Optical device, reception module and coherent receiver
The optical device uses metasurfaces to perform spatial mode and polarization separation in a miniaturized form, addressing the need for compact coherent receivers by efficiently separating and interfering multiplexed light components.
Patent Information
- Application Number
- JP2023215158
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-12-20
- Publication Date
- 2025-07-02
AI Technical Summary
Existing coherent receivers require separate components for spatial mode and polarization separation, as well as optical circuits like 90° optical hybrids, which hinder miniaturization.
A miniaturized optical device comprising multiple metasurfaces arranged in stages that perform both spatial mode and polarization separation by controlling the phase and polarization of incident light, allowing for efficient separation and interference of multiplexed light components.
Achieves spatial mode and polarization separation with a compact configuration, enabling efficient generation of interference light for complex amplitude detection while minimizing device size.
Smart Images
Figure 2025098788000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to an optical device, a receiving module, and a coherent receiver.
Background Art
[0002] As optical communication aiming at increasing the transmission capacity, polarization multiplexing coherent communication is known. In polarization multiplexing coherent communication, on the transmission side, a pair of signal lights modulated with independent data are respectively converted into polarizations orthogonal to each other, and the signal light multiplexing each polarization is transmitted through an optical fiber. In the coherent receiver of polarization multiplexing coherent communication, as in Non-Patent Document 1, the signal light is polarization-separated into signal light components for each polarization by a polarization separator, and for each signal light component, for example, generation of two sets of interference wave pairs by multiplexing with local light (local emission) by a 90° optical hybrid, and detection of the interference wave pairs by two sets of balanced photodiodes are performed, and the complex amplitude of the transmitted signal light is obtained from the detection result.
[0003] As optical communication aiming at further increasing the transmission capacity, spatial multiplexing coherent communication multiplexing a plurality of spatial modes is known. In spatial multiplexing coherent communication, signal light components of a plurality of spatial modes modulated with independent data are multiplexed, and the multiplexed signal light is transmitted through a multimode optical fiber, a multicore optical fiber, or the like. In the case of spatial multiplexing coherent communication, in the coherent receiver, as in Non-Patent Document 2, the signal light is separated into each signal light component by a mode separator, and for each signal light component, generation and detection of interference wave pairs are performed in the same manner as above. Also in this spatial multiplexing coherent communication, polarization multiplexing is possible, and in this case, in the coherent receiver, a polarization separator is provided to perform polarization separation of the optical signal.
Prior Art Documents
Non-Patent Documents
[0004]
Non-Patent Document 1
Non-Patent Document 2
Summary of the Invention
Problems to be Solved by the Invention
[0005] As described above, in a coherent receiver, a mode separator for separating spatial modes and a polarization separator for performing polarization separation are required. In addition, for one signal light component separated by spatial mode and polarization, an optical circuit such as a 90° optical hybrid for generating an interference wave pair is required.
[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a miniaturized optical device, a receiving module, and a coherent receiver capable of performing spatial mode separation and polarization separation.
Means for Solving the Problems
[0007] The optical device of the present invention is composed of a plurality of metasurfaces arranged in multiple stages at intervals, each of which controls the phase and polarization of incident light. The plurality of metasurfaces are configured such that multiplexed light that is spatially mode multiplexed and polarization multiplexed is incident on the first-stage metasurface, and the optical components of the multiplexed light that has passed through or been reflected by the previous-stage metasurface are incident on the next-stage metasurface, thereby separating the multiplexed light into a plurality of multiplexed light components that are spatially mode separated and polarization separated and emitting them from the last-stage metasurface.
[0008] The optical device of the present invention is composed of a plurality of metasurfaces arranged in multiple stages at intervals, each of which controls the phase and polarization of incident light. The plurality of metasurfaces are configured such that signal light that is spatially mode multiplexed and polarization multiplexed with a plurality of signal light components and local light are incident, and the optical components of the signal light and local light that have passed through or been reflected by the previous-stage metasurface are incident on the next-stage metasurface, thereby separating the signal light into a plurality of signal light components that are spatially mode separated and polarization separated, dividing each of the plurality of signal light components into a plurality of first split lights and condensing them at a plurality of different output positions, dividing the local light into a plurality of second split lights and condensing them at the plurality of output positions respectively, and emitting a plurality of first split lights and a plurality of second split lights from the last-stage metasurface, where the first split lights and the second split lights interfere with each other at different phase differences at each output position of each of the plurality of signal light components.
[0009] The optical device of the present invention is composed of a plurality of metasurfaces arranged in multiple stages at intervals, each of which controls the phase and polarization of incident light. The plurality of metasurfaces are configured such that multiplexed light that multiplexes one or more signal light components of the signal light and local light having a different mode from the signal light component is incident on the first-stage metasurface, and the optical components of the multiplexed light that has passed through or been reflected by the previous-stage metasurface are incident on the next-stage metasurface, thereby dividing the signal light component into a plurality of first split lights and condensing them at a plurality of different output positions, dividing the local light into a plurality of second split lights and condensing them at the output positions of the first split lights respectively, and emitting a plurality of first split lights and a plurality of second split lights from the last-stage metasurface, where the first split lights and the second split lights interfere with each other at different phase differences at each output position for each signal light component.
