JP2017026672A - Optical fiber for optical fiber grating and fiber laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber for an optical fiber grating capable of achieving favorable beam quality.SOLUTION: There is provided an optical fiber 10 for an optical fiber grating that has a core 4 through which at least two LP modes different in order can be propagated. The core 4 is doped with Ge, and a concentration of Ge in a central part 8 of the core 4 is higher than a concentration of Ge in an outer peripheral edge 9 of the core 4.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical fiber for an optical fiber grating and a fiber laser device.

FIG. 9 is a schematic configuration diagram illustrating an example of a fiber laser device.
The fiber laser device 101 includes an excitation light source 102, an optical combiner 103, and an optical resonator 104.
The optical resonator 104 includes an amplification optical fiber 107 having an amplification coil 121, a first reflection unit 122, and a second reflection unit 123. An input side optical fiber 105 is connected to one end of the amplification optical fiber 107, and an output side optical fiber 106 is connected to the other end of the amplification optical fiber 107. For the amplification optical fiber 107, for example, a Yb-doped core fiber is used.

The excitation light source 102 includes a plurality of laser diodes 11 and outputs excitation light. The laser diode 11 is connected to the optical combiner 103 via the optical fiber 12.
The optical combiner 103 combines a plurality of excitation lights from a plurality of laser diodes 11.

The first reflecting portion 122 is provided in a part of the input side optical fiber 105. The first reflection unit 122 reflects the laser light propagating through the amplification optical fiber 107.
The second reflecting portion 123 is provided in a part of the output side optical fiber 106. The second reflecting portion 123 reflects a part of the laser light propagating through the amplification optical fiber 107. The laser beam resonates and is amplified between the first reflection unit 122 and the second reflection unit 123.

The 1st reflection part 122 and the 2nd reflection part 123 are comprised by the fiber Bragg grating (FBG: Fiber Bragg Grating). The FBG is a reflector in which a portion (grating) whose refractive index changes periodically in the longitudinal direction (light propagation direction) of the core is formed. Thereby, the FBG reflects only light of a specific wavelength corresponding to the grating period.
The first reflecting portion 122 has a high reflectance (for example, approximately 100%), and the second reflecting portion 123 has a low reflectance (for example, approximately 10%).

FIG. 10A is a cross-sectional view showing an optical fiber 110 for optical fiber grating (hereinafter simply referred to as an optical fiber 110) that can be used for the input side optical fiber 105, for example. FIG. 10B is a diagram showing the refractive index distribution of the optical fiber 110.
As shown in FIG. 10A, the optical fiber 110 is formed on the optical fiber bare wire 111, the polymer cladding layer 2 formed on the outer peripheral surface of the optical fiber bare wire 111, and the outer peripheral surface of the polymer cladding layer 2. And a protective coating layer 3. The bare optical fiber 111 has a core 114 and a clad 5 surrounding it.
Ge is added to the core 114. In FIG. 10B, hatched hatching is added to the region to which Ge is added. As shown in this figure, Ge is added to the entire core 114, and the concentration of Ge is uniform in the radial direction of the core 114.

The characteristics required for the fiber laser device include the following (1) and (2).
(1) Even if high-power laser light is output, wavelength conversion due to nonlinear optical effects (particularly stimulated Raman scattering) does not occur.
(2) The laser beam to be output is easily condensed by the lens. In other words, M ² (indicating beam quality) is a small value.

Regarding (1), the magnitude of the nonlinear optical effect is proportional to the optical path length and inversely proportional to the effective area (A _eff ).
Therefore, in order to suppress the nonlinear optical effect, it is effective to shorten the optical path length of the amplification optical fiber (Yb-doped core fiber).
However, in order to shorten the optical path length, it is necessary to increase the Yb concentration of the amplification optical fiber. When the Yb concentration is increased, the laser output power may be reduced due to a phenomenon called photodarkening. Therefore, there is a limit to suppressing the nonlinear optical effect by this method.

In order to suppress the nonlinear optical effect, it is also effective to increase the effective area (A _eff ). In order to increase the effective area, a method of decreasing the refractive index of the core or increasing the core diameter can be considered.
However, if the refractive index of the core is reduced, it becomes difficult to confine light and bending loss tends to occur, so that the optical fiber cannot be accommodated in the fiber laser device in a compact manner. Generally, it is considered that actual use becomes difficult when the refractive index of the core is less than 0.1%.
On the other hand, when the core diameter is increased, the cut-off wavelength becomes longer and the single mode condition cannot be satisfied. For example, when the refractive index of the core is 0.12%, in order to satisfy the single mode condition at a wavelength of 1060 nm, the core diameter can only be increased to about 12 μm (effective cross-sectional area is about 130 μm ² ).
With such an effective area, a nonlinear optical effect occurs even in a CW fiber laser device of several hundred watts class.

