Patent Description:
Recently, there has been a resurgence in the interest in virtual reality (VR) and augmented reality (AR) devices and other such near eye devices. These devices typically include a video transmitter of some sort, such as a light engine, and optics couple to the video transmitter configured to transmit images to the eyes of the user using the devices. In particular, a user will wear a headset or similar device that includes a video transmitter optically coupled to one or more waveguides where the waveguides are configured to optically couple images out to a user.

One problem that has needed to addressing by manufacturers of such devices is a problem related to limited Field of View (FoV). In the contexts illustrated herein, the FoV is the number of degrees of visual high angle assuming a fixed eye position. Horizontally, the FoV for a human is around <NUM>°. However, often virtual reality and augmented reality devices will have a much lower FoV available. The lower the FoV available from the device, the less realistic the experience with the device.

Technologies have been implemented which attempt to widen the FoV. One such technology is the use of diffraction gratings which spread the light by wavelength to increase the FoV. That is, a diffraction grating is dispersive, which means that it creates diffraction orders such that the colors of all non-zero orders propagate in different directions. While this behavior is highly beneficial, e.g., in spectroscopic applications, in AR/VR devices based on diffractive waveguides it is unwanted, since carrying and expanding the image content in the waveguide requires three (or in some cases two) separate waveguides unless the FoV is very small.

Having multiple waveguides greatly complicates the manufacturing process. Not only one must manufacture several waveguides but manufacturing tolerances become much tighter. In addition, one must accurately put the multiple plates in a grating stack, which adds additional manufacturing steps which require high accuracy, and increased cost.

<CIT> describes an optical device is disclosed for expanding input light in two dimensions in an augmented reality display. The device comprises a waveguide (<NUM>) and three linear diffraction gratings H0, H1, H2. An incident beam from a projector illuminates an input grating H0 with polychromatic light, and the light is coupled into the waveguide (<NUM>). The other two gratings H1, H2 are overlaid on top of one another. Light can be diffracted by one grating H1 into a first diffracted order and towards the other grating H2 which can couple the light out of the waveguide (<NUM>) towards a viewer. In another arrangement the crossed gratings H1, H2 may be replaced by a photonic crystal (<NUM>) having a regular array of pillars (<NUM>) which create a number effective diffraction gratings.

<CIT> describes a first optical arrangement is configured to couple into an apparatus a first component of a light beam having a wavelength within a first spectral range; a second optical arrangement configured to couple a second component of the light beam having a wavelength within a different second spectral range; a third optical arrangement configured to expand the first component in a first dimension to create an expanded first component; a fourth optical arrangement configured to expand, in a second dimension, the expanded first component to create a further expanded first component, and to output the further expanded first component; a fifth optical arrangement configured to expand the second component in the second dimension to create an expanded second component; and a sixth optical arrangement configured to expand, in the first dimension, the expanded second component to create a further expanded second component, and to output the further expanded second component.

In order to describe the manner in which the above-recited and other advantages and features can be obtained, a more particular description of the subject matter briefly described above will be rendered by reference to the appended drawings:.

The aspects herein -below are to be considered in all respects only as illustrative, while being nevertheless useful for understanding the invention. Some aspects illustrated herein may include or may be used to implement a diffractive waveguide based AR/VR device that <NUM>) carries virtual content from a light engine to the front of a user's eye and <NUM>) expands the pupil, thus enlarging the eye box. In particular, some aspects, can support a large FoV (e.g., <NUM> x <NUM>°) in a single waveguide plate for multiple different wavelengths. Aspects are configured to support red, green, and blue (RGB) wavelengths with a large FoV, in a single waveguide. Thus, aspects may carry large FoV RGB content through a single waveguide. This can be accomplished in some aspects as now illustrated.

Some aspects include various diffractive optical elements (DOEs) in a waveguide to accomplish the functionality described herein. In one aspect an incoupling grating (referred to herein as DOE1) diffracts light into two or more directions such that one spectrum of wavelengths, e.g., the red light, is diffracted primarily in a different direction(s) than another spectrum of wavelengths e.g., blue light. Green light is split between these directions. This can be accomplished, by using a single-sided crossed grating (doubly-periodic grating) or by using linear gratings on the two surfaces of the waveguide. While the examples illustrated herein refer to the two (or more) paths through the waveguide as the red path and the blue path, it should be appreciated that other color spectrum paths may be implemented. Further, it should be appreciated that in practice, part of red light (or other colors) naturally goes through the blue path (or other colors), and vice versa.

