DE102018110003A1 - OPTICAL DEVICE, COMMUNICATION SYSTEM AND METHOD FOR MULTIPLEXING AND DEMULTIPLEXING AN OPTICAL SIGNAL - Google Patents

Abstract

An object of the present invention is to provide a mode group multiplex communication system and demultiplex and multiplex groups of modes. An optical device comprises a main optical fiber (10); a mode conversion optical fiber (40) continuous with the main optical fiber and consisting of a bent SI-MMF waveguide; a first optical fiber (20) continuous with the mode conversion optical fiber and consisting of an SI-MMF waveguide extending in the bending direction of the mode conversion optical fiber; and a second optical waveguide (30) consisting of an SI-MMF waveguide and connected to a peripheral curved surface of the mode conversion optical waveguide.

Description

BACKGROUND OF THE INVENTION

Field of the invention:

The present invention relates to an optical device, a communication system and a method of multiplexing and demultiplexing an optical signal, wherein mode group multiplex communication is realized.

State of the art:

There is known a conventional mode group multiplexing method as disclosed in the non-patent literature 1 is disclosed in which a gradient multimode optical fiber (hereinafter referred to as "GI-MMF") is used to cause a lower order mode group signal to propagate at a radial position closer to the central axis of the core and propagate a higher order mode group signal at a radial position farther from the central axis of the core.

For demultiplexing and detecting the lower order mode group signal and the higher order mode group signal from the GI-MMF, the GI-MMF is split into two branches. The lower order mode group signal is extracted by arranging a single-mode fiber (hereinafter referred to as "SMF") at the output end face of a (first) branch of the GI-MMF such that the center axis of the SMF coincides with the central axis of the branch , The mode group signal of higher order is obtained by converting the light from the output end face of the other or second branch of the GI-MMF into parallel light using an optical lens, removing the light near the center axis by using a mask, condensing the light using a Coming lens and coupling the collimated light extracted in another GI-MMF.

As in the patent literature 1 and Non-Patent Literature 2, a technique for bending an optical fiber for branching light from the peripheral curved surface of the bent portion is known. Furthermore, a self-written optical waveguide is known from Patent Literature 2.

State of the art

patent literature

• patent literature 1 : JP H10-54915
• Patent Literature 2: JP 2000-347043

Non-patent literature

Non-patent literature 1 : Y. Li, JD Ingham, VF Olle, G. Gordon, RV Penty and IH White "20 Gb / s Fashion Group Division Multiplexing Employing Hermite-Gaussian Launches Over Worst Case Multimode Fiber Links," OFC, Optical Society of America , 2014 ,

Non-patent literature 2: M. Kagami, Y. Sakai and H. Okada, "Variable Ratio Tao for Plastic Optical Fiber," Applied Optics, Vol. 30, No. 6 pp. 645-649, 1991 ,

In the mode group multiplexing method disclosed in the above-referenced non-patent literature 1 is disclosed and in which a GI-MMF is used, to extract the higher order mode group signal, the lower order mode group signal is removed from the light from the end face of the second branch of the GI-MMF using an optical system. Therefore, the configuration for optical demultiplexing becomes complicated, which makes the mode group multiplexing method difficult to realize. Further, one possible method for demultiplexing the higher order mode group signal having an angular distribution on the light output end face of the second branch of the GI-MMF is arranging an end face of an SMF at a portion of the light output end face. However, in this case, there is a problem that the efficiency of coupling is low, and therefore the reception performance of the higher order mode group is low. In addition, positioning is difficult.

At a signal transmitting end, where the lower order and higher order mode group signals are multiplexed, an optical signal propagated by an SMF and output from the end face of the SMF is controlled by controlling the propagation angle of the optical signal using a single lens optical system , a mask, etc. are converted to the higher-order mode group signal, and the higher-order mode group signal is supplied to a GI-MMF. Using a beam splitter, the higher order mode group signal is then multiplexed with another optical signal propagated by another SMF and used as the lower order mode group signal. Since an optical system having a lens, a mask and a beam splitter is used on the side where light is transmitted, the mechanism becomes complicated. Accordingly, the conventional proposed mode group multiplex communication system using the GI-MMF can not be practically used as a signal transmission line in a car or the like used under difficult conditions, since an optical system having a lens, etc. both the transmission side as well as on the receiver side is used.

Although there is a demand for a low-cost step-index multi-mode optical fiber (hereinafter referred to as "SI-MMF"), as a multiplexing method using an SI-MMF, only wave front multiplexing is known. A mode group multiplexing method using an SI-MMF is not known, and a method for demultiplexing or multiplexing mode groups is also not known.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-described problem, and it is an object of the present invention to realize a mode group multiplex communication system as well as demultiplexing and multiplexing mode groups using a simple configuration, more specifically, using an SI-MMF.

A first invention is an optical apparatus for performing multiplexing or demultiplexing an optical signal for a main optical fiber. The optical device comprises the main optical waveguide, which is a step index multimode optical waveguide and forms a mode group multiplex transmission path in which channels for signals of plural mode groups are formed; a mode conversion optical fiber which is continuous with the main optical waveguide and consists of a curved step index multimode optical waveguide; a first optical fiber continuous with the mode conversion optical fiber and consisting of a step index multi-mode optical fiber extending in a bending direction of the mode conversion optical fiber; and a second optical fiber composed of a step index multi-mode optical waveguide and connected in an axial direction of the main optical waveguide to a peripheral curved surface of the mode conversion optical waveguide. When the plurality of mode groups in the main optical fiber are divided into a higher-order mode group and a lower-order mode group, the mode conversion optical fiber causes a lower-order mode group signal to propagate therethrough and a higher-order mode group signal to emit the second optical fiber side ) and propagates as a signal of a lower order mode group to the second optical fiber, and a lower order mode group signal propagating through the second optical fiber to the mode conversion optical fiber is converted into a higher order mode group signal coupled to the mode conversion optical fiber.

The term "mode" means a propagation mode that represents an optical path of light propagating through the main optical fiber at a certain propagation angle (an angle with respect to the central axis of the optical fiber). Therefore, the term "mode group" means a luminous flux having a propagation angle within a predetermined range. The order of the mode corresponds to the propagation angle. The larger the propagation angle, the higher the order of the mode. Accordingly, the higher-order mode group and the lower-order mode group correspond to regions of the propagation angle of light in the main optical fiber.

