## 3 OASLM Structure  
The structure of the OASLM [17] is shown in Figure 2. It is composed of a photoconductive layer, a transparent electrode layer, an alignment layer, a liquid crystal molecule layer, and a glass substrate. The photoconductive layer was connected to the liquid crystal layer in series, and an AC voltage of approximately 29 V was applied during operation. When a write-in optical signal (blue, 470 nm) is incident on the photoconductive layer, with a change in the resistance of the photoconductive layer, the partial pressure on the  
Figure 1: Layout of the optical system of the additive manufacturing scheme of the surface exposure (SE) technology. PBS\({}_{1}\), and PBS\({}_{2}\): Polarizing beam splitter; L\({}_{1}\): concave lens; L\({}_{2}\), L\({}_{3}\), and L\({}_{4}\): convex lenses; OASLM: optically addressed spatial light modulator; M\({}_{1}\): mirror. liquid crystal layer changes, which results in different deflection states of the liquid crystal molecules. When a read-out light signal is incident, its optical parameters change correspondingly [13]. Transmissive OASLM has a high transmittance in the range of 85-90%. Compared with a reflective structure, the transmissive OASLM is more convenient in the construction of optical path. The OASLM can be divided into two types: amplitude type and phase type. When the director difference of the liquid crystal molecule on the front and back surface is 90\({}^{\circ}\), the transmittance of the read-out light signal has a monotonous, nearly linear relationship with the intensity of blue light on the photoconductive layer. When the liquid crystal molecular twist is 0\({}^{\circ}\), the phase delay of the output light varies with the intensity of blue light on the photoconductive layer, which can be used for phase modulation. When using the phase type modulator to realize the intensity modulation, it is usually placed at the front focus of the lens. In this way, the intensity distribution of the output light after lens or the intensity distribution after the phase type modulator and then the far-field transmission can be realized by loading the phase hologram on the modulator, and the phase hologram should be calculated by Fourier transform [18]. In addition, [PERSON] has reported that the Fourier transform is in need to using the phase type modulator [19]. However, the amplitude modulator can be directly loaded with an optical signal for the corresponding modulation, which is more convenient to operate. The device was integrated into a volume of 90 mm \(\times\) 74 mm \(\times\) 134 mm, and the aperture was 22 mm \(\times\) 22 mm. The damage threshold of the OASLM was tested to be 521 mJ/cm\({}^{2}\) @ 1064 nm, 4 ns) using the \(N\)-on-1 method. 4 Requirement Analysis of Laser Energy Density for Surface Irradiated Tin Laser Additive Fabrication  
The melting threshold of tin was simulated using a finite-element method. The dimensions of the powder were 34 \(\times\) 20 \(\times\) 1 mm\({}^{3}\), and the laser was irradiated at the center of the powder. Owing to the symmetrical nature of the problem, we established a 1/4 model (17 \(\times\) 10 \(\times\) 1 mm\({}^{3}\)). The 3D heat conduction model can be expressed as:  
\[\rho C_{p}\frac{\partial T}{\partial t}+\
abla\cdot(-k\
abla T)=Q \tag{1}\]  
where \(\rho\), \(C_{p}\), and \(k\) are the density, specific heat capacity, and thermal conductivity, respectively, and \(Q\) is the input source, which is the pulsed laser incident along the \(z\)-axis. Benchmarking the experiment, the input source had a uniform heat flux and the time domain obeys a Gaussian distribution with full width at half maximum of 8 ns. The beam waist radius was 0.42 mm. The absorption coefficient of material at 1064 nm was set to be  
Figure 2: Schematic diagram of the OASLM. 0.2. The initial temperature was 100 \({}^{\circ}\)C. In addition, it was assumed that the heat source was focused on center of the model and there was no radiant heat loss or melting. The powder layer is a mixture of solid (Sn) and gas (air) phases.