Optical fiber amplifier

Abstract

An optical fiber amplifier (50) comprises: a double-clad optical fiber formed from quartz glass and including a plurality of cores (51) to which a rare earth element is added, an inner cladding (52a) that includes the plurality of cores (51), and an outer cladding (52b) that includes the inner cladding (52a); and an excitation light source that outputs lateral single-mode excitation light to the double-clad optical fiber. The total area of the plurality of cores (51) is 10% or more with respect to the area of the inner cladding (52a).

Description

Optical Fiber Amplifier

[0001] This application claims priority to Japanese Patent Application No. 2024-046683, filed on March 22, 2024, and incorporates by reference the entire contents of said Japanese application.

[0002] Non-Patent Documents 1 to 3 each describe a cladding pumping type multi-core optical fiber amplifier in which an outer cladding is formed of a resin material and a multimode laser is used as pump light.

[0003] Each of Patent Documents 1 and 2 discloses a rare-earth-doped multi-core optical fiber amplifier in which a rare-earth element is doped in a ring shape in a rare-earth-doped optical fiber.

[0004] Japanese Patent Application Laid-Open No. 2022-033477

[0005] KS Abedin et al., “Clading-pumped erbium-doped multicore fiber amplifier”, Optics Express, 20, 18(2012).S. Takasaka et al., “Clading-pumped seven-core EDFA using a multimode pump light coupler,” 39th European Conference and Exhibition on Optical Communication (ECOC 2013), London, 2013, pp. 1-3, doi: 10.1049 / cp.2013.1470.Y. Mimura et al., "Batch multicore amplification with cladding-pumped multicore EDF," 2012 38th European Conference and Exhibition on Optical Communications, Amsterdam, Netherlands, 2012, pp. 1-3, doi: 10.1364 / ECEOC.2012.Tu.4.F.1.

[0006] The present disclosure provides an optical fiber amplifier comprising: a double-clad optical fiber formed of silica glass, the double-clad optical fiber including a plurality of cores doped with a rare earth element, an inner cladding including the plurality of cores, and an outer cladding including the inner cladding; and a pumping light source that outputs transverse single-mode pumping light to the double-clad optical fiber, wherein the total area of ​​the plurality of cores is 10% or more of the area of ​​the inner cladding.

[0007] FIG. 1 is a diagram showing an outline of an optical fiber amplifier according to a first embodiment. FIG. 2 is a cross-sectional view showing an outline of a double-clad optical fiber in the optical fiber amplifier according to the first embodiment. FIG. 3 is a diagram showing the relationship between the core-clad area ratio and pumping efficiency in a double-clad optical fiber. FIG. 4 is a diagram showing an outline of an optical fiber amplifier according to a second embodiment. FIG. 5 is a cross-sectional view showing an outline of a double-clad optical fiber in the optical fiber amplifier according to the second embodiment. FIG. 6 is a diagram showing an outline of an optical fiber amplifier according to a third embodiment. FIG. 7 is a diagram showing an outline of an optical fiber amplifier according to a fourth embodiment. FIG. 8 is a diagram showing an outline of an optical fiber amplifier according to a fifth embodiment.

[0008] [Problem to be Solved by the Present Disclosure] In optical fiber amplifiers, there is a demand for improving the pumping efficiency in rare-earth doped optical fibers.

[0009] The present disclosure provides an optical fiber amplifier with increased pumping efficiency.

[0010] [Effects of the Present Disclosure] According to the optical fiber amplifier of the present disclosure, it is possible to increase the pumping efficiency.

[0011] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0012] (1) The optical fiber amplifier of the present disclosure includes a double-clad optical fiber formed of silica glass, the double-clad optical fiber including a plurality of cores doped with a rare earth element, an inner cladding including the plurality of cores, and an outer cladding including the inner cladding. The optical fiber amplifier of the present disclosure also includes a pumping light source that outputs transverse single-mode pumping light to the double-clad optical fiber. Furthermore, the total area of ​​the plurality of cores in the optical fiber amplifier of the present disclosure is 10% or more of the area of ​​the inner cladding.

