JPH05249329A - High-input optical fiber and production of its base material - Google Patents

Abstract

PURPOSE:To provide the high-input optical fiber having high long-term reliability and the process for production of its base material. CONSTITUTION:The dopant concn. of the core and clad of the optical fiber consisting of the core and the clad enclosing the core changes in the axial direction of the optical fiber and the relative refractive index distribution standardized by the max. refractive index of the core is kept always constant within the section in the arbitrary position in the axial direction of the optical fiber. The threshold value of generation of an induced Brillouin scattering is increased by changing the dopant concn. in the axial direction of the optical fiber to intensity the light which can be made incident on the optical fiber. Consequently, the transmission distance is greatly increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-input optical fiber for long-distance transmission and a method for manufacturing a base material thereof.

[0002]

2. Description of the Related Art Generally, in order to realize long-distance optical transmission, for example, it is necessary to (1) reduce the transmission loss of the optical fiber, (2) strengthen the light incident on the optical fiber, etc. Are known.

As for the point (1) of reducing the transmission loss of the optical fiber, research on a practical level is underway.
Currently, the optical fiber with GeO ₂ as the core is 0.2 dB /
0.1 km or less in an optical fiber having a cladding of SiO ₂ as a core and F doped in SiO ₂
A low loss of / km or less is obtained.

On the other hand, regarding the point (2) of increasing the intensity of light incident on the optical fiber, the output of the semiconductor laser is improved.
Alternatively, progress in optical amplification technology using an erbium-doped optical fiber amplifier or the like is remarkable, but it is known that there is a limit to the light intensity that can be incident on the optical fiber. That is, when a strong light of about 7 dBm or more is incident, a sound wave is excited in the optical fiber, and an optical nonlinear phenomenon in which the light wave is scattered by the sound wave, that is, stimulated Brillouin scattering occurs. As a result, most of the incident light is reflected toward the light incident end of the optical fiber and does not reach the light receiving end.

Regarding this phenomenon, D. "Observation of stimulus" by Cotter
ted Brillouin scattering
inlow-loss silica fibre a
t 1.3 μm ”(Electronics lett
ers, vol. 18, No. 12, 495-49
6, page 1982). Therefore, the amount of light that can be effectively incident on the optical fiber is limited. In order to increase the transmission distance, it is necessary to suppress the occurrence of stimulated Brillouin scattering, but it is necessary to increase the stimulated Brillouin gain width, that is, the stimulated Brillouin frequency shift (frequency of incident light and scattered light frequency). It is necessary to try to make the difference) non-uniform.

As a conventional technique for suppressing stimulated Brillouin scattering in an optical fiber, a method of causing mechanical strain in the optical fiber has been reported. This utilizes the fact that the frequency shift amount of stimulated Brillouin scattering changes when strain is generated in the optical fiber.

Regarding this phenomenon, "Tensile Strain Effect" by Kurashima, Horiguchi, and Tateda is described.
s on Brillouin Frequency
Shift in Single-mode Fiber
rs Having Pure Silica and
GeO ₂ -doped Cores "(IOOC'8
9, 21C4-2). As a method of increasing the threshold value of stimulated Brillouin scattering generation by utilizing this phenomenon, a method of spirally winding an optical fiber in an optical fiber cable and changing the strain in the optical fiber axial direction to widen the induced Brillouin gain width Is N, Yoshizawa,
T. Horiguchi, T .; "Proposal for stimulus" by Kurashima et al.
ed Brillouin scatterings
uppress by fiber cabli
ng ", (Electronics Letters,
vol. 27, No. 12, pp. 1100-110
1, 1991).

In the structure of this optical fiber cable, for example, as shown in FIG. 12, the optical fiber 11 has a double spiral structure in the axial direction, and the mechanical strain applied to the optical fiber 11 changes from a compressed state to a stretched state. Alternating alternately.
Along with this, the amount of stimulated Brillouin frequency shift changes depending on the location of the cable, and as a result, the Brillouin scattering gain width is increased, and thus the threshold for the occurrence of stimulated Brillouin scattering is increased.

As another method, a method of changing the residual stress in the axial direction by changing the tension at the time of drawing the optical fiber is described by Nozawa, Sakai, Wada and Yamauchi in "Optical fiber suppressing generation of stimulated Brillouin scattering" ( It is described in a paper entitled The 1991 Autumn Meeting of The Institute of Electronics, Information and Communication Engineers, B-546). Fig. 13 shows FS with SiO ₂ as a core
3 shows a tension distribution in the optical fiber axial direction of a Brillouin scattering suppression optical fiber having iO ₂ as a clad.

