Optical device, reception module, and coherent receiver

Abstract

Provided are an optical device, a reception module, and a coherent receiver which are capable of performing spatial mode and polarization separation, and are advantageous for miniaturization. A reception module 13 has a mode separation multiplexer 14 and a light receiving element array 15. The mode separation multiplexer 14 has six metasurfaces 21A to 21F arranged apart from each other in a plurality of stages, and receives signal light S and local light Lo subjected to spatial mode multiplexing and polarization multiplexing. Through the metasurfaces 21A to 21F, a plurality of divided signal light components are generated for each signal light component of the signal light S, the local light Lo is divided into a plurality of divided local light beams, and interference light in which the divided signal light components and the divided local light beams are caused to interfere with each other is generated. Through the metasurfaces 21A to 21F, a phase is controlled so that, at the output positions of the divided signal light components for each signal light component, the divided signal light components and the divided local light beams interfere with each other in different phase differences. For each signal light component, the complex amplitude of the signal light component is obtained from each interference light.

Description

Optical device, receiving module and coherent receiver

[0001] The present invention relates to an optical device, a receiving module, and a coherent receiver.

[0002] Polarization-multiplexed coherent communication is known as an optical communication method aimed at increasing transmission capacity. In polarization-multiplexed coherent communication, a transmitter converts a pair of signal lights, each modulated with independent data, into mutually orthogonal polarizations, and transmits the multiplexed signal light through an optical fiber. In a coherent receiver for polarization-multiplexed coherent communication, as described in Non-Patent Document 1, a polarization separator separates the signal light into signal light components of each polarization. Each signal light component is then multiplexed with local light (local light) using a 90° optical hybrid to generate two pairs of interference waves, and two balanced photodiodes detect the interference waves, and the complex amplitude of the transmitted signal light is obtained from the detection results.

[0003] As an optical communication method aiming at further increasing transmission capacity, spatial multiplexing coherent communication in which multiple spatial modes are multiplexed is known. In spatial multiplexing coherent communication, signal light components of multiple spatial modes, each modulated with independent data, are multiplexed, and the multiplexed signal light is transmitted via a multimode optical fiber, a multicore optical fiber, or the like. In the case of spatial multiplexing coherent communication, as in Non-Patent Document 2, a coherent receiver separates the signal light into each signal light component using a mode separator, and generates and detects interference wave pairs for each signal light component in the same manner as described above. Polarization multiplexing is also possible in this spatial multiplexing coherent communication, and in this case, a polarization separator is provided in the coherent receiver to perform polarization separation of the optical signal.

[0004] Hideki Yagi et al., “InP-Based Photodetectors Monolithically Integrated with 90° Hybrid toward Over 400Gb / s Coherent Transmission Systems.” IEICE Trans. Electron. 102-C(4): 347-356 (2019)D. Soma et al., “2.05 Peta-bit / s super-Nyquist-WDM SDM transmission using 9.8-km 6-mode 19-core fiber in full C band,” in Proc. ECOC'15, PDP3.2, 2015.

[0005] As described above, a coherent receiver requires a mode separator to separate spatial modes, a polarization separator to separate polarizations, and an optical circuit such as a 90° optical hybrid to generate an interference wave pair for each signal light component separated by spatial mode and polarization.

[0006] The present invention has been made in view of the above circumstances, and aims to provide an optical device, a receiving module, and a coherent receiver that are capable of performing spatial mode separation and polarization separation and that are advantageous for miniaturization.

[0007] The optical device of the present invention consists of a plurality of metasurfaces arranged at intervals in multiple stages, each of which controls the phase and polarization of incident light.The multiple metasurfaces are configured so that spatially mode and polarization multiplexed multiplexed light is incident on the first stage metasurface, and the optical components of the multiplexed light that have passed through or reflected from the previous stage metasurface are incident on the next stage metasurface, thereby separating the multiplexed light into multiple spatial mode and polarization separated multiplexed light components and outputting them from the final stage metasurface.

[0008] The optical device of the present invention consists of a plurality of metasurfaces arranged at intervals in multiple stages, each of which controls the phase and polarization of incident light.The multiple metasurfaces receive signal light obtained by spatial mode multiplexing and polarization multiplexing of multiple signal light components, and local light.The signal light and local light components that have passed through or reflected from the metasurface of the previous stage are incident on the metasurface of the next stage, whereby the signal light is spatial mode separated and polarization separated, and each of the multiple signal light components is divided into a plurality of first divided light beams and focused at a plurality of different output positions, and the local light is divided into a plurality of second divided light beams and focused at a plurality of output positions, and a plurality of first divided light beams and a plurality of second divided light beams in which the first divided light and the second divided light beam interfere with each other with different phase differences at each output position of each of the multiple signal light components are output from the metasurface of the final stage.

[0009] The optical device of the present invention consists of a plurality of metasurfaces arranged at intervals in multiple stages, each of which controls the phase and polarization of incident light.The multiple metasurfaces are configured such that a multiplexed light, which is a multiplex of one or more signal light components of a signal light and local light of a different mode from the signal light components, is incident on the metasurface of the first stage, and the light components of the multiplexed light that have passed through or reflected from the metasurface of the previous stage are incident on the metasurface of the next stage, thereby dividing the signal light components into a plurality of first divided lights and focusing them at a plurality of different output positions, dividing the local light into a plurality of second divided lights and focusing them respectively at the output positions of the first divided lights, and outputting a plurality of first divided lights and a plurality of second divided lights from the metasurface of the final stage, in which the first divided lights and the second divided lights interfere with each other with different phase differences at each output position for each signal light component.

[0010] The receiving module of the present invention comprises the optical device described above and a plurality of light receiving elements arranged at a plurality of output positions for receiving the interference light.

