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.
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.
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.
In 2013, a serial sky test has been held on 1.8 meter telescope in Yunnan observation site after 2011-2012 Laser guide star photon return test. In this test, the long-pulsed sodium laser and the launch telescope have been upgraded, a smaller and brighter beacon has been observed. During the test, a sodium column density lidar and atmospheric coherence length measurement equipment were working at the same time. The coupling efficiency test result with the sky test layout, data processing, sodium beacon spot size analysis, sodium profile data will be presented in this paper.
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