In the first case, only phase distortions caused by atmospheric turbulence are studied. 1(a)) (ii) compensation for a distorted link with both atmospheric turbulence and obstruction ( Fig. Two cases are considered here: (i) compensation for a distorted link with only atmospheric turbulence ( Fig. The compensation effects on inter-channel crosstalk and bit-error rate (BER) performance of a two Bessel beams multiplexing system each carrying a 10-Gbaud (40-Gbit/s) 16-ary quadrature amplitude modulation (16-QAM) data are studied.įigure 1 shows the block diagram of turbulence compensation for distorted Bessel beam propagation links. Using this scheme, we experimentally demonstrate compensation of multiple Bessel beams through emulated atmospheric turbulence and obstruction. By measuring the phase distortions of a probe Gaussian beam through a wavefront sensor (WFS), correction masks are created and loaded to another SLM to compensate the distortions of multiplexed Bessel beams. A spatial light modulator (SLM) loaded with Kolmogorov phase mask added by an obstruction phase mask is used to simultaneously emulate atmospheric turbulence and obstruction in the laboratory environment. In this paper, we propose an obstruction- and turbulence-tolerant FSO OAM multiplexing link by using self-healing Bessel beams assisted by adaptive compensation techniques. However, so far there has been limited report in studying the performance of Bessel beam multiplexing link through atmospheric turbulence and compensation block. In order to mitigate the influence of turbulence, adaptive optics based compensation techniques have been proposed for OAM-carrying LG beam multiplexing systems, showing favorable performance 19, 20, 21. Although OAM-carrying Bessel beam can reconstruct itself after encountering obstruction, the strong turbulence will also impair the nondiffracting property causing phase and intensity distortions. Moreover, for OAM multiplexing systems, the turbulence may also cause crosstalk between multiple channels, resulting in significant performance degradation of signal 18.
These fluctuations can lead to an increase in the link error probability, limiting the performance of communications systems 17. In FSO links, a critical challenge is the atmospheric turbulence which will cause random fluctuations in amplitude and phase of the laser beams. Very recently, by using the self-healing mechanism (reconstruct itself after encountering an obstruction) and orthogonality of Bessel beams, it has shown the possibility to establish the obstruction-free or obstruction-tolerant FSO OAM communication links 15, 16. In particular, Bessel beams having the property of propagation-invariant or diffraction-free have generated considerable interest 12, 13, 14. Although all these modal sets are orthogonal and complete, there exit differences between LG beams with other OAM modal sets which may lead to different potential applications. However, the LG beam is not the only example of OAM-carrying beams Bessel beams, Mathieu beams, and Ince–Gaussian beams can also carry orbital angular momentum 11.
Most attention in the literature regarding OAM-carrying beams has been dedicated to LG beams. Typically, a well-known representation of helically phased beam is the so-called Laguerre–Gaussian (LG) laser beam 10. In particular, one such set is orbital angular momentum (OAM) 8.Īn OAM-carrying light beam is featured by a helical phase front of exp ( ilφ), where l is the topological charge number and φ refers to the azimuth angle 9. Several different types of orthogonal modal basis sets have been presented as potential candidates for FSO systems. Very recently, using orthogonal spatial modes in FSO systems has shown the potential to tremendously increase system capacity and spectral efficiency by multiplexing data-carrying beams in the same spectral band 3, 4, 5, 6, 7. Favored by bandwidth, spectrum and security issues, it is expected that FSO links can be widely used in the future military and civilian networks, such as communications between spacecraft, temporary network installation for events or other purpose as disaster recovery, up-and-down links between space platforms and ground platforms, deep-space probes, Last-Mile access, and security applications 2. Over the past decades, free-space optical communication (FSO) which provides an easy means of high-bit-rate communications has attracted more and more interest as an adjunct or alternative to radio relay link line of sight (LOS) communications 1.
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