JP2005070189A - Optical link for luminous flux multiplex communication, and optical link for two-way optical communication - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an LFM communication which can be realized with light emitting elements of only the single wavelength without requiring light emitting elements of different light emission wavelengths. <P>SOLUTION: A transmission side terminal 10 comprises the first light emitting element 11, the second light emitting element 12, and a first optical coupler 13, and a reception side terminal 20 comprises a first light receiving element 18, a second light receiving element 19 and a second optical coupler 17. A communication line connecting the transmission side terminal 10 and the reception side terminal 20 is an optical link for LFM communication constituted by a step index type multimode optical fiber 16. The first optical coupler converts the exit light from an LD into parallel luminous fluxes and makes the parallel luminous fluxes incident on the step index type multimode optical fiber 16 at respective different angles with its optical axes. The second optical coupler 17 condenses the exit luminous flux from the optical fiber 16 and makes the exit luminous flux of a lower order propagation mode incident on the light receiving element 18 and the exit luminous flux of a higher order propagation mode incident on the light receiving element 19, respectively. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  In the present invention, communication is performed by allocating an optical signal of one channel for each of a plurality of propagation modes of a step index type multi-mode optical fiber (hereinafter also abbreviated as “multi-mode optical fiber” or “optical fiber”). The present invention relates to light flux multiplexing (LFM) communication. The present invention also relates to bidirectional optical communication in which a plurality of propagation modes of a multimode optical fiber are different and communication is performed by assigning a transmission signal and a reception signal.

  In particular, a multiplexer capable of bundling and entering a plurality of incident light beams propagating in a multi-mode optical fiber in different propagation modes, and a plurality of emitted light beams emitted from the multi-mode optical fiber in a propagation mode It is related with the splitter which can be separated for every.

  Both have the function of causing an output light beam output from the light emitting element as a transmission light beam to enter the multimode optical fiber, and on the other hand, inputting a light beam emitted from the multimode optical fiber as a reception light beam to the light receiving element. The present invention relates to a directional coupler.

  The present invention also relates to a mode converter that is arranged at one end or in the middle of a multi-mode optical fiber and performs mode conversion of a propagation mode from a higher order to a lower order propagation mode or from a lower order to a higher order propagation mode.

  In recent years, inexpensive optical links using plastic optical fibers as communication lines have attracted attention as communication means for in-vehicle and indoor communication devices. The length of the communication line of these optical links is about 3m to 4m. In communication using this plastic optical fiber, as a method of increasing communication channels, wavelength division multiplexing (WDM) using multiple lights with different wavelengths and multi-level amplitude modulation that takes multiple levels of light amplitude intensity (PAM: Pulse Amplitude Modulation) method and mode multiplexing method.

  In WDM communication, a plurality of light emitting elements having different emission wavelengths are prepared and propagated through a single optical fiber. Light emitted from the optical fiber is separated by a wavelength separation filter and received by separate light receiving elements (for example, photodiodes). Therefore, the wavelength band necessary for communication needs to be larger than the value obtained by multiplying the number of channels to be multiplexed by the wavelength bandwidth occupied by one channel.

  In the case of transmission using the PAM system, the light emitting elements may be monochromatic, and the number of required light receiving and emitting elements does not need to be increased as in the WDM system. Data exchanged by communication is incident on an optical fiber as an optical pulse signal through a circuit that converts a binary digital signal into a multi-value signal. The optical pulse signal emitted from the optical fiber is converted into an electric signal to be a multi-value digital signal. This multilevel signal is further converted into a binary digital signal. Thereby, for example, even if the same communication band as that used in communication using a four-value digital signal is used, the transmission speed can be doubled.

  Also in the case of transmission using the mode multiplexing method, the light emitting elements may be of a single color, and the number of required light receiving and emitting elements does not need to be increased as in the case of the WDM method. As an example in which transmission using a mode multiplexing system can be implemented, there is an invention of time division multiplexing optical communication using a multimode optical fiber as a communication line (see, for example, Patent Document 1). In this example, time-division multiplexed optical pulse signals of a plurality of channels are transmitted by changing the incident angle with respect to the propagation axis of the multimode optical fiber for each channel. On the receiving side, the multiplexed optical pulse signal is gated by a gate circuit to be divided into optical pulse signals for each channel and received.

  This system is essentially optical time division multiplexing (OTDM). In order to multiplex and enter multiple channels into a multimode optical fiber, which is the transmission path, only the method of changing the incident angle with respect to the propagation axis of the multimode optical fiber is used, and it propagates through the optical fiber. This is not a method that positively utilizes the characteristics of the propagation mode of the optical pulse signal. In order to multiplex and enter multiple channels into a multimode optical fiber that is a transmission path, optical pulse signals that have been multiplexed and time-division multiplexed in advance are multiplexed and transmitted. As long as the light can be incident on the multimode optical fiber that is the path, it is not necessary to adopt a method of changing the incident angle with respect to the propagation axis of the multimode optical fiber.

  Therefore, at present, in order to obtain an optical pulse signal that is time-division multiplexed by multiplexing the optical pulse signals for a plurality of channels to be transmitted as described above, it is realized using a planar optical waveguide or the like. A single mode optical fiber is used as the optical transmission line, and OTDM is realized.

  In addition, there is an example in which transmission using a mode multiplexing method is performed by actively utilizing the propagation mode characteristics of an optical pulse signal propagating in an optical fiber (see, for example, Patent Document 2). In this example, a multimode optical waveguide having a rectangular cross section is used as a transmission line. Then, an optical pulse signal is transmitted using a propagation mode in which reflection is repeatedly propagated on two parallel surfaces facing each other in the optical transmission path. However, in this transmission method using the mode multiplexing method, a special waveguide having a square cross-section must be used as the optical transmission line, which is not practical. In addition, since there are only two sets of two parallel planes of this optical transmission line, basically two-channel multiplex transmission can only be realized unless special measures are taken.

  On the other hand, a method of using two optical fibers for two-way communication has been mainly performed, but a two-way communication method using one optical fiber has been proposed in order to reduce the size and cost.

  For example, a bidirectional communication method has been proposed in which light emitted from an optical fiber is condensed on a light receiving element by a mirror and transmitted light is incident on the optical fiber from a part of the end face of the optical fiber (for example, Patent Document 3). reference). In this two-way communication system, the internally scattered light is formed by forming a divergent part on the outer periphery of the lens for condensing the incident light to the optical fiber and installing a thin film reflecting mirror for condensing the received light. The device which reduces the interference by is given.

  In addition, a device has been devised to reduce the optical coupling loss by making the core area required to make the optical pulse signal incident on the optical fiber sufficiently smaller than the core area required to obtain the received light. A bidirectional communication method has also been proposed (see, for example, Patent Document 4, Patent Document 5, and Patent Document 6).

  In the bidirectional optical communication device disclosed in Patent Document 4, the above-described problem is solved by a component called an optical separation element that has a part of a concave mirror and a part that guides outgoing signal light to an optical fiber. It has been solved. In the bidirectional optical communication device disclosed in Patent Document 5, a plane mirror for guiding outgoing signal light to an optical fiber and a concave mirror for guiding received light, which is light emitted from the optical fiber, to a light receiver are used. Thus, the above-mentioned problems are solved. Further, in the bidirectional optical communication module disclosed in Patent Document 6, by using a polarizing film, light emitted from a light emitting element that emits a transmission optical signal is reflected by the end face of the optical fiber and does not reach the light receiving element. By taking offense, the above-mentioned problems are solved.

In addition, when transmission light from the light emitting element is reflected by the end face of the optical fiber and the reflected return light incident on the light receiving element becomes strong, crosstalk occurs and communication quality is deteriorated. Therefore, a structure for weakening the reflected return light has been proposed. For example, there is an apparatus having a structure that prevents reflection at an end face of an optical fiber by sandwiching a refractive index matching agent between optical fiber connection portions (see, for example, Patent Document 7). In addition, there is an apparatus that employs a structure that reduces the intensity of reflected return light by processing the end face of an optical fiber into a convex surface (see, for example, Patent Document 8).
USP 3,633,034 USP 4,516,828 Japanese Patent Laid-Open No. 2003-167616 JP 2001-188148 JP 2001-235660 Japanese Patent Laid-Open No. 2001-249254 Japanese Patent Laid-Open No. 11-174283 JP 2002-323643 A

  However, in order to construct an optical communication system using WDM, a plurality of light emitting elements having different emission wavelengths are required, which makes the system expensive. Further, since a multiplexer that multiplexes light having different wavelengths and a demultiplexer that divides light are expensive, it becomes an expensive system.

  In addition, an optical communication system using PAM requires an expensive circuit for realizing complex conversion between a binary digital signal and a multilevel digital signal. Further, in the optical communication system that has been realized so far by mode multiplexing, it is necessary to use an expensive optical coupler corresponding to each mode, and moreover, the incidence to the optical fiber is efficiently performed. In addition, it is necessary to align the incident light and the end face of the optical fiber with high accuracy.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical link for LFM communication by a mode multiplexing method using a step index type multimode optical fiber that solves the above-mentioned problems.

  Specifically, it is to realize multiplex communication that can be realized with only one wavelength without requiring a plurality of light emitting elements having different emission wavelengths, and that does not require the use of a multi-value digital signal.

  Another object of the present invention is to realize an optical link for bidirectional optical communication that can realize bidirectional communication using a single optical fiber with only one wavelength without requiring a plurality of light emitting elements having different emission wavelengths.

  Another object of the present invention is to realize a mode converter for realizing an optical link capable of transmission / reception using communication terminals having the same configuration in performing bidirectional communication.

  In the following description, the optical axis of the optical fiber means a locus in which the center of the core of the optical fiber is connected in the light propagation direction. The incident angle or exit angle of the light beam with respect to the optical axis of the optical fiber means the angle formed by the propagation direction of the light beam with respect to the optical axis of the optical fiber in the vicinity of the incident end or the exit end surface of the optical fiber. To do.

  When the outgoing light beam from the light emitting element is incident as an incident light beam parallel to the optical axis of this optical fiber at a certain angle, it is emitted as a conical outgoing light beam from the output end of this optical fiber at the same angle as this incident angle. Is done. Since the incident angle and the outgoing angle are equal, hereinafter, the incident angle and the outgoing angle may be collectively referred to as an incoming and outgoing angle.

  The incident / exit angle of the light beam corresponds to the propagation mode of the light beam propagating through the optical fiber. The apex angle of the cone, which is the shape of the light beam emitted from the output end of the optical fiber, is equal to twice the incident / exit angle.

  In addition, a light beam that is incident at a small angle (incident angle close to parallel to the optical axis) with respect to the optical axis of this optical fiber and propagates through this optical fiber is a light beam that propagates in a low-order propagation mode or simply a low-order propagation. Sometimes referred to as mode light flux. Conversely, a light beam that is incident at a large angle (incident angle away from a direction parallel to the optical axis) with respect to the optical axis of the optical fiber and propagates in the optical fiber is propagated in a higher-order propagation mode. Or it may simply be a light beam in a higher-order propagation mode.

  The optical link for LFM communication according to the present invention has the following structural features. That is, an LFM comprising a plurality of light emitting elements, a plurality of light receiving elements corresponding to each of the plurality of light emitting elements via a step index type multimode optical fiber, and first and second optical couplers An optical link for communication. The first optical coupler has a function of causing the emitted light beams of the plurality of light emitting elements to enter at different angles with respect to the optical axis of the step index type multimode optical fiber. The second optical coupler has a function of causing light beams emitted at different angles with respect to the optical axis of the step index type multi-mode optical fiber to enter different light receiving elements.

According to the optical link for LFM communication of the present invention configured as described above, an LFM communication method including the following steps can be realized. That is, in a method of performing LFM communication using a step index type multimode optical fiber,
(1) A light emitting step for outputting a plurality of light beams, which are optical carriers of a plurality of channels, from a plurality of separate light emitting elements, respectively (2) Step index type multimode light by combining the plurality of light beams output as described above Input step to input into fiber (3) Waveguide step to guide a plurality of light beams combined and input through step index type multimode optical fiber in different propagation modes (4) Outgoing from this optical fiber A demultiplexing step for demultiplexing a plurality of light beams for each propagation mode. (5) A light receiving step for receiving the demultiplexed light beams by a plurality of separate light receiving elements.

