Broadband polarization rotator and optical switch

Abstract

Reflective polarization rotators based on twisted nematic (TN) liquid crystal (LC) configurations may optimize achromatic performance in bright and dark states through wavelength dependencies of polarization conversion compensated by TN LC layers. Under linear achromatic input polarization, the purpose of compensation is to provide circular achromatic polarization at the reflector, which ensures achromatism of 90 degree polarization rotation at two passes. The compensation notes to maintain linear polarization at the reflector when the LC device is driven to a vertical LC director configuration. The method ensures that the performance of the polarization rotator is achromatic in the driving state and the non-driving state of the LC device.

Description

Technical Field

[0001] This disclosure relates to polarization rotators and optical switches, as well as visual displays based on polarization rotator arrays. Background Technology

[0002] Visual displays provide information, including still images, videos, data, etc., to one or more viewers. Visual displays are used in a variety of fields, including entertainment, education, engineering, science, training, and advertising, to name just a few. Some visual displays (e.g., televisions) can display images to multiple users, while some visual display systems (e.g., near-eye displays (NEDs)) are intended for a single viewer. NEDs, which can be worn on a user's head, can be standalone or connected to a controller or console that provides video feed and / or the necessary control functions.

[0003] Artificial reality systems can include NEDs (Network Edge Displays) configured to present content to users, such as headsets or glasses. NEDs can display virtual objects or combine images of real objects with virtual objects in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. Artificial images can be generated by miniature displays, which can include scanning or microdisplay-based image projectors.

[0004] Since HMD displays are worn on the user's head, large, bulky, unbalanced, and / or heavy display devices are cumbersome and uncomfortable for the user. Therefore, head-mounted display devices can benefit from compact and efficient configurations, including efficient multi-color light sources and image projectors, high-throughput eyepieces, light switches, high-contrast microdisplay panels, diffractive optics, and other compact optical elements and modules in the image forming chain. Summary of the Invention

[0005] According to a first aspect of this disclosure, a polarization rotator is provided, comprising: a reflective liquid crystal (LC) device including an LC layer coupled to a reflector, wherein the LC layer is switchable between an undriven state having a distorted LC director configuration and a driven state having a substantially vertical LC director configuration, wherein, in the undriven state, the LC layer converts a first achromatic polarization state of the light beam into a first wavelength distribution of the polarization state in a single pass of the light beam; and a passive compensation structure (PCS) disposed upstream of the LC device and including a stack of fixed retardation layers configured to convert a second achromatic polarization state of the light beam into a second wavelength distribution of the polarization state matching the first wavelength distribution in a single pass of the light beam.

[0006] In some embodiments, the twist angle of the twisted LC pointer configuration may be less than 90 degrees.

[0007] In some embodiments, when the LC layer is in an undriven state, the polarization rotator can switch between orthogonal achromatic polarization states of the beam propagating through the PCS, the LC layer, reflected by the reflector, and propagating back through the LC layer and the PCS.

[0008] In some embodiments, when the LC layer is in an undriven state and the polarization state of the beam illuminating the PCS is achromatic linear polarization, the polarization state of the beam at the reflector can be achromatic circular polarization.

[0009] In some embodiments, when the LC layer is in a driven state, the polarization rotator can substantially maintain the achromatic polarization state of the beam incident on the polarization rotator at the output end of the polarization rotator.

[0010] In some embodiments, when the LC layer is in a driven state and the polarization state of the beam illuminating the PCS is an achromatic linear polarization state, the polarization state of the beam at the reflector can be a non-achromatic linear polarization state.

[0011] In some embodiments, when the LC layer is in an undriven state and the light beam illuminating the PCS is in a second achromatic polarization state, the polarization state of the light beam at the reflector can be a first achromatic polarization state.

[0012] In some embodiments, during operation, a non-zero voltage can be applied to the LC layer in an undriven state to obtain a better match between the first wavelength distribution and the second wavelength distribution of the polarization state compared to the match between the first wavelength distribution and the second wavelength distribution when zero voltage is applied to the LC layer.

[0013] In some embodiments, the stack of fixed delay layers may include a plate A.

[0014] In some embodiments, all A-plates in the stack may have substantially the same in-plane delay and different optical axis orientations.

[0015] In some embodiments, the stack may further include a C-plate for improving the off-axis performance of the polarization rotator.

[0016] In some implementations, the stacking of the fixed delay layers may include cyclic olefin polymers.

[0017] In some embodiments, when the LC layer is in an undriven state, the polarization rotator can switch between orthogonal achromatic polarization states of the beam within a spectral bandwidth between 450 nm and 700 nm; and when the LC layer is in a driven state, the polarization rotator can substantially maintain the achromatic polarization state of the beam within a spectral bandwidth between 450 nm and 700 nm.

[0018] According to a second aspect of this disclosure, an optical switch is provided, comprising: a front illuminator for providing a light beam; a polarization rotator located downstream of the front illuminator for receiving the light beam and changing the direction of the light beam to propagate back to the front illuminator; and a polarizer located downstream of the polarization rotator for transmitting or blocking the light beam according to the polarization state of the light beam. The polarization rotator includes: a reflective liquid crystal (LC) device including an LC layer coupled to a reflector, wherein the LC layer is switchable between an undriven state having a distorted LC director configuration and a driven state having a substantially vertical LC director configuration, wherein, in the undriven state, the LC layer converts a first achromatic polarization state of the light beam into a first wavelength distribution of polarization state in a single pass of the light beam; and a passive compensation structure (PCS) disposed upstream of the LC device and including a stack of fixed retardation layers configured to convert a second achromatic polarization state of the light beam into a second wavelength distribution of polarization state matching the first wavelength distribution in a single pass of the light beam.

[0019] In some embodiments, when the LC layer is in an undriven state and the polarization state of the beam incident on the PCS is a second achromatic polarization state, the polarization state of the beam at the reflector of the reflective LC device can be a first achromatic polarization state.

[0020] In some embodiments, when the LC layer is in an undriven state, the polarization rotator can switch between orthogonal achromatic polarization states of the beam; and when the LC layer is in a driven state, the polarization rotator can maintain the achromatic polarization state of the beam.

[0021] In some embodiments, the LC layer of the polarization rotator may include an array of LC pixels that can be individually controlled.

[0022] In some embodiments, the front illuminator may include at least one of an evanescent coupler or a replica light guide, the evanescent coupler or the replica light guide being used to provide a portion of the light beam to each LC pixel of the array.

[0023] According to a third aspect of this disclosure, a passive compensation structure (PCS) for an LC layer is provided, the LC layer being switchable between an undriven state having a distorted LC director configuration and a driven state having a substantially vertical LC director configuration, wherein, when in the undriven state, the LC layer converts a first achromatic polarization state of the beam into a first wavelength distribution of polarization state in a single pass of the beam, the PCS comprising: a stack of fixed retardation layers configured to convert a second achromatic polarization state of the beam into a second wavelength distribution of polarization state matching the first wavelength distribution in a single pass of the beam.

[0024] In some embodiments, when the LC layer is in an undriven state and the beam incident on the PCS is in a second achromatic polarization state, the polarization state of the beam at the output end of the LC layer can be a first achromatic polarization state.

[0025] It will be appreciated that any feature described herein that is suitable for incorporation into one or more aspects or embodiments of this disclosure is intended to be generalizable to any and all aspects and embodiments of this disclosure. Other aspects of this disclosure will be understood by those skilled in the art based on the specification, claims, and drawings of this disclosure. The foregoing general overview and the following detailed description are exemplary and illustrative only and are not intended to limit the scope of the claims. Attached Figure Description

