Optical amplifier

Abstract

An optical amplifier (10) comprises at least one amplification unit (11) and a photoelectric conversion unit (21). The amplification unit (11) includes: an amplification optical fiber (12A); a first light source (13A) that generates first excitation light; a first optical coupler (14A) that outputs the first excitation light and signal light to the amplification optical fiber (12A); and a light recovery device (15) that extracts, as recovery light, light in a predetermined band from the light that is output from the amplification optical fiber (12A). The photoelectric conversion unit (21) includes at least one photoelectric converter (22) that converts the recovered light into electric power.

Description

Optical amplifier

[0001] The present disclosure relates to an optical amplifier.

[0002] Internet traffic has continued to increase in recent years due to the diversification of services. The transmission capacity of traffic has increased dramatically due to the increase in transmission speed and the number of wavelengths multiplexed by wavelength division multiplexing (WDM) technology. In addition, further expansion of the transmission capacity is expected by digital coherent technology, which has been actively studied in recent years. Digital coherent technology improves frequency utilization efficiency by using multi-level phase modulation signals, while a higher signal-to-noise ratio is required.

[0003] In a conventional transmission system using single mode fiber (SMF), in addition to the theoretical limit, there is a limit on the input power due to non-linear effects. Therefore, it is expected that the transmission capacity will saturate at the boundary of 100 Tbit / sec, and it has become difficult to increase the capacity further. In the future, in order to further increase the transmission capacity, a medium that can achieve innovative transmission capacity expansion is required.

[0004] Therefore, in recent years, space division multiplexing (SDM) has attracted attention. SDM is a transmission technology that uses multiple cores in an optical fiber as channels, and an improvement in space utilization efficiency can be expected. However, in order to perform long-distance transmission using SDM, an optical amplifier installed in the middle of the transmission line is essential, as in the conventional system. Regarding the optical amplifier for SDM, not only the same core excitation method as the conventional one but also the clad excitation method capable of amplifying a plurality of spatial channels simultaneously has been studied (see Non-Patent Document 1).

[0005] M. Wada et al., “Two-LP-Mode Six-Core Cladding Pumped EDFA With High Pump Power Density”, JLT, vol. 36, no. 2, 2017.

[0006] In the core excitation method, the residual light component (light intensity), such as excitation light, increases in proportion to the number of spatial channels. In the cladding excitation method, multiple spatial channels can be amplified simultaneously with a single light source. However, because the excitation light propagates through the cladding portion of the optical fiber, the excitation light that is not absorbed by the core doped with rare earth ions is removed as residual light. In other words, the power spent to generate the excitation light is wasted as it is removed as residual light.

[0007] This disclosure is made in view of these circumstances and aims to provide an optical amplifier that can substantially improve the efficiency of the power used to generate excitation light.

[0008] An optical amplifier according to an aspect of the present disclosure comprises at least one amplification unit including an amplification optical fiber, a first light source that generates first excitation light, a first optical coupler that combines the first excitation light and signal light and outputs it to the amplification optical fiber, and an optical recoverer that extracts light of a predetermined band from the output light of the amplification optical fiber as recovered light, and a photoelectric conversion unit including at least one photoelectric converter that converts the recovered light into power.

[0009] According to this disclosure, it is possible to provide an optical amplifier that can substantially improve the efficiency of the power consumed in generating excitation light.

[0010] Figure 1 shows an example of the configuration of an optical amplifier according to this embodiment. Figure 2 shows the configuration of an optical amplifier according to a first modification of this embodiment. Figure 3 shows the configuration of an optical amplifier according to a second modification of this embodiment. Figure 4 shows the configuration of an optical amplifier according to a third modification of this embodiment. Figure 5A is a graph showing the analysis results of the third modification. Figure 5B is a graph showing the analysis results of the third modification. Figure 6 shows a first application example of the optical amplifier. Figure 7 shows a second application example of the optical amplifier. Figure 8 shows a third application example of the optical amplifier.

[0011] The optical amplifier according to the embodiment of this disclosure will be described below with reference to the drawings. In the drawings, the same parts are denoted by the same reference numerals and their descriptions are omitted.