  The first optical coupler is preferably configured as follows. That is, a plurality of convex lenses are arranged with a step index type multi-mode optical fiber for each of a plurality of light emitting elements at different angles between the optical axes of the plurality of convex lenses and the optical axis of the optical fiber. This constitutes the first optical coupler. And this 1st optical coupler is set as the structure which inject | emits the emitted light beam of a several light emitting element in the state of a parallel light beam to this optical fiber.

  Alternatively, the first optical coupler is configured by disposing a plurality of concave mirrors for each of the plurality of light emitting elements at different angles between the optical axis of the concave mirror and the optical axis of the step index type multimode optical fiber. . The first optical coupler is configured to reflect the emitted light beams of the plurality of light emitting elements so as to be incident on the optical fiber in a parallel light beam state.

  Alternatively, the emitted light beams of the plurality of light emitting elements are converted into a parallel light flux state on the step index type multimode optical fiber, and for each of the emitted light fluxes of the plurality of light emitting elements in the parallel light beam state, The first optical coupler is constituted by a single convex lens arranged at a position where the incident angle is different from the optical axis of the optical fiber and incident on the optical fiber.

  In addition, the emitted light beams of the plurality of light emitting elements are reflected to be converted into a parallel light beam in the step index type multimode optical fiber, and for each of the emitted light beams from the plurality of light emitting elements in the parallel light beam state. The first optical coupler is composed of a single concave mirror disposed at a position where the optical fiber is incident on the optical fiber at a different angle from the optical axis of the optical fiber.

  If the first optical coupler having the above-described configuration is used, it is possible to combine the incident light beams of a plurality of light emitting elements at different angles with respect to the optical axis of the optical fiber.

  The second optical coupler is preferably configured as follows. That is, a ring-shaped multifocal lens configured by concentrically arranging a plurality of ring-shaped lenses having different diameters, and a plurality of cone-shaped lenses that are emitted from step index type multimode optical fibers with different apex angles. The second optical coupler is configured by concentrating the luminous fluxes at respective positions where they are collected and incident on separate light receiving elements for each apex angle.

  Alternatively, it is a composite concave mirror in which a plurality of concave mirrors having different focal lengths and reflection directions are connected, and a plurality of light beams spread in a conical shape emitted from step index type multimode optical fibers with different apex angles. However, the second optical coupler is configured by reflecting the light at each apex angle and condensing the light on separate light receiving elements to be arranged at the incident positions.

  Alternatively, it is a multifocal element configured by combining a lens and a concave mirror, and a plurality of light beams spread in a conical shape with different apex angles are emitted from step index type multimode optical fibers. The second optical coupler is configured by being arranged at a position where the light is reflected and refracted for each apex angle to be collected and incident on separate light receiving elements.

  If the second optical coupler having the above-described configuration is used, the light beams emitted at different angles with respect to the optical axis of the step index type multimode optical fiber can be separately demultiplexed for each propagation mode.

  The bidirectional optical communication optical link of the present invention has the following structural features. That is, this is an optical link for bidirectional optical communication that performs bidirectional optical communication using a step index type multimode optical fiber. The optical link for bidirectional optical communication includes a light emitting element, a light receiving element, and a bidirectional optical coupler. The bidirectional optical coupler causes a light beam output as a transmission light beam from the light emitting element to enter the step index type multimode optical fiber. The optical fiber has a function of causing the outgoing light, which is output as a received light beam and guided through the optical fiber in the opposite direction to the transmission mode different from the transmitted light beam, to enter the light receiver.

  The bidirectional optical coupler is preferably configured as follows. That is, a ring-shaped multifocal lens configured by concentrically arranging a plurality of ring-shaped lenses having different diameters, and a light beam emitted from a light-emitting element is in a state of a parallel light beam on a step index type multimode optical fiber. The light beam emitted from the light emitting element in a parallel light beam is incident on the optical fiber, and the light beam emitted from the light emitting element spreads in a conical shape that is emitted at different angles with respect to the optical axis of the optical fiber. The two-way optical coupler is configured by placing the emitted light beam at a position where the emitted light beam is collected and incident on the light receiving element.

  Alternatively, it is a composite concave mirror in which a plurality of concave mirrors having different focal lengths and reflection directions are connected, and the luminous flux emitted from the light emitting element is in a state of a parallel luminous flux on the step index type multimode optical fiber, and The light beam emitted from the light emitting element in the state of a parallel light beam enters the optical fiber, and the light beam emitted from the light emitting element spreads in a conical shape, which is emitted at a different angle with respect to the optical axis of the optical fiber. A bidirectional optical coupler is configured by arranging the emitted light beam at a position where the emitted light beam is condensed and incident on the light receiving element.

  Alternatively, it is a multifocal element configured by combining a lens and a concave mirror, and the light beam emitted from the light emitting element is made into a parallel light beam state on the step index type multimode optical fiber, and the parallel light beam The light beam emitted from the light emitting element in the state is incident on the optical fiber, and the light beam emitted from the light emitting element is a light beam emitted from the optical fiber at a different angle with respect to the optical axis. A bidirectional optical coupler is configured by disposing the light incident on the light receiving element.

  If the bidirectional optical coupler having the above-described configuration is used, a light beam output as a transmission light beam from a light emitting element is incident on a step index type multimode optical fiber, and is output as a reception light beam from this optical fiber. The outgoing light guided through this optical fiber in the opposite direction in the propagation mode different from the transmission light beam can be made incident on the light receiving element.

  In the optical link for bidirectional optical communication according to the present invention, it is preferable to include any one of the following mode converters.

  That is, it is a mode converter configured with a compound lens that converts a low-order propagation mode light beam into a high-order propagation mode light beam. In addition, the mode converter includes a compound lens that converts a high-order propagation mode light beam into a low-order propagation mode light beam. This mode converter can be set as a part of the bidirectional coupler.

  Alternatively, the mode converter is converted into a middle part of a step index type multimode optical fiber used in the optical link for bidirectional optical communication according to the present invention, and a low-order propagation mode light beam is converted into a high-order propagation mode light beam. You may install the mode converter comprised with a compound lens. In addition, a mode converter configured with a compound lens that converts a high-order propagation mode light beam into a low-order propagation mode light beam may be installed in an intermediate portion of the step index type multimode optical fiber.

  According to the optical link for LFM communication of the present invention, the emitted light beams of the plurality of light emitting elements are incident at different angles with respect to the optical axis of the step index type multimode optical fiber by the first optical coupler, The emitted light beam is incident on a plurality of separate light receiving elements corresponding to each light emitting element on a one-to-one basis by the second optical coupler.

  The first optical coupler allows incident light beams from a plurality of light emitting elements to be incident as light beams having different angles with respect to the optical axis of the step index type multimode optical fiber. It is possible to propagate through the optical fiber in a propagation mode corresponding to each incident angle. That is, it is possible to realize a waveguide step in which a plurality of light beams that are combined and input are guided through the step index type multimode optical fiber in different propagation modes.

  In addition, the second optical coupler can demultiplex the light beams emitted at different angles with respect to the optical axis of the step index type multimode optical fiber for each propagation mode. That is, a demultiplexing step for demultiplexing a plurality of light beams emitted from the optical fiber for each propagation mode can be realized.

  These demultiplexed light beams can be incident on the corresponding light receiving elements for each light beam. That is, it is possible to realize a light receiving step of receiving a plurality of demultiplexed light beams by a plurality of separate light receiving elements.

  As described above, since a plurality of light beams having different propagation modes can be simultaneously guided through the step index type multi-mode optical fiber, the plurality of light beams can be used as carrier waves for transmitting transmission signals of different channels. If a role is given, optical multiplex communication using a single step index type multimode optical fiber as a transmission line can be performed. As a result, optical multiplex communication can be realized with only one wavelength without requiring a plurality of light emitting elements having different emission wavelengths. Also, multiplex communication can be realized without using a multi-value digital signal.

  On the other hand, the bidirectional optical communication optical link of the present invention comprises a light emitting element, a light receiving element, and a bidirectional optical coupler. This bidirectional optical coupler allows a light beam output as a transmission light beam from a light emitting element to enter a step index type multimode optical fiber, and a propagation mode different from the transmission light beam output from this optical fiber as a reception light beam. The light emitted from the optical fiber is incident on the light receiving element in the opposite direction.

  With the bidirectional optical coupler, the light beam output as the transmission light beam from the light emitting element can be incident on the step index type multimode optical fiber. In addition, the bidirectional optical coupler allows the outgoing light guided through the optical fiber in the opposite direction in the propagation mode different from the transmission light beam output as the reception light beam to be incident on the light receiving element.

  That is, according to the optical link for bidirectional optical communication of the present invention, the step of performing transmission from the transmitting / receiving terminal A toward the transmitting / receiving terminal B using the light beam propagating through the optical fiber in the first propagation mode can be realized. . In addition, it is possible to realize a step of performing transmission from the transmission / reception terminal B to the transmission / reception terminal A using a light beam propagating through the optical fiber in the second propagation mode that is different from the first propagation mode.

  Therefore, if one step index type multi-mode optical fiber is used as an optical communication line, this single optical fiber does not require a plurality of light emitting elements having different emission wavelengths, and is used for bidirectional optical communication with only one wavelength. An optical link can be realized.

  Furthermore, according to the mode converter of the present invention, when performing bidirectional optical communication of the present invention, transmission / reception can be performed using communication terminals having the same configuration.

  That is, in the optical link for bidirectional optical communication according to the present invention, a bidirectional optical coupler comprising a mode converter configured with a compound lens that converts a low-order propagation mode light beam into a high-order propagation mode light beam. Is used.

  Further, in the optical link for bidirectional optical communication according to the present invention, a bidirectional optical coupler comprising a mode converter configured with a compound lens that converts a high-order propagation mode light beam into a low-order propagation mode light beam. It is good also as a structure using.

  Further, in the optical link for bidirectional optical communication according to the present invention, the intermediate portion of the step index type multimode optical fiber is configured with a compound lens for converting a low-order propagation mode light beam into a high-order propagation mode light beam. A mode converter is provided.

  Also, in the optical link for bidirectional optical communication according to the present invention, the intermediate portion of the step index type multimode optical fiber is configured with a compound lens that converts a high-order propagation mode light beam into a low-order propagation mode light beam. A mode converter may be provided.

  If the above-mentioned mode converter is provided and the optical link for bidirectional optical communication according to the present invention is configured, both terminals A and B of the bidirectional optical communication (the transmitter and receiver are set as a set of the optical fiber transmission line). The transmission / reception units installed at both ends can be the same.

  If the transmitted luminous flux is transmitted from one terminal A to the terminal B in the higher-order propagation mode, the other terminal B that receives this transmitted luminous flux transmits the luminous flux transmitted in this higher-order propagation mode to the lower order. Receive in propagation mode. In addition, when the transmitted light flux is transmitted from one terminal to the terminal A in the high-order propagation mode, the other terminal A side that receives this transmission light flux reduces the light flux transmitted in this high-order propagation mode. Receive in next propagation mode.

  In this case, the above-described mode converter that converts the high-order propagation mode to the low-order propagation mode may be used.

  That is, both the terminal A and the terminal B can be configured to transmit the transmitted light beam in the high-order propagation mode and receive the received light beam in the low-order propagation mode. In other words, the light emitting element that emits the transmitted light beam is arranged so that it can enter the optical fiber in the higher order propagation mode, and the light receiver that receives the received light beam is unified so that it can receive the light beam emitted from the optical fiber in the lower order propagation mode. Can be set. As a result, the same transceiver can be used for the terminal A and the terminal B, and multiple types of terminals are not required. As a result, it is convenient when an optical link having a star configuration is formed.

  In the above description, the configuration of transmitting in the high-order propagation mode and receiving in the low-order propagation mode has been described, but conversely, the configuration of transmitting in the low-order propagation mode and receiving in the high-order propagation mode may be used. It is obvious that it is possible. In this case, the above-described mode converter that converts the low-order propagation mode to the high-order propagation mode may be used.

