MULTI-CORE FIBER OPTICAL AMPLIFIER AND OPTICAL AMPLIFICATION METHOD

A multi-core fiber optical amplifier includes: a multi-core excitation fiber configured to include a first core and a second core; and a clad excitation circuit configured to inject excitation light into a clad of the multi-core excitation fiber, wherein signal light input to one end of the first core is output from the other end of the first core, the signal light output from the other end of the first core is input to one end of the second core, and the signal light input to one end of the second core is output from the other end of the second core.

This application is based upon and claims the benefit of priority from Japanese patent application No. 2023-160131, filed on Sep. 25, 2023, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present disclosure relates to a multi-core fiber optical amplifier and an optical amplification method.

BACKGROUND ART

Practical application of space division multiplexing systems using multi-core fibers (MCF) is under way. An MCF is an optical fiber including a plurality of cores. Use of an MCF in place of a single core fiber (SCF) enables expansion of transmission capacity of an optical transmission system. An MCF optical amplifier using an MCF as an excitation fiber (multi-core excitation fiber) is also known. A rare-earth element such as erbium (Er) is doped into a core of an MCF in such an excitation fiber. Two types of excitation being “core excitation” by which excitation light is directly injected into a core and “clad excitation” by which erbium in a plurality of cores is collectively excited by injecting excitation light into a clad are applicable to an MCF optical amplifier.

Wavelength division multiplexed signal light is normally used as signal light transmitted by an MCF. The C-band and the L-band are known as bands of wavelength division multiplexed signal light. The band of the C-band is roughly 1520 to 1560 nm, and the band of the L-band is roughly 1570 to 1610 nm. Wavelength division multiplexed signal light is hereinafter described as WDM light.

In relation to the present disclosure, an optical amplifier described in Japanese Unexamined Patent Application Publication No. 2013-058651 [patent literature 1: (PTL1)] is configured to include an excitation core in the central part and reflect signal light propagating through a signal light core around the excitation core by using a lens array and a mirror.

SUMMARY

Signal light in the C-band is mainly used in transmission of a WDM signal. Then, transmission of a WDM signal using signal light in the L-band is also under study for further expansion of transmission capacity. However, efficiency in the L-band in a common optical fiber amplifier is lower than efficiency in the C-band. Therefore, an excitation fiber several times as long as that for the C-band needs to be used for amplification of signal light in the L-band using an optical fiber amplifier.

Each technology described in PTL 1 attempts to provide an effect of extending an excitation fiber by propagating one beam of signal light through a plurality of cores in an optical amplifier including an MCF as an excitation fiber. However, the technology described in PTL 1 requires use of a special MCF including a core for excitation at the center.

The present disclosure describes a technology for providing a multi-core fiber optical amplifier and an optical amplification method that enable effective utilization of a core of an MCF without using an excitation fiber using a special MCF.

A multi-core fiber optical amplifier according to the present disclosure is a multi-core fiber optical amplifier including: a multi-core excitation fiber configured to include a first core and a second core; and a clad excitation means for injecting excitation light into a clad of the multi-core excitation fiber, whereinsignal light input to an one end of the first core is output from an other end of the first core,the signal light output from the other end of the first core is input to an one end of the second core, andthe signal light input to the one end of the second core is output from an other end of the second core.

An optical amplification method according to the present disclosure is an optical amplification method used in a multi-core fiber optical amplifier including a multi-core excitation fiber including a first core and a second core, the method including a procedure for:injecting excitation light into a clad of the multi-core excitation fiber;outputting signal light input to an one end of the first core from an other end of the first core;inputting the signal light output from the other end of the first core to an one end of the second core; andoutputting the signal light input to the one end of the second core from an other end of the second core.

The present disclosure provides a multi-core fiber optical amplifier and an optical amplification method that enable effective utilization of a core of an MCF without using an excitation fiber using a special MCF.

EXAMPLE EMBODIMENT

First Example Embodiment

FIG.1is a diagram illustrating a configuration example of an MCF optical amplifier1000according to the present disclosure. The MCF optical amplifier1000is a multi-core fiber optical amplifier and includes an MCF including two or more cores as an excitation fiber100. An excitation fiber is a multi-core excitation fiber in which a rare-earth element such as erbium is doped into cores. A case of the excitation fiber100including four cores (a core1to a core4) will be described inFIG.1. However, the number of cores in the excitation fiber100has only to be more than one and is not limited to four. End faces of the core1to the core4appearing at an end face101being an one end of the excitation fiber100are respectively represented by an end face1A to an end face4A inFIG.1. End faces of the core1to the core4appearing at an end face102being an another end of the excitation fiber100are respectively represented by an end face1B to an end face4B. The end face101and the end face102face each other in parallel. The central axis of the excitation fiber100at the end face101and the central axis of the excitation fiber100at the end face102match. Furthermore, as will be described later, the end face102is positioned to be rotated 90 degrees around the central axis relative to the end face101inFIG.1.

