Spatial light modulator and communication system

Abstract

The objective of the present invention is to provide a spatial light modulator capable of high-speed operation. The spatial light modulator 1 comprises: a ferroelectric liquid crystal element having a first surface, a second surface disposed to face the first surface, and a liquid crystal layer disposed between the first surface and the second surface, operating in an in-plane mode, the ferroelectric liquid crystal element capable of controlling a phase amount applied to light transmitted through the liquid crystal layer by a voltage applied between the first surface and the second surface; and a reflection mechanism optically connected to the ferroelectric liquid crystal element. The ferroelectric liquid crystal element emits incident light, which is circularly polarized and incident on the second surface, from the first surface to the reflection mechanism. The reflection mechanism reflects the incident light incident from the ferroelectric liquid crystal element and emits, to the ferroelectric liquid crystal element, reflected light obtained by inverting the rotation direction of the incident light, which is circularly polarized when viewed from the ferroelectric liquid crystal element. The ferroelectric liquid crystal element emits, from the second surface, the reflected light incident on the first surface.

Description

Spatial Light Modulator and Communication System

[0001] The present disclosure relates to a spatial light modulator and a communication system.

[0002] Spatial light modulators using LCOS (Liquid Crystal On Silicon) are known. For example, Non-Patent Document 1 describes a spatial light modulator capable of realizing polarization-independent phase modulation by having a superstructure surface.

[0003] "Metasurface-enabled polarization-independent LCoS spatial light modulator for 4K resolution and beyond" by Zhaoxiang Zhu et al., June 19, 2023, https: / / www.nature.com / articles / s41377-023-01202-6

[0004] However, the liquid crystal contained in the liquid crystal element of the spatial light modulator described in Non-Patent Document 1 is a nematic liquid crystal with a relatively slow response speed, and it is not easy to improve the response speed.

[0005] The present disclosure solves such problems and aims to provide a spatial light modulator capable of high-speed operation.

[0006] The spatial light modulator according to the present disclosure has a first surface, a second surface disposed opposite to the first surface, and a liquid crystal layer disposed between the first surface and the second surface and operating in an in-plane mode. It has a ferroelectric liquid crystal element capable of controlling the amount of phase imparted to the light transmitted through the liquid crystal layer by the voltage applied between the first surface and the second surface, and a reflection mechanism optically connected to the ferroelectric liquid crystal element; the ferroelectric liquid crystal element emits incident light, which is circularly polarized light incident on the second surface, from the first surface to the reflection mechanism, the reflection mechanism reflects the incident light incident from the ferroelectric liquid crystal element, and emits reflected light, whose rotation direction of the incident light, which is circularly polarized light as viewed from the ferroelectric liquid crystal element, is reversed, to the ferroelectric liquid crystal element, and the ferroelectric liquid crystal element emits the reflected light incident on the first surface from the second surface.

[0007] Furthermore, the spatial light modulator according to this disclosure preferably further includes an optical element that emits incident light to a ferroelectric liquid crystal element by converting linearly polarized light to circularly polarized light.

[0008] Furthermore, in the spatial light modulator according to this disclosure, it is preferable that the ferroelectric liquid crystal element has the characteristic of continuously changing its molecular longitudinal axis in response to the applied voltage.

[0009] Furthermore, in the spatial light modulator according to this disclosure, the ferroelectric liquid crystal element preferably contains a polymer-stabilized ferroelectric liquid crystal.

[0010] Furthermore, in the spatial light modulator according to this disclosure, the reflection mechanism is a reflection element having a metamaterial structure formed on the surface facing the ferroelectric liquid crystal element, and the metamaterial structure preferably has a plurality of unit structures arranged in an aligned manner.

[0011] Furthermore, in the spatial light modulator according to this disclosure, it is preferable that each of the multiple unit structures is arranged along the rubbing direction of the ferroelectric liquid crystal element.

[0012] The communication system according to this disclosure comprises a spatial light modulator according to this disclosure, a plurality of light-emitting units each emitting linearly polarized light having different wavelengths, a multiplexer emitting combined light obtained by combining the plurality of linearly polarized light emitted from the plurality of light-emitting units, an incident transmission path for transmitting the combined light to the spatial light modulator, and a plurality of outgoing transmission paths for transmitting reflected light. The spatial light modulator further comprises a single electrode and a plurality of pixel electrodes arranged two-dimensionally opposite each other with a liquid crystal layer in between, and is characterized in that the voltage applied to each of the plurality of pixel electrodes is controlled to change periodically at a predetermined pitch along the direction of arrangement of the plurality of outgoing transmission paths, thereby causing each of the plurality of light-emitting units to function as a diffraction grating that diffracts the component of the reflected light corresponding to the linearly polarized light, according to the wavelength of the linearly polarized light from that light-emitting unit, toward the outgoing transmission path corresponding to that light-emitting unit.

[0013] The communication system according to this disclosure comprises a spatial light modulator according to this disclosure, a plurality of light-emitting units each emitting linearly polarized light having different wavelengths, a multiplexer emitting combined light obtained by combining the plurality of linearly polarized light emitted from the plurality of light-emitting units, an incident transmission path for transmitting the combined light to the spatial light modulator, and a plurality of outgoing transmission paths for transmitting reflected light. The spatial light modulator further comprises a single electrode and a plurality of pixel electrodes arranged two-dimensionally opposite to the single electrode with a liquid crystal layer in between. Furthermore, for each of the plurality of light-emitting units, the voltage applied to each of the plurality of pixel electrodes is controlled to change periodically along the direction of the arrangement of the plurality of outgoing transmission paths at a pitch corresponding to the light-emitting unit, thereby functioning as a diffraction grating that diffracts the component of the reflected light corresponding to the linearly polarized light toward the outgoing transmission path corresponding to the light-emitting unit, according to the wavelength of the linearly polarized light from the light-emitting unit.

[0014] The spatial light modulator and communication system described herein enable high-speed operation.

