# Four-Dimensional Thermal Analysis of 888 nm Pumped Nd:YVO4 Dual-Rod Acousto-Optic Q-Switched Laser
## 3 Experimental Setup  
The experimental setup of the dual-rod AO Q-switched resonator is shown in Figure 2. Two identical a-cut \(2\times 2\times 25\) mm\({}^{3}\) low doping concentration of 0.5 at.% Nd:YVO\({}_{4}\) crystals water-cooled by copper heat sinks were used as the gain medium. Both end faces of the crystals were dichroic anti-reflective (AR) coated at 1064 nm and 888 nm with transmissivity higher than 99.9% and 98%, respectively. The crystals were pumped at 888 nm (\({}^{4}l_{9/2}\rightarrow^{4}F_{3/2}\)) and lasing at 1064 nm (\({}^{4}F_{3/2}\rightarrow^{4}I_{11/2}\)) as a quasi-three-level system for reducing thermal effect. The dual-rod and dual-end pumped geometry was chosen to enhance the pump power. A symmetrical Z-shape cavity with the AOM at the center for intracavity Q-switching is designed to obtain similar mode matching in both crystals. The AOM is with module of Ante I-QS027-2C4G-U5-ST1. The fiber-coupled laser diode module of nLIGHT NL-P4-110-0888-2-A-R01 with numerical aperture of 0.22 and core diameter of 400 \(\upmu\)m was used in the experiment as the pump source. The temperature of the laser diode was controlled by the thermoelectric cooling module (TECM) and the center wavelength of the laser diode could be temperature-tuned by adjusting the temperature of the laser diode (LD) with TECM. The dichroic mirrors were AR coated at 888 nm and high-reflective (HR) coated at 1064 nm for lasers at the incidence angle of 22.5\({}^{\circ}\) with the transmissivity more than 98% and reflectivity higher than 99.9%, respectively. A set of near aberration-free lenses were used to image the pump mode from the fiber onto the Nd:YVO\({}_{4}\) crystal with the pump waist radius of about \(\omega_{\text{p0}}=450\)\(\upmu\)m. The pump waist is at a distance of about 1/3 crystal length from the incident end face for each crystal. A symmetric planar-planar resonator was used with the totally reflecting mirror M1 and the output coupler (OC) M2, which has the transmissivity of 57% at 1064 nm. Actually, our resonator is operating as a compound stable cavity due to the thermal compensation management of dual-crystal, which is designed for not only reaching a good mode matching to reach high power but also outputting a stable near TEM\({}_{00}\) mode. The transmissivity of the output coupler is also fully optimized, which is around the maximum output case and cavity stability cannot be broken. These ideas and designs are referred to in Refs. [1; 2]. The parameters used in our numerical simulations are listed in Table 1 corresponding to our experimental systems. The output power of the oscillator was measured by the OPHIR FL250A-LP1-DIF power meter. The SPIRICON M2-200s laser beam analyzer was used to measure the beam quality. In Q-switching operation, the pulse signal was detected by a high-speed silicon photoelectric detector as well as a Tektronix TDS 3202B digital phospnor oscilloscope with 300 MHz bandwidth. The thermal imager (FLIR: T420, TLIR-T62101, with a \(320\times 240\) imaging resolution is used to measure the average temperature on the surface of the crystal, the measured data of which is the important guide of our numerical simulations. Because the resolution of our thermal imaging is limited and the crystal is small, it is impossible to measure the detailed temperature distribution. In our experiment, we oriented to the crystal at a tiny angle and adjusted the thermal imaging to a proper measured distance that a resolution area can approximately cover the end face of crystal. The measured temperature is approximately the average temperature at the end surface of crystal within a tolerable error.