JP6136164B2 - Optical fiber and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical fiber and an optical fiber manufacturing method.

  An optical fiber in which a core region is made of pure silica glass and a desired refractive index difference is generated between the core region and the cladding region by adjusting the amount of fluorine added to the cladding region is known (patent) Reference 1). In such an optical fiber, a clad region having a depressed layer adjacent to the core region and a jacket layer on the outer periphery of the depressed layer is known. Desired optical characteristics can be obtained by setting the refractive index of the depressed layer to be lower than the refractive indexes of the core region and the jacket layer.

Japanese Patent No. 4080164

  By the way, in such an optical fiber, the jacket layer lowers the refractive index of the jacket layer by adding an additive such as fluorine that lowers the refractive index of silica to the silica.

  However, the amount of the additive may vary due to manufacturing errors, and the refractive index of the jacket layer may vary. Then, the refractive index of the depressed layer and the jacket layer hardly change, and even if the depressed layer is provided, an optical fiber having desired optical characteristics may not be obtained.

  Accordingly, an object of the present invention is to provide an optical fiber that is easy to manufacture and stably exhibits excellent optical characteristics, and a method for manufacturing the optical fiber.

The optical fiber of the present invention capable of solving the above problems is
A core region made of pure silica;
A cladding region provided around the core region and having a lower refractive index than the core region;
The cladding region comprises a depressed layer adjacent to the core region, and a jacket layer provided on the outer periphery of the depressed layer,
The depressed layer has a lower refractive index than the jacket layer,
When the relative refractive index difference of the innermost peripheral portion of the depressed layer is Δn3a and the relative refractive index difference of the outermost peripheral portion of the depressed layer is Δn3b,
A = Δn3a−Δn3b
The difference A [%] of the relative refractive index difference in the depressed layer represented by the formula satisfies −0.02 ≦ A <0.
“Pure silica” refers to quartz glass that does not contain germanium (it may contain a small amount of other elements (K, Cl, F, etc.)).

The method for producing an optical fiber of the present invention capable of solving the above problems
The core material to be the core region is inserted and collapsed into a glass pipe to be the depressed layer, and the jacket layer is sooted and sintered on the outside, and then drawn. A method of manufacturing the optical fiber of
In the step of sintering the glass body to be the glass pipe, a heater capable of heating a part of the glass body in the axial direction is sintered while relatively moving in the axial direction of the glass body in an atmosphere containing fluorine. The relative movement speed of the heater with respect to the glass body is 3.0 mm / min or less.

  According to the optical fiber of the present invention, the refractive of the depressed layer is such that the difference A of the relative refractive index difference between the innermost peripheral portion and the outermost peripheral portion of the depressed layer is −0.02% or more and less than 0%. It is inclined so that the rate decreases toward the outer peripheral side. As a result, even when the refractive index of the jacket layer varies somewhat, the refractive index of the outermost periphery of the depressed layer can be made lower than the refractive index of the jacket layer, and an optical fiber having excellent characteristics can be stably provided. can do.

  Moreover, according to the manufacturing method of the optical fiber which concerns on this invention, the relative moving speed with respect to the glass body of a heater shall be 3.0 mm / min or less in the process of sintering the glass body used as a depressed layer. For this reason, the glass body can be sintered so that the difference A in the relative refractive index difference of the depressed layer is −0.02% or more and less than 0%.

(A) is sectional drawing of the optical fiber which concerns on one Embodiment of this invention, (b) is a figure which shows the refractive index distribution of an optical fiber. It is a figure explaining the manufacturing method of the optical fiber which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the relative moving speed of a glass fine particle deposition body and a heater, and the difference of the relative refractive index difference of the innermost peripheral part and outermost peripheral part of a depressed layer.

Hereinafter, an example of an embodiment of an optical fiber according to the present invention will be described with reference to the drawings.
FIG. 1A is a cross-sectional view of an optical fiber 1 according to an embodiment of the present invention. As shown to (a) of FIG. 1, the optical fiber 1 has the core area | region 2 in the center, and has the clad area | region 5 in the outer periphery. The clad region 5 has a depressed layer 3 positioned on the outer periphery of the core region 2 and a jacket layer 4 positioned on the outer periphery of the depressed layer 3.

