Abstract:
A system of beam steering using electrical operation. A first system provides a grating and a liquid crystal material. When the liquid crystal is unenergized, there is a mismatch between the liquid crystal and the grating, causing the grating to diffract the light in a specified direction. The liquid crystal is energized to match its index of refraction to the grating. Then, the light is not diffracted by the grating, and hence travels in a different direction then it would when the liquid crystal was not energized. Another, finer system, forms electrically generated gratings using a liquid crystal material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 60/192,646, filed Mar. 27, 2000. 
    
    
     BACKGROUND 
     Gratings can be used for many purposes, including as optical filters. A grating can be formed as a hologram on a substrate. Light which matches the grating is then deflected in a specified way. 
     Many optical applications such as optical networking, optical switching, projection displays, optical data storage and optical holographic applications, may need to steer an optical beam in a desired direction. 
     SUMMARY 
     The present application teaches forming a plurality of stacked adjustable gratings which can be used for beam steering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will now be described in detail with reference to the accompanying drawings, wherein: 
     FIG. 1 shows a single layer crystal beam deflector; 
     FIG. 2 shows a stacked, positive grating with a number of layers; 
     FIG. 3 shows an electronically controlled grating system, which forms a virtual grating; and 
     FIG. 4 shows a composite system of blazed grating/liquid crystal layers, mixed with a liquid crystal virtual grating layer. 
    
    
     DETAILED DESCRIPTION 
     The present system forms an addressable beam deflector. The addressing may deflect an incoming optical beam to one of a plurality of different steering angles. 
     A first embodiment comprising a single-layer liquid crystal beam deflector is shown in FIG.  1 . This device includes a substrate of glass or other optically transparent material  100 . The glass is covered with an Indium Tin Oxide (ITO) layer  102 . A layer of Polymethyl methacrylate (PMMA)  104  is formed as blazed grating on the ITO layer  102 . The blazed grating can be formed by direct E beam lithography on the substrate. The E beam lithography may form the PMMA layer into the shape of a grating, having a specified period  131 . 
     Either one or a number of fiber spacers  112  may cover the PMMA grating. These fiber spacers may be configured to leave a space of 0.25 to 3 microns as the width of cavity used to form the space  126 . 
     Glass substrate  120 , also having an ITO layer  122 , may cover the spacers  112 . The spacers define a cavity  126 , along with the substrate  120  and substrate  100 . 
     The cavity is filled with liquid crystal material  128 , preferably a nematic type liquid crystal material  128 . For example, the liquid crystal material may be Merck E7, whose refractive indices at a 633 nm wavelength for extraordinary and ordinary light are respectively 1.737 and 1.5815. 
     An electrical field is applied between the ITO layer  102 , which is under the grating, and the other ITO layer  122 , which is above the grating. 
     The electro-optic affect of the nematic liquid crystal  128  changes the orientation of the liquid crystal, and hence the refractive index for extraordinary light, according to the modulation of the driving voltage applied between the ITO layers  102 ,  122 . Therefore, the phase information which is applied to the grating  104  depends on the electric field applied between the ITO gratings. 
     The system can be operated in a binary mode. When an electric field is present, the refractive indices of the PMMA grating  104  and liquid crystal  128  are different. Hence, a strong diffraction is produced by the refractive index/phase difference between the grating and liquid crystal when the voltage is in the off state. The effective diffraction efficiency may be determined by the parameters of the blazed grating, such as grating depth, grating period, and blaze profile. 
     An electric control element  131  is used to apply an electric field between the substrates  100 ,  120 . When the electric field is applied between the electrodes, the refractive index of the liquid crystal is decreased. At a specified driving voltage, “index matching” occurs between the PMMA material  104 , and the liquid crystal  128 . When this happens, the entire liquid crystal/PMMA composite grating structure can then be considered as an optically flat plate. Little or no diffraction occurs in this state. 
     Hence, the device can be viewed as an electrically controlled binary switch. The incident beam can either be deflected when in the off state, or undeflected when in the on state. 
     Moreover, this device may work preferentially for extraordinary light, and hence form a polarized beam deflector. The incident light  130  needs to have a polarization direction that is the same as the liquid crystal extraordinary light direction, which is also the “rubbing” direction for the homogeneous alignment configuration. The rubbing direction can be established by rubbing one of the glass plates, to cause the liquid crystal to align along the specified direction. 
