Optical combiner and laser device

Abstract

Provided is an optical combiner capable of suppressing light loss. An optical combiner (1) comprises: input optical fibers (10) each having a core (11) through which light propagates; divergence angle reduction parts (20) each having an incidence end (21) on which the light propagating through the core (11) of the input optical fiber (10) is incident, and an emission end (22) from which the light is emitted in a state where the divergence angle is smaller than that when the light is incident on the incidence end (21); and a bridge fiber (40) that includes intermediate optical fibers (30) each having a core (31) on which the light emitted from the emission end (22) of the divergence angle reduction part (20) is incident, and that has a tapered part (42) in which the outer shape of the bundled intermediate optical fibers (30) decreases toward the downstream side. The diameter of the core (31) at an upstream end of each of the intermediate optical fibers (30) is greater than the diameter of the core (11) at a downstream end of each of the input optical fibers (10).

Description

Optical combiner and laser device

[0001] The present invention relates to an optical combiner and a laser device.

[0002] Optical combiners are widely used to combine laser beams from multiple light sources to obtain high-power laser beams. One known example of such optical combiners involves bundling multiple optical fibers on the input side, melting and stretching them to reduce their diameters, and then connecting the reduced-diameter bundle of optical fibers to an output side optical fiber (see, for example, Patent Document 1). In such optical combiners, the diameter or numerical aperture of the core located closer to the output side optical fiber is larger than the diameter or numerical aperture of the core located closer to the input side optical fiber (see, for example, paragraphs

[0030] -

[0034] of Patent Document 1). Therefore, the core diameter or numerical aperture changes discontinuously with respect to the light propagation direction at the fusion splice point with the output side optical fiber, resulting in light with a large divergence angle (the angle in the direction in which the light spreads relative to the optical axis of the core), resulting in light loss.

[0003] JP 2011-186267 A

[0004] The present invention has been made in consideration of the problems of the conventional technology, and aims to provide an optical combiner that can reduce light loss and a laser device equipped with such an optical combiner.

[0005] A first aspect of the present invention is an optical combiner comprising: a plurality of input optical fibers each having a first core through which light propagates; a plurality of divergence angle reduction sections each having an input end into which the light propagating through the first core of a corresponding input optical fiber among the plurality of input optical fibers is incident, and an output end from which the light is emitted with a smaller divergence angle than when it was incident at the input end; and a first bridge fiber including a plurality of intermediate optical fibers each having a second core into which the light emitted from the output end of a corresponding one of the plurality of divergence angle reduction sections is incident, the first bridge fiber having a tapered section in which the outer dimensions of the bundled intermediate optical fibers become smaller toward the downstream side, wherein the diameter of the second core at the upstream end of each of the plurality of intermediate optical fibers is larger than the diameter of the first core at the downstream end of each of the plurality of input optical fibers.

[0006] A second aspect of the present invention is the optical combiner according to the first aspect, further comprising an output optical fiber having a third core into which the light propagating through the second core of the first bridge fiber is incident, wherein the third core of the output optical fiber is sized at the upstream end of the output optical fiber to accommodate the second cores of the plurality of intermediate optical fibers at the downstream end of the first bridge fiber.

[0007] Aspect 3 of the present invention is the optical combiner according to Aspect 1 or 2, further comprising a second bridge fiber having a fourth core into which the light propagating through the second core of the first bridge fiber is incident, wherein the fourth core at the upstream end of the second bridge fiber is sized to accommodate the second cores of the plurality of intermediate optical fibers at the downstream end of the bridge fiber, and the fourth core of the second bridge fiber has a tapered core portion whose diameter decreases downstream.