[0011] The receiving module of the present invention has a propagation layer through which light propagates, and a first reflection film and a second reflection film arranged opposite each other on either side of the propagation layer, and is equipped with a reflection section that repeatedly reflects the optical components of signal light and local light incident from one end of the propagation layer between the first reflection film and the second reflection film while shifting the reflection position toward the other end of the propagation layer, and then emits the optical components from the other end of the propagation layer, and the multiple metasurfaces include the above-mentioned optical device arranged in the optical path of the optical components of the signal light and local light in the propagation layer, and a photodetector array provided on the other end of the propagation layer and having multiple photodetectors that each receive the interference light of the first divided light and the second divided light emitted from the propagation layer.

[0012] A coherent receiver according to the present invention comprises the above-described receiving module and a processing unit that obtains complex amplitudes for a plurality of signal light components based on the light reception results of a plurality of light receiving elements.

[0013] According to the present invention, multiplexed light is transmitted through or reflected by multiple metasurfaces arranged in multiple stages at intervals, so that the multiplexed light is output as multiple multiplexed light components that have been spatially separated and polarized, thereby enabling spatial mode separation and polarization separation in a configuration that is advantageous for miniaturization.

[0014] Furthermore, according to the present invention, signal light in which the signal light components are spatially mode multiplexed and polarization multiplexed and local light, or multiplexed light in which the signal light components of the signal light and local light of a different mode are multiplexed, is incident on multiple metasurfaces arranged in multiple stages at intervals, and the signal light components and local light are each split into multiple parts, and each signal light component is made to interfere by changing the phase difference with the local light, thereby generating interference light that obtains a complex amplitude for each signal light component while achieving compactness.

[0015] 1 is a perspective view showing the configuration of a coherent receiver of a first embodiment; FIG. 2 is a perspective view showing meta-atoms formed on a metasurface; FIG. 3 is an explanatory diagram showing the arrangement of meta-atoms on a metasurface; FIG. 4 is an explanatory diagram showing the output state of multiple split signal light components emitted from a mode splitting / combining device; FIG. 5 is an explanatory diagram showing Jones vectors in forward propagation and backward propagation on each metasurface; FIG. 6 is an image showing the distribution of meta-atom parameters on each optimized metasurface; FIG. 7 is an image showing the optical intensity and phase of each split signal light component and split local light by numerical simulation; FIG. 8 is an explanatory diagram showing the configuration of a coherent receiver of a second embodiment in which the receiving module is unitized; FIG. 9 is an explanatory diagram showing the configuration of a coherent receiver in which a metasurface is arranged in the middle of an optical block.

[0016] [First Embodiment] In FIG. 1 , a coherent receiver (hereinafter simply referred to as a receiver) 10 includes a local light source 12, a mode demultiplexer / multiplexer 14 and a photodetector array 15 that constitute a receiving module 13, and a processing unit 17. This receiver 10 is a receiving device for coherent communications, and signal light S is incident on the mode demultiplexer / multiplexer 14 from a multimode or multicore optical fiber 18. The signal light S is multiplexed light that has been spatially multiplexed and polarization multiplexed. That is, the signal light S is light obtained by multiplexing multiple signal light components (multiplexed light components) with different combinations (modes) of spatial modes and polarization modes. Each signal light component has been quadrature-modulated (IQ-modulated) based on data to be transmitted. The wavelength of the signal light component is, for example, 1.55 μm. As shown in the figure, the direction of incidence of the signal light S on the mode demultiplexer / multiplexer 14 is the Z direction, and two directions that are orthogonal to the Z direction and mutually orthogonal are the X direction and the Y direction.

[0017] In this example, the signal light S is in the fundamental mode LP ０１ and higher mode LP １１ａ , LP １１ｂThe signal light S is multiplexed in three spatial modes, namely, X-polarized wave and Y-polarized wave, and polarization multiplexed in the first and second polarization modes, which are orthogonal to each other, for each of the three spatial modes. Specifically, the first and second polarizations in this example are X-polarized wave and Y-polarized wave, which are linearly polarized waves. Therefore, the signal light S is multiplexed in the fundamental mode LP of the X-polarized wave. Ｘ０１、 Higher-order mode LP Ｘ１１ａ , LP Ｘ１１ｂ The signal light component S Ｘ０１ , S Ｘ１１ａ , S Ｘ１１ｂ and the fundamental mode LP of Y polarization Ｙ０１ , higher mode LP Ｙ１１ａ , LP Ｙ１１ｂ The signal light component S Ｙ０１ , S Ｙ１１ａ , S Ｙ１１ｂ In the following description, the signal light component S Ｘ０１ , S Ｘ１１ａ , S Ｘ１１ｂ , S Ｙ０１ , S Ｙ１１ａ , S Ｙ１１ｂ When there is no need to particularly distinguish between them, they are collectively referred to as signal light components.

[0018] Although the first and second polarizations are linearly polarized X and Y, the orthogonal polarization modes are not limited to this and may be right-handed and left-handed circularly polarized waves or elliptical polarizations, etc. Furthermore, the spatial modes multiplexed into the signal light S are not limited to those described above and may include, for example, higher-order spatial modes or spatial modes at different positions, as in the case of using a multi-core type optical fiber.

[0019] The local light source 12 outputs local light Lo. The receiver 10 in this example employs a homodyne detection system, and the wavelength of the local light Lo is the same as that of the signal light component. The local light source 12 also outputs, for example, X-polarized local light Lo, which is then transmitted via an optical fiber 19 to the fundamental mode LP of X-polarized light. Ｘ０１ The local light Lo is incident on the mode splitter / multiplexer 14. The local light Lo is split into a plurality of split local light beams by the mode splitter / multiplexer 14, as will be described later.

[0020] The mode multiplexer / demultiplexer 14, which serves as an optical device, includes six metasurfaces 21A to 21F. The metasurfaces 21A to 21F are, for example, plate-shaped, with each side having a length of approximately 400 μm, and are arranged parallel to the XY plane. In the following description, when there is no need to distinguish between the metasurfaces 21A to 21F, they will be referred to as metasurface 21.

