Lcos device for optical communication

Abstract

A liquid crystal on silicon (LCoS) device (100) for optical communication, sequentially comprising, from top to bottom: a top substrate (110) made of transparent glass; a transparent conductive layer (120) serving as a common electrode; an upper alignment layer (130) patterned for uniform alignment of liquid crystal molecules; a liquid crystal layer (140); a lower alignment layer (150) patterned for uniform alignment of liquid crystal molecules, wherein the liquid crystal layer (140) is arranged between the upper alignment layer (130) and the lower alignment layer (150); a lower conductive layer (160) serving as a metallic reflector; and a complementary metal oxide semiconductor (CMOS) backplane (170) comprising a silicon substrate. At least one of the upper alignment layer (130) and the lower alignment layer (150) is patterned as a periodically varying multi-domain alignment structure, in which adjacent domains have different orientations, such that the device acts as a polarization-independent optical element when an electric field is applied.

Description

LCoS devices for optical communication

[0001] Cross-reference to related applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 725,527, filed November 26, 2024, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] This disclosure generally relates to the field of liquid crystal on silicon (LCoS) technology. Background Technology

[0004] Wavelength selective switches (WSS) are core components of modern reconfigurable optical add-drop multiplexers and optical cross-connects. LCoS technology plays a central role as a "smart optical switch" and "optical path manager" in optical communications, and is mainly used in core nodes and backbone networks of communication networks.

[0005] LCoS devices combine the liquid crystal electro-optic effect with complementary metal-oxide-semiconductor (CMOS) technology. The basic structure of an LCoS device includes: a silicon substrate at the bottom, a reflective layer attached to the silicon substrate, a liquid crystal layer above the reflective layer, and a transparent electrode (typically a transparent conductive film made of indium tin oxide (ITO)) above the liquid crystal layer. The silicon substrate is essentially a silicon chip integrating CMOS driving circuitry and numerous pixel electrodes (one electrode per pixel), unlike traditional liquid crystal displays (LCDs) which use glass substrates and thin-film transistors. The reflective layer is a very flat metal film above the pixel electrodes. The liquid crystal layer is a thin layer of liquid crystal material located above the reflective layer. Above the liquid crystal layer are the transparent electrodes, and the ITO film can be attached to a transparent glass plate. Its working principle is as follows: light enters from above, passes through the transparent electrodes and the liquid crystal layer, and reaches the bottom reflective layer, which reflects the light back along its original path. During this process, the CMOS circuitry on the silicon substrate precisely controls the voltage of each pixel electrode according to the image signal. Changes in voltage alter the alignment of liquid crystal molecules, thus acting as a "light valve" to control the polarization of incident light after reflection. In this process, the liquid crystal changes its orientation by applying voltage, thereby altering the phase of the incident light. However, this phase change is only effective for light rays with a specific polarization direction (e.g., linearly polarized light). For light rays with a polarization direction perpendicular to the liquid crystal's orientation, the modulation effect of the liquid crystal is almost "invisible."

[0006] Figure 1 illustrates a conventional optical fiber communication system. ROADM, OAN, OXC, OMCN, and ONU refer to a reconfigurable optical add-drop multiplexer, optical access network, optical cross-connect, optical metropolitan area core node, and optical network unit, respectively. The ROADM resembles a traffic hub, consisting of WSSs in different directions, power dividers (PS), and arrayed waveguide gratings (AWGs). The silicon-based liquid crystal spatial light modulator (LCoS-SLM) device discussed in this paper is located within the WSS. In conventional optical fiber communication, the polarization state (SOP) of real-world optical signals is random and constantly changing. However, when applying LCoS to the WSS, the light must first pass through a polarization diversity optics system (shown by the dashed box in Figure 1), resulting in a 50% signal loss. This loss is inherent in current WSS setups based on polarization-phase modulation and poses a significant challenge to network systems.

[0007] Various approaches have been explored. US2017 / 0371217A1 introduced a reconfigurable optical spatial mode multiplexing system with a polarization-independent phase modulator, although it lacks inherent polarization independence. Zhu et al. recently made progress by integrating metasurfaces onto a CMOS backplane, enhancing polarization independence, but also introducing fabrication complexity and reducing reflectivity and diffraction efficiency. Zachmann et al. proposed a quarter-wave plate based on a liquid crystal polymer to achieve polarization independence, but faced limitations in diffraction efficiency and circuit complexity. Zhang et al. improved the driving circuit of the above devices, but lacked detailed analysis for WSS applications. Hyman et al.'s polymer-stabilized blue phase liquid crystal device achieved high-speed switching, but required high voltage. A similar approach by Sun et al. achieved phase modulation at lower voltages, but lacked performance analysis in the C-band and L-band. Li et al.'s metasurface-based device demonstrated high polarization conversion efficiency, but raised concerns about voltage requirements.

[0008] As can be seen, traditional polarization-independent LCoS devices face many limitations, such as reduced diffraction efficiency, increased device complexity, and a lack of detailed research on wavelength selective switching. To overcome these shortcomings, this disclosure proposes a high-efficiency and polarization-independent LCoS device for optical communication. Summary of the Invention

[0009] Various aspects and advantages of this disclosure will be set forth in part in the description which follows, or may become apparent from the description, or may be learned by practice of the art.

