Patent Description:
Head-Mounted Display (HMD) has been widely used in, e.g., video playback, gaming, and sports. One major application of HMD is to realize virtual reality (VR) or augmented reality (AR).

Some VR or AR applications require an eye tracking function that monitors the eye of the user and/or the region surrounding the eye of the user. By monitoring the eye and/or the surrounding region, the HMD can determine a gaze direction of the user, which can be used for improving display quality, performance, and/or user experience, and can be used to address vergence/accommodation conflict. Further, by monitoring the eye and/or the surrounding region, the HMD can estimate the psychological state and/or changes in the psychological state of the user, as well as physical characteristics of the user. The above information can be used by the HMD to, e.g., determine what content to provide to the user. For example, if the user is concentrating on a particular task or activity, the HMD may determine the user prefers not to be interrupted with other information unless such information would be important to the user. However, in conventional technologies, because of the small size of HMD, especially smart glasses, it is difficult to arrange various extra components for eye tracking.

<CIT> describes a polarization volume grating (PVG) that includes a bulk, birefringent medium characterized by a plurality of helical structures with helix axes and a periodicity Ay and an anisotropic alignment material having a rotatable optical axis, disposed on a top or bottom surface of the medium. The PVG is characterized in that the optical axis of the alignment material has a continuously rotated optical axis orientation in a plane of the material surface and a periodicity Ax, wherein the helix axes are normal to the optical axes in the alignment material surface, further wherein the birefringent medium is characterized by a plurality of controllably slanted refractive index planes having a slant angle φ=±arctan (Ay/Ax) and a Bragg period ΛB. Fabrication methods are disclosed.

<CIT> describes an optical device that includes a stack of multiple grating structures, each of which includes a plurality of sublayers of liquid crystal material. Each sublayer of liquid crystal material includes laterally extending repeating units, each formed of a plurality of liquid crystal molecules. The repeating units of the liquid crystal layers are lateral offset from one another, and defined a tilt angle. The grating structures forming the stack of grating structure have tilt angles of different magnitudes. The grating structures may be configured to redirect light of visible or infrared wavelengths. The different tilt angles of the stack of grating structures allows for highly efficient diffraction of light incident on the grating structures at a wide range of incident angles.

In accordance with the disclosure, there is provided an optical system according to claim <NUM>.

In some embodiments, each of the first PVH layer and the second PVH layer may be configured to reflect infrared (IR) light.

In some embodiments, the optical system may further comprise: a light source emitting a light beam to be reflected by a target toward the first PVH layer and the second PVH layer, the light beam having a wavelength in IR spectrum.

In some embodiments, the light source is a first light source, the light beam is a first light beam, and the wavelength is a first wavelength; the optical system further comprising: a second light source emitting a second light beam to be reflected by the target toward the first PVH layer and the second PVH layer, the second light beam having a second wavelength in the IR spectrum and different from the first wavelength.

In some embodiments, the first wavelength corresponds to a first Bragg period of a first Bragg grating formed by liquid crystal (LC) molecules in the first PVH layer; and the second wavelength corresponds to a second Bragg period of a second Bragg grating formed by LC molecules in the second PVH layer.

In some embodiments, the first image and the second image are different perspective views of a same region of a target.

In some embodiments, the first image and the second image are images of different regions of a target.

In some embodiments, the optical system may further comprise: an optical switch arranged between the PVH composite film and the optical sensor, and configured to switch between: a first state transmitting the polarized light reflected by the first PVH layer and blocking the polarized light reflected by the second PVH layer, and a second state transmitting the polarized light reflected by the second PVH layer and blocking the polarized light reflected by the first PVH layer.

In some embodiments, the optical switch includes: a quarter-wave plate; and a switchable linear polarizer configured to switch between two orthogonal polarization directions.

In some embodiments, the first image and the second image are superimposed on each other; the optical system further comprising: a processor configured to separate the first image and the second image based on features in the first image and the second image.

In some embodiments, the optical system may further comprise: a temple arm connected to the substrate; wherein the optical sensor is mounted on the temple arm and faces the PVH composite film.

In some embodiments, the first PVH layer and the second PVH layer have different optical powers.

In some embodiments, the first PVH layer has a first field of view (FOV); the second PVH layer has a second FOV; and one of the first FOV and the second FOV encompasses another one of the first FOV and the second FOV.

In some embodiments, an optical axis of the first PVH layer and an optical axis of the second PVH layer point toward an approximately same direction.

In some embodiments, an optical axis of the first PVH layer and an optical axis of the second PVH layer point toward different directions.

In some embodiments, a field of view (FOV) of the first PVH layer and an FOV of the second PVH layer are approximately same as each other.

In some embodiments, a field of view (FOV) of the first PVH layer and an FOV of the second PVH layer do not encompass each other.

Hereinafter, embodiments consistent with the disclosure will be described with reference to drawings, which are merely examples for illustrative purposes and are not intended to limit the scope of the disclosure. In the drawings, the shape and size may be exaggerated, distorted, or simplified for clarity. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts, and a detailed description thereof may be omitted.

Further, in the present disclosure, the disclosed embodiments and the features of the disclosed embodiments may be combined under conditions without conflicts. The described embodiments are some but not all of the embodiments of the present disclosure. Based on the disclosed embodiments, persons of ordinary skill in the art may derive other embodiments consistent with the present disclosure, all of which are within the scope of the present disclosure, which is set forth in the appended claims.

