Multi-core fiber amplifier, optical amplification method, and recording medium

Abstract

This multi-core fiber amplifier comprises: a first optical fiber amplifier having a first excitation MCF that is cladding-pumped by excitation and backward excitation; an excitation light source that outputs an excitation light that excites the first excitation MCF; a second optical fiber amplifier having a second excitation MCF that is cladding-pumped by forward excitation and backward excitation using the excitation light output from the first excitation MCF; and a control circuit that performs control relating to the excitation light power of at least one of the first optical fiber amplifier and the second optical fiber amplifier.

Description

Multi-core fiber amplifier, optical amplification method, and recording medium

[0001] The present disclosure relates to a multi-core fiber amplifier, an optical amplification method, and a recording medium.

[0002] The practical application of spatial optical multiplexing transmission systems using multicore fibers (MCFs) is progressing. MCFs have multiple cores in a single optical fiber. Therefore, using MCFs instead of single-core fibers (SCFs) can increase the transmission capacity of optical transmission systems. MCF amplifiers (multicore fiber amplifiers) that use MCFs as the optical amplification medium are also known. The optical amplification medium is an optical fiber whose core is doped with a rare-earth element such as erbium (Er). The rare-earth element doped in the core of the optical amplification medium is excited by pump light, amplifying the signal light. Hereinafter, an optical fiber whose core is doped with a rare-earth element will be referred to as a "pump fiber." Furthermore, an MCF whose core is doped with a rare-earth element will be referred to as a "pump MCF." An erbium-doped pump fiber is sometimes called an EDF (Er-doped fiber). An optical amplifier using an EDF is sometimes called an EDFA (Er-doped fiber amplifier).

[0003] There are two known pumping methods for MCF amplifiers: "core pumping," in which pumping light is directly injected into the core, and "clad pumping," in which pumping light is injected into the clad to simultaneously pump the rare earth elements in multiple cores.

[0004] Wavelength division multiplexed signal light is usually used as the signal light transmitted by MCF. The C-band and L-band are known as wavelength division multiplexed signal light bands. The C-band band is approximately 1520-1560 nm, and the L-band band is approximately 1570-1610 nm. Hereinafter, wavelength division multiplexed signal light will be referred to as WDM (wavelength division multiplexing) light.

[0005] In relation to the present disclosure, Patent Document 1 describes a multi-core EDFA in which pump light (residual pump light) output from a multi-core EDF used for amplifying a C-band signal is supplied to a multi-core EDF used for amplifying a L-band signal.

[0006] International Publication No. 2022 / 181294

[0007] A multi-band MCF transmission system is known that transmits C-band signal light and L-band signal light using an MCF transmission line. In a multi-band MCF transmission system, WDM light in which the power of each carrier is approximately equal is generally input to an MCF amplifier. The power of each carrier is also required to remain approximately equal after amplification. Therefore, it is preferable that the gain of the MCF amplifier has a flat wavelength characteristic. In other words, it is preferable that the gain of the MCF amplifier has a small wavelength-dependent fluctuation range. However, the multi-band EDFA described in Patent Document 1 has a problem in that it cannot flatten the gain. Furthermore, a single-core EDFA using a single-core EDF has a problem in that it cannot amplify signal light propagating through multiple cores of the MCF, and also cannot simultaneously amplify signal light in the C-band and L-band.

[0008] This disclosure describes techniques that can improve the gain flatness of an MCF amplifier that supports multi-band amplification.

[0009] The multicore fiber amplifier according to the present disclosure comprises: a first optical fiber amplifying means having a first pumping MCF (multicore fiber) that is clad-pumped by forward pumping and backward pumping; a pumping means that outputs pumping light that pumps the first pumping MCF; a second optical fiber amplifying means having a second pumping MCF that is clad-pumped by forward pumping and backward pumping using the pumping light output from the first pumping MCF; and a control means that controls the pumping light power of at least one of the first optical fiber amplifying means and the second optical fiber amplifying means.

[0010] An optical amplification method according to the present disclosure includes the steps of: cladding-pumping a first pumping MCF with pumping light; cladding-pumping a second pumping MCF using the pumping light output from the first pumping MCF; and controlling the pumping light power of at least one of the first pumping MCF and the second pumping MCF.

[0011] The recording medium of the present disclosure has recorded thereon a program for causing a computer to execute the following steps: a procedure for clad-pumping a first pumping MCF with pumping light; a procedure for clad-pumping a second pumping MCF with residual pumping light output from the first pumping MCF; and a procedure for controlling the power of the pumping light.

[0012] The present disclosure makes it possible to improve the gain flatness of an MCF amplifier that supports multi-band amplification.

[0013] FIG. 1 is a diagram illustrating an example of the configuration of an MCF amplifier; FIG. 2 is a diagram illustrating an optical path of a first pumping light; FIG. 3 is a diagram illustrating an optical path of a second pumping light; FIG. 4 is a diagram illustrating an example of the wavelength dependency of the gain of a cladding-pumped EDFA; FIG. 5 is a block diagram illustrating an example of the configuration of a control circuit; FIG. 6 is a diagram illustrating an example of the configuration of an MCF amplifier; FIG. 7 is a diagram illustrating an example of the configuration of an MCF amplifier; FIG. 8 is a diagram illustrating another example of the configuration of an MCF amplifier; FIG. 9 is a diagram illustrating an example of the configuration of an MCF amplification system; and FIG. 10 is a flowchart illustrating an example of a control procedure for pumping light power.

