Abstract:
The present invention is directed to a method for making infrared transmitting graded index optical elements by selecting at least two different infrared-transmitting materials, each with a different refractive index, having similar thermo-viscous behavior; assembling the infrared-transmitting materials into a stack comprising one or more layers of each infrared-transmitting material resulting in the stack having a graded index profile; and forming the stack into a desired shape. Also disclosed is the related optical element made by this method.

Description:
[0001]    PRIORITY CLAIM 
         [0002]    The present application is a nonprovisional application of U.S. Provisional Application No. 61/441,778 filed on Feb. 11, 2011 by Daniel J. Gibson et al., entitled “Infrared Transmitting Graded Index Optical Elements and Method for Making Same,” the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates generally to optical elements and more specifically to infrared transmitting graded index optical elements. 
         [0005]    2. Description of the Prior Art 
         [0006]    Graded index (GRIN) optics focus light through the use of an index profile in the lens instead of refracting the light of each surface as is the case in conventional lenses. For example, the flat ends of a GRIN lens enable more efficient coupling to fiber optics. GRIN lenses can reduce aberrations, and replace multiple lens elements in a lens system thereby permitting compact, lightweight lenses to replace bulky heavy lenses in existing systems. 
         [0007]    Currently, GRIN optics are made from monovalent ion-doped silicate glasses or polymers. In both cases, there is no light transmission in the infrared beyond a wavelength of about two microns, an important wavelength region for many applications, such as sensing. The method used to make the silicate GRIN optics relies on ionic diffusion from the outside surface to the inside to create the index profile. This is a slow process since the diffusion lengths are large. For example, it can take as long as 200 hours to ion exchange a 2 mm thick silicate glass to make a GRIN lens. Additionally, diffusion limits the complexity of the profiles that can be made using traditional ion exchange. The polymer systems suffer from a small index difference (˜0.1). This limits the usefulness of polymer GRIN lenses. 
         [0008]    A microlayering/molding process has been used to fabricate plano-convex GRIN lenses from alternating layers of two immiscible polymers in a structure that mimics the ocular lens of fish (Beadie et al., “Optical properties of a bio-inspired gradient refractive index polymer lens,”  Opt. Exp.,  16 (15), 11540-11547 (2008), the entire contents of which is incorporated herein by reference). These lenses are not suitable for IR light due to the high IR absorption of polymers. IR light is defined as a wavelength beyond about 2 μm. For example in the 2-5 μm region, the 8-12 μm region, and the 2-12 μm region. Further, those skilled in the art, will quickly see that the process is not applicable to IR materials since such materials (IR crystals and ceramics) do not exhibit the plasticity needed for microlayer extrusion. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The aforementioned problems are overcome in the present invention which provides a method for making infrared transmitting graded index optical elements by selecting at least two different infrared-transmitting materials, each with a different refractive index, having similar thermo-viscous behavior; assembling the infrared-transmitting materials into a stack comprising one or more layers of each infrared-transmitting material, resulting in the stack having a graded index profile; and forming the stack into a desired shape. Also disclosed is the related optical element made by this method. 
         [0010]    These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows the plano-convex IR GRIN lens of Example 1 comprising alternating layers of As 39%-S 61% (n=2.38) and As 40%-S 45%-Se 15% (n=2.46). Layer pair thicknesses are ˜50 μm, and the ratio of the thicknesses of the 2 glasses is graded throughout the lens. 
           [0012]      FIG. 2(   a ) shows the IR grin lens of Example 2 demonstrating that the proportion of high-index glass to low-index glass varies linearly along the height of the lens blank resulting in a graded index optic in the mid-IR (2-5 μm).  FIG. 2(   b ) shows the IR GRIN optic of Example 2 shaped in a spherical mold and finished to produce a plano-convex GRIN lens optic. 
           [0013]      FIG. 3  shows the process of making the GRIN optic of Example 4 from only a few sheets of glass with large thicknesses (up to 1 mm and thicker). 
           [0014]      FIG. 4  shows the process of extruding a stack of 3 different IR glass compositions into a rod having a radially-stepped refractive index profile and stretching the rod on a fiber draw tower to achieve a small diameter (200 μm) rod with a radially graded index. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention pertains to IR GRIN optical elements and a method for creating IR GRIN optical elements. The optical elements are comprised of multiple IR-transmitting glasses, crystals, and ceramics, each with a different refractive index. The IR-transmitting materials are placed in a specific arrangement to achieve the desired optical performance for the intended application, including beam focusing, beam diverging, optical fiber coupling, beam combining/splitting and waveguiding. The actual arrangement of the glass components in the optical element depends on the optical design and may be fabricated using the method described. 