[0026] Exemplary embodiments will now be described in conjunction with the accompanying drawings, in which: Figure 1 This is a schematic cross-sectional view of an optical switch based on a reflective twisted nematic (TN) liquid crystal (LC) polarization rotator coupled to a polarizer; Figure 2 This is a schematic cross-sectional view of an optical switch based on a reflective TN LC polarization rotator coupled with a polarization beamsplitter (PBS); Figure 3A and Figure 3B yes Figure 2 The light switch is in the bright state ( Figure 3A ) and dark state ( Figure 3BSpectral transmission diagram under ) Figure 4A and Figure 4B Is Figure 2 The light beam propagating in the optical switch is in the bright state of the optical switch ( Figure 4A ) and dark state ( Figure 4B Polarization evolution diagram under ( ); Figure 5 This is a Poincaré spherical view showing the polarization path of an achromatic circularly polarized beam propagating through a TN LC polarization rotator; Figure 6 This is a Poincaré sphere view showing the polarization path of an achromatic circularly polarized beam that propagates from the reflector via a TN LC polarization rotator and propagates back via the TN LC polarization rotator. Figure 7A and Figure 7B The optical switch based on the compensated polarization rotator according to one or more embodiments of this disclosure is in the bright state of the optical switch ( Figure 7A ) and dark state ( Figure 7B Schematic cross-sectional view below; Figure 8A and Figure 8B It is based on one or more embodiments of this disclosure Figure 7A and Figure 7B The light switch is in the on state ( Figure 8A , corresponding to Figure 7A ) and dark state ( Figure 8B , corresponding to Figure 7B Spectral transmittance diagram; Figure 9A and Figure 9B The compensated optical switch according to one or more embodiments of this disclosure is in the bright state of the optical switch ( Figure 9A ) and dark state ( Figure 9B Polarization evolution diagram under ( ); Figure 10 According to one or more embodiments of this disclosure Figure 7B A schematic cross-sectional view of the equivalent configuration of the compensated optical switch in the dark (i.e., driven) state; Figure 11 The propagation is illustrated according to one or more embodiments of this disclosure. Figure 7A and Figure 7B Poincaré sphere view of the polarization path of an achromatic linearly polarized beam in a passive compensation structure (PCS) of a compensated optical switch. Figure 12A and Figure 12B Examples of optical switches including polarizers according to one or more embodiments of this disclosure are compensated for in the dark state of the optical switch. Figure 12A ) and bright state ( Figure 12BSchematic cross-sectional view below; Figure 13 This is a graph showing the relationship between contrast and cell gap error in a compensated optical switch having a zero-voltage undriven state according to one or more embodiments of the present disclosure. Figure 14 It is for use with one or more embodiments of this disclosure. Figure 13 A comparison of the relationship between the maximum contrast and the cell gap error in a compensated optical switch with a non-zero voltage undriven state. Figure 15 This is a graph showing the contrast ratio versus the angle of incidence (or viewing angle) of a compensated optical switch without thickness delay compensation according to one or more embodiments of this disclosure. Figure 16 This is a schematic diagram of a light switch having a fleeting light illuminator according to one or more embodiments of the present disclosure; Figure 17 This is a schematic diagram of a light switch having a pupil-replicating front illuminator according to one or more embodiments of the present disclosure; Figure 18 This is a flowchart of a method for reducing the wavelength dependence of flux in an optical switch including a switchable LC layer, according to one or more embodiments of the present disclosure. Figure 19 This is a view of an augmented reality (AR) display having shape elements of a pair of glasses according to one or more embodiments of this disclosure; and Figure 20 This is a three-dimensional view of a head-mounted display (HMD) according to one or more embodiments of the present disclosure. Detailed Implementation

[0027] Although this teaching has been described in conjunction with various embodiments and examples, it is not intended to be limited to these embodiments. Rather, those skilled in the art will recognize that this teaching encompasses a variety of alternatives and equivalents. All statements regarding the principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to cover their structural and functional equivalents. Furthermore, such equivalents are intended to include both currently known equivalents and those developed in the future, i.e., any elements developed that perform the same function, regardless of the structure of those elements.

[0028] As used herein, unless explicitly stated otherwise, the terms "first," "second," etc., are not intended to suggest order, but rather to distinguish one element from another. Similarly, unless explicitly stated otherwise, the order of method steps does not imply the order in which they are performed. Figure 7A and Figure 7B , Figure 9A and Figure 9B , Figure 10 as well as Figure 12A and Figure 12B In the figures, similar reference numerals indicate similar elements.

[0029] An optical switch based on a polarization rotator between a pair of polarizers can have brightness and / or contrast that depends on the wavelength of the illumination light. For a display implementation of such an optical switch (e.g., a liquid crystal (LC) display), the display brightness and / or contrast can depend on the displayed color; that is, the display brightness and / or contrast can differ for the red, green, and blue channels. For an LC display including a polarization-rotated pixel array, a delay film can be added to the display to improve the performance of the excited state by compensating for the residual birefringence of the near-electrode boundary LC sublayers, which maintain primarily in-plane orientation even in the excited state.

[0030] Since contrast is often a more important parameter than color uniformity in bright states, the excitation state of the LC polarization rotator is assigned to black level or zero output; this configuration is called "drive-to-black" and is compensated for with a delay plate. However, the bright states are still not compensated. In the opposite "drive-to-white" configuration, the bright field is color uniform, but the contrast is poor because the undriven state is not compensated.

[0031] According to the present invention, the characteristics of the retarder stack in a dual-pass reflective configuration can be used to provide dispersion compensation for the LC polarization rotator in both driven and undriven states. Dispersion compensation in both states allows for the construction of optical switches and visual display panels that simultaneously possess uniform color brightness and high contrast. Achromatic bright and dark fields appear in the drive-to-black and drive-to-white configurations of optical switches and / or visual displays. To achieve achromaticity, the polarization rotator based on twisted nematic (TN) liquid crystals can be equipped with a passive compensation structure (PCS) comprising a stack of multiple fixed retardation layers with pre-configured retardation and optical axis orientation. When the TN LCs are predominantly vertically aligned, the PCS "cancels" the wavelength selection retardation of the TV LC cells in a manner that does not degrade performance in the driven state (i.e., the state in which an electric field is applied), thereby allowing both the driven and undriven states to be achromatic.

[0032] According to the present invention, a polarization rotator is provided, comprising a reflective liquid crystal (LC) device, the reflective LC device including an LC layer coupled to a reflector. The LC layer is switchable between an undriven state having a distorted LC director configuration and a driven state having a substantially vertical LC director configuration. When in the undriven state, the LC layer converts a first achromatic polarization state of the light beam into a first wavelength distribution of the polarization state during a single pass of the light beam.

[0033] The polarization rotator also includes a passive compensation structure (PCS) disposed upstream of the LC device, also referred to as a "dispersion rotator". The PCS may include a stack of fixed retardation layers configured to convert a second achromatic polarization state of the beam into a second wavelength distribution matching the polarization state of a first wavelength distribution during a single pass of the beam. In some embodiments, when the LC layer is in an undriven state and the beam incident on the PCS is in the second achromatic polarization state, the polarization state of the beam at the reflector is the first achromatic polarization state.

[0034] In operation, a non-zero voltage can be applied to the LC layer in an undriven state to obtain a better match between the first and second wavelength distributions in a polarization state compared to the match between the first and second wavelength distributions when zero voltage is applied to the LC layer. The twist angle of the twisted LC director configuration can be less than 90 degrees.

[0035] When the LC layer is in an undriven state, the polarization rotator can switch between multiple orthogonal achromatic polarization states of the beam propagating through the PCS, the LC layer, reflected by the reflector, and propagating back through the LC layer and the PCS. When the LC layer is in an undriven state and the polarization state of the beam incident on the PCS is achromatic linear polarization, the polarization state of the beam at the reflector can be achromatic circular polarization.

[0036] When the LC layer is in the driven state, the polarization rotator can maintain the achromatic polarization state of the beam incident on the polarization rotator at its output end. When the LC layer is in the driven state and the polarization state of the beam incident on the PCS is achromatic linear polarization, the polarization state of the beam at the reflector is non-achromatic linear polarization.

[0037] In some embodiments, when the LC layer is in an undriven state, the polarization rotator switches between multiple orthogonal achromatic polarization states of the beam (e.g., within a spectral bandwidth between 450 nm and 700 nm). When the LC layer is in a driven state, the polarization rotator substantially maintains the achromatic polarization state of the beam, for example, within a spectral bandwidth between 450 nm and 700 nm. In embodiments where the stack of fixed retardation layers includes an A-plate, all A-plates in the stack may have substantially the same in-plane retardation and different optical axis orientations. The stack may also include a C-plate for improving the off-axis performance of the polarization rotator. The stack of fixed retardation layers may include, for example, a cyclic olefin polymer.

[0038] According to this disclosure, an optical switch is provided, comprising: a front illuminator for providing a light beam; a polarization rotator of this disclosure disposed downstream of the front illuminator for receiving the light beam and changing its direction to propagate it back to the front illuminator; and a polarizer located downstream of the polarization rotator for transmitting or blocking the light beam according to its polarization state. The LC layer of the polarization rotator may include an array of individually controllable LC pixels. The front illuminator may include at least one of an evanescent output coupler or a replicated light guide for providing a portion of the light beam to each LC pixel of the array.

[0039] According to this disclosure, a passive compensation structure (PCS) for an LC layer is provided, which can switch between an undriven state with a distorted LC director configuration and a driven state with a substantially vertical LC director configuration. In the undriven state, the LC layer converts a first achromatic polarization state of the beam into a first wavelength distribution of polarization state during a single pass of the beam.

[0040] The PCS may include a stack of fixed delay layers configured to convert a second achromatic polarization state of the beam into a second wavelength distribution with a polarization state matching a first wavelength distribution during a single pass of the beam. In some embodiments, when the LC layer is in an undriven state and the beam incident on the PCS is in the second achromatic polarization state, the polarization state of the beam at the output of the LC layer is the first achromatic polarization state.

[0041] In reflective liquid crystal displays (LCDs) or optical switching devices, input light propagates forward, is reflected by a mirror, and then passes back through an LC cell. The LC cell includes an LC layer, which can be pixelated to allow for spatially selective switching, for example, in visual display applications. In reflective polarization-selective LC devices, the input beam needs to be polarized. Polarization of the input beam can be achieved using, for example, a polarizer or polarizing beam splitter (PBS) placed upstream of the reflective LC cell.

