JPH10194769A - Production of optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method of optical fiber provided with such a structure as to effectively restrain any influences due to a non-linear optical effect with increased MFD and downsize the diameter of the inside core of the optical fiber. SOLUTION: This method is to make an interference between a flame from the second burner 552 for forming the second porous glass 652 bound to become the outside core and another flame from the first burner 551 for forming the first porous glass 651 bound to become the inside core, and to dispose the second burner 552 in a specific position so as to enable downsizing the diameter of the first porous glass 651 viz. the inside core of the resulting optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber applicable to a long-distance, large-capacity optical communication system.
The present invention relates to a method of manufacturing a dispersion-shifted fiber suitable for a wavelength division multiplexing (WDM) communication system and having a zero dispersion wavelength set within a desired range.

[0002]

2. Description of the Related Art Conventionally, in an optical communication system to which a single mode optical fiber (hereinafter referred to as SM optical fiber) is applied as a transmission line, a signal light for communication is 1.3 μm.
In many cases, light in the wavelength band or the 1.55 μm wavelength band is used. However, recently, use of light in a 1.55 μm wavelength band has been increasing from the viewpoint of reducing transmission loss in a transmission line. In particular, an SM optical fiber (hereinafter referred to as 1.55 μm S
Since the transmission loss of a silica-based SM optical fiber is minimized with respect to light in the 1.55 μm wavelength band, the chromatic dispersion (a phenomenon in which a pulse wave spreads because the light propagation speed varies depending on the wavelength). ) Is also designed to be zero for light in the 1.55 μm wavelength band. The 1.55 μm SM optical fiber having the zero dispersion wavelength shifted to the vicinity of the 1.55 μm wavelength band as described above is generally called a dispersion shifted fiber.

A conventional dispersion-shifted fiber is disclosed in, for example, Japanese Patent No. 2533083 (first conventional example).
A cross-sectional structure, a composition, and a manufacturing method of a dispersion-shifted fiber whose zero-dispersion wavelength is shifted to around 1.55 μm are disclosed. The dispersion-shifted fiber of the first conventional example is GeO ₂ —SiO ₂ (SiO ₂ containing germanium).
An inner core made of SiO ₂ , an outer core made of SiO ₂ , and F
A cladding made of SiO ₂ (SiO ₂ containing fluorine). However, the refractive index profile of the dispersion-shifted fiber of the first conventional example is a so-called Matched type profile in which a portion corresponding to a clad does not have a depression, and an optical fiber having this Matched type profile is referred to as a Matched type profile in this specification. It is called fiber. on the other hand,
A refractive index profile in which a concave portion is provided in a portion corresponding to the clad is called a depressed cladding type profile. In this specification, an optical fiber having this depressed cladding type profile is called a depressed type fiber. In the structure of the dispersion-shifted fiber of the first conventional example, only the setting of the zero-dispersion wavelength near 1.55 μm can be realized.

Japanese Patent Application Laid-Open No. 63-208005 (second conventional example) discloses a first clad having a lower refractive index than the core, and a first clad having a lower refractive index than the core. Dispersion shifted fibers having a depressed cladding type profile with a second cladding having a higher refractive index than the cladding are disclosed. The purpose of the dispersion-shifted fiber of the second conventional example is 1.3 μm to 1.5 μm.
The object is to suppress chromatic dispersion over a wide range of m wavelength bands.

[0005]

In recent years, construction of a high-speed and large-capacity transmission system has been actively studied, and among them, a transmission system of a wavelength division multiplexing (WDM) system has been particularly noticed. This method is a method for simultaneously transmitting a plurality of signal lights having different wavelengths through one transmission line, and is a technique for dramatically increasing the amount of information that can be transmitted.

In order to realize such a transmission system,
Various new specifications are required for an optical fiber used as a transmission line, and the conventional dispersion-shifted fiber as described above cannot meet such requirements.

Particularly, the mode field diameter (MFD) of the conventional dispersion-shifted fiber is about 8 μm, and the influence of the nonlinear optical effect is likely to occur as the power of the signal light increases. Also, since the dispersion of the chromatic dispersion is large among the dispersion-shifted fibers applied to the transmission system, if the signal light wavelength and their zero-dispersion wavelengths match, four-wave mixing, one of the nonlinear optical effects, occurs. There is a problem that noise due to noise is easily generated.

