JP2010001193A - Method for manufacturing optical fiber preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide: a multi-clad fiber in which no high refractive index portion exists in the core center and which is easy to control the refractive index profile; an optical fiber preform suitable for manufacturing the multi-clad fiber; a method for stably manufacturing the optical fiber preform; an optical fiber laser provided with the multi-clad fiber; and an optical amplifier. <P>SOLUTION: The method for manufacturing the optical fiber preform includes a first process for forming a first glass layer 11 containing Ge along the inner surface of a quartz tube 12, a second process for forming a second glass layer 15 containing no Ge along the inner surface of the first glass layer 11, a third process for forming a silica soot 10 on the inner side of the second glass layer 15, a fourth process for adding a dopant to the silica soot 10 and sintering the soot 10, and a fifth process for collapsing the tube obtained in the fourth process. The optical fiber preform is manufactured by the method. The multi-clad fiber is obtained by spinning the optical fiber preform and covering the outer surface of the resulting fiber with a polymer layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing an optical fiber preform.

An optical fiber in which a rare earth element is added to the core has an optical amplification function, and is used for applications such as an optical fiber laser and an optical amplifier. In particular, the importance of optical fiber lasers has increased in recent years, and has been actively researched.
Hereinafter, an ytterbium (Yb) -doped optical fiber will be described as an example of a rare earth element-doped optical fiber for an optical fiber laser.
Examples of Yb-doped optical fibers used for optical amplification of optical fiber lasers include optical fibers having a plurality of claddings. These are generally collectively referred to as multi-clad fibers. The multi-clad fiber is configured by concentrically arranging a plurality of clads outside the core. And what has a clad two layers is called a double clad fiber, and what a clad has three layers is called a triple clad fiber (for example, refer to patent documents 1). FIG. 5 is a diagram illustrating a cross section of a double clad fiber and a preferable refractive index profile. The double clad fiber 5 illustrated here has two layers of clads (a first clad 51 and a second clad 52) outside the core 50. ) Are arranged concentrically, and the second clad 52 is covered with a protective resin layer 54. In FIG. 5, Δ1 represents the relative refractive index difference between the core and the first cladding. On the other hand, FIG. 6 is a diagram illustrating a cross section of a triple-clad fiber and a preferable refractive index profile. The triple-clad fiber 6 illustrated here has a three-layer clad (first clad 61, second clad 61) outside the core 60. The clad 62 and the third clad 63) are arranged concentrically, and the third clad 63 is covered with a protective resin layer 64. In FIG. 6, Δ1 indicates the relative refractive index difference between the core and the second cladding, and Δ2 indicates the relative refractive index difference between the first cladding and the second cladding.
For example, deterioration of laser output over time called photodarkening can be effectively suppressed by using a triple clad fiber.

It is known that photodarkening can be suppressed by adding aluminum (Al) to the core, and the suppression effect increases as the amount of Al added increases (see Non-Patent Document 1, for example). On the other hand, the addition of Al increases the refractive index of the glass.
By the way, it is desirable that the optical fiber for a fiber laser is designed so that the signal light is guided through the core in a single mode from the viewpoint of the beam quality. On the other hand, the optical fiber is designed so that the effective cross-sectional area (A _eff ) of the laser light guided through the core is increased in order to suppress a decrease in energy conversion efficiency due to nonlinear optical effects such as stimulated Raman scattering. It is desirable. For this reason, for example, in the case of a Yb-doped multi-clad fiber having a core diameter of about 20 to 30 μm, the relative refractive index difference between the core and the first cladding is designed to be about 0.10%. It is generally desirable.

  On the other hand, in the case of a double clad fiber, since the first clad is usually made of pure quartz, in order to make the relative refractive index difference between the core and the first clad about 0.10%, the core The amount of Al that can be added to is limited, and it is difficult to suppress photodarkening. For example, it is possible to increase the amount of Al added by co-adding a dopant that lowers the refractive index of glass, such as fluorine (F) or boron (B), to the core. From the viewpoint of production suitability, it is not preferable.

On the other hand, in the case of a triple clad fiber, as a preferred example, the refractive index of the first clad is such that the relative refractive index difference (Δ1) between the core and the second clad is about 0.40%, and The relative refractive index difference (Δ2) between the first clad and the second clad is designed to be about 0.30%. As a result, the relative refractive index difference between the core and the first cladding can be reduced to about 0.10% without co-adding F or B to the core, and the amount of Al added to the core can be increased. It becomes possible. Note that Δ2 does not necessarily have to be designed to be about 0.30%, but may be designed so that an amount of Al necessary for suppressing photodarkening can be added to the core.
For this reason, the triple-clad fiber is easy to achieve both suppression of laser output deterioration with time due to photodarkening and control of the refractive index profile.