[0012] Figure 1 shows an example of the configuration of the optical amplifier 10 according to this embodiment. As shown in Figure 1, the optical amplifier 10 comprises at least one amplification unit 11 and a photoelectric conversion unit 21. The amplification unit 11 amplifies the signal light 2 propagating through the transmission line 1 using a first excitation light 3A, and extracts the first excitation light 3A that did not contribute to the amplification and / or a portion of the amplified signal light 2 as recovered light 4. The photoelectric conversion unit 21 converts the recovered light 4 output from the amplification unit 11 into electricity.

[0013] The power generated by the photoelectric conversion unit 21 is used as driving power for devices such as IoT sensors, or as at least a portion of the driving power for the light source of the optical amplifier 10. In either case, the light energy that would otherwise be wasted is used as driving power for other devices. Consequently, the efficiency of the power consumed by the first light source 13A that generates the first excitation light 3A is substantially improved.

[0014] As shown in Figure 1, the amplification unit 11 comprises an amplification optical fiber (first amplification optical fiber) 12, a first light source 13A, a first optical coupler 14A, and an optical collector 15. An optical isolator 17 is provided upstream of the first optical coupler 14A in the propagation path of the signal light 2. An optical isolator 18 is also provided downstream of the optical collector 15. For convenience of explanation, the amplification optical fiber 12A will be referred to as optical fiber 12A below.

[0015] The optical fiber 12A is a so-called rare-earth doped fiber in which rare-earth ions are added to the core (not shown) of the optical fiber 12A. The rare-earth doped material is, for example, erbium (Er), thulium (Tm), neodymium (Nd), or ytterbium (Yb), and is selected according to the wavelength of the signal light 2 that propagates and is amplified within the optical fiber 12A. For example, when erbium (Er) is used as the rare-earth doped material, the signal light 2 is, for example, light in the C band (1530-1565 nm), which is a wavelength band for long-distance communication. In addition, co-doped materials such as germanium (Ge) or aluminum (Al) may be further added to the core.

[0016] The optical fiber 12A is, for example, an SDM fiber such as a multi-core fiber, a multi-mode fiber, or a multi-mode multi-core fiber. Alternatively, the optical fiber 12A may be a single-mode fiber or a photonic crystal fiber. The cladding structure of the optical fiber 12A is selected according to the excitation method. That is, when a cladding excitation method is adopted, the optical fiber 12A has a double-cladding structure in which the first excitation light 3A is guided through its cladding. On the other hand, when a core excitation method is adopted, the optical fiber 12A has a single-cladding structure in which the first excitation light 3A is guided through its core.

[0017] The first light source 13A is a laser light source that generates the first excitation light 3A. For example, a semiconductor laser is used as the laser light source. The oscillation method of the first light source 13A is selected according to the cladding excitation method. That is, when the cladding excitation method is adopted, the first light source 13A is a multimode laser, and when the core excitation method is adopted, the first light source 13A is a single-mode laser. For example, when the signal light 2 is in the C band and the optical fiber 12A is an erbium-doped fiber (EDF), the first light source 13A generates the first excitation light 3A having a wavelength of around 980 nm.

[0018] The first optical coupler 14A combines the first excitation light 3A and the signal light 2 and outputs it to the optical fiber 12A. In the optical fiber 12A, rare earth ions are excited by the first excitation light 3A, and stimulated radiation is generated when the signal light 2 is incident on them. As a result, the signal light 2 is amplified within the optical fiber 12A and input to the recovered light 4.

[0019] The optical recoverer 15 extracts light of a predetermined bandwidth from the output light of the optical fiber 12A as recovered light 4, and outputs the signal light 2 to the optical isolator 18. Here, the light of a predetermined bandwidth is the first excitation light 3A that did not contribute to optical amplification (excitation). The optical recoverer 15 is composed of, for example, a fiber coupler or a spatial optical element such as a WDM filter, and outputs the extracted recovered light 4 to the photoelectric conversion unit 21.

[0020] The optical recovery unit 15 may also include a configuration as a gain equalizer 16. The gain equalizer 16 is composed of a long-period grating, an etalon filter, or a dielectric multilayer filter, and flattens the intensity of the signal light 2 amplified by the optical fiber 12A in the wavelength band. As a result, the gain of the optical amplifier is equalized within the wavelength band, and the removed light (i.e., a portion of the signal light 2) is output to the photoelectric conversion unit 21 as recovered light 4 together with the first excitation light 3A.