In the invention described above, the second optical waveguide is connected in a region before the mode conversion optical fiber is reached in the direction of the central axis of the main optical waveguide to the peripheral curved surface of the mode conversion optical waveguide. Although it is desirable for the second optical fiber and the main optical fiber to be coaxial, the axis of the second optical fiber may be slightly different from the axis of the main optical fiber. When the second optical fiber and the main optical fiber are coaxial with each other, the degree of coupling between the mode conversion optical fiber and the second optical fiber becomes approximately maximum. In a region where a tangential plane of the peripheral curved surface perpendicular to a radial direction is parallel to the axis of the main optical fiber compared to other regions, the degree of coupling between the mode conversion optical fiber and the second mode group optical waveguide is larger than mode conversion in other areas.

In the present invention, the optical device may be one of the following devices. First, as in 1 1, the optical device comprises an optical signal demultiplexing or multiplexing device consisting of a main optical waveguide, a mode converting optical waveguide, a first optical waveguide, and a second optical waveguide and having connectors for connecting external optical waveguides, if necessary. Further, as in 5 and 6 10, the optical device is used for forming parts of a communication system in which an A-end terminal and a B-end terminal are connected by a main optical fiber, the parts multiplexing or demultiplexing an optical signal at the A-end terminal and the B-end terminal carry out. Moreover, the optical device is an optical device at an end terminal in which the part for multiplexing or demultiplexing the optical signal and the main optical fibers extends to the other end terminal.

The invention described above is not limited to the case where each of the higher-order mode group and the lower-order mode group has a single channel. Each mode group may have multiple multiplexed channels. In particular, when a dendritic path is formed by repeating a connection of the optical device of the present invention, the first optical waveguide and / or the second optical waveguide being considered the main optical waveguide, optical signals can be demultiplexed into signals of individual channels and signals of individual channels be multiplexed in stages.

In the invention described above, the peripheral bent surface of the mode conversion optical fiber is preferably bent such that the lower order mode group in the mode conversion optical fiber remains a guided mode and the higher order mode group in the mode conversion optical fiber becomes an outgoing mode.

The peripheral curved surface of the mode conversion optical fiber may have a constant curvature or a curvature that changes in the propagation direction of light.

When a single light beam propagates through the main optical fiber and has a constant propagation angle, the angle of incidence of the light beam that is the interface between the core and the cladding (hereinafter simply referred to as "the curved surface") is the angle with respect to the normal of the curved beam Area), the smaller, the larger the curvature ( 1 / Radius of curvature) of the peripheral curved surface of the mode conversion optical fiber. When the incident angle becomes smaller than a critical angle, the light exits through the peripheral curved surface to the outside, so that the light does not propagate through the mode conversion optical fiber.

The propagation angle of the light beam in the main optical waveguide that causes the angle of incidence with respect to the peripheral curved surface to become equal to the critical angle is defined as a mode separation propagation angle. That is, the mode separation propagation angle is the maximum propagation angle of the light beam in the main optical fiber which can propagate through the mode conversion optical fiber.

For this reason, the larger the curvature of the peripheral curved surface, the smaller the mode separation propagation angle. That is, as the curvature of the peripheral curved surface becomes larger, the propagation angle range of the lower-order mode group in the main optical fiber becomes narrower, and the propagation angle range of the higher-order mode group in the main optical fiber becomes wider. Accordingly, the larger the curvature of the peripheral curved surface of the mode conversion optical waveguide, the greater the likelihood that light of a lower-order mode exits the peripheral curved surface toward the center axis of the second optical waveguide. Since the light emerges from the peripheral curved surface toward the center axis of the second optical fiber and propagates through the second optical fiber, the higher-order mode is converted to the low-order mode. Since light of a mode that does not leak has an angle of incidence (with respect to the curved surface) greater than the critical angle, the guided mode is maintained so that the light propagates through the mode conversion fiber.

Accordingly, the guided mode and mode-out modes of the mode conversion optical fiber are determined by the curvature of the peripheral curved surface of the mode conversion optical fiber. The larger the curvature, the lower the order of the propagation mode (guided mode), and the higher the order of the outgoing mode. The concept of the guided mode and the outgoing mode includes not only the case where propagation and exit at a certain order are completely separated, but also the case where propagation and exit are separated to a certain extent, so that no crosstalk occurs and demodulation is possible.

In the invention described above, the second optical waveguide may be connected to the peripheral curved surface of the mode conversion optical waveguide by a self-written optical waveguide extending from a distal end of the second optical waveguide. The self-written optical fiber is an optical fiber which is formed as follows. A straight light beam is applied to a photo-curing resin liquid whose refractive index increases after curing, so that a hardened and extending portion is formed while confining light to the newly formed cured portion. A technique for producing a self-written optical waveguide is known, and it can be determined by analysis whether a formed optical waveguide is a self-written optical waveguide. Therefore, the self-written optical fibers have the properties of an optical waveguide, and the term "self-written optical waveguide" is a well-known term that describes its structure. With this structure, the connection between the mode conversion waveguide and the second optical waveguide can be easily and reliably realized. A portion of the peripheral curved surface to which the second optical fiber is connected may or may not have a cladding. When the core of the second optical waveguide is directly connected to the core of the mode conversion waveguide, the refractive index of the core of the second optical waveguide must be made smaller than the refractive index of the core of the mode conversion waveguide.

Moreover, the second optical fiber can be connected in a state to the peripheral curved surface of the mode conversion optical fiber in which a self-written optical fiber extending from the peripheral curved surface of the mode conversion optical fiber and a self-written optical fiber extending from a distal end of the second optical fiber extends, are interconnected. The portion of the peripheral curved surface of the mode conversion optical fiber to which the second waveguide is connected may or may not have a cladding. When the self-written waveguide is directly connected to the core of the mode conversion optical waveguide, the refractive index of the self-written optical waveguide (the core) is made smaller than the refractive index of the core of the mode conversion optical waveguide. Also, in the case where no cladding is provided on the peripheral curved surface, when the refractive index of the photocuring resin is smaller than the refractive index of the core of the mode conversion optical waveguide, higher order mode light exits to the photocuring resin to a greater extent the resin serves as a jacket. During manufacture, when the refractive index of the photo-curing resin liquid is smaller than the refractive index of the core of the mode conversion optical fiber, light for curing a higher-order mode from the peripheral curved surface to the outer resin liquid exits, and the exiting light becomes a lower-order mode and propagated through the photo-curing resin liquid in the direction of the center axis of the main optical fiber. As a result, the self-written optical waveguide, which is formed to extend outwardly from the peripheral curved surface, is formed in a straight line.

It is desirable that the core of the self-written optical waveguide connected to the peripheral curved surface is directly connected to the core of the mode conversion optical waveguide and the core of the self-written optical waveguide connected to the peripheral curved surface has a refractive index which is smaller is as the core of the mode conversion fiber. The core of the self-written optical waveguide serves as a clad in a region in which the mode conversion optical fiber is connected to the self-written optical waveguide.

