Pumping light generating device, optical amplifier, and pumping light generating method

Abstract

This excitation light generation device comprises: a first multi-mode laser that outputs first excitation light; a second multi-mode laser that outputs second excitation light; a first excitation current / temperature controller that controls the temperature and excitation current of the first multi-mode laser; a second excitation current / temperature controller that controls the temperature and excitation current of the second multi-mode laser; a first polarization-maintaining variable optical attenuator that adjusts the intensity of light with the polarized state thereof maintained in linear polarization, and outputs the resulting light; a second polarization-maintaining variable optical attenuator that adjusts the intensity of light with the polarized state thereof maintained in linear polarization, and outputs the resulting light; and a polarization-multiplexing circuit that performs polarization-multiplexing and outputs the result. The first and second excitation current / temperature controllers perform control such that the longitudinal modes of the first excitation light and the second excitation light do not overlap. The first and second polarization-maintaining variable optical attenuators perform control so as to provide equal intensities.　

Description

[Technical Field]

[0001] The present invention relates to a pumping means for an optical amplifier. This application claims priority based on PCT / JP2022 / 007892, filed in Japan on February 25, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In the design of high-speed, large-capacity optical transmission systems, it is important to reduce the degradation of the signal-to-noise (SN) ratio of the received signal due to transmission line loss. For this reason, various configurations have been devised to compensate for transmission line loss by optically amplifying the signal in repeaters or in the optical transmission line itself. Among these, optical amplifiers using erbium-doped fiber as a gain medium have been widely used due to their simplicity.

[0003] On the other hand, Raman amplifiers that utilize the Raman effect can realize a wide gain bandwidth, so active efforts are being made to apply them to wavelength division multiplexing transmission systems. In particular, distributed Raman amplification, which uses the optical fiber transmission line itself as the gain medium, has the great advantage of being able to use existing optical fiber as the gain medium, and is therefore expected to be applied to next-generation high-speed, large-capacity optical communications.

[0004] FIG. 10 is a diagram illustrating a configuration example of a conventional optical transmission system 1000 using distributed Raman amplification. The optical transmission system 1000 illustrated in FIG. 10 includes an optical transmitter 100, an optical receiver 200, a forward pumping light generating unit 300, a backward pumping light generating unit 400, a forward pumping light multiplexing unit 310, and a backward pumping light multiplexing unit 410. The optical transmitter 100 and the optical receiver 200 are connected via an optical transmission line 500. The optical transmission system 1000 is designed to perform bidirectional pumping. Therefore, in the optical transmission line 500 of the optical transmission system 1000 illustrated in FIG. 10, forward pumping is performed by the pumping light output by the forward pumping light generating unit 300, and backward pumping is performed by another pumping light output by the backward pumping light generating unit 400. As a result, the optical signal transmitted from the optical transmitter 100 is amplified and reaches the optical receiver 200.

[0005] In the case of Raman amplification, the wavelength of the pump light is set to be approximately 0.1 μm shorter than the wavelength of the optical signal. Since the pump light typically propagates through the core of the optical transmission line 500 in the same manner as the optical signal, the forward pumping light multiplexing unit 310 must multiplex the pump light traveling in the same direction as the optical signal onto the optical signal. Meanwhile, the backward pumping light multiplexing unit 410 must send the pump light traveling in the opposite direction to the optical signal onto the optical transmission line 500 and separate only the optical signal to send it to the optical receiver 200. These multiplexing and demultiplexing operations can be achieved using a wavelength division multiplexing coupler or a circulator. While bidirectional pumping has been described in Figure 10, the pumping direction may be forward-only or backward-only.

[0006] The gain of Raman amplification is determined by the optical intensity of the pump light output from the pump light source. Therefore, fine adjustment of the gain is possible by fine-tuning the optical intensity of the pump light. On the other hand, the gain bandwidth of Raman amplification is determined by the wavelength of the pump light output from the pump light source. A semiconductor laser is usually used as the pump light source for Raman amplification, and its optical intensity and wavelength can be adjusted by adjusting the pump current and temperature.

[0007] Incidentally, semiconductor lasers used as pump light sources for Raman amplification are often multimode lasers. The output of a multimode laser is not a single wavelength, but rather emits light of multiple wavelengths simultaneously. These multiple lights are called longitudinal modes. The intensity and wavelength of each longitudinal mode change with changes in the pump current and temperature. However, the optical frequency spacing of the longitudinal modes remains roughly the same, as it is determined by the cavity length of the multimode laser.

[0008] Another factor that determines the gain of Raman amplification is the polarization of the pump light. Because Raman amplification is an optical effect that is polarization-dependent, if the pump light is single-polarized or if the pump light is depolarized but not ideally depolarized, the gain of the optical signal becomes polarization-dependent. That is, the gain changes depending on the polarization state of the optical signal when it enters the optical transmission line 500, and the optical intensity of the amplified optical signal also changes. This gain fluctuation is called PDG (Polarization Dependent Gain). PDG is particularly pronounced in configurations that use only forward pumping. In backward pumping, polarization fluctuations within the optical transmission line differ significantly due to the difference in the propagation direction of the optical signal and the pump light. Therefore, although PDG is smaller than in forward pumping, some means are required to completely suppress it.

[0009] As one means for suppressing PDG, the outputs of an even number of multimode lasers are polarization-multiplexed and made unpolarized inside the forward pumping light generating unit 300 or the backward pumping light generating unit 400. Fig. 11 is a diagram showing a configuration example in which two multimode lasers are provided inside the forward pumping light generating unit 300 or the backward pumping light generating unit 400. Fig. 11 shows an example in which two multimode lasers are provided inside the forward pumping light generating unit 300. Note that the configuration shown in Fig. 11 may also be provided inside the backward pumping light generating unit 400.

[0010] The forward pumping light generating unit 300 includes a first multimode laser 10, a second multimode laser 11, a first pumping current / temperature controller 12, a second pumping current / temperature controller 13, a first polarization-maintaining optical waveguide 14, a second polarization-maintaining optical waveguide 15, and a PBC (Polarization Beam Combiner) 16. The wavelengths and optical intensities of the first multimode laser 10 and the second multimode laser 11 are controlled by the first pumping current / temperature controller 12 and the second pumping current / temperature controller 13, respectively. The first multimode laser 10 outputs first pumping light having a wavelength and optical intensity controlled by the first pumping current / temperature controller 12. The second multimode laser 11 outputs second pumping light having a wavelength and optical intensity controlled by the second pumping current / temperature controller 13.

[0011] The first pump light output from the first multimode laser 10 propagates through the first polarization-maintaining optical waveguide 14 and is input to the PBC 16. Furthermore, the second pump light output from the second multimode laser 11 propagates through the second polarization-maintaining optical waveguide 15 and is input to the PBC 16. The PBC 16 polarization-multiplexes the input first pump light and second pump light, and outputs depolarized pump light.

[0012] As another means for suppressing PDG, it is also possible to use a depolarizer using a passive optical circuit as described in Non-Patent Document 1. A detailed description thereof will be omitted here.

[0013] Here, in order to perform stable Raman amplification, the following three conditions must be met for the first pump light and the second pump light. (First condition) The central wavelengths of the first and second pump lights must be approximately the same. (Second condition) The light intensities of the first and second excitation lights must be the same. (Third condition) The longitudinal modes of the first pump light and the second pump light must be arranged so as not to overlap.

