# High-Power, High-Beam-Quality, Long-Pulse-Width 532 nm Laser Based on a 4f Optical System
## 1 Introduction  
High-power 532 nm lasers are widely used in laser machining [1], medical laser treatments [2], and laser displays [3] due to their high efficiency, stability, and output energy. For specific applications, such as laser welding, adjustments and optimizations based on the material properties are necessary to achieve the best welding results. By adjusting the laser's output pulse width, the degree of material melting and heating depth can be controlled, allowing for more precise parameter control [4]. In laser medical applications, the differences in skin tissue composition and characteristics require the selection of an appropriate laser output pulse width based on the patient's skin type and condition [5]. Therefore, researching long-pulse all-solid-state 532 nm lasers is of great significance. In Q-switched lasers, the pulse width is determined by parameters such as gain, loss, and cavity length. The most effective method for tuning pulse width in such lasers is to adjust the cavity length, thereby increasing the output laser pulse width and achieving tunable pulse widths. In 2002, [PERSON] from NASA's Glenn Research Center used three ring cavities to achieve simple passive long nanosecond pulse laser stretching, converting input laser pulses of 2.8 ns to output pulses of 75 ns with a conversion efficiency of 83% [6]. In 2018, [PERSON] and colleagues from Andric Powerlase in the UK proposed a 400 W Q-switched 532 nm laser that controls pulse width and pulse shape by varying the repetition frequency and trigger delay of each cavity's Q-switch, as well as adjusting the delay and attenuation of each cavity. By extending the cavity length, the pulse duration was increased from 66 ns to 140 ns at a repetition rate of 8 kHz without significant power loss, though thebeam quality was poorer with an M\({}^{2}\) < 30 [7]. In 2019, [PERSON] and colleagues from the Institute of Physical Chemistry, Chinese Academy of Sciences, used a multi-pass cavity (MPC) combined with pump power technology to achieve adjustable pulse widths ranging from 160 to 1000 ns at a repetition rate of 10 kHz. At a pulse width of 560 ns, they achieved an output power of 10.6 W with a beam quality factor of M\({}^{2}\) = 1.45 [8]. Nd:YAG is one of the most commonly used solid-state laser materials. Due to the excellent match between Nd:YAG's activation ions and the LD (laser diode) emission spectrum, along with its narrow fluorescence line and good physical and mechanical properties, it is highly favorable for laser generation. Using Nd:YAG as the laser medium allows for high-gain, low-threshold lasers and is suitable for high-energy pumping, which helps in achieving high optical quality. Consequently, Nd:YAG lasers pumped by an LD are the most common DPL devices [9; 10; 11]. In the frequency doubling of Nd:YAG lasers, KTP (Potassium Titanyl Phosphate) and LBO (Lithium Triborate) crystals are the most frequently used doubling crystals. KTP crystals are widely used due to their excellent properties and large non-linear coefficient. However, KTP crystals have some limitations, including a relatively low damage threshold, making them susceptible to damage under high cavity power density. Additionally, after long-term operation, KTP crystals may develop gray spots, increasing loss and reducing laser output power. In contrast, LBO crystals have a significantly higher damage threshold, making them better suited for high-power lasers and reducing the risk of crystal damage. Moreover, LBO crystals have lower hygroscopicity and are less prone to issues like gray spots. Thus, in high-power green laser systems, selecting LBO crystals for frequency doubling can effectively enhance the performance and reliability of the laser [12]. The q-parameter of a Gaussian beam is used to describe the physical properties of the beam during propagation, including amplitude distribution, phase distribution, and spot size. As the Gaussian beam propagates through space, its q-parameter changes accordingly.