In the above-described invention, each of the second optical fiber and the main optical fiber may be a step index multi-mode optical fiber, the first optical fiber and the mode conversion optical fiber may be formed as portions of the main optical fiber, and the second optical fiber may be connected through the self-written optical fiber to the cladding of the mode conversion optical fiber.

Further, the core of the self-written optical waveguide connected to the peripheral curved surface preferably has a refractive index smaller than that of the core of the mode conversion optical waveguide. In this case, the higher-order mode light exiting the peripheral curved surface is converted to light of a lower-order mode propagating through the second optical fiber, which is desirable.

In the invention described above, the first optical waveguide may be a step index multimode optical fiber, and the mode conversion optical waveguide may be a curved self-written optical waveguide continuous with a distal end of the first optical waveguide.

Further, the first optical waveguide may be a step index multimode optical fiber, and the mode conversion optical waveguide may be a curved self-written optical waveguide formed by connecting a self-written optical waveguide continuous with a distal end of a first optical waveguide and a self-written optical waveguide continuous with a distal end of the main optical waveguide is formed, and by bending the connected self-written optical waveguide.

A second invention is a mode group multiplex communication system using the optical device described above.

In particular, the second invention is a communication system which is characterized in that the above-described optical Device is used as an A-end device, and the above-described optical device is used as a B-end device; the main optical fiber of the A-end device and the main optical fiber of the B-end device are continuous and constitute a main optical fiber; a lower-order mode group signal, which is converted by the mode conversion optical fiber to the higher-order mode group, is transmitted to the second optical fiber of the A-end device and the second optical fiber of the B-end device; and transmitting a signal of the lower-order mode group to the first optical fiber of the A-end device and the first optical fiber of the B-end device, so that mode group-multiplex communication is performed between the A-end device and the B-end device.

A third invention is a mode group multiplex communication system characterized by an optical waveguide comprising a main optical waveguide consisting of a step index multimode optical waveguide, a first step index multimode optical waveguide connected to a peripheral curved surface of a first curved section of the main optical waveguide, and a second step index multimode optical waveguide comprising a demultiplexing or multiplexing an optical signal is performed by the order of the mode group between the main optical fiber and the first step index multimode optical waveguide and between the main optical waveguide and the second step index multimode optical waveguide is changed so that a mode groups multiplex communication is performed for signals propagating between the first bent portion and the second bent portion through the main optical fiber.

As in 5 and 6 As shown in the above-described communication systems of the present invention, the main optical fiber of the A-end device and the main optical fiber of the B-end device are continuously connected. In the communication systems, signals of different channels are multiplexed in the A-end device or a bent section, and the signals of different channels are demultiplexed in the B-end device or the other bent section, so that communication is performed. Further, a signal of a particular a-channel may be multiplexed in the A-end device or a bent portion, and a signal of a different b-channel may be demultiplexed there, and the signal of the a-channel may be in the B-end device or the other bent portion can be demultiplexed, and the signal of the b channel can be multiplexed there. In this case, two-way full-duplex communication can be performed using the a-channel and the b-channel.

A fourth invention is a method for multiplexing / demultiplexing an optical signal used in a mode group multiplex communication method, characterized in that a part of a main optical waveguide consisting of a step index multimode optical waveguide is bent to form a bent portion, and a second one Step index multimode optical fiber is connected to a peripheral curved surface of the bent portion; using the bent portion of a mode group multiplex signal propagating through the main optical fiber, demultiplexing a signal of a lower order mode group for propagating in the direction of the main optical fiber, and demultiplexing a higher order mode group signal for propagation toward the second step index multi-mode optical waveguide, while is modulated to a lower order mode group, or by using the bent portion to propagate a lower order mode group signal for multiplexing an optical signal toward the main optical fiber and a lower order mode group signal propagating through the second step index multimode optical waveguide to a higher order mode group modengewandelt and is propagated for multiplexing an optical signal in the direction of the main optical waveguide.

Since multiplexing and demultiplexing of the lower order mode group and the higher order mode group can be realized by this method, a mode group multiplex communication method using inexpensive step index multimode optical waveguides is practically enabled.

The present invention can realize mode group multiplex communication using step index multimode optical waveguides with a simple configuration.

list of figures

• 1 Fig. 10 is a schematic diagram of an optical device according to a first embodiment of the present invention;
• 2 Fig. 12 is an explanatory diagram showing groups of modes to be multiplexed in a main optical fiber of the device of the first embodiment;
• 3 Fig. 15 is a diagram showing another example of the self-written optical waveguide of the first embodiment;
• 4A and 4B Figs. 15 are diagrams showing still another example of the self-written optical waveguide of the first embodiment;
• 5 Fig. 10 is a schematic diagram of a mode group multiplex communication system according to a second embodiment of the present invention;
• 6 Fig. 10 is a schematic diagram of another mode group multiplex communication system according to the second embodiment of the present invention;
• 7A to 7D Fig. 11 are diagrams showing a measured change in the width of a lower-order mode group with a change in the length of the main optical fiber of the mode group multiplex communication system according to the second embodiment;
• 8A and 8D Fig. 11 are diagrams showing a measured change in the width of a higher-order mode group in a change in the length of the main optical fiber of the mode group multiplex communication system according to the second embodiment;
• 9 Fig. 12 is a diagram showing increases in the numerical aperture of a step index multi-mode fiber and a mode group width in a communication system according to a third embodiment; and
• 10 FIG. 10 is an explanatory diagram showing increases in the numerical aperture of a step-indexed multi-mode optical fiber and a mode group width in a communication system according to the third embodiment. FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiments.

First embodiment:

Configuration of the optical device

1 shows the configuration of an optical device 1 according to a first embodiment of the present invention. A main fiber optic cable 10 , a first optical fiber 20 and a mode conversion fiber 40 are formed by a single step index multimode optical fiber (hereinafter referred to as "SI-MMF") 11. The SI-MMF 11 is bent, and a bent portion of the SI-MMF 11 serves as the mode conversion optical fiber 40 , The core of a self-written fiber optic cable 32 extending from a distal end of a second optical fiber 30 extends, is with a coat 42 a peripheral curved surface (outer surface) 41 of the mode conversion optical fiber 40 connected. The second optical fiber 30 is an SI-MMF 31. The central axis 15 of the main optical fiber 10 falls with the central axis 35 of the second optical waveguide 30 together. The core diameter of the SI-MMF 11 and 31 is 200 μm, and the diameter of the SI-MMF 11 and 31 with sheath is 230 μm. Each of the SI-MMF 11 and 31 is a so-called large diameter optical fiber (optical fiber). The numerical aperture NA of a large diameter optical fiber which can be used in the present invention is 0.2 to 0.65.

