JP2022039849A - Multi-core fiber coupling system - Google Patents

Abstract

To suppress reduction of the coupling efficiency due to displacement of the focal position of each optical propagation core and a lens in a spatial coupling system of a multi-core fiber.SOLUTION: A multi-core fiber coupling system 100 includes: a first multi-core fiber 10 for emitting light from a plurality of light propagation cores; a second multi-core fiber 20 for receiving the emitted light; and two single lenses between the first and second multi-core fibers 10, 20 for passage of the light. The two single lenses are a first lens 30 located near the first multi-core fiber 10 and a second lens 40 located near the second multi-core fiber 20. An exit end surface of the first multi-core fiber 10 and an incidence end surface of the second multi-core fiber 20 are inclined to a surface O perpendicular to a main axis L of the single lenses 30 and 40. The exit end surface and the incidence end surface are arranged line-symmetrically to a middle point between the first lens 30 and the second lens 40.SELECTED DRAWING: Figure 4

Description

The present invention relates to a multi-core fiber coupling system in which two multi-core fibers are optically coupled.

Space division multiplexing transmission (SDM) has been proposed in order to meet the demand for an increase in the amount of traffic in an optical fiber network, and multi-core fiber (MCF) has been proposed as one of the methods. As an MCF, one having a plurality of optical propagation cores in one optical fiber is known.

In order to bond the optical propagation cores of MCF to each other, typically, as shown in FIG. 1, two optically equivalent single lenses 30 and 40 are typically inserted between the two MCFs 10 and 20 to be coupled. The arranged multi-core fiber coupling system 100 is adopted. The light emitted from each light propagation core of the first MCF 10 on the emitting side travels through the space while expanding, but is condensed by passing through the first lens 30 and theoretically becomes parallel light to form a space. Go further. Further, this light is further condensed by passing through the second lens 40, and is coupled to each light propagation core of the second MCF 20 on the incident side. At this time, for example, when spatially coupling MCFs having a cross-sectional structure in which a central core is provided at the center of one optical fiber and a plurality of outer peripheral cores are provided around the central core, the central core is 2 for each central core. It may be arranged so that light enters and exits on the main axis (optical axis) of the lenses 30 and 40. Further, in the outer peripheral core, the same as the image formation of an object called the 4f system (an optical system that transfers an image by connecting the rear focal position and the front focal position of the lens) due to the basic effect of the lenses 30 and 40. The light of the object height of the height h is inverted to the height −h and imaged, so that the MCFs can be imaged with each other.

Here, as shown in FIG. 2, in the spatial coupling of the MCF, as in the case of the single mode fiber (SMF), due to the difference in the refractive index between the quartz fiber and the air, the Fresnel on the end face (emission end face) of the MCF on the exit side. It is known that reflection occurs. When the reflected light flows back through the optical fiber, there is a concern that the signal may be deteriorated due to interference generated by the interaction of the light and noise of the transmission signal, or the laser of the signal source may be damaged. Therefore, by polishing the end face of the optical fiber diagonally, the reflected light is emitted to the clad layer outside the light propagation core to prevent the reflected light from flowing back into the light propagation core (). Patent Document 1 etc.). The reflection amount T of Fresnel reflection is obtained by 4n ₁ n ₂ / (n ₁ + n ₂ ) ² . Note that n ₁ is the reflectance of the substance 1 (for example, quartz), and n ₂ is the reflectance of the substance 2 (for example, air). As for the amount of Fresnel reflection, the return loss decreases as the end face angle of the optical fiber increases. The angle is generally set to about 8 degrees so that the reflection angle is larger than the NA (numerical aperture) of the light propagation core and the return loss is a sufficiently low value.