[0015] Figure 1 is a block diagram of a communication system having a wavelength-selective switch according to an embodiment. Figure 2 is a diagram showing the electro-optical properties of the liquid crystal layer shown in Figure 1. Figure 3 is a diagram showing the electro-optical properties of the liquid crystal layer shown in Figure 2. Figure 4 is a diagram showing the relationship between the voltage applied to the pixel electrode and the amount of phase imparted to the light transmitted through the liquid crystal element. Figure 5(A) shows the propagation direction of incident and reflected light transmitted through the wavelength-selective switch, Figure 5(B) shows the polarization state of the incident light indicated by arrow A in (A), Figure 5(C) shows the polarization state of the incident light indicated by arrow B in (A), Figure 5(D) shows the polarization state of the incident light indicated by arrow C in (A), Figure 5(E) shows the polarization state of the incident light indicated by arrow D in (A), Figure 5(F) shows the polarization state of the incident light indicated by arrow E in (A), Figure 5(G) shows the polarization state of the incident light indicated by arrow F in (A), and Figure 5(H) shows the polarization state of the incident light indicated by arrow G in (A). Figure 6(A) shows the propagation direction of incident and reflected light passing through the wavelength selector switch; Figure 6(B) shows the polarization state of the incident light indicated by arrow A in (A); Figure 6(C) shows the polarization state of the incident light indicated by arrow B in (A); Figure 6(D) shows the polarization state of the incident light indicated by arrow C in (A); Figure 6(E) shows the polarization state of the incident light indicated by arrow D in (A); Figure 6(F) shows the polarization state of the incident light indicated by arrow E in (A); Figure 6(G) shows the polarization state of the incident light indicated by arrow F in (A); and Figure 6(H) shows the polarization state of the incident light indicated by arrow G in (A). Figure 7 is a cross-sectional view of an LCOS element according to a modified example. Figure 8(A) is a plan view of the reflector element 28, and Figure 8(B) is a cross-sectional view along the line A-A in (A). Figure 9 is a diagram showing the relationship between the rubbing direction of the liquid crystal element and the arrangement of the unit structure.

[0016] A wavelength-selective switch, which is an example of a spatial light modulator according to this disclosure, will be described below with reference to the drawings. However, it should be noted that the technical scope of the present invention is not limited to those embodiments, but extends to the invention described in the claims and its equivalents.

[0017] (Configuration and function of the wavelength selector switch according to the embodiment) Figure 1 is a block diagram of a communication system having a wavelength selector switch according to the embodiment, and Figure 2 is a cross-sectional view of the wavelength selector switch shown in Figure 1.

[0018] The communication system 100 includes a plurality of light-emitting units 101, a multiplexer 102, an incident transmission line 103, a plurality of output transmission lines 104, and a wavelength selection switch 1, and realizes WDM type communication. Each of the plurality of light-emitting units 101 has a light-emitting element such as a semiconductor laser, an isolator, and a polarizer, and polarizes the light emitted from the light-emitting element to linearly polarized light by the polarizer and emits it as incident light. The incident light emitted from each of the plurality of light-emitting units 101 has different wavelengths from each other. For example, the plurality of light-emitting units 101 output incident light that exists between approximately 1530 nm and 1560 nm at 100 GHz intervals. The multiplexer 102 is formed of optical elements such as a prism, an interference film filter, and a diffraction grating, and combines the incident light emitted from each of the plurality of light-emitting units 101 and emits the combined light obtained by combining the multiple incident lights to the incident transmission line 103. Each of the incident transmission line 103 and the multiple outgoing transmission lines 104 is an optical fiber. The incident transmission line 103 transmits the combined light, which is obtained by combining multiple incident light beams by the multiplexer 102, to the wavelength selector switch 1.

[0019] The wavelength selective switch 1 includes an optical element 10 and an LCOS (Liquid crystal on silicon) element 11. When combined light combined by the multiplexer 102 is incident on the wavelength selective switch 1, it emits each of the multiple incident light beams forming the combined light as reflected light in a direction corresponding to its wavelength. The wavelength selective switch 1 is a demultiplexer that separates each of the multiple incident light beams forming the combined light transmitted by the incident transmission line 103 by emitting the incident light as reflected light in an emission direction corresponding to the wavelength of the incident light. The multiple reflected light beams emitted from the wavelength selective switch 1 are incident on each of the multiple emission transmission lines 104. Each of the multiple emission transmission lines 104 transmits the reflected light incident on the wavelength selective switch 1 to a photoelectric conversion element (not shown), such as a photocoupler.

[0020] The optical element 10 is a λ / 4 wave plate (Quarter Wave Plate, QWP) and is placed between the incident transmission line 103 and the LCOS element 11. The optical element 10 applies a 90-degree phase difference (hereinafter also referred to as retardation) between the leading axis and lagging axis of each of the multiple incident light beams, which are linearly polarized light beams that form the combined light transmitted by the incident transmission line 103, and converts them into circularly polarized light before emitting them to the LCOS element 11.

[0021] The LCOS element 11 comprises a semiconductor substrate 20, a first electrode 21, a second electrode 22, a transparent substrate 23, a liquid crystal (LC) element 24, a conversion element 25, and a reflecting element 26. The LCOS element 11 functions as a diffraction grating that, when circularly polarized incident light is incident on it, emits circularly polarized reflected light in a desired direction according to a voltage applied to the liquid crystal element 24 via the first electrode 21 and the second electrode 22 from a control device (not shown).

