Issue #2/2020
G. I. Greysukh, V. A. Danilov, E. G. Ezhov, A. I. Antonov, B. A. Usievich
DIFFRACTION ELEMENTS IN OPTICAL SYSTEMS OF MIDDLE AND DOUBLE IR RANGE
DIFFRACTION ELEMENTS IN OPTICAL SYSTEMS OF MIDDLE AND DOUBLE IR RANGE
DOI: 10.22184/1993-7296.FRos.2020.14.2.160.169
It is shown that placing a diffraction microstructure on the flat surface of one of the refractive lenses of a high-aperture triplet can simultaneously satisfy the correction conditions for both chromatic and monochromatic aberrations and obtain lenses designed for the middle and double infrared ranges having sufficiently high optical characteristics.
It is shown that placing a diffraction microstructure on the flat surface of one of the refractive lenses of a high-aperture triplet can simultaneously satisfy the correction conditions for both chromatic and monochromatic aberrations and obtain lenses designed for the middle and double infrared ranges having sufficiently high optical characteristics.
Теги: chromatic and monochromatic aberrations diffraction microstructure lens middle and double ir ranges дифракционная микроструктура объектив средний и двойной ик- диапазоны хроматические и монохроматические аберрации
DIFFRACTION ELEMENTS IN OPTICAL SYSTEMS OF MIDDLE AND DOUBLE IR RANGE
G. I. Greysukh1, V. A. Danilov2, E. G. Ezhov1, A. I. Antonov1, B. A. Usievich3
Penza State University of Architecture and Construction, www.pguas.ru, Penza, Russia
Scientific and Technological Center for Unique Instrumentation of the Russian Academy of Sciences, www.ntcup.ru, Moscow, Russia
A. M. Prokhorov Institute of General Physics of the Russian Academy of Sciences, www.gpi.ru, Moscow, Russia
It is shown that placing a diffraction microstructure on the flat surface of one of the refractive lenses of a high-aperture triplet can simultaneously satisfy the correction conditions for both chromatic and monochromatic aberrations and obtain lenses designed for the middle and double infrared ranges having sufficiently high optical characteristics.
Key words: middle and double IR ranges, lens, diffraction microstructure, chromatic and monochromatic aberrations
Received: 26.12.2019
Accepted: 21.01.2020
INTRODUCTION
Currently, one of the areas of optical instrumentation, the development of which is most in demand, is the direction associated with the development and creation of monofocal optics and optics with variable focal length, designed to work in extended spectral ranges. In the visible and near infrared (IR) ranges, this is primarily the mass optics of mobile phones, smartphones and tablets, as well as security systems and technical vision (in particular, unmanned land, underwater and air vehicles). In the dual IR range, including medium and far infrared radiation (3.7–11 microns), this is the optics of thermal imagers and night vision devices for various purposes. High-quality optics of all the above ranges is also necessary for medical instrumentation. At the same time, it is obvious that in order to meet the increasing requirements for the overall dimensions and technical characteristics of optical systems, it is necessary to expand the elemental base and the range of optical materials.
One of the possible ways to expand the elemental base involves the use of diffractive optical elements (DOE). Moreover, the unique aberration properties of DOEs give the greatest effect in the infrared range. Indeed, the introduction of such an element into the refractive lens of a thermal imager allows us to simplify its optical design and achieve the required optical characteristics (see, for example, [1, 2]). So, in particular, in the mid-IR range (3 ≤ λ ≤ 5 μm), the placement of the diffraction microstructure on the flat surface of one of the refractive lenses of a high-aperture triplet allows simultaneous fulfillment of the correction conditions for both chromatic and monochromatic aberrations. Equally important is the ability to refuse to use germanium in a triplet, which has a high and non-linear temperature coefficient of refractive index. When germanium is replaced by chalcogenide glass, the diffraction microstructure makes it possible to maintain apochromatization and a low level of spherochromatism, and a slight thermal change in the refractive index of chalcogenide glass avoids thermal defocusing. Moreover, the use of DOEs, as will be shown in this article, is very effective in dual IR range lenses, both monofocal and mechanically variable focal length (zoom lenses).
