JP7527114B2 - Optical Fiber - Google Patents

Description

The present invention relates to optical fibers.

Conventionally, optical fibers with low transmission loss at a wavelength of 1550 nm are known. This type of optical fiber includes a core made of pure silica glass or silica glass doped with fluorine, chlorine, or an alkali metal. For example, only a small amount of fluorine is doped, and for example, chlorine is doped in high concentration. Examples of alkali metals include potassium (K) and sodium (Na). Note that fluorine, chlorine, or an alkali metal may be co-doped. In addition, for example, fluorine is doped in the cladding layer as an additive that reduces the refractive index of the silica glass (Patent Documents 1 to 3).

This type of low transmission loss optical fiber can be manufactured, for example, as follows. First, a silica glass rod (core rod) that will become the core of the optical fiber is manufactured, for example, by the VAD (Axial Vapor Deposition) method. Next, soot that will become the cladding layer is formed around the outer periphery of the core rod by an external deposition method called OVD (Outside Vapor Deposition). Next, the soot is sintered and vitrified in a fluorine-containing gas atmosphere to manufacture an optical fiber preform. Note that the formation of soot and vitrification of soot are usually performed multiple times until the cladding layer of the optical fiber preform has the required outer diameter. After that, an optical fiber is drawn from the manufactured optical fiber preform.

Special Publication No. 2008-536190 Special Publication No. 2013-512463 Special table 2019-526073 publication

Optical fibers are required to have even lower transmission loss. It is also preferable for them to have low transmission loss over a wide wavelength band.

The present invention has been made in view of the above, and its purpose is to provide an optical fiber with low transmission loss.

In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is an optical fiber comprising a core portion and a cladding layer formed on the outer periphery of the core portion and having a refractive index lower than the maximum refractive index of the core portion, the maximum refractive index of the core portion having a relative refractive index difference of 0% or more and 0.2% or less with respect to pure silica glass, the refractive index at the interface between the core portion and the cladding layer changes continuously in the radial direction of the optical fiber, and there are step portions in the cladding layer where the refractive index changes discontinuously at two or less positions spaced a predetermined distance radially from the interface with the core portion, and the step in the relative refractive index difference of the step portions with respect to pure silica glass is 0.005% or less.

The core portion may include a portion made of pure silica glass or silica glass doped with fluorine, chlorine, or an alkali metal, and the cladding layer may be made of silica glass doped with fluorine.

The optical fiber may have a transmission loss of less than 0.175 dB/km at a wavelength of 1550 nm and a transmission loss of 1.0 dB/km or less in the 1380 nm wavelength band.

If the core radius of the core portion is r1 [μm], then 3.0 μm≦r1 [μm]≦5.0 μm, and if the distance from the center of the core portion to the step portion closest to the core portion is r2 [μm], then 18 μm≦r2 [μm] or r2 [μm]≦24 μm, and 6<r2/r1 may be satisfied.

The optical fiber may have a transmission loss of 0.40 dB/km or less in the 1380 nm wavelength band.

The optical fiber may have a mode field diameter at a wavelength of 1310 nm of 8.6 μm or more and 9.5 μm or less, an effective cutoff wavelength of 1260 nm or less, a zero dispersion wavelength of 1300 nm or more and 1324 nm or less, and a dispersion slope at the zero dispersion wavelength of 0.092 ps/nm ² /km or less.

The present invention has the advantage of low transmission loss.

FIG. 1 is a schematic cross-sectional view of an optical fiber according to a first embodiment. FIG. 2 is a diagram showing a refractive index profile of the optical fiber shown in FIG. 3A to 3C are diagrams showing an example of a method for manufacturing the optical fiber shown in FIG. FIG. 4 is a diagram showing a refractive index profile of the optical fiber according to the second embodiment. FIG. 5 is a diagram showing a refractive index profile of the optical fiber according to the third embodiment.

