JP2006292843A - Manufacturing method of optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber capable of raising a threshold value of SBS at a low cost, and of making high energy signal light incident thereon, and to provide a manufacturing method therefor. <P>SOLUTION: The optical fiber of this invention is a single mode optical fiber having an operating wavelength band in 1.3 to 1.6 μm and is characterized by having a variation in which a radial distribution of a relative refractive-index difference in the core part of the optical fiber varies ≥±10% to an average value of the whole core part, and is further characterized in that the radial distribution form of the relative refractive-index difference varies in the longitudinal direction, and the manufacturing method therefor is characterized in that a flow rate of a dopant raw material for raising the refractive index of the core is made to vary at intervals of tens of seconds to a few minutes when manufacturing a core rod for preforming an optical fiber by supplying a glass raw material and the dopant raw material to a burner by a VAD method and depositing glass particles formed in the flame. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a method of manufacturing an optical fiber used in an optical communication application in which signal light having relatively high light energy is input.

  In an application such as a PON system, a signal is sent to the vicinity of a subscriber through a single optical fiber, and branched according to the number of subscribers. To this end, the farther the branch point is from the station and the greater the number of branches, the higher the light energy that needs to be input on the transmitting station side.

In recent years, with the development of amplifiers such as EDFA, it has become possible to input such high optical energy. Generally, when such high energy signal light is input to an optical fiber, stimulated Brillouin scattering (SBS) ) Occurs, the optical energy actually transmitted is limited. The threshold value [mW] at which SBS occurs is proportional to the effective area (A _eff ) of the optical fiber, as described in Non-Patent Document 1, for example, and the Brillouin gain coefficient (gB) and the effective interaction length ( L _eff ).

  SBS is caused by a periodic density distribution formed in an optical fiber by acoustic phonons, which acts as a grating and scatters signal light. Since the acoustic phonon is also moving, the scattered light has a slightly lower frequency than the signal light due to the Doppler effect. The acoustic phonon is further excited by the interference between the scattered light and the signal light, and the intensity of the scattering is increased.

  SBS has a threshold, and the signal light intensity is hardly affected by scattering below this threshold. However, when the threshold is exceeded, the scattering intensity suddenly increases. Above a certain level, the intensity of the input signal light is increased. However, only the scattered light increases, and the transmitted signal intensity does not increase. In addition, there is an adverse effect that noise increases and the signal deteriorates.

Therefore, various methods have been proposed to increase the SBS threshold.
It is known that the spectral shape of scattered light varies depending on the material composition and strain of the optical fiber, and the broader the spectrum, the higher the SBS threshold.
For example, Patent Document 1 discloses a method of increasing the SBS threshold by changing the core diameter, refractive index, and strain in the longitudinal direction of the optical fiber.
In Patent Document 2, when a preform for an optical fiber is manufactured by a two-step method, in a step of manufacturing a part of a core part and a cladding part, for example, by changing a dopant concentration such as fluorine in the longitudinal direction. A method for increasing the SBS threshold is disclosed.
Patent Document 3 also discloses a method of changing the dopant concentration in the longitudinal direction. Further, Patent Document 4 discloses a method of simultaneously changing the core diameter and the relative refractive index difference in the longitudinal direction.
The above is a method for increasing the SBS threshold by changing the longitudinal direction of the optical fiber.

In addition, Patent Document 5 is different in thermal expansion coefficient and viscosity doped with a dopant, adjusted so as not to affect the transmission characteristics determined by the refractive index distribution in the vicinity of the boundary between the core part and the clad part. Disclosed is a method of forming a strain distribution in the radial direction of an optical fiber by providing a plurality of uniform thin annular regions in the longitudinal direction, thereby suppressing SBS.
Patent Document 6 discloses a method of suppressing SBS by making the distribution of acoustic modes different from normal by increasing the refractive index of the core portion, particularly the central portion.

On the other hand, there is a VAD method as a method for manufacturing an optical fiber preform. This is one of the widely used technologies. A soot deposit is formed on the tip of a starting member that is pulled up while rotating, and the soot deposit is dehydrated and vitrified in an electric furnace. This is a method for obtaining a material.
In general, an optical fiber preform is manufactured by first forming a part of a core part and a clad part, and further applying an additional clad part by another means. For example, a dopant such as GeO ₂ that increases the refractive index is added to the core, and a refractive index distribution is formed by the distribution of the dopant.