1.
DESIGN PARAMETERS AND OPTICAL CHARACTERISTICS OF THE MIDDLE IR RANGE REFRACTIVE-DIFFRACTIVE LENS
We will demonstrate the above-described capabilities using an IR lens as an example, two refractive lenses of which are made of AMTIR3 glass (refractive index at a wavelength of λ = 4 μm nAMTIR = 2.621003, and a dispersion coefficient of λmin = 3 μm and λmax = 5 μm, νAMTIR = 173.38), while the third lens, bearing the diffraction microstructure, is made of zinc sulfide (nZnS = 2.250382; νZnS = 112.20). The refractive indices and dispersion coefficients given here and below were calculated using the dispersion formulas of the INFRARED catalog of the ZEMAX optical design program [3] and work [4].
The optical scheme of the lens is shown in Fig. 1. Its focal length is fʹ = 40 mm, the aperture value is K = 0.84, and the angular field in the space of objects is 2ω = 24°. The distance from the vertex tangent plane of the front lens to the image plane is L = 73,7 mm.
Tables 1 and 2 show the design parameters of the lens obtained after preliminary dimensional and aberration calculations by optimization using the ZEMAX optical design program [3]. Moreover, each of the non-planar surfaces of the refractive lenses of the lens was a so-called even aspherical surface, described in ZEMAX by the equation:
,
where z(ρ) is the coordinate of a surface point spaced apart from the optical axis by a distance ρ in the coordinate system whose XOY plane touches the top of this surface; c is the curvature of the surface at its apex; κ is a conical constant; αp – surface asphericity coefficients.
As for the diffraction microstructure placed on the flat frontal surface of the second refractive lens, it was modeled in the framework of ZEMAX by a Binary2 type surface with a phase delay of the form
,
where m is the number of the working diffraction order, Aj are constant coefficients.
This IR lens with a relative aperture of 1:0.84 provides a resolution of 50 mm–1 with a contrast of at least 0.5 and 35 mm–1 with a contrast of at least 0.65 within a field angle of 2ω ≤ 24°. The residual position chromatism in the range from λmin = 3 μm to λmax = 5 μm does not exceed 3.4 μm with a tolerance limited by diffraction equal to 11.3 μm. The chromaticity of the increase is 4.1 μm, which is slightly smaller than the radius of the main maximum of the Airy picture. Lens distortion is less than 0.25%.
2.
DESIGN PARAMETERS AND OPTICAL CHARACTERISTICS OF THE DOUBLE IR RANGE REFRACTIVE-DIFFRACTION LENS
Aberration calculation and subsequent optimization using ZEMAX optical design programs have shown that the refractive-diffraction triplet, after a corresponding modification of the optical scheme, is able to successfully operate in the double IR range [5]. The modification of the optical scheme included, in particular, the replacement of the used optical materials with GASIR1 (refractive index at a wavelength of λ = 7.35 μm nGASIR1 = 2.501489, and the dispersion coefficient at λmin = 3.7 μm and λmax = 11 μm νGASIR1 = 74.84) and ZnS_broad (nZnS_broad = 2.228805; νZnS_broad = 18.03).
For the obtained lens (see Fig. 2, Tables 3 and 4), the apochromatic correction of longitudinal chromatism with a tertiary spectrum not exceeding the tolerance limited by diffraction and correction of chromaticity increase to a value on the order of the radius of the main maximum of the Airy picture are preserved in the double IR range.
However, chromatic aberrations of higher orders with an expansion of the spectral range increased significantly, which led to a decrease in aperture (K = 1.1) and resolution. Within the same angular field for which the above-presented mid-IR lens (2ω ≤ 24°) is designed, this dual-IR lens provides a resolution of 35 mm–1 with a contrast of at least 0.3 and 25 mm–1 with contrast not lower than 0.5. The focal length of the lens is still fʹ = 40 mm, and the distance from the vertex tangent plane of the front lens to the image plane is L = 69.03 mm. Distortion does not exceed 0.3%.