The base material of a triple clad fiber in which Yb is added to the core is generally manufactured by a method using the following MCVD method and liquid immersion method together.
That is, using a quartz tube (preferably a pure quartz tube) to be the second cladding, a glass layer to be the first cladding is deposited by the MCVD method. At this time, germanium (Ge), phosphorus (P), or the like is added to the glass layer so that the Δ2 is about 0.30%. Next, the core is deposited. The core is deposited in a silica soot (porous body) state (that is, at a temperature at which sintering is not performed), and Yb and Al are added by a liquid immersion method. In the immersion method, an immersion solution prepared so that the Δ1 is about 0.40% is used, the core soot is impregnated with the Yb compound and the Al compound, and then the core soot is dried, dehydrated and dehydrated. Sintering and collapsing to solidify the tube. In collapses, for example, a part of the tube is first heated and crushed with an oxyhydrogen burner on the gas downstream side. Thereafter, the burner is moved in the direction of the central axis of the tube while keeping the pressure in the tube at an appropriate level, and the tube is sequentially crushed. In this way, an optical fiber preform in which Yb and Al are added to the core is obtained. Then, after spinning the optical fiber preform, it is coated with a polymer layer serving as a third cladding, thereby obtaining a triple-clad fiber in which Yb and Al are added to the core.
Special table 2006-519495 gazette T. T. Kitabayashi, et al. , “Population Inversion Factor Dependence of Yb-doped Fibers and its Suppression by High Aluminum Doping”, OFC 2006, Anaheim, USA, Tap.

However, the triple clad fiber manufactured by the above method has a problem that an undesirably high refractive index portion exists at the core center. The refractive index profile of such a triple clad fiber is illustrated in FIG. 4 together with a cross-sectional view of the fiber. In the triple clad fiber 4 illustrated here, three layers of clads (a first clad 41, a second clad 42, and a third clad 43) are disposed concentrically outside the core 40, and the third clad 43 is a protective resin layer. 44 is covered. Here, the refractive index of the core 40 is not uniform, and the refractive index at the center is high. In such an optical fiber having a high refractive index portion at the center of the core, the electric field is concentrated at the center of the core, the effective area (A _eff ) of the laser light guided through the core is increased, and the single mode operation It is impossible to make it compatible. In addition, the refractive index of the high refractive index portion is likely to change greatly depending on the heating method in the sintering process, the collapse process, etc., and there is a problem that it is difficult to control the refractive index profile.

Therefore, the present inventors have considered the cause of the high refractive index portion at the core center of the triple clad fiber. Specifically, the cross-sections of the following two types of triple-clad fiber base materials ((a) and (b)) having a high refractive index portion at the core center were analyzed by EPMA (Electron Probe Micro Analyzer).
(A) Base material prepared by adding Ge, Al and Yb to the core soot and adding Ge and P to the first glass layer. (B) Adding Al and Yb to the core soot and adding Ge and P to the first glass layer. As a result, in both (a) and (b), a high concentration of Ge was detected at the core center of the base material. Therefore, it was found that the high refractive index portion at the core center is caused by the presence of Ge. In addition, since Ge is not added to the core soot in the preparation of (b), Ge added to the first glass layer diffuses in the subsequent base material manufacturing process, and as a result, Ge is concentrated at the core center. It was found to remain.

Therefore, in order to solve the above problems, the present inventors have added the following three types of base materials ((c), (d) and (e)) without adding Ge to any of the core soot and the first glass layer. I tried to make.
(C) Base material in which Al and Yb are added to the core soot and P is added to the first glass layer (d) Base material in which Al and Yb are added to the core soot and Al is added to the first glass layer (e) Core soot A base material in which Al and Yb are added to the first glass layer, and Al and P are added to the first glass layer. However, in the production of (c), the first glass layer serving as the first cladding and the line of the quartz tube serving as the second cladding. The expansion coefficients differed greatly, and the glass was distorted during the cooling process, causing the problem that the tube broke.
In the production of (d), by reacting Al and Cl ₂ to generate AlCl ₃ gas, it was added Al by conveying it in a carrier gas, because of low vapor pressure of the _{AlCl 3} gas, gas In addition to the necessity of heating the pipe used for the conveyance to about 200 ° C., there is a problem that it is difficult to control the amount of Al added. Furthermore, the necessity of keeping the pipes at a high temperature has caused a problem that cost increases are inevitable.
Also in the production of (e), the same problems as in the above (c) and (d) occurred, and this could not be solved by changing the ratio of Al and P.
From the above, it was confirmed that it is very difficult to manufacture a base material of a triple clad fiber without adding Ge to the first glass layer.

