Light beams with orbital angular momentum for free space optics

The light＇s orbital angular momentum （OAM） is a consequence of the spiral flow of the electromagnetic energy. In this paper, an analysis of light beams with OAM used for free space optics （FSO） is conducted. The basic description and conception of light＇s OAM are reviewed. Both encoding information into OAM states of single light beam and encoding information into spatial structure of the mixed optical vortex with OAM are discussed, and feasibility to improve the FSO＇s performance of security and obstruction of line of sight is examined.

Outage Probability Analysis of Free Space Communication System Using Diversity Combining Techniques

Recently,free space optical(FSO)communication is gaining much attention towards the research community.The reason for this attention is the promises of high data-rate,license-free deployment,and non-interfering links.It can,however,give rise to major system difficulties concerning alignment and atmospheric turbulence.FSO is the degradation in the signal quality because of atmospheric channel impairments and conditions.The worst effect is due to fog particles.Though,Radio Frequency(RF)links are able to transmit the data in foggy conditions but not in rain.To overcome these issues related to both the FSO and RF links.A free space communication system(FSCS)is proposed,in which the hybrid technology is based on the individual FSO and RF channel.FSCS is a capable solution to overcome the difficulties of the existing systems(FSO and RF)as well as to enhance the overall link reliability and availability.In this paper,FSCS is investigated in terms of performance throughput(i.e.,outage probability and bit-error-rate(BER))by implementing the receive diversity combining techniques.An analysis of the outage probability of the proposed system along with the individual FSO and RF system is developed.Simulation results are presented to support the analysis.It is shown that the proposed system outperforms the individual FSO andRF system and gives a power gain of 3dB over a distinct number of receive antennas.

Millimeter-Wave Absorption Properties of Thin Wave Absorber in Free Space with New Porous Carbon Material

In this paper, we focus on PHYTOPOROUS, a porous carbon material made entirely from plant-based ingredients, as a new broadband-wave absorber material that acts in the millimeter wave band. We created prototypes of thin rubber-sheet wave absorbers that contain porous carbon (PHYTOPOROUS) made from rice chaff and soybean hulls, which are both agricultural residue products that are generated in large quantities. We investigated the permittivity and reflectance characteristics of this material using the free-space time-domain method. The thin rubber-sheet wave absorber that contained PHYTOPOROUS made from soybean hulls exhibited a frequency band that was approximately 18 GHz wide and centered at 90 GHz. The return loss for this material was greater than −20 dB. This demonstrates that the material provides nearly constant reflection absorption over a wide frequency band.

MIMO free space optical communication based on orthogonal space time block code

An appropriate coding method that can reduce the error rate of communication system is especially important to the free space optical communication. STBC （space-time block code） is an orthogonal encoding method integrating space domain and time domain. The technology can combat fading effectively and improve error rate performance. In this paper, first, an STBC fit for optical communication with intensity modulation and direct detection （IM/DD） is proposed by combining the orthogonality of the Alamouti space-time code and the QPPM modulation. Then, the error rate performance of the system is analyzed under four cases： with or without channel fading, with or without background radiation. At last, this scheme is confirmed by Monte Carlo approach. It is shown that this method not only realizes the full speed rate transmission, and can improve the error rate performance of the system effectively, but also overcomes the scintillation effect exerted by atmosphere turbulence. When the symbol error probability （SEP） is 2×10^-3, according to the 1× 1 system, the sending power of the 2×1 system is nearly reduced by 3.5 dBJ, and the 2×2 system is nearly reduced by 9 dBJ.