The present disclosure provides an optical system using one or more polarization volume hologram (PVH) layer configured to reflect infrared (IR) light for, e.g., eye tracking purposes. A PVH layer includes a plurality of liquid crystal (LC) molecules which are spatially orientated to enable at least one optical function of the PVH layer, and is also referred to as, e.g., "polarization sensitive gratings," "polarization sensitive optical elements," "liquid crystal gratings," or "chiral liquid crystal elements. " <FIG> schematically show an example PVH layer <NUM> consistent with the disclosure. <FIG> is a perspective view of the PVH layer <NUM>. <FIG> is a cross-sectional view of the PVH layer <NUM> in the y-z plane. <FIG> is a plan view of the PVH layer <NUM> in the x-y plane. <FIG> is a partial plan view of the PVH layer <NUM> in the x-y plane along the y-axis from a center region of the PVH layer <NUM> to an edge region of the PVH layer <NUM>.

The optical function of a PVH layer can be determined based on the manipulation of optic axes of the LC molecules in the PVH layer. Hereinafter, an orientation of the optic axis of an LC molecule is also referred to as an orientation or alignment of the LC molecule. The manipulation of optic axes of the LC molecules in the PVH layer is a <NUM>-dimensinoal (3D) alignment of the LC molecules. A PVH layer consistent with the disclosure can deflect, light via Bragg diffraction. The Bragg grating in the PVH layer can be created by adding a chiral dopant to induce helical twist along the vertical direction, e.g., the z-axis direction shown in <FIG>.

As shown in <FIG>, in the z-axis direction of the PVH layer <NUM>, the LC molecules twist and the rotating angle changes continuously and periodically along the z-axis with a period of Λz. The period Λz (or pitch length p = 2Λz) can be adjusted by controlling the helical twist power (HTP) and concentration of the chiral dopant. Similarly, an in-plane periodicity in the x-y plane is also introduced into the PVH layer <NUM> by, e.g., modifying the surface alignment of the PVH layer <NUM> to provide a rotation of the LC molecules in the x-y plane. As a result, the Bragg planes in the PVH layer <NUM> become slanted, as indicated by the slanted lines in <FIG>. The distance between neighboring slanted lines is the Bragg period ΛB of the Bragg grating formed by the LC molecules in the PVH layer <NUM>. The Bragg period ΛB can depend on the z-axis period Λz of the LC molecules and a slanting angle of the Bragg planes with respect to a surface of the PVH layer <NUM>.

The slanted Bragg planes can allow the PVH layer <NUM> to redirect incident light to be converged or diverged in reflection or in transmission. Thus, through further manipulation of the orientation of the LC molecules in the x-y plane, the PVH layer <NUM> can be configured to function as a lens, such as a reflective lens, that can, e.g., converge or diverge incident light. The PVH layer is thus also referred to as a "PVH lens.

Consistent with the disclosure, as shown in <FIG> and <FIG>, the PVH layer <NUM> creates a respective lens profile via the in-plane (x-y plane) orientation (azimuth angle θ) of the LC molecules, in which the phase difference T = <NUM>θ. In the PVH layer <NUM>, the azimuth angles of LC molecules change continuously from a center <NUM> to an edge <NUM> of the PVH layer <NUM>, with a varied period Λ, i.e., a distance between two LC molecules whose azimuth angles differ from each other by <NUM>°.

The lens of the PVH layer <NUM> may include a certain symmetry in the arrangement of the LC molecules about an optical axis of the PVH layer <NUM>, which, for example, may pass through the center <NUM> of the PVH layer <NUM>. As shown in <FIG> and <FIG>, the LC molecules in at least a portion of the PVH layer <NUM> are orientated or aligned rotationally-symmetrically (e.g., three-fold, four-fold, six-fold, or eight-fold) about the optical axis of the PVH layer <NUM>. In some embodiments, in the center portion of the PVH layer <NUM>, the LC molecules are aligned rotationally-symmetrically about the optical axis of the PVH layer <NUM>. In some embodiments, the rotational-symmetry of the LC molecules can be axisymmetry, i.e., the LC molecules in the at least one portion can be aligned axisymmetrically about the optical axis of the PVH layer <NUM>.

The change of LC orientation from the center <NUM> to the edge <NUM> of the PVH layer is more clearly seen in the partial plan view of <FIG>. As shown in <FIG>, for the LC orientation, a rate of period variation from the center <NUM> to the edge <NUM> of the PVH layer <NUM> is a function of distance from the center <NUM>, and increases with the distance from the center <NUM>. For example, the period at the center <NUM> (Λ<NUM>) is the longest, the period at the edge <NUM> (Λr) is the shortest, and the period in between (e.g., Λ<NUM>) is moderate, i.e., Λ<NUM> > Λ<NUM> >.

<FIG> is a schematic view showing a portion of an example optical system <NUM> consistent with the disclosure. The optical system <NUM> includes a substrate <NUM> and a PVH layer <NUM>. The substrate <NUM> provides support to the PVH layer <NUM>, and can be, for example, a piece of rigid material, such as glass, a piece of flexible material, such as plastic, or a functional device, such as a display screen. For illustrative purposes, in <FIG>, the substrate <NUM> and the PVH layer <NUM> are shown as spaced apart from each other. In actual implementation, they can contact each other or be spaced apart from each other by, for example, one or more spacing members, or by being held at different places of a frame or a housing of the optical system <NUM>. In some embodiments, additional layer(s), such as protection layer(s) and/or buffer layer(s), can be arranged between the substrate <NUM> and the PVH layer <NUM>.