3.
DIFFRACTION EFFICIENCY OF IR KINOFORM AND WAYS TO INCREASE IT
The diffraction microstructure modeled in Zemax by an infinitely thin banner that performs a given phase delay is actually performed in the form of a kinoform, i. e. sawtooth relief on the surface of the optical material. Today, a well-established technology of diamond turning allows the industrial production of hybrid elements of the IR range with a kinoform microstructure made on a flat, spherical or even aspherical surface of a refractive lens [6].
The diffraction efficiency (DE) of a kinoform microstructure can approach unity, but only at a single wavelength and at a single angle of incidence of radiation, and only a strict theory of diffraction based on the solution of the Maxwell equation system gives a reliable estimate of the decrease in DE depending on the wavelength and angle of incidence. The DE of kinoform microstructures of both lenses described in this work was calculated by rigorous coupled-wave analysis (RCWA) [7]. In this case, the calculation of the microstructures of the mid-IR lens was performed using the RCWA-PSUACE computer program developed by the authors, and the microstructures of the dual-IR lens using the computer program presented in [8].
The kinoform microstructure of the middle IR range lens consists of 33 ring zones. In this case, the width of the narrowest zone (the minimum period of the kinoform) P > 400 μm, and the maximum angle of incidence of radiation on the microstructure from the air θ ≤ 13.5°. The calculation showed that the DE of this kinoform at the optimal relief depth (h = 3.2 μm), due to the significant ratio of the spatial period of the microstructure to the relief depth (P / h > 125), turned out to be close to the limit predicted by the scalar diffraction theory, i. e. not lower than 0.8 in the entire working spectral range. At the same time, twenty percent of the energy incident on kinoforms attributable to secondary diffraction orders will inevitably lead to a decrease in contrast in the image formed by the lens and, in some cases, to an unacceptable decrease in the signal-to-noise ratio.
It is possible to raise the DE in the entire spectral range and for the entire range of angles of incidence of radiation to the level of 0.85 and even to 0.95 by switching to two- or three-layer kinoform microstructures. So, in particular, a two-layer single-relief microstructure (see Fig. 3) composed of Al2O3 and ZnS (n1 = 1.660731, n2 = 2.250382) with an optimal relief depth (h = 6.72 μm), angles of incidence of radiation –15 ≤ θ ≤ 15° and in the spectral range of 3 ≤ λ ≤ 5 μm provides DE ≤ 0.87 at P / h ≤ 5 and DE ≥ 0.90 at λ / h ≥ 30.
Here, we immediately note that modern technologies make it possible, using the flat surface of a refractive lens as a substrate, to apply a layer of another IR transparent material with a thickness sufficient to make a sawtooth relief in it, and in turn to cover it with another layer, which will complete the formation of a two-layer single-relief sawtooth microstructure. The analysis showed that Al2O3 and AgCl (n2 = 1.99996) can serve as the best pair of materials for such a microstructure in the spectral range of 3 ≤ λ ≤ 5 μm. With an optimal relief depth (h = 12.22 microns) and radiation angles of incidence of –15 ≤ θ ≤ 15°, this microstructure provides DE ≤ 0.9 at P / h ≤ 5 and DE ≥ 0.92 at P / h ≥ 30.
The three-layer microstructure (see Fig. 4) at the optimal depths of two reliefs (h1 = 18.76 μm, h2 = 10.96 μm), composed of Al2O3, MgF2 and ZnS (n1 = 1,660731, n2 = 1,348829, n3 = 2.250382) in the same angular and spectral ranges provides DE ≤ 0.92 for P / (h1 + h2) ≤ 5 and DE ≥ 0.95 for P / (h1 + h2) ≥ 30.
Kinoform microstructure of the double IR lens presented above consists of only 5 annular zones and is performed on a flat surface of the optical material GASIR1 (refractive index at a wavelength of λ = 7.35 μm nGASIR1 = 2.501489, and the dispersion coefficient at λmin = 3,7 μm and λmax = 11 μm νGASIR1 = 74.84). The width of the narrowest zone, i. e. the minimum kinoform period is P > 2.8 mm, and the maximum angle of incidence of radiation on the microstructure from the air is θ ≤ 12°.
