Light source device

Abstract

A light source device 1 comprises: a light source 16; a multicore fiber 11 for amplification having a plurality of cores doped with rare-earth ions; a fan-in device 17 that couples light emitted by the light source 16 to each of the cores of the multicore fiber 11 for amplification; a multimode LD 18 that outputs excitation light for exciting rare-earth ions used to dope the multicore fiber 11 for amplification; and an excitation light coupler 19 that couples the excitation light to the cladding of the multicore fiber 11 for amplification. The core radius is determined so that the multicore fiber 11 for amplification operates in single mode, while the core pitch and area ratio of the core and the cladding are determined so that intermodal coupling in the multicore fiber 11 for amplification is -20 dB or less.

Description

light source device

[0001] The present disclosure relates to a light source device.

[0002] Single-mode operation of 0.98 μm light is used as a pumping source for erbium-doped fiber amplifiers and as an optical power supply for optical fibers. To increase the output power of 0.98 μm light, the use of ytterbium (Yb)-doped fiber has been investigated. Yb-doped fiber generally has a high emission spectrum at 1.03 μm. To achieve a high population inversion while suppressing stimulated emission at 1.03 μm, efforts are being made to shorten the fiber length and increase the core-to-clad area ratio (Rcc). Non-Patent Document 1 proposes a single-mode 976 nm amplifier using a Yb-doped fiber with a core diameter of 14 μm, a cladding diameter of 45 μm, and an Rcc of 0.08.

[0003] L. Kotov et al., “Efficient single-mode 976 nm amplifier based on a 45 micron outer diameter Yb-doped fiber,” Optics Letters, 2020, Vol. 45, No.15, pp. 4292-4295

[0004] When amplifying light using a cladding pumping configuration, increasing Rcc is effective in increasing the pumping efficiency. When creating a single-mode operating 0.98 μm light source using Yb-doped fiber, there was a limit to how much Rcc could be increased with a single-core structure, from the perspective of achieving both a single-mode operating core and a manageable cladding diameter.

[0005] The present disclosure has been made in view of the above, and aims to provide a more efficient light source.

[0006] A light source device according to one aspect of the present disclosure includes a light source and a rare-earth-doped multicore fiber having a plurality of cores doped with rare-earth ions, the rare-earth-doped multicore fiber combining light emitted from the light sources and outputting light from each of the plurality of cores, wherein the radius of the cores is determined so that the rare-earth-doped multicore fiber operates in single mode, and the core pitch and the core-to-clad area ratio are determined so that inter-mode coupling in the rare-earth-doped multicore fiber is −20 dB or less.

[0007] The present disclosure provides a more efficient light source.

[0008] FIG. 1 is a diagram showing an example of the configuration of a light source device in an amplifier configuration. FIG. 2 is a diagram showing an example of the configuration of a light source device in a fiber laser configuration. FIG. 3 is a cross-sectional view showing an example of the configuration of an amplification multicore fiber. FIG. 4 is a diagram showing an example of a single mode operation region. FIG. 5 is a diagram showing an example of the relationship between core pitch and inter-core crosstalk. FIG. 6 is a diagram showing an example of the relationship between the number of cores and the core-to-clad area ratio. FIG. 7 is a diagram showing an example of a system using a light source device as a pumping light source for an optical amplifier. FIG. 8 is a diagram showing an example of a system using a light source device for optical power feeding.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] [Light Source with Amplifier Configuration] An example of the configuration of a light source device 1 with an amplifier configuration according to this embodiment will be described with reference to FIG. 1 . The light source device 1 is a light source that amplifies seed light in an amplifier configuration using an amplification fiber doped with rare-earth ions and outputs light. A fiber with a multi-core structure is used as the amplification fiber to increase Rcc compared to a single-core structure. The light source device 1 shown in FIG. 1 includes an amplification multi-core fiber 11, optical isolators 12 and 13, a fan-out device 14, a power coupler 15, a light source 16, a fan-in device 17, a multimode laser diode (LD) 18, and a pumping light coupler 19.

[0011] The light source 16 outputs seed light to be amplified in the amplification multi-core fiber 11. For example, when the amplification multi-core fiber 11 is a Yb-doped fiber, the light source 16 outputs seed light with a wavelength of 0.98 μm.

[0012] The fan-in device 17 serves to couple the seed light output from the light source 16 to each core of the multi-core fiber.

[0013] The optical isolators 12 and 13 are arranged in front and rear of the amplification multi-core fiber 11, respectively, and serve to realize unidirectional transmission of seed light.

