JP7371900B2 - Bulk monitor and monitoring method for multi-core fiber - Google Patents

Description

The present invention relates to a monitoring device and a monitoring method for collectively detecting optical information of beams emitted from each core of a multi-core fiber.

In existing single-mode fiber (SMF) transmission systems (Figure 3 (a)) and optical equipment such as fiber amplifiers that use SMF (Figure 3 (b)), optical beams are split at relay points using split couplers. The light is branched and optical information is monitored using a photodetector (PD) for photoelectric conversion. This ensures the quality of the transmission path by monitoring disconnections, light intensity, etc. in the transmission path, and providing feedback based on the monitoring results.

In addition, space division multiplexing (SDM) has been proposed to meet the increasing traffic volume in optical fiber networks. Multi-core fibers (MCF) with a propagation core have been proposed.

International publication WO2014/119270 pamphlet

By the way, there are various types of MCF such as coupled 4 cores, non-coupled 4 cores, non-coupled 5 cores, and coupled/non-coupled 7 cores, but all types have a core pitch of 15 to 15 cores. They are in close contact with each other at a distance of about 50 μm (Fig. 4(a)). For this reason, the MCF has a problem in that the individual beams overlap due to the spread of each beam when emitted into space from each core, making it difficult to separate the beams (FIG. 4(b)).

It is also possible to separate and detect each beam by bringing the MCF and PD close together until the beams overlap.In that case, the PD element spacing must be narrowed to around the core pitch of the MCF, but the size of one light receiving element itself It is not easy to narrow the distance between PDs, which range from several tens of μm to several mm. Further, when the MCF and PD are brought close to each other, it is assumed that light reflected from the light receiving surface returns to the core of the MCF. If such return light is generated in the transmission path, it may cause noise, and therefore a device (such as an isolator) for preventing reflection is required, resulting in a complicated device configuration.

Due to the above-mentioned circumstances, in order to individually monitor the beams emitted from each core of the MCF, it is necessary to separate the beams emitted from each core using a fan-out device, input them to each SMF, and then separate each beam emitted from each core using a fan-out device. It was necessary to adopt a configuration in which the transmitted beam was detected by a PD (Figure 5). Also in the optical receiver described in Patent Document 1, based on the same idea, the beams emitted from each core of the MCF are physically separated and introduced into the light receiving element.

However, when adopting a configuration in which an SMF or the like is coupled to each core of the MCF to physically separate each beam, it is necessary to arrange SMFs and couplers corresponding to the number of cores in addition to the photodetector. Therefore, there is a problem that the configuration of the monitor device becomes complicated and the entire device becomes relatively large. In addition, since it is necessary to bond each SMF to each minute core of the MCF, not only does the processing cost increase, but there is a risk that the bond between the core and the SMF may be misaligned or the SMF itself may become defective. There is also a concern that the accuracy of beam transmission and photodetection may decrease.

SUMMARY OF THE INVENTION Therefore, the main object of the present invention is to provide an apparatus and method that can collectively monitor optical information of beams emitted from each core of an MCF with a compact configuration and high precision.

The inventors of the present invention, as a result of intensive study on means for solving the problems of conventional inventions, spatially separate the beams from each densely packed core of the MCF using a spatial optical system, and optically detect each beam. We obtained the knowledge that by introducing this into a device, it becomes possible to collectively monitor the optical information of the beams emitted from each core of the MCF with a compact configuration and with high precision. Based on the above knowledge, the present inventors came up with the idea that the problems of the conventional invention could be solved, and completed the present invention. Specifically, the present invention has the following configurations or steps.

A first aspect of the present invention relates to a batch monitor for multi-core fibers. A monitor according to the present invention includes a lens system and a plurality of photodetectors. The lens system has a configuration in which one or more lenses are arranged so as to give an angular difference to a plurality of beams emitted from each core of a multi-core fiber and widen the interval between each beam. A plurality of photodetectors are arranged so that they can each receive each beam that has passed through the lens system. In this way, by spatially separating the multiple beams emitted from each core of the MCF using a lens system, each beam can be physically separated by coupling an SMF or the like to each core of the MCF. Since this is not necessary, the overall configuration of the device can be simplified and downsized. Furthermore, since there is no need to couple an SMF or the like to each core of the MCF, it is easy to maintain the accuracy of beam transmission and photodetection. As described above, according to the present invention, it is possible to collectively monitor the optical information of the beams of each core directly from the MCF without converting transmission by MCF to transmission by SFM.

