Optical Fiber Amplifier

Abstract

The purpose of the present invention is to provide an optical fiber amplifier which reduces a loss of excitation light in an excitation light combiner to increase amplification efficiency.　This optical fiber amplifier according to the present invention is provided with: an amplification optical fiber 13 having a plurality of cores doped with rare earth element in a cladding; an excitation light source 11 for outputting, to a multi-mode optical fiber 16-1, excitation light for exciting the rare earth element; a core size-converting part 14 for reducing the diameter of the core of the multi-mode optical fiber 16-1 to the diameter of the cladding of the amplification optical fiber 13; and an excitation light combiner 12 for making signal light Ls transmitted through each core of the multi-core optical fiber 15 incident to the each core of the amplification optical fiber 13 and making excitation light Le output from the excitation light source 11 and passing through the core size-converting part 14 incident to the cladding of the amplification light fiber 13 containing the cores.

Description

[Technical Field]

[0001] The present disclosure relates to optical fiber amplifiers. [Background technology]

[0002] In optical fiber communication systems, the loss of light propagating through an optical fiber is reduced by amplifying it at regular intervals using optical amplifiers and relaying it for long-distance transmission. Amplification within the optical amplifier is achieved by injecting signal light and pumping light (mainly 980nm or 1480nm light in the case of EDF) into an amplifying optical fiber with rare earth elements added to its core (mainly erbium-doped optical fiber: EDF), and amplifying the light without converting it to electricity.

[0003] In current communications using single-mode optical fibers (SMF), core-pumped optical amplifiers are used, which amplify signal light propagating through the core by guiding pumping light into the core in the same manner.On the other hand, in recent years, in order to expand the transmission capacity of optical fibers, multicore fibers having multiple cores in the cross section of the optical fiber or optical fibers for space division multiplexing (SDM) using few-mode fibers with two or more modes propagating within the core have been considered, and amplifiers for these optical fibers in which multiple spatial modes propagate through a single optical fiber have been considered (for example, see Non-Patent Document 1).

[0004] For these SDM optical fibers, SDM optical fiber amplifiers capable of simultaneously amplifying multiple spatial modes have been investigated. Cladding-pumped optical fiber amplifiers, which differ from core-pumped amplifiers in that they guide pump light through the cladding region of the optical fiber and simultaneously amplify multiple cores or multiple modes, are being investigated (see, for example, Non-Patent Document 2). Cladding-pumped optical fiber amplifiers can use a multimode light source for pump light, which is superior in power efficiency to the single-mode light source typically used in core-pumped amplifiers. They also do not necessarily require the temperature control required for single-mode light sources, such as Peltier devices. Therefore, cladding-pumped optical fiber amplifiers are expected to exhibit superior amplification efficiency. Compared to core-pumped optical fiber amplifiers, cladding-pumped optical fiber amplifiers have a lower overlap between the region through which the pump light propagates and the rare-earth-doped core region, resulting in a lower amount of pump light absorbed in the amplification optical fiber. However, studies have been conducted to increase the amount of pump light absorbed in the optical fiber by increasing the core-cladding ratio (Rcc), which is the ratio of the total area of ​​the cores in the optical fiber to the cladding area including the core region, and high amplification efficiency has been demonstrated (see, for example, Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Y. Tsuchida et al., “Amplification characteristics of a multi-core erbium-doped fiber amplifier,” in Proc. of OFC2012, paper OM3C.3 (2012) [Non-patent document 2] KS Abedin et al., “Clading-pumped erbium-doped multicore fiber amplifier,” Opt. Express, vol.20, No. 18, pp.20191-20200 (2012) [Non-patent document 3] T. Sakamoto et al., “Characteristics of Randomly Coupled 12-core Erbium-Doped Fiber Amplifier,” J. of Lightw. Technol., vol. 39, no. 4, pp. 1186-1193 (2021) Summary of the Invention [Problem to be solved by the invention]

[0006] A smaller cladding diameter is preferable to increase Rcc. However, in optical fiber amplifier configurations, the optical fiber connected to the pump light source is typically a multimode fiber with a core diameter of 105 μm. When such a multimode fiber is used with a pump optical fiber with a cladding diameter of 105 μm or less, the spot size of the multimode fiber does not match the cladding diameter of the amplification optical fiber in the pump light combiner, resulting in a decrease in amplification efficiency due to loss of pump light.

