Photonic crystal fiber and optical fiber amplifier

Abstract

A photonic crystal fiber (10) comprises: a core region (11) having a rare earth element added thereto; and a cladding region (12) including a plurality of refractive index reduction parts (13) provided around the core region (11). Each of the refractive index reduction parts (13) is formed of a glass rod having a refractive index lower than that of the cladding region (11) or a hole. The mode field diameter of the basic mode at a wavelength of 1.55 µm is not less than 16.4 µm.

Description

Photonic crystal fibers and optical fiber amplifiers

[0001] The present disclosure relates to photonic crystal fibers and optical fiber amplifiers.

[0002] Cut-off shifted optical fiber (CSF), which has an expanded core and a cutoff wavelength increased to 1.53 μm, is specified in ITU-T G.654 (Non-Patent Document 1). Compared to the currently used standard single-mode optical fiber (SMF), CSF has lower loss and nonlinearity, making it suitable for long-distance transmission. Non-Patent Document 2 states that in order to obtain low loss and low nonlinearity, the effective area (mode area) Aeff should be set to 150 μm. ２ There have been reports of cases where the condition has expanded to this extent.

[0003] In recent years, hollow-core fibers (HCFs) have also attracted attention. In particular, nested anti-resonant nodeless hollow-core fibers (NANFs), which are one type of HCF, have the characteristics of low latency, low nonlinearity, and low loss, and are being studied for various applications, including information communications (Non-Patent Documents 3 and 4). The NANF described in Non-Patent Document 3 comprises multiple large-diameter tubular structures enclosing smaller-diameter tubular structures. The large-diameter tubular structures are arranged circumferentially within a hollow cladding structure. At the center of the hollow cladding structure, a circle inscribed on the outer surface of the large-diameter tubular structure functions as a waveguide region. The diameter Dcore of this waveguide region largely determines the mode field diameter (MFD) of the NANF.

[0004] ITU-T G.654: Characteristics of a cut-off shifted single-mode optical fiber and cableYoshiaki Tamura, Hirotaka Sakuma, Keisei Morita, Masato Suzuki, Yoshinori Yamamoto, Kensaku Shimada, Yuya Honma, Kazuyuki Sohma, Takashi Fujii, and Takemi Hasegawa, “The First 0.14-dB / km Loss Optical Fiber and Its Impact on Submarine Transmission,” J. Lightwave Technol. 36, 44-49 (2018)Francesco Poletti, “Nested antiresonant nodeless hollow core fiber,” Opt. Express 22, 23807-23828 (2014)GT Jasion, H Sakr, JR Hayes, SR Sandoghchi, L Hooper, EN Fokoua, A Saljoghei, HC Mulvad, M Alonso, A Taranta, TD Bradley, IA Davidson, Y Chen, DJ Richardson, and F Poletti, “0.174 dB / km Hollow Core Double Nested Antiresonant Nodeless Fiber (DNANF),” in S. Matsuo, D. Plant, J. Shan Wey, C. Fludger, R. Ryf, and D. Simeonidou, eds., Technical Digest Series (Optica Publishing Group, 2022), paper Th4C.7.Taiji Sakamoto, Takayoshi Mori, Takashi Yamamoto, Lin Ma, Nobutomo Hanzawa, Shinichi Aozasa, Kyozo Tsujikawa, and Shigeru Tomita, “Transmission over large-core few-mode photonic crystal fiber using distance-independent modal dispersion compensation technique,” ​​Opt. Express 19, B478-B485 (2011).

[0005] The MFD of a typical NANF is larger than that of a standard SMF. According to Non-Patent Document 4, the diameter Dcore of the NANF for which the minimum loss is reported is 34.5 μm. This value is approximately four times larger than the core diameter of the standard SMF specified in ITU-T G.652. In other words, existing rare-earth doped fibers conforming to ITU-T G.652 have smaller core diameters than NANFs. Therefore, there is concern that splice loss due to MFD mismatch at the splice point with the NANF may become a problem.

[0006] The present disclosure has been made in consideration of the above circumstances, and aims to provide a photonic crystal fiber and an optical fiber amplifier that can suppress an increase in connection loss when connected to an optical fiber with a large MFD.

