Fiber laser device

Abstract

To provide a fiber laser apparatus capable of improving beam properties while maintaining light focusing performance of output laser light.SOLUTION: A fiber laser apparatus 1 includes: an amplification optical fiber 10 including a core 11 doped with an active element; a pumping light source 3 capable of emitting pumping light pumping the active element; and a delivery fiber 5 including a core 50 capable of allowing light of a single mode or a few mode to propagate therethrough in a wavelength of laser light generated in the amplification optical fiber 10, the delivery fiber having an output end 6 capable of outputting laser light. The delivery fiber 5 includes: a center core area 61 located in a center at least at the output end 6; an outer core area 62 covering around the center core area 61 and having a refractive index higher than a refractive index of the center core area 61; and a cladding area 63 covering around the outer core area 62 and having a refractive index lower than a refractive index of the outer core area 62.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a fiber laser device, and more particularly to a fiber laser device including a delivery fiber that outputs laser light. [Background technology]

[0002] Fiber lasers have been increasingly used in recent years for cutting, welding, machining, and other processes on metal materials, due to their superior beam quality, ability to produce smaller beam spots, and high power densities compared to carbon dioxide lasers, which have traditionally been used for processing metal materials. However, materials such as copper and aluminum have lower absorptance for light at the oscillation wavelength of fiber lasers than materials such as iron. Therefore, processing these materials requires the irradiation of a beam with a high power density, which makes processing difficult with the conventionally widely used multimode fiber lasers. For this reason, when processing materials such as copper and aluminum, it is considered to use a single-mode fiber laser, which emits a laser beam with a majority of its fundamental mode and can emit a laser beam with a higher power density than a multimode fiber laser (see, for example, Patent Document 1).

[0003] Typically, laser light output from a single-mode fiber laser has a Gaussian beam profile, as shown in Figure 4A. In such a Gaussian beam profile, the power density is particularly high in the center of the beam. However, the high power density in the center makes it more likely that metal melted by the laser beam irradiated during laser processing will further evaporate and scatter around the processing spot (a phenomenon known as sputtering). Such sputtering reduces the strength of the metal material and results in poor appearance, degrading the quality of the laser processing. Therefore, there is a need for technology that can improve the beam characteristics of the laser light output from a fiber laser while maintaining high focusing performance and preventing the power of the laser light from becoming excessively high in the center. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-139857 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in consideration of the problems of the conventional technology, and aims to provide a fiber laser device that can improve beam characteristics while maintaining high focusing of the output laser light. [Means for solving the problem]

[0006] According to one aspect of the present invention, there is provided a fiber laser device capable of improving beam characteristics while maintaining high focusing of output laser light. The fiber laser device includes an amplification optical fiber including a core doped with an active element, a pumping light source capable of emitting pumping light for exciting the active element, and a delivery fiber including a core capable of propagating single-mode or multi-mode light at the wavelength of laser light generated in the amplification optical fiber and having an output end capable of outputting the laser light. The delivery fiber has a multi-step refractive index structure at least at the output end, which is composed of multiple regions whose refractive index varies along a radial direction. The multi-step refractive index structure includes: a central core region located at the center of the multi-step refractive index structure and having a first refractive index; an outer core region surrounding the central core region and having a second refractive index higher than the first refractive index; and a cladding region surrounding the outer core region and having a third refractive index lower than the first refractive index. The relative refractive index difference of the outer core region with respect to the cladding region is 0.1% to 0.15%. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram schematically illustrating the configuration of a fiber laser device according to a first embodiment of the present invention. [Figure 2]FIG. 2 is a cross-sectional view schematically showing the structure of an amplification optical fiber in the fiber laser device shown in FIG. [Figure 3] FIG. 3 is a cross-sectional view schematically showing the structure of the delivery fiber in the fiber laser device shown in FIG. 1 together with the refractive index. [Figure 4A] FIG. 4A is a diagram schematically illustrating an example of a beam profile of laser light output from a single-mode fiber laser. [Figure 4B] FIG. 4B is a diagram schematically illustrating an example of a beam profile of the laser light output from the fiber laser device illustrated in FIG. [Figure 4C] FIG. 4C is a diagram schematically showing another example of the beam profile of the laser light output from the fiber laser device shown in FIG. [Figure 5] FIG. 5 is a schematic diagram for explaining differences in beam profiles of laser beams having the same power. [Figure 6] FIG. 6 is a diagram schematically illustrating a modification of the fiber laser device shown in FIG. [Figure 7] FIG. 7 is a diagram schematically illustrating the configuration of a fiber laser device according to the second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of a fiber laser device according to the present invention will be described in detail with reference to FIGS. 1 to 7. In FIGS. 1 to 7, identical or corresponding components are denoted by the same reference numerals, and redundant description will be omitted. In addition, in FIGS. 1 to 7, the scale and dimensions of each component may be exaggerated or some components may be omitted. In the following description, unless otherwise specified, terms such as "first" and "second" are used only to distinguish components from one another and do not indicate a specific order or sequence.