The calculation showed that at the optimal relief depth h = 3.7 μm, the DE at the edges of the working spectral range (3.7 ≤ λ ≤ 11 μm) will drop to 0.4 even with very large ratios of the spatial period of the microstructure to the depth of the relief.
In the case of a two-layer single-relief microstructure composed of GASIR1 and ZnS_broad, ДЭ ≤ 0.52. And even if two layers of CdTe and ZnS_broad materials separated by one sawtooth relief are applied to the flat surface of the first refractive lens, it will not be possible to raise the DE above 0.7.
The situation becomes fundamentally different when composing a microstructure from the same materials GASIR1 and ZnS_broad, but with two reliefs of different depths (h1 = 132 μm, h2 = 155.8 μm). Despite such a significant total depth of the relief and due to the large ratio of the spatial period of the microstructure to the total depth (P / h ≈ 10) DE ≥ 0.9.
CONCLUSION
The research results presented in this paper clearly demonstrate the effectiveness of using diffraction elements in the lenses of the middle and double IR ranges. It is shown that the placement of a diffractive microstructure on the flat surface of one of the refractive lenses of a high aperture triplet can simultaneously satisfy the correction conditions of both chromatic and monochromatic aberrations and achieve sufficiently high optical characteristics. At the same time, a significant width of the working spectral range forces us to switch to two-layer single- or two-relief microstructures to ensure an acceptable DE sawtooth relief microstructure. At the same time, the depths of reliefs increase many times, which significantly limits the permissible minimum spatial period of the microstructure and the angles of incidence of radiation on it. This in turn imposes certain restrictions on the placement of the microstructure within the optical circuit of the lens [9]. However, as shown in this work, it is possible to compose the optical scheme of the lens, which provides the proper correction of aberrations at an acceptable minimum spatial period of the diffraction microstructure and angles of incidence of radiation on it, even in the simplest case of a triplet.
CONTRIBUTION OF TEAM MEMBERS
TO THE WORK
All authors declare an equal contribution of each to the preparation, discussion and writing of the article.
The study is carried out with the support RSF project № 20-19-00081.
ABOUT AUTHORS
Grigory Greisukh, Dr. of the Eng. Sc., subscribing_2002@mail.ru
ORCID: 0000-0003-1905-1513
Ezhov Evgeny
ORCID: 0000-0001-9281-5394
Artem Antonov
ORCID: 0000-0003-1532-2750
Victor Danilov
ORCID: 0000-0002-1766-5223
Boris Usievich
ORCID: 0000-0001-5071-3058
G. I. Greysukh1, V. A. Danilov2, E. G. Ezhov1, A. I. Antonov1, B. A. Usievich3
Penza State University of Architecture and Construction, www.pguas.ru, Penza, Russia
Scientific and Technological Center for Unique Instrumentation of the Russian Academy of Sciences, www.ntcup.ru, Moscow, Russia
A. M. Prokhorov Institute of General Physics of the Russian Academy of Sciences, www.gpi.ru, Moscow, Russia
It is shown that placing a diffraction microstructure on the flat surface of one of the refractive lenses of a high-aperture triplet can simultaneously satisfy the correction conditions for both chromatic and monochromatic aberrations and obtain lenses designed for the middle and double infrared ranges having sufficiently high optical characteristics.
Key words: middle and double IR ranges, lens, diffraction microstructure, chromatic and monochromatic aberrations
Received: 26.12.2019
Accepted: 21.01.2020
INTRODUCTION
Currently, one of the areas of optical instrumentation, the development of which is most in demand, is the direction associated with the development and creation of monofocal optics and optics with variable focal length, designed to work in extended spectral ranges. In the visible and near infrared (IR) ranges, this is primarily the mass optics of mobile phones, smartphones and tablets, as well as security systems and technical vision (in particular, unmanned land, underwater and air vehicles). In the dual IR range, including medium and far infrared radiation (3.7–11 microns), this is the optics of thermal imagers and night vision devices for various purposes. High-quality optics of all the above ranges is also necessary for medical instrumentation. At the same time, it is obvious that in order to meet the increasing requirements for the overall dimensions and technical characteristics of optical systems, it is necessary to expand the elemental base and the range of optical materials.
