GB2571389A

GB2571389A - Optical structure for augmented reality display

Info

Publication number: GB2571389A
Application number: GB1813535.0A
Authority: GB
Inventors: Salim Valera Mohmed; Woods David; Crosby David
Original assignee: Wave Optics Ltd
Current assignee: Wave Optics Ltd
Priority date: 2018-08-20
Filing date: 2018-08-20
Publication date: 2019-08-28
Also published as: GB201813535D0

Abstract

An optical structure for use in an augmented reality display has two waveguides 4, 6, and a chromatic filter 7 configured to receive coloured light from a projector 2 and to split the coloured light into at least two wavelength ranges which are directed respectively towards the first and second waveguides in directions that are offset from one another. The waveguides have input diffractive optical elements 8, 10 configured to couple light into the respective waveguides and output diffractive optical elements 12, 14 configured to receive and diffract totally internally reflected light within the waveguides so that light can be coupled out towards a viewer such as by projector 2. The filter may be a dichroic filter and reflective surface 9 may reflect the second wavelength range to diffractive element 10.

Description

OPTICAL STRUCTURE FOR AUGMENTED REALITY DISPLAY

The present invention relates to an optical structure for use in a full-colour augmented reality or virtual reality display such as a headset or a head-up display. In particular, the invention relates to a stack of waveguides in an arrangement that causes first and second primary colours from a projector to be split between at least two waveguides.

An augmented reality display allows a user to view their surroundings as well as projected images. In military or transportation applications the projected images can be overlaid on the real world perceived by the user. Other applications for these displays include video games and wearable devices, such as glasses. By contrast, in a virtual reality display a user can only perceive projected images and light from their real world surroundings is obscured.

In a normal augmented reality set-up a transparent display screen is provided in front of a user so that they can continue to see the physical world. The display screen is typically a glass waveguide, and a projector is provided to one side. Light from the projector is coupled into the waveguide by a diffraction grating (an input grating). The projected light is totally internally reflected within the waveguide. The light is then coupled out of the waveguide by another diffraction grating (an output grating) so that it can be viewed by a user. The projector can provide information and/or images that augment a user's view of the physical world.

One challenge in the field of augmented reality displays is to provide full colour images. To date this has proven to be difficult without introducing undesirable optical effects. In one approach coloured light is split into three primary components: red, green and blue. Each primary component is then coupled towards a viewer separately via a dedicated waveguide. This requires a stack of three waveguides which may be undesirably bulky for some applications.

WO 2011/131978 describes an optical element that attempts to provide full colour images using only two waveguides. According to this technique three primary colour components are split between two waveguides and are re-combined in light provided towards a viewer in order to provide full colour viewing. This technique requires the use of diffraction gratings having small periods which can be difficult to manufacture. In addition, optical quality may be higher from the front waveguide in a stack than from a rear waveguide.

An object of the present invention is to provide an alternative technique for providing full colour images using a stack of waveguides.

According to an aspect of the present invention there is provided an optical structure for use in an augmented reality display, the optical structure comprising a first waveguide and a second waveguide, wherein the optical structure comprises: a chromatic filter configured to receive coloured light from a projector and to split the coloured light into at least first and second wavelength ranges so that the first and second wavelength ranges are directed respectively towards the first and second waveguides in directions that are offset from one another; wherein the first waveguide comprises: a first input diffractive optical element configured to receive light in at least the first wavelength range and to diffract the received light so that diffracted wavelengths of the light are coupled into the first waveguide, to be totally internally reflected within the first waveguide; and a first output diffractive optical element configured to receive and diffract totally internally reflected light within the first waveguide in order to couple the totally internally reflected light out of the first waveguide towards a viewer; wherein the second waveguide comprises: a second input diffractive optical element configured to receive light in at least the second wavelength range and to diffract the received light so that diffracted wavelengths of the light are coupled into the second waveguide, to be totally internally reflected within the second waveguide; and a second output diffractive optical element configured to receive and diffract totally internally reflected light within the second waveguide in order to couple the totally internally reflected light out of the second waveguide towards the viewer.