[0009] FIG. 1 is a diagram schematically illustrating the configuration of a fiber laser device 1 according to a first embodiment of the present invention. As shown in FIG. 1, the fiber laser device 1 includes an optical resonator 2, a plurality of pumping light sources 3 that supply pumping light to the optical resonator 2 from one end (upstream side) of the optical resonator 2, an optical combiner 4 that combines the pumping light output from the plurality of pumping light sources 3 and introduces the combined light into the optical resonator 2, and a delivery fiber 5 that is connected to the other end (downstream side) of the optical resonator 2. The delivery fiber 5 has an output end 6 that can output laser light L. In this specification, unless otherwise specified, the direction in which the laser light propagates from the optical resonator 2 toward the output end 6 of the delivery fiber 5 will be referred to as the "downstream side," and the opposite direction will be referred to as the "upstream side."

[0010] The optical resonator 2 includes an amplifying optical fiber 10 capable of amplifying laser light, a high-reflection portion 21 that reflects light in a predetermined wavelength band (e.g., 1070 nm) with a high reflectance (e.g., a reflectance close to 100%), and a low-reflection portion 22 that reflects light of this wavelength with a lower reflectance (e.g., a reflectance of 10%) than the high-reflection portion 21. The high-reflection portion 21 and the low-reflection portion 22 are formed, for example, by a fiber Bragg grating (FBG) or a mirror formed by periodically changing the refractive index of an optical fiber along the propagation direction of the light. In the example shown in FIG. 1, the high-reflection portion 21 and the low-reflection portion 22 are formed by a fiber Bragg grating.

[0011] The high-reflection portion 21 and the amplification optical fiber 10 are fusion-spliced ​​to each other at a fusion splice portion 31, and the high-reflection portion 21 and the optical fiber 4A of the optical combiner 4 are fusion-spliced ​​to each other at a fusion splice portion 32. The low-reflection portion 22 and the amplification optical fiber 10 are fusion-spliced ​​to each other at a fusion splice portion 33, and the low-reflection portion 22 and the delivery fiber 5 are fusion-spliced ​​to each other at a fusion splice portion 34.

[0012] FIG. 2 is a cross-sectional view schematically showing the structure of an amplification optical fiber 10. As shown in FIG. 2, the amplification optical fiber 10 has a core 11, an inner cladding layer 12 that covers the periphery of the core 11, and an outer cladding layer 13 that covers the periphery of the inner cladding layer 12. The core 11 is formed, for example, by doping quartz with an element such as aluminum that increases the refractive index, and further doping at least a portion of the core 11 with an active element. Examples of the active element that can be doped into the core 11 include rare earth elements such as ytterbium (Yb), erbium (Er), thulium (Tm), and neodymium (Nd), bismuth (Bi), and chromium (Cr). In this embodiment, an example in which Yb is doped into the core 11 of the amplification optical fiber 10 will be described, but the present invention is not limited to this.

