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EP-4216380-B1 - MULTI-CORE OPTICAL AMPLIFYING FIBER, MULTI-CORE OPTICAL FIBER AMPLIFIER, AND OPTICAL COMMUNICATION SYSTEM

EP4216380B1EP 4216380 B1EP4216380 B1EP 4216380B1EP-4216380-B1

Inventors

  • TAKASAKA, SHIGEHIRO
  • MAEDA, KOICHI
  • SUGIZAKI, RYUICHI

Dates

Publication Date
20260506
Application Date
20210309

Claims (9)

  1. A multi-core optical amplifying fiber (1, 2) comprising: a plurality of core portions (laa, 1ab, 2aa, 2ab, 2ac) doped with a rare-earth element; an inner cladding portion (1b, 2b) surrounding the core portions (laa, 1ab, 2aa, 2ab, 2ac) and having a refractive index lower than a maximum refractive index of the core portions (laa, 1ab, 2aa, 2ab, 2ac); and an outer cladding portion (1c, 2c) surrounding the inner cladding portion (1b, 2b) and having a refractive index lower than the refractive index of the inner cladding portion (1b, 2b), wherein a mode field diameter of each of the core portions (laa, 1ab, 2aa, 2ab, 2ac) at a wavelength at which the rare-earth element performs optical amplification is 5 µm to 11 µm, a relative refractive-index difference of the maximum refractive index of each of the core portions (laa, 1ab, 2aa, 2ab, 2ac) with respect to the inner cladding portion is 0.35% to 2%, a core-to-core distance that is a separation distance between centers of the respective core portions (laa, 1ab, 2aa, 2ab, 2ac) on a section perpendicular to a longitudinal direction is set such that total inter-core crosstalk is -40 dB/100 m or lower in an optical amplification wavelength band subjected to the optical amplification, a cladding thickness that is a distance from a center of a core portion (laa, 1ab, 2aa, 2ab, 2ac) closest to an outer end of the inner cladding portion (1b, 2b) out of the core portions (laa, 1ab, 2aa, 2ab, 2ac) to the outer end is smaller than a value obtained by adding the mode field diameter to a minimum value of the core-to-core distance, and characterised in that a ratio of a total sectional area of the plurality of core portions (laa, 1ab, 2aa, 2ab, 2ac) to a sectional area of the inner cladding portion (1b, 2b) is 1.9% or more.
  2. The multi-core optical amplifying fiber (1, 2) according to claim 1, wherein the minimum value of the core-to-core distance is 27 µm or larger.
  3. The multi-core optical amplifying fiber (1, 2) according to claim 1 or 2, wherein the core-to-core distance is set such that the total inter-core crosstalk is -50 dB/100 m or lower in the optical amplification wavelength band.
  4. The multi-core optical amplifying fiber (1,2) according to claim 3, wherein the minimum value of the core-to-core distance is 29 µm or larger.
  5. The multi-core optical amplifying fiber (1, 2) according to any one of claims 1 to 4, wherein the core-to-core distance is set such that the total inter-core crosstalk is -60 dB/100 m or lower in the optical amplification wavelength band.
  6. The multi-core optical amplifying fiber (1, 2) according to claim 5, wherein the minimum value of the core-to-core distance is 31 µm or larger.
  7. The multi-core optical amplifying fiber (1, 2) according to any one of claims 1 to 6, wherein the cladding thickness is 10 µm or larger.
  8. A multi-core optical fiber amplifier (100) comprising: the multi-core optical amplifying fiber (1, 2) according to any one of claims 1 to 7; a pumping light source (30) configured to output pumping light for photoexciting the rare-earth element of the multi-core optical amplifying fiber (1, 2); and an optical coupler (40) configured to optically couple the pumping light to the inner cladding portion.
  9. An optical communication system (1000) comprising the multi-core optical fiber amplifier (100) according to claim 8.