Method of manufacturing the optical device

The self-written optical fiber 32 is formed as follows. The SI-MMF 11 is bent at a predetermined position to have a predetermined curvature, and the bent SI-MMF 11 is attached to a frame 50 fixed. The SI-MMF 31 becomes such on the frame 50 fixed that center axis 35 the SI-MMF 31 with the central axis 15 of the main optical fiber 10 coincides and the end face of the distal end of the SI-MMF 31 of the peripheral curved surface 41 of the mode conversion optical fiber 40 opposite. Subsequently, the frame becomes 50 filled with a light-curing resin liquid, and light having a wavelength suitable for optically curing the photo-curing resin liquid is applied to the end surface of the proximal end of the SI-MMF 31. The light for curing is emitted from the end surface of the distal end of the SI-MMF 31, whereby the resin cures along the optical path of the light. The photocuring resin liquid is selected such that the cured resin has a refractive index greater than that of the resin liquid. The light-curing resin liquid may be a mixture of different light-curing resin liquids which are different from each other in wavelength for curing and refractive index. In this case, only the resin liquid of a certain kind can be cured so that the cured resin has a refractive index larger than that of the liquid mixture. In addition to the method described above, other methods of making the self-written optical fiber are known.

Since the refractive index of the photo-curing resin after curing is larger than the refractive index of the uncured light-curing resin liquid around the cured photo-curing resin, the photo-cured resin serves as a core, and the photo-curing resin liquid around it serves as a clad, so that the light is straight propagated while remaining confined to the optically cured resin. As a result, the core formed of the optically cured resin extends in a straight line from the end face of the distal end of the SI-MMF 31 and reaches the sheath 42 the peripheral curved surface 41 of the mode conversion optical fiber 40 , This optically cured linear extending resin serves as the core of the self-written optical fiber 32 , Subsequently, the in the frame 50 Resists remaining light-curing resin liquid by another light-curing resin liquid having a lower refractive index, and the light-curing resin liquid in the frame 50 is optically cured. Consequently, around the core of the self-written optical fiber 32 formed a coat. Also, in the case where a mixture of different light-curing resin liquids is used, the materials are selected such that the refractive index of the resin produced as a result of curing the liquid mixture becomes smaller than the refractive index of the core of the self-written optical fiber 32 is. In this case, all the resin liquid in the frame 50 cured without replacing the resin liquid. Furthermore, a cladding may only be around the core of the self-written optical waveguide 32 be formed.

Optical connectors 16 . 26 . 36 are for the main fiber optic cable 10 , the first optical fiber 20 and the second optical fiber 30 provided, which are formed in the manner described above. More specifically, the optical connector 16 provided at a position at which one end of the main optical fiber 10 on the frame 50 is fixed; the optical connector 26 is provided at a position at which one end of the first optical waveguide 20 on the frame 50 is fixed; and the connector 36 is provided at a position at which one end of the second optical waveguide 30 on the frame 50 is fixed. As a result, an optical device serving as a component for multiplexing and demultiplexing optical signals can be obtained, enabling mode group-division multiplexing communication.

Effect of Mode Conversion Fiber Optic Cable

A lower-order mode group signal whose propagation angle is about 7 ° or less and a higher-order mode group signal whose propagation angle is about 14 ° to 21 ° become the end surface of the main optical fiber 10 such that the lower order, higher order mode group signals are directed toward the mode conversion fiber 40 propagate. The value of the propagation angle is the value of the angle of incidence of light propagating from air into the incident end surface of an optical fiber, or the value of the angle of emergence of light propagating from the exit end surface of the optical fiber in air. The propagation angle in the optical fiber corresponding to the angle of incidence or the angle of reflection is a smaller value determined according to Snell's law. The propagation angle is defined by the angle between the propagation direction of light causing multiple reflection and the center axis of an optical waveguide. 21 ° is the maximum propagation angle of the SI-MMF 11 and 31 and corresponds to the maximum number of modes. An area of the propagation angle between 7 ° and 14 ° is a buffer area (propagation angle area) for avoiding interference.

2 FIG. 12 shows the relationship between the propagation angle θ and an encircled angular flux (EAF) of propagating light whose propagation angle is θ or a smaller angle. As in 2 Shown are mode groups of all orders in a lower order mode group whose propagation angle 10 , 5 ° or smaller, and a higher-order mode group whose propagation angle 10 , 5 ° to 21 °, is divided. The propagation angle of 10.5 ° is the above-mentioned mode separation propagation angle. The curvature of the peripheral curved surface 41 is set to a value determined so that in the mode conversion optical fiber 40 the angle of incidence of the lower order mode group with respect to the curved surface is greater than or equal to a critical angle and the angle of incidence of the higher order mode group with respect to the curved surface is less than the critical angle. Accordingly, the lower-order mode group keeps a guided mode in the mode conversion optical fiber 40 at, and the lower order mode group signal whose propagation angle is about 7 ° or less propagates through the mode conversion optical fiber 40 ,

The incident angle of the higher order mode group signal with respect to the curved surface of the mode conversion optical fiber 40 becomes smaller than the critical angle, and therefore, the higher order mode group signal enters the cladding 42 of the mode conversion optical fiber 40 out. Therefore, the higher order mode group signal does not propagate to the first optical fiber 20 , That is, in the Mode conversion optical fiber 40 becomes the mode group signal of higher order to an exiting mode.

It becomes a tangential plane of the peripheral curved surface 41 assumed orthogonal to the radial direction of the curvature of the peripheral curved surface 41 of the mode conversion optical fiber 40 is. An area where the tangential plane is approximately parallel to the central axis 15 of the main optical fiber 10 is, ie, a section 41 in which the peripheral curved surface 41 begins to be bent, in an area that is continuous with the main fiber 10 is (a part of the contact surface between the peripheral curved surface 41 and the self-written optical fiber 32 who is in 1 on the upper side of the central axes 15 and 35 is) is considered. In this range, the propagation angle of the higher order mode group signal that goes to the cladding increases 42 of the mode conversion optical fiber 40 exits, and its propagation direction becomes parallel to the central axis 35 of the second optical waveguide 30 , That is, the higher order mode group signal is mode-converted to a lower order mode group signal. In this area of the peripheral curved surface 41 For example, the efficiency of conversion from the higher-order mode to the lower-order mode becomes high.

