JP5657274B2 - Rare earth element-doped optical fiber and manufacturing method thereof - Google Patents

Description

  The present invention relates to a rare earth element-doped optical fiber that is an optical fiber having a core portion doped (doped) with a rare earth element, and a method for manufacturing the same.

Optical fibers with rare earth elements such as ytterbium (Yb), erbium (Er), thulium (Tm), neodymium (Nd) added to the core, especially fiber lasers and optical fiber amplifiers, have high conversion efficiency and excellent beam quality. Has attracted great interest in the field of laser processing technology.
In order to realize a high-quality fiber laser with rare-earth-doped optical fibers, physical properties such as a wide gain spectrum, low reabsorption, laser stability, high light-to-light conversion efficiency, low photodarkening, and low loss are required. Required. Among these physical properties, in particular, high conversion efficiency is more required from the viewpoint of energy utilization efficiency and weight reduction of the apparatus. If the optical fiber suppresses photodarkening, the cooling device does not need to be separately configured, and thus the laser device is further reduced in weight.

  Examples of a method for manufacturing an optical fiber preform in which a rare earth element is added to the core include a VAD (Vapor phase Axial Deposition) method, an OVD (Outside Vapor Deposition) method, and an MCVD (Modified Chemical Vapor Deposition) method. As the MCVD method, a sintered MCVD method and a vapor phase MCVD method are known.

  Since the rare earth element-doped optical fiber is used as a high-power laser or an amplifier, high reliability is required for the temperature change of the optical fiber. In particular, photodarkening caused by the color center formation mechanism is known to occur in proportion to approximately the seventh power of the excitation inversion distribution or the excitation concentration of rare earth ions, which is a major cause of optical fiber deterioration. (For example, Non-Patent Document 1).

It is generally known that photodarkening is caused by clustering of ions of added rare earth elements. This clustering of ions of rare earth elements is a phenomenon in which doped rare earth ions cannot sufficiently diffuse (separate) and become oxygen defects.
When the concentration of rare earth element ions is increased, clustering also appears prominently. Therefore, as the amount of rare earth element added increases, photodarkening tends to increase.
As a technique for suppressing this photodarkening, a method of co-adding rare earth elements and aluminum (Al) to the core has been proposed (for example, Non-Patent Document 2).
In addition, in order to suppress clustering, a DND method (Direct Nano Particle Deposition Method) in which rare earth elements are dissolved in water or alcohol, and the solution is directly atomized in an oxyhydrogen flame and added to the glass fine particle layer. Has been proposed (for example, Non-Patent Document 3).
Furthermore, a method for suppressing clustering of rare earth elements by adding boron (B), fluorine (F), germanium (Ge) or the like to the core to which rare earth elements are added has been proposed (for example, patents). Reference 1). Both methods require sufficient diffusion to reduce the rare earth clustering, and it is advantageous to make the glass softer.

However, when the present inventors studied in more detail about the manufacturing method of the optical fiber which has the core which added the rare earth elements as mentioned above, it turned out that each has a problem.
First, there is a problem that adding a high concentration of aluminum to the core makes it difficult to control the numerical aperture (NA) of the core. In order to reduce the refractive index, elements such as B and F must be added. I must.
Also, in the DND method, in order to spray liquid-phase mist-like droplets in the size of nanometer order in the flame, expensive equipment and advanced technology are necessary, and from the viewpoint of low cost and manufacturing stability. It is not preferable.
Furthermore, even in the method of adding the third additive (dopant) to the core, since it is difficult to control the refractive index profile, stable core production is not easy.

US Patent No. 7006752 (US 7006752 B2)

SPIE Photonics West 2007, Vol. 6453-50 OFC2006, OThC5, 2006 SPIE Photonics West 2006, Vol. 6116-16 (2006)

  An object of the present invention is to solve the above-described problems and provide a rare earth element-doped fiber in which photodarkening is suppressed and a manufacturing method thereof.

As a result of intensive studies to solve the above problems, the present inventors have found that if the concentration of rare earth elements from the central part of the core to the outer part is changed according to the light intensity distribution, photodarkening The present invention has been completed.
That is, the present invention has the following characteristics.

(1) A method for producing a rare earth element-doped optical fiber,
Glass fine particles are deposited on the inner surface of a quartz tube while adding rare earth elements or by adding rare earth elements after glass fine particles are deposited to form a glass fine particle layer to which rare earth elements are added. A layer forming step;
A vitrification step of heating the glass fine particle layer to form a glass layer;
A collapsing step in which the quartz tube having the glass layer formed on the inner surface by the step is collapsed into a core base material;
One or both of the following (B) and (C) are processed so that the concentration of the rare earth element in the core in the core base material after the collapse process satisfies the following change in the concentration (A): And
A method for producing the rare earth element-doped optical fiber.
(A) The concentration of the rare earth element in the core is a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then decreases toward the core center. It is the lowest point, and the ratio of the core center concentration to the maximum concentration value is 5% to 95%.
(B) Controlling the amount of rare earth element added in the glass fine particle layer forming step.
(C) To reduce the amount of rare earth element added by introducing Cl ₂ gas into the quartz tube after the glass fine particle layer forming step and bringing the Cl ₂ gas into contact with the inner surface of the tube.
(2) The method of forming the glass fine particle layer in the glass fine particle layer forming step is a sintered MCVD method,
In the glass fine particle layer forming step, after the glass fine particle layer is deposited a plurality of times on the inner surface of the quartz tube, a solution containing at least a rare earth element is brought into contact with the inner surface of the glass fine particle layer so that the solution penetrates into the layer. Adding a rare earth element in the layer, drying by heating,
The manufacturing method according to (1), wherein in the collapse step, Cl ₂ gas is introduced into the quartz tube to satisfy the change in the concentration of (A).
(3) The method of forming the glass fine particle layer is a gas phase MCVD method,
In the glass fine particle layer forming step and the vitrification step, glass fine particles and at least a rare earth element are simultaneously deposited on the inner surface of the quartz tube, and the amount of rare earth element sent into the quartz tube is changed as the layer thickness increases. To satisfy the change in the concentration of (A).
(4) After the glass fine particle layer forming step and before the final collapsing step, Cl ₂ gas is introduced into the quartz tube, and the additive element is etched by bringing the Cl ₂ gas into contact with the inner surface of the tube. And the manufacturing method in any one of said (1)-(3) which further reduces the density | concentration of the rare earth element of the center of the core in the core base material after a collapse process.
(5) In the glass fine particle layer forming step, the rare earth element is added to the glass fine particle layer so as to satisfy the change in the concentration of (A), and at the same time, the following change in the concentration of (D) or (E) is satisfied. Thus, the manufacturing method in any one of said (1)-(3) which co-adds aluminum in a glass fine particle layer.
(D) The concentration of aluminum in the core of the core base material after the collapse has a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then decreases toward the core center. It is the lowest point of descent at the core center, and the ratio of the core center concentration to the maximum concentration value is 5% to 90%.
(E) The concentration of aluminum in the core in the core base material after the collapse has a maximum value at least between the core outer peripheral surface of the core and the center of the core, and the maximum value from there to the center of the core It remains constant.
(6) The aluminum concentration satisfies the change in the concentration in (D) above,
After the glass fine particle layer forming step, heat drying is performed, and during the collapse step, Cl ₂ gas is introduced into the quartz tube, and the Cl ₂ gas is brought into contact with the inner surface of the glass fine particle layer or the inner surface of the glass layer. The method according to (5), wherein the concentration of aluminum at the center of the core in the core base material after the collapse is further reduced.
(7) A rare earth element-doped optical fiber obtained by the production method according to any one of (1) to (6) above,
Having a core to which at least a rare earth element is added, and a surrounding quartz cladding layer,
The concentration of the rare earth element in the core is the maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then descends toward the core center. The ratio of the concentration of the core center to the maximum value of the concentration is 5% to 95%.
(8) A rare earth element and aluminum are co-added to the core,
The concentration of aluminum in the core is a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then descends toward the core center. The rare earth element-doped optical fiber according to (7), wherein the concentration ratio of the core center to the maximum concentration value is 5% to 90%.
(9) A rare earth element and aluminum are co-added to the core,
The concentration of aluminum in the core is a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and the maximum value is constant from there to the center of the core (7 ) Rare earth element-doped optical fiber.