  Therefore, when a base material was produced by adding Ge to the first glass layer, an attempt was made to etch away the high refractive index portion at the center of the core immediately before crushing the tube in the collapse process. However, in the manufacturing method using both the MCVD method and the liquid immersion method, it is difficult in terms of manufacturing method to increase the cross-sectional area of the core. Further, there has been a problem that cost increases cannot be avoided by further reducing the core by etching. In addition, there is a problem that the etching conditions vary greatly and the yield is significantly reduced.

  The present invention has been made in view of the above circumstances, and a multi-clad fiber that does not have a high refractive index portion at the center of the core and can easily control the refractive index profile, and an optical fiber suitable for manufacturing the multi-clad fiber. It is an object of the present invention to provide a base material, a manufacturing method capable of stably manufacturing the optical fiber base material, and an optical fiber laser and an optical amplifier including the multi-clad fiber.

To solve the above problem,
The invention according to claim 1 includes a first step of forming a first glass layer containing germanium along the inner surface of the quartz tube, and a first step not containing germanium along the inner surface of the first glass layer. A second step of forming two glass layers, a third step of forming silica soot inside the second glass layer, a fourth step of adding a dopant to the silica soot and sintering it,
And a fifth step of collapsing the tube obtained in the fourth step.
The invention according to claim 2 is the method for producing an optical fiber preform according to claim 1, wherein the thickness of the second glass layer at the end of the second step is 10 to 500 μm.
The invention according to claim 3 is the method for producing an optical fiber preform according to claim 1 or 2, wherein the dopant contains a rare earth element.
The invention according to claim 4 is the method for producing an optical fiber preform according to claim 3, wherein the rare earth element is ytterbium.
The invention according to claim 5 is the method for producing an optical fiber preform according to claim 3 or 4, wherein the rare earth element is added by a liquid immersion method.
The invention according to claim 6 is the method for producing an optical fiber preform according to any one of claims 1 to 5, wherein the second glass layer contains aluminum and / or phosphorus.
The invention according to claim 7 is the method for producing an optical fiber preform according to claim 6, wherein the refractive indexes of the first glass layer and the second glass layer are substantially equal.

  According to the present invention, a multi-clad fiber that does not have a high refractive index portion at the center of the core and can easily control the refractive index profile is obtained. An optical amplifier is obtained.

Hereinafter, the present invention will be described in detail with reference to the drawings.
<Optical fiber preform and manufacturing method thereof>
The method for producing the optical fiber preform of the present invention includes:
(1) a first step of forming a first glass layer containing germanium along the inner surface of the quartz tube;
(2) a second step of forming a second glass layer not containing germanium along the inner surface of the first glass layer;
(3) a third step of forming silica soot inside the second glass layer;
(4) a fourth step of adding a dopant to the silica soot and sintering;
(5) a fifth step of collapsing the pipe obtained in the fourth step;
It is characterized by including.
An optical fiber preform obtained by such a manufacturing method has a multi-clad structure. The quartz tube forms a second cladding, the first glass layer forms a first cladding, and the silica soot forms a core.

  The manufacturing method of the optical fiber preform of the present invention is different from the conventional manufacturing method in that the second glass layer is formed. The second glass layer functions as a barrier layer that suppresses the diffusion of Ge from the first glass layer to the silica soot in the manufacturing process of the optical fiber preform. Thereby, even if Ge is added to the first glass layer, an optical fiber preform in which Ge does not exist in the core is obtained.

(1) First Step The first step can be performed by a known MCVD method. However, the first glass layer contains germanium (Ge). Ge is a dopant that increases the refractive index of the glass layer, and liquid germanium tetrachloride (GeCl ₄ ) may be vaporized by bubbling and added.
For example, the first step is preferably performed as follows. That is, while rotating the quartz tube, silicon tetrachloride (SiCl ₄ ) gas, GeCl ₄ gas, O ₂ gas or the like is circulated from one open end to the hollow portion. At this time, a heat source such as an oxyhydrogen burner is disposed outside the quartz tube, and the quartz tube is heated while moving the heat source in the direction of the central axis of the quartz tube. As a result, the temperature rises in the hollow portion of the quartz tube, and SiCl ₄ and GeCl ₄ are oxidized to produce fine particulate soot, which is deposited on the inner surface of the quartz tube, sintered, and the glass particulate layer is formed. It is formed. The heating temperature at this time may be set so that the deposited glass fine particle layer is sintered.
Thus, a tubular first glass layer is formed substantially concentrically with the quartz tube along the inner surface of the quartz tube.