[0042] Figure 1This is an illustrative example of a reflective LC optical switch 100, which includes a reflective LC device 102 and a transmissive polarizer 104 optically coupled to the reflective LC device 102 upstream of it. The latter includes an LC layer 106 between a transparent electrode 107 and a reflective electrode 108. In an undriven or low-voltage state, the director of the LC layer 106 is in a twisted configuration 110, the twist angle of which can be, for example, between 60 and 75 degrees, thereby providing an optical delay of approximately one-quarter wavelength during a single pass of the incident beam 112. In a driven or high-voltage state, the director is in a near-vertical configuration, i.e., the twisted configuration 110 becomes a near-vertical orientation of the LC molecules throughout the LC layer 106, thereby providing a near-zero optical delay for the incident beam 112.

[0043] Beam 112 propagates through and is polarized by the transmission polarizer 104, thus having a polarization perpendicular to the light beam. Figure 1 Linear polarization in a plane. The polarized beam 112 propagates through the LC layer 106, is reflected by the reflecting electrode 108, and propagates back through the LC layer 106. When the LC layer 106 is in an undriven state, the beam 112 experiences approximately a half-wave delay during its double-pass propagation through the LC layer 106, and... Figure 1 The light beam becomes linearly polarized in the plane of light. The light beam 112 is blocked by the transmissive polarizer 104. When the liquid crystal layer 106 is in the driven state, the polarization of the light beam 112 is maintained, thereby allowing the light beam 112 to propagate through the transmissive polarizer 104 and form the output light beam 114. Therefore, the optical switch 100 operates in a drive-to-white configuration.

[0044] exist Figure 1 In this method, the input and output beams are separated by a slightly angled incident beam 112. Using a PBS instead of a polarizer allows for the separation of input and output beams propagating in opposite directions at orthogonal (i.e., perpendicular) incident angles, or illumination and image light in an imaging system. (Reference) Figure 2 An illustrative example, the optical switch 200 is similar to Figure 1 The optical switch 100 includes the same components, except that the sheet-like transmission polarizer 104 is replaced by a PBS 204. In operation, a beam 112 is incident on the polarization interface 205 of the PBS 204, which polarizes beams perpendicular to... Figure 2 The plane-polarized light is reflected. When the LC layer 106 is in an undriven or low-voltage state, the beam 112 receives approximately a half-wave delay during its double-pass propagation through the LC layer 106, and... Figure 2 In the plane, it becomes linearly polarized, just like in... Figure 1 The same as in optical switch 100. Then, beam 112 ( Figure 2The light beam 112 is transmitted through the polarization interface 205 to form an output beam 114. When the LC layer 106 is in the driven state, the polarization of the beam 114 is maintained, causing the beam 114 to be reflected back by the polarization interface 205. Therefore, the optical switch 200 operates in a drive-to-black configuration.

[0045] Figure 2 The optical switch 200 configuration can be used in a pixelated embodiment of a liquid crystal-on-silicon (LCoS) image projector having a reflective LC device 102, which provides an image beam (i.e., output beam 114) when illuminated by a beam 112. The LCoS image projector can provide a color image by providing a color filter matrix to the reflective LC device 102, or by configuring the timing illumination of the reflective LC device 102 to provide individual color channels of the displayed image in a sequential manner. It should also be noted that the term "optical switch" as used herein also includes visual displays and image projectors where the reflective LC device is pixelated to provide spatially varied light modulation for forming an image in a linear domain for display by the visual display / image projector.

[0046] Figure 1 and Figure 2 One drawback of optical switch / monitor / projector configurations is their reliance on color performance. (Reference) Figure 3A and Figure 3B Illustrative examples, and further references Figure 2 , drew Figure 2 The relationship between the luminous flux or dual-pass transmittance of the optical switch 200 in the undriven (low voltage) and driven (high voltage) states and wavelength. Figure 3A The dual-pass spectral transmittance of the optical switch 200 in the undriven state shown is... It exhibits a strong wavelength dependence. This occurs because the wavelength of blue light is only about 65% of that of red light, so the optical retardation, in wavelength units, varies from blue to green and then to red. The single-pass optical retardation in the undriven state is optimized to be one-quarter of the wavelength of green light, which makes it longer than one-quarter of the wavelength of blue light (which is shorter than green) and shorter than one-quarter of the wavelength of red light (which is longer than green). Material dispersion also plays a role in the wavelength-dependent optical retardation, thus increasing... Figure 3A Bell-shaped wavelength dependence shown .

[0047] exist Figure 3B The dual-pass spectral transmittance of the optical switch 200 under the driving state or high voltage state shown. Low values ​​are observed for all three colors because, under high voltage, the LC director is mostly perpendicular to the plane of the LC layer 106, thus eliminating most of the delay. Therefore, the high voltage state of the optical switch 200 is more achromatic due to the near-zero optical delay of the three colors. Since the near-electrode LC sublayer is thin and the in-plane director is essentially unaffected by the applied electric field, any residual non-zero delay can be at least partially compensated or canceled by using one or more appropriately oriented retardation plates in the optical path of beam 112. Black-level achromatic performance is generally more important than white-level achromatic performance because black-level performance affects the contrast or extinction ratio of the optical switch or display. Therefore, drive-to-black configurations are used more frequently than drive-to-white configurations. Drive-to-white configurations (e.g., Figure 1 The configuration shown will provide more than Figure 2 The drive-to-black configuration produces a more uniform white field, but the contrast will be quite low due to the inherent chromaticity of the undriven LC layer 106.

[0048] Figure 4A and Figure 4B It shows Figure 1 and Figure 2 The origin of the chromaticity of the light switch 100. Figure 4A It exhibits a non-driven state or a low-voltage state. V L The polarization evolution diagram 400A shows that, in this example, the LC director exhibits a twisted configuration 110 with a twist angle of 70 degrees. Figure 2 This configuration, along with other configurations with a twisted LC director, is referred to herein as a "hybrid twisted nematic" or MTN. The incident beam 112 ( Figure 2 It is horizontally polarized, as shown in 402. Figure 4A On the forward path, beam 112 passes through... arrive The LC pointing angle propagates through the LC layer 106 and obtains the first elliptic polarization represented at 404, with an ellipticity of Depends on wavelength , In the reverse path, beam 112 passes through LC layer 106. arrive The LC pointing angle propagates through the LC layer 106 and obtains a second elliptic polarization represented at 406, where the transmittance through PBS 204 is... T Depends on ellipticity Therefore, it depends on the wavelength. As shown in the reference above Figure 3A As mentioned above.

[0049] Turning Figure 4BThe polarization evolution diagram 400B corresponds to the driving state or the high-voltage state. V H The LC director is predominantly vertical. Residual birefringence caused by the boundary LC sublayer is ignored or assumed to be compensated. The incident horizontally polarized beam 112 propagates through the LC layer 106, maintaining its original polarization state 402; it is reflected by the reflecting electrode 107. Figure 2 And it propagates back through LC layer 106, maintaining the original polarization state 402, because the delay encountered on the return journey remains zero, thus ensuring... Figure 3B The achromatic dark-state performance is shown.

[0050] This disclosure aims to compensate for the dispersion of the polarization transformation of incident light in the low-voltage state of an MTN LC device (i.e., an LC device with a twist angle not limited to 90 degrees) in a manner that does not affect the achromatic nature of the high-voltage state, thereby allowing both states of the polarization rotator to be achromatic. The following characteristics A through E of a delayer stack operating in a double-pass manner can be considered to facilitate understanding how a polarization rotator based on a reflective LC device achieves achromatic performance in both the low-voltage and high-voltage states of the reflective LC device.

[0051] Characteristic A. Two-pass structures (e.g., delayed film stacks and / or (M)TN LC structures) can be unfolded and analyzed as equivalent single-pass structures, consisting of a forward-pass structure and a subsequent equivalent reverse-pass structure. The equivalent reverse-pass structure has reverse-order (RO) symmetry relative to the forward-pass structure, where the orientation angles of the layers in the stack remain unchanged.

[0052] Characteristic B. In a stack with RO symmetry, the round-trip transition from the first polarization state (SOP) to an orthogonal SOP can be achieved using only a delay. For a complete transition, a half-wave (HW) delay is required during the round trip (at all wavelengths of interest). More specifically, this requires accumulating a quarter-wave (QW) delay during the first and second passes of the structure. As long as the linear input SOP becomes circular at the reflector (independent of the stack's structural details), the outgoing light is guaranteed to have an orthogonal linear SOP.

[0053] Characteristic C. For a linear input SOP, a complete transition between the linear polarization states of each orthogonal polarization requires the effective optical axis of the half-wave delay to be relative to the input. .

[0054] Characteristic D.For stacks with RO symmetry, the polarization rotation accumulated in the first pass is canceled out in the second pass. This is a property of reciprocal systems. Therefore, as long as the linear SOP remains linear at the reflector (regardless of orientation), it is guaranteed that the outgoing light will maintain the linear SOP of the input light.

