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When conducting interferometric tests of freeform optical surfaces, additional optical components, such as computer-generated holograms or deformable mirrors, are often necessary to achieve a null or quasi-null. These additional optical components increase both the cost and the difficulty of interferometric tests of freeform optical surfaces. In this paper, designs using off-axis segments of conics as base surfaces for freeforms are explored. These off-axis conics are more complex base surfaces than typically-used base spheres but remain null-testable. By leveraging off-axis conics in conjunction with additional orthogonal polynomial departures, designs were found with up to an order-of-magnitude of improvement in testability estimates relative to designs that use base spheres. Two design studies, a three-mirror telescope and a wide field-of-view four-mirror telescope, demonstrate the impact of using off-axis conics as the base surface.

 

Significant advances for optical systems in terms of both performance and packaging are enabled by freeform optical components. Yet, surface form metrology for freeform optics remains a challenge. We developed and investigated a point-cloud cascade optical coherence tomography (C-OCT) technique to address this metrology challenge. The mathematical framework for the working principle of C-OCT is presented. A novel detection scheme is developed to enable high-speed measurements. Experimental results validate the C-OCT technique with the prototype setup demonstrating single-point precision of ±26 nm (∼λ/24 at the He-Ne wavelength), paving the way towards full surface measurements on freeform optical components.

 

Driven by the development of freeform imaging systems, we have combined several concepts and techniques from the literature to analytically generate unobscured freeform starting point designs that are corrected through the third-order image degrading aberrations. The surfaces used in these starting point designs are described as a base off-axis conic that images stigmatically for the central field point, also known as a Cartesian reflector, with an aspheric departure “cap” (quartic with the aperture) added to the base off-axis conic to correct for the third-order image degrading aberrations. Once the aspheric caps are added to the surfaces, the system is then optimized using higher order freeform terms while leaving second-order terms frozen to preserve the focal length of the system during optimization. This technique is used to survey the three-mirror freeform imager solution space. Several systems that are the result of this technique are shown, with different numbers of internal images, internal pupil conjugates and folding geometries.