In this way, coloured light can be separated into different wavelength components and these can be directed towards respective waveguides in directions that are offset from one another. The first and second input diffractive optical elements can therefore be optimised for the wavelengths of light that they receive. By providing the respective colours at offset positions the first input diffractive optical element can be isolated from wavelengths that are intended for the second input diffractive optical element, and vice-versa. This optical isolation may improve performance by avoiding cross-talk between layers. Preferably the chromatic filter directs the first and second wavelength ranges in parallel respective directions which are offset from one another. The first and second wavelength ranges preferably include first and second primary colours respectively.

Preferably the first and second input diffractive optical elements are also offset from one another with respect to the respective directions in which the first and second wavelength ranges are directed towards the first and second waveguides. Preferably there is no active overlapping area between the first and second input diffractive optical elements in order to avoid cross-talk between adjacent waveguide layers. This can also improve optical quality for light in the second waveguide because this light does not need to be transmitted through the first input diffractive optical element in the first waveguide. It has been found that transmission through an input diffractive optical element can cause some wavefront distortion, and this can be avoided by offsetting the first and second input diffractive optical elements.

The chromatic filter may be a dichroic filter. This may provide an effective short pass or long pass filter that can separate the first and second wavelength ranges.

The chromatic filter may be configured to reflect at least the first wavelength range towards the first input diffractive optical element, and to transmit the second wavelength range. A reflective surface may be provided to receive the second wavelength range that is transmitted by the chromatic filter and to reflect the received light towards the second input diffractive optical element.

The chromatic filter and the reflective surface can be offset from one another. Thus, they can provide reflected light in parallel respective directions that are offset from one another laterally. The reflective surface may be an uncoated prism surface that is angled to allow total internal reflection. Alternatively the reflective surface may have a metallised coating.

The reflective surface is preferably arranged in a plane that is parallel to the plane of the chromatic filter. Thus, the reflective surface may be arranged as one surface in a rhombic prism having four parallel sides.

In another arrangement the chromatic filter may be configured to transmit at least the first wavelength range towards the first input diffractive optical element, and to reflect the second wavelength range towards the second input diffractive optical element.

Preferably there are two waveguides only for receiving light in the first and second wavelength ranges. In another embodiment the present technique may be applied with a stack of three waveguides and first and second chromatic filters that can separate light into first, second and third wavelength ranges. In this way, the first, second and third wavelength ranges can be directed towards the respective waveguides at offset positions, to be received at the respective waveguides by first, second and third input diffractive optical elements.

Preferably the first and second input diffractive optical elements are reflection diffraction gratings. Preferably the first and second wavelength ranges are split at a green wavelength. Green wavelengths may be selected by the chromatic filter. In this way, green wavelengths can be received at both the first and second waveguides. Longer green wavelengths can be received by the red waveguide and shorter green wavelengths can be received by the blue waveguide.

Preferably the first and second input diffractive optical elements are provided with a reflective coating, such as a metallised coating. This can improve the diffraction efficiency of the first and second input diffractive optical elements. The first and second input diffractive optical elements can therefore be rendered optically opaque, which is only possible if they are offset from one another with respect to the directions in which the first and second wavelength ranges are directed towards the first and second waveguides.

In some embodiments the optical structure may further comprise a third waveguide which comprises a third input diffractive optical element configured to receive light in at least a third wavelength range and to diffract the received light so that diffracted wavelengths of the light are coupled into the third waveguide, to be totally internally reflected within the third waveguide. The third waveguide may further comprise a third output diffractive optical element configured to receive and diffract totally internally reflected light within the third waveguide in order to couple the totally internally reflected light out of the third waveguide towards the viewer. There may be first and second chromatic filters. The first chromatic filter may be configured to receive coloured light from a projector and to direct a first wavelength range towards the first waveguide. The second chromatic filter may be configured to direct a second wavelength range towards the second waveguide. A reflector may be provided to direct a third wavelength range towards the third waveguide so that the first, second and third wavelength ranges are directed respectively towards the first, second and third waveguides in directions that are mutually offset from one another.

According to another aspect of the present invention there is provided an augmented reality display comprising: the optical structure as previously defined; and a projector configured to direct light with at least first and second wavelength ranges towards the chromatic filter.

Embodiments of the invention are now described, by way of example, with reference to the drawings, in which:

Figure 1 is a side view of an augmented reality display in an embodiment of the invention;

Figure 2 is a side view of an augmented reality display in another embodiment of the invention; and

Figure 3 is a side view of an augmented reality display in another embodiment of the invention.

