JP2003344687A - Optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber having a low loss in the wavelength range of 1450-1530 nm. <P>SOLUTION: In the optical fiber having a core and a clad in which the core is composed of a central first core 11 and an outer second core 12, the clad is composed of an inner first clad 13 and an outer second clad 14. The ratio of the coefficient of viscosity when the first clad 13 is drawn to the coefficient of viscosity when the second core 12 is drawn is ≥1.2 and ≤5.0. The coefficient of viscosity when the second clad 14 is drawn to the coefficient of viscosity when the first clad 13 is drawn is ≥1.1 and ≤10.0. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber used for optical communication and the like.

[0002]

2. Description of the Related Art Recently, as a technique for increasing the transmission capacity in optical transmission using an optical fiber, wavelength division multiplexing (W
Research and development on (DM) optical transmission have been actively conducted, and many studies have been made on optical fibers that are preferably used for WDM optical transmission.

As an optical fiber used for WDM optical transmission, a dispersion shifted optical fiber (NZDSF) having no zero dispersion in a used wavelength band has been developed. This NZDSF
Is designed so that four-wave mixing hardly occurs and the nonlinearity is sufficiently small.

Recently, as a wavelength band used in a WDM optical transmission system, optical transmission outside the conventional wavelength band of 1530 to 1570 nm has been studied for the purpose of increasing the transmission capacity. For example, wavelengths of 1450 to 1530n.
There is a demand for an optical fiber that can be used in the range of m. Also, in the conventional WDM optical transmission system of 1530 to 1570 nm, which is the wavelength band used, in order to realize an optical transmission system using Raman amplification, the wavelength of the pumping light source is about 100 nm shorter than the wavelength band used. Although the wavelength is shortened to 1430 nm, there is a demand for an optical fiber usable there. Such an optical fiber has a wavelength of at least 1450 to 1530 nm, preferably 1400 to 1530 nm.
It is required to have low loss at 1530 nm. Further, in forming a cable, it is also required that the bending loss be not more than a predetermined value.

By the way, the optical fiber has a wavelength of 1380.
In the vicinity of nm, loss due to absorption of hydroxyl groups is likely to occur,
Due to the influence, there is a problem that the transmission loss on the shorter wavelength side than the wavelength of 1530 nm becomes large. Therefore, the wavelength 138
It is necessary to reduce the loss due to the absorption of hydroxyl groups near 0 nm.

As a method for reducing the loss due to the absorption of hydroxyl groups in the vicinity of the wavelength of 1380 nm, US Pat. No. 6,131,415 discloses that by reducing the concentration of the hydroxyl groups, optical transmission in the entire wavelength range of 1200 to 1600 nm is possible. The following technologies are disclosed.

Also, US Pat. Nos. 5,838,866 and 6
No. 129928, in the clad portion adjacent to the core,
A technique is disclosed in which the loss due to the absorption of hydroxyl groups in the vicinity of the wavelength of 1380 nm is reduced by adding germanium to the extent that the refractive index is not substantially increased.

However, all of the techniques disclosed in the above-mentioned US patents aim at improving the characteristics of an optical fiber having a simple rectangular refractive index distribution in which there is no change in the refractive index distribution in the cladding region and the core region. I am trying.

In other words, the wavelength is 1450 to 1530 nm.
An optical fiber having low loss in the vicinity is known for an optical fiber having a simple refractive index distribution such as a single mode optical fiber, but is known for an optical fiber having a complicated refractive index distribution such as NZDSF. Absent.

[0010]

An optical fiber having a complicated refractive index distribution such as the conventional NZDSF has a wavelength of 14 nm.
When it is used in the range of 50 to 1530 nm, it has the following problems. That is, 1) The transmission loss at a wavelength of 1380 nm is generally 1d.
Larger than B / km. When the transmission loss at this wavelength becomes 0.5 dB / km or more, the transmission loss at wavelengths shorter than 1500 nm, particularly at 1400 to 1450 nm, increases. 2) The so-called hydrogen loss is likely to increase near the wavelength of 1520 nm. This is because there are many places that become interfaces at the time of manufacturing the optical fiber preform, and defects due to residual stress during drawing (stress due to thermal expansion stress and tension during drawing) are likely to occur. is there. Therefore, in the present invention,
An optical fiber having a complex refractive index distribution having a core structure of two or more layers, and having a smaller absolute value of dispersion as compared with a conventional standard single mode optical fiber,
It is an object of the present invention to provide an optical fiber having a low dispersion in the wavelength range of 1450 to 1530 nm with respect to an optical fiber used for a submarine laid or land-based transmission line having a low dispersion slope or both low dispersion slopes.