[0010]

However, the conventional method of applying strain in the optical fiber axial direction has a problem that the optical fiber is easily broken. For example, if the strain is increased from 0.1% to 0.2% with respect to an ordinary optical fiber, the breakage rate per 1 km increases by 10 ⁷ times. That is, Yutaka Mitsui, Yutaka Katsuyama, and Keikazu Kobayashi. , Ishida Yukinori et al., "Optical Fiber Strength Guarantee Method by Screening Test" (IEICE Transactions, vol. J66-B, N.
o. 7.1983). Therefore, in order to avoid this, it is indispensable to use a technique of increasing the strength by coating the surface of the optical fiber with a carbon film. Further, in the conventional method, there is a problem that the cable structure is limited because a special strain state must be maintained, and it is very difficult to use it in an actual optical fiber line.

In view of the above circumstances, it is an object of the present invention to provide a high-input optical fiber for long-distance transmission, which has high long-term reliability, and a method of manufacturing a base material thereof.

[0012]

To achieve the above object, the present invention provides an optical fiber comprising a core and a cladding surrounding the core, wherein the dopant concentration of the core and the cladding changes in the optical fiber axial direction. And the relative refractive index distribution standardized by the maximum refractive index of the core is always constant in the cross section at any position in the axial direction of the optical fiber. In the optical fiber, the dopant to be doped into the core and the clad is characterized by being one kind selected from GeO ₂ , P ₂ O ₅ , Al ₂ O ₃ and F or a combination thereof, and the above high-input optical fiber in, characterized in that the core and cladding are made of GeO ₂ doped quartz, and in the high input optical fiber, the core and the cladding are GeO ₂ de An optical fiber consisting flop quartz, dopant doped in the core and cladding is F
In the above high-input optical fiber, the core is made of GeO ₂ -doped quartz and the clad is made of pure silica, and the dopant doped in the core and the clad is F. In the above high-input optical fiber, the core and the clad are made of F-doped quartz, and the dopant for doping the core and the clad is F, and In high-input optical fiber, the core is pure quartz and the cladding is F
An optical fiber made of doped quartz, wherein the dopant for doping the core and the clad is F, and a method for producing a high-input optical fiber preform,
Quartz glass particles or Ge synthesized by gas phase reaction
In a method for producing an optical fiber preform by subjecting an O ₂ -doped quartz glass particle body to heat treatment for dehydration and / or fluorine addition in a heating furnace, and then producing an optical fiber preform, The GeO ₂ dopant distribution is made uniform in the longitudinal direction, and the glass particles are heated in a zone furnace in a heat treatment step for adding fluorine while varying the concentration of fluorine added depending on the position in the longitudinal direction at which the particles are vitrified. It is characterized by being made transparent glass.

[0013]

According to the present invention, the frequency range of stimulated Brillouin scattering is widened by changing the dopant concentrations of the core and the clad in the axial direction of the optical fiber, thereby increasing the threshold value of incident light amount for the occurrence of stimulated Brillouin scattering. it can. At this time, the refractive index distribution in the optical fiber cross section standardized by the maximum refractive index of the core is kept constant.

That is, taking a step type optical fiber as an example, the relative refractive index difference Δ (= (n ₁ −
By keeping n ₂ ) / n ₁ ) constant, the field distribution of the guided mode is kept constant, and it is possible to obtain transmission characteristics equivalent to those of an ordinary optical fiber.