[0055] Feature E. RO symmetry applies to both discrete structures (e.g., retarder stacks) and continuous phase difference structures (e.g., active LC devices (TN, MTN, etc.) or passive reactive liquid crystal pre-coatings). The distorted LC director distribution can be discretized into... A single-axis delay plate. For a total in-plane delay of... LC devices, among which It is birefringence. d It is the unit gap. It is the wavelength, and the delay of each homogeneous plate is... If the device has a torsion angle Each homogeneous plate then has an orientation given by sampling continuous pointing angles along the thickness direction. Based on RO symmetry, the second pass of the device therefore has effective orientation. For a suitable number of boards, the behavior of a stack of RO symmetric retarders is virtually identical to that of consecutive corresponding sections.

[0056] We will now consider single-pass polarization transformation via MTN LC units. Figure 5 This illustrates the Poincaré transforms of the MTN LC layer for red, green, and blue wavelengths, assuming an achromatic circularly polarized input is incident on the LC layer. Figure 5 In the example, the twist angle is The delay at a wavelength of 550 nm is 235 nm. This indicates that a circularly polarized input (“achromatic circular input”) maps to a quasi-linearly polarized output (“linearly polarized”), where the orientation of the quasi-linear polarization is the wavelength. The function is represented as "R", "G", and "B" for red light at 650 nm, green light at 550 nm, and blue light at 450 nm, respectively. The polarization path of red light is shown as a dashed curve, the polarization path of green light as a solid curve, and the polarization path of blue light as a dotted line.

[0057] Table 1 below shows the ellipticity field ratios and linear polarization orientations for red, green, and blue light. This also applies to the inverse transform, where wavelength-dependent orientations exist. The quasi-linear polarization input is mapped to an achromatic circular SOP. Regardless of the method, this transformation represents a delay of approximately one-quarter of a wavelength per wavelength, i.e., it represents an embodiment of inverse dispersion. Figure 5The limitations of a single MTN device are also shown, stemming from the fact that the SOP (Side Optimizer) typically cannot be mapped between the poles corresponding to circular polarization and the equator corresponding to linear polarization with different orientations. Red indicates north of the equator, signifying a delay less than a quarter wavelength, while green indicates south of the equator, signifying a delay greater than a quarter wavelength. Although the MTN LC unit exhibits some anti-dispersion relative to a linear retarder, it is not a perfect anti-dispersion quarter-wavelength retarder in the visible light band. This can be an important consideration, for example, when higher-order compensation is required to maximize contrast.

[0058] Table 1. Output of single-pass and double-pass reflective MTN polarization rotators.

[0059]

[0060] Figure 6 This demonstrates that passing the light through an MTN LC layer twice can produce a change in the cycloidity of circularly polarized incident light. In other words, a double-pass MTN LC layer is functionally equivalent to an approximate half-wave delayer for circularly polarized input. (As shown in...) Figure 5 Similarly, the red light polarization path is in Figure 6 The polarization paths of green light and blue light are represented by dashed curves, respectively. The wavelength dependence of the rotation is a mechanism utilized by the MTN LC layer, which forces shorter wavelength light (e.g., blue light) to travel a more tortuous path than longer wavelength light, as a means of compensating for the relatively longer path length. Figure 6 As shown, the limitation of the MTN transform in failing to map all wavelengths to the equator is exacerbated in the reverse pass. This is demonstrated by observation... Figure 6 The diversity of the endpoints near the south pole of the middle Poincaré sphere is clear. Table 1 above summarizes the single-pass and double-pass SOP values ​​for beams propagating through the MTN LC layer. It can be seen that, overall, the double-pass performance of the MTN LC structure is not achromatic.

[0061] According to this disclosure, a passive compensation structure (PCS) can be added to a reflective LC polarization rotator to compensate for the chromaticity of the polarization transformation under low-pressure conditions (e.g., the chromaticity shown in Table 1 above) without degrading the achromaticity under high-pressure conditions. Reference Figure 7A and Figure 7B , Figure 7A and Figure 7BAs a non-limiting example of such an achromatic polarization rotator, the optical switch 700 includes a polarization rotator 750 coupled to the PBS 204. The polarization rotator 750 includes a reflective LC device 702. The reflective LC device includes an LC layer 706 coupled to a reflector 708. An electric field can be applied to the LC layer 706 by providing a voltage between the transparent electrode 707 and the reflector 708. Therefore, the reflector 708 can serve as a second electrode of the reflective LC device 702. The reflector 708 can be pixelated to provide spatially varying polarization rotation / switching of the incident light beam 112, such that the LC layer 706 includes an individually controllable array of LC pixels. This allows the optical switch 700 to become a visual display panel.

[0062] The LC layer 706 of the optical switch 700 can be configured with a twisted LC director 710A ( Figure 7A The undriven or low-voltage state and the LC pointing configuration of the 710B ( Figure 7B The LC layer 706 switches between a driving state and a high-voltage state. In this paper, when the term "substantially" is applied to the LC director orientation, it means the directional angle of the LC layer 706 (i.e., the directional angle relative to the LC director orientation). Figure 7A and Figure 7B The geometric angle formed by the XY plane in the LC layer 706 can deviate from 90 degrees to 10 degrees. The term "substantially" also means that the thin boundary sublayer of the LC layer 706 (i.e., the submicron-thickness sublayer adjacent to the orientation layer defining the boundary LC molecular orientation of the LC layer 706) can have a non-vertical orientation because the interaction between the thin boundary sublayer and the orientation layer adjacent to the LC layer 706 is stronger than that between the electric field applied to the LC layer 706. In some embodiments, the twisted LC director configuration 710A can have a twist angle of less than 90 degrees.

[0063] When in a low-voltage or non-driven state ( Figure 7A When beam 112 passes through LC layer 706 once, LC layer 706 converts the first achromatic polarization state of beam 112 into a first wavelength distribution of polarization state. For example, as considered above... Figure 5 In this context, the first achromatic state is the circular achromatic polarization state at the top of the Poincaré sphere, denoted as "achromatic circular input," and the first wavelength distribution of the polarization state is the distribution of the quasi-linear polarization state around the equator of the Poincaré sphere, denoted as... Figure 5 Linear polarization is indicated by the black dots "R", "G", and "B". Polarization rotator 750 ( Figure 7AThe system also includes a PCS 752 positioned upstream of the reflective LC device 702 relative to the incident beam 112. The PCS 752 is configured to convert a second achromatic polarization state of the beam 112 into a second wavelength distribution with a polarization state that matches the first wavelength distribution during a single pass of the beam 112. The purpose of this matching is to compensate for the chromaticity of the LC layer 706 in its undriven state, which will be explained in more detail below. The second achromatic polarization state can be, for example, a linear achromatic polarization state.

[0064] In operation, the incident beam 112 is polarized along the X-axis by reflection from the polarization interface 205 of the PBS 204. The linearly X-polarized beam 112 propagates through the PCS 752, and according to the reciprocity principle (characteristic D above), the PCS 752 generates a second wavelength distribution of the polarization state from the achromatic linear polarization state. Then, the beam 112 propagates through the LC layer 706. Because the second wavelength distribution of the polarization state closely matches the first wavelength distribution of the polarization state, the polarization state of the beam 112 propagating through the LC layer 706 (i.e., at the reflector 708) will be very close to the achromatic circular polarization state. Therefore, according to characteristic B above of the structure with RO symmetry, as long as the linear input polarization state becomes circular at the reflector 708, it can be guaranteed that the output beam 114 has an orthogonal linear polarization state across the entire wavelength band of the beam 112, thereby allowing the output beam 114 to propagate through the PBS 204 along the output optical path 714. Therefore, when the LC layer 706 is in a low-voltage state or an undriven state, the polarization rotator 750 switches between orthogonal achromatic polarization states of the beam 112 propagating through PCS 752, LC layer 706, reflected by reflector 708, and propagating back through LC layer 706 and PCS 752. In the undriven state, the polarization rotator 750 of the optical switch 700 exhibits achromatic behavior, such as... Figure 8A As shown.

[0065] When under high pressure or driving conditions ( Figure 7B The LC layer 706 exhibits virtually no in-plane delay, thus maintaining the second wavelength distribution of the polarization state. According to the aforementioned property D of the stack with RO symmetry, for all colors, the polarization rotation accumulated in the first pass is canceled out in the second pass, resulting in the output beam 114 having a substantially identical polarization state to the incident beam 112, thus reflecting from the polarization interface 205, making it virtually impossible for light to propagate along the output optical path 714. Therefore, the polarization rotator 750 maintains the achromatic polarization state of the beam 112 incident on it at its output end. In other words, in the undriven state, the polarization rotator 750 of the optical switch 700 also exhibits achromatic behavior, such as... Figure 8B As shown.

[0066] In summary, in the undriven state of the LC layer 706, the PCS 752 converts the achromatic linear input into a wavelength-dispersion linear state, preprocessing the SOP for modulation by the undriven MTN LC layer 706. Using, for example... Figure 5 The data of the Poincaré sphere mapping will be appropriate The input to the undriven MTN LC layer 706 is such that, for the wavelength range of interest (e.g., between 450 nm and 700 nm), the ellipticity field ratio at reflector 708 is as close to 1 as possible (i.e., as close to circular polarization as possible). This produces a flat spectral response when analyzed by PBS 204 after two passes. In the driven state of the LC layer 706, the PCS 752 provides a linear SOP at reflector 708 as long as the excited LC layer 706 has zero in-plane delay, and due to the characteristics of the aforementioned RO system, reflector 708 maintains a linear SOP after the round trip.