FIG.2is a diagram illustrating an example of a section of the excitation fiber100. The excitation fiber100is an MCF with a circular section. The core1to the core4are equidistantly spaced on a circumference around the central axis of the excitation fiber100. Therefore, when the excitation fiber100is rotated around the central axis, the positions of the core1to the core4on the section of the excitation fiber100overlap every 90 degrees. The position of the central axis in a longitudinal direction of an excitation fiber at an end face of the excitation fiber is hereinafter simply described as the “center of the excitation fiber.” The center of the excitation fiber100is represented by a black dot inFIG.2.

The excitation fiber100illustrated inFIG.1includes an excitation light coupling unit110for injecting excitation light into a clad of the excitation fiber100. The excitation light coupling unit110is a clad excitation circuit configured to inject excitation light into the clad of the multi-core excitation fiber. A widely known configuration may be applied to the excitation light coupling unit110. For example, excitation light can be injected into the clad of the excitation fiber100by winding an optical fiber for propagating the excitation light around the excitation fiber100with the covering removed. The excitation fiber100amplifies signal light propagating through the core1to the core4by excitation of rare-earth ions doped into the core1to the core4by the excitation light injected into the clad (clad excitation light).

The excitation fiber100is configured in such a way that one end face (the end face101) of the fiber and the other end face (the end face102) face each other, and cores appearing at the end faces101and102are optically coupled on a one-to-one basis. Then, by applying rotation (a twist) to the end face101or the end face102, signal light propagating through a core (such as the core1) can be caused to propagate through the space between the end face101and the end face102and be input to another core (such as the core2). Placement of cores illustrated inFIG.1is acquired by twisting the excitation fiber10090 degrees (that is, rotating the end face101or10290 degrees in a direction perpendicular to the central axis of the excitation fiber100). The excitation fiber100may include a maintenance part in order to maintain the positional relation of the cores between the end face101and the end face102. For example, the maintenance part is a member for simultaneously holding the neighborhood of the end face101of the excitation fiber100and the neighborhood of the end face102in a fixed manner.

When signal light is input to the end face1A of the core1at the end face101, the signal light propagates through the core1and is output from the end face1B of the core1at the end face102. As illustrated inFIG.1, the signal light output from the end face1B is input to the end face2A of the core2. Then, the signal light propagates through the core2and is output from the end face2B of the core2at the end face102. The signal light output from the end face2B is input to the end face3A of the core3. The signal light propagates through the core3and is output from the end face3B of the core3at the end face102. The signal light output from the end face3B is input to the end face4A of the core4. The signal light input to the end face4A of the core4propagates through the core4and is output from the end face4B of the core4at the end face102. The signal light output from the end face4B is output to outside the excitation fiber100.

Japanese Unexamined Patent Application Publication No. 2014-021225 describes an optical amplifier configured to input signal light and excitation light from the outer periphery of an MCF by using a coupler. However, the optical amplifier described in Japanese Unexamined Patent Application Publication No. 2014-021225 inputs signal light and excitation light to an excitation fiber by using the coupler and therefore can only use a core close to the outer periphery of the excitation fiber for optical amplification. On the other hand, the MCF optical amplifier1000can utilize a core at any position in the excitation fiber for optical amplification as long as the cores are optically connected between the end face101and the end face102.

With such a configuration, the MCF optical amplifier1000described in the present example embodiment enables effective utilization of a core of an MCF without using an excitation fiber using a special MCF.

Signal light input to the core1propagates through the core1, the core2, the core3, and the core4in this order and is output from the core4. Then, by injection of clad excitation light into the excitation fiber100, the signal light is amplified in each core while propagating through the core1to the core4. In other words, the MCF optical amplifier1000amplifies the signal light by using four cores of one excitation fiber100, and therefore the length of an excitation fiber through which the signal light propagates is the quadruple of that of an excitation fiber using an SCF of the same length as the excitation fiber100. As a result, the MCF optical amplifier1000can shorten the length of an excitation fiber required for optical amplification compared with an optical amplifier using an SCF as an excitation fiber.

Another Expression of First Example Embodiment

The effects of the MCF optical amplifier1000described above are provided also by a multi-core fiber optical amplifier with the following configuration. A reference sign inFIG.1related to each component is indicated in parentheses.

A multi-core fiber optical amplifier (1000) includes a multi-core excitation fiber (100) including a first core and a second core, and a clad excitation circuit (110) configured to inject excitation light into a clad of the multi-core excitation fiber (100). The multi-core fiber optical amplifier (1000) is configured to output signal light input to one end (1A) of the first core from the other end (1B) of the first core. The multi-core fiber optical amplifier (1000) is further configured to input signal light output from the other end (1B) of the first core to one end (2A) of the second core and outputs the signal light input to the one end (2A) of the second core from the other end (2B) of the second core.