The PVH layer <NUM> can be any PVH layer consistent with the disclosure, such as the PVH layer <NUM> described above in connection with <FIG>. As shown in <FIG>, the LC molecules are aligned in a helix twist with helix axis along the z-direction. The helix twist can be either left-handed or right-handed. A PVH layer may allow deflection for one circularly polarized light while the other polarization may transmit through. In some embodiments, a PVH layer can deflect circularly polarized light having a same handedness as the helix twist of the PVH layer and transmit circularly polarized light having an orthogonal handedness. In some embodiments, depending on how the LC molecules in the PVH layer are aligned, the PVH layer can either converge or diverge the incident light.

For illustrative purpose and as an example, in the description below, the PVH layer <NUM> is described as having a helix twist of right handedness (indicated by hollow block in the figure). In some other embodiments, the PVH layer <NUM> can have a helix twist of left handedness.

As shown in <FIG>, incident light <NUM> includes two components that are polarized in mutually perpendicular (orthogonal) directions, i.e., a first incident light ray <NUM> having a right-handed circular polarization (indicated by hollow arrow in the figure) and a second incident light ray <NUM> having a left-handed circular polarization (indicated by solid arrow in the figure). Because the first incident light ray <NUM> has a same handedness as the helix twist of the PVH layer <NUM>, the first incident light ray <NUM> is reflected by the PVH layer <NUM> to form a reflected light ray <NUM>. Further, the PVH layer <NUM> does not change the handedness of the polarization of the first incident light ray <NUM>, and hence the reflected light ray <NUM> retains the handedness of the polarization, i.e., also having a right-handed circular polarization. On the other hand, because the second incident light ray <NUM> has a different handedness than the PVH layer <NUM>, the second incident light ray <NUM> passes through the PVH layer <NUM> without being reflected and without changing the handedness of the polarization.

As described above, a PVH layer can reflect incident light by the Bragg grating formed by the LC molecules in the PVH layer. The angle between the incident light ray and the reflected light ray can depend on the wavelength of the light and the Bragg period of the Bragg grating in the PVH layer. Therefore, an angle α between the first incident light ray <NUM> and the reflected light ray <NUM> can depend on the Bragg period of the Bragg grating in the PVH layer <NUM> and the wavelength of the first incident light ray <NUM>.

The optical power of a PVH layer determines a degree to which the PVH layer can converge or diverge light and can be inversely proportional to a focal length or effective focal length of the PVH layer. The optical power of the PVH layer can be adjusted by changing the alignment of the LC molecules in the PVH layer to change the angle of reflection at different points of the PVH layer. Changing the optical power of a PVH layer may also change a field of view (FOV) of the PVH layer.

Similar to the optical power, the optical axis of a PVH layer can also be adjusted by changing the alignment of the LC molecules in the PVH layer. The direction of the optical axis of the PVH layer may or may not be perpendicular to the surface of the PVH layer.

<FIG> is another schematic view of the optical system <NUM>. In <FIG>, the optical system <NUM> is shown as a head-mounted display, such as smart glasses. <FIG> shows a portion of the optical system <NUM>, where the un-shown portion of the optical system <NUM> can be symmetric to the illustrated portion of the optical system <NUM>.

As shown in <FIG>, the optical system <NUM> further includes an optical sensor <NUM> configured to generate an image using polarized light reflected by the PVH layer <NUM>. In some embodiments, the optical sensor <NUM> can be sensible to light having a wavelength within a spectrum that includes IR spectrum. In some embodiments, the optical sensor <NUM> can be sensible to IR light but not visible light. The optical sensor <NUM> can be a camera and can include, for example, a charge-coupled device (CCD) sensor, a complementary metal-oxide-semiconductor (CMOS) sensor, or an N-type metal-oxide-semiconductor (NMOS) sensor.

The optical sensor <NUM> can be mounted at any suitable part of the optical system <NUM>, so long as the optical sensor <NUM> can be arranged to face the PVH layer <NUM> to receive light reflected by the PVH layer <NUM>. In some embodiments, the optical system <NUM> can include a frame or a housing, and the optical sensor <NUM> can be mounted on the frame or the housing. As shown in <FIG>, the optical system <NUM> further includes a mounting member <NUM> for mounting the optical system <NUM> to an object, such as a user's head. The optical sensor <NUM> can be mounted at the mounting member <NUM>. In some embodiments, the optical system <NUM> can include smart glasses and the mounting member <NUM> can include one or more temple arms. The optical sensor <NUM> can be mounted at one of the one or more temple arms and faces the PVH layer <NUM>.

The optical sensor <NUM> can generate images of a region bounded by marginal rays indicated by the dashed lines in <FIG>. Besides the characteristics of the optical sensor <NUM> itself, some other factors can also affect the region being imaged by the optical sensor <NUM>, such as the optical power of the PVH layer <NUM> and the direction of the optical axis of the PVH layer <NUM>. Both the optical power and the optical axis direction of the PVH layer <NUM> can be configured by configuring the alignment of the LC molecules in the PVH layer <NUM>. Therefore, consistent with the disclosure, the optical system <NUM> can be easily designed to image different regions of an object. For example, the object can be the user's head, and the PVH layer <NUM> can be designed in such a manner that the optical system <NUM> can image a portion, such as, a pupil area, of the user's eye, the entire eye of the user, an area near, such as above, below, left to, or right to, the eye of the user, or an area including the eye and the area near the eye. Thus, eye tracking can be realized.