[0013] According to the optical fiber amplifier of the present disclosure, the pumping efficiency can be increased.

[0014] (2) In the above (1), the diameter of the inner cladding may be 30 μm or more and 80 μm or less, because this can improve the pumping efficiency when the optical fiber amplifier amplifies the signal light.

[0015] (3) In the above (1) or (2), the number of the plurality of cores may be 3 or more and 8 or less. This is because the pumping efficiency can be improved when the optical fiber amplifier amplifies the signal light.

[0016] (4) In any of (1) to (3) above, the inter-core distance of the multiple cores may be 25 μm or less. This is because the optical fiber amplifier can accommodate coupled transmission when amplifying signal light. Also, when the optical fiber amplifier amplifies signal light, the cores can be densely arranged inside the cladding, thereby improving pumping efficiency.

[0017] (5) In any of (1) to (4) above, the number of the cores may be four, and the diameter of the inner cladding may be 50 μm or less. This is because the diameter of the inner cladding can be reduced when the optical fiber amplifier amplifies the signal light.

[0018] (6) In any of the above (1) to (5), the diameter of the outer cladding may be 125 μm. This is because this is a standard outer diameter when connecting the optical fiber amplifier to the outside, thereby improving connectivity.

[0019] (7) In any of the above (1) to (6), the relative refractive index difference between the inner cladding and the outer cladding may be 0.5% or more and 2% or less, because this allows the double-clad optical fiber to increase the acceptance angle of light received from the pumping light source when the optical fiber amplifier amplifies signal light.

[0020] (8) In any of (1) to (7) above, two pumping light sources, a first pumping light source and a second pumping light source, may be provided. The first pumping light output from the first pumping light source and the second pumping light output from the second pumping light source may be combined by a polarization combiner, and the combined pumping light may be input to the double-clad optical fiber. This is because the intensity of the pumping light can be increased when the optical fiber amplifier amplifies the signal light.

[0021] (9) In any of (1) to (7) above, two pumping light sources, a first pumping light source and a second pumping light source, may be provided. The first pumping light output from the first pumping light source and the second pumping light output from the second pumping light source may be multiplexed by an optical multiplexer, and the resulting pumping light may be input to the double-clad optical fiber. This is because the intensity of the pumping light can be increased when the optical fiber amplifier amplifies the signal light.

[0022] (10) In any of (1) to (9) above, two pumping light sources, a first pumping light source and a second pumping light source, may be provided, and the first pumping light source may be connected to the front of the double-clad optical fiber, and the second pumping light source may be connected to the rear of the double-clad optical fiber. This is because, when the optical fiber amplifier amplifies the signal light, the double-clad optical fiber is pumped from the front and rear, thereby facilitating the amplification of the signal light in the double-clad optical fiber.

[0023] [Details of the embodiments of the present disclosure] Specific examples of the optical fiber amplifier of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0024] In the description of the specification and drawings relating to each embodiment, components having substantially the same or corresponding functions may be designated by the same reference numerals to avoid redundant explanation. For ease of understanding, the scale of each part in the drawings may differ from the actual scale.

[0025] The following embodiments will be described, and at least some of the embodiments described below may be combined in any manner.

[0026] First Embodiment An optical fiber amplifier according to a first embodiment will be described. The optical fiber amplifier includes a double-clad multi-core optical fiber formed of silica glass, the double-clad multi-core optical fiber including a plurality of cores doped with a rare-earth element, an inner cladding including the plurality of cores, and an outer cladding including the inner cladding. The optical fiber amplifier includes a pumping light source that outputs transverse single-mode pumping light to the double-clad optical fiber. In the double-clad optical fiber, the total area of ​​the plurality of cores in a cross section perpendicular to the longitudinal direction is 10% or more of the area of ​​the inner cladding in the same cross section.

[0027] The number of cores of the double-clad optical fiber in the optical fiber amplifier is 4. Signal light is transmitted through at least one of the four cores.

[0028] 1 is a diagram showing an outline of an optical fiber amplifier 1, which is an example of an optical fiber amplifier according to a first embodiment. In the optical fiber amplifier 1, an erbium-doped optical fiber (EDF) doped with erbium (Er) is used as a double-clad optical fiber.