For optical fibers doped with rare earth elements, the present invention controls the concentration of rare earth elements added to the core, and the ratio of the core center concentration to the maximum value of the rare earth element concentration in the core is 5% to 95%. Thus, the concentration at the center of the core is reduced.
With this configuration, when a fiber laser is configured using the optical fiber, even if the electric field distribution of the fundamental mode of laser oscillation is high at the center of the core, the Yb concentration at the core center is relatively low, so that rare earth ions are excited. The density becomes low, and photodarkening hardly occurs.

In the present invention, photodarkening is more effectively suppressed by further adding aluminum (hereinafter, Al) to the core.
Here, in the embodiment in which the Al concentration at the center of the core is lowered, the formation of rare earth element clusters is suppressed by reducing the Al concentration so that the molar ratio of Al to the rare earth element is 3 or more. By reducing the refractive index at the center, the electric field distribution at the core center is lowered, the inversion distribution of rare earth ions is reduced, and the effect of suppressing photodarkening is obtained.
On the other hand, in an embodiment in which the Al concentration at the center of the core is not reduced, the inversion distribution is reduced even if the electric field distribution at the core center is high by reducing the rare earth element concentration so that the molar ratio of Al to the rare earth element is 3 or more. As a result, the effect of suppressing photodarkening is obtained, and the effect of maintaining high beam quality is also obtained. Also, in multimode fiber lasers, high-order transverse mode gain saturation is unlikely to occur, so self-pulses are likely to occur, and the fiber end face is destroyed. Low-order mode gain saturation is reduced by reducing the central concentration of rare earth elements. Can be eliminated by stabilizing the gain.

FIG. 1 is a schematic diagram illustrating the steps in order for explaining an example of the manufacturing method according to the present invention. FIG. 2 is a graph schematically showing the concentration and refractive index profiles of additives in the core base material produced in Example 1 of the present invention. 2A shows a Yb concentration profile, FIG. 2B shows an Al concentration profile, and FIG. 2C shows a refractive index profile. In each graph, the horizontal axis represents each position in the core base material from the outermost surface of the core base material to the core outer surface on the opposite side, and the addition of Yb at each position. The values of concentration, Al addition concentration, and refractive index are plotted on the vertical axis. The profile (distribution curve) referred to in the present invention is a graph curve showing how various physical quantities such as concentration, refractive index, and light intensity are distributed in the fiber and the core preform. FIG. 3 is a graph showing the change of the inversion distribution with respect to the width of the center depression in the single mode fiber. FIG. 4 is a graph showing the change of the inversion distribution with respect to the center depression in the multimode fiber. FIG. 5 is a graph showing the correlation between the depth of the central depression of the refractive index profile and the electric field strength of the fundamental mode (LP01) for a single mode fiber having a core outer diameter of 12.5 μm and a cladding outer diameter of 125 μm. FIG. 6 is a graph showing the correlation between the depth of the central depression of the refractive index profile and the electric field strength of the fundamental mode (LP01) for a multimode fiber having a core outer diameter of 40 μm and a cladding outer diameter of 125 μm. FIG. 7 is a graph showing a refractive index profile, an electric field distribution, an X-axis cross section of the electric field distribution, and a Y-axis cross section of the electric field distribution when the depth of the refractive index central depression is 38% with respect to the core diameter of 40 μm. FIG. FIG. 8 is a schematic diagram showing a configuration of an apparatus used for evaluating photodarkening in an example of the present invention. FIG. 9 is a graph showing the result of evaluation of photodarkening in Example 1 of the present invention. FIG. 10 is a graph plotting the photodarkening rate with respect to the population inversion in Example 1 of the present invention. FIG. FIG. 11 is a graph schematically showing the additive concentration and refractive index profiles in the core preform produced in Example 2 of the present invention. FIG. 11A shows a Yb concentration profile, FIG. 11B shows an Al concentration profile, and FIG. 11C shows a refractive index profile.

In the present invention, when the rare earth element-doped fiber is manufactured, a glass fine particle layer is formed on the inner surface of the quartz tube by using a gas phase MCVD method or a sintered MCVD method (glass fine particle layer forming step).
The gas phase MCVD method is a method in which glass fine particles and additives (rare earth elements, Al, etc.) are simultaneously deposited on the inner surface of a quartz tube to form a glass fine particle layer while containing the additives. In this method, it is possible to provide a gradient in the concentration of the additive by controlling the amount of the supply gas and the raw material.
Further, in the process after depositing the glass particles (for example, while collapsing), a Cl ₂ gas (chlorine gas) is applied to the reaction tube (a tubular intermediate having a glass particle layer formed on the inner surface of the quartz tube). It is possible to remove at least the surface layer additive on the inner surface of the reaction tube, thereby providing a gradient of the additive concentration. Hereinafter, the process of introducing Cl ₂ gas into the reaction tube to provide the additive concentration gradient is also referred to as “chlorine treatment”.
The sintered MCVD method is a method in which a glass fine particle layer is first deposited, and a solution containing the additive is brought into contact with the layer to infiltrate the layer, thereby adding the additive into the layer later. In this method, after the glass fine particle layer is deposited, the additive permeates and diffuses the additive, thereby providing a gradient in the additive concentration. Also in this method, chlorination may be performed on the inner surface of the reaction tube.
In the following description, the present invention will be described in detail mainly on the procedure of the sintered MCVD method, and the vapor phase MCVD method will be referred to as necessary.

  The unit of flow [SLM] used in the following explanation is an abbreviation of [Standard Liter / Min; standard liter per minute], and is a unit expressed in liters per minute at 1 atm and 0 ° C. is there. The unit [SCCM] of the flow rate is an abbreviation of [standard cc / min; standard cc per minute], and is a unit in which the flow rate per minute at 1 atm and 0 ° C. is expressed in cc.

In the production method of the present invention, first, as a preferred ground treatment for the inner surface of the anhydrous quartz tube, as shown in FIG. 1A, sulfur hexafluoride (SF) is formed on the inner surface of the anhydrous quartz tube from one end to the other end. ₆ ) Helium (He) and oxygen (O ₂ ) are supplied as a mixed gas, and the inner wall surface of the anhydrous quartz tube is etched at a heating temperature of 700 to 2000 ° C.
An anhydrous quartz tube means a hollow tube made of quartz, and the amount of OH groups contained in the quartz is 1 ppm or less which is the limit of measurement by an infrared spectrometer.
The dimensions of the anhydrous quartz tube are not particularly limited, but in the example of FIG. 1, the outer diameter is 28 mm, the wall thickness is 1.5 mm, and the length is 500 mm.
In the MCVD method, transmission loss (absorption) increases due to OH groups contained in the raw material and OH groups mixed during the production of the core base material, and the laser characteristics deteriorate. The use of an anhydrous quartz tube aims to further reduce the OH group content.
In the above etching, the flow rate of SF ₆ is 0.01 to 0.2 [SLM], the flow rate of He is 0.1 to 0.5 [SLM], and the flow rate of O ₂ is 0.5 to 3.0 [SLM]. Are preferred for each.
Examples of the heating include heating by the burner 2 illustrated in FIG. 1A and heating by an electric furnace. A preferable burner includes an oxyhydrogen burner. The following heating is the same.

