Optical amplification device

Abstract

The present invention provides an optical amplifier with high gain and improved flatness of gain and NF. [Solution] The optical amplification device comprises a plurality of Raman optical amplifiers, each having a Raman amplification optical fiber and an excitation light source unit that supplies excitation light to the Raman amplification optical fiber. The plurality of Raman optical amplifiers are connected in series. The product of the maximum value of the Raman gain efficiency of the Raman amplification optical fiber of the second Raman optical amplifier and its fiber length is smaller than the product of the maximum value of the Raman gain efficiency of the Raman amplification optical fiber of the first Raman optical amplifier, which is located before the second Raman optical amplifier, and its fiber length. The number of excitation light sources in the excitation light source unit of the second Raman optical amplifier is smaller than the number of excitation light sources in the excitation light source unit of the first Raman optical amplifier. At least one wavelength of the excitation light output by the excitation light source unit of the second Raman optical amplifier is different from at least one wavelength of the excitation light output by the excitation light source unit of the first Raman optical amplifier.

Description

Technical Field

[0001] The present invention relates to an optical amplification device.

Background Art

[0002] A centralized Raman optical amplifier is known as an optical amplifier used in an optical communication network. Currently, the most commonly used communication wavelength bands are the C band (for example, wavelengths from 1530 nm to 1565 nm) and the L band (for example, wavelengths from 1565 nm to 1625 nm). However, using only these wavelength bands may result in insufficient transmission capacity. Therefore, the S band (for example, wavelengths from 1460 nm to 1530 nm) has attracted attention as a wavelength band to compensate for insufficient transmission capacity. The Raman optical amplifier can also be used as an optical amplifier in the S band.

[0003] As a centralized Raman optical amplifier, for example, the configurations disclosed in Non-Patent Documents 1 and 2 are known.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

[0005] However, known technologies still have room for improvement in terms of gain height and the flatness of the gain and noise figure (NF). Here, flatness refers to the flatness of the spectrum in the gain spectrum and NF spectrum. Therefore, for example, if the difference in gain per wavelength is small, it means that the gain flatness is high.

[0006] The present invention has been made in view of the above, and aims to provide an optical amplifier that has high gain and improved flatness of gain and NF. [Means for solving the problem]

[0007] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention provides an optical amplification device comprising a plurality of Raman optical amplifiers, each having a Raman amplification optical fiber and an excitation light source unit that supplies excitation light to the Raman amplification optical fiber, wherein the plurality of Raman optical amplifiers are connected in series, the product of the maximum value of the Raman gain efficiency of the Raman amplification optical fiber of the second Raman optical amplifier and the fiber length is smaller than the product of the maximum value of the Raman gain efficiency of the Raman amplification optical fiber of the first Raman optical amplifier located before the second Raman optical amplifier and the fiber length, the number of excitation light sources in the excitation light source unit of the second Raman optical amplifier is smaller than the number of excitation light sources in the excitation light source unit of the first Raman optical amplifier, and at least one wavelength of the excitation light output by the excitation light source unit of the second Raman optical amplifier is different from at least one wavelength of the excitation light output by the excitation light source unit of the first Raman optical amplifier.

[0008] At least one of the plurality of Raman light amplifiers may have a termination device for terminating residual excitation light.

[0009] At least one of the Raman amplification optical fibers in the plurality of Raman light amplifiers may be a dispersion compensation optical fiber.

[0010] The optical amplification device may include an input-side photodetector for monitoring the power of the input light to be amplified, and an output-side photodetector for monitoring the power of the output amplified light.

[0011] The number of the aforementioned Raman light amplifiers may be as many as two.

[0012] The wavelength range of the excitation light output by the excitation light source unit of the second Raman light amplifier may overlap, at least in part, with the wavelength range of the excitation light output by the excitation light source unit of the first Raman light amplifier.

