Multicore fiber, optical coupler, and optical amplifier

Abstract

This multicore fiber is provided with: a plurality of core parts made of glass; an inner cladding part which is made of glass having a refractive index lower than the maximum refractive index of the core parts, and which surrounds the outer periphery of the plurality of core parts; and an outer cladding part which is made of glass having a refractive index lower than that of the inner cladding part, and which surrounds the inner cladding part. The outer cladding part may be made of glass to which fluorine is added. The multicore fiber may further include a coating layer which is made of resin having a refractive index lower than that of the outer cladding part, and which surrounds the outer cladding part.

Description

Multicore fiber, optical coupler and optical amplifier

[0001] The present invention relates to a multicore fiber, an optical coupler, and an optical amplifier.

[0002] For example, in applications such as undersea optical communications, it is expected that the power consumption of optical amplifiers will be reduced by using multi-core erbium-doped optical fiber amplifiers (EDFAs) as optical amplifiers.

[0003] A known multi-core EDFA is configured to use a double-clad multi-core EDF and optically pump erbium (Er), an optical amplification medium contained in the core, by a cladding pumping method (see Non-Patent Documents 1 and 2).

[0004] In a multi-core EDFA, when optically pumping an optical amplification medium included in a core portion, an optical coupler called a lateral coupling type or a transverse coupling type may be used (see Patent Document 1). In a lateral coupling type optical coupler, for example, a pumping optical fiber that propagates pumping light is fused to the inner cladding portion of the multi-core fiber and contacts the inner cladding portion at a contact region. As a result, at least a part of the pumping light propagating through the pumping optical fiber is coupled from the contact region to the inner cladding portion of the multi-core fiber.

[0005] Japanese Patent Application Laid-Open No. 2021-163814

[0006] 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.Kazi S Abedin et al, “Clading-pumped erbium-doped multicore fiber amplifier”, OPTICS EXPRESS Vol.20,No.18 27 August 2012 pp.20191-20200.

[0007] When heat is applied to a multi-core fiber when fusing an optical fiber such as an optical fiber for pumping light to the multi-core fiber, the heat may affect the multi-core fiber. For example, the heat may cause the dopant added to the cores to increase the refractive index to diffuse. In this case, the refractive index of the cores decreases, which may increase inter-core crosstalk between multiple cores. The heat may also cause the cores to deform, which may increase inter-core crosstalk.

[0008] The present invention has been made in view of the above, and aims to provide a multi-core fiber that can suppress the influence of heat when welding an optical fiber to a multi-core fiber, and an optical coupler and an optical amplifier using the same.

[0009] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a multicore fiber including: a plurality of core portions made of glass; an inner cladding portion made of glass having a refractive index lower than the maximum refractive index of the core portions and surrounding the outer peripheries of the plurality of core portions; and an outer cladding portion made of glass having a refractive index lower than the refractive index of the inner cladding portion and surrounding the inner cladding portion.

[0010] The outer cladding may be made of fluorine-doped glass.

[0011] The multi-core fiber may further include a coating layer made of a resin having a refractive index lower than that of the outer cladding portion, the coating layer surrounding the outer cladding portion.

[0012] The softening point of the outer cladding may be lower than the softening point of the inner cladding.

[0013] The outer cladding may have a thickness of 10 μm or more.

[0014] At the wavelength of light input to the core portion and propagating through the core portion, inter-core crosstalk between adjacent core portions among the plurality of core portions may be −50 dB or less per 1 km length.

[0015] The distance between the central axis of the core portion closest to the outer peripheral surface of the outer cladding portion and the outer peripheral surface may be 20 μm or more.

[0016] At the wavelength of light input to the inner cladding portion and propagating through the inner cladding portion, a numerical aperture determined by the refractive index of the inner cladding portion and the refractive index of the outer cladding portion may be 0.16 or more and 0.28 or less.

[0017] The multicore fiber may be made of a resin having a refractive index equal to or higher than that of the outer cladding portion, and may include a coating layer surrounding the outer cladding portion, and the thickness of the outer cladding portion may be 10 times or more the wavelength of light input to the inner cladding portion and propagating through the inner cladding portion.

[0018] One aspect of the present invention is an optical coupler including the multicore fiber and an optical fiber for optical coupling that is fused to the outer cladding portion of the multicore fiber and propagates light to be coupled to the inner cladding portion of the multicore fiber.

[0019] The coupling efficiency of the light propagating through the optical coupling optical fiber to the inner cladding portion may be 60% or more.

[0020] At the wavelength of light input to the plurality of core portions of the multicore fiber and propagating through the plurality of core portions, inter-core crosstalk between adjacent core portions among the plurality of core portions may be −50 dB or less.

[0021] The optical coupling optical fiber may be configured as a side-coupling type optical coupler, with a portion in the longitudinal direction of the optical coupling optical fiber being welded along the outer clad portion.

[0022] One aspect of the present invention is an optical amplifier comprising: the optical coupler; an optically amplifying multi-core fiber having a plurality of optically amplifying core sections made of glass and doped with an optical amplification medium; and an optically amplifying inner cladding section made of glass having a refractive index lower than the maximum refractive index of the optical amplification core sections and surrounding the outer peripheries of the plurality of optical amplification core sections; and an excitation light source that inputs excitation light that optically excites the optical amplification medium into the optical fiber for optical coupling of the optical coupler, wherein each of the plurality of optically amplifying core sections of the optically amplifying multi-core fiber is optically connected to each of the plurality of core sections of the multi-core fiber of the optical coupler, and the optically amplifying inner cladding section of the optically amplifying multi-core fiber is optically connected to the inner cladding section of the multi-core fiber of the optical coupler.