[0014] First Embodiment Fig. 1 is a diagram showing a configuration example of an MCF amplifier (multi-core fiber amplifier) ​​1000 according to the present disclosure. The MCF amplifier 1000 includes pumping MCFs 100 and 200. Each of the pumping MCFs 100 and 200 is a pumping MCF having N cores, where N is an integer equal to or greater than 2. The cores of the pumping MCFs 100 and 200 are doped with erbium.

[0015] The MCF amplifier 1000 receives C-band signal light and L-band signal light propagating through an input optical fiber 10. The C-band signal light and the L-band signal light may be wavelength-multiplexed in one core. The input optical fiber 10 is, for example, a single-core fiber (SCF) with N cores. However, the input optical fiber 10 may also be an MCF with N cores (N-core MCF). Each core of the input optical fiber 10 is connected to an optical demultiplexer (DMUX) 11. The optical demultiplexer 11 wavelength-separates the C-band signal light and the L-band signal light for each core. The output of the optical demultiplexer 11 is connected to fan-ins (FIs) 12 and 22 via N SCFs. The C-band signal light output from the optical demultiplexer 11 is input to the MCF 101 via the fan-in 12. The L-band signal light output from the optical demultiplexer 11 is input to the MCF 201 via the fan-in 22. Both the MCFs 101 and 201 are N-core MCFs.

[0016] The C-band signal light is amplified by the pumping MCF 100. The amplified C-band signal light is input to the fan-out (FO) 13 via the MCF 102. The fan-out 13 outputs the amplified C-band signal light to N SCFs. Similarly, the L-band signal light is amplified by the pumping MCF 200. The amplified L-band signal light is input to the fan-out 23 via the MCF 202. The fan-out 23 outputs the amplified L-band signal light to N SCFs. The optical multiplexer (MUX) 14 wavelength-multiplexes the C-band signal light and the L-band signal light for each core and outputs the multiplexed signal to the output optical fiber 15. The output optical fiber 15 is, for example, N SCFs. However, the output optical fiber 15 may also be an N-core MCF.

[0017] The configuration for pumping the pumping MCFs 100 and 200 will be described. The pumping light source 300 outputs pumping light (first pumping light and second pumping light) that pumps the pumping MCFs 100 and 200. The pumping light source 300 includes two pumping LDs (laser diodes) 301 and 302. The pumping LDs 301 and 302 are both multimode LDs. The pumping LD 301 outputs the first pumping light, and the pumping LD 302 outputs the second pumping light. The wavelengths of the first pumping light and the second pumping light are both, for example, in the 980 nm band. The control circuit 380 controls the power of the pumping light (pumping light power) output by the pumping LDs 301 and 302. The first pumping light is input to an optical circulator 311. The second pumping light is input to an optical circulator 312. The first pumping light and the second pumping light may each be generated by mode combining pumping light output from a plurality of multi-mode LDs.

[0018] 2 is a diagram illustrating the optical path of the first pumping light. The first pumping light is input to the optical circulator 311 via the MMF 351. The first pumping light output from the optical circulator 311 is input to the clad of the pumping MCF 100 via the pumping light coupling unit 321. Therefore, the core of the pumping MCF 100 is forward pumped by the first pumping light input to the clad. The first pumping light not consumed in the pumping MCF 100 is output from the pumping light coupling unit 322 to the optical circulator 312 as first residual pumping light. The first residual pumping light is input to the pumping light coupling unit 323 via the optical circulator 312 and the MMF 352. The pumping light coupling unit 323 inputs the first residual pumping light to the clad of the pumping MCF 200. As a result, the core of the pumping MCF 200 is forward pumped by the first residual pumping light input to the clad. The first residual pump light that has not been consumed in the pump MCF 200 is output from the pump light coupling unit 324 to the optical circulator 311 via the MMF 354 .

[0019] Here, the first residual pump light output from the pump light coupling unit 324 may be terminated in the optical circulator 311. When the first residual pump light is terminated in the optical circulator 311, the first residual pump light is not output to the MMF 351. This reduces the influence of the first residual pump light on the pump LD 301. Alternatively, an optical isolator may be provided between the pump LD 301 and the optical circulator 351. Note that these configurations for terminating the residual pump light input to the optical isolator 311 can also be applied to the case of terminating the residual pump light input to another optical isolator in the following embodiments.

[0020] 3 is a diagram illustrating the optical path of the second pumping light. The second pumping light is input to the optical circulator 312 via the MMF 353. The second pumping light output from the optical circulator 312 is input to the clad of the pumping MCF 100 via the pumping light coupling unit 322. Therefore, the core of the pumping MCF 100 is backward pumped by the second pumping light input to the clad. The second pumping light not consumed in the pumping MCF 100 is output from the pumping light coupling unit 321 to the optical circulator 311 as second residual pumping light. The second residual pumping light is input to the pumping light coupling unit 324 via the optical circulator 311 and the MMF 354. The pumping light coupling unit 324 inputs the second residual pumping light to the clad of the pumping MCF 200. That is, the core of the pumping MCF 200 is backward pumped by the second residual pumping light input to the clad.

[0021] With this configuration, the pumping MCF 100 amplifies WDM light in the C band. The pumping MCF 100 operates as a cladding-pumped MCF amplifier using the first pumping light and the second pumping light. The first pumping light pumps the pumping MCF 100 by forward pumping, and the second pumping light pumps the pumping MCF 100 by backward pumping. The pumping MCF 200 amplifies WDM light in the L band. The pumping MCF 200 operates as a cladding-pumped MCF amplifier using the first residual pumping light and the second residual pumping light. The first residual pumping light pumps the pumping MCF 200 by forward pumping, and the second residual pumping light pumps the pumping MCF 200 by backward pumping. That is, both the pumping MCF 100 and the pumping MCF 200 operate by bidirectional pumping.