The first core (1) and the second core (2) may be equidistantly spaced on a circumference around the center of an end face of the multi-core excitation fiber (100). By maintaining one end (102) of the multi-core excitation fiber (100) in a state of being rotated by a predefined angle relative to the other end (101) of the multi-core excitation fiber, the other end (1B) of the first core may be optically coupled to the one end (2A) of the second core.

The multi-core fiber optical amplifier (1000) may include a third core. The multi-core fiber optical amplifier (1000) may be configured to input signal light output from the other end (2B) of the second core to one end (3A) of the third core and output the signal light input to the one end (3A) of the third core from the other end (3B) of the third core.

Second Example Embodiment

A configuration example of an optical path between an end face101and an end face102of an excitation fiber100will be described as a second example embodiment below. Each ofFIG.3toFIG.6illustrates part of the configuration of the MCF optical amplifier1000inFIG.1, and configurations inFIG.3toFIG.6are applicable to the MCF optical amplifier1000.

FIG.3is a diagram illustrating a configuration example of an optical path between the end face101and the end face102of the excitation fiber100in the MCF optical amplifier1000. InFIG.3, the end face1A of the core1and the end face4B of the core4are placed in such a way as to face each other between the end face101and the end face102. Similarly, the end face1B and the end face2A face each other, the end face2B and the end face3A face each other, and the end face3B and the end face4A face each other.

Optical coupling is performed between the end face1B and the end face2A by two optical collimators1021and1012. The optical collimator1021is placed on the end face102side, and the optical collimator1012is placed on the end face101side. The optical collimator1021converts signal light output from the end face1B into collimated light. The optical collimator1012condenses the collimated light output from the optical collimator1021and inputs the condensed light to the end face2A. The optical axes of optical collimator1021and the optical collimator1012are adjusted in such a way that the core1is optically coupled to the core2between the end face1B and the end face2A. The signal light propagates between the optical collimator1021and the optical collimator1012as collimated light.

Two each of optical collimators are similarly used in coupling of the core2to the core3and coupling of the core3to the core4, respectively, between the end face101and the end face102. Specifically, an optical collimator1022and an optical collimator1013couple the core2to the core3, and an optical collimator1023and an optical collimator1014couple the core3to the core4. With such a configuration, signal light input to the end face1A of the core1propagates through the core1to the core4and is output from the end face4B of the core4. Then, by injection of excitation light into the excitation fiber100from the excitation light coupling unit110, the signal light is excited over a length quadruple to that of the excitation fiber100. The optical collimators1012to1014and the optical collimators1021to1023may couple collimated light to each core by using a lens array.

InFIG.3, the end face1A of the core1and the end face4B of the core4face each other between the end face101and the end face102. However, the end face1A is used for input of signal light, and the end face4B is used for output. Accordingly, the end face1A is not optically connected to the end face4B in the space between the end face101and the end face102. Any configuration is applicable as the configuration for inputting signal light to the end face1A and the configuration for outputting signal light output from the end face4B to the outside.

FIG.4is a diagram illustrating another configuration example of an optical path between the end face101and the end face102.FIG.4toFIG.6illustrate only parts necessary for description. InFIG.4, the end face1A of the core1and the end face4B of the core4face each other between the end face101and the end face102, similarly toFIG.3. As described above, the end face1A is used for input of signal light, and the end face4B is used for output of signal light.FIG.4illustrates a configuration including a mirror801for separating signal light input to the core1from signal light output from the core4.

The mirror801is provided between the end face1A and the end face4B. Both faces of the mirror801reflect signal light. For example, the mirror801is a reflective film using a thin metal film. The end face1A and the end face4B face each other with an optical collimator1011and an optical collimator1024interposed in between. Then, the mirror801is placed in such a way as to make 45 degrees with an optical axis connecting the end face1A to the end face4B. With such a configuration, signal light introduced from outside the MCF optical amplifier1000as collimated light can be reflected off the mirror801and be guided to the end face1A. Furthermore, signal light output from the end face4B can be reflected off the mirror801and be taken out to outside the MCF optical amplifier1000.

The reflective film in the mirror801may reflect signal light and excitation light. Then, when the signal light is guided to the end face1A, signal light multiplexed with the excitation light may be input to the end face1A through the mirror801and the optical collimator1011. In general, wavelength bands of excitation light and signal light do not overlap each other. For example, the wavelength of excitation light is in the 980 nm band, and the wavelength of signal light is in the C-band or the L-band (that is, roughly 1520 nm to 1610 nm). Accordingly, by wavelength division multiplexing excitation light and signal light by using a wavelength filter and converting the signal light wavelength division multiplexed with the excitation light into collimated light, the excitation light and the signal light can be input to the core1on the same optical path. Excitation light directly input to a core is hereinafter described as core excitation light.

