Apodization of refractive index profile in volume gratings

Abstract

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.

What is claimed is: 
     
         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 at least the first surface of the photopolymer layer to a reactive agent for reducing an amplitude of the periodic refractive index variations proximate the first surface.   
     
     
         2 . The method of  claim 1 , wherein a photopolymer of the photopolymer layer comprises a photopolymerizable group connected to an end group by an acid-cleavable linker group. 
     
     
         3 . The method of  claim 2 , wherein a local refractive index is defined, at least in part, by the end group. 
     
     
         4 . The method of  claim 3 , wherein the reactive agent comprises an acid for separating the end group from the photopolymerizable group. 
     
     
         5 . The method of  claim 4 , wherein in operation, the end group diffuses away upon being separated from the corresponding photopolymerizable group by application of the acid to the at least first surface, thereby reducing the amplitude of the periodic refractive index variations proximate the at least first surface. 
     
     
         6 . A grating coupler manufactured by:
 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 at least the first surface of the photopolymer layer to a reactive agent for reducing an amplitude of the periodic refractive index variations proximate the first surface.   
     
     
         7 . The grating coupler of  claim 6 , wherein a photopolymer of the photopolymer layer comprises a photopolymerizable group connected to an end group by an acid-cleavable linker group. 
     
     
         8 . The grating coupler of  claim 7 , wherein a local refractive index is defined, at least in part, by the end group. 
     
     
         9 . The grating coupler of  claim 8 , wherein the reactive agent comprises an acid for separating the end group from the photopolymerizable group. 
     
     
         10 . The grating coupler of  claim 9 , wherein in manufacturing, the end group diffuses away upon being separated from the corresponding photopolymerizable group by application of the acid to the at least first surface, thereby reducing the amplitude of the periodic refractive index variations proximate the at least first surface. 
     
     
         11 . A display device comprising:
 a projector for providing image light; and   a grating coupler for at least one of in-coupling or out-coupling the image light, the grating coupler manufactured by:   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 at least the first surface of the photopolymer layer to a reactive agent for reducing an amplitude of the periodic refractive index variations proximate the first surface.   
     
     
         12 . The display device of  claim 11 , wherein a photopolymer of the photopolymer layer comprises a photopolymerizable group connected to an end group by an acid-cleavable linker group. 
     
     
         13 . The display device of  claim 12 , wherein a local refractive index is defined, at least in part, by the end group. 
     
     
         14 . The display device of  claim 13 , wherein the reactive agent comprises an acid for separating the end group from the photopolymerizable group. 
     
     
         15 . The display device of  claim 14 , wherein in manufacturing, the end group diffuses away upon being separated from the corresponding photopolymerizable group by application of the acid to the at least first surface, thereby reducing the amplitude of the periodic refractive index variations proximate the at least first surface.