[0022] The semiconductor substrate 20 is, for example, a silicon substrate, on which various elements such as MOSFETs and capacitors are formed, and the first electrode 21 is arranged to cover the elements formed on the surface. The first electrode 21 is made of a metal such as copper and is arranged to cover the entire surface of the semiconductor substrate 20. The second electrode 22 is a transparent electrode such as an ITO (Indium Tin Oxide) electrode and is arranged on the surface of the transparent substrate 23 facing the liquid crystal element 24, sandwiching the liquid crystal element 24 together with the first electrode 21. The first electrode 21 is formed as a plurality of pixel electrodes, and the second electrode 22 is formed as a single electrode. The plurality of pixel electrodes are arranged in a two-dimensional manner on the surface facing the liquid crystal element 24. That is, the plurality of pixel electrodes are arranged in a two-dimensional manner together with the single electrode, facing each other with the liquid crystal layer 24c, which will be described later, in between. Note that the first electrode 21 may be formed as a single electrode, and the second electrode 22 may be formed as a plurality of pixel electrodes. By forming either the first electrode 21 or the second electrode 22 as multiple pixel electrodes, the liquid crystal element 24 can have different voltages applied to each of the multiple pixel regions. The transparent substrate 23 is made of a transparent material such as glass, and the second electrode 22 is formed on the surface facing the liquid crystal element 24. The transparent substrate 23 seals the liquid crystal element 24 by sandwiching it together with the conversion element 25.

[0023] The liquid crystal element 24 has a first surface 24a, a second surface 24b, and a liquid crystal layer 24c. The phase amount applied to the light transmitted through the liquid crystal layer 24c can be controlled by controlling the voltage applied between the first surface 24a and the second surface 24b via the first electrode 21 and the second electrode 22. The first surface 24a is the surface in contact with the conversion element 25, and the second surface 24b is the surface in contact with the second electrode 22.

[0024] The liquid crystal layer 24c is a ferroelectric liquid crystal element containing a ferroelectric liquid crystal (FLC) that operates in in-plane mode and is capable of high-speed response. In in-plane mode, the orientation of the long axis of the liquid crystal molecules is oriented parallel to the first surface 24a and the second surface 24b. In in-plane mode, depending on the voltage applied between the first surface 24a and the second surface 24b, the orientation of the long axis of the liquid crystal molecules changes so that they rotate in a predetermined direction around an axis perpendicular to the first surface 24a and the second surface 24b (the direction of the electric field), while maintaining the state in which the liquid crystal molecules are oriented parallel to the first surface 24a and the second surface 24b. The direction in which the liquid crystal molecules rotate is determined by the direction of the electric field formed by the voltage. The FLC contained in the liquid crystal layer 24c is, for example, a polymer-stabilized ferroelectric liquid crystal. Among these, PSV-FLC (Polymer-stabilized V-mode FLC) is formed from FLC and a photopolymerized polymer that holds the FLC, and has the characteristic that the molecular long axis direction changes continuously in response to the voltage applied between the first surface 24a and the second surface 24b.

[0025] Figure 3 shows the electro-optical properties of the liquid crystal layer 24c. In Figure 3, the horizontal axis represents the electric field corresponding to the voltage applied between the first surface 24a and the second surface 24b, and the vertical axis represents the tilt of the slow axis, i.e., the molecular long axis, of the liquid crystal contained in the liquid crystal layer 24c. In Figure 3, the origin represents the state in which a predetermined voltage is applied to the liquid crystal layer 24c. For example, in Figure 3, the origin represents the state in which the tilt of the slow axis of the liquid crystal contained in the liquid crystal layer 24c is 0 degrees when no voltage is applied.

[0026] The tilt of the slow axis of the liquid crystal contained in the liquid crystal layer 24c increases as the applied voltage value increases. The slow axis of the liquid crystal contained in the liquid crystal layer 24c is tilted within a range of ±45 degrees.

[0027] The liquid crystal contained in the liquid crystal layer 24c may be formed by polymerizing a liquid crystalline polymer and a photopolymerizable polymer in a smectic C (SmC) phase by applying an alternating electric field. The application conditions, such as the frequency, amplitude, and waveform of the alternating electric field applied to the liquid crystalline polymer, are determined as appropriate. Alternatively, the liquid crystal contained in the liquid crystal layer 24c may be formed by polymerizing a liquid crystalline polymer and a photopolymerizable polymer in a smectic A (SmA) phase without applying an alternating electric field. The materials and polymerization conditions when polymerizing the liquid crystalline polymer in a smectic A (SmA) phase are determined as appropriate.

[0028] In the LCOS element 11 having a liquid crystal element 24, the voltage applied to each pixel electrode of the first electrode 21, which is formed as a plurality of pixel electrodes, is controlled. By controlling the voltage applied between the first surface 24a and the second surface 24b, the amount of phase given to the light transmitted through the liquid crystal element 24 is controlled, and the liquid crystal element 24 gives the emitted light a desired phase difference distribution.

[0029] Figure 4 shows the relationship between the voltage applied to the pixel electrode and the amount of phase imparted to the light transmitted through the liquid crystal element 24. As shown in Figure 4, the amount of phase imparted to the light transmitted through the liquid crystal element 24 changes according to the voltage applied to the pixel electrode. As mentioned above, the tilt of the slow axis of the liquid crystal contained in the liquid crystal layer 24c increases as the value of the applied voltage increases. Also, as mentioned above, it is possible to apply different voltages to each of the multiple pixel regions in the liquid crystal element 24. Therefore, the liquid crystal element 24 makes it possible to form different phase differences for each of the multiple pixel regions.

[0030] Therefore, in the LCOS element 11 including the liquid crystal element 24, the voltage applied to each of the multiple pixel electrodes is controlled so that, at a predetermined pitch P, the amount of phase that the liquid crystal element 24 imparts to the emitted light changes periodically to 0 to 2π during the round trip period from when the incident light enters the liquid crystal element 24 until the emitted light is emitted. The pitch P indicates the period of the phase distribution that functions as a diffraction grating and determines the direction of emission of the diffracted light DL. By controlling the voltage applied to each of the multiple pixel electrodes, that is, by controlling the voltage distribution, the period of the phase distribution, i.e., the pitch P, changes. By changing the pitch P, the LCOS element 11 is able to change the direction of emission of the diffracted light DL relative to the incident light.