[0014] The first condition must be satisfied because, as the first and second pump lights propagate through the optical transmission line 500, slight anisotropy of the optical transmission line 500 causes polarization rotation. However, because polarization rotation is wavelength-dependent, if the center wavelengths of the first and second pump lights differ, the polarization orthogonality between the two cannot be maintained. The second condition must be satisfied because PDG occurs if the optical intensities of the first and second pump lights differ. The third condition must be satisfied because, if the longitudinal modes of the first and second pump lights overlap, a large amount of noise is superimposed on the light amplified by Raman amplification (see, for example, Non-Patent Document 2). The cause of this noise can be explained by fluctuations in synthesized polarization, as discussed in Non-Patent Document 1, but details will not be discussed here. Non-Patent Document 2 shows that in order to suppress the generation of this noise, the longitudinal modes of the first multimode laser 10 and the second multimode laser 11 should be arranged alternately, as shown in FIG. 12.

[0015] 12 is a schematic diagram of the optical spectrum output from each of the first multimode laser 10 and the second multimode laser 11. In FIG. 12, the optical frequency of the longitudinal mode output from the first multimode laser 10 is represented as f 1_1 , f 1_2 , f 1_5 Similarly, in FIG. 12, the optical frequency of the longitudinal mode output from the second multimode laser 11 is expressed as f 2_1 , f 2_2 , f 2_5 It is written as follows. [Prior art documents] [Non-patent literature]

[0016] [Non-Patent Document 1] Hiroto Kawakami et al., “Suppression of Intensity Noises in Forward-pumped Raman Amplifier Utilizing Depolarizer for Multiple Pump Laser Sources,” J. Lightw. Technol., Vol.39, PP.7417-7426, 2021. [Non-patent document 2] Catherine Martinelli et al., “RIN Transfer in Copumped Raman Amplifiers Using Polarization-Combined Diodes,” Photonics. Technol. Lett., Vol.17, PP.1836-1838, 2005. Summary of the Invention [Problem to be solved by the invention]

[0017] However, the conventional configuration shown in Figure 11 has the following problems. As mentioned above, the optical frequency and intensity of each longitudinal mode of a multimode laser must satisfy three conditions. It is relatively easy to satisfy one or two of the three conditions by selecting a laser and adjusting the pump current and temperature. However, because adjusting the pump current and temperature affects both the optical frequency and intensity of each longitudinal mode, it is difficult to simultaneously satisfy all three of the above conditions. Even if all three conditions could be satisfied, if the gain of the Raman amplifier needs to be changed, fine adjustments would need to be made again.

[0018] Another problem is that when the longitudinal modes are arranged alternately, four optical signals are mixed (four-wave mixing) between the multiple longitudinal modes of the pump light and the optical signal, causing signal degradation inside the optical transmission line 500. As mentioned above, the optical signal and the pump light are 0.1 μm apart, and four-wave mixing that occurs at wavelengths this far apart can usually be ignored, but the pump light used in Raman amplification is generally extremely high-power, so there is a problem in that the impact on signal quality cannot be ignored.

[0019] In view of the above circumstances, an object of the present invention is to provide a technology that can suppress degradation in the signal quality of an amplified optical signal when Raman amplification is performed using pump light obtained by polarization multiplexing the outputs of an even number of multimode lasers. [Means for solving the problem]

[0020] One aspect of the present invention includes a first multimode laser that outputs first pump light, a second multimode laser that outputs second pump light, a first pump current / temperature controller that controls the temperature and pump current of the first multimode laser, a second pump current / temperature controller that controls the temperature and pump current of the second multimode laser, a first polarization-maintaining variable optical attenuator that receives the first pump light as input, adjusts the optical intensity of the first pump light while maintaining the polarization state of the first pump light as linearly polarized, and outputs the adjusted optical intensity, a second polarization-maintaining variable optical attenuator that receives the second pump light as input, adjusts the optical intensity of the second pump light while maintaining the polarization state of the second pump light as linearly polarized, and a control circuit for controlling the first polarization-maintaining variable optical attenuator. and a polarization multiplexing circuit that polarization-multiplexes and outputs the first pumping light, the light intensity of which has been adjusted by the second polarization-maintaining optical variable attenuator, and the second pumping light, the light intensity of which has been adjusted by the second polarization-maintaining optical variable attenuator, wherein the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping currents or temperatures of the first multimode laser and the second multimode laser so that the longitudinal modes contained in the first pumping light and the longitudinal modes contained in the second pumping light do not overlap, and the first polarization-maintaining optical variable attenuator and the second polarization-maintaining optical variable attenuator control the intensities of the first pumping light and the second pumping light so that they are equal.

[0021] One aspect of the present invention includes a first multimode laser that outputs first pump light, a second multimode laser that outputs second pump light, a first pump current / temperature controller that controls the temperature and pump current of the first multimode laser, a second pump current / temperature controller that controls the temperature and pump current of the second multimode laser, a first polarization-maintaining optical amplifier that receives the first pump light as an input, amplifies the optical intensity of the first pump light while maintaining the polarization state of the first pump light as linearly polarized, and outputs the amplified optical intensity, a second polarization-maintaining optical amplifier that receives the second pump light as an input, and amplifies the optical intensity of the second pump light while maintaining the polarization state of the second pump light as linearly polarized, and a polarization-maintaining optical amplifier operated by the first polarization-maintaining optical amplifier. and a polarization multiplexing circuit that polarization-multiplexes and outputs the first pumping light, the light intensity of which has been amplified by the second polarization-maintaining optical amplifier, and the second pumping light, the light intensity of which has been amplified by the second polarization-maintaining optical amplifier, wherein the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping currents or temperatures of the first multimode laser and the second multimode laser so that a longitudinal mode contained in the first pumping light and a longitudinal mode contained in the second pumping light do not overlap, and the first polarization-maintaining optical amplifier and the second polarization-maintaining optical amplifier control so that the intensities of the first pumping light and the second pumping light are equal.

[0022] One aspect of the present invention includes a first multimode laser that outputs first pump light having a longitudinal mode frequency interval of Δf1, a second multimode laser that outputs second pump light having a longitudinal mode frequency interval of Δf2, a first pump current / temperature controller that controls the temperature and pump current of the first multimode laser, a second pump current / temperature controller that controls the temperature and pump current of the second multimode laser, a polarization multiplexing circuit that polarization-multiplexes the first pump light and the second pump light and outputs the resultant signal, and a polarization multiplexing circuit that multiplexes an optical signal and a signal output from the polarization multiplexing circuit. and a gain medium into which all of the first pumping light and the second pumping light inputted thereto are inputted and which amplifies and outputs the optical signal, wherein the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping currents and temperatures of the first multimode laser and the second multimode laser so that a longitudinal mode included in the first pumping light and a longitudinal mode included in the second pumping light do not overlap, and the optical signal amplified by the first pumping light and the second pumping light is outputted at a baud rate f B When the digital signal is a signal of the longitudinal mode, the frequency intervals δf1 and δf2 are equal to the baud rate f B It is an optical amplifier device larger than