[0029] The optical fiber amplifier 1 amplifies the signal light Ls and outputs it as the signal light La. The optical fiber amplifier 1 includes a pumping light source 10, an optical multiplexer 20, an optical isolator 30, and an EDF 50. The EDF 50 is provided between the optical multiplexer 20 and the optical isolator 30.

[0030] The signal light Ls amplified by the optical fiber amplifier 1 is input to the optical multiplexer 20. The pumping light source 10 is connected to the optical multiplexer 20. The pumping light Le output from the pumping light source 10 is multiplexed with the signal light Ls in the optical multiplexer 20. The optical multiplexer 20 outputs multiplexed light Lc, which is obtained by multiplexing the signal light Ls and the pumping light Le, to the EDF 50. The EDF 50 amplifies the signal light Ls included in the input multiplexed light Lc, and outputs it to the optical isolator 30. The multiplexed light Lc that has passed through the optical isolator 30 is output as signal light La.

[0031] The pumping light source 10 outputs a transverse single mode light beam having a wavelength of 980 nm, for example. The pumping light source 10 is connected to the optical multiplexer 20 by an optical fiber, which is, for example, a single-core optical fiber having a core diameter of 5.3 μm and a cladding diameter of 125 μm. The numerical aperture (NA) of the optical fiber connecting the pumping light source 10 to the optical multiplexer 20 is, for example, 0.14.

[0032] A double-clad optical fiber in an optical fiber amplifier according to a first embodiment will be described. The double-clad optical fiber includes a plurality of cores doped with a rare earth element, an inner cladding including the plurality of cores, and an outer cladding including the inner cladding. The double-clad optical fiber is made of silica glass. In other words, the cores, the inner cladding, and the outer cladding are each made of silica glass.

[0033] FIG. 2 is a diagram showing a schematic structure of the EDF 50 in the optical fiber amplifier 1, and is a cross-sectional view taken along a plane perpendicular to the longitudinal direction of the EDF 50. As shown in FIG.

[0034] The EDF 50 has a plurality of cores 51 and a cladding 52. The cladding 52 has an inner cladding 52 a and an outer cladding 52 b. The EDF 50 has a waveguide structure for signal light Ls formed by the cores 51 and the inner cladding 52 a, and a waveguide structure for pump light Le formed by the inner cladding 52 a and the outer cladding 52 b. In the EDF 50, each of the plurality of cores 51 is doped with erbium.

[0035] The core 51 is made of silica glass co-doped with germanium and aluminum. The core 51 has a higher refractive index than pure silica glass due to the co-doping of germanium and aluminum, with a relative refractive index difference of 0.9% higher. The diameter of the core 51 is, for example, 8.9 μm. The core 51 contains erbium at an atomic concentration of 5×10 ２４ m －３ The core 51 is doped with about 10 μm of EDFA. The mode field diameter of light at a wavelength of around 1550 nm in the core 51 is, for example, 10 μm. The centers of the multiple cores 51 are arranged on the vertices of a square lattice with 20 μm spacing in a cross section perpendicular to the longitudinal direction of the EDF 50. In other words, the distance between the centers of the multiple cores 51 may be 20 μm.

[0036] The inter-core distance among the multiple cores 51 is not limited to the above example and may be, for example, 25 μm or less. If the inter-core distance is 25 μm or less, the optical fiber amplifier 1 can also accommodate coupled transmission when amplifying signal light. When the optical fiber amplifier 1 amplifies signal light, the cores can be densely arranged inside the cladding, thereby improving pumping efficiency.

[0037] The inner cladding 52a is made of aluminum-doped silica glass. The aluminum-doped silica glass increases the refractive index of the inner cladding 52a compared to pure silica glass, with a relative refractive index difference of 0.5% higher. The diameter of the inner cladding 52a is, for example, 50 μm.

[0038] The diameter of the inner cladding 52a is not limited to the above example and may be, for example, 30 μm or more and 80 μm or less. The diameter of the inner cladding 52a is not limited to the above example and may be, for example, 30 μm or more and 50 μm or less.