[0013] The wavelength range of the excitation light output by the excitation light source unit of the second Raman light amplifier may be within the wavelength range of the excitation light output by the excitation light source unit of the first Raman light amplifier. [Advantages of the Invention]

[0014] According to the present invention, an optical amplification device with high gain and improved gain and NF flatness can be realized. [Brief Description of the Drawings]

[0015] [Figure 1] FIG. 1 is a schematic configuration diagram of an optical amplification device according to Embodiment 1. [Figure 2] FIG. 2 is a diagram for explaining the Raman gain efficiency. [Figure 3] FIG. 3 is a diagram for explaining an example of the relationship between the wavelengths of pump light. [Figure 4] FIG. 4 is a diagram for explaining another example of the relationship between the wavelengths of pump light. [Figure 5] FIG. 5 is a diagram showing the gains of Example 1 and Comparative Example. [Figure 6] FIG. 6 is a diagram showing the gains and NF of Examples 2 to 4. [Figure 7] FIG. 7 is a schematic configuration diagram of an optical amplification device according to Embodiment 2. [Modes for Carrying Out the Invention]

[0016] Hereinafter, embodiments will be described with reference to the drawings. Note that the present invention is not limited by this embodiment. In the description of the drawings, the same or corresponding elements are appropriately given the same reference numerals, and redundant descriptions are appropriately omitted. Also, the drawings are schematic, and it should be noted that the dimensional relationships between elements, the ratios of elements, etc. may be different from reality. There may also be parts where the dimensional relationships and ratios between the drawings are different.

[0017] (Embodiment 1) [Configuration of Optical Amplification Device] FIG. 1 is a schematic configuration diagram of an optical amplification device according to Embodiment 1. The optical amplification device 1000 is used, for example, by being inserted into a signal transmission line. The optical amplification device 1000 includes Raman optical amplifiers 101 and 102. The optical amplification device 1000 also includes an optical coupler 1001, an optical isolator 1002, an optical coupler 1003, photodetectors (PDs) 1004 and 1005, a controller 1006, an optical coupler 202, and a PD 302. The two Raman optical amplifiers 101 and 102 are an example of a plurality of Raman optical amplifiers connected in series. The Raman optical amplifier 102 is an example of a second Raman optical amplifier, and the Raman optical amplifier 101 is an example of a first Raman optical amplifier located in front of the second Raman optical amplifier.

[0018] The optical coupler 1001 branches a part of the amplified light L1 input to the optical amplification device 1000 as detection light. The optical coupler 1001 is an optical coupler that branches a part (for example, 1% or more and 10% or less of the power) of the input light, such as a Tap coupler. The amplified light L1 is, for example, a WDM (Wavelength Division Multiplexing) signal light.

[0019] The Raman optical amplifier 101 optically amplifies the amplified light L1 input via the optical coupler 1001 and outputs it as first amplified light. The configuration of the Raman optical amplifier 101 will be described in detail later.

[0020] The optical coupler 202 branches a part of the first amplified light input from the Raman optical amplifier 101 as detection light. The optical coupler 202 has a configuration similar to that of the optical coupler 1001, for example.

[0021] The Raman optical amplifier 102 optically amplifies the first amplified light input via the optical coupler 202 and outputs it as amplified light L2. The configuration of the Raman optical amplifier 102 will be described in detail later.

[0022] The optical isolator 1002 allows the amplified light L2 to pass through and blocks the light traveling in the opposite direction to the amplified light L2.

[0023] The optical coupler 1003 branches off a portion of the amplified light L2 input from the Raman light amplifier 101 via the optical isolator 1002 as detection light. The optical coupler 1003 has a configuration similar to, for example, the optical coupler 1001.

[0024] PD1004 receives the detection light branched by the optical coupler 1001 and outputs an electrical signal corresponding to the power of the received detection light. PD302 receives the detection light branched by the optical coupler 202 and outputs an electrical signal corresponding to the power of the received detection light. PD1005 receives the detection light branched by the optical coupler 1003 and outputs an electrical signal corresponding to the power of the received detection light. PD1004 is an example of an input-side photodetector for monitoring the power of the amplified light L1 input to the optical amplifier 1000, and PD1005 is an example of an output-side photodetector for monitoring the power of the amplified light L2 output from the optical amplifier 1000.

