MULTI-CORE FIBER MODULE AND MULTI-CORE FIBER AMPLIFIER

A multi-core fiber module includes a transmission MCF configured to be used as a transmission path for an optical signal, a connection MCF having a core arrangement similar to a core arrangement of a core of the transmission MCF, and a relay lens system interposed between the transmission MCF and the connection MCF. A relay magnification of the relay lens system is equal to a ratio of a core interval of the connection MCF to a core interval of the transmission MCF. A core at a leading end surface of the connection MCF is expanded such that a ratio between the core interval and a mode field diameter of the connection MCF is equal to a ratio between the core interval and a mode field diameter of the transmission MCF.

TECHNICAL FIELD

The present disclosure relates to a multi-core fiber module and a multi-core fiber amplifier.

This application claims priority based on Japanese Patent Application No. 2020-125668 filed on Jul. 22, 2020, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

PTL 1 describes a configuration in which light passing through a transmission multi-core optical fiber (MCF) and a multi-core optical amplifier arranged in a transmission section is decomposed into a plurality of single-core optical fibers (SCFs) by fan-in and fan-out.

PTL 2 describes a technique in which a connection loss between a pair of optical fibers having different mode field diameters (MFDs) is reduced by a thermal expanded core (TEC). In the technique described in PTL 2, a cladding excitation method is employed.

PTL 3 describes a technique for increasing the core diameter of a multi-core erbium doped optical fiber (MC-EDF) and reducing the mismatch of the MFD with a transmission MCF.

PRIOR ART DOCUMENT

Patent Literature

SUMMARY OF INVENTION

A multi-core fiber module according to an embodiment includes a transmission optical waveguide assembly configured to be used as a transmission path for an optical signal, a connection optical waveguide assembly having a core arrangement similar to a core arrangement of a core of the transmission optical waveguide assembly, and a relay lens system interposed between the transmission optical waveguide assembly and the connection optical waveguide assembly. A relay magnification of the relay lens system is equal to a ratio of a core interval of the connection optical waveguide assembly to a core interval of the transmission optical waveguide assembly. A core at a leading end surface of the connection optical waveguide assembly is expanded such that a ratio between the core interval and a mode field diameter of the connection optical waveguide assembly is equal to a ratio between the core interval and a mode field diameter of the transmission optical waveguide assembly. At least one of the transmission optical waveguide assembly and the connection optical waveguide assembly is a multi-core fiber.

A multi-core fiber module according to another aspect includes a transmission optical waveguide assembly configured to be used as a transmission path for an optical signal, a connection optical waveguide assembly having a core arrangement similar to a core arrangement of a core of the transmission optical waveguide assembly, and a relay lens system interposed between the transmission optical waveguide assembly and the connection optical waveguide assembly. A relay magnification of the relay lens system is equal to a ratio of a core interval of the connection optical waveguide assembly to a core interval of the transmission optical waveguide assembly. A coma aberration on an output side of the relay lens system is non-negative and at least one of the transmission optical waveguide assembly and the connection optical waveguide assembly is a multi-core fiber.

A multi-core fiber amplifier according to an embodiment is a multi-core fiber amplifier includes the multi-core fiber module and a rare-earth element-doped multi-core fiber in which the connection optical waveguide assembly is doped with a rare earth element. The multi-core fiber amplifier includes the transmission optical waveguide assembly that is a first transmission optical waveguide assembly on a signal input side, the transmission optical waveguide assembly that is a second transmission optical waveguide assembly on a signal output side, the multi-core fiber module that is a first multi-core fiber module and the multi-core fiber module that is a second multi-core fiber module. The rare-earth element-doped multi-core fiber is connected to the connection optical waveguide assembly of the first multi-core fiber module and the connection optical waveguide assembly of the second multi-core fiber module. The transmission optical waveguide assembly of the first multi-core fiber module is connected to the first transmission optical waveguide assembly, and the transmission optical waveguide assembly of the second multi-core fiber module is connected to the second transmission optical waveguide assembly.

DESCRIPTION OF EMBODIMENTS

The transmission MCF for signal transmission has a relatively large mode field diameter (hereinafter may be referred to as MFD) (9 to 11 μm) in order to suppress loss or nonlinearity. On the other hand, in the MC-EDF, the MFD is relatively small (6 μm or less) in order to increase the excitation efficiency and the amplification efficiency. As described above, the transmission MCF and the MC-EDF have different MFDs. Therefore, when the transmission MCF is directly connected to the MC-EDF or an MCF having the same core arrangement with the MC-EDF (hereinafter may be referred to as a connection MCF), a connection loss of light may occur due to mismatching of the MFDs.

