Optical Amplification Device and Excitation Light Generation 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

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

[0001] This application claims priority to PCT / JP2022 / 034368, filed in Japan on September 14, 2022, the contents of which are incorporated herein by reference.

[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 losses. Therefore, various configurations have been devised to compensate for transmission line losses 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. 14 is a diagram showing an example of the configuration of a conventional optical transmission system 1000 using distributed Raman amplification. The optical transmission system 1000 shown in FIG. 14 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 be bidirectionally pumped. Therefore, in the optical transmission line 500 of the optical transmission system 1000 shown in FIG. 14, 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 sent 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 pump 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 pump 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 by a wavelength division multiplexing coupler or a circulator. While bidirectional pumping has been described in FIG. 14 , 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, resulting in a change in the optical intensity of the amplified optical signal. This gain fluctuation range 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 differences in the propagation directions of the optical signal and the pump light. Therefore, although PDG is smaller than that of 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 put into a non-polarized state inside the forward pumping light generating unit 300 or the backward pumping light generating unit 400. Fig. 15 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. 15 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. 15 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 first pump light and the second pump light must satisfy the following three conditions: (First condition) The center wavelengths of the first pump light and the second pump light must be approximately the same. (Second condition) The optical intensities of the first pump light and the second pump light 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 reason why the first condition must be satisfied is that, during the propagation of the first pump light and the second pump light through the optical transmission line 500, polarization rotation occurs due to slight anisotropy of the optical transmission line 500. However, since polarization rotation is wavelength-dependent, if the center wavelengths of the first pump light and the second pump light are different, the polarization orthogonality between the two cannot be maintained. The reason why the second condition must be satisfied is that PDG occurs if the optical intensities of the first pump light and the second pump light are different. The reason why the third condition must be satisfied is that if the longitudinal modes of the first pump light and the second pump light overlap, large 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 be omitted 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. 16.

[0015] 16 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. 16, 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. 16, 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.

[0016] 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. Catherine Martinelli et al., “RIN Transfer in Copumped Raman Amplifiers Using Polarization-Combined Diodes,” Photonics. Technol. Lett., Vol.17, PP.1836-1838, 2005.

[0017] However, the conventional configuration shown in Figure 15 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 Raman amplification 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 spaced 0.1 μm apart, and four-wave mixing that occurs at wavelengths this far apart is usually negligible. However, since the pump light used in Raman amplification is generally extremely high-power, 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.

[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 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 variable attenuator and the second polarization-maintaining optical variable attenuator control the intensities of the first pumping light and the second pumping light to be 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 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 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 using the first polarization-maintaining optical amplifier. and a polarization multiplexing circuit that polarization-multiplexes 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, and outputs the polarization-multiplexed 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 first polarization-maintaining optical amplifier and the second polarization-maintaining optical amplifier control the intensities of the first pumping light and the second pumping light to be equal.

[0022] In one aspect of the present invention, the frequency interval of the longitudinal mode is δf １ a first multimode laser that outputs first pump light having a longitudinal mode frequency interval of δf ２a first pumping current / temperature controller that controls a temperature and a pumping current of the first multimode laser; a second pumping current / temperature controller that controls the temperature and the pumping current of the second multimode laser; a polarization multiplexing circuit that polarization-multiplexes the first pumping light and the second pumping light and outputs the polarization-multiplexed light; and a gain medium that receives an optical signal and all of the first pumping light and the second pumping light output from the polarization multiplexing circuit and 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 or 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 amplified at a baud rate f Ｂ When the digital signal is １ and δf ２ is the baud rate f Ｂ It is an optical amplifier device larger than

[0023] In one aspect of the present invention, the optical frequency is f ｃ１ a first single-mode laser that outputs continuous wave light having an optical frequency of f ｃ１ different values ​​of f ｃ２ a second single-mode laser that outputs continuous wave light having an optical frequency of f ｃ１ + n × δf １ a first wavelength number changing unit that generates first excitation light having a plurality of emission line spectra where n is an integer including negative numbers; and a second wavelength number changing unit that changes the output light of the second single mode laser so that the optical frequency is f ｃ２ + n × δf ２ and generating second excitation light having a plurality of emission line spectra in which the frequency interval of the longitudinal modes is δf ２ The frequency interval of the longitudinal mode is δf １a polarization multiplexing circuit that polarization-multiplexes the first pump light and the second pump light and outputs the polarization-multiplexed light; and a gain medium that receives as input an optical signal and all of the first pump light and second pump light output from the polarization multiplexing circuit, and that amplifies and outputs the optical signal.