Long-distance quantum teleportation assisted with free-space entanglement distribution

Faithful long-distance quantum teleportation necessitates prior entanglement distribution between two communicated locations. The particle carrying on the unknown quantum information is then combined with one particle of the entangled states for Bell-state measurements, which leads to a transfer of the original quantum information onto the other particle of the entangled states. However in most of the implemented teleportation experiments nowadays, the Bell-state measurements are performed even before successful distribution of entanglement. This leads to an instant collapse of the quantum state for the transmitted particle, which is actually a single-particle transmission thereafter. Thus the true distance for quantum teleportation is, in fact, only in a level of meters. In the present experiment we design a novel scheme which has overcome this limit by utilizing fiber as quantum memory. A complete quantum teleportation is achieved upon successful entanglement distribution over 967 meters in public free space. Active feed-forward control techniques are developed for real-time transfer of quantum information. The overall experimental fidelities for teleported states are better than 89.6%, which signify high-quality teleportation.

Sensitivity analysis of an HgCdTe based photovoltaic receiver for long-wavelength free space optical communication systems

We examine theoretically the performance of an Hg0.77Cd0.23Te based p-n photodetector/HFET optical receiver due to its possible application at 10.6 μm free space optical communication system at high bit rate.A rigorous noise model of the receiver has been developed for this purpose.We calculate the total noise and sensitivity of the receiver.The front-end of the receiver exhibits a sensitivity of -45 dBm at a bit rate of 1 Gb/s and -30 dBm at a bit rate of 10 Gb/s,and the total mean-square noise curren t〈i2n〉=5×10-15 A2 at a bit rate of 1 Gb/s an d〈i2n〉 =10-12 A2 at a bit rate of 10 Gb/s,and a 3-dB bandwidth of 10 GHz.

Climate effects on performance of free space optical communication systems in Yemen

Free space optical （FSO） communication has been considered as an alternative to radio relay link line-of- sight （LOS） communication systems. The total attenuation is a combination of atmospheric attenuation in the atmosphere and geometric losses. The purpose of this paper is to study the geometric loss versus link range （in kin）, divergence angle, transmitter aperture diameter, and receiver aperture diameter. Total attenuation versus low visibility, average visibility, beam divergence, link range and rainfall rate were presented in this paper. Atmospheric attenuation （in dB） and scattering coefficient （in km-~） for several Yemeni main cities were explored. The study was concentrated on received power versus low and average visibilities and link range. Series of related simulation results were illustrated and discussed in this paper about the climate effects on performance of FSO communication systems in Yemen.

Dynamic spot tracking system based on 2D galvanometer in free space optical communication for short distance

Dynamic tracking of laser spot is a key process in the establishment of free space optical communication. In this paper, a dynamic tracking system was presented. In this system, a two-dimensional （2D） galvanometer was used to change the angle of the optical axis of the incident beam at a certain scanning frequency as optical signal jitter simulator, and another galvanometer was used to track the jitter with quadrant detector （QD） and data processing module to acquire the position information of laser spot. Results indicated that the tracking accuracy of this system mainly composed of 2D galvanometer was as high as 27,8 ~rad, and its linear deviation was less than 0.013. The system could still keep the dynamic tracking of the spot stable when the jitter frequency of the optical signal was less than 1000 Hz. Those results suggested that this system could be suitable for the short distance in free space communication due to its simple structure, easy to control and low cost compared with conventional system.

Experimental realization of quantum cryptography communication in free space

Utilizing linear optical devices, the principle of B92 quantum key distribution (QKD) protocol is demonstrated in free space with a distance of transmission of 2.2 meters. The faint laser pulses with 650 nm wavelength are used as the single photon sources. The experimental results show that the eavesdropping behavior in the signal transmission can be detected. We also discuss the problems and solutions in using the quantum cryptography communication practically. It is pointed out that one of the approaches to increasing the distance of the quantum communication is to overcome the attenuation of the single photon in transmission. This could not be solved by the use of single photon source, and new quantum communication protocols are needed to solve these problems.