[0011]

The present invention has been made to achieve the above object, and the invention according to claim 1 has a core and a clad, and the core is composed of at least two layers. In the fiber, the cladding has at least 2
And a viscosity ratio of the innermost layer of the clad at the time of drawing to the outermost layer of the core at a rate of 1.2 or more and 5.0 or less. It is a thing.

The invention according to claim 2 is the invention according to claim 1, wherein the outermost layer of the clad has a higher viscosity during drawing, and the outermost layer of the clad has a maximum viscosity during drawing. The viscosity ratio of the inner layer to the viscosity at the time of drawing is 1.1 or more and 10.0 or less.

The invention described in claim 3 is the optical fiber according to the invention described in claim 1 or 2, characterized in that the transmission loss in the wavelength band of 1380 nm is 0.4 dB / km or less.

According to a fourth aspect of the present invention, in an optical fiber having a core and a clad, wherein the core is composed of at least two layers, the transmission loss in the wavelength 1380 nm band is 0.4 dB / km or less. , And wavelength 145
The absolute value of the dispersion value in the range of 0 to 1570 nm (unit: ps / nm / km) is 2 or more and 12 or less, and the absolute value of the dispersion slope in the same wavelength range (unit: ps / km)
nm ² / km) is 0.1 or less and the zero dispersion wavelength is 1
An optical fiber having a wavelength shorter than 400 nm or a wavelength longer than 1640 nm.

The invention described in claim 5 is the optical fiber according to the invention described in claim 3 or 4, characterized in that the bending loss at a bending diameter of 20 mm is 20 dB / m or less.

Further, the invention according to claim 6 is the same as claim 3.
In the invention described in any one of 1 to 5, the core or a part of the core and the clad is formed by vitrifying a continuously synthesized porous glass base material or a collectively synthesized porous glass base material. It is characterized by that. Here, the "continuously synthesized porous glass base material" means,
As in the OVD method, it is a porous glass base material in which one layer and one layer are successively synthesized. It is a porous glass base material synthesized in the direction.

The inventions according to claims 1 and 2 are obtained as a result of earnestly examining. That is, as described in claim 1, when the viscosity of the innermost layer of the clad at the time of drawing is higher than the viscosity of the outermost layer of the core at the time of 1.2 to 5.0, The strain remaining in the core of the outermost layer when the fiber is drawn can be reduced, and the defects generated in the core can be reduced, so that the transmission loss due to the hydroxyl group in the 1380 nm band can be reduced. If the viscosity ratio is less than 1.2, the residual strain of the outermost layer of the core during drawing becomes large, defects are likely to occur, and transmission loss increases. Further, when the viscosity ratio is larger than 5.0,
The viscosity of the core becomes too small, a large number of foams are generated during the synthesis of the porous glass preform, and during drawing, which lowers the yield of the optical fiber and worsens the transmission loss.

Further, as described in claim 2, the outermost layer of the clad has a higher viscosity at the time of drawing, and the outermost layer of the clad has a viscosity at the time of drawing which is the innermost layer of the clad. On the other hand, if the viscosity ratio is increased, the addition amount of the additive such as fluorine, chlorine or germanium in the innermost clad layer is increased in order to achieve this viscosity condition. Therefore, the temperature at the time of synthesizing the porous glass base material and the vitrification temperature can be lowered, and foaming generated in the glass base material can be suppressed. In addition, since the drawing temperature is absolutely lowered, residual stress due to drawing is not concentrated in the core, but also disperses at the interface in the clad consisting of multiple layers to reduce defects in the core that cause loss. be able to. Further, when the viscosity ratio is 1.1 or more, this effect is substantially obtained, but 10.0
When it exceeds, the amount of foaming increases during the synthesis and drawing of the porous glass preform, and the yield decreases. The viscosity conditions in claims 1 and 2 are to be applied in the temperature range of the glass base material at the time of drawing of 1400 ° C to 1800 ° C.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. 1A and 1B are diagrams respectively showing a refractive index distribution and a viscosity distribution in a radial direction of an embodiment of an optical fiber according to the present invention. The viscosity in the present embodiment is a viscosity in the temperature range of 1400 to 1800 ° C. In the present embodiment, the first core 11 whose core is the center core and the second core 12 outside the first core 11 are two cores.
The first clad 1 is composed of layers and the clad is an inner layer.
3 and the second cladding 14 on the outside thereof.

