JPH09251121A - Distributed refractive index optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber which has a wide band adequate for communication, is easily joinable to an existing quartz GI type optical fiber having an optical fiber diameter of 125μm and has an excellent low-temp. characteristic. SOLUTION: This optical fiber is composed of a core 1 of quartz glass having a refractive index distribution, a clad 2 of quartz glass disposed in tight contact with the outer periphery of this clad 2, a polymer coating layer 3 disposed in tight contact with the outer periphery of this clad 2 and primary coating layers 4, 5 consisting of one or plural layers disposed in tight contact with the outer periphery of this polymer coating layer 3. The polymer coating layer 3 consists of a rigid polymer having >=5μm thickness, Shore hardness D of >=55 and glass transition temp. of >=60 deg.C. The innermost layer 4 of the primary coating layers consists of a soft polymer having a modulus of elasticity in tension of <=1kg/mm<2> at 23 deg.C and glass transition temp. of <=-20 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradient index optical fiber which can be suitably used for optical transmission in the information communication field such as voice, video and data by taking advantage of high-speed signal transmission. Is.

[0002]

2. Description of the Related Art As a communication optical fiber for constructing a high-speed and large-capacity data link, there are a silica-based multimode optical fiber using silica-based glass for a core and a clad, and a silica-based single-mode optical fiber. Industrial standard (JIS) C6832, 6835 or 6831
The standards are defined in. In these optical fiber core wires, a primary coating layer of ultraviolet (UV) curable resin or silicone resin is formed adjacent to the outside of the quartz glass clad, and a secondary coating layer of nylon or the like is further formed outside thereof.

Optical fiber capable of high-speed optical signal transmission (for example, optical fiber having a band of 200 MHz · km or more)
Are a refractive index distribution (graded index) type optical fiber (hereinafter referred to as quartz GI type optical fiber) and a quartz type single mode optical fiber among the silica type multimode optical fibers.

In the silica GI type optical fiber, the refractive index distribution in the core region is controlled so as to reduce the mode dispersion which is a factor limiting the transmission band, and the core diameter is usually 50 to 100 μm (particularly 50 μm, 62.5 μm).

In the case of this silica GI type optical fiber, since the core and the cladding are composed of silica glass, if the primary coating layer and the secondary coating layer made of polymer are removed and the crimp type connector is crimped and crimped, the optical Fiber cracks and chips occur. Further, if the crimp-type connector is crimped and crimped while the primary coating layer or the secondary coating layer is left, the core is misaligned, resulting in a connection loss that is unbearable for practical use.

Therefore, in order to avoid these problems, conventionally, the primary coating layer and the secondary coating layer made of a polymer are removed, and then a thermosetting resin, an ultraviolet curable resin, a thermoplastic resin, or a two-component mixed curable resin is used. It was necessary to attach an adhesive connector that secured the center of the connector with an adhesive.

However, the adhesive-type connector has a problem that it takes a long time to attach the connector, which is not desirable in terms of cost and labor.

On the other hand, the crimp type connector has an advantage that it can be mounted much easier and in a shorter time than the adhesive type connector, and is used in a polymer clad quartz optical fiber and a plastic optical fiber. . However, these have not only a small transmission band but also a large diameter, which makes it difficult to join with an existing quartz GI type optical fiber standardized by the standard of an optical fiber diameter of 125 ± 3 μm. There is a problem that it is difficult to have compatibility with the GI type optical fiber.

Further, by providing a coating layer made of a hard polymer such as a thermoplastic fluorinated acrylate resin having a Shore hardness of D65 or more in close contact with the outer periphery of the cladding of the quartz fiber, the crimp type connector can be crimped and crimped. An attempt has been proposed in Japanese Patent Laid-Open No. 2-151821. This also has a similar problem because the cladding diameter of the optical fiber is 125 μm and the coating diameter of the hard polymer is 140 μm or more. Also,
Transmission of thick optical fiber strands and core wires with a hard polymer coating diameter of 140 μm or more, compared to existing 125 μm diameter quartz GI optical fiber strands and core wires without coating with hard polymer Although the loss is a little worse, it is at a comparable level. However, the coating diameter of the hard polymer is 125
When the thickness of the clad layer is reduced to ± 3 μm, problems such as transmission loss and its low temperature characteristics are rapidly deteriorated, which makes practical application difficult.