  The first glass layer preferably further contains phosphorus (P). P also increases the refractive index of the glass layer, but also lowers the sintering temperature of the glass layer. When the sintering temperature of the first glass layer is too high, the diameter of the quartz tube gradually decreases when the first cladding is deposited a plurality of times, and it may be difficult to continue the deposition. Therefore, it is preferable to contain an appropriate amount of P in the first glass layer.

The content of Ge and P in the first glass layer may be adjusted so that the relative refractive index difference (Δ2) between the first cladding and the second cladding becomes a desired value. It is preferable to adjust so that it may become larger than the refractive index of a quartz tube.
However, when there is too much content of P, the linear expansion coefficient of a 1st glass layer and a 2nd glass layer will differ greatly, and it will become easy to break a glass tube. Therefore, the content of P is preferably adjusted so as to facilitate the continuation of deposition while taking into consideration the linear expansion coefficient of the second glass layer.

As a method for containing P in the first glass layer, for example, liquid phosphorus oxychloride (POCl ₃ ) is vaporized by bubbling at the time of forming the first glass layer, and POCl ₃ gas is converted into the above-mentioned SiCl ₄ gas, GeCl. ₄ gas, the O ₂ gas or the like, it is sufficient to flow through the quartz tube.

The quartz tube is preferably made of a material having a refractive index smaller than that of the first glass layer, and is preferably made of pure quartz glass.
In addition, the outer diameter of the quartz tube is not particularly limited, and may be appropriately adjusted according to the purpose, but when the obtained optical fiber preform is used for an application such as an optical fiber laser or an optical amplifier, usually, It is preferably 10 to 50 mm, and more preferably 15 to 35 mm. For the same purpose, the thickness of the quartz tube is preferably 1.0 to 5.0 mm, and more preferably 1.5 to 3.0 mm.

What is necessary is just to adjust the thickness of a 1st glass layer suitably according to the objective. For example, when the outer diameter or thickness of the quartz tube is within the above-mentioned preferable range, the thickness of the first glass layer is usually preferably 0.5 to 2.0 mm, 0.8 to More preferably, it is 1.2 mm.
The thickness of the first glass layer can be adjusted by adjusting the flow rate of the source gas, the heating temperature, and the like.

(2) Second step The second step can also be performed by the MCVD method. For example, SiCl ₄ gas and O ₂ gas are circulated in the hollow portion in the glass tube obtained in the first step (that is, radially inside the first glass layer) without flowing Ge source gas such as GeCl _4. The second glass layer not containing Ge is formed by heating. The heating temperature at this time may be set so that the glass fine particle layer deposited on the inner surface of the first glass layer is sintered.
As described above, the tubular second glass layer is formed substantially concentrically with the quartz tube and the first glass layer along the inner surface of the first glass layer.

The second glass layer may be made of pure quartz glass, but preferably has a refractive index substantially equal to the refractive index of the first glass layer. When the second glass layer is made of pure quartz glass, the refractive index of the second glass layer is smaller than the refractive index of the first glass layer. As will be described later, the thickness of the second glass layer is usually extremely thin, and the influence of the refractive index of the second glass layer on the refractive index profile of the optical fiber is negligible or negligible. No problem. However, even if the effect is slight, in an optical fiber manufactured from such an optical fiber preform, a low refractive index portion is generated near the boundary between the core and the first cladding, and the light guided through the core The effective area (A _eff ) tends to be small. On the other hand, if the refractive index of the second glass layer is made substantially equal to the refractive index of the first glass layer, the generation of a low refractive index portion can be suppressed, and an optical fiber that can further increase A _eff can be obtained.

In order to adjust the refractive index of the second glass layer as described above, for example, it is preferable to contain an appropriate amount of P and / or aluminum (Al) in the second glass layer.
Note that the thickness of the second glass layer is usually extremely thin. Therefore, even if the second glass layer has a linear expansion coefficient different from that of the first glass layer by containing P, a large strain is hardly generated in the glass, so that the optical fiber preform can be manufactured stably.
The method of containing P in the second glass layer may be the same as in the case of the first glass layer, and may be a method of circulating POCl ₃ gas in the glass tube when the second glass layer is formed.

As a method for containing Al in the second glass layer, when the second glass layer is formed, Al and Cl ₂ are reacted to generate aluminum trichloride (AlCl ₃ ) gas, which is then used as SiCl ₄ gas or O ₂ gas. The method of making it distribute | circulate in a glass tube with these can be illustrated.
At this time, since the AlCl ₃ gas has a low vapor pressure, the Al content of the second glass layer tends to vary. However, as described above, since the influence of the refractive index of the second glass layer on the refractive index profile of the optical fiber is negligible or slight, there is no practical problem even if the Al content varies.

