Optical communication technology is the future direction for realizing high-speed broadband networks in space. However, the performance of optical communication will be limited by atmospheric turbulence in the pathway. Adaptive optics have been proved effective in suppressing atmospheric turbulence, but adaptive optics have reduced efficacy in strong atmospheric turbulence due to light intensity scintillation. When light intensity scintillates, current wavefront recovery algorithms will have reduced spatial resolution and poor wavefront recovery accuracy. In this paper, a novel wavefront recovery algorithm is proposed to set up a light gatherer for every sub-aperture of the Schack-Hartmann wavefront sensor influenced by light intensity scintillation. Distributed light gatherer is set for every sub-aperture with insufficient light intensity, and gatherer will keep ADU accumulation until the set accuracy threshold is satisfied. Then, use mode method to get the distribution of aberration phase. This reconstruction algorithm is numerically simulated and experimentally verified under the condition of light intensity scintillation. The results show that, when the atmospheric turbulence has a scintillation factor of 0.50, the recovered wavefront processed by the distributed light gathering method is significantly improved compared with the results of the conventional recovery algorithm: the wavefront recovery error is reduced from 1.34λ to 0.82λ, which is 0.52λ less, and the accuracy effect is improved by 39%. The proposed algorithm can effectively enhance the accuracy of center-of-mass computation under the light intensity scintillation condition caused by atmospheric turbulence and improve the performance of adaptive optics system.
Using the optical vortex beams (OVBs) can further improve the communication rate and spectrum utilization of the conventional space optical communication systems. In order to suppress the atmospheric turbulence effects in the space OVBs communication links, as well as to make the communication links simpler. We introduced the pin-like optical vortex beams (POVBs) that can dynamically self-focus during the free-space propagation. The POVBs have ability to counteract diffraction effects and atmospheric turbulence effects, and can maintain stable propagation with constant shape over long distances in the free-space. We theoretically analyzed the POVBs and experimentally demonstrated their stable propagation performance in the simulated atmospheric turbulence.
We studied ring width of the Laguerre–Gaussian vortex beams, which is the outer radius minus the inner radius. Combining theoretical analysis and numerical simulation, the approximate expression of the ring width is obtained. From the expression, we find that the ring width is independent of the topological charge, and increases with the increase of the waist radius at z=0 of the host Gaussian beam and the propagation distance. This research has good application prospects in optical tweezers technology and OAM optical communication.
We presented a new method to measure the topological charge (TC) of the vortex beam (VB) by use a Shack-Hartmann wavefront sensor with sector microlens array (SMLA). The SMLA is circularly symmetric, which perfectly corresponds to helical phase and ring intensity of the VB. On the focal plane, there are spot array with circular distribution due to the special arrangement of the SMLA. In this letter, we also analyzed the error, and concluded that the error of the outer ring is smaller. As a result, we only employed the data of the outermost ring. This method easily can obtain the sign and magnitude of the TC by calculating offset direction and offset of the spots respectively. The simulation results show that our method can determine large TCs and has high accuracy. While this method has a simple structure and can be used in optical communications in the future.
We analyze the topological charge of a Laguerre-Gaussian(LG) vortex beam transmitted in atmospheric turbulence. The analytic expression of the topological charge detection probability is derived to analyze the influences of turbulence and beam parameters. This theoretical model is applied to illustrate the OAM modal crosstalk of a LG vortex beam propagating in atmospheric turbulence. The scattering width of topological charge is defined, and its analytic expression is derived. The relationship between the refractive index structure constant of atmospheric turbulence and scattering width has been studied. Furthermore, the linear expression of the scattering variance, coherence parameter and the original topological charge of the vortex beam are obtained. In order to reduce the bit error rate (BER) caused by crosstalk and improve the performance of the communication system, we provide a theoretical basis for the selection of the topological charge spacing and the appropriate beam waist in different turbulence.
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