[0023] In one aspect of the present invention, a first multimode laser outputs first pump light, a second multimode laser outputs second pump light, a first pump current / temperature controller controls the temperature and pump current of the first multimode laser, a second pump current / temperature controller controls the temperature and pump current of the second multimode laser, a first light intensity changer receives the first pump light as an input, changes the light intensity of the first pump light while maintaining the polarization state of the first pump light as linearly polarized, and outputs the changed light, a second light intensity changer receives the second pump light as an input, changes the light intensity of the second pump light while maintaining the polarization state of the second pump light as linearly polarized, and a polarization multiplexing circuit controls the The first pumping light, the light intensity of which has been changed by a first light intensity changer, and the second pumping light, the light intensity of which has been changed by a second light intensity changer, are polarization-multiplexed and output; the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping currents and temperatures of the first multimode laser and the second multimode laser so that the longitudinal modes contained in the first pumping light and the longitudinal modes contained in the second pumping light do not overlap; and the first light intensity changer and the second light intensity changer control so that the intensities of the first pumping light and the second pumping light are equal. [Effects of the Invention]

[0024] According to the present invention, when Raman amplification is performed using pump light obtained by polarization multiplexing the outputs of an even number of multimode lasers, it is possible to suppress degradation of the signal quality of the amplified optical signal. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 2 is a diagram illustrating an example of the configuration of an excitation light generating unit in the first embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the arrangement of longitudinal modes of the first pump light and the second pump light. [Figure 3] 10 is a schematic diagram showing another example of the arrangement of longitudinal modes of the first pumping light and the second pumping light. FIG. [Figure 4]5 is a flowchart showing a processing flow of an excitation light generator in the first embodiment. [Figure 5] FIG. 10 is a diagram illustrating a configuration example of an excitation light generating section in a modified example of the first embodiment. [Figure 6] FIG. 10 is a diagram illustrating a configuration example of an excitation light generating unit in a second embodiment. [Figure 7] FIG. 10 is a diagram illustrating a configuration example of an excitation light generating unit in a third embodiment. [Figure 8] FIG. 10 is a diagram illustrating an example of the configuration of an optical amplifier according to a fourth embodiment. [Figure 9] FIG. 13 is a diagram illustrating an example of the configuration of an optical amplifier according to a modification of the fourth embodiment. [Figure 10] FIG. 1 is a diagram illustrating an example of the configuration of a conventional optical transmission system using distributed Raman amplification. [Figure 11] FIG. 10 is a diagram showing a configuration example in which two multimode lasers are provided inside the forward pumping light generating unit or the backward pumping light generating unit. [Figure 12] 3 is a schematic diagram of the optical spectra output from a first multimode laser and a second multimode laser. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The system configuration of the optical transmission system according to the present invention is the same as the system configuration shown in Fig. 10. What differs from the conventional optical transmission system is the internal configuration of the forward pumping light generating unit 300 or the backward pumping light generating unit 400. Therefore, in the following description, the configuration that characterizes the present invention will be described.

[0027] (First embodiment) Fig. 1 is a diagram showing an example of the configuration of an excitation light generating unit 50 in the first embodiment. The excitation light generating unit 50 is either a forward excitation light generating unit 300 or a backward excitation light generating unit 400. The excitation light generating unit 50 is one aspect of an excitation light generating device. In the excitation light generating unit 50 shown in Fig. 1, components that are common to the configuration shown in Fig. 11 are assigned the same numbers.

[0028] The pump light generating unit 50 includes a first multimode laser 10, a second multimode laser 11, a first pump current / temperature controller 12, a second pump current / temperature controller 13, a first polarization-maintaining optical waveguide 14, a second polarization-maintaining optical waveguide 15, a PBC 16, a first polarization-maintaining VOA (variable optical attenuator) 20, and a second polarization-maintaining VOA 21.

[0029] The wavelength and light intensity of the first multimode laser 10 and the second multimode laser 11 are controlled by a first excitation current / temperature controller 12 and a second excitation current / temperature controller 13, respectively. The first multimode laser 10 outputs first excitation light having a wavelength and light intensity controlled by the first excitation current / temperature controller 12. The second multimode laser 11 outputs second excitation light having a wavelength and light intensity controlled by the second excitation current / temperature controller 13. The first multimode laser 10 and the second multimode laser 11 output approximately the same wavelength.

[0030] The first excitation current / temperature controller 12 controls the first multimode laser 10. Specifically, the first excitation current / temperature controller 12 aligns the cavity length of the first multimode laser 10 with the cavity length of the second multimode laser 11, and controls the excitation current and temperature of the first multimode laser 10 to be approximately the same as the excitation current and temperature of the second multimode laser 11. The first excitation current / temperature controller 12 may control at least either the excitation current or the temperature of the first multimode laser 10.

[0031] The second excitation current / temperature controller 13 controls the second multimode laser 11. Specifically, the second excitation current / temperature controller 13 aligns the cavity length of the second multimode laser 11 with the cavity length of the first multimode laser 10, and controls the excitation current and temperature of the second multimode laser 11 to be approximately the same as the excitation current and temperature of the first multimode laser 10. The second excitation current / temperature controller 13 may control at least either the excitation current or the temperature of the second multimode laser 11.

[0032] Of the three conditions for stable Raman amplification, the first condition (the central wavelengths of the first pump light and the second pump light must be approximately the same) can be relatively easily achieved by aligning the cavity lengths of the first multimode laser 10 and the second multimode laser 11 as described above, and by using the first pump current / temperature controller 12 and the second pump current / temperature controller 13 to align the pump currents and temperatures of the first multimode laser 10 and the second multimode laser 11 to be approximately the same.

[0033] Next, the third of the three conditions (the longitudinal modes of the first and second pumping lights must be arranged so as not to overlap) can be achieved by slightly differentiating the temperatures of the first multimode laser 10 and the second multimode laser 11 using the first pumping current / temperature controller 12 and the second pumping current / temperature controller 13. The pumping currents and temperatures obtained in the above explanation are fixed and will not be changed in the subsequent fine adjustments.

[0034] The first polarization-maintaining VOA 20 is disposed in the first polarization-maintaining optical waveguide 14, and adjusts the optical intensity of the first pump light output from the first multimode laser 10. The first polarization-maintaining VOA 20 is one aspect of the first optical intensity changer.

[0035] The second polarization-maintaining VOA 21 is disposed in the second polarization-maintaining optical waveguide 15, and adjusts the optical intensity of the second pump light output from the second multimode laser 11. The second polarization-maintaining VOA 21 is one aspect of the second optical intensity changer.

[0036] The second of the three conditions (the optical intensities of the first and second pump lights must be the same) is realized by fine-tuning the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21. The first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21 change only the optical intensity of the pump lights and do not affect the optical frequencies of each longitudinal mode, so the first and third conditions remain satisfied as described above.

[0037] However, a high gain in Raman amplification is not necessarily a good thing. Too high a gain can cause nonlinear optical effects in the optical signal, resulting in degradation of signal quality. To avoid this, the intensity of the pump light can be reduced. In this case, rather than changing the pump current as is often done in conventional technology, the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21 are simultaneously changed to increase the optical loss by the same amount, thereby adjusting the intensity of the pump light.

