JP6182098B2 - Mode separation device, mode multiplexing device, mode separation system, and mode multiplexing system - Google Patents

Description

  The present invention relates to a mode multiplexing and demultiplexing technique in the field of optical communication.

  In recent years, in optical fiber communication, spatial multiplexing (mode multiplexing) transmission technology has attracted attention in addition to wavelength multiplexing and polarization multiplexing as a technology for increasing the transmission capacity compared to the prior art. An existing optical network using a single-mode fiber (SMF) separates an optical signal having a required number of wavelengths from a wavelength-multiplexed optical signal transmitted in the SMF, and converts it into a wavelength-multiplexed optical signal. On the other hand, it is constituted by an optical wavelength demultiplexing device capable of multiplexing optical signals of a necessary number of wavelengths.

  In the future, when the mode multiplexing transmission technology is applied to the network, each network base separates the required number of mode optical signals from the mode multiplexed optical signal, and the mode multiplexed optical, as well as the wavelength multiplexed optical signal. It is required that a required number of modes of optical signals can be multiplexed with the signal. As techniques for performing such mode multiplexing and separation, techniques described in Non-Patent Documents 1 and 2 have been known so far. Non-Patent Document 1 describes a technique of multiplexing mode light generated by a mode converter using a beam splitter (BS). As described in Non-Patent Document 2, a technique for spatially separating light in a specific mode in an arbitrary angle direction using a volume hologram is described.

C. Koebele, M. Salsi, L. Milord, R. Ryf, C. Bolle, P. Sillard, S. Bigo, and G. Charlet, "40km transmission of five mode division multiplexed data streams at 100Gb / s with low MIMO -DSP complexity ", ECOC2011, Th.13.C.3, (2011). A. Okamoto, K. Morita, Y. Wakayama, J. Tanaka, and K. Sato, "Mode division multiplex communication technique based on dynamic volume hologram and phase conjugation," Photonics in Europe 2010, proc. Vol.7716, (2010 ). B. E. A. Saleh, M. C. Teich, "Basic Optical Engineering 1", Morikita Publishing, June 2006

  However, in the above-described prior art, multiplexing and demultiplexing of modes capable of realizing low loss and low crosstalk is not realized unlike the optical wavelength demultiplexing device. Here, FIG. 1 is a diagram illustrating a configuration example of a conventional mode multiplexing apparatus. In the mode multiplexing apparatus 10 of FIG. 1, optical signals emitted from SMFs prepared for the number of modes to be multiplexed are converted into desired modes by a mode converter. Thereafter, optical signals of a plurality of modes output from the mode converter are combined into one beam using a plurality of beam splitters (BS), and the obtained beam is transmitted to a multimode fiber (MMF :) for transmission. Multi-mode fiber). In this way, optical signals in a plurality of modes can be multiplexed and transmitted through a single optical fiber.

  In the mode multiplexing apparatus 10, since an optical signal of each mode causes a power loss of about 3 dB every time it passes through one BS, it is desirable that the number of BSs through which the optical signal passes is small. However, in the mode multiplexing apparatus 10, as the number of modes to be multiplexed increases, the number of necessary BSs also increases. As a result, a power loss of 1 mode (the number of passing BSs) × 3 dB occurs. It should be noted that such power loss can occur not only in mode multiplexing but also in mode separation as the number of necessary BSs increases.

  The present invention has been made in view of the above-described problems. An object of the present invention is to provide a technique for reducing a loss that occurs when mode multiplexing and demultiplexing of an optical signal is performed in a mode demultiplexer and a mode demultiplexer.

