The robustness of spatial modes of light for long distance free-space propagation

Abstract

Turbulence remains a major challenge for applications that rely on laser beam transmission through the atmosphere. The properties of laser beams are susceptible to degradation due to variations in the refractive index structure mostly as a result of temperature

uctuations. Spatial uctuations in the refractive index induce phase distortions, which subsequently modify the propagation of the laser beam through the atmosphere. The modi cation can be ascribed to three detrimental e ects; beam wander, spreading and scintillation. Beam wander causes de ections in the beam's centroid and can result in the beam missing the target. It occurs primarily due to large-scale uctuations such as atmospheric tip-tilt aberrations. Beam spreading, on the other hand, results from small-scale uctuations, which causes the beam power to distribute over a large surface area. This spread reduces the intensity and power that is received by optical systems. Finally, scintillation causes temporal and spatial uctuations in the beam's intensity. It is therefore imperative to constantly look for ways to optimize the performance of beams as they traverse through inhomogeneous media. In this work, we investigate the feasibility of using spatial modes of light as alternative optical beam sources for use in long distance laser propagation applications. We exploit novel properties of various spatial mode sets such as Hermite-Gaussian, Laguerre- Gaussian and Bessel-Gaussian modes to determine the robustness of each mode in improving the performance of free-space laser propagation systems. We show that Hermite- Gaussian modes are more resilient to lateral displacements such as tip/tilt aberrations in optical systems when compared to modes carrying orbital angular momentum due to their Cartesian symmetry. It is also shown that over a normal distribution of lateral displacements, the Hermite-Gaussian modes experience less mode-crosstalk and less mode-dependent losses.We also study the self-healing behaviour of Bessel beams by propagating them through aberrated obstacles and show that the self-healing is not guaranteed, but rather a function of the severity of the aberration. The modes are then passed through laboratory simulated turbulence determined by combination of appropriately weighted Zernike aberrations to show that the modes are not resilient to such perturbations as previously claimed. Finally, we develop a real-world turbulence link and propagate modi ed Bessel beams which preserve the Bessel structure for much longer distance compared to the traditional Bessel beams. These modes show comparable performance to Gaussian modes with the added bene t of self-healing associated with Bessel beams. From this research, we can deduce that various properties of spatial modes can be meticulously tailored to mitigate some of the detrimental e ects associated with laser beam transmission through turbulence. This can pave way for e cient transmission to much longer distances which remain challenging to this day such as ranging to satellites and the Moon.