Objective Passively Q-switched laser is widely used in laser ignition, laser processing, nonlinear frequency conversion, and other fields owing to its short pulse width, high peak power, and miniaturization. A composite crystal can further shorten the length of the cavity to compress the pulse width, reduce the loss, eliminate the air gap, and avoid the damage to crystal surface coating caused by air-break. Although the heat can be dissipated via the end face YAG, strong thermal effect remains inside the Yb∶YAG/Cr∶YAG/YAG composite crystal. With the increase in temperature, the number of particles in the lower level of Yb∶YAG increases, augmenting the self-absorption effect of Yb∶YAG at wavelength of 1030 nm, resulting in its low efficiency. In addition, Yb, Cr∶YAG dielectric layer, which has detrimental effect on the laser output performance, will be formed in the center of the composite crystal owing to the preparation technology. The present study proposes a four-layer-bonded composite crystal, Yb∶YAG/YAG/Cr∶YAG/YAG. Among these layers, the outermost one, YAG, plays the role of cooler. YAG as the intermediate layer can dissipate heat and reduce the thermal effect, thus reducing the self-absorption effect of Yb∶YAG, as well as avoid the dielectric layer produced between Yb∶YAG and Cr∶YAG. Experimentally, the good output parameters can be obtained by the Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal.
Methods In the present study, passively Q-switched lasers based on Yb∶YAG/YAG/Cr∶YAG/YAG and Yb∶YAG/Cr∶YAG/YAG composite crystals are studied. First, the optimal transmittance of output coupler (OC) is selected to achieve the highest output power. Then, the effect of pump beam radius on the output power is investigated using different collimation-focus ratios. Thereafter, the relationship among pump power of the two types of passively Q-switched lasers, repetition frequency, pulse width, single pulse energy, peak power, and spectrum are studied by optimizing the OC and pump spot. Furthermore, the quality factors (M2) of the laser beams with the two kinds of crystals are measured.
Results and Discussions The optimal transmittance of OC for passively Q-switched lasers based on Yb∶YAG/YAG/Cr∶YAG/YAG and Yb∶YAG/Cr∶YAG/YAG composite crystals is 50%, and the output power increases approximately linearly with the increase of pump power. Passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal acquires a higher output power of 7.41 W and the slope efficiency of 57% (Fig. 2). When using 1∶2 collimation-focus ratio (diameter of pump spot is 210 μm), the maximum output power is achieved and the output power of passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal increases more rapidly as the diameter of pump spot increases as compared with that of passively Q-switched laser based on Yb∶YAG/Cr∶YAG/YAG composite crystal (Fig. 3). At the maximum pump power (17 W), the repetition rate, pulse width, single pulse energy, and peak power of passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal are 25.5 kHz, 4.6 ns, 290.6 μJ, and 63.2 kW, respectively. Overall, compared with passively Q-switched laser based on Yb∶YAG/Cr∶YAG/YAG composite crystal, passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal has the lower repetition rate and pulse width, higher single pulse energy and peak power (Fig. 4), and better beam quality at the same output power (Fig. 8).
Conclusions We experimentally demonstrate a highly efficient passively Q-switched laser with Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal pumped by a LD with a wavelength of 940 nm. The output performances are investigated based on Yb∶YAG/YAG/Cr∶YAG/YAG and Yb∶YAG/Cr∶YAG/YAG composite crystals with different transmittances of OC and diameters of pump spot. Based on the Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal, the maximum average output power of 7.41 W is obtained at the pump power of 17 W. The slope efficiency is 57.0%, while the optical-to-optical efficiency is 43.6%. The laser pulse has a pulse width of 4.6 ns, a repetition rate of 25.5 kHz, pulse energy of 290.6 μJ , and peak power of 63.2 kW. Compared with the passively Q-switched laser based on of Yb∶YAG/Cr∶YAG/YAG composite crystal, Output power, pulse energy, and peak power of passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal are raised by 5.6%, 14.7%, and 22.2%, respectively. In the meantime, the M2factors of passively Q-switched lasers based on two kinds of composite crystals at the same output power are investigated, the passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal has a lower M2 factor when producing the same amount of power. By comparing the experimental results, passively Q-switched laser based on Yb∶YAG/YAG/Cr∶YAG/YAG composite crystal can achieve better laser output parameters.
