Multilevel diffractive lens in the MWIR with extended depth-of-focus and wide field-of-view

Optics in the mid-wave-infra-red (MWIR) band are generally heavy, thick and expensive. Here, we demonstrate multi-level diffractive lenses; one designed using inverse design approaches and another using the conventional propagation phase (the Fresnel zone plate or FZP) with diameter = 25um and focal length = 25um operating at $\lambda=4\thinspace\mu$m. We fabricated the lenses by optical lithography and characterized their performance. We show that the inverse-designed MDL achieves larger depth-of-focus and better off-axis performance when compared to the the FZP at the expense of larger spot size and reduced focusing efficiencies. Both lenses are flat with thickness <=0.5mm and weigh <= 3.63g, which are far smaller than their conventional refractive counterparts.