Holographic display and method

Abstract

A holographic display is provided. The display comprises (i) an illumination source configured to emit at least partially coherent light, (ii) a liquid crystal layer comprising an array of elements arranged to be illuminated by the light. The layers are configured such that (a) when a first electric field is applied on an element of the liquid crystal layer, the light from the element has a first polarization state, (b) when a second electric field is applied on the element, the light has a second polarization state, and (c) when a third electric field is applied on the element, the light has a third polarization state. The display further includes (iii) a controller configured to control an electric field applied on the elements, and (iv) an output polarizing element configured to: (a) remove a portion of the light having the first polarization state and also transmit a portion of the light having the first polarization state, and (iv) an output polarizing element configured to: (iv) transmit a portion of the light having the second polarization state; (b) removing substantially all of the light having the second polarization state, and (c) removing a portion of the light having the third polarization state and also transmitting a portion of the light having the third polarization state. In this display, one of: (1) the second electric field is greater than the first electric field, and the third electric field is greater than the second electric field; and (2) the second electric field is lower than the first electric field and the third electric field is lower than the second electric field.

Description

Technical Field

[0001] The present invention relates to holographic displays and methods of operating holographic displays. Background Art

[0002] Computer generated holography (CGH) is well known. A holographic light field is determined for a holographic display using coherent or at least partially coherent light and is defined in terms of the amplitude and phase of each element (e.g., pixel) of the display. This combination produces a light field that can be perceived by a viewer and has depth information.

[0003] An ideal spatial light modulator (SLM) in a holographic display is capable of full complex modulation, where the amplitude and phase values ​​of each pixel of the hologram can be varied to closely resemble a determined light field amplitude and phase. In other words, an ideal SLM is able to display all possible combinations of phase and amplitude.

[0004] In practice, SLMs used for CGHs cannot achieve perfect full complex modulation. Typically, an SLM may only have a very limited number of values ​​that it can display. For example, an SLM may only be able to modulate one of the amplitude or phase. Resolution may also be limited, perhaps only 8 bits of resolution (256 or fewer values ​​may be displayed), or even only 2 values ​​in the case of a binary display.

[0005] Therefore, the pixels of the fully complex holographic image are quantized to values ​​that can be reproduced by the SLM for display. For example, in the extreme case of a binary SLM (e.g., a digital micromirror device, DMD), each pixel in the display can only be in one of two states. Therefore, all points on the fully complex Argand diagram need to be mapped to one of the two states. This quantization process degrades the image quality, which manifests itself in the perceived image as reduced contrast and / or reduced noise.

[0006] Fourier plane filtering can be used to encode complex CGH data for display on an amplitude-only SLM. In this case, the complex conjugate is added to the full complex value to create a real value, and an additional constant term is added to create a positive real value. The conjugate and constant terms are then filtered in the Fourier plane by a spatial filter so that the remaining components are related only to the original full complex value. However, the constant term that must be added in this approach can result in a significant zeroth order component, which requires accurate spatial filtering and results in reduced brightness.

[0007] Methods have been proposed to encode a full complex hologram by interfering light from two elements, such as dual phase amplitude coding (DPAC), but it is difficult to achieve an amplitude close to zero and achieve proper interference between pixels.

[0008] Therefore, it is desirable to improve the image quality of holographic displays. Summary of the invention

[0009] Spatial light modulators (SLMs) can modulate light in a variety of ways. One approach is to use liquid crystals arranged in an array of elements / pixels that change the polarization state of incident light as it passes through the elements. The liquid crystal layer is birefringent, meaning that the refractive index is a function of the polarization state of the incident light relative to the crystal axis. By controlling the voltage applied to the element electrodes, the orientation of the crystals within each element can be changed, thereby applying an electric field to the liquid crystal. This voltage-controlled change in crystal orientation thus controls the polarization / rotation of light by the element. Thus, applying a voltage / field to a particular element causes the incident light to rotate by an angle that depends on the applied voltage / field. It should be understood that control of the effective intensity of the applied electric field can be achieved by analog control of the voltage (i.e., high / low electric field), pulse width modulation (i.e., an applied voltage with a high / low duty cycle), or any other means.

[0010] In addition to the liquid crystal layer, the SLM may also include an output polarization element (e.g., an output polarizer) disposed after the liquid crystal layer. In some examples, the output polarization element is separate from the SLM. Under certain specific electric fields / voltages, the polarization state of light after passing through the liquid crystal layer is such that substantially all light can be transmitted through the output polarization element. Under different electric fields / voltages, the polarization state of light after passing through the liquid crystal layer is such that the light is blocked / removed by the output polarization element. Therefore, the amplitude of the light can be controlled by controlling the voltage applied to the elements of the SLM (as well as the electric field applied to the elements). It is well known that the transformation of light as it propagates through the liquid crystal layer can be represented by the Jones matrix that characterizes the liquid crystal layer.

[0011] An SLM may be either reflective or transmissive, and references to "SLM" herein are not limited to reflective or transmissive SLMs. A transmissive SLM may be configured such that light is primarily transmitted through the SLM from one side to the other. A reflective SLM may be configured such that light is primarily reflected by the SLM. A reflective SLM includes a mirror so that light passes through a liquid crystal layer element, is reflected, and then passes through the element a second time in the opposite direction. Examples of reflective SLMs include liquid crystal on silicon (LCoS) displays. For a reflective SLM, such as an LCoS, where light passes through the layer twice due to reflection, it will be appreciated that the Jones matrix associated with the liquid crystal layer represents the transformation after passing through the liquid crystal layer twice. Therefore, the Jones matrix associated with the liquid crystal layer represents the transformation of light when it passes through once in the case of a transmissive SLM or twice in the case of a reflective SLM.

[0012] Existing amplitude modulated SLMs (i.e., SLMs that include a liquid crystal layer and an output polarization element) modulate the amplitude of light between "off" and "on" states. For example, when light is completely blocked / filtered / deflected, the amplitude of the light can be said to be "0" (i.e., the SLM in the element / pixel area may be "off"). When substantially all light is transmitted, the amplitude of the light can be said to be "1" (i.e., the SLM in the element / pixel area may be "on"). Therefore, conventional SLMs modulate amplitude in the range of 0 to 1.

[0013] As mentioned above, the holograms computed in a CGH are usually complex-valued, where these complex values ​​are quantized to some discrete values ​​for display on the SLM. An ideal SLM is able to modulate the complex values ​​- i.e. the modulation associated with each pixel / element of the SLM will be given by Ae iθ , where A is the amplitude and θ is the phase, and each element of the SLM can be used to individually control A in the range of 0 to 1 (i.e., [0,1]), and θ in the range of 0 to 2π (i.e., [0,2π]).

[0014] However, unlike these ideal SLMs, commercially available SLMs usually modulate only the phase or only the amplitude. That is, a phase-only SLM applies a modulation factor of e to θ in the range [0,2π]. iθ The modulation is defined by Ae, while the amplitude-only SLM is applied to A in the range [0,1] if(A) Modulation is defined as f(A) where f(A) is a function of amplitude. f(A) is ideally constant, but in some cases an amplitude-only SLM may partially modulate the phase over a small range as the amplitude varies. Since this phase depends on the amplitude, the phase is not independent of the amplitude modulation through the relationship of f(A).

[0015] There are methods for encoding a fully complex hologram using either a phase-only or amplitude-only SLM. For example, dual phase amplitude coding (DPAC) coherently interferes two elements of a phase-only SLM together to provide a signal with the same amplitude as the e iθ1 +e iθ2 Proportional modulation, which spans the entire complex range [0,2π], where θ1 is the phase introduced by the first element and θ2 is the phase introduced by the second element. DPAC can be effective, but is very sensitive to precise control of the phase value to achieve near-zero amplitude, and achieving proper interference of two phase-only elements is difficult.

[0016] The inventors have realized that, when represented on an Argand diagram, the SLM can be modified so that it modulates the amplitude of light in a range spanning either side of "0" (i.e., zero amplitude), rather than modulating in a range from 0 to 1 (i.e., [0,1]). A "negative" amplitude has the same intensity as a corresponding "positive" amplitude, but can be thought of as having a phase shift relative to light of the same positive amplitude, such that the negative amplitude light and the positive amplitude light appear in different quadrants on the Argand diagram. In other words, the SLM is configured so that an output amplitude of substantially zero is obtained at intermediate modulation values, rather than at the ends of the modulation range as is conventionally the case.

[0017] For example, the SLM can be modified to modulate the Argand map In one specific example, light with negative amplitude may have a phase shift of π radians relative to light with positive amplitude.

[0018] The inventors have discovered that operating the SLM via "zero" modulation amplitude has several benefits.

[0019] The first benefit is that the improved SLM can achieve amplitudes close to zero more accurately than existing methods, such as DPAC. For example, a single element can be operated more precisely to achieve an amplitude of zero without causing the light to interfere with light from another element.