[0007] For example, Non-Patent Document 3 discloses that an erbium-doped optical fiber with a cladding diameter of 90 μm is used as an amplification optical fiber, but loss occurs at the connection point with the optical fiber output from the pumping light combiner due to a mismatch in the cladding region.

[0008] In other words, SDM optical fiber amplifiers have a problem in that the spot size of the pump light from the pump light source in the pump light combiner does not match the cladding diameter of the amplification optical fiber, making it difficult to increase the amplification efficiency due to loss of the pump light. SUMMARY OF THE INVENTION In order to solve the above problems, an object of the present invention is to provide an optical fiber amplifier that reduces the loss of pumping light in a pumping light combiner and improves amplification efficiency. [Means for solving the problem]

[0009] To achieve the above objective, a core size conversion section that narrows the core diameter of the optical fiber that propagates the pumping light from the pumping light source is placed in front of the pumping light combiner, and the spot size of the pumping light and the cladding diameter of the amplification optical fiber are made to match in the pumping light combiner.

[0010] Specifically, the optical fiber amplifier according to the present invention comprises: an amplifying optical fiber having a plurality of cores doped with rare earth elements in a cladding; an excitation light source that outputs excitation light for exciting the rare earth element to a multimode optical fiber; a core size conversion unit that reduces the diameter of the core of the multimode optical fiber to the diameter of the cladding of the amplification optical fiber; a pumping light combiner that inputs signal light propagated through each core of a multi-core optical fiber into each core of the amplification optical fiber, and inputs the pumping light output from the pumping light source and passed through the core size conversion unit into the cladding including the core of the amplification optical fiber; Equipped with.

[0011] By arranging the core size conversion section, the spot size of the pumping light in the pumping light combiner can be made to match the cladding diameter of the amplification optical fiber, thereby reducing the loss of the pumping light. Furthermore, the spot size of the pumping light can be made compatible with an amplification fiber with a small cladding diameter, thereby improving the amplification efficiency in terms of increasing the Rcc. Therefore, the present invention can provide an optical fiber amplifier that reduces the loss of the pumping light in the pumping light combiner and improves the amplification efficiency.

[0012] The amplification optical fiber of the optical fiber amplifier according to the present invention has a resin on the outer periphery of the cladding, and the relationship between the refractive index n1 of the core, the refractive index n2 of the cladding, and the refractive index n3 of the resin satisfies n1>n2>n3.

[0013] The amplification optical fiber of the optical fiber amplifier according to the present invention may have an outer cladding on the outer periphery of the cladding, and a coating on the outer periphery of the outer cladding, and the relationship between the refractive index n1 of the core, the refractive index n2 of the cladding, and the refractive index nc of the outer cladding may be n1>n2>nc.

[0014] In the optical fiber amplifier according to the present invention, the reduction ratio of the core by the core size converter is preferably greater than 0.19 and less than 0.76. [Effects of the Invention]

[0015] The present invention can provide an optical fiber amplifier that reduces the loss of pumping light in a pumping light combiner and improves amplification efficiency. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an optical fiber amplifier. [Figure 2] 1 is a diagram illustrating the configuration of an optical fiber amplifier according to the present invention; [Figure 3] 1 is a diagram illustrating a core size conversion section of an optical fiber amplifier according to the present invention. [Figure 4] FIG. 1 is a diagram illustrating an experimental system. [Figure 5] 10A and 10B are diagrams illustrating the relationship between the core diameter and the transmittance of excitation light. [Figure 6] 1 is a diagram illustrating a pump light combiner of an optical fiber amplifier according to the present invention; [Figure 7] 1 is a cross-sectional view illustrating an amplifying optical fiber of an optical fiber amplifier according to the present invention. [Figure 8] 1 is a cross-sectional view illustrating an amplifying optical fiber of an optical fiber amplifier according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0018] (Related Technology) FIG. 1 illustrates the configuration of cladding-pumped optical fiber amplifiers (300a, 300b). A pumping light combiner 12, which combines pumping light Le, is connected to one end of a rare-earth-element-doped pumping optical fiber 13, amplifying the signal light Ls propagating through the core. The amplified signal light is labeled "Lsa." Typically, an isolator is connected to the other end of the pumping optical fiber 13 in line with the propagation direction of the signal light Ls, but this is omitted in this figure. A residual pumping light remover may also be installed to emit pumping light Le that is not absorbed by the amplification optical fiber 13 out of the pumping optical fiber 13. FIGS. 1(a) and 1(b) respectively illustrate a forward pumping type in which pumping light Le is incident from the input side of the signal light Ls, and a backward pumping type in which pumping light Le is incident from the output side of the amplified signal light Lsa. In general, the pumping light Le output from the pumping light source 11 is propagated to the pumping light combiner 12 through a multimode optical fiber 16 having a core diameter of 105 μm.