[0007] A photonic crystal fiber according to a first aspect of the present disclosure comprises a core region doped with a rare earth element, and a cladding region including a plurality of refractive index reduction portions arranged around the core region, the refractive index reduction portions being formed by air holes or glass rods having a refractive index lower than that of the cladding region, and the mode field diameter of the fundamental mode at a wavelength of 1.55 μm is 16.4 μm or greater.

[0008] An optical fiber amplifier according to a second aspect of the present disclosure comprises an excitation light source, an optical combiner that combines signal light with excitation light from the excitation light source, a photonic crystal fiber according to the first aspect into which the signal light and excitation light output from the optical combiner are input, and an optical isolator that blocks the excitation light from the light output from the photonic crystal fiber.

[0009] According to the present disclosure, it is possible to provide a photonic crystal fiber and an optical fiber amplifier that can suppress an increase in connection loss when connecting to an optical fiber with a large MFD.

[0010] FIG. 1A is a cross-sectional view of an optical fiber according to this embodiment. FIG. 1B is a cross-sectional view of an optical fiber according to this embodiment. FIG. 2A is a graph showing the relationship between center-to-center spacing Λ and d / Λ for several MFDs. FIG. 2B is a graph showing the relationship between center-to-center spacing Λ and d / Λ, showing two V values. FIG. 3A is a diagram for explaining possible values ​​of center-to-center spacing Λ and d / Λ. FIG. 3B is a diagram for explaining possible values ​​of center-to-center spacing Λ and d / Λ. FIG. 4 is a configuration diagram of an optical fiber amplifier according to this embodiment. FIG. 5 is a diagram for explaining the incidence of pumping light on an optical fiber when core pumping is adopted.

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, identical parts are designated by the same reference numerals, and description thereof will be omitted. As described below, the optical fiber according to this embodiment is a rare-earth-doped photonic crystal fiber that can be used in an optical fiber amplifier. For convenience of explanation, the rare-earth-doped photonic crystal fiber according to this embodiment will be referred to as the optical fiber, the mode field diameter as MFD, and the single-mode optical fiber as SMF.

[0012] 1A is a cross-sectional view of an optical fiber 10 according to this embodiment. As shown in FIG. 1A, the optical fiber 10 includes a core region (core) 11 and a cladding region (cladding) 12 provided around the core region 11.

[0013] The core region 11 is doped with a rare earth element. The rare earth element is selected depending on the wavelength band of the signal light 18 to be amplified. For example, when amplifying signal light in the C band (1.530 to 1.565 μm), erbium is selected as the rare earth element.

[0014] The cladding region 12 is provided around the core region 11. The cladding region 12 includes a plurality of refractive index reduction portions 13 around the core region 11. The refractive index reduction portions 13 are formed as holes or glass rods having a refractive index lower than that of the cladding region 12. The glass rods are, for example, material-doped glass having a refractive index lower than that of pure silica.

[0015] The refractive index reduced portions 13 are arranged in a triangular lattice pattern with the core region 11 at the center. That is, the centers of the refractive index reduced portions 13 are located on lattice points of the triangular lattice in a cross section perpendicular to the central axis of the optical fiber 10. In a cross section perpendicular to the central axis of the optical fiber 10, the refractive index reduced portions 13 have a circular cross section with a diameter d. The length of one side of the triangular lattice is Λ. That is, in the cross section, the centers of two adjacent refractive index reduced portions 13 among the plurality of refractive index reduced portions 13 are separated by a distance Λ. For convenience, this distance Λ may be referred to as the center-to-center distance.

[0016] The number of cells in the core region 11 and the number of layers in the refractive index reduced portions 13 are arbitrary. The number of cells is the number of two-dimensional unit cells occupying the core region 11 and is equal to the number of lattice points of a triangular lattice. The number of layers is the number of refractive index reduced portions 13 arranged radially outward from the center of the optical fiber 10 and is equal to the number of similar hexagons formed by the refractive index reduced portions 13 surrounding the core region 11. For example, as shown in FIG. 1A, the number of cells may be one and the number of layers may be two. Alternatively, as shown in FIG. 1B, the number of cells may be seven and the number of layers may be three.