Figure 1 is a side view of an augmented reality display. A colour projector 2 is provided which emits an image in a narrow beam comprising three wavelength ranges in primary colours: red, green and blue. A pair of waveguides 4, 6 is provided in the path of the projected beam. The waveguides 4, 6 are optically transparent and, in this embodiment, are made of glass having a refractive index, n, of around 1.7. The waveguides 4, 6 have first and second major surfaces that are parallel to one another and which extend out of the plane of the page as viewed in Figure 1. For the sake of simplicity the waveguides are referred to as the “blue” waveguide 4 and the “red” waveguide 6. This does not refer to the colour of the waveguides themselves, since they are optically transparent. Rather, it refers to the predominant colour of the light that the respective waveguides are configured to carry.

The ray along the optical axis of projector 2 is transmitted towards a dichroic filter 7. The dichroic filter 7 is configured to reflect red light and to transmit blue light. This can be achieved using a short-pass filter that selectively transmits light having a wavelength below a predetermined value, such as 540nm. In this way, the dichroic filter 7 can split light in the green part of the spectrum so that longer wavelengths of green are reflected and shorter wavelengths of green are transmitted.

The dichroic filter 7 is angled at 45° to the beam of light emitted by the projector 2. In this way, the dichroic filter 7 can reflect red wavelengths of light so that they are turned through

90° into a direction that is substantially parallel to the surface normal vectors of the blue and red waveguides 4, 6.

Light that is reflected by the dichroic filter 7 is transmitted directly through the transparent blue waveguide 4 and through a first surface of the red waveguide 6 at an angle that is approximately parallel to the surface normal of the blue waveguide 4. Light is transmitted directly through the blue waveguide 4. The major faces of the blue waveguide 4 are chosen so that they are smooth and flat so that they introduce minimal wavefront distortion to the transmitted light.

On the rear surface of the red waveguide 6 from the perspective of the projector 2 is positioned a reflective input diffraction grating 8 having a period of 440nm where the period is defined as the distance between respective grating grooves. The reflective input diffraction grating 8 is configured to diffract the light that is incident upon it. The angle of diffraction is, of course, wavelength dependent, according to the grating equation. Therefore a spectrum of colours is provided, following diffraction, in an angular spread emanating away from the reflective input diffraction grating 8.

The period of the reflective input diffraction grating 8 is chosen so that, following diffraction, red wavelengths are totally internally reflected within the red waveguide 6. A proportion of green wavelengths are also totally internally reflected within the red waveguide 6. Preferably very few blue wavelengths are received at the reflective input diffraction grating 8 since these are filtered out by the dichroic filter 7.

A rhombic prism 11 is cemented to the dichroic filter 7 and receives light that is transmitted by the dichroic filter 7. The rhombic prism 11 includes a reflective surface 9 that is arranged at 45° to the direction in which light is emitted by the projector 2. The surface 9 may be arranged to reflect light through total internal reflection, depending on the choice of refractive index for the prism 11. Alternatively, the surface 9 may have a metallised coating to make it reflective for all of the received light. The reflective surface 9 can therefore turn blue wavelengths of light that are transmitted by the dichroic filter 7 towards the second input diffraction grating 10 on the blue waveguide 4.

The second input diffraction grating 10 is provided with a period of around 335nm. The second input diffraction grating 10 is configured to diffract the received light so that the received blue and green wavelengths are coupled into the blue waveguide 4 at an angle that is higher than the critical angle for total internal reflection.

The first and second input diffraction gratings 8, 10 are offset from one another with respect to the respective (parallel) directions in which light is reflected by the dichroic filter 7 and the reflective surface 9. In one arrangement the blue waveguide 4 can be offset from the red waveguide 6 so that reflected light from the dichroic filter 7 can be received directly at the red waveguide 6 without first being transmitted through the blue waveguide 4. The first and second input diffraction gratings 8, 10 are provided with a metallised coating in order to improve their diffraction efficiency. A metallised coating can only be provided where the first and second input diffraction gratings 8, 10 are offset from one another with respect to the direction in which the red and blue wavelengths are coupled towards the waveguides 4, 6 since otherwise the second input diffraction grating 10 in the blue waveguide 4 could prevent light from reaching the red waveguide 6.