[0038] The PBC 16 polarization-multiplexes the first pump light whose optical intensity has been adjusted by the first polarization-maintaining VOA 20 and the second pump light whose optical intensity has been adjusted by the second polarization-maintaining VOA 21, and outputs depolarized pump light. The PBC 16 is one aspect of a polarization multiplexing circuit.

[0039] Next, the longitudinal mode arrangement of the first pump light and the second pump light will be described with reference to Figs. 2 and 3. Fig. 2 is a schematic diagram of the longitudinal mode arrangement of the first pump light and the second pump light to be achieved by the first pump current / temperature controller 12 and the second pump current / temperature controller 13. As in Fig. 10, the optical frequency of the longitudinal mode output from the first multimode laser 10 is expressed as f 1_1 ,f 1_2 ,···,f 1_5and the optical frequency of the longitudinal mode output from the second multimode laser 11 is expressed as f 2_1 ,f 2_2 ,···,f 2_5 It is written as follows.

[0040] First, let us focus on one of the longitudinal modes. In FIG. 2, the optical frequency f 2_3 Next, an optical frequency f 2_3 Larger than the optical frequency f 2_3 and the longitudinal mode closest to the optical frequency f 2_3 smaller than the optical frequency f 2_3 In Figure 2, the optical frequency f 1_4 and the optical frequency f 1_3 These correspond to:

[0041] Here, the optical frequency f 2_3 -f 1_3 Δf 2+ , optical frequency f 2_3 -f 1_4 Δf 2- In this embodiment, |Δf 2+ | and |Δf 2- Each longitudinal mode is set so that the optical frequency f 2_3 The explanation focused on the optical frequency f 2_3 When focusing on any longitudinal mode other than |Δf 2+ | and |Δf 2- Set it so that it is not equal to |.

[0042] The above conditions are satisfied when the first multimode laser 10 and the second multimode laser 11 are interchanged and Δf 1+ and Δf 1- Similarly, when we define |Δf 1+ | and |Δf 1- By setting it in this way, the light generated by four-wave mixing will not be spaced equally, and it will be possible to disperse the optical noise.

[0043] (Another example of the arrangement of the longitudinal modes of the first pump light and the second pump light) In Fig. 2, the explanation has been given assuming that the longitudinal mode spacing of the output of the first multimode laser 10 is equal to the longitudinal mode spacing of the output of the second multimode laser 11. Next, with reference to Fig. 3, a case will be explained in which the longitudinal mode spacing of the output of the first multimode laser 10 is wider than the longitudinal mode spacing of the output of the second multimode laser 11. In Fig. 3, the optical frequency f 2_4 is selected for the illustration, but here too |Δf 2+ | and |Δf 2- The first excitation current / temperature controller 12 and the second excitation current / temperature controller 13 control the first multimode laser 10 and the second multimode laser 11 so that | and | are not equal.

[0044] In Figure 3, the optical frequency f 2_4 Select based on f 2+ and f 2- However, f 2_3 Based on |Δf 2+ | and |Δf 2- |, the optical frequency f 2_4 Compared to the case where |Δf 2+ | decreases and |Δf 2- In this embodiment, regardless of which longitudinal mode is selected, |Δf 2+ | and |Δf 2- | and |Δf 2+ | and |Δf 2- The longitudinal mode arrangement is selected so that the number of combinations where | is equal is minimized.

[0045] Incidentally, in FIGS. 2 and 3, the total number of longitudinal modes output from the first multi-mode laser 10 and the total number of longitudinal modes output from the second multi-mode laser 11 are each set to 5. However, in an actual multi-mode laser, particularly a multi-mode laser that does not use a fiber Bragg grating, an extremely large number of longitudinal modes are generated. Therefore, when the longitudinal mode intervals of the outputs of the first multi-mode laser 10 and the second multi-mode laser 11 are not equal, it becomes very difficult to set them such that |Δf 2+ | and |Δf 2- | are always different values.

[0046] In such a case, a constant R satisfying 0 < R < 1 is determined in advance. When expressing the maximum optical power among the optical frequencies f 2_1 , f 2_2 , ··· as P 2_max , regarding the longitudinal modes of the output of the second multi-mode laser 11 having an optical power lower than P 2_max ×R, it is allowed that |Δf 2+ | and |Δf 2- | are equal. And when expressing the maximum optical power among f 1_1 , f 1_2 , ··· as P 1_max , regarding the longitudinal modes of the output of the first multi-mode laser 10 having an optical power lower than P 1_max ×R, the condition may be relaxed to allow |Δf 1+ | and |Δf 1- | to be equal. Here, how to set the value of R is not obvious because it strongly depends on the spectrum of the multi-mode laser. As one guideline, it may be selected such that the relative intensity noise (RIN) of the Raman-amplified light is minimized. Alternatively, as a more convenient method, an optical band-pass filter may be placed at the output of the PBC16 to suppress the longitudinal modes around the first multi-mode laser 10 and the second multi-mode laser 11 and reduce the number of longitudinal modes.

[0047] FIG. 4 is a flowchart showing the flow of processing by the excitation light generating unit 50 in the first embodiment. The first excitation current / temperature controller 12 and the second excitation current / temperature controller 13 control the first multimode laser 10 and the second multimode laser 11 (step S101). Specifically, the first excitation current / temperature controller 12 aligns the cavity length of the first multimode laser 10 and controls the excitation current and temperature of the first multimode laser 10 to be approximately the same as the excitation current and temperature of the second multimode laser 11. The second excitation current / temperature controller 13 aligns the cavity length of the second multimode laser 11 and controls the excitation current and temperature of the second multimode laser 11 to be approximately the same as the excitation current and temperature of the first multimode laser 10.

[0048] The first multimode laser 10 and the second multimode laser 11 output pumping light (step S102). Specifically, the first multimode laser 10 is controlled by the first pumping current / temperature controller 12, and then outputs the first pumping light to the first polarization-maintaining optical waveguide 14. The second multimode laser 11 is controlled by the second pumping current / temperature controller 13, and then outputs the second pumping light to the second polarization-maintaining optical waveguide 15.

[0049] The first pump light propagating through the first polarization-maintaining optical waveguide 14 is input to the first polarization-maintaining VOA 20. The second pump light propagating through the second polarization-maintaining optical waveguide 15 is input to the second polarization-maintaining VOA 21. The first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21 adjust the optical intensity of the input pump light (step S103). Specifically, the first polarization-maintaining VOA 20 adjusts the optical intensity of the input first pump light, and the second polarization-maintaining VOA 21 adjusts the optical intensity of the input second pump light. To satisfy the second condition, the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21 adjust the optical intensity of the first pump light and the optical intensity of the second pump light so that they are the same.

[0050] The first polarization-maintaining VOA 20 outputs the first pump light whose optical intensity has been adjusted to the PBC 16. The second polarization-maintaining VOA 21 outputs the second pump light whose optical intensity has been adjusted to the PBC 16. The PBC 16 polarization-multiplexes the first pump light whose optical intensity has been adjusted by the first polarization-maintaining VOA 20 and the second pump light whose optical intensity has been adjusted by the second polarization-maintaining VOA 21 (step S104). As a result, the PBC 16 generates depolarized pump light. The PBC 16 outputs the depolarized pump light.

