Fiber laser device

Abstract

To provide a fiber laser device that can suppress the occurrence of stimulated Raman scattering and photodarkening while suppressing a decrease in output power.SOLUTION: A fiber laser device includes a first amplifying optical fiber 61, a first FBG 51 and a second FBG 52 sandwiching the first amplifying optical fiber 61, a second amplifying optical fiber 62 arranged on the opposite side of the second FBG 52 from the first amplifying optical fiber 61 side, a first excitation light source 11 that introduces excitation light into the first amplifying optical fiber from the first FBG 51 side, and a second excitation light source 12 that introduces excitation light into the second amplifying optical fiber 62 from the opposite side to the second FBG 52 side, where the concentration of an active element added to the core of the second amplifying optical fiber 62 is higher than the concentration of an active element added to the core of the first amplifying optical fiber 61, and a portion of the excitation light from the first excitation light source 11 transmits through the first amplifying optical fiber 61 and propagates to the second amplifying optical fiber 62.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fiber laser device. [Background technology]

[0002] Fiber laser devices are used in various fields such as laser processing and medical fields. An example of a fiber laser device is described in the following Patent Document 1. This fiber laser device includes a seed light source and an optical amplifier connected to the seed light source and amplifying light from the seed light source. The seed light source includes a resonator formed of a high reflectance mirror, a low reflectance mirror, and an amplification optical fiber arranged between these mirrors, and an excitation light source that introduces excitation light into the amplification optical fiber from the high reflectance mirror side. The optical amplifier includes an amplification optical fiber optically coupled to the seed light source, and an excitation light source that introduces excitation light into the amplification optical fiber from the side opposite to the seed light source side.

[0003] In this fiber laser device, light resonates between a high reflectance mirror and a low reflectance mirror, the light is amplified in an amplification optical fiber between these mirrors, and a part of the amplified light is emitted from the seed light source. The emitted light enters the amplifier, where it is further amplified in the amplification optical fiber and emitted. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-190834 A Summary of the Invention [Problem to be solved by the invention]

[0005] In such a fiber laser device, since there exists light propagating forward and light propagating backward in the resonator, the power density of the light increases, and there is a concern that stimulated Raman scattering may occur. If stimulated Raman scattering occurs, there is a concern that light of an unintended wavelength may be amplified, causing an unstable output. Therefore, it is conceivable to suppress the occurrence of stimulated Raman scattering by shortening the amplification optical fiber in the resonator. However, since a fiber laser device with a higher output power is required, it is preferable to suppress a decrease in the output power. For this reason, when high-power pumping light is input to the resonator, photodarkening may occur, in which the transmission loss of light in the core of the amplification optical fiber increases. Therefore, an object of the present invention is to provide a fiber laser device that can suppress the occurrence of stimulated Raman scattering and photodarkening while suppressing a decrease in output power. [Means for solving the problem]

[0006] In order to solve the above problems, a fiber laser device of the present invention includes a first amplifying optical fiber having a core and a cladding to which an active element is doped, a first mirror on one side of the first amplifying optical fiber, which is optically coupled to the core of the first amplifying optical fiber and which reflects light of at least a portion of the wavelengths of light emitted by the excited active element, and a second mirror on the other side of the first amplifying optical fiber, which is optically coupled to the core of the first amplifying optical fiber and which reflects light of at least a portion of the wavelengths of light reflected by the first mirror with a reflectance lower than that of the first mirror, and a core and cladding to which an active element is doped, the core being disposed on the opposite side of the second mirror to the first amplifying optical fiber, The optical fiber amplifier comprises a second amplification optical fiber optically coupled to a second mirror, the cladding of which is optically coupled to the cladding of the first amplification optical fiber, a first pumping light source that introduces pumping light into the cladding of the first amplification optical fiber from the first mirror side, and a second pumping light source that introduces pumping light into the cladding of the second amplification optical fiber from the side opposite to the second mirror side, wherein a concentration of the active element doped into the core of the second amplification optical fiber is higher than a concentration of the active element doped into the core of the first amplification optical fiber, and a portion of the pumping light from the first pumping light source transmits through the cladding of the first amplification optical fiber and propagates to the cladding of the second amplification optical fiber.