As will be illustrated in further detail below, there are different expansion gratings (illustrated herein as DOE2) for the blue path and the red path. Both have at least one distinct wing of DOE2 but may also have more. The number of wings for DOE2 can also be unequal for these two paths.

The out-coupling grating (illustrated herein as DOE3) has two different periods and orientations (or more if multiple colors are handled separately) for the red and blue. Again, this can be done by crossed grating on one side of a grating or "crossed" linear gratings, one on each of the different surfaces of the waveguide.

Note that in components where light reaches DOE3 by multiple possible paths, each path obeys a zero summation rule separately such that the summations of vectors for each path sums to approximately zero, as illustrated in more detail below.

Thus, in general, aspects split the FoV of different colors into two (or more) paths, carry the partial FoVs to DOE3 while expanding the pupil by pupil replication, and at DOE3 recombining the different contributions of each color.

<FIG> shows a near-eye display device in which aspects can be practiced. The near-eye display device <NUM> may be a virtual reality (VR) and/or augmented reality (AR) device that can provide a VR or AR experience with the user. In a VR experience, essentially the entire visual experience is provided by the VR device's light engine. In an AR experience, the light engine is used to transmit images onto a transparent protective visor. In this way, the visual experience includes elements provided by the light engine of the VR device as well as objects that can be seen visually by the user through the transparent protective visor. In the examples illustrated herein, the near-eye display device <NUM> is designed for AR visualization, but VR devices can be implemented using the principles illustrated.

In the illustrated aspect, the near-eye display device <NUM> includes a chassis <NUM>, a transparent protective visor <NUM> mounted to the chassis <NUM>, and left and right side arms <NUM> mounted to the chassis <NUM>. The visor <NUM> forms a protective enclosure for various display elements shown in <FIG>.

A display assembly <NUM> (see <FIG>) that can generate images for AR/VR visualization is also mounted to the chassis <NUM> and enclosed within the protective visor <NUM>. The visor assembly <NUM> and/or chassis <NUM> may also house electronics to control the functionality of the display assembly <NUM> and other functions of the near-eye display device <NUM>. The near-eye display device <NUM> further includes an adjustable headband <NUM> attached to the chassis <NUM>, by which the near-eye display device <NUM> can be worn on a user's head.

<FIG> shows a side view of display components that may be contained within the visor <NUM> of the near-eye display device <NUM>, in some aspects of the invention. During operation of the near-eye display device <NUM>, the display components are positioned relative to the user's left eye <NUM>L or right eye <NUM>R. The display components are mounted to the interior surface of the chassis <NUM>. The chassis <NUM> is shown in cross-section in <FIG>.

In an AR application, the display components are designed to overlay three-dimensional images on the user's view of a real-world environment viewable through the transparent protective visor <NUM>, e.g., by projecting light into the user's eyes. Accordingly, the display components include a display module <NUM> that houses a light engine including components such as: one or more light sources (e.g., one or more light emitting diodes (LEDs)); one or more microdisplay imagers, such as liquid crystal on silicon (LCOS), liquid crystal display (LCD), digital micromirror device (DMD); and one or more lenses, beam splitters and/or waveguides. The microdisplay imager(s) (not shown) within the display module <NUM> may be connected via a flexible circuit connector <NUM> to a printed circuit board <NUM> that has image generation/control electronics mounted on it.

The display components further include a transparent waveguide carrier <NUM> to which the display module <NUM> is mounted, and one or more output waveguides <NUM> on the user's side of the waveguide carrier <NUM>, for each of the left eye and right eye of the user. A single waveguide implements the functionality described herein. The waveguide carrier <NUM> has a central nose bridge portion <NUM>, from which its left and right waveguide mounting surfaces extend. Waveguides <NUM> are implemented on each of the left and right waveguide mounting surfaces of the waveguide carrier <NUM>, to project light emitted from the display module and representing images into the left eye <NUM>L and right eye <NUM>R, respectively, of the user. The display assembly <NUM> can be mounted to the chassis <NUM> through a center tab <NUM> located at the top of the waveguide carrier <NUM> over the central nose bridge section <NUM>.

