Rare-earth-doped optical fiber and fiber laser device

Abstract

Provided is a rare-earth-doped optical fiber capable of suppressing mode coupling between a fundamental mode and a high-order mode when light is transmitted through a long-period grating due to a TMI phenomenon. A rare-earth-doped optical fiber (10) includes a core (20) having a radius R, and a cladding (30, 40) covering the periphery of the core (20) and having a refractive index lower than that of the core (20). A rare-earth element is added to an addition region (22) occupying a range from the center of the core (20) to the radius R re The normalized frequency (V) at the operating wavelength of the rare-earth-doped optical fiber is 2.4 or more and 4.5 or less, and satisfies 0 < R re < (0.0247V 2 - 0.3353V + 1.6474)R.

Description

Technical Field

[0001] This invention relates to a rare-earth-added optical fiber and a fiber laser device, and more particularly to a rare-earth-added optical fiber used in a fiber laser device. Background Technology

[0002] Compared to traditional laser devices, fiber lasers offer superior focusing capabilities and can achieve higher power densities. Furthermore, they allow for smaller beam points, making them suitable for various fields such as laser processing and medicine. In recent years, the demand for high-power lasers has increased, leading to a push towards higher output in fiber laser devices. However, this increased output also increases the power density of light propagating in the fiber, making it more susceptible to wavelength shifts caused by stimulated Raman scattering (SRS). Considering that SRS can reduce the output of light at wavelengths designed for amplification, the fiber laser's output becomes unstable, and optical components like fiber Bragg gratings (FBGs) cannot function adequately in wavelengths beyond the target wavelength, SRS-induced wavelength shifts could cause these components to malfunction, leading to fiber laser device failure. Therefore, in high-output fiber laser devices, to prevent SRS-induced wavelength shifts, multimode fibers with increased core diameters are used to reduce the power density of light propagating in the fiber.

[0003] It is known that when multimode fiber is used as a rare-earth-based additive fiber for fiber laser devices, a phenomenon known as Transverse Mode Instability (TMI) occurs if the optical power exceeds a certain threshold. This TMI phenomenon is the formation of periodic refractive index variations in the rare-earth-based additive fiber, corresponding to the difference in propagation constants between the fundamental mode and higher-order modes.

[0004] The TMI phenomenon is believed to arise from the following process: When multiple modes propagate in a rare-earth-added fiber, interference between these modes occurs. Specifically, for rare-earth-added fibers undergoing laser oscillation, stimulated emission (SEMI) is more prevalent in regions producing constructive interference, while less so in regions producing destructive interference. When SEMI occurs, heat is generated due to quantum defects, resulting in a heat distribution along the length corresponding to the period of interference between modes. This leads to periodic refractive index variations.

[0005] When this TMI phenomenon occurs, the periodic refractive index variation along the length direction functions as a long-period grating, potentially causing intermode coupling between the fundamental mode and higher-order modes. Such intermode coupling leads to a deterioration in the beam quality of the output laser, impairing the advantages of the aforementioned fiber laser device, namely its focusing characteristics. For example, as shown in Non-Patent Document 1, if the optical power exceeds a certain threshold, the beam quality deteriorates drastically.

[0006] Existing technical documents

[0007] Patent documents

[0008] Non-patent document 1: T.Eidam, C.Wirth, C.Jauregui, F.Stutzki, F.Jansen, H.-J.Otto, O.Schmidt, T.Schreiber, J.Limpert, and A.Tunnermann, "Experimental observations of the threshold-like onset of mode instabilities in high power fiberamplifiers," Optics express 19,13218-13224(2011). Summary of the Invention

[0009] (a) Technical problems to be solved

[0010] This invention addresses the problems of the prior art. Its primary objective is to provide a rare-earth-based optical fiber that can suppress intermode coupling between the fundamental mode and higher-order modes when light passes through a long-period grating generated by the TMI phenomenon.

[0011] Furthermore, a second objective of the present invention is to provide a fiber laser device capable of suppressing the degradation of the beam quality of the output laser.

[0012] (II) Technical Solution

[0013] According to a first aspect of the present invention, a rare-earth-added optical fiber is provided, which is capable of suppressing intermode coupling between the fundamental mode and higher-order modes when light passes through a long-period grating caused by the TMI phenomenon. This rare-earth-added optical fiber comprises: a core of radius R, and a cladding covering the core and having a refractive index lower than that of the core. re Rare earth elements are added to the specified range of regions. The normalized frequency V at the operating wavelength of the aforementioned rare earth-added optical fiber is above 2.4 and below 4.5, satisfying 0 < R. re<(0.0247V) 2 -0.3353V+1.6474)R.