Preferably, the monitor according to the present invention is configured such that the beams emitted from each core of the MCF are incident non-perpendicularly on the light receiving surface of the photodetector. For example, the arrangement of the lenses included in the lens system may be adjusted so that the optical axis of the beam incident on the photodetector is non-perpendicular to the light receiving surface of the photodetector. Alternatively, the light-receiving surface of the photodetector may be tilted with respect to the optical axis of the beam so that the beam is incident non-perpendicularly on the light-receiving surface. When a beam is perpendicularly incident on the light receiving surface of a photodetector, there is a possibility that the reflected light returns to the MCF via the lens system (this phenomenon is referred to as "return light" in this specification). Return light must be prevented because it can cause noise, but by making the beam incident non-perpendicularly to the photodetector as in the above configuration, the reflected light is diverged toward the outside of the device. Therefore, return light can be effectively prevented.

In the monitor according to the present invention, the lens system may be configured such that the optical axis of the beam emitted from the core and the optical axis of the beam incident on the photodetector are non-parallel. In this way, by making the optical axis of the light emitted from the core and the optical axis of the light incident on the photodetector non-parallel, the above-mentioned return light can be effectively prevented.

In the monitor according to the present invention, the lens system may consist of a single condensing lens. In this case, the light emitted from each core of the MFC passes through different positions of a single condensing lens. This creates an angular difference between each beam, increasing the distance between the beams. In this way, the lens system can be configured with a single condensing lens, so the entire device becomes compact.

In the monitor according to the present invention, the lens system includes a first lens that gives an angular difference to a plurality of beams emitted from each core of the MCF to collimate each beam, and a first lens that collimates each beam that has passed through the first lens. It may also include a second lens that focuses light on the photodetector. In this way, by configuring a lens system with two lenses, a parallel collimated beam is created between the lenses.For example, by placing a half mirror or an optical filter between the first lens and the second lens, the beam is emitted from a certain MCF. It also becomes possible to introduce the beam into other MCFs. This makes it possible to simultaneously monitor and transmit beam optical information.

In the monitor according to the present invention, the lens system is configured such that the focal position P2 on the input side of the second lens is closer to the optical axis direction than the focal position P1 on the output side of the first lens (that is, closer to the first lens). may be configured. By arranging the second lens close to the first lens in this manner, the optical axis of the beam emitted from the core and the optical axis of the beam entering the photodetector can be made non-parallel. Thereby, as described above, return light can be effectively prevented. Further, by making the distance between the condenser lens and the PD shorter than the focal length (f) of the lens, further shortening is possible.

A second aspect of the present invention relates to a batch monitoring method for multi-core fibers. In the method according to the present invention, first, in the first step, a lens system gives an angular difference to a plurality of beams emitted from each core of a multi-core fiber to widen the interval between each beam. Next, in a second step, each beam that has passed through the lens system is received by a plurality of photodetectors.

According to the present invention, the optical information of the beams emitted from each core of the MCF can be monitored all at once with a compact configuration and with high precision.

FIG. 1 shows a first embodiment of a monitor according to the present invention. FIG. 2 shows a basic form (a) and an improved example (b) of a second embodiment of the monitor according to the present invention. FIG. 3 shows an existing SMF transmission system and a fiber amplifier using SMF. FIG. 4 schematically shows the core pitch of an existing MCF and the divergence state of the emitted beam. FIG. 5 schematically shows a conventional monitor device combining an MCF and an SMF.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated using drawings. The present invention is not limited to the embodiments described below, but also includes modifications from the following embodiments as appropriate within the range obvious to those skilled in the art.

FIG. 1 shows a first embodiment of a monitor 10 according to the invention. The monitor 10 is an optical device for collectively detecting optical information (light amount, wavelength, phase, frequency, etc.) of a plurality of beams emitted from a multi-core fiber (MCF) 20. The MCF is a fibrous member made of quartz glass, plastic, or the like, and has a plurality of optical signal propagation cores 22 formed in a cladding 21 . The core 22 is designed to have a higher refractive index than the cladding 21, and the light introduced into the core 22 is completely reflected due to the difference in refractive index with the cladding layer, thereby being confined within the core 22. propagate. As the MCF, for example, a known one such as a coupled 4-core, a non-coupled 4-core, a non-coupled 5-core, or a coupled/non-coupled 7-core as shown in FIG. 4(a) can be used. However, the present invention does not depend on the number of cores or core pitch of the MCF. In other words, since the lens used in the present invention is a single lens, a configuration that is not limited by the number of cores, core arrangement, core spacing, etc. is possible.