[0008] A fourth aspect of the present invention is characterized in that each of the plurality of divergence angle reduction sections is composed of a GRIN lens whose refractive index gradually decreases from the central axis toward the radially outer side, and the refractive index distribution constant g of the GRIN lens is expressed as follows, where n0 is the refractive index at the central axis, λ is the wavelength of the light, ω is the mode field diameter of the fundamental mode of the input optical fiber, and ω' is the mode field diameter of the fundamental mode of the intermediate optical fiber: 4. The optical combiner according to any one of aspects 1 to 3, wherein the optical combiner is in a range defined by:

[0009] Aspect 5 of the present invention is characterized in that each of the plurality of divergence angle reduction sections is composed of a GRIN lens whose refractive index gradually decreases from the central axis toward the radially outward direction, and the length L of the GRIN lens along the optical axis direction is expressed as follows, where m is an integer equal to or greater than 0 and g is a refractive index distribution constant of the GRIN lens: 5. The optical combiner according to any one of aspects 1 to 4, wherein:

[0010] Aspect 6 of the present invention is characterized in that each of the plurality of divergence angle reduction sections comprises: a first GRIN lens connected to the input optical fiber, the refractive index of which gradually decreases from the central axis toward the radially outward direction; and a second GRIN lens connected to the first GRIN lens and the intermediate optical fiber, the refractive index of which gradually decreases from the central axis toward the radially outward direction; wherein, assuming that the refractive index distribution constant of the first GRIN lens is g1, the refractive index distribution constant of the second GRIN lens is g2, the mode field diameter of the fundamental mode of the input optical fiber is ω, and the mode field diameter of the fundamental mode of the intermediate optical fiber is ω', then The optical combiner according to any one of aspects 1 to 3, wherein the following relationship holds:

[0011] A seventh aspect of the present invention is that the length L1 of the first GRIN lens along the central axis is expressed as follows, where j is an integer equal to or greater than 0: The length L2 of the second GRIN lens along the central axis is in a range defined by: where k is an integer equal to or greater than 0, 7. The optical combiner according to claim 6, wherein the optical combiner is in a range defined by:

[0012] Aspect 8 of the present invention is an optical combiner according to any one of Aspects 1 to 7, wherein an outer diameter from a portion of the plurality of input optical fibers located upstream of the divergence angle reduction section to the tapered section of the bridge fiber decreases downstream.

[0013] A ninth aspect of the present invention is a laser device comprising: a plurality of laser light sources that generate laser light; and an optical combiner according to any one of aspects 1 to 8, wherein the first cores of the plurality of input optical fibers of the optical combiner are optically coupled to the plurality of laser light sources.

[0014] FIG. 1 is a perspective view showing an optical combiner according to a first embodiment of the present invention. FIG. 2 is an exploded perspective view of the optical combiner of FIG. 1. FIG. 3 is a cross-sectional view taken along the optical axis direction of the optical combiner of FIG. 1. FIG. 4 is a cross-sectional view taken along the optical axis direction of an optical combiner according to a second embodiment of the present invention. FIG. 5 is a cross-sectional view taken along the optical axis direction of an optical combiner according to a third embodiment of the present invention. FIG. 6 is a cross-sectional view taken along the optical axis direction of an optical combiner according to a fourth embodiment of the present invention. FIG. 7 is a schematic diagram showing an example of a laser device including an optical combiner according to the present invention. FIG. 8 is a schematic diagram showing another example of a laser device including an optical combiner according to the present invention.

[0015] Hereinafter, an embodiment of an optical combiner and a laser device using the same according to the present invention will be described in detail with reference to FIGS. 1 to 8. In FIGS. 1 to 8, identical or corresponding components are denoted by the same reference numerals, and redundant description will be omitted. In addition, in FIGS. 1 to 8, the scale and dimensions of each component may be exaggerated, and 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 each other, and do not represent a specific order or sequence.

[0016] Fig. 1 is a perspective view showing an optical combiner 1 according to a first embodiment of the present invention, Fig. 2 is an exploded perspective view, and Fig. 3 is a cross-sectional view taken along the optical axis. In this specification, unless otherwise specified, the direction in which light propagates from a light source, such as a laser light source (described later), is referred to as the "downstream side," and the opposite direction is referred to as the "upstream side." In Fig. 3, the left side is the upstream side, and the right side is the downstream side. As shown in Figs. 1 to 3, the optical combiner 1 includes a plurality of input optical fibers 10, a plurality of divergence angle reducers 20 connected to the downstream ends of the corresponding input optical fibers 10, a bridge fiber 40 (first bridge fiber) including intermediate optical fibers 30 connected to the downstream ends of the corresponding divergence angle reducers 20, and an output optical fiber 50 connected to the downstream end of the bridge fiber 40.