The near-eye display device can provide light representing an image to an optical receptor (e.g., an eye) of a user. The user may be, e.g., a human, an animal or a machine.

<FIG> shows an example of an output waveguide that can be mounted on the waveguide carrier <NUM> to convey light to one eye of the user. A similar waveguide can be designed for the other eye (or eyes), for example, as a (horizontal) mirror image of the waveguide shown in <FIG>. The waveguide <NUM> is transparent (although diffractive) and, as can be seen from <FIG>, would normally be disposed directly in front of the eye of the user during operation of the near-eye display device, e.g., as one of the waveguides <NUM> in <FIG>. The waveguide <NUM> is, therefore, shown from the user's perspective during operation of the near-eye display device <NUM>.

The waveguide <NUM> includes a single input port <NUM>, which is a DOE indicated as DOE1 (also called in-coupling element). The input port <NUM> may be formed from a surface diffraction grating a volume diffraction grating or a reflective component.

In the example illustrated herein, the input port <NUM> is configured to diffract input light into two or more spectra (with some leakage of the other specta) and to diffract those two or more spectra in different directions. This causes the different spectra to take different paths on the transmission channel <NUM> illustrated in <FIG>.

This is illustrated in one detailed example illustrated in <FIG> illustrates a wave vector space representation. <FIG> shows a transverse wave vector space representation of light waves being diffracted by DOE1, input port <NUM> in the waveguide <NUM>. The inner solid circle <NUM> represents the border of total internal refraction (TIR) condition. The outer solid circle <NUM> represents the border of evanescent waves.

Therefore, any light waves in the doughnut shaped portion between concentric circles <NUM> and <NUM> propagate in the waveguide <NUM> by total internal reflection (TIR). Any light waves in the inner circle <NUM> are waves propagate in the waveguide and then exit into the air. In other words, those light waves propagate in the waveguide and then exit from the waveguide. Any light waves outside of the outer circle <NUM> are evanescent waves that are not coupled into the waveguide.

<FIG> shows two grating vectors for DOE1 and the FOVs diffracted by DOE1 of the waveguide <NUM>. In particular, <FIG> shows a DOE1 blue path, -<NUM> order, a DOE1 red path, -<NUM> order, a DOE1 red path, +<NUM> order and a DOE1 blue path, +<NUM> order.

The DOE1 includes a linear grating with a first grating orientation and period on the front surface of the grating and a second grating orientation and period on the back of the grating. The first grating diffracts one spectrum of light, and the second grating diffracts a second spectrum of light.

Referring once again to <FIG>, the waveguide <NUM> includes a transmission channel <NUM>. The transmission channel includes a DOE, referred to herein as DOE2. Note that DOE2 has several different wings, including DOE2 top left, DOE2 top right, DOE2 bottom left and DOE2 bottom right. As noted previously, DOE2 comprises a number of expansion gratings. The functionality of DOE2, will be explained in more detail below in conjunction with the description of <FIG>.

However, reference is now made to <FIG> which illustrates DOE2 grating vectors and FOVs diffracted by DOE2.

Note that the various wings of DOE2 are implemented on a grating with a first wing on the front of the grating, and a second wing on the back of the grating. In an aspect, these first and second wings can overlap. In another aspect, DOE2 is a linear grating with first and second wings on the front of the waveguide. In another aspect, DOE2 is a linear grating with first and second wings on the back of the waveguide.

Referring once again to <FIG>, The waveguide <NUM> further includes a single output port <NUM>, which is a DOE indicated as DOE3 (also called out-coupling element).

Referring now to <FIG>, DOE3 grating vectors and FOVs diffracted by DOE3 are shown.

During operation, the display module <NUM> (see <FIG>) outputs light representing an image for an eye from its output port into the input port <NUM> of the waveguide <NUM>.

The transmission channel <NUM> conveys light from the input port <NUM> to the output port <NUM> and is a surface diffraction grating, polarization grating, a volume diffraction grating or a reflective component. The transmission channel <NUM> may be designed to accomplish this by use of total internal reflection (TIR). Light representing the image is then projected from the output port <NUM> to the user's eye.

Thus, in general, aspects split the FoV of different colors into two (or more) paths, carry the partial FoVs to DOE3 while expanding the pupil by pupil replication, and at DOE3 recombining the different contributions of each color. Two or more paths may be identical in some parts of the path.