[0021] The gain equalizer 16 may be provided independently of the optical recoverer 15. In this case, the gain equalizer 16 is provided between the optical fiber 12A and the optical recoverer 15.

[0022] The optical collector 15 and the photoelectric conversion unit 21 are connected to each other via an optical fiber 6 or a spatial optical element (not shown) such as a lens. The photoelectric conversion unit 21 includes at least one photoelectric converter 22. The photoelectric converter 22 is composed of a photoelectric conversion element such as a photodiode.

[0023] The first excitation light 3A generated by the first light source 13A enters the optical fiber 12A via the first optical coupler 14A. The first excitation light 3A excites rare earth ions inside the optical fiber 12A, and the signal light 2 is amplified by the stimulated emission of the excited rare earth ions.

[0024] In the cladding excitation method using an Er-doped fiber (EDF) as the optical fiber 12A, depending on the structure of the optical fiber 12A, more than 50% of the first excitation light 3A may be transmitted without being absorbed by the core. That is, a portion of the first excitation light 3A is absorbed by the core of the optical fiber 12A and used to amplify the signal light 2, while the remainder is not completely absorbed by the core and is emitted from the optical fiber 12A.

[0025] The light collector 15 recovers at least the first excitation light 3A and outputs it to the photoelectric converter 22. The photoelectric converter 22 then converts the first excitation light 3A into electricity. In other words, in this embodiment, power for supply to devices and the like can be obtained from the first excitation light 3A. Furthermore, the efficiency of the power consumed by the first light source 13A that generates the first excitation light 3A can be substantially improved.

[0026] Figure 2 shows the configuration of an optical amplifier 10 according to a first modified example of this embodiment. As shown in Figure 2, the optical amplifier 10 according to the first modified example includes a plurality of amplification units 11 that amplify the signal light 2 propagating through each transmission line 1. The excitation method of each amplification unit 11 may be either a core excitation method or a cladding excitation method. The photoelectric conversion unit 21 includes a plurality of photoelectric converters 22 that convert the recovered light 4 from each amplification unit 11 into power. The outputs (positive and negative sides) of the plurality of photoelectric converters 22 may be connected in series or in parallel.

[0027] In the first modified example, the recovered light 4 from multiple amplification units 11 is converted into power by multiple corresponding photoelectric converters 22 and then combined. Generally, it is not possible to collect multiple lights of the same mode and wavelength in such a way that the sum of their respective light energies is obtained. On the other hand, in this example, each recovered light 4 is first converted into power and then combined. Therefore, the conversion efficiency from recovered light 4 to power can be improved.

[0028] Figure 3 shows the configuration of an optical amplifier 10 according to a second modification of this embodiment. As shown in Figure 3, the optical amplifier 10 according to the second modification comprises a plurality of amplification units 11 that amplify the signal light 2 propagating through each transmission line 1. The photoelectric conversion unit 21 also includes a multiplexer 23. The multiplexer 23 combines the recovered light 4 from each amplification unit 11 and outputs it to a photoelectric converter 22.

[0029] The recovered light 4 collected by each optical collector 15 is input to the multiplexer 23 via each optical fiber 6 and combined. The combined light is input to the photoelectric converter 22 via the optical fiber 7 and converted into electricity.

[0030] The optical fiber 7 propagates the recovered light 4 output from each optical collector 15 to the photoelectric converter 22. Therefore, the number of propagation modes of the optical fiber 7 is greater than or equal to the product of the number of optical fibers 6 and the number of propagation modes. For example, if the optical fibers 6 are single-mode fibers, the optical fiber 7 is composed of multimode fibers. Specifically, if each of the four optical fibers 6 is a single-mode fiber, the optical fiber 7 becomes a four-mode fiber capable of propagating four modes of light. In this example, all optical fibers 6 may be bundled together and connected directly to the photoelectric converter 22 without using a multiplexer 23.

[0031] Figure 4 shows the configuration of an optical amplifier 10 according to a third modification of this embodiment. As shown in Figure 4, the optical amplifier 10 according to the third modification further includes, in addition to the amplification unit 11 and the photoelectric conversion unit 21, an amplification optical fiber (second amplification optical fiber) 12B, a second light source 13B, and a second optical coupler 14B. For convenience of explanation, the amplification optical fiber 12B will be referred to as optical fiber 12B below.