Consistent with the disclosure, because the optical power and the optical axis direction of the PVH layer <NUM> depend on the alignment of the LC molecules in the PVH layer <NUM>, the overall shape and dimensions of the PVH layer <NUM> can remain the same for different optical powers and/or optical axis directions. Further, because the optical power and the optical axis direction of the PVH layer <NUM> do not depend on the orientation of the surface of the PVH layer <NUM>, the PVH layer <NUM> can be designed to reflect an incident light ray at a large angle even if the incident light ray has a zero or small incident angle onto the PVH layer <NUM>. This provides more freedom in arranging the optical sensor <NUM>, and a more compact design of the optical system <NUM> can be achieved.

In some embodiments, the optical system <NUM> can generate images by utilizing IR light emitted or reflected by the target being tracked, such as the user's eye. In some embodiments, as shown in <FIG>, the optical system <NUM> further includes a light source <NUM> configured to emit a light beam to be reflected by the target toward the PVH layer <NUM>. The light beam emitted by the light source <NUM> can include a narrow spectrum or a relatively broad spectrum, and one or more wavelengths of the light beam are in the IR spectrum, i.e., the spectrum of the light source <NUM> can be within, overlap, or encompass the IR spectrum. In some embodiments, at least one wavelength in the spectrum of the light source <NUM> corresponds to the Bragg period of the Bragg grating formed by the LC molecules in the PVH layer <NUM>. In some embodiments, the light beam emitted by the light source <NUM> has a wavelength in the IR spectrum and corresponding to the Bragg period of the Bragg grating in the PVH layer <NUM>. The wavelength of the light beam can be, e.g., from about <NUM> to about <NUM>, such as about <NUM>, about <NUM>, or about <NUM>. The Bragg period of the Bragg grating in the PVH layer <NUM> can be, e.g., from about <NUM> to about <NUM>, or centered at about <NUM> or about <NUM>. In some embodiments, the Bragg period can be longer, such as about <NUM>, about <NUM>, or about <NUM>.

<FIG> is a schematic view showing a portion of another example optical system <NUM> consistent with the disclosure. The optical system <NUM> includes a substrate <NUM> and a PVH composite film <NUM> formed over the substrate <NUM>. The substrate <NUM> provides support to the PVH composite film <NUM>, and can be, for example, a piece of rigid material, such as glass, a piece of flexible material, such as plastic, or a functional device, such as a display screen. As shown in <FIG>, the PVH composite film <NUM> includes a first PVH layer <NUM> formed over the substrate <NUM> and a second PVH layer <NUM> coupled to the first PVH layer <NUM>. For illustrative purposes, in <FIG>, the substrate <NUM>, the first PVH layer <NUM>, and the second PVH layer <NUM> are shown as spaced apart from each other. In actual implementation, they can contact each other or be spaced apart from each other by, for example one or more spacing members, or by being held at different places of a frame or a housing of the optical system <NUM>. In some embodiments, additional layer(s), such as protection layer(s) and/or buffer layer(s), may be arranged between each neighboring pair of the substrate <NUM>, the first PVH layer <NUM>, and the second PVH layer <NUM>.

Each of the first PVH layer <NUM> and the second PVH layer <NUM> can be a PVH layer consistent with the disclosure, such as the PVH layer <NUM> described above in connection with <FIG>. In some embodiments, the handedness of the helix twist of the first PVH layer <NUM> can be different from (orthogonal to) the handedness of the helix twist of the second PVH layer <NUM>. For example, one of the first PVH layer <NUM> and the second PVH layer <NUM> can have a left-handed helix twist and the other one of the first PVH layer <NUM> and the second PVH layer <NUM> can have a right-handed helix twist. For illustrative purposes and as examples, in the description below, the first PVH layer <NUM> is described as having a left handedness (indicated by solid block in the figure) and the second PVH layer <NUM> is described as having a right handedness (indicated by hollow block in the figure). In some other embodiments, the first PVH layer <NUM> can have a right handedness and the second PVH layer <NUM> can have a left handedness.

As shown in <FIG>, incident light <NUM> includes two components that are polarized in mutually perpendicular (orthogonal) directions: - a first incident light ray <NUM> having a right-handed circular polarization (indicated by hollow arrow in the figure) and a second incident light ray <NUM> having a left-handed circular polarization (indicated by solid arrow in the figure). Because the first incident light ray <NUM> has a same handedness as the helix twist of the second PVH layer <NUM>, the first incident light ray <NUM> is reflected by the second PVH layer <NUM> to form a first reflected light ray <NUM>. Further, the second PVH layer <NUM> does not change the handedness of the polarization of the first incident light ray <NUM>, and hence the first reflected light ray <NUM> retains the handedness of the polarization, i.e., also having a right-handed circular polarization.

On the other hand, because the second incident light ray <NUM> has a different handedness than the second PVH layer <NUM>, the second incident light ray <NUM> passes through the second PVH layer <NUM> without being reflected and without changing the handedness of the polarization. When the second incident light ray <NUM> hits the first PVH layer <NUM>, it is reflected by the first PVH layer <NUM> that has a same handedness, forming a second reflected light ray <NUM> having a left-handed circular polarization. The second reflected light ray <NUM> passes through the second PVH layer <NUM> without being reflected and without changing the handedness of the polarization.