Regarding (2), as described above, since it is difficult to increase the effective area under the constraints of the single mode, the core diameter must be further increased to make a multimode optical fiber. For example, when the core refractive index is 0.12%, if the core diameter is 18 μm, the effective area can be increased to about 210 μm ^2, but the modes that can be propagated are LP ₀₁ and LP ₁₁ .
Further, when the core diameter is increased, for example, 28 μm, the effective area can be increased to about 410 μm ² , which is suitable for suppressing the nonlinear optical effect. At this time, there are six modes LP ₀₁ , LP ₁₁ , LP ₂₁ , LP ₀₂ , LP ₃₁ , and LP _{12 that} can propagate at a wavelength of 1060 nm. When stored under a bending condition of about 100 mm in diameter, LP ₂₁ , LP ₀₂ , LP ₃₁ and LP ₁₂ are substantially cut off, but LP ₀₁ and LP ₁₁ propagate stably.
Since LP ₁₁ which is the first higher-order mode propagates stably, if a resonator is configured with this core, LP ₁₁ is mixed into the laser output light, and the value of M ² becomes large.

In order to overcome this problem, optical fibers in which the Yb-doped region of the Yb-doped core fiber that is an amplification optical fiber is limited have been proposed (for example, Patent Documents 1 to 6).
In the optical fiber, the concentration distribution of the active element Yb is brought close to the electric field intensity distribution of the fundamental mode, so that the fundamental mode is preferentially amplified by suppressing the gain of the higher order mode.
However, if the Yb-added region is limited, the absorptance of the cladding pump light may be reduced, so that the Yb-added core fiber needs to be long. Also, the fusion point of different optical fibers in the resonator (for example, in FIG. 9, the fusion point 108 between the input side optical fiber 105 and the amplification optical fiber 107, and between the amplification optical fiber 107 and the output side optical fiber 106). There has been a problem that the LP ₁₁ mode generated at the fusion point 109 or the like cannot be sufficiently suppressed, and an improvement in beam quality has been desired.

Japanese Patent No. 46667535 Japanese Patent No. 5159956 Japanese Patent No. 5124701 Japanese Patent No. 5468666 Japanese Patent No. 5468667 JP 2014-179404 A

  This invention is made | formed in view of the said situation, and makes it a subject to provide the optical fiber for optical fiber gratings which can make beam quality favorable.

One aspect of the present invention is an optical fiber for an optical fiber grating having a core capable of propagating at least two LP modes having different orders, wherein Ge is added to the core, and a Ge concentration in a central portion of the core is An optical fiber for an optical fiber grating having a higher Ge concentration in the outer peripheral edge of the core is provided.
The core preferably includes an inner region including the central portion and an outer region on the outer peripheral side of the inner region, and the Ge concentration in the inner region may be higher than that in the outer region.
The outer region may not be added with Ge.
The inner region may be co-doped with Ge and B.
The Ge concentration distribution in the radial direction of the core may be a distribution along the power distribution of the LP ₀₁ mode which is a fundamental mode among the at least two LP modes.
B may be added to the core.
The refractive index of the core is preferably constant in the radial direction of the core.

  One aspect of the present invention is an optical fiber grating provided with an excitation light source that outputs excitation light, an amplification optical fiber to which the excitation light is input, and one end side of the amplification optical fiber. An input side optical fiber having a reflection part; and an output side optical fiber provided on the other end side of the amplification optical fiber and having a second reflection part which is an optical fiber grating; and At least one of the output side optical fibers provides a fiber laser device including the optical fiber for optical fiber grating.

According to one aspect of the present invention, since the Ge concentration at the core center is higher than the Ge concentration at the outer periphery of the core, the LP ₀₁ mode having a high power at the center of the core is reflected, but the higher order LP mode has a power distribution. Since it spreads outside, it becomes difficult to reflect. Therefore, it is possible to sufficiently suppress higher-order LP modes that occur at the fusion point of different optical fibers in the optical resonator. Therefore, a laser beam with good beam quality can be obtained.

