Optical amplifier

Abstract

This optical amplifier comprises: a rare-earth-doped optical fiber; an excitation light coupler that inputs signal light and excitation light to the rare-earth-doped optical fiber; a multimode excitation light source that outputs the excitation light to a multimode fiber; and a spatial multiplexing optical fiber that is connected to the multimode fiber, that introduces the excitation light into the excitation light coupler, and that has two or more spatial channels.

Description

Optical amplifier

[0001] This disclosure relates to an optical amplifier.

[0002] Non-Patent Document 1 discloses a technique using a clad excitation method using a multimode laser in an optical amplifier for mode division multiplexing (MDM). According to this technique, it has been reported that the mode gain difference is low because the mode distribution in the fiber cross-section is flat.

[0003] M. Wada et al., “Full C-band and Power Efficient Coupled-multi-core Fiber amplifier”, Proc. OFC, M4C.3, 2020.

[0004] According to the technique described in Non-Patent Document 1, since a multimode laser, which is a light source that couples multimode-oscillated light to a multimode fiber, is used, the heat resistance is higher and cooling of the laser element can be easily performed compared to the case of using a single-mode laser. Therefore, the power consumption per unit optical output can be reduced.

[0005] However, since the core area doped with rare-earth ions is about one digit smaller than the clad area, according to the clad excitation method using a multimode laser, a lot of light is transmitted and output, and there is a problem that high amplification efficiency cannot be achieved with low power consumption.

[0006] This disclosure has been made in view of the above problems. The object is to provide an optical amplifier capable of achieving high amplification efficiency with low power consumption.

[0007] In order to solve the above-described problems, an optical amplifier according to one aspect of this disclosure includes a rare-earth-doped optical fiber, an excitation light coupler that inputs signal light and excitation light into the rare-earth-doped optical fiber, a multimode excitation light source that outputs excitation light to a multimode fiber, and a spatial multiplexing optical fiber having two or more spatial channels that is connected to the multimode fiber and introduces the excitation light into the excitation light coupler.

[0008] According to this disclosure, it is possible to provide an optical amplifier that can achieve high amplification efficiency with low power consumption.

[0009] This is a schematic diagram showing a first configuration example of the optical amplifier according to this disclosure. This is a diagram showing the evaluation results of the coupling efficiency at the connection point. This is a schematic diagram showing a second configuration example of the optical amplifier according to this disclosure. This is a schematic diagram showing a third configuration example of the optical amplifier according to this disclosure.

[0010] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, identical components are denoted by the same reference numerals, and redundant explanations will be omitted.

[0011] [First Configuration Example] Figure 1 is a schematic diagram showing a first configuration example of an optical amplifier according to the present disclosure. The optical amplifier 1 comprises a rare-earth doped optical fiber EDF, an excitation optical coupler CP, a multimode excitation light source MLD, and a spatially multiplexed optical fiber SDM. In addition, the optical amplifier 1 may also include isolators IS1 and IS2.

[0012] Rare-earth doped optical fiber (EDF) receives both signal light and excitation light and has the function of amplifying the signal light. A rare-earth doped optical fiber (EDF) is an optical fiber having a core region in which rare-earth ions are added. Examples of rare-earth ions that can be added include Er ３＋ , Pr ３＋ , Tm ３＋ , Yb ３＋ , Nd ３＋ , Dy ３＋ These are some examples, but are not limited to these. Rare earth ions are selected according to the bandwidth of the signal light to be amplified. Multiple types of rare earth ions may be added. In addition, Ge and Al may be co-added.

[0013] For example, erbium may be added to the core of a rare-earth doped optical fiber (EDF) for amplification in the C band (1530-1565 nm), which is a wavelength band used for long-distance communication.

[0014] The rare-earth-doped optical fiber EDF may be a spatially multiplexed optical fiber such as a multi-core optical fiber or a multi-mode multi-core optical fiber, or it may be a photonic crystal fiber. In that case, the fiber connected to the transmission line and the multi-mode fiber may be the same type of optical fiber as the rare-earth-doped optical fiber EDF. The rare-earth-doped optical fiber EDF is not limited to the examples given herein.

