Diffractive optical waveguide and display device

1. A diffractive optical waveguide for optical pupil expansion, comprising:

a waveguide substrate;

a coupling-out grating disposed on or in the waveguide substrate and configured to couple input light out of the waveguide substrate by diffraction, wherein,

the coupling-out grating comprises a plurality of grating lines with widths; the plurality of grating lines are spaced in a cycle of a first predetermined period along a first direction and are spaced in a cycle of a second predetermined period along a second direction; each of the grating lines comprises a plurality of periodic structures in continuous and connected arrangement,

wherein, each of the periodic structures comprises a first edge and a second edge spaced apart in the first direction; the first predetermined period is defined as the distance between the first edge and the second edge in the first direction; the width of the first edge is equal to the width of the second edge; the width of the first edge and the width of the second edge are each smaller than the second predetermined period; the distance between the center of the first edge and the center of the second edge in the second direction is defined 0.5 times the second predetermined period,

each of the periodic structures further comprises a first side edge connecting the first edge and the second edge, and a second side edge connecting the first edge and the second edge; the first side edge is opposite to the second side edge; at least one of the first side edge and the second side edge is a non-linear side edge.

2. The diffractive optical waveguide of claim 1 , wherein the second direction is orthogonal or non-orthogonal to the first direction.

3. The diffractive optical waveguide of claim 1 , wherein the shape of the non-linear side edge is one of: a polyline, an arc, or a combination of a straight line and an arc.

4. The diffractive optical waveguide of claim 1 , wherein the first side edge and the second side edge each is a non-linear side edge, wherein,

the first side edge and the second side edge each is in the shape of a polyline, or the first side edge and the second side edge each is in the shape of an arc, or the first side edge and the second side edge each is in the shape of a combination of a straight line and an arc.

5. The diffractive optical waveguide of claim 4 , wherein a center line is defined as a line connecting the midpoint of the first edge and the midpoint of the second edge, and the distance from the first side edge to the center line in the second direction is equal to the distance from the second side edge to the center line in the second direction.

6. The diffractive optical waveguide of claim 4 , wherein a center line is defined as a line connecting the midpoint of the first edge and the midpoint of the second edge; a symmetry midpoint is defined as a point at equal distance to the first edge and to the second edge in the center line along the first direction; the periodic structure is in a center-symmetrical shape and is symmetrical with respect to the symmetry midpoint.

7. The diffractive optical waveguide of claim 4 , wherein the width of each of the grating lines is defined by the distance between the first side edge and the second side edge of the periodic structures in the second direction, and the width of the grating lines is constant.

8. The diffractive optical waveguide of claim 1 , wherein the plurality of grating lines are configured such that a predetermined condition for uniformity of photon energy distribution of a project optical field, which is resulted from the input light in different incident directions being coupled out of the coupling-out grating, is met.

9. The diffractive optical waveguide of claim 8 , wherein the predetermined condition includes that the value of the uniformity is lower than 10%, wherein the value of the uniformity is calculated based on uni=(maxeffi−mineff)/(maxeff+mineff), uni representing the value of the uniformity, maxeff representing maximum relative coupling-out efficiency, and mineff representing minimum relative coupling-out efficiency.

10. The diffractive optical waveguide of claim 1 , further comprising:

a coupling-in grating disposed on or in the waveguide substrate and configured to couple the input light into the waveguide substrate to allow it to be coupled to the coupling-out grating by total reflection.

11. A display device comprising a diffractive optical waveguide of claim 1 .

12. The display device of claim 11 , wherein the display device is a near-eye display device, comprising: a lens and a frame for holding the lens near eyes, the lens including said diffractive optical waveguide.

13. The display device of claim 11 , wherein the display device is an augmented reality display device or a virtual reality display device.