Optical amplifier and optical amplification method

Abstract

To provide an optical amplifier that can perform Raman amplification while reducing power consumption and size.SOLUTION: A fiber Raman amplifier 10 includes a light source 130 and a multi-core fiber 110. The light source 130 outputs pumping light PL used to amplify a multiplexed optical signal SIG. The multi-core fiber 110 constitutes a distributed amplifier for the multiplexed optical signal SIG by cladding pumping using pumping light PL. The light source 130 outputs multiplexed multimode laser light.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an optical amplifier and an optical amplification method. [Background technology]

[0002] In optical communications, it is desirable to increase the capacity of optical fiber links. This can be achieved by increasing the Spectral Efficiency (SE) of the signal transmitted over the optical fiber link. A common way to achieve this is to use a more efficient modulation format for the information being transmitted. This modulation format can be used in combination with Wavelength Division Multiplexing (WDM). Space Division Multiplexing (SDM) is also used to increase the transmission capacity of a single fiber while still maintaining the possibility of long distance transmission.

[0003] Non-Patent Document 1 discloses SDM implemented with a multi-core fiber (MCF) containing seven cores, which is used for transmitting 40-wavelength 128 Gb / s PM-QPSK (Polarization Multiplexed - Quadrature Phase Shift Keying) signals over a distance of 6,160 km or more. The MCF consists of several cores that transmit optical signals in the same fiber, and a multi-core (MC)-Erbium Doped Fiber Amplifier (EDFA) consisting of a fiber amplifier with MCF as a gain medium. The MC-EDFA pumps each core containing one MCF gain medium with a separate pumping light source by the direct core pumping method. Patent Document 1 also proposes another multi-core fiber amplifier in which the cores are doped with rare earth elements.

[0004] According to the system of Non-Patent Document 1, the system capacity can be increased by a factor of the number of cores of the MCF, i.e., seven times the number of cores of Non-Patent Document 1. By using MCF, in addition to WDM in each core, it is possible to spatially multiplex optical signals using multiple cores. This makes it possible to increase the transmission capacity of the fiber without sacrificing the transmission distance.

[0005] Non-Patent Document 2 discloses various different amplification methods such as Individual Core Pumping (ICP), Shared Core Pumping (SCP) and Common Cladding Pumping (CCP), and applies these techniques to MC-EDFA.

[0006] Furthermore, Raman amplification is also a well-known amplification process with excellent noise characteristics. An example of a Raman amplification method is proposed by Non-Patent Document 3. This method relies on stimulated Raman scattering (SRS), in which short-wavelength pump light (high frequency) amplifies long-wavelength (low frequency) signals in a non-linear state due to phonon emission in the fiber. Typically, pump light in the range of 1430 nm to 1490 nm is used to amplify one or both of the C- and L-band signals. Raman amplification occurs over a wide area in the transmission fiber, so it is a distributed amplification. The noise characteristics of Raman amplification are superior to those of EDFA. To realize hybrid EDFA / Raman amplification, Raman amplification can be applied to EDFA. However, as described in Non-Patent Document 3, Raman amplification consumes more power than EDFA. Therefore, when the power supply is limited, the use of Raman amplification with low noise characteristics is limited.

[0007] Furthermore, two problems are known related to Raman amplification. The first is that pump lights of different wavelengths are wavelength multiplexed (i.e. WDM). However, since high-power pump lights have a wide spectrum, the spectrum of the multiplexed pump lights may overlap in some regions. Multiplexing in the wavelength region of the overlapped spectral parts leads to suppression of the overlap of different pump lights, which are not used for amplification. This leads to inefficient power consumption and increases the power consumption.

[0008] Secondly, Raman amplification depends on the power density of pump light depending on the fiber core, which is the ratio of pump light power by the signal effective area. Therefore, the fiber core diameter cannot be adjusted due to the severe degradation of the transmitted optical signal, and Raman amplification cannot be adjusted according to the transmission distance. Therefore, the design of Raman amplification cannot be optimally designed in terms of signal-to-noise and power consumption from the viewpoint of the transmission distance parameter.