[0010] The receiving module of the present invention includes the above optical device and a plurality of light receiving elements arranged at a plurality of output positions for receiving the interfering 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 to each other with the propagation layer interposed therebetween, and reflects the optical components of the signal light and the local light incident from one end side of the propagation layer while shifting the reflection position toward the other end side of the propagation layer, and repeatedly reflects between the first reflection film and the second reflection film and then emits from the other end side of the propagation layer. The plurality of metasurfaces include the above optical device arranged in the optical paths of the optical components of the signal light and the local light in the propagation layer, 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 the interfering light of the first split light and the second split light emitted from the propagation layer, respectively.
[0012] The coherent receiver of the present invention includes the above receiving module and a processing unit for obtaining the complex amplitudes of a plurality of signal light components based on the light receiving results of the plurality of light receiving elements.
Advantages of the Invention
[0013] According to the present invention, multiplexed light is output as a plurality of multiplexed light components that are spatially mode-separated and polarization-separated by transmitting or reflecting the multiplexed light through or from a plurality of metasurfaces arranged in a plurality of stages with intervals therebetween. Therefore, spatial mode separation and polarization separation can be achieved with a configuration that is advantageous for miniaturization.
[0014] Further, according to the present invention, multiplexed light in which signal light components are spatially mode-multiplexed and polarization-multiplexed with local light, or multiplexed light in which signal light components of the signal light and local light having different modes are multiplexed, is incident on a plurality of metasurfaces arranged in a plurality of stages with intervals therebetween, the signal light components and the local light are each divided into a plurality, and each signal light component is interfered with by changing the phase difference with the local light. Therefore, interference light for obtaining the complex amplitude of each signal light component can be generated while achieving miniaturization.
Brief Description of the Drawings
[0015]
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Mode for Carrying Out the Invention
[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 separation / combiner 14 and a light receiving element array 15 that constitute a receiving module 13, and a processing unit 17. This receiver 10 is a receiving device for coherent communication, and signal light S is incident on the mode separation / combiner 14 from a multimode or multicore type optical fiber 18. The signal light S is multiplexed light that is space-division multiplexed and polarization multiplexed. That is, the signal light S is light obtained by multiplexing a plurality of signal light components (multiplexed light components) having different combinations (modes) of a spatial mode and a polarization mode. Each signal light component is quadrature modulated (IQ modulation) 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 incident direction of the signal light S to the mode separation / combiner 14 is defined as the Z direction, and two directions orthogonal to the Z direction and orthogonal to each other are defined as the X direction and the Y direction.
[0017] In this example, for the signal light S, the fundamental mode LP 01 and the higher-order mode LP 11a , LP 11b are space-division multiplexed in three types of spatial modes, and for each of the three types of spatial modes, polarization multiplexing is performed in a first polarization and a second polarization, which are polarization modes orthogonal to each other. Specifically, the first polarization and the second polarization in this example are the X polarization and the Y polarization, which are linear polarizations. Therefore, the signal light S includes signal light components S X01、 of the fundamental mode LP X11a , LP X11b of the higher-order mode, X01 S X11a , S X11b and signal light components S of the fundamental mode LP Y01 of the Y polarization, Y11a of the higher-order mode LP Y11b , LP Y01 S Y11a , S Y11b . In the following description, when there is no need to particularly distinguish the signal light components S X01 , S X11a , S X11b , S Y01 , S Y11a , S Y11b , they are collectively referred to as signal light components.
[0018] Note that the first polarization wave and the second polarization wave are the X polarization wave and the Y polarization wave which are linear polarization waves. However, the orthogonal polarization modes are not limited to this, and can be, for example, right-handed and left-handed circular polarization waves or elliptical polarization waves. Also, the spatial mode multiplexed with the signal light S is not limited to the above, and for example, may include higher-order spatial modes or may be 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 is of the homodyne detection method in this example, and the wavelength of the local light Lo is the same as the wavelength of the signal light component. Also, from the local light source 12, for example, local light Lo of the X polarization is output and enters the mode separation / combiner 14 as light of the fundamental mode LP X01 in the X polarization through the optical fiber 19. The local light Lo is split into a plurality of split local lights by the mode separation / combiner 14 as will be described later.
[0020] The mode separation / combiner 14 as an optical device includes six metasurfaces 21A to 21F. The metasurfaces 21A to 21F are, for example, plate-shaped with each side length of about 400 μm and are arranged parallel to the XY plane. In the following description, when it is not necessary to distinguish the metasurfaces 21A to 21F, they are referred to as metasurface 21.
[0021] Meta-surfaces 21A to 21F are arranged in a plurality of stages (six stages in this example) at intervals in the Z direction. Signal light S and local light Lo are incident on one surface of the first-stage (the first layer) meta-surface 21A. A plurality of divided signal light components converted from the signal light S and a plurality of divided local lights obtained by dividing the local light Lo are emitted from the final-stage meta-surface 21F, and interference light is generated by interfering each divided signal light component with the divided local light. From the viewpoint of improving the accuracy of mode separation of the optical components separated and output by the mode separation multiplexer 14, it is preferable that the number of meta-surfaces 21 is equal to or greater than the number of mode separations (the number of separated modes). In this example, since three types of spatial modes (transmission modes) and two polarization modes are separated, the number of mode separations is six.
[0022] An optical fiber 18 for incident signal light S and an optical fiber 19 for incident local light Lo on the first-stage meta-surface 21A are arranged adjacent to each other. Therefore, the incident positions of the signal light S and the local light Lo on the incident surface of the meta-surface 21A are close but different. The distance between the axial centers of the optical fiber 18 and the optical fiber 19 in this example is about 127 μm. In this example, each emission end of the optical fiber 18 and the optical fiber 19 and the incident surface of the meta-surface 21A are separated by a predetermined interval, and the beams of the signal light S and the local light Lo are expanded and incident on the meta-surface 21A. Since the beams of the signal light S and the local light Lo can be expanded by transmitting through the first-stage meta-surface 21A, each emission end of the optical fiber 18 and the optical fiber 19 and the incident surface of the meta-surface 21A may be arranged close to each other. Also, instead of the closely arranged optical fibers 18 and 19, a multi-core type optical fiber may be used, and the signal light S and the local light Lo may be incident on the meta-surface 21A from different cores.