[0031] In this embodiment, each of the multiple output transmission lines 104 is arranged at a predetermined pitch P such that, according to the wavelength of linearly polarized light from each of the multiple light-emitting units 101, a component of the reflected light corresponding to the linearly polarized light from each of the multiple light-emitting units 101 is incident on it. Therefore, by controlling the voltage applied to each of the multiple pixel electrodes to change periodically at a predetermined pitch P along the direction of alignment of the multiple output transmission lines 104, the LCOS element 11, including the liquid crystal element 24, separates each of the multiple incident light beams that form the combined light CL. That is, the LCOS element 11, including the liquid crystal element 24, functions as a diffraction grating that emits a component of the reflected light corresponding to the linearly polarized light from each light-emitting unit 101 toward each output transmission line 104 corresponding to that light-emitting unit 101, according to the wavelength of the linearly polarized light from that light-emitting unit 101.

[0032] The conversion element 25 is a λ / 4 wave plate and is positioned between the liquid crystal element 24 and the reflecting element 26. The conversion element 25 converts incident circularly polarized light into linearly polarized light and linearly polarized reflected light into circularly polarized light by applying a 90-degree retardation between the phase-advancing axis and the phase-lagging axis of the λ / 4 wave plate to the light that passes through the conversion element 25. The conversion element 25 converts the incident circularly polarized light that has passed through the liquid crystal element 24 into linearly polarized light and emits it to the reflecting element 26, and also converts the linearly polarized reflected light incident from the reflecting element 26 into circularly polarized light and emits it to the liquid crystal element 24.

[0033] The reflective element 26 is made of a highly reflective material such as aluminum and is stacked on the first electrode 21. The reflective element 26 reflects the incident light, which has been converted to linear polarization by the conversion element 25, and emits the reflected light back to the conversion element 25. The conversion element 25 and the reflective element 26 form a reflection mechanism 27 that, when circularly polarized incident light is incident on the liquid crystal element 24, emits reflected light, which is circularly polarized with its rotation direction reversed as seen from the liquid crystal element 24, back to the liquid crystal element 24.

[0034] Figures 5(A) and 6 show the polarization states of incident and reflected light transmitted through the wavelength selective switch 1. Figures 5(A) to 5(H) show the polarization states of incident and reflected light when the tilt angle of the slow axis of the liquid crystal contained in the liquid crystal element 24 with respect to the polarization direction of the incident light is 45 degrees, and Figures 6(A) to 6(H) show the polarization states of incident and reflected light when the tilt angle of the slow axis of the liquid crystal contained in the liquid crystal element 24 with respect to the polarization direction of the incident light is 90 degrees. Referring to Figures 5(A) to 5(H) and Figures 6(A) to 6(H), it is explained that the wavelength selective switch 1 functions as a diffraction grating by controlling the voltage applied between the first electrode 21 and the second electrode 22.

[0035] As shown in Figure 5(A), the incident light indicated by arrow A is incident from the surface of the optical element 10 facing the incident transmission path 103, as shown in Figure 1. For the sake of explanation, in the following description, the direction of propagation of the incident light will be defined as the z-axis direction, and the two orthogonal axes in the plane perpendicular to the z-axis will be defined as the x-axis and y-axis. As shown in Figure 5(B), the incident light indicated by arrow A is incident on the optical element 10 as linearly polarized light. In Figure 5(B), for the sake of explanation, the polarization direction of the incident linearly polarized light is defined as the y-axis direction, and the direction perpendicular to the y-axis is defined as the x-axis direction. The relationship between the x-axis and y-axis is the same for Figures 5(C) to 5(H) thereafter.

[0036] The incident light indicated by arrow A, incident on the optical element 10, passes through the optical element 10, which is a λ / 4 wave plate, and a 90-degree retardation is given between the leading axis and the lagging axis of the λ / 4 wave plate. In this embodiment, the lagging axis of the λ / 4 wave plate is tilted at 45 degrees with respect to the x-axis, so the incident light indicated by arrow A is given a 90-degree retardation between the leading axis and the lagging axis of the λ / 4 wave plate, and is converted into right-handed circularly polarized light indicated by arrow B (i.e., circularly polarized light in which the direction of the maximum value of the combined amplitude of the components along the leading axis and the lagging axis changes clockwise), as shown in Figure 5(C), and propagates from the end of the leading axis tilted at 135 degrees from the x-axis toward the end of the axis tilted at 45 degrees. The incident light, which is circularly polarized light indicated by arrow B, is emitted to the liquid crystal element 24.

[0037] The incident light, which is circularly polarized and is incident on the liquid crystal element 24, as indicated by arrow B, advances in phase by 180 degrees as the slow axis of the liquid crystal contained in the liquid crystal element 24 (LC slow axis) is tilted 45 degrees with respect to the x-axis. In the example shown in Figures 5(A) to 5(H), the voltage applied between the first electrode 21 and the second electrode 22 is 0, and the slow axis of the liquid crystal contained in the liquid crystal element 24 is tilted 45 degrees from the x-axis. Therefore, the incident light, which is circularly polarized and is indicated by arrow B, advances in phase by 180 degrees and is emitted from the liquid crystal element 24 as counterclockwise circularly polarized light, indicated by arrow C, which propagates from the end of the phase-advancing axis tilted -45 degrees from the x-axis toward the end of the axis tilted 45 degrees, as shown in Figure 5(D).

[0038] The incident light, indicated by arrow C, which is circularly polarized, enters the conversion element 25, which is a λ / 4 wave plate. The slow axis of the conversion element 25, i.e., the λ / 4 wave plate, is tilted at a 45-degree angle with respect to the x-axis. Therefore, the incident light, indicated by arrow C, which is circularly polarized, passes through the conversion element 25, which is a λ / 4 wave plate, and as shown in Figure 5(E), a 90-degree retardation is given between the leading axis and the slow axis of the λ / 4 wave plate, and it is converted into linearly polarized light, indicated by arrow D. The linearly polarized light indicated by arrow D has a polarization direction in the y-axis direction. The phase of the linearly polarized light indicated by arrow D is the same as the phase of the linearly polarized light indicated by arrow A. The linearly polarized light indicated by arrow D, i.e., the incident light, is reflected by the reflecting element 26 and enters the conversion element 25 as reflected light.