[0020] A second benefit is that, unlike encoding methods that add constant values ​​mapped to only positive amplitudes, the zeroth order diffracted light in the light field is greatly reduced (i.e., the energy of the zeroth order is much lower than when modulated in the range [0,1]). This approach can produce holograms with higher image quality than conventional holographic displays, where the zeroth order light is higher in energy and must be removed in an additional spatial filtering step. For example, as briefly mentioned, in many holographic displays, after the light is modulated by the SLM, the unwanted components of the light can be removed by positioning the spatial filter in the Fourier plane of the optical system. Thus, after the light passes through the SLM, it is incident on the lens and is focused in the Fourier plane where the spatial filter is located. The spatial filter can physically block specific areas of the light field to remove unwanted components of the light, including the complex conjugate of the light field and the zeroth order of the light field. When a large amount of power is contained in the zeroth order, the filtering effect is usually poor due to stray light and imperfect filtering. This results in a degradation of the image quality of the displayed hologram. By operating the SLM to modulate the amplitude on either side of the zero amplitude, the zero order does not necessarily need to be removed, resulting in a hologram with higher image quality than a conventional holographic display. If the zero order noise is still removed, then depending on the situation, there will be far fewer filtering issues due to the lower energy of the zero order noise, which in turn results in lower overall noise.

[0021] Therefore, in a first aspect of the invention, there is provided a holographic display comprising: (i) an illumination source configured to emit at least partially coherent light, (ii) a liquid crystal layer comprising an array of elements arranged to be illuminated by the light. The liquid crystal layer is configured so that: (a) when a first electric field is applied to the elements of the liquid crystal layer, the light from the elements has a first polarization state, (b) when a second electric field is applied to the elements of the liquid crystal layer, the light from the elements has a second polarization state, and (c) when a third electric field is applied to the elements of the liquid crystal layer, the light from the elements has a third polarization state. The display further comprises (iii) a controller configured to control the electric field applied to the elements of the liquid crystal layer, and (iv) an output polarization element. The output polarization element is configured to: (a) remove a portion of light having a first polarization state and also transmit a portion of light having the first polarization state, (b) remove substantially all of the light having a second polarization state, and (c) remove a portion of light having a third polarization state and also transmit a portion of light having the third polarization state, wherein one of the following: (1) the second electric field is greater than the first electric field and the third electric field is greater than the second electric field, and (2) the second electric field is lower than the first electric field and the third electric field is lower than the second electric field.

[0022] In some examples, the output polarization element is (or includes) a transmissive polarizer and is therefore configured to remove light of a specific polarization state by blocking the light. That is, the light cannot pass through the polarizer and is therefore removed. Similarly, the output polarization element is configured to transmit light of a specific polarization state by allowing the light to pass through the polarizer.

[0023] In other examples, the output polarization element is (or includes) a reflective polarizer and is therefore configured to remove light of a specific polarization state by directing / reflecting light in a specific direction so that the light is not present in the displayed hologram. For example, light can be directed to an absorptive element of the output polarization element, which absorbs the light. Since this removed light is absorbed, it is not "transmitted" out by the output polarization element. Similarly, the output polarization element is configured to transmit light of a specific polarization state by directing / reflecting light in a different direction (different from the direction of the removed light) so that the light is present in the displayed hologram. Therefore, in some examples, the output polarization element includes a polarizer and an absorptive element. They work together to remove or transmit light incident on the output polarization element. In some examples, the absorptive element is an inner wall or cavity of the housing of the holographic display.

[0024] In another example, the output polarization element is (or includes) a prism, such as a Wollaston prism. Light passes through the prism and is directed to a specific direction according to the polarization state of the light. Therefore, this prism is configured to remove light of a specific polarization state by guiding / reflecting light in a specific direction when the light passes through the prism, so that the light does not exist in the displayed hologram. In this case, the light can still pass through the prism, but is blocked by the absorption element of the output polarization element and is not transmitted by the output polarization element. Similarly, the output polarization element is configured to transmit light of a specific polarization state by guiding / reflecting light in a different direction (different from the direction of the light removed), so that the light exists in the displayed hologram.

[0025] In some specific examples, the electric field is controlled by controlling the current. However, in general, the electric field is controlled by controlling the voltage applied to the elements of the liquid crystal layer. Therefore, in some examples, the controller is configured to control the electric field applied to the elements of the liquid crystal layer by controlling the voltage applied to the elements of the liquid crystal layer. Therefore, the liquid crystal layer is configured so that: (a) when the element is operated at a first voltage level, a first electric field is applied to the element, (b) when the element is operated at a second voltage level, a second electric field is applied to the element, and (c) when the element is operated at a third voltage level, a third electric field is applied to the element.

[0026] In this document, control of the electric field may be described as being achieved by operating an element at a specific voltage level. However, it should be understood that any reference to operating a liquid crystal layer element or applying a voltage level to a liquid crystal layer element may also be referred to as applying a specific electric field across the element.

[0027] As described above, the electric field can be varied by analog control of the voltage to the element, or by digital control of the voltage and adjusting the duty cycle. It should therefore be understood that when an electric field is applied to an element, this may mean a pulse width modulated (PWM) electric field where the duty cycle is varied, rather than an electric field with a smoothly varying duty cycle. Similarly, when one electric field is greater / less than another, this may mean a PWM controlled electric field with a higher / lower duty cycle. Therefore, even if the instantaneous electric field changes rapidly, changing the duty cycle over a certain time period will apply the average electric field for that time period. Therefore, the average electric field over a first time period may be greater or lower than the average electric field over a second time period. Most LCoS displays are digitally driven rather than analog, but this is not always the case. Thus, in some examples, the liquid crystal layer is thus configured such that: (a) a first electric field is applied to the element when the element operates at a first duty cycle during a first time period, (b) a second electric field is applied to the element when the element operates at a second duty cycle during a second time period, and (c) a third electric field is applied to the element when the element operates at a third duty cycle during a third time period.

[0028] In some examples, the liquid crystal layer and the output polarization element form at least part of the SLM. For example, this may be the case with a transmissive SLM. In other examples, the liquid crystal layer forms at least part of the SLM, and the output polarization element is separate from the SLM. For example, this may be the case with a reflective SLM. However, in some examples, the output polarization element may be part of a reflective SLM. An SLM having both a liquid crystal layer and an output polarization element means that the SLM itself can achieve amplitude modulation.

[0029] In the above aspects, when the first and third electric fields are applied to the element (e.g., the element is operated at the first and third voltage levels), only a portion of the total light intensity incident on the element is transmitted by the output polarization element. The portion may be less than 80%, or less than 70%, or less than 60% of the total light intensity incident on the element. In an example, the remaining light intensity portion is about 50% of the total light intensity incident on the element, which may be the case when zero amplitude is obtained at the midpoint of the modulation amplitude range.

[0030] In one example, when the second electric field is greater than the first electric field, and the third electric field is greater than the second electric field: the first voltage level is zero voltage, the second voltage level is non-zero voltage, and the third voltage level is non-zero voltage. Therefore, the voltage of the second voltage level is higher than the first voltage level, and the voltage of the third voltage level is higher than the second voltage level. Therefore, when the voltage (and the electric field) changes from the first voltage level to the third voltage level via the second voltage level, it can be said that the voltage (and the electric field) is "increasing". The first voltage level can be the minimum voltage, and the third voltage level can be the maximum voltage. When the element is operating at zero voltage, it can be said that the element is in a static state.

[0031] Alternatively, in another example, when the second electric field is lower than the first electric field, and the third electric field is lower than the second electric field: the first voltage level is a non-zero voltage, the second voltage level is a non-zero voltage, and the third voltage level is a zero voltage. Therefore, the voltage of the second voltage level is lower than the first voltage level, and the voltage of the third voltage level is lower than the second voltage level. Therefore, when the voltage (and the electric field) changes from the first voltage level to the third voltage level via the second voltage level, it can be said that the voltage (and the electric field) is "decreased". The third voltage level can be a minimum voltage, and the first voltage level can be a maximum voltage.

[0032] In some examples, the first voltage may be referred to as an initial voltage, the third voltage may be referred to as a final voltage, and the second voltage may be referred to as an intermediate voltage.

[0033] In some examples, when the first electric field is applied to the element, the light has a first phase, and when the third electric field is applied to the element, the light has a second phase, and the first phase and the second phase are located in different quadrants on the Argant diagram. Therefore, light transmitted by / through the output polarization element at the first voltage level has a phase located in the first quadrant of the Argant diagram, and light transmitted by the output polarization element at the third voltage level has a phase located in the second quadrant of the Argant diagram, where the first quadrant and the second quadrant are different.

[0034] Thus, the liquid crystal layer and the output polarization element are configured such that when the voltage changes (increases or decreases) from a first level via a second (intermediate) level to a third level, the amplitude of the light changes from the first quadrant of the Argant diagram (e.g., "positive" or "negative" amplitude) via zero amplitude to the second quadrant of the Argant diagram (e.g., "negative" or "positive" amplitude). As the voltage applied to the element changes, the viewer sees the light change from on to off and back on. This is different from a conventional amplitude modulated SLM, which works in that the amplitude of the light changes within the range of [0,1] as the voltage increases (or decreases) from a first level via a second level to a third level. As the voltage applied to the element changes, the viewer of a conventional SLM sees the light change from off to on at different brightness levels.

