JP3098828B2 - Dispersion shifted fiber and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a quartz optical fiber for communication and a method of manufacturing the same, and more particularly to a single mode fiber (hereinafter referred to as "dispersion shift") having a zero-dispersion wavelength shifted to a wavelength of 1.5 .mu.m. (Referred to as "fiber") and a method for producing the preform.

[0002]

2. Description of the Related Art A dispersion-shifted fiber in which a zero-dispersion wavelength is shifted to a 1.5 μm band, which is the lowest loss wavelength region, of a silica-based optical fiber has been put into practical use as an optical communication transmission line having a long distance and a large transmission capacity. In. Among the dispersion-shifted fibers, those having a step-like refractive index distribution as shown in FIG. 2 have a smaller bending loss and larger practical advantages than dispersion-shifted fibers having a simple step-type refractive index distribution. Studies are ongoing (Ref. 1:
"Dispersion-shifted convex-index single-mode fibers" Kwaki et al., Electronics Letters December 5, 1985, 21 , No. 25/26, p.
7). In the step-type refractive index distribution shown in FIG. 2, a portion 21 (referred to as an inner core) having the highest refractive index at the center and a portion 22 having a lower refractive index than the inner core 21 surrounding the inner core 21 (the outer core and the inner core 21). ), And a refractive index distribution structure is formed from the clad portion 23 having the lowest refractive index surrounding the outer core 22. Regarding the dispersion-shifted fiber having such a step-type refractive index, the inner core is made of GeO ₂ —SiO as a glass composition for forming the refractive index distribution.
_{2. A} proposal has been made in which the outer core is made of SiO ₂ and the cladding is made of F-SiO ₂ (Ref. 2: “Dispersion-shifted fibers with fluorin-added cladding by the vapor phase axial deposition method”). ．Yokota and other technical digests
Topical Meeting on Optical Fiber Communication (Atlanta, 1986) Paper WF2).

[0003] The refractive index distribution of an optical fiber is most commonly obtained by adding GeO ₂ to SiO ₂ glass as a refractive index increasing component. However, G
When the amount of added eO ₂ is increased, Rayleigh scattering of the glass increases to increase the transmission loss, or the electron transition absorption in the ultraviolet region, which is considered to be based on the reduction of GeO ₂ → GeO, increases. The transmission loss extends to the 1.5 μm band which is the band. Therefore, in the above composition, Ge is added to the clad portion to lower the refractive index of the clad portion, and GeO ₂ is added only to the inner core, thereby making GeO _2.
Attempts are being made to reduce the amount of addition to reduce transmission loss. Previously the inventors have, based on this idea, the inner core 31 is GeO ₂ -SiO ₂ as shown in FIG. 3, the outer core 32
Is SiO ₂ , the cladding part 33 is F-SiO ₂ ,
For example, a dispersion-shifted fiber having a refractive index distribution and a composition shown in FIG. 3 has been experimentally manufactured, and the transmission loss at a wavelength of 1.55 μm can be reduced to 0.23 dB / km.
In addition, a, b, and c in FIG. 3 represent the diameter of each part, and in this example, a is 3 μm, b is 9 μm, and 1c is 125 μm (Ref. 3: Transmission characteristics of 1.5 μm band dispersion shift single mode fiber). Shigematsu, Kanamori, Tanaka, Tanaka, Watanabe, Suzuki, IEICE Technical Report OQE86-9
9).

[0004]

As described above, in a dispersion-shifted fiber having a step-type refractive index distribution in which the inner core is made of GeO ₂ —SiO ₂ , the outer core is made of SiO ₂ , and the cladding is made of F—SiO ₂ , as described above. The portion of the outer core made of SiO ₂ sandwiched between the inner core containing GeO ₂ and the clad portion containing F has a higher viscosity in the high-temperature heating process at the time of drawing than the other portions. The tension applied at the time of drawing is concentrated on the portion of the outer core, and a tensile stress remains in the spun fiber. As a result, the refractive index of the outer core SiO ₂ portion of the fiber after spinning (after drawing) decreases due to the photoelastic effect, and as shown in FIG. 4, the refractive index distribution before drawing (solid line) and the refractive index after drawing. There is a problem that the shape is different from the distribution (broken line). If the refractive index distribution in the preform stage and the refractive index distribution in the fiber are different, the mode field diameter zero dispersion wavelength predicted in the preform stage will be different from the actually manufactured fiber, which has been a major production problem. . In addition, GeO ₂ —F—SiO ₂ glass has a problem that structural defects exist and a low-loss fiber cannot be obtained. An object of the present invention is to provide a structure of a dispersion-shifted fiber that solves the above-mentioned problems and a method for manufacturing the same.