[0067] In practice, to maximize contrast, the PCS 752 can be combined with additional compensation to address the residual delay from the LC layer 706. Alternatively, the compensation for the PCS 752 and the LC layer 706 can be modular, allowing the PCS to map an achromatic linear SOP to a wavelength-dispersion linear SOP. The compensation between the PCS 752 and the reflective LC device 702 eliminates the residual delay from the fully excited LC layer 706. As long as the compensated polarization rotator 750 behaves as a simple mirror (i.e., with zero recombination delay), the optimized PCS 752 ideally produces linear SOPs at all wavelengths of interest.

[0068] Figure 9A and Figure 9B Further shown Figure 7A and Figure 7B The polarization rotator 750 is at a low voltage in the LC layer 706. V L ( Figure 9A ) state and high voltage V H ( Figure 9B Polarization conversion in the ) state. Figure 9A Presented along beam 112 through polarization rotator 750 ( Figure 7A A schematic diagram 900A shows the SOP evolution of the unfolded optical path, where the LC layer 706 is in a low-voltage state characterized by a twisted director configuration 710A. In this example, the twist angle of the unexcited (low-voltage state) LC layer 706 is 70 degrees. The incident beam 112 is horizontally polarized, as shown in 902. Figure 9A The PCS 752 operates as a dispersion rotator, mapping an achromatic linear polarization input to a linear SOP with appropriate wavelength dispersion, as shown in 904. For example, taking into account Figure 5 The polarization transformation of the unexcited LC layer is presented in the figure. PCS 752 "anticipates" the desired orientation for each wavelength, providing an unachromatic linear polarization state for the beam 112 incident on the LC layer 706, so that the SOP at the reflector 708 is as close as possible to circular polarization.

[0069] If the combination of PCS 752 and reflective LC device 702 produces near-circular polarization (ellipticity) at reflector 708 As shown in 906A, the polarization rotator 750 behaves as an achromatic half-wave delayer in a two-pass configuration according to the aforementioned RO symmetry. The LC layer 706 converts the polarization of the backpropagating beam 112 into orthogonal linear polarization for each wavelength, as shown in 908A, which is then converted into vertical linear polarization, as shown in 910A. Therefore, when the LC layer 706 is in an undriven state, the polarization rotator 750 switches between orthogonal achromatic polarization states of the beam 112.

[0070] Figure 9B It shows the same structure (i.e. Figure 7B A schematic diagram 900B of the SOP evolution of the polarization rotator 750 is shown, where the LC layer 706 is in a fully excited or driven state. At sufficient voltage, the in-plane conversion from the LC layer 706 is small, and its effect on bright-state transmittance is negligible. As previously described, the rotation generated during forward pass is canceled out during reverse pass. The initial SOP 902 is converted to a wavelength-dispersive SPO 904, which is maintained through the optical path until the reverse pass returns the SPO 904 to the initial SOP 902. Therefore, the initial SOP 902 remains unchanged after the round trip, and the driven state is also achromatic. In other words, the polarization rotator 750 maintains the achromatic polarization state of the beam 112. It is noteworthy that in the configuration considered herein, the dispersion rotator makes the polarization conversion of the MTN LC cell achromatic under low-voltage conditions without compromising the polarization conversion under high-voltage conditions. As a first approximation, the PCS 752 is used to convert an achromatic linear input into a wavelength-dispersive or wavelength-dependent linear SOP. The PCS 752 may also include a compensation element for reducing the residual incremental ellipticity as needed to meet desired specifications, such as maximizing contrast.

[0071] refer to Figure 10 Example structure of PCS or dispersion rotator 752 in Figure 7B The optical switch 700 is explicitly shown in its equivalent configuration in the driven state. It is assumed that the LC layer 706 in the driven state has no remaining in-plane birefringence, or that such in-plane birefringence is fully compensated, therefore in Figure 10The LC layer 706 is not shown. PCS 752 comprises a stack of five fixed retardation layers 1017, which have the same in-plane retardation of 130 nm at a wavelength of 520 nm, but with different optical axis orientations of -9.0°, -4.4°, -2.9°, -43.3°, and -0.4°, respectively. This is assumed to be birefringent dispersion of a cyclic olefin polymer.

[0072] Cycloolefin polymers are a non-limiting example material for the fixed retardation layer 1017. Many other materials and configurations can be used to construct the PCS 752 stack. Typically, the PCS 752 can include any technique for converting the input SOP into the desired output SOP, including +A plates or positive in-plane birefringent plates, -A plates or negative in-plane birefringent plates, etc. Meeting the tolerance constraints for polarization conversion at each wavelength may require two to ten (or more) birefringent layers, including non-twisted or linear retarders, such as stacks of stretched polymers and crosslinked reactive liquid crystal polymers. Linear retarders can also form twisted birefringent layers. Stretched polymers can be cut at a suitable angle relative to the stretching direction and laminated together using pressure-sensitive adhesives or solvent lamination. Reactive mesocrystalline stacks can be formed in a similar manner, either by transfer from roll-to-roll coating or by using, for example, direct coating stacks aligned with ultraviolet (UV) light. The composite transformation of the stack represents engineered dispersion, as the endpoints (ellipticity and orientation) are determined by the sequence of transformations experienced at each wavelength.

[0073] Table 2 below shows the SOP at reflector 708 for a selected wavelength, and the transmission leakage at the output port in the dual-pass circuit (i.e., at output optical path 714). The PCS output angle can be extracted by inputting an achromatic circularly polarized input into a specific MTN LC cell and recording the direction of the major axis of the ellipse at each wavelength of interest. Then, the PCS can be configured as an achromatic linear input to transform the input to produce this polarization orientation distribution.

[0074] Table 2. PCS performance for polarization distributions from linear to dispersive linear.

[0075]

[0076] As can be seen from Table 2, dual-pass transmission provides high suppression in the dark state and near-achromatic contrast in a wavelength range of at least 450 nm to 700 nm. This is a result of the polarization rotator 750 switching between the orthogonal achromatic polarization states of beam 112 in the non-driven state of LC layer 706, and maintaining the achromatic polarization state of beam 112 in the aforementioned wavelength range of 450 nm to 700 nm in the driven state of LC layer 706.

[0077] Figure 11 It shows Figure 10 The Poincaré sphere mapping of the PCS configuration. This transformation sequence maps the red (dashed) wavelength, green (solid) wavelength, and blue (dotted) wavelength from the common linear polarization state 1102 to a low ellipticity state with the rotation angles shown in Table 2. The low ellipticity state is as follows: Figure 11 As shown, “R”, “G” and “B” refer to the polarization states of red light, green light and blue light, respectively. The numbers in brackets (1), (2, 3), (4) and (5) refer to the sequence number of the delay layer 1017 in the stack of PCS 752 from left to right.

[0078] Global optimization algorithms can be used to identify the delay stack with optimal performance. For example, optimization criteria could be based on the angle of incidence (e.g., ...). to ), azimuth ( to Leakage of orthogonal polarization averaged over wavelengths (e.g., 440 nm to 650 nm) and incident angles. The optimization process can use a set of sparse wavelengths covering this range (e.g., 450 nm, 550 nm, and 650 nm). For each wavelength and incident angle, the leakage can be calculated. Directional leakage power. Configurations that minimize overall leakage across the wavelength and incident angle range can exhibit inherently low recombination delay in the thickness direction. Even if the stack can be composed of multiple uniaxial materials (each of the multiple uniaxial materials has a non-zero value). The retardation in the thickness direction can become significant when deviating from the vertical normal angle, thereby disrupting the normal incident transformation, as discussed further below.

[0079] One aspect of this disclosure is that it enables the display to have high contrast in a normally dark configuration while benefiting from achromatic aberration in a white-driven state. (Reference) Figure 12A and Figure 12B In a non-limiting example, the optical switch 1200 includes a polarizer 1204 or a transmission polarizer coupled to a polarization rotator 1250. The polarization rotator 1250 includes a PCS stack 1252 optically coupled to a reflective LC device 1202, which includes an LC layer 1206 coupled to a reflector 1208, and other layers omitted for brevity. The LC layer 1206 has a distorted LC director configuration in the undriven state, with a distortion angle of 68 degrees, changing from a 90-degree orientation on one side of the LC layer 1206 to a 158-degree orientation on the other side. The PCS stack 1252 includes five in-plane fixed retarders 1217. Figure 12A and Figure 12B The center point points to 71.7 from left to right. o 73.4 o -12.5 o 38.3 o and 74.3 o At green light wavelengths, it has zero thickness delay (“0R”). th ") and an in-plane retardation of 130 nm. The polarizer 1204 is oriented at -47.2 o As described above, the purpose of the PCS stack 1252 is to compensate for or counteract the low chromaticity of the polarization rotation of the reflective LC device 1202.