[0021] The metasurfaces 21A-21F are arranged in multiple stages (six stages in this example) spaced apart in the Z direction. The signal light S and local light Lo are incident on one surface of the first stage metasurface 21A. The final stage metasurface 21F outputs multiple split signal light components converted from the signal light S and multiple split local lights split from the local light Lo, generating interference light by interfering each of the split signal light components with the split local lights. The number of metasurfaces 21 is preferably equal to or greater than the number of mode separations (the number of modes to be separated) in order to improve the accuracy of mode separation of the light components separated and output by the mode separation multiplexer 14. In this example, the number of mode separations is six, since three types of spatial modes (transmission modes) and two polarization modes are separated.

[0022] The optical fiber 18 that inputs the signal light S to the first-stage metasurface 21A and the optical fiber 19 that inputs the local light Lo are arranged in close proximity. Therefore, the signal light S and the local light Lo are incident at different positions on the incident surface of the metasurface 21A, although they are close to each other. In this example, the distance between the axial centers of the optical fibers 18 and 19 is approximately 127 μm. In this example, the exit ends of the optical fibers 18 and 19 are spaced apart from the incident surface of the metasurface 21A by a predetermined distance, and the beams of the signal light S and the local light Lo are expanded and incident on the metasurface 21A. Note that, because the beams of the signal light S and the local light Lo can be expanded by passing them through the first-stage metasurface 21A, the exit ends of the optical fibers 18 and 19 may be arranged in close proximity to the incident surface of the metasurface 21A. Alternatively, instead of the optical fibers 18 and 19 arranged in close proximity, a multi-core optical fiber may be used, and the signal light S and the local light Lo may be incident on the metasurface 21A from different cores.

[0023] The split signal light components are emitted from the metasurface 21F so as to be focused at different output positions set on the light-receiving surface 15a of the light-receiving element array 15 as an output surface. The split local light beams are also emitted from the metasurface 21F so as to be focused at the output positions of the split signal light components. This generates interference light on the light-receiving surface 15a, where the split local light beams interfere with each other for each split signal light component.

[0024] The metasurface 21 is a two-dimensional array of microscopic (subwavelength order) metaatoms (scatterers) that locally change the phase, amplitude, and polarization of incident light, and the two-dimensional distribution of the phase, amplitude, and polarization of light is controlled by adjusting the shape of each metaatom. In this example, the metasurface 21 is a transmissive type that controls the amplitude distribution, phase distribution, and polarization distribution of light that passes through the metasurface 21.

[0025] In this example, the meta-atoms 25 are provided in an elliptical cylindrical shape on the surface of a transparent substrate 26, as shown in FIG. 2. Furthermore, as shown in FIG. 3, the meta-atoms 25 are arranged in a square array on the substrate 26, at each vertex of a square. The meta-atoms 25 are made of a dielectric material (e.g., silicon). While the meta-atoms 25 may be made of other materials, a metasurface 21 in which the meta-atoms 25 are formed of a dielectric material has low optical loss and is advantageous for constructing a mode splitter / multiplexer 14. The substrate 26 is made of a material with high transmittance for the signal light S, such as a glass substrate or a quartz substrate. The period Λ of the meta-atoms 25 (the length of one side of the square in the square array) is set to be equal to or less than the wavelength of the signal light.

[0026] The shape of the meta-atom 25 is not limited to an elliptical cylinder, as long as it can shift the phase of each of the polarization components in two orthogonal directions. For example, it may be a prism with a rectangular or diamond cross section. Also, in this example, the meta-atoms 25 are arranged in a square, but the arrangement is not limited to this and may be, for example, a regular triangular arrangement.

[0027] By adjusting the shape of the meta-atom 25 provided as described above, i.e., the length Da of the major axis, the length Db of the minor axis, and the height h, it is possible to impart a phase difference φa to the polarization component along the major axis of light incident at the position of the meta-atom 25, and a phase difference φb to the polarization component along the minor axis. "Providing a phase difference" here means imparting a phase difference based on the phase of the original light, i.e., phase shifting. Furthermore, by adjusting the tilt θ of the major axis of each meta-atom 25 distributed on the substrate 26 and the phase differences φa and φb imparted by the meta-atom 25, it is possible to convert light incident at each position on the metasurface 21 into light with the desired polarization components.

[0028] As described above, the metasurfaces 21A to 21F are arranged at intervals, and the optical components of the signal light S and local light Lo emitted from the metasurface 21 of the previous stage are incident on the metasurface 21 of the next stage. The interval between the metasurfaces 21 of the previous and next stages is a distance at which the optical components emitted from the previous stage can cause spatial mixing due to free space propagation, and is, for example, a length equivalent to several hundred wavelengths of the signal light S. In this example, the interval between the metasurfaces 21 is 1 mm for the signal light S with a wavelength of 1.55 μm. Note that intermediate layers made of transparent glass, quartz, or the like, which allow light to propagate in free space, may be arranged between the metasurfaces 21 and between the metasurface 21F and the photodetector array 15.

[0029] The mode splitter / multiplexer 14 uses the metasurfaces 21A to 21F to perform mode splitting, light splitting, phase difference control, and polarization conversion, thereby generating multiple split signal light components with equal light intensities from the signal light S, and generating multiple split local lights with equal light intensities from the local light Lo, so that each split signal light component interferes with the split local light under specified conditions.

[0030] In this example, the mode separation is a separation of the signal light S into components of each mode, that is, a separation of the spatial mode components (spatial mode separation) and the polarization mode components (polarization separation) contained in the signal light S. Specifically, in this example, the spatial mode separation is a separation of the fundamental mode LP ０１ and higher mode LP １１ａ , LP １１ｂThe polarization splitting means splitting the signal light S into the X polarization (first polarization) component and the Y polarization (second polarization) component. Therefore, by this mode splitting, the signal light S is split into the signal light component S Ｘ０１ , S Ｘ１１ａ , S Ｘ１１ｂ , S Ｙ０１ , S Ｙ１１ａ , S Ｙ１１ｂ Separate into.