[0010]

Therefore, even if the coating diameter of the hard polymer is 125 ± 3 μm or less, which is the same as the general outer diameter of the silica GI type optical fiber, the loss deterioration is small and the low temperature characteristics are good. Moreover, it has been required that not only the fiber cracking and chipping do not occur even when crimping and crimping the crimp type connector, but also that the crimping force is large and the crimping loss is small.

Therefore, the present invention solves the above-mentioned drawbacks of the prior art, and even if the crimping type connector is crimped and crimped so that the crimping connector has a sufficient crimping force, the generation of fiber cracks and chips can be prevented. The loss is small, the connection loss can be reduced, the optical fiber diameter is 125 μm, the splicing is easy with the existing silica GI optical fiber, the compatibility with the existing silica GI optical fiber is excellent, and the low temperature characteristics The main object of the present invention is to provide a good graded index optical fiber.

[0012]

In order to achieve this object, a gradient index optical fiber of the present invention is provided with a core made of silica glass having a refractive index distribution and closely attached to the outer periphery of the core. At least a clad made of silica-based glass, a polymer coating layer provided in close contact with the outer periphery of the clad, and a primary coating layer formed of one or more layers provided in close contact with the outer periphery of the polymer cover layer. In the gradient index optical fiber described above, the polymer coating layer has a thickness of 5 μm or more and a Shore hardness of D55.
As described above, the innermost layer of the primary coating layer has a glass transition temperature of 60 ° C. or higher, and the innermost layer has a tensile elastic modulus at 23 ° C. of 1 kg / mm ² or less and a glass transition temperature of −20.
It is characterized by comprising a soft polymer having a temperature of ℃ or less. Particularly, it is effective when the outer diameter of the polymer coating layer is 125 ± 3 μm.

Further, in the optical fiber of the present invention, the primary coating layer is composed of at least two layers, and the second layer from the inside has a glass transition temperature of 60 ° C. or higher and a Shore hardness of D55 or higher and does not contain a fluorine atom. It is preferable that

In the present invention, an optical fiber consisting only of a glass core, a glass clad, and a polymer coating layer of a hard polymer provided in close contact with the outer periphery thereof is provided.
The above-mentioned optical fiber is hereinafter referred to as a bare optical fiber or simply an optical fiber because it is handled in the same manner as a conventional bare optical fiber composed of only a glass core and a glass clad. In addition, a primary coating layer (1) such as an ultraviolet curable resin (hereinafter referred to as a UV curable resin) is provided on the outer circumference of the bare optical fiber.
Or a plurality of layers) is referred to as an optical fiber elemental wire, and the optical fiber elemental wire is composed of a single core or a plurality of cores and a secondary coating layer such as nylon or UV curable resin on the outer periphery of the optical fiber. The core line.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a core made of silica-based glass is a central portion of an optical fiber in which incident optical power is confined and transmitted, and a silica glass having a refractive index distribution formed by a dopant. Consists of. The clad surrounds the outer periphery of the core and has a lower refractive index than the core, and is made of silica glass.

The clad diameter is the diameter of the circle that most closely approximates the outer surface of the clad, and the core diameter is the diameter of the circle that most closely approximates the core region. The core region is a region having a refractive index n represented by the following formula and occupies the inside of the optical fiber. n ₁ ≧ n ≧ n ₂ + 0.05 × (n ₁ −n ₂ ) (where n ₁ = maximum refractive index of the core and n ₂ = refractive index of the cladding).

