Fiber laser device

Abstract

A fiber laser device (1) comprises: a first excitation light source (12) that emits first excitation light; a first amplification optical fiber (11) to which an active element that absorbs the first excitation light has been added and which amplifies and emits light of a first wavelength; a second amplification optical fiber (21) to which the same active element as the active element added to the first amplification optical fiber (11) has been added, and which absorbs the light of the first wavelength and amplifies and emits light of a second wavelength; combiners (31), (32); and a light reduction section (60) which is provided on an optical path between the combiners (31), (32) and the first amplification optical fiber (11), transmits light from the first amplification optical fiber (11), and suppresses the transmission of light from the combiners (31), (32).

Description

Fiber laser device

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

[0002] Fiber laser devices are used in various fields such as laser processing and medicine. Patent Document 1 listed below describes an example of a fiber laser device. 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 optical amplifier includes an amplification optical fiber optically connected to the seed light source, and a pumping light source that introduces pumping light into the amplification optical fiber from the front and rear sides. Light from the seed light source that enters the amplification optical fiber is amplified by the pumping light from the pumping light source and then emitted. In Patent Document 1, a laser oscillator is used as the pumping light source.

[0003] In a typical fiber laser device, when the active element added to the amplifying optical fiber used in the optical amplifier is ytterbium, the wavelength of the pump light is 915 nm to 976 nm, and the wavelength of the amplified and output light is 1060 nm to 1080 nm. Therefore, there is a difference of at least 84 nm between the wavelength of the pump light and the wavelength of the amplified light. Such a large difference between the wavelength of the pump light and the wavelength of the amplified light can cause transverse mode instability (TMI) due to quantum defects. Transverse mode instability can cause coupling with higher-order modes, making the amplified light more susceptible to leakage, which can make thermal management difficult when emitting high-power light. Therefore, there is a demand for suppressing the occurrence of quantum defects and thereby suppressing degradation of beam quality while emitting high-power light. To meet this demand, it is considered to bring the wavelength of the pump light closer to the wavelength of the amplified light. In Patent Document 1, the wavelength of the light amplified by the optical amplifier is 1080 nm, and the wavelength of the pump light is 1018 nm.

[0004] Ruixian Li, Hanshuo Wu, Hu Xiao, Jinyong Leng, Liangjin Huang, AND Pu Zhou, “More than 6 kW near single-mode fiber amplifierbased on a bidirectional tandem pumping scheme” Vol. 61, No. 23 / 10 August 2022 / Applied Optics

[0005] When the wavelength of the light to be amplified and the wavelength of the pumping light are made closer as in the fiber laser device of Patent Document 1, it is possible to use, as the pumping light source, a fiber laser in which light emitted from a semiconductor laser or the like is input into an amplifying optical fiber doped with the same active element as the active element doped in the amplifying optical fiber of the optical amplifier. By configuring the pumping light source in this way, it is possible to make the wavelength of the light from the semiconductor laser within the pumping light source closer to the wavelength of the light to be amplified by the optical amplifier.

[0006] In a fiber laser device, light of a wavelength amplified in an amplification optical fiber may be incident on a pumping light source. In a backward-pumping light source in which a combiner is provided on the front side, i.e., the emission direction of the laser light from the amplification optical fiber, and pumping light is introduced from the combiner toward the rear side, the light may be incident on the pumping light source as follows. That is, when light from a seed light source is amplified and the amplified light is introduced, part of the light may be incident on the pumping light source due to reflection at the combiner or the like. In a forward-pumping light source in which a combiner is provided on the rear side of the amplification optical fiber and pumping light is introduced from the combiner toward the front side, the light may be incident on the pumping light source as follows. That is, when light amplified in the amplification optical fiber is introduced from the fiber laser device, the light may be reflected at the emission end or the workpiece and enter the amplification optical fiber as returned light, and part of the returned light may enter the pumping light source from the combiner. As described above, when the active element added to the amplification optical fiber of the pumping light source is the same as the active element added to the amplification optical fiber of the optical amplifier, if a portion of the light of the wavelength amplified in the amplification optical fiber enters the pumping light source, as in these examples, the amplification efficiency of the pumping light in the pumping light source may deteriorate, resulting in a decrease in the power of the pumping light and a decrease in the power of the light emitted from the fiber laser device.

[0007] Therefore, an object of the present invention is to provide a fiber laser device that can emit high-power light.

[0008] In order to solve the above-described problems, a first aspect of the present invention is a fiber laser device comprising: a first pumping light source capable of emitting first pumping light; a first amplification optical fiber doped with an active element and capable of absorbing the first pumping light and amplifying and emitting light of a first wavelength; a second amplification optical fiber doped with the same active element as that doped in the first amplification optical fiber and capable of absorbing light of the first wavelength as pumping light and amplifying and emitting light of a second wavelength; a combiner capable of inputting light of the first wavelength into the second amplification optical fiber; and a light reducing unit provided on an optical path between the combiner and the first amplification optical fiber, which transmits light from the first amplification optical fiber and suppresses transmission of light from the combiner.

[0009] In this way, the active element doped in the first amplification optical fiber and the active element doped in the second amplification optical fiber are the same, and the first pumping light is converted to light of the first wavelength using the first amplification optical fiber, so the first wavelength can be closer to the second wavelength than the wavelength of the first pumping light. Therefore, according to the fiber laser device of this aspect, compared to when the first pumping light is incident on the second amplification optical fiber, the occurrence of quantum defects in the second amplification optical fiber can be suppressed, and degradation of the beam quality of light of the second wavelength output from the second amplification optical fiber can be suppressed. Furthermore, in this fiber laser device, even when light of the second wavelength amplified in the second amplification optical fiber travels toward the first amplification optical fiber via the combiner as described above, the light from the combiner is reduced by the light reducing unit. Therefore, propagation of light of the second wavelength to the first core of the first amplification optical fiber can be suppressed, and degradation of the amplification efficiency of light of the first wavelength in the first amplification optical fiber can be suppressed. Therefore, a decrease in the power of the light of the first wavelength incident as pump light into the second amplification optical fiber can be suppressed, and a decrease in the amplification efficiency of the light of the second wavelength in the second amplification optical fiber can be suppressed. Thus, the fiber laser device of Aspect 1 can emit high-power light.