  The core region 2 is quartz glass that does not contain germanium, and is substantially pure quartz, but may contain trace amounts of chlorine, potassium, fluorine, and the like during the manufacturing process. The cladding region 5 is quartz glass to which fluorine is added, and has a refractive index lower than that of the core region 2.

  As an example of the specific size of each part, the diameter d1 of the core region 2 can be 9.6 μm, the diameter d2 of the depressed layer 3 can be 48 μm, and the diameter d3 of the jacket layer 4 can be 125 μm.

  FIG. 1B is a schematic diagram showing a refractive index distribution in a cross section of the optical fiber 1. As shown in FIG. 1B, the refractive index n2 of the central core region 2 is the highest in the optical fiber 1, and the refractive index of the surrounding cladding region 5 is lower than the refractive index n2 of the core region 2. . The refractive index n2 of the core region 2 is substantially uniform regardless of the radial direction.

  Further, the refractive index n 4 of the jacket layer 4 in the cladding region 5 is smaller than the refractive index n 2 of the core region 2. The refractive index n4 of the jacket layer 4 is substantially uniform regardless of the radial direction.

  The refractive index of the depressed layer 3 in the cladding region 5 varies from the refractive index n3a in the innermost periphery of the depressed layer 3 adjacent to the core region 2 to the refractive index n3b in the outermost periphery of the depressed layer 3 adjacent to the jacket layer 4. Is inclined so as to decrease substantially uniformly.

  The refractive index n3b at the outermost periphery of the depressed layer 3 is smaller than the refractive index n3a at the innermost periphery of the depressed layer 3, and smaller than the refractive index n4 of the jacket layer 4.

Here, the relative refractive index difference of the innermost circumference in the depressed layer 3 is Δn3a, the relative refractive index difference of the outermost circumference is Δn3b, and the difference A [%] of the relative refractive index difference is
A = Δn3a−Δn3b
The refractive index n3a of the innermost periphery of the depressed layer and the refractive index n3b of the outermost periphery of the depressed layer 3 are set so as to satisfy −0.02 ≦ A <0.

  As the refractive index of each part, for example, the relative refractive index difference Δn3a of the refractive index n3a in the innermost circumference of the depressed layer 3 is 0.295% with respect to the refractive index n2 of the core region 2, and the outermost circumference of the depressed layer 3 The relative refractive index difference Δn3b of the refractive index n3b can be 0.315%, and the relative refractive index difference Δn4 of the refractive index n4 of the jacket layer 4 can be 0.292%.

  In general, as optical characteristics of an optical fiber, each of a mode field diameter MFD (Mode Field Diameter), a cutoff wavelength λc, a MAC value, and a zero dispersion wavelength λ0 is required to be within a desired range. The optical fiber 1 according to the present embodiment is intended to satisfy the requirement by adjusting the shape of the refractive index distribution with high accuracy.

  The cutoff wavelength λc changes due to the difference between the refractive index n4 of the jacket layer 4 and the refractive index of the depressed layer 3. Therefore, in order to design the refractive index distribution so as to satisfy the desired cutoff wavelength λc, a sufficient difference can be taken between the refractive index n4 of the jacket layer 4 and the refractive index n3b of the outermost layer of the depressed layer 3. It is necessary to do so. That is, if the refractive index n4 of the jacket layer 4 is sufficiently different from the refractive index n3b of the outermost layer of the depressed layer 3, the adjustment range of the cutoff wavelength λc is widened.

  In addition, the bending loss, which is one of the transmission characteristics of the optical fiber 1, can be managed using as an index the MAC value that is a value obtained by dividing the mode field diameter MFD of the optical fiber by the cutoff wavelength λc of the optical fiber. The smaller the MAC value, the smaller the bending loss. Since the MFD is determined to some extent by the structure of the core, it is necessary to adjust the cutoff wavelength λc with high accuracy in order to set the MAC value to a desired value or less.