[0024] 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 outputs the changed light, a first pumping light whose intensity has been changed by a first light intensity changer and a second pumping light whose intensity has been changed by the second light intensity changer, and outputs the first pumping light and the second pumping light whose intensity has been changed by the second light intensity changer; 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 first light intensity changer and the second light intensity changer control the intensities of the first pumping light and the second pumping light to be equal.

[0025] 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.

[0026] FIG. 1 is a diagram showing an example of the configuration of a pumping light generating unit in the first embodiment. FIG. 2 is a schematic diagram of the arrangement of longitudinal modes of the first pumping light and the second pumping light. FIG. 3 is a schematic diagram of another example of the arrangement of longitudinal modes of the first pumping light and the second pumping light. FIG. 4 is a flowchart showing the processing flow of the pumping light generating unit in the first embodiment. FIG. 5 is a diagram showing an example of the configuration of a pumping light generating unit in a modified example of the first embodiment. FIG. 6 is a diagram showing an example of the configuration of a pumping light generating unit in a modified example of the second embodiment. FIG. 7 is a diagram showing an example of the configuration of a pumping light generating unit in a third embodiment. FIG. 8 is a diagram showing an example of the configuration of an optical amplifier in a fourth embodiment. FIG. 9 is a diagram showing an example of the configuration of an optical amplifier in a modified example of the fourth embodiment. FIG. 10 is a diagram for explaining problems in the first to fourth embodiments. FIG. 11 is a diagram showing an example of the configuration of an optical amplifying device in a fifth embodiment. FIG. 12 is a diagram showing an example of the configuration of an optical amplifying device in a modified example 1 of the fifth embodiment. FIG. 13 is a diagram showing another example of the configuration of the pumping light generating unit in a modified example 1 of the fifth embodiment. FIG. 14 is a diagram showing an example of the configuration of a conventional optical transmission system using distributed Raman amplification. FIG. 15 is a diagram showing an example of the configuration when two multimode lasers are provided inside the forward pumping light generating unit or the backward pumping light generating unit. 3A and 3B are schematic diagrams of optical spectra output from a first multimode laser and a second multimode laser, respectively.

[0027] An embodiment of the present invention will be described below with reference to the drawings. The system configuration of an optical transmission system according to the present invention is similar to the system configuration shown in Fig. 14. What differs from conventional optical transmission systems is the internal configuration of the forward pumping light generating unit 300 or the backward pumping light generating unit 400. Therefore, the following description will focus on the configuration that characterizes the present invention.

[0028] First Embodiment Fig. 1 is a diagram showing an example of the configuration of an excitation light generating unit 50 in a 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.

[0029] 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.

[0030] The wavelength and light intensity of the first multimode laser 10 and the second multimode laser 11 are controlled, respectively, by a first excitation current / temperature controller 12 and a second excitation current / temperature controller 13. 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.

[0031] The first pumping current / temperature controller 12 controls the first multimode laser 10. Specifically, the first pumping 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 pumping current and temperature of the first multimode laser 10 to be approximately the same as the pumping current and temperature of the second multimode laser 11. The first pumping current / temperature controller 12 may control at least either the pumping current or the temperature of the first multimode laser 10.

[0032] 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.

[0033] Of the three conditions for stable Raman amplification, the first condition (the central wavelengths of the first and second pumping lights must be approximately the same) can be relatively easily achieved by matching the cavity lengths of the first multimode laser 10 and the second multimode laser 11 as described above, and by using the first pumping current / temperature controller 12 and the second pumping current / temperature controller 13 to match the pumping currents and temperatures of the first multimode laser 10 and the second multimode laser 11 to approximately the same.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] The second of the three conditions (the optical intensities of the first pump light and the second pump light 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 light and do not affect the optical frequency of each longitudinal mode, so the first and third conditions remain satisfied as described above.

[0038] 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, instead of 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.

[0039] The PBC 16 polarization-multiplexes the first pump light, the optical intensity of which has been adjusted by the first polarization-maintaining VOA 20, and the second pump light, the optical intensity of which 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.

[0040] Next, the longitudinal mode arrangement of the first pumping light and the second pumping light will be described with reference to Figures 2 and 3. Figure 2 is a schematic diagram of the longitudinal mode arrangement of the first pumping light and the second pumping light to be achieved by the first pumping current / temperature controller 12 and the second pumping current / temperature controller 13. As in Figure 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＿５ and 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.

[0041] First, let us focus on one of the longitudinal modes. In FIG. 2, the optical frequency f ２＿３Next, the optical frequency f ２＿３ is greater than the optical frequency f ２＿３ and the longitudinal mode closest to the optical frequency f ２＿３ and the optical frequency f ２＿３ In Figure 2, the optical frequency f １＿４ and optical frequency f １＿３ These correspond to:

[0042] 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 has been given with a focus on the optical frequency f ２＿３ Regardless of which longitudinal mode you focus on, |Δf ２＋ | and |Δf ２－ Set it so that it is not equal to |.