[0010] A second aspect of the present invention is the fiber laser device of the first aspect, wherein the light reducing section is a filter that transmits light of the first wavelength and suppresses transmission of light of the second wavelength.

[0011] According to the configuration of aspect 2, the light of the second wavelength traveling from the combiner toward the first amplification optical fiber can be more appropriately reduced by the light reducing section.

[0012] A third aspect of the present invention is the fiber laser device according to the second aspect, wherein the filter is a mirror that reflects the light of the second wavelength.

[0013] According to the configuration of Aspect 3, when light of the second wavelength travels through the combiner toward the first amplification optical fiber, the light of the second wavelength can be reflected in a desired direction. For example, by setting a mirror to reflect the light of the second wavelength traveling toward the first amplification optical fiber toward the combiner, the light of the second wavelength can be incident again on the second core of the second amplification optical fiber and amplified in the second core. In this case, a decrease in the amplification efficiency of the second amplification optical fiber can be further suppressed.

[0014] A fourth aspect of the present invention is the fiber laser device according to the third aspect, characterized in that the mirror is an FBG (Fiber Bragg Grating).

[0015] In this case, the light of the second wavelength traveling toward the first amplification optical fiber can be reflected toward the combiner, so that the light of the second wavelength can be incident again on the second core of the second amplification optical fiber and amplified in the second core, thereby further suppressing a decrease in the amplification efficiency in the second amplification optical fiber.

[0016] A fifth aspect of the present invention is the fiber laser device according to the fourth aspect, wherein the FBG is a slant type.

[0017] A sixth aspect of the present invention is the fiber laser device according to the second aspect, characterized in that the filter is a photonic bandgap fiber.

[0018] The photonic bandgap fiber has excellent filtering performance, allowing light of a desired wavelength to pass through while blocking light of other wavelengths, and therefore can further suppress propagation of light of the second wavelength into the first amplification optical fiber.

[0019] A seventh aspect of the present invention is the fiber laser device of the fifth or sixth aspect, characterized in that the filter and the first amplification optical fiber are optically connected by an optical fiber including a core and a cladding, and a cladding mode stripper is provided midway along the optical fiber.

[0020] When a slant FBG or a photonic bandgap fiber is used as a filter, light of the second wavelength may enter the cladding of the optical fiber. Even in this case, the cladding mode stripper can cause the light propagating through the cladding to leak out of the cladding of the optical fiber. Therefore, it is possible to further prevent the light propagating through the cladding from entering the first amplification optical fiber.

[0021] The light reducing unit may be configured other than a filter. In this case, for example, the fiber laser device of Aspect 1 is characterized in that the optical path between the first amplification optical fiber and the combiner includes a first optical fiber optically connected to the first amplification optical fiber and a second optical fiber connected to the first optical fiber and optically connected to the combiner, the light reducing unit includes a connection portion between the first optical fiber and the second optical fiber, and a core diameter of the second optical fiber is larger than a core diameter of the first optical fiber.

[0022] In this embodiment, the light reducing portion is formed by providing a difference in the diameter of the cores of the two optical fibers, and therefore the light reducing portion can be easily configured.

[0023] An eighth aspect of the present invention is the fiber laser device according to any one of the first to seventh aspects, further comprising: a second pumping light source capable of emitting second pumping light; and a third amplifying optical fiber doped with the same active element as the active element doped in the second amplifying optical fiber, absorbing the second pumping light, and amplifying and emitting light of the second wavelength, wherein the light of the second wavelength emitted from the third amplifying optical fiber is incident on one end side of the second amplifying optical fiber.

[0024] Alternatively, Aspect 9 of the present invention is the fiber laser device according to any one of Aspects 1 to 7, further comprising: a first mirror optically connected to the second amplification optical fiber at one end thereof and reflecting light of the second wavelength; and a second mirror optically connected to the second amplification optical fiber at the other end thereof and reflecting light of the second wavelength with a reflectance lower than that of the first mirror.

[0025] As described above, the present invention provides a fiber laser device that can emit high-power light.

[0026] 1 is a diagram showing a fiber laser device according to a first embodiment of the present invention; 2 is a diagram showing a pumping light source; 3 is a diagram showing an amplification optical fiber; 4 is a diagram showing a seed light source; 5 is a diagram showing a fiber laser device according to a second embodiment of the present invention; 6 is a diagram showing a first modified example of a light reducing section; 7 is a diagram showing a second modified example of a light reducing section; 8 is a diagram showing a photonic bandgap fiber; and 9 is a diagram showing a third modified example of a light reducing section.

[0027] Preferred embodiments of a fiber laser device according to the present invention will be described in detail below 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 within the scope of the claims without departing from the spirit of the invention. Note that, for ease of understanding, the scales of the drawings may differ from those used in the following description.

[0028] Fig. 1 is a diagram showing a laser device according to this embodiment. As shown in Fig. 1, the fiber laser device 1 according to this embodiment mainly comprises a seed light source MO and an optical amplifier PA. In other words, the fiber laser device 1 according to this embodiment is a MO-PA (Master Oscillator-Power Amplifier) ​​type fiber laser device.

[0029] First, the optical amplifier PA will be described. The optical amplifier PA mainly comprises a plurality of pumping light sources 10, a second amplification optical fiber 21, combiners 31 and 32, and a plurality of light reducing sections 60.

[0030] Fig. 2 is a diagram showing the pumping light source 10. As shown in Fig. 2, the pumping light source 10 mainly comprises a first amplification optical fiber 11, a first pumping light source 12, an HR-FBG 16, and an OC-FBG 17.

[0031] The first pumping light source 12 is composed of a plurality of laser diodes 12D and emits first pumping light having a wavelength that excites the active element doped in the first amplification optical fiber 11. The wavelength of this first pumping light is, for example, 976 nm when the active element doped in the first amplification optical fiber 11 is ytterbium (Yb), as described below. The first pumping light source 12 is connected to a predetermined power supply and emits first pumping light with a power corresponding to the current from the power supply. It is preferable that this power supply be capable of varying the output current. Each laser diode 12D of the first pumping light source 12 is connected to one end of an optical fiber 121. The other end of each optical fiber 121 is connected to the combiner 13.

[0032] One end of an optical fiber 14 (described later) is connected to the combiner 13 , and one end of a first amplifying optical fiber 11 is connected to the other end of the optical fiber 14 .