[0017] Each of the input optical fibers 10 has a core 11 (first core) and a cladding 12 surrounding the core 11, with the cladding 12 having a lower refractive index than the core 11. For example, the core 11 may be formed from silica glass (SiO2), and the cladding 12 may be formed by adding a dopant (e.g., fluorine (F) or boron (B)) that lowers the refractive index to the silica glass. Alternatively, the cladding 12 may be formed from silica glass (SiO2), and the core 11 may be formed by adding a dopant (e.g., germanium (Ge)) that increases the refractive index. This forms an optical waveguide within the core 11 of the input optical fiber 10, through which light propagates. Note that in portions not shown in FIGS. 1 to 3 , the cladding 12 of the input optical fiber 10 is covered with a coating (not shown) made of, for example, resin.

[0018] The divergence angle reduction section 20 is a cylindrical section that functions to reduce the exit angle of light propagating inside. The divergence angle reduction section 20 has an input end 21 into which light propagating through the core 11 of the input optical fiber 10 is incident, and an output end 22 from which the light propagating inside exits. The divergence angle reduction section 20 is configured so that the light incident on the input end 21 exits from the output end 22 with a smaller divergence angle than when it entered the input end 21. In other words, by propagating the light from the core 11 of the input optical fiber 10 inside the divergence angle reduction section 20, the light exits from the output end 22 with a smaller divergence angle than when it entered the input end 21.

[0019] For example, a GRIN (Graded Index or Gradient Index) lens, whose refractive index gradually decreases from the central axis toward the radially outward direction, can be used as the divergence angle reduction portion 20. Such a GRIN lens can be formed by, for example, adding a high concentration of a dopant such as germanium (Ge) to the center of a cylindrical glass made of quartz. The refractive index distribution of such a GRIN lens is expressed by the following equation (1), where n is the refractive index at the central axis, r is the radial distance from the central axis, and g is the refractive index distribution constant.

[0020] The bridge fiber 40 is formed by bundling a plurality of intermediate optical fibers 30, and has a tapered portion 42 in which the outer shape of the bundled intermediate optical fibers 30 becomes smaller toward the downstream side. The cross-sectional area of ​​the tapered portion 42 becomes smaller toward the downstream side. Such a tapered portion 42 can be formed by heating the bundle of intermediate optical fibers 30 and melt-drawing it.

[0021] 2 and 3 , each intermediate optical fiber 30 has a core 31 (second core) and a cladding 32 surrounding the core 31, with the cladding 32 having a lower refractive index than the core 31. For example, the core 31 may be made of silica glass (SiO2), and the cladding 32 may be formed by adding a dopant (e.g., fluorine (F) or boron (B)) that has the property of lowering the refractive index to the silica glass. Alternatively, the cladding 32 may be made of silica glass (SiO2), and the core 31 may be formed by adding a dopant (e.g., germanium (Ge)) that has the property of increasing the refractive index. As a result, an optical waveguide through which light propagates is formed inside the core 31 of the intermediate optical fiber 30.

[0022] The output optical fiber 50 has a core 51 (third core) and a cladding 52 surrounding the core 51, with the cladding 52 having a lower refractive index than the core 51. For example, the core 51 may be formed from silica glass (SiO2), and the cladding 52 may be formed by adding a dopant (e.g., fluorine (F) or boron (B)) that has the property of lowering the refractive index to the silica glass. Alternatively, the cladding 52 may be formed from silica glass (SiO2), and the core 51 may be formed by adding a dopant (e.g., germanium (Ge)) that has the property of increasing the refractive index. As a result, an optical waveguide through which light propagates is formed inside the core 51 of the output optical fiber 50.