[0009] In addition, Patent Document 2 and Non-Patent Document 4 propose Raman amplification for SDM using MCF. In this case, pump light is provided to each core in the MCF, and Raman amplification is performed in each core. Therefore, Raman amplification for SDM using MCF can realize large capacity and parallelization. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US Patent Publication No. 2008 / 0018989 [Patent Document 2] US Patent Publication No. 2007 / 0268569 [Non-patent literature]

[0011] [Non-Patent Document 1] H. Takahashi et al., "First Demonstration of MC-EDFA-Repeatered SDM Transmission of 40 x 128-Gbit / s PDM-QPSK Signals per Core over 6,160-km 7-core MCF", ECOC 2012, paper Th.3.C.3. [Non-Patent Document 2] E. Le Taillandier de Gabory et al., "Transmission of 256Gb / s PM-16QAM Signal through 7-Core MCF and MC-EDFA with Common Cladding and Variable Shared Core Pumping for Reduction of Power Consumption", ECOC 2017, paper M.1.E.2 [Non-Patent Document 3] JX. Cai et al., "49.3 Tb / s Transmission Over 9100 km Using C+L EDFA and 54 Tb / s Transmission Over 9150 km Using Hybrid-Raman EDFA", Journal of Lightwave technology, Vol. 33, No. 13, pages 2724-2734. [Non-Patent Document 4] T. Mizuno et al., "Hybrid Cladding-pumped EDFA / Raman for SDM Transmission Systems Using Core-by-core Gain Control Scheme", ECOC 2017, paper M.1.E.3 Summary of the Invention [Problem to be solved by the invention]

[0012] However, the above-mentioned Raman amplification has some problems. As described in Non-Patent Document 3, Raman amplification consumes more power than EDFA. Therefore, when the power supply is limited, the use of Raman amplification with low noise characteristics is limited. In addition, the Raman amplification for SDM using MCF in Non-Patent Document 4 can realize large capacity and parallelization. However, since it is necessary to supply pump light to each core, a large number of devices for supplying pump light are required. As a result, the cost is high and the installation area of ​​the device is large. Therefore, there is a demand for reducing the power consumption, cost, and size of Raman optical amplifiers.

[0013] The present invention has been made in view of the above circumstances, and has an object to provide an optical amplifier capable of performing Raman amplification while suppressing power consumption and size. [Means for solving the problem]

[0014] An optical amplifier according to one aspect of the present invention includes a pumping light source that outputs pumping light used to amplify signal light, and a multi-core fiber that constitutes a distributed amplifier for the signal light by cladding pumping using the pumping light, and the pumping light source outputs multiplexed multimode laser light.

[0015] An optical amplification method according to one aspect of the present invention outputs pumping light used to amplify signal light, and by cladding pumping using the pumping light, a multicore fiber constitutes a distributed amplifier for the signal light, and multiplexed multimode laser light is used as the pumping light. Effect of the Invention

[0016] According to the present invention, it is possible to provide an optical amplifier capable of performing Raman amplification while suppressing power consumption and size. [Brief description of the drawings]

[0017] [Figure 1] 1 is a block diagram illustrating an optical communication system according to a first embodiment. [Diagram 2] FIG. 1 is a diagram illustrating a configuration of a fiber Raman amplifier according to a first embodiment. [Diagram 3] FIG. 1 is a diagram illustrating a configuration of a fiber Raman amplifier according to a first embodiment. [Figure 4] FIG. 2 is a diagram showing a configuration of FRAs arranged repeatedly according to the first embodiment; [Diagram 5] 5 is a diagram showing simulation results of the fiber Raman amplifier according to the first embodiment and a comparative example. [Figure 6] FIG. 13 is a diagram illustrating a configuration of a fiber Raman amplifier according to a second embodiment. [Figure 7] FIG. 2 is a diagram showing the spectrum of light emitted from a laser and excitation light. [Figure 8] FIG. 13 is a diagram showing a comparison of power consumption of lasers. [Figure 9] FIG. 13 is a diagram illustrating a configuration of a fiber Raman amplifier according to a third embodiment. [Figure 10] FIG. 13 is a diagram illustrating a configuration of a fiber Raman amplifier according to a fourth embodiment. [Figure 11] FIG. 13 is a diagram showing a simulation of the power of a multiplexed optical signal. [Figure 12] FIG. 13 is a diagram illustrating a configuration of a fiber Raman amplifier according to a fifth embodiment. [Figure 13] FIG. 13 is a diagram showing a simulation of the power of a multiplexed optical signal. [Figure 14] FIG. 13 is a diagram illustrating a configuration of a fiber Raman amplifier according to a sixth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0019] First embodiment A fiber Raman amplifier according to the embodiment 1 will be described. Fig. 1 is a diagram illustrating an optical communication system 100 according to the embodiment 1. The optical communication system 100 includes optical communication devices 101 and 102, and a fiber Raman amplifier (FRA) 10.