[0010] According to one aspect of this disclosure, an LCoS device for optical communication is provided, comprising, from top to bottom: a top substrate made of transparent glass; a transparent conductive layer serving as a common electrode; an upper alignment layer patterned to uniformly align liquid crystal molecules; a liquid crystal layer; a lower alignment layer patterned to uniformly align liquid crystal molecules, wherein the liquid crystal layer is disposed between the upper and lower alignment layers; a lower conductive layer serving as a metal mirror; and a CMOS backplane comprising a silicon substrate. The first alignment layer of the upper and lower alignment layers is patterned into a periodically varying multi-domain alignment structure, with adjacent domains having different orientations, such that when an electric field is applied, the silicon-based liquid crystal LCoS device functions as a polarization-independent optical element.

[0011] In some embodiments, the LCoS device can be used for unpolarized incident light.

[0012] In some embodiments, the multi-domain alignment structure includes a mixed pattern of vertical alignments with a first large pretilt angle and vertical alignments with a second large pretilt angle arranged periodically, wherein the vertical alignment with a large pretilt angle means that the long axis of the liquid crystal molecules forms an angle greater than 45 degrees relative to the plane of the first alignment layer in the initial state of being energized, and the angle between the first large pretilt angle and the second large pretilt angle is greater than 0 degrees and less than 45 degrees; and the second alignment layer in the upper alignment layer and the lower alignment layer are patterned in a consistent manner with the first alignment layer.

[0013] In some embodiments, the multi-domain alignment structure provides a first large pretilt angle vertical alignment for pixels located in odd-numbered rows, and a second large pretilt angle vertical alignment for pixels located in even-numbered rows; alternatively, the multi-domain alignment structure provides a first large pretilt angle vertical alignment for pixels located in odd-numbered columns, and a second large pretilt angle vertical alignment for pixels located in even-numbered columns. The long axis of the liquid crystal molecules vertically aligned with the first large pretilt angle and the long axis of the liquid crystal molecules vertically aligned with the second large pretilt angle form a non-zero angle angle when projected onto the first alignment layer.

[0014] In some embodiments, the multi-domain alignment structure includes a mixed pattern of periodically alternating first-angle parallel alignments and second-angle parallel alignments, wherein the first-angle parallel alignment aligns the liquid crystal molecules in a plane parallel to the first alignment layer along the first angle orientation, and the second-angle parallel alignment aligns the liquid crystal molecules in the plane along the second angle orientation, the included angle between the first angle and the second angle being greater than 0 degrees and less than or equal to 90 degrees; and the second alignment layer in the upper alignment layer and the lower alignment layer aligns the liquid crystal molecules perpendicularly.

[0015] In some embodiments, the second alignment layer is patterned to vertically align the liquid crystal molecules in the following manner: a mixed pattern of vertical alignments with a first large pretilt angle and vertical alignments with a second large pretilt angle arranged periodically, wherein the vertical alignment with a large pretilt angle means that the long axis of the liquid crystal molecules forms an angle greater than 45 degrees relative to the plane of the first alignment layer in the initial state of being energized, and the angle between the first large pretilt angle and the second large pretilt angle is greater than 0 degrees and less than 45 degrees.

[0016] In some embodiments, the multi-domain alignment structure provides a first-angle parallel alignment for pixels located in odd-numbered rows and a second-angle parallel alignment for pixels located in even-numbered rows; or the multi-domain alignment structure provides a first-angle parallel alignment for pixels located in odd-numbered columns and a second-angle parallel alignment for pixels located in even-numbered columns.

[0017] In some embodiments, the upper and lower orientation layers comprise azo dye materials.

[0018] In some embodiments, the multi-domain alignment structure is formed by irradiating a photomask with polarized light multiple times.

[0019] In some embodiments, the upper and lower alignment layers are aligned by polarized light, with an irradiation dose ranging from 0.1 joules to 2 joules.

[0020] In some embodiments, the multi-domain alignment structure is formed by irradiation with an interference pattern of polarized light.

[0021] In some embodiments, the size of each domain in the multi-domain alignment structure is on the micrometer scale.

[0022] In some embodiments, the optical element is a blazed phase grating.

[0023] In some embodiments, the LCoS device is suitable for the visible spectrum of light.

[0024] In some embodiments, the LCoS device is suitable for optical fiber communication bands, including O, E, S, C, L, and U bands.

[0025] These and other features, aspects, and advantages of this disclosure will become more readily understood with reference to the description below and the appended claims. The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the technology and, together with the specification, serve to explain the principles of the technology. Attached Figure Description

[0026] The present disclosure is made in its entirety and in practice with reference to the accompanying drawings. This disclosure is intended for those skilled in the art and includes the best mode of making and using the system and method.

[0027] Figure 1 illustrates a conventional optical fiber communication system in the prior art.

[0028] Figure 2 is a schematic diagram of the structure of an LCoS device for optical communication according to an embodiment of the present disclosure.

[0029] Figure 3 is a schematic diagram of a parallel-aligned multi-domain alignment structure according to an embodiment of the present disclosure.

[0030] Figure 4 is a schematic diagram of the preparation of a multi-domain alignment structure according to an embodiment of the present disclosure.

[0031] Figure 5 is a comparative experiment of conventional single-domain alignment and dual-domain alignment according to an embodiment of the present disclosure.

[0032] Figure 6 illustrates an LCoS device including a multi-domain alignment structure according to an embodiment of the present disclosure.

[0033] Figure 7 shows the relationship between diffraction efficiency and polarization angle according to an embodiment of the present disclosure.

[0034] Figure 8A shows a side view of a conventional uniformly aligned (single domain) LCoS design.

[0035] Figure 8B shows a side view of a multi-domain alignment LCoS design according to an embodiment of the present disclosure.