The nonlinear optical effect is a phenomenon in which a signal light pulse is distorted with an increase in the density of light intensity and the like, and is a major limiting factor of the transmission speed.

[0009] On the other hand, a conventional dual core shape comprising an inner core and an outer core is used.
In manufacturing a dispersion-shifted fiber having a mold structure, if the outer diameter of the outer core is made sufficiently larger than the outer diameter of the inner core (for example, about 5 to 10 times), the outer diameter of the inner core is inevitable. Therefore, the outer diameter of the outer core has to be increased. In particular, if GeO ₂ is added to the outer core to increase the refractive index, it becomes difficult to flatten the refractive index profile of the outer core in the radial direction. For this reason, when adjusting the refractive index by adding germanium to the outer core, radially flattening the refractive index profile in the outer core in a state where the outer diameter ratio of the inner core and the outer core is maintained at a desired value. To achieve this, it is desired to further reduce the diameter of the inner core. In addition, reducing the diameter of the inner core is an important technical problem in order to avoid concentration of signal power in the central region of the core region (including the inner core and the outer core) and to effectively suppress the influence of the nonlinear optical effect. It is.

The present invention has been made to solve the above-mentioned problems, and is a manufacturing method for obtaining an optical fiber having a structure in which the MFD is increased and the effect of the nonlinear optical effect is effectively suppressed. Accordingly, it is an object of the present invention to provide a method of manufacturing an optical fiber for realizing a reduction in the diameter of an inner core of the optical fiber.

[0011]

According to a method for manufacturing an optical fiber according to the present invention, the MFD is at least 8.6 μm, preferably at least 9 μm, and its zero-dispersion wavelength is a typical signal light wavelength. This is a manufacturing method for obtaining a single-mode optical fiber which is a dispersion-shifted fiber shifted to a longer wavelength side or a shorter wavelength side than 55 μm and mainly composed of quartz glass. In particular, in this dispersion-shifted fiber, the chromatic dispersion is intentionally generated by shifting the zero-dispersion wavelength from the signal light wavelength by a predetermined amount to suppress the influence of the nonlinear optical effect. It is possible to construct a transmission system that allows dispersion wavelength dispersion.

Optical fiber 1 obtained by the manufacturing method
As shown in FIG. 1, for example, a first core 10 (inner core) mainly composed of quartz glass and having a predetermined refractive index n _1, and a first core 10 provided on the outer periphery of the inner core 10 and having a refractive index a second core 20 (outer core) having n ₂ (<n ₁ );
It is provided on the outer periphery of the outer core 20 and has a refractive index n ₃ (<
n ₂ ) and a jacket 40 provided on the outer periphery of the clad 30. In addition,
The jacket portion 40 is a glass region that does not substantially contribute to signal light propagation (a glass region where the signal light hardly propagates), and is mainly composed of the inner core 10 and the outer core 2.
0, and are provided to physically reinforce the cladding 30. For this reason, the jacket part 40 may be called a physical clad.

In order to obtain an optical fiber having the above-described structure, the method for manufacturing an optical fiber according to the present invention comprises the steps of: forming a first porous glass body 651 to be the inner core 10; A second porous glass body 652 provided on the outer periphery of the outer core 20 and serving as the outer core 20, and a third porous glass body provided on the outer periphery of the second porous glass body 652 and serving as the clad 30 And a first step for obtaining a porous preform 50 (soot preform) comprising

In the first step, as shown in FIGS. 2 and 3, for example, the first to third porous glass bodies extend from the tip of the rotating starting rod 501 along the longitudinal direction of the starting rod 501. By depositing the glass fine particles for constituting 651 to 653 respectively, the porous base material 50 grows along the longitudinal direction of the starting rod 501. In the first step, glass particles for forming each of the porous glass bodies 651 to 653 are synthesized.
To the third burners 551 to 553 are installed at predetermined positions.