[0031] The optical division splits each signal light component of the signal light S into a plurality of split signal light components. Furthermore, the optical division splits the local light Lo into a plurality of split local light components. The optical division of this local light Lo generates split local light components in the same number as the split signal light components generated. In this example, for six signal light components, four split signal light components are generated for each signal light component, generating 24 split local light components.

[0032] The phase difference control controls the phase of one or both of the split signal light components and the split local light so that the phase difference between the split signal light components and the split local light focused at the same output position is different for each output position for each signal light component. The phase difference is the relative phase difference between the split signal light components and the split local light when they interfere at the output position. Ideally, it is preferable that the phase difference at each output position for each signal light component be shifted at equal intervals.

[0033] In this example, the phase difference is shifted at intervals of 90° (=π / 2), so that the phase differences between the split signal light components and the split local light at each output position for each signal light component are different. That is, at each output position for each signal light component, if the phase difference of the interference between one split signal light component and the split local light is expressed as a relative phase difference with reference (0°), each split signal light component interferes with the split local light at phase differences of 0°, 90°, 180°, and 270°. Accordingly, four split signal light components are generated for each signal light component as described above.

[0034] In addition, in order to cause the split signal light components and the split local light to interfere with each other with the above-mentioned phase differences, in this example, the phases of the four split signal light components focused at each output position are shifted from one another so that the split local light is focused at each output position in the same phase. Therefore, for each signal light component, when the phase of one split signal light component is expressed as the reference (0°), four split signal light components are generated that are focused at the output positions with phases of 0°, 90°, 180°, and 270°.

[0035] In addition, as long as the divided signal light components and the divided local light can be made to interfere with each other with different phase differences at each output position of the signal light components, the phase of either the divided signal light components or the divided local light may be controlled, or both phases may be controlled. For example, the divided signal light components may be focused at each output position in the same phase, and the four divided local light beams for each signal light component may be focused at the output positions with their phases shifted from one another. In this example, the divided signal light components are the first divided light beams, and the divided local light beams are the second divided light beams.

[0036] Polarization conversion is a conversion of polarization modes, and converts the split signal light components and split local light output from the mode splitter / multiplexer 14 into the same polarization mode. This allows the split signal light components and split local light to interfere when multiplexed, and eliminates the influence of differences in sensitivity of the photodetector array 15 due to differences in polarization mode. In this example, when splitting the local light Lo to generate split local light, the mode splitter / multiplexer 14 maintains the polarization mode of the local light Lo and converts the polarization mode of the polarization component of the signal light S to match the polarization mode of the split local light. That is, in this example, since the local light Lo is X-polarized, the Y-polarized component of the signal light S is X-polarized, and each output split signal light component is X-polarized.

[0037] From the viewpoint of causing interference between the split signal light component and the split local light, the polarization mode of the polarization component of the local light Lo may be converted to make the split signal light component and the split local light have the same polarization mode. Also, the split local light to be multiplexed with the split signal light component of the X polarization component may maintain the X polarization of the local light Lo, and the split signal light component of the Y polarization component may maintain the Y polarization, and the split local light to be multiplexed with the split signal light component of the Y polarization component may be converted from X polarization to Y polarization.

[0038] As described above, it is sufficient that the polarization modes of the split signal light components and split local light emitted from the mode splitter / multiplexer 14 are the same, and therefore there are no particular limitations on the polarization mode of the local light Lo at the stage of entering the mode splitter / multiplexer 14. On the other hand, from the viewpoint of simplifying the configuration and design of the mode splitter / multiplexer 14, it is preferable to generate split local light while maintaining the polarization mode of the local light Lo as described above.

[0039] In the above, for the sake of convenience, mode separation, light division, phase difference control, and polarization conversion are described individually, but in the mode separation multiplexer 14, when light passes through the metasurfaces 21A to 21F, multiple split signal light components and multiple split local lights are obtained that have been subjected to mode separation, light division, phase difference control, and polarization conversion for the incident signal light S and local light Lo, and there is no need for mode separation, light division, phase difference control, and polarization conversion to be performed individually or sequentially.

[0040] The function of the mode splitter / multiplexer 14 to focus the split signal light components and split local light at the output position is realized by forming a wavefront using the metasurface 21 similar to that when light passes through a convex lens.

[0041] In this example, the above-mentioned mode demultiplexer / multiplexer 14 demultiplexes the signal light S into signal light components S of six modes, each of which is a combination of spatial modes and polarization modes. Ｘ０１ , S Ｘ１１ａ , S Ｘ１１ｂ , S Ｙ０１ , S Ｙ１１ａ , S Ｙ１１ｂFour split signal light components are generated for each of the split signal light components, and 24 split local light beams, which is equal to the total number of split signal light components, are generated from the local light Lo. The split signal light components and each split local light beam are all X-polarized.

[0042] As shown in Fig. 4, in this example, the mode splitter / multiplexer 14 focuses the split signal light components by arranging them in a matrix on the light receiving surface 15a as an output surface. Ｘ０１ The signal light component S Ｘ０１ From the optical fiber 1, a split signal light component S1a with a relative phase of 0°, a split signal light component S1b with a relative phase of 90°, a split signal light component S1c with a relative phase of 180°, and a split signal light component S1d with a relative phase of 270° are respectively collected.