     FIG. 2 shows a system that allows controlling the system to deflect the beam to multiple angles. Several layers  200 ,  210  of the LC/PMMA composite blazed grating are formed. Each of these layers may be of the general structure shown in FIG.  1 . Each of a plurality of the gratings may have different grating periods ( 131  in FIG.  1 . One embodiment may use a stack that has the period of the top grating  250  being double the period of the bottom grating  252 . This may make all steering angles clearly resolvable. 
     Each layer may effectively have different driving conditions selected by the electronic control structure that is associated with the layer. Later  200  includes an associated electronic control structure  201 . Layer  210  includes an associated electronic control structure  211 . The two control structures can be the same so long as they can provide separate driving voltages to the respective gratings. By driving the layers in this way, multiple steering angles may be achieved. The available number of steering angles is 2 N , where N is the number of stacked layers. In a dual layer system such as in FIG. 2, the output can be in one of four different directions  220 ,  222 ,  224 ,  226 , as shown. The direction of the outputs depends on the driving condition combinations. The first layer  220  deflects the light  230  into one of two different directions  232 ,  234 . The second layer  210  deflects each of these two beams in one of two different directions. Beam  232  can be deflected as either direction  220  or  224 . Seam  234  can deflect into either direction  222  or  226 . 
     Similarly, a four layer embodiment may provide 16 dynamically addressable angles. 
     In order to increase the number of layers beyond the four layers which have been described above, the performance of each individual layer may need to be further optimized. The optimization can be done by fine-tuning the PMMA blazed grating fabrication process. Also, improving the blaze profile and depth control can allow an increase the number of layers that may be deposited. 
     In another embodiment, anti reflection coatings may be deposited on each layer in order to reduce scattering inside the stacked layers. Another improvement may use a specific liquid crystal material that has improved index matching with the PMMA. 
     Another embodiment is shown in FIG.  3 . In this embodiment, a grating is formed electrically. The electrically formed grating is called a virtual grating. This may use a cascading approach to form an electrically generated blazed grating as described in Resler et al “High Efficiency Liquid Crystal Optical Phased array beam steering: Optics Letter, Vol 21, pp 689-691, and Wang et al, “Liquid Crystal on Silicon Beam deflector, SPIE, Vol 3633, PP 160-169. 
     FIG. 3 shows the operation. Two cover glass substrates  300 ,  310  are formed with patterned electrodes  302 ,  304  thereon. 
     A liquid crystal layer is used to build up a virtual blazed grating inside the liquid crystal medium. Appropriate voltages are assigned along the electrodes to form virtual blazed gratings in the liquid crystal. The assignment of appropriate voltages may generate a device which can be addressed to deflect beams into multiple angles. 
     It may be easiest to make this electrically generated blazed grating with a relatively fine scanning and hence a relatively small angle. Therefore, this system may operate best when used as a “fine” scanning component. In contrast, the liquid crystal/PMMA blazed grating may form a normally small period. This may be used as the coarse scanning component. 
     The embodiment shown in FIG. 4 combines a four stacked layer blazed grating/LC coarse scanning component, with the virtual grating layer fine scanning component described with reference to FIG.  3 . The composite structure includes 4 layers of PMMA blazed grating/LC materials  400 ,  402 ,  404 ,  406 , and a single layer  410  of the virtual electrically generated blazed grating. This may yield a steering device with a large number of addressable angles. 
     For example, The period of the electrically generated grating can be programmed to 80 microns, 160 microns, 320 microns, 640 microns and 1280 microns. This system forms addressable angles which is 16·2 5 =512. In the embodiment of FIG. 2, these many directions would require nine layers of liquid crystal/PMMA grating. The hybrid approach may reduce the number of layers, hence increasing the throughput and simplifying the final device. 
     Although only a few embodiments have been disclosed in detail above, other modifications are possible. For example, while the above has described using nematic liquid crystal, ferroelectric liquid crystals may be used in order to provide a faster switching speed. While the grating is described as being formed from PMMA, any material can be used to form the gratings, but preferably an etchable material. 
     All such modifications are intended to be encompassed within the following claims, in which: