Optical fiber

Abstract

The present invention provides an optical fiber that can suppress an increase in light loss even if stored in a moist and hot environment.　An optical fiber (10) comprises: a core (11) which has an average refractive index value of n1; an inner cladding (12) which surrounds the outer circumferential surface of the core (11), is made of silica glass having fluorine added thereto, and has an average refractive index value of n2; and an outer cladding (13) which surrounds the outer circumferential surface of the inner cladding (12), has a lower added fluorine concentration than the inner cladding (12), has an average refractive index value of n3, and has a thickness of T3, where n1>n2, n3>n2, and T3≥1.40 μm are satisfied.

Description

optical fiber

[0001] The present invention relates to optical fibers.

[0002] An optical fiber is known in which the outer surface of a core is surrounded by a fluorine-doped inner cladding, and the outer surface of the inner cladding is surrounded by a silica cladding. Such an optical fiber is described in Patent Document 1 listed below.

[0003] JP 2002-82250

[0004] The present inventors have confirmed that in optical fibers having a cladding made of fluorine-doped quartz, the number of OH groups in the cladding increases in a humid and hot environment. When the number of OH groups in the cladding increases, the OH groups may absorb light propagating through the cladding, generating heat. Furthermore, when a portion of high-power light propagating through the optical fiber propagates through the cladding, the light may be absorbed by foreign matter adhering to the surface of the cladding, generating heat. For this reason, a fluorine-free quartz layer may be provided at the outermost periphery of the cladding, as in the optical fiber disclosed in Patent Document 1. However, Patent Document 1 focuses on optical fibers for communication applications and does not consider the propagation of high-power light through optical fibers. Therefore, Patent Document 1 does not consider the increase in OH groups in the cladding or the thickness of the quartz cladding. Therefore, there is a concern that the loss of propagating light may increase if the optical fiber disclosed in Patent Document 1 is stored in a humid and hot environment.

[0005] Therefore, an object of the present invention is to provide an optical fiber that can suppress an increase in light loss even when stored in a humid and hot environment, and can suppress heat generation even when foreign matter adheres to the cladding surface.

[0006] In order to solve the above problems, the present invention provides an optical fiber comprising a core having an average refractive index of n1, an inner cladding surrounding the outer surface of the core, made of fluorine-doped silica glass, and having an average refractive index of n2, and an outer cladding surrounding the outer surface of the inner cladding, having a lower fluorine doping concentration than the inner cladding, an average refractive index of n3, and a thickness of T3, wherein n1>n2 n3>n2 T3≧1.40 μm are satisfied.

[0007] By making the fluorine doping concentration of the outer cladding lower than that of the inner cladding, an increase in OH groups in the cladding can be suppressed compared to when the fluorine doping concentration of the outer cladding is equal to or greater than that of the inner cladding. Furthermore, by making the thickness T3 of the outer cladding 1.40 μm or greater, the penetration of OH groups into the inner cladding, which has a higher fluorine doping concentration, can be suppressed. Therefore, with the optical fiber of the present invention, even when stored in a humid and hot environment, an increase in OH groups in the cladding can be suppressed, and an increase in optical loss can be suppressed.

[0008] As described above, the present invention provides an optical fiber that can suppress an increase in optical loss even when stored in a humid and hot environment.

[0009] FIG. 1 is a diagram showing a fiber laser device according to an embodiment of the present invention. FIG. 2 is a diagram showing an optical fiber connector. FIG. 3 is a diagram showing a cross section perpendicular to the longitudinal direction of a pumping optical fiber. FIG. 4 is a diagram showing the refractive index profile of a pumping optical fiber. FIG. 5 is a diagram showing a combiner. FIG. 6 is a diagram showing the relationship between the storage time of an optical fiber stored in a humid and hot environment and the loss of light of a given power propagating through the optical fiber. FIG. 7 is a diagram showing the relationship between the fluorine doping concentration of the outer cladding and the increase in light loss per unit time. FIG. 8 is a diagram showing the relationship between the thickness of the outer cladding and the ratio of the power of light propagating through the outer cladding to the power of light propagating through the entire pumping optical fiber. FIG. 9 is a diagram showing the results of a theoretical calculation of the relationship between the thickness of the inner cladding and the power of evanescent light. FIG. 10 is a diagram showing the calculated relationship between the diameter of an optical fiber and the cumulative fracture probability after 35,000 hours.

[0010] Preferred embodiments of the optical fiber according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved within the scope of the claims without departing from the spirit thereof. Note that, for ease of understanding, the scales of the drawings may differ from those used in the following description.

[0011] Fig. 1 is a diagram showing a fiber laser device according to this embodiment. As shown in Fig. 1, the fiber laser device 1 according to this embodiment mainly comprises a plurality of pumping light sources 30, an amplification optical fiber 40, a combiner 50, a high reflectivity fiber bragg grating (HR-FBG) 55, an output coupler fiber bragg grating (OC-FBG) 56, and a plurality of optical fiber connectors OFC connected to the pumping light sources 30. The fiber laser device 1 according to this embodiment is a resonator-type fiber laser device.

[0012] Each pumping light source 30 is a light source that emits pumping light that is absorbed by an active element doped in the amplification optical fiber 40 (described later). The pumping light source 30 is configured, for example, by a laser diode (LD). The wavelength of the pumping light emitted from the pumping light source 30 is, for example, 976 nm when the active element doped in the amplification optical fiber 40 is ytterbium (Yb).

[0013] One end of an optical fiber connector OFC is connected to the pumping light source 30, and the other end of the optical fiber connector OFC is connected to a combiner 50. In Fig. 1, to avoid complication, only one optical fiber connector OFC is labeled with a reference numeral. Fig. 2 is a diagram showing the optical fiber connector OFC. The optical fiber connector OFC includes an LD pigtail fiber 20 and a pumping optical fiber 10.

[0014] The LD pigtail fiber 20 mainly comprises a core 21 and a cladding 22. The LD pigtail fiber 20 has a tapered portion 24 that is tapered midway along its length, with one end of the LD pigtail fiber 20 being a thick portion 23 and the other end being a thin portion 25. The thick portion 23 is connected to a pumping light source 30, and pumping light from the pumping light source 30 enters the core 21 from the end of the thick portion 23. By connecting the thick portion 23 to the pumping light source 30 in this manner, high-power light can be input while suppressing loss.

[0015] One end of the excitation optical fiber 10 is connected to the end of the small-diameter section 25. Fig. 3 is a diagram showing a cross section of the excitation optical fiber 10 perpendicular to the longitudinal direction. The excitation optical fiber 10 includes a core 11, an inner cladding 12 tightly surrounding the outer peripheral surface of the core 11, an outer cladding 13 tightly surrounding the outer peripheral surface of the inner cladding 12, and a coating layer 14 covering the outer cladding 13. The inner cladding 12 and the outer cladding 13 may be simply referred to as the cladding. The core 11 is connected to a core 21 in the small-diameter section 25 of the LD pigtail fiber 20. The diameter D1 of the core 11 is equal to or greater than the diameter of the core 21 in the small-diameter section 25.

[0016] FIG. 4 is a diagram showing the refractive index profile of the excitation optical fiber 10. In FIG. 4, the region showing the refractive index of the core 11 is designated 11n, the region showing the refractive index of the inner cladding 12 is designated 12n, the region showing the refractive index of the outer cladding 13 is designated 13n, and the region showing the refractive index of the coating layer 14 is designated 14n. The refractive index n1 of the core 11 is higher than the refractive index n2 of the inner cladding 12. That is, n1 > n2. The refractive index n3 of the outer cladding 13 is higher than the refractive index of the inner cladding 12. That is, n3 > n2. In this embodiment, the refractive index n1 of the core 11 and the refractive index n3 of the outer cladding 13 are equal. However, the refractive index n3 of the outer cladding 13 may be lower or higher than the refractive index n1 of the core 11. The refractive index n4 of the coating layer 14 is lower than the refractive index n3 of the outer cladding 13. That is, n3 > n4. Furthermore, in this embodiment, the refractive index n4 is lower than the refractive index n2 of the inner cladding 12. That is, n2 > n4.

[0017] In this embodiment, the core 11 is made of pure silica glass. The inner cladding 12 is made of silica glass doped with fluorine. The outer cladding 13 is made of silica glass doped with less fluorine than the inner cladding 12. A low fluorine doping level also includes non-doped fluorine. In this embodiment, the outer cladding 13 is made of pure silica. Therefore, as described above, the refractive index n1 of the core 11 and the refractive index n3 of the outer cladding 13 are equal. Note that as long as the above refractive index profile is maintained, the core 11, inner cladding 12, and outer cladding 13 may be doped with a dopant that changes the refractive index.

[0018] The diameter D1 of the core 11, the thickness T2 of the inner cladding 12, the thickness T3 of the outer cladding 13, the outer diameter D3 of the outer cladding 13, etc. will be described in detail later.