In particular, the second burner 552 sets the maximum outer diameter of the first porous glass body 651 to 2Rc (the starting rod 501).
The outer diameter of the second porous glass body is set to 2Rs (the outer diameter along the rotation axis AX1 of the starting rod 501). Is constant), the central axis AX2 is equal to or more than 0.3 × (Rs−Rc) + Rc,
And 0.5 × (Rs−Rc) + Rc or less,
The second porous glass body 652 is provided so as to intersect with the glass particle deposition area 660a (the surface of the growth area 660 in the second porous glass body 652). The growth region is a region in each porous glass body in which the outer diameter of the porous glass body itself changes with time due to the deposition of glass particles on the surface. Further, the glass particle deposition region is the surface of the growth region.

As described above, by installing the second burner 552 for forming the second porous glass body 652 to be the outer core 20 at the above-mentioned specific position, the first porous glass to be the inner core 10 is formed. The flame from the first burner 551 for forming the porous glass body 651 and the flame from the second burner 552 interfere with each other, and the diameter of the first porous glass body 651, that is, the inner core 10 can be reduced. ing. In addition, the outer core 20
Can also be reduced (the ratio of the outer diameter of the inner core 10 to the outer core 20 is constant), and even if germanium is added to the outer core 20,
Flattening of the refractive index profile in the radial direction can be realized. Also, by reducing the diameter of the inner core 10, the concentration of signal power in the central region of the core region (including the inner core 10 and the outer core 20) can be avoided, and the effect of the nonlinear optical effect can be effectively suppressed.

In addition, in the method of manufacturing an optical fiber according to the present invention, the second burner 552 starts at the central axis AX2 from the viewpoint of the efficiency of depositing the glass particles on the growing porous preform 50. The rod 501 is set to be inclined at a predetermined angle θ (50 ° to 70 °) with respect to the rotation axis AX1 of the rod 501 (the rotation axis of the growing porous base material 50).

In the above-mentioned first step, the third burner 553 for forming the third porous glass body 653 to be the clad 30 has at least fluorine for adjusting the refractive index of the clad 30. In particular, the supply amount of the fluorine-based gas supplied to the third burner 553 is such that the relative refractive index difference of the clad 30 of the optical fiber 1 obtained by the manufacturing method with respect to pure quartz is zero. 0.03% to 0.1%.

In the method of manufacturing an optical fiber according to the present invention, the first step further includes a step of sintering the obtained porous preform 50 to obtain an intermediate glass preform 51 (FIG. 5).

Further, in the method of manufacturing an optical fiber according to the present invention, since the above-described jacket portion 40 is formed, FIGS.
As shown in FIG. 8, after stretching the intermediate glass base material 51 obtained in the first step to a predetermined outer diameter, VAD (Vapor ph) is applied.
ase axial deposition), OVD (Outside vapor ph
A fourth porous glass body 654 to be the jacket portion 40 covering the outer periphery of the clad 30 is formed on the outer periphery of the stretched intermediate glass base material 51 by a vapor phase synthesis method such as ase deposition) method. A second step of obtaining a material 52 and sintering the composite preform 52 to obtain an optical fiber preform 53;

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for manufacturing an optical fiber according to the present invention will be described below with reference to FIGS. In the drawings, the same portions are denoted by the same reference numerals, and description thereof is omitted.

FIG. 1 is a diagram showing a cross-sectional structure of an optical fiber (dispersion shift fiber) obtained by the manufacturing method and a refractive index profile thereof. In particular, the optical fiber 1 has a mode field diameter (MFD) of 8.6 μm or more (preferably 9 μm or more) and a zero dispersion wavelength of 1.5.
This is a dispersion-shifted fiber shifted to a longer wavelength side or a shorter wavelength side than 5 μm, and is a single mode optical fiber mainly composed of silica glass.

In this figure, an optical fiber 1 has, as a core region, an inner core 10 having an outer diameter a having a _first refractive index n ₁ and a _first refractive index provided on the outer periphery of the inner core 10. It includes an outer core 20 having a lower second refractive index n ₂ than n _1. Further, the outer diameter ratio Ra (= a / b) of the inner core 10 (outer diameter a) to the outer core 20 (outer diameter b).
Is 0.1 to 0.2.