[0051] The pump light generating unit 50 configured as described above can suppress degradation of the signal quality of the amplified optical signal when Raman amplification is performed using pump light obtained by polarization multiplexing the outputs of an even number of multimode lasers. Specifically, all three conditions must be satisfied to perform stable Raman amplification. The pump light generating unit 50 can satisfy the first and third conditions by controlling the first pump current / temperature controller 12 and the second pump current / temperature controller 13, and can also satisfy the second condition by adjusting the optical intensity using the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21. As a result, degradation of the signal quality of the amplified optical signal can be suppressed.

[0052] (Modification 1 of the first embodiment) In the above embodiment, if it is known that the optical intensity of the output of the first multimode laser 10 will always be higher (or lower) than the optical intensity of the output of the second multimode laser 11 when the first and second conditions out of the three conditions are satisfied, the second polarization-maintaining VOA 21 (or the first polarization-maintaining VOA 20) can be omitted from the configuration of the pump light generating unit 50. However, in this case, it becomes difficult to change the intensity of the pump light after polarization multiplexing.

[0053] (Modification 2 of the first embodiment) The first excitation current / temperature controller 12 and the second excitation current / temperature controller 13 may be configured to control at least one of the excitation current and the temperature of the first multimode laser 10 and the second multimode laser 11 so that the longitudinal modes contained in the first excitation light and the longitudinal modes contained in the second excitation light do not overlap and the first excitation light has higher power than the second excitation light.

[0054] (Modification 3 of the first embodiment) The excitation light generating section 50 may be modified to the configuration shown in Fig. 3. Fig. 5 is a diagram showing an example of the configuration of an excitation light generating section 50a in a modification of the first embodiment. The excitation light generating section 50a is either a forward excitation light generating section 300 or a backward excitation light generating section 400. The excitation light generating section 50a is one aspect of an excitation light generating device. In the excitation light generating section 50a shown in Fig. 5, components common to the configuration shown in Fig. 1 are assigned the same numbers.

[0055] The pump light generating unit 50a includes a first multimode laser 10, a second multimode laser 11, a first pump current / temperature controller 12, a second pump current / temperature controller 13, a first polarization-maintaining optical waveguide 14, a second polarization-maintaining optical waveguide 15, a PBC 16, a first polarization-maintaining optical amplifier 30, and a second polarization-maintaining optical amplifier 31.

[0056] The pump light generating unit 50a differs in configuration from the pump light generating unit 50 in that it includes a first polarization-maintaining optical amplifier 30 and a second polarization-maintaining optical amplifier 31 instead of the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21. The differences from the pump light generating unit 50 will be described below.

[0057] The first polarization-maintaining optical amplifier 30 adjusts the optical intensity of the first pump light. The second polarization-maintaining optical amplifier 31 adjusts the optical intensity of the second pump light. To satisfy the second condition, the first polarization-maintaining optical amplifier 30 and the second polarization-maintaining optical amplifier 31 adjust the optical intensity of the first pump light to be the same as the optical intensity of the second pump light. As the first polarization-maintaining optical amplifier 30 and the second polarization-maintaining optical amplifier 31, for example, a semiconductor optical amplifier can be used.

[0058] The intensities of the first and second pump light can be made equal by fine-tuning the gain of either the first polarization-maintaining optical amplifier 30 or the second polarization-maintaining optical amplifier 31. The intensity of the pump light after polarization multiplexing can be changed by adjusting the gain of both the first polarization-maintaining optical amplifier 30 and the second polarization-maintaining optical amplifier 31. Generally, designing a high-power laser involves technical difficulties. However, in this embodiment, the pump light is amplified by the first polarization-maintaining optical amplifier 30 and the second polarization-maintaining optical amplifier 31, so the specifications for the first multimode laser 10 and the second multimode laser 11 can be relaxed. Unlike the above-described embodiment, the absence of first and second polarization-maintaining VOAs, which act as loss media, makes it possible to minimize the loss of the pump light.

[0059] (Second embodiment) Fig. 6 is a diagram showing an example of the configuration of an excitation light generating unit 50b in the second embodiment. The excitation light generating unit 50b is either a forward excitation light generating unit 300 or a backward excitation light generating unit 400. The excitation light generating unit 50b is one aspect of an excitation light generating device. In the excitation light generating unit 50b shown in Fig. 6, components common to the configuration shown in Fig. 1 are assigned the same numbers.

[0060] The pump light generating unit 50b includes a first multimode laser 10, a second multimode laser 11, a first pump current / temperature controller 12, a second pump current / temperature controller 13, a first polarization-maintaining optical waveguide 14, a second polarization-maintaining optical waveguide 15, a PBC 16, a first polarization-maintaining VOA 20, a second polarization-maintaining VOA 21, a first isolator 22, and a second isolator 23.

[0061] The pumping light generating section 50b differs in configuration from the pumping light generating section 50 in that it further includes a first isolator 22 and a second isolator 23. The differences from the pumping light generating section 50 will be described below.

[0062] The first isolator 22 is provided between the first multimode laser 10 and the first polarization-maintaining VOA 20, and blocks input of reflected light from the first polarization-maintaining VOA 20. In this way, the first isolator 22 blocks input of reflected light from the first polarization-maintaining VOA 20 to the first multimode laser 10. The first isolator 22 is one aspect of a first optical intensity modifier.

[0063] The second isolator 23 is provided between the second multimode laser 11 and the second polarization-maintaining VOA 21, and blocks the input of reflected light from the second polarization-maintaining VOA 21. In this way, the second isolator 23 blocks the input of reflected light from the second polarization-maintaining VOA 21 to the second multimode laser 11. The second isolator 23 is one aspect of the second optical intensity changer.

[0064] Due to the configuration of the optical circuit, it is difficult to completely eliminate optical reflection from the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21. However, it is known that the output of a semiconductor laser can become unstable due to reflected light flowing back from the outside. By blocking this reflected light with the first isolator 22 and the second isolator 23, it becomes possible to improve the stability of the first multimode laser 10 and the second multimode laser 11.

[0065] (Modification 1 of the second embodiment) The pump light generating unit 50b may be configured to include a first polarization-maintaining optical amplifier 30 and a second polarization-maintaining optical amplifier 31 instead of the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21, as in the first embodiment.

[0066] (Third embodiment) 7 is a diagram showing a configuration example of an excitation light generating unit 50c in the third embodiment. The excitation light generating unit 50c is either a forward excitation light generating unit 300 or a backward excitation light generating unit 400. The excitation light generating unit 50c is one aspect of an excitation light generating device. In the excitation light generating unit 50c shown in FIG. 7, components common to the configuration shown in FIG. 1 are assigned the same numbers.

[0067] The pump light generating unit 50c includes a first multimode laser 10, a second multimode laser 11, a first pump current / temperature controller 12, a second pump current / temperature controller 13, a first polarization-maintaining optical waveguide 14, a second polarization-maintaining optical waveguide 15, a PBC 16, a first polarization-maintaining VOA 20, a second polarization-maintaining VOA 21, a first polarizer 24, and a second polarizer 25.

[0068] The excitation light generating section 50c differs in configuration from the excitation light generating section 50 in that it further includes a first polarizer 24 and a second polarizer 25. The differences from the excitation light generating section 50 will be described below.

