Atmospheric turbulence causes refractive index fluctuations, and atmospheric molecules absorb and scatter light wave energy, causing the power density of the laser to reach the target to decrease, restricting the application performance of advanced optoelectronic systems. Precision optoelectronic equipment has problems such as long development cycles and difficulty in quality control. Digital twins have low cost to establish digital models, can be quickly iterated, and have great potential in system analysis and diagnosis. According to the laser atmospheric transmission physical model and performance evaluation requirements, the laser transmission model is constructed and integrated in the digital space, and the C++ language, Qt (Q Toolkit) platform and multi-threaded parallel processing technology is used to develop and test the software system. Provides practical software tools for laser transmission performance evaluation and system parameter optimization design. With the help of the software system, the functional relationship of the refractive index structure constant, wind speed, absorption coefficient and extinction coefficient profile with height changes as well as the energy intensity distribution of the target surface spot can be calculated and visualized.
Due to variations of influencing factors and atmospheric effects, the propagation efficiency (transmittance, thermal distortion parameter and 63.2% encircled average power density) of high-energy laser propagating in the atmosphere is uncertain. In this paper, aiming to evaluate the uncertainty of propagation efficiency and identify the main influencing factors, the following research is made. (1) The scaling law is established through numerical simulation, which is suitable for the Gaussian waveform laser and considers the interaction between different effects. (2) The probability distribution characteristics and uncertainty of propagation efficiency are evaluated in the horizontal propagation scenario by the Latin hypercube sampling method. (3) The Elementary Effect Test is applied, with the aim to give the parameters prioritization and identify the crucial parameters affecting encircled average power density. The results show that the uncertainty and parameters prioritization of propagation efficiency vary with the propagation distance. Considering the results of the Elementary Effect Test at different distances, the crucial parameters for 63.2% encircled average power density are transverse wind speed, absorption coefficient, power, and initial beam quality. This research is of great significance for the application of laser systems.
Scaling law and artificial intelligence model are two methods to quickly evaluate the far-field spot characteristics of laser propagation through turbulence. On the one hand, it is necessary to compare the evaluation accuracy of both models. On the other hand, the comparison between different models is only meaningful if each has their best accuracy. For specific scaling law model and artificial intelligence model, scaling exponents and hyperparameters determine the evaluation accuracy of the model to a certain extent. This paper discusses how to search better scaling exponents and hyperparameters to construct each model and compare the evaluation accuracy of both models. This paper first introduces the MRSS (Modified-Radius-Square-Sum) scaling law model and FT-Transformer (Feature Tokenizer + Transformer) model, and 3 hyperparameter (scaling exponent) optimization algorithms. Then, the accuracy of scaling exponents and hyperparameters obtained by different optimization algorithms is compared. Finally, the best scaling exponents and hyperparameters are used to construct each model. The results show that the TPE algorithm achieves better search results in fewer iterations for the FT-Transformer model, and the CmaEs algorithm achieves higher accuracy in more iterations for the scaling law model. The FT-Transformer model has better accuracy compared to the scaling law model, with the mean relative error of the far field effective radius and mean intensity is 1.32% and 2.66%, while those of scaling law model is 1.97% and 3.91% respectively.
Atmospheric coherence length represents the diffraction limit of light wave propagation through atmospheric turbulence, which characterises the integrated optical turbulence intensity in the atmospheric transmission path of light waves, and the measurement of atmospheric coherence length is of great significance. A light column turbulence lidar for atmospheric coherence length measurements has been developed based on differential image motion method and oblique range imaging for laser light column, which can obtain atmospheric coherence length profiles at a certain height in the vertical direction with a high spatial resolution, and can overcome the problem of non-active turbulence measurement devices that are not able to measure without suitable beacons or beacons that are occluded. The basic principle and system structure of differential light column lidar are introduced, and the whole lidar system can be divided into laser transmitting unit, signal receiving unit and control unit. The laser transmitter unit adjusts the direction of laser emission through the reflector, CCD camera, diaphragm and stepper motor. The signal receiving unit receives the beam backward scattering signals through the telescope and CCD camera to obtain the light column image, and adjusts the position of the CCD camera through the stepper motor to achieve focusing. The control unit receives and sends signals to control the operation of the entire system. A preliminary detection experiment was carried out with this lidar, and the height distribution profile of atmospheric coherence length was obtained. The variation trend of atmospheric coherence length with altitude is consistent with the theoretical trend, which verified the functionality of the lidar system.
