Optical fiber and optical fiber amplifier

Abstract

An optical fiber (10) comprises: a core (11) which is doped with a rare earth element; and a cladding (12) which has a refractive index that is lower than the core (11) by a relative refractive index difference Δ. The radius a of the core (11) and the relative refractive index difference Δ have values such that the mode field diameter of not less than 16.4 μm is obtained at a wavelength of 1.55 μm.

Description

Optical fiber and optical fiber amplifier

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

[0002] In recent years, hollow-core nested antiresonant nodeless fiber (NANF) has achieved a new record for the lowest optical loss among optical fibers, drawing attention as an optical fiber for future long-distance, high-capacity transmission. NANF is known to have a larger mode field diameter (MFD) than conventional single-mode optical fiber (SMF). Furthermore, the MFD of an optical fiber is an important parameter in determining splice characteristics. A large difference in MFD between optical fibers being spliced ​​together results in increased splice loss. Non-Patent Document 1 reports that the MFD of NANF is approximately 22 to 25 μm. This is more than twice the value of conventional SMF.

[0003] GT Jasion et al., “Hollow core NANF with 0.28 dB / km attenuation in the C and L bands,” in Proc. Opt. Fiber Commun. Conf 2020, Paper Th4B-4.K. Nakajima et al., “Single-mode hole-assisted fiber as a bending-loss insensitive fiber,” Optical Fiber Technology 16 (2010) 392-398.

[0004] Existing optical fiber amplifiers (OFAs) are designed to be connected to SMF. In other words, conventional OFAs are designed with an MFD equivalent to that of SMF. When considering the future application of NANF to large-capacity trunk lines, there is a concern that the connection loss may increase due to MFD mismatch.

[0005] The present disclosure has been made in view of the above circumstances, and aims to provide an optical fiber and an optical fiber amplifier that can suppress an increase in connection loss when connected to an optical fiber with a large MFD.

[0006] An optical fiber according to a first aspect of the present disclosure comprises a core doped with a rare earth element and a cladding having a refractive index lower than that of the core by a relative refractive index difference, wherein the core radius and the relative refractive index difference have values ​​that result in a mode field diameter of 16.4 μm or more at a wavelength of 1.55 μm.

[0007] An optical fiber amplifier according to a second aspect of the present disclosure includes the optical fiber according to the first aspect.

[0008] According to the present disclosure, it is possible to provide an optical fiber and an optical fiber amplifier that can suppress an increase in connection loss when connected to an optical fiber with a large MFD.

[0009] FIG. 1 is a diagram showing the cross section and refractive index profile of an optical fiber according to this embodiment. FIG. 2 is a graph showing the change in connection loss of optical fibers connected to each other. FIG. 3 is a graph showing the relationship between the core radius and the relative refractive index difference value and the MFD. FIG. 4 is a configuration diagram of an optical amplifier equipped with the optical fiber according to this embodiment. FIG. 5 is a diagram showing the cross section and refractive index profile of an optical fiber according to a modified example of this embodiment.

[0010] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the description of the drawings, the same parts are designated by the same reference numerals and the description will be omitted. As described below, the optical fiber according to this embodiment is a rare-earth doped optical fiber that can be used in an optical fiber amplifier. For convenience of explanation, the mode field diameter will be abbreviated as MFD and single-mode optical fiber as SMF.

[0011] FIG. 1 is a diagram showing the cross section and refractive index profile of an optical fiber 10 according to this embodiment. As shown in FIG. 1, the optical fiber 10 includes a core (core region) 11 and a cladding (cladding region) 12 provided around the core 11. The core 11 is doped with a rare earth element at a certain concentration. The rare earth element is selected depending on the wavelength band of the signal light 18 to be amplified. For example, when the signal light 18 in the C band or L band is amplified by the pump light 19, erbium is selected as the rare earth element.

[0012] The refractive index profile of the core 11 is constant. That is, the optical fiber 10 according to this embodiment is a step-index (SI) optical fiber. The cladding 12 has a refractive index that is lower than that of the core 11 by a relative refractive index difference Δ.

[0013] The splice loss between optical fibers can be estimated by the difference in their MDF. Figure 2 is a graph showing the change in splice loss when the MFD of one of the two optical fibers being spliced ​​together is changed. In this analysis, the MFD of the other of the two optical fibers is set to a constant value of 10.3 μm. This value is the standard value for amplification optical fibers used in existing optical fiber amplifiers. The wavelength assumed in the analysis is 1.55 μm.

[0014] As shown in Figure 2, to keep the connection loss per connection point to 1 dB or less, the MFD must be set to 16.4 μm or less. For example, if the MFD of the optical fiber connected to the optical fiber amplifier is 16.4 μm or more, the MFD of the amplification optical fiber must be set to a value greater than 10.3 μm.

