Multicore optical fiber amplifier and optical amplification method using multicore optical fiber amplification medium

A multicore optical fiber amplifier according to an exemplary aspect of the present invention includes a multicore optical fiber amplification medium including, in a clad, a plurality of cores doped with a rare earth element; signal light introduction means for introducing, into each of the plurality of cores, signal light with a wavelength included in a gain band of the multicore optical fiber amplification medium; excitation light introduction means for introducing, into the clad, excitation light for exciting the multicore optical fiber amplification medium; and control light introduction means for introducing control light into each of the plurality of cores, wherein the control light introduction means introduces the control light into a non-signal core into which the signal light is not being introduced, among the plurality of cores, only when the excitation light is being introduced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2018/001602, filed on Jan. 19, 2018, which claims priority from Japanese Patent Application No. 2017-009725, filed on Jan. 23, 2017, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to multicore optical fiber amplifiers and optical amplification methods using multicore optical fiber amplification media and, in particular, to a multicore optical fiber amplifier and an optical amplification method using a multicore optical fiber amplification medium that are used for an optical communication system or an optical interconnect system.

BACKGROUND ART

In an optical fiber communication system, in order to cope with expansion of a communication capacity, technologies for time-multiplexing or wavelength-multiplexing optical signals transmitted through a core of an optical fiber have been researched and developed. These days, a limit of optical signal power for each core is becoming actual, and an approach to a spatial multiplexing technology is being intensified to further expand the communication capacity.

The spatial multiplexing technology uses a multicore optical fiber in which a plurality of cores are formed in a clad. Transmitting optical signals through each of the plurality of cores enables a signal transmission capacity with respect to each optical fiber to increase.

In order to make an optical communication system using a multicore optical fiber, an optical amplifier suitable for a multicore optical fiber is required. Examples proposed as such an optical amplifier include an optical amplifier in which optical excitation is performed with a rare-earth element such as erbium (Er) doped in a plurality of cores of a multicore optical fiber. Examples of an excitation method for optically exciting a plurality of cores with an amplification medium doped include a core excitation method in which excitation light is individually inputted into each core, and a clad excitation method in which a plurality of cores are optically excited together with excitation light inputted into a clad. The clad excitation method makes it possible to use a single excitation light source and suppress heat generation by expanding the width of a light emitting section, which is caused by making a transverse mode of excitation light a multimode. This makes it possible to fabricate an optical amplifier with low electric power consumption.

Patent Literature 1 discloses an example of a multicore optical fiber amplifier in which the above-mentioned clad excitation method is used with the core excitation method. A related multicore optical fiber amplifier described in Patent Literature 1 includes an amplification multicore fiber, a clad excitation section, a core excitation section, and a gain equalizer.

The amplification multicore fiber is configured in a double-clad structure and includes a plurality of cores doped with erbium ions. The clad excitation section is placed between the amplification multicore fiber and a first transmission multicore fiber connected to an input end of the multicore optical fiber amplifier. The core excitation section is placed between the amplification multicore fiber and a second transmission multicore fiber connected to an output end of the multicore optical fiber amplifier. The gain equalizer is placed between the core excitation section and the second transmission multicore fiber, and flattens the gain.

In the related multicore optical fiber amplifier, the clad excitation section mainly excites wavelength division multiplexing (WDM) signals. The excitation light is compensated for by controlling a core excitation light source independently in each of the core excitation sections in such a way as to correspond to wavelength dependence of output power and gain that arises due to a change in the number of WDM signals to be inputted into each core of the amplification multicore fiber. It is said that the related multicore optical fiber amplifier with above-described configuration can achieve gain flatness.

As the related technologies, there are technologies described in Patent Literature 2 and Patent Literature 3

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In a transmission line using a multicore optical fiber, signal light is not always inputted into all cores. Accordingly, it could be that signal light is inputted into only one part of the plurality of cores, and that signal light is not inputted into the other cores, resulting in a non-signal-light state. In this case, in a multicore optical fiber amplifier based on the clad excitation method in which a plurality of cores are optically excited together with excitation light inputted into a clad, strong population inversion is formed because stimulated emission due to signal light does not arise without signal light.

A case will be described with reference toFIG. 7AandFIG. 7Bwhere signal light is inputted with the strong population inversion formed in an optical fiber amplifier. When signal light having an input waveform as illustrated inFIG. 7Ais inputted into the optical fiber amplifier in which strong population inversion is formed, an optical pulse having an extremely large optical intensity arises for some hundreds of microseconds (μs) as illustrated inFIG. 7B. Such an optical pulse is called an optical surge. When the optical surge occurs, an optical component and an optical receiver are damaged that are positioned on the output side of the optical fiber amplifier.

If the input of the excitation light is started after the input of the signal light, an optical surge does not occur because strong population inversion is not formed. In other words, the input of the excitation light is once stopped, signal light is newly inputted into a core into which signal light is not inputted, and then the input of the excitation light is restarted; consequently, an optical surge does not occur. However, if a plurality of cores are optically excited together with the excitation light inputted into a clad, the gain in the core into which the signal light has already been inputted also largely fluctuates due to such an operation for changing the excitation light intensity.

As described above, there is the problem that, in a multicore optical fiber amplifier based on a clad excitation method, it is difficult to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

The object of the present invention is to provide a multicore optical fiber amplifier and an optical amplification method using a multicore optical fiber amplification medium that solve the above-mentioned problem.

Solution to Problem

A multicore optical fiber amplifier according to an exemplary aspect of the present invention includes a multicore optical fiber amplification medium including, in a clad, a plurality of cores doped with a rare earth element; signal light introduction means for introducing, into each of the plurality of cores, signal light with a wavelength included in a gain band of the multicore optical fiber amplification medium; excitation light introduction means for introducing, into the clad, excitation light for exciting the multicore optical fiber amplification medium; and control light introduction means for introducing control light into each of the plurality of cores, wherein the control light introduction means introduces the control light into a non-signal core into which the signal light is not being introduced, among the plurality of cores, only when the excitation light is being introduced.

An optical amplification method using a multicore optical fiber amplification medium according to an exemplary aspect of the present invention includes introducing signal light into at least one of a plurality of cores, the signal light having a wavelength included in a gain band of the multicore optical fiber amplification medium including, in a clad, the plurality of cores doped with a rare earth element; generating excitation light to be introduced into the clad in order to excite the multicore optical fiber amplification medium; generating control light to be introduced into the plurality of cores; and introducing the control light into a non-signal core into which the signal light is not being introduced, among the plurality of cores, only when the excitation light is being introduced.

Advantageous Effects of Invention

According to the multicore optical fiber amplifier of the present invention, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

According to the optical amplification method using a multicore optical fiber amplification medium of the present invention, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

EXAMPLE EMBODIMENT

First Example Embodiment

FIG. 1is a block diagram illustrating a configuration of a multicore optical fiber amplifier100according to a first example embodiment of the present invention.

The multicore optical fiber amplifier100includes a multicore optical fiber amplification medium110, a signal light introduction section (signal light introduction means)120, an excitation light introduction section (excitation light introduction means)130, and a control light introduction section (control light introduction means)140.

The multicore optical fiber amplification medium110includes, in a clad112, a plurality of cores111doped with a rare earth element. Erbium (Er) can be typically used as the rare earth element. This enables the multicore optical fiber amplification medium110to amplify signal light with a wavelength in the 1.55 micrometer (μm) band used in an optical communication system. The clad112can have a double clad structure in which the clad is double-layered.

The signal light introduction section120is configured to introduce, into each of the plurality of cores111, signal light11with a wavelength included in the gain band of the multicore optical fiber amplification medium110.

