JP2006133496A - Optical fiber and method for manufacturing optical fiber preform used for same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber having both satisfactory bend loss and dispersion property and suitable for a line of coarse wavelength division multiplexing (CWDM) transmission and for indoor wiring in Fiber to the Home (FTTH), and to provide a method for manufacturing an optical fiber preform capable of easily manufacturing the optical fiber in a high yield. <P>SOLUTION: The optical fiber has a mode field diameter at 1,310 nm wavelength of 8.2-9.0 μm, a cable cutoff wavelength λcc of ≤1,260 nm, an absolute value of a dispersion value at 1,285 nm wavelength of ≤3.5 ps/nm/km, a zero dispersion wavelength of 1,300-1,320 nm, and a dispersion slope at the zero dispersion wavelength of ≤0.090 ps/nm<SP>2</SP>/km. When the optical fiber is wound 100 times on a cylinder having 30 mm diameter, a loss increase at 1,625 nm wavelength is ≤2 dB. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention mainly relates to an optical fiber suitable for low-density wavelength division multiplexing (CWDM) transmission lines and in-home wiring in fiber-to-the-home (FTTH), and an optical fiber preform used for manufacturing the same. It is related with the manufacturing method.

  With the rapid spread of the Internet in recent years, the amount of communication information has increased dramatically, and the expansion of optical fiber line networks has been progressing. As an example, the construction of regional information networks by local governments is proceeding at a rapid pace, and these network constructions require system designs that can cope with future increases in transmission capacity. In particular, for optical fibers, there is a growing need to reduce the allowable bending radius in order to reduce bending loss in the long wavelength range considering adaptation to CWDM transmission and to increase the work efficiency of home wiring in FTTH. ing.

However, if the bending loss is reduced while maintaining the simple step type that is the refractive index profile of the conventional single mode optical fiber (SMF),
1) The mode field diameter (MFD) is reduced, and the connection loss with the conventional SMF is increased.
2) The cut-off wavelength shifts to the long wavelength side, and single mode transmission at a desired wavelength becomes impossible.
3) Dispersion characteristics deteriorate and signal waveform deterioration increases.
Problems arise.
As a means for solving these problems, a method is known in which the refractive index profile of an optical fiber is slightly changed from the conventional simple step type, and a region having a refractive index lower than that of the cladding is provided on the outer peripheral portion of the core.

As an optical fiber using this technique, a light having a low refractive index portion at the outer peripheral portion of the core whose refractive index is lower by 1 × 10 ^{−5 to} 3 × 10 ⁻⁴ than the refractive index of the cladding portion Patent Document 1 discloses a method for manufacturing a fiber preform. According to Patent Document 1, by setting such a refractive index profile, all of the zero dispersion wavelength, MFD, and cutoff wavelength can be within a desired numerical range, and an optical fiber having extremely excellent dispersion characteristics. Can be realized.
JP 2002-047027 A

However, an optical fiber used for CWDM transmission and FTTH is required to have an allowable bending radius that is two or more times smaller than that of a general optical fiber. The refractive index profile specified in Patent Document 1 satisfies this requirement. There is a problem that can not be. More specifically, when an optical fiber with a small bending loss is maintained while maintaining the refractive index profile defined in Patent Document 1, the difference in refractive index between the low refractive index portion and the cladding portion is 1 × 10 ^−5. Since it is as small as ˜3 × 10 ⁻⁴ , there is a problem that the shift of the zero dispersion wavelength toward the long wavelength side becomes obvious. Furthermore, if a / b, which is the diameter ratio between the core having the outer diameter a and the low refractive index portion having the outer diameter b, is too small, there is a problem that the dispersion slope at the zero dispersion wavelength increases. As a result, when performing CWDM transmission in a wide wavelength range, the refractive index profile defined in Patent Document 1 increases the dispersion value at the desired wavelength and increases the signal waveform deterioration. It is impossible to achieve both characteristics.

