Designer vector beams maintaining a robust intensity profile on propagation through turbulence

Abstract

We report experiments on propagation of scalar and vector optical beams through random phase screens mimicking turbulence and show that the intensity profile of the beam containing a Cpoint polarization singularity shows maximally robust behavior. This observation is explained in terms of the polarization and orbital angular momentum (OAM) diversity in the beam. The l = 0 and l = 1 OAM states whose vector combination leads to the C-point singularity are seen to produce complementary speckle intensity patterns with significant negative correlation on propagation through a random phase screen. This unique property of C-point singularity makes it superior to other inhomogeneous polarization states as demonstrated in our experiments. The results provide an important generic guideline for designing beams that can maintain optimally robust beam intensity profile on passing through random phase fluctuations and are expected to have a number of applications.

Phase Singularities to Polarization Singularities

“…Irradiance [573] and scintillation [574,575] of radial/azimuthally polarized beams and Poincaré beams [576] propagating through atmosphere were examined. Use of C-point beams in turbulence-resistant robust beam generation has been proposed [577][578][579][580]. Reports on the use of polarization singularities for enhancing chiral light matter interaction are also there [88,[581][582][583].…”

Revealing the invariance of vectorial structured light in perturbing media

“…Careful inspection of the studies that report vectorial robustness in noisy channels reveal that the distances propagated were short and the strength of perturbation low, mimicking a phase-only near-field effect where indeed little change is expected, and hence these are not true tests for robustness or invariance. Studies that claim enhanced resilience of vector modes over distances comparable to the Rayleigh length [44,50] have used the variance in the field's intensity as a measure, a quantity that one would expect to be robust due to the fact that each polarisation component behaves independently and so will have a low covariance.…”

“…Given the importance of these structured light fields, much attention has focused on their propagation through optical systems that are paraxial [35], guided [36] and tight focussing [37,38], as well as in perturbing media such as turbid [39][40][41][42], turbulent [43][44][45][46][47][48][49][50] and underwater [51][52][53]. The conclusions are seemingly contradictory, with compelling evidence that the vectorial structure is stable, and equally compelling evidence that it is not, while the specific nature of each study prohibits making general conclusions on the robustness of vectorial light in arbitrary complex media.…”

Robust structured light in atmospheric turbulence

“…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]. Careful inspection of the studies that report vectorial robustness in noisy channels reveal that the distances propagated were short and the strength of perturbation low, mimicking a phase-only near-field effect where indeed little change is expected, and hence these are not true tests for robustness or invariance.…”