For free space optical transmission, vortex beam can greatly improve the channel capacity, but it is easy to be affected by atmospheric turbulence. To solve this problem, in this paper, a radially aligned Gaussian beam array is loaded with a discrete vortex phase-coherent synthetic vortex beam. Based on the multi-phase screen numerical simulation method, the transmission of the synthetic vortex beam in Von Karman spectrum atmospheric turbulence is simulated, and the intensity distribution, drift and flicker characteristics of the beams under different turbulence intensity are studied. The effect of topological charge on light intensity flicker and beam drift is also discussed. The simulation results show that when the coherent vortex beam is transmitted in atmospheric turbulence, the turbulence makes the intensity distribution at the receiving end disordered and the phase distribution distorted. With the increase of turbulence intensity and transmission distance, the scintillation index and drift mean square error will increase, but when increasing to a certain extent, the scintillation index will tend to be flat. Under the same transmission conditions, the more topological charge of the beam, the better the transmission quality of the coherent synthetic vortex beam.
KEYWORDS: Data transmission, Chlorophyll, Monte Carlo methods, Signal attenuation, Photon transport, Data modeling, Laser scattering, Particles, Absorption, Scattering
With the development of photoelectric detection technology, blue-green laser underwater transmission and target detection have become a hot research field. At present, most of the underwater laser attenuation channel models regard seawater as a homogeneous medium, and a few layered models only consider the change of chlorophyll concentration with seawater depth, and the number of layers is small. This paper not only considers the influence of chlorophyll concentration, but also introduces the influence of temperature and salinity, increases the number of stratification intervals, and improves the seawater stratification model. Based on the measured data of temperature, salinity and chlorophyll concentration in the vertical direction of seawater provided by the Array for Real-time Geostrophic Oceanography data center website of China, the vertical distribution of seawater attenuation coefficient is given. Using this distribution, the seawater is stratified, and a more accurate vertical stratification model of seawater in the Pacific near Japan is established. Using the vertical stratification model of seawater established in this paper, the transmission process of blue-green laser underwater is simulated based on Monte Carlo method. The results show that with the increase of transmission distance, the number of photon packets on the receiving plane in layered seawater is always larger than that in uniform seawater. The photon packet energy on the receiving plane in the case of stratified seawater and the photon packet energy in the case of uniform seawater increase alternately, which provides a reference for the transmission characteristics of laser in actual seawater.
The characteristics of aerosol optical depth (AOD), Ångström exponent α, water vapor content and turbidity coefficient were analyzed by using the latest observation data of CE318 Sun-photometer in the southern suburb of Xi’an between December 2019 and November 2020. In addition, the frequency distribution and aerosol types were studied, and the influencing factors were analyzed briefly. The annual mean value of AOD in the southern suburb of Xi'an was 0.529±0.358, and the distribution of frequency was unimodal. The values of AOD in winter and spring were higher than that in summer and autumn. The annual mean value of α was 0.894±0.373, and the distribution of frequency was multipeak distribution. The number of the α value lower than 0.5 in spring was significantly higher than that in the other three seasons, which was caused by the influence of coarse mode particles, while the other seasons were affected both by coarse and fine mode particles. It can be seen that different types of aerosols are mixed in Xi’an. The influence of Desert Dust Aerosols in spring is greater than that in other seasons. Biomass Burning/Urban Industry Aerosols (BB/UI) are the main types in autumn and winter, and all types of aerosols are relatively balanced in summer.
Based on the unified theory of coherence and polarization of stochastic electromagnetic beams and extended Huygens-Fresnel principle, an analytical expression of beam width for radial stochastic electromagnetic Gaussian-Schell model (EGSM) array beams in non-Kolmogorov turbulence is obtained. The effects of the beam parameters, exponent value α, inner and outer scale parameters on the spreading of array beams are studied in detail. It is shown that the effect of turbulence on spreading of radial stochastic EGSM array beams can be reduced by choosing the suitable array beam parameters.
High imaging resolution can be achieved by using synthetic aperture ladar (SAL) with laser radiation source. The
destruction of the signal phase information caused by atmospheric turbulence makes the optical heterodyne detection
efficiency reduce. Therefore the imaging performance of SAL degraded seriously. The study on the influence of
atmospheric turbulence on SAL imaging is of great significance and an effective compensation method of image is
necessary to be found. Research shows that conventional phase gradient autofocus (PGA) algorithm has some
improvement on SAL imaging only in weak turbulence. The mixed phase compensation method combining Rayleigh
laser guide star (LGS) with PGA algorithm is presented based on the real-time detection of optical wavefront phase
distortion with Rayleigh LGS and the phase compensation method of the SAL images. The phase distortion caused by
different turbulence intensities with von Karman spectrum is estimated with Rayleigh LGS. SAL echo signals are
compensated with the estimated phase and the PGA algorithm is implemented in the final imaging data. The results show
that significant improvements of the SAL images in moderate turbulence are obtained and the images can be identified
basically by using the mixed phase compensation method in strong turbulence. The focusing effect of the SAL images is
improved effectively, and a higher SAL resolution is gained in azimuth. In addition, the research of SAL imaging
compensation in atmospheric turbulence in a slant path is carried out for the first time, which is of great significance to
the practical application of SAL.
Based on the theory of optical wave propagation in the slant path and the ITU-R turbulence structure constant model
which is dependent on altitude, the on-axis scintillation index of the flat-topped Gaussian beam at the receiver plane in
slant path turbulence was given by using Kolmogorov atmospheric turbulence power spectrum model. The influences of
the link altitudes, atmospheric refractive index structure constant C0 at the ground,the source size and the beam order on scintillation index of the flat-topped Gaussian beam are discussed in detail. The result shows that the scintillation index
increased first and then decreased with the increase of the beam order. The advantage of a flat-topped Gaussian beam
over a single Gaussian beam is restricted to small source sizes, which is consistent with the case of the horizontal path.
To find the average bit error rate under weak slant path turbulence, the log-normal distribution model of the intensity
fluctuation was used. The influence of beam order and source size on BER was discussed. The result indicates that the
smaller sized flat-topped Gaussian beam will bring average bit error rate advantage over the same size Gaussian beam.
Our results correctly reduce to the result of the horizontal path with atmospheric structure constant fixed.
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