[0036] Figure 9A shows a top view of a conventional uniformly aligned (single domain) LCoS design.

[0037] Figure 9B shows a top view of a multi-domain alignment LCoS design according to an embodiment of the present disclosure. Detailed Implementation

[0038] Embodiments of this disclosure will now be described in detail with reference to the accompanying drawings, one or more examples of which are illustrated. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as being more preferred or advantageous than other implementations. Furthermore, each example is provided by way of explanation rather than limitation. Indeed, it will be apparent to those skilled in the art that modifications and variations can be made to this technology without departing from the scope or spirit of the claimed technology. For example, features shown or described as part of one embodiment may be used with another embodiment to produce yet another embodiment. Therefore, this disclosure is intended to cover such modifications and variations within the scope of the appended claims and their equivalents. Numerical and alphabetic designations are used in the detailed description to denote features in the drawings. Similar or analogous designations in the drawings and specification are used to refer to similar or analogous parts in this disclosure.

[0039] The singular expressions “a,” “an,” and “the” also include the plural cases, unless the context explicitly specifies otherwise. The terms “comprising,” “including,” “comprising,” “having,” or any other variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, article, or apparatus that includes a set of features is not necessarily limited to those features, but may include features not expressly listed or other features inherent to such a process, method, article, or apparatus. Furthermore, unless explicitly stated to the contrary, “or” is inclusive rather than exclusive. For example, any of the following satisfy conditions A or B: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); and both A and B are true (or exist).

[0040] Terms indicating approximation, such as "approximately," "basically," etc., include values ​​up to 10% larger or smaller than the described value. When the terms "parallel" or "perpendicular" are used in the context, these terms include angles or directions up to 10 degrees larger or smaller than the described angle or direction. Furthermore, directional terms such as top, bottom, upper, and lower in this disclosure are used only to describe the relative positions between elements and are not intended to limit their specific orientation. For example, after a device is rotated 180 degrees, the top becomes the bottom, the bottom becomes the top, and so on.

[0041] The benefits, other advantages, and solutions to problems are described below with reference to specific embodiments. However, the benefits, advantages, solutions to problems, and any features that may cause any benefit, advantage, or solution to occur or become more apparent should not be construed as key, necessary, or essential features of any or all claims.

[0042] LCoS devices primarily operate in two modes: amplitude modulation (AM) and phase modulation (PM). Amplitude modulation, also known as intensity modulation, controls the light intensity emitted by each pixel to form a grayscale image. AM is commonly used in microdisplays, augmented reality (AR), virtual reality (VR) displays, and various other applications. Phase modulation does not change the intensity of light but precisely controls the wavefront phase of the light wave. Phase modulation is commonly used in fiber optic communication networks, holographic displays, adaptive optics, beam shaping, scientific instruments, and various other applications. Phase modulation plays a crucial role in tasks such as beam shaping and beam control. This invention focuses specifically on the phase modulation capability of LCoS devices. The basic process of phase modulation involves the uniform alignment of liquid crystal molecules across the entire panel. When polarized light shines on the device, the polarization azimuth angle remains parallel to the fast axis of the liquid crystal alignment direction. The fast axis of the liquid crystal refers to the direction of polarized light with the fastest propagation speed in the liquid crystal molecule arrangement structure. The structure of an LCoS device typically includes a CMOS backplane. A metal mirror serves as the bottom substrate on the backplane. The liquid crystal layer is sandwiched between a top glass layer and the bottom substrate. The top glass layer includes an ITO layer used as a common electrode.

[0043] In reflection mode, the phase modulation capability of an LCoS device is determined by the thickness of the liquid crystal layer and its birefringence, thus determining the degree of phase modulation according to the retardation equation. In the absence of an electric field, the incident light undergoes 2π phase modulation, a key requirement for achieving high diffraction efficiency. Beam steering is achieved by realizing a blazed grating phase profile, where a linear phase change within a period determines the diffraction angle. Different periods can be used for different wavelengths to achieve different steering angles. Such beam steering devices can be used as optical switching engines or wavelength selective switches (WSS), essential components in reconfigurable optical add-drop multiplexer (ROADM) systems for telecommunications applications. However, it is important to note that although optical signals in the network are unpolarized, they must pass through a polarization diversity optics system (shown in Figure 1) for use with the WSS, resulting in a 50% signal loss. This loss is inherent in current WSS setups based on polarized phase modulation and poses a significant challenge to network systems. Existing research has attempted to mitigate these losses and explored various possible solutions, which will be discussed below in a non-chronological order. Research on polarization-independent LCoS devices is diverse and constantly evolving.

[0044] US2017 / 0371217A1 discloses a multimode reconfigurable optical spatial mode multiplexing system that uses a beam combiner to combine input beams to produce an optical output. One of its key components is a polarization-independent reconfigurable phase modulator that can change the spatial mode order of the input beam. Control inputs are used to adjust the phase distribution using ferroelectric liquid crystals, thereby achieving mode switching. Regardless of the polarization state of the input beams, the combined output contains more spatial modes than a single input beam. The authors introduce a beam combiner technique aimed at reducing polarization dependence in wavelength division multiplexing (WDM) systems. While this approach is innovative, it is bulky and complex to implement. The patent does not provide explicit details regarding the device's associated losses or operating voltage requirements. Furthermore, the method is not inherently polarization-independent, which may limit its practical applications.

