Development of cubic freeform optical surface for wavefront coding application for extended depth of field Infrared camera

Freeform surfaces are being developed for number of applications like illumination systems, compact projection systems, head up display, ophthalmic and automotive industries. One of the important use of such surfaces in optical imaging application is wavefront coding for large depth of focus camera. The thermal induced defocus is the well-known problem in infrared (IR) imaging systems. The efficient technique to mitigate the effect of defocus is wavefront coding, The phase profile for wavefront coding application with IR camera is a cubic freeform profile, that enables the focus invariant across the region near the focal plane. This work presents the development of a high precision cubic freeform profile on Germanium (Ge) substrate and its application for extended depth of focus for IR imaging camera. The cubic freeform profile on the surface is generated using single point diamond turning (SPDT) in slow tool servo (STS) configuration. It is further polished by using bonnet polisher with polyurethane and uninap-13 pads in seven axis polishing machine. The precise work piece alignment during the generation and characterization of the surface is very important aspect which is taken care by introducing fiducials from the beginning of the process of realization of the component. Full field measurement of grinded surface is carried out by using long wavelength interferometer. Computer generated hologram (CGH) in Fizeau interferometer is used for corrective polishing metrology and final testing of the surface profile. Precise work piece alignment and full field measurement starting from the grinding stage helped to achieve surface with residual error of 0.2 µm PV and 1.4 nm RMS roughness (Rq). The developed cubic profile is used in infrared camera for wavefront coding in an experimental setup for the verification of the intended results.