Waveguide display element with an intermediate layer between an in-coupling grating and a waveguide

Abstract

A waveguide display element includes waveguide layers stacked on top of each other and an in-coupler associated with each waveguide layer for coupling light within a predefined wavelength band into the waveguide layer. Each of the in-couplers includes an intermediate layer arranged on the waveguide layer, the intermediate layer having intermediate layer properties. Each of the in-couplers further includes an in-coupling grating arranged on the intermediate layer, the grating having grating properties. The combination of the intermediate layer properties and grating properties of each in-coupler is different with respect to other in-couplers.

Claims

1. A waveguide display element comprising: a plurality of waveguide layers stacked on top of each other, and an in-coupler associated with each waveguide layer for coupling light within a predefined wavelength band into the waveguide layer, wherein: each of the in-couplers includes: an intermediate layer arranged on the waveguide layer, the intermediate layer having intermediate layer properties, and an in-coupling grating arranged on the intermediate layer, the grating having grating properties, and the combination of intermediate layer properties and grating properties of each in-coupler is different with respect to other in-couplers.

2. The element according to claim 1, wherein the in-couplers have the same intermediate layer properties and different grating properties with respect to other in-couplers.

3. The element according to claim 2, wherein in-couplers associated with different waveguide layers have different in-coupling grating periods.

4. The element according to claim 3, wherein the grating period increases from a top waveguide layer on the incoming light side of the element towards a bottom waveguide layers on the opposite side of the element.

5. The element according to claim 2, wherein the grating period increases from a top waveguide layer on the incoming light side of the element towards a bottom waveguide layers on the opposite side of the element.

6. The element according to claim 2, wherein the intermediate layers of each in-coupler have a thickness of 20 μm or less, in particular 10 μm or less, such as 1-10 μm.

7. The element according to claim 2, wherein: the waveguide layers have an index of refraction which is 1.7 or more, such as 2.0 or more, and the intermediate layers have an index of refraction which is less than 1.8, such as 1.7 or less.

8. The element according to claim 2, wherein the in-couplers have the same shape and are aligned with each other in the lateral plane of the element.

9. The element according to claim 1, wherein in-couplers associated with different waveguide layers have different in-coupling grating periods.

10. The element according to claim 9, wherein the grating period increases from a top waveguide layer on the incoming light side of the element towards a bottom waveguide layers on the opposite side of the element.

11. The element according to claim 9, wherein the grating period increases from a top waveguide layer on the incoming light side of the element towards a bottom waveguide layers on the opposite side of the element.

12. The element according to claim 1, wherein the in-couplers have the same grating properties and different intermediate layer properties with respect to other in-couplers.

13. The element according to claim 12, further comprising one or more optical bandpass filters or dichroic mirrors between the waveguide layers.

14. The element according to claim 1, wherein the intermediate layers of each in-coupler have a thickness of 20 μm or less, in particular 10 μm or less, such as 1-10 μm.

15. The element according to claim 1, wherein: the waveguide layers have an index of refraction which is 1.7 or more, such as 2.0 or more, and the intermediate layers have an index of refraction which is less than 1.8, such as 1.7 or less.

16. The element according to claim 1, wherein the in-couplers have the same shape and are aligned with each other in the lateral plane of the element.

17. A personal display device, such as a head-mounted display or head-up display, comprising: a waveguide display element according to claim 1, and a multicolor laser projector adapted to project light rays in different angles of incidence to in-couplers of the display element.

18. A method of coupling propagating light rays into a waveguide display, comprising: providing a waveguide display element according to claim 1, and directing light rays in different angles of incidence to the display element in order to selectively couple said rays into the different waveguide layers of the display element.

19. The method according to claim 18, wherein said light rays comprise light rays in at least three different wavelength bands, each of the wavelength bands being directed to the display element in a different angle of incidence.

20. The element according to claim 1, wherein each intermediate layer has a thickness and index of refraction smaller than that of the waveguide layer it is associated with.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a cross-sectional detail side view of a single waveguide layer and in-coupler arranged thereon.

(2) FIG. 2 shows a cross-sectional side view of a stack of three waveguide layers and in-couplers provided thereon.

(3) FIGS. 3A and 3B shows wave vector diagram for the first and second in-couplers of a stack of three waveguides, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS

(4) FIG. 1 shows a section of a planar waveguide layer 12 having an index of refraction n.sub.1 and thickness of t.sub.1. The waveguide layer 12 is made of transparent material and allows light rays to propagate therein laterally via total internal reflections. On a top surface of the waveguide layer 12, there is an intermediate layer 14 having an index of refraction n.sub.2 and thickness of t.sub.2.