One of the possible ways to expand the elemental base involves the use of diffractive optical elements (DOE). Moreover, the unique aberration properties of DOEs give the greatest effect in the infrared range. Indeed, the introduction of such an element into the refractive lens of a thermal imager allows us to simplify its optical design and achieve the required optical characteristics (see, for example, [1, 2]). So, in particular, in the mid-IR range (3 ≤ λ ≤ 5 μm), the placement of the diffraction microstructure on the flat surface of one of the refractive lenses of a high-aperture triplet allows simultaneous fulfillment of the correction conditions for both chromatic and monochromatic aberrations. Equally important is the ability to refuse to use germanium in a triplet, which has a high and non-linear temperature coefficient of refractive index. When germanium is replaced by chalcogenide glass, the diffraction microstructure makes it possible to maintain apochromatization and a low level of spherochromatism, and a slight thermal change in the refractive index of chalcogenide glass avoids thermal defocusing. Moreover, the use of DOEs, as will be shown in this article, is very effective in dual IR range lenses, both monofocal and mechanically variable focal length (zoom lenses).
1.
DESIGN PARAMETERS AND OPTICAL CHARACTERISTICS OF THE MIDDLE IR RANGE REFRACTIVE-DIFFRACTIVE LENS
We will demonstrate the above-described capabilities using an IR lens as an example, two refractive lenses of which are made of AMTIR3 glass (refractive index at a wavelength of λ = 4 μm nAMTIR = 2.621003, and a dispersion coefficient of λmin = 3 μm and λmax = 5 μm, νAMTIR = 173.38), while the third lens, bearing the diffraction microstructure, is made of zinc sulfide (nZnS = 2.250382; νZnS = 112.20). The refractive indices and dispersion coefficients given here and below were calculated using the dispersion formulas of the INFRARED catalog of the ZEMAX optical design program [3] and work [4].
The optical scheme of the lens is shown in Fig. 1. Its focal length is fʹ = 40 mm, the aperture value is K = 0.84, and the angular field in the space of objects is 2ω = 24°. The distance from the vertex tangent plane of the front lens to the image plane is L = 73,7 mm.
Tables 1 and 2 show the design parameters of the lens obtained after preliminary dimensional and aberration calculations by optimization using the ZEMAX optical design program [3]. Moreover, each of the non-planar surfaces of the refractive lenses of the lens was a so-called even aspherical surface, described in ZEMAX by the equation:
,
where z(ρ) is the coordinate of a surface point spaced apart from the optical axis by a distance ρ in the coordinate system whose XOY plane touches the top of this surface; c is the curvature of the surface at its apex; κ is a conical constant; αp – surface asphericity coefficients.
As for the diffraction microstructure placed on the flat frontal surface of the second refractive lens, it was modeled in the framework of ZEMAX by a Binary2 type surface with a phase delay of the form
,
where m is the number of the working diffraction order, Aj are constant coefficients.
This IR lens with a relative aperture of 1:0.84 provides a resolution of 50 mm–1 with a contrast of at least 0.5 and 35 mm–1 with a contrast of at least 0.65 within a field angle of 2ω ≤ 24°. The residual position chromatism in the range from λmin = 3 μm to λmax = 5 μm does not exceed 3.4 μm with a tolerance limited by diffraction equal to 11.3 μm. The chromaticity of the increase is 4.1 μm, which is slightly smaller than the radius of the main maximum of the Airy picture. Lens distortion is less than 0.25%.
2.