[0039] The reflected light incident on the conversion element 25 from the reflecting element 26 passes through the conversion element 25, which is a λ / 4 wave plate, thereby creating a 90-degree retardation between the leading and lagging axes of the λ / 4 wave plate. As shown in Figure 5(F), it is converted into clockwise circularly polarized light, indicated by arrow E, which propagates from the end of the leading axis, which is tilted 135 degrees from the x-axis, toward the end of the axis, which is tilted 45 degrees. The reflected light, which is circularly polarized light indicated by arrow E, is then incident on the liquid crystal element 24 from the conversion element 25.

[0040] The reflected light indicated by arrow E, incident on the liquid crystal element 24, advances in phase by 180 degrees as the slow axis of the liquid crystal contained in the liquid crystal element 24 tilts by 45 degrees. As the phase of the reflected light indicated by arrow E advances by 180 degrees, it is emitted from the liquid crystal element 24 as counterclockwise circularly polarized light indicated by arrow F, propagating from the end of the fast axis tilted at -45 degrees from the x-axis toward the end of the axis tilted at 45 degrees, as shown in Figure 5(G).

[0041] The reflected light indicated by arrow F, which is circularly polarized, is incident on the optical element 10, which is a λ / 4 wave plate. The reflected light indicated by arrow F, which is circularly polarized, passes through the optical element 10, and as shown in Figure 5(H), a 90-degree retardation is given between the leading axis and the lagging axis of the λ / 4 wave plate, and it is converted to linearly polarized light indicated by arrow G and emitted from the wavelength selection switch 1. The linearly polarized light indicated by arrow G has a polarization direction in the y-axis direction.

[0042] When the tilt angle of the slow axis of the liquid crystal contained in the liquid crystal element 24 is 45 degrees, the phase of the incident light, which is linearly polarized light indicated by arrow G emitted from the wavelength selection switch 1, is the same as the phase of the incident light, which is linearly polarized light indicated by arrow A incident on the wavelength selection switch 1. When the tilt angle of the slow axis of the liquid crystal contained in the liquid crystal element 24 is 45 degrees, the incident light incident on the wavelength selection switch 1 and the reflected light emitted from the wavelength selection switch 1 are in the same phase state, and the reflected light having the same phase as the incident light is emitted from the wavelength selection switch 1.

[0043] Since the polarization states of the incident light indicated by arrows A and B in FIGS. 6(A) to 6(C) have been described with reference to FIGS. 5(A) to 5(C), detailed description thereof will be omitted here. The relationships of the x-axis, y-axis, and z-axis in FIGS. 6(A) to 6(H) are the same as the relationships of the x-axis, y-axis, and z-axis in FIGS. 5(A) to 5(H).

[0044] In the example shown in FIGS. 6(A) to 6(H), the voltage applied between the first electrode 21 and the second electrode 22 is controlled so that the slow axis (LC slow axis) of the liquid crystal contained in the liquid crystal element 24 is inclined by 90 degrees from the x-axis. Therefore, the incident light, which is circularly polarized light indicated by arrow B incident on the liquid crystal element 24, advances its phase by 270 degrees as the slow axis of the liquid crystal contained in the liquid crystal element 24 is inclined by 90 degrees. The incident light, which is circularly polarized light indicated by arrow B, advances its phase by 270 degrees and is emitted from the liquid crystal element 24 as left-handed circularly polarized light indicated by arrow C, which travels from one end inclined by 45 degrees from the x-axis across the slow axis toward the other end inclined by 135 degrees.

[0045] The incident light indicated by arrow C, which is circularly polarized light, is incident on the conversion element 25, which is a λ / 4 wavelength plate. The incident light indicated by arrow C, which is circularly polarized light, passes through the conversion element 25, which is a λ / 4 wavelength plate. As shown in FIG. 6(E), a retardation of 90 degrees is imparted between the fast axis and the slow axis of the λ / 4 wavelength plate, and it is converted into linearly polarized light indicated by arrow D. The linearly polarized light indicated by arrow D has a polarization direction in the y-axis direction. The phase of the linearly polarized light indicated by arrow D is different by 180 degrees compared with the incident light indicated by arrow A. In FIG. 6(E), the difference in phase of 180 degrees of the linearly polarized light indicated by arrow D is indicated by the direction of the arrow. The linearly polarized light indicated by arrow D, that is, the incident light, is reflected by the reflection element 26 and incident on the conversion element 25 as reflected light.

[0046] The reflected light incident on the conversion element 25 from the reflection element 26 passes through the conversion element 25, and a retardation of 90 degrees is imparted between the fast axis and the slow axis of the λ / 4 wavelength plate. As shown in FIG. 6(F), starting from the end of the fast axis inclined 45 degrees from the x-axis, it propagates toward the end of the axis inclined -45 degrees, and is converted into right-handed circularly polarized light indicated by arrow E. The reflected light, which is circularly polarized light indicated by arrow E, is incident on the liquid crystal element 24 from the conversion element 25.

[0047] The reflected light indicated by arrow E incident on the liquid crystal element 24 advances in phase by 90 degrees in accordance with the 90-degree inclination of the slow axis of the liquid crystal contained in the liquid crystal element 24. The reflected light indicated by arrow E advances in phase by 90 degrees. As shown in FIG. 6(G), starting from the end of the fast axis inclined 135 degrees from the x-axis, it propagates toward the end of the axis inclined 225 degrees, and is emitted from the liquid crystal element 24 as left-handed circularly polarized light indicated by arrow F.

[0048] The reflected light indicated by arrow F, which is circularly polarized light, is incident on the optical element 10, which is a λ / 4 wavelength plate. The reflected light indicated by arrow F, which is circularly polarized light, passes through the optical element 10. As shown in FIG. 6(H), a retardation of 90 degrees is imparted between the fast axis and the slow axis of the λ / 4 wavelength plate, and it is converted into linearly polarized light indicated by arrow G and emitted from the wavelength selection switch 1. The linearly polarized light indicated by arrow G has a polarization direction in the y-axis direction.

