Patent Application
20230275390
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.
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.
PTL 1: K. Takeshima, et al, “51.1-Tbit/s MCF Transmission Over 2520 km Using Cladding-Pumped Seven-Core EDFAs,” Journal of Light. Technol. 34 (2016), 761
PTL 2: Japanese Unexamined Patent Application Publication No. 2003-98378
PTL 3: M. Wada, et al “Full C-band Low Mode Dependent and Flat Gain Amplifier using Cladding Pumped Randomly Coupled 12-core EDF,” ECOC2017, -Th.PDP.A.5
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.
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.
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.
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.
For example, multi-core fiber module 1 inputs light L1 to an optical amplifier through transmission MCF 10, relay lens system R, and connection MCF 20. In this case, transmission MCF 10 is an input-side optical waveguide assembly and connection MCF 20 is an output optical waveguide assembly. Relay lens system R includes, for example, a first lens 30 which is an input-side lens facing a leading end surface 14 of transmission MCF 10 and a second lens 40 which is an output-side lens facing a leading end surface 24 of connection MCF 20.
For example, an antireflection film is provided on each of leading end surface 14 and leading end surface 24. The normal line of each of leading end surface 14 and leading end surface 24 may be inclined (for example, about 8°) with respect to the direction in which transmission MCF 10 and connection MCF 20 extend. In this case, it is possible to suppress reflection of light L1 on each of leading end surface 14 and leading end surface 24. For example, in multi-core fiber module 1, transmission MCF 10, first lens 30, second lens 40, and connection MCF 20 are arranged in this order. Transmission MCF 10 and connection MCF 20 are optically coupled via a space (spatial coupling).
An arrangement shape of a plurality of cores 11 of transmission MCF 10 and an arrangement shape of a plurality of cores 21 of connection MCF 20 are similar to each other. For example, when a core interval of cores 11 of transmission MCF 10 is P1 (μm) and a core interval of cores 21 of connection MCF 20 is P2 (μm), P1 is equal to P2.
For example, connection MCF 20 has a core expansion portion 23 at leading end surface 24. Core expansion portion 23 denotes a portion where core 21 is expanded. The expansion of core 21 is performed, for example, by heating core 21. As illustrated in
For example, an MFD at the specific wavelength at an output end of core 11 of transmission MCF 10 is MFD1 (μm), and an MFD of the specific wavelength at an output end of core 21 of connection MCF 20 is MFD2 (μm). At this time, core 21 at leading end surface 24 of connection MCF 20 is expanded so that a ratio between the core interval P2 and the MFD2 of connection MCF 20 is equal to a ratio between the core interval P1 and the MFD1 of transmission MCF 10.
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 MFD2 of connection MCF 20 in which core 21 is expanded is 7 μm or more and 30 μm or less, for example.
In relay lens system R, for example, first lens 30 converts light L1 emitted from each of the plurality of cores 11 of transmission MCF 10 into collimated light, and second lens 40 condenses light L1 on core 21 of connection MCF 20. When a relay magnification of relay lens system R (for example, first lens 30 and second lens 40) is r, the value of r is equal to the value of (P2/P1), that is, a ratio of core interval P2 of connection MCF 20 to core interval P1 of transmission MCF 10.
Meanwhile, coma aberration may occur on an output side of relay lens system R.
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 core 21, 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 cores 21 is realized. A refractive index, a shape, and a position of each of first lens 30 and second lens 40 are determined such that coma aberration is outward at leading end surface 24 of connection MCF 20. Examples of the refractive index, the shape, and the position will be described below.