Japanese Unexamined Patent Publication No. 2018-31917

Since the influence of the reflected light of Fresnel reflection becomes a problem in MCF as well as SMF, it is necessary to slant the fiber end face to let the reflected light escape to the outside of the path also in the space-coupled optical system of MCF. However, in that case, as shown in FIG. 2, with respect to the outer peripheral core of the MCF, there is a problem that the emission end surface (interface) of the outer peripheral core and the focal position of the lens deviate from the ideal state due to the oblique polishing of the fiber end face. Occur. That is, since the central core is arranged so that the emission end surface is aligned with the focal length f of the lens and the light emitted from the central core passes through the center of the lens, the light emitted from the central core is collected by the lens. It becomes a parallel beam. On the other hand, if the end face of the MCF is polished diagonally, the emission end face of the outer peripheral core shifts back and forth by α with respect to the focal length f of the lens. In this case, the emitted light from the outer peripheral core arranged at the position of f−α with respect to the focal length of the lens cannot be completely focused by the lens and becomes a divergent beam. Further, the light emitted from the outer peripheral core arranged at the position of f + α with respect to the focal length of the lens is excessively focused by the lens and becomes a focused beam. As described above, when the focal position shift occurs in the outer peripheral core of the MCF, the spatially propagated light is not a completely parallel beam but a divergent beam or a focused light, which lowers the coupling efficiency. As a result, there will be variations in the loss of coupling between the central core and the outer peripheral core. In addition, when the coupling efficiency is low, the light that could not be coupled to the optical propagation core propagates through the clad layer and recombines to an optical propagation core different from the original optical propagation core, thereby deteriorating the crosstalk characteristics between the cores. It will deteriorate the signal quality.

Therefore, the main object of the present invention is to suppress a decrease in coupling efficiency due to a shift in the focal position between each light propagation core and a lens in an MCF spatial coupling system in which the end face is diagonally polished to prevent backflow of reflected light. And.

As a result of diligent studies on means for solving the above-mentioned problems of the prior art, the inventors of the present invention set the polishing direction of the input / output end faces of each MCF at the midpoint between the two lenses in the space coupling system of the MCF. It was found that by arranging them symmetrically, it is possible to cancel the deviation of the focal length between the outer peripheral core and the lens and suppress the variation in the coupling efficiency. Then, the present inventors have come up with the idea that the problems of the prior art can be solved based on the above findings, and have completed the present invention. Specifically, the present invention has the following configurations.

The present invention relates to an MCF coupling system in which two multi-core fibers (MCF) are spatially coupled by two single lenses. The MCF coupling system according to the present invention includes a first MCF 10 that emits light, a second MCF 20 that emits light, and two single lenses 30 that are interposed between the first and second MCFs 10 and 20. Includes 40. The two single lenses 30 and 40 are a first lens 30 located on the first MCF 10 side and a second lens 40 located on the second MCF 20 side. Here, the emission end surface of the first MCF 10 and the incident end surface of the second MCF 20 are inclined with respect to the surface O orthogonal to the main axis (optical axis) L of the single lens. Further, these emission end faces and incident end faces are arranged line-symmetrically with respect to the midpoint between the first lens 30 and the second lens 40. The MCF may have a plurality of propagation cores in one optical fiber, or may be a densely bundled optical fiber having one propagation core in one fiber. .. Further, each optical propagation core may be a core propagating in a single mode or a core propagating in a multimode.

In the present invention, the plurality of light propagation cores included in the first MCF 10 have a corresponding relationship with the light propagation core of the second MCF 20 to which the light emitted from the light propagation core is incident. Here, when the focal lengths of the two single lenses 30 and 40 are f, the distance between the emission end face and the first lens 30 of the paired light propagation core is f + α, and the incident end face and the second lens It is preferable that the distance from 40 is arranged so as to be f−α, and f + α and f−α are in a canceling relationship. The value of α is the same in a certain optical path. Further, the value of α may be a negative value. In this way, the outer peripheral core arranged at the position of the focal length f + α of the first lens 30 on the emitting side and the outer peripheral core arranged at the position of the focal length f−α of the second lens 40 on the incident side are paired. By doing so, the deviation of the focal length is corrected for each other, so that it is possible to suppress the deterioration of the coupling loss for the light passing through these two outer peripheral cores. This effect becomes higher as the core pitch of the MCF is wider and the number of cores is larger, that is, the light propagation core is arranged farther from the main axis of the lens.

