The diffractive zone thicknesses of conventional diffractive optical elements (DOEs) are generally obtained using the thin element approximation (TEA). However, the TEA yields inaccurate results in the case of thick multilayer DOEs (MLDOEs). The extended scalar theory (EST) is an alternative thickness optimization method that depends on the diffractive order and the optimization wavelength. We developed an algorithm to research suitable EST input parameters. It combines ray-tracing and Fourier optics to provide a performance estimate for each EST parameter pair. The resulting “best”MLDOEdesigns for three different material combinations are analyzed using rigorous finite-difference time-domain. Compared to the TEA, the proposed algorithm can provide performing zone thicknesses. @en

The polychromatic integral diffraction efficiency (PIDE) metric is generally used to select the most suitable materials for multilayer diffractive optical elements (MLDOEs). However, this method is based on the thin element approximation, which yields inaccurate results in the case of thick diffractive elements such as MLDOEs. We propose a new material selection approach, to the best of our knowledge, based on three metrics: transmission, total internal reflection, and the optical component’s total thickness. This approach, called “geometric optics material selection method” (GO-MSM), is tested in mid-wave and long-wave infrared bands. Finite-difference time-domain is used to study the optical performance (Strehl ratio) of the “optimal” MLDOE combinations obtained with the PIDE metric and the GO-MSM. Only the proposed method can provide MLDOE designs that perform. This study also shows that an MLDOE gap filled with a low index material (air) strongly degrades the image quality.@en

Infrared (IR) remote sensing offers a huge range of applications, mostly addressing make-or-break issues of our century (wildfires, irrigation monitoring, etc.). Multispectral spaceborne instruments require bulky optical systems designed for a specific scientific goal and have very low revisit time. Thereby, constellations of small satellites embarking compact dual-band IR imagers are very promising solutions. We study a dual-band IR diffractive element called multilayer diffractive optical elements (MLDOE). It replaces classical diffractive lenses (DOEs) that cannot operate simultaneously in two distinct wavebands. An MLDOE design is studied using the rigorous finite difference time domain (FDTD) method. Its performance at the ”best” focal plane is deduced using free-space Fourier optics wave propagation. The presented MLDOE design has over 80% Strehl ratio in both bands, outperforming classical DOEs. Its chromatic focal shift has a negative variation, in opposition to refractive lenses, allowing efficient and compact dual-band hybrid lenses.@en

Hi-5 is a proposed L' band high-contrast nulling interferometric instrument for the visitor focus of the Very Large Telescope Interferometer (VLTI). As a part of the ERC consolidator project called SCIFY (Self-Calibrated Interferometry For exoplanet spectroscopY), the instrument aims to achieve sufficient dynamic range and angular resolution to directly image and characterize the snow line of young extra-solar planetary systems. The spectrometer is based on a dispersive grism and is located downstream of an integrated optics beam-combiner. To reach the contrast and sensitivity specifications, the outputs of the I/O chip must be sufficiently separated and properly sampled on the Hawaii-2RG detector. This has many implications for the photonic chip and spectrometer design. We present these technical requirements, trade-off studies, and phase-A of the optical design of the Hi-5 spectrometer in this paper. For both science and contract-driven reasons, the instrument design currently features three different spectroscopic modes (R=20, 400, and 2000). Designs and efficiency estimates for the grisms are also presented as well as the strategy to separate the two polarization states.@en