It is sectional drawing which shows 1st Embodiment of the optical fiber for optical fiber gratings which concerns on this invention. It is a figure which shows the refractive index distribution of the optical fiber for optical fiber gratings shown in FIG. It is a schematic block diagram which shows the fiber laser apparatus using the optical fiber for optical fiber gratings shown in FIG. Is a diagram illustrating the power distribution of the LP ₀₁ mode and the _{LP 11} mode. (A) It is a figure which shows the refractive index distribution of 2nd Embodiment of the optical fiber for optical fiber gratings which concerns on this invention. (B) It is a figure explaining the contribution to the refractive index of an additive in the optical fiber for optical fiber gratings of (A). (A) It is a figure which shows the refractive index distribution of 3rd Embodiment of the optical fiber for optical fiber gratings which concerns on this invention. (B) It is a figure explaining the contribution to the refractive index of an additive in the optical fiber for optical fiber gratings of (A). (A) It is a figure which shows the refractive index distribution of 4th Embodiment of the optical fiber for optical fiber gratings which concerns on this invention. (B) It is a figure explaining the contribution to the refractive index of an additive in the optical fiber for optical fiber gratings of (A). (A) It is a figure which shows the refractive index distribution of 5th Embodiment of the optical fiber for optical fiber gratings which concerns on this invention. (B) It is a figure explaining the contribution to the refractive index of an additive in the optical fiber for optical fiber gratings of (A). It is a schematic block diagram which shows a fiber laser apparatus. (A) It is sectional drawing which shows the optical fiber for optical fiber gratings used for the fiber laser apparatus of a previous figure. (B) It is a figure which shows the refractive index distribution of the optical fiber for optical fiber gratings of (A).

[First Embodiment]
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a cross-sectional view showing an optical fiber 10 for an optical fiber grating (hereinafter simply referred to as an optical fiber 10), which is a first embodiment of an optical fiber for an optical fiber grating according to the present invention. FIG. 2 is a diagram showing the refractive index distribution of the optical fiber 10. FIG. 3 is a schematic configuration diagram showing a fiber laser device 100 using the optical fiber 10. FIG. 4 is a diagram showing power distributions in the LP ₀₁ mode and the LP ₁₁ mode.

[Fiber laser equipment]
As shown in FIG. 3, the fiber laser device 100 includes a pumping light source 102, an optical combiner 103, and an optical resonator 104.
The optical resonator 104 includes an amplification optical fiber 107 having an amplification coil 121, a first reflection unit 22, and a second reflection unit 23.
An input side optical fiber 25 is connected to one end of the amplification optical fiber 107, and an output side optical fiber 26 is connected to the other end of the amplification optical fiber 107.
For example, a Yb-doped core fiber is used as the amplification optical fiber 107.

The excitation light source 102 includes a plurality of laser diodes 11 and outputs excitation light. The laser diode 11 is connected to the optical combiner 103 via the optical fiber 12.
The optical combiner 103 combines a plurality of excitation lights from a plurality of laser diodes 11. Excitation light from the optical combiner 103 is input to the amplification optical fiber 107 through the input side optical fiber 25.

The first reflecting portion 22 is provided in a part of the input side optical fiber 25. The first reflecting unit 22 reflects the laser light propagating through the amplification optical fiber 107.
The second reflecting portion 23 is provided in a part of the output side optical fiber 26. The second reflecting unit 23 reflects a part of the laser light propagating through the amplification optical fiber 107. The laser beam resonates and is amplified between the first reflecting portion 22 and the second reflecting portion 23.

The 1st reflection part 22 and the 2nd reflection part 23 are comprised by the fiber Bragg grating (FBG: Fiber Bragg Grating). The FBG is a reflector in which a portion (grating) whose refractive index changes periodically in the longitudinal direction (light propagation direction) of the core is formed. Thereby, the FBG reflects only light of a specific wavelength corresponding to the grating period.
The reflectance of the first reflecting portion 22 is higher than the reflectance of the second reflecting portion 23. The reflectance of the first reflecting portion 22 can be set to 90% or more, for example, and preferably 99% or more. The reflectance of the 2nd reflection part 23 can be 5-50%, for example, and 5-10% is preferable.

[Optical fiber]
FIG. 1 is a cross-sectional view showing an optical fiber 10 of the present embodiment.
The optical fiber 10 can be used as at least one of the input side optical fiber 25 and the output side optical fiber 26 in the fiber laser device 100 shown in FIG. The optical fiber 10 shown here is a polymer clad optical fiber and is preferably used as at least the input side optical fiber 25.
As the output-side optical fiber 26, an optical fiber having the same configuration as that of the optical fiber 10 can be used except that the polymer cladding layer 2 is not provided.

