Abstract:
A device for coupling a multicore/multimode fiber that can transmit a large quantity of information. This multicore/multimode fiber coupling device has a first fiber group ( 11 ), a first light converging system ( 13 ), a first mode converter ( 15 ), a second fiber group ( 21 ), a second light converging system ( 23 ), and light converging system ( 33 ) for multicore fibers. By means of the first mode converter ( 15 ), light from the first fiber group ( 11 ) is subjected to mode conversion en masse. A space coupling system ( 33 ) for multicore fibers transmits light derived from the second fiber group ( 21 ) and light derived from the first fiber group ( 11 ) subjected to mode conversion to the multicore fibers ( 31 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a multi-core multi-mode fiber coupling device configured to simultaneously convert light beams in a single mode from a plurality of fibers into a higher order mode to thereby have ability to effectively couple the plurality of single mode fibers and a multi-core multi-mode fiber, and a method of coupling a plurality of optical fibers and a multi-core multi-mode fiber using such a device. 
       BACKGROUND ART 
       [0002]    In recent years, the limit in transmission capacity of optical fibers has become a problem, and studies on Space Division Multiplexing (SDM) are thus being intensively made in order to solve such a problem. For this reason, multi-core fibers having a plurality of cores in a single fiber, and/or multi-mode fibers capable of transmitting a plurality of propagation modes through a single core are subject to study. 
         [0003]    In the Japanese Patent Application Laid Open No. 2013-182222 publication (the following Patent reference 1), there is a multi-core fiber coupling device disclosed. This multi-core fiber coupling device is configured to couple a plurality of single mode fibers and a multi-core fiber. 
         [0004]    In a non-Patent reference 1, there is a technology adapted for stretching a bundle fiber in a tapered shape to couple a multi-core fiber of 7-core and single mode fibers disclosed. With this technology, a plurality of single mode fibers are bundled and stretched to implement fusion splicing to the multi-core fiber. 
       PRIOR ART REFERENCE 
     Patent Reference 
       [0005]    Patent reference 1: Japanese Patent Application Laid Open No. 2013-182222 publication 
       Non-Patent Reference 
       [0006]    Non-patent reference 1: B. Zhu, et. al “Space-, Wavelength-, Polarization-Division Multiplexed Transmission of 56-Tb/s over a 76.8-km Seven-Core Fiber,” in Optical Fiber Communication Conference, OSA Technical Digest (CD)(Optical Society of America, 2011), paper PDPB7. 
       SUMMARY OF THE INVENTION 
     The Problems to be Solved by the Invention 
       [0007]    As described above, the limit in the transmission capacity of the optical fiber is considered to be a problem. For this reason, it is desirable to develop a multi-core multi-mode fiber coupling device which is able to transmit a large number of information, and to propose a multi-core multi-mode fiber coupling method using such a device. 
       Means for Solving the Problems 
       [0008]    The present invention is fundamentally based on the finding that outgoing light beams from a plurality of multi-core couplers are subjected to mode multiplexing using an optical coupler (optical mode combiner) so that the multi-core multi-mode fiber coupling can be made. 
         [0009]    The present invention relates to a multi-core multi-mode fiber coupling device. This device comprises: a first group  11  of fibers, which will be hereinafter simply referred to as a first fiber group  11 ; a first light converging system  13 , a first mode converter  15 ; a second group  21  of fibers, which will be hereinafter simply referred to as a second fiber group  21 ; a second light converging system  23 ; and a spatial coupling system  33  for multi-core fiber, which will be hereinafter simply referred to as a multi-core fiber spatial coupling system  33 . The first mode converter  15  simultaneously implements thereat mode conversion to light beams from the first fiber group  11 . The multi-core fiber spatial coupling system  33  multiplexes light beams originating from the first fiber group  11 , which have been subjected to mode conversion, and light beams originating from the second fiber group  21  to transmit the light beam or beams thus obtained to a multi-core fiber  31 . 
