Atmospheric turbulence causes fluctuations in the local refractive index of air that accumulatively disturb a waves phase and amplitude distribution as it propagates. This impairs the effective range of laser weapons as well as the performance of fre
e space optical (FSO) communication systems. Adaptive optics (AO) can be applied to effectively correct wavefront distortions in weak turbulence situations. However, in strong or deep turbulence, where scintillation and beam breakup are common phenomena, traditional wavefront sensing techniques such as the use of Shack-Hartmann sensors lead to incorrect results. Consequently, the performance of AO systems will be greatly compromised. We propose a new approach that can determine the major phase distortions in a beam instantaneously and guide an AO device to compensate for the phase distortion in a few iterations. In our approach, we use a plenoptic wavefront sensor to image the distorted beam into its 4D phase space. A fast reconstruction algorithm based on graph theory is applied to recognize the phase distortion of a laser beam and command the AO device to perform phase compensation. As a result, we show in our experiments that an arbitrary phase distortion with peak to peak value up to 22{pi} can be corrected within a few iteration steps. Scintillation and branch point problems are smartly avoided by the plenoptic sensor and its fast reconstruction algorithm. In this article, we will demonstrate the function of the plenoptic sensor, the fast reconstruction algorithm as well as the beam correction improvements when our approach is applied to an AO system.
We have designed a plenoptic sensor to retrieve phase and amplitude changes resulting from a laser beams propagation through atmospheric turbulence. Compared with the commonly restricted domain of (-pi, pi) in phase reconstruction by interferometers,
the reconstructed phase obtained by the plenoptic sensors can be continuous up to a multiple of 2pi. When compared with conventional Shack-Hartmann sensors, ambiguities caused by interference or low intensity, such as branch points and branch cuts, are less likely to happen and can be adaptively avoided by our reconstruction algorithm. In the design of our plenoptic sensor, we modified the fundamental structure of a light field camera into a mini Keplerian telescope array by accurately cascading the back focal plane of its object lens with a microlens arrays front focal plane and matching the numerical aperture of both components. Unlike light field cameras designed for incoherent imaging purposes, our plenoptic sensor operates on the complex amplitude of the incident beam and distributes it into a matrix of images that are simpler and less subject to interference than a global image of the beam. Then, with the proposed reconstruction algorithms, the plenoptic sensor is able to reconstruct the wavefront and a phase screen at an appropriate depth in the field that causes the equivalent distortion on the beam. The reconstructed results can be used to guide adaptive optics systems in directing beam propagation through atmospheric turbulence. In this paper we will show the theoretical analysis and experimental results obtained with the plenoptic sensor and its reconstruction algorithms.
Inspired by the analysis of several empirical online social networks, we propose a simple reaction-diffusion-like coevolving model, in which individuals are activated to create links based on their states, influenced by local dynamics and their own i
ntention. It is shown that the model can reproduce the remarkable properties observed in empirical online social networks; in particular, the assortative coefficients are neutral or negative, and the power law exponents are smaller than 2. Moreover, we demonstrate that, under appropriate conditions, the model network naturally makes transition(s) from assortative to disassortative, and from sparse to dense in their characteristics. The model is useful in understanding the formation and evolution of online social networks.
Do scientists follow hot topics in their scientific investigations? In this paper, by performing analysis to papers published in the American Physical Society (APS) Physical Review journals, it is found that papers are more likely to be attracted by
hot fields, where the hotness of a field is measured by the number of papers belonging to the field. This indicates that scientists generally do follow hot topics. However, there are qualitative differences among scientists from various countries, among research works regarding different number of authors, different number of affiliations and different number of references. These observations could be valuable for policy makers when deciding research funding and also for individual researchers when searching for scientific projects.