[0035] In an example, the output polarization element is arranged so that when the applied voltage is equal to a first voltage level, the light has a maximum amplitude, and increasing or decreasing the voltage to a second voltage level reduces the amplitude. At the second voltage level, the amplitude of the light is zero (i.e., substantially no light is transmitted from the output polarization element), and continuing to increase or decrease the voltage to a third voltage level increases the amplitude of the light again to a maximum value.

[0036] In some examples, the amplitude of the transmitted light may be "mirrored" on either side of zero. Thus, for example, if light with a specific amplitude is desired to generate a holographic image, one of two voltages may be applied to the element, and of the two voltages, the actual voltage chosen depends on the desired phase modulation.

[0037] In some examples, the holographic display is configured such that light from a first element does not interfere with light from a second element. Thus, a desired amplitude and / or phase of light from one element can be achieved without interfering light from two or more elements of the liquid crystal layer. Thus, light from the element is transmitted to the observer / viewer without interfering with light from another element (e.g., light from an adjacent or neighboring element).

[0038] In some examples, the controller is further configured to control an illumination source. In one example, the controller can control the brightness / intensity of the illumination source such that the magnitude depends on the operation of the illumination source and the liquid crystal layer.

[0039] In some arrangements, when the second electric field is applied to the element of the liquid crystal, the polarization state of the light is adjusted relative to the first polarization state. Similarly, when the third electric field is applied to the element, the polarization state of the light is adjusted relative to the second polarization state. Thus, in some examples, the first, second, and third polarization states are different.

[0040] In an example, when the first electric field is applied, the polarization state of the light is adjusted by the element relative to the initial polarization state of the light incident on the element. In another example, when the first electric field is applied, the polarization state is unchanged relative to the initial polarization state of the light incident on the element.

[0041] As described above, the liquid crystal layer may include a plurality of elements / pixels, such as an array of elements. An element may correspond to a pixel or a sub-pixel (eg, a sub-pixel of an RGB pixel).

[0042] The illumination source may include a single light emitter or a plurality of light emitters and have an illumination area sufficient to illuminate the liquid crystal layer.

[0043] In some examples, there is a single output polarization element that receives light from the entire area of ​​the liquid crystal layer. In other examples, there are multiple output polarization elements that receive light from one or more elements of the liquid crystal layer. In a particular example, there is a separate output polarization element for each element of the liquid crystal layer.

[0044] In the example, the illumination source, the liquid crystal layer and the output polarization element are arranged along the optical axis / optical path. The arrangement along the optical axis can ensure that the light in the display can be transmitted through the various components of the display. Therefore, the output polarization element is arranged so that the light passes through the liquid crystal layer and is incident on the output polarization element. Therefore, in terms of the light path starting from the illumination source, it can be said that the output polarization element is located "after" the liquid crystal layer. The liquid crystal layer can sometimes be physically arranged between the illumination source and the output polarization element.

[0045] In some examples, there may also be an input polarization element (also referred to as an input polarizer) disposed "before" the liquid crystal layer, with respect to the light path starting from the illumination source, to polarize the light incident on the liquid crystal layer. Thus, the input polarization element may be disposed between the illumination source and the liquid crystal layer. The input polarization element may also be disposed on the optical axis of the holographic display. In other examples, the illumination source itself is configured to output polarized light, so that the input polarization element may be omitted. The input polarization element may still be used with a polarized illumination source, for example to change the polarization to one that best suits the characteristics of the liquid crystal layer.

[0046] In some examples, the output polarization element is arranged at a specific angle relative to the initial polarization of the light incident on the liquid crystal layer. In some examples, the angle can be predetermined. The angle of the output polarization element relative to the polarization state of the light can enable the SLM to achieve "zero-crossing" modulation.

[0047] More generally, the relationship between the output polarization element, the initial polarization state of light incident on the liquid crystal layer, and the liquid crystal layer itself can be defined using Jones matrices and vectors. Specific matrices / vectors can therefore be configured to achieve zero-crossing modulation.

[0048] As briefly mentioned above, the "behavior" (i.e., transformation) of light as it passes through a liquid crystal layer can be represented by a Jones matrix and a Jones vector. The liquid crystal layer (and each element of the liquid crystal layer) is associated with a Jones matrix, which can be determined. The Jones matrix for each element is a function of the electric field applied to the element. Similarly, the polarization state of light can be described by a Jones vector, and the output polarization element can also be described by a Jones vector. Therefore, the rotation / polarization change of an element passing through the liquid crystal layer can be mathematically quantified, and the transmittance through the output polarization element can be predicted. It can be useful to determine the Jones matrix of the liquid crystal layer so that specific voltages and / or output polarization elements can be determined so that the SLM can be appropriately configured to modulate light through zero.

[0049] Thus, light incident on an element of the liquid crystal layer has an initial polarization associated with a first Jones vector, the output polarization element is associated with a second Jones vector, the element of the liquid crystal layer is associated with a Jones matrix, wherein the Jones matrix of the element is a function of an electric field applied to the element, and the second Jones vector is based on the Jones matrix and the first Jones vector. In a specific example, the second Jones vector can be adjusted by adjusting an angle of the output polarization element relative to the initial polarization state of the light incident on the liquid crystal layer.

[0050] In an example, light incident on an element of the liquid crystal layer is linearly polarized, and the output polarization element comprises a linear output polarizer. The incident light may be polarized using the linear input polarization element.

[0051] In a specific example where the light is linearly polarized, the liquid crystal layer is a plane switching (IPS) type liquid crystal layer. In such an IPS liquid crystal layer, the liquid crystals remain within a plane defined by the liquid crystal layer and rotate within the plane as different electric fields / voltages are applied. Thus, the liquid crystal layer may include liquid crystals configured to align parallel to the plane of the liquid crystal layer when a first electric field, a second electric field, and a third electric field are applied to the element (i.e., when the element is operated at a first voltage level, a second voltage level, and a third voltage level). Such a liquid crystal layer may be associated with a Jones matrix that is approximately proportional to a real matrix.

[0052] It should be understood that the terms "parallel" and "perpendicular" in relation to liquid crystal alignment should be understood as "approximately parallel" and "approximately perpendicular". Those skilled in the art will appreciate that a typical liquid crystal layer will have liquid crystals that are not completely parallel or perpendicular to the plane defined by the liquid crystal layer. Furthermore, liquid crystals aligned parallel to a plane means that the axis defined by each liquid crystal lies within the plane, rather than being perpendicular to the plane.

[0053] In a particular arrangement, the orientation of the output polarization element is the same as the incident polarization orientation. Thus, the light incident on the liquid crystal layer has a polarization state, and the output polarization element is oriented to transmit substantially all light having a specific polarization state. In a particular example, this allows the use of a reflective SLM, so that the light passes through the polarization element on both the input and output paths. Therefore, the output polarization element can also be used as an input polarization element. Therefore, the liquid crystal layer can form at least a part of a reflective spatial light modulator SLM, and the light is configured to pass through the output polarization element in both the following cases: (i) before being incident on the element of the liquid crystal layer, and (ii) after passing through the element of the liquid crystal layer. Therefore, the light passes through the output polarization element in a first direction before being incident on the element, and passes through the output polarization element in a second direction after passing through the element, wherein the first direction and the second direction are opposite to each other. Because the input and output polarization directions are the same, the output polarization element can be part of a reflective SLM (i.e., because the incident light can pass through the output polarizer without being changed).

[0054] In another example where the light is linearly polarized, the liquid crystal layer is a twisted nematic (TN) type liquid crystal layer. In a TN liquid crystal layer, the liquid crystals are parallel to a plane defined by the liquid crystal layer when zero voltage is applied, and rotate away from the plane as different electric fields / voltages are applied. Thus, the liquid crystal layer may include liquid crystals configured to: (i) align parallel to the plane of the liquid crystal layer when the electric field applied to the element is zero (i.e., operating at a zero voltage level), and (ii) rotate away from the plane (towards perpendicular to the plane) when the electric field applied to the element is non-zero (i.e., operating at a non-zero voltage level).

[0055] In an alternative example, light incident on an element of the liquid crystal layer is circularly polarized and the output polarization element comprises a linear output polarizer.The incident light may be polarized using a circular input polarization element such as a wave plate (often referred to as a polarization modifying element).

[0056] In a specific example where light is circularly polarized, the liquid crystal layer is a vertically aligned (VA) type liquid crystal layer. In such a VA liquid crystal layer, the liquid crystals are perpendicular to a plane defined by the liquid crystal layer when zero voltage is applied, and rotate toward the plane as different electric fields / voltages are applied. Thus, the liquid crystal layer may include liquid crystals configured to: (i) align perpendicular to the plane of the liquid crystal layer when the electric field applied to the element is zero (i.e., operating at a zero voltage level), and (ii) rotate toward the plane when the electric field applied to the element is non-zero (i.e., operating at a non-zero voltage level).