With core excitation light input from the mirror801, the core1can amplify signal light by core excitation in addition to clad excitation by the excitation light coupling unit110. Core excitation light remaining in the core1without being used for amplification may be input to the core2through the end face1B, the optical collimators1021and1012, and the end face2A. The core excitation light input to the core2may further propagate to the core3and the core4with signal light. In the configuration in which excitation light and signal light are multiplexed and are input to the core1, the mirror801is optional. In other words, the configuration is applicable to the configurations without the mirror801that are illustrated inFIG.1andFIG.3. Furthermore, the configuration in which signal light multiplexed with excitation light is input to a core from the end face101is also applicable to configurations inFIG.5andFIG.6to be described later.

FIG.5is a diagram illustrating another configuration example of an optical path between the end face101and the end face102. The configuration inFIG.5differs from the configuration inFIG.4in further including a mirror802. The mirror802is provided between the end face3A of the core3and the end face2B of the core2. Both faces of the mirror802reflect signal light. For example, the mirror802is a reflective film using a thin metal film. The end face3A and the end face2B face each other with the optical collimator1013and the optical collimator1022interposed in between. The mirror802is placed in such a way as to make 45 degrees with an optical axis connecting the end face3A to the end face2B.

With such a configuration, second signal light introduced from outside the MCF optical amplifier1000as collimated light can be reflected off the mirror802and be input to the core3. The second signal light input to the core3propagates through the core3and the core4and is output to the outside by the mirror801. On the other hand, first signal light input to the core1by the mirror801propagates through the core1and the core2, is output from the end face2B of the core2, and is output to the outside by the mirror802.

FIG.5illustrates a configuration example of placing mirrors according to the number of beams of signal light to be amplified. In the example inFIG.5, by using the four cores1to4by twos, two beams of signal light can be amplified by using one excitation fiber100. Further, by changing the positions of the mirrors, the lengths of cores through which two beams of signal light propagate can be changed. InFIG.5, the first signal light propagates through the core1and the core2, and the second signal light propagates through the core3and the core4.

However, for example, when the mirror802is placed between the optical collimator1012and the optical collimator1021, the first signal light propagates only through the core1, and the second signal light on the other hand propagates through the core2to the core4. Changing the position of the mirror801similarly allows the length of a core through which each of a plurality of beams of signal light propagates to be changed. Therefore, the configuration inFIG.5enables selection of the length of a core according to an amplification factor required of signal light and therefore can amplify each of a plurality of beams of signal light under a preferable condition.

FIG.6is a diagram illustrating another configuration example of an optical path between the end face101and the end face102.FIG.6illustrates a configuration including a mirror803in place of the mirror801inFIG.4. The mirror803is formed of a reflective film and an optical filter. The reflective film is formed only in a part necessary for reflecting signal light input to the end face1A and signal light output from the end face4B. The other part of the mirror803is the optical filter. The optical filter transmits light at the wavelength of signal light and reflects light at the wavelength of excitation light. For example, the optical filter is formed of a dielectric multilayer film. The reflective film may reflect both signal light and excitation light.

The mirror803is placed in such a way as to make 45 degrees with collimated light connecting the end face1A to the end face4B. In this case, the mirror803also makes an angle of 45 degrees with collimated light connecting other cores between the end face101and the end face102. The optical axis of the optical collimator1012and the optical axis of the optical collimator1021are adjusted in such a way that the collimators are optically coupled by signal light passing through the optical filter in the mirror803. The same holds for the optical axis of the optical collimator1013and the optical axis of the optical collimator1022, and the optical axis of the optical collimator1014and the optical axis of the optical collimator1023. With such a configuration, the mirror803reflects excitation light incident from a direction perpendicular to an optical axis connecting cores between the end face101and the end face102. The excitation light incident on the mirror803can be generated by converting light output from a common excitation light source into collimated light with roughly the same diameter as that of the excitation fiber100by an optical collimator.

Excitation light reflected off the mirror803passes between the optical collimators1011to1014and is input to the clad of the excitation fiber100at the end face101. On the other hand, the optical filter part of the mirror803transmits signal light, and therefore signal light propagating through the core1to the core4in a sequential order is not affected by the mirror802. As a result, the excitation fiber100amplifies the signal light propagating through the core1to the core4by clad excitation.

Part of the excitation light reflected off the mirror803is input to the end faces1A to4A through the optical collimators1011to1014. The excitation light input to the end face1A to4A excites the core1to the core4as core excitation light. In other words, the configuration inFIG.6can simultaneously achieve clad excitation and core excitation of the excitation fiber100with one beam of excitation light. When core excitation is not necessary, an optical filter blocking excitation light and transmitting signal light may be formed at the lens of an optical collimator. Thus, excitation light being reflected off the mirror803and traveling in the direction of the end face101can be used only for clad excitation of the excitation fiber100.

