JP2003075293A - Method for evaluating loss of single mode optical fiber and single mode optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating the loss of a single mode optical fiber in a wavelength region of 1380 nm band and to provide a single mode optical fiber in which increase of loss due to diffusion of hydrogen can be reduced using that method for evaluating the loss. SOLUTION: A quartz rod comprising a core 1 and a first clad 2 is applied externally with microparticles of SiO2 to produce a glass preform in which a second clad 3 is formed and then the glass preforms are spun to produce an optical fiber. When an increase of loss in the wavelength of 630 nm band caused by bonding defect of silicon and oxygen in the quartz glass is limited to 4 dB/km or less, an increase of loss in the wavelength of 1380 nm band caused by the diffusion of oxygen into the quartz glass can be limited to 0.1 dB/km or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single mode optical fiber for optical communication and a loss evaluation method therefor, and more particularly to 1.
The present invention relates to a method for evaluating transmission loss in a wavelength band of 380 nm and a single mode optical fiber having a small transmission loss in a wavelength band of 1380 nm and excellent in hydrogen resistance.

[0002]

2. Description of the Related Art With the increase in demand for communication capacity, a wavelength multiplexing transmission technique for transmitting light of a large number of wavelengths in one optical fiber has become important. Therefore, O which has been used conventionally
-Band (wavelength band 1260 nm to 1360 nm),
C-Band (wavelength band 1530nm to 1565nm)
Development of optical fibers that can be used in wavelength bands other than the above is underway. A typical optical fiber has a large transmission loss in the 1380 nm band because of a small amount of OH groups present in its structure. Therefore, if the OH groups in this optical fiber can be reduced, optical transmission in a wider wavelength band becomes possible. In JP-A-11-171575, by controlling the ratio of core / deposited cladding of the core rod within a predetermined range, 1385 is brought about by the presence of OH groups.
It has been reported that the loss in the nm band can be reduced. The structure of quartz glass, which is the material of the optical fiber, has a network structure in which SiO ₄ is three-dimensionally randomly bonded. However, when impurities or defects are present in the structure, a new bond is generated. , Disappears, which causes light absorption. Of this light absorption, the loss in the 1380 nm band is
This is attributed to the OH groups present in the quartz glass. Therefore, the transmission loss in the 1380 nm band increases as the amount of OH groups contained increases.

[0003]

[Patent Document 1] Japanese Patent Application Laid-Open No. 11-1715
If the method disclosed in Japanese Patent No. 75 is used, 138
It is possible to manufacture an optical fiber having a loss of less than 0.33 dB / km in the 5 nm band. However, even if this method is used, the increase in loss in the 1380 nm band cannot always be suppressed to a small extent when hydrogen diffuses from the environment outside the optical fiber. The mechanism of this loss can be explained as follows. In the drawing process of the optical fiber, Si-O-Si in the silica glass structure is cut by the reaction represented by the formula (1).

[Chemical 1] The bond defect between silicon and oxygen (hereinafter referred to as “defect”), which is represented by Si—O.
It causes light absorption in the band, but depending on the drawing conditions, S
i-O. reacts with hydrogen diffused from the outside to newly generate a bond represented by the formula (2).

[Chemical 2] However, when the drawing speed is high, that is, when the optical fiber is cooled earlier than the reaction according to the equation (2), Si—O. Remains in the optical fiber without reacting with hydrogen. I will end up. This remaining Si-O.
When hydrogen again diffuses from the outside, Si—OH is produced by the reaction shown in the equation (2). That is,
Even if the loss in the 1380 nm band is small when manufacturing the optical fiber, the loss in the 1380 nm band may increase when hydrogen diffuses from the outside depending on the drawing conditions.

