Cascade optical coherence tomography (C-OCT) towards freeform metrology

Surface form metrology for freeform optical components is a significant challenge that impacts the entire process chain. One of the main aims is that the metrology technique provides sufficiently low measurement uncertainty within a reasonable measurement time. We developed a non-contact, non-part-specific cascade optical coherence tomography (COCT) technique within this metrology context. C-OCT utilizes a primary and a secondary interferometer in a cascade that transforms a difficult-to-measure optical path difference (OPD), which encodes the freeform sag in this case, into a more readily measured OPD in the secondary interferometer. To enable high-speed measurements, we developed the secondary interferometer based on a rotating optical cube. A custom telecentric objective lens and a custom pupil relay enable optical scanning over the sample surface. Experimental results validate the C-OCT technique with consecutive single-point measurements showing precision of ±26 nm (~λ/24 at the He-Ne wavelength) and a preliminary flat surface measurement demonstrating 22 nm RMS (~λ/28) over a central 20 mm diameter region. Developments on the system are underway towards surface measurements on freeform optical components.