  The present invention can be realized as a mode separation device, for example. The mode separation device according to one aspect of the present invention includes a first beam splitter that divides incident light into light that is transmitted in the direction of the first optical path and light that is reflected in the direction of the second optical path; The light transmitted through the first beam splitter and propagating through the first optical path is incident, and the light reflected by the first beam splitter and propagated through the second optical path is transmitted through the first beam splitter. The light that is provided at a position that is incident from a different direction from the light that has propagated through the first optical path and reflects the light that has propagated through the first optical path toward the first output port and the direction of the second output port The light that has propagated through the second optical path is divided into light that is transmitted in the direction of the first output port and light that is reflected in the direction of the second output port. A second beam splitter, wherein the first and second optical paths A spatial distribution of light propagating through the first or second optical path provided on the second beam splitter and the first or second optical path for combining and outputting the propagated light, Reversing means for reversing with respect to a predetermined axis, and when the spatial distribution is reversed by the reversing means, the light in the first mode in which the phase distribution does not change, and the spatial distribution is reversed by the reversing means. In this case, the light of the second mode whose phase distribution is reversed is incident on the first beam splitter in the direction of the first optical path in a multiplexed state, and the light of the first mode and the second mode is incident on the first beam splitter. The light is output from the first and second output ports in a separated state by the multiplexing by the second beam splitter.

  The present invention can be realized as a mode multiplexing device, for example. The mode multiplexing device according to one aspect of the present invention divides light incident in the direction of the first optical path into light that is transmitted in the direction of the first optical path and light that is reflected in the direction of the second optical path. A first beam splitter that splits light incident in the direction of the second optical path into light reflected in the direction of the first optical path and light transmitted in the direction of the second optical path; The light transmitted through the first beam splitter and propagating through the first optical path is incident, and the light reflected by the first beam splitter and transmitted through the second optical path is transmitted through the first optical path. It is provided at a position that is incident from a different direction from the light that has propagated through the optical path, and reflects the light that has propagated through the first optical path toward the first output port and the second output port. The light that has been divided into transmitted light and propagated through the second optical path is converted into the first light. A second beam splitter that divides the light transmitted in the direction of the output port and the light reflected in the direction of the second output port, and combines the light propagating through the first and second optical paths; The spatial distribution of the light propagating through the first or second optical path provided on the second beam splitter and the first or second optical path is output with respect to a predetermined axis. Reversing means for reversing the light, and the first mode light whose phase distribution does not change when the spatial distribution is reversed by the reversing means is directed in the direction of the first optical path with respect to the first beam splitter. And the second mode light whose phase distribution is inverted when the spatial distribution is inverted by the inverting means is incident on the first beam splitter in the direction of the second optical path, and The light in the first and second modes is By multiplexing by second beam splitter, and wherein the output from said first output port in a multi-state.

  ADVANTAGE OF THE INVENTION According to this invention, the loss which arises when performing mode multiplexing and isolation | separation of an optical signal in a mode separation apparatus and a mode multiplexing apparatus can be reduced.

The figure which shows the structural example of a mode multiplexing apparatus. The figure for demonstrating the principle of operation of a Mach-Zehnder interferometer. The figure which shows notionally the structure and operation | movement of the mode multiplexing apparatus and mode separation apparatus which concern on one Embodiment. The figure which shows an example of operation | movement in the case of isolate | separating the structure of the mode separation apparatus which concerns on one Embodiment, and the light of the same LP mode. The figure which shows an example of operation | movement in the case of isolate | separating the structure of the mode separation apparatus which concerns on one Embodiment, and the light of the same LP mode. The figure which shows the phase change example of the light of each propagation mode at the time of image inversion by the image inversion function based on one Embodiment. The figure which shows an example of the operation | movement in the case of isolate | separating the light of the structure of the mode separation apparatus which concerns on one Embodiment, and different LP mode. The figure which shows an example of a structure and operation | movement of the mode multiplexing apparatus which concerns on one Embodiment. The figure which shows an example of the mode separation system which isolate | separates the light of five modes based on one Embodiment. The figure which shows an example of the mode multiplexing system which multiplexes the light of five modes based on one Embodiment. The figure which shows an example of the mode separation system which isolate | separates the light of five modes based on one Embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

<Overview>
The embodiment of the present invention is characterized in that a Mach-Zehnder interferometer (for example, see Non-Patent Document 3) is used for mode multiplexing and demultiplexing. Specifically, with respect to one of the two optical paths in the Mach-Zehnder interferometer, a function for inverting the spatial distribution of the propagating light with respect to a predetermined axis (hereinafter also referred to as “image inversion function”). .) Is added. As a result, it is possible to multiplex and separate modes without causing any loss in principle for light input (incident) to the Mach-Zehnder interferometer.