[0019] FIG. 5 is a diagram showing a combiner 50. In the combiner 50, the other end of the pumping optical fiber 10 is connected to one end of an optical fiber 51. Although not shown, the optical fiber 51 mainly comprises a core, an inner cladding tightly surrounding the outer surface of the core, an outer cladding surrounding the outer surface of the inner cladding, and a coating layer covering the outer cladding. In other words, from a structural standpoint, the optical fiber 51 is a double-clad optical fiber. However, the inner cladding may be composed of multiple cladding layers. The refractive index of the inner cladding is lower than that of the core, and the refractive index of the outer cladding is lower than that of the inner cladding. The pumping optical fiber 10 is connected to the inner cladding.

[0020] One end of an optical fiber 70 having a core and a cladding is connected to the core at one end of the optical fiber 51. The other end of the optical fiber 70 is connected to a terminating member 71 that converts light into heat, as shown in FIG.

[0021] The other end of the optical fiber 51 is connected to the optical fiber 53 shown in FIG. 1 . The optical fiber 53 has the same configuration as the optical fiber 51, with the core of the optical fiber 51 and the core of the optical fiber 53 connected, and the inner clad of the optical fiber 51 and the inner clad of the optical fiber 53 connected. An HR-FBG 55 is provided in the core of the optical fiber 53. The HR-FBG 55 reflects, for example, light with a wavelength of 1064 nm with a reflectance of 99% or more. Note that the optical fiber 51 may be extended to form the optical fiber 53.

[0022] One end of the amplification optical fiber 40 is connected to the other end of the optical fiber 53. The amplification optical fiber 40 has the same configuration as the optical fiber 53, except that an active element is doped in the core. The core of the optical fiber 53 and the core of the amplification optical fiber 40 are connected, and the inner cladding of the optical fiber 53 and the inner cladding of the amplification optical fiber 40 are connected.

[0023] The active element added to the core of the amplification optical fiber 40 is an element that is excited by the pumping light emitted by the pumping light source 30. An example of such an active element is ytterbium. Other examples of active elements include thulium (Tm), cerium (Ce), neodymium (Nd), europium (Eu), erbium (Er), etc. Further examples of active elements include bismuth (Bi) in addition to rare earth elements.

[0024] One end of an optical fiber 54 is connected to the other end of the amplification optical fiber 40. The optical fiber 54 has a configuration similar to that of the optical fiber 53. The core of the amplification optical fiber 40 and the core of the optical fiber 54 are connected, and the inner clad of the amplification optical fiber 40 and the inner clad of the optical fiber 54 are connected. An OC-FBG 56 is provided in the core of the optical fiber 54. The OC-FBG 56 reflects light of at least a portion of the wavelengths of the light reflected by the HR-FBG 55 with a reflectivity lower than that of the HR-FBG 55. The OC-FBG 56 reflects the light with, for example, a reflectivity of 10%. In this way, a resonator is formed by the HR-FBG 55, the amplification optical fiber 40, and the OC-FBG 56. A portion of the light resonating in this resonator is transmitted through the OC-FBG 56.

[0025] One end of a delivery fiber 61 is connected to the other end of the optical fiber 54. The delivery fiber 61 has a core and a clad, and the core is connected to the core of the optical fiber 54.

[0026] An output section 62 is connected to the other end of the delivery fiber 61. The output section 62 is made of, for example, a glass rod having a diameter larger than that of the core of the delivery fiber 61. Therefore, the light incident on the output section 62 from the core of the delivery fiber 61 is expanded in diameter before being output from the output section 62.

[0027] Next, the operation of the fiber laser device 1 will be described.

[0028] First, pumping light is emitted from each pumping light source 30. As described above, the wavelength of this pumping light is, for example, 976 nm. The pumping light emitted from the pumping light source 30 is incident on the core 21 of the large-diameter portion 23 of the LD pigtail fiber 20. The effective cross-sectional area of ​​the pumping light propagating through the core 21 of the large-diameter portion 23 is reduced in the small-diameter portion 24, increasing the optical power density, and the pumping light propagates to the core 21 of the small-diameter portion 25. In the small-diameter portion 24, the light is reflected at the boundary between the core 21 and the cladding 22, increasing the NA (numerical aperture) of the propagating light. Therefore, the NA of the light emitted from the small-diameter portion 25 is greater than the NA of the light incident on the large-diameter portion 23.