[0014] The multimode LD 18 outputs pumping light having a wavelength corresponding to the absorption band of the rare earth ions doped in the amplification multicore fiber 11. For example, when the amplification multicore fiber 11 is a Yb-doped fiber, the multimode LD 18 outputs pumping light having a wavelength of 0.915 μm.

[0015] The pump light coupler 19 couples the pump light into the cladding of the multi-core fiber.

[0016] The multi-core fiber in which the seed light is coupled to the core and the pump light is coupled to the cladding is fusion-spliced ​​to the amplification multi-core fiber 11. The seed light propagates through the core, and the pump light propagates through the amplification multi-core fiber 11 while being reflected at the boundary between the inner cladding and the outer cladding.

[0017] The amplification multicore fiber 11 is a rare-earth doped fiber with a multicore structure in which Yb ions are doped into the cores of the multicore fiber. The amplification multicore fiber 11 is a double-clad fiber, and pumping light is incident on the inner clad and propagates through the amplification multicore fiber 11 while being reflected at the boundary with the outer clad. When the pumping light propagates through the amplification multicore fiber 11, Yb ions are excited, causing stimulated emission of light having the same wavelength as the seed light, thereby amplifying the seed light. The rare-earth ions doped into the amplification multicore fiber 11 can be selected from Yb ions, Er ions, co-doping of Er ions and Yb ions, etc. The rare-earth ions are selected depending on the seed light to be amplified. The wavelength bands of the light source 16 and the multimode LD 18 are selected based on the absorption and emission characteristics of the rare-earth ions.

[0018] Because the amplification multicore fiber 11 is short, at about 1 m, the core pitch can be set narrower than that of a multicore fiber used as a transmission line. To keep the output fluctuation at 1% or less, a design that also includes crosstalk requirements is required. Design examples of the core radius, core pitch, cladding diameter, and core-to-cladding area ratio of the amplification multicore fiber 11 will be described later.

[0019] Each core of the amplification multicore fiber 11 is used as an independent light source. The seed light (output light) amplified by each core of the amplification multicore fiber 11 may be directly coupled to the multicore fiber, or may be branched into a plurality of optical fibers by a fan-out device 14, and each of the branched output lights may be used as a light source. If necessary, the branched output light may be further branched by a power coupler 15.

[0020] [Light Source with Fiber Laser Configuration] With reference to FIG. 2 , an example of the configuration of a light source device 2 with a fiber laser configuration according to this embodiment will be described. The light source device 2 is similar to the light source device 1 in that it uses a fiber with a multi-core structure as the amplification fiber. Unlike the light source device 1, the light source device 2 is a light source that uses a fiber laser configuration to oscillate 0.98 μm light stimulated and emitted by pumping light, without injecting seed light for amplification into each core of the multi-core fiber. The light source device 2 shown in FIG. 2 includes an amplification multi-core fiber 11, optical isolators 12 and 13, a fan-out device 14, a power coupler 15, a multi-mode LD 21, and reflectors 22 and 23. The same components as those in the light source device 1 in FIG. 1 are designated by the same reference numerals, and redundant description will be omitted here.

[0021] The multimode LD 21 outputs pumping light having a wavelength corresponding to the absorption band of the rare earth element doped in the amplification multicore fiber 11. The pumping light output from the multimode LD 21 is coupled to the clad of the amplification multicore fiber 11 via the optical isolator 12.

[0022] Reflectors 22 and 23 are provided at both ends of the amplification multicore fiber 11, and 0.98 μm light stimulated by pumping light is extracted by the reflectors 22 and 23 and resonated to be amplified. Fiber Bragg gratings or dielectric multilayer filters can be used for the reflectors 22 and 23. The reflector 22 has high reflectivity, and the reflector 23 has low reflectivity, and light is output from the reflector 23 side.

[0023] The amplification multicore fiber 11 is a rare-earth doped fiber with a multicore structure in which the cores of the multicore fiber are doped with Yb ions. The amplification multicore fiber 11 is a double-clad fiber, and pumping light is incident on the inner clad and propagates through the amplification multicore fiber 11 while being reflected at the boundary with the outer clad. The pumping light is absorbed in the core doped with rare-earth ions, causing population inversion and emitting light. The emitted light is repeatedly reflected between reflectors 22 and 23, amplified, and laser oscillation occurs. The rare-earth ions doped in the amplification multicore fiber 11 can be selected from Yb ions, Er ions, co-doping of Er ions and Yb ions, etc. The rare-earth ions are selected depending on the seed light to be amplified. The wavelength bands of the light source 16 and the multimode LD 18 are selected based on the absorption and emission characteristics of the rare-earth ions.

[0024] The design of the amplification multi-core fiber 11 is the same as that of the light source device 1 .