  First, the operation principle of the Mach-Zehnder interferometer will be described with reference to FIG. As shown in FIG. 2, in the Mach-Zehnder interferometer 20, the light input from the input port and incident on the beam splitter (BS) 1 is divided into transmitted light and reflected light, and is divided into two optical paths (optical path 1 and optical path 2). Are propagated respectively. Further, the two lights respectively propagated in the optical paths 1 and 2 are combined again at the BS 2 and interfere with each other.

  In the Mach-Zehnder interferometer 20, the power of light emitted from the output ports A and B changes due to the effect of interference depending on the phase difference between the two lights when they are multiplexed by the BS2. Here, when the phase difference between the two lights propagating through the optical paths 1 and 2 from the output port A is 0, the two lights are strengthened in the same phase at the output port A. . In this case, the phase difference between the two lights when the two lights propagating through the optical paths 1 and 2 are emitted from the output port B is π. Fit (cancel each other). As a result, in principle, light is emitted only from the output port A, and light is not emitted from the output port B (that is, the intensity of light emitted from the output port B is 0).

  On the other hand, when the phase difference between the two lights propagating through the optical paths 1 and 2 from the output port A is π, the two lights are weakened in the output port A with opposite phases ( Cancel each other). In this case, the phase difference between the two lights when the two lights propagating through the optical paths 1 and 2 are emitted from the output port B is 0. Therefore, at the output port B, the two lights have the same phase and become stronger. Fit. As a result, in principle, light is emitted only from the output port B, and no light is emitted from the output port A (that is, the intensity of the light emitted from the output port A is 0).

  In the present embodiment, the characteristics of the Mach-Zehnder interferometer described above are used for mode multiplexing and separation. Specifically, when two modes of light are input to the Mach-Zehnder interferometer, for one mode (the first mode), the optical paths 1 and 2 (the first and second optical paths) propagated 2 respectively. The phase difference between the two lights when the two lights exit from the output port A is set to be zero. For the other mode (second mode), the phase difference between the two lights when the two lights propagating through the optical paths 1 and 2 are emitted from the output port A is set to π. As a result, light in the first mode is emitted only from the output port A, and light in the second mode is emitted only from the output port B. In principle, mode separation is realized without causing loss. it can.

  In this embodiment, in order to make the Mach-Zehnder interferometer function as a mode separation device, an image inversion function is added to the optical path 2 of the Mach-Zehnder interferometer, as shown in FIG. Further, as the first mode, a mode in which the phase distribution does not change when the spatial distribution of light is inverted by the image inversion function is selected, and as the second mode, the phase distribution is acquired when the spatial distribution is inverted by the image inversion function. Select the mode in which is reversed. Thus, for the first mode, the phase difference when the two lights from the optical paths 1 and 2 are emitted from the output port A is set to 0, while for the second mode, the light from the optical paths 1 and 2 It is possible to realize the phase difference when the two lights are emitted from the output port A as π.

  Further, as shown in FIG. 3B, the Mach-Zehnder interferometer can function as a mode multiplexing device with the same configuration as when the Mach-Zehnder interferometer functions as a mode separation device. In this case, the first mode light may be incident on the BS 1 in the direction of the optical path 1, and the second mode light may be incident on the BS 1 in the direction of the optical path 2. As a result, the light in the first and second modes is emitted in a state of being multiplexed from the optical path 2 to the output port in the direction that transmits the BS2.