[0025] The controller 1006 receives electrical signals output from PDs 1004, 1005, and 302, detects the power of the amplified light L1, the first amplified light, and the amplified light L2 based on these electrical signals, and calculates the gain characteristics of the Raman light amplifiers 101 and 102 based on the detection results. Such a controller 1006 is composed of a microcomputer including a processor such as a CPU (Central Processing Unit) and memory, an AD converter, and a DA converter. The functions of the controller 1006 are realized through the cooperation of hardware and software, for example, by the processor executing a program read from memory. The controller 1006 may also be composed of an FPGA (Field Programmable Gate Array), for example. The controller 1006 also has the function of controlling the excitation light source units 101c and 102c, which will be described later, based on the calculated gain characteristics.

[0026] [Configuration of a Raman light amplifier] The Raman light amplifier 101 includes an optical isolator 101a, a Raman amplification optical fiber 101b, an excitation light source unit 101c, WDM couplers 101d and 101e, and a terminator 101f.

[0027] The optical isolator 101a allows the amplified light L1 input to the Raman light amplifier 101 to pass through, while blocking light traveling in the opposite direction from the amplified light L1.

[0028] When the light to be amplified L1 is input to the Raman amplification optical fiber 101b while excitation light is supplied, it performs Raman amplification on the input and outputs it as the first amplified light. In this embodiment, the Raman amplification optical fiber 101b is a dispersion compensation optical fiber. A dispersion compensation optical fiber is an optical fiber used to compensate for the chromatic dispersion of optical fibers constituting a signal transmission line, for example, at C-band or L-band wavelengths. Such a dispersion compensation optical fiber has high Raman gain efficiency and is suitable as a Raman amplification optical fiber. However, the Raman amplification optical fiber 101b is not limited to a dispersion compensation optical fiber, and other optical fibers with high nonlinearity may also be used.

[0029] The excitation light source unit 101c has N1 excitation light sources, which supply excitation light to the Raman amplified optical fiber 101b. Here, N1 is a positive integer greater than N2, which will be described later. The excitation light sources output excitation light to excite the Raman amplified optical fiber 101b. The wavelengths of the excitation light output by the excitation light sources are different from each other.

[0030] The WDM coupler 101d combines the excitation light output by the excitation light source unit 101c into multiple wavelengths and outputs it to the Raman amplified optical fiber 101b.

[0031] The WDM coupler 101e is positioned on the opposite side of the Raman amplification optical fiber 101b from the WDM coupler 101d. The WDM coupler 101e branches the excitation light (residual excitation light) that has passed through the Raman amplification optical fiber 101b to the terminator 101f. The terminator 101f is a known terminator configured to terminate the residual excitation light.

[0032] On the other hand, the Raman light amplifier 102 includes an optical isolator 102a, a Raman amplification optical fiber 102b, an excitation light source unit 102c, WDM couplers 102d and 102e, and a terminator 102f. The optical isolator 102a and the terminator 102f have the same configuration as the corresponding elements in the Raman light amplifier 101, so their description is omitted.

[0033] When the first amplified light is input to the Raman-amplifying optical fiber 102b while excitation light is being supplied, it performs Raman amplified light and outputs amplified light L2 as the output light of the optical amplifier 1000. In this embodiment as well, the Raman-amplifying optical fiber 102b is a dispersion-compensating optical fiber.

[0034] The excitation light source unit 102c has at least one excitation light source, with N2 excitation light sources supplying excitation light to the Raman amplified optical fiber 102b. Here, N2 is a positive integer less than N1. The excitation light sources output excitation light to excite the Raman amplified optical fiber 102b. The wavelengths of the excitation light output by the excitation light sources are different from each other.