As shown in FIG. 1, an optical fiber 10 includes an optical fiber bare wire 1, a polymer clad layer 2 formed on the outer peripheral surface of the optical fiber bare wire 1, and a protective coating formed on the outer peripheral surface of the polymer clad layer 2. And layer 3. The bare optical fiber 1 has a core 4 and a clad 5 surrounding it.
The bare optical fiber 1 is made of quartz glass or the like.
The polymer cladding layer 2 has a refractive index lower than that of the bare optical fiber 1.

The core 4 has an inner region 6 including a central portion 8 and an outer region 7 on the outer peripheral side of the inner region 6.
The inner region 6 is a circular region that is concentric with the core 4. The outer diameter of the inner region 6 can be set to, for example, 30 to 80% with respect to the outer diameter of the core 4.
The center portion 8 is a region including the center 4 a of the core 4. The central portion 8 is a concentric area with the core 4 and is a circular cross-sectional area having an outer diameter of, for example, 5 to 20% with respect to the outer diameter of the core 4.

  As shown in FIG. 2, the refractive index of the core 4 is constant in the radial direction of the core 4. Therefore, the refractive index of the inner region 6 and the refractive index of the outer region 7 are equal to each other. Hereinafter, “radial direction” means the radial direction of the core.

Germanium (Ge) is added to the inner region 6. The concentration of Ge in the inner region 6 can be, for example, 0.5 to 2.0 mol% in terms of germanium dioxide (GeO ₂ ).
In FIG. 2, hatched hatching is added to the region where Ge is added in the core 4. As shown in this figure, Ge is added only to the inner region 6. Ge is added to the entire inner region 6, and the concentration is preferably uniform in the radial direction of the inner region 6.

Ge is not added to the outer region 7. The outer region 7 has a refractive index equal to the refractive index of the inner region 6 by adding a refractive index increasing dopant other than Ge.
Examples of the refractive index increasing dopant other than Ge include aluminum (Al) and phosphorus (P). A dopant other than Al and P may be used as long as it is a dopant that increases the refractive index and has a smaller refractive index change than Ge when an FBG is formed by irradiation with ultraviolet rays. In FIG. 2, the outer region 7 to which the refractive index increasing dopant other than Ge is added is hatched.

  In the optical fiber 10, Ge is added to the inner region 6 and Ge is not added to the outer region 7, so that the Ge concentration in the central portion 8 (for example, the average concentration of Ge in the central portion 8) is the core 4. Higher than the Ge concentration at the outer peripheral edge 9.

  The optical fiber 10 can be manufactured by adding Ge so as to be concentrated and distributed only in the inner region 6. In forming the optical fiber grating, for example, ultraviolet rays may be irradiated by a known method.

Two or more LP modes can propagate to the core 4. For example, when the core diameter is 28 μm and the relative refractive index difference is 0.12%, modes that can propagate at a wavelength of 1060 nm include LP ₀₁ , LP ₁₁ , LP ₂₁ , LP ₀₂ , LP ₃₁ , LP _{12 and the} like. Among these, LP ₀₁ that is the fundamental mode and LP ₁₁ that is the first higher-order mode stably propagate to the core 4.

FIG. 4 is a diagram showing the radial power distribution of the LP ₀₁ mode and the LP ₁₁ mode. FIG. 4 shows that the total power of each mode is the same, that is, the values obtained by integrating the power distribution are equal. As shown in this figure, the power of the LP ₀₁ mode is distributed so as to form a smooth mountain-shaped curve having a peak at the center. On the other hand, the power of the LP ₁₁ mode is distributed so that the power is low in the center and has a peak in the peripheral portion.
Thus, the power distribution is different between the basic mode (LP ₀₁ mode) and the higher order mode (for example, LP ₁₁ mode), and the power distribution spreads outward in the higher order mode.

As shown in FIG. 10, when the optical fiber 110 having the core 114 having a uniform Ge concentration in the radial direction is used, not only the LP ₀₁ mode but also the LP ₀₁ mode is used in the reflectors 122 and 123 (see FIG. 9). _{Since the 11} mode also reflects with high efficiency, the LP ₁₁ mode also resonates in the optical resonator 104.
Therefore, the fusion points of the different optical fibers in the optical resonator 104 in FIG. 9 (the fusion point 108 between the input side optical fiber 105 and the amplification optical fiber 107, the amplification optical fiber 107 and the output side optical fiber 106). The LP ₁₁ mode generated at the fusion point 109 or the like cannot be sufficiently suppressed. For this reason, it has been difficult to obtain laser light with good beam quality.