<Mode separation>
Next, FIG. 4 is a diagram illustrating an example of the configuration and operation of the mode separation device according to the embodiment. As shown in FIG. 4, the mode separation device 40 corresponds to the configuration of the Mach-Zehnder interferometer with the image inversion function added to the optical path 2 as described above. In the present embodiment, the image inversion function corresponds to a function of inverting the spatial distribution of light propagating in the optical path 2 with respect to a predetermined axis as will be described later. This image inversion function can be realized by a plurality of mirrors (mirrors M2 and M3) that sequentially reflect light propagating in the optical path 2, as shown in FIG. Below, with reference to FIG. 5, the structure of the mode separation apparatus 40 is demonstrated more concretely.

  The mode separation device 40 includes BS1 and BS2, a mirror M1 on the optical path 1, a mirror M2 on the optical path 2, and an image inverting unit 50 provided on the optical path 2. BS 1 divides incident light (input light) into light that is transmitted in the direction of optical path 1 and light that is reflected in the direction of optical path 2. BS2 is a position where light transmitted through BS1 and propagating through optical path 1 is incident, and light reflected by BS1 and propagated through optical path 2 is different from the direction of light propagating through optical path 1 It is provided at the incident position. The BS 2 divides the light that has propagated through the optical path 1 into light that is reflected toward the output port A and light that is transmitted toward the output port B. Further, the BS 2 divides the light propagating in the optical path 2 into light that is transmitted in the direction of the output port A and light that is reflected in the direction of the output port B. In this way, the BS 2 combines and outputs the light propagated through the optical paths 1 and 2. The image inverting unit 50 is provided on the optical path 2 and performs image inverting with respect to light propagating in the optical path 2. Reflected by the mirrors M1, M2, M3, and BS2 provided in the optical paths 1 and 2 and the multiplexing portion thereof, the light propagating through the optical path 1 and the light propagating through the optical path 2 are output ports at the same position and angle. The light is emitted to A and the output port B and interferes at both output ports.

  Further, in the mode separation device 40, the optical path difference between the optical paths 1 and 2 between BS1 and BS2 is that the two lights in the first mode described above that have propagated through the optical paths 1 and 2 are output port A. It is adjusted (determined) so that the phase difference between the two lights when combined is zero.

  In the mode separation device 40, the first and second mode lights described above are incident on the BS 1 in the direction of the optical path 1 in a multiplexed state. As a result, the light in the first and second modes propagates along the optical paths 1 and 2 and is output from the output ports A and B in a separated state by multiplexing by the BS 2.

  Here, FIG. 5 shows a case where light of two modes, LP11a and LP11b, which are a mode and b mode of the same propagation mode (LP mode), is input to the mode separation device 40 from the transmission fiber. It is shown as an example. Note that LP11a and LP11b correspond to two degenerate modes in the same propagation mode.

  Further, the image inverting unit 50 has an image inverting function in the horizontal direction, that is, as shown in FIG. 5, the spatial distribution of light in the LP11a and LP11b modes with respect to the axis along the vertical direction. Invert each one. As a result, the phase distribution of the LP11a mode light corresponding to the first mode does not change before and after the image inversion, and the phase distribution of the LP11b mode light corresponding to the second mode is inverted by the image inversion. Thus, the axis used for image inversion by the image inverting unit 50 is such that the light phase distribution does not change when the spatial distribution of light in the LP11a mode (first mode) is inverted with respect to the axis, and The phase distribution of light is determined to be reversed when the spatial distribution of light in the LP11b mode (second mode) is reversed with respect to the axis.

  As a result, when the LP11a mode light is multiplexed at BS2 (output port A), the two lights from the optical paths 1 and 2 have the same phase and are output while strengthening each other. On the other hand, at the output port B, since these two lights are in opposite phases, the output from the output port B is zero. In addition, when the light in the LP11b mode is multiplexed at BS2 (output port A), the two lights from the optical paths 1 and 2 have opposite phases, and the output from the output port A becomes zero. On the other hand, at the output port B, these two lights have the same phase and are output while strengthening each other.

  As described above, according to the present embodiment, it is possible to separate light of two modes of LP11a and LP11b, which are a mode and b mode of the same LP mode, without causing any loss in principle. .