The following describes in detail an embodiment of the present invention with reference to the drawings. Note that the present invention is not limited to the embodiment described below. In addition, in each drawing, the same or corresponding components are appropriately labeled with the same reference numerals, and the description is omitted as appropriate. In this specification, the cutoff wavelength is the effective cutoff wavelength, and means the cable cutoff wavelength defined in ITU-T (International Telecommunication Union) G. 650.1. In addition, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G. 650.1 and G. 650.2.

(Embodiment 1)
1 is a schematic cross-sectional view of an optical fiber according to embodiment 1. The optical fiber 10 includes a core 1 and a cladding layer 2 formed on the outer periphery of the core 1.

The core section 1 includes a portion made of, for example, pure silica glass or silica glass doped with fluorine, chlorine, or an alkali metal. Pure silica glass is extremely high-purity silica glass that contains substantially no dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm. Pure silica glass has an extremely low concentration of impurities that cause Rayleigh scattering loss, so by using it as a constituent material for the core section 1, a reduction in transmission loss can be expected. In addition, silica glass doped with fluorine, chlorine, or an alkali metal has a reduced glass viscosity, so by using it as a constituent material for the core section 1, a further reduction in Rayleigh scattering loss can be expected.

The cladding layer 2 has a refractive index lower than the maximum refractive index of the core portion 1. The cladding layer 2 is, for example, silica glass doped with fluorine.

Figure 2 is a diagram showing the refractive index profile of optical fiber 10. The horizontal axis shows the radial direction of the optical fiber when the central axis of the core portion 1 is set to zero, and the horizontal axis shows the relative refractive index difference with respect to the refractive index of pure silica glass. In the figure, the refractive index of pure silica glass is indicated by ns, and the relative refractive index difference is zero at the refractive index ns. In the following, unless otherwise specified, the relative refractive index difference is taken to be the relative refractive index difference with respect to pure silica glass at a wavelength of 1550 nm.

If the relative refractive index difference of the portion of the core portion 1 with the maximum refractive index located at or near the central axis is Δcore, Δcore is 0% or more and 0.2% or less. If the maximum refractive index of the core portion 1 is made of pure silica glass, Δcore is 0%. In addition, Δcore is adjusted to 0.2% or less depending on the amount of fluorine, chlorine, or alkali metal added to the core portion 1. Furthermore, the relative refractive index difference around the portion with the maximum refractive index of the core portion 1 decreases in the radial direction, for example, depending on the amount of fluorine.

As shown in FIG. 2, the core radius of the core portion 1 is r1. The core radius r1 can be defined from various viewpoints. For example, it can be defined as the radius at the position where the relative refractive index difference of the core portion 1 is 1/10 or 1/ ^e2 . Here, e is the base of the natural logarithm. The core radius r1 can also be defined as the radius at the position where the relative refractive index difference is zero. Regarding which definition to select, for example, the optical characteristics (cutoff wavelength and chromatic dispersion) of the optical fiber 10 can be actually measured, and the value closest to the value of the core radius derived from the actual measured value using a theoretical formula can be selected. The core radius derived using a theoretical formula may also be the core radius r1. In FIG. 2, the core radius r1 is defined as the radius at the position where the relative refractive index difference is zero, as an example.

The core radius r1 satisfies, for example, 3.0 μm≦r1≦5.0 μm.

As shown in FIG. 2, the cladding layer 2 has a first region 2a, a second region 2b, and a third region 2c arranged from the side closer to the core portion 1. The first region 2a, the second region 2b, and the third region 2c are all regions having a substantially cylindrical shape. The outer diameter of the third region 2c corresponds to the cladding diameter of the cladding layer 2. The cladding diameter is, for example, 125 μm ± 1 μm, which complies with the standard for a standard single-mode optical fiber defined by ITU-T G.652.

The first region 2a is a region that constitutes the interface between the cladding layer 2 and the core portion 1. As shown in FIG. 2, the relative refractive index difference of the optical fiber 10 changes continuously in the radial direction of the optical fiber 10 at the interface between the core portion 1 and the cladding layer 2. A continuous change in the relative refractive index difference corresponds to a continuous change in the refractive index. "Continuous change" means, for example, that the curve of the refractive index or the relative refractive index difference measured by an inspection device such as a known profile analyzer becomes a curve that can be differentiated with respect to the diameter. In addition, it is preferable that the change in the relative refractive index difference at the interface is gradual. "Gradual" means, for example, that the change in the relative refractive index difference when the radial position changes by 0.1 μm is 0.01% or less.