In some embodiments, the first incident light ray <NUM> and the second incident light ray <NUM> can have an approximately same wavelength. In these embodiments, the deflection angle α<NUM> between the first incident light ray <NUM> and the first reflected light ray <NUM> can depend on the Bragg period of the Bragg grating in the second PVH layer <NUM>; and the deflection angle α<NUM> between the second incident light ray <NUM> and the second reflected light ray <NUM> can depend on the Bragg period of the Bragg grating in the first PVH layer <NUM>. In some embodiments, the first PVH layer <NUM> and the second PVH layer <NUM> can have different Bragg periods so that the angles α<NUM> and α<NUM> can be different from each other.

In some embodiments, the LC molecules of the first PVH layer <NUM> and the LC molecules of the second PVH layer <NUM> can be arranged such that the first PVH layer <NUM> and the second PVH layer <NUM> have an approximately same optical power. In some other embodiments, the LC molecules of the first PVH layer <NUM> and the LC molecules of the second PVH layer <NUM> can be arranged in such a manner that the first PVH layer <NUM> and the second PVH layer <NUM> have different optical powers. Changing the optical power of a PVH layer may also change an FOV of the PVH layer. Therefore, the first PVH layer <NUM> and the second PVH layer <NUM> can be configured to have different FOVs. In some embodiments, one of the FOV of the first PVH layer <NUM> and the FOV of the second PVH layer <NUM> can encompass another one of the FOV of the first PVH layer <NUM> and the FOV of the second PVH layer <NUM>.

In some embodiments, the LC molecules of the first PVH layer <NUM> and the LC molecules of the second PVH layer <NUM> can be arranged such that the optical axis of the first PVH layer <NUM> and the optical axis of the second PVH layer <NUM> point toward an approximately same direction. In some other embodiments, the LC molecules of the first PVH layer <NUM> and the LC molecules of the second PVH layer <NUM> can be arranged such that the optical axis of the first PVH layer <NUM> and the optical axis of the second PVH layer <NUM> point toward different directions.

With different arrangements of the LC molecules in the first PVH layer <NUM> and the arrangement of the LC molecules in the second PVH layer <NUM>, different combinations of optical powers, FOVs, and optical axis directions can be achieved. For example, the first PVH layer <NUM> and the second PVH layer <NUM> can have an approximately same optical power, and their optical axes can point toward different directions. As another example, the first PVH layer <NUM> and the second PVH layer <NUM> can have different optical powers, and their optical axes can point toward different directions. As a further example, the optical axes of the first PVH layer <NUM> and the second PVH layer <NUM> can point toward an approximately same direction, but the first PVH layer <NUM> and the second and the second PVH layer <NUM> can have different optical powers so that the FOV of one of the first PVH layer <NUM> and the second PVH layer <NUM> can encompass the FOV of the other one of the first PVH layer <NUM> and the second PVH layer <NUM>. As a further example, the optical axes of the first PVH layer <NUM> and the second PVH layer <NUM> can point toward different directions, and the FOV of the first PVH layer <NUM> and the FOV of the second PVH layer <NUM> may or may not encompass each other, or may or may not overlap with each other. Various other combinations are possible but not listed here.

As shown in <FIG>, the optical system <NUM> further includes an optical sensor <NUM> configured to generate a first image using polarized light reflected by the first PVH layer <NUM> and to generate a second image using polarized light reflected by the second PVH layer <NUM>. In some embodiments, the optical sensor <NUM> can be sensible to light having a wavelength within a spectrum that includes IR spectrum. In some embodiments, the optical sensor <NUM> can be sensible to IR light but not visible light. The optical sensor <NUM> can be a camera and can include, for example, a charge-coupled device (CCD) sensor, a complementary metal-oxide-semiconductor (CMOS) sensor, or an N-type metal-oxide-semiconductor (NMOS) sensor.

The optical sensor <NUM> can be mounted at any suitable part of the optical system <NUM>, so long as the optical sensor <NUM> can be arranged to face the PVH composite film <NUM> to receive light reflected by the first PVH layer <NUM> and the light reflected by the second PVH layer <NUM>. In some embodiments, the optical system <NUM> can include a frame or a housing, and the optical sensor <NUM> can be mounted on the frame or the housing. As shown in <FIG>, the optical system <NUM> further includes a mounting member <NUM> for mounting the optical system <NUM> to an object, such as a user's head. The optical sensor <NUM> can be mounted at the mounting member <NUM>. In some embodiments, the optical system <NUM> can include smart glasses and the mounting member <NUM> can include one or more temple arms. The optical sensor <NUM> can be mounted at one of the one or more temple arms and faces the PVH composite film <NUM>.

As described above, optical powers and optical axis directions of the first PVH layer <NUM> and the second PVH layer <NUM> can be configured by manipulating the arrangements of the LC molecules in the first PVH layer <NUM> and the second PVH layer <NUM>. With different combinations of the arrangements of the LC molecules in the first PVH layer <NUM> and the second PVH layer <NUM>, different combinations of imaging regions can be realized.

<FIG> and <FIG> show two examples of imaging regions resulting from different combinations of arrangements of the LC molecules in (and hence different optical powers and/or optical axis directions of) the first PVH layer <NUM> and the second PVH layer <NUM>. In <FIG> and <FIG>, the short-dashed lines indicate marginal rays bounding the imaging region of the first PVH layer <NUM> and the long-dashed lines indicate marginal rays bounding the imaging region of the second PVH layer <NUM>.

