Free Space Media Research Articles

Multi-element full-duplex FSO transceiver design using optimum tiling

With ever-growing demand of high-speed mobile data and in vision of smart cities, optical wireless, a.k.a. free-space optical (FSO), communication deem to be a critical technology due to its significantly faster data transfer rate, higher security, lower costs, and reduced power usage. These beneficial effects have led to interest in exploring FSO technologies for mobile platforms such as drone-based cell tower to provide Internet services to remote areas, or even for wireless communications on the ground with jammed radio channels as in battlefields. Direct line-of-sight (LOS) is required to establish secure directional FSO communication (FSOC) links which are highly susceptible to random and erratic movements of the mobile nodes as well as the turbulence in the free-space medium. The performance of FSOC links can be improved by designing multi-element tiling of the laser-based transceivers which is capable of in-band full-duplex (IBFD) communication. In this work, we propose a genetic algorithm framework to explore optimized multi-element FSO transceiver tiling patterns to ensure maximal signal-to-interference and noise ratio (SINR) and minimize the effects of vibration of the mobile platform and atmospheric turbulence.

Experimental free-space quantum secure direct communication and its security analysis

We report an experimental implementation of free-space quantum secure direct communication based on single photons. The quantum communication scheme uses phase encoding, and the asymmetric Mach–Zehnder interferometer is optimized so as to automatically compensate phase drift of the photons during their transitions over the free-space medium. At a 16 MHz pulse repetition frequency, an information transmission rate of 500 bps over a 10 m free space with a mean quantum bit error rate of 0.49 % ± 0.27 % is achieved. The security is analyzed under the scenario that Eve performs the collective attack for single-photon state and the photon number splitting attack for multi-photon state in the depolarizing channel. Our results show that quantum secure direct communication is feasible in free space.

Modulation Techniques of Free Space Optics using Various Link Ranges

Free space optics is the present trend, which enables the communication to achieve the expected data rates for the increasing user demands. The utility of FSO in free space medium enables higher data rates than the conventional microwave links for efficient communication. Several researchers have performed the analysis of free space optics by use of different modulation techniques and optimization of the link budget in free space optical links. In this work, CW laser transmitting source ranging to 10dB of output level, link ranges of 500, 1000 and 1500 mts with attenuation values of 0.2, 0.7 and 0.9 dB/Km respectively are considered. The analysis of free space optics is carried out by using various modulation techniques such as ASK, PSK, FSK, MZ, EA, Dual Drive MZ and it is observed that Dual Drive MZ modulation performs well when compared with the other modulation techniques.

An Analytic study for effect of Antenna Height on Line-of-Sight VHF/UHF Communications Coverage Distance applied to Baghdad City

A radio system conveys information from transmitter to the receiver. The information can be sent by any antenna that transforms the RF signal into an electromagnetic wave through free space medium for electromagnetic wave propagation. Long-distance data communication has been higher in effect through wireless networks. Generally, VHF and UHF radio frequencies are termed as line-of-sight (LOS). In point of fact, the considered horizon for visual LOS is not the identical as the horizon for radio wave propagation as geographic obstacles and the earth curvature limits LOS transmission. This research provides an overview, study, analysis, and implementing simulations with single-antenna and two-antenna systems using MATLAB programming language related to antenna height effect on determining the distance range that can be expected in a radio line of sight data link. Also, the effect of weather and earth terrain changes effects on coverage distance is also taken in consideration when doing analysis. The city of Baghdad (from many locations in the world) is assumed as the place where the research ideas are applied.

An Analytic study for effect of Antenna Height on Line-of-Sight VHF/UHF Communications Coverage Distance applied to Baghdad City

A radio system conveys information from transmitter to the receiver. The information can be sent by any antenna that transforms the RF signal into an electromagnetic wave through free space medium for electromagnetic wave propagation. Long-distance data communication has been higher in effect through wireless networks. Generally, VHF and UHF radio frequencies are termed as line-of-sight (LOS). In point of fact, the considered horizon for visual LOS is not the identical as the horizon for radio wave propagation as geographic obstacles and the earth curvature limits LOS transmission. This research provides an overview, study, analysis, and implementing simulations with single-antenna and two-antenna systems using MATLAB programming language related to antenna height effect on determining the distance range that can be expected in a radio line of sight data link. Also, the effect of weather and earth terrain changes effects on coverage distance is also taken in consideration when doing analysis. The city of Baghdad (from many locations in the world) is assumed as the place where the research ideas are applied.