[0057] In another example where the light is circularly polarized, the liquid crystal layer is a twisted nematic (TN) type liquid crystal layer. As previously described, in a TN liquid crystal layer, the liquid crystals are parallel to a plane defined by the liquid crystal layer when zero voltage is applied, and rotate away from the plane as different electric fields / voltages are applied. Thus, the liquid crystal layer may include liquid crystals configured to: (i) align parallel to the plane of the liquid crystal layer when the electric field applied to the element is zero (i.e., operating at a zero voltage level), and (ii) rotate away from the plane (towards perpendicular to the plane) when the electric field applied to the element is non-zero (i.e., operating at a non-zero voltage level). Both VA and TN liquid crystal layers may be associated with a Jones matrix having complex entries.

[0058] In some examples, the display further includes a beam splitter. In a specific example, the display includes a polarizing beam splitter. In this way, the polarizing beam splitter may include an output polarizing element. Therefore, the polarizing beam splitter may be used as an input polarizing element and an output polarizing element. As described above, the SLM may be reflective, and the beam splitter may be useful in a display in which the liquid crystal layer forms at least a portion of the reflective SLM. In some cases, the display further includes a polarization modifying element, such as a waveguide. The light from the polarizing beam splitter passes through the polarization modifying element and is incident on the liquid crystal layer. After passing through the liquid crystal layer and reflecting, the light may pass through the polarization modifying element for a second time from the opposite direction. Similarly, the light is incident on the polarizing beam splitter again. Therefore, the polarization modifying element may modify the polarization state of the light both before the light is incident on the element and after the light passes through the element. Therefore, the polarization modifying element is located between the output polarizing element and the liquid crystal layer along the optical axis / optical path. The polarization modifying element may be selected so that when the second electric field is applied to the element of the liquid crystal layer, the polarization state of the light after passing through the polarization modifying element causes substantially all the light to be removed / blocked by the polarization beam splitter.

[0059] Thus, in an example, the display further comprises: a polarization modifying element arranged between the output polarizing element and the liquid crystal layer, the polarization modifying element being configured to modify the polarization state of the light; and a polarization beam splitter comprising the output polarizing element and being configured to remove substantially all light having the second polarization state p. The liquid crystal layer forms at least a portion of the reflective SLM, and elements of the liquid crystal layer are associated with a Jones matrix J(B), wherein the Jones matrix of the elements is a function of an electric field B applied to the elements. The polarization modifying element is associated with a Jones matrix R, and RJ(B')Rp=p, wherein J(B') is the Jones matrix associated with elements of the liquid crystal layer at the second electric field B'. Thus, the polarization modifying element is selected such that RJ(B')Rp=p.

[0060] Therefore, when the light has a second polarization state, p (the same as the incident polarization state), the light returns along the incident path - the remaining light transmitted to the viewer is the component of the reflected light that is orthogonal to p (which is zero when the light has the second polarization state). By configuring the display in this way so that the outgoing polarization is orthogonal to the incident polarization, a polarizing beam splitter can be used to separate the incident and outgoing paths, thereby achieving higher optical efficiency.

[0061] In a particular case, the polarization modifying element is a wave plate having half the retardation of J(B') and the slow axis is aligned with the fast axis of J(B'). J(B) can be described by pure retardation.

[0062] As described above, after the light is modulated using the liquid crystal layer and the output polarization element, the light can be filtered to improve image quality. Therefore, in some examples, the holographic display also includes: (i) a lens arranged to receive light from the output polarization element and configured to generate a Fourier transform F(H) of the target light field at the focal plane of the lens, and (ii) a spatial filter located at the focal plane and configured to remove the Fourier transform F(H*) of the complex conjugate of the target light field. In some examples, the spatial filter is also configured to remove the zero order of the target light field. As described above, due to the "zero-crossing" amplitude modulation scheme, the zero-order energy is much lower than that of conventional systems. This means that the impact of any imperfect filtering of this zero order is much smaller than that of an SLM modulated in the range [0,1].

[0063] In some examples, the spatial filter is configured to remove the complex conjugate Fourier transform F(H*) of the target light field without filtering out the zero order of the target light field. Therefore, as described above, since the SLM is configured to modulate the amplitude in a range spanning zero, the amount of power contained in the zero order is small and therefore does not require filtering, resulting in higher image quality. The filter can be configured to allow the zero order of the target light field to pass.

[0064] In some examples, the lens has a focal length, and the SLM is spaced apart from the lens by a distance equal to the focal length of the lens. In some examples, the lens is spaced apart from the SLM by a distance greater than or less than the focal length. In this case, additional optical elements further along the optical path may be used to collimate the converging / diverging light. It may be desirable to separate the lens and SLM by focal length to provide a more compact display (because additional optical elements may be omitted).

[0065] In one example, the focal plane of the lens is spaced apart from the lens by a distance equal to the focal length of the lens. Thus, the lens is spaced apart from the filter by a distance equal to the focal length of the lens.

[0066] The filter may physically block a portion of the light received from the lens, such as the portion containing the complex conjugate Fourier transform F(H*) of the target light field.

[0067] According to a second aspect of the invention, there is provided a method for modulating the amplitude of light in a holographic display, comprising: (i) operating an illumination source to emit at least partially coherent light to an element of a liquid crystal layer, (ii) applying a first electric field to the element so that light from the element has a first polarization state, (iii) applying a second electric field to the element so that light from the element has a second polarization state, (iv) applying a third electric field to the element so that light from the element has a third polarization state, (v) using an output polarization element, removing a portion of the light having the first polarization state and transmitting a portion of the light having the first polarization state, (vi) using an output polarization element, removing substantially all of the light having the second polarization state, and (vii) using an output polarization element, removing a portion of the light having the third polarization state and transmitting a portion of the light having the third polarization state, wherein one of the following: (1) the second electric field is greater than the first electric field, and the third electric field is greater than the second electric field, and (2) the second electric field is lower than the first electric field, and the third electric field is lower than the second electric field.

[0068] In some examples, applying a first electric field across the element includes operating the element at a first voltage level, applying a second electric field across the element includes operating the element at a second voltage level, and applying a third electric field across the element includes operating the element at a third voltage level.

[0069] In some examples, the method further includes: (i) determining a Jones matrix of the liquid crystal layer, and (ii) determining the angle based on the Jones matrix and an initial polarization of the light.

[0070] In some examples, the method further includes: (i) passing light representing a target light field H of the hologram through a lens to generate a Fourier transform F(H) of the target light field at a focal plane of the lens, and (ii) passing the light through a spatial filter located at the focal plane to remove a complex conjugate Fourier transform F(H*) of the target light field. In some examples, passing the light through the spatial filter includes filtering a zero order of the target light field. In other examples, passing the light through the spatial filter includes passing the light through the spatial filter without filtering the zero order of the target light field.

[0071] Further features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 is a schematic diagram of an example holographic display according to an embodiment;

[0073] Figure 2 yes Figure 1 Schematic diagram of a holographic display;

[0074] Figure 3 is an Argand diagram showing the range of phase and amplitude modulation of a holographic display using the first embodiment;

[0075] Figure 4 is a schematic diagram of another holographic display according to an example;

[0076] Figure 5 is an Argand diagram showing the range of phase and amplitude modulation of a holographic display using the second embodiment; and

[0077] Figure 6 is a schematic diagram of a holographic display including a lens and a filter. DETAILED DESCRIPTION

[0078] Figure 1 and Figure 2 A system 100 is depicted that forms at least a portion of a holographic display according to an example. The system 100 includes an illumination source 102 configured to emit at least partially coherent light. In this example, the illumination source 102 is a single light emitter that emits light toward a liquid crystal layer 104 of an SLM 106. In other examples, the illumination source 102 includes multiple light emitters that collectively illuminate the liquid crystal layer 104. Examples using multiple light emitters may also have the ability to control the light emitters individually or by region, thereby enabling reduced power consumption and / or improved contrast. In this example, the illumination source 102 is controlled using time division multiplexing to emit red light, green light, or blue light. Other examples may use an illumination source 102 that emits light having a single wavelength or light having a wavelength within a single wavelength range.

[0079] Light emitted by the illumination source 102 propagates at least from the illumination source 102 to the liquid crystal layer 104. The light follows an optical path along the optical axis 116 from the illumination source 102 toward the eye of an observer viewing the holographic display.

[0080] The liquid crystal layer 104 includes an array of pixels / elements 104a. A voltage / bias may be applied individually to each element 104a to control the electric field applied to the element 104a. The presence of the electric field causes the liquid crystal to rotate and thus may change the polarization state of light as it passes through the element 104a.

[0081] The SLM 106 of this example also includes an output polarization element 110 located after the liquid crystal layer 104 along the optical path.

[0082] Although in this example, the SLM 106 includes the liquid crystal layer 104 and the output polarization element 110, in other examples, the SLM 106 may not include the output polarization element 110. Then, the output polarization element 110 may be separated from the SLM 106.

[0083] Thus, light emitted by illumination source 102 propagates through liquid crystal layer 104 and along an optical path incident on output polarization element 110. Depending on the polarization of the light when it reaches output polarization element 110 (which may be affected by the voltage applied to a particular element 104a), the light may be blocked or may partially pass through output polarization element 110.