DESIGN PARAMETERS AND OPTICAL CHARACTERISTICS OF THE DOUBLE IR RANGE REFRACTIVE-DIFFRACTION LENS
Aberration calculation and subsequent optimization using ZEMAX optical design programs have shown that the refractive-diffraction triplet, after a corresponding modification of the optical scheme, is able to successfully operate in the double IR range [5]. The modification of the optical scheme included, in particular, the replacement of the used optical materials with GASIR1 (refractive index at a wavelength of λ = 7.35 μm nGASIR1 = 2.501489, and the dispersion coefficient at λmin = 3.7 μm and λmax = 11 μm νGASIR1 = 74.84) and ZnS_broad (nZnS_broad = 2.228805; νZnS_broad = 18.03).
For the obtained lens (see Fig. 2, Tables 3 and 4), the apochromatic correction of longitudinal chromatism with a tertiary spectrum not exceeding the tolerance limited by diffraction and correction of chromaticity increase to a value on the order of the radius of the main maximum of the Airy picture are preserved in the double IR range.
However, chromatic aberrations of higher orders with an expansion of the spectral range increased significantly, which led to a decrease in aperture (K = 1.1) and resolution. Within the same angular field for which the above-presented mid-IR lens (2ω ≤ 24°) is designed, this dual-IR lens provides a resolution of 35 mm–1 with a contrast of at least 0.3 and 25 mm–1 with contrast not lower than 0.5. The focal length of the lens is still fʹ = 40 mm, and the distance from the vertex tangent plane of the front lens to the image plane is L = 69.03 mm. Distortion does not exceed 0.3%.
3.
DIFFRACTION EFFICIENCY OF IR KINOFORM AND WAYS TO INCREASE IT
The diffraction microstructure modeled in Zemax by an infinitely thin banner that performs a given phase delay is actually performed in the form of a kinoform, i. e. sawtooth relief on the surface of the optical material. Today, a well-established technology of diamond turning allows the industrial production of hybrid elements of the IR range with a kinoform microstructure made on a flat, spherical or even aspherical surface of a refractive lens [6].
The diffraction efficiency (DE) of a kinoform microstructure can approach unity, but only at a single wavelength and at a single angle of incidence of radiation, and only a strict theory of diffraction based on the solution of the Maxwell equation system gives a reliable estimate of the decrease in DE depending on the wavelength and angle of incidence. The DE of kinoform microstructures of both lenses described in this work was calculated by rigorous coupled-wave analysis (RCWA) [7]. In this case, the calculation of the microstructures of the mid-IR lens was performed using the RCWA-PSUACE computer program developed by the authors, and the microstructures of the dual-IR lens using the computer program presented in [8].
The kinoform microstructure of the middle IR range lens consists of 33 ring zones. In this case, the width of the narrowest zone (the minimum period of the kinoform) P > 400 μm, and the maximum angle of incidence of radiation on the microstructure from the air θ ≤ 13.5°. The calculation showed that the DE of this kinoform at the optimal relief depth (h = 3.2 μm), due to the significant ratio of the spatial period of the microstructure to the relief depth (P / h > 125), turned out to be close to the limit predicted by the scalar diffraction theory, i. e. not lower than 0.8 in the entire working spectral range. At the same time, twenty percent of the energy incident on kinoforms attributable to secondary diffraction orders will inevitably lead to a decrease in contrast in the image formed by the lens and, in some cases, to an unacceptable decrease in the signal-to-noise ratio.
It is possible to raise the DE in the entire spectral range and for the entire range of angles of incidence of radiation to the level of 0.85 and even to 0.95 by switching to two- or three-layer kinoform microstructures. So, in particular, a two-layer single-relief microstructure (see Fig. 3) composed of Al2O3 and ZnS (n1 = 1.660731, n2 = 2.250382) with an optimal relief depth (h = 6.72 μm), angles of incidence of radiation –15 ≤ θ ≤ 15° and in the spectral range of 3 ≤ λ ≤ 5 μm provides DE ≤ 0.87 at P / h ≤ 5 and DE ≥ 0.90 at λ / h ≥ 30.