Spatial Operator Algebra for Free-floating Space Robot Modeling and Simulation

As the dynamic equations of space robots are highly nonlinear,strongly coupled and nonholonomic constrained,the efficiency of current dynamic modeling algorithms is difficult to meet the requirements of real-time simulation.This paper combines an efficient spatial operator algebra（SOA） algorithm for base fixed robots with the conservation of linear and angular momentum theory to establish dynamic equations for the free-floating space robot,and analyzes the influence to the base body＇s position and posture when the manipulator is capturing a target.The recursive Newton-Euler kinematic equations on screw form for the space robot are derived,and the techniques of the sequential filtering and smoothing methods in optimal estimation theory are used to derive an innovation factorization and inverse of the generalized mass matrix which immediately achieve high computational efficiency.The high efficient SOA algorithm is spatially recursive and has a simple math expression and a clear physical understanding,and its computational complexity grows only linearly with the number of degrees of freedom.Finally,a space robot with three degrees of freedom manipulator is simulated in Matematica 6.0.Compared with ADAMS,the simulation reveals that the SOA algorithm is much more efficient to solve the forward and inverse dynamic problems.As a result,the requirements of real-time simulation for dynamics of free-floating space robot are solved and a new analytic modeling system is established for free-floating space robot.

THE ROBUST CONTROL SCHEME FOR FREE-FLOATING SPACE MANIPULATOR TO TRACK THE DESIRED TRAJECTORY IN JOINTSPACE

The control of a free-floating space manipulator system isdiscussed. With the augmentation approach, the nonlinearparameterization problem of the dynamic equations of the spacemanipulator system is overcome. Based on the results, the robustcontrol scheme for free-floating space manipulator with uncer- tainpayload parameters to track the desired trajectory in jointspace isproposed, and the global convergence of the tracking is verified byusing the Lyapunov method.

Interaction of two edge dislocations in free-space propagation

This paper studies in detail the interaction of two edge dislocations nested in a Gaussian beam propagating in free space. It shows that in free-space propagation the edge dislocations are unstable and vanish, and two noncanonical vortices with opposite topological charge take place when off-axis distances cl and c2 of two edge dislocations are nonzero, and the condition k2w08＋ 32c1c2（w02- 2C1C2）Z2 〉 0 is fulfilled （k-wave number, w0-waist width）. A noncanonical vortex appears when one off-axis distance is zero. However, one edge dislocation is stable when two edge dislocations are perpendicular and one off-axis distance is zero. Two perpendicular edge dislocations both with zero off-axis distance are also stable. The analytical results are illustrated by numerical examples.

Free-Space Optical communication using visible light

The possibility of visible red light laser being used as signal light source for Free-Space Optical (FSO) communication is proposed. Based on analysis of transmission in atmospheric channel concerning 650 nm laser beam, performance of wireless laser communication link utilizing a low power red laser diode was evaluated. The proposed system can achieve a maximum range of 300 m at data rate 100 Mb/s theoretically. An experimental short-range link at data rate 10 Mb/s covering 300 m has been implemented in our university. It is feasible to enhance the system performance such as link range and data rate by increasing transmitting power and decreasing laser beam divergence angle or through other approaches.

Impact dynamics analysis of free-floating space manipulator capturing satellite on orbit and robust adaptive compound control algorithm design for suppressing motion

The impact dynamics, impact effect, and post-impact unstable motion sup- pression of free-floating space manipulator capturing a satellite on orbit are analyzed. Firstly, the dynamics equation of free-floating space manipulator is derived using the sec- ond Lagrangian equation. Combining the momentum conservation principle, the impact dynamics and effect between the space manipulator end-effector and satellite of the cap- ture process are analyzed with the momentum impulse method. Focusing on the unstable motion of space manipulator due to the above impact effect, a robust adaptive compound control algorithm is designed to suppress the above unstable motion. There is no need to control the free-floating base position to save the jet fuel. Finally, the simulation is proposed to show the impact effect and verify the validity of the control algorithm.