Further, in order to manufacture the optical fiber of the present invention, a manufacturing method for changing the concentration of the dopant in the longitudinal direction is required. FIG. 8 shows a heating furnace for fluorine addition relating to the manufacturing method of the present invention, and a method for changing the F dopant amount in the longitudinal direction will be described with reference to this drawing. The heating furnace 1 as shown in the figure is provided with a heater portion 2, and the length Lh of this heater is Lh << L with respect to the length Lb of the base material.
It is a zone furnace having a relationship of b. In this zone furnace, fluorine-containing gases SF ₆ and H
Introduce e. Other examples of the fluorine-containing gas that replaces SF ₆ include CF ₄ , C ₂ F ₆ , SiF ₄ , and F ₂ . While rotating the particle body 4 manufactured by the VAD method or the like, the particle body 4 is lowered at a constant speed. The atmosphere of the heating furnace 1 is SF
₆ + He. The final dopant concentration of F is determined by the partial pressure of SF ₆ in this atmosphere. When the particles descend and reach the heater 2, the heater 2 raises the temperature to 14
When the temperature is raised to 00 degrees or more, the amount of F in the particles is fixed and vitrified. At this time, the amount of F fixed on the particles can be controlled by adjusting the partial pressure of F in the heating furnace 1. In this case, when the heater portion is larger than the length of the particle body, the amount of F becomes uniform in the longitudinal direction of the particle body, so that Lh << Lb shown above is particularly necessary. By doing so, it becomes possible to fabricate optical fiber preforms having different F doping amounts in the longitudinal direction. As a result, when this optical fiber preform is drawn into an optical fiber, a high input optical fiber can be realized.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

(Embodiment 1) FIG. 1 shows changes in the refractive index of the core and cladding of an optical fiber according to the first embodiment of the present invention in the axial direction of the optical fiber. In this embodiment, the refractive index distribution is shown. Has a step shape, and GeO ₂ is used as a dopant in both the core and the clad.

At one end (z = 0) of the optical fiber of this embodiment, the cladding has the same structure as a normal optical fiber in which the cladding contains no dopant.

1, 3, 4, and 5, Le is an effective length for generating Brillouin scattering, and when the optical fiber length is sufficiently long, it is approximately 4.3 / α (α: loss (dB / k
m)) is given in the above paper by Cotter. Optical fiber loss is 0.21 dB
When / km, the effective length Le is 20.5 km. Length L
In the section e, Brillouin scattering is likely to occur due to the interaction between the sound wave and the light wave. Therefore, it is possible to increase the threshold value for the occurrence of Brillouin scattering by changing the dopant concentration in the optical fiber in this section. FIG. 2 shows the refractive index distribution in the cross section of the optical fiber of this example at an arbitrary length z. The core diameter is 10 regardless of the axial position.
μm, the clad diameter is constant at 125 μm, and FIG.
As shown in, the relative refractive index difference Δ (= (n ₁
-N ₂ ) / n ₁ ; n ₁ : refractive index of core, n ₂ : refractive index of clad) are also set to a constant value of 0.3%.

FIG. 4 shows a dopant concentration distribution in the axial direction of the optical fiber for realizing such an optical fiber structure. When GeO ₂ is used as a dopant, 1 mo
The relative refractive index to SiO ₂ by l% of GeO ₂ is increased by about 0.1%.

In the optical fiber of FIG. 3, the amount of dopant in the core is 3 mol% at z = 0 and 4.5 at z = 20.5 km.
The amount of dopant in the cladding is 0 mol% at z = 0 and 1.5 mol% at z = 20.5 km. The amount of these dopants is continuously changed so that the refractive index difference Δ between the core and the cladding at an arbitrary position z is maintained at 0.3%.

In this embodiment, the threshold of optical input is set to be 10 times higher than that of a normal optical fiber, and it is designed as follows. The frequency shift amount of stimulated Brillouin scattered light changes in proportion to the amount of dopant added to the core. Shibata, Y .; Azuma, T .; Ho
riguchi, M .; Tateda et al., “Bri
llouin-gain spectrum fors
single-modefibers having v
unusual core / cladding mater
ial compositions "(ECOC'8
8). It has been reported by Cotter that the stimulated Brillouin gain width of the quartz material is about 16 MHz at an optical wavelength of 1.55 μm.

Therefore, the input threshold level is set to 10d.
To increase B, the amount of induced Brillouin frequency shift is 1
It suffices if the frequency changes by 60 (= 16 × 10) MHz. Ge
The frequency shift amount due to O ₂ is 107 MHz / mol%. Reported in a paper by Shibata et al.

Therefore, about 1.5 mol% of GeO ₂ may be changed in the axial direction of the optical fiber over the effective length Le. At this time, since the amount of GeO _{2 to be} doped is small, the increase in loss is about 0.01 dB / km as compared with the conventional optical fiber having a loss of 0.20 dB / km, and the loss of the optical fiber is 0.21 dB / km. It becomes a degree. Therefore,
For incident light increase of 10 dB, 48 (= 10 / 0.2
1) The optical fiber of this embodiment having a length of km can be connected to a conventional optical fiber.