[0014] According to a second aspect of the present invention, a fiber laser device is provided that can suppress the degradation of the beam quality of the output laser. The fiber laser device includes: the aforementioned rare-earth-based fiber, and at least one excitation light source, which generates excitation light to excite the rare-earth elements added to the aforementioned rare-earth-based fiber and guides the excitation light into the aforementioned rare-earth-based fiber. Attached Figure Description

[0015] Figure 1 This is a schematic cross-sectional view of a rare-earth-based optical fiber according to an embodiment of the present invention.

[0016] Figure 2A This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP11 modes at the normalized frequency V = 2.5 of the optical fiber.

[0017] Figure 2B This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP11 modes when the normalized frequency V=3 of the optical fiber.

[0018] Figure 2C This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP11 modes at the normalized frequency V = 3.5 of the optical fiber.

[0019] Figure 2D This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP11 modes when the normalized frequency V=4 of the optical fiber.

[0020] Figure 2E This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP11 modes at the normalized frequency V = 4.45 of the optical fiber.

[0021] Figure 3 This is a graph showing the curves when the normalized mode coupling constant between the LP01 and LP11 modes begins to be less than 1 at each normalized frequency V.

[0022] Figure 4A It is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP21 modes when the normalized frequency V=4 of the optical fiber.

[0023] Figure 4B This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP21 modes at the normalized frequency V = 4.45 of the optical fiber.

[0024] Figure 5A This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP02 modes when the normalized frequency V=4 of the optical fiber.

[0025] Figure 5B This is a graph showing the relationship between the addition region of rare earth elements and the normalized mode coupling constant between the LP01 and LP02 modes at the normalized frequency V = 4.45 of the optical fiber.

[0026] Figure 6 It is a graph representing the bending diameter of the optical fiber when the normalized propagation constant b of the LP11 and LP02 modes is 0 relative to the normalized frequency V.

[0027] Figure 7 This represents the required length L' of the rare-earth-based fiber and the radius R of the added region. re A graph showing the relationship between the two.

[0028] Figure 8 This is a block diagram schematically illustrating the structure of a fiber laser device according to an embodiment of the present invention. Detailed Implementation

[0029] The following is for reference Figures 1 to 8 The embodiments of the rare-earth-based optical fiber and fiber laser device of the present invention will be described in detail. Figures 1 to 8 In this drawing, identical or equivalent constituent elements are labeled with the same reference numerals, and redundant descriptions are omitted. Additionally, in... Figures 1 to 8 In this text, there are instances where the scale and dimensions of each component are exaggerated, and instances where some components are omitted. Unless otherwise specified in the following explanation, terms such as "first" and "second" are used only to distinguish the components from each other, and do not indicate a specific order or sequence.

[0030] Figure 1 This is a schematic cross-sectional view illustrating a rare-earth-based optical fiber 10 according to an embodiment of the present invention. Figure 1As shown, the rare-earth-added optical fiber 10 includes: a core 20 of radius R, an inner cladding 30 covering the periphery of the core 20, and an outer cladding 40 covering the periphery of the inner cladding 30. The refractive index of the inner cladding 30 is lower than that of the core 20, and the refractive index of the outer cladding 40 is lower than that of the inner cladding 30. Furthermore, in this embodiment, an example of a double-clad optical fiber with two claddings is described, but it is not limited to this; the rare-earth-added optical fiber 10 may have a single cladding, or it may have three or more claddings.

[0031] This rare-earth-added optical fiber 10, for example, is used as an amplification fiber in a fiber laser device. It includes: an added region 22 containing rare-earth elements such as ytterbium (Yb), erbium (Er), thulium (Tr), and neodymium (Nd), and a non-added region 24 without rare-earth elements. The added region 22 extends from the center of the fiber core 20 to a radius R. re (0 < R) re The inventors discovered that by designing the radius R of the added region 22 of the rare-earth-based fiber 10 as follows... re This allows for the suppression of inter-mode coupling in rare-earth-added optical fibers 10.

[0032] Generally, the coupling constant κ between modes based on a long-period grating is represented by the following equation (1). The smaller the coupling constant κ, the less optical power is transferred between modes. That is, when the coupling constant κ is small, inter-mode coupling can be suppressed.