[0023] The diameter of the input end 21 of the divergence angle reducing section 20 is equal to or greater than the diameter of the core 11 of the input optical fiber 10. Furthermore, the diameter of the core 31 at the upstream end of the intermediate optical fiber 30 is greater than the diameter of the core 11 at the downstream end of the input optical fiber 10. The size (diameter) of the core 51 at the upstream end of the output optical fiber 50 is set to a size (diameter) that can accommodate all of the cores 31 (shown by dotted lines in FIG. 2 ) of the intermediate optical fiber 30 at the downstream end of the bridge fiber 40, and the bridge fiber 40 and the output optical fiber 50 are fusion spliced ​​such that all of the cores 31 of the intermediate optical fiber 30 in the tapered section 42 are located within the region of the cores 51 at the upstream end of the output optical fiber 50. That is, the diameter of the core 51 at the upstream end of the output optical fiber 50 is equal to or larger than the diameter of the circumscribing circle of the cores 31 of the plurality of intermediate optical fibers 30 at the downstream end of the bridge fiber 40, and the core 51 at the upstream end of the output optical fiber 50 contains all of the cores 31 of the plurality of intermediate optical fibers 30 at the downstream end of the bridge fiber 40. Even when the plurality of intermediate optical fibers 30 have cores 31 with different diameters, the core 51 at the upstream end of the output optical fiber 50 is sized to contain all of the cores 31 of the plurality of intermediate optical fibers 30 at the downstream end of the bridge fiber 40.

[0024] When light propagates from an optical fiber with a smaller core diameter to an optical fiber with a larger core diameter, light with a large divergence angle is generated at the fusion splice point, resulting in light loss. Light propagating downstream through the core 11 of the input optical fiber 10 enters the core 31 of the intermediate optical fiber 30 of the bridge fiber 40 with its divergence angle reduced by the divergence angle reducer 20. This reduces light loss compared to when optical fibers with different core diameters are directly fusion-spliced. Furthermore, when the diameter of the core 31 is reduced in the tapered portion 42, the divergence angle of light emitted from the tapered portion 42 to the core 51 of the output optical fiber 50 is kept small. This reduces light loss at the fusion splice point between the bridge fiber 40 and the output optical fiber 50.

[0025] Furthermore, because the outer diameter of the tapered portion 42 of the bridge fiber 40 becomes smaller toward the downstream side, the intermediate optical fibers 30 in this portion can be closely packed to reduce the center-to-center distance of the cores 31, thereby increasing the brightness of the light incident on the output optical fiber 50 from the bridge fiber 40. Furthermore, because the downstream end face of the bridge fiber 40 can be formed by cutting the ends of the bundle of intermediate optical fibers 30 after melt-drawing, there is no need to align the end faces of multiple intermediate optical fibers with an accuracy of, for example, 10 μm for fusion splicing.

[0026] Here, when the divergence angle reduction unit 20 is configured using a GRIN lens, if the refractive index distribution constant g of the GRIN lens and the length L of the GRIN lens along the optical axis direction are set to values ​​expressed by the following equations (2) and (3), it is possible to perform adiabatic beam diameter conversion (it is possible to prevent the fundamental mode from being coupled to higher-order modes). In the above equations (2) and (3), n is the refractive index at the central axis of the GRIN lens, λ is the wavelength of light propagating through the GRIN lens, ω is the mode field diameter of the fundamental mode of the input optical fiber 10, ω′ is the mode field diameter of the fundamental mode of the intermediate optical fiber 30, and m is an integer equal to or greater than 0.

[0027] Therefore, in order to perform adiabatic beam diameter conversion, it is preferable that the refractive index distribution constant g of the GRIN lens and the length L of the GRIN lens along the optical axis direction are within the range of ±10% of the above formulas (2) and (3), respectively. It is preferable that:

[0028] 4 is a cross-sectional view taken along the optical axis of an optical combiner 101 according to a second embodiment of the present invention. In this embodiment, not only the tapered portion 42 of the bridge fiber 40 but also the outer diameter of the portion upstream thereof, i.e., from a portion P of the input optical fiber 10 located upstream of the divergence angle reducer 20 to the tapered portion 42 of the bridge fiber 40, decreases downstream.

[0029] Generally, discontinuous bending is likely to occur at the starting end of integration by melt-drawing due to factors such as sagging of the optical fiber during melting and non-axial symmetry of the heat source. When discontinuous bending occurs in the optical fiber, higher-order modes including cladding modes are more likely to occur, but the smaller the core diameter of the optical fiber, the less these higher-order modes are generated. Therefore, as in this embodiment, by setting the starting end of integration by melt-drawing to a portion of the input optical fiber 10 that has a smaller core diameter than the intermediate optical fiber 30, it is possible to suppress the generation of higher-order modes due to discontinuous bending that occurs at the starting end of integration by melt-drawing.