The grating vectors of DOEs <NUM>, <NUM> and <NUM> satisfy D<NUM> + D<NUM> + D<NUM> = <NUM>.

Specifically, the two grating vectors of DOE1 (for the +<NUM> order) are denoted by D1r and D1b for "red" and "blue" paths as illustrated in the Figures above.

DOE2 grating vectors are denoted by D2tr, D2br, D2tl, D2bl for top-right, bottom-right, top-left, bottom-left, respectively.

Grating vectors of DOE3 are denoted by D3b and D3r.

Then the path equations are:
<MAT>
<MAT>
<MAT>
<MAT>.

<FIG> presents an example of a k-vector map enabling this type of solution. Note that in <FIG>, a part of both red and blue FoV appear to be leaky but this is not necessary the case in all aspects.

The waveguide <NUM> includes multiple diffraction optical elements (DOEs), in order to control the directions of the light propagating in the near-eye display device via multiple occurrences of optical diffraction. The DOEs are surface diffraction gratings or volume diffraction gratings. Various components of the waveguide <NUM> are designed to contain one or more of the DOEs.

According to the invention, the waveguide <NUM> includes three DOEs. The input port <NUM> of the waveguide <NUM> is a DOE1 for coupling light into the waveguide <NUM> and controlling the direction of light path after the light reaches the input port <NUM>.

The transmission channel <NUM> of the waveguide <NUM> is a DOE2 for controlling the direction of light path in the transmission channel <NUM> and ensuring the light propagating inside of the transmission channel <NUM> through total internal reflection (TIR). Further, DOE2 is configured homogenize light signals in a horizontal direction.

The output port <NUM> is a DOE3 for controlling the direction of the light path after the light exits the output port <NUM>. DOE3 configured to diffract light into an eye box keeping output propagation angles within some predetermined threshold of the input propagation angle.

The propagation directions of the expanded light waves are substantially parallel to each other (within some predetermined threshold). The expanded light waves are spaced or distributed along the particular direction.

In other words, the expanded light waves are translated along the particular direction (or coordinate axis) in an output waveguide before exiting the output waveguide. Each of the expanded light waves has a relatively narrow range of propagation angles or FoV. Each expanded light wave has a "propagation vector" representing the average propagation direction of the light wave and denoting a center axis of the prorogation energy of the expanded light wave. Translation of a light wave means shifting the corresponding propagation vector of the light wave along a particular direction (or coordinate axis) that is not parallel to the propagation vector itself.

Thus, according to the invention, the light waves exiting the output waveguide have the same direction as (i.e., are substantially parallel, within some threshold, to) the light waves entering the output waveguide for light of any given wavelength, to have the light waves follow the desired path to the optical receptor of a user. This condition is called achromatic imaging.

The following illustrates details with respect to expanding light and is directed to a single light path. However, it should be appreciated that the concepts illustrated can be applied to the different paths described above, such that expansion and the summation rules apply for each distinct path of light.

The waveguide including three DOEs can expand the light waves in two dimensions. The expansion process is also referred to as exit pupil expansion. <FIG> shows an output waveguide that expands the exit pupil of a near-eye display device. The waveguide <NUM> includes three DOEs <NUM>, <NUM> and <NUM> to expand the exit pupil. The DOEs <NUM>, <NUM> and <NUM> are successive in a common light path. The DOEs <NUM>, <NUM> and <NUM> can be, e.g., arranged on a planar substrate.

The imager <NUM> (e.g., an LCOS device) outputs a light wave <NUM> that is incident upon the first DOE <NUM> in a Z direction. The DOE <NUM> directs the light wave <NUM> toward the second DOE <NUM>. As shown in <FIG>, the DOE <NUM> expands the light wave <NUM> in a first dimension (X dimension). As shown in <FIG>, during the expansion, each propagation vector of the expanded light waves <NUM> is shifted along the X coordinate axis such that the expanded light waves are spaced or distributed in the X dimension.

The DOE <NUM> further redirects the expanded light wave <NUM> to a third DOE <NUM>. The third DOE <NUM> further expands the light wave <NUM> in a second dimension (Y dimension), and redirects the expanded light wave <NUM> outward in the Z direction.