The excitation light introduction section130is configured to introduce, into the clad112, excitation light12for exciting the multicore optical fiber amplification medium110. When the cores111are doped with erbium (Er) as a rare earth element, laser light with a wavelength of 0.98 micrometers (μm) or 1.48 micrometers (μm) can be used as the excitation light12.

The control light introduction section140is configured to introduce control light13into each of the plurality of cores111. Only when the excitation light12is being introduced, the control light introduction section140introduces the control light13into a non-signal core into which the signal light11is not being introduced, among the plurality of cores111.

As described above, in the multicore optical fiber amplifier100according to the present example embodiment, the control light13is introduced into the non-signal core only when the excitation light12is being introduced. Consequently, the control light13is amplified, which makes it possible to avoid having strong population inversion due to the excitation light12formed. At this time, the gain of a signal core into which the signal light11is being introduced among the plurality of cores111does not fluctuate because the excitation light12is being introduced.

As mentioned above, according to the multicore optical fiber amplifier100of the present example embodiment, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

Next, an optical amplification method using a multicore optical fiber amplification medium according to the present example embodiment will be described.

In the optical amplification method using a multicore optical fiber amplification medium of the present example embodiment, first, signal light is introduced into at least one of a plurality of cores, and the signal light has a wavelength included in the gain band of a multicore optical fiber amplification medium including, in a clad, a plurality of cores doped with a rare earth element. In order to excite the multicore optical fiber amplification medium, excitation light to be introduced into the clad is generated. Control light to be introduced into the plurality of cores is generated. Only when the excitation light is being introduced, the control light is introduced into a non-signal core into which the signal light is not being introduced, among the plurality of cores.

As mentioned above, according to the optical amplification method using a multicore optical fiber amplification medium of the present example embodiment, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

Second Example Embodiment

Next, a second example embodiment of the present invention will be described.FIG. 2illustrates a configuration of a multicore optical fiber amplifier200according to the second example embodiment of the present invention.

The multicore optical fiber amplifier200according to the present example embodiment includes an amplification optical fiber222serving as a multicore optical fiber amplification medium, a clad excitation introduction section241serving as an excitation light introduction means, a signal light introduction section221, and a dummy light introduction section225serving as a control light introduction means.

In an example illustrated inFIG. 2, the amplification optical fiber222has a configuration in which a multicore optical fiber including seven cores C01to C07, for example, is doped with a rare earth element. As the rare earth element, erbium (Er) can be typically used. This enables the amplification optical fiber222to amplify signal light with a wavelength in the 1.55 micrometers (μm) band used in an optical communication system.

The clad excitation introduction section241inputs, into a clad of the amplification optical fiber222, clad excitation light240output from a clad excitation light source230. The signal light introduction section (signal light introduction means)221inputs signal light beams201to207into the respective cores of the amplification optical fiber222. The dummy light introduction section225inputs, into the respective cores, dummy light261to267output from dummy light sources251to257. The wavelength of the dummy light (control light) is included in the gain band of the amplification optical fiber222.

As each of the clad excitation introduction section241and the dummy light introduction section225, an optical multiplexer such as an optical fiber coupler can be typically used.

The multicore optical fiber amplifier200according to the present example embodiment is configured to further include a signal light output section223for outputting, from the respective cores, after-propagation signal light beams211to217each of which is signal light after having been propagated through the amplification optical fiber222.

Next, the operation of the multicore optical fiber amplifier200according to the present example embodiment will be described. Here, a case will be described as an example where the signal light beams201to206are being inputted into the multicore optical fiber amplifier200, but the signal light beam207is not being inputted, that is, the core C07is in a state of no signal light.

The multicore optical fiber amplifier200puts the excitation light source230into an on-state, and the excitation light240is inputted into the clad of the amplification optical fiber222. This optical excitation causes the signal light beams201to206to be amplified. The dummy light beams261to267are inputted into the cores C01to C07from the dummy light sources251to257. This makes the dummy light beam267from the dummy light source257inputted into the core C07with no signal light, and amplified; therefore, it is possible to avoid having strong population inversion formed. This makes it possible to suppress the production of the optical surge effect when it is started in this state to input the signal light beam207into the core C07.