  The total content of dopants such as P and Al in the second glass layer may be adjusted so that the refractive index of the second glass layer has a desired value. Usually, when only P is added, the content is 3.5. It is preferable that it is -4.5 wt%, and when adding only Al, it is preferable that it is 2.0-3.0 wt%. When P and Al are co-added, the addition concentration may be adjusted as appropriate so that the refractive index of the second glass layer becomes a desired value.

  The magnitude of the barrier function that suppresses the diffusion of Ge in the second glass layer depends on the thickness of the second glass layer. Furthermore, the degree of the barrier function required for the second glass layer varies depending on the outer diameter and inner diameter of the glass tube after the first step (that is, the inner diameter of the first glass layer). This is because the degree of Ge diffusion depends on the heating conditions and required time during the collapse in the fifth step, and the heating time and required time depend on the size of the glass tube before the collapse.

Therefore, the thickness of the second glass layer is preferably adjusted as appropriate according to the outer diameter and inner diameter of the glass tube after the first step. For example, when the outer diameter and thickness of the quartz tube after the end of the first step and the thickness of the first glass layer are within the above preferred range, the thickness of the second glass layer is usually 10 to 10. It is preferable that it is 500 micrometers, and it is more preferable that it is 10-100 micrometers. When the thickness of the second glass layer is thinner than the lower limit of the above range, the effect of suppressing Ge diffusion from the first glass layer to silica soot may be reduced. On the other hand, when the thickness of the second glass layer becomes thicker than the upper limit of the above range, the influence of the refractive index of the second glass layer on the refractive index profile of the optical fiber finally increases. For example, when the second glass layer has a refractive index smaller than that of the first glass layer, a wide low refractive index portion may be generated near the boundary between the core and the first cladding in the manufactured optical fiber. Further, when the linear expansion coefficients of the first glass layer and the second glass layer are greatly different, a large strain is generated in the glass, so that the tube is easily broken in the manufacturing process of the optical fiber preform.
When the outer diameter and thickness of the quartz tube and the thickness of the first glass layer are outside the above preferred ranges, the thickness of the second glass layer should be appropriately adjusted in consideration of the above preferred ranges. It ’s fine.
The thickness of the second glass layer can be adjusted by adjusting the flow rate of the source gas, the heating temperature, and the like.

  Thus, in the optical fiber manufacturing process, the diffusion of Ge from the first glass layer to the silica soot is suppressed by forming the second glass layer. As a result, there can be obtained a multi-clad fiber in which the high refractive index portion does not exist at the center of the core and the core has a uniform refractive index.

(3) Third Step The third step can also be performed by the MCVD method. For example, silica soot (core soot) is formed by circulating SiCl ₄ gas and O ₂ gas through the hollow part (that is, the radially inner side of the second glass layer) obtained in the second step and heating. Let The heating temperature at this time is set so that the silica soot maintains a porous state and is not sintered. In this step, the dopant source gas is not circulated during the formation of silica soot.
Thus, silica soot is formed inside the second glass layer. The cross section of the glass tube 1 at this time is illustrated in FIG. In FIG. 1, reference numeral 10 indicates silica soot, reference numeral 15 indicates a second glass layer, reference numeral 11 indicates a first glass layer, and reference numeral 12 indicates a quartz tube.

(4) Fourth Step In the fourth step, first, a dopant is added to the silica soot formed in the third step. When the obtained optical fiber preform is used for an application such as an optical fiber laser or an optical amplifier, the dopant to be added contains at least a rare earth element. Here, as the rare earth element, ytterbium (Yb) is preferable.
The dopant may be added by a known method. For example, when a rare earth element is added, it is preferably performed by a liquid immersion method. The reason is that a compound containing a rare earth element used at the time of addition (hereinafter referred to as a rare earth compound) usually has a low vapor pressure. Therefore, when the rare earth compound is vaporized, it is necessary to heat at a high temperature of several hundred degrees Celsius, and it is necessary to keep the piping at a high temperature even in the process of transporting the generated gas to silica soot. In such a method, it is difficult to stabilize the content of the rare earth element in the silica soot, and the production cost also increases.

  The liquid immersion method can be performed as follows, for example. First, a rare earth compound, preferably a rare earth chloride, is dissolved in water or ethanol to prepare an immersion solution. Next, silica soot is immersed in the immersion solution by introducing the obtained immersion solution into the inside of the glass tube obtained in the third step from the open end. The immersion solution is then withdrawn from the tube, the tube is dried and further dehydrated, and then the silica soot is sintered to make it transparent.

