Liquid crystal on silicon and wavelength selective switch

Abstract

A liquid crystal on silicon (LCoS) and a wavelength selective switch (100), which are used for providing an LCoS device capable of realizing polarization independence, and can also ameliorate the problem of secondary crosstalk occurring due to a residual of unconverted deflection, thereby ensuring the application of the LCoS. In the LCoS, an extra phase is pre-added in a dispersion direction on the basis of structures such as a meta-surface design or a phase film; and by means of the pre-added extra phase, unconverted polarization light forms a high-frequency deflection or a scattering phase deviates the energy of the unconverted polarization light from a crosstalk level, so as to prevent the crosstalk of the unconverted polarization light and converted polarization light, thereby implementing the suppression of secondary crosstalk in a port direction.

Description

Silicon-based liquid crystal and wavelength selective switch

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the State Intellectual Property Office of China on June 21, 2023, with application number 202310748707.X and application name “A Silicon-Based Liquid Crystal and Wavelength Selection Switch”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The present application relates to the field of optical communication technology, and in particular to a silicon-based liquid crystal and a wavelength selective switch. Background Art

[0004] Liquid crystals are a class of materials that possess both the fluidity of liquids and the orderliness of crystals, exhibiting anisotropy in their physical properties. Optically, by adjusting the electric field applied to the liquid crystal molecules, the orientation of the liquid crystal molecules can be altered, changing their refractive index for light polarized parallel to the liquid crystal molecular orientation, thus achieving voltage-controlled phase modulation.

[0005] A commonly used architecture is to sandwich liquid crystal between a layer of indium tin oxide (ITO) electrodes and a metal reflector pixel array to form a sandwich-like structure. Silicon-based array liquid crystal phase modulators based on this architecture are generally referred to as liquid crystal on silicon (LCoS) or LCoS-spatial light modulation (SLM). The LCoS-based optical wavelength selective switch is one of the most core components of the wavelength selective switch (WSS), which can realize the wavelength crossover function. The key step to achieve this function is to modulate the two-dimensional array of the LCoS liquid crystal screen. LCoS is composed of millions of pixels. The main method of modulating LCoS is to adjust the phase of different LCoS pixels, and the phase of the pixel depends on the size of the voltage applied to the pixel.

[0006] In practical applications of WSS, it is usually necessary to modulate polarization in both directions. The current solution is to use a polarization beam splitter to decompose a beam of light with any polarization direction into two perpendicular polarization beams, and then use a half-wave plate (HWP) or a quarter-wave plate (QWP) plus a reflector to turn the polarization of one beam parallel to the other beam, and then inject it into the LCoS. However, this method adds additional components, which makes the optical path complicated, hindering the evolution of WSS to large ports and small size, and also increasing the cost of the system. Therefore, how to realize a polarization-independent LCoS device is of great research significance.

[0007] Summary of the Invention

[0008] The embodiments of the present application provide a liquid crystal on silicon and a wavelength selective switch, which are used to provide an LCoS device that can achieve polarization independence and reduce the secondary crosstalk problem caused by residual deflection that is not converted, thereby ensuring the application of LCoS.

[0009] In a first aspect, a silicon-based liquid crystal is provided for use in a wavelength selective switch. The silicon-based liquid crystal comprises: a first panel and a second panel disposed opposite each other; a liquid crystal layer located between the first and second panels; a drive circuit for generating an electric field to control liquid crystal deflection in the liquid crystal layer; and two alignment films, one located on opposite sides of the liquid crystal layer; a reflector layer located above the first panel for reflecting a light beam; and a polarization conversion layer located above the reflector and below the two alignment layers for converting the polarization of the light beam after passing through the liquid crystal layer. The polarization conversion layer comprises at least two types of metasurface microstructures, which are periodically and alternately arranged.

[0010] By using a metasurface structure including at least two types of microstructures, different modulation effects can be achieved on polarization-converted light and polarization-unconverted light, so that the polarization-unconverted light passing through the LCoS can deviate from the polarization-converted light, thereby reducing the secondary crosstalk effect of the polarization-unconverted light on the polarization-converted light.

[0011] In one possible design, the silicon-based liquid crystal has N pixel areas, the reflector layer includes N reflectors corresponding one-to-one to the N pixel areas, and N is a positive integer; the N reflectors correspond one-to-one to the N groups of metasurface microstructures; wherein, a group of metasurface microstructures includes a class of metasurface microstructures.

[0012] With this design, more accurate processing of polarization-unconverted light can be achieved, thereby better reducing the secondary crosstalk effect of polarization-unconverted light on polarization-converted light.

[0013] In one possible design, the light beam after polarization conversion includes polarization-converted light and polarization-unconverted light; the silicon-based liquid crystal has N pixel areas, the driving circuit has a first voltage control value in the first pixel area, and has a second voltage control value in the second pixel area; the second pixel area is an adjacent pixel area of ​​the first pixel; the first voltage control value and the second voltage control value are determined based on the phase modulation values ​​of the first type of metasurface microstructure and the second type of metasurface structure.

[0014] By using the phase compensation of the phase modulation by the liquid crystal layer, the polarization converted light can be restored to the phase at the time of incidence after passing through the LCoS, thereby reducing the impact on the polarization converted light.