[0015] The excitation light coupler CP inputs the signal light and excitation light to the rare-earth doped optical fiber EDF. The excitation light coupler CP is connected to the optical fiber FB, and introduces the signal light that has propagated through the optical fiber FB into the rare-earth doped optical fiber EDF. Furthermore, the excitation light coupler CP is connected to the spatially multiplexed optical fiber SDM, which will be described later, and introduces the excitation light that has propagated through the spatially multiplexed optical fiber SDM into the rare-earth doped optical fiber EDF.

[0016] The structure of the excitation light coupler CP can be a spatial optical element type composed of spatial filters and lenses, a Planar Lightwave Circuit (PLC), or a fiber coupler type, among others. The excitation light coupler CP is not limited to the examples given here.

[0017] The multimode excitation light source (MLD) outputs excitation light to the multimode fiber MFB (MFB). For example, the multimode excitation light source (MLD) may be coupled to a multimode fiber MFB having a core diameter of 50 μm or 100 μm. The multimode fiber MFB has a larger core diameter than optical fibers used for long-distance transmission.

[0018] Therefore, when a typical single-mode fiber (SMF) is connected after a multimode fiber MFB, there is a concern about significant connection loss due to mode-field (MFD) mismatch or mismatch in the number of modes.

[0019] In this disclosure, by connecting a spatially multiplexed optical fiber (SDM) after a multimode fiber MFB, it becomes possible to connect and guide the light from the multimode fiber MFB with relatively low loss.

[0020] Furthermore, multimode excitation light sources (MLDs) offer higher output configurations compared to single-mode laser light sources. In addition, the power consumption required for cooling the laser light source per unit of optical output is kept low in multimode excitation light sources (MLDs). Therefore, by achieving efficient optical coupling between the core of the rare-earth doped optical fiber (EDF) and the multimode excitation light source (MLD), a power-saving configuration can be realized. Moreover, since the signal light is amplified by excitation light containing higher-order modes, the gain difference between modes is reduced.

[0021] The spatially multiplexed optical fiber (SDM) is connected to a multimode fiber (MFB) at connection point CN, and the excitation light is introduced into the excitation light coupler (CP). Furthermore, the spatially multiplexed optical fiber (SDM) has two or more spatial channels.

[0022] For example, a spatially multiplexed optical fiber (SDM) is an optical fiber having two or more spatial channels, such as a multi-mode fiber (FMF) or a multi-core optical fiber (MCF). The examples given here are not the only examples of spatially multiplexed optical fiber (SDM). In the following explanation, we will assume that the spatially multiplexed optical fiber (SDM) is a multi-mode fiber (FMF).

[0023] When connecting a spatially multiplexed optical fiber (SDM) and a multimode fiber (MFB), the mode field of the SDM may be expanded by heat treatment. Alternatively, the mode field of the multimode fiber MFB may be adjusted by tapering the end of the MFB. These methods may suppress mode field mismatch at the connection point CN between the SDM and the MFB. The methods for suppressing mode field mismatch are not limited to those given herein.

[0024] In addition, spatially multiplexed optical fibers (SDM) and rare-earth doped optical fibers (EDF) may also have a double-clad structure. Here, a double-clad structure is a structure in which a multi-mode fiber is coated with a low refractive index coating. By using a double-clad structure, light that is not coupled to the core of the multi-mode fiber can be propagated in the cladding of the multi-mode fiber.

[0025] A double-clad rare-earth doped optical fiber (EDF) is used, and the light propagating through the core and cladding of the spatially multiplexed optical fiber (SDM) is coupled to the rare-earth doped optical fiber (EDF) via an excitation optical coupler (CP). This configuration makes it possible to achieve even higher efficiency for the optical amplifier 1.

[0026] The isolator IS1 allows light to propagate from the optical fiber FB towards the excitation optical coupler CP, while blocking light propagation in the reverse direction. This suppresses the propagation of unwanted light from the excitation optical coupler CP towards the optical fiber FB.

[0027] The isolator IS2 allows light to propagate in the direction output from the rare-earth doped optical fiber EDF, while blocking light propagation in the reverse direction. This prevents unwanted light other than signal and excitation light from being input to the rare-earth doped optical fiber EDF.