[0007] As described above, the part where stimulated Raman scattering is likely to occur is inside the resonator. In this fiber laser device, the resonator is composed of the first mirror, the first amplifying optical fiber, and the second mirror. However, according to this fiber laser device, a part of the pumping light from the first pumping light source is transmitted through the first amplifying optical fiber, so that the power density of the light propagating through the core in the resonator does not become too large compared to the case where all of the pumping light from the first pumping light source is absorbed by the first amplifying optical fiber, and the occurrence of stimulated Raman scattering can be suppressed. Incidentally, the likelihood of photodarkening occurring is influenced by the value obtained by dividing the power density of the light propagating through the core of the amplifying optical fiber by the power density of the pumping light. Therefore, the greater the power of the pumping light from the first pumping light source, the more likely photodarkening occurs in the first amplifying optical fiber. However, the concentration of the active element added to the core of the first amplifying optical fiber in this fiber laser device is lower than the concentration of the active element added to the core of the second amplifying optical fiber. Therefore, the occurrence of photodarkening in the first amplifying optical fiber is suppressed compared to the case where the concentration of the active element added to the core of the first amplifying optical fiber is as high as the concentration of the active element added to the core of the second amplifying optical fiber. In addition, in this fiber laser device, a part of the pumping light from the first pumping light source propagates to the second amplifying optical fiber, which has a higher concentration of the active element than the first amplifying optical fiber. Therefore, the amplification factor of light in the second amplifying optical fiber is higher than when the pumping light from the first pumping light source does not propagate to the second amplifying optical fiber. Therefore, the fiber laser device of the present invention can suppress a decrease in output power. As described above, the fiber laser device of the present invention can suppress a decrease in output power while suppressing the occurrence of stimulated Raman scattering and photodarkening.

[0008] Moreover, it is preferable that the concentration of the active element doped in the core of the second amplifying optical fiber is two to four times the concentration of the active element doped in the core of the first amplifying optical fiber.

[0009] It is also preferable that the first amplification optical fiber and the second amplification optical fiber absorb 99% or more of the pumping light emitted from the first pumping light source and the second pumping light source.

[0010] With this configuration, the power of the leaking pump light is small, the light propagating through the core is efficiently amplified, and damage to components due to the leaking pump light can be suppressed.

[0011] In this case, it is preferable that the second amplification optical fiber absorbs 98% or more of the pumping light incident thereon.

[0012] With this configuration, the power of the pump light emitted from the second amplifying optical fiber is small, and the pump light emitted from the second amplifying optical fiber propagates to the first amplifying optical fiber, suppressing excessive amplification of the light propagating through the core in the first amplifying optical fiber, and thus the occurrence of stimulated Raman scattering can be further suppressed. Also, the pump light transmitted through the first amplifying optical fiber and incident on the second amplifying optical fiber can be used efficiently.

[0013] It is also preferable that 33% to 66% of the pumping light from the first pumping light source propagates to the second amplifying optical fiber.

[0014] With this configuration, it is possible to stably amplify the light propagating through the cores of the first amplifying optical fiber and the second amplifying optical fiber while suppressing the occurrence of stimulated Raman scattering in the first amplifying optical fiber.

[0015] Moreover, it is preferable that the power of the excitation light emitted by the first excitation light source and the power of the excitation light emitted by the second excitation light source are approximately equal.

[0016] With this configuration, pumping light with as much power as possible can be introduced into the first and second amplification optical fibers, and high-power light can be emitted from the fiber laser device.

[0017] Furthermore, a diameter of the core of the first amplifying optical fiber and a diameter of the core of the second amplifying optical fiber may be approximately the same. Effect of the Invention

[0018] As described above, according to the present invention, a fiber laser device is provided that can suppress the occurrence of stimulated Raman scattering and photodarkening while suppressing a decrease in output power. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram showing a fiber laser device according to an embodiment of the present invention. [Diagram 2] 1 is a diagram showing a cross section perpendicular to the longitudinal direction of an amplification optical fiber; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, preferred embodiments of a fiber laser device according to the present invention will be described in detail with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved without departing from the spirit and scope of the present invention. For ease of understanding, the scales of the drawings may differ from those described in the following description.

[0021] Fig. 1 is a diagram showing a laser device according to this embodiment. As shown in Fig. 1, the fiber laser device 1 of this embodiment mainly includes a first pumping light source 11, a second pumping light source 12, a first FBG (Fiber Bragg Grating) 51 as a first mirror, a second FBG 52 as a second mirror, a first amplifying optical fiber 61, and a second amplifying optical fiber 62.