[0045] Zhu et al. published a paper titled "Metasurface-enabled polarization-independent LCoS spatial light modulator for 4K resolution and beyond." In it, they developed a polarization-independent LCoS device by integrating a thin gold metasurface onto a CMOS backplane. While this represents an advancement in polarization-independent LCoS technology, this approach also presents several challenges. For example, modifications to the CMOS backplane and the addition of the metasurface introduce new complexities to the manufacturing process. Furthermore, the reflectivity of the original LCoS device, typically exceeding 70% in the C-band, drops to approximately 65% ​​after adding the metasurface, implying increased losses. In contrast, the reflectivity of state-of-the-art LCoS devices currently exceeds 90%. Another key parameter of LCoS devices is diffraction efficiency; this device achieved only 53%, far below the expected 80% or more.

[0046] Zachmann et al. published a paper titled "Design and fabrication of polarization-independent LCoS phase modulators with polymer waveplate and analog driving," which proposes a method to achieve polarization independence in LCoS devices by incorporating a quarter-wave plate (QWP) based on liquid crystal polymer (LCP) using optical alignment technology. Furthermore, they developed a custom pixel circuit capable of handling higher driving voltages and providing high pixel grayscale. This is achieved by utilizing an analog-driven frame-buffered pixel circuit to meet the voltage requirements introduced by the QWP layer. While this method makes a significant contribution to realizing polarization-independent LCoS devices, it still faces some limitations. The device's diffraction efficiency is approximately 50%, far below the standard requirement of over 80%. Moreover, the addition of the QWP layer requires significant modifications to the circuit design and voltage requirements, increasing the complexity of the LCoS device. Additionally, there is a lack of detailed research on whether this device is suitable for C-band and L-band wavelength selective switching (WSS) applications crucial to telecommunications networks.

[0047] Zhang et al. published a paper titled "Novel Frame Buffer Pixel Circuits and Silicon Backplane Development for Polarization-Independent LCOS," in which they improved the driving and pixel circuits based on the method of Zachmann et al., to enhance scalability and reduce cost. While this optimization represents a step forward, the study lacks a detailed analysis of the device's suitability for WSS applications, particularly in the C-band and L-band. These aspects are crucial for evaluating its practical application in telecommunications networks.

[0048] The paper "Polarization-independent phase modulation using a blue-phase liquid crystal over silicon device" by Hyman et al. proposes a novel LCoS device based on polymer-stabilized blue-phase liquid crystal. This device simultaneously provides phase modulation and high-speed switching within a silicon backplane, and is essentially independent of the input polarization state. The device achieves continuous phase modulation of light with sub-millisecond switching times and is insensitive to the polarization direction of the input light. However, the main limitation of this approach is the requirement for a high electric field of approximately 20 V / μm, significantly higher than the 1-2 V / μm typically required by conventional LCoS devices. This high voltage requirement poses a challenge for practical applications.

[0049] Sun et al. published a paper titled "A polarization-independent blue phase liquid crystal on silicon with low operating voltage," which describes how polarization-independent phase modulation was achieved using a blue phase liquid crystal. Their device achieved complete 2π phase modulation at a relatively low voltage of approximately 6V. However, this method requires specially designed LCoS devices and additional optical design, increasing system complexity. Furthermore, the study does not provide information on key performance parameters such as diffraction efficiency or C-band and L-band losses, which are crucial for WSS applications.

[0050] Li et al. published a paper titled "Broadband Phase-Polarization Co-Modulated Dielectric Metasurface for Polarization-Independent LCoS Device," which demonstrated a polarization-independent LCoS device by combining a half-wave plate based on a dielectric metasurface. This device achieved a polarization conversion rate exceeding 90% and exhibited high diffraction efficiency. However, adding a metasurface layer to a conventional LCoS structure presents several challenges, such as increased voltage requirements and potential losses. These factors need to be carefully addressed to improve the practicality of this method.

[0051] US9065707B2 describes a polarization-independent LCoS device that utilizes an electrode grid fabricated on a CMOS backplane as a structured grating element. While this method demonstrates the device's polarization independence, its performance in terms of loss and diffraction efficiency in the critical C-band and L-band regions remains unclear, limiting its evaluation for WSS applications.

[0052] The paper "High-efficiency, liquid-crystal-based, controllable diffraction grating" by Zhang et al. proposes a diffraction grating element based on a liquid crystal with periodic polymer walls. This design helps reduce edge field effects and enables the generation of polarization-independent gratings. However, this study primarily focuses on projection display systems and does not explore the applicability of this design to WSS applications. Furthermore, all experiments were conducted in the visible spectrum, and performance analysis in the C-band and L-band regions, which are crucial for telecommunications, is not provided.

[0053] Patents and research exploring structured gratings and diffractive elements have also shown potential in reducing edge effects and improving polarization independence. This disclosure provides a method for achieving polarization independence in conventional LCoS devices by utilizing photoalignment technology to realize a periodic multi-domain alignment structure of liquid crystals. In liquid crystal displays, a "domain" refers to a "region" where liquid crystal molecules have the same alignment direction within a local area. A "multi-domain structure" refers to multiple "domains" with different alignment directions of liquid crystal molecules existing in space. In WSS applications, using a blazed phase grating and a multi-domain alignment structure only changes the fast axis of the liquid crystal within each pixel, thereby maintaining phase consistency among adjacent pixels. One domain can correspond to one or more pixels. The pretilt angle is the angle formed by the long axis of the liquid crystal molecules relative to the alignment layer plane in the initial state of being energized. In this disclosure, pretilt angles greater than 0 degrees are referred to as vertical alignment. Pretilt angles equal to 0 degrees are referred to as parallel alignment. "Periodic alternation" refers to the alternation of two orientations at periodic intervals. For example, elements in odd-numbered rows are arranged in the first orientation, while elements in even-numbered rows are arranged in the second orientation; or, elements in odd-numbered columns are arranged in the first orientation, while elements in even-numbered columns are arranged in the second orientation. "Uniformity" refers to the consistent orientation and orderly arrangement of liquid crystal molecules within a local region of the defined orientation (e.g., within each row or column).