The optical path of light coming from the second optical fiber 30 to the peripheral curved surface 41 of the mode conversion optical fiber 40 is the same as the optical path of light, that of the main optical fiber 10 to the second optical fiber 30 propagated. Accordingly, in this area, the lower order mode group signal passing through the second optical fiber becomes 30 propagated through the mode conversion fiber optic cable 40 efficiently converted to a higher order mode group signal, and the higher order mode group signal propagates to the main optical fiber 10 ,

Consider the case where the multiplexed signal having a lower-order mode group signal and a higher-order mode group signal written in 2 is shown by the main optical fiber 10 to the mode conversion fiber 40 propagated. Due to the above-described effect of the mode conversion optical fiber 40 only the lower order mode group signal remains in the guided mode and propagates to the first optical fiber 20 , The higher order mode group signal exits the cladding 42 the peripheral curved surface 41 of the mode conversion optical fiber 40 which converts the higher order mode group signal to a lower order mode group signal, and the lower order converted mode group signal propagates through the second optical fiber 30 , In this way, the optical device 1 demultiplexing (splitting) the multiplexed mode group signals into the lower order mode group signal and the higher order mode group signal.

Next, a lower order mode group signal passing through the first optical fiber 20 to the mode conversion fiber 40 and a lower order mode group signal passing through the second optical fiber 30 to the mode conversion fiber 40 propagated, considered. Due to the effect described above, the lower order mode group signal passing through the first optical fiber passes 20 propagated through the mode conversion fiber optic cable 40 while it remains the guided mode, and propagates through the main fiber 10 , In contrast, due to the effect described above, the lower order mode group signal transmitted through the second optical fiber 30 propagated through the peripheral curved surface 41 of the mode conversion optical fiber 40 is converted to a higher order mode group signal, and the converted higher order mode group signal propagates through the main optical fiber 10 , In this way, the optical device 1 converting one of the lower order mode group signals of two channels to a higher order mode group signal and the lower order mode group signal and the higher order mode group signal in the main optical fiber 10 multiplex. That is, the present optical device 1 can be used as an optical signal multiplexing device in a mode group multiplex communication.

Next, consider a case where a lower order mode group signal is transmitted through the main optical fiber 10 to the mode conversion fiber 40 propagated. This lower order mode group signal remains a guided mode, passing through the mode conversion fiber 40 and propagated through the first optical fiber 20 , In this state, a lower order mode group signal is caused to flow from the second optical fiber 30 to the mode conversion fiber 40 propagated. This lower order mode group signal is passed through the mode conversion fiber 40 is converted to a higher order mode group signal, and the converted higher order mode group signal propagates through the main optical fiber 10 , In this case, full duplex communication between two terminal devices can be realized. The present optical device 1 can be used as a multiplexing device / Demultiplexing of an optical signal can be used.

Further, consider a case where a higher order mode group signal is transmitted through the main optical fiber 10 in the direction of the mode conversion fiber 40 propagated. This higher order mode group signal is passed through the mode conversion fiber 40 is converted to a lower-order mode group signal, and the lower-order converted mode group signal propagates through the second optical fiber 30 , In this state, a lower order mode group signal is caused from the first optical fiber 20 to the mode conversion fiber 40 propagated. This lower order mode group signal remains a guided mode, passing through the mode conversion fiber 40 and propagated through the main optical fiber 10 , In this case, full duplex communication between two end devices can also be realized. The present optical device 1 can be used as an apparatus for multiplexing / demultiplexing an optical signal.

The refractive index of the core of the self-written optical waveguide 32 is arbitrary, as long as the coat 42 of the mode conversion optical fiber 40 has a refractive index smaller than the refractive index of its core. The refractive index of the core of the self-written optical waveguide 32 however, it is preferably set to be less than or equal to the refractive index of the cladding 42 is. That is, the refractive index of the core of the self-written optical waveguide 32 is preferably set to be smaller than the refractive index of the core of the mode conversion optical fiber 40 is. When the refractive index of the core of the self-written optical waveguide 32 As described above, the conversion between the higher order mode of the mode conversion optical fiber 40 and a lower-order mode. This is because light input to a medium having a lower refractive index is refracted in a direction in which the propagation angle decreases.

Light emission to the first optical fiber

The mode conversion fiber 40 can not completely block the higher order mode group signal. Therefore, when a mode order signal of lower order by the mode conversion optical fiber 40 is demultiplexed, a notch filter for a higher order mode may be used. In this case, the lower-order mode group signal can be demultiplexed (separated) with high accuracy.

Other examples of the self-written optical fiber

The self-written optical fiber 32 can by a in 3 shown method are formed. More specifically, a self-written optical waveguide 32b is formed to extend from the peripheral curved surface 41 of the mode conversion optical fiber 40 extends; another self-written optical waveguide 32a is formed so as to extend from the distal end of the second optical waveguide 30 extends; and the self-written optical waveguides 32a and 32b are connected to each other so that the self-written optical waveguide 32 is trained. The main fiber optic cable 10 , the mode conversion fiber optic cable 40 and the first optical fiber 30 are formed by a single SI-MMF 11. In this case, the self-written optical waveguide 32b is formed by causing a curing light of a higher order mode group of the main optical waveguide 10 in the mode conversion optical fiber 40 propagated. The curing light of the higher order mode group is transmitted through the mode conversion fiber 40 converted into curing light of a lower order mode group, and the curing light propagates straight along the center axis of the second optical fiber 30 (the SI-MMF 31). The self-written optical waveguide 32b is formed by this curing light. At the same time, a curing light of a lower order mode group becomes the second optical waveguide 30 supplied, so that the self-written optical waveguide 32a from the distal end of the second optical waveguide 30 out grows. When the distal end of the self-written optical waveguide 32a comes in contact with the self-written optical waveguide 32b, the single self-written optical waveguide becomes 32 formed by the effect of an optical confinement effect. A technique for connecting self-written optical fibers by emitting curing light from the opposite ends of the two optical fibers is already known.

Other examples of the mode conversion fiber

In the embodiment described above, the mode conversion optical fiber is 40 is formed as a bent portion of the SI-MMF 11. This mode conversion fiber 40 can be formed by a self-written optical waveguide. As in 4A shown are two SI-MMF 11a and SI-MMF 11b coaxially arranged so that their distal end surfaces face each other. Curing light is emitted from the end faces of the two SI-MMF 11a and 11b emitted, so that self-written optical fiber 40a and 40b are formed, and the straight self-written optical fibers 40a and 40b are connected at their distal ends, whereby a single straight self-written optical waveguide is formed. Subsequently, as in 4B shown, this self-written optical waveguide for forming the mode conversion optical waveguide 40 bent.

As described above, the mode conversion optical fiber 40 be designed as a self-written optical waveguide. Alternatively, the mode conversion fiber 40 as a molded product or an optical component formed by pattern transfer.

