Home – Home – Jornals – Atmospheric and Oceanic Optics 2016 number 11

In this paper, we present new results obtained within Russian Science Foundation project N 15-19-20013. The project was started in the middle of 2015. It is devoted to the solution of actual problems of the optoelectronic industry on the basis of the use of adaptive optics in astronomy, laser, and other applications. Recent achievements in this industry are completely determined by the progress in priority directions of science and critical technologies. One of these directions is the design of adaptive optics (AO) systems, which allow overcoming random distortions and, thus, achieving diffraction-limited performance for systems operating in a turbulent atmosphere. We describe the progress in the development of the newest AO system for the Russian largest solar telescope, i.e., 1-m Big Solar Vacuum Telescope of the Baikal Astrophysical Observatory, and approaches to the development of such systems operating in strong day turbulence conditions.

Structure of air turbulent motion inside the dome room (Primary mirror closed shaft) at Siberian lidar station of V.E. Zuev Institute of Atmospheric Optics of SB RAS has been experimentally and theoretically studied. The researches are needed to forecast the laser radiation distortion. Experimental measurements have been performed with the portable compact ultrasonic meteorological station. The major heat-exchange directions of air flows inside the dome have been determined. Theoretical results have been obtained by numerical solving of the boundary value problem for Navier-Stokes equations. Solitary large vortices (coherent structures, topological solitons) are observed indoors. Coherent decay of these vortices leads to the coherent turbulence. One may expect the weakening of optical radiation phase fluctuations inside the dome and, therefore, the enhancement of optical images. It increases the efficiency of lidar station.

In this report, the results of simulation are presented of multichannel radiation propagation in the atmosphere. Correction of turbulent distortions on the basis of the beam phase control is considered. The results demonstrate the dependence of the correction effectiveness on the number of channels and on precision of reference beam phase reconstruction. Additional increase of the effectiveness is possible with adjustment of amplification in the channels of the optical system, i.e., with the use of amplitude-phase control of radiation.

The comparison of analytical models of vertical profile of the structure parameter of atmospheric refractive index is performed as one of the key points for the development and investigation of the effectiveness of adaptive optics systems. The possibilities of the development of the models are analyzed. The model of atmospheric turbulence profile for the Baikal Astrophysical Observatory is developed.

The paper presents a new algorithm for generating random phase screens, which are used in the numerical solution of the problem of laser radiation propagation in a turbulent atmosphere. Developed on the basis of this algorithm, a numerical model of the evolution of phase fluctuations of a light field at the entrance aperture of an adaptive optics system allows us to estimate the temporal transformation of atmospheric inhomogeneities, which include small-scale wind fluctuations, which occur in the real atmosphere, along with the wind transfer of turbulent inhomogeneities. Using this numerical model, a correlation method for calculation of the speed of the crosswind turbulence transfer from measurements of a wavefront Shack-Hartmann sensor is analyzed.

A number of aspects are considered at phase correction of regular and vortical (speckled) laser beams by flexible adaptive mirror, the surface control of which is performed with the help of stochastic parallel gradient (SPG) algorithm, without the use of the wavefront sensor. It is shown that optimal choice of the criterion functional (metric) and basis function set allows one to improve the phase correction accuracy and convergence speed of SPG algorithm. A possibility is demonstrated of partial phase correction of the incoherent multimode radiation that can be realized, for example, in а laser cavity with optically inhomogeneous gain medium.

The paper considers the performance of an adaptive optics system and its components at an artificial atmospheric beamlet, which can provide the controlled and reproducible influence onto the radiation beam along its path. The paper outlines the results of experimental investigation of the polychrome radiation wavefront distortions measured with the use of two Shack-Hartmann sensors. Qualitative and quantitative comparisons of wavefront parameters were carried out. The relationships between the wavefront parameters and the radiation wavelength were revealed for various lengths of the atmospheric path.