[0030] Equation (2) in the first embodiment described above is based on the assumption that the refractive index distribution constant g of the GRIN lens serving as the divergence angle reduction section 20 is constant along the axial direction. However, in this embodiment, the diameter of the divergence angle reduction section 20 varies along the axial direction. Therefore, when a GRIN lens is used as the divergence angle reduction section 20, it is necessary to take into account that the refractive index distribution constant g varies along the axial direction.

[0031] 5 is a cross-sectional view taken along the optical axis direction of an optical combiner 201 according to a third embodiment of the present invention. The divergence angle reduction section 220 in this optical combiner 201 is composed of a first GRIN lens 221 connected to the input optical fiber 10, and a second GRIN lens 222 connected to the first GRIN lens 221 and the intermediate optical fiber 30. Each of the GRIN lenses 221 and 222 has the same specifications as the GRIN lenses described in the first embodiment.

[0032] When such a divergence angle reduction section 220 is used, the relationship expressed by the following equation (4) holds between the refractive index distribution constant g of the first GRIN lens 221 and the refractive index distribution constant g of the second GRIN lens 222, and if the refractive index distribution constant g of the first GRIN lens 221, the refractive index distribution constant g of the second GRIN lens 222, the length L of the first GRIN lens 221 along the optical axis direction, and the length L of the second GRIN lens 222 along the optical axis direction are set to values ​​expressed by the following equations (5) and (6), the input optical fiber 10 and the intermediate optical fiber 30 can be connected adiabatically. In the above equations (4) to (6), ω is the mode field diameter of the fundamental mode of the input optical fiber 10, ω' is the mode field diameter of the fundamental mode of the intermediate optical fiber 30, and j and k are each integers of 0 or greater.

[0033] Therefore, in order to perform adiabatic beam diameter conversion, it is preferable that the refractive index distribution constant g of the first GRIN lens 221, the refractive index distribution constant g of the second GRIN lens 222, the length L of the first GRIN lens 221 along the optical axis direction, and the length L of the second GRIN lens 222 along the optical axis direction are within the range of ±10% of the above formulas (4) to (6). Preferably, where n 01 is the refractive index of the first GRIN lens 221 at the central axis, n 02 is the refractive index of the second GRIN lens 222 at the central axis, and λ is the wavelength of light propagating through the first GRIN lens 221.

[0034] By using a divergence angle reduction section 220 including two GRIN lenses 221, 222 as in this embodiment, it may be possible to more effectively suppress the occurrence of higher-order modes, including cladding modes, compared to using a divergence angle reduction section 20 including a single GRIN lens.

[0035] FIG. 6 is a cross-sectional view along the optical axis direction of an optical combiner 301 according to a fourth embodiment of the present invention. In this embodiment, a second bridge fiber 340 is disposed between the bridge fiber 40 (first bridge fiber) and the output optical fiber 50 according to the first embodiment. The second bridge fiber 340 has a core 341 (fourth core) and a cladding 342 surrounding the core 341. The cladding 342 has a lower refractive index than the core 341. For example, the core 341 may be formed from silica glass (SiO ), and the cladding 342 may be formed by adding a dopant (e.g., fluorine (F) or boron (B)) that lowers the refractive index to the silica glass. Alternatively, the cladding 342 may be formed from silica glass (SiO ), and the core 341 may be formed by adding a dopant (e.g., germanium (Ge)) that increases the refractive index to the silica glass. As a result, an optical waveguide through which light propagates is formed inside the core 341 of the second bridge fiber 340 .