At this time, at the connection point with the conventional optical fiber 48 km away from the incident end, the light intensity is sufficiently attenuated to the optical input threshold of the conventional optical fiber, so that stimulated Brillouin scattering does not occur. ..

As described above, the optical fiber 48 of the present invention
By connecting km to a conventional optical fiber, light (17 dB) 10 times stronger than the input threshold (7 dBm) of a normal optical fiber is used.
When m) is incident from the side of the optical fiber of the present invention, the light intensity at the connection point between the optical fiber of the present invention and the conventional optical fiber is exactly the optical input threshold value of the conventional optical fiber.
The length of the optical fiber capable of communication can be extended by 48 km.

It is obvious that the same effect can be obtained by using FIG. 5 as a dopant distribution other than that described in FIG. Further, according to the completely same principle, generally, within the effective length Le, the Brillouin scattering frequency shift amount is a Brillouin scattering gain width (16 in the above example) in an ordinary optical fiber.
If the dopant amount changes in the direction of the optical fiber axis so as to be G times (MHz) or more (10 times in the above example), even if the refractive index distribution shape in the cross section is an arbitrary shape other than the step shape, It can be seen that the effect of increasing the incident light amount threshold value by G times is obtained.

(Embodiment 2) A case where F is used as a dopant for controlling a waveguide structure will be described as a second embodiment with reference to FIGS. 6 and 7 show the structure of the optical fiber according to the present embodiment (change in dopant concentration in the optical fiber axial direction).

When SiO ₂ is doped with F, the refractive index with respect to SiO ₂ is reduced by 0.4% per 1 wt% of F. Using this, a waveguide structure is realized by adding more F to the clad than to the core. As in the first embodiment, the core type has a diameter of 10 μm, the cladding diameter is 125 μm,
The relative refractive index difference Δ was set to 0.3%.

On the other hand, when F is added as a dopant to the core of the optical fiber, 356 MH per 1 wt% of F doping amount.
It is known that the amount of z-induced Brillouin scattering frequency shift varies (N. Shibata et al., ECOC'8.
8, 115-118). In accordance with the same design principle as in the first embodiment, the input level is changed by changing the Brillouin frequency shift amount by 30 times the gain width, that is, 480 MHz.
In order to increase 8 dB, about 1.35 wt% of F may be changed in the optical fiber axial direction. At this time, the relative refractive index Δ of the core / clad is kept at 0.3%, and F in the optical fiber axial direction is kept.
The amount of change in the doping amount is set to be the same in the core and the clad.

As described above, the loss of the pure silica core optical fiber in which only the cladding is doped with F has a loss of 0.18d.
B / km, but F is 1.3 at maximum in the longitudinal direction.
When doped at 5 wt%, the average loss becomes about 0.21 dB / km due to an increase in Rayleigh scattering. Therefore, 70
(= 14.8 / 0.21) km of the optical fiber of the present embodiment is connected to the conventional optical fiber, and if 21.8 dBm of light is incident from the end face of the optical fiber of the present embodiment, the optical fiber of the present embodiment is The light intensity at the point of incidence on the conventional optical fiber is
The optical input limit of the conventional optical fiber is just 7 dBm.
After all, the length of the optical fiber capable of communication is extended by 70 km.

In both the first and second embodiments described above, the refractive index structure at one end of the optical fiber of the present invention can be matched with that of a normal optical fiber. There is no adverse effect such as reflection at the connection point with the fiber. Further, since it does not include mechanical strain, it is possible to provide a highly reliable long-distance optical transmission line, unlike the conventional strain-doped optical fiber.

Next, specific combinations of dopants of the high-input optical fiber according to the present invention will be listed below, including the cases of the above-mentioned embodiments.

(1) When the core and the clad are made of GeO ₂ -doped quartz.

(2) An optical fiber having a core and a clad made of GeO ₂ -doped quartz, and the dopant for doping the core and the clad is F.

(3) In the case where the core is an optical fiber having GeO ₂ -doped quartz and the cladding is made of pure quartz, and the dopant for doping the core and the cladding is F.

(4) The core and the clad are optical fibers made of F-doped quartz, and the dopant for doping the core and the clad is F.

(5) An optical fiber having a core made of pure quartz and a clad made of F-doped quartz, and the dopant for doping the core and the clad is F.