Next, as shown in FIG. 1B, a step of supplying a Cl ₂ gas into the reaction tube at a heating temperature of 1000 to 2000 ° C. to dehydrate the inner wall surface of the anhydrous quartz tube 1 may be added. .

Next, in the sintered MCVD method, as shown in FIGS. 1 (c) and 1 (d), a glass fine particle layer is deposited in a reaction tube at a heating temperature of 1200 to 1500 ° C., and penetration of a solution containing at least a rare earth element and The additive concentration is graded by ion diffusion of the additive. Further, in the vapor phase MCVD method, an added glass fine particle layer is deposited in the reaction tube so as to have a gradient in the concentration of at least the rare earth element (not shown).
Whether the sintering MCVD method or the vapor phase MCVD method is used, it is important that the concentration of the rare earth element in the glass fine particle layer satisfies the concentration change according to the concentration profile of FIG. is there. The density profile refers to a curve representing a change in density from the outermost surface of the core to the core outermost surface passing through the core center.
As in the concentration profile shown in FIG. 2A, in the present invention, the concentration of the rare earth element in the core in the core base material after the collapse is at least a position (intermediate) between the core outer peripheral surface of the core and the center of the core. (Referred to as a position), and descends from the intermediate position toward the core center, and is the lowest point of descent at the core center. Since the concentration profile is symmetric with respect to the central axis of the core, the concentration profile over the entire core diameter draws a curve in which the protruding convex center is depressed as in the concentration profile shown in FIG.
The maximum value of the density does not need to be an increase / decrease pattern having a peak at the intermediate position as in the example shown in FIG. 2A, and the maximum value is constant from the core body outer peripheral surface to the intermediate position. , It may descend from the intermediate position toward the core center, and may be the lowest point of descent at the core center. “Constant at the maximum value” includes a state in which the vicinity of the maximum value is finely increased and decreased around the maximum value and generally remains at the maximum value.
The ratio of the core center concentration to the maximum concentration value is 5% to 95%. A preferred range of the ratio of the core center concentration to the maximum concentration value will be described later.

In the sintered MCVD method, while the anhydrous quartz tube 1 is heated to 1200 to 1500 ° C., various gases such as a glass raw material gas, a gas, and a reactive gas are introduced into the tube from one end thereof, thereby the inner surface of the tube 1. A glass fine particle layer 3 is formed by depositing glass fine particles. The glass fine particle layer is also called soot.
Of the various gases sent into the tube in this step, the glass raw material gas is essential, and examples thereof include SiCl ₄ and SiF ₄ .
The gas is a gas serving as a carrier for transporting the element to be added into the tube, and examples thereof include O ₂ , He, and Ar. In the case of the gas phase MCVD method, elements carried by the gas in this step are Al, Er, Yb, Tm, P, and the like.
In the sintered MCVD method, the rare earth element and Al are added from the solution in the subsequent immersion process. Phosphorus (P) is added when the glass fine particle layer is deposited.

Flow rate of each gas, sintering MCVD method, slightly different in the case of the gas phase MCVD method, also, etc. but a quartz tube or a rare earth element to be added to be used, generally, a glass material gas (such as SiCl ₄₎ is 0.1-0.6 [SLM], gases (He, etc.) is 0.3 to 0.6 [SLM], (such as O ₂₎ the reaction gas is 0.4 to 3.0 [SLM], preferred are Flow rate.

  In the sintered MCVD method, the glass fine particle layer is deposited at a relatively low temperature of 1200 to 1500 ° C. In the sintered MCVD method, when the glass fine particle layer is deposited at a temperature higher than 1500 ° C., the size of the glass fine particles becomes excessively fine or directly vitrified, which will be described later. In the immersion process, the addition concentration and refractive index profile of the rare earth element soaked into the glass fine particle layer are affected, which is not preferable. Further, when the heating temperature is less than 1200 ° C., the adhesion between the glass fine particles and the inner surface of the quartz tube is weak, and the glass fine particle layer may be peeled off during liquid permeation or natural drying.

  In both cases of the sintered MCVD method and the vapor phase MCVD method, when the glass fine particles are deposited, the internal pressure of the quartz tube influences the flow of the gas in the reaction tube and the glass fine particles. It is desirable to control the pressure so that it is about 4 to 10 Pa lower than the pressure outside the reaction tube (atmospheric pressure).

On the other hand, in the gas phase MCVD method, in the glass fine particle layer forming step, rare earth elements, Al, and all other additives are sent into the tube by gas, so that the gradient of the concentration is formed in the glass fine particle layer. Added.
The temperature at which the glass fine particle layer is deposited and vitrified by the vapor phase MCVD method is generally 1800 to 2000 ° C. For the detailed technique itself in the case of forming the glass fine particle layer by the vapor phase MCVD method, the prior art may be referred to.

In both of the sintered MCVD method and the vapor phase MCVD method, in the step of depositing the glass fine particles, not only the gas and the reactive gas, but also phosphorus (P) and boron trifluoride (BF ₃ ) in addition to these. Etc. may be supplied. For example, phosphorus is supplied by gas, and BF ₃ is supplied directly.
Since the glass to which BF ₃ is added has a lower refractive index than quartz, the refractive index of the core to which a rare earth such as Yb is added can be reduced. Instead of BF ₃ , B, F, silicon tetrafluoride (SiF ₄ ), boron trichloride (BCl ₃ ), boron tribromide (BBr ₃ ), etc. can be substituted, but the deposition efficiency and handling ease From the viewpoint of BF _3, BF ₃ is desirable.

The glass fine particle layer may be a single layer by a single deposition or a laminated structure by a plurality of depositions, and the total layer thickness depends on the inner diameter of the quartz tube used and the structure of the core of the fiber to be manufactured. However, 0.02 mm to 0.2 mm is general purpose. In particular, in the case of producing a single-mode, air-hole type double clad optical fiber in which a rare earth element and Al are added to the core, the total layer thickness of the glass fine particle layer is preferably 0.02 mm to 0.2 mm. It is a range.
In the sintered MCVD method, when a glass fine particle layer is deposited a plurality of times to form a laminated structure, soot is continuously deposited, and the concentration of the additive is graded by chlorination in an ion diffusion or collapse stage.
When the glass fine particle layer is deposited multiple times to form a laminated structure, for example, the thickness of one deposition is set to 0.01 mm to 0.02 mm, and this is repeated 2 to 5 times to obtain the above total layer thickness. This is a preferred embodiment. These values may be appropriately determined according to the core diameter of the core base material after the collapse.
For example, as an example when the core diameter of the core base material is thin, when the thickness is about 1 mm to 1.5 mm, the layer thickness of one deposition is preferably about 0.01 mm to 0.03 mm, and it is 3 times to 5 times. It is preferable that the total layer thickness is 0.03 mm to 0.15 mm by repeating the process. Moreover, as an example when the core diameter of the core base material is thick, when the thickness is about 2 mm to 3 mm, the layer thickness of one deposition is preferably about 0.02 mm to 0.05 mm, and this is repeated 3 to 5 times. Thus, the total layer thickness is preferably 0.06 mm to 0.25 mm.
On the other hand, in the gas phase MCVD method, by changing the content of the additive in the supply gas, it is possible to freely form a concentration gradient of the additive while depositing the glass fine particle layer. There is no particular need to distinguish the laminated structure.