In the present invention, the arrangement of the plurality of light propagation cores contained in the first MCF 10 and the plurality of light propagation cores contained in the second MCF 20 is located at the midpoint between the first lens 30 and the second lens 40. On the other hand, it is preferable that they are arranged point-symmetrically. In this case, the arrangement is such that each light propagation core of the MCF is point-symmetrical, and the polishing orientation and polishing angle of the end face of the MCF are line-symmetrical, regardless of the number and arrangement of MCF cores and the orientation of the MCF. It suffices if the relationship is satisfied.

In the present invention, it is preferable that the exit end surface of the first MCF 10 and the incident end surface of the second MCF 20 have an inclination angle of 1 to 10 degrees with respect to a surface orthogonal to the main axis of the single lens.

In the present invention, the paired light propagation cores pass through the first lens 30 of the emitted light, except that the emitted light passes through the center of the first lens 30 and the second lens 40 (that is, the central core). It is preferable that the distance from the point to the center of the first lens 30 is equal to the distance from the passing point of the second lens 40 to the center of the second lens 40.

According to the present invention, in the spatial coupling system of MCF in which the end face of the optical fiber is polished diagonally, it is possible to suppress a decrease in coupling efficiency due to a deviation between the focal positions of each optical propagation core and the lens.

FIG. 1 schematically shows the overall configuration of the MCF binding system according to the present invention. FIG. 2 schematically shows the cross-sectional structure of the vicinity of the emission end surface of the first MCF and the first lens. FIG. 3 schematically shows the cross-sectional structure of the conventional MCF coupling system. FIG. 4 schematically shows the cross-sectional structure of the MCF coupling system according to the present invention. FIG. 5 shows an example of the types of MCF that can be adopted in the present invention. FIG. 6 shows comparative experimental data of the MCF coupling system according to the conventional point-symmetrical arrangement (parallel arrangement) and the MCF coupling system according to the line-symmetrical arrangement of the present invention. FIG. 7 shows comparative experimental data of the MCF coupling system according to the conventional point-symmetrical arrangement (parallel arrangement) and the MCF coupling system according to the line-symmetrical arrangement of the present invention.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The present invention is not limited to the form described below, and includes those appropriately modified from the following forms to the extent apparent to those skilled in the art.

As shown in FIG. 1, the multi-core fiber (MCF) coupling system 100 according to the present invention comprises two MCFs 10 and 20 propagating light and at least two single lenses 30 and 40 for spatially coupling them. It is composed of including. The light emitted from each core of the first MCF 10 is collimated by the first lens 30 to reach the second lens 40, and the second lens 40 to the paired core of the second MCF 20. It is focused. In the embodiment shown in FIG. 1, the first MCF10 and the second MCF20 are 7 core types including one central core (1) and six outer cores (2 to 6), respectively. In the cross-sectional view of the first MCF 10 and the second MCF 20 shown in FIG. 1, the cores assigned the same number are in a paired relationship with each other. That is, the light emitted from each core of the first MCF 10 is guided by the two lenses 30 and 40 so as to be incident on the paired core of the second MCF 20, respectively. As shown in FIG. 1, the core of the first MCF10 and the core of the second MCF20 spatially coupled by the first and second lenses 30 and 40 are arranged point-symmetrically with respect to each other with respect to the main axis. ..

The MCF is a fibrous member made of quartz glass, resin, or the like, and a plurality of cores for propagating optical signals are formed in the cladding. The core is designed to have a higher refractive index than that of the clad, and the light introduced into the core propagates in a state of being confined in the core by being totally reflected by the difference in the refractive index from the clad layer. As the MCF, for example, 4 cores of the coupled type, 4 cores of the unbound type, 5 cores of the unbound type, or 7 cores of the bonded / unbound type, 12 cores of the unbound type, and 19 cores of the unbound type as shown in FIG. However, since the present invention utilizes the action of image transfer by a single lens, it does not depend on the number of cores of the MCF, the core pitch, or the like. That is, since the lenses 30 and 40 used in the present invention are single lenses, the configuration is not limited by the number of cores, the arrangement of cores, the core spacing, and the like.