[0032] The optical fiber 12B is provided between the optical isolator 17 and the first optical coupler 14A. The second optical coupler 14B is provided between the optical isolator 17 and the optical fiber 12B. That is, the optical fiber 12B is provided upstream of the first optical coupler 14A, and the second optical coupler 14B is provided upstream of the optical fiber 12B. The optical isolator 17 is provided upstream of the second optical coupler 14B.

[0033] Optical fiber 12B is a rare-earth doped fiber with the same rare-earth additives as optical fiber 12A. The second optical coupler 14B outputs the second excitation light 3B and the signal light 2 to the first optical coupler 14A. The second light source 13B generates the second excitation light 3B and outputs the generated second excitation light 3B to the second optical coupler 14B. The wavelength band of the second excitation light 3B is the same as the wavelength band of the first excitation light 3A.

[0034] In this example, a multimode laser is used as the first light source 13A, and a cladding excitation method is used for the excitation of the optical fiber 12A. Therefore, the optical fiber 12A has a double-cladding structure. On the other hand, a single-mode laser is used as the second light source 13B, and a core excitation method is used for the excitation of the optical fiber 12B. Therefore, the optical fiber 12B has a single-cladding structure.

[0035] The first light source 13A excites rare earth ions in the optical fiber 12A, and the second light source 13B, located upstream of the first light source 13A, excites rare earth ions in the optical fiber 12B. Due to these excitations, the signal light 2 is amplified in the optical fiber 12B and then further amplified in the optical fiber 12A. In other words, the optical amplifier 10 in this example has a two-stage amplification configuration.

[0036] In the first stage of amplification, a core-excited optical fiber 12B is used, and in the second stage of amplification, a clad-excited optical fiber 12A, as shown in Figure 1, is used. The power output from the photoelectric conversion unit 21 is supplied to the second light source 13B as part of the driving power for the second light source 13B.

[0037] Generally, clad excitation methods tend to have a higher noise figure than core excitation methods. Therefore, by placing a core-excited optical fiber 12B before the clad-excited optical fiber 12A and performing two-stage amplification, it is possible to improve amplification efficiency and suppress the noise figure.

[0038] In this example, the first excitation light 3A and the second excitation light 3B are recovered by the optical recoverer 15 and converted into electricity by the photoelectric converter 22. This electricity is used to supplement the power of the second light source 13B. Therefore, the efficiency of the power consumed by the second light source 13B that generates the second excitation light 3B is substantially improved. Note that the optical fiber 12B may be an optical fiber without rare earth elements added. In this case, the second excitation light 3B output from the second light source 13B is incident on the optical fiber 12A via the first optical coupler 14A.

[0039] Figures 5A and 5B are graphs showing the analysis results of the third modified example. In this analysis, uncoupled 4-core multicore fibers were assumed for the transmission line 1 and optical fibers 12A and 12B. The output of the first light source 13A was assumed to be 3W, the intensity of the recovered light 4 that was not absorbed by the optical fiber 12A and output from the optical recoverer 15 was assumed to be 2.4W, the coupling / recovery efficiency of each optical coupler 14A, 14B and the optical recoverer 15 was assumed to be 95%, the conversion efficiency of the photoelectric converter 22 was assumed to be 50%, and the electro-to-optical conversion efficiency of the second light source 13B was assumed to be 30%. The signal light 2 was assumed to be light of 8 wavelengths set at 5nm intervals in the range of 1530 to 1565nm.

[0040] Figure 5A shows the calculated output strength for single-stage amplification using the first light source 13A and for two-stage amplification using the first light source 13A and the second light source 13B. As shown in Figure 5A, two-stage amplification can be confirmed to improve the output by approximately 3 dB compared to single-stage amplification. Note that this improvement varies depending on the structure of the second optical fiber 12A.

[0041] Figure 5B shows the change in total output power in a two-stage amplification configuration, depending on the length of the first-stage optical fiber (rare-earth doped fiber) 12B (referred to as the EDF length for convenience). As shown in Figure 5B, the total output power changes depending on the EDF length, and in this analysis, if there are no requirements for the spectral shape, the maximum output power can be obtained by setting the EDF length to approximately 0.6 m.