[0035] The WDM coupler 101d combines the excitation light output by the excitation light source unit 102c into multiple wavelengths and outputs it to the Raman amplified optical fiber 102b.

[0036] The WDM coupler 102e is positioned on the opposite side of the Raman amplification optical fiber 102b from the WDM coupler 102d. The WDM coupler 102e branches the excitation light (residual excitation light) that has passed through the Raman amplification optical fiber 101b to the terminator 102f.

[0037] Next, we will specifically explain the differences between Raman optical amplifier 101 and Raman optical amplifier 102. First, the product of the maximum Raman gain efficiency of the Raman amplification optical fiber 102b in Raman optical amplifier 102 and the fiber length is smaller than the product of the maximum Raman gain efficiency of the Raman amplification optical fiber 101b in Raman optical amplifier 101 and the fiber length.

[0038] Figure 2 illustrates the Raman gain efficiency of a Raman-amplified optical fiber. In Figure 2, the horizontal axis represents the wavelength shift from the excitation wavelength when the excitation light is a single wavelength (e.g., 1447 nm). As shown in Figure 2, the Raman gain efficiency generally reaches its maximum value at a wavelength shifted by approximately 100 nm from the excitation wavelength.

[0039] Furthermore, the number of excitation light sources N2 in the excitation light source section 102c of the Raman light amplifier 102 is smaller than the number of excitation light sources N1 in the excitation light source section 101c of the Raman light amplifier 101. That is, N2 <N1である。

[0040] Furthermore, at least one of the wavelengths of the excitation light output by the excitation light source unit 102c of the Raman light amplifier 102 is different from at least one of the wavelengths of the excitation light output by the excitation light source unit 101c of the Raman light amplifier 101.

[0041] Figure 3 illustrates an example of the relationship between the wavelength of excitation light output by excitation light source unit 102c and the wavelength of excitation light output by excitation light source unit 101c. In Figure 3, solid arrows indicate the wavelength of excitation light output by excitation light source unit 101c, and dashed arrows indicate the wavelength of excitation light output by excitation light source unit 102c. Figure 3(a) shows the case where N1 is 2 and N2 is 1, and Figure 3(b) shows the case where N1 is 3 and N2 is 2. In the cases of Figures 3(a) and (b), the wavelength of excitation light output by excitation light source unit 102c and the wavelength of excitation light output by excitation light source unit 101c are all different.

[0042] In the optical amplifier 1000 configured as described above, the product of the maximum Raman gain efficiency of the Raman amplification optical fiber 102b and its fiber length is smaller than the product of the maximum Raman gain efficiency of the Raman amplification optical fiber 101b and its fiber length. That is, if the maximum Raman gain efficiency is g and the fiber length is L, then the later Raman amplification optical fiber 102b has a smaller gL. As a result, since the later Raman amplification optical fiber 102b, which receives higher optical power, has a smaller gL, nonlinearities such as SBS (Stimulated Brillouin Scattering) and laser oscillation, which are gain limiting factors in the Raman amplification optical fiber 102b, are suppressed. Consequently, a higher gain can be obtained in the optical amplifier 1000.

[0043] Furthermore, since the number of excitation light sources N2 in the excitation light source unit 102c is less than the number of excitation light sources N1 in the excitation light source unit 101c, it is possible to supply excitation light of appropriate power to the Raman amplification optical fiber 102b, which has a small gL.

[0044] Furthermore, since at least one wavelength of the excitation light output by the excitation light source unit 102c is different from at least one wavelength of the excitation light output by the excitation light source unit 101c, a dip in the gain spectrum generated in one Raman light amplifier is compensated for by the other Raman light amplifier. As a result, the flatness of the gain in the optical amplifier 1000 is improved, and at the same time, the flatness of the NF is also improved.