[0022] If the powers of the first pumping light and the second pumping light at the time of output from the pumping light source 300 are P1 and P2, respectively, then the control parameter η (eta) can be defined as η = P1 / P2. That is, η is the ratio between P1 and P2. P1 corresponds to the power of forward pumping, and P2 corresponds to the power of backward pumping. Note that in control using the control parameter η, P2 is not 0 (zero). Note that values ​​in which passive losses between the pumping light source 300 and the MCF 100 and MCF 200 are compensated for may be used for P1 and P2.

[0023] Figure 4 shows an example of the wavelength dependence of the gain of a cladding-pumped EDFA. The vertical axis indicates the gain of each wavelength relative to the wavelength at which the maximum gain is achieved (approximately 1538 nm). For example, "-3 dB" on the vertical axis indicates a gain 3 dB lower than the maximum gain. In Figure 4, "UD-BWD" stands for backward pumping (unidirectional and backward), "BD" stands for bidirectional pumping, and "UD-FWD" stands for forward pumping (unidirectional and forward). For example, "BD (0.14 / 0.86)" indicates bidirectional pumping with a ratio of 0.14:0.86 between the power of the forward pumping pump light and the power of the backward pumping pump light. Figure 4 shows that the gain flatness of the EDFA can be controlled by changing the ratio of P1 to P2. For example, by increasing the ratio of the backward pumping power P2, the gain flatness around the wavelength of 1558 nm is improved by approximately 0.8 dB compared to forward pumping alone (UD-FWD). That is, by changing the parameter η (=P1 / P2), the flatness of the gain of each of the pumping MCFs 100 and 200 can be improved.

[0024] An optical equalizer is generally used to flatten the gain of an optical amplifier that amplifies WDM light. The optical equalizer has wavelength characteristics that offset the wavelength characteristics of the gain of the optical amplifier. The wavelength characteristics of the optical equalizer are set so that loss is minimized for the lowest-power carrier contained in the WDM light. Therefore, if the wavelength characteristics of the WDM light output from the optical amplifier are not flat, part of the power of the WDM light is lost by the optical equalizer. Therefore, by improving the flatness of the gain characteristics of the optical amplifier, it is possible to reduce the power of the WDM light lost in the optical equalizer. This contributes to a reduction in the gain required for the optical fiber amplifier, i.e., a reduction in the power required to generate pump light.

[0025] 5 is a block diagram showing an example configuration of the control circuit 380. The control circuit 380 includes a central processing unit (CPU) 381, a recording device (memory) 382, ​​a drive circuit 383, and a bus 384. The central processing unit 381 is connected to the memory 382 and the drive circuit 383 via the bus 384. The drive circuit 383 is connected to the pump light source 300. The drive circuit 383 adjusts the power of the pump light output from the pump light source 300 in accordance with a control signal generated in the control circuit 380. The central processing unit 381 is a computer, and generates the control signal by executing a program recorded in the memory 382. The recorder 382 is a tangible, non-transitory recording medium for the program.

[0026] As described above, the MCF amplifier 1000 makes it possible to improve the gain flatness of a multi-core fiber amplifier that is compatible with multi-band amplification.

[0027] 1 can also be described as follows. That is, the optical amplifier including the pumping MCF 100 and the pumping light coupling units 321 and 322 serves as a first optical fiber amplifying means. Also, the optical amplifier including the pumping MCF 200 and the pumping light coupling units 323 and 324 serves as a second optical fiber amplifying means. The optical circulators 311 and 312 can be included in either the first optical amplifying means or the second optical fiber amplifying means.

[0028] The pumping MCF 100 can be called a first pumping MCF. The first pumping MCF is clad-pumped by forward pumping and backward pumping. The pumping light source 300 serves as pumping means that outputs pumping light that pumps the first pumping MCF. The pumping MCF 200 can be called a second pumping MCF. The second pumping MCF is clad-pumped by forward pumping and backward pumping using the pumping light output from the first pumping MCF. The control circuit 380 serves as control means that controls the power of the pumping light. The control means controls the pumping light power of at least one of the first optical fiber amplifying means and the second optical fiber amplifying means. A multi-core fiber amplifier having such a configuration can control the gain flatness of the EDFA by controlling the pumping light power.

[0029] The first pumping MCF may amplify light in a first wavelength band (C band), and the second pumping MCF may amplify light in a second wavelength band (L band).

[0030] The optical demultiplexer 11 functions as an optical demultiplexing means. The optical demultiplexing means wavelength-separates the light input to the MCF amplifier 1000 into light of a first wavelength band (e.g., C band) and light of a second wavelength band (e.g., L band). Furthermore, the optical demultiplexing means outputs the light of the first wavelength band to the first optical fiber amplifying means and outputs the light of the second wavelength band to the second optical fiber amplifying means.

[0031] The optical multiplexer 14 functions as an optical multiplexing means, which wavelength-multiplexes the light output from the first optical fiber amplifying means and the light output from the second optical fiber amplifying means.