As an example, first lens 30 and second lens 40 are plano-convex lenses. For example, the refractive index of first lens 30 is 1.68 or more (as an example, about 1.69), and a radius of curvature of an incidence surface of first lens 30 is 10 times or more of a radius of curvature of an exit surface of first lens 30. In the embodiment, the value of the refractive index indicates a value in a wavelength band of 1520 nm or more and 1570 nm or less (C band) or 1520 nm or more and 1630 nm or less (C+L band) each of which is a communication wavelength band of the optical fiber. The incidence surface of first lens 30 is substantially planar. A distance between an emitting end of the light L of transmission MCF 10 and a principal point of first lens 30 is 0.99 times or more and 1.01 times or less of a focal distance of first lens 30. The refractive index of second lens 40 is 1.70 or less, and a radius of curvature of an exit surface of second lens 40 is 10 times or more of a radius of curvature of an incidence surface of second lens 40. The exit surface of second lens 40 is substantially planar. A distance between a light incidence end of connection MCF 20 and a principal point of second lens 40 is 0.99 times or more and 1.01 times or less of a focal distance of second lens 40.
In multi-core fiber module 1A, core interval P1 of cores 11A of transmission MCF 10A is smaller than core interval P2 of cores 21A of connection MCF 20A. Connection MCF 20A has a core expansion portion 23A at leading end surface 24A. Core 21A at leading end surface 24A of connection MCF 20A is expanded such that a ratio between core interval P2 and the MFD2 of connection MCF 20A is equal to a ratio between core interval P1 and the MFD1 of transmission MCF 10A.
For example, a light L2 emitted from core 11A of transmission MCF 10 is condensed to core 21A of connection MCF 20A via relay lens system R. In this case, transmission MCF 10 is the input-side optical waveguide assembly and connection MCF 20A is the output optical waveguide assembly. As in the case of multi-core fiber module 1 described above, the relay magnification r of relay lens system R is equal to the ratio of core interval P2 of connection MCF 20 to core interval P1 of transmission MCF 10. In multi-core fiber module 1A, the ratio is larger than that of multi-core fiber module 1.
Faraday rotator 52 and half-wave plate 53 are sandwiched between a pair of birefringent crystals 51, for example. In addition, optical function element 50 may be an optical isolator. A light L3 of
Excitation MCF 60 has a core arrangement similar to the core arrangement of connection MCF 20. In addition, a relay magnification of the relay lens system including a lens 70, dichroic mirror 71, and second lens 40 located between excitation MCF 60 and connection MCF 20 and an expansion ratio of core 61 in core expansion portion 63 are determined from a relationship between a core interval P3 of core 61 of excitation MCF 60 and an MFD3 that is an MFD of core 61, as described above. Therefore, the relay magnification of the relay lens system is equal to a ratio of core interval P3 of excitation MCF 60 to core interval P2 of connection MCF 20. A ratio of core interval P3 to MFD3 of excitation MCF 60 is equal to the ratio of core interval P2 to MFD2 of connection MCF 20.
Multi-core fiber amplifier 80 includes a plurality of transmission MCFs 10, a plurality of connection MCFs 20, a plurality of excitation MCFs 60, and a plurality of splicing points S. Splicing point S is provided at each of a boundary between a pair of transmission MCFs 10, a boundary between a pair of excitation MCFs 60, and a boundary between connection MCF 20 and rare-earth element-doped MCF 85.
Multi-core fiber amplifier 80 includes, for example, multi-core fiber module 1C (first multi-core fiber module) including transmission MCF 10, connection MCF 20 and excitation MCF 60, multi-core fiber module 1B (second multi-core fiber module), and rare-earth element-doped MCF 85.
Rare-earth element-doped MCF 85 is connected to connection MCF 20 of multi-core fiber module 1C and connection MCF 20 of multi-core fiber module 1B. Transmission MCF 10 on the signal input side is connected to transmission MCF 10 of multi-core fiber module 1C, and transmission MCF 10 on the signal output side is connected to transmission MCF 10 of multi-core fiber module 1B.