Further, on the outer periphery of the core region (including the inner core 10 and the outer core 20), a clad 30 having an outer diameter c having a _third refractive index n ₃ lower than the second refractive index n ₂ is provided. And an outer periphery of the inner cladding 30 has a fourth refractive index n ₄ higher than the third refractive index n ₃ and a fourth refractive index n ₄ lower than the second refractive index n _2. A part 40 is provided. In addition, this jacket part 4
0 is a glass region where signal light hardly propagates, and is a glass region provided mainly for physically reinforcing the inner core 10, the outer core 20, and the clad 30 described above. It is called cladding.

The horizontal axis of the refractive index profile 150 shown in FIG. 1 corresponds to each position on the line L in the cross section of the optical fiber 1 (a plane perpendicular to the traveling direction of the propagating signal light). ing. Further, the refractive index profile 15
0, the area 101 is the refractive index (n ₁ ) of each part on the line L of the inner core 10, and the area 102 is the outer core 2
0, the refractive index (n ₂ ) of each part on the line L, the area 103 is the refractive index (n ₃ ) of each part on the line L of the clad 30, and the area 104 is each refractive index (n ₂ ) It corresponds to the refractive index (n ₄ ) of the site.

The basic composition of the optical fiber 1 is as follows.
Is GeO ₂ —SiO ₂ , and the outer core 20 is GeO ₂ —Si
O ₂ and the cladding 30 are F-SiO ₂ . Further, since the jacket portion 40 does not substantially contribute to the propagation of the signal light, the jacket portion 40 is made of SiO ₂ (a glass region intentionally containing no impurities).
It is.

However, in the optical fiber 1, since the inner core 10, the outer core 20, and the clad 30 contain chlorine as described later, the actual composition is as follows. Incidentally, chlorine (Cl) is known as an additive for increasing the refractive index, and the change in the refractive index due to the Cl is caused by a concentration of the Cl of 10%.
0.01% per 00 ppm.

Inner core: SiO ₂ + GeO ₂ + Cl Outer core: SiO ₂ + GeO ₂ + Cl Clad: SiO ₂ + F + Cl Jacket portion: SiO _{2 In} this specification, the relative refractive index difference Δ of each glass region is pure quartz glass. It is defined as follows on the basis of:

[0029] ^{_{Δ = (n t 2 -n 0}} 2) / 2n 0 2 ≒ (n t -n 0) / n 0 , where, n ₀ is the refractive index of pure silica glass as a reference, n _t is the glass Indicates the refractive index of the region. Therefore, the relative refractive index difference in a glass region to which fluorine or the like is added and the refractive index of which is lower than that of pure silica glass is expressed as a negative value. Further, in this specification, the relative refractive index difference between the respective glass regions is a difference (a positive value obtained by subtracting a smaller value from a larger value) of a relative refractive index difference with respect to each pure silica glass as follows. Is represented by

The relative refractive index difference between the inner core and the outer core:
0.7% -0.8% Relative refractive index difference between outer core and clad: 0.1% -0.
25% relative refractive index difference between clad and jacket: 0.03% ~
0.1% The relative refractive index difference between the jacket portion 40 and the pure quartz glass is 0%.

Next, the manufacturing method for obtaining the optical fiber 1 having the above-described structure will be described with reference to FIGS.

In the manufacturing method, the inner core 1 is formed in the first step.
The core region including the outer core 20 and the outer core 20 and the porous base material 50 to be the clad 30 are formed by VAD (Vapor phase axial).
The porous base material 5 formed and obtained by the deposition method
0 is sintered to obtain an intermediate glass base material 51.

FIG. 2 is a view for explaining the production of the porous preform 50 in the first step by the VAD method, and FIG. 3 is a view for explaining a burner installation position in the first step shown in FIG. FIG.

First, in the first step, the soot forming apparatus shown in FIG.
To produce a porous base material 50 extending along the longitudinal direction.
This soot forming apparatus includes a container 500 having at least an exhaust port 504 and a support mechanism 503 for supporting the porous base material 50. The support mechanism 503 is provided with a rotatable support rod 502.
A starting rod 501 for growing the porous base material 50 is attached to the tip of the second.

The soot forming apparatus shown in FIG. 2 further includes a first burner 551 for depositing a first porous glass body 651 (soot body) to be the inner core 10 and an outer core 20.
A second burner 552 for depositing a second porous glass body 652 (soot body) to be formed, and a third burner 553 for depositing a third porous glass body 653 (soot body) to be the clad 30. Are provided at predetermined positions, and the gas supply system 600 supplies desired burner gases (for example, GeCl ₄ , SiCl _4, etc.) and combustion gases (H ₂ and O ₂ ) to the burners 551, 552, 553. ), And a carrier gas such as Ar or He is supplied.