On the other hand, as shown in FIG. 2, when the optical fiber 10 in which Ge is added only to the inner region 6 is used, the LP ₀₁ mode having a high power at the center is the reflecting portions 22 and 23 (see FIG. 3). but reflected power is low LP ₁₁ mode at the center is hardly reflected.
Therefore, the fusion points of different optical fibers in the optical resonator 104 (the fusion point 108 between the input-side optical fiber 25 and the amplification optical fiber 107 and the fusion point between the amplification optical fiber 107 and the output-side optical fiber 26). the _{LP 11} mode occurs at 109, etc.) can be sufficiently suppressed. Therefore, a laser beam with good beam quality can be obtained. For example, a laser beam having M ² of 1.3 or less, preferably 1.1 or less can be obtained.

[Second Embodiment]
FIG. 5A is a diagram showing the refractive index distribution of the optical fiber according to the second embodiment of the present invention. FIG. 5B is a diagram illustrating the degree of contribution of the additive to the refractive index in this optical fiber.
In addition, in the following description, about the common point with the optical fiber 10 of 1st Embodiment, the same code | symbol is attached | subjected and the description is abbreviate | omitted.

  The optical fiber of the second embodiment is different from the optical fiber 10 of the first embodiment in that Ge and boron (B) are co-added to the inner region 16 of the core 14. Other configurations may be the same as those of the optical fiber 10.

  As shown in FIG. 5B, B has a function of lowering the refractive index, but in the inner region 16, the refractive index is increased by the action of increasing the refractive index by Ge and the action of decreasing the refractive index by B. Is equal to the refractive index of the outer region 7. Therefore, the refractive index of the core 14 is constant in the radial direction.

As with the optical fiber 10 of the first embodiment, the optical fiber of the second embodiment suppresses the LP ₁₁ mode and obtains laser light with good beam quality.
Furthermore, in the optical fiber of the second embodiment, since B is added to the inner region 16, the photosensitivity can be improved. Therefore, the exposure time for forming the optical fiber grating can be shortened, and the efficiency of forming the optical fiber grating can be increased.

[Third Embodiment]
FIG. 6A is a diagram showing the refractive index distribution of the optical fiber according to the third embodiment of the present invention. FIG. 6B is a diagram illustrating the degree of contribution of the additive to the refractive index in this optical fiber.

In the optical fiber according to the third embodiment, the concentration of Ge in the radial direction in the core 24 is distributed so as to form a smooth mountain-shaped curve having a peak at the center. This Ge concentration distribution follows the power distribution of the LP ₀₁ mode shown in FIG.
The fact that the Ge concentration distribution follows the power distribution of the LP ₀₁ mode means that the radial position of the Ge concentration peak coincides (or substantially coincides) with the radial position of the LP ₀₁ mode peak.
Since the concentration distribution of Ge in the radial direction of the core 24 has a peak at the center like the power distribution of the LP ₀₁ mode shown in FIG. 4, it can be said that the Ge concentration distribution of the core 24 follows the power distribution of the LP ₀₁ mode. .
In the optical fiber of the third embodiment, the other configuration may be the same as that of the optical fiber 10.

  As shown in FIG. 6B, the core 24 is added by adding a refractive index increasing dopant (Al, P, etc.) other than Ge so as to form a valley-shaped curve that is lowest at the center. The refractive index of the core 24 is constant in the radial direction.

In the optical fiber according to the third embodiment, since the Ge concentration distribution follows the power distribution of the LP ₀₁ mode, the efficiency of suppressing the LP ₁₁ mode can be increased. Therefore, the beam quality of the laser beam can be further improved.

[Fourth Embodiment]
FIG. 7A is a diagram showing the refractive index distribution of the optical fiber according to the fourth embodiment of the present invention. FIG. 7B is a diagram illustrating the degree of contribution of the additive to the refractive index in this optical fiber.

In the optical fiber of the fourth embodiment, Ge and B are co-added to the inner region 36 of the core 34, and only Ge is added to the outer region 37. In this optical fiber, no refractive index increasing dopant other than Ge is used.
Since the refractive index of the inner region 36 is equal to the refractive index of the outer region 37, the refractive index of the core 34 is constant in the radial direction.
In the optical fiber of the fourth embodiment, the other configuration may be the same as that of the optical fiber 10.

