Polarization separation element and optical reception device

Abstract

To provide a polarization separation element capable of suitably separating and focusing polarized components of target light with a simple configuration.SOLUTION: A polarization separation element 10 includes a substrate 11 and a polarization separation layer 15 having a structure pattern 20 including a plurality of structures each having two-fold rotational symmetry and formed on an upper surface 12 of the substrate 11. The polarization separation layer 15 separates first to sixth polarized components included in target light L0, and focuses the light on first to sixth focusing positions, which are different from each other, on a focusing surface 35, respectively. The structure pattern 20 has a first structure pattern that focuses the first and second polarized components, a second structure pattern that focuses the third and fourth polarized components, and a third structure pattern that focuses the fifth and sixth polarized components, uses a pattern consisting of the first structure, the second structure, and the third structure as a unit pattern, and is formed by arranging a plurality of unit patterns in a two-dimensional manner.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a polarization separation element that separates polarization components of target light and condenses them at different condensing positions, and an optical receiver using the same.

Background Art

[0002] Non-Patent Document 1 discloses a technique for measuring six polarization components of target light, namely, the x-direction linearly polarized component, the y-direction linearly polarized component, the +45° linearly polarized component, the -45° linearly polarized component, the left-handed circularly polarized component, and the right-handed circularly polarized component, and performing imaging of Stokes parameters. In the configuration described in Non-Patent Document 1, separation and condensation of each polarization component included in the target light are performed using a sub-wavelength metasurface structure.

[0003] Non-Patent Document 2 discloses a technique for separating and condensing, for example, two polarization components of left-handed circular polarization and right-handed circular polarization. In the configuration described in Non-Patent Document 2, a reflection-type configuration that separates the polarization components of the target light by a metasurface structure formed on a metal layer that reflects the target light is used. Also, Patent Document 1 and Non-Patent Document 3 describe control of phase and polarization using a metasurface structure.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0006] In recent years, due to the expansion of cloud services and the spread of the concept of IoT (Internet of Things), short-distance communication traffic, including that between data centers, has been increasing. In such short-distance communication, for example, an intensity modulation - direct detection (IM-DD) method is used, in which information is carried on the intensity of light used for communication, and the light intensity is directly detected by a light-receiving element.

[0007] In addition, in order to enable further high-capacity transmission, the development of an optical communication system with high spectral utilization efficiency has been promoted (see, for example, Non-Patent Document 4). As one such optical communication system, a pulse amplitude modulation (PAM) method has been studied. However, in the PAM method, there is a problem that it is difficult to increase the modulation level because only one-dimensional modulation with respect to the light intensity can be performed.

[0008] On the other hand, as a communication method that can utilize a multi-dimensional space while using direct detection, a Stokes-vector modulation - direct detection (SVM-DD) method that uses the polarization state of light has attracted attention (see, for example, Non-Patent Documents 4 and 5). In the SVM-DD method, information is transmitted by modulating the polarization components of light and assigning values to the Stokes vectors (Stokes parameters) corresponding to each polarization state.

[0009] According to such an SVM-DD method, three-dimensional optical modulation using a three-dimensional Stokes space becomes possible by using the intensities of two orthogonal polarizations and the three degrees of freedom of their relative phase difference. In addition, since only the relative phase information between polarizations is used, the signal can be received by direct detection, and a coherent system is not required.

[0010] For example, FIG. 4 of Non-Patent Document 5 shows examples of the arrangement of signal points in the case of binary, quaternary, and octal values in a three-dimensional Stokes space. As can be understood from this figure, since the SVM-DD method uses a three-dimensional space, the spectral efficiency can be improved compared to conventional methods even though a simple direct detection method is used.

[0011] In the optical signal receiver in the SVM-DD method, for example, as shown in FIG. 1 of Non-Patent Document 4, there are problems such as the complexity of the configuration of the optical system and the optical circuit. In contrast, in the optical signal receiver, it is conceivable to use polarization control by a metasurface structure. However, for example, in the configuration described in Non-Patent Document 1, the metasurface structure body groups corresponding to each polarization component are separately arranged in different sections. In such a configuration, considering its application in optical communication, there is a problem that the symmetry is poor because the functions are divided by position. Also, in the configuration described in Non-Patent Document 2, only two polarization components can be condensed, and all Stokes parameters cannot be obtained.

[0012] An object of the present invention is to provide a polarization separation element and an optical receiving device capable of suitably performing separation and condensation of polarization components of target light with a simple configuration.

Means for Solving the Problems

[0013] The polarization separation element according to the present invention includes: (1) a substrate that is transparent to target light; and (2) a polarization separation layer formed on the upper surface of the substrate, made of a material that is transparent to target light and has a higher refractive index than the substrate, and having a structure pattern including a plurality of rotationally symmetric structures with two-fold rotational symmetry. One of the lower surface of the substrate and the upper surface of the polarization separation layer serves as the light incident surface, and the other serves as the light exit surface, having a transmissive configuration. (3) The polarization separation layer separates the first to sixth polarization components included in the target light incident from the light incident surface side and focuses them on different first to sixth focusing positions on the focusing surface set on the light exit surface side. (4) The structure pattern includes a plurality of first structures, a first structure pattern that focuses the first and second polarization components on the first and second focusing positions, respectively, a plurality of second structures, a second structure pattern that focuses the third and fourth polarization components on the third and fourth focusing positions, respectively, and a plurality of third structures, a third structure pattern that focuses the fifth and sixth polarization components on the fifth and sixth focusing positions, respectively. A linear or triangular pattern composed of the first structure, the second structure, and the third structure is used as a unit pattern, and a plurality of unit patterns are two-dimensionally arranged on the upper surface of the substrate.

[0014] In the polarization separation element having the above configuration, in the polarization separation layer on the substrate, a structure pattern including a plurality of structures is formed, and using this structure pattern, the first to sixth polarization components included in the target light incident from the light incident surface side are separated and focused on different first to sixth focusing positions on the focusing surface. Further, the structure pattern is constituted by a first structure pattern that separates and focuses the first and second polarization components, a second structure pattern that separates and focuses the third and fourth polarization components, and a third structure pattern that separates and focuses the fifth and sixth polarization components.

[0015] And in such a configuration, regarding the arrangement of a plurality of structures including the first to third structures on the upper surface (pattern formation surface) of the substrate, a pattern consisting of three structures, namely the first structure, the second structure, and the third structure, is defined as a unit pattern, and a plurality of unit patterns are two-dimensionally arranged on the substrate. According to such a configuration, the first to third structures can be uniformly and dispersedly arranged on the substrate, and the first to sixth polarization components of the target light incident on the element can be separated and condensed with good symmetry in a simple configuration.