As described above, the optical fiber composed of the gradient index silica-based glass core and the silica-based glass clad may be obtained by heating and drawing a precursor quartz glass rod having a desired refractive index distribution. . The precursor quartz glass rod can be manufactured by subjecting a glass main raw material made of a silicon compound and a glass dope raw material made of a germanium compound to a gas phase reaction. Although the method is known,
There is no particular limitation as long as an appropriate refractive index distribution and low loss can be obtained.

The silica-based glass core according to the present invention comprises:
Having a refractive index profile parameter α of 8 to 2.2,
It is preferable to realize a signal transmission band of 150 MHz · km, and particularly preferably in the range of 1.9 to 2.1 in order to further widen the signal transmission band.

The core diameter in the present invention is the same as that of the existing silica type G
From the viewpoint of ensuring compatibility and compatibility with the I-type optical fiber, it is preferably 65.5 μm or less, and particularly 5 μm or less.
0 ± 3 μm or 62.5 ± 3 μm is preferable. Further, the clad diameter is 11 from the viewpoint that the outer diameter and the thickness of the polymer coating layer made of a hard polymer are set to desired values.
It is preferably 8 μm or less, and particularly preferably 97 to 118 μm.

The numerical aperture (NA) of the optical fiber is calculated from the maximum refractive index of the core and the refractive index of the clad according to the following equation. The numerical aperture is 0.30 or less, and particularly 0.
The range of 18 to 0.29 is preferable for obtaining a sufficient wide band. NA = (n ₁ ² −n ₂ ² ) ^1/2 (where n _{1 is} the maximum refractive index of the core, and n ₂ is the refractive index of the clad.)

Among them, from the viewpoint of ensuring compatibility and compatibility with the existing silica-based GI optical fiber, the numerical aperture (that is, core diameter / bareness) specified in C6832 of Japanese Industrial Standard (JIS) is used. NA = 0.20 ± 0.02 when optical fiber diameter = 50/125 μm, NA = 0.275 ± 0.0 when core diameter / naked optical fiber diameter = 62.5 / 125 μm
It is preferable to make it about the same as 15).

The polymer coating layer adhered to the outer periphery of the optical fiber clad of the present invention must be made of a hard polymer having a Shore hardness of D55 or more and a glass transition temperature of 60 ° C. or more and a thickness of 5 μm or more.

When the Shore hardness of the polymer coating layer is D55 or more, the connection loss when the crimping type connector is caulked and attached can be suppressed, and at the same time, the core axis deviation can be reduced, and further the optical axis can be reduced. A clean cut surface or a polished surface can be obtained during fiber cutting or polishing.
The Shore hardness D is a value measured by the D method of ASTM-D2240, and the Shore hardness of a hard polymer is a polymer plate obtained by curing it alone into a plate under the same curing conditions as when manufacturing an optical fiber. The hardness may be obtained by measuring the hardness of the above.

In the case of a bare optical fiber having a polymer coating layer as the outermost layer, the hardness of the polymer coating layer is determined by a dynamic hardness (a micro compression tester (manufactured by Shimadzu Corporation, MCTE-500)) by indentation of a diamond. Use, diamond regular triangular indenter (ridge interval = 115 °)
Can be measured with an indenter used) or micro Vickers hardness. There is a substantially proportional relationship between the value of the dynamic hardness or the micro Vickers hardness thus measured and the Shore hardness D as shown in FIG. Therefore, the hardness level of the Shore hardness D55 or more of the hard polymer constituting the polymer coating layer corresponds to 5 or more of the dynamic hardness or the micro Vickers hardness.

The polymer coating layer made of this hard polymer needs to have a thickness of 5 μm or more, and particularly preferably 5 to 15 μm. When crimping the crimp type connector by caulking, a thickness of 5 μm or more is necessary to prevent stress concentration due to caulking force and to prevent cracking or chipping in the glass clad or the glass core. Further, the thicker the polymer coating layer is, the more the above effect is enhanced. However, if the polymer coating layer is too thick, the clad becomes too thin to maintain a small diameter of 128 μm or less, resulting in an increase in microbend loss. Since an increase in axis deviation easily occurs, it is preferable that the thickness is 15 μm or less.

