Patent Publication Number: US-2009231677-A1

Title: Variable Focal Point Optical Assembly Using Zone Plate and Electro-Optic Material

Description:
ORIGIN OF THE INVENTION  
     The invention was made in part by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION  
     1. Field of the Invention 
     This invention relates to variable focal point optical assemblies. More specifically, the invention is an optical assembly having a variable focal point where the assembly includes a zone plate and electro-optical material. 
     2. Description of the Related Art 
     As illustrated in a plan view in  FIG. 1 , a conventional optical zone plate  10  is a planar arrangement of concentric, spaced-apart opaque rings (e.g., four rings  13 ,  15 ,  17 ,  19 ) centered on an opaque central circular region  11 . The resulting ring-shaped regions (e.g., regions  12 ,  14 ,  16 ,  18 ) between the opaque rings are transparent to light. Light impinging on one face of zone plate  10  passes through the transparent ring-shaped regions to form Fresnel diffraction patterns. As is known in the art, the Fresnel diffraction patterns constructively interfere at fixed, spaced-apart locations along an axis extending perpendicularly from the central portion of zone plate  10 . Accordingly, zone plate  10  essentially provides a fixed-focus lens effect. To change the focus points, some type of positioning mechanism is required to either move the light source generating the light impinging on the zone plate, the zone plate, or both. 
     SUMMARY OF THE INVENTION  
     Accordingly, it is an object of the present invention to provide a variable-focal-point optical assembly using a zone plate. 
     Another object of the present invention is to provide a variable-focal-point optical assembly based on a zone plate that does not require the use of any moving parts or assemblies. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, an optical assembly includes a zone plate having concentric spaced-apart rings of opaque material. Between the opaque rings, light is transmitted through the zone plate. Electro-optic material is disposed on one side of the zone plate. Coupled to the electro-optic material is the means to control an index of refraction thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a plan view of a conventional optical zone plate; 
         FIG. 2  is a cross-sectional schematic view of a variable-focal-point optical assembly in accordance with an embodiment of the present invention 
         FIG. 3  is a cross-sectional view of a variable-focal-point optical assembly in accordance with another embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of a variable-focal-point optical assembly in accordance with yet another embodiment of the present invention; and 
         FIG. 5  is an isolated schematic view of a multi-layer electro-optic material arrangement in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Referring again to the drawings and more particularly to  FIG. 2 , an optical assembly in accordance with an embodiment of the present invention is shown and is referenced by numeral  20 . In general, optical assembly  20  is controllable/programmable in terms of the assembly&#39;s focal point when light passes therethrough. Thus, optical assembly  20  can be incorporated as a sub-assembly in a variety of optical systems such as a programmable micro-lens and a programmable micro-spectrometer. Accordingly, it is to be understood that the ultimate end use of optical assembly  20  is not a limitation thereof. 
     Optical assembly  20  includes a zone plate  30  that is defined by a planar arrangement of concentric and spaced-apart opaque rings as is known in the art. In the illustrated embodiment, zone plate  30  has a circular and opaque central region  31  and four concentric and spaced-apart rings  33 ,  35 ,  37  and  39  to thereby define four concentric and spaced-apart transparent ring regions  32 ,  34 ,  36 , and  38 . It is to be understood that this pattern (i.e., starting with opaque central region  31 ) could be reversed (i.e., starting with a transparent central region) without departing from the scope of the present invention. In addition, the number of opaque rings/transparent ring regions is not a limitation of the present invention. Still further, the size and spacing used for zone plate  30  can be tailored for a particular application without departing from the scope of the present invention. 
     Disposed adjacent to one face of zone plate  30  is an optically transparent electrode  40  (e.g., made from a material such as indium tin oxide or zinc oxide). Adjacent and coupled to electrode  40  is a planar face of one or more layers/types of an electro-optic material  42 . Electro-optic material  42  is any of a variety of materials that can experience a change in refractive index or absorption coefficient by one or multiples in the presence of an applied electric field, current, or magnetic field. Choices for electro-optic material  42  include, but are not limited to, non-linear optical crystals, ferroelectric materials, piezoelectric materials, electro-active polymers, and liquid crystals. 