[0033] 3 is a diagram showing the cross section of the first amplification optical fiber 11 shown in FIG. 2. As shown in FIG. 2, the first amplification optical fiber 11 mainly comprises a first core 111, a first clad 112 as an inner clad that tightly surrounds the outer peripheral surface of the first core 111, an outer clad 113 that surrounds the outer peripheral surface of the first clad 112, and a coating layer 114 that coats the outer clad 113. In other words, from a structural point of view, the first amplification optical fiber 11 is a double-clad optical fiber. The refractive index of the first clad 112 is lower than the refractive index of the first core 111, and the refractive index of the outer clad 113 is lower than the refractive index of the first clad 112. The diameter of the first core 111 is, for example, 28 μm, and the outer diameter of the first clad 112 is, for example, 400 μm. In this embodiment, the first amplification optical fiber 11 is a multimode optical fiber from an optical viewpoint, and light of, for example, 2LP mode propagates through the first core 111. Furthermore, first pumping light, as described below, is incident on the first cladding 112, and the first pumping light propagates mainly through the first cladding 112. The length of the first amplification optical fiber 11 is, for example, 20 m.

[0034] The first core 111 is doped with an active element that is excited by the first excitation light emitted from the first excitation light source 12. In this embodiment, ytterbium is doped as the active element. It is preferable that aluminum and phosphorus are further doped into the first core 111 to increase resistance to photodarkening. Furthermore, a dopant such as fluorine (F) or boron (B) may be doped into at least a portion of the first core 111 to adjust the refractive index. Unlike this embodiment, the active element doped into the first core 111 may be an active element other than ytterbium. Examples of such active elements include, in addition to ytterbium, thulium (Tm), cerium (Ce), neodymium (Nd), europium (Eu), and erbium (Er). Furthermore, examples of active elements include, in addition to rare earth elements, bismuth (Bi).

[0035] The first cladding 112 is made of, for example, pure quartz with no dopant added, or quartz with a dopant such as fluorine added to reduce the refractive index. The outer cladding 113 is made of, for example, a resin with a lower refractive index than the first cladding 112, or quartz with an element such as fluorine added to reduce the refractive index. The coating layer 114 is made of, for example, a thermosetting or ultraviolet-curing resin, and if the outer cladding 113 is made of resin, the coating layer 114 is made of a resin different from that of the outer cladding 113. The first cladding 112 may be made of multiple layers with different refractive indices or dopants.

[0036] The optical fiber 14 connected to one end of the first amplification optical fiber 11 has the same configuration as the first amplification optical fiber 11, except that the core is not doped with an active element. Therefore, the optical fiber 14 has a core, an inner cladding surrounding the core, an outer cladding surrounding the inner cladding, and a coating layer covering the outer cladding. The first core 111 of the first amplification optical fiber 11 is connected to the core of the optical fiber 14, and the first cladding 112 of the first amplification optical fiber 11 is connected to the inner cladding of the optical fiber 14.

[0037] An HR-FBG 16 is provided in the core of the optical fiber 14. The HR-FBG 16 is provided on one end of the first amplification optical fiber 11 and optically connected to the first core 111. The FBG is configured by repeating high-refractive-index regions and lower-refractive-index regions at a constant period along the longitudinal direction of the core, and is capable of reflecting light of a predetermined wavelength and transmitting light of other wavelengths. By adjusting this period, the HR-FBG 16 reflects at least light of a first wavelength emitted by the active element of the first amplification optical fiber 11 in an excited state. The first wavelength is longer than the wavelength of the first pumping light. When the active element doped in the first amplification optical fiber 11 is ytterbium as described above, the first wavelength is, for example, 1020 nm, and the HR-FBG 16 reflects light of the first wavelength with a reflectance of, for example, 99% or more.

[0038] In the combiner 13, the core of each optical fiber 121 is optically connected to the inner cladding of the optical fiber 14. Therefore, the first pumping light emitted from the first pumping light source 12 can propagate to the first cladding 112 of the first amplification optical fiber 11 via the optical fiber 121, the combiner 13, and the inner cladding of the optical fiber 14.

[0039] One end of an optical fiber 15 is connected to the other end of the first amplification optical fiber 11. The optical fiber 15 is an optical fiber having a core and a cladding. A first core 111 of the first amplification optical fiber 11 and the core of the optical fiber 15 are connected, and a first cladding 112 of the first amplification optical fiber 11 and the cladding of the optical fiber 15 are connected.

[0040] An OC-FBG 17 is provided in the core of the optical fiber 15. In this way, the OC-FBG 17 is provided on the other end side of the first amplification optical fiber 11 and is optically connected to the first core 111. The OC-FBG 17 is configured to reflect the light of the first wavelength reflected by the HR-FBG 16 with a reflectance lower than that of the HR-FBG 16. The OC-FBG 17 is configured to reflect the light of the first wavelength with a reflectance of, for example, 10%. In this way, the HR-FBG 16, the first amplification optical fiber 11, and the OC-FBG 17 form a resonator that resonates with the light of the first wavelength. A portion of the light of the first wavelength that resonates in this resonator is transmitted through the OC-FBG 17.

[0041] The excitation light source 10 having the above configuration emits excitation light, which is light of the first wavelength.

[0042] The other end of the optical fiber 15 is connected to one end of an optical fiber 19 having the same configuration as the optical fiber 15. The optical fiber 15 may be extended to form the optical fiber 19. The other end of the optical fiber 19 is connected to the combiner 31 or the combiner 32.

[0043] A light reducing unit 60 is provided midway along the optical fiber 19, i.e., on the optical path between the combiners 31, 32 and the first amplification optical fiber 11. The light reducing unit 60 of this embodiment is a filter that transmits light of a first wavelength and suppresses transmission of light of a second wavelength, which is longer than the first wavelength. In this embodiment, the filter is composed of an FBG 61. The FBG 61 constituting the light reducing unit 60 reflects light of the second wavelength and transmits light of the first wavelength. In the FBG 61 of the light reducing unit 60 of this embodiment, the interface between a high-refractive-index region and a lower-refractive-index region, which are repeatedly formed at a constant interval, is approximately perpendicular to the longitudinal direction of the core of the optical fiber 19. Therefore, the light reducing unit 60 of this embodiment reflects light of the second wavelength incident on the light reducing unit 60 in a direction opposite to the traveling direction of the light of the second wavelength incident on the light reducing unit 60. The second wavelength is, for example, 1064 nm when the active element doped in the second amplification optical fiber 21 is ytterbium, as described below. The FBG 61 constituting the light reducing section 60 reflects the light of the second wavelength with a reflectance of, for example, 99% or more.