[0049] When the tilt angle of the slow axis of the liquid crystal contained in the liquid crystal element 24 is 90 degrees, the phase of the linearly polarized incident light indicated by arrow G emitted from the wavelength selection switch 1 differs by 180 degrees from the phase of the linearly polarized incident light indicated by arrow A incident on the wavelength selection switch 1. When the tilt angle of the slow axis of the liquid crystal contained in the liquid crystal element 24 is 90 degrees, the wavelength selection switch 1 emits reflected light that is shifted in phase by 180 degrees from the incident light.

[0050] The wavelength-selective switch 1 functions as a diffraction grating by applying a voltage between the first electrode 21 and the second electrode 22 so as to arrange a unit pattern in which the phase difference distribution of reflected light changes linearly in a predetermined direction from 0 degrees to 360 degrees (2π). The wavelength-selective switch 1 adjusts the arrangement pitch of the unit pattern by controlling the voltage applied between the first electrode 21 and the second electrode 22, thereby directing the reflected light outward so that it is incident on one of the multiple outward transmission lines 104.

[0051] (Effects of the Wavelength Selective Switch According to the Embodiment) In the wavelength selective switch 1, the liquid crystal layer 24c contains a ferroelectric liquid crystal capable of high-speed response, enabling faster operation than a wavelength selective switch using a liquid crystal element containing a nematic liquid crystal. That is, because the ferroelectric liquid crystal has spontaneous polarization, when a voltage is applied, a force is generated corresponding to the relative angle between the spontaneous polarization and the electric field generated by the voltage, causing the liquid crystal molecules to rotate so as to orient their polarization toward the direction of the electric field. As a result, the ferroelectric liquid crystal enables high-speed response, and the wavelength selective switch 1 containing the ferroelectric liquid crystal enables high-speed operation.

[0052] Furthermore, since the wavelength selective switch 1 contains a PSV-FLC in the liquid crystal layer 24c, which allows for relatively easy orientation control, it is possible to manufacture a liquid crystal element 24 with a thickness that allows for the formation of a unit pattern in which the phase difference continuously changes from 0 to 2π with high precision. Because the wavelength selective switch 1 can manufacture a liquid crystal element 24 with a desired thickness with high precision, manufacturing costs can be reduced.

[0053] Furthermore, since the wavelength select switch 1 incidents the FLC element, which operates in in-plane mode, with circularly polarized light for both the incident and reflected light, the polarization state can be maintained and phase modulation can be achieved by rotating the cone angle of the FLC. On the other hand, when the incident and reflected light are incident on the FLC element, which operates in in-plane mode, it is not possible to simultaneously maintain the polarization state and achieve phase modulation by rotating the cone angle of the FLC.

[0054] Furthermore, the nematic liquid crystal in the wavelength-selective switch described in Patent Document 1 exhibits a change in retardation rate as the applied voltage increases. When the applied voltage is small, the retardation of the nematic liquid crystal hardly changes; when the applied voltage exceeds a first threshold, the retardation changes significantly; and when the applied voltage further exceeds a second threshold, the retardation saturates and the change in retardation rate becomes small. Since the nematic liquid crystal is typically used between the first and second thresholds where the change in retardation rate is large, the film thickness, which is proportional to the retardation rate, becomes thicker in proportion to the size of the unused regions outside the first and second thresholds.

[0055] On the other hand, in the wavelength selective switch 1, the film thickness of the liquid crystal element 24 only needs to be such that the retardation of the liquid crystal layer is 2π, and it can be made thinner than the wavelength selective switch described in Patent Document 1 which uses nematic liquid crystal. The wavelength selective switch 1 can be miniaturized by making the film thickness of the liquid crystal element 24 thinner than the wavelength selective switch described in Patent Document 1 which uses nematic liquid crystal.

[0056] Furthermore, the wavelength selective switch 1 allows for a thinner film thickness of the liquid crystal element 24 than the wavelength selective switch described in Patent Document 1, thereby reducing the voltage applied between the first surface 24a and the second surface 24b. By reducing the voltage applied between the first surface 24a and the second surface 24b, the wavelength selective switch 1 can reduce power consumption.

[0057] Furthermore, in the communication system 100 having the wavelength selector switch 1, the voltage applied to each of the multiple pixel electrodes is controlled. That is, the voltage is controlled so that the pitch P is such that the diffracted light of a predetermined order (e.g., first order) for each incident light matches the direction to the corresponding output transmission path 104, according to the wavelength of each incident light and the direction to the individual output transmission path 104 corresponding to each incident light. Moreover, the voltage is controlled so that the phase difference periodically applied to the incident light at the pitch P changes from 0 to 2π during the round trip from when the incident light enters the liquid crystal element 24 until the output light is emitted, as shown in Figure 4. As a result, the wavelength selector switch 1 operates as a demultiplexer. As described above, since the wavelength selector switch 1 contains a ferroelectric liquid crystal capable of high-speed response, the communication system 100 enables high-speed switching.

[0058] (Modified example of the wavelength-selective switch according to the embodiment) The wavelength-selective switch 1 has an optical element 10, but the wavelength-selective switch according to the embodiment does not have an optical element 10. When the wavelength-selective switch according to the embodiment does not have an optical element 10, circularly polarized incident light is incident on the LCOS element 11.

[0059] Furthermore, in the wavelength selective switch 1, the liquid crystal element 24, the conversion element 25, and the reflecting element 26 are integrally arranged inside the LCOS element 11. However, in the wavelength selective switch according to this embodiment, the liquid crystal element 24, the conversion element 25, and the reflecting element 26 may be arranged separately and optically connected by spatial coupling.