[0028] Figure 2 shows the evaluation results of coupling efficiency at the connection point. Graph L1 in Figure 2 shows the evaluation results of coupling efficiency when a single or up to 10-mode optical fiber is connected after a multimode fiber MFB. On the other hand, graph L2 shows the evaluation results when a single-mode laser light source is used instead of a multimode excitation light source MLD.

[0029] As shown in Graph L1, it can be confirmed that the coupling efficiency increases as the number of modes increases, and it can be seen that the coupling efficiency is about 40% for 10-mode fibers. The fact that the coupling efficiency does not change significantly between 6-mode and 10-mode fibers is thought to be due to the fact that the core diameter of the 6-mode fiber and the 10-mode fiber used in this evaluation are almost the same in terms of design.

[0030] Next, we will compare the power consumption of a single-mode laser light source with that of a multi-mode excitation light source (MLD). For example, we will refer to the power consumption of the Lumentum S32 series (600 mW output) as a single-mode laser light source.

[0031] When the coupling efficiency exceeds 31%, the power consumption of a multimode excitation light source (MLD) falls below that of a single-mode laser light source. Further efficiency improvements can be achieved by using a multimode fiber with a mode field close to that of a multimode fiber, or by using a TEC fiber (Thermally-diffused Expanded Core Fiber). Similar improvements in coupling efficiency can be expected with structures that have many spatial channels, such as multicore fibers and multimode multicore fibers.

[0032] [Second Configuration Example] Figure 3 is a schematic diagram showing a second configuration example of the optical amplifier according to the present disclosure. In Figure 3, the optical amplifier 1 further comprises an excitation light collector RC and an auxiliary excitation light coupler SP.

[0033] The excitation light collector RC collects the excitation light propagating through the cladding of the spatial multiplexed optical fiber SDM. For example, the excitation light collector RC may be integrated with the cladding of the spatial multiplexed optical fiber SDM, or it may be installed on the end or side of the cladding of the spatial multiplexed optical fiber SDM.

[0034] The auxiliary excitation light coupler SP is connected to the excitation light recoverer RC, and the excitation light recovered by the excitation light recoverer RC is input to the rare earth doped optical fiber EDF. The auxiliary excitation light coupler SP and the excitation light recoverer RC may be connected by an optical fiber or the like, and the excitation light input to the excitation light recoverer RC from the cladding of the spatially multiplexed optical fiber SDM may propagate to the auxiliary excitation light coupler SP.

[0035] In the configuration shown in Figure 3, the excitation light coupler CP inputs the excitation light propagating through the core of the spatially multiplexed optical fiber SDM to the rare-earth doped optical fiber EDF, and the auxiliary excitation light coupler SP inputs the excitation light propagating through the cladding of the spatially multiplexed optical fiber SDM to the rare-earth doped optical fiber EDF.

[0036] [Third Configuration Example] Figure 4 is a schematic diagram showing a third configuration example of the optical amplifier according to the present disclosure. In Figure 4, the optical amplifier 1 further comprises an excitation light collector RC and a photoelectric converter EC.

[0037] The pump light recovery device RC recovers the pump light propagating in the cladding of the space division multiplexed optical fiber SDM, similar to that shown in FIG. 3.

[0038] The photoelectric converter EC is connected to the pump light recovery device RC and converts the pump light recovered by the pump light recovery device RC into electric power. The electric power obtained by the photoelectric converter EC may be supplied to a device (not shown) or the like.

[0039] [Effect of Embodiment] As described in detail above, the optical amplifier according to the present disclosure includes a rare earth doped optical fiber, a pump light coupler that inputs signal light and pump light to the rare earth doped optical fiber, a multimode excitation light source that outputs pump light to a multimode fiber, and a space division multiplexed optical fiber having two or more spatial channels that is connected to the multimode fiber and introduces pump light into the pump light coupler.