[0043] The above condition is satisfied when the first multimode laser 10 and the second multimode laser 11 are interchanged and Δf １＋ and Δf １－ Similarly, when |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.

[0044] (Another example of the arrangement of longitudinal modes of the first pumping light and the second pumping 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＿４is selected to illustrate the diagram, 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.

[0045] In FIG. 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－ When | is found, 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.

[0046] 2 and 3, the total number of longitudinal modes output from the first multimode laser 10 and the second multimode laser 11 is set to 5. However, in actual multimode lasers, particularly multimode lasers that do not use a fiber Bragg grating, an extremely large number of longitudinal modes are generated. Therefore, when the longitudinal mode spacing of the output of the first multimode laser 10 and the longitudinal mode spacing of the output of the second multimode laser 11 are not equal, |Δf ２＋ | and |Δf ２－ It would be very difficult to set it so that | and | are always different values.

[0047] In such a case, a constant R that satisfies 0<R<1 is determined in advance, and the optical frequency f ２＿１ , f ２＿２ , ... the maximum optical power is P ２＿ｍａｘ When written as P ２＿ｍａｘFor the longitudinal modes of the output of the second multimode laser 11 having optical power lower than |Δf ２＋ | and |Δf ２－ | and f １＿１ , f １＿２ , ... the maximum optical power is P １＿ｍａｘ When written as P １＿ｍａｘ For longitudinal modes of the output of the first multimode laser 10 having optical power lower than |Δf １＋ | and |Δf １－ It is also possible to relax the condition that | is allowed to be equal to |. How to set the value of R here is not obvious because it strongly depends on the spectrum of the multimode laser. However, as a guideline, it may be selected so that the relative intensity noise (RIN) of the Raman-amplified light is minimized. Alternatively, as a simpler method, an optical bandpass filter may be placed at the output of the PBC 16 to suppress the peripheral longitudinal modes of the first multimode laser 10 and the second multimode laser 11, thereby reducing the number of longitudinal modes.

[0048] 4 is a flowchart showing the processing flow of 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 equal to 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 equal to the excitation current and temperature of the first multimode laser 10.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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, to perform Raman amplification stably, all three conditions must be satisfied. 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.

[0053] (First Modification of the First Embodiment) In the above-described 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 of the three conditions are satisfied, it is possible to omit the second polarization-maintaining VOA 21 (or the first polarization-maintaining VOA 20) 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.

[0054] (Second Modification of 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 mode included in the first excitation light and the longitudinal mode included in the second excitation light do not overlap and the first excitation light has higher power than the second excitation light.

[0055] (Third Modification of First Embodiment) The excitation light generating section 50 may be modified to the configuration shown in Fig. 5. Fig. 5 is a diagram showing a configuration example 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.

[0056] The pumping light generating unit 50a 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, a PBC 16, a first polarization-maintaining optical amplifier 30, and a second polarization-maintaining optical amplifier 31.

[0057] 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.

[0058] 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.

[0059] The intensities of the first and second pump light can be made equal by finely adjusting 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.

[0060] Second Embodiment Fig. 6 is a diagram showing an example of the configuration of an excitation light generating unit 50b in a 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 that are common to the configuration shown in Fig. 1 are assigned the same numbers.

[0061] 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.

[0062] 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.

[0063] 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 manner, the first isolator 22 prevents 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 the first optical intensity changer.

[0064] The second isolator 23 is provided between the second multimode laser 11 and the second polarization-maintaining VOA 21, and blocks input of reflected light from the second polarization-maintaining VOA 21. In this manner, the second isolator 23 prevents reflected light from the second polarization-maintaining VOA 21 from being input to the second multimode laser 11. The second isolator 23 is one aspect of the second optical intensity changer.

[0065] 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 is possible to improve the stability of the first multimode laser 10 and the second multimode laser 11.

[0066] (Variation 1 of the Second Embodiment) As in the first 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.

[0067] Third Embodiment Fig. 7 is a diagram showing a configuration example of an excitation light generating unit 50c in a 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.

[0068] 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.

[0069] The excitation light generating unit 50c differs in configuration from the excitation light generating unit 50 in that it further includes a first polarizer 24 and a second polarizer 25. The differences from the excitation light generating unit 50 will be described below.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] (Variation 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.

[0075] (Variation 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.

[0076] (Variations 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.

[0077] In each embodiment, a configuration using two multimode lasers outputting approximately the same wavelength has been described. In each embodiment, for example, two multimode lasers outputting a wavelength of 1.45 μm and two multimode lasers outputting 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.