To optimize the application of the array fibers in the measurement of large angle laser parameters, the fiber loss is analyzed specifically. And the quantitative method of the actual and available numerical aperture about the fiber is studied, which also represent the allowable incident angle of the laser of the measurement system. Firstly, the theoretical model of fiber loss is constructed, and the influences of incident angle, fiber bend and theoretical numerical aperture on loss are discussed. Secondly, the basic loss, which be seen as the background value, is measured experimentally; by which it is considered that the all-glass fiber with coating and protective layers is suitable for this application. Thirdly, combined with experiment and simulation data, the tolerable bend of the fiber is quantified: when the bend radius is more than 300 times the core radius, the loss can be unaffected by the degree of bend. Finally, how to deduce the actual numerical aperture from the theoretical numerical aperture is discussed. It is verified that there is a good linear correlation between them, and the fitting goodness reaches 0.9970. This paper provides pertinent and effective standards for the selection and arrangement of the array fibers, and offers a theoretical foundation for constructing large angle laser parameters measurement.
The random change of the atmospheric temperature field causes the fluctuation of the refractive index, which produces the optical turbulence effect. This will affect the results of atmospheric light transmission experiments. In order to study the effect of light transmission under different conditions, an air convection turbulence simulation device was developed. This device can simulate the turbulence field under different temperature conditions. The temperature pulsation method is used to measure the refractive index structure constant and one-dimensional temperature spectrum distribution inside the turbulence simulation device in detail under different temperature difference conditions. The results show that the refractive index structure constant distribution trend on the path of multiple measurements under the same conditions is relatively consistent. In general, the turbulence intensity on both sides is relatively large, and the middle is uniform. The range of equivalent coherence length generated on the optical transmission path is about 2.75cm~109.72cm. The outer scale is about 0.55m under the condition of 100 degrees Celsius temperature difference. The temperature fluctuation spectrum satisfies the -5/3 scale rate in the range of 0.3Hz~20 Hz. The basic turbulence characteristics simulated by this device are close to the actual atmospheric turbulence performance. It has the advantages of controllable experimental conditions, good repeatability and long-term stable operation, etc. Therefore, the device can be used for experimental research on light transmission under different turbulent conditions.
Knowledge of the atmospheric optical turbulence profile (AOTP) is critical for atmospheric optics studies. Meteorological sounding of long-term AOTP observations at seas often comes at an outrageous cost. It is necessary to establish a mathematical model driven by conventional meteorological parameters to predicate the AOTPs at high altitudes. Conventional meteorological parameters TUH (i.e., temperature, wind speed and relative humidity), have an important impact on the sea surface turbulence. AOTPs together with TUHs in Maoming were obtained. Based on the artificial neural network (NN) algorithm, an NN model is established according to the data to predict the upper atmospheric turbulence profile. The AOTPs measurements were used to validate the model predictions with the existing estimation theory. Cross-validation between these methods are performed and evaluated with mean absolute error (MAE), mean variance (MSE) and root mean square variance (RMSE). The results show that the predicted values simulated by the NN algorithm agree well with the real values, which proves that it is feasible and reliable to use the NN to simulate the atmospheric turbulence profile.