[0016] In the above polarization separation element, the size of each of the plurality of structures constituting the structure pattern on the upper surface of the substrate and the height from the upper surface of the substrate may each be set to be less than the wavelength of the target light. Also, the arrangement interval of the plurality of structures in the structure pattern may be set to be less than the wavelength of the target light. Thereby, a sub-wavelength metasurface structure for separating and condensing the polarization components of the target light can be suitably configured by the structure pattern including the plurality of structures. Also, for the structures with two-fold rotational symmetry in the structure pattern, specifically, for example, elliptical cylinders can be used.

[0017] In the above polarization separation element, regarding the first to sixth polarization components of the target light, specifically, they may each be an x-direction linearly polarized component, a y-direction linearly polarized component, a +45° linearly polarized component, a -45° linearly polarized component, a left-handed circularly polarized component, and a right-handed circularly polarized component. According to such a configuration, by detecting each separated and condensed polarization component, for example, the Stokes parameters of the target light can be suitably obtained.

[0018] In the above polarization separation element, regarding the specific configurations of the unit pattern and the structure pattern in the polarization separation layer, the unit pattern may be an equilateral triangle pattern, and the structure pattern may be configured in a hexagonal lattice. Also, the unit pattern may be a right-angled isosceles triangle pattern, and the structure pattern may be configured in a square lattice.

[0019] Further, the structure pattern may be configured to include, as a plurality of unit patterns, a first unit pattern including a structure used for condensing one of the first polarization component and the second polarization component in the first structure, a structure used for condensing one of the third polarization component and the fourth polarization component in the second structure, and a structure used for condensing one of the fifth polarization component and the sixth polarization component in the third structure, and a second unit pattern including a structure used for condensing the other of the first polarization component and the second polarization component in the first structure, a structure used for condensing the other of the third polarization component and the fourth polarization component in the second structure, and a structure used for condensing the other of the fifth polarization component and the sixth polarization component in the third structure.

[0020] Further, the structure pattern may be configured to include, as a plurality of unit patterns, a plurality of types of unit patterns in which the arrangements (arrangement order, pattern shape, etc.) of the first structure, the second structure, and the third structure are different from each other.

[0021] In the above polarization separation element, the refractive index difference between the substrate and the material of the plurality of structures may be 0.25 or more. Thereby, separation and condensation of the polarization components of the target light by the structure pattern including the plurality of structures can be preferably realized.

[0022] In the above polarization separation element, regarding the materials constituting the plurality of structures, the materials of the plurality of structures may be configured to be dielectric materials. Further, the materials of the plurality of structures may be configured to be semiconductor materials or oxide materials.

[0023] In the above polarization separation element, the first to sixth condensing positions where the first to sixth polarization components of the target light are respectively condensed may be arranged at the vertices of a regular hexagon set on the condensing surface. According to such a configuration, condensation of each polarization component of the target light and detection by the photodetector can be preferably performed.

[0024] The optical receiver according to the present invention includes a polarization separation element having the above configuration, and a light detection unit that detects the first to sixth polarization components of the target light condensed at the first to sixth condensing positions on the condensing surface. According to such a configuration, the light intensities of the first to sixth polarization components of the target light separated and condensed by the above polarization separation element can be suitably detected by the light detection unit.

[0025] In the above optical receiver, the light detection unit may be configured to include first to sixth photodetectors arranged at the first to sixth condensing positions and detecting the first to sixth polarization components respectively. Also, for the light detection unit, in addition to the configuration of providing a photodetector that individually detects each polarization component as described above, for example, a configuration of providing a single photodetector such as an imaging device that detects the first to sixth polarization components may also be used.

[0026] The above optical receiver may further include an analysis unit that analyzes the detection results of the first to sixth polarization components by the light detection unit to obtain the Stokes parameters of the target light. Such a configuration is suitable, for example, when applying the optical receiver to optical communication using the SVM-DD method.

Advantages of the Invention

[0027] According to the polarization separation element and the optical receiver of the present invention, it is possible to suitably perform the separation and condensation of the polarization components of the target light with a simple configuration.

Brief Description of the Drawings

[0028]

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Embodiments for Carrying Out the Invention

[0029] Hereinafter, embodiments of a polarization separation element and an optical receiver will be described in detail with reference to the drawings. In the description of the drawings, the same reference numerals are assigned to the same elements, and redundant descriptions are omitted. Also, the dimensional ratios in the drawings do not necessarily match those in the description.

[0030] First, the polarization components of light to be polarization-separated by the polarization separation element, the Stokes parameters, etc. will be briefly described. Hereinafter, the propagation direction of the target light is defined as the z-axis, and the two axes orthogonal to the z-axis are defined as the x-axis and the y-axis. The polarization state of light can be represented by a Stokes vector S consisting of Stokes parameters S1, S2, and S3 defined by the following equation (1).

Equation

[0031] Also, the three-dimensional space defined by the above Stokes parameters is called the Stokes space. When the Stokes vector S = (S1, S2, S3) is plotted in the Stokes space, any polarization state can be corresponded to a point in the space. In particular, when each Stokes parameter is normalized by the light intensity, the points corresponding to the polarization states of light are distributed on the unit sphere in the Stokes space. This unit sphere is called the Poincaré sphere.

[0032] In the Stokes vector S, the parameter S1 corresponds to the intensity difference between the linearly polarized component in the x direction and the linearly polarized component in the y direction, the parameter S2 corresponds to the intensity difference between the +45° linearly polarized component and the -45° linearly polarized component, and the parameter S3 corresponds to the intensity difference between the left-handed circularly polarized component and the right-handed circularly polarized component. Fig. 1(a) shows the Poincaré sphere in the Stokes space and the above six polarization components corresponding to each Stokes parameter, and Fig. 1(b) shows the polarization state of light corresponding to the point P on the Poincaré sphere.

[0033] As understood from the above, for the target light, for example, the light intensities of six polarization components, namely, the x-direction linearly polarized light component, the y-direction linearly polarized light component, the +45° linearly polarized light component, the -45° linearly polarized light component, the left-handed circularly polarized light component, and the right-handed circularly polarized light component, are detected, and by obtaining the differences thereof, the Stokes parameters S1, S2, and S3 of the target light can be obtained. The polarization separation element described below enables the separation and condensing of such polarization components of the target light.

[0034] FIG. 2 is a schematic diagram showing the configuration of an embodiment of an optical receiving device including a polarization separation element. Here, in each of the following figures, for ease of explanation, an xyz orthogonal coordinate system is illustrated as appropriate. In this coordinate system, the z-axis indicates the propagation direction of the target light L0 to be polarization-separated as described above, and the x-axis and y-axis indicate two axes orthogonal to the z-axis.

[0035] The optical receiving device 1A according to this embodiment includes a polarization separation element 10, a photodetection unit 30, and an analysis unit 50. Further, the polarization separation element 10 separates the polarization components included in the target light L0 and condenses them at predetermined condensing positions, and is configured to include a substrate 11 and a polarization separation layer 15. In FIG. 2, as an example of the target light L0, light having a predetermined wavelength output from the output end face of the optical fiber 60 is shown. The wavelength λ of the target light L0 is, for example, 1.55 μm used in the communication wavelength band.

