Compact liquid crystal beam steering devices including multiple polarization gratings

Abstract

Systems, methods, and apparatus are disclosed for attenuating an incident polarized light beam using a plurality of LCPGs and one or more switchable liquid crystal layers. When four LCPGs are used, a spacing between first and second LCPGs can be equal to a spacing between third and fourth LCPGs. Pi and FCL cells can also be used in place of more traditional LC switches. Switching of the LC switch can be imparted via an AC bias.

1 . A liquid crystal beam steering device, comprising:
 a first polarization grating configured to direct incident light into first and second beams having different directions of propagation than that of the incident light, the first and second beams having substantially orthogonal circular polarizations with respect to each other;   a liquid crystal layer configured to receive the first and second beams from the first polarization grating, the liquid crystal layer being switchable between first and second states for introducing a first and second retardance, respectively, to the first and second beams;   a second polarization grating spaced apart from the first polarization grating by a distance D and configured to receive the first and second beams from the liquid crystal layer and to alter the respective directions of propagation of the first and second beams according to the first or second retardance introduced to the first and second beams;   a third polarization grating configured to receive the first and second beams from the second polarization grating and to further alter the respective directions of propagation thereof;   an intermediate region configured to transmit the first and second beams from the third polarization grating therethrough;   a fourth polarization grating configured to receive the first and second beams from the intermediate region and to additionally alter the respective directions of propagation thereof to provide output light; and   an aperture configured to transmit a first portion of both the first and second beams from the fourth polarization grating when the liquid crystal layer is in the first state, and to transmit a second portion of both the first and second beams from the fourth polarization grating therethrough when the liquid crystal layer is in the second state, the first portion being greater than the second portion,   wherein at least one of the first and second polarization gratings are arranged on a substrate, the substrate arranged between the first and second polarization gratings, and   wherein the distance D between the first and second polarization gratings is substantially equal to a distance D′ between the third and fourth polarization gratings.   
     
     
         2 . The liquid crystal beam steering device of  claim 1 , wherein the output light from the fourth polarization grating propagates in a direction substantially:
 parallel to that of the incident light when the liquid crystal layer is in the first state; and   oblique to that of the incident light when the liquid crystal layer is in the second state.   
     
     
         3 . The liquid crystal beam steering device of  claim 1 , wherein the first, second, third, and fourth polarization gratings exhibit substantially similar diffractive properties. 
     
     
         4 . The liquid crystal beam steering device of  claim 1 , wherein in the first state the liquid crystal layer introduces a retardance of nλ, and wherein in the second state the liquid crystal layer introduces a retardance of 
       
         
           
             
               
                 m 
                 + 
                 
                   λ 
                   2 
                 
               
               , 
             
           
         
       
       where n and m are selected from the set including integers and 0. 
     
     
         5 . The liquid crystal beam steering device of  claim 4 , further comprising an AC bias device configured to selectively apply an AC bias to the liquid crystal layer in order to switch between the first and second state. 
     
     
         6 . The liquid crystal beam steering device of  claim 4 , wherein a thickness of the liquid crystal layer is such that in the first state the liquid crystal layer introduces a retardance of nλ, and wherein in the second state the liquid crystal layer introduces a retardance of 
       
         
           
             
               
                 m 
                 + 
                 
                   λ 
                   2 
                 
               
               , 
             
           
         
       
       where n and m are selected from the set including integers and 0. 
     
     
         7 . The liquid crystal beam steering device of  claim 1 , further comprising one or more trim retarders arranged to one or both sides of the liquid crystal layer and between the first and second polarization gratings, the trim retarders shaped and arranged so as to, in combination with the liquid crystal layer, impart no retardance to the first and second beams when an AC bias device imparts a finite AC bias to the liquid crystal layer, thereby placing the liquid crystal layer in the first state. 
     
     
         8 . The liquid crystal beam steering device of  claim 1 , wherein the third polarization grating is configured to output off-axis beams when the liquid crystal is in the second state. 
     
