Apodized optical elements for optical artifact reduction
Techniques disclosed herein relate to a near-eye display system. One example of an optical device of a near-eye display includes a substrate and holographic grating conformally coupled to a surface of the substrate. The substrate is transparent to visible light and infrared light and is configured to be placed in front of an eye of a user of the near-eye display. A refractive index modulation of the holographic grating is apodized in a surface-normal direction of the substrate to reduce optical artifacts in the visible light.
1. An optical device for a near-eye display, the optical device comprising:
a substrate transparent to visible light and infrared light and configured to be placed in front of an eye of a user of the near-eye display; and
a holographic grating conformally coupled to a surface of the substrate and configured to transmit the visible light and reflectively diffract infrared light in a first wavelength range for eye tracking, wherein the holographic grating includes a photopolymer layer on the surface of the substrate, and wherein a refractive index modulation of the holographic grating is apodized in a direction along a thickness of the photopolymer layer to reduce optical artifacts in the visible light.
2. The optical device of claim 1 , wherein a magnitude of the refractive index modulation of the holographic grating in the direction along the thickness of the photopolymer layer is characterized by a bell-shaped curve.
3. The optical device of claim 1 , wherein a magnitude of the refractive index modulation of the holographic grating is asymmetrical in the direction along the thickness of the photopolymer layer.
4. The optical device of claim 1 , wherein the holographic grating includes a reflective holographic grating configured to transmit the visible light and reflectively diffract the infrared light in the first wavelength range for eye tracking.
5. The optical device of claim 4 , wherein the holographic grating is configured to reflectively diffract the infrared light in the first wavelength range from an infrared light source to the eye of the user or from the eye of the user to an infrared camera.
6. The optical device of claim 1 , wherein:
the substrate is configured to guide display light within the substrate through total internal reflection; and
the holographic grating includes a grating coupler configured to couple at least a portion of the display light out of the substrate.
7. The optical device of claim 1 , wherein:
the thickness of the photopolymer layer is greater than 15 μm; and
a maximum refractive index modulation of the holographic grating is at least 0.035.
8. A method of fabricating an optical device for a near-eye display, the method comprising:
obtaining a photopolymer layer on a support substrate, wherein the photopolymer layer is sensitive to light in a first wavelength range; and
projecting, from a first side of the photopolymer layer, a first light beam in the first wavelength range on the photopolymer layer,
wherein a first wavelength of the first light beam, a first intensity of the first light beam, and a light absorption rate of the photopolymer layer are configured such that the first intensity of the first light beam gradually decreases in the photopolymer layer in a direction along a thickness of the photopolymer layer from the first side according to a predetermined light intensity profile to apodize the photopolymer layer in the direction along the thickness of the photopolymer layer.
9. The method of claim 8 , wherein projecting the first light beam on the photopolymer layer comprises:
positioning a prism adjacent to the photopolymer layer, wherein a refractive index of the prism is greater than 1.33; and
projecting the first light beam on the prism,
wherein the prism refracts the first light beam on the photopolymer layer.
10. The method of claim 8 , further comprising recording a holographic grating in the apodized photopolymer layer, wherein:
the holographic grating is apodized in the direction along the thickness of the photopolymer layer; and
the holographic grating is configured to transmit light in the first wavelength range and reflectively diffract infrared light for eye tracking.
11. The method of claim 8 , further comprising projecting, from a second side of the photopolymer layer, a second light beam in the first wavelength range on the photopolymer layer, wherein:
a second wavelength of the second light beam, a second intensity of the second light beam, and the light absorption rate of the photopolymer layer are configured such that the second intensity of the second light beam gradually decreases in the photopolymer layer in the direction along the thickness of the photopolymer layer from the second side to apodize the photopolymer layer in the direction along the thickness of the photopolymer layer.
12. The method of claim 11 , wherein the first wavelength and the second wavelength are identical or are different.
13. The method of claim 11 , wherein the photopolymer layer is asymmetrically apodized in the direction along the thickness of the photopolymer layer.
14. The method of claim 11 , wherein:
the first light beam and the second light beam are coherent; and
the first light beam and the second light beam interfere in the photopolymer layer to form an apodized holographic grating in the photopolymer layer.
15. The method of claim 14 , wherein a magnitude of a refractive index modulation of the apodized holographic grating in the direction along the thickness of the photopolymer layer is characterized by a bell-shaped curve.
16. The method of claim 14 , wherein the apodized holographic grating is configured to perform at least one of:
reflectively diffracting infrared light for eye tracking; or
reducing optical artifacts in visible light.
17. The method of claim 11 , further comprising:
recording a holographic grating in the apodized photopolymer layer; or
desensitizing the photopolymer layer such that the photopolymer layer is not photosensitive to light in the first wavelength range.
18. The method of claim 8 , wherein the photopolymer layer includes:
a photosensitive material configured to absorb the first light beam to initiate polymerization in the photopolymer layer; and
a light absorption material configured to absorb the first light beam without initiating the polymerization in the photopolymer layer.
19. The method of claim 8 , wherein the photopolymer layer includes a plurality of layers of different photopolymer materials that have different maximum achievable amplitudes of refractive index modulation.