[0040] Thereby, in the optical amplifier, high amplification efficiency can be realized with low power consumption. In particular, while using a multimode excitation light source that realizes a higher output configuration compared to a single mode laser light source, the power consumption required for cooling the laser light source per unit optical output can be kept low. Furthermore, since efficient optical coupling between the core of the rare earth doped optical fiber and the multimode excitation light source can be realized, a power saving configuration can be realized. Furthermore, since the signal light is amplified by pump light including higher order modes, the gain difference between modes is reduced. In addition, since a space division multiplexed optical fiber is connected after the multimode fiber, it is possible to connect and waveguide the light from the multimode fiber with relatively low loss.

[0041] Also, in the optical amplifier according to the present disclosure, the space division multiplexed optical fiber and the rare earth doped optical fiber may have a double clad structure. By adopting the double clad structure, the light that has not been coupled to the core of the multimode fiber can be propagated in the cladding of the multimode fiber. As a result, in the optical amplifier, even higher amplification efficiency can be realized with low power consumption.

[0042] Furthermore, the optical amplifier according to the present disclosure may further include an excitation light recovery unit that recovers excitation light propagating through the cladding of the space-division multiplexed optical fiber, and an auxiliary excitation light coupler that inputs the excitation light recovered by the excitation light recovery unit into the rare-earth-doped optical fiber. Thereby, the excitation light propagating through the core of the space-division multiplexed optical fiber can be input into the rare-earth-doped optical fiber, and further, the excitation light propagating through the cladding of the space-division multiplexed optical fiber can be input into the rare-earth-doped optical fiber. As a result, in the optical amplifier, higher amplification efficiency can be realized with lower power consumption. The excitation light that has propagated through the cladding and has been difficult to contribute to the amplification of the signal light can be made to contribute to the amplification of the signal light.

[0043] In addition, the optical amplifier according to the present disclosure may further include an excitation light recovery unit that recovers excitation light propagating through the cladding of the space-division multiplexed optical fiber, and a photoelectric converter that converts the excitation light recovered by the excitation light recovery unit into electric power. Thereby, the energy of the excitation light that has propagated through the cladding and has been difficult to contribute to the amplification of the signal light can be recovered and converted into electric power, and supplied to other devices or the like. Thereby, the energy efficiency can be improved.

[0044] As described above, the content of the present disclosure has been described in accordance with the embodiments. However, it is obvious to those skilled in the art that the present disclosure is not limited to these descriptions, and various modifications and improvements are possible. It should not be understood that the arguments and drawings forming a part of this disclosure limit the present disclosure. Various alternative embodiments, examples, and operation techniques will be apparent to those skilled in the art from this disclosure.

[0045] Of course, the present disclosure includes various embodiments and the like not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specific matters according to the reasonable claims based on the above description.

[0046] 1. Optical Amplifier CN (Connection Point) CP (Excitation Optical Coupler) EC (Photoelectric Converter) EDF (Rare Earth Doped Optical Fiber) FB (Optical Fiber) IS1, IS2 (Isolators) MFB (Multimode Fiber) MLD (Multimode Excitation Light Source) RC (Excitation Optical Collector) SDM (Spatial Multiplexed Optical Fiber) SP (Auxiliary Excitation Optical Coupler)

Claims

1. An optical amplifier comprising: a rare-earth-doped optical fiber; an excitation light coupler that inputs signal light and excitation light to the rare-earth-doped optical fiber; a multimode excitation light source that outputs the excitation light to a multimode fiber; and a spatially multiplexed optical fiber having two or more spatial channels that is connected to the multimode fiber and introduces the excitation light to the excitation light coupler.

2. The optical amplifier according to claim 1, wherein the spatial multiplexed optical fiber and the rare earth-doped optical fiber have a double-clad structure.

3. The optical amplifier according to claim 1 or 2, further comprising: an excitation light recoverer for recovering the excitation light propagating through the cladding of the spatial multiplexed optical fiber; and an auxiliary excitation light coupler for inputting the excitation light recovered by the excitation light recoverer into the rare earth-doped optical fiber.

4. The optical amplifier according to claim 1 or 2, further comprising: an excitation light recoverer for recovering the excitation light propagating through the cladding of the spatial multiplexed optical fiber; and a photoelectric converter for converting the excitation light recovered by the excitation light recoverer into power.

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