The present invention has been made to solve the above problems, and the optical fiber according to claim 1 of the present invention is located in the center, has a relative refractive index difference of Δ1 with respect to the cladding, and has a diameter of a. The first core, the outer periphery of the first core, the relative refractive index difference with respect to the clad is Δ2, the diameter is b, the outer periphery of the second core is covered, and the relative refractive index with respect to pure quartz In an optical fiber composed of a clad having a difference of ΔC, the relative refractive index difference and the diameter ratio in each region are
0.4% ≦ Δ1 ≦ 0.5%
-0.08% <Δ2 <-0.02%
0% <ΔC ≦ 0.05%
4.7 ≦ b / a ≦ 4.9
The filling,
The mode field diameter at a wavelength of 1310 nm is 8.2 μm or more and 9.0 μm or less, the cable cutoff wavelength λcc is 1260 nm or less, the absolute value of the dispersion value at a wavelength of 1285 nm is 3.5 ps / nm / km or less, and zero The dispersion wavelength is 1300 nm to 1320 nm, the dispersion slope at zero dispersion wavelength is 0.090 ps / nm ² / km or less, and the increase in loss at a wavelength of 1625 nm when an optical fiber is wound 100 times on a cylinder with a diameter of 30 mm is 2 dB. It is characterized by the following.

According to the optical fiber according to claim 1 of the present invention thus configured, the connection loss with the conventional SMF can be reduced by setting the MFD to 8.2 μm or more and 9.0 μm or less.
Further, by setting the cable cutoff wavelength λcc to 1260 nm or less, single mode operation is ensured in a wavelength region longer than 1260 nm. That is, the present invention can be applied to CWDM in a wavelength region longer than 1260 nm.
In addition, the absolute value of the dispersion value at a wavelength of 1285 nm is 3.5 ps / nm / km or less, the zero dispersion wavelength is 1300 nm to 1320 nm, and the dispersion slope at the zero dispersion wavelength is 0.090 ps / nm ² / km or less. The dispersion value at a desired wavelength in the wavelength range can be suppressed to be small, and deterioration of the signal waveform can be suppressed in a wide wavelength range.

Furthermore, although it is known that the bending loss of an optical fiber increases as the wavelength increases, the optical fiber of the present invention is wound 100 times around a cylinder having a diameter of 30 mm at a wavelength of 1625 nm, which is assumed as the longest wavelength of CWDM transmission. By making the loss increase at this time 2 dB or less, it is ensured that the bending loss is small in the entire wavelength range of CWDM transmission.
Further, by reducing the bending loss in this way, an optical fiber suitable as an optical fiber for FTTH in-home wiring can be obtained.

  The method for producing an optical fiber preform according to claim 2 of the present invention is a method for producing an optical fiber preform used for producing the optical fiber according to claim 1, wherein the core soot is synthesized by the VAD method. A first baking burner for flowing a combustion gas mainly composed of oxygen and hydrogen gas is disposed adjacent to a first core burner for synthesizing a soot serving as a first core located in the center, and the first baking burner. A second core burner for synthesizing soot as a second core is disposed adjacent to the second core burner, and a second baking burner for flowing a combustion gas mainly composed of oxygen and hydrogen gas is disposed adjacent to the second core burner. The core soot is synthesized.

  According to the optical fiber preform manufacturing method of the present invention as described above, the density of the soot serving as the first core is increased by providing the baking burner adjacent to the first core burner. As a result, generation of cracks can be prevented and soot formation rate can be improved. In addition, by providing a burned burner adjacent to the second core burner, the density of the soot that becomes the second core is also increased, and not only is cracking suppressed, but it is also possible to prevent the soot outer diameter from becoming excessively large. As a result, the core soot formation rate is improved.