During the production of the porous preform 50, the first burner 5
In the flame of 51, the second burner 552, and the third burner 553, glass fine particles are generated by a hydrolysis reaction of the raw material gas supplied from the gas supply system 600, and these glass fine particles are deposited on the tip of the starting rod 501. To go. During this time, the support mechanism 503 rotates the support rod 502 provided at the tip thereof in the direction indicated by the arrow S1 (the longitudinal direction of the porous preform 50 and the longitudinal direction of the starting rod 501) while rotating the support rod 502 in the direction indicated by the arrow S1. (The directions respectively correspond to the directions of S2). By this operation, the porous glass body grows downward from the starting rod 501 (in the longitudinal direction of the starting rod 501), and becomes the inner core 10 at the central portion along the longitudinal direction of the starting rod 501. First porous glass body 651 to be used
A second porous glass body 652 to be the outer core 20 in a peripheral region surrounding the central portion, and a third porous glass to be a clad in a peripheral region surrounding the outer periphery of the second porous glass body 652. The bodies 653 grow respectively, and the porous preform 50 (soot preform) is obtained. Since the growing porous base material 50 is directly held by the starting rod 501, it rotates about the rotation axis AX1 with the rotation of the starting rod 501.

The second burner 552 sets the maximum outer diameter of the first porous glass body 651 to 2Rc (the starting rod 501).
The outer diameter of the second porous glass body is set to 2Rs (the outer diameter along the rotation axis AX1 of the starting rod 501). Is constant), the central axis AX2 is equal to or more than 0.3 × (Rs−Rc) + Rc,
And 0.5 × (Rs−Rc) + Rc or less,
The second porous glass body 652 is provided so as to intersect with the glass particle deposition region 660a (the surface of the growth region 660 in the second porous glass body 652) (see FIG. 3). In addition, from the viewpoint of the deposition efficiency of the glass microparticles on the growing porous base material 50, the second burner-552 includes:
The center axis AX2 is at a predetermined angle θ with respect to the rotation axis AX1 of the starting rod 501 (the rotation axis of the growing porous base material 50).
(50 ° to 70 °). here,
The growth region (especially, the growth region 660 of the second porous glass body 652 is a hatched portion in the figure) is a porous glass body in which the fine glass particles are deposited on the surface of each porous glass body. This is a region where the outer diameter of itself changes over time. In addition, the glass particle deposition area is:
The surface of the growth region.

As described above, by installing the second burner 552 for forming the second porous glass body 652 to be the outer core 20 at the above-described specific position, the first porous glass to be the inner core 10 can be formed. The flame from the first burner 551 for forming the porous glass body 651 and the flame from the second burner 552 interfere with each other to form the first porous glass body 651, that is, the inner core 10 of the obtained optical fiber 1.
The diameter can be reduced. In addition, by reducing the diameter of the inner core 10, the outer diameter of the outer core 20 can be reduced (the outer diameter ratio Ra of the inner core 10 and the outer core 20 is constant), and germanium is added to the outer core 20. Even in such a case, it is possible to realize the radial flattening of the refractive index profile of the outer core 20 in the radial direction. Also,
By reducing the diameter of the inner core 10, concentration of signal power in the central region of the core region (including the inner core 10 and the outer core 20) can be avoided, and the effect of the nonlinear optical effect can be effectively suppressed.

In the first step, the cladding 3
In order to adjust the refractive index of the clad 30, at least a fluorine-based gas (specifically, CF ₄ ) is supplied to the third burner 553 for forming the third porous glass body 653 to be zero. In particular, the third burner 55
The supply amount of the fluorine-based gas supplied to the optical fiber 3 is adjusted such that the relative refractive index difference of the clad 30 of the optical fiber 1 obtained by the manufacturing method with respect to pure quartz is 0.03% to 0.1%. I have.

