Fiber laser

Abstract

A fiber laser is realized which reduces the possibility of occurrence of a reduction in light beam output caused by TMI and reduces the possibility of occurrence of stimulated Raman scattering. A fiber laser (1) is provided with a gain fiber (11) in which Yb is added in a core, and a front excitation light source group (15a) which is capable of generating front excitation light which is capable of being input to the gain fiber (11) and belongs to a 976 nm band. The amount of absorption of the front excitation light in a length of 1 m interval (I1) including an end face of the gain fiber (11) at which the front excitation light is incident is 253 W or more and 1100 W or less.

Description

Technical Field

[0001] This invention relates to a fiber laser having a gain fiber with Yb added to the core. Background Technology

[0002] A widely used type of fiber laser employs a double-clad fiber with Yb (ytterbium) added to the core as the gain fiber. Yb in glass has the 915nm and 976nm bands as its main absorption bands. Therefore, this type of fiber laser uses the following structure: by supplying excitation light belonging to the 915nm or 976nm band to the cladding of the gain fiber, the Yb added to the core is excited (transformed into a flipped distribution state). To promote high output in this type of fiber laser, sufficient amplification can be achieved in the gain fiber by increasing the power of the excitation light or increasing the concentration of Yb.

[0003] Existing technical documents Non-patent literature Non-patent literature 1: C. Jauregui et al., "Physical origin of mode instabilities in high-power fiber laser systems", Opt. Express 20(12) 12912-12925 (2012) Summary of the Invention

[0004] (a) Technical problems to be solved For the aforementioned fiber laser, increasing the power of the excitation light or increasing the Yb concentration may cause problems due to TMI (Transverse Mode Instability): the proportion of higher-order modes in the laser output from the fiber laser increases, i.e., the beam output of the laser output from the fiber laser decreases (see Non-Patent Document 1). Furthermore, if a mode filter that removes higher-order modes is used to avoid beam output reduction, the power of the laser output from the fiber laser may decrease.

[0005] Here, the mechanism by which beam output decreases due to TMI can be explained, for example, as follows: Intermodal interference occurs between the fundamental and higher-order modes propagating along the fiber core, resulting in a spatial variation in the signal light quantity along the length of the gain fiber. Consequently, the stimulated emission quantity also varies spatially along the length of the gain fiber, resulting in a spatial variation in the heat generated by quantum defects along the length of the gain fiber. This temperature variation along the length of the gain fiber further leads to a spatial variation in the refractive index. This variation in refractive index promotes the transition from the fundamental mode to higher-order modes, resulting in the aforementioned decrease in beam output.

[0006] To maintain the total Yb content while avoiding beam output reduction caused by TMI, one could consider increasing the overall length of the gain fiber. However, increasing the overall length of the gain fiber may introduce other problems, such as stimulated Raman scattering (SRS), which can lead to excitation source failure.

[0007] The present invention addresses the above-mentioned problems by providing a fiber laser that reduces the likelihood of beam output reduction caused by TMI and also reduces the likelihood of stimulated Raman scattering.

[0008] (II) Technical Solution In one aspect of the fiber laser of the present invention, the following structure is adopted: comprising: a gain fiber with Yb added to the fiber core, and a front excitation light source group, the front excitation light source group being capable of generating front excitation light, the front excitation light being input into the gain fiber and belonging to the 976nm frequency band, the absorption of the front excitation light in a 1m interval of length including the end face of the gain fiber where the front excitation light is incident, the absorption of the front excitation light in the interval calculated according to ∫P(λ)A(λ)dλ is 253W or more and 1100W or less.

[0009] Here, P(λ) [W] is the power spectrum of the forward excitation light, and A(λ) [% / m] is the absorptivity spectrum of Yb added to the core of the gain fiber.

[0010] In another aspect of the fiber laser of the present invention, the following structure is adopted: comprising: a gain fiber with Yb added to the fiber core, and a front excitation light source group, which is capable of generating front excitation light, which is input to the gain fiber and belongs to the 976nm frequency band, and the absorption of the front excitation light in a 1m interval of the gain fiber including the end face of the incident front excitation light, wherein the measured value of the absorption of the front excitation light in the interval is 253W or more and 1100W or less.

[0011] (III) Beneficial Effects According to one aspect of the present invention, a fiber laser can be realized that reduces the likelihood of beam output reduction caused by TMI and reduces the likelihood of stimulated Raman scattering. Attached Figure Description

[0012] Figure 1 This is a block diagram illustrating the structure of a fiber laser according to an embodiment of the present invention.

[0013] Figure 2 This is a graph showing the relationship between the amount of excitation light absorbed and the proportion of non-compliance in interval I1 for multiple fiber lasers prepared as samples.