         [0010]    The first light converging system  13  is an optical system for converging a group of outgoing light beams from the first fiber group  11 . The first mode converter  15  is an optical device for converting the mode of the group of outgoing light beams from the first fiber group  11 , which have been converged by means of the first light converging system  13 , into a first mode. The second light converging system  23  is an optical system for converging a group of outgoing light beams from the second fiber group  21 . 
         [0011]    The multi-core fiber spatial coupling system  33  is an optical system for multiplexing the group of light beams from the first mode converter  15  and the group of light beams from the second mode converter  23  to guide the multiplexed light beam or beams into the multi-core fiber  31 . 
         [0012]    When the first mode is the base mode, respective light beams, which have been converged by means of the first light converging system  13 , are passed through the first mode converter  15  which provides a distribution to allow a difference between phases adjacent in a spatial light in correspondence with the propagation mode within the optical fiber to be π (180°) to thereby have ability to perform conversion into a higher order mode. The multi-core multi-mode fiber coupling device of the present invention simultaneously implements mode conversion to a plurality of outgoing light beams from the first fiber group  11  by converging light beams into the first mode converter  15 . It is preferable that the first mode converter  15  is a phase plate arranged at a position in which the group of outgoing light beams from the first fiber group  11  are allowed to coincide with each other by the first light converging system  13 . This position may be also a position for realizing an optimum mode conversion efficiency as will be described later. 
         [0013]    A more preferred example of the multi-core multi-mode fiber coupling device is a device further comprising: a third group  41  of fibers, which will be hereinafter simply referred to as a third fiber group  41 ; a third light converging system  43 ; and a third mode converter  45 . The third light converging system  43  is an optical system for converging a group of outgoing light beams from the third fiber group  41 . The third mode converter  45  is an optical device for performing mode conversion into a second mode of the group of outgoing light beams from the third fiber group  41 , which have been converged by means of the third light converging system  43 . 
         [0014]    In the case of the above-mentioned example, the multi-core fiber spatial coupling system  33  guides the group of outgoing light beams from the first mode converter  15 , the group of outgoing light beams from the second light converging system  23  and the group of outgoing light beams from the third mode converter  45 , into the multi-core fiber  31 . 
         [0015]    The present invention also provides a multi-core multi-mode fiber coupling method using the above-described multi-core multi-mode fiber coupling device. This method comprises steps described below. 
         [0016]    A group of outgoing light beams are emitted from the first fiber group  11 . The group of outgoing light beams from the first fiber group  11  are converged by means of the first light converging system  13 . The group of outgoing light beams from the first fiber group  11 , which have been converged by means of the first light converging system  13 , are subjected to mode conversion into the first mode by means of the first mode converter  15 . 
         [0017]    The group of outgoing light beams are emitted from the second fiber group  21 . The group of outgoing light beams from the second fiber group  21  are converged by means of the second light converging system  23 . The group of outgoing light beams from the first mode converter  15  and the group of outgoing light beams from the second light converging system  23  are guided into the multi-core fiber  31  by means of the multi-core fiber spatial coupling system  33 . 
       Advantageous Effects 
       [0018]    In accordance with the present invention, since multi-core multi-mode coupling device can be provided by means of a plurality of multi-core couplers and optical coupler (optical mode combiner), it is possible to realize multi-core multi-mode fiber coupling with lesser number of parts or components. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a block diagram illustrating an example of the basic configuration of a multi-core multi-mode fiber coupling device. 
           [0020]      FIG. 2  is a view for explaining an example of a mode converter. 
           [0021]      FIG. 3  is a block diagram illustrating a preferred example of the multi-core multi-mode fiber coupling device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    Preferred embodiments for carrying out the present invention will now be described with reference to the attached drawings. It is to be noted that the present invention is not limited to embodiments described below, but may include an embodiment or embodiments which has or have been modified or changed as occasion demands within the scope that the person skilled in the art knows from the embodiments described below. 