[0044] Furthermore, in this embodiment, cladding mode strippers 70 are provided between the light reducing unit 60, which is a filter, and the combiner 31, and between the light reducing unit 60 and the combiner 32. The cladding mode stripper 70 is a member that causes light propagating through the cladding of the optical fiber 19 to leak out of the cladding. The configuration of the cladding mode stripper 70 is not particularly limited, and examples include providing a member having a refractive index equal to or higher than the refractive index of the cladding of the optical fiber 19 so as to be in contact with the cladding of the optical fiber 19. Note that the cladding mode stripper 70 may be provided in other positions as long as it removes light propagating through the cladding of the optical fiber 19. Providing such a cladding mode stripper 70 can suppress light from propagating through the cladding of the optical fiber 19 from the combiners 31, 32 to the first amplification optical fiber 11. This can suppress a decrease in the light amplification efficiency of the pumping light source 10 and damage to the first pumping light source 12 in the pumping light source 10.

[0045] The second amplification optical fiber 21 in the optical amplifier PA is an amplification optical fiber that amplifies and outputs light emitted from the seed light source MO. Therefore, the seed light source MO side of the second amplification optical fiber 21 is the rear side, and the side opposite the seed light source MO side of the second amplification optical fiber 21 is the front side. The direction from the rear side to the front side is the direction in which light emitted from the fiber laser device 1 propagates, and therefore this direction is the forward direction.

[0046] Although not particularly shown, the second amplification optical fiber 21 has the same configuration as the first amplification optical fiber 11. That is, the second amplification optical fiber 21 mainly comprises a second core having the same configuration as the first core 111, a second cladding as an inner cladding having the same configuration as the first cladding 112 and tightly surrounding the outer peripheral surface of the second core, an outer cladding having the same configuration as the outer cladding 113 and surrounding the outer peripheral surface of the second cladding, and a coating layer that coats the outer cladding. The second core is doped with the same active element as the active element doped in the first core.

[0047] The combiner 31 is provided at one end of the second amplification optical fiber 21, which is on the seed light source MO side, and the combiner 32 is provided at the other end of the second amplification optical fiber 21. The combiners 31 and 32 are, for example, made of optical fibers having a core and a cladding surrounding the core. In this case, the cores of the combiners 31 and 32 are connected to the second core of the second amplification optical fiber 21, and the claddings of the combiners 31 and 32 are connected to the second cladding of the second amplification optical fiber 21. Note that when the combiners 31 and 32 have this configuration, the combiners 31 and 32 may be tapered fibers having a smaller diameter on the second amplification optical fiber 21 side and a larger diameter on the side opposite to the second amplification optical fiber 21 side. As described above, each optical fiber 19 is connected to the combiner 31 or the combiner 32. Specifically, the cores of some of the optical fibers 19 among the respective optical fibers 19 are optically connected to the second clad of the second amplification optical fiber 21 in the combiner 31, and the cores of the other part of the optical fibers 19 are optically connected to the second clad of the second amplification optical fiber 21 in the combiner 32. When the combiners 31 and 32 are made of optical fibers as described above, the cores of some of the optical fibers 19 are connected to the clad of the combiner 31, and the cores of the other part of the optical fibers 19 are connected to the clad of the combiner 32. It is preferable that the number of pumping light sources 10 optically connected to the second clad via the optical fibers 19 and the combiner 31 is equal to the number of pumping light sources 10 optically connected to the second clad via the optical fibers 19 and the combiner 32, from the viewpoint of enabling pumping light of greater power to be incident on the second amplification optical fiber 21. The pumping light source 10 optically connected to the second clad via the combiner 31 is a forward pumping light source that introduces first light as pumping light from the rear side toward the front side of the second amplification optical fiber 21, and the pumping light source 10 optically connected to the second clad via the combiner 32 is a backward pumping light source that introduces first light as pumping light from the front side toward the rear side of the second amplification optical fiber 21. The configuration of the combiners 31 and 32 is not limited to the above, and for example, the core of the optical fiber 19 may be directly connected to the second clad of the second amplification optical fiber 21, and this connection may be the combiners 31 and 32.

[0048] One end of a delivery fiber 40 having a core and a cladding is connected to the combiner 32. The core of the delivery fiber 40 is optically connected to the second core of the second amplification optical fiber 21 in the combiner 32.

[0049] An output section 41 is connected to the other end of the delivery fiber 40. The output section 41 is made of, for example, a glass rod having a diameter larger than that of the core of the delivery fiber 40. Therefore, the light incident on the output section 41 from the core of the delivery fiber 40 has its diameter expanded before it is output from the output section 41.

[0050] Next, the seed light source MO will be described.

[0051] 4 is a diagram showing an example of a seed light source MO. As shown in Fig. 4, the seed light source MO mainly comprises a third amplification optical fiber 51, a second pumping light source 52, an HR-FBG 56, and an OC-FBG 57. The seed light source MO has a configuration generally similar to that of the pumping light source 10.

[0052] Although not specifically shown, the third amplification optical fiber 51 has a similar configuration to the second amplification optical fiber 21. That is, the third amplification optical fiber 51 mainly comprises a third core having a similar configuration to the second core, a third cladding as an inner cladding having a similar configuration to the second cladding and tightly surrounding the outer peripheral surface of the third core, an outer cladding having a similar configuration to the outer cladding of the second amplification optical fiber 21 and surrounding the outer peripheral surface of the third cladding, and a coating layer that coats the outer cladding. The third core is doped with the same active element as the active element doped in the second core. That is, in this embodiment, the first core 111 of the first amplification optical fiber 11, the second core of the second amplification optical fiber 21, and the third core of the third amplification optical fiber 51 are each doped with the same active element. From a structural standpoint, the third amplification optical fiber 51 is a double-clad optical fiber. Note that the third core of the third amplification optical fiber 51 propagates light in, for example, a 2LP mode or less.

