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

Zhe Zhao,Cong Liu,Nanzhe Hu,Amir Minoofar,Huibin Zhou,Alan E Willner,Ahmed Almaiman,Andreas F Molisch,Runzhou Zhang,Kai Pang,Doohwan Lee,Haoqian Song,Moshe Tur,Xinzhou Su,Hao Song,Hirofumi Sasaki

doi:10.1038/s41598-020-80179-3

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.

Highlights

Orbital-angular-momentum (OAM) multiplexing has been utilized to increase the channel capacity in both millimeter-wave and optical domains
Previous works have shown that there are multiple fundamental issues in wireless OAM links: (a) the divergence of an OAM beam scales approximately with the square root of |l| and wavelength. It indicates that an OAM beam with a higher order or/and lower frequency has a larger beam size at the receiver, which leads to higher power attenuation[8]; (b) when an OAM beam propagates through atmospheric turbulence, the power of the transmitted OAM mode is leaked to neighbouring modes, which is proportional to the strength of phasefront distortion D/r0, where D is the beam size and r0 is the Fried parameter
Simulation results show that: (i) the power leakage to the neighbouring mode remains < − 15 dB when we transmit OAM + 4 at 0.5 THz with a beam waist of 1 m through a 1-km link under calm weather (Cn2 < 5 × 10–13 m−2/3); (ii) for 3-OAM-multiplexed THz links, the signal-to-interference ratio (SIR) increases by ~ 5–7 dB if the mode spacing increase 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 (Cn2 < 1 × 10–13 m −2/3), while it tends to be unobtrusive under bad weather (Cn2 = 1 × 10–11 m−2/3)

Summary

Introduction

Orbital-angular-momentum (OAM) multiplexing has been utilized to increase the channel capacity in both millimeter-wave and optical domains. We simulate and analyze the effects of divergence, turbulence, limited-size aperture, and misalignment on the signal power and crosstalk of THz OAM links. Previous works have shown that there are multiple fundamental issues in wireless OAM links: (a) the divergence of an OAM beam scales approximately with the square root of |l| and wavelength It indicates that an OAM beam with a higher order or/and lower frequency has a larger beam size at the receiver, which leads to higher power attenuation[8]; (b) when an OAM beam propagates through atmospheric turbulence, the power of the transmitted OAM mode is leaked to neighbouring modes, which is proportional to the strength of phasefront distortion D/r0, where D is the beam size and r0 is the Fried parameter. Since the THz wave wavelength is shorter than that of mm-wave but longer than that of optical wave, divergence and turbulence effects might degrade THz OAM links in their own m anners[14]

Methods

Results

Conclusion