[0013] The inner cladding layer 12 is made of, for example, quartz to which no dopant is added. The refractive index of the inner cladding layer 12 is lower than that of the core 11, and an optical waveguide is formed inside the core 11. The outer cladding layer 13 is made of, for example, an ultraviolet curing resin. The refractive index of the outer cladding layer 13 is lower than that of the inner cladding layer 12, and an optical waveguide is also formed inside the inner cladding layer 12.

[0014] Each of the pumping light sources 3 includes a Fabry-Perot type semiconductor laser element made of, for example, a GaAs-based semiconductor, and generates pumping light with a center wavelength of, for example, 915 nm. An optical fiber 3A extending from each of the pumping light sources 3 is fusion-spliced ​​to an optical fiber 4B of the optical combiner 4 at a fusion splice 35. The optical combiner 4 is configured to combine the pumping light output from the multiple pumping light sources 3 and introduce this pumping light into the inner cladding layer 12 of the amplification optical fiber 10.

[0015] Fig. 3 is a cross-sectional view schematically showing the structure of the delivery fiber 5 together with the refractive index. As shown in Fig. 3, the delivery fiber 5 has a core 50, a cladding 51 that surrounds the core 50, and a coating layer 52 that surrounds the cladding 51. The refractive index of the cladding 51 is lower than that of the core 50, and an optical waveguide is formed inside the core 50. For example, the core 50 is formed by adding an element such as aluminum that increases the refractive index to quartz, and the cladding 51 is made of quartz. The refractive index of the cladding 51 is, for example, 1.45.

[0016] The delivery fiber 5 in this embodiment has a multi-step refractive index structure 60 composed of a plurality of regions whose refractive index changes along the radial direction. Specifically, as shown in Fig. 3, the delivery fiber 5 has a multi-step refractive index structure 60 composed of a central core region 61 located at the center of the core 50 and having a refractive index equal to or higher than that of the cladding 51, an outer core region 62 surrounding the central core region 61 and having a refractive index higher than that of the central core region 61, and a cladding region 63 surrounding the outer core region 62 and having a refractive index lower than that of the central core region 61. For example, the relative refractive index difference of the central core region 61 with respect to the cladding region 63 is 0.01% to 0.09%, and the relative refractive index difference of the outer core region 62 with respect to the cladding region 63 is 0.1% to 0.15%.

[0017] In the optical resonator 2, the pumping light propagating through the inner cladding layer 12 of the amplification optical fiber 10 is absorbed by Yb as it passes through the core 11, and this Yb is excited to generate spontaneous emission light. The spontaneous emission light generated by the excitation of Yb is retroreflected between the high-reflection portion 21 and the low-reflection portion 22, and light of a specific wavelength (for example, 1064 nm) is amplified to generate laser oscillation. The laser light amplified in the optical resonator 2 propagates within the core 11 of the amplification optical fiber 10, and a portion of it transmits through the low-reflection portion 22 and propagates downstream. The laser light L that has transmitted through the low-reflection portion 22 passes through the delivery fiber 5 and is emitted from the output end 6 toward the workpiece W, such as a metal material.

[0018] The core 50 of the delivery fiber 5 in this embodiment is configured to allow single-mode light or few-mode light to propagate when light of the wavelength (e.g., 1064 nm) of the laser light to be amplified in the amplification optical fiber 10 propagates. For example, if the core 50 is configured to allow few-mode light to propagate, in addition to light in the LP01 mode, which is the fundamental mode, light in the LP11 mode also propagates through the core 50. Here, few modes means LP (Linearly Polarized) modes of about 2 to 10. By propagating single-mode light or few-mode light through the core 50 of the delivery fiber 5 in this way, the power density of the output laser light can be increased compared to when multimode light is propagated through the core 50 of the delivery fiber 5.