Therefore, the outer diameter of the polymer coating layer is 1
The thickness is preferably 28 μm or less, and particularly preferably 125 ± 3 μm. Such a small diameter facilitates splicing with an existing quartz GI type optical fiber standardized by an optical fiber diameter of 125 ± 3 μm, and further provides compatibility with the existing quartz GI type optical fiber. be able to. That is, it becomes easy to ensure compatibility and compatibility with the existing silica GI type optical fiber. The outer diameter of the polymer coating layer is the diameter of the circle that most closely approximates the outer surface of the polymer coating layer.

This hard polymer has a linear expansion coefficient (AST
M × D696) is 0.6 × 10 ^{−4 to} 2.0 ×
It is preferably 10 ⁻⁴ / deg. 0.6 x 10 ^-4
If it is smaller than / deg, it is too fragile and easily chipped when cutting or polishing the optical fiber, which is not preferable for obtaining a clean cut surface or a polished surface. On the contrary, if it is larger than 2.0 × 10 ⁻⁴ / deg, the microbend loss tends to be large.

The hard polymer of the polymer coating layer preferably has a refractive index sufficiently higher than that of the silica glass forming the clad. That is, since the silica glass of the clad generally has a refractive index of 1.458, the refractive index of the hard polymer is preferably 1.48 or more. When the refractive index is lower than this range, the internal surface of the polymer coating layer is also reflected to the inside of the transmitted light to generate different modes, and the band is narrowed, which is not preferable.

UV curable resins are preferred as the hard polymer of the present invention. In the case of a thermoplastic resin, it is necessary to melt-coat the outer circumference of the quartz clad, or to coat it as a solution in a solvent and then dry it, but the adhesion with the clad is not high, the wire diameter is difficult to stabilize, and However, there are drawbacks such as microbend loss, poor low temperature characteristics, and difficulty in specular fracture when the optical fiber is stress-ruptured.

Among UV curable resins, acrylate type U
V-curable resin is preferable because of its fast curing speed. Since the curing rate is not sufficient with other UV curable resins, there is a problem that the drawing rate must be slowed down in order to sufficiently cure to a desired hardness level, which is not preferable.

The curable monomer (composition) for forming the acrylate-based UV curable resin is mainly composed of a curable monomer having an acryloyl group and / or a methacryloyl group having a UV-curable double bond. ,
As long as it has a Shore hardness of D55 or more after curing, it may be composed of a single monomer or a mixed monomer, and the number of double bonds in the molecule may be any, and further, amide in the molecule. Group, imide group, urethane group, ester group, ether group, epoxy group, hydroxyl group,
It may have a bond such as a carbonate group, a ketone group, a sulfone group, a sulfide group, a melamine bond, or a siloxane group.

It is also possible for the resin to contain halogen atoms. However, the inclusion of fluorine atoms is not preferable because the surface friction is reduced, the caulking crimping force of the connector is reduced, and the connection loss during caulking crimping tends to be large. Further, if the content of fluorine in the resin is large, the refractive index becomes low, and it becomes difficult to obtain a preferable refractive index level, which is not preferable. Further, the hard polymer may contain a silane coupling agent or the like that can enhance the adhesion to the quartz-based clad.

The above-mentioned polymer coating layer and primary coating layer may be formed by the following method.

Following the step of drawing a GI type quartz matrix having a quartz glass core and a quartz glass clad, an uncured curable composition for the hard polymer of the polymer coating layer is applied to the surface of the drawn optical fiber substrate. To a predetermined thickness, followed by UV irradiation to cure to a desired hardness and refractive index to form a polymer coating layer.
Further, a primary coating layer is provided in close contact with the outer periphery of the polymer coating layer.

The innermost layer among them must be a soft polymer having a tensile elastic modulus at 23 ° C. of 1 kg / mm ² or less. If the innermost layer contains a hard polymer coating having a tensile elastic modulus of more than 1 kg / mm ² , there is a problem that microbend loss increases. For this soft polymer, an acrylate-based or silicon-based UV curable resin used in conventional optical fibers can be preferably used.