Thus, the waveguide <NUM> receives the input light wave <NUM> incident in the Z direction, expands the light wave in both X and Y dimensions, and redirects the expanded light waves in the same Z-direction. In other words, the waveguide <NUM> expanded light distribution in two dimensions while maintains the direction of the light wave. Thus, the waveguide <NUM> can be referred to as a beam-expanding device or an exit pupil expander.

The waveguide, as a beam-expanding device, can expand light waves in, e.g., an odd-order expansion process or an even-order process. <FIG> shows an output waveguide conducting an odd-order expansion. The waveguide <NUM> includes DOEs <NUM>, <NUM> and <NUM>.

Each of the DOEs <NUM>, <NUM> and <NUM> has a diffraction grating. A diffraction grating is an optical component with a periodic structure, which splits and diffracts an incident light beam into several beams travelling in different directions. The periodic structure can include linear grooves arranged in a periodic pattern. The distance between nearby grooves is called grating period d.

The diffraction grating has a property of grating vector D (also referred to as diffraction pattern vector). The grating vector D represents the direction and spacing of the grating pattern (also referred to as periodic diffraction pattern). The length of a grating vector is D = 2π / d. The direction of the grating vector D is perpendicular to ("normal to" or "orthogonal to") center axes of the periodic linear grooves, where the center axes are perpendicular to the cross sections of the periodic linear grooves.

Light is incident upon the waveguide <NUM> in a Z direction, which is perpendicular to the X and Y directions. The first DOE <NUM> couples light from an imager (not shown) into the waveguide <NUM>. The second DOE <NUM> expands the light in the X direction. The third DOE <NUM> further expands the light in the Y direction and couples the expanded light out from the waveguide <NUM> in the same Z direction.

As shown in <FIG>, the second DOE <NUM> receives the light wave from the first DOE <NUM> at a left edge (as the reader views the figure) of the DOE <NUM>. The light wave is reflected by the grating pattern in the DOE <NUM> for one or more times before the light wave exits the DOE <NUM> at a bottom edge of the DOE <NUM>. Because the odd-order expansion enables the second DOE <NUM> to receive the light wave at a side edge, a waveguide of an odd-order expansion configuration usually occupies less space than a waveguide of an even-order expansion configuration (which is discussed later).

During the odd-order expansion process, the second DOE <NUM> reflects (i.e., changes the direction of) the light for an odd number of times before redirecting the light into the third DOE <NUM>. Over the process of multiple reflections between <NUM> and +<NUM> diffraction orders, a greater portion of the light energy is converted to +<NUM> order, which is redirected toward the third DOE <NUM>.

<FIG> shows the wave vectors of light propagating in the waveguide and grating vectors of DOEs of the waveguide. The incident light has a pair of transverse wave vector components kx0 and ky0. The magnitude of the wave vector is the wave number k = 2π / λ, where λ is the wavelength of the light. The wave number of the incident light in the air is denoted as k<NUM>. The wave number of the light propagating in the waveguide is denoted as k = k<NUM> * n, where n is the refractive index of the waveguide material.

The grating vectors of the DOE1, <NUM>, and <NUM> (<NUM>, <NUM> and <NUM> in <FIG>) are denoted as Dj = (Dxj, Dyj). The DOE <NUM> with a wave vector of (Dx1, Dy1) redirects the incident light (kx0, ky0) toward the second DOE <NUM>. Therefore, (kx1, ky1) = (kx0 + Dx1, ky0 + Dy1).

The DOE <NUM> with a wave vector of (Dx2, Dy2) receives light (kx1, ky1) and redirects the light (kx1, ky1) toward the third DOE <NUM>. Therefore, (kx2, ky2) = (kx1 + Dx2, ky1 + Dy2) = (kx0 + Dx1 + Dx2, ky0 + Dy1 + Dy2).

The DOE <NUM> with a wave vector of (Dx3, Dy3) receives light (kx2, ky2) and couples the light (kx2, ky2) out in a Z direction. Therefore, (kx3, ky3) = (kx2 + Dx3, ky2 + Dy3) = (kx0 + Dx1 + Dx2 + Dx3, ky0 + Dy1 + Dy2 + Dx3).

