## 2 Materials and Methods
### Structural Design of Microchip Laser  
Illustrated in Figure 1, a semiconductor laser diode (LD) serves as the initial energy source. The energy is coupled to the laser crystal through a lens assembly, functioning as an imaging optical system. The laser crystal comprises a solid gain medium and a saturable absorber, wherein the saturable absorber passively adjusts the quality factor (Q) to emit laser pulses with a narrow bandwidth and high energy. Typically, when the Q value is high, cavity loss is low. Thus, during the pumping phase, electrons transfer to the upper energy level. At this juncture, due to the characteristics of the saturable absorber, the Q value inside the cavity is low, resulting in high losses, preventing laser formation and retaining the electrons in the upper energy level. Once a sufficient number of electrons accumulate in the upper energy level, the Q value experiences a rapid increase. Subsequently, the electrons undergo a swift transition to the lower energy level, resulting in an instantaneous high-power laser output. This operational principle highlights the role of the saturable absorber in passive Q adjustment and the dynamic electron transitions that lead to precise and controlled pulsed laser emissions in deep space exploration TOF mass spectrometers. Considering the imperative for small size, low power consumption, and a compact structure, the chosen pumping source is a semiconductor laser with a maximum power of 10 W and a wavelength of 808 nm. It possesses a threshold current of 1.8 A and operates at a voltage below 2.2 V. The advantageous low current emission requirement facilitates the development of a low-power drive system. To ensure optimal conversion efficiency, end-pumping is implemented, aligning the pump light co-linearly with the resonant cavity. Significant overlap between the pumped volume and the TEM\({}_{00}\) mode enhances efficient energy transfer. Utilizing an imaging optical system, the pump light is collimated and focused, effectively suppressing optical distortions. The distribution of pump light aligns with the volume of the resonant cavity mode, ensuring precise incidence within the laser crystal, as depicted in Figure 2. This meticulous design approach contributes to the microchip laser's efficiency, addressing the specific demands of deep space exploration missions for a compact, low-power laser system with optimal performance. The conventional approach to preparing laser crystals involves various methods, such as bonding Nd\({}^{3+}\):YAG gain media to saturable absorbing crystals, including Cr\({}^{4+}\):YAG [18]. Alternatively, one material can be grown on the other to form a composite crystal [19], or a saturable absorbing material can be coated onto the gain medium [20]. The composite structure comprising Nd\({}^{3+}\):YAG and Cr\({}^{4+}\):YAG is characterized by its simplicity, good thermal stability, and extended service life, rendering it a high-quality passive Q-switched laser crystal [21]. To leverage the absorption characteristics of Cr\({}^{4+}\):YAG at 1 \(\upmu\)m, meticulous polishing of two planes perpendicular to the optical axis is necessary during production. The pumping-side surface requires coating with a dielectric material to facilitate pump light transmission and exhibit high reflectivity at the oscillation wavelength, ensuring the efficient absorption of pump light and the effective operation of the laser crystal at the desired wavelength. This methodical preparation enhances the performance and reliability of the laser crystal, making it a valuable component for passive Q-switching in laser systems. Leveraging the operational characteristics of the semiconductor laser and laser crystal, a meticulously designed microchip laser structure that meets the specified requirements has been crafted, as shown in Figure 3. The pumping source and laser crystal find their place on a baseplate crafted from oxygen-free copper, serving for collimation and alignment while facilitating rapid temperature conduction.