The properties of long plasma-channel generated by terawatt (TW) femtosecond laser propagating in natural environmental air are investigated. It shows that the plasma filamentation with length of 2 km can be achieved when 2 TW femtosecond laser propagates in natural environment, and the electron densityis measured to be 1011 cm-3 at different location of filament, which shows the good conductivity of plasma channel even at the end of channel. Moreover, high voltage discharge test also confirms that, compared with the case without plasma channel, the high voltage of discharging can be reduced by 30% when plasma channel exists, which indicates the effectiveness of fs laser induced discharging. This work paves away for generation of several kilometer and long lifetime filamentation in air, laser induced lightning, environmental monitoring, and artificial intervention of climate.
The propagation of high energy laser in the real atmosphere is a complex process involving the mechanism of light propagation, the optical properties of atmosphere and the interaction between laser andatmosphere. To evaluate the propagating performance of high energy laser, it is necessary not only to study the mechanism of laser propagation to establish the mathematical model of high-energy laser propagation in atmosphere, but also to measure and analyze the optical properties of the atmosphere in the actual transmission path. In view of the urgent need of engineering application of high energy laser systems, the methods and development of the evaluation of high energy laser propagating performance in atmosphere are briefly introduced, with emphasis on the discussion of the uncertainty of atmospheric parameter measurement which affects the accuracy of high energy laser propagation in atmosphere. The research methods, the atmospheric parameters and the development trend of evaluation technology, which should be paid more attention to, are suggested.
The propagation characteristics of laser beam passing through turbulent atmosphere and scattering medium are reviewed. As a laser beam propagates through turbulent atmosphere, its propagation properties will be affected. In this case, the laser beam still keeps the beam-like characteristic, therefore the propagation of laser in the turbulent atmosphere based on the extended Huygens-Fresnel diffraction integral canbe investigated. It is shown that some parameters of the incident light beam, such as beam width, polarization, spatial coherence, and scintillation factor, will experience significant change as the light beam propagates in the turbulent atmosphere. The change of these parameters is not only dependent on the properties of atmospheric turbulence, but also on the characteristics of the incident light beam. Therefore, the influence of the atmospheric turbulence on beam propagation can be reduced by choosing suitable parameters of the incident light beam. It is shown that the speckle pattern will be generated as the coherent laser beam passes through the scattering medium, and by modulating the wavefront of the incident laser beam, the laser beam can be focused after passing through the scattering medium. The techniques to realize focus as a laser beam passes through the scattering medium, including feedback-based wavefront shaping, transmission matrix, and digital phase conjugation, have been introduced. Finally, the applications of these techniques in laser propagation through realistic atmosphere is prospected, for example the propagation through fogs and clouds.
Through the method of multi-layer phase screen, a large number of numerical simulations have been carried out for the laser propagation in the marine atmospheric environment, and 2000 sets of the simulation data of laser initial parameters, atmospheric environmental parameters, atmospheric optical parameters and light field evaluation factors at the receiving surface are obtained. Based on support vector machine, an evaluation method of laser atmospheric propagation efficiency is proposed. By inputting parameters such as initial laser radius, initial laser power, visibility, atmospheric refractive index structure constant and so on, light field evaluation factors such as spot radius, energy circle, and energy circle rate at the receiving surface can be quickly output to realize evaluation of laser atmospheric propagation efficiency. The research results show that the support vector machine can well fit the multiple regression relationship between the input and output, the average relative error between the prediction result and the simulation data is basically below 2%, and the prediction accuracy is high. It is believed that this work can provide a certain theoretical basis for the evaluation of the laser propagation efficiency in the atmospheric environment.
The atmospheric coherent laser communication technology has developed to the stage of practical deployment in the engineering terminals, but its theory is far away from perfect. The theory of atmospheric coherent laser communication and its relating research on adaptive optics and atmospheric scintillation are reviewed and analyzed. The precise analyses for the random process of beam perturbation and adaptive optical phase correction in atmospheric turbulence are the main challenges in completing the theory of coherent laser communication, and the probability method should be adopted in related research. At present, the basic framework of probabilistic analysis of adaptive optical phase correction has been formed, and the concept of scintillation for atmospheric coherent laser communication must be extended to include the variations of speckle and intensity, which should be investigated more fundamentally in future.
Based on the interaction between laser photons and atmosphere, light detection and range (lidar) can measure the atmospheric parameters by active remote sensing, so it plays an increasingly important role in the fields of atmospheric science research, environmental monitoring, weather forecasting and so on. The atmospheric density decreases exponentially with altitude, and the atmospheric density at high altitude is far lower than that at low altitude.Subject to that, the detection for upper atmosphere is of difficulty and the detection data is insufficient. With the development of lidar technology in recent years, the detection ability of lidar to measure air density, temperature, wind field and other parameters of upper atmosphere has been gradually improved, and it has been applied to the development of mesospheric model and near space environment monitoring. The detection mechanism of lidar for upper atmosphere, as well as the detection capability and station construction of lidar in China is introduced systematically, and the development trend of lidar is also briefly prospected.
Using optical Doppler effect, Doppler lidar can obtain atmospheric wind parameters with high temporal and spatial resolution by detecting the frequency drift of the emitted laser. The technology has provided a wide range of applications, such as aircraft wake detection, atmospheric turbulence detection, wind shear detection near the airport, and application in wind farms. Firstly, the basic principle of Doppler effect and wind lidar technology is introduced, and then the Doppler wind lidar is classified based on principle and practical application. Then different technologies, such as coherent detection and direct detection, continuous lidar detection and pulse lidar detection, single-edge technology and double-edge technology, FP fringe technology and Fizeau fringe technology, VAD and DBS scanning, are systematically introduced. Finally, some examples of ground-based, airborne and spaceborne wind lidar, as well as the new wind lidar technology, are briefly introduced, and the Doppler wind lidar technology is summarized and prospected.
The characteristics of light scattering by particles play an important role in various scientific fields. The early calculation of particle light scattering is mostly based on the hypothesis of spheroidal particles. However, the shape of the particles is not always theoretically spherical, and the aggregating of the particles makes the shape more complex. Lorentz-Mie scattering theory is usually used to calculate the scattering properties of spheroidal particles, but the calculation of scattering phase function of non-spherical and aggregated particles with the theory will cause larger biases. In addition, the hypothesis of spheroidal particles also fails to effectively analyze the depolarizationeffect. With the increase of computational power and the improvement of numerical methods, the scattering solutions of non-spherical particles are mostly solved by numerical methods in addition to the limited laboratory experiments. The numerical methods for scattering of non-spherical particles are reviewed, and their advantages and disadvantages are analyzed. The laboratory measurement methods of light scattering by non-spherical particles are also introduced, as well as their applications in the fields of optics, geophysics, remote sensing, astrophysics, engineering, medicine and biology.
The propagation properties of partially coherent beam can be controlled by modulating the spatial correlation structure when the beam propagates in turbulent atmosphere. Based on the basic theories of partially coherent beam with special correlation functions propagating in turbulent atmosphere, the methods of generating partially coherent beam with special correlation functions and measuring correlation functions are introduced, respectively, and thepropagation properties of the beam are outlined, which is of significance for the laser propagation in turbulent atmosphere.