[0036] The size (diameter) of the core 341 at the upstream end of the second bridge fiber 340 is set to a size (diameter) that can contain all of the cores 31 (indicated by dotted lines in FIG. 2 ) of the intermediate optical fibers 30 at the downstream end of the first bridge fiber 40, and the first bridge fiber 40 and the second bridge fiber 340 are fusion spliced ​​such that all of the cores 31 of the intermediate optical fibers 30 in the tapered portion 42 of the first bridge fiber 40 are located within the region of the core 341 at the upstream end of the second bridge fiber 340. In other words, the diameter of the core 341 at the upstream end of the second bridge fiber 340 is equal to or greater than the diameter of the circumscribing circle of the cores 31 of the multiple intermediate optical fibers 30 at the downstream end of the first bridge fiber 40, and the core 341 at the upstream end of the second bridge fiber 340 contains all of the cores 31 of the multiple intermediate optical fibers 30 at the downstream end of the first bridge fiber 40. Even if the multiple intermediate optical fibers 30 have cores 31 with different diameters, the core 341 at the upstream end of the second bridge fiber 340 is sized to contain all of the cores 31 of the multiple intermediate optical fibers 30 at the downstream end of the first bridge fiber 40.

[0037] The core 341 of this second bridge fiber 340 includes a tapered core portion 343 whose diameter decreases toward the downstream side. A second bridge fiber 340 including such a tapered core portion 343 can be formed by heating a large-diameter optical fiber and melt-drawing it. Unlike the first bridge fiber 40, which is formed by melt-drawing a bundle of multiple intermediate optical fibers 30, this second bridge fiber 340 is easier to design so that the divergence angle of the output light approaches an ideal value, and it is easier to increase the diameter reduction ratio (the ratio of the outer diameter of the most tapered portion to the outer diameter of the non-tapered portion) compared to the first bridge fiber 40. Therefore, by using a second bridge fiber 340 including such a tapered core portion 343, the diameter reduction ratio of the tapered portion 42 of the first bridge fiber 40 can be reduced. This reduces the likelihood of crosstalk between adjacent cores 31 in the tapered portion 42 of the first bridge fiber 40, making it easier to improve the optical characteristics of the light output to the output optical fiber 50.

[0038] In the second bridge fiber 340 of this embodiment, a cladding 342 is formed around the core 341, but since it is sufficient that a medium (e.g., air) with a lower refractive index than the core 341 is present outside the core 341, the second bridge fiber 340 does not necessarily have to include the cladding 342.

[0039] In the above-described embodiment, the output optical fiber 50 is connected to the downstream side of the bridge fiber 40 or the second bridge fiber 340. However, in cases where light is emitted into space from the bridge fiber 40 or the second bridge fiber 340, it is not necessary to connect the output optical fiber 50 to the downstream side of the bridge fiber 40 or the second bridge fiber 340.

[0040] 7 is a diagram schematically illustrating a fiber laser device 401 as a laser device including the above-described optical combiner. As shown in Fig. 7, the fiber laser device 401 includes an optical resonator 412 including an amplification optical fiber 410 capable of amplifying laser light, a plurality of pumping light sources 420 (laser light sources) that supply pumping light to the optical resonator 412 from one end side of the optical resonator 412, an optical combiner 430 that combines the pumping light output from the plurality of pumping light sources 420 and introduces the combined light into the optical resonator 412, a delivery fiber 440 extending from the optical resonator 412, and a laser output unit 450 provided at the downstream end of the delivery fiber 440. Each pumping light source 420 and the optical combiner 430 are connected by an optical fiber 460, and the optical combiner 430 and the optical resonator 412 are connected by an optical fiber 470.

[0041] The amplification optical fiber 410 of the optical resonator 412 has a core doped with rare earth element ions such as ytterbium (Yb), erbium (Er), thulium (Tm), or neodymium (Nd), and is composed of, for example, a double-clad fiber having an inner cladding formed around the core and an outer cladding formed around the inner cladding.

[0042] The optical resonator 412 includes a high-reflection portion 414 that reflects light in a predetermined wavelength band (e.g., 1060 nm to 1100 nm) with a high reflectance, and a low-reflection portion 416 that reflects light in this wavelength band with a lower reflectance than the high-reflection portion 414. The high-reflection portion 414 and the low-reflection portion 416 are configured, for example, by a fiber Bragg grating (FBG) or a mirror formed by periodically changing the refractive index of an optical fiber along the light propagation direction. In the example shown in FIG. 7 , the high-reflection portion 414 and the low-reflection portion 416 are configured by a fiber Bragg grating.