[0023] The optical amplification medium may be erbium, and the wavelength of the pumping light may be 1450 nm or more and 1520 nm or less.

[0024] The optical amplifier may include a plurality of forward pumping light sources that output forward pumping light having a wavelength of 970 nm or more and 990 nm or less, which optically pumps the optical amplification medium, and a guide unit that guides each of the forward pumping light output from the plurality of forward pumping light sources to each of the plurality of optical amplification core units, wherein the optical amplification medium is erbium, the optical coupler and the pumping light source are configured to backward pump the optically amplifying multi-core fiber, and the guide unit propagates the forward pumping light in a single mode.

[0025] The optical amplifier may include two of the optical couplers and two of the pumping light sources, wherein the wavelength of the pumping light output by a first pumping light source of the two pumping light sources is 1450 nm or more and 1520 nm or less, and the wavelength of the pumping light output by a second pumping light source of the two pumping light sources is 970 nm or more and 990 nm or less, and the two optical couplers and the two pumping light sources may be configured such that the first pumping light source backward pumps the optically amplifying multi-core fiber, and the second pumping light source forward pumps the optically amplifying multi-core fiber.

[0026] According to the present invention, it is possible to realize a multi-core fiber that can suppress the influence of heat when an optical fiber is fused to the multi-core fiber, and an optical coupler and an optical amplifier that use the same.

[0027] Fig. 1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment. Fig. 2 is a schematic diagram showing a refractive index profile of the multi-core fiber shown in Fig. 1. Fig. 3 is a schematic configuration diagram of an optical coupler according to a second embodiment. Fig. 4 is a schematic cross-sectional view of a multi-core fiber according to a third embodiment. Fig. 5 is a schematic configuration diagram of an optical amplifier according to a fourth embodiment. Fig. 6 is a schematic cross-sectional view of the multi-core EDF shown in Fig. 5. Fig. 7 is a schematic configuration diagram of an optical amplifier according to a first modification of the fourth embodiment. Fig. 8 is a schematic configuration diagram of an optical amplifier according to a second modification of the fourth embodiment.

[0028] The following describes embodiments with reference to the drawings. Note that the present invention is not limited to these embodiments. In the drawings, identical or corresponding elements are appropriately designated by the same reference numerals, and redundant explanations are omitted. It should be noted that the drawings are schematic, and the dimensional relationships and ratios of elements may differ from the actual figures. The dimensional relationships and ratios may differ between drawings. In this specification, the term "cutoff wavelength" or "effective cutoff wavelength" refers to the cable cutoff wavelength defined in ITU-T G.650.1 of the International Telecommunications Union (ITU). Other terms not specifically defined in this specification are defined and measured according to the definitions and measurement methods in G.650.1 and G.650.2.

[0029] (First and Second Embodiments) In the following, a multi-core fiber according to a first embodiment will be first described, and then an optical coupler including the multi-core fiber according to the first embodiment will be described.

[0030] FIG. 1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment, taken along a plane perpendicular to the longitudinal direction of the multi-core fiber 1. As shown in FIG.

[0031] The multi-core fiber 1 includes 19 cores 1a, an inner cladding 1b, and an outer cladding 1c. The 19 cores 1a are an example of a plurality of cores.

[0032] The 19 core portions 1a have a circular cross section. The 19 core portions 1a are made of glass such as silica-based glass, and are arranged in a triangular lattice pattern in the cross section of the multicore fiber 1. The inner cladding portion 1b has a circular cross section. The inner cladding portion 1b is made of glass such as silica-based glass, and surrounds the outer periphery of the 19 core portions 1a. The outer cladding portion 1c has a circular cross section. The outer cladding portion 1c is made of glass such as silica-based glass, and surrounds the outer periphery of the inner cladding portion 1b.

[0033] Fig. 2 is a schematic diagram showing the refractive index profile of the multicore fiber 1, illustrating the refractive index profile in a cross section taken along line A-A in Fig. 1. In Fig. 2, profile P1 is the refractive index profile of the core 1a, profile P2 is the refractive index profile of the inner cladding 1b, and profile P3 is the refractive index profile of the outer cladding 1c. As shown in Fig. 2, the refractive index n0 of the inner cladding 1b is lower than the maximum refractive index n1 of the core 1a. Furthermore, the refractive index n2 of the outer cladding 1c is lower than the refractive index n0 of the inner cladding 1b. Furthermore, the refractive indices of the inner cladding 1b and the outer cladding 1c are approximately constant regardless of the radial position.

[0034] For example, the core 1a is made of silica glass doped with germanium, the inner cladding 1b is made of pure silica glass, and the outer cladding 1c is made of silica glass doped with fluorine. This achieves the refractive index profile shown in Figure 2. Note that pure silica glass is extremely high-purity silica glass that does not substantially contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0035] 3 is a schematic configuration diagram of an optical coupler according to embodiment 2. The optical coupler 10 includes the multi-core fiber 1 shown in FIG.

[0036] The optical fiber 2 for optical coupling includes a constant diameter portion 2a, a tapered portion 2b, and a small diameter portion 2c, arranged in this order in the longitudinal direction. The optical fiber 2 for optical coupling is a multimode optical fiber in the constant diameter portion 2a. This multimode optical fiber is a step-index type with a core diameter / cladding diameter of, for example, 105 μm / 125 μm, and a numerical aperture (NA) of, for example, 0.16 or more and 0.28 or less. The NA may also be 0.22 or less.