[0080] exist Figure 12A In the dark state depicted, LC layer 1206 is under low pressure. V L The incident beam 112 is linearly polarized by polarizer 1204, propagates through the PCS stack 1252 pre-adjusted for the input polarization of the LC device 1202, propagates through the LC unit 1206, and becomes circularly polarized. In the reverse path, the beam 112 acquires orthogonal linear polarization and is blocked from propagating along the output optical path 1214. The PCS stack 1252 ensures... Figure 12A The dark state is achromatic.

[0081] exist Figure 12B In the bright state depicted, LC layer 1206 is under high voltage. V H The incident beam 112 is linearly polarized by polarizer 1204, propagates through the PCS stack 1252 pre-adjusted for the input polarization of the LC device 1202, and becomes linearly polarized again, with the polarization angle depending on the wavelength. The beam 112 propagates through the LC layer 1206, and due to being in a driven or high-voltage state, the beam 112 essentially does not change its polarization. On the reverse path, the beam 112 folds its polarization back to its original state and couples out as the output beam 114. Due to the RO symmetry of the PCS stack 1252 and the LC device 1202, Figure 12B The white state is also achromatic.

[0082] In the undriven state, the PCS stack 1252, combined with the MTN configuration of the LC layer 1206, causes the beam 112 to produce nearly circularly polarized light at the reflector 1208 across the entire visible wavelength range. The PCS stack 1252 is configured to deliver optimal polarization orientation to the LC layer 1206 at each wavelength and compensate for the... Figure 5 The residual ellipticity associated with the MTN geometry is shown. Compensation and Figure 5The residual ellipticity associated with the MTN geometry shown has a significant benefit to contrast while having almost no impact on the flux in the bright state. The PCS stack 1252 can be optimized for the normal incident angle. For PCS using a base retarder with zero thickness directional delay, compensation and... Figure 5 The residual ellipticity related to the MTN geometry shown can also be considered the optimal case. The performance is actually unaffected by the angle of incidence.

[0083] Table 3 shows the performance of the optical switch 1200.

[0084] Table 3. Performance of MTN LC cell and PCS stacking for normally dim light switch.

[0085]

[0086] For benchmarking purposes, column 2 of Table 3 shows the single-pass ellipticity field ratio of the MTN LC layer 1206 with uniform linear input polarization. Figure 12A and Figure 12B The configuration of the optical switch 1200 shown optimizes circular polarization at 550 nm (i.e., maximizes...). Although it remains below 1 (unity) due to the conversion efficiency limitations of the MTN LC cell configuration, the ellipticity is clearly a strong function of wavelength, which causes a rapid increase in leakage of the dual-pass parallel polarizer (column 3). Columns 4 and 5 show the output orientation and ellipticity of the PCS with uniform linear polarization input, respectively. The ellipticity of the PCS effectively cancels out the ellipticity introduced by the MTN configuration. Column 6 shows the ellipticity of the combined effect of the PCS and MTN at reflector 1208, showing near-circular polarization across the entire visible band. Column 7 shows the double-pass leakage, where contrast is the reciprocal of leakage. This verifies that a PCS matched to the MTN LC cell can provide the acceptable contrast required by most display products in a parallel polarizer configuration. Column 8 verifies that there is almost no loss in double-pass transmission when the MTN LC cell is fully excited. In practice (e.g., using LCoS display panels), voltage limitations reduce modulation depth, although it is important to note that the PCS and achromatic QW delay of the MTN device are, in principle, capable of achieving near-perfect achromatic switching across the entire visible band.

[0087] In practice, the contrast ratio of ambient light switches or visual displays is a strong function of the match between the PCS and the MTN LC device. Manufacturing uncertainties in the MTN cell gap and PCS performance can lead to mismatches between components, resulting in reduced contrast. Given the tight tolerances for retardation and slow-axis orientation of typical stretched polymer retarders (e.g., Zeon retardation films manufactured by Zeon Specialty Materials, Inc., California, USA), MTN cell gap uncertainty is a primary cause of this mismatch. This includes spatially uniform cell gap uncertainties (e.g., part-to-part or wafer-to-wafer) as well as intra-part spatial inhomogeneities due to factors such as internal stress and wafer warping. Intra-part inhomogeneities can also be a function of temperature.

[0088] Figure 13 It was shown that increasing the cell gap by only 6% (relative to the optimal value) would cause the contrast ratio to drop from 14000:1 to below 100:1, which is impractical. The way to overcome this is to slightly increase the cell gap relative to the optimal value and then apply a small voltage to actively minimize dark-state leakage. In other words, a non-zero voltage can be applied to an undriven LC layer to better match the wavelength distribution of the LC layer's polarization state to the wavelength distribution of the PCS polarization state, compared to the matching of the LC and PCS wavelength distributions when zero voltage is applied to the LC layer. For example, refer to... Figure 14 , Figure 14 This illustrates the relationship between contrast ratio and cell gap non-uniformity under optimal voltage conditions. Although the contrast ratio decreases, it remains above 1000:1, with a cell gap error exceeding 12%. This tolerance is achievable for LCoS panel manufacturing. For pixelated optical switch / visual display panels, spatial variation voltage correction can also be applied to address deterministic spatial non-uniformity in the cell gap.

[0089] In most projection and direct-view display systems, normal incidence performance must be maintained over large illumination / frust angles. Generally, modulation of in-plane delay is combined with modulation of thickness-direction delay. When optimizing contrast over the field-of-view (FOV), dark states are compensated for at the expense of angular inhomogeneities in bright states. The performance of nematic LC devices can be described by the in-plane distribution of the LC director and the thickness-direction distribution of the LC director. The in-plane distribution of the LC director determines the functional polarization transformation near the normal incidence angle, while the thickness-direction distribution of the LC director introduces distortion in the transformation as the incidence angle increases. In reflective mode optical switches or displays, LCs can operate in analog or digitally addressed modes. MTN LC devices can provide the maximum in-plane functionality (e.g., quarter-wavelength delay) and the minimum in-plane functionality (e.g., zero in-plane delay) when the electric field amplitude reaches its limit.

[0090] For the normally-on configuration of the polarization rotator, optical switch, and / or visual display of this disclosure, dark-state leakage can be determined by the non-zero ellipticity at the mirror. Near-normal incidence (e.g., < The angle of incidence (AOI), and the associated leakage, can be given by the remaining in-plane delay of the MTN LC panel compensated for in the fully excited state. For the normally dark configuration, the contrast can be determined by the non-uniform ellipticity at the mirror. This can depend on the matching quality between the PCS and the MTN device in the low-voltage state. As mentioned earlier, this matching can take into account the optimal input orientation and ellipticity input of the MTN to transmit circular polarization at the mirror.

[0091] Under extreme electric field amplitudes, the LC device of this disclosure can have a pointing vector field that is substantially in-plane in a functional (unexcited or low voltage) state and substantially orthogonal to the device in an alternating (e.g., fully excited) state. In practice, it is preferable to minimize the degree of tilt of the pointing vector near the boundary by using a low pre-tilt orientation layer and by operating the LC cell at a sufficiently high voltage.

[0092] Under ideal conditions, a fully in-plane MTN pointing vector field produces the following transformation in the functional state: this transformation maps all wavelengths to a circular SOP. Assuming a single-axis LC, the in-plane delay of this element can be described as...

[0093] in, It is a very high refractive index. It is the refractive index of normal light. d It is the unit gap. This is the wavelength-dependent birefringence. Although the pointing vector field of the MTN is a distorted arrangement, this is the total retardation (or optical path length difference) of the LC. For an LC with zero distortion angle, it can be considered as an A-plate. The thickness retardation of the A-plate is...

[0094] In fact, when the angle of incidence increases relative to the normal, nonzero This tends to limit the performance of LC devices. If If it is not zero, then the in-plane function of the LC device is essentially preserved with respect to the incident angle. For devices with a tortuous director configuration, The definitions are the same, although the distortion in polarization conversion deviating from the normal differs from that of the A-plate due to the azimuth distribution in the slow axis. For a normally dark configuration, the low-voltage state is likely the most critical for determining the impact of the incident angle on contrast performance, and compensation can be included in or outside the PCS to minimize the distortion of the polarization conversion due to the incident angle. Compensation can include +A plates, -A plates, +C plates, and -C plates, or combinations thereof. C-plates are uniaxial retarders orthogonal to the substrate, so they only contribute to polarization conversion when the incident angle is significantly deviated from the normal. Note that the PCS can have non-zero composite values. For a traditional positive single-axis delayer, this non-zero composite value The tendency is towards positive. Ultimately, the objective could be interpolation compensation that minimizes the combined value of the PCS and MTN combination under low-voltage conditions. .

[0095] Figure 15 It shows Figure 12A and Figure 12B The relationship between the contrast ratio and the angle of incidence of the optical switch 1200. The PCS stack 1252 has zero composite value. Therefore, the decrease in contrast is mainly due to the LC layer 1206. Decision. Although there exists an azimuth angle where the contrast is very stable with the angle of incidence, Figure 15 The worst-case azimuth contrast is shown. Without off-axis performance compensation, when the angle of incidence (AOI) > At that time, the contrast ratio will drop from over 14000:1 to below 1000:1.

