Zoned optical waveplate
View Patent ↗A zoned waveplate has a series of transversely stacked birefringent zones alternating with non-birefringent zones. The birefringent and non-birefringent zones are integrally formed upon an AR-coated face of a single substrate by patterning the AR coated face of the substrate with zero-order sub-wavelength form-birefringent gratings configured to have a target retardance. The layer structure of the AR coating is designed to provide the target birefringence in the patterned zones and the reflection suppression.
1. A zoned optical waveplate (ZWP) comprising:
a substrate that is transparent to light of a target wavelength λ; and
an anti-reflection (AR) coating disposed over the substrate and defining an AR coated face of the substrate,
wherein the AR coated face of the substrate comprises a series of spaced apart patterned zones alternating with non-patterned zones or differently-patterned zones in a transverse direction, each patterned zone, of the series of spaced apart patterned zones, comprising a first zero-order form-birefringent sub-wavelength (ZOFBSW) grating that is configured to provide a first target retardance for light of the target wavelength.
2. The ZWP of claim 1 , wherein the first ZOFBSW grating is a first ZOFBSW relief grating defined by a plurality of parallel trenches formed in the AR coated face of the substrate.
3. The ZWP of claim 2 , wherein the plurality of parallel trenches are oriented at 45 degrees to the transverse direction.
4. The ZWP of claim 1 , wherein
the series of spaced apart patterned zones alternate with the non-patterned zones,
the series of spaced apart patterned zones and the non-patterned zones are of substantially equal width, and
the AR coated face of the substrate, in the non-patterned zones, is substantially flat and absent of trenches.
5. The ZWP of claim 2 , wherein
the AR coating comprises a spacer layer of a first dielectric material disposed over the substrate and an index matching layer of a second dielectric material disposed upon the spacer layer, and
the plurality of parallel trenches extend through the AR coating.
6. The ZWP of claim 5 , further comprising:
an etch stop layer of a third dielectric material disposed between the spacer layer and the substrate, wherein the plurality of parallel trenches extend through the spacer layer up to the etch stop layer.
7. The ZWP of claim 5 , wherein the substrate comprises fused silica, and the spacer layer comprises one of:
tantala,
hafnia, or
silicon nitride.
8. The ZWP of claim 5 , further comprising:
an etch-stop layer of aluminum oxide deposited between the spacer layer and the substrate.
9. The ZWP of claim 5 , wherein
the spacer layer has a thickness in a range of 1.8 to 2.2 microns (μm), and
the first ZOFBSW relief grating has a pitch in a range of 0.5 to 0.9 μm and a fill factor in a range of 0.55 to 0.75 so as to provide a half-waveplate retardance at the target wavelength of 1.55 μm.
10. The ZWP of claim 5 , wherein
the substrate comprises fused silica and the spacer layer comprises silicon, and
the index matching layer comprises one of:
tantala,
hafnia, or
silicon nitride.
11. The ZWP of claim 5 , wherein
the spacer layer has a thickness in a range of 0.4 to 0.55 microns (μm), and
the first ZOFBSW grating is characterized by a pitch in a range of 0.5 to 0.9 μm and a fill factor in a range of 0.55 to 0.85 so as to provide a half-waveplate retardance at the target wavelength of 1.55 μm.
12. The ZWP of claim 5 , further comprising:
an etch-stop layer of aluminum oxide between the spacer layer and the substrate.
13. The ZWP of claim 2 , wherein
the substrate comprises crystalline silicon,
the AR coating comprises tantala, and
the plurality of parallel trenches extend into the substrate.
14. The ZWP of claim 2 , wherein the plurality of parallel trenches comprise between 60% to 80% of the first ZOFBSW relief grating in area.
15. The ZWP of claim 5 , wherein the spacer layer has a thickness that is selected to provide a half-waveplate birefringence for light of the target wavelength at normal incidence.
16. The ZWP of claim 2 , wherein:
the series of spaced apart patterned zones alternate with the differently-patterned zones in the transverse direction, and
each differently-patterned zone, of the differently-patterned zones, comprises a second ZOFBSW relief grating that is defined by a plurality of parallel trenches and configured to provide a second target retardance for light of the target wavelength.
17. The ZWP of claim 16 , wherein
the plurality of parallel trenches of the first ZOFBSW relief grating are directed at 45 degrees to the plurality of parallel trenches of the second ZOFBSW relief grating, and
the first and second ZOFBSW relief gratings are configured to provide a half-wave retardance at normal incidence.
18. A method of fabricating a zoned optical waveplate, comprising:
depositing an antireflection (AR) coating upon a substrate of optically transmissive material to form a wafer having an AR coated face; and
defining, on the AR coated face, a series of patterned zones alternating with non-patterned zones or differently-patterned zones, wherein each patterned zone, of the series of patterned zones, comprises a first zero-order form-birefringent sub-wavelength (ZOFBSW) relief grating providing a target retardance for light of a target wavelength.
19. The method of claim 18 , wherein depositing an antireflection (AR) coating comprises:
depositing an etch-stop layer over the substrate;
depositing a spacer layer over the etch-stop layer; and
depositing an index-matched layer over the spacer layer.
20. The method of claim 18 , wherein defining the series of patterned zones comprises:
forming a grating mask upon the AR coating comprising a plurality of parallel sub-wavelength strips defining a plurality of grating lines of the first ZOFBSW relief grating;
forming a zone mask over the grating mask, the zone mask having a plurality of zone openings defining the series of patterned zones;
subjecting the wafer to an anisotropic etch process so as to form a plurality of trenches within the series of patterned zones; and
removing the grating mask and the zone mask.
21. The method of claim 20 , wherein
the grating mask and the zone mask are hard metal masks, and
forming the zone mask comprises:
forming the plurality of zone openings using photo-lithographic photoresist patterning and a lift-off process.
22. The method of claim 18 , wherein defining the series of patterned zones comprises:
forming a two-level hard mask using a lift-off process.