[0084] exist Figure 1 and Figure 2 In the example, the output polarization element 110 is shown as a transmissive polarizer, wherein the polarizer itself can absorb at least a portion of the light incident thereon (and / or allow at least a portion of the light to pass through). It should be understood that the same concept also applies to other types of polarizers. In some examples, the output polarization element 110 includes a polarization element and an absorption element, wherein the absorption element absorbs light reflected from the polarization element and / or light transmitted through the polarization element. For example, the polarization element can be a reflective polarizer, which splits the light into two separate beams and directs the "unwanted" light to the absorption element. Regardless of the type, the output polarization element 110 is configured to: (i) remove a portion of the light having a specific polarization state (e.g., the first or third polarization state) and transmit a portion of the light having a specific polarization state, or (ii) remove substantially all light having a different polarization state (e.g., the second polarization state). In this example, the output polarization element 110 is a linear polarizer. The output polarization element 110 is located after the liquid crystal layer 104 along the optical path of the light.

[0085] Similarly, in Figure 1 and Figure 2, the SLM 106 is shown as being transmissive, such that light enters one side of the layer (e.g., the front side) and exits the other side of the layer (e.g., the back side). Again, it should be understood that the same concepts apply to other types of SLMs. For example, the SLM can be a reflective liquid crystal layer, such that light enters and exits from the same side it entered (e.g., the front side). As previously discussed, in the case where the SLM is reflective, the output polarization element 110 is generally not considered to be part of the SLM. Examples of reflective SLMs will be discussed later, such as Figure 4 shown.

[0086] Thus, the output polarization element 110, together with the liquid crystal layer 104, modulates the amplitude of light from the liquid crystal layer 104. For example, if light linearly polarized along the vertical axis 112 propagates in a direction along the optical axis 116 (and therefore has a particular Jones vector) and is incident on a vertically oriented output polarization element (also having a particular Jones vector), the light transmitted by the output polarization element 110 may have substantially 100% of its initial intensity (i.e., the amplitude of the transmitted light is unchanged and / or is at a maximum). Such a vertically oriented output polarization element can be said to be aligned with the linearly polarized light and therefore arranged at an angle of 0 radians relative to the linearly polarized light incident on the output polarization element. Conversely, if light is linearly polarized along the horizontal axis 114 and is incident on the vertically oriented output polarization element, the light may have substantially 0% of its initial intensity after interacting with the output polarization element (i.e., the amplitude of the transmitted light is minimal or zero). Such a vertically oriented output polarization element can be said to be "crossed" with the horizontally polarized light and therefore arranged at an angle of π / 2 radians relative to the linearly polarized light incident on the output polarization element. When substantially all light incident on an output polarization element is blocked / removed by the output polarization element, it can be said that the output polarization element removes substantially all light having a particular polarization state.

[0087] In another example, if a light ray linearly polarized along the vertical axis 112 propagates in a direction along the optical axis 116 and is incident on an output polarizing element disposed at an angle of π / 4 radians relative to the vertical axis 112, the light ray may have approximately 50% (i.e., 0.5) of its initial intensity after interacting with the output polarizing element (i.e., the amplitude of the transmitted light ray is ). Thus, the output polarization element of this orientation is arranged at an angle of π / 4 radians relative to the linearly polarized light incident on the output polarization element. When a portion (e.g., 50% of the light intensity) is transmitted by the output polarizer, it can be said that the output polarization element removes a portion of the light having a specific polarization state and also transmits a portion of the light having a specific polarization state. The removed portion can be absorbed by the polarization element or the absorption element, for example.

[0088] In some examples, system 100 includes an input polarization element 108 disposed between illumination source 102 and liquid crystal layer 104. Input polarization element 108 can polarize light so that the polarized light is incident on liquid crystal layer 104. In some examples, input polarization element 108 can be omitted if the light emitted by illumination source 102 is already polarized in a desired manner. In some examples, input polarization element 108 is a linear polarizer (e.g., Figure 1 ), but in other examples, the input polarization element 108 is a different type of polarizer, such as a circular polarizer.

[0089] Figure 2 A controller 120 is shown which can control the electric field / voltage applied to each element of the liquid crystal layer 104 to fully control the amplitude (and possibly phase) modulation of the light output by the system 100. The controller 120 can also control the operation of the illumination source 102.

[0090] Figure 1 and Figure 2 A linear arrangement of holographic displays is also depicted, but other arrangements may include image folding components. For example, a folded optical path may be provided.

[0091] As discussed above, the inventors have appreciated that an optical system may be configured such that when element 104a is controlled at a particular voltage level, the amplitude of the light may be modulated to "cross zero" as the voltage increases or decreases.

[0092] To illustrate this more clearly, several implementations will be discussed in turn.

[0093] In-plane switching (IPS) type liquid crystal layer

[0094] In the first embodiment, the liquid crystal layer 104 is an in-plane switching (IPS) type liquid crystal layer 104 . Figure 3 An example Argand diagram / phasor diagram of an example system including an IPS-type liquid crystal layer 104 is shown. The phasor diagram represents the amplitude and phase of the light after interaction with the output polarizing element 110. The amplitude of the light after interaction with the output polarizing element 110 is given by the magnitude of the value on the real axis (Re). The sign of the amplitude corresponds to the sign of the projection of the real part of the complex number on the real axis. The phase of the light is given by the azimuth between the positive real axis and the position of a point on the diagram. By controlling the voltage applied to the element 104a of the liquid crystal layer 104, the position of the point on the phasor diagram can be adjusted.

[0095] As mentioned above, Figure 3The phasor diagram of represents an example system including an IPS type liquid crystal layer 104, a specific polarization of light incident on layer 104, and a specific orientation of output polarization element 110. In this case, the light incident on liquid crystal layer 104 is linearly polarized, and output polarization element 110 includes a linear polarizer.

[0096] The IPS-type liquid crystal layer 104 can be characterized as having a Jones matrix that contains only real terms, or having a Jones matrix that is approximately proportional to the real matrix. More generally, the IPS liquid crystal layer 104 includes liquid crystal molecules that: (i) align parallel to the layer / display (in-plane), and (ii) reorient (due to an applied voltage) while remaining substantially parallel to the layer. For example, in the absence of an applied voltage, the liquid crystal can be aligned parallel to the plane of the layer 104, and in the presence of an applied voltage, the liquid crystal remains parallel to the plane of the layer 104, but is in a different corresponding orientation about an axis perpendicular to the plane of the layer. Reference Figure 1 , the longitudinal axis of the liquid crystal can be aligned in the plane defined by the axes 112 and 114 and thus rotated about the optical axis 116. Therefore, the optical axis 116 is perpendicular to the plane defined by the liquid crystal layer 104.

[0097] In one specific example, the liquid crystal layer 104 has a Jones matrix given by a rotation matrix approximately proportional to the following matrix:

[0098]

[0099] Among them, the angle is the fast axis orientation angle measured relative to the orientation in the quiescent state and is a function of the voltage applied to a particular element of the liquid crystal layer 104. In this particular example, In the range [0,π / 4]. This particular Jones matrix describes the angle at which a half-wave plate is oriented from its rest position. Rotation model. For example, linearly polarized light with an angle of θ (measured from the fast axis orientation of the liquid crystal in a static state) will be transformed into linearly polarized light with an angle of 2π-θ, so if θ is set to 0, the liquid crystal layer 104 can therefore rotate the polarization of the light in the range of [0,π / 2], where the rotation angle of the polarized light is twice the fast axis orientation angle.

[0100] It should be understood that the Jones matrix representation described above (as well as any other example representations discussed herein) is an approximate model used to inform candidate polarizer configurations, rather than an exact description of a real liquid crystal layer. These examples have been found to be sufficiently accurate models for the purposes described, but are idealized representations.

[0101] In a similar manner, the polarization state of light can be represented by a Jones vector, and the specific orientation / configuration of the output polarization element can be represented by a Jones vector. Thus, light incident on the liquid crystal layer 104 can have a first Jones vector, and the output polarization element 110 can have a second Jones vector.

[0102] It can be seen that, for linearly polarized light (incident on the IPS liquid crystal layer 104), applying a voltage to a specific element 104a of the liquid crystal layer 104 results in a pure rotation of the polarization state of the light. For example, the incident light may be linearly polarized by the linear input polarization element 108 .

[0103] In this example system, the output polarization element 110 is arranged at a specific angle 118 relative to the initial polarization of the light so that as the voltage applied to the element 104a increases, the amplitude of the light passes through zero. For any particular type of liquid crystal layer 104, this can be achieved by selecting an appropriate angle 118 and initial polarization. The azimuth angle 118 of the output polarization element 110 can be defined as an angle extending around the optical axis 116 (relative to a reference point, such as the vertical axis 112). The inherent characteristics of the output polarization element 110 and the angle 118 are represented by the Jones vector of the output polarization element 110.