  In addition, any of the above-described mode converters can be set as an optical coupler constituting the optical link for bidirectional optical communication of the present invention, or set in the middle of a step index type multimode optical fiber. It is clear that an effect is obtained that the configurations of both terminals of the above-described bidirectional optical communication can be made the same.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each drawing merely schematically shows the shape, size, and arrangement relationship of each component to the extent that the present invention can be understood, and the present invention is not limited to the illustrated examples. In the following description, specific materials and conditions may be used. However, these materials and conditions are only one preferred example, and are not limited to these. Moreover, in each figure, the same number is shown about the same component, The overlapping description may be abbreviate | omitted regarding those functions.

<Configuration of optical link for beam multiplex communication>
The basic principle of LFM communication will be described by taking as an example a two-channel LFM communication system that uses two light emitting elements and two light receiving elements. It is assumed that a first optical coupler that receives light emitted from two light emitting elements arranged in a line is placed and light emitted from each light emitting element is incident on a step index type multimode optical fiber at different angles. That is, the light beam emitted from one of the two light emitting elements is incident parallel to the optical axis of the optical fiber, and the light beam emitted from the other light emitting element is at an angle with respect to the optical axis of the optical fiber. To enter.

  If a second optical coupler configured with a lens is placed at a distance from the output end face of the optical fiber, the light beam having a propagation component parallel to the optical axis of the optical fiber is transmitted to the lens on the rear focal plane of the lens. It is condensed near the intersection with the optical axis (in the center of the focal plane). On the other hand, a light beam incident at an angle parallel to the optical axis of the optical fiber is condensed on a peripheral portion away from the vicinity of the intersection of the rear focal plane of the lens and the optical axis of the lens.

  When the light receiving element is placed on the rear focal plane of the above-mentioned second optical coupler at a position where each light beam is condensed, the light beam incident in parallel to the optical axis of the optical fiber is located on the optical axis. The light beams incident in the inclined direction are received by different light receiving elements.

  That is, the above-described system composed of the two light emitting elements, the first optical coupler, the optical fiber, the second optical coupler, and the two light receiving elements is a light flux two-multiplex communication system.

  It is possible to construct an LFM communication system having not less than the above-described two sets of light-emitting elements and light-receiving elements, but three or more sets, with the same configuration as that of the above-described two-beam luminous flux multiplex communication system. That is, as a structure in which light beams emitted from a plurality of light emitting elements are incident at different angles with respect to the optical axis of the optical fiber, the light beams propagated in different propagation modes are respectively provided behind the output end of the optical fiber. A light receiving element may be placed at a position where each light beam is condensed so that light can be separated and received.

  With reference to FIGS. 1A and 1B, the characteristics of the far-field image of the incident light beam and the emitted light beam in the case of performing two-beam light multiplex communication capable of two-channel communication will be described. As will be described later, a light beam is incident on the optical fiber at an incident angle of 0 ° (a direction parallel to the optical axis of the optical fiber), and one channel is not allocated to this light beam. Therefore, since the light flux with an incident / exit angle of 0 ° shown in FIGS. 1 (A) and (B) is not used, by assigning 1 channel to each of the remaining two light fluxes with different incident angles, a total of 2 channels The figure assumes communication.

  FIG. 1A shows a far-field image of a light beam incident on an optical fiber, and FIG. 1B shows a far-field image of a light beam emitted from the optical fiber. The horizontal axis in FIG. 1 (A) indicates the incident angle with respect to the optical axis of the optical fiber, and the horizontal axis in FIG. 1 (B) indicates the emission angle with respect to the optical axis of the optical fiber. In both figures, the vertical axis indicates the light intensity on an arbitrary scale.

  In both the figures, only the values in the positive direction away from the direction parallel to the optical axis direction from the incident / exit angle of 0 ° (the direction parallel to the optical axis of the optical fiber) are shown. Actually, since the outgoing light beam has a conical shape with the optical axis of the optical fiber as the axis of symmetry, the figure showing the far-field image is symmetrical with the vertical axis (incident / outgoing angle 0 °) as the axis of symmetry. However, the portion corresponding to the negative value on the horizontal axis is omitted.

  FIG. 1 (A) shows far-field images of three types of light beams: a light beam a having an incident / exit angle of 0 °, a light beam b having a small incident angle, and a light beam c having a large incident angle. Further, FIG. 1B shows far-field images of three types of light fluxes: a light flux a ′ having an exit angle of 0 °, a light flux b ′ having a small exit angle, and a light flux c ′ having a large exit angle. A light beam a having an incident angle of 0 ° exits the optical fiber as a light beam a ′ having an exit angle of 0 °, a light beam b having a small incident angle exits the optical fiber as a light beam b ′ having a small exit angle, and the incident angle is A large light beam c is emitted from the optical fiber as a light beam c ′ having a large emission angle.

  When a plurality of types of incident light having different incident angles are incident on the step index type multimode optical fiber, the propagation modes of the respective light beams change while these light beams propagate through the optical fiber. As is apparent from a comparison between FIG. 1A and FIG. 1B, the shape of the incident light beam to the optical fiber is different from the shape of the outgoing light beam from the optical fiber. For example, the full width at half maximum of the light beam b with a small incident angle (interval between the arrows in FIG. 1 (A)) and the full width at half maximum of the light beam b ′ that propagates through the optical fiber and exits from multiple ends (FIG. 1 (B Obviously, the latter is wider. That is, it is shown that the propagation mode changes so that crosstalk does not occur with the light beam propagating in the adjacent propagation mode while the light beam propagates through the optical fiber.

  When designing an LFM communication system, it is necessary to consider the increase in the full width at half maximum of the far-field image of the emitted light beam described above so that the light beams of adjacent channels are separated on the receiving side and can be received by each receiving element. is there. In order to separate the transmitted light beam on the receiving side, it is only necessary to enter the incident beam with a sufficiently different incident angle on the incident side. There are few limited channels.

  Therefore, on the receiving side, even if there is a slight overlap of adjacent outgoing light beams, a device that can sufficiently separate and receive the light beams uses a light transmittance filter having a light amount attenuation characteristic indicated by a broken line in FIG. That is. That is, the amount of light having the characteristic indicated by the broken line p for the outgoing light beam a ′, the characteristic indicated by the broken line q for the outgoing light beam b ′, and the characteristic indicated by the broken line r for the outgoing light beam c ′. A light transmittance filter having attenuation characteristics may be set on the incident surface of the light receiving element. By applying such a device, even on the receiving side, even if there is a slight overlap between the outgoing light beams, the light beams can be sufficiently separated and received.

  With reference to FIG. 2, the configuration of the optical link for LFM communication of the present invention will be described. FIG. 2 is a conceptual diagram of the structure of an optical link for 2LFM optical communication. Since it is a multiplex communication that uses two types of light fluxes, two-multiplex optical communication is realized.

  The transmission-side terminal 10 includes a first light emitting element 11, a second light emitting element 12, and a first optical coupler 13. The reception-side terminal 20 includes a first light receiving element 18, a second light receiving element 19, and a second optical coupler 17. The communication line connecting the transmission side terminal 10 and the reception side terminal 20 is configured with a step index type multimode optical fiber 16.

  As the first light emitting element 11 and the second light emitting element 12, for example, a semiconductor laser (LD: Semiconductor Laser Diode) can be used. The first optical coupler 13 converts the emitted light from these LDs into parallel light fluxes and makes them incident at different angles with respect to the optical axis of the step index type multimode optical fiber 16. The first optical coupler 13 can be configured using an optical component such as a collimating lens, as will be described later.

  The outgoing light beam from the first light emitting element 11 that is incident at an angle close to parallel to the optical axis of the optical fiber 16 propagates through the optical fiber 16 in the low-order propagation mode, and is close to parallel to the optical axis of the optical fiber 16. The luminous flux emitted from On the other hand, the outgoing light beam from the second light emitting element 12 that is incident at an angle deviating from the parallel to the optical axis of the optical fiber 16 propagates through the optical fiber 16 in a higher-order propagation mode and is parallel to the optical axis of the optical fiber 16. The luminous flux is emitted at an angle deviating from the direction.

  As described above, the light beam propagated through the optical fiber 16 in the low-order propagation mode incident at an angle close to the optical axis of the optical fiber 16 (small incident angle) is an angle close to the optical axis of the optical fiber 16 ( It has a conical shape with a small incident angle) as the apex angle (twice the incident angle). On the other hand, the light beam propagating through the optical fiber 16 in a higher-order propagation mode that is incident at an angle deviated from the direction parallel to the optical axis of the optical fiber 16 (large incident angle) is parallel to the optical axis of the optical fiber 16. This is a conical shape with the angle deviating from the angle (large incident angle) as the apex angle (twice the incident angle).

  The second optical coupler 17 condenses these outgoing light beams from the optical fiber 16, and causes the outgoing light beam in the low order propagation mode to enter the light receiving element 18, and the outgoing light beam in the higher order propagation mode to enter the light receiving element 19. It has a function. The second optical coupler 17 can be configured using an optical component such as a ring-shaped multifocal lens as will be described later.

  As the first light receiving element 18 and the second light receiving element 19, for example, a photodiode can be used.

<Effect of optical fiber bending on propagation mode>
As described above, in order for the light beam incident so as to propagate through the optical fiber 16 in the low-order propagation mode to be preserved in the propagation mode and to be emitted in the low-order propagation mode, the optical fiber 16 is bent largely in the middle. It is necessary not to include a part. This is because it is known that the propagation mode of light propagating through the step index type multimode optical fiber changes its propagation mode when propagating through a large bent portion of the optical fiber.

  Therefore, an experiment was conducted to determine how much the propagation mode of light propagating through the optical fiber is affected by the degree of bending of the step index type multimode optical fiber. This is because it is necessary to clarify to what extent the bending of the optical fiber needs to be stored in order to realize the optical link for LFM communication of the present invention.

  With reference to FIGS. 3 (A), (B), and (C), the above experimental results will be described. These figures show the results of experiments conducted using a step index type multimode optical fiber formed using PMMA (polymethyl methacrylate) as a core material. A step index type multimode optical fiber with a core diameter of 980μm, a cladding thickness of 10μm, and an aperture ratio (NA: Numerical Aperture) = 0.3 is wound 20 times with a bending radius of 25mm and a length of 3.8m. It is the result of having measured the far-field image of the emitted light of low order propagation mode and high order propagation mode.

  In FIGS. 3 (A), (B) and (C), the horizontal axis is the emission angle (° C.), and the vertical axis is scaled with the maximum value of light intensity normalized to 100%. In each of FIGS. 3A, 3B, and 3C, the upper diagram shows the far-field image in the x direction, and the lower diagram shows the far-field image in the y direction. The xy plane was set parallel to the light receiving surface that receives the light beam emitted from the optical fiber. The fact that the upper and lower figures are symmetric with respect to the emission angle of 0 ° on the horizontal axis means that the intensity is distributed concentrically in a plane perpendicular to the emission direction of the light beam. .

  FIG. 3 (A) is a diagram showing a far-field image measured by placing the center position of the light receiving element on the optical axis of the optical fiber. A far-field image of the emitted light beam in the low-order propagation mode is observed. FIG. 3B is a diagram showing a far-field image measured by placing the center position of the light receiving element in a direction inclined by 5 ° with respect to the optical axis of the optical fiber. A far-field image of the emitted light beam in the same low-order propagation mode as the far-field image in FIG. 3A is observed. From this figure, it can be seen that the outgoing light beam in the low-order propagation mode is a substantially parallel light beam rather than a conical light beam.

  FIG. 3C is a diagram showing a far-field image measured by placing the center position of the light receiving element in a direction inclined by 15 ° with respect to the optical axis of the optical fiber. FIG. 3C shows a far-field image of a high-order propagation mode light beam emitted as a conical light beam from the optical fiber. That is, the light beam in the low-order propagation mode shown in FIGS. 3A and 3B and the light beam in the high-order propagation mode shown in FIG. It can be seen that the light is received.

  As a result of the above experiment, when performing LFM communication using a step index type multimode optical fiber with a core diameter of 980 μm, a cladding thickness of 10 μm, and an aperture ratio (NA: Numerical Aperture) = 0.3, the length is 25 mm at the bending radius. If the optical fiber is bent about 20 times of 3.8 m, the light receiving element is placed about 15 ° away from the optical axis of the optical fiber, so that the light beam in the low-order propagation mode and the high-order propagation mode are installed. It was confirmed that the light beam could be received with sufficient separation.