[0054] This disclosure presents a novel and ingenious method for achieving polarization independence in conventional LCoS devices by periodically aligning liquid crystals with multiple domains using photoalignment technology. At the pixel level, the photoalignment technology ensures that each pixel has a uniform alignment, while adjacent pixels have different alignment axes, effectively eliminating the polarization requirement of the input light, i.e., eliminating the need for a polarization diversity optics system in the optical communication system of Figure 1. This method eliminates the need for additional layers or design changes in the LCoS device, thus avoiding increased complexity, increased voltage requirements, or changes to pixel circuitry. In WSS applications that typically use blazed phase gratings, the multi-domain alignment structure only changes the fast axis of the liquid crystal within each pixel, thus maintaining phase coherence among adjacent pixels. This disclosure simplifies WSS optical design by pairing two pixels to produce a step change in the phase profile of the blazed grating, eliminating the need for polarization diversity optics and ultimately improving system efficiency.

[0055] Figure 2 is a schematic diagram of the structure of an LCoS device 100 for optical communication according to an embodiment of the present disclosure. The LCoS device 100 includes, from top to bottom, a top substrate 110, a transparent conductive layer 120, an upper alignment layer 130, a liquid crystal layer 140, a lower alignment layer 150, a lower conductive layer 160, and a CMOS backplane 170. Each component will be described in detail below.

[0056] The top substrate 110 is typically a transparent glass cover plate, primarily serving a sealing and protective function. It can be encapsulated with the CMOS backplane 170 using sealant to form a thin cell housing the liquid crystal. The top substrate 110 needs to possess high light transmittance and good flatness. The CMOS backplane 170 is a silicon substrate made of single-crystal silicon. This is the core of LCoS technology. It is not merely a supporting substrate; its interior integrates complex active driving circuitry using mature CMOS technology. Each circuit unit corresponds to a pixel and can independently and precisely control the voltage. Simultaneously, the high flatness of silicon lays the foundation for forming a high-quality reflective surface.

[0057] A transparent conductive layer 120 is located below the top substrate 110. In the prior art, the transparent conductive layer 120 is mainly implemented using ITO. ITO is a conductive and transparent indium tin oxide, which serves as a common electrode to form a unified voltage reference plane across the entire display surface. ITO technology is mature and has high light transmittance and low resistance. Transparent conductive layers 120 made of other materials are also within the scope of this invention, such as graphene and aluminum-doped zinc oxide. When light is incident from above, it passes through the transparent conductive layer 120 to reach the liquid crystal layer 140, is reflected by the pixel electrode below, and then passes through the transparent conductive layer 120 again before exiting. Therefore, its high light transmittance is crucial. Above the CMOS backplane 170, there is a metal mirror serving as the lower conductive layer 160. The lower conductive layer 160 is both the point where electrical signals are applied (i.e., the pixel electrode) and a reflector.

[0058] An upper alignment layer 130 is coated below the transparent conductive layer 120, and a lower alignment layer 150 is coated above the lower conductive layer 160. Arrows schematically indicate the alignment directions in the upper alignment layer 130 and the lower alignment layer 150. The upper alignment layer 130 and the lower alignment layer 150 can be patterned to uniformly align liquid crystal molecules. The alignment layer is typically a polymer. The alignment layer controls the behavior of the liquid crystal and has an orienting effect. Through processes such as friction or photoalignment, fine grooves are formed on the polymer surface, forcing the liquid crystal molecules in contact with it to align along a specific direction. The alignment layer can also provide a pretilt angle for the liquid crystal molecules, so that the long axis of the liquid crystal molecules forms a predetermined angle relative to the plane of the alignment layer in the initial state of energization. This pretilt angle ensures that when a voltage is applied, all liquid crystal molecules can tilt in a desired, uniform direction, preventing alignment defects. In this disclosure, to achieve a high-performance LCoS with a wider viewing angle and faster response, a multi-domain alignment structure is designed using photoalignment technology. The different orientations of adjacent domains cause liquid crystal molecules to tilt in different directions within different domains, macroscopically compensating for color shift and contrast reduction caused by viewing angle. The multi-domain alignment structure will be discussed in detail later.

[0059] Liquid crystal molecules are filled between the upper alignment layer 130 and the lower alignment layer 150. A liquid crystal layer 140 is sealed between these layers, with a very uniform thickness, typically only a few micrometers. The liquid crystal layer 140 acts as the "executor" of light modulation, implementing electrically controlled birefringence of incident light. Since liquid crystal molecules are anisotropic, their refractive index varies with orientation. Without an electric field, the liquid crystals align according to the guidance of the alignment layers, changing the polarization state of the incident light. In LCoS, the incident light passes through the liquid crystal layer 140, is reflected by the pixel electrodes on the lower surface, and then passes through the liquid crystal layer 140 again before exiting. This process involves the light passing through the liquid crystal layer 140 twice, doubling the modulation effect and resulting in high efficiency. By applying different voltages to each pixel electrode using CMOS circuitry, the tilt angle of the liquid crystal molecules is changed, thereby precisely controlling their ability to change the polarization state of light.