The method for synthesizing an optical fiber soot according to claim 3 of the present invention is characterized in that in the method for manufacturing an optical fiber preform according to claim 2, a gas containing fluorine is allowed to flow through the second core burner. .
According to the method for manufacturing an optical fiber preform of the present invention, the fluorine is added to the second core at the soot stage by flowing a gas containing fluorine to the second core burner. Therefore, the refractive index can be controlled minutely, and the controllability of the profile is improved.
As a result, the optical fiber preform can be manufactured with a high yield without significantly changing the conventional SMF manufacturing method.

  As described above, according to the present invention, an optical fiber suitable for CWDM transmission lines and in-home wiring in FTTH, and the optical fiber can be easily manufactured at a high yield. The manufacturing method of the optical fiber preform which can be provided can be provided.

Embodiments of the present invention will be described below. FIG. 1 is a schematic diagram of a refractive index profile in an optical fiber of the present invention.
The optical fiber of the present invention is located in the center, has a relative refractive index difference of Δ1 with respect to the clad 30, and has a diameter a, covers the outer periphery of the first core 10, and has a ratio with respect to the clad 30. The second core 20 having a refractive index difference of Δ2 and a diameter of b is further covered with the outer periphery of the second core 20 and a clad 30 having a relative refractive index difference of ΔC with respect to pure quartz.

Further, the relative refractive index difference and the diameter ratio in each region satisfy the following expressions.
0.4% ≦ Δ1 ≦ 0.5%
-0.08% <Δ2 <-0.02%
0% <ΔC ≦ 0.05%
4.7 ≦ b / a ≦ 4.9
Here, the relative refractive index differences Δ1, Δ2, and ΔC used in the present invention are defined by the following equations (1) to (3).
Δ1 = [(n _C1 −n _C ) / n _C1 ] × 100% (1)
Δ2 = [(n _C2 −n _C ) / n _C2 ] × 100% (2)
_{ΔC = [(n C -n g} ) / n C] × 100% (3)
Where n _C1 is the maximum refractive index of the first core, n _C2 is the minimum refractive index of the second core, n _C is the refractive index of the cladding, and _ng is the refractive index of pure silica glass.

In the present invention, the diameter of each part of the optical fiber is defined as follows. In FIG. 1, the diameter a of the first core 10 is a diameter at a position having a relative refractive index difference of ½ of Δ1 at the boundary between the first core 10 and the second core 20. The diameter b of the second core is a diameter at a position having a relative refractive index difference of ½ of Δ2 at the boundary between the second core and the clad.
Further, in this specification, the cable cutoff wavelength λcc is the ITU-T (International Telecommunication Union) G.I. The cable cutoff wavelength λcc defined by 650.1. For other terms not specifically defined in this specification, see ITU-T G.C. It shall follow the definition and measurement method in 650.1.

  In the optical fiber preform of the present invention, a core soot is manufactured by the VAD method using the apparatus shown in FIG. That is, a first baking burner 4 for flowing a combustion gas mainly composed of oxygen and hydrogen gas is disposed adjacent to the first core burner 3 for synthesizing the soot that becomes the first core 10. Adjacent to the second core burner 5 is a second baking burner in which a combustion gas mainly composed of oxygen and hydrogen gas is flowed. Distribute 6. The soot is deposited by blowing the raw material gas and the combustion gas toward the starting base material 1 with each burner to form the core soot 2.

Further, a gas containing Ge as an additive for increasing the refractive index is supplied to the first core burner 3, and a gas containing fluorine as an additive for reducing the refractive index is supplied to the second core burner 5. preferable. SiF ₄ , SF ₆ , CF ₄ or the like is used as the gas containing fluorine to be added.

Here, the relative refractive index difference Δ1 of the first core 10 with respect to the clad 30 can be adjusted by increasing or decreasing the amount of gas containing Ge added to the first core burner 3, and the second core 20 with respect to the clad 30. The relative refractive index difference Δ2 can be adjusted by increasing or decreasing the amount of fluorine-containing gas added to the second core burner 5. Further, for both Δ1 and Δ2, both Ge and fluorine may be added to adjust the refractive index to an appropriate value.
Further, the ratio b / a between the diameter b of the second core 20 and the diameter a of the first core 10 can be increased or decreased by adjusting the position of each burner or the gas flow rate. Further, b / a is preferably 4.6 or more from the viewpoint of an increase in loss due to OH groups and hydrogen resistance.