At this time, each gain of the cores C01to C06into which the signal light beams201to206are being introduced does not fluctuate because the excitation light240is being introduced.

As described above, the multicore optical fiber amplifier200according to the present example embodiment avoids forming a strong population inversion state by using the dummy light when the signal light is inputted into the core with no signal light, with the plurality of cores excited together by the excitation light inputted into the clad. This causes the optical surge effect to be suppressed. That is to say, according to the multicore optical fiber amplifier200of the present example embodiment, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

Third Example Embodiment

Next, a third example embodiment of the present invention will be described.FIG. 3illustrates a configuration of a multicore optical fiber amplifier300according to the third example embodiment of the present invention. The same constituents as those of the multicore optical fiber amplifier200according to the second example embodiment are represented by the same reference signs, and their detailed descriptions may not be repeated.

The multicore optical fiber amplifier300according to the present example embodiment includes an amplification optical fiber222serving as a multicore optical fiber amplification medium, a clad excitation introduction section241serving as an excitation light introduction means, a signal light introduction section221, and a core excitation light introduction section324serving as a control light introduction means.

In an example illustrated inFIG. 3, the amplification optical fiber222has a configuration in which a multicore optical fiber including seven cores C01to C07, for example, is doped with a rare earth element. As the rare earth element, erbium (Er) can be typically used.

The clad excitation introduction section241inputs, into a clad of the amplification optical fiber222, clad excitation light240output from a clad excitation light source230. The signal light introduction section221inputs signal light beams201to207into the respective cores of the amplification optical fiber222.

The core excitation light introduction section324inputs, into the respective cores, core excitation light beams341to347output from core excitation light sources331to337serving as excitation light sources. Here, the core excitation light sources331to337generate core excitation light beams each of which is control light and is used for exciting the amplification optical fiber222.

The multicore optical fiber amplifier300according to the present example embodiment further includes a first control section (first control means)371that controls the operations of the clad excitation light source230and the core excitation light sources331to337. The first control section371controls the clad excitation light source230and the core excitation light sources331to337in such a way as to stop introducing the clad excitation light240into the clad, with the core excitation light introduced into only a signal core into which signal light is being introduced among the plurality of cores C01to C07. That is to say, the first control section371controls the clad excitation light source230and the core excitation light sources331to337when inputting signal light into a core with no signal light.

As each of the clad excitation introduction section241and the core excitation light introduction section324, an optical multiplexer such as an optical fiber coupler can be typically used.

FIG. 3illustrates a configuration including a signal light output section223that outputs, from the respective cores, after-propagation signal light beams211to217each of which is signal light after having been propagated through the amplification optical fiber222.

Next, the operation of the multicore optical fiber amplifier300according to the present example embodiment will be described. Here, a case will be described as an example where the signal light beams201to206are being inputted into the multicore optical fiber amplifier300, but the signal light beam207is not being inputted, that is, the core C07is in a state of no signal light.

The first control section371included in the multicore optical fiber amplifier300puts the excitation light source230into an on-state. This causes the excitation light240to be inputted into the clad of the amplification optical fiber222. By this optical excitation, the signal light beams201to206are amplified.

When it is started in this state to input the signal light beam207, the first control section371puts the core excitation light sources331to336into on-states, and the core excitation light source337into an off-state. Then the first control section371puts the clad excitation light source230into an off-state temporarily. This prevents the clad excitation light240from being inputted into the clad. However, the amplification operation on the signal light beams201to206is maintained because the core excitation light beams341to346are being inputted into the respective cores C01to C06.

After the signal light beam207is inputted into the core C07, the first control section371puts the clad excitation light source230into an on-state again. At this time, the first control section371also puts the core excitation light source337into an on-state, and controls the optical power of the core excitation light beam347and the core excitation light beams341to346, which makes it possible to reduce a difference, between the plurality of cores, in the gain caused by the amplification optical fiber222.