[0005]

As a means for solving the above-mentioned problems, according to the present invention, an outer core having a lower refractive index than the inner core is formed on the outer periphery of the inner core, and an outer layer having a lower refractive index than the outer core is formed on an outer layer of the outer core. A first clad is formed and has a stepped clad distribution, and a first clad is formed on an outer layer of the first clad.
Cladding is formed a second cladding having a higher refractive index becomes than the dispersion-shifted fiber having a stepped refractive index distribution, the inner core GeO ₂ -SiO _2, the outer core is SiO _2, the first cladding F- SiO _2, second cladding provides a dispersion-shifted fiber made of SiO _2. The present invention also provides an inner core of GeO ₂ —SiO ₂ and SiO _2.
2 is inserted into the hollow portion of the pipe-shaped first cladding glass body made of F-SiO ₂ by heating and integrated by inserting the glass composite consisting of the outer core of No. ₂ into GeO.
_2- SiO ₂ , outer core is SiO ₂ , first cladding is F
To form a glass composite consisting -SiO _2, to provide a method of manufacturing a dispersion-shifted fiber and forming a second cladding comprising a further SiO ₂ on the outer periphery of the glass composite.

[0006]

According to the present invention, the inner core (center core) is made of GeO ₂ -Si in order to suppress a change in the refractive index distribution before and after spinning.
O ₂ , outer core is SiO ₂ , first cladding is F-Si
Consist O _2, is obtained by a dispersion shifted fiber structure having a stepped refractive index distribution having a second cladding portion made of SiO ₂ on the outside of the first cladding portion, flop of one embodiment of the present invention in FIG. 1 4 shows a refractive index distribution in a reforming stage.
In FIG. 1, 11 is an inner core, 12 is an outer core, 13
Denotes a first clad and 14 denotes a second clad. That is, the tension of the drawing tension during spinning is not limited to the outer core SiO ₂ alone, but the tension is concentrated on the SiO ₂ portion of the second clad portion having a larger sectional area than the outer core portion, and the tension of the outer core portion is increased. By reducing the concentration, the change in the refractive index distribution after spinning is suppressed. In the composition of the fiber of the present invention, the inner core is GeO ₂ —SiO ₂ , the outer core is SiO ₂ , the first clad is F—SiO ₂ ,
By making the cladding of SiO ₂ , coexistence of Ge and F is avoided, and the loss of the dispersion-shifted fiber is reduced. The inner core diameter, outer core diameter, first clad diameter,
Assuming that the second cladding diameters are a, b, c, and d, respectively,
Typical diameter ratios in the single mode fiber of the present invention include the following. Inner core diameter / outer core diameter (a / b) = 0.25 to 0.4
5 0.3 <c / d <1.0 (this is to prevent the mode cutoff on the long wavelength side by having a depressed structure)

By employing the refractive index distribution structure of the present invention, the tensile tension concentrated on the SiO ₂ portion of the outer core during drawing can be reduced, so that the refractive index distribution after spinning is almost the same as that of the preform before spinning. It can be shaped. Therefore, the mode field diameter and the zero-dispersion wavelength of the spun fiber almost match the predicted values at the preform stage, and are very effective in manufacturing and quality-controlling the dispersion-shifted fiber.
Conventionally, glass containing GeO ₂ in the core is heated at a high temperature during drawing, so that Ge is 4 as shown in the following equation (1).
Is reduced from divalent to divalent. GeO ₂ → GeO + ／ O ₂ ... (1) It is known that divalent Ge has a large absorption band in the ultraviolet region, and this absorption adversely affects the communication wavelength band of 1.55 μm. Therefore, drawing at a lower temperature, in other words, with a high tension, is more advantageous from the viewpoint of transmission loss. In the present invention, specifically, it is preferable to draw with a tension of 40 g or more. Further, in the structure of the present invention, Ge
Since F and F do not coexist, it is also effective in suppressing structural defects.