As described in the embodiment described above, the self-written optical waveguide 32 that is different from the second optical fiber 30 extends, with the peripheral curved surface 41 of the mode conversion optical fiber 40 connected. At this time, since the mode conversion fiber 40 is formed only by a core and has no cladding, the refractive index of the core of the self-written optical waveguide 32 Using the mode conversion fiber optic cable 40 is less than the refractive index of the core of the mode conversion optical fiber 40 be. The core of the self-written optical fiber 32 acts as a jacket for the core of the mode conversion fiber 40 , Subsequently, a light-curing resin liquid that covers the entire space of the frame 50 fills, optically cured, leaving a cladding around an exposed portion of the core of the mode conversion fiber 40 and the self-written optical fiber 32 is trained.

It should be noted that when the mode conversion fiber 40 is formed by a curved self-written optical waveguide, after forming the straight self-written optical waveguide 40a and 40b and connecting them at their distal ends, as in FIG 4A shown, a coat can be formed around this.

Other examples of the main optical fiber, the first optical fiber, and the second optical fiber

In the embodiment described above, each of the main optical fiber, the first optical fiber, and the second optical fiber is formed by an SI-MMF. However, each of the main optical fiber, the first optical fiber, and the second optical fiber may be an optical fiber formed by curing a resin or all the optical fibers in the frame 50 can be formed by self-written optical fibers and with the connectors 16 . 26 and 36 get connected.

miscellaneous

The present invention includes optical devices in which the frame 50 or the connectors 16 . 26 and 36 are not provided.

Second embodiment:

The present embodiment is a communication system in which mode group multiplex communication is realized by using the optical devices described above.

5 and 6 show the configuration of the communication system. Two optical devices according to the first embodiment are used as an A-end device and a B-end device. A single common main fiber optic cable 10 is considered the main fiber 10 the A-end device and the main optical fiber 10 the B-end device used. In this communication system, the optical connectors 16 for connecting the main optical fibers 10 be provided or omitted. That is, the main fiber 10 The A-end device can be replaced by a long common main optical fiber, and the other end of the common main optical fiber can be used in the B-end device. In such a case, a second optical fiber is 30 with the peripheral curved surface 41 of the mode conversion optical fiber 40 the A-end device connected, and another second optical fiber 30 is with the peripheral curved surface 41 of the mode conversion optical fiber 40 connected to the B-end device. In a similar way, the frame 50 and the optical connectors 26 and 36 be provided or omitted.

In the in 5 As shown, the A-end apparatus is a device for multiplexing an optical signal (light signal), and the B-end apparatus is an apparatus for demultiplexing an optical signal. Through the effect of the mode conversion fiber optic cable 40 In the A-end apparatus and the B-terminal apparatus, mode group multiplex communication can be realized in the following manner. The A-end device is a transmitting terminal, and the B-end device is a receiving terminal. Lower order mode group signals are received from port # 2 (the first fiber optic cable) 20 ) and a port # 3 (the second optical fiber 30 ) of the A-end device transfer. The signal from port # 3 is passed through the mode conversion fiber 40 the A-end device converted to a mode group signal of higher order. The signal (lower order mode group signal) from the port # 2 becomes unchanged to the main optical fiber 10 output. As a result, in the main optical fiber 10 the lower-order mode group signal and the higher-order mode group signal are each multiplexed as a lower-order mode group and a higher-order mode group, and propagate to the B-end device.

In the B-end device, the lower order mode group signal passes through the mode conversion optical fiber 40 the optical device and is unchanged from the port # 2 (the first optical fiber 20 ). On the other hand, the higher order mode group signal is transmitted through the mode conversion optical fiber 40 is converted to a lower order mode group signal, and the lower order mode group signal is output from port # 3 (the second optical fiber 30 ). Accordingly, mode group multiplex communication is realized between the A-end device serving as a transmitting device and the B-end device serving as a receiving device.

In the in 6 In the communication system shown, both the A-end device and the B-end device serve as transmitting / receiving terminals.

For a lower order mode group signal, the B-end device serves as a transmitting terminal, and the A-end device serves as a receiving terminal. For a higher order mode group signal, the A-end device serves as a transmitting terminal, and the B-end device serves as a receiving terminal. That from port # 2 (the first fiber optic cable 20 ), the lower order mode group signal input to the B-end device passes through the mode conversion optical fiber 40 propagated through the main fiber optic cable 10 , goes through the mode conversion fiber optic cable 40 the A-end device and is from the port # 2 (the first optical fiber 20 ). As a result, a transmission from the B-end device to the A-end device is realized.

On the other side, one of the port # 3 (the second fiber optic cable 30 ) of the A-end device, the lower order mode group signal is input through the mode conversion optical fiber 40 converted to a mode group signal of higher order. The resulting higher order mode group signal propagates through the main optical fiber 10 and is passed through the mode conversion fiber 40 the B-end device is converted to a lower order mode group signal. The lower order mode group signal is output from port # 3 (the second optical fiber 30 ). As a result, a transmission from the A-end device to the B-end device is realized. That is, a full-duplex communication can be performed between the two terminals using a single SI-MMF.

In the examples described above, two-channel multiplexing is performed; however, as explained below, multichannel multiplexing may be performed. Each of the A-end device and the B-end device includes the optical connectors 16 . 26 and 36 , The main fiber optic cable 10 another C-end device is connected to port # 2 of the first optical fiber 20 the A-end device and / or port # 3 of the second optical fiber 30 connected to the A-end device. Similarly, the main fiber is 10 another D-end device with the port # 2 of the first optical fiber 20 the B-end device and / or port # 3 of the second optical fiber 30 connected to the B-end device. The curvatures of the peripheral curved surfaces 41 the mode conversion fiber 40 However, the A-end device, the B-end device, the C-end device, and the D-end device must be adapted so as to be divided into mode groups according to the channel number as shown in FIG 2 shown is possible.

Next in the system in 5 the extent of separation of multiplexed mode groups in the main fiber 10 in the case where lower-order mode group signals were input from the ports # 2 and # 3 of the A-terminal device. 7A to 7D are graphs of measurements showing the relationship between EAF and the propagation angle θ of the lower-order mode group in the case where the length of the main optical fiber 10 was changed, show. 8A to 8D are graphs showing measurements showing the relationship between EAF and the propagation angle θ of the higher order mode group in the case where the length of the main fiber 10 was changed, show. The core diameter of the SI-MMF is 200 μm. It was confirmed that the lower order mode group and the higher order mode group can be separated when the length of the main optical fiber 10 50 m or less.

Third embodiment:

Change the mode group width

In the system in 5 a change in width of a propagating mode group was measured.

A HPCF (Hard Plastic Cladding Fiber, Model No. HPCF-M0200T) from OFS Fitel, LLC was used as the SI-MMF. The core diameter of the HPCF is 200 μm, and the NA of the HPCF is 0.37. The maximum propagation angle with respect to the optical axis (the maximum mode group width) θ _{MAX of} the fiber is 21.7 ° since the maximum propagation angle θ _{MAX is given} by sin ^-1 (NA).