As shown in FIG. 1, the monitor 10 according to the first embodiment includes one condenser lens 11 and a plurality of photodetectors (PD) 12. The number of photodetectors 12 is at least the same as the number of cores of the MCF 20 to be measured. In the example shown in FIG. 1, since the MCF 20 to be measured has four cores 22, four photodetectors 12 are also provided, but since FIG. 1 is a side view, the drawing It is abbreviated into two parts above.

The lens 11 is arranged immediately after the exit surface of the MCF 20 at a position where all the plurality of beams emitted from each core 22 are incident. For example, when the focal length of the lens 11 is f, it is preferable that the distance between the exit surface of the MCF 20 and the lens 11 be f. However, the distance between the MCF 20 and the lens 11 may be such that a spot diameter smaller than the PD light receiving element size can be obtained. Further, the position of the lens 11 is adjusted so that the beams emitted from each core 20 of the MCF 20 pass through different positions of the lens 11, respectively. Specifically, when the MCF 20 is a four-core type, each beam may pass through a different position outside the main axis of the lens 11. Alternatively, if the MCF 20 is a 5-core type or a 7-core type, the beam from the central core should pass through the main axis of the lens 11, and the beams from the other four cores should all pass outside the main axis of the lens 11. good. As a result, each beam that has entered the lens 11 is condensed by the lens 11, and then proceeds while creating an angular difference in the optical axis of each beam. Therefore, as the exit surface of the MFC 20 approaches the lens 11, the angular difference between the beams increases.

The photodetector 12 is a light receiving element for photoelectric conversion. The photodetector 12 is placed behind the lens 11 at a position where the distance between each beam is sufficiently wide. Preferably, the light receiving surface of the photodetector 12 is aligned with the focal point of each beam that has passed through the lens 11. This allows the photodetector 12 to receive each beam with a sufficient amount of light. However, it is not always necessary to align the photodetector 12 with the focal point of the beam, and there is no problem as long as the diameter of the beam focused by the lens 11 falls within the light-receiving surface of the photodetector 12. Although the distance between the lens 11 and the photodetector 12 is not particularly limited, the photodetector 12 may be placed at a distance that is at least less than the focal length f of the lens 11, or at least 1/2 times the focal length f. preferable.

In the optical system shown in FIG. 1, the beams emitted into space from each core 22 of the MCF 20 proceed through space while diverging. Thereafter, each beam is condensed by passing through the lens 11 and proceeds. Furthermore, by passing outside the main axis of the lens 11, the beams emitted from each core 22 pass through a point where the front focal length of the lens intersects the lens main axis and proceed obliquely. As a result, the distance between each beam increases. Furthermore, a photodetector 12 is placed near the focal point where a sufficient beam spacing is obtained. Thereby, the light emitted from each core can be monitored all at once without converting transmission by MCF to SMF.

By configuring the monitor 10 as described above, the distance d2 between each beam at the light receiving position (focusing point) of the photodetector 12 can be increased with respect to the distance d1 between each core 22 at the output end of the MCF 20. I can do it. For example, the distance d2 between the beams is preferably twice or more, more preferably 5 times or more or 8 times or more, and 10 times or more or 12 times or more the distance d1 between the cores 22. Particularly preferred. In this way, by expanding the interval between each beam using the single condensing lens 11, the photodetectors 12 can be arranged with sufficient separation.

Further, in FIG. 1, the optical axis of the beam (outgoing light) emitted from the MCF 20 is indicated by the symbol L1, and the optical axis of the beam (incident light) entering the photodetector 12 is indicated by the symbol L2. In the first embodiment, a spatial optical system for widening the interval between each beam is formed by a single condensing lens 11, but in this case, the optical axis L1 of the emitted light and the optical axis L2 of the incident light are Become non-parallel. Specifically, when the angle formed by the optical axis L1 and the optical axis L2 is θ, θ is preferably 1 to 80 degrees, preferably 3 to 60 degrees or 5 to 45 degrees, and 10 to 45 degrees. Particularly preferred is 30 degrees. In this way, since the optical axis L2 of the incident light is inclined with respect to the optical axis L1 of the emitted light, when the output surface of the MCF 20 and the light receiving surface of the photodetector 12 are parallel, the photodetector The beam enters the 12 light receiving surfaces in a non-perpendicular state. Thereby, as shown in FIG. 1, the beam reflected by the light receiving surface of the photodetector 12 is diverged toward the outside of the spatial optical system. Therefore, it is possible to avoid a situation in which the reflected light passes through the lens 11 and is reintroduced into each core of the MCF 20.