[0017] The optical fiber 10 according to this embodiment can be applied to an amplifying optical fiber of an optical fiber amplifier 20, which will be described later. In this case, the pumping method for the optical fiber 10 may be either core pumping or cladding pumping. However, in the case of cladding pumping, it is preferable that the outer periphery of the optical fiber 10 is covered with a coating layer 14 having a lower refractive index than the cladding region 12. In the optical fiber 10 according to this embodiment, the cladding occupancy is reduced by the refractive index reduced portions 13, such as holes. Therefore, in the case of cladding pumping, an improvement in amplification efficiency can be expected. Furthermore, an improvement in amplification efficiency can be expected due to Mie scattering of the pumping light by the refractive index reduced portions 13, such as holes.

[0018] FIG. 2A is a graph showing the relationship between center-to-center spacing Λ and d / Λ for several MFDs. The values ​​on the curves in the graph indicate MFD [μm]. Meanwhile, FIG. 2B is a graph showing the relationship between center-to-center spacing Λ and d / Λ, showing two V values. The values ​​on the curves in the graph indicate V values. The finite element method was used for the analysis, and it was assumed that the core region 11 of the optical fiber 10 has a single-cell structure. In both analyses, the assumed wavelength was 1.55 μm.

[0019] Figure 2A shows that the MFD tends to increase as the spacing Λ increases. Meanwhile, curve G2 in Figure 2B, which indicates a V value of 2.405, represents the normalized frequency of the first higher-order mode. Curve G1 in the same figure, which indicates a V value of 3.83, represents the normalized frequency of the second higher-order mode. Region R2 below curve G2 represents the region in which single-mode operation is possible. Region R1 below curve G1 represents the region in which quasi-single-mode operation is possible.

[0020] The MFD of existing rare-earth-doped fibers conforming to ITU-T G.652 is considered to be approximately 10 μm at a wavelength of 1.55 μm. When splicing such rare-earth-doped fibers with expanded-core fibers such as NANFs, analysis using the finite element method (FEM) indicates that when the MFD of the expanded-core fiber is 16.4 μm or greater at λ = 1.55 μm, the splice loss between the two fibers is 1 dB or greater at λ = 1.55 μm. On the other hand, the optical fiber 10 according to the present embodiment is a photonic crystal fiber, and as shown in FIG. 2A , by adjusting the center spacing Λ and d / Λ, the MFD of the fundamental mode at a wavelength of 1.55 μm can be 16.4 μm or greater. In other words, by applying the optical fiber 10 according to the present embodiment to an optical fiber spliced ​​with the expanded-core fiber, the splice loss between the two fibers can be reduced.

[0021] 3A and 3B are diagrams illustrating possible values ​​of the center-to-center spacing Λ and d / Λ. In addition to the curves G1 and G2 shown in FIG. 2B, both diagrams also show curves G3 and G4. Curve G3 indicates the values ​​of the center-to-center spacing Λ and d / Λ at which the MFD of an optical fiber 10 having a single-cell core region 11 (see FIG. 1A) is 16.4 μm. That is, the region below curve G3 in FIG. 3A (FIG. 3B) indicates the values ​​of the center-to-center spacing Λ and d / Λ at which the MFD of an optical fiber 10 having a single-cell core region 11 is 16.4 μm. On the other hand, curve G4 indicates the values ​​of the center-to-center spacing Λ and d / Λ at which the MFD of an optical fiber 10 having a seven-cell core region 11 (see FIG. 1B) is 16.4 μm. That is, the region below curve G4 in FIG. 3A (FIG. 3B) shows the center spacing Λ and d / Λ values ​​at which the MFD of the optical fiber 10 having the core region 11 with a 7-cell structure is 16.4 μm.

