Modal coupling and crosstalk due to turbulence and divergence on free space THz links using multiple orbital angular momentum beams

Abstract

Orbital-angular-momentum (OAM) multiplexing has been utilized to increase the channel capacity in both millimeter-wave and optical domains. Terahertz (THz) wireless communication is attracting increasing attention due to its broadband spectral resources. Thus, it might be valuable to explore the system performance of THz OAM links to further increase the channel capacity. In this paper, we study through simulations the fundamental system-degrading effects when using multiple OAM beams in THz communications links under atmospheric turbulence. We simulate and analyze the effects of divergence, turbulence, limited-size aperture, and misalignment on the signal power and crosstalk of THz OAM links. We find through simulations that the system-degrading effects are different in two scenarios with atmosphere turbulence: (a) when we consider the same strength of phasefront distortion, faster divergence (i.e., lower frequency; smaller beam waist) leads to higher power leakage from the transmitted mode to neighbouring modes; and (b) however, when we consider the same atmospheric turbulence, the divergence effect tends to affect the power leakage much less, and the power leakage increases as the frequency, beam waist, or OAM order increases. Simulation results show that: (i) the crosstalk to the neighbouring mode remains < − 15 dB for a 1-km link under calm weather, when we transmit OAM + 4 at 0.5 THz with a beam waist of 1 m; (ii) for the 3-OAM-multiplexed THz links, the signal-to-interference ratio (SIR) increases by ~ 5–7 dB if the mode spacing increases by 1, and SIR decreases with the multiplexed mode number; and (iii) limited aperture size and misalignment lead to power leakage to other modes under calm weather, while it tends to be unobtrusive under bad weather.

Adaptive demodulation by deep-learning-based identification of fractional orbital angular momentum modes with structural distortion due to atmospheric turbulence

“…The laser beams propagating through atmospheric media experience different phase distortions depending on their wavelength and beam size 16 . So, it is proper to use the phasefront distortion , where is the beam diameter, and is the coherence length, for providing comprehensive criteria 16 , 30 . For turbulence environments that gives rise to the phasefront distortion of , the proposed scheme can exploit the mode spacing of 0.05.…”

“…Simulation of laser beams propagating through AT channels is implemented using the split-step Fourier method and random phase plates placed along the propagation direction 16 , 32 – 34 . As shown in Fig.…”

“…To investigate the effects of AT strength, we prepared data sets for 5 different turbulence levels . Here, we selected the range of and the sampling distance by considering the conditions for the Rytov variance 16 , 32 , and . The random phase screen is set with window size , the number of grid points , grid interval , inner scale , and outer scale .…”

“…The number of addressable OAM modes is physically restricted by the system, even though the range of the OAM mode itself is unbounded 6 , 15 . Besides, the higher-order mode with a larger beam size is relatively more vulnerable to atmospheric distortion, resulting in higher modal crosstalk 16 . Hence, OAM modes separated by fractional intervals have drawn attention as a potential solution to the problems.…”

Propagation and Transformation of Vortexes in Linear and Nonlinear Radio-photon Systems

“…x y y z z f x y z dx dy dz (35) and where the Green's function is determined by expression (9) in which it should be assumed…”