Optical: systems and elements – Diffraction – From zone plate
Reexamination Certificate
1999-08-27
2002-10-08
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
From zone plate
C359S569000, C359S570000, C359S900000
Reexamination Certificate
active
06462874
ABSTRACT:
I. FIELD OF THE INVENTION
This invention relates to optical systems employing stepped diffractive surfaces (SDSs).
More specifically, in accordance with certain of its aspects, the invention relates to the color correction of optical systems using SDSs, and, in particular, to the use of SDSs to correct axial and/or lateral chromatic aberration.
In accordance with other aspects, the invention relates to correction of monochromatic aberrations of optical systems, as well as balancing the monochromatic aberrations of non-SDS elements of an optical system against the monochromatic aberrations of SDS elements.
In accordance with additional aspects, the invention relates to correction of both chromatic and monochromatic aberrations of optical systems employing SDSs.
In accordance with still further aspects, the invention relates to:
(1) SDSs in which the surface's step heights are determined based on the curvature of the propagating wavefront;
(2) SDSs in which the surface step widths are determined based on the shape of the base curve and the local step height;
(3) SDSs in which the surface's step widths and step heights are determined using the grating equation (see, for example, equation (10.3) below);
(4) SDSs in which the wavefront incident on the SDS is non-planar (e.g., converging or diverging) or is planar but oriented at an angle to the optical system's optical axis;
(5) methods for tracing rays through SDSs using the grating equation (see, for example, equation (10.3) below);
(6) methods for designing optical systems which employ SDSs;
(7) optimization of the diffraction efficiency of an SDS; and/or
(8) the use of SDSs having a radially variable step height within a diffraction order.
These latter aspects of the invention are applicable to color corrected and non-color corrected (e.g., monochromatic) optical systems.
II. BACKGROUND OF THE INVENTION
A. Diffractive Optical Elements (DOEs)
The demand for a higher level of correction of aberrations in optical systems, preferably at a lower cost, has always exist. The modern optical designer possesses numerous design tools and techniques to correct aberrations, including aspherical surfaces, a wide variety of optical materials, diffractive optical elements, etc.
Diffractive optical elements (DOEs) have proven themselves as effective tools in the correction of aberrations of optical systems, including both monochromatic and chromatic aberrations. The advantages of a DOE come at the price of contrast reduction due to a portion of the light going into spurious diffraction orders. The efficiency of a DOE is used as the measure of the amount of light that leaks into spurious orders and causes the reduction in contrast. There are several factors that reduce the efficiency of a DOE, including manufacturing imperfections and the fundamental nature of a DOE. Improving the fabrication process can reduce the factor due to imperfections. The fundamental nature of a DOE causes reduction in efficiency due to the finite spectral band of light used in the system, as well as through “light shadowing” in the DOE.
One of the widely used diffractive surface configurations for a DOE is a kinoform. Kinoform DOEs have optical power and affect (change) the direction of propagation of light for the entire wavelength band and range of incidence angles. This optical power needs to be accounted for during the design stage of the optical system and affects the paraxial properties of the entire system, as well as the aberration contribution of the refractive part of the system in the case of hybrid diffractive-refractive systems. The diffraction order of a kinoform DOE in most cases is equal to unity, but can also be equal to a larger integer. The kinoform phase profile is blazed at an angle that varies as a function of aperture, i.e., as a function of radial distance from the system's optical axis. See G. J. Swanson, Binary Optical Technology: Theoretical Limits on the Diffraction Efficiency of Multilevel Diffractive Optical Elements, MIT Lincoln Laboratory Tech. Rep. 914, 1991; D. W. Sweeney and G. E. Sommargren, Harmonic diffractive lenses, Appl. Opt., V. 34 (14), pp. 2469-2475, 1995; D. Faklis and M. Morris, Spectral properties of multiorder diffractive lenses, Appl. Opt., V. 34 (14), pp. 2462-2468, 1995; and D. Faklis and M. Morris, Polychromatic diffractive lens, U.S. Pat. No. 5,589,982, 1996.
Except for a very limited wavelength band and range of incidence angles, the diffraction efficiency (DE) of even a perfectly manufactured (theoretical) kinoform is less than unity. As a result, a certain amount of light is redistributed into diffraction orders differing from the working order, thus producing a halo in the image and a reduction in image contrast. See G. J. Swanson, 1989, supra; C. Londono and P. Clark, Modeling diffraction efficiency effects when designing hybrid diffractive lens systems, Appl. Opt., V. 31 (13), pp. 2248-2252, 1992; and G. J. Swanson, 1991, supra.
The use of DOEs, specifically, kinoform DOEs, to correct chromatic aberrations of optical systems has been discussed in detail in several articles. See T. Stone and N. George, Hybrid diffractive-refractive lenses and achromats, Appl. Opt., V. 27(14), pp. 2960-2971, 1988; M. M. Meyers and J. R. Bietry, Hybrid refractive/diffractive achromatic camera lens and camera using such, U.S. Pat., No. 5,581,405, 1996; and G. J. Swanson, Binary optics technology: the theory and design of multi-level diffractive optical elements, MIT Lincoln Laboratory Tech. Rep. 854, 1989. An ideal DOE would effectively correct the chromatism of an optical system without a significant decrease in DE.
One of the fundamental limitations on the DE of a kinoform is the “light shadowing” phenomenon referred to above which causes a DOE to have a duty cycle. See G. J. Swanson, 1991, supra. For a kinoform lens, the duty cycle grows with an increase in the lens' optical power and usually increases with an increase in radial coordinate. It follows that the DE of a kinoform decreases with an increase in optical power and/or clear aperture size.
Another issue with kinoform DOEs is fabrication. See H. Welch, Fabrication Issues for DOE Design, CODE V News Supplement, Summer 1996. Because the blaze angle of the kinoform varies as a function of radial coordinate, control of this fundamental feature of a kinoform is very difficult. In many cases, the blaze profiles of kinoform DOEs are approximated with binary step profiles. See G. J. Swanson, 1989, supra; and Gary J. Swanson and Wilfrid B. Weldkamp, Diffractive optical elements for use in infrared systems, Opt. Eng., V. 28 (6), pp. 605-608, 1989. When the continuous blazed profile is approximated by 16 steps, the diffraction efficiency at the primary wavelength is reduced by about 1% from its theoretical maximum value of 100%. See Swanson and Weldkamp, supra.
The fabrication of a binary kinoform DOE typically involves lithographic projection of masks onto the surface of the DOE or a DOE master. See Swanson and Weldkamp, supra; and Gary J. Swanson and Wilfrid B. Weldkamp, Binary lenses for use at 10.6 micrometers, Opt. Eng., V. 24 (5), pp. 791-795, 1985. Such projection techniques have limitations in terms of resolution and alignment tolerances, which lead to a minimum feature size which can be fabricated without substantial loss in accuracy.
If, for example, the minimum feature size for given equipment is 2 micrometers, then for a zone width of 16 micrometers, only 8 steps (features) can be fabricated. As discussed above, 16 steps lead to a reduction in diffraction efficiency at the primary wavelength of 1%. If only 8 steps can be used, the reduction in diffraction efficiency increases to 5%. This reduction in DE means that more light is diffracted into orders other than the working order, leading to a greater contrast reduction in the image plane.
Because every step boundary implies some imperfections (e.g., due to mask alignment and/or the etching process), the binary approximation, with its increased number of boundaries, leads to increased scatter and
Jr. John Juba
Klee Maurice M.
KSM Associates, Inc.
Spyrou Cassandra
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