[0022] The first pumping light source 11 is composed of a plurality of laser diodes 11d, and emits pumping light of a wavelength that excites an active element added to each of the first amplifying optical fiber 61 and the second amplifying optical fiber 62 described later. The wavelength of this pumping light is, for example, 976 nm when the active element added to the first amplifying optical fiber 61 and the second amplifying optical fiber 62 is ytterbium (Yb) as described later. The first pumping light source 11 is connected to a predetermined power source, and emits pumping light of a power corresponding to the current from the power source. It is preferable that this power source can vary the output current. Each laser diode 11d of the first pumping light source 11 is connected to one end of an optical fiber 21 for pumping light. The optical fiber 21 for pumping light is, for example, an optical fiber having a core with a diameter of 125 μm. The other end of each optical fiber 21 for pumping light is connected to a pumping light combiner 31.

[0023] One end of an optical fiber 41 (described later) is connected to the pumping light combiner 31, and one end of a first amplifying optical fiber 61 is connected to the other end of the optical fiber 41.

[0024] FIG. 2 is a diagram showing a cross section of the first amplification optical fiber 61 shown in FIG. 1. As shown in FIG. 2, the first amplification optical fiber 61 mainly includes a core 611, an inner clad 612 surrounding the outer peripheral surface of the core 611 without any gaps, an outer clad 613 covering the outer peripheral surface of the inner clad 612, and a coating layer 614 covering the outer clad 613. That is, the first amplification optical fiber 61 is a double-clad optical fiber from a structural point of view. The refractive index of the inner clad 612 is lower than that of the core 611, and the refractive index of the outer clad 613 is lower than that of the inner clad 612. The diameter of the core 611 is, for example, 28 μm, and the outer diameter of the inner clad 612 is, for example, 400 μm. In this embodiment, the first amplification optical fiber 61 is a multimode optical fiber from an optical point of view, and light of, for example, 4LP mode propagates through the core 611. The length of the first amplification optical fiber 61 is, for example, 20 m. The core 611 may be configured to be capable of propagating light of, for example, 2 LP mode or more and 10 LP mode or less. Pumping light propagates through the inner cladding 612 as described below, and the inner cladding through which such pumping light propagates may be simply called a cladding.

[0025] The core 611 is doped with an active element that is excited by the excitation light emitted from the first excitation light source 11. In this embodiment, ytterbium is doped as the active element. It is preferable that aluminum and phosphorus are further doped in the core 611 to increase the resistance to photodarkening. In addition, a dopant such as fluorine (F) or boron (B) may be doped at least partially in the core 611 to adjust the refractive index. Also, unlike this embodiment, the active element doped in the core 611 may be an active element other than ytterbium. In addition to ytterbium, examples of such active elements include thulium (Tm), cerium (Ce), neodymium (Nd), europium (Eu), and erbium (Er). In addition to rare earth elements, examples of the active element include bismuth (Bi).

[0026] The inner cladding 612 is made of, for example, pure quartz to which no dopant is added, or quartz to which a dopant such as fluorine that reduces the refractive index is added. The outer cladding 613 is made of, for example, a resin having a lower refractive index than the inner cladding 612, or quartz to which an element such as fluorine that reduces the refractive index is added. The coating layer 614 is made of, for example, a thermosetting or ultraviolet curing resin, and if the outer cladding 613 is made of resin, it is made of a resin different from that of the outer cladding 613.

[0027] The optical fiber 41 connected to one end of the first amplifying optical fiber 61 has the same configuration as the first amplifying optical fiber 61, except that an active element is not added to the core. Therefore, the optical fiber 41 is a double-clad optical fiber from a structural point of view, and a multimode optical fiber from an optical point of view. A core 611 of the first amplifying optical fiber 61 and a core of the optical fiber 41 are optically coupled, and an inner clad 612 of the first amplifying optical fiber 61 and an inner clad of the optical fiber 41 are optically coupled.

[0028] The first FBG 51, which is a first mirror, is provided in the core of the optical fiber 41. Thus, the first FBG 51 is provided on one end side of the first amplifying optical fiber 61 and is optically coupled to the core 611. The first FBG 51 is configured by repeating portions with a high refractive index at a constant period along the longitudinal direction of the optical fiber 41. By adjusting this period, the first FBG 51 reflects light of at least a part of the wavelengths of light emitted by the active element of the first amplifying optical fiber 61 in an excited state. When the active element doped in the first amplifying optical fiber 61 is ytterbium as described above, the first FBG 51 reflects light with a wavelength of 1070 nm±3 nm, for example, with a reflectance of 99% or more.