In the example shown in <FIG>, the optical power of the first PVH layer <NUM> can be smaller than the optical power of the second PVH layer <NUM>, and the optical axes of the first PVH layer <NUM> and the second PVH layer <NUM> can point to an approximately same direction or slightly different directions. The FOV of the second PVH layer <NUM> encompasses the FOV of the first PVH layer <NUM>. Thus, as shown in <FIG>, the second PVH layer <NUM> can image a larger region than the first PVH layer <NUM>. For example, the first PVH layer <NUM> can image a pupil area of the user's eye and the second PVH layer <NUM> can image the entire eye of the user.

In the example shown in <FIG>, the optical powers of the first PVH layer <NUM> and the second PVH layer <NUM> can be approximately the same as each other, and the optical axes of the first PVH layer <NUM> and the second PVH layer <NUM> can point to different directions. The FOVs of the first PVH layer <NUM> and the second PVH layer <NUM> can be approximately the same as each other. Thus, as shown in <FIG>, the first PVH layer <NUM> and the second PVH layer <NUM> can image an approximately same region from different perspectives. That is, the image generated by the optical sensor <NUM> using the polarized light reflected by the first PVH layer <NUM> and the image generated by the optical sensor <NUM> using the polarized light reflected by the second PVH layer <NUM> can be approximately a same region of the target. For example, the first PVH layer <NUM> can image the pupil area from the left perspective (the lower perspective in the figure as presented in the drawing sheet, indicated by the short-dashed lines) and the second PVH layer <NUM> can image the pupil area from the right perspective (the upper perspective in the figure as presented in the drawing sheet, indicated by the long-dashed lines).

When imaging is performed from only one perspective, accuracy of eye tracking may decrease when the user looks away from an image of the optical sensor <NUM> (formed by the first PVH layer <NUM> and/or the second PVH layer <NUM>). On the other hand, consistent with the disclosure, using two PVH layers to allow imaging the user's eye from different perspectives can increase the accuracy of eye tracking when the user's eye moves. For example, as shown in <FIG>, when the user looks to the left, the first PVH layer <NUM> can provide a higher tracking accuracy, and when the user looks to the right, the second PVH layer <NUM> can provide a higher tracking accuracy.

In the example shown in <FIG>, the FOVs of the first PVH layer <NUM> and the second PVH layer <NUM> are approximately the same as each other. In some other embodiments, with the approximately same optical powers, the optical axis directions of the first PVH layer <NUM> and the second PVH layer <NUM> can be configured in such a manner that the FOVs of the first PVH layer <NUM> and the second PVH layer <NUM> are different from each other but do not encompass each other.

In some embodiments, the optical system <NUM> can generate images by utilizing IR light emitted or reflected by the target being tracked, such as the user's eye. In some embodiments, as shown in, e.g., <FIG>, the optical system <NUM> further includes a light source <NUM> configured to emit a light beam to be reflected by the target toward the PVH composite film <NUM>. The light beam emitted by the light source <NUM> can include a narrow spectrum or a relatively broad spectrum, and one or more wavelengths of the light beam are in the IR spectrum, i.e., the spectrum of the light source <NUM> can be within, overlap, or encompass the IR spectrum. In some embodiments, the light source <NUM> can have a relatively broad spectrum, and at least one wavelength in the spectrum of the light source <NUM> corresponds to the Bragg period of the Bragg grating formed by the LC molecules in the first PVH layer <NUM> and/or the Bragg period of the Bragg grating formed by the LC molecules in the second PVH layer <NUM>. In some embodiments, the light beam emitted by the light source <NUM> can have a relatively narrow spectrum having a peak wavelength in the IR spectrum, and the peak wavelength can correspond to the Bragg period of the Bragg grating in the first PVH layer <NUM> and/or the Bragg period of the Bragg grating in the second PVH layer <NUM>. The wavelength of the light beam can be, e.g., from about <NUM> to about <NUM>, such as about <NUM>, about <NUM>, or about <NUM>. The Bragg period of the Bragg grating in the first PVH layer <NUM> can be, e.g., from about <NUM> to about <NUM>, or centered at about <NUM> or about <NUM>. The Bragg period of the Bragg grating in the second PVH layer <NUM> can be the same as or different from the Bragg period of the Bragg grating in the first PVH layer <NUM>, and can be, e.g., from about <NUM> to about <NUM>, or centered at about <NUM> or about <NUM>. In some embodiments, the Bragg period in one or both of the first PVH layer <NUM> and the second PVH layer <NUM> can be longer, such as about <NUM>, about <NUM>, or about <NUM>.

In some embodiments, as shown in, e.g., <FIG>, the light source <NUM> is a first light source <NUM> and the light beam emitted by the light source <NUM> is a first light beam, and the optical system <NUM> further includes a second light source <NUM> configured to emit a second light beam to be reflected by the target toward the PVH composite film <NUM>. The second light beam emitted by the second light source <NUM> can include a narrow spectrum or a relatively broad spectrum, and one or more wavelengths of the light beam are in the IR spectrum, i.e., the spectrum of the light source <NUM> can be within, overlap, or encompass the IR spectrum.