[0042] FIG. 6 is a diagram showing a first application example of the optical amplifier 10. As shown in FIG. 6, the optical amplifier 10 is used as a network repeater, and the power generated by the photoelectric converter 22 is supplied to an external device 30 installed near the optical amplifier 10. The external device 30 is, for example, an IoT sensor such as a vibration sensor that detects earthquakes, a monitoring camera, a module for detecting abnormalities at the installation location of the optical amplifier 10, and the like. That is, the optical amplifier 10 operates as these optical power supply systems. The driving power of the external device 30 is supplied from the optical amplifier 10 installed in the vicinity thereof. Therefore, even in a place where power supply by power lines is difficult, an external device 30 driven by power can be installed, and necessary power can be supplied to the external device 30. Note that the power generated by the photoelectric converter 22 may be supplied to a storage battery (not shown) that stores the driving power of the external device 30.

[0043] FIG. 7 is a diagram showing a second application example of the optical amplifier 10. As shown in FIG. 7, the optical amplifier 10 is used as a network repeater, and the power generated by the photoelectric converter 22 is supplied to an external device 30 such as an IoT sensor installed near the optical amplifier 10. However, in the second application example, the photoelectric converter 22 is arranged sufficiently far (for example, several km) from the optical recovery device 15. Therefore, the optical fiber 6 has a length (for example, several km) that can connect between them. Also in this example, the optical amplifier 10 operates as an optical power supply system. Further, the power generated by the photoelectric converter 22 may be supplied to a storage battery (not shown) that stores the driving power of the external device 30.

[0044] FIG. 8 is a diagram showing a third application example of the optical amplifier 10. In the third application example, the optical amplifier 10 of the first modification example (see FIG. 2) or the optical amplifier 10 of the second modification example (see FIG. 2) is applied. That is, as shown in FIG. 8, the power output from the plurality of photoelectric conversion units 21 is combined and used as the driving power of another optical amplifier 31. Alternatively, the recovered light 4 output from the plurality of optical recovery devices 15 is multiplexed and then converted into power and used as the driving power of another optical amplifier 31. Therefore, the optical amplifier 31 can be driven without supplying power from an external power source (not shown).

[0045] As described above, this embodiment provides an optical amplifier that can substantially improve the efficiency of the power consumed in generating excitation light. The recipient of the photoelectric converter 22 is not limited to the equipment described above and can be changed as appropriate.

[0046] 1 Transmission line 2 Signal light 3A First excitation light 3B Second excitation light 4 Recovery light 6 Optical fiber 7 Optical fiber 10 Optical amplifier 11 Amplifier 12A First amplification optical fiber 12B Second amplification optical fiber 13A First light source 13B Second light source 14A First optical coupler 14B Second optical coupler 15 Optical recovery unit 16 Gain equalizer 21 Photoelectric conversion unit 22 Photoelectric converter 23 Multiplexer

Claims

1. An optical amplifier comprising: at least one amplification unit including an amplification optical fiber, a first light source for generating first excitation light, a first optical coupler for outputting the first excitation light and signal light to the amplification optical fiber, and an optical recoverer for extracting light of a predetermined bandwidth from the light output from the amplification optical fiber as recovered light; and a photoelectric conversion unit including at least one photoelectric converter for converting the recovered light into power.

2. The optical amplifier according to claim 1, wherein at least one of the amplification units comprises a plurality of amplification units, and at least one of the photoelectric converters converts the recovered light from each of the amplification units into power, and comprises a plurality of photoelectric converters whose outputs are connected in series or in parallel.

3. The optical amplifier according to claim 1, wherein at least one of the amplification units comprises a plurality of amplification units, and the photoelectric conversion unit comprises a multiplexer that combines the recovered light from each of the amplification units and outputs it to the photoelectric converter.

4. The optical amplifier according to claim 1, further comprising: a second light source that generates a second excitation light; and a second optical coupler that outputs the second excitation light and the signal light to the first optical coupler, wherein the first light source is a multimode laser, the second light source is a single-mode laser, and the power output from the photoelectric conversion unit is supplied to the second light source as part of the driving power for the second light source.

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