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Introduction
Orbital angular momentum (OAM) is an intrinsic property of light and is identified by the transverse phase distribution of the wave front1. Generally, a vortex beam with a helical phase front, i.e., containing a phase term of exp(ilθ), carries an OAM quantum number of lħ on each of its photons, where l is an unbounded integer indicating the topological charge, θ is the azimuthal angle, and ħ is Plank's constant. Laguerre-Gauss (LG) laser modes were the first to be identified as carrying OAM2. Akin to the spin angular momentum, also known as left- and right-handed circular polarization, OAM is a spatial (orbital) degree of freedom common to both classical and quantum waves3. The exotic property therefore enables OAM beams to have a range of unprecedented uses, e.g., for rotational Doppler metrology4, optical spanners5, quantum key distribution systems6, high density data storage7, astrophysics8, and telecommunications9-15, as well as finding applications in optical interferometers for the detection of gravitational waves16, 17. In particular, the advantages of OAM have been explored in depth for high-capacity optical communication applications, because OAM can enhance the channel information capacity considerably owing to extensively diverse mode multiplexing without an increase in the spectral bandwidth9-15. In principle, various OAM modes are mutually orthogonal and consequently there is no interference or crosstalk between the multiplexing channels.
Despite free-space optical (FSO) communications systems that use OAM encoding/multiplexing technology having numerous advantages over conventional systems, such as being cost-effective, license-free, having access to a high bandwidth, and having been shown to be viable on a terabit/second scale in a laboratory environment9-15, 18, the widespread use of such systems still faces obstacles in complexes environments. In an open environment, intensity fluctuations caused by obstacles are introduced and become intractable challenges for FSO communications, causing a degradation of the systems' capacities.
To address these issues, the vast majority of prevalent solutions focus on finding ways of improving the laser source. Previous efforts that adopted non-diffraction beams (e.g. hypergeometric-Gaussian, Bessel-Gauss, and Hankel-Bessel beams) instead of LG beams were able to mitigate the effects of disturbances owing to their self-healing and partial...