[0014] Figure 3 This is a graph representing the standard absorbance spectrum of Yb [dB / m].

[0015] Figure 4 This is a graph showing the distribution of qualified and unqualified products for multiple fiber lasers used as samples. The graph uses the excitation light absorption per unit length [dB / m] of the gain fiber at a wavelength of 978nm as the horizontal axis and the absorption of the front excitation light from the front excitation source (LDM front excitation light absorption) as the vertical axis.

[0016] Figure 5 This is a graph showing the elemental concentration distribution of the gain fiber for the embodiment shown in Table 8.

[0017] Figure 6 This is a graph showing the distribution of the relative refractive index difference of the gain fiber for the embodiment shown in Table 8.

[0018] Figure 7 This is a graph showing the cladding absorption spectrum for the embodiment shown in Table 8. Detailed Implementation

[0019] (Structure of a fiber laser) Reference Figure 1 The structure of a fiber laser 1 according to an embodiment of the present invention will be described. Figure 1 This is a block diagram showing the structure of fiber laser 1.

[0020] like Figure 1 As shown, the fiber laser 1 includes: a gain fiber 11, a high-reflection mirror 12, a low-reflection mirror 13, a front excitation combiner 14a, a rear excitation combiner 14b, a front excitation light source group 15a, a rear excitation light source group 15b, an input fiber 16a, and an output fiber 16b.

[0021] Gain fiber 11 is an optical fiber capable of amplifying laser light using the energy of excitation light. In this embodiment, Yb-added double-clad fiber is used as gain fiber 11, which has: a cylindrical core with added Yb (ytterbium), an inner cladding that surrounds the core and is cylindrical, and an outer cladding that surrounds the inner cladding and is cylindrical. Furthermore, the cross-sectional shape of the inner cladding can be polygonal or D-shaped. Therefore, even when the length of gain fiber 11 is long, the absorptivity of the excitation light is not reduced, and excitation light can be absorbed efficiently. Furthermore, the refractive index distribution of gain fiber 11 is determined to allow the propagation of one fundamental mode and at least one higher-order mode. Additionally, gain fiber 11 does not contain fusion splices, and the refractive index distribution and Yb concentration are the same throughout its length. Therefore, no connection loss occurs, and the total loss in gain fiber 11 can be minimized.

[0022] A high-reflectivity mirror 12 is connected (in this embodiment, by fusion splicing) to one end of the gain fiber 11. A low-reflectivity mirror 13 is connected (in this embodiment, by fusion splicing) to the other end of the gain fiber 11. In this embodiment, an FBG (Fiber Bragg Grating) is used as both the high-reflectivity mirror 12 and the low-reflectivity mirror 13.

[0023] In the fiber laser 1, at least a portion of the reflection wavelength band of the high-reflectivity mirror 12 overlaps with at least a portion of the reflection wavelength band of the low-reflectivity mirror 13. Thus, the gain fiber 11, the high-reflectivity mirror 12, and the low-reflectivity mirror 13 constitute a resonator O that recursively amplifies laser light of wavelength λ belonging to the overlapping portion of these two reflection wavelength bands. At wavelength λ, the reflectivity of the low-reflectivity mirror 13 (e.g., less than 15%) is lower than that of the high-reflectivity mirror 12 (e.g., more than 95%). Therefore, the laser light of wavelength λ recursively amplified in the resonator O is primarily output to the outside of the resonator O via the low-reflectivity mirror 13.

[0024] The front excitation light source group 15a contains ma excitation light sources (ma is any natural number). Each excitation light source in the front excitation light source group 15a is a structure for generating excitation light belonging to the 976nm frequency band. Here, the 976nm frequency band refers to a wavelength band of 967.9nm and above and 983.0nm and below, which includes the absorption band of Yb. The front excitation combiner 14a is a structure for inputting the excitation light generated by each excitation light source in the front excitation light source group 15a into the gain fiber 11. The front excitation combiner 14a includes at least ma excitation light input ports 14a1, at least one visible light input port 14a2, and at least one resonator-side port 14a3. Each excitation light input port 14a1 is connected to an excitation light source constituting the front excitation light source group 15a. The visible light input port 14a2 is connected (in this embodiment, fused) to the input fiber 16a for visually confirming the irradiation position of the laser. Furthermore, inputting visible light into the visible light input port 14a2 is not mandatory, and it can be used as a port for inputting excitation light, or as a port for monitoring the amount of reflected light from the object being processed. The resonator-side port 14a3 is connected (in this embodiment, fused together) to the end opposite to the gain fiber 11 side of the high-reflectivity mirror 12.