Thus, the mirror803has a function of inputting excitation light to the clad of the excitation fiber100in addition to a function of inputting and outputting signal light. Accordingly, the mirror803has the function of the excitation light coupling unit110illustrated inFIG.1. The mirror803may cause residual excitation light output from the clad of the end face102to reflect and take the light out of the MCF optical amplifier1000. The residual excitation light is excitation light not being used for excitation in the excitation fiber100. The residual excitation light may be used for excitation of another excitation fiber.

The reflective film in the mirror803may have the functions of both mirrors801and802illustrated inFIG.5. Specifically, the mirror803includes a reflective film at the position where an optical path on which signal light to be reflected propagates as collimated light and the mirror803cross each other. Then, a part of the mirror803other than the reflective film may be formed of an optical filter transmitting signal light and reflecting excitation light.

Both signal light and excitation light travel in the direction of the end face101inFIG.6. Therefore, the excitation fiber100operates by forward excitation. However, the direction of excitation light input to the excitation fiber100may be opposite to that inFIG.6. By inputting excitation light to the end face102, the excitation fiber100can operate by backward excitation.

The MCF optical amplifier1000using an excitation fiber including four cores has been described inFIG.1toFIG.6. However, the aforementioned configuration in which the end face101and the end face102are optically connected is also applicable to an excitation fiber with two or more cores by increasing or decreasing the number of optical collimators in parallel.

The configurations illustrated inFIG.3toFIG.6are applicable to the MCF optical amplifier1000. Accordingly, the MCF optical amplifiers1000including the configurations enable effective utilization of a core of an MCF without using an excitation fiber using a special MCF.

Third Example Embodiment

Excitation fibers with various types of core placement applicable to the MCF optical amplifier1000will be described as a third example embodiment below with reference toFIG.7toFIG.14. Each of excitation fibers illustrated inFIG.7toFIG.14is applicable to the MCF optical amplifier1000inFIG.1. Accordingly, MCF optical amplifiers1000to which the following configurations are applied also enable effective utilization of a core of an MCF without using an excitation fiber using a special MCF.

FIG.7toFIG.14are diagrams illustrating examples of an optical path of signal light between end faces of an excitation fiber out of excitation fibers with various types of core placement. The configurations described inFIG.3toFIG.6for optically connecting cores are also applicable to the excitation fibers illustrated inFIG.7toFIG.14. Note that illustration of optical components such as an optical collimator and a mirror is omitted in the following drawings.

FIG.7illustrates a scene in which an end face101and an end face102of an excitation fiber100including four cores face each other.FIG.7illustrates a case of the end face101and the end face102of the excitation fiber100facing each other illustrated inFIG.1again. The end faces101and102inFIG.7represent a diagram of end faces of the excitation fiber100viewed from outside the excitation fiber100, similarly toFIG.1. The four cores are equidistantly spaced on a circumference around the center of the excitation fiber at a section of the excitation fiber100. In other words, the position of each of adjacent cores of the excitation fiber100is at the position of the core when the section of the excitation fiber100is rotated 90 degrees.

An arrow inFIG.7schematically indicates a propagation path of signal light between cores. Signal light input to an end face1A of a core1is output from an end face1B of the core1. Then, the excitation fiber100is rotated 90 degrees around the central axis of the excitation fiber100when the end face101and the end face102face each other. As a result, the end face1B faces an end face2A. The signal light output from the end face1B is input to a core2from the end face2A. Similarly, the signal light output from an end face2B is input to an end face3A, and the signal light output from an end face3B is input to an end face4A. Then, the signal light propagating through the core1to a core4is output from an end face4B.

FIG.8is a diagram illustrating an example of core placement of an excitation fiber200. The excitation fiber200includes a core (a core5) along the central axis. The core5is not included in an optical path of signal light propagating through a core1to a core4. The core5may be singly used as an optical amplifier with the length of one excitation fiber100. For example, the core1to the core4may be used for amplification of signal light in the L-band, and the core5may be used for amplification of signal light in the C-band. An end face5A and an end face5B of the core5face each other between an end face101and an end face102. Therefore, signal light can be input to and output from the core5by providing a mirror reflecting signal light propagating through the core5on an optical path connecting the end face5A to the end face5B. The mirror provides a function similar to that of the mirror801inFIG.4. When the configuration inFIG.5is applied to the excitation fiber200, the mirror on the optical path connecting the end face5A to the end face5B may be provided in a part of the mirror802.

