Layered infrared transmitting optical elements and method for making same

Description (excerpt)

PRIORITY CLAIM The present application is a divisional application of U.S. application Ser. No. 14/210,828, filed on Mar. 14, 2014 by Dan J. Gibson et al., entitled “LAYERED INFRARED TRANSMITTING OPTICAL ELEMENTS AND METHOD FOR MAKING SAME,” which claimed the benefit of U.S. Provisional Application No. 61/787,365, filed on Mar. 15, 2013 by Dan J. Gibson et al., entitled “Layered Infrared Transmitting Optical Elements and Method for Making Same,” the entire contents of both are incorporated herein by reference. BACKGROUND OF THE INVENTION Field of the Invention This disclosure pertains to the field of infrared optics and, more specifically, to infrared lens elements and multi-element infrared imaging lens systems. Description of the Prior Art It is common to refer to an optical glass as having a refractive index at a certain wavelength and to describe the shape of the dispersion function using the Abbe number, V (or v)=(n d −1)/(n F −n C ), and various partial dispersion values, P x,y =(n x −n y )/(n F −n C ), as dictated by the precision of the optical design. Since infrared transmitting glasses often have poor transmission for visible wavelengths, a ‘modified’ Abbe number is used where the visible wavelengths, λ F , λ d , and λ C , are replaced with more suitable infrared wavelengths. Two common examples are the mid-wave infrared (MWIR), where the wavelengths 3, 4 and 5 μm are used and the long-wave infrared (LWIR) where the wavelengths 8, 10 and 12 μm are used to define the MWIR dispersion, V (3−5) (or V MWIR )=(n 4 −1)/(n 3 −n 5 ) and LWIR dispersion, V (8−12) or (V LWIR )=(n 10 −1)/(n 8 −n 12 ) respectively. While these dispersion parameters describe the wavelength dependent refractive index of IR-transmitting materials sufficiently to aid the selection of materials for a lens design, they lack the precision required for modern high performance optical design software. As a result, the refractive index is also represented in either tabular form (a list of indices at specific wavelengths) or more precisely by Sellmeier coefficients that permit interpolation and extrapolation of refractive index values. Refractive optical imaging systems typically utilize multiple refractive optical elements to manipulate light and create an image. Commonly, these individual optical elements are comprised of different optical materials with different optical properties, including refractive indices, dispersions, or thermo-optic coefficients, in such combinations that attempt to reduce or eliminate problems associated with using a single material, including for example chromatic dispersion and thermal drift. For various reasons, including reducing system size, weight and complexity or improving performance and reliability, optical designers can use specialized optical elements, for example achromatic doublets or gradient index (GRIN) optics. Achromatic doublets and triplets are comprised of separate optical elements of dissimilar materials, with different optical properties that have been bonded to each other using transparent adhesives. This is a common practice for visible imaging systems, but the lack of suitable IR-transparent optical cements limits application of this technology to IR optical elements. GRIN optics are single optical elements wherein the optical properties vary in a controlled way within the bulk of the optical element. GRIN optics are also limited to primarily visible wavelengths as the methods used in their fabrication are not well-suited to IR transparent materials. The majority of GRIN optics are fabricated using an ion-exchange process wherein the optical element is submerged in a hot salt bath for an extended time such that the ions in the element diffuse through the element into the bath and ions from the bath diffuse into the element, imparting a compositional concentration gradient and thereby a gradient in the optical properties of the optical element. This process is typically not possible with IR transparent materials, especially those used beyond 2 μm. Furthermore, the thermodynamics of diffusion limit the size of optical elements fabricated via the method under reasonable times to about 1 inch in diameter. Layered optical elements have been fabricated with gradient index using polymer sheets (U.S. Pat. No. 7,002,754 to Baer et al. (2006)) which are bonded to one another using pressure (Beadie et al., “Optical properties of a bio-inspired gradient refractive index polymer lens,” Optics Express, 16, 15, 11540-47 (2008)) and later molded and/or machined. Hagerty et al. describe a method of stacking