  The bending radius of 25 mm is the minimum bending radius in design as the bending radius of the step index type multimode optical fiber. If a bend with a bending radius smaller than this is applied to the optical fiber, the optical fiber is broken, and the performance cannot be guaranteed. Therefore, it is not necessary to assume that the optical fiber is deformed beyond this experimental condition, that is, the bending of the optical fiber in which the bending radius is 25 mm and the length of 3.8 m is wound 20 times.

  Since the far-field image shown in FIG. 3 (A) is a far-field image observed by the light receiving element installed on the optical axis of the optical fiber, the observed light beam is installed on the optical axis of the optical fiber. A light beam emitted from the light emitting element and incident perpendicularly to the end face of the optical fiber (parallel to the optical axis of the optical fiber) is a far-field image of the light beam that has propagated through the optical fiber and emitted. When a light beam is incident perpendicularly to the end face of the optical fiber, part of the reflected light from the end face returns to the light emitting element. For this reason, so-called return light noise in which the light emission intensity of the light emitting element fluctuates irregularly may be generated.

  Therefore, in order to prevent the occurrence of this return optical noise, it is general that a configuration in which a light beam enters the optical fiber from a direction perpendicular to the end face of the optical fiber cannot be taken. Also in the optical link of the present invention, even when a light beam in the low-order propagation mode is incident on the optical fiber, the light beam emitted from the light emitting element is made incident on the optical fiber at an angle of about several degrees. Therefore, light flux multiplex communication is possible by assigning one channel to each of the two light fluxes having a light incident / exit angle of about 5 ° as a light beam in a low-order propagation mode and a light flux in a high-order propagation mode of about 15 °. It has been shown.

That is, if the light receiving element is disposed on the optical axis of the optical fiber at a position about 5 ° from the optical axis of the optical fiber and at a position about 15 ° from the optical axis of the optical fiber, the above-mentioned bending degree is obtained. It has been shown that 2-channel LFM communication is possible even when optical fibers are added.

  If the communication distance is about 3.8 m, the above-mentioned step index type multimode optical fiber will be used for LFM communication, and even if this optical fiber is slightly bent, problems such as crosstalk between channels will occur. It has been confirmed that does not occur.

<Configuration example of the first optical coupler>
With reference to FIG. 4, an example of a first optical coupler configured with a collimating lens will be described. FIG. 4 is a schematic diagram of the transmitting side terminal 10 of the 2-channel LFM communication method. The light emitted from the first light emitting element 11 that outputs the signal light of the first channel is made into a parallel light flux by the collimating lens 14 and is incident on the incident end 16a of the step index type multimode optical fiber 16. Further, the output light from the second light emitting element 12 that outputs the signal light of the other second channel is also made into a parallel light beam by the collimating lens 15 and is incident on the incident end 16a of the step index type multimode optical fiber 16. .

  In this example, the collimating lenses 14 and 15 correspond to the first optical coupler 13 in FIG. According to the configuration shown in FIG. 4, two collimating lenses (convex lenses) are arranged between the optical fiber 16 and the optical axis of the collimating lens and the optical axis of the optical fiber 16 for each of the two light emitting elements. The structure is arranged with different angles. The light beams emitted from the two light emitting elements are incident on the optical fiber 16 as a parallel light beam.

  An example of a first optical coupler configured with a concave mirror will be described with reference to FIG. FIG. 5 is a schematic diagram of the transmitting side terminal 10 of the 2-channel LFM communication method. The light emitted from the first light emitting element 11 that outputs the signal light of the first channel is made into a parallel light flux by the concave mirror 54 and is incident on the incident end 16a of the step index type multimode optical fiber 16. Further, the output light from the second light emitting element 12 that outputs the signal light of the other second channel is also made into a parallel light beam by the concave mirror 55 and is incident on the incident end 16a of the step index type multimode optical fiber 16.

  In this example, concave mirrors 54 and 55 correspond to the first optical coupler 13 in FIG. According to the configuration shown in FIG. 5, the two concave mirrors are respectively reflected between the two light emitting elements and the optical fiber 16, and the propagation directions of the parallel light fluxes reflected by the concave mirrors to be in the state of the parallel light fluxes. In this structure, the angles formed with the optical axis of the optical fiber 16 are different from each other. The light beams emitted from the two light emitting elements are incident on the optical fiber 16 as a parallel light beam.

  Using a convex lens or concave mirror in the first optical coupler to convert the emitted light beam from the light emitting element into a parallel light beam, the amount of light that enters the optical fiber even if the distance from the light emitting element to the incident end of the optical fiber is sufficiently separated Does not decrease. That is, the distance from the light emitting element to the incident end of the optical fiber can be sufficiently separated. As a result, a sufficient space for installing a plurality of light emitting elements can be secured in the transmission side terminal 10, so that many light emitting elements can be installed, which is convenient for increasing the multiplicity of LFM communication. .

  An example of a first optical coupler configured with a single collimating lens (convex lens) 22 will be described with reference to FIG. FIG. 6 is a schematic diagram of the transmitting side terminal 10 of the 2-channel LFM communication method. The first light emitting element that outputs the signal light of the first channel and the second light emitting element that outputs the signal light of the second channel are integrated into an array to form an array light emitting element 21. The light emitted from the two-channel light emitted from the array light emitting element 21 is made into two parallel light beams by the collimating lens 22 and is incident on the incident end 16a of the step index type multimode optical fiber 16.

  In this example, a single collimating lens (convex lens) 22 corresponds to the first optical coupler 13 in FIG. According to the configuration shown in FIG. 6, the first optical coupler converts the two outgoing light beams from the arrayed light emitting element 21 into a state of a parallel light beam on the step index type multimode optical fiber 16, and the parallel light beam A single convex lens 22 arranged so as to be incident at a different incident angle with respect to the optical axis of the optical fiber 16 corresponds to each of the two emitted light beams in a state.

  With reference to FIG. 7, an example of a first optical coupler configured with a single concave mirror 52 will be described. FIG. 7 is a schematic diagram of the transmitting side terminal 10 of the 2-channel LFM communication method. The first light emitting element that outputs the signal light of the first channel and the second light emitting element that outputs the signal light of the second channel are integrated into an array to form an array light emitting element 21. The light emitted from the two-channel light emitted from the array light emitting element 21 is made into two parallel light beams by the concave mirror 52 and is incident on the incident end 16a of the step index type multimode optical fiber 16.

  In this example, a single concave mirror 52 corresponds to the first optical coupler 13 in FIG. According to the configuration shown in FIG. 7, the first optical coupler converts the two outgoing light beams from the array-like light emitting element 21 into a state of a parallel light beam on the step index type multimode optical fiber 16, and the parallel light beam A single concave mirror 52 arranged so as to be incident at a different incident angle with respect to the optical axis of the optical fiber 16 corresponds to each of the two emitted light beams in a state.

  Even if a single convex lens or concave mirror is used for the first optical coupler and the light beam emitted from the light emitting element is converted into a parallel light beam, it is incident on the optical fiber as in the case of using the plurality of convex lenses and concave mirrors described above. The amount of light does not decrease. That is, the distance from the light emitting element to the incident end of the optical fiber can be sufficiently separated. As a result, a sufficient space for installing a plurality of light emitting elements can be secured in the transmission side terminal 10, so that many light emitting elements can be installed, which is convenient for increasing the multiplicity of LFM communication. . Further, according to the configuration shown in FIG. 6 and FIG. 7 in which a single convex lens or concave mirror is used as the first optical coupler, when an arrayed light emitting element in which a plurality of light emitting elements are arrayed is used as the light emitting element. Convenient configuration.

  If the single collimating lens (convex lens) 22 shown in FIG. 6 or the single concave mirror 52 shown in FIG. 7 is used, the size of the convex lens 22 and the concave mirror 52 is large even when the number of light emitting elements or light receiving elements is large. By appropriately selecting and adjusting the geometric position of the light emitting element or the light receiving element, it is possible to cope with the problem without increasing the number of convex lenses 22 and concave mirrors 52.

<Configuration example of the second optical coupler>
With reference to FIG. 8, an example of a second optical coupler configured with a ring-shaped multifocal lens 37 will be described. FIG. 8 is a schematic diagram of a transmitting side terminal of a 2-channel LFM communication system. The first light receiving element 18 that receives the signal light of the first channel and the second light receiving element 19 that receives the signal light of the second channel are set in the receiving terminal 20. The output light for two channels emitted from the step index type multimode optical fiber 16 is condensed by the ring-shaped multifocal lens 37 into two parallel light beams on the first light receiving element 18 and the second light receiving element 19, respectively. Incident.

  A light beam that propagates through the optical fiber 16 in the high-order propagation mode and is output from the exit end of the optical fiber 16 has a shape that spreads and propagates in a conical shape. Therefore, a ring-shaped lens is required to collect the light beam that spreads in a conical shape on the light receiving element surface.

  The ring-shaped multifocal lens 37 shown in FIG. 8 includes a peripheral portion lens 37b having a donut-shaped peripheral portion through which the center portion of the ring-shaped multifocal lens 37 penetrates, and a center lens 37a at the center portion of the ring-shaped multifocal lens 37. Is a composite type lens composed of If a light beam that spreads in a conical shape is incident on the ring-shaped multifocal lens 37 that is such a composite lens, the light beam is condensed by the outer peripheral lens 37b of the ring-shaped multifocal lens 37.

  At this time, light energy does not reach the center lens 37a of the ring-shaped multifocal lens 37. This is because the energy of light is zero on the propagation axis of the light beam propagating in a conical shape. Here, the propagation axis of a light beam spreading and spreading in a conical shape is an axis included in the light beam and connected in a direction parallel to the propagation vector of the light beam. That is, the electric field energy of the light beam spreading and propagating in a conical shape is distributed so as to surround the propagation axis of this light beam.

  Since the light beam that propagates through the optical fiber 16 in the higher-order propagation mode and is output from the output end of the optical fiber 16 has a shape that spreads and propagates in a conical shape, the light beam in the higher-order propagation mode is a ring-shaped multifocal. The light is condensed on the second light receiving element 19 by the outer peripheral lens 37 b of the lens 37. On the other hand, the light flux that propagates through the optical fiber 16 in the low-order propagation mode and is output from the output end of the optical fiber 16 reaches all the central lens 37a of the ring-shaped multifocal lens 37. Therefore, the light beam in the low-order propagation mode is condensed on the first light receiving element 18 by the central lens 37a of the ring-shaped multifocal lens 37.

  Similarly, when there are three or more light fluxes having different propagation modes propagating through the optical fiber 16, the higher the propagation mode, the larger the apex angle of the light flux that propagates conically and propagates. Therefore, when there are three or more light beams having different propagation modes, ring-shaped lenses corresponding to the sizes of the respective apex angles may be overlapped on a concentric circle.

  The central lens 37a and the outer peripheral lens 37b of the ring-shaped multifocal lens 37 need to be set with different optical axes. This is because the light beam in the low-order propagation mode and the light beam in the high-order propagation mode can be received separately by the first light receiving element and the second light receiving element, respectively. If the optical axis of the central lens 37a and the outer peripheral lens 37b of the ring-shaped multifocal lens 37 is the same, the light beam in the low-order propagation mode and the light beam in the high-order propagation mode are respectively sent to the first light receiving element and the second light-receiving element. It cannot be arranged so that it can receive light with the light receiving element. Similarly, when there are three or more light beams having different propagation modes, it is necessary to set different optical axes for the ring-shaped lenses formed by concentric overlapping.

  The feature of the ring-shaped multifocal lens 37 used in the present invention is a lens formed in a state in which lenses having different focal lengths are stacked on the same optical axis, such as a multifocal lens used in glasses or the like. Instead, the ring-shaped lenses are formed in different directions of the optical axes. Although the direction of the optical axis can be continuously changed to form a ring-shaped lens, in the present invention, this light is limited to the number corresponding to the number of channels in order to reduce crosstalk between channels. It is preferable to set the direction of the axis.