[0037] The tapered portion 2b and the narrow-diameter portion 2c extend along the outer cladding portion 1c of the multicore fiber 1 and are fused to the outer peripheral surface of the outer cladding portion 1c. The tapered portion 2b is a portion whose outer diameter tapers down from the outer diameter of the fixed-diameter portion 2a in the longitudinal direction. The narrow-diameter portion 2c is a portion whose outer diameter extends in the longitudinal direction with approximately the same outer diameter as the narrowed outer diameter of the tapered portion 2b. The optical coupling optical fiber 2 configured as described above can be fabricated, for example, by processing the tip end of a multimode optical fiber by thermal deformation or the like to form the tapered portion 2b and the narrow-diameter portion 2c. The tapered portion 2b and the narrow-diameter portion 2c are fused to the outer cladding portion 1c by heating, for example, using a burner. The tapered portion 2b and the narrow-diameter portion 2c are an example of a portion of the optical coupling optical fiber in the longitudinal direction.

[0038] The optical coupling optical fiber 2 is an optical fiber that propagates light to be coupled to the inner cladding portion 1b of the multicore fiber 1. Specifically, in this embodiment, the pumping light source 3 inputs pumping light L2 to the optical coupling optical fiber 2. Here, the pumping light source 3 is, for example, a transverse multimode semiconductor laser, and outputs the pumping light L2. The optical coupling optical fiber 2 propagates the pumping light L2 in multimode. The pumping light L2 leaks from the optical coupling optical fiber 2 at the tapered portion 2b and the small-diameter portion 2c, passes through the outer cladding portion 1c, and is coupled to the inner cladding portion 1b. The pumping light L2 coupled to the inner cladding portion 1b propagates through the inner cladding portion 1b. The pumping light L2 is an example of light that is input to the inner cladding portion and propagates through the inner cladding portion.

[0039] Furthermore, signal light L1 is input to the core 1a of the multicore fiber 1. The core 1a propagates the signal light. Note that, although Fig. 3 illustrates a state in which the signal light L1 is input to one core 1a, signal light may be input to each of two or more cores 1a. The signal light L1 is an example of light that is input to a core and propagates through the core. Note that the cutoff wavelength of the core 1a of the multicore fiber 1 is set so that the signal light L1 propagates in a single mode.

[0040] Hereinafter, in this specification, it is assumed that the cutoff wavelength of the core of the optical fiber into which the signal light is input is set so that the input light such as the signal light propagates in a single mode.

[0041] As described above, the optical coupler 10 has the function of combining the signal light L1 and the pumping light L2, and is configured as a side-coupling type optical coupler. Note that the coupling efficiency of the pumping light L2 to the inner cladding portion 1b is preferably 60% or more.

[0042] As described above, the tapered portion 2b and the narrowed diameter portion 2c of the optical coupling optical fiber 2 are welded to the outer peripheral surface of the multi-core fiber 1. When performing such welding, if the multi-core fiber 1 does not have an outer cladding portion 1c, the tapered portion 2b and the narrowed diameter portion 2c of the optical coupling optical fiber 2 will be welded to the outer peripheral surface of the inner cladding portion 1b. In this case, since the outer peripheral surface of the inner cladding portion 1b and the cores 1a are relatively close to each other, the cores 1a are easily affected by the heat applied during welding. In particular, the cores 1a closer to the outer peripheral surface of the inner cladding portion 1b are more easily affected by the heat.

[0043] If the outer diameter of the inner cladding 1b is increased to increase the distance between the outer peripheral surface of the inner cladding 1b and the core 1a in order to suppress the influence of heat, the cross-sectional area of ​​the inner cladding 1b will increase, and as a result, the optical density of the pumping light L2 in the inner cladding 1b will decrease.

[0044] In contrast, in the multi-core fiber 1, the tapered portion 2b and the narrow-diameter portion 2c of the optical coupling optical fiber 2 are fused to the outer peripheral surface of the outer cladding portion 1c. As a result, the outer cladding portion 1c functions to increase the distance between the outer peripheral surface to which heat is applied during fusion and the core portion 1a. Furthermore, since the heat capacity of the entire multi-core fiber 1 is larger than that of the inner cladding portion 1b alone, the influence of heat on the core portion 1a is suppressed. Furthermore, since the refractive index n2 of the outer cladding portion 1c is lower than the refractive index n0 of the inner cladding portion 1b, when the pumping light L2 propagates through the multi-core fiber 1, the pumping light L2 does not spread much to the outer cladding portion 1c but is mainly confined and propagated within the inner cladding portion 1b. As a result, a decrease in the light density of the pumping light L2 is also suppressed.

[0045] As described above, in the multi-core fiber 1 according to the first embodiment and the optical coupler 10 using the same, the influence of heat when the optical coupling optical fiber 2 is fused to the multi-core fiber 1 can be suppressed, and a decrease in the optical density of the excitation light L2 in the multi-core fiber 1 can be suppressed.

[0046] In the multicore fiber 1, at the wavelength of the signal light L1, the inter-core crosstalk between adjacent core portions 1a among the 19 core portions 1a is preferably −50 dB or less per 1 km of length. This allows the multicore fiber 1 to provide good crosstalk characteristics for the signal light L1 propagating through each core portion 1a. The wavelength of the signal light L1 is not particularly limited as long as it is the wavelength of signal light used in optical fiber communications, but is, for example, a wavelength in the 1550 nm band. When the wavelength of the signal light L1 is in the 1550 nm band, for example, if the refractive index profile of the core portions 1a is a step-index type, the relative refractive index difference of the core portions 1a with respect to the inner cladding portion 1b is 0.35% or more and 2% or less, the mode field diameter at the wavelength of the signal light L1 is 5 μm or more and 11 μm or less, and the center-to-center distance of adjacent core portions 1a is 27 μm or more (preferably 29 μm or more), the inter-core crosstalk of the multicore fiber 1 will be −50 dB or less per 1 km of length.