[0045] In Figure 3(a), the wavelength range R21 of the excitation light output by the excitation light source unit 102c of the Raman light amplifier 102 lies within the wavelength range R11 of the excitation light source unit 101c of the Raman light amplifier 101. Similarly, in Figure 3(b), the wavelength range R22 of the excitation light output by the excitation light source unit 102c of the Raman light amplifier 102 lies within the wavelength range R12 of the excitation light output by the excitation light source unit 101c of the Raman light amplifier 101. Thus, when the wavelength range of the excitation light output by the excitation light source unit of the second Raman light amplifier lies within the wavelength range of the excitation light output by the excitation light source unit of the first Raman light amplifier, the flatness of the gain and NF is further improved.

[0046] However, the relationship between the wavelength of the excitation light output by the excitation light source unit 102c and the wavelength of the excitation light output by the excitation light source unit 101c is not limited to the case shown in Figure 3. For example, Figure 4 illustrates another example of the relationship between the wavelength of the excitation light output by the excitation light source unit 102c and the wavelength of the excitation light output by the excitation light source unit 101c. As in the case of Figure 4(a), the wavelength range R13 of the excitation light output by the excitation light source unit 101c and the wavelength range R23 of the excitation light output by the excitation light source unit 102c do not have to overlap. Also, as in the case of Figure 4(b), the wavelength range R14 of the excitation light output by the excitation light source unit 101c and the wavelength range R24 of the excitation light output by the excitation light source unit 102c may overlap at least in part.

[0047] Furthermore, the wavelength range of the excitation light output by the excitation light source of the first Raman light amplifier, which is located further up the stage, may also be within the wavelength range of the excitation light output by the excitation light source of the second Raman light amplifier.

[0048] (Example 1, Comparative Example) As Example 1, an optical amplifier with the same configuration as the optical amplifier according to Embodiment 1 was constructed, and its gain characteristics were simulated. The settings for the excitation light wavelength and power in the Raman optical amplifier 101 of Example 1 are shown below. Wavelength: 1390nm, Power: 492mW Wavelength: 1405nm, Power: 310mW Wavelength: 1430nm, Power: 290mW Total power: 1092mW Furthermore, the settings for the excitation light wavelength and power in the Raman light amplifier 102 are shown below. Wavelength: 1395nm, Power: 440mW Wavelength: 1421nm, Power: 298mW Total power: 738mW Furthermore, for the Raman amplification optical fiber in Raman light amplifier 101, a dispersion-compensated optical fiber with a maximum Raman gain efficiency of 2.4 [1 / W / km] and a fiber length of 4 km was used. Similarly, for the Raman amplification optical fiber in Raman light amplifier 102, a dispersion-compensated optical fiber with a maximum Raman gain efficiency of 2.4 [1 / W / km] and a fiber length of 3 km was used.

[0049] Furthermore, as a comparative example, in Example 1, the wavelength of the excitation light was changed to be the same for both Raman light amplifiers 101 and 102, and the gain characteristics were simulated. The settings for the excitation light wavelength and power in the comparative example Raman light amplifier 101 are shown below. Wavelength: 1390nm, Power: 492mW Wavelength: 1405nm, Power: 310mW Wavelength: 1430nm, Power: 290mW Total power: 1092mW Furthermore, the settings for the excitation light wavelength and power in the Raman light amplifier 102 are shown below. Wavelength: 1390nm, Power: 332.5mW Wavelength: 1405nm, Power: 209.5mW Wavelength: 1430nm, Power: 196mW Total power: 738mW In Example 1 and the Comparative Example, the total power of the excitation light in each Raman light amplifier was set to be equal.

[0050] Furthermore, eight WDM signal lights were set as the light to be amplified, input to the optical amplifier in Example 1 and the Comparative Example. The wavelengths of the eight channels are 1489nm, 1494nm, 1500nm, 1505nm, 1510nm, 1515nm, 1520nm, and 1525nm, respectively. The power of all eight channels is -24dBm.