FIG. 2 shows an enlarged cross-sectional structure of the first MCF 10 and the first lens 30 included in the MCF coupling system 100 according to the present invention. As described above, the emission end surface (interface) of the first MCF 10 is obliquely polished in order to prevent the reflected light reflected by Fresnel on the emission end surface from flowing back into the core. In this case, the central core of the first MCF 10 aligns the emission end surface with the focal length f of the first lens 30, and the light emitted from the central core is obliquely emitted by refraction of the first lens 30. It is arranged so as to be parallel to the center (main axis). Therefore, the light emitted from the central core is focused by the first lens 30 to form a parallel beam. On the other hand, due to the fact that the end face of the first MCF 10 is polished diagonally, the emission end face of the outer peripheral core is displaced back and forth by α with respect to the focal length f of the first lens 30. Become. In this case, the emitted light from the outer peripheral core arranged at the position of f−α with respect to the focal length of the lens cannot be completely focused by the lens and becomes a divergent beam. Further, the light emitted from the outer peripheral core arranged at the position of f + α with respect to the focal length of the lens is excessively focused by the lens and becomes a focused beam. When the end face of the first MCF 10 is diagonally polished as shown in FIG. 2, the optical axis itself of the emitted light from the MCF becomes slanted due to the difference in the refractive index between the glass constituting the light propagation core and the air. .. Similarly, the optical axis of the incident light on the second MCF 20 is also slightly slanted. However, in the drawings of the present application, the refraction of the incoming / outgoing light is not taken into consideration, and the light entering / exiting each MCF is simply expressed as going straight.

Here, FIG. 3 shows the conventional structure of an MCF coupling system with an obliquely polished end face. As shown in FIG. 3, conventionally, the exit end surface of the first MCF 10 and the incident end surface of the second MCF 20 are point-symmetrical with respect to the midpoint between the first lens 30 and the second lens 40. The polishing direction and the polishing angle were set so as to be. Further, the correspondence between the cores of the first and second MCFs 10 and 20 is also point-symmetrical with the main axis as the base point as shown in FIG. Therefore, when the same core arrangement and diagonal polishing angle are used for both the first and second MCFs 10 and 20, the end face diagonal polishing directions of the MCFs 10 and 20 are arranged in parallel as shown in FIG.

By the way, in the case of the prior art relating to such a parallel arrangement, the outer peripheral core located at a position deviated from the main axis of the lens is formed on either the first MCF10 on the exit side or the second MCF20 on the incident side. A deviation will occur with respect to the focal lengths of the lenses 30 and 40. That is, the outer peripheral core (6) of the first MCF 10 is paired with the outer peripheral core (6) of the second MCF 20, but the emission end face of the outer peripheral core (6) of the first MCF 10 is the first lens. It is arranged at a position α minutes shorter than the focal length f of 30 (f−α). Therefore, the light emitted from the outer peripheral core (6) of the first MCF 10 cannot be completely focused by the first lens 30 and becomes a divergent beam. Further, the emission end surface of the outer peripheral core (6) of the second MCF 20 paired with the outer peripheral core (6) of the first MCF 10 is also arranged at a position α minutes shorter than the focal length f of the second lens 40. (F-α). Therefore, the emitted light (divergent beam) from the outer peripheral core (6) of the first MCF 10 is incident on the outer peripheral core (6) of the second MCF 20 without being completely focused even in the second lens 40. It will be. On the other hand, the relationship between the outer peripheral core (3) of the paired first MCF 10 and the outer peripheral core (3) of the second MCF 20 is opposite to that described above. That is, the emission end surface of the outer peripheral core (3) of the first MCF 10 is arranged at a position α minutes longer than the focal length f of the first lens 30 (f + α). Therefore, the light emitted from the outer peripheral core (3) of the first MCF 10 is too focused by the first lens 30 and becomes a focused beam. Further, the emission end surface of the outer peripheral core (3) of the second MCF 20 paired with the outer peripheral core (3) of the first MCF 10 is also arranged at a position α minutes longer than the focal length f of the second lens 40. (F + α). Therefore, the emitted light (focused beam) from the outer peripheral core (3) of the first MCF 10 is transferred to the outer peripheral core (3) of the second MCF 20 in a state of being too focused even in the second lens 40. It will be incident. As described above, in the case of the prior art relating to the parallel arrangement, the deviation from the focal length generated on the emission end face side also occurs on the incident end face side, and this deviation is further amplified. Therefore, the variation in the loss of the connection between the central core and the outer peripheral core becomes large. This tendency becomes more remarkable as the core pitch of the MCF is wide or the number of cores is large.