[0020] The optical communication devices 101 and 102 are configured as optical transceivers having a plurality of transponders. For the sake of simplicity, this embodiment will be described with reference to an example in which the optical communication device 101 outputs a multiplexed optical signal SIG to the optical communication device 102 via the FRA 10. It goes without saying that the optical communication device 102 may output a multiplexed optical signal to the optical communication device 101 via the FRA.

[0021] The optical communication device 101 outputs a multiplexed optical signal SIG obtained by multiplexing optical signals output from the transponders. Here, the optical signals output from the transponders are multiplexed by a wavelength multiplexing method (for example, wavelength division multiplexing (WDM)) and a specific multiplexing method (for example, space division multiplexing (SDM)).

[0022] The multiple optical signal SIG output from the optical communication device 101 is attenuated while being transmitted to the optical communication device 102. Therefore, in this configuration, at least one FRA 10 is provided between the optical communication device 101 and the optical communication device 102 to compensate for the attenuation of the multiple optical signal SIG.

[0023] The FRA 10 is configured as an optical amplifier that amplifies the multiplexed optical signal SIG by stimulated Raman scattering (SRS). Amplification by SRS is called Raman amplification. The FRA 10 amplifies the multiplexed optical signal SIG and outputs it to the optical communication device 102. This allows the optical communication device 102 to receive the multiplexed optical signal SIG with sufficient power to perform appropriate demodulation.

[0024] The following describes the configuration of the FRA 10. Fig. 2 and Fig. 3 show the configuration of the FRA 10 according to the first embodiment. The FRA 10 includes a multicore fiber (MCF) 110, a WDM coupler 120, and a light source 130.

[0025] The MCF 110 has a double clad structure and has seven cores C1 to C7. The length of the MCF 110 is generally several tens of kilometers, for example, 80 kilometers. The multiplexed optical signal SIG output from the optical communication device 101 is transmitted via the cores C1 to C7. That is, the optical signals in the multiplexed optical signal SIG multiplexed by WDM are distributed to the cores C1 to C7, and are then multiplexed by SDM in the MCF 110.

[0026] In this configuration, the cores C1 to C7 are included in the inner cladding CL1, and the inner cladding CL1 is included in the outer cladding CL2. The refractive index of the outer cladding CL2 is lower than that of the inner cladding CL1. For example, the outer cladding CL2 may be formed by applying a layer of low refractive index resin onto the surface of the inner cladding CL1.

[0027] The double clad structure may also be composed of a single clad and air (air layer or air hole) surrounding the single clad. In this case, since the refractive index of air is generally lower than that of the single clad, the single clad can function as the inner clad CL1 and the air can function as the outer clad CL2.

[0028] The light source 130 (also referred to as a first light source) has a plurality of lasers such as laser devices and laser diodes. In this embodiment, the light source 130 has three lasers 131 to 133 which are multimode lasers. The lasers 131 to 133 output laser beams L1 to L3 (also referred to as a first multimode laser beam) which are multimode laser beams supplied to the MCF 110 as excitation beams, respectively. The number of lasers can be changed as appropriate.

[0029] For example, the central wavelengths of the laser beams L1 to L3 emitted from the lasers 131 to 133 differ within a predetermined range. This range is typically 1430 nm to 1490 nm. The power of each of the laser beams L1 to L3 is typically several watts. Each of the lasers 131 to 133 has higher power efficiency than a single mode laser such as the lasers used in Non-Patent Documents 3 and 4. All or part of the spectrum of the laser beam may be overlapped.

[0030] An output (also referred to as a first end) of the MCF 110 and an output of the light source 130 are connected to the WDM coupler 120. The WDM coupler 120 can output the multiplexed optical signal SIG transmitted through the MCF 110 to the optical communication device 102. The WDM coupler 120 multiplexes the laser beams L1 to L3 emitted from the lasers 131 to 133. In this case, the laser beams L1 to L3 can be wavelength-multiplexed. Furthermore, the laser beams L1 to L3 may be polarization-multiplexed before being wavelength-multiplexed.