[0019] The delivery fiber 5 in this embodiment has the multi-step refractive index structure 60 as described above, and the relative refractive index difference of the central core region 61 with respect to the cladding region 63 is smaller than the relative refractive index difference of the outer core region 62, so that the light confinement effect in the outer core region 62 is higher than the light confinement effect in the central core region 61. Therefore, when laser light is propagated through such a multi-step refractive index structure 60, the power density in the central core region 61 can be made lower than that of a Gaussian beam profile as shown in Fig. 4A. Therefore, laser light L having a ring-shaped beam profile as shown in Fig. 4B or a top-hat-shaped beam profile as shown in Fig. 4C can be emitted from the output end 6 of the delivery fiber 5.

[0020] Fig. 5 is a schematic diagram illustrating the difference in beam profile (light intensity distribution) between laser beams having the same power. Fig. 5 shows a Gaussian beam profile emitted from a conventional single-mode fiber laser and a ring-shaped beam profile emitted from a fiber laser device 1 using a delivery fiber 5 having the above-mentioned multi-stage refractive index structure 60. As shown in Fig. 5, in laser beams having a Gaussian beam profile, the light intensity in the central portion is particularly higher than the light intensity in the radially outer portions (see the portion indicated by A in Fig. 5). As described above, this high power density in the central portion leads to sputtering and a deterioration in the quality of laser processing.

[0021] On the other hand, when laser light having a ring-shaped beam profile is irradiated at the same power, the power density in the central portion is lower than that of a Gaussian beam profile, and the power density in the radially outer portion is higher than that of a Gaussian beam profile (see the part marked B in Figure 5). In this way, with a ring-shaped beam profile, the power density peak can be made ring-shaped. Therefore, the area irradiated with most of the laser energy can be made lower than with a Gaussian beam profile without significantly changing the area irradiated with most of the laser energy. Therefore, the beam characteristics can be improved so that the power in the central portion of the laser light does not become excessively high while maintaining high focusing ability of the laser light. This suppresses the occurrence of sputtering.

[0022] The ring-shaped beam profile described above also has an advantage over a Gaussian beam profile in terms of its tail. If the tail slope is small, the irradiated laser energy tends to spread to the surrounding area, which may cause heat from the irradiated laser light to affect the surrounding area. On the other hand, if the tail slope is large, the irradiated laser energy is less likely to spread to the surrounding area, allowing the laser light to be irradiated in a concentrated manner on the intended area. As shown in Figure 5, the tail of the ring-shaped beam profile has a steeper slope than the tail of the Gaussian beam profile (see the area indicated by C in Figure 5). Therefore, by using laser light with such a ring-shaped beam profile, the laser energy can be irradiated in a concentrated manner on the intended area, and the thermal impact on the surrounding area can be reduced.

[0023] In the above-described embodiment, the delivery fiber 5 has been described as having the above-described multi-step refractive index structure 60 over its entire length. However, in order to improve the beam characteristics while maintaining high focusing of the laser light L output from the fiber laser device 1 as described above, it is sufficient that the delivery fiber 5 has the above-described multi-step refractive index structure 60 at least at the output end 6. Furthermore, as shown in FIG. 6 , a delivery fiber 7 that does not have the multi-step refractive index structure 60 may be connected between the delivery fiber 5 and the low-reflection section 22. In the example shown in FIG. 6 , the delivery fiber 7 and the low-reflection section 22 are fusion-spliced ​​to each other at a fusion splice section 36, and the delivery fiber 7 and the delivery fiber 5 are fusion-spliced ​​to each other at a fusion splice section 37.

[0024] The fiber laser device 1 in the above-described embodiment is a forward pumping type fiber laser device in which pumping light is introduced from the upstream side of the optical resonator 2, but the present invention can also be applied to a backward pumping type fiber laser device in which pumping light is introduced from the downstream side of the optical resonator 2. Furthermore, the present invention can also be applied to a dual pumping type fiber laser device 201 in which pumping light is introduced from both sides of the optical resonator 2 as shown in FIG.