  In this example, the ring-shaped multifocal lens 37 corresponds to the second optical coupler 17 in FIG. According to the configuration shown in FIG. 8, the second optical coupler includes a plurality of ring-shaped lenses having different diameters, which are installed between the optical fiber 16, the first light receiving element 18, and the second light receiving element 19. This is a ring-shaped multifocal lens configured concentrically. A plurality of light beams spread in a conical shape emitted from the optical fiber 16 at different apex angles are arranged so as to be condensed and incident on separate light receiving elements for each apex angle.

With reference to FIG. 9, an example of a second optical coupler configured with a composite concave mirror 57 will be described. FIG. 9 is a schematic diagram of a transmitting side terminal of a 2-channel LFM communication system. The first light receiving element 18 that receives the signal light of the first channel and the second light receiving element 19 that receives the signal light of the second channel are set in the receiving terminal 20. The output light for two channels emitted from the step index type multimode optical fiber 16 is reflected by the composite concave mirror 57, and the two parallel light beams are condensed on the first light receiving element 18 and the second light receiving element 19, respectively. Is incident.
The composite concave mirror 57 is composed of two types of concave mirrors, a first reflecting portion 57a and a second reflecting portion 57b, which have different concave curvatures and reflection directions.

  Since the light beam that propagates through the optical fiber 16 in the high-order propagation mode and is output from the exit end of the optical fiber 16 has a shape that spreads and propagates in a conical shape, the light beam in the high-order propagation mode is combined with the composite concave mirror 57. The light is condensed on the second light receiving element 19 by the second reflecting portion 57b. On the other hand, the light beam that propagates through the optical fiber 16 in the low-order propagation mode and is output from the output end of the optical fiber 16 is condensed on the first light receiving element 18 by the first reflecting portion 57a of the composite concave mirror 57. The

  Although the curvature (focal length) of the concave surface of the composite concave mirror 57 used in the present invention and the reflection direction can be continuously changed, as in the case of the ring-shaped multifocal lens 37 described above, In order to reduce the crosstalk between the channels, it is preferable to set a concave mirror that is limited to a number corresponding to the number of channels.

  In this example, the composite concave mirror 57 corresponds to the second optical coupler 17 in FIG. According to the configuration shown in FIG. 9, the second optical coupler is a composite concave mirror 57 in which a plurality of concave mirrors having different focal lengths and reflection directions are connected to each other. A plurality of light beams that are differently emitted and spread in a conical shape are reflected at each apex angle so as to be condensed and incident on separate light receiving elements.

  An example of the second optical coupler configured with the multifocal element 58 will be described with reference to FIG. FIG. 10 is a schematic diagram of a transmitting side terminal of a 2-channel LFM communication system. The first light receiving element 18 that receives the signal light of the first channel and the second light receiving element 19 that receives the signal light of the second channel are set in the receiving terminal 20. The light emitted from the two channels emitted from the step index type multimode optical fiber 16 is reflected or refracted by the multifocal element 58, and two parallel light beams are respectively transmitted to the first light receiving element 18 and the second light receiving element 19. Condensed and incident.

  The multifocal element 58 includes a reflecting portion 58b formed of a concave mirror and a refracting portion 58a formed of a convex lens.

  Since the light beam that propagates through the optical fiber 16 in the high-order propagation mode and is output from the output end of the optical fiber 16 has a shape that spreads and propagates in a conical shape, the light beam in the high-order propagation mode is the multifocal element 58. The light is condensed on the second light receiving element 19 by the reflecting portion 58b. On the other hand, the light beam that propagates through the optical fiber 16 in the low-order propagation mode and is output from the output end of the optical fiber 16 is condensed on the first light receiving element 18 by the refracting portion 57a of the multifocal element 58.

  In this example, the multifocal element 58 corresponds to the second optical coupler 17 in FIG. According to the configuration shown in FIG. 10, the second optical coupler is a composite concave mirror in which a convex lens and a concave mirror are combined, and a plurality of second optical couplers spread in a conical shape emitted from the optical fiber 16 with different apex angles. Are reflected and refracted for each apex angle so as to be condensed and incident on separate light receiving elements.

  A number of optical systems have been described as the second optical coupler with reference to FIGS. 8 to 10, but it is obvious that these can also be used as the first optical coupler. When these are used as the first optical coupler, the first and second light receiving elements (18 and 19) may be replaced with the first and second light emitting elements (11 and 12).

  The configuration examples of the transmitting side terminal and the receiving side terminal described with reference to FIGS. 4 to 10 described above are based on two-channel LFM communication in principle, but in multi-channel LFM communication of three or more channels. However, it is obvious that the same can be realized.

  When the multifocal element 58 is used, the reflection part and the refraction part can be alternately formed in a ring shape, and the direction of the adjacent light beam (adjacent propagation mode light beam) can be greatly changed, so that crosstalk is achieved. It is convenient to reduce the size.

  An optical coupler using a plurality of concave mirrors shown in FIGS. 5 and 9 is disclosed in Patent Document 5. Further, Patent Document 4 discloses an optical coupler in which a convex lens and a concave mirror shown in FIG. However, these inventions are different from the present invention in that the incident light to the optical fiber and the outgoing light from the optical fiber are not designed to correspond to the propagation mode propagating through the optical fiber.

<LFM communication method>
As an LFM communication method for realizing the above-described optical link for LFM communication, the following method can be adopted. That is, in the method of performing LFM communication using a step index type multimode optical fiber, the LFM communication method includes the following steps.
(1) A light emitting step for outputting a plurality of light beams, which are optical carriers of a plurality of channels, from a plurality of separate light emitting elements, respectively (2) Step index type multimode light by combining the plurality of light beams output as described above Input step to input to fiber (3) Waveguide step to guide a plurality of light beams that have been combined and input in different propagation modes through step index type multimode optical fiber (4) Multiple to output from this optical fiber (5) A light receiving step of receiving a plurality of demultiplexed light beams by a plurality of separate light receiving elements.

  As described above, the light emission step can be realized by using, for example, an LD as a light emitting element. The input step can be realized by using the first optical coupler having the configuration described with reference to FIGS. The waveguide step can be realized, for example, by using a step index type multimode optical fiber formed using PMMA as a core material. The demultiplexing step can be realized by using the second optical coupler having the configuration described with reference to FIGS. As already described, the light receiving step can be realized by using, for example, a photodiode as a light receiving element.

<Configuration of optical link for bidirectional optical communication>
The configuration of the optical link for bidirectional optical communication according to the present invention will be described with reference to FIG. FIG. 11 is a conceptual diagram of the configuration of an optical link for bidirectional optical communication. Two-way optical communication that uses one type of light flux for each of transmission and reception, and uses a total of two types of light flux, realizing bi-directional optical communication for each channel for transmission and reception To do.

  This optical link for bidirectional optical communication is configured by connecting a pair of transmission / reception terminals including a light emitting element, a light receiving element, and a bidirectional optical coupler via a step index type multimode optical fiber. .

  For convenience of the following description, one set of transmission / reception terminals may be distinguished by indicating one transmission / reception terminal A and the other as transmission / reception terminal B. In addition, elements constituting the transmission / reception terminal A and the transmission / reception terminal B may be distinguished by adding the A and B.

  The transmitting / receiving terminal A 30 includes a light emitting element A 110, a light receiving element A 180, and a bidirectional optical coupler A 23. The transmission / reception terminal B 40 includes a light emitting element B 120, a light receiving element B 190, and a bidirectional optical coupler B 27. A communication line connecting the transmission / reception terminal A 30 and the transmission / reception terminal B 40 is configured by a step index type multimode optical fiber 16.

  The bi-directional optical coupler A 23 and the bi-directional optical coupler B 27 convert the emitted light from these LDs into parallel luminous fluxes and make them incident at different angles with respect to the optical axis of the step index type multimode optical fiber 16. At the same time, the light beams propagating through the optical fiber 16 are condensed and incident on the light receiving element A 180 and the light receiving element B 190, respectively.

  The bidirectional optical coupler A 23 and the bidirectional optical coupler B 27 can be configured using optical components such as a ring-type multifocal lens as will be described later. The light emitting element A 110 and the light emitting element B 120 can use, for example, an LD. The light receiving element A 180 and the light receiving element B 190 can use, for example, a photodiode.

  In the optical link for bidirectional optical communication according to the present invention, the propagation mode in which the transmission light flux propagates through the optical fiber 16 from the transmission / reception terminal A 30 toward the transmission / reception terminal B 40 and the transmission / reception terminal B 40 toward the transmission / reception terminal A 30 The propagation modes in which all the transmitted light beams propagate through the optical fiber 16 are set to different propagation modes. For example, the propagation mode of the transmission light flux from the transmission / reception terminal A 30 to the transmission / reception terminal B 40 is set to the low-order propagation mode, and the propagation mode of the transmission light flux from the transmission / reception terminal B 40 to the transmission / reception terminal A 30 is set to the higher order. Set to propagation mode. In the following description, the case where such a setting is assumed will be described. However, it is obvious that the propagation mode may be set in the opposite direction.

  The bi-directional optical communication described above has been described for bi-directional optical communication of 1 channel each for transmission and reception, but if the number of pairs of light emitting elements and light receiving elements is increased and the propagation mode propagating in the optical fiber 16 is increased, 2 Obviously, bi-directional multiplex communication over channels is possible.

  The configuration of the optical link for bidirectional optical communication of the present invention described with reference to FIG. 11 is used in a system configured to distinguish the upstream side from the downstream side, such as USB (Universal Serial Bus). Suitable for doing.

<Configuration example of bidirectional optical coupler>
With reference to FIG. 12, an example of a bidirectional optical coupler constituted by ring-shaped multifocal lenses 33 and 37 will be described. FIG. 12 is a schematic diagram of an optical link for bidirectional optical communication including bidirectional optical couplers 23 and 27 configured using ring-shaped multifocal lenses 33 and 37. The central lens 33a and the outer peripheral lens 33b of the ring-shaped multifocal lens 33 need to be set with different optical axes. Similarly, it is necessary to set the central lens 37a and the outer peripheral lens 37b of the ring-shaped multifocal lens 37 with different optical axes. By setting different optical axes in this way, the light emitting element and the light receiving element can be spatially separated and installed.

  In the description with reference to FIG. 12, for convenience of explanation, the propagation mode of the transmitted light beam from the transmission / reception terminal A 30 to the transmission / reception terminal B 40 is set to the low-order propagation mode, and the transmission / reception terminal B 40 to the transmission / reception terminal A The propagation mode of the transmitted light beam toward 30 is set to a higher-order propagation mode.

  A light beam emitted from the light emitting element A 110 is converted into a parallel light beam by the central lens 33a of the ring-shaped multifocal lens 33 and is incident on the input / output end 16a of the step index type multimode optical fiber 16. The light beam emitted from the light-emitting element A 110 incident on the optical fiber 16 propagates through the optical fiber 16 in the low-order propagation mode, and is output from the input / output end 16b of the optical fiber 16, and is the central portion of the ring-shaped multifocal lens 37. The light is incident on the lens 37a, collected by the light receiving element B 190, and received.

  On the other hand, the light beam emitted from the light emitting element B 120 becomes a parallel light beam by the outer peripheral lens 37b of the ring-shaped multifocal lens 37 and is incident on the input / output end 16b of the step index type multimode optical fiber 16. The light beam emitted from the light emitting element B 120 incident on the optical fiber 16 propagates through the optical fiber 16 in a high-order propagation mode, is output from the input / output end 16a of the optical fiber 16, and is the outer periphery of the ring-shaped multifocal lens 33. The light is incident on the lens 33b, collected by the light receiving element A 180, and received.

  In this example, the ring-shaped multifocal lenses 33 and 37 correspond to the bidirectional optical couplers 23 and 27 in FIG. 11, respectively. According to the configuration shown in FIG. 12, the bidirectional optical couplers 23 and 27 have a function to play the following roles.

  That is, the central lens 33a of the ring-shaped multifocal lens 33 has a parallel light flux emitted from the light emitting element A 110, and the input / output end 16a is propagated to the optical fiber 16 in the low-order propagation mode. To enter. In addition, the peripheral lens 33b of the ring-shaped multifocal lens 33 causes the emitted light beam from the optical fiber 16 to be condensed and incident on the light receiving element A 180.