Next, FIG. 4 shows the structure of the MCF binding system 100 according to the embodiment of the present invention. The emitted light from each core emitted from the emission end surface of the first MCF 10 into the space travels through the space while spreading. The light emitted from each core is collected by passing through the first lens 30. At this time, the emitted light from the central core (1) whose emission point is arranged at the focal position of the first lens 30 is collimated by the first lens 30 and proceeds as parallel light. Further, the emitted light from the outer peripheral cores (2 to 6) arranged at a position away from the main axis L of the first lens 30 passes outside the axis of the first lens 30, so that the first lens 30 is used. Proceed diagonally so as to pass the point where the front focal length of the lens and the main axis L intersect. After that, the emitted light from each core of the first MCF 10 is collected again by passing through the second lens 40 arranged line-symmetrically with the front focal position of the first lens 30 as the base point, and this second lens is collected. It is coupled to the corresponding core of the second MCF 20 located at the posterior focal position of the lens 40 of 2.

At this time, as shown in the lower part of FIG. 4, the core of the first MCF10 and the core of the second MCF20 have a correspondence relationship with the core arranged at the point-symmetrical position with respect to the main axis. Here, as described above, when the emission end surface of the first MCF 10 is diagonally polished, when the focal length f of the first lens 30 is adjusted to the central core on the main axis, the focal length f is applied to the outer peripheral core. There will be a core that is α minutes shorter and a core that is α minutes longer than the focal length f. Therefore, in the MCF coupling system 100 according to the present invention, in order to correct the deviation between the outer peripheral core and the focal length of the first lens 30, as shown in FIG. 4, the oblique polishing angle of the incident end surface of the second MCF 20 The polishing direction is line-symmetrical with the emission end surface of the first MCF10. Specifically, the exit end surface of the first MCF 10 and the incident end surface of the second MCF 20 are line-symmetrical with respect to the midpoint between the first lens 30 and the second lens 40. By arranging the emission end surface and the incident end surface line-symmetrically in this way, the optical path that was f−α on the emission side becomes f + α on the incident side. Similarly, the optical path that was f + α on the emitting side becomes f−α on the incident side. By arranging the emitting end surface and the incident end surface line-symmetrically in this way, it is possible to correct the deviation of the focus of each of the lenses 30 and 40.

In FIG. 4, the polishing angle between the exit end surface of the first MCF 10 and the incident end surface of the second MCF 20 is shown by θ. The polishing angle θ is an inclination angle of the exit end surface and the incident end surface with respect to the surface O orthogonal to the main axis L of the lens. The polishing angle θ is preferably 1 to 10 degrees, and particularly preferably 5 to 8 degrees.

Further, as shown in FIG. 4, each core of the first MCF 10 is arranged parallel to the main axis L of the lens, and light is emitted from the emission end surface along the main axis L. Similarly, each core of the second MCF 20 is also arranged parallel to the main axis L of the lens, and light along the main axis L is incident on the incident end face. Here, for the outer peripheral core arranged at a position away from the main axis L of the lens, the distance from the passing point of the light of the first lens 30 to the center of the first lens 30 and the light of the second lens 40. The distances from the passing point of the second lens 40 to the center of the second lens 40 are arranged to be equal. Specifically, the light from the outer peripheral core (6) of the first MCF 10 shown in FIG. 4 passes through the passing point (P1) of the first lens 30 and the passing point (P3) of the second lens 40. Then, it is incident on the outer peripheral core (6) of the second MCF 20, at this time, the distance from the passing point (P1) to the center (C1) of the first lens 30 and the passing point (P2) of the second lens 40. ) To the center (C2) are D1 and equal. Similarly, the light from the outer peripheral core (3) of the first MCF 10 passes through the passing point (P2) of the first lens 30 and the passing point (P4) of the second lens 40, and is passed through the second MCF 20. At this time, the distance from the passing point (P2) of the first lens 30 to the center (C1) and the passing point (P4) to the center (C2) of the second lens 40 are incident on the outer peripheral core (3) of the lens 40. The distances of are D2 and are equal to each other. By satisfying such conditions, the above-mentioned focal length correction effect of the lens is more accurately exhibited. The distance D1 and the distance D2 may be the same distance, but may be different distances. Whether or not the distance D1 and the distance D2 are equal depends on the core arrangement and pitch of each of the MCFs 10 and 20.