  Note that image reversal by the image reversing unit 50 can be realized by controlling the number of reflections of light by the mirrors provided in the optical paths 1 and 2 and BS1 and BS2. Specifically, the number of reflections in the optical path 1 by the BS1 and BS2 and the mirror until the light incident on the BS1 is output from the output port A or B, and the number of reflections in the optical path 2 by the BS1, BS2 and the mirror, Is determined to be an odd number. For example, in FIG. 5, since the light emitted from the output port A is reflected twice in the optical path 1 and three times in the optical path 2, the difference in the number of reflections is 1 (odd number). Further, since the light emitted from the output port B is reflected once in the optical path 1 and four times in the optical path 2, the difference in the number of reflections is 3 (odd number).

  In addition, the mode separation device 40 is not limited to the LP11a and LP11b modes, and mode separation can be realized in the same manner as long as it is a combination of the same LP mode a and b modes. As shown in FIG. 6, when image inversion is performed on light in the same LP mode (LP11, LP21, and LP31 modes in FIG. 6) in the a and b modes, one of the a and b modes is obtained. The phase distribution of light does not change, and the other phase is inverted. Therefore, not only the LP11a and LP11b modes, but the combination of the a and b modes of the same LP mode, the mode separation can be performed by the mode separation device 40 without causing any loss in principle.

  Further, the mode separation device 40 can similarly realize mode separation not only for two degenerate modes in the same LP mode, such as the LP11a and LP11b modes, but also for different LP modes. FIG. 7 shows an example in which the mode separation device 40 separates the LP11 and LP21 modes as examples of different LP modes. As shown in FIG. 7, when image inversion is performed on LP11 and LP21 mode light, the phase distribution of LP11 mode light does not change, and the phase distribution of LP21 mode light is inverted. In this way, any combination of different LP modes can be used as long as the phase distribution does not change in the light of one mode due to image inversion and the phase distribution of the light of the other mode is reversed. It is possible to perform mode separation without causing any loss.

<Mode multiplexing>
Next, FIG. 8 is a diagram illustrating an example of the configuration and operation of the mode multiplexing apparatus according to an embodiment. As shown in FIG. 8, the mode separation device 80 corresponds to the configuration of the Mach-Zehnder interferometer with an image inversion function added to the optical path 2 as described above. Is equivalent to

  In the mode multiplexer 80, the above-described first mode light is incident on the BS 1 in the direction of the optical path 1 (from the input port A), and the above-described second mode light is in the direction of the optical path 2 with respect to the BS 1 (From input port B). As a result, the light in the first and second modes propagates in the optical paths 1 and 2 and is output from the output port (in the direction of transmitting from the optical path 2 to BS2) in a multiplexed state by multiplexing by the BS2. .

  According to the present embodiment, as in the case of mode separation, mode multiplexing (multiplexing) can be performed for the a and b modes of the same LP mode and different LP modes without causing loss in principle. Is possible.

<Other embodiments>
By using a plurality of mode separation devices 40 and mode multiplexing devices 80 described in the above embodiments, it is possible to separate and multiplex light in three or more modes. FIG. 9 shows a mode separation system that separates light of five modes (LP01, LP11a, LP11b, LP21a, LP21b) emitted from an optical fiber (multimode fiber (MMF)) through which light of a plurality of modes can propagate. An example is shown. In FIG. 9, the mode separation device 40 is shown as an image inversion MZI (Mach-Zehnder interferometer).

  In addition to the two image inversion MZIs 40, the mode separation system shown in FIG. 9 includes BS91 and BS92 for dividing the light of the five modes into a number of lights corresponding to the number of different LP modes. The light split (branched) at BS 91 and BS 92 is input to two image inversion MZIs 40. Thereby, a and b mode light in the same LP mode can be separated and output without causing any loss in principle.

  Further, by switching the input and output of the mode separation system shown in FIG. 9, a mode multiplexing system that multiplexes light of five modes (LP01, LP11a, LP11b, LP21a, LP21b) is realized as shown in FIG. it can. The mode multiplexing system shown in FIG. 10 multiplexes (multiplexes) the light of the same LP mode a and b mode out of the five modes of light without causing any loss in principle. Can be output to the MMF.

