Technical Field

The present invention relates to holographic displays and methods of operating holographic displays.

Background

Computer-Generated Holograms (CGH) are known and produce a hologram by interfering light to produce interference patterns. Unlike an image displayed on a conventional display, which is modulated only for amplitude, CGH displays also modulate phase to result in an image which preserves depth information at a viewing position. The amplitude and phase modulation in a CGH display can be achieved using a Spatial Light Modulator (SLM), typically in the form of a liquid crystal layer, like those found in liquid crystal displays (LCDs).

CGH generally requires phase modulation across the full range of 0 to 2p radians, which is achievable using an SLM having a liquid crystal layer based on the Twisted Nematic (TN) technology. However, TN-based LCDs are becoming less common for consumer LCDs, and are being replaced by a liquid crystal layer technology generally referred to as ‘In Plane Switching’ (IPS) technology. However, IPS-based LCDs have liquid crystal layers that present challenges for use in CGH displays and other applications that require modulation of amplitude and phase because they do not permit phase modulation covering the full range of 0 to 2p radians in the way that a TN display does.

As an example of a system using a nematic based SLM, US 5,719,650 describes a light modulator having two nematic polarization-rotating layers where the layers are specifically manufactured to have liquid crystals aligned orthogonal to each other, such that the crystals in the first layer are aligned along a first axis, and crystals in the second layer are aligned along a second axis orthogonal to the first axis. The liquid crystals in the first layer rotate the polarization of the incident optical field from linear to elliptical.

Accordingly, what is needed is a method of adapting IPS based LCDs for use in CGH displays. Summary

In a typical amplitude-only display, an IPS liquid crystal layer is used in conjunction with “crossed” linear input and output polarizers. Applying a bias/voltage/electric field to a particular element/pixel within the liquid crystal layer causes the liquid crystals to rotate. The degree of rotation can be controlled by controlling the applied voltage. Accordingly, applying a voltage to a particular element causes an incident light ray (linearly polarized from a linear input polarizer) to rotate by an angle dependent on the applied voltage. The amplitude of the light ray is controlled by the interaction of the light ray with the output polarizer. The amplitude can therefore be controlled by rotating the light ray relative to the linear output polarizer.

In typical liquid crystal displays, the rotation angle is zero for an applied voltage of zero, and because the input and output polarizers are “crossed” (such as being arranged at an angle of p/2 radians relative to each other) the amplitude of the transmitted light ray is zero. Applying a non-zero voltage causes the rotation angle to be non-zero, which in turn increases the amplitude of the transmitted light ray.

Linearly polarized light can be expressed by a Jones vector having only real terms. The transformation to the light as it travels through a liquid-crystal layer can be expressed in terms of a Jones matrix characterising the liquid-crystal layer. Some liquid crystal layers, such as IPS liquid crystal layers do not modulate the phase of linearly polarized light by a variable amount (i.e. within a range), unlike TN liquid crystal layers, because they have a Jones matrix comprising only real elements/terms. A liquid crystal layer having a Jones matrix comprising only real elements (i.e. a Jones matrix that is proportional to a real matrix) includes IPS liquid crystal layers. In contrast, TN liquid crystal layers have a Jones matrix comprising one or more imaginary/complex terms, which means a TN liquid crystal layer can modulate the phase of linearly polarized light by a variable amount because of the presence of the imaginary/complex components.

In certain embodiments, phase modulation is enabled on a liquid crystal layer characterised by a real Jones matrix, such as an IPS liquid crystal layer, by using circularly polarized light. Circularly polarized light can be expressed by a Jones vector having an imaginary term. Thus, passing a circularly polarized light ray through a liquid crystal layer having a Jones matrix comprising only real elements (such as an IPS liquid crystal layer) would cause the phase of the circularly polarized light ray to be modulated by a variable amount, based on the applied voltage. According to a first aspect of the present invention, there is provided a method of modulating an amplitude and a phase of a light ray in a holographic display. The method comprises: modulating an amplitude of the light ray based on a voltage applied to an element of a first liquid crystal layer and modulating a phase of the light ray based on a voltage applied to an element of a second liquid crystal layer. The light ray passing through the first liquid crystal layer is linearly polarized and the light ray passing through the second liquid crystal layer is circularly polarized.

Accordingly, the method involves passing light through two liquid crystal layers/displays. The first liquid crystal layer modulates the amplitude of the linearly polarized light ray and the second liquid crystal layer modulates the phase of the circularly polarized light ray.

In a specific example, the first and second liquid crystal layers both have Jones matrices comprising only real elements (for example, they may both be IPS liquid crystal layers). Put another way, the liquid crystal layers are associated with a Jones matrix that is proportional to a real matrix. In more general terms, the first and second liquid crystal layers comprise liquid crystals that are configured to align parallel to a plane of the liquid crystal layer when a voltage is applied to elements of the liquid crystal layer, and are configured to align parallel to the plane in absence of a voltage applied to the elements. The liquid crystal molecules are therefore: (i) aligned parallel to the layer/display (in-plane), and (ii) reoriented (as a result of the applied voltage) while remaining essentially parallel to the layer. For example, in absence of an applied voltage, the liquid crystals may be aligned parallel to the plane of the layer, and in the presence of an applied voltage, the liquid crystals remain parallel to the plane of the layer, but at different respective orientations about an axis that is perpendicular to the plane of the layer.

Accordingly, in some examples, modulating an amplitude of the light ray based on a voltage applied to an element of a first liquid crystal layer, comprises: applying a voltage to the element of the first liquid crystal layer such that liquid crystals within the element rotate in a plane parallel to a plane of the first liquid crystal layer, wherein the liquid crystals are configured to align parallel to the plane in absence of a voltage applied to the element. Similarly, modulating a phase of the light ray based on a voltage applied to an element of a second liquid crystal layer, comprises: applying a voltage to the element of the second liquid crystal layer such that liquid crystals within the element rotate in a plane parallel to a plane of the second liquid crystal layer, wherein the liquid crystals are configured to align parallel to the plane in absence of a voltage applied to the element.

In some examples, the first and second liquid crystal layers are the same. That is, the liquid crystals within each layer are aligned in substantially the same direction (such as along an axis) in absence of a voltage applied to the elements. In some examples, the liquid crystals from both layers rotate in the same direction when a voltage is applied. Thus, when the first and second liquid crystal layers are arranged in use, the liquid crystals are aligned in substantially the same direction in absence of a voltage applied to the elements.

Through the use of two IPS liquid crystal layers and linearly and circularly polarized light, a holographic display can be provided that allows both amplitude and phase to be modulated. It will be appreciated that this method and other methods described herein are applicable to not only IPS-based liquid crystal layers, but also liquid crystal layers that act as a rotating waveplate. A liquid crystal layer may comprise a plurality of elements/pixels, such as an array of elements. In an example, each element can be controlled by applying a voltage to the element. An element may correspond to a pixel or a sub-pixel (such as a sub-pixel of an RGB pixel).

Modulating an amplitude of the light ray may comprise modulating the amplitude by a variable amount based on the voltage applied to the element of the first liquid crystal layer. Modulating a phase of the light ray may comprise modulating the phase by a variable amount based on the voltage applied to the element of the second liquid crystal layer. Thus, a desired amplitude and/or phase may be selected within a range of amplitudes and/or phases based on the applied voltages. This is in contrast to modulating phase by a fixed/discrete/predetermined additional amount.

In one example, the method further comprises selectively adjusting the phase of the light ray by a predetermined additional amount. Accordingly, in addition to the phase modulation provided by the second liquid crystal layer, a fixed additional phase shift can be introduced (as needed) in order to increase the range over which the phase modulation is achievable. For example, as discussed above, it may be desirable for the phase to be modulated within the full range of 0 to 2p radians.

In a first embodiment, the first liquid crystal layer provides the predetermined additional amount of phase modulation. Accordingly, selectively adjusting the phase of the light ray by the predetermined additional amount is based on the voltage applied to the element of the first liquid crystal layer. The first liquid crystal layer can therefore provide both amplitude modulation and a fixed phase modulation.

In a particular example, the predetermined additional amount is p radians. Some types of IPS liquid crystal layers (which includes the first liquid crystal layer) permit the rotation of light by between 0 and p radians based on the voltage applied to the element, which results in a phase modulation of either 0 radians or p radians but nothing in between. The phase modulation of p radians is obtained because a light ray rotated by p radians is identical to a light ray that has undergone a phase modulation of p radians. Accordingly, such a liquid crystal layer can adjust/modulate the phase of a light ray by a predetermined additional amount, where the predetermined additional amount is p radians.