Due to such hydrogen diffusion, 1380
When the increase in loss in the nm band is 0.1 dB / km or more, the increase in loss in the wavelength band around the 1380 nm band, that is, the spread of the loss becomes remarkable. For this reason, when the optical fiber is used for a long period of time, the transmission loss deteriorates with time, and optical transmission in a wide wavelength band becomes impossible. Increase in loss in the 630 nm band due to generation of Si-O., And 138 due to hydrogen diffusion
Regarding the relationship with the loss increase in the 0 nm band, only the qualitative discussion has been made so far about the mechanism of defect generation, and a quantitative criterion for suppressing the loss increase in the 1380 nm band due to hydrogen diffusion is low. Was not disclosed. Further, since the loss at 630 nm is not discussed as the loss amount due to the defect itself from which the loss specific to silica glass such as Rayleigh scattering is subtracted, the difference due to the manufacturing conditions cannot be distinguished. Further, as a method of evaluating the amount of increase in loss in the 1380 nm band due to diffusion of hydrogen, a method called a hydrogen test in which an optical fiber of a certain fixed length is exposed to hydrogen for a long time was used. According to this method, It took a long time and there was a problem in terms of cost. The present invention has been made in consideration of such circumstances, and provides a loss evaluation method for a single mode optical fiber in the wavelength region of 1380 nm band, and using this loss evaluation method, loss increase due to hydrogen diffusion is provided. It is an object of the present invention to provide a single-mode optical fiber that can reduce

[0005]

In order to solve the above-mentioned problems, the invention according to claim 1 is a method for evaluating the loss of a single mode optical fiber made of quartz glass, in which a bond between silicon and oxygen in the quartz glass is bonded. A loss evaluation method for a single-mode optical fiber, characterized in that the loss increase in the wavelength 1380 nm band caused by hydrogen diffusion into silica glass is evaluated based on the loss increase in the wavelength 630 nm band caused by a defect. . As a result, the loss increase in the 630 nm wavelength band caused by the coupling defect,
Wavelength 1380 produced by diffusion of hydrogen into quartz glass
By correlation with the increase in loss in the nm band, wavelength 1380
The increase in loss in the nm band can be accurately evaluated.
According to a second aspect of the present invention, in the loss evaluation method for a single mode optical fiber according to the first aspect, a second cladding is externally attached to a quartz rod composed of a core and a first cladding to form a glass preform. By measuring the loss in the wavelength 630 nm band of the single mode optical fiber formed by spinning this glass preform, and then exposing the single mode optical fiber in hydrogen to measure the loss in the wavelength 1380 nm band. The feature is that the loss of the single mode optical fiber is evaluated.

According to a third aspect of the present invention, in a single mode optical fiber made of silica glass, a wavelength 630 generated by a bond defect between silicon and oxygen in the silica glass.
By setting the loss increase in the nm band to 4 dB / km or less, the wavelength of 1
The single mode optical fiber is characterized in that the increase in loss in the 380 nm band is 0.1 dB / km or less. This makes it possible to suppress an increase in loss in the wavelength of 630 nm band and reduce an increase in loss in the wavelength of 1380 nm band due to hydrogen diffusion.

According to a fourth aspect of the present invention, in a single mode optical fiber made of silica glass, a wavelength 630 generated by a bond defect between silicon and oxygen in the silica glass.
By setting the loss increase in the nm band to 2 dB / km or less, the wavelength of 1 caused by hydrogen diffusion into silica glass
The single mode optical fiber is characterized in that the increase in loss in the 380 nm band is 0.05 dB / km or less. This makes it possible to further suppress the loss increase in the wavelength 630 nm band and reduce the loss increase in the wavelength 1380 nm band due to hydrogen diffusion to a more preferable value.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
FIG. 1 shows a cross section of a glass preform for producing a single mode optical fiber of the present invention in a direction perpendicular to the longitudinal direction. In FIG. 1, reference numeral 1 is a core that is a high refractive index portion, and reference numeral 2 is a first cladding that has a lower refractive index than the core 1. Reference numeral 3 is a second clad having the same refractive index as the first clad 2. The quartz rod composed of the core 1 and the first cladding 2 is formed into a porous body composed of fine particles of GeO ₂ and SiO ₂ by using a general vapor phase deposition (VAD) device, and then dehydrated and baked. It is formed by tying. At this time, core 1 (diameter:
d) and the first cladding 2 (diameter: D) diameter ratio (D / d)
Is preferably about 4 or more. If this ratio is less than 4, the initial loss at 1380 nm tends to be high, and the original purpose cannot be achieved. The dehydration step is performed in chlorine-based gas at a temperature of about 1200 ° C. The sintering process is performed at about 1450 ° C. in a helium atmosphere to vitrify. The second cladding 3 is, for example,
On the quartz rod composed of the core 1 and the first cladding 2, SiO
₂ Formed by externally attaching fine particles. The thickness of the second clad 3 varies depending on the diameter of the quartz rod to be manufactured.
When the optical fiber has a thickness of 5 μm, it is preferable to externally attach the SiO ₂ fine particles so that the thickness of the second cladding 3 becomes 43 μm or less. If the thickness of the second cladding 3 is thicker than 43 μm, the initial loss at 1380 nm tends to be high, which is not preferable. In this way, the quartz rod to which the second cladding 3 is externally attached is dehydrated by using a chlorine-based gas if necessary, and sintered in a helium atmosphere to produce a glass preform. This glass preform is spun using a spinning device to produce an optical fiber. Light of wavelength 630 nm band is incident on the obtained optical fiber,
The loss in the 630 nm band is measured.