Subsequently, the porous preform 50 obtained by the above-described VAD method is placed in the heating vessel 700 shown in FIG. 4, and is subjected to a dehydration treatment in an atmosphere containing a halogen gas. The heating container 700 is provided with an inlet 702 and an outlet 701 for supplying the halogen gas. During the dehydration process, the support mechanism 503 operates to move the porous preform 50 in the direction shown by the arrow S3 while rotating the porous preform 50 in the direction shown by the arrow S3 (by this operation, the porous base material 50 becomes porous). The entire quality preform 50 is heated).

The temperature in the container during the dehydration treatment is 1000 ° C. to 1 ° C.
About 300 ° C., preferably 1100 ° C. to 1200
° C. In this embodiment, the concentration is 20,000 ppm.
The dehydration process was performed while supplying chlorine gas (Cl ₂ ) of (2%) from the inlet 702, but the chlorine gas concentration was 100%.
The effect can be sufficiently obtained if it is about 00 ppm to 50000 ppm (1% to 5%).

As the gas for dehydration treatment, chlorine gas and S
A similar effect can be obtained with a halogen gas such as iCl ₄ .
In particular, SiCl ₄ increases the amount of chlorine added, and the outer core 2
It can be a means for increasing the difference in the refractive index between 0 and the inner cladding 30.

In the first step, the raw material gas supplied to each of the first and second burners 551 and 552 is supplied to the inner core 10 (outer diameter b) with respect to the outer core 20 (outer diameter b) of the obtained optical fiber. a) outer diameter ratio Ra (= a /
b) is adjusted to be 0.1 to 0.2.

The porous base material 50 obtained through the above processing
Is subsequently sintered in the heating vessel 700 described above.
That is, FIG. 5 shows only a main part of the heating container shown in FIG. As shown in the drawing, the support mechanism 503 operates to move the porous preform 50 in the direction shown by the arrow S6 while rotating in the direction shown by the arrow S5. By this operation, the porous base material 50 is fed into the heater 750 from the tip thereof (the temperature in the container at the time of sintering is 1500 ° C to 1650 ° C, preferably 1550 ° C to 1 ° C).
650 ° C.), and a transparent intermediate glass base material 51 is obtained.

The production, dehydration, and sintering of the porous base material 50 can be performed in the same container.

Next, in the second step of the method for manufacturing an optical fiber according to the present invention, first, the transparent intermediate glass base material 51 obtained in the above-mentioned first step is desirably processed by the stretching apparatus shown in FIG. (Finished outer diameter). Prior to stretching, the intermediate glass base material 51 is subjected to end treatments at both ends, and rods 61 and 62 are attached for easy handling.

The stretching apparatus shown in FIG. 6 includes an upper chuck 63 movable in a direction indicated by an arrow S7 and a lower chuck 64 movable in a direction indicated by an arrow S8. These upper chuck 63 and lower chuck 6
4 are driven by drive motors 65 and 66 at S7 and S, respectively.
8 along. The intermediate glass preform 51 having been subjected to the edge treatment is attached to the stretching device shown in FIG. 6 by the rod 61 being gripped by the upper chuck 63 and the rod 62 being gripped by the lower chuck 64.

The upper chuck 63 is moved in the direction of S7, so that the intermediate glass base material 51 is
8 (for example, a vertical resistance heating furnace). On the other hand, the lower chuck 64 moves along the direction of S8, so that the intermediate glass base material 51
It works to draw out. Since the intermediate glass preform 51 sent into the heater 68 is partially softened, in this stretching apparatus, the moving speed of the upper chuck 63 (the speed at which the intermediate glass preform 51 is fed into the heater 68) is reduced. By increasing the moving speed of the lower chuck 64 (the speed at which the intermediate glass preform 51 is drawn out of the heater 68) and applying a tensile stress to the softened portion of the intermediate glass preform 51, the intermediate glass preform 51 is desirably formed. Stretched to the finished outer diameter.

The controller 67 constantly monitors the outer diameter of a predetermined portion of the softened portion heated by the outer diameter measuring device 69, and controls the drive motors 65 and 66 to obtain a desired finished outer diameter. Controlling.

Subsequently, in the second step, a fourth porous glass body 654 to be the jacket portion 40 (physical cladding) is further deposited on the outer periphery of the intermediate glass base material 51 stretched by the above stretching device. , A composite base material 52 is obtained.
The production of the composite base material 52 is performed by the VAD method or the O
Any of the VD methods may be applied.