As described above, in the embodiment, GeO is used as the dopant.
_Although the case of using ₂ and F has been described, Al ₂ O ₃ and P are used.
_The same effect can be obtained by using ₂ O ₅ or the like as a dopant. Further, the effects described here can be obtained in the same manner even for an optical fiber having a special structure such as a dispersion shift optical fiber or a polarization maintaining optical fiber.

(Embodiment 3) An embodiment relating to a method of manufacturing a high input optical fiber will be described. Heat treatment and transparent vitrification were carried out using a particle body in which the core portion synthesized by the VAD method was GeO ₂ -doped quartz and the cladding portion was quartz. Transparent vitrification was performed in a F atmosphere using a zone furnace. These particles were gradually moved in the transparent vitrification zone furnace shown in FIG. 8 with a constant descending speed, and SF ₆ and He were used as introduced gases. When the particles come to the heater portion 2 of the zone furnace, the concentration of F doped in the particles is fixed by the concentration of F in the furnace and the vitrification becomes transparent. In this case, the concentration of F doped in the particle body is changed by changing the amount of gas to be introduced depending on the location of the particle body.

FIG. 9 shows an example of the refractive index distribution in the longitudinal direction of the high input optical fiber preform of the present invention. The length of the prototyped optical fiber is about 29 km. From the figure, F in the longitudinal direction
The change in the dopant is about 0.2% in the relative refractive index difference. Further, the relative refractive index difference between the core and the clad in the longitudinal direction is unchanged. When SiO ₂ is doped with F, the refractive index with respect to SiO ₂ is reduced by 0.4% per 1 wt% of F. Therefore, in this case, there is a change of about 0.5 wt% in the F dopant amount between the beginning and the end of the optical fiber. The transmission loss of the optical fiber of this embodiment is 0.43d at 1.3 and 1.55 μm, respectively.
B / km and 0.23 dB / km, which are almost the same as the loss of a normal optical fiber.

On the other hand, when F is added as a dopant to the core of the optical fiber, 356 MH per 1 wt% of F doping amount.
z-induced Brillouin scattering frequency shift amount is wavelength 1.55μ
It has been shown to change in m. This corresponds to 424 MHz at a wavelength of 1.3 μm.

FIG. 10 shows the measurement results of the transmission power and the scattering power with respect to the input power at 1.3 μm in the optical fiber shown in FIG. 9 and the conventional optical fiber. From the figure, the input power level at which the scattered light due to SBS starts to increase rapidly is about 13 dBm. In a conventional optical fiber, it is about 7 dBm. Therefore, the optical fiber of this embodiment has an input level of about 6 dB as compared with the conventional optical fiber.
I was able to improve.

As described above, the threshold value of the incident light amount can be improved by changing the F dopant amount in the longitudinal direction of the optical fiber.

(Embodiment 4) FIG. 11 is an embodiment in which the present invention is applied to a dispersion-shifted optical fiber having a three-layer structure. When manufacturing the optical fiber of this embodiment, first, Ge is used.
A silica glass particle body composed of a center core 5 containing only O ₂ as a dopant, a side core 6 and a cladding 7 containing no dopant is synthesized by a vapor phase reaction method. Thereafter, sintering is performed while changing the F concentration in the atmosphere of the heating furnace 1 in FIG.
Vitrify. FIG. 11 shows the refractive index distribution of the glass base material thus obtained. The solid line and the alternate long and short dash line represent the refractive index distribution at both ends of the base material. The broken line represents the refractive index distribution in the center of the base material. When the change in the refractive index due to the change in the F doping amount in the longitudinal direction is 0.7%, the Brillouin frequency shift is about 300 MHz at 1.3 μm. At this time, the SBS suppression effect is about 8 dB because the Brillouin gain width of the optical fiber is usually 50 MHz. As described above, the refractive index distribution of the optical fiber of the present invention is not limited to the step shape, and can be applied to any refractive index distribution.

[0046]

As described above, according to the present invention, the property that the induced Brillouin frequency shift amount changes depending on the dopant concentration added to SiO ₂ glass for forming the waveguide structure is utilized. , While keeping the relative refractive index distribution standardized by the maximum refractive index of the core constant in the optical fiber axial direction, by changing the dopant concentration in the optical fiber axial direction, the threshold value for stimulated Brillouin scattering is increased, The light that can be incident on the optical fiber can be strengthened. As a result, the transmission distance by the optical fiber can be greatly extended.