[0025] The light amplified in each core of the amplification multicore fiber 11 may be coupled to the multicore fiber as it is, or may be branched into a plurality of optical fibers by the fan-out device 14, and each of the branched output lights may be used as a light source. If necessary, the branched output light may be further branched by the power coupler 15.

[0026] Both of the light source devices 1 and 2 may include a wavelength coupler for diverting unnecessary spontaneous emission light (for example, 1.03 μm light that is easily amplified in an Yb-doped fiber) generated in the amplification multicore fiber 11. In addition, in both of the light source devices 1 and 2, if the light output from each core may operate in multimode, the devices constituting the light source devices 1 and 2 may have a multimode configuration.

[0027] [Design of Amplification Multicore Fiber] Figure 3 shows a cross section of the amplification multicore fiber 11. The amplification multicore fiber 11 shown in the figure includes four cores 111, a first cladding 112, and a second cladding 113. Pumping light is incident on the first cladding 112 and propagates through the amplification multicore fiber 11 while being reflected at the boundary with the second cladding. In the light source device 1, seed light propagates through each core 111. In the light source device 2, light stimulated and emitted by the pumping light propagates through each core 111. Hereinafter, as shown in Figure 3, the distance between adjacent cores is referred to as the core pitch, and the diameter of the first cladding 112 is referred to as the cladding diameter.

[0028] When amplifying light using a cladding pumping configuration, increasing Rcc is an effective way to increase the pumping efficiency. However, in light sources that require single-mode operation, there is a limit to how much the core can be enlarged.

[0029] Figure 4 shows the single-mode operating region when the bending radius is 50 mm and the fiber length is 1 m. In Figure 4, the horizontal axis represents the core radius (μm) and the vertical axis represents the relative refractive index difference (%). The relative refractive index difference is the difference between the refractive index of the core 111 and the refractive index of the first cladding 112. The shaded area below the solid line in the figure indicates the single-mode operating region. If a connection loss of up to 0.5 dB with a general-purpose single-mode fiber is allowed, the maximum allowable core radius is 7 μm.

[0030] In the light source devices 1 and 2, an amplifying multicore fiber 11 with a multicore structure was used to increase Rcc compared to an amplifying fiber with a single-core structure. Multicore fibers used in general transmission paths may be subject to high requirements, such as inter-core crosstalk of -50 dB / km or less, to ensure transmission quality for each core. Because the output stability of commercially available semiconductor LDs is about 1%, the crosstalk requirements can be relaxed when used as a light source, and crosstalk (inter-mode coupling) per light source can be -20 dB or less.

[0031] Figure 5 shows the relationship between core pitch and inter-core crosstalk when the light source wavelength is 0.98 μm, the fiber length is 1 m, the core radius is 7 μm, and the relative refractive index difference Δ is 0.15%. In Figure 5, the horizontal axis represents core pitch (μm) and the vertical axis represents inter-core crosstalk (dB). With the above parameters, if the core pitch is set to 19 μm or more, inter-core crosstalk can be set to -20 dB or less. The core pitch is set to meet the required conditions depending on the light source wavelength and core structure.

[0032] Figure 6 shows the relationship between the number of cores and Rcc when the core diameter is 14 μm, the distance from the core to the cladding is 10 μm, and the core pitch is 20 μm and 36 μm. For an even number of cores (4 or more), the cores are arranged in a square lattice pattern, while for an odd number of cores, the cores are arranged in a hexagonal grid pattern. By setting the core pitch to 36 μm or less, the Rcc is higher than the maximum value of 0.08 reported in previous studies. A core pitch of 20 μm can improve the Rcc to 0.25, approximately three times that of previous studies, while still meeting the crosstalk requirements.

[0033] In this way, by determining the core radius for single-mode operation and finding the core pitch for inter-core crosstalk of −20 dB or less, it is possible to design an amplification multi-core fiber 11 that maximizes Rcc while satisfying the required conditions. Note that, when multi-mode operation is permitted as a light source, the amplification multi-core fiber 11 may also be designed to have a core structure in which a set number of propagation modes propagate, such that inter-core crosstalk is −20 dB or less.

[0034] [Example] With reference to Figs. 7 and 8, an example using the light source devices 1 and 2 of the present disclosure will be described.

[0035] 7 shows an embodiment in which light source devices 1 and 2 are used as pumping light sources for a plurality of optical amplifiers 110. Either light source device 1 or 2 may be used.

[0036] The optical amplifier 110 is a device that amplifies the signal light by combining the pumping light output from the light source devices 1 and 2 with the signal light propagating through the single-mode fiber 100, and amplifies the signal light using a rare-earth doped fiber. The rare-earth elements doped in the fiber are excited by the pumping light, and the signal light is amplified.