Here, we immediately note that modern technologies make it possible, using the flat surface of a refractive lens as a substrate, to apply a layer of another IR transparent material with a thickness sufficient to make a sawtooth relief in it, and in turn to cover it with another layer, which will complete the formation of a two-layer single-relief sawtooth microstructure. The analysis showed that Al2O3 and AgCl (n2 = 1.99996) can serve as the best pair of materials for such a microstructure in the spectral range of 3 ≤ λ ≤ 5 μm. With an optimal relief depth (h = 12.22 microns) and radiation angles of incidence of –15 ≤ θ ≤ 15°, this microstructure provides DE ≤ 0.9 at P / h ≤ 5 and DE ≥ 0.92 at P / h ≥ 30.
The three-layer microstructure (see Fig. 4) at the optimal depths of two reliefs (h1 = 18.76 μm, h2 = 10.96 μm), composed of Al2O3, MgF2 and ZnS (n1 = 1,660731, n2 = 1,348829, n3 = 2.250382) in the same angular and spectral ranges provides DE ≤ 0.92 for P / (h1 + h2) ≤ 5 and DE ≥ 0.95 for P / (h1 + h2) ≥ 30.
Kinoform microstructure of the double IR lens presented above consists of only 5 annular zones and is performed on a flat surface of the optical material GASIR1 (refractive index at a wavelength of λ = 7.35 μm nGASIR1 = 2.501489, and the dispersion coefficient at λmin = 3,7 μm and λmax = 11 μm νGASIR1 = 74.84). The width of the narrowest zone, i. e. the minimum kinoform period is P > 2.8 mm, and the maximum angle of incidence of radiation on the microstructure from the air is θ ≤ 12°.
The calculation showed that at the optimal relief depth h = 3.7 μm, the DE at the edges of the working spectral range (3.7 ≤ λ ≤ 11 μm) will drop to 0.4 even with very large ratios of the spatial period of the microstructure to the depth of the relief.
In the case of a two-layer single-relief microstructure composed of GASIR1 and ZnS_broad, ДЭ ≤ 0.52. And even if two layers of CdTe and ZnS_broad materials separated by one sawtooth relief are applied to the flat surface of the first refractive lens, it will not be possible to raise the DE above 0.7.
The situation becomes fundamentally different when composing a microstructure from the same materials GASIR1 and ZnS_broad, but with two reliefs of different depths (h1 = 132 μm, h2 = 155.8 μm). Despite such a significant total depth of the relief and due to the large ratio of the spatial period of the microstructure to the total depth (P / h ≈ 10) DE ≥ 0.9.
CONCLUSION
The research results presented in this paper clearly demonstrate the effectiveness of using diffraction elements in the lenses of the middle and double IR ranges. It is shown that the placement of a diffractive microstructure on the flat surface of one of the refractive lenses of a high aperture triplet can simultaneously satisfy the correction conditions of both chromatic and monochromatic aberrations and achieve sufficiently high optical characteristics. At the same time, a significant width of the working spectral range forces us to switch to two-layer single- or two-relief microstructures to ensure an acceptable DE sawtooth relief microstructure. At the same time, the depths of reliefs increase many times, which significantly limits the permissible minimum spatial period of the microstructure and the angles of incidence of radiation on it. This in turn imposes certain restrictions on the placement of the microstructure within the optical circuit of the lens [9]. However, as shown in this work, it is possible to compose the optical scheme of the lens, which provides the proper correction of aberrations at an acceptable minimum spatial period of the diffraction microstructure and angles of incidence of radiation on it, even in the simplest case of a triplet.
CONTRIBUTION OF TEAM MEMBERS
TO THE WORK
All authors declare an equal contribution of each to the preparation, discussion and writing of the article.
The study is carried out with the support RSF project № 20-19-00081.
ABOUT AUTHORS
Grigory Greisukh, Dr. of the Eng. Sc., subscribing_2002@mail.ru
ORCID: 0000-0003-1905-1513
Ezhov Evgeny
ORCID: 0000-0001-9281-5394
Artem Antonov
ORCID: 0000-0003-1532-2750
Victor Danilov
ORCID: 0000-0002-1766-5223
Boris Usievich
ORCID: 0000-0001-5071-3058
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