The loss in the short wavelength region of an optical fiber is
It consists of a loss due to Rayleigh scattering, a loss due to a structural irregularity of the optical fiber material, a loss due to an ultraviolet absorption, and a loss due to a defect in the structure, and can be expressed by the formula (3).

[Equation 1] In equation (3), the first term is the loss due to Rayleigh scattering,
The second term is loss due to structural irregularity, the third term is loss due to ultraviolet absorption, and the fourth term is loss due to defects. In the equation (3), λ is the wavelength, w is the GeO ₂ concentration [wt%], K
_UV, C _{U V} is a constant. FIG. 2 illustrates the loss of the optical fiber. In FIG. 2, the straight line 1 is the sum of the loss due to Rayleigh scattering and the loss due to structural irregularity, and the curve 1 is
It is the sum of the loss due to Rayleigh scattering, the loss due to structural irregularity, and the loss due to ultraviolet absorption. Curve 2 is the total loss,
Therefore, the region surrounded by the curve 1 and the curve 2 represents the loss increase due to the presence of the defect.

In this way, the loss increase due to the presence of defects in the 630 nm band is obtained from the measured total loss value, excluding the loss due to Rayleigh scattering, the loss due to structural irregularity, and the loss due to ultraviolet absorption. Then, this optical fiber is exposed to a hydrogen atmosphere to measure the loss increase in the 1380 nm band. FIG. 3 shows the results of measurement after exposure to a hydrogen atmosphere of 0.01 atm at room temperature for 10 days. Figure 3
As can be seen from the above, in order to reduce the loss increase due to hydrogen diffusion in the 1380 nm band to 0.1 dB / km or less, 6
It is necessary that the increase in loss due to defects in the 30 nm band is 4 dB / km or less. Further, in order for the loss increase due to hydrogen diffusion in the 1380 nm band to become a more preferable value of 0.05 dB / km or less, it is necessary that the loss increase due to defects in the 630 nm band be 2 dB / km or less. Become. If the loss increase due to hydrogen diffusion in the 1380 nm band is 0.1 dB / km or less, it will be used satisfactorily in a normal optical fiber cable installation environment even if the loss increases due to hydrogen diffusion due to the external environment. be able to. On the other hand, if the loss increase due to defects in the 630 nm band is 4 dB / km or more, the loss increase due to hydrogen diffusion in the 1380 nm band exceeds 0.1 dB / km, which is not preferable for use.

According to the loss evaluation method of the single mode optical fiber of this example, hydrogen diffusion into the silica glass is performed based on the loss increase in the wavelength 630 nm band caused by the bond defect between silicon and oxygen in the silica glass. By evaluating the increase in loss in the wavelength 1380 nm band, loss can be evaluated by shortening the strip length of the optical fiber used as the measurement sample, and the loss evaluation of the single mode optical fiber in a short time and at low cost can be performed. Is possible. Further, according to the single mode optical fiber of this example, the loss increase in the wavelength of 630 nm band caused by the bond defect between silicon and oxygen in the silica glass is set to 4 dB / km or less, so that the hydrogen diffusion into the silica glass is achieved. The loss increase in the 1380 nm wavelength band caused by 0.1 dB /
The single-mode optical fiber whose quality is guaranteed can be provided by a simple loss evaluation method. Further, by setting the loss increase in the wavelength 630 nm band caused by the bond defect between silicon and oxygen in the silica glass to 2 dB / km or less, the loss increase in the wavelength 1380 nm band caused by hydrogen diffusion into the silica glass. Can be 0.05 dB / km or less,
A single mode optical fiber having more preferable quality can be provided by a simple loss evaluation method.

