Xiaoping Ma

Performance analysis of pupil-matching optical differential receivers in space-to-ground laser communication

In the paper, the principle and structure of a pupil-matching optical differential receiver consisting of double 4f confocal lens groups is introduced to overcome atmosphere turbulences in space-to-ground laser communication. Using the scalar diffraction theory, a systematic analysis of 4f lens groups is formulated mathematically. Based on Seidel aberration, lens aberrations produced by the inherent unideal lens and mutual alignment errors of double 4f lens groups primarily caused by relative axial displacement of the foci and vertical position change of the optical axes are studied mathematically and detailed. Under the effects of varying aberrations on the double 4f lens groups, we evaluate the performance of this receiving system by the model of power penalty for a given 10(-9) bit error ratio. Simulated results of the relationship between power penalty and the different root-mean-square errors are concluded in order to put forward the requirement of machining precision of individual components. That will be helpful in optimizing the design of these groups in the optical receiver.

Speckle reduction of synthetic aperture imaging ladar based on wavelength characteristics

We propose a speckle-reduction method based on wavelength characteristics of speckle effect in synthetic aperture imaging ladar (SAIL). The return signal, which is the back scattering field with speckle effect from the rough surface of target, can be integrated over N chirp periods and heterodyne detected with a local-oscillator signal. After performing image processing respectively, the final image can be regarded as the incoherent superposition of the N sub-images. Numerical simulations indicate the effectiveness of this method. Our research may facilitate practical applications of SAIL.

Performance of multi-hop parallel free-space optical communication over gamma-gamma fading channel with pointing errors

Multi-hop parallel relaying is considered in a free-space optical (FSO) communication system deploying binary phase-shift keying (BPSK) modulation under the combined effects of a gamma-gamma (GG) distribution and misalignment fading. Based on the best path selection criterion, the cumulative distribution function (CDF) of this cooperative random variable is derived. Then the performance of this optical mesh network is analyzed in detail. A Monte Carlo simulation is also conducted to demonstrate the effectiveness of the results for the average bit error rate (ABER) and outage probability. The numerical result proves that it needs a smaller average transmitted optical power to achieve the same ABER and outage probability when using the multi-hop parallel network in FSO links. Furthermore, the system use of more number of hops and cooperative paths can improve the quality of the communication.

The performance of coherent receiver controlled by the phase lock loop in dual rate free-space laser communication

The technique of differential phase shift keying(DPSK) modulation is applied into demodulating phase information in the coherent optical receiver. The dual rate free-space receiving structure on the base of Mach-Zehnder delay interferometer with the lens is used suitably for differential delay which is equal to the one bit corresponding to a certain data rate. Delay distance at the interference receiver is varied with transmission rata from satellite to ground. Differential information is obtained by the subtraction of the two successive wave-front phases when made to interfere. The phase demodulation is extremely sensitive to phase fluctuation. Because of the incident light through atmospheric turbulence, the wave-front of optical signal became jittered in the temporal and spatial domain rapidly. In the paper, the dual rate free-space laser communication receiver for phase lock to stable signal light phase is proposed, increasing the homodyne efficiency and decreasing the bit error rate.

Orthogonal phase modulation with self homodyne detect laser communication method for the satellite-to-ground link

Signal laser propagation will pass through the random atmosphere turbulence channel in satellite-to-ground laser communication application. The turbulence will cause the wavefront distortion in the receiver telescope front. For direct detection laser communication system, atmospheric turbulence can affect the coupling efficiency from space laser to the detector. For coherent detection laser communication system, Atmosphere turbulence not only affects the coupling efficiency, but also can seriously reduce the heterodyne detection efficiency. Coherent detection communication receiver must use small aperture telescopes or large aperture telescope with adaptive wavefront compensation system. To mitigate the influence of the atmosphere turbulence, we proposed a new method base orthogonal phase modulation with self homodyne detect. This method can not only mitigate the influence of the turbulence, but also adjust the communication date rate steplessly. The experiment results show that the method is very suitable to the satellite-to-ground link.