As described above, in the sintered MCVD method, as a liquid immersion process, a solution containing at least a rare earth element is brought into contact with the inner surface of the glass fine particle layer, the solution is penetrated into the layer, and the rare earth element is added to the layer. To do.
Since the clustering of rare earth ions causes photodarkening and adversely affects fiber laser characteristics, in a preferred embodiment of the present invention, rare earth elements and Al are co-added to the core, and the rare earth ions are added by Al. To suppress clustering.
Therefore, the addition amount of Al is important, and the molar ratio between Al and rare earth element is also an important parameter for the characteristics of the rare earth element doped fiber.
For example, in order to suppress the clustering of Yb, it is desirable that the molar ratio R (= Al / Yb) between Al and Yb is 3 or more. A more preferable range of the molar ratio R of Al and Yb is 3-10. The molar ratio R is the same when Al is combined with other rare earth elements.

In the glass fine particle layer forming step, when adding rare earth elements to the glass fine particle layer and adding Al at the same time, the Al in the glass fine particle layer can be obtained by either the sintered MCVD method or the vapor phase MCVD method. It is important to satisfy the density change according to the density profile shown in FIG. 2B or the density profile shown in FIG. 11B.
The change pattern of the concentration profile shown in FIG. 2 (b) is the same as the concentration profile of the rare earth element shown in FIG. 2 (a), and the aluminum concentration in the core in the core base material after the collapse is at least that of the core. It has a maximum value at a position between the outer peripheral surface of the fuselage and the center of the core, descends from the center toward the core center, and is the lowest point of descent at the core center. Therefore, the concentration profile over the entire core diameter draws a curve in which the protruding convex center is depressed like the concentration profile shown in FIG.
Similarly to the concentration profile of the rare earth element, the maximum concentration value does not need to be an increase / decrease pattern having a peak at the intermediate position as in the example shown in FIG. The maximum value from the outer peripheral surface of the body to the intermediate position may be constant and may be lowered from the intermediate position toward the core center, and may be the lowest point of descent at the core center.
The ratio of the core center concentration to the maximum concentration value will be described later.
In addition, the density profile change pattern shown in FIG. 11B has no density depression in the center as compared to the density profile change pattern shown in FIG. That is, the Al concentration has a maximum value at an intermediate position between the core outer peripheral surface of the core and the center of the core, and remains constant at the maximum value from there to the center of the core. This change pattern is not only a pattern in which the central portion of the core has a peak as shown in FIG. 11B, but also a rectangular wave shape in which the concentration is constant from the outer peripheral surface of the core to the center of the core. It may be a pattern.

In order to bring the solution into contact with the inner surface of the glass particle layer, for example, as shown in FIG. 1 (d), a method of injecting the solution into the reaction tube and bringing it into contact with the inner surface of the glass particle layer is an effective method. It is done. At this time, in order to uniformly bring the solution 4 into contact with the inner surface of the glass fine particle layer and to uniformly infiltrate the entire inner surface of the layer, the reaction tube 1 is kept in a state where the central axis of the reaction tube 1 is kept horizontal. It is desirable to rotate around the central axis at a speed of about 2-20 revolutions / minute.
The method of rotating the reaction tube is not particularly limited, and examples thereof include a method of holding the reaction tube by a lathe chuck and rotating the lathe.

The co-additives such as rare earth elements and Al are preferably dissolved in the solvent as compounds such as chlorides and oxides or hydrates thereof.
The compound or a hydrate of the rare earth element, erbium chloride _{(ErCl 3, ErCl 3 · 6H} 2 O), neodymium chloride (NdCl _3), ytterbium chloride _{(YbCl 3, YbCl 3 · 6H} 2 O), thulium chloride ( TmCl ₃ ), lanthanum chloride (LaCl ₃ ) and the like. These additives may be a single additive or a combination thereof. From the viewpoint of ease of handling, hydrates such as YbCl ₃ .6H ₂ O are preferred materials.
Further, Al or other elemental compounds that are co-additives or hydrates thereof include AlCl ₃ , AlCl ₃ .6H ₂ O, Al (NO ₃ ) ₃ , P ₂ O ₅ , H ₃ PO _{4 and the} like. Can be mentioned. These additives may be a single additive or a combination thereof. As for Al, from the viewpoint of ease of handling, AlCl ₃ .6H ₂ O is mentioned as a preferable material.

As a solvent for dissolving the additive, for example, alcohols such as water, ethanol and methanol are preferable. These solvents are convenient because they can sufficiently dissolve compounds such as chlorides and salts, and can be easily and sufficiently removed by a reactive gas such as Cl ₂ in addition to being almost volatilized by natural drying. From these points, a particularly preferred solvent is high-purity ethanol.

The light-light conversion efficiency of an optical fiber to which a rare earth element is added is greatly affected by the concentration of the additive contained in the solution.
For example, when Yb and Al are added together, the solute concentrations [wt%] are about 0.05 to 1.5 [wt%] for Yb and about 0.05 to 2 [wt%] for Al. desirable. As a specific example, when dissolving and Yb and Al in ethanol, to ethanol 50cc, YbCl ₃ · 6H ₂ O is 0.1~0.15g, AlCl ₃ · 6H ₂ O is from 0.33 to 0. 5g is a preferred amount.

  Optimum conditions for the immersion process vary depending on the thickness of the glass fine particle layer, but examples include conditions such as a temperature of 20 to 60 ° C. and an immersion time of 20 minutes to 2 hours.

Next, in order to discharge the solution in the reaction tube while supplying He and O ₂ , natural drying is performed for 1 to 3 hours, and then the temperature of the burner is gradually increased to 50 to 1800 ° C. Heat drying and ion diffusion.
YbCl ₃ · 6H ₂ O and AlCl ₃ · 6H ₂ O, when above about 900 ° C., during the heat drying, is decomposed into _{Yb (OH) 3, Al (} OH) 3 and HCl. Further, after Yb and Al become simple substances, they diffuse in the fine particles with the temperature. The diffusion situation varies greatly depending on the pipe temperature and chlorine flow rate. For example, it is preferable to increase the chlorine flow rate in order to promote ion diffusion. However, care must be taken because etching of the added ions is promoted.

  The difference between the loss of 1.3 μm wavelength and the loss of 1.248 μm wavelength is 2.2 to 20 dB / km, and the difference between the loss of 1.3 μm wavelength and the loss of 1.38 μm wavelength is 58 to 1160 dB / km. Residual moisture (OH group) is removed so that the distance becomes km.

Next, as shown in FIG. 1 (e), the internal pressure of the reaction tube is made lower by about 100 Pa to 200 Pa than the pressure outside the reaction tube (atmospheric pressure) to remove residual traces of moisture and foreign matter, O ₂ and Cl ₂ gases are supplied, the temperature of the anhydrous quartz tube is raised to 1500 to 1900 ° C., and Yb and Al are diffused into the fine particles.
The flow rate of each gas at this time is 0.1 to 1.0 [SLM] for He, 0.1 to 5.0 [SLM] for O ₂ , and 0.01 to 0.5 [SLM] for Cl _2. desirable. In particular, the flow rate of Cl ₂ is important to promote the decomposition of chloride with temperature.

