center and is balanced with the refractive index rising distribution in the radial direction of the clad by outward diffusion of the dopant of the core to the clad. The variation of specific refractive index is <=5×10-5 in a clad zone of >= twice the core diameter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber core member, an optical fiber base material (preform), and more particularly to a chlorine-doped single mode optical fiber core member having high dispersion characteristics and an optical fiber base material.

[0002]

2. Description of the Related Art A wavelength at which light passing through an optical fiber has only a fundamental mode is called a cutoff wavelength. When light with a wavelength longer than the cutoff wavelength enters this optical fiber, the higher-order modes disappear and only the fundamental mode propagates. On the other hand, in a single mode optical fiber, signal degradation occurs due to dispersion. The wavelength at which dispersion disappears is called the zero dispersion wavelength. Since the used wavelength is fixed, the upper limit of the zero-dispersion wavelength required in the market is determined.

When connecting an optical fiber, if the region where the light propagates is narrow, the loss of the optical signal at this connection point tends to increase. The region where this light propagates is represented by the mode field diameter (MFD). Since the system generally has a plurality of connection points, the optical fiber needs to have an MFD in which the loss falls within an allowable range. In this way, the minimum MFD value required in the market is determined.

The light propagating in the optical fiber leaks out due to the bending of the optical fiber, thus degrading the signal. This is called bending loss. It has been found that this bending loss correlates with the ratio of the cutoff wavelength λ _C of the optical fiber to the MFD. Since it is necessary to keep the bending loss below the allowable range, the lower limit of the cutoff wavelength and the upper limit of the mode field diameter are limited.

Strictly speaking, the light passing through the optical fiber does not have a single wavelength, so that the difference in the wavelengths causes a difference in the transmission speed and spreads the signal. This phenomenon is called dispersion. The wavelength at which the dispersion is 0 as described above is called the zero dispersion wavelength.
Ideally, it is desirable that the dispersion at the used wavelength is 0 in the entire system. That is, in a normal system that does not use a dispersion compensating fiber, it is desirable that the used wavelength and the zero dispersion wavelength match. In this way, the range of zero dispersion wavelength required on the market is determined.

From the above constraints, the design area of the optical fiber as shown in FIG. 2A is determined. here,
2, can be determined by drawing showing a relation between a cutoff wavelength lambda _C and MFD, the lambda _C and MFD corresponding to the allowable range of the bending loss. Usually, the refractive index distribution of the core and the thickness of the clad are determined so as to fit within such a design area. Here, the work of determining the clad thickness when there is a core member having a certain refractive index distribution is referred to as “designing”.

For example, in a general optical fiber having a working wavelength of 1310 nm, as is well known, the ideal refractive index distribution is a perfect rectangle, but the VAD method (vapor phase axis method) and the OVD method (external method) are used. It is very difficult to make a rectangle by an actual manufacturing method such as a chemical vapor deposition method. This is because, in general, in these manufacturing methods, a gentle increase in the refractive index appears in the clad from the outer periphery to the center. This increase in the refractive index is caused by the diffusion of Ge, which is a dopant, from the core to the clad by being heated in the dehydration / vitrification process.

Since it is usually possible to manufacture only a profile having such a gradual increase in the refractive index, as shown in FIG.
As shown in (c), the conventional "designable area" that can be actually designed is only a very narrow portion in the "design area" of FIG. 2 (a) determined by the market requirements. In addition, if only within such a “designable area”, as shown in FIG. 3B, the center of the zero dispersion wavelength distribution becomes a value larger than the used wavelength, and the fiber having a large zero dispersion wavelength is obtained. It becomes difficult to match the zero-dispersion wavelength of the entire system to the used wavelength by combining the fiber with a wavelength having a small zero-dispersion wavelength.

As a method of adjusting the refractive index distribution of the clad, for example, as described in Japanese Patent No. 2659066, a method of doping a fluorine compound to lower the refractive index is known. This method is known. According to the report, it is necessary to dope a large amount of fluorine near the core having a high refractive index. However, the increase of the dopant in the region where the light propagates causes a loss, which is not preferable.

[0010]

Therefore, the present invention has been made in view of the above problems, and a method and a method for flattening the refractive index distribution of the optical fiber core member and the cladding of the optical fiber preform. The main purpose is to provide an improved core member and preform, expand the designable area of the fiber, improve the quality and yield, and stabilize the operation and improve the productivity.