  Furthermore, as shown in FIG. 11, by providing (connecting) a plurality of image inversion MZIs 40 in multiple stages, not only the a and b modes of the same LP mode but also different LP modes can be separated without using a BS. As a result, it is possible to separate light of a plurality of modes propagating in the MMF in principle without causing loss.

  The various embodiments described above can contribute to the realization of a spatial multiplexing (mode multiplexing) transmission technique that increases the transmission capacity by improving the space utilization efficiency of the optical fiber. By using these embodiments, it is possible to modulate and demodulate independent signals for each mode in the optical fiber. As a result, the theoretical communication capacity is (transmission capacity per mode) × (number of modes), and an increase in the capacity of the optical network can be expected.

  In addition, according to the mode demultiplexing technique described in the various embodiments described above, a required number of modes of optical signals are separated from the mode multiplexed optical signal at each network site, and the mode multiplexed optical signal is separated from the mode multiplexed optical signal. A required number of modes of optical signals can be multiplexed. That is, it is possible to handle the mode in the mode multiplexed optical signal in the same manner as the wavelength in the conventional wavelength multiplexed optical signal.

Claims (19)

A first beam splitter that divides incident light into light that is transmitted in the direction of the first optical path and light that is reflected in the direction of the second optical path;
The light transmitted through the first beam splitter and propagating through the first optical path is incident, and the light reflected by the first beam splitter and propagated through the second optical path is transmitted through the first beam splitter. The light that is provided at a position that is incident from a different direction from the light that has propagated through the first optical path and reflects the light that has propagated through the first optical path toward the first output port and the second output port. The light that is transmitted in the direction is divided, and the light that has propagated through the second optical path is divided into light that is transmitted in the direction of the first output port and light that is reflected in the direction of the second output port A second beam splitter that combines and outputs the light propagating through the first and second optical paths; and
Reversing means provided on the first or second optical path and for reversing the spatial distribution of light propagating through the first or second optical path with respect to a predetermined axis,
The first mode light whose phase distribution does not change when the spatial distribution is inverted by the inverting means and the second mode light whose phase distribution is inverted when the spatial distribution is inverted by the inverting means are multiplexed. Is incident on the first beam splitter in the direction of the first optical path,
The first and second mode lights are respectively output from the first and second output ports in a separated state by multiplexing by the second beam splitter. . The optical path difference between the first and second optical paths between the first and second beam splitters is that the two lights in the first mode that have propagated through the first and second optical paths, respectively, The mode separation device according to claim 1, wherein the phase difference between the two lights when combined at the first output port is determined to be zero. The axis used for reversing the spatial distribution of light by the reversing means is such that the phase distribution of light does not change when the spatial distribution of light in the first mode is reversed with respect to the axis, and the second The mode separation device according to claim 1, wherein the phase distribution of the light is determined to be reversed when the spatial distribution of the light in the mode is reversed with respect to the axis.   4. The mode separation device according to claim 1, wherein the inversion unit includes a plurality of mirrors that sequentially reflect light propagating in the second optical path. 5. The number of mirrors provided in the inverting means is such that the light incident on the first beam splitter is output from the first or second output port and the first and second beam splitters and the first beam splitter. The number of reflections in the first optical path by the mirror provided on the first optical path, and the second frequency by the first and second beam splitters and the mirror provided on the second optical path. The mode separation device according to claim 4, wherein the difference from the number of reflections in the optical path is determined to be an odd number.   6. The mode separation device according to claim 1, wherein the first and second modes are two degenerate modes in the same propagation mode. 6.   The mode separation device according to any one of claims 1 to 5, wherein the first and second modes are different propagation modes.   The first and second output ports are configured such that the light propagating through the first optical path and the light propagating through the second optical path are emitted to the first and second output ports at the same position and angle. The mode separation device according to claim 1, wherein a mirror on a second optical path and the second beam splitter are provided. A mode separation system that separates three or more modes of light,