Most IPS liquid crystal layers are configured during normal use to rotate light by between 0 and p/2 radians. However, some of these can allow a rotation of up to p radians by applying a voltage above a threshold voltage to the element within the layer. This technique may be known as “overbiasing” because the element is driven to a voltage above its normal operating range. Thus, in some examples, the predetermined additional amount is p radians and is added by applying a voltage above a threshold voltage to the element of the first liquid crystal layer. Accordingly, at voltages below the threshold, there is no additional phase modulation and at voltages above the threshold, there is a predetermined additional amount of phase modulation of p radians. Overbiasing the first liquid crystal layer therefore allows the liquid crystal layer to be adapted to provide an additional fixed phase modulation.

As mentioned above, the first liquid crystal layer cannot provide a variable amount of phase modulation because the light ray is linearly polarized and the first liquid crystal layer has a Jones matrix comprising only real terms. This predetermined additional phase modulation may be said to be a “fixed” additional phase modulation because it cannot be selected from within a range. There is either a fixed additional phase modulation of p radians or there is no additional phased modulation provided by the first liquid crystal layer. This is in contrast to the variable phase modulation achievable using the second liquid crystal layer, where the light incident upon the second liquid crystal layer is circularly polarized, which allows the phase modulation to be “selected” from within a range. In this first embodiment, the second liquid crystal layer also permits the rotation of light by between 0 and p radians (using the overbiasing technique) so that any phase modulation within the range of 0 and p radians can be obtained by applying a particular voltage. In some examples, the second liquid crystal layer is configured to modulate the phase of the light by a variable amount of between 0 radians and p radians.

As mentioned above, the amplitude of the transmitted light ray depends on the polarization orientation of the light ray (after it has passed through the liquid crystal layer) relative to the linear output polarizer. The light ray passes through the linear output polarizer after traveling through the first liquid crystal layer. In an example, the linear output polarizer is arranged such that when the applied voltage is zero, the light ray has a maximum amplitude, and increasing the voltage towards the threshold voltage decreases the amplitude. At the threshold voltage, the amplitude of the light ray is zero (i.e. substantially no light passes through the linear output polarizer) and increasing the voltage towards a maximum voltage increases the amplitude of the light ray to its maximum again. As discussed above, at voltages above the threshold voltage to achieve overbiasing, the light ray also has a fixed additional phase modulation of p radians. Accordingly, in an example, modulating the amplitude of the light ray based on the voltage applied to the element of the first liquid crystal layer comprises: arranging a linear output polarizer at an angle relative to the linearly polarized light ray incident upon the first liquid crystal layer, such that: (i) applying a first voltage to the element of the first liquid crystal layer causes the light ray passing through the linear output polarizer to have a first amplitude, (ii) applying a second voltage to the element of the first liquid crystal layer causes the light ray passing through the linear output polarizer to have a second amplitude substantially equal to the first amplitude, and (ii) applying a threshold voltage to the element of the first liquid crystal layer causes substantially no light to pass through the linear output polarizer, wherein the threshold voltage is between the first voltage and the second voltage. This specific orientation of the linear output polarizer allows the first liquid crystal layer to act as fixed phase modulator and a variable amplitude modulator.

Following the example above, the first voltage is below the threshold voltage (more generally referred to as the threshold voltage to achieve a fixed additional phase modulation of p radians) and the second voltage is above the threshold voltage. The amplitudes of the transmitted light rays may be “mirrored” either side of the threshold voltage. So, for example, if a light ray having specific amplitude is desired to generate a holographic image, one of two voltages can be applied to the element, and of these two voltages, the actual voltage selected depends upon the phase modulation required (i.e. whether the additional fixed phase modulation is required). Accordingly, applying the second voltage to the element of the first liquid crystal layer causes the light ray passing through the linear output polarizer to have the predetermined additional amount of phase modulation.

In the examples above, substantially equal to the first amplitude may mean that the magnitude of the second amplitude is substantially equal to the magnitude of the first amplitude (i.e. ignoring any sign conventions). In the above example, the threshold voltage is a non-zero voltage. In a specific example the first voltage is zero and the first amplitude is a maximum amplitude. The second voltage may be a maximum voltage and the second amplitude is therefore a maximum amplitude. In one example, the threshold voltage is about 50% of maximum voltage.

In a second embodiment, the first and second liquid crystal layers are multi-domain layers having first and second element types, such that: (i) applying a particular voltage to the first element type causes liquid crystals having the first element type to rotate in a first direction and (ii) applying the particular voltage to the second element type causes liquid crystals having the second element type to rotate in a second direction, opposite to the first direction. In this second embodiment, the method further comprises interfering a first light ray from a first element of the first element type with a second light ray from a second element of the second element type. Elements of the first element type may be known as elements within a first domain and elements of the second element type may be known as elements within a second domain.

Thus, light rays from the first and second domains of one of the layers can be interfered. Preferably, the light rays are interfered after they have passed through the final liquid crystal layer positioned along an optical path. For example, if the first liquid crystal layer is positioned before the second liquid crystal layer (along an optical path from an illumination source to the viewer’s eye), the light rays from the second liquid crystal layer are interfered. However, if the second liquid crystal layer is positioned before the first liquid crystal layer, the light rays from the first liquid crystal layer are interfered. As will become apparent from the detailed discussion found herein, interfering light rays from two elements increases the phase range over which light can be modulated.

In the first embodiment discussed above, the first liquid crystal layer selectively provides the predetermined additional amount of phase modulation. However, in the second embodiment, a predetermined additional amount of phase modulation can be provided by a fixed phase modulating element. Accordingly, the method may further comprise passing the second light ray (from the second element of the second element type) through a fixed phase modulating element to additionally adjust the phase of the second light ray by a predetermined additional amount. The predetermined additional amount may be a predetermined additional amount relative to the first light ray. The fixed phase modulating element may be an optical element, such as a lens, configured to introduce the predetermined additional phase modulation. In some examples, both the first and second light rays pass through the optical element but only the second light ray is modulated by the predetermined additional amount. The fixed phase modulating element can be manufactured to provide a desired fixed phase modulation. In a particular example the predetermined additional amount is p/2 radians.

As above, the fixed additional phase shift is introduced in order to increase the range over which the phase modulation is achievable. For example, as discussed above, it is desirable for the phase to be modulated within the full range of 0 to 2p radians.

In an example, the phase of the second light ray is adjusted by the predetermined additional amount before being interfered with the first light ray.

In a specific example, unlike the first embodiment, the second liquid crystal layer is configured to modulate the phase of a light ray by a variable amount of: between 0 radians and p/2 radians for elements of the first element type; and between 0 radians and -p/2 radians for elements of the second element type. Thus, unlike the first embodiment, the first and second liquid crystal layers are configured to rotate light by between 0 and p/2 radians and 0 and - p/2 radians, respectively, rather than being overbiased to p radians. As a result of this reduced ability to modulate the phase, light needs to be both: (i) modulated using the fixed phase modulating element, and (ii) interfered in order to allow the phase to be modulated within the full range of 0 to 2p radians.

In an example, a linear output polarizer is arranged such that when the voltage applied to the element is zero, the light ray has a zero amplitude, and increasing the voltage towards a maximum voltage increases the amplitude. Accordingly, in an example, modulating the amplitude of the light ray comprises: arranging a linear output polarizer at an angle relative to the linearly polarized light ray incident upon the first liquid crystal layer such that: applying a first voltage to the element of the first liquid crystal layer causes the light ray passing through the linear output polarizer to have a first amplitude; and applying a zero-voltage to the element causes substantially no light to pass through the linear output polarizer. In preferred examples of the first and second embodiments, the first liquid crystal layer is positioned before the second liquid crystal layer. Accordingly, the method comprises modulating the phase of the light ray after modulating the amplitude of the light ray. It has been found that the amount of stray light can be reduced by modulating the phase after modulating the amplitude.

In some examples, the method comprises converting a linearly polarized light ray into a circularly polarized light ray using a circular polarizer. The circular polarizer is positioned such that the circularly polarized light ray is incident upon the second liquid crystal layer. In some examples, the circular polarizer is a quarter-wave plate configured to convert linearly polarized light rays into circularly polarized light rays. The circular polarizer therefore polarizes the light so that the second liquid crystal layer can modulate the phase of the light ray incident upon it. As discussed above, the Jones vector for a circularly polarized light ray contains an imaginary term, so that a liquid crystal layer with a Jones matrix comprising only real terms can modulate the phase of the light ray. The circular polarizer is therefore positioned in front of (i.e., before) the second liquid crystal layer along the optical path so that circularly polarized light is incident upon the second liquid crystal layer. In some examples, the circular polarizer is positioned between the first and second liquid crystal layers and the first liquid crystal layer is between the circular polarizer and the illumination source. In such a case, the linearly polarised light travelling from the first liquid crystal layer therefore passes through the circular polarizer.