  As described above, the cutoff wavelength λc varies depending on the difference in refractive index between the refractive index n3b at the outermost periphery of the depressed layer 3 and the refractive index n4 of the jacket layer 4. In order to obtain the effect of the depressed layer 3, it is necessary to lower the refractive index of the depressed layer 3 than the jacket layer 4, so that the refractive index n4 of the jacket layer 4 is greatly increased to satisfy the desired cutoff wavelength λc. Even if you try to lower it, there are cases where it cannot be lowered.

  Therefore, in the present embodiment, the refractive index distribution of the depressed layer 3 is inclined so as to be lowered from the inner peripheral side to the outer peripheral side, thereby making it easy to make a refractive index difference between the depressed layer 3 and the jacket layer 4, thereby reducing the cutoff. The wavelength λc is easily adjusted with high accuracy.

  As a result, even if the refractive index n4 of the jacket layer 4 varies somewhat, the outermost refractive index n3b of the depressed layer 3 is made lower than the refractive index n4 of the jacket layer 4 as compared with the case where there is no inclination. Therefore, it becomes easy to manufacture each of the mode field diameter MFD, the cutoff wavelength λc, the MAC value, and the zero dispersion wavelength λ0 within a desired value range.

  Thus, according to the optical fiber 1 according to the present embodiment, the refractive index of the depressed layer 3 is such that the difference A of the relative refractive index difference of the depressed layer 3 is not less than −0.02% and less than 0%. It is inclined so as to become lower toward the side. Thereby, the refractive index n4 of the jacket layer 4 can be set to a value greatly different from the refractive index n3b of the outermost periphery of the depressed layer 3. For this reason, even when the refractive index n4 of the jacket layer 4 varies due to a manufacturing error or the like, the refractive index n3b of the outermost periphery of the depressed layer 3 tends to be lower than the refractive index n4 of the jacket layer 4 and has excellent characteristics. An optical fiber can be provided stably.

Next, a method for manufacturing such an optical fiber will be described.
First, a core material that becomes the core region 2 of the optical fiber 1 is prepared. In order to produce the core material, for example, a glass fine particle deposit of pure silica is produced by, for example, the VAD method, dehydrated and sintered in a chlorine atmosphere, transparentized, and then stretched to have a desired outer diameter. . The core material is made of pure silica, but may contain a very small amount of impurities such as chlorine.

  Also, a cylindrical depressed pipe material having a hole penetrating in the axial direction at the center of the optical fiber 1 serving as the depressed layer 3 is prepared. A method for producing the depressed pipe material will be described later.

  Further, the core material is inserted into the central hole of the depressed pipe material. In this state, the core material and the depressed pipe material are integrated by heating and collapsing the depressed pipe material. Thereby, the primary intermediate body which has the part used as the core area | region 2 and the part used as the depressed layer 3 of an optical fiber is formed.

  And the glass of the part used as the jacket layer 4 of the optical fiber 1 is formed in the outer periphery of this primary intermediate body, and it is set as a secondary intermediate body. For example, glass particles of pure silica are deposited on the outer periphery of the primary intermediate by the OVD method, and the deposit is grown in the radial direction.

  Further, the secondary intermediate is subjected to dehydration treatment in a heating furnace. The dehydration treatment is performed by heating the secondary intermediate in a heating furnace having a chlorine-containing gas and an inert gas atmosphere. Thereby, the OH group adhering to the secondary intermediate is eliminated.

  Next, the secondary intermediate is sintered in a heating furnace having an atmosphere of silicon fluoride (SiF) and an inert gas inside the heating furnace. At this time, fluorine permeates the outer periphery of the secondary intermediate, and the glass of the portion that becomes the jacket layer 4 is sintered while fluorine is mixed into the glass of the portion that becomes the jacket layer. Thereby, the refractive index of the glass of the part used as the jacket layer 4 falls.