[0043] The optical fibers 460 connected to the pumping light sources 420 each have a core and a cladding that surrounds the core and has a refractive index lower than that of the core. An optical waveguide is formed inside the core of each of these optical fibers 460, through which the pumping light generated by the pumping light sources 420 propagates. As the pumping light source 420, for example, a laser module including a high-power multimode semiconductor laser element capable of emitting laser light with a wavelength of 975 nm is used. The pumping light generated by each pumping light source 420 propagates through the core of the optical fiber 460 toward the optical combiner 430, where it is combined and introduced into the optical resonator 412.

[0044] Pumping light introduced from the pumping light source 420 into the optical resonator 412 via the optical combiner 430 propagates through the inner cladding and core of the amplification optical fiber 410. As this pumping light passes through the core, it is absorbed by rare earth element ions doped in the core, exciting the rare earth element ions and generating spontaneous emission light. This spontaneous emission light is retroreflected between the high-reflection portion 414 and the low-reflection portion 416, amplifying light of a specific wavelength (e.g., 1070 nm) and generating laser oscillation. The laser light thus amplified by the optical resonator 412 propagates through the core of the amplification optical fiber 410, and a portion of it transmits through the low-reflection portion 416. The laser light transmitted through the low-reflection portion 416 propagates through the core of the delivery fiber 440 and is output from the laser output unit 450.

[0045] The optical combiner 1, 101, 201, or 301 of the above-described embodiment can be used as the optical combiner 430 in such a fiber laser device 401. In this case, a portion of an optical fiber 460 extending from the pumping light source 420 constitutes the input optical fiber 10 of the above-described optical combiner 1, 101, 201, or 301, or the input optical fiber 10 of the optical combiner 1, 101, 201, or 301 is connected to the optical fiber 460, and the core 11 of the input optical fiber 10 is optically coupled to the pumping light source 420. Also, a portion of an optical fiber 470 constitutes the output optical fiber 50 of the optical combiner 1, 101, 201, or 301, or the output optical fiber 50 of the optical combiner 1, 101, 201, or 301 is connected to the optical fiber 470.

[0046] In addition to the configuration shown in Figure 7, a MOPA fiber laser device is also known as a fiber laser device that amplifies seed light from a seed light source using pump light from a pump light source.It goes without saying that the above-mentioned optical combiners 1, 101, 201, and 301 can also be used in such MOPA fiber laser devices.

[0047] 8 is a schematic diagram showing a laser device 501 including an optical combiner according to the present invention. The laser device 501 includes a plurality of laser generation units 510 (laser light sources) that generate laser light, optical fibers 520 that propagate the laser light output from each of the laser generation units 510, an optical combiner 530 that combines the laser light propagating through each of the optical fibers 520, a delivery fiber 540 extending from the optical combiner 530, and a laser output unit 550 provided at the downstream end of the delivery fiber 540. The laser generation unit 510 may be, for example, a fiber laser device 401 as shown in FIG. 7. The laser device 501 can output even higher power laser light by combining outputs from a plurality of such fiber laser devices 401.

[0048] The optical combiner 1, 101, 201, or 301 of the above-described embodiment can be used as the optical combiner 530 in such a laser device 501. In this case, a portion of an optical fiber 520 extending from the laser generating unit 510 constitutes the input optical fiber 10 of the above-described optical combiner 1, 101, 201, or 301, or the input optical fiber 10 of the optical combiner 1, 101, 201, or 301 is connected to the optical fiber 520, and the core 11 of the input optical fiber 10 is optically coupled to the laser generating unit 510. Also, a portion of a delivery fiber 540 constitutes the output optical fiber 50 of the optical combiner 1, 101, 201, or 301, or the output optical fiber 50 of the optical combiner 1, 101, 201, or 301 is connected to the delivery fiber 540.

[0049] 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.

[0050] This application is based on and claims priority from Japanese Patent Application No. 2024-014016, filed February 1, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0051] The present invention is suitable for use in optical combiners and laser devices.