[0051] Figure 5 shows the gains of Example 1 and the Comparative Example. The Net Gain on the vertical axis refers to the ratio of the input (amplified light: L1) power to the output (amplified light: L2) power in the optical amplifier. In the Comparative Example, the average gain (Net Gain) was 34.48 dB and the gain flatness was 2.36 dB. On the other hand, in Example 1, the average gain was 35.25 dB and the gain flatness was 1.34 dB, which was higher gain and flatter than the Comparative Example. The gain flatness was calculated as {(gain in the channel with the highest gain) - (gain in the channel with the lowest gain)}.

[0052] (Examples 2-4) As Examples 2 to 4, optical amplifiers with the same configuration as the optical amplifier according to Embodiment 1 were fabricated, and their gain and NF were measured.

[0053] The settings for the excitation light wavelength and power in the Raman light amplifier 101 of Example 2 are shown below. Note that the power is the power input to the Raman amplification optical fiber. Example 2 is a case where the wavelength range of the excitation light output by the excitation light source unit 101c and the wavelength range of the excitation light output by the excitation light source unit 102c overlap in part. Wavelength: 1386nm, Power: 242mW Wavelength: 1395nm, Power: 337.5mW Wavelength: 1430nm, Power: 197.5mW Total power: 777mW Furthermore, the settings for the excitation light wavelength and power in the Raman light amplifier 102 are shown below. Wavelength: 1382nm, Power: 184mW Wavelength: 1411nm, Power: 158.5mW Total power: 342.5mW

[0054] The settings for the excitation light wavelength and power in the Raman light amplifier 101 of Example 3 are shown below. Note that in Example 3, the wavelength range of the excitation light output by the excitation light source unit 102c is within the wavelength range of the excitation light output by the excitation light source unit 101c. Wavelength: 1395nm, Power: 296mW Wavelength: 1405nm, Power: 134.5mW Wavelength: 1430nm, Power: 212mW Total power: 642.5mW Furthermore, the settings for the excitation light wavelength and power in the Raman light amplifier 102 are shown below. Wavelength: 1395nm, Power: 328.4mW Wavelength: 1421nm, Power: 251mW Total power: 579.4mW

[0055] The settings for the excitation light wavelength and power in the Raman light amplifier 101 of Example 4 are shown below. Note that in Example 4, the wavelength range of the excitation light output by the excitation light source unit 102c is within the wavelength range of the excitation light output by the excitation light source unit 101c. Wavelength: 1390nm, Power: 492mW Wavelength: 1405nm, Power: 330mW Wavelength: 1430nm, Power: 302mW Total power: 1124mW Furthermore, the settings for the excitation light wavelength and power in the Raman light amplifier 102 are shown below. Wavelength: 1395nm, Power: 464mW Wavelength: 1421nm, Power: 298mW Total power: 762mW

[0056] Furthermore, in all of Examples 2 to 4, a dispersion-compensated optical fiber with a maximum Raman gain efficiency of 2.4 [1 / W / km] and a fiber length of 4 km was used as the Raman amplification optical fiber in Raman light amplifier 101. In addition, a dispersion-compensated optical fiber with a maximum Raman gain efficiency of 2.4 [1 / W / km] and a fiber length of 3 km was used as the Raman amplification optical fiber in Raman light amplifier 102.

[0057] Furthermore, the light to be amplified input to the optical amplifiers in Examples 2-4 was 8-channel WDM signal light. The wavelengths of the 8 channels were 1489nm, 1494nm, 1500nm, 1505nm, 1510nm, 1515nm, 1520nm, and 1525nm, respectively. The power of all 8 channels was -24dBm.

[0058] Figure 6 shows the gain and NF for Examples 2-4. Figure 6(a) shows the gain (Net Gain), and Figure 6(b) shows the NF.

