# Passively Q-Switched Operation of a Tm,Ho:LuVO\({}_{4}\) Laser with a Graphene Saturable Absorber
## 2 Experimental Setup  
The experimental setup for the PQS mode operation of a Tm,Ho:LuVO\({}_{4}\) laser is shown in Figure 1. A \({}^{5}\)\(I_{7}\)\(\rightarrow\)\({}^{5}\)\(I_{8}\) laser transition of Ho\({}^{3+}\) in a Tm,Ho:LuVO\({}_{4}\) crystal was used to achieve a laser emission with a 2 \(\upmu\)m wavelength range. A Tm,Ho:LuVO\({}_{4}\) crystal was used in a resonator cavity with a 150-mm physical cavity length, and the physical length from a dichroic mirror (M5) to a 300-mm concave radius mirror (M6) was 60 mm. An LD (nLight Corp., Vancouver, WA, USA, NL-PPS50-10027) with a center output wavelength of 798.6 nm, corresponding to the output power of 13 W, was used as the pump source for the Tm,Ho:LuVO\({}_{4}\) laser. The temperature of the LD was selected at 298.15 K in the experiment, and the output power of LD was coupled to a fiber (a core diameter was 400 \(\upmu\)m and numerical aperture (NA) was 0.22). Tm,Ho:LuVO\({}_{4}\) with the mass percentages of 5% and 0.5% for Tm and Ho was cut along _a_-axis, of which the dimension was 4 mm\(\times\)4 mm\(\times\)8 mm. The end-faces of the laser crystal were coated at pump wavelengths from 790 to 810 nm and its laser wavelength range from 1.9 to 2.2 \(\upmu\)m with a transmission efficiency over 99.5% was used. The laser crystal was cooled at 77 K. In addition, the laser crystal was located in the middle of M5 and M6.  
A configuration with double-end pumping was chosen to relieve the thermal loading of the laser crystal. The pumped laser from the LD was divided into two beams after being collimated by a collimation lens (L1) and a split light mirror (M1). Then, One of the divided pumped laser was refocused on one end-face of the laser crystal by a focusing lens (L2), and the other pumped laser was reflected by the mirrors (M2, M3 and M4) and refocused on one end-face of the laser crystal by a focusing lens (L3). The focal lengths of L1, L2 and L3 were 25, 50 and 50 mm, respectively, and the pump spots with 800 \(\upmu\)m in diameter were placed at the input surfaces of the laser crystal. A flat 45\({}^{\circ}\) mirror was used as a splitting light mirror (M1), which was coated at 790-810 nm with a 50%-transmission-efficiency and 50%-reflection-efficiency material on one face and another high-transmission material (\(T>\) 99.0%) on the other face. Three flat 45\({}^{\circ}\) mirrors were used as reflection mirrors (M2, M3 and M4), which were coated at 790-810 nm with a high-reflectivity (\(R>\) 99.5) material on one face. A dichroic mirror (M5) was a plano mirror placed at 45\({}^{\circ}\) with respect to the incident light, and it was coated with a high-transmission material working at 798.6 nm on two faces and another high-reflection material working at 2000-2100 nm on one face. A concave mirror with a radius of 300 mm (M6) was coated with a high-transmission material working at 798.6 nm on two faces, and was coated at a high-reflectivity material working at 2000-2100 nm on its concave face. A plane mirror coated with a 2% or 5% transmittance at 2000-2100 nm was used as an output coupler (OC) mirror of the laser. The mirror made from a CaF\({}_{2}\) crystal was used as the substrate of the SA, and a graphene crystal was chosen as the material for the SA. The graphene material dissolved in ethyl alcohol was coated onto the surface of one face at the CaF\({}_{2}\) mirror with a spin coating machine (KW-4A, Chinese Academy of Sciences, Beijing, China).