In an optical fiber amplifier using an erbium-doped excitation fiber, the L-band generally has lower amplification efficiency than the C-band. Therefore, in order to bring the gain of signal light in the L-band close to the gain of signal light in the C-band, an excitation fiber several times as long as an excitation fiber for the C-band needs to be separately prepared and be used as an excitation fiber for the L-band. By amplifying signal light by using the core1to the core4, the excitation fiber200can provide an effect of extending the length of the excitation fiber and improving amplification efficiency. Accordingly, the excitation fiber200can amplify signal light in the C-band and signal light in the L-band with one excitation fiber and can reduce the difference in excitation efficiency between the C-band and the L-band. In other words, the MCF optical amplifier1000using the excitation fiber200enables downsizing of an optical amplifier. A core place at the center of an excitation fiber can be used for amplification of signal light in the C-band in and afterFIG.9described below. Further, the core used for amplification in the C-band may not be the core at the center.

FIG.9is a diagram illustrating an example of core placement of an excitation fiber300. The excitation fiber300includes six cores (a core1to a core6) on the circumference of a circle around the center of the fiber and includes a core7along the central axis of the excitation fiber300. The core1to the core6are equidistantly spaced on the circumference around the center of the excitation fiber300. The excitation fiber300undergoes 120-degree left-hand rotation viewing an end face302. Therefore, for example, an end face3A of the core3faces an end face1B of the core1. Accordingly, first signal light being input to an end face1A of the core1at the end face301and being output from the end face1B is input to the end face3A of the core3. Then, the first signal light propagates through the core3and the core5and is output from an end face5B of the core5.

On the other hand, second signal light input to an end face2A of the core2at the end face301is output from an end face2B at an end face302and is input to an end face4A of the core4. The light propagates through the core4and the core6and is output from an end face6B of the core6. Thus, the cores of the excitation fiber300are divided into two groups (the cores1,3, and5and the cores2,4, and6) and different signal light can be amplified for each group. The core7is not included in optical paths of light propagating through the cores1to6. The core7may be singly used as an optical amplifier with the length of one excitation fiber300, similarly to the excitation fiber300inFIG.8. For example, the cores1,3, and5and the cores2,4, and6may be respectively used for amplification of different beams of signal light in the L-band, and the core7may be used for amplification of signal light in the C-band. The excitation fiber300can also amplify signal light in the C-band and signal light in the L-band with one excitation fiber and reduce the difference in excitation efficiency between the C-band and the L-band.

FIG.10is a diagram illustrating an example of core placement at an end face401of an excitation fiber400. Illustration of core placement at the other end face of an excitation fiber is omitted inFIG.10andFIG.11. The excitation fiber400includes12cores (a core1to a core12). The core1to the core4are placed on a circumference around the center of the excitation fiber400and are equidistantly spaced on the circumference. The core5to the core8are equidistantly spaced on the same circumference as the core1to the core4. The positions of the core1to the core8do not overlap each other. The core9to the core12are equidistantly spaced on the circumference of a circle with a smaller radius than that for the core1to the core8. Accordingly, application of 90-degree rotation to the excitation fiber400can cause first signal light input from an one end face of the core1to propagate through the core1to the core4and be output from an other end face of the core4, similarly to the excitation fibers100to300. Similarly, second signal light input to an end face of the core5can be caused to propagate through the core5to the core8and be output from an end face of the core8. Furthermore, third signal light input to an end face of the core9can be caused to propagate through the core9to the core12and be output from an end face of the core12. Thus, by dividing the12cores into three groups by fours, the excitation fiber400can independently amplify the first to third signal light.

FIG.11is a diagram illustrating an example of core placement at an end face501of an excitation fiber500. The excitation fiber500includes19cores (a core1to a core19). The core1to the core6are placed on a circumference around the center of the excitation fiber500and are equidistantly spaced on the circumference. The core7to the core12are equidistantly spaced on the same circumference as the core1to the core6. The positions of the core1to the core12do not overlap each other. The core13to the core18are equidistantly spaced on the circumference of a circle with a smaller radius than that for the core1to the core12. Application of 60-degree rotation to the excitation fiber500can cause first signal light input to the core1from one end face of the excitation fiber500to propagate through the core1to the core6and be output from the core6at the other end face of the excitation fiber500. Similarly, second signal light input to the core7can be caused to propagate through the core7to the core12and be output from the core12. Furthermore, third signal light input to the core13can be caused to propagate through the core13to the core18and be output from the core18.

Thus, by dividing the18cores into3groups by sixes, the excitation fiber500can independently amplify the first to third signal light. Each beam of the first to third signal light propagates six cores (that is, the core1to the core6, the core7to the core12, or the core13to the core18) in the excitation fiber500. Therefore, the excitation fiber500provides a greater effect of extending the length of a core for amplification than the excitation fiber400.