(5) On top of the intermediate layer, there is an in-coupling grating 16 which comprises periodically arranged grating features 17. The grating can be a one-dimensional grating, i.e. a line grating having a single period p, or a two-dimensional grating having two main periods in different directions (whereby references to “period” herein apply to either of the periods). A binary grating is shown in FIG. 1, but the grating profile may be any. Other examples include blazed gratings and slanted gratings.

(6) The waveguide layer 12, the intermediate layer 14 and the grating 16 together form a color/angle-sensitive layer entity 10.

(7) In general, n.sub.2<n.sub.1 and t.sub.2<t.sub.1. Typically, t.sub.2<<t.sub.1.

(8) In one example, n.sub.1=1.5 . . . 2.5. Preferably, n.sub.1≥1.7, such as 2.0.

(9) In one example, n.sub.2=1.1 . . . 1.8. Preferably, n.sub.2≤1.7.

(10) In one example, t.sub.2<t.sub.1/10. Typically, t.sub.2<t.sub.1/20.

(11) In one example, t.sub.1=0.2 . . . 1.2 mm, such as 0.3 . . . 0.7 mm.

(12) In one example, t.sub.2=0.5 . . . 50 μm. Typically, t.sub.2=1 . . . 20 μm.

(13) FIG. 2 shows three superimposed layer entities 10A-C, i.e. waveguide layers 12A-C and intermediate layers 14A-C and in-coupling gratings 16A-C arranged thereon, respectively. In one example, all waveguide layers 12A-C and intermediate layers 14A-C are essentially identical, but the periods of the in-coupling gratings 16A-C differ from each other. This causes different wavelengths λ.sub.A, λ.sub.B, λ.sub.C directed to the stack at different angles, to selectively couple to different waveguide layers 12A-C, respectively, as propagating waves.

(14) In one example, the period of the second grating 16B is longer then the period of the first grating 16A and the period of the third grating 16C is longer than the period of the second grating 16B.

(15) The periods of the neighboring in-coupling gratings 16A/16B, 16B/16C can differ e.g. by 100-400 nm. In one example, the period of the first grating 16A is 200-300 nm, the period of the second grating 16B is 350-450 nm and the period of the third grating 16C is 750-850 nm.

(16) The above embodiments allow for keeping diffraction angles of all wavelengths arriving at the element low with respect to the angle of incidence, whereby the hop length of light rays remains short.

(17) FIG. 3A shows the wave vector diagram in the (k.sub.x,k.sub.y) plane for the first in-coupler of a stack of three waveguides. The primary colors (blue, green, red) are incident at non-overlapping angles on the in-coupling grating (the three central FOV boxes). Also partially overlapping configuration is possible. The image has diagonal field of view of 52 deg with 16:9 aspect ratio. Here, n.sub.1=2.0, n.sub.2=1.7 for all waveguides. The first diffraction order of the in-coupling grating moves the FOV boxes along the y-direction from center to upwards. The FOV points inside the annulus, defined by the inner radius 1.0 and the outer radius n.sub.2, represent modes that propagate inside the main waveguide via total internal reflection. The FOV points outside the n.sub.1-circle are forbidden modes that never exist. It can be seen that only blue light in-couples to the 1.0/n.sub.2-annulus. When the n.sub.2-layer below the in-coupling gratin is thick enough, no transmissive diffraction take place into the n.sub.1/n.sub.2-annulus and thus the green image does not in-couple. After the first waveguide, a long pass filter can be used to remove the blue light, and thus the in-coupler of the second lightguide receives only green and red light. The wave vector diagram for the in-coupler of the second lightguide is shown in FIG. 3B. After the second lightguide, the green light is removed by a bandpass filter. The last guide receive only red light and thus conventional in-couplers can be used for it. Instead of band-bass absorbers, also dichroic mirrors or other wavelength selective components can be used.

(18) The waveguide layers can be planar pieces of transparent material, typically plastic or glass, having two parallel main surfaces. The intermediate layers are also transparent and typically plastic or glass layers. In one example, the intermediate layers are provided as coating layers. The gratings can be fabricated for example as surface relief gratings (SRGs) or by providing additional material onto the surface as diffractive features, or other diffractive optical elements (DOEs). In one example, the gratings comprise linear features made of at least one oxide or nitride material, such as TiO.sub.2, Si.sub.3N.sub.4, and HfO.sub.2.