Optimal design of the computational flat diffractive optical system.

A design method of the computational flat diffractive computational flat diffractive optical system is presented to simplify the optical system structure and achieve high image quality. The aberration expression of the flat diffractive optical element (FDOE) is derived, and then computational imaging methods are used to eliminate the influence of off-axis aberration on image quality, so the field of view is expanded. Based on theoretical analysis, the FDOE is designed, and the field of view has been expanded from 2° to 5°. The results show that the detail resolution of the edge field of view is enhanced after restoration, and the modulation transfer function (MTF) of different subareas calculated using the slanted-edge method improved by an average of 0.17. The diffraction efficiency of the FDOE is greater than 95.75%. This method realizes the miniaturization and lightweight of the optical system, and provides new ideas for the integration of optical systems.

Optical design of a monolithic compressed folding imaging lens for infrared/laser dual-band.

In order to realize the miniaturization of the dual-band system, the monolithic compressed folding imaging lens (CFIL) is designed for infrared/laser dual-band in this paper. The relationship among the back focal length, field of view, pupil diameter, and central obscuration of the CFIL are derived. The design method of the dual-band CFIL is given, and the stray light of the CFIL can be suppressed by the double-layer hood structure. According to the design method of the CFIL, the infrared/laser dual-band can be applied by a monolithic optical element. The design results show that the minimum MTF for all fields of view in the infrared band is greater than 0.125 at 42lp/mm, the spot uniformity in the laser band is greater than 90%, and the total system length is only 0.305 times the focal length. After tolerance analysis, the MTF of CFIL is greater than 0.1, and the spot diagram is less than 880µm. The working temperature of the system is -20∼50°C, and the compensation distance is given. After stray light optimization, The point source transmittance (PST) value in the infrared band is reduced by 2 to 4 orders of magnitude, and the PST value in the laser band is reduced by 1 to 5 orders of magnitude. Compared with the traditional coaxial reflective system, the infrared/laser dual-band CFIL has only one lens, and the optical structure is compact. It provides a new idea for the integration and miniaturization of the multi-band system.

Design of achromatic annular folded lens with multilayer diffractive optics for the visible and near-IR wavebands.

In this paper, the annular folded lens (AFL) is applied to the realization of a miniaturized system for the visible and near-IR spectrums (0.45-1.1µm). In order to correct the chromatic aberration, a hybrid AFL is designed with the multilayer diffractive optical element (MLDOE) in which the substrate materials are precision molded glasses. We propose a new design method of the MLDOE to improve the polychromatic integral diffraction efficiency (PIDE) that makes it suitable for the optical path of the AFL. By comparing the characteristic angle weighted PIDE (CAW-PIDE), the optimal microstructure heights of the MLDOE can be obtained, and the effect of diffraction efficiency on image quality can be minimized for the entire incident angle range. The design results show that the ratio of total length to the focal length is only 0.332, and comprehensive modulation transfer function considering the diffraction efficiency is larger than 0.26 at 166 lp/mm. This study can provide a new idea for designing a broadband, miniaturized, and low-cost imaging system.

Dynamic aberration correction for conformal aircraft windows using the inner window surface and a fixed lens array.

A static solution to dynamic aberrations for conformal aircraft windows with a large range of look angles is reported. The solution uses the inner window surface and a fixed lens array near the image plane to correct the dynamic window aberrations at different look angles. The inner window surface is first used to balance the dynamic aberrations of the full field of regard. Then a lens array is implanted in front of the image, and the dynamic aberrations are corrected by designing the surface shape and rotation direction of each lens unit in the array. The principle of the solution is discussed, and a design example shows that the solution can realize the dynamic aberration correction introduced by the conformal window with a field of regard of ±42∘. Compared to other dynamic or static correctors, this solution takes into account the large field of regard, the light weight and the stability of the optoelectronic system.

Achromatic annular folded lens with reflective-diffractive optics.

This paper proposes an achromatic annular folded lens (AFL) with a reflective-diffractive optical element (RDOE). We derive novel mathematical models of the diffraction efficiency and polychromatic integral diffraction efficiency (PIDE) of the RDOE and an expression for its microstructure height. An AFL with an RDOE made of an optical plastic substrate material is designed in the visible waveband. To minimize the influence of incident angle on the diffraction efficiency and PIDE, the microstructure height is optimized. The design results indicate that the lateral color of the AFL is corrected, the modulation transfer function considering the diffraction efficiency is larger than 0.25 at 111 cycles/mm for all field of views. The hybrid AFL outperforms the conventional refractive imaging system in terms of the system size, volume, and image quality under the same specifications. It can be used in new-generation miniaturized imaging systems.

Design of dual-band infrared zoom lens with multilayer diffractive optical elements.

