In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power, and low mass requirements. FSO communication uses optical carrier in the near infrared band to establish either terrestrial links within the Earth’s atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links. It also finds its applications in remote sensing, radio astronomy, military, disaster recovery, last mile access, backhaul for wireless cellular networks, and many more. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering, and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of this paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. The latter part of this paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers (link, network, or transport layer) to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high capacity and low cost backhaul solutions.

/pdf/optical-communication-in-space-challenges-and-mitigation-41d8by09lh.pdf

Optical Communication in Space: Challenges and Mitigation Techniques

In recent years, free space optical communication has gained significant importance owing to its unique features: large bandwidth, license-free spectrum, high data rate, easy and quick deployability, less power and low mass requirements. FSO communication uses the optical carrier in the near infrared band to establish either terrestrial links within the Earth's atmosphere or inter-satellite or deep space links or ground-to-satellite or satellite-to-ground links. However, despite the great potential of FSO communication, its performance is limited by the adverse effects viz., absorption, scattering, and turbulence of the atmospheric channel. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of the paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. The latter part of the paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers i.e., link, network or transport layer to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of the optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high-capacity and low-cost backhaul solutions.

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Since the early 1990s, a large number of chaos-based communication systems have been proposed exploiting the properties of chaotic waveforms. The motivation lies in the significant advantages provided by this class of non-linear signals. For this aim, many communication schemes and applications have been specially designed for chaos-based communication systems where energy, data rate, and synchronization awareness are considered in most designs. Recently, the major focus, however, has been given to the non-coherent chaos-based systems to benefit from the advantages of chaotic signals and non-coherent detection and to avoid the use of chaotic synchronization, which suffers from weak performance in the presence of additive noise. This paper presents a comprehensive survey of the entire wireless radio frequency chaos-based communication systems. First, it outlines the challenges of chaos implementations and synchronization methods, followed by comprehensive literature review and analysis of chaos-based coherent techniques and their applications. In the second part of the survey, we offer a taxonomy of the current literature by focusing on non-coherent detection methods. For each modulation class, this paper categorizes different transmission techniques by elaborating on its modulation, receiver type, data rate, complexity, energy efficiency, multiple access scheme, and performance. In addition, this survey reports on the analysis of tradeoff between different chaos-based communication systems. Finally, several concluding remarks are discussed.

Wireless Chaos-Based Communication Systems: A Comprehensive Survey

The emerging graphene-based material, an atomic layer of aromatic carbon atoms with exceptional electronic and optical properties, has offered unprecedented prospects for developing flat two-dimensional displaying systems. Here, we show that reduced graphene oxide enabled write-once holograms for wide-angle and full-colour three-dimensional images. This is achieved through the discovery of subwavelength-scale multilevel optical index modulation of athermally reduced graphene oxides by a single femtosecond pulsed beam. This new feature allows for static three-dimensional holographic images with a wide viewing angle up to 52 degrees. In addition, the spectrally flat optical index modulation in reduced graphene oxides enables wavelength-multiplexed holograms for full-colour images. The large and polarization-insensitive phase modulation over π in reduced graphene oxide composites enables to restore vectorial wavefronts of polarization discernible images through the vectorial diffraction of a reconstruction beam. Therefore, our technique can be leveraged to achieve compact and versatile holographic components for controlling light.

/pdf/athermally-photoreduced-graphene-oxides-for-three-4pedaj3yu4.pdf

Athermally photoreduced graphene oxides for three-dimensional holographic images

The direct algebraic method for constructing travelling wave solutions on nonlinear evolution and wave equations has been generalized and systematized The class of solitary wave solutions is extended to analytic (rather than rational) functions of the real exponential solutions of the linearized equation Expanding the solutions in an infinite series in these real exponentials, an exact solution of the nonlinear PDE is obtained, whenever the series can be summed Methods for solving the nonlinear recursion relation for the coefficients of the series and for summing the series in closed form are discussed The algorithm is now suited to solving nonlinear equations by any symbolic manipulation program This direct method is illustrated by constructing exact solutions of a generalized KdV equation, the Kuramoto-Sivashinski equation and a generalized Fisher equation

https://inside.mines.edu/~whereman/papers/Hereman-Takaoka-JPA-v23(21)-(1990)4805-4822.pdf