[0031] The WDM coupler 120 (also referred to as the first optical coupler) couples the combined laser light to the inner cladding CL1 of the MCF 110, so that the combined laser light is transmitted to the input of the MCF 110 via the inner cladding CL1 (also referred to as the second end). This allows all of the cores C1 to C7 to be excited simultaneously using the combined laser light, which is the pump light PL (also referred to as the first pump light). Since the MCF 110 has a double cladding structure, the pump light PL can be transmitted via the inner cladding CL1 without being wasted outside the inner cladding CL1, similar to an MCF that does not have a double cladding structure.

[0032] Here, the transmission direction of the multiplexed optical signal SIG is defined as the forward direction (also referred to as the first direction). The direction opposite to the forward direction is defined as the reverse direction (also referred to as the second direction). In the MCF 110, the pump light PL is transmitted in the reverse direction.

[0033] In this embodiment, the optical communication devices 101 and 102 can transmit and receive 200 Gb / s optical signals using a PM-16QAM (Phase Modulation - 16 Quadrature Amplitude Modulation) modulation scheme with a channel width of 37.5 GHz. The multiplexed optical signal output from each optical communication device includes optical signals of 100 wavelengths. Each of the cores C1 to C7 can transmit optical signals of up to 20 Tb / s, so the total capacity of the MCF 110 is up to 140 Tb / s.

[0034] Next, in order to consider the advantages of the FRA10, a comparative example in which pump light is transmitted by the core will be described. In this example, as in Non-Patent Document 4, Raman amplification is performed directly in the core of the MCF. In this case, three laser lights per core are wavelength-multiplexed to achieve broadband and high gain. Therefore, in the comparative example, three lasers are required per core. Therefore, to achieve a total capacity of 140 Tb / s using the MCF110, 3 x 7 = 21 lasers are required.

[0035] In contrast, the FRA 10 can achieve Raman amplification of the multiplexed optical signal SIG with a smaller number of pump lasers, which is advantageous in terms of reducing the overall size of the FRA and manufacturing it at low cost.

[0036] Furthermore, when the distance between optical communication device 101 and optical communication device 102 is several hundred kilometers or more, a plurality of FRAs may be arranged in series between optical communication device 101 and optical communication device 102. Fig. 4 shows a configuration of FRAs arranged repeatedly according to the first embodiment. As shown in Fig. 4, N (N is an integer equal to or greater than 2) FRAs 10_1 to 10_N are arranged in series.

[0037] As described above, one FRA 10 can cover one MCF 110 of 80 km. As a result, FRAs 10_1 to 10_N can cover N×80 km. For example, when the distance between the optical communication device 101 and the optical communication device 102 is 400 km, N is set to 5. According to this configuration, the multiplexed optical signal SIG is repeatedly amplified by FRAs 10_1 to 10_N, so that attenuation of the multiplexed optical signal SIG due to long-distance transmission can be appropriately compensated for.

[0038] Simulation results of this configuration and a comparative example are shown in Fig. 5. In Fig. 5, the solid line indicates the number of lasers included in the FRA according to the first embodiment. The dashed line indicates the number of lasers when the FRA according to the comparative example is arranged in series.

[0039] In the comparative example, when the number of FRAs increases by one, the number of lasers increases by 21. When N=5, the number of lasers is as high as 105. In contrast, with this configuration, when the number of FRAs increases by one, the number of lasers increases by only three. When N=5, the number of lasers is only 15.

[0040] Therefore, even if the FRAs are arranged in series, in this configuration, the number of excitation lasers can be reduced, and the overall size and cost of the FRA can be suppressed.

[0041] Embodiment 2 Next, a fiber Raman amplifier according to the second embodiment will be described. Fig. 6 shows a schematic configuration of an FRA 20 according to the second embodiment. The FRA 20 has a configuration in which the WDM coupler 120 and the light source 130 of the FRA 10 are replaced with a space division multiplex (SDM) coupler 220 (also referred to as a first optical coupler) and a light source 230 (also referred to as a first light source).