[0015] Based on the above design, in a possible embodiment, the first voltage control value includes phase compensation for a first phase modulation value; wherein the first phase modulation value is used to indicate the phase modulation value of the first type of microstructure to the polarization-converted light; the second voltage control value includes phase compensation for a second phase modulation value; wherein the second phase modulation value is used to indicate the phase modulation value of the second type of microstructure to the polarization-converted light.

[0016] Based on the above design, in another possible embodiment, the polarization conversion layer includes two types of metasurface microstructures; the second voltage control value includes phase compensation for a third phase modulation value; wherein the third phase modulation value is used to indicate the sum of the first phase modulation value of the polarization conversion light by the first type of microstructure and the second phase modulation value of the polarization conversion light by the second type of microstructure.

[0017] In one possible design, the metasurface microstructure is an elliptical cylindrical microstructure, and different types of metasurface microstructures have different sizes.

[0018] In a second aspect, another silicon-based liquid crystal is provided for use in a wavelength selective switch. The silicon-based liquid crystal includes: a first panel and a second panel arranged opposite to each other; a liquid crystal layer located between the first panel and the second panel; a driving circuit for generating an electric field to control the deflection of the liquid crystal in the liquid crystal layer; and two alignment films, the two alignment films being located on opposite sides of the liquid crystal layer; a reflector layer located above the first panel for reflecting the light beam; a polarization conversion layer located above the reflector and below the two alignment layers for performing polarization conversion on the light beam after passing through the liquid crystal layer; wherein the light beam after polarization conversion includes polarization-converted light and polarization-unconverted light; and a phase film layer located above the reflector for performing the same phase modulation on the polarization-converted light and the polarization-unconverted light.

[0019] In this LCOS, considering that unpolarized light has twice the optical path length of polarized light, a phase film is used to introduce an additional pre-phase in the dispersion direction. This phase is then compensated for using a phase diagram. This reduces the secondary crosstalk caused by unpolarized light. Furthermore, this structure is easy to manufacture.

[0020] In one possible design, the phase film is a high-frequency phase film. In one possible design, the high-frequency phase film is a sinusoidal grating phase film, a triangle wave phase film, or a square wave phase film.

[0021] In a third aspect, an embodiment of the present application further provides a wavelength selective switch, which may include the silicon-based liquid crystal introduced in the first aspect or any possible design in the first aspect, or may include the silicon-based liquid crystal introduced in the second aspect or any possible design in the second aspect.

[0022] Since the wavelength selective switch in the third aspect includes the silicon-based liquid crystals of each design in the first aspect, it also has the technical effects brought about by each design in the first aspect, or includes the silicon-based liquid crystals of each design in the second aspect, it also has the technical effects brought about by each design in the second aspect, which will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG1 is a schematic diagram of a wavelength selective switch structure applicable to an embodiment of the present application;

[0024] FIG2 is a schematic diagram of the structure of the liquid crystal on silicon (LCOS) of the wavelength selective switch shown in FIG1 ;

[0025] FIG3 is a schematic diagram of liquid crystal rotation in the LCOS shown in FIG2 ;

[0026] FIG4 is a schematic structural diagram of a silicon-based liquid crystal;

[0027] FIG5 is a schematic diagram of the structure of another type of silicon-based liquid crystal;

[0028] FIG6 is a system architecture diagram of a possible WSS provided in an embodiment of the present application;

[0029] FIG7 is a schematic diagram of a structure of a PI-LCoS provided in an embodiment of the present application;

[0030] FIG8 is a schematic structural diagram of a metasurface structure provided in an embodiment of the present application;

[0031] FIG9 is a second structural diagram of a PI-LCoS provided in an embodiment of the present application;

[0032] FIG10 is a third structural diagram of a PI-LCoS provided in an embodiment of the present application;

[0033] FIG11 is a fourth structural diagram of a PI-LCoS provided in an embodiment of the present application. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be described in detail below in conjunction with the drawings in the embodiments of the present application.

[0035] LCoS has a wide range of applications. It is not only a key technology for display devices like projectors and projection TVs, but also plays a crucial role as a phase modulation device in applications like WSS. With the development of optical network communications, the WSS application market is expanding, and the demand for lower-cost and higher-performance WSS is becoming increasingly urgent.

[0036] As an example, Figure 1 is a schematic diagram of the structure of a wavelength selective switch (WSS) 100 applicable to embodiments of the present application. The WSS 100 can be used in a reconfigurable optical add-drop multiplexer (ROADM). The WSS 100 includes liquid crystal on silicon (LCoS) 1, which serves as the optical switching engine of the WSS 100 and is configured to achieve a phase modulation effect to independently control the diffraction and deflection directions of light beams incident on different regions of the LCoS 1.