The above-mentioned operation of the first control section371makes it possible to suppress the optical surge effect when starting to input the signal light beam207.

Next, an optical amplification method using a multicore optical fiber amplification medium according to the present example embodiment will be described.

In the optical amplification method using a multicore optical fiber amplification medium of the present example embodiment, first, signal light is introduced into at least one of a plurality of cores, and the signal light has a wavelength included in the gain band of a multicore optical fiber amplification medium including, in a clad, a plurality of cores doped with a rare earth element. In order to excite the multicore optical fiber amplification medium, excitation light to be introduced into the clad is generated. Control light to be introduced into the plurality of cores is generated. Only when the excitation light is being introduced, the control light is introduced into a non-signal core into which the signal light is not being introduced, among the plurality of cores.

The above-described control light is core excitation light for exciting the multicore optical fiber amplification medium. Then the introduction of the excitation light into the clad is stopped, with the core excitation light being introduced into only a signal core into which the signal light is being introduced among the plurality of cores.

As mentioned above, in the multicore optical fiber amplifier300and the optical amplification method using the multicore optical fiber amplification medium of the present example embodiment, the signal light is inputted into the core with no signal light, with the plurality of cores excited together by the excitation light inputted into the clad. At this time, the operation of each excitation light source is controlled. This makes it possible to suppress the optical surge effect. That is to say, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

Fourth Example Embodiment

Next, a fourth example embodiment of the present invention will be described.FIG. 4illustrates a configuration of a multicore optical fiber amplifier400according to the fourth example embodiment of the present invention. The same constituents as those of the multicore optical fiber amplifier200according to the second example embodiment and the multicore optical fiber amplifier300according to the third example embodiment are represented by the same reference signs, and their detailed descriptions may not be repeated.

The multicore optical fiber amplifier400according to the present example embodiment includes an amplification optical fiber222serving as a multicore optical fiber amplification medium, a clad excitation introduction section241serving as an excitation light introduction means, a signal light introduction section221, and a signal light output section223.

In an example illustrated inFIG. 4, the amplification optical fiber222has a configuration in which a multicore optical fiber including seven cores C01to C07, for example, is doped with a rare earth element. As the rare earth element, erbium (Er) can be typically used.

The clad excitation introduction section241inputs, into a clad of the amplification optical fiber222, clad excitation light240output from a clad excitation light source230. The signal light introduction section221inputs signal light beams201to207into the respective cores of the amplification optical fiber222. The signal light output section223outputs, from the respective cores, after-propagation signal light beams211to217each of which is signal light after having been propagated through the amplification optical fiber222.

The multicore optical fiber amplifier400further includes dummy light sources251to257serving as light sources for dummy light, core excitation light sources331to337, and a control light introduction section (control light introduction means). The dummy light sources251to257generate dummy light beams261to267each of which is control light with a wavelength included in the gain band of the amplification optical fiber222. The core excitation light sources331to337generate core excitation light beams341to347each of which is control light and is used for exciting the amplification optical fiber222. The control light introduction section is configured to introduce the dummy light beams261to267and the core excitation light beams341to347into the plurality of cores, respectively.

Specifically, as illustrated inFIG. 4, the control light introduction section can be configured to include a first control light introduction section (first control light introduction means)425and a second control light introduction section (second control light introduction means)424, for example. Here, the first control light introduction section425is configured to introduce the dummy light beams261to267into the plurality of cores C01to C07, respectively. The second control light introduction section424is configured to introduce the core excitation light beams341to347into the plurality of cores C01to C07, respectively. As each of the first control light introduction section425and the second control light introduction section424, an optical multiplexer such as an optical fiber coupler can be typically used.

Next, the operation of the multicore optical fiber amplifier400according to the present example embodiment will be described. Here, a case will be described as an example where the signal light beams201to206are being inputted into the multicore optical fiber amplifier400, but the signal light beam207is not being inputted, that is, the core C07is in a state of no signal light.

