Free space laser telecommunication through fog

Abstract

Atmospheric clearness is a key issue for free space optical communications (FSO). We present the first active method to achieve FSO through clouds and fog, using ultrashort high intensity laser filaments. The laser filaments opto-mechanically expel the droplets out of the beam and create a cleared channel for transmitting high bit rate telecom data at 1.55 µm. The low energy required for the process allows considering applications to Earth-satellite FSO and secure ground based optical communication, with classical or quantum protocols. http://dx.Significant efforts are currently dedicated to establish free space optical communications (FSO) [1], both classical [2-5] and quantum [6][7][8][9][10], within a network of satellites [11,12], between the Earth and satellites [5,10] and between the Earth and drones [13]. For instance, the first free space quantum communication satellite from the Chinese Aerospace Agency demonstrated quantum key distribution over 1200 km between the satellite and the Earth [9]. Space agencies also support laser telecommunication programs [14] aiming at significantly increasing the data rates as compared to radio-frequencies (RF) and reducing the problem of the availability of RF bands.The major limitation for optical free space telecommunication is the availability of a clear sky, i.e. free from fog and clouds. This critical issue is currently only addressed by the multiplication of networked ground stations, which is complex and expensive. No active methods, with the objective of creating a clear channel within clouds or fogs to allow data transmission, have been proposed and implemented so far. This is the aim of the present paper.Early attempts to clear the sky from fog and clouds with high power CO2 lasers took place in the 70's and 80's, mainly for increasing visibility on the battlefield. However, the very high energy required to vaporize and shatter water drops (lasers of typically 10 kW.cm −2 continuous wave [15] and 10-1000 MW.cm −2 pulsed [16,17]) was prohibitive for applications with fogs extending over 100 m thickness. Moreover, the laser energy was deposited in the first meters of the optical path, according to the usual Beer-Lambert exponential decay.The 2000's saw the emergence of femtosecond TW-class lasers (1 TW = 10 12 W), hence the opportunity to reconsider laser transmission through fog with a fundamentally different approach: non-linear propagation in the atmosphere and laser filamentation [18]. Due to the high peak-power, focusing (Kerr effect) and defocusing (Kerr saturation, plasma generation) non-linearities involved in the propagation lead to the formation of laser filaments [19][20][21][22][23]. Laser filaments are self-sustained light structures of typically 100 µm diameter (at 800 nm) and up to hundreds of meters in length, widely extending the traditional linear diffraction limit. They bear high intensities (10-100 TW.cm -2 ) and generate a low density plasma in air with typically 10 16 charges per cm 3 . A laser filament also survives interaction with water d...

Molecular quantum wakes for clearing fog

“…When only one pulse of the train is used (bearing 0.48 mJ as in the pulse train), no transmission gain is observed, which suggests that the plasma generation of individual pulses of the train is negligible. Conversely if the whole energy of the pulse train, 3.8 mJ, is concentrated in a single pulse, plasma generation and filamentation are initiated, and we observe the associated acoustic wave clearing process seen previously [8,9], i.e. −8 dB, which exceeds the quantum wake clearing process with 8 pulses.…”

“…Until recently no active method for opening clear optical transmission channels through clouds or fog was available. The propagation of ultrashort and high intensity lasers through the atmosphere provided a radically new concept, based on plasma generation inherent to laser filamentation [7][8][9]. Laser filaments are self-sustained light structures with a typical core diameter d core ∼ 100 µm (at 800 nm) and up to hundreds of meters in length, orders of magnitude beyond the linear diffraction limit.…”

“…Over a time window of 0.1 − 1 ms, this leads to a reduced air density channel significantly wider than the filament core [13][14][15][16][17]. When launched in fog (water droplet diameter centered around 5 µm), this filament-induced shockwave radially ejects the scatterers out of the beam [8,9]. This fog clearing mechanism requires modest energy deposition, of the order of less than 1 mJ/m [18,19].…”

Spatio-temporal measurement of ionization-induced modal index changes in gas-filled PCF by prism-assisted side-coupling

“…The precise timescales of these different processes depend on the experimental conditions. Investigations of photoionization have led to a better understanding of plasmas [1,2] and filamentation [3,4], as well as the development of plasma and air waveguides [4][5][6][7] that might be exploited for laser-based free space communication in the presence of fog [8]. A good understanding of plasma dynamics is also crucial in experiments at MHz pulse repetition rates in the strong field regime, for example in high harmonic generation within femtosecond enhancement cavities [9] and the generation of broadband deep and vacuum ultraviolet via dispersive wave emission in gas-filled hollow-core photonic crystal fibers (HC-PCFs) [10].…”

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

“…The long-distance transmission of the filament offers many potential applications. Wolf et al [179] reported a free space laser telecommunication through the fog. They use a filament-induced shock wave to radially expel the droplets out of the beam from the air it sweeps, then providing a clean channel for telecom laser transmission, which has the potential to be applied to Earth-satellite free-space optical communications and secure ground-based optical communications.…”