[0008]

EXAMPLES Example 1 1) Production of Glass Body for Inner Core-Outer Core A soot body for a core was produced with a configuration as shown in FIG. 5
Reference numeral 1 denotes a burner for synthesizing glass fine particles for the inner core (referred to as an inner core burner), 52 denotes a burner for synthesizing glass fine particles for the outer core (referred to as an outer core burner), and the inner core burner 51 includes GeCl ₄ and SiCl. ₄ ,
H ₂ , O ₂ and inert gas are supplied, and GeCl ₄ , SiC
l ₄ are reacted in an oxyhydrogen flame to obtain GeO ₂ -containing sulfur.
The iO ₂ glass fine particles are generated, and the inner core soot body 53 is deposited on the leading end of the starting material 55. The starting material 55 is pulled upward while rotating as the inner core soot body 53 grows. On the other hand, the outer core burner 52 includes:
By supplying SiCl ₄ , H ₂ , O ₂ , and an inert gas, the outer core soot body 54 made of SiO ₂ glass fine particles is formed so as to surround the inner core soot body 53. In the present embodiment, H ₂ .
0 l / min, O ₂ 10 l / min, SiCl ₄ 8
5 cc / min, 4.2 cc / min of GeCl ₄ and 3.5 liter / min of Ar were supplied to the burner 52 for the outer core.
₂ 8.0 l / min, O ₂ 5 l / min, SiCl
₄ By supplying 300 cc / min and Ar 2 liter / min, the outer diameter is 80 mmφ (the inner core diameter is 25 mmφ),
A core soot body having a length of 500 mm was obtained at a pulling speed of 50 mm / hour. This soot body for a core was first inserted into a furnace having a ring-shaped carbon heater, heated to 1050 ° C., and heated and dehydrated by setting the atmosphere in the furnace to Cl ₂ : He = 3: 100. Next, the core soot body is
Transparent vitrification was performed by heating to 1600 ° C. in a 100% atmosphere. As a result, outer diameter 35mm
A transparent glass body for a core having an inner core diameter of 12 mmφ was obtained. The transparent glass body for a core thus obtained was heated to about 1900 ° C. in an electric resistance furnace to have a diameter of 3.8 mmφ.
The glass was stretched to a transparent glass body having an inner core and an outer core. Here, it is not preferable that the surface of the transparent glass body for the core is contaminated with OH groups and the transmission loss after fiberization is remarkably increased if the substrate is heated with a flame containing an OH component such as an oxyhydrogen burner during the stretching.

2) Preparation of First Transparent Glass Body for Cladding A single soot body for cladding consisting of SiO ₂ was produced by a VAD method using one burner for synthesizing glass fine particles. The burner, H ₂ 30 liters / min, O ₂ 25
Liter / minute, Ar 15 liter / minute, SiCl ₄ 16
00cc / min, outer diameter 110mmφ, length 550
mm was obtained. This soot body for cladding is inserted into a furnace having an atmosphere of Cl ₂ : He = 3: 100, heated to 1050 ° C. and subjected to a dehydration treatment.
It is heated to 1200 ° C. in an atmosphere of SiF ₄ : He = 8: 100 to perform an F addition treatment, and further, SiF ₄ : He =
The glass was heated to 1600 ° C. in an atmosphere of 8: 100 to perform transparent vitrification. As a result, outer diameter 50mmφ, length 270m
m was obtained. 8 mm at the center of the transparent glass body for cladding using an ultrasonic punch.
After making a pipe with a hole of φ, it was stretched to 25 mmφ (the inner diameter became about 4.0 mmφ at this time). Next, by heating the inside of the transparent glass body for cladding with an oxyhydrogen burner while flowing SF ₆ therein, the inner surface was gas-etched until the inner diameter became about 7 mmφ. By this gas etching, scratches and irregularities generated on the inner surface at the time of perforation were eliminated, and a smooth inner surface was obtained.

3) Integration of the core transparent glass body and the first clad transparent glass body As shown in FIG. 6, the core transparent glass body (3.8 mmφ) comprising the inner core and the outer core obtained in 1) above. ) 61
Is inserted into the hollow portion of the first transparent glass body for cladding (outside diameter 25 mmφ, inside diameter 7 mmφ) 62 obtained in the above 2), and the first first oxyhydrogen burner 63 is used from outside.
By heating so that the cladding glass body surface temperature becomes 1700 to 1800 ° C., the first cladding glass body 62 is contracted, and the first cladding glass inner wall and the core glass body surface are fused to each other. Was integrated. In addition,
Reference numeral 64 denotes a dummy tube. The inner core diameter at this time is 1.3m
mφ, outer core diameter is 3.8 mmφ, glass base material outer diameter is 2
It was 2.8 mmφ.