[0037] The wavelength selective switch 100 may also include at least one input port 101 (e.g., an input optical fiber), at least one output port group corresponding to the input port 101, a polarization conversion unit 103, a wavelength division multiplexer 104 (e.g., a diffraction grating), and a lens 105. Each output port group includes at least two output ports 102 (e.g., output optical fibers). As shown in FIG1 , the beam transmission path is a reversible path. A multi-wavelength signal forms an input beam through the input port 101. The input beam is converted by the polarization conversion unit 103 into linearly polarized light corresponding to the operating polarization state of the silicon liquid crystal 1. The linearly polarized light is incident on the wavelength division multiplexer 104, which disperses the individual wavelengths at different angles. The lens 105 then converts the dispersed beams into parallel beams for incident on different locations of the silicon liquid crystal 1. The beams of different wavelengths are focused onto different regions of the silicon liquid crystal 1 and are diffracted and deflected. The diffracted beams are then restored to their original polarization states by the polarization conversion unit 103, and the beams of different wavelengths are coupled into their respective target output ports 102. Since light beams of different wavelengths are diffracted and deflected in different directions by different regions on the silicon liquid crystal 1, the silicon liquid crystal 1 can switch any wavelength combination to any of the output ports 102. The wavelength selective switch 100 further includes a reflector 106 for reflecting light.

[0038] In addition to the aforementioned structural units, the wavelength selective switch may also include a controller or processor for selectively configuring a bit sequence to drive pixels in the silicon-based liquid crystal, thereby controlling the optical output in the port direction. The controller or processor may be implemented using hardware, software, firmware, or any combination thereof. For example, the controller or processor may utilize one or more processors, digital signal processors, application-specific integrated circuits, field-programmable gate arrays, discrete logic, or any combination thereof. When the controller or processor is partially implemented in software, the device may store computer-executable instructions for the software in a suitable, non-transitory computer-readable storage medium and may use one or more processors in hardware to execute the instructions to implement the technical solution of the present invention.

[0039] Figure 2 provides a schematic diagram of the physical structure of the aforementioned LCOS. The LCOS can be used in the wavelength selective switch 100. The LCOS 1 can be used to diffract a linearly polarized incident light beam to form a deflected light beam. The LCOS 1 is a polarization-sensitive device that can only operate in one polarization direction (i.e., the operating polarization direction). The polarization direction of the incident light beam is the operating polarization direction of the LCOS 1. The LCOS 1 includes a first panel 11, a second panel 12, a liquid crystal layer 13, a drive circuit 14, and two layers of alignment films 15. The first panel 11 and the second panel 12 are arranged opposite each other. The first panel 11 is parallel to the second panel 12. The first panel 11 can be a silicon backplane panel, and the second panel 12 can be a transparent glass substrate. The liquid crystal layer 13 is located between the first panel 11 and the second panel 12 and contains numerous liquid crystal molecules. The liquid crystal molecules deflect when a voltage is applied to the electrode layer. The drive circuit 14 is used to generate an electric field to control the deflection of the liquid crystal in the liquid crystal layer 13. Two alignment films 15 are located on opposite sides of the liquid crystal layer 13. Specifically, one alignment film 15 is located between the liquid crystal layer 13 and the first panel 11, and the other alignment film 15 is located between the liquid crystal layer 13 and the second panel 12. The alignment films 15 are used to provide an initial alignment for the liquid crystals in the liquid crystal layer 13.

[0040] As an example, the driving circuit 14 includes a first electrode layer 141 and a second electrode layer 142. The first electrode layer 141 is located between the liquid crystal layer 13 and the first panel 11. The second electrode layer 142 is located between the liquid crystal layer 13 and the second panel 12. The first electrode layer 141 and the second electrode layer 142 can be indium tin oxide (ITO) electrode layers. Specifically, the first electrode layer 141 is formed on the side of the first panel 11 facing the liquid crystal layer 13, and the second electrode layer 142 is formed on the side of the second panel 12 facing the liquid crystal layer 13. The two layers of the alignment film 15 are located between the first electrode layer 141 and the second electrode layer 142. When the first electrode layer 141 and the second electrode layer 142 are energized, the liquid crystal deflection in the liquid crystal layer 13 is controlled by a vertically aligned (VA) driving method.

[0041] Please refer to Figures 2 and 3 in combination. When a voltage is applied to the first electrode layer 141 and the second electrode layer 142 and an electric field is formed between the first electrode layer 141 and the second electrode layer 142, the liquid crystal in the liquid crystal layer 13 will deflect (the deflection plane is the XY plane in Figure 3, that is, the plane perpendicular to the first panel 11), where X represents the wavelength direction (which can also be called the "dispersion direction" in the embodiment of the present application), Y represents the phase period 2π (or can also be understood as the direction of the optical path), and Z represents the port direction (or can also be called the "switching direction", "exchange direction", etc.). In one example, the liquid crystal in the liquid crystal layer 13 is deflected from the state in the left figure in Figure 3 to the state in the right figure in Figure 3, that is, the long axis of the liquid crystal is deflected from a direction roughly parallel to the first panel 11 to a direction roughly perpendicular to the first panel 11. This application uses this as an example for explanation. In another embodiment, the long axis of the liquid crystal in the liquid crystal layer 13 can also be deflected from a direction roughly perpendicular to the first panel 11 to a direction roughly parallel to the first panel 11. Because liquid crystal is a birefringent material, its deflection causes a change in its equivalent refractive index, thereby achieving a phase modulation effect. Furthermore, the angle of liquid crystal molecule deflection is related to the magnitude of the voltage applied to the first electrode layer 141 and the second electrode layer 142. Therefore, varying the applied voltage can achieve different phase modulation amounts. This is also the principle behind phase modulation of pixels in a two-dimensional liquid crystal on silicon array.