The excitation fiber500includes the core19at the center. The core19is not included in optical paths of light propagating through the cores1to18. The core19may be singly used as an optical amplifier with the length of one excitation fiber500. For example, the cores1to18may be used for amplification of signal light in the L-band, and the core19may be used for amplification of signal light in the C-band.

FIG.12is a diagram illustrating an example of core placement of an excitation fiber600. The excitation fiber600includes a core1to a core4. The core1to the core4of the excitation fiber600are equidistantly spaced on a straight line passing through a section of the excitation fiber600. In the example inFIG.13, the core1to the core4are equidistantly spaced on a straight line passing through the center of the excitation fiber600at positions point-symmetric with respect to the center of the excitation fiber600. By causing an end face602of such an excitation fiber600to rotate 180 degrees and face an end face601, first signal light input to an end face1A of the core1can be caused to propagate through the core1and the core4and be output from an end face4B of the core4. Furthermore, second signal light input to an end face2A of the core2can be caused to propagate through the core2and the core3and be output from an end face3B of the core3.

FIG.13is a diagram illustrating another facing example of the excitation fiber600. For ease of understanding, the end face601indicates core placement viewed from outside the excitation fiber600, and the end face602indicates core placement at the end face602virtually viewed from inside the excitation fiber600in this diagram. Further, in order to clearly indicate the positions of the end faces of the cores1to4at the end faces601and602, the end face602is shifted downward in the illustration inFIG.13.

InFIG.13, when the end face601and the end face602of the excitation fiber600face each other, the position of the end face602in the lateral direction is laterally shifted relative to the end face101by a spacing of one core. Specifically, in a state of the end face601facing the end face602, an end face1B and the end face2A, an end face2B and an end face3A, and the end face3B and an end face4A respectively face each other.

Signal light is input to the end face1A of the core1. The input signal light is input to the end face2A from the end face1B and propagates through the core2. The signal light propagates through the core2to the core4and is output from the end face4B of the core4. The end face1A of the core to which the signal light is input does not face the end face4B of the core from which the signal light is output in the configuration inFIG.13; and therefore, the mirror801inFIG.4or the like for input and output of signal light is not necessary, which simplifies the configuration of the MCF optical amplifier1000.

FIG.14is a diagram illustrating a facing example of an excitation fiber700. The excitation fiber700includes a core1to a core16. The core1to the core16of the excitation fiber700are placed in four rows and four columns on a section of the excitation fiber700. The rows are equidistantly spaced, and the columns are also equidistantly spaced. An end face701of the excitation fiber700indicates core placement viewed from outside the excitation fiber700, and an end face702indicates core placement virtually viewed from inside the excitation fiber700in this diagram as well. In order to clearly indicate the positions of end faces of the cores1to16at the end faces701and702, the end face702is shifted downward in the illustration inFIG.14. Reference signs of the end faces of the core5to the core16are omitted in the drawing.

The end face701and the end face702are placed in such a way that an end face1B to an end face3B and an end face2A to an end face4A respectively face each other on a one-to-one basis. Similarly, an end face5B to an end face7B and an end face6A to an end face8A respectively face each other on a one-to-one basis, an end face9B to an end face11B and an end face10A to an end face12A respectively face each other on a one-to-one basis, and an end face13B to an end face15B and an end face14A to an end face16A respectively face each other on a one-to-one basis.

First signal light is input to the end face1A of the core1. The first signal light is input to the end face2A from the end face1B and propagates through the core2. Then, the first signal light propagates through the core2to the core4and is output from the end face4B of the core4. Second signal light input to the core5is input to the end face6A from the end face5B and propagates through the core6. The second signal light propagates through the core6to the core8and is output from the end face8B of the core8. Similarly, third signal light input to the core9propagates through the cores9to12and is output from the end face12B of the core12. Fourth signal light input to the core13propagates through the cores13to16and is output from the end face16B of the core16. End faces of the core for inputting signal light and the core for outputting signal light do not face each other in the configuration using the excitation fiber700inFIG.14, similarly toFIG.13. Therefore, optical paths to the outside for input and output of signal light can be more easily configured. Further, the excitation fiber700can simultaneously and independently amplify four beams of signal light.

The example embodiments of the present disclosure may also be described as, but not limited to, the following Supplementary Notes.

Supplementary Note 1

A multi-core fiber optical amplifier including:a multi-core excitation fiber configured to include a first core and a second core; anda clad excitation means for injecting excitation light into a clad of the multi-core excitation fiber, whereinsignal light input to an one end of the first core is output from an other end of the first core,the signal light output from the other end of the first core is input to an one end of the second core, andthe signal light input to one end of the second core is output from an other end of the second core.