The waveguide <NUM> satisfies the achromatic imaging condition, which means that when the light waves with different wavelengths are expanded by the waveguide <NUM> and exit the waveguide <NUM>, the exit directions of the light waves are the same as the input directions in which the light waves enter the waveguide <NUM>. In other words, the incident light wave number (kx0, ky0) matches the out-coupled light wave number (kx3, ky3): (kx0, ky0) = (kx3, ky3). Therefore, the grating vectors of the waveguide <NUM> satisfy Dx1 + Dx2 + Dx3 = Dy1 + Dy2 + Dx3) = <NUM>. Alternatively, in a vector form, a vector summation of the grating vectors equals zero: D<NUM> + D<NUM> + D<NUM> = <NUM> (also referred to as the "summation rule").

Note that the grating vectors D<NUM>, D<NUM>, D<NUM> depend on grating periods but do not depend on wavelengths of the light waves. Therefore, once the grating vectors satisfy the summation rule, the achromatic imaging condition is satisfied for light waves with any wavelengths (hence the term "achromatic imaging").

To satisfy the achromatic imaging condition, it is not necessary to restrict the diffraction gratings of first DOE <NUM> and the DOE <NUM> to have the same grating period. The summation rule relaxes the design limitations of those diffraction gratings. The relaxed design limitations enable a waveguide <NUM> to have a larger FoV.

Furthermore, the waveguide <NUM> keeps the light diffracted by DOEs <NUM> and <NUM> inside the waveguide <NUM>. Thus, the light propagating inside of the waveguide <NUM> is not evanescent and satisfies condition of total internal reflection (TIR). In other words, light diffracted by DOE <NUM> satisfies the TIR condition inside of the waveguide: kx1<NUM> + ky1<NUM> > k<NUM><NUM>. Light diffracted by DOE <NUM> is not evanescent: kx1<NUM> + ky1<NUM> < k<NUM>. Light diffracted by DOE <NUM> also satisfies the TIR condition inside of the waveguide: kx2<NUM> + ky2<NUM> > k<NUM><NUM>. Light diffracted by DOE <NUM> is not evanescent: kx2<NUM> + ky2<NUM> < k<NUM>.

Although <FIG>, <FIG> and 6C shows a waveguide including three DOEs, a waveguide according to the disclosed technology can have any arbitrary number of DOEs. For example, if a waveguide includes N number of DOEs, the condition of achromatic imaging is Dx1 + Dx2 + Dx3 +. + DxN = Dy1 + Dy2 + Dx3 +. + DyN = <NUM>. Alternatively, in a vector form: D<NUM> + D<NUM> + D<NUM> +. + DN = <NUM>. The DOEs also satisfy the conditions for TIR and non-evanescence.

In some aspects, the achromatic imaging condition can be expressed as a weighted vector summation of the grating vectors: mD<NUM> + nD<NUM> +lD<NUM> = <NUM>, where the values m, n, and l in the addends are integer weight values that represent diffraction orders to which the periodic diffraction patterns are designed to concentrate light energy. In some aspects, the integer weight values can be <NUM>, negative, or positive.

Furthermore, the waveguide, as a beam-expanding device, can expand light waves in an even-order expansion process as well. <FIG> shows an output waveguide conducting an even-order expansion. The waveguide <NUM> includes DOEs <NUM>, <NUM> and <NUM>.

Light is incident upon the waveguide <NUM> in a Z direction, which is perpendicular to the X and Y directions. The first DOE <NUM> couples light into the waveguide <NUM>, and redirects the light wave into the second DOE <NUM> at a top edge of DOE <NUM>. The second DOE <NUM> expands the light in the X direction. The third DOE <NUM> further expands the light in the Y direction and couples the expanded light out from the waveguide <NUM> in the same Z direction.

As shown in <FIG>, the second DOE <NUM> receives the light wave from the first DOE <NUM> at the top edge of the DOE <NUM>. Note that in the odd-order expansion illustrated in <FIG>, the second DOE <NUM> receives the light wave at the left side edge. The choice of either odd-order expansion or even-order expansion depends on various design factors for the waveguide. Typically, a waveguide of odd-order expansion configuration tends to be smaller. An even-order expansion configuration, on the other hand, enables supplying the light wave at the top edge of the second DOE, which may be advantageous when there is a limitation on the width of the waveguide.