[0043] These split signal light components S1a to S1d are focused so as to be aligned linearly at predetermined intervals in the column direction (vertical direction in FIG. 4), and are aligned in the order of split signal light component S1a, split signal light component S1c, split signal light component S1b, and split signal light component S1d. Ｙ０１ The signal light component S Ｙ０１ Similarly, divided signal light components S2a to S2d are generated from the optical fiber 11a and 11b, and are condensed so as to be aligned linearly in the row adjacent to the divided signal light components S1a to S1d.

[0044] Hereafter, high-order mode LP Ｘ１１ａ The signal light component S Ｘ１１ａ The split signal light components S3a to S3d generated from the higher-order mode LP Ｙ１１ａ The signal light component S Ｙ１１ａ The split signal light components S4a to S4d generated from the higher-order mode LP Ｘ１１ｂ The signal light component S Ｘ１１ｂ The split signal light components S5a to S5d generated from the higher-order mode LP Ｙ１１ｂ The signal light component S Ｙ１１ｂ The split signal light components S6a to S6d generated from the split signal light components S6a to S6d are similarly focused so as to be aligned in a straight line. The arrangement of the split signal light components can be determined arbitrarily.

[0045] 1, the photodetector array 15 is disposed on the output side of the mode splitter / multiplexer 14 at a predetermined distance from the final-stage metasurface 21F, with its photodetector surface 15a facing the exit surface of the metasurface 21F. Photodetectors 23 are disposed at each output position of the split signal light components on the photodetector surface 15a of the photodetector array 15. The photodetectors 23 are, for example, composed of photodiodes, and receive interference light resulting from interference between the split signal light components and the split local light, and convert the intensity of the interference light into an electrical photodetection signal.

[0046] The processing unit 17 calculates the complex amplitude of each signal light component from the received signal of four interference lights obtained by combining the divided signal light components and the divided local light obtained from one signal light component. For example, the signal light component S Ｘ０１ Regarding the signal light component S, the signal light component S is calculated from a received light signal obtained by receiving interference light corresponding to the divided signal light component S1a whose relative phase is 0° and a received light signal obtained by receiving interference light corresponding to the divided signal light component S1c whose relative phase is 180°. Ｘ０１ In addition, the signal light component S is obtained from a received light signal obtained by receiving interference light corresponding to the divided signal light component S1b whose relative phase is 90° and a received light signal obtained by receiving interference light corresponding to the divided signal light component S1d whose relative phase is 270°. Ｘ０１ The quadrature component (Q component) of

[0047] The shape of each metaatom 25 in the metasurfaces 21A-21F can be determined using the adjoint method (back propagation). Hereinafter, the combination of the spatial mode and polarization mode combining the signal light and the local light will be referred to as mode m (m = 1, 2, ... M (in this example, M = 7)), and k will be the number of stages of the metasurface 21 (k = 1, 2, ... K (in this example, K = 6)). Furthermore, the light consisting of the signal light S and the local light Lo emitted from the optical fibers 18 and 19 will be referred to as input light. In this example, m = 7 is the local light.

[0048] As shown in FIG. 5, the input Jones vector a in(m) (x, y), and the Jones vector at the position (x, y) of the output surface when the input light is propagated (forward propagation) through the metasurface 21 (hereinafter referred to as the output Jones vector) a out (m) The relationship between (x, y) can be expressed as in equation (1).

[0049]

[0050] J in formula (1) k As shown in equation (2), (x, y) is a Jones matrix corresponding to the meta-atom 25 present at each position (x, y) on the kth stage metasurface 21. R(θ(x, y)) in equation (2) is a rotation matrix with the tilt θ of the major axis of the meta-atom 25 at the position (x, y) as the rotation angle. Also, F k is a propagation function representing the free space on the output side of the k-th stage metasurface 21. This propagation function F k is calculated by, for example, the angular spectrum method or the Rayleigh-Sommerfeld method. ０ is the propagation function representing the free space between the optical fibers 18, 19 and the metasurface 21A.

[0051] The shape of each meta-atom 25 in the metasurface 21 can be determined by optimizing it using the adjoint method (backpropagation) so as to maximize the objective function ε shown in Equation (3). tar (m) (x, y) is the target Jones vector at the position (x, y) of the output surface (hereinafter referred to as the target Jones vector). This target Jones vector is the Jones vector that serves as the starting point when backpropagating the metasurface 21. † " denotes the adjoint matrix (complex conjugate transpose) of matrix A.

[0052]

[0053] Output Jones vector a out (m) (x,y), target Jones vector a tar (m)The inner product of the (x,y) adjoint matrix is ​​a out,x (m) (x,y), a tar,x (m) (x, y), and the Y-polarized component is a out,y (m) (x,y), a tar,y (m) When (x, y) is used, it becomes as shown in equation (4). * " denotes the complex conjugate of the complex number A.

[0054]

[0055] The objective function ε is maximized by calculating the gradient shown in Equation (5) and the meta-atom parameter p k (x, y) and iteratively update the meta-atom parameter p that maximizes the objective function ε. k (x,y) is calculated. k (x, y) is the metaatom parameter (φ a,k , φ b,k , θ k The function G() determines the amount of change in the meta-atom parameters, and can be based on an optimization method such as steepest descent, momentum, RMSProp, Adam, etc.

[0056]

[0057] Specifically, the gradient of the objective function ε is the output Jones vector a out (m) (x,y), target Jones vector a tar (m) (x, y), the forward propagation Jones vector a of the kth metasurface 21 k (m) (x,y) and the backpropagation Jones vector b k (m) It can be calculated using equation (8) using (x, y).

[0058]

[0059] The forward propagation Jones vector a in equation (8) k(m) (x, y) is the input Jones vector a of the signal light S and the local light Lo emitted from the optical fibers 18 and 19. in (m) The Jones vector when (x, y) is propagated forward to the input side surface of the kth metasurface 21 is expressed as in equation (9). In addition, the backward propagation Jones vector b k (m) (x,y) is the target Jones vector a tar (m) This is the Jones vector when (x, y) is propagated (back-propagated) to the output side surface of the kth metasurface 21, and is expressed as in equation (10). Note that F in equation (10) k -1 is a propagation function indicating the free space on the output side of the k-th stage metasurface 21 in back propagation.