According to a second aspect of the present invention, there is provided a method of modulating an amplitude and a phase of light in a holographic display. The method comprises: (i) applying a first voltage to a first element of a liquid crystal layer to cause a first light ray passing through the first element to have a first amplitude, and a phase of 0 or p, (ii) applying a second voltage to a second element of the liquid crystal layer to cause a second light ray passing through the second element to have a second amplitude, and a phase of 0 or p, (iii) additionally adjusting the phase of the second light ray by a predetermined additional amount, and (iv) interfering the first light ray with the second light ray.

Accordingly, in this third embodiment (unlike the first and second embodiments) the method involves passing light through a single liquid crystal layer. This third embodiment also involves interfering light from two elements and introducing a fixed phase modulation to provide a holographic display that allows both amplitude and phase to be modulated. In a specific example, the liquid crystal layer has a Jones matrix comprising only real elements (such as an IPS liquid crystal layer). Put another way, the liquid crystal layer is associated with a Jones matrix that is proportional to a real matrix. In more general terms, the liquid crystal layer comprises liquid crystals that are configured to align parallel to a plane of the liquid crystal layer when a voltage is applied to elements of the liquid crystal layer, and are configured to align parallel to the plane in absence of a voltage applied to the elements. The liquid crystal molecules are therefore: (i) aligned parallel to the layer/display (in-plane), and (ii) reoriented (as a result of the applied voltage) while remaining essentially parallel to the layer. For example, in absence of an applied voltage, the liquid crystals may be aligned parallel to the plane of the layer, and in the presence of an applied voltage, the liquid crystals remain parallel to the plane of the layer, but at different respective orientations about an axis that is perpendicular to the plane of the layer.

Accordingly, in some examples, applying a first voltage to a first element of a liquid crystal layer causes liquid crystals within the first element to rotate in a plane parallel to a plane of the liquid crystal layer, wherein the liquid crystals are configured to align parallel to the plane in absence of a voltage applied to the first element. Similarly, applying a second voltage to a second element of the liquid crystal layer causes liquid crystals within the second element to rotate in the plane, wherein the liquid crystals are configured to align parallel to the plane in absence of a voltage applied to the second element.

In an example, the first and second light rays are linearly polarized. Accordingly, a variable phase modulation cannot be achieved using the liquid crystal layer due to the Jones matrix of the liquid crystal layer having only real elements. However, the liquid crystal layer can selectively provide a fixed phase modulation of p radians as desired (resulting in the first and second light rays have a phase of 0 or p) by arranging one or more linear output polarizers at a specific angle relative to the liquid crystal layer. As in the second embodiment, this third embodiment uses a liquid crystal layer that is configured to rotate light by between 0 and p/2 radians. In other embodiments however, the liquid crystal layer is configured to modulate the phase of light by a fixed amount of 0 or p radians using the overbiasing technique discussed above.

In an example, the method comprises: arranging a linear output polarizer at an angle relative to the first element such that: (i) when the first voltage is zero, the first amplitude is non-zero, and (ii) when the first voltage is an intermediate voltage, the first amplitude is zero. The method further comprises arranging the linear output polarizer or a second linear output polarizer at the angle relative to the second element such that: (i) when the second voltage is zero, the second amplitude is non-zero and (ii) when the second voltage is the intermediate voltage, the second amplitude is zero. In addition, when the first voltage is above the intermediate voltage, the first amplitude is non-zero and when the second voltage is above the intermediate voltage, the second amplitude is non-zero. Further, when the first voltage is below the intermediate voltage, the first light ray has a phase of 0 and when the first voltage is above the intermediate voltage, the first light ray has a phase of p. Similarly, when the second voltage is below the intermediate voltage, the second light ray has a phase of 0 and when the second voltage is above the intermediate voltage, the second light ray has a phase of p. The angle of the linear output polarizer can therefore control amplitude and introduce a fixed phase shift of p radians based on the applied voltages.

In a specific example, the angle is p/4 radians relative to the polarization of the light ray that has passed through the first/second element of the liquid crystal layer in absence of a voltage applied to the element. Applying a voltage to the element below the intermediate voltage rotates the light ray by a total amount of between 0 and p/4 radians and results in the light ray having a phase of 0, and applying a voltage above the intermediate voltage rotates the light ray between p/4 radians and p/2 radians, and results in the light ray having a phase of p. Accordingly, in this example the liquid crystal layer is configured to rotate light by between 0 and p/2 radians yet still provide an additional phase modulation of p radians. In other examples, the angle is within ± 25% of p/4 radians, or within ± 20% of p/4, or within ± 15% of p/4, or within ± 10% of p/4, or within ± 5% of p/4. By arranging the linear output polarizer at p/4 radians, the first and second amplitudes are substantially equal in magnitude when the first and second voltages are zero.

In a specific example, the amplitudes of the light rays in absence of a voltage applied to the respective element is about 50% of a maximum amplitude that would be achievable in absence of the linear output polarizer. This is because a linear output polarizer arranged at p/4 radians relative to the polarization of the light ray only allows about 50% of the light ray to be transmitted.

According to a third aspect of the present invention there is provided a holographic display, comprising: (i) an illumination source configured to emit at least partially coherent light, (ii) a first liquid crystal layer, and (iii) a second liquid crystal layer positioned relative to the first liquid crystal layer such that a light ray passes through an element in the first liquid crystal layer and an element in the second liquid crystal layer. Such a holographic display can implement the methods of the first and second embodiments discussed above.

In an example, such as in the first embodiment, the second liquid crystal layer is configured to modulate the phase of the light by a variable amount of between 0 radians and p radians. In one example, the first liquid crystal layer is configured to selectively modulate the phase of the light by a predetermined additional amount of p radians. In other examples, the first liquid crystal layer does not modulate the phase of the light by a predetermined additional amount.

In the second embodiment, the second liquid crystal layer is configured to modulate the phase of the light by a variable amount of between 0 radians and p/2 radians.

An illumination source configured to emit partially coherent light preferably has sufficient coherence that the light from elements within the display can interfere with each other at a viewing distance. The illumination source may comprise a single light emitter or a plurality of light emitters and has an illumination area sufficient to illuminate the first and/or second liquid crystal layers. A suitably sized illumination area may be formed by enlarging the light emitter(s) such as by (i) pupil replication using a waveguide / Holographic Optical Element, (ii) a wedge, or (iii) localised emitters, such as localised diodes.

In a specific example, the display further comprises a controller configured to control voltages applied to elements of the first and second liquid crystal layers, to: (i) modulate a phase of the light ray, and (ii) modulate an amplitude of the light ray.

In some examples, the display further comprises a linear output polarizer arranged at an angle relative to the light ray incident upon the first liquid crystal layer. In certain examples, there is a single output polarizer that receives light from the entire area of the first liquid crystal layer. In other examples there a plurality of output polarizers that receive light from one or more elements of the first liquid crystal layer. In a specific example, there is a separate linear output polarizer for each element of the first liquid crystal layer.

The linear output polarizer may be arranged/positioned after the first liquid crystal layer along an optical path from the illumination source to the viewer’s eye. Thus, the first liquid crystal layer is positioned between the illumination source and the linear output polarizer.

In some examples the display further comprises a linear input polarizer arranged between the illumination source and the first liquid crystal layer such that the linearly polarized light is incident upon the first liquid crystal layer. In other examples, the illumination source is itself configured to output linearly polarized light so that the linear input polarizer is omitted.

In some examples, such as in the second embodiment discussed above, the first and second liquid crystal layers are multi-domain displays, in which liquid crystals within a first element type of the display are configured to rotate in a first direction when a particular voltage is applied to the first element type, and liquid crystals within a second element type of the display are configured to rotate in a second direction, opposite to the first direction, when the particular voltage is applied to the second element type.

In some examples, such as in the second embodiment discussed above, the display further comprises at least one of: (i) an interference element positioned to interfere a first light ray from a first element of the first element type with a second light ray from a second element of the second element type, and (ii) a fixed phase modulating element configured to adjust a phase of the second light ray by a predetermined additional amount. After light has passed through the interference element, the first and second light rays may combine to generate an output light ray.

In a particular example, the fixed phase modulating element and the interference element are integrally formed. For example, the fixed phase modulating element and interface element may be part of an optical element. Integrally forming the fixed phase modulating element and the interference element can simplify construction and may also reduce the overall size of the display. In an example, both the first and second light rays are incident upon the optical element, but only the second light ray passes through the fixed phase modulating element before both light rays are interfered by the interference element. In some examples, the interference itself occurs external to the interference element (and/or optical element). For example, the interference element may alter the trajectory of the light rays so that they are interfered at a certain distance from the interference element.