[0060] Furthermore, while the wavelength selection switch 1 is a demultiplexer that separates each of the multiple incident light beams forming the multiplexed light CL, the wavelength selection switch in the embodiment does not have to be a demultiplexer. For example, in the wavelength selection switch according to the embodiment, the wavelength component of the incident light from one of the multiple light-emitting units 101 (hereinafter referred to as the "target light-emitting unit" for convenience) is diffracted by an LCOS element 11 controlled to form a diffraction grating having a pitch corresponding to the target light-emitting unit, and then incident on the output transmission path 104 corresponding to that light-emitting unit. The wavelength components of the incident light from the other light-emitting units are arranged such that they are not incident on the other output transmission paths after being diffracted by the LCOS element 11 controlled to form a diffraction grating having a pitch corresponding to the target light-emitting unit. Therefore, the LCOS element 11 is controlled to configure a diffraction grating having a pitch corresponding to each light-emitting unit 101, making it possible to emit light from only one of the multiple light-emitting units 101 to the corresponding output transmission path. In the communication system according to this modified example, the voltage applied to each of the multiple pixel electrodes is controlled as follows. That is, the voltage is controlled so that, according to the wavelength of the incident light from any of the light-emitting units, the diffraction angle of the diffracted light of a predetermined order relative to the incident light matches the direction to the output transmission path corresponding to the incident light among the multiple output transmission paths 104. Furthermore, the voltage is controlled so that the phase difference periodically applied to the incident light at this pitch changes from 0 to 2π during the round trip from when the incident light enters the liquid crystal element 24 until the output light is emitted. By controlling the voltage in this way, in the wavelength selection switch according to this modified example, the diffracted light (reflected light) corresponding to the wavelength of incident light other than the specific incident light is diffracted in a direction that does not enter the other output transmission paths 104. In other words, the communication system according to this modified example makes it possible to direct only the wavelength component corresponding to the linear polarization of the selected light-emitting part into the desired output transmission path. In this case, the LCOS element 11 functions as a diffraction grating that diffracts the component of the reflected light corresponding to the linear polarization from the light-emitting part toward the output transmission path corresponding to the light-emitting part of interest among the plurality of output transmission paths 104, according to the wavelength of the linear polarization from the light-emitting part.

[0061] Furthermore, the wavelength selective switch 1 uses a conversion element 25 and a reflective element 26 to form a reflection mechanism 27 that, as viewed from the liquid crystal element 24, reflects incident light and emits reflected light to the liquid crystal element 24 with the rotation direction of the circularly polarized incident light reversed. However, in the embodiment, the wavelength selective switch may also form the reflection mechanism with a single element.

[0062] Figure 7 is a cross-sectional view of an LCOS element according to a modified example. The LCOS element 12 can be placed in the wavelength selective switch 1 in place of the LCOS element 11 in the communication system 100.

[0063] The LCOS element 12 differs from the LCOS element 11 in that it has a reflective element 28 instead of the conversion element 25 and the reflective element 26. The configuration and function of the components of the LCOS element 12 other than the reflective element 28 are the same as those of the components of the LCOS element 11, which are given the same reference numerals, so a detailed explanation is omitted here.

[0064] The reflective element 28 is formed from a highly reflective material such as aluminum and is laminated on the semiconductor substrate 20. In the reflective element 28, a metamaterial structure is formed on the surface facing the liquid crystal element 24 that, when incident light is incident from the liquid crystal element 24, provides a 90-degree retardation between the leading axis and the lagging axis and emits reflected light back to the liquid crystal element 24. The metamaterial structure has, for example, an aligned unit structure of rectangular columns rotated with respect to the alignment direction, which is placed on the surface of a spacer layer laminated on the reflective layer.

[0065] Figure 8(A) is a plan view of the reflective element 28, and Figure 8(B) is a cross-sectional view along the line A-A in Figure 8(A).

[0066] The reflective element 28 has a reflective layer 28a, a spacer layer 28b, and a plurality of unit structures 28c. The reflective layer 28a is a metal layer formed of a highly reflective material such as aluminum, and is laminated on the first electrode 21. The length of the reflective layer 28a in the lamination direction is 150 nm. The spacer layer 28b is silica (SiO ２ ) and silicon nitride (SiN ＸIt is formed from a dielectric material with high transmittance, such as ). The spacer layer 28b has a length of 110 nm in the stacking direction. Each of the multiple unit structures 28c has a rectangular columnar shape. In the unit structure 28c, the length in the stacking direction is 80 nm, and in the rectangular columnar shape, excluding the side in the stacking direction, the length of the short side is 165 nm and the length of the long side is 390 nm. The multiple unit structures 28c are aligned by rotating them by a predetermined angle, which in one example is 45 degrees with respect to the alignment direction. The predetermined angle is the rubbing direction of the liquid crystal element 24. The rubbing direction is the direction of the fine grooves formed by rubbing the alignment film, and is the reference direction that determines the initial alignment direction of the liquid crystal molecules in the liquid crystal layer 24c. The alignment film is a thin film formed to align the orientation of the liquid crystal molecules in the liquid crystal layer 24c in a certain direction, and is formed on the first electrode 21 and the second electrode 22, respectively.

[0067] Figure 9 shows the relationship between the rubbing direction RD of the liquid crystal element 24 and the arrangement of the unit structure 28c. In Figure 9, the liquid crystal molecules 24d in the liquid crystal element 24 are conceptually shown as elliptical shapes. When no voltage is applied between the first surface 24a and the second surface 24b, the direction of the molecular long axis of the liquid crystal molecules 24d is the rubbing direction RD. When a voltage is applied between the first surface 24a and the second surface 24b, the direction of the molecular long axis of the liquid crystal molecules 24d changes according to the applied voltage, as shown by the dotted line in Figure 9.

[0068] In the unit structure 28c, the longer side of the rectangular columnar shape is arranged along the rubbing direction RD of the liquid crystal element 24. That is, each of the multiple unit structures 28c is arranged along the rubbing direction RD of the ferroelectric liquid crystal element.

[0069] As described above, the reflective element 28 has a metamaterial structure formed on the surface facing the liquid crystal element 24. That is, in the wavelength switch according to this embodiment, the reflection mechanism is a reflective element 28 formed of a metamaterial structure, and the reflective element 28 has a plurality of unit structures 28c arranged in an aligned manner. As a result, the reflective element 28 functions as an element having the functions of both a conversion element 25 and a reflective element 26, thus contributing to the miniaturization of the LCOS element 12 having the reflective element 28.