[0011]

In order to solve the above-mentioned problems, according to the invention described in claim 1 of the present invention, in the core member of a silica-based optical fiber, the radial refractive index distribution of the clad due to chlorine is at the outer periphery. A core member of an optical fiber characterized in that the distribution is high and decreases toward the center, and is balanced with the refractive index increase distribution in the radial direction of the cladding due to the outward diffusion of the core dopant into the cladding. .

As described above, according to the present invention, since the refractive index distribution of the chlorine which is inclined from the outside to the inside in the radial direction of the cladding and the refractive index distribution of the reverse inclination in the cladding which is caused by the dopant of the core are balanced, It is a core member with an extremely flat rate distribution, and the zero-dispersion wavelength, which has deviated from the used wavelength, becomes almost the same, and it is possible to greatly expand the designable range of the optical fiber, which was conventionally narrower. .

According to a second aspect of the present invention, there is provided an optical fiber characterized in that the fluctuation of the relative refractive index is 5 × 10 ^-5 or less in the cladding region having a core diameter twice or more. It is a core member. In this way, if the fluctuation of the relative refractive index in the clad is suppressed to 5 × 10 ⁻⁵ or less, the zero dispersion wavelength λ ₀ becomes almost the same as the used wavelength within the allowable range, and the dispersion characteristics are improved. Thus, the designable range of the optical fiber can be expanded.

Then, as described in claim 3, the dopant doped in the core can be Ge. Thus, when the core dopant is Ge,
The present invention can correct the fact that the refractive index distribution becomes high on the core side due to outward diffusion from the core to the clad.

The invention described in claim 4 of the present invention is an optical fiber preform manufactured from the core member of the optical fiber according to any one of claims 1 to 3.
In this way, the remaining clad is deposited on the core member having the above-mentioned characteristics, dehydrated with chlorine, and the optical fiber preform produced by transparently vitrifying with chlorine to produce the desired characteristics like the core member. It becomes an optical fiber preform.

Further, the invention described in claim 5 of the present invention is an optical fiber manufactured from the optical fiber preform described in claim 4. When the optical fiber manufactured by stretching the optical fiber preform having the above characteristics is manufactured, the optical fiber has desired properties, like the optical fiber preform.

The invention according to claim 6 of the present invention is as follows.
A core made of SiO ₂ doped with a dopant and a part of a clad made of SiO ₂ are deposited on the starting member to manufacture a soot deposit body, which is dehydrated with chlorine to be transparent vitrified to form an optical fiber core member. In the method for producing, the chlorine concentration remaining in the soot deposit is controlled to be higher as it is closer to the outer circumference, and the distribution of the refractive index in the radial direction of the clad due to chlorine is high at the outer circumference and becomes lower toward the center. The method for producing an optical fiber core member is characterized in that the core dopant is produced so as to be balanced with the refractive index increase distribution in the radial direction of the clad due to the outward diffusion of the core dopant into the clad.

When the core member is manufactured in this manner, the refractive index distribution in the clad is extremely flattened, the zero dispersion wavelength deviated from the used wavelength becomes substantially the same, and the dispersion characteristic is improved. The designable range of the narrowed optical fiber can be greatly expanded, and the yield and the productivity can be improved.

In this case, as described in claim 7, it is desirable to suppress the fluctuation of the relative refractive index in the cladding region having a diameter twice or more the core diameter of 5 × 10 ⁻⁵ or less. In this way, if the variation range of the relative index of refraction in the cladding is determined, the zero dispersion wavelength λ ₀ will be almost the same as the operating wavelength within the allowable range, improving the dispersion characteristics and expanding the designable range of the optical fiber. can do.

In this case, the core dopant may be Ge.
By doing this, G diffused outward from the core to the clad
The increase in the refractive index due to e and the increase in the refractive index due to chlorine diffused into the clad from the outside can be balanced to flatten the refractive index of the clad.