Above-low-cloud aerosol (ACA) has important impacts on low clouds bellow. Based on the satellite data from 2007 to 2010, this study analyzed the relationship between ACA optical depth (OD), ACA occurrences and low cloud integrated color ratio (CR) over tropic Atlantic region where ACA frequently occurs. The results show that, the integrated attenuated CR (IACR) of low cloud is about 30%-50% larger over smoke region in smoke outbreak seasons than other regions or seasons. However, the IACR of low cloud over dust region shows small difference between dust outbreak seasons and other seasons. It indicates that above-low-cloud smoke aerosol can introduce stronger color effect than dust. The integrated corrected CR (ICCR) of low cloud tends to decrease with increasing above-cloud dust OD, while the low cloud ICCR shows weak relationship with above-low-cloud smoke OD. And, the above-low-cloud dust aerosol could introduce strong microphysics effect, that is, the low cloud droplet size may decrease with increasing burden of dust aerosol above.
The altitude of atmospheric medium involved in atmospheric optics has a height range of 100km, and the most complicated variation of atmospheric properties is mainly in the atmospheric boundary layer (ABL). The variety of ABL height is of considerable significance to the distribution of aerosol, cloud, and other processes. Since the research of Chinese marine ABL analysis is limited, in this study, we improved the algorithm by using 532nm total attenuated backscattering (TAB) for retrieving atmospheric boundary layer height (ABLH) from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and verified the results gained from micro-pulse lidar (MPL) and radiosonde over the South China Sea. Finally, we used the validated ABLH algorithm model to retrieve the ABLH against CALIPSO data from Mar. 2018 to Feb.2019 over the South China Sea.
Based on the solution to the rate equations of interactions between laser and sodium atoms, the excitation probability of sodium atoms is achieved. The return photons at 330 nm are numerically calculated for the 1 W laser power at the sodium layer in the mesosphere. Results show that a long pulse laser with a few microsecond durations and 1000 Hz repetition rates can well excite the polychromatic laser guide star to obtain no less return photons at 330 nm than that by the continuous-wave laser in a given condition. In order to obtain enough return photons to satisfy the requirement of tip-tilt detection, the parameters of linewidth, durations, repetition rates, polarization and launch diameters of the laser should be properly chosen. And effects of the laser launch diameters and the atmospheric turbulence on the return photons are slight. For the wonderful vertical atmospheric transmittance T0=0.5 at the Starfire Optical Range, Mauna Kea and Paranal, considering the effects of geomagnetic field, when the power of launch laser varies from 10 W to 25 W with the zenith angles from 0° to 40°, the return photons at 330 nm reach 104 ph/m2/s.
In this study, we investigate the fractal properties of optical turbulence profiles. Through rescaled range analysis, optical turbulence profiles roughly exhibit three different regimes. The effects of stratifications are the underlying mechanism contributing to this phenomenon. The results of detrended fluctuation analysis and multifractal detrended fluctuation analysis indicate that optical turbulence profiles have multifractal structure.
The long-term measurement of atmospheric aerosol is constructed via such equipment as visibility meter, optical particle counter, solar radiometer, automatic weather station, aerosol laser radar and aerosol scattering absorption coefficient measurer and so on during the year of 2010 and 2017 in the coastal areas of Guangzhou, China to study the optical parameter characteristics of atmospheric aerosol and establish the aerosol optical parameter mode in such areas. The effects of temperature and humidity on aerosol concentration, extinction and absorption coefficient are analyzed and the statistical characteristics of atmospheric temperature and humidity, visibility, extinction profiles and other parameters in different months are tallied, preliminarily establishing the atmospheric aerosol optical parameter pattern in Guangzhou coastal areas.