By the way, even when the TEC process is performed as in the PTL 2 described above, the MFDs of the transmission MCF and the MC-EDF or the connection MCF may not match each other due to a difference between the refractive index distribution of the transmission MCF and the refractive index distribution of the MC-EDF or the connection MCF. Furthermore, in order to match the MFDs, matching of the core interval may also be required. Therefore, even when the TEC process is performed, it may be difficult to obtain the effect of reducing the connection loss. Since the MFD of the MC-EDF or the connection MCF used inside the optical amplifier is small, end face reflection may occur in an optical module that performs spatial coupling by a lens system such as an optical isolator. Furthermore, since the utilization efficiency of the excitation light may be low when the cladding excitation method is employed as in the PTL 2 described above, there is room for improvement in the utilization efficiency of the excitation light.

An object of the present disclosure is to provide a multi-core fiber module and a multi-core fiber amplifier capable of reducing connection loss of light.

According to the present disclosure, the connection loss of light can be reduced.

DESCRIPTION OF EMBODIMENTS OF PRESENT DISCLOSURE

Embodiments of the present disclosure are listed below. A multi-core fiber module according to an embodiment includes a transmission optical waveguide assembly configured to be used as a transmission path for an optical signal, a connection optical waveguide assembly having a core arrangement similar to a core arrangement of a core of the transmission optical waveguide assembly, and a relay lens system interposed between the transmission optical waveguide assembly and the connection optical waveguide assembly. A relay magnification of the relay lens system is equal to a ratio of a core interval of the connection optical waveguide assembly to a core interval of the transmission optical waveguide assembly. A core at a leading end surface of the connection optical waveguide assembly is expanded such that a ratio between the core interval and a mode field diameter of the connection optical waveguide assembly is equal to a ratio between the core interval and a mode field diameter of the transmission optical waveguide assembly. At least one of the transmission optical waveguide assembly and the connection optical waveguide assembly is a multi-core fiber.

In this multi-core fiber module, the core arrangement of the transmission optical waveguide assembly is similar to the core arrangement of the connection optical waveguide assembly connected to the transmission optical waveguide assembly via the relay lens system. The relay magnification of the relay lens system is equal to the ratio of the core interval of the connection optical waveguide assembly to the core interval of the transmission optical waveguide assembly. The core at a leading end surface of the connection optical waveguide assembly is expanded such that a ratio between the core interval and a mode field diameter of the connection optical waveguide assembly is equal to a ratio between the core interval and a mode field diameter of the transmission optical waveguide assembly. Therefore, the ratio between the core interval and the mode field diameter is matched between the transmission optical waveguide assembly and the connection optical waveguide assembly, and the ratio between the core interval of the transmission optical waveguide assembly and the core interval of the connection optical waveguide assembly is equal to the relay magnification. Therefore, it is possible to connect the transmission optical waveguide assembly and the connection optical waveguide assembly with low loss via the relay lens system.

Both the transmission optical waveguide assembly and the connection optical waveguide assembly may be multi-core fibers.

The relay magnification may be 0.5 times or more and 2.0 times or less. In this case, since the relay magnification is 0.5 times or more and 2.0 times or less, it is possible to suppress the occurrence of aberration of the relay lens system between the transmission optical waveguide assembly and the connection optical waveguide assembly.

The mode field diameter at a leading end surface of the connection optical waveguide assembly may be 7 μm or more. In this case, since the mode field diameter at the leading end surface of the connection optical waveguide assembly is 7 μm or more, the connection loss due to reflection of light at the leading end surface can be more reliably suppressed.

A coma aberration on an output side of the relay lens system may be non-negative. In this case, even though a coma aberration occurs on the output side of the relay lens system, the coma aberration can be directed outward. Therefore, optical coupling to an adjacent core can be avoided, and occurrence of excessive crosstalk can be suppressed.