[0060] In some embodiments, one of the upper alignment layer 130 and the lower alignment layer 150 is patterned into a periodically varying multi-domain alignment structure, and the other alignment layer vertically aligns the liquid crystal molecules (e.g., vertical alignment with a pretilt angle of 90 degrees). The inconsistent alignment of the upper alignment layer 130 and the lower alignment layer 150 can produce a torsional effect on the liquid crystal molecules. This multi-domain alignment structure includes a mixed pattern of periodically alternating first-angle parallel alignments and second-angle parallel alignments. The first-angle parallel alignment aligns the liquid crystal molecules in a plane parallel to the upper alignment layer 130 and the lower alignment layer 150 along a first-angle direction, and the second-angle parallel alignment aligns the liquid crystal molecules in the same plane along a second-angle direction. The angle between the first and second angles is greater than 0 degrees and less than or equal to 90 degrees. Figure 3A is a schematic diagram of a parallel-aligned multi-domain alignment structure according to an embodiment of the present disclosure. Each square represents a domain, and each domain covers the same number of pixels, which may correspond to one or more pixels. The upper alignment layer 130 and the lower alignment layer 150 work together to enable the device to function as a polarization-independent optical element. This multi-domain alignment structure can provide a first angular parallel alignment for pixels located in odd-numbered columns and a second angular parallel alignment for pixels located in even-numbered columns. Alternatively, the multi-domain alignment structure can provide a first angular parallel alignment for pixels located in odd-numbered rows and a second angular parallel alignment for pixels located in even-numbered rows. In the embodiment of FIG. 3A, the angle between the first angle (along the x-direction in the xy alignment plane) and the second angle (along the y-direction in the xy alignment plane) is 90 degrees. The angle between the first angle and the second angle can also be any angle greater than 0 degrees and less than or equal to 90 degrees. In a further embodiment, the vertical alignment of another alignment layer in the upper alignment layer 130 and the lower alignment layer 150 with the liquid crystal molecules can be as follows: a mixed pattern of vertical alignments with a first large pretilt angle and vertical alignments with a second large pretilt angle arranged periodically. The vertical alignment with a large pretilt angle refers to the formation of an angle greater than 45 degrees between the long axis of the liquid crystal molecules and the plane of the upper alignment layer 130 or the lower alignment layer 150 in the initial state of being energized. The first large pretilt angle is different from the second large pretilt angle, and the angle between them is greater than 0 degrees and less than 45 degrees.

[0061] In another embodiment, both the upper alignment layer 130 and the lower alignment layer 150 are uniformly patterned as a periodically varying multi-domain alignment structure. This multi-domain alignment structure includes a mixed pattern of vertical alignments with a first large pretilt angle and a second large pretilt angle arranged in a periodic alternation. The first large pretilt angle differs from the second large pretilt angle, with the included angle between them ranging from greater than 0 degrees to less than 45 degrees. This multi-domain alignment structure provides a first large pretilt angle vertical alignment for pixels located in odd-numbered rows and a second large pretilt angle vertical alignment for pixels located in even-numbered rows. Alternatively, the multi-domain alignment structure provides a first large pretilt angle vertical alignment for pixels located in odd-numbered columns and a second large pretilt angle vertical alignment for pixels located in even-numbered columns. For example, the multi-domain alignment structure provides a pretilt angle of 50 degrees for pixels located in odd-numbered rows and a pretilt angle of 60 degrees for pixels located in even-numbered rows. In this embodiment, it should be noted that the liquid crystal molecules within each row are uniformly aligned, meaning that the long axes of the liquid crystal molecules within each row coincide with the projections of the first alignment layer, embodying the meaning of uniform alignment. The long axes of the liquid crystal molecules perpendicularly aligned by the first large pretilt angle and the long axes of the liquid crystal molecules perpendicularly aligned by the second large pretilt angle form a non-zero angle with the projections of the liquid crystal molecules in the first alignment layer. This feature can also be used in combination with the preceding embodiments, provided it is feasible.

[0062] The multi-domain alignment in this disclosure is achieved using an upper alignment layer 130 and a lower alignment layer 150, such that the device functions as a polarization-independent optical element when an electric field is applied. This polarization independence allows the LCoS device 100 to be used with unpolarized incident light. Both the upper alignment layer 130 and the lower alignment layer 150 can be patterned into a periodically varying multi-domain alignment structure using photo-alignment techniques. In a preferred embodiment of this disclosure, the upper alignment layer 130 and the lower alignment layer 150 are photo-alignment layers containing an azo dye. The multi-domain alignment structure is formed by irradiating a photomask with multiple polarized light. The upper alignment layer 130 and the lower alignment layer 150 are aligned with polarized light, with an irradiation dose ranging from 0.1 joules to 2 joules. In a preferred embodiment of this disclosure, the multi-domain alignment structure is formed by irradiating with an interference pattern of the polarized light. The size of each domain in the multi-domain alignment structure is on the micrometer scale, for example, ranging from 1 micrometer to several micrometers. The alignment direction of each domain can vary between 0 degrees and 90 degrees. Multi-domain alignment structures form periodically changing alignment directions, configured to have at least two different orientation directions, such that adjacent domains have different alignment directions. In the embodiment of Figure 3, the alignment directions in adjacent domains are perpendicular to each other. In some embodiments, other photoalignment techniques may also be employed, such as photoisomerization doping or photocrosslinking polymers.