[0053] An optical fiber 54 is connected to one end of the third amplification optical fiber 51. The optical fiber 54 has the same configuration as the third amplification optical fiber 51, except that the core is not doped with an active element. Therefore, the optical fiber 54 has a core, an inner cladding surrounding the core, an outer cladding surrounding the inner cladding, and a coating layer covering the outer cladding. The third core of the third amplification optical fiber 51 is connected to the core of the optical fiber 54, and the third cladding of the third amplification optical fiber 51 is connected to the inner cladding of the optical fiber 54.

[0054] An HR-FBG 56 is provided in the core of the optical fiber 54. In this way, the HR-FBG 56 is provided on one end side of the third amplifying optical fiber 51, and is optically connected to the third core of the third amplifying optical fiber 51. The HR-FBG 56 reflects light of the second wavelength. As described above, the second wavelength is, for example, 1064 nm, and the HR-FBG 56 reflects light of the second wavelength with a reflectance of, for example, 99% or more.

[0055] The second pumping light source 52 is composed of a plurality of laser diodes 52D similar to the plurality of laser diodes 12D of the first pumping light source 12, and emits second pumping light of a wavelength that excites the active element doped in the third amplification optical fiber 51. The power of the second pumping light is, for example, the same as the power of the first pumping light emitted from the first pumping light source 12, and the wavelength of the second pumping light is, for example, the same as the wavelength of the first pumping light emitted from the first pumping light source 12. The second pumping light source 52 is connected to a predetermined power supply and emits second pumping light of a power corresponding to the current from the power supply. It is preferable that this power supply be capable of varying the output current. Each laser diode 52D of the second pumping light source 52 is connected to one end of an optical fiber 521. The optical fiber 521 has a configuration similar to that of the optical fiber 121. The other end of each optical fiber 521 is connected to a combiner 53. The power of the second excitation light does not have to be the same as the power of the first excitation light emitted from the first excitation light source 12, and the wavelength of the second excitation light does not have to be the same as the wavelength of the first excitation light emitted from the first excitation light source 12.

[0056] The combiner 53 is connected to an end of the optical fiber 54 opposite to the end connected to the third amplification optical fiber 51. In the combiner 53, the core of each optical fiber 521 is optically connected to the inner clad of the optical fiber 54. Therefore, the pumping light emitted from the second pumping light source 52 can propagate to the third clad of the third amplification optical fiber 51 via the core of the optical fiber 521 and the inner clad of the optical fiber 54. In this way, the second pumping light source 52 is a light source that introduces the second pumping light from the HR-FBG 56 side into the third clad of the third amplification optical fiber 51.

[0057] One end of an optical fiber 55 is connected to the other end of the third amplification optical fiber 51. The optical fiber 55 has a configuration similar to that of the optical fiber 54, and an OC-FBG 57 is provided in the core of the optical fiber 54. In this way, the OC-FBG 57 is provided on the other end side of the third amplification optical fiber 51 and is optically connected to the third core of the third amplification optical fiber 51. The OC-FBG 57 reflects the light of the second wavelength with a reflectance lower than that of the HR-FBG 56, for example, 10%. In this way, a resonator is formed by the HR-FBG 56, the third amplification optical fiber 51, and the OC-FBG 57. A portion of the light of the second wavelength that resonates in this resonator passes through the OC-FBG 57 and is emitted from the seed light source MO.

[0058] One end of an optical fiber 35 having a configuration similar to that of the optical fiber 55 is connected to the optical fiber 55 of the seed light source MO. Note that the optical fiber 55 may be extended to form the optical fiber 35. The optical fiber 35 propagates the light of the second wavelength emitted from the seed light source MO. The other end of the optical fiber 35 is optically connected to the second amplification optical fiber 21 in the combiner 31, and the core of the optical fiber 35 is optically connected to the second core of the second amplification optical fiber 21. Therefore, the light of the second wavelength emitted from the seed light source MO can be incident on the second core of the second amplification optical fiber 21 via the core of the optical fiber 35.

[0059] Next, the operation of the fiber laser device 1 will be described.

[0060] First, in the seed light source MO, second pumping light is emitted from each laser diode 52D of the second pumping light source 52. As described above, the wavelength of this pumping light is, for example, 976 nm. The second pumping light emitted from the second pumping light source 52 travels from the optical fiber 521 through the combiner 53 and the optical fiber 54, enters the third cladding, which is the inner cladding of the third amplification optical fiber 51, from one end of the third amplification optical fiber 51, and mainly propagates through the third cladding. At least a portion of the second pumping light propagating through the third cladding is absorbed by the active element doped in the third core as it passes through the third core surrounded by the third cladding. This excites the active element. The light emitted by the excited active element causes the light of the second wavelength to resonate between the HR-FBG 56 and the OC-FBG 57 and be amplified. A portion of the light of the second wavelength passes through the OC-FBG 57 and is emitted as seed light from the seed light source MO. In this way, the seed light of the second wavelength is emitted from the seed light source MO. As described above, the second wavelength is longer than the first wavelength, and is, for example, 1064 nm.

[0061] Furthermore, in each pumping light source 10, first pumping light is emitted from each laser diode 12D of the first pumping light source 12. As described above, the wavelength of this pumping light is, for example, 976 nm. The first pumping light emitted from the first pumping light source 12 travels from the optical fiber 121 through the combiner 13 and the optical fiber 14, enters the first clad 112, which is the inner clad of the first amplification optical fiber 11, from one end of the first amplification optical fiber 11, and propagates mainly through the first clad 112. At least a portion of the first pumping light propagating through the first clad 112 is absorbed by the active element doped in the first core 111 as it passes through the first core 111 surrounded by the first clad 112, and the active element is excited. The light emitted by the excited active element resonates between the HR-FBG 16 and the OC-FBG 17, and is amplified. A portion of the light of the first wavelength passes through the OC-FBG 17 and is emitted as pump light from the pump light source 10. In this way, the pump light of the first wavelength is emitted from the pump light source 10. As described above, the first wavelength is a wavelength longer than the wavelength of the first pump light, and is, for example, 1020 nm.

[0062] The seed light of the second wavelength emitted from the seed light source MO passes through the optical fiber 35, enters the combiner 31, and enters the second core of the second amplification optical fiber 21, and propagates through the second core. Furthermore, the pumping light of the first wavelength emitted from each pumping light source 10 passes through the optical fiber 19 and the light reducing unit 60, enters the second clad of the second amplification optical fiber 21, and enters the combiners 31 and 32, and propagates through the second clad. The pumping light propagating through the second clad is absorbed by the active element doped in the second core when passing through the second core surrounded by the second clad. This excites the active element. The excited active element causes stimulated emission due to the seed light propagating through the second core, and the seed light is amplified. In other words, the light of the second wavelength is amplified.