[0029] The pumping light combiner 31 is connected to the core of each of the pumping light optical fibers 21, and optically couples the core of each of the pumping light optical fibers 21 to the inner clad of the optical fiber 41. Therefore, the pumping light emitted from the first pumping light source 11 can propagate to the inner clad 612 of the first amplification optical fiber 61 via the pumping light optical fiber 21, the pumping light combiner 31, and the optical fiber 41. Therefore, the first pumping light source 11 is a light source that introduces pumping light into the inner clad 612 of the first amplification optical fiber 61 from the first FBG 51 side.

[0030] One end of the optical fiber 42 is connected to the other end of the first amplification optical fiber 61. The optical fiber 42 has the same configuration as the optical fiber 41. Therefore, the optical fiber 42 is a double-clad optical fiber from a structural point of view, and a multimode optical fiber from an optical point of view. A core 611 of the first amplification optical fiber 61 and a core of the optical fiber 42 are optically coupled, and an inner clad 612 of the first amplification optical fiber 61 and an inner clad of the optical fiber 42 are optically coupled.

[0031] The second FBG 52, which is a second mirror, is provided in the core of the optical fiber 42. Thus, the second FBG 52 is provided on the other end side of the first amplifying optical fiber 61 and is optically coupled to the core 611. The second FBG 52 has a portion with a high refractive index repeated at a constant interval along the longitudinal direction of the optical fiber 42, and is configured to reflect light of at least a part of the wavelength of the light reflected by the first FBG 51 with a reflectance lower than that of the first FBG 51. The second FBG 52 is configured to reflect light of the same wavelength as the light reflected by the first FBG 51 with a reflectance of, for example, 10%. In this way, a resonator is formed by the first FBG 51, the first amplifying optical fiber 61, and the second FBG 52. A part of the light resonating in this resonator is transmitted through the second FBG 52.

[0032] One end of the second amplifying optical fiber 62 is connected to the other end of the optical fiber 42. The second amplifying optical fiber 62 has the same configuration as the first amplifying optical fiber 61, except for the concentration of the active element added to the core, which will be described later. Therefore, the second amplifying optical fiber 62 is a double-clad optical fiber from a structural point of view, and a multimode optical fiber from an optical point of view. In addition, in this embodiment, the diameter of the core and the outer diameter of the inner cladding of the second amplifying optical fiber 62 are equal to the diameter of the core 611 and the outer diameter of the inner cladding 612 of the first amplifying optical fiber 61, and the length of the second amplifying optical fiber 62 is equal to the length of the first amplifying optical fiber 61. The core of the optical fiber 42 and the core of the second amplifying optical fiber 62 are optically coupled, and the inner cladding of the optical fiber 42 and the inner cladding of the second amplifying optical fiber 62 are optically coupled. Therefore, the core of the second amplifying optical fiber 62 is optically coupled to the second FBG 52, and light passing through the second FBG 52 propagates to the core of the second amplifying optical fiber 62. In addition, the inner cladding 612 of the first amplifying optical fiber 61 and the inner cladding of the second amplifying optical fiber 62 are optically coupled via the inner cladding of the optical fiber 42, so that when light is emitted from the inner cladding 612 of the first amplifying optical fiber 61, the light is incident on the inner cladding of the second amplifying optical fiber 62.

[0033] The second pumping light source 12 is a light source that introduces pumping light into the inner clad of the second amplification optical fiber 62 from the opposite side to the second FBG 52 side. The second pumping light source 12 is composed of a plurality of laser diodes 12d having a configuration similar to that of the plurality of laser diodes 11d of the first pumping light source 11, and emits the same light as the first pumping light source 11. Therefore, the power of the pumping light emitted by the first pumping light source 11 and the power of the pumping light emitted by the second pumping light source 12 are approximately equal. In addition, the light emitted from the second pumping light source 12 has a wavelength approximately equal to that of the pumping light emitted by the first pumping light source 11, and excites the active element added to the second amplification optical fiber 62. The second pumping light source 12 is connected to a power source similar to the power source connected to the first pumping light source 11. Each laser diode 12d of the second pumping light source 12 is connected to one end of a pumping light optical fiber 22 having a configuration similar to that of the pumping light optical fiber 21. The other end of each of the pumping light optical fibers 22 is connected to a pumping light combiner 32.