Observation of a large, resonant, cross-Kerr nonlinearity in a cold Rydberg gas

We report the experimental observation of a cross-Kerr nonlinearity in a free-space medium based on resonantly-excited, interacting Rydberg atoms and electromagnetically induced transparency. The nonlinearity is used to implement cross-phase modulation between two optical pulses. The nonlinear phase written onto the probe pulse is measured to be as large as $8$ mrad per nW of signal power, corresponding to a $\chi^{(3)}$ of $10^{-8}\textrm{m}^2/\textrm{V}^2$. Potential applications range from optical quantum information processing to quantum non-demolition measurement of photon number.

Radially dependent angular acceleration of twisted light.

While photons travel in a straight line at constant velocity in free space, the intensity profile of structured light may be tailored for acceleration in any degree of freedom. Here we propose a simple approach to control the angular acceleration of light. Using Laguerre-Gaussian modes as our twisted beams carrying orbital angular momentum, we show that superpositions of opposite handedness result in a radially dependent angular acceleration as they pass through a focus (waist plane). Due to conservation of orbital angular momentum, we find that propagation dynamics are complex despite the free-space medium: the outer part of the beam (rings) rotates in an opposite direction to the inner part (petals), and while the outer part accelerates, the inner part decelerates. We outline the concepts theoretically and confirm them experimentally. Such exotic structured light beams are topical due to their many applications, for instance in optical trapping and tweezing, metrology, and fundamental studies in optics.

Compensation of non-ideal beam splitter polarization distortion effect in Michelson interferometer

A composite optical structure consisting of two quarter-wave plates and a single half-wave plate is proposed for compensating for the polarization distortion induced by a non-ideal beam splitter in a Michelson interferometer. In the proposed approach, the optimal orientations of the optical components within the polarization compensator are determined using a genetic algorithm (GA) such that the beam splitter can be treated as a free-space medium and modeled using a unit Mueller matrix accordingly. Two implementations of the proposed polarization controller are presented. In the first case, the compensator is placed in the output arm of Michelson interferometer such that the state of polarization of the interfered output light is equal to that of the input light. However, in this configuration, the polarization effects induced by the beam splitter in the two arms of the interferometer structure cannot be separately addressed. Consequently, in the second case, compensator structures are placed in the Michelson interferometer for compensation on both the scanning and reference beams. The practical feasibility of the proposed approach is introduced by considering a Mueller polarization-sensitive (PS) optical coherence tomography (OCT) structure with three polarization controllers in the input, reference and sample arms, respectively. In general, the results presented in this study show that the proposed polarization controller provides an effective and experimentally-straightforward means of compensating for the polarization distortion effects induced by the non-ideal beam splitters in Michelson interferometers and Mueller PS-OCT structures.

Optical cloaking using alternate Raman gain and free-space media in the presence of spatially distributed pump fields

We discuss an optical ``cloaking'' scheme using three-level active Raman gain media in tandem in the presence of a spatially distributed pump field. Using eikonal approximation we first derive analytically the deflection angle and show that the trajectory of light can be controlled dynamically using a spatially distributed pump. We further show an excellent agreement of analytical results with a full numerical calculation of the Maxwell equation for the probe field propagating across regions where an object is embedded. Our results show that the probe light travels around the object under detection, achieving the desired spatial cloaking of the object.

Implementation of two-way free space quantum key distribution

We report an implementation of a two-way, four-state quantum key distribution (QKD) protocol (namely, the LM05 protocol) over a free space medium. A stable quantum bit error rate of 3.34% and maximum tolerable channel loss for secure key generation of 5.68 dB in the absence of a photon number splitting attack were recorded. The result strongly indicates the feasibility of a two-way QKD protocol implementation over a free space medium.

Implementation of two way Quantum Key Distribution protocol with decoy state

We report on the first demonstration of a two way Quantum Key Distribution (QKD) protocol with decoy state. The experiment was conducted over free space medium and exhibits a significant increase in the maximum secure distance of a two way QKD protocol specifically the LM05 protocol.

Advanced Power Amplifier Technologies [From the Guest Editor's Desk

Wireless communications consists of a set of technologies used to transmit information through a free-space medium that is common to all users. But, as happens in many other fields, this medium, for many years believed costless and inexhaustible, is far from being an unlimited resource. According to what is predicted by information theory, a wireless channel's capacity (the number of transmitted bits per second) is proportional to the transmitted signal-to-noise-power ratio and to the bandwidth used in the transmission. Thus, for a certain amount of noise power, error-free communication can only be guaranteed at the expense of spectrum consumption, power consumption, or both.