[0042] Limited by the optoelectronic properties of liquid crystal itself, general LCoS devices are polarization-dependent, and only light with a polarization direction parallel to the orientation of the liquid crystal molecules can be modulated. However, in the actual application of WSS, we need to modulate the polarization in both directions. The current solution is mainly to use a polarization beam splitter to decompose a beam of light with any polarization direction into two perpendicular polarization beams, and then use a half-wave plate (HWP) or a quarter-wave plate (QWP) plus a reflector to turn the polarization of one beam parallel to the other, and then incident on the LCoS. This method adds additional components, which makes the optical path complicated; it hinders the evolution of WSS to large ports and small sizes; at the same time, it also increases the cost of the system. Therefore, how to realize a polarization-independent LCoS device has become a very critical issue.

[0043] In one possible embodiment, Figure 4 is a schematic diagram of the physical structure of another type of silicon-based liquid crystal. Conventional LCoS typically employs a structure in which liquid crystal is placed between a layer of ITO electrodes and a metal reflector pixel array, forming a sandwich-like structure as shown in Figure 4. In another possible embodiment, Figure 5 is a schematic diagram of the physical structure of yet another type of silicon-based liquid crystal. This structure differs from the silicon-based liquid crystal schematic shown in Figure 4 in that a subwavelength grating structure, metasurface structure, or polymer is added above the reflector layer included in the silicon-based liquid crystal shown in Figure 4 to form a QWP polarization conversion layer, with the optical axis of this QWP oriented at 45 degrees to the LCoS orientation. Through the structure shown in Figure 5, incident polarized light is reflected and converted by the QWP into vertically polarized light.

[0044] As a possible example, when the light that the liquid crystal can modulate is s light, the incident light with polarization s can be modulated when passing through the liquid crystal, but after being converted into p light by the QWP polarization conversion layer, the reflected p light cannot be modulated by the liquid crystal; it can be understood that the optical path modulation of this beam of incident light is a phase modulation of twice the height of the liquid crystal.

[0045] As another possible example, when the light modulated by the liquid crystal is s-light, incident light with p-polarization cannot be modulated when passing through the liquid crystal. However, after being converted to s-light by the QWP polarization conversion layer, the reflected s-light can be modulated by the liquid crystal. It can be understood that the optical path modulation of this incident light is also a phase modulation of one liquid crystal height. It can be understood that regardless of the polarization state of the incident light, when passing through the LCoS structure shown in Figure 5, it will experience phase modulation of one liquid crystal height, thus achieving polarization-independent phase modulation.

[0046] However, due to limitations in micro-nanostructure fabrication, the wavelength bandwidth required for WSS, and micro-nanostructure design, the actual QWP polarization conversion layer in the LCoS structure shown in Figure 5 cannot completely convert incident light into vertically polarized reflected light. Instead, a portion of polarization remains unconverted, which can introduce a secondary crosstalk problem in WSS applications. For example, when the light modulated by the liquid crystal is s-light, and the incident light is also s-light, after passing through the QWP polarization conversion layer, some polarization-unconverted s-light may still be modulated by the liquid crystal after reflection. The optical path of this portion of s-light is modulated to a phase modulation of twice the height of the liquid crystal, resulting in a secondary crosstalk problem.

[0047] In view of this, an embodiment of the present application provides an LCoS. In this LCoS, an additional phase (referred to as "pre-phase" in the present embodiment) is pre-added in the dispersion direction based on a metasurface design or a phase film structure. This pre-added phase creates a high-frequency deflection or scattering phase for the unconverted polarization light, shifting its energy away from the crosstalk level. This prevents crosstalk between the unconverted polarization light and the converted polarization light, thereby suppressing secondary crosstalk in the port direction.

[0048] Figure 6 is a system architecture diagram of a possible WSS provided in an embodiment of the present application. The system may include at least: a fiber array (FA), two dispersion direction lenses, a port direction main lens, a grating and a polarization-independent (PI)-LCoS. Among them, the light beam coming out of the FA port is shaped by the dispersion direction lens unit and then incident on the PI-LCoS. In addition, the light of different wavelengths in the light beam can be split by the grating and thus hit different areas on the PI-LCoS; then, the corresponding phase diagram is loaded on each area on the PI-LCoS to achieve that the light of a certain wavelength channel is deflected to the designated port after passing through the port direction main lens, thereby realizing wavelength switching.