  Through the above steps, an optical fiber glass preform having a portion that becomes the jacket layer 4, a portion that becomes the depressed layer 3, and a portion that becomes the core region 2 of the optical fiber 1 is obtained. The optical fiber 1 is obtained by drawing this glass preform for an optical fiber with a drawing device.

  Next, the manufacturing method of the depressed pipe material used as the depressed layer 3 is explained in full detail using FIG. FIG. 2 shows a sintering furnace for sintering the depressed pipe material.

  First, as a starting material for a depressed pipe material, a glass fine particle deposit 20 of pure silica is produced by, for example, the VAD method. In the VAD method, glass particles are deposited on a starting rod suspended in a reaction vessel by a glass particle generating burner. For example, the glass particulate deposit 20 can be grown in the axial direction of the starting rod by pulling up the starting rod so that the position of the deposition surface is constant while detecting the deposition surface of the glass particulate.

  The glass fine particle deposit 20 thus produced is subjected to dehydration and sintering in a heating furnace 10 as shown in FIG. The heating furnace 10 includes a fixed heater 11 that can heat a part of the glass particulate deposit 20 in the axial direction Ax, and a gas supply pipe 12 that supplies silicon fluoride into the heating furnace 10.

  The dehydration process is performed by setting the inside of the heating furnace 10 to an atmosphere containing chlorine gas and inert gas, and heating the glass particulate deposit 20 with the heater 11. By heating with the heater 11, the OH group adhering to the glass fine particles constituting the glass fine particle deposit 20 is desorbed.

  Subsequently, the glass fine particle deposit 20 is subjected to a sintering process. This sintering process is performed after replacing the atmosphere inside the heating furnace 10 with an atmosphere of a silicon fluoride (SiF) gas and an inert gas (such as helium).

  In the sintering process, the heater 11 heats a part of the glass particulate deposit 20 in the axial direction while moving the glass particulate deposit 20 in the axial direction by moving the starting bar 21 up and down. Then, in the heated part, many glass fine particles are densified and it becomes a transparent glass body. In addition, fluorine in the atmosphere is mixed into the glass fine particle deposit 20 between the heated glass fine particles.

  By the sintering process, the glass fine particle deposit 20 becomes a transparent glass body, and a portion of the central axis of the transparent glass body is perforated to obtain a depressed pipe material.

  In the step of sintering the depressed pipe material, the refractive index distribution of the portion that becomes the depressed layer 3 can be adjusted by adjusting the relative moving speed of the glass fine particle deposit 20 and the heater 11. As the refractive index distribution of the portion that becomes the depressed layer 3, the above-described difference A of the relative refractive index difference can be adjusted by the relative movement speed during sintering. FIG. 3 is a graph showing the relationship between the relative movement speed of the glass particulate deposit 20 and the heater 11 and the difference A of the relative refractive index difference of the depressed layer 3 of the optical fiber 1.

  As shown in FIG. 3, when the relative movement speed between the glass fine particle deposit 20 and the heater 11 is set to a low speed, the difference A in the relative refractive index difference of the depressed layer 3 becomes small. When the relative movement speed is reduced, the time for heating the vicinity of the same position in the axial direction by the heater 11 is increased, and fluorine sufficiently penetrates and sinters to the inner peripheral end of the portion that becomes the depressed layer 3, This is considered to be because the difference in fluorine concentration between the inner peripheral side and the outer peripheral side becomes small.

  On the other hand, when the relative movement speed is set to a high speed, the difference A in the relative refractive index difference of the depressed layer 3 increases. This is because when the relative movement speed is increased, the time for heating the vicinity of the same position in the axial direction by the heater 11 is shortened, and the fluorine is not sufficiently permeated to the inner peripheral end of the depressed layer 3 and sintered. It is considered that this is because the difference in fluorine concentration between the inner peripheral side and the outer peripheral side becomes large.

  In the present embodiment, the relative movement speed between the glass fine particle deposit 20 and the heater 11 is set to 3.0 mm / min or less. Thereby, the difference A of the relative refractive index difference between the inner and outer peripheral portions of the depressed layer 3 of the optical fiber 1 is set to −0.02 or more and less than 0, and the refractive index of the depressed layer 3 is increased from the inner peripheral side toward the outer peripheral side. It can be tilted down.