[0047] Also in the optical coupler 10, it is preferable that the inter-core crosstalk between adjacent core portions 1a among the 19 core portions 1a of the multicore fiber 1 is −50 dB or less at the wavelength of the signal light L1. Such good crosstalk characteristics are realized in the optical coupler 10 because the influence of heat during welding is suppressed. Note that, since the length of the multicore fiber 1 in the optical coupler 10 is, for example, several meters to several tens of meters, it is preferable that the structural parameters of the multicore fiber 1 (refractive index profile, relative refractive index difference, core diameter, center-to-center distance, etc.) be set so that the inter-core crosstalk is −50 dB or less, taking into consideration the length and the influence of heat.

[0048] Furthermore, in the multi-core fiber 1, the thickness T of the outer cladding portion 1c is preferably 10 μm or more. This makes it possible to suitably suppress the influence of heat during welding. Specifically, for example, it is easy to ensure appropriate processing conditions with a burner that suppress problems such as diffusion of additives due to sudden application of heat to the core portion 1a. Furthermore, the thickness T is preferably 50 μm or less. This prevents the outer diameter of the multi-core fiber 1 from becoming excessively large, which reduces the reliability of mechanical strength.

[0049] Furthermore, it is preferable that the softening point of the outer cladding 1 c is lower than that of the inner cladding 1 b, so that the amount of heat required to weld the optical coupling optical fiber 2 to the outer peripheral surface of the outer cladding 1 c is reduced, thereby further suppressing the influence of heat during welding.

[0050] Furthermore, the distance D between the central axis of the core 1a closest to the outer peripheral surface of the outer cladding 1c is preferably 20 μm or more. This makes it possible to reduce leakage loss from the core 1a to a level that does not pose a practical problem, and is suitable for reducing inter-core crosstalk to −50 dB or less per 1 km of length. Furthermore, D is preferably 60 μm or less, for example.

[0051] Furthermore, at the wavelength of the pumping light L2, the numerical aperture (=√(n0 ２ -n2 ２)) is preferably 0.16 or more and 0.28 or less. If the numerical aperture is 0.16 or more and 0.28 or less (or 0.22 or less), the pumping light L2 is sufficiently confined in the inner cladding portion 1b. The numerical aperture is defined as the relative refractive index difference (=(n2 ２ -n0 ２ ) / (2n0 ２ )), the relative refractive index difference is -1.87% or more and -0.7% or less. In this case, it is preferable that the thickness T of the outer cladding portion 1c is 10 times or more the wavelength of the pumping light L2, from the viewpoint of optical confinement of the pumping light L2 in the inner cladding portion 1b. For example, when the wavelength of the pumping light L2 is 980 nm, the thickness T is preferably 9.8 μm or more. When the wavelength of the pumping light L2 is 1480 nm, the thickness T is preferably 14.8 μm or more.

[0052] (Embodiment 3) Fig. 4 is a schematic cross-sectional view of a multi-core fiber according to embodiment 3. A multi-core fiber 1A has a configuration in which a coating layer 1d is added to the multi-core fiber 1 shown in Fig. 1. The coating layer 1d has a circular cross section, is made of a resin having a refractive index lower than that of the outer cladding portion 1c, and surrounds the outer cladding portion 1c. The resin constituting the coating layer 1d is not particularly limited as long as it is a known resin that constitutes a coating of an optical fiber, and is made of, for example, an ultraviolet-curable resin or a thermosetting resin.

[0053] In the multi-core fiber 1A configured as above, the outer cladding portion 1c can be protected by the coating layer 1d. When configuring an optical coupler as shown in Fig. 3 using the multi-core fiber 1A, the coating layer 1d can be removed from a portion of the multi-core fiber 1A to which the optical coupling optical fiber 2 is to be welded, and the optical coupling optical fiber 2 can be welded to the outer peripheral surface of the exposed outer cladding portion 1c.

[0054] As a modified example of the multicore fiber according to the third embodiment, the coating layer 1d of the multicore fiber 1A may be replaced with a coating layer having a circular cross section, made of a resin having a refractive index equal to or greater than that of the outer cladding 1c, and surrounding the outer cladding 1c. The resin constituting the coating layer is not particularly limited as long as it is a known resin constituting a coating for an optical fiber. In this case, from the viewpoint of optical confinement of the pumping light L2 in the inner cladding 1b, the numerical aperture determined by the refractive index n0 of the inner cladding 1b and the refractive index n2 of the outer cladding 1c at the wavelength of the pumping light L2 is preferably 0.16 or more and 0.28 or less (or 0.22 or less). That is, the relative refractive index difference corresponding to the numerical aperture is preferably −1.87% or more and −0.7% or less. Furthermore, the thickness T of the outer cladding 1c is preferably 10 times or more the wavelength of the pumping light L2.

[0055] 5 is a schematic configuration diagram of an optical amplifier according to embodiment 4. The optical amplifier 100 according to embodiment 2 includes an optical coupler 10 including a multicore fiber 1 and an optical fiber for optical coupling 2, a pumping light source 3, 19 optical isolators 4, an optical fiber fan-in (FAN IN) 5, a multicore EDF 6, a pump stripper 7, an optical fiber fan-out (FAN OUT) 8, and 19 optical isolators 9. In the figure, the symbol "x" indicates a fusion splice point of the optical fibers.