In certain examples, the display further comprises a circular polarizer (such as a quarter- wave plate) configured to convert a linearly polarized light ray into a circularly polarized light ray and positioned such that the circularly polarized light ray is incident upon the second liquid crystal layer. The quarter-wave plate therefore polarizes the light so that the second liquid crystal layer can modulate the phase of the light rays incident upon it. As discussed above, the Jones vector for a circularly polarized light ray contains an imaginary term, so that a liquid crystal layer with a Jones matrix comprising only real terms can modulate the phase of the light ray. The quarter-wave plate is therefore positioned in front of (i.e. before) the second liquid crystal layer along the optical path so that circularly polarized light is incident upon the second liquid crystal layer. In some examples, the quarter-wave plate is positioned between the first and second liquid crystal layers and the first liquid crystal layer is between the quarter-wave plate and the illumination source. This specific arrangement of entities along the optical path means that the phase modulation of a particular light ray (performed by the second liquid crystal layer) is performed after the amplitude modulation (performed by the first liquid crystal layer), thereby reducing stray light.

In some examples, the first and second liquid crystal layers comprise liquid crystals that are configured to align parallel to a plane of the liquid crystal layer when a voltage is applied to elements of the liquid crystal layer, and are configured to align parallel to the plane in absence of a voltage applied to the elements.

According to a fourth aspect of the present 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 a first element and a second element, (iii) an interference element configured to interfere a first light ray from the first element with a second light ray light from the second element to generate an output light ray, and (iv) a fixed phase modulating element configured to adjust a phase of the second light ray by a predetermined additional amount. Such a holographic display can implement the method of the third embodiment discussed above.

In some examples, the display further comprises a controller configured to control voltages applied to the first and second elements, to modulate a phase and an amplitude of the output light ray.

In some examples, the fixed phase modulating element and the interference element are integrally formed.

In a particular example, the display further comprises a linear output polarizer arranged at an angle relative to the light incident upon the liquid crystal layer. In a particular example, the display further comprises a linear input polarizer to linearly polarize light emitted by the illumination source.

In some examples, the liquid crystal layer comprises liquid crystals that are configured to align parallel to a plane of the liquid crystal layer when a voltage is applied to elements of the liquid crystal layer, and are configured to align parallel to the plane in absence of a voltage applied to the elements.

According to a fifth aspect of the present invention there is provided a holographic display configured to implement a method according to the first aspect. In a particular example, the holographic display comprises an illumination source, a first liquid crystal layer, a second liquid crystal layer and a controller configured to control the illumination source, the first liquid crystal layer and the second liquid crystal layer to implement a method according to the first and second aspects. In another example, the holographic display comprises an illumination source, a liquid crystal layer and a controller configured to control the illumination source and the liquid crystal layer to implement a method according to the second aspect.

As discussed above, control of the elements of the liquid crystal layers are described as being achieved by applying a voltage to the elements. In general, however, the voltage applies an electric field across the element, which causes the liquid crystals to rotate. It will be appreciated that any reference to operating or applying a voltage level to an element of a liquid crystal layer can be equally referred to as applying a particular electric field across the element. In some other examples, the electric fields can be controlled by controlling a current.

Within this document, the output polarizer is described and shown as being a transmissive polariser, in which the polariser itself can absorb at least a portion of the light incident upon it (and/or allow at least a portion of the light to pass through). It will be appreciated that the same concepts are applicable to other types of polarisers also. In some examples, the output polariser comprises a polarising element and an absorbing element, where the absorbing element absorbs light reflected from and/or transmitted through the polarising element. For example, the polarising element may be a reflective polariser that separates light into two separate beams and directs “unwanted” light towards an absorbing element.

Similarly, liquid crystal layers can be reflective or transmissive and references herein to a “liquid crystal layer” are not limited to reflective or transmissive liquid crystal layers. A transmissive liquid crystal layer may be configured such that light is primarily transmitted through the liquid crystal layer from one side to the other. A reflective liquid crystal layer may be configured such that light is primarily reflected by the liquid crystal layer. Reflective liquid crystal layers contain a mirror such that light passes through the element of the liquid crystal layer, is reflected, and then passes through the element a second time in the opposite direction. Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Figure l is a diagrammatic representation of an example holographic display according to a first embodiment;

Figure 2 is a schematic diagram of the holographic display of Figure 1;

Figure 3 A is a diagram showing the amplitude modulation of an example light ray for an applied voltage below a threshold voltage;

Figure 3B is a diagram showing the amplitude modulation of an example light ray for an applied voltage above a threshold voltage;

Figure 3C is a diagram showing the phase modulation of the example light ray of Figure 3 A;

Figure 3D is a diagram showing the phase modulation of the example light ray of Figure 3B;

Figure 3E is a diagram showing the full range of phase and amplitude modulation using the holographic display of the first embodiment;

Figure 4 is a diagrammatic representation of an example holographic display according to a second embodiment;

Figure 5 is a schematic diagram of the holographic display of Figure 4;

Figure 6A is a diagram showing the amplitude modulation of an example first light ray from an element of a first element type;

Figure 6B is a diagram showing the amplitude modulation of an example second light ray from an element of a second element type;

Figure 6C is a diagram showing the phase modulation of the example light ray of Figure 6 A;

Figure 6D is a diagram showing the phase modulation of the example light ray of Figure 6B;

Figure 6E is a diagram showing an additional fixed phase modulation of the example light ray of Figure 6D; Figure 6F is diagram showing the full range of phase and amplitude modulation using the holographic display of the second embodiment after interfering the first and second example light rays of Figures 6C and 6E;

Figure 7 is a diagrammatic representation of an example holographic display according to a third embodiment;

Figure 8 is a schematic diagram of the holographic display of Figure 7;

Figure 9A is a diagram showing the amplitude modulation of an example first light ray from a first element of a liquid crystal layer;

Figure 9B is a diagram showing the amplitude modulation of an example second light ray from a second element of a liquid crystal layer;

Figure 9C is a diagram showing an additional fixed phase modulation of the example light ray of Figure 9B;

Figure 9D is diagram showing the full range of phase and amplitude modulation using the holographic display of the third embodiment after interfering the first and second example light rays of Figures 9A and 9C;

Figure 10 is an example method that can be used with the display of Figures 1 and 2;

Figure 11 is an example method that can be used with the display of Figures 4 and 5; and

Figure 12 is an example method that can be used with the display of Figures 7 and 8. Detailed Description

Figures 1 and 2 depict a holographic display 100 according to a first embodiment. The display 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 rays towards a first liquid crystal layer 104. In other examples, the illumination source 102 comprises a plurality of light emitters which together illuminate the first liquid crystal layer 104. Examples using a plurality of light emitters may also have the ability to control light emitters individually or by region, enabling reduced power consumption and/or increased contrast. In this example, the illumination source 102 is controlled to emit Red, Green or Blue light using time division multiplexing. Other examples may use an illumination source 102 that emits light having a single wavelength or light having wavelengths within a range of a single wavelength.

The display 100 includes the first liquid crystal layer 104 and also includes a second liquid crystal layer 106. In this example, the first liquid crystal layer 104 is arranged between the illumination source 102 and the second liquid crystal layer 106. Light rays, emitted by the illumination source 102, travel from the illumination source 102 to the first liquid crystal layer 104 and from the first liquid crystal layer 104 to the second liquid crystal layer 106. The light rays follow an optical path along an optical axis, from the illumination source 102 towards the eyes of an observer that is viewing the holographic display 100.

Each liquid crystal layer 104, 106 comprises an array of pixels/elements 104a, 106a. A voltage/bias can be applied to each element individually to control the rotation of the liquid crystals and therefore the rotation of light as it passes through the element. In the example of Figure 1, the first and second liquid crystal layers 104, 106 are IPS based liquid crystal layers 104, 106. In more general terms, both liquid crystal layers 104, 106 have a Jones matrix comprising only real terms (or have a Jones matrix that is proportional to a real matrix). In a first example, both liquid crystal layers 104, 106 have a Jones matrix given by a rotation matrix proportional to:

The rotation angle, f is a function of the applied voltage to the particular element of the liquid crystal layer 104, 106. In a second example, both liquid crystal layers 104, 106 are approximated by a rotated half waveplate, and have a Jones matrix proportional to:

The liquid crystal layers 104, 106 may be the same type of liquid crystal layer (i.e. they have the same Jones matrix), but in other examples the liquid crystal layers are of different types.

Thus, in either example, it can be shown that for linearly polarized light, applying a voltage to a particular element of the liquid crystal layer 104, 106 causes a pure rotation of the polarization state of the light by an angle of cp. For circularly polarized light, applying a voltage to a particular element of the liquid crystal layer 104, 106 causes a pure phase modulation/retardation of cp. Returning to Figure 1, the display further comprises a linear input polarizer 108 that linearly polarizes light such that the linearly polarized light is incident upon the first liquid crystal layer 104. In some examples the linear input polarizer 108 is omitted if the light emitted by the illumination source 102 is already linearly polarized.

