The present invention relates to an optical device for use in a head-up display, an augmented reality display or a virtual reality display. In particular, the invention relates to a system with improved uniformity of illumination across a user's field of view.

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. In a virtual reality display a user can typically only see projected images

In a normal 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. The projected light is totally internally reflected within the waveguide. The light is then coupled out of the waveguide by another diffraction 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.

A number of techniques are known for expanding light in at least one dimension within a waveguide. In this way, a thin input beam can be coupled into the waveguide by an input diffraction grating. The light can then be expanded in order to fill a user's field of view. This is typically achieved by providing diffraction gratings with lines that are angled with respect to incoming rays of light.

WO2016/020643 describes an optical device for expanding input light in two dimensions in an augmented reality display. In this arrangement an input diffractive optical element is configured to couple input light into a waveguide and two diffractive optical elements are overlaid on one another in or on the waveguide. The lines in each of the overlaid diffractive optical elements are symmetrically angled with respect to rays from the input diffraction grating. Each of the overlaid diffractive optical elements can receive light from the input diffractive optical element and couple it towards the other diffractive optical element in the pair, which can then act as an output diffractive optical element, coupling light out of the waveguide towards a viewer. In this way the optical device can achieve two-dimensional expansion of an input light source while simultaneously coupling light out of the waveguide so that it can be viewed by a user.

A problem can arise in techniques for expanding light in two dimensions in a waveguide. Specifically, it has been found that known expansion techniques can create differences in the uniformity across a user's field of view. An object of the present invention is to reduce and mitigate these problems.

According to an aspect of the invention there is provided an optical device for expanding light in two dimensions within a waveguide, the optical device comprising: a waveguide; an input diffractive optical element configured to couple light into the waveguide in a first direction; and at least two beam expanding diffractive optical elements, overlaid on one another in or on the waveguide, wherein the two beam expanding diffractive optical elements comprise respective diffractive features that are angled symmetrically with respect to the first direction so that the two beam expanding diffractive optical elements respectively diffract light received from the input diffractive optical element in second and third directions within the waveguide; wherein the at least two beam expanding diffractive optical elements are arranged with an increasing diffraction efficiency in the first direction, away from the input diffractive optical element. Each of the two beam expanding diffractive optical elements is configured to receive light from the input diffractive optical element and couple it towards the other beam expanding diffractive optical element which can then act as an output diffractive optical element providing outcoupled orders towards a viewer. In this way, the overlaid beam expanding diffractive optical elements can expand light in two directions simultaneously, symmetrically disposed about the first direction. By increasing diffraction efficiency away from the input diffractive optical element, the beam expanding diffractive optical elements can diffract a higher proportion of light at positions along their respective lengths. In use, the absolute amount of light decreases in the first direction in the beam expanding diffractive optical elements as light is diffracted into the second and third directions and is coupled out of the waveguide towards a viewer. Therefore, a diffraction efficiency can be selected for each position in the beam expanding diffractive optical elements so that substantially similar amounts of light are diffracted into the second and third directions, and towards a viewer, at each position. This can advantageously improve uniformity of the expanded beam.

The overlaid diffractive optical elements can simultaneously expand light from the input diffractive optical element in two dimensions and couple light out of the waveguide towards a viewer. The diffractive optical elements are provided with carefully selected diffraction efficiencies at positions along their length in order to improve uniformity of the output, and therefore improve the user experience.

The two beam expanding diffractive optical elements may be provided with increasing diffraction efficiencies, away from the input diffractive optical element, in a direction that is substantially perpendicular to the first direction and/or substantially parallel to the second and/or third directions. This can enhance uniformity of the output beam across two dimensions. The waveguide may be provided with a coating at the location of the at least two beam expanding diffractive optical elements, and the coating may have variable properties to achieve the increasing diffraction efficiency in the first direction, away from the input diffractive optical element. In particular, the coating thickness may increase in the first direction, away from the input diffractive optical element.

The at least two beam expanding diffractive optical elements may be surface relief diffraction gratings. Ridges or rulings may be provided in the diffraction gratings, and the feature height of the at least two beam expanding diffractive optical elements may increase in the first direction, away from the input diffractive optical element.

The diffractive feature in the at least two beam expanding optical elements may comprise differences in refractive index. In one arrangement the beam expanding diffractive optical elements may be holographic diffraction gratings. The differences in refractive index may be arranged to increase in the first direction, away from the input diffractive optical element.

In one arrangement the at least two beam expanding diffractive optical elements may be provided in a photonic crystal.

Embodiments of the invention are now described, by way of example, with reference to the drawings, in which: Figure 1 is a perspective view of a waveguide in an embodiment of the invention;

Figure 2A is a graph showing diffraction efficiency at different positions along a diffraction grating; Figure 2B is a graph showing the optical power diffracted into the first diffraction order at different positions along a diffraction grating; and

Figure 3 is a top view of a photonic crystal in another embodiment of the invention. Figure 1 is a perspective view of a waveguide 12 including three linear gratings HO, H1 , H2. The grooves of input grating HO lie in the x-y plane on a first surface of the waveguide 12, are oriented parallel to the y-axis and have a grating pitch p. The linear grating H1 is laterally separated from the input grating HO in the x-y plane and it lies on a second surface of the waveguide 12. The grooves of grating H1 lie in the x-y plane, are oriented at 30 ° to the x-axis, and have a grating pitch p. The linear grating H2 is superimposed on H1 in the x-y plane and lies on the first surface of the waveguide 12, opposite to the grating H2. The crossed gratings H1 , H2 are therefore separated by the thickness of the waveguide 12 in the z-axis. The grooves of grating H2 lie in the x-y plane, are oriented at -30° to the x-axis, and have a grating pitch p.