By using wave optics numerical simulation, the scintillation of pseudo-partially coherent Gaussian beam propagating in atmospheric turbulence is investigated. The effects of partial coherence on scintillation index are analyzed as a function of the correlation length of beam source. The reduction of the aperture averaging scintillation index, on-axis and off-axis scintillation are shown for a horizontal propagation path. The aperture averaging factor of pseudo-partially coherent beam is
compared with that of fully-coherent beam. And how the pseudo-partially coherent Gaussian beam behaves like partially coherent Gaussian Schell-model beam is also discussed. It was found that the on-axis scintillation index and off-axis scintillation index of pseudo-partially coherent beam can be reduced greatly by decreasing the coherence degree of beam source. The results of aperture averaging scintillation index also revealed the advantage of using pseudo-partially coherent beam compared to fully coherent beam. However, the aperture averaging factor of a pseudo-partially coherent beam is smaller than that of the fully coherent beam at the same receiving aperture diameter. This implies that the aperture averaging effect of scintillation index may be weakened by reducing the coherence degree of beam source. This work may
provide a basis for the utilization of pseudo-partially coherent beam in free-space optical communications.
This paper designs a style of particle counter which may measure the aerodynamic size and the
scattering intensity of two scattering angles of the aerosol particles. The scattering intensity can also be
calculated from the size and the refractive index according to the Mie theory. When the aerodynamic size is
equal to the optics size approximatively, we can inverse the refractive index of individual aerosol particles by
combining the relative results of the measurements.
The intermittency in optical scintillation was theoretically studied and experimentally observed by laser propagation through the turbulent atmosphere over sea surface. It is found that the intermittent characteristics of the atmospheric turbulence repeat itself in optical irradiance fluctuations, then a method of the singularity measurement analysis was introduced and the intermittency index was presented to quantitatively evaluate the intermittency strength. Experiment results show that the intermittency index in laser scintillation is around 0.1 and varies a little compared with the significant variation of the turbulence strength, which had been approximately estimated at two orders of magnitude in a whole day.
The dependence of the intermittency on the wavelength was further studied and the relationship was obtained using a multi-wavelength scintillometer. Generally, the intermittency indices display some differences at the different optical wavelengths, especially in strong turbulence. However, the wavelength dependence is not so notable that the intermittency indices are assumed to be equal within less than 10%, even the maximum error is no more than 20%. Moreover, some statistical results of the intermittency strength are also obtained from a long-term plan of laser propagating in coastal surroundings.
This paper presents a speckle image restoration algorithm using wavelet transform. Based on the wavelet theory, a new gauss PSF accurate estimation is put forward. Firstly, wavelet with varied scales is transformed, after which the local maxima of the modulus of the wavelet are computed respectively. Secondly, on the basis of the relation deduced among the local maxima of the modulus of the wavelet at different scales, Lipschitz exponent and variance, the variance of a Gaussian point spread function is computed. According to Fried theory, the Fried parameter can be deduced from the variance. From the Fried parameter we could estimate the optical transfer function of the turbulence. Row action projection Method is applied to restore the image through atmosphere over a distance of 1 km. Primary results are obtained.
The spot dancing of the focused laser beam in the turbulent atmosphere was studied using a two dimensional position sensitive photomultiplier tube (PSPMT). The centroid position of the laser spot was evaluated by means of current-dividing center-of-gravity detection. The system has advantage over detector array system in spatial resolution and over the imaging system in dynamic range and sampling rate. Laser propagation experiments were carried out over 1000m path above the sea level and the fluctuations of laser intensity were measured simultaneously. The frequency spectra were calculated by fast Fourier tansform and the standard deviation of the spot dancing were analyzed.
Laser scintillation in the atmosphere exhibits fractal behavior. Two fractal characteristic parameters, the fractal dimension and the Hurst coefficient, are analyzed and compared with two traditional parameters, the scintillation index and the scaling exponent of the power spectrum. Results show that the scintillation is a kind of non-stationary process with long-run dependence; under the saturated scintillation conditions, the values of the two-fractal parameters are mostly smaller than those under weak scintillation conditions. According to the statistical behavior of the fractal parameters, it is possible to find new ways to deal with some problems concerning the scintillation saturation.
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