[0032] Second Embodiment Fig. 6 is a diagram illustrating an exemplary configuration of an MCF amplifier 2000 according to the present disclosure. The MCF amplifier 2000 includes a pumping light source 303 instead of the pumping light source 300 in the MCF amplifier 1000. The pumping light source 303 includes a pumping LD 304 and a variable branching coupler 305. The variable branching coupler 305 is a general optical device. The variable branching coupler 305 distributes input light in two directions at a predetermined branching ratio in response to an instruction from a control circuit 380. Specifically, the variable branching coupler 305 distributes the pumping light output from the pumping LD 304 to optical circulators 311 and 312 in response to an instruction from the control circuit 380. That is, the control circuit 380 controls the branching ratio of the variable branching coupler 305 in response to a control parameter η.

[0033] The pumping light input from the pumping light source 303 to the optical circulator 311 is the first pumping light described in FIGS. 1 to 3 , and the pumping light input to the optical circulator 312 is the second pumping light. The configurations of the paths of the signal light input and output to and from the pumping MCFs 100 and 200, and the configurations of the paths of the first and second pumping lights, are the same as those in FIGS. 1 to 3 . Therefore, the MCF amplifier 2000 also makes it possible to improve the gain flatness of a multi-core fiber amplifier compatible with multi-band amplification. Furthermore, the pumping light source 303 can reduce the number of pumping LDs, thereby simplifying the configuration of the MCF amplifier 2000.

[0034] 7 is a diagram showing an example of the configuration of an MCF amplifier 3000 according to the present disclosure. The MCF amplifier 3000 differs from the MCF amplifier 1000 shown in FIG. 1 in that it includes variable attenuators 331 and 332.

[0035] The variable attenuator 331 is disposed between the optical circulator 312 and the pumping light coupling unit 323. The variable attenuator 332 is disposed between the optical circulator 311 and the pumping light coupling unit 324. The variable attenuator 331 adds a predetermined attenuation to the first residual pumping light that is output from the pumping MCF 100 and propagated through the pumping light coupling unit 322 and the optical isolator 312, and inputs the light to the pumping MCF 200. The variable attenuator 332 adds a predetermined attenuation to the second residual pumping light that is output from the pumping MCF 100 and propagated through the pumping light coupling unit 321 and the optical isolator 311, and inputs the light to the pumping MCF 200. The attenuation amounts of the variable attenuators 331 and 332 may be controlled by the control circuit 380.

[0036] The MCF amplifier 3000 having such a configuration also improves gain flatness by controlling the power of the pump light output from the pump light source 300. In particular, by including the variable attenuators 331 and 332, the MCF amplifier 3000 can independently adjust the power of the first residual pump light and the power of the second residual pump light. Furthermore, the attenuation amounts of the variable attenuators 331 and 332 do not affect the power of the first pump light and the second pump light. Therefore, the MCF amplifier 3000 can change the pumping state of the pumping MCF 200 (e.g., the magnitude and ratio of the forward pumping pump light power and the backward pumping pump light power) without affecting the operation of the pumping MCF 100. For example, when the pumping MCF 100 amplifies a C-band signal light and the pumping MCF 200 amplifies an L-band signal light, the MCF amplifier 3000 can independently adjust the wavelength characteristics of the pumping MCF 100 for the C-band signal light and the wavelength characteristics of the pumping MCF 200 for the L-band signal light.

[0037] Fourth Embodiment FIG. 8 is a diagram illustrating a configuration example of an MCF amplifier 4000 according to the present disclosure. The MCF amplifier 4000 includes a pumping light source 306 having four outputs, pumping light combiners 325-328, and optical switches 341-342. The optical switches 341 and 342 are both 2×2 optical switches, and are switched between a bar state and a cross state by a control circuit 380. FIG. 8 illustrates a case where the optical switches 341 and 342 are in the cross state. The pumping light combiners 325-328 are optical devices that combine two input pumping lights and output the combined light. The pumping light combiners 325-328 may be optical devices that mode combine multimode light.

[0038] The pumping light source 306 outputs pumping light 471-474. Four pumping LDs may generate the pumping light 471-474, respectively. The control circuit 380 can independently control the power of each of the pumping light 471-474. The pumping light 471 is input to the clad of the pumping MCF 100 via the pumping light coupler 325, the optical circulator 311, and the pumping light coupler 321. The pumping light 471 that has pumped the pumping MCF 100 is input as pumping light 481 to the clad of the pumping MCF 200 via the pumping light coupler 322, the optical circulator 312, the optical switch 341, the pumping light coupler 327, the optical circulator 313, and the pumping light coupler 323. The pumping light 481 is residual pumping light corresponding to the pumping light 471. The pumping light 481 that pumped the pumping MCF 200 is input again as pumping light 491 into the clad of the pumping MCF 100 via the pumping light coupling unit 324, the optical circulator 314, the optical switch 342, the pumping light coupling unit 325, the optical circulator 311, and the pumping light coupling unit 321. The pumping light 491 is residual pumping light corresponding to the pumping light 481.

[0039] In this way, the pumping light 471 output from the pumping light source 306 can pump the pumping MCF 100 and the pumping MCF 200, and then again pump the pumping MCF 100. As a result, the MCF amplifier 4000 exhibits an advantage of being able to more effectively utilize the energy of the pumping light than the MCF amplifiers 1000, 2000, and 3000.