[0023] Each split signal light component is emitted from the metasurface 21F so as to be respectively focused on different output positions set on the light receiving surface 15a of the light receiving element array 15 as the output surface. Further, a plurality of split local lights are emitted from the metasurface 21F so as to be respectively focused on each output position of the split signal light components. Thereby, interference light is generated on the light receiving surface 15a by interfering the split local light with each of the split signal light components.
[0024] The metasurface 21 is formed by arranging meta-atoms (scatterers) of a minute size (sub-wavelength order) that locally change the phase, amplitude, and polarization of incident light on a two-dimensional plane, and controls the two-dimensional distribution of the phase, amplitude, and polarization of light by adjusting the shape of each meta-atom. In this example, the metasurface 21 is of a transmissive type, and controls the amplitude distribution, phase distribution, and polarization distribution of the light transmitted through the metasurface 21.
[0025] As shown in FIG. 2, the meta-atom 25 in this example is provided to protrude in an elliptical column shape on the surface of a transparent substrate 26. Further, as shown in FIG. 3, the meta-atoms 25 are arranged in a square array so as to be provided at each vertex of a square on the substrate 26. The meta-atom 25 is formed of a dielectric (for example, silicon). Although the meta-atom 25 may be formed of other materials, the metasurface 21 formed of the dielectric for the meta-atom 25 has low light loss and is advantageous in configuring the mode demultiplexer 14. The substrate 26 is made of a material having a high transmittance for the signal light S, for example, 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 made 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 the elliptical column shape as long as it can shift the phase for each of the two polarization components in orthogonal directions. For example, a prism shape having a rectangular or rhombic cross-sectional shape may be used. Further, in this example, the meta-atoms 25 are arranged in a square array, but the arrangement pattern is not limited to this, and for example, a regular triangular array or the like may be used.
[0027] By adjusting the shape of the meta-atom 25 provided as described above, that is, the length Da of the major axis, the length Db of the minor axis, and the height h, it is possible to give a phase difference φa to the polarization component in the major axis direction of the light incident on the position of the meta-atom 25 and a phase difference φb to the polarization component in the minor axis direction. Here, giving a phase difference means giving a phase difference based on the phase of the original light, that is, performing a phase shift. Further, by adjusting the inclination θ of the major axis of each meta-atom 25 distributed on the substrate 26 and the phase differences φa and φb given by the meta-atom 25, the light incident on each position of the metasurface 21 can be converted into light having a desired polarization component.
[0028] As described above, the metasurfaces 21A to 21F are arranged at intervals, and the optical components of the signal light S and the local light Lo emitted from the previous-stage metasurface 21 are incident on the next-stage metasurface 21. The interval between the metasurfaces 21 in the previous stage and the next stage is a distance at which the optical components emitted from the previous stage can cause spatial mixing by free-space propagation, for example, a length corresponding to several hundreds of wavelengths of the signal light S. In this example, for the signal light S with a wavelength of 1.55 μm, each interval between the metasurfaces 21 is set to 1 mm. An intermediate layer formed of transparent glass, quartz, or the like through which light propagates in free space may be arranged between the metasurfaces 21 and between the metasurface 21F and the light-receiving element array 15.
[0029] The mode separation multiplexer 14 performs mode separation, optical splitting, phase difference control, and polarization conversion by the metasurfaces 21A to 21F, and generates a plurality of split signal optical components having equal optical intensities from the signal light S and a plurality of split local lights having equal optical intensities from the local light Lo so that each split signal optical component interferes with the split local light under predetermined conditions.
[0030] In this example, the mode separation separates the signal light S into components of each mode, which is the separation of the spatial mode component (spatial mode separation) and the polarization mode component (polarization separation) included in the signal light S. Specifically, in this example, the spatial mode separation is the fundamental mode LP 01 and the higher-order mode LP 11a 、LP11b means separating into each spatial mode component, and polarization separation means separating into an X-polarized wave (first polarization wave) component and a Y-polarized wave (second polarization wave) component. Therefore, by this mode separation, the signal light S is separated into signal light components S X01 、S X11a 、S X11b 、S Y01 、S Y11a 、S Y11b .
[0031] Optical splitting splits each signal light component into a plurality of split signal light components with respect to the signal light S. Also, optical splitting splits the local light Lo itself into a plurality of split local lights with respect to the local light Lo. The optical splitting with respect to this local light Lo generates the same number of split local lights as the generated split signal light components. In this example, for six signal light components, four split signal light components are generated for each signal light component, and 24 split local lights are generated.
[0032] Phase difference control controls the phase of one or both of the split signal light component and the split local light so that the phase difference between the split signal light component and the split local light that are condensed at the same output position is different for each output position of the signal light component. The above phase difference is the difference in the relative phase between the split signal light component and the split local light at the time of interference at the output position. Ideally, it is preferable that the phase differences at each output position for each signal light component are shifted at equal intervals.