Positioned after the first liquid crystal layer 104 is a linear output polarizer 110. The linear output polarizer 110, together with the first liquid crystal layer 104, controls and therefore modulates the amplitude of light rays passing through the first liquid crystal layer 104. For example, if a light ray that is linearly polarized along a vertical axis 112 is traveling in a direction along the optical axis 116, and is incident upon a vertically orientated output polarizer 110, the transmitted light ray may have substantially 100% of its initial intensity (i.e. the amplitude of the transmitted light ray is unchanged and/or is at maximum). The vertically orientated output polarizer 110 may be said to be aligned with the linearly polarized light ray and is therefore arranged an angle of 0 radians relative to the linearly polarized light ray incident upon the polarizer 110. Conversely, if the light ray is linearly polarized along the horizontal axis 114 and is incident upon the vertically orientated output polarizer 110, the transmitted light ray may have substantially 0% of its initial intensity (i.e. the amplitude of the transmitted light ray is at a minimum, or zero). The vertically orientated output polarizer 110 may be said to be “crossed” with the horizontally polarized light ray and is therefore arranged at an angle of p/2 radians relative to the linearly polarized light ray incident upon the polarizer 110. If the light ray is linearly polarized along the vertical axis 112 and is incident upon an output polarizer 110 that is arranged at an angle of p/4 radians relative to the linearly polarized light ray incident upon the polarizer 110, the transmitted light ray may have approximately 50% of its initial intensity (i.e. the amplitude of the transmitted light ray is at half maximum).

In the display 100, the polarization orientation of the light ray incident upon the polarizer 110 is controlled by varying the voltage applied to the element 104a of the first liquid crystal layer 104 through which the light ray passes. Accordingly, it can be seen that varying the voltage applied to the element controls/modulates the amplitude of the transmitted light ray by varying the angle between the polarizer 110 and the polarization state of the linearly polarized light ray. Thus, in the example of Figure 1, the first liquid crystal layer 104 acts as an amplitude modulator in conjunction with the linear output polarizer 110.

Arranged in front of the second liquid crystal layer 106 is a quarter-wave plate 118 configured to convert linearly polarized light rays into circularly polarized light rays. Circularly polarized light is therefore incident upon the second liquid crystal layer 106. As discussed above, when circularly polarized light interacts with a liquid crystal layer 106 having a Jones matrix with only real terms, the effect is to modulate the phase of the light by a variable amount based on the voltage applied to the element 106a through which the light ray has passed. Because there is no output polarizer positioned after the second liquid crystal layer 106, the light ray has substantially 100% of its initial intensity (i.e. there is no amplitude modulation).

Figure 2 depicts a controller 120 that can control the voltages applied to each element of the first and second liquid crystal layers 104, 106 to provide full control over the amplitude and phase modulation of a light ray output by the display 100. The controller 120 may also control operation of the illumination source 102.

It should be noted that the schematic depictions in Figures 1 and 2 are to aid understanding, and that spacing between entities is not necessarily required. For example, the illumination source 102, the first liquid crystal layer 104, the linear input and output polarizers 108, 110, the quarter-wave plate 118 and the second liquid crystal layer 106 may have substantially no space between them. It will also be appreciated that the first and second liquid crystal layers (and the polarizers 108, 118) may be arranged in any order along the optical path. For example, the second liquid crystal layer 106 and the corresponding quarter wave plate 118 may be arranged before the linear input polarizer 108, the first liquid crystal layer 104 and the linear output polarizer 110. However, it has been found that the specific ordering in Figures 1 and 2 reduces the amount of stray light.

Figures 1 and 2 also depict a linear arrangement of the holographic display but other arrangements may include image folding components. For example, a folded optical path may be provided.

In holographic displays, it is desirable to control the phase modulation across the full range of 0 to 2p radians. Some liquid crystal layers can control the degree of rotation, cp, from 0 to p/2 radians. However, other liquid crystal layers can control the degree of rotation from 0 to p radians by overbiasing an element by applying a voltage above a threshold voltage which causes rotation beyond the normal operating range of 0 to p/2 radians. In the example of Figures 1 and 2, both the first and second liquid crystal layers 104, 106 are capable of rotating the polarization state of the light ray from 0 to p radians using this overbiasing technique. Figures 3A-3E depict how the arrangement of Figures 1 and 2 allows phase to be modulated across the full range of 0 to 2p. Figure 3 A depicts a phasor diagram for an example light ray that has passed through an element 104a of the first liquid crystal layer 104 and the linear output polarizer 110. The amplitude of a light ray is given by the magnitude of the value. The phase of the light ray is given by the azimuthal angle between the positive real axis and the line drawn on the diagram. In Figure 3A, the line is drawn along the positive real axis so the phase is 0. Figure 3B also depicts a phasor diagram for another example light ray that has passed through an element 104a of the first liquid crystal layer 104 and the linear output polarizer 110. In Figure 3B, the line is drawn along the negative real axis, so the phase is p.

Figure 3A is applicable for voltages below the threshold voltage required to overbias the element 104a. Due to the orientation of the linear output polarizer 110, the diagram shows that for voltages within the range of between 0V and the threshold voltage, the amplitude varies between +1 and 0 (i.e. between a first amplitude and zero). In this particular example, when the voltage applied is 0V, the linear output polarizer 110 is orientated such that the light ray is transmitted substantially unimpeded so that the light ray has substantially 100% of its initial intensity (i.e. there is no amplitude modulation). When an element has a zero voltage applied, the element may be said to be in a “quiescent state”. When the voltage applied is equal to the threshold voltage, the linearly polarized light ray is rotated by an angle such that substantially no light is transmitted through the linear output polarizer 110. For example, the light ray may have rotated by an angle of p/2 radians relative to the light ray in the quiescent state and the output polarizer 110 is arranged at an angle relative to the light ray such that substantially no light can pass through the polarizer 110. The light ray therefore has substantially 0% of its initial intensity (i.e. has a zero amplitude). Varying the applied voltage between 0V and the threshold voltage results in a transmitted light ray with an amplitude of between a maximum amplitude (such as +1) and 0. The dark line on Figure 3 A therefore shows the range of amplitudes a light ray may have when the element 104a is driven between 0V and the threshold voltage. The first liquid crystal layer 104 can therefore modulate the amplitude by a variable amount based on the applied voltage.

Figure 3B is applicable for when the voltage applied to the element 104a is above the threshold voltage. Due to the orientation of the linear output polarizer 110, the diagram shows that for voltages within the range of between the threshold voltage and a maximum voltage, the amplitude varies between -1 and 0 (i.e. between a second amplitude, substantially equal to the first amplitude of Figure 3 A, and zero). In this particular example, when the voltage applied is at a maximum, the linear output polarizer 110 is orientated such that the light ray is transmitted unimpeded so that the light ray has substantially 100% of its initial intensity (i.e. there is no amplitude modulation). In this scenario, the light ray is rotated by p radians relative to the light ray in the quiescent state. When the voltage applied corresponds to the threshold voltage, the linearly polarized light ray is rotated by an angle such that substantially no light is transmitted through the linear output polarizer 110. The light ray therefore has substantially 0% of its initial intensity (i.e. has a zero amplitude). Varying the applied voltage between a maximum voltage and the threshold voltage results in a transmitted light ray with an amplitude between a maximum and 0. The dark line on Figure 3B therefore shows the range of amplitudes a light ray may have when the element 104a is driven between a maximum voltage and the threshold voltage.

As shown in Figure 3B, the linearly polarized light ray has been rotated by p radians compared to the light ray in Figure 3Adue to overbiasing. Overbiasing an element 104a therefore selectively adjusts the phase of a light ray by a fixed/predetermined amount of p radians.

After the light ray of Figure 3 A has passed through the quarter wave plate 118, the light ray passes through an element 106a of the second liquid crystal layer 106. Circularly polarized light is therefore incident upon the element 106a of the second liquid crystal layer 106 and undergoes a phase modulation based on the voltage applied to the element 106b.

Figure 3C depicts a phasor diagram for showing the possible range of phases of an example light ray that has passed through an element 104a of the first liquid crystal layer 104 (that was operated in the voltage range of 0V to the threshold voltage, as in Figure 3A), the linear output polarizer 110, the quarter wave plate 118 and the element 106a of the second liquid crystal layer 106. The shaded region shows that the phase of the light ray can be modulated within a range of 0 to p radians. For voltages within the range of between 0V and the threshold voltage, the phase can be modulated between 0 and p/2 radians. In this particular example, when the voltage applied is 0V, the transmitted light ray is substantially unimpeded so that the light ray is not modulated in phase. When the voltage applied corresponds to the threshold voltage, the phase modulation of the transmitted light ray is p/2. When the voltage applied is greater than the threshold voltage, the phase modulation of the transmitted light ray is between p/2 and p. When the voltage applied corresponds to a maximum voltage, the phase modulation of the transmitted light ray is p. Varying the applied voltage between zero and a maximum voltage therefore results in a transmitted light ray with a phase shift of between 0 and p radians. Unlike the fixed phase modulation discussed in respect of Figure 3B, the phase modulation may take any value within the range of 0 to p radians.