[0056] The optical fiber fan-in 5 includes 19 single-mode optical fibers bundled at one end and one multi-core fiber having 19 cores. Each core on the bundled side of the 19 single-mode optical fibers is optically coupled to a corresponding core on the multi-core fiber. In this multi-core fiber, the cores are arranged in a triangular lattice pattern, similar to the core 1a of the multi-core fiber 1 shown in FIG. 1 . The 19 single-mode optical fibers are standard single-mode optical fibers defined, for example, in ITU-T (International Telecommunication Union) G.652, and each is provided with an optical isolator 4. The optical isolators 4 and 9 allow light to pass in the direction indicated by the arrows and block light from passing in the opposite direction. Each core on the multi-core fiber of the optical fiber fan-in 5 is connected to a corresponding core 1a of the multi-core fiber 1 of the optical coupler 10. In the optical fiber fan-in, the end faces on the bundled side of the 19 single-mode optical fibers and the optically coupled end face of the multi-core fiber are processed at an angle to the optical axis to suppress reflection, but may also be perpendicular to the optical axis.

[0057] When signal light is input to each single-mode optical fiber of the optical fiber fan-in 5, each optical isolator 4 passes the signal light. Each core of the multi-core fiber propagates the signal light and outputs the signal light to the connected core 1 a in the multi-core fiber 1. The core 1 a propagates the input signal light.

[0058] As described above, the pumping light source 3 is, for example, a transverse multimode semiconductor laser, and outputs pumping light. The wavelength of the pumping light is 975 nm, which is approximately the same as the wavelength of the absorption peak of erbium ions in the 900 nm wavelength band (for example, a first wavelength band of 970 nm or more and 990 nm or less). Therefore, the pumping light can optically pump erbium ions. The optical coupling optical fiber 2 propagates the pumping light output from the pumping light source. As described above, the pumping light is coupled to the inner cladding 1 b of the multicore fiber 1 in the optical coupler 10 and propagates through the inner cladding 1 b.

[0059] Fig. 6 is a schematic cross-sectional view of the multi-core EDF shown in Fig. 5. The multi-core EDF 6 comprises 19 optical amplifying cores 6a, an inner cladding 6b, and an outer cladding 6c. The 19 optical amplifying cores 6a are an example of a plurality of optical amplifying cores. The inner cladding 6b is an example of an optical amplifying inner cladding.

[0060] The 19 optical amplification cores 6a have a circular cross section. The 19 optical amplification cores 6a are made of glass such as silica-based glass doped with erbium ions as an optical amplification medium, and are arranged in a triangular lattice pattern, similar to the cores 1a of the multicore fiber 1 shown in FIG. 1. The inner cladding 6b has a circular cross section. The inner cladding 6b is made of glass such as silica-based glass, and surrounds the outer periphery of the 19 optical amplification cores 6a. The outer cladding 6c has a circular cross section. The outer cladding 6c is made of glass such as silica-based glass or resin, and surrounds the outer periphery of the inner cladding 6b.

[0061] The refractive index of the inner cladding 6b is lower than the maximum refractive index of the optical amplifying core 6a, and the refractive index of the outer cladding 6c is lower than the refractive index of the inner cladding 6b.

[0062] The multi-core EDF 6 has a known cladding pumping type configuration and is an example of an optically amplifying multi-core fiber.

[0063] Each of the optical amplification cores 6a of the multi-core EDF 6 is optically connected to each of the cores 1a of the multi-core fiber 1 of the optical coupler 10. Furthermore, the inner cladding 6b of the multi-core EDF 6 is optically connected to the inner cladding 1b of the multi-core fiber 1. Therefore, when the signal light and pumping light propagating through the multi-core EDF 6 are input to the multi-core EDF 6, they propagate in the same direction through the optical amplification cores 6a and the inner cladding 6b. The pumping light optically pumps erbium in each of the optical amplification cores 6a while propagating through the inner cladding 6b. Each of the signal light propagating through each of the optical amplification cores 6a is optically amplified by the action of stimulated emission of erbium. The multi-core EDF 6 outputs each of the optically amplified signal light and pumping light (residual pumping light) that did not contribute to the optical amplification.

[0064] The pump stripper 7 is a known device that removes residual pump light while passing each signal light output from the multi-core EDF 6. The pump stripper 7 has a configuration in which, for example, a part of the outer cladding of a double-clad multi-core fiber having 19 cores arranged in a triangular lattice pattern is removed, and the residual pump light is extracted from the surface of the inner cladding of the removed part, irradiated onto a heat sink or the like, and absorbed, converting the energy of the residual pump light into thermal energy and dissipating it. The pump stripper 7 passes each signal light through the multi-core fiber, while reducing the power of the residual pump light to a level that will not cause any problems even if it is output from the optical amplifier 100.

[0065] Similar to the optical fiber fan-in 5, the optical fiber fan-out 8 includes 19 single-mode optical fibers bundled at one end and one multi-core fiber having 19 cores arranged in a triangular lattice pattern, and is configured so that each core on the bundled side of the 19 single-mode optical fibers is optically coupled to each core on the multi-core fiber. Each single-mode optical fiber is provided with an optical isolator 9. The multi-core fiber is connected to a pump stripper 7. Note that the end faces of the bundled side of the 19 single-mode optical fibers and the optically coupled end face of the multi-core fiber are processed at an angle with respect to the optical axis to suppress reflection, but may also be perpendicular to the optical axis.

[0066] When signal light is input from each core of the multi-core fiber of the pump stripper 7 to each core of the optical fiber fan-out 8, each signal light propagates through each core of each single-mode optical fiber and is output through the optical isolator 9.

[0067] The optical amplifier 100 configured in this manner has, for example, good crosstalk characteristics because it uses the optical coupler 10 in which the influence of heat when fusing the optical coupling optical fiber 2 to the multicore fiber 1 is suppressed. Furthermore, the optical amplifier 100 uses the optical coupler 10 in which a decrease in the optical density of the pumping light is suppressed, so that a decrease in gain and energy efficiency is suppressed.