A plurality of beam splitters for dividing the light of the three or more modes into a number of lights corresponding to the number of different propagation modes of the three or more modes;
A plurality of mode separation devices according to any one of claims 1 to 8,
The light split by the plurality of beam splitters is incident on the first beam splitter of each of the plurality of mode separation devices,
The plurality of mode separation devices separate and output light of two degenerated modes in different propagation modes. A mode separation system that separates three or more modes of light,
A plurality of mode separation devices according to any one of claims 1 to 8,
The mode separation system, wherein the plurality of mode separation devices are provided in multiple stages, and sequentially separate and output two lights of different modes. Light that is incident in the direction of the first optical path is divided into light that is transmitted in the direction of the first optical path and light that is reflected in the direction of the second optical path, and is incident in the direction of the second optical path A first beam splitter that splits light into light reflected in the direction of the first optical path and light transmitted in the direction of the second optical path;
The light transmitted through the first beam splitter and propagating through the first optical path is incident, and the light reflected by the first beam splitter and propagated through the second optical path is transmitted through the first beam splitter. The light that is provided at a position that is incident from a different direction from the light that has propagated through the first optical path and reflects the light that has propagated through the first optical path toward the first output port and the second output port. The light that is transmitted in the direction is divided, and the light that has propagated through the second optical path is divided into light that is transmitted in the direction of the first output port and light that is reflected in the direction of the second output port A second beam splitter that combines and outputs the light propagating through the first and second optical paths; and
Reversing means provided on the first or second optical path and for reversing the spatial distribution of light propagating through the first or second optical path with respect to a predetermined axis,
The light of the first mode whose phase distribution does not change when the spatial distribution is reversed by the reversing means is incident on the first beam splitter in the direction of the first optical path, and the spatial distribution is obtained by the reversing means. The second mode light whose phase distribution is inverted when is inverted is incident on the first beam splitter in the direction of the second optical path,
The mode multiplexing device, wherein the light in the first and second modes is output from the first output port in a multiplexed state by multiplexing by the second beam splitter. The optical path difference between the first and second optical paths between the first and second beam splitters is that the two lights in the first mode that have propagated through the first and second optical paths, respectively, The mode multiplexing device according to claim 11, wherein the phase difference between the two lights when combined at the first output port is determined to be zero. The axis used for reversing the spatial distribution of light by the reversing means is such that the phase distribution of light does not change when the spatial distribution of light in the first mode is reversed with respect to the axis, and the second The mode multiplexing device according to claim 11 or 12, wherein the phase distribution of the light is determined to be reversed when the spatial distribution of the light in the mode is reversed with respect to the axis.   14. The mode multiplexing device according to claim 11, wherein the reversing unit includes a plurality of mirrors that sequentially reflect light propagating in the second optical path. The number of mirrors provided in the inverting means is such that the light incident on the first beam splitter is output from the first or second output port and the first and second beam splitters and the first beam splitter. The number of reflections in the first optical path by the mirror provided on the first optical path, and the second frequency by the first and second beam splitters and the mirror provided on the second optical path. The mode multiplexing apparatus according to claim 14, wherein the difference from the number of reflections in the optical path is determined to be an odd number.   16. The mode multiplexing apparatus according to claim 11, wherein the first and second modes are two modes that are degenerated in the same propagation mode.   The mode multiplexing apparatus according to claim 11, wherein the first mode and the second mode are different propagation modes.   The first and second output ports are configured such that the light propagating through the first optical path and the light propagating through the second optical path are emitted to the first and second output ports at the same position and angle. The mode multiplexer according to any one of claims 11 to 17, wherein a mirror on a second optical path and the second beam splitter are provided. A mode multiplexing system that multiplexes three or more modes of light,
A plurality of mode multiplexing devices according to any one of claims 11 to 18,
A plurality of beam splitters for combining and outputting the light output from the plurality of mode multiplexing devices,
The plurality of mode multiplexing devices multiplex and output light of two degenerated modes in different propagation modes, respectively.

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