[0034] The other end of the second amplification optical fiber 62 is connected to the pumping light combiner 32, and the core of each pumping light optical fiber 22 is optically coupled to the inner clad of the second amplification optical fiber 62. Therefore, the pumping light emitted from the second pumping light source 12 can propagate to the inner clad of the second amplification optical fiber 62 via the pumping light optical fiber 22 and the pumping light combiner 32.

[0035] One end of a delivery optical fiber 71 is connected to the pumping light combiner 32. The delivery optical fiber 71 has a core and a clad, and the core and clad have the same diameter as, for example, the core and the inner clad of the second amplification optical fiber 62. The core of the delivery optical fiber is optically coupled to the core of the second amplification optical fiber 62 via the pumping light combiner 32. In addition, an emission part 75 is connected to the other end of the delivery optical fiber 71. The emission part 75 is formed of, for example, a glass rod having a diameter larger than that of the core of the delivery optical fiber 71. Therefore, the light incident on the emission part 75 from the core of the delivery optical fiber 71 is expanded in diameter and emitted from the emission part 75.

[0036] Next, the concentration of the active element doped in the core of the first amplifying optical fiber 61 and the second amplifying optical fiber 62 will be described.

[0037] The first amplification optical fiber 61 is configured such that a part of the pumping light incident on the inner cladding 612 of the first amplification optical fiber 61 is absorbed by the active element doped in the core 611, and another part of the pumping light is emitted from the first amplification optical fiber 61. The concentration of the active element doped in the core 611 of the first amplification optical fiber 61 and the length of the first amplification optical fiber 61 are adjusted so that the pumping light is absorbed in this manner. The pumping light emitted from the first amplification optical fiber 61 is incident on the second amplification optical fiber 62 via the optical fiber 42.

[0038] Moreover, the first amplification optical fiber 61 is preferably configured to emit 33% or more and 66% or less of the pumping light incident on the first amplification optical fiber 61. With this configuration, it is possible to efficiently amplify the light propagating through the core 611 while suppressing the occurrence of stimulated Raman scattering in the first amplification optical fiber 61. In this case, the 33% or more and 66% or less of the pumping light is incident on the second amplification optical fiber 62. Furthermore, it is more preferable that the first amplification optical fiber 61 is configured to emit approximately 50% of the pumping light incident on the first amplification optical fiber 61, and in this case, approximately 50% of the pumping light is incident on the second amplification optical fiber 62.

[0039] In this embodiment, 99% or more of the pumping light emitted from the first pumping light source 11 and the second pumping light source 12 is absorbed by the first amplification optical fiber 61 and the second amplification optical fiber 62. Therefore, the power of the leaking pumping light is small, the light propagating through the core is efficiently amplified, and damage to components due to the leaking pumping light can be suppressed.

[0040] Moreover, the concentration of the active element added to the core of the second amplification optical fiber 62 is higher than the concentration of the active element added to the core 611 of the first amplification optical fiber 61. Specifically, the concentration of the active element added to the core of the second amplification optical fiber 62 is preferably two to four times, and more preferably approximately three times, the concentration of the active element added to the core 611. For this reason, the second amplification optical fiber 62 is configured to absorb more pumping light than the first amplification optical fiber 61, and in this embodiment, the second amplification optical fiber 62 is configured so that most of the pumping light incident on the inner clad of the second amplification optical fiber 62 is absorbed by the second amplification optical fiber 62. Specifically, the second amplification optical fiber 62 preferably absorbs 98% or more of the incident pumping light, and more preferably absorbs 99% or more. In this way, the second amplification optical fiber 62 absorbs most of the incident pumping light, and thus the power of the pumping light emitted from the second amplification optical fiber 62 is small. Therefore, the excitation light emitted from the second amplification optical fiber 62 propagates to the first amplification optical fiber 61, which prevents the light propagating through the core 611 within the first amplification optical fiber 61 from being excessively amplified, thereby further suppressing the occurrence of stimulated Raman scattering.

[0041] Next, the operation of the fiber laser device 1 will be described.