[0068] In the above embodiments, the number of cores in the multi-core fiber such as the multi-core fiber 1 or the multi-core EDF 6 is 19, but the number of cores is not particularly limited. The arrangement of the cores is also not limited to a triangular lattice pattern. In the above embodiments, the optical amplifying medium in the optical amplifying fiber is erbium, but other optical amplifying media such as ytterbium may be used. The wavelength of the pumping light may be appropriately set to a wavelength that can optically pump the optical amplifying medium. In the above embodiments, there is one pumping light source and one optical fiber for optical coupling in the optical coupler, but there may be multiple pumping light sources and multiple optical fibers for optical coupling in the optical coupler. In the above embodiments, the optical amplifier is a forward pumping type, but it may be a backward pumping type or a bidirectional pumping type. To configure a backward pumping type optical amplifier, for example, in the optical amplifier 100, the optical coupler 10 may be connected to the optical fiber fan-out 8 side of the multi-core EDF 6 and the pump stripper 7 may be connected to the optical fiber fan-in 5 side. Furthermore, if the power of the pump light output from the multi-core EDF 6 that has not contributed to optical amplification is relatively low, there may be cases where the pump stripper 7 need not be provided.

[0069] (First Modification of Optical Amplifier) ​​Fig. 7 is a schematic diagram of an optical amplifier according to a first modification of embodiment 4. The optical amplifier 100A has a configuration in which an optical coupler 10A, a pumping light source 3A, a pump stripper 7A, and pumping light blocking units 11 and 11A are added to the optical amplifier 100 according to embodiment 4 shown in Fig. 5.

[0070] In the optical amplifier 100A, the pumping light source 3 is an example of the second pumping light source of the two pumping light sources. As described above, the pumping light output from the pumping light source 3 propagates in the same direction as the signal light in the multi-core EDF 6, and therefore the pumping light source 3 forward pumps the multi-core EDF 6.

[0071] The optical coupler 10A is provided between the multi-core EDF 6 and the pump stripper 7. The optical coupler 10A has the same configuration as the optical coupler 10. That is, the optical coupler 10A includes a multi-core fiber 1A and an optical coupling optical fiber 2A. The multi-core fiber 1A has the same configuration as the multi-core fiber 1. The optical coupling optical fiber 2A has the same configuration as the optical coupling optical fiber 2.

[0072] The pumping light source 3A is, for example, a transverse multimode semiconductor laser, and outputs pumping light. The wavelength of the pumping light is included in a wavelength band of 1450 nm to 1520 nm (second wavelength band), which is an example of a pumping wavelength band for erbium ions. Therefore, the pumping light can optically pump erbium ions. The optical coupling optical fiber 2A propagates the pumping light output from the pumping light source 3A in multiple modes. The pumping light is coupled to the inner cladding portion of the multi-core fiber 1A in the optical coupler 10A, propagates through the inner cladding portion, and is input to the inner cladding portion 6b of the multi-core EDF 6.

[0073] The pumping light output from the pumping light source 3A propagates in the multi-core EDF 6 in the opposite direction to the signal light, and therefore the pumping light source 3A counter-pumps the multi-core EDF 6. That is, the pumping light source 3A is an example of a first pumping light source of the two pumping light sources, and the optical coupler 10A and the pumping light source 3A are configured so that the pumping light source 3A counter-pumps the multi-core EDF 6.

[0074] Each signal light optically amplified by the multi-core EDF 6 propagates through each core of the multi-core fiber 1A of the optical coupler 10A and is output to the pump stripper 7.

[0075] The pump stripper 7A is provided between the optical fiber fan-in 5 and the optical coupler 10. The pump stripper 7A is a known device that passes each signal light output from the optical fiber fan-in 5 while removing residual pump light resulting from the pump light from the pump light source 3A output from the multi-core EDF 6. The pump stripper 7A may have a similar configuration to the pump stripper 7.

[0076] In this embodiment, the pumping light blocking section 11 is provided midway through the optical fiber for optical coupling 2. When a portion of the residual pumping light resulting from the pumping light from the pumping light source 3A propagates through the optical fiber for optical coupling 2, the pumping light blocking section 11 has the function of blocking the portion before it enters the pumping light source 3. The pumping light blocking section 11 is an optical element, such as a bandpass filter or a wavelength coupler, that transmits the pumping light output from the pumping light source 3 and blocks the pumping light output from the pumping light source 3A. Alternatively, instead of the pumping light blocking section 11, a filter may be provided on the output end face of the semiconductor chip of the pumping light source 3, and used as the pumping light blocking section.

[0077] In this embodiment, the pumping light blocking section 11A is provided midway through the optical fiber 2A for optical coupling. When a portion of residual pumping light resulting from the pumping light from the pumping light source 3 propagates through the optical fiber 2A for optical coupling, the pumping light blocking section 11A has the function of blocking the portion before it enters the pumping light source 3A. The pumping light blocking section 11A is an optical element, such as a bandpass filter or a wavelength coupler, that transmits the pumping light output from the pumping light source 3A and blocks the pumping light output from the pumping light source 3. Alternatively, instead of the pumping light blocking section 11A, a filter may be provided on the output end face of the semiconductor chip of the pumping light source 3A and used as the pumping light blocking section.

[0078] In the optical amplifier 100A configured as described above, in addition to the effects obtained by the optical amplifier 100, the multi-core EDF 6 is forward pumped with pumping light in the first wavelength band (970 nm or more and 990 nm or less) and backward pumped with pumping light in the second wavelength band (1450 nm or more and 1520 nm or less), thereby achieving low noise characteristics and high saturated output characteristics.