[0060]

[0061] Furthermore, the meta-atom parameter p in Eq. (8) k At each of (x,y), the Jones matrix J k Differentiating (x, y) gives equations (11) to (13).

[0062] Specifically, by repeating the following steps 1 to 4 until it is determined that the value of the objective function ε has converged to its maximum value, the meta-atom parameter p k (x, y) is calculated. When you first perform step 1, p k The initial values ​​are assigned to (x, y). In addition, three meta-atom parameters (φ a,k , φ b,k , θ k ) is substantially updated, it may be determined that the value of the objective function ε has reached its maximum value.

[0063] Step 1: According to Equation (9), the forward propagation Jones vector a on the input side surface of each metasurface 21 (k = 1, 2, ... K) in forward propagation k (m) (x,y) and the output Jones vector aout (m) (x, y) is calculated. In equation (9), "k = K + 1" is used to calculate the output Jones vector a out (m) (x, y) (= a K+1 (m) (x, y)) can be calculated. Also, the value of the objective function ε is calculated. Step 2: Using equation (10), the backpropagation Jones vector b k (m) Step 3: Using the results of steps 1 and 2, three meta-atom parameters (φ a,k , φ b,k , θ k Step 4: Based on Equation (6), the gradients of the three meta-atom parameters (φ a,k , φ b,k , θ k ) are updated.

[0064] By the above optimization, the metaatom parameters p k (x, y), that is, the phase differences φa, φb and the major axis inclination θ are determined, and the phase differences φa, φb and the major axis inclination θ determined for each meta-atom 25 are converted into the actual shape (size).

[0065] Figure 6 shows the distribution of meta-atom parameters for each metasurface 21 optimized using the above method. Figure 7 also shows the optical intensities and phases of the split signal light components of each signal light component (m = 1 to 6) and the split local light from the local light Lo (m = 7) at the output surface, which were numerically simulated using the distribution of the obtained meta-atom parameters. From these results, it can be seen that each signal light component is separated into split signal light components with relative phases of 0°, 90°, 180°, and 270° and output. It can also be seen that each split signal light component and each split local light have the same optical intensity.

[0066] In the above example, the interval by which the phase difference between the divided signal light components and the divided local light during interference is shifted is 90°, but the interval when the phase difference is shifted at equal intervals is not limited to this and can be, for example, 72° (=2π / 5), 120° (=2π / 3), etc. Furthermore, the interval by which the phase difference is shifted can be different for each mode, for example, for the X-polarized signal light component and the Y-polarized signal light component.

[0067] [Second Embodiment] In the second embodiment, the receiving module is unitized. A multiplexed light obtained by multiplexing signal light and local light is input to a mode demultiplexer / multiplexer. Note that, except for the following description, the second embodiment is the same as the first embodiment, and the same reference numerals are used to designate substantially the same components, and detailed descriptions thereof will be omitted.

[0068] In FIG. 8 , the receiver 50 includes a mode splitter / multiplexer 54 and a light-receiving element array 15, a processing unit 17, and a local light source (not shown), which constitute a receiving module 53. The mode splitter / multiplexer 54 includes an optical block 56, a metasurface array 57, and a reflective film 58. The optical block 56, which serves as a propagation layer, is formed of a transparent material, such as quartz or glass, through which the signal light S, the local light Lo, and their optical components propagate in free space. In this example, the optical block 56 has a rectangular parallelepiped shape with a predetermined thickness. A metasurface array 57 is fixed to one surface 56a of a pair of surfaces facing each other in the thickness direction (the vertical direction in FIG. 8 ) of the optical block 56, and a total reflection type reflective film 58 is provided on the other surface 56b.

[0069] The metasurface array 57 is a single, planar array of reflective metasurfaces 61A-61F. The metasurface array 57 includes a plurality of metaatoms 25 that constitute each of the reflective metasurfaces 61A-61F, a thin film 57a that is common to the reflective metasurfaces 61A-61F, and a total-reflection reflective film 57b. The film 57a is made of, for example, polymer, quartz, or glass. The metaatoms 25 are provided on the surface of the film 57a that faces the optical block 56. The reflective film 57b is provided on the surface of the film 57a that faces away from the optical block 56.

[0070] The reflective metasurfaces 61A to 61F configured as described above can control the two-dimensional distribution of the phase, amplitude, and polarization of light incident on and reflected from them. These reflective metasurfaces 61A to 61F differ only in that they are reflective from the transmissive metasurfaces 21A to 21F of the first embodiment, but have the same function. In the following description, when there is no need to distinguish between the reflective metasurfaces 61A to 61F, they will be referred to as the reflective metasurface 61.

[0071] In the mode demultiplexer / multiplexer 54, the signal light S and the local light Lo are incident on the inside of the optical block 56 from the surface 56b on one end side (left side in FIG. 8 ) of the optical block 56 toward the first-stage reflective metasurface 61A. In this example, the signal light S and the local light Lo are incident on the reflective metasurface 61A at an incident angle greater than 0°. The reflective metasurface 61A reflects the incident optical components of the signal light S and the local light Lo at a reflection angle equal to the incident angle.

[0072] The optical components of the signal light S and local light Lo reflected by the reflective metasurface 61A are reflected by the reflective film 58 and enter the next-stage reflective metasurface 61B. The optical components of the signal light S and local light Lo that enter the reflective metasurface 61B are reflected by the reflective metasurface 61B. Thereafter, the optical components of the signal light S and local light Lo that are reflected by the reflective metasurface 61 are similarly reflected by the reflective film 58 and enter the subsequent-stage reflective metasurface 61. In this way, the optical components of the signal light S and local light Lo are repeatedly reflected between the reflective film 58 and the metasurface array 57 within the optical block 56, shifting the reflection position toward the other end (the right side in Figure 8), and sequentially enter the reflective metasurfaces 61A to 61F.