The relative refractive index difference of the first region 2a decreases continuously in the radial direction to become Δclad. Δclad is the average value of the relative refractive index difference in the radial direction of the cladding layer 2. Δclad is set in relation to Δcore and r1 so that the optical characteristics of the optical fiber 10 become the desired characteristics. Δclad is set so that its absolute value, for example (Δcore + Δclad), is 0.33% or more and 0.40% or less.

The desired optical characteristics of the optical fiber 10 are, for example, a mode field diameter at a wavelength of 1310 nm of 8.6 μm or more and 9.5 μm or less, an effective cutoff wavelength of 1260 nm or less, a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less, and a dispersion slope at the zero-dispersion wavelength of 0.092 ps/ ^nm2 /km or less. These characteristics comply with the standard defined by ITU-T G.652. If the core radius r1 satisfies 3.0 μm≦r1≦5.0 μm, it is easy to realize such various characteristics.

In the cladding layer 2, the second region 2b is a region that exists adjacent to the outer periphery of the first region 2a. Here, at the boundary between the second region 2b and the first region 2a, there is a first step portion 2b1 where the relative refractive index difference changes discontinuously. The step Δs1 of the relative refractive index difference in the first step portion 2b1 is, for example, 0.005% or less. The relative refractive index difference of the second region 2b gradually decreases from the first step portion 2b1 toward the outer periphery in the radial direction.

The position of the first step 2b1 in the radial direction is a position that is a predetermined distance away in the radial direction from the interface between the cladding layer 2 and the core portion 1. Hereinafter, the position of the first step 2b1 is represented as r2, which is the distance from the center of the core portion 1. The relationship between the core radius r1 and r2 can be expressed as an inequality, for example, 6<r2/r1.

The third region 2c is a region that exists adjacent to the outer periphery of the second region 2b. At the boundary between the third region 2c and the second region 2b, there is a second step portion 2c1 where the relative refractive index difference changes discontinuously. The step Δs2 of the relative refractive index difference in the second step portion 2c1 is, for example, 0.005% or less. The relative refractive index difference of the third region 2c gradually decreases from the second step portion 2c1 toward the outer periphery in the radial direction.

The position of the second step 2c1 in the radial direction is a position that is a predetermined distance away in the radial direction from the interface between the cladding layer 2 and the core portion 1. In this optical fiber 10, there are two steps: the first step 2b1 and the second step 2c1. The first step 2b1 is an example of a step that is closest to the core portion 1.

Such a step occurs, for example, when the manufacturing process is switched during the manufacture of the optical fiber preform used in the manufacture of the optical fiber 10. An example of a case where the manufacturing process is switched is when soot formation and soot vitrification are alternately performed multiple times, and then the vitrification process is switched to the next soot formation process. When such a step occurs, the minimum value of the steps Δs1 and Δs2 is equal to or greater than the minimum value observable by a known inspection device such as a profile analyzer, for example, equal to or greater than 0.0001%.

In the optical fiber 10 according to the first embodiment, the relative refractive index difference at the interface between the core 1 and the cladding layer 2 changes continuously in the radial direction of the optical fiber 10. As a result, the optical fiber 10 has low transmission loss because the increase in transmission loss caused by discontinuity when the relative refractive index difference at the interface changes discontinuously is suppressed.

