Abstract
Most of the application scenarios of techniques related to electromagnetic wave transmission of photoelectric system are located in the earth's atmosphere or through the atmospheric path. The electromagnetic wave will change state through the atmosphere when encounters the inhomogeneities of refractive index caused by turbulence. Optical turbulence causes degradation of beam quality and energy to laser transmission, and brings image deterioration to astronomical observation. The refractive index structure coefficient is an important parameter describing the turbulence strength. For visible and near infrared band, the refractive index structure coefficient mainly depends on temperature structure coefficient. Using the theory of Wheatstone bridge, the micro-thermometer is designed and self-developed. To avoid interference from human and buildings, the wireless control for the micro-thermometer is realized based on CC1100. The observation of turbulence strength of representative test point, for more than one month, is implemented at Yangmeikeng near the South China Sea. Compared to ultrasonic anemometer, there is a sensitive lower measurement limit of micro-thermometer whose effective refractive index structure coefficient of system noise is less than 10 -18 m -2/3 . There is obvious `Sombrero' structure diurnal variation of turbulence near South China Sea, whose strength is mainly brought out by buoyancy heat bubble in day, and by wind shear at night. Monin-Obukhov length is positive at night and negative in day, and the scaling exponent is near -5/3 for temperature power spectrum, which is similar to wind power spectrum except for periods when wind from inland. The diurnal variation and scaling exponent of power spectrum analysis indicate that the measurement range and the sample rate of micro-thermometer are enough to response the turbulence measurement encountered in most laser transmission and astronomical observation fields. The turbulence characteristics information gained from micro-thermometer measurement data analysis brings good reference to optimal design and operation for photoelectric system.
Highlights
Turbulence transports matter and energy between the earth’s surface and the atmosphere, which is an important reason for the change of boundary layer state [1]–[4]
Turbulence strength is driven to some extent by temperature gradients, and during crossover the gradients drop to near zero and so does Cn2.The monthly averaged temperature and wind speed all display homologous diurnal variation with the single day, so the buoyancy heat bubble from solar radiation is the main source of intensive turbulence in day is further confirmed from a statistical point of view
Most of the transmission paths of the photoelectric system pass through the atmosphere, and optical turbulence is always the inevitable factor causing degradation of transmission performance
Summary
Turbulence transports matter and energy between the earth’s surface and the atmosphere, which is an important reason for the change of boundary layer state [1]–[4]. Trinqueta et al [21] presented in situ technique to measure the microstructure of the temperature field in atmosphere, which gained the structure function of refractive index along with atmospheric pressure, temperature, humidity, and wind speed. This cross-calibrated technique gave a higher spatiotemporal resolution than before. To visible and near infrared light waves, the fluctuation of atmospheric refractive index is mainly caused by temperature fluctuation [29], the micro-thermometer based on temperature fluctuation is aimed and self-developed, observations from a field campaign over Yangmeikeng implemented for validation of the instrument in the paper.
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