Solitary wave solutions of nonlinear evolution and wave equations using a direct method and MACSYMA

The Harry Dym equation, which is related to the classical string problem, is derived in three different ways. An implicit cusp solitary-wave solution is constructed via a simple direct method. The existing connections between the Harry Dym and the Korteweg-de Vries equations are uniformised and simplified, and transformations between their respective solutions are carried out explicitly. Whenever possible, physical insights are provided.

http://iopscience.iop.org/article/10.1088/0305-4470/22/3/009/pdf

Derivation and implicit solution of the Harry Dym equation and its connections with the Korteweg-de Vries equation

Generation of chaos from acousto-optic (A-O)Bragg cell mod- ulators with an electronic feedback has been studied for over 3 decades. Since an acousto-optic Bragg cell with zeroth- and first-order feedback exhibits chaotic behavior past the threshold for bistability, such a system was recently examined for possible chaotic encryption of simple mes- sages (such as a low-amplitude sinusoidal signal) applied via the bias input of the sound cell driver. Subsequent recovery of the message signal was carried out via a heterodyne-type strategy employing a locally gener- atedchaoticcarrier,withthresholdparametersmatchedtothetransmitting Bragg cell. In this paper, we present numerical results and detailed inter- pretations for signal encryption and recovery under hybrid A-O electronic feedback using a heterodyne strategy. Important features of this setup, such as the system robustness in terms of parameter matching (feedback gain, dc bias, and time delay) are also examined in some detail. C 2011

/pdf/examination-of-chaotic-signal-encryption-and-recovery-for-2cn40ng620.pdf

Examination of chaotic signal encryption and recovery for secure communication using hybrid acousto-optic feedback

Numerical modeling of cylindrically symmetric nonlinear self-focusing using an adaptive fast Hankel split-step method

We discuss the design of an acousto-optic cell based free space optical communication link where the data
beam is made secure through chaos encryption. Using external signal modulation of the diffracted light from
a hybrid acousto-optic cell chaos (or directly via incorporation in the sound-cell driver's bias voltage)
encryption of data is possible. We have shown numerically that decryption of the encoded data is possible by
using an identical acousto-optic system in the receiver.

/pdf/design-of-acousto-optic-chaos-based-secure-free-space-3u0ih086je.pdf

Design of Acousto-optic Chaos based Secure Free-space Optical Communication Links

By using a true phasor approach and slowly varying envelope approximations along with Maxwell's equations and the constitutive relations in their respective domains (time and frequency, respectively), we derive expressions for the allowable propagation vector(s), electromagnetic fields, Poynting vectors, phase, energy, and group velocities in a medium where independent material parameters such as permittivity, permeability, and chirality are frequency dependent, including not only the carrier (e.g., optical) frequency but excursions around the carrier. One definition of negative index, viz., contradirection of the propagation and Poynting vector, demands a large value for the chirality parameter, which may not be physically attainable. We show that by incorporating dispersions in these (independent) material parameters, it may be possible to achieve negative index as defined through contradirected phase and group velocities for a range of carrier frequencies that are lower than the resonant frequency for the aforementioned material parameters as described through the Lorenz (for permittivity and permeability) and Condon (for chirality) models, and without violating the upper bound on the chirality. This has the added advantage that losses will be minimal, and further justifies our approach of using real functions for the material parameters.

Monish Ranjan Chatterjee

Papers

Derivation and implicit solution of the Harry Dym equation and its connections with the Korteweg-de Vries equation

Examination of chaotic signal encryption and recovery for secure communication using hybrid acousto-optic feedback

Numerical modeling of cylindrically symmetric nonlinear self-focusing using an adaptive fast Hankel split-step method

Design of Acousto-optic Chaos based Secure Free-space Optical Communication Links

Negative index in the presence of chirality and material dispersion