[0040] The pumping light 472 is input to the clad of the pumping MCF 100 via the pumping light coupling unit 326, the optical circulator 312, and the pumping light coupling unit 322. The pumping light 472 that pumped the pumping MCF 100 is input as pumping light 482 to the clad of the pumping MCF 200 via the pumping light coupling unit 321, the optical circulator 311, the optical switch 342, the pumping light coupling unit 328, the optical circulator 314, and the pumping light coupling unit 324. The pumping light 482 is residual pumping light corresponding to the pumping light 472. The pumping light 482 that pumped the pumping MCF 200 is input as pumping light 492 to the clad of the pumping MCF 100 again via the pumping light coupling unit 323, the optical circulator 313, the optical switch 341, the pumping light coupling unit 326, the optical circulator 312, and the pumping light coupling unit 322. The pump light 492 is the residual pump light corresponding to the pump light 482 .

[0041] That is, the pumping light 472, like the pumping light 471, pumps the pumping MCF 100 and the pumping MCF 200 in this order, and then again pumps the pumping MCF 100. That is, the MCF amplifier 4000 can also effectively utilize the energy of the pumping light 472.

[0042] The pumping light 473 and 474 differ from the pumping light 471 and 472 in that the pumping light 473 and 474 first pump the pumping MCF 200. That is, the pumping light 473 is input to the clad of the pumping MCF 200 via the pumping light coupling unit 327, the optical circulator 313, and the pumping light coupling unit 323. The pumping light 473 that has pumped the pumping MCF 200 is input as pumping light 483 to the clad of the pumping MCF 100 via the pumping light coupling unit 324, the optical circulator 314, the optical switch 342, the pumping light coupling unit 325, the optical circulator 311, and the pumping light coupling unit 321. The pumping light 483 is residual pumping light corresponding to the pumping light 473. The pumping light 483 that pumped the pumping MCF 100 is input again as pumping light 493 into the clad of the pumping MCF 200 via the pumping light coupling unit 322, the optical circulator 312, the optical switch 341, the pumping light coupling unit 327, the optical circulator 313, and the pumping light coupling unit 323. The pumping light 493 is residual pumping light corresponding to the pumping light 483.

[0043] The pumping light 474 is input to the clad of the pumping MCF 200 via the pumping light coupling unit 328, the optical circulator 314, and the pumping light coupling unit 324. The pumping light 474 that has pumped the pumping MCF 200 is input as pumping light 484 to the clad of the pumping MCF 100 via the pumping light coupling unit 323, the optical circulator 313, the optical switch 341, the pumping light coupling unit 326, the optical circulator 312, and the pumping light coupling unit 322. The pumping light 484 is residual pumping light corresponding to the pumping light 474. The pumping light 484 that has pumped the pumping MCF 100 is input as pumping light 494 to the clad of the pumping MCF 200 again via the pumping light coupling unit 321, the optical circulator 311, the optical switch 342, the pumping light coupling unit 328, the optical circulator 314, and the pumping light coupling unit 324. The pump light 494 is the residual pump light corresponding to the pump light 484 .

[0044] In this way, the pumping lights 473 and 474 pump the pumping MCF 200 and the pumping MCF 100 in this order, and then pump the pumping MCF 200 again. That is, the MCF amplifier 4000 can effectively utilize the energy of the pumping lights 473 and 474. The MCF amplifier 4000 shown in Fig. 8 also makes it possible to improve the gain flatness of the multi-core fiber amplifier.

[0045] FIG. 9 is a diagram illustrating another exemplary configuration of an MCF amplifier 4000 according to the present disclosure. FIG. 9 illustrates a case in which the optical switches 341 and 342 are in the bar state. In FIG. 9 , pumping light 471 is input to the clad of the pumping MCF 100 via the pumping light coupling unit 325, the optical circulator 311, and the pumping light coupling unit 321. The pumping light 471 that has pumped the pumping MCF 100 is input as pumping light 481 to the optical circulator 313 via the pumping light coupling unit 322, the optical circulator 312, and the optical switch 341. In FIG. 9 , the pumping light 481 is terminated in the optical circulator 313. Because the pumping light 481 is terminated in the optical circulator 313, the pumping light 471 pumps only the pumping MCF 100 once in FIG. 9 .

[0046] Furthermore, the pumping light 472 is input to the clad of the pumping MCF 100 via the pumping light coupling unit 326, the optical circulator 312, and the pumping light coupling unit 322. The pumping light 472 that has pumped the pumping MCF 100 is input as pumping light 482 to the optical circulator 314 via the pumping light coupling unit 321, the optical circulator 311, and the optical switch 342. In Fig. 9, the pumping light 482 is terminated in the optical circulator 314. Since the pumping light 481 is terminated in the optical circulator 313, the pumping light 472 pumps only the pumping MCF 100 once in Fig. 9.

[0047] Furthermore, the pumping light 473 is input to the clad of the pumping MCF 200 via the pumping light coupling unit 327, the optical circulator 313, and the pumping light coupling unit 323. The pumping light 473 that has pumped the pumping MCF 200 is input as pumping light 483 to the optical circulator 311 via the pumping light coupling unit 324, the optical circulator 314, and the optical switch 342. In Fig. 9, the pumping light 483 is terminated in the optical circulator 311. By terminating the pumping light 483 in the optical circulator 311, the pumping light 473 pumps only the pumping MCF 200 once in Fig. 9.

[0048] In addition, the pumping light 474 is input to the clad of the pumping MCF 200 via the pumping light coupling unit 328, the optical circulator 314, and the pumping light coupling unit 324. The pumping light 474 that has pumped the pumping MCF 200 is input as pumping light 484 to the optical circulator 312 via the pumping light coupling unit 323, the optical circulator 313, and the optical switch 341. In Fig. 9, the pumping light 484 is terminated in the optical circulator 312. Since the pumping light 481 is terminated in the optical circulator 312, the pumping light 474 pumps only the pumping MCF 200 once in Fig. 9.