In some embodiments, the spectrum of the second light beam can be different from the spectrum of the first light beam. In some embodiments, the first light beam can have a first wavelength in the IR spectrum, the second light beam can have a second wavelength in the IR spectrum, and the first wavelength and the second wavelength can be different from each other. In some embodiments, the first wavelength can correspond to the Bragg period of the Bragg grating formed by the LC molecules in the first PVH layer <NUM>, and the second wavelength can correspond to the Bragg period of the Bragg grating formed by the LC molecules in the second PVH layer <NUM>. For example, the first wavelength can be about <NUM> and the Bragg period of the Bragg grating in the first PVH layer <NUM> can be about <NUM>, and the second wavelength can be about <NUM> and the Bragg period of the Bragg grating in the second PVH layer <NUM> can be about <NUM>.

In the embodiments described above in connection with <FIG>, the optical sensor <NUM> generates a first image using polarized light reflected by the first PVH layer <NUM> and a second image using polarized light reflected by the second PVH layer <NUM>. The two images can be images of two different areas (which may be areas that one encompasses another, one overlaps another, and one separated from another), or images of the same area from different perspective. The polarized light reflected by the two PVH layers may be projected to an approximately same area of the optical sensor <NUM>. Therefore, if the polarized light reflected by the two PVH layers is received by the optical sensor <NUM> at the same time, the two images may superimpose on each other and the resulting image may also be referred to as a superimposed image.

The superimposed image can be processed to obtain the two individual images. In some embodiments, the superimposed image can be processed based on characteristics of the target to separate the two images. For example, when the target is the user's eye, cues of the user's eye can be used in processing the superimposed image. The cues of the user's eye can include binocular cues, such as stereopsis, eye convergence, disparity, and yielding depth from binocular vision through exploitation of parallax, and/or monocular cues, such as size, grain, and motion parallax of the optokinetic response.

Another approach is to temporally separate the first image and the second image, i.e., allowing the light reflected by the first PVH layer <NUM> and the light reflected by the second PVH layer <NUM> to enter the optical sensor <NUM> at different times. In some embodiments, as shown in, e.g., <FIG>, the optical system <NUM> further includes an optical switch <NUM> arranged between the PVH composite film <NUM> and the optical sensor <NUM>. The optical switch <NUM> can be attached to the optical sensor <NUM> or attached to another part of the optical system <NUM>, such as the mounting member <NUM>. In some embodiments, the optical switch <NUM> can be configured to be a part of the optical sensor <NUM>.

The optical switch <NUM> can be configured to switch from a first state to a second state and vice versa. In the first state, the optical switch <NUM> can transmit the polarized light reflected by the first PVH layer <NUM> and block the polarized light reflected by the second PVH layer <NUM>. In the second state, the optical switch <NUM> can transmit the polarized light reflected by the second PVH layer <NUM> and block the polarized light reflected by the first PVH layer <NUM>.

In some embodiments, the optical switch <NUM> can include a quarter-wave plate and a switchable linear polarizer. The quarter-wave plate can be configured to convert circularly polarized light reflected by the first PVH layer <NUM> into first linearly polarized light, and convert second circularly polarized light reflected by the second PVH layer <NUM> into second linearly polarized light. Because the first circularly polarized light and the second circularly polarized light have orthogonal polarization directions, the polarization direction of the first linearly polarized light and the polarization direction of the second linearly polarized light can also be orthogonal to each other.

Correspondingly, the switchable linear polarizer can be configured to switch between the two orthogonal polarization directions. As such, when the polarization direction of the switchable linear polarizer is along the polarization direction of the first linearly polarized light, the optical switch <NUM> can transmit the light reflected by the first PVH layer <NUM> and block the light reflected by the second PVH layer <NUM>. On the other hand, when the polarization direction of the switchable linear polarizer is along the polarization direction of the second linearly polarized light, the optical switch <NUM> can transmit the light reflected by the second PVH layer <NUM> and block the light reflected by the first PVH layer <NUM>.

In some embodiments, the linear polarizer can be rotated to switch between the two orthogonal polarization directions. In some embodiments, the linear polarizer can include two pieces of polarizing materials having orthogonal linear polarization directions, and the polarization direction of the linear polarizer can be switched by mechanically moving one of the two pieces of polarizing materials into the optical path between the quarter-wave plate and the optical sensor <NUM>. In some embodiments, the linear polarizer can include a switchable material that can change polarization direction under an external actuation. For example, the linear polarizer can include an LC film and the LC molecules in the LC film can rotate to different directions when different external voltages are applied, e.g., to form a half-wave plate.

A quarter-wave plate can convert circularly polarized light into linearly polarized light when the following condition is satisfied: d × Δn = (<NUM>+<NUM>)λ / <NUM>, where d and Δn denote the thickness and the birefringence of the quarter-wave plate, λ denotes the wavelength of the light in the vacuum, and m is a non-negative integer. Therefore, when the optical switch <NUM> includes a quarter-wave plate and a switchable linear polarizer, the light reflected by the first PVH layer <NUM> and the light reflected by the second PVH layer <NUM> may need to have an approximately same wavelength. In these embodiments, the optical system <NUM> may either have one light source, such as the light source <NUM>, or have multiple light sources, such as the light source <NUM> and the light source <NUM>, that emit light beams having approximately same wavelength.

As described above, in some embodiments, the optical system <NUM> have two light sources - the light source <NUM> and the light source <NUM>, and the two light sources can emit light beams having different wavelengths (the first wavelength and the second wavelength) that can be reflected by the first PVH layer <NUM> and the second PVH layer <NUM>, respectively. In these embodiments, the optical switch can include a switchable absorber that can switch between two states. In one of the two states, the absorber can absorb light having the first wavelength but not the light having the second wavelength, and in the other one of the two states, the absorber can absorb light having the second wavelength but not the light having the first wavelength. The switchable absorber can switch between the two states under an external control.