[0104] At a first point in time, a first voltage level is applied to element 104a. In this example, the first voltage level is zero (or 0V). Therefore, element 104a is in a static state because no electric field is applied to the liquid crystal in element 104a. In this case, when the element is in a static state, The light passing through the element has a first polarization state (which may remain unchanged from its initial polarization state). As described above, the linearly polarized light, voltage level, and angle 118 relative to the initial polarization are such that the intensity of the light is attenuated by the output polarization element 110 by approximately 50% (thus, a portion of the light is removed and a portion of the light is transmitted). This can be achieved by orienting the polarizer 110 at an angle of π / 4 radians relative to the polarization state of the linearly polarized light passing through the element 104a. More generally, the output polarizer 110 can be arranged at an angle between π / 8 radians and 3π / 8 radians (although this will affect the intensity of the attenuated light).

[0105] Due to the initial polarization (characterized by the first Jones vector) and the type of liquid crystal layer 104 (characterized by the Jones matrix), it can be seen that at such a voltage level, the light does not experience phase modulation. Therefore, the light is represented by point 202 on the phasor diagram, where the light has 50% of its initial incident intensity ( like Figure 3 shown) and has no phase modulation.

[0106] Thereafter, if the voltage is increased from the first voltage level (0V in this example) to a second (higher, non-zero) voltage level, the linearly polarized light is rotated within element 104a. Thus, the light passing through element 104a has a second polarization state that is different from the state when element 104a is operated at the first voltage level. In this case, radians. This rotation increases the angle between the output polarization element 110 and the light. At this second voltage level, the angle between the output polarization element 110 and the light may be approximately π / 2 radians, such that light having the second polarization state is blocked / absorbed by the output polarization element 110 and thus has an amplitude of approximately zero. For example, at the second voltage, the light passing through element 104a may be rotated by π / 4 radians relative to the light at rest and thus perpendicular to the polarizer 110, such that the light has an angle of π / 2 radians between it and the polarizer 110. Thus, the light is represented on the phasor diagram by point 204, where the light has approximately 0% of its initial intensity and amplitude (represented by Figure 3 The position of the origin of the Argand diagram is shown in Figure 1).

[0107] Thereafter, if the applied voltage continues to increase from the second voltage level to a third (higher, non-zero) voltage level (possibly a maximum voltage), the light is further rotated and has a third polarization state that is different from the second polarization state. In this case, radians. This rotation further increases the angle between the output polarization element 110 and the light. At this third voltage, the angle between the output polarization element 110 and the light may be approximately 3π / 4 radians, so that the intensity of the output light is again approximately 50% of its maximum intensity (thus, a portion of the light is removed and a portion of the light is transmitted). For example, at the third voltage level, the light passing through element 104a may be rotated by π / 4 radians relative to the light at the second voltage level, and therefore rotated by π / 2 radians relative to the light at rest. Since the degree of rotation exceeds π / 2 radians, the light is represented in a different quadrant on the phasor diagram. This is actually represented as light with a "negative" amplitude. Therefore, the light is represented by point 206 on the phasor diagram, where the light has approximately 50% of its maximum intensity (such as Figure 3 As shown, the amplitude is ) and π radians of effective phase modulation.

[0108] therefore, Figure 3It is shown how the amplitude of light can be modulated through zero by arranging the output polarization element 110 at a particular angle 118 so that when the voltage applied to the element increases from a first voltage level through a second voltage level to a third voltage level, the intensity decreases from a particular level to zero and then increases again. Zero amplitude is achieved at this second intermediate voltage level. As described above, this is achieved by aligning the linear input polarization state with the static orientation of the liquid crystal (i.e., the first Jones vector can be (1,0)) and aligning the linear output polarization element at π / 4 radians from the input polarization (i.e., the second Jones vector can be (1,-1;-1,1)).

[0109] In summary, in this embodiment, light incident on the liquid crystal layer has linear polarization (in any orientation), and the linear output polarizer is at an intermediate value. at (for example, in ) is orthogonal to the light from the liquid crystal layer. This will be based on Modulate the output amplitude to achieve A specific example has an output polarizer that matches the input polarization state, meaning that the polarizer can be part of a reflective SLM and light passes through both on the input and output paths. For example, when , the input orientation of (cos(-π / 8), sin(-π / 8)) will be transformed into (cos(3π / 8), sin(3π / 8)), which is orthogonal to the output polarizer passing through (cos(-π / 8), sin(-π / 8)).

[0110] It should be understood that the same effect can also be achieved when the voltage is reduced from the first voltage level to the third voltage level. For example, the first voltage level may be a maximum voltage level, and it is reduced to a third zero voltage level via the second intermediate voltage level.

[0111] In a variation of the above embodiment, for a custom IPS type liquid crystal layer, In the range [0,π / 2]. This can be achieved by driving the IPS-type liquid crystal layer with a larger electric field to achieve more rotation. In this variation, the light incident on the liquid crystal layer has linear polarization (in any orientation), and the linear output polarizer is at an intermediate value at (for example, in ) is orthogonal to the light from the liquid crystal layer. This will be based on Modulate the output amplitude to achieve pure amplitude modulation in the range [-1, +1]. Figure 3The Argand plot has an amplitude of +1 at point 202, an amplitude of 0 at point 204, and an amplitude of -1 at point 206. Another example has an output polarizer that matches the input polarization state, meaning that the polarizer can be part of a reflective SLM and light passes through on both the input and output paths. For example, when When , the input orientation of (1,0) will be transformed to (0,1), which is orthogonal to the output polarizer passing through (1,0).

[0112] Twisted nematic (TN) liquid crystal layer

[0113] Figure 3 Also represented is an example system 100 in which the liquid crystal layer 104 is a TN-type liquid crystal layer 104. As in the first embodiment discussed with an IPS-type liquid crystal layer, in this second embodiment, the light incident on the liquid crystal layer 104 is linearly polarized (e.g., with a Jones vector (1,0)), and the output polarization element 110 is arranged at an angle of π / 4 radians relative to the reference axis 112.

[0114] The TN-type liquid crystal layer 104 can be characterized as having liquid crystal molecules that are (i) aligned parallel to the plane defined by the liquid crystal layer 104 in the absence of an applied electric field / voltage (when the electric field / voltage is zero), and (ii) reoriented away from the plane (due to the application of a non-zero electric field / voltage). Reorientation away from the plane can be referred to as rotation to be perpendicular to the plane.

[0115] In one specific example, the liquid crystal layer 104 has a Jones matrix given by a rotation matrix approximately proportional to the following matrix:

[0116]

[0117] For the IPS type liquid crystal layer changes discussed above, In the range of [0,π / 2], when the elements of the liquid crystal layer are in a stationary state, And when the electric field applied to the element is at its maximum, In this embodiment, the light incident on the liquid crystal layer has a linear polarization (in any orientation), and the linear output polarizer is arranged at π / 4 degrees relative to the input polarization. This will be based on Modulate the output amplitude to achieve Pure amplitude modulation within the range. Therefore, the reference Figure 3 The Argand plot of point 202 is The amplitude at point 204 is 0, and the amplitude at point 206 is

[0118] Reflective SLM

[0119] Figure 4 A schematic diagram of another system 150 that forms at least a portion of a holographic display is depicted according to an example. System 150 includes an illumination source 152 configured to emit at least partially coherent light. Illumination source 152 can correspond to illumination source 102 discussed previously. Light rays emitted by illumination source 152 propagate from at least illumination source 152 to liquid crystal layer 154. The light rays follow a nonlinear optical path from illumination source 152 toward an eye of an observer viewing the holographic display.

[0120] The liquid crystal layer 154 includes an array of pixels / elements. Figure 1 and Figure 2 In contrast to the example of FIG. 1 , the liquid crystal layer 154 is reflective, so that light passes through the liquid crystal layer 154 for the first time, is reflected, and then passes through the liquid crystal layer 154 for the second time. As described above, a voltage / bias can be applied to each element individually to control the electric field applied to the element. The presence of the electric field causes the liquid crystal to rotate and, therefore, may change the polarization state of the light as it passes through the element.

[0121] The system of this example also includes a polarizing beam splitter 160, which splits the light beam from the illumination source 152 into a transmitted light beam and a reflected light beam, which is reflected toward the liquid crystal layer 154 and then incident on the liquid crystal layer. The transmitted light beam may be absorbed elsewhere in the system 150. After passing through the liquid crystal layer 154, the light is incident on the polarizing beam splitter 160 and is again split into a transmitted light beam and a reflected light beam, which is reflected back to the illumination source 152, while the transmitted light beam can ultimately be seen by an observer of the holographic display. In addition to splitting the light beam, the polarizing beam splitter 160 can also polarize the light incident thereon. In this example, the polarizing beam splitter 160 is a linear polarizer.

[0122] The system of this example also includes a wave plate 156. In this example, the wave plate 156 is a circular polarizer / elliptical polarizer, so that circularly polarized light or elliptically polarized light is incident on the liquid crystal layer 154, and then, the circularly polarized light or elliptically polarized light is incident on the polarization beam splitter 160 for a second time after passing through the liquid crystal layer 154 and the wave plate 156.

[0123] Figure 4 Also shown is a controller 158 which can control the electric field / voltage applied to each element of the liquid crystal layer 154 to fully control the amplitude (and possibly phase) modulation of the light output by the system 150. The controller 158 can also control the operation of the illumination source 152.