[0025] 1 , the fiber laser device 201 shown in FIG. 7 includes, in addition to the configuration shown in FIG. 1 , a plurality of pumping light sources 8 that supply pumping light to the optical resonator 2 from the downstream side of the optical resonator 2, and an optical combiner 9 that combines the pumping light output from the plurality of pumping light sources 8 and introduces the combined pumping light into the optical resonator 2. The low-reflection section 22 and an optical fiber 9A of the optical combiner 9 are fusion-spliced ​​to each other at a fusion splice section 234. The optical fiber 8A extending from the pumping light source 8 is fusion-spliced ​​to an optical fiber 9B of the optical combiner 9 at a fusion splice section 235. The optical fiber 9C of the optical combiner 9 and the above-mentioned delivery fiber 5 are fusion-spliced ​​to each other at a fusion splice section 236. The optical combiner 9 is configured to combine the pumping light output from the plurality of pumping light sources 8 and introduce the combined pumping light into the inner cladding layer 12 of the amplification optical fiber 10, and to introduce the laser light amplified in the optical resonator 2 and transmitted through the low-reflection section 22 into the core 50 of the delivery fiber 5. Even in such a configuration, by adopting the above-mentioned multi-stage refractive index structure 60 at least at the output end 6 of the delivery fiber 5, it is possible to improve the beam characteristics so that the power of the central part of the laser light does not become excessively high while maintaining high focusing of the laser light.

[0026] In each of the above-described examples, in addition to the delivery fiber 5, the amplification optical fiber 10 may also have a multi-step refractive index structure. That is, the core 11 of the amplification optical fiber 10 may be composed of a central core region located at the center of the core 11 and having a refractive index equal to or higher than that of the inner cladding layer 12, and an outer core region surrounding the central core region and having a refractive index higher than that of the central core region, and the inner cladding layer 12 of the amplification optical fiber 10 may be composed of a cladding region surrounding the outer core region of the core 11 and having a refractive index lower than that of the central core region of the core 11. In this way, by having the multi-step refractive index structure in the amplification optical fiber 10, the light propagating through the core 11 can be drawn toward the outer periphery, thereby increasing the effective core area. Therefore, the power density of the light propagating through the core 11 can be reduced, and the influence of nonlinear optical effects can be suppressed.

[0027] Furthermore, the laser light propagates through a waveguide from the high-reflection portion 21 of the optical resonator 2 to the output end 6 of the delivery fiber 5, and this waveguide is configured by connecting multiple optical fiber components. Since optical loss is expected to occur between these different optical fiber components, the entire waveguide from the high-reflection portion 21 of the optical resonator 2 to the output end 6 of the delivery fiber 5 may have the above-mentioned multi-step refractive index structure to reduce optical loss between the optical fiber components. Specifically, in the example shown in FIG. 1 , the optical fiber including the high-reflection portion 21, the amplification optical fiber 10, the optical fiber including the low-reflection portion 22, and the delivery fiber 5 may have a multi-step refractive index structure. Furthermore, in the example shown in FIG. 7 , the optical fiber including the high-reflection portion 21, the amplification optical fiber 10, the optical fiber including the low-reflection portion 22, the optical combiner 9, and the delivery fiber 5 may have a multi-step refractive index structure.

[0028] As described above, one aspect of the present invention provides a fiber laser device that can improve beam characteristics while maintaining high focusing of output laser light. The fiber laser device includes an amplification optical fiber including a core doped with an active element, a pumping light source capable of emitting pumping light for exciting the active element, and a delivery fiber including a core capable of propagating single-mode or multi-mode light at the wavelength of laser light generated in the amplification optical fiber and having an output end capable of outputting the laser light. The delivery fiber has a multi-step refractive index structure at least at the output end, which is composed of multiple regions whose refractive index varies along a radial direction. The multi-step refractive index structure includes: a central core region located at the center of the multi-step refractive index structure and having a first refractive index; an outer core region surrounding the central core region and having a second refractive index higher than the first refractive index; and a cladding region surrounding the outer core region and having a third refractive index lower than the first refractive index.