  On the other hand, the peripheral lens 37b of the ring-shaped multifocal lens 37 converts the light beam emitted from the light emitting element B 120 into a parallel light beam, and propagates the optical fiber 16 to the input / output end 16b to the optical fiber 16 in a high-order propagation mode. Make it incident. Further, the central lens 37a of the ring-shaped multifocal lens 37 causes the light beam emitted from the optical fiber 16 to be condensed and incident on the light receiving element 190.

  With reference to FIG. 13, an example of a bidirectional optical coupler configured with composite concave mirrors 53 and 57 will be described. FIG. 13 is a schematic diagram of an optical link for bidirectional optical communication including a bidirectional optical coupler configured using composite concave mirrors 53 and 57. The first reflecting portion 53a and the second reflecting portion 53b of the composite concave mirror 53 need to be set with different optical axes. Similarly, the first reflecting portion 57a and the second reflecting portion 57b of the composite concave mirror 57 need to be set with different optical axes. By setting different optical axes in this way, the light emitting element and the light receiving element can be spatially separated and installed.

  In the description with reference to FIG. 13, for convenience of explanation, the propagation mode of the transmission light flux from the transmission / reception terminal A 30 to the transmission / reception terminal B 40 is set to the low-order propagation mode, and the transmission / reception terminal B 40 to the transmission / reception terminal A The propagation mode of the transmitted light beam toward 30 is set to a higher-order propagation mode.

  The emitted light beam from the light emitting element A 110 is converted into a parallel light beam by the first reflecting portion 53a of the composite concave mirror 53 and is incident on the input / output end 16a of the step index type multimode optical fiber 16. The light beam emitted from the light emitting element A 110 incident on the optical fiber 16 propagates through the optical fiber 16 in the low-order propagation mode, and is output from the input / output end 16b of the optical fiber 16, and the first reflecting portion of the composite concave mirror 57. The light enters the light 57a, is collected by the light receiving element 190, and is received.

  On the other hand, the light beam emitted from the light emitting element B 120 becomes a parallel light beam by the second reflecting portion 57b of the composite concave mirror 57 and is incident on the input / output end 16b of the step index type multimode optical fiber 16. The light beam emitted from the light emitting element B 120 incident on the optical fiber 16 propagates through the optical fiber 16 in a high-order propagation mode, and is output from the input / output end 16a of the optical fiber 16. The second reflecting portion of the composite concave mirror 53 The light enters the light beam 53b and is collected by the light receiving element A 180 and received.

  In this example, the composite concave mirrors 53 and 57 correspond to the bidirectional optical couplers 23 and 27 in FIG. 11, respectively. According to the configuration shown in FIG. 13, the bidirectional optical couplers 23 and 27 have a function to play the following roles.

  That is, the first reflecting portion 53a of the composite concave mirror 53 converts the light beam emitted from the light emitting element A 110 into a parallel light beam, and propagates the optical fiber 16 to the input / output end 16a so as to propagate in the low-order propagation mode. Make it incident. Further, the second reflecting portion 53b of the composite concave mirror 53 causes the emitted light beam from the optical fiber 16 to be condensed and incident on the light receiving element A 180.

  On the other hand, the peripheral lens 57b of the composite concave mirror 57 makes the light beam emitted from the light-emitting element B 120 a parallel light beam, and makes the optical fiber 16 enter the input / output end 16b so as to propagate in the higher-order propagation mode. . Further, the central portion 57a of the composite concave mirror 57 collects the incident light beam from the optical fiber 16 and makes it incident on the light receiving element B 190.

  Referring to FIG. 14, an example of a bidirectional optical coupler configured with multifocal elements 58 and 59 will be described. FIG. 14 is a schematic diagram of an optical link for bidirectional optical communication including bidirectional optical couplers 30 and 40 configured using multifocal elements 58 and 59. The reflecting portion 59b and the refracting portion 59a of the multifocal element 59 need to be set with different optical axes. Similarly, the reflection part 58b and the refraction part 58a of the multifocal element 58 need to be set with different optical axes. By setting different optical axes in this way, the light emitting element and the light receiving element can be spatially separated and installed.

  In the description with reference to FIG. 14, for convenience of explanation, the propagation mode of the transmission light flux from the transmission / reception terminal A 30 to the transmission / reception terminal B 40 is set to the higher-order propagation mode, and the transmission / reception terminal B 40 to the transmission / reception terminal A The propagation mode of the transmitted light beam toward 30 is set to the low-order propagation mode.

  The emitted light beam from the light emitting element A 110 is converted into a parallel light beam by the reflecting portion 59b of the multifocal element 59 and is incident on the input / output end 16a of the step index type multimode optical fiber 16. The light beam emitted from the light emitting element A 110 that has entered the optical fiber 16 propagates through the optical fiber 16 in a high-order propagation mode, and is output from the input / output end 16b of the optical fiber 16 to the reflecting portion 58b of the multifocal element 58. Incident light is collected and received by the light receiving element B 190.

  On the other hand, the light beam emitted from the light emitting element B 120 becomes a parallel light beam by the refracting portion 58a of the multifocal element 58 and is incident on the input / output end 16b of the step index type multimode optical fiber 16. The light beam emitted from the light emitting element B 120 incident on the optical fiber 16 propagates through the optical fiber 16 in the low-order propagation mode, and is output from the input / output end 16a of the optical fiber 16 to the refracting portion 59b of the multifocal element 59. Incident light is collected on the light receiving element A 180 and received.

  In this example, the multi-focal elements 59 and 58 correspond to the bidirectional optical couplers 23 and 27 in FIG. 11, respectively. According to the configuration shown in FIG. 14, the bidirectional optical couplers 23 and 27 have a function to play the following roles.

  That is, the reflecting portion 59b of the multifocal element 59 converts the light beam emitted from the light emitting element A 110 into a parallel light beam, and causes the optical fiber 16 to enter the input / output end 16a so as to propagate in the high-order propagation mode. . In addition, the transmission part 59a of the multifocal element 59 collects the incident light beam from the optical fiber 16 and makes it incident on the light receiving element A 180.

  On the other hand, the transmission part 58a of the multifocal element 58 converts the light beam emitted from the light emitting element B 120 into a parallel light beam, and causes the optical fiber 16 to enter the input / output end 16b so as to propagate in the low-order propagation mode. . In addition, the reflecting portion 58b of the multifocal element 58 focuses the incident light beam from the optical fiber 16 on the light receiving element B 190 and makes it incident.

<Bidirectional optical communication method>
As a bidirectional optical communication method using the above-described optical link for bidirectional optical communication, the following method can be adopted. That is, in a method for performing bidirectional optical communication using a step index type multimode optical fiber, the bidirectional optical communication method includes the following steps.

  Transmitting from the transmitter / receiver terminal A to the transmitter / receiver terminal B using a step index type multimode optical fiber using a light beam propagating in the first propagation mode, and transmitting from the transmitter / receiver terminal B to the transmitter / receiver terminal A Performing a step index type multimode optical fiber using a light beam propagating in a second propagation mode. In this bidirectional optical communication method, the first propagation mode and the second propagation mode are set to different propagation modes.

The step of transmitting from the transmitting / receiving terminal A to the transmitting / receiving terminal B may be configured to include the following steps.
(1) In the transmitting / receiving terminal A, a light emission step of outputting a light beam, which is an optical carrier wave of a transmission channel from the transmitting / receiving terminal A to the transmitting / receiving terminal B, from the light emitting element A provided in the transmitting / receiving terminal A (2) Output described above Input step of inputting the light flux into the step index type multimode optical fiber (3) Waveguide step for guiding the above light flux through the step index type multimode optical fiber (4) Demultiplexing step to separate the luminous flux emitted from the transmission terminal and the optical carrier wave of the transmission channel from the transmission / reception terminal end to the transmission / reception terminal end (5) A light receiving step of receiving a light beam, which is an optical carrier wave of the transmission channel, by a light receiving element provided in the transmitting and receiving terminal.

Further, the step of performing transmission from the transmission / reception terminal B to the transmission / reception terminal A may include the following steps.
(1) In the transmitting / receiving terminal B, a light emission step in which a light beam, which is an optical carrier wave of a transmission channel from the transmitting / receiving terminal B to the transmitting / receiving terminal A, is output from the light emitting element B provided in the transmitting / receiving terminal B. (2) Output described above Input step for inputting the processed light beam into the step index type multimode optical fiber (3) Waveguide step for guiding the above light beam through the step index type multimode optical fiber (4) Demultiplexing step to separate the luminous flux emitted from the optical fiber and the optical carrier wave of the transmission channel from the transmission / reception terminal A to the transmission / reception terminal B (5) From the transmission / reception terminal B emitted from this optical fiber toward the transmission / reception terminal A A light receiving step of receiving a light beam, which is an optical carrier wave of the transmission channel, by a light receiving element A provided in a transmitting / receiving terminal A.

  As described above, the light emission step can be realized by using, for example, an LD as a light emitting element. The input step and the demultiplexing step can be realized by using the bidirectional optical coupler having the configuration described with reference to FIGS. The waveguide step can be realized, for example, by using a step index type multimode optical fiber formed using PMMA as a core material. As already described, the light receiving step can be realized by using, for example, a photodiode as a light receiving element.

<Configuration example of mode converter>
The transmitting / receiving terminals constituting the optical link for bidirectional optical communication according to the present invention described with reference to FIGS. 11 to 14 do not have the same configuration with respect to the arrangement of the light emitting elements and the light receiving elements. If the optical link for bidirectional optical communication can have the same configuration with respect to the arrangement of the light emitting element and the light receiving element, one type of transmitting / receiving terminal may be manufactured, which is highly convenient for industrial use. In particular, this is advantageous when, for example, realizing a star configuration optical link constructed by using three or more transmission / reception terminals.

  Specifically, in a system in which the transmission / reception terminals are designed to be equal, such as Ethernet (registered trademark) and IEEE1394, the transmission / reception terminal having the same configuration with respect to the arrangement of the light emitting element and the light receiving element is provided. It is suitable to do.

  Therefore, a mode converter for realizing an optical link capable of transmission / reception using communication terminals having the same configuration in performing bidirectional communication will be described with reference to FIGS. 15 to 18. FIG.

  Referring to FIG. 15, the configuration of mode converter 60 in the case of converting a low-order propagation mode light beam into a high-order propagation mode light beam will be described. The mode converter 60 includes a ring-shaped multifocal lens 41 and a mode conversion lens 42. As shown in FIG. 16, the mode conversion lens 42 is a composite lens configured by combining a first partial lens 43 and a second partial lens 44. In FIG. 15, the mode conversion lens 42 is arranged so that the first partial lens 43 comes to a portion indicated by l and the second partial lens 44 comes to a portion indicated by a position h.

  17A to 17D show lens shapes that can be used as the first partial lens 43 and the second partial lens 44 constituting the mode conversion lens 42. FIG. In each figure, the left figure is a diagram showing a cross-sectional shape taken along a plane including the optical axis of the lens and parallel to the optical axis. Moreover, the figure on the right side is a view showing a cross-sectional shape cut along a plane that passes through the center of the lens and is perpendicular to the optical axis.

  When the light beam L in the low-order propagation mode is incident on the center lens 41a of the ring-shaped multifocal lens 41 from the left side of FIG. 15, the center lens 41a is converted into a parallel light beam, and the second partial lens 44 of the mode conversion lens 42. The light is incident at a small incident angle with respect to the optical axis of the optical fiber 16. As the second partial lens 44, for example, a lens shown in FIG. 17B can be used. By arranging the lens shown in FIG. 17B with its optical axis inclined with respect to the optical axis of the optical fiber 16, the role of the second partial lens can be realized.

  The second partial lens 44 changes the light beam emitted from the central lens 41a of the ring-shaped multifocal lens 41 into a parallel light beam and makes it incident on the optical fiber 16 at a large angle with respect to the optical axis of the optical fiber 16. At this time, the light beam L incident on the optical fiber 16 is made into a thin light beam and is incident on the second partial lens 44 of the mode conversion lens.

  In order to cope with the case where the light beam L incident on the optical fiber 16 is not a parallel light beam but a conical light beam, the second partial lens 44 of the mode conversion lens 42 is shown in FIG. A lens or a lens shown in FIG. Furthermore, instead of the second partial lens 44 of the mode conversion lens 42, the position of the mode conversion lens 42 where the second partial lens 44 is installed is hollow, and the same function is realized by using a concave mirror instead of the lens. It is also possible.