[0078] 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 do not mention the bandwidth 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 device that takes these values ​​into consideration will be described.

[0079] 8 is a diagram showing an example of the configuration of an optical amplifier according to the fourth embodiment. The optical amplifier includes a 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 an optical amplifier with a compact configuration using an optical waveguide with a relatively short length.

[0080] The internal configuration of the pumping light generating unit 50 is a polarization multiplexed configuration of the first multimode laser 10 and the second multimode laser 11. In the example shown in Fig. 8, the internal configuration of the 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.

[0081] 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 the backward pumping light multiplexing unit 410 and the 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 absorbs a large amount of pump light, the optical filter 502 may be omitted.

[0082] 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 has before being multiplexed. These pumping lights can be considered as a collection of multiple CW lights with a constant frequency interval, as shown in Figure 2. Here, the longitudinal mode interval of the first multimode laser 10, i.e., f １_ｎ＋１ -f １_ｎ δf １ Similarly, the longitudinal mode spacing of the second multimode laser, i.e., f ２_ｎ＋１ -f ２_ｎ δf ２ As is clear from FIG. １ = Δf ２＋ +Δf ２－ and δf ２ = Δf １＋ +Δf １－ This becomes:

[0083] Let us assume that the first multimode laser 10 is a mode-locked laser. In a mode-locked laser, the relative relationship between the optical phases of the longitudinal modes is strictly controlled, and the optical output is regulated at a time interval of 1 / δf. １ Therefore, the output of the mode-locked laser has a very strong intensity modulation component, and its fundamental frequency is δf １ When Raman amplification is performed using such pumping light, the intensity noise RIN of the pumping light is transferred to the amplified light, and the amplified light has a frequency δf １ This is called RIN transfer. To suppress RIN transfer, mode-locked lasers are not usually used as pump light sources for Raman amplification. In this case, the relative relationship between the optical phases of each longitudinal mode becomes random, and the intensity of the pump light is not pulsed but is almost constant, with a frequency of δf １ However, even if the laser is not a mode-locked laser, the optical phases of the longitudinal modes may coincide (or almost coincide) momentarily. １ It is not necessarily true that the RIN can be ignored.

[0084] Frequency δf of the excitation light１ or δf ２ Consider a case where the RIN of the optical signal output from the optical transmitter 100 is large and the RIN transfer to the amplified light is not negligible. １ or δf ２ The noise spectrum is the frequency of the optical signal from the carrier frequency of the optical signal. １ or δf ２ The bandwidth of the optical signal is δf １ or δf ２ If the signal width is wider than 1 / 2, these noise components can degrade the signal quality.

[0085] To solve this problem, the bandwidth of the optical signal output from the optical transmitter 100 should be set to δf １ and δf ２ The cavity lengths of the first multimode laser 10 and the second multimode laser 11 can be designed so that δf is high. When receiving an optical signal, frequencies higher than the signal band are not required for demodulation and can be removed by a filter in the demodulator. １ and δf ２ The noise components are also removed, and the demodulation results are not affected.

[0086] The bandwidth of the optical signal output from the optical transmitter 100 is not simple because it strongly depends on the signal format. However, if the optical signal is a digital signal, one guideline is to use δf １ and δf ２ The first multimode laser 10 and the second multimode laser 11 are designed so that

[0087] According to the fourth embodiment configured as above, it is possible to remove the influence of noise components caused by RIN transfer.

[0088] (Modification of the Fourth Embodiment) In FIG. 8, an optical signal output from the optical transmitter 100 is a signal having a single carrier wavelength and a baud rate of f ＢOn the other hand, as shown in Fig. 9, it is also possible to adopt a configuration in which wavelength-multiplexed signals using a plurality of carrier wavelengths are amplified collectively. Optical signals having three types of 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. Normally, in a wavelength-multiplexed signal, many carrier frequencies are arranged at equal intervals on the optical spectrum, but if the intervals between these carrier frequencies are δf １ or δf ２ When the frequency is equal to δf, the frequency is superimposed on each carrier frequency. １ and δf ２ The noise components of the adjacent carrier frequencies overlap with each other, degrading the signal quality of the entire wavelength multiplexed signal. To avoid this problem, the frequency spacing of adjacent optical channels and δf １ and δf ２ It is desirable to design it so that it is different from the above.

[0089] The noise components generated by Raman amplification can be caused by various physical phenomena other than the RIN transfer described above. For example, the four-wave mixing caused by each longitudinal mode and the optical signal, as described in paragraph 0018, also becomes noise. When four-wave mixing occurs, the frequency component Δf １＋ , Δf １－ , Δf ２＋ and Δf ２－ These noise components also cause signal degradation, similar to the noise components caused by RIN transfer described above. To avoid these effects, it is necessary to suppress the noise components by a factor of Δf rather than the baud rate of the signal, similar to the method for suppressing noise components caused by RIN transfer described above. 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.