[0124] In another example, the system does not have a polarizing beam splitter, but rather includes a non-polarizing beam splitter, an input polarizer, and an output polarizer, where both polarizers are circular polarizers / elliptical polarizers. Light propagates from an illumination source, through the input polarizer, through the beam splitter, through the liquid crystal layer, through the beam splitter, and then through the output polarizer.

[0125] The use of a polarizing beam splitter provides a benefit by reducing efficiency losses compared to a non-polarizing beam splitter. In both examples, the SLM includes a liquid crystal layer and the polarizer is separate from the SLM.

[0126] Mathematically, the SLM (i.e., the reflective liquid crystal layer 154) can be described by a Jones matrix J(B), where B is the electric field applied to a particular element of the liquid crystal layer 154, and J is a function of B. The wave plate 156 can have a Jones matrix R, and the polarizing beam splitter 160 can be described as removing polarization p from the output.

[0127] R can be chosen so that for some intermediate value of B (labeled B'), RJ(B')Rp=p (by operating the elements of the liquid crystal layer 154 under the second / intermediate electric field). This can be achieved by setting RJ(B')R=I, where I is the unit matrix. This means that the polarization of the wave plate cancels the polarization effect of the SLM at B'. Therefore, when the element is operated at B', the light incident on the polarization beam splitter 160 (after passing through the element) has the same polarization state as when it was originally incident on the polarization beam splitter 160 (i.e., before it was incident on the element). This means that if light is reflected by polarizing beam splitter 160 toward the element on its first pass, it is reflected again on its second pass (i.e., it is reflected back toward where it originally came from (i.e., back toward the illumination source). This means that the viewer cannot see any of the light because the light is "removed" by polarizing beam splitter 160. Instead, to be seen by the viewer, the light needs to pass through the beam splitter (i.e., be transmitted by polarizing beam splitter 160) in a direction other than that from which it originally came. This is achieved by operating the liquid crystal layer at other values ​​of B, which changes the polarization state of the light, thereby allowing at least some of the light to be routed to the output (and seen by the viewer).

[0128] As previously described, for a reflective liquid crystal layer 154, the Jones matrix J of the liquid crystal layer 154 describes a double pass through the SLM. In this example, the light experiences a pure delay when passing through the SLM (it can be seen that a double pass in the opposite direction would cause the rotation introduced by the liquid crystal layer to cancel, leaving only a delay).

[0129] In one example, R is a wave plate having half the retardation of J(B') and having its slow axis aligned with the fast axis of J(B').

[0130] Thus, light emitted by illumination source 152 propagates through liquid crystal layer 154 and along an optical path incident on polarizing beam splitter 160. Depending on the polarization of the light when it reaches polarizing beam splitter 160 (which may be affected by the voltage applied to a particular element), the light may be blocked (when B=B') or may partially pass through polarizing beam splitter 160.

[0131] Vertically aligned (VA) liquid crystal layer

[0132] Figure 5 An example Argand / phasor diagram is shown for an example system 150 in which the liquid crystal layer 154 is a VA type liquid crystal layer 154. In this second embodiment, the light incident on the liquid crystal layer 154 is circularly polarized (due to the wave plate 156), and the polarizing beam splitter 160 comprises a linear polarizer.

[0133] The VA-type liquid crystal layer 154 is typically present in a liquid crystal on silicon (LCoS) SLM. The VA-type liquid crystal layer 154 can be characterized as having liquid crystal molecules that are (i) aligned perpendicular to the plane defined by the liquid crystal layer 154 (i.e., vertically aligned), and (ii) reoriented toward the plane (due to an applied voltage). For example, in the absence of an applied voltage, the liquid crystal can be aligned in a vertical arrangement perpendicular to the plane of the layer 154, and in the presence of an applied voltage, the liquid crystal can be rotated toward the plane. This results in the Jones matrix of the VA liquid crystal layer 154 having complex entries.

[0134] In one specific example, the liquid crystal layer 154 has a Jones matrix given by a rotation matrix proportional to the following matrix:

[0135]

[0136] in is a function of the voltage applied to a particular element of the liquid crystal layer 154. Thus, light polarized along a given axis undergoes radians phase delay, and The magnitude of is controlled by the magnitude of the applied electric field. Here, the polarization orientation that is not retarded when the electric field is applied is referred to as the 'fast axis', and the polarization orientation that is retarded is referred to as the 'slow axis'. In this example, the fast and slow axes are aligned with axes 112 and 114, but in general the fast and slow axes do not need to be aligned with the x-axis and y-axis of the display. In this particular example, In the range [0,π].

[0137] Thus, it can be seen that for circularly polarized incident light, such as right-handed circularly polarized light (i.e., proportional to (1; -i)), and a linear output polarization element 160 arranged at π / 4 degrees relative to the fast axis of the liquid crystal layer, applying a voltage to a particular element of the liquid crystal layer 154 will cause amplitude modulation as well as phase modulation, such as Figure 5 The phase modulation depends on the amplitude, which gives Figure 5 The relationship shown.

[0138] As described in the first embodiment, the output polarization element 160 is arranged in a particular configuration (and therefore has a particular Jones vector) such that the amplitude of the light passes through zero as the voltage applied to the element increases (or decreases).

[0139] At a first point in time, a first voltage level is applied to the element. In this example, the first voltage level is zero (or 0V). Therefore, the element is in a quiescent state because no electric field is applied to the liquid crystal in the element. When the element is in its quiescent state, It can be seen that at this voltage level, the light interacts with the output polarization element of the polarization beam splitter 160 and has a specific non-zero amplitude (i.e., a percentage of the maximum amplitude in the absence of the output polarization element) and a specific phase modulation. Therefore, the light is represented by point 302 on the phasor diagram, where the light has a specific percentage of its maximum intensity and a phase modulation.

[0140] Thereafter, if the voltage is increased from the first voltage level (0V in this example) to a second (higher, non-zero) voltage level, the polarized light undergoes a rotation within the element. This rotation adjusts the polarization state, and when the light interacts with the output polarization element of the polarization beam splitter 160, the amplitude of the light is approximately zero and is therefore blocked by the output polarization element. For example, at this second voltage level, radians, right-handed circularly polarized light (denoted as (1; -i)) is converted to linearly polarized light (denoted as (1; 1), i.e., +π / 4 radians), and the angle of the output polarization element may be set to -π / 4 radians. Thus, at this second voltage level, the angle between the polarization state of the light and the output polarization element is π / 2 radians, and thus the light is blocked by the output polarization element (i.e., the amplitude is 0). Thus, the light is represented by point 304 on the phasor diagram, where the light has approximately 0% of its maximum intensity.

[0141] Thereafter, if the applied voltage continues to increase from the second voltage level to a third (higher, non-zero) voltage level (possibly the maximum voltage), the light will rotate further. For example, at this third voltage level, radians. This rotation adjusts the polarization state, and when the light interacts with the output polarization element of polarizing beam splitter 160, the transmitted light has a non-zero amplitude (i.e., a percentage of the maximum amplitude in the absence of the output polarization element) and a specific phase modulation. Due to the degree of rotation, the light is represented in a different quadrant on the phasor diagram. This is actually represented as light with "negative" amplitude. Therefore, the light is represented by point 306 on the phasor diagram, where the light has a percentage of its maximum intensity and a specific phase modulation.

[0142] therefore, Figure 5It is shown how the amplitude of light can be modulated through zero by arranging an output polarization element with a specific Jones vector so that when the voltage applied to the element increases from a first voltage level via a second voltage level to a third voltage level, the amplitude decreases from a specific level to zero and then increases again. Zero amplitude is achieved at this second intermediate voltage level. It should be understood that the same effect can also be achieved when the voltage is reduced from the first voltage level to the third voltage level. For example, the first voltage level can be a maximum voltage level, and it is reduced to the third zero voltage level via a second intermediate voltage level. The overall modulation scheme adopted by the example system is given by (1+ie iφ ) / 2 is given.

[0143] The commercially available LCoS SLM is Sony TM SXRD241A, which has a VA mode. A simplified model of the VA liquid crystal layer suggests circular input polarization and linear output polarization. It follows that to achieve zero-crossing operation, the SLM is illuminated by circularly polarized light (e.g., via wave plate 156), and then the orientation of the linear output polarizer (e.g., polarizing beam splitter 160) is set so that the modulation amplitude at zero voltage is approximately equal to the modulation amplitude at maximum voltage and passes through zero at intermediate voltages. For this particular Sony TM For LCoSSLM, the output polarizer is oriented approximately 25 degrees to the y-axis of the SLM (ie, axis 112).

[0144] "Low twist" twisted nematic (TN) liquid crystal layer

[0145] Figure 5 Also represented is an example system 150 in which the liquid crystal layer 154 is a "low twist" TN-type liquid crystal layer 154. Some TN-type liquid crystal layer examples (e.g., a dual-pass reflective TN layer) can be described as purely retarded, with the twist of the TN having little effect on it. Such TN-type liquid crystal layers can be referred to as "low twist" TN-type liquid crystal layers (relative to more typical TN liquid crystal layers, such as the liquid crystal layers described previously). As with the second embodiment discussing the VA-type liquid crystal layer, in this third embodiment, the light incident on the liquid crystal layer 154 is circularly polarized (due to the wave plate 156), and the polarization beam splitter 160 includes a linear polarizer.