The lower-order mode group signal input from the port # 2 of the A-terminal serving as a transmitting terminal is considered.

For the lower order mode group signal, a propagation angle (a mode group width) at which EAF becomes 75% is defined as 0 _{# 2} . This lower order mode group signal is passed through the mode conversion fiber 40 to the main optical fiber 10 output. When the lower order mode group signal is passed through the mode conversion fiber 40 For example, the mode group width increases due to scattering of light, for example. The extent of this increase is represented by Δθ _bending . Even if the lower order mode group signal is transmitted through the main optical fiber 10 propagated to the B-end device, the mode group width increases due to scattering on optical connectors and / or optical fibers on the path. The full extent of these increases is given by Δθ _hniedrig. The mode group width θ _{# 2 of} the lower-order mode group signal inputted from the port # 2 of the A-terminal changes as shown in FIG 9 is shown before the lower order mode group signal is output from the port # 2 of the B terminal.

Accordingly, when the mode group width occurs at the input end of the mode conversion optical fiber 40 the B-end device is less than or equal to 1/2 the maximum mode group width, no crosstalk between the lower-order mode group and the higher-order mode group. Accordingly, it suffices that the mode group width θ _{# 2 of} the lower order mode group signal transmitted from the port # 2 of the A-end apparatus satisfies the following equation. $${\mathrm{<figure-callout\; id="2"\; label="\theta \; \#"\; filenames="DE102018110003A1\_0008.png,DE102018110003A1\_0009.png"\; state="\{\{state\}\}">\theta \; </figure-callout>}}_{\#}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{bending}}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{hniedrig}}\le {\mathrm{\theta }}_{\text{MAX}}/2$$

Next, the lower-order mode group signal input from the port # 3 of the A-end device serving as a transmission terminal is considered. For the lower order mode group signal, a propagation angle (a mode group width) at which EAF becomes 75% is defined as θ _{# 3} . This lower order mode group signal becomes the mode conversion fiber 40 and converted into a higher order mode group signal corresponding to the main optical fiber 10 is issued. The amount of increase of the mode group width due to the mode conversion by the mode conversion optical fiber 40 is represented by Δθ _tran . Even if the mode group signal higher order by the main optical fiber 10 propagated to the B-end device, the mode group width of the higher order mode group signal due to scattering on optical connectors and / or optical fibers on the path increases. The total extent of this increase is indicated by Δθ _hhoch . However, since the value of Δθ _hhigh increases as EAF decreases, the value of Δθ _hhigh when EAF is 25% is used. The mode group width θ _{# 3 of} the lower-order mode group signal inputted from the port # 3 of the B terminal changes as in FIG 10 before the mode group signal of lower order, the input end of the mode conversion optical fiber 40 reaches the B-end device.

Accordingly, when the mode group width of the higher order mode group occurs at the input end of the mode conversion optical fiber 40 the B-end device is less than or equal to 1/2 of the maximum propagation angle (the maximum mode group width) θ _{MAX of} the main optical fiber 10 is no crosstalk between the lower order mode group and the higher order mode group. Accordingly, it is sufficient that the mode group width θ _{# 3 of} the lower-order mode group signal transmitted from the port # 3 of the A-end apparatus satisfies the following equation. $${\mathrm{<figure-callout\; id="3"\; label="\theta \; \#"\; filenames="DE102018110003A1\_0008.png,DE102018110003A1\_0009.png"\; state="\{\{state\}\}">\theta \; </figure-callout>}}_{\#}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{tran}}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{hhoch}}\le {\mathrm{\theta }}_{\text{MAX}}/2$$

Equations (1) and (2) are conditions under which the lower-order mode group width and the higher-order mode group width are equal and whose occupation mode group width is equal to or smaller than 1/2 of the maximum propagation angle (the maximum mode group width) θ _MAX . However, the mode separation propagation angle for separating the lower-order mode group and the higher-order mode group is not necessarily equal to θ _MAX / 2. When one of the lower order mode group width and the mode group width higher order is less than θ _MAX / 2, the other mode group width may be greater than θ _MAX / 2. In view of this, it is sufficient that the lower order mode group widths θ _{# 2} and θ _{# 3 of} the lower order mode group signals input from the ports # 2 and # 3 of the A-end apparatus satisfy the following equation by adding the equations (1) and (2) is obtained. $${\mathrm{<figure-callout\; id="2"\; label="\theta \; \#"\; filenames="DE102018110003A1\_0008.png,DE102018110003A1\_0009.png"\; state="\{\{state\}\}">\theta \; </figure-callout>}}_{\#}+{\mathrm{<figure-callout\; id="3"\; label="\theta \; \#"\; filenames="DE102018110003A1\_0008.png,DE102018110003A1\_0009.png"\; state="\{\{state\}\}">\theta \; </figure-callout>}}_{\#}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{bending}}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{tran}}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{hniedrig}}+\mathrm{\Delta }{\mathrm{\theta }}_{\text{hhoch}}\le {\mathrm{\theta }}_{\text{MAX}}$$

The above-mentioned mode group widths and their increases were in the system in 5 measured, with the length of the main optical fiber 10 was set at 15 m, the refractive index of the core was set to 1.41, and the radius of curvature of the peripheral curved surface 41 of the mode conversion optical fiber 40 was set to 5 mm. The results of the measurements show that 0 _{# 2} = θ _{# 3} = 4.2 °, Δθ _bending = 1.2 °, Δθ _hlow = 1.5 °, Δθ _hhigh = 1.7 ° and Δθ _tran = 1.5 °. Therefore, the left side in equation (3) becomes 14.3 °. Since this value is less than or equal to the maximum propagation angle (θ _MAX ), ie, 21.7 °, the main optical fiber passes 10 no crosstalk between the lower order mode group signal and the higher order mode group signal.

Industrial Applicability

The present invention can be applied to mode group multiplex communication systems.

It is explicitly pointed out that all features disclosed in the description and / or the claims are considered separate and independent of each other for the purpose of original disclosure as well as for the purpose of limiting the claimed invention independently of the feature combinations in the embodiments and / or the claims should. It is explicitly stated that all range indications or indications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, in particular also as the limit of a range indication.