The outer periphery of this soft polymer can be coated with a harder polymer. This polymer may be selected from the hard polymers mentioned above for the polymer coating layer, and may be the same polymer as the hard polymer used for the polymer coating layer.

The outer diameter of these primary coating layers can be freely selected, but usually about 250 to 500 μm is suitable.

It is important that the polymer used as the innermost layer of the polymer coating layer and the primary coating layer has a glass transition temperature (Tg) outside the usable temperature range of the optical fiber. Specifically, it is necessary that the hard polymer of the polymer coating layer has a Tg of 60 ° C. or higher and the soft polymer of the innermost layer of the primary coating layer has a Tg of −20 ° C. or lower. Furthermore, when a hard polymer layer is provided as the second primary coating layer on the outer periphery of the innermost layer (soft polymer), the Tg is preferably 60 ° C. or higher. Furthermore, it is more preferable that the respective glass transition temperatures are 70 ° C. or higher, −30 ° C. or lower, and 70 ° C. or higher in order. If the glass transition temperature of each coating polymer is within the operating temperature range (-20 to 60 ° C), the hardness and the tensile elastic modulus change significantly near that temperature, which causes an increase in transmission loss of the optical fiber. May break the optical fiber.

Further, on the outside of the primary coating layer, a single core or two cores may be provided with a secondary coating to form an optical fiber core wire, or a tape shape may be used in which three or more cores are provided with a secondary coating. . Secondary coating materials include heat-resistant fluororesins such as tetrafluoroethylene / hexafluoropropylene copolymer, ethylene / tetrafluoroethylene copolymer, polytetrafluoroethylene, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer Polyamide resin such as nylon 11 or nylon 12; polyimide resin or UV curable resin can be used, and may be arbitrarily selected according to the application and required characteristics.

A polymer sheath layer is further formed on this optical fiber core wire so as to have a tensile strength, so that the optical fiber cord is in an actually used state. In order to provide a tensile strength, a tensile strength body such as steel fiber or aramid fiber may be interposed, or a composite cable may be used together with a copper electric wire.

Like the conventional quartz GI type optical fiber core wire and cord, the optical fiber core wire and the optical fiber cord of the present invention can be attached to the terminal at the time of use. It is preferable to use a crimp type connector that can perform crimp crimping.

That is, in the case of the optical fiber core wire, the crimp connector is inserted in a state where the primary coating layer and the secondary coating layer of the terminal portion to which the connector is to be attached are removed, and the polymer coating layer is not removed. Then, they may be fixed by caulking, and then the connection end surface treatment may be carried out by a usual method if necessary.

In the case of an optical fiber cord, the crimping connector is inserted in a state where the primary coating layer, the secondary coating layer and the polymer sheath layer of the terminal portion to which the connector is to be attached are peeled off and the polymer coating layer is not peeled off. It may be attached, fixed by caulking, and then, if necessary, a connection end surface treatment may be performed by a usual method.

As described above, according to the present invention, when the crimp connector is attached, the polymer coating layer is left, and the crimp connector is attached by caulking.

Therefore, it is possible to assemble the connector in a shorter time than the mounting of the adhesive type connector, and moreover, it is possible to mount it without using an adhesive, so that when laying the optical fiber core wire, cord or cable, Connector installation work at the laying site is greatly simplified.

In the present invention, since the polymer coating layer made of a specific hard polymer is provided, the connection end face treatment after the crimp type connector is attached to the optical fiber cord is a sufficiently clean mirror surface only by the end face formation by stress rupture. But you can
End face polishing may be performed. End face formation by stress rupture
The end face formation by polishing is excellent in simplicity and the reduction of connection loss and reproducibility. The end surface polishing may be flat surface polishing or spherical surface polishing.

Each physical property value specified in the present invention is a value measured by the following method.