  If the relative movement speed is set to be larger than 3.0 mm / min, the difference A in the relative refractive index difference between the inner and outer peripheral portions of the depressed layer 3 can be further increased, but to the fluorine depressed layer 3 during sintering. There will be a large variation in the mixing of the. For this reason, it is difficult to form the depressed layer 3 so that the refractive index decreases substantially uniformly toward the outer peripheral side.

  Preferably, the relative movement speed is set to 1.5 mm / min or more. If the relative moving speed is lower than 1.5 mm / min, fluorine will permeate too much to the inner peripheral edge of the portion that becomes the depressed layer 3. For this reason, the refractive index of the depressed layer 3 becomes uniform from the inner peripheral side to the outer peripheral side, and the refractive index distribution cannot be tilted.

  According to the method of manufacturing the optical fiber 1 according to this embodiment, the mode field diameter MFD of 1550 nm is 10.0 to 11.0 μm, the cutoff wavelength λc is 1250 nm or less, the MAC value is 9.2 or less, and zero An optical fiber 1 having excellent dispersion characteristics with a dispersion wavelength λ0 of 1300 to 1324 nm can be provided.

It should be noted that the inspection yield when the difference A in the relative refractive index difference between the inner and outer peripheral portions of the depressed layer 3 becomes 0 or a positive value without considering the relative moving speed during sintering of the depressed pipe material (above The ratio at which a fiber satisfying the characteristics was obtained was 0%. On the other hand, the relative moving speed during sintering of the depressed pipe material is set to 3.0 mm / min or less, and the difference A of the relative refractive index difference between the inner and outer peripheral portions in the depressed layer 3 is set to −0. Inspection yield when the 0 2 or less than 0, was 71.6%. The relative moving speed during sintering of the depressed pipe material is made larger than 3.0 mm / min, and the difference A in the relative refractive index difference between the inner and outer peripheral portions in the depressed layer 3 is set to −0. 0 2 than when smaller, it is difficult to substantially uniform refractive index change of the depressed layer 3, for thereby variations in the outermost layer of the refractive index n3b, inspection yield was 62.3%.

  The optical fiber 1 and the manufacturing method thereof according to the present invention are not limited to the above-described embodiments, and appropriate modifications and improvements can be made.

1: optical fiber, 2: core region, 3: depressed layer, 4: jacket layer, 5: clad region, 10: sintering furnace, 11: heater, 20: glass fine particle deposit, n2, n3a, n3b, n4 : Refractive index, A: Difference in specific refractive index difference

Claims (1)

A core region made of pure silica;
A cladding region provided around the core region and having a lower refractive index than the core region;
The cladding region comprises a depressed layer adjacent to the core region, and a jacket layer provided on the outer periphery of the depressed layer,
The depressed layer has a lower refractive index than the jacket layer,
The refractive index of the depressed layer is inclined so as to decrease substantially uniformly from the refractive index in the innermost circumference of the depressed layer to the refractive index in the outermost circumference of the depressed layer,
In the depressed layer represented by A = Δn3a−Δn3b, where Δn3a is the relative refractive index difference of the innermost peripheral portion of the depressed layer and Δn3b is the relative refractive index difference of the outermost peripheral portion of the depressed layer. The difference A [%] of the relative refractive index difference satisfies −0.02 ≦ A <0, and is a method of manufacturing an optical fiber ,
The core material to be the core region is inserted and collapsed into the glass pipe to be the depressed layer, and the jacket layer is sooted and sintered on the outside, and then the optical fiber is manufactured by drawing. A way to
In the step of sintering the glass body to be the glass pipe, a heater capable of heating a part of the glass body in the axial direction is sintered while relatively moving in the axial direction of the glass body in an atmosphere containing fluorine. At the time, the relative movement speed of the heater with respect to the glass body is set to 3.0 mm / min or less.

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