[0025] For the forward excitation light generated in each excitation source of the forward excitation light source group 15a, it is guided to the cladding of the gain fiber 11 via the forward excitation combiner 14a and the high-reflectivity mirror 12, which is used to change the Yb added to the core of the gain fiber 11 to a flipped distribution state. For the light of wavelength λ in the naturally emitted light generated in the gain fiber 11 that is consistent with the reflection wavelength of the high-reflectivity mirror 12 and the low-reflectivity mirror 13, it is amplified in the core of the gain fiber 11 by the resonator O. In this embodiment, the excitation light source group 15a is adopted as an excitation light source group (ma=78) obtained by connecting six LD modules, each containing 13 LD (laser diode) chips. Each LD chip is designed with a center wavelength of approximately 975 nm (the absorption wavelength of Yb) in the oscillation wavelength band and a wavelength width of 7 nm or more and 12 nm or less in the oscillation wavelength band. In addition, Figure 1 The diagram shows six LD modules constituting the front excitation light source group 15a. For each LD module, the excitation light from 13 LD chips (not shown) is coupled to a single optical fiber. In other words, the excitation light from a total of 78 LD chips is coupled in the front excitation combiner 14a and then coupled to a single optical fiber at the resonator side port 14a3.

[0026] The rear excitation light source group 15b contains mb (mb is any natural number) excitation light sources. Each excitation light source in the rear excitation light source group 15b is a structure for generating excitation light belonging to the 976nm frequency band. The rear excitation combiner 14b is a structure for inputting the excitation light generated in each excitation light source in the rear excitation light source group 15b into the gain fiber 11. The rear excitation combiner 14b includes: at least mb excitation light input ports 14b1, at least one signal light output port 14b2, and at least one resonator-side port 14b3. Each excitation light input port 14b1 is connected to the excitation light source constituting the rear excitation light source group 15b. The signal light output port 14b2 is connected (in this embodiment, fused) to the output fiber 16b. The resonator-side port 14b3 is connected (in this embodiment, fused) to the end opposite to the gain fiber 11 side of the low-reflection mirror 13.

[0027] For the rear excitation light generated in each excitation source of the rear excitation light source group 15b, it is guided to the cladding of the gain fiber 11 via the rear excitation combiner 14b and the low-reflection mirror 13, which is used to change the Yb added to the core of the gain fiber 11 to a flipped distribution state. For the signal light (laser) amplified in the core of the gain fiber 11, it is guided to the output fiber 16b via the low-reflection mirror 13 and the rear excitation combiner 14b, and output to the outside via the output fiber 16b. In this embodiment, the rear excitation light source group 15b is an excitation light source group (mb=78) obtained by connecting six LD modules, each containing 13 LD (laser diode) chips. Each LD chip is designed with a center wavelength of approximately 975nm (the absorption wavelength of Yb) in the oscillation wavelength band, and a wavelength width of 7nm or more and 12nm or less in the oscillation wavelength band. Furthermore, in Figure 1 The diagram shows six LD modules constituting the rear excitation light source group 15b. For each LD module, the excitation light from 13 LD chips (not shown) is coupled to a single optical fiber. In other words, the excitation light from a total of 78 LD chips is coupled to the rear excitation combiner 14b and to a single optical fiber at the resonator side port 14b3.

[0028] Furthermore, in this embodiment, the fiber laser 1 is implemented as a bidirectional excitation type fiber laser having a front excitation light source group 15a and a rear excitation light source group 15b, but the present invention is not limited thereto. That is, the fiber laser 1 can also be implemented as a unidirectional excitation type fiber laser having only the front excitation light source group 15a.

[0029] (Characteristics of fiber lasers) Transverse Mode Instability (TMI) primarily occurs within a 1-meter-long section I1 of the gain fiber 11, encompassing the end face of the incident front excitation light. This is because the absorption of the front excitation light within section I1 is greater than that outside of it, resulting in more heat generation within I1. Therefore, to suppress the beam output reduction caused by TMI, it is necessary to reduce the absorption of the front excitation light within section I1.

[0030] Therefore, the inventors of this case conducted an inspection on 436 fiber lasers 1 prepared as samples using the following qualification criteria: the maximum output of the laser obtained based on the total output of the front excitation light source group 15a and the rear excitation light source group 15b is 2070W or more. Figure 2 This is a graph showing the relationship between the excitation light absorption and the proportion of non-conforming products in interval I1. According to... Figure 2 The chart shows that by reducing the absorption of the excitation light in interval I1 to below 1100W, the defect rate can be suppressed to below 1 / 2. Furthermore, according to... Figure 2 The chart shows that by reducing the absorption of the excitation light in interval I1 to below 1050W, the defect rate can be suppressed to below 4 / 10. Furthermore, according to... Figure 2 As shown in the chart, by reducing the absorption of the excitation light in interval I1 to below 1000W, the defect rate can be suppressed to below 3 / 10.