In addition to the rare earth element, Al is preferably added as a dopant to the silica soot. By co-adding Al, the optical characteristics of the optical fiber preform are improved, for example, photodarkening can be suppressed. Al may be added by dissolving a compound containing Al, preferably aluminum chloride (AlCl ₃ ), together with the rare earth compound in the immersion solution, and using this immersion solution.

  The dopant content of silica soot can be adjusted by adjusting the dopant concentration in the immersion solution. The dopant content of the silica soot may be adjusted so that the relative refractive index difference (Δ1) between the core and the second cladding becomes a desired value, and the refractive index after sintering of the core is the first glass layer. It is preferable to adjust so that it may become larger than the refractive index of this.

(5) Fifth Step In the fifth step, the pipe obtained in the fourth step is collapsed and solidified. Thereby, an optical fiber preform in which a dopant is added to the core is obtained. The collapse may be performed by a known method.
According to the present invention, even when the first glass layer contains Ge, an optical fiber preform in which Ge does not diffuse into the core and Ge does not exist in the core is obtained by the barrier function of the second glass layer. Therefore, there is no high refractive index portion at the center of the core, and the refractive index of the core is uniform in the radial direction.

  Up to this point, the case where the optical fiber preform is manufactured by a method using both the MCVD method and the immersion method has been described. However, the present invention is not limited to this, and other methods such as the VAD method, the OVD method, the PCVD method, etc. It may be manufactured by applying a known method.

<Multi-clad fiber>
The multi-clad fiber in the present invention is obtained by spinning the optical fiber preform of the present invention and then coating the outer surface with a polymer layer.
For example, such a multi-clad fiber can be obtained by applying a known method except that the optical fiber preform of the present invention is used.
Specifically, for example, after completion of the fifth step, a clad is produced on the outer side as much as necessary by the OVD method, the obtained optical fiber preform is spun, and the outer surface of the obtained optical fiber is covered with a polymer layer. Just do it. At this time, a double clad fiber can be obtained by using a protective resin as a polymer, and a triple clad fiber can be obtained by using a polymer that forms a third clad as a polymer and then using a protective resin. As the polymer, for example, a thermosetting resin or a UV curable resin can be used, and these may be coated and cured in an appropriate amount so that the polymer layer has a desired thickness.

When producing a triple clad fiber, the refractive index of the polymer layer is preferably smaller than the refractive index of the second clad.
In the multi-clad fiber, the protective resin layer preferably has a higher refractive index than the core and the clad. Furthermore, the refractive index of the second cladding is preferably smaller than the refractive index of the first cladding, and the refractive index of the first cladding is preferably smaller than the refractive index of the core. And in a triple clad fiber, it is preferred to set Δ1-Δ2 to be about 0.1. For example, when Δ2 is about 0.3, it may be adjusted so that Δ1 is about 0.4.

<Optical fiber laser, optical amplifier>
An optical fiber laser or an optical amplifier according to the present invention includes the multi-clad fiber. Such an optical fiber laser or optical amplifier is obtained by applying a known method except that the multi-clad fiber is used, and has good optical characteristics.