[0029] The pumping light propagating through the core 21 of the LD pigtail fiber 20 enters the core 11 of the pumping optical fiber 10 and propagates through the core 11. At this time, a portion of the pumping light may leak from the core 11. Even in this case, the leaked pumping light propagates through the outer cladding 13 because the refractive index n4 of the coating layer 14 is lower than the refractive index n3 of the outer cladding 13. Then, in the combiner 50, the pumping light enters the inner cladding of the optical fiber 51 from the core 11. At this time, the pumping light propagating through the outer cladding 13 also enters the inner cladding of the optical fiber 51. The light propagating through the inner cladding of the optical fiber 51 enters the inner cladding of the amplification optical fiber 40 via the inner cladding of the optical fiber 53 and propagates mainly through the inner cladding. At least a portion of the pumping light propagating through the inner cladding is absorbed by the active element doped in the core of the amplification optical fiber 40 as it passes through the core of the amplification optical fiber 40, exciting the active element. The light emitted by the excited active element resonates between the HR-FBG 55 and the OC-FBG 56, and the resonating light is amplified. A portion of the amplified light passes through the OC-FBG 56, passes through the core of the delivery fiber 61, and is emitted from the emission unit 62. In this way, light is emitted from the fiber laser device 1.

[0030] Next, the excitation optical fiber 10 will be described in detail.

[0031] First, the effect of fluorine added to the cladding will be explained.

[0032] Figure 6 shows the relationship between storage time and the loss of light of a given power propagating through an optical fiber when the optical fiber is stored in a humid and hot environment. In Figure 6, Samples 1 and 2 are pumping optical fibers in which the entire cladding is doped with fluorine at a generally uniform concentration and has a resin coating layer. In Samples 1 and 2, 82 / 90 and 86 / 90 refer to the core diameter (μm) / cladding diameter (μm). Samples 3 and 4 are pumping optical fibers in which the core is made of pure silica glass and is coated with a polymer cladding having a refractive index lower than that of the silica glass. In Samples 3 and 4, - / 105 and - / 175 indicate core diameters of 105 μm and 175 μm, respectively. Samples 5 to 8 are the pumping optical fibers 10 of this embodiment shown in Figure 3. In Samples 5 to 8, 80 / 90 / 105, 93 / 97 / 105, 160 / 168 / 175, and 98 / 101 / 105 mean the diameter (μm) of the core 11 / the diameter (μm) of the inner cladding 12 / the diameter (μm) of the outer cladding 13, respectively.

[0033] In addition, in Samples 5 to 8, the propagation of light to the outer cladding 13 is suppressed. Therefore, even if foreign matter adheres to the surface of the outer cladding 13, the foreign matter is prevented from absorbing light and generating heat. In contrast, in Samples 1 to 4, the propagation of light to the surface of the optical fiber is not suppressed as compared to Samples 5 to 8. Therefore, in Samples 1 to 4, if foreign matter adheres to the surface, the foreign matter may absorb light and generate heat.

[0034] As shown in Figure 6, in Samples 1 and 2, in which the entire cladding is doped with fluorine, the optical loss increases with storage time. As explained above, the pumping optical fiber can also use light leaking into the cladding as pumping light. In Samples 1 and 2, the number of OH groups in the cladding increases with storage time, which is thought to increase the absorption of cladding light by the OH groups, resulting in the optical loss increasing with storage time. Furthermore, measurement of the optical absorption spectrum of Samples 1 and 2 and analysis by Raman spectroscopy revealed that the number of OH groups in the cladding increased.

[0035] On the other hand, in Samples 3 to 8, compared with Samples 1 and 2, the loss of light did not increase over time.

[0036] Next, the fluorine doping concentration in the outer cladding 13 of a pumping optical fiber having the same configuration as the pumping optical fiber 10 was changed, and the relationship between the fluorine concentration and the increase in optical loss per unit time was measured. Table 1 below shows the relative refractive index difference Δ (%) of the outer cladding 13 with respect to pure silica glass, the fluorine concentration [wt %] in the outer cladding 13, and the increase in optical loss per unit time [db / h], and Fig. 7 shows the above relationships. As is clear from Table 1 and Fig. 7, the increase in optical loss per unit time increased as the fluorine doping concentration in the outer cladding 13 increased.

[0037] 6 , Table 1, and 7 show that when the fluorine concentration added to the outer cladding 13 is high, the loss of light propagating through the optical fiber increases. Furthermore, when the fluorine concentration added to the outer cladding 13 is low, the increase in the loss of light propagating through the optical fiber is suppressed. Therefore, when the fluorine concentration added to the outer cladding 13 is lower than the fluorine concentration added to the inner cladding 12, the increase in OH groups in the inner cladding 12 and the outer cladding 13 can be suppressed compared to when the fluorine concentration added to the outer cladding 13 is equal to or higher than the fluorine concentration added to the inner cladding 12. In other words, the increase in OH groups in the claddings can be suppressed, and the increase in light loss can be suppressed.

[0038] Next, the thickness of the outer cladding 13 of the excitation optical fiber 10 will be described.