[0070] In the wavelength switch according to this embodiment, each of the multiple unit structures 28c is arranged along the rubbing direction RD of the liquid crystal element 24. This makes the wavelength switch according to this embodiment less prone to cracking of the reflective element 28. Furthermore, the wavelength switch according to this embodiment facilitates the alignment and arrangement of the multiple unit structures 28c.

[0071] In the wavelength selective switch according to this embodiment, the conversion element 25 can be omitted by arranging the LCOS element 12 in place of the LCOS element 11, and the separation distance between the first electrode 21 and the second electrode 22 can be shortened. Since the separation distance between the first electrode 21 and the second electrode 22 can be shortened in the wavelength selective switch according to this embodiment, the voltage applied between the first electrode 21 and the second electrode 22 to control the liquid crystal element 24 can be reduced, and power consumption can be further reduced.

[0072] The length, the length of the short side, and the length of the long side of the unit structure 28c described above do not have to be the lengths described above. The unit structure 28c does not have to be rectangular in shape; for example, it may be elliptical in shape. The unit structure 28c does not have to be made of metal; for example, it may be made of silicon.

[0073] In the reflective element 28 described above, the lengths of the reflective layer 28a and the spacer layer 28b do not necessarily have to be the lengths described above.

[0074] The predetermined angle does not have to be the rubbing direction of the liquid crystal layer 24c. In other words, each of the multiple unit structures 28c does not have to be arranged along the rubbing direction of the liquid crystal element 24.

[0075] Here, the wavelength selective switch 1 was described as an example of a spatial light modulator according to the embodiment. However, the spatial light modulator according to the embodiment can be used for applications other than wavelength selective switches, such as laser processing machines, optical microscopes, and marking with black and white patterns.

[0076] 1 Wavelength selective switch (spatial light modulator) 10 Optical elements 11, 12 LCOS elements 24 Liquid crystal elements 25 Conversion elements 26, 28 Reflecting elements 27 Reflection mechanism 100 Communication system

Claims

1. A spatial light modulator comprising: a ferroelectric liquid crystal element having a first surface, a second surface positioned opposite the first surface, and a liquid crystal layer positioned between the first surface and the second surface and operating in in-plane mode, wherein the amount of phase imparted to light transmitted through the liquid crystal layer can be controlled by a voltage applied between the first surface and the second surface; and a reflection mechanism optically connected to the ferroelectric liquid crystal element, wherein the ferroelectric liquid crystal element emits incident light, which is circularly polarized, incident light incident on the second surface, from the first surface to the reflection mechanism; the reflection mechanism reflects the incident light incident on the ferroelectric liquid crystal element and emits reflected light, which is circularly polarized as seen from the ferroelectric liquid crystal element, with its rotation direction reversed, to the ferroelectric liquid crystal element; and the ferroelectric liquid crystal element emits the reflected light incident on the first surface from the second surface.

2. The spatial light modulator according to claim 1, further comprising an optical element that emits the incident light to the ferroelectric liquid crystal element by converting linearly polarized light to circularly polarized light.

3. The spatial light modulator according to claim 1 or 2, wherein the ferroelectric liquid crystal element has the characteristic of continuously changing the molecular longitudinal axis direction in response to the applied voltage.

4. The spatial light modulator according to claim 3, wherein the ferroelectric liquid crystal element contains a polymer-stabilized ferroelectric liquid crystal.

5. The spatial light modulator according to claim 1 or 2, wherein the reflection mechanism is a reflection element having a metamaterial structure formed on a surface facing the ferroelectric liquid crystal element, and the metamaterial structure has a plurality of unit structures arranged in an aligned manner.

6. The spatial light modulator according to claim 5, wherein each of the plurality of unit structures is arranged along the rubbing direction of the ferroelectric liquid crystal element.

7. A communication system comprising: a spatial light modulator according to claim 1 or 2; a plurality of light-emitting units each emitting linearly polarized light having different wavelengths from each other; a multiplexer emitting combined light obtained by combining a plurality of linearly polarized light emitted from each of the plurality of light-emitting units; an incident transmission path for transmitting the combined light to the spatial light modulator; and a plurality of outgoing transmission paths corresponding to each of the plurality of light-emitting units, wherein the spatial light modulator further comprises a single electrode and a plurality of pixel electrodes arranged two-dimensionally facing each other with respect to the liquid crystal layer, and the voltage applied to each of the plurality of pixel electrodes is controlled to change periodically at a predetermined pitch along the direction of arrangement of the plurality of outgoing transmission paths, thereby functioning as a diffraction grating for each of the plurality of light-emitting units, diffracting the component of the reflected light corresponding to the linearly polarized light toward the outgoing transmission path corresponding to the light-emitting unit, according to the wavelength of the linearly polarized light from the light-emitting unit.

8. A communication system comprising: a spatial light modulator according to claim 1 or 2; a plurality of light-emitting units each emitting linearly polarized light having different wavelengths from each other; a multiplexer emitting combined light obtained by combining a plurality of linearly polarized light emitted from each of the plurality of light-emitting units; an incident transmission path for transmitting the combined light to the spatial light modulator; and a plurality of output transmission paths corresponding to each of the plurality of light-emitting units, wherein the spatial light modulator further comprises a single electrode and a plurality of pixel electrodes arranged two-dimensionally facing each other with respect to the single electrode and the liquid crystal layer in between, and for each of the plurality of light-emitting units, the voltage applied to each of the plurality of pixel electrodes is controlled to change periodically along the direction of arrangement of the plurality of output transmission paths at a pitch corresponding to the light-emitting unit, thereby functioning as a diffraction grating that diffracts the component of the reflected light corresponding to the linearly polarized light toward the output transmission path corresponding to the light-emitting unit, according to the wavelength of the linearly polarized light from the light-emitting unit.

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