# Research on Microhole Processing Technology Based on the Femtosecond-Laser Spiral Trepanning Method
## 2 Experimental Setup  
The femtosecond-laser processing equipment used in the experiment (Figure 2) mainly included a femtosecond laser, reflector, beam-scanning system, focusing lens, and five-axis positioning platform. The model of the femtosecond laser was Light Conversion l17771, which emits a wavelength of 1064 nm with the maximum average power of 20 W at 30 kHz repetition rate and a pulse width of 200 fs. The focusing spot diameter was 20 \(\upmu\)m. The beam scanning module was mainly composed of two wedges (Figure 2), which were used to control the laser beam scanning path. The motion accuracy of the PI platform was 1 \(\upmu\)m. The air pressure is set to 0.3 Pa. The sample size was 40 \(\times\) 40 \(\times\) 2 mm\({}^{2}\) of  
Figure 1: Different laser processing methods.  
304 stainless steel. The sample was fixed on the fixture, which was installed on the five-axis positioning platform. All experiments were carried out in a clean room and at constant temperature.  
First, we mainly studied the influence of the increment of radius angle, the number of circles, and the power of each circle on microhole machining. Before laser processing, the sample was immersed in ethanol for 10 min for ultrasonic cleaning and impurities removal; then, the sample was taken out to dry. The specific laser parameters were as follows: the laser scanning speed was 2400 mm/s, the starting angle was adjusted to 3\({}^{\circ}\) through the wedge, and each layer was scanned for 4 cycles. The laser power of the first cycle was 45%, the increment of radius angle was 1.1\({}^{\circ}\), and the number of circles was 2. The laser power of the second circle was 50%, the increment of radius angle was 0.8\({}^{\circ}\), and the number of circles was 2. The laser power of the third circle was 55%, the increment of radius angle was 0.3\({}^{\circ}\), and the number of circles was 10. The laser power of the fourth circle was 60%, the increment of radius angle was 0.1\({}^{\circ}\), and the number of circles was 18. After the first layer was scanned, the vertical feed was adjusted to 0.01 mm (the \(Z\)-axis was adjusted by the five-axis motion platform to make the laser focus drop by 0.01 mm), and the number of feeding layers was adjusted according to the material thickness, with the maximum of 400 layers. The microhole diameter was determined by the incident beam deflection angle and the focal length f of the focusing lens, and the corresponding relationship is D = d + d0 = 2F \(\cdot\) tan \(\theta\) + d0, where D is the processing aperture, d is the scanning diameter of the spot, F is the focusing lens focal length, \(\theta\) is the deflection angle between the beam incident to the focusing lens and the focusing lens optical axis, and d0 is the laser spot diameter. The schematic diagram and trajectory of laser spiral scanning are shown in Figure 3. Finally, an inverted cone hole with an entrance diameter of 0.30 \(\pm\) 0.01 mm and an exit diameter of 0.35 \(\pm\) 0.01 mm was required to be machined on a 2-mm-thick stainless steel sample.  
Figure 3: Scanning path of the microhole in femtosecond-laser processing.  
Figure 2: Experimental setup of femtosecond-laser processing.