The orbital angular momentum of a turbulent atmosphere and its impact on propagating structured light fields

Abstract

When structured light is propagated through the atmosphere, turbulence results in modal scattering and distortions. An extensively studied example is that of light carrying orbital angular momentum (OAM), where the atmosphere is treated as a phase distortion and numerical tools extract the resulting modal cross-talk. This approach focuses on the light itself, perturbed by the atmosphere, yet does not easily lend itself to physical insights, and fails to ask a pertinent question: where did the OAM that the beam gained or lost come from? Here, we address this by forgoing the beam and instead calculating the OAM of the atmosphere itself. With this intuitive model we are able to draw general conclusions on the impact of atmospheric turbulence on OAM beams, which we confirm experimentally. Our work alters the perspective on this problem, opening new insights into the physics of OAM in turbulence, and is easily extended to other structured light fields through arbitrary aberrations.

Universal crosstalk of twisted light in random media

“…bottom left plots in Fig. 3(a) and (b)], consistent with analytic findings for Kolmogorov turbulence [34]. Furthermore, the numerical results are in agreement with experimental data (circles in Fig.…”

“…Crosstalk among pure OAM modes [top left plots in Fig. 3(a) and (b)] is distributed diagonally, evidence of the absence of coherent coupling between modes with opposite OAM, confirming previous findings [34]. The remaining phase information expressed by the diagonal's broadening is due to the disorder-induced coupling to neighboring modes = ± 1, 2... On the other hand, pure amplitude modes [top right plots in Fig.…”

“…Consequently, higher transmission fidelities correspond to weaker crosstalk, i.e., to matrices C , (L) with prevailing diagonals. Figure 1(c) sketches the crosstalk amplitudes for a fixed source LG mode with initial OAM = 3 after propagation through a channel of length L for three distortion regimes quantified by w. As observed previously [34], under weak distortion (top row) the detection probability of the original OAM = 3 (solid black line) is highest and crosstalk is limited to neighboring modes. In contrast, under strong distortion [bottom row of Fig.…”

Turbulence-resistant self-focusing vortex beams

“…This means that though it's a good idea to construct high dimensional Hilbert space by increasing the value of topological charge m, it may not be efficient in improving free space optical communication. In addition, it's to be noted that the OAM spectrum becomes asymmetric and shifts towards zero under increasing turbulence, which is due to the beam losing angular momentum to the turbulence [45]. Mathematically, this can be explained from the turbulence correlated coordinates a s and ξ s , as the shifts of the coordinates in x and y directions are asymmetric (see Supplement 1 for more information).…”

Robust structured light in atmospheric turbulence

“…Arguments have been put forward for one mode family being more robust than another, with studies covering Bessel-Gaussian [42][43][44][45][46][47][48][49][50][51][52][53], Hermite-Gaussian [54][55][56][57], Laguerre-Gaussian [58][59][60][61][62] and Ince-Gaussian [63] beams, with mixed and contradictory results. In the context of OAM, since the atmosphere itself can be thought of giving or taking OAM from the beam, it has been shown theoretically and experimentally that atmospheric turbulence distortions are independent of the original OAM mode [64], all susceptible to the deleterious effects of atmospheric turbulence, and indeed OAM has been suggested as not the ideal modal carrier through turbulence [65]. Vectorial structured light has been suggested to improve resilience because of the invariance of the polarisation degree of freedom, but numerous studies in turbulence [66][67][68][69][70][71][72][73] have been inconclusive, with some reporting that the vectorial structure is stable [67,68,73], and others not [69][70][71][72]74].…”