[0069] The first polarizer 24 is provided between the first multimode laser 10 and the first polarization-maintaining VOA 20, and transmits only the single linearly polarized wave of the first pump light output from the first multimode laser 10.

[0070] The second polarizer 25 is provided between the second multimode laser 11 and the second polarization-maintaining VOA 21, and transmits only the single linearly polarized wave of the second pump light output from the second multimode laser 11.

[0071] Generally, the optical output of a semiconductor laser is a single linearly polarized wave, and the first polarization-maintaining optical waveguide 14 and the second polarization-maintaining optical waveguide 15 maintain this linear polarization while propagating the light. However, because the polarization extinction ratio is finite, each longitudinal mode cannot maintain a single polarization perfectly, and slight polarization rotation may occur. When a polarization other than linear polarization is input to the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21, the optical output is often not guaranteed, which may cause operational instability of the pump light.

[0072] Therefore, in the third embodiment, linear polarization is ensured by installing a first polarizer 24 in front of the first polarization-maintaining VOA 20 and a second polarizer 25 in front of the second polarization-maintaining VOA 21. As a result, more stable pump light output is possible.

[0073] (Modification 1 of the third embodiment) Depending on the configuration of the polarizer, it may also function as an isolator, so the pump light generating unit 50c may be configured to include an isolator as in the second embodiment. In such a configuration, the isolator may be installed, for example, between the polarizer and the polarization-maintaining VOA.

[0074] (Modification 2 of the third embodiment) The pump light generating unit 50c may be configured to include a first polarization-maintaining optical amplifier 30 and a second polarization-maintaining optical amplifier 31 instead of the first polarization-maintaining VOA 20 and the second polarization-maintaining VOA 21, as in the first embodiment.

[0075] (Modifications common to the first to third embodiments) The pump light generating unit in each embodiment has been described as generating pump light in an optical transmission system using Raman amplification, but it may also be used for purposes other than Raman amplification, for example, to generate pump light for pumping an optical fiber doped with a rare earth element.

[0076] In each embodiment, a configuration using two multimode lasers that output approximately the same wavelength has been described. In each embodiment, for example, two multimode lasers that output a wavelength of 1.45 μm and two multimode lasers that output a wavelength of 1.49 μm may be combined using a wavelength division multiplexing coupler, thereby configuring to use an even number of multimode lasers greater than two.

[0077] (Fourth embodiment) In the first to third embodiments described above, the distance between the longitudinal modes of the first multimode laser 10 or the second multimode laser 11 arranged inside the forward pumping light generating unit 300 or the backward pumping light generating unit 400 shown in FIG. 10, i.e., Δf 1+ , Δf 1- , Δf 2+ and Δf 2- However, the first to third embodiments described above do not mention the band of the optical signal output from the optical transmitter 100 shown in Fig. 10 or the wavelength spacing when the optical signal output from the optical transmitter 100 is a wavelength multiplexed signal. Therefore, in the fourth embodiment, a configuration of an optical amplifier that takes these values ​​into consideration will be described.

[0078] 8 is a diagram showing an example of the configuration of an optical amplifier according to the fourth embodiment. The optical amplifier includes a forward pumping light generating unit 50, a forward pumping light multiplexing unit 310, a gain medium 501, and an optical filter 502. An optical signal output from the optical transmitter 100 is input to the gain medium 501 via the forward pumping light multiplexing unit 310. This optical signal is a single carrier wavelength signal having a baud rate of f b The gain medium 501 may perform distributed amplification using an optical transmission line as shown in Fig. 10, or may be a compact optical amplifier using a relatively short optical waveguide.

[0079] The forward pumping light generating unit 50 has an internal configuration in which the first multimode laser 10 and the second multimode laser 11 are polarization-multiplexed. In the example shown in Fig. 8, the internal configuration of the forward pumping light generating unit 50 uses the configuration shown in Fig. 1 described in the first embodiment, but is not limited to this, and for example, the configuration shown in Fig. 6 described in the second embodiment may also be used.

[0080] The polarization-multiplexed pump light is input to the gain medium 501 via the forward pumping light multiplexing unit 310. In this embodiment, optical amplification is performed by forward pumping only. However, as shown in FIG. 10, bidirectional pumping may be performed using a backward pumping light multiplexing unit 410 and a backward pumping light generating unit 400, or optical amplification may be performed by backward pumping only. In such a configuration, the optical amplifying device also includes components such as the backward pumping light multiplexing unit 410 and the backward pumping light generating unit 400. The light amplified by the gain medium 501 passes through the optical filter 502. The optical filter 502 blocks the remaining pump light. If the gain medium 501 has a high absorption of the pump light, the optical filter 502 may be omitted.

[0081] Here, let us consider what kind of noise components the first pumping light output from the first multimode laser 10 or the second pumping light output from the second multimode laser 11 each has before being multiplexed. As shown in Figure 2, these pumping lights can be considered as a collection of multiple CW lights that maintain a constant frequency interval. Here, the longitudinal mode interval of the first multimode laser 10, that is, f 1_n+1 -f 1_n Similarly, the longitudinal mode spacing of the second multimode laser, i.e., f 2_n+1 -f 2_n is defined as δf2. As is clear from Figure 2, δf1 = Δf 2+ +Δf 2- and δf2=Δf 1+ +Δf 1- This becomes:

[0082] Let us assume that the first multimode laser 10 is a mode-locked laser. In a mode-locked laser, the relative optical phases of the longitudinal modes are strictly controlled, producing a pulsed optical output with a time interval of 1 / δf1. Therefore, the output of a mode-locked laser contains a very strong intensity-modulated component with a fundamental frequency of δf1. When Raman amplification is performed using such pump light, the intensity noise (RIN) of the pump light is transferred to the amplified light, resulting in the superposition of intensity noise with a frequency of δf1 on the amplified light. This phenomenon is called RIN transfer. To suppress RIN transfer, a mode-locked laser is not typically used as the pump light source for Raman amplification. In this case, the relative optical phases of the longitudinal modes are randomized, and the intensity of the pump light is nearly constant rather than pulsed. This suppresses RIN transfer at the frequency δf1. However, even in a laser that is not mode-locked, the possibility cannot be denied that the optical phases of the longitudinal modes may momentarily coincide (or nearly coincide). Therefore, the RIN of the pump light at the frequency δf1 cannot necessarily be ignored.

[0083] Consider a case where the RIN of the pump light at frequency δf1 or δf2 is large and the RIN transfer to the amplified light cannot be ignored. In this case, intensity noise at frequency δf1 or δf2 is superimposed on the optical signal output from optical transmitter 100, and this noise spectrum occurs at a distance δf1 or δf2 from the carrier frequency of the optical signal. If the bandwidth of the optical signal is wider than δf1 or δf2, these noise components can degrade signal quality.

[0084] To solve this problem, the cavity lengths of the first multimode laser 10 and the second multimode laser 11 can be designed so that Δf1 and Δf2 are higher than the bandwidth of the optical signal output from the optical transmitter 100. When receiving an optical signal, frequencies higher than the signal bandwidth are not required for demodulation and can be removed by a filter in the demodulator. Therefore, the noise components of Δf1 and Δf2 superimposed on the signal light by RIN transfer are also removed, and the demodulation results are not affected.