Unified Modeling Approach of Kinematics, Dynamics and Control of a Free-Flying Space Robot Interacting with a Target Satellite

In this paper a unified control-oriented modeling approach is proposed to deal with the kinematics, linear and angular momentum, contact constraints and dynamics of a free-flying space robot interacting with a target satellite. This developed approach combines the dynamics of both systems in one structure along with holonomic and nonholonomic constraints in a single framework. Furthermore, this modeling allows consid-ering the generalized contact forces between the space robot end-effecter and the target satellite as internal forces rather than external forces. As a result of this approach, linear and angular momentum will form holonomic and nonholonomic constraints, respectively. Meanwhile, restricting the motion of the space robot end-effector on the surface of the target satellite will impose geometric constraints. The proposed momentum of the combined system under consideration is a generalization of the momentum model of a free-flying space robot. Based on this unified model, three reduced models are developed. The first reduced dynamics can be considered as a generalization of a free-flying robot without contact with a target satellite. In this re-duced model it is found that the Jacobian and inertia matrices can be considered as an extension of those of a free-flying space robot. Since control of the base attitude rather than its translation is preferred in certain cases, a second reduced model is obtained by eliminating the base linear motion dynamics. For the purpose of the controller development, a third reduced-order dynamical model is then obtained by finding a common solution of all constraints using the concept of orthogonal projection matrices. The objective of this approach is to design a controller to track motion trajectory while regulating the force interaction between the space robot and the target satellite. Many space missions can benefit from such a modeling system, for example, autonomous docking of satellites, rescuing satellites, and satellite servicing, where it is vital to limit the con-tact force during the robotic operation. Moreover, Inverse dynamics and adaptive inverse dynamics control-lers are designed to achieve the control objectives. Both controllers are found to be effective to meet the specifications and to overcome the un-actuation of the target satellite. Finally, simulation is demonstrated by to verify the analytical results.

OpenFIow-Based Control Architecture for the Mobile FreeSpace Optical Networks

Mobile free space optical networks have aroused much attention due to the ability of providing high speed connectivity over long distance using the wireless laser links,while requiring relatively high available bandwidth resource and less energy consumption.However,maintaining the network with laserlinks is quite challenging due to a number of issues,such as the link fragility,the difficulty in pointingand tracking of the link,which also raises the great difficulty in the control of the network.In this paper,we present the methodology for the deployment of the mobile freespace optical networks based on our proposed OpenFlow-based control architecture.In addition,a new routing scheme is proposed and demonstrated on the testbed based on this control architecture.Delivery ratio,average delivery delay and time complexity are given to verify the performance of the OpenFlow-based control architecture.

Effect of normalized plasma frequency on electron phase-space orbits in a free-electron laser

Irregular phase-space orbits of the electrons are harmful to the electron-beam transport quality and hence deteriorate the performance of a free-electron laser （FEL）. In previous literature, it was demonstrated that the irregularity of the electron phase-space orbits could be caused in several ways, such as varying the wiggler amplitude and inducing sidebands. Based on a Hamiltonian model with a set of self-consistent differential equations, it is shown in this paper that the electron- beam normalized plasma frequency functions not only couple the electron motion with the FEL wave, which results in the evolution of the FEL wave field and a possible power saturation at a large beam current, but also cause the irregularity of the electron phase-space orbits when the normalized plasma frequency has a sufficiently large value, even if the initial energy of the electron is equal to the synchronous energy or the FEL wave does not reach power saturation.

Research on Task Planning Algorithm of Retrieving Invalid Satellite for Free-Flying Space Robot

A free-flying space robot will accomplish manufacturing, assembling and repair instead of astronauts in the future unmanned space flight hbecause of its flexible maneuverability in space. This paper presents a task planning algorithm of retrieving invalid satellite for free-fiving space robot. First we discuss kinematics model and deduct cinematics equations of dual-arm space robot. Then the process of retrieving an invalid satellite, which is divided into eleven motion procedures. At the same time, we have developed a free-flying space robot task planning simulation system and the experimental results show that this algorithm is feasible and correct.