That is, as shown in FIG. 7, in the flame of the burner 900, glass fine particles are generated by a hydrolysis reaction of the raw material gas supplied from the gas supply system 600, and the intermediate glass fine particles are stretched. It is deposited on the outer periphery of the glass base material 51. During this time, the stretched intermediate glass base material 51 is moving in the direction shown by the arrow S10 while rotating in the direction shown by the arrow S9.
By this operation, the fourth porous glass body 654 (soot body) is deposited on the outer periphery of the intermediate glass base material 51, and the composite base material 52 is obtained.

Next, in the second step, the composite preform 52 is sintered as shown in FIG. 8 in order to obtain a transparent optical fiber preform 53. This sintering process is continuously performed in the heating container 700 described above. Further, the support mechanism 50
3 operates to move the composite base material 52 along the direction indicated by the arrow S12 while rotating the composite base material 52 in the direction indicated by the arrow S11. With this operation, the composite base material 52 is fed into the heater 750.

Specifically, the sintering temperature of the composite base material 52 in the heating vessel 700 (temperature in the vessel) is 1450 ° C. to 1
650 ° C (preferably 1500 ° C to 1600 ° C).

The optical fiber preform 5 manufactured as described above
3, an inner core glass 100 to be the inner core 10 of the optical fiber 1 and an outer core 20 as shown in FIG.
An outer core glass 200 to be formed, a clad glass 300 to be the clad 30, and a glass region 400 to be the jacket 40 are provided.

In the final step (drawing step), the heater 950 is used.
The optical fiber preform 53 having the above structure
By drawing the optical fiber preform 53 while heating one end of the optical fiber 1, the optical fiber 1 having an outer diameter of 125 μm shown in FIG. 1 is obtained.

The characteristics of the optical fiber 1 obtained by the above-described manufacturing method are shown below.

(Composition) Inner core: SiO ₂ + GeO ₂ + Cl Outer core: SiO ₂ + GeO ₂ + Cl Clad: SiO ₂ + F + Cl Jacket portion: SiO ₂ (Refractive index profile) Relative refractive index difference between outer core and inner core: 0 .73% Relative refractive index difference between outer core and clad: 0.18% Relative refractive index difference between clad and jacket: 0.08% It is a difference (a positive value obtained by subtracting a smaller value from a larger value) of a relative refractive index difference from pure silica glass in a region.

Finally, the MFD of the obtained optical fiber 1
Has a cutoff wavelength of 8.6 μm and a reference length of 2 m, and has a cutoff wavelength of 1.
70 μm, and the zero dispersion wavelength was 1.58 μm.

Here, the cut-off wavelength of the dispersion-shifted fiber selected for the optical transmission in the 1.55 μm wavelength band is usually less than the signal light wavelength at a reference length of 2 m (measurement method according to CCITT-G.653). 1.55 μm or less is selected.

With a short length of 2 m, which is a standard for the general evaluation of the cutoff wavelength, the dispersion-shifted fiber propagates not only the fundamental mode but also higher-order modes of the transmission light. However, although the cutoff wavelength of this embodiment is longer than the signal light wavelength (1.55 μm),
Since the higher-order mode light has a higher attenuation factor in propagation in the dispersion-shifted fiber than the fundamental-mode light, a propagation length of several km is sufficiently smaller than the fundamental-mode light. Therefore, when the propagation distance ranges from several hundred to several thousand km as in the case of a submarine communication cable, the problem due to the higher-order mode does not occur.

[0061]

As described above, according to the present invention, the second porous glass body to be the outer core is formed.
The first place where the burner is to be installed in a specific location and is to be the inner core
By causing the flame from the first burner for forming the porous glass body to interfere with the flame from the second burner, the diameter of the first porous glass body, that is, the inner core of the obtained optical fiber is reduced. This has the effect of making it possible.

In addition, the outer diameter of the outer core can be reduced by reducing the diameter of the inner core (the outer diameter ratio of the inner core to the outer core is constant), and when germanium is added to the outer core. Even so, there is an effect that the refractive index profile in the radial direction of the outer core can be flattened.
Further, by reducing the diameter of the inner core 10, there is also an effect that signal power concentration in the central region of the core region (including the inner core 10 and the outer core 20) is avoided, and the effect of the nonlinear optical effect is effectively suppressed. .