[0039] The thickness T3 of the outer cladding 13 for Samples 1 and 2 and Samples 5, 6, 8, and 9 shown in Figure 6, and the loss results for Figure 6 are shown in Table 2. The NA of each optical fiber was 0.28. An NA of 0.28 for an optical fiber means that the core of each sample is made of pure silica glass, while Samples 1 and 2 have the cladding doped with fluorine at the highest possible concentration, and Samples 5, 6, 8, and 9 have the inner cladding doped with fluorine at the highest possible concentration.

[0040] As is clear from Table 2, if the thickness T3 of the outer cladding 13 satisfies at least T3≧1.40 μm, the optical loss does not increase even when the optical fiber is stored in a humid and hot environment.

[0041] Fig. 8 shows the relationship between the thickness T3 of the outer cladding 13 and the ratio of the power of light propagating through the outer cladding 13 to the power of light propagating through the entire pumping optical fiber 10 when 25-mode light is propagated through the pumping optical fiber 10 with an NA of 0.28. This light ratio is a value expressed as a percentage of the power of light propagating through the outer cladding 13 when the total power of light propagating through the pumping optical fiber 10 is 100. In deriving this relationship, the diameter D1 of the core 11 of the pumping optical fiber 10 whose refractive index profile is shown in Fig. 4 was set to 80 µm, the outer diameter D2 of the inner cladding 12 was set to 90 µm, and the thickness T3 of the outer cladding 13 was set to 1.0 µm, 3.5 µm, 5.0 µm, and 10.0 µm. The values ​​shown in Fig. 8 are shown in Table 3.

[0042] As shown in Figure 8 and Table 3, if the thickness T3 of the outer cladding 13 satisfies T3 ≤ 5 μm, the power of light propagating through the outer cladding 13 can be reduced by more than 15%. Furthermore, if the thickness T3 of the outer cladding 13 satisfies T3 ≤ 3.5 μm, the power of light propagating through the outer cladding 13 can be reduced by more than 5%, thereby stabilizing the power of light propagating through the pumping optical fiber 10. Therefore, the cladding thickness T3 is preferably 5 μm or less, and more preferably 3.5 μm or less. Furthermore, in Table 3, there is no difference in the leakage light rate between when the thickness T3 is 3.5 μm and when it is 1.0 μm. However, the leakage light rate is actually smaller to two decimal places when the thickness T3 is 1.0 μm.

[0043] Next, the thickness of the inner cladding 12 will be described.

[0044] FIG. 9 shows the results of theoretical calculations of the relationship between the thickness T2 of the inner cladding 12 and the power of evanescent light. In this calculation, a power distribution of less than NA 0.28 was used as the light source, based on actual data from a high-NA light source in which 95% of the light was NA 0.435. The vertical axis of FIG. 9 represents the ratio of the power of evanescent light reaching the outer cladding 13 to the power of light at the interface between the core 11 and the inner cladding 12. As shown in FIG. 9 , when the thickness T2 of the inner cladding 12 is 1.3 μm or greater, the ratio of evanescent light reaching the outer cladding 13 can be reduced to 5% or less. Furthermore, when the thickness T2 of the inner cladding 12 is 2.3 μm or greater, the ratio of evanescent light reaching the outer cladding 13 can be reduced to 1% or less. Therefore, from the viewpoint of suppressing light leakage into the outer cladding 13, the thickness T2 of the inner cladding 12 preferably satisfies T2≧1.3 μm, and more preferably satisfies T2≧2.3 μm.

[0045] Next, the NA of the light incident on the excitation optical fiber 10 will be described.

[0046] The NA of the light incident on the pumping optical fiber 10 is determined by the divergence angle θ of the light emitted from the LD pigtail fiber 20 into space when the LD pigtail fiber 20 connected to the pumping optical fiber 10 is separated from the pumping optical fiber 10, and the refractive index n ０ The divergence angle here is the angle between the longitudinal direction of the core 21 at the output end of the LD pigtail fiber 20 and the traveling direction of the light that spreads most outward out of the light emitted into space. The NA of the light incident on the pumping optical fiber 10 is defined by the following formula: NA=n ０ sinθ

[0047] Therefore, if the NA of the light incident on the core 11 of the pumping optical fiber 10 is 0.46 or less, it is possible to prevent the light from leaking from the outer cladding 13. For the above reasons, it is preferable that the NA of the light incident on the pumping optical fiber 10 satisfies NA≦0.46.

[0048] Next, the outer diameter D3 of the outer cladding 13 will be described.

