Apodization of refractive index profile in volume gratings
A grating coupler may be fabricated by exposing a photopolymer layer to grating forming light for forming periodic refractive index variations in the photopolymer layer. The photopolymer layer may be exposed to apodization light for reducing an amplitude of the periodic refractive index variations in a spatially-selective manner. The apodization may also be achieved or facilitated by subjecting outer surface(s) of the photopolymer layer to a chemically reactive agent that causes the refractive index contrast to be reduced near the surface(s) of application. The apodized refractive index profile of the gratings facilitates the reduction of optical crosstalk between different gratings of the grating coupler.
1. A method of fabrication of a grating coupler, the method comprising:
exposing a photopolymer layer having a thickness between opposed first and second surfaces to grating forming light for forming periodic refractive index variations in the photopolymer layer; and
exposing the first surface of the photopolymer layer to apodization light for reducing an amplitude of the periodic refractive index variations proximate the first surface.
2. The method of claim 1 , further comprising exposing the second surface of the photopolymer layer to apodization light for reducing an amplitude of the periodic refractive index variations proximate the second surface.
3. The method of claim 1 , wherein the first surface of the photopolymer layer is exposed to the apodization light before the photopolymer layer is exposed to the grating forming light.
4. The method of claim 1 , wherein a wavelength of the grating forming light is different from a wavelength of the apodization light, whereby the apodization light is absorbed in the photopolymer layer stronger than the grating forming light.
5. The method of claim 4 , wherein a transmittance of the photopolymer layer at the wavelength of the apodization light is no greater than 5%.
6. The method of claim 4 , wherein the periodic refractive index variations in the photopolymer layer are formed due to a photoreactive agent of the photopolymer layer being sensitive to illumination with the grating forming light, wherein the amplitude of the periodic refractive index variations is reduced due to the photoreactive agent being sensitive to illumination with the apodization light.
7. The method of claim 4 , wherein a duration of the exposure of the photopolymer layer to the apodization light is shorter than a duration of the exposure of the photopolymer layer to the grating forming light.
8. The method of claim 1 , wherein the exposure of the photopolymer layer to the apodization light is performed concurrently with the exposure of the photopolymer layer to the grating forming light.
9. The method of claim 8 , wherein the photopolymer layer comprises a photoreactive agent for forming the periodic refractive index variations by a photoreaction to the grating forming light, and a photoinhibitor agent for impeding the photoreaction when illuminated with the apodization light;
wherein a wavelength of the grating forming light is different from a wavelength of the apodization light.
10. The method of claim 9 , wherein the grating forming light substantially does not activate the photoinhibitor agent, and the apodization light substantially does not activate the photoreactive agent.
11. The method of claim 9 , wherein:
the photoreaction comprises photopolymerization;
the photoinhibitor agent undergoes photolysis when illuminated with the apodization light to produce a radical for impeding the photopolymerization; and
the photoinhibitor agent comprises at least one of butyl nitrite, hexaarylbiimidazole, or tetraethylthiuram disulfide.
12. The method of claim 1 , wherein the exposure of the photopolymer layer to the apodization light is performed after the exposure of the photopolymer layer to the grating forming light.
13. The method of claim 12 , wherein the photopolymer layer comprises a photoreactive group that reduces the amplitude of the periodic refractive index variations upon illumination with the apodization light by at least one of photoisomerization, photoelimination, photopolymerization, or photolocking.
14. The method of claim 13 , wherein the photoreactive group comprises at least one of azobenzene, stilbene, spiropyran, diarylethene, a diazo group, or an azido group.
15. The method of claim 13 , wherein the photoreactive group is on a polymer backbone of the photopolymer layer.
16. A grating coupler for a waveguide, the grating coupler comprising:
a photopolymer layer having a thickness between opposed first and second surfaces, the photopolymer layer comprising periodic refractive index variations due to exposure to grating forming light;
wherein an amplitude of the periodic refractive index variations proximate the first surface is reduced by exposing the first surface the photopolymer layer to apodization light.
17. The grating coupler of claim 16 , wherein an amplitude of the periodic refractive index variations proximate the second surface is reduced by exposing the second surface the photopolymer layer to apodization light.
18. The grating coupler of claim 16 , wherein the periodic refractive index variations in the photopolymer layer are formed due to a photoreactive agent of the photopolymer layer being sensitive to illumination with the grating forming light, wherein the amplitude of the periodic refractive index variations is reduced due to the photoreactive agent being sensitive to illumination with the apodization light.