The relay lens system may include an input-side lens and an output-side lens. A refractive index of the input-side lens may be 1.68 or more, and a radius of curvature of an incidence surface of the input-side lens may be 10 times or more of a radius of curvature of an exit surface of the input-side lens. One of the transmission optical waveguide assembly and the connection optical waveguide assembly may be an input-side optical waveguide assembly, and another one of the transmission optical waveguide assembly and the connection optical waveguide assembly may be an output optical waveguide assembly, and the input-side optical waveguide assembly may be disposed such that a distance between a light exit end of the input-side optical waveguide assembly and a principal point of the input-side lens is 0.99 times or more and 1.01 times or less of a focal distance of the input-side lens. A refractive index of the output-side lens may be 1.70 or less, and a radius of curvature of an exit surface of the output-side lens may be 10 times or more of a radius of curvature of an incidence surface of the output-side lens. The output optical waveguide assembly may be disposed such that a distance between a light incidence end of the output optical waveguide assembly and a principal point of the output-side lens is 0.99 times or more and 1.01 times or less of a focal distance of the output-side lens. In this case, coma aberration may be directed outward in the relay lens system including a plano-convex lens.

The relay lens system may include an input-side lens and an output-side lens, a refractive index of the input-side lens may be 1.62 or more, and a radius of curvature of an incidence surface of the input-side lens may be 10 times or more of a radius of curvature of an exit surface of the input-side lens. One of the transmission optical waveguide assembly and the connection optical waveguide assembly may be an input-side optical waveguide assembly, and another one of the transmission optical waveguide assembly and the connection optical waveguide assembly may be an output optical waveguide assembly, and the input-side optical waveguide assembly may be disposed such that a distance between a light exit end of the input-side optical waveguide assembly and a principal point of the input-side lens is 0.99 times or more and 1.01 times or less of a focal distance of the input-side lens. A refractive index of the output-side lens may be 1.51 or less, and a radius of curvature of an exit surface of the output-side lens may be 10 times or more of a radius of curvature of an incidence surface of the output-side lens. The output optical waveguide assembly may be disposed such that a distance between a light incidence end of the output optical waveguide assembly and a principal point of the output-side lens is 0.99 times or more and 1.01 times or less of a focal distance of the output-side lens. In this case, coma aberration may be directed outward in a relay lens system including a plano-convex lens.

A multi-core fiber module according to another aspect includes a transmission optical waveguide assembly configured to be used as a transmission path for an optical signal, a connection optical waveguide assembly having a core arrangement similar to a core arrangement of a core of the transmission optical waveguide assembly, and a relay lens system interposed between the transmission optical waveguide assembly and the connection optical waveguide assembly. A relay magnification of the relay lens system is equal to a ratio of a core interval of the connection optical waveguide assembly to a core interval of the transmission optical waveguide assembly. A coma aberration on an output side of the relay lens system is non-negative, and at least one of the transmission optical waveguide assembly and the connection optical waveguide assembly is a multi-core fiber. In this case, even though coma aberration occurs on the output side of the relay lens system, coma aberration can be directed outward. Therefore, optical coupling to an adjacent core can be avoided, and occurrence of excessive crosstalk can be suppressed.

A core at a leading end surface of an optical waveguide of at least one of the transmission optical waveguide assembly and the connection optical waveguide assembly may be expanded. In this case, mismatching of the mode field diameter can be suppressed.

The transmission optical waveguide assembly and the connection optical waveguide assembly may be multi-core fibers of types identical to each other. The transmission optical waveguide assembly and the connection optical waveguide assembly may be multi-core fibers of types differing from each other. One of the transmission optical waveguide assembly and the connection optical waveguide assembly may be an assembly of single-core fibers. At least one of the transmission optical waveguide assembly and the connection optical waveguide assembly may be an assembly of multi-core fibers.

A multi-core fiber amplifier according to an embodiment is a multi-core fiber amplifier includes the multi-core fiber module described above and a rare-earth element-doped multi-core fiber in which the connection optical waveguide assembly is doped with a rare earth element. The multi-core fiber amplifier includes a first transmission optical waveguide assembly on a signal input side, a second transmission optical waveguide assembly on a signal output side, a first multi-core fiber module, and a second multi-core fiber module. The rare-earth element-doped multi-core fiber is connected to the connection optical waveguide assembly of the first multi-core fiber module and the connection optical waveguide assembly of the second multi-core fiber module. The transmission optical waveguide assembly of the first multi-core fiber module is connected to the first transmission optical waveguide assembly, and the transmission optical waveguide assembly of the second multi-core fiber module is connected to the second transmission optical waveguide assembly.

