Free-Space Orbital Angular Momentum Comb Generation via Second-Harmonic Generation.

At the end of the last century, Allen et al. showed that light has angular orbital momentum in addition to spin. This discovery contributed to the active development of free-space optical communication. In the modern world, free-space optical communication is the main competitor to fiber optical communication lines. However, they are still inferior in throughput to fiber systems. In this work, we experimentally demonstrate the implementation of a free-space orbital angular momentum comb, as an analogue of a frequency comb for free-space optical communication. We present a simple approach to the generation of the orbital angular momentum comb, based on the generation of the second harmonic of the Hermite-Gauss mode with its subsequent conversion into the orbital comb of the Laguerre-Gauss mode. Our results are supported by analytical theory simulations and experimental evidence.