[0029] In such a fiber laser device, at least the output end of the delivery fiber has a multi-step refractive index structure, so that the light confinement effect in the outer core region at least at the output end of the delivery fiber is higher than the light confinement effect in the central core region. Therefore, by propagating laser light through such a multi-step refractive index structure, it is possible to emit laser light with a lower power density in the central core region compared to a Gaussian beam profile. This makes it possible to improve the beam characteristics by preventing the power of the central portion of the laser light from becoming excessively high while maintaining high focusing ability of the laser light.

[0030] The delivery fiber may have the multi-step refractive index structure over the entire length of the delivery fiber.

[0031] The amplification optical fiber may have the multi-step refractive index structure. By using an amplification optical fiber having such a multi-step refractive index structure, the light propagating through the core of the amplification optical fiber can be drawn toward the outer periphery, thereby increasing the effective core area. Therefore, the power density of the light propagating through the core of the amplification optical fiber can be reduced, and the influence of nonlinear optical effects can be suppressed.

[0032] The fiber laser device may further include a high-reflection portion connected to the upstream side of the amplification optical fiber and reflecting the light amplified by the amplification optical fiber at a first reflectance, and a low-reflection portion connected to the downstream side of the amplification optical fiber and reflecting the light amplified by the amplification optical fiber at a second reflectance lower than the first reflectance. At least the optical waveguide from the high-reflection portion to the output end of the delivery fiber may have the multi-step refractive index structure. By making the optical waveguide from at least the high-reflection portion to the output end of the delivery fiber have the same structure in this way, optical loss between different optical fiber components can be reduced.

[0033] Although the preferred embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the above-described embodiments and may be embodied in various different forms within the scope of the technical concept thereof. [Explanation of symbols]

[0034] 1,201 Fiber laser device 2 Optical resonator 3,8 Excitation light source 4,9 Optical Combiner 5,7 Delivery fiber 6 Output terminal 10 Amplifying optical fiber 11 cores 12 Inner cladding layer 13 Outer cladding layer 21 Highly reflective part 22 Low reflection section 31~37, 234~236 Fusion splicer 50 cores 51 Clad 52 Covering layer 60 Multi-stage refractive index structure 61 Central Core Region 62 outer core region 63 Cladding region

Claims

1. an amplifying optical fiber including a core doped with an active element; an excitation light source capable of emitting excitation light for exciting the active element; a delivery fiber including a core capable of propagating single-mode or multi-mode light at the wavelength of the laser light generated in the amplification optical fiber, and having an output end capable of outputting the laser light; Equipped with the delivery fiber has, at least at the output end, a multi-step refractive index structure formed by a plurality of regions whose refractive index changes along a radial direction; The multi-stage refractive index structure is a central core region located at the center of the multi-stage refractive index structure and having a first refractive index; an outer core region surrounding the central core region and having a second refractive index higher than the first refractive index; a cladding region surrounding the outer core region and having a third refractive index lower than the first refractive index; Including, the relative refractive index difference of the outer core region with respect to the cladding region is 0.1% to 0.15%; Fiber laser device.

2. The fiber laser device according to claim 1 , wherein the delivery fiber has the multi-step refractive index structure over the entire length of the delivery fiber.

3. 3. The fiber laser device according to claim 1, wherein the amplification optical fiber has the multi-stage refractive index structure.

4. a high-reflection portion connected to the upstream side of the amplification optical fiber and reflecting the light amplified by the amplification optical fiber at a first reflectance; a low-reflection portion connected to the downstream side of the amplification optical fiber and reflecting the light amplified by the amplification optical fiber at a second reflectance lower than the first reflectance; Furthermore, 4. The fiber laser device according to claim 1, wherein at least an optical waveguide from the high-reflection portion to the output end of the delivery fiber has the multi-stage refractive index structure.

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