Supplementary Note 2

The multi-core fiber optical amplifier according to Supplementary Note 1, whereinthe first core and the second core are equidistantly spaced on a circumference around a center of an end face of the multi-core excitation fiber, andthe other end of the first core is optically coupled to the one end of the second core by maintaining an other end of the multi-core excitation fiber in a state of being rotated by a predefined angle relative to an one end of the multi-core excitation fiber.

Supplementary Note 3

The multi-core fiber optical amplifier according to Supplementary Note 1 or 2, whereinthe multi-core excitation fiber includes a third core,the signal light output from the other end of the second core is input to an one end of the third core, andthe signal light input to the one end of the third core is output from an other end of the third core.

Supplementary Note 4

The multi-core fiber optical amplifier according to Supplementary Note 1 or 2, whereinthe other end of the first core is optically coupled to the one end of the second core by shifting a center of the one end of the multi-core excitation fiber from a center of the other end of the multi-core excitation fiber.

Supplementary Note 5

The multi-core fiber optical amplifier according to any one of Supplementary Notes 1 to 4, whereinoptical coupling is performed between the other end of the first core and the one end of the second core by optical collimators facing each other.

Supplementary Note 6

The multi-core fiber optical amplifier according to Supplementary Note 5, further including,between the optical collimators facing each other, a mirror configured to couple the signal light to the first core by reflecting the signal light.

Supplementary Note 7

The multi-core fiber optical amplifier according to Supplementary Note 6, whereinthe mirror couples the first core to the excitation light by reflecting the excitation light.

Supplementary Note 8

The multi-core fiber optical amplifier according to Supplementary Note 6 or 7, whereinthe clad excitation means includes a wavelength filter configured to be placed between the optical collimators facing each other and guide the excitation light input from outside the multi-core excitation fiber to a clad of the multi-core excitation fiber.

Supplementary Note 9

The multi-core fiber optical amplifier according to Supplementary Note 8, whereinthe wavelength filter causes the signal light to propagate from the other end of the first core to end of the second core by transmitting light at a wavelength of the signal light.

Supplementary Note 10

The multi-core fiber optical amplifier according to Supplementary Note 8 or 9, whereinthe wavelength filter is formed in a part of the mirror.

Supplementary Note 11

An optical amplification method used in a multi-core fiber optical amplifier including a multi-core excitation fiber including a first core and a second core, the method including:injecting excitation light into a clad of the multi-core excitation fiber;outputting signal light input to an one end of the first core from an other end of the first core;inputting the signal light output from another end of the first core to one end of the second core; andoutputting the signal light input to the one end of the second core from the other end of the second core.

Supplementary Note 12

The optical amplification method according to Supplementary Note 11, further including:equidistantly spacing the first core and the second core on a circumference around a center of an end face of the multi-core excitation fiber;rotating an other end of the multi-core excitation fiber by a predefined angle relative to an one end of the multi-core excitation fiber; andoptically coupling the other end of the first core to the one end of the second core.

Supplementary Note 13

The optical amplification method according to Supplementary Note 11 or 12, further including:inputting the signal light output from the other end of the second core to an one end of a third core of the multi-core excitation fiber; andoutputting the signal light input to the one end of the third core from an other end of the third core.

Supplementary Note 14

The optical amplification method according to Supplementary Note 11 or 12, further including:shifting a center of an one end of the multi-core excitation fiber from a center of an other end of the multi-core excitation fiber; andoptically coupling the other end of the first core to the one end of the second core.

Supplementary Note 15

The optical amplification method according to any one of Supplementary Notes 11 to 14, further includingperforming optical coupling between the other end of the first core and the one end of the second core by optical collimators facing each other.

Supplementary Note 16

The optical amplification method according to Supplementary Note 15, further including,by a mirror provided between the optical collimators facing each other, coupling the signal light to the first core by reflecting the signal light.

Supplementary Note 17

The optical amplification method according to Supplementary Note 16, further including,by the mirror, coupling the first core to the excitation light by reflecting the excitation light.

Supplementary Note 18

The optical amplification method according to Supplementary Note 16 or 17, further including,by a wavelength filter placed between the optical collimators facing each other, guiding the excitation light input from outside the multi-core excitation fiber to a clad of the multi-core excitation fiber.

Supplementary Note 19

The optical amplification method according to Supplementary Note 18, further including, by the wavelength filter:transmitting light at a wavelength of the signal light; andcausing the signal light to propagate from the another end of the first core to the one end of the second core.

Supplementary Note 20

The optical amplification method according to Supplementary Note 18 or 19, whereinthe wavelength filter is formed in a part of the mirror.While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. For example, the multi-core fiber optical amplifier described in each example embodiment also discloses an optical amplification method applicable to the multi-core fiber optical amplifier.

The configurations described in the example embodiments are not necessarily exclusive to each other. The advantageous effects of the present invention may be provided by configurations acquired by combining the aforementioned example embodiments in whole or in part.