[0052] 1, 101, 201, 301 Optical combiner 10 Input optical fiber 11 Core (first core) 12 Cladding 20, 220 Divergence angle reduction section 21 Incident end portion 22 Emitting end portion 30 Intermediate optical fiber 31 Core (second core) 32 Cladding 40 Bridge fiber (first bridge fiber) 42 Tapered section 50 Output optical fiber 51 Core (third core) 52 Cladding 221 First GRIN lens 222 Second GRIN lens 340 Second bridge fiber 341 Core (fourth core) 342 Cladding 343 Reduced diameter core section 401 Fiber laser device (laser device) 420 Pumping light source (laser light source) 430, 530 Optical combiner 501 Laser device 510 Laser generation unit (laser light source)

Claims

1. An optical combiner comprising: a plurality of input optical fibers, each having a first core through which light propagates; a plurality of divergence angle reducers, each having an input end into which the light propagating through the first core of a corresponding one of the plurality of input optical fibers is incident, and an output end from which the light is emitted with a smaller divergence angle than when it was incident on the input end; and a first bridge fiber including a plurality of intermediate optical fibers, each having a second core into which the light emitted from the output end of a corresponding one of the plurality of divergence angle reducers is incident, the first bridge fiber having a tapered portion in which the outer dimensions of the bundled plurality of intermediate optical fibers become smaller toward the downstream side, wherein the diameter of the second core at the upstream end of each of the plurality of intermediate optical fibers is larger than the diameter of the first core at the downstream end of each of the plurality of input optical fibers.

2. The optical combiner according to claim 1, further comprising an output optical fiber having a third core into which the light propagating through the second core of the first bridge fiber is incident, wherein the third core at the upstream end of the output optical fiber is sized to accommodate the second cores of the plurality of intermediate optical fibers at the downstream end of the first bridge fiber.

3. An optical combiner according to claim 1 or 2, further comprising a second bridge fiber having a fourth core into which the light propagating through the second core of the first bridge fiber is incident, wherein the fourth core of the second bridge fiber is sized at the upstream end of the second bridge fiber to accommodate the second cores of the plurality of intermediate optical fibers at the downstream end of the first bridge fiber, and the fourth core of the second bridge fiber has a tapered core portion whose diameter decreases downstream.

4. Each of the plurality of divergence angle reduction sections is composed of a GRIN lens whose refractive index gradually decreases from the central axis toward the radially outward direction, and the refractive index distribution constant g of the GRIN lens is given by: where n0 is the refractive index at the central axis, λ is the wavelength of the light, ω is the mode field diameter of the fundamental mode of the input optical fiber, and ω' is the mode field diameter of the fundamental mode of the intermediate optical fiber. The optical combiner according to claim 1 , wherein the optical combiner is in a range defined by:

5. Each of the plurality of divergence angle reduction sections is composed of a GRIN lens whose refractive index gradually decreases from the central axis toward the radially outward direction, and the length L of the GRIN lens along the optical axis direction is, where m is an integer equal to or greater than 0 and g is the refractive index distribution constant of the GRIN lens, The optical combiner according to claim 1 , wherein the optical combiner is in a range defined by:

6. Each of the plurality of divergence angle reduction sections is composed of: a first GRIN lens connected to the input optical fiber, the refractive index of which gradually decreases from the central axis toward the radially outward direction; and a second GRIN lens connected to the first GRIN lens and the intermediate optical fiber, the refractive index of which gradually decreases from the central axis toward the radially outward direction; wherein, assuming that the refractive index distribution constant of the first GRIN lens is g1, the refractive index distribution constant of the second GRIN lens is g2, the mode field diameter of the fundamental mode of the input optical fiber is ω, and the mode field diameter of the fundamental mode of the intermediate optical fiber is ω', then: The optical combiner according to claim 1 , wherein the following relationship holds:

7. The length L1 of the first GRIN lens along the central axis is expressed as: The length L2 of the second GRIN lens along the central axis is in a range defined by: where k is an integer equal to or greater than 0, 7. The optical combiner of claim 6.

8. An optical combiner as described in any one of claims 1 to 7, wherein the outer diameter from the portion of the plurality of input optical fibers located upstream of the divergence angle reduction section to the tapered section of the first bridge fiber becomes smaller toward the downstream side.

9. A laser device comprising: a plurality of laser light sources that generate laser light; and an optical combiner according to any one of claims 1 to 8, wherein the first cores of the plurality of input optical fibers of the optical combiner are optically coupled to the plurality of laser light sources.

Images