[0049] According to the configuration of Fig. 9, none of the pumping lights 471-474 circulates around a looped optical path including the pumping MCFs 100 and 200. Therefore, the configuration of Fig. 9 simplifies the optical path of the pumping lights, and the operation of the MCF amplifier 4000 can be stabilized. Furthermore, each of the pumping lights 471-474 pumps the pumping fiber 100 or 200 only once by forward pumping or backward pumping. Therefore, according to the configuration of Fig. 9, the pumping power of the forward pumping and backward pumping of the pumping fibers 100 and 200 can be controlled independently.

[0050] 8 and 9, the optical switches 341 and 342 can be switched between a cross state and a bar state. When the optical switches 341 and 342 are in the cross state, the energy of the pump light can be utilized more effectively. When the optical switches 341 and 342 are in the bar state, the operational stability of the MCF amplifier 4000 can be improved.

[0051] In the MCF amplifier 4000, when the powers of the pumping lights 471, 472, 473, and 474 are P1, P2, P3, and P4, respectively, the following two types of control parameters can be used to control the power of the pumping lights of the MCF amplifier 4000.

[0052] η1=P1 / P2 η2=P3 / P4 η1 is the ratio between the power of the pumping light 471 and the power of the pumping light 472, and controls the ratio between the pumping light power of forward pumping and the pumping light power of backward pumping in the pumping MCF 100. Furthermore, η2 is the ratio between the power of the pumping light 473 and the power of the pumping light 474, and controls the ratio between the pumping light power of forward pumping and the pumping light power of backward pumping in the pumping MCF 200. The control circuit 380 controls the four outputs of the pumping light source 306 in accordance with the control parameters η1 and η2.

[0053] Fifth Embodiment Fig. 10 is a diagram showing an example configuration of an MCF amplification system 5000 according to the present disclosure. The MCF amplification system 5000 includes an MCF amplifier 1000 and a wavelength monitor 390. The configuration of the MCF amplifier 1000 has been described using Fig. 1. In Fig. 10, the configuration of the MCF amplifier 1000 is shown in a simplified form. WDM light having substantially the same carrier power is input to the input optical fiber 10. Therefore, the wavelength characteristics of the WDM light propagating through the output optical fiber 15 reflect the wavelength characteristics of the gain of the MCF amplifier 1000. Therefore, by flattening the wavelength characteristics of the gain of the MCF amplifier 1000, it is possible to reduce the difference in the power of the WDM light output from the MCF amplifier 1000 depending on the wavelength.

[0054] The wavelength monitor 390 monitors the wavelength characteristics of the WDM light propagating through the output optical fiber, and notifies the control circuit 380 of the resulting data as monitoring information. The control circuit 380 controls the pumping light source 300 using the monitoring information notified from the wavelength monitor 390. A general optical channel monitor (OCM) may be used as the wavelength monitor.

[0055] The wavelength monitor 390 may generate monitor information of the signal light propagating through at least one core included in the output optical fiber 15. The monitor information may include, for example, the power at multiple wavelengths included in the wavelength band of the WDM light to be monitored. The monitor information may include data on the power at the upper end and the lower end of the wavelength band of the WDM light to be monitored. The monitor information may further include data on the power between the upper and lower ends of the wavelength band of the WDM light (e.g., the central wavelength of the wavelength band). The wavelength monitor 390 serves as wavelength monitoring means that outputs the results of monitoring the power of the light output from the MCF amplifier 1000 for each wavelength as monitor information. The monitor information is the result of monitoring the power of the light output from at least one of the pumping MCF 100 and the pumping MCF 200 for each wavelength.

[0056] The control circuit 380 controls the power of the pump light output by the pump light source 300 so that the wavelength characteristics of the WDM light indicated by the monitor information approach flat. The control circuit 380 may control the power of the pump light by changing the control parameter η described above. When the monitor information includes the wavelength characteristics of the C-band signal light and the wavelength characteristics of the L-band signal light, the power of the pump light may be controlled so that the wavelength characteristics of both of these signal lights approach flat.

[0057] 11 is a flowchart showing an example of a procedure for controlling pump light power in the MCF amplifier system 5000. The wavelength monitor 390 acquires monitor information of the WDM light output from the MCF amplifier 1000 (step S01 in FIG. 11 ) and notifies the monitor information to the control circuit 380 (step S02). Based on the monitor information, the control circuit 380 controls the control parameter η so as to improve the flatness of the wavelength characteristics of the WDM light. Through such control, the MCF amplifier system 5000 can improve the flatness of the wavelength characteristics of the WDM light output from the MCF amplifier 1000.

[0058] 10, the MCF amplifier 1000 has been described as an example. However, the configuration for controlling the wavelength characteristics of WDM light using the wavelength monitor 390 can also be applied to the MCF amplifiers 2000, 3000, and 4000 already described.

[0059] The embodiments of the present disclosure can also be described as follows, but are not limited to the following:

[0060] (Supplementary Note 1) A multicore fiber amplifier comprising: a first optical fiber amplifying means comprising a first pumping MCF (multicore fiber) that is clad-pumped by forward pumping and backward pumping; a pumping means that outputs pumping light that pumps the first pumping MCF; a second optical fiber amplifying means comprising a second pumping MCF that is clad-pumped by forward pumping and backward pumping using the pumping light output from the first pumping MCF; and a control means that controls the pumping light power of at least one of the first optical fiber amplifying means and the second optical fiber amplifying means.