In addition, it is preferable that r2, which is the distance from the center of the core portion 1 of the first step portion 2b1, satisfies 18 μm≦r2 [μm] or r2 [μm] ≦ 24 μm. If 18 μm ≦ r2 [μm], the distance between the core portion 1 and the first step portion 2b1 can be made sufficiently large, and the increase in transmission loss caused by the first step portion 2b1 is suppressed. Furthermore, if the first step portion 2b1 is close to the core portion 1, it may affect the mode field diameter and cutoff wavelength of the optical fiber 10, and even if the refractive index profile of the core portion 1 is designed to satisfy the ITU-T G. 652 standard, it may deviate from the standard. However, if 18 μm ≦ r2 [μm], the effect of the presence of the first step portion 2b1 on the mode field diameter and cutoff wavelength can be reduced. Also, if r2 [μm] ≦ 24 μm, the thickness of the first region 2a is relatively thin, so the manufacturability of the optical fiber 10 is increased.

In particular, as described below, if the relative refractive index difference at the interface between the core portion 1 and the cladding layer 2 changes continuously in the radial direction of the optical fiber 10, the concentration of hydroxyl groups (OH groups) at the interface is extremely reduced, so that the transmission loss in the 1380 nm wavelength band caused by OH groups can be reduced. The 1380 nm wavelength band is a wavelength band that includes the peak of light absorption by OH groups at a wavelength of approximately 1383 nm, for example, a wavelength band of 1383 nm ± 3 nm.

For example, the optical fiber 10 can achieve a transmission loss of less than 0.175 dB/km at a wavelength of 1550 nm and a transmission loss of 1.0 dB/km or less in the 1380 nm wavelength band. Such an optical fiber 10 can be used as a communication band over a wide band, for example, from 1310 nm to 1625 nm.

Furthermore, the optical fiber 10 can achieve characteristics such as a transmission loss of 0.40 dB/km or less from wavelengths 1310 nm to 1625 nm, a transmission loss of 0.30 dB/km or less from wavelengths 1530 nm to 1565 nm, and a transmission loss of 0.40 dB/km or less from 1383 nm ± 3 nm after hydrogen aging. Such an optical fiber 10 complies with the standard defined by ITU-T G. 652D and can be used as a communication band over a wide band, for example, from wavelengths 1310 nm to 1625 nm.

An example of a method for manufacturing the optical fiber 10 will be described with reference to FIG.
First, in step S101, a core soot is prepared. Then, in step S102, the core soot is vitrified. Then, in step S103, cladding soot is deposited and vitrified. Then, in step S104, cladding soot is deposited and vitrified. Then, in step S105, the optical fiber preform prepared by carrying out steps S101 to S104 is drawn.

Each step will now be described in detail.
In step S101, a core soot is prepared to form a portion of the optical fiber preform that will be the core portion 1 of the optical fiber 10 and the first region 2a in the cladding layer 2 (hereinafter, sometimes referred to as a core equivalent portion or a first region equivalent portion). The core soot is prepared by the VAD method using, for example, a VAD device. At this time, a first burner is used to inject a gas containing hydrogen gas, oxygen gas, and a first raw material gas that is a raw material for forming the core equivalent portion, a second burner is used to inject a gas containing hydrogen gas, oxygen gas, and a second raw material gas that is a raw material for forming the first region equivalent portion, and a third burner is used to inject a gas containing hydrogen gas and oxygen gas for annealing the core equivalent portion. The first raw material gas is, for example, silicon chloride gas or a mixed gas of a gas containing silicon such as silicon chloride gas and chlorine gas. The second raw material gas is, for example, a gas containing silicon such as silicon chloride gas.

In step S102, the core soot produced in step S101 is heated and vitrified using a vitrification furnace to produce a core rod. This vitrification is performed in a fluorine-containing gas atmosphere by flowing a fluorine-containing gas containing fluorine into the vitrification furnace. The fluorine-containing gas is, for example, a mixed gas of silicon tetrafluoride (SiF ₄ ) gas and an inert gas (for example, nitrogen (N ₂ ) gas). The flow rate ratio of SiF ₄ gas to N ₂ gas is, for example, 1:1. By performing the vitrification in a fluorine-containing gas atmosphere, fluorine can be penetrated into the core-corresponding portion and the first region-corresponding portion to add fluorine. In addition, by appropriately annealing the core-corresponding portion in step S101, the amount of fluorine that penetrates into the core-corresponding portion can be adjusted.