[0039] The outer cladding 52b is formed of fluorine-doped silica glass. The doping of fluorine into the silica glass lowers the refractive index of the outer cladding 52b compared to pure silica glass, with a relative refractive index difference of 0.7% lower. The diameter of the outer cladding 52b is, for example, 125 μm. Since the standard outer diameter of the outer cladding in a typical optical fiber is 125 μm, it is preferable that the diameter (outer diameter) of the outer cladding 52b be 125 μm. Setting the diameter (outer diameter) of the outer cladding 52b to 125 μm improves connectivity with other elements.

[0040] The outer cladding diameter of 125 μm does not necessarily mean that the outer cladding diameter is strictly 125 μm. For example, the outer cladding diameter of 125 μm may be within the manufacturing tolerance range.

[0041] The refractive index difference between the inner cladding 52 a and the outer cladding 52 b is, for example, 0.8% in terms of relative refractive index difference. The numerical aperture (NA) of the waveguide structure formed by the inner cladding 52 a and the outer cladding 52 b is, for example, 0.19, assuming that the refractive index of pure silica is 1.5.

[0042] The refractive index difference between the inner cladding 52 a and the outer cladding 52 b is not limited to the above example, and may be, for example, a relative refractive index difference of 0.5% to 2%. By setting the relative refractive index difference between the inner cladding 52 a and the outer cladding 52 b to 0.5% to 2%, the acceptance angle of light received by the double-clad optical fiber from the pump light source 10 can be increased when the optical fiber amplifier 1 amplifies signal light.

[0043] Here, we will explain the relationship between the pumping efficiency and the ratio of the total area of ​​the core to the area of ​​the inner cladding in a cross section perpendicular to the longitudinal direction in a double-clad optical fiber. Figure 3 is a diagram showing the relationship between the core-cladding area ratio and the pumping efficiency in a double-clad optical fiber. Figure 3 shows the pumping efficiency in a double-clad optical fiber as a result of changing the intensity of the pump light.

[0044] The horizontal axis in Fig. 3 represents the core-clad area ratio, which is the area ratio between the core and the clad. To explain the core-clad area ratio using the EDF 50 as an example, the area ratio RS, which is the ratio between the total area S1, which is the sum of the cross-sectional areas of the multiple cores 51, and the area S2, which is the cross-sectional area of ​​the inner clad 52a, is the core-clad area ratio. In other words, the area ratio RS represents the proportion of the total area S1 of the multiple cores 51 to the area of ​​the inner clad 52a. The area ratio RS, which is the core-clad area ratio, is calculated based on Equation 1.

[0045] RS = S1 / S2...Formula 1

[0046] The vertical axis in Fig. 3 represents pumping efficiency. Lines L1, L2, L3, and L4 in Fig. 3 represent the results when the pumping light intensity per core is 18, 23, 26, and 29 dBm / core, respectively.

[0047] In the case of a single mode laser, the laser output can be, for example, 1000 mW. In the optical fiber amplifier 1, in the case of four cores in the EDF 50, for example, when 1000 mW of pumping light Le is output from the pumping light source 10, the light intensity per core is 250 mW.

[0048] From the above considerations, it is considered that the realistic intensity of the pumping light is line L2 (23 dBm / core ≈ 200 mW / core) in Figure 3. In Figure 3, when the pumping light is 23 dBm / core (line L2), the pumping efficiency is 5, 10, and 20% when the core-clad area ratio is 0.01, 0.025, and 0.083, respectively. In Figure 3, when the pumping light is 23 dBm / core (line L2), the pumping efficiency can be made 20% or more by setting the core-clad area ratio to 0.083, i.e., 8.3% or more.

[0049] Taking the variations in pump light into consideration based on the above study, for example, the core-clad area ratio may be 8.5% or more, preferably 10% or more, and even more preferably 15% or more, with the upper limit being 100% or less. In other words, in the optical fiber amplifier according to the first embodiment, the total area of ​​the multiple cores in a cross section perpendicular to the longitudinal direction of the double-clad optical fiber may be 8.5% or more of the area of ​​the inner cladding, preferably 10% or more, and even more preferably 15% or more, with the upper limit being 100% or less.