Description

Field The present invention relates to a multi-core optical amplifying fiber, a multi-core optical fiber amplifier, and an optical communication system. Background In undersea optical communications, for example, the use of a multi-core EDFA (erbium-doped optical fiber amplifier) as an optical amplifier is expected to reduce the power consumption of the optical amplifier. Widely known is a multi-core EDFA obtained by using a double-cladding multi-core EDF as a multi-core optical amplifying fiber and photoexciting erbium (Er) serving as a rare-earth element included in the core portions by a cladding pumping system (refer to Non-Patent Documents 1 and 2). Typically, the length of the multi-core EDF used for the multi-core EDFA is approximately 100 m or shorter. Citation List Patent Literature Non-Patent Document 1: Kazi S Abedin et al., "Multimode Erbium Doped Fiber Amplifiers for Space Division Multiplexing Systems", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 16, AUGUST 15, 2014 pp. 2800-2808.Non-Patent Document 2: Kazi S Abedin et al., "Cladding-pumped erbium-doped multicore fiber amplifier", OPTICS EXPRESS Vol. 20, No. 18 27 August 2012 pp. 20191-20200. US 2015/318659 A1 discloses an amplifying optical fibre and optical amplifier that can amplify communication light while suppressing crosstalk. Summary Technical Problem Multi-core optical amplifying fibers need to have more suitable characteristics to increase the communication capacity because communication traffic is constantly increasing. In particular, it is preferable to improve the excitation efficiency of multi-core optical amplifying fibers from the viewpoint of reducing the power consumption of multi-core optical fiber amplifiers. The excitation efficiency is expressed, for example, by the ratio of the energy of pumping light used for optical amplification to the energy of pumping light input to a multi-core optical amplifying fiber. In view of the background described above, an object of the present invention is to provide a multi-core optical amplifying fiber with improved excitation efficiency, and a multi-core optical fiber amplifier and an optical communication system using the multi-core optical amplifying fiber. Solution to Problem To solve the above-described problem and achieve the object, a multi-core optical amplifying fiber according to one aspect of the present invention includes: a plurality of core portions doped with a rare-earth element; an inner cladding portion surrounding the core portions and having a refractive index lower than a maximum refractive index of the core portions; and an outer cladding portion surrounding the inner cladding portion and having a refractive index lower than the refractive index of the inner cladding portion, wherein a mode field diameter of each of the core portions at a wavelength at which the rare-earth element performs optical amplification is 5 µm to 11 µm, a relative refractive-index difference of the maximum refractive index of each of the core portions with respect to the inner cladding portion is 0.35% to 2%, a core-to-core distance that is a separation distance between centers of the respective core portions on a section perpendicular to a longitudinal direction is set such that total inter-core crosstalk is -40 dB/100 m or lower in an optical amplification wavelength band subjected to the optical amplification, and a cladding thickness that is a distance from a center of a core portion closest to an outer end of the inner cladding portion out of the core portions to the outer end is smaller than a value obtained by adding the mode field diameter to a minimum value of the core-to-core distance. The minimum value of the core-to-core distance may be 27 µm or larger. The core-to-core distance may be set such that the total inter-core crosstalk is -50 dB/100 m or lower in the optical amplification wavelength band. The minimum value of the core-to-core distance may be 29 µm or larger. The core-to-core distance may be set such that the total inter-core crosstalk is -60 dB/100 m or lower in the optical amplification wavelength band. The minimum value of the core-to-core distance may be 31 µm or larger. The cladding thickness may be 10 µm or larger. A multi-core optical fiber amplifier according to one aspect of the present invention includes: the multi-core optical amplifying fiber; a pumping light source configured to output pumping light for photoexciting the rare-earth element of the multi-core optical amplifying fiber; and an optical coupler configured to optically couple the pumping light to the inner cladding portion. An optical communication system according to one aspect of the present invention includes the multi-core optical fiber amplifier. Advantageous Effects of Invention The present invention can provide a multi-core optical amplifying fiber with improved excitation efficiency, and a multi-core optical fiber amplifier and an optical communication system using the multi-core optic