[0031] Furthermore, to suppress stimulated Raman scattering, it is preferable to shorten the overall length of the gain fiber 11. For example, when the effective cross-sectional area of ​​the gain fiber 11 is 400 μm... 2 Ideally, the total length of the gain fiber 11 should be 27m or less. Here, the effective cross-sectional area is 400μm. 2 At that time, there is a tendency to properly take into account the following (1) to (3).

[0032] (1) It can make the number of waveguide modes of light two, namely LP01 mode light and LP11 mode light, and can suppress the reduction in beam quality caused by the number of waveguide modes of light to a minimum. (2) It can reduce the bending loss of LP01 mode and LP11 mode; (3) The effective cross-sectional area is relatively large, which can effectively suppress nonlinear optical effects.

[0033] Increasing the core diameter of the gain fiber 11 increases its effective cross-sectional area. However, this also increases the V-value, allowing LP02 mode light to propagate even when the gain fiber 11 is bent. Propagating LP02 mode light into the gain fiber 11 in this way could potentially lead to TMI (Transient Motion Injection). Conversely, decreasing the relative refractive index difference of the core of the gain fiber 11 also increases the effective cross-sectional area. However, this also increases the bending loss of the gain fiber 11. LP01 and LP11 mode light, which can be emitted as laser output light, are more susceptible to this bending loss, potentially increasing their propagation losses.

[0034] Based on the above, the effective cross-sectional area value that can appropriately take into account (1) to (3) above can be 400 μm. 2 Furthermore, when the total length of the gain fiber 11 is set to L and the effective cross-sectional area to Aeff, the effective cross-sectional area of ​​the gain fiber 11 is 400 μm. 2 To achieve a 2kW fiber laser output, in order to suppress nonlinear optical effects, the L / Aeff ratio, which is proportional to the nonlinear optical effects, needs to be 27m / 400μm. 2 In other words, from the perspective of suppressing nonlinear optical effects, the total length of the gain fiber 11 needs to be less than 27m.

[0035] Furthermore, to suppress overheating in various parts, it is preferable to reduce residual excitation light, i.e., increase the excitation light absorption of the gain fiber 11. For example, in an embodiment of the fiber laser 1 described later, the total output of the front excitation light source group 15a is 1611 W. From the viewpoint of suppressing heat generation caused by residual excitation light and ensuring long-term reliability, it is preferable to keep the power of the residual excitation light below 16 W, that is, to keep the excitation light absorption of the gain fiber 11 above 20 dB. This value is obtained by using the output Pout of the residual excitation light and the total output Pin of the front excitation light source group 15a, and calculating 10×log(16 / 1611) ≒20 dB using the formula 10×log(Pout / Pin). Here, in order to obtain the above-mentioned effective cross-sectional area of ​​400 μm... 2The desired value is to achieve an excitation light absorption of 20 dB or more in a gain fiber 11 with a total length of less than 27 m. Therefore, the excitation light absorption per unit length needs to be 0.74 dB / m or more, and the excitation light absorption rate per unit length needs to be 15.68% or more. This value is obtained by using the excitation light absorption per unit length AA (dB / m) and calculating (1 - 1 / 10^(AA / 10)) × 100 to obtain (1 - 1 / 10^(0.74 / 10)) × 100 ≒ 15.68%. At this time, the excitation light absorption in the aforementioned interval I1 is 253 W or more based on the above results. This value is obtained by using the total output W of the front excitation light source group 15a and the absorption AB (%) of the excitation light per unit length, and calculating 1611×15.68 / 100≒253W using the formula W×AB / 100.

[0036] Based on the above, in the fiber laser 1 of this embodiment, a structure is adopted in which the absorption of the forward excitation light in interval I1 is 253W or more and 1100W or less. Therefore, the fiber laser 1 of this embodiment achieves the following effects: it reduces the possibility of beam output reduction caused by TMI, prevents overheating of various parts, and reduces the possibility of stimulated Raman scattering.

[0037] Furthermore, in fiber laser 1, (1) the absorption of the forward excitation light in interval I1 can be calculated to be 253W or more, up to 1100W; (2) the measured value of the absorption of the forward excitation light in interval I1 can be 253W or more, up to 1100W. The above-mentioned effects are achieved in all cases.