FIG. 2 shows a second embodiment of a monitor 10 according to the invention. FIG. 2(a) shows the basic configuration of the second embodiment, and FIG. 2(b) shows an improved example thereof. The second embodiment is a telecentric optical configuration in which two lenses, a first lens 13 and a second lens 14, are used instead of the single condensing lens 11 in the first embodiment.

As shown in FIG. 2A, the beams emitted from each core 22 of the MCF 20 pass through different positions of the first lens 13, respectively. The first lens 13 is a collimating lens that converts incident light into parallel light. The first lens 13 is arranged immediately after the exit surface of the MCF 20 at a position where all the plurality of beams emitted from each core 22 are incident. For example, when the focal length of the first lens 13 is f, it is preferable that the distance between the exit surface of the MCF 20 and the first lens 13 be f. However, the distance between the MCF 20 and the first lens 13 may be such that a spot diameter smaller than the PD light receiving element size can be obtained. Further, the position of the first lens 13 is adjusted so that the beams emitted from each core 22 of the MCF 20 pass through different positions of the first lens 13. Specifically, when the MCF 20 is of a four-core type, each beam may pass through different positions outside the main axis of the first lens 13. Alternatively, if the MCF 20 is a 5-core type or a 7-core type, the beam from the central core passes through the main axis of the first lens 13, and the beams from the other four cores all pass outside the main axis of the first lens 13. Just let it pass. As a result, each beam that has entered the first lens 13 is collimated, and then advances while creating an angular difference in the optical axis of each beam. Therefore, the distance between each beam increases as the distance from the first lens 13 increases.

Each beam that has passed through the first lens 13 passes through the second lens 14. The second lens 14 is a condenser lens that condenses each collimated beam toward the photodetector 12 . The second lens 14 may be placed at a position where the distance between the beams passing through the first lens 13 is sufficiently wide. Specifically, the second lens 14 is preferably arranged at a position away from the first lens 13 by at least the focal length f of the first lens 13, and is arranged at a position at least twice the focal length f. It is particularly preferable. Furthermore, a photodetector 12 is arranged behind the second lens 14 . Preferably, the light receiving surface of the photodetector 12 is aligned with the focal point of each beam that has passed through the second lens 14. This allows the photodetector 12 to receive each beam with a sufficient amount of light. However, it is not always necessary to align the photodetector 12 with the focal point of the beam, and there is no problem as long as the diameter of the beam focused by the second lens 14 falls within the light-receiving surface of the photodetector 12.

In the optical system shown in FIG. 2, first, a beam that is emitted into space from the end face of the MCF 20 and spreads out is converted into parallel light by the first lens 13. Each parallel light intersects at a point where the front focal length of the first lens 13 intersects with the main axis of the lens, and travels diagonally. Further, a second lens 14 is arranged at a position where the beams of each core are sufficiently spaced apart, and the parallel light is condensed by this second lens 14. Since the photodetector 12 is individually arranged at the condensing position of the second lens 14, the beams from each core can be monitored at once.

In addition, in the basic form shown in FIG. 2(a), each lens 13, 14 is arranged so that the focal position P1 on the output side of the first lens 13 and the focal position P2 on the input side of the second lens 14 match. The placement has been adjusted. In this case, the optical axis L1 of the beam (outgoing light) emitted from the MCF 20 and the optical axis L2 of the beam (incoming light) entering the photodetector 12 are almost parallel, and the beam is received by the photodetector 12. It will be incident almost perpendicularly to the surface. In such a case, there is a concern that the reflected light from the photodetector 12 may pass through the lenses 13 and 14 and be reintroduced into each core of the MCF 20.

Therefore, in the improved example of the second embodiment shown in FIG. 2(b), as a countermeasure against the above-mentioned return light, the position of the second lens 14 is shifted to a position closer to the first lens 13. . In FIG. 2, the symbol g indicates the deviation between the position of the second lens 14 in the basic form (FIG. 2(a)) and the position of the second lens 14 in the improved example (FIG. 2(b)). Specifically, in the improved example shown in FIG. 2(b), the focal position P2 on the input side of the second lens 14 is positioned closer to the front side in the optical axis direction than the focal position P1 on the output side of the first lens 13. In addition, the second lens 14 is arranged at a position closer to the first lens 13. Thereby, the optical axis L1 of the emitted light and the optical axis L2 of the incident light become non-parallel. Specifically, when the angle formed by the optical axis L1 and the optical axis L2 is θ, θ is preferably 1 to 80 degrees, preferably 3 to 60 degrees or 5 to 45 degrees, and 10 to 45 degrees. Particularly preferred is 30 degrees. In this way, since the optical axis L2 of the incident light is inclined with respect to the optical axis L1 of the emitted light, when the output surface of the MCF 20 and the light receiving surface of the photodetector 12 are parallel, the photodetector The beam enters the 12 light receiving surfaces in a non-perpendicular state. Thereby, as shown in FIG. 2(b), the beam reflected by the light receiving surface of the photodetector 12 is diverged toward the outside of the spatial optical system. Therefore, generation of return light can be avoided.