[0061] (Supplementary Note 2) The multi-core fiber amplifier according to Supplementary Note 1, wherein the control means controls a ratio between forward pumping and backward pumping in the first pumping MCF.

[0062] (Supplementary Note 3) The multi-core fiber amplifier according to Supplementary Note 2, wherein the control means controls a ratio between forward pumping and backward pumping in the second pumping MCF.

[0063] (Supplementary Note 4) The multi-core fiber amplifier according to Supplementary Note 2 or 3, wherein the control means controls the power of the pumping light so as to reduce the wavelength dependence of the gain of the first pumping MCF and the wavelength dependence of the gain of the second pumping MCF.

[0064] (Supplementary Note 5) The multi-core fiber amplifier according to any one of Supplementary Notes 1 to 4, wherein the first pumping MCF amplifies light in a first wavelength band, and the second pumping MCF amplifies light in a second wavelength band.

[0065] (Supplementary Note 6) A multi-core fiber amplifier according to Supplementary Note 5, comprising: optical demultiplexing means for wavelength-separating input light into light of the first wavelength band and light of the second wavelength band, and outputting the light of the first wavelength band to the first optical fiber amplifying means and outputting the light of the second wavelength band to the second optical fiber amplifying means; and optical multiplexing means for wavelength-multiplexing the light output from the first optical fiber amplifying means and the light output from the second optical fiber amplifying means.

[0066] (Supplementary Note 7) The multi-core fiber amplifier according to Supplementary Note 5 or 6, wherein the first wavelength band is a C-band and the second wavelength band is an L-band.

[0067] (Supplementary Note 8) The multi-core fiber amplifier according to any one of Supplementary Notes 1 to 7, further comprising: wavelength monitoring means for monitoring the power of light output from the multi-core fiber amplifier for each wavelength and outputting the result as monitor information; and the control means for controlling the power of the pumping light in accordance with the monitor information.

[0068] (Supplementary Note 9) An optical amplification method comprising: cladding-pumping a first pumping MCF with pumping light; cladding-pumping a second pumping MCF using the pumping light output from the first pumping MCF; and controlling the pumping light power of at least one of the first pumping MCF and the second pumping MCF.

[0069] (Supplementary Note 10) The optical amplification method according to Supplementary Note 9, further comprising controlling a ratio between forward pumping power and backward pumping power in the first pumping MCF.

[0070] (Supplementary Note 11) The optical amplification method according to Supplementary Note 10, further comprising controlling a ratio between forward pumping light power and backward pumping light power in the second pumping MCF.

[0071] (Supplementary Note 12) The optical amplification method according to Supplementary Note 10 or 11, further comprising controlling the power of the pumping light so as to reduce the wavelength dependence of the gain of the first pumping MCF and the wavelength dependence of the gain of the second pumping MCF.

[0072] (Supplementary Note 13) The optical amplification method according to any one of Supplementary Notes 9 to 12, wherein the first pumping MCF amplifies light in a first wavelength band, and the second pumping MCF amplifies light in a second wavelength band.

[0073] (Supplementary Note 14) An optical amplification method according to Supplementary Note 13, comprising: wavelength-separating input light into light of the first wavelength band and light of the second wavelength band; outputting the light of the first wavelength band to the first pumping MCF; outputting the light of the second wavelength band to the second pumping MCF; and wavelength-multiplexing the light output from the first pumping MCF and the light output from the second pumping MCF.

[0074] (Supplementary Note 15) The optical amplification method according to Supplementary Note 13 or 14, wherein the first wavelength band is a C-band and the second wavelength band is an L-band.

[0075] (Supplementary Note 16) An optical amplification method according to any one of Supplementary Notes 9 to 14, comprising: generating monitor information as a result of monitoring the power of light output from at least one of the first pumping MCF and the second pumping MCF for each wavelength; and controlling the power of the pumping light in accordance with the monitor information.

[0076] (Supplementary Note 17) A recording medium having recorded thereon a program for causing a computer to execute the following steps: a procedure for cladding-pumping a first pumping MCF with pumping light; a procedure for cladding-pumping a second pumping MCF with residual pumping light output from the first pumping MCF; and a procedure for controlling the power of the pumping light.

[0077] (Supplementary Note 18) The recording medium according to Supplementary Note 17, further comprising a procedure for controlling a ratio between forward pumping light power and backward pumping light power in the first pumping MCF.

[0078] (Supplementary Note 19) The recording medium according to Supplementary Note 18, wherein a ratio between forward pumping light power and backward pumping light power in the second pumping MCF is controlled.

[0079] (Supplementary Note 20) The recording medium according to Supplementary Note 18 or 19, wherein the power of the pumping light is controlled so as to reduce the wavelength dependence of the gain of the first pumping MCF and the wavelength dependence of the gain of the second pumping MCF.

[0080] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure.

[0081] In addition, in each embodiment, light in the C band and L band is described as "signal light." However, the light amplified in the MCF amplifier is not limited to signal light. For example, the MCF amplifier described in each embodiment can also amplify light that does not contain a signal (e.g., continuous light). Furthermore, the multi-core fiber amplifier of each embodiment also discloses an optical amplification method according to each configuration.

[0082] Furthermore, the configurations described in the respective embodiments are not necessarily mutually exclusive, and the functions and effects of the present disclosure may be achieved by a configuration that combines all or part of the above-described embodiments.