Subwavelength-resolution microwave tomography using wire grid models and enhanced regularization techniques

This paper presents a new approach to handle the nonlinear microwave tomography equations for dielectric materials. The electromagnetic tomography equations are rewritten in the framework of an equivalent wire grid model loaded with resistors and capacitors to represent the permittivity and conductivity of the material. The validation of the model is performed using near-field measurements at a frequency of 2.45 GHz on devices under test made of dielectric plates of various shapes surrounded by an unbounded free-space medium. The reconstruction algorithm is based on the contrast source inversion (CSI) technique. Here, we introduce an enhanced version of the CSI cost function by adding extra regularization terms; in addition, minimization is carried out using a logarithmic barrier constraint in order to avoid nonphysical permittivity and conductivity values. The distributions of those physical properties retrieved with experimental data, for nonhomogeneous dielectric structures of known composition, are in good agreement with the expected ones. The imaging resolution is limited by the density of the wire grid meshing.

Analysis of Radiation Phenomena in a Wedge Ended Dielectric Slab Waveguide

Radiation phenomena observed in a wedge shape ended dielectric slab waveguide are analyzed using mode matching technique. The case of transverse electric polarization (TE) being parallel to dielectric slab waveguide is assumed. In order to describe the fields in the wedge region, a stack of dielectric plates is assumed and in each layer the fields are expanded in terms of the mixed spectrum of guided and radiated modes. A similar expansion is used in the constant thickness slab waveguide while in free space medium a continuous-radiation mode expansion is used. Then a mode matching approach is applied, incorporating the orthogonality properties of mixed spectrum modes, in order to compute the wave fields inside the dielectric slab waveguide and wedge medium. Mode matching is achieved by discretizing the continuous radiation mode spectrum leading into a numerically stable solution provided a sufficient large number of points are used to convert integrals into finite summations. Numerical computations are carried out for various wedge geometries and shapes including linear and exponential profiles.

Electroacoustic Radiation Properties of Microstrip Linear Array Antenna on YIG Ferrite

In this paper, the electroacoustic (EA) and electromagnetic (EM) characteristics of a four element linear array of circular patch microstrip antenna (CPMA) designed on a typical YIG (Y3Fe5O12) ferrite substrate are presented at 10 GHz. The far-zone field expressions for the EM and EA mode are obtained using well established hydrodynamic theory and vector wave function technique. Computation of maximum field intensity, half Power beam width (HPBW) and other important antenna parameter like directive gain, radiation efficiency and conductance has been made for the plasma and the free space medium.

Adaptive Multiscale Moment Methods for Analyzing Em Scattering From Electrically Large Perfectly Conducting Objects

Three types of adaptive multiscale moment methods are presented here for analyzing electromagnetic scattering from perfectly conducting two-dimensional electrically large objects. The conducting structures are assumed to be in a free-space medium and is illuminated by a TM plane wave. The three algorithms decompose the conventional method of moment matrix into a scaled-block matrix through a matrix multiplication. The numbers of unknowns corresponding to the linear equations at increasing scales can be automatically reduced by using a suitable threshold and the solution obtained from previous scales. Numerical results for the monostatic as well as bistatic radar cross section of several electrically large objects are presented. Comparison has been made with respect to the numerical solution obtained by the moment method. The objective of this paper is to illustrate that a possibility exist for efficiently reducing the size of the linear equations in an automatic fashion utilizing the adaptive multiscale moment methods.

Simple analytical solution to electromagnetic scattering by two-dimensional conducting object with edges and corners. I. TM polarization

A simple and approximate analytical solution is presented by invoking on-surface radiation condition theory for the analysis of electromagnetic scattering by a perfectly conducting object. The scattering object is assumed to be placed in a free space medium and is excited by a time harmonic plane wave having transverse magnetic (TM) polarization. The closed form analytical result for the monostatic as well as bistatic radar cross section is approximate. It is applicable only for the case of a convex conducting object having arbitrary two dimensional cross section with arbitrary edges and corners. Canonical scattering objects, such as a triangular scatterer and a thin strip scatterer, are analyzed for the transverse magnetic excitation to evaluate the usefulness of the analytical results. Numerical data for the monostatic and bistatic radar cross section are presented by comparing them with respect to the numerical solution obtained by solving an electric field integral equation based on the method of moments technique. >

Wave propagation through curved shells and the validity of the plane-slab approximation

A variety of results is presented for the transmission of microwaves through curved shells. Cylindrical shells with line sources and spherical shells with dipole sources, located in the shell cavity, are considered, and ray theory is applied in systematic computation following G.H. Deschamps' procedure (1972). The ray tracing procedure implemented in the program includes the contribution of multiple reflections between the shell dielectric and free-space medium interface. These results are compared with the local plane-slab approximation described by G. Tricoles (1964). Errors 10 lambda /sub 0/, and thickness >