Inhibiting Effect of Topological Charge on Laser-Induced Breakdown of Gases: Miniaturization of an SRS Laser with Vortex Raman Outputs

Stimulated Raman scattering (SRS) lasers equipped with bulky gas cells have been widely criticized. The special "donut″-shaped intensity distribution of vortex light (VL) is expected to reduce the pump laser intensity, suppress laser-induced breakdown, and fulfill the laser miniaturization. In this work, spiral phase plates of different topological charges (l = 1,2,4) have been employed to convert the l = 0 flattened Gaussian beam (FGB) into VLs, which served as pump sources. Lenses with focal lengths of F = 1000, 400, and 200 mm were chosen and coupled with Raman cells of L = 1.8, 0.9, and 0.23 m, respectively, to investigate the effects of l on photon conversion efficiency (PCE), beam quality, and the output orbital angular momentums (OAMs). The optimal l under different focusing parameters was proved to be dependent on the pump energy and gas pressure. By focusing the l = +4 laser via an F = 200 mm lens into a L = 0.23 m Raman cell, a first-order Stokes (S1) PCE of up to 61.5% was achieved at a pump energy of 62.1 mJ, which is comparable with the 66.4% obtained with an F = 1000 mm lens focusing the l = 0 FGB in a L = 1.8 m cell. Next, the S1 Raman beam quality at high pressure was verified to be significantly improved from βo0 = 6.3 (l = 0) to βo2 = 3.6 (l = 2) at 0.8 MPa H2. Furthermore, moduli and signs of average Raman OAMs were visualized with an F = 1.5 m cylindrical lens, and the results verified the law of OAM conservation in our SRS scheme.