[0073] In this example, the metasurface array 57 (reflective metasurfaces 61A to 61F) is the first reflective film, and the reflective film 58 is the second reflective film, which together form the reflective section. As described above, the reflective metasurfaces 61A to 61F are arranged in the optical paths of the optical components of the signal light S and local light Lo that are repeatedly reflected and propagated between the metasurface array 57 and the reflective film 58, or the optical components thereof.

[0074] The signal light S and the optical components of the local light Lo reflected by the reflective metasurface 61F, i.e., the multiple split signal light components generated from the signal light S and the multiple split local light beams generated from the local light Lo, are emitted to the outside from the surface 56b on the other end side of the optical block 56. A photodetector array 15 is attached to the portion of the surface 56b from which each split signal light component and each split local light beam are emitted, and interference light between the split signal light components and the split local light beams is received by each photodetector of the photodetector array 15. The photodetector array 15 receives each interference light beam resulting from interference between the split signal light component and the local light Lo. As a result, the complex amplitude of each signal light component is determined by the processing unit 17.

[0075] The above configuration allows for a more compact configuration of the mode separation / multiplexer 54. Furthermore, multiple metasurfaces can be realized with a single metasurface array 57, and the mode separation / multiplexer 54 and the receiving module 53 and receiver 50 including it can be configured with fewer parts.

[0076] In this example, the optical block 56 is used as the propagation layer, but the space between the metasurface array 57 and the reflective film 58 may be hollow. Furthermore, the reflection direction may be controlled by the reflective metasurfaces 61A-61F. For example, the multiplexed light may be incident perpendicularly on the reflective metasurface 61A, reflected by the reflective metasurface 61A in a direction tilted from the normal direction of the incident signal light S and local light Lo components, and then incident on the next-stage reflective metasurface 61B via the reflective film 58. Alternatively, the multiplexed light may be reflected perpendicularly from the reflective metasurface 61F and incident on the photodetector array 15. Alternatively, the metasurface array 57 may be provided on the surface 56b on which the multiplexed light is incident, and only a reflective film may be provided on the surface 56a. Furthermore, in this example, the photodetector array 15 is provided on the surface 56b on the incident side of the multiplexed light, but the split signal light components and split local light may be configured to exit to the outside from the surface 56a opposite the surface 56b. In this case, the light receiving element array 15 may be provided on the surface 56a.

[0077] In this example, the metasurface array 57 is provided only on one surface 56a of the optical block 56, but a metasurface array may also be provided on the other surface 56b. In this case, the surface array provided on the other surface 56b serves as a second reflective film.

[0078] In the metasurface array 57, each reflective metasurface 61 is formed independently of the other on the film 57a, but adjacent reflective metasurfaces 61 may share a partial area with each other. In this case, the beams of the optical components of the signal light S and the local light Lo are incident twice on a partial area shared by one reflective metasurface 61 and the next reflective metasurface 61.

[0079] As shown in FIG. 9, a mode splitter / multiplexer 54A may be configured with multiple transmissive metasurfaces 71A-71F arranged midway through the thickness of an optical block 66. The mode splitter / multiplexer 54A in this example includes an optical block 66, a metasurface array 67, and reflective films 68 and 69. The metasurface array 67 is configured by arranging metasurfaces 71A-71F in a planar, integrated configuration, with multiple metaatoms (not shown) constituting each of the metasurfaces 71A-71F provided on a substrate 67a. The metasurface array 67 is disposed, for example, between a pair of blocks 72 and 73 that constitute the optical block 66, thereby being positioned midway through the thickness of the optical block 66.

[0080] A reflective film 68 serving as a first reflective film is provided on one surface 66a facing the optical block 66 in the thickness direction, and a reflective film 69 serving as a second reflective film is provided on the other surface 66b. Metasurfaces 71A-71F are disposed in the optical path along which the signal light S, local light Lo, and their optical components incident from one end side (left side in FIG. 9 ) of the optical block 66 are repeatedly reflected between the reflective films 68 and 69 while shifting their reflection positions toward the other end side (right direction in FIG. 9 ). As a result, the incident signal light S, local light Lo, and their optical components are sequentially transmitted through the metasurfaces 71A-71F.

[0081] In the above embodiments, the case where the propagation mode of a multi-mode type optical fiber is used as the spatial mode to be multiplexed has been described, but the spatial mode may be, for example, the propagation mode of each core of a multi-core optical fiber or the propagation mode in free space. That is, it is possible to separate light incident from a multi-core optical fiber by the propagation mode of each core, or to separate light incident from free space by the propagation mode.

[0082] In addition, in each of the above embodiments, the signal light is multiplexed light, but it is also possible to configure the mode splitter / multiplexer, i.e., the first-stage metasurface, to multiplex one or more signal light components of the signal light with local light having a different mode from the one or more signal light components, i.e., a different spatial mode, polarization mode, or a combination thereof. For example, the fundamental mode LP of the X-polarized light may be input to the mode splitter / multiplexer, i.e., the first-stage metasurface. Ｘ０１ The local light Lo and the higher-order mode LP of X polarization Ｘ１１ａ and Y-polarized higher-order mode LP Ｙ１１ａ It is also possible to input the multiplexed light obtained by multiplexing each of the signal light components of the first and second optical components into a mode separation multiplexer, and to separate them by mode.

[0083] In each of the above embodiments, an optical device consisting of multiple metasurfaces is described as being configured as a mode separation / multiplexer, but by omitting some functions, it is possible to configure it so that, for example, multiplexed light that has been spatially multiplexed and polarization multiplexed is converted into multiple multiplexed light components that have been spatially separated and polarization separated and emitted from the final stage metasurface, or so that the multiple multiplexed light components are split and emitted from the final stage metasurface.