[0036] The substrate 11 is made of a material having transmissivity with respect to the target light L0 having the wavelength λ. The polarization separation element 10 shown in FIG. 2 is configured as a transmissive type, the lower surface 13 of the substrate 11 is a light incident surface on which the target light L0 is incident, and the upper surface 12 is a pattern formation surface on which the structure pattern 20 of the polarization separation layer 15 described later is formed.

[0037] As the material constituting the substrate 11, any material may be used as long as it can form and support the polarization separation layer 15 and has transparency with respect to the target light L0. As such a material for the substrate 11, for example, an oxide material or a fluoride material can be used. Specifically, as the oxide material, quartz (SiO2), BK7, Al2O3, etc. can be used. Also, specifically, as the fluoride material, MgF2, CaF2, LiF, etc. can be used.

[0038] The polarization separation layer 15 is made of a material that has transparency with respect to the target light L0 and has a refractive index higher than that of the substrate 11, and is formed on the upper surface 12 of the substrate 11. Also, the polarization separation layer 15 has a structure pattern 20 including a plurality of structures each having two-fold rotational symmetry, and separates the first to sixth polarization components included in the target light L0 incident from the lower surface 13 side of the substrate 11, and is formed so as to condense at different first to sixth condensing positions on the condensing surface 35 set on the side opposite to the substrate 11. In such a configuration, the upper surface (the surface opposite to the substrate 11) of the polarization separation layer 15 is the light emission surface from which the target light L0 is emitted.

[0039] As the material constituting the plurality of structures in the polarization separation layer 15, any material may be used as long as it has transparency with respect to the target light L0 and has a refractive index higher than that of the substrate 11, but it is preferable to use a dielectric material, and it is also preferable to use a semiconductor material or an oxide material. Specifically, as the semiconductor material, Si which is a group IV semiconductor material, GaAs, InP, GaN, InGaAs, etc. which are group III-V semiconductor materials can be used. Also, specifically, as the oxide material, TiO2, Ta2O5, Nb2O5, HfO2, ZrO2, Y2O3, Gd2O3, CeO2, Al2O3, etc. can be used. Also, Si3N4 etc. which are nitrides may be used as the material of the polarization separation layer 15.

[0040] Also, regarding the first to sixth polarization components of the target light L0 separated by the structure pattern 20 of the polarization separation layer 15, in the following, as a specific example, the first polarization component is the x-direction linearly polarized light component L11 is defined as the second polarization component being a linearly polarized light component L in the y direction 12 is defined as the third polarization component being a linearly polarized light component L at +45° 21 is defined as the fourth polarization component being a linearly polarized light component L at -45° 22 is defined as the fifth polarization component being a left-handed circularly polarized light component L 31 is defined as the sixth polarization component being a right-handed circularly polarized light component L 32 By setting the first to sixth polarization components as described above, the Stokes parameters S1, S2, and S3 of the target light L0 can be suitably obtained from their measurement results. Note that the configuration of the structure pattern 20 for separating the polarization components of the target light L0 will be specifically described later.

[0041] The light detection unit 30 detects the polarization components L 11 of the target light L0 that are condensed at the first to sixth condensation positions on the condensation surface 35 by the polarization separation element 10 12 and L 21 and L 22 and L 31 and L 32 respectively. In the light receiving device 1A of the present embodiment, the light detection unit 30 is arranged corresponding to the first to sixth condensation positions on the condensation surface 35, and includes first to sixth photodetectors 311, 312, 321, 322, 331, and 332 that detect the above polarization components respectively. In FIG. 2, for convenience of explanation, the above photodetectors are shown arranged in a one-dimensional manner, but the actual arrangement of the photodetectors in the light detection unit 30 is, for example, as shown in FIG. 3.

[0042] FIG. 3 is a plan view showing the setting of the first to sixth condensation positions on the condensation surface 35 and the arrangement configuration of the photodetectors in the light detection unit 30. In the present embodiment, as shown in FIG. 3, the polarization components L 11 of the target light L0 12 and L 21 and L 22 and L 31 and L 32The condensing positions where the lights are respectively condensed are arranged at the vertices of a regular hexagon set on the condensing surface 35, and the photodetectors 311, 312, 321, 322, 331, and 332 that constitute the light detection unit 30 are arranged at the corresponding condensing positions respectively.

[0043] In the configuration shown in FIG. 3, specifically, the photodetector 311 corresponding to the linearly polarized light component L in the x direction is arranged upward on the condensing surface 35. The linearly polarized light component L in the y direction 11 The corresponding photodetector 312 is arranged in the upper right. The photodetector 321 corresponding to the +45° linearly polarized light component L 12 Is arranged in the lower right. The photodetector 322 corresponding to the -45° linearly polarized light component L 21 Is arranged downward. The photodetector 331 corresponding to the left-handed circularly polarized light component L 22 Is arranged in the lower left. The photodetector 332 corresponding to the right-handed circularly polarized light component L 31 Is arranged in the upper left. 32 Is arranged in the upper left.

[0044] Thus, in the light detection unit 30, in the configuration where photodetectors are provided individually for each polarization component of the target light L0, as the photodetector, a photodetector such as a 0-dimensional photodetector that operates at high speed can be used. As such a photodetector, for example, a photodiode (e.g., a pin photodiode), a photomultiplier tube, etc. can be used. Each photodetector of the light detection unit 30 detects the corresponding polarization component and outputs a detection signal indicating the detected light intensity.

[0045] Note that in the above configuration, the condensing positions of each polarization component of the target light L0 and each photodetector of the corresponding light detection unit 30 are arranged at the vertices of a regular hexagon on the condensing surface 35. However, the condensing positions on the condensing surface 35 and the arrangement of the photodetectors are not limited to such a configuration, and specifically, various arrangement configurations can be used.

[0046] Referring back to FIG. 2, the analysis unit 50 inputs the detection signals from the respective photodetectors of the light detection unit 30, and performs necessary analysis on the detection results of the respective polarization components of the target light L0 by the light detection unit 30. Specifically, for example, the analysis unit 50 obtains the Stokes parameters of the target light L0 based on the detection results by the light detection unit 30.

[0047] In the configuration shown in FIG. 2, in the analysis unit 50, based on the detection result of the linearly polarized component L in the x direction by the photodetector 311 11 and the detection result of the linearly polarized component L in the y direction by the photodetector 312 12 , the Stokes parameter S1 is obtained. Also, based on the detection result of the linearly polarized component L at +45° by the photodetector 321 21 and the detection result of the linearly polarized component L at -45° by the photodetector 322 22 , the Stokes parameter S2 is obtained. Further, based on the detection result of the left-handed circularly polarized component L by the photodetector 331 31 and the detection result of the right-handed circularly polarized component L by the photodetector 332 32 , the Stokes parameter S3 is obtained.

[0048] A configuration including such an analysis unit 50 is suitable, for example, when applying the optical reception device 1A to optical communication using the SVM-DD method. Note that such an analysis unit 50 may not be provided if it is unnecessary.