Figure 3D depicts a phasor diagram showing the possible range of phases for an example light ray that has passed through an element 104a of the first liquid crystal layer 104 (that was operated in the voltage range of the threshold voltage to the maximum voltage, as in Figure 3B), the linear output polarizer 110, the quarter wave plate 118 and the element 106a of the second liquid crystal layer 106. The shaded region shows that the phase of the light ray can be modulated within a range of 0 to p radians. For voltages within the range of between 0V and the threshold voltage, the phase can be modulated between 0 and p/2 radians which results in a total phase modulation of between p and 3p/2 radians. In this particular example, when the voltage applied is 0V, the transmitted light ray is substantially unimpeded so that the light ray is not further modulated in phase. The total phase modulation of the light ray is therefore p radians. When the voltage applied corresponds to the threshold voltage, the phase modulation of the transmitted light ray is p/2, resulting in a total phase modulation of 3p/2 radians. When the voltage applied is greater than the threshold voltage, the phase modulation of the transmitted light ray is between p/2 and p, resulting in a total phase modulation of between 3p/2 and 2p radians. When the voltage applied corresponds to a maximum voltage, the phase modulation of the transmitted light ray is p, resulting in a total modulation of 2p radians.

Accordingly, it can be seen that applying particular voltages to the different elements 104a, 106a generates a light ray with a particular amplitude and phase within the full range of 0 to 2p. Figure 3E depicts a phasor diagram showing the total range of achievable amplitudes and phases of the output light ray.

Figures 4 and 5 depict a holographic display 200 according to a second embodiment. In general, the display and its components are similar to those discussed in Figures 1 and 2, but the first and second liquid crystal layers 204, 206 are configured to rotate light by between 0 and p/2 radians, in contrast to the 0 and p radian range achieved by the liquid crystal layers of the first embodiment. For example, in this second embodiment, the liquid crystal layers cannot be overbiased. As will be explained below in more detail, in this embodiment the output of two elements are interfered to provide a full range of modulation of amplitude and phase.

The display 200 therefore includes an illumination source 202 configured to emit at least partially coherent light, a first liquid crystal layer 204 and a second liquid crystal layer 206. In this example, the first liquid crystal layer 204 is arranged between the illumination source 202 and the second liquid crystal layer 206.

Each liquid crystal layer 204, 206 comprises an array of pixels/elements 204a, 204b, 206a, 206b. In this example the first and second liquid crystal layers 204, 206 are IPS based liquid crystal layers 204, 206 and have a Jones matrix comprising only real terms. The liquid crystal layers 204, 206 may be the same type of liquid crystal layer (i.e. they have the same Jones matrix), but in other examples the liquid crystal layers are of different types.

In this example, each liquid crystal layer 204, 206 is a multi-domain layer having first and second element types (also known as first and second domains) which rotate in opposition directions to an applied voltage. In the example of Figures 4 and 5, the first element 204a of the first liquid crystal layer 204 is an element of a first type and the second element 204b of the first liquid crystal layer 204 is an element of the second type. Similarly, the first element 206a of the second liquid crystal layer 206 is an element of the first type and the second element 206b of the second liquid crystal layer 206 is an element of the second type. Figures 4 and 5 show a first light ray passing through elements of the first type in both the first and second liquid crystal layers 204, 206 and a second light ray passing through elements of the second type in both the first and second liquid crystal layers 204, 206. As mentioned, applying a particular voltage to the first element type causes liquid crystals within the first element type to rotate in a first direction (such as clockwise) and applying the same particular voltage to the second element type causes liquid crystals within the second element type to rotate by the same amount in a second direction, opposite to the first direction (i.e. anticlockwise). Thus, for a given voltage, the degree of rotation is equal but in opposite directions.

The display 200 further comprises a linear input polarizer 208 that linearly polarizes light such that the linearly polarized light is incident upon the first liquid crystal layer 204. In some examples the linear input polarizer 208 is omitted if the light emitted by the illumination source 202 is already linearly polarized.

Positioned after the first liquid crystal layer 204 is a linear output polarizer 210. The linear output polarizer 210, together with the first liquid crystal layer 204, controls and therefore modulates the amplitude of light rays passing through the first liquid crystal layer 204. Positioned in front of the second liquid crystal layer 206 is a quarter-wave plate 218 configured to convert linearly polarized light rays into circularly polarized light rays. Circularly polarized light is therefore incident upon the second liquid crystal layer 206.

Figure 5 depicts a controller 220 that can control the voltages applied to each element of the first and second liquid crystal layers 204, 206 to provide full control over the amplitude and phase modulation of a light ray output by the display 200. The controller 220 may also control operation of the illumination source 202.

As mentioned, in holographic displays, it is desirable to control the phase modulation across the full range of 0 to 2p radians. In this example, both the first and second liquid crystal layers 204, 206 can control the degree of rotation, cp, from 0 to p/2 radians.

Accordingly, further elements are needed to permit phase modulation within the range of 0 to 2p radians. The display 200 therefore also includes an interference element 222 positioned to interfere a first light ray from a first element 204a, 206a of the first element type with a second light ray from a second element 204b, 206b of the second element type. In this particular example, the interference element 222 is positioned after the second liquid crystal layer 206 and receives and interferes a first light ray from the first element 206a of the second liquid crystal layer 206 with the second light ray from the second element 206b of the second liquid crystal layer 206. Light from two different element types (i.e. domains) are therefore interfered. In this example, the interference element 222 is part of an optical element 226 and may take the form of a lens. The optical element 226 may also be part of a lens array that is positioned over the entire second liquid crystal layer. The lens array may comprise half the number optical elements 222 as there are pixels/elements in the entire second liquid crystal layer 206 (i.e. there is one optical element 222 for each pixel/element pair). As will be apparent from the following discussion, interfering two light rays increases the range over which the phase can be modulated.

The display 200 also includes a fixed phase modulating element 224 to additionally adjust the phase of the second light ray (from the second element 204b, 206b) by a predetermined additional amount. In this example, the fixed phase modulating element 224 is positioned after the second liquid crystal layer 206 and receives the second light ray from the second element 206b of the second liquid crystal layer 206. In the example of Figure 5, the fixed phase modulating element 224 is part of part of the optical element 226 and only one of the two light rays receives the additional amount of phase modulation. The second light ray from the second element 206b receives an additional phase modulation relative to the first light ray from the first element 206a due to an increased path length through the fixed phase modulating element 224. The increased path length is achieved by providing the optical element 226 with a stepped profile. In this particular example, the path length is increased by an amount to modulate the phase of the second light ray by a fixed additional amount of p/2 radians. In Figure 5, the interference element 222 and the fixed phase modulating element 224 are integrally formed as part of the optical element 226.

Figures 6A-6E depict how the arrangement of Figures 4 and 5 allows phase to be modulated across the full range of 0 to 2p.

Figure 6A depicts a phasor diagram for an example light ray that has passed through a first element 204a of the first element type (in the first liquid crystal layer 204), and the linear output polarizer 210.

Since the first liquid crystal layer 204 is configured to rotate light in the range of 0 to p/2 radians, Figure 6A is applicable for the full range of voltages that can be applied to the element 204a. Due to the orientation of the linear output polarizer 210, when the voltage applied to the first element 204a is 0V, the first light ray (which is linearly polarized) cannot pass through the linear output polarizer 210. The light ray therefore has substantially 0% of its initial intensity (i.e. has a zero amplitude). When the voltage applied to the first element 204a is non-zero, the linearly polarized light ray is rotated by an angle such that the amplitude is increased. At a particular voltage, which may be a maximum operating voltage, the transmitted first light ray will have a non-zero amplitude, which may be a maximum amplitude. Thus, at a particular voltage, the linear output polarizer 210 is orientated such that the light ray is transmitted substantially unimpeded so that the first light ray has substantially 100% of its initial intensity (i.e. there is no amplitude modulation). Varying the applied voltage between 0V and a particular voltage results in the transmitted first light ray with an amplitude between 0 and a maximum amplitude (such as +1). The dark line on Figure 6 A therefore shows the range of amplitudes the first light ray may have when the element 204a is driven between 0V and a particular voltage. The first liquid crystal layer 204 can therefore modulate the amplitude by a variable amount based on the applied voltage.