[0059] The average gain and flatness of the gain in Examples 2-4 are as follows. Example 2: Average value: 28.8 dB, Flatness: 1.9 dB Example 3: Average value: 34.0 dB, Flatness: 2.0 dB Example 4: Average value: 35.0 dB, Flatness: 1.5 dB

[0060] The average values ​​of NF and the flatness of NF in Examples 2-4 are as follows. However, the flatness of NF was calculated as {(NF in the channel with the highest NF) - (NF in the channel with the lowest NF)}. Example 2: Average value: 4.91 dB, Flatness: 1.06 dB Example 3: Average value: 4.70 dB, Flatness: 0.58 dB Example 4: Average value: 5.06 dB, Flatness: 0.42 dB

[0061] As described above, in Examples 2 to 4, the average gain was high, and the flatness of the gain and the flatness of the noise figure (NF) were low.

[0062] (Embodiment 2) Figure 7 is a schematic diagram of the optical amplifier according to Embodiment 2. The optical amplifier 1000A includes Raman light amplifiers 101, 102, ..., 10K, where K is an integer of 3 or more. Raman light amplifiers 101, 102, ..., 10K are an example of multiple Raman light amplifiers connected in series.

[0063] Furthermore, the optical amplifier 1000A includes an optical coupler 1001, an optical isolator 1002, an optical coupler 1003, PDs 1004 and 1005, a controller 1006, optical couplers 202, ..., 20K, and PDs 302, ..., 30K.

[0064] The optical coupler 20K is positioned between the Raman light amplifier 10K and the Raman light amplifier immediately preceding it, and branches off a portion of the amplified light (the (K-1) amplified light) amplified in the stage before the Raman light amplifier 10K to be used as detection light. The optical coupler 20K has a configuration similar to, for example, the optical coupler 202.

[0065] The Raman light amplifier 10K optically amplifies the (K-1) amplified light input via the optical coupler 20K and outputs it as amplified light L2.

[0066] PD30K receives the detection light branched by the optical coupler 20K and outputs an electrical signal to the controller 1006 corresponding to the power of the received detection light. In this case, the controller 1006 receives the electrical signals output from PD1004, 1005, 302, and 30K as input, and based on these electrical signals, detects the power of the amplified light L1, the first amplified light, ..., the (K-1) amplified light, and the amplified light L2, and calculates the gain characteristics of the Raman light amplifiers 101, 102, ..., and 10K based on the detection results. The controller 1006 also has the function of controlling the excitation light sources 101c, 102c, ..., and 10Kc based on the calculated gain characteristics.

[0067] Next, the Raman light amplifier 10K includes an optical isolator 10Ka, a Raman amplification optical fiber 10Kb, an excitation light source 10Kc, WDM couplers 10Kd and 10Ke, and a terminator 10Kf.

[0068] The optical isolator 10Ka allows the (K-1) amplified light input to the Raman light amplifier 10K to pass through, while blocking the light traveling in the opposite direction from the (K-1) amplified light.

[0069] When the Raman amplification optical fiber 10Kb receives the (K-1) amplification light while the excitation light is supplied, it performs Raman amplification on this light and outputs the amplified light L2 as the output light of the optical amplifier 1000A. In this embodiment, the Raman amplification optical fiber 10Kb is a dispersion-compensating optical fiber. However, the Raman amplification optical fiber 10Kb is not limited to a dispersion-compensating optical fiber, and other optical fibers with high nonlinearity may also be used.

[0070] The excitation light source unit 10Kc has at least one excitation light source, comprising NK excitation light sources, which supply excitation light to the Raman amplified optical fiber 101Kb. Here, NK is a positive integer at least smaller than N2. The excitation light sources output excitation light to excite the Raman amplified optical fiber 10Kb. The wavelengths of the excitation light output by the excitation light sources are different from each other.

[0071] The WDM coupler 10Kd outputs the excitation light output by the excitation light source unit 10Kc to the Raman amplified optical fiber 10Kb by multi-wavelength superposition.

[0072] The WDM coupler 10Ke is positioned on the opposite side of the Raman amplification optical fiber 10Kb from the WDM coupler 10Kd. The WDM coupler 10Ke branches the excitation light (residual excitation light) that has passed through the Raman amplification optical fiber 10Kb to the terminator 10Kf.