[0049] The specific configuration of the structure pattern 20 in the polarization separation layer 15 of the polarization separation element 10 will be described with reference to FIG. 4. FIG. 4 is a plan view schematically showing the configuration of the structure pattern 20 in the polarization separation element 10.

[0050] Each of the plurality of structures constituting the structure pattern 20 has a two-fold rotational symmetry shape on the upper surface 12 of the substrate 11. In FIG. 4, an elliptic cylinder is shown as a preferred example of such a structure. The length in the major axis direction, the length in the minor axis direction, and the inclination angle with respect to the x-axis that define the shape of each elliptic cylinder are individually and appropriately set corresponding to the function of the structure pattern 20.

[0051] Also, in the structure pattern 20, the size of each of the plurality of structures on the upper surface 12 of the substrate 11 and the height from the upper surface 12 are preferably set to be less than the wavelength λ of the target light L0 respectively. Here, regarding the size of the structure, for example, when the structure is an elliptical column, it is preferable to set the length in the major axis direction to be less than the wavelength λ of the target light L0. Also, in the structure pattern 20, the arrangement interval of the plurality of structures is preferably set to be less than the wavelength λ of the target light L0. By setting the size, height, and arrangement interval of the plurality of structures in the structure pattern 20 as described above with respect to the wavelength λ, this structure pattern 20 functions as a sub-wavelength metasurface structure.

[0052] The polarization separation layer 15 separates each polarization component included in the target light L0 as described above and condenses them at the corresponding condensing positions on the condensing surface 35. In order to realize the functions of separating and condensing such polarization components, the sub-wavelength structure pattern 20 including a plurality of structures is composed of three types of structure patterns, namely, a first structure pattern 21, a second structure pattern 22, and a third structure pattern 23, as shown in FIG. 4.

[0053] The first structure pattern 21 is composed of a plurality of first structures 26, and condenses the x-direction linearly polarized light component L 11 and the y-direction linearly polarized light component L 12 at the first and second condensing positions where the photodetectors 311 and 312 are respectively arranged.

[0054] The second structure pattern 22 is composed of a plurality of second structures 27, and condenses the +45° linearly polarized light component L 21 and the -45° linearly polarized light component L 22 at the third and fourth condensing positions where the photodetectors 321 and 322 are respectively arranged.

[0055] The third structure pattern 23 is composed of a plurality of third structures 28, and condenses the left-handed circularly polarized light component L 31 and the right-handed circularly polarized light component L 32They are respectively condensed onto the fifth and sixth light condensing positions where the photodetectors 331 and 332 are arranged.

[0056] Regarding the arrangement on the substrate 11 of the first to third structures 26 to 28 that constitute the first to third structure patterns 21 to 23, as shown by the solid lines connecting the structures in FIG. 4, the structure pattern 20 has a linear or triangular pattern composed of the first structure 26, the second structure 27, and the third structure 28 as a unit pattern 25, and a plurality of unit patterns 25 are two-dimensionally arranged on the upper surface 12 of the substrate 11.

[0057] In the configuration shown in FIG. 4, the unit pattern 25 is an equilateral triangular pattern, with the first structure 26 arranged at the upper vertex, the second structure 27 arranged at the lower left vertex, and the third structure 28 arranged at the lower right vertex. Also, as shown in FIG. 5, the structure pattern 20 shown in FIG. 4 is configured in a hexagonal lattice (regular triangular lattice) together with the hexagonal lattice A to show the arrangement of the first to third structures 26 to 28.

[0058] Regarding the arrangement of the first to third structures 26 to 28 shown in FIGS. 4 and 5, as shown by the straight lines B1 to B3 extending in the y-axis direction in FIG. 5, the straight line B2 where the second structures 27 are arranged at equal intervals, the straight line B1 where the first structures 26 are arranged at equal intervals, and the straight line B3 where the third structures 28 are arranged at equal intervals are arranged at equal intervals in the x-axis direction and in an alternating configuration of the structure arrangements.

[0059] Regarding the separation and condensation of each polarization component by the structure pattern 20, specifically, the plurality of first structures 26 of the first structure pattern 21 include a structure used for condensing the linearly polarized light component L in the x direction 11 and a structure used for condensing the linearly polarized light component L in the y direction 12 . The plurality of second structures 27 of the second structure pattern 22 include a structure used for condensing the +45° linearly polarized light component L 21 and a structure used for condensing the -45° linearly polarized light component L 22 . The plurality of third structures 28 of the third structure pattern 23 include the left-handed circularly polarized light component L31 a structure used for condensing 31 , and a right-handed circular polarization component L 32 and a structure used for condensing 32 .

[0060] Regarding the arrangement of the above six types of structures in the first to third structure patterns 21 to 23, for example, it can be arranged as shown in FIG. 6. The structure pattern 20 shown in FIG. 6 is composed of a plurality of unit patterns 25, including a first unit pattern 251 shown by a solid line and a second unit pattern 252 shown by a broken line.

[0061] The first unit pattern 251 includes a structure 261 used for condensing the x-direction linearly polarized light component L among the first structures 26, a structure 271 used for condensing the +45° linearly polarized light component L among the second structures 27, and a structure 281 used for condensing the left-handed circular polarization component L among the third structures 28. The second unit pattern 252 includes a structure 262 used for condensing the y-direction linearly polarized light component L among the first structures 26, a structure 272 used for condensing the -45° linearly polarized light component L among the second structures 27, and a structure 282 used for condensing the right-handed circular polarization component L among the third structures 28. 11 a structure 261 used for condensing 11 , a structure 271 used for condensing the +45° linearly polarized light component L among the second structures 27, and a structure 281 used for condensing the left-handed circular polarization component L among the third structures 28. 21 a structure 271 used for condensing 21 , and a structure 281 used for condensing the left-handed circular polarization component L among the third structures 28. 31 a structure 281 used for condensing 31 . The second unit pattern 252 includes a structure 262 used for condensing the y-direction linearly polarized light component L among the first structures 26, a structure 272 used for condensing the -45° linearly polarized light component L among the second structures 27, and a structure 282 used for condensing the right-handed circular polarization component L among the third structures 28. 12 a structure 262 used for condensing 12 , a structure 272 used for condensing the -45° linearly polarized light component L among the second structures 27, and a structure 282 used for condensing the right-handed circular polarization component L among the third structures 28. 22 a structure 272 used for condensing 22 , and a structure 282 used for condensing the right-handed circular polarization component L among the third structures 28. 32 a structure 282 used for condensing 32 .