[0042] First, pumping light is emitted from each laser diode 11d of the first pumping light source 11 and each laser diode 12d of the second pumping light source 12. The pumping light emitted from the first pumping light source 11 is incident on the inner clad 612 from one end of the first amplification optical fiber 61 through the pumping light optical fiber 21, the pumping light combiner 31 and the optical fiber 41, and mainly propagates through the inner clad 612. A part of the pumping light propagating through the inner clad 612 is absorbed by the active element added to the core 611 when passing through the core 611, and excites the active element. The excited active element emits spontaneous emission light in a wavelength band including a predetermined wavelength. Starting from this spontaneous emission light, light including a predetermined wavelength that is reflected in common by the first FBG 51 and the second FBG 52 resonates between the first FBG 51 and the second FBG 52. When the resonating light propagates through the core 611 of the first amplifying optical fiber 61, the excited active element causes stimulated emission, and the resonating light is amplified. A part of the resonating light passes through the second FBG 52. Then, when the gain and loss in the resonator including the first FBG 51, the first amplifying optical fiber 61, and the second FBG 52 become equal, a laser oscillation state is reached, and light of a certain power is incident on the core of the second amplifying optical fiber 62.

[0043] Another part of the pumping light incident on the first amplifying optical fiber 61 and propagating through the inner cladding 612 is not absorbed by the first amplifying optical fiber 61, exits from the other end of the first amplifying optical fiber 61, passes through the inner cladding of the optical fiber 42, enters the inner cladding of the second amplifying optical fiber 62 from one end of the second amplifying optical fiber 62, and mainly propagates through the inner cladding. Furthermore, the pumping light emitted from the second pumping light source 12 passes through the pumping light optical fiber 22 and the pumping light combiner 32, enters the inner cladding of the second amplifying optical fiber 62 from the other end of the second amplifying optical fiber 62, and mainly propagates through the inner cladding in the opposite direction to the traveling direction of the pumping light incident from the first amplifying optical fiber 61. The pumping light propagating through the second amplifying optical fiber 62 is absorbed by the active element added to the core of the second amplifying optical fiber 62. As described above, in this embodiment, most of the pumping light incident on the inner cladding of the second amplifying optical fiber 62 is absorbed by the active element doped in the core of the second amplifying optical fiber 62. This causes the active element to enter an excited state, and the active element causes stimulated emission due to the light propagating from the core 611 of the first amplifying optical fiber to the core of the second amplifying optical fiber 62, and the light propagating through the core is amplified.

[0044] The light amplified by the second amplification optical fiber 62 enters the core of the delivery optical fiber 71 from the second amplification optical fiber 62, propagates from the core to the exit portion 75, has its diameter expanded at the exit portion 75, and exits from the exit portion 75.

[0045] As described above, in the fiber laser device 1 of this embodiment, the concentration of the active element added to the core of the second amplifying optical fiber 62 is higher than the concentration of the active element added to the core 611 of the first amplifying optical fiber 61, and a part of the pumping light from the first pumping light source 11 transmits through the first amplifying optical fiber 61 and propagates to the second amplifying optical fiber 62. Thus, according to the fiber laser device 1 of this embodiment, since a part of the pumping light from the first pumping light source 11 transmits through the first amplifying optical fiber 61, the power density of the light propagating through the core in the resonator does not become too large compared to the case where all of the pumping light from the first pumping light source 11 is absorbed by the first amplifying optical fiber 61, the occurrence of stimulated Raman scattering can be suppressed.

[0046] As described above, when the output power of the fiber laser device increases, photodarkening may occur, and the likelihood of photodarkening is affected by the value obtained by dividing the power density of the light propagating through the core of the amplification optical fiber by the power density of the pumping light and the concentration of the active element. Therefore, the higher the concentration of the active element added to the core, the more likely photodarkening occurs, and the higher the power density of the pumping light is relative to the power density of the light propagating through the core, the more likely photodarkening occurs. In the fiber laser device 1 of this embodiment, at one end of the first amplification optical fiber 61, which is the first pumping light source 11 side, the power density of the light propagating through the core 611 is small, and the power density of the pumping light propagating through the inner cladding 612 is large. Therefore, in the first amplification optical fiber 61, photodarkening is most likely to occur at one end. However, the concentration of the active element added to the core 611 of the first amplification optical fiber 61 is lower than the concentration of the active element added to the core of the second amplification optical fiber 62. Therefore, compared to a case where the concentration of the active element doped to the core 611 of the first amplification optical fiber 61 is as high as the concentration of the active element doped to the core of the second amplification optical fiber 62, the occurrence of photodarkening in the first amplification optical fiber 61 is suppressed.