[0050] For example, if the diameter of the core 51 is 8.9 μm and the diameter of the inner cladding 52 a is 50 μm in the EDF 50, the core-cladding area ratio is 12.6% or more and 100% or less. Therefore, for example, when 1000 mW pumping light from the pumping light source 10 is amplified in the EDF 50, the pumping efficiency can be 20% or more.

[0051] According to the optical fiber amplifier of the first embodiment, by making the total area of ​​the multiple cores 10% or more of the area of ​​the inner cladding, the ratio of the area of ​​the cores to the area of ​​the inner cladding can be increased, thereby improving the pumping efficiency. Also, according to the optical fiber amplifier of the first embodiment, by forming the double-clad optical fiber from silica glass, the diameter of the inner cladding can be reduced. By reducing the diameter of the inner cladding, the ratio of the area of ​​the cores to the area of ​​the inner cladding can be increased, thereby improving the pumping efficiency.

[0052] For example, in a cladding pumping optical fiber amplifier pumped by a multimode laser, a multimode laser with a core diameter of 105 μm is generally used. In this case, if the inner cladding is made small, a difference in the core diameter between the inner cladding and the pumping light source occurs, deteriorating the coupling efficiency and resulting in low pumping efficiency.

[0053] According to the optical fiber amplifier of the first embodiment, by reducing the diameter of the inner cladding and amplifying with transverse single mode pumping light, it is possible to increase the coupling efficiency compared to when pumping light from a multimode laser is used, and to improve the pumping efficiency.

[0054] Second Embodiment An optical fiber amplifier according to a second embodiment will be described. The optical fiber amplifier according to the second embodiment differs from the optical fiber amplifier according to the first embodiment in the number of cores in the double-clad optical fiber. The number of cores in the double-clad optical fiber in the optical fiber amplifier is seven. Signal light is transmitted through at least one of the seven cores.

[0055] FIG. 4 is a diagram showing an outline of an optical fiber amplifier 2 as an example of an optical fiber amplifier according to the second embodiment.

[0056] The optical fiber amplifier 2 amplifies the signal light Ls and outputs it as signal light La. The optical fiber amplifier 2 includes a pumping light source 110, an optical multiplexer 120, an optical isolator 130, and an EDF 150. The EDF 150 is provided between the optical multiplexer 120 and the optical isolator 130. The pumping light source 110, the optical multiplexer 120, and the optical isolator 130 have the same functions and configurations as the pumping light source 10, the optical multiplexer 20, and the optical isolator 30 in the optical fiber amplifier 1, respectively. Therefore, the description of the optical fiber amplifier 1 should be referred to and detailed description thereof will be omitted.

[0057] FIG. 5 is a diagram showing a schematic structure of the EDF 150 in the optical fiber amplifier 2, and is a cross-sectional view taken along a plane perpendicular to the longitudinal direction of the EDF 150. As shown in FIG.

[0058] The EDF 150 has a plurality of cores 151 and a cladding 152. The cladding 152 has an inner cladding 152 a and an outer cladding 152 b. The EDF 150 has a waveguide structure for signal light Ls formed by the cores 151 and the inner cladding 152 a, and a waveguide structure for pump light Le formed by the inner cladding 152 a and the outer cladding 152 b. In the EDF 150, each of the plurality of cores 151 is doped with erbium.

[0059] The core 151 is made of silica glass co-doped with germanium and aluminum. By doping the silica glass with germanium and aluminum, the refractive index of the core 151 is higher than that of pure silica glass, with a relative refractive index difference of 0.9% higher. The diameter of the core 151 is, for example, 8.9 μm. The core 151 contains erbium at an atomic concentration of 5×10 ２４ m －３ The mode field diameter of light in the core 151 at a wavelength of around 1550 nm is, for example, 10 μm. The multiple cores 151 are arranged in a hexagonal close-packed lattice with an interval of 20 μm.

[0060] The inter-core distance among the multiple cores 151 is not limited to the above example, and may be, for example, 25 μm or less.