Here, FIGS. 6 and 7 show a combination when the exit end face and the incident end face are arranged in parallel as in the prior art (when arranged in point symmetry) and when they are arranged in line symmetry as in the present invention. The result of calculating the difference in efficiency by changing the focal length and core pitch of the lens is shown. From this calculation result, it can be seen that the line-symmetrical arrangement according to the present invention has better coupling efficiency due to the focal length correction effect. Further, it can be seen that this effect is effective regardless of the focal length of the lens, and that the larger the core pitch, the higher the effect.

As described above, in the present specification, in order to express the content of the present invention, the embodiments of the present invention have been described with reference to the drawings. However, the present invention is not limited to the above embodiment, and includes modifications and improvements which are obvious to those skilled in the art based on the matters described in the present specification.

In the specification of the present application, α is used to simply express the amount of deviation between the end face of the MCF and the focal length of the lens. In the present invention, the value of α in one optical path needs to be equal, but the value of α in one optical path and α in another optical path do not necessarily have to be equal (may be equal or equal). It does not have to be). That is, in the example shown in FIG. 4, the outer peripheral core (6) of the first MCF 10 is arranged at the position of f−α with respect to the focal length f, and the outer peripheral core (6) of the second MCF 20 is the focal length f. Although it is arranged at the position of f + α with respect to, it is necessary that the values of these α are equal. On the other hand, the outer peripheral core (6) of the first MCF 10 is arranged at the position of f−α with respect to the focal length f, and the outer peripheral core (3) of the first MCF 10 is arranged at the position of f + α with respect to the focal length f. Although arranged, the values of these α do not necessarily have to be equal.

10 ... First multi-core fiber (MCF)
20 ... Second multi-core fiber (MCF)
30 ... 1st lens 40 ... 2nd lens 100 ... Multi-core fiber (MCF) coupling system

Claims (5)

A first multi-core fiber that emits light from multiple optical propagation cores,
The second multi-core fiber to which the light is incident and
Two single lenses through which the light passes between the first multi-core fiber and the second multi-core fiber, the first lens located on the first multi-core fiber side, and the first lens. Includes a second lens located on the multi-core fiber side of 2
The exit end face of the first multi-core fiber and the incident end face of the second multi-core fiber are inclined with respect to a plane orthogonal to the main axis of the single lens.
The emitting end surface and the incident end surface are multi-core fiber coupling systems arranged line-symmetrically with respect to a midpoint between the first lens and the second lens. The plurality of optical propagation cores included in the first multi-core fiber are each paired with the optical propagation core of the second multi-core fiber to which the light emitted from the optical propagation core is incident.
When the focal lengths of the two single lenses are f, in the paired core, the distance between the emission end surface and the first lens is f + α, and the distance between the incident end surface and the second lens. The multi-core fiber coupling system according to claim 1, wherein is arranged so as to be f−α, and f + α and f−α are in a canceling relationship. The arrangement of the plurality of light propagation cores contained in the first multi-core fiber and the plurality of light propagation cores contained in the second multi-core fiber is located at a midpoint between the first lens and the second lens. The multi-core fiber coupling system according to claim 1 or 2, which is arranged point-symmetrically with respect to the other. The multi-core fiber coupling system according to any one of claims 1 to 3, wherein the exit end surface and the incident end surface have an inclination angle of 1 to 10 degrees with respect to a surface orthogonal to the main axis. The paired light propagation core is the center of the first lens from the passing point of the light of the first lens, except that the emitted light passes through the center of the first lens and the second lens. The multi-core fiber coupling according to any one of claims 1 to 4, wherein the distance to the second lens is equal to the distance from the light passing point of the second lens to the center of the second lens. system.

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