[0047] Furthermore, in the fiber laser device 1 of this embodiment, a portion of the pumping light from the first pumping light source 11 that passes through the first amplifying optical fiber 61 is incident on the second amplifying optical fiber. Since the concentration of the active element in the second amplifying optical fiber 62 is higher than that in the first amplifying optical fiber 61, the pumping light from the first pumping light source 11 is efficiently absorbed in the second amplifying optical fiber 62 and used to amplify the light propagating through the core. Therefore, the fiber laser device of this embodiment suppresses a decrease in output.

[0048] The concentration of the active element added to the core of the second amplifying optical fiber 62 is higher than the concentration of the active element added to the core 611 of the first amplifying optical fiber 61. However, at one end of the second amplifying optical fiber 62 on the side of the first amplifying optical fiber 61, the power density of the pumping light propagating from the first pumping light source is smaller because a part of the pumping light is absorbed by the first amplifying optical fiber 61, and the power density of the pumping light from the second pumping light source is smaller because a part of the pumping light is absorbed by the second amplifying optical fiber 62. Therefore, the occurrence of photodarkening at one end of the second amplifying optical fiber 62 is suppressed. At the other end of the second amplifying optical fiber, the light propagating through the core is amplified, and the power density of the light is large. Therefore, the occurrence of photodarkening is also suppressed at the other end of the second amplifying optical fiber 62. In this way, according to the fiber laser device 1 of this embodiment, the occurrence of photodarkening is suppressed.

[0049] As described above, when the power of the pumping light emitted from the first pumping light source 11 and the power of the pumping light emitted from the second pumping light source are approximately equal to each other and the second amplification optical fiber 62 absorbs 99% or more of the incident pumping light, it is preferable that the concentration of the active element doped in the core of the second amplification optical fiber 62 is two to four times the concentration of the active element doped in the core 611 of the first amplification optical fiber 61, and the concentration of the pumping light from the first pumping light source 11 that is output from the first amplification optical fiber 61 and incident on the second amplification optical fiber 62 is 33% to 66%. With such a balance, the fiber laser device 1 can be made more powerful while further suppressing the occurrence of photodarkening. Furthermore, from the viewpoint of further suppressing the occurrence of photodarkening, it is more preferable that the concentration of the active element added to the core of the second amplifying optical fiber 62 is approximately three times that of the active element added to the core 611 of the first amplifying optical fiber 61, and that the excitation light from the first excitation light source 11 emitted from the first amplifying optical fiber 61 and incident on the second amplifying optical fiber 62 is approximately 50%.

[0050] The present invention has been described above using an embodiment as an example, but the present invention should not be construed as being limited to the above embodiment, and the configuration can be changed as appropriate within the scope of achieving the object of the present invention.

[0051] For example, the concentration of the active element doped to the core of the second amplifying optical fiber 62 does not have to be more than two times and less than four times the concentration of the active element doped to the core 611, as long as it is higher than the concentration of the active element doped to the core 611 of the first amplifying optical fiber 61.

[0052] Furthermore, the first amplification optical fiber 61 and the second amplification optical fiber 62 may absorb less than 99% of the pumping light emitted by the first pumping light source 11 and the second pumping light source 12. However, in order to efficiently emit high-power light, it is preferable that the first amplification optical fiber 61 and the second amplification optical fiber 62 absorb 99% or more of the pumping light.

[0053] Furthermore, the second amplification optical fiber 62 may absorb the incident pumping light with an efficiency lower than 98%. For example, the second amplification optical fiber 62 may absorb the incident pumping light with an efficiency of 97%. In this case, approximately 3% of the pumping light from the second pumping light source 12 is output from the second amplification optical fiber 62 and enters the first amplification optical fiber 61.

[0054] Furthermore, as long as a portion of the pumping light from the first pumping light source 11 propagates to the second amplification optical fiber 62, pumping light with a power lower than 33% of the pumping light from the first pumping light source 11 or pumping light with a power higher than 66% may propagate to the second amplification optical fiber 62.

[0055] Furthermore, the power of the pumping light emitted by the first pumping light source 11 and the power of the pumping light emitted by the second pumping light source 12 may be different from each other. However, from the viewpoint of introducing pumping light with as high a power as possible into the amplification optical fiber and emitting high-power light from the fiber laser device 1, it is preferable that the power of the pumping light emitted by the first pumping light source 11 and the power of the pumping light emitted by the second pumping light source 12 are equal to each other.