[0063] Figure 4 is a schematic diagram of the preparation of a multi-domain alignment structure according to an embodiment of the present disclosure. Subfigure (a) shows a schematic diagram of the alignment layer coating and irradiation. In this embodiment, an azo dye is used as the alignment layer material coated on top of other layers. For example, a sulfonated azo dye (SD-1) can be used. SD-1 is a photoisomerization material with a photoreorientation mechanism. Under linearly polarized ultraviolet light irradiation, the photoabsorbing oscillators of SD-1 molecules reorient themselves to be perpendicular to the polarization direction of the incident light. This process is mainly a physical rotation rather than a photochemical reaction. Typically, the optimal thickness of the SD-1 layer is 3-5 nm. This thickness is a trade-off between two instabilities (focal conic domains and ferroelectric domains), providing the highest stability for multiple addressing operations and high contrast. The thickness (0-12 nm) can be controlled by changing the spin coating speed and the concentration of SD-1 in the DMF solvent (0.2%-1.3%). In the embodiment of subfigure (a), irradiation is performed using 365 nm linearly polarized ultraviolet light. The intensity can be 6 mW / cm². 2 The exposure time needs to be sufficiently long (e.g., 60 minutes). Experiments show that shorter exposure times (e.g., 10 minutes) result in lower contrast and noticeable dislocation lines; while sufficient exposure eliminates these defects, yielding a uniform pattern. Subfigure (b) illustrates the specific exposure steps. After the first exposure, a consistent alignment orientation is obtained, followed by a second exposure using a photomask. After the second exposure, an alignment layer with periodically alternating parallel alignments along the x-direction and y-direction is obtained, i.e., an alignment layer including multi-domain alignment structures. Other periodically alternating hybrid patterns can be obtained in a similar manner.

[0064] Figure 5 shows a comparative experiment of conventional single-domain alignment and dual-domain alignment according to an embodiment of the present disclosure. Subfigure (a) shows a schematic diagram of liquid crystal orientation and phase change in conventional single-domain alignment. Subfigure (b) shows a schematic diagram of liquid crystal orientation and phase change in dual-domain alignment according to an embodiment of the present disclosure. It can be seen that the phase depth change of the dual-domain alignment structure is not significant. Experimental verification shows that the LCoS device of the present disclosure is applicable to a wide spectral range, including visible light and fiber optic communication bands, such as O, E, S, C, L, and U bands. Figure 6 shows an LCoS device including a multi-domain alignment structure according to an embodiment of the present disclosure. The orientation of the multiple domains allows the liquid crystal to periodically maintain an angular relationship with the long axis of the panel. In Figure 6, the imperfect liquid crystal alignment is due to a mismatch caused by manual alignment in the laboratory, resulting in the top glass and bottom CMOS not being perfectly aligned in the same position, resulting in an offset. In the intended application, this mismatch problem can be solved by more advanced processing technology. Figure 7 shows the relationship between diffraction efficiency and polarization angle according to an embodiment of the present disclosure. The diffraction efficiency of a single-domain conventional LCoS decreases significantly with increasing angle to the long axis of the panel. In contrast, the LCoS with a dual-domain alignment structure in this disclosure exhibits a gradual increase in diffraction efficiency with increasing angle to the long axis of the panel, without a significant degradation in diffraction efficiency.

[0065] Figure 8A shows a side view of a conventional uniformly aligned (single-domain) LCoS design. In contrast, Figure 8B shows a side view of a multi-domain aligned LCoS design according to an embodiment of the present disclosure. Figure 9A shows a top view of a conventional uniformly aligned (single-domain) LCoS design. In contrast, Figure 9B shows a top view of a multi-domain aligned LCoS design according to an embodiment of the present disclosure. Figures 8A to 9B respectively show the simulated director distribution results of the side and top views of the conventional single-domain structure and the mutually perpendicular aligned structure, and compare them with the blazed grating phase profile generated using a linearly varying voltage over 12 pixels. The results show that both structures exhibit the same phenomenon in terms of director distribution, and the multi-domain aligned structure is suitable for polarization-independent blazed phase gratings.

[0066] This disclosure presents a novel method for achieving polarization independence in conventional liquid crystal on silicon (LCoS) devices by utilizing photoalignment technology to achieve novel periodic multi-domain alignment of the liquid crystal. Unlike existing methods, this method requires no additional layers, design modifications, increased complexity, voltage adjustments, or changes to pixel circuitry. It significantly improves system efficiency by simplifying the design and eliminating 50% of the signal loss in existing polarization-phase-modulated light-based soft-switching (WSS) devices. Through this innovative approach, this disclosure not only enhances the polarization independence of LCoS devices but also optimizes system performance for critical applications such as optical switching engines and wavelength-selective switches, which are particularly advantageous for telecommunications networks.

[0067] This disclosure addresses the challenge of polarization dependence in LCoS-based WSS systems, which currently leads to significant signal loss and design complexity. The innovation of this disclosure lies in utilizing optical alignment techniques to create periodic multi-domain structures, thereby achieving polarization-independent phase modulation without the need for additional layers or voltage requirements.

[0068] This disclosure introduces a novel polarization-independent LCoS modulator designed to improve the optical efficiency of WSS and other optical systems. The LCoS panel employs a multi-domain alignment structure, achieving polarization independence without altering the phase distribution, a fact confirmed by experimental results using simulated panels. The inventors demonstrate improved diffraction efficiency over a wide polarization angle and emphasize compatibility with existing CMOS backplanes, simplifying the manufacturing process and avoiding increased complexity. This disclosure achieves the desired accuracy through multi-domain alignment without altering the fundamental configuration of conventional LCoS systems.