Soot deposited by the VAD method is fine SiO ₂ particles produced by introducing SiCl ₄ as a glass material into a burner that reacts oxygen and hydrogen to form a flame. When GeCl ₄ is added to the raw material SiCl ₄ , GeO ₂ is added into the SiO ₂ fine particles according to the concentration.
At present, the refractive index distribution of a widely used single mode optical fiber is substantially rectangular in the radial direction, and the refractive index difference between the clad part and the core part is about 0.3 to 0.4%. The optical fiber diameter is 125 μm, and the core diameter is slightly over 8 μm. Such an optical fiber is used in various systems, and its operating wavelength ranges from 1.3 to 1.6 μm.

‘Optical Fiber Telecommunications IIIA’, Academic Press, p.200 Patent No.2584151 Patent No. 2753426 Japanese Patent Laid-Open No. 9-301738 JP-A-10-96828 U.S. Pat.No. 6,542,683 US Published Patent US2004 / 0218882A1 Patent No. 3580640

When the characteristic parameter of the optical fiber is changed in the longitudinal direction, a remarkable effect can be obtained by a change in a short span of about 1 km or less. However, the drawing speed of the optical fiber is 1 km / min or higher, and with a large preform, the length of the optical fiber preform is about 1.5 to 5 mm to draw an optical fiber of 1 km. Therefore, it is difficult to change in such a short span in a low-cost manufacturing method using a large-sized preform for manufacturing an optical fiber.
Therefore, for example, Patent Document 7 dares to improve characteristics by manufacturing a small preform.

In addition, as in Patent Document 5, the method of forming a thin annular region with a plurality of dopants in the radial direction of the optical fiber is extremely difficult to manufacture because the dopant diffuses in the radial direction during the manufacturing. Further, such an optical fiber has a loss larger than that of a normal optical fiber, and as a result, the effective interaction length L _eff becomes small. Therefore, it is difficult to increase a desired effect. In addition, there is a problem that light energy to be input must be further increased.

An object of the present invention is to provide an optical fiber manufacturing method that can increase the threshold value of SBS at low cost and enables high-energy signal light to be input.

The optical fiber manufacturing method of the present invention is a method of supplying a core material of an optical fiber preform by supplying a glass raw material and a dopant raw material to a burner and depositing glass fine particles generated in a flame by a VAD method. It is characterized in that the flow rate of the dopant raw material for increasing the refractive index is changed at intervals of several tens of seconds to several minutes. The dopant material is GeCl _Four Is preferably used. The flow rate of the dopant material is periodically changed, and the amount of change is 30% or more of the average value, preferably 50% or more.

  According to the manufacturing method of the present invention, the radial distribution of the relative refractive index difference in the core portion of the optical fiber has a variation value that varies ± 10% or more with respect to the average value of the entire core portion, and A single mode optical fiber having an operating wavelength band in the range of 1.3 to 1.6 μm, in which the radial distribution shape of the relative refractive index difference periodically changes in the longitudinal direction, is obtained.

  According to the present invention, a single mode optical fiber having an operating wavelength band of 1.3 to 1.6 μm and a high SBS threshold can be manufactured at low cost, and high energy signal light can be input.

  The present inventors pay attention to the fact that the tip of the core on which the glass fine particles generated by the flame hydrolysis reaction of the burner are deposited by the VAD method has a curved surface as shown in FIG. By changing the dopant concentration in accordance with the growth, the distribution of the relative refractive index difference of the core portion can be changed both in the radial direction and in the longitudinal direction. Above the burner 2, a cladding deposition burner (not shown) is provided, and a cladding part 3 is formed. As a result, it was found that both changes work synergistically and the SBS threshold is effectively increased, thereby achieving the present invention.

That is, the manufacturing method of the present invention is to change the flow rate of the dopant raw material for increasing the refractive index of the core at intervals of several tens of seconds to several minutes when the core rod is manufactured by the VAD method.
Preferably, GeCl ₄ is used as the dopant material, and the flow rate is 30% or more of the average flow rate, preferably 50% or more, for example, by periodically changing the deposition. Note that if the amount of change in the flow rate of the dopant material is less than 30%, the increase in the SBS threshold is small and not effective.
According to this method, it is possible to easily cope with the production of a large-size preform, which was difficult with the past method.

  As a result, the radial distribution of the relative refractive index difference in the core portion of the optical fiber has a variation value that varies by ± 10% or more with respect to the average value of the entire core portion, and the radial direction of the relative refractive index difference The single-mode optical fiber having an operating wavelength band of 1.3 to 1.6 μm is obtained.