[0061] The inner cladding 152a is made of aluminum-doped silica glass. The aluminum-doped silica glass increases the refractive index of the inner cladding 152a compared to pure silica glass, with a relative refractive index difference of 0.5% higher. The diameter of the inner cladding 152a is, for example, 80 μm.

[0062] The diameter of the inner cladding 152a is not limited to the above example and may be, for example, 30 μm or more and 80 μm or less. The diameter of the inner cladding 152a is not limited to the above example and may be, for example, 30 μm or more and 50 μm or less.

[0063] The outer cladding 152b is made of fluorine-doped silica glass. The fluorine-doped silica glass lowers the refractive index of the outer cladding 152b compared to pure silica glass, with a relative refractive index difference of 0.7% lower. The diameter of the outer cladding 152b is, for example, 125 μm.

[0064] The relative refractive index difference between the inner cladding 152 a and the outer cladding 152 b is, for example, 0.8%. The numerical aperture (NA) of the waveguide structure formed by the inner cladding 152 a and the outer cladding 152 b is, for example, 0.14, calculated assuming that the wavelength emitted from the pumping light source 110 is 980 nm and the refractive index of pure silica is 1.5.

[0065] The refractive index difference between the inner cladding 152a and the outer cladding 152b is not limited to the above example, and may be, for example, 0.5% to 2%.

[0066] According to the optical fiber amplifier of the second embodiment, as in the optical fiber amplifier of the first embodiment, the diameter of the inner cladding can be reduced and the ratio of the area of ​​the core to the area of ​​the inner cladding can be increased, thereby improving the pumping efficiency.

[0067] In the examples of the optical fiber amplifiers according to the first and second embodiments, four-core or seven-core double-clad optical fibers are shown, but the number of cores in the double-clad optical fiber is not limited to the above examples. The number of cores in the double-clad optical fiber may be, for example, 3 or more and 8 or less.

[0068] Third Embodiment An optical fiber amplifier according to a third embodiment will be described. The optical fiber amplifier according to the third embodiment differs from the optical fiber amplifier according to the first embodiment in the portion related to the pumping light source.

[0069] FIG. 6 is a diagram showing an outline of an optical fiber amplifier 3 as an example of an optical fiber amplifier according to the third embodiment.

[0070] The optical fiber amplifier 3 amplifies the signal light Ls and outputs it as signal light La. The optical fiber amplifier 3 includes pumping light sources 210a and 210b, a polarization beam combiner 240, an optical multiplexer 20, an optical isolator 30, and an EDF 50. The EDF 50 is provided between the optical multiplexer 20 and the optical isolator 30. In other words, the optical fiber amplifier 3 includes pumping light sources 210a and 210b, and a polarization beam combiner 240, instead of the pumping light source 10 in the optical fiber amplifier 1. Regarding the configuration of the optical fiber amplifier 3 that is common to the optical fiber amplifier 1, the description of the optical fiber amplifier 1 should be referred to, and detailed description thereof will be omitted here.

[0071] Each of the pumping light sources 210a and 210b outputs transverse single mode light with a wavelength of, for example, 980 nm. The pumping light Lea output from the pumping light source 210a and the pumping light Leb output from the pumping light source 210b are polarization-combined in the polarization combiner 240. The pumping light Le obtained by polarization-combining the pumping light Lea and the pumping light Leb by the polarization combiner 240 is output from the polarization combiner 240 to the optical multiplexer 20.

[0072] In the above example, the optical fiber amplifier 3 includes two pumping light sources, but the number of pumping light sources is not limited to two. The optical fiber amplifier according to the third embodiment may include three or more pumping light sources. In other words, the optical fiber amplifier according to the third embodiment may include a plurality of pumping light sources.

[0073] According to the optical fiber amplifier of the third embodiment, as in the optical fiber amplifier of the first embodiment, the diameter of the inner cladding can be reduced and the ratio of the core area to the inner cladding area can be increased to improve the pumping efficiency. Furthermore, according to the optical fiber amplifier of the third embodiment, the intensity of the pumping light can be further increased by increasing the number of pumping light sources.