[0037] The light source devices 1 and 2 can drive multiple optical amplifiers 110. Since the light source devices 1 and 2 efficiently output high-power pumping light, they are expected to realize a power-saving communication network compared to pumping light sources that use commonly used semiconductor LDs.

[0038] 8 shows an embodiment in which the light source devices 1 and 2 are used as a light source for power supply light. Either the light source device 1 or 2 may be used.

[0039] For example, communication equipment arranged on an optical transmission path is driven by power supply light from light source devices 1 and 2 arranged in a communication building. The communication equipment performs photoelectric conversion on the power supply light and operates on the electrical energy obtained by the photoelectric conversion. The light source devices 1 and 2 and the communication equipment are connected by a multicore fiber 200. For example, each of a plurality of communication equipment is connected to a respective core of the multicore fiber 200. The light source devices 1 and 2 are directly connected to the multicore fiber 200 without being provided with a fan-out device. The communication equipment is provided with a photodetector 210 and performs photoelectric conversion on the power supply light propagating through the multicore fiber 200 to obtain electrical energy.

[0040] The communication building may be provided with a transmitter 220 and a wavelength coupler 240 to superimpose a control signal for controlling the communication equipment onto the power supply light. The communication equipment is provided with a receiver 230 and a wavelength coupler 250 to receive the control signal superimposed on the power supply light and operate in accordance with the control signal.

[0041] Optical power supply systems using 1 μm band light are an attractive application field due to their high photoelectric conversion efficiency. Commercially available single-mode light sources that output 0.98 μm wavelengths are limited to an output intensity of approximately 1 W. Using light source devices 1 and 2 makes it possible to achieve both high power output and power savings.

[0042] As described above, the light source device 1 of this embodiment includes the light source 16, the amplification multicore fiber 11 having a plurality of cores doped with rare earth ions, the fan-in device 17 that couples light emitted by the light source 16 to each of the cores of the amplification multicore fiber 11, the multimode LD 18 that outputs pumping light for exciting the rare earth ions doped in the amplification multicore fiber 11, and the pumping light coupler 19 that couples the pumping light to the cladding of the amplification multicore fiber 11. The core radius is determined so that the amplification multicore fiber 11 operates in single mode, and the core pitch and the core-to-clad area ratio are determined so that inter-mode coupling in the amplification multicore fiber 11 is −20 dB or less.

[0043] The light source device 2 of this embodiment includes a multimode LD 21, an amplification multicore fiber 11 having a plurality of cores doped with rare earth ions, and reflectors 22, 23 that are arranged on both ends of the amplification multicore fiber 11 and reflect light that is stimulated and emitted when light emitted from the multimode LD 21 is coupled to the amplification multicore fiber 11. The core radius is determined so that the amplification multicore fiber 11 operates in single mode, and the core pitch and the core-to-clad area ratio are determined so that inter-mode coupling in the amplification multicore fiber 11 is −20 dB or less.

[0044] This makes it possible to increase the core-to-clad area ratio Rcc of the amplification multicore fiber 11 and provide a more efficient light source.

[0045] REFERENCE SIGNS LIST 1, 2 Light source device 11 Amplification multicore fiber 12, 13 Optical isolator 14 Fan-out device 15 Power coupler 16 Light source 17 Fan-in device 18 Multimode laser diode 19 Pumping optical coupler 21 Multimode laser diode 22, 23 Reflector

Claims

1. A light source device comprising: a light source; and a rare-earth-doped multi-core fiber having a plurality of cores doped with rare-earth ions, the rare-earth-doped multi-core fiber having a plurality of cores doped with rare-earth ions and combining light emitted from the light source to output light from each of the plurality of cores, wherein a radius of the cores is determined so that the rare-earth-doped multi-core fiber operates in single mode, and a core pitch and an area ratio of the core to the cladding are determined so that inter-mode coupling in the rare-earth-doped multi-core fiber is -20 dB or less.

2. A light source device according to claim 1, comprising: a fan-in device that couples light emitted by said light source to each of the cores of said rare-earth doped multi-core fiber; an excitation light source that outputs excitation light for exciting the rare earth doped in said rare-earth doped multi-core fiber; and an excitation light coupler that couples said excitation light to the cladding of said rare-earth doped multi-core fiber.

3. A light source device according to claim 1, comprising: reflectors arranged on both ends of the rare-earth-doped multi-core fiber and configured to reflect light that is stimulated and emitted when light emitted by the light source is coupled to the rare-earth-doped multi-core fiber.

4. A light source device according to any one of claims 1 to 3, wherein the area ratio of the core to the cladding is greater than 0.08.

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