Feedback-control-based laser multi-focus energy manipulation using a spatial light modulator

Laser multi-focus techniques enable the splitting of the laser energy into multiple focus positions and thus play a key role in enabling parallel laser processing and high-speed imaging applications. In such techniques, the multiple focus positions are generated using spatial light modulators (SLMs) with specifically designed phase modulation patterns. These patterns can be produced directly using a phase-segmentation method without the need for an iterative phase-retrieval process. However, the energy uniformity of the multi-focus points is sensitive to the incident laser quality, optical system imperfections, and external disturbances. To address this issue, the present study proposes a multi-focus energy manipulation with feedback-control algorithm (MEMFA). In the proposed method, feedback from the multi-focus image is employed to iteratively adjust the angular proportions of the sectors of the SLM phase pattern in such a way as to achieve the desired multi-focus energy distribution. The experimental results show that the 3 × 3 multi-focus uniformity is greatly improved, with a reduction in the root-mean-square deviation (RMSD) from 46.1 % to 1.06 %. Moreover, convergence is obtained within four time steps. It is shown that MEMFA also provides the ability to achieve target non-uniform energy distributions within the multi-focus array, with RMSD values of 1.85 % and 1.35 % for 3 × 3 and 5 × 5 arrays, respectively. Finally, a large-scale multi-focus capability is demonstrated by scanning the 5 × 5 multi-focus array over an array consisting of 2500 laser points within an area of 2.5 × 2.5 mm². Overall, the results confirm that MEMFA has the ability to manipulate and maintain the multi-focus energy quality necessary for advanced parallel laser processing applications.