Next, the obtained core soot 2 is dehydrated in an electric furnace or the like using a dehydrated gas, for example, a gas containing chlorine, and then transparentized to obtain a core glass rod (hereinafter referred to as a core rod).
Further, the obtained core rod is heated and stretched to a predetermined diameter, and a soot to be the clad 30 is synthesized on the outer periphery of the stretched core rod by the OVD method. An optical fiber preform is obtained by dehydrating this with dehydrating gas and then making it transparent.
At this time, when the soot synthesized by the OVD method is dehydrated and transparentized, the ratio of chlorine gas in the gas atmosphere in the electric furnace is set to 3% to 4%, whereby the relative refraction of the cladding 30 to pure silica glass is made. The rate difference can be 0% <ΔC ≦ 0.05%.
Furthermore, the optical fiber of the present invention can be obtained by drawing the obtained optical fiber preform.

  Table 1 shows the result of examining the refractive index profile of an optical fiber satisfying the characteristics of the present invention by transmission characteristic simulation.

In Table 1, chromatic dispersion is a dispersion value at a wavelength of 1285 nm, MFD is a mode field diameter at a wavelength of 1310 nm, λcc is a cable cutoff wavelength, λ ₀ is a zero dispersion wavelength, S ₀ is a dispersion slope at a zero dispersion wavelength, and bending loss is light. The figure shows the increase in loss at a wavelength of 1625 nm when the fiber is wound 100 times around a cylinder with a diameter of 30 mmφ.

Sample A is a conventional SMF. According to this, it can be seen that the conventional SMF has a large bending loss and cannot be used as an optical fiber for CWDM transmission or a fiber for home wiring of FTTH.
Therefore, starting from the conventional SMF, Δ1 was first increased as shown in Sample B in order to reduce bending loss. As a result, it was found that with the same MFD as in the prior art, the cable cut-off wavelength λcc becomes too large and single-mode transmission in the desired wavelength region is impossible. In response to this, a sample C in which the center value of the MFD was set to 8.6 μm that could be connected to a conventional single mode optical fiber without any problem was examined. As a result, it has been found that bending loss and dispersion (dispersion value and zero dispersion wavelength) cannot be compatible.

Based on the results up to this point, we decided that it was difficult to reduce the bending loss by adjusting the parameters of each refractive index profile while maintaining the conventional simple step type. In addition, a refractive index profile shown in FIG. 1 in which a core having a lower refractive index than that of the clad, that is, a second core is provided was examined.
As a result of examining the sample D having the refractive index profile shown in FIG. 1, it was found that almost desired transmission characteristics can be obtained except for the zero dispersion slope. Therefore, by optimizing the refractive index of the second core 20 and the diameter ratio b / a between the second core 20 and the first core 10, an optical fiber having all the desired values can be obtained (sample). E). It was found that the bending loss increased (allowable bending radius increased) as b / a was decreased (Sample F). Further, the material F in which b / a is too small is not desirable from the viewpoint of increased loss due to OH groups and hydrogen resistance as described above. FIG. 3 shows the relationship between the relative refractive index difference Δ2 of the second core 20 relative to the cladding 30 and the zero dispersion wavelength λ ₀ , and the dispersion slope S _{0 at} the zero dispersion wavelength, obtained by simulation, and the diameters of the second core and the first core. FIG. 4 shows the relationship between the ratio b / a, the zero dispersion wavelength λ ₀ , and the dispersion slope S _{0 at} the zero dispersion wavelength.