[0033] In this example, by shifting the phase difference at intervals of 90° (=π / 2), the phase difference between the split signal light component and the split local light at each output position for each signal light component is made different. That is, when the phase difference of the interference between one split signal light component and the split local light is used as a reference (0°) and represented by the relative phase difference at each output position for each signal light component, each split signal light component interferes with the split local light at phase differences of 0°, 90°, 180°, and 270°. Along with this, four split signal light components are generated for each signal light component as described above.
[0034] Also, in order to interfere each divided signal light component and each divided local light with the respective phase differences as described above, in this example, the phases of the four divided signal light components focused on each output position are shifted from each other, and the divided local light is focused on each output position with the same phase. Therefore, for each signal light component, when the relative phase is represented with the phase of one divided signal light component as a reference (0°), four divided signal light components that are focused on the output position with the phases of 0°, 90°, 180°, and 270° are generated.
[0035] Note that as long as the divided signal light component and the divided local light can be interfered with each other at different phase differences at each output position of the signal light component, either the phase of the divided signal light component or the phase of the divided local light may be controlled, or both phases may be controlled. For example, the divided signal light components may be focused on each output position with the same phase, and the four divided local lights for each signal light component may be focused on the output position with their phases shifted from each other. In this example, the divided signal light component is the first divided light, and the divided local light is the second divided light.
[0036] Polarization conversion is conversion of a polarization mode, and makes the divided signal light component and the divided local light emitted from the mode separation multiplexer 14 have the same polarization mode. Thereby, when the divided signal light component and the divided local light are multiplexed, they are made to interfere with each other, and the influence of the difference in sensitivity of the light receiving element array 15 due to the difference in polarization mode is eliminated. In this example, when the mode separation multiplexer 14 divides the local light Lo to generate the divided local light, it maintains the polarization mode of the local light Lo, and converts the polarization mode of the polarization component of the signal light S so as to be the polarization mode of the divided local light. That is, in this example, since the local light Lo is X polarization, the Y polarization component of the signal light S is made X polarization, and each divided signal light component to be emitted is made X polarization.
[0037] From the perspective of causing the divided signal light component and the divided local light to interfere with each other, the polarization mode of the polarization component of the local light Lo may be converted so that the divided signal light component and the divided local light are in the same polarization mode. Further, for the divided local light combined with the divided signal light component of the X polarization component, the X polarization of the local light Lo is maintained, and for the divided signal light component of the Y polarization component, the Y polarization is maintained. For the divided local light combined with the divided signal light component of the Y polarization component, the polarization may be converted from the X polarization to the Y polarization.
[0038] As described above, it is only necessary that the polarization modes of the divided signal light component and the divided local light emitted from the mode separation multiplexer 14 are the same. Therefore, the polarization mode of the local light Lo at the stage of entering the mode separation multiplexer 14 is not particularly limited. On the other hand, from the viewpoint of simplifying the configuration and design of the mode separation multiplexer 14, it is preferable to generate the divided local light while maintaining the polarization mode of the local light Lo as described above.
[0039] In the above description, for the sake of convenience of explanation, mode separation, optical division, phase difference control, and polarization conversion are described individually. However, in the mode separation multiplexer 14, when light passes through the metasurfaces 21A to 21F, a plurality of divided signal light components and a plurality of divided local lights subjected to mode separation, optical division, phase difference control, and polarization conversion are obtained for the incident signal light S and local light Lo. Therefore, it is not necessary for mode separation, optical division, phase difference control, and polarization conversion to be performed individually and sequentially.
[0040] The function of condensing the divided signal light component and the divided local light of the mode separation multiplexer 14 at the output position is realized by forming a wavefront similar to the case where light passes through a convex lens by the metasurface 21.
[0041] In this example, by the above-described mode separation multiplexer 14, signal light components S of six modes combining the spatial mode and the polarization mode are obtained from the signal light S X01 、S X11a 、S X11b 、S Y01 、S Y11a 、S Y11bFor each of them, four split signal light components are generated, and 24 split local lights equal to the total number of the split signal light components are generated from the local light Lo. Both the split signal light components and each split local light are in the X polarization state.
[0042] As shown in FIG. 4, in this example, the mode demultiplexer 14 condenses the split signal light components arranged in a matrix on the light receiving surface 15a as the output surface. The fundamental mode LP of the X polarization state X01 of the signal light component S X01 produces the split signal light component S1a with a relative phase of 0°, the split signal light component S1b with a relative phase of 90°, the split signal light component S1c with a relative phase of 180°, and the split signal light component S1d with a relative phase of 270°, which are condensed respectively.
[0043] These split signal light components S1a to S1d are condensed so as to be linearly arranged at a predetermined interval in the column direction (the vertical direction in FIG. 4), and are arranged in the order of the split signal light component S1a, the split signal light component S1c, the split signal light component S1b, and the split signal light component S1d. The fundamental mode LP of the Y polarization state Y01 of the signal light component S Y01 similarly produces split signal light components S2a to S2d, which are condensed so as to be linearly arranged in the adjacent column to the split signal light components S1a to S1d.