[0085] The bandwidth of the optical signal output from the optical transmitter 100 is not simple, as it depends heavily on the signal format. However, if the optical signal is a digital signal, one guideline is to design the first multimode laser 10 and the second multimode laser 11 so that δf1 and δf2 are greater than the baud rate of the signal.

[0086] According to the fourth embodiment configured as above, it is possible to remove the influence of noise components caused by RIN transfer.

[0087] (Modification of the fourth embodiment) In FIG. 8, an optical signal output from an optical transmitter 100 has a single carrier wavelength and a baud rate of f b The digital optical signal is shown in Fig. 9. Alternatively, as shown in Fig. 9, a wavelength-multiplexed signal using multiple carrier wavelengths can be collectively amplified. Optical signals with three different carrier wavelengths output from the first optical transmitter 100a, the second optical transmitter 100b, and the third optical transmitter 100c are wavelength-multiplexed by the wavelength multiplexing circuit 503. Typically, in a wavelength-multiplexed signal, multiple carrier frequencies are arranged at equal intervals on the optical spectrum. However, if the interval between these carrier frequencies is equal to δf1 or δf2, the noise components of δf1 and δf2 superimposed on each carrier frequency will overlap with adjacent carrier frequencies, degrading the signal quality of the entire wavelength-multiplexed signal. To avoid this problem, it is desirable to design the frequency intervals of adjacent optical channels so that δf1 and δf2 are different.

[0088] The noise components generated by Raman amplification can be caused by various physical phenomena other than the RIN transfer mentioned above. For example, the four-wave mixing that occurs between each longitudinal mode and the optical signal, as mentioned in paragraph 0018, also becomes noise. When four-wave mixing occurs, the frequency component is divided by Δf 1+ , Δf 1- , Δf 2+ and Δf 2-Noise components occur at optical frequencies that are separated by Δf. These noise components, like the noise components caused by RIN transfer described above, can also cause signal degradation. To avoid these effects, it is necessary to suppress the noise components caused by RIN transfer described above by setting the frequency Δf higher than the baud rate of the signal. 1+ , Δf 1- , Δf 2+ and Δf 2- When the optical signal output from the optical transmitter 100 is a wavelength multiplexed signal, the frequency interval between adjacent optical channels and Δf 1+ , Δf 1- , Δf 2+ and Δf 2- It is desirable to design it so that it is different from the above.

[0089] According to the fourth embodiment configured as above, it is possible to remove the influence of noise components caused by four-wave mixing.

[0090] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Industrial Applicability]

[0091] The present invention can be applied to the technology of optical amplifiers that use pumping light. [Explanation of symbols]

[0092] 10...first multimode laser, 11...second multimode laser, 12...first pump current / temperature controller, 13...second pump current / temperature controller, 14...first polarization-maintaining optical waveguide, 15...second polarization-maintaining optical waveguide, 16...PBC, 20...first polarization-maintaining VOA, 21...second polarization-maintaining VOA, 22...first isolator, 23...second isolator, 24...first polarizer, 25...second polarizer, 30...first polarization-maintaining optical amplifier, 31...second polarization-maintaining optical amplifier, 50, 50a, 50b, 50c...pump light generating unit, 501...gain medium, 502...optical filter

Claims

1. a first multimode laser that outputs a first excitation light; a second multimode laser that outputs a second excitation light; a first pumping current / temperature controller that controls each longitudinal mode of the first pumping light output from the first multimode laser by changing a temperature and a pumping current of the first multimode laser; a second pumping current / temperature controller that controls each longitudinal mode of the second pumping light output from the second multimode laser by changing the temperature and pumping current of the second multimode laser; a first polarization-maintaining optical variable attenuator that receives the first pump light as an input, adjusts the optical intensity of the first pump light while maintaining the polarization state of the first pump light as a linearly polarized wave, and outputs the adjusted optical intensity; a second polarization-maintaining optical variable attenuator that receives the second pump light as an input, adjusts the optical intensity of the second pump light while maintaining the polarization state of the second pump light as a linearly polarized wave, and outputs the adjusted optical intensity; a polarization multiplexing circuit that polarization-multiplexes the first pump light, the optical intensity of which has been adjusted by the first polarization-maintaining optical variable attenuator, and the second pump light, the optical intensity of which has been adjusted by the second polarization-maintaining optical variable attenuator, and outputs the polarization-multiplexed light; Equipped with the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping current and the temperature of the first multimode laser and the second multimode laser so that the center wavelengths of the first multimode laser and the second multimode laser are substantially the same and a longitudinal mode included in the first pumping light and a longitudinal mode included in the second pumping light do not overlap; the first polarization-maintaining optical variable attenuator and the second polarization-maintaining optical variable attenuator are controlled so that the intensities of the first pump light and the second pump light are equal to each other; Excitation light generator.

2. the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping currents and temperatures of the first multimode laser and the second multimode laser so that a longitudinal mode included in the first pumping light and a longitudinal mode included in the second pumping light do not overlap, and the optical intensity of the first pumping light output from the first multimode laser is higher than the optical intensity of the second pumping light output from the second multimode laser. The excitation light generating device according to claim 1 .

3. a first multimode laser that outputs a first excitation light; a second multimode laser that outputs a second excitation light; a first pumping current / temperature controller that controls each longitudinal mode of the first pumping light output from the first multimode laser by changing a temperature and a pumping current of the first multimode laser; a second pumping current / temperature controller that controls each longitudinal mode of the second pumping light output from the second multimode laser by changing the temperature and pumping current of the second multimode laser; a first polarization-maintaining optical amplifier that receives the first pump light as an input, amplifies the optical intensity of the first pump light while maintaining the polarization state of the first pump light as linearly polarized, and outputs the amplified optical intensity; a second polarization-maintaining optical amplifier that receives the second pump light as an input, amplifies the optical intensity of the second pump light while maintaining the polarization state of the second pump light as linearly polarized, and outputs the amplified optical intensity; a polarization multiplexing circuit that polarization-multiplexes the first pumping light whose optical intensity has been amplified by the first polarization-maintaining optical amplifier and the second pumping light whose optical intensity has been amplified by the second polarization-maintaining optical amplifier, and outputs the polarization-multiplexed light; Equipped with the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping current and the temperature of the first multimode laser and the second multimode laser so that the center wavelengths of the first multimode laser and the second multimode laser are substantially the same and a longitudinal mode included in the first pumping light and a longitudinal mode included in the second pumping light do not overlap; the first polarization-maintaining optical amplifier and the second polarization-maintaining optical amplifier are controlled so that the intensities of the first pump light and the second pump light are equal to each other; Excitation light generator.

4. a first isolator that blocks input of reflected light is disposed at the output of the first multimode laser; a second isolator that blocks input of reflected light is disposed at the output of the second multimode laser; The excitation light generating device according to any one of claims 1 to 3.

5. a first polarizer that transmits a single linearly polarized wave is disposed at the output of the first multimode laser; a second polarizer that transmits a single linearly polarized wave is disposed at the output of the second multimode laser; The excitation light generating device according to any one of claims 1 to 3.