The multicore optical fiber amplifier400puts the excitation light source230into an on-state. This causes the excitation light240to be inputted into the clad of the amplification optical fiber222. By this optical excitation, the signal light beams201to206are amplified. At this time, the multicore optical fiber amplifier400puts the core excitation light sources331to336into an on-state, and controls the optical power of the core excitation light beams341to346, which makes it possible to reduce a difference, between the plurality of cores, in the gain caused by the amplification optical fiber222.

The dummy light beam267from the dummy light source257is being inputted into at least the core C07among the cores C01to C07. That is to say, the dummy light beam267from the dummy light source257is inputted into the core C07with no signal light and amplified; therefore, it is possible to avoid having strong population inversion formed. This makes it possible to suppress the production of the optical surge effect when it is started in this state to input the signal light beam207into the core C07.

At this time, each gain of the cores C01to C06into which the signal light beams201to206are being introduced does not fluctuate and can be kept constant because the excitation light240and the core excitation light beams341to346are being introduced.

After the signal light beam207has been inputted into the core C07, the dummy light source257is put into an off-state. At this time, the core excitation light source337is put into an on-state, and the optical power of the core excitation light347is also controlled, which makes it possible to reduce a difference, between the plurality of cores, in the gain caused by the amplification optical fiber222.

As described above, the multicore optical fiber amplifier400according to the present example embodiment avoids forming a strong population inversion state by using the dummy light when the signal light is inputted into the core with no signal light, with the plurality of cores excited together by the excitation light inputted into the clad. This causes the optical surge effect to be suppressed. That is to say, according to the multicore optical fiber amplifier400of the present example embodiment, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

In the above description, the control light introduction section is configured to include the first control light introduction section425and the second control light introduction section424. However, the configuration is not limited to this; as a multicore optical fiber amplifier401illustrated inFIG. 5, the multicore optical fiber amplifier may be configured to include a piece of synthesized-control-light introduction section426serving as a control light introduction section, instead of the first control light introduction section425and the second control light introduction section424. The multicore optical fiber amplifier401further includes synthesis sections481to487and a light source control section472.

The synthesis sections481to487combine the dummy light beams261to267and the core excitation light beams341to347, and generate synthesized control light beams491to497, respectively. Each of the synthesis sections481to487is typically composed of a multiplexer. The synthesized-control-light introduction section426is configured to introduce the synthesized control light beams491to497into the plurality of cores C01to C07, respectively. As the synthesized-control-light introduction section426, an optical multiplexer such as an optical fiber coupler can be typically used. As mentioned above, the light source control section472performs on-off control on the core excitation light sources331to337and the dummy light sources251to257, depending on the presence or absence of the signal light beams201to207inputted into the cores C01to C07.

The multicore optical fiber amplifier401having such a configuration can also suppress the optical surge effect by performing an operation similar to that of the above-mentioned multicore optical fiber amplifier400. Furthermore, the multicore optical fiber amplifier401is configured to use the synthesized-control-light introduction section426in common, in order to input each of the dummy light beams261to267and the core excitation light beams341to347into the amplification optical fiber222. Consequently, according to the multicore optical fiber amplifier401, it becomes possible to simplify a device structure and become miniaturized.

Next, an optical amplification method using a multicore optical fiber amplification medium according to the present example embodiment will be described.

In the optical amplification method using a multicore optical fiber amplification medium of the present example embodiment, first, signal light is introduced into at least one of a plurality of cores, and the signal light has a wavelength included in the gain band of a multicore optical fiber amplification medium including, in a clad, a plurality of cores doped with a rare earth element. In order to excite the multicore optical fiber amplification medium, excitation light to be introduced into the clad is generated. Control light to be introduced into the plurality of cores is generated. Only when the excitation light is being introduced, the control light is introduced into a non-signal core into which the signal light is not being introduced, among the plurality of cores.