[0019] (Embodiment 1) FIG. 2 is a diagram illustrating the configuration of the optical fiber amplifiers (301a, 301b) of this embodiment. The optical fiber amplifiers (301a, 301b) are an amplifying optical fiber 13 having a plurality of cores doped with rare earth elements in a cladding; an excitation light source 11 that outputs excitation light for exciting the rare earth element to a multimode optical fiber 16-1; a core size conversion unit 14 that reduces the diameter of the core of the multimode optical fiber 16-1 to the diameter of the cladding of the amplification optical fiber 13; a pumping light combiner 12 that inputs signal light Ls propagated through each core of a multi-core optical fiber 15 into each core of an amplification optical fiber 13, and inputs pumping light Le output from a pumping light source 11 and passed through a core size conversion unit 14 into the cladding including the core of the amplification optical fiber 13; Equipped with. The optical fiber amplifier 301a is a forward pumping type, and the optical fiber amplifier 301b is a backward pumping type.

[0020] The optical fiber amplifiers (301a, 301b) differ from the optical fiber amplifiers (300a, 300b) in Fig. 1 in that they include a core size converter 14 between the pumping light source 11 and the pumping light combiner 12. The core size converter 14 and the pumping light source 11 are connected by a multimode optical fiber 16-1. The core size converter 14 and the pumping light combiner 12 are connected by a multimode optical fiber 16-2.

[0021] 3 is a diagram illustrating an example of the configuration of the core size converter 14. The core size converter 14 comprises a large core section 31, a down taper section 32, and a small core section 33. The large core section 31 is connected to the multimode optical fiber 16-1, and the small core section 33 is connected to the multimode optical fiber 16-2. If the cladding diameter of the multimode optical fiber 16-1 and the large core section 31 is Dd0 and the core diameter is Dc0, then typically Dd0 = 125 μm and Dc0 = 105 μm. The cladding diameter of the multimode optical fiber 16-2 and the small core section 33 is Dd1 and the core diameter is Dc1. The down taper section 32 connects the large core section 31 and the small core section 33, and reduces the cladding diameter from Dd0 to Dd1 and the core diameter from Dc0 to Dc1.

[0022] Note that FIG. 3 is just one example of the configuration, and the core size converter 14 may be configured to output the pumping light Le from the multimode optical fiber 16-1 into space, control the spot size using a lens or the like, and input it into the multimode optical fiber 16-2.

[0023] The core diameter Dc1 that can be reduced by the core size converter 14 will be explained. Figure 4 is a diagram illustrating a taper experimental system using a multimode optical fiber 140 with a core diameter of 105 μm and an NA of 0.22, which is typically connected to a pumping light source. NA is the numerical aperture, and the refractive index of the core is expressed as n core , the refractive index of the cladding is n clad Then, it is defined as follows:

number

[0024] The multimode optical fiber 140 is covered with a coating 42 having a higher refractive index than the core 35 and the cladding 36, and light coupled from the core 35 to the cladding 36 is immediately absorbed by the coating 42 and lost.

[0025] The coating was removed from the middle part of the multimode optical fiber 140, and the multimode optical fiber 140 was heated and stretched using a ceramic heater to form the tapered portion 43. An excitation light source was connected to one end of the multimode optical fiber 140, and excitation light was input. The excitation light output from the other end was received by a power meter to measure the transmittance.

