# Passively Q-Switched Operation of a Tm,Ho:LuVO\({}_{4}\) Laser with a Graphene Saturable Absorber
## 3 Experimental Results and Discussion  
Two kinds of OC mirrors (with transmittances of 2% and 5%) were chosen in CW and PQS mode operation of an _a_-cut Tm,Ho:LuVO\({}_{4}\) laser to achieved optimal output performances. In CW and PQS mode operations, the output power and the average output power of the laser are shown in Figure 2. Under the CW mode operation, output powers of 2310 and 2398 mW were achieved with 2% and 5% transmittances of the OC mirror, and optical-optical conversion efficiencies were 17.8% and 18.5%, respectively. In the PQS mode operation, a graphene SA mirror placed between M5 and OC mirror was located near the OC mirror. The transmittance of a few-layer graphene material at  
Figure 1: Experimental setup of the passively Q-switched Tm,Ho:LuVO\({}_{4}\) laser. 2075.72 nm was measured and found to have been approximately 88.6%, and the modulation depths were 69.0% (\(T\) = 2%) and 58.3% (\(T\)= 5%). The laser spot radii on the surface of M6, left end face of laser crystal, right end face of laser crystal, M5, graphene SA and the OC mirror were calculated to be approximately 445, 416, 395, 367, 310 and 310 \(\upmu\)m, respectively, by the ABCD matrix without considering the thermal lens effect. The average output powers of 604 and 1034 mW were achieved with 2% and 5% transmittances of the OC mirror, respectively, corresponding to optical-optical conversion efficiencies of 6.0% and 8.0%. In the PQS mode operation of Tm,Ho:LuVO\({}_{4}\) laser, the pump threshold power was approximately 5 W with a 2% or 5% transmittance of the OC mirror. The laser was operated under a stable condition, and the pump power of 10.03 W or 13 W was injected into the laser cavity with a 2% or 5% transmittance of the OC mirror to avoid the damage of the graphene SA. Compared with the high transmittance of the OC mirror, a low transmittance of OC was chosen to acquire a stable PQS mode operation, due to more energy being stored in the crystal, but the damage threshold of the SA was easily reached, and the graphene SA was damaged at a peak power density of approximately 0.019 MW/cm\({}^{2}\) at 2057.03 nm with a 2% transmittance of the OC mirror in the experiment. Under the PQS mode operation, the pulse repetition frequency (PRF) of the Tm,Ho:LuVO\({}_{4}\) laser is shown in Figure 3. As can be seen, the PRF increased with increasing pump power, and the highest PRFs were 35.92 kHz (\(T\) = 2%) and 54.5 kHz (\(T\) = 5%), respectively, corresponding to energies per pulse of 16.82 and 18.97 \(\upmu\)J. In addition, the maximum pulse energy of 40.4 \(\upmu\)J was achieved with a PRF of 24.01 kHz (\(T\) = 5%) and an average output power of 970 mW (\(T\) = 5%), corresponding a pulse width of 1.429 \(\upmu\)s. The pulse widths of Tm,Ho:LuVO\({}_{4}\) laser are shown in Figure 4, and the narrowest pulse width of 300 ns was acquired under a 2% transmittance of the OC mirror. The pulse widths were 2.521 and 8.250 \(\upmu\)s with the pump powers of 6.96 and 5.01 W, respectively for a 5% transmittance of the OC mirror. With a detector (Thorlabs, Newton, NJ, USA, PDA10 PT-EC) and an oscilloscope with a bandwidth of 1 GHz (Tektronix, Beaverton, OR, USA, DPO4104), the typical Q-switched pulse trains were recorded in 40 and 200 \(\upmu\)s time scales (Figure 5). Therefore, the highest pulse energy and the peak power of the Tm,Ho:LuVO\({}_{4}\) laser were calculated to be 40.4 \(\upmu\)J and 56.07 W based on the findings in Figure 2.  
A 721A IR laser wavelength meter with a measuring range of 1300-5000 nm (Bristol Instruments Inc., Victor, NY, USA) was used to obtain an output wavelength of Tm,Ho:LuVO\({}_{4}\) laser in CW and PQS modes operations (Figure 6). An output wavelength of 2075.72 nm was achieved in the CW mode operation, and 2057.03 nm was achieved in the PQS mode operation. Figure 2: Output power of Tm,Ho:LuVO\({}_{4}\) laser in CW and PQS mode operations verse the pump power. Figure 4: Pulse widths of Tm,Ho:LuVO4 laser verse the pump power. Figure 5: The pulse trains in 40 \(\upmu\)s and 200 \(\upmu\)s time scales.