[0038] The absorption amount X [W] of the front excitation light in interval I1 can be calculated, for example, using the power spectrum P (λ) [W] of the front excitation light output from the front excitation light source group 15a, and the absorptivity spectrum A (λ) [% / m] of Yb added to the core of the gain fiber 11, according to X = ∫P (λ)A (λ)dλ.

[0039] Here, the absorptivity spectrum A(λ) of Yb can be obtained through calculation or measurement. When calculated, the absorptivity spectrum A(λ) of Yb can be calculated, for example, using the standard absorptivity spectrum A1(λ) of Yb normalized to 978 nm (theoretical value) and the absorptivity A2 [% / m] of Yb added to the core of gain fiber 11 at 978 nm, according to A(λ) = A1(λ) × A2. In this case, the absorption amount X[W] of the forward excitation light in interval I1 can be calculated according to X = ∫P(λ)A1(λ)A2dλ. Figure 3The standard absorbance spectrum of Yb is shown in [dB / m]. Figure 3 The absorption spectrum [dB / m] shown is converted into an absorption rate spectrum [% / m]. The obtained absorption rate spectrum [% / m] is then normalized with the value at a wavelength of 978 nm as 100% to obtain the standard absorption rate spectrum A(λ) mentioned above.

[0040] Alternatively, instead of using the absorbance of Yb at a wavelength of 978 nm to calculate the absorption spectrum A(λ) of Yb, a structure using the absorbance of Yb at other absorption wavelengths (966 nm, 915 nm, etc.) can be used. Here, the reason for using the absorbance of Yb at a wavelength of 978 nm is that the difference from the wavelength of the actual excitation light is small, thus allowing for a more accurate calculation of the absorption of the excitation light in interval I1 compared to other absorption wavelengths. In fact, while using the absorbance of Yb at other absorption wavelengths allows for higher accuracy in the absorption measurement itself compared to using the absorbance at 978 nm, the absorption of the excitation light in interval I1 is inaccurate due to the larger difference from the wavelength of the actual excitation light. Furthermore, the reason for being able to measure the absorbance of Yb at other absorption wavelengths with high accuracy is that, since the absorption is smaller compared to 978 nm, measurements using longer gain fibers can be performed, reducing the impact of errors caused by trimming measurements.

[0041] Furthermore, the power spectrum P(λ) of the front excitation light can be calculated or obtained through actual measurement. When calculated, the power spectrum P(λ) of the front excitation light can, for example, be obtained by using the output spectra Pi(λ) of each front excitation light source included in the front excitation light source group 15a according to P(λ) = Σ i=1,2,...,ma Pi(λ) is used for calculation. The output spectrum Pi(λ) of each front excitation light source can be a measured distribution or an approximate distribution (a Gaussian distribution that reproduces the center wavelength and wavelength width of the actual power spectrum). In addition, as in this embodiment, when an LD module containing multiple LD chips is used as a front excitation light source, based on taking the representative value (average, median, etc.) of the center wavelength of each LD chip as the center wavelength of the LD module and taking the representative value of the wavelength width of each LD chip as the wavelength width of the LD module, the output spectrum Pi(λ) of the LD module can be obtained by simply taking the Gaussian distribution that reproduces its center wavelength and its wavelength width.

[0042] Figure 4 For the fiber laser 1 used as a sample, the excitation light absorption per unit length [dB / m] of the gain fiber 11 at a wavelength of 978 nm is used as the horizontal axis (in Figure 4The figure shows "Yb absorption at 978 nm" [dB / m], and the absorption of the front excitation light from the front excitation source (LDM front excitation light absorption) is used as the vertical axis (in Figure 4 The “forward absorption” [W] shown in the figure represents the distribution of conforming and nonconforming products.