[0084] 10, 50 Coherent receiver 12 Local light source 13, 53 Receiving module 14, 54, 54A Mode splitter / multiplexer 15 Light receiving element array 15a Light receiving surface 17 Processing unit 21A to 21F, 71A to 71D Metasurface 56, 66 Optical block 57, 67 Metasurface array 57b, 58, 68, 69 Reflective film 61A to 61F Reflective metasurface

Claims

1. An optical device comprising a plurality of metasurfaces arranged at intervals in multiple stages, each of which controls the phase and polarization of incident light, wherein the plurality of metasurfaces are arranged such that spatial-mode-multiplexed and polarization-multiplexed multiplexed light is incident on the first stage metasurface, and optical components of the multiplexed light that have been transmitted through or reflected by the previous stage metasurface are incident on the next stage metasurface, thereby separating the multiplexed light into multiple spatial-mode-separated and polarization-separated multiplexed light components and outputting them from the final stage metasurface.

2. The optical device described in claim 1, characterized in that the multiple metasurfaces convert each of the multiple multiple light components into multiple first split lights, and emit the multiple first split lights from the final stage metasurface and focus them at multiple output positions that are different from each other.

3. An optical device comprising a plurality of metasurfaces arranged at intervals in a plurality of stages, each of which controls the phase and polarization of incident light, wherein signal light obtained by spatial-mode multiplexing and polarization multiplexing of a plurality of signal light components and local light are incident on the plurality of metasurfaces, and the optical components of the signal light and the local light that have transmitted or reflected from the metasurface of the previous stage are incident on the metasurface of the next stage, whereby the signal light is spatial-mode-separated and polarization-separated into a plurality of signal light components, each of which is divided into a plurality of first divided light beams and focused at a plurality of different output positions, the local light is divided into a plurality of second divided light beams and focused at each of the plurality of output positions, and the plurality of first divided light beams and the plurality of second divided light beams in which the first divided light and the second divided light interfere with each other with different phase differences at each output position of each of the plurality of signal light components are output from the metasurface of the final stage.

4. The optical device according to claim 3, wherein the phase difference between the first divided light and the second divided light at each output position of each of the plurality of signal light components is shifted at equal intervals.

5. The optical device according to claim 3, characterized in that the phase difference between the first split light and the second split light at each output position of each of the plurality of signal light components is 0°, 90°, 180°, or 270° relative to one of the phase differences as a reference.

6. The optical device described in claim 3, characterized in that the local light is in a single polarization mode, and the multiple metasurfaces maintain the polarization mode of the local light to emit the multiple second split lights, and convert the multiple first split lights into the same polarization mode as the local light and emit them.

7. The optical device according to claim 3, characterized in that the signal light and the local light are incident on different positions of the metasurface in the first stage.

8. The optical device described in claim 3, characterized in that the local light, which has a spatial mode different from that of the signal light, is spatial mode multiplexed onto the signal light, and the multiple metasurfaces spatial mode separate and polarization separate the signal light into the multiple signal light components and spatial mode separate the local light.

9. An optical device comprising a plurality of metasurfaces arranged at intervals in a plurality of stages, each of which controls the phase and polarization of incident light, wherein the plurality of metasurfaces are arranged such that a multiplexed light obtained by multiplexing one or more signal light components of a signal light with local light having a different mode from the signal light components is incident on a first-stage metasurface, and the optical components of the multiplexed light that have passed through or been reflected by the previous-stage metasurface are incident on a next-stage metasurface, thereby splitting the signal light components into a plurality of first divided lights and focusing them at a plurality of different output positions, splitting the local light into a plurality of second divided lights and focusing them respectively at the output positions of the first divided lights, and emitting the plurality of first divided lights and the plurality of second divided lights in which the first divided lights and the second divided lights interfere with each other with different phase differences at each output position for each signal light component from the final-stage metasurface.

10. The optical device according to claim 9, wherein the phase difference between the first divided light and the second divided light at each output position for each of the signal light components is shifted at equal intervals.

11. The optical device according to claim 9, characterized in that the phase differences between the first and second split lights at each output position for each signal light component are 0°, 90°, 180°, or 270° relative to one of the phase differences as a reference.

12. The optical device described in claim 9, characterized in that the multiplexed light has multiple signal light components multiplexed in spatial mode or polarization, and the multiple metasurfaces generate the multiple first split lights for each of the multiple signal light components obtained by spatial mode separation or polarization separation of the multiplexed light.

13. A receiving module comprising: an optical device according to any one of claims 3 to 12; and a plurality of light receiving elements arranged at the plurality of output positions for receiving interference light.

14. A coherent receiver comprising: a receiving module according to claim 13; and a processing unit for obtaining complex amplitudes for the plurality of signal light components based on the light receiving results of the plurality of light receiving elements.

15. An optical device as described in any one of claims 3 to 12, characterized in that it comprises: a propagation layer through which light propagates; and a reflecting section having a first reflecting film and a second reflecting film arranged opposite each other across the propagation layer, and which repeatedly reflects the optical components of the signal light and the local light incident from one end side of the propagation layer between the first reflecting film and the second reflecting film while shifting the reflection position of the optical components of the signal light and the local light toward the other end side of the propagation layer, and emits the optical components from the other end side of the propagation layer; and the multiple metasurfaces are arranged in the optical paths of the optical components of the signal light and the local light in the propagation layer.

16. A receiving module comprising: an optical device as described in claim 15; and a light receiving element array provided on the other end side of the propagation layer and having a plurality of light receiving elements for receiving interference light between the first divided light and the second divided light emitted from the propagation layer.

17. A coherent receiver comprising: a receiving module according to claim 15; and a processing unit for obtaining complex amplitudes of the plurality of signal light components based on the light receiving results of the plurality of light receiving elements.

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