Light that is totally internally reflected within the red waveguide 6 extends towards a first output diffractive optical element 14. The first output diffractive optical element 14 is provided with relatively low diffraction efficiency in order to act as an expansion grating and provide expansion of the beam in at least one dimension, and preferably two dimensions. The second output diffractive optical element 12 is similar to the first output diffractive optical element 14, but is provided with a different period. In this example the first output diffractive optical element 14 has a period of around 440nm and the second output diffractive optical element 12 has a period of around 335nm.

The first and second output diffractive optical elements 12, 14 in this embodiment are expanding diffractive optical exit pupils, as described in WO 2016/020643. The first and second output elements 12, 14 therefore comprise a pair of crossed linear gratings, or a photonic crystal structure. In both cases, the output elements 12, 14 each comprise two diffractive optical elements overlaid on one another in or on the waveguide 2. Each diffractive optical element within the respective output elements 12, 14 can then couple the received light towards the other diffractive optical element in the pair which can then act as an output diffractive optical element which couples light out of the red of blue waveguide 4, 6 towards a viewer. The grating period for the overlaid diffractive optical elements is 440nm for the first output diffractive optical element 14 in the red waveguide 6 and 335nm for the second output diffractive optical element 12 in the blue waveguide 4.

In an alternative configuration the first and second output diffractive optical elements 12, 14 can be simple linear diffraction gratings in order to provide a one-dimensional expansion of the totally internally reflected light within the red and blue waveguides 4, 6.

Figure 2 is a side view of an augmented reality display in another embodiment of the invention. In this embodiment light from the projector 2 is incident on the dichroic filter 7 which is arranged to transmit blue wavelengths of light and to reflect red wavelengths. The transmitted blue wavelengths propagate towards the blue waveguide 4 and are incident on the second input diffraction grating 10, which is provided as a transmission grating. The blue wavelengths are diffracted by the second input diffraction grating 10 and coupled into the blue waveguide 4 to be totally internally reflected within the blue waveguide 4.

The red wavelengths of light are reflected by the dichroic filter 7 and turned through 90° to be incident on a mirror 13. The mirror 13 reflects the red wavelengths towards the red waveguide 6 and the first input diffraction grating 8, which is provided at a transmission grating. The reflected light is turned through 90° by the mirror 13 so that the reflected light propagates in a direction that is parallel to the light that is transmitted by the dichroic filter 7. Thus, these parallel beams are offset from one another laterally and extend respectively towards the first and second input diffraction gratings 8, 10.

The first input diffraction grating 8 in this embodiment receives red wavelengths of light (and some green wavelengths) and diffracts the received light so that it is coupled into the red waveguide 6. The diffracted light is then totally internally reflected within the red waveguide

6. The red waveguide 6 includes a first output diffractive optical structure 12 and the blue waveguide 4 includes a second output diffractive optical structure 14, and these have similar properties to those described in relation to previous embodiments.

In this embodiment the red waveguide 6 is offset from the blue waveguide 4 so that light is received at the first input diffraction grating 8 without first being transmitted through the blue waveguide 4. This may improve optical quality of the light output from the red waveguide 6 because it does not experience any wavefront distortion due to transmission through the blue waveguide 4.

Figure 3 is an example of another embodiment having three waveguides 4, 6, 18: the blue waveguide 4, the red waveguide 6 and a green waveguide 16. In this example embodiment light from the projector 2 is incident on a first dichroic filter 7 which is arranged to transmit blue wavelengths of light and to reflect red and green wavelengths. The transmitted blue wavelengths propagate towards the blue waveguide 4 and are incident on the second input diffraction grating 10, which is provided as a reflection grating with a metallised coating. The blue wavelengths are diffracted with high efficiency by the second input diffraction grating 10 and coupled into the blue waveguide 4 to be totally internally reflected within the blue waveguide 4.

The red and green wavelengths of light are reflected by the first dichroic filter 7 and turned through 90° to be incident on a second dichroic filter 11. The second dichroic filter 11 is arranged to reflect red wavelengths and to transmit green wavelengths. Light reflected by the second dichroic filter 11 is turned through 90° so that it propagates in a direction that is parallel to, but offset from, the light that is transmitted by the first dichroic filter 7.