[0043] according to Figure 4 It can be seen that for a gain fiber 11 that satisfies the following condition, the defect rate can be suppressed to a level of 1 / 2, wherein the absorption of the front excitation light from the front excitation source (i.e., Figure 4 The "forward excitation light absorption (in other words, LDM forward excitation light absorption)" is above 206W and below 358W, and the excitation light absorption per unit length of the gain fiber 11 at a wavelength of 978nm (i.e., Figure 4 The "excitation light absorption (in other words, Yb absorption at 978 nm)" is above 2.58 dB / m and 3.07 dB / m. Furthermore, in the case of a gain fiber 11 that meets this condition, the following is taken: Figure 4 The average output of the fiber laser 1, based on the total output of the front excitation light source group 15a and the rear excitation light source group 15b, is calculated by averaging the output values ​​of each qualified product. The result is 2110W. In contrast, in the case of a gain fiber 11 that does not meet this condition, the average output of the fiber laser 1 is calculated by averaging the output values ​​of each qualified product. Figure 4 The average output of the fiber laser 1, based on the total output, is calculated by averaging the output values ​​of each defective product, resulting in an average output of 2068W. Therefore, the gain fiber 11 that meets this condition is advantageous in terms of improved average output (and reduced power consumption when operating at rated output) compared to the gain fiber 11 that does not meet this condition. Furthermore, in Figure 4 In the diagram, a solid line represents the curve where the absorption of the excitation light in front, calculated using ∫P(λ)A1(λ)A2dλ, is 253W. Figure 4 The curve representing the excitation light absorption of 253W is shown in the figure. The curve representing the absorption of the excitation light in front, calculated using ∫P(λ)A1(λ)A2dλ, is shown in the figure. The curve representing the absorption of the excitation light in front, calculated using ∫P(λ)A1(λ)A2dλ, is shown in the figure. Figure 4 The curve shown is represented by an excitation light absorption of 1100 W. According to... Figure 4It can be seen that the region satisfying the above conditions falls between these two curves. That is, it can be seen that if the above conditions are met, the following condition is automatically satisfied: "The absorption of the front excitation light calculated using ∫P(λ)A1(λ)A2dλ is above 253W and below 1100W". In addition, regarding the value of the absorption of the front excitation light from the above-mentioned front excitation source (LDM front excitation light absorption), the calculated value of the absorption of the front excitation light calculated according to ∫P(λ)A1(λ)dλ can be used, or the measured value of the absorption of the front excitation light can be divided by A2. Here, A1(λ) (theoretical value) is the standard absorption spectrum of Yb after normalization based on a wavelength of 978nm. A2 [% / m] (measured value) is the absorption rate of Yb added to the core of the gain fiber at a wavelength of 978nm.

[0044] (Example of a fiber laser) Regarding the embodiments and comparative examples of fiber laser 1, for 11 fiber lasers 1 selected from 436 fiber lasers 1 prepared as samples, the following quantities are shown in Tables 1 to 11 respectively.

[0045] (1) The center wavelength of each LD module in the front excitation light source group 15a [nm] ("LD center wavelength [nm]" in Tables 1-11), (2) Wavelength width of each LD module in the front excitation light source group 15a [nm] ("LD wavelength width [nm]" in Tables 1-11), (3) Excitation light absorption per unit length of gain fiber 11 at a wavelength of 978 nm [dB / m] (Yb "Yb 978 nm absorption [dB / m]" in Tables 1-11) (4) Absorption amount [W] in interval I1 of the front excitation light output from each LD module of the front excitation light source group 15a (“Front excitation × 978 absorption amount [W]” in Tables 1-11). (5) Absorption amount [W] of the front excitation light output from the front excitation light source group 15a in interval I1 (calculated according to ∫P(λ)A1(λ)A2dλ) ("Total Absorption [W]" in Tables 1-11) (6) Output of front excitation light source group 15a [W] (“Excitation light output [W]” in Tables 1-11) (7) Output of fiber laser 1 [W] ("Output [W]" in Tables 1-11), (8) Qualification results ("Qualification" in Tables 1-11).

[0046] Furthermore, regarding (4) and (5), in Tables 1 to 11 below, the following situation exists: the total value of the absorption amount in interval I1 of the front excitation light output from each LD module of the front excitation light source group 15a is inconsistent with the value of the absorption amount in interval I1 of the front excitation light output from the front excitation light source group 15a. However, this is due to the following effect: the value of the absorption amount in interval I1 of the front excitation light output from each LD module is rounded up or discarded. Therefore, for this embodiment and comparative example, it can be explained that the total value of the absorption amount in interval I1 of the front excitation light output from each LD module of the front excitation light source group 15a is consistent with the value of the absorption amount in interval I1 of the front excitation light output from the front excitation light source group 15a.

[0047] [Table 1] [Table 2] [Table 3] [Table 4] [Table 5] [Table 6] [Table 7] [Table 8] [Table 9] [Table 10] [Table 11] The fiber lasers 1 shown in Tables 1 to 9, as examples, satisfy the condition that the absorption of the forward excitation light in interval I1 is 253W or more and 1100W or less. The fiber lasers 1 shown in Tables 10 and 11 are comparative examples and do not satisfy the condition that the absorption of the forward excitation light in interval I1 is 253W or more and 1100W or less. Therefore, it can be seen that the fiber laser 1 of the examples is a qualified product, while the fiber laser 1 of the comparative examples is a defective product.

[0048] Furthermore, in the qualified fiber laser 1, the absorption in region I1 of the front excitation light output from the front excitation light source group 15a is 634W or more. Therefore, based on these embodiments, it can be seen that, in order to reduce the defect rate (reduce the possibility of beam output reduction caused by TMI), it is preferable to make the absorption of the front excitation light in region I1 634W or more.

