# High-Power, High-Beam-Quality, Long-Pulse-Width 532 nm Laser Based on a 4f Optical System
## 3 Experimental Setup  
The device diagram of the 532 nm acousto-optic Q-switched laser is shown in Figure 2. The laser head is an Nd:YAG module pumped by a LD, with Nd\({}^{3+}\) ion doping concentration of 0.6 at%, and the crystal dimensions are \(\Phi\)4 \(\times\) 100 mm. The structure of LD pump is a 5-way ring pump, with 5 bars in each direction and a maximum power of 40 W per bar. The end faces of the laser crystal are coated with an anti-reflection film for 1064 nm to reduce Fresnel reflection. To ensure temperature stability of the Nd:YAG crystal, the laser pumping module uses a 5.2 kW water chiller for cooling, with the cooling water temperature set to 25 \({}^{\circ}\)C. An acousto-optic Q-switch (QS27-5C4G-US-ST1) manufactured by GOOCH & HOUSEGO (Ilminster, UK) is used. The working medium is molten silicon, and the crystal surface is coated with multiple layers of dielectric hard film. This film not only increases the transmittance at 1064 nm to over 99.6%, but also raises the damage threshold (>1 GW/cm\({}^{2}\)), effectively protecting the crystal from high-power laser damage. Additionally, this design ensures that the insertion loss remains at a low level (<6%), which is crucial for achieving high-power pulsed lasers. The device's overheat protection mechanism is also noteworthy, with a protection threshold set at 55 \({}^{\circ}\)C, ensuring stable operation within a safe temperature range. Two acousto-optic Q-switches are placed perpendicularly on either side of the Nd:YAG laser module. This arrangement ensures precise matching of the acoustic and oscillation beam apertures and effectively achieves precise laser shutdown under high-power conditions by enhancing the acousto-optic diffraction capability. To ensure signal synchronization between the two acousto-optic Q-switches, they are driven by the same acousto-optic driver, which is crucial for stable laser output. The LBO frequency doubling crystal has dimensions of 4 \(\times\) 4 \(\times\) 16 mm\({}^{3}\). To improve the frequency doubling efficiency, the LBO crystal is placed close to the M\({}_{3}\) position. To optimize the temperature control of the crystal, the LBO crystal is placed in a copper heat sink with a water-cooling channel and wrapped with indium foil to ensure comprehensive contact between the crystal and the heat sink for effective heat dissipation. A thermistor is installed at one end of the heat sink near the crystal to monitor the temperature in real time. The LBO crystal is precisely temperature-controlled by a Thermo Electric Cooler (TEC), with temperature fluctuations maintained within \(\pm\)0.1 \({}^{\circ}\)C, which effectively enhances the frequency doubling efficiency and stability of the laser. After a series of tests, it was found that the laser output power reaches its peak at a crystal temperature of 27 \({}^{\circ}\)C, so the TEC temperature is set to 27 \({}^{\circ}\)C. During the experiment, the angle and position of the LBO crystal are continuously adjusted to achieve optimal phase matching and maximize laser output. The mirror M\({}_{1}\) is a plane mirror with a surface coating of high-reflectivity film for 1064 nm (R \(>\) 99.5%@1064 nm). The mirror M\({}_{3}\) is a plane output mirror, with high-transmission coatings for 532 nm and high-reflectivity coatings for 1064 nm on both surfaces (T \(>\) 99%@532 nm, R \(>\) 99.5%@1064 nm). The mirror M\({}_{2}\) is a plane dichroic mirror with a high-reflectivity coating for 532 nm and a high-transmission coating for 1064 nm on the surface near the LBO crystal (R \(>\) 99.5%@6532 nm, T \(>\) 95%@1064 nm), and the other surface is coated with an anti-reflection film for 1064 nm. The function of M\({}_{2}\) is to allow the fundamental frequency light to pass through the frequency doubling crystal twice, effectively improving the frequency doubling efficiency. And a sub-cavity for frequency doubling is formed between M\({}_{2}\) and M\({}_{3}\).