[0033] [Number 1]

[0034]

[0035] Here, x and y represent the two-dimensional coordinates of the optical fiber on a cross-section perpendicular to the optical axis, and Δn represents the amplitude of the refractive index variation along the length direction, E FM and E HOM These represent the electric field distributions of the basic mode and higher-order modes, respectively.

[0036] As described above, the refractive index variation caused by the TMI phenomenon originates from the heating caused by quantum defects. These quantum defects can occur in regions where rare-earth elements are added; therefore, the inventors believe that the aforementioned refractive index variation will occur in regions in the optical fiber where rare-earth elements are added. Therefore, the coupling constant κ between the LP01 and LP11 modes was calculated at different normalization frequencies V when the portion of Δn in equation (1) above was replaced with regions where rare-earth elements were added, and rare-earth elements were added to the core 20 of the optical fiber 10. The results are shown in… Figures 2A-2E .exist Figures 2A-2EIn the diagram, the horizontal axis represents the radius R of the added region 22, which is made up of the radius R of the fiber core. re Standardized value R re / R, the vertical axis is represented by R. re When / R=1, the coupling constant is the normalized value of the calculated coupling constant κ (normalized mode coupling constant). Figure 2A This represents the normalized mode coupling constant when V = 2.5. Figure 2B This represents the normalized mode coupling constant when V=3. Figure 2C This represents the normalized mode coupling constant when V = 3.5. Figure 2D This represents the normalized mode coupling constant when V=4. Figure 2E This represents the normalized mode coupling constant when V = 4.45.

[0037] according to Figures 2A-2E It can be seen that the radius R of the added region 22 re The smaller the value, the smaller the normalized mode coupling constant between the LP01 and LP11 modes. At each normalized frequency V, the R value is the value at which the normalized mode coupling constant begins to be less than 1. re Set as R' re ,

[0038] When V = 2.5, R' re / R = 0.95,

[0039] When V=3, R' re / R=0.9,

[0040] When V = 3.5, R' re / R = 0.75,

[0041] When V=4, R' re / R=0.7,

[0042] When V = 4.45, R' re / R = 0.65.

[0043] If we make it into a line graph, then as follows Figure 3 As shown. According to this Figure 3 It can be seen that when the normalized frequency V increases, if the radius R of the added region 22 is not increased... re If it becomes smaller, the normalized mode coupling constant cannot be smaller than 1.

[0044] Here, if we use a quadratic function for fitting... Figure 3 R' shown re / R, then we get the following equation (2).

[0045] [Number 2]

[0046]

[0047] Therefore, if the radius R of the added region 22 is increased... re R' as defined by equation (2) above re If the value is small, then the normalized mode coupling constant can be smaller than 1. That is, if 0 < R is satisfied... re <(0.0247V) 2 The radius R of the added region 22 is set in the manner of -0.3353V+1.6474)R···(3). re Compared to adding rare earth elements to the entire region of the fiber core 20 (with a normalized mode coupling constant of 1), this method can suppress the coupling from the LP01 mode to the LP11 mode.

[0048] Similarly, the normalized mode coupling constant between the LP01 and LP21 modes can be expressed as follows: Figure 4A and Figure 4B Perform the calculation as shown. Figure 4A This represents the normalized mode coupling constant when V=4. Figure 4B This represents the normalized mode coupling constant when V = 4.45. According to... Figure 4A and Figure 4B It can be seen that the normalized mode coupling constant between the LP01 and LP21 modes represents the relationship with... Figures 2A-2E The normalized mode coupling constants between the LP01 and LP11 modes shown in the figure exhibit the same tendency. Furthermore, if compared... Figure 4A and Figure 2D , Figure 4B and Figure 2E Therefore, if the radius R of region 22 is added... re If they are the same, then the normalized mode coupling constant between the LP01 mode and the LP21 mode is smaller than the normalized mode coupling constant between the LP01 mode and the LP11 mode. Therefore, if the radius R of the added region 22 is set in a manner that satisfies the above equation (3), re It can also suppress inter-mode coupling between LP01 mode and LP21 mode.