A mid-wave infrared (MWIR)/long-wave infrared (LWIR) dual-band zoom lens design with multilayer diffractive optical elements (MLDOEs) is presented. The mathematical relationship between the substrate material selection for dual-band MLDOE and polychromatic integral diffraction efficiency (PIDE) is deduced in the oblique incident situation, and further, a method for optimal selection of substrate material is proposed to obtain the high PIDE in an incident angle range. In the optimization process, the optimal substrate material combination is selected based on the proposed method, and the principle of lens material replacement is discussed. After optimization, the 5× hybrid dual-band infrared zoom system is obtained, which consists of seven lenses. The modulation transfer function values in all configurations are larger than 0.5 and 0.3 in MWIR and LWIR, respectively. The distortion values are less than 2% both in MWIR and LWIR for all configurations.

Optimization method of multilayer diffractive optical elements with consideration of ambient temperature.

A method for the optimal design of multilayer diffractive optical elements (MLDOEs) with consideration of ambient temperature is presented to improve the image quality over the entire temperature range. The relationship between diffraction efficiency and temperature is analyzed, and an optimization process of surface relief height for the MLDOEs is given. A practical 3-5 µm athermal hybrid optical system with a double-layer diffractive optical element is designed in the temperature range from -20°C to 60°C, and the image quality of two hybrid optical systems with optimized MLDOE and original MLDOE is compared. The result shows that the comprehensive modulation transfer function is obviously improved in the whole working temperature range. This method can be used during the passive athermalization hybrid optical system design with MLDOEs.

PSF model for diffractive optical elements with improved imaging performance in dual-waveband infrared systems.

Single-layer diffractive optical elements (SLDOEs) have advantages in terms of configuration, fabrication, range of angles and cost; however, the diffraction efficiency decreases sharply with wavelength deviating from the design wavelength, especially for dual-waveband imaging, causing apparent image blur. We propose a point spread function (PSF) model affected by the diffraction efficiency, which is called PSFDOE, and a method of restoring the blurred image to improve imaging performance in dual-waveband infrared systems with an SLDOE. Then, a design example of cooled MWIR and LWIR is presented. Furthermore, imaging simulations with different grades noises and restorations are conducted. Results reveal that the PSFDOE model can significantly improve the image blur caused by the decreased diffraction efficiency.

Optimal design method on diffractive optical elements with antireflection coatings.

The effect of antireflection coatings on diffraction efficiency of diffractive optical elements (DOEs) was studied and the mathematical model of diffraction efficiency affected by antireflection coatings for DOEs is presented. We found antireflection coatings can cause a significant reduction on diffraction efficiency at the designed, or the central wavelength. In order to solve this problem, we proposed a method to keep 100% diffraction efficiency at the designed wavelength by ensuring the 2π phase induced by DOEs and the antireflection coatings. Diffraction efficiency affected by antireflection coatings for DOEs with consideration of antireflection coatings are simulated. Analysis results can be utilized for refractive-diffractive hybrid imaging optical system optimal design and image quality evaluation.

Substrate material selection method for multilayer diffractive optics in a wide environmental temperature range.

We present a substrate material selection method for multilayer diffractive optical elements (MLDOEs) to obtain high polychromatic integral diffraction efficiency (PIDE) in a wide environmental temperature range. The extended expressions of the surface relief heights for the MLDOEs are deduced with consideration of the influence of the environmental temperature. The PIDE difference Δη¯(λ) and PIDE change factor F are introduced to select a reasonable substrate material combination. A smaller value of Δη¯(λ) or F indicates a smaller decrease of the PIDE in a wide temperature range, and the corresponding substrate material combination is better. According to the deduced relation, double-layer and three-layer DOEs with different combinations are discussed. The results show that IRG26 and zinc sulfide is the best substrate material combination in the infrared waveband for double-layer DOEs, and polycarbonate is more reasonable than polymethyl methacrylate as the middle filling optical material for three-layer DOEs when the two substrate materials are the same.

High diffraction efficiency of three-layer diffractive optics designed for wide temperature range and large incident angle.

A mathematical model of diffraction efficiency and polychromatic integral diffraction efficiency affected by environment temperature change and incident angle for three-layer diffractive optics with different dispersion materials is put forward, and its effects are analyzed. Taking optical materials N-FK5 and N-SF1 as the substrates of multilayer diffractive optics, the effect on diffraction efficiency and polychromatic integral diffraction efficiency with intermediate materials POLYCARB is analyzed with environment temperature change as well as incident angle. Therefore, three-layer diffractive optics can be applied in more wide environmental temperature ranges and larger incident angles for refractive-diffractive hybrid optical systems, which can obtain better image quality. Analysis results can be used to guide the hybrid imaging optical system design for optical engineers.

Tolerance analysis of multilayer diffractive optics based on polychromatic integral diffraction efficiency.

Multilayer diffractive optical elements (MLDOEs) can achieve high diffraction efficiency for broadband wavelength. Polychromatic integral diffraction efficiency (PIDE) is the key concern for evaluating diffraction efficiency over the waveband. The modulation transfer function of a hybrid refractive-diffractive optical system is directly affected by the PIDE. The relationship between PIDE and continuous manufacturing errors for microstructure heights and periodic widths of MLDOEs is studied theoretically in this paper, and an example of MLDOEs is discussed in the visible waveband. The analysis results can be used for manufacturing error control in microstructure heights and periodic widths.