[0049] In other possible embodiments, some traditional WSS structures also add an additional polarization splitter component to the right of the FA or to the left of the PI-LCoS to first split the incident polarized light into two beams of orthogonal polarization. The polarization of the two beams is then adjusted to be parallel to the liquid crystal orientation through an HWP or QWP plus a reflector. The two beams are then incident on different areas of the PI-LCoS, thereby achieving modulation of the two polarizations. However, after the WSS structure introduces the PI-LCoS as shown in Figure 6, the WSS architecture removes the polarization splitter component and can more effectively utilize the PI-LCoS to achieve a lower cost and smaller size WSS structure. As the core component in the ROADM, the WSS can dynamically add or drop service wavelengths through remote reconfiguration without manual fiber adjustment, achieving wavelength reconstruction of the wavelength division multiplexing (WDM) system and realizing flexible service scheduling. This is of great significance for realizing all-optical networking, flexible scheduling of ultra-large capacity services, and achieving completely transparent service transmission.

[0050] The PI-LCoS provided in the embodiment of the present application can be applied in a WSS without a polarization splitting component, such as the WSS system architecture shown in FIG6 .

[0051] In an optional embodiment, the pre-phase in the dispersion direction of the PI-LCoS proposed in the embodiments of the present application can be achieved by using the structure of a metasurface polarization conversion layer. The metasurface structure provided in the embodiments of the present application can be composed of at least two microstructures, thereby achieving a gradient phase deviation of the polarization of unconverted light, thereby reducing secondary crosstalk. Examples include the following descriptions of Examples 1 and 2.

[0052] In another optional embodiment, the pre-phase in the dispersion direction of the PI-LCoS proposed in the present embodiment can be achieved by using a phase film structure. Since unpolarized light has twice the optical path length, while polarized light has one optical path length, the phase film can deflect the unpolarized light, thereby reducing secondary crosstalk. This is exemplified by the following third embodiment.

[0053] FIG7 is a schematic diagram of the structure of a PI-LCoS provided in an embodiment of the present application. The PI-LCoS may include at least: glass, ITO electrodes, two alignment layers, a liquid crystal layer, a reflector, a silicon-based backplane, and a metasurface polarization conversion layer 701; it can be understood that the metasurface polarization conversion layer 701 is used to represent the QWP polarization conversion layer shown in FIG5 using a metasurface structure.

[0054] The glass surface has an ITO electrode that is transparent to selected wavelengths. This ITO electrode, along with a metal reflector electrode in a pixel array above the silicon backplane, forms a pair of voltage-driven electrodes for the liquid crystal. The liquid crystal layer, after being aligned by an alignment layer, is sandwiched between these electrodes. In some possible embodiments, adjusting the voltage across each pixel in the pixel array modulates the orientation of the liquid crystal above each pixel, thereby achieving phase adjustment for each pixel. Each pixel corresponds to a corresponding reflector.

[0055] The metasurface polarization conversion layer 701 is located above the metal reflector of the pixel array. The metasurface polarization conversion layer 701 shown in FIG7 can adopt a metasurface structure, which can have the following phase constraints:

[0056] (1) The metasurface structure can include two types of microstructures and have an ABAB periodic arrangement as shown in FIG7 .

[0057] In some possible embodiments, the metasurface on each column of pixel array in the dispersion direction is the same type A microstructure, or the same type B microstructure; it can also be understood that the system period is 2 pixel widths in the dispersion direction.

[0058] In other embodiments, the correspondence between the metasurface structure in the metasurface polarization conversion layer 701 and the pixel array is not limited.

[0059] By using a metasurface structure that includes two types of microstructures, different modulation effects can be achieved on polarization-converted light and polarization-unconverted light, so that the polarization-unconverted light passing through the LCoS can deviate from the polarization-converted light, thereby reducing the secondary crosstalk effect of the polarization-unconverted light on the polarization-converted light.

[0060] (2) For both type A and type B metasurface microstructures, the optical axis forms an angle of 45 degrees with the liquid crystal direction, so that the two components of the incident light with polarization parallel to and perpendicular to the metasurface optical axis have a phase difference of about 180 degrees after being reflected, and most of the incident light is converted into another polarization perpendicular to the polarization and then reflected, thereby playing a role in polarization conversion.

[0061] (3) The phase modulation of reflected light by the metasurface microstructures of type A and type B is different. Moreover, as shown in FIG7 , after the polarization conversion, the incident light includes polarization-converted light and polarization-unconverted light.

[0062] For example, the phase modulation of polarization-converted light by the type A metasurface microstructure can be expressed as The phase modulation of the polarization-unconverted light by the type A metasurface microstructure can be expressed as The phase modulation of polarization-converted light by the type B metasurface microstructure can be expressed as The phase modulation of the type B metasurface microstructure for polarization-unconverted light can be expressed as In an optional embodiment, the phase difference of the polarization-converted light may satisfy And, the phase difference of the polarization unconverted light can satisfy

[0063] (4) Considering that the phase modulation of polarization-converted light by the A-type metasurface microstructure can be expressed as The polarization conversion light can be compensated by voltage control of the liquid crystal.

[0064] In an optional embodiment, the voltage control of the liquid crystal in the pixel column where the type A metasurface structure is located can be expressed as In order to achieve phase compensation for the phase modulation of the type A metasurface microstructure. Similarly, the voltage control of the liquid crystal by the pixel column where the type B metasurface structure is located can be expressed as To achieve phase compensation for the phase modulation of the Class B metasurface microstructure.