[0042] The light source 230 includes lasers 131 and 132 and a mode coupler 230A. In other words, compared to the light source 130, the light source 230 has a configuration in which the laser 133 is removed and a mode coupler 230A is added. The laser beams L1 and L2 emitted from the lasers 131 and 132 are mode-multiplexed by the mode coupler 230A, which performs space division multiplexing at the input. The multiplexed light, which is the pump light PL, is supplied to the input of the SDM coupler 220. The SDM coupler 220 couples the pump light PL to the inner cladding CL1 of the MCF 110.

[0043] Therefore, according to this configuration, similarly to the first embodiment, the multiplexed optical signal SIG transmitted through the MCF is appropriately amplified by the SRS.

[0044] Next, advantages of the FRA 20 will be described. Fig. 7 shows the spectra of the light L1 and L2 and the pumping light PL. In Fig. 7, the pumping light PL in the FRA 20 is indicated as PL_2. Fig. 7 also shows, as a comparative example A, pumping light PL_C in which the laser lights L1 and L2 are wavelength-multiplexed by the WDM coupler 120, for example.

[0045] As shown in Fig. 7, the peaks of the pumping lights PL_2 and PL_C are lower than the peaks of the laser lights L1 and L2 due to the insertion loss of the coupler. Also, the peaks of the pumping lights PL_2 and PL_C are almost the same. However, in the central region between the two peaks, the power of the pumping light PL_2 is greater than the power of the pumping light PL_C. This is because the pumping light PL_2 is mode multiplexed rather than wavelength multiplexed, and the laser lights L1 and L2 can be multiplexed without additional loss.

[0046] As a result, according to this configuration, the power of the laser light emitted by the laser can be reduced compared to FRA 10. FIG. 8 shows a comparison of the power consumption of the lasers. In FIG. 8, the power consumption of the two lasers in Non-Patent Document 4 is shown as Comparative Example B. As shown in FIG. 8, FRA 20 can further reduce the power consumption.

[0047] Third embodiment Next, a fiber Raman amplifier according to the third embodiment will be described. Fig. 9 shows the configuration of an FRA 30 according to the third embodiment. The FRA 30 has a configuration in which the MCF 110 of the FRA 10 is replaced with an MCF 310, and an MC-EDFA (Multi Core - Erbium Doped Fiber Amplifier) ​​340 is added. In this configuration, the MC-EDFA 340 further amplifies the multiplexed optical signal SIG amplified by SRS via the MCF 310. Note that, for simplification, the outer cladding CL2 is omitted in Fig. 9.

[0048] Like the MCF110, the MCF310 also has cores C1 to C7. However, the diameter D1 of the inner cladding CL1 at the input end of the MCF310 is smaller than the diameter D2 at the output end. The diameter of the inner cladding CL1 changes continuously in the forward (or reverse) direction. Specifically, the diameter of the inner cladding CL1 increases continuously from the input end to the output end.

[0049] In this case, the pump light PL is transmitted in the reverse direction, as in the first embodiment. The power density of the pump light PL at the output end of the MCF 310 is reduced due to the large diameter D2, and this reduces the gain of the Raman amplification. Furthermore, as the distance from the output end of the MCF 310 increases, the power of the pump light decreases due to consumption by SRS.

[0050] That is, in the backward direction, as the power of the pump light PL decreases, the diameter of the inner cladding CL1 also decreases. Therefore, the power density of the pump light PL (i.e., the gain of Raman amplification) is averaged over a long distance in the backward direction. Also, by appropriately designing the change in the diameter of the inner cladding CL1, the power density of the pump light PL (i.e., the amplitude gain due to SRS) can be kept constant over a long distance.

[0051] Since the power density of the pump light PL is averaged or constant, the transmission characteristics of the multiplexed optical signal SIG can be kept stable without change. Therefore, according to this configuration, the multiplexed optical signal SIG can be amplified with higher quality by SRS.

[0052] Fourth embodiment Next, a description will be given of a fiber Raman amplifier according to the fourth embodiment. A configuration of an FRA 40 according to the fourth embodiment is shown in Fig. 10. The FRA 40 has a configuration in which the MCF 310 of the fiber Raman amplifier 30 is replaced with an MCF 410.

[0053] The MCF 410 includes MCFs 410A and 410B. The MCF 410A is disposed on the input side of the MCF 410. The MCF 410B is disposed on the output side of the MCF 410. The MCFs 410A and 410B are spliced ​​together at a splice point 410C.