         [0023]    The present invention relates to a multi-core multi-mode fiber coupling device. The multi-core multi-mode fiber coupling device is configured to couple light beams from a plurality of light sources with a multi-core multi-mode fiber. Namely, such a device refers to a device adapted to guide light beams caused to be in a multi-mode including a plurality of higher order mode into respective cores of a multi-core fiber having a plurality of cores in a single optical fiber. There is no need for all cores included in the multi-core fiber to be used for optical information communication, but, e.g., any one of the central core and/or peripheral cores may be used for detection, and any feedback may be employed in advance as occasion demands. 
         [0024]      FIG. 1  is a block diagram illustrating an example of the basic configuration of a multi-core multi-mode fiber coupling device of the present invention. As illustrated in  FIG. 1 , this device comprises first fiber group  11 , first light converging system  13 , first mode converter  15 , second fiber group  21 , second light converging system  23 , and multi-core fiber spatial coupling system  33 . 
         [0025]    The first fiber group  11  refers to a group of two optical fibers or more which are provided at positions spatially away from each other. An example of the optical fibers constituting the first fiber group is a single mode fiber. 
         [0026]    The first light converging system  13  is an optical system for converging a group of outgoing light beams from the first fiber group  11 . An example of the first light converging system  13  is a prism or a mirror for guiding a plurality of outgoing light beams from the first fiber group  11  into the first mode converter  15  (e.g., phase plate). When the first light converging system  13  is a mirror, an optical path is adjusted so that light beams from a plurality of optical fibers existing spatially away from each other arrive at a predetermined position of a wavelength plate. In this way, a plurality of light beams from the first fiber group  11  are guided to a predetermined position of the first mode converter  15  by means of the first light converging system  13 . However, in the case of an arrangement which can conduct a guide to a predetermined position of the first mode converter  15  without relying on the first light converging system  13 , it is not necessarily required to use the first light converging system  13 . 
         [0027]    The first mode converter  15  is an optical device for converting the mode of a group of outgoing light beams from the first fiber group  11 , which have been converged by means of the first light converging system  13 , into the first mode. An example of the first mode converter  15  is a phase plate. It is preferable that the first mode converter  15  is a phase plate arranged in a position where the group of outgoing light beams from the first fiber group  11  coincide with each other by means of the first light converging system  13 . In the first mode converter  15 , light beams included in the group of outgoing light beams from the first fiber group  11  may be simultaneously subjected to mode conversion, thereby making it possible to easily attain the multi-core multi-mode fiber coupling. 
         [0028]    The mode converter is known as disclosed in, e.g. the Japanese Patent Application Laid Open No. 2009-047784 publication, and the Japanese Patent Application Laid Open No. 2010-122688 publication. The mode converter can convert light beams of the base mode into light beams of any higher order mode. An output from the first fiber has ordinarily basic (base) mode (TEM 00 ). Light beams of the basic mode are subjected to mode conversion as occasion demands at the first mode converter  15 . An example of the mode after it has undergone mode conversion is a first order mode (TEM 01  or TEM 10 ). Any mode (e.g., TEM 11  or TEM 02 ) except for the above may be used. On the other hand, it is preferable that a group of outgoing light beams from the first mode converter  15 , which arrives at a multi-core multi-mode fiber, are different in the mode from any other group of light beams. 
         [0029]    For example, when three kinds of groups of light beams are input into the multi-core multi-mode fiber, it is preferable to employ three modes of the base mode (the case where, e.g. an output from the single mode fiber is not subjected to mode conversion), TEM 01  and TEM 10 . Such modes are introduced into the multi-core multi-mode fiber, and any mode may be employed if it can be separated by using known means in a multi-core multi-mode fiber of the receiving side. 