[0074] In the example of the optical fiber amplifier 3, the optical fiber amplifier 3 includes the EDF 50 as a double-clad optical fiber, but the optical fiber amplifier 3 may include an EDF 150 instead of the EDF 50.

[0075] Fourth Embodiment An optical fiber amplifier according to a fourth embodiment will be described. The optical fiber amplifier according to the fourth embodiment differs from the optical fiber amplifier according to the first embodiment in the portion related to the pumping light source.

[0076] FIG. 7 is a diagram showing an outline of an optical fiber amplifier 4 as an example of an optical fiber amplifier according to the fourth embodiment.

[0077] The optical fiber amplifier 4 amplifies the signal light Ls and outputs it as signal light La. The optical fiber amplifier 4 includes pumping light sources 310a and 310b, optical multiplexers 320 and 321, an optical isolator 30, and an EDF 50. The EDF 50 is provided between the optical multiplexers 320 and 321. In other words, the optical fiber amplifier 4 includes the pumping light source 310a and the optical multiplexer 320 instead of the pumping light source 10 and the optical multiplexer 20 in the optical fiber amplifier 1, and further includes the pumping light source 310b and the optical multiplexer 321.

[0078] In the optical fiber amplifier 4, detailed descriptions of the components common to the optical fiber amplifier 1 are omitted here, and reference is made to the description of the optical fiber amplifier 1. Furthermore, the pumping light source 310a and the optical multiplexer 320 have the same functions and configurations as the pumping light source 10 and the optical multiplexer 20, and reference is made to the description of the optical fiber amplifier 1, and descriptions thereof are omitted.

[0079] The pumping light source 310b outputs a transverse single mode light beam having a wavelength of 980 nm, for example. The pumping light beam Leb output from the pumping light source 310b is combined with the combined light beam Lc in the optical combiner 321.

[0080] In the optical fiber amplifier 4, the pumping light sources are connected to the EDF 50 from the front and rear. More specifically, the pumping light source 310a is connected to the EDF 50 from the front by an optical multiplexer 320. The pumping light Lea output from the pumping light source 310a propagates through the EDF 50 in the same direction as the signal light Ls. The pumping light source 310b is connected to the EDF 50 from the rear by an optical multiplexer 321. The pumping light Leb output from the pumping light source 310b propagates through the EDF 50 in the opposite direction to the signal light Ls.

[0081] According to the optical fiber amplifier of the fourth embodiment, as in the optical fiber amplifier of the first embodiment, the diameter of the inner cladding can be reduced and the ratio of the core area to the inner cladding area can be increased, thereby improving the pumping efficiency. Furthermore, according to the optical fiber amplifier of the fourth embodiment, the double-clad optical fiber can be pumped from the front and rear, thereby promoting the amplification of signal light in the double-clad optical fiber.

[0082] In the example of the optical fiber amplifier 4, the optical fiber amplifier 4 includes the EDF 50 as a double-clad optical fiber, but the optical fiber amplifier 4 may include an EDF 150 instead of the EDF 50.

[0083] Fifth Embodiment An optical fiber amplifier according to a fifth embodiment will be described. The optical fiber amplifier according to the fifth embodiment differs from the optical fiber amplifier according to the first embodiment in the portion related to the pumping light source.

[0084] FIG. 8 is a diagram showing an outline of an optical fiber amplifier 5 as an example of an optical fiber amplifier according to the fifth embodiment.

[0085] The optical fiber amplifier 5 amplifies the signal light Ls and outputs it as signal light La. The optical fiber amplifier 5 includes pumping light sources 410a and 410b, a wavelength combiner 440, an optical combiner 20, an optical isolator 30, and an EDF 50. The EDF 50 is provided between the optical combiner 20 and the optical isolator 30. In other words, the optical fiber amplifier 5 includes pumping light sources 410a and 410b and a wavelength combiner 440 instead of the pumping light source 10 in the optical fiber amplifier 1. Regarding the configuration of the optical fiber amplifier 5 that is common to the optical fiber amplifier 1, the description of the optical fiber amplifier 1 should be referred to and detailed description thereof will be omitted here.

