Design of the cooled free-form diffractive lens for infrared waveband

Cooled infrared optical systems are suitable for long-distance and high-sensitivity detection. In order to achieve 100% cold stop efficiency, traditionally cooled infrared optical systems result in a large number of lenses and complex structures. In addition, the imaging quality is also affected by the Narcissus. To simplify the optical path structure of cooled infrared optical systems, the design of a cooled free-form diffractive lens (CFDL) is investigated to achieve single-element imaging with the cooled infrared detector. Based on the scalar diffraction theory, the diffraction formulas for the off-axis fields of view of the CFDL and the corresponding weighted focusing efficiency across the entire waveband are derived. The CFDL is designed for a mid-infrared range of 3.7 to 4.8µm, an F number of 2, a microstructure size of 7.6µm × 7.6µm and a half field of view angle of 9°. The average focusing efficiency of the entire waveband and entire fields of view is 70% and all the modulation transfer function (MTF) is larger than 0.277 at the spatial frequency of 33lp/mm. Compared to current cooled infrared optical systems, the CFDL provides a new idea for the design of miniaturization.