[0062] In addition, regarding the arrangement of the above six types of structures, in addition to the configuration shown in FIG. 6, specifically, various configurations can be used. Generally, the structure pattern 20 includes, as a plurality of unit patterns 25, a structure used for condensing one of the x-direction linearly polarized light component L 11 and the y-direction linearly polarized light component L 12 among the first structures 26, a structure used for condensing one of the +45° linearly polarized light component L 21 and the -45° linearly polarized light component L 22 among the second structures 27, and a structure used for condensing one of the left-handed circular polarization component L 31 and the right-handed circular polarization component L 32The first unit pattern composed of a structure used for one of the light condensations, and the x-direction linearly polarized light component L among the first structures 26 11 and the y-direction linearly polarized light component L 12 The structure used for the other light condensation, the +45° linearly polarized light component L among the second structures 27 21 and the -45° linearly polarized light component L 22 The structure used for the other light condensation, and the left-handed circularly polarized light component L among the third structures 28 31 and the right-handed circularly polarized light component L 32 It is preferably configured to include a second unit pattern composed of a structure used for the other light condensation.

[0063] FIG. 7 is an optical microscope image showing the overall configuration of a specific configuration example of the polarization separation element 10. FIG. 8 is an electron microscope image showing a partial enlargement of the configuration of the structure pattern 20 in the polarization separation element 10. This polarization separation element 10 is formed in a circular shape with a diameter of 500 μm. The specific structure, material, etc. of the configuration example of the polarization separation element shown in FIGS. 7 and 8 will be described in detail later.

[0064] The effects of the polarization separation element 10 according to the above embodiment and the optical reception device 1A using the polarization separation element 10 will be described.

[0065] In the polarization separation element 10 shown in FIGS. 2 and 4, in the polarization separation layer 15 on the substrate 11, a structure pattern 20 including a plurality of structures is formed, and using this structure pattern 20, the first to sixth polarization components included in the target light L0 incident from the lower surface 13 side of the substrate 11 are separated and condensed at different first to sixth condensation positions on the condensing surface 35, respectively. Further, the structure pattern 20 on the substrate 11 is composed of a first structure pattern 21 that separates and condenses the first and second polarization components, a second structure pattern 22 that separates and condenses the third and fourth polarization components, and a third structure pattern 23 that separates and condenses the fifth and sixth polarization components.

[0066] And in such a configuration, regarding the arrangement of a plurality of structures on the upper surface 12 of the substrate 11, a pattern consisting of three structures, namely the first structure 26, the second structure 27, and the third structure 28, is defined as the unit pattern 25, and a plurality of unit patterns 25 are two-dimensionally arranged on the substrate 11 in a predetermined arrangement pattern. According to such a configuration, as shown in FIG. 4, the first to third structures 26 to 28 are uniformly and dispersedly arranged on the substrate 11, and the first to sixth polarization components of the target light L0 incident on the polarization separation element 10 can be separated and condensed with good symmetry in a simple configuration.

[0067] Further, the optical receiver 1A shown in FIG. 2 is configured to include the polarization separation element 10 having the above configuration and a photodetection unit 30 that detects each of the first to sixth polarization components of the target light L0 condensed at the first to sixth condensing positions on the condensing surface 35. According to such a configuration, the light intensity of each polarization component of the target light L0 separated and condensed by the polarization separation element 10 can be suitably detected by the photodetection unit 30.

[0068] In the polarization separation element 10 having the above configuration, as described above, it is preferable to set the size, height, and arrangement interval of each of the plurality of structures constituting the structure pattern 20 to be less than the wavelength of the target light L0. Thereby, a sub-wavelength metasurface structure for separating and condensing the polarization components of the target light L0 can be suitably realized by the structure pattern 20 including a plurality of structures on the substrate 11.

[0069] Regarding the material of the plurality of structures in the polarization separation layer 15, as described above, a material having a refractive index higher than that of the substrate 11 is used. Specifically, it is preferable to set the refractive index difference between the material of the substrate 11 and the material of the plurality of structures in the polarization separation layer 15 to be 0.25 or more. Thereby, separation and condensation of the polarization components of the target light L0 by the structure pattern 20 including a plurality of structures can be suitably realized.

[0070] For reference, regarding the materials described above for the substrate 11 and the polarization separation layer 15, the refractive index values for light with a wavelength of 1.55 μm are as follows. For the substrate 11, the refractive index of quartz (SiO2) is 1.44, that of BK7 is 1.50, that of Al2O3 is 1.75, that of MgF2 is 1.36, that of CaF2 is 1.42, and that of LiF is 1.38.

[0071] Also, for the polarization separation layer 15, the refractive index of Si is 3.478, that of GaAs is 3.374, that of InP is 3.167, that of GaN is 2.3, that of TiO2 is 2.4, that of Ta2O5 is 2.09, that of Nb2O5 is 2.17, that of HfO2 is 1.82, that of ZrO2 is 2.07, that of Y2O3 is 1.90, that of Gd2O3 is 1.7, that of CeO2 is 1.7, that of Al2O3 is 1.75, and that of Si3N4 is 1.989.

[0072] The specific configuration of the structure pattern 20 in the polarization separation element 10 and the design method of the structure (elliptical cylinder) constituting the structure pattern 20 will be described. The polarization separation element 10 composed of the substrate 11 and the polarization separation layer 15 can be manufactured, for example, by using a SOQ (Silicon on Quartz) substrate in which a silicon (Si) layer is formed on a quartz (SiO2) substrate and subjecting the Si layer to meta-surface structuring by microfabrication technology. Also, in the following, it is assumed that the structure constituting the structure pattern 20 is an elliptical cylinder.

[0073] The first structure pattern 21 is designed such that the light transmission phase distribution φ(x, y) thereby has the x-direction linearly polarized component L 11 and the y-direction linearly polarized component L 12 focused at the first and second focusing positions. That is, the first structure pattern 21 is designed to operate as a polarization separation and focusing lens based on the S1 axis of the Poincaré sphere.

[0074] The second structure pattern 22 is designed such that the light transmission phase distribution φ(x, y) thereby has the +45° linearly polarized component L 21 and the -45° linearly polarized component L 22is designed to condense light at the third and fourth light-condensing positions. The phase distribution by this second structure pattern 22 can be obtained, for example, by rotating the phase distribution of the above-described first structure pattern 21 by 45° from the x-axis.

[0075] The third structure pattern 23 is such that the light transmission phase distribution φ(x, y) thereby has a left-handed circular polarization component L 31 and a right-handed circular polarization component L 32 and is designed to condense light at the fifth and sixth light-condensing positions. The phase distribution by this third structure pattern 23 can be obtained, for example, by creating a structure in which the phase difference between the x-direction and the y-direction is π and tilting it by an angle θ with respect to the x-axis.

[0076] FIG. 9 and FIG. 10 are diagrams showing the setting of the shape of each structure in the structure pattern 20. As shown in FIGS. 9(a) and 9(b), the elliptical column shape of the structure is D indicating the length of the major axis in the x-direction u and D indicating the length of the minor axis in the y-direction v and is determined by the rotation angle θ of the elliptical column from the x-axis. Also, in FIG. 9(a), d indicates the arrangement interval of the elliptical columns that are the structures. For example, in the case of the hexagonal lattice configuration shown in FIG. 4, the arrangement interval d is constant among all the elliptical columns.

