Developing compact and innovative dual-band thermal imagers using multi-layer diffractive optical elements

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.