The light wave is reflected by the grating pattern in the DOE <NUM> multiple times before the light wave exits the DOE <NUM> at a bottom edge of the DOE <NUM>. During the even-order expansion process, the second DOE <NUM> reflects the light an even number of times (including zero time) before redirecting the light into the third DOE <NUM>. Similar to the odd-order expansion, over the process of multiple reflections between <NUM> and +<NUM> orders, more of the light energy is converted to +<NUM> order, which is redirected toward the third DOE <NUM>.

As shown in <FIG>, the second DOE <NUM> expands the light wave in the X direction. However, the second DOE <NUM> maintains the direction of its output light as the same of the direction of its input light. In other words, in the even-order expansion, the wave vectors of light waves before and after second DOE <NUM> are identical. Thus, the grating vector for the diffraction grating of the second DOE <NUM> does not impose limitation to diffraction vectors of other DOEs in the waveguide <NUM>.

In the even-order expansion, the first DOE <NUM> has linear diffraction gratings on two sides of the DOE <NUM> (also referred to as "dual-sided linear grating"). The first diffraction grating on a first side (e.g., top side) of DOE <NUM> has a grating vector of D1a = (Dx1a, Dy1a). The second diffraction grating on a second side (e.g., bottom side) of DOE <NUM> has a grating vector of D1b = (Dx1b, Dy1b). The diffraction grating of the third DOE <NUM> has a grating vector of D<NUM> = (Dx3, Dy3).

The waveguide <NUM> satisfies the achromatic imaging condition, which means the incident light (kx0, ky0) matches the out-coupled light (kx3, ky3). The achromatic imaging condition is satisfied, if mD1a + nD1b = ±D<NUM>, wherein m and n are integer order numbers.

In some aspects, the achromatic imaging condition can be expressed as a weighted vector summation of the grating vectors: mD1a + nD1b +lD<NUM> = <NUM>, where the values m, n and l in the addends are integer weight values that represent diffraction orders to which the periodic diffraction patterns are designed to concentrate light energy (also referred to as "weighted summation rule"). In some aspects, the integer weight values can be <NUM>, -<NUM> or + <NUM>. Higher diffraction orders, corresponding to integer numbers whose absolute values are larger than <NUM>, are usually suppressed by the grating patterns.

In some aspects, m=<NUM> and n=<NUM>, or m=<NUM> and n=<NUM>. Thus, the first DOE <NUM> has one diffraction grating with a wave vector D<NUM> = ±D<NUM>. In other words, if the first DOE <NUM> and the third DOE <NUM> have the same length for the grating vectors (or the same grating period), the achromatic imaging condition is satisfied.

The design limitation of the grating vectors can be further relaxed, because the grating periods for first DOE <NUM> and third DOE <NUM> do not need to be equal. In some aspects, m=<NUM> and n=<NUM>, which means the first diffraction grating of the first DOE <NUM> reflects the light wave to +<NUM> diffraction order, and then the second diffraction grating of the first DOE <NUM> reflects the light wave again to +<NUM> diffraction order. Diffraction orders higher than the +<NUM> diffraction order usually are less efficient and can create ghost image effects. Thus, when m=<NUM> and n=<NUM>, a vector sum of the grating vectors of the diffraction gratings of the first DOE <NUM> either equals the grating vector of the third DOE <NUM>, or is the exact opposite to the grating vector of the third DOE <NUM>: D1a + D1b = ±D<NUM>. Particularly, in case of -D<NUM>, the first and second diffraction gratings of the first DOE <NUM> and the diffraction grating of the third DOE <NUM> satisfy the summation rule: D1a + D1b + D<NUM> = <NUM>.

Besides dual-sided linear grating, the first DOE <NUM> can have, e.g., crossed diffraction gratings on two sides of the DOE <NUM> (also referred to as "dual-sided crossed grating"). Thus, the first DOE <NUM> effectively have four diffraction gratings with four grating vectors. On a first side (e.g., top side) of the first DOE <NUM>, there are two diffraction gratings that are crossed to each other and have grating vectors of D1a = (Dx1a, Dy1a) and D1b = (Dx1b, Dy1b). In other words, the grating pattern is periodic in two directions on the first side. On a second side (e.g., bottom side) of the first DOE <NUM>, there are two diffraction gratings that are crossed to each other and have grating vectors of D1c = (Dx1c, Dy1c) and D1d = (Dx1d, Dy1d).