The invention described in claim 9 of the present invention is a method for producing an optical fiber preform for forming an optical fiber preform from the optical fiber core member produced in any one of claims 6 to 8. In this way, the remaining porous clad (hereinafter, sometimes referred to as the second clad) is deposited using the core member as a starting material, dehydrated with chlorine, doped with chlorine to form transparent glass, and has good characteristics. An optical fiber preform can be formed.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The embodiments of the present invention will be specifically described below, but the present invention is not limited thereto. When the present inventors flatten the refractive index distribution of the cladding, chlorine used for dehydration raises the refractive index of silica (SiO ₂ ) like Ge, and reverses the direction of the refractive index distribution. Focusing on this, the present invention was completed by confirming various conditions.

That is, Ge doped into the core is diffused outward toward the clad to increase the refractive index of the clad, and the distribution of the refractive index is gradually inclined from the core side to the outer periphery. On the other hand, chlorine increases the refractive index of the clad, and at the same time, the distribution of the refractive index is inclined so as to gradually decrease from the outer circumference toward the center of the core. In other words, by giving the refractive index increase due to chlorine, which is completely equivalent to the refractive index increase due to Ge in the clad and only has the opposite slope, the refractive index increase due to Ge is balanced with the refractive index increase due to chlorine, and as a result, The refractive index distribution can be made flat.

Here, since the increase in refractive index due to chlorine that can be actually given is gentle, it is extremely difficult to completely cancel the sudden increase in refractive index due to the diffusion of Ge in the vicinity of the core. The vicinity of the core referred to here is
This is an area about twice the core diameter. When the increase in the refractive index in the vicinity of the core is similar to the conventional one, the relationship between the variation in the refractive index in the cladding region other than the vicinity of the core and at least twice the core diameter and the "designable range" was examined. as a result,
As shown in FIG. 4, practically, the variation of the relative refractive index is 5 with respect to the used wavelength (zero dispersion wavelength λ ₀ ) 1310 ± 1 nm.
It has been found that a value of × 10 ^-5 or less is sufficient.

Specifically, the processing temperature range in the dehydration and vitrification process of the soot deposit body for core member is 1000.
The chlorine concentration in the chlorine / helium mixed gas corresponding to ˜1400 ° C. may be set so as to be within the fluctuation range of the relative refractive index.

According to the present invention as described above, the amount of chlorine, which is a dopant for adjusting the refractive index of the clad, becomes smaller as it gets closer to the core, and therefore, compared with the conventional adjustment method of adding a fluorine compound. The risk of increased loss is extremely low.

The present invention will now be described in more detail with reference to the accompanying drawings. 6A is a schematic diagram showing a configuration example of a core member manufacturing apparatus, FIG. 6B is a sintering reaction furnace, and FIG. 6C is a schematic diagram showing a configuration example of a clad manufacturing apparatus.

An example of an apparatus for producing an optical fiber core member having desired fiber characteristics as in the present invention is, for example, VA.
It can be manufactured by the manufacturing apparatus of the core member by the D method (gas phase axial method). This device is shown in FIG.
As shown in (a), a quartz substrate (core) 3 as a substrate
Is provided with a chamber 4 into which is inserted, an exhaust pipe, and a core burner 1 arranged with its tip facing a target portion at the lower end of the quartz base material 3. The burner 1 is provided with a raw material of an optical fiber core member. Raw material lines such as silicon tetrachloride (SiCl ₄ ), germanium tetrachloride (GeCl ₄ ) as a dopant for controlling the refractive index, H ₂ for oxyhydrogen flame
Gas lines such as gas and O ₂ gas are connected. Further, the first cladding burner 2 arranged toward the side surface of the quartz substrate 3 is provided and is connected to the silicon tetrachloride gas and the oxyhydrogen gas line.

Then, by using such a device, the quartz substrate 3
The raw material and the gas are blown from the burner 1 and the burner 2 toward the target portion while rotating, and a flame hydrolysis reaction is caused to occur, soot-like reaction products (Si
O ₂ ) is deposited in the axial direction and the quartz substrate 3 is pulled up to manufacture a porous core member 5 which is a soot deposit body including a core having a high refractive index and a first cladding having a low refractive index.

Next, as shown in FIG. 6B, the porous core member 5 is placed in a sintering reaction furnace 7 equipped with a mixed gas introduction pipe and an exhaust pipe of helium gas and chlorine gas and a heating device 6. The mixture is heated to 1100 ° C. while adjusting the chlorine gas concentration, dehydrated, doped with chlorine, and further sintered to form a transparent glass to prepare a core member. At this time, in the present invention, it is important to control the chlorine gas concentration in the atmosphere so as to be balanced with the increase in the refractive index due to Ge in the vicinity of the core. This determines the chlorine concentration so as to be in balance with the dopant concentration of the core and the amount of outward diffusion due to the sintering temperature.