4) Preparation of Transparent Glass Body for Second Clad The glass base material obtained in the above 3) is stretched to an outer diameter of 16 mmφ to form an inner core-outer core-first clad glass composite. 7 is formed on the inner core-outer core-first cladding glass composite 71 by using a VAD apparatus of the form shown in FIG.
The porous glass body 72 made of iO ₂ was deposited, and then subjected to the same heat dehydration and transparency treatment as in 1) to obtain a preform having an outer diameter of 55 mmφ. FIG. 1 shows the refractive index distribution of the glass base material of the present invention thus obtained.

5) Fiberization The glass base material obtained in 4) was drawn to an outer diameter of 25 mmφ, and then drawn to an outer diameter of 125 μm with a drawing tension of 70 g.
The refractive index distribution after spinning was measured for the zero dispersion wavelength and the mode field diameter.

6) Evaluation of Fiber Characteristics Table 1 shows the zero-dispersion wavelength and the mode field diameter of the fiber drawn with the preform structure of the present invention in comparison with those of the fiber drawn with the conventional preform structure.

[Table 1] By using the preform structure of the present invention, it was confirmed that the zero-dispersion wavelength and the mode field diameter after spinning almost matched the predicted values. Also, the transmission loss was as good as 0.20 dB / km at 1.55 μm.

Example 2 In Example 1, a transparent glass body for the first clad was prepared by the following alternative method, and otherwise, a preform for a dispersion-shifted fiber was prepared in the same manner as in Example 1. Evaluation was performed in the same manner as in Example 1. The result was equivalent to that of Example 1. 2 ') Preparation of transparent glass body for first clad By VAD method, SiCl ₄ and CF ₄ were supplied as raw materials using one burner for synthesizing glass fine particles, and FS
A soot body for cladding made of iO ₂ was prepared. The burner contains 30 liters / minute H ₂ , 25 liters / minute O ₂ ,
Ar 15 liters / minute, SiCl ₄ 1600 cc / minute,
By supplying CF _{4 at} 300 cc / min, a soot body for cladding having an outer diameter of 110 mmφ and a length of 550 mm was obtained. This soot body for cladding was inserted into a furnace having an atmosphere of Cl ₂ : He = 3: 100, heated to 1050 ° C. to perform a dehydration treatment, and then vitrified in an atmosphere of 100% He. . As a result, a columnar transparent glass body for the first clad having an outer diameter of 50 mmφ and a length of 270 mm was obtained. 8 mm at the center of the transparent glass body for the first clad using an ultrasonic punch.
After making a pipe with a hole of φ, it was stretched to 25 mmφ (the inner diameter became about 4.0 mmφ at this time). Next, the inner surface was gas-etched by heating with an oxyhydrogen burner from the outside while flowing SF ₆ inside the first cladding transparent glass body until the inner diameter became about 7 mmφ. By this gas etching, scratches and irregularities generated on the inner surface at the time of perforation were eliminated, and a smooth inner surface was obtained.

Comparative Example 1 The inner core was GeO ₂ -F-SiO, and the outer core was F-Si
O ₂ , first cladding is F—SiO ₂ , second cladding is S
A dispersion-shifted fiber made of iO ₂ was manufactured and its characteristics were evaluated. Although the mode field diameter and the zero-dispersion wavelength could almost match the predicted values, the transmission loss at 1.55 μm was only 0.22 dB / km.

In the present invention, the method of synthesizing the outer cladding (second cladding) is not limited to the structure shown in FIG.
As shown in the figure, the inner glass-outer core-first clad composite glass body 82 is set in a horizontal direction or a vertical direction, and the method of moving the composite glass body 82 and the burner 81 relatively left, right, up and down is used. You may. In FIG. 8, reference numeral 83 denotes a dummy quartz rod, and reference numeral 84 denotes a porous glass body serving as an outer clad portion.