[0079] Here, the inventors compared the optical amplification characteristics obtained with the optical amplifier 100A with those obtained with an optical amplifier according to a comparative embodiment. The optical amplifier according to the comparative embodiment is an optical amplifier configured by replacing the optical couplers 10 and 10A in the configuration of the optical amplifier 100A with an outer cladding made of resin instead of the glass outer cladding. This resin is a resin commonly used in double-clad optical fibers. As a result, it was confirmed that the optical amplification gain obtained with the optical amplifier 100A was higher than that obtained with the optical amplifier according to the comparative embodiment under the same operating conditions. Thus, a configuration using the optical couplers 10 and 10A, such as the optical amplifier 100A, is considered to be suitable for optical pumping in the second wavelength band.

[0080] (Modification 2 of Optical Amplifier) ​​Fig. 8 is a schematic configuration diagram of an optical amplifier according to Modification 2 of Embodiment 4. The optical amplifier 100B has a configuration in which the optical coupler 10 and the pumping light source 3 in the optical amplifier 100A according to Modification 1 shown in Fig. 7 are replaced with an optical coupler 20, a plurality of forward pumping light sources 23, and a pump stripper 7B, and the pump strippers 7, 7A and pumping light blocking units 11, 11A are deleted.

[0081] The plurality of forward pumping light sources 23 are, for example, transverse single-mode semiconductor lasers, and output forward pumping light. The wavelength of the forward pumping light falls within a first wavelength band of 970 nm to 990 nm. The number of forward pumping light sources 23 is 19, which is the same as the number of optical amplification cores 6 a in the multi-core EDF 6.

[0082] The optical coupler 20 includes a single-clad multi-core fiber 21 and 19 optical coupling single-mode optical fibers 22. Each optical coupling single-mode optical fiber 22 is connected to a corresponding forward pumping light source 23, and propagates the forward pumping light output from each forward pumping light source 23 in a single mode and outputs it to the single-clad multi-core fiber 21. The single-clad multi-core fiber 21 has a configuration similar to that of the multi-core fiber 1, but does not have an outer clad portion and has a single clad structure with only an inner clad portion. Note that instead of the single-clad multi-core fiber 21, a multi-core fiber 1 having an outer clad portion may be used.

[0083] Furthermore, the cutoff wavelength of the 19 core portions of each of the optical coupling single-mode optical fibers 22 and the single-clad multi-core fiber 21 is set so that the forward pumping light propagates in a single mode, and the cutoff wavelength is, for example, 970 nm.

[0084] The optical coupler 20 is configured to wavelength-multiplex each signal light output from the optical fiber fan-in 5 and the forward pumping light output from each optical coupling single-mode optical fiber 22, and output the multiplexed signal light to each core portion of the single-clad multi-core fiber 21. That is, the optical coupler 20 is configured as a core-pumping type optical coupler. Such an optical coupler 20 may be configured to include, for example, a plurality of known core-pumping WDM (Wavelength Division Multiplexing) couplers.

[0085] The pump stripper 7B is provided between the optical coupler 20 and the multi-core EDF 6. Like the pump stripper 7A, the pump stripper 7B is a known device that passes each signal light output from the optical coupler 20 while removing residual pump light resulting from the pump light from the pump light source 3A output from the multi-core EDF 6. The pump stripper 7B may have a configuration similar to that of the pump stripper 7A. However, in this embodiment, the cutoff wavelength of each core of the pump stripper 7B is set so that the forward pump light propagates in a single mode.

[0086] The optical coupling single-mode optical fiber 22 and the pump stripper 7B function as guide sections that guide the forward pumping light beams output from the forward pumping light source 23 to the optical amplification core sections 6a, respectively. As described above, the guide sections propagate the forward pumping light beams in a single mode.

[0087] In the optical amplifier 100B configured as described above, forward pumping light having a wavelength included in the first wavelength band of 970 nm or more and 990 nm or less is input in single mode to the optical amplifier core 6a. As a result, the optical amplifier core 6a is selectively excited in single mode (fundamental mode), and therefore the pumped state of the optical amplifier core 6a is more stable over time than when the optical amplifier core 6a is excited in multiple modes, and the obtained optical amplification gain is also more stable over time.

[0088] Furthermore, in the optical amplifier 100B, each optical amplification core 6a is individually forward pumped by each forward pumping light source 23, and therefore, by individually adjusting the power of the pumping light output from each forward pumping light source 23, the gain deviation between the optical amplification cores 6a can be reduced.

[0089] Here, if the optical amplifier core 6a propagates the signal light in single mode, the cutoff wavelength may be set so that the forward pumping light propagates in multimode. For example, if the wavelength of the signal light is 1530 nm or more, the cutoff wavelength of the optical amplifier core 6a may be 1530 nm or less. Note that if the cutoff wavelength is set so that the optical amplifier core 6a propagates the signal light in single mode and the forward pumping light in multimode, it is easier to set a larger core diameter for the optical amplifier core 6a. The larger the core diameter of the optical amplifier core 6a, the easier it is to absorb the pumping light for cladding pumping output by the pumping light source 3A. As a result, it is easier to increase the amplified light output of the optical amplifier 100B.

[0090] In the optical amplifier 100A according to the first modification, the optical coupler 10, the pumping light source 3, and the pump stripper 7 may be eliminated to form a backward pumping type configuration using pumping light in the second wavelength band of 1450 nm to 1520 nm. Also, the configuration of the optical amplifier 100 according to the fourth embodiment may be modified to form a forward pumping type configuration using pumping light in the second wavelength band.