[0012]

As described above, according to the present invention,
Based on the loss increase in the wavelength 630 nm band caused by the bond defect between silicon and oxygen in the silica glass, the wavelength 1380 nm generated by the hydrogen diffusion into the silica glass.
By evaluating the increase in loss in the band, it is possible to shorten the length of the optical fiber used as the measurement sample and perform loss evaluation, enabling loss evaluation of single-mode optical fiber in a short time and at low cost. Become. Further, according to the present invention, the loss increase in the wavelength of 630 nm band caused by the bond defect between silicon and oxygen in the silica glass is 4 dB / k.
By setting it to be equal to or smaller than m, the loss increase in the wavelength 1380 nm band caused by hydrogen diffusion into the silica glass can be reduced to 0.
It can be 1 dB / km or less, and a single mode optical fiber with guaranteed quality can be provided by a simple loss evaluation method. Furthermore, the loss increase in the wavelength 630 nm band caused by the bond defect between silicon and oxygen in the silica glass is set to 2 dB / km or less, so that the wavelength of 1380 nm generated by hydrogen diffusion into the silica glass.
The loss increase in the band can be set to 0.05 dB / km or less, and a single mode optical fiber having more preferable quality can be provided by a simple loss evaluation method.

[Brief description of drawings]

FIG. 1 is a view showing a cross section in a direction perpendicular to a longitudinal direction of a glass preform for producing a single mode optical fiber of the present invention.

FIG. 2 is a diagram showing an increase in loss caused by a bond defect between silicon and oxygen.

FIG. 3 is a diagram showing a relationship between a loss increase in a wavelength of 630 nm band caused by a bond defect between silicon and oxygen and a loss increase in a wavelength of 1380 nm band caused by hydrogen diffusion.

[Explanation of symbols] 1 ... Core, 2 ... 1st clad, 3 ... 2nd clad

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Manabu Saito             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office (72) Inventor Munehisa Fujimaki             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office (72) Inventor Koichi Harada             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office F term (reference) 2G086 BB01                 2H050 AB04Y AB05X AC09 AC36                       AC71

Claims (4)

[Claims] 1. A method for evaluating the loss of a single mode optical fiber made of quartz glass, wherein the loss in the quartz glass is increased based on an increase in loss in the wavelength 630 nm band caused by a bond defect between silicon and oxygen in the quartz glass. 1380 produced by hydrogen diffusion of hydrogen
A loss evaluation method for a single mode optical fiber, characterized by evaluating an increase in loss in the nm band. 2. The loss of the single mode optical fiber is evaluated by forming a glass preform by externally attaching a second cladding to a quartz rod composed of a core and a first cladding, and spinning the glass preform. And measuring the loss in the wavelength 630 nm band of the formed single mode optical fiber, and then exposing the single mode optical fiber to hydrogen to measure the loss in the wavelength 1380 nm band. Item 1. A single-mode optical fiber loss evaluation method according to Item 1. 3. In a single-mode optical fiber made of quartz glass, the loss increase in the wavelength of 630 nm band caused by a bond defect between silicon and oxygen in the quartz glass is set to 4 dB / km or less, whereby the quartz glass A single-mode optical fiber characterized in that a loss increase in a wavelength band of 1380 nm caused by hydrogen diffusion into the inside is 0.1 dB / km or less. 4. A single mode optical fiber made of quartz glass, wherein the loss increase in the wavelength of 630 nm band caused by a bond defect between silicon and oxygen in the quartz glass is set to 2 dB / km or less, A single-mode optical fiber characterized in that an increase in loss in the wavelength 1380 nm band caused by hydrogen diffusion into the inside is 0.05 dB / km or less.