Skew mirrors, methods of use, and methods of manufacture
An optical reflective device referred to as a skew mirror, having a reflective axis that need not be constrained to surface normal, is described. Examples of skew mirrors are configured to reflect light about a constant reflective axis across a relatively wide range of wavelengths. In some examples, a skew mirror has a constant reflective axis across a relatively wide range of angles of incidence. Exemplary methods for making and using skew mirrors are also disclosed. Skew mirrors include a grating structure, which in some examples comprises a hologram.
1. Apparatus comprising:
a grating structure residing in a grating medium, wherein:
the grating structure is configured to reflect first incident light, the first incident light being incident upon the grating medium at a specific site and having a first wavelength and a first internal angle of incidence relative to a surface normal of the grating medium;
the first incident light is principally reflected by the grating medium as first reflected light, the first reflected light having the first wavelength and a first internal angle of reflection relative to the surface normal;
the first incident light and the first reflected light are bisected by a first reflective axis having a first reflective axis angle relative to the surface normal;
the grating structure is further configured to reflect second incident light, the second incident light being incident on the grating medium at the specific site and having a second wavelength and a second internal angle of incidence relative to the surface normal;
the second incident light is principally reflected by the grating medium as second reflected light, the second reflected light having the second wavelength and a second internal angle of reflection relative to the surface normal;
the second incident light and the second reflected light are bisected by a second reflective axis having a second reflective axis angle relative to the surface normal;
the first and second reflective axis angles are each non-zero relative to the surface normal;
the first wavelength differs from the second wavelength; and
the first reflective axis angle differs from the second reflective axis angle.
2. The apparatus of claim 1 , wherein the first reflective axis angle differs from the second reflective axis angle by 0.25 degrees or less.
3. The apparatus of claim 2 , wherein the first reflective axis angle relative to the surface normal is at least 1.0 degree.
4. The apparatus of claim 1 , wherein the first reflective axis angle relative to the surface normal is at least 1.0 degree.
5. The apparatus of claim 1 , wherein the first incident light is offset from the first reflective axis by at least 1.0 degree.
6. The apparatus of claim 1 , wherein:
the grating structure comprises a plurality of volume holograms;
each of the volume holograms in the plurality of volume holograms spatially overlaps at least one other volume hologram in the plurality of volume holograms; and
the grating medium is at least 70 μm thick.
7. The apparatus of claim 6 , wherein:
the plurality of volume holograms includes at least four volume holograms.
8. The apparatus of claim 1 , wherein:
the grating structure comprises at least 9 volume holograms;
each of the at least 9 volume holograms at least partially spatially overlaps all others of the at least 9 volume holograms; and
the grating medium is at least 200 μm thick.
9. The apparatus defined in claim 1 , wherein the first wavelength differs from the second wavelength by a wave fraction of at least 0.005.
10. The apparatus defined in claim 9 , wherein the first reflective axis angle relative to the surface normal is at least 1.0 degree.
11. The apparatus defined in claim 9 , wherein the first internal angle of incidence is the same as the second internal angle of incidence.
12. The apparatus defined in claim 9 , wherein each of the first internal angle of incidence and the second internal angle of incidence includes multiple angles spanning a range.
13. The apparatus defined in claim 1 , wherein the first internal angle of incidence is the same as the second internal angle of incidence.
14. The apparatus defined in claim 13 , wherein each of the first internal angle of incidence and the second internal angle of incidence includes multiple angles spanning a range.
15. Apparatus comprising:
a grating structure residing in a grating medium, wherein:
the grating structure is configured to reflect first incident light, the first incident light being incident upon the grating medium at a specific site and having a first internal angle of incidence relative to a surface normal of the grating medium;
the first incident light is principally reflected by the grating medium as first reflected light, the first reflected light having a first internal angle of reflection relative to the surface normal;
the first incident light and the first reflected light are bisected by a first reflective axis having a first reflective axis angle relative to the surface normal;
the grating structure is further configured to reflect second incident light, the second incident light being incident on the grating medium at the specific site and having a second internal angle of incidence relative to the surface normal;
the second incident light is principally reflected by the grating medium as second reflected light, the second reflected light having a second internal angle of reflection relative to the surface normal;
the second incident light and the second reflected light are bisected by a second reflective axis having a second reflective axis angle relative to the surface normal;
the first incident light has the same wavelength as the first reflected light;
the second incident light has the same wavelength as the second reflected light;
the first internal angle of incidence differs from the second internal angle of incidence by a multiple of Δθ B , wherein Δθ B is an angular Bragg selectivity calculated for an incident light angle of incidence residing at a midpoint between the first incident light and the second incident light;
the first reflective axis angle is non-zero relative to the surface normal;
each of the first incident light and the second incident light are offset from the first reflective axis; and
the first reflective axis angle differs from the second reflective axis.
16. The apparatus of claim 15 , wherein:
the first reflective axis angle differs from the second reflective axis angle by 0.25 degrees or less.
17. The apparatus of claim 15 , wherein:
the first internal angle of incidence differs from the second internal angle of incidence by 20 times Δθ B .
18. The apparatus of claim 15 , wherein: the grating medium is at least 70 μm thick.
19. The apparatus of claim 15 , wherein:
each of the first incident light and the second incident light are offset from the first reflective axis by at least 1.0 degree.
20. The apparatus of claim 15 , wherein:
the first incident light, the first reflected light, the second incident light, and the second reflected light all have the same wavelength.