Next, the core member 8 is provided with the second cladding burner 9 to which the SiCl ₄ gas and the oxyhydrogen flame are supplied and the clad manufacturing apparatus 1 shown in FIG. 6 (c).
The second clad is formed on the core member 8 so that desired fiber characteristics can be obtained while the burner 9 is moved to the left and right by the OVD method. After that, the glass rod 11 with the second clad was attached again to the sintering reaction furnace 7 shown in FIG. 6B, dehydrated at 1100 ° C. in a mixed gas atmosphere of helium gas and chlorine gas, and doped with chlorine, Further, it is sintered into transparent glass to manufacture an optical fiber preform.

[0032]

EXAMPLES Specific embodiments of the present invention will be described below with reference to examples, but the present invention is not limited to these. (Example) Using the core member manufacturing apparatus shown in FIG. 6A, SiCl ₄ gas and GeCl ₄ gas as a dopant were burned with an oxyhydrogen flame to deposit soot on a quartz substrate to form a porous core. Is formed, and at the same time, SiCl ₄ gas is decomposed by oxyhydrogen flame to deposit soot to form a porous clad to manufacture a porous core member.

Then, the porous core member was introduced into the sintering reaction furnace shown in FIG. 6 (b) in a helium gas containing 5 mol% of chlorine at a pulling rate of 5 mm / min for 1100.
It was passed through a sintering furnace at a temperature of ℃, dehydrated, doped with chlorine and made into a transparent glass to prepare an optical fiber core member.

When this core member was analyzed, FIG.
As shown in (c), it was possible to form a clad having a flat refractive index distribution in which the fluctuation of the relative refractive index was 1 × 10 ⁻⁵ or less in the clad region having twice the core diameter or more. This value is zero dispersion wavelength λ ₀ , as is apparent from FIG.
= 1309.4 nm, which was within the allowable range (± 1 nm) of the used wavelength of 1310 nm.

Here, FIG. 1A shows the refractive index distribution due to only the dopant Ge doped into the core. Figure 1
(B) shows the refractive index distribution due to only chlorine diffused in the clad. Then, in FIG. 1C, in the cladding region, the distributions of (a) and (b) are canceled out, resulting in a flat distribution, and the refractive index itself is the dopant concentration (Ge + C).
It can be seen that it is rising almost in proportion to l).

Further, when the distribution of the zero-dispersion wavelength λ ₀ of the optical fiber was obtained, the center of the distribution was almost coincident with the used wavelength as shown in FIG. It can be seen that it is extremely easy to construct a fiber with zero dispersion. Further, when the core member manufactured under these conditions is used, the designable range of the optical fiber is greatly expanded as compared with the following comparative example (see FIG. 2C) (see FIG. 2B).

(Comparative Example) The porous core member produced in the example was immersed in helium gas containing 1 mol% of chlorine at 5 mm / m.
It was passed through a sintering furnace at 1100 ° C. at a pull-down rate of in, dehydrated, doped with chlorine, sintered, and made into a transparent glass to prepare an optical fiber core member. When this core member was analyzed, as shown in FIG. 5C, a gentle increase in the refractive index was observed as the refractive index increased toward the core. In this case, the fluctuation of the relative refractive index in the cladding region which is twice the core diameter or more is as large as 8 × 10 ⁻⁵ , which is largely outside the allowable range of the zero dispersion wavelength in FIG. Figure 3
The distribution of the zero-dispersion wavelength is shown in (b), but it did not match the used wavelength at all and was shifted to the larger side.

FIG. 5A shows the refractive index distribution only with the dopant Ge doped in the core. FIG. 5B shows a refractive index distribution due to only chlorine diffused in the clad. As described above, in FIG. 5C, the distributions of (a) and (b) in the cladding region cannot be canceled out, and there is a gentle downward slope from the core to the outer periphery.