The control algorithm for adaptive optics system, which works using focal spot of the light beam, is proposed. Algorithm is based on the analytical relationship between spot-radius and the changing of the deformable mirror surface. A numerical modeling, which confirms this dependence and the opportunity of its usage for wave front correction, has been carried out. Some experimental results that point out the opportunity of using the algorithm in practice have been presented.

TM-modes of surface plasmon polaritons (SPP) can be excited on the surface of the metal layer of an adaptive mirror at falling of a bulk electromagnetic wave. A part of the energy of the electromagnetic wave is involved in excitation of the SPP modes. The E-modes of the SPP are excited at reflection of the TM-modes from the boundaries of deformed areas on the adaptive mirror surface. The superposition of TM-modes and E-modes leads to the formation of SPP vortices at singular points of the interference field. The topology of the SPP vortices changes depending on the curvature of the boundaries of deformed areas on the adaptive mirror surface. In this case, the SPP vortices appear and disappear in the components of the Poynting vector, and the screw dislocations emerge at the wavefront at the singular points of the field. Emergence of SPP vortices on the metal surface of a mirror should be considered when calculating the wavefront correction parameters in the adaptive systems.

The results of two-site measurements of aerosol optical depth (AOD) of the atmosphere in Primorye (Vladivostok - village Gornotayezhnoe, a distance of 60 km), near Tomsk (Akademgorodok - observatory “Fonovaya”, 60 km), the western and eastern coasts of the lake Baikal ( 69 km) are considered. It is shown that the level of turbidity of the atmosphere over Vladivostok is higher than over the remote inland area. Moreover, the main contribution to the differences is caused by high content of the coarse aerosol, obviously of marine origin, in the atmosphere over Vladivostok (compared with village Gornotayezhnoe). The difference between the measurement sites in other two regions does not exceed the error in determining the AOD.

The temperature series from 818 weather stations of the Northern hemisphere and the Wolf numbers for the period from 1955 to 2010 are considered. Components of the series, acting in opposite directions and having extreme properties, are introduced. The presence of various connections between the solar activity and the temperature are confirmed. Conditions of occurrence of the connections are discovered. The developed approach is advisable to apply for the analysis of observations and to analytical transformations.

The paper proposes and investigates the method of measuring the height of the cloud base using the stereo pair of images taken using two digital cameras. The method of determination of camera orientation parameters required to solve the problem uses images of the night star sky. To calculate the distance to the cloud, the image shift of a fragment of the cloud as a whole is used. The selection of the fragment in the photos is performed using methods of morphological image analysis. When the stereo base is 60 m and taking pictures with a resolution of 1200 pixels in the field of view of 60 the error does not exceed 10% at the height of the cloud base less than 4 km. Optimization of the parameters of photography, including the increase of the stereo base, can significantly improve the accuracy of the estimation the height of the cloud base. Examples of estimation of the height of cloud base on the layout of the experimental setup are compared with the measurements of the laser range finder.

The applicability of the formulas obtained in the first part of the article [1] is investigated in a numerical experiment. It is shown that the geometric optics approximation is applicable only for short distances hundreds of times smaller than the corresponding length of the diffraction. Limitations, caused by the oscillations of the original integral equation kernel, lead to strong oscillations of the third derivative of the measured data. Relationships based on the asymptotic formula for a point receiver are insensitive to the oscillations of the measured data. The application of formulas for point receiver in the case of finite radius of receivers leads to smoothing inversed distributions and their shift relative to the given ones. Method of accounting for these factors is developed. The use the approximation of a point receiver together with smoothing the restored distributions leads to a partial loss in the restored data information about the turbulence spectrum. This makes it possible to simplify the solution of the inverse problem, reducing it to the calculation of the second order derivatives.

A method for measuring vertical profiles of meteorological parameters using portable automatic electronic weather station mounted on an unmanned aerial vehicle (PEWS-UAV) is proposed. Facility description and the results of the field test are presented, and PEWS-UAV development prospects are discussed.