[0083] Furthermore, some or all of the procedures described in Supplementary Notes 10 to 16, which are dependent on Supplementary Note 9, may also be dependent on Supplementary Note 16 in the same dependent relationship as Supplementary Notes 10 to 16. Furthermore, not limited to Supplementary Note 9, but within the scope of each of the above-described embodiments, some or all of the configurations described as Supplements may be made dependent on various hardware, software, and various recording means or systems for recording software.

[0084] 10 Input optical fiber 11 Optical demultiplexer (DMUX) 12, 22 Fan-in (Fan-in, FI) 13, 23 Fan-out (Fan-out, FO) 14 Optical multiplexer (MUX) 15 Output optical fiber 100, 200 Pumping MCF 101, 102, 201, 201 MCF 300, 303, 306 Pumping light source 301, 302, 304 Pumping LD 305 Variable branching coupler 311-314 Optical circulator 321-328 Pumping light coupling section 331, 332 Variable attenuator 341, 342 Optical switch 380 Control circuit 471-474, 481-484, 491-494 Pumping light 1000, 2000, 3000, 4000 MCF amplifier

Claims

1. A multicore fiber amplifier comprising: a first optical fiber amplifying means having a first pumping MCF (multicore fiber) that is clad pumped by forward pumping and backward pumping; a pumping means that outputs pumping light that pumps the first pumping MCF; a second optical fiber amplifying means having a second pumping MCF that is clad pumped by forward pumping and backward pumping using the pumping light output from the first pumping MCF; and a control means that controls the pumping light power of at least one of the first optical fiber amplifying means and the second optical fiber amplifying means.

2. A multi-core fiber amplifier according to claim 1, wherein the control means controls a ratio between forward pumping light power and backward pumping light power in the first pumping MCF.

3. A multi-core fiber amplifier according to claim 1 or 2, wherein the control means controls a ratio between forward pumping light power and backward pumping light power in the second pumping MCF.

4. A multi-core fiber amplifier according to claim 2 or 3, wherein the control means controls the power of the pumping light so as to reduce the wavelength dependence of the gain of the first pumping MCF and the wavelength dependence of the gain of the second pumping MCF.

5. A multi-core fiber amplifier according to any one of claims 1 to 4, wherein the first pumping MCF amplifies light in a first wavelength band, and the second pumping MCF amplifies light in a second wavelength band.

6. A multi-core fiber amplifier according to claim 5, comprising: optical demultiplexing means for wavelength-separating input light into light of said first wavelength band and light of said second wavelength band, outputting the light of said first wavelength band to said first optical fiber amplifying means and outputting the light of said second wavelength band to said second optical fiber amplifying means; and optical multiplexing means for wavelength-multiplexing the light output from said first optical fiber amplifying means and the light output from said second optical fiber amplifying means.

7. A multi-core fiber amplifier according to claim 5 or 6, wherein the first wavelength band is the C-band and the second wavelength band is the L-band.

8. A multi-core fiber amplifier according to any one of claims 1 to 7, further comprising wavelength monitoring means for outputting monitor information based on a result of monitoring the power of light output from said multi-core fiber amplifier for each wavelength, and said control means controls the power of said pumping light in accordance with said monitor information.

9. An optical amplification method comprising: cladding pumping a first pumping MCF with pumping light; cladding pumping a second pumping MCF using the pumping light output from the first pumping MCF; and controlling the pumping light power of at least one of the first pumping MCF and the second pumping MCF.

10. The optical amplification method according to claim 9, further comprising controlling a ratio between forward pumping light power and backward pumping light power in said first pumping MCF.

11. The optical amplification method according to claim 9 or 10, further comprising controlling a ratio between forward pumping light power and backward pumping light power in said second pumping MCF.

12. The optical amplification method according to claim 10 or 11, further comprising controlling the power of the pumping light so as to reduce the wavelength dependence of the gain of the first pumping MCF and the wavelength dependence of the gain of the second pumping MCF.

13. An optical amplification method according to any one of claims 9 to 12, wherein the first pumping MCF amplifies light in a first wavelength band, and the second pumping MCF amplifies light in a second wavelength band.

14. An optical amplification method according to claim 13, comprising the steps of: wavelength-separating input light into light of the first wavelength band and light of the second wavelength band; outputting the light of the first wavelength band to the first pumping MCF; outputting the light of the second wavelength band to the second pumping MCF; and wavelength-multiplexing the light output from the first pumping MCF and the light output from the second pumping MCF.

15. The optical amplification method according to claim 13 or 14, wherein the first wavelength band is the C band, and the second wavelength band is the L band.

16. An optical amplification method according to any one of claims 9 to 14, further comprising generating monitor information which is a result of monitoring the power of light output from at least one of the first pumping MCF and the second pumping MCF for each wavelength, and controlling the power of the pumping light in accordance with the monitor information.

17. A recording medium having recorded thereon a program for causing a computer to execute the steps of: cladding pumping a first pumping MCF with pumping light; cladding pumping a second pumping MCF with residual pumping light output from the first pumping MCF; and controlling the power of the pumping light.

18. The recording medium according to claim 17, further comprising a step of controlling a ratio between forward pumping light power and backward pumping light power in said first pumping MCF.

19. The recording medium according to claim 18, wherein a ratio between forward pumping light power and backward pumping light power in said second pumping MCF is controlled.

20. The recording medium according to claim 18 or 19, wherein the power of the pumping light is controlled so as to reduce the wavelength dependence of the gain of the first pumping MCF and the wavelength dependence of the gain of the second pumping MCF.

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