Here, Cl ₂ shows the effect of removing excess rare earth elements and Al existing on the inner surface of the glass layer and the effect of promoting the decomposition of chloride in addition to showing the dehydration effect of the glass. Further, the control of the refractive profile at the core center, the rare earth element concentration at the core center, and the aluminum concentration can be further reduced.
Therefore, by controlling the amount of Cl ₂ gas introduced into the quartz tube and the pipe temperature, the concentration profile of the rare earth element at the center of the core base material and the concentration profile of Al can be controlled.
For example, as the flow rate of Cl ₂ gas is increased, the depressions at the core centers of the rare earth element concentration profile and the Al concentration profile become deeper. Further, as the Cl ₂ gas flow rate and temperature are increased, the decomposition of chloride is promoted, and the refractive index profile has no abnormal shape such as corners in the vicinity of the outermost surface of the fuselage, and rises sharply from the outermost surface of the fuselage, A round shape that draws a parabolic curve is obtained up to the center of the core.
If the Cl ₂ flow rate is increased too much, the center of the refractive index profile will be recessed too much, and this will have an adverse effect on the light-light conversion efficiency and beam quality.

  Regarding the range of the rare earth element concentration in the core, the ratio of the rare earth element concentration in the core center to the maximum value of the rare earth element concentration in the middle position should be 5 to 95%, from the viewpoint of conversion efficiency. 5 to 60% is more preferable, and 5 to 25% is a particularly preferable range. In order for the Yb core to function as a gain medium, it is important that the conditions are such that the inversion distribution (excitation ion concentration) does not vary greatly.

FIG. 3 is a graph showing the change in the inversion distribution with respect to the width of the center depression in the single mode fiber, and FIG. 4 is a graph showing the change in the inversion distribution with respect to the center depression in the multimode fiber. In these drawings, a indicates the core outer diameter, b indicates the width of the central depression of the Yb concentration profile, and the refractive index profile has a step shape.
3 and 4, in the step index profile, the Yb concentration that the central depression does not have is set to 0% [(b / a) × 100%], and the width of the central depression of the Yb concentration (that is, the concentration is maximum). (2 times the distance from the intermediate position to the core center) and the inversion distribution.
The excitation conditions are a fiber length of 15 M, an excitation wavelength of 976 nm, and an input power of 500 W.
As is apparent from the graph of FIG. 3B, in the single mode fiber having the core outer diameter of 10 μm, the cladding outer diameter of 125 μm, and NA of 0.08, the change in the inversion distribution is reduced until the width b of the recess becomes 2 μm (17%). can not see.
As is clear from the graph of FIG. 4B, in a multimode fiber having a core outer diameter of 40 μm, a cladding outer diameter of 125 μm, and an NA of 0.08, the inversion distribution is obtained until the central recess width b becomes 9 μm (23%). No change is seen.
From this result, it can be seen that even if the Yb density at the center is reduced, the inversion distribution is not affected and photodarkening can be effectively suppressed.
The width b of the dent varies slightly depending on the single mode fiber and the multimode fiber, but 5 to 50% of the LP01 (basic mode) MFD (mode filled diameter) is a preferable range.
As for the range of Al concentration in the core, the ratio of the Al concentration at the core center to the maximum value of Al concentration at the intermediate position is preferably 0% to 90%. From the viewpoint of beam quality and conversion efficiency. From 0% to 60% is more preferable.
The Al concentration profile is almost the same as the refractive index profile, but is somewhat affected by the amount of phosphorus (P) and rare earth doping. Conditions that do not significantly change the beam quality and beam profile are important.
In the case of the single mode, the ratio of the Al concentration at the core center to the maximum value of the Al concentration at the intermediate position is particularly preferably 40%. The ratio of the Al concentration at the core center to the maximum value of the Al concentration at the intermediate position in the case of the multimode is a particularly preferable value of 20%.

FIG. 5 shows a single mode fiber having a core outer diameter of 12.5 μm and a cladding outer diameter of 125 μm, and FIG. 6 shows a depth of the central depression of the refractive index profile for a multimode fiber having a core outer diameter of 40 μm and a cladding outer diameter of 125 μm. It is a graph which shows the correlation of the electric field intensity | strength of fundamental mode (LP01).
FIG. 5A is a graph showing the change in electric field strength depending on the depth of the central depression of the refractive index profile of the single mode optical fiber, and FIG. 6A is the center of the refractive index profile of the multimode optical fiber. It is a graph which shows the change of the electric field strength by the depth of a hollow. The ratio of the depth of the central recess of the refractive index profile [(b / a) × 100%] is shown in FIGS. 5 (b) and 6 (b). Expressed as a percentage of depth b.
In the single mode, the product of the electric field distribution of all modes, the electric field distribution of the fundamental mode, and the electric field distribution at the center of the dent are separated, and the behavior of the change in the dent depth and the electric field distribution is calculated. The electric field distribution at the center of the depression showed the same behavior as in the basic mode until the intensity was changed, and the intensity change was only 7%.
In the case of the multimode, the electric field distribution at the center of the depression shows the same behavior as the basic mode up to a ratio of the depression depth of about 20%, and the change is about 28%.
From this result, the ratio of the depth of the center depression without changing the beam quality is most preferably in the single mode up to 60% and in the multi mode up to 20%.

FIG. 7 shows the refractive index profile, the electric field distribution, the X-axis cross section of the electric field distribution, and the Y-axis cross section of the electric field distribution when the depth of the refractive index central depression is 38% with respect to the core outer diameter of 40 μm. FIG.
FIG. 7A shows a refractive index profile in which the depth of the refractive index central depression is 38% with respect to the core outer diameter of 40 μm.
FIG. 7B is a graph showing three-dimensionally the product of the electric field strengths of all mode products.
FIG. 7C is a graph showing the X-axis cross section of the electric field distribution, and FIG. 7D is a graph showing the Y-axis cross section of the electric field distribution. The X-axis cross section is a cross section when the optical fiber is cut along a plane including the central axis and the X axis, with one axis orthogonal to the central axis of the optical fiber as the X axis. The Y-axis cross section is a cross section when the optical fiber is cut along a plane including the central axis and the Y axis, with the axis orthogonal to the central axis and the X axis of the optical fiber as the Y axis.
These graphs of FIGS. 7C and 7D show that there is no depression at the center of the electric field strength even when the depth of the central depression is 38%.

The width of the depression at the center of the concentration profile of the rare earth element (that is, twice the distance from the intermediate position where the concentration is maximum to the center of the core) is the case of the single mode optical fiber and the case of the multimode optical fiber. Different from each other.
In the case of a single mode optical fiber, the width of the central depression is preferably 40% or less of the LP01 mode diameter formed in the optical fiber when the central depression of the concentration profile of the rare earth element is 0%, preferably 20%. The following is more preferable, and 15% or less is particularly preferable.
In the case of a multimode optical fiber, the width of the central depression is preferably 60% or less of the LP01 mode diameter formed in the optical fiber when the central depression of the concentration profile of the rare earth element is 0%, and 40% The following is more preferable, and 30% or less is particularly preferable.
In any mode, the lower limit of the width of the central depression of the concentration profile of the rare earth element is not particularly limited, but is preferably in the range of 5 to 50% of the LP01 mode diameter.
Also, the width of the central depression of the Al concentration profile is different between the case of the single mode optical fiber and the case of the multimode optical fiber, as in the case of the rare earth element.
In the case of a single mode optical fiber, the width of the central depression is preferably 40% or less of the LP01 mode diameter formed in the optical fiber when the central depression of the Al concentration profile is 0%, and preferably 30% or less. Is more preferable, and 20% or less is particularly preferable.
In the case of a multimode optical fiber, the width of the central depression is preferably 60% or less of the LP01 mode diameter formed in the optical fiber when the central depression of the Al concentration profile is 0%, and preferably 40% or less. Is more preferable, and 35% or less is particularly preferable.
In any mode, the lower limit of the width of the central depression of the Al concentration profile is not particularly limited, but is preferably in the range of 5 to 40% of the LP01 mode diameter.