Measurement of polarization parameters of the targets in synthetic aperture imaging LADAR

In Synthetic aperture imaging ladar (SAIL), the polarization state change of the backscattered light will affect the imaging. Polarization state of the reflected field is always determined by the interaction of the light and the materials on the target plane. The Stokes parameters, which can provide the information on both light intensity and polarization state, are the ideal quantities for characterizing the above features. In this paper, a measurement system of the polarization characteristic for the SAIL target materials is designed. The measurement results are expected to be useful in target identification and recognition.

New coherent laser communication detection scheme based on channel-switching method

A new coherent laser communication detection scheme based on the channel-switching method is proposed. The detection front end of this scheme comprises a 90° optical hybrid and two balanced photodetectors which outputs the in-phase (I) channel and quadrature-phase (Q) channel signal current, respectively. With this method, the ultrahigh speed analog/digital transform of the signal of the I or Q channel is not required. The phase error between the signal and local lasers is obtained by simple analog circuit. Using the phase error signal, the signals of the I/Q channel are switched alternately. The principle of this detection scheme is presented. Moreover, the comparison of the sensitivity of this scheme with that of homodyne detection with an optical phase-locked loop is discussed. An experimental setup was constructed to verify the proposed detection scheme. The offline processing procedure and results are presented. This scheme could be realized through simple structure and has potential applications in cost-effective high-speed laser communication.

Phase-space distribution of optical field intensity of laser beam propagation through atmospheric turbulence

Phase-space optics has its advantage over the research of laser beam propagating through atmospheric turbulence channel due to its special joint space-spatial frequency representation, which can be used to describe the property of optical signals between variables that form a Fourier transform pair. In this paper, we introduce the effect of atmospheric turbulence in terms of matrix into the phase-space analysis of laser beam propagation through turbulence. The far field optical intensity in the channel will be given in terms of phase-space representation and the phase-space distribution of received optical intensity can be plotted. It may be a new method analyzing optical scintillation under the turbulent condition.

The effect of the light size and telecommunication rate on homodyne detection efficiency in the satellite-to-ground laser communication

Atmospheric turbulence influences the wave-front, and reduces homodyne detection efficiency and bit error rate in the Satellite-to-Ground Laser Communication. Free-space differential interference structure based on differential phase shift keying (DPSK) is applied in the optical signal receiver. The free-space Mach-Zehnder delay interferometer without lens is suited for differential delay which is equal to the one bit period. Differential information is obtained by the subtraction of the two successive wave-front phases when made to interfere. Differential distance at the interference receiver is varied with transmission rate from satellite to ground. And through the receiving telescope, the spot size of incident signal light within the interference became small than before, which influences the interference efficiency of the two unequal branches. So that, it is significant for increasing homodyne efficiency to determine the optical signal rate and choose the magnification of receiving telescope. In this paper, the effect of the spot size of incident light and transmission data rate on homodyne detection efficiency is analysed. By the simulation result of efficiency in different spot size and transmission date of incident light, the homodyne efficiency will be predicted in the given data rate and light spot size on the basis of experiment setup. And application condition of free-space differential structure at DPSK differential receiver is proposed.

The impact of space-time speckle to the resolution in range and azimuth direction on synthetic aperture imaging ladar

As synthetic aperture imaging ladar employs the linear chirp laser signal, it is inevitably impacted by the space-time varying speckle effect. In many SAIL two-dimensional reconstructed images, the laser speckle effect severely reduces the image quality. In this paper, we analyze and simulate the influence of space-time speckle effect to the resolution element imaging both in range direction and in azimuth direction. Expressions for two-dimensional data collection contained space-time speckle effect are obtained, and computer simulation results of resolution degradation both in range direction and in cross-range direction are presented.