Shore D of Polymer D: ASTM-D
2240 is a value measured based on the D method, in which case the curable composition for forming the polymer coating layer is cured under the same curing conditions as when the polymer coating layer was formed, and the thickness is 1 mm. It may be measured using a sample of the polymer plate of In the case of a soft polymer for the primary coating layer, a polymer plate prepared by curing the curable composition for forming the polymer under the same curing conditions as when forming the primary coating layer may be used as a sample.

Glass transition temperature Tg (° C): "TMA"
(Manufactured by Seiko Denshi KK), and is measured under the condition of 10 ° C./min.

Tensile elastic modulus (kg / cm ² ): JIS K
Measured using a No. 2 dumbbell tester based on -7113.

Low temperature characteristics (dB): Sample length 1k in constant temperature bath
m optical fiber was put in, both ends 1m were taken out of the thermostatic chamber, left at 60 ° C for 1 hr, then lowered to -20 ° C and left for 1 hour, and the loss increase amount measured by the 850 nm LED was measured for low temperature characteristics. The value.

Ferrule Caulking Loss (dB): The amount of transmitted light measured by an 850 nm LED for an optical fiber having a sample length of 3 m and having no connector and no caulking and a connector attached to one end was taken as the initial amount of light. Then, the connector was attached to the other end by caulking while adjusting the pulling force of the optical fiber from the connector to be 2 kgf, and the amount of transmitted light in that state was measured in the same manner as above. The difference between the value of the transmitted light amount and the initial light amount was obtained, and the average value of the number of measurements 3 was taken as the ferrule caulking loss.

[0053]

【Example】

Example 1 A silica-based GI type glass preform was continuously supplied to a heating furnace at 2200 ° C. and drawn so that the cladding diameter was 100 μm. At this time, the core diameter was 50 μm. On the other hand, the curable composition for UV-curable urethane acrylate resin having Tg = 86 ° C. and Shore D hardness of 64 after curing was filtered through a 0.1 μm filter and supplied to a coating die.

After the curable composition of the coating die was applied to the surface of the drawn optical fiber, 3
A bare optical fiber having an outer diameter of the polymer coating layer of 125 μm was manufactured by irradiating light from an ultraviolet lamp having a center wavelength of 60 nm to cure the resin, and winding it on a bobbin at a constant speed by a roller.

Subsequently, two kinds of UV-curable urethane acrylate resins for primary coating (each having Tg = -42 ° C.,
Soft with a tensile modulus of 0.09 kg / cm ² , Tg = 104
℃, Shore D60 hardness) each 250μ outer diameter
m and 400 μm, apply and cure,
Further, nylon 12 for secondary coating was melt-coated so as to have an outer diameter of 900 μm to obtain an optical fiber core wire. This core wire was covered with a polymer sheath layer having an outer diameter of 4 mm to obtain an optical fiber cord. 850 of the obtained optical fiber cord
The values of the transmission loss and the transmission band in nm were 2.5 dB / km and 390 MHz · km, respectively, and the optical fiber was excellent in both the transmission loss and the transmission band. The numerical aperture (NA) was 0.20.

The outer layer up to the primary coating was removed from the end portion of the obtained optical fiber cord to form a bare optical fiber having an outer diameter of 125 μm, and this bare optical fiber portion was a crimp type connector for PCF (manufactured by Toshiba Corp .; Model TOCP101QK;
Clad diameter 230μm), the same structure, clad diameter 1
The connector modified for 25 μm was crimped by crimping only the polymer coating layer, and the fiber end portion protruding from the connector was stress-ruptured with a fiber cutter to expose the end face.

The optical fiber to which the connector was attached was not damaged by crimping, and the ferrule caulking loss was as small as 0.04 dB.

The increase in loss at a low temperature of -20 ° C after being left at 60 ° C was as small as 0.2 dB.

[Comparative Examples 1 and 2] An optical fiber cord was prepared in the same manner as in Example 1 except that the polymer coating layer and the primary coating soft / hard polymer were the ultraviolet-curable urethane acrylate resins shown in Table 1, respectively. Obtained.