Since B added to the inner region 36 has an effect of lowering the refractive index, more Ge is added to the inner region 36 than to the outer region 37. Therefore, similarly to the optical fiber 10 of the first embodiment, the LP ₁₁ mode is suppressed and a laser beam with good beam quality can be obtained.
Furthermore, since B is added to the inner region 36, the exposure time for forming the optical fiber grating can be shortened and the efficiency of forming the optical fiber grating can be increased, as in the second embodiment.

[Fifth Embodiment]
FIG. 8A is a diagram showing a refractive index distribution of an optical fiber according to the fifth embodiment of the present invention. FIG. 8B is a diagram illustrating the degree of contribution of the additive to the refractive index in this optical fiber.

In the optical fiber of the fifth embodiment, Ge, B, and a refractive index increasing dopant other than Ge are added to the core 44.
As shown in FIG. 8B, the radial Ge concentration distribution in the core 44 forms a smooth mountain-shaped curve having a peak at the center. This Ge concentration distribution follows the power distribution of the LP ₀₁ mode shown in FIG.
B and the refractive index increasing dopant other than Ge are added so as to form a distribution having a valley-shaped curve that is lowest at the center.
The refractive index of the core 44 is constant in the radial direction.
The other configuration of the optical fiber according to the fifth embodiment may be the same as that of the optical fiber 10.

In the optical fiber of the fifth embodiment, since the Ge concentration distribution follows the power distribution of the LP ₀₁ mode, the efficiency of suppressing the LP ₁₁ mode can be increased. Therefore, the beam quality of the laser beam can be further improved.
Further, since B is added, the exposure time for forming the optical fiber grating can be shortened and the efficiency of forming the optical fiber grating can be increased, as in the second embodiment.
Furthermore, since the refractive index increasing dopant other than Ge is also added, the addition amount of B can be adjusted.

The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above embodiment, the refractive index of the core is constant in the radial direction, but the configuration in which the refractive index of the core is not constant in the radial direction, for example, the configuration in which the refractive index of the inner region and the refractive index of the outer region are different from each other. It is included in the technical scope of the present invention.
In addition, the specific description regarding the shape, arrangement, material, and the like of each component of the fiber laser device is not limited to the above embodiment. The present invention is applicable to both CW fiber laser devices and pulse fiber laser devices.

  DESCRIPTION OF SYMBOLS 1 ... Bare optical fiber, 4, 14, 24, 34, 44 ... Core, 5 ... Cladding, 6, 16, 36 ... Inner area | region, 7, 37 ... Outer area | region, 8 ... Center part, 9 ... Outer rim, 10 ... Optical fiber for optical fiber grating, 22 ... First reflection part, 23 ... Second reflection part, 102 ... For excitation Light source, 107... Amplification optical fiber.

Claims (8)

An optical fiber for an optical fiber grating having a core capable of propagating at least two LP modes having different orders,
An optical fiber for an optical fiber grating, wherein Ge is added to the core, and a Ge concentration in a central portion of the core is higher than a Ge concentration in an outer peripheral edge of the core. The core has an inner region including the central portion, and an outer region on the outer peripheral side of the inner region,
The optical fiber for an optical fiber grating according to claim 1, wherein a Ge concentration in the inner region is higher than a Ge concentration in the outer region.   The optical fiber for an optical fiber grating according to claim 2, wherein the outer region is not doped with Ge.   4. The optical fiber for an optical fiber grating according to claim 2, wherein Ge and B are co-doped in the inner region.   2. The optical fiber for an optical fiber grating according to claim 1, wherein the Ge concentration distribution in the radial direction of the core is along a power distribution of a fundamental LP mode of the at least two LP modes.   The optical fiber for an optical fiber grating according to claim 5, wherein B is added to the core.   The optical fiber for an optical fiber grating according to any one of claims 1 to 6, wherein a refractive index of the core is constant in a radial direction of the core. An excitation light source that outputs excitation light;
An amplification optical fiber to which the excitation light is input;
An input side optical fiber provided on one end side of the amplification optical fiber and having a first reflecting portion which is an optical fiber grating;
An output side optical fiber provided on the other end side of the amplification optical fiber and having a second reflecting portion which is an optical fiber grating;
8. The fiber laser device according to claim 1, wherein at least one of the input side optical fiber and the output side optical fiber includes the optical fiber for an optical fiber grating according to claim 1.

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