Figure 6B depicts a phasor diagram for a second example light ray that has passed through a second element 204b of the second element type (in the first liquid crystal layer 204), and the linear output polarizer 210. As mentioned above, when a voltage is applied to elements of the second element type, the liquid crystals rotate in the opposite direction. This effectively results in the second light ray having an additional phase modulation of p radians relative the first light ray in Figure 6A. This is shown schematically in Figure 6B because the dark line is drawn along the negative real axis. Other than this additional fixed phase modulation of p radians, the second element 204b otherwise operates in the same manner as the first element 204a. That is, when the voltage applied to the second element 204b is 0V, the second light ray (which is linearly polarized) cannot pass through the linear output polarizer 210. The second light ray therefore has substantially 0% of its initial intensity (i.e. has a zero amplitude). When the voltage applied to the second element 204b is non-zero, the linearly polarized light ray is rotated by an angle such that the amplitude is increased. At a particular voltage, which may be a maximum operating voltage, the transmitted second light ray will have a non-zero amplitude, which may be a maximum amplitude. Thus, at a particular voltage, the linear output polarizer 210 is orientated such that the light ray is transmitted substantially unimpeded so that the second light ray has substantially 100% of its initial intensity (i.e. there is no amplitude modulation). Varying the applied voltage between 0V and a particular voltage results in the transmitted second light ray with an amplitude between 0 and a maximum amplitude (such as -1). The dark line on Figure 6B therefore shows the range of amplitudes the second light ray may have when the second element 204b is driven between 0V and a particular voltage.

After the first light ray of Figure 6 A has passed through the quarter wave plate 218, the first light ray passes through a first element 206a of the first element type in the second liquid crystal layer 206. Circularly polarized light is therefore incident upon the first element 206a of the second liquid crystal layer 206 and therefore undergoes a phase modulation based on the voltage applied to the element 206a. As was the same for the first liquid crystal layer 204, the second liquid crystal layer 206 is also capable of rotating the light ray from 0 to p/2 radians. This results in a phase modulation of between 0 to p/2 radians based on the applied voltage.

Figure 6C depicts a phasor diagram for an example first light ray that has passed through a first element 204a of the first liquid crystal layer 204, the linear output polarizer 210, the quarter wave plate 218 and the first element 206a of the second liquid crystal layer 206. Figure 6C is applicable for the full range of voltages that can be applied to the element 206a. The shaded region shows that the phase of the light ray can be modulated within a range of 0 to p/2 radians. In this particular example, when the voltage applied is 0V, the transmitted light ray is substantially unimpeded so that the light ray is not modulated in phase. When the voltage applied corresponds to a particular voltage, which may be a maximum operating voltage, the phase modulation of the transmitted light ray is p/2. Varying the applied voltage between zero and a maximum voltage therefore results in a transmitted first light ray with a phase shift of between 0 and p/2 radians. Unlike the fixed phase modulation discussed in respect of Figure 6B, the phase modulation may take any value within the range of 0 to p/2 radians.

After the second light ray of Figure 6B has passed through the quarter wave plate 218, the second light ray passes through a second element 206b of the second element type in the second liquid crystal layer 206. Circularly polarized light is therefore incident upon the second element 206b of the second liquid crystal layer 206 and therefore undergoes a phase modulation based on the voltage applied to the element 206b. As was the same for the first liquid crystal layer 204, the second liquid crystal layer 206 is also capable of rotating the light ray from 0 to p/2 radians. This results in a phase modulation of between 0 to -p/2 radians based on the applied voltage. Here, the sign is negative because the second element 206b is an element of the second type, and therefore rotates the light in the opposite direction to elements of the first type (as in Figure 6C).

Figure 6D depicts a phasor diagram for an example second light ray that has passed through a second element 204b of the first liquid crystal layer 204, the linear output polarizer 210, the quarter wave plate 218 and the second element 206b of the second liquid crystal layer 206. Figure 6D is applicable for the full range of voltages that can be applied to the element 206b. The shaded region shows that the phase of the light ray can be modulated within a range of 0 to -p/2 radians from the starting phase of p radians, so that the resulting phase, as depicted in Figure 6D is from p to p/2 radians. In this particular example, when the voltage applied is 0V, the transmitted light ray is substantially unimpeded so that the light ray is not modulated in phase. When the voltage applied corresponds to a particular voltage, which may be a maximum operating voltage, the phase modulation of the transmitted light ray is -p/2. Varying the applied voltage between zero and a maximum voltage therefore results in a transmitted second light ray with a phase shift of between 0 and -p/2 radians. Unlike the fixed phase modulation discussed in respect of Figure 6B, the phase modulation may take any value within the range of 0 to -p/2 radians.

Together, Figures 6C and 6D show that the range of phase modulation achievable at this point is between 0 and p radians. Further steps are therefore required to permit phase modulation over the full range of 0 to 2p radians. Accordingly, after the second light ray of Figure 6D has passed through the second element 206b of the second element type in the second liquid crystal layer 206, the second light ray passes through the fixed phase modulating element 224 to additionally adjust the phase of the second light ray by a predetermined additional amount. In this example, the predetermined additional amount is p/2 radians. Figure 6E therefore depicts a phasor diagram for an example second light ray that has passed through a second element 204b of the first liquid crystal layer 204, the linear output polarizer 210, the quarter wave plate 218, the second element 206b of the second liquid crystal layer 206 and the fixed phase modulating element 224. The phasor diagram is similar to that of Figure 6D, but has been rotated by p/2 radians.

To achieve the full range of phase modulation, the first light ray from the first element 206a of the second liquid crystal layer is interfered with the second light ray from the second element 206b of the second liquid crystal layer by the interference element 222. Figure 6F therefore depicts a phasor diagram for the total range of amplitudes and phases of the output light ray that can be achieved by interfering these two light rays. Selecting specific voltages of the elements can allow an output light ray to have any amplitude and phase modulation shown in Figure 6F.

Figures 7 and 8 depict a holographic display 300 according to a third embodiment. In general, the display 300 is similar to that discussed in Figures 4 and 5, but the display 300 comprises a single liquid crystal layer 304, rather than two liquid crystal layers. As in the display of Figures 4 and 5, the liquid crystal layer 304 is configured to rotate light by between 0 and p/2 radians, in contrast to the 0 and p radian range achieved by the liquid crystal layers of the first embodiment.

The display 300 includes an illumination source 302 configured to emit at least partially coherent light, and a liquid crystal layer 304. The liquid crystal layer 304 comprises an array of pixels/elements 304a, 304b. In this example the liquid crystal layer 304 is an IPS based liquid crystal layer 304 and has a Jones matrix comprising only real terms. Figures 7 and 8 show a first light ray passing through a first element 304a and a second light ray passing through a second element 304b.

The display 300 further comprises a linear input polarizer 308 that linearly polarizes light such that the linearly polarized light is incident upon the liquid crystal layer 304. In some examples the linear input polarizer 308 is omitted if the light emitted by the illumination source 302 is already linearly polarized. Positioned after the liquid crystal layer 304 is a linear output polarizer 310. The linear output polarizer 310, together with the liquid crystal layer 304, controls and therefore modulates the amplitude of light rays passing through the liquid crystal layer 304. In this particular example, the linear output polarizer 310 is positioned at an angle such that when the voltage applied to the elements 304a, 304b is 0V (i.e. when the element is in a quiescent state), the output light ray has an amplitude of approximately 50% of its maximum amplitude (where the maximum amplitude is the amplitude of the light ray in the absence of the linear output polarizer 310). In one example, this can be achieved by orientating the polarizer at an angle of p/4 radians relative to the polarization state of the linearly polarized light ray that has passed through the element 304a, 304b (when the applied voltage is 0V). Figure 7 depicts the output polarizer 310 arranged at an angle of p/4 radians relative to the vertical axis 112. In the quiescent state, the light ray passing through the element 304a, 304b may be linearly polarized along the vertical axis 112 such that the angle of p/4 radians is subtended between the light ray and the polarizer 310.

Increasing the applied voltage from 0V to an intermediate voltage rotates the light ray and therefore increases the angle between the polarizer 310 and the light ray. At the intermediate voltage the angle between the polarizer 310 and the light ray may be approximately p/2 radians such that the light ray has an amplitude of approximately zero and does not pass through the polarizer 310. For example, at the intermediate voltage, the light ray passing through the element 304a, 306a may have rotated by p/4 radians relative to the light ray in the quiescent state and therefore be perpendicular to the polarizer 310 such that an angle of p/2 radians is subtended between the light ray and the polarizer 310.

Increasing the applied voltage from the intermediate voltage to a maximum voltage further rotates the light ray and further increases the angle between the polarizer 310 and the light ray. At the maximum voltage the angle between the polarizer 310 and the light ray may be approximately 3p/4 radians such that the output light ray again has an amplitude of approximately 50% of its maximum amplitude. For example, at the maximum voltage, the light ray passing through the element 304a, 306a may have rotated by p/4 radians relative to the light ray at the intermediate voltage and therefore p/2 radians relative to the light ray in the quiescent state. In the example of Figure 7, the light ray would therefore be linearly polarized along the horizontal axis 114. Figure 8 depicts a controller 320 that can control the voltages applied to each element of the liquid crystal layer 304 to provide full control over the amplitude and phase modulation of a light ray output by the display 300. The controller 320 may also control operation of the illumination source 302.