[0049] FIG. 10 is a graph showing the calculated relationship between the diameter of an optical fiber and the cumulative fracture probability after 35,000 hours. The calculations were performed assuming a proof survival length of 2,000 m, a bending diameter φ of 100 mm, and two turns. As shown in FIG. 10 , if the outer diameter of the optical fiber cladding is 560 μm or less, the cumulative fracture probability can be reduced to 1.00E-6 or less. Therefore, it is preferable that the diameter of the glass portions of the optical fiber 51, the optical fiber 53 connected to the optical fiber 51, the amplification optical fiber 40, and the optical fiber 42 of the combiner 50 shown in FIG. 5 , i.e., the diameter of the inner cladding, be 560 μm or less. Furthermore, as shown in FIG. 5 , when the pump optical fiber 10 and the optical fiber 70 have the same diameter, the optical fiber 70 is arranged in the center, and six pump optical fibers 10 surround the optical fiber 70, i.e., a 1-6 arrangement, the optical fibers 70 and the pump optical fibers 10 can be arranged closely together. In this arrangement, when the optical fiber 70 and the excitation optical fiber 10 are connected to the optical fiber 51, the excitation optical fiber 10, the optical fiber 70, and the excitation optical fiber 10 are aligned in a straight line, so if the outer diameter of the inner cladding of the optical fiber 51 is 560 μm, it is preferable that the outer diameter D3 of the outer cladding 13 of the excitation optical fiber 10 is smaller than 1 / 3 of 560 μm, and therefore it is preferable that 186 μm≧D3 is satisfied.

[0050] Next, we will explain the microbend loss of the excitation optical fiber 10. Table 4 below shows the results of measuring the microbend loss of an optical fiber having a core and a cladding. To measure this microbend loss, light with a wavelength of 1180 nm was propagated, and the optical loss was measured using an OTDR (Optical Time Domain Reflectometer).

[0051] As shown in Table 4, the cladding diameters of the optical fibers of Samples 10 to 14 were 105 μm, the cladding diameters of the optical fibers of Samples 15 to 18 were 90 μm, and the cladding diameters of the optical fibers of Samples 19 and 20 were 95 μm. For the optical fibers with a cladding diameter of 90 μm, the average microbend loss was 9.33 dB / km, with a variation of 0.91 dB / km. In contrast, for the optical fibers with a cladding diameter of 105 μm, the average microbend loss was 0.84 dB / km, with a variation of 0.11 dB / km. Furthermore, for the optical fibers with a cladding diameter of 95 μm, the average microbend loss was 4.58 dB / km, with a variation of 0.78 dB / km. Thus, in an optical fiber having a cladding diameter of 105 μm, the average microbend loss is below 1.0 dB / km, and compared to an optical fiber having a cladding diameter of 90 μm, the microbend loss characteristics are superior to the optical fiber having a cladding diameter of 90 μm by an order of magnitude. Furthermore, optical fibers having a cladding diameter greater than 105 μm also have superior microbend loss characteristics to the extent that the microbend loss value is superior to the optical fiber having a cladding diameter of 105 μm by an order of magnitude. Therefore, from the viewpoint of reducing microbend loss in the pumping optical fiber 10, it is preferable that the outer diameter D3 of the outer cladding 13 satisfies D3≧105 μm. Furthermore, in an optical fiber having a cladding diameter of 95 μm, the average microbend loss is below 6.0 dB / km, and compared to an optical fiber having a cladding diameter of 90 μm, the microbend loss characteristics are superior.

[0052] Next, the diameter D1 of the core 11 will be described.

[0053] As described above, the outer diameter D3 of the outer cladding 13 is preferably 186 μm or less, and the thickness T2 of the inner cladding 12 is preferably 1.3 μm or more. The thickness T3 of the outer cladding 13 is 1.84 μm or more. Therefore, the diameter D1 of the core 11 is D1=D3-2×(T2+T3), and therefore the maximum value D1 of the diameter D1 is ｍａｘ is D1 ｍａｘ= 186 - 2 × (1.3 + 1.84) = 179.72 ≈ 179, so it is preferably 179 μm. In other words, it is preferable that the diameter D1 of the core 11 satisfies D1 ≦ 179 μm. Note that, when decimal points are taken into consideration, the diameter D1 of the core 11 may be D1 ≦ 179.72 μm. When the outer diameter D3 of the outer cladding 13 is 186 μm or less, the thickness T2 of the inner cladding 12 may be 1.3 μm or more, and the thickness T3 of the outer cladding 13 may be 1.84 μm or more, which can suppress leakage of light propagating through the core 11 to the outer cladding 13 and suppress the cumulative breakage probability to 1.00E-6 or less, and splice in a 1-6 configuration to an optical fiber having a cladding diameter of 560 μm or less. Furthermore, the thickness T3 of the outer cladding 13 may be 1.40 μm or more. In this case, the maximum value D1 of the diameter D1 ｍａｘ is D1 ｍａｘ = 186 - 2 × (1.3 + 1.40) = 180.6, so it is preferable that it is 180.6 μm. In other words, it is preferable that the diameter D1 of the core 11 satisfies D1≦180.6 μm. In this case, the thickness T2 of the inner cladding 12 can be set to 1.3 μm or more, and the thickness T3 of the outer cladding 13 can be set to 1.40 μm or more, which can prevent light propagating through the core 11 from leaking into the outer cladding 13 and can suppress the cumulative breakage probability to 1.00E-6 or less, and the 1-6 configuration can be spliced ​​to an optical fiber having a cladding diameter of 560 μm or less. It is more preferable that the diameter D1 satisfies D1≦180 μm.