[0026] Figure 5 is a diagram illustrating the relationship between the core diameter at the taper waist portion of the tapered portion 43, where the cladding diameter is the smallest, and the transmittance of the pumping light. Figure 5 shows that when the core diameter is 20 μm or greater, the transmittance remains almost constant, achieving a value close to 100%. Since the transmittance drops rapidly when the core diameter is smaller than 20 μm, it can be seen that the lower limit of the core diameter Dc1 reduced by tapering is 0.19 times the core diameter Dc0 of the multimode optical fiber 16-1. In other words, if an optical fiber with a core diameter Dc0 of 105 μm, which is typical, is used as the multimode optical fiber 16-1, the core diameter Dc1 of the multimode optical fiber 16-2 must be 20 μm or greater.

[0027] 6 is a diagram illustrating the structure of the pumping light combiner 12. The pumping light combiner 12 has a port 63a to which the signal light Ls is input and a port 63b to which the pumping light Le is input. These light beams are wavelength-multiplexed by a dichroic mirror 61, a lens 62, or the like, and output to a single output port 63c. The signal light Ls is guided through each core of the multi-core optical fiber 15 and is incident on each core of the amplification optical fiber 13. The pumping light Le propagated in multimode through the core of the multimode optical fiber 16-2 is incident on the entire cladding region including the core of the amplification optical fiber 13.

[0028] The spot size of the pumping light Le can be changed by adjusting the lens 62 of the pumping light combiner 12, and the pumping light combiner 12 can also perform the function of the core size conversion unit .

[0029] 7 is a diagram illustrating the cross-sectional configuration of a rare-earth doped optical fiber, which is the amplification optical fiber 13. The amplification optical fiber 13 of this configuration is composed of a core 71 with a refractive index of n1, a cladding 72 that surrounds it with a refractive index of n2, and a resin 73 that surrounds the cladding 72 with a refractive index of n3. The relationship between these refractive indices is n1>n2>n3 is. "Λ" represents the center-to-center distance of the cores 71 (core-to-core distance). Although the drawing illustrates an amplification optical fiber 13 having two cores 71, the number of cores in the amplification optical fiber 13 may be three or more.

[0030] When the amplification optical fiber 13 is connected to the output side of the pumping light combiner 12 via an optical fiber, the optical fiber also has the structure shown in Fig. 7. Similarly, the coating of the optical fiber, which is a resin 73, is set to have a lower refractive index than the cladding 72. The optical fiber inputs the pumping light, which has been input from the pumping light combiner 12 to the cladding region (region including both the core 71 and the cladding 72), into the cladding region of the amplification optical fiber 13.

[0031] 8 is a diagram illustrating the cross-sectional configuration of a rare-earth doped optical fiber, which is another example of the amplifying optical fiber 13. The amplifying optical fiber 13 of this configuration is composed of a core 71 with a refractive index of n1, a cladding 72 surrounding it with a refractive index of n2, an outer cladding 72a surrounding the cladding 72 with a refractive index of nc, and a coating 74 surrounding the outer cladding 72a with a refractive index of n4. The relationship between these refractive indices is n1>n2>nc is. Although the drawing illustrates an amplification optical fiber 13 having two cores 71, the number of cores in the amplification optical fiber 13 may be three or more.

[0032] The diameter of the cladding 72 of the amplification optical fiber 13 of this structure is equal to Dc1 (the core diameter of the small core portion 33 of the core size converter 14 and the multimode optical fiber 16-2). Therefore, the pumping light combiner 12 can input the pumping light having a spot size of Dc1, which is input from the multimode optical fiber 16-2, into the cladding 72.

[0033] Since the pumping light propagating through the cladding region (region including both the core 71 and the cladding 72) is confined within the cladding region due to the difference in refractive index with the outer cladding 72a, n4 may be any refractive index.

[0034] As mentioned above, reducing the cladding diameter (increasing Rcc) is effective in improving amplification efficiency. However, as explained in Fig. 5, when the core diameter of the multimode optical fiber 16-1 is 105 µm, the core diameter Dc1 of the multimode optical fiber 16-2 needs to be 20 µm or more (the lower limit of the core reduction ratio of the core size converter 14 is 0.19). Therefore, the diameter of the cladding 72 of the amplification optical fiber 13, which is Dc1, also needs to be 20 µm or more.