[0044] Hereinafter, the split signal light components S3a to S3d generated from the signal light component S of the higher order mode LP X11a the split signal light components S4a to S4d generated from the signal light component S of the higher order mode LP X11a the split signal light components S5a to S5d generated from the signal light component S of the higher order mode LP Y11a the split signal light components S6a to S6d generated from the signal light component S of the higher order mode LP Y11a are condensed so as to be linearly arranged in the same way. Note that the arrangement of the split signal light components can be arbitrarily determined. X11b of the signal light component S X11b the split signal light components S5a to S5d generated from the signal light component S of the higher order mode LP Y11b of the signal light component S Y11b the split signal light components S6a to S6d generated from the signal light component S of the higher order mode LP
[0045] As shown in FIG. 1, the light receiving element array 15 is arranged on the output side of the mode demultiplexer / combiner 14 with a predetermined interval from the final-stage metasurface 21F, and its light receiving surface 15a faces the emitting-side surface of the metasurface 21F. Light receiving elements 23 are respectively arranged at the output positions of the divided signal light components on the light receiving surface 15a of the light receiving element array 15. The light receiving element 23 is composed of, for example, a photodiode, receives the interference light obtained by interfering the divided signal light component and the divided local light, and converts its intensity into an electrical light receiving signal.
[0046] For each signal light component, the processing unit 17 obtains the complex amplitude of the signal light component from the light receiving signals of the four interference lights obtained by combining each divided signal light component and the divided local light obtained from one signal light component. For example, for the signal light component S X01 from the light receiving signal obtained by receiving the interference light corresponding to the divided signal light component S1a with a relative phase of 0° and the light receiving signal obtained by receiving the interference light corresponding to the divided signal light component S1c with a relative phase of 180°, the in-phase component (I component) of the signal light component S X01 is obtained. Also, from the light receiving signal obtained by receiving the interference light corresponding to the divided signal light component S1b with a relative phase of 90° and the light receiving signal obtained by receiving the interference light corresponding to the divided signal light component S1d with a relative phase of 270°, the quadrature component (Q component) of the signal light component S X01 is obtained.
[0047] The shape of each meta-atom 25 in the metasurfaces 21A to 21F can be determined using the adjoint method (backward propagation). Hereinafter, the combination of the spatial mode and the polarization mode of the signal light and the local light is referred to as mode m (m = 1, 2 ··· M (in this example, M = 7)), and k is the number of stages of the metasurface 21 (k = 1, 2 ··· K (in this example, K = 6)) for explanation. Also, the light composed of the signal light S and the local light Lo emitted from the optical fibers 18 and 19 is described as the input light. In this example, m = 7 is the local light.
[0048] As shown in Fig. 5, the input Jones vector a representing the signal light component or local light of mode m at the output ends of the optical fibers 18 and 19 in (m) (x, y) and the Jones vector (hereinafter referred to as the output Jones vector) a at the position (x, y) on the output surface when the input light is propagated (forward propagated) to the metasurface 21 out (m) The relationship with (x, y) can be expressed as in Equation (1).
[0049] [Number]
[0050] J in Equation (1) k (x, y) is the Jones matrix corresponding to the meta-atoms 25 existing at each position (x, y) on the k-th stage metasurface 21 as shown in Equation (2). R(θ(x, y)) in Equation (2) is a rotation matrix with the inclination 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 indicating the free space on the output side of the k-th stage metasurface 21. This propagation function F k is calculated, for example, by the angular spectrum method or the Rayleigh - Sommerfeld method. Note that F0 is a propagation function indicating the free space between the optical fibers 18 and 19 and the metasurface 21A.
[0051] Regarding the shape of each individual meta-atom 25 on the metasurface 21, it can be determined by optimizing using the adjoint method (backward propagation) so as to maximize the objective function ε shown in Equation (3). a in Equation (3) tar (m) (x, y) is the target Jones vector (hereinafter referred to as the target Jones vector) at the position (x, y) on the output surface. This target Jones vector is the Jones vector that serves as the starting point when the metasurface 21 is backward propagated. Note that " † " indicates the adjoint matrix (complex conjugate transpose) of matrix A.
[0052] [Mathematics]
[0053] Output Jones vector a out (m) (x, y), target Jones vector a tar (m) The inner product with the adjoint matrix of (x, y) is the component of these X-polarized waves as a out,x (m) (x, y), a tar,x (m) Taking (x, y) as, the component of the Y-polarized wave as a out,y (m) (x, y), a tar,y (m) When taking (x, y), it becomes as shown in Equation (4). Note that "A * " represents the complex conjugate of the complex number A.
[0054] [Mathematics]
[0055] Maximizing the objective function ε is to repeatedly calculate the gradient shown in Equation (5) and update the meta-atom parameters p k (x, y) to obtain the meta-atom parameters p k (x, y) that maximize the objective function ε. Note that p k (x, y) are the meta-atom parameters (φ a,k , φ b,k , θ k ) of the meta-atom 25 existing at the position (x, y) on the k-th stage meta-surface 21. The function G() is a function that determines the change amount of the meta-atom parameters and can be made according to optimization methods such as the steepest descent method, momentum, RMSProp, Adam, etc.
[0056] [Mathematics]
[0057] The gradient of the objective function ε can be specifically calculated by Equation (8) using the output Jones vector a out (m) (x, y), the target Jones vector a tar (m) (x, y), the forward propagation Jones vector a of the k-th meta-surface 21 k (m) (x, y) and the backward propagation Jones vector b k (m) (x, y).
[0058] [Equation]
[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) (x, y) when propagated forward to the input side surface of the k-th meta-surface 21, and is expressed as Equation (9). Also, the backward propagation Jones vector b in Equation (8) k (m) (x, y) is the Jones vector when the target Jones vector a tar (m) (x, y) is propagated (backward) to the output side surface of the k-th meta-surface 21 and is expressed as Equation (10). Note that F in Equation (10) k -1 is the propagation function indicating the free space on the output side of the k-th meta-surface 21 in backward propagation.