For example, multi-core fiber module 1C may include excitation light combiner 82, and multi-core fiber module 1B may include optical isolator 86. Optical isolator 81 is connected to transmission MCF 10 on the signal input side, and is connected to excitation light combiner 82 via transmission MCF 10. Transmission MCF 10 is connected to both the signal input side and the signal output side of optical isolator 81. Connection MCF 20 is connected to the signal input side of optical isolator 86, and transmission MCF 10 is connected to the signal output side of optical isolator 86.
For example, excitation light combiner 82 is connected to an excitation light output portion 83 and a driver 84 via excitation MCF 60. The signal light output from excitation light combiner 82 via connection MCF 20 and the excitation light are input to rare-earth element-doped MCF 85. A plurality of cores of rare-earth element-doped MCF 85 have a core arrangement similar to transmission MCF 10, connection MCF 20, and excitation MCF 60.
For example, rare-earth element-doped MCF 85 may collectively excite signal lights passing through a plurality of cores and collectively amplify the signal lights. Rare-earth element-doped MCF 85 may 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 MCF 85 has 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 MCF 85, for example, the Er element doped in the core of rare-earth element-doped MCF 85 is excited and the signal light is amplified.
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 module 1, the core arrangement of transmission MCF 10 is similar to the core arrangement of connection MCF 20 connected to transmission MCF 10 via relay lens system R. Relay magnification r of relay lens system R is equal to the ratio of core interval P2 of connection MCF 20 to core interval P1 of transmission MCF 10. Core 21 of leading end surface 24 of connection MCF 20 is expanded so that the ratio between core interval P2 and MFD2 of connection MCF 20 is equal to the ratio between core interval P1 and MFD1 of transmission MCF 10. Therefore, the ratios of core intervals P1 and P2 and MFD1 and MFD2 are matched between transmission MCF 10 and connection MCF 20, and the ratio of core interval P1 of transmission MCF 10 and core interval P2 of connection MCF 20 is equal to relay magnification r. Therefore, transmission MCF 10 and connection MCF 20 can 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 MCF 10 and connection MCF 20.
MFD2 at leading end surface 24 of connection MCF 20 may be 7 μm or more. In this case, since the MFD2 at leading end surface 24 of connection MCF 20 is 7 μm or more, the connection loss due to the reflection of light at leading end surface 24 can be more reliably suppressed.
Multi-core fiber amplifier 80 includes multi-core fiber module 1C, multi-core fiber module 1B, and rare-earth element-doped MCF 85. Rare-earth element-doped MCF 85 is connected to connection MCF 20 of multi-core fiber module 1C and connection MCF 20 of multi-core fiber module 1B. Transmission MCF 10 on the signal input side is connected to transmission MCF 10 of multi-core fiber module 1C, and transmission MCF 10 for signal output is connected to transmission MCF 10 of multi-core fiber module 1B. Core intervals P1, P2 and MFD1, MFD2 are matched between each transmission MCF 10 and each connection MCF 20, and the ratio of core intervals P1, P2 in each transmission MCF 10 and each connection MCF 20 coincides with relay magnification r. Therefore, the MFDs of transmission MCF 10 and rare-earth element-doped MCF 85 can be matched.
Multi-core fiber module 1C may include excitation light combiner 82, and multi-core fiber module 1B may include optical isolator 86. In this case, since core intervals P1, P2 and MFD1, MFD2 of respective transmission MCF 10 and connection MCF 20 are matched, it is possible to suppress the end face reflection in the optical connection via rare-earth element-doped MCF 85 or connection MCF 20 having a small MFD. In addition, it is possible to increase the utilization efficiency of the excitation light output from excitation MCF 60.
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 in
As shown in
As shown in
As shown in
As shown in
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.
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, Rc is 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, S1 represents a distance between the image plane and the pupil plane, S0 represents a distance between the object plane and the pupil plane, r1 represents a radius of curvature of an object-side surface of the lens, and r2 represents a radius of curvature of an image-side surface of the lens. In Equation (2), when one of the absolute values of r1 and r2 is 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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-125668 | Jul 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/023217 | 6/18/2021 | WO |