     
         9 . A liquid crystal beam steering device, comprising:
 a first polarization grating configured to direct incident light into first and second beams having different directions of propagation than that of the incident light, the first and second beams having substantially orthogonal circular polarizations with respect to each other;   a liquid crystal layer configured to receive the first and second beams from the first polarization grating, the liquid crystal layer being switchable between first and second states for introducing a first and second retardance, respectively, to light traveling therethrough;   a second polarization grating spaced apart from the first polarization grating by a distance D1 and configured to receive the first and second beams from the liquid crystal layer to alter the respective directions of propagation of the first and second beams in response to each of the first and second states of the liquid crystal layer;   a third polarization grating configured to receive the first and second beams from the second polarization grating to further alter the respective directions of propagation thereof;   an intermediate region having a thickness D2 and configured to transmit the first and second beams from the third polarization grating therethrough;   a fourth polarization grating spaced apart from the third polarization grating by a distance D2 and configured to receive the first and second beams from the third polarization grating to additionally alter the respective directions of propagation thereof to provide output light that propagates in a direction substantially parallel to that of the first and second beams output from the second polarization grating; and   an aperture configured to block both first and second beams when the liquid crystal layer is in the first state, and to transmit both first and second beams therethrough when the liquid crystal layer is in the second state,   wherein at least one of the first and second polarization gratings are arranged on a substrate, the substrate arranged between the first and second polarization gratings, and   wherein the distance D1 between the first and second polarization gratings is substantially equal to distance D2 between the third and fourth polarization gratings.   
     
     
         10 . The liquid crystal beam steering device of  claim 9 , wherein the intermediate region comprises a refractive component. 
     
     
         11 . The liquid crystal beam steering device of  claim 9 , wherein D1 is equal to D2. 
     
     
         12 . The liquid crystal beam steering device of  claim 9 , wherein D1 is not equal to D2. 
     
     
         13 . A liquid crystal beam steering device, comprising:
 a first polarization grating configured to direct incident light into first and second beams having different directions of propagation than that of the incident light, the first and second beams having substantially orthogonal circular polarizations with respect to each other;   a liquid crystal layer configured to receive the first and second beams from the first polarization grating, the liquid crystal layer being switchable between first and second states for introducing a first and second retardance, respectively, to light traveling therethrough;   a second polarization grating spaced apart from the first polarization grating and configured to receive the first and second beams from the liquid crystal layer to alter the respective directions of propagation of the first and second beams in response to each of the first and second states of the liquid crystal layer;   a third polarization grating configured to receive the first and second beams from the second polarization grating to further alter the respective directions of propagation thereof;   an intermediate region configured to transmit the first and second beams from the third polarization grating therethrough while modifying the respective directions of propagation thereof;   a fourth polarization grating configured to receive the first and second beams from the intermediate region to additionally alter the respective directions of propagation thereof to provide output light that propagates in a direction substantially parallel to that of the first and second beams output from the second polarization grating; and   an aperture configured to block both first and second beams when the liquid crystal layer is in the first state, and to transmit both first and second beams therethrough when the liquid crystal layer is in the second state,   wherein at least one of the first and second polarization gratings are arranged on a substrate, the substrate arranged between the first and second polarization gratings, and   wherein a first distance between the first and second polarization gratings is substantially equal to a second distance between the third and fourth polarization gratings.   
     
     
         14 . The liquid crystal beam steering device of  claim 13 ,
 wherein the first and second polarization gratings are spaced apart by a distance D, and   wherein the intermediate region has a thickness configured to separate the third and fourth polarization gratings by the distance D.   
     
     
         15 . The liquid crystal beam steering device of  claim 13 ,
 wherein the incident light is characterized by a wavelength λ, the liquid crystal layer exhibits a first refractive index n1(λ) at the wavelength λ, the intermediate region exhibits a second refractive index n2(λ) at the wavelength λ, and D1 and D2 are related by the equation D1*λ*n1(λ)=D2*λ*n2(λ).   
     
     
         16 . The liquid crystal beam steering device of  claim 15 , and wherein the first, second, third, and fourth polarization gratings exhibit substantially similar diffractive properties. 
     
     
         17 . The liquid crystal beam steering device of  claim 15 , wherein the first and second polarization gratings exhibit similar, first diffractive properties, and wherein the third and fourth polarization gratings exhibit similar, second diffractive properties. 
     
     
         18 . The liquid crystal beam steering device of  claim 13 ,
 wherein the incident light is incident on the first polarization grating at a first angle with respect to an optical axis,   wherein the first and second beams exit the fourth polarization grating at a second angle with respect to the optical axis when the liquid crystal layer is in the second state, wherein the second angle is not equal to the first angle.