Since no distortion is applied to the optical fiber of the present invention, it is possible to provide a long-distance transmission line having high long-term reliability.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing an example of refractive indices of a core and a clad in an axial direction z of an optical fiber according to the present invention.

FIG. 2 is a characteristic diagram showing an example of a refractive index distribution in a cross section of an optical fiber according to the present invention.

FIG. 3 is a characteristic diagram showing an example of an axial change of the relative refractive index difference Δ of the core / clad according to the present invention.

FIG. 4 is a characteristic diagram showing an example of an axial change in the amount of dopant added to the core / clad according to the present invention.

FIG. 5 is a characteristic diagram showing a dopant distribution of an example different from FIG. 4 according to the present invention.

FIG. 6 is a characteristic diagram showing an example of an axial dopant amount when F is used as a dopant according to the present invention.

FIG. 7 is a characteristic diagram showing an example of an axial dopant amount when F is used as a dopant according to the present invention.

FIG. 8 is a configuration diagram showing an example of a fluorine-adding heating furnace in the method for producing a high-input optical fiber according to the present invention.

FIG. 9 is a characteristic diagram showing an example of a refractive index distribution in the longitudinal direction of the optical fiber preform according to the present invention.

FIG. 10 is a characteristic diagram showing an example of measurement results of transmitted light and backscattered light of an optical fiber according to the present invention and a conventional optical fiber.

FIG. 11 is a characteristic diagram showing a refractive index distribution in the longitudinal direction of a second embodiment in which the present invention is applied to a dispersion shift optical fiber.

FIG. 12 is a structural diagram showing a conventional Brillouin scattering suppression optical fiber cable having a double spiral structure.

FIG. 13 is a characteristic diagram showing a change in residual tension in the axial direction of a Brillouin scattering suppression optical fiber that utilizes residual strain during drawing of a conventional optical fiber.

[Explanation of symbols]

1 ... Heating furnace, 2 ... Heater part, 3 ... Inlet gas supply port, 4
... Particles, 5 ... Center core, 6 ... Side core, 7 ... Clad.

Front Page Continuation (72) Inventor Katsusuke Tajima 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (8)

[Claims] 1. An optical fiber comprising a core and a clad surrounding the core, wherein the dopant concentration of the core and the clad changes in the axial direction of the optical fiber, and the relative relative to the maximum refractive index of the core is standardized. A high-input optical fiber characterized in that the refractive index distribution is always constant in a cross section at an arbitrary position in the optical fiber axial direction. 2. The high-input optical fiber according to claim 1, wherein the dopant doped into the core and the clad is Ge.
A high-input optical fiber comprising one or a combination of O ₂ , P ₂ O ₅ , Al ₂ O ₃ and F, or a combination thereof. 3. The high input optical fiber according to claim 1, wherein the core and the clad are made of GeO ₂ -doped quartz. 4. The high-input optical fiber according to claim 1, wherein the core and the clad are made of GeO ₂ -doped quartz, and the dopant doped into the core and the clad is F. Input optical fiber. 5. The high-input optical fiber according to claim 1, wherein the core is an optical fiber made of GeO ₂ -doped quartz and the cladding is made of pure quartz, and the dopant for doping the core and the cladding is F. And high input optical fiber. 6. The high-input optical fiber according to claim 1, wherein the core and the clad are made of F-doped quartz, and the dopant doped in the core and the clad is F. Optical fiber. 7. The high-input optical fiber according to claim 1, wherein the core is pure silica and the clad is F-doped silica, and the dopant doped into the core and the clad is F. High input optical fiber. 8. A quartz glass particle body synthesized by a gas phase reaction or a GeO ₂ -doped quartz glass particle body is subjected to a heat treatment for dehydration and / or fluorine addition in a heating furnace, and then made into a transparent glass to form an optical fiber. In the method for manufacturing a base material, Ge in the cross section in the quartz glass particle body is used.
The O ₂ dopant distribution is uniform in the longitudinal direction, a zone furnace is used as a heating furnace, and in the heat treatment step for adding fluorine, it is transparent while changing the concentration of fluorine added depending on the position in the longitudinal direction at which the glass particles are vitrified. A method for producing a high-input optical fiber preform characterized by vitrification.