**5. Systems and methods of femtosecond laser fabrication**

Two important types of FSL micromachining include direct writing, parallel method, laser pulse shaping in the temporal and spatial domain, laser frequency modulation, and laser-pulse-coordinated shaping in temporal/spatial/frequency domain based on electrons dynamics control. The direct writing processing method, the commonly used basic processing method, is a serial fabrication process, and the increased numerical aperture results in achieving augmented resolution. The high resolution, excellent flexibility, and quality make this method suitable for fabricating 3D microstructures within transparent materials, while in the parallel method, a slitoriented parallel is placed in the scanning direction before the objective lens, and the slit's orientation should be adjusted along with the scanning direction; therefore, the aspect ratio of the hollow microchannel and the laser efficiency is low. This method can only fabricate periodic structures. Another difference between the two methods is the throughputs; parallel microprocessing can realize high throughputs and is appropriate for large-scale FSL micromachining. But direct writing method has the limitation of low throughputs, which has been improved by the use of high-average-power and high-repetition-rate FSLs in recent years [3, 6, 9].

The choice of method to be used depends on what is required in that particular application. For each specific application, we have to consider the relation between working distance and fabrication resolution, as well. The fabrication resolution depends on their numerical aperture (NA). The focal spot diameter is inversely proportional to the NA (in the lateral direction, 1.22 λ/NA). Consequently, we require an objective lens with NA of ~0.5 for 3D machining. Then, a high-NA objective

lens (e.g., oil immersion objectives), which usually has a short working distance, is applicable for surface nanostructuring or TPP (which involves surfaces and thin samples); while they are not appropriate for the fabrication of 3D microstructures deep in a substrate [6].

If the spatial and temporal aspects of laser pulses are simultaneously focused, the laser components will be separated spatially in space before they enter the objective lens and then overlap at the spatial focal point focus of the objective lens. For processing a symmetric spherical light intensity distribution and improved axial resolution, the shortest pulse duration should be confined to the spatial focus; this approach increases the resolution. In contrast, for application in microfluidics, we require an improved aspect ratio of a hollow microchannel, achieved by combining temporallyand spatially-shaped laser pulses to enhance the etching depth of the microchannel by a factor of 13, enabling high-throughput fabrication of ultra-high-aspect-ratio hollow microchannel [9]. In the following, we explain the FSL processing systems in detail.