         [0030]    A preferred example of the multi-core multi-mode fiber coupling device is an example in which light beams are converged on a predetermined position of the first mode converter  15  by means of the first light converging system  13 .  FIG. 2  is a view for explaining an example of the mode converter. As illustrated in  FIG. 2 , this phase plate  16  indicates that there is a transparent medium having a specific refractive index in an adjacent intensity distribution in a spatial light in correspondence with the propagation mode within an optical fiber arranged, in order to allow a difference between phases of intensities adjacent in the spatial light in correspondence with the propagation mode within the optical fiber to be π (180°) to provide a physical optical path difference corresponding to a phase difference at that wavelength. In this drawing, since light beams  19 , which are arranged so that they are uniformly irradiated onto respective thin and thick parts  17  and  18 , have optical path lengths different from each other, mode conversion will be performed. The configuration of the mode converter is not limited to the configuration described above. However, with this method, the traveling direction of light beams in the base mode from the single mode fiber is adjusted by means of the optical system, thus making it possible to easily and simultaneously perform mode conversion of these plural light beams in the base mode. 
         [0031]    Ordinarily, the multi-core fiber has a plurality of cores in positions symmetrical with respect to the central core. Moreover, the phase plate is such that the boundary between the thin part  17  and the thick part  18  linearly exists. In the present invention, light beams from the fiber, which have been once converged at this boundary, are guided into respective cores of the multi-core. 
         [0032]    The second light converging system  23  is an optical system for converging a group of outgoing light beams from the second fiber group  21 . In this example, such a second mode converter adapted to perform mode conversion of the mode of the group of outgoing light beams from the second fiber group  21 , which have been converged by the second light converging system  23 , is not indispensable. This is because it is sufficient that any light signal in the base mode is included in the multi-core multi-mode fiber. On the other hand, the mode of a group of outgoing light beams from the second fiber group  21 , which have been converged by means of the second light converging system  23 , may be subjected to mode change or conversion by means of the second mode converter. 
         [0033]    The multi-core fiber spatial coupling system  33  is an optical system for guiding a group of outgoing light beams from the first mode converter  15  and a group of outgoing light beams from the second light converging system  23  into the multi-core fiber  31 . The multi-core fiber spatial coupling system  33  guides respective plural light beams, which are included in the group of rays of outgoing light beams from the first mode converter  15  and the group of outgoing light beams from the second light converging system  23 , into a corresponding one or ones of a plurality of cores of the multi-core multi-mode fiber. An example of such an optical system is an optical system in the multi-core fiber coupling device disclosed in the Japanese Patent Application Laid Open No. 2013-182222 publication. 
         [0034]    An example of the multi-core fiber spatial coupling system  33  is configured to comprise, as illustrated in  FIG. 1 , a first relay lens to which a group of outgoing light beams from the first mode converter  15  are incident, and a beam splitter which guides light beams from the first relay lens and a group of outgoing light beams from the second light converging system  23  into the multi-core multi-mode fiber. An output from the beam splitter is propagated into the multi-mode fiber via a multi-mode fiber coupling lens, and respective ones of the group of light beams will be thus propagated into objective cores. The relay lens refers to a lens or a lens system for transmitting an image which has been formed at an optical system located forward towards further backward. Mismatch in size between a predetermined higher order mode in which multi light beams have been converted by the phase plate and the homogeneous mode in the fiber is optimized by means of the relay lens so that the maximum coupling efficiency can be provided. 
         [0035]    As described above, light beams included in the group of outgoing light beams from the first fiber group  11  are simultaneously subjected to mode conversion at the first mode converter  15 , thereby making it possible to easily attain multi-core multi-mode fiber coupling. 
         [0036]    The light beams which have been emitted from the multi-core multi-mode fiber  31  are separated by a multi-core fiber coupling lens (multi-core fiber separation lens)  61  so that they are transmitted into a plurality of fibers  65  by means of an optical system  63 . 
         [0037]    The present invention also provides a multi-core multi-mode fiber coupling method using the above-described multi-core multi-mode fiber coupling device. This method comprises steps described below. 