The multi-core fiber amplifier includes the first and second multi-core fiber modules described above and the rare-earth element-doped multi-core fiber. The rare-earth element-doped multi-core fiber is connected to the connection optical waveguide assembly of the first multi-core fiber module and the connection optical waveguide assembly of the second multi-core fiber module. The transmission optical waveguide assembly of the first multi-core fiber module is connected to the first transmission optical waveguide assembly of the signal input side, and the transmission optical waveguide assembly of the second multi-core fiber module is connected to the second transmission optical waveguide assembly of the signal output side. The core interval and the mode field diameter are matched between each transmission optical waveguide assembly and each connection optical waveguide assembly, and the ratio of the core interval between each transmission optical waveguide assembly and each connection optical waveguide assembly is equal to the relay magnification. Therefore, the mode field diameters of the transmission optical waveguide assembly and the rare-earth element-doped multi-core fiber can be matched.

The first multi-core fiber module may include an excitation light combiner, and the second multi-core fiber module may include an optical isolator. In this case, since the core interval and the mode field diameter of each multi-core fiber are matched, it is possible to reduce the end face reflection in the optical connection through the rare-earth element-doped multi-core fiber having a small mode field diameter or the connection optical waveguide assembly. Thus, the utilization efficiency of the excitation light can be enhanced.

Details of Embodiments of Present Disclosure

Specific examples of a multi-core fiber module and a multi-core fiber amplifier according to an embodiments of the present disclosure will be described. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted as appropriate. The drawings may be partially simplified or exaggerated for ease of understanding, and dimensional ratios and the like are not limited to those illustrated in the drawings.

FIG.1is a diagram showing a multi-core fiber module1according to an embodiment. In the following description, the multi-core fiber may be referred to as MCF, and the mode field diameter may be referred to as MFD. Multi-core fiber module1includes a transmission MCF10, which is an example of a transmission optical waveguide assembly, and a connection MCF20, which is an example of a connection optical waveguide assembly. In this embodiment, multi-core fiber module1includes transmission MCF10, connection MCF20, and a relay lens system R interposed between transmission MCF10and connection MCF20. Transmission MCF10is used as a transmission path of a light L1which is an optical signal. Transmission MCF10includes a plurality of (for example, seven) cores11and a cladding12. Connection MCF20includes a plurality of (for example, seven) cores21and a cladding22. Connection MCF20has a core arrangement similar to cores11of transmission MCF10.

For example, multi-core fiber module1inputs light L1to an optical amplifier through transmission MCF10, relay lens system R, and connection MCF20. In this case, transmission MCF10is an input-side optical waveguide assembly and connection MCF20is an output optical waveguide assembly. Relay lens system R includes, for example, a first lens30which is an input-side lens facing a leading end surface14of transmission MCF10and a second lens40which is an output-side lens facing a leading end surface24of connection MCF20.

For example, an antireflection film is provided on each of leading end surface14and leading end surface24. The normal line of each of leading end surface14and leading end surface24may be inclined (for example, about 8°) with respect to the direction in which transmission MCF10and connection MCF20extend. In this case, it is possible to suppress reflection of light L1on each of leading end surface14and leading end surface24. For example, in multi-core fiber module1, transmission MCF10, first lens30, second lens40, and connection MCF20are arranged in this order. Transmission MCF10and connection MCF20are optically coupled via a space (spatial coupling).

An arrangement shape of a plurality of cores11of transmission MCF10and an arrangement shape of a plurality of cores21of connection MCF20are similar to each other. For example, when a core interval of cores11of transmission MCF10is P1(μm) and a core interval of cores21of connection MCF20is P2(μm), P1is equal to P2.

For example, connection MCF20has a core expansion portion23at leading end surface24. Core expansion portion23denotes a portion where core21is expanded. The expansion of core21is performed, for example, by heating core21. As illustrated inFIG.14, as core21is heated, the MFD of connection MCF20is expanded.

For example, an MFD at the specific wavelength at an output end of core11of transmission MCF10is MFD1(μm), and an MFD of the specific wavelength at an output end of core21of connection MCF20is MFD2(μm). At this time, core21at leading end surface24of connection MCF20is expanded so that a ratio between the core interval P2and the MFD2of connection MCF20is equal to a ratio between the core interval P1and the MFD1of transmission MCF10.