[0060] [Equation]
[0061] Furthermore, when differentiating the Jones matrix J k (x, y) with respect to each of the meta-atom parameters p k (x, y) in Equation (8), it becomes as shown in Equations (11) to (13).
Number
[0062] Specifically, until it is determined that the value of the objective function ε has converged to the maximum value, the following steps 1 to 4 are repeated to optimize the meta-atom parameter p so as to maximize the objective function ε k (x, y) is obtained. When performing step 1 for the first time, p k is given an initial value appropriately determined for (x, y). Also, when it is determined that the value of the objective function ε has reached the maximum value by virtue of the substantial update of the three types of meta-atom parameters (φ a,k , φ b,k , θ k ), it may be acceptable.
[0063] Step 1: According to Equation (9), the forward-propagation Jones vector a k (m) (x, y) on the input-side surface of each meta-surface 21 (k = 1, 2 ··· K) in forward propagation and the output Jones vector a out (m) (x, y) are obtained. In Equation (9), when "k = K + 1", the output Jones vector a out (m) (x, y) (= a K+1 (m) (x, y)) can be obtained. Also, the value of the objective function ε is obtained. Step 2: According to Equation (10), the backward-propagation Jones vector b k (m) (x, y) on the output-side surface of each meta-surface 21 (k = 1, 2 ··· K) in backward propagation is obtained. Step 3: Using the results of steps 1 and 2, according to Equation (8), for each of the meta-surfaces 21 (k = 1, 2 ··· K), the gradients of the three types of meta-atom parameters (φ a,k , φ b,k , θ k ) are obtained. Step 4: Based on Equation (6), for the three types of meta-atom parameters (φ a,k , φb,k , θ k ) are each updated.
[0064] By the above optimization, the meta-atom parameter p of each individual meta-atom 25 in each stage of the meta-surface 21 k (x, y), that is, the phase difference φa, φb and the inclination θ of the major axis are obtained, and the phase difference φa, φb and the inclination θ of the major axis obtained for each individual meta-atom 25 are converted into the actual shape (size).
[0065] FIG. 6 shows the distribution of the meta-atom parameters in each optimized meta-surface 21 by the above method. Also, FIG. 7 shows the light intensity and phase of the split signal light component of each signal light component (m = 1 to 6) and the split local light from the local light Lo (m = 7) on the output surface obtained by numerical simulation using the obtained distribution of the meta-atom parameters. From this result, it can be seen that each signal light component is separated and output into split signal light components having relative phases of 0°, 90°, 180°, and 270°. Also, it can be seen that each split signal light component and each split local light have the same light intensity.
[0066] In the above example, the interval for shifting the phase difference between the split signal light component and the split local light during interference is set to 90°, but the interval in the case of shifting the phase difference at equal intervals is not limited to this, and can be, for example, 72° (= 2π / 5), 120° (= 2π / 3), etc. Also, for each mode, the intervals for shifting the phase difference can be made different, for example, for the signal light component of X polarization and for the signal light component of Y polarization.
[0067] [Second Embodiment] The second embodiment is a unitized reception module. Multiplexed light obtained by multiplexing signal light and local light is input to a mode separation multiplexer. Note that, other than what will be described below, it is the same as the first embodiment, and the same reference numerals are given to substantially the same constituent members, and the detailed description thereof is omitted.
[0068] In FIG. 8, the receiver 50 includes a mode separation multiplexer 54 and a light receiving element array 15 that constitute a receiving module 53, a processing unit 17, and a local light source (not shown). The mode separation multiplexer 54 has an optical block 56, a metasurface array 57, and a reflective film 58. The optical block 56 as a propagation layer is formed of a transparent material such as quartz or glass in which signal light S, local light Lo, and their light 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 (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 formed by arranging reflective metasurfaces 61A to 61F in a plane and integrally providing them. This metasurface array 57 has a plurality of meta-atoms 25 that constitute each of the reflective metasurfaces 61A to 61F, a thin film 57a common to the reflective metasurfaces 61A to 61F, and a total reflection type reflective film 57b. The film 57a is formed of, for example, a polymer, quartz, or glass. The meta-atoms 25 are provided on the surface of the film 57a on the side of the optical block 56. The reflective film 57b is provided on the surface of the film 57a on the side opposite to 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 the light incident on and reflected by them. These reflective metasurfaces 61A to 61F are different only in that they are reflective from the transmissive metasurfaces 21A to 21F of the first embodiment, and they have the same function. In the following description, when it is not necessary to distinguish between the reflective metasurfaces 61A to 61F, they are referred to as reflective metasurfaces 61.
[0071] In the mode demultiplexer / combiner 54, the signal light S and the local light Lo are incident into the optical block 56 from the surface 56b on one end side (the 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 light components of the incident signal light S and local light Lo at a reflection angle equal to the incident angle.
[0072] The light components of the signal light S and the local light Lo reflected by the reflective metasurface 61A are reflected by the reflection film 58 and are incident on the next-stage reflective metasurface 61B. The light components of the signal light S and the local light Lo incident on the reflective metasurface 61B are reflected by the reflective metasurface 61B. Thereafter, similarly, the light components of the signal light S and the local light Lo reflected by the reflective metasurface 61 are reflected by the reflection film 58 and are incident on the subsequent-stage reflective metasurface 61. In this way, the light components of the signal light S and the local light Lo are repeatedly reflected between the reflection film 58 and the metasurface array 57 while shifting the reflection position to the other end side (the right side in FIG. 8) inside the optical block 56, and are sequentially incident on the reflective metasurfaces 61A to 61F.