[0049] Furthermore, in the qualified fiber laser 1, the total power of the front excitation light, i.e., the output of the front excitation light source group 15a, is 1509W or more. Therefore, based on these embodiments, it can be seen that, in order to reduce the defect rate (reduce the possibility of beam output reduction caused by TMI), it is preferable to make the total power of the front excitation light 1509W or more.

[0050] Figure 5 This is a graph showing the elemental concentration distribution of the gain fiber 11 in the embodiment shown in Table 8. Figure 6 This is a graph showing the refractive index distribution of the gain fiber 11 in the embodiment shown in Table 8. In the gain fiber 11, as... Figure 5 As shown, the following local addition structure was adopted: there are addition regions of Yb, Al, P, and B in the center, and addition regions of Ge on the outer side. Figure 5 As shown, the concentration of these elements was adjusted in a way that kept the overall refractive index of the fiber core constant. Figure 7 This is a graph showing the cladding absorption spectrum [dB / m] of the gain fiber 11 in the embodiment shown in Table 8. The shape of the cladding absorption spectrum is similar to... Figure 3 The standard absorbance spectra of Yb shown have roughly the same shape.

[0051] (Summarize) In the fiber laser of Embodiment 1 of the present invention, the following structure is adopted: it comprises: a gain fiber with Yb added to the fiber core, and a front excitation light source group, which is capable of generating front excitation light, which is input to the gain fiber and belongs to the 976nm frequency band, and the absorption of the front excitation light in a 1m interval of the gain fiber including the end face of the incident front excitation light, the absorption of the front excitation light in the interval calculated according to ∫P(λ)A(λ)dλ is more than 253W and less than 1100W.

[0052] Here, P(λ) [W] is the power spectrum of the forward excitation light, and A(λ) [% / m] is the absorptivity spectrum of Yb added to the core of the gain fiber.

[0053] Regarding the fiber laser of embodiment 2 of the present invention, based on the structure of embodiment 1, the following structure is adopted: the absorption spectrum A(λ) is calculated according to A(λ) = A1(λ) × A2.

[0054] Here, A1(λ) is the standard absorption spectrum of Yb after normalization based on a wavelength of 978 nm, and A2[% / m] is the absorption rate of Yb added to the core of the gain fiber at a wavelength of 978 nm.

[0055] Regarding the fiber laser of embodiment 3 of the present invention, based on the structure of embodiment 1 or 2, the following structure is adopted: the absorption amount of the forward excitation light in the interval is 634W or more.

[0056] Regarding the fiber laser of embodiment 4 of the present invention, based on the structure of any one of embodiments 1 to 3, the following structure is adopted: the total power of the front excitation light is 1509W or more.

[0057] Regarding the fiber laser of embodiment 5 of the present invention, based on the structure of any one of embodiments 1 to 4, the following structure is adopted: the value of the absorption of the front excitation light from the front excitation light source group is 206W or more and 358W or less, calculated by ∫P(λ)A1(λ)dλ, or the value of the measured value of the absorption of the front excitation light ÷ A2, and the excitation light absorption per unit length of the gain fiber at a wavelength of 978nm is 2.58dB / m or more and 3.07dB / m or less.

[0058] Here, A1(λ) is the standard absorption spectrum of Yb after normalization based on a wavelength of 978 nm, and A2[% / m] is the absorption rate of Yb added to the core of the gain fiber at a wavelength of 978 nm.

[0059] Regarding the fiber laser of embodiment 6 of the present invention, based on the structure of any one of embodiments 1 to 5, the following structure is adopted: the gain fiber does not contain fusion splices, and the refractive index distribution and Yb concentration are the same throughout the entire length.

[0060] In the fiber laser of Embodiment 7 of the present invention, the following structure is adopted: it comprises: a gain fiber with Yb added to the fiber core, and a front excitation light source group, which is capable of generating front excitation light, which is input to the gain fiber and belongs to the 976nm frequency band, and the absorption of the front excitation light in a 1m long interval including the end face of the gain fiber where the front excitation light is incident, is measured to be 253W or more and 1100W or less.

[0061] Regarding the fiber laser of embodiment 8 of the present invention, based on the structure of embodiment 7, the following structure is adopted: the absorption amount of the forward excitation light in the interval is 634W or more.

[0062] Regarding the fiber laser of embodiment 9 of the present invention, based on the structure of embodiment 7 or 8, the following structure is adopted: the total power of the front excitation light is 1509W or more.