[0049] Furthermore, the normalized mode coupling constant between the LP01 and LP02 modes can be as follows: Figure 5A and Figure 5B Perform the calculation as shown. Figure 5A This represents the normalized mode coupling constant when V=4. Figure 5B This represents the normalized mode coupling constant when V = 4.45. According to... Figure 5A and Figure 5B It can be seen that the radius R of the added region 22 reWhen the radius R of the core 20 is smaller than that of the fiber core 20, the normalized mode coupling constant between the LP01 and LP02 modes is larger than when rare earth elements are added to the entire region of the core 20. Therefore, the inventors considered that the radius R of the added region 22 should not be smaller than that of the core 20. re Instead of making adjustments, the normalized frequency V of the rare-earth fiber 10 is adjusted, thereby suppressing intermode coupling between the LP01 and LP02 modes.

[0050] Generally, the rare-earth-added optical fibers used in fiber laser devices are several meters to tens of meters long. These fibers are wound into coils and housed within a housing. Typically, if the normalized frequency V exceeds 3.9, both the LP21 and LP02 modes can function as waveguides. However, if the normalized frequency V of the rare-earth-added fiber decreases, the transmission loss of the LP02 mode increases, potentially leading to a situation where the LP02 mode does not actually propagate within the rare-earth-added fiber.

[0051] Figure 6 This represents the bend diameter of the optical fiber when the normalized propagation constant b for both the LP11 and LP02 modes is 0 relative to the normalized frequency V. A normalized propagation constant b of 0 indicates that the mode cannot propagate in the fiber core. For example, at V = 4.5, the LP02 mode cannot propagate in a bent optical fiber with a diameter of approximately 100 mm or less. Furthermore, at V = 4.2, the LP02 mode cannot propagate in a bent optical fiber with a diameter of approximately 200 mm or less.

[0052] Therefore, if the radius R of the added region 22 is set in a manner that satisfies the above equation (3), re Furthermore, if the normalized frequency V of the rare-earth-based fiber 10 at the operating frequency (oscillation frequency) is 4.5 or less, preferably 4.2 or less, then when light passes through the long-period grating generated by the TMI phenomenon, the coupling between the LP01 mode and the LP11 mode, the coupling between the LP01 mode and the LP21 mode, and the coupling between the LP01 mode and the LP02 mode in the waveguide in the fiber core 20 can be suppressed, thereby suppressing the degradation of beam quality.

[0053] However, the amount of excitation light absorbed in a rare-earth-added fiber is directly proportional to the product of the amount of rare-earth elements contained in the cross-section of the rare-earth-added fiber and the length of the rare-earth-added fiber. Therefore, if the length of the rare-earth-added fiber 10 required to absorb a specified amount of excitation light when adding rare-earth elements to the entire region of the fiber core 20 is set to L, then the length of the rare-earth-added fiber 10 extending from the center of the fiber core 20 to a radius R... re The length L' of the rare earth-added optical fiber 10 required to absorb the same amount of excitation light when adding rare earth elements to the added region 22 is expressed by the following formula (4).

[0054] [Number 3]

[0055]

[0056] According to equation (4), if the radius R of the addition region 22 containing rare earth elements is increased... re If the length is too small, the entire length of the rare-earth-based fiber 10 needs to be increased in order to fully absorb the excitation light. If the entire length of the rare-earth-based fiber 10 is increased, stimulated Raman scattering is more likely to occur, resulting in a reduction in the amplification effect in the fiber core 20.

[0057] Here, the length L' of the added optical fiber 10 and the radius R of the added region are compared with those of the rare earth element mentioned above. re The relationship was examined. Figure 7 It means R re A graph showing the relationship between / R and L' / L. (Example) Figure 7 As shown, if R re If R ≥ 0.1R, then the required length L' of the rare-earth-added fiber 10 can be less than 100 times the length L required when adding rare-earth elements to the entire region of the fiber core 20. Furthermore, if R re If the value is ≥0.3R, then the required length L' of the rare-earth-added optical fiber 10 can be less than about 10 times the length L required when adding rare-earth elements to the entire region of the fiber core 20. Therefore, in order to avoid making the rare-earth-added optical fiber 10 too long, it is preferable to have 0.1R < R. re More preferably, 0.3R < R re .

[0058] However, the normalized frequency V of the optical fiber is defined by the following equation (5). In this embodiment, the normalized frequency V of the rare-earth-added optical fiber 10 is 2.4 or higher.

[0059] [Number 4]

[0060]

[0061] Here, λ is the operating wavelength of the optical fiber (the oscillation wavelength of the laser), n1 is the refractive index of the fiber core, n2 is the refractive index of the cladding, and Δ is the relative refractive index difference between the fiber core and the cladding.