[0054] Furthermore, the diameter D1 of the core 11 will be considered from the perspective of light coupling efficiency. A dummy fiber consisting of only a core with a diameter of 105 μm was connected to a light source, and three samples were prepared in which the core 11 of the pumping optical fiber 10 was connected to this dummy fiber. In each pumping optical fiber 10, the ratios of the core 11 diameter (μm) / the inner cladding 12 diameter (μm) / the outer cladding 13 diameter (μm) were 80 / 90 / 105, 93 / 97 / 105, and 98 / 101 / 105. Therefore, in each sample, the thickness T2 of the inner cladding 12 was 1.3 μm or more, and therefore leakage of light propagating through the core 11 of each sample into the outer cladding 13 can be suppressed. For each sample, before the pumping optical fiber 10 was fused to the dummy fiber, light was incident on the dummy fiber from the light source, and the intensity of the light emitted from the dummy fiber was calculated from the difference between the intensity of the light incident on the dummy fiber from the light source and the intensity of the light emitted from the pumping optical fiber 10 after the pumping optical fiber 10 was fused to the dummy fiber. The results are shown in Table 5. The NA of the light at 95% of the power of the light emitted from the light source was set to 0.46.

[0055] As shown in Table 5, when the core diameter is 93 μm or more, there is no significant variation in the connection loss and the connection loss tends to be approximately minimal. Therefore, when a high-output laser diode is used as the pumping light source 30 and high-power light is propagated through the core 11, the diameter D1 of the core 11 is preferably 93 μm or more from the viewpoint of suppressing deterioration in the coupling efficiency of the pumping light. In other words, it is preferable that the diameter D1 of the core 11 satisfy 93 μm≦D1.

[0056] Although the present invention has been described above using the embodiments as examples, the present invention is not limited to the above embodiments.

[0057] For example, in the above embodiment, the optical fiber is the excitation optical fiber 10 , but the optical fiber of the present invention is not limited to the excitation optical fiber 10 .

[0058] 1 may have other configurations. For example, the fiber laser device 1 shown in Fig. 1 has been described as an example of forward pumping in which pumping light is incident on the amplification optical fiber 40 from the rear side toward the front side, but it may also have a backward pumping configuration in which pumping light is incident on the amplification optical fiber 40 from the front side toward the rear side, or may have a bidirectional pumping configuration in which pumping light is incident on both the front side and the rear side of the amplification optical fiber 40. Also, the fiber laser device 1 shown in Fig. 1 has a resonator type configuration, but it may also be, for example, a MO-PA (Master Oscillator - Power Amplifier) ​​type fiber laser device.

[0059] The following aspects of the invention can be derived from the above-described embodiment.

[0060] That is, aspect 1 of the present invention is an optical fiber comprising a core 11 having an average refractive index of n1, an inner cladding 12 surrounding the outer surface of the core 11, made of fluorine-doped silica glass, and having an average refractive index of n2, and an outer cladding 13 surrounding the outer surface of the inner cladding 12, having a lower fluorine doping concentration than the inner cladding 12, an average refractive index of n3, and a thickness of T3, wherein n1>n2 n3>n2 T3≧1.40 μm are satisfied.

[0061] In the optical fiber of this embodiment, the fluorine doping concentration of the outer cladding 13 is lower than that of the inner cladding 12, thereby suppressing the increase in OH groups in the cladding compared to when the fluorine doping concentration of the outer cladding 13 is equal to or higher than that of the inner cladding 12. Furthermore, the thickness T3 of the outer cladding 13 is 1.40 μm or greater, thereby suppressing the penetration of OH groups into the inner cladding 12, which has a high fluorine doping concentration. Therefore, even when the optical fiber is stored in a humid and hot environment, the increase in OH groups in the cladding can be suppressed, thereby suppressing an increase in optical loss. Furthermore, because light is less likely to propagate through the outer cladding 13, heat generation can be suppressed even when foreign matter adheres to the surface of the outer cladding 13.

