Study on Characteristics of Q-switched Vortex Solid-state Laser with Spiral Chirality Controlled by Injection Seeding

Optical vortex beam has wide application prospect in areas such as optical communication, lidar detection and optical trapping. To increase the operating distance, a high-power vortex laser source is necessary in these applications. Control of the spiral chirality of the Laguerre-Gaussian (LG) mode has become a key problem in Q-switched pulsed solid-state vortex lasers. We put forward the injection seeding method to control the spiral chirality of the LG mode in Q-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 optical field has the same spiral chirality with the injected beam will surpass the optical field has the opposite spiral chirality and the chirality purity increases as the injected power increasing. The threshold injected signal-to-noise ratio increases with the angular order of the LG mode, this is due to the decreasing of the overlapping of the standing wave pattern of the opposite chirality beam. The threshold injected signal-to-noise ratio also increases with the pumping power and the reflectivity of the output mirror. The ratio of the pulse energy under injection to the pulse energy under free running decreases with the rising angular order. This ratio increases with the rising pumping power and decreases with the increasing reflectivity of the output mirror. The seeding beam, which is generated by spiral phase modulation on the fundamental mode beam, always has a wide radial spectrum. The radial spectrum of the beam generated by second order spiral phase modulation on the fundamental mode beam is shown in Fig. (b). Under proper ring width of the pumping beam, this radial spectrum can be purified in the Q-switched laser cavity, as is shown in Fig. (c). Therefore, the spiral phase modulated beam can be used as the seeding source to generate high-purity vortex pulse.