Laser-filamentation-assisted 1.25 Gb/s video communication under harsh conditions

Highlights

• 1.25 Gb/s video communication assisted by filamentation is performed in harsh environment.

• Within certain range, the higher deposited energy, the better the auxiliary performances of filaments would be.

• The set up process of quasi-steady-state region is characterized and discussed in detail.

• Fundamental step, limitations and future applications of this proposal are discussed.

Abstract

Beam scattering and wavefront distortion have always been an arduous hurdle for free-space optical communications. We demonstrate video communication assisted by laser filamentation through a cloud chamber, benefiting from the shock-wave-induced low-density channel. The optical power measured at the receiver exhibits periodic fluctuations dependent on the repetition rate of ultrashort pulse laser. Thus, to stabilize the communication quality, an automatic gain control unit is necessary. The analyses of receiver power, intensity distribution and eye diagram prove the effectiveness of this method. A scheme for long-distance applications has also been proposed.

Introduction

Owing to their large-bandwidth, high data security, license-free spectrum and ease of deployment, free-space optical (FSO) communications have been applied to various areas including laser satellite communications, secure communication and 5G back-haul wireless networks. One of the inevitable problems that should be considered for practical applications is that harsh environments (such as the cloudy and foggy ones) are about to severely degrade the system performances. Unfortunately, conventional optical communication methods, such as high power transmission [1], relay-assisted systems [2], [3], [4], adaptive optics [5], RF/FSO hybrid systems [6], multiple-input multiple-output (MIMO) [7], are normally accompanied with the unwanted issues such as high power induced nonlinearity [8] and system complexity [9]. Some studies attempted to employ high-power CO₂ lasers to evaporate water droplets in foggy environment. However, the required high laser power limits the applications of this method [10], [11], [12], [13]. More interestingly, femtosecond laser may provide a promising scheme to tackle the above issue [11]. As a femtosecond pulse is propagating in air, the dynamic balance between Kerr self-focusing and defocusing in plasma will lead to the emergence of filamentation [14]. Due to its highly clamped intensity (~5 × 10¹³ W·cm⁻² @800 nm), low average power and unique nonlinear process, laser filaments has found various applications in the fields of atmospheric remote sensing [15], [16], [17], weather control [18], guiding lightning [19], [20], intense few-cycle laser fields [21], [22] and terahertz radiation sources [23], [24], etc.

The most exciting fact in this aspect is that the filaments onset and length can be both controlled for a distance up to several kilometers with current technologies [25], [26], [27], [28], [29]. Moreover, the plasma formation in cloud droplets is prior to the generation of filaments in the air due to droplet focusing effect [30], [31]. The filamentation-induced channels significantly ease wireless laser communications in harsh environments. In 2014, Alexandru Hening et al. firstly proposed the concept of laser filaments for optical communication [32]. Recently, Jean-Pierre Wolf et al. studied the interaction mechanism between femtosecond laser pulses and water droplets and drew the conclusion that the quasi-steady-state regime would benefit from laser pulses of kHz repetition rate [11]. Zhang et al. utilized the transient coupling model to investigate the evolution of laser-induced plasma in water cloud [31]. Although these innovations overcame major difficulties in the implementation of filamentation clearance technique, the progress in applying laser filamentation for optical communications is rather slow, and many related issues, such as demand for high-performance laser (peak power, repetition rate, pulse duration, chirp), tunability of filament features (filament length, self-focusing distance, spatial distribution) and low beam divergence, have not been thoroughly resolved.

The main objective of this paper is to provide and experimentally verify the fundamental step required for FSO communications by using the filamentation clearance technique to evaluate the performances of telecom signal during transmission. We demonstrate laser-filamentation-assisted wireless laser video communication in an optical-attenuation-tunable cloud chamber and address some key issues. In addition, we prospect its future applications and propose a solution for long-distance cases. The work presented in this paper would be of importance for facilitating laser communications under harsh conditions.