From the results of FIGS. 3 and 4, the wavelength when the optical fiber is wound 100 times around a cylinder having a zero dispersion wavelength of 1300 nm to 1320 nm, a dispersion slope at the zero dispersion wavelength of 0.090 ps / nm ² / km or less, and a diameter of 30 mm. In order to satisfy the loss increase at 1625 nm of 2 dB or less, Δ2 is set to −0.08% <Δ2 <−0.02%, and b / a is set to 4.7 ≦ b / a ≦ 4.9. I found it necessary.

  Further, in order to study the allowable range of Δ1, Δ2 is set to −0.08% <Δ2 <−0.02% and b / a is set to a range of 4.7 ≦ b / a ≦ 4.9. Δ1 was changed and a simulation was performed. Table 2 shows the results.

  FIG. 5 shows the relationship between Δ1 obtained from Table 2, the cable cutoff wavelength λcc, and the bending loss. As described above, there is a trade-off relationship between the cable cutoff wavelength λcc and the bending loss, and the allowable range of Δ1 is mainly governed by this relationship. That is, as shown in FIG. 5, when Δ1 is small, the cable cutoff wavelength λcc is small, but the bending loss becomes large. From Table 2 and FIG. 5, it is clear that 0.4% ≦ Δ1 ≦ 0.5% is allowed as the range of Δ1.

Based on the results of this simulation, optical fibers corresponding to Sample E and Sample I were actually manufactured.
First, a core soot was manufactured using the VAD apparatus shown in FIG. At this time, the synthesis was performed using four burners, the raw material gas silicon tetrachloride was added to the first core burner 3 at 0.37 liter / minute, and germanium tetrachloride as an additive for increasing the refractive index was added at 0.02 liter / minute. The glass fine particles produced by the flame hydrolysis reaction were supplied to the second core burner 5 at 1.75 liter / minute of silicon tetrachloride and 0.10 liter / minute of silicon tetrafluoride as an additive to lower the refractive index. Soot) was deposited on the starting matrix. In order to adjust the density of the deposited soot and improve the handleability, the first baking burner 4 adjacent to the first core burner 3 has a hydrogen gas of 9 liters / minute, an oxygen gas of 4 liters / minute, and a second core burner. The second baking burner 6 adjacent to 5 was supplied with hydrogen gas at 3 liters / minute and oxygen gas at 4 liters / minute.

  The obtained core soot was dehydrated and made transparent in an electric furnace to produce a core rod. At this time, the proportion of chlorine gas in the gas atmosphere in the electric furnace is 1.2%, the maximum temperature in the electric furnace is 1215 ° C., dehydration is performed for 5 hours, and then the maximum temperature in the electric furnace is set in the helium atmosphere. It was heat treated at 1540 ° C. for 5 hours to be transparent, and then heated and stretched in an electric furnace to obtain a core rod having an outer diameter of 40 mm and a length of 1400 mm.

Then, the soot which becomes a clad was synthesize | combined by the OVD method on the outer periphery of the extended core rod.
Furthermore, this was dehydrated and made transparent to obtain an optical fiber preform.
When the soot synthesized by the OVD method was dehydrated and made transparent, the ratio of chlorine gas in the gas atmosphere in the electric furnace was set to 3% to 4%. Thereby, the refractive index of the clad 30 could be increased by 0% to 0.05% than that of pure silica glass.

  The optical fiber of the present invention was obtained by drawing the obtained optical fiber preform. Table 3 shows the characteristics of the obtained optical fiber. An optical fiber corresponding to the sample E is referred to as a fiber E, and an optical fiber corresponding to the sample I is referred to as a fiber I.

