The present subject matter relates generally to optical elements, especially optical elements which diffract optical radiation.
Conventional optical detection apparatuses may include diffractive elements to focus optical beams. These diffractive elements are most efficient at a particular wavelength, and performance degrades the further the wavelength gets from the peak wavelength. Thus, these diffractive elements are most useful for a narrow band of wavelengths around the peak wavelength. This may be acceptable for optical detection apparatuses designed to operate in a narrow band of frequencies, but is not acceptable for a broadband detector.
The present invention broadly comprises an optical element and a method for making an optical element.
In one embodiment, the optical element includes a first lens and a second lens. The first lens has a first refractive index and a first power. The first lens also has a first surface and a second surface. The first surface of said first lens is a continuous surface for optical radiation to enter. The second surface of said first lens has multiple facets connected by passive facets providing physical offsets. The second lens has a second refractive index different from the first refractive index and a second power. The second lens has a first surface and a second surface. The first surface of the second lens is in contact with the second surface of the first lens. Both surfaces of the second lens have multiple facets connected by passive facets providing physical offsets. The optical element has a peak diffraction efficiency at a first wavelength and at a second wavelength different from the first wavelength.
A full and enabling disclosure of the present subject matter, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:
Reference is presently made in detail to exemplary embodiments of the present subject matter, one or more examples of which are illustrated in or represented by the drawings. Each example is provided by way of explanation of the present subject matter, not limitation of the present subject matter. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present subject matter without departing from the scope or spirit of the present subject matter. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter covers such modifications and variations as being within the scope of the disclosure and equivalents thereof.
First lens 20 has a first surface 22 through which optical radiation enters. In the cross-sectional view of an exemplary optical element 10 shown in
Optical element 10 is optimized to have a peak diffraction efficiency at a first wavelength and at a second wavelength different than the first wavelength.
As shown in
In other embodiments, the passive facets 27, 37, 39 may not be perfectly vertical, but defined by a slope angle. The passive facets may also not be perfectly flat. These modifications are within the scope of the invention as claimed.
In one embodiment, the optical element 10 is parameterized by the intended optical phase offset across the optical element. One example of a phase offset is parabolic, and may be defined by the following equation
ϕ(r)=Ar2
Phase is cyclical with period 2π, and starting from r=0, the phase is an integer multiple of a at cut locations ri=√{square root over (2πi/|A|)}, where i is an integer. The discontinuities are when the phase cycles past an integer multiple of 2π.
In one embodiment, each facet 26, 36, 38 is a parabola. In such a case, the surfaces 24, 32, 34 may be defined by the following surface sag equations, ignoring constant offsets and the passive facets:
Surfaces 24/32: z1(r)=D1r2
Surface 34: z2(r)=D2r2.
These equations may be determined by choosing two design wavelengths λ1 and λ1 and design diffractive mode m, then solving the following (relating values shown in
(n1 cos θn1−n2 cos θn2)tan α1+(n2 cos θn2−n3 cos θn3)tan α2=mλ1/Λ
(n1 cos θn1−n2 cos θn2)tan α1+(n2 cos θn2−n3 cos θn3)tan α2=mλ2/Λ
by realizing the local pitch Λ must be the same at each wavelength simultaneously to find the ratio of tan α1 and tan α2, the derivatives of the surface sag equations. Then use the following equations
(n1 cos θn1−n2 cos θn2)d1+(n2 cos θn2−n3 cos θn3)d2=mλ1
(n1 cos θn1−n2 cos θn2)d1+(n2 cos θn2−n3 cos θn3)d2=mλ2
to solve for heights of vertical discontinuities d1 and d2 simultaneously. The passive facet locations are where the surface height difference equal d1 and d2 respectively. Not all design wavelength choices may lead to solutions with all or any material choices.
In one embodiment, the heights of the vertical discontinuities are constant across the lens. Equating the passive facet locations using the surface sag equations to the phase cut locations determines the surface sag coefficients D1 and D2.
In one embodiment, the heights of the vertical discontinuities vary across the lens. The surface sag equations must be adjusted or expanded to fit to the endpoint locations of each facet.
In one embodiment, the phase offset is defined by a generalized polynomial. The surface sag equations will be at the same polynomial order or higher.
In one embodiment, first lens 20 and second lens 30 form a monolithic double diffractive kinoform doublet having a focal length between 0 and infinity and corresponding optical power.
In one embodiment, first lens 20 and second lens 30 form a monolithic double diffractive kinoform doublet having a focal length between minus infinity and 0 and corresponding optical power.
In one embodiment, the first lens is made from ZnSe. The second lens may be made with ZnS. However, other materials may also be used and these alternative materials are within the scope of the invention as disclosed.
In one embodiment, the manufacturing process may be as follows:
1. Use diamond-turning to cut second surface 24 from a substrate of first optical quality material having an index of refraction of n1;
2. Add second material having an index of refraction of n2 onto second surface 24 to fill in the cuts, plus an additional “thick” layer, all of optical quality, by a deposition or growth process; and
3. Cut the second surface 34 into the second layer of material having an index of refraction n2.
The present written description uses examples to disclose the present subject matter, including the best mode, and also to enable any person skilled in the art to practice the present subject matter, including making and using any devices or systems and performing any incorporated and/or associated methods. While the present subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.
This application claims priority to U.S. Provisional Patent Application No. 62/768,242, filed on Nov. 16, 2018, the disclosure of which is incorporated herein by reference in its entirety.
The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America.
Number | Name | Date | Kind |
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4895790 | Swanson et al. | Jan 1990 | A |
5260828 | Londono | Nov 1993 | A |
6157488 | Ishii | Dec 2000 | A |
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Y. G. Soskind, “Field Guide to Diffractive Optics,” SPIE Press (2011), p. 96. |
T. Nakai, H. Ogawa, “Research on multi-layer diffractive optical elements and their application to camera lenses,” Optical Society of America/DOMO 2002. |
Number | Date | Country | |
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20200158929 A1 | May 2020 | US |
Number | Date | Country | |
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62768242 | Nov 2018 | US |