Tg of the coating polymer constituting the composition is -20 to 6
Since it is in the range of 0 ° C, the loss increase at a low temperature of -20 ° C after being left at 60 ° C is large, and the Shore D of the polymer coating layer is further increased.
In Comparative Example 2 having low hardness, the ferrule caulking loss was large.

[Examples 2 and 3, Comparative Examples 3 and 4] As shown in Table 1, in addition to the Tg, Shore D hardness or tensile modulus of the coating polymer constituting the core, the outer diameter of the core, the clad and the polymer coating layer. Optical fiber cords having different (polymer coating layer thickness), polymer coating layer type and the like were obtained in the same manner as in Example 1.

The results of Examples 2 and 3 satisfying the conditions of the present invention were good. However, the composition of the coating polymer T
6 in Comparative Examples 3 and 4 in which g is in the range of −20 to 60 ° C.
The loss increase at a low temperature of −20 ° C. after being left at 0 ° C. was large (Comparative Example 3 was broken), and the tensile elastic modulus of the primary coated soft polymer was 1.
There was also a problem that the transmission loss was large in Comparative Example 3 higher than kg / cm ^{2 and the} ferrule caulking loss was large in Comparative Example 4 in which the polymer coating layer was less than 5 μm.

[0063]

[Table 1]

[0064]

According to the present invention, even if the diameter of the silica GI type bare optical fiber having the polymer coating layer is reduced to 125 ± 3 μm or less, both the transmission loss and the transmission band are excellent, and the pressure bonding is performed. Even if the mold connector is crimped to have a sufficient crimping force, no fiber cracks or chips occur, the loss due to crimping is small, the connection loss can be reduced, and the low temperature characteristics can be good ports.

Further, even if a bare fiber having the same outer diameter as the standardized existing quartz GI type optical fiber is used, it is possible to obtain an optical fiber core wire and a cord with a connector having excellent characteristics. It is easy to splice with optical fiber and has excellent compatibility.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing an example of a gradient index optical fiber of the present invention.

FIG. 2 is a diagram showing a correlation between Shore hardness and dynamic hardness of the hard polymer used in the polymer coating layer in the present invention.

[Explanation of symbols]

1: a core made of quartz glass having a refractive index distribution,
2: Cladding made of quartz glass, 3: Polymer coating layer made of hard polymer, 4: Innermost layer of primary coating layer (made of soft polymer), 5: Second layer of primary coating layer (made of hard polymer)

Claims (4)

[Claims] 1. A core made of silica-based glass having a refractive index distribution, a cladding made of silica-based glass provided in close contact with the outer periphery of the core, a polymer coating layer provided in close contact with the outer periphery of the clad, And a gradient index optical fiber comprising at least a primary coating layer composed of one or a plurality of layers provided in close contact with the outer periphery of the polymer coating layer, wherein the polymer coating layer has a thickness of 5 μm or more. A Shore hardness D55 or higher, a glass transition temperature of 60 ° C. or higher, and the innermost layer of the primary coating layer has a tensile elastic modulus at 23 ° C. of 1 kg / mm ² or less,
A gradient index optical fiber comprising a soft polymer having a glass transition temperature of -20 ° C or lower. 2. The primary coating layer is composed of at least two layers, and the second layer from the inside is composed of an ultraviolet curable resin having a glass transition temperature of 60 ° C. or higher and a Shore hardness of D55 or higher. Graded index optical fiber. 3. The gradient index optical fiber according to claim 1, wherein the polymer coating layer and the primary coating layer are made of an ultraviolet curable resin containing no fluorine atom. 4. A core diameter of 50 ± 3 μm or 62.
The gradient index optical fiber according to claim 1, 2 or 3, wherein the outer diameter of the cladding is 5 ± 3 µm, the outer diameter of the cladding is 97 to 118 µm, and the outer diameter of the polymer coating layer is 125 ± 3 µm.