[0096] Under another ideal condition, the pointing vector field of the fully excited LC layer 1206 is actually orthogonal to the device, forming a virtual +C board. For this example,

[0097] and

[0098] When the normal is incident, the projection of the pointing vector field onto the device plane is close to zero, thus it can be functionally approximated as isotropic. However, larger negative... Alternatively, the +C board delay may introduce abnormal delays that could degrade device performance. For a constant-on architecture, the contrast on the FOV depends on compensation for the fully energized state of the MTN device. This compensation can serve the dual purpose of eliminating the residual in-plane delay of the MTN and the +C board delay. The +C board delay can be compensated for with the -C board delay, which can be a single layer or a pair of crossed +A boards. The objective in this case could be interpolation compensation to minimize the combined effect of the PCS and MTN under high-voltage conditions. .

[0099] In some embodiments of the optical switch disclosed herein, for example Figure 7A and Figure 7B The optical switch 700 can be replaced with a more compact light guide-based front illuminator by replacing the front PBS. (Reference) Figure 16 Non-restrictive examples, and further references Figure 7A and Figure 7B Optical switch 1600 ( Figure 16 )include Figure 7A and Figure 7B The optical switch 700 includes a polarization rotator 750, an evanescent illuminator 1604, and a reflective polarizer 1605. The evanescent illuminator 1604 includes a substrate 1660, a plate waveguide core 1662 supported by the substrate 1660, and an evanescent coupler 1664 supported by the plate waveguide core 1662. In operation, the plate waveguide core 1662 receives and guides a beam 112 parallel to the XY plane. The beam 112 is gradually coupled outwards towards the reflective polarizer 1605 via the evanescent coupler 1664.

[0100] The evanescent output 1664 may include a low-refractive-index wedge 1663 that generates a leakage propagation mode for the beam 112. The thickness of the low-refractive-index wedge 1663 gradually decreases to compensate for optical power loss due to upstream coupling. The coupled beam 112, after being reflected by the reflective polarizer 1605, is linearly polarized parallel to the X-axis. The X-polarized beam illuminates the polarization rotator 750. The reflected light propagates through the evanescent output 1664 and, depending on its polarization, is reflected and / or transmitted by the reflective polarizer 1605. This is a drive-to-black configuration. Other polarizer and polarization rotator configurations of this disclosure can be used.

[0101] Go to Figure 17 and further reference Figure 7A and Figure 7B Optical switch 1700 ( Figure 17 )include Figure 7A and Figure 7BThe optical switch 700 includes a polarization rotator 750, a replica light guide front illuminator 1764, and a transmission polarizer 1605. The replica light guide front illuminator 1764 comprises a plate 1760 of transparent material that supports a volume Bragg grating (VBG) 1762 or another angle-sensitive grating. In operation, a beam 112 propagates in the plate 1760 along a sawtooth optical path as shown, via a series of alternating internal reflections (e.g., total internal reflection) from its opposing surfaces. The VBG 1762 couples out a portion of the beam 112 polarized by the transmission polarizer 1605. The reflected light propagates through the front illuminator 1764 and, depending on its polarization, is absorbed and / or transmitted by the transmission polarizer 1705. This is a drive-to-white configuration. Other polarizer and polarization rotator configurations can be used. Furthermore, in some embodiments, a front illuminator based on polarized or unpolarized geometry waveguides can be used.

[0102] Now for reference Figure 18 The method 1800 for reducing the wavelength dependence of flux of an optical switch disclosed herein includes: providing (1802) an LC layer switchable between an undriven state having a distorted LC director configuration and an driven state having a substantially vertical LC director configuration. When in the undriven state, the LC layer (e.g., Figure 7A and Figure 7B The LC layer 706 of the polarization rotator 750 converts the first achromatic polarization state of the beam into the first wavelength distribution of the polarization state during a single pass of the beam.

[0103] A passive compensation structure (PCS) is provided and optically coupled (1804) to an LC layer, for example, PCS 752 is optically coupled to LC layer 706. PCS 752 may include a stack of fixed delay layers, such as... Figure 10 and Figure 12A , Figure 12B As shown. The PCS can be configured (1803) for example, through global optimization, to convert the second achromatic polarization state of the beam into a second wavelength distribution with a polarization state matching the first wavelength distribution during a single pass of the beam. When the LC layer is in an undriven state and the beam illuminating the polarization rotator is in the second achromatic polarization state, the polarization state of the beam at the reflector is the first achromatic polarization state. For example, when linear achromatic polarization is converted into circular achromatic polarization, the achromatic performance of the polarization rotator in a single pass will ensure achromatic conversion between orthogonal linear polarizations of characteristics A to E based on the aforementioned reversed-order (RO) stack. Method 1800 may also include providing (1806) a reflector (e.g., reflector 708) to configure double-pass propagation through the PCS-LC layer stack.

[0104] This document provides an embodiment 1 of method 800 for reducing the wavelength dependence of optical switch flux. The optical switch includes an LC layer switchable between an undriven state having a distorted LC director configuration and a driven state having a substantially perpendicular LC director configuration. In the undriven state, the LC layer converts a first achromatic polarization state of the beam into a first wavelength distribution of polarization state in a single pass of the beam. Embodiment 1 includes providing a PCS optically coupled to the LC layer, the PCS including a stack of fixed retardation layers configured to convert a second achromatic polarization state of the beam into a second wavelength distribution of polarization state matching the first wavelength distribution in a single pass of the beam.

[0105] This document provides an embodiment 2 of method 800. Embodiment 2 includes all the features of embodiment 1. In embodiment 2, when the LC layer is in an undriven state and the beam incident on the PCS is in a second achromatic polarization state, the polarization state of the beam at the output end of the LC layer is a first achromatic polarization state.

[0106] Turning Figure 19 The augmented reality (AR) near-eye display 1900 includes a frame 1901 with the shape elements of a pair of glasses. The frame 1901 supports for each eye: a projector 1908 including any of the light switches / display panels disclosed herein, a pupil replication waveguide 1910 optically coupled to the projector 1908, an eye-tracking camera 1904, multiple illuminators 1906, and an eye-tracking camera controller 1907. The illuminators 1906, which may be supported by the pupil replication waveguide 1910, illuminate an eyebox 1912. The projector 1908 provides a beam of light carrying an image to be projected onto a angular region in the user's eye. The pupil replication waveguide 1910 receives the beam and provides multiple laterally offset parallel copies of each beam in the beam, thereby extending the projected image onto the eyebox 1912.

[0107] For AR applications, the pupil replication waveguide 1910 can be transparent or semi-transparent to allow users to simultaneously view the external world and an image projected into each eye and superimposed on the view of the external world. The image projected into each eye can include objects set to simulate parallax so that it appears immersive in the real-world view.

[0108] The purpose of the eye-tracking camera 1904 is to determine the position and / or orientation of the user's eyes. Once the position and orientation of the user's eyes are known, the gaze convergence distance and direction can be determined. The image displayed by the projector 1908 can be dynamically adjusted to adapt to the user's gaze, allowing the user to be realistically immersed in the displayed augmented reality scene, and / or providing specific functions for interacting with augmented reality. In operation, the illuminator 1906 illuminates the eye at the corresponding eye-fitting frame 1912 to enable the eye-tracking camera to acquire an image of the eye, and to provide a reference reflection, i.e., a flash. The flash can serve as a reference point in the acquired eye image, facilitating the determination of the eye gaze direction by determining the position of the pupil image relative to the flash image. To avoid the illumination light distracting the user, the illumination light can be made invisible to the user. For example, infrared light can be used to illuminate the eye-fitting frame 1912.

[0109] The eye-tracking camera controller 1907 processes images acquired by the eye-tracking camera 1904 to determine the user's gaze direction in real time. In some embodiments, the image processing and eye position / orientation determination functions may be performed by a central controller (not shown) of the AR near-eye display 1900. The central controller may also provide control signals to the projector 1908 to generate an image to be displayed to the user based on the determined eye position, eye orientation, gaze direction, eye vergence, etc.

[0110] Now for reference Figure 20 The HMD 2000 is an example of an AR / VR wearable display system. The HMD 2000 surrounds the user's face for greater immersion in the AR / VR environment. The HMD 2000 can generate fully virtual 3D images. The HMD 2000 may include a front body 2002 and a strap 2004 that can be secured around the user's head. The front body 2002 is configured to be placed reliably and comfortably in front of the user's eyes. A display system 2080 may be disposed within the front body 2002 for presenting AR / VR images to the user. The display system may include any of the light switches, polarization rotators, and display panels disclosed herein that have achromatic performance. The side 2006 of the front body 2002 may be opaque or transparent.

