A novel common aperture optical system for MWIR/SWIR polarization imager

Abstract

A novel common aperture dual waveband imaging optical system for a MWIR/SWIR polarization imager is presented in this paper. The optical system consists of an all-reflective telescope with focal ratio 4, a dichroic beam-splitter, a MWIR refractive optical arm with effective focal length 60 mm, a SWIR refractive optical arm with effective focal length 60 mm. The effective focal length of the whole optical system is 240 mm, the f-number is 2 and the efficiency of the cold stop is 100%. The MWIR arm includes a secondary refractive telescope, a MWIR polarimetric analyzer, a re-imaging objective and a MWIR 320×256 FPA. The SWIR arm includes a secondary refractive telescope, a SWIR polarimetric analyzer, a focusing lens and a SWIR 320×256 FPA. The common axis all-reflective telescope has a minimal central obstacle ratio of 0.3, which is composed of a first paraboloid, a mirror with two operational faces, a second paraboloid. In order to minimize the apertures of the first paraboloid, the beam-splitter and the polarizer, the thrice imaging technique is taken to design the MWIR arm, that is, the remote scene is imaged three times and the cold stop is imaged twice from the image space to the object space. The first real image of the remote scene is focused on the common focus of the first paraboloid and the second paraboloid, i.e., the small hole centered on the mirror with two operational faces, the second real image is focused on the intermediate real image plane of the re-imaging objective, and the third real image is focused on the FPA. The first real image of the cold stop is located nearby the beam-spiltter, the second real image (the entrance pupil) is located nearby the first paraboloid, so that the clear apertures of the first paraboloid, the beam-splitter and the polarizer are compressed within Φ 125mm, Φ 45mm and Φ 30mm respectively. The image quality of the dual waveband optical system is optimized perfect well, and it can fulfill the requirements and the specifications of the dual waveband IR polarization imager.