  According to the present invention, even when the first glass layer contains Ge, an optical fiber preform having no Ge in the core can be obtained by the barrier function of the second glass layer. In such an optical fiber preform, the amount of dopant added to the core can also be adjusted as appropriate. Therefore, there can be obtained a multi-clad fiber in which a high refractive index portion does not exist in the center of the core, the refractive index of the core is uniform in the radial direction, and the refractive index profile can be easily controlled. According to such a multi-clad fiber, since it is easy to suppress photodarkening, an optical fiber laser and an optical amplifier having good optical characteristics such as suppression of deterioration of laser output with time can be obtained.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples.
Example 1
The first glass layer was formed by the MCVD method using a pure quartz glass tube having an inner diameter of 27.0 mm and an outer diameter of 32.0 mm (first step). At this time, SiCl ₄ gas, GeCl ₄ gas, POCl ₃ gas, and O ₂ gas were circulated through the hollow part of the quartz tube and heated with an oxyhydrogen burner installed outside the quartz tube. When the first step was finished, the outer diameter of the quartz tube was 29.0 mm. The thickness of the first glass layer was 0.9 mm, the Ge content was 3.0 wt%, and the P content was 1.0 wt%.
Next, the second glass layer is formed along the inner surface of the first glass layer by the MCVD method in which SiCl ₄ gas and O ₂ gas are circulated and heated in the hollow portion of the glass tube in which the first glass layer is formed. (Second step). The thickness of the second glass layer was 0.1 mm.
Next, silica soot was formed inside the second glass layer by the MCVD method in which SiCl ₄ gas and O ₂ gas were circulated and heated in the hollow portion of the glass tube in which the first and second glass layers were formed ( Third step).
Next, Yb and Al were added to the silica soot formed in the third step by a liquid immersion method. As the immersion solution, a solution containing ytterbium trichloride (YbCl ₃ ) and AlCl ₃ was used. Then, the immersion solution was extracted from the tube, the tube was dried and further dehydrated, and then silica soot was sintered to make it transparent (fourth step). The Yb content in the sintered core was 1.0 wt%, and the Al content was 4.0 wt%.
Next, the obtained tube was collapsed and solidified to obtain an optical fiber preform in which Yb and Al were added to the core (fifth step).
Next, a clad was deposited by the OVD method to obtain an appropriate clad diameter.
Further, by spinning an optical fiber preform, coating the outer surface of the obtained optical fiber with a polymer layer, and further coating with a protective resin, a triple clad fiber having a cross section as shown in FIG. 2 can be obtained. It was. In the triple clad fiber 2, the diameter of the core 20 is 20 μm, the thickness of the first clad 21 is 80 μm, the thickness of the second clad 22 is 300 μm, the thickness of the third clad 23 is 50 μm, and the thickness of the protective resin layer 24. Was 100 μm.

Further, the refractive index profile of the obtained triple clad fiber 2 is shown in FIG. As shown in FIG. 2, there was no high refractive index portion at the core center of the fiber. On the other hand, although a low refractive index portion was present near the boundary between the core and the first cladding, the A _{eff of} this optical fiber was 225 μm ² , which was sufficiently large and confirmed to have good optical characteristics.

(Example 2)
A triple clad fiber was produced in the same manner as in Example 1 except that POCl ₃ gas was circulated together with SiCl ₄ gas and O ₂ gas in the second step, and P was further contained in the second glass layer. The content of P in the second glass layer was 4.0 wt%. FIG. 3 shows a cross-sectional view and a refractive index profile of the obtained triple clad fiber. When the optical fiber preform was manufactured, the tube was not broken because the second glass layer was thin.
In the obtained triple clad fiber 3, the core 30, the first clad 31, the second clad 32, the third clad 33, and the protective resin layer 34 were all the same size as in the first embodiment. Also, as shown in FIG. 3, the triple-clad fiber 3 is different from the fiber of Example 1 in that there is no low refractive index portion near the boundary between the core and the first clad, and A _eff is 250 μm ² . It was confirmed that the optical properties were even better.

(Example 3)
A triple clad fiber was prepared in the same manner as in Example 1 except that AlCl ₃ gas was circulated together with SiCl ₄ gas and O ₂ gas in the second step, and Al was further contained in the second glass layer. The Al content in the second glass layer was 2.5 wt%. When the optical fiber preform was manufactured, the tube was not broken because the second glass layer was thin.
The obtained triple-clad fiber had the same core, first clad, second clad, third clad, and protective resin layer size as in Example 1. As in the case of the fiber of Example 2, there is no low refractive index portion near the boundary between the core and the first cladding, and A _eff is 250 μm ^2. It was confirmed that it had characteristics.

(Comparative Example 1)
A triple clad fiber was produced in the same manner as in Example 1 except that the second step was not performed. That is, a first glass layer was formed, and then silica soot was formed inside the first glass layer, and dopant addition, sintering, and tube collapse were performed. FIG. 4 shows a cross-sectional view and a refractive index profile of the obtained triple clad fiber.
In the obtained triple clad fiber 4, the sizes of the core 40, the first clad 41, the second clad 42, the third clad 43, and the protective resin layer 44 were all the same as those in Example 1. However, as shown in FIG. 4, a high refractive index portion was present at the center of the core 40 of the triple clad fiber 4. In this fiber 4, the electric field concentrated at the center of the core 40, A _eff was as small as 120 μm ^2, and the optical characteristics were not good.