[0077] Also, as shown in FIG. 10, for the target light L0 to be subjected to polarization separation by the polarization separation element 10, the equivalent amplitude of the light polarized in the x-direction is |t u |, its transmission phase is φ u , the equivalent amplitude of the light polarized in the y-direction is |t v |, and its transmission phase is φ v . Also, in FIG. 10, t indicates the thickness of the substrate 11, and h indicates the height of the polarization separation layer 15 including the structure pattern 20. In one example, the thickness of the SiO2 substrate 11 is t = 625 μm, the height of the Si polarization separation layer 15 is h = 1050 nm, and the arrangement interval of the elliptical columns is d = 700 nm.

[0078] In the design of the plurality of structures constituting the structure pattern 20, first, the lengths D u and Dv The phase φ obtained when u φ v is changed is obtained by the rigorous coupled wave analysis (RCWA), which is an electromagnetic field calculation method. The specific calculation method is the same as that described in Non-Patent Document 1, for example.

[0079] FIG. 11 shows the transmission phase φ of light in the structure pattern 20 u φ v , and the length D, which is the shape parameter of the elliptical cylinder u D v The relationship between u φ v is shown. FIG. 11(a) shows the length D u corresponding to each value of the phase φ u φ v , and FIG. 11(b) shows the length D v corresponding to each value of the phase φ u φ v . Here, the wavelength λ of the target light L0 is set to 1.55 μm, the arrangement interval (lattice constant) d of the elliptical cylinders in the hexagonal lattice structure pattern 20 is set to 700 nm, and the rotation angle θ with respect to the x-axis is set to 0° for the calculation. By using the calculation results shown in FIGS. 11(a) and 11(b), the lengths D u D v of the elliptical cylinders for obtaining the required transmission phases φ

[0080] In the above, the calculation was performed with the rotation angle of the elliptical cylinder being θ = 0°. For the ±45° linearly polarized components, the above elliptical cylinder may be rotated by ±45° from the x-axis. For the left-handed circularly polarized component, the required transmission phase is φ L = φ u + 2θ, and for the right-handed circularly polarized component, the required transmission phase is φ R = φ u - 2θ. Also, the necessary condition at this time is that D v = φ u + π, and D u D v are determined.

[0081] ​ Regarding the focusing of each polarization component of the target light L0 by the structure pattern 20, in order to realize the function as a metalens that focuses the polarization component to the focusing position on the focusing surface 35, the metasurface structure in the first to third structure patterns 21 to 23 is given a phase distribution represented by the following formula (2). [Number] Here, (x0, y0) are the coordinates of the focusing position of each polarization component on the focusing surface 35, and f is the focal length.

[0082] Specifically, the focal length f is set to 2.5 mm (NA~0.10), and the distance between the focusing positions on the focusing surface 35 (the distance between the vertices of the hexagon, see FIG. 3) is set to 50 μm. In this case, for example, taking the center position of the polarization separation element 10 as the origin and μm as the unit, the x-direction linearly polarized light component L 11 The center coordinates of the first structure pattern 21 that focuses are (0, 50), and the y-direction linearly polarized light component L 12 The center coordinates of the first structure pattern 21 that focuses are (43.3, 25).

[0083] The +45° linearly polarized light component L 21 The center coordinates of the second structure pattern 22 that focuses are (43.3, -25), and the -45° linearly polarized light component L 22 The center coordinates of the second structure pattern 22 that focuses are (0, -50). The left-handed circularly polarized light component L 31 The center coordinates of the third structure pattern 23 that focuses are (-43.3, -25), and the right-handed circularly polarized light component L 32 The center coordinates of the third structure pattern 23 that focuses are (-43.3, 25).

[0084] FIG. 12 is a diagram showing the phase distribution corresponding to each polarization component (wavelength 1.55 μm) in the structure pattern 20. FIGS. 12(a) and (b) show the phase distribution in the first structure pattern 21 (MS1), FIGS. 12(c) and (d) show the phase distribution in the second structure pattern 22 (MS2), and FIGS. 12(e) and (f) show the phase distribution in the third structure pattern 23 (MS3). Note that, regarding the shape and size of the polarization separation element 10, in consideration of the coupling with the optical fiber, it is a circle with a diameter of φ500 μm. In this way, the configuration of the structure pattern 20 including a plurality of structures in the polarization separation element 10 can be determined.

[0085] An example of the manufacturing method of the polarization separation element 10 according to the above embodiment will be briefly described. First, a SOQ substrate having a SiO2 layer to be the substrate 11 and a Si layer to be the polarization separation layer 15 is prepared, and the SOQ substrate is cleaned by a general organic cleaning. Here, for example, ultrasonic cleaning of the SOQ substrate is performed in acetone, IPA, or ethanol. Also, in the SOQ substrate, the thickness t of the SiO2 layer is, for example, 625 μm, and the height h of the Si layer is, for example, 1050 nm.

[0086] Subsequently, for the purpose of improving the wettability and adhesion between the Si layer and the resist, a surfactant (for example, Tokyo Ohka Kogyo OAP) is applied, and after spin coating at, for example, 3000 rpm for 30 sec, baking is performed at 120° C. for 1 minute. Then, an EB resist (for example, ZEP520A) is applied on the Si layer, and by spin coating, a thickness of about 200 nm is obtained.

[0087] After that, by performing drawing using an electron beam lithography apparatus and development, a resist pattern of the metasurface structure corresponding to the structure pattern 20 is formed. Next, by using the resist pattern as a protective film and performing dry etching of the Si layer, a metasurface structure of the Si layer that becomes the structure pattern 20 is formed. As the gas for dry etching, for example, SF6, C4F8, or Ar can be used. After that, by removing the EB resist used as a mask when etching the Si layer by O2 ashing, the polarization separation element 10 having the above configuration can be manufactured.

[0088] The characteristics of the polarization separation element 10 according to the above-described configuration example will be described together with specific measurement data. FIG. 13 is a diagram showing the configuration of a measurement system 2A for evaluating the characteristics of the polarization separation element 10. The measurement system 2A shown in FIG. 13 is composed of a laser light source 101, a polarization controller 102, a fiber collimator 103, a polarizer 104, a half-wave plate 105, a quarter-wave plate 106, a diaphragm 107, an objective lens 108, a tube lens 109, and an imaging device 110.

[0089] In the above measurement system 2A, the polarization separation element 10 to be evaluated is disposed between the diaphragm 107 and the objective lens 108. As the laser light source 101, for example, a wavelength-variable laser can be used. As the imaging device 110, for example, an InGaAs camera (C12741-03 manufactured by Hamamatsu Photonics, light-receiving surface size: 12.8 mm (H) × 10.24 mm (V)) can be used.

[0090] The target light L0 having a wavelength λ = 1.55 μm output from the laser light source 101 reaches the polarization separation element 10 through the polarization controller 102, the fiber collimator 103, the polarizer 104, the half-wave plate 105, the quarter-wave plate 106, and the diaphragm 107. At this time, by rotating the polarizer 104, the half-wave plate 105, and the quarter-wave plate 106, the target light L0 can be set to a desired polarization state.