The control light includes dummy light having a wavelength included in the gain band of the multicore optical fiber amplification medium, and core excitation light for exciting the multicore optical fiber amplification medium. Here, the dummy light is introduced into at least a non-signal core into which the signal light is not being introduced, among the plurality of cores, and the core excitation light is introduced into a signal core into which the signal light is being introduced among the plurality of cores. The optical power of the core excitation light is controlled in such a way as to reduce a difference, between the plurality of cores, in the gain caused by the multicore optical fiber amplification medium.

As mentioned above, in the multicore optical fiber amplifier400or401, and the optical amplification method using the multicore optical fiber amplification medium of the present example embodiment, the signal light is inputted into the core with no signal light, with the plurality of cores excited together by the excitation light inputted into the clad. At this time, it is avoided to form a strong population inversion state by using the dummy light, which makes it possible to suppress the optical surge effect. That is to say, according to the multicore optical fiber amplifier400of the present example embodiment, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

Fifth Example Embodiment

Next, a fifth example embodiment of the present invention will be described.FIG. 6illustrates a configuration of a multicore optical fiber amplifier500according to the fifth example embodiment of the present invention. The same constituents as those of the multicore optical fiber amplifier200according to the second example embodiment and the multicore optical fiber amplifier300according to the third example embodiment are represented by the same reference signs, and their detailed descriptions may not be repeated.

The multicore optical fiber amplifier500according to the present example embodiment includes an amplification optical fiber222serving as a multicore optical fiber amplification medium, a clad excitation introduction section241serving as an excitation light introduction means, a signal light introduction section221, and a signal light output section223.

In an example illustrated inFIG. 6, the amplification optical fiber222has a configuration in which a multicore optical fiber including seven cores C01to C07, for example, is doped with a rare earth element. As the rare earth element, erbium (Er) can be typically used.

The clad excitation introduction section241inputs, into a clad of the amplification optical fiber222, clad excitation light240output from a clad excitation light source230. The signal light introduction section221inputs signal light beams201to207into the respective cores of the amplification optical fiber222. The signal light output section223outputs, from the respective cores, after-propagation signal light beams211to217each of which is signal light after having been propagated through the amplification optical fiber222.

The multicore optical fiber amplifier500further includes dummy light sources251to257serving as light sources for dummy light, core excitation light sources331to337, and a control light introduction section (control light introduction means). The dummy light sources251to257generate dummy light beams261to267each of which is control light with a wavelength included in the gain band of the amplification optical fiber222. The core excitation light sources331to337generate core excitation light beams341to347each of which is control light and is used for exciting the amplification optical fiber222. The control light introduction section is configured to introduce the dummy light beams261to267and the core excitation light beams341to347into the plurality of cores, respectively.

The control light introduction section is configured to include a first control light introduction section (first control light introduction means)525and a second control light introduction section (second control light introduction means)524. Here, the first control light introduction section525is configured to introduce the dummy light beams261to267into the plurality of cores C01to C07, respectively. The second control light introduction section524is configured to introduce the core excitation light beams341to347into the plurality of cores C01to C07, respectively. As each of the first control light introduction section525and the second control light introduction section524, an optical multiplexer such as an optical fiber coupler can be typically used.

In the multicore optical fiber amplifier500according to the present example embodiment, the first control light introduction section525is located on the input side of the signal light beams201to207with respect to the amplification optical fiber222, and the second control light introduction section524is located on the output side. Here, the second control light introduction section524includes a dummy light separation section (dummy light separation means) configured to separate after-propagation dummy light that is dummy light after having been propagated through the amplification optical fiber222.

The multicore optical fiber amplifier500further includes light intensity monitors (monitoring means)591to597each of which monitors a light intensity of the after-propagation dummy light, demultiplexing sections581to587that introduce the after-propagation dummy light into the light intensity monitors591to597, and a second control section (second control means)573. Here, the second control section573controls the output optical power of the clad excitation light source230and the core excitation light sources331to337based on the monitored light intensities in such a way as to reduce a difference, between the plurality of cores C01to C07, in the gain caused by the amplification optical fiber222.