[0015] As described above, the optical fiber 10 according to this embodiment is a step-index optical fiber. The MFD(2W) of a step-index optical fiber can be approximately expressed by the following formula (1) using the wavelength λ, the core radius a, the refractive index n of the core 11, the relative refractive index difference Δ, and the V value (V parameter).

[0016] Figure 3 is a graph showing the relationship between the core radius a and the relative refractive index difference Δ versus the MFD. Curve G in Figure 3 shows the core radius a and the relative refractive index difference Δ calculated using Equation (1) for an MFD of 16.4 μm when the wavelength λ is 1.55 μm. Therefore, by setting the core radius a and the relative refractive index difference Δ in region R below curve G, an MFD of 16.4 μm or greater can be obtained. In other words, the core radius a and the relative refractive index difference Δ in region R satisfy the relationship shown in Equation (2) below.

[0017] By selecting the core radius a and the relative refractive index difference Δ within region R, the optical fiber 10 has an MFD of 16.4 μm or more. Therefore, splice loss can be reduced compared to when using conventional SMF. In other words, an increase in splice loss can be suppressed when splicing with optical fibers such as NANF, which have a larger MFD than general SMF.

[0018] The value substituted for W on the left side of Equation (1) or Equation (2) is not limited to 16.4 μm. That is, W may be less than 16.4 μm or greater than 16.4 μm, as long as it is greater than the MFD of a typical SMF. In either case, it is possible to reduce the difference in MFD with the optical fiber at the splice destination and suppress an increase in splice loss when splicing with an optical fiber having a larger MFD than a typical SMF.

[0019] 4 is a configuration diagram of an optical amplifier 20 including an optical fiber 10 according to this embodiment. The optical amplifier 20 includes the optical fiber 10 as an amplifying optical fiber, and an optical coupler (pump light combiner) 21. The optical coupler 21 combines signal light 18 and pump light 19 and outputs the combined light to the optical fiber 10. The optical fiber 10 amplifies and outputs the signal light 18 by stimulated emission from the population inversion of rare earth ions.

[0020] For example, if the optical fiber 10 is an erbium-doped optical fiber, the signal light 18 is light in the 1.5 μm band that is amplified within the optical fiber 10. On the other hand, the pumping light 19 is light with a wavelength of 1.48 μm or 0.98 μm that is output from a light source 22 such as a semiconductor laser. Optical isolators (not shown) are inserted between the optical coupler 21 and the pumping light source and on the downstream side of the optical fiber 10 to suppress oscillation of the pumping light 19.

[0021] The optical amplifier 20 includes the optical fiber 10 according to this embodiment as the amplification optical fiber. That is, the optical fiber 10 has an MFD of 16.4 μm or more at a wavelength of 1.55 μm. Therefore, compared to existing optical amplifiers that include SMF as the amplification optical fiber, it is possible to suppress an increase in connection loss when connecting to an optical fiber with a large MFD.

[0022] FIG. 5 is a diagram showing the cross section and refractive index profile of an optical fiber 10 according to a modification of this embodiment. As shown in FIG. 5, the cladding 12 may have a plurality of holes 13 arranged around the core 11. The plurality of holes 13 are arranged at equal angular intervals on the same circle centered on the core 11 and extend parallel to the core 11. For example, as shown in FIG. 1, six holes 13 are arranged on the same circle at angular intervals of 60°. In other words, the optical fiber 10 according to this modification is a hole-assisted optical fiber (HAF) having a step-index core 11. Note that the number of holes 13 is not limited to six as shown in FIG. 1. The holes 13 may also be formed on multiple concentric circles.

[0023] Generally, bending loss increases as the core radius a decreases. On the other hand, by providing holes 13 in the cladding 12, bending loss characteristics can be improved. According to Non-Patent Document 2, when six holes 13 as shown in FIG. 5 are provided in the cladding 12, bending loss can be improved by more than two orders of magnitude compared to an optical fiber without holes 13. Also, according to the same document, it is possible to reduce the cutoff wavelength to 1.53 μm or less. Therefore, the optical fiber 10 according to this modification can also be applied to the optical amplifier 20 according to this embodiment.

[0024] 10 Optical fiber 11 Core 12 Cladding 13 Hole 18 Signal light 19 Pumping light 20 Optical amplifier 21 Optical coupler

Claims

1. An optical fiber comprising: a core doped with a rare earth element; and a cladding having a refractive index lower than that of said core by a relative refractive index difference, wherein the core radius and the relative refractive index difference have values ​​that result in a mode field diameter of 16.4 μm or more at a wavelength of 1.55 μm.

2. The optical fiber according to claim 1, wherein the core radius and the relative refractive index difference satisfy the relationship shown in the following formula: a: the core radius Δ: the relative refractive index difference λ=1.55 μm n: the refractive index of the core 3. The optical fiber according to claim 1 or 2, wherein the cladding has a plurality of holes arranged around the core.

4. An optical fiber amplifier comprising the optical fiber according to any one of claims 1 to 3 as an amplifying optical fiber.

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