         [0038]    A group of outgoing light beams are emitted from the first fiber group  11 . The group of outgoing light beams from the first fiber group  11  are converged by means of the first light converging system  13 . The group of outgoing light beams from the first fiber group  11 , which have been converged by means of the first light converging system  13 , are subjected to mode conversion into the first mode by means of the first mode converter  15 . 
         [0039]    A group of outgoing light beams are emitted from the second fiber group  21 . The group of outgoing light beams from the second fiber group  21  are converged by means of the second light converging system  23 . The group of outgoing light beams from the first mode converter  15  and the group of outgoing light beams from the second light converging system  23  are guided into the multi-core fiber  31  by means of the multi-core fiber spatial coupling system  33 . 
         [0040]      FIG. 3  is a block diagram illustrating a preferred example of the multi-core multi-mode coupling device. The example illustrated in  FIG. 3  further comprises, in addition to the components of the previously mentioned multi-core multi-mode coupling device, a third group of fibers  41  which will be hereinafter simply referred to as a third fiber group  41 , a third light converging system  43  and a third mode converter  45 . By using the same principle as described above, it is possible to guide a group of light beams having a larger number of modes into the multi-core multi-mode fiber. The third light converging system  43  is an optical system for converging a group of outgoing light beams from the third fiber group  41 . The third mode converter  45  is an optical device for converting the mode into a second mode of a group of outgoing light beams from the third fiber group  41 , which have been converged by means of the third light converging system  43 . The third fiber group  41 , the third light converging system  43  and the third mode converter  45  have configurations similar to those of the first fiber group  11 , the first light converging system  13  and the first mode converter  15 , respectively. 
         [0041]    In the case of this example, the multi-core fiber spatial coupling system  33  guides a group of outgoing light beams from the first mode converter  15 , a group of outgoing light beams from the second light converging system  23 , and a group of outgoing light beams from the third mode converter  45  into the multi-core fiber  31 . 
         [0042]    The present invention also provides a multi-core multi-mode fiber coupling method using the above-described multi-core multi-mode fiber coupling device. This method comprises steps described below. 
         [0043]    A group of outgoing light beams are emitted from the first fiber group  11 . The group of outgoing light beams from the first fiber group  11  are converged by means of the first light converging system  13 . The group of outgoing light beams from the first fiber group  11 , which have been converged by means of the first light converging system  13 , are subjected to mode conversion into the first mode by means of the first mode converter  15 . 
         [0044]    A group of outgoing light beams are emitted from the second fiber group  21 . The group of outgoing light beams from the second fiber group  21  are converged by means of the second light converging system  23 . 
         [0045]    A group of outgoing light beams are emitted from the third fiber group  41 . The group of outgoing light beams from the third fiber group  41  are converged by means of the third light converging system  43 . The group of outgoing light beams from the third fiber group  41 , which have been converged by means of the third light converging system  43 , are subjected to mode conversion into the second mode (the third mode when light beams of the second fiber group have been subjected to mode conversion into the second mode) by means of the third mode converter  45 . The group of outgoing light beams from the first mode converter  15 , the group of outgoing light beams from the second light converging system  23  and the group of outgoing light beams from the third mode converter  45  are guided into the multi-core fiber  31  by means of the multi-core fiber spatial coupling system  33 . 
       INDUSTRIAL APPLICABILITY 
       [0046]    The present invention can be utilized in the field of the optical fiber communication using spatial division multiplexing and multi-core multi-mode fiber. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0047]      11  . . . First fiber group 
         [0048]      13  . . . First light converging system 
         [0049]      15  . . . First mode converter 
         [0050]      21  . . . Second fiber group 
         [0051]      23  . . . Second light converging system 
         [0052]      31  . . . Multi-core fiber 
         [0053]      33  . . . Multi-core fiber spatial coupling system 
         [0054]      41  . . . Third fiber group 
         [0055]      43  . . . Third light converging system 
         [0056]      45  . . . Third mode converter 
         [0057]      61  . . . Multi-core fiber coupling lens 
         [0058]      63  . . . Optical system 
         [0059]      65  . . . Fiber