Accordingly, an optical fiber having a larger desired MFD and having a structure for effectively suppressing the influence of the nonlinear optical effect can be obtained by the above-described manufacturing method.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross-sectional structure of an optical fiber obtained by a manufacturing method according to the present invention and a refractive index profile thereof.

FIG. 2 is a view for explaining a first step (VAD method) in the method for manufacturing an optical fiber according to the present invention.

FIG. 3 is a view for explaining a burner installation position in a first step shown in FIG. 2;

FIG. 4 is a view for explaining a dehydration treatment during a first step in the method for manufacturing an optical fiber according to the present invention.

FIG. 5 is a view for explaining a sintering process during a first step in the method for manufacturing an optical fiber according to the present invention.

FIG. 6 is a view for explaining a stretching process in a second step in the method for manufacturing an optical fiber according to the present invention.

FIG. 7 is a diagram showing a method of manufacturing a composite preform (VAD method or OVD method) during a second step in the method for manufacturing an optical fiber according to the present invention.
FIG.

FIG. 8 is a view for explaining a sintering process in a second step in the method for manufacturing an optical fiber according to the present invention.

FIG. 9 is a view for explaining a drawing step in the method for manufacturing an optical fiber according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Dispersion shift fiber, 10 ... 1st core (inner core), 20 ... 2nd core (outer core), 30 ... clad,
40: jacket part (physical cladding), 50: porous base material, 51: intermediate glass base material, 52: composite base material, 53: optical fiber base material, 501: starting rod, 551: first burner, 552: No. 2 burners, 553 ... 3rd burner, 6
51: first porous glass body, 652: second porous glass body, 653: third porous glass body, AX1: rotation axis of the growing porous base material, AX2: central axis of the second burner.

Claims (6)

[Claims] At least a first core having a predetermined refractive index n ₁ and a refractive index n provided on an outer periphery of the first core
₂ (<n ₁ ) and a cladding provided around the second core and having a refractive index n ₃ (<n ₂ ). A method for manufacturing, comprising: a first burner for forming a first porous glass body to be the first core; and a second porous glass body for forming the second core to be the second core. And a third burner for forming a third porous glass body to be the clad on the outer circumference of the second porous glass body are respectively provided at predetermined positions. By depositing the glass fine particles synthesized in the flames from the first to third burners on the leading end portion of the rotating starting rod, the glass fine particles extend along the longitudinal direction of the starting rod from the leading end of the starting rod. The first to third porous glass bodies grown by A first step of obtaining a porous base material, wherein the second burner has a maximum outer diameter of the first porous glass body of 2Rc and a maximum outer diameter of the second porous glass body of 2Rc.
s, the central axis is 0.3 × (Rs−Rc) + Rc or more and 0.5 × (R
A method for manufacturing an optical fiber, wherein the optical fiber is installed so as to intersect a glass fine particle deposition region on the surface of the second porous glass body, defined as s-Rc) + Rc or less. 2. The method for manufacturing an optical fiber according to claim 1, wherein the second burner is installed such that a center axis thereof is inclined by 50 ° to 70 ° with respect to a rotation axis of the starting rod. . 3. The method according to claim 1, wherein at least a fluorine-based gas is supplied to the third burner. 4. The supply amount of the fluorine-based gas supplied to the third burner is adjusted so that the relative refractive index difference of the cladding of the obtained optical fiber with respect to pure quartz is 0.03% to 0.1%. 4. The method of manufacturing an optical fiber according to claim 3, wherein the method is performed. 5. The method according to claim 1, wherein the first step further includes a step of sintering the obtained porous base material to obtain an intermediate glass base material. Manufacturing method of optical fiber. 6. After stretching the intermediate glass base material obtained in the first step to a predetermined outer diameter, the outer circumference of the intermediate glass base material stretched by a vapor phase synthesis method covers the outer circumference of the clad. A second step of forming a fourth porous glass body to be a jacket portion to obtain a composite preform, and sintering the composite preform to obtain an optical fiber preform. Item 6. The method for producing an optical fiber according to Item 5.