The first input diffraction grating 8 in this embodiment receives red wavelengths of light and diffracts the received light so that it is coupled into the red waveguide 6. The diffracted light is then totally internally reflected within the red waveguide 6.

The mirror 13 is provided in this embodiment to reflect green wavelengths that are reflected by the first dichroic filter 7 and transmitted by the second dichroic filter 11. The mirror 13 reflects the green wavelengths towards the green waveguide 16 where the light is received at a third input diffraction grating 18, which is provided as a reflection grating with a metallised coating. Thus, parallel beams of light are offset from one another and extend respectively towards the first, second and third input diffraction gratings 8, 10, 18 in the blue, red and green waveguides 4, 6, 16.

The red waveguide 6 includes a first output diffractive optical structure 12, the blue waveguide 4 includes a second output diffractive optical structure 14, and the green waveguide 16 includes a third output diffractive optical structure 20. These output diffractive optical structures are arranged to expand the light in two-dimensions and provide output light towards a viewer, as described in relation to previous embodiments.

Claims

1. An optical structure for use in an augmented reality display, the optical structure comprising a first waveguide and a second waveguide, wherein the optical structure comprises:

a chromatic filter configured to receive coloured light from a projector and to split the coloured light into at least first and second wavelength ranges so that the first and second wavelength ranges are directed respectively towards the first and second waveguides in directions that are offset from one another;

wherein the first waveguide comprises:

a first input diffractive optical element configured to receive light in at least the first wavelength range and to diffract the received light so that diffracted wavelengths of the light are coupled into the first waveguide, to be totally internally reflected within the first waveguide; and a first output diffractive optical element configured to receive and diffract totally internally reflected light within the first waveguide in order to couple the totally internally reflected light out of the first waveguide towards a viewer;

wherein the second waveguide comprises:

a second input diffractive optical element configured to receive light in at least the second wavelength range and to diffract the received light so that diffracted wavelengths of the light are coupled into the second waveguide, to be totally internally reflected within the second waveguide; and a second output diffractive optical element configured to receive and diffract totally internally reflected light within the second waveguide in order to couple the totally internally reflected light out of the second waveguide towards the viewer.

2. The optical structure of claim 1, wherein the chromatic filter is a dichroic filter.

3. The optical structure of claim 1 or claim 2, wherein the chromatic filter is configured to reflect at least the first wavelength range towards the first input diffractive optical element, and to transmit the second wavelength range.

4. The optical structure of claim 3, wherein a reflective surface is arranged to receive the second wavelength range that is transmitted by the chromatic filter and to reflect the received light towards the second input diffractive optical element.

5. The optical structure of claim 1 or claim 2, wherein the chromatic filter is configured to transmit at least the first wavelength range towards the first input diffractive optical element, and to reflect the second wavelength range towards the second input diffractive optical element.

6. The optical structure of any of the preceding claims, wherein the first and second input diffractive optical elements are reflection diffraction gratings.

7. The optical structure of claim 6, wherein the first and second input diffractive optical elements are provided with a reflective coating.

8. The optical structure of claim 8, wherein the reflective coating is a metallised coating.

9. The optical structure of any of the preceding claims, wherein the first and second wavelength ranges are non-overlapping and are split at a wavelength that is green in colour.

10. The optical structure of any of the preceding claims, further comprising a third waveguide, wherein the third waveguide comprises a third input diffractive optical element configured to receive light in at least a third wavelength range and to diffract the received light so that diffracted wavelengths of the light are coupled into the third waveguide, to be totally internally reflected within the third waveguide; and a third output diffractive optical element configured to receive and diffract totally internally reflected light within the third waveguide in order to couple the totally internally reflected light out of the third waveguide towards the viewer.

11. The optical structure of claim 10, comprising first and second chromatic filters, wherein the first chromatic filter is configured to receive coloured light from a projector and to direct a first wavelength range towards the first waveguide, and wherein the second chromatic filter is configured to receive light from the first chromatic filter and direct a second wavelength range towards the second waveguide, and wherein a reflector is provided to receive light from the second chromatic filter and direct a third wavelength range towards the third waveguide so that the first, second and third wavelength ranges are directed respectively towards the first, second and third waveguides in directions that are offset from one another.

12. An augmented reality display comprising:

the optical structure of any of the preceding claims; and a projector configured to direct light with at least first and second wavelength ranges

5 towards the chromatic filter.