Advanced approaches to laser induced fluorescence for plasma diagnostic

Structured light,[1] with tailored spatial and spatiotemporal structures, such as orbital angular momentum, have led to innovations in several areas. Yet, its applications in plasma spectroscopy are scarce. This study unveils Annular Beam Confocal Laser-Induced Fluorescence (ABC-LIF), a novel application of structured light. Conventional LIF necessitates dual-sided plasma access for ion velocity distribution functions analysis, which is not always feasible. This issue is addressed by the confocal LIF configurations, of which ABC-LIF is a variant. ABC-LIF employs a Laguerre-Gaussian laser beam, produced via axicons, providing spatial resolution control through beam parameters and high signal-to-noise ratios (SNR). Achieving 5mm resolution at a 300mm focal distance, it has the potential to reach 1mm, akin to conventional LIF. ABC-LIF maintains a small depth of field comparable to large-diameter Gaussian beams, ensures spatial separation between fluorescence and laser lights, and resolves beam back reflection issues. The study further explores Wavelength Modulation (WM) spectroscopy with applications to LIF.[2] ABC-LIF and the WM approach were validated through argon ion VDF measurements in an industrial DC plasma source.

[1] A. Forbes, M. De Oliveira, and M.R. Dennis, Nat. Photonics 15, 253 (2021).

[2] E. I. Moses, C. L. Tang. Opt. Lett. 1, 115 (1977)