LIST OF REFERENCE NUMBERS

1:

optical device

10:

Main optical waveguide

15:

central axis

20:

first optical fiber

30:

second optical fiber

32:

self-written optical fiber

35:

central axis

40:

Mode conversion optical fiber

41:

peripheral curved surface

42:

coat

50:

frame

16, 26, 36:

optical connector

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• JP H1054915 [0004]
• JP 2000347043 [0004]

Cited non-patent literature

• Y. Li, JD Ingham, VF Olle, G. Gordon, RV Penty and IH White "20 Gb / s Fashion Group Division Multiplexing Employing Hermite-Gaussian Launches Over Worst Case Multimode Fiber Links," OFC, Optical Society of America , 2014 [0005]
• M. Kagami, Y. Sakai and H. Okada, "Variable Ratio Tao for Plastic Optical Fiber," Applied Optics, Vol. 30, No. 6 pp. 645-649, 1991 [0006]

Claims (12)

An optical apparatus for performing multiplexing or demultiplexing an optical signal for a main optical waveguide, characterized in that the optical device (1) comprises: the main optical waveguide (10) which is a step index multimode optical waveguide and forms a mode group multiplex transmission path in which channels are formed for signals of plural mode groups are; a mode conversion optical fiber (40) which is continuous with the main optical fiber (10) and consists of a bent step index multimode optical waveguide; a first optical fiber (20) continuous with the mode conversion optical fiber (40) and consisting of a step index multi-mode optical fiber extending in a bending direction of the mode conversion optical fiber (40); and a second optical waveguide (30) consisting of a step index multimode optical waveguide and connected in an axial direction of said main optical waveguide (10) to a peripheral curved surface (41) of said mode conversion optical waveguide (40), wherein, when said plurality of mode groups in said main optical waveguide (10) 10) are divided into a higher-order mode group and a lower-order mode group, the mode conversion optical fiber (40) causes a lower-order mode group signal to propagate therethrough and a higher-order mode group signal to the second optical fiber side, and as a signal a lower-order mode group propagates to the second optical fiber (30), and a lower-order mode signal propagating through the second optical fiber (30) to the mode conversion optical fiber (40) is converted into a higher-order mode group signal; which is coupled to the mode conversion fiber (40). Optical device after Claim 1 in that the peripheral curved surface (41) of the mode conversion optical fiber (40) is bent so that the lower order mode group in the mode conversion optical fiber (40) remains a guided mode and the higher order mode group in the mode conversion optical fiber (40) becomes an outgoing mode. Optical device after Claim 1 or 2 wherein the second optical fiber (30) is connected to the peripheral curved surface (41) of the mode conversion optical fiber (40) by a self-written optical fiber (32) extending from a distal end of the second optical fiber (30). Optical device after Claim 1 or 2 wherein the second optical waveguide (30) is in a state in which a self-written optical waveguide (32b) extending from the peripheral curved surface (41) of the mode conversion optical waveguide (40) and a self-written optical waveguide (32a) extending from a distal end of the second optical fiber (30) are connected to each other, with the peripheral curved surface (41) of the mode conversion optical fiber (40) is connected. Optical device after Claim 3 or 4 in which a core of the self-written optical waveguide (32) connected to the peripheral curved surface (41) is directly connected to a core of the mode conversion optical fiber (40) and the core of the self-written optical waveguide (32) connected to the peripheral curved one Surface (41) has a refractive index which is smaller than a refractive index of the core of the mode conversion optical fiber (40). Optical device after Claim 3 or 4 wherein each of the second optical waveguide (30) and the main optical waveguide (10) is a step index multimode optical fiber, the first optical waveguide (20) and the mode conversion optical waveguide (40) are formed as portions of the main optical waveguide (10) and the second optical waveguide (30) the self-written optical waveguide (32) is connected to a cladding of the mode conversion optical waveguide (40). Optical device according to one of Claims 3 to 6 wherein a core of the self-written optical waveguide (32) connected to the peripheral curved surface (41) has a refractive index smaller than a refractive index of a core of the mode conversion optical waveguide (40). Optical device according to one of Claims 1 to 7 in which the first optical waveguide (20) is a step index multimode optical fiber and the mode conversion optical waveguide (40) is a curved self-written optical waveguide (40) continuously connected to a distal end of the first optical waveguide (20). Optical device according to one of Claims 1 to 7 wherein the first optical fiber (20) is a step index multimode optical fiber and the mode conversion optical fiber (40) is a curved self-written optical fiber (40) formed by connecting a self-written optical fiber (40b) continuously from a distal end of the first optical fiber (20) , and a self-written optical waveguide (40a) continuously extending from a distal end of the main optical waveguide (10) and bending the connected self-written optical waveguides. Communication system, characterized in that an optical device (1) according to one of Claims 1 to 9 is used as an A-end device and an optical device (1) according to any one of Claims 1 to 9 is used as a B-end device; the main optical fiber (10) of the A-end device and the main optical fiber (10) of the B-end device are continuous and constitute a main optical fiber (10); a lower-order mode group signal, which is converted by the mode conversion optical fiber (40) to the higher-order mode group, is transmitted to the second optical fiber (30) of the A-end device and the second optical fiber (30) of the B-end device; and transmitting a signal of the lower-order mode group to the first optical fiber (20) of the A-end device and the first optical fiber (20) of the B-end device in which mode group multiplex communication is performed between the A-end device and the B-end device. A mode group multiplex communication system, characterized in that the system comprises: an optical waveguide comprising a main optical waveguide (10) formed of a step index multimode optical waveguide (11); a first step index multimode optical waveguide (31) connected to a peripheral curved surface (41) of a first curved section main optical waveguide (10), and a second step index multimode optical waveguide (31) connected to a peripheral curved surface (41) of a second bent portion (40) of the main optical waveguide (10), at the first bent portion and the second bent portion, demultiplexing or multiplexing an optical signal is performed by multiplying the order of the mode group between the main optical waveguide (10) and the first step index multimode optical waveguide (31) and between the main optical waveguide (10) and the second step index optical fiber (31) is changed, thereby performing mode group multiplex communication for signals propagating between the first bent portion and the second bent portion through the main optical fiber (10). A method of multiplexing / demultiplexing an optical signal used in a mode group multiplex communication method, characterized in that a part of a main optical waveguide (10) consisting of a step index multimode optical waveguide (11) is bent to form a bent portion, and second step index multimode optical fiber (31) is connected to a peripheral curved surface (41) of the bent portion; by demultiplexing a signal of a lower-order mode group for propagation toward the main optical fiber (10) by using the bent portion of a modal group multiplex signal propagating through the main optical fiber (10) and a higher-order mode group signal for propagating toward the second step-index multi-mode optical fiber (10) 31) is demultiplexed while being mode-modulated into a lower-order mode group, or by using the bent portion, a lower-order mode group signal is propagated to multiplex an optical signal toward the main optical fiber (10) and a lower-order mode group signal propagated through the second step index multimode optical waveguide (31), mode modulated into a higher order mode group, and propagated to multiplex an optical signal toward the main optical waveguide (10).

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