[0063] The amplified light of the second wavelength is output from the combiner 32 via the delivery fiber 40 and output from the output unit 41. In this way, the light of the second wavelength is output from the fiber laser device 1.

[0064] Incidentally, a portion of the light of the second wavelength amplified by the second amplification optical fiber 21 may propagate to the optical fiber 19 due to reflection or the like at the combiner 32. Furthermore, the light of the second wavelength may be reflected when it is emitted from the emission portion 41, or the light of the second wavelength emitted from the emission portion 41 may be reflected by the workpiece, causing the light of the second wavelength to enter the second amplification optical fiber 21 as reflected light and propagate backward. In this case, the light of the second wavelength propagating backward may propagate from the combiner 31 to the optical fiber 19. However, a light reducing unit 60 is provided on the optical path of each optical fiber 19. The light reducing unit 60 suppresses transmission of the light of the second wavelength. This prevents the light of the second wavelength from propagating to the first amplification optical fiber 11 of the pumping light source 10. In addition, in this embodiment, the light reducing unit 60 is made of an FBG 61. As described above, the FBG 61 reflects the light of the second wavelength. Therefore, the light of the second wavelength propagating from the combiners 31 and 32 toward the pumping light source 10 is reflected by the light reducing section 60 toward the combiners 31 and 32 .

[0065] Furthermore, even if the light of the second wavelength reflected by the light reduction section 60 leaks into the clad of the optical fiber 19, the clad mode stripper 70 causes at least a portion of the light to leak outside the optical fiber 19 and be removed from the optical fiber 19.

[0066] As described above, the fiber laser device 1 of this embodiment includes the first pumping light source 12 that emits first pumping light, the first core 111 that is doped with an active element that absorbs the first pumping light, and the first cladding 112 that surrounds the first core 111 and into which the first pumping light is incident, and the first amplification optical fiber 11 that amplifies and emits light of a first wavelength in the first core 111, a second core that absorbs light of the first wavelength and is doped with the same active element as the active element doped in the first amplification optical fiber 11, and The optical fiber includes a second amplification optical fiber (21) that includes a second cladding that surrounds the second core and amplifies and emits light of a second wavelength in the second core; combiners (31, 32) that optically connect the second cladding and the first core (111) and cause light of the first wavelength to enter the second cladding; and a light reduction section (60) that is provided on the optical path between the combiners (31, 32) and the first amplification optical fiber (11) and transmits light from the first amplification optical fiber (11) and suppresses transmission of light from the combiners (31, 32).

[0067] In this way, the active element doped in the first amplification optical fiber 11 and the active element doped in the second amplification optical fiber 21 are the same, and the first pumping light is converted to light of the first wavelength using the first amplification optical fiber 11, so the first wavelength can be made closer to the second wavelength than the wavelength of the first pumping light. Therefore, according to the fiber laser device 1 of this embodiment, compared to a case in which the first pumping light is incident on the second amplification optical fiber 21, it is possible to suppress the occurrence of a quantum defect in the second amplification optical fiber 21 and to suppress deterioration in the beam quality of light of the second wavelength output from the second amplification optical fiber 21. Furthermore, in this fiber laser device, as described above, even when light of the second wavelength amplified in the second amplification optical fiber 21 travels toward the first amplification optical fiber 11 via the combiners 31 and 32, the light reducing unit 60 reduces the light from the combiners 31 and 32. This can suppress propagation of light of the second wavelength into the first core 111 of the first amplification optical fiber 11, and can suppress a decrease in the efficiency of amplification of light of the first wavelength in the first amplification optical fiber 11. Therefore, it can suppress a decrease in the power of light of the first wavelength that is incident as pumping light on the second amplification optical fiber 21, and can suppress a decrease in the amplification efficiency of light of the second wavelength in the second amplification optical fiber 21. Therefore, the fiber laser device 1 of this embodiment can emit high-power light.

[0068] From the viewpoint of suppressing quantum defects, the difference between the first wavelength and the second wavelength is preferably 50 nm or less, and more preferably 45 nm or less. Furthermore, in the light reducing unit 60 using the FBG 61, from the viewpoint of transmitting light of the first wavelength and suppressing transmission of light of the second wavelength, the difference between the first wavelength and the second wavelength is preferably 5 nm or more, and more preferably 10 nm or more.

[0069] Second Embodiment Next, a second embodiment of the present invention will be described in detail. Note that components that are the same as or equivalent to those in the first embodiment will be assigned the same reference numerals and redundant descriptions will be omitted unless otherwise specified.

[0070] Fig. 5 is a diagram showing a fiber laser device according to this embodiment. As shown in Fig. 5, the fiber laser device 1 according to this embodiment differs from the fiber laser device 1 according to the first embodiment mainly in that it does not include a seed light source MO, but includes an HR-FBG 26 and an OC-FBG 27. In other words, the fiber laser device 1 according to this embodiment is a resonator-type fiber laser device.

[0071] In this embodiment, an optical fiber 24 is connected to one end of the second amplification optical fiber 21, and the end of the optical fiber 24 opposite to the second amplification optical fiber 21 side is connected to a combiner 31. The optical fiber 24 has a configuration similar to that of the second amplification optical fiber 21, except that an active element is not doped in the core. Therefore, the optical fiber 24 has a core, an inner cladding surrounding the core, an outer cladding surrounding the inner cladding, and a coating layer that coats the outer cladding. The second core of the second amplification optical fiber 21 is connected to the core of the optical fiber 24, and the second cladding of the second amplification optical fiber 21 is connected to the inner cladding of the optical fiber 24. Furthermore, in this embodiment, in the combiner 31, the cores of some of the optical fibers 19 are optically connected to the inner cladding of the optical fiber 25.

[0072] An HR-FBG 26 serving as a first mirror is provided in the core of the optical fiber 24. In this way, the HR-FBG 26 is provided on one end side of the second amplifying optical fiber 21, and is optically connected to the second core of the second amplifying optical fiber 21. The HR-FBG 26 reflects light of the second wavelength described above. As described above, the second wavelength is, for example, 1064 nm, and the HR-FBG 26 reflects light of the second wavelength with a reflectance of, for example, 50 to 99%.