Next, the operation of the multicore optical fiber amplifier500according to the present example embodiment will be described. Here, a case will be described as an example where the signal light beams201to206are being inputted into the multicore optical fiber amplifier500, but the signal light beam207is not being inputted, that is, the core C07is in a state of no signal light.

The second control section573included in the multicore optical fiber amplifier500puts the excitation light source230into an on-state. This causes the excitation light240to be inputted into the clad of the amplification optical fiber222. By this optical excitation, the signal light beams201to206are amplified. At this time, the second control section573puts the core excitation light sources331to336into an on-state, and controls the optical power of the core excitation light beams341to346, which makes it possible to reduce a difference, between the plurality of cores, in the gain caused by the amplification optical fiber222.

The dummy light beams261to267from the dummy light sources251to257are being inputted into the cores C01to C07. That is to say, the dummy light beam267from the dummy light source257is inputted into the core C07with no signal light and amplified; therefore, it is possible to avoid having strong population inversion formed. This makes it possible to suppress the production of the optical surge effect when it is started in this state to input the signal light beam207into the core C07.

At this time, each gain of the cores C01to C06into which the signal light beams201to206are being introduced does not fluctuate and can be kept constant because the excitation light240and the core excitation light beams341to346are being introduced.

The after-propagation dummy light, which has been propagated through the amplification optical fiber222and has reached the second control light introduction section524, is separated by the dummy light separation section included in the second control light introduction section524, and introduced into the light intensity monitors591to597through the demultiplexing sections581to587. At this time, the second control section573controls the clad excitation light source230and the core excitation light sources331to337based on the monitored light intensities, which makes it possible to reduce a difference in the gain between the cores C01to C07.

As mentioned above, the multicore optical fiber amplifier500according to the present example embodiment avoids forming a strong population inversion state by using the dummy light when the signal light is inputted into the core with no signal light, with the plurality of cores excited together by the excitation light inputted into the clad. This causes the optical surge effect to be suppressed. That is to say, according to the multicore optical fiber amplifier500of the present example embodiment, it is possible, even when a clad excitation method is used, to suppress the occurrence of an optical surge due to input of signal light into a core with no signal light, without the occurrence of fluctuation in the gain of a core into which signal light is inputted.

Furthermore, the multicore optical fiber amplifier500is configured to use the second control light introduction section524in common, in order to input the core excitation light beams341to347into the amplification optical fiber222and separate the amplified after-propagation dummy light. Consequently, according to the multicore optical fiber amplifier500, it becomes possible to simplify a device structure and become miniaturized.

The above-mentioned respective example embodiments are described using, as an example, the amplification optical fiber222in which the multicore optical fiber including the seven cores C01to C07is doped with the rare earth element. However, the number of cores is not limited to this, and advantageous effects can be achieved that are similar to those of the multicore optical fiber amplifier according to each of the above-mentioned example embodiments, even though the number of cores differs from the above.

REFERENCE SIGNS LIST

100,200,300,400,401,500Multicore optical fiber amplifier110Multicore optical fiber amplification medium111Core112Clad120Signal light introduction section130Excitation light introduction section140Control light introduction section201to207Signal light beam211to217After-propagation signal light beam221Signal light introduction section222Amplification optical fiber223Signal light output section225Dummy light introduction section230Clad excitation light source240Clad excitation light241Clad excitation introduction section251to257Dummy light source261to267Dummy light beam324Core excitation light introduction section331to337Core excitation light source341to347Core excitation light beam371First control section424,524Second control light introduction section425,525First control light introduction section426Synthesized-control-light introduction section472Light source control section481to487Synthesis section491to497Synthesized control light beam573Second control section581to587Demultiplexing section591to597Light intensity monitor