In the production method of the present invention, after forming the glass fine particle layer containing the additive, as shown in FIG. 1 (e), the glass fine particle layer 5 containing the rare earth element and further Al is heated to be vitrified. The glass layer 6 to which the element is added is obtained (vitrification process). This figure shows a state in the middle of processing, and a part of the glass fine particle layer 5 is a glass layer 6.
The heating temperature for vitrification is preferably 1900 ° C. to 2300 ° C., particularly 1950 ° C. to 2200 ° C., from the viewpoint of controlling the refractive index profile and suppressing core flatness.

After the vitrification step, as shown in FIG. 1 (f), the anhydrous quartz tube is heated at 1900 to 2300 ° C. to be collapsed to obtain a core base material (collapse step). “Collapse” means that the glass layer is shrunk and integrated at the center of the reaction tube to form a core having no space inside (making it solid).
In the collapse process, Cl ₂ gas is allowed to flow through the quartz tube while collapsing, whereby dehydration, profile control, and additive concentration control are possible. The flow rate of the Cl ₂ gas is 5 SCCM to 100 SCCM, preferably 5 [SCCM] to 10 [SCCM].
By the collapsing process, a core base material having a central rare earth element-added core and a surrounding quartz cladding layer is obtained.
In order to match the core diameter and the cladding diameter around the obtained core preform, an additional glass layer coating and an outer layer (for example, a second cladding layer or a support layer) are added as necessary to form an optical fiber preform. Manufacture and drawing to obtain a rare earth element-doped fiber.

The rare earth element-doped optical fiber according to the present invention is a result obtained by the manufacturing method of the present invention described above, and the structural features thereof are at least a core doped with a rare earth element and a surrounding quartz cladding. The layer is characterized by the concentration profile of the rare earth element in the core, and the concentration of the rare earth element is at least as shown in FIGS. 2 (a) and 11 (a). It is the maximum value at a position between the center of the core and descends from the center toward the core center, and is the lowest point of descent at the core center.
The preferable ratio of the concentration at the core center to the maximum value of the concentration and the range of the depth and width of the depression at the center of the core of the concentration profile are as described in the description of the manufacturing method.

In a preferred embodiment of the rare earth element-doped optical fiber according to the present invention, a rare earth element and aluminum are co-doped in the core. In this aspect, as shown in FIG. 2B, the concentration of aluminum in the core has a maximum value at least at an intermediate position between the core outer peripheral surface of the core and the center of the core. It descends as it goes to, and is the lowest point of descent at the core center. That is, there is a dent in the center of the core of the concentration profile.
In another preferred embodiment in which a rare earth element and aluminum are co-added to the core, the concentration of aluminum in the core is at least between the core outer peripheral surface of the core and the center of the core, as shown in FIG. It has a maximum value at an intermediate position, and the maximum value is constant from there to the center of the core. That is, there is no depression at the center of the core of the concentration profile.

  The rare earth element-doped optical fiber according to the present invention can be used for constructing laser devices and optical amplifiers used in various fields such as communication, welding, cutting, marking, illumination, medical, and the like. A preferable structure is a double clad optical fiber, a polymer double clad optical fiber, a polarization maintaining optical fiber, or the like.

Example 1
In this example, a rare earth element-doped optical fiber was formed by depositing a glass fine particle layer by a sintered MCVD method.
(Etching of the inner surface of anhydrous quartz tube)
An anhydrous quartz tube having an outer diameter of 28 mm, a thickness of 1.5 mm, and a length of 500 mm is attached to a lathe. The inner surface of the anhydrous quartz tube is directed from one end to the other, He is 0.5 [SLM], and O ₂ is 1 The inner surface of the anhydrous quartz tube was etched with an oxyhydrogen burner at 1800 to 2000 ° C. while supplying SF ₆ at 0.2 [SLM] at 0.8 [SLM].

(Formation of glass fine particle layer)
SiCl ₄ is supplied at 0.2 [SLM], He is supplied at 0.4 [SLM], and O ₂ is supplied at 2.0 [SLM]. Of glass particles were deposited. When depositing glass particles, the pressure in the reaction tube was controlled so that the internal pressure of the quartz tube was 5 Pa lower than the atmospheric pressure. At this stage, there is no change in the concentration profile because there is no additive.

(Penetration of YbCl ₃ and AlCl ₃ into glass fine particle layer)
0.1 g of YbCl ₃ .6H ₂ O and 0.33 g of AlCl ₃ .6H ₂ O are dissolved in 50 ml of ethanol as a solvent (Al: Yb molar ratio = 5.3: 1). The glass fine particle layer was poured into the quartz tube formed on the inner wall of the reaction tube, and the reaction tube was held horizontally on a lathe chuck and rotated at a speed of 5 rotations / minute at room temperature for 1 hour.

(Drying process)
In order to eliminate ethanol in the reaction tube while maintaining the internal pressure of the reaction tube at 1 Pa lower than atmospheric pressure and supplying He at 0.5 [SLM] and O ₂ at 3.0 [SLM] Natural drying was performed for a time.
Then, in order to decompose YbCl ₃ · 6H ₂ O and AlCl ₃ · 6H ₂ O and diffuse them into the inside of the fine particles, the temperature of the oxyhydrogen burner is raised stepwise to 50-1800 ° C., and the quartz tube is heated and dried. It was. At this stage, since the Cl ₂ flow rate is 0.12 [SLM] along with He and O ₂ , the decomposition of chlorides and additives is promoted as the temperature rises, and the additive ions are uniformly diffused into the soot particles. Therefore, the density profile and the refractive index profile do not have an abnormal shape such as a corner in the vicinity of the outermost peripheral surface of the fuselage, but rise steeply from the outermost peripheral surface of the fuselage and have a pseudo step shape that is a parabolic curve up to the center.
Chlorides such as YbCl ₃ .6H ₂ O and AlCl ₃ .6H ₂ O are decomposed into Yb (OH) ₃ and HCl and Al (OH) ₃ and HCl, respectively, at about 900 ° C. or higher. Moreover, after Yb and Al become simple substances and oxides, they tend to harden and do not readily diffuse uniformly. However, by flowing Cl ₂ , the additive is finely decomposed as the temperature rises and diffuses in the fine particles. Although the diffusion state varies greatly depending on the pipe temperature and chlorine flow rate, the higher the temperature, the higher the chlorine flow rate, the smaller the additive clusters and the more uniform diffusion is promoted.
When checking refractive index profile, but the core refractive index profile which does not shed Cl ₂ showed a profile there is no depression of the interface to the peak (square) and the center of the pipe, the core was flushed with Cl ₂ torso outermost peripheral surface As a result, the core center showed a pseudo step shape with a round core, and a uniform diffusion was confirmed. In addition, by flowing Cl ₂ , there is an effect of removing water mixed in soot deposition and liquid permeation, so that the OH concentration in the core can be controlled.

(Collapse)
In order to make the internal pressure of the reaction tube higher than the atmospheric pressure, the differential pressure between the internal pressure and the atmospheric pressure is maintained to be a differential pressure of +30 Pa, He is 0.6 [SLM], and O ₂ is 3. At 0 [SLM], each gas is supplied while Cl ₂ is gradually changed from 100 [SCCM] to 8 [SCCM], and the temperature of the anhydrous quartz tube is raised to 1800 to 2300 ° C. to form a glass fine particle layer. The glass base layer was collapsed to shrink the pipe tube to obtain a core base material.
In the final stage of the collapse, the Cl ₂ flow rate was set to 8 [SCCM], and the control was performed so that the additive concentration and the depth of the refraction profile were 40% of the total height. In order to confirm the Cl ₂ effect at the collapse stage, the refractive index profile of the core that was collapsed with Cl ₂ stopped has no abnormal shape such as corners near the outermost surface of the fuselage, and rises sharply from the outermost surface of the fuselage. A pseudo step shape that draws a parabolic curve up to the core center where there is no depression at the core center was shown.