The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

[0040]

As described above, according to the present invention, the refractive index distribution of the slanted cladding can be flattened by the dopant diffused outward from the core, so that the zero dispersion wavelength deviated from the used wavelength can be reduced. It becomes almost the same, the dispersion characteristic is improved, the designable range of the optical fiber which has been narrowed in the past can be greatly expanded, and the yield and the productivity can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a radial direction refractive index distribution in an optical fiber core member of the present invention. (A) Refractive index distribution due to Ge doped in the core, (b)
Refractive index distribution due to chlorine diffused in the clad, (c):
The refractive index distributions synthesized in (a) and (b).

FIG. 2 Cutoff wavelength λ when designing an optical fiber
It is a figure which shows the designable area determined by the correlation of _C and MFD. (A) Design area determined by market requirements, (b) Designable area of the present invention, (c) Designable area by conventional technology.

FIG. 3 is a diagram showing a frequency distribution of zero-dispersion wavelength λ ₀ . (A) The optical fiber of the present invention, (b) the conventional optical fiber.

FIG. 4 is a diagram showing a relationship between an average value of zero-dispersion wavelength λ ₀ when designed in a designable area and a change in relative refractive index in a cladding region having a core diameter twice or more.

FIG. 5 is a diagram showing a radial direction refractive index distribution in a conventional optical fiber core member. (A) refractive index distribution due to Ge doped in the core, (b) refractive index distribution due to chlorine diffused into the clad,
(C): Refractive index distribution synthesized in (a) and (b).

FIG. 6 is a schematic diagram showing a configuration example of an optical fiber preform manufacturing apparatus used in the present invention. (A) Core member manufacturing apparatus, (b) Sintering reaction furnace, (c) Clad manufacturing apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Burner for core, 2 ... Burner for 1st clad, 3 ... Quartz base material (core), 4 ... Chamber, 5 ... Porous core member,
6 ... Heating device, 7 ... Sintering reaction furnace, 8 ... Core member, 9 ... Burner for second clad, 10 ... Clad manufacturing device, 11 ...
A glass rod with a second cladding.

Continued front page    (72) Inventor Tadakatsu Shimada             2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu             Gaku Kogyo Co., Ltd. Precision Materials Research Laboratory (72) Inventor Hideo Hirasawa             2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu             Gaku Kogyo Co., Ltd. Precision Materials Research Laboratory F-term (reference) 2H050 AA01 AB05X AB18Y AC35                       AC36                 4G062 AA06 BB02 LA03 LB01 MM04                       NN01

Claims (9)

[Claims] 1. A core member of a silica-based optical fiber,
The refractive index distribution of chlorine in the radial direction of the clad is high at the outer circumference and becomes lower toward the center, which is in balance with the radial refractive index increase distribution of the clad due to outward diffusion of the core dopant into the clad. An optical fiber core member characterized in that 2. A core member for an optical fiber, characterized in that the fluctuation of the relative refractive index is 5 × 10 ⁻⁵ or less in a cladding region having a core diameter twice or more. 3. The dopant doped into the core is G
It is e, The core member of the optical fiber of Claim 1 or Claim 2 characterized by the above-mentioned. 4. An optical fiber preform manufactured from the core member of the optical fiber according to any one of claims 1 to 3. 5. An optical fiber manufactured from the optical fiber preform according to claim 4. 6. The starting member is a Si doped with a dopant.
A method for producing an optical fiber core member by depositing a core made of O ₂ and a part of a clad made of SiO _{2 on the} core to produce a soot deposit body, dehydrating this with chlorine, and vitrifying it into a transparent glass to produce an optical fiber core member. The chlorine concentration remaining in the deposit is controlled so that it becomes higher as it gets closer to the outer circumference, and the distribution of the refractive index in the radial direction of the clad due to chlorine is high at the outer circumference and becomes lower toward the center, and the core dopant is A method of manufacturing an optical fiber core member, characterized in that the optical fiber core member is manufactured so as to be balanced with the radial refractive index increase distribution of the cladding due to the outward diffusion. 7. The optical fiber core member according to claim 6, wherein the optical fiber core member is manufactured so that the fluctuation of the relative refractive index in the cladding region that is twice or more the core diameter is 5 × 10 ⁻⁵ or less. Production method. 8. The method of manufacturing an optical fiber core member according to claim 6, wherein the dopant is Ge. 9. A method of manufacturing an optical fiber preform, which comprises forming an optical fiber preform from the optical fiber core member produced in any one of claims 6 to 8.