Three-dimensional holographic parallel focusing with feedback control for femtosecond laser processing.

• Three-dimensional holographic parallel focusing with feedback control is proposed for the first time. • Implementations of measurement of three-dimensional focusing intensities and a recalculation of the corresponding computer-generated hologram from the measurement simultaneously. • Significant improvement of the uniformity for the 3D intensities and the consistency for the laser processed micro-holes proved that the effectiveness of this method and has the great potential to be applied to high-precision machining. We demonstrate feedback control of three-dimensional (3D) parallel focusing for automatic compensation of imperfections in an optical system and long-term stability of parallel focusing. These are realized by a computer-generated hologram (CGH) displayed on a spatial light modulator (SLM), and an iterative calculation based on the observation of the 3D focusing intensities with an image sensor on a programmable linear stage. We demonstrate our parallel laser processing method using 40 parallel beams focused at four different planes, with a maximum uniformity of 0.96, applied to single-shot 3D fabrication in transparent glass. [ABSTRACT FROM AUTHOR]