In step S103, in order to form a portion of the optical fiber preform that will become the second region 2b of the optical fiber 10 (hereinafter, sometimes referred to as the second region equivalent portion), cladding soot is deposited on the outer periphery of the core rod. The cladding soot is produced by the OVD method using, for example, an OVD device. At this time, a burner is used that sprays gas containing hydrogen gas, oxygen gas, and a raw material gas that is the raw material for forming the second region equivalent portion. The raw material gas is, for example, a gas containing silicon such as silicon chloride gas. At this time, a step portion corresponding to the first step portion 2b1 is formed in the second region equivalent portion by switching the process. In addition, OH groups may be added to the outer periphery of the core rod when the process is switched from step S102 to step S103. Next, the cladding soot is heated using a vitrification furnace to vitrify. This vitrification is also performed in a fluorine-containing gas atmosphere, as in step S102.

During this vitrification, the OH groups on the outer periphery of the core rod are removed to some extent, but generally are not completely removed, which causes transmission loss in the 1380 nm wavelength band after the optical fiber is formed. However, according to this manufacturing method, even if OH groups remain on the outer periphery of the core rod, the outer periphery of the core rod corresponds to the interface between the first region 2a and the second region 2b in the cladding layer 2, which is located radially away from the outer periphery of the core portion 1 of the optical fiber 10. Therefore, the OH groups remain in a position far from the position where the field of light propagating through the core portion 1 is strong, thereby reducing transmission loss in the 1380 nm wavelength band.

In step S104, cladding soot is deposited on the outer periphery of the core rod to form a portion of the optical fiber preform that will become the third region 2c of the optical fiber 10 (hereinafter, sometimes referred to as the portion corresponding to the third region). The cladding soot can be produced in the same manner as in step S103. At this time, a step portion corresponding to the second step portion 2c1 is formed in the portion corresponding to the third region by switching the process. The cladding soot is then heated in a vitrification furnace to vitrify it. This vitrification is also carried out in a fluorine-containing gas atmosphere, as in step S102.

In step S105, the optical fiber preform is drawn using a known drawing device to produce the optical fiber 10.

(Embodiment 2)
4 is a diagram showing a refractive index profile of the optical fiber according to embodiment 2. The optical fiber 10A includes a core 1A and a cladding layer 2A formed on the outer periphery of the core 1A. The cross-sectional structure of the optical fiber 10A is similar to that of the optical fiber 10 according to embodiment 1, and therefore a description thereof will be omitted as appropriate.

The core portion 1A has the same structure as the core portion 1 of the optical fiber 10, and is made of the same constituent materials. The cladding layer 2A is made of the same constituent materials as the cladding layer 2 of the optical fiber 10, but differs from the cladding layer 2 in that the cladding layer 2A is made of two regions, a first region 2Aa and a second region 2Ab, arranged from the side closer to the core portion 1A.

The first region 2Aa is a region that forms the interface between the cladding layer 2A and the core portion 1A. At the interface between the core portion 1A and the cladding layer 2A, the relative refractive index difference of the optical fiber 10A changes continuously in the radial direction of the optical fiber 10A. The relative refractive index difference of the first region 2Aa decreases continuously in the radial direction, becoming Δclad.

In the cladding layer 2A, the second region 2Ab is a region that exists adjacent to the outer periphery of the first region 2Aa. At the boundary between the second region 2Ab and the first region 2Aa, there is a first step portion 2Ab1 where the relative refractive index difference changes discontinuously. The step ΔsA1 of the relative refractive index difference in the first step portion 2b1 is, for example, 0.005% or less. The relative refractive index difference of the second region 2Ab decreases gently from the first step portion 2Ab1 toward the outer periphery in the radial direction. The outer diameter of the second region 2Ab corresponds to the cladding diameter of the cladding layer 2A. The cladding diameter is, for example, 125 μm ± 1 μm.