The refractive index distribution structure of this embodiment is so-called W.
As shown in FIG. 1A, the refractive index of the first core 11 is higher than that of the first cladding 13, and the refractive index of the second core 12 is lower than that of the first cladding 13. Has become. The first cladding 13 and the second cladding 14 have almost the same refractive index. Here, the almost equal refractive index means that the relative refractive index difference is within a range of ± 0.05%.

The viscosity distribution of this embodiment is shown in FIG.
As shown in (b), the height increases from the center to the outside, and the first core 11, the second core 12, the first clad 13,
The viscosity increases in the order of the second cladding 14.

This embodiment is different from the conventional one in that the clad is composed of two layers of the first clad 13 and the second clad 14, and the viscosity of the first clad 13 adjacent to the second core 12 is The viscosity ratio with respect to the viscosity of the second core 12 is in the range of 1.2 or more and 5.0 or less, and
The viscosity ratio of the second clad 14 to the viscosity of the first clad 13 is 1.1 or more and 10.0 or less.

FIGS. 2 (a) and 2 (b) are views respectively showing the refractive index distribution and the viscosity distribution in the radial direction of another embodiment of the optical fiber according to the present invention. The viscosity in the present embodiment is a viscosity in the temperature range of 1400 to 1800 ° C. In this embodiment, the core is the first core 2 from the inside.
It is composed of three layers of the first, second core 22 and the third core 23, and the clad is composed of two layers of the first clad 24 and the second clad 25 from the inside.

The refractive index distribution structure of the present embodiment is a so-called segment type, and as shown in FIG. 2A, the refractive index of the first core 21 and the refractive index of the third core 23 are the same as those of the first cladding 24. It is higher than the refractive index, and the refractive index of the second core 22 is lower than that of the first cladding 24. The first clad 24 and the second clad 25 have the same refractive index.

The viscosity distribution of this embodiment is shown in FIG.
As shown in (b), the height increases from the center to the outside, and the first core 21, the second core 22, the third core 23, and the first core 21
The viscosity increases in the order of the clad 24 and the second clad 25.

Also in this embodiment, the first cladding 24
Third core 23 (adjacent to the first cladding 24) with a viscosity of
Of the first clad 2 to the viscosity of the second clad 25 is in the range of 1.2 or more and 5.0 or less.
The viscosity ratio is 1.1 or more and 10.
It is in the range of 0 or less.

Example 1 The following optical fiber having the refractive index distribution structure and the viscosity distribution structure shown in FIGS. 1 (a) and 1 (b) was prototyped. That is, the first core 11 (outer diameter 6.8 μ
The relative refractive index difference of m) with respect to the first cladding 13 is 0.6.
%, The relative refractive index difference of the second core 12 (outer diameter 16.2 μm) with respect to the first cladding 13 is −0.15%. Also,
The first cladding 13 (outer diameter 32 to 48 μm) and the second cladding 14 (outer diameter 125 μm) have almost the same relative refractive index. The viscosity of the first cladding 13 is 1.7 times the viscosity of the second core 12, and the viscosity of the second cladding 14 is 1.1 times the viscosity of the first cladding 13.

The optical fiber of this example was manufactured by the following steps. That is, 1) Using the VAD method, the parts to be the first core 11, the second core 12, and the first cladding 13 are collectively synthesized to produce a porous glass preform. At this time, germanium is added to the portions to be the first core 11 and the first cladding 13,
No additive is added to the portion that becomes the second core 12. 2) Next, the porous glass base material is dehydrated, added with fluorine, and sintered in a dehydration sintering furnace to obtain a glass rod to which fluorine is added almost uniformly throughout. At this time, the amount of fluorine added is adjusted so that the refractive index distribution after drawing will be the above distribution. 3) Next, the glass rod is stretched to prepare a target rod, and a portion containing water or a hydroxyl group is removed by a usual method when the target rod is stretched. 4) Next, glass particles to be the second cladding 14 are deposited on the surface of the target rod. 5) Then, dehydration and sintering are performed to obtain an optical fiber preform. 6) The above optical fiber preform is drawn with a wire drawing device to an outer diameter of about 125.
An optical fiber having a diameter of .mu.m is drawn, and two layers of coating are applied to form an optical fiber core wire having an outer diameter of about 250 .mu.m.

