Micro-refractive optical elements fabricated by multi-exposure lithography for laser speckle reduction.

Laser speckle reduction with a macro refractive optical element (mROE) is restricted by the limited entrance facet size of light pipe. Here, we have fabricated a micro-ROE (µROE) that incorporates three-dimensional micro-optical structures. The µROE with 2 × 2 duplicated multi-level cells is made of SU-8 photoresist with the help of multi-exposure lithography process. When the µROE works together with the mROE, objective speckle contrast is reduced to 0.2, where the light source is a low-coherence multimode laser diode. In principle, more speckle reduction can be obtained by fabricating µROEs with more cells and larger height differences among the cells.