[0022] Curves G1, G2, G3, and G4 can be expressed by the following approximate formulas using exponential functions: G1: d / Λ = 0.9271*exp(-0.9091*Λ) + 0.4985*exp(-0.006675*Λ) G2: d / Λ = 0.4391*exp(-0.5248*Λ) + 0.6805*exp(-0.002708*Λ) G3: d / Λ = 0.5262*exp(-0.9301*Λ) + 0.1053*exp(0.1107*Λ) G4: d / Λ = 0.7765*exp(-1.223*Λ) + 0.143*exp(0.2334*Λ) Then, by combining either one of the curves G1 and G2 with either one of the curves G3 and G4, it is possible to calculate the range of values ​​that the center distance Λ and d / Λ can take under various conditions.

[0023] 3A , by combining curves G2 and G3, a region R3 is obtained in which the center-to-center spacing Λ and d / Λ values ​​simultaneously satisfy the following two equations: d / Λ ≦0.4391*exp(−0.5248*Λ) + 0.6805*exp(−0.002708*Λ) d / Λ ≦0.5262*exp(−0.9301*Λ) + 0.1053*exp(0.1107*Λ). When the center-to-center spacing Λ and d / Λ are set to values ​​within region R3, an optical fiber 10 having an MFD of 16.4 μm or greater and a core region 11 with a single-cell structure can be obtained. In other words, an optical fiber 10 having an MFD of 16.4 μm or greater and a core region 11 with a single-cell structure can be designed.

[0024] 3B , by combining curves G1 and G4, a region R4 is obtained in which the center-to-center spacing Λ and d / Λ values ​​simultaneously satisfy the following two equations: d / Λ ≦0.9271*exp(−0.9091*Λ) + 0.4985*exp(−0.006675*Λ) d / Λ ≦0.7765*exp(−1.223*Λ) + 0.143*exp(0.2334*Λ). When the center-to-center spacing Λ and d / Λ are set to values ​​within region R4, an optical fiber 10 having an MFD of 16.4 μm or greater and a core region 11 with a seven-cell structure can be obtained. In other words, an optical fiber 10 having an MFD of 16.4 μm or greater and a core region 11 with a seven-cell structure can be designed. Region R4 encompasses region R3. Therefore, in the region where the regions R3 and R4 overlap each other, an MFD of 16.4 μm or more can be obtained in both the one-cell structure 7 and the one-cell structure.

[0025] 4 is a configuration diagram of an optical fiber amplifier 20 according to this embodiment. As shown in FIG. 4, the optical fiber amplifier 20 includes a pumping light source 21, a first optical isolator 22, an optical combiner (optical coupler) 23, the optical fiber 10 according to this embodiment, and a second optical isolator 24. For ease of explanation, the optical fiber amplifier 20 will be described using an example in which signal light 18 in the 1.55 μm band is amplified by pumping light 19 with a wavelength of 1.48 μm. Therefore, in this example, erbium is doped into the core region 11 (see FIG. 1A ) of the optical fiber 10.

[0026] The pumping light source 21 outputs pumping light 19. In this example, the pumping light source 21 is, for example, a semiconductor laser in the 1.48 μm band. The first optical isolator 22 passes the pumping light 19 from the pumping light source 21 to the optical combiner 23, and blocks the pumping light 19 that is reflected from the optical combiner 23 or the like and travels back to the pumping light source 21.

[0027] The optical combiner 23 is formed by a part of the optical fiber 10. That is, the optical fiber 10 functions as the optical combiner 23, combining the signal light 18 from the transmission fiber 31 with the pumping light 19 from the pumping light source 21. Furthermore, the optical fiber 10 functions as an optical amplifying fiber, amplifying the signal light 18 with the pumping light 19. The signal light 18 from the transmission fiber 31 is incident on the core region 11 on the end face of the optical fiber 10.

[0028] When cladding pumping is employed as the pumping method for the signal light 18, pumping light 19 is incident on the cladding region 12 on the end face of the optical fiber 10. The pumping light 19 is repeatedly reflected and scattered within the cladding region 12, exciting the rare earth elements in the core region 11 and generating a population inversion of the rare earth elements in the core region 11. The signal light 18 traveling through the core region 11 is amplified by stimulated emission from this population inversion and is output to the second optical isolator 24.