As a usual method for removing the above-mentioned water and hydroxyl group, there are a method of dipping in a hydrofluoric acid aqueous solution, a method of mechanically shaving, a method of etching by the heat of plasma flame, and the like. Further, in the above step 2), if the density of the first core 11 is increased to 0.4 g / cm ³ or more when synthesizing the porous glass preform so that fluorine does not enter the first core 11, It is not necessary to dope the core 11 with an excessive dopant (germanium), and the transmission loss of the entire core can be reduced. Furthermore, the cores 11 and 1
Regarding the control of the viscosity of each layer of 2 and the clads 13 and 14, the fact that the relationship between the doping amount of the dopant (fluorine and germanium) and the viscosity is known is utilized to control the relative refractive index difference, The amount of dope, in other words, the viscosity can be controlled.

The optical fiber of this example has the following characteristics. That is, the zero dispersion wavelength is 1375 nm and the wavelength 145
Dispersion value at 0 nm is 4.5 ps / nm / km, wavelength 145
Average dispersion slope from 0 to 1570 nm is 0.052 ps
/ Nm ² / km. The transmission loss (unit: dB / km) of the optical fiber at wavelengths of 1380 nm, 1450 nm, and 1550 nm is 0.
38-0.40, 0.24-0.27, 0.20-0.
It was 22. Furthermore, the bending loss at a bending diameter of 20 mm was 18 to 20 dB / m.

Therefore, the optical fiber of this embodiment has a transmission loss of 0.40 dB / km at a wavelength of 1380 nm.
The dispersion value at the wavelength of 1450 nm is 2 to 12 ps.
/ Nm / km range, wavelength 1450 to 1570n
It can be seen that the average dispersion slope of m is 0.1 ps / nm ² / km or less, and the zero dispersion wavelength is on the shorter wavelength side than 1400 nm, which is suitable for NZDSF.

Example 2 The following optical fiber having the refractive index distribution structure and the viscosity distribution structure shown in FIGS. 2 (a) and 2 (b) was experimentally manufactured. That is, the first core 21 (outer diameter 8.9 μ
m) is 0.5 relative to the first cladding 24.
%, The relative refractive index difference of the second core 22 (outer diameter 12.4 μm) with respect to the first cladding 24 is −0.2%, the third core 23
The relative refractive index difference with respect to the first cladding 24 (outer diameter 16 μm) is 0.3%. In addition, the first clad 24 (outer diameter 3
2 to 48 μm) and the second clad 25 (outer diameter 125 μm)
Have the same relative refractive index. The viscosity ratio of the first clad 24 to the viscosity of the third core 23 is 1.
4. The viscosity ratio of the second clad 25 to the viscosity of the first clad 24 is 1.1.

In this embodiment, a porous glass base material to be the first core 21 is synthesized, vitrified and stretched, and subsequently successively produced to synthesize a porous glass base material to be the second core 22. , Vitrification and stretching, and thereafter, a porous glass base material to be the third core 23 is synthesized, vitrified and stretched. Next, a porous glass base material to be the first cladding 24 is synthesized, vitrified and stretched. After each stretching, as described in Example 1, a treatment to prevent the inclusion of water and hydroxyl groups is carefully performed. Then the second
The porous glass base material that becomes the clad 25 is synthesized and dehydrated.
Sintered into an optical fiber preform. The viscosity of each layer is controlled in the same manner as in Example 1. The optical fiber preform thus obtained is drawn into an optical fiber having an outer diameter of about 125 μm by a drawing device, and two layers of coating are applied to give an outer diameter of about 250 μm.
The optical fiber core wire of μm is used. If necessary, especially in the case of a dispersion compensating optical fiber (DCF), the diameter of the optical fiber or the coating diameter is changed. For example, the optical fiber diameter is 70μ
m to 200 μm, the coating diameter is 100 to 300 μm, and it is made resistant to miniaturization and bending depending on required characteristics.

The optical fiber of this embodiment is the following optical fiber.
The characteristics as a bar are shown. That is, the zero dispersion wavelength is 139
6nm, dispersion value of wavelength 1450nm is 3.45ps / n
Average dispersion slot of m / km and wavelength of 1450 to 1570 nm
Loop is 0.058 ps / nm ² It was / km. Also,
The wavelength of this optical fiber is 1380 nm, the wavelength is 1450 n
m, transmission loss at wavelength 1550 nm (unit: dB /
km) is about 0.38, about 0.25, and about 0.2, respectively.
It was 0. Therefore, the optical fiber of the present embodiment is
As in Example 1, the conditions suitable for NZDSF are satisfied.
I understand that

The present invention is not limited to the above embodiment. For example, the following refractive index distribution structure and viscosity distribution structure may be used. That is, the refractive index distribution structure is as shown in FIG.
As shown in, the first core 3 whose core is the center core
1, a second core 32 serving as a first depressed core, a third core 33 serving as a segment core, and a fourth core 34 serving as a second depressed core, and the clads from the inside are a first clad 35 and a second clad. And 36. Further, the viscosity distribution structure is as shown in FIG.
The viscosity of the first clad 35 is 1.2 times that of the fourth core 34.
The viscosity of the second clad 36 is set to be 1.1 times or more and 10.0 times or less of the viscosity of the first cladding 35 so that the viscosity of the second clad 36 is not less than 5.0 times.

