Experimental investigation of the performance of different modulation techniques under controlled FSO turbulence channel

This paper experimentally investigates and compares the performance of the free space optics system employing three different modulation schemes, on-off keying (OOK) with nonreturn- to-zero (NRZ) and return-to-zero (RZ) and the binary phase shift keying (BPSK) operating under the turbulent atmosphere. The received average signal is measured and used to characterize the strength of the turbulence. The experiment is performed with a temperature gradient of 4 degrees at a wind velocity of 4 m/s. The temperature gradient within the controlled channel results in turbulence of a log irradiance variance of 0.002, which is classified as a very weak turbulence. The received signal eye diagram and power histograms are presented and analyzed for performance evaluation of the selected modulation schemes in the weak turbulence model

FSO通信システムの新高度化技術に関する研究

早大学位記番号:新7795早稲田大

Orbital Angular Momentum Waves: Generation, Detection and Emerging Applications

Orbital angular momentum (OAM) has aroused a widespread interest in many fields, especially in telecommunications due to its potential for unleashing new capacity in the severely congested spectrum of commercial communication systems. Beams carrying OAM have a helical phase front and a field strength with a singularity along the axial center, which can be used for information transmission, imaging and particle manipulation. The number of orthogonal OAM modes in a single beam is theoretically infinite and each mode is an element of a complete orthogonal basis that can be employed for multiplexing different signals, thus greatly improving the spectrum efficiency. In this paper, we comprehensively summarize and compare the methods for generation and detection of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications and technical challenges of OAM in communications, including free-space optical communications, optical fiber communications, radio communications and acoustic communications. To complete our survey, we also discuss the state of art of particle manipulation and target imaging with OAM beams

Vector Beams for Fundamental Physics and Applications

Lights salient degrees of freedom are the independent parameters that completely de- scribe an electromagnetic wave (in the paraxial approximation) and include polarization, wavelength, and time. Most recently, lights space degree of freedom has received sig- nificant attention via the sub-discipline of optics that can be referred to as complex light or structured light. The study of complex light is a veritable renaissance of optics; us- ing lights space degree of freedom many classical optics phenomena have been revisited with novel results. In this thesis, a novel form of structured light referred to as vector beams will be investigated. It will be shown that vector beams have properties very much connected to fundamental physical phenomena. In Chapters 1 and 2, it will be shown that Laguerre-Gaussian modes and cylindrical vector beams are inherently connected. The con- nection can be illustrated by their representation on what is referred to as a higher-order Poincare sphere. In analogy to the representation of linear and circular polarizations on the well-known Poincare sphere, on the higher-order Poincare sphere, radially and azimuthally polarized cylindrical vector beams are represented by points on the equator, and orthogonal circularly polarized Laguerre-Gaussian modes whose azimuthally varying phases have op- posite handedness are represented by North and South poles. In Chapter 3, it will be shown that vector beam can be generated using an optical element referred to as a q-plate inside. Using the q-plate, any state of polarization on the well-known Poincare sphere could be converted into any Laguerre-Gaussian mode or cylindrical vector beam on the higher-order Poincare sphere, effectively breaking the spatial modes degeneracy. In chapters 4 and 5, it will be shown that vector beams can be used for optical communication, i.e., they can be used to encode or carry that information. The transmitted vector beams’ spatial patterns must not change as they propagate so that the information they encode or carry can be re- ceived with minimal error. Equally important, solutions to the paraxial Helmholtz wave equation make up a complete and orthogonal set. As solutions to the paraxial Helmholtz wave equation, with regard to optical communication, the complete and orthogonal set of vector beams can be used as an ”N” dimensional state space with which to encode data, or as N channels to carry information. In chapter 6, it will be shown that vector Bessel beams can be generated by combining liquid crystal q-plates with ”Durnin’s” ring method. Similar to scalar Bessel beams, it will be shown that the spatially inhomogeneous states of polarization of vector Bessel beams can self-heal after they are obstructed. The polarization of vector Bessel beams was measured using Stokes polarimetry before and after they were obstructed. It will be experimentally verified that their spatially inhomogeneous states of polarization can self-heal

New challenges in wireless and free space optical communications

AbstractThis manuscript presents a survey on new challenges in wireless communication systems and discusses recent approaches to address some recently raised problems by the wireless community. At first a historical background is briefly introduced. Challenges based on modern and real life applications are then described. Up to date research fields to solve limitations of existing systems and emerging new technologies are discussed. Theoretical and experimental results based on several research projects or studies are briefly provided. Essential, basic and many self references are cited. Future researcher axes are briefly introduced

Integrated Approach to Free Space Optical Communications in Strong Turbulence

The propagation of a free space optical communication signal through atmospheric turbulence experiences random fluctuations in intensity, including signal fades which negatively impact the communications link performance. This research develops an analytical probability density function (PDF) to model the best case scenario of using multiple independent beams to reduce the intensity fluctuations. The PDF was further developed to account for partially correlated beams, such as would be experienced by beams having finite separation. The PDF was validated with results obtained from digital simulations as well as lab experiments. The research showed that as the number of transmitted beams increases the probability of fade decreases. While fade probability is reduced by adding more beams, using more than four transmitters does little to improve the overall performance. Additionally, the use of pulse position modulation (PPM) provided significant improvement over traditional fixed threshold on/off keying with the impact of signal fading reduced. Combining PPM with multiple transmitters produced the best overall bit error rate results