As mentioned, in holographic displays, it is desirable to control the phase modulation across the full range of 0 to 2p radians. In this example, the liquid crystal layer 304 can control the degree of rotation, cp, from 0 to p/2 radians.

Accordingly, further elements are needed to permit phase modulation within the range of 0 to 2p radians. The display 300 therefore also includes an interference element 322 positioned to interfere a first light ray from a first element 304a with a second light ray from a second element 304b. In this particular example, the interference element 322 is positioned after the liquid crystal layer 304 and receives and interferes a first light ray from the first element 304a with the second light ray from the second element 304b. In this example, the interference element 322 is part of an optical element 326 and may take the form of a lens. The optical element 326 may also be part of a lens array that is positioned over the entire liquid crystal layer 304. The lens array may comprise half the number optical elements 322 as there are pixels/elements in the entire liquid crystal layer 304 (i.e. there is one optical element 322 for each pixel/element pair). As will be apparent from the following discussion, interfering two light rays increases the range over which the phase can be modulated.

The display 300 also includes a fixed phase modulating element 324 to additionally adjust the phase of the second light ray (from the second element 304b) by a predetermined additional amount. In this example, the fixed phase modulating element 324 is positioned after the liquid crystal layer 304 and receives the second light ray from the second element 304b of the liquid crystal layer 304. In the example of Figure 8, the fixed phase modulating element 324 is part of part of the optical element 326 and only one of the two light rays receives the additional amount of phase modulation. The second light ray from the second element 304b receives an additional phase modulation relative to the first light ray from the first element 304a due to an increased path length through the fixed phase modulating element 324. The increased path length is achieved by providing the optical element 326 with a stepped profile. In this particular example, the path length is increased by an amount to modulate the phase of the second light ray by a fixed additional amount of p/2 radians. In Figure 8, the interference element 322 and the fixed phase modulating element 324 are integrally formed as part of the optical element 326.

Figures 9A-9D depict how the arrangement of Figures 7 and 8 allows phase to be modulated across the full range of 0 to 2p.

Figure 9A depicts a phasor diagram for an example light ray that has passed through a first element 304a of the liquid crystal layer 304, and the linear output polarizer 310.

Since the liquid crystal layer 304 is configured to rotate light in the range of 0 to p/2 radians, Figure 9A is applicable for the full range of voltages that can be applied to the element 304a. As discussed above, due to the orientation of the linear output polarizer 310, when the voltage applied to the first element 304a is 0V, the first light ray (which is linearly polarized) has an amplitude of approximately 50% of its maximum amplitude (shown as +0.5 in Figure 9 A). When the voltage applied to the first element 304a is non-zero, the linearly polarized light ray is rotated by an angle such that the amplitude is decreased. At a particular intermediate voltage, the transmitted first light ray is rotated relative to the output polarizer 310 such that it has an amplitude of approximately zero and does not pass through the polarizer 310. Varying the applied voltage between 0V and the intermediate voltage results in the transmitted first light ray having an amplitude between +0.5 and 0. The dark line along the positive real axis in Figure 9 A therefore shows the range of amplitudes the first light ray may have when the element 304a is driven between 0V and the intermediate voltage. Increasing the applied voltage from the intermediate voltage to a maximum voltage further rotates the light ray and further increases the angle between the polarizer 310 and the light ray. At a maximum voltage the angle between the polarizer 310 and the light ray is such that the output light ray again has an amplitude of approximately 50% of its maximum amplitude (shown as -0.5 in Figure 9A). Varying the applied voltage between the intermediate voltage and a maximum voltage results in the transmitted first light ray having an amplitude between 0 and -0.5. The dark line along the negative real axis in Figure 9A therefore shows the range of amplitudes the first light ray may have when the element 304a is driven between the intermediate voltage and a maximum voltage. As shown in Figure 9A, when the voltage is between 0V and the intermediate voltage the light ray has a phase of 0 and when the voltage is between the intermediate voltage and the maximum voltage, the light ray has a phase of p radians. Accordingly, depending on the voltage applied to the first element 304a, the light ray may have a phase of either 0 or p radians. The liquid crystal layer 304 can therefore selectively adjust the phase of the light ray by a predetermined additional amount of p radians.

Figure 9B depicts a phasor diagram for a second example light ray that has passed through a second element 304b of the liquid crystal layer 304, and the linear output polarizer 310. Since the first and second elements 304a, 304b are of the same type/domain (or the linear output polarizer 310 associated with (i.e. covering) the first element 304a is perpendicular to the linear output polarizer 310 covering the second element 304b), the phasor diagram of Figure 9B corresponds to the phasor diagram of Figure 9A.

After the second light ray of Figure 9B has passed through the second element 304b of the liquid crystal layer 304, the second light ray passes through the fixed phase modulating element 324 to additionally adjust the phase of the second light ray by a predetermined additional amount. In this example, the predetermined additional amount is p/2 radians. Figure 9C therefore depicts a phasor diagram for an example second light ray that has passed through a second element 304b of the liquid crystal layer 304, the linear output polarizer 310 and the fixed phase modulating element 324. The phasor diagram is similar to that of Figure 9B, but has been rotated by p/2 radians.

To achieve the full range of phase modulation, the first light ray from the first element 304a of the liquid crystal layer is interfered with the second light ray from the second element 304b of the liquid crystal layer by the interference element 322. Figure 9D therefore depicts a phasor diagram for the total range of amplitudes and phases of the output light ray that can be achieved by interfering these two light rays. Accordingly, unlike the displays of Figures 1, 2, 4 and 5, the phase is not modulated by a variable amount by a liquid crystal layer. Instead, the full range of phase modulation is achieved using other techniques.

An overall method of modulating the display of Figures 1 and 2 is depicted in Figure 10. More particularly, the method 400 is a method of modulating an amplitude and a phase of a light ray in a holographic display. The method comprises, in block 402, modulating an amplitude of a light ray based on a voltage applied to an element of a first liquid crystal layer, where the light ray passing through the first liquid crystal layer is linearly polarized. Next, the method comprises, at block 404, modulating a phase of the light ray based on a voltage applied to an element of a second liquid crystal layer, where the light ray passing through the second liquid crystal layer is circularly polarized. In some examples, block 502 also comprises selectively adjusting the phase of the light ray by a predetermined additional amount. In some examples, blocks 402 and 404 may be carried out by a processor/controller of the display by selectively controlling the voltages applied to the elements. In other examples, blocks 402 and 404 may be carried out elsewhere, for example by a processing system of an attached computing system.

An overall method of operating the display of Figures 4 and 5 is depicted in Figure 11. More particularly, the method 500 is a method of modulating an amplitude and a phase of a light ray in a holographic display. The method comprises, in block 502, modulating an amplitude of a first light ray based on a voltage applied to a first element of a first element type in a first liquid crystal layer, and modulating an amplitude of a second light ray based on a voltage applied to a second element of a second element type in the first liquid crystal layer, where the light rays passing through the first liquid crystal layer are linearly polarized. Next, the method comprises, at block 504, modulating a phase of the first light ray based on a voltage applied to a first element of a second element type in a second liquid crystal layer, and modulating a phase of the second light ray based on a voltage applied to a second element of a second element type in the second liquid crystal layer, where the light rays passing through the second liquid crystal layer are circularly polarized. In block 506, the method further comprises interfering the first light ray from the first element of the first element type with the second light ray from the second element of the second element type. In some examples, block 506 comprises passing the second light ray through a fixed phase modulating element to additionally adjust the phase of the second light ray by a predetermined additional amount.

In some examples, blocks 502 and 504 may be carried out by a processor/controller of the display by selectively controlling the voltages applied to the elements. In other examples, blocks 502 and 504 may be carried out elsewhere, for example by a processing system of an attached computing system.

An overall method of operating the display of Figures 7 and 8 is depicted in Figure 12. More particularly, the method 600 is a method of modulating an amplitude and a phase of a light ray in a holographic display. The method comprises, in block 602, applying a first voltage to a first element of a liquid crystal layer to cause a first light ray passing through the first element to have a first amplitude, and a phase of 0 or p. In block 604, the method comprises applying a second voltage to a second element of the liquid crystal layer to cause a second light ray passing through the second element to have a second amplitude, and a phase of 0 or p. In block 606, the method comprises additionally adjusting the phase of the second light ray by a predetermined additional amount. In block 608, the method comprises interfering the first light ray with the second light ray.

In some examples, blocks 602 and 604 may be carried out by a processor/controller of the display by selectively controlling the voltages applied to the elements. In other examples, blocks 602 and 604 may be carried out elsewhere, for example by a processing system of an attached computing system.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one 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 of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.