A differential phase-shift keying modulation for free-space optical (FSO) communication is considered in atmospheric turbulence modeled by the exponentiated Weibull distribution. The selection combining (SelC) spatial diversity is used to mitigate the effects of atmospheric turbulence. We analyze the average bit error rate (BER) of the system using SelC spatial diversity by Gauss–Laguerre approximation. The effect of aperture averaging and spatial diversity on the outage probability is also studied. The numerical results show that it requires a smaller level of signal-to-noise ratio to reach the same BER when large aperture and SelC spatial diversity are deployed in the FSO system. Moreover, it is proved that aperture averaging and SelC spatial diversity are effective for improving the performance of the system’s outage probability.
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.
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.
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.
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.
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.
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.
The phase-space description of signals is a joint space-spatial frequency representation in local domain.
It employ the phase-space representation functions to describe the property of optical signals between
two variables that form a Fourier transform pair and provide a valuable analysis tool for signals when
this new mathematical analysis tool applied to an optical system. This method can also be applied to
the analysis of propagation of laser beam within free space or atmosphere condition. In this paper, we
give the analytical formula of optical field distribution of laser beam after propagation within
turbulence in the domain of phase-space. The results show an instructing aspect of optical signal
propagation and are helpful to the future application of phase-space method to the laser
communications.
Laser communication links between satellite and ground remains a bottleneck problem that has not been solved in free
space communication network now. Atmospheric disturbances have badly influenced the wave-front of signal light and
destroyed the integrality of optical phase, so the bit error rate (BER) is increased at the receiving terminal in the
space-to-ground laser communication. With conventional coherent reception, the contrast of coherent light increased
dramatically, and transmission efficiency of Space to ground laser communication decreased. Receiving technology
based on differential phase shift keying (DPSK) is proposed here to overcome the effects of atmosphere here.
Differential phase shift keying without the integrality and compensation of the optical phases, is suited for high rate
space to ground communication links due to its immunity of the wavefront of a beam passing atmosphere. A
Mach-Zehnder delay interferometer is used for differential delay which is equal to the one bit period. The differential
data information can be obtained from the optical phase changes. Differential phase modulation technique can be a
promising optical receiving technology.
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