6. Any one of the optical frequencies of the longitudinal modes included in the first pump light is designated as f １＿ｎ and among the longitudinal modes contained in the second pump light, f １＿ｎ is smaller than f １＿ｎ The difference in optical frequency between the longitudinal mode closest to １－ and among the longitudinal modes contained in the second pump light, f １＿ｎ is greater than f １＿ｎ The difference in optical frequency between the longitudinal mode closest to １＋ In this case, The first excitation current and temperature controller |Δf １＋ |=|Δf １－ and setting at least one of the excitation current and the temperature of the first multimode laser so as to minimize the number of combinations in which | Any one of the optical frequencies of the longitudinal modes included in the second pump light is designated as f ２＿ｎ and among the longitudinal modes contained in the first pump light, f ２＿ｎ is smaller than f ２＿ｎ The difference in optical frequency between the longitudinal mode closest to ２－ and among the longitudinal modes contained in the second pump light, f ２＿ｎ is greater than f ２＿ｎ The difference in optical frequency between the longitudinal mode closest to ｆ２＋ In this case, The second excitation current and temperature controller ２＋ |=|Δf ２－ and setting at least one of the excitation current and the temperature of the second multimode laser so as to minimize the number of combinations where | The excitation light generating device according to any one of claims 1 to 5.

7. Let R be a predetermined constant greater than 0 and smaller than 1, and let P be the longitudinal mode having the maximum optical power in the first pump light. １＿ｍａｘ When this is done, P １＿ｍａｘ For longitudinal modes with intensities less than or equal to ×R, |Δf １＋ |=|Δf １－ | is allowed, and the longitudinal mode having the maximum optical power in the second pump light is defined as P ２＿ｍａｘ When this is done, P ２＿ｍａｘ For longitudinal modes with an intensity equal to or less than ×R, |Δf ２＋ |=|Δf ２－ | 7. The excitation light generating device according to claim 6.

8. The frequency interval of the longitudinal mode is δf １ a first multimode laser that outputs a first excitation light, The frequency interval of the longitudinal mode is δf ２ a second multimode laser that outputs second excitation light, a first pumping current / temperature controller that controls each longitudinal mode of the first pumping light output from the first multimode laser by changing a temperature and a pumping current of the first multimode laser; a second pumping current / temperature controller that controls each longitudinal mode of the second pumping light output from the second multimode laser by changing the temperature and pumping current of the second multimode laser; a first polarization-maintaining optical variable attenuator or a first polarization-maintaining optical amplifier that receives the first pump light as an input, adjusts the optical intensity of the first pump light while maintaining the polarization state of the first pump light as a linearly polarized wave, and outputs the adjusted optical intensity; a second polarization-maintaining optical variable attenuator or a second polarization-maintaining optical amplifier that receives the second pump light as an input, adjusts the optical intensity of the second pump light while maintaining the polarization state of the second pump light as a linearly polarized wave, and outputs the adjusted optical intensity; a polarization multiplexing circuit that polarization-multiplexes the first pump light, the optical intensity of which has been adjusted by the first polarization-maintaining optical variable attenuator or the first polarization-maintaining optical amplifier, and the second pump light, the optical intensity of which has been adjusted by the second polarization-maintaining optical variable attenuator or the second polarization-maintaining optical amplifier, and outputs the polarization-multiplexed light; a gain medium to which the optical signal and the first pump light and the second pump light output from the polarization multiplexing circuit are input, and which amplifies and outputs the optical signal; Equipped with the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping current and the temperature of the first multimode laser and the second multimode laser so that the center wavelengths of the first multimode laser and the second multimode laser are substantially the same and a longitudinal mode included in the first pumping light and a longitudinal mode included in the second pumping light do not overlap; The optical signal amplified by the first pumping light and the second pumping light has a baud rate f Ｂ When the digital signal is １ and δf ２ is the baud rate f Ｂ An optical amplifier larger than

9. The optical signal has an optical frequency spacing of f ＷＤＭ and the frequency interval of the longitudinal mode is δf １ and δf ２ is the optical frequency spacing f of the adjacent wavelength channels. ＷＤＭ 9. The optical amplifier device according to claim 8, wherein the value is different from

10. Any one of the optical frequencies of the longitudinal modes included in the first pump light is designated as f １_ｎ and among the longitudinal modes included in the second pump light, the optical frequency f １_ｎ and the optical frequency f １＿ｎ The difference in optical frequency between the longitudinal mode closest to １－ and among the longitudinal modes included in the second pump light, the optical frequency f １_ｎ and the optical frequency f １_ｎ The difference in optical frequency between the longitudinal mode closest to １＋ In this case, the first excitation current / temperature controller is １＋ |=|Δf １－ and setting at least one of the excitation current and the temperature of the first multimode laser so as to minimize the number of combinations in which | Any one of the optical frequencies of the longitudinal modes included in the second pump light is designated as f ２_ｎ and among the longitudinal modes included in the first pump light, the optical frequency f ２_ｎ and the optical frequency f ２_ｎ The difference in optical frequency between the longitudinal mode closest to ２－ and among the longitudinal modes included in the second pump light, the optical frequency f ２_ｎ and the optical frequency f ２_ｎ The difference in optical frequency between the longitudinal mode closest to ２＋ In this case, the second excitation current / temperature controller is ２＋ |=|Δf ２－ 9. The optical amplifying device according to claim 8, wherein at least one of the excitation current and the temperature of said second multimode laser is set so as to minimize the number of combinations where |

11. The optical signal has a baud rate of f Ｂ When the digital signal is a signal of the optical frequency difference Δf １＋ , Δf １－ , Δf ２＋ and Δf ２－ is the baud rate f Ｂ The optical amplifier device according to claim 10, wherein the optical amplifier device is greater than

12. The optical signal is a wavelength multiplexed signal in which the optical frequency spacing of adjacent wavelength channels is f WDM , and the optical frequency difference Δf １＋ , Δf １－ , Δf ２＋ and Δf ２－ are the optical frequency intervals f of the adjacent wavelength channels. ＷＤＭ 11. The optical amplifier device according to claim 10, wherein the value is different from

13. a first multimode laser outputs a first excitation light; a second multimode laser outputs a second pump light; a first pumping current / temperature controller for controlling each longitudinal mode of the first pumping light output from the first multimode laser by changing a temperature and a pumping current; a second pumping current / temperature controller for controlling each longitudinal mode of the second pumping light output from the second multimode laser by changing a temperature and a pumping current; a first light intensity changer that receives the first pump light as an input, changes the light intensity of the first pump light while maintaining the polarization state of the first pump light as linear polarization, and outputs the changed light; a second light intensity changer receives the second pump light as an input, changes the light intensity of the second pump light while maintaining the polarization state of the second pump light as linear polarization, and outputs the changed light intensity; a polarization multiplexing circuit polarization-multiplexing the first pumping light whose light intensity has been changed by the first light intensity changer and the second pumping light whose light intensity has been changed by the second light intensity changer, and outputting the polarization-multiplexed light; the first pumping current / temperature controller and the second pumping current / temperature controller control at least one of the pumping current and the temperature of the first multimode laser and the second multimode laser so that the center wavelengths of the first multimode laser and the second multimode laser are substantially the same and a longitudinal mode included in the first pumping light and a longitudinal mode included in the second pumping light do not overlap; the first light intensity changing unit and the second light intensity changing unit control the intensities of the first excitation light and the second excitation light to be equal to each other; Excitation light generation method.

Images