Freeform mirror fabrication and metrology using a high performance test CGH and advanced alignment features

Abstract

The metrology of mirrors with an off-axis aspheric or freeform shape can be based on optical testing using a Computer Generated Hologram as wavefront matching element in an interferometric setup. Since the setup can be understood as optical system consisting of multiple elements with six degrees of freedom each, the accuracy strongly depends on the alignment of the surface under test with respect to the transmission element of the interferometer and the micro optics of the CGH. A novel alignment approach for the relative positioning of the mirror and CGH in six degrees of freedom is reported. In the presented work, a proper alignment is achieved by illuminating alignment elements outside the Clear Aperture (CA) of the optical surface with the help of auxiliary holograms next to the test CGH on the substrate. The peripheral holograms on the CGH substrate are used to generate additional phase maps in the interferogram, that indicate positioning errors. Since the reference spheres represent the coordinate system of the mirror and are measured in the same precision as the optical surface, the registration and shape has to be appropriate to embody the mirrors coordinate system. The alignment elements on the mirror body are diamond machined using freeform turning or micro milling processes in the same machine setup used for the mirror manufacturing. The differences between the turning and milling of alignment lenses is discussed. The novel approach is applied to correct the shape error of a freeform mirror using ultra precision machining. The absolute measurement of the quality of freeform mirror shapes including tilt and optical power is possible using the presented alignment concept. For a better understanding, different metrology methods for aspheres and freeforms are reviewed. To verify the novel method of alignment and the measurement results, the freeform surface is also characterized using ultra high accuracy 2½D profilometry. The results of the different techniques for the absolute measurement of freeforms are compared

Calibrating CGH Substrate TWE and Simulating Adjustment Error in Cylindrical Surface Measurement

“…For more information, see https://creativecommons.org/licenses/by/4.0/ second passes though the CGH, which carries the information of deviations from measured cylinder, and returns into the interferometer. Obviously, the entire measurement system requires an exact alignment between the interferometer, the CGH and the measured cylinder [3], [8]. If we use six-dimensional adjustment frame to adjust the relative position between CGH and interferometer, it will be complex and introduce some alignment errors simultaneously [9], [10].…”

An insight on optical metrology in manufacturing

“…As another great advantage of CGH, diverse holographic alignment patterns can be designed and included outside of the metrology nulling CGH pattern to assist the test-configuration alignment and positioning [122]. Since the auxiliary alignment patterns are lithographically printed on the CGH glass substrate at the same time as the main metrology null pattern, all the holographic patterns are precisely aligned to one another at almost no additional cost.…”

Optics technology for large-aperture space telescopes: from fabrication to final acceptance tests

“…Over the 8.4 m clear aperture of the segment, 20 nm RMS residual error is achieved with the null test [101]. Alignment features can be put outside the primary CGH to make sure all the components are correctly positioned [161]. The external reference patterns are printed on the CGH at the same time as the main correction null pattern, which means they are aligned to the precision of the lithographic printing method.…”