[0073] An optical fiber 25 is connected to the other end of the second amplification optical fiber 21, and the end of the optical fiber 25 opposite to the second amplification optical fiber 21 side is connected to a combiner 32. The optical fiber 25 has a configuration similar to that of the optical fiber 24, and the second core of the second amplification optical fiber 21 is connected to the core of the optical fiber 25, and the second clad of the second amplification optical fiber 21 is connected to the inner clad of the optical fiber 25. In this embodiment, the cores of some of the other optical fibers 19 are optically connected to the inner clad of the optical fiber 25 in the combiner 32.

[0074] An OC-FBG 27 serving as a second mirror is provided in the core of the optical fiber 25. In this way, the OC-FBG 27 is provided on the other end side of the second amplifying optical fiber 21, and is optically connected to the second core of the second amplifying optical fiber 21. The OC-FBG 27 reflects light of the second wavelength with a reflectance lower than that of the HR-FBG 26, for example, 10%. In this way, the HR-FBG 26, the second amplifying optical fiber 21, and the OC-FBG 27 form a resonator.

[0075] Next, the operation of the fiber laser device 1 of this embodiment will be described.

[0076] As in the fiber laser device 1 of the first embodiment, pumping light of a first wavelength is emitted from each pumping light source 10. Pumping light emitted from some of the pumping light sources 10 passes through a combiner 31 and the inner clad of the optical fiber 24 and enters the second clad of the second amplification optical fiber 21. Pumping light emitted from other of the pumping light sources 10 passes through a combiner 32 and the inner clad of the optical fiber 25 and enters the second clad of the second amplification optical fiber 21.

[0077] At least a portion of the pumping light of the first wavelength propagating through the second cladding of the second amplification optical fiber 21 is absorbed by the active element doped in the second core as it passes through the second core surrounded by the second cladding. This excites the active element, and the light emitted by the excited active element causes the light of the second wavelength to resonate between the HR-FBG 26 and the OC-FBG 27 and be amplified. A portion of the light of the second wavelength is transmitted through the OC-FBG 27. The light of the second wavelength that has passed through the OC-FBG 27 is emitted from the combiner 32 via the delivery fiber 40 and the output unit 41. In this way, the light of the second wavelength is emitted from the fiber laser device 1.

[0078] Incidentally, in the present embodiment as well, similar to the first embodiment, light of the second wavelength may propagate from the combiners 31, 32 to the optical fiber 19. However, in the present embodiment as well, the light reducing section 60 made of the FBG 61 is provided on the optical path of each optical fiber 19, and therefore, similar to the first embodiment, the light reducing section 60 suppresses transmission of light of the second wavelength. Therefore, in the present embodiment as well, propagation of light of the second wavelength to the first amplification optical fiber 11 of the pumping light source 10 is suppressed.

[0079] As described above, the fiber laser device 1 of this embodiment can also emit high-power light in the same manner as the fiber laser device 1 of the first embodiment.

[0080] (Modifications) Next, modifications of the light reduction unit 60 will be described. Note that components that are the same as or equivalent to those in the above embodiment will be assigned the same reference numerals and redundant description will be omitted unless otherwise specified. Note that in the following modifications, one optical path from the combiner 32 to the excitation light source 10 will be described as an example, but other optical paths from the combiner 32 to the excitation light source 10 and the optical path from the combiner 31 to the excitation light source 10 are also configured in the same way.

[0081] (First Modification) FIG. 6 is a diagram illustrating this modification. As shown in FIG. 6 , the light reduction unit 60 of this modification is a filter that transmits light of a first wavelength and suppresses transmission of light of a second wavelength longer than the first wavelength, similar to the above embodiment. The filter is an FBG, but differs from the above embodiment in that the FBG is a slant type. In the slant FBG 62, the interface between a high-refractive-index region and a lower-refractive-index region, which are repeatedly formed at a constant interval, is oblique to the longitudinal direction of the core of the optical fiber 19. Therefore, the light reduction unit 60 of this modification reflects light of the second wavelength incident on the light reduction unit 60 in a direction different from the direction opposite to the traveling direction of the light of the second wavelength incident on the light reduction unit 60.

[0082] Furthermore, in this modification, a cladding mode stripper 70 is provided midway along the optical fiber 19 between the light reducing unit 60, which is a filter, and the pumping light source 10. In this modification, the slant FBG 62 can reflect the light of the second wavelength that is incident on the light reducing unit 60 in a desired direction. When the slant FBG 62 is used as a filter, the light of the second wavelength may be incident on the cladding of the optical fiber 19. However, in this modification, the cladding mode stripper 70 can cause the light propagating through the cladding of the optical fiber 19 to leak out of the cladding. Therefore, it is possible to further prevent the light propagating through the cladding from being incident on the first amplification optical fiber 11.

[0083] The angle of the interface in the slant FBG 62 may be adjusted so that, when light of the second wavelength is incident on the light reducing section 60, the light of the second wavelength reflected by the slant FBG 62 leaks from the cladding of the optical fiber 19. For example, by setting the angle to 45 degrees with respect to the longitudinal direction of the optical fiber 19, even when light of the second wavelength is incident on the light reducing section 60, the light can be reflected in a direction at 90 degrees with respect to the longitudinal direction of the optical fiber 19, and the light of the second wavelength can be leaked to the outside of the optical fiber 19 in the light reducing section 60.

[0084] (Second Modification) Fig. 7 is a diagram showing this modification. As shown in Fig. 7, the light reducing unit 60 of this modification is a filter that transmits light of a first wavelength and suppresses transmission of light of a second wavelength that is longer than the first wavelength, as in the above embodiment, but the filter is made of a photonic bandgap fiber 63. Therefore, in this modification, the optical fiber 19 is divided into a first optical fiber 19a connected to the pumping light source 10 and a second optical fiber 19b connected to the combiner 32, and the light reducing unit 60 made of the photonic bandgap fiber 63 is connected between the first optical fiber 19a and the second optical fiber 19b.