[0062] A second aspect of the present invention is the optical fiber according to the first aspect, characterized in that the thickness T2 of the inner cladding 12 satisfies the following relationship: T2≧1.3 μm.

[0063] According to this aspect, it is possible to prevent light propagating through the core 11 from leaking into the outer cladding 13 .

[0064] A third aspect of the present invention is the optical fiber according to the first or second aspect, characterized in that the NA of the light incident on the optical fiber satisfies NA≦0.46.

[0065] According to this aspect, it is possible to prevent light incident on the optical fiber from leaking from the outer cladding 13 .

[0066] A fourth aspect of the present invention is the optical fiber according to any one of the first to third aspects, wherein the outer diameter D3 of the outer cladding 13 satisfies the following relationship: 186 μm≧D3≧95 μm.

[0067] In this embodiment, by setting the outer diameter D3 to 186 μm or less, splicing can be performed in a 1-6 configuration to an optical fiber having a cladding diameter of 560 μm or less, which can suppress the cumulative breakage probability to 1.00E-6 or less. Furthermore, by setting the outer diameter D3 to 95 μm or more, microbending loss in the excitation optical fiber 10 can be reduced.

[0068] A fifth aspect of the present invention is the optical fiber according to any one of the first to fourth aspects, wherein the outer diameter D3 of the outer cladding 13 satisfies the following relationship: 186 μm≧D3≧105 μm.

[0069] In this embodiment, by setting the outer diameter D3 to 186 μm or less, splicing can be performed in a 1-6 configuration to an optical fiber having a cladding diameter of 560 μm or less, which can suppress the cumulative breakage probability to 1.00E-6 or less. Furthermore, by setting the outer diameter D3 to 105 μm or more, microbending loss in the excitation optical fiber 10 can be reduced.

[0070] A sixth aspect of the present invention is the optical fiber according to any one of the first to fourth aspects, wherein the diameter D1 of the core 11 satisfies the following condition: D1≧80 μm.

[0071] According to this aspect, even when a high-power laser diode is used as the pumping light source 30, deterioration of the coupling efficiency of the pumping light can be suppressed.

[0072] A seventh aspect of the present invention is the optical fiber according to any one of the first to sixth aspects, wherein the diameter D1 of the core 11 satisfies the following relationship: D1≦180.6 μm.

[0073] According to this aspect, when the outer diameter D3 of the outer cladding 13 is 186 μm or less as described above, the thickness T2 of the inner cladding 12 can be 1.3 μm or more, and the thickness T3 of the outer cladding 13 can be 1.40 μm or more.

[0074] A seventh aspect of the present invention is the optical fiber according to any one of the first to sixth aspects, wherein the thickness T3 of the outer cladding 13 satisfies the following relationship: T3≦5 μm.

[0075] According to this embodiment, the power of the light propagating through the outer cladding 13 can be made smaller than 15% of the total power of the light propagating through the optical fiber.

[0076] As described above, according to the present invention, a fiber laser device capable of emitting high-power light is provided, and it is expected to be used in laser devices for processing, etc.

Claims

1. A core having an average refractive index of n1, an inner cladding surrounding the outer peripheral surface of the core, made of silica glass doped with fluorine and having an average refractive index of n2, and an outer cladding surrounding the outer peripheral surface of the inner cladding, having a lower fluorine doping concentration than the inner cladding, having an average refractive index of n3, and having a thickness of T3, wherein n1 > n2, n3 > n2, and T3 ≥ 1.40 μm. A fiber optic cable characterized by satisfying the above conditions.

2. The fiber optic cable according to claim 1, wherein the thickness T2 of the inner cladding satisfies T2 ≥ 1.3 μm.

3. The fiber optic cable according to claim 1, wherein the outer diameter D3 of the outer cladding satisfies 186 μm ≥ D3 ≥ 95 μm.

4. The fiber optic cable according to claim 3, wherein the outer diameter D3 of the outer cladding satisfies 186 μm ≥ D3 ≥ 105 μm.

5. The fiber optic cable according to claim 1, wherein the diameter D1 of the core satisfies D1 ≥ 93 μm.

6. The fiber optic cable according to claim 1, wherein the diameter D1 of the core satisfies D1 ≤ 180.6 μm.

7. The fiber optic cable according to claim 1, wherein the thickness T3 of the outer cladding satisfies T3 ≤ 5 μm.

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