[0090] According to the fourth embodiment configured as above, it is possible to eliminate the influence of noise components caused by four-wave mixing.

[0091] Fifth Embodiment In the first to fourth embodiments described above, the configuration in which pump light is generated by two multimode lasers is shown. In the first to fourth embodiments, the optical frequency of the longitudinal mode of each multimode laser is set to f １＿１ , f １＿２ , ... and f ２＿１ , f ２＿２ , ..., the longitudinal mode spacing δf １ = f １＿２ -f １＿１ and δf ２ = f ２＿２ -f ２＿１ How to set the frequency interval (Δf １－ , Δf １＋ , Δf ２－ , Δf ２＋ However, the first to fourth embodiments involve the following difficulties.

[0092] Let T be the time it takes for light to propagate through the laser cavity. １＿１ and optical frequency f １＿２ is roughly expressed as follows: T = m / f １＿１ = (m + 1) / f １＿２ f １＿１ = m / T f １＿２ =(m+1) / T δf １ = 1 / T

[0093] Here, m is a positive integer. Generally, the change in 1 / T due to the change in the excitation current or temperature is small, but m is very large, so the optical frequency f １＿１ and optical frequency f １＿２ On the other hand, the longitudinal mode spacing δf １is hardly adjustable by the pump current or temperature, and is determined almost entirely by the cavity length of the laser, as shown at the end of paragraph 0007. ２ The same is true for .

[0094] As described above, the longitudinal mode spacing δf １ and δf ２ To set the desired value, it is necessary to start from the design of the multimode laser, and individual differences due to manufacturing errors are １ and δf ２ It is necessary to consider in advance the influence on the longitudinal mode spacing δf １ and longitudinal mode spacing δf ２ When the longitudinal mode on the short wavelength side is used as a reference, Δf １－ , Δf １＋ , Δf ２－ , Δf ２＋ The reason for this is shown in Fig. 10. Fig. 10 is a diagram for explaining the problems in the first to fourth embodiments.

[0095] As in paragraph 0040, the optical frequency f ２＿３ Focusing on f ２＋ and f ２－ When determining f ２－ is f ２＋ However, the optical frequency f ２＿１ Focusing on f ２＋ and f ２－ When determining f as shown in FIG. ２－ is f ２＋ That is, the longitudinal mode spacing δf １ and longitudinal mode spacing δf ２ If different from ２＋ and f ２－ depends on the optical frequency of interest. Since the wavelength is a quantity determined by the optical frequency, the longitudinal mode spacing δf １ and longitudinal mode spacing δf ２ If different from ２＋ and f ２－ In other words, f has wavelength dependency.１＋ and f １－ Similarly, the wavelength dependence of the optical amplifier is also wavelength dependent. Although this wavelength dependence does not immediately cause adverse effects, the possibility of unexpected effects occurring when longitudinal modes are generated over a wide wavelength range cannot be denied. Therefore, in the fifth embodiment, a configuration of an optical amplifier that takes this problem into consideration will be described.

[0096] 11 is a diagram showing an example of the configuration of an optical amplifier according to the fifth embodiment. The optical amplifier includes a pumping light generating unit 50d, a forward pumping light multiplexing unit 310, a gain medium 501, and an optical filter 502. The pumping light generating unit 50d is one aspect of a pumping light generating device. The pumping light generating unit 50d includes 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, a PBC 16, a first polarization-maintaining VOA 20, a second polarization-maintaining VOA 21, a first single-mode laser 35, a second single-mode laser 36, a first oscillator 37, a second oscillator 38, a first wavelength number changer 39, and a second wavelength number changer 40.

[0097] The difference between the excitation light generating unit 50 in the fourth embodiment and the excitation light generating unit 50 in the fourth embodiment is that the first multimode laser 10 is replaced with a first single-mode laser 35, a first oscillator 37, and a first wavelength number changing unit 39, and the second multimode laser 11 is replaced with a second single-mode laser 36, a second oscillator 38, and a second wavelength number changing unit 40.

[0098] The first single mode laser 35 emits light at an optical frequency f ｃ１ The second single-mode laser 36 outputs light (continuous wave light) in a single mode at an optical frequency f ｃ２ where the optical frequency is f ｃ１ and f ｃ２ are different values. ｃ１ The value of is controlled by the first excitation current and temperature controller 12. ｃ２ The value of is controlled by the second excitation current and temperature controller 13.

[0099] The first oscillator 37 generates a frequency δf １ The second oscillator 38 outputs a signal of frequency δf ２ The signal is output.

[0100] The optical frequency f output from the first single mode laser 35 ｃ１ The light of the single mode of f is input to the first wavelength number changer 39. When n is a negative integer, the first wavelength number changer 39 changes the optical frequency to f ｃ１ + n × δf １ In this embodiment, the light having a plurality of emission line spectra converted by the first wavelength number changer 39 is used as the first excitation light.

[0101] In this way, the first wavelength number changer 39 changes the optical frequency f ｃ１ and the output from the first oscillator 37, the output light of the first single mode laser 35 is changed to have an optical frequency f ｃ１ + n × δf １ The first wavelength number changing unit 39 generates first excitation light having a plurality of emission line spectra, where:

[0102] The optical frequency f output from the second single mode laser 36 ｃ２ The light of the single mode of f is input to the second wavelength number changing unit 40. When n is a negative integer, the second wavelength number changing unit 40 changes the optical frequency to f ｃ２ + n × δf ２ In this embodiment, the light having a plurality of emission line spectra converted by the second wavelength number changer 40 is used as the second excitation light.

[0103] In this way, the second wavelength number changer 40 changes the optical frequency f ｃ２ and the output from the second oscillator 38, the output light of the second single mode laser 36 is changed to have an optical frequency f ｃ２ + n × δf ２The second wavelength number changing unit 40 generates second excitation light having a plurality of emission line spectra in which:

[0104] There are several methods for converting light of a single wavelength into light of multiple wavelengths with a constant optical frequency spacing δf. One method is to apply periodic optical modulation of the frequency δf. The other method is to use an optical frequency comb using a nonlinear medium or the like. However, these methods result in optical intensity modulation as the number of wavelengths increases. As mentioned above, changes in the intensity of the pump light can cause RIN transfer, so this optical intensity modulation must be suppressed. Therefore, the first wavelength number changer 39 and the second wavelength number changer 40 are configured to combine multiple different modulation methods, such as optical intensity modulation and optical phase modulation, to simultaneously use optical intensity modulation to offset undesirable intensity changes that occur as the number of wavelengths increases, thereby maintaining an approximately constant optical intensity.

[0105] The modulation frequency of the optical modulation performed inside the first wavelength number changer 39 is controlled by the first oscillator 37. The modulation frequency of the optical modulation performed inside the second wavelength number changer 40 is controlled by the second oscillator 38. The output frequencies of these oscillators are basically δf １ and δf ２ However, depending on the modulation means and amplitude, δf １ and δf ２ It is also possible to divide by an integer.

[0106] As an example, consider a case where the first wavelength number changer 39 is configured with an optical phase modulator and an optical intensity modulator. １ When driven by a drive signal of f, the generated phase-modulated light has an optical frequency of f ｃ１ + n × δf １ However, since the optical phase difference of these emission line spectra is strictly determined, the frequency δf １This generates beats, resulting in undesired intensity modulation. However, if the phase and amplitude of the drive signal applied to the optical phase modulator are known, the phase and amplitude of this undesired intensity modulation can also be predicted in advance. Using the optical intensity modulator included in the first wavelength number changing unit 39, it is possible to apply intensity modulation that suppresses this undesired intensity modulation, thereby setting the total optical intensity to be approximately constant.

[0107] By using such a modulation means, the first pump light and the second pump light each have an optical frequency of f ｃ１ + n × δf １ and f ｃ２ + n × δf ２ The intensity modulation of the spectrum is suppressed. １ and δf ２ By equating the above, Δf １－ , Δf １＋ , Δf ２－ , Δf ２＋ As with the cavity length of a laser, the characteristics of the oscillator also have the problem of manufacturing errors, but fine-tuning the oscillation frequency of an oscillator can be achieved much more easily and with higher precision than fine-tuning the cavity length of a laser.

[0108] It is technically difficult to widen the optical frequency intervals of the bright line spectra generated by such a method and completely suppress the intensity modulation. Therefore, it is necessary to allow some intensity modulation components to remain, and to suppress the optical signal output from the optical transmitter 100 at a baud rate of f Ｂ In the case of a digital signal of n×δf, in order to minimize the influence of RIN transfer, １ and n × δf ２ is the baud rate f Ｂ It is desirable to set it to a value different from

[0109] The first pump light output from the first wavelength number changer 39 is input to the first polarization-maintaining VOA 20. 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 wavelength number changer 39.

[0110] The second pump light output from the second wavelength number changer 40 is input to the second polarization-maintaining VOA 21. 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 wavelength number changer 40.

[0111] The PBC 16 polarization-multiplexes the first pump light, the optical intensity of which has been adjusted by the first polarization-maintaining VOA 20, and the second pump light, the optical intensity of which 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.

[0112] The polarization-multiplexed pump light is input to the gain medium 501 via the forward pump light multiplexing unit 310. The gain medium 501 receives the optical signal output from the optical transmitter 100 and the pump light (first pump light and second pump light) output from the PBC 16, amplifies the optical signal with the input pump light, and then outputs the amplified optical signal.

[0113] According to the fifth embodiment configured as above, it is possible to improve the problems in the first to fourth embodiments.

[0114] 11, the light output from the first single mode laser 35 is externally modulated by a first wavelength number changer 39, and the light output from the second single mode laser 36 is externally modulated by a second wavelength number changer 40. On the other hand, as shown in FIG. 12, it is also possible to configure part of the modulation means to perform direct modulation.

[0115] 12 is a diagram showing an example of the configuration of an optical amplifier according to a first modification of the fifth embodiment. The optical amplifier includes a pumping light generating unit 50e, a forward pumping light multiplexing unit 310, a gain medium 501, and an optical filter 502. The pumping light generating unit 50e is one aspect of a pumping light generating device. The pumping light generating unit 50e includes 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, a PBC 16, a first polarization-maintaining VOA 20, a second polarization-maintaining VOA 21, a first single-mode laser 35, a second single-mode laser 36, a first oscillator 37, a second oscillator 38, a first intensity modulation suppression unit 41, and a second intensity modulation suppression unit 42.

[0116] The first oscillator 37 generates a signal having the same frequency δf １ to the first excitation current / temperature controller 12 and the first intensity modulation suppression unit 41. The first excitation current / temperature controller 12 controls either or both of the excitation current and the temperature of the first single mode laser 35 to a frequency δf １ It is directly modulated with the signal.

[0117] The optical frequency of the output of the single mode laser depends on the excitation current or temperature. Therefore, by the above-mentioned direct modulation, the output light of the first single mode laser 35 has an optical frequency of f ｃ１ + n × δf １ The output intensity of a single-mode laser also responds nonlinearly to the pump current and temperature. Therefore, although the original goal of generating multiple emission lines is achieved, the total light intensity of the pump light is subject to complex intensity modulation, which is an undesirable problem. However, if the average values ​​of the pump current and temperature, and the frequency and modulation depth of the direct modulation applied to them are constant, it is possible to predict the modulation waveform of this undesirable intensity modulation.

[0118] The frequency δf output from the first oscillator 37 １The signal of the frequency δf is input to the first intensity modulation suppressor 41. The first intensity modulation suppressor 41 suppresses the input signal of the frequency δf １ In this way, the first intensity modulation suppression unit 41 cancels out the undesired intensity modulation caused by the direct modulation.

[0119] The second oscillator 38 and the second intensity modulation suppression unit 42 are used in the same manner as the first oscillator 37 and the first intensity modulation suppression unit 41. The first intensity modulation suppression unit 41 and the second intensity modulation suppression unit 42 in this embodiment perform only simple intensity modulation, unlike the first wavelength number changer 39 and the second wavelength number changer 40 in the embodiment shown in Fig. 11. This makes it possible to reduce optical loss of the first pump light and the second pump light associated with modulation.

[0120] Although the first intensity modulation suppression unit 41 and the second intensity modulation suppression unit 42 can be realized by an intensity modulator using, for example, a Mach-Zehnder interferometer, an intensity modulator using a Mach-Zehnder interferometer is not practical due to its large insertion loss. A more realistic configuration is to configure the first intensity modulation suppression unit 41 and the second intensity modulation suppression unit 42 by combining a polarizer and a polarization controller.

[0121] 13 shows a specific example of an excitation light generating unit 50f having a first intensity modulation suppression unit 41 and a second intensity modulation suppression unit 42, in which the first intensity modulation suppression unit 41 and the second intensity modulation suppression unit 42 are configured by combining a polarizer and a polarization controller as described above. The first intensity modulation suppression unit 41 is configured with a first polarization change unit 43 and a first polarizer 24. The second intensity modulation suppression unit 42 is configured with a second polarization change unit 44 and a second polarizer 25.

[0122] The output of the first single-mode laser 35 is input to the first polarization change unit 43. Since the output of a single-mode laser is usually linearly polarized, if the first polarization change unit 43 transmits the light without changing the polarization, the output of the first polarization change unit 43 will also be linearly polarized. The output of the first polarization change unit 43 is input to the first polarizer 24. Here, the plane of polarization that maximizes the transmittance of the first polarizer 24 is set to match the plane of polarization that is obtained when the first polarization change unit 43 transmits the light without changing the polarization.