[0062] The relative refractive index difference Δ is defined by the following equation (6).

[0063] [Number 5]

[0064]

[0065] According to equations (5) and (6) above, in order to keep the normalized frequency V of the optical fiber below a certain level, it is necessary to reduce the radius R of the fiber core or reduce the relative refractive index difference Δ between the fiber core and the cladding. If the relative refractive index difference Δ between the fiber core and the cladding is too small, the light containment effect in the fiber core will be reduced. Therefore, even for optical power below the threshold for TMI phenomenon, intermode coupling is easily generated due to minor external interference. The inventors have discovered that, for example, by reducing the radius R of the fiber core 20 of the rare-earth-based fiber 10 to R[μm]≦V / 0.34, the light containment effect in the fiber core 20 is maintained, and the normalized frequency V is reduced.

[0066] Next, an embodiment of the fiber laser device using the rare earth-based fiber 10 described above will be explained. Figure 8 This is a block diagram schematically illustrating the structure of a fiber laser device 100 using the aforementioned rare-earth-based fiber 10. Figure 8 As shown, the fiber laser device 100 includes: an optical resonator 110, which comprises the aforementioned rare-earth-based additive fiber 10 as an amplifying fiber capable of amplifying laser light; a plurality of front excitation light sources 120A that supply excitation light to the optical resonator 110 from one end (front); a plurality of rear excitation light sources 120B that supply excitation light to the optical resonator 110 from the other end (rear); a front optical combiner 130A that couples the excitation light output from the plurality of front excitation light sources 120A and guides it into the optical resonator 110; a rear optical combiner 130B that couples the excitation light output from the plurality of rear excitation light sources 120B and guides it into the optical resonator 110; a transmission fiber 140 that extends from the rear optical combiner 130B; and a laser output section 150 disposed at the downstream end of the transmission fiber 140. Each front excitation light source 120A is connected to the front optical combiner 130A via optical fiber 160A, and each rear excitation light source 120B is connected to the rear optical combiner 130B via optical fiber 160B.

[0067] The optical resonator 110 includes: a high-reflectivity section 170A that reflects light of a predetermined oscillation wavelength (e.g., 1070 nm) with a high reflectivity; and a low-reflectivity section 170B that reflects light of the same wavelength with a lower reflectivity than the high-reflectivity section 170A. The high-reflectivity section 170A and the low-reflectivity section 170B are, for example, constructed using a fiber Bragg grating (FBG) formed by periodically varying the refractive index of the optical fiber along the direction of light propagation, and a mirror. Figure 8 In the example shown, the high-reflectivity section 170A and the low-reflectivity section 170B are composed of fiber Bragg gratings.

[0068] The optical fibers 160A and 160B connected to the excitation light sources 120A and 120B respectively have a core and a cladding covering the core with a refractive index lower than that of the core. Optical waveguides for propagation of the excitation light generated by the excitation light sources 120A and 120B are formed inside the cores of these optical fibers 160A and 160B.

[0069] The excitation light sources 120A and 120B use laser modules that include, for example, high-output multimode semiconductor laser elements capable of emitting laser light with a wavelength of 975 nm. The excitation light generated by each front excitation light source 120A propagates in the core of optical fiber 160A and is directed toward the front optical combiner 130A, where it is coupled and guided into the optical resonator 110. Similarly, the excitation light generated by each rear excitation light source 120B propagates in the core of optical fiber 160B and is directed toward the rear optical combiner 130B, where it is coupled and guided into the optical resonator 110. Furthermore, the wavelengths of the excitation light generated by the front excitation light source 120A and the excitation light generated by the rear excitation light source 120B can be the same or different.

[0070] Excitation light from excitation sources 120A and 120B, respectively, is introduced into optical resonator 110 via optical combiners 130A and 130B, and propagates within the inner cladding 30 and core 20 of rare-earth-added optical fiber 10. When this excitation light passes through the added region 22 of core 20, it is absorbed by rare-earth element ions, which are excited to generate spontaneous emission light. This spontaneous emission light undergoes recursive reflection between high-reflectivity section 170A and low-reflectivity section 170B, amplifying light of a specific wavelength (e.g., 1070 nm) to generate laser oscillation. The laser amplified by optical resonator 110 propagates within the core 20 of rare-earth-added optical fiber 10, with a portion passing through low-reflectivity section 170B. The laser light that has passed through low-reflectivity section 170B is introduced into the core of transmission optical fiber 140 from the rear optical combiner 130B and propagates within the core of transmission optical fiber 140, such as... Figure 8 As shown, the laser output unit 150 emits the laser P as an output laser towards, for example, the object to be processed.