The optical fiber of this embodiment is manufactured by the following steps. That is, 1) using the VAD method, the first core 31, the second core 32,
The third core 33, the fourth core 34, and the portion to be the first cladding 35 are collectively synthesized to produce a porous glass preform. At this time, germanium and fluorine are added to the first core 31 and the third core 33, and fluorine or fluorine and a small amount of germanium are added to the second core 32 and the fourth core 34. 2) Dehydration, addition of fluorine, and sintering of the porous glass base material in a dehydration sintering furnace to obtain a glass rod to which fluorine is added almost uniformly throughout. At this time, the amount of fluorine added is adjusted so that the refractive index distribution after drawing will be the above distribution. 3) Next, the glass rod is stretched to prepare a target rod, and a portion containing water or a hydroxyl group is removed by a usual method when the target rod is stretched. 4) Next, glass particles to be the second cladding 36 are deposited on the surface of the target rod. 5) After that, dehydration and sintering are performed to obtain an optical fiber preform. 6) The above optical fiber preform is drawn with a wire drawing device to an outer diameter of about 125.
An optical fiber having a diameter of .mu.m is drawn, and two layers of coating are applied to form an optical fiber core wire having an outer diameter of about 250 .mu.m. In the above steps 4) and 5), instead of depositing the glass particles and dehydrating and sintering, the target rod may be covered with a silica pipe.

[0037]

As described above, according to the present invention, there is an excellent effect that an optical fiber having a low loss in a wavelength range of 1450 to 1530 nm and suitable for WDM optical transmission can be obtained. Further, the present invention is effective even when the variance value is negative.

[Brief description of drawings]

1A and 1B are diagrams respectively showing a refractive index distribution and a viscosity distribution of an embodiment of an optical fiber according to the present invention.

FIGS. 2A and 2B are diagrams showing a refractive index distribution and a viscosity distribution of another embodiment, respectively.

3A and 3B are diagrams showing a refractive index distribution and a viscosity distribution of still another embodiment, respectively.

[Explanation of symbols]

11, 21, 31 1st core 12, 22, 32 2nd core 13, 24, 35 First clad 14, 25, 36 Second clad 23, 33 Third core 34 4th core

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H050 AB05 AB10 AC14 AC15 AD16                 4G021 EA01 EA03 HA01 HA05

Claims (6)

[Claims] 1. An optical fiber having a core and a clad, wherein the core is composed of at least two layers, the clad is composed of at least two layers, and the innermost layer of the clad has a viscosity at the time of drawing the core. The viscosity ratio is 1.2 or more with respect to the viscosity at the time of drawing in the outermost layer of
An optical fiber which is 5.0 or less. 2. The outermost layer of the clad has a higher viscosity at the time of drawing, and the outermost layer of the clad has a viscosity ratio of 1 to that of the innermost layer of the clad at the time of drawing. It is 1 or more and 10.0 or less, The optical fiber of Claim 1 characterized by the above-mentioned. 3. The transmission loss in the wavelength 1380 nm band is 0.4 dB / km or less.
Or the optical fiber described in 2. 4. An optical fiber having a core and a clad, wherein the core is composed of at least two layers and has a wavelength of 1
The transmission loss in the 380 nm band is 0.4 dB / km or less, and the absolute value (unit: ps / nm / km) of the dispersion value in the wavelength range of 1450 to 1570 nm is 2 or more and 12 or less, in the same wavelength range. An optical fiber having an absolute value (unit: ps / nm ² / km) of the dispersion slope in 0.1 or less and a zero dispersion wavelength on the shorter wavelength side than 1400 nm or on the longer wavelength side than 1640 nm. 5. The bending loss at a bending diameter of 20 mm is 20.
It is below dB / m, The claim 3 or 4 characterized by the above-mentioned.
The described optical fiber. 6. The core or a part of the core and the clad is formed by vitrifying a continuously synthesized porous glass base material or a collectively synthesized porous glass base material. 6. The optical fiber according to any one of 1 to 5.