JP7804482B2 - Fiber laser device - Google Patents

Description

The present invention relates to a fiber laser device, and more particularly to a fiber laser device that includes a delivery fiber that outputs laser light.

Compared to carbon dioxide lasers, which have traditionally been used to process metal materials, fiber lasers offer better beam quality, allow for a smaller beam spot, and have higher power density. Therefore, in recent years, they have been increasingly used to process metal materials, including cutting, welding, and machining. However, materials such as copper and aluminum have a lower absorption rate for light at the oscillation wavelength of fiber lasers than materials such as iron. Processing these materials requires the irradiation of a high-power-density beam, which makes processing difficult with the 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 higher power density than a multimode fiber laser, with the majority of the emitted laser light consisting of the fundamental mode (see, for example, Patent Document 1).

Generally, 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 laser light irradiation during laser processing will further evaporate and scatter around the processing spot (sputtering). Such sputtering reduces the strength of the metal material, results in cosmetic defects, and reduces the quality of the laser processing. Therefore, there is a need for technology that can improve beam characteristics so that the power in the center of the laser light does not become excessively high while maintaining high focusing capabilities of the laser light output from a fiber laser.

Japanese Patent Application Laid-Open No. 2007-139857

The present invention was made in consideration of these problems with the prior art, and aims to provide a fiber laser device that can improve beam characteristics while maintaining high focusing of the output laser light.

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%.

FIG. 1 is a diagram schematically illustrating the configuration of a fiber laser device according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing the structure of an amplification optical fiber in the fiber laser device shown in FIG. 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. FIG. 4A is a diagram schematically illustrating an example of a beam profile of laser light output from a single-mode fiber laser. 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. 1 . FIG. 4C is a diagram schematically illustrating another example of the beam profile of the laser light output from the fiber laser device shown in FIG. 1 . FIG. 5 is a schematic diagram for explaining differences in beam profiles of laser beams having the same power. FIG. 6 is a diagram schematically illustrating a modification of the fiber laser device shown in FIG. FIG. 7 is a diagram schematically illustrating the configuration of a fiber laser device according to the second embodiment of the present invention.

Embodiments of a fiber laser device according to the present invention will be described in detail below with reference to Figures 1 to 7. In Figures 1 to 7, identical or corresponding components are designated by the same reference numerals, and duplicate descriptions will be omitted. Furthermore, in Figures 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 merely to distinguish components from one another, and do not represent a specific order or ranking.

Figure 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 Figure 1, the fiber laser device 1 includes an optical resonator 2, multiple 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 multiple pumping light sources 3 and introduces it into the optical resonator 2, and a delivery fiber 5 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."

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

The high-reflection section 21 and the amplification optical fiber 10 are fusion-spliced together at the fusion splice section 31, and the high-reflection section 21 and the optical fiber 4A of the optical combiner 4 are fusion-spliced together at the fusion splice section 32. The low-reflection section 22 and the amplification optical fiber 10 are fusion-spliced together at the fusion splice section 33, and the low-reflection section 22 and the delivery fiber 5 are fusion-spliced together at the fusion splice section 34.

Figure 2 is a cross-sectional view schematically illustrating the structure of the amplification optical fiber 10. As shown in Figure 2, the amplification optical fiber 10 has a core 11, an inner cladding layer 12 that surrounds the core 11, and an outer cladding layer 13 that surrounds 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 then doping at least a portion of the core with an active element. Examples of active elements that can be doped into the core 11 include rare earth elements such as ytterbium (Yb), erbium (Er), thulium (Tm), and neodymium (Nd), as well as bismuth (Bi) and chromium (Cr). In this embodiment, an example is described in which Yb is doped into the core 11 of the amplification optical fiber 10, but the present invention is not limited to this.

The inner cladding layer 12 is formed, for example, from quartz without any dopants. 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 formed, for example, from 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.

Each of the pumping light sources 3 includes a Fabry-Perot semiconductor laser element made of, for example, a GaAs-based semiconductor, and generates pumping light with, for example, a center wavelength of 915 nm. The optical fiber 3A extending from the pumping light source 3 is fusion-spliced to the optical fiber 4B of the optical combiner 4 at the fusion splicer 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.

Figure 3 is a cross-sectional view showing the structure of the delivery fiber 5 along with its refractive index. As shown in Figure 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.

The delivery fiber 5 in this embodiment has a multi-step refractive index structure 60 composed of multiple 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%.

In the optical resonator 2, 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, exciting the Yb and generating spontaneous emission. The spontaneous emission light generated by the excitation of Yb is retroreflected between the high-reflection section 21 and the low-reflection section 22, amplifying light of a specific wavelength (e.g., 1064 nm) and generating laser oscillation. The laser light amplified by the optical resonator 2 propagates within the core 11 of the amplification optical fiber 10, with a portion of it passing through the low-reflection section 22 and propagating downstream. The laser light L that has passed through the low-reflection section 22 passes through the delivery fiber 5 and is emitted from the output end 6 toward the workpiece W, such as a metal material.

In this embodiment, the core 50 of the delivery fiber 5 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 the propagation of few-mode light, in addition to the fundamental mode LP01 mode light, LP11 mode light also propagates through the core 50. Here, few modes refers to LP (Linearly Polarized) modes of approximately 2 to 10. In this way, by propagating single-mode light or few-mode light through the core 50 of the delivery fiber 5, the power density of the output laser light can be increased compared to when multi-mode light is propagated through the core 50 of the delivery fiber 5.

