Evaluation of wavefront aberrations induced by overlay errors in stitching computer-generated holograms

The emergence of large-aperture telescopes has fueled the need for testing and assisted alignment of large-scale optical systems. Computer-generated holograms (CGHs) have proven to be a useful method for aspheric optical testing and assisted alignment, and large-sized CGHs are required for large-scale aspherical systems. However, the maximum size of CGH is generally in the range of 6 to 9 inches owing to its current lithography capability. Stitching technology is a promising method for realizing larger-sized CGHs. As a proof of concept, a CGH with an aperture of 80 mm is fabricated using multistep lithography to investigate the overlay errors in stitching. Theoretical derivations reveal that additional tip/tilt aberrations are introduced owing to the overlay errors, which commonly couple with aberrations from adjustment errors. Therefore, to budget the effects of overlay errors, an evaluation method is proposed in this study. The evaluation results guide the separation and compensation of coupling aberrations in measurement. It is verified that the accuracy of a stitching CGH is considerably improved from 0.156λ to 0.017λ in terms of RMS after compensation. The findings obtained in this study during the development of stitching method and error evaluation are expected to promote the technology development and improvement of testing accuracy in stitching CGHs.