[0091] Each polarized component separated and condensed by the polarization separation element 10 is made into parallel light by the objective lens 108 with a magnification of 50 times, and is imaged on the light receiving surface of the imaging device 110 by the tube lens 109. The characteristics of the polarization separation element 10 can be evaluated based on the light intensity of each polarized component in the two-dimensional image acquired by the imaging device 110.

[0092] FIG. 14 is a diagram showing the intensity distribution of the target light L0 acquired by the imaging device 110. FIG. 14(a) shows the measurement result when the Stokes vector (S1, S2, S3) of the target light L0 to be polarization-separated is (+1, 0, 0), FIG. 14(b) shows the measurement result when it is (-1, 0, 0), FIG. 14(c) shows the measurement result when it is (0, +1, 0), FIG. 14(d) shows the measurement result when it is (0, -1, 0), FIG. 14(e) shows the measurement result when it is (0, 0, +1), and FIG. 14(f) shows the measurement result when it is (0, 0, -1).

[0093] Also, FIGS. 15(a) to (f) are graphs showing the Stokes parameters S1, S2, and S3 of the target light L0 respectively obtained from the measurement results of FIGS. 14(a) to (f). As shown in FIGS. 14 and 15, according to the polarization separation element 10 having the above configuration, each polarized component included in the target light L0 can be preferably separated and condensed, and its Stokes parameters can be accurately measured.

[0094] FIG. 16 is a diagram showing the Stokes vector reproduced from the intensity distribution of the target light L0 acquired by the imaging device 110. In FIG. 16, the sphere shown in the Stokes space represents the Poincare sphere, the data shown by the solid line represents the theoretically obtained polarization state, and the data shown by the plotted points represents the result of actually measuring the polarization state of the target light.

[0095] Also, in FIG. 16, data C1 represents the data when the half-wave plate (HWP) 105 and the quarter-wave plate (QWP) 106 are rotated while maintaining the relationship of θ QWP =2θ HWP , and data C2 represents the data when the half-wave plate 105 is rotated at θHWP shows the data when θ of the quarter-wave plate 106 is rotated with it fixed at 0°, and data C3 shows the data when θ of the quarter-wave plate 106 is rotated with the half-wave plate 105 fixed at θ QWP = 45°. As shown in these data C1 to C3, the theoretically obtained polarization state and the polarization state of the measurement result are in good agreement. HWP = 45°. As shown in these data C1 to C3, the theoretically obtained polarization state and the polarization state of the measurement result are in good agreement. QWP = 45°. As shown in these data C1 to C3, the theoretically obtained polarization state and the polarization state of the measurement result are in good agreement.

[0096] Another configuration example of the structure pattern 20 shown in FIGS. 4 and 5 will be described. In the above embodiment, for the structure pattern 20, the unit pattern 25 is a regular triangular pattern, and the structure pattern 20 is configured in a hexagonal lattice. For the structure pattern 20 used for separating and condensing each polarization component of the target light L0, various configurations other than the above-described configuration can be specifically used. In FIGS. 17 to 19 below, each structure is illustrated as a circle, and the arrangement pattern of a plurality of structures is schematically shown.

[0097] FIG. 17 is a diagram showing a first modification of the configuration of the structure pattern 20 in the polarization separation element 10. In the configuration shown in FIG. 17, an isosceles triangular pattern composed of a first structure 26, a second structure 27, and a third structure 28 is used as the unit pattern 25a. Also, the overall configuration of the structure pattern 20 composed of a plurality of unit patterns 25a is configured in a hexagonal lattice, similar to the configuration of FIG. 5.

[0098] FIG. 18 is a diagram showing a second modification of the configuration of the structure pattern 20 in the polarization separation element 10. In the configuration shown in FIG. 18, as a plurality of unit patterns, it includes a plurality of types (two types in the example of FIG. 18) of unit patterns 25b and 25c in which the arrangements of the first structure 26, the second structure 27, and the third structure 28 are different from each other. The unit patterns 25b and 25c are both regular triangular patterns.

[0099] In the unit pattern 25b, similar to the unit pattern 25 in FIG. 4, the first structure 26 is arranged at the upper vertex of the equilateral triangular pattern, the second structure 27 is arranged at the lower left vertex, and the third structure 28 is arranged at the lower right vertex. On the other hand, in the unit pattern 25c, the first structure 26 is arranged at the upper vertex of the equilateral triangular pattern, the third structure 28 is arranged at the lower left vertex, and the second structure 27 is arranged at the lower right vertex. The arrangements of the second structure 27 and the third structure 28 are interchanged compared to the unit pattern 25b.

[0100] In the structure pattern 20, in a configuration where a plurality of types of unit patterns are provided as the plurality of unit patterns, generally, it is preferable to use unit patterns in which the arrangement order or pattern shape, etc. of the first structure 26, the second structure 27, and the third structure 28 are different from each other. Also, regarding the number of types of unit patterns, in the above configuration, it is two types, but it may be three or more types.

[0101] FIG. 19 is a diagram showing a third modified example of the configuration of the structure pattern 20 in the polarization separation element 10. In the configuration shown in FIG. 19, a right-angled isosceles triangular pattern composed of the first structure 26, the second structure 27, and the third structure 28 is used as the unit pattern 25d. Also, regarding the overall configuration of the structure pattern 20 composed of a plurality of unit patterns 25d, it is different from the configuration in FIG. 5 and is configured in a square lattice.

[0102] As described above, regarding the structure pattern 20 in the polarization separation layer 15 formed on the substrate 11, specifically, various pattern configurations can be used. Note that, regarding the overall configuration of the structure pattern 20, in the configurations of FIGS. 5, 17, and 18, it is a hexagonal lattice pattern, and in the configuration of FIG. 19, it is a square lattice pattern. Regarding these patterns, in the square lattice pattern of FIG. 19, the arrangement interval between the second structure 27 and the third structure 28 is longer than the arrangement interval between the first structure 26 and the second structure 27 or the third structure 28. Considering such a point, it is more preferable that the overall configuration of the structure pattern 20 is a hexagonal lattice pattern in which the arrangement intervals of all the structures are equal.

[0103] The polarization separation element and the optical reception device are not limited to the above-described embodiments and configuration examples, and various modifications are possible. For example, regarding the first to sixth polarization components of the target light to be separated and condensed by the polarization separation element, in the above embodiment, they are the x-direction linearly polarized component, the y-direction linearly polarized component, the +45° linearly polarized component, the -45° linearly polarized component, the left-handed circularly polarized component, and the right-handed circularly polarized component, but it is not limited to such a configuration, and specifically, various combinations of polarization components may be used. Further, regarding the shape of the two-fold rotationally symmetric structure constituting the structure pattern, it is not limited to the above-described elliptical cylinder, and specifically, structures of various shapes may be used.