The transmission characteristics of each optical fiber almost coincide with the results obtained by simulation.
Moreover, when the loss spectrum in wavelength 1260nm-1625nm was measured with respect to the obtained optical fiber, the transmission loss was 0.39 dB / km or less in the whole wavelength range. The transmission loss at 1383 nm was 0.29 dB / km or less. As a representative, the loss spectrum of Document I is shown in FIG.
Further, the obtained optical fiber was treated in a 20% deuterium atmosphere for 4 hours, and then a hydrogen test was performed. The hydrogen test conditions were IEC 60793-2-50 C.I. It shall be in accordance with 3.1 and the conditions shown below.
An optical fiber is exposed to hydrogen in an atmosphere with a hydrogen concentration of 0.01 atm at room temperature, and the transmission loss at a wavelength of 1240 nm is increased by 0.03 dB / km or more compared to the transmission loss before hydrogen exposure (initial value). Keep that state until you do. After that, it is taken out into the atmosphere and left for 14 days or more, and the transmission loss is measured.

  After the hydrogen test, the transmission loss at a wavelength of 1383 nm was measured, and all were 0.29 dB / km or less. That is, transmission loss hardly increased due to hydrogen diffusion. Therefore, the optical fiber of the present invention can maintain a low transmission loss stably for a long time in a wide wavelength band of 1260 nm to 1625 nm including around 1380 nm, and is suitable as an optical fiber suitable for CWDM transmission lines and home wiring in FTTH. Can be used.

It is a schematic diagram which shows an example of the refractive index profile of the optical fiber of this invention. It is the schematic of the manufacturing apparatus (VAD method) of the optical fiber preform | base_material of this invention. The relative refractive index difference Δ2 with zero-dispersion wavelength lambda ₀ with respect to the cladding of the second core, and is a graph showing the relationship between dispersion slope S ₀ at the zero dispersion wavelength. 6 is a graph showing the relationship between the diameter ratio b / a of the second core to the first core, the bending loss, and the dispersion slope S _{0 at} the zero dispersion wavelength. It is the graph which showed the relative refractive index difference (DELTA) 1 with respect to the clad | crud of a 1st core, cable cutoff wavelength (lambda) cc, and the bending loss. It is a graph which shows the spectrum of the monochromatic loss of the optical fiber of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Starting base material 2 Core soot 3 1st core burner 4 1st baking burner 5 2nd core burner 6 2nd baking burner 10 1st core 20 2nd core 30 Clad

Claims (3)

The first core having a relative refractive index difference with respect to the cladding of Δ1 and a diameter of a is located at the center, covers the outer periphery of the first core, has a relative refractive index difference of Δ2 with respect to the cladding, and has a diameter of b And an optical fiber made of a clad covering the outer periphery of the second core and having a relative refractive index difference with respect to pure quartz of ΔC, the relative refractive index difference and the diameter ratio in each region are:
0.4% ≦ Δ1 ≦ 0.5%
-0.08% <Δ2 <-0.02%
0% <ΔC ≦ 0.05%
4.7 ≦ b / a ≦ 4.9
The filling,
The mode field diameter at a wavelength of 1310 nm is 8.2 μm or more and 9.0 μm or less,
The cable cutoff wavelength λcc is 1260 nm or less,
The absolute value of the dispersion value at a wavelength of 1285 nm is 3.5 ps / nm / km or less,
The zero dispersion wavelength is from 1300 nm to 1320 nm,
The dispersion slope at zero dispersion wavelength is 0.090 ps / nm ² / km or less,
An optical fiber, wherein an increase in loss at a wavelength of 1625 nm when an optical fiber is wound 100 times around a cylinder having a diameter of 30 mm is 2 dB or less. A method for producing an optical fiber preform used for producing the optical fiber according to claim 1, comprising:
In the step of synthesizing the core soot by the VAD method, a first baking burner for flowing a combustion gas mainly composed of oxygen and hydrogen gas adjacent to the first core burner for synthesizing the soot serving as the first core located at the center portion. A second core burner for synthesizing a soot as a second core adjacent to the first baking burner, and a combustion gas mainly composed of oxygen and hydrogen gas adjacent to the second core burner The manufacturing method of the optical fiber preform characterized by arrange | positioning the 2nd baking burner which circulates, and synthesize | combining a core soot. 3. The method of manufacturing an optical fiber preform according to claim 2, wherein a gas containing fluorine is allowed to flow through the second core burner.

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