[0065] For example, after the polarization-converted light passes through the metasurface polarization conversion layer 701 shown in FIG. 7 , it can be expressed as the following formula 1-1:

[0066] For example, after the polarization-unconverted light passes through the metasurface polarization conversion layer 701 shown in FIG. 7 , it can be expressed as the following formula 1-2:

[0067] According to the above formulas 1-1 and 1-2, the phase of the polarization-converted light after passing through the metasurface polarization conversion layer 701 can be restored to The total phase modulation of the polarization-unconverted light is It can be understood that for polarization-unconverted light, the The phase of the crosstalk order is achieved by deviating the energy of the crosstalk order from the zero order in the dispersion direction, and the deviation of the polarization-unconverted light relative to the polarization-converted light is achieved, thereby reducing the secondary crosstalk.

[0068] In another optional embodiment, the voltage control of the liquid crystal by the pixel column where the type A metasurface structure is located can also be expressed as At this time, the voltage control of the liquid crystal by the pixel column where the type B metasurface structure is located can be expressed as It can be understood that when the selected metasurface structure satisfies There is no need to perform phase compensation and the phase diagram can be loaded normally.

[0069] In some possible embodiments, Figure 8 is a schematic diagram of a metasurface structure provided in an embodiment of the present application. For example, the metasurface structure may be in the shape of an elliptical cylinder and made of silicon material.

[0070] Optionally, the period of the Class A metasurface microstructures can be d = 850 nm, the height can be h = 400 nm, the angle between the optical axis and the X-direction can be 45°, and every four structures can form a class in the Y-direction and be located in the same column of the pixel array. In one possible embodiment, the major and minor axes of the Class A structures can be implemented as: r1 = 140 nm, r2 = 270 nm.

[0071] Optionally, the period of the Class B metasurface microstructures can be d = 850 nm, the height can be h = 400 nm, the angle between the optical axis and the X-direction can be 45°, and every four structures can form a class in the Y-direction and be located in the same column of the pixel array. In one possible embodiment, the major and minor axes of the Class B structures can be implemented as: r1 = 170 nm, r2 = 250 nm.

[0072] As can be seen from FIG8 , the metasurface structure can be periodically arranged in the Y direction by multiple type A microstructures and multiple type B microstructures, and can be expanded in the X direction.

[0073] In the above optional embodiment, the electromagnetic simulation results of the phase difference between the type A metasurface microstructure and the type B metasurface microstructure for polarization-converted light are: The electromagnetic simulation results for the phase difference of the polarization-unconverted light are: Furthermore, electromagnetic simulation results show that even without liquid crystal compensation, the additional insertion loss introduced by this structure is only approximately 0.4dB, but the isolation of secondary crosstalk can be improved by over 30dB, exceeding the 25dB isolation requirement for WSS applications. With the addition of liquid crystal compensation, the insertion loss can be further optimized to approximately 0.2dB.

[0074] It should be noted that FIG8 is only an example of a possible metasurface structure. The number of type A microstructures and type B microstructures can be determined according to the period, microstructure size, LCoS size, etc., and this application does not limit this.

[0075] Through the metasurface structure provided in the above embodiment 1, by performing parameter design on the metasurface structure as shown in Figure 8, a periodic arrangement is formed by using Class A and Class B metasurface structures, and the Class A and Class B metasurface structures have different modulation effects on polarization-converted light and polarization-unconverted light, thereby effectively reducing the secondary crosstalk caused by the presence of polarization-unconverted light.

[0076] FIG9 is a schematic diagram of the structure of a PI-LCoS provided in an embodiment of the present application. The PI-LCoS may include at least: glass, an ITO electrode, two alignment layers, a liquid crystal layer, a reflector, a silicon-based backplane, and a metasurface polarization conversion layer 901. It is understood that the metasurface polarization conversion layer 901 is used to indicate that the QWP polarization conversion layer shown in FIG5 adopts a metasurface structure. The same or similar aspects as FIG7 are not repeated here. The difference is that the metasurface polarization conversion layer 901 can also adopt four types of microstructures including Class A, Class B, Class C, and Class D as shown in FIG9, and are arranged in a periodic manner. It is understood that in this embodiment, every four columns of pixels form a period in the dispersion direction.

[0077] The phase constraints of the metasurface polarization conversion layer 901 may include: the phase difference of the four types of metasurface structures ABCD for polarization-converted light is less than 0.5π; and the phase of the polarization-unconverted light forms a gradient from 0 to 2π, which can also be understood as the blazed grating phase in the dispersion direction, thereby achieving the deflection of the polarization-unconverted light to the negative first order in the dispersion direction to reduce the secondary crosstalk.

[0078] It should be noted that although FIG7 describes a metasurface structure comprising two types of microstructures and FIG9 describes a metasurface structure comprising four types of microstructures, the embodiments of the present application do not limit the number of types of metasurface structures included. For example, more types of microstructures may be included, such as three, five, or more types. In addition, the phase of the polarization-unconverted light provided in the embodiments of the present application is not limited to the blazed grating phase, but may also be other high-frequency phase changes such as the sinusoidal grating phase, to achieve the effect of deviating the polarization-unconverted light energy from the zero order of the dispersion direction.