In the present disclosure, the term “equal” is not limited to a case where the values completely coincide with each other, but also includes a case where the values are substantially the same to the extent that there is no functional difference (for example, a case of ±10% or less). The MFD2of connection MCF20in which core21is expanded is 7 μm or more and 30 μm or less, for example.

In relay lens system R, for example, first lens30converts light L1emitted from each of the plurality of cores11of transmission MCF10into collimated light, and second lens40condenses light L1on core21of connection MCF20. When a relay magnification of relay lens system R (for example, first lens30and second lens40) is r, the value of r is equal to the value of (P2/P1), that is, a ratio of core interval P2of connection MCF20to core interval P1of transmission MCF10.

FIG.1shows an example where MFD1is equal to MFD2. That is, in multi-core fiber module1, transmission MCF10and connection MCF20each having the same core interval are connected to each other through an equal-magnification relay lens system. Optical electric fields in core11of transmission MCF10and in core21of connection MCF20are shown as bell-shaped symbols M inFIG.1. As indicated by the mark M, for example, the optical electric field at leading end surface24in core21of connection MCF20coincides with the optical electric field of core11of transmission MCF10. An expansion ratio of the MFD at leading end surface24of connection MCF20is, for example, equal to a ratio of the MFD of transmission MCF10to the MFD of core21which is not subjected to core expansion, and is, for example, about ±10%.

Meanwhile, coma aberration may occur on an output side of relay lens system R.FIG.2shows an example in which coma aberration occurs in an outward direction with respect to an optical axis (outward coma aberration), andFIG.3shows an example in which coma aberration occurs in an inward direction with respect to the optical axis (inward coma aberration). In a configuration in which the plurality of cores21are arranged to form an annular shape in a cross section of connection MCF20orthogonal to the optical axis, a spread of the optical electric field due to the inward coma aberration may cause excessive crosstalk between cores21. When a doublet lens or a triplet lens is used as relay lens system R, coma aberration can be suppressed. However, from the viewpoint of cost reduction, it is preferable to use a singlet lens as relay lens system R. According to the embodiment, first lens30and second lens40are singlet lenses.

In the embodiment, the singlet lens of relay lens system R is designed such that coma aberration to be outward. Since the optical electric field expanded by the outward coma aberration is not coupled to a waveguide mode of adjacent core21, it does not cause excessive crosstalk. By making the coma aberration of the output side of relay lens system R non-negative, the coma aberration becomes outward, and the suppression of excessive crosstalk between cores21is realized. A refractive index, a shape, and a position of each of first lens30and second lens40are determined such that coma aberration is outward at leading end surface24of connection MCF20. Examples of the refractive index, the shape, and the position will be described below.

FIG.4is a diagram showing a multi-core fiber module1A according to another embodiment. Hereinafter, description common to multi-core fiber module1described above will be omitted as appropriate. In multi-core fiber module1A, a transmission MCF10A having narrow core interval P1and a connection MCF20A having a relatively wide core interval P2are connected to each other via relay lens system R. Transmission MCF10A includes a core11A, a cladding12A and a leading end surface14A, and connection MCF20A includes a core21A, a cladding22A and a leading end surface24A.

In multi-core fiber module1A, core interval P1of cores11A of transmission MCF10A is smaller than core interval P2of cores21A of connection MCF20A. Connection MCF20A has a core expansion portion23A at leading end surface24A. Core21A at leading end surface24A of connection MCF20A is expanded such that a ratio between core interval P2and the MFD2of connection MCF20A is equal to a ratio between core interval P1and the MFD1of transmission MCF10A.

For example, a light L2emitted from core11A of transmission MCF10is condensed to core21A of connection MCF20A via relay lens system R. In this case, transmission MCF10is the input-side optical waveguide assembly and connection MCF20A is the output optical waveguide assembly. As in the case of multi-core fiber module1described above, the relay magnification r of relay lens system R is equal to the ratio of core interval P2of connection MCF20to core interval P1of transmission MCF10. In multi-core fiber module1A, the ratio is larger than that of multi-core fiber module1.

FIG.5is a diagram showing a multi-core fiber module1B according to another embodiment. In multi-core fiber module1B, an optical function element50(or an optical function element group) is arranged in a region including a confocal point of relay lens system R. For example, optical function element50includes a birefringent crystal51, a faraday rotator52, and a half-wave plate53arranged at a confocal portion of relay lens system R.