[0071] As an example, the core 20 of the rare-earth-added optical fiber 10 has a radius R of 9.25 μm and a refractive index of 1.4523. Furthermore, the refractive index of the inner cladding 30 is 1.45, and the relative refractive index difference Δ between the core 20 and the inner cladding 30 is 0.16%. The normalized frequency V of this rare-earth-added optical fiber 10 is V = 4.44 according to the above equation (5). The total length of the rare-earth-added optical fiber 10 is 15 m, and its bending diameter is 100 mm. Ytterbium is added to the addition region 22 of the core 20, and the radius R of the addition region 22 is... re It is 5.5 μm. In this example, since equation (3) above is 0 < Rre <5.97μm, therefore equation (3) is satisfied. Furthermore, since 0.3R = 2.78μm, 0.3R < R is satisfied. re .

[0072] Since the bending diameter of the rare-earth-added fiber 10 is 100 mm, as described above, the LP02 mode cannot propagate in the core 20 of the rare-earth-added fiber 10. Therefore, the waveguide modes that can be implemented in the core 20 are the LP01 mode, LP11 mode, and LP21 mode. At this time, the normalized mode coupling constants of the LP01 and LP11 modes, which are the basic modes, are 0.98, and the normalized mode coupling constants of the LP01 and LP21 modes are 0.96. Therefore, by using this rare-earth-added fiber 10 as the amplification fiber, compared with the case where rare-earth elements are added to the entire region of the core 20, the coupling from the LP01 mode to the LP11 mode and the coupling from the LP01 mode to the LP21 mode can be suppressed, and the beam quality degradation of the output laser P output from the laser output section 150 can be suppressed.

[0073] exist Figure 8 In the example shown, excitation light sources 120A and 120B and optical combiners 130A and 130B are arranged on both sides of the optical resonator 110, forming a bidirectional excitation type fiber laser device. Alternatively, the excitation light source and optical combiner can be arranged only on one side of the optical resonator 110. Furthermore, as a fiber laser device, the MOPA fiber laser device, which uses excitation light from the excitation light source to amplify seed light from the seed light source, is also known. The aforementioned rare-earth-based fiber 10 can also be used as the amplification fiber in such a MOPA fiber laser device.

[0074] As described above, according to a first aspect of the present invention, a rare-earth-added optical fiber can be provided that can suppress intermode coupling between the fundamental mode and higher-order modes when light passes through a long-period grating caused by the TMI phenomenon. This rare-earth-added optical fiber comprises: a core of radius R, and a cladding covering the core and having a refractive index lower than that of the core. The cladding occupies the area from the center of the core to radius R. re Rare earth elements are added to the specified range of regions. The normalized frequency V at the operating wavelength of the aforementioned rare earth-added optical fiber is above 2.4 and below 4.5, satisfying 0 < R. re <(0.0247V) 2 -0.3353V+1.6474)R.

[0075] By adding such rare-earth elements to optical fibers, the coupling constant between the fundamental and higher-order modes in the waveguide core can be reduced, thereby suppressing intermode coupling between the fundamental and higher-order modes when light passes through a long-period grating generated by the TMI phenomenon.

[0076] In addition, it is preferable to satisfy 0.1R < R re <(0.0247V) 2 -0.3353V + 1.6474)R. By making the radius R of the added region... re A value greater than 0.1R prevents excessively long rare-earth fiber optic cables and suppresses intermode coupling between the fundamental and higher-order modes. Furthermore, by satisfying 0.3R < R... re <(0.0247V) 2 -0.3353V+1.6474)R, which enables the rare earth-added optical fiber to be relatively short and suppresses intermode coupling between the fundamental mode and higher-order modes.

[0077] The normalized frequency V can be above 2.4 and below 4.2. By making the normalized frequency V fall within this range, it becomes difficult for the LP02 mode to be waveguided in the core of rare-earth-added fibers, thus suppressing intermode coupling between the fundamental mode and the LP02 mode. Furthermore, the normalized frequency V can also be above 2.4 and below 3.9. By making the normalized frequency V fall within this range, the LP02 mode is essentially not waveguided in the core of rare-earth-added fibers, or does not waveguide at all, thus substantially eliminating intermode coupling between the fundamental mode and the LP02 mode.