[0079] FIG10 is a schematic diagram of the structure of a PI-LCoS provided by an embodiment of the present application. In addition to including the various structures shown in FIG5 , the PI-LCoS may also insert a phase film at any position above the metal reflector of the pixel array. For example, FIG10 shows an example of inserting a phase film between the glass and the ITO electrode, or, for example, a phase film may be inserted above the reflector (or below the QWP polarization conversion layer), above the QWP polarization conversion layer, below the ITO electrode, etc. Among them, the phase modulation of the phase film on the polarization-converted light and the polarization-unconverted light is the same. Assume that the phase modulation of the phase film on the polarization-converted light is expressed as The phase modulation of the phase film on the polarization unconverted light is expressed as but In addition, the structure has the following phase constraints: the phase in the dispersion direction is approximately continuously changing and high-frequency; and the phase difference between adjacent pixels in each column should be less than 0.5π to facilitate liquid crystal compensation.

[0080] Considering that the light beam passes through the phase film twice, the phase that the liquid crystal needs to compensate should be twice the phase added by the phase film. Therefore, the control phase added by the liquid crystal can be adjusted as shown in the following formula 2-1:

[0081] It can be understood that the total phase modulation of the polarization converted light is still The total phase modulation for polarization-unconverted light becomes as shown in the following formula 2-2:

[0082] It can also be understood that there is still residual in the dispersion direction The high-frequency phase of the polarization-unconverted light can be deviated from the polarization-converted light, reducing the influence of the polarization-unconverted light on the polarization-converted light, thereby reducing the secondary crosstalk.

[0083] In one possible embodiment, the phase film 1001 may be a sinusoidal grating phase film. For example, FIG11 is a schematic diagram of the structure of a PI-LCoS provided in an embodiment of the present application. The sinusoidal grating phase film shown in FIG11 has high transmittance for the selected wavelength, which can be expressed as the following formula 3-1:

[0084] Here, T is larger than the size of a pixel.

[0085] The liquid crystal is controlled by voltage, and the adjusted phase is shown in the following formula 3-2:

[0086] Based on the above formulas 3-1 and 3-2, and considering that the light beam passes through the phase film twice, the total modulation phase of the polarization converted light after passing through the liquid crystal with the phase shown in formula 3-2 and the sinusoidal grating phase film twice as shown in formula 3-1 is

[0087] Based on the above formula 3-1 and formula 3-2, and considering that the light beam passes through the phase film twice, the total phase modulation of the polarization unconverted light is It can be understood that for polarization-unconverted light, a sinusoidal grating phase in the dispersion direction is still retained on the basis of the secondary crosstalk, so as to deviate the energy of the crosstalk order from the zero order in the dispersion direction, realize the deviation of the polarization-unconverted light relative to the polarization-converted light, and thus reduce the secondary crosstalk.

[0088] It should be noted that, in some other possible embodiments, the phase film 1001 may also be a grating phase film in other shapes such as a triangular wave or a square wave, with a continuously changing high-frequency phase.

[0089] It should also be noted that the QWP polarization conversion layer in Example 3 can adopt a metasurface structure such as that in Example 1 or Example 2, or can adopt a module having a metasurface structure with a uniform microstructure, a subwavelength grating structure or a polymer that can realize the polarization conversion function, and this application does not limit this.

[0090] The structure provided in Example 3 can reduce secondary crosstalk caused by unpolarized light by introducing an additional pre-phase in the dispersion direction through a sinusoidal grating phase film and then compensating this phase with a phase diagram. In addition, this structure is easy to manufacture.

[0091] The present application also provides a WSS, comprising at least the silicon-based liquid crystals described in the aforementioned embodiments. In an optional embodiment, the WSS may further include an input port, an output port, a polarization conversion unit, a wavelength division multiplexer, a lens, a reflector, etc. In another optional embodiment, the WSS may further include an FA, a dispersion-direction lens unit, a port-direction main lens, a grating, etc.

[0092] An embodiment of the present application further provides an optical communication device, comprising the silicon-based liquid crystal as described in the aforementioned embodiments.

[0093] It should be noted that, in the description of this application, "at least one" refers to one or more, wherein, "a plurality" refers to two or more. In view of this, in the embodiments of this application, "a plurality" may also be understood as "at least two". "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / ", unless otherwise specified, generally indicates that the previous and next associated objects are in an "or" relationship. In addition, it should be understood that, in the description of this application, words such as "first" and "second" are only used to distinguish the purpose of description, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying order.

[0094] It should be noted that in the embodiments of the present application, "coupling" can be understood as an electrical connection, and the coupling of two elements can be direct or indirect coupling between the two elements. For example, the connection between A and B can be either direct coupling between A and B, or indirect coupling between A and B through one or more other elements. For example, A and B can be coupled, or A and C can be directly coupled, and C can be directly coupled to B, with A and B being coupled through C. In some scenarios, "coupling" can also be understood as connection.

[0095] In the various embodiments of the present application, unless otherwise specified or there is a logical conflict, the terms and / or descriptions between different embodiments are consistent and can be referenced by each other. The technical features in different embodiments can be combined to form new embodiments according to their inherent logical relationships.