  As described above, the light beam L propagating from the left side to the mode converter 60 shown in FIG. 15 in the low-order propagation mode is converted into the central lens 41a of the ring-shaped multifocal lens 41 and the second partial lens of the mode conversion lens 42. 44 enables conversion to a mode in which the optical fiber 16 propagates in a higher-order propagation mode.

On the other hand, the light beam propagating through the optical fiber 16 from the right side in FIG. 15 in the low-order propagation mode is emitted from the optical fiber 16 and propagated in a conical shape by the first partial lens 43 of the mode conversion lens 42. The propagation mode is converted to. Lenses that can be used as the first partial lens 43 of the mode conversion lens 42 are, for example, the lens shown in FIG. 17 (A) and the lens shown in FIG. 17 (C).
The light beam H converted as a conical light beam is condensed by the peripheral lens 41b of the ring-shaped multifocal lens 41 at a position (region) indicated by the left side M of the mode converter 60, and is in a higher-order propagation mode. The light is output from the mode converter 60 as a light beam.

  If the mode converter 60 is placed in the middle of the optical fiber 16 serving as a transmission line, the input / output end of the optical fiber 16 may be installed at the position indicated by M. That is, the optical fiber 16 as shown in this figure is arranged on the right side of the mode converter 60 shown in FIG. 15, and another step index type multimode light similar to the optical fiber 16 is also provided at the position indicated by M. What is necessary is just to arrange | position the incident / exit end of a fiber. By arranging in this way, mode conversion can be executed in the middle of the communication line.

  If the mode converter 60 is placed at the exit end of the optical fiber 16 that is a transmission line, a bidirectional optical coupler may be disposed on the right side of the position indicated by M, that is, more than the ring-shaped multifocal lens 41. . That is, the bidirectional optical coupler may be configured to include a mode conversion lens 42 that is a compound lens that converts a low-order propagation mode light beam into a high-order propagation mode light beam.

  With this arrangement, the light beam H and the light beam L can be coupled to the light emitting element and the light receiving element, or the light receiving element and the light emitting element, respectively, by the bidirectional optical coupler after performing the mode conversion.

  As described above, the mode converter 60 has a function of converting the propagation mode of the light beam incident in the low-order propagation mode from both the left and right sides of the mode converter 60 into the light beam of the high-order propagation mode. Therefore, if the mode converter 60 described with reference to FIG. 15 is arranged in the middle of the transmission line of the optical link for bidirectional optical communication, the structures of the transmission / reception terminals can be made the same.

  That is, the structure of all the transmitting and receiving terminals constituting the optical link for bidirectional optical communication, so that the transmission signal can be transmitted as a low-order propagation mode light beam and the high-order propagation mode light beam can be received as a reception signal. A configuration in which a light-emitting element that outputs a transmission signal and a light-receiving element that receives a reception signal are arranged.

  Further, it is obvious that the mode converter 60 described above may be disposed at both ends of the optical fiber 16 that is a transmission line, or may be disposed at an intermediate portion of the optical fiber 16. That is, when configuring an optical link for bidirectional optical communication, one mode converter 60 is connected to each pair of transmission / reception terminals, and the optical fiber 16 that is a transmission line connecting the one set of transmission / reception terminals is connected. What is necessary is just to arrange | position to both ends or an intermediate part.

  Next, the configuration of the mode converter 60 in the case of converting a light beam in a high-order propagation mode into a light beam in a low-order propagation mode will be described with reference to FIG. Also in this case, the mode converter 60 includes a ring-shaped multifocal lens 41 and a mode conversion lens. Further, as shown in FIG. 16, the mode conversion lens 42 is a compound lens configured by combining a first partial lens 43 and a second partial lens 44. However, the lens shape that can be used as the first partial lens 43 and the second partial lens 44 is different from the mode converter in the case of converting the light beam in the low-order propagation mode to the light beam in the high-order propagation mode. Further, in FIG. 15, reference is made assuming that the direction of the arrow indicating the traveling direction of the light beam is reversed.

  When the high-order propagation mode light beam H is incident on the peripheral lens 41b of the ring-shaped multifocal lens 41 from the left side of FIG. 15, the peripheral lens 41b is converted into a parallel light beam and the second partial lens 44 of the mode conversion lens 42 is obtained. The light is incident in a direction that is incident at a large incident angle with respect to the optical axis of the optical fiber 16. As the second partial lens 44, for example, a lens shown in FIG. 17C can be used. The role of the second partial lens 44 can be realized by disposing the lens shown in FIG. 17C with its optical axis inclined with respect to the optical axis of the optical fiber 16.

  The second partial lens 44 changes the light beam emitted from the peripheral lens 41b of the ring-shaped multifocal lens 41 into a parallel light beam and makes it incident on the optical fiber 16 at a small angle with respect to the optical axis of the optical fiber 16. At this time, the light beam H incident on the optical fiber 16 is made into a thin light beam and is incident on the second partial lens 44 of the mode conversion lens 42.

  As described above, the light beam H propagating from the left side to the mode converter 60 shown in FIG. 15 in the high-order propagation mode is converted into the peripheral lens 41b of the ring-shaped multifocal lens 41 and the second partial lens of the mode conversion lens 42. 44 enables the mode to be propagated through the optical fiber 16 in the low-order propagation mode.

  On the other hand, the light beam that has propagated through the optical fiber 16 from the right side of FIG. The propagation mode is converted to. A lens that can be used as the first partial lens 43 of the mode conversion lens 42 is, for example, a lens (ordinary convex lens) shown in FIG.

  The light beam L converted as a substantially parallel light beam is condensed by the center lens 41a of the ring-shaped multifocal lens 41 at the position indicated by the left side M of the mode converter 60, and as a light beam in a low-order propagation mode. This is output from the mode converter 60.

  As described above, the mode converter 60 has a function of converting the propagation mode of the light beam incident in the high-order propagation mode from both the left and right sides of the mode converter 60 into the light beam of the low-order propagation mode.

  If the mode converter 60 is placed at the exit end of the optical fiber 16 serving as a transmission line, a bidirectional optical coupler may be disposed on the right side of the position indicated by M, that is, more than the ring-shaped multifocal lens 41. That is, the bidirectional optical coupler may be configured to include a mode conversion lens 42 that is a compound lens that converts a high-order propagation mode light beam into a low-order propagation mode light beam. If comprised in this way, it will become possible to make the structure of a transmission / reception terminal the same.

  Further, by arranging the mode converter 60 described with reference to FIG. 15 in the middle of the transmission line of the optical link for bidirectional optical communication, the structure of the transmission / reception terminal can be made the same.

  That is, the structure of all transmission / reception terminals constituting the optical link for bidirectional optical communication, so that the transmission signal can be transmitted as a high-order propagation mode light beam, and the low-order propagation mode light beam can be received as a reception signal. A configuration in which a light-emitting element that outputs a transmission signal and a light-receiving element that receives a reception signal are arranged.

  Similarly to the mode converter 60 in the case where the light beam in the low-order propagation mode described above is converted into the light beam in the high-order propagation mode, the light beam in the high-order propagation mode described above is converted into the light beam in the low-order propagation mode. The same applies to the mode converter 60 for conversion as follows. That is, it may be arranged at both ends of the optical fiber 16 that is a transmission line, or may be arranged at an intermediate portion of the optical fiber 16. That is, when configuring an optical link for bidirectional optical communication, one mode converter 60 is connected to each pair of transmission / reception terminals, and the optical fiber 16 that is a transmission line connecting the one set of transmission / reception terminals is connected. What is necessary is just to arrange | position to both ends or an intermediate part.

  A mode converter that converts a light beam in a higher-order propagation mode into a light beam in a lower-order propagation mode, and a mode conversion for converting a light beam in a lower-order propagation mode into a light beam in a higher-order propagation mode, as described with reference to FIG. There is a difference in energy loss of luminous flux due to the conversion. That is, the energy loss due to the propagation mode conversion between the light beam passing through the first partial lens and the light beam passing through the second partial lens is inversely proportional to the area ratio.

  Therefore, when the mode converter is incorporated in the optical link, it is necessary to consider the above-described difference in energy loss. It is necessary to determine the area ratio and the shape of the first partial lens and the second partial lens so that the energy of the signal beam does not fall below the minimum required for reception due to energy loss due to the mode converter.

  Specifically, it is necessary to determine the areas and shapes of the first partial lens 43 and the second partial lens 44 according to the propagation mode of the light beam incident on the mode conversion lens 42.

  When a parallel light beam is incident on a step index type multimode optical fiber while being inclined with respect to the optical axis of the optical fiber, a conical light beam is formed when the light beam is emitted from the optical fiber. However, if the length from the incident end to the exit end of this optical fiber is short, the light intensity on a plane perpendicular to the propagation direction of the emitted light beam is not distributed in a uniform circle. A propagation mode in which light intensity is localized only in a part of the circumference (sometimes referred to as non-uniform propagation mode).

  On the other hand, if the length from the entrance end to the exit end of the optical fiber is sufficiently long, the light intensity on a plane perpendicular to the propagation direction of the outgoing light beam is distributed in a uniform circular shape. This is a propagation mode in which the light intensity is uniformly distributed over the entire circumference (sometimes referred to as a uniform propagation mode).

  Therefore, in the place where the mode converter is inserted, the arrangement of the first lens and the second lens and the light beam propagation mode are different depending on whether the propagation mode of the light flux is the non-uniform propagation mode or the uniform propagation mode as described above. It is necessary to determine the lens shape and the like.

  Next, the shape of the lens shown in FIG. 17 (A) will be described with reference to FIGS. 18 (A) to 18 (C) and FIGS. 19 (A) to 19 (C). FIG. 18A is a three-dimensional cross section called a donut-shaped normal torus. A solid that can be surrounded by the envelope drawn when a circle with a radius r is rotated around a point at a position different from the center of the circle so that the center of the circle draws the circumference of the radius R It is. The torus shown in FIG. 18A is a case where R> r. On the other hand, the torus shown in FIG. 18B is a case where R ≦ r. In the case of R <r, although it is not strictly a torus, for convenience of explanation, here, it will be described as a torus. In addition, when R = r, a square torus is used. In this case, too, a torus is simply described for convenience of explanation.

  Further, when the torus shown in FIG. 18 (B) is cut in half along a plane including a circle with a radius R connecting the centers of circles with a radius r, a solid having the cross-sectional shape shown in FIG. 18 (C) is obtained.

  FIGS. 19 (A) to 19 (D) are three-dimensional cross sections having a deformed shape of the torus having the cross sectional shape shown in FIGS. 18 (A) to 18 (C). Assuming the cross-sectional shape shown in FIGS. 18 (A) to (C), a solid having the cross-sectional shape shown in FIGS. 19 (A) to (D) is assumed as a figure similar to this, and a lens having this shape is The lens is used as shown in FIG. The straight lines indicated by a, b, and c shown in FIGS. 19A to 19D are optical axes of lenses having these shapes.

  When a parallel light beam is incident on a lens having the cross-sectional shape shown in FIGS. 19A to 19D, each part of the light beam is bent in different directions depending on the incident position. In this case, it is the optical axes indicated by a, b and c that determine the traveling direction of the light flux. By adjusting the directions of the optical axes indicated by a, b, and c, the incident angle with respect to the optical axis of the optical fiber 16 can be adjusted, and the function of converting the propagation mode of the conical light beam can be adjusted. Can be given.

  As the lens illustrated in FIG. 17C, a lens having a cross-sectional shape illustrated in FIG. 19D may be used. The lens having the cross-sectional shape shown in FIGS. 19A to 19C is a lens having a condensing function, whereas the lens having the cross-sectional shape shown in FIG. It is a lens that has. In each case of the conversion from the low-order propagation mode to the high-order propagation mode and the conversion from the high-order propagation mode to the low-order propagation mode, a lens having a condensing function is appropriately used, Decide whether to use a lens that has the function of expanding the luminous flux.

  As is clear from the above description, according to LFM communication in which an optical signal of one channel is assigned and communicated for each of a plurality of propagation modes of the multimode optical fiber of the present invention, a plurality of light emitting elements having different emission wavelengths are connected. It is possible to realize multiplex communication that can be realized with only one wavelength without needing to use a multilevel digital signal.

