Sensible heat flux (H) is an important physical quantity to characterize the turbulent energy transport in the near-surface atmosphere, which is benefit for understanding the spread and diffusion of urban atmospheric pollutants and weather changes. Referring to the principle of the large aperture scintillometer (LAS) to obtain H, we propose a method for detecting H by using laser speckle imaging (LSI) system based on a cooperative target. The experiment system used 3M microcrystalline reflective film to effectively reduce the requirement of laser power. Besides, it could overcome the limitation of the complex terrain of the test field, such as islands and hills where the transportation and electricity supply are not convenient. The refractive index structure parameter 2 Cn , which is required to calculate the H, inversion from image scintillation method and centroid drift method are compared to the 2 Cn inversion by coherence length meter. The comparison results show that 2 Cn obtained by the image scintillation method are not reasonable, while the results obtained by the centroid drift method are in good agreement with the coherence length meter, especially under the circumstances of laser beam near collimation.
In this paper, we investigate that the effects of weather, turbulence and pointing errors on laser beams, and establish a joint channel statistical model, then derive the performance parameters of free space optical communication system based on OOK modulation, such as average bit error rate, average channel capacity and outage probability, and finally derive their new expressions by using Meijer’s G function and H function. For an atmospheric laser communication system with a wavelength of 1550 nm and a link length of 1 km, the performance parameters of the system are simulated and analyzed. The results show that when the standard deviation of jitter is small, the communication system is obviously affected by turbulence, and the performance of communication system with different turbulence can be significantly improved by the aperture average effect; aperture averaging can also significantly compensate for the deterioration of the communication system performance due to the decrease of atmospheric visibility; when the standard deviation of jitter is large, the system performance is limited by the average aperture effect.
Laser ablation absorption spectroscopy (LAAS) is an analytical technique by combining diode laser absorption spectroscopy (DLAS) with laser ablation (LA) technique. It has been developed to atomize various samples and distinguish elements and isotopes directly without chemical separation. This article reviews the principle and recent research highlights of LAAS technique.
To get the knowledge of sky background spectral radiance and its usage in retrieving atmospheric parameters, a wide-spectrum fiber spectrophotometer was calibrated with the field view of one degree. An experiment was performed at the fine days after a heavy snow weather process, and the horizontal spectral radiance direct to the south was measured periodically during the whole days. We found that when the sky was fine, the short-wavelength sky radiance was significantly higher than the long-wave band. This is because the intensity of the scattered light is inversely proportional to the fourth power of the wavelength. A sunny day with red sky in the morning, the snow began to melt., the short-wavelength sky background radiance was lower than the long-wavelength band. The absorption band of water vapor in the spectral data is apparent, which implies the possibility of retrieving atmospheric water vapor content from these data. Using MODTRAN, the Continuum Interpolated Band Ratio (CIBR) near the 940nm water absorption bandwidth was estimated, and the water vapor column concentration was retrieved. Comparison the results with what was measured by the POM2 sun radiometer show satisfied consistence. The method of retrieving water vapor content from the background spectral radiance provide us a way to have a deeper understanding of the absolute radiance of the sky background.
A theory about scales in atmospheric optical turbulence vortex from the point view of spatial correlation function is described. Then an experiment is carried out to prove this theory by the fiber optical turbulence sensor array near the ground. Results show that the outer scale has a mean value of 0.62m and varies from 0.34m to 0.95m by doing a nonlinear fitting on spatial correlation functions. With this method, the value of the outer scale can be given directly without any hypothesis when the optical turbulence is well-developed. A question about how the trend of the spatial correlation function show when the displacement approaches the outer scale is solved. This research can be regarded as a progress about understanding the characters of spatial correlation function in optical turbulence.
The performance of satellite-to-ground laser communication is affected by the atmospheric environment seriously, so it is a key issue for system design and operation to analyze the link availability quantitatively according to the channel characteristics. Firstly, the basic concept of laser communication link availability is put forward. Secondly, the preliminary theory model and calculation method of geostationary satellite-to-ground laser communication link availability are established by analyzing influencing factors. Lastly, the link availability of 77°E GEO satellite-to-ground laser communication is analyzed, combining with the atmospheric statistics data of typical optical ground stations in China. The results show that, it is feasible to achieve more than 90% joint link availability by implementing two stations, or to achieve more than 95% joint link availability by implementing three stations.