[0146] The low twist TN liquid crystal layer 154 can be characterized as having liquid crystal molecules that are (i) aligned parallel to the plane defined by the liquid crystal layer 104 in the absence of an applied electric field / voltage (when the electric field / voltage is zero), and (ii) reoriented / rotated away from the plane (due to the application of a non-zero electric field / voltage). Reorienting / rotating away from the plane can be referred to as rotating to be perpendicular to the plane. In a specific example, the liquid crystal layer 154 has a Jones matrix given by a rotation matrix that is the same as the Jones matrix for the VA liquid crystal layer provided above. For the VA liquid crystal layer, is in the range of [0,π], but unlike the VA type liquid crystal layer, when the elements of the liquid crystal layer are in a stationary state, And when the electric field applied to the element is at its maximum, Therefore, the behavior of the TN-type liquid crystal layer 154 is similar to that of the VA-type liquid crystal layer, and the overall modulation scheme applied by the exemplary system is given by (1+ie iφ ) / 2 is given.

[0147] Filtering

[0148] Figure 6 At least a portion of a holographic display 400 according to an example is depicted. The holographic display 400 includes Figure 1 and Figure 2 System 100 or Figure 4 150. In addition to the components of systems 100, 150, display 400 also includes a lens 402. Lens 402 has a focal length f, and in this particular example, it is located one focal length away from the SLM of system 100, 150. In other examples, the distance between lens 402 and the SLM may be different. Display 400 also includes a filter 404 that defines an aperture, such as a spatial filter. Filter 404 is located one focal length away from lens 402, on the side of lens 402 opposite the SLM. As shown, the SLM, lens 402, and filter 404 are coaxial along optical axis 116. Other configurations may also be used, such as a folded optical path that can achieve a more compact display.

[0149] The SLM is configured to generate a light field that is a quantized representation of a target light field H, as described above. The arrangement of the holographic optical display 400 is such that the Fourier transform F(H) of the light field is formed on a plane that coincides with the position of the filter 404. This plane is the Fourier plane of the SLM imaged by the lens 402. The target light field is determined so that the Fourier transform F(H) of the target light field does not overlap with the complex conjugate Fourier transform F(H*) of the target light field at least in the Fourier plane of the SLM. In addition, the aperture in the filter 404 is positioned so that portions of the target light field are blocked. For example, the filter is arranged to remove the complex conjugate Fourier transform F(H*) of the target light field. This can effectively reduce the impact of quantization noise. As described above, due to Figures 1 to 5 For the specific modulation schemes discussed in , the filter does not necessarily need to filter the zeroth order of the target light field. However, as mentioned above, in some examples, the zeroth order may still be filtered, but because the zeroth order has lower energy, it is easier to filter than in traditional systems. Therefore, the zeroth order can still be filtered to remove any residual zeroth order.

[0150] The above embodiments are to be understood as illustrative examples of the present invention. Additional embodiments of the present invention are contemplated. It should be understood that any feature described with respect to any embodiment may be used alone or in combination with other features described, and may also be used in combination with one or more features of any other embodiment in the embodiment or any combination of any other embodiment in the embodiment. In addition, equivalents and modifications not described above may also be adopted without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A holographic display, comprising: an illumination source configured to emit at least partially coherent light; a liquid crystal layer comprising an array of elements arranged to be illuminated by the light, and configured such that: When a first electric field is applied to an element of the liquid crystal layer, the light from the element has a first polarization state; When a second electric field is applied to the element of the liquid crystal layer, the light from the element has a second polarization state; and When a third electric field is applied to the element of the liquid crystal layer, the light from the element has a third polarization state; a controller configured to control an electric field applied to the element of the liquid crystal layer; as well as An output polarization element, the output polarization element being configured as: removing a portion of the light having the first polarization state and also transmitting a portion of the light having the first polarization state; removing substantially all light having the second polarization state; and removing a portion of the light having the third polarization state and also transmitting a portion of the light having the third polarization state; Among them, one of the following: The second electric field is greater than the first electric field, and the third electric field is greater than the second electric field; and The second electric field is lower than the first electric field, and the third electric field is lower than the second electric field.

2. The holographic display according to claim 1, wherein: The controller is configured to control an electric field applied to the element of the liquid crystal layer by controlling a voltage applied to the element of the liquid crystal layer; and The liquid crystal layer is configured such that: The first electric field is applied to the element when the element is operated at a first voltage level; the second electric field is applied to the element when the element is operated at a second voltage level; and When the element operates at a third voltage level, the third electric field is applied to the element.

3. The holographic display according to claim 2, wherein: When the second electric field is greater than the first electric field, and the third electric field is greater than the second electric field: The first voltage level is zero voltage; The second voltage level is a non-zero voltage; and The third voltage level is a non-zero voltage; and When the second electric field is lower than the first electric field, and the third electric field is lower than the second electric field: The first voltage level is a non-zero voltage; The second voltage level is a non-zero voltage; and The third voltage level is zero voltage.

4. A holographic display according to any preceding claim, wherein: Light incident on the element of the liquid crystal layer is linearly polarized, and wherein the output polarization element comprises a linear output polarizer.

5. The holographic display according to claim 4, wherein: The liquid crystal layer includes liquid crystals configured to be aligned parallel to a plane of the liquid crystal layer when the first electric field, the second electric field, and the third electric field are applied to the element.

6. The holographic display according to claim 4 or 5, wherein: The light incident on the liquid crystal layer has a polarization state, and the output polarization element is oriented to transmit substantially all light having a particular polarization state.

7. The holographic display according to claim 6, wherein: The liquid crystal layer forms at least part of a reflective spatial light modulator (SLM), and the light is configured to pass through the output polarizing element in both the following cases: before incident on the element of the liquid crystal layer; and After passing through the element of the liquid crystal layer.

8. The holographic display according to claim 4, wherein: The liquid crystal layer includes liquid crystals configured to be aligned parallel to a plane of the liquid crystal layer when the electric field applied to the element is zero, and to rotate away from the plane when a non-zero electric field is applied.

9. The holographic display according to any one of claims 1 to 3, wherein: Light incident on the element of the liquid crystal layer is circularly polarized, and wherein the output polarizing element comprises a linear output polarizer.

10. The holographic display according to claim 9, wherein: The liquid crystal layer includes liquid crystals, and the liquid crystals are configured as follows: is aligned perpendicular to the plane of the liquid crystal layer when the electric field applied to the element is zero; and Rotation towards the plane occurs when the electric field applied to the element is non-zero.

11. The holographic display according to claim 9, wherein: The liquid crystal layer includes liquid crystals configured to be aligned parallel to a plane of the liquid crystal layer when the electric field applied to the element is zero, and to rotate away from the plane when a non-zero electric field is applied.

12. A holographic display according to any preceding claim, wherein: light incident on said element of said liquid crystal layer has an initial polarization associated with a first Jones vector; The output polarization element is associated with a second Jones vector; The elements of the liquid crystal layer are associated with a Jones matrix, wherein the Jones matrix of the elements is a function of the electric field applied to the elements; and The second Jones vector is based on the Jones matrix and the first Jones vector.

13. The holographic display according to any one of claims 1 to 11, wherein: The display further comprises: a polarization modifying element disposed between the output polarizing element and the liquid crystal layer and configured to modify the polarization state of light; and a polarizing beam splitter comprising the output polarizing element and configured to remove substantially all light having the second polarization state p; and in: The liquid crystal layer forms at least a part of a reflective SLM; said elements of said liquid crystal layer being associated with a Jones matrix J(B), wherein said Jones matrix of said elements is a function of said electric field B applied to said elements; The polarization modifying element is associated with a Jones matrix R; and RJ(B')Rp=p, where J(B') is the Jones matrix associated with the elements of the liquid crystal layer at the second electric field B'.

14. The holographic display according to claim 13, wherein: The polarization modifying element is a wave plate having half the retardation of J(B') and having a slow axis aligned with the fast axis of J(B').

15. A holographic display according to any preceding claim, further comprising: a lens arranged to receive light transmitted by the output polarization element and configured to generate a Fourier transform F(H) of the target light field at a focal plane of the lens; as well as A spatial filter is located at the focal plane and is configured to remove the complex conjugate Fourier transform F(H*) of the target light field.

16. The holographic display according to claim 15, wherein: The spatial filter is configured to remove the complex conjugate of the Fourier transform F(H*) of the target light field and the zeroth order of the target light field.

17. The holographic display according to claim 15, wherein: The spatial filter is configured to remove the complex conjugate of the Fourier transform F(H*) of the target light field without filtering the zeroth order of the target light field.

18. A holographic display according to any preceding claim, wherein: When the first electric field is applied to the element, the light has a first phase; When the third electric field is applied to the element, the light has a second phase; The first phase and the second phase are located in different quadrants on the Argand diagram.

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