[0091] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

[0092] DESCRIPTION OF SYMBOLS 1, 1A: Multi-core fiber 1a: Core portion 1b: Inner cladding portion 1c: Outer cladding portion 1d: Coating layer 2: Optical fiber for optical coupling 2a: Fixed diameter portion 2b: Tapered portion 2c: Thin diameter portion 3, 3A: Pumping light source 4: Optical isolator 5: Optical fiber fan-in 6: Multi-core EDF 6a: Optical amplification core portion 6b: Inner cladding portion 6c: Outer cladding portion 7, 7A, 7B: Pump stripper 8: Optical fiber fan-out 9: Optical isolator 10, 10A, 20: Optical coupler 11, 11A: Pumping light blocking portion 21: Single-clad multi-core fiber 22: Single-mode optical fiber for optical coupling 23: Forward pumping light source 100, 100A, 100B: Optical amplifier D: Distance L1 : Signal light L2 : Excitation light P1, P2, P3 : Profile T : Thickness

Claims

1. A multicore fiber comprising: a plurality of cores made of glass; an inner cladding made of glass having a refractive index lower than the maximum refractive index of the cores and surrounding the outer periphery of the plurality of cores; and an outer cladding made of glass having a refractive index lower than the refractive index of the inner cladding and surrounding the inner cladding.

2. The multi-core fiber according to claim 1, wherein the outer cladding portion is made of glass doped with fluorine.

3. The multicore fiber according to claim 1, further comprising a coating layer made of a resin having a refractive index lower than that of the outer cladding portion and surrounding the outer cladding portion.

4. The multi-core fiber according to claim 1, wherein the softening point of the outer cladding portion is lower than the softening point of the inner cladding portion.

5. The multi-core fiber according to claim 1, wherein the thickness of the outer cladding portion is 10 μm or more.

6. The multicore fiber according to claim 1, wherein inter-core crosstalk between adjacent core portions among the plurality of core portions is -50 dB or less per km length at the wavelength of light input to and propagating through the core portions.

7. The multicore fiber according to claim 1, wherein the distance between the central axis of the core portion closest to the outer peripheral surface of the outer cladding portion among the plurality of core portions and said outer peripheral surface is 20 μm or more.

8. The multicore fiber according to claim 1, wherein a numerical aperture determined by the refractive index of said inner cladding portion and the refractive index of said outer cladding portion is 0.16 or more and 0.28 or less at the wavelength of light input to said inner cladding portion and propagating through said inner cladding portion.

9. A multicore fiber according to claim 8, comprising a coating layer made of a resin having a refractive index equal to or higher than that of said outer cladding portion and surrounding said outer cladding portion, wherein the thickness of said outer cladding portion is 10 times or more the wavelength of light input to said inner cladding portion and propagating through said inner cladding portion.

10. An optical coupler comprising: a multicore fiber according to any one of claims 1 to 9; and an optical coupling optical fiber fused to the outer cladding portion of the multicore fiber, for propagating light to be coupled to the inner cladding portion of the multicore fiber.

11. The optical coupler according to claim 10, wherein the coupling efficiency of the light propagating through the optical coupling optical fiber to the inner cladding portion is 60% or more.

12. The optical coupler according to claim 10, wherein inter-core crosstalk between adjacent core portions among the plurality of core portions is -50 dB or less at the wavelength of light input to the plurality of core portions of the multicore fiber and propagating through the plurality of core portions.

13. The optical coupler according to claim 10, wherein a portion of the optical fiber for optical coupling in the longitudinal direction is welded along the outer clad portion, and the optical coupler is configured as a side-coupling type optical coupler.

14. An optical amplifier comprising: the optical coupler according to claim 10; an optically amplifying multicore fiber having a plurality of optically amplifying core sections made of glass and doped with an optical amplification medium, and an optically amplifying inner cladding section made of glass having a refractive index lower than the maximum refractive index of the optical amplifying core sections and surrounding the outer periphery of the plurality of optically amplifying core sections; and an excitation light source that inputs excitation light for optically exciting the optical amplification medium into the optical fiber for optical coupling of the optical coupler, wherein each of the plurality of optically amplifying core sections of the optically amplifying multicore fiber is optically connected to each of the plurality of core sections of the multicore fiber of the optical coupler, and the optically amplifying inner cladding section of the optically amplifying multicore fiber is optically connected to the inner cladding section of the multicore fiber of the optical coupler.

15. An optical amplifier according to claim 14, wherein the optical amplification medium is erbium, and the wavelength of the pumping light is 1450 nm or more and 1520 nm or less.

16. An optical amplifier according to claim 14, comprising: a plurality of forward pumping light sources which output forward pumping light having a wavelength of 970 nm or more and 990 nm or less and which optically pumps the optical amplification medium; and a guide section which guides each of the forward pumping lights output from the plurality of forward pumping light sources to each of the plurality of optical amplification core sections, wherein the optical amplification medium is erbium, the optical coupler and the pumping light source are configured to backward pump the optical amplification multi-core fiber, and the guide section propagates the forward pumping light in a single mode.

17. An optical amplifier according to claim 14, comprising two of the optical couplers and two of the pumping light sources, wherein a wavelength of the pumping light output by a first of the two pumping light sources is 1450 nm or more and 1520 nm or less, and a wavelength of the pumping light output by a second of the two pumping light sources is 970 nm or more and 990 nm or less, and the two optical couplers and the two pumping light sources are configured such that the first pumping light source backward pumps the optically amplifying multi-core fiber, and the second pumping light source forward pumps the optically amplifying multi-core fiber.

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