[0078] Preferably, the radius R of the fiber core is V / 0.34 [μm] or less. By making the radius R of the fiber core less than V / 0.34 [μm], the refractive index of the fiber core is not excessively reduced, and the normalized frequency V can be reduced.

[0079] Ytterbium can also be used as one of the aforementioned rare earth elements. When ytterbium is used as a rare earth element, it can be incorporated into optical fibers for use in fiber laser devices for materials processing.

[0080] According to a second aspect of the present invention, a fiber laser device capable of suppressing the degradation of the beam quality of the output laser is provided. The fiber laser device comprises: the aforementioned rare-earth-based fiber, and at least one excitation light source that generates excitation light to excite the aforementioned rare-earth elements added to the aforementioned rare-earth-based fiber and guides it into the aforementioned rare-earth-based fiber.

[0081] According to such a fiber laser device, since the intermode coupling between the fundamental mode and the higher-order mode in the rare-earth-added fiber can be suppressed, the beam quality of the output laser can be suppressed, the high focusing ability can be maintained, and the high output laser can be output.

[0082] Preferably, at least a portion of the aforementioned rare-earth-added optical fiber is bent with a bending diameter of 200 mm or less, more preferably with a bending diameter of 100 mm or less. By bending at least a portion of the rare-earth-added optical fiber in this way, the LP02 mode in the waveguide within the core of the rare-earth-added optical fiber can be reduced, thereby suppressing intermode coupling between the fundamental mode and the LP02 mode.

[0083] The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments, and various different methods can be implemented within the scope of its technical concept.

[0084] This application is based on and claims priority to Japanese Patent Application No. 2021-014119, filed on February 1, 2021. The disclosure of that application is incorporated herein by reference in its entirety.

[0085] Industrial applicability

[0086] This invention is applicable to rare-earth-based optical fibers used in fiber laser devices.

[0087] Explanation of reference numerals in the attached figures

[0088] 10 - Rare earth-added fiber; 20 - Fiber core; 22 - Added area; 24 - Unadded area; 30 - Inner cladding; 40 - Outer cladding; 100 - Fiber laser device; 110 - Optical resonator; 120A, 120B - Excitation source; 130A, 130B - Optical combiner; 140 - Transmission fiber; 150 - Laser output section; 170A - High reflectivity section; 170B - Low reflectivity section.

Claims

1. A rare-earth-based optical fiber, which possesses: The fiber core, having a radius of R, occupies a radius from the center of the fiber core to a radius of R. re Rare earth elements were added to the area within the range; and A cladding layer, which covers the periphery of the fiber core, has a refractive index lower than that of the fiber core. The normalized frequency V at the operating wavelength of the rare-earth-added optical fiber is above 2.4 and below 4.5, satisfying 0 < R. re < (0.0247V) 2 -0.3353V + 1.6474)R.

2. The rare-earth-based optical fiber according to claim 1, characterized in that, Further satisfying 0.1R < R re < (0.0247V) 2 -0.3353V + 1.6474)R.

3. The rare-earth-based optical fiber according to claim 2, characterized in that, Further satisfying 0.3R < R re < (0.0247V) 2 -0.3353V + 1.6474)R.

4. The rare-earth-based optical fiber according to any one of claims 1 to 3, characterized in that, The normalized frequency V is greater than 2.4 and less than 4.

2.

5. The rare-earth-based optical fiber according to claim 4, characterized in that, The normalized frequency V is greater than 2.4 and less than 3.

9.

6. The rare-earth-based optical fiber according to any one of claims 1 to 3, characterized in that, The radius R of the fiber core is less than V / 0.34 [μm].

7. The rare-earth-based optical fiber according to any one of claims 1 to 3, characterized in that, The rare earth element mentioned is ytterbium.

8. A fiber laser device comprising: The rare-earth-based optical fiber according to any one of claims 1 to 7; and At least one excitation light source generates excitation light that excites the rare earth element added to the rare earth-added optical fiber and guides the excitation light into the rare earth-added optical fiber.

9. The fiber laser device according to claim 8, characterized in that, At least a portion of the rare-earth-added optical fiber is bent with a bending diameter of less than 200 mm.

10. The fiber laser device according to claim 9, characterized in that, At least a portion of the rare-earth-added optical fiber is bent with a bending diameter of less than 100 mm.

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