With the growing demand for fine imaging and processing in industrial, medical, and scientific fields, the field of optical technology is gradually entering the era of femtosecond laser. Femtosecond laser(fs-laser) has the advantages of high peak power, high processing accuracy, and very low thermal damage, but suffers from fluence limitations due to the inherent form of point-scanning. Previous studies have shown that multifocal parallel scanning can effectively improve the fluence for fs-laser based imaging and processing. Conventional spatial light modulator (SLM) only has a low refresh rate (~100 Hz), and generally needs to be coupled with a galvanometer or other fast scanning devices to improve the fluence. Most of these existed methods can only realize periodic scanning trajectories, which becomes a disadvantage in terms of flexibility. In advanced application case, it is still challenging to realize non-periodic scanning trajectories, especially when field-of-view stitching is required.

To solve the above problems, Prof. Shaoqun Zeng's team at Huazhong University of Science and Technology (HUST) proposed a multi-focus nonperiodic parallel scanning strategy with DMD combined with galvanometer. This method realizes flexible nonperiodic trajectories while improving the scanning throughput. The article called "Multi-focus non-periodic scanning method for femtosecond lasers based on DMD and galvanometer scanners" are published in Chinese Optics Letters 2024, Vol. 22, Issue. 5.

Fig 1. Proposed scanning process schematic and processed result demonstrated on in-vitro porcine cornea

The research group used a modified GSW-B algorithm generate holograms with multiple foci. An aberration term is introduced into the hologram, and the specific focus is turned on or "off" by changing the aberration term. Specifically, by loading different binary holograms, the DMD can generate and control single to tens of foci simultaneously. During galvanometer scanning, complex non-periodic structures can be realized by rapidly switching the displayed holograms on the DMD.

Further onwards, in order to maintain the uniformity of each processed spot while multi-point switching, this work proposed a high uniformity multi-focus generation and regulation method. In this process, the actual number of spots generated by each hologram does not change, and the focus "off" effect can be realized by introduced aberrations, degrading the non-essential spots. With the help of the nonlinear and threshold effects in fs-laser processing, the energy density of the non-essential spots is suppressed below the processing threshold. The necessary spots keep in focal plane and the energy is not sacrificed drastically, thus maintaining the focus "on" state. Compared to conventional methods, this method does not require additional energy modulation devices to correct for the energy difference of the processed spots under different holograms.

This work shows the method can effectively improve the scanning fluence while realizing non-periodic scanning pattern. In the paper, the research group demonstrated the proposed scanning strategy in fs-laser cornea surgery to successfully fabricating corneal flap on an in-vitro porcine eye. It provided the applicable potential in ultrafast laser processing and laser medicine.