  Also, the output light beam output from the light emitting element as the transmission light beam is incident on the multimode optical fiber, and the outgoing light beam emitted from the multimode optical fiber as the reception light beam is incident on the light receiving element. According to the bidirectional optical coupler having a function, bidirectional communication using a single optical fiber can be realized with only one wavelength without requiring a plurality of light emitting elements having different emission wavelengths.

  Further, according to the mode converter which is arranged at one end or in the middle of the multimode optical fiber of the present invention and converts the propagation mode from the higher order to the lower order propagation mode or from the lower order to the higher order propagation mode, this mode converter In performing bidirectional communication of the invention, transmission and reception can be performed using communication terminals having the same configuration. As a result, it is convenient when an optical link having a star configuration is formed.

It is a figure which shows the far-field image of an incident light beam and an emitted light beam. FIG. 3 is a conceptual diagram illustrating the configuration of an optical link for beam multiplexing communication according to the first embodiment. It is a figure which shows the result of having measured the far-field image of the emitted light of the low order propagation mode and high order propagation mode from an optical fiber. FIG. 3 is a schematic diagram of a first optical coupler configured using a plurality of convex lenses. FIG. 3 is a schematic diagram of a first optical coupler configured using a plurality of concave mirrors. FIG. 3 is a schematic diagram of a first optical coupler configured using a single convex lens. FIG. 3 is a schematic diagram of a first optical coupler configured using a single concave mirror. It is the schematic of the 2nd optical coupler comprised using the ring-shaped multifocal lens. FIG. 6 is a schematic view of a second optical coupler configured using a composite concave mirror. It is the schematic of the 2nd optical coupler comprised using a multifocal element. 6 is a conceptual diagram illustrating a configuration of an optical link for bidirectional optical communication according to Embodiment 2. FIG. It is a conceptual diagram explaining the structure of the optical link for bidirectional | two-way optical communication comprised using the ring-shaped multifocal lens. It is a conceptual diagram explaining the structure of the optical link for bidirectional | two-way optical communication comprised using the composite type concave mirror. It is a conceptual diagram explaining the structure of the optical link for bidirectional | two-way optical communication comprised using the multifocal element. It is a block diagram of a mode converter. It is a composition conceptual diagram of a compound lens. It is a figure which shows the cross-sectional shape of a partial lens. It is a figure which shows the cross-sectional shape of a torus. It is a figure which shows the cross-sectional shape of a partial lens.

Explanation of symbols

10: Sending terminal
11, 110: 1st light emitting element
12, 120: Second light emitting element
13: First optical coupler
14, 15, 22: Collimating lens
16: Step index type multimode optical fiber
17: Second optical coupler
18, 180: First light receiving element
19, 190: Second light receiving element
20: Receiver terminal
21: Array light emitting device
23, 27: Bidirectional optical coupler
33, 37, 41: Ring-shaped multifocal lens
30, 40: Transmission / reception terminal
42: Mode conversion lens
43: 1st partial lens
44: Second part lens
52, 54, 55: concave mirror
53, 57: Composite concave mirror
58, 59: Multifocal element
60: Mode converter

Claims (19)

In an optical link for optical communication that performs optical multiplex communication using a step index type multimode optical fiber,
A plurality of light emitting elements, and a plurality of light receiving elements corresponding to each of the plurality of light emitting elements via the step index type multimode optical fiber,
A first optical coupler that makes the light beam emitted from the light emitting element incident at different angles with respect to the optical axis of the step index type multimode optical fiber;
A second optical coupler for causing light beams emitted at different angles to the optical axis of the step index type multi-mode optical fiber to enter the respective light receiving elements, respectively. Optical link. In the optical link for light flux multiplex communication according to claim 1,
The first optical coupler includes a plurality of convex lenses provided corresponding to the plurality of light-emitting elements, and the plurality of convex lenses includes a respective optical axis and an optical axis of the step index type multimode optical fiber. The light beam for multiplex communication is characterized in that the light beams emitted from the light emitting elements are incident on the step index type multimode optical fiber in the form of parallel light beams, which are arranged at different angles. Link. In the optical link for light flux multiplex communication according to claim 1,
The first optical coupler includes a plurality of concave mirrors provided corresponding to the plurality of light emitting elements, and the concave mirror is formed by an angle formed between each optical axis and the optical axis of the step index type multimode optical fiber. And the concave mirror reflects the outgoing light beam of the light emitting element and makes it incident on the step index type multimode optical fiber in the state of a parallel light beam. Optical link. In the optical link for light flux multiplex communication according to claim 1,
The first optical coupler converts the emitted light beams of the plurality of light emitting elements into parallel light beams, and the parallel light beams are incident on the optical axis of the step index type multimode optical fiber with different incident angles. An optical link for beam multiplexing communication, which is a single convex lens that is incident on a multimode optical fiber. In the optical link for light flux multiplex communication according to claim 1,
The first optical coupler converts the emitted light beams of the plurality of light emitting elements into parallel light beams, and the parallel light beams are incident on the optical axis of the step index type multimode optical fiber with different incident angles. An optical link for beam multiplexing communication, which is a single concave mirror that is incident on a multimode optical fiber. In the optical link for light flux multiplex communication according to claim 1,
The second optical coupler is configured by concentrically arranging a plurality of ring-shaped lenses having different diameters, and spreads from the step index type multimode optical fiber in a conical shape with different apex angles. An optical link for light beam multiplex communication, characterized in that it is a ring-shaped multifocal lens that focuses a plurality of light beams on light receiving elements arranged at different positions. In the optical link for light flux multiplex communication according to claim 1,
The second optical coupler is formed by connecting a plurality of concave mirrors having different focal lengths and reflection directions, and spreads from the step index type multimode optical fiber in a conical shape with different apex angles. An optical link for beam multiplex communication, characterized in that it is a composite concave mirror that reflects a plurality of beams and collects them on separate light receiving elements. In the optical link for light flux multiplex communication according to claim 1,
The second optical coupler is configured by combining a lens and a concave mirror, and reflects and refracts a plurality of light beams spread in a conical shape emitted from the step index type multimode optical fiber with different apex angles. An optical link for beam multiplex communication, characterized in that the optical link is a multifocal element that is focused and incident on separate light receiving elements. In a bidirectional optical communication optical link that performs bidirectional optical communication using a step index type multimode optical fiber,
A light emitting element, a light receiving element, and a light beam output as a transmission light beam from the light emitting element is incident on the step index type multimode optical fiber, and is output from the step index type multimode optical fiber as a received light beam. And a bi-directional optical coupler for allowing the outgoing light beam guided through the step index type multimode optical fiber in the opposite direction in the propagation mode different from the transmission light beam to enter the light receiving element. Optical link for optical communication. The optical link for bidirectional optical communication according to claim 9,
The bidirectional optical coupler is a ring-shaped multifocal lens configured by concentrically arranging a plurality of ring-shaped lenses having different diameters, and the light beam emitted from the light emitting element is used as the step index type multimode optical fiber. Incident light is incident as a parallel light flux, and the light flux emitted from the light-emitting element is focused on the light-receiving element. The light flux is emitted in a conical shape and emitted at a different angle with respect to the optical axis of the step index type multimode optical fiber. An optical link for bidirectional optical communication, wherein the optical link is a bidirectional optical coupler. The optical link for bidirectional optical communication according to claim 9,
The bidirectional optical coupler is a composite concave mirror in which a plurality of concave mirrors having different focal lengths and reflection directions are connected to each other, and the light beam emitted from the light-emitting element is parallel to the step index type multimode optical fiber. The light beam emitted from the light-emitting element is condensed and incident on the light-receiving element with a light beam that is spread in a conical shape and is emitted at a different angle with respect to the optical axis of the step index type multimode optical fiber. An optical link for bidirectional optical communication, which is a directional coupler. The optical link for bidirectional optical communication according to claim 9,
The bidirectional optical coupler is a multifocal element configured by combining a lens and a concave mirror, and the light beam emitted from the light emitting element is incident on the step index type multimode optical fiber as a state of a parallel light beam, The light beam emitted from the light emitting element is a bi-directional optical coupler that condenses and enters the light beam, which is spread in a conical shape, emitted at different angles with respect to the optical axis of the step index type multimode optical fiber. An optical link for bidirectional optical communication, characterized in that The optical link for bidirectional optical communication according to claim 9,
The bidirectional optical coupler is a bidirectional optical coupler including a mode converter configured with a composite lens that converts a low-order propagation mode light beam into a high-order propagation mode light beam. An optical link for bidirectional optical communication. The optical link for bidirectional optical communication according to claim 9,
The bidirectional optical coupler is a bidirectional optical coupler including a mode converter configured with a compound lens that converts a high-order propagation mode light beam into a low-order propagation mode light beam. An optical link for bidirectional optical communication. The optical link for bidirectional optical communication according to claim 9,
A mode converter composed of a compound lens for converting a low-order propagation mode light beam into a high-order propagation mode light beam is provided in the middle portion of the step index type multimode optical fiber. Optical link for optical communication. The optical link for bidirectional optical communication according to claim 9,
A mode converter comprising a compound lens for converting a high-order propagation mode light beam into a low-order propagation mode light beam is provided at an intermediate portion of the step index type multimode optical fiber. Optical link for optical communication. In a method of performing light beam multiplex communication using a step index type multimode optical fiber,
A light emission step of outputting a plurality of light beams, which are optical carriers of a plurality of channels, from a plurality of separate light emitting elements, and
An input step for combining the output light fluxes and inputting the resultant light into a step index type multimode optical fiber;
A waveguide step for guiding the plurality of light beams that are combined and input through the step index type multimode optical fiber in different propagation modes;
A demultiplexing step of demultiplexing a plurality of light beams emitted from the step index type multimode optical fiber for each propagation mode; and a light receiving step of receiving the demultiplexed light beams by a plurality of separate light receiving elements, respectively. A light flux multiplex communication method. In a method of performing bidirectional optical communication using a step index type multimode optical fiber,
Performing transmission from the transmitting / receiving terminal A to the transmitting / receiving terminal B using a light beam propagating in the first propagation mode through the step index type multimode optical fiber;
A step of performing transmission from the transmission / reception terminal B to the transmission / reception terminal A using a light beam propagating in a second propagation mode, which is a propagation mode different from the first propagation mode, on the step index type multimode optical fiber. And a bidirectional optical communication method. The bidirectional optical communication method according to claim 18,
The step of sending from the sending / receiving terminal A to the sending / receiving terminal B is
In the transmission / reception terminal A, a light emission step for outputting a light beam, which is an optical carrier wave of a transmission channel from the transmission / reception terminal A to the transmission / reception terminal B, from a light emitting element A provided in the transmission / reception terminal A;
An input step for inputting the output light flux described above into a step index type multimode optical fiber;
A waveguide step for guiding the above-mentioned light flux through a step index type multimode optical fiber;
In the transmission / reception terminal end, a demultiplexing step for separating the light beam emitted from the optical fiber and the light flux that is the optical carrier wave of the transmission channel from the transmission / reception terminal end toward the transmission / reception terminal back,
Including a light receiving step of receiving a light beam, which is an optical carrier wave of a transmission channel from the transmission / reception terminal A to the transmission / reception terminal B, emitted from the optical fiber by a light receiving element B provided in the transmission / reception terminal B;
The step of sending from the sending / receiving terminal B to the sending / receiving terminal A,
In the transmission / reception terminal B, a light emission step of outputting a light beam, which is an optical carrier wave of a transmission channel from the transmission / reception terminal B to the transmission / reception terminal A, from a light emitting element B included in the transmission / reception terminal B;
An input step for inputting the output light flux described above into a step index type multimode optical fiber;
A waveguide step for guiding the above-mentioned light flux through a step index type multimode optical fiber;
In the transmission / reception terminal A, a demultiplexing step for separating the luminous flux emitted from the optical fiber and the luminous flux that is the optical carrier wave of the transmission channel from the transmission / reception terminal A to the transmission / reception terminal B;
A light receiving step of receiving a light beam, which is an optical carrier wave of a transmission channel from the transmitting / receiving terminal B to the transmitting / receiving terminal A, emitted from the optical fiber by a light receiving element A provided in the transmitting / receiving terminal A Mukko communication method.

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