[0073] Note that in this example, the metasurface array 57 (reflective metasurfaces 61A to 61F) is the first reflection film, and the reflection film 58 is the second reflection film, and these constitute the reflection portion. As described above, the reflective metasurfaces 61A to 61F are arranged in the optical path of the light components of the signal light S and the local light Lo that are repeatedly reflected and propagated between the metasurface array 57 and the reflection film 58.
[0074] The optical components of the signal light S and the local light Lo reflected by the reflective metasurface 61F, that is, the plurality of divided signal light components generated from the signal light S and the plurality of divided local lights 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 light receiving element array 15 is attached to a portion of the surface 56b from which each divided signal light component and each divided local light are emitted, and the interference light between the divided signal light component and the divided local light is received by each light receiving element of the light receiving element array 15. The light receiving element array 15 receives the interference light in which the divided signal light component and the local light Lo interfere with each other. Thereby, the complex amplitude of each signal light component is obtained by the processing unit 17.
[0075] By configuring as described above, the mode separation multiplexer 54 can be configured more compactly. Further, a plurality of metasurfaces can be realized by a single metasurface array 57, and the mode separation multiplexer 54, the receiving module 53 including the same, and the receiver 50 can be configured with fewer component 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. Further, the reflection direction may be controlled by the reflective metasurfaces 61A to 61F. For example, multiplexed light is incident perpendicularly to the reflective metasurface 61A, and the optical components of the signal light S and the local light Lo incident on the reflective metasurface 61A are reflected in a direction inclined from the normal direction, and are incident on the next-stage reflective metasurface 61B via the reflective film 58, or may be reflected in a direction perpendicular to the reflective metasurface 61F and incident on the light receiving element array 15. Further, a metasurface array 57 may be provided on the surface 56b where the multiplexed light is incident, and only a reflective film may be provided on the surface 56a. Further, in this example, the light receiving element array 15 is provided on the surface 56b side on the incident side of the multiplexed light, but the divided signal light component and the divided local light may be configured to be emitted to the outside from the surface 56a side facing the surface 56b. In this case, the light receiving element array 15 may be provided on the surface 56a.
[0077] In this example, the meta-surface array 57 is provided only on one surface 56a of the optical block 56, but a meta-surface array may also be provided on the other surface 56b. In this case, the surface array provided on the other surface 56b serves as the second reflective film.
[0078] In the above meta-surface array 57, on the film 57a, each reflective meta-surface 61 is formed in an independent manner, but adjacent reflective meta-surfaces 61 may also share a partial region with each other. In this case, the beams of the optical components of the signal light S and the local light Lo will be incident twice on a partial region shared by one reflective meta-surface 61 and the next-stage reflective meta-surface 61.
[0079] As in the mode demultiplexer / combiner 54A shown in FIG. 9, a configuration may be adopted in which a plurality of transmissive meta-surfaces 71A to 71F are arranged in the middle of the optical block 66 in the thickness direction. The mode demultiplexer / combiner 54A in this example includes an optical block 66, a meta-surface array 67, and reflective films 68 and 69. The meta-surface array 67 is formed by arranging the meta-surfaces 71A to 71F in a planar manner and integrally providing them, and has a configuration in which a plurality of meta-atoms (not shown) constituting each of the meta-surfaces 71A to 71F are provided on a substrate 67a. The meta-surface array 67 is arranged in the middle of the optical block 66 in the thickness direction by being provided, for example, between a pair of blocks 72 and 73 constituting the optical block 66.
[0080] A reflective film 68 as the first reflective film is provided on one surface 66a facing in the thickness direction of the optical block 66, and a reflective film 69 as the second reflective film is provided on the other surface 66b. The signal light S, the local light Lo, and their optical components incident from one end side (the left side in FIG. 9) of the optical block 66 are arranged such that the reflection position is shifted to the other end side (the right direction in FIG. 9), and the meta-surfaces 71A to 71F are arranged in the optical path where they are repeatedly reflected between the reflective film 68 and the reflective film 69. Thereby, the incident signal light S, the local light Lo, and their optical components sequentially pass through the meta-surfaces 71A to 71F.
[0081] In each of the above embodiments, the case where the spatial mode is the propagation mode of a multimode type optical fiber as the multiplexed spatial mode has been described. However, the spatial mode may be, for example, the propagation mode of each core of a multicore optical fiber or the propagation mode in free space. That is, it is possible to separate the light incident from the multicore optical fiber in the propagation mode of each core, or to separate the light incident from free space in the propagation mode.
[0082] Also, in each of the above embodiments, the signal light is multiplexed light. However, a multiplexed light obtained by multiplexing one or more signal light components of the signal light and local light having a different mode from the one or more signal light components, that is, different in one of the spatial mode and the polarization mode or a combination thereof, may be incident on the mode separation multiplexer, that is, the first-stage metasurface. For example, local light Lo of the fundamental mode LP of the X polarization X01 and the higher-order mode LP of the X polarization X11a and each signal light component of the higher-order mode LP of the Y polarization Y11a may be multiplexed and incident on the mode separation multiplexer to separate these modes.
[0083] In each of the above embodiments, the case where an optical device including a plurality of metasurfaces is configured as a mode separation multiplexer has been described. However, by omitting some functions, for example, it can be configured to separate the multiplexed light that is spatially and polarization multiplexed into a plurality of multiplexed light components that are spatially and polarization separated and emit them from the final-stage metasurface, or to divide a plurality of multiplexed light components and emit them from the final-stage metasurface.