In this article, a high quality fiber optical turbulence sensing array contains several sensors is proposed to obtain time series of air refractive index fluctuations. A fixed sensor is supposed to be set as the origin and others to be arranged in Cartesian coordinates respectively. Under the spatial configurations above, two-point correlation algorithm is used to give two order structure parameters and multipoint correlation algorithm for more structure information about scalar turbulence. For each direction, two-point spatial correlation coefficients varying with distance are provided. Meanwhile spatial power spectrum and outer-scales according to the data are introduced. Multipoint correlations can give more structure information such as the interactions between scales and the spatial structure of relevant fluctuations. In the one-dimension circumstance for instance the x-axis, spatial correlation coefficient tends to take oscillation. After a short-time averaging, it tends to decrease with the increase of spatial displacement, and then tends to zero after outer scale. Further study show that within the limit of outer scale, diurnal variation of the spatial correlation coefficient and intensity reveal a higher similarity, the relevancy is about 60% and keeps stable; once the distance goes across the outer scale, they are uncorrelated. In short, utilizing the fiber optical turbulence sensing array is a new method for measuring spatial correlation of optical turbulence. It can overcome some problems from single-point measurement, especially when using Taylor’s frozen-turbulence hypothesis. Some structural information of optical turbulence not only makes the theory of scalar field more abundant, but also in favor of some problems about optical propagation.
Effects of light propagation in random atmospheric optical turbulence are critical problems for ground based high
resolution optical imaging. To get further knowledge of turbulence intensity or structure properties, the concept of fiber
optic sensing system is proposed and realized. Different to fine-wire platinum resistance thermometer or laser
scintillometer, the system has the ability to make non-contact measurement of optical turbulence up to the frequency of
500Hz during the air gap of 100mm, and has the dynamic range of 10−18 ~ 10−12 . The optic fiber’s merit of corrosion
resistance is sufficiently demonstrated by one month field test on the seacoast. Some properties of high frequency
turbulence power spectrum that have never been observed before in the range of 10cm are reported. In the end, prospects
of optical turbulence research by the usage of fiber optic turbulence sensing system are also discussed.
A novel fiber optical turbulence sensing system based on phase generated carrier technique is developed for measuring the intensity and structure of local atmospheric optical turbulence. The turbulence-induced air refractive index fluctuations are detected through fiber optical turbulence sensor array and demodulated by the algorithm of correlation. The system’s dynamic range is 10-18 ~ 10-12, which satisfies most of the sceneries in the boundary atmosphere. Time series of the measured refractive index fluctuations and its power spectrum up to the frequency of 500Hz in different conditions such as sealed box or free turbulent atmosphere are illuminated and analyzed. In the end, some of its applications are briefly outlined.
Optical turbulence degrades the quality of laser beam propagation and the quality of the image of optical system, limiting the spatial resolution that can be obtained. A novel single-air-gap fiber optical interferometric system useful for non-contact measurement of the fine structure of optical turbulence is presented. The main idea of this system is based on the application of a specially constructed optical fiber Mach-Zehnder interferometer to measure the phase fluctuations effected by the random fluctuations of refractive index in the turbulent atmosphere. The light source is a long coherence length infrared laser operating at the wavelength of 1.31μm and the optical path exposed to the atmosphere can be adjusted to a most suitable value according to the operational environment. Theoretical estimation illustrates that the system can measure the minimal atmospheric refractive index fluctuation up to 10-10 during a 2cm propagation path. It is easy to have a calibration of the system and the result shows that the voltage refractive index sensitivity is about 2.1x10-6 V-1. The system is integrated and well fixed in a burly airproof box with only the sensing arm exposed to the air. It follows that the system is suitable for the measurement of atmospheric turbulence over land and ocean surfaces.
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