An input projector (not shown) can provide input light in a direction that is orthogonal to the plane of the waveguide 12. The input grating HO can diffract the input light so that the first diffraction order is coupled into the waveguide 12 in a first direction. The captured light can travel within the waveguide 12 by total internal reflection towards the crossed gratings H1 , H2. Light is coupled into the waveguide 12 by the input grating HO in a first direction. When light from the input grating HO encounters the crossed gratings H1 , H2 it is either transmitted or diffracted. Diffracted light encounters the two gratings in the crossed gratings H1 , H2 simultaneously so that light is diffracted in two directions which are at equal and opposite angles to the first direction, within the plane of the waveguide 12.

The crossed gratings H1 , H2 are overlaid on one another in the waveguide 12. Each grating H1 , H2 can receive light from the input grating HO in the first direction and can diffract it toward the other grating in the overlaid pair. The other grating can then couple light out of the waveguide towards a viewer, providing simultaneous beam expansion and out-coupling, in the manner described in WO2016/020643.

The (diffraction) efficiency of a diffraction grating is a measure of how much optical power is diffracted into a designated direction compared to the power incident on the grating. The input grating HO is provided with high diffraction efficiency to maximise the amount of light from the input projector that is coupled into the waveguide 12. The crossed gratings H1 , H2 are provided with a relatively low diffraction efficiency to enable respective a two-dimensional expansion of light. At each point of interaction with the crossed gratings H1 , H2 light can either be transmitted or diffracted. The use of a relatively low diffraction efficiency can ensure that a reasonable amount of light is transmitted to the ends of the crossed gratings H1 , H2. This can allow illumination across the full two-dimensional area of the crossed gratings H1 , H2, which represents a user's field of view.

In this arrangement the crossed gratings H1 , H2 are provided with increasing diffraction efficiency in the first direction, away from the input grating HO. This advantageously improves uniformity of the output from the crossed gratings H1 , H2, towards a viewer. The diffraction efficiency is selected so that, at each position in the crossed gratings H1 , H2, a similar optical power is diffracted into the first orders. In other words the proportion of light diffracted by the crossed gratings H1 , H2 increases in the first direction so that the absolute amount of light coupled out of the waveguide towards a viewer remains substantially the same across the two- dimensional area covered by the crossed gratings H1 , H2. In another arrangement the crossed gratings H1 , H2 are provided with increasing diffraction efficiency in the first direction and a second (orthogonal) direction within the plane of the waveguide 12. The combined effect is therefore crossed gratings H1 , H2 with a diffraction efficiency that increases in radial bands, expanding outwardly from the point at which rays from the input grating HO first encounter the crossed gratings H1 , H2.

In this arrangement the diffraction gratings HO, H1 , H2 are surface relief gratings on a surface of the waveguide 12. The diffraction efficiency of the gratings HO, H1 , H2 may be influenced by a number of factors such as coating properties and feature height. A variable diffraction efficiency may be provided in the crossed gratings H1 , H2 by providing increasing feature height in the grating and/or increased coating thickness. In other embodiments the 'grooves' in the diffraction gratings HO, H1 , H2 may be provided by variations in refractive index, and an increased diffraction efficiency may be provided by using an increasing refractive index mismatch and/or increased coating thickness.

Figure 2A is a graph showing the diffraction efficiency of the crossed gratings H1 , H2 in the first direction, away from the input grating 4. Figure 2B is a graph showing the optical power in the first diffracted order from the crossed gratings H1 , H2, away from the input grating. It can be seen from these graphs that the diffraction efficiency increases linearly in the direction in which light is transmitted from the input grating HO so that the optical power in the first diffracted order can remain substantially the same at every position along the length of the crossed gratings H1 , H2.

Figure 2B is a graph showing the optical power diffracted into the first order at different positions in the crossed gratings H1 , H2 in the first direction, away from the input grating HO. It can be seen that the optical power diffracted into the first order is substantially the same at all positions in the crossed gratings H1 , H2. This is achieved by careful selection of the diffraction efficiency at different positions in the crossed gratings H1 , H2.

Figure 4 is a top view of a photonic crystal 19 having circular pillars 20 that have an increased refractive index relative to the waveguide 12. In this example the photonic crystal is in the form of a triangular lattice. The pillars 20 are arranged in a regular pattern and they are all provided in the same x-y plane. It is possible to define a grating H1 with pillars 20 aligned along the y-axis with adjacent rows of pillars 20 separated by a distance p. Grating H2 is arranged with rows of pillars 20 at an angle of +30° to the x-axis, with adjacent rows separated by a distance p. Finally, grating H3 is arranged with rows of pillars 20 at an angle of -30 ° to the x-axis, with adjacent rows separated by a distance p. It is noted that gratings H1 and H2 have the same properties as the corresponding gratings in the crossed grating

embodiment shown in Figure 1 . When light from the input grating HO is incident on the photonic crystal 19 it undergoes multiple simultaneous diffractions by the various diffractive optical elements in the manner described in WO 2016/020643. An increasing diffraction efficiency is provided for the photonic crystal in the first direction, away from the input grating HO. Alternatively, the photonic crystal is provided with a diffraction efficiency that increases in radial bands, expanding outwardly from the point at which rays from the input grating HO first encounter the photonic crystal 19. In a photonic crystal an increased diffraction efficiency can be provided by an increased difference in refractive index between the pillars 20 and the gaps between them and/or by providing a coating having an increased thickness.