[0096] In this application, the word "exemplary" is used to mean an example, illustration, or explanation. Any embodiment or design described in this application as "exemplary" should not be construed as preferred or advantageous over other embodiments or designs. Alternatively, it can be understood that the use of the word "exemplary" is intended to present concepts in a concrete way and does not constitute a limitation of this application.

[0097] It will be appreciated that the various numerals involved in this application are merely for the purpose of describing the distinctions made, and are not intended to limit the scope of the embodiments of the present application. The sequence numbers of the above-mentioned processes do not necessarily indicate the order of execution, and the order of execution of each process should be determined by its function and inherent logic. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusions, for example, encompassing a series of steps or units. Methods, systems, products, or devices are not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or that are inherent to these processes, methods, products, or devices.

[0098] Although the present application has been described with reference to specific features and embodiments thereof, it is apparent that various modifications and combinations thereof may be made without departing from the spirit and scope of the present application. Accordingly, this specification and the drawings are intended to be illustrative only of the solutions defined by the appended claims and are to be construed as covering any and all modifications, variations, combinations or equivalents within the scope of the present application.

[0099] Obviously, those skilled in the art may make various changes and modifications to the present application without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the embodiments of the present application fall within the scope of the claims of the present application and their equivalents, the present application is intended to include such modifications and variations.

Claims

1. A silicon-based liquid crystal, applied to a wavelength selective switch, characterized in that: The silicon-based liquid crystal comprises: A first panel and a second panel arranged opposite to each other; a liquid crystal layer, located between the first panel and the second panel; a driving circuit for generating an electric field to control the deflection of liquid crystal in the liquid crystal layer; and Two layers of alignment films, the two layers of alignment films are respectively located on opposite sides of the liquid crystal layer; A reflector layer, located above the first panel, for reflecting the light beam; A polarization conversion layer is located above the reflector and below the two alignment layers, and is used to perform polarization conversion on the light beam after passing through the liquid crystal layer; wherein the polarization conversion layer includes at least two types of supersurface microstructures, and the at least two types of supersurface microstructures are periodically alternately arranged.

2. The silicon-based liquid crystal according to claim 1, characterized in that: The silicon-based liquid crystal has N pixel areas, and the reflector layer includes N reflectors corresponding to the N pixel areas one by one, where N is a positive integer; The N reflectors correspond one-to-one to the N groups of metasurface microstructures; wherein a group of metasurface microstructures includes a type of metasurface microstructures.

3. The silicon-based liquid crystal according to claim 1 or 2, characterized in that: The light beam after polarization conversion includes polarization-converted light and polarization-unconverted light; The silicon-based liquid crystal has N pixel areas, the driving circuit has a first voltage control value in the first pixel area and a second voltage control value in the second pixel area; the second pixel area is an adjacent pixel area of ​​the first pixel; The first voltage control value and the second voltage control value are determined based on the phase modulation values ​​of the first type of metasurface microstructure and the second type of metasurface structure.

4. The liquid crystal on silicon according to claim 3, characterized in that: The first voltage control value includes a phase compensation for a first phase modulation value; wherein the first phase modulation value is used to indicate a phase modulation value of the first type of microstructure on the polarization converted light; The second voltage control value includes a phase compensation for a second phase modulation value; wherein the second phase modulation value is used to indicate a phase modulation value of the second type of microstructure on the polarization converted light.

5. The silicon-based liquid crystal according to claim 3, characterized in that: The polarization conversion layer includes two types of metasurface microstructures; The second voltage control value includes a phase compensation for a third phase modulation value; wherein the third phase modulation value is used to indicate the sum of a first phase modulation value of the polarization converted light by the first type of microstructure and a second phase modulation value of the polarization converted light by the second type of microstructure.

6. The liquid crystal on silicon according to any one of claims 1 to 5, characterized in that: The super surface microstructure is an elliptical columnar microstructure, and different types of super surface microstructures have different sizes.

7. A silicon-based liquid crystal used in a wavelength selective switch, characterized in that: The silicon-based liquid crystal comprises: A first panel and a second panel arranged opposite to each other; a liquid crystal layer, located between the first panel and the second panel; a driving circuit for generating an electric field to control the deflection of liquid crystal in the liquid crystal layer; and Two layers of alignment films, the two layers of alignment films are respectively located on opposite sides of the liquid crystal layer; A reflector layer, located above the first panel, for reflecting the light beam; A polarization conversion layer, located above the reflector and below the two alignment layers, for performing polarization conversion on the light beam after passing through the liquid crystal layer; wherein the light beam after polarization conversion includes polarization-converted light and polarization-unconverted light; The phase film layer is located above the reflector and is used to perform the same phase modulation on the polarization-converted light and the polarization-unconverted light.

8. The liquid crystal on silicon according to claim 7, characterized in that: The phase film is a high-frequency phase film.

9. The liquid crystal on silicon according to claim 8, characterized in that: The high-frequency phase film is a sinusoidal grating phase film, a triangle wave phase film, or a square wave phase film.

10. A wavelength selective switch, characterized in that: include: The liquid crystal on silicon according to any one of claims 1 to 6 or the liquid crystal on silicon according to any one of claims 7 to 9.

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