  As a result of evaluation by the preform analyzer, the core diameter in the core base material was about 2.1 mm.

As shown in FIG. 2, it can be seen that the refractive index of the central region of the core base material is locally lower than the outside of the core base material. This means that the additive concentrations of Al and Yb at the center of the core base material are lower than in other regions.
As a result, the electric field distribution of the fundamental mode of laser oscillation becomes low at the center of the core and causes a low inversion distribution, so that photodarkening hardly occurs at the core center.
As a result of calculation in a multimode optical fiber having a core outer diameter of 40 μm, it was found that even if the depth of the central depression of the refractive index profile is 50%, the beam shape is not affected.

In order to confirm that the core preform obtained in this example suppresses the occurrence of photodarkening, an air-hole type double clad optical fiber was produced using the core preform, and photodark Ning evaluation was performed.
The main configuration of the fiber is a core outer diameter of 40 μm, a first cladding outer diameter of 600 μm, a second cladding (air hole) outer diameter of 30 μm, and an optical fiber outer diameter of 1000 μm.
Detailed evaluation methods are described in the reference (SPIE Photonics West 2007, Vol. 6453-50).
In order to cause a uniform inversion distribution over the entire optical fiber, the length of the optical fiber was set to 125 mm, and the polymer jacket covering the air hole type double clad optical fiber was removed from the rear end by about 20 mm.
FIG. 8 shows an optical system constructed for the evaluation. As shown in the figure, a laser beam having a wavelength of 675 nm and an excitation light (laser beam) having a wavelength of 915 nm are emitted from a light source 11 and passed through a collimator 12 to the core of an air hole type double clad optical fiber 13 (see FIG. The laser light having a wavelength of 675 nm output from the other end side is received by the light receiving element 14 and the power thereof is measured, and the temporal change in the transmittance of the laser light is observed. Thus, photodarkening was evaluated. In FIG. 8, the thick arrow indicated by the symbol L means the mixed light of the two wavelengths of laser light.
In addition, the end face of the input side and the output side of the air hole type double clad optical fiber 13 was cut with the clothes being peeled off so as not to cause abnormal reflection. In FIG. 8, reference numeral 13a is a jig for fixing the fiber connector, and reference numeral 13a is a copper cooling stand for preventing the temperature of the fiber from rising.

The main values of the characteristics of the optical fiber structure of Example 1 are as follows.
Core outer diameter: 40 [μm]
Pump guide outer diameter: 600 [μm]
Yb concentration: about 5300 [ppm-wt]
Al concentration: about 6100 [ppm-wt]
Absorption coefficient: 0.49 [dB / m]
The Yb doping concentration is about 5300 [ppm-wt].

FIG. 9 is a graph showing the results of photodarkening evaluation. The photodarkening evaluation shown in the figure shows the temporal change in the transmittance of 675 nm wavelength light in each case when the pump power (output wattage) of the pump light source laser diode (PLD) is changed. Yes. The output wattage of the PLD is 30 W, 50 W, 90 W, and 150 W, and the change in transmittance with respect to each output is indicated by a solid line, a dotted line, a broken line, and a one-dot chain line.
As is apparent from the graph of FIG. 9, photodarkening largely depends on the excitation power (or inversion distribution), and the higher the inversion distribution, the faster the photodarkening occurs.
The fitting parameter β (stretching parameter) estimated from the following formula was 0.67.
In the above formula, T (t) is a function representing the transmittance of an optical fiber, where t is a measurement time, A is a constant, (1-A) is a steady-state transmittance, and τ is a photodarkening time constant (photo (Darkening rate), β is a fitting parameter, which means a change in the transmittance of the fiber due to photodarkening.

FIG. 10 is a log plot of the inversion distribution, where the horizontal axis is the log value of the inversion distribution and the vertical axis is the log value of the photodarkening time constant.
The exponential function slope n (linier slope by power law) obtained from the following equation is 2.2, and photodarkening occurs in proportion to the power of 2.2 of the inversion distribution (or excited Yb concentration). It will be.
In the above formulas, I is an inversion distribution, n is the slope of the exponential function, and τ is a photodarkening time constant (photodarkening rate).

In general, the inversion distribution of the CW fiber laser is about 5% and the inversion distribution of the pulse laser is about 30%, so that the pulse laser deteriorates about 52 times faster than the CW laser.
The exponential function n is influenced by complicated factors such as Yb concentration and glass defects, and is difficult to determine, but it is known that it is mainly influenced by Yb concentration and Al concentration.
The exponential function n = 2.2 obtained from Example 1 is reported in the reference (OFC 2008, OThC5), n = 7 reported in the reference (SPIE Photonics West 2007, Vol. 6453-50). N is smaller than 2.6.
Therefore, the optical fiber manufactured using the core preform obtained in Example 1 shows that photodarkening can be suppressed as compared with the conventional case.

Example 2
In this example, a rare earth element-doped optical fiber was formed by depositing a glass fine particle layer by vapor phase MCVD.
The vapor phase MCVD method differs from the sintered MCVD method in that it is doped with additives such as rare earth and Al together with the soot volume, and vitrified with the deposition, so that the rare-earth element is formed without the depression of the center of the refractive index profile. In this method, only the concentration can be changed.
The vapor phase MCVD method, unlike the sintered MCVD method, can simultaneously supply rare earth elements, AlCl _{3 and the} like together with SiCl ₄ which is a glass material. is there.

(Etching of the inner surface of anhydrous quartz tube)
An anhydrous quartz tube having an outer diameter of 28 mm, a thickness of 1.5 mm, and a length of 500 mm is attached horizontally to the chuck of the lathe, and the He is 0.5 [SLM] on the inner surface of the anhydrous quartz tube from one end to the other end. O ₂ was supplied at 1.8 [SLM] and SF ₆ was supplied at 0.2 [SLM], and the inner wall surface of the anhydrous quartz tube was etched at 1800 to 2000 ° C. with an oxyhydrogen burner.

(Glass fine particle layer formation structure and vitrification process)
SiCl ₄ at 0.2 [SLM], He at 0.4 [SLM], O ₂ at 2.0 [SLM], He gas as gas in Yb (DPM) ₃ tank and AlCl ₃ tank Was supplied at 0.2 [SLM], and glass particles with a thickness of 0.06 mm were deposited and vitrified four times in succession at a reaction tube temperature of 1950 ° C. Here, it is also possible to separate the fine particle volume and vitrification.
DPM is dipivaloylmethanato (also called 2,2,6,6-tetramethyl-3,5-heptanedionate).

In Example 2, He was used as a carrier gas for transporting Yb (DPM) ₃ and AlCl ₃ to the reaction tube.
In order to form a refractive index profile that varies continuously, the flow rate of AlCl ₃ was not changed throughout, and only Yb (DPM) ₃ was reduced to 0.1 [SLM] in the fourth deposition.