     Adjacent and coupled to the opposing planar face of electro-optic material  42  is another optically transparent electrode  44 . The combination of electrode  40 , material  42  and electrode  44  span an area that, at a minimum, covers the area defined by transparent ring regions  32 ,  34 ,  36 , and  38 . In the illustrated embodiment, the combination of electrode  40 , material  42  and electrode  44  span the entire area defined by zone plate  30 . Although not shown, the above-described assembly can be fabricated on a rigid and transparent substrate such as glass, quartz or sapphire. 
     A controllable voltage source  50  is electrically coupled to electrodes  40  and  44 . When a voltage is applied to electrodes  40  an  44 , an electric field is developed across electro-optic material  42 . The electric field causes a circular gradient refractive index zone to be formed inside electro-optic material  42 . This gradient refractive index changes the phase of photons of light  100  passing therethrough thereby changing the focal point  102  of light  100  passing through optical assembly  20 . The focal distance F is controlled by the electric field in electro-optic material  42 . 
     By itself, a zone plate is known to have a strong dispersion relation with the wavelength of light passing therethrough. Therefore, conventional zone plates can only serve as a focusing element with a specific monochromatic wavelength. However, in the optical assembly of the present invention, the circular gradient refractive index caused by the applied electric field also changes the properties (i.e., focal point at a specific wavelength, distribution of intensity of passing photons, and phase of passing photons) of the zone plate such that the zone plate can be optimized for a new wavelength. Thus, the present invention can also be used to control the designated wavelength of operation by controlling the electric field in the electro-optic material. 
     The present invention is not limited to the optical assembly construction illustrated in  FIG. 2 . For example, another embodiment of the present invention is illustrated in  FIG. 3  where optical assembly  60  includes a zone plate  70  whose opaque central region  71  and concentric, spaced-apart opaque rings  73 ,  75 ,  77 , and  79  are made from an electrically conductive material (e.g., a metal). Electro-optic material  42  is adjacent and coupled to one face of zone plate  70  while transparent electrode  44  is adjacent and coupled to the opposing face of electro-optic material  42  as in the previous embodiment. In this embodiment, controllable voltage source  50  is electrically coupled to each ring of zone plate  70  and transparent electrode  44 . The applied voltages can be the same or different to suit a particular application. 
     Still another embodiment of the present invention is illustrated in  FIG. 4  where optical assembly  80  includes a zone plate  70  whose opaque central region  71  and concentric, spaced-apart opaque rings  73 ,  75 ,  77 , and  79  are made from an electrically conductive material (e.g., a metal). Electro-optic material  42  is adjacent and coupled to one face of zone plate  70 . In this embodiment, controllable voltage source  50  is electrically coupled only to each ring of zone plate  70 . That is, the rings of zone plate  70  define the electrodes for optical assembly  80 . The applied voltages between rings will be different to generate an electric field in electro-optic material  42 . 
     As mentioned briefly above, electro-optic material  42  can be realized by a single homogeneous layer of electro-optic material or multiple distinct layers of electro-optic material with each of such layers having unique electro-optic properties. For example, as shown in  FIG. 5 , electro-optic material  42  could comprise distinct layers  42 A- 42 D of different electro-optic materials. Further, as illustrated, the thickness of the various layers can be different to satisfy particular application requirements. Still further, the same or unique voltages could be applied to the various electro-optic material layers in order to fine tune the wavelength of operation and/or focal point positioning. Accordingly, it is to be understood that the particular choice of electro-optic material(s) and arrangement thereof are not limitations of the present invention. 
     The advantages of the present invention are numerous. The optical assembly can be adjusted in terms of focal point location without the use of any moving parts. The assembly can be used in a neutral state (i.e., no voltage applied) to provide a fixed focal point for a specific wavelength of operation. However, the assembly can also be used in an active state (i.e., voltage applied) to adjust the wavelength of operation and/or the location of the assembly&#39;s focal point. The optical assembly can function as a programmable micro-lens whose focal distance changes with the applied voltage. The optical assembly can also form part of a micro-spectrometer that transmits a specific wavelength of light to a photo-detector where the specific wavelength is controlled by the applied voltage. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, the controllable voltage source could be replaced with a controllable current or magnetic field source without departing from the scope of the present invention It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.