Faraday rotator52and half-wave plate53are sandwiched between a pair of birefringent crystals51, for example. In addition, optical function element50may be an optical isolator. A light L3ofFIG.3represents a principal ray in multi-core fiber module1B, and the dashed line ofFIG.3represents an example of extraordinary ray. Multi-core fiber module1B is arranged, for example, on an output side of an optical amplifier (MC-EDF) to be described in detail later.

FIG.6shows a multi-core fiber module1C in which a dichroic minor71is disposed at the confocal portion of relay lens system R and an excitation multi-core fiber (excitation MCF)60is connected to connection MCF20through dichroic minor71. Excitation MCF60includes a core61having a core expansion portion63at a leading end surface64and a cladding62. Excitation MCF60is, for example, an MCF of the same type as connection MCF20.

Excitation MCF60has a core arrangement similar to the core arrangement of connection MCF20. In addition, a relay magnification of the relay lens system including a lens70, dichroic mirror71, and second lens40located between excitation MCF60and connection MCF20and an expansion ratio of core61in core expansion portion63are determined from a relationship between a core interval P3of core61of excitation MCF60and an MFD3that is an MFD of core61, as described above. Therefore, the relay magnification of the relay lens system is equal to a ratio of core interval P3of excitation MCF60to core interval P2of connection MCF20. A ratio of core interval P3to MFD3of excitation MCF60is equal to the ratio of core interval P2to MFD2of connection MCF20.

FIG.7shows a multi-core fiber amplifier80according to an embodiment. Multi-core fiber amplifier80includes the above-described transmission MCF10and connection MCF20, an optical isolator81, an excitation light combiner82, a rare-earth element-doped MCF85, an optical isolator86, and a gain flattening filter87.

Multi-core fiber amplifier80includes a plurality of transmission MCFs10, a plurality of connection MCFs20, a plurality of excitation MCFs60, and a plurality of splicing points S. Splicing point S is provided at each of a boundary between a pair of transmission MCFs10, a boundary between a pair of excitation MCFs60, and a boundary between connection MCF20and rare-earth element-doped MCF85.

Rare-earth element-doped MCF85is connected to connection MCF20of multi-core fiber module1C and connection MCF20of multi-core fiber module1B. Transmission MCF10on the signal input side is connected to transmission MCF10of multi-core fiber module1C, and transmission MCF10on the signal output side is connected to transmission MCF10of multi-core fiber module1B.

For example, multi-core fiber module1C may include excitation light combiner82, and multi-core fiber module1B may include optical isolator86. Optical isolator81is connected to transmission MCF10on the signal input side, and is connected to excitation light combiner82via transmission MCF10. Transmission MCF10is connected to both the signal input side and the signal output side of optical isolator81. Connection MCF20is connected to the signal input side of optical isolator86, and transmission MCF10is connected to the signal output side of optical isolator86.

For example, excitation light combiner82is connected to an excitation light output portion83and a driver84via excitation MCF60. The signal light output from excitation light combiner82via connection MCF20and the excitation light are input to rare-earth element-doped MCF85. A plurality of cores of rare-earth element-doped MCF85have a core arrangement similar to transmission MCF10, connection MCF20, and excitation MCF60.

For example, rare-earth element-doped MCF85may collectively excite signal lights passing through a plurality of cores and collectively amplify the signal lights. Rare-earth element-doped MCF85may constitute, for example, a multi-core erbium (Er)-doped optical fiber amplifier (coupled amplifier) doped with erbium (Er). In this case, rare-earth element-doped MCF85has a plurality of cores doped with Er and a cladding surrounding the plurality of cores. When the excitation light and the signal light are input to rare-earth element-doped MCF85, for example, the Er element doped in the core of rare-earth element-doped MCF85is excited and the signal light is amplified.

FIG.8shows a multi-core fiber amplifier80A according to another embodiment. Multi-core fiber amplifier80A is different from the above-described multi-core fiber amplifier80in that transmission MCF10is connected to the signal input side of optical isolator81and connection MCF20is connected to the signal output side of optical isolator81. Multi-core fiber amplifier80A is also different from multi-core fiber amplifier80in that connection MCF20is connected to both the signal input side and the signal output side of optical isolator86.

Next, effects obtained from the multi-core fiber module and the multi-core fiber amplifier according to the embodiment will be described. In multi-core fiber module1, the core arrangement of transmission MCF10is similar to the core arrangement of connection MCF20connected to transmission MCF10via relay lens system R. Relay magnification r of relay lens system R is equal to the ratio of core interval P2of connection MCF20to core interval P1of transmission MCF10. Core21of leading end surface24of connection MCF20is expanded so that the ratio between core interval P2and MFD2of connection MCF20is equal to the ratio between core interval P1and MFD1of transmission MCF10. Therefore, the ratios of core intervals P1and P2and MFD1and MFD2are matched between transmission MCF10and connection MCF20, and the ratio of core interval P1of transmission MCF10and core interval P2of connection MCF20is equal to relay magnification r. Therefore, transmission MCF10and connection MCF20can be connected with low loss via relay lens system R.

Relay magnification r may be 0.5 times or more and 2.0 times or less. In this case, since relay magnification r is 0.5 times or more and 2.0 times or less, it is possible to suppress the occurrence of aberration of relay lens system R between transmission MCF10and connection MCF20.

MFD2at leading end surface24of connection MCF20may be 7 μm or more. In this case, since the MFD2at leading end surface24of connection MCF20is 7 μm or more, the connection loss due to the reflection of light at leading end surface24can be more reliably suppressed.

Multi-core fiber amplifier80includes multi-core fiber module1C, multi-core fiber module1B, and rare-earth element-doped MCF85. Rare-earth element-doped MCF85is connected to connection MCF20of multi-core fiber module1C and connection MCF20of multi-core fiber module1B. Transmission MCF10on the signal input side is connected to transmission MCF10of multi-core fiber module1C, and transmission MCF10for signal output is connected to transmission MCF10of multi-core fiber module1B. Core intervals P1, P2and MFD1, MFD2are matched between each transmission MCF10and each connection MCF20, and the ratio of core intervals P1, P2in each transmission MCF10and each connection MCF20coincides with relay magnification r. Therefore, the MFDs of transmission MCF10and rare-earth element-doped MCF85can be matched.

Multi-core fiber module1C may include excitation light combiner82, and multi-core fiber module1B may include optical isolator86. In this case, since core intervals P1, P2and MFD1, MFD2of respective transmission MCF10and connection MCF20are matched, it is possible to suppress the end face reflection in the optical connection via rare-earth element-doped MCF85or connection MCF20having a small MFD. In addition, it is possible to increase the utilization efficiency of the excitation light output from excitation MCF60.

The embodiments of the multi-core fiber module and the multi-core fiber amplifier according to the present disclosure have been described above. However, the multi-core fiber module and the multi-core fiber amplifier according to the present disclosure are not limited to the above-described embodiments and can be appropriately modified. Hereinafter, further modification of the multi-core fiber module will be described.

As shown inFIG.9, a multi-core fiber module1E according to a modification is different from multi-core fiber module1C ofFIG.6in that a plurality of excitation single-core fibers (excitation SCFs)90are provided instead of excitation MCF60. Each excitation SCF90includes a core91, a cladding92, a core expansion portion93and a leading end surface94, similar to core61, cladding62, core expansion portion63and leading end surface64of excitation MCF60. In this way, the configuration of the excitation light combiner that outputs excitation light can be changed as appropriate.

As shown inFIG.10, a multi-core fiber module1F according to another modification includes lens70and a lens101as relay lens system R and a dichroic minor102. Dichroic minor102reflects the light input from core11of transmission MCF10via lens101and transmits the excitation light input from core61of excitation MCF60via lens70. Dichroic minor102inputs the signal light from transmission MCF10and the excitation light from excitation MCF60to connection MCF20via lens101.

As shown inFIG.11, a multi-core fiber module1G according to a further modification includes first lens30and a lens111as relay lens system R and a dichroic minor112. Dichroic minor112transmits the light input from core11of transmission MCF10through first lens30and reflects the excitation light input from core61of excitation MCF60through lens111. Dichroic minor112inputs the signal light from first lens30to connection MCF20via lens111together with the excitation light from excitation MCF60.

As shown inFIG.12, a multi-core fiber module1H according to a modification includes first lens30which is an input-side lens of relay lens system R and lens111which is an output-side lens of relay lens system R. Further, multi-core fiber module1H has a plurality of multi-core fibers120which are bundled and disposed on the output side of lens111. In this case, transmission MCF10is the input-side optical waveguide assembly, and the plurality of bundled multi-core fibers120which are bundled is the output optical waveguide assembly. Like the above-described multi-core fibers, multi-core fiber120includes a core121and a cladding122. For example, a core expansion portion123is formed on an end surface of each core21on lens111side. The plurality of multi-core fibers120are slidable in a direction orthogonal to the optical axis. Multi-core fiber module1H has an optical system as an optical switch for switching connection by sliding the bundled multi-core fiber120.

As shown inFIG.13, a multi-core fiber module1J according to a modification has a fan-in fan-out optical system. Multi-core fiber module1J includes the above-described transmission MCF10, first lens30which is an input-side lens of relay lens system R, lens70which is an output-side lens of relay lens system R, and a plurality of single-core fibers130. For example, the plurality of single-core fibers130which are bundled may be provided on the output side of lens70. Single-core fiber130includes a core131and a cladding132, and a core expansion portion133is formed on an end surface of core131on lens70side. In this case, transmission MCF10is the input-side optical waveguide assembly, and the plurality of bundled single-core fibers130is the output optical waveguide assembly.

Various examples of the multi-core fiber module have been described above. In each of the above-described examples, the core expansion portion may be formed on the lens-side end surface of the core.FIG.14is a graph showing a relationship between a heating time of a core of an optical fiber and a mode field diameter of the optical fiber. As shown inFIG.14, the longer the heating time of the core of the optical fiber is, the larger the mode field diameter of the optical fiber can be made.

As described above, in the multi-core fiber module according to the embodiment of the present disclosure, the coma aberration of the output side of relay lens system R is non-negative. Therefore, even if the coma aberration occurs on the output side of relay lens system R, the core aberration can be directed outward. Therefore, optical coupling to an adjacent core can be avoided, and occurrence of excessive crosstalk can be suppressed.

The coma aberration will be described in detail. First, when a radius of a circle formed by the coma aberration is Rc, Rcis expressed by Equation (1).

Where H is a distance from the optical axis to a ray of light on an image plane, p is a distance from the optical axis to a ray of light on a pupil plane, and f is the focal distance of the lens. C is a coma coefficient expressed by Equation (2), and when the value of C is positive, the outward coma aberration occurs, and when the value of C is negative, the inward coma aberration occurs.

Here, n represents a refractive index of a glass material of a lens, S1represents a distance between the image plane and the pupil plane, S0represents a distance between the object plane and the pupil plane, r1represents a radius of curvature of an object-side surface of the lens, and r2represents a radius of curvature of an image-side surface of the lens. In Equation (2), when one of the absolute values of r1and r2is extremely large as in the plano-convex lens, it is difficult to distinguish the convex surface, the concave surface, and the flat surface. In the scale of the spatial optical module for multi-core fiber, when the radius of curvature exceeds the 100 mm, even the convex surface or the concave surface cannot be distinguished from the flat surface.

FIG.15is a graph showing the relationship between a coma coefficient and a refractive index when a parallel light is emitted from a plane in a plano-convex lens.FIG.16is a graph showing a relationship between a coma coefficient and a refractive index when a parallel light is incident on a plane in a plano-convex lens.FIG.17shows various examples of ray of light when a coma aberration occurs. The uppermost row ofFIG.17shows the case where the outward coma aberration occurs in an unit conjugated system, the second row from the top ofFIG.17shows the case where the inward coma aberration occurs in the unit conjugated system, the third row from the top ofFIG.17shows the case where the outward coma aberration occurs in a relay system, and the first row from the bottom ofFIG.17shows the case where the inward coma aberration occurs in the relay system. In the embodiment, excessive crosstalk can be suppressed by adjusting the lens so that the coma aberration that occurs is outward.

Various examples of the multi-core fiber module and the multi-core fiber amplifier have been described above. However, the multi-core fiber module and the multi-core fiber amplifier according to the present disclosure are not limited to the above-described examples. That is, it is easily recognized by those skilled in the art that various modifications and changes can be made to the present invention within the scope of the gist described in the claims. For example, the configuration, function, material, and arrangement mode of each part of the multi-core fiber module and the multi-core fiber amplifier can be appropriately changed within the scope of the gist described above.

REFERENCE SIGNS LIST