Design and fabrication of long focal length microlens arrays

We present microlens array (MLA) with various structure and arrangement in long focal length (millimeter range) based on thermal reflow process. The microlens focal length is usually in the same order of lens diameter or several hundred microns. To extend the focal length, we made a photoresist MLA covered by Polydimethysiloxane (PDMS) film on a glass substrate. Because the refractive index difference between PDMS and MLA interface is lower than that of air and MLA interface, the light is less bended when passing through it and focuses in a far distance. The experimental result shows 235 µm diameter and 135 µm diameter MLA with 5.27 mm focal length and 1.81 mm focal length, which is around 6 times longer than the conventional thermal reflow method. Besides, other specific focal lengths could be realized by modifying the refractive index difference.