[0104] Also, regarding the configuration of the transmissive polarization separation element, in the above embodiment, the lower surface of the substrate is the light incident surface and the upper surface of the polarization separation layer is the light emission surface. However, it is not limited to such a configuration, and the upper surface of the polarization separation layer may be the light incident surface and the lower surface of the substrate may be the light emission surface. Generally, the polarization separation element has a transmissive configuration in which one of the lower surface of the substrate and the upper surface of the polarization separation layer is the light incident surface and the other is the light emission surface. The polarization separation layer may be configured to separate the first to sixth polarization components included in the target light incident from the light incident surface side and condense them at different first to sixth condensing positions on the condensing surface set on the light emission surface side. Further, in the configuration in which the lower surface of the substrate is the light emission surface, for example, a light detector such as a photodiode serving as a light detection unit may be directly bonded to the lower surface of the substrate.

[0105] Also, in the optical receiver including the polarization separation element, regarding the configuration of the light detection unit that detects each polarization component of the target light, in the above embodiment, the configuration is to provide the first to sixth light detectors that detect the first to sixth polarization components respectively. However, it is not limited to such a configuration. For example, as shown in the evaluation measurement system of FIG. 13, a single imaging device may be used as the light detection unit. As such an imaging device, for example, a CCD camera, a CMOS camera, etc. can be used.

Industrial Applicability

[0106] The present invention can be used as a polarization separation element and an optical receiver that can suitably perform separation and condensation of polarization components of target light with a simple configuration.

Explanation of Reference Numerals

[0107] 1A... Optical receiver, 10... Polarization separation element, 11... Substrate, 12... Upper surface, 13... Lower surface, 15... Polarization separation layer, 20... Structure pattern, 21... First structure pattern, 22... Second structure pattern, 23... Third structure pattern, 25, 251, 252, 25a to 25d... Unit pattern, 26, 261, 262... First structure, 27, 271, 272... Second structure, 28, 281, 282... Third structure, 30…Light detection unit, 311, 312, 321, 322, 331, 332…Photodetectors, 35…Condensing surface, 50…Analysis unit, 60…Optical fiber, 2A…Measurement system, 101…Laser light source, 102…Polarization controller, 103…Fiber collimator, 104…Polarizer, 105…Half-wave plate, 106…Quarter-wave plate, 107…Aperture, 108…Objective lens, 109…Tube lens, 110…Imaging device, L0…Target light, L 11 …x-direction linearly polarized light component, L 12 …y-direction linearly polarized light component, L 21 …+45° linearly polarized light component, L 22 …-45° linearly polarized light component, L 31 …Left-handed circularly polarized light component, L 32 …Right-handed circularly polarized light component.

Claims

1. A substrate having transparency to target light, and a polarization separation layer formed on the upper surface of the substrate, having a structure pattern including a plurality of structures that are each rotationally symmetric about two axes, made of a material having transparency to the target light and a refractive index higher than that of the substrate comprising a transmissive configuration in which one of the lower surface of the substrate and the upper surface of the polarization separation layer is a light incident surface and the other is a light exit surface, the polarization separation layer separates first to sixth polarization components included in the target light incident from the light incident surface side, and condenses them at first to sixth condensing positions different from each other on a condensing surface set on the light exit surface side, and the structure pattern includes a plurality of first structures, and a first structure pattern that condenses the first polarization component and the second polarization component at the first condensing position and the second condensing position, respectively, includes a plurality of second structures, and a second structure pattern that condenses the third polarization component and the fourth polarization component at the third condensing position and the fourth condensing position, respectively, includes a plurality of third structures, and a third structure pattern that condenses the fifth polarization component and the sixth polarization component at the fifth condensing position and the sixth condensing position, respectively and has a linear or triangular pattern composed of the first structure, the second structure, and the third structure as a unit pattern, and a plurality of the unit patterns are two-dimensionally arranged on the upper surface of the substrate, a polarization separation element.

2. The size of each of the plurality of structures constituting the structure pattern on the upper surface of the substrate and the height from the upper surface of the substrate are each set to be less than the wavelength of the target light, The polarization separation element according to claim 1.

3. The arrangement interval of the plurality of structures in the structure pattern is set to be less than the wavelength of the target light, The polarization separation element according to claim 1 or 2.

4. The structure that is rotationally symmetric about two axes is an elliptical cylinder, The polarization separation element according to any one of claims 1 to 3.

5. The first to sixth polarization components are, respectively, an x-direction linearly polarized component, a y-direction linearly polarized component, a +45° linearly polarized component, a -45° linearly polarized component, a left-handed circularly polarized component, and a right-handed circularly polarized component, The polarization separation element according to any one of claims 1 to 4.

6. The unit pattern is an equilateral triangular pattern, and the structure pattern is configured in a hexagonal lattice, The polarization separation element according to any one of claims 1 to 5.

7. The unit pattern is a right-angled isosceles triangular pattern, and the structure pattern is configured in a square lattice. The polarization separation element according to any one of claims 1 to 5.

8. The structure pattern includes, as a plurality of the unit patterns, a first unit pattern including a structure used for condensing one of the first polarization component and the second polarization component in the first structure, a structure used for condensing one of the third polarization component and the fourth polarization component in the second structure, and a structure used for condensing one of the fifth polarization component and the sixth polarization component in the third structure, and a second unit pattern including a structure used for condensing the other of the first polarization component and the second polarization component in the first structure, a structure used for condensing the other of the third polarization component and the fourth polarization component in the second structure, and a structure used for condensing the other of the fifth polarization component and the sixth polarization component in the third structure The polarization separation element according to any one of claims 1 to 7.

9. The structure pattern includes, as a plurality of the unit patterns, a plurality of types of unit patterns in which the arrangements of the first structure, the second structure, and the third structure are different from each other. The polarization separation element according to any one of claims 1 to 8.

10. The refractive index difference between the substrate and the material of the plurality of structures is 0.25 or more. The polarization separation element according to any one of claims 1 to 9.

11. The material of the plurality of structures is a dielectric material. The polarization separation element according to any one of claims 1 to 10.

12. The material of the plurality of structures is a semiconductor material or an oxide material. The polarization separation element according to any one of claims 1 to 10.

13. The first to sixth condensing positions are respectively arranged at the vertices of a regular hexagon set on the condensing surface. The polarization separation element according to any one of claims 1 to 12.

14. A polarization separation element according to any one of claims 1 to 13, and a light detection unit that detects each of the first to sixth polarization components of the target light condensed at the first to sixth condensing positions on the condensing surface A light receiving device comprising.

15. The light detection unit is arranged at the first to sixth condensing positions and has first to sixth photodetectors that respectively detect the first to sixth polarization components. The light receiving device according to claim 14.

16. The optical receiver according to claim 14 or 15, further comprising an analysis unit that analyzes the detection results of the first to sixth polarization components by the light detection unit to obtain the Stokes parameters of the target light.

Images