[0054] Like the MCF 310, the MCFs 410A and 410B also have cores C1 to C7. The diameter of the inner cladding of each of the MCFs 410A and 410B is constant. However, the diameter D3 of the inner cladding of the MCF 410A is smaller than the diameter D4 of the inner cladding of the MCF 410B.

[0055] As with the MCF 310, the power density of the pump light PL can be controlled within the MCF 410 by changing the diameter of the inner cladding. Therefore, as with the third embodiment, the power density of the pump light PL can be averaged or made constant. Therefore, with this configuration, the optical signal can be amplified with higher quality by SRS.

[0056] In this configuration, the diameter of the inner cladding changes in a stepwise manner, which allows for more coarse control of the power density of the pump light PL than FRA30.

[0057] A simulation of the power of the multiplexed optical signal is shown in Fig. 11. Fig. 11 shows the cases of FRA30, FRA40, EDFA only (EDFA-ONLY), and a comparative example. The EDFA only case is a case where amplification is performed only by the MC-EDFA 340 of FRA30. The comparative example is the case of Non-Patent Document 4.

[0058] Here, by arranging three FRAs with 80 km long MCFs in series, the power of the multiplexed optical signal SIG is amplified by the MC-EDFA to a maximum every 80 km. The loss in the 80 km MCF is 0.2 dB / km.

[0059] As shown in Fig. 11, in the case of only EDFA, since attenuation is not compensated for by FRA, the power of the multiplexed optical signal SIG is the smallest and the dynamic range is the largest. In the comparative example of Non-Patent Document 4, the multiplexed optical signal SIG is amplified by SRS so that the power is higher after 40 km than in the case of only EDFA. In general, the higher the minimum power during transmission, the higher the quality of the multiplexed optical signal SIG, such as the OSNR (Optical Signal to Noise Ratio) at the receiving point. Therefore, the quality of the multiplexed optical signal SIG in the comparative example is higher than that in the case of only EDFA.

[0060] Furthermore, in the FRAs 30 and 40, the Raman amplification in the backward direction is controlled by the diameter of the inner cladding of the MCF. As a result, in the FRAs 30 and 40, the power of the multiplexed optical signal SIG is more suitably compensated than in the comparative example. As a result, the minimum power of the multiplexed optical signal SIG in each of the FRAs 30 and 40 is greater than in the comparative example. Therefore, the FRAs 30 and 40 can further improve the quality of the multiplexed optical signal SIG at the receiving point.

[0061] However, as described above, the power density of the pump light PL in the FRA 40 is controlled more roughly than in the FRA 30, so that the power of the multiplexed optical signal SIG in the FRA 40 is reduced more at the bottom than in the FRA 30.

[0062] However, the structure of the MCF310 in the FRA30 is more complicated than that of the MCF410. For this reason, it is relatively difficult to manufacture the MCF310 whose diameter changes continuously. In contrast, the MCF410 has a simple structure in which MCF410A and 410B, which have different diameters, are spliced ​​together. Therefore, although the effect of averaging the power density of the pump light PL is inferior to that of the MCF310, the MCF410 can be manufactured more easily than the MCF310.

[0063] Therefore, with this configuration, it is possible to provide an FRA that can achieve both low-cost manufacturing and high-quality amplification of multiplexed optical signals.

[0064] Fifth embodiment Next, a fiber Raman amplifier according to the fifth embodiment will be described. Fig. 12 shows the configuration of an FRA 50 according to the fifth embodiment. The FRA 50 has a configuration in which the MCF 310 of the FRA 30 is replaced with an MCF 110, the light source 130 is replaced with light sources 530 and 550, and a WDM coupler 560 is added. The WDM coupler 560 is disposed at the input end of the MCF 110.

[0065] The light source 530 (also referred to as the first light source) has lasers 531 and 532 (also referred to as the first laser) which are identical to the lasers L131 and L132. The lasers 531 and 532 emit laser beams L111 and L12, respectively. The laser beams L11 and L12 are multiplexed by the WDM coupler 120, and the multiplexed pump light PL1 (also referred to as the first pump light) is coupled to the inner cladding CL1. The pump light PL1 is transmitted in the reverse direction.