The flow rate of the dopant GeCl _{4 to be} added was periodically changed by a VAD method using an apparatus for manufacturing a core rod having a normal rectangular refractive index difference distribution. The raw material supply conditions (steady conditions) to the core deposition burner before the change were SiCl ₄ 300 sccm and GeCl ₄ 16 sccm. On the other hand, the flow rate was changed by changing only the flow rate of GeCl ₄ , and after flowing 8 sccm for 1 minute, flowing 24 sccm for 1 minute, and again flowing 8 sccm for 1 minute, the GeCl ₄ flow rate was changed periodically. .

The thickness of the core deposited per minute was about 2 km in terms of the length of the optical fiber finally obtained by drawing it.
As schematically shown in FIG. 1, in the VAD method, the tip of the core deposition part is a curved surface. Therefore, when the dopant concentration is finely changed, the distribution shape of the relative refractive index difference of the core portion in each cross section of the obtained optical fiber is not substantially rectangular like the conventional profile shown in FIG. 3 and 4, both the high refractive index portion and the low refractive index portion appear in the core, and the profile changes in the longitudinal direction. 3 and 4 are profiles of the relative refractive index difference of the core measured at different positions in the longitudinal direction of the same fiber.

  In the profile of the relative refractive index difference of the core part in FIGS. 3 and 4, the fluctuation value (height difference) of the relative refractive index difference was about ± 15% with respect to the average value of the entire core part. Despite changing the dopant material at ± 50% under steady conditions, the variation in the relative refractive index difference remained at about ± 15% because of the heat treatment during the dehydration and vitrification process in the electric furnace. This is probably because the dopant diffused.

  The core rod thus obtained was further provided with a necessary amount of clad to form a preform, and then drawn in a drawing furnace to obtain an optical fiber having a diameter of 125 μm. When the SBS characteristic of this optical fiber 30 km was examined, the threshold value at a wavelength of 1510 nm was 8.5 dBm, which is 2 dB higher than 6.5 dBm of the normal product.

Such an effect is prominent when the change in the flow rate of GeCl ₄ is set to ± 30% or more, and a change in the relative refractive index difference of ± 10% or more is observed. The effect was great when the time was from several tens of seconds to several minutes. If this time interval is shorter than 10 seconds, a large change in relative refractive index difference is not observed due to dopant diffusion. Conversely, if the time interval exceeds 10 minutes and becomes too long, the period of change in the refractive index distribution when it is made into a fiber is too long, so that a sufficient effect cannot be obtained.

  Single mode optical fiber with a high SBS threshold can be supplied at low cost.

It is the schematic which shows typically the deposition state of a core part by VAD method, and distribution of dopant concentration. It is the profile of the conventional optical fiber which shows the distribution shape of the relative refractive index difference of a core part. It is the profile of the optical fiber of this invention which shows the distribution shape of the relative refractive index difference of a core part. It is another example which shows the profile of the core part of this invention.

Explanation of symbols

1 ...... Core part,
2 …… Burner,
3 …… Clad part.

Claims (7)

A single-mode optical fiber having an operating wavelength band of 1.3 to 1.6 μm, and the radial distribution of the relative refractive index difference in the core portion of the optical fiber varies by ± 10% or more with respect to the average value of the entire core portion. An optical fiber characterized in that the radial distribution shape of the relative refractive index difference varies in the longitudinal direction. The optical fiber according to claim 1, wherein the radial distribution shape of the relative refractive index difference periodically changes in the longitudinal direction. When the core rod of an optical fiber preform is manufactured by supplying glass raw material and dopant raw material to the burner by the VAD method and depositing glass fine particles generated in the flame, the flow rate of the dopant raw material for increasing the refractive index of the core is set. A method of manufacturing an optical fiber, wherein the optical fiber is changed at intervals of several tens of seconds to several minutes. The method for producing an optical fiber according to claim 3, wherein the dopant raw material is GeCl ₄ . The manufacturing method of the optical fiber of Claim 3 or 4 which changes the flow volume of the said dopant raw material periodically. The method of manufacturing an optical fiber according to any one of claims 3 to 5, wherein a change amount of the flow rate of the dopant material is 30% or more of the average flow rate. The method for manufacturing an optical fiber according to any one of claims 3 to 6, wherein a change amount of the flow rate of the dopant raw material is 50% or more of the average flow rate.

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