Design and fabrication of long-focal-length microlens arrays for Shack-Hartmann wavefront sensors

Presented is a microlens array (MLA) with long focal length (millimetre range) in various structures and arrangements using thermal reflow process for Shack-Hartmann wavefront sensors (SHWSs). The microlens focal length is usually of the same order of the lens diameter or several 100--m. To extend the focal length, the authors made a photoresist MLA covered by polydimethysiloxane (PDMS) film on a glass substrate. Since the refractive index difference between the PDMS and MLA interface is lower than that of air and the MLA interface, the light is less bended when passing through it and focuses in a far distance. The exerimental result shows a 235 and 135--m diameter MLA with 5.27 and 1.81-mm focal lengths, which is around six times longer than the conventional thermal reflow method. Other specific focal lengths could be realised by modifying the refractive index difference. After the long-focal-length MLA film was fabricated, it could be integrated with an image sensor to build a SHWS. The authors evaluated the performance of their sensor by measuring a progressive addition lens wavefront aberration.