The amount of Yb (DPM) ₃ and AlCl ₃ reaching the reaction tube depends not only on the He gas flow rate, but also on the temperature of the constant temperature bath for vaporizing Yb (DPM) ₃ and AlCl ₃ and the temperature of the raw material piping. Different. The temperatures of the Yb (DPM) ₃ tank and the AlCl ₃ tank were set to 230 ° C. and 160 ° C., respectively. The AlCl ₃ pipe temperature was set to 160 ° C., the Yb (DPM) ₃ pipe temperature was set to 300 ° C., and a Hastelloy nozzle having a two-layer structure was used to supply the raw material to the quartz pipe.
When depositing the glass fine particles, the internal pressure of the glass was controlled so that the differential pressure between the internal pressure of the reaction tube and the atmospheric pressure was −5 Pa.

(Collapse)
Thereafter, the internal pressure of the reaction tube is maintained to be lower by +30 Pa than the external pressure (atmospheric pressure) of the reaction tube, He is 0.6 [SLM], O ₂ is 3.0 [SLM], While supplying each gas at 5 [SCCM] with Cl ₂ , the temperature of the reaction tube is raised to 1800 to 2300 ° C., the glass fine particle layer is made into a glass layer, the reaction tube is collapsed, and the core base material is Obtained. Here, since Cl ₂ is similar to the effect of sintered MCVD, the profile is affected. That is, the additive by flowing Cl ₂ is a Cl ₂ to be etched stepwise lowered in the final collapse stage reduced the flow rate of Cl ₂ to 0 [SLM].

As a result of the evaluation by the preform analyzer, the core diameter of the core base material was about 2.2 mm.
As shown in FIG. 11, the EPMA (Electron Probe Microscopy Analyzer) evaluation results show that only the Yb concentration is depressed and the Al concentration is not depressed in the core central region.

In order to confirm that the core base material obtained in Example 2 suppresses the occurrence of photodarkening, the air base type double clad is used using the core base material in the same manner as in Example 1. A fiber was prepared and evaluated for photodarkening.
The main configuration and evaluation method of the fiber are the same as those in Example 1.
An air hole type double clad optical fiber having a core outer diameter of 40 μm, a first cladding outer diameter of 600 μm, a second cladding (air hole) outer diameter of 30 μm, and an optical fiber outer diameter of 1000 μm was fabricated, and photodarkening evaluation was performed. In order to obtain a uniform inversion distribution over the entire optical fiber, the length of the optical fiber was set to 125 mm, and the polymer jacket covering the air hole type double clad optical fiber was removed from the rear end by about 20 mm. The detailed conditions are the same as in Example 1.
The main values of the characteristics of the optical fiber structure of Example 2 are as follows.
Core outer diameter: 40 [μm]
Pump guide outer diameter: 600 [μm]
Yb concentration: about 5000 [ppm-wt]
Al concentration: about 5500 [ppm-wt]
Absorption coefficient: 0.51 [dB / m]
The Yb doping concentration is about 5000 [ppm-wt].
Excellent photodarkening characteristics were exhibited with the exponential function n = 2.5 obtained from Example 2.

  According to the present invention, it is possible to provide a method capable of preferably producing a rare earth element-doped optical fiber with suppressed photodarkening, and thereby provide a rare earth element doped optical fiber with suppressed photodarkening. It became possible to do.

DESCRIPTION OF SYMBOLS 1 Anhydrous quartz tube 2 Burner 3 Glass fine particle layer 4 Solution containing rare earth element 5 Glass fine particle layer infiltrated with rare earth element 6 Glass layer containing rare earth element 7 Core containing rare earth element Core core material

Claims (8)

A method of manufacturing a rare earth element-doped optical fiber,
Glass fine particles are deposited on the inner surface of a quartz tube while adding rare earth elements or by adding rare earth elements after glass fine particles are deposited to form a glass fine particle layer to which rare earth elements are added. A layer forming step;
A vitrification step of heating the glass fine particle layer to form a glass layer;
A collapsing step in which the quartz tube having the glass layer formed on the inner surface by the step is collapsed into a core base material;
One or both of the following (B) and (C) are processed so that the concentration of the rare earth element in the core in the core base material after the collapse process satisfies the following change in the concentration (A): And
A method for producing the rare earth element-doped optical fiber.
(A) The concentration of the rare earth element in the core is a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then decreases toward the core center. It is the lowest point, and the ratio of the core center concentration to the maximum concentration value is 5% to 95%.
(B) Controlling the amount of rare earth element added in the glass fine particle layer forming step.
(C) To reduce the amount of rare earth element added by introducing Cl ₂ gas into the quartz tube after the glass fine particle layer forming step and bringing the Cl ₂ gas into contact with the inner surface of the tube. The method of forming the glass fine particle layer in the glass fine particle layer forming step is a sintered MCVD method,
In the glass fine particle layer forming step, after the glass fine particle layer is deposited a plurality of times on the inner surface of the quartz tube, a solution containing at least a rare earth element is brought into contact with the inner surface of the glass fine particle layer so that the solution penetrates into the layer. Adding a rare earth element in the layer, drying by heating,
The manufacturing method according to claim 1, wherein in the collapsing step, a change in the concentration of (A) is satisfied by introducing Cl ₂ gas into the quartz tube. The method of forming the glass fine particle layer is a gas phase MCVD method,
In the glass fine particle layer forming step and the vitrification step, glass fine particles and at least a rare earth element are simultaneously deposited on the inner surface of the quartz tube, and the amount of rare earth element sent into the quartz tube is changed as the layer thickness increases. The manufacturing method according to claim 1, wherein the change in the concentration of (A) is satisfied. The method of forming the glass fine particle layer is a gas phase MCVD method,
In the glass fine particle layer forming step, while adding a rare earth element to the glass fine particle layer so as to satisfy the change in the concentration of (A), at the same time, so as to satisfy the change in the concentration of (D) or (E) below. The production method according to claim 1 or 3 , wherein aluminum is co-added into the glass fine particle layer.
(D) The concentration of aluminum in the core of the core base material after the collapse has a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then decreases toward the core center. It is the lowest point of descent at the core center, and the ratio of the core center concentration to the maximum concentration value is 5% to 90%.
(E) The concentration of aluminum in the core in the core base material after the collapse has a maximum value at least between the core outer peripheral surface of the core and the center of the core, and the maximum value from there to the center of the core It remains constant. The method of forming the glass fine particle layer is a sintered MCVD method,
In the glass fine particle layer forming step, the rare earth element is added to the glass fine particle layer so as to satisfy the change in the concentration of the above (A), and at the same time, in the glass fine particle layer so as to satisfy the change in the concentration of the following (D). The manufacturing method according to claim 1 or 2, wherein aluminum is added together.
(D) The concentration of aluminum in the core of the core base material after the collapse has a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then decreases toward the core center. It is the lowest point of descent at the core center, and the ratio of the core center concentration to the maximum concentration value is 5% to 90%. A rare earth element-doped optical fiber obtained by the production method according to any one of claims 1 to 5 ,
Having a core to which at least a rare earth element is added, and a surrounding quartz cladding layer,
The concentration of the rare earth element in the core is the maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then descends toward the core center. The ratio of the concentration of the core center to the maximum value of the concentration is 5% to 95%. A rare earth element and aluminum are co-added to the core,
The concentration of aluminum in the core is a maximum value at least at a position between the core outer peripheral surface of the core and the center of the core, and then descends toward the core center. The rare earth element-doped optical fiber according to claim 6 , wherein the concentration ratio of the core center to the maximum value of the concentration is 5% to 90%. A rare earth element and aluminum are co-added to the core,
The concentration of aluminum in the core, is constant at least has a maximum value at a location between the center of the body outer circumferential surface and the core of the core, while the maximum value from there to the center of the core, according to claim 6 The rare earth element-doped optical fiber described.