Description of Reference Numerals
[0084] 10, 50 Coherent Receiver 12 Local Light Source 13, 53 Receiver Module 14, 54, 54A Mode Separation Multiplexer 15 Light-Receiving Element Array 15a Light-Receiving Surface 17 Processing Unit 21A to 21F, 71A to 71D metasurfaces 56, 66 optical blocks 57, 67 metasurface arrays 57b, 58, 68, 69 reflective films 61A to 61F reflective metasurfaces
Claims
1. Comprising a plurality of metasurfaces arranged in multiple stages at intervals, each controlling the phase and polarization of incident light, wherein the plurality of metasurfaces, spatially mode multiplexed and polarization multiplexed multiplexed light is incident on the first-stage metasurface, and the light components of the multiplexed light transmitted or reflected by the previous-stage metasurface are incident on the next-stage metasurface, thereby separating the multiplexed light into a plurality of multiplexed light components by spatial mode separation and polarization separation and emitting them from the final-stage metasurface An optical device characterized by the above.
2. The plurality of metasurfaces, for each of the plurality of multiplexed light components, convert them into a plurality of first split lights, emit the plurality of first split lights from the final-stage metasurface, and condense them at a plurality of different output positions The optical device according to claim 1, characterized by the above.
3. Comprising a plurality of metasurfaces arranged in multiple stages at intervals, each controlling the phase and polarization of incident light, wherein the plurality of metasurfaces, a signal light obtained by spatially mode multiplexing and polarization multiplexing a plurality of signal light components and a local light are incident, and the light components of the signal light and the local light transmitted or reflected by the previous-stage metasurface are incident on the next-stage metasurface, thereby separating the signal light into a plurality of signal light components by spatial mode separation and polarization separation, each of which is divided into a plurality of first split lights and condensed at a plurality of different output positions, the local light is divided into a plurality of second split lights and condensed at the plurality of output positions respectively, and the plurality of first split lights and the plurality of second split lights, in which the first split light and the second split light interfere at different phase differences at each output position of each of the plurality of signal light components, are emitted from the final-stage metasurface An optical device characterized by the above.
4. 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 shifted at equal intervals.
5. 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 such that the relative phase difference when one of the phase differences is used as a reference is 0°, 90°, 180°, 270° The optical device according to claim 3, characterized by the above.
6. The local light is in a single polarization mode, wherein the plurality of metasurfaces, maintain the polarization mode of the local light and emit the plurality of second split lights, Converting the plurality of first divided lights into the same polarization mode as the local light and emitting them The optical device according to claim 3, characterized in that.
7. The signal light and the local light are incident on different positions of the first metasurface in the first stage The optical device according to claim 3, characterized in that.
8. The local light having a different spatial mode from the signal light is spatially mode multiplexed with the signal light, The plurality of metasurfaces spatially mode separate and polarization separate the signal light into the plurality of signal light components and spatially mode separate the local light The optical device according to claim 3, characterized in that.
9. Comprising a plurality of metasurfaces arranged in a plurality of stages at intervals, each controlling the phase and polarization of the incident light, The plurality of metasurfaces are, Multiplexed light in which one or more signal light components of the signal light and local light having a different mode from the signal light component are incident on the first-stage metasurface, and the light components of the multiplexed light transmitted or reflected by the previous-stage metasurface are incident on the next-stage metasurface, thereby splitting the signal light component into a plurality of first divided lights and condensing them at a plurality of different output positions, splitting the local light into a plurality of second divided lights and condensing them at the output positions of the first divided lights, respectively, and causing the first divided light and the second divided light to interfere with each other at different phase differences at each output position for each signal light component, and emitting the plurality of first divided lights and the plurality of second divided lights from the last-stage metasurface An optical device characterized by that.
10. The optical device according to claim 9, characterized in that the phase difference between the first divided light and the second divided light at each output position for each signal light component is shifted at equal intervals.
11. The phase difference between the first divided light and the second divided light at each output position for each signal light component is such that the relative phase difference with respect to one of the phase differences is 0°, 90°, 180°, 270° The optical device according to claim 9, characterized in that.
12. The multiplexed light has a plurality of signal light components spatially or polarization multiplexed, The plurality of metasurfaces are, Generating the plurality of first divided lights for each of the plurality of signal light components obtained by spatially mode separating or polarization separating the multiplexed light The optical device according to claim 9, characterized in that.
13. The optical device according to any one of claims 3 to 12, and Comprising a plurality of light receiving elements arranged at the plurality of output positions for receiving the interference light A receiving module characterized by this.
14. The receiving module according to claim 13, and A processing unit for obtaining complex amplitudes of the plurality of signal light components based on the light reception results of the plurality of light receiving elements A coherent receiver characterized by comprising this.
15. A propagation layer through which light propagates, Having a first reflection film and a second reflection film arranged opposite to each other with the propagation layer interposed therebetween, and repeatedly reflecting between the first reflection film and the second reflection film while shifting the reflection positions of the light components of the signal light and the local light incident from one end side of the propagation layer toward the other end side of the propagation layer, and a reflection unit that emits light from the other end side of the propagation layer Comprising The plurality of metasurfaces are arranged in the optical paths of the light components of the signal light and the local light in the propagation layer. The optical device according to any one of claims 3 to 12, characterized by this.
16. The optical device according to 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 the interference light of the first divided light and the second divided light emitted from the propagation layer A receiving module characterized by comprising this.
17. The 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 reception results of the plurality of light receiving elements A coherent receiver characterized by comprising this.