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.
Ranging distance was one of the most important index of laser rangefinders. However, it could be affected by the atmospheric turbulence. Effect on maximum distance critical value of laser rangefinders with atmospheric turbulence was discussed. Firstly, the atmospheric structure constant of refractive index (C2n) of specific area was monitoring and the variable regularity was analyzed. With the assumption that the target was the small target, the power of small target was calculated. Then, the receiving power of detector was obtained. Lastly, the maximum distance critical value with the same visibility and different turbulence intensity was got. When the refractive index (C2n) was 5×10-15m-2/3, 5×10-14 m-2/3, 5×10-13 m-2/3 and 5×10-12 m-2/3, the maximum distance critical value was 1900m, 1850m, 1850-1810m and 1810m. It could be generalized that the maximum distance critical value of laser rangefinders could be affected by the atmospheric turbulence.
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.
The circular-polarized laser can excite Sodium Laser Guide Star (SLGS) to obtain a large number of the return photons. Re-pumping technology can further enhance the return photon flux of SLGS. But laser propagating in the atmosphere suffers the atmospheric turbulence which causes the stochastic distributions of laser intensity in mesosphere. This leads to the fluctuations of return photon flux as the strength of atmospheric turbulence. We study effects of atmospheric turbulence on the return photon flux of SLGS under the Hufnagle-vally5/7(HV5/7), Greenwood and Mod-HV models by numerical simulation. Results show that both the return photon flux and fluctuations under the strong atmospheric turbulence are more than those under the weak one. Comparing re-pumping with no re-pumping, increment of the return photon flux under the three atmospheric turbulence models increase with the decreasing strength of atmospheric turbulence. But the fluctuations of the return photon flux greatly decrease for re-pumping.
Based on the relative intensity distributions of Sodium Laser Beacon (SLB) and analysis of the on-axis imaging of incoherent light, considering the effects of atmospheric turbulence and the changes of telescope receiving diameter on the short-exposure SLB imagings on the focal plane, imagings of an extended source SLB are simulated under the three atmospheric turbulence models. Results indicate that sharpness and peak strehl ratio of SLB imagings increase but sharpness radius decrease with the decrease of atmosphere turbulence strengths. Moreover, the changes of telescope diameter from 3.0m to 1.5m cause the decrease of sharpness and peak strehl ratio but the increase of sharpness radius.
As one of the main weapons, impulse laser rangefinders have become the main object of the electro-optical countermeasures. So its real maximum range (defined as utmost operating range in the paper) becomes the most concerned index to evaluate the performance of electro-optical countermeasure weapons. A method for calculating laser rangefinders′ utmost operating range by its sensitivity in different weather is obtained. Then a method by experiment for getting the sensitivity is supplied. By analyzing the experiment data which the detectivity is 40%-60%, the laser rangefinders′ sensitivity is in the range of 1.7×10-5 W to 9.8×10-5 W. For the reason that in order to get an exact utmost operating range, the experiment accuracy of sensitivity is very important, in the last part of paper, the factors which influence the experiment accuracy of sensitivity are analyzed, such as circuit of automatic gain control, the fluctuation of laser power, incident angle of laser.
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.
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