[0056] Also, the diameter of the core 611 of the first amplification optical fiber 61 and the diameter of the core of the second amplification optical fiber 62 may be different from each other, and the outer diameter of the inner clad 612 of the first amplification optical fiber 61 and the outer diameter of the inner clad 612 of the second amplification optical fiber 62 may be different from each other. In this case, it is preferable that the diameter of the core of the second amplification optical fiber 62 is larger than the diameter of the core 611 of the first amplification optical fiber 61 from the viewpoint that the light emitted from the core 611 of the first amplification optical fiber 61 efficiently enters the core of the second amplification optical fiber 62. Also, it is preferable that the outer diameter of the inner clad of the second amplification optical fiber 62 is larger than the outer diameter of the inner clad 612 of the first amplification optical fiber 61 from the viewpoint that the light emitted from the inner clad 612 of the first amplification optical fiber 61 efficiently enters the inner clad of the second amplification optical fiber 62. Also, the length of the first amplification optical fiber 61 and the length of the second amplification optical fiber 62 may be different from each other. [Industrial Applicability]

[0057] As described above, according to the present invention, a fiber laser device is provided that can suppress the occurrence of stimulated Raman scattering and photodarkening and suppress a decrease in output power, and is expected to be used in laser devices for processing, etc. [Explanation of symbols]

[0058] 1. Fiber laser device 11. First excitation light source 12 Second excitation light source 51 1st FBG (1st mirror) 52 2nd FBG (2nd mirror) 61 First amplifying optical fiber 62 Second amplifying optical fiber 71...Delivery optical fiber

Claims

1. A first amplification optical fiber having a core to which an active element is added, and a cladding, On one side of the first amplification optical fiber, a first mirror is optically coupled to the core of the first amplification optical fiber and reflects light of at least some wavelengths of light emitted by the excited active element, On the other side of the first amplification optical fiber, a second mirror is optically coupled to the core of the first amplification optical fiber and reflects at least some wavelengths of light reflected by the first mirror with a lower reflectivity than the first mirror. A second amplification optical fiber is positioned on the opposite side of the second mirror from the first amplification optical fiber side, and has a core and cladding to which an active element is added, wherein the core is optically coupled to the second mirror, and the cladding is optically coupled to the cladding of the first amplification optical fiber. A first excitation light source that introduces excitation light into the cladding of the first amplification optical fiber from the first mirror side, A second excitation light source that introduces excitation light into the cladding of the second amplification optical fiber from the side opposite to the second mirror, Equipped with, The concentration of the active element added to the core of the second amplification optical fiber is higher than the concentration of the active element added to the core of the first amplification optical fiber. A portion of the excitation light from the first excitation light source passes through the cladding of the first amplification optical fiber and propagates to the cladding of the second amplification optical fiber. A fiber laser apparatus characterized by the following features.

2. The concentration of the active element added to the core of the second amplification optical fiber is two times or more and four times or less the concentration of the active element added to the core of the first amplification optical fiber. The fiber laser apparatus according to feature 1.

3. The first amplification optical fiber and the second amplification optical fiber absorb 99% or more of the excitation light emitted by the first and second excitation light sources. The fiber laser apparatus according to feature 1 or 2.

4. The second amplification optical fiber absorbs 98% or more of the incident excitation light. The fiber laser apparatus according to feature 3.

5. 33% to 66% of the excitation light from the first excitation light source propagates into the second amplification optical fiber. The fiber laser apparatus according to feature 1 or 2.

6. The power of the excitation light emitted by the first excitation light source and the power of the excitation light emitted by the second excitation light source are approximately equal. The fiber laser apparatus according to feature 1 or 2.

7. The diameter of the core of the first amplification optical fiber and the diameter of the core of the second amplification optical fiber are approximately the same. The fiber laser apparatus according to feature 1 or 2.

8. 33% to 66% of the excitation light from the first excitation light source propagates to the second amplification optical fiber. The fiber laser apparatus according to feature 3.

9. The power of the excitation light emitted by the first excitation light source and the power of the excitation light emitted by the second excitation light source are approximately equal. The fiber laser apparatus according to feature 3.

10. The diameter of the core of the first amplification optical fiber and the diameter of the core of the second amplification optical fiber are approximately the same. The fiber laser apparatus according to feature 3.

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