[0085] 8 is a diagram showing a photonic bandgap fiber 63. The photonic bandgap fiber 63 includes a core 631, a plurality of high-refractive-index members 633 arranged to surround the core, and a cladding 632 tightly surrounding the core 631 and each of the high-refractive-index members 633. The photonic bandgap fiber 63 has excellent filtering performance, transmitting light of a desired wavelength and blocking light of other wavelengths. In this modification, when light of a first wavelength is incident on the core 631 of the photonic bandgap fiber 63, the light of the first wavelength propagates through the core 631, and when light of a second wavelength is incident on the core 631, the light of the second wavelength leaks out of the core 631. Therefore, this modification can further suppress propagation of light of the second wavelength through the first amplification optical fiber 11.

[0086] When light of the second wavelength is incident on the light reducing section 60, the light of the second wavelength may leak from the photonic bandgap fiber 63 into the cladding of the first optical fiber 19 a. However, even in this case, the cladding mode stripper 70 can cause the light propagating through the cladding of the first optical fiber 19 a, which is part of the optical fiber 19, to leak out of the cladding. Therefore, it is possible to further prevent the light propagating through the cladding from entering the first amplification optical fiber 11.

[0087] When a photonic bandgap fiber 63 is used as a filter as in this modified example, from the viewpoint of transmitting light of the first wavelength and suppressing transmission of light of the second wavelength in the light reduction section 60, the difference between the first wavelength and the second wavelength is preferably 5 nm or more, and more preferably 10 nm or more.

[0088] (Third Modification) Fig. 9 is a diagram showing this modification. As shown in Fig. 9, this modification differs from the second modification in that a first optical fiber 19a connected to the pumping light source 10 and a second optical fiber 19b connected to the combiner 32 are directly connected at a connection portion 64. The first optical fiber 19a has a core 191a and a clad 192a, and the second optical fiber 19b has a core 191b and a clad 192b. Furthermore, in this modification, the diameter of the core 191a of the first optical fiber 19a is larger than the diameter of the core 191b of the second optical fiber 19b. In other words, in this modified example, the optical path between the excitation light source 10 and the combiner 32 includes an optical fiber 19 consisting of a first optical fiber 19a and a second optical fiber 19b, the light reduction section 60 includes a connection section 64 between the first optical fiber 19a and the second optical fiber 19b, and the diameter of the core 191b of the second optical fiber 19b is larger than the diameter of the core 191a of the first optical fiber 19a.

[0089] In this modified example, the above configuration allows a portion of the light of the second wavelength propagating from the combiner 32 to leak from the core 191 b of the second optical fiber 19 b to the outside of the core 191 a of the first optical fiber 19 a at the connection unit 64. Note that at least a portion of the leaking light of the second wavelength may enter the cladding 192 a of the first optical fiber 19 a, but even in this case, at least a portion of the light can be removed by the cladding mode stripper 70. According to this modified example, the light reducing unit 60 is formed by providing a difference in the diameters of the cores of the two optical fibers, and therefore the configuration of the light reducing unit 60 can be simplified.

[0090] 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.

[0091] For example, although a resonator-type fiber laser is used as the pumping light source 10, the pumping light source 10 may have another configuration as long as the first pumping light source 12 and the first amplification optical fiber 11 are used.

[0092] Furthermore, the light reducing section 60 may have another configuration, such as an isolator, as long as it transmits light from the first amplification optical fiber and suppresses transmission of light from the combiner.

[0093] The cladding mode stripper 70 is not an essential component. However, it is preferable that the fiber laser device 1 includes the cladding mode stripper 70, since the unnecessary light of the second wavelength can be removed from the optical fiber 19.

[0094] Furthermore, the fiber laser device 1 of the above embodiment is a dual-pumped fiber laser device that includes the pumping light source 10 optically connected to the second amplification optical fiber 21 via the combiners 31 and 32. However, the fiber laser device 1 of the present invention may be a backward-pumped fiber laser device that does not include the pumping light source 10 optically connected to the second amplification optical fiber 21 via the combiner 31, but includes the pumping light source 10 optically connected to the second amplification optical fiber 21 via the combiner 32, or may be a forward-pumped fiber laser device that does not include the pumping light source 10 optically connected to the second amplification optical fiber 21 via the combiner 32, but includes the pumping light source 10 optically connected to the second amplification optical fiber 21 via the combiner 31.

[0095] As described above, according to the present invention, a fiber laser device capable of emitting high-power light is provided, and it is expected to be used in laser devices for processing, etc.

Claims

1. A fiber laser device comprising: a first excitation light source capable of emitting a first excitation light; a first amplification optical fiber doped with an active element and capable of absorbing the first excitation light and amplifying and emitting light of a first wavelength; a second amplification optical fiber doped with the same active element as the active element doped in the first amplification optical fiber and capable of absorbing the light of the first wavelength as excitation light and amplifying and emitting light of a second wavelength; a combiner capable of inputting the light of the first wavelength into the second amplification optical fiber; and a light reducing section provided on an optical path between the combiner and the first amplification optical fiber, for transmitting light from the first amplification optical fiber and suppressing transmission of light from the combiner.

2. The fiber laser device according to claim 1, characterized in that the light reducing section is a filter that transmits light of the first wavelength and suppresses transmission of light of the second wavelength.

3. The fiber laser device according to claim 2, wherein the filter is a mirror that reflects the light of the second wavelength.

4. The fiber laser device according to claim 3, characterized in that the mirror is an FBG (Fiber Bragg Grating).

5. The fiber laser device according to claim 4, wherein the FBG is a slant type.

6. The fiber laser device according to claim 2, characterized in that the filter is a photonic bandgap fiber.

7. A fiber laser device as described in claim 5 or 6, characterized in that the filter and the first amplification optical fiber are optically connected by an optical fiber including a core and a cladding, and a cladding mode stripper is provided midway along the optical fiber.

8. A fiber laser device as claimed in any one of claims 1 to 7, further comprising: a second excitation light source capable of emitting a second excitation light; and a third amplification optical fiber doped with the same active element as that doped in the second amplification optical fiber, absorbing the second excitation light, and amplifying and emitting light of the second wavelength, wherein the light of the second wavelength emitted from the third amplification optical fiber is incident on one end side of the second amplification optical fiber.

9. A fiber laser device as described in any one of claims 1 to 7, further comprising: a first mirror optically connected to the second amplifying optical fiber at one end thereof and reflecting light of the second wavelength; and a second mirror optically connected to the second amplifying optical fiber at the other end thereof and reflecting light of the second wavelength with a reflectance lower than that of the first mirror.

Images