(Comparative Example 2)
An attempt was made to produce a triple clad fiber in the same manner as in Example 1 except that GeCl ₄ gas was not circulated in the first step. That is, an attempt was made to produce an optical fiber preform by containing only P but not Ge in the dopant first glass layer. At this time, Δ2 was adjusted to 0.3 by adjusting the P content.
As a result, after adding the dopant to the silica soot in the fourth step, the tube was cracked during sintering and an optical fiber preform could not be produced. This is because the Δ content is adjusted only by the P content, so the P content increases to 4.0 wt%, the linear expansion coefficient of the first glass layer becomes too large, and a large strain is generated between the quartz tube and the quartz tube. It was thought that this was caused by

(Comparative Example 3)
A triple clad fiber was produced in the same manner as in Example 1 except that AlCl ₃ gas was circulated without flowing GeCl ₄ gas and POCl ₃ gas in the first step. That is, the first glass layer was not doped with Ge as a dopant, but only Al was contained to produce an optical fiber preform. At this time, Δ2 was adjusted to 0.3 by adjusting the Al content. The AlCl ₃ gas generated by reacting Al and Cl ₂ was used, but the vapor pressure of the AlCl ₃ gas is low, and the pipe used for transporting the gas is heated to about 200 ° C. to generate the gas. Circulated. However, it has been difficult to adjust the Al content of the first glass layer to a desired value. As a result, an optical fiber preform was repeatedly produced by the same method, but it was difficult to control the refractive index of the first cladding, the production reproducibility was poor, and the yield was remarkably reduced. In addition, it was confirmed that the manufacturing cost increases because high temperature heating is required.

(Comparative Example 4)
An attempt was made to produce a triple-clad fiber in the same manner as in Example 1 except that AlCl ₃ gas was allowed to flow without flowing GeCl ₄ gas in the first step. That is, an attempt was made to fabricate a fiber in which P and Al were not contained as the dopant in the first glass layer. At this time, Δ2 was adjusted to 0.3 by adjusting the contents of P and Al.
As a result, the tube was cracked in the same manner as in Comparative Example 2, and the optical fiber preform could not be produced. Furthermore, as in Comparative Example 3, it was difficult to adjust the Al content of the first glass layer.

Example 4
Triple clad fibers were prepared by the methods of Examples 1 to 3. However, in order to suppress skew light (excitation light that does not cross the core), the second cladding was processed so that the shape of its radial cross section was a substantially regular heptagon. The produced triple clad fiber has a core diameter of 20 μm, a thickness of the first clad of 80 μm, a thickness of the second clad of 300 μm, a thickness of the third clad of 50 μm, and a thickness of the protective resin layer of 100 μm. It was. And using these triple clad fibers, a fiber laser was produced.
As a result, a fiber laser capable of single mode operation with a sufficiently large A _eff was obtained. And compared with the case where the fiber produced with the method of the comparative example 1 was used, the fall of the energy conversion efficiency by a nonlinear optical effect was able to be suppressed.

  The present invention is applicable to optical fiber lasers, optical amplifiers, and the like in the field of optical communications.

It is a figure which illustrates the cross section of the glass tube at the time of completion | finish of the 3rd process of this invention. It is a figure which shows the cross section and refractive index profile of the triple clad fiber of Example 1. FIG. It is a figure which shows the cross section and refractive index profile of the triple clad fiber of Example 2. It is a figure which shows the cross section and refractive index profile of the triple clad fiber of the comparative example 1. It is a figure which illustrates the cross section of a double clad fiber, and a preferable refractive index profile. It is a figure which illustrates the cross section of a triple clad fiber, and a preferable refractive index profile.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Glass tube, 10 ... Silica soot, 11 ... 1st glass layer, 12 ... Quartz tube, 15 ... 2nd glass layer, 2, 3, 6 ... Triple clad fiber, 20, 30, 60 ... core, 21, 31, 61 ... first clad, 22, 32, 62 ... second clad, 23, 33, 63 ... third clad, 24, 34, 64 ... Protective resin layer

Claims (7)

A first step of forming a first glass layer containing germanium along the inner surface of the quartz tube;
A second step of forming a second glass layer not containing germanium along the inner surface of the first glass layer;
A third step of forming silica soot inside the second glass layer;
A fourth step of adding a dopant to the silica soot and sintering;
A fifth step of collapsing the tube obtained in the fourth step;
An optical fiber preform manufacturing method comprising:   The method for producing an optical fiber preform according to claim 1, wherein the thickness of the second glass layer at the end of the second step is 10 to 500 µm.   The method for manufacturing an optical fiber preform according to claim 1 or 2, wherein the dopant contains a rare earth element.   The method for producing an optical fiber preform according to claim 3, wherein the rare earth element is ytterbium.   The method of manufacturing an optical fiber preform according to claim 3 or 4, wherein the rare earth element is added by a liquid immersion method.   The method for producing an optical fiber preform according to any one of claims 1 to 5, wherein the second glass layer contains aluminum and / or phosphorus.   The optical fiber preform manufacturing method according to claim 6, wherein the refractive indexes of the first glass layer and the second glass layer are substantially equal.

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