Characteristics of <i>Q</i>-switched vortex solid-state laser with spiral chirality controlled by injection seeding

Optical vortex beam has wide application prospect in fields such as optical communication, lidar detection and optical trapping. To increase the operating distance, a high-power vortex laser source are required in these applications. Control of the spiral chirality of the Laguerre-Gaussian (LG) mode has become a key problem in <i>Q</i>-switched pulsed solid-state vortex lasers. In this work, we present an injection seeding method to control the spiral chirality of the LG mode in <i>Q</i>-switched laser cavity. The schematic of the method is shown in Fig. (a). A small power continuous wave vortex beam with determined chirality is injected into the laser cavity, with the gain medium pumped by a ring-shaped beam. The light field with the same spiral chirality as the injected beam will exceed the light field with the opposite spiral chirality, and the chirality purity will increase as the injected power increases. The threshold injected signal-to-noise ratio increases with the angular order of the LG mode increasing, this is due to the reduced overlap of the standing wave patterns of the opposite chiral beams. The signal-to-noise ratio of threshold injection also increases as the pumping power and the reflectivity of the output mirror increase. The ratio of the pulse energy under injection to the pulse energy under free running decreases with the angular order rising. This ratio increases with the pumping power rising, and decreases with the reflectivity of the output mirror increasing. The seeding beam generated by spiral phase modulation of the fundamental mode beam always has a wide radial spectrum. The radial spectrum of the beam generated by second order spiral phase modulation of the fundamental mode beam is shown in Fig. (b). Under an appropriate ring width of the pumping beam, this radial spectrum can be purified in the <i>Q</i>-switched laser cavity as shown in Fig. (c). Therefore, the spiral phase modulated beam can be used as a seeding source to generate high-purity vortex pulse.