[0029] The second optical isolator 24 passes the signal light 18 output from the optical fiber 10 to the downstream transmission fiber 32. The second optical isolator 24 also blocks the pump light 19 traveling toward the transmission fiber 31.

[0030] As described above, the optical fiber 10 also serves as the optical combiner 23 connected to the transmission fiber 31. Therefore, the optical combiner 23 has an MFD equivalent to that of the optical fiber 10, which is 16.4 μm or more. Therefore, even if the transmission fiber 31 is an expanded-core fiber such as NANF, an increase in connection loss can be suppressed, for example, to 1 dB or less. Since the increase in connection loss is suppressed, the amplification efficiency of the optical fiber amplifier 20 also improves.

[0031] It should be noted that core pumping can also be adopted as a pumping method for the signal light 18. Fig. 5 is a diagram for explaining the incidence of pumping light 19 into the optical fiber 10 when core pumping is adopted. As shown in Fig. 5, when core pumping is adopted, the pumping light 19 is incident on the side surface of the bent optical fiber 10.

[0032] When the optical fiber 10 is bent, bending loss occurs according to the curvature. In other words, bending the optical fiber 10 causes light to leak from the core region 11 through the side surface of the optical fiber 10. Conversely, by bending the optical fiber 10 with an appropriate curvature radius, the pump light 19 can be made to enter the core region 11 from the side surface of the optical fiber 10. In other words, core pumping can be performed. Note that, because the pump light 19 enters the side surface of the optical fiber 10, some of the pump light 19 may be reflected or scattered within the cladding region 12. In this case, the reflected or scattered pump light 19 contributes to cladding pumping.

[0033] As described above, the optical fiber 10 is a photonic crystal fiber. It is known that one of the characteristic properties of photonic crystal fiber is that bending loss increases as the wavelength becomes shorter. In this example, the wavelength of the pump light 19 is shorter than the wavelength of the signal light 18. Therefore, by imparting a bend with an appropriate curvature to the optical fiber 10, the signal light 18 can be confined in the core region 11 while the pump light 19 can be incident on the side of the optical fiber 10 into the core region 11.

[0034] REFERENCE SIGNS LIST 10 Optical fiber 11 Core region 12 Cladding region 13 Refractive index reduced portion 14 Coating layer 18 Signal light 19 Pumping light 20 Optical fiber amplifier 21 Pumping light source 22 First optical isolator 23 Optical combiner 24 Second optical isolator

Claims

1. A photonic crystal fiber comprising: a core region doped with a rare earth element; and a cladding region including a plurality of refractive index reduction sections arranged around the core region, wherein the refractive index reduction sections are formed by air holes or glass rods with a refractive index lower than that of the cladding region; and wherein the mode field diameter of the fundamental mode at a wavelength of 1.55 μm is 16.4 μm or greater.

2. The photonic crystal fiber according to claim 1, wherein d is the diameter of each of the refractive index reduced portions and Λ is the center-to-center distance between two adjacent refractive index reduced portions among the plurality of refractive index reduced portions, and d and Λ have values ​​within a region that simultaneously satisfies the following two equations: d / Λ ≦ 0.9271*exp(-0.9091*Λ) + 0.4985*exp(-0.006675*Λ) d / Λ ≦ 0.7765*exp(-1.223*Λ) + 0.143*exp(0.2334*Λ) 3. The photonic crystal fiber according to claim 1, wherein d is the diameter of each of the refractive index reduced portions and Λ is the center-to-center distance between two adjacent refractive index reduced portions among the plurality of refractive index reduced portions, and d and Λ have values ​​within a range that simultaneously satisfies the following two equations: d / Λ ≦ 0.4391*exp(-0.5248*Λ) + 0.6805*exp(-0.002708*Λ) d / Λ ≦ 0.5262*exp(-0.9301*Λ) + 0.1053*exp(0.1107*Λ) 4. An optical fiber amplifier comprising: a pumping light source; a photonic crystal fiber according to any one of claims 1 to 3, into which signal light and pumping light from said pumping light source are input; and an optical isolator that blocks said pumping light from among the light output from said photonic crystal fiber.

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