In the embodiments described above, the light reflected by the first PVH layer <NUM> and the light reflected by the second PVH layer <NUM> may be projected to an approximately same area of the optical sensor <NUM> and hence the image formed by the light from the first PVH layer <NUM> and the image formed by the light from the second PVH layer <NUM> may be superimposed on each other. In some other embodiments, the first PVH layer <NUM> and the second PVH layer <NUM> can be configured such that the light reflected by the first PVH layer <NUM> and the light reflected by the second PVH layer <NUM> can be projected to two different areas of the optical sensor <NUM>, so as to avoid the images being superimposed on each other. In these embodiments, electric signals from the two different areas of the optical sensor <NUM> can be processed separately to obtain the images from the two PVH layers. In some embodiments, the optical sensor <NUM> may be longer in one dimension as compared to the other dimension.

In some embodiments, the optical system <NUM> may include two optical sensors, referred to as a first optical sensor and a second optical sensor, arranged side by side and each being associated with one circular polarizer covering an aperture of the corresponding optical sensor. A first circular polarizer associated with the first optical sensor can have a same handedness of polarization as the light reflected by the first PVH layer <NUM>. As such, light reflected by the second PVH layer <NUM> may be blocked by the first circular polarizer, while the light reflected by the first PVH layer <NUM> can transmit through the first circular polarizer and form image in the first optical sensor. Similarly, a second circular polarizer associated with the second optical sensor can have a same handedness of polarization as the light reflected by the second PVH layer <NUM>. As such, the light reflected by the first PVH layer <NUM> may be blocked by the second circular polarizer, while the light reflected by the second PVH layer <NUM> can transmit through the second circular polarizer and form image in the second optical sensor.

In some embodiments, the optical system <NUM> may further include a geometric phase lens arranged between the PVH composite film <NUM> and the optical sensor <NUM>. The geometric phase lens can be configured to further divert light from one or both of the first PVH layer <NUM> and the second PVH layer <NUM>, and hence effectively alter the focal length or effective focal length of the first PVH layer <NUM> and/or the focal length or effective focal length of the second PVH layer <NUM>. As a result, a relative focal length of the first PVH layer <NUM> relative to the second PVH layer <NUM> can be altered. For example, the first PVH layer <NUM> may have a relatively short focal length and the second PVH layer <NUM> may have a relatively long focal length. Therefore, an effective depth of field of the optical system <NUM> as a whole may be increased.

The geometric phase lens can be arranged at any suitable location along the optical path from the PVH composite film <NUM> to the optical sensor <NUM>. For example, the geometric phase lens can be arranged in front of the composite film <NUM>, between the first PVH layer <NUM> and the second PVH layer <NUM>, in front of the optical sensor <NUM>, or integrated within the optical sensor <NUM>.

The operation of the optical system consistent with the disclosure, such as the optical system <NUM> or the optical system <NUM> described above, can be controlled locally by a controller of the optical system <NUM>. <FIG> shows a block diagram of an example controller <NUM> consistent with the disclosure. As shown in <FIG>, the controller <NUM> includes one or more processors <NUM> and one or more memories <NUM> coupled to the one or more processors <NUM>. The one or more memories <NUM> can store instructions that, when executed by the one or more processors <NUM>, cause the one or more processors <NUM> to perform a method consistent with the disclosure, such as one of the example functions described above. For example, the instructions can cause the one or more processors <NUM> to process and record the images generated by the optical sensor <NUM> or <NUM>, or to control the light source <NUM> or the light sources <NUM> and <NUM> to turn on or off. In the optical system <NUM>, the instructions can cause the one or more processors <NUM> to separate the superimposed image to obtain the two individual images according to, e.g., the example method described above. The instructions can also cause the one or more processors <NUM> to control the optical switch to switch between the first state and the second state.

Each of the one or more processors <NUM> can include any suitable hardware processor, such as a microprocessor, a micro-controller, a central processing unit (CPU), a graphic processing unit (GPU), a network processor (NP), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or another programmable logic device, discrete gate or transistor logic device, discrete hardware component. Each of the one or more memories <NUM> can include a non-transitory computer-readable storage medium, such as a random access memory (RAM), a read only memory, a flash memory, a hard disk storage, or an optical media.

The foregoing description of the embodiments of the disclosure have been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

Some portions of this description describe the embodiments of the disclosure in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof.

Embodiments of the disclosure may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability.

Claim 1:
An optical system (<NUM>) comprising:
a substrate (<NUM>); and
a polarization volume hologram, PVH, composite film (<NUM>) formed over the
substrate, the PVH composite film including:
a first PVH layer (<NUM>) formed over the substrate and having a helix twist of a first handedness, the first PVH layer being configured to reflect and converge circularly polarized light having the first handedness; and
a second PVH layer (<NUM>) coupled to the first PVH layer and having a helix twist of a second handedness orthogonal to the first handedness, the second PVH layer being configured to reflect and converge circularly polarized light having the second handedness; and
an optical sensor (<NUM>) arranged to receive polarized light reflected from the first PVH layer and polarized light reflected from the second PVH layer and
configured to generate a first image using polarized light reflected by the first PVH layer and to generate a second image using polarized light reflected by the second PVH layer.