Waveguide array illuminator with light scattering mitigation

1. A waveguide illuminator comprising:

an input waveguide for guiding an input light beam therein;

a waveguide splitter coupled to the input waveguide for splitting the input light beam into a plurality of sub-beams;

a waveguide array coupled to the waveguide splitter for propagating the plurality of sub-beams therein, waveguides of the waveguide array running parallel to one another, each waveguide being configured to guide a sub-beam of the plurality of sub-beams; and

an array of out-coupling gratings comprising rows of spaced apart out-coupling gratings, each row being coupled to a corresponding waveguide of the waveguide array along a length of the waveguide, wherein the out-coupling gratings of each row are configured for out-coupling portions of the sub-beam propagating in the corresponding waveguide, whereby in operation, a two-dimensional array of the sub-beam portions is out-coupled from the waveguide array.

2. The waveguide illuminator of claim 1 , wherein the out-coupling gratings of each row of the array out-coupling gratings have an out-coupling efficiency gradually increasing with distance from the waveguide splitter, for flattening a spatial distribution of optical power of the two-dimensional array of the out-coupled sub-beam portions.

3. The waveguide illuminator of claim 1 , wherein out-coupling gratings of the array of out-coupling gratings are apodized in a direction of length of corresponding waveguides, such that at least one of:

a duty cycle of each out-coupling grating is less than 0.1 or greater than 0.9 at opposite ends of the out-coupling grating, and is greater than 0.1 and less than 0.9 between the opposite ends of the out-coupling grating; or

a groove depth of each out-coupling grating is less at the opposite ends of the out-coupling grating than at the middle of the out-coupling grating.

4. The waveguide illuminator of claim 1 , wherein:

the waveguide array comprises an array of ridge waveguides;

the array of out-coupling gratings is formed in the array of ridge waveguides;

the out-coupling gratings of each row comprise grooves etched in a corresponding ridge waveguide of the array of ridge waveguides to a grating etch depth D GR ; and

gaps between neighboring out-coupling gratings of each row are uniformly etched to a gap etch depth D GAP =aD GR , wherein 0<a<1.

5. The waveguide illuminator of claim 4 , wherein a<0.5.

6. The waveguide illuminator of claim 1 , wherein:

the waveguide array comprises an array of ridge waveguides;

the array of out-coupling gratings is formed in the array of ridge waveguides;

the out-coupling gratings of each row comprise grooves in a corresponding ridge waveguide of the array of ridge waveguides at a grating pitch P GR ; and

gaps between neighboring out-coupling gratings of each row comprise gap gratings.

7. The waveguide illuminator of claim 6 , wherein the gap gratings comprise grooves running non-parallel to the waveguides of the waveguide array at a gap pitch P GAP =bP GR , wherein 0<b<0.5.

8. The waveguide illuminator of claim 6 , wherein the gap gratings comprise grooves running parallel to the waveguides of the waveguide array.

9. A display device comprising:

a waveguide illuminator comprising:

an input waveguide for guiding an input light beam therein;

a waveguide splitter coupled to the input waveguide for splitting the input light beam into a plurality of sub-beams;

a waveguide array coupled to the waveguide splitter for propagating the plurality of sub-beams therein, waveguides of the waveguide array running parallel to one another, each waveguide being configured to guide a sub-beam of the plurality of sub-beams; and

an array of out-coupling gratings comprising rows of spaced apart out-coupling gratings, each row being coupled to a corresponding waveguide of the waveguide array along a length of the waveguide, wherein the out-coupling gratings of each row are configured for out-coupling portions of the sub-beam propagating in the corresponding waveguide, whereby in operation, a two-dimensional array of the sub-beam portions is out-coupled from the waveguide array; and

a display panel comprising an array of display pixels disposed and configured to receive the two-dimensional array of the out-coupled sub-beam portions.

10. The display device of claim 9 , wherein the out-coupling gratings of each row of the array out-coupling gratings have an out-coupling efficiency gradually increasing with distance from the waveguide splitter, for flattening a spatial distribution of optical power of the two-dimensional array of the out-coupled sub-beam portions.

11. The display device of claim 9 , wherein out-coupling gratings of the array of out-coupling gratings are apodized in a direction of length of waveguides of the waveguide array to reduce light scattering by the array of out-coupling gratings.

12. The display device of claim 9 , wherein a pitch of the display pixels is substantially equal to a pitch of the array of out-coupling gratings.

13. The display device of claim 9 , further comprising a light source for providing the input light beam to the input waveguide.

14. The display device of claim 13 , wherein the light source is a polarized light source, wherein the input light beam and the out-coupled sub-beam portions are polarized; and

wherein the array of display pixels comprises an array of tunable polarization rotators for tuning polarization of individual sub-beam portions.

15. The display device of claim 13 , wherein the light source comprises a monochromatic light source, wherein the input light beam has a wavelength of a first color channel.

16. A method for reducing light scattering in a waveguide illuminator comprising a waveguide splitter for splitting an input light beam into a plurality of sub-beams, a waveguide array coupled to the waveguide splitter for propagating the plurality of sub-beams therein, each waveguide being configured to guide a sub-beam of the plurality of sub-beams, and an array of out-coupling gratings comprising rows of spaced apart out-coupling gratings formed in corresponding waveguides for out-coupling portions of the sub-beams propagating in the corresponding waveguides, the method comprising:

matching impedance of out-coupling gratings of a row of the out-coupling gratings to impedance of gaps between the out-coupling gratings of the row of the out-coupling gratings.

17. The method of claim 16 , wherein matching the impedance of the out-coupling gratings comprises apodizing out-coupling gratings of the array of out-coupling gratings in a direction of length of corresponding waveguides, such that a duty cycle of each out-coupling grating at opposite ends of the out-coupling grating is less than 0.1 or greater than 0.9, being between 0.1 and 0.9 between the opposite ends of the out-coupling grating.

18. The method of claim 16 , wherein the waveguide array comprises an array of ridge waveguides, and the array of out-coupling gratings is formed in the array of ridge waveguides by etching the ridge waveguides to a grating etch depth D GR ; and

wherein matching impedance of the out-coupling gratings comprises uniformly etching gaps between neighboring out-coupling gratings of each row a gap etch depth D GAP =a D GR , wherein 0<a<1.

19. The method of claim 16 , wherein the waveguide array comprises an array of ridge waveguides, and the array of out-coupling gratings is formed in the array of ridge waveguides; and

wherein matching impedance of the out-coupling gratings comprises forming gap gratings in gaps between neighboring out-coupling gratings, at a pitch at least twice less than a pitch of the out-coupling gratings.

20. The method of claim 16 , wherein the waveguide array comprises an array of ridge waveguides, and the array of out-coupling gratings is formed in the array of ridge waveguides; and

wherein matching impedance of the out-coupling gratings comprises forming gap gratings in gaps between neighboring out-coupling gratings, the gap gratings comprising grooves running parallel to the ridge waveguides of the array of ridge waveguides.