[0066] The light source 550 (also referred to as the second light source) has lasers 551 and 552 (also referred to as the second laser) which are the same as the lasers L131 and L132. The lasers 551 and 552 respectively emit laser beams L21 and L22 (also referred to as the second multimode laser beam). The laser beams L21 and L22 are multiplexed by a WDM coupler 560 (also referred to as the second optical coupler), and the multiplexed pumping light PL2 (also referred to as the second pumping light) is coupled to the inner cladding CL1. The pumping light PL2 is transmitted in the forward direction.

[0067] As described above, the power density of the pump light PL1 decreases as the distance from the output end of the MCF 110 (or the WDM coupler 120) increases. On the other hand, the power density of the pump light PL2 increases as the distance from the output end of the MCF 110 (or the WDM coupler 120) increases. In other words, a decrease in the power density of the pump light PL1 can be compensated for by an increase in the pump light PL2. This makes it possible to average the total power density of the pump light PL1 and PL2. Also, by appropriately designing the MCF 110 and setting the power of the pump light, the total power density of the pump light PL1 and PL2 (i.e., the gain of Raman amplification) can be kept constant over a longer distance.

[0068] A simulation of the power of the multiplexed optical signal is shown in Fig. 13. Fig. 13 shows the cases of FRAs 30, 40, and 50, and EDFA only (EDFA-ONLY). As shown in Fig. 11, the EDFA only case is the case where the signal is amplified only by the MC-EDFA 340 of FRA 30. As shown in Fig. 13, by arranging three FRAs having MCFs with a length of 80 km in series, the power of the multiplexed optical signal SIG is amplified to the maximum every 80 km.

[0069] As shown in Fig. 13, in the case of only EDFA, since attenuation is not compensated by the FRA, the dynamic range of the power of the multiplexed optical signal SIG is the smallest. In the FRA 50, the power of the multiplexed optical signal up to the input end (0 km) 15 km, where Raman amplification is excited by PL2, is high. Since the reduction in power can be compensated for by Raman amplification excited by PL2, the power of the multiplexed optical signal SIG can be reduced. Note that the reduction in power can be realized by appropriately controlling the output power of the optical communication device 101 and the amplification by the MC-EDFA 340 of the previous stage FRA.

[0070] In general, signal distortion due to nonlinear degradation caused by the Kerr effect can be reduced by suppressing the amplification of optical signals. Therefore, according to this configuration, the quality of the multiplexed optical signal at the receiving point can be further improved.

[0071] Sixth embodiment Next, a fiber Raman amplifier according to the sixth embodiment will be described. A configuration of an FRA 60 according to the sixth embodiment is shown in Fig. 14. The FRA 60 has a configuration in which the MCF 110 of the FRA 10 is replaced with an MCF 610.

[0072] The MCF610 has an MCF610A, an MC-EDFA610B, and an MCF610C, and the MCF610A, the MC-EDFA610B, and the MCF610C are connected in series in the forward direction. The MCF610A and the MCF610C have the same configuration as the MCF110. The MC-EDFA610B has cores C1 to C7, similar to the MCF110. The length of the MC-EDFA610B is usually several tens of meters. The length of the MCF610C is usually several tens of kilometers.

[0073] In the light source 130, the center wavelength of the laser 131 is 1480 nm, which is suitable for amplification by an EDFA. The output power of the laser 131 is set higher than the output power of the lasers 132 and 133.

[0074] Next, optical amplification in the MCF 610 will be described. The pump light PL transmitted in the reverse direction is attenuated by the MCF 610C due to Raman amplification. However, the output power of the laser 131 is higher than the output power of the lasers 132 and 133, and the laser light L1 is not completely attenuated by the MCF 610C, so the remaining laser light L1 enters the MC-EDFA 610B. Therefore, the MC-EDFA 610B is pumped by the remaining laser light L1, and the multiplexed optical signal SIG is amplified by the MC-EDFA 610B.

[0075] According to this configuration, the multiplexed optical signal SIG can be amplified not only by the MCF 610C but also by the MC-EDFA 610B without providing an additional laser for exciting the MC-EDFA 610B.

[0076] If an additional laser is required to pump the MC-EDFA 610B, the additional laser is placed at a location tens of kilometers away from the light source 130. In this case, since it is necessary to supply power to a remote location, the power supply configuration becomes relatively large. In contrast, according to the present configuration, since an additional laser is not required to pump the MC-EDFA 610B, the size of the FRA can be suppressed.