[0111] In some embodiments, the front body 2002 includes a positioner 2008, an inertial measurement unit (IMU) 2010 for tracking the acceleration of the HMD 2000, and a position sensor 2012 for tracking the position of the HMD 2000. The IMU 2010 is an electronic device that generates data indicating the position of the HMD 2000 based on measurement signals received from one or more position sensors 2012, which generate one or more measurement signals in response to motion of the HMD 2000. Examples of position sensors 2012 include: one or more accelerometers; one or more gyroscopes; one or more magnetometers; another suitable type of sensor for detecting motion; a type of sensor for error correction of the IMU 2010; or some combination thereof. The position sensor 2012 may be located external to the IMU 2010, internal to the IMU 2010, or some combination thereof.

[0112] The locator 2008 is tracked by the external imaging device of the virtual reality system, enabling the virtual reality system to track the position and orientation of the entire HMD 2000. Information generated by the IMU 2010 and position sensor 2012 can be compared with the position and orientation obtained by tracking the locator 2008 to improve the tracking accuracy of the HMD 2000's position and orientation. Accurate position and orientation are crucial for presenting the appropriate virtual scene to the user as they move and rotate in 3D space.

[0113] The HMD 2000 may also include a depth camera assembly (DCA) 2011, which acquires data describing depth information of part or all of the local area surrounding the HMD 2000. This depth information can be compared with information from the IMU 2010 to more accurately determine the position and orientation of the HMD 2000 in 3D space.

[0114] The HMD 2000 may also include an eye-tracking system 2014 for determining the orientation and position of the user's eyes in real time. The obtained eye position and orientation also allow the HMD 2000 to determine the user's gaze direction and adjust the image generated by the display system 2080 accordingly. The determined gaze direction and convergence angle can be used to adjust the display system 2080 to reduce convergence-accommodation conflict. Orientation and convergence can also be used for the exit pupil steering of the display, as disclosed herein. Furthermore, the determined convergence angle and gaze angle can be used for user interaction, highlighting objects, bringing objects to the foreground, creating additional objects or pointers, etc. An audio system may also be provided, including, for example, a set of small speakers built into the front body 2002.

[0115] Various embodiments of this disclosure may include or incorporate artificial reality systems. An artificial reality system modulates sensory information about the external world obtained through the senses, such as visual, auditory, tactile (body-sensing) information, acceleration, balance, etc., and then presents it to the user. As a non-limiting example, artificial reality may include virtual reality (VR), augmented reality (AR), mixed reality (MR), or some combination and / or derivative thereof. Artificial reality content may include entirely generated content or may include generated content combined with acquired (e.g., real-world) content. Artificial reality content may include video, audio, haptic or tactile feedback, or some combination thereof. Any such content may be presented in a single channel or multiple channels, such as stereoscopic video that produces a three-dimensional effect for the viewer. Furthermore, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof for, for example, creating content in artificial reality and / or otherwise using it in artificial reality (e.g., performing activities in artificial reality). Artificial reality systems that deliver artificial reality content can be implemented on a variety of platforms, including wearable displays such as HMDs connected to a host computer system, stand-alone HMDs, near-eye displays with eyeglass shape elements, mobile devices or computing systems, or any other hardware platform capable of delivering artificial reality content to one or more viewers.

[0116] The scope of this disclosure is not limited to the specific embodiments described herein. In fact, various other embodiments and modifications will be apparent to those skilled in the art, in addition to those described herein, based on the foregoing description and drawings. Therefore, such other embodiments and modifications are intended to fall within the scope of this disclosure. Furthermore, although this disclosure has been described in the context of specific implementations for specific purposes in specific environments, those skilled in the art will recognize that its utility is not limited thereto, and that the contents of this disclosure can be advantageously practiced in any number of environments for any number of purposes. Therefore, the following claims should be interpreted according to the full scope of this disclosure as set forth herein.

Claims

1. A polarization rotator, comprising: a reflective liquid crystal (LC) device, the reflective LC device comprising an LC layer coupled to a reflector, wherein the LC layer is switchable between an un-driven state having a twisted LC director configuration and a driven state having a substantially vertical LC director configuration, wherein, when in the un-driven state, the LC layer converts a first achromatic polarization state of a light beam in a single pass of the light beam to a first wavelength distribution of polarization states; and a passive compensation structure (PCS) disposed upstream of the LC device and comprising a stack of fixed retardation layers configured to convert a second achromatic polarization state of the light beam in a single pass of the light beam to a second wavelength distribution of polarization states matching the first wavelength distribution.

2. The polarization rotator of claim 1, wherein, a twist angle of the twisted LC director configuration is less than 90 degrees.

3. The polarization rotator of claim 1 or 2, wherein, when the LC layer is in the un-driven state, the polarization rotator converts between orthogonal achromatic polarization states of a light beam propagating through the PCS, the LC layer, being reflected by the reflector, and propagating back through the LC layer and the PCS; and preferably, wherein, when the LC layer is in the un-driven state and a polarization state of a light beam impinging on the PCS is an achromatic linear polarization state, a polarization state of the light beam at the reflector is an achromatic circular polarization state.

4. The polarization rotator of any of the preceding claims, wherein, when the LC layer is in the driven state, the polarization rotator substantially maintains an achromatic polarization state of a light beam incident on the polarization rotator at an output of the polarizer; and preferably, wherein, when the LC layer is in the driven state and a polarization state of a light beam impinging on the PCS is an achromatic linear polarization state, a polarization state of the light beam at the reflector is a non-achromatic linear polarization state.

5. The polarization rotator of any of the preceding claims, wherein, when the LC layer is in the un-driven state and a polarization state of a light beam impinging on the PCS is the second achromatic polarization state, a polarization state of the light beam at the reflector is the first achromatic polarization state.

6. The polarization rotator of any of the preceding claims, wherein, in operation, a non-zero voltage is applied to the LC layer in the un-driven state to obtain a better match of the first wavelength distribution of polarization states and the second wavelength distribution of polarization states compared to a match of the first wavelength distribution and the second wavelength distribution when a zero voltage is applied to the LC layer.

7. The polarization rotator of any of the preceding claims, wherein, the stack of fixed retardation layers comprises A-plates, preferably wherein all A-plates of the stack have substantially the same in-plane retardation and different optic axis directions; and / or wherein the stack further comprises C-plates for improving off-axis performance of the polarization rotator.

8. The polarization rotator of any of the preceding claims, wherein, the stack of fixed retardation layers comprises a cyclo-olefin polymer.

9. The polarization rotator according to any of the preceding claims, wherein: when the LC layer is in the un-driven state, the polarization rotator converts between orthogonal achromatic polarization states of the light beam within a spectral bandwidth between 450 nm and 700 nm; and, The polarization rotator substantially maintains a achromatic polarization state of the light beam within a spectral bandwidth between 450 nm and 700 nm when the LC layer is in the driven state.

10. An optical switch comprising: a front light illuminator for providing a light beam; a polarization rotator downstream of the front light illuminator for receiving the light beam and changing a direction of the light beam to cause the light beam to propagate back to the front light illuminator; and a polarizer downstream of the polarization rotator for transmitting or blocking the light beam depending on a polarization state of the light beam; The polarization rotator comprises: a reflective liquid crystal (LC) device comprising an LC layer coupled to a reflector, wherein the LC layer is switchable between an un-driven state having a twisted LC director configuration and a driven state having a substantially vertical LC director configuration, wherein the LC layer converts a first achromatic polarization state of a light beam to a first wavelength distribution of polarization states in a single pass of the light beam when in the un-driven state; and a passive compensation structure (PCS) disposed upstream of the LC device and comprising a stack of fixed retardation layers configured to convert a second achromatic polarization state of the light beam to a second wavelength distribution of polarization states matching the first wavelength distribution in a single pass of the light beam. The polarization state of the light beam at the reflector of the reflective LC device is the first achromatic polarization state when the LC layer is in the un-driven state and the polarization state of the light beam incident on the PCS is the second achromatic polarization state.

11. The optical switch of claim 10, wherein, 12. The optical switch of claim 10 or 11, wherein: the polarization rotator converts between orthogonal achromatic polarization states of the light beam when the LC layer is in the un-driven state; and the polarization rotator maintains the achromatic polarization state of the light beam when the LC layer is in the driven state. The LC layer of the polarization rotator comprises an array of individually controllable LC pixels; and preferably wherein the front light illuminator comprises at least one of an evanescent out-coupler or a replicated light guide for providing a portion of the light beam to each LC pixel of the array.

13. The optical switch of any of claims 10 to 12, wherein, The LC layer converts a first achromatic polarization state of a light beam to a first wavelength distribution of polarization states in a single pass of the light beam when in the un-driven state, the PCS comprises:

14. A passive compensation structure (PCS) for an LC layer, the LC layer being switchable between an un-driven state having a twisted LC director configuration and a driven state having a substantially vertical LC director configuration, wherein, a stack of fixed retardation layers configured to convert a second achromatic polarization state of the light beam to a second wavelength distribution of polarization states matching the first wavelength distribution in a single pass of the light beam. ​ 15. The PCS of claim 14, wherein, when the LC layer is in the un-driven state and the light beam incident on the PCS is in the second achromatic polarization state, a polarization state of the light beam at an output end of the LC layer is the first achromatic polarization state.

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