The delivery fiber 5 in this embodiment has the multi-step refractive index structure 60 described above, and because 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, the light confinement effect in the outer core region 62 is greater 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 lowered compared to the Gaussian beam profile shown in FIG. 4A. As a result, laser light L with a ring-shaped beam profile as shown in FIG. 4B or a top-hat beam profile as shown in FIG. 4C can be emitted from the output end 6 of the delivery fiber 5.

Figure 5 is a schematic diagram illustrating the difference in beam profile (light intensity distribution) between laser beams having the same power. Figure 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 Figure 5, in laser beams having a Gaussian beam profile, the light intensity in the central portion is particularly high compared to the light intensity at the radially outer edges (see the portion indicated by A in Figure 5). As mentioned above, this high power density in the central portion leads to sputtering and reduced quality in laser processing.

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 in a Gaussian beam profile, and the power density in the radially outer portion is higher than that in 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 where most of the laser energy is irradiated does not change significantly from that in the case of a Gaussian beam profile, and the local power density in the central portion can be lowered compared to a Gaussian beam profile. Therefore, while maintaining high focusing ability of the laser light, the beam characteristics can be improved so that the power in the central portion of the laser light does not become excessively high. This suppresses the occurrence of sputtering.

The ring-shaped beam profile described above also has an advantage over a Gaussian beam profile in terms of its base. If the base slope is small, the irradiated laser energy tends to spread to the surrounding area, which could potentially affect the heat from the irradiated laser light. On the other hand, if the base 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 base of a ring-shaped beam profile has a larger slope than the base of a 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.

In the above-described embodiment, the delivery fiber 5 has been described as having the above-described multi-step refractive index structure 60 along 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 for the delivery fiber 5 to have the above-described multi-step refractive index structure 60 at least at the output end 6. Also, 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 the fusion splice section 36, and the delivery fiber 7 and the delivery fiber 5 are fusion-spliced to each other at the fusion splice section 37.

The fiber laser device 1 in the above-described embodiment is a forward-pumped fiber laser device in which pumping light is introduced from the upstream side of the optical resonator 2. However, the present invention can also be applied to a backward-pumped 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-pumped fiber laser device 201 in which pumping light is introduced from both sides of the optical resonator 2, as shown in Figure 7.

The fiber laser device 201 shown in Figure 7 includes, in addition to the configuration shown in Figure 1, multiple pumping light sources 8 that supply pumping light to the optical resonator 2 from downstream of the optical resonator 2, and an optical combiner 9 that combines the pumping light output from the multiple pumping light sources 8 and introduces it into the optical resonator 2. The low-reflection section 22 and the 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 the 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 multiple pumping light sources 8 and introduce this pumping light into the inner cladding layer 12 of the amplification optical fiber 10, and to introduce 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 this configuration, by employing the above-described multi-stage refractive index structure 60 at least at the output end 6 of the delivery fiber 5, it is possible to maintain high focusing of the laser light while improving the beam characteristics so that the power in the central portion of the laser light does not become excessively high.

In each of the above 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 greater 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 amplification optical fiber 10 also have a multi-step refractive index structure, light propagating through the core 11 can be drawn toward the outer periphery, thereby increasing the effective core cross-sectional area. This allows the power density of light propagating through the core 11 to be reduced, suppressing the effects of nonlinear optical effects.

In addition, 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 constructed by connecting multiple optical fiber components. Because optical loss is likely 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 multi-stage refractive index structure described above 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-stage refractive index structure. 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-stage refractive index structure.

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. This 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 that excites the active element, and 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. 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 the 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.

With this type of fiber laser device, at least the output end of the delivery fiber has a multi-step refractive index structure, so the light confinement effect in the outer core region at least at the output end of the delivery fiber is greater 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 improves the beam characteristics by preventing the power of the central portion of the laser light from becoming excessively high while maintaining high focusing of the laser light.

The delivery fiber may have the multi-stage refractive index structure over the entire length of the delivery fiber.

The amplification optical fiber may have the multi-step refractive index structure. By using an amplification optical fiber with 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 cross-sectional area. This allows the power density of the light propagating through the core of the amplification optical fiber to be reduced, and the effects of nonlinear optical effects to be suppressed.

The fiber laser device may further include a high-reflection section 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 section 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. The optical waveguide extending from at least the high-reflection section to the output end of the delivery fiber may have the multi-step refractive index structure. By providing the same structure for the optical waveguides extending from at least the high-reflection section to the output end of the delivery fiber in this manner, optical loss between different optical fiber components can be reduced.

While 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 a variety of different forms within the scope of its technical concept.

1, 201 Fiber laser device 2 Optical resonator 3, 8 Pumping light source 4, 9 Optical combiner 5, 7 Delivery fiber 6 Output end 10 Amplification optical fiber 11 Core 12 Inner cladding layer 13 Outer cladding layer 21 High reflectivity portion 22 Low reflectivity portion 31 to 37, 234 to 236 Fusion splice portion 50 Core 51 Cladding 52 Coating layer 60 Multi-stage refractive index structure 61 Central core region 62 Outer core region 63 Cladding region

Claims (4)

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;
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 ;
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. The fiber laser device of claim 1, wherein the delivery fiber has the multi-step refractive index structure throughout the entire length of the delivery fiber. The fiber laser device of claim 1 or 2, wherein the amplification optical fiber has the multi-stage refractive index structure. 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 a 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,
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.