The optical fiber 10A according to the second embodiment has the same effect as the optical fiber 10, for example, low transmission loss over a wide wavelength band. The optical fiber 10A can be manufactured in the same manner as the optical fiber 10 by the manufacturing method shown in FIG. 3. However, step S104 can be omitted.

(Embodiment 3)
5 is a diagram showing a refractive index profile of an optical fiber according to the third embodiment. The optical fiber 10B includes a core 1B and a cladding layer 2B formed on the outer periphery of the core 1B. The cross-sectional structure of the optical fiber 10B is similar to that of the optical fiber 10 according to the first embodiment, and therefore the description will be omitted as appropriate. The cladding layer 2B has a first region 2Ba, a second region 2Bb, and a third region 2Bc arranged from the side closer to the core 1B. The second region 2Bb has a first step portion 2Bb1 with a step difference ΔsB1, and the third region 2Bc has a second step portion 2Bc1 with a step difference ΔsB2.

Optical fiber 10B differs from optical fiber 10 in that the refractive index profile of core 1B is a so-called α type. Because the refractive index profile of core 1B is an α type, the optical characteristics of optical fiber 10B, such as the cutoff wavelength, are highly controllable.

The optical fiber 10B according to the third embodiment has the same effect as the optical fiber 10, and has low transmission loss over a wide wavelength band, for example. The optical fiber 10B can be manufactured in the same manner as the optical fiber 10, by the manufacturing method shown in FIG. 3. However, in step S101, in order to make the refractive index profile of the core portion 1B an α type, the flow rate of the oxygen gas and hydrogen gas during annealing is reduced to, for example, about 1/3 to reduce the degree of annealing.

The present invention is not limited to the above-mentioned embodiment. The present invention also includes configurations in which the above-mentioned components are appropriately combined. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-mentioned embodiment, and various modifications are possible.

1, 1A, 1B Core portion 2, 2A, 2B Cladding layer 2a, 2Aa, 2Ba First region 2b, 2Ab, 2Bb Second region 2b1, 2Ab1, 2Bb1 First step portion 2c, 2Bc Third region 2c1, 2Bc1 Second step portion 10, 10A, 10B Optical fiber

Claims (3)

A core portion;
a clad layer formed on the outer periphery of the core portion and having a refractive index lower than the maximum refractive index of the core portion;
Equipped with
The maximum refractive index of the core portion has a relative refractive index difference of 0% or more and 0.2% or less with respect to pure silica glass,
a refractive index at an interface between the core and the cladding layer varies continuously in a radial direction of the optical fiber;
the cladding layer has step portions in which the refractive index changes discontinuously at two or less positions spaced a predetermined distance from the interface with the core in the radial direction, and the step portions have a relative refractive index difference of 0.005% or less with respect to pure silica glass;
When the core radius of the core portion is r1 [μm], 3.0 μm≦r1 [μm]≦5.0 μm;
If the distance from the center of the core portion to the step portion closest to the core portion is r2 [μm], 18 μm≦r2 [μm] and r2 [μm] ≦24 μm;
6<r2/r1,
The transmission loss at a wavelength of 1550 nm is less than 0.175 dB/km,
The transmission loss in the 1380 nm wavelength band is 1.0 dB/km or less,
a mode field diameter at a wavelength of 1310 nm is 8.6 μm or more and 9.5 μm or less, an effective cutoff wavelength is 1260 nm or less, a zero dispersion wavelength is 1300 nm or more and 1324 nm or less, and a dispersion slope at the zero dispersion wavelength is 0.092 ps/nm ² /km or less;
From the step portion toward the outer periphery in the radial direction, the relative refractive index difference with respect to pure silica glass decreases such that the change in the relative refractive index difference is greater than 0% and is 0.01% or less when the radial position changes by 0.1 μm.
Optical fiber. the core portion includes a portion made of pure silica glass or silica glass doped with fluorine, chlorine, or an alkali metal;
The optical fiber according to claim 1 , wherein the cladding layer is made of fluorine-doped silica glass. 3. The optical fiber according to claim 1, wherein the transmission loss in the 1380 nm wavelength band is 0.40 dB/km or less.

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