[0077] As described above, according to this configuration, it is possible to effectively amplify an optical signal with a compact configuration.

[0078] Other embodiments The present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present invention. For example, the number of cores can be any number equal to or greater than two.

[0079] In the first, second and sixth embodiments, the MC-EDFA can be arranged in the same manner as in the third to fifth embodiments. The number of MC-EDFAs is not limited to one, and a plurality of MC-EDFAs according to the above-mentioned embodiments may be arranged in the FRA. In this case, the MC-EDFA may be arranged in series between the FRA and the optical communication device. Also, instead of the EDFA, another fiber amplifier doped with a rare earth element other than erbium may be used.

[0080] The light sources 130 and 530 may be configured similarly to the light source 230 that mode-multiplexes laser light. In this case, the WDM coupler 120 may be replaced with an SDM coupler such as the SDM coupler 220.

[0081] In the fifth embodiment, the light source 550 may have a configuration similar to that of the light source 230 which mode-multiplexes laser light. In this case, the WDM coupler 560 may be replaced with an SDM coupler such as the SDM coupler 220. In addition, the spectra of the laser light in the light source 560 may be entirely or partially overlapped. [Explanation of symbols]

[0082] 10, 10_1~10_N, 20, 30, 40, 50, 60 Fiber Raman Amplifier (FRA) 100 Optical communication system 101, 102 Optical communication device 110, 310, 410, 410A, 410B, 610, 610A, 610C Multicore Fiber (MCF) 120, 560 WDM coupler 130, 230, 550 light sources 131~132, 531, 532, 551, 552 Laser 220 SDM coupler 230A mode coupler 340, 610B Multi-core Erbium-doped Fiber Amplifier (MC-EDFA) C1~C7 Core CL1 Inner cladding CL2 Outer Cladding L1~L3, L11, L12, L21, L22 laser light PL, PL1, PL2 excitation light

Claims

1. a pumping light source that outputs pumping light used for amplifying signal light; a multi-core fiber constituting a distributed amplifier for the signal light by cladding pumping using the pumping light, the pumping light source outputs multiplexed multimode laser light; The inner cladding of the multicore fiber increases in diameter along the propagation direction of the signal light. Optical amplifier.

2. a lumped amplifier for the signal light, The lumped amplifier includes a multi-core fiber doped with a rare earth element other than erbium.

2. The optical amplifier of claim 1.

3. a lumped amplifier for the signal light, The lumped amplifier is disposed in series with the distributed amplifier or is included within the distributed amplifier.

2. The optical amplifier of claim 1.

4. A pumping light source that outputs pumping light used for amplifying signal light; a multi-core fiber constituting a distributed amplifier for the signal light by cladding pumping using the pumping light; a lumped amplifier for the signal light, the pumping light source outputs multiplexed multimode laser light; The multiplexed multimode laser light from the pump light source is used for both the distributed amplifier and the lumped amplifier. Optical amplifier.

5. The pump light source includes a plurality of laser diodes and an optical coupler that multiplexes a plurality of output lights from the plurality of laser diodes.

5. An optical amplifier according to claim 1.

6. The pump light is multiplexed by at least one of wavelength multiplexing and mode multiplexing.

6. An optical amplifier according to claim 1.

7. The signal light is multiplexed by at least one of wavelength division multiplexing and space division multiplexing.

7. An optical amplifier according to claim 1.

8. The distributed amplifier is capable of supporting multiple bands of optical communication wavelength bands, The multi-band includes at least one of an L band, a C band, and an S band.

8. An optical amplifier according to claim 1.

9. Outputting pump light used for amplifying the signal light; By cladding pumping using the pumping light, the multi-core fiber constitutes a distributed amplifier for the signal light, A multiplexed multimode laser beam is used as the pumping beam, The inner cladding of the multicore fiber increases in diameter along the propagation direction of the signal light. Optical amplification method.

10. An optical amplifier for amplifying a signal light, comprising: By cladding pumping using the pumping light, the multi-core fiber constitutes a distributed amplifier for the signal light, A multiplexed multimode laser beam is used as the pumping beam, The multiplexed multimode laser light is used for both the distributed amplifier and the lumped amplifier for the signal light. Optical amplification method.

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