[0069] The polarization-independent LCoS with a multi-domain alignment structure disclosed herein can be applied to nematic LCoS and WSS. It also has potential applications in consumer electronics (projectors, head-up displays, and head-mounted displays, etc.), automotive, aerospace, optical 3D measurement, medical, and military fields.

[0070] This specification uses examples to disclose, including best practices, and to enable any person skilled in the art to practice this disclosure, including making and using any device or system and methods of performing any combination. The patent scope of this disclosure is defined by the claims, but may include other examples that would occur to a person skilled in the art. The scope of the claims covers such other examples if they include structural elements that are not distinct from the literal expression of the claims, or if they include equivalent structural elements that are not substantially different from the literal expression of the claims.

Claims

1. A liquid crystal on silicon (LCoS) device for optical communication, comprising, in order from top to bottom: a top substrate made of transparent glass; a transparent conductive layer as a common electrode; an upper alignment layer patterned to uniformly align liquid crystal molecules; a liquid crystal layer; a lower alignment layer patterned to uniformly align the liquid crystal molecules, wherein the liquid crystal layer is disposed between the upper and lower alignment layers; a lower conductive layer as a metal mirror; and a complementary metal-oxide-semiconductor (CMOS) backplane comprising a silicon substrate, wherein a first of the upper and lower alignment layers is patterned as a periodically varying multi-domain alignment structure with adjacent domains having different orientations such that, upon application of an electric field, the LCoS device functions as a polarization-independent optical element.

2. The LCoS device of claim 1, wherein, The LCoS device can be used for non-polarized incident light. 3.The LCoS device of claim 1, wherein: the multi-domain alignment structure comprises a hybrid pattern periodically alternating vertical alignment with a first large pre-tilt angle and vertical alignment with a second large pre-tilt angle, wherein vertical alignment with a large pre-tilt angle refers to a long axis of the liquid crystal molecules forming an angle greater than 45 degrees with a plane of the first alignment layer in an initial state with power on, and an angle between the first and second large pre-tilt angles ranges from greater than 0 degrees to less than 45 degrees; and a second of the upper and lower alignment layers is patterned identically to the first alignment layer. 4.The LCoS device of claim 3, wherein: the multi-domain alignment structure provides vertical alignment with the first large pre-tilt angle for pixels in odd rows and vertical alignment with the second large pre-tilt angle for pixels in even rows, or the multi-domain alignment structure provides vertical alignment with the first large pre-tilt angle for pixels in odd columns and vertical alignment with the second large pre-tilt angle for pixels in even columns, wherein a projection of the long axis of the liquid crystal molecules vertically aligned with the first large pre-tilt angle and the long axis of the liquid crystal molecules vertically aligned with the second large pre-tilt angle forms an angle other than 0 degrees with the plane of the first alignment layer. 5.The LCoS device of claim 1, wherein: the multi-domain alignment structure comprises a hybrid pattern periodically alternating parallel alignment with a first angle and parallel alignment with a second angle, wherein the parallel alignment with the first angle aligns the liquid crystal molecules in a plane parallel to the plane of the first alignment layer along an orientation with a first angle, and the parallel alignment with the second angle aligns the liquid crystal molecules in the plane along an orientation with a second angle, and an angle between the first and second angles ranges from greater than 0 degrees to less than or equal to 90 degrees; and a second of the upper and lower alignment layers vertically aligns the liquid crystal molecules.

6. The LCoS device of claim 5, wherein, The second alignment layer is patterned to vertically align liquid crystal molecules in a mixed pattern periodically alternating between a first large pre-tilt angle vertical alignment and a second large pre-tilt angle vertical alignment, wherein the large pre-tilt angle vertical alignment refers to a pre-tilt angle of the long axis of the liquid crystal molecules with respect to the plane of the first alignment layer in an initial state after being powered on, and the angle between the first large pre-tilt angle and the second large pre-tilt angle ranges from greater than 0 degrees to less than 45 degrees.

7. The LCoS device of claim 5, wherein: the multi-domain alignment structure provides a first angular parallel alignment for pixels located in odd rows and a second angular parallel alignment for pixels located in even rows; or the multi-domain alignment structure provides a first angular parallel alignment for pixels located in odd columns and a second angular parallel alignment for pixels located in even columns.

8. The LCoS device of claim 1, wherein, the upper and lower alignment layers are patterned into a multi-domain alignment structure by a photo-alignment technique.

9. The LCoS device of claim 8, wherein, the upper and lower alignment layers comprise an azo dye material.

10. The LCoS device of claim 9, wherein, the multi-domain alignment structure is formed by multiple polarized light irradiations using a photo mask.

11. The LCoS device of claim 10, wherein, the upper and lower alignment layers are aligned by the polarized light with an irradiation dose ranging from 0.1 Joule to 2 Joule.

12. The LCoS device of claim 10, wherein, the multi-domain alignment structure is formed by irradiation with an interference pattern of the polarized light.

13. The LCoS device of claim 1, wherein, each domain in the multi-domain alignment structure is in a micron scale.

14. The LCoS device of claim 1, wherein, the optical element is a blazed phase grating.

15. The LCoS device of claim 1, wherein, the LCoS device is suitable for the visible spectrum of light.

16. The LCoS device of claim 1, wherein, the LCoS device is suitable for the fiber communication band spectrum, including O, E, S, C, L and U bands.

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