[0017]

According to the present invention, as described above, F-SiO
By forming a second clad made of SiO ₂ outside the first clad made of ₂ glass, Si of the outer core is formed.
Since the drawing tension applied to the O ₂ portion is reduced, the change in the refractive index distribution before and after spinning is suppressed, and the characteristic predicted value in the preform stage and the characteristic value of the spun fiber can be almost matched, so that the quality of the product can be improved. It is extremely effective in management. Also,
Since the change in the refractive index distribution can be reduced even when the drawing tension is set to a high tension, the transmission loss deterioration factor having absorption in the ultraviolet region caused by GeO can be reduced, and the loss deterioration factor caused by the structural defect due to Ge-F coexistence can be reduced. Can also be reduced, which is also effective in reducing the loss.

[Brief description of the drawings]

FIG. 1 is a diagram showing a refractive index distribution structure of a specific example of the dispersion-shifted fiber of the present invention.

FIG. 2 is a diagram illustrating a step-like refractive index distribution of a conventional dispersion-shifted fiber.

FIG. 3 is a diagram illustrating a step-like refractive index distribution of a conventional dispersion-shifted fiber.

FIG. 4 is a diagram showing a change in a refractive index distribution structure before and after drawing in a conventional dispersion-shifted fiber.

FIG. 5 is a diagram illustrating a method for producing a porous glass body for an inner core in an example of the present invention.

FIG. 6 is an explanatory view of a method of heating and integrating a transparent glass body for an inner core and an outer core and a transparent glass body for a pipe-shaped first clad in an embodiment of the present invention.

FIG. 7 is an explanatory view of a composite having inner and outer cores and a first clad and a method of depositing a porous glass body on an outer peripheral portion in an embodiment of the present invention.

FIG. 8 is a diagram illustrating a composite having an inner and outer core and a first clad and another method of depositing a porous glass body on an outer peripheral portion in the present invention.

FIG. 9 is a view showing a refractive index distribution structure of a preform obtained in Comparative Example 1.

[Explanation of symbols]

Reference Signs List 11 inner core (GeO ₂ —SiO ₂ ) 12 outer core (SiO ₂ ) 13 first clad (F-SiO ₂ ) 14 second clad (SiO ₂ ) 21 inner core 22 outer core 23 clad 31 inner core (GeO ₂ −) SiO ₂ ) 32 Outer core (SiO ₂ ) 33 Cladding (F-SiO ₂ ) 51 Burner for synthesizing glass fine particles for inner core 52 Burner for synthesizing glass fine particles for outer core 53 Soot body for inner core 54 Soot body for outer core 55 Starting Material 61 Inner Core-Outer Core Composite Transparent Glass Body 62 First Clad Transparent Glass Body 63 Hydrogen Oxygen Burner 64 Dummy Quartz Pipe 71 Inner Core-Outer Core-First Clad Glass Composite 72 Glass body 81 Hydrogen oxyhydrogen burner 82 Inner core-Outer core-Composite glass body for first clad 83 Dummy quartz rod 84 Porous glass body to be the second clad

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshio Torazuka 1 Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Hiroshi Yokota 1 Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric (72) Inventor Shoji Ohashi 1-6-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (58) Field surveyed (Int. Cl. ⁷ , DB name) C03C 13/04 C03B 37/012 G02B 6/00 356 G02B 6/16 321 G02B 6/22

Claims (2)

(57) [Claims] An outer core having a lower refractive index than the inner core is formed on an outer periphery of the inner core, and a first clad having a lower refractive index than the outer core is formed on an outer layer of the outer core, and has a step-like cladding distribution. Further, a second clad having a higher refractive index than the first clad is formed in an outer layer of the first clad. In the dispersion-shifted fiber having a step-like refractive index distribution, the inner core is GeO ₂ —SiO ₂ , and the outer The core is S
A dispersion-shifted fiber comprising iO ₂ , a first cladding made of F-SiO ₂ , and a second cladding made of SiO ₂ . 2. A glass body having an inner core made of GeO ₂ —SiO ₂ and an outer core made of SiO ₂ is F
-SiO by heating integrally inserted into the hollow portion of the glass body ₂ first cladding part pipe-shaped consisting of, GeO ₂ -SiO ₂ inner core, the outer core SiO _2, first
A clad is formed of a glass composite made of F-SiO _2, and a _second composite made of SiO ₂ is further formed around the glass composite.
A method for manufacturing a dispersion-shifted fiber, comprising forming a clad.