[0035] Next, we will explain the upper limit of Dc1. The diameter of the optical fiber core specified in the IEC optical fiber standard document IEC60793-2-60 is at least 80 μm, and if it is made smaller than that, problems will arise in terms of handling and reliability. When the diameter of the cladding 72 is 80 μm or more, there are no problems with handling and reliability, and the amplification fiber 13 may have the structure described with reference to FIG. On the other hand, when the diameter of the cladding 72 is less than 80 μm to improve Rcc, in order to eliminate problems with handling and reliability, the amplification fiber 13 has a double-cladding structure as described in Fig. 8, and the diameter of the outer cladding 72a is set to 80 μm or more. In other words, if the cladding outer diameter Dd1 of the multimode optical fiber 16-2 is less than 80 μm, the core diameter Dc1 of the multimode optical fiber 16-2 will necessarily be less than 80 μm, and the diameter of the cladding 72 of the amplification optical fiber 13 will also be less than 80 μm.

[0036] As described above, the diameter of the cladding 72 of the amplifying optical fiber 13 in FIG. 20 μm < Dc1 < 80 μm The above numerical values ​​are for the case where the core diameter of the multimode optical fiber 16-1 is 105 μm, and expressed as a ratio, the ratio is greater than 0.19 and smaller than 0.76 with respect to the core diameter of the multimode optical fiber 16-1.

[0037] If the diameter of the outer cladding 72a is set to, for example, 80 μm to 125 μm, the handling ability and reliability of the amplification optical fiber 13 can be made equivalent to that of conventional optical fibers.

[0038] On the other hand, when the core diameter (Dc1) of the multimode optical fiber 16-2 is 80 μm to 105 μm, the amplification optical fiber 13 of FIG. 7 may be used and the diameter of the cladding 72 may be designed to match Dc1. [Explanation of symbols]

[0039] 11: Excitation light source 12: Pump light combiner 13: Amplifying optical fiber 14: Core size conversion unit 15: Multi-core optical fiber 16, 16-1, 16-2: Multimode optical fiber 31: Large core 32: Down taper section 33: Small core part 35: Core 36: Clad 42: Covering 43: Tapered section 61:Dichroic mirror 62: Lens 63a, 63b, 63c: Ports 71: Core 72: Clad 72a: outer cladding 73: Resin 74: Covering 140: Multimode optical fiber 301a, 301b: Optical fiber amplifier

Claims

1. an amplifying optical fiber having a plurality of cores doped with rare earth elements in a cladding; a pumping light source that outputs pumping light for exciting the rare earth element to a first multimode optical fiber; a core size converter that reduces the core diameter of one of the first multimode optical fibers to the core diameter of one of the second multimode optical fibers; a pumping light combiner that inputs signal light propagated through each core of a multi-core optical fiber into each core of the amplification optical fiber, and inputs the pumping light output from the pumping light source and passing through a core of one of the first multi-mode optical fibers, the core size converter, and a core of one of the second multi-mode optical fibers into the entire cladding region including the core of the amplification optical fiber; Equipped with An optical fiber amplifier, wherein the diameter of the core of the second multimode optical fiber is the same as the diameter of the cladding of the amplification optical fiber.

2. the amplification optical fiber has a resin on the outer periphery of the cladding, 2. The optical fiber amplifier according to claim 1, wherein the relationship between the refractive index n1 of the core, the refractive index n2 of the cladding, and the refractive index n3 of the resin satisfies n1>n2>n3.

3. the amplification optical fiber has an outer cladding around the cladding, and a coating around the outer cladding, 2. An optical fiber amplifier according to claim 1, wherein the relationship between the refractive index n1 of said core, the refractive index n2 of said cladding, and the refractive index nc of said outer cladding satisfies n1>n2>nc.

4. 4. The optical fiber amplifier according to claim 1, wherein the reduction ratio of the core by the core size converter is greater than 0.19 and smaller than 0.76.

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