Generation of Multiple Obstruction-Free Channels for Free Space Optical Communication

Plasma filaments generated by ultrafast beams with orbital angular momentum can simultaneously clear two cylindrical transmission channels, within which an annular beam can propagate free of obstructions.

Free-Space Optical communication (FSO) is a line-of-sight communication method that provides unguided wireless optical data in free space [1].FSO communication systems typically use very narrow spectrum laser beams as carrier signals that provide high-speed data communication between two fixed nodes over distances up to a few kilometers [1][2][3].They are appealing for a wide range of applications such as metropolitan and local area network connectivity, fiber back-up, wireless cellular networks, disaster recovery, wireless video surveillance/monitoring, and quantum key distribution [4,5].Despite the multiple advantages of FSO systems over a wide range of applications, the performance of FSO links suffers from link reliability and high sensitivity to some limiting factors, including outdoor weather conditions (e.g., heavy rain, fog, smoke, storms, deep clouds, snow, and scintillation), atmospheric turbulence, and physical obstructions.
Here we demonstrate a new approach that could allow effective FSO through air obstructed by clouds and fog.The approach is based on the use of optimized multi-filament structures to clear a dual channel in the air.An inner channel (in between the filaments) is created by a combination of thermal heating of the air and the shock waves released due to the creation of the filaments, and an outer channel (around the filaments) is cleared by the shock waves only.This can be used to guide both traditional Gaussian beams and Laguerre-Gaussian (LG) beams with an annular spatial distribution as the information signal.
Figure 1-1 shows the experimental setup.As a proof of concept, we guide a 543 nm continuous wave (CW) information signal carried by LG 0,1 and LG 4,0 beams generated by applying LG phase masks into a spatial light modulator (SLM) through a 1-m long cloud chamber.The filament driver used in our experiments is a custombuilt Ti:sapphire chirped-pulse amplified laser, operating at 480-Hz repetition rate and generating pulses of ∼40-fs duration at a ∼800-nm center wavelength capable of delivering a pulse energy of ∼20 mJ.The diameter of the filament driver beam before passing through a spiral phase plate (SPP) is set by an iris, and the beam is then weakly focused using a lens with a focal length f =2.5 m.The SPP used to generate the vortex beam of order imposes a modulation in the form of exp(i Φ), modulo 2π, onto the flat phase front of the filament driver, where Φ represents the azimuthal angle.For this experiment vortical beams of topological charge =1 and =5 were used to generate multi-filaments.A dichroic mirror (DM1) is used to couple the information signal with the filament driver through cloudy atmospheric conditions made by an ultrasonic nebulizer.The multi-filaments propagating through the cloud chamber are imaged in Figure 1-1 inset (a).
The cloud drastically attenuated the information signal, resulting in an attenuation coefficient at λ =543 nm equal to 14.26 dB.m −1 .The information signal is collected with a f = 15 cm lens and imaged using a Thorlabs sCMOS camera.For each measurement, the information signal is imaged before and after the filament is introduced into the cloud chamber.The camera uses an exposure time of 30 ms, capturing multi-shot images of the information signal.
Figure 1-2 shows the results of the experiment while using a vortical beam filament driver with topological charge =1.LG 0,1 and LG 4,0 beams were used as the information signals.The LG 0,1 signal utilizes the inner channel, while the LG 4,0 propagates through the outer channel cleared by the acoustic wave.The images in the left [Figure 1 A comparison of the performance of each filament driver is shown in the bar plot in Figure 1-3 .A Gaussian beam (LG 0,0 ) filament driver producing a single filament was used as the control experiment.The LG 1,0 filament driver performed the best in all experiments (Figure 1-2 ).This is attributed to the fact that the multi-filament structure is both relatively wide and long.The shorter multi-filament structure created by the LG 5,0 beam is limited by the reduction of size of the quasi-transparent channel (Figure 1-3 ).This is especially challenging with information signals with larger topological charges such as the LG 6,0 .In the context of data transmission through a dynamic system such as the cloud, it is evident that multi-filaments outperform single-filament structures.These results demonstrate the feasibility of the use of this method to enhance FSO in cloudy environments.It is expected that high data rates can be achieved with a high-frequency modulator operating at the standard telecom wavelength (1.55 μm).This wavelength also suffers less Mie scattering when compared to visible light.

Fig. 1 . 1 -
Fig. 1. 1 -Schematic of the experimental setup.The inset shows the multi-filament beam propagating through the cloud chamber. 2 -Experimental results with the multi-filament structure generated by the LG 1,0 SPP.The images in the left and the middle column show the information signal carried by LG 0,1 (a, b) and LG 4,0 (d, e) before (I 0 ) and after (I) the filament is introduced respectively.The right column (c, f) shows the enhancement factor (γ = I/I 0 ) calculated for each pair of information signals in the respective row. 3 -Average enhancement of each information signal coupled to different filament driver beams.