The present study utilizes a turbulence radiosonde to obtain profiles of atmospheric temperature, pressure, wind field, and optical turbulence at different time periods in the northwest region of China. Three turbulence parametric models based on Tatarski theory (Dewan model, Kondo model, and HMNSP99 model) are selected for comparison and validation with measured turbulence profiles. According to the radiosonde data, wind shear, potential temperature gradient and Richardson number are calculated, and the corresponding relationship between them and the measured C2n (atmospheric refractive index structure constant) profile is analyzed, which confirms the feasibility of Tatarski theory in the local area. The C2n profiles calculated by the three models were compared with the measured C2n profiles at different time periods. The results show that Tatarski theory has a large error in the estimation of nearsurface layer and boundary layer turbulence, but has a better estimation effect on the free atmosphere turbulence dominated by wind shear. We also analyze the applicability and limitations of the three models, and the results of root mean squared error (RMSE) and correlation coefficient (Rxy) show that the HMNSP99 model has the best application effect in this area, which provides method verification for the application research of atmospheric optical turbulence model in typical region.
Using the sun-sky radiometer CE318 data in 2013~2019 observed in typical dust regions including Kashgar, Zhangye and Minqin, the variation characteristics of Aerosol Optical Depth(AOD), Angstrom exponent, single scattering albedo and asymmetry factor were analyzed. The results show as follows. There are significant differences in the variability characteristics of AOD at three typical dust-type stations. The seasonal variation of AOD in Kashgar is the most obvious. The highest AOD is in spring with more dispersed KDE, and the lowest in winter with more concentrated KDE. In Zhangye and Minqin, the seasonal difference of AOD is small, with monthly means below 0.9 and more concentrated distribution in the range of 0.5~0.8. The value of Angstrom exponent in Kashgar is higher in autumn and winter than spring and summer. In spring, summer and autumn, Zhangye and Minqin have significantly higher Angstrom exponent than Kashgar. The single scattering albedo in all three stations is above 0.85, and the asymmetry factor ranges from 0.63~0.8, indicating that the aerosol scattering effects are evident throughout the year.
This paper illustrates an adaptive mesh grid scaling algorithm, which can be applied in the simulation of laser propagation. The algorithm can effectively improve the far-filed laser spot resolution in the simulation of focused laser propagation. And the algorithm’s effectiveness is verified by the simulation of various laser beam type’s diffraction. The phenomenon of diffraction exits in all the laser beam propagation, in which the divergence angle is directly proportional to the laser wavelength, and inversely proportional to the laser aperture. The proportion coefficients will vary depending on the type of laser beam, such as gaussian beam and flat-topped beam with different obscuration, which have accurately theoretical value. Moreover, the simulation results of adaptive mesh scaling algorithm agree well with theoretical values, which effectively checked the algorithm’s feasibility and accuracy.
High power fiber lasers can be incoherently combined to form the basis for high energy laser applications. Incoherent combining of fiber lasers has a number of advantages over other laser beam combining methods. However, the far-field beam quality of the incoherently combined laser array can still be significantly degraded by atmospheric optical turbulence. In this article, a general scaling law for propagation of incoherently combined laser array through atmosphere is developed by employing theoretical analysis and the common stochastic wave optics technique, and mainly focus on the effects of diffraction and atmospheric optical turbulence. The scaling law developed in the present work differs from standard scaling laws in its definition of irradiance. We show that the far-field irradiance and beam dispersion of any incoherently combined laser array, regardless of near-field beamlets geometry, can be obtained in terms of four basic parameters: laser power, field effective area, pupil field factor, and the Fried parameter.The results show that the formula is simple but predicts peak irradiance and beam dispersion accurately in the far field with varying levels of atmospheric turbulence, regardless of the near-field beamlets geometry.
Atmospheric transmittance can critically affect the accuracy of measuring infrared characteristics exhibited by targets. On the whole, the existing measurement of atmospheric transmittance has complied with the engineering calculation results of the MODTRAN software by applying several vital parameters (e.g., temperatures, air pressures and water-vapor content). In general, the error of such a method exceeds 20%, and it is significantly impacted by local weather. In this study, a ratio correction method was adopted to decrease the error in measuring atmospheric transmittance. The correction factor was determined by comparing the directly measured value from the infrared images of reference blackbody at different temperatures with the calculated value of the MODTRAN. Subsequently, the correction factor could be exploited to correct atmospheric transmittance. The experiment for measuring infrared radiation was performed, and the radiance inversion error was reduced by more than 10% after the correction of atmospheric transmittance. Furthermore, the correction factor calculated from LWIR images could be extrapolated to other bands. Besides, the inversion accuracy of the infrared radiation characteristics significantly increased. Thus, the multi-band applicability of the correction method was verified.
Thermal blooming can have a major impact on high energy laser (HEL) beam propagation in the atmosphere. Previous analyses about this effect are mainly focus on monolithic beam. In this paper, the numerical framework of atmospheric propagation of coherently combined beams is established. The corresponding time-dependent wave optics simulation code is also developed and utilized to investigate the propagation properties of coherently combined beams under various thermal blooming conditions. For the purpose of comparison, the simulations of atmospheric propagations of incoherently combined and monolithic beam are also included. In the end, the effect of fill factor is investigated in a preliminary manner. The beam propagation efficiency (BPE) as the performance metric of coherently combined beam is employed in this paper. The results show that the thermal blooming places a significant limit on the ability of coherently combined beams. The BPE is degraded significantly under the strong thermal blooming condition. Consequently, coherent beam combining is ineffective under typical atmospheric conditions.
The optical vortex (OV) is one type of optical singularity which has a spiral wavefront around a point where the intensity of light is zero and phase is undefined. Exact determination of the OV properties, involving location and sign, are very significant. In this paper, a novel OV detection method using the fractional Fourier system is presented. The fractional Fourier system is employed to provide high-sampling density phase gradient data. The closed form formula for the Hertz potential in terms of phase gradient measurement is given. It is shown that the presence of optical vortices could be visualized as the peaks and valleys of the Hertz potential where peaks correspond to the positive optical vortices and valleys correspond to the negative ones. Therefore it allows the determination of the OV location and sign in a very straightforward way. The validity and reliability are demonstrated through several numerical examples including noisy signals with different signal-to-noise ratio (SNR) levels. The outstanding merits possessed by the proposed technique are its low experiment effort and high detection precision.
In this paper, a novel Modal decomposition (MD) method based on the Wigner representation of fiber modal field is developed. An important distinction between this work and some others is that it can allow either fully or partially spatial coherent modal contents to be decomposed into guided modes. When the Wigner representation is reconstructed, based on the orthogonal property of guided modes and Moyal identity rule, the modal weights and possible relative phases can be obtained in an exact way. Further, the validity and reliability of the method are demonstrated with the numerical simulations.
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