In the improved example shown in FIG. 2(b), the position of the second lens 14 in the optical axis direction is set in front of the focal length f position. In this way, by moving the second lens 14 closer to the first lens 13 in the optical axis direction, the light is focused so that the optical axis angle swings outward without changing the focal length. Therefore, the angle of incidence on the photodetector 12 is no longer perpendicular, and the reflected light from the light receiving surface of the photodetector 12 does not return to the original optical path. In addition, since the condensing optical axes from each core are symmetrically angled outward, the optical path length required to obtain the desired distance between the photodetectors 12 is shortened, making it possible to miniaturize the device.

Furthermore, as can be seen by comparing FIGS. 2(a) and 2(b), by shifting the position of the second lens 14 to a position closer to the first lens 13, the light receiving position of the photodetector 12 (light condensing position) is changed as a result. The distance d2 between each beam at point ) can be further widened. Further, the size of the monitor 10 in the optical axis direction can be reduced by the amount by which the second lens 14 is shifted (distance g). Therefore, it can be said that the improved example shown in FIG. 2(b) is more advantageous in terms of beam separation performance and size reduction.

Although not shown, in the second embodiment, a half mirror or an optical filter is arranged between the first lens 13 and the second lens 14 to separate the beam heading towards the photodetector 12 and the outside of the device. It is also possible to branch into a beam toward which the beam is directed. In this case, the beam directed to the outside of the device can also be connected to another MCF. Thereby, it is possible to simultaneously realize optical transmission by the MCF and monitoring of information on the light being transmitted by the MCF.

Further, in the above-described embodiment, as a measure against returning light, the optical axis of the beam is tilted by adjusting the lens so that the beam does not enter the light receiving surface of the photodetector perpendicularly. However, measures against returning light are not limited to this, and it is also possible to prevent the beam from perpendicularly entering the light receiving surface by, for example, tilting the light receiving surface of the photodetector with respect to the optical axis of the beam. In this case, also in the embodiment shown in FIG. 2A, for example, countermeasures against returning light can be taken by tilting the light receiving surface of the photodetector 12.

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

10... Monitor 11... Lens 12... Photodetector 13... First lens 14... Second lens 20... Multi-core fiber 21... Clad 22... Core

Claims (7)

a lens system in which one or more lenses are arranged so as to give an angular difference to the plurality of beams emitted from each core of the multi-core fiber and widen the interval between each beam;
comprising a plurality of photodetectors each receiving each beam that has passed through the lens system,
The position of the lens included in the lens system is adjusted so that the beam emitted from each core of the multi-core fiber is incident non-perpendicularly to the light-receiving surface of the photodetector. . The monitor according to claim 1, wherein the output surface of the multi-core fiber and the light receiving surface of the photodetector are parallel. The lens system is configured such that the optical axis of the beam emitted from the core and the optical axis of the beam incident on the photodetector are non-parallel. monitor. The monitor according to any one of claims 1 to 3, wherein the lens system includes a single condensing lens. The lens system includes a first lens that gives an angular difference to a plurality of beams emitted from each core of the multi-core fiber to collimate each beam, and an individual photodetector for each beam that has passed through the first lens. The monitor according to any one of claims 1 to 3, further comprising a second lens that condenses light onto the monitor. The lens system is configured such that the focal position (P2) on the input side of the second lens is closer to the optical axis direction than the focal position (P1) on the output side of the first lens. 5. The monitor described in 5. a step of widening the interval between each beam by giving an angular difference to the plurality of beams emitted from each core of the multi-core fiber using a lens system;
a step of receiving each beam that has passed through the lens system by a plurality of photodetectors,
The position of the lens included in the lens system is adjusted so that the beam emitted from each core of the multi-core fiber is incident non-perpendicularly to the light-receiving surface of the photodetector. Bulk monitoring for multi-core fiber Method.

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