         [0016]    The method includes the selection of appropriate IR glass compositions spanning a wide range of refractive indices (2.2 to 3.4), and having similar thermo-viscous behavior. For example, their glass transition temperatures should be within +/−20° C. of one another, and typically within +/−10° C., and more typically within +/−5° C. of each other. The number of compositions, as well as the refractive index difference between each composition, called the n-step, depends on the design of the optic. The set of glasses will span a wide range of refractive index, called the delta-n, as required by the design. The glasses can them be assembled into the desired shape using several approaches. 
         [0017]    The main approach is to assemble multiple layers (essentially thin sheets) of varying glass compositions and therefore varying refractive index. A large number of sheets, are fabricated in a stack. The stack, with an associated graded index profile, is then pressed inside a shaped mold, which, for example, may be shaped like a ball and socket, resulting in a thin plate with spherical curvature. The radius of the curvature is relatively large compared to the thickness of the stack. Different lens geometries can then be obtained by trimming, for example, the concave portion of the molded plate can be ground flat, resulting in a plano-convex lens with a graded index. Instead of molding, the plate can be shaped into a curved surface using hydrostatic pressure or blow molding. Alternatively, the stack could be extruded through an orifice resulting in a thin rod with a radially graded index. Sections of this rod can be sliced and polished to form graded index optical elements. 
         [0018]    The multi-layered glass stacks can be fabricated by pressing them into thin sheets under load (under vacuum or inert gas or reactive gas), drawing thin ribbons of glass from a melt crucible, extruding a stack of glass plates into a ribbon or extruding multiple glasses into a sheet/ribbon using a co-extrusion die. The multilayered glass stacks are comprised of individual glass sheets, each with a different composition and refractive index. These sheets can be fabricated by similar means, except care must be taken to control the composition/refractive index of the different sheets. This can be achieved by precise metering of the constituent elements during sheet fabrication, or by mixing two or more baseline glass compositions, in precisely controlled ratios, during the sheet fabrication process. 
         [0019]    While this disclosure describes GRIN optics and lenses operating in the infrared, the methods and products disclosed are not limited to only IR glasses and optics. The methods can also be used for other glasses and polymers to create optics that function at other wavelengths/frequencies and wavelength/frequency ranges including visible, near-IR, mid-IR, far-IR and THz wavelengths/frequencies. 
       EXAMPLE 1 
       [0020]    A plano-convex IR GRIN lens was comprised of alternating layers of As 39%-S 61% (n=2.38) and As 40%-S 45%-Se 15% (n=2.46). Layer pair thicknesses were ˜50 μm, and the ratio of the thicknesses of the 2 glasses was graded throughout the lens ( FIG. 1 ). 
       EXAMPLE 2 
       [0021]    An IR grin lens comprised of a mixture of As 40%-S 40%-Se 20% (n=2.53) and As 35.5%-S 64.5% (n=2.30.) The lens was fabricated from glass ribbons/plates made using extrusion. The ribbons were comprised of a mixture of high-index and low-index glass in varying proportion. The glasses were mixed during extrusion and the ratio of high-index glass to low index glass was precisely controlled. Ribbons at the top of the lens blank were comprised of 85% of the high-index glass and 15% of the low-index glass for a weighted average index of 2.496. Ribbons at the bottom of the lens blank were comprised of 15% of the high-index glass and 85% of the low-index glass for a weighted average index of 2.334. The proportion of high-index glass to low-index glass varied linearly along the height of the lens blank resulting in a graded index optic in the mid-IR (2-5 μm), as shown in  FIG. 2   a.    
       EXAMPLE 3 
       [0022]    The IR Grin optic of example 2 was shaped in a spherical mold and finished as shown in  FIG. 2   b  to produce a plano-convex GRIN lens optic. 
       EXAMPLE 4 
       [0023]    This example consisted of a GRIN optic made from only a few sheets of glass with large thicknesses (up to 1 mm and thicker) as shown in  FIG. 3 . The three different IR glass compositions were, As 39%-S 61% (n=2.386), As 38%-S 62% (n=2.380), and As 37%-S 63% (n=2.369), respectively. The part was made by fusing three polished glass sheets together above their glass transition temperatures. 
       EXAMPLE 5 
       [0024]    A stack of 3 different IR glass compositions, As 39%-S 61% (n=2.386), As 38%-S 62% (n=2.380), and As 37%-S 63% (n=2.369), was extruded into a rod having a radially-stepped refractive index profile. This rod was then stretched on a fiber draw tower to achieve a small diameter (200 μm) rod with a radially graded index ( FIG. 4 ). 
       EXAMPLE 6 
       [0025]    A stack of plates or ribbons, each having a different average refractive index, due to its composition, was extruded through a die resulting in a preform having a radially graded refractive index profile. This preform was then stretched on a fiber draw tower to achieve a small diameter (200 μm) rod or cane with a radially graded index. A portion can be cut from this cane and polished into a cylindrical IR GRIN lens, with a radial GRIN profile. 
         [0026]    The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” are not to be construed as limiting the element to the singular.