[0123] The first oscillator 37 periodically controls the first polarization changer 43. This control slightly changes the polarization state of the output light of the first polarization changer 43, resulting in low-degree polarization modulation. This polarization modulation may be achieved by periodically slightly changing the angle of the polarization plane of linear polarization. Alternatively, it may be achieved by periodically changing the linear polarization to an elliptical polarization and then back to the original linear polarization. This polarization modulation is converted into intensity modulation at the output of the first polarizer 24. As already mentioned, this intensity modulation is set to cancel out the undesired intensity modulation caused by direct modulation. The second intensity modulation suppressor 42 operates in a similar manner.

[0124] (Variation 2 of the Fifth Embodiment) Generally, the output light intensity of a single-mode laser is smaller than that of a multimode laser, and therefore, it is predicted that the light intensity will be insufficient for use as pump light. To compensate for this drawback, semiconductor optical amplifiers may be disposed between the first single-mode laser 35 and the first polarization-maintaining VOA 20, and between the second single-mode laser 36 and the second polarization-maintaining VOA 21. Although the first oscillator 37 and the second oscillator 38 have been described as two separate oscillators, they may also be configured as the same oscillator, with its output branched into multiple outputs.

[0125] 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.

[0126] The present invention can be applied to the technology of optical amplifiers that use pumping light.

[0127] REFERENCE SIGNS LIST 10...first multimode laser, 11...second multimode laser, 12...first pumping current / temperature controller, 13...second pumping 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, 35...first single-mode laser, 36...second single-mode laser, 37...first oscillator, 38...second oscillator, 39...first wavelength number change unit, 40...second wavelength number change unit 41...first intensity modulation suppression unit, 42...second intensity modulation suppression unit, 43...first polarization change unit, 44...second polarization change unit, 50, 50a, 50b, 50c, 50d...pump light generation unit, 501...gain medium, 502...optical filter

Claims

1. A first single-mode laser that outputs continuous light with an optical frequency of f ｃ１ and a second single-mode laser that outputs continuous light with an optical frequency of f The optical frequency is f ｃ１ f having a value different from ｃ２ a second single-mode laser that outputs continuous light having Apply a change to the output light of the first single-mode laser, and the optical frequency is f ｃ１ + n × δf １ A first wavelength number changing unit that generates first excitation light having a plurality of emission line spectra where (n is an integer including negative numbers), To change the output light of the second single-mode laser so that the optical frequency is f ｃ２ + n × δf ２ To generate a second excitation light having a plurality of emission line spectra, and the frequency interval δf of the longitudinal mode ２ To set the frequency interval δf of the longitudinal mode １ To a second wavelength number changing unit that is set to be as close as possible to δf A polarization multiplexing circuit that polarization multiplexes and outputs the first excitation light and the second excitation light; A gain medium into which all of an optical signal, the first excitation light, and the second excitation light output from the polarization multiplexing circuit are input, and which amplifies and outputs the optical signal; An optical amplification device comprising the above.

2. The first wavelength number changing unit generates a plurality of emission line spectra by applying a periodic modulation to the output light of the first single-mode laser; The second wavelength number changing unit generates a plurality of emission line spectra by applying a periodic modulation to the output light of the second single-mode laser. The optical amplification device according to Claim 1. The optical amplification device according to Claim 1.

3. When the optical signal is a digital signal with a baud rate fB, the n×δf １ and n×δf ２ are different from the baud rate fB The optical amplification device according to Claim 1.

4. The optical signal is a wavelength multiplexed signal of fWDM with an optical frequency interval between adjacent wavelength channels, and the frequency intervals δf １ and δf ２ are values different from the optical frequency interval fWDM between the adjacent wavelength channels. The optical amplification device according to claim 1.

5. The first single-mode laser outputs continuous light having an optical frequency of f c1; The second single-mode laser outputs continuous light having an optical frequency of f c2, which is a value different from f c1; The first wavelength number changing unit changes the output light of the first single-mode laser to generate first excitation light having a plurality of emission line spectra with an optical frequency of f c1 + n×δf 1 (n is an integer including negative numbers); The second wavelength number changing unit changes the output light of the second single-mode laser to generate second excitation light having a plurality of emission line spectra with an optical frequency of f c2 + n×δf 2, and sets the frequency interval δf 2 between longitudinal modes to be as close as possible to the frequency interval δf 1 between longitudinal modes; The polarization multiplexing circuit polarization multiplexes and outputs the first excitation light and the second excitation light; The gain medium inputs all of the optical signal, the first excitation light, and the second excitation light output from the polarization multiplexing circuit, and amplifies and outputs the optical signal. An excitation light generation method. An excitation light generation method.