The waveguide <NUM> satisfies the achromatic imaging condition, which means the incident light matches the out-coupled light. The achromatic imaging condition is satisfied, if mD1a + nD1b + oD1a + pD1b = ±D<NUM>, wherein m, n, o, and p are integer order numbers.

Therefore, the weighted vector summation rule can be used to design DOEs of output waveguides. The diffraction gratings of DOEs follow the summation rule or the weighted summation rule, and therefore satisfies the achromatic imaging order. The summation rule or the weighted summation rule enables relaxed degrees of freedom for designing the configuration of the output waveguides with various properties of DOEs.

Referring now to <FIG>, a method <NUM> which relates to combinations of features which are not in the claims but are considered as useful for understanding the invention. The method <NUM> includes acts for combining RGB optical signals in a single waveguide. The waveguide includes a plurality of DOEs. The method incudes directing an optical signal at a first DOE at input propagation angles (act <NUM>).

The method <NUM> further includes at the first DOE, diffracting the optical signal based on spectrum such that predominately one spectrum of light is diffracted in a first direction and predominately a second spectrum of light is diffracted in a second different direction such that different portions of optical signal take different paths, including at least two different paths (act <NUM>).

The method <NUM> further includes at the first DOE, diffracting the different portions into a second DOE (act <NUM>).

The method <NUM> further includes at the second DOE, diffracting the different portions into a third DOE (<NUM>).

The method <NUM> further includes at the second DOE and the third DOE expanding the optical signal in a substantially non-parallel fashion; Expansions at DOE2 and DOE3 are substantially non-parallel. Aspects may expand the pupil at DOE2 essentially in the vertical direction, and then at DOE3 in the horizontal direction during the outcoupling process (act <NUM>).

The method <NUM> further includes at the third DOE, diffracting the different portions into an eye box keeping output propagation angles within some predetermined threshold of the input propagation angles. That is, an attempt is made to keep the output propagation angles substantially parallel to the input propagation angles to prevent distortion and/or other side-effects (act <NUM>).

The plurality of DOEs are associated with grating vectors. The acts of method <NUM> are performed such that a summation of grating vectors for each of the paths in the at least two different paths is substantially equal to zero (act <NUM>).

Note that being 'substantially equal to zero' is dependent on the display resolution of a device. In particular, the summation is substantially equal to zero so long as some predefined resolution is maintained. In some aspects, this may mean that the output resolution of an outgoing optical signal must be the same as the input resolution of an incoming optical signal.

The method of the invention is defined in claim <NUM>.

Claim 1:
An optical device (<NUM>) for combining RGB optical signals in a single waveguide (<NUM>), the single waveguide comprising a plurality of diffractive optical elements DOEs, including:
a first DOE (<NUM>) comprising a first linear grating on a front surface of the grating and having a first grating period and a second linear grating on the back of the grating and having a second grating period, the first DOE being configured to receive an optical signal at input propagation angles and to diffract the optical signal based on spectrum such that a +<NUM> diffraction order of a predominately first spectrum of light is diffracted by the first linear grating in a first direction and a -<NUM> diffraction order of the predominantly first spectrum of light is diffracted in a second direction by the first linear grating; and a +<NUM> diffraction order of a predominately second spectrum of light is diffracted by the second linear grating and a -<NUM> diffraction order of the predominantly second spectrum of light is diffracted by the second linear grating, such that different portions of optical signal take different paths, including four different paths;
a second DOE (<NUM>) and a third DOE (<NUM>), the second DOE comprising four wings with different expansion gratings, each expansion grating being oriented to diffract light toward the third DOE, wherein the first DOE is configured to diffract the optical signal of the predominantly first spectrum of light diffracted in the first direction toward a first wing, the optical signal of the predominantly first spectrum of light diffracted in the second direction toward a second wing, the +<NUM> diffraction order of the predominately second spectrum of light toward a third wing, and the -<NUM> diffraction order of the predominantly second spectrum of light toward a fourth wing;
the third DOE (<NUM>) configured to diffract light into an eye box keeping output propagation angles within some predetermined threshold of the input propagation angles;
wherein the second and third DOE are configured to cause expansions that are substantially non-parallel;
wherein the plurality of DOEs are associated with grating vectors and wherein a summation of grating vectors for each of the paths in the four different paths is substantially equal to zero.