[0063] Regarding the fiber laser of embodiment 10 of the present invention, based on the structure of any one of embodiments 7 to 9, the following structure is adopted: the value of the absorption of the front excitation light from the front excitation light source group is 206W or more and 358W or less, calculated by ∫P(λ)A1(λ)dλ, or the value of the measured value of the absorption of the front excitation light ÷ A2, and the excitation light absorption per unit length of the gain fiber at a wavelength of 978nm is 2.58dB / m or more and 3.07dB / m or less.

[0064] Here, A1(λ) is the standard absorption spectrum of Yb after normalization based on a wavelength of 978 nm, and A2[% / m] is the absorption rate of Yb added to the core of the gain fiber at a wavelength of 978 nm.

[0065] (Appendix items) This invention is not limited to the above embodiments. Various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical solutions disclosed in the above embodiments are also included in the technical scope of this invention.

[0066] Explanation of reference numerals in the attached figures 1-Fiber laser; 11-Gain fiber; 12-High-reflectivity mirror; 13-Low-reflectivity mirror; 14a-Front excitation combiner; 14b-Rear excitation combiner; 15a-Front excitation light source group; 15b-Rear excitation light source group; 16a-Input fiber; 16b-Output fiber.

Claims

1. A fiber laser, characterized in that, It comprises: a gain fiber with Yb added to its core, a high-reflectivity mirror and a low-reflectivity mirror disposed at both ends of the gain fiber, and a front excitation light source group capable of generating front excitation light that can be input into the gain fiber and belongs to the 976nm frequency band. The reflection wavelength bands of the high-reflectivity mirror and the low-reflectivity mirror include the wavelength of the laser. The forward excitation light is input into the gain fiber via the high-reflectivity mirror. Regarding the absorption of the forward excitation light in a 1m interval encompassing the end face of the gain fiber where the forward excitation light is incident, the absorption of the forward excitation light in this interval, calculated according to ∫P(λ)A(λ)dλ, is above 253W and below 1100W. Regarding the absorption value of the front excitation light from the aforementioned front excitation light source group, the calculated value of the absorption value of the front excitation light calculated according to ∫P(λ)A1(λ)dλ, or the value of the measured value of the absorption value of the front excitation light ÷ A2, is 206W or more and 358W or less. The excitation light absorption per unit length of the aforementioned gain fiber at a wavelength of 978 nm is above 2.58 dB / m and below 3.07 dB / m. Here, P(λ) [W] is the power spectrum of the forward excitation light, A(λ) [% / m] is the absorptivity spectrum of Yb added to the core of the gain fiber, A1(λ) is the theoretical value of the absorptivity spectrum of Yb after normalization based on a wavelength of 978 nm, and A2 [% / m] is the absorptivity of Yb added to the core of the gain fiber at a wavelength of 978 nm.

2. The fiber laser according to claim 1, characterized in that, The absorption of the forward excitation light in the specified interval is above 634W.

3. The fiber laser according to claim 1 or 2, characterized in that, The total power of the forward excitation light is above 1509W.

4. The fiber laser according to claim 1 or 2, characterized in that, The gain fiber has no fusion splices and its refractive index distribution and Yb concentration are the same throughout its entire length.

5. A fiber laser, characterized in that, It comprises: a gain fiber with Yb added to its core, a high-reflectivity mirror and a low-reflectivity mirror disposed at both ends of the gain fiber, and a front excitation light source group capable of generating front excitation light that can be input into the gain fiber and belongs to the 976nm frequency band. The reflection wavelength bands of the high-reflectivity mirror and the low-reflectivity mirror include the wavelength of the laser. The forward excitation light is input into the gain fiber via the high-reflectivity mirror. Regarding the absorption of the forward excitation light in a 1m long interval encompassing the end face of the gain fiber where the forward excitation light is incident, the measured absorption value of the forward excitation light in this interval is above 253W and below 1100W. Regarding the absorption value of the front excitation light from the aforementioned front excitation light source group, the calculated value of the absorption value of the front excitation light calculated according to ∫P(λ)A1(λ)dλ, or the value of the measured value of the absorption value of the front excitation light ÷ A2, is 206W or more and 358W or less. The excitation light absorption per unit length of the aforementioned gain fiber at a wavelength of 978 nm is above 2.58 dB / m and below 3.07 dB / m. Here, A1(λ) is the theoretical value of the absorption spectrum of Yb after normalization based on a wavelength of 978 nm, and A2[% / m] is the absorption rate of Yb added to the core of the gain fiber at a wavelength of 978 nm.

6. The fiber laser according to claim 5, characterized in that, The absorption of the forward excitation light in the specified interval is above 634W.

7. The fiber laser according to claim 5 or 6, characterized in that, The total power of the forward excitation light is above 1509W.

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