[0086] The pumping light source 410a and the pumping light source 410b output transverse single mode light having wavelengths of, for example, 1460 nm and 1485 nm, respectively. The pumping light Lea output from the pumping light source 410a and the pumping light Leb output from the pumping light source 410b are wavelength-multiplexed in a wavelength multiplexer 440. The pumping light Lea and the pumping light Leb are polarization-multiplexed by the wavelength multiplexer 440, and the resulting pumping light Le is output from the wavelength multiplexer 440 to the optical multiplexer 20.

[0087] In the above example, the optical fiber amplifier 5 includes two pumping light sources, but the number of pumping light sources is not limited to two. The optical fiber amplifier according to the fifth embodiment may include three or more pumping light sources. In other words, the optical fiber amplifier according to the fifth embodiment may include a plurality of pumping light sources.

[0088] According to the optical fiber amplifier of the fifth embodiment, as in the optical fiber amplifier of the first embodiment, the diameter of the inner cladding can be reduced and the ratio of the core area to the inner cladding area can be increased to improve the pumping efficiency. Furthermore, according to the optical fiber amplifier of the fifth embodiment, the intensity of the pumping light can be further increased by increasing the number of pumping light sources.

[0089] In the example of the optical fiber amplifier 5, the optical fiber amplifier 5 includes the EDF 50 as a double-clad optical fiber, but the optical fiber amplifier 5 may include an EDF 150 instead of the EDF 50.

[0090] 1, 2, 3, 4, 5 Optical fiber amplifier 10, 110, 210a, 210b, 310a, 310b, 410a, 410b Pumping light source 20, 120, 320, 321 Optical multiplexer 30, 130 Optical isolator 240 Polarization multiplexer 440 Wavelength multiplexer 50, 150 EDF 51, 151 Core 52, 152 Cladding 52a, 152a Inner cladding 52b, 152b Outer cladding L1, L2, L3, L4 Lines La, Ls Signal light Lc Multiplexed light Le, Lea, Leb Pumping light

Claims

1. An optical fiber amplifier comprising: a double-clad optical fiber formed of silica glass, the double-clad optical fiber having a plurality of cores doped with a rare earth element, an inner cladding including the plurality of cores, and an outer cladding including the inner cladding; and a pumping light source that outputs transverse single-mode pumping light to the double-clad optical fiber, wherein the total area of ​​the plurality of cores is 10% or more of the area of ​​the inner cladding.

2. The optical fiber amplifier according to claim 1, wherein the diameter of the inner cladding is 30 μm or more and 80 μm or less.

3. An optical fiber amplifier according to claim 1 or claim 2, wherein the number of cores is 3 or more and 8 or less.

4. An optical fiber amplifier according to any one of claims 1 to 3, wherein the inter-core distance among the plurality of cores is 25 μm or less.

5. An optical fiber amplifier according to any one of claims 1 to 4, wherein the number of cores is four and the diameter of the inner cladding is 50 µm or less.

6. An optical fiber amplifier according to any one of claims 1 to 5, wherein the diameter of the outer cladding is 125 µm.

7. An optical fiber amplifier according to any one of claims 1 to 6, wherein the relative refractive index difference between the inner cladding and the outer cladding is 0.5% or more and 2% or less.

8. An optical fiber amplifier according to any one of claims 1 to 7, comprising two pumping light sources, a first pumping light source and a second pumping light source, wherein the first pumping light output from the first pumping light source and the second pumping light output from the second pumping light source are combined by a polarization combiner to produce pumping light, which is input to the double-clad optical fiber.

9. An optical fiber amplifier according to any one of claims 1 to 7, comprising two pumping light sources, a first pumping light source and a second pumping light source, wherein the first pumping light output from the first pumping light source and the second pumping light output from the second pumping light source are multiplexed by an optical multiplexer to produce pumping light, which is input to the double-clad optical fiber.

10. An optical fiber amplifier according to any one of claims 1 to 9, comprising two pumping light sources, a first pumping light source and a second pumping light source, wherein the first pumping light source is connected to the front of the double-clad optical fiber, and the second pumping light source is connected to the rear of the double-clad optical fiber.

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