[0073] Here, one Raman optical amplifier selected from Raman optical amplifiers 101, 102, ..., 10K is designated as the second Raman optical amplifier, and the Raman optical amplifier located before the second Raman optical amplifier in Raman optical amplifiers 101, 102, ..., 10K is designated as the first Raman amplifier. Then, the product of the maximum value of the Raman gain efficiency of the Raman amplification optical fiber in the second Raman optical amplifier and the fiber length is smaller than the product of the maximum value of the Raman gain efficiency of the Raman amplification optical fiber in the first Raman optical amplifier and the fiber length. Also, the number of excitation sources in the excitation source section of the second Raman optical amplifier is smaller than the number of excitation sources in the excitation source section of the first Raman optical amplifier. Furthermore, at least one wavelength of the excitation light output by the excitation source section of the second Raman optical amplifier is different from at least one wavelength of the excitation light output by the excitation source section of the first Raman optical amplifier.

[0074] The optical amplifier 1000A configured as described above can achieve the same effects as the optical amplifier 1000 according to Embodiment 1. In particular, it can achieve a higher gain than the optical amplifier 1000.

[0075] It should be noted that the present invention is not limited to the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of Symbols]

[0076] 101, 102, ..., 10K: Raman light amplifier 101a, 102a, ..., 10Ka, 1002: Optical isolators 101b, 102b, ..., 10Kb: Raman-amplified optical fiber 101c, 102c, ..., 10Kc: Excitation light source 101d, 102d, ..., 10Kd, 101e, 102e, ..., 10Ke: WDM coupler 101f,102f,...,10Kf:Terminator 202,···,20K,1001,1003: Optical coupler 302,···,30K,1004,1005:PD 1000, 1000A: Optical Amplifier 1006: Controller L1: Amplified light L2: Amplified light R11, R12, R13, R14, R21, R22, R23, R24: Range

Claims

1. The system comprises a plurality of Raman light amplifiers, each having a Raman amplification optical fiber and an excitation light source unit that supplies excitation light to the Raman amplification optical fiber. The aforementioned multiple Raman light amplifiers are connected in series, The product of the maximum Raman gain efficiency of the Raman amplification optical fiber in the second Raman optical amplifier among the plurality of Raman optical amplifiers and its fiber length is smaller than the product of the maximum Raman gain efficiency of the Raman amplification optical fiber in the first Raman optical amplifier located before the second Raman optical amplifier and its fiber length. The number of excitation light sources in the excitation light source section of the second Raman light amplifier is smaller than the number of excitation light sources in the excitation light source section of the first Raman light amplifier. At least one of the wavelengths of the excitation light output by the excitation light source of the second Raman light amplifier is different from at least one of the wavelengths of the excitation light output by the excitation light source of the first Raman light amplifier. Optical amplification device.

2. At least one of the plurality of Raman light amplifiers has a terminator for terminating residual excitation light. The optical amplification device according to claim 1.

3. At least one of the Raman amplification optical fibers of the plurality of Raman light amplifiers is a dispersion compensation optical fiber. The optical amplification device according to claim 1.

4. The optical amplification device includes an input-side photodetector for monitoring the power of the input light to be amplified, and an output-side photodetector for monitoring the power of the output amplified light. The optical amplification device according to claim 1.

5. The number of the aforementioned Raman light amplifiers is two. The optical amplification device according to claim 1.

6. The wavelength range of the excitation light output by the excitation light source unit of the second Raman light amplifier overlaps, at least in part, with the wavelength range of the excitation light output by the excitation light source unit of the first Raman light amplifier. The optical amplification device